Electrochemical CO2 conversion can be coupled with a photovoltaic cell and provide a pathway to utilize solar energy for the chemical synthesis. Ideally, such artificial photosynthesis system want to use CO2 and H2O as feed-stock molecules to produce value-added chemicals such as fuels or raw chemicals. My research team reported a monolithic and stand-alone device composed of a photovoltaic cell module, an Au CO2 reduction, a cobalt oxide anode accomplishing over 4 % conversion efficiency for CO2 conversion to CO production. To improve the solar to chemical conversion efficiency and to increase the feasibility further, we have developed efficient electrocatalysts and replaced the photovoltaic cell with Si modules, achieving ~ 8% of solar-to-CO conversion efficiency.

In addition, in this talk, metal-based electrocatalysts interacting with p-block elements or surface mediated molecules will be discussed for selective CO or C2+ (i.e. ethylene) production from CO2 reduction. The experimental results and theoretical simulation with various different types of metal catalysts (Ag, Zn, and Cu) give insights how to suppress the hydrogen evolution reaction (HER) is crucial to achieve efficient CO2 reduction catalysts. Monodispersed Ag nanoparticles are suggested to have the special interaction between the surface Ag and the surface mediated molecules which can modify the local electronic structure favoring for the selective CO production (up to 95 % of Faradaic efficiency). In addition, in the case of selective ethylene production, special Cu nanostructure formed by in-situ electrochemical fragmentation is demonstrated to be effective for increasing C-C bond coupling (up to 73 % of Faradaic efficiency) and selective ethylene production (up to ~ 60 % of Faradaic efficiency). In-situ X-ray absorption spectroscopy (XAS) studies are performed to understand the catalyst activity. Our series of studies suggests the modification of the metal nanoparticle surface by oxygen atom or surface mediated molecules can be effective strategies to increase CO2 reduction reaction activity and stability.

Chemically synthesized quantum dots (QDs) can potentially enable new classes of highly flexible, spectrally tunable lasers processible from solutions [1,2]. Despite a considerable progress over the past years, colloidal-QD lasing, however, is still at the laboratory stage and an important challenge - realization of lasing with electrical injection - is still unresolved. A major complication, which hinders the progress in this field, is fast nonradiative Auger recombination of gain-active multicarrier species such as trions (charged excitons) and biexcitons [3,4]. Recently, we explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of core/multi-shell QDs with a radially graded composition that allow for considerable (nearly complete) suppression of Auger recombination by “softening” the electron and hole confinement potentials [5]. Using these specially engineered QDs, we have been able to realize optical gain with direct-current electrical pumping [6], which has been a long-standing goal in the field of colloidal nanostructures. Further, we apply these dots to practically demonstrated the viability of a “zero-threshold-optical-gain” concept using not neutral but negatively charged particles wherein the pre-existing electrons block either partially or completely ground-state absorption [7]. Such charged QDs are optical-gain-ready without excitation and, in principle, can exhibit lasing at vanishingly small pump levels. All of these exciting recent developments demonstrate a considerable promise of colloidal nanomaterials for implementing solution-processible optically and electrically pumped laser devices operating across a wide range of wavelengths and fabricated on virtually any substrate using a variety of optical-cavity designs.

[1] Klimov, V. I.et al., Optical gain and stimulated emission in nanocrystal quantum dots. Science290, 314 (2000).

[2] Klimov, V. I.et al., Single-exciton optical gain in semiconductor nanocrystals. Nature447, 441 (2007).

[3] Klimov, V. I. et al., Quantization of multiparticle Auger rates in semiconductor quantum dots. Science287, 1011 (2000).

[4] Robel, I., et al., Universal Size-Dependent Trend in Auger Recombination in Direct-Gap and Indirect-Gap Semiconductor Nanocrystals. Phys. Rev. Lett.102, 177404 (2009).

[5] Y.-S. Park, et al., Effect of Interfacial Alloying versus “Volume Scaling” on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots. Nano Lett. 17, 5607 (2017).

[6] Lim, J., et al., Optical Gain in Colloidal Quantum Dots Achieved by Direct-Current Charge Injection. Nat. Mater.17, 42 (2018).

[7] Wu, K., et al., Towards zero-threshold optical gain using charged semiconductor quantum dots. Nat. Nanotechnol.12, 1140 (2017).

Colloidal semiconductor nanocrystals have gained interest since their optical and electronic properties can be tuned by varying their shape, size and composition. Recently, 2D square and honeycomb superlattice of lead- and cadmium-chalcogenide quantum dots (QDs) have been prepared. These superstructures are formed by assembling PbSe nanocrystals in a monolayer at the toluene suspension air/interface after which the nanocrystals attach via their four vertical {100} facets [1],[2]. Afterward, cation exchange transforms PbSe into zinc blend CdSe. Theoretical studies show that these 2-D systems have distinct band structures compared to continuous nanosheets, with the appearance of Dirac cones in the case of the honeycomb [3]. Strong electronic coupling via the atomic connections of the QDs in the superstructure may result in a higher mobility compared to the self-assembled lead chalcogenide QDs that are less strongly coupled due to the (in) organic ligands [4].  

In our research, we use electrolyte-gated transistors to study the optoelectronic properties and transport characteristics of 2-D PbSe and CdSe superstructures [5]. The potential of the gate electrode determines the Fermi level with respect to the conduction band (CB) or valence band (VB) of the superstructure. First, to monitor the stability of the superlattice under electron injection we measure the differential capacitance as a function of gate voltage. Second, the conductivity of the network is measured as a function of the Fermi level position. To quantify band occupation into the superlattice, the optical absorption quenching employed. Finally, the mobility of the system is calculated from conductivity and charge density.

We reported the first study of electron transport in a 2-D PbSe system with a square geometry in which band occupation is assured by the electron density of 8 electrons per nanocrystal . The electron mobility between 5 and 18 cm²/Vs is observed for these supersructures [6].­­

In our recent work, we study the electron transport of CdSe superlattices with square and honeycomb geometry. The band occupation is assured by the number of 2 electrons per nanocrystal. The electron mobility of 1 and 10 cm²/Vs is achieved for square and honeycomb geometry respectively.   

1) W.H. Evers et al., Nano Lett., 13, (2013).

2) M.P. Boneschanscher et al., Science, (2014).

3) E. Kalesaki et al., Phys. Rev. B 88, (2013).

4) W.H. Evers et al., Nature Communications 6, (2015).

5) D. Vanmaekelbergh et al., Electrochemica Acta, 53, (2007).

6) M. Alimoradi Jazi et al., Nano Lett., 17, (2017)

Nanocrystal building blocks can be assembled to make an artificial, nanocrystal solid. The choice of building block and the way they are assembled set up pathways to make new and unique materials with tailored properties. A case in point are superlattices of semiconductor nanocrystals or quantum dots (QDs), which find applications in, e.g., photodetectors, solar cells and field-effect transistors. Quantum dots offer the appealing combination of a tunable band gap, a high absorption coefficients, and a suitability for solution-based processing. QD films are typically produced through, e.g., spincoating, dropcasting or spraycoating. This results in disordered nanocrystal stacks, where poor electronic transport can be caused by excessive surface defects or restricted dot-to-dot hopping. To disentangle such effects, we analyzed the delocalization and transport of charge carriers in 2D superlattices of epitaxially connected QDs. In the case of PbS and PbSe QDs, such superlattices can be formed over several square micrometer. Using elemental analysis and structural analysis by in-situ XRF and GISAXS, respectively, we show that such lattices keep their structural integrity in a wide temperature window, ranging up to 310 ºC and more; an ideal starting point to assess the effect of gentle thermal annealing on the superlattice properties. We find that annealing such superlattices at temperatures ranging from 75-150 ºC induces a marked redshift of the QD band-edge transition. In fact, the band-edge found after annealing agrees, opposite from state-of-the-art literature, with theoretical predictions on charge carrier delocalization in such epitaxially connected superlattices. In addition, we observe a 1000-fold increases of the charge carrier mobility after mild annealing. While the superstructure remains intact at these temperatures, an XRD rocking curve analysis indicates that annealing markedly decreases the density of grain boundaries. This indicates that the presumably epitaxial connections between QDs in as-synthesized superlattices still form a major source of grain boundaries and defects, to an extend that carrier delocalization over multiple QDs is prevented and dot-to-dot transport remains strongly restricted.

Cadmium chalcogenide (CdX, X=Se, S, Te) colloidal quantum wells or nanoplatelet (NPLs) have atomically precise thickness of a few CdX layers (1-2 nm) and uniform exciton confinement energy across lateral dimensions of 10s of nanometer and larger. This unique property has led to speculation of coherent delocalization of the exciton center-of-mass over the entire NPL, which would have profound effect on fundamental properties of excitons (including its oscillator, transport mechanism and Auger annihilation) and their applications (such as lasing threshold). In this talk, I will summarize a series of recent studies on fundamental exciton properties in 2D NPLs. We show that at room temperature, exciton transport can be described by 2D diffusion with diffusion constants near the bulk crystal values and hot exciton diffusion completes with exciton relaxation. In CdSe NPLs, the biexciton Auger recombination lifetime does not depend linear on its volume, deviating from the “Universal Volume” scaling law that has been reported for 0D quantum dots. Instead, the Auger lifetime scales linearly with the lateral size because of the size dependent collision frequency, and the Auger lifetime depends sensitively (nonlinearly) on the NPL thickness due to change in the degree of quantum confinement. We suggest Auger lifetime in for other 2D NPLs and 1D nanorods can also be expected to deviate from the volume scaling law because of the different dependences on the quantum confined and non-confined dimensions. We have developed an optical gain model that accounts for the nature of 1D confinement in NPLs and revealed the origin of low gain thresholds in these materials. Finally, we will also discuss conditions under which coherent delocalization of excitons and the Giant Oscillator Strength Transition (GOST) effect can be observed.

Hybrid nanoparticles (HNPs) combine disparate materials onto a single nanosystem thus providing a powerful approach for bottom-up design of novel architectures. Beyond the fundamental development in synthesis, the interest in HNPs arises from their combined and often synergetic properties exceeding the functionality of the individual components. These ideas are well demonstrated in hybrid semiconductor-metal nanoparticles, which are the focus of this talk. The synergistic optical and chemical properties of hybrid nanoparticles resulting in light-induced charge separation and charge transfer, allow photocatalytic activity which can promote surface chemistry redox reactions, and open a pathway for converting solar energy to chemical energy stored in a fuel. An additional area of interest is in use of the HNPs for light-induced generation of radicals opening options for light-induced on-demand radicals formation.

We will report on the effects of the surface coating and the co-catalyst metal size on the photocatalytic function of metal tipped semiconductor nanorods as a model hybrid nanoparticle system. Both tested parameters were found to influence the photocatalytic efficiency and charge transfer dynamics. The work combines advances in synthesis of well-controlled hybrid nanoparticles, hydrogen evolution efficiency measurements, steady state and time resolved emission measurements, as well as ultrafast transient absorption measurements to gain a complete view on the effects of these parameters on photocatalysis with metal tipped semiconductor nanorods. A model was devised to capture the essential effects of the size of the metal tip on the photoctalytic efficiency. An additional effect concerns the HNPs functionality under high excitation fluence in the regime of multiexcitons. The understanding of the effects of the hybrid nanosystems properties on the photocatalytic processes contribute to the rational design of hybrid nanostructures in photocatalytic applications. Moreover, use of the HNPs in generation of reactive hydrogen species and its application for controlling enzymatic activity and additional processes will also be highlighted.

A considerable portion of photonics based research to enhance thin film solar cell performance considers the incorporation of plasmonic nano-particles to enhance light absorption and consequently increase the short circuit current. This approach, which has produced some interesting results, suffers from, essentially, two shortcomings: It has a positive effect in only one of the three photovoltaic parameters that determine the power conversion efficiency (PEC), while it may also have a negative impact on the electronic performance of the device. In the work we present, we will discuss several novel photonic approaches, having a minimal impact on electronic aspects of the solar cell not directly linked to light absorption or emission, capable of enhancing either the short circuit current or open circuit voltage for organic and perovskite solar cells.

In one configuration the standard ITO based transparent electrode is replaced by a 1-dimensional multilayer containing, at least, a TiO2 and a Ag layers [1]. This combination forms with the back metal electrode in thin film cells two coupled optical cavities with a resonance degeneracy which can be broken to produce a broadband light trapping. When applied to non-fullerene acceptor based single junction polymer cells we show that PECs close to 14% can be reached.

In a perovskite cell, by naturally transferring the random nano-texturing inherent of the perovskite layer to the back semiconductor/metal interface, where the contrast in the imaginary part of the refractive index is very large, we demonstrate that backscattering reduces light escape leading to an optimal light absorption bringing the PCE from 19.3% to 19.8%. Such path towards an ergodic behavior for maximum light absorption in perovskite cells may lead to the most effective light absorption in such cells [2].

To bring perovskite solar cells towards the Shockley-Queisser limit requires lowering the bandgap while simultaneously increasing the open circuit voltage. This, to some extent divergent objective, may demand the use of large cations to obtain a perovskite with larger lattice parameter together with a large crystal size to minimize interface non-radiative recombination. We successfully incorporated such large cations in larger than 1 μm perovskite crystals and fabricated cells that exhibited a largely increased fluorescence quantum yield and an open circuit voltage equivalent to 93% of the corresponding radiative limit one.

1 Quan Liu et al., Adv. Energy Mater. 7, Art. No. 1700356 (2017).

2 Hui Zhang et al., ACS Photonics (2018).

Organic semiconductors based on conjugated polymers and fullerene acceptors are a large class of materials that have been broadly explored for various applications that allow printed, flexible, stretchable, and large-area electronics like organic field-effect transistors (OFETs) and bulk-heterojunction organic photovoltaics (OPVs). The commercialization of large area photovoltaic devices relies on the capability of coating thick active layers in ambient condition without losing the efficiency. After surpassing the 13% threshold, OPVs becomes a promising approach in the field of thin film photovoltaic technology  but it is limited to thin film only. Till now the performance of scalable printed solar cells is quite low as compared to spin coated devices. In OPVs the photoactive layer, which consists of donor and acceptor materials plays an important role. Initially, the focus of OPV research was limited to fullerene based acceptor PC61BM and PC71BM ([6,6]-phenyl C61-/C71-butyric acid methyl ester). The fullerene based acceptors have some limitations to achieve high efficiency like low absorption coefficient and narrow visible absorption window limiting the light-to-current generation, thus inhibiting the improvement of device performance . From the last few years, non - fullerene acceptors (NFAs) have emerged as a new concept to overcome the limitations associated with fullerene based acceptors.

In this work, we used blend of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl) benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2 carboxylate-2-6-diyl)] (PBDTTT-EFT, or more commonly PCE10) : PC71BM (fullerene based) and PBDTTT-EFT: EH-IDTBR (NFA) as an active layer material. We used the wire-bar (WB) coating as well as spin coating (SC) to deposit the active layer of PCE10:PC71BM and PCE10: EH-IDTBR . Thicker active layer (>100 nm) devices based on WB coated shows good performance as compared to devices based on thicker active layer coated by SC. Comparison of the film properties as well as device performance has been carried out when we change the process form lab to scalable (SC to WB) technique. Solar cell (ITO/ZnO/PCE10:PC71BM/MoO3/Ag) based on WB coated PCE10:PC71BM results the PCE of 10.22 % comparable to the device based on SC PCE10:PC71BM with PCE of 10.10%. Devices based on NFAs (ITO/ZnO/PCE10: EH-IDTBR/MoO3/Ag) shows comparatively good performance with PCE of 10.77% for WB coated and PCE of 10.60% for SC devices. Reported WB coating technique approximating scalable fabrication methods and hold great promise for the development of low-cost and high-efficiency OSCs by high-throughput production.

Halide Perovskites may be mostly normal (inorganic) semiconductors and, yes, we should be careful to describe them with concepts from organic electronics. HOWEVER, it is remarkable that a material with over-all high quality optoelectronic properties can result from fast, low temperature, solution preparation. Understanding the reason(s) behind this, may help answer the question if this is because Pb is so unique or if we can generalize to find other materials like these. I will consider apparent inconsistencies in what we think we know about their defects, to zoom in on what remains special, and maybe even unique.

work done with Gary Hodes (Weizmann) and many others

Halide Perovskites may be mostly normal (inorganic) semiconductors and, yes, we should be careful to describe them with concepts from organic electronics. HOWEVER, it is remarkable that a material with over-all high quality optoelectronic properties can result from fast, low temperature, solution preparation. Understanding the reason(s) behind this, may help answer the question if this is because Pb is so unique or if we can generalize to find other materials like these. I will consider apparent inconsistencies in what we think we know about their defects, to zoom in on what remains special, and maybe even unique.

Impressive photovoltaic conversion efficiencies have been achieved within a very short time span with solar cells based on metal halide perovskites. Such a development is especially remarkable given the low temperature preparation methods used for these materials.¹ It is generally accepted that defects must be “benign” and thus allow for high-quality devices despite low temperature synthesis. Relatedly, doping in halide perovskites has been achieved by intrinsic defects, however with very limited control over doping levels and spatial profiles.

We will discuss how the soft nature of halide perovskites contributes to the unusual optoelectronic properties.² For doping via intrinsic defects, we developed a procedure to control and determine the exact stoichiometry of halide perovskite films from infrared spectroscopy, which allows us to quantify the MA content in the films. We present the first perovskite-perovskite homojunction obtained by vacuum deposition of stoichiometrically-tuned methylammonium lead iodide films and devices based thereon.³ We analyzed the resulting thin film junctions by cross-sectional scanning Kelvin probe microscopy, and found a pronounced contact potential difference at the interface between the two differently doped perovskite layers.

[1] David Egger, Achintya Bera, David Cahen, Gary Hodes, Thomas Kirchartz, Leeor Kronik, Robert Lovrincic, Andrew Rappe, David Reichman, and Omer Yaffe. What Remains Unexplained about the Properties of Halide Perovskites? Advanced Materials, 30 (20):800691, 2018.

[2] Michael Sendner, Pabitra K. Nayak, David A. Egger, Sebastian Beck, Christian Müller, Bernd Epding, Wolfgang Kowalsky, Leeor Kronik, Henry J. Snaith, Annemarie Pucci, and Robert Lovrincic. Optical Phonons in Methylammonium Lead Halide Perovskites and Implications for Charge Transport. Materials Horizons, 3:613–620, 2016.

[3] Benedikt Dänekamp, Christian Müller, Michael Sendner, Pablo P. Boix, Michele Sessolo, Robert Lovrincic, and Henk Bolink. Perovskite-Perovskite homojunctions via Compositional Doping. The Journal of Physical Chemistry Letters, 9:2770, 2018.

Perovskite cells pose an intriguing modelling challenge as the electrical cell properties are governed by both electronic and ionic charge transport and the optical cell properties need to be carefully optimized when seeking record efficiencies in tandem cell configurations with silicon wafer cells. In this contribution, we give an update on recent advances in both electrical and optical modeling and discuss experimental results.

Negative capacitance and inductive loops in impedance spectroscopy in perovskite solar cells have been described in several recent reports, though their origin remained unclear so far. The negative capacitance and inductive loop may be related to one another as they appear in the same samples but at different applied biases. Similarly, we have demonstrated that ion migration is present even in high-efficiency low-hysteresis perovskite cells [1]. We shed light on the likely physical mechanisms behind these observations and compare devices in the frequency domain at different applied bias by employing a mixed electronic-ionic device model that naturally produces inductive loops and negative capacitance allowing us to study correlations with relevant material parameters.

Moreover we present an optical model implemented in the software SETFOS 4.6 [2] for simulating perovskite/silicon monolithic tandem solar cells that exploit light scattering structures [3]. We validate the model with experimental data of tandem solar cells that either use front- or rear-side textures. The software is used to investigate the potential of different monolithic tandem structures. The p-i-n solar cell architecture is the most promising with respect to achievable photocurrent for both flat and textured wafers. Finally, cesium-formamidinium-based perovskite materials with several bandgaps were synthetized, optically characterized [4] and their potential in tandem devices was quantified by simulations. The most promising tandem has a potential of reaching a power conversion efficiency of 31% [5].

[1] M. Neukom, S. Züfle, E. Knapp, M. Makha, R. Hany, B. Ruhstaller, Solar En. Mat. & Solar Cells, 169 159ff (2017)

[2] T. Lanz, B. Ruhstaller, C. Battaglia, and C. Ballif, J. Appl. Phys. 110, 33111 (2011) and SETFOS 4.6 by Fluxim AG, https://www.fluxim.com, Switzerland

[3] S. Altazin, L. Stepanova, K. Lapagna, P. Losio, J. Werner, B. Niesen, A. Dabirian, M. Morales-Masis, S. de Wolf, C. Ballif, B. Ruhstaller, Proc. 32nd Eur. Photovolt. Sol. Energy Conf. 1276 (2016)

[4] J. Werner et al., ACS Energy Lett. 3, 742–747 (2018)

[5] S. Altazin, L. Stepanova, J. Werner, B. Niesen, C. Ballif, and B. Ruhstaller, Optics Express 26 (10), A579 (2018)

Metal halide perovskites are mixed ionic-electronic conductors extremely efficient for making solar cells, due to its strong absorption in the visible and their relatively slow recombination. Processes like transport, recombination, charge accumulation, hysteresis, etc. occur at very different time scales and determine the photovoltaic performance of the solar cell. Small-perturbation, frequency-modulated optoelectronic techniques such as impedance spectroscopy (IS) or intensity-modulated photocurrent spectroscopies (IMPS/IMVS) are especially suited to detect, deconvolute and quantify all these processes.

Nowadays, a full understanding of the typical features observed in the IS and IMPS/IMVS spectra in perovskite solar cells is still missing. Hence, it is not clear yet how properties like the magnitude, locus and the nature of the recombination loss can be identified or quantified from the analysis of the data. Besides, low frequency signals, associated to hysteresis phenomena should also be unambiguously assessed in the spectra. In this talk I discuss the interpretation of the spectra and the use of simple models to rationalize the small-perturbation response of the device and its impact on efficiency-determining dynamic processes.

The need for better energy storage and conversion devices has never been so urgent in order to enable the rapid deployment of renewable energies and reduce our use of fossil fuels. While batteries spurred the spread of portable electronics, their limited energy density hampers their use for large scale applications. Hydrogen has long been envisioned as a viable energy carrier owing to its very large energy density, nevertheless its electrochemical production by electrolysis, the most efficient carbon-free production route at large scale, greatly suffers from the slow kinetics associated with the oxygen evolution reaction (OER). The key challenges that need to be addressed to improve the OER kinetics are well-spotted and researchers eagerly pushed to better understand the reaction leading to numerous progresses since our early vision of this reaction. Hence, while some works were devoted to finding physical descriptors capable of describing the OER activity following a Sabatier principle, recent developments in the field point towards the complexity of such reaction. Indeed, a substantial body of evidence now points towards the involvement of the bulk chemistry of the most active transition metal oxides in the OER mechanism. Hence, we recently found that bulk oxygen atoms are evolved under OER conditions for cobalt-based perovskites materials,¹ eventually triggering a new mechanism into which chemical steps and proton exchange are rate limiting. We could then demonstrate that the origin for the often observed activity-stability relationship for OER electrocatalysts is nested into the existence of a common intermediate which is reactive surface oxygen in the form of oxyl-group.² Overall, the line becomes blurrier between heterogeneous and homogeneous catalysis when using transition metal oxides as OER catalysts for which complex surface dynamics are at play.³ Hence, efforts must be paid to understanding and stabilizing these intermediates in order to break this relationship and further enhance the stability of OER electrocatalysts. We will therefore discuss in this talk our recent efforts at designing chemical approach to counterbalance the chemical reactivity of the most active transition metal oxides through a combined crystallographic and electrolyte engineering approach.

The increasing replacement of fossil fuels by renewable energies from wind and sun requires large energy storage capabilities, because of the only intermittent availability of these sources. Such big capacities can only be provided by the storage in chemical bonds, the simplest being the hydrogen molecule formed by direct electrochemical water splitting. For this purpose, expensive and rare electrocatalysts made from Platinum series metals will have to be replaced by cheaper, more abundant and hazard-free materials. Nickel metal and its oxides are known since many years for their good activity for water splitting.

In our studies Nickeloxide nanoparticles and thin films are prepared by electrochemical deposition as well as by magnetron sputtering. The composition is optimised towards the achievement of an optimum activity for the hydrogen evolution reaction (HER) as well as the oxygen evolution reaction (OER). In order to gain a deeper understanding of the critical parameters for the catalytic activity during the electrochemical reaction, we investigate the chemical composition by XPS and SEM before and after electrochemical testing as well as by in-operando Raman-spectroscopy. The achieved activities are comparable to Platinum for the HER and to RuO2 for the OER. They depend strongly on the chemical composition of the catalyst, which in turn is heavily influenced by the chosen preparation method. Furthermore, we investigate the stability of our catalysts in relation to their activity and composition. The highly active Nickel (oxide/hydroxide) mixture for the HER degrades over time by the complete transformation to the less active pure Nickel-dihydroxide compound. For the OER the pre-treatment of the Nickel compound is of extreme importance in order to form a large amount of the catalytically active NiO(OH) species on the surface during the electrochemical reaction. These nanoparticles show a nearly constant activity for a testing period of 26 hours at a current density of 10 mA/cm2.

The study of electrocatalysts for water splitting is pivotal to improving the efficiency of fuel production. This is an attractive field since electrocatalysts can be used: (1) as part of a photoelectrode, for direct sunlight to fuel transformation, or (2) can be used in an electrolyser to transform the electricity generated through a solar panel to produce a fuel. In a complete system it is thought that water oxidation is the limiting process. Therefore, a lot of effort is devoted to finding a better catalyst for this reaction in order to enhance the final performance of the system.

Recently, iron and nickel oxyhydroxides have been appointed as good candidates for the water oxidation reaction in basic conditions. In this context, interest for understanding the origin of their behaviour has grown in recent years. In 2014, Boettcher and co-workers published for the first time that the high activity towards water oxidation of NiOOH was due to Fe incorporation from the electrolyte.[1] Since then, there has been a lot of effort to determine which species are involved in the catalytic reaction, with no consensus reached. Some experiments suggest the presence of Fe(IV) during the catalysis,[2] while on the contrary, other works cannot detect such iron species and suggest that nickel centres are the active sites.[3] Despite this debate, some structural differences have been found due to iron incorporation which leads to an improved performance. However, the mechanistic and kinetic analysis of metal oxide based electrocatalysts, in general, is hampered by the non-ideal nature of these materials. Most of these electrocatalysts present different redox states and a non-planar and dense structure. Thus the interpretation of the traditional electrochemical techniques, such as tafel plots, is more complicated and sometimes not possible.[4]

In this work, we used spectroelectrochemical methods to analyse three different samples: Pure FeOOH, a mixed FeOOHNiOOH sample composed of different layers, and finally Ni(Fe)OOH with spontaneous Fe incorporation. From these measurements, we are able to determine the active species and study the kinetics under catalytic conditions to yield a basic mechanistic picture.

References

[1]          L. Trotochaud, et al. JACS, 2014, 136, 6744-6753.

[2]          J. Y. C. Chen, et al. JACS, 2015, 137, 15090-15093.

[3]          M. Görlin, et at, JACS, 2017, 139, 2070-2082.

[4]          E. Pastor, et al, Nat. Commun., 2017, 8, 14280.

The dynamics of photoluminescence (PL) from nanocrystal quantum dots (QDs) is significantly affected by reversible trapping of photo-excited charge carriers. This process occurs after up to 50% of the absorption events, depending on the type of QD considered, and can extend the time between photo-excitation and relaxation of the QD by orders of magnitude. Although many opto-electronic applications require QDs assembled into a QD solid, until now reversible trapping has been studied only in (ensembles of) spatially separated QDs. Here, we study the influence of reversible trapping on the excited-state dynamics of CdSe/CdS core/shell QDs when they are assembled into close-packed “supraparticles”. Time- and spectrally resolved PL measurements reveal competition between spontaneous emission, reversible charge carrier trapping, and Förster resonance energy transfer between the QDs. While Förster transfer causes the PL to redshift over the first 20–50 ns after excitation, reversible trapping stops and even reverses this trend at later times. We can model this behavior with a simple kinetic Monte Carlo simulation by considering that charge carrier trapping leaves the QDs in a state with zero oscillator strength from which no energy transfer can occur. Our results highlight that reversible trapping significantly affects the energy and charge carrier dynamics for applications where QDs are assembled into a QD solid.

Hot carriers refer to electrons (holes) that are formed from the thermalization of non-equilibrium photoexcited carrier populations following the absorption of above-bandgap photons. These hot carriers (HCs) later equilibrate within few picoseconds with the semiconductor lattice through carrier cooling processes such as carrier phonon scattering, Auger process, etc. The mechanisms and dynamics of HC cooling in semiconductors are of fundamental importance for enhancing device functionalities. Colloidal lead halide perovskite nanocrystals recently emerged as promising candidate materials for many optoelectronic applications. Building efficient and long-lasting devices from perovskite nanocrystals however remains a challenge. In this work, we study the size and temperature dependent cooling process of HC in CsPbBr₃ nanocrystals using time-domain density functional theory. We provide detailed insights into the mechanism of cooling by analyzing the effect of the passivating ligands (alkyl ammonium) in this process. We demonstrate that the inorganic lattice plays a much larger role in mediating the thermalization to the band edge.

Colloidal semiconductor nanocrystals, specifically quantum dots (QDs), are of interest to numerous scientific disciplines due to their highly tunable optical and electronic properties. For many years, various chemical treatments have been developed to fabricate conductive QD arrays for use in solar cells.  While such investigations have lead to increased solar cell performance there is much about how the chemical treatments modify the optical and electrical properties that are not well understood.  We have developed a simple, robust, and scalable solution-phase X-type ligand exchange method for PbS QDs that replaces native surface ligands with functionalized cinnamate ligands, yielding highly tunable, well-defined organic/inorganic hybrid chemical systems. We explore a library of functionalized cinnamic acid molecules to systematically tune PbS QD surface chemistry, and find that thin films of fully ligand exchanged QDs exhibit remarkable band edge shifts: the band edge position of QDs can be tuned over 2.0 eV.  We have also developed simple methods to impurity dope PbS and PbSe QDs with both n and p-type dopants.  We show how the dopants are incorporated and result in doped QDs with well behaved properties.   

Delayed exciton emission has been extensively studied in several types of low dimensional semiconductors but is still not fully understood. The photon energy of delayed emission is identical to that of the spontaneous exciton emission, but it occurs on an extended time scale, from the life time for spontaneous emission (typical 20 ns) to the microsecond and even millisecond regime. In a heuristic way, we might say that the exciton is stored as non-radiative state in the nanocrystal for a variable time without (much) energy loss. The stored energy finally returns to the radiative state and emission can occur.

I will review our work on the characterization of delayed emission of low dimensional CdSe systems (core-shell quantum dots, rods, and platelets) which was performed on the ensemble level as well as on the single-dot level. I will also relate the phenomenon of delayed emission to another extensively studied phenomenon in these systems, namely blinking. In another contribution from our group the interplay between spontaneous emission, delayed emission and energy transfer as observed in QD supraparticles will be presented.

Reduced Auger Recombination in Single CdSe/CdS Nanorods by One-Dimensional Electron Delocalization." Nano Letters 13(10): 4884-4892.

Delayed Exciton Emission and Its Relation to Blinking in CdSe Quantum Dots." Nano Letters 15(11): 7718-7725.

Dynamics of Intraband and Interband Auger Processes in Colloidal Core-Shell Quantum Dots." Acs Nano 9(10): 10366-10376.

Temporary Charge Carrier Separation Dominates the Photoluminescence Decay Dynamics of Colloidal CdSe Nanoplatelets." Nano Letters 16(3): 2047-2053.

Composite Supraparticles with Tunable Light Emission." Acs Nano 11(9): 9136-9142.

Generating multiple excitons by a single high-energy photon is a promising third generation solar energy conversion strategy.  I will discuss recent progress within the Center for Advanced Solar Photophysics (CASP) on both improving the multple exciton generation (MEG) efficiency in heterostructured nanostructures as well as in the using MEG in the production of solar fuels.  We are exploring MEG in PbE|CdE (E = S, Se) Janus-like hetero-nanostructures and find that MEG is enhanced over that of single-component and core/shell nanocrystal architectures. The enhanced MEG arrises due to the asymmetric nature of the hetero-nanostructure that results in an increase in the effective Coulomb interaction that drives MEG and a reduction of the competing hot exciton cooling rate. We find that slowed cooling occurs through effective trapping of hot-holes by a manifold of valence band interfacial states having character of both PbS and CdS. The Janus-like NCs retain their symmetric structure and thus can be easily incorporated as the main absorber layer in functional solid-state solar cell architectures. Finally, based upon our analysis, we provide design rules for the next generation of engineered nanocrystals to further improve the MEG characteristics. In addition, we have developed a PbS QD photoelectrochemical cell that is able to drive a hydrogen evolution reaction with a peak external quantum efficiency (EQE) of over 100%, with the highest EQE at 114±1.3%.  Our results show that the extra carriers produced via MEG can be used to drive a chemical reaction with above unity quantum efficiency thus demonstrating a new direction in exploring high efficiency approaches for solar fuels. I will discuss the potential for using MEG to drive a photochemical reaction as opposed to use in a solar cell. 

Ternary CuInX₂ (X= S, Se) nanocrystals (NCs) have attracted increasing attention as promising alternatives for CdX and PbX NCs due to their low toxicity, large absorption cross-sections across a broad spectral range, and unparalleled photoluminescence tunability, spanning a spectral window that extends from the green to the NIR (~550 to ~1100 nm for X= S). To achieve properties that are inaccessible to single component NCs (such as high PL quantum yields, spatial charge carrier separation, etc.), researchers have been synthesizing colloidal CuInX₂-based hetero-NCs (HNCs) (e.g., CuInS₂/ZnS concentric core/shell HNCs, CuInSe₂/CuInS₂ dot-in-rod HNCs). Anisotropic CuInX₂-based HNCs are particularly interesting, since they are expected to exhibit novel properties, such as polarized NIR photoluminescence (PL) and spatial charge separation, which are attractive for many applications (e.g., polarized LEDs, photocatalysis and artificial photosynthesis, luminescent solar concentrators, solar cells). Nevertheless, reports on the synthesis of anisotropic CuInX₂-based HNCs are scarce. 

In this work, we report a novel two-step pathway that yields CuInS₂/ZnS dot core/ rod shell heteronanorods. The wurtzite CuInS₂ NCs used as seeds are obtained by cation exchange in template Cu_2-xS NCs. The CuInS₂ NC seeds are injected together with the S precursor into a hot solution of the Zn precursor and suitable coordinating ligands, which leads to heteroepitaxial growth of ZnS primarily on the cation-rich polar facet of the seeds, as demonstrated by high-angle annular dark-field scanning transmission electron microscopy and electron tomography. The colloidal wurtzite CuInS₂/ZnS dot-in-rod heteronanorods have large molar extinction coefficients, and photoluminescence in the NIR (~800 nm) with PLQYs ~20%. Moreover, they exhibit multi-exponential PL decay that is initially rather fast (a few ns), and then slows down to several hundreds of ns, similar to the behavior previously reported for both chalcopyrite and wurtzite isotropic CuInS₂/ZnS core/shell HNC, which has been attributed to radiative recombination of a conduction band electron with a hole localized at a Cu ion. The slow radiative recombination dynamics are potentially beneficial for photovoltaic and photocatalytic applications, since long carrier lifetimes are of great importance for effectively extracting charge carriers. 

research results will be presented on nanomaterials as selective emitters and for near field radiative transfer for thermophotovoltaics and on tailored photonic glasses as non-iridescent structural colors.

of thermal radiation is a fundamental physical process defined by the dielectric properties of the thermally excited materials. Radiation into far field is described by Planck’s law and is limited by the blackbody emission. In near field, additional thermal energy transfer can be achieved due to evanescent fields, which are orders of magnitude larger than in far field. In the far field, emission, e.g. of long wavelengths below the energy of a semiconductor receiver band gap, can be suppressed in band edge emitters from nanostructured hyperbolic optical metamaterials as well as with resonantly coupled dielectric particle layers on top of plasmonic substrates. We demonstrate selective band edge emitters for thermophotovoltaic devices stable up to 1400°C based on W-HfO2 refractive metamaterials as well as ZrO2 based ceramic particles on tungsten. We further report on ceramic photonic structures as high-temperature compatible structural colors. A careful choice of the interplay between lattice and motif parameters of the photonic glass allows for structural colors with strong color saturation.

References

Shang, G.; Maiwald, L.; Renner, H.; Jalas, D.; Dosta, M.; Heinrich, S.; Petrov, A.; & Eich, M.; Photonic glass for high contrast structural color, Scientific Reports, 8, 7804 (2018)

Dyachenko, P.N.; Molesky, S.; Petrov, A.Y.; Stormer, M.; Krekeler, T.; Lang, S.; Ritter, M.; Jacob, Z.; and Eich, M.; Controlling thermal emission with refractory epsilon-near-zero metamaterials via topological transitions, Nature Communications, vol. 7, no. 11809, pp. 1–8, June 2016

Lang, S.; Sharma, G.; Molesky, S.; Kränzien, P.U.; Jalas, T.; Jacob, Z.; Petrov, A.Y.; and Eich, M.; Dynamic measurement of near-field radiative heat transfer, Scientific Reports, vol. 7, no. 1, p. 13916–13916, October 2017

Leib, E.W.; Pasquarelli, R.M.; do Rosario, J.J.; Dyachenko, P.N.; Doring, S.; Puchert, A.; Petrov, A.Y.; Eich, M.; Schneider, G.A.; Janssen, R.; Weller, H.; and Vossmeyer, T.; Yttria-stabilized zirconia microspheres: novel building blocks for high-temperature photonics, Journal of Materials Chemistry C, vol. 4, no. 1, pp. 62–74, January 2016

More than 30% of the production cost of commercial solar modules can be attributed to the cost of silicon and wafering. As a result, silicon wafers have reduced their thickness from 350 µm down to 180 µm. However, current sawing technology is reaching its inherent limits due to reduced yield and excessive kerf losses. Here we report on a method that can produce multiple free-standing ultra-thin (<20 µm) silicon layers from one silicon wafer and in a single technological process. This method, that we call “Silicon Millefeuille”, is based on the reorganization of close-packed arrays of pores under a high-temperature annealing in Ar:H ambient. The transformation process takes place in solid phase by surface diffusion, what results in the formation of high-quality monocrystalline thin layers that can later be peeled off into independent ultra-thin substrates. Pores are produced by electrochemical dissolution of n-type silicon in HF solution under back-side illumination. This allows to introduce an in-depth periodic modulation of the pore diameter that defines the number and thickness of the layers produced after the annealing. Furthermore, we show that the precise control of the initial in-depth pore profile has a profound impact on the pore reorganization dynamics, allowing to control the morphology of the thin layers obtained through annealing.

Photovoltaic (PV) cells are efficient heat engines, converting incident photon energy to electrical energy. In the case of solar PV, the cell receives radiation from the Sun – which can be approximated as a black body at 5500^oC – and converts part of the received radiation to electricity. In an ideal solar PV cell, the maximum energy conversion efficiency is ~33.5%, the famous Shockley-Queisser limit. Entropic losses, thermalization, and unused below-bandgap photons are the main limits of solar PV.

If instead of relying on the Sun, a local black body is used as the source of photons, many of the limitations mentioned can be minimized. This idea, known as thermo-photovoltaics (TPV), has been known since 1960. With a local thermal emitter at a suitable temperature these losses can be minimized and high efficiencies are attainable. Previous efforts have attempted to tune the emissivity spectrum of the emitter, minimizing the amount of below‑bandgap radiation reaching the photovoltaic cell.  This approach, however, bring some challenges due to the difficulty of developing a high quality spectral filter that remains stable at high temperatures.

We report on a thermo‑photovoltaic device that relies on the band‑edge of a photovoltaic absorber to spectrally filter the incoming radiation. Photons above the bandgap are converted to electricity, while the unused and unabsorbed photons below the bandgap are reflected by a ~94% reflective rear electrode and recovered by the source of radiation. Our system uses graphite as the blackbody emitter, and In_0.55Ga_0.45As (bandgap of 0.74eV) as the photovoltaic absorber.  For an emitter temperature of ~1200°C, we report a power conversion efficiency of 28.1%.

An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CH₃NH₃PbI₃ are strongly affected by photon reabsorption that masks a much larger intrinsic bimolecular recombination rate constant.

We have used optical-pump THz-probe spectroscopy to study the charge-carrier dynamics in a set of dual-source vapour-deposited CH₃NH₃PbI₃ films whose thicknesses vary between 50 and 533 nm [1]. We find that the bimolecular charge recombination rate appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon reabsorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient of value 6.8 × 10^–10 cm³s^–1 is common to all samples irrespective of film thickness [2].

We therefore postulate that the wide range of literature values reported for such coefficients is partly to blame on differences in photon out-coupling between samples with crystal grains or mesoporous scaffolds of different sizes influencing light scattering, whereas thinner films or index-matched surrounding layers can reduce the possibility for photon reabsorption. We discuss the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers and highlight an approach to assess the success of such schemes from transient spectroscopic measurement.

References:

1)  Crothers, Timothy W., et al. "Photon reabsorption masks intrinsic bimolecular charge-carrier recombination in CH3NH3PbI3 perovskite." Nano letters 17.9 (2017): 5782-5789.

2)  Davies, Christopher L., et al. "Bimolecular recombination in methylammonium lead triiodide perovskite is an inverse absorption process." Nature communications 9.1 (2018): 293.

Hybrid halide perovskites are being intensively studied as active layers in photovoltaic cells. They combine cost efficiency due to solution processability and high power conversion efficiency due to electrical transport properties and proper band gap. The chemical stability, which is often affected by quality of surfaces and interfaces, still remains a concern. Meanwhile, lead sulfide (PbS) is rocksalt-structured semiconductor with low band gap, which can be easily made as large single crystals. PbS is also well known for quantum dot material with high radiative efficiency. Recently, quantum-dot-in-perovskite crystals reported as promising optoelectronic material as well. Therefore, understanding the interface between lead sulfide and halide perovskite is important. In this study, we perform first-principles density-functional theory (DFT) calculations to investigate atomic contact properties (e.g. interface geometry) and electronic contact properties (e.g. charge redistribution and band offset). We focus on interface between PbS and CsPbBr₃ where epitaxial interface between the both of materials with low lattice strain is possible. Our results predict spontaneous forming of CsPbBr3 - PbS interface is feasible, and consequentially this interface will have a type I band alignment.

The structural phase behavior of high quality single crystals of methylammonium lead iodide (CH₃NH₃PbI₃ or MAPbI₃) was revisited by combining Raman scattering and photoluminescence (PL) measurements under high hydrostatic pressure up to ca. 10 GPa. Both PL and Raman spectra show simultaneous changes in their profiles that indicate the occurrence of three phase transitions subsequently at around 0.4 GPa, 2.7 GPa and 3.3 GPa. At the second phase transition, the Raman spectra exhibit a pronounced reduction in linewidth of the phonon modes of the inorganic cage, similar to the changes observed at the tetragonal-to-orthorhombic phase transition occurring at around 160 K but ambient pressure [1]. This behavior is interpreted as evidence for the locking of the organic cations in the cage voids above 2.7 GPa, due to the reduced volume and symmetry of the unit cell. At the third phase transition, reported here for the first time, the PL is greatly affected, whereas the Raman experiences only subtle changes related to a splitting of some of the peaks. This behavior may indicate a change mostly in the electronic structure with little effect on the crystal structure. Strikingly, no amorphization of the sample was observed up to the highest pressure which reached close to 10 GPa, in frank discrepancy with most of the high-pressure (x-ray) data of the literature [2], which established an onset of 3 GPa for the set in of an amorphous phase in MAPI₃.

[1] Leguy, A.M.A. et al., Phys. Chem. Chem. Phys. 2016, 18, 27051–27066.
[2] Postorino, P. & Malavasi, L., J. Phys. Chem. Lett. 2017, 8, 2613–2622 and references therein.

Perovskite solar cells have been extensively developed for few years but still the ultrafast and fast processes occurring in this system are not fully understood. The main tools providing wide view of a charge transport behavior are transient absorption and time-resolved emission spectroscopies. In the femtoseconds to nanoseconds time range two main transient absorption features dominate. The first is the long-wavelength band edge bleach assigned to a state filling and decaying in nanosecond range [1]. The second one is caused by cooling of charges, it takes place in hundreds of femtoseconds and usually has a shape of a band edge bleach first derivative [2].

Comparison of transient absorption signals for differently sensitized methylammonium lead iodide (MAPbI₃) solar cells (non-stoichiometric and stoichiometric ratio of PbI₂:MAI in precursor solution) will be presented. It was found that modification of the precursor ratio from non-stoichiometric to stoichiometric leads to the drastic changes of transient absorption signal from the typical strong long-wavelength band-edge bleach to the weak derivative-like signal due to the bandgap shift [3].

Transient absorption and emission measurements allows determination of charge injection kinetics for different charge transporting materials and recombination rates within perovskite [2]. Our studies of the hole injection from MAPbI₃ to spiro-OMeTAD and its xanthene derivative X60 [4] as well as the recent results of electron injection from MAPbI₃ to PCBM, PenPTC and SPPO13 will be presented. Moreover, proper determination of the first and the second order recombination rate constants in perovskite materials will be proposed. In particular, the influence of different transient absorption data treatment (band integral, global analysis based on singular value decomposition and bleach minimum amplitude) on the obtained rate constants will be shown.

Acknowledgements

The study was supported by Polish Ministry of Science and Higher Education under project 0019/DIA/2017/46.

[1]  J. Peng, Y. Chen, K. Zheng, T. Pullerits, Z.Liang, Chem. Soc. Rev., 2017, 46, 5714-5729.

[2] K. Pydzińska, J. Karolczak, I. Kosta, R. Tena-Zaera, A. Todinova, J. Idigoras, J.A. Anta, M. Ziółek, ChemSusChem, 2016, 9, 1547-1659.

[3] K. Pydzińska, J. Karolczak, M. Szafrański, M. Ziółek, RCS Advances, 2018, 8, 6479-6487.

[4] K. Pydzińska,  P. Florczak, G. Nowaczyk, M. Ziółek, Synthetic Metals, 2017, 232, 181-187.

Thermodynamic calculations revealed that single junction solar cell conversion efficiencies can exceed the Shockley-Queisser limits and reach around 66% under 1-sun illumination if the excess energy of hot photogenerated carriers is utilized before they cool down to the lattice temperature (i.e., hot-carrier solar cells).¹ Organic–inorganic lead halide perovskite semiconductors have recently emerged as the leading contender in low-cost high-performance solar cells.^2,3 The key for the realization of hot-carrier solar cell include the slow hot-carrier cooling and effective extraction of hot-carrier energies which requires fast hot-carrier injection into charge collection layer before hot-carrier cooling down to the lattice temperature. Emulating semiconductor nanoscience, some interesting questions would be if the hot-carrier cooling rate in halide perovskites could be further modulated through confinement effects, and if these hot-carriers can be efficiently extracted. Here, the hot-carrier cooling dynamics and mechanisms in colloidal CH₃NH₃PbBr₃ nanocrystals of different sizes (with mean radius ~2.5–5.6 nm) and their bulk-film counterpart were compared using room-temperature transient absorption spectroscopy. Our results revealed that the weakly quantum confined CH₃NH₃PbBr₃ nanocrystals are very promising hot-carrier absorber materials (~ 2 orders slower hot-carrier cooling times and around 4 times larger hot-carrier temperatures than their bulk-film counterparts). This is attributed to their intrinsic phonon bottleneck and Auger-heating effects at low and high carrier densities, respectively. Importantly, we demonstrate efficient room temperature hot-electrons extraction (up to about 83%) by an energy-selective electron acceptor layer within ~1 ps from surface-treated perovskite nanocrystal very thin films (~30 nm). These new insights would allow the development of extremely thin absorber and concentrator-type hot-carrier perovskite solar cells. ⁴

References:

[1] Ross, R.T. & Nozik, A.J. Efficiency of Hot-Carrier Solar-Energy Converters. J. Appl. Phys. 53, 3813-3818 (1982).

[2] Lee, M.M., Teuscher, J., Miyasaka, T., Murakami, T.N. & Snaith, H.J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 338, 643-647 (2012).

[3] Zhou, H.P. et al. Interface engineering of highly efficient perovskite solar cells. Science 345, 542-546 (2014).

[4] Li, M. et al. Slow cooling and highly efficient extraction of hot carriers in colloidal perovskite nanocrystals. Nat. Commun. 8, 14350 doi: 10.1038/ncomms14350 (2017).

The phenomenom of hysteresis in the current-voltage characteristics of perovskite solar cells has been discussed by numerous authors. It is agreed by many that mobile ions or ion vacancies play a significant role in the underlying processes causing this hysteresis. Calao et al. have shown that hysteresis will only occur if ion migration alters the rate of surface recombination. The latter is a loss mechanism which occurs at the interfaces of the perovskite absorber with the adjacent materials, usually electron (ETM) and hole (HTM) transporting material and it depends sensitively on the concentration of electrons and holes in the vicinity of such an interface. As Calao et al. could show there is minimal hysteresis if ETM or HTM layers passivate the corresponding interface to the absorber such that surface recombination becomes insignificant regardless of the distribution of mobile ions or ion vacancies.
We built inverted planar p-i-n perovskite solar cells and observed a high open-circuit voltage (Voc) when NiOx was employed as HTM. In contrast, Voc was significantly reduced if NiOx was replaced by PEDOT:PSS. This reduction of Voc could clearly be attributed to surface recombination occurring at the absorber/HTM interface as confirmed by photoluminescence measurements. Interestingly, the PEDOT:PSS based perovskite solar cells nevertheless did not show any hysteresis. To answer the question why the migrating ions do not cause hysteresis in these kind of devices we performed numerical simulations. We can reproduce the experimental finding when implementing interface traps at the perovskite/PEDOT:PSS interface. These states can lead to a fermi level pinning effect such that the ionic distribution has only very little influence on the concentrations of electrons and holes in the vicinity of that recombination active interface.

[1] P. Calado, A. M. Telford, D. Bryant, X. Li, J. Nelson, B. C. O’Regan, P. R.F. Barnes, Nature Communications 7 (2016), 13831

In perovskite solar cells, achieving consistency in device fabrication is very difficult. Although it easy to measure whether a solar cell performs well, it is not always easy to determine what the problem is in case of a bad solar cell. Many devices that show less than ideal performance show a feature called an ‘s-shape’, where in the JV-characteristic a region exists where the second derivative of the current with respect to voltage is negative. If this s-shape occurs below Voc, this will dramatically decrease the fill factor. Until now some possible causes are known. It has been shown using drift diffusion simulations that in perovskite solar cells, s-shapes can exist under forward bias. Also from organic solar cells it is well known that poor charge transport can lead to s-shapes, but no comprehensive explanation has been shown on these s-shapes in perovskites as of yet.

In this contribution we identify all the processes and parameters that yield s-shapes in the JV-characteristic and can therefore give a complete story on the phenomenon of s-shapes in perovskite solar cells. Furthermore, the occurrence of s-shapes is linked to device characteristics such as transport layer mobility and effects such as charge buildup at certain bias voltages. This is done using very large amounts of simulated solar cells with different parameters, where parameters are chosen such that the whole parameter space of solar cells yielding s-shapes is swept. Each individual solar cell simulation is a drift diffusion simulation including mobile ions where n-i-p or p-i-n structures are assumed. The conclusions from the modeling can be used to pinpoint weak spots of fabricated solar cells and help improve device recipes and fabrication.

During the last years, three-dimensional hybrid perovskites were in the spotlight because of their very promising properties as semiconductor materials for solar cells, but also for optoelectronic applications in general. Indeed, with a low temperature and precursor solution based synthesis and certified 20% photovoltaic efficiency, 3D hybrid perovskites have been claimed as the new big thing in photovoltaics¹. Further studies must be conducted to understand in detail the properties of this material, but especially to overcome one of its main flaws: degradation issues.

Currently, two-dimensional hybrid perovskites are getting attention of the scientific community mainly due to two reasons: the improvement of the stability and the chemical composition versatility, much higher than for the 3D-peovoskites. In fact, in the 3D case, the size of the “cages” created by the octahedral network limits the candidates as organic/inorganic cation, which is not the case of layered 2D hybrid perovskites. With these materials, we are expending the field of possibilities and it can lead to better tunability of the photophysical properties.

We are reporting here (time-dependent) density functional theory calculations [(TD)-DFT] on alkyl-ammonium lead iodide perovskites, and more specifically, the influence of the alkyl chain length of the spacer cation on the electronic structure and optical properties of the material. We predicted significant changes using long or short chains. Indeed, if the inorganic layers fix the electronic properties, the length of the organic cation showed to have an indirect effect. With long alkyl chains, as dodecyl chains, an opening of the electronic band gap occurs, due to the influence of the supramolecular packing on the structure organization of the octahedra network. For the case of long organic spacer dodecylammonium lead iodide perovskites, organic chains adopt a polyethylene-like packing, causing distortions in the inorganic frame and leading to the observed electronic band gap opening. These theoretical results are in agreement with experimental data and demonstrate that organic saturated chains can modify the optoelectronic properties of layered halide perovskite semiconductors².

¹ J. Bisquert, Journal of Physical Chemistry Letters 4(15):2597, 2013

² C.Quarti, et al., Journal of Physical Chemistry Letters, DOI: 10.1021/acs.jpclett.8b01309

Photoelectrochemical water splitting is one of the most interesting alternatives to produce hydrogen in a clean way by solar energy conversion.(1) Despite the huge potential and great advances, new materials need to be developed in order to take this technology to a commercial level. At present, different materials as oxides, oxisulfides and metal chalcogenides are being investigated as photoelectrodes in photoelectrochemical cells. However, achieving high energy conversion efficiencies by using a single material is a very tricky objective. Therefore, hybrid materials are getting a lot of attention lately. (2)

In this work, we present a hybrid material formed by the heterojunction of a novel synthesized organic conductive polymer and TiO2 nanocrystals. The nanostructured conjugated porous polymer is based on dithiothiophene moiety (Nano-CMPDTT) and was synthesized by Sonogashira cross coupling reaction from precursors in mini-emulsion conditions. In order to elucidate the electronic structure and the ability of this material to be used as a photocatalyst, HOMO and LUMO positions were determined by cyclic voltammetry. The energy diagram shows an ideal position of the energy bands in order to use the synthesized polymer as an electron injector to TiO2 in photocatalytic reactions. TiO2 NCs and organic polymer suspensions have been deposited by spin coating in ITO glasses. The formed films have been characterized by X-ray diffraction, SEM, EDX and AFM. Photoelectrochemical measurements were performed in a three electrode cell configuration, using the hybrid material as the working electrode. The hybrid material presents an enhancement in photovoltages and photocurrents values. Electrochemical Impedance Spectroscopy (EIS) was performed to confirm the improved charge transfer observed when illuminating the hybrid material in comparison to the TiO2 nanocrystals alone. In fact, a decrease in the resistance associated with this phenomenon was found. This confirms that the presence of the polymer in the hybrid material increases the absorption of light, charge transfer and reduces electron-hole recombination, making this hybrid a good candidate to be used as a photoelectrode for the hydrogen evolution reaction. In fact, recent results show an improvement in the energy conversion efficiency by using this new hybrid material as electrode compared with regular TiO2.

[1] Z. Chen, H. N. Dinh, E. Miller, Photoelectrochemical Water Splitting (Springer New York, New York, NY, 2013; http://link.springer.com/10.1007/978-1-4614-8298-7), SpringerBriefs in Energy.

[2] M. P. Arciniegas et al., Self-Assembled Dense Colloidal Cu2Te Nanodisk Networks in P3HT Thin Films with Enhanced Photocurrent. Adv. Funct. Mater. 26, 4535–4542 (2016).

Transition metal oxides are amongst the most widely studied materials for solar water oxidation owing to their earth abundance, good aqueous stability and facile syntheses.¹ Understanding the dynamics of the photogenerated charge carriers in these materials is key to highlighting properties that need to be targeted to improve water splitting performance. With a narrower band gap than TiO₂, tungsten trioxide (WO₃) can absorb a larger proportion of the solar spectrum and has a deeper valence band energy to provide a large thermodynamic driving force for water oxidation.^2-3 WO₃ is also often reported to have high electrical conductivity, leading to its frequent implementation as an electron transporting layer in various heterojunction photoanodes.^2-4 This high conductivity is considered to be a result of a large density of charge carriers, caused by intrinsic oxygen vacancies which can act as n-type dopants.⁵ However, deviations from stoichiometry have also been suggested to introduce chemical defects that can result in increased trapping of charges which, rather than boost performance, can often introduce additional recombination pathways.⁶

Using transient diffuse reflectance spectroscopy and transient photocurrent measurements, I will discuss the dynamics of the photogenerated charges in WO₃ photoanodes, synthesised by chemical vapour deposition (CVD). The synthesis method generates heavily doped monoclinic WO_3-x needles, which are annealed in air at elevated temperature to remove most of the oxygen vacancies. These photoanodes exhibit an early photocurrent onset and a high faradaic efficiency (>85%) for water oxidation. Compared to other transition metal oxide photoanodes, we observed rapid water oxidation (>1 ms) but found the rate of electron extraction is significantly slower (>10 ms). We investigated the effect of oxygen vacancy states on electron transport and found that electron trapping in the needles is significant, proposing a trap-mediated mechanism of electron transport to the back contact. We then used ultrafast transient absorption spectroscopy to examine the bias dependence on bulk recombination processes. Finally, we altered the oxygen vacancy content to determine the overall effect of these defects on charge transport and performance.

[1] G. Wang, et al. Energy Environl Sci, 2012, 5, 6180–6187.

[2] Z. Huang, et al. Advanced Materials, 2015, 27 (36), 5309-5327.

[3] X. Liu, et al. PCCP, 2012, 14 (22), 7894-7911.

[4] C.X. Kronawitter, et al. Energy Environl Sci 2011, 4 (10), 3889-3899.

[5] T. Zhu, et al. ChemSusChem, 2014, 7, 2974-2997

[6] M.B. Johansson, et al. J Phys Condens Matter, 2016, 28, 475802.

Organometallic complexes with reactive metal centers are promising candidates for photocatalysis. To improve their performance, insight into the nature of excited states and control thereof is crucial. Ultrafast x-ray spectroscopy is a powerful method to achieve mechanistic insight into processes at catalytic metal moieties and is particularly useful to detect light-induced changes in oxidation state and coordination geometry. It can also be applied to probe the essential charge transfer step to the catalyst in real-time, including the potential involvement of atomic rearrangements.
We recently developed a series of new Ru-metal photocatalysts, of which the RuPt derivative showed a H2 turnover number of 80 after 6 h of irradiation at 470 nm. The photodynamics of RuPt studied by femtosecond transient absorption are highly complex and differ significantly from the RuPd analogue [1], indicating an important role of the catalytic moiety and motivating ultrafast x-ray absorption studies performed at the Advanced Photon Source at Argonne. This work particularly focuses at light-induced redox processes at the Pt moiety and the timescales thereof. Another question to be answered involves the local structure of the catalytic moiety and possible temporal or permanent changes.
The earliest differential absorption spectrum at 30 ps indicates that the Pt moiety is already reduced at this timescale. The signal intensity partially decays on a timescale of ca. 930 ps. The observation of a differential absorption signal at timescales far beyond indicates branching. The spectra at late timescales can be modelled by a hexa-coordinated PtIV (or possibly PtIII) species. This intermediate species may be formed by oxidative addition of iodine, is long-lived (>10 microseconds) and ultimately recovers into the original ground state structure. Hence, activation of the catalyst by a single photon may lead to withdrawal of two electrons. This mechanism presents a new paradigm for H2 generation by supramolecular photocatalysts.

[1] Q. Pan, F. Mecozzi, J.P. Korterik, J.G. Vos, W.R. Browne, A. Huijser, "The Critical Role Played by the Catalytic Moiety in the Early-Time Photodynamics of Hydrogen-Generating Bimetallic Photocatalysts", ChemPhysChem 17, 2654-2659 (2016).   

Developing energy storage mechanisms is a crucial step to secure a sustainable energy economy. This is particularly important for solar power because there is a mismatch between the periods of peak energy harvesting and peak consumption. Photo-electrocatalytic energy storage has traditionally been dominated by inorganic systems. In particular, transition metal oxides are attractive due to their stability in liquid electrolytes and the remarkable catalytic flexibility of metallic centres. However, inorganic systems often underperform and only poor quantum yields are achieved, even when synthetic care is taken to eliminate defects and impurities.  In the dark, oxides like Fe₂O₃ and BiVO₄ are poor conductors due to the localization of charges on specific atomic sites forming small polarons. Under illumination, transport improves but the mechanisms behind it are unknown. Recent x-ray and terahertz studies have hinted at the existence of light-induced small polarons and their link with structural properties. ^1-3 However, these measurements lack device sensitivity and it is difficult to know if such states really influence performance. In this talk, I will present ultrafast optical studies with photocurrent detection of photoelectrochemical devices. These measurements are capable of probing light-initiated changes that control actual device activity.⁴ I will present data demonstrating that, in oxides such as efficient Fe₂O₃ thin-films,⁵ charges intrinsically self-trap within femtoseconds of photo-generation hindering transport. Importantly, I will show how in efficient systems, crossover between molecular-type, localised transport and band-like transport is critical and necessary for activity. This behavior is remarkably similar to that observed in molecular crystals and some of the strategies used to improve molecular semiconductors might apply to metal oxides. I will discuss the material implication of these findings and how it might be possible to achieve synthetic control of charge hopping and delocalisation rates in oxides.

References:

¹Carneiro L. et al. Nat Mater. 2017, 16, 8, 819 

²Biswas S. et al. Nano Lett. 2018,18,1228

³Butler  K.T. et al. J.Mat.Chem.A, 2016,4,18516

⁴Bakulin A. et al. J.Phys.Chem. Lett. 2016, 7, 2, 250

⁵Steier L. et al. ACS Nano, 2015,9,11775

Nanocrystals can be used to achieve transition in the mid and far infrared. This is in particular promising for the design of low cost infrared photodetectors. However more need to be understood on the dynamic of carriers in narrow band gap nanocrystals. This question is nevertheless quite challenging since many conventional experimental methods such as time resolved photoluminescence cannot be used because of the low PL efficiency in the infrared nanocrystals and because of difficulties to build infrared optical setup. We have investigated two complementary ways to probe carrier dynamics in narrow band gap colloidal materials which are time resolved photoemission and transient photocurrent. To illustrate these experiments, two systems have been investigated. First I will show that pump probe experiment can be conducted while exciting 2D HgTe nanoplatelets in the near IR and while looking at relaxation with a photoemission probe. This method is very efficient to determine without contact majority and minority carrier lifetime.In the second part of the talk, I will discuss how it is possible to determine band structure parameter such as the Urbach energy from transient photocurrent measurements conducted on HgTe nanocrystals. To do so, we have built a broad bandwidth setup (from ns to ms) and bring evidence for the multi-trapping transport regime.These two methods are extremely complementary to optical time resolved spectroscopy often limited to short dynamics (ns and less)

Nonresonant excitation of colloidal quantum dots (QDs) creates hot carriers that subsequently cool down to the band edges or are trapped in localized states. Carrier cooling and trapping typically happens on timescales from femtoseconds to picoseconds, orders of magnitude faster than the nanosecond to microsecond timescales of radiative recombination. Understanding cooling and trapping is relevant for (hot) carrier extraction in photovoltaics and to increase the luminesence output of QDs used as phosphor.

We investigate carrier cooling and trapping in InP QDs with a ZnSe_1-xS_x shell. Undoped QDs are compared to Cu⁺-doped QDs, where the Cu⁺ ion serves as a designed hole trap. Using pump probe transient absorption spectroscopy with femtosecond time resolution, we are able to monitor the population of electrons in the conduction band.

Our comparative study shows that hot electron cooling is almost an order of magnitude slower in the Cu⁺-doped QDs than in the undoped QDs. We ascribe this to rapid hole trapping on the Cu⁺ ion. This confirms the model in which hot electron cooling goes via an Auger-like process, where the hot electron transfers its excess energy to the hole which subsequently relaxes by phonon coupling. In our Cu⁺-doped QDs the hole is trapped on a Cu⁺ ion on sub-picosecond timescales, so the Auger cooling pathway is unavailable to the hot electron. Instead it must cool down via another, slower, pathway, most likely by coupling to high-energy vibrations at the surface of the QDs. This must also mean that hole trapping on the Cu⁺ ion is faster than the Auger cooling timescale of the undoped QDs, which is of the order of 300 fs. Our results provide insight in the behaviour of hot electrons and holes in the short time period after excitation of both Cu⁺-doped and undoped InP QDs.

The operational lifetime and scale-up of quantum dot sensitized solar cells (QDSCs) are largely limited by the hole transport layer, which is usually an aqueous polysulfide solution. The liquid, and alkaline nature of this electrolyte makes it difficult to develop laminated sealed devices. Further, the polysulfide electrolyte composition alters with use (there are observable color and viscosity variations!) thus impacting the cell performance. Besides the photoanode, the counter electrode (CE) also plays a key role in controlling cell response. While a variety of CEs have been attempted in the past: metallic coatings, carbon nanomaterials and conducting polymers, poly(3,4-ethylenedioxypyrrole) or PEDOP has rarely been used in QDSCs.

In view of the above described issues, in this report, QDSCs with a photoanode comprising of N-doped graphene quantum dots (N-GQDs) and cadmium sulfide (CdS)/titania (TiO₂) based solar cells with electropolymerized PEDOP@carbon cloth as CE and a polysulfide/SiO₂ gel with an electrolyte additive, namely, sodium poly(styrene sulfonate) or NaPSS were developed. The proportion of NaPSS is optimized on the basis of cell performance. A significant improvement in QDSC performance is obtained by incorporating NaPSS in the gel. By using impedance spectroscopy, the role of NaPSS in improving the cell performance is determined. NaPSS increases the recombination resistance for back electron transfer at the photoanode/electrolyte interface, thus increasing the power conversion efficiency to nearly 7% from 6.1% (when no NaPSS is present). NaPSS also imparts an enhanced operational life to the QDSC. Apart from NaPSS, the effectivity of PEDOP as a CE for QDSCs is studied by comparing impedance parameters, electrocatalytic activities and electrical conductivities of PEDOP films with different dopants. Ionically conducting and electrically conducting dopants were attempted. These studies provide a deeper understanding of factors that limit QDSC performances, and help in overcoming them.

Control over the energy alignment of Quantum Dots (QD) heterostructures can be used to unlock new functionalities for QD based optoelectronic devices: from improved carrier separation in type-II heterostructure, to control over hot-carrier transfer in type I heterostructures and lowered Carrier Multiplication threshold in quasi-type-II heterostructures.

We investigated the carrier dynamics in QD heterojunction films composed of PbSe and CdSe QDs. We demonstrate that such films tend to form a type I band alignment in which fast and efficient hot electron transfer from PbSe QDs to CdSe QDs is observed by transient absorption (TA) measurements. ­ The efficiency of the hot electron transfer process increases with excitation energy as a result of the more favorable competition between hot-electron transfer and electron cooling. The experimental picture is supported by time-domain density functional theory calculations, showing that electron density is transferred from lead selenide to cadmium selenide quantum dots on the sub-picosecond timescale. Hot-electron solar cells have been proposed as a route towards higher efficiency solar cells, and our observation reveals the possibility to achieve and control hot-electron transfer via energy-structure engineering in QD heterojunctions.

We next attempted to switch the energy alignment between the PbSe QDs and CdSe QDs, using the size-dependence of their energy structure as well as using tailored ligands to shift the energy levels through their surface dipoles. Spectroelectrochemical measurements reveal that we can shift the type I alignment to a type II alignment and TA measurements demonstrate a much-improved efficiency of “cold” electron transfer.

We thus proved that a combination of size-variation and control over surface-passivation allows to span the range between type-I and type-II alignment. One particularly interesting configuration is that of quasi-type-II alignment, where the conduction electron levels are resonant, as this could potentially be used for optimal Carrier Multiplication.

Metal-organic frameworks (MOFs) are coordination polymers consisting of metal ions connected by organic ligands. Besides the traditional applications in gas storage and separation as well as catalysis, the long-range crystalline order in MOFs and the tunable coupling between their organic and inorganic constituents have recently led to the design and synthesis of (semi-)conducting MOFs, opening the path for their application in opto-electronics. Yet, despite being a critical aspect for the development of MOF based electronics, the true nature of charge transport in MOFs, i.e. whether hopping or band-like transport occurs, has remained unresolved to date.

Here we report a time-resolved high-frequency (terahertz) conductivity study of a newly developed Fe₃(THT)₂ (THT=2,3,6,7,10,11-hexathioltriphenylene) two-dimensional MOF.  The novel π-d conjugated samples, synthesized through interfacial method at room temperature, are obtained as a large-area, free-standing film with tunable geometry (size and thickness). The Fe₃(THT)₂ films are conductive (~1 S/cm), porous (specific surface area of 526±5 m²/g) and semiconducting (with a ~250 meV direct bandgap). We demonstrate for the first time band-like charge transport in MOFs. This finding is directly apparent from the Drude-type high-frequency (terahertz) photo-conductivity response obtained in the samples; revealing free-moving, delocalized charge carriers displaying ~200 cm²/Vs mobilities at room temperature; a record mobility for MOFs. The temperature dependence of the mobility reveals that the main scattering mechanism limiting the mobility and hence band-like charge transport in this material is related to impurity scattering, so that material improvements may further increase the mobility.

The demonstration of band-like charge transport in MOFs reveal the potential of (porous) conductive MOFs to be employed as active materials in opto-electronics devices.

III-V semiconductor quantum wells have obtained a central place in advanced logics and opto-electronics. In more recent research directions heading towards materials with entirely new functions, the effects of a nano scale geometry forming a periodic scattering potential in the lateral directions of the quantum well has been discussed and calculated. In case of a nano scale honeycomb geometry, and entirely new band structure emerges in which the highest valence and lowest conduction bands become Dirac cones at the K-points, while the semiconductor quantum well band gap remains nearly unaltered.

In this presentation we report on the fabrication of a 10 nm thick InGaAs quantum well (QW) on a n-type InP substrate with a honeycomb symmetry structure by creating a triangular anti-lattice inside the QW using high-resolution electron beam lithography. The morphology of the samples are studied using atomic force microscopy (AFM), elementary diffraction spectroscopy (EDS) and cross-section transmission electron microscopy (TEM). The quality of the samples is characterized using scanning electron microscopy (SEM), which is used for an extensive statistical analysis to determine the disorder inside the lattices. The results are supported by theoretical simulations on the bandstructure and density of states (DOS).

Nanocrystals with type-II heterostructures are attractive candidates for applications in which separation of excited charge carriers are exploited, such as photovoltaics [1], upconversion [2], or quantum dot lasing [3]. These properties are in competition with processes involving interfacial trap states, usually associated with the particle surface or strained interfaces. To use semiconductor nanocrystals in technical applications it is of utmost importance to characterise and control these effects.

Here we present a model system made of CdTe-tipped CdSe/CdS seeded nanorods, in which the CdTe particle is separated in space from the CdSe seed while being in electronic contact. The CdTe/CdS interface has type-II character, but charge carrier delocalisation is likely affected by the large lattice mismatch of 10% between CdTe and CdS [4]. We performed ultrafast (femto to picosecond) transient absorption spectroscopy to map carrier dynamics after exciting one or both recombination centres, with seed size and rod length as variable parameters. Conduction band electrons, after exciting the CdTe/CdS interface, are expected to localise in the CdSe seed, which has the lowest energy band edge and acts as a reporter particle. However, while states in the CdS rod get bleached by the electron no signal corresponding to CdSe is detected. We employ electron and hole scavengers to determine the roles of the individual charge carriers, and by this the contribution of Coulomb interactions and interfacial trap states.

This experiment gives fundamental insight into the behaviour of strained nano-heterointerfaces, as well as the effects from size quantisation and mean free path of the carriers. The experimental data is compared to effective mass approximation-based simulations for free carriers. While the basic electronic structure is predicted with sufficient accuracy the comparison highlights the importance to simulate carrier interactions and interfaces on the atomic scale.

[1] Itzhakov, S.; Shen, H.; Buhbut, S.; Lin, H.; Oron, D.; J. Phys. Chem. C 2013, 117 (43), 22203–22210.

[2] Deutsch, Z.; Neeman, L.; Oron, D.; Nat. Nanotech. 2013, 8, 649–653.

[3] Klimov, V. I.; Ivanov, S. A.; Nanda, J.; Achermann, M.; Bezel, I. V.; Mcguire, J. A.; Piryatinski, A.; Nature 2007, 447, 441–446.

[4] Jing, L.; Kershaw, S. V.; Kipp, T.; Kalytchuk, S.; Ding, K.; Zeng, J.; Jiao, M.; Sun, X.; Mews, A.; et al.; J. Am. Chem. Soc. 2015, 137, 2073–2084.

It was recently shown that single colloidal quantum dots (QDs) may exhibit large sensitivities to external electric fields in the form of a spectral shift of the emission wavelength. This opens the possibility of designing QDs’ based local electric field sensors, for example for neuronal membrane action potential (AP) sensing. One of the hurdles on the way to such a goal is fast detection. Typically, slow spectrometers are used to detect the spectral shifts however, AP sensing requires significantly faster detection rates of the order of a millisecond.

The underlying mechanism for the QDs’ response to external electric fields is the quantum confined Stark effect. From symmetry considerations spherical QDs exhibit only a red-shifted emission spectrum in the presence of an electric field. APs consist of a change from negative to positive potential thus, symmetric QDs are of limited value. Asymmetric type II ZnSe/CdS nanorods, on the other hand, exhibit a large linear response, namely both red- and blue-shifts, depending on the orientation of the QD in the electric field. Moreover, the spectral shifts are expected to be correlated with changes in the decay rates.

Here, we demonstrate an experimental setup designed at achieving shot-noise limited sensitivity to emission spectral shifts on time scale suitable for AP sensing. We present experimental results of these phenomena as well as characterize the performance of single QDs as sensors for short millisecond voltage pulses comparable to APs in both duration and amplitude.

Two-dimensional colloidal nanomaterials represent very exciting optoelectronic properties. The formation of ordered and densely packed surface layers of amphiphilic ligand molecules on certain crystal facets can drive the normally isotropic into a two-dimensional crystal growth, resulting in semiconducting nanosheets. Such structures combine good lateral conductivity with solution-processability and electronic confinement in height, which allows tuning the effective bandgap of the materials. I will present the syntheses of the materials and the analyses of the optoelectronic transport through these materials in field-effect transistors, solar cells, and spintronic devices. It turns out that the electronic confinement allows for an optimization of their performances.

Two-dimensional colloidal nanomaterials represent very exciting optoelectronic properties. The formation of ordered and densely packed surface layers of amphiphilic ligand molecules on certain crystal facets can drive the normally isotropic into a two-dimensional crystal growth, resulting in semiconducting nanosheets. Such structures combine good lateral conductivity with solution-processability and electronic confinement in height, which allows tuning the effective bandgap of the materials. I will present the syntheses of the materials and the analyses of the optoelectronic transport through these materials in field-effect transistors, solar cells, and spintronic devices. It turns out that the electronic confinement allows for an optimization of their performances.

The solar-driven photocatalytic splitting of water into hydrogen and oxygen is a potential source of clean and renewable fuels. However, four decades of global research have proven this multi-step reaction to be highly challenging. The design of effective artificial photo-catalytic systems will depend on our ability to correlate the photocatalyst structure, composition, and morphology with its activity.

I will present our strategies, and most recent results, in taking photocatalyst production to new and unexplored frontiers. I will focus on unique design of innovative nano scale particles, which harness nano phenomena for improved activity, and methodologies for the construction of sophisticated heterostructures. I will share our design rules and accumulated insights, which enabled us to obtain a perfect 100% photon-to-hydrogen production efficiency, under visible light illumination, for the photocatalytic water splitting reduction half reaction. Finally, I will describe our future designs of systems capable of overall water splitting and genuine solar-to-fuel energy conversion.

In order to consider the effective structure of silicon solar cells for perovskite/silicon tandem solar cells, the optic and photovoltaic properties of textured and flat silicon surfaces were compared using mechanical-stacking-tandem of 2- and 4- terminal structures by perovskite layers on crystal silicon wafers.  The reflectance of the texture silicon surface in the range of 750-1050 nm could be reduced more than that of the flat silicon surface (from 2.7% to 0.8%), which resulted in increases in average IPCE values (from 83.0% to 88.0%) and current density (from 13.7 mA cm-2 to 14.8 mA cm-2).  Using the texture surface of silicon heterojunction (SHJ) solar cells, the significant conversion efficiency of 21.4% was achieved by 4-terminal device, which was 2.4%-up from that of SHJ solar cells alone.

And, for the progress of perovskite/silicon tandem solar cells, we introduce a totally vacuum-free cost-efficient crystalline silicon solar cells. Solar cells were fabricated based on low-cost techniques including spin coating, spray pyrolysis and screen-printing. A best efficiency of 17.51% was achieved by non-vacuum process with a basic structure of <AI/p+/p-Si/n+/SiO₂/TiO₂/Ag> CZ-Si p-type solar cells. J_SC and V_OC of the best cell were measured as 38.1 mAcm^-2 and 596.2 mV, respectively with FF of 77.1%. Suns-Voc measurements were carried out and the detrimental effect of the series resistance on the cell performance was revealed. It is concluded that higher efficiencies are achievable by the improvements of the contacts and by utilizing good quality starting wafers.

Finally, for the study of stability test on perovskite solar cells, Carbon-based triple-porous-layer perovskite solar cells without any hole transporting material were selected for investigation to reduce internal degradation issues about thermal stress.  The sealed perovskite solar cells which kept at 100 ˚C performed slow degradation in the power conversion efficiency until 4500 h, but the degradation speed was accelerated after that.  By analyzing the perovskite solar cells aged for 7000 h at 100 ˚C, the results of energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy suggest that, although Pb²⁺ and I- were sealed inside of the devices, the most of CH₃NH₃⁺ diffused out of the sealant UV-curable adhesive at 100 ˚C, which is the reason of the thermal degradation for the sealed perovskite solar cells.

Understanding and controlling the relaxation process of optically excited charge carriers in solids with strong correlations is of great interest in the quest for new strategies to exploit solar energy. Usually, optically excited electrons in a solid thermalize rapidly on a femtosecond to picosecond timescale due to interactions with other electrons and phonons. New mechanisms to slow down thermalization would thus be of great significance for efficient light energy conversion, e.g. in photovoltaic devices. Ultrafast optical pump probe experiments in the manganite Pr0.65Ca0.35MnO3, a photovoltaic and electro-catalytic material with strong polaronic correlations, reveal an ultra-slow recombination dynamics on a nanosecond-time scale [1]. The photo-diffusion of excited electron-hole polaron pairs gives rice to a pronounced photovoltaic effect [1,2] and is studied by electron beam induced current (EBIC) on nanometer length scales [3]. Theoretical considerations suggest that the excited charge carriers are trapped in a hot polaron state. The dependence of the lifetime on charge order implies that strong correlation between the excited polaron and the octahedral dynamics of its environment appears to be substantial for stabilizing the hot polaron [4]. Furthermore, modification of the interfacial atomic and electronic structure of the manganite-titanite junctions gives insights into the processes underlying the transfer of a small polaron across the interface [5].

[1] Evolution of hot polaron states with a nanosecond lifetime in a manganite, D. Raiser, S. Mildner, B. Ifland, M. Sotoudeh, P. Blöchl, S. Techert, C. Jooss, Advanced Energy Materials, 2017, 1602174, DOI: 10.1002/aenm.201602174

[2] Polaron absorption for photovoltaic energy conversion in a manganite-titanate pn-heterojunction, G. Saucke, J. Norpoth, D. Su, Y. Zhu and Ch. Jooss, Phys. Rev. B 85, 165315 (2012).

[3]  Low energy scanning transmission electron beam induced current for nanoscale characterization of p-n junctions, Patrick Peretzki, B. Ifland, C. Jooss, M.Seibt, Phys. Status Solidi RRL 11,1600358 (2017)

[4] Contribution of Jahn-Teller and charge transfer excitations to the photovoltaic effect of manganite/titanite heterojunctions, B. Ifland, J. Hoffmann, B. Kressdorf, V. Roddatis, M. Seibt and Ch. Jooss, New Journal of Physics, 19 (2017) 063046

[5 ] Current–voltage characteristics of manganite–titanite perovskite junctions, B. Ifland, P. Peretzki, B. Kressdorf, Ph. Saring, A. Kelling, M. Seibt and Ch. Jooss, Beilstein Journal of Nanotechnology, 2015, 6, 1467–1484

As an effective way to surpass the Shockley-Queisser efficiency limit multijunction solar cells have been designed and developed for many years now. Silicon, in particular, is very appealing as the low bandgap material in a tandem design since we already have a mature understanding of its optical and electronic properties and its fabrication technology is widely available. For the high bandgap, III-V materials are shown to be promising in both light management and carrier management. However, fabricating monolithic III-V on Si multijunction is still challenged by various limiting factors such as lattice mismatching, high fabrication cost, and the size/weight of the tandem designs which makes them unsuitable for many applications. One way to tackle these issue is epitaxial growth of vertically standing III-V nanowires on Si ultrathin substrate. This design has many advantages: Not only the wires can overcome the mismatching problem thanks to their intrinsic strain relaxation properties, they also create a natural anti-reflection coating.

In this work, we study light-matter interactions in GaAs-based nanowire arrays on ultrathin silicon film with the dual goal of obtaining large absorption in the array and improving light trapping in the bottom thin film cell. By performing FDTD simulations we show that the coupling of the incident light to the HE11 waveguiding mode of the wires not only boosts the absorption in the wires themselves, but also efficiently transfers the non-absorbed light to Si bottom cell. Moreover, the grating properties of the array is capable of changing the momentum of the transmitted light. In this way, light is trapped in the Si thin film. As a result, we induce higher absorption for the wavelengths close to the bandgap of silicon where the absorption coefficient is very low.

To conclude, by optimizing the geometry of both each wire and the grating we are able to firstly couple the light into waveguiding modes of each wire and later couple the transmitted/scattered light into waveguiding modes of the ultrathin silicon layer underneath. By combining these 1D and 2D waveguiding properties a high efficiency ultrathin and flexible tandem cell is designed.

In the last years semiconductor organometallic halide (CH₃NH₃PbX₃, X=Cl, Br, I) perovskites (MHP) have emerged as an outstanding material to develop a new generation of photonics and electronics devices [1]. MHPs present excellent conductivity, light detection and emission properties, which have been successfully exploited in the implementation of broad range of optoelectronic devices. Examples include solar cells with efficiencies higher than 22%, broad band photodetectors, efficient optical sources, or optical amplifiers with low thresholds. Nevertheless, in spite of this important progress, most of this works use a rigid substrate to fabricate the device, while the integration of MHPs in flexible substrate is still at its very beginning. These new kinds of substrates, however, represent an important trend in optoelectronics, not only due to their wide range of applications, but also to the possibility to implement wearable devices directly in contact with the skin or clothes. In this work, MHP materials are successfully incorporated on a nanocellulose (NC) substrate with the intention to to construct wearable devices with new functionalities based on the light emission/detection properties of MHPs. In particular, a bilayer Poly(methyl methacrylate) /MHP deposited on NC  resulted in a suitable waveguide to demonstrate amplification of the spontaneous emission (ASE) with a threshold as low as 3-4 nJ [2-3].  Moreover, when a photodetector system is integrated within the waveguide, the device provides a photocurrent useful to monitor the light/ASE propagated/generated along the structure [4]. This approximation paves the road of new wearable systems with a broad range of applications.

[1] I. Suárez: Active photonic devices based on colloidal semiconductor nanocrystals and organometallic halide perovskites, Eur. Phys. J. Appl. Phys., vol. 75, 30001, 2016.

[2] T.T. Ngo, et al.: Enhancement of the Performance of Perovskite Solar Cells, LEDs, and Optical Amplifiers by Anti‐Solvent Additive Deposition," Adv. Mater., vol.  29, pp.  1604056, Dec. 2016

[3] I. Suárez et al.: Polymer/perovskite amplifying waveguides for active hybrid silicon photonics. Adv. Mater., vol. 27, 6157-6162, 2015.

[4] I. Suárez et al.: Integrated Optical Amplifier-Photodetector on a Wearable Nanocellulose Substrate. Adv. Opt. Matter., online available.

The electrical conversion and storage of solar energy is a crucial world target in the long-term scenario. Therefore, the use and integration of photovoltaic (PV) technologies into different electrochemical processes for obtaining solar fuels have been strongly developed in the last years. Following this approach, the integration of PV and other energy storage systems such as batteries is a logical approach that pursues simplification of capture and storage of the solar energy through direct conversion into (electro)chemical energy. These so-called solar batteries offer the advantage of carrying out in a single device, a process normally done in several steps in two independent units, which result expensive, bulky and inefficient. Among several other configurations, the application of this concept to redox flow batteries has attracted attention considering their advantages, including decoupling of energy and power and large-scale development.

Despite the obvious interest in these systems, the direct influence of the redox potential (i.e., selected redox pair) into the operation point of the PV, constitutes an important challenge in the design of such devices. Therefore, some studies have focused on metal oxides and/or on PV tandem configurations, for instance CdS/DSSC, being applied to organic redox pairs and/or to vanadium redox flow batteries (VRFB) reaching limited state of charge (SoC).

In this work, we report the adaptation and integration of thin film PV to VRFB in a single device. Copper Indium Gallium Selenide (CIGS) modules have been adapted by the interconnection of CIGS commercial cells deposited on flexible metallic substrates. This way, three and four-cell modules leading to different open circuit voltages (OCV) and i-V performances were integrated in VRFB with modified carbon felt (CF) electrodes. Two kinds of batteries reaching different cell voltages were evaluated.

A very close dependence between the VRFB cell potential and the photocurrent of the photovoltaic system has been observed, ultimately influencing the overall capacity of the battery. In particular, there is a clear difference between using 3 or 4 CIGS cells, due to the different operation points. Therefore, it is obvious that adapting of the PV is necessary before its integration. Ultimately, the four-cell module has provided enough photovoltage for carrying out the unbiased photo-assisted charge of the battery up to SoC 100%, with acceptable charge time and overall round-trip energy conversion efficiency of around 4%.

We have used time-resolved transient spectroscopies to better understand both bulk carrier dynamical processes as well as surface carrier dynamics. We employed transient reflection spectroscopy to measure the surface carrier dynamics in methylammonium lead iodide perovskite single crystals and polycrystalline films. We find that the surface recombination velocity (SRV) in polycrystalline films is nearly an order of magnitude smaller than that in single crystals, likely due to unintended surface passivation of the films during synthesis. In spite of the low SRV, surface recombination limits the total carrier lifetime in polycrystalline thin films, meaning that recombination inside grains and at grain boundaries is less important than top and bottom surface recombination. The suppressed SRV in the polycrystalline films appears to be related to an excess of methylammonium compared to the single crystals surfaces, determined by X-ray photoelectron spectroscopy analysis.  We studied the charge carrier dynamics in 2D Ruddlesden-Popper perovskites PEA2PbI4·(MAPbI3)n−1(PEA, phenethylammonium; MA, methylammonium; n = 1, 2, 3, 4) single crystals. We have also studied carrier dynamics in Perovskite QD samples and will discuss recent results. 

Recently, the low-temperature solution-processed metal-halide perovskites have demonstrated great potential in light harvesting applications. The light to electricity power conversion efficiency up to 23.25% has been achieved. The primary advantages of these lead based perovskites for solar cells are the large photon absorption coefficient, long-range balanced charge carrier diffusion lengths, low trap density, high charge carrier mobility, fast and efficient photo-generated exciton dissociation and slow electron-hole bimolecular recombination. Additional to these properties, the tunable optical direct bandgap over a wide range also makes these perovskites good candidates for using in light emission applications (Lasing & Electroluminescence). However, the relatively slow electron-hole bimolecular recombination in the traditional three-dimensional perovskites that drives their outstanding photovoltaic performance is a fundamental limitation for the light emission. Here, we show that the perovskite light emission efficiency could be greatly enhanced by tailoring the bimolecular-type recombination in three-dimensional crystals to excitonic-type recombination in low-dimensional crystals.

References

[1] Guichuan Xing, Bo Wu, Xiangyang Wu, Mingjie Li, Bin Du, Qi Wei, Jia Guo, Edwin K. L. Yeow, Tze Chien Sum, Wei Huang, Nature Communications, 8, 14558 (2017).

[2] Guichuan Xing, Mulmudi Hemant Kumar, Wee Kiang Chong, Xinfeng Liu, Yao Cai, Hong Ding, Mark Asta, Michael Grätzel, Subodh Mhaisalkar, Nripan Mathews, Tze Chien Sum, Advanced Materials, 28, 8181-8196 (2016).

[3] Guichuan Xing, Nripan Mathews, Swee Sien Lim, Natalia Yantara, Xinfeng Liu, Dharani Sabba, Michael Grätzel, Subodh Mhaisalkar, Tze Chien Sum, Nature Materials, 13, 476-480 (2014).

[4] Guichuan Xing, Nripan Mathews, Shuangyong Sun, Swee Sien Lim, Yeng Ming Lam, Michael Grätzel, Subodh Mhaisalkar, Tze Chien Sum, Science, 342, 344-347 (2013).

[5] Fei Yan, Jun Xing, Guichuan Xing, Lina Quan, Swee Tiam Tan, Jiaxin Zhao, Rui Su, Lulu Zhang, Shi Chen, Yawen Zhao, Alfred Huan, Edward H Sargent, Qihua Xiong, Hilmi Volkan Demir, Nano Lett., 18, 3157-3164 (2018)

Acknowledgements: Financial support from the Macau Science and Technology Development Fund (FDCT-116/2016/A3 and FDCT-091/2017/A2), Research Grant (SRG2016-00087-FST and MYRG2018-00148-IAPME) from the University of Macau, the Natural Science Foundation of China (91733302, 61605073 and 2015CB932200) is gratefully acknowledged.

In less than 10 years, hybrid organic-inorganic perovskites have emerged as a new generation of absorber materials for high-efficiency and low-cost solar cells [1], [2]. More recently, fully inorganic perovskite quantum dots (QD) also led to promising efficiencies [3], [4] and then become a serious alternative to hybrid organic-inorganic perovskites. Currently, the record efficiency for QD solar cells is obtained with CsPbI₃. High resolution in-situ synchrotron XRD measurements have been performed on CsPbI₃ as a function of the temperature and revealed a highly anisotropic variation of the lattice parameters. Moreover, CsPbI₃ can be temporarily maintained in a perovskite-like structure down to room temperature, stabilizing a metastable perovskite polytype (black-phase) crucial for photovoltaic applications. Structural, vibrational and electronic properties of the three experimentally observed black phases are further scrutinized using theoretical approaches [5], [6]. A symmetry-based tight-binding model, calibrated with self-consistent GW calculations including spin-orbit coupling, affords further insight into their electronic properties. A Rashba effect is thus predicted for both cubic and tetragonal phases when using the symmetry breaking structures obtained through frozen phonon calculations.

The ab initio simulations have been performed on HPC resources of CINES under the allocation 2017-[x2017096724] made by GENCI (Grand Equipement National de Calcul Intensif).

[1] A. Kojima, et al., Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J.

Am. Chem. Soc. 2009, 131, 6050−6051.

[2] Best research-cell efficiencies; https://www.nrel.gov/pv/assets/images/efficiency-chart.png (accessed Nov 7, 2017).

[3] H. Bian et al., Graded Bandgap CsPbI2+xBr1-x Perovskite Solar Cells with a Stabilized Efficiency of 14.4%, Joule (2018), https://doi.org/10.1016/j.joule.2018.04.012

[4] E. M. Sanehira, et al., Enhanced Mobility CsPbI3 Quantum Dot Arrays for Record-efficiency, High-voltage Photovoltaic Cells. Sci. Adv. 2017, 3, eaao4204.

[5] A. Marronnier et al., Structural Instabilities Related to Highly Anharmonic Phonons in Halide Perovskites. . J. Phys. Chem. Lett. 2017, 8, 2659−2665

[6] A. Marronnier et al., Anharmonicity and Disorder in the Black Phases of Cesium Lead Iodide Used for Stable Inorganic Perovskite Solar Cells. ACS Nano 2018, 12, 3477−3486

Due to their sizable refractive index, reflectivity of visible light off the lead halide perovskite - air interface exceeds 15%. This has prompted a number of investigations into the prominence of photo-reflective contributions to pump-probe data in these materials, with conflicting results. Here we report experiments aimed at assessing this by comparing transient transmission from lead halide perovskite films and weakly quantum confined nanocrystals of cesium lead iodide (CsPbI3) perovskite. The rationale is to compare pump-probe measurements samples where the relative contributions of changes in the real and imaginary components of the refractive index are very different. The absorption cross section of a nanocrystal of volume v in terms of its complex refractive index and that of its (non-absorbing) surroundings n0 is

is a local field factor which must be included since it depends on the same refractive indices. For a polycrystalline film or a crystal, reflectivity at the interface with a non-absorbing dielectric whose refractive index is n0, is given by the Fresnel relation {2}:

The first derivatives of sabs or of R (both of which reduce transmission) with respect to n or k quantify their sensitivity to variations in either constant. By analyzing how complex refractive index changes impact the two experiments we can show that reflectivity changes due to variations in n would not only differ in amplitude but even in sign in both experiments. The results presented in the figure below demonstrate virtually identical TA data for both samples proving that changes in absorption and not reflection dominate transient transmission measurements in thin films of these materials. None of the characteristic spectral signatures reported in such experiments is exclusively due to, or even strongly affected by changes in sample reflectivity. This finding is upheld by another experiment where a methyl ammonium lead iodide perovskite film was formed on high index flint glass, and probed after pump irradiation from either face of the sample. We conclude that interpretations of ultrafast pump-probe experiments on thin perovskite films in terms of photo-induced changes in absorption alone are qualitatively sound, requiring relatively minor adjustments to factor in photo-reflective effects.

Layered halide hybrid organic−inorganic perovskites [1] have been the subject of intense investigation before the rise of three-dimensional (3D) halide perovskites and their impressive performance in solar cells. Recently, layered perovskites have also been proposed as attractive alternatives for photostable solar cells [2] and revisited for light-emitting devices. Interestingly, these performances can be traced back to extremely efficient internal exciton dissociation through edge states identified on thin films and single crystals [3].

Layered perovskites present fascinating features with inherent quantum and dielectric confinements imposed by the organic layers sandwiching the inorganic core, and computational approaches have successfully help rationalized their properties (excitonic, Rashba effects, etc.) [4-6]. Here, we propose a joint spectroscopic and computational investigation to unravel the origin of the recently identified layer-edge states in layered Ruddlesden-Popper phases with inorganic layers containing n = 1 to 4 octahedra. We show that for n > 2, the system presents a localized surface state within the band gap.

Based on our conclusion, we propose an elastic model providing design principles for future layered perovskites with optimized properties for photovoltaics or light emission.

References

[1] L. Pedesseau et al., ACS Nano (2016), 10, 9776.

[2] H. Tsai et al.,Nature (2016), 536, 312.

[3] J.-C. Blancon et al., Science (2017), 355, 1288.

[4] M. Kepenekian et al., ACS Nano (2015), 12, 11557.

[5] D. Sapori, M. Kepenekian, L. Pedesseau, C. Katan, J. Even, Nanoscale (2016), 8, 6369.

[6] M. D. Smith et al., Chem. Sci. (2017), 8, 1960.

The analysis of perovskite solar cells by impedance spectroscopy has provided a rich variety of behaviors that demand adequate interpretation. Two main features have been reported: First, different impedance spectral arcs vary in combination; second, inductive loops and negative capacitance characteristics appear as an intrinsic property of the current configuration of perovskite solar cells. Here we adopt a previously developed surface polarization model based on the assumption of large electric and ionic charge accumulation at the external contact interface. Just from the equations of the model, the impedance spectroscopy response is calculated and reproduces and explains the mentioned general features. The inductance element in the equivalent circuit is the result of the delay of the surface voltage and depends on the kinetic relaxation time. The model is therefore able to quantitatively describe exotic features of the perovskite solar cell and provides insight into the operation mechanisms of the device.

Recently, hybrid perovskite solar cells (HPSCs) have achieved conversion efficiencies > 22 % and
have revived the search for clean, affordable and efficient energy. However, the practical realization
of this hope is pending due to problems related to the stability of HPSCs in moisture, heat,
ultraviolet light and oxygen rich environments [1]. Thus, the search is underway for protective coating materials to protect HPSCs. A proper coating should have a wide band gap in order to serve as a good window
material, exhibit minimum lattice mismatch at the coating-perovskite interface and most
importantly, be resistance to environmental conditions. In this study, we consider a series of ABX3 perovskites, with  A = MA/Cs, B = Sn/Pb and X = Cl/Br/I. To select possible protective-coating candidates, we
filtered the large Aflow [2] database to collect materials with wide band gap (≥ 3 eV) and further
sorted them based on their solubility in water, toxicity and abundance. To avoid large lattice
mismatch, we only considered rectangular surfaces of coating materials and limited the mismatch to be
within -5 and 5 %. For instance, for MAPI3, we found the following promising coating candidates, NiO, PbTiO3, BaZrO3 , ZnO and GaN, whose lattice mismatch are -0.35, -0.10, 0.37, -0.04 and 0.64%, respectively.
In addition to protecting the stability of HPSCs, our collected coating materials have the potential to serve as efficient transport layers in the HPSC architecture. Our search will not only improve the stability of HPSCs but also serve as a starting point in the search of novel device materials for emergent HPSC technologies.
Keywords
hybrid, perovskites, resistance, lattice mismatch

Reference
[1] M. A. Green, A. Ho-Baille, and H. J Snaith, Nature Photon. 8, 506(2014).
[2] R. H. Taylor, F. Rose, C. Tohler, O. Levy, K. Yang, M. B. Nardelli, and S. Curtarolo,
Computational Materials Science 93, 178 (2014).

Solution-processed halide perovskites have demonstrated remarkable performances in optoelectronic devices and applications. Despite the extraordinary progress associated with perovskite materials, many questions about the fundamental photophysical processes taking place in these devices remain open. Here we report the results from an in-depth computational study of small polaron formation utilizing information from electronic structure, charge density, and reorganization energy calculations on isolated structures. Local lattice symmetry, electronic structure, and electron phonon coupling are interrelated in polaron formation in hybrid halide perovskites. To illustrate these aspects, first principles calculations are performed on CsPbI₃, CsSnI₃, CsPbBr₃, MAPbI₃, FAPbI₃, MAPbBr₃, FAPbBr₃, MASnI₃, and FASnBr₃. This study will focus on how ionic exchange changes the geometry and polaron binding energy in the material. It is found that in all cases that hole polaron formation is associated with smaller binding energies and lattice contraction, while electron polaron formation exhibits larger polaron binding energies, lattice expansion, and Jahn Teller like distortions.

The development of organic-inorganic lead halide perovskites with very large efficiency requires us to understand the operation of the solar cell. The application of frequency techniques is a major tool for the analysis of perovskite solar cells. Here we describe the general theory and methods of small perturbation frequency modulated techniques. These methods connect a broad range of experimental tools that interrogate the system in specific steady-state conditions.¹ the first method we discuss is the simulation of the dynamic response of the perovskite/contact interface. Here we find that it is necessary to establish the main polarization conditions, and how they are coupled to recombination and charge transfer, eventually leading to a typical behaviour of negative capacitance. Secondly we investigate the physical meaning of light modulated techniques as IMPS and IMVS when applied to charge collection in perovskite solar cells. Third we introduce a light-to-light impedance that is able to provide information on phenomena of photon diffusion as in photon recycling regime.²

(1) Bertoluzzi, L.; Bisquert, J. Investigating the Consistency of Models for Water Splitting Systems by Light and Voltage Modulated Techniques, J. Phys. Chem. Lett. 2017, 8, 172-180.

(2) Ansari-Rad, M.; Bisquert, J. Theory of Light-Modulated Emission Spectroscopy, J. Phys. Chem. Lett. 2017, 8, 3673-3677.

The fundamental nature of charge carrier transport (band-like or polaronic), and the influence thereupon of various scattering mechanisms and defect distributions are of central importance to the operation of semi-conductor based devices. While there have been numerous investigations aiming to understand these effects in hybrid halide perovskites, there remains much to be understood [1,2]. The structural and compositional complexity of perovskite based solar cells renders it extremely difficult to disentangle these effects, and theoretical simulations can provide valuable insights and predictions. So far modelling has focused on atomistic [3] and continuum [4] length scales, but a model bridging these scales, while taking into account all of the aspects described above, is lacking.

Here, we will describe a “device Monte Carlo” meso-scale model, based on well established semi-classical transport theory, which takes into account the band structure of the material, phonon and defect scattering, and electrostatic fields arising from inhomogeneities in defect and carrier concentrations, using parameters derived from experiment and ab initio calculations. We will present the results of the application of this model to charge carrier transport in hybrid halide perovskites, with a particular emphasis on current–voltage characteristics and the experimentally observed effects of changing defect distributions under illuminatation [5].

References

[1] T. Brenner et al., Nat. Rev. Mater. 1 (2016) 15007

[2] L. M. Herz, ACS Energy Lett. 2 (2017) p1539

[3] C. Motta and S. Sanvito, J. Phys. Chem. C 122 (2018) p1361

[4] S. E. J. O’ Kane et al., J. Mater. C 5 (2017) p452

[5] G. Y. Kim et al., Nat. Mater. 17 (2018) p445

Knowledge of the vibrational structure of a semiconductor is essential for explaining its optical and electronic properties and enabling optimized materials selection for optoelectronic devices. However, experimental measurement of the vibrational density of states of nanomaterials is particularly challenging. In this contribtion, I will describe recent work my group has carried out, investigating electron-phonon interactions through a variety of computational and experimental techniques. In particular, we have performed ab-initio molecular dynamics simulations on CsPbBr₃ perovskite nanocrystals in order to gain insight in the electronic and vibrational structure of these materials. We haved studied the influence of the large surface to volume ratio in nanocrystals versus bulk crystals, and shown that our computational results match the first experimental measurements of the vibrational denisty of states of CsPbBr₃ perovskite nanocrystals obtained using inelastic x-ray scattering (IXS).  We also show how the power spectrum of the electron and hole wavefunction dephasing can be used to investigate which phonons couple strongly to electrons and holes and mediate non-radiative transitions. 

Mimicking photosynthesis and producing solar fuels is an appealing way to store the huge amount of renewable energy from the sun in a durable and sustainable way. Hydrogen production through water splitting has been set as a primary target for artificial photosynthesis,¹ which requires the development of efficient and stable catalytic systems, only based on earth abundant elements, for the reduction of protons from water to molecular hydrogen. We will report on our contribution to the development of various series of catalysts for H₂ evolution,^2-4 including the reinvestigation of amorphous molybdenum sulfide⁵ and to the establishment of methodologies towards the rational benchmarking of their catalytic activity. Besides, we will also describe our effort towards the combination of such catalysts with various photoactive motifs for the preparation of photoelectrode materials^6-10 that can be implemented into photoelectrochemical (PEC) cells for water splitting.

References

1.         N. Queyriaux, N. Kaeffer, A. Morozan, M. Chavarot-Kerlidou and V. Artero, J. Photochem. Photobiol. C, 2015, 25, 90-105.

2.         D. Brazzolotto, M. Gennari, N. Queyriaux, T. R. Simmons, J. Pécaut, S. Demeshko, F. Meyer, M. Orio, V. Artero and C. Duboc, Nat. Chem., 2016, 8, 1054-1060.

3.         N. Kaeffer, M. Chavarot-Kerlidou and V. Artero, Acc. Chem. Res., 2015, 48, 1286–1295.

4.         T. N. Huan, R. T. Jane, A. Benayad, L. Guetaz, P. D. Tran and V. Artero, Energy Environ. Sci., 2016, 9, 940-947.

5.         P. D. Tran, T. V. Tran, M. Orio, S. Torelli, Q. D. Truong, K. Nayuki, Y. Sasaki, S. Y. Chiam, R. Yi, I. Honma, J. Barber and V. Artero, Nat. Mater., 2016, 15, 640-646.

6.         J. Massin, M. Bräutigam, N. Kaeffer, N. Queyriaux, M. J. Field, F. H. Schacher, J. Popp, M. Chavarot-Kerlidou, B. Dietzek and V. Artero, Interface Focus, 2015, 5, 20140083.

7.         N. Kaeffer, J. Massin, C. Lebrun, O. Renault, M. Chavarot-Kerlidou and V. Artero, J. Am. Chem. Soc., 2016, 138, 12308−12311.

8.         T. Bourgeteau, D. Tondelier, B. Geffroy, R. Brisse, C. Laberty-Robert, S. Campidelli, R. de Bettignies, V. Artero, S. Palacin and B. Jousselme, Energy Environ. Sci., 2013, 6, 2706-2713.

9.         T. Bourgeteau, D. Tondelier, B. Geffroy, R. Brisse, R. Cornut, V. Artero and B. Jousselme, ACS Appl. Mater. Interfaces, 2015, 7, 16395–16403.

10.       A. Morozan, T. Bourgeteau, D. Tondelier, B. Geffroy, B. Jousselme and V. Artero, Nanotechnology, 2016, 27, 355401.

One of the most promising future sources of alternative energy involves water-splitting photoelectrochemical cells (PECs) – a technology that could potentially convert sunlight and water directly to a clean, environmentally-friendly, and cheap hydrogen fuel. Practical PEC-mediated hydrogen production requires robust and highly efficient semiconductors, which should possess good light-harvesting properties, a suitable energy band position, stability in harsh condition, and a low price. Despite great progress in this field, new semiconductors that entail such stringent requirements are still sought after. Over the past few years, graphitic carbon nitride (g-CN) has attracted widespread attention due to its outstanding electronic properties, which have been exploited in various applications – including in photo- and electro-catalysis, heterogeneous catalysis, CO2 reduction, water splitting, light-emitting diodes, and PV cells. gCN comprises only carbon and nitrogen, and it can be synthesized by several routes. Its unique and tunable optical, chemical, and catalytic properties, alongside its low price and remarkably high stability to oxidation (up to 500 °C), make it a very attractive material for photoelectrochemical applications. However, to date, only a few reports regarded the utilization of g-CN in PECs, due to the difficulty in acquiring a homogenous g-CN layer on a conductive substrate and to our lack of basic understanding of the intrinsic layer properties of g-CN. In this talk I will introduce new approaches to grow g-CN layers with altered properties on conductive substrates for photoelectrochemical application. The growth mechanism as well as their chemical, photophysical, electronic and charge transfer properties will be discussed.

While the field of sunlight-driven fuel generation has traditionally been dominated by inorganic materials, organic photocatalysts are currently gaining substantial momentum - particularly due to their much higher synthetic flexibility. For instance, their optical band gap can be tuned continuously throughout large parts of the solar spectrum by copolymerising suitable monomers in defined ratios.¹ This tunability has sparked intense research interest in organic photocatalysts,^2,3 however, the fundamental understanding of photoinduced processes in these systems has stayed behind the rapid development of new materials. Some parallels can be drawn to organic photovoltaics where comparable materials are used, but especially the aqueous environment makes polymer photocatalysts distinct from other applications. To understand what dictates their performance and how structurally similar polymers can exhibit very different degrees of hydrogen evolution activity,⁴ photophysical processes in these materials require further investigation.

The combined study presented here is the first in-depth investigation of hydrogen evolution activity of linear conjugated polymers and combines materials development with spectroscopic characterisation and computational modelling. We investigate a series of polymers with strikingly different hydrogen evolution activity, including some of the highest performing photocatalysts reported to date in this class of materials. A comparison to structurally related polymers with significantly lower activity allows us to identify the key determinants of hydrogen evolution activity in this series. To this end, we use transient absorption spectroscopy to monitor photogenerated reaction intermediates on time sales of femtoseconds to seconds after light absorption and correlate the type and yield of observed intermediates with the hydrogen evolution activity of the respective polymer. Computational simulations and calculations build on this transient data and extend the observations to the role of the solvent environment in the photoinduced reaction sequence. The presented results can provide design strategies for new materials and thus have implications for the development of more efficient organic photocatalysts.

References

1. Sprick, R. S. et al. Tunable Organic Photocatalysts for Visible-Light-Driven Hydrogen Evolution. J. Am. Chem. Soc. 137, 3265–3270 (2015).

2. Zhang, G., Lan, Z.-A. & Wang, X. Conjugated Polymers: Catalysts for Photocatalytic Hydrogen Evolution. Angew. Chemie Int. Ed. 55, 15712–15727 (2016).

3. Vyas, V. S., Lau, V. W. & Lotsch, B. V. Soft Photocatalysis: Organic Polymers for Solar Fuel Production. Chem. Mater. 28, 5191–5204 (2016).

4. Sprick, R. S. et al. Visible-Light-Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts. Angew. Chemie Int. Ed. 55, 1792–1796 (2016).

The concept of dye-sensitized  photoelectrosynthesis cells (DS-PECs) has recently been proposed as an alternative water splitting cell. The DS-PEC architecture involves an oxide semiconductor sensitized by an organic/inorganic chromophore and a water oxidation catalyst (WOC) either connected to the chromophore (a dyad assembly) or directly to the semiconductor. In this assembly, chromophore produces an electron-hole pair upon excitation with visible light followed by the injection of electrons to the semiconductor and then to the cathode compartment while holes are transferred to the catalytic site to activate the catalyst for water oxidation.

Despite the recent promising studies, achieving high stability for both the catalyst and the chromophore under DS-PEC operating conditions, extending the light absorption to visible and near IR region, and developing entirely earth abundant assemblies still remain as key challenges. 

Our recent research efforts have recently been devoted to developing dye-sensitized photoanodes involving cyanide-based assemblies. For this purpose, pentacyanoferrate building block was used not only as donor system for the construction of a new donor-acceptor chromophore but also as a cyanide precursor to produce a heterogeneous cobalt-iron Prussian blue electrocatalyst film. The photoelectrochemical performance and superior stability of the Prussian blue film coupled with TiO₂ will be presented in detail.

Integrated wireless monolithic solar water splitting devices, i.e. monoliths submerged in the electrolyte, is a promising approach for low-cost photoelectrochemical solar water splitting [1]. However, such a device design poses a significant limitation: the ion transport distances around the monolith are long and consequently, the ionic Ohmic losses become high. This turns out to be a bottleneck for reaching high efficiencies and maintain optimum performance when it comes to up-scaling.

In this work, we present a novel approach to tackle the aforementioned limitation. Our device design is based on the concept of porosity as an ionic shortcut, implemented in silicon-based monoliths for low-cost and scalable solar water splitting [2, 3]. Simulation and experimental results towards enabling the proof of concept device consisting of porous multi-junction thin-film silicon solar cells on perforated substrates are presented. Based on simulations, we highlight how porous monoliths can benefit from lower ionic Ohmic losses compared to dense monoliths for various pore geometries and monolith thicknesses. As a result, the overpotentials to drive the water splitting reaction can be reduced by more than 400 mV. Experimentally, the impact of porosity (square array of holes with a period of 100 μm and a diameter of 20 μm) on single-junction and multi-junction amorphous and microcrystalline thin-film silicon solar cells is explained. A minimal decrease in V_OC is seen, with porous triple-junction thin-film silicon solar cells reaching a value of 1.98 V. Additionally, we discuss the implementation of surface coatings by atomic layer deposition to i) alleviate the material degradation that occurs during silicon etching (electrical passivation) and ii) enable a chemically stable operation (protection against material corrosion). Finally, to demonstrate the beneficial effect of porosity on the hydrogen production we focus on two systems: i) a well-known simplified system based on platinum nanoparticle decorated porous silicon photocathodes (V_on ≈ 475 mV) immersed in a sulfite scavenger (cell potential ΔΕ⁰ = 170 mV) in absence of electrical bias and ii) the envisage device of porous multi-junction silicon solar cell monoliths in unbiased solar water splitting conditions. Overall, keeping a device oriented point of view, we discuss the results and challenges in our approach as well as some design guidelines.

References:

[1] S. Y. Reece et al., Science 334, 645-648 (2011).

[2] T. Bosserez et al., J. Phys. Chem. C 120 (38), 21242 – 21247 (2016)

[3] C. Trompoukis et al., Sol. Energy Mater. Sol. Cells 182, 196–203 (2018).

Photocatalytic conversion of CO2 and H2O is an interesting route to produce fuels and chemicals [1]; this process is also known as Artificial Photosynthesis (AP). In last years, extensive efforts have been made to develop efficient catalytic systems capable of harvesting light absorption and reducing CO2 especially when using water as the electron donor. Several modification pathways of inorganic semiconductors have been performed to improve the reaction efficiencies, including tailoring of the band structure, doping with metals and non-metallic elements and deposition of metal nanoparticles, etc [1-2].

Herein, we report different strategies and modifications of photocatalysts to increase process performance. Among them, an interesting approach to improve charge separation in photocatalytic systems is the use of heterojunctions. In this line, the combination of different semiconductors with noble metal nanoparticles or organic semiconducting polymers leads to a separation of the photogenerated charge carriers and thus to increasing their life time, facilitating charge transfer to adsorbed molecules.

The main products, using bare TiO2, were CO and H2, with low concentrations of CH4. The deposition of surface plasmon nanoparticles (SP-NPs) leads to changes in the selectivity to higher electron-demanding products, such as CH4. TAS measurements confirm that this behaviour is due to the electron scavenging ability of SP-NPs.

The H2 evolution rates of organo-inorganic hybrid materials, is increased with the polymer content reaching the optimum with IEP-1@T-10 which improves the activity of TiO2 by a factor of 40. These hybrid materials also show a dramatic reactivity improvement in CO2 photoreduction. Hybrid materials also show a great change in the selectivity, enhancing the relative production of methane vs. carbon monoxide, and largely promoting selectivity towards hydrogen. Meanwhile, bare TiO2 gives rise to the formation of CO, a small amount of CH4 and H2 and traces of CH3OH and higher hydrocarbons (C2 mainly). To explain this behaviour a combination of in-situ NAP-XPS, FTIR, TAS spectroscopies and theoretical tools has been used, showing a more efficient light absorption and charge transfer in the hybrid photocatalyst compared with bare materials.

References

[1] V. A. de la Peña O’Shea, D. P. Serrano, J. M. Coronado, “Current challenges of CO2 photocatalytic reduction over semiconductors using sunlight”, in Molecules to Materials—Pathway to Artificial Photosynthesis, Ed. E. Rozhkova, K. Ariga (Eds.), Springer, London, 2015.

[2] L. Collado, A. Reynal, J.M. Coronado, D.P. Serrano, J.R. Durrant, V.A. de la Peña O'Shea, Appl. Catal. B: Environ. 178, 177, 2015

Understanding the charge transport rates across molecules and molecule-electrode interfaces  is important in many areas of research including chemistry, biology, and nanoscience.^1-2 A crucial parameter is the tunnelling decay coefficient (β, in Å^-1 or n_C^-1) which determines how quickly the current across the junction decreases as a function of the length of the molecule. Usually, the value of β can be changed by changing the chemical structure of the molecular backbone,^3-4 but β also depends on the type of the binding with the electrodes for conjugated systems.³ We have reported that SAMs of S(CH₂)_nX where X = H, F, Cl, Br, or I, have increasingly high currents with increasing polarizability of X.⁵ Here we report a new approach to tune β by simply changing X. In this system, eutectic gallium-indium alloy (EGaIn) was used as the top electrode, a monolayer of S(CH₂)_nX was self-assembled on a Ag surface which also served as the bottom electrode. We found that as the polarizability of X increases from X = F to I, β decreased from 0.97 ± 0.04 n_C^-1 to 0.34 ± 0.01 n_C^-1 and the dielectric constant ε_r increased from 2.5 ± 0.6 to 8.9 ± 1.6, respectively. DFT calculations show that the electrostatic potential profile of the SAM depends on X. More specifically, we found that the HOMO-1 is the dominant conduction orbital that is highly effected by X resulting in the tunnelling barrier height and thus the decay coefficient. In other words, the value of β can be controlled by using one polarizable atom without the need to change the molecular backbone.

References:

(1)           Stubbe, J. et al. Chem. Rev. 2003, 103, 2167.

(2)           Heitzer, H. M. et al. Acs Nano 2014, 8, 12587.

(3)           Kim, B. et al. Am. Chem. Soc. 2011, 133, 19864.

(4)           Xie, Z. et al. Acs Nano 2015, 9, 8022.

(5)           Wang, D. et al. Adv. Mater. 2015, 27, 6689.

Among many other applications, room temperature ionic liquids (ILs) are used as electrolytes for storage and energy conversion devices. In this context, rechargeable batteries are extended and useful devices to store energy. Since their discovery in 1970s, Li-ion batteries have become popular energy storage solutions and nowadays represent a promising alternative to conventional devices. The increasing requirements and power of these batteries, and disadvantages such as degradation or high flammability, make essential the search of alternative materials. Thanks to the high abundancy of Na as a raw material, Na-ion batteries have attracted intense attention as potential candidates for the replacement of Li-ion batteries. Other alternatives to Li-ion batteries based on multivalent metal cations are emerging in recent years. The ability of these metal species to transfer more than one electron can be useful to obtain faster charge rates. In this work, we investigate at microscopic level the structural and dynamical properties of 1-methyl-1-butyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide [C₄PYR]⁺[Tf₂N]^- IL-based electrolytes for metal-ion batteries. We carried out molecular dynamics simulations of electrolytes mainly composed of [C₄PYR]⁺[Tf₂N]^- IL with the addition of Mⁿ⁺-[Tf₂N]^- metal salt (M = Li⁺, Na⁺, Ni²⁺, Zn²⁺, Co²⁺, Cd²⁺, and Al³⁺, n = 1, 2, and 3) dissolved in the IL. The addition of low salt concentration lowers the charge transport and conductivity of the electrolytes. This effect is due to the strong interaction of the metal cations with the [Tf₂N]^- anions, which allows for molecular aggregation between them. We analyze how the conformation of the [Tf₂N]^- anions surrounding the metal cations determine the charge transport properties of the electrolyte. We found two main conformations based on the size and charge of the metal cation: monodentate and bidentate (number of oxygen atoms of the anion pointing to the metal atoms). The microscopic local structure of the Mⁿ⁺-[Tf₂N]^- aggregates influences the microscopic charge transport as well as the macroscopic conductivity of the total electrolyte.

Acknowledgements. The research leading to these results has received funding from the Andalucía Region (FQM-1851).

While charge transport in highly ordered system is generally well understood, correct modelling of conductivity in highly disordered system quite often presents an important theoretical but also experimental challenge. Slow conductance relaxation has been studied in many disordered insulators using field effect measurements. After a quench at low temperatures, a change in the gate voltage is accompanied by a sudden increase in the conductivity, slowly decreases with a roughly logarithmic dependence on time. Memory effects and aging are often seen in the same type of experiments. These glassy behaviour has been interpreted in different ways, but there is a growing tendency to explain them in terms of electron glasses, i.e., systems with states localized by the disorder and long-range Coulomb interactions between carriers.

The use of local probe techniques, such as scanning Kelvin probe microscopy (SKPM), presents two advantages as compared with the conductance measurements performed so far: i) it allows a study of the phenomena at the nanometer scale, and ii) samples with larger resistances can be measured. In the present work Dynamic AFM is used to characterize the electronic properties of two quite different nanoscale highly disordered and low conductivity systems. On the one hand highly resistive granular metal grains[1], and on the other Graphene Oxide islands[2].

We apply Electrostatic and Kelvin Force Microscopy to these samples. In addition, their time evolution is studied using “movies” where topography and surface potential are acquired simultaneously. Our AFM studies suggest evidence of the formation of an electron glass on the materials studied. This evidence includes the presence of domains on the surface potential, uncorrelated with topograph. The fluctuations of the surface potential are compatible with variations of the Coulomb energy of a single charge over the distance between domains. At the same time, the fact that the fluctuations are larger than kT and that time correlations are dominated by a broad distribution of characteristic times can be naturally explained within the electron glass model. When the conducting polymers are excited with light the surface potential relaxes logarithmically with time, as usually observed in electron glasses.

[1] M. Ortuño, E. Escasaín, E. López-Elvira, A. Somoza, J. Colchero and E. Palacios-Lidón. Scientific Reports, Scientific Reports 6, article #21647 (2016).

[2]M.F. Orihuela, A.M. Somoza, J. Colchero, M. Ortuño, E. Palacions-Lidón, Physical Review B 95(20) 205427 (2017).

One of the potential pathways for exceeding the Shockley-Queisser efficiency limit of photovoltaic cells is via incoherent upconversion of low-energy photons. An ‘ideal’ upconversion system would have high efficiency, a broad response and a low saturation intensity, all encapsulated in an easily deposited system. Past colloidal upconversion nanocrystals were based on rare-earth doped oxides or on organic triple-triplet annihilation polymers. Several years ago we introduced double quantum dots as an alternative upconversion mechanism. In this system a low bandgap dot is coupled to a high band gap dot through a tunneling barrier. Interband absorption in the low bandgap dot is followed by intraband absorption which enables to transfer the excitation to the higher bandgap dot, eventually leading to emission of a higher energy photon. The formation of the latter system and recent advances in the design of double quantum dots will be discussed. We also introduce a new hybrid organic-inorganic particle where upconversion is mediated by ligand induced mid-gap states. Importantly, this latter system relies on direct excitation of charge transfer state and can therefore be potentially extended deeper into the infrared. The state of the art of all these designs will be discussed, as well as the potential for low cost upconversion add-ons to present day solar cells.

The maximum efficiency reachable by a photovoltaic device based on a single absorber is thermodynamically limited to ~33%; the Shockley−Queisser (SQ) limit. To a large extent, this efficiency limit is determined by waste heat in the absorber; waste heat which is generated by the thermalization of “hot” charge carriers generated in the material following the absorption of above-bandgap high-energy photons. Several approaches have been proposed in order to bypass thermal losses in solar cell devices, among them, hot carrier solar cells (HCSCs) are distinctly the most promising concept when considering photo-conversion efficiency limits (reaching ~74%). However, practical implementation of operational HCSCs prototypes remains a big challenge; this is in part due to the difficulties on finding/engineering systems where hot carriers are efficiently collected at room temperature.

In this communication, by employing time resolved THz spectroscopy (TRTS), we demonstrate highly-efficient room-temperature hot electron transfer (HET) at QD/mesoporous oxide interfaces. The emergence of HET is directly apparent from photon-energy dependent TRTS measurements. When the samples are irradiated with photon energies matching the QD bandgap, the ET dynamics are monophasic and defined by a time constant of ~10ps. When the samples are irradiated with above QD bandgap photon energies, the ET dynamics become bi-phasic with characteristic time constants of ~10ps and <1ps respectively (representing cold and hot ET components respectively). For even higher photon energies (~3eV photons onto ~1eV bandgap PbS QDs) the ET dynamics become again monophasic with sub-ps time constants (≤0.1ps, limited by the TRST setup resolution). In this case, the HET collection efficiency for photo-generated carries reaches unity quantum yield. Finally, from temperature dependent analysis of interfacial QD-oxide dynamics, we resolve that HET rates (and hence efficiency) are substantially enhaced as the temperature of the system is reduced. This observation is fully consistent with the “a pirori” expectation that HET efficiency is determined by kinetic competition between QD-to-oxide HET rate and hot electron cooling rate within the QD.

Our results reveal the effect and interplay of key parameters governing hot electron transfer at QD-oxide interfaces. The foreseen potential and constraints for the analyzed systems for the realization of hot carrier solar cells prototypes will be briefly discussed.

Nanocrystals of heavily-doped semiconductors have recently emerged as very promising materials for plasmonics. In contrast to nanocrystals of noble metals, their Localized Surface Plasmon Resonance (LSPR) can be easily tuned in energy by controlling the carrier concentration through doping. In addition, due to the low concentration of carriers compared to metals, the LSPR can be extended to infrared and near-infrared ranges. Recent experimental studies have demonstrated the existence of LSPR in doped nanocrystals of Si and different types of oxides (ZnO, SnO₂, In₂O₃). However, the physics of the LSPR in these NCs is not totally understood. In this talk, I will review recent theoretical studies performed to clarify a certain number of issues. The evolution with doping concentration of the optical processes from single-electron transitions to collective excitations will be described. The conditions required for the emergence of plasmonic modes will be discussed. The results of atomistic calculations will be compared with those of more classical approaches. The intrinsic mechanisms at the origin of plasmon damping in doped ZnO nanocrystals will be analyzed. In this case, the theoretical simulations show that the intrinsic line width of the LSPR can be below 80 meV, in agreement with recent experiments [1]. These results confirm that doped ZnO nanocrystals are very promising for the development of IR plasmonics.

[1] Delerue C. “Minimum Line Width of Surface Plasmon Resonance in Doped ZnO Nanocrystals”. Nano Letters 17 (12), 7599-7605 (2017).

In the present talk we discuss the opportunities and challenges for Kesterite based high band gap absorber layers to act as functional layers in tandem or semi transparent thin film solar cells. Materials based on the kesterite crystal structure have already shown their good properties as thin film solar cell absorbers. Depending on the elements in the crystal, a high band gap value can be achieved, which could present an opportunity for these type of materials to be used as a top cell in tandem devices or as an active layer in a semi-transparent thin film solar cell. After a general introduction we present the properties of Cu2ZnGe(S,Se)4 based solar cell devices, which have a band gap in the 1.4 to 2 eV range depending on the S content in the layer. The crystallization process is analyzed and the solar cell properties of such devices are presented in detail. Solar cell conversion efficiencies in excess of 8 % could be achieved with this material system but some challenges remain. We will compare these results to other high band gap chalcogenide thin film absorber materials.

Thin-film technologies represent one step forward in the range of applicability of photovoltaics (PV) since they can be manufactured onto virtually any surface or material giving them an enormous versatility compared to Si-based solar cells. Flexible and light-weight substrates like thin metallic or polymeric foils are particularly attractive for thin-film PV since they allow powering portable electronics, wearables and IoT devices adapting to any shape and without adding extra weight. These characteristics also make them interesting for applications in the space and transportation industry. In addition, flexible substrates are expected to reduce PV fabrication costs through roll-to-roll high-throughput industrial processes. Another interesting niche of application that can benefit from the versatility of thin-film PV is building-integrated photovoltaics (BIPV). Solar devices can be directly deposited on materials commonly used in construction such as ceramics or glazing so that solar devices are made integral parts of buildings reducing manufacturing and installation costs as well as avoiding the need of extra land allocation for power generation which are often regarded as two critical factors that may restrict the massive deployment of photovoltaics. Among thin-film technologies, Cu₂ZnSn(S_1-xSe_x)₄ compounds, also known as Kesterites, stand out by the earth-abundancy and non-toxicity of its constituent elements that makes them compatible with a future mass deployment of PV. Thus, this work explores the implementation of a Cu₂ZnSnSe₄ (CZTSe) sequential fabrication process based on the selenization of sputtered metallic stack precursors onto different substrates: polyimide (PI), stainless steel (SS) and ceramic tiles. However, the increased applicability range of these substrates has a dark side since they lack several favorable properties of soda-lime glass (SLG): favourable thermomechanical properties and beneficial alkali (Na and K) composition that diffuse into the absorber during its synthesis and are fundamental for high efficiency kesterite devices. This way, SS and ceramic possess rough surfaces that complicate the deposition of thin films and detrimental impurities in their composition that can hinder the performance of the devices. As for PI it has a low thermal robustness that limits the synthesis temperature below 500ºC. In addition, none of them contain alkalis thus requiring the development of extrinsic doping procedures. In this work, we present different strategies to overcome the specific problems of each of the substrates and demonstrate their suitability as substrates for CZTSe solar cells.

In a solar cell, an ideally selective electron (hole) contact would only exchange electrons (holes) with the conduction (valence) band of the absorber material. This would ensure that no losses occur due to surface recombination
and the open-cicuit voltage would be determined solely by generation and recombination in the bulk. We had shown that selectivity depends on the conductivity of the minority carriers in the vicinity of the contacts [1].
Introducing this general concept allows also to show that a solar cell does not require an electric field in the dark und thus that Voc can be much larger than the built-in voltage. After that different approaches will be discussed how to achieve a high degree of contact selectivity in solar cells such as the mobility junction, the pn junction, the heterojunction and more recent concepts such as thin layers with strong dipole moment altering the effective work function of the electrode [2,3]. Results from intensity and temperature-dependent measurements in combination with numerical simulations will be discussed in detail. Finally, examples from our own labs shall demonstrate which phenomena can arise from the selectivcity of the contacts in the case of silicon, organic and perovskite solar cells [4,5].

[1] U. Würfel, A. Cuevas, P. Würfel, IEEE J. Photovoltaics 5 (2015), 461
[2] U. Würfel et al., Adv. Energy Mater. 6 (2016), 1600594
[3] C. Reichel, U. Würfel et al., J. Appl. Phys. 123 (2018), 024505
[4] J. Reinhardt, M. Grein, C. Bühler, M. Schubert and U. Würfel, Adv. Energy Mater. 4 (2014), 1400081
[5] A. Spies, T. Sarkar, M. List and U. Würfel Adv. Energy Mater. 7 (2017), 1601750

In recent years, the interest in hybrid organic - inorganic perovskites rose at a rapid pace due to their tremendous success in the field of photovoltaic; but other fields, like light emitting diodes, show great potential as well. In such devices, the function and performance depend crucially on the proper alignment of the energy level landscape throughout the device, i.e. allowing for efficient charge transport across the various interfaces. Here, an advantage of these novel semiconductors is that the electronic structure and band gap energy can be readily tuned by changing the compositions of the perovskite.

In this talk, I will discuss recent findings regarding the variations in electronic structure of hybrid perovskites, covering all lead and tin based halide systems using a combined DFT and UV-/ inverse/ x-ray photoelectron spectroscopy study. Furthermore, with these surface sensitive techniques, the energetic alignment at interfaces between different layers can be probed in-situ by performing a stepwise film preparation. Looking at various bottom contacts I will show that chemical interactions, band bending, and interface dipole formation play an important role.

Lead halide materials have seen a recent surge of interest from the photovoltaics community following the observation of surprisingly high photovoltaic performance, with optoelectronic properties similar to GaAs. This begs the question: What is the limit for the efficiency of these materials? It has been known that under 1-sun illumination the efficiency limit of crystalline silicon is ∼29%, despite the Shockley–Queisser (SQ) limit for its bandgap being ∼33%: the discrepancy is due to strong Auger recombination. In this article, we show that methyl ammonium lead iodide (MAPbI3) likewise has a larger than expected Auger coefficient. Auger nonradiative recombination decreases the theoretical external luminescence efficiency to ∼95% at open-circuit conditions. The Auger penalty is much reduced at the operating point where the carrier density is less, producing an oddly high fill factor of ∼90.4%. This compensates the Auger penalty and leads to a power conversion efficiency of 30.5%, close to ideal for the MAPbI3 bandgap.

The effects of transport layers on perovskite solar cell performance, in particular anomalous hysteresis, are investigated. A model for coupled ion vacancy motion and charge transport is formulated and solved in a three-layer planar perovskite solar cell. Its results are used to demonstrate that the replacement of standard transport layer materials (spiro-OMeTAD and TiO2) by materials with lower permittivity and/or doping leads to a shift in the scan rates at which hysteresis is most pronounced to rates higher than those commonly used in experiment. These results provide a cogent explanation for why organic electron transport layers can yield seemingly “hysteresis-free” devices but which nevertheless exhibit hysteresis at low temperature. In these devices the decrease in ion vacancy mobility with temperature compensates for the increase in hysteresis rate with use of low permittivity/doping organic transport layers. Simulations are used to classify features of the current-voltage curves that distinguish between cells in which charge carrier recombination occurs predominantly at the transport layer interfaces and those where it occurs predominantly within the perovskite. These characteristics are supplemented by videos showing how the electric potential, electronic and ionic charge profiles evolve across a planar perovskite solar cell during a current-voltage scan. Design protocols to mitigate the possible effects of high ion vacancy distributions on cell degradation are discussed. Finally, features of the steady-state potential profile for a device held near the maximum power point are used to suggest ways in which interfacial recombination can be reduced, and performance enhanced, via tuning transport layer properties. 

Perovskite solar cells have gathered a large interest in the last years as a very compelling and promising photovoltaic technology thanks to many interesting properties such as a wide spectrum of deposition techniques, a simple integration with both organic and inorganic materials and, most important of all, a high light power conversion efficiency.

Perovskite materials have also challenged the scientific community due to the many different physical processes that concur to set the optical and electrical properties: from ferroelectricity [1], to ion migration [2], defects and different recombination processes [3]. An important aspect of perovskite films is the presence of interfaces, both due to grain boundaries as well as due to interfaces between the perovskite layer and the charge selective contacts. Although many progresses have been obtained in the quality of the film, still grain boundaries within the perovskite film in fabricated devices are present. These interfaces can play a major role in setting device performances and hysteresis effects.

The effect of these grain boundaries and interfaces have been investigated by many groups, we refer here to just one reference [4], but the effect to free charges and ion migration is still under debate.

In the present work we theoretically investigate the effect of ion migration with the presence of grain boundaries. The analysis is performed using two different models: a drift-diffusion model to study the role of the mesoporous electron selective contact [5] and kinetic Monte Carlo [6] for the effect of grains and grain boundaries.

References

[1] A. Pecchia et al., Nano Lett., 16, 988 (2016)

[2] J. M. Azpiroz et al., Energy & Environmental Science, 8, 2118-2127 (2015)

[3] L. M. Herz, Annual Rev. Phys. Chemistry, 67, 65-89 (2016)

[4] B. Roose et al., Nano Energy, 39, 24-29 (2017)

[5] A. Gagliardi and A. Abate, ACS Energy Lett., 3, 163 (2018)

[6] T. Albes, A. Gagliardi, Physical Chemistry Chemical Physics, 19 (31), 20974-20983 (2017)

Lead halide perovskites are promising materials for new generation photovoltaics, exceeding 22% of efficiency in solar cells devices.[1] The presence of native defects, however, can strongly affect their efficiency due to charge trapping processes which can limit the lifetime of the photogenerated charge carriers. In this work a state of the art Density Functional Theory (DFT) study of native defects in MAPbI₃ and MAPbBr₃ is presented, aimed to unveil the nature of deep charge traps in these materials and the associated defects chemistry. The technical aspects of the computational modelling of defects are illustrated, with particular emphasis on the role of theory in the accurate evaluation of defects properties. The role of spin-orbit and self interactions corrections are discussed, as well as the performance of different corrections schemes in the supercell approach.[2] Good practices and open issues in the technical modelling of defects in these materials are discussed. Thus, a global picture of the defects chemistry in these perovskites is provided by the analysis of the associated formation energies in different conditions of growth and of the thermodynamic ionization levels. Our discussion shows that the defects chemistry of these materials is intrinsically dominated by halide chemistry, that is at the heart of their high defects tolerance.[3]     

References

[1] Wehrenfennig et al. Adv. Mater. 2014, 26, 1584-1589.

[2] Komsa et al. Phys. Rev. B 2012, 86, 045112.

[3] Meggiolaro et al. Energy Environ. Sci. 2018, 11, 702-713.

There is a need to rationalise the catalytic mechanisms occurring during solar fuels production if the design rules for improved materials are to be identified. With both direct solar to fuels materials (e.g. photoelectrodes) and indirect systems (e.g. PV driven electrolysis) there is a common challenge – how to characterise short-lived intermediates that are present only at the electrode surface in the presence of a high concentration of bulk solvent/reagent.

IR-Vis Sum-Frequency Generation (SFG) spectroscopy specifically probes molecules at interfaces offering a sensitive probe of only the catalytically relevant species at the electrode surface. SFG has been widely used to study model electrochemical systems but rarely to explore the mechanisms of carbon dioxide reduction and water oxidation. Here I will describe both early experiments to examine photoelectrode surfaces and our recent reports on the mechanisms of carbon dioxide reduction by group 6 and 7 transition metal electrocatalysts which allow for the identification of new catalytic intermediates and the first in-situ observation of the “protonation-first” pathway by [Mn(bpy)(CO)₃Br]. We will also discuss how the potential and light dependence of the non-resonant response recorded during the SFG experiment can provide important insights into the changing nature of the double layer structure during the study of solar fuels materials.

Storing the energy of sunlight into feedstock chemicals or energy-rich compounds, such hydrogen, appears as an enticing option to broaden the utilization of solar energy far beyond classical photovoltaics. Among different emerging technologies, photoelectrochemical (PEC) water splitting stands out with the promise of competitive solar-to-hydrogen conversion efficiencies (predicted to be above 20 %) and a conveniently simple design.¹ These devices, comprising two photoactive electrodes wired-stacked together, to leverage their complementary light absorption, and directly immersed in an aqueous electrolyte, generate under illumination a voltage greater than 1.23 V, driving the photoelectrosynthetic reactions of H₂ and O₂ separately at the different electrode-electrolyte interfaces. Unfortunately, current conversion efficiencies are far below the expectations. In recent years, there has been an encouraging progress on the refinement of the bulk properties of the semiconducting electrodes (viz. nanostructuring, doping, band gap engineering, etc.) as well as on the design of more active electrocatalyst, both contributing to an improved performance. However, another key component of these devices, the semiconductor-liquid junction (SCLJ), where the complex reactions occur and therefore, whose electrochemical and catalytic properties (namely, surface potential, overpotential, kinetics of charge transfer and recombination) control the conversion efficiency, remains hazy posing a major bottleneck for enhancing the conversion efficiency. A better understanding on the electrocatalytic properties of the SCLJ could not only nail down the processes limiting the performance of these devices but also provide specific routes to patch them.

Over the last few years, a wide variety of in-operando electrochemical based techniques have burst in the field of electrochemical-based solar fuel production offering new insights on the nature of intermediate species, the functioning of electrocatalysts, the carrier dynamics within the electrodes, etc.² Here, we introduce a new technique where a network-type electrical-contact at the electrode-electrolyte interface afford direct probing of the surface carrier dynamics when combined with a transient photocurrent/photovoltage technique. This technique applied to model semiconducting materials incorporating state-of-the-art electrocatalyst demonstrates to provide unprecedented access to steady-state (interfacial energetics) and transient (interfacial kinetics) electrochemical information of the SCLJ in-operando. We believe that these new tools will help to forge a better understanding on the SCLJ and to establish the designing principles for a next generation of more efficient electrodes.

[1] M. S. Prévot, K. Sivula. J. Phys. Chem. C, 2013, 117, 17879-17893

[2] W. A. Smith, I. D. Sharp, N. C. Strandwitz, J. Bisquert. Energy Environ. Sci. 2015, 8, 2851-2862

Due to their potential long term stability, ease of synthesis, and low production cost, semiconducting metal oxide materials have received much attention for use as photoanode materials in photoelectrochemical water splitting devices. To date, most research has focused on binary semiconducting oxide materials. Since no binary oxide material has currently met all the criteria listed above, researchers have expanded the materials search database to include more complex multinary oxides. Among the multinary oxides investigated, bismuth vanadate, BiVO₄, is the highest performing material. However, charge carrier recombination at the BiVO₄/electrolyte interface remains a limitation. Reactions at the BiVO₄/electrolyte interface may give rise to surface states that can act as relay sites for charge injection into the electrolyte, but also as electron and hole traps that can enhance recombination rates. A detailed understanding of the chemical composition at the BiVO₄/electrolyte interface and its dependence on specific conditions (applied potential and illumination) would provide valuable input for strategies to suppress surface recombination and to further optimize BiVO₄-based photoanode materials.

We have used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to gain a molecular-level understanding of the BiVO₄/aqueous electrolyte interface. With soft X-rays water adsorption from the gas phase at pressures up to a few Torr can be studied providing information about the early stages of solid/electrolyte interface formation. The tender X-ray form (AP-HAXPES) can be used to directly interrogate a solid surface under a bulk-like electrolyte film that is tens of nanometers thick. Our AP-XPS measurements on the BiVO₄(010) single crystal surface indicate that the surface is significantly hydroxylated by ~0.5 Torr. Surface hydroxylation is accompanied by reduced vanadium at the surface which leads to occupied states above the valence band maximum. These states may act as recombination centers on BiVO₄-based photoanodes. Using AP-HAXPES we have studied the open-circuit behaviour of the thin-film BiVO₄/potassium phosphate electrolyte interface when illuminated with a solar simulator. Upon illumination we observe spectral changes consistent with the formation of a thin bismuth phosphate layer and significant restructuring of the electrolyte near the interface. Bismuth phosphate formation under illumination may quench surface states that have been observed in capacitance versus voltage scans. Surface state suppression by bismuth phosphate formation is a also potential explanation for the increase in performance of BiVO₄ photoanodes that have undergone light soaking. In general, these results provide fundamental information about the complex chemical behaviour of semiconductor/electrolyte interfaces in water splitting devices.

The development of efficient photoelectrochemical (PEC) cells for solar hydrogen production is necessary for a prospective renewable energy supply of mankind. Semiconducting films in PEC cells are used as absorber layers in a triple system of electrolyte-catalyst-semiconductor for generating hydrogen which can be stored and afterwards reburned to water in gas turbines, fuel cells or for the production of chemicals [1]. Due to the low efficiency and poor stability of the absorber layers in the electrolyte their performance must be considerably improved.

We were successful in the preparation of highly (001)-textured polycrystalline WSe_2+x thin films with a Pd-promoter on TiN:O back contact substrates [2]. In this process, in a first step X-ray amorphous Se-rich WSe_x films have been deposited at room temperature by a reactive magnetron sputtering onto a thin metal-promoter Pd film which were afterwards annealed in an H₂S(e)/Ar atmosphere. During the crystallization process a liquid promoter PdSe_x forms and migrates to the surface of the growing WSe₂ film. It acts as a solvent agent and accumulates at the non van-der-Waals-planes finally initiating the lateral growth of WSe₂ platelets and their coalescence. A maximum hole mobility of 70 cm² V^-1s^-1 was reached for our polycrystalline films, which is close to the value for a single crystalline WSe₂ (100 cm² V^-1s^-1).

Recently, we improved the photoelectrochemical performance using Pt [3], Rh and ammonium thiomolybdate (ATM: (NH₄)₂Mo₃S₁₃) layers on top of the WSe₂ photocathode for hydrogen evolution reaction in an acidic solution. The surface states of WS₂ and WSe₂ crystallites can be passivated by photodeposition of the metal catalysts such as Pt, Rh and Ag. We show that the ATM can be used instead of expensive conventional metal catalysts on the WSe₂ photocathode. In addition, the ATM layer forms a heterojunction with the WSe₂ film, which improves the charge carrier separation, and allows lowering the prize for preparation of catalyst layers on top of different types of semiconductors.

1. Fichtner, M. J. Alloys Comp. 509S, S529-S534 (2012).

2. Bozheyev, F., Friedrich, D., Nie, M., Rengachari, M. & Ellmer, K. Phys. Stat. Sol. A 211, 2013-2019 (2014).

3. Bozheyev, F., Harbauer, Zahn, C., Friedrich D., & Ellmer, K. Sci. Rep. 7, 16003 (2017).

Constructing a materials-specific theory of charge dynamics in organic single materials is a complex prob- lem, where the computation of accurate structural and vibrational properties needs to be coupled to ways of determining the charge mobility characteristics. In particular one needs an accurate method for describing excitations, which is also scalable to reasonably large systems. Here I will discussed how different flavours of constrained density functional theory (CDFT) can achieve such goal.

Firstly I will consider the most conventional form of CDFT, which allows one to calculate the energy of systems with displaced electron densities (e.g. in a charge transfer process). Such scheme can be used to extract a number of quantities important for charge dynamics. Here I will make examples of the calculation of 1) the charge transfer energies of molecules on surfaces, so to derive accurate level alignments [1,2]; 2) the quasi-particle gap renormalisation in molecular crystals [3]; 3) the reorganisation energy of molecules in the gas phase and on surfaces [4].

Then I will move to show a recently implemented scheme, which uses CDFT to compute elementary excitations in molecules [5]. This method, which we have named excitonic DFT (XDFT), calculates the M-particle excited state of an N-electron system, by optimizing a constraining potential to confine N−M electrons within the ground-state Kohn-Sham valence subspace. The efficacy of XDFT will be demonstrated by calculating the lowest single-particle singlet and triplet excitation energies of the well-known Thiel molecular test set, with results which are in excellent agreement with time-dependent density functional theory (TDDFT).

[1]A.M.Souza, I.Rungger, C.D.Pemmaraju, U.Schwingenschloegl and S.Sanvito, Constrained-DFTmethod for accurate energy-level alignment of metal/molecule interfaces, Phys. Rev. B 88, 165112 (2013).

[2]  Subhayan Roychoudhury, Carlo Motta and Stefano Sanvito, Charge transfer energies of benzene physisorbed on a graphene sheet from constrained density functional theory, Phys. Rev. B 93, 045130 (2016).

[3] A. Droghetti, I. Rungger, C.D. Pemmaraju and S. Sanvito, Fundamental gap of molecular crystals via constrained Density Functional Theory, Phys. Rev. B 93, 195208 (2016).

[4] Subhayan Roychoudhury, David D. O’Regan and Stefano Sanvito, Wannier-function-based constrained DFT with nonorthogonality-correcting Pulay forces in application to the reorganization effects in graphene- adsorbed pentacene, Phys. Rev. B 97, 205120 (2018).

[5] Subhayan Roychoudhury, Stefano Sanvito and David D. ORegan, XDFT: an efficient first-principles method for neutral excitations in molecules, arXiv:1803.01421 (2018).

The development of robust, chemically-sensitive techniques is crucial for the advancement of single-molecule electronics. Studies in single-molecule junctions largely rely on indirect electrical characterization to statistically evaluate the chemistry and quality of the established circuits. One fundamental challenge is the direct, quantitative determination of charge-vibrational coupling for well-defined single-molecule junctions. The ability to record molecular charge-vibrational coupling for individual species grants access to the determination of maximal charge transport efficiencies for specific molecular configurations and currents. Here we explore the charge-vibrational coupling for current-carrying tethered molecules by combined vibrational and metal-molecule-metal junction current-voltage spectroscopy. By inspecting the steady-state vibrational distribution during charge transport in a bis-phenyl-ethynyl-anthracene derivative by Raman scattering, we deduce a coupling constant of ≈0.35 vibrational excitations per charge carrier. Furthermore we follow the conformational response of a two-state molecular switch. Specifically, we remove the ground state polarizability and symmetry of a known p-terphenyl-4,4´´-dithiol (TPD) molecule by employing the 2,2´,5´,2´´-tetramethylated (TM-TPD) derivate. Whereas the highly sterically hindered, non-planar TM-TPD, lacking π-conjugation, in its pristine conformation does not exhibit a Raman signature, a marked on/off modulation of the single-molecule Raman signal exceeding a factor of 100 is achieved via redox state control by means of the applied voltage.

Support by the Deutsche Forschungsgemeinschaft (DFG) via SPP 1234 (Grant RE2592) & Munich Centre for Advanced Photonics (MAP), the European Research Council via Advanced Grant MolArt (n° 247299) and Chinese Scholarhip Council (H.B., Y.G.) is gratefully acknowledged.

Key refs.: JACS 140 (2018) 4835 | Nature Comm. 7 (2016) 10700 | Nature Nanotechn. 7 (2012), 673

Electron transport (ETp), i.e., electronic conduction, across protein monolayers in a solid state–like configuration is surprisingly efficient, comparable, length-normalized, to completely conjugated molecules. This is amazing as apparently nature does not use this capability, except for electron transfer, ET, within and some times between redox proteins, a process that is coupled to ionic transport.

Nature regulates ET via redox chemistry, while for ETp a redox process is not a necessary condition. This allows studying ETp via non-redox proteins, such as rhodopsins and albumins; remarkably, ETp is quite efficient also across these proteins.

If contact to the external world does not limit ETp, i.e., intra-protein transport dominates, there seems to be no barrier for transport. As ETp is temperature-independent, it may well be coherent…. This may be important also for electron transfer, ET, which involves injection and extraction of electrons, the analogue of which for ETp is the coupling to the electrodes. Then, efficient ETp via non-redox proteins suggests why there are redox centres in ET proteins, ito protect it from high reducing power.

I will discuss experimental data that illustrate our main results as well as some more recent ones on multi-heme proteins and on protein multilayers, which raise even more questions.^1,2

* work with Mordechai Sheves & Israel Pecht, Weizmann Inst.

References

C. Bostick et al.  Rep. Prog.Phys., 81 (2018) 026601, “Protein bioelectronics:a review of what we do and do not know” doi.org/10.1088/1361-6633/aa85f2 ,

N. Amdursky et al., Adv. Mater. 42, (2014) 7142 “Electronic Transport via Proteins” 10.1002/adma.201402304

Colloidally prepared, quantum-confined, semiconductor nanocrystals offer tunable energy gaps, strong photoluminescence, and, in some cases, optical gain and lasing [1]. We report ultrafast optical pump, X-ray diffraction probe experiments performed at Argonne National Lab’s Advanced Photon Source with CdSe nanocrystal (NC) colloidal dispersions as functions of particle size, polytype, and pump intensity. Shifts of diffraction peaks relate lattice heating and peak amplitude reduction conveyed transient lattice disordering (or melting). For smaller NCs, melting was observed upon absorption of as few as ∼15 electron–hole pair excitations per NC on average (0.89 excitations/nm³ for a 1.5 nm radius) with a similar electron-hole pair density inducing disordering for all examined NCs. Diffraction intensity recovery kinetics, attributable to recrystallization, occur over hundreds of picoseconds with slower recoveries for larger particles. Zincblende and wurtzite NCs revert to initial structures following intense photoexcitation suggesting melting occurs primarily at the surface, as supported by simulations. These findings suggest a need to take into account nanomaterial physical stability and transient electronic structure for high intensity excitation applications such as lasing and solid-state lighting.
[1] Klimov et al. Science, 290, 314 (2000).; Kazes et al. Adv. Mater. 14, 317 (2002).
[2] Kirschner et al. Nano Lett. 17, 5314 (2017).

Decades of investigation show that inter-band photoexcitation of quantum dots is followed by rapid relaxation of hot carriers to the quantized band edge states within one or two picoseconds. Due to the large oscillator strength and low degeneracy of the band edge exciton transition, evolution in its intensity and spectrum have played a pivotal part in probing quantum dot exciton cooling. These changes start as a bi-exciton spectral shift while carriers are hot, changing to a bleach due to state filling once the exciton relaxes. Accordingly, kinetics of the BE bleach buildup has served to characterize the final stages of carrier cooling, and its amplitude per cold exciton the degeneracy of underlying electronic states.

Hot multi-excitons (MX) add a new relaxation process to this scenario. Auger recombination (AR) reduces an N exciton state to N-1 plus heat, initially. Again, investigation of AR dynamics is based on the amplitude and decay kinetics of the BE bleach. Interpretation of such data was based on the following assumptions: 1) that ultrafast cooling of hot excitons leads directly to occupation of the lowest electron and hole states (in accordance with the lattice temperature and the state degeneracy), and 2) that aside from mild spectral shifts induced in the remaining band edge transitions, after carrier cooling is over the BE bleach increases linearly with N until state filling is complete.

To test these assumptions, three pulse pump-probe experiments were conducted in our lab, measuring fs transient transmission (TT) of PbSe nanocrystals in the presence and absence of single cold spectator excitons. Results show that the bleach introduced by a second hot exciton falls significantly from that introduced by the spectator. Later we will describe a recent extension to CdSe which shows that adding an additional hot exciton to the cold spectator reduces the band edge bleach merely by a half! We conclude that the source of incomplete bleaching by the second exciton are hitherto unrecognized random spin orientation conflicts between the two conduction electrons in the crystallites. The presence of this effect both in lead salts and in CdSe NCs demonstrates its generality. This new discovery imposes new restrictions on the utility of the BE exciton transition as a universal “exciton counter” in experiments with all kinds of semiconductor NCs
 

Using a combination of steady-state and ultrafast deep-ultraviolet (UV) we have identified the nature of the transitions at the optical gap of the much studied anatase TiO₂ nanoparticles. The first excitation is a strongly bound 2-dimensional exciton in the 3D lattice of the material.[1] We also find that coherent acoustic phonons confined in the nanoparticles selectively modulate the oscillator strength of the 2D exciton, and theory shows that the deformation potential is at the origin of the coherent phonon wavepackets.[2] Further studies of the charge electron and hole trapping in anatase TiO2[3] and in ZnO[4] using ultrafast X-ray spectroscopy will be presented.

Finally, time resolved X-ray studies of CsPbBr₃ perovskite NPs reveal the nature of the electron-hole recombination across the band gap, with the electrons being delocalized in the conduction band, while holes form small polarons in the valence band.[5]

[1] Strongly bound excitons in anatase TiO₂ single crystals and nanoparticles
E. Baldini, L. Chiodo, S. Moser, J. Levallois, E. Pomarico, G. Auböck, A. Magrez, L. Forro, M. Grioni, A. Rubio and M. Chergui. Nature Communications 8 (2017) 13

[2] Phonon-Driven Selective Modulation of Exciton Oscillator Strengths in Anatase TiO₂ Nanoparticles

E. Baldini, T. Palmieri, A. Dominguez, P. Ruello, A. Rubio and M. Chergui. Nanoletters (under review)

[3] Femtosecond X-ray absorption study of electron localization in photoexcited anatase TiO₂

F. G. Santomauro, A. Lübcke, J. Rittmann, E. Baldini, A. Ferrer, M. Silatani, P. Zimmermann, S. Grübel, J. A. Johnson, S. O. Mariager, P. Beaud, D. Grolimund, C. Borca, G. Ingold, S.L. Johnson, M. Chergui

Scientific Reports 5 (2015) 14834-1-6

[4] Revealing hole trapping in ZnO nanoparticles by time-resolved X-ray spectroscopy

Thomas J. Penfold, Jakub Szlachetko, Fabio G. Santomauro, Alexander Britz, Wojciech Gawelda, Gilles Doumy, Anne Marie March, Stephen H. Southworth, Jochen Rittmann, Rafael Abela, Majed Chergui and Christopher J. Milne. Nature Communications 9 (2018) 478

[5] Localized holes and delocalized electrons in photoexcited inorganic perovskites: Watching each atomic actor by picosecond X-ray absorption spectroscopy

Fabio G. Santomauro, Jakob Grilj, Lars Mewes, Georgian Nedelcu, Sergii Yakunin, Thomas Rossi, Gloria Capano, André Al Haddad, James Budarz, Dominik Kinschel, Dario S. Ferreira, Giacomo Rossi, Mario Gutierrez Tovar, Daniel Grolimund, Valerie Samson, Maarten Nachtegaal, Grigory Smolentsev, Maksym V. Kovalenko, and Majed Chergui. Structural Dynamics 4 (2017) 044002

Several emerging photovoltaic (PV) applications are currently requiring the development and implementation of transparent conductive materials (TCMs) beyond their traditional use as front electrodes in solar cells, and not only letting the light reaching the junction. Next generation PV devices with higher functionalities as transparent/semitransparent devices for glass-based building integration elements and windows require the integration of TCMs at several levels of the device architecture, with functions as charge transport or even light absorber layers. This is also required for advanced device architectures aiming towards improving the device efficiency as bifacial cells and tandem/multijunction concepts. The TCMs family includes thus electrodes, selective contacts, and even absorber layers presenting a certain degree of transparency (defined by a wide bandgap).

The transparent conductive oxides (TCOs) are the most common TCMs encountered in literature, thanks to their high electrical conductivity and light transmittance, and are often used as high-performance n-type conductors in a plethora of transparent devices (LEDs, gas sensors, solar cell, etc). Less is found regarding p-type conductors, even if recently Cu-based materials (oxides and chalcogenides) have gained interest in the field, especially thinking in their integration as interlayer electrodes in tandem devices. Another example of TCMs for PV relies in the selective contacts, generally based on metallic oxides/chalcogenides, exploiting kinetics at the interfaces with the absorber, establishing different conductivities for electrons and holes in different regions of the device, thus creating effective separation and selective transport. Finally, wide bandgap absorbers, also respond to the criteria of (semi-) transparent conductive material, thus completing the TCMs family.

After making a brief review of the most common uses of TCMs for PV applications, summarizing their main potential and challenges, a special focus will be put on their integration with existing thin film technologies, in particular with kesterite. The main results of the optimization of the replacement of the Mo back contact (commonly used in the kesterite technology) by TCOs will be presented. In particular the required functionalization (involving TCMs as interlayers improving the valence band alignment) of the TCOs for several anionic compositions of the kesterite absorbers (i.e. bandgaps) will be presented, such as the first results of integration in a tandem structure.

The manufacturing cost of thin film solar cells and modules can be significantly reduced through the replacement of vacuum deposition steps by solution-based chemical routes. In the case of kesterite PV technologies, use of solution-based processes for the preparation of the device absorbers has demonstrated the possibility to achieve device efficiencies similar to those obtained with vacuum-based processes that are used for the industrial implementation of chalcogenide technologies. Kesterites are Cu2Zn,Sn(S,Se)4 compounds that are receiving an increasing interest for the development of PV devices free of critical raw materials (CRM), which are strongly relevant for a sustainable mass-deployment of the proposed applications. Kesterite based technologies benefit also from a high level of technological compatibility with technologies that are already at industrial production scale, as CIGS. In this case, relevant challenges to achieve a full exploitation of the cost reduction potential of these technologies are related to: i) the use of processes scalable to industrial production scale; and ii) avoiding the use of highly toxic or hazardous reagents as hydrazine. This gives a strong interest to printing based processes that are compatible with very high throughput processes. Between them, ink-jet printing processes have an additional advantage related to the possibility for the definition of spatially resolved processes, which provides with a higher degree of device design flexibility.

Nowadays, Building integrated photovoltaics (BIPV) has acquired a great interest within the possible applications for thin film photovoltaic devices. It is for this reason that it is vitally important to develop technologies and processes compatible with their integration on substrates alternative to glass, as ceramic based architectural substrates.

This work is focused on the analysis of the manufacturing cost reduction of thin film solar cells and modules by replacing vacuum process steps involved on the precursor preparation by in-air screen printing based processes on ceramics substrates. Solution-based in conjunction of inkjet technology were used for the preparation of the PV active layers on molybdenum coated vitro-ceramic substrates made from recovered material in order to demostrate the viability of these techlogogies for the preparation of more ecofriendly and sustainable opto-electronic devices for BIPV applicationa. With this in mind, promising results were obtanained on cell device prototypes getting around 75% of efficientcy on ceramic substrate comparing with the average value on soda-lime reference substrate, and their scalability for the developmnet of medium size module prototypes has been investigated.

With this approach, the aim is add value to non-vacuum technologies for their future transition for the industrial production of sustainable cost efficient BIPV elements and systems.

Sb₂(S,Se)₃, is a relevant semiconductor free of critical raw materials that is receiving an increasing interest for photovoltaic (PV) applications, demonstrating solar cells in superstrate configuration with a record efficiency of 6.5%. The semiconductor is characterised by a 1D crystalline formation which in principle favours formation of benign grain boundaries and anisotropic conduction properties. Additionally Sb₂Se₃ has a high degree of flexibility in terms of substrate type, due to its relatively low synthesis temperature (300-400 ºC). This allows the use of different substrates as polymers, steels, ceramics and TCO/glass. The compound has also the possibility to tune the band gap between 1.1 and 1.9 eV, which opens very interesting perspectives for wide band gap solar cells suitable for energy harvesting in indoor applications, semi-transparent devices or high efficiency tandem devices. This high versatility make this compound very promising for ubiquitous applications based on the integration of PV devices in products and systems requiring light weight, mechanical flexibility and/or optical transparency. 

This work reports a systematic study of Sb₂Se₃ layers and substrate configuration solar cells that were fabricated using a reactive annealing treatment of Sb evaporated precursors. The study analyses the dependence of the physico-chemical properties of the layers and the optoelectronic characteristics of the devices on the pressure and temperature used during the Se reactive annealing process, as well as on the characteristics of the substrate layers, having used different kinds of substrates including both transparent (FTO) and non-transparent (Mo, Al, Au, Ag, W) back contact materials. The results obtained have allowed us to achieve  reproducible functional devices using near atmospheric pressure at 320ºC on Mo coated soda lime glass substrate with a promising efficiency of 5.6% close to the 6.5% certified world record.

This study involved a deep characterization of the fundamental properties of the synthesised layers that that have been correlated with the optoelectronic characterization of the devices. The results allow us to observe the formation of continuous layers with large and homogeneous crystals, reporting for the first time a weak PL close to 1.3eV in agreement with the band gap value obtained by IQE. Finally, the systematic vibrational characterization of the layers performed under resonant and non- resonant Raman conditions –including measurements on reference single crystal Sb2Se3 – has allowed the identification of 15 Raman peaks of the compound.

The development of organic-inorganic lead halide perovskites with very large efficiency requires us to understand the operation of the solar cell. This class of semiconductors presents remarkable bulk electronic and optical properties, but the contacts to the device are a key aspect of the operation and show important dynamic interactions. We describe the results of analysis of kinetic phenomena using frequency modulated techniques. First with impedance spectroscopy we provide an interpretation of capacitances as a function of frequency both in dark and under light, and we discuss the meaning of resistances and how they are primarily related to the operation of contacts in many cases.¹ The capacitance reveals a very large charge accumulation at the electron contact, which has a great impact in the cell measurements, both in photovoltage decays, recombination, and hysteresis. We also shows the identification of the impedance of ionic diffusion by measuring single crystal samples.² Working in samples with lateral contacts, we can identify the effect of ionic drift on changes of photoluminescence, by the creation of recombination centers in deffects of the structure.³ We also address new methods of characterization of the optical response by means of light modulated spectroscopy. The IMPS is able to provide important influence on the measured photocurrent. We describe important insinghts to the measurement of EQE in frequency modulated conditions, which shows that the quantum efficiency can be variable at very low frequencies.⁴

(1)         Lopez-Varo, P.; Jiménez-Tejada, J. A.; García-Rosell, M.; Ravishankar, S.; Garcia-Belmonte, G.; Bisquert, J.; Almora, O. Device Physics of Hybrid Perovskite Solar cells: Theory and Experiment, Adv. Energy Mater. 2018, 1702772.

(2)         Peng, W.; Aranda, C.; Bakr, O. M.; Garcia-Belmonte, G.; Bisquert, J.; Guerrero, A. Quantification of Ionic Diffusion in Lead Halide Perovskite Single Crystals, ACS Energy Lett. 2018.

(3)         Li, C.; Guerrero, A.; Zhong, Y.; Gräser, A.; Luna, C. A. M.; Köhler, J.; Bisquert, J.; Hildner, R.; Hüttner, S. Real-Time Observation of Iodide Ion Migration in Methylammonium Lead Halide Perovskites, Small 2017, 1701711.

(4)         Ravishankar, S.; Aranda, C.; Boix, P. P.; Anta, J. A.; Bisquert, J.; Garcia-Belmonte, G. Effects of Frequency Dependence of the External Quantum Efficiency of Perovskite Solar Cells, J. Phys. Chem. Lett. 2018, 3099-3104.

Triple mesoporous layer devices containing a TiO₂ electron transport layer, a ZrO₂ insulating layer and carbon as the hole transporting contact show great promise for scale-up and wide spread implementation. To improve these devices and begin to challenge inorganic PV record efficiencies a deeper understanding of their operation, and in particular sources of performance loss, is needed.
The current state-of-the-art devices use a mixed cation perovskite, consisting of methylammonium and 5-aminovaleric acid (5-AVA). The AVA containing perovskite has been shown to give greater stability and performance – linked to 2D/3D structuring of the perovskite as well as interfacial modifications at the TiO₂ surface. The cells undergo a slow light soaking effect during which time the JV performance of the device is vastly improved. They also show improvement when exposed to a high relative humidity.
A striking feature observed using TPV measurements is the presence of a negative photovoltage transient, comparable to that observed in our previous work on planar TiO₂ devices at low temperature. This behaviour suggests the presence of high rates of interfacial recombination at the TiO₂ surface. In carbon based cells the phenomena is observed at room temperature and is very slow to disappear under continued illumination. For the planar devices the negative transient was shown to diminish over time as ions in the perovskite redistributed, leading to a reduction in the recombination rate. We show that in the carbon devices the exceptionally slow dynamic behaviour observed at room temperature has a similar origin linked to the effects of ion migration – activation energy calculated to be 0.4 eV (in the range of many literature values for iodide migration). However, it takes place at a much slower rate due to the 2D AVA based perovskite hindering iodide ion migration – attempt frequency reduced by several orders of magnitude compared to pure MAPI devices. We show that the inhibited ion migration is the dominant affect rather than the AVA having a direct impact at the TiO₂ interface by adsorption via the carboxylic acid group. This inhibition of iodide migration is also linked to the increased stability demonstrated for these devices.

Lead halide perovskites have long been known to be ionic and electronic semiconductors.^{1, 2} Recently, lead halide perovskite have revolutionized the photovoltaics field with impressive efficiencies reaching ~23 %. Unlike most photoactive materials, ionic conductivity plays a key role in perovskites during photovoltaic device operation. In this presentation it is described how to characterize the ionic properties of lead halide perovskites by advanced electrical and optical techniques.³ Approaches to minimize the electronic contribution to the measured current are used so the ionic current can be probed. For example, Impedance Spectroscopy (IS)  reveals the characteristic signature of ionic diffusion in monocrystalline devices which do not contain HTL. This is the Warburg element and transmission line equivalent circuit in MAPbBr₃ and ion accumulation at the MAPbBr₃/Au interface typical for non-reactive contacts. Alternatively, measurements of confocal photoluminescence (PL) in interdigitated electrodes as a function of the applied bias reveals that the ionic movement dramatically modifies the PL emission properties that also allows the calculation of the diffusion coefficient of the material.⁴

References

1.         Kuku, T. A.; Salau, A. M., Electrical conductivity of CuSnI3, CuPbI3 and KPbI3. Solid State Ionics 1987, 25, 1-7.

2.         Kuku, T. A., Ionic transport and galvanic cell discharge characteristics of CuPbI3 thin films. Thin Solid Films 1998, 325, 246-250.

3.         Peng, W.; Aranda, C.; Bakr, O. M.; Garcia-Belmonte, G.; Bisquert, J.; Guerrero, A., Quantification of Ionic Diffusion in Lead Halide Perovskite Single Crystals. ACS Energy Lett. 2018, 1477-1481.

4.         Li, C.; Guerrero, A.; Zhong, Y.; Gräser, A.; Luna, C. A. M.; Köhler, J.; Bisquert, J.; Hildner, R.; Huettner, S., Real-Time Observation of Iodide Ion Migration in Methylammonium Lead Halide Perovskites. Small 2017, 13, 1701711.

Perovskite solar cells (PSCs) are the current rockstar of photovoltaic research attracting more and more attention. With efficiency now reaching up to 23% PSCs are on the way of catching up with classical inorganic solar cells. However, PSCs have not reached their full potential yet. In fact, their efficiency is limited, on the one hand, by non-radiative recombination, mainly via trap states located either at the grain boundaries or at the interface between the perovskite and the transport layers. On the other hand, it is limited by losses due to the poor transport properties of the commonly used transport layers.  Indeed, state-of-the-art transport layers (e.g. TiO₂, PCBM and Spiro-OMeTAD…) suffer from rather low mobilities, typically within 10^-4 – 10^-2 cm² V^-1 s^-1, when compared to the high mobilities, 1 – 10 cm² V^-1 s^-1, measured for perovskite using field-effect transistors or space-charge-limited-current measurement.

In this work, the effect of the mobility, thickness and doping density of the transport layers was investigated by means of a combined experimental and modeling analysis. For the experiment, two sets of devices made of a triple-cation perovskite were studied, including n-i-p and  p-i-n structures demonstrating efficiencies of up to 20%. For the two structures, the thickness and doping density of one of the transport layers were varied in order to understand their effect on the performance and especially on the FF. In addition, we performed a transient extraction experiment to look at the influence of the transport layers properties on the rate of extraction. The experimental results were then reproduced using drift-diffusion simulations to explain how and by how much every single parameter influences the extraction and the performance. A new and simple formula was also introduced to easily calculate the amount of doping necessary to counterbalance the low mobility of the transport layer.

In conclusion, this work presents a comprehensive analysis of the effects of the different properties of a transport layer on the efficiency of PSCs. We also present general guidelines on how to optimize a transport layer to avoid losses.

Experimental and theoretical studies show that the presence of mobile ions in perovskite solar cells (PSCs) modifies the electronic operation of the device [1-3]. The effects of the ion migration on the PSC performance can be enhanced or attenuated with the selective contacts (charge-transport-layer /perovskite heterojunctions) [2]. Thus, the knowledge of the mechanisms that take place at the selective contacts is crucial for the optimization of PSCs.

Anomalous high values of the low-frequency capacitance at open-circuit (OC) and short-circuit (SC) indicate a high accumulation of charge at the heterojunctions, which could hinder the extraction of charge and increase hysteresis in current-voltage curves [2]. This accumulation of charge can be affected by the presence of ionic species. Our goal is to quantify this accumulation of charge as a function of the different physical mechanisms that take place along its bulk and heterojunctions [2]. 

To investigate this issue, we developed a simulation model based on the drift-diffusion equations with specific boundary conditions at the heterojunctions [1-2]. The effect of ion migration on the charge and energy profile distributions along the PSC in OC and SC conditions was analyzed. We conclude that the accumulation of charge at the interfaces is strongly affected by the specific contact materials, and critically depends on a compromise among the presence of ions, the values of the carrier mobility, and the interfacial and bulk recombination parameters.

1. López-Varo, P. et al. ACS Energy Letters 1450-1453(2017)

2. García-Rosell, M. et al. J. Phys. Chem. C. (2018)

3. Reenen, S. et al. J. Phys. Chem. Lett. 6, 3808-3814(2015)

In this work, we have synthesized and characterized a totally inorganic, lead-free and non-soluble in water perovskite with catalytic properties, demonstrated by the photodegradation of an organic dye as crystal violet used, for example, in textile industries. From characterization, XRD revealed the presence of cubic (Pm-3m) and tetragonal (I4/mcm) CsSnBr₃ perovskite. The formation of the perovskite is supported by the Goldschmidt’s tolerance factor and octahedral factor calculations. From XPS, we have observed the presence of Sn²⁺, Sn⁴⁺ and the formation of non-stoichiometric tin oxide on the surface. UV-Vis spectroscopy showed high absorption of light in the visible range of the electromagnetic spectrum, with an optical band gap of 1.74 eV. Adsorption and photocatalysis tests have been performed under photovoltaic standard conditions (1 sun, AM1.5G solar spectrum and 25 ºC). The evolution of the system, followed by UV-Vis spectroscopy, showed a photodegradation of the dye in presence of the perovskite. A maximum of 73.1% of photodegradation of the crystal violet has been reached so the synthesized perovskite is, a priori, a suitable and eco-friendly material for removing dyes and other contaminants from the environment. A possible mechanism for the photocatalytic process has been proposed.

The synthesis of solar fuels and chemicals through artificial photosynthesis does not only require the coupling of solar light absorption and charge separation, but also the direct pairing with chemical redox processes. This approach is a one-step and versatile alternative to the more indirect coupling of a photovoltaic cell with electrolysis and enables potentially the synthesis of a wide range of fuels and feedstock chemicals. A common drawback in most artificial photosynthetic systems and organic photocatalysis is their reliance on expensive materials and device architectures, which challenges the development of ultimately scalable devices. Another limitation in many approaches is their inefficiency and reliance on sacrificial redox reagents, which may be system damaging and often prevent truly energy-storing chemistry to proceed. This presentation will give an overview about our recent progress in developing semiconductor hybrid materials to perform efficient full redox cycle solar fuel catalysis with inexpensive components, and our first steps in extending this approach for sustainable biomass photoreforming and fine chemical synthesis.

Representative recent references

(1) “Solar Hydrogen Generation from Lignocellulose”

Kuehnel, Reisner, Angew. Chem. Int. Ed., 2018, 57, 3290.

(1) “Photocatalytic CO₂ Reduction in Water through Anchoring of a Molecular Ni Catalyst on CdS Nanocrystals”

Kuehnel, Orchard, Dalle, Reisner, J. Am. Chem. Soc., 2017, 139, 7217.

(2) “Solar-driven reforming of lignocellulose to H₂ with a CdS/CdO_x photocatalyst”

Wakerley, Kuehnel, Orchard, Ly, Rosser, Reisner, Nature Energy, 2017, 2, 17021.

(3) “Enhancing Light Absorption and Charge Transfer in Carbon Dots through Graphitization and Core N-doping”

Martindale, Hutton, Caputo, Prantl, Godin, Durrant, Reisner, Angew. Chem. Int. Ed., 2017, 56, 6459.

(4) “Carbon Dots as Versatile Photosensitizers for Solar-Driven Catalysis with Redox Enzymes”

Hutton, Reuillard, Martindale, Caputo, Lockwood, Butt, Reisner, J. Am. Chem. Soc., 2016, 138, 16722.

(5) “Solar-driven Reduction of Protons Coupled to Alcohol Oxidation with a Carbon Nitride-Catalyst System”

Kasap, Caputo, Martindale, Godin, Lau, Lotsch, Durrant, Reisner, J. Am. Chem. Soc., 2016, 138, 9183.

(6) “Clean Donor Oxidation Enhances H₂ Evolution Activity of a Carbon Dot-Catalyst Photosystem”

Martindale, Joliat, Bachmann, Alberto, Reisner, Angew. Chem. Int. Ed., 2016, 55, 9402.

(7) “Electrocatalytic and Solar-driven CO₂ Reduction with a Mn Catalyst Immobilized on Mesoporous TiO₂”

Rosser, Windle, Reisner, Angew. Chem. Int. Ed., 2016, 55, 7388.

Semiconductor nanocrystals (quantum dots) emerged in the last years as an appealing alternative to molecular photosensitizers, owing to their superior stability, intense light absorption and bright luminescence. In particular, non-cadmium quantum dots seem to be a particularly interesting choice as they display good optoelectronic properties while containing no toxic elements with respect to CdSe and CdTe.[1] In the field of artificial photosynthesis, very efficient ”hybrid” photocatalytic systems for hydrogen production in water were obtained by associating Cd-based quantum dots as photosensitizers with molecular H₂-evolving catalysts in presence of a sacrificial reductant. [2,3]

In this communication, we describe new hybrid systems associating environmentally friendly (Cd-free) quantum dots with molecular catalysts based on earth-abundant metals, in order to perform photocatalytic H₂ production in purely aqueous environment.

Core-shell CuInS₂/ZnS nanocrystals capped with glutathione were synthesized in the aqueous phase, and their structural and optical properties were fully characterized. The nanocrystals exhibit a broad absorption throughout the visible range with orange luminescence in aqueous solution. For the photocatalysis process the colloidal solution was mixed with a cobalt macrocyclic catalyst and a sacrificial reductant, and the H₂ production under irradiation was quantified by gas chromatography. This hybrid system exhibited remarkable activity for hydrogen production under visible light irradiation at pH 5.0 with up to 7700 and 1010 turnover numbers versus catalyst and QDs, respectively. The CIS/ZnS nanoparticles were also compared to widely studied CdSe nanocrystals, using the same catalyst, and the former give remarkably better performances in terms of TON. [4]

1. M. Sandroni et al. ACS Energy Lett., 2017, 2, 1076–1088.

2. C. Gimbert-Suriñach et al., J. Am. Chem. Soc. 2014, 136, 7655-7661.

3. Z.J. Han et al., Science 2012, 338, 1321-1324

4. M. Sandroni et al. Energy Environ. Sci., 2018, DOI: 10.1039/c8ee00120k.

Artificial photosynthesis is a very promising method to turn solar energy into fuels.¹ In a photoelectrochemical (PEC) cell, the generation of H₂ by water splitting takes place on two different electrodes, which facilitates the separation of the products by the addition of a proton exchange membrane.  PEC cells have led to some of the highest solar-to-hydrogen efficiencies achieved to date.² The setup of a successful PEC cell requires the preparation of efficient and stable photoelectrodes.

Subestequiometric copper telluride, Cu_2-xTe, is a very interesting material that, as far as we know, has not been used in a PEC cell yet. It has already been used in photovoltaic cells due to its small band gap of around 1.5 eV and its high conductivity.³ In this work, we used Cu_2-xTe nanocrystals synthesized by a colloidal method⁴ to prepare thin films and use them as photoelectrodes. After their organic capping was successfully removed by a thermal treatment in Ar, two different crystallographic phases were obtained depending on the heating temperature. Then, the photoelectrochemical and optical properties of both phases were measured. Electrochemical impedance spectroscopy was used to build the Mott-Schottky diagrams and determine the flat band potential, charrier density and conduction type, which is p-type for both phases. This conductivity type was ratified by photopotential measurements. Both electrochemical impedance spectroscopy and UV-Vis-nIR spectroscopy allowed us to build the bands energy diagram, which confirms the ability of this material to reduce water to hydrogen. This means that Cu_2-xTe thin films are good candidates to be used as photocathodes, which was also confirmed by the measurement of their photocurrents. As they show some stability problems, two strategies were proved to solve this: the addition of an amorphous TiO₂ layer and the change of the electrolyte pH. Moreover, H₂ generation by water splitting in a two-electrode PEC cell was observed and quantified.

References

1.           Sivula, K. & van de Krol, R. Semiconducting materials for photoelectrochemical energy conversion. Nat. Rev. Mater. 1, 1–16 (2016).

2.           Young, J. L. et al. Direct solar-to-hydrogen conversion via inverted metamorphic multijunction semiconductor architectures. Nat. Energy 17028, 1–8 (2017).

3.           Rungtaweechai, N. & Tubtimtae, A. Cu_2-xTe/MnTe co-sensitized near-infrared absorbing liquid-junction solar cells. Mater. Lett. 158, 70–74 (2015).

4.           Arciniegas, M. P. et al. Self-Assembled Dense Colloidal Cu₂Te Nanodisk Networks in P3HT Thin Films with Enhanced Photocurrent. Adv. Funct. Mater. 26, 4535–4542 (2016).

All-Inorganic Halide Perovskite Quantum Dots (QDs) have emerged as a new class of fascinating nanomaterials with outstanding optoelectronic properties, with promise to revolutionize different disciplines like photovoltaics, lasing and emission.[1] In the present talk, we will discuss about the application of these materials for solar-driven environmental remediation. To that aim, it is essential to elucidate the energy levels of CsPbxSn1-xX3 QDs by different techniques and to identify candidate target molecules, which can be oxidized with photogenerated holes in these QDs. As a proof of concept, we demonstrate the efficient photocatalytic and photoelectrochemical oxidation of 2-Mercaptobenzothiazol with CsPbBr3 QDs.

We report recent experimental and theoretical results for molecular junctions based on \pi conjugated wires. These wires represent a class of linear molecules whose transport properties can be understood in the framework of the topological Su-Schrieffer-Heeger model for polyacetylene. We present an in-depth theoretical analysis based on tight-binding and ab-initio simulations of their coherent transport properties and show that, under certain conditions and depending on the chain parity (even /odd) and length, the conductance at the Fermi level can depend very weakly or even increase with the wire length. For short odd chains, we also provide experimental evidence of the role of the external environment in their charge transport properties: we study conductance trends in single-molecular junctions of polymethine dyes and prove that the trends can also be altered by the choice of the embedding solvent. Overall, our results suggest a way of enabling efficient electron transport at the nanoscale with one-dimensional wires.Iryna Davydenko

The extraordinary absorption cross-section and high photoluminescence (PL) quantum yield of perovskite nanocrystals make this type of material attractive to a variety of applications in optoelectronics. For the same reasons, nanocrystals are also ideally suited to function as nanoantennae to excite nearby single dye molecules by fluorescence resonant energy transfer (FRET). Here, we demonstrate that FAPbBr3 perovskite nanocrystals, of cuboidal shape and approximately 10 nm in size, are capable of selectively exciting single Cyanin 3 molecules at a concentration a hundredfold higher than standard single-molecule concentrations. This FRET antenna mechanism increases the effective brightness of the single dye molecules one hundredfold. Photon statistics and emission polarization measurements provide evidence for the FRET process by revealing photon-antibunching with unprecedented fidelity and highly polarized emission stemming from single dye molecules. Remarkably, the quality of single-photon emission improves two-fold compared to emission collected directly from the nanocrystals since the higher excited states of the dye molecule act as effective filters to multiexcitons. The same process gives rise to efficient deshelving of the molecular triplet state by reversed intersystem crossing, translating into a reduction of the PL saturation of the dye, thereby increasing the maximum achievable PL intensity of the dye by a factor of five.

Perovskite based solar cells have shown impressive progress in recent years. Now that high efficiencies are in reach, more and more effort is put into understanding the underlying device physics and to improve the devices intrinsic and extrinsic stability.

Touching on both of these subjects, I want to first discuss recent findings regarding the implementation of metal oxide layers into perovskite solar cell device stacks. These can be used to form impenetrable barrier layers which prevent the ingress of humidity as well the egress of perovskite decomposition products. Using this strategy, the overall decomposition of perovskite can be significantly suppressed, leading to outstanding solar cell device stability.

On the other hand, we find for a variety of systems that directly interfacing the perovskite to metal oxide layers can trigger a complex variety of reactions, which significantly alter the composition of the perovskite at the interface and lead to the presence of degradation products.  These thin interlayers play an important role for film formation and charge extraction and will therefore influence the overall device performance.

Photovoltaic devices based on hybrid metal halide perovskites are rapidly improving in power conversion efficiency. While these materials are generally viewed as three-dimensional, the effects of nano-scale interfaces within the bulk films are still poorly understood. Such interfaces may appear between the perovskite layers and the charge extraction layers, or within the perovskite layers, at grain boundaries, of when passivating or hydrophobic interlayers are included.

We show here that photon reabsorption in lead iodide perovskite layers is strongly influenced by layer thickness and interfaces formed with charge-extraction layers^[1]. Such photon re-absorption is found to reduce the apparent bi-molecular charge-carrier recombination rate constant with increasing film thickness, while the intrinsic value can be fully explained as the inverse process of absorption^[2].

In addition, we discuss the role of nanoscale interfaces^[3,4], energetic disorder^[5], and passivating interlayers^[6] on the dynamics of charge-carriers in various metal halide perovskites. Lowering of the perovskite dimensionality is shown to have effects similar to those known for classic inorganic semiconductors, such as enhancing bimolecular and Auger recombination and reducing trap-mediated recombination through surface passivation. Such extrinsic effects will also reduce the charge-carrier mobility below the maximum attainable value near 100cm²/(Vs) for MAPbI₃^[7].

[1] T. W. Crothers, R. L. Milot, J. B. Patel, E. S. Parrott, J. Schlipf, P. Müller-Buschbaum, M. B. Johnston, and L. M. Herz,Nano Lett. 17, 5782 (2017).

[2] C. L. Davies, M. R. Filip, J. B. Patel, T. W. Crothers, C. Verdi, A. D. Wright, R. L. Milot, F. Giustino, M. B. Johnston, and L. M. Herz, Nature Communications 9, 293 (2018).

[3] D. P. McMeekin, Z. Wang, W. Rehman, F. Pulvirenti, J. B. Patel, N. K. Noel, S. R. Marder, M. B. Johnston, L. M. Herz, and H. J. Snaith, Adv. Mater., 29 (2017), p. 1607039.

[4] R. L. Milot, R. J. Sutton, G. E. Eperon, A. A. Haghighirad, J. M. Hardigree, L. Miranda, H. J. Snaith, M. B. Johnston, and L. M. Herz, Nano Letters, 16 (2016), pp. 7001-7007.

[5] A. D. Wright, R. L. Milot, G. E. Eperon, H. J. Snaith, M. B. Johnston, and L. M. Herz, Adv. Func. Mater., 27 (2017), p. 1700860.

[6] Z. Wang, Q. Lin, F. P. Chmiel, N. Sakai, L. M. Herz, and H. J. Snaith, Nature Energy, 2 (2017), p. 17135.

[7] L. M. Herz, ACS Energy Lett., 2 (2017), pp. 1539-1548.

The optical and transport properties of lead-halide perovskites (LHPs) have been used as a basis for new solar cell technologies showing record improvements in efficiencies. In the search for the microscopic origins of this success, many recent studies suggest that structurally dynamic effects are active already at room temperature and standard operating conditions and may affect device performance and/or stability. Here, we explore this issue using first-principles calculations based on density functional theory. In particular, we focus on ion migration and dynamic distortions.

The optical and transport properties of lead-halide perovskites (LHPs) have been used as a basis for new solar cell technologies showing record improvements in efficiencies. In the search for the microscopic origins of this success, many recent studies suggest that structurally dynamic effects are active already at room temperature and standard operating conditions and may affect device performance and/or stability. Here, we explore this issue using first-principles calculations based on density functional theory. In particular, we focus on ion migration and dynamic distortions.

This presentation will focus on the excited state behavior of semiconductor nanocrystals as light absorbers that, coupled with redox catalysts, drive light-driven multi-electron transfer reactions. Reactions of interest include hydrogen generation, carbon dioxide reduction, nitrogen fixation, and water oxidation. We have demonstrated that nanocrystal excited state behavior, charge transfer dynamics, and surface chemistry play a governing role in the overall photochemistry of nanocrystal-catalyst complexes. This presentation will describe our most recent findings about how the reactions of interest can be driven and controlled through manipulation of nanocrystal excited state dynamics. In particular, the presentation will focus on: (i) measurement (by transient absorption spectroscopy), kinetic modeling, and control of electron transfer kinetics for injection of photoexcited electrons into redox enzymes; and (ii) elucidation of the motion of photoexcited holes on nanocrystal surfaces, using a combination of transient absorption measurements, modeling, and theory, and the implications of this motion on oxidation photochemistry.

The efficient production of solar fuels requires catalysts capable of accelerating complex multi-electron reactions at electrified interfaces. These reactions can be carried out at the metallic surface sites of heterogeneous electrocatalysts or via redox mediation at molecular electrocatalysts. Molecular catalysts yield readily to synthetic alteration of their redox properties and secondary coordination sphere, permitting systematic tuning of their activity and selectivity. Similar control is difficult to achieve with heterogeneous electrocatalysts because they typically exhibit a distribution of active site geometries and local electronic structures, which are recalcitrant to molecular-level synthetic modification. However, metallic heterogeneous electrocatalysts benefit from a continuum of electronic states which distribute the redox burden of a multi-electron transformation, enabling more efficient catalysis. We have developed a simple synthetic strategy for conjugating well-defined molecular catalyst active sites with the extended states of graphitic solids. Electrochemical and spectroscopic data indicate that these graphite-conjugated catalysts do not behave like their molecular analogues, but rather as metallic active sites with molecular definition, providing a unique bridge between the traditionally disparate fields of molecular and heterogeneous electrocatalysis. Our efforts to deploy these new hybrid materials for solar fuels production will be discussed.

Dimensionally stable electrodes that are able to sustain the harsh chemical environment involved during electrochemical reactions is a requirement for materials to reach industrial applications. Dimensionally stable anodes (DSA) have been obtained previously for the electrochemical generation of chlorine gas and for water splitting applications.¹ So far the most successful approach has been the generation of a thin layer of a stable electrocatalytic and conductive coating deposited on a base metal, usually Ti. Unfortunately, this approach renders materials with a small contact surface area between catalyst and electrolyte which could limit the overall activity of the reaction. The current presentation will focus on how DSA can be prepared from powder metallurgy processes. The composition can be controlled to modify the porosity of the electrode. This type of electrodes can be used in combination with photovoltaic materials by using different photoelectrochemical configurations for generation of solar fuels.^2-4

References

1.         Mousty, C.; Fóti, G.; Comninellis, C.; Reid, V., Electrochemical behaviour of DSA type electrodes prepared by induction heating. Electrochim. Acta 1999, 45, 451-456.

2.         Guerrero, A.; Bisquert, J., Perovskite semiconductors for photoelectrochemical water splitting applications. Current Opinion in Electrochemistry 2017, 2, 144-147.

3.         Guerrero, A.; Haro, M.; Bellani, S.; Antognazza, M. R.; Meda, L.; Gimenez, S.; Bisquert, J., Organic photoelectrochemical cells with quantitative photocarrier conversion. Energy Environ. Sci. 2014, 7, 3666-3673.

4.         Haro, M.; Solis, C.; Blas‐Ferrando Vicente, M.; Margeat, O.; Dhkil Sadok, B.; Videlot‐Ackermann, C.; Ackermann, J.; Di Fonzo, F.; Guerrero, A.; Gimenez, S., Direct Hydrogen Evolution from Saline Water Reduction at Neutral pH using Organic Photocathodes. ChemSusChem 2016, 9, 3062-3066.

The morphology of copper electrodes significantly affects the performance of electrochemical CO₂ reduction devices. Porous morphologies are important to increase the active reaction sites and to tune the selectivity towards desired products, but it can also limit the mass transport through the pore space. A better understanding of morphology-induced performance limitations or enhancements in porous electrodes is needed.

We used a coupled experimental-numerical approach to quantitatively characterize morphologically-complex porous electrodes (micrometer thick macroporous films with mesoporous structural details). We utilized a 3D-microscopy method, FIB-SEM tomography [1] with a high resolution of 4x4x4 nm³, to obtain a grey value array representing the photoelectrode morphology. The digital structure was segmented based on trainable machine-learning algorithms to subsequently quantify performance-related morphological parameters.

We applied this method to a hierarchically structured indium−tin oxide (ITO) electrode, covered by electrodeposited copper. The ITO scaffold has a macroporous inverse opal (IO) architecture and a mesoporous skeleton to increase the effective surface area [2]. Various samples with IO film thickness of 10 – 30 μm and pore diameters of 0.5 – 2 μm were scanned.

The digitalized electrode morphologies were then used in direct pore-level simulations to understand the mass transport within the macro- and mesopores. The diffusive transport of reactants and products in the electrolyte were investigated with a finite volume solver on a meshed representation of the exact geometries. Local current densities at the solid-liquid interface and pH distributions in the pore space were determined for the different film thickness and pore diameters.

The FIB-SEM tomography, with its nanometer-scale resolution, and the advanced pore-level simulations allowed for direct linking of the multi-physical transport to the morphological parameters of the porous ITO/Cu films. This study lays the ground for the optimization of the CO₂ reduction efficiency by tuning the morphological parameters on digitally modified electrode representations (e.g. modified pore diameters, pore distribution and film thickness).

References

[1]         M. Cantoni and L. Holzer, “Advances in 3D focused ion beam tomography,” MRS Bull., vol. 39, no. 4, pp. 354–360, 2014.

[2]         D. Mersch et al., “Wiring of Photosystem II to Hydrogenase for Photoelectrochemical Water Splitting,” 2015.

The electrocatalytic reduction of CO2 has attracted high attention recently, not only to reduce the carbon footprint, but for the possibility to produce value-added chemicals, such as carbon monoxide, formic acid, methanol or more complex organic molecules like ethanol or ethylene, under ambient temperature and pressure conditions. However, the real application of these electrochemical techniques are still a challenge, and efforts have to be made in decrease the overpotential, improve the product selectivity and stability of the electrocatalyst. Among all the CO2 reduction products that can be obtained, we have focused our research in formic acid, as it has been receiving great industrial attention for its several applications in fuel cells, textile or chemical industries.

We have studied two different metals and the combination of both as electrocatalysts for CO2 reduction to formic acid: tin (Sn)1 and palladium (Pd)2. Electrodeposited Sn has shown a potential around -0,8 VRHE at 10 mA/cm2 and a faradaic efficiency to formate around 70% using KHCO3 as electrolyte. On the other hand, under the same experimental conditions, the electrode containing Pd nanoparticles supported on carbon black shows a potential around -0.25 VRHE at 10 mV/cm2 and more than 80% of faradaic efficiency. The cathode potential obtained with Pd electrodes in comparison with Sn is 500 mV lower however, the catalyst working at such high current density is rapidly poisoned with CO (subproduct of the CO2 reduction) and the faradaic efficiency to formic is decreased dramatically. In this work, we present how the combination of both catalysts can be used to modulate the overpotential of the CO2 electroreduction reaction, the selectivity and the electrode stability under continuous operation. Also, a novel technique of co-electrodeposition of both metals is achieved, showing an alternative method to produce easily tunable electrodes, which facilitates and enhance the possible coupling to a photo-absorbing element in a complete photoelectrochemical cell.

References:

1. J. Mater. Chem. A,2016, 4, 13582–13588

2. ACS Energy Lett. 2016, 1, 764−770

The transformation of solar energy into chemical energy stored in the form of hydrogen, through photoelectrochemical water splitting is a promising method that has the important advantages of being environment friendly and free from carbon dioxide emission. Metal oxides are promising candidates for photoanode but the strong electron-hole recombination may explain their low efficiency. It has been recently proposed to use the spontaneous electric field of a ferroelectric compound to separate photogenerated charges in photoanode. In our laboratory we have been developing model oxide thin films for solar water splitting prepared by oxygen assisted molecular beam epitaxy for several years, in order to understand the relevant parameters to improve photoanodes performance. In this context, in order to unravel the role of ferroelectricity we have been investigating epitaxial TiO₂ films and TiO₂/BaTiO₃ heterojonctions deposited on Pt(001) where BaTiO₃ is the prototypical ferroelectric material. We have studied the growth, the electronic structure, the ferroelectric properties and the photoelectrochemical performance (photocurrent and impedance spectroscopy) as a function of the position of the ferroelectric layer in the film (above, in the middle and below) and of the orientation of ferroelectric polarization. Thereby, we show that the TiO₂ films adopt a TiO₂-B (001) structure for thickness lower than 3 nm and an anatase (001) structure for higher thicknesses, while the BaTiO₃ films adopt a tetragonal (001) structure with a natural electrical polarization perpendicular to the surface. We show that the performance of the TiO₂ photoanode can be improved by a polarized layer of BaTiO₃, the best improvement is when the ferroelectric film is below the TiO₂ film and when the electrical polarization is downward polarized. We explain this finding by the presence of an internal electric field which favors the separation of photo-generated charges, and explain how the electronic structure is modified by the electrical polarization in each case.

In this contribution, we report on strategies for high yield syngas production from solar CO₂ recycling. To make CO₂ re-use available at industry level, versatile and cost-effective processes at large-scale are in dire need. Therefore, we focused our study on efficient processes, which are scalable to large areas and compatible with state-of-the-art photovoltaics and electrocatalysts. Efficient processes include low overpotentials for the CO₂ reduction reaction (CO₂RR) and the oxygen evolution reaction (OER), respectively, along with excellent selectivity for the desired products (in particular for the case of CO₂RR). We demonstrate that the coating of three-dimensional metallic foam electrodes with adapted nanosized catalysts resulted in high yield CO₂ conversion to syngas. Furthermore, we provide evidence that photovoltaic structures based on cheap and mature silicon technology can be adapted to provide the required photovoltage to break the CO₂ molecule into the targeted product.

In particular, we have investigated the application of silicon heterojunction photovoltaic cells as photoanodes, which requires meeting challenges, such as increasing the photovoltage without impairing the photovoltaic efficiency; protection of the solar cell by robust coatings to increase the stability in aqueous electrolytes; and the decoration with catalysts ensuring an efficient OER. We show that photovoltages up to 2.5 V with photocurrent densities up to 7.5 mA/cm² can be reached by connecting four HIT cells in series. Furthermore, for the rear contact of the HIT cells we explored the applicability of metallic foams (e.g. Ni and Cu) loaded with metallic particles as OER catalyst. We demonstrate OER overpotentials below 300 mV (for 10 mA/cm²).

The CO₂RR was performed by using large-scale metallic foam electrodes as highly conductive catalyst scaffolds. In this context, we developed a deposition process, which enables tunable coating of Cu and Ni foams, respectively, with highly active nanosized metal catalysts, such as Zn or Ag, for selective syngas production. The performance of the as-produced electrodes has been evaluated in terms of product selectivity, Faradaic efficiency, overpotentials, and stability. The developed electrodes exhibit overpotentials below 400 mV for CO₂RR. Furthermore, stable and tunable H₂:CO ratios between 5 and 1 along with high CO Faradaic efficiencies of up to 96% and CO current densities of 40 mA/cm² were measured. Finally, we demonstrate a bias-free operation of the complete  device (2-electrode configuration) providing a photocurrent density of 5.0 mA/cm² measured under 100 mW/cm² illumination. This corresponds to a solar-to-syngas conversion efficiency of 4.3%.

The relatively weak bond of metal-halide perovskites  (MHPs) gives rise to an inherently soft crystal lattice which is naturally prone to disorder, [1] associated to formation of defects. Defects introducing levels in the material’s band-gap may act as traps and recombination centers for photogenerated charge carriers, limiting the device performance and possibly impacting the device temporal stability. Defects may also introduce ionic mobility channels in MHPs. Ion migration is boosted by the presence of vacancies and interstitial defects, acting as shuttles for ion hopping.[2] If the migrating defects are also charge traps, as it occurs for iodine defects in MAPbI₃, one has migrating traps which can respond to the action of an electric field [3] and to the presence of photogenerated carriers.[4, 5] Some of the traps may also undergo photochemical reactions, such as the reported release of molecular iodine under light irradiation[6, 7]. Defects may also lay behind the reported material transformation under light exposure, followed by very slow relaxation to initial conditions.[8,9]

Theoretical and computational modeling is a complementary tool for rationalizing experimental results, on the one hand, and to direct experiments and device fabrication towards innovative concepts, on the other hand.  Several computational studies have already been carried out on native defects in MHPs, employing Density Functional Theory. The complex interplay of electronic structure and dynamical features of MHPs, however, poses challenging problems to the accuracy and reproducibility of these calculations.[10] Here we present what we believe are the “best practices” in defect modeling of metal-halide perovskites with selected examples of applications related to the effect of electric fields and charge carriers on the structural and electronic properties of perovskites relevant to stability and solar cell operation.

References:

[1] Conings, B. et al. Adv. Energy Mater. 2015, 5, 1500477.

[2] Mosconi, E.; De Angelis, F. ACS Energy Lett. 2016, 1, 182-188.

[3] Chen, B. et al. Nat. Mater., 2018, in press.

[4] Birkhold, S.T.  et al. ACS Energy Lett. 2018, 3, 1279−1286

[5] Meggiolaro, D. et al. Energy Environ. Sci. 2018, 11, 702-713.

[6] Meggiolaro, D. et al. ACS Energy Lett., 2018, 3, 447–451.

[7] Kim, G.Y. et al. Nat Mater 2018, 17, 445-449.

[8] Gottesman, R. et al.  J. Phys. Chem. Lett. 2014, 5, 2662-2669.

[9] Tsai, H. et al. Science 2018, 360, 67-70.

[10] Meggiolaro, D.; De Angelis, F. ACS Energy Lett. 2018, 2206-2222.

The high performance of recently emerged lead halide perovskite-based photovoltaic

devices has been attributed to remarkable carrier properties in this kind of material:

long carrier diffusion length, long carrier lifetime, and low electron-hole recombination

rate. However, the mechanism of the charge separation is still not fully understood. In my talk, it will be demonstrated that the charge separation is induced by the structural fluctuation of the inorganic lattice using first-principles molecular dynamics simulations [1]. On the other hand, charge carrier trapping at defects on surfaces or grain boundaries is detrimental for the performance of perovskite solar cells. In practice, it is one of the main limiting factors for carrier lifetime. 　Surface defects responsible for carrier trapping are clarified by comprehensive first-principles investigations and it is proposed that PbI2-rich condition is preferred to MAI-rich one, while intermediate condition has possibility to be the best choice [2].

References

[1] H. Uratani and K. Yamashita, J. Phys. Chem. C, 121, 26648−26654 (2017).

[2] H. Uratani and K. Yamashita, J. Phys. Chem. Lett., 8, 742−746 (2017).

While 3D perovskites are the materials currently leading the field for photovoltaics, 2D hybrid organic/inorganic layered materials have a much broader versatility in accommodating a vast variety of organic molecules and are providing a long-term devices stability. In particular, we obtained a one-year stable perovskite device by engineering an ultra-stable 2D/3D perovskite junction with a PCE of 14.6% in standard mesoporous solar cells.[1] Hybrid organic-inorganic multidimensional perovskites, also known as Ruddlesden-Popper perovskites, are composed of 3D domains separated by large organic cations. The mixing of the two mainly studied types of perovskites supposes the combination of the good properties from each one. Due to the 3D domains, Ruddlesden-Popper perovskites can absorb radiation in a wide range of the electromagnetic spectrum. Moreover, the environmental stability issue characteristic of 3D perovskites is solved thanks to the good stability provided by the 2D domains, in which the larger amount of organic phase acts as barrier against water and moisture penetration.[1] The main problem of this material arises from its photoexcitation and the consequent generation of the electron-hole pairs. Holes and electrons keep confined in the inorganic layers due to the electric isolation of the organic cations in 2D domains, i.e.: while their diffusion along the 3D domains is excellent, it is quite poor across the 2D ones. In order to find a solution, we will explore the possibility of improving the conductivity of charge carriers across 2D domains by the insertion of specifically designed conducting organic cations. The chemical structure of these cations should be suitable for the purpose, for instance by embedding aromatic rings or conjugated multiple bonds and allowing the selective transport of a single carrier type. The desired cation properties will be finely computed in order to realize a Ruddlesden-Popper perovskite with high and selective vertical conduction, see Fig. 1.

[1] Grancini, G.; Roldán-Carmona, C.; Zimmermann, I.; Mosconi, E.; Lee, X.; Martineau, D.; Narbey, S.; Oswald, F.; De Angelis, F.; Graetzel, M., et al. Nat Commun, 8 (2017) 15684.

Both all inorganic and hybrid halide perovskites have undeniably remarkable characteristics for next-generation photovoltaics, which deserve to be better understood. There are many different perovskite structures that are currently widely explored as absorber materials among which 3D AMX₃ and 2D A₂ A’_n-1 M_n X_3n+1 frameworks, where A, A’ are cations, M is a metal, X is a halide. Here, through a couple of recent examples including newly discovered halide perovskite phases [1], we will discuss their optoelectronic properties based on first-principles calculations and semi-empirical modelling. Impact of interfaces [2], structural fluctuations [3], quantum and dielectric confinements [4] on charge carriers and excitons will be inspected. Particular attention will be paid on excitonic effects comparing the results of model calculations with low temperature optical spectroscopy and 60-Tesla magneto-absorption [5]. Theoretical inspection of low energy states associated with electronic states localized on the edges of the perovskite layers [6] will also be shown to provide guidance for the design of new synthetic targets [7] taking advantage of experimentally determined elastic constants [8].

[1] C. M. M. Soe et al. JACS, 139, 16297, 2017; L. Mao et al. JACS, 140, 3775, 2018; X. Li et al. submitted.

[2] W. Nie et al. Adv. Mater. 30, 1703879, 2018.

[3] M. A. Carignano et al. J. Phys. Chem. C, 121, 20729, 2017; A. Marronnier et al. ACS Nano, 12, 3477, 2018; L. Zhou et al. ACS Energy Lett., 3, 787–793, 2018; C. Katan et al. Nature Materials, 17, 377, 2018.

[4] B. Traore et al. ACS Nano, 12, 3321, 2018.

[5] J.-C. Blancon et al. Nature Com. in press (arXiv:1710.07653).

[6] J.-C. Blancon et al. Science, 355, 1288, 2017.

[7] M. Kepenekian et al. arXiv:1801.00704.

[8] A. C. Ferreira et al. arXiv:1801.08701.

Sample preparation of perovskite materials has a crucial impact on their optoelectronic properties and has indeed been a dominant factor for the rapid advances in photoconversion efficiencies. However, very little is currently known about the microcopic details that determine the nucleation and crystal growth proces. Such a knowledge could be a starting point to enable control over the crystallization process and a rational optimization of preparation conditions.

In principle, molecular dynmaics simulations can provide atomistic insight into complex phenomena but direct simulations of the nucleation process are highly challenging due to the large activation barriers that are involved and the high-dimensionality of the available phase space. Here, we present enhanced sampling molecular dynamics simulations based on well-tempered metadynamics simulations about the nucleation process of lead halide perovskites from solution. Choosing appropriate collective varaibles, it has been possible for the first time to monitor the nucleation and growth of such a multicomponent system (containing, lead ions, halide anions, monovalent cations and solvent molecules). These simulations demonstrate  the influence that different solvents play in this process and reveal a pivotal role of the monovalent cations.

In this talk, I will discuss our group’s recent experimental and computational work on understanding electronic and phononic structure nanocrystal thin films and charge transport in these thin films. Using electrochemical-based approaches, we show that we can quantify the electronic density of states and also examine charge-transfer processes across interfaces. Using inelastic x-ray scattering, we quantify the phononic denisty of states. We combine density functional theory calculations and kinetic Monte Carlo simulations to develop a first-principles model for charge transport in nanocrystals solids. We show that these simulations explain temperature-dependent time-of-flight measurements of electron and hole mobility performed on lead sulfide nanocrystal thin films. The combination of experimental and computational work highlights the importance of electron-phonon interactions in nanoscale transport and enables us to determine the relative impact of energetic and positional disorder on transport, providing us with design guidelines on parameters to consider when optimizing nanocrystal synthesis, nanocrystal surface treatments, and nanocrystal thin film preparation for different device applications.

For the fabrication of an integrated solar-to-chemical system, different components should be interfaced together in an orchestrated manner. Photoelectrodes need to absorb in the visible range, with a valence and a conduction band suited for the target reaction. Moreover, the presence of catalysts is required to manage the intrinsic energetic hurdle. Herein, we address the study of the major challenges, namely performance, stability, and interfaces to enable fabrication of integrated solar-to chemical systems. Novel scientific directions for the synthesis of functional interfaces and the development of new tools for their characterization will be addressed. Specifically, we will present a methodology for evaluating corrosion mechanisms and apply it to bismuth vanadate, a state-of-the-art photoanode. Analysis of changing morphology and composition under solar water splitting conditions reveals chemical instabilities that are not predicted from thermodynamic considerations of stable solid oxide phases, as represented by the Pourbaix diagram for the system. These findings are confirmed by in situ electrochemical atomic force microscopy (EC-AFM), which reveals that degradation under operating conditions occurs via dissolution of the film, starting at exposed facets of grains in polycrystalline thin films. In addition, we will present the correlation between morphological and functional heterogeneity in this material by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate.

Insights into corrosion mechanisms and nanoscale heterogeneity aid development of protection strategies and provide information on how local functionality affects the macroscopic performance. 

Metal-halide perovskites are promising materials for opto-electronic applications. Their mechanical and electronic properties are directly connected to the nature of their lattice vibrations. Whereas the mid infrared (IR) range contains mainly information on the internal vibrations of the methylammonium cation, the lead-halide lattice vibrations are located in the far IR.

We will report far-IR spectroscopy measurements of lead halide perovskite thin films and single crystals at room temperature and a detailed quantitative analysis of the spectra.¹ We find strong broadening and anharmonicity of the lattice vibrations for all three halide perovskites. We determine the frequencies of both the transversal and longitudinal optical phonons, and use them to calculate the static dielectric constants, polaron masses, and upper limits for the phonon-scattering limited charge carrier mobilities. Furthermore, we compare our experimental results to molecular dymanics simulations of lead halide perovskites. Our findings are important for the basic understanding of charge transport processes and mechanical properties in metal halide perovskites.

(1) Sendner, M.; Nayak, P. K.; Egger, D. A.; Beck, S.; Müller, C.; Epding, B.; Kowalsky, W.; Kronik, L.; Snaith, H. J.; Pucci, A.; Lovrincic, R. Optical Phonons in Methylammonium Lead Halide Perovskites and Implications for Charge Transport. Mater Horiz 2016, 3 (6), 613–620.

Solar cells based on metal-halide perovskite absorber layers have resulted in outstanding photovoltaic devices with long non-radiative lifetimes as a crucial feature enabling high efficiencies. Long non-radiative lifetimes occur if the transfer of the energy of the electron-hole pair into vibrational energy is slow, due to, e.g., a low density of defects, weak electron phonon coupling or the release of a large number of phonons needed for a single transition. Here, we discuss the implications of the known material properties of metal-halide perovskites (such as permittivities, phonon energies and effective masses) and combine those with basic models for electron-phonon coupling and multiphonon-transition rates in polar semiconductors. We find that the low phonon energies of MAPbI₃ lead to a strong dependence of recombination rates on trap position, which can be readily deduced from the underlying physical effects determining non-radiative transitions. Here, we show that this is important for the non-radiative recombination dynamics of metal-halide perovskites, as it implies that these systems are rather insensitive to defects that are not at midgap energy. This can lead to long lifetimes, which indicates that the low phonon energies are likely an important factor for the high performance of optoelectronic devices with metal halide perovskites. 

We use tunable nanoscale defect cavities to create zero-dimensional exciton-polaritons [1]. Data on the strong coupling as well as mode engineering is shown as a function of cavity detuning. At ambient conditions, we observe non-equilibrium exciton-polariton condensation [2] with strong lateral confinement on the wavelength scale [3]. Threshold and line narrowing are analyzed as a function of excitation density. Both, in real and momentum space we observe the distinct signature of strong transversal confinement in the condensation. First order coherence properties are investigated by means of a Michelson interferometer.

As a building block towards extended lattices, we realize two coupled cavities by focused ion beam milling and thermal scanning probe lithography. These photonic molecules are investigated by means of atomic force microscopy and optical measurements, to compare both fabrication methods. Furthermore, we investigate different ways to improve the optical properties of the active material e.g., photo-degradation and inhomogeneity.

These are the initial steps towards studying quantum fluids in extended, arbitrary potential landscapes at ambient conditions.

References

[1] D. Urbonas et al., ACS Photonics 3 (9), 1542–1545 (2016).

[2] J. D. Plumhof et al., Nat. Mater. 13, 247 (2014).

[3] F. Scafirimuto et al., ACS Photonics 5 (1), 85–89 (2018).

Spectroscopy of single colloidal quantum dots (QDs), especially at cryogenic temperature, helps to understand the inherent properties of nanocrystals that are often hidden in ensemble level studies. This applies in particular to InP-based QDs, which attract increasing interest as Cd-free alternative nanocrystals yet were hardly investigated at the single QD level. Here, we discuss the photoluminescence properties of single InP/ZnSe QDs, both at room temperature and at cryogenic temperature. While ensemble level measurements feature a luminescent linewidth of around 50 nm, we find that emission spectrum of single InP/ZnSe QDs can have a linewidth as narrow as 14 nm (50 meV). Hence, the relatively broad emission line that characterizes ensembles of InP-based QDs is by no means an intrinsic material property. In addition, we found that InP/ZnSe QDs combine a nearly blinking free emission with a high purity single photon emission (g²(0)<0.03), also well beyond the saturation intensity.[1] Cryogenic single QD spectra, on the other hand, consist of zero-phonon lines that can be as narrow as 40 µeV. Polarization resolved spectra point to a linearly polarized spectral doublet from the bright exciton. At lower excitation intensities, jitter was negligible and spectra could be integrated for tens of seconds without erasing the doublet splitting of 1.2 meV. At higher excitation intensities, jitter becomes more severe and switching between emission from the exciton-doublet and the trion-singlet can be observed. This indicates that spectral jitter finds its origin in changes of the local electric field caused by the temporal trapping of one charge carrier. In summary, we find conclude that single InP QDs have emission characteristics similar to the extensively studied CdSe-based QDs. Moreover, the narrow emission lines, limited jitter and fluorescence intermittency of single InP/ZnSe QDs holds great promise to further explore these materials as a solution-processable single-photon emitter and improve the ensemble level characteristics of these materials.

Reference:
[1] Chandrasekaran, V.; Tessier, M. D.; Dupont, D.; Geiregat, P.; Hens, Z.; Brainis, E. Nano Letters 2017, 17 (10), 6104–6109.

Metal-halide perovskites have emerged as promising solution-processable semiconductors for optoelectronics applications. These materials show unexpectedly high luminescence yields and long carrier lifetimes under operating conditions. Facile changes in composition during fabrication can be employed to control their optical properties, and the nature of electronic states. Recently, the ad-mixture of monovalent cations to the precursor solutions has demonstrated enhanced luminescence yields and optoelectronic performance, which harvests photon-recycling effects.

The properties and dynamics of the perovskites’ electronic states are controlled by the crystal structure and symmetry. Strong spin-orbit coupling is predicted to introduce Rashba-type state splitting in the electronic band structure. Together with the soft crystal structure of the perovskite lattice, it is likely that dynamic changes occur in the electronic states during their lifetime. So far, it is not understood how such effects change after optical excitation and how they proceed during relaxation of electronic states.

In this talk I will present how we use spectroscopic techniques to study the dynamics of electronic states and crystal structure in metal-halide perovskites on ultrafast timescales. I will report results on layered and bulk lead-halide perovskites, but also on more sustainable lead-free variants. I will discuss how the crystal structure affects the properties of electronic states, and how we can use these modifications to create novel optoelectronic devices.

Three-dimensional lead-halide perovskites combine solution processing with outstanding optoelectronic properties. Despite their soft ionic nature these materials demonstrate a surprisingly low level of electronic disorder. Understanding how structural and dynamic disorder impacts the optoelectronic properties of these perovskites is important for many applications. Here we combine a number of bulk-sensitive and surface-selective spectroscopic techniques to eluscidate the structure and dynamics of organic cations.

We use ultrafast two-dimensional vibrational spectroscopy and molecular dynamics simulations to study the dynamics of the organic cation orientation in a films of pure and mixed tri-halide perovskite materials. For pure MAPbX3 (X=I, Br, Cl) perovskite films, we observe that the cation dynamics accelerate with decreasing size of the halide atom. Much slower dynamics, up to partial immobilisation of the organic cation, are observed in the mixed MAPb(ClxBr1-x)3 and MAPb(BrxI1-x)3 alloys, which we associate with symmetry breaking within the perovskite unit cell.

We also applied surface sensitive vibrtational sum-frequency generation spectroscopy (VSFG) to address the orientation of cations at the perovrkite active layer interfaces with different electron- and hole-extracting materials.We found that at perovskite spiro interface cations are patially immobilised that can be and evidence of high interfacial charge density. 

The observed dynamics and structural information are essential for understanding the effects of structural and dynamical disorder in perovskite-based optoelectronics.

During this presentation we will introduce the A-LEAF project: "An artificial leaf: a photo-electro-catalytic cell from earth-abundant materials for sustainable solar production of CO2-based chemicals and fuels" (FET-PROACT-732840). This project, funded by the European Union, is a collaborative effort by thirteen partners from eight European countries, coordinated by the Institute of Chemical Research of Catalonia (ICIQ), in Spain.

Our A-LEAF initiative aims to design and develop a fully functional scheme to transform sunligh, water and carbon dioxide into useful, sustainable and environmentally neutral fuels and/or fine chemicals. We have gathered a truly multidisciplinary consortium to cover all scientific expertise needed to overcome the major challenges: physicists to achieve sunlight capture; surface scientists to use this energy to oxidize water, extracting protons and electrons; electrochemists to use these equivalents to reduce carbon dioxide into useful stock; and system engineers to implement and fine-tune all components into a viable and cost-effective process. Because, in addition to the general scientific challenge, we have committed to use, exclusively, scalable processes, earth abundant non-critical raw materials and inexpensive platforms. Only with these added values, we may dream of bringing artificial photosynthesis, and solar fuels, to the future Energy pull, where societal impact is the ultimate target.

Our  thirteen partner institutions: ICIQ, ETH Zürich, Universitet Leiden, IMDEA Nanociencia, Technische Universität Wien, Universitat Jaume I, Imperial College, Technische Universität Darmstadt, Forschungzentrum Jülich, Université de Montpellier, INSTM and COVESTRO; started the collaborative work in Januay 2017, and the major results and current state of the project (successful achievements and unexpected problems) will be highlighted and discussed.

Efforts in the solar water splitting field for the past decade have established BiVO₄ as the highest performing metal oxide photoanode. Within this period, the AM1.5 photocurrents of BiVO₄ have been increased from hundreds of mA/cm² to ~7 mA/cm².^[1] However, high photocurrents (> 5 mA/cm²) are only achieved with nanostructuring, which poses additional complexities (e.g., light scattering) for the design and scalability of a tandem device for water splitting. Non-nanostructured BiVO₄ is limited by its poor carrier transport properties. In addition, with reported photocurrents already within 90% of the theoretical maximum, the solar-to-hydrogen (STH) efficiency of BiVO₄ is limited to < 9% due to its relatively wide bandgap of 2.4-2.5 eV.

Here, we demonstrate that these limiting factors can be alleviated by controlled anion and cation substitution in BiVO₄. To overcome the bandgap limitation, we incorporated sulfur into BiVO₄ by post-annealing in sulfur-rich atmosphere. As a result, the bandgap is reduced by up to ~0.3 eV, which increases the theoretical maximum STH efficiency to ~12%. We confirmed that sulfur substitutes oxygen in the lattice of BiVO₄ by a series of structural and chemical characterization (e.g., XRD, Raman, XPS). Moreover, hard X-ray photoelectron spectroscopy (HAXPES) reveals that the bandgap decrease is a result of an upward shift of the valence band maximum. Simultaneously, time-resolved microwave conductivity (TRMC) measurements reveal an improvement of charge carrier transport by the incorporation of sulfur; the mobility increases by a factor of ~5. Wavelength-dependent TRMC measurements confirm the photoactivity of the sulfur-incorporated BiVO₄ up to 560 nm, well beyond the bandgap of typical BiVO₄ films. In addition, we successfully introduced calcium into BiVO₄ thin films;^[2] calcium substitutes bismuth and acts as an acceptor-type dopant. Although it seems counterintuitive to introduce an acceptor dopant into n-type BiVO₄, this would allow the fabrication of p-type, and eventually p-n homojunctions based on BiVO₄. HAXPES measurements reveal that calcium out-diffuses towards the surface of the film, thereby creating a spontaneous p-n homojunction within the film. As a result of the internal electric field, the carrier separation efficiency was enhanced by a factor of ~2. Overall, this work underlines the importance of controlled ionic substitution in complex metal oxides, which may not only improve the performance but also enable new device architectures.

[1] Abdi and Berglund, J. Phys. D. Appl. Phys. (2017)

[2] Abdi et al. ChemPlusChem (2018)

In conventional solar cells (SCs), above-bandgap “hot” carriers (HCs) rapidly lose their excess energy to vibrations in the semiconductor lattice via electron-phonon coupling. This thermalisation to the band edge forms the main part of the Shockley-Queisser limiting efficiency. Semiconductors with reduced carrier cooling rates are desirable to exceed this limit via a hot-carrier architecture, where the hot carriers are extracted before they are fully cooled. Lead-halide perovskites are solution processible solar cell materials, which exhibit exceptional power conversion efficiencies and a tunable band gap. Recent measurements indicate slow cooling at high carrier densities in these material systems. Here we use ultrafast infrared intraband spectroscopy to directly compare the dynamics of carrier cooling in a range of five commonly studied lead-halide perovskites: FAPbI₃, FAPbBr₃, MAPbI₃, MAPbBr₃ and CsPbBr₃. We measure this cooling as occuring within ~100-900 fs, depending on both the carrier density, n^hot (slower cooling at higher n^hot) and choice of cation (with the slowest cooling in the all-inorganic Cs-based system). These observations support the existence of a “hot-phonon bottleneck” and assert the role of lattice vibrations towards HC cooling.

Metal-halide perovskites show exceptional optoelectronic properties for next generation photovoltaics and light-emitting diodes. Recently, monovalent cation substitution has been reported to generate luminescence very efficiently, yet the underlying photo-physics remain to be understood.

Here, we study the origin of this increased brightness by combining transient absorption and photoluminescence (PL) to track charge carrier dynamics in thin films. Unexpectedly, we find that the recombination regime changes from the previously-reported second to first order regime dynamically within tens of nanoseconds after excitation, in line with fluence-dependent PLQE measurements. In temperature-dependent PL we find a redshift of the luminescence with decreasing temperature, directly mapping localized shallow traps. Supported by DFT calculations and transistor measurements we propose that energetic disorder in the distribution of electronic states leads to spatial accumulation of charges, creating n- and p-type regions. Our results indicate that strong luminescence can be achieved in mixed-cation perovskites even at low carrier densities and thereby provides a roadmap for highly efficient LEDs.

Colloidal semiconductor quantum dots (CQDs) have been at the forefront of scientific research for more than two decades, based on their size tunable properties. Although implementation of CQDs in opto-electronic devices already occurs, various fundamental issues with a direct impact on technology are left as open questions. Recent years showed an interest in the investigation of magneto-optical properties of various CQDs with substantial importance for opto-electronic and spin-based devices.

Here we include the study of two different CQD platforms: (1) Synthesis and magneto-optical characterization of spectrally stable pure and diluted magnetic semiconductor CQDs from the II-VI semiconductor family (e.g., CdTe/CdSe and Mn@CdTe/CdSe); (2) Magneto-optical characterization of perovskite CQDs of the type APbBr3 (A - methylamonium or Cs+). Both systems show intriguing spin properties of special scientific and technological interests. The uniqueness of the spin properties and their novelty will be the focus during the talk.

CdTe/CdSe colloidal quantum dots with special composition, including soft boundary (alloying) at the core/shell interface or a giant core or a shell, possess quasi type-II configuration and show blinking-free behavior. The Mn+2 doping induces internal spin interactions between photo-generated species and the dopant spins, leading to giant magnetization or to an internal energy transfer into the dopant orbitals, and consequence emission from host-dopant hybrid- or from dopant atomistic-states. The current study developed a method to position the Mn ions selectively either at the core or at the shell. The magneto-optical measurements, including the use of optically detected magnetic resonance, exhibited resonance transitions related to the coupling of the Mn electron and the nuclear spins with the individual photo-generated carriers. The work was done in collaboration with the laboratory of Prof. Volkan Hilmi Demir from Bilkent and University and NTU.

The perovskites are minerals that have been studied extensively in the past. They are the focus of new interest in recent years, due to their exceptional performance in photovoltaic cells. Perovskites semiconductors possess high absorption coefficients as well as long-range transport properties. Currently, they are also prepared in the form of CQDs with very interesting properties including ferroelectricity, magnetism and exciton effects. The magneto-optical measurements of excitons in CsPbBr3 as individuals were investigated by monitoring the micro-photoluminescence spectra in the presence of an external magnetic field, while monitoring either the circular or linear polarization components.

Fully inorganic cesium lead halide (CsPbX₃, where X = Cl, Br, I) perovskite-type nanocrystals are colloidal quantum dots with bright, narrowband emission that is tunable by composition and size over a wide spectral range with room-temperature photoluminescence quantum yield of up to 90%^[1]. A pecularity of this material is the almost blinking-free emission at low temperature^[2] that originates from a triplet state with exceptionally high oscillator strength^[3].

We use densely packed arrays of up to several millions of these nanocrystals, known as superlattices, produced by means of solvent-drying-induced spontaneous assembly^[4]. Such ensemble of emitters behaves dramatically different from its individual constituents due to coherent interaction enabled by the strong light-matter interaction and the excellent monodispersity of the quantum dots. The collective coupling gives rise to an intriguing many-body quantum phenomenon, resulting in short, intense bursts of light: so-called superfluorescence. We observe a comprehensive set of key signatures like dynamically red-shifted emission with more than twenty-fold accelerated radiative decay, extension of the first-order coherence time by more than a factor of four, photon bunching, and delayed emission pulses with Burnham–Chiao ringing behaviour at high excitation density.

This is the first demonstration of optical collective behaviour and extended coherent states with nanocrystals, opening up new opportunities for high-brightness and multi-photon quantum light sources.

References:
[1] Protesescu et al., Nano Lett. 15, 3692–3696 (2015)
[2] Rainò et al., ACS Nano 10, 2485–2490 (2016)
[3] Becker et al., Nature, 553, 187-193 (2018)
[4] Rainò et al., arXiv 1804.01873 (2018)

Coherent quantum dynamics of excitons in semiconductor quantum dots (QDs) are of key interest, besides fundamental physics, for many applications ranging from quantum computing to advanced photonic devices. With the advances in colloidal synthesis, high-quality semiconductor nanocrystals can be fabricated at lower cost, and more flexibility in size, shapes and compositions. Despite its importance, measuring the exciton dephasing time in colloidal nanocrystals is technically challenging. Using three-beam transient resonant four-wave mixing technique in heterodyne detection not affected by spectral diffusion, we have measured the temperature-dependent ground-state exciton dephasing dynamics in CdSe/ZnS wurtzite QDs, CdSe/CdS spherical zincblende and rod-shape wurtzite QDs with variable core diameter and shell thickness / rod length, CdSe nanoplatelets, PbS QDs, InP/ZnSe QDs, and perovskite (CsPbBr₂Cl) QDs.

In these structures, the importance of phonon-assisted transitions, and the zero-phonon line (ZPL) dephasing by phonon-mediated spin-relaxation and radiative decay at low temperature are vastly varying. In PbS QDs, the peculiar band structure allows coupling with phonons at the zone edge (X-point), resulting in dominating phonon assisted transitions [1] even at low temperatures with sub-picosecond dephasing and a ZPL weight of less than 7%. In CdSe QDs [2,3] and dots in rods of similar size, the ZPL weight instead is above 50% since only zone-center phonons can couple, and the ZPL dephasing is limited by spin-relaxation into dark states on a 10-1000ps time scale, faster than the radiative lifetime of about 10ns. In InP QDs, a non-toxic alternative to CdSe, a similar behaviour is observed. In CdSe nano-platelets, the quasi two-dimensional confinement leads to quantum-well type behaviour, with a large exciton coherence area, such that the exciton dephasing is dominated by a fast radiative decay in the 1ps range [4]. In perovskite CsPbBr₂Cl QDs, the exciton ground state is bright, and a radiatively limited dephasing in the 10-100ps range is observed at low temperatures.  

[1] F. Masia et. al., Phys. Rev. B 83, 201309(R) (2011) DOI:10.1103/PhysRevB.83.201309
[2] F. Masia et al. Phys. Rev. Lett. 108, 087401 (2012) DOI:10.1103/PhysRevLett.108.087401
[3] N. Accanto et al., ACS Nano 6, 5227-5233 (2012) DOI:10.1021/nn300992a
[4] A. Naeem et al., Phys. Rev. B 91, 121302(R) (2015)  DOI 10.1103/PhysRevB.91.121302

Within five years, methylammonium lead iodide (MAPbI3) solar cells have quickly reached remarkable power conversion efficiencies rivaling those of established technologies. However, arguably, the toxic and water-soluble lead compound may be an obstacle on their way to market. The quest for alternative, non-toxic photo harvesters is partly hampered by a lack of fundamental understanding of the crystal grain’s characteristics and energy conversion mechanisms. As part of this process, the scientific community controversially discusses the importance of ferroic properties for the exceptional performance of MAPbI3 light-harvesting layers, including claims of non-ferroelectricity, anti-ferroelectricity, ferroelectricity and ferroelasticity. Simulations have predicted ferroelectricity in MAPbI3 with alternating polarized domains ruling the charge carrier transport [1]. Understanding the crystallographic cause and the effects of the crystal’s ferroelectricity would therefore provide helpful guidance for the quest to find non-toxic MAPbI3 replacements.

We explore the ferroic properties of methylammonium lead iodide perovskite solar cells by piezoresponse force microscopy (PFM) [2][3]. In vertical and horizontal PFM imaging, we find 90 nm wide ferroelectric domains of alternating polarization. High-resolution photo-conductive atomic force micrographs under illumination also show alternating charge carrier extraction patterns which we attribute to the local vertical polarization components within the ferroelectric domains.

We apply these techniques to investigate formation of polarized domains during thermal treatment and study their influence on the performance of perovskite solar cells. Annealing steps, commonly only viewed as a means of crystal growth and precursor conversion, prove to directly influence the formation, shape and polarization direction of ferroelectric domains in perovskite thin films.

References:

[1] D. Rossi, A. Pecchia, M. Auf der Maur, T. Leonhard, H. Röhm, M. J. Hoffmann, A. Colsmann, A. D. Carlo, Nano En. (2018).

[2] H. Röhm, T. Leonhard, M.J. Hoffmann, A. Colsmann, Energy Environ. Sci. (2017).

[3] T. Leonhard, A. Schulz, H. Röhm, S. Wagner, F. Altermann, W. Rheinheimer, M.J. Hoffmann, A. Colsmann, submitted.

Slow carrier recombination in metal halide perovskites (MHP) is considered the origin of the observed large charge carrier diffusion length. However, the reasons for this slow charge carrier recombination remains unclear. In this report, on the basis of novel experimental data and the results of modeling, we propose that the observed long luminescence lifetimes in MHP are due to the extremely fast capture of carriers by shallow non-quenching traps, followed by their much slower release back to the conduction band. Multiple (tens to thousands of times) repetition of this loss-free process can explain the observed slow luminescence decay after pulsed photoexcitation (the so-called "delayed luminescence") [1, 2]. In the case of polycrystalline layers and in single crystals of MHP, the result of this multiple capture of carriers by shallow non-quenching traps are rather low values of the diffusion coefficient found for this type of materials [2].

REFERENCES

[1] Chirvony, V. S.; González-Carrero, S.; Suárez, I.; Galian, R. E.; Sessolo, M.; Bolink, H. J.; Martínez-Pastor, J. P.; Pérez-Prieto, J. Delayed luminescence in lead halide perovskite nanocrystals. J. Phys. Chem. C 2017, 121, 13381-13390.

[2] Chirvony, V. S.; Martínez-Pastor, J. P. Trap-limited dynamics of excited carriers and delayed luminescence in metal halide perovskites. J. Phys. Chem. Lett. 2018, accepted.

Recently, Ham et al. reported that aluminum incorporation into SrTiO₃ microparticles followed by modification with Rh_2-yCryO₃ produces a superior overall water splitting catalyst with 30% apparent quantum efficiency at 350 nm. Based on transient IR spectroscopy, the improved activity was attributed to the removal of Ti³⁺ recombination sites. Here we use results from photoelectron spectroscopy (XPS) and density functional theory to show that Al³⁺ incorporation into the perovskite lattice not only reduces the number of Ti³⁺ deep recombination sites and but also promotes the formation of oxygen vacancies in the material. The oxygen vacancies form a mini-band 0.2 eV above the valence band whose energy position strongly depends on the spatial separation between Al³⁺ sites and oxygen vacancies. The vacancy band is believed to aid photochemical charge separation in this metal oxide. Particle suspensions of the material inside of a baggie reactor promote overall water splitting under direct sunlight illumination with 0.1% solar to hydrogen efficiency.

References

Ham, Y., T. Hisatomi, Y. Goto, Y. Moriya, Y. Sakata, A. Yamakata, J. Kubota, and K. Domen, Flux-Mediated Doping of SrTiO₃ Photocatalysts for Efficient Overall Water Splitting. Journal of Materials Chemistry A, 2016. 4(8): p. 3027-3033. http://dx.doi.org/10.1039/C5TA04843E

Cu(I)-based delafossites (CuMO2) have garnered significant attention for their small optical bandgaps, strong light absorption characteristics, and native p-type conductivities, which make them suitable photocathode candidates for solar fuel production. While resolving the inherently complex electronic structures of these ternary oxides is often challenging, doing so is imperative for understanding optical excitations and carrier transport mechanisms, as well as for furthering material design and discovery. Combining density functional theory (DFT) calculations and X-ray spectroscopy techniques, this research gives a detailed portrait of the band structure of rhombohedral 3R-CuFeO2, prepared by reactive co-sputtering. In particular, element-specific contributions from 3d orbitals of Fe and Cu, as well as 2p orbitals of O, in the valence and conduction bands are revealed by resonant inelastic X-ray scattering (RIXS) measurement. By combining this knowledge of electronic structure, with measurements of optical properties, carrier relaxation dynamics, and photoelectrochemical performance characteristics, this research provides insights into current limitations of this novel photocathode material and informs strategies to overcome them.

The nanostructured W-doped BiVO₄ photoanodes were prepared by electrospinning. The role of surface states (SS) during water oxidation for the as-prepared photoanodes was investigated by using electrochemical, photoelectrochemical, and impedance spectroscopy measurements. An optimum 2% doping is observed in voltammetric measurements with the highest photocurrent density at 1.23 V_RHE under back side illumination. It has been found that a high PEC performance requires an optimum ratio of density of surface states (N_SS) with respect to the charge donor density (N_d), to give both good conductivity and enough surface reactive sites. The optimum doping (2%) shows the highest N_d and SS concentration, which leads to the high film conductivity and reactive sites. The reason for SS acting as reaction sites (i-SS) is suggested to be the reversible redox process of V⁵⁺/V⁴⁺ in semiconductor bulk to form water oxidation intermediates by electron trapping process. Otherwise, the irreversible surface reductive reaction of VO₂⁺ to VO²⁺ by electron trapping process raises the surface recombination. W doping does have an effect on the surface properties of BiVO₄ electrode. It can tune the electron trapping process to obtain high concentration of i-SS and less surface recombination. This work gives a further understanding for the enhancement of PEC performance caused by W doping in the field of charge transfer at the semiconductor/electrolyte interface.

One of the critical challenges for efficient solar water splitting is the identification of a stable photoelectrode material with a bandgap of 1.6 - 1.9 eV that can be used as a top absorber in an efficient D4 tandem device. Recently, n-type α-SnWO₄ was identified as a potential photoanode candidate due to the combination of its ideal bandgap (~1.9 eV) and a very negative photocurrent onset potential (~0 V vs. RHE) [1-3]. However, up to now, α-SnWO₄ photoelectrodes have shown very low photoconversion efficiencies. The reason for this is not fully understood, and many of the essential material parameters are still elusive.

In this study, we identify the major limiting parameters of pulsed laser-deposited α-SnWO₄ photoanodes: (I) the oxidation of Sn²⁺ to Sn⁴⁺ at the surface creating a hole blocking layer and pushing the photoconversion efficiency to almost zero, and (II) the relatively poor charge carrier dynamics resulting in a mismatch between the charge carrier diffusion length and the light penetration depth. To address these challenges, we explored several strategies such as the deposition of a hole-conducting NiO_x protection layer resulting in a new benchmark sulfite oxidation photocurrent of 0.75 mA cm^-2 at 1.23 V vs. RHE. We furthermore show that a high-temperature treatment enhances the charge carrier transport by improving the film crystallinity, resulting in a charge carrier diffusion length comparable to state-of-the-art BiVO₄ photoanodes. Finally, the remaining challenges for oxygen evolution using our α-SnWO₄ films will be discussed.

Our findings provide important insights into the limitations and key properties of α-SnWO₄ photoelectrodes and will help to further improve the performance of this promising photoanode material.

[1] Ziani et al., APL Materials, 3 (2015) 096101

[2] Zhu et al., ACS Appl. Mater. Interfaces, 9 (2017) 1459-1470

[3] Pyper et al., Chin. Chem. Lett., 26 (2015) 474-478

The development of technologies for H2 production or CO2 reduction strongly relies on an abundant supply of protons and electrons liberated by water oxidation. [1-2] Therefore, photoelectrochemical (PEC) water oxidation is an important anodic half-cell process in the development of a sustainable artificial solar fuel system. In the PEC devices design, coupling water oxidation catalysts with active photoanode materials has become the most promising methodology, since the attachment/integration of the catalyst on the semiconductor light absorbers could kinetically facilitate interfacial charge transfer reactions.

In this contribution, we have fabricated ITO/Fe2O3/Fe2TiO5/FeNiOOH multi-layers nanowire heterostructures via combination of sputtering, hydrothermal, ALD, photo-electrodepositon methods for photoelectrochemical (PEC) oxygen evolution application. Structural, spectroscopic and electrochemical investigations disclose that the origin of the superior catalytic performance is owing to the interfacial coupling effect of ITO underlayer (Sn doping and conductivity promoter), ultrathin Fe2TiO5 coating (Ti doping, energetics and surface state density modulation) and FeNiOOH eletrocatalyst (varying surface state energy level). [2]

Meanwhile, an alternative earth-abundant CoFe prussian blue analogues (CoFe PBA) is incorporated in Fe2O3/Fe2TiO5 core-shell type II heterojunction nanowires as photoanodes for PEC water oxidation. The observed photocurrent is improved from 0.12 mA cm-2 to 1.25 mA cm-2 at 1.23 V vs. RHE under illumination by involvement of ultrathin Fe2TiO5 layer and CoFe PBA WOCs coating. Further investigation of the PEC mechanisms via photoelectrochemical impedance spectroscopy unveils that the enhanced PEC performance is attributed to the enhanced charge transfer efficiency owing to the tuned energy level and density of surface state. [3-4]

References

[1] Félix Urbain, Pengyi Tang, Nina M. Carretero, Teresa Andreu, Luís G. Gerling, Cristóbal Voz, Jordi Arbiol, Joan R. Morante, Energy & Environmental Science, 10, 2256-2266 (2017).

[2] Pengyi Tang, HaiBing Xie, Carles Ros, LiJuan Han, Martí Biset-Peiró, Yongmin He, Wesley Kramer, Alejandro Perez-Rodriguez, Edgardo Saucedo, Jose Galan-Mascaros, Teresa Andreu, Joan R. Morante, Jordi Arbiol, Energy & Environmental Science, 10, 2124-2136 (2017).

[3] Lijuan Han, Pengyi Tang, Alvaro Reyes-Carmona, Barbara Rodriguez-Garcia, Mabel Torrens, Joan Ramon Morante, Jordi Arbiol, Jose Ramon Galan-Mascaros, Journal of the American Chemical Society, 138, 16037-16045 (2016).

[4] PengYi Tang, LiJuan Han, Paul Paciok, Marti Biset Peiro, Hong-Chu Du, Xian-Kui Wei, Lei Jin, Hai-Bing Xie, Qin Shi, Teresa Andreu, Joan Ramon Morante, Mónica Lira-Cantú, Marc Heggen, Rafal E. Dunin-Borkowski, José Ramón Galán-Mascarós, Jordi Arbiol, to be submitted.

Recent progress in colloidal chemistry have led to the synthesis of new types of two-dimensional (2D) lattices of PbX (X=Se, S, Te) nanocrystals [1,2]. In these lattices, each nanocrystal is epitaxially connected to its neighbors, the 2D materials are single crystalline in absence of disorder. Remarkably, square and honeycomb lattices can be synthesized using the same initial nanocrystals, allowing to investigate the role of the lattice geometry on the electronic properties. In addition, cation exchange processes can be used to transform PbX lattices into CdX ones. Theoretical studies accompanying these experiments have already demonstrated that these 2D lattices are characterized by very interesting band structures, including in some cases Dirac and non-trivial flat bands. In the present talk, theoretical works on the optical properties of 2D lattices will be presented. The effects of the epitaxial bonds between neighbor nanocrystals on the optical properties will be discussed. The theoretical calculations will be compared to recent experimental studies [3]. The differences between IV-VI (PbX) and II-VI (CdX) materials, between square and honeycomb lattices, will be reviewed.

[1] W. H. Evers, B. Goris, S. Bals, M. Casavola, J. de Graaf, R. van Roij, M. Dijkstra, and D. Vanmaekelbergh, Nano Lett. 13, 2317 (2013).

[2] M. P. Boneschanscher, W. H. Evers, J. J. Geuchies, T. Altantzis, B. Goris, F. T. Rabouw, S. A. P. van Rossum, H. S. J. van der Zant, L. D. A. Siebbeles, G. Van Tendeloo, I. Swart, J. Hilhorst, A. V. Petukhov, S. Bals, and D. Vanmaekelbergh, Science 344, 1377-1380 (2014).

[3] M. Alimoradi Jazi et al., Nano Lett. 17, 9 5238-5243 (2017).

Ever since the exfoliation of graphite into an atomically thin monolayer, known today as graphene [1], two dimensional (2D) materials have been of central interest for a variety of electronic applications. 2D materials belong to a large family of anisotropically active compounds which have strong, covalent bonds within a layer while in between layers there are only weak van der Waals interactions that can be overcome, obtaining molecularly thin sheets. Such a reduction of dimensionality has a profound impact on properties, known to vary strongly with respect to the number of atomic layers.

As graphene applications in electronics has thus far been hindered by its non-existent band gap, layered semiconductors are studied as potential candidates for future devices. Many Transition Metal Dichalcogenides (TMDs) including MoS₂, MoSe_2, WS₂, WSe₂ have been thoroughly investigated, however in order to meet rising demands new families of 2D semiconductors are studied. One family of such is the transition metal thiophosphates, denoted MPS_x, for x=3 or 4; for example bulk crystals of CrPS₄ - chromium thiophosphate – which has been examined in the past for applications in lithium batteries. Nowadays, this compound has once again gained scientific interest due to its optical anisotropic properties and the possibility to obtain and study its few- and monolayer systems [2].

In this work, bulk crystals of CrPS₄ were obtained by vapor transport synthesis (furnace method), followed by structure and composition confirmation via different techniques, for example Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (SEM/EDX), Powder X-Ray Diffraction (PXRD) and Raman spectroscopy. Optical properties, such as band gap and optical transitions were investigated by Solid State UV-VIS Spectroscopy, PhotoAcoustic Spectroscopy (PAS) [3] and Modulated Spectroscopy (MS) [3]. Later, bulk crystals of chromium thiophosphate (CrPS₄) were exfoliated in liquid to obtain few layers systems and photoconductivity measurements were used to ascertain photoactive properties, both of re-stacked films and bulk crystals.

Acknowledgments:
This work was supported by the European Comission via the Marie-Sklodowska Curie action Phonsi (H2020-MSCA-ITN-642656)
This work was performed within the grant of the National Science Centre Poland (OPUS 11 no. 2016/21/B/ST3/00482).
S.J.Z. also acknowledges the support within the ETIUDA 5 grant from National Science Center Poland (no. 2017/24/T/ST3/00257).

References:
[1] Novoselov et.al., Science, vol 306, no. 5696 (2004)
[2] Lee et.al., ACS Nano vol. 11, no. 11 (2017)
[3] Zelewski & Kudrawiec, Scientific Reports, vol. 7 no. 15365 (2017)

Colloidal nanoplatelets are atomically flat, quasi-two-dimensional sheets of semiconductor that can exhibit efficient, spectrally pure fluorescence. These materials were discovered when liquid-phase chemical syntheses of spherical nanocrystals were modified. However, despite intense interest in their properties, the mechanism behind their highly anisotropic shape and precise atomic-scale thickness remained unclear, and even counter-intuitive for commonly studied nanoplatelets that arise from isotropic crystal structures (such as zincblende CdSe and lead halide perovskites). Here we describe an intrinsic instability in growth kinetics that can lead to such highly anisotropic shapes. By combining experimental results on the synthesis of CdSe nanoplatelets with theory predicting enhanced growth on narrow surface facets, we develop a model that explains nanoplatelet formation as well as observed dependencies on time and temperature. Based on standard concepts of volume, surface, and edge energies, the resulting growth instability criterion can be directly applied to other crystalline materials. Thus, knowledge of this previously unknown mechanism for controlling shape at the nanoscale can lead to broader libraries of quasi-two-dimensional materials.

Two-dimensional (2D) semiconductors are finding a renewed interest in recent years, due to their combination of physical properties, e.g., mobility, fluorescence and spin selectivity, with a potential implementation in new and emerging opto-electronic and spin-electronic devices. The current work discusses various magneto-optical phenomena, found in two different 2D systems: The transition metal phosphorous trichalcogenides, indium chalcogenides and magnetically doped colloidal nanoplatelets. The magneto-optical properties are investigated by following optical polarization in the presence of an external magnetic field, as various temperatures, as well as implementing the use of optically detected magnetic resonance spectroscopy.

The transition metal phorphorous trichalcogenides resemble the most common layered dichalcogenides, but one-third of the metals are replaced with a P-P pair; hence, the chemical formula is written as M2/3(P-P)1/3X2 or M2P2X6. The dilution of metal site by non-magnetic atoms, endows a column like arrangement of the remaining metals, leading to a special magnetic properties, from a full antiferromagnetic Neel, through antiferromagnetic zigzag to ferromagnetic character. The various arrangements can be tuned by variation of the metal cations (among the first row of transitions metal atoms). The work focuses on the influence of the created magnetism on the magneto-optical properties of the M2P2X6 semiconductors. In addition, the talk will report about the synthesis and characterization of In2S3 layered compounds, exploring the various possible structural phases and their magnetically doped derivatives.

Transition metal dopant embedded in colloidal semiconductor nanoplatelets (NPLs) exhibit special magnetic properties, resemble the bulk diluted magnetic semiconductors. However, the NPLs confined thickness induces an extremely intense spin-exchange interaction between the resident photo-excited carriers and the guest magnetic spins. Such an interaction leads to a giant magnetization and g-factor, and consequently endows the materials with special magneto-optical properties. The work emphases the investigation of the spin-exchange interaction while varying the magnetic dopants, by following variation in the magneto-optical properties, when detecting either an ensemble of NPLs or a focus on a single platelet. **

** Magnetically doped NPLs project was carried out in collaboration with Prof. Volkan Hilmi Demir and his groups from Nanyang Technological University – NTU Singapore 639798, Singapore and from Bilkent University, Ankara 06800, Turkey.

We introduce the electronic and exciton dynamic properties of these nanoplatelets and their hetero structures and demonstrate for example that ligand induced strain in these colloidal quantum wells with finite size results in a strong alteration of the transition energies. We show that intrinsically directional light emitters are potentially important for applications in photonics including lasing and energy efficient display technology and propose a new route to overcome intrinsic efficiency limitations in light-emitting devices by studying a CdSe nanoplatelets monolayer that exhibits strongly anisotropic and directed photoluminescence.  Our analysis of the two-dimensional k-space distribution of the nanoplatelet absorption and emission reveals the underlying internal transition dipole distribution. The observed directed emission is related to the anisotropy of the electronic Bloch states governing the exciton transition dipole moment and forming a bright plane. The strongly directed emission perpendicular to the platelet is further enhanced by the optical local density of states and local fields. In contrast to the emission directionality, the off-resonant absorption into the energetically higher 2D-continuum of states is isotropic. These contrasting optical properties make the oriented CdSe nanoplatelets, or superstructures of parallel-oriented platelets, an interesting and potentially useful class of semiconductor-based emitters for photonic applications. We also demonstrate by 2D k-space spectroscopy that two-photon absorption (TPA) is highly anisotropic in CdSe nanoplatelets, thus promoting them as a new class of directional two-photon absorbers with extremely large cross sections.

References:                                                                                                                     1. Scott and Achtstein et al., Nature Nanotechnology 12, 1155 (2017)                        2. Heckmann and Achtstein et al., Nano Letters 17, 6321, (2017)                                3. Antanovich and Achtstein et al., Nanoscale, (2017); DOI: 10.1039/c7nr05065h

Solution-processed semiconductor nanocrystals have attracted great interest in photonics including high-purity color conversion and enrichment in quality lighting and display backlighting. These nanocrystals span different types and heterostructures of semiconductors in the forms of colloidal quantum dots and rods to a more recently emerging class of colloidal quantum wells. Here we will talk about colloidal photonics using the family of quasi-2D, tightly-confined, atomically flat nanocrystals. Here we will show that custom-design 2D heteronanoplatelets uniquely offer record high optical gain coefficients and ultra-low threshold stimulated emission. In addition, we will show that controlled stacking of these nanoplatelets provides us with the ability to further tune and master their excitonic properties. Also, we will discuss doping of these nanoplatelets with Cu for high-flux solar concentration properties and with Mn for precise wavefunction-engineered magnetic properties. Given their most recent accelerating progress, these solution-processed quantum materials hold great promise to challenge their epitaxial counterparts in semiconductor optoelectronics in the near future.

Colloidal PbS nanosheets represent an important infrared 2D nanomaterial. They have tunable energy gaps and much higher charge carrier mobility than quantum-dot films, which makes them ideal for optoelectronic and electronic applications including photovoltaic devices and field-effect transistors.

The growth of PbS nanosheets is confirmed by electron microscopy and photoluminescence spectroscopy. Their thickness can be tuned by changing the reaction temperature during the synthesis. Recent research also demonstrates that the lateral size of the nanosheets can be systematically tuned between 20 nm to a few hundred nanometers. Core/shell PbS/CdS nanosheets can be synthesized using a cation exchange method. This method can protect the PbS core as well as further tuning the core thickness. Surface passivation of the nanosheets using organic molecules is also proven to be very effective to suppress the surface defects and improve the optical properties.

The energy gap of the PbS nanosheets can be tuned by changing their thickness. The thickness dependent energy gap is a unique feature of the exciton under one-dimensional confinement. In contrast to quantum dots, the confinement energy can be achieved in a typical nanosheet is smaller than a quantum dot. However, the maximum energy gap of the nanosheet can still reach 1 eV which is about more than twice of the energy gap of the bulk PbS.

The optical absorption of typical PbS nanosheets shows a step-like spectrum. The step edge is nearly coincident with the photoluminescence peak, indicating a negligible Stokes shift. The absorption spectrum of the nanosheets of 20 nm in lateral size shows an excitonic peak similar to quantum dots, which is possibly due to the additional lateral confinement.

The absolute photoluminescence quantum yield of the PbS nanosheets can be measured accurately by using an integrating-sphere technique. Well-passivated PbS nanosheets typically show 20% to 30% photoluminescence quantum yield. The maximum photoluminescence quantum yield has reached 60%, which is about twice of the quantum yield from PbS quantum dots of the same energy gap.

The time-resolved photoluminescence of the PbS nanosheets shows a distinct fast decay followed by a slow decay. The fast decay accounts for more than 90% of the total luminescence intensity. The exciton radiative lifetime derived from the time-resolved photoluminescence and the quantum yield is much shorter than the PbS quantum dots. It is an indication of giant oscillation strength transition which is a consequence of large exciton coherence volume in a nanosheet.

The n-type semiconductor bismuth vanadate (BiVO₄) is a promising material as photoanode for light induced water oxidation. Its absorption in the visible range (band gap energy of 2.4 eV), its suitable band edge positions compared to the OER half reaction, its stability against photo-corrosion as well as its low cost make BiVO₄ one of the most interesting ternary oxide materials for light-induced oxygen evolution from water.¹ One major drawback for BiVO₄ is its poor electronic conductivity, which can however be overcome by applying three strategies: i) adjustment of thin film properties (especially thickness), ii) n-type doping by cation and more recently anion substitution and iii) heterojunction design (type II). Within the present talk, I will give an overview of the improvements we achieved for novel sol-gel based BiVO₄ thin film photoanodes following each of these strategies allowing us to go all the way from low to high performance BiVO₄ photoanodes.

First we developed a new, sol-gel-based synthesis involving simple dip-coating and calcination allowing the easy and reproducible fabrication of porous BiVO₄ and Mo‑doped BiVO₄ thin film photoanodes.² The obtained thin films crystallize in the monoclinic scheelite structure in micrometre large, two-dimensional, single-crystalline porous domains with wall features in the range of the hole diffusion length of BiVO₄. Optimization of the electron transport properties resulting in higher PEC performance was realized by cation and anion doping using Molybdenum and Fluorine, respectively.³ Finally, our new synthesis approach could be easily applied for the fabrication of BiVO₄/WO₃ type II heterojunctions following a simple layer-by-layer deposition. It is shown that precise control of the layer morphology and the overlapping interface between the layers is an indispensable prerequisite for high performance WO₃/BiVO₄ heterojunction photoanodes.

This work was funded by the DFG SPP1613 program.

¹ Z.-F. Huang, L. Pan, J.-J. Zou, X. Zhang, L. Wang, Nanoscale 2014, 6, 14044.

² M. Rohloff, B. Anke, S. Zhang, U. Gernert, C. Scheu, M. Lerch, A. Fischer, Sustainable Energy Fuels 2017, 1, 1830.

³ B. Anke, M. Rohloff, M. G. Willinger, W. Hetaba, A. Fischer, M. Lerch, Solid State Sci. 2017, 63, 1.

Recently, due to promising results in the fields of photocatalysis and photoelectrocatalysis, the attention has been focused on ferrites as new visible light-active materials [1,2]. Properties such as narrow band gap energies (≈ 2 eV), high stability, abundance, and low cost make ferrites promising for solar energy production and solar remediation. Zinc ferrite (ZnFe₂O₄) is one of the most widely studied compounds belonging to the spinel ferrite family. However, various and even contradictory results regarding the photocatalytic activity of ZnFe₂O₄ have been reported in the scientific literature [3]. ZnFe₂O₄ crystallize in a phase-centered cubic spinel structure with Fe³⁺ and Zn²⁺ ions occupying tetrahedral or octahedral sites[1]. When the Fe³⁺ and Zn²⁺ ions are arranged in octahedral and tetrahedral sites, respectively, the ferrite exhibits a so-called normal spinel structure (^T[Zn]^O[Fe₂]O₄). However, when all the Zn²⁺ ions at the tetrahedral sites are exchanged by Fe³⁺ ions from octahedral sites, the compound adopts a so-called inverse spinel structure (^T[Fe]^O[ZnFe]O₄). The degree of inversion, x, defined as the fraction of Zn²⁺ ions occupying octahedral sites, can consequently adopt values from 0 (normal structure) to 1 (inverse structure) according to ^T[Zn_1-xFe_x]^O[Zn_xFe_2-x]O₄ with 0 ≤ x ≤ 1. The degree of inversion closely depends on the synthetic route.

For the first time, the effect of the degree of inversion on the physicochemical properties affecting the photocatalytic behavior of ZnFe₂O₄ has been investigated. Interestingly, as the degree of inversion increases, the conductivity of the material increases exponentially and an influence on the photocatalytic activity is observed. Thus, the degree of inversion plays a fundamental role and is a parameter of utmost importance to be investigated in order to get a meaningful approach regarding the photocatalytic performance of spinel ferrites.

[1] R. Dillert, D. H. Taffa, M. Wark, T. Bredow and D. W. Bahnemann, APL Materials, 2015, 3, 104001.

[2] D. H. Taffa, R. Dillert, A. C. Ulpe, K. C. Bauerfeind, T. Bredow, D. W. Bahnemann and M. Wark, Journal of Photonics for Energy, 2016, 7, 012009.

[3] A. Arimi, L. Megatif, L. I. Granone, R. Dillert and D. W. Bahnemann, Journal of Photochemistry and Photobiology A: Chemistry, 2018, DOI: 10.1016/j.jphotochem.2018.03.014.

Using a combination of experimental and theoretical methods we explore the beneficial effect of Sn(IV) doping in ultrathin hematite photoanodes for water oxidation. A series of hematite photoanodes with tailored Sn-doping profiles were prepared by alternating atomic layer deposition. Using data from spectrophotometry and intensity-modulated photocurrent spectroscopy we deconvoluted the overall efficiency and obtained the individual process efficiencies for light harvesting, charge separation and charge transfer. Photoanodes with Sn-doping both on the surface and in the subsurface region show the best performance with enhanced charge separation and charge transfer efficiency. Density functional theory calculations with a Hubbard U parameter were performed to investigate the causes of the efficiency improvement considering both Fe₂O₃ (0001), as well as Fe₂O₃ (11-26) surface orientation, as identified from micrographs at atomic resolution. The energetics of surface intermediates during the oxygen evolution reaction reveal that while Sn-doping decreases the overpotential on the (0001) surface, the Fe₂O₃  (11-26) orientation shows a significantly lower overpotential, one of the lowest reported for hematite so far. Electronic structure calculations demonstrate that Sn-doping on the surface also enhances the charge transfer efficiency by elimination of surface hole trap states (passivation). Moreover, the subsurface Sn-doping introduces a band bending that helps to improve the charge separation efficiency.

We acknowledge funding by SPP1613 and computational time at MagntUDE.

[1] A. G. Hufnagel, H. Hajiyani, S. Zhang, T. Li, O. Kasian, B. Gault, B. Breitbach, T. Bein, D. Fattakhova-Rohlfing, C. Scheu and R. Pentcheva, Adv. Funct. Mater. (accepted).

We report on the carrier transport properties of CVD-grown CuWO₄ films for solar water splitting with varying copper-to-tungsten stoichiometry within the borders of the binary metal oxides CuO and WO₃. Time-resolved terahertz (TRTC) and microwave conductivity (TRMC) measurements addresses the bulk dynamics in different time windows from sub-ps to ms. In addition, photo-induced absorption (PIA) in a photo-electrochemical cell is applied to study the temporal behavior of excited carriers in the vicinity of the semiconductor/electrolyte interface under varying bias. A charge carrier mobility of ∼6× 10⁻³ cm² V⁻¹ s⁻¹ and a diffusion length of and 30 nm is determined for a CuWO₄ absorber film deposited with a copper-to-tungsten ratio (Cu/W) of 1.1 which also provides the best photocurrents in assembled photo-electrochemical cells. This value is comparable to undoped BiVO₄ where poor carrier transport is governed by small polaron formation leading to slow mobility values. We compare the experimental results with measurements performed on dip-coated CuWO₄ samples and other metal oxides such as BiVO₄ and Cu₂O. Our findings establish new insights into the advantages and limits of CuWO₄-based photoanodes that can be possibly used in a tandem configuration on top of a highly absorbing semiconductor with optimal electronic properties.

[1] D. Peeters,  O. Mendoza Reyes,  L. Mai,  A. Sadlo,  S. Cwik,  D. Rogal, lH.-W. Becker, H. M.  Schütz, J. Hirst, S. Müller, D. Friedrich, D. Mitoraj, M. Nagli, M. Caspary Toroker, R.Eichberger, R. Beranek, A. Devi, J. Mater. Chem. A, adv. article (2018)

[1] M. Ziwritsch, S. Müller, H. Hempel, T. Unold, F. F. Abdi, R. v. d. Krol, D. Friedrich, R. Eichberger, ACS Energy Lett., 1 , 888 (2016)

[2] F. F. Abdi, T. J. Savenije, M. May, B. Dam, R. van de Krol, J. Phys. Chem. Lett. 4, 2752 (2013)

[4] J. Ravensbergen, F. F. Abdi, J. H. van Santen, R. N. Frese, B. Dam, R. van de Krol, J. T. M. Kennis, J. Phys. Chem. C, 118, 27793 (2014)

The application of multijunction solar cells in photoelectrochemical (PV-EC) devices for hydrogen production is addressed. Integrated PV-EC devices are composed of a ‘traditional’ photovoltaic (PV) cell combined with an electrochemical (EC) cell, presenting a promising approach to produce hydrogen and other fuels. The requirements for PV-EC devices and strategies for the solar cell development will be discussed. The results on the photovoltaic development of multijunction silicon based cells will be presented, focusing on a wide range of photovoltages and photocurrents in various systems, including the adoption of the cells to function on either cathode or anode sides of the system. A prototype integrated PV-EC system based on silicon multijunction solar cells can yield solar-to-hydrogen efficiencies (STH) of 9.5%.

The paths of device upscaling beyond laboratory size and the influence of varied illumination conditions close to obtained outdoor will also be discussed. This includes the effects of spectral quality, intensity and incident angle on the performance of both photovoltaic cells and PV-EC devices.

The efficient catalysis of the four-electron oxidation of water to molecular oxygen (oxygen evolution reaction, OER) is a central challenge for the development of devices for the conversion of electrical energy, ideally from renewable sources, into storable chemical energy (e.g. by electrolysis or “artificial leaves”). Up to now, a plethora of earth-abundant, non-toxic catalysts (e.g. Ni/Fe, Co, Mn oxides)¹ for OER is known. Most of these possess high activities and stabilities under strongly alkaline pH conditions. In contrast, the only convincing materials for OER in acidic media are based on the scarce elements Ru and Ir, while especially Ni and Co based electrodes suffer from corrosion and/or show little activity.²

Inspired by the oxygen evolving complex (OEC) of Photosystem II, the biological catalyst for this reaction, several manganese oxide (MnO_x) polymorphs have been tested as heterogeneous water oxidation (electro-)catalysts. Here we found that amorphous layered or tunnelled manganese oxides show good activities and stabilities also for strong acidic reaction media.³ Recently, we developed a route to directly prepare coatings of amorphous MnO_x on free-standing carbon based electrode supports (e.g. carbon-fiber-paper, CFP) by a simple, scalable redox deposition approach.⁴

This presentation will deal with a study where the electrocatalytic performance of such MnO_x/CFP-anodes was tested under different electrolyte conditions. A special focus was laid on the influence of the pH on activity and stability. Additionally, the MnO_x/CFP-electrodes were characterized before and after electrolysis by means of XRD, SEM/EDX, vibrational- and X-ray absorption spectroscopy. Details of the preparation, characterization, electrocatalytic performance and corrosion stability will be discussed together with possible reasons for the different behavior of the electrocatalyst at different pHs. We will report on an efficient catalyst for OER at acidic and near-neutral solution, a pH range highly desirable for (photo-)electrochemical devices, such as polymer electrolyte membrane (PEM) electrolysers or artificial leaves.

References

¹ N.-T. Suen, S.-F. Hung, Q. Quan, N. Zhang, Y.-J. Xu and H. M. Chen, Chem. Soc. Rev., 2017, 46, 337–365.

² L. C. Seitz, C. F. Dickens, K. Nishio, Y. Hikita, J. Montoya, A. Doyle, C. Kirk, A. Vojvodic, H. Y. Hwang, J. K. Nørskov and T. F. Jaramillo, Science, 2016, 353, 1011–1014.

³ C. E. Frey and P. Kurz, Chem. A Eur. J., 2015, 21, 14958–14968.

⁴ J. Melder, W. L. Kwong, D. Shevela, J. Messinger and P. Kurz, ChemSusChem, 2017, 4491–4502.

Semiconducting metal oxide thin films are promising candidates for photoelectrochemical (PEC) solar water-splitting applications due to their abundance, light absorption properties and stability in aqueous media. To identify materials with optimized properties, thin-film materials libraries, exhibiting combined thickness and compositional gradients, were synthesized by combinatorial reactive co-sputtering from elemental targets on platinized 100 mm diameter wafers in several complex multinary oxide systems: Fe-W-Ti-O, Fe-Cr-Al-O, Cu-Si-Ti-O, V-X-O, and Bi-V-X-O. High-throughput measurements of compositional, structural and functional data on the materials libraries were performed by automated thickness measurements, energy-dispersive x-ray analysis (EDX), X-ray diffraction (XRD) and PEC analyses using an optical scanning droplet cell in 342 measurement areas on each of the materials libraries. Furthermore, the microstructure of selected thin films was characterized by electron and atomic force microscopy. The analysis of the obtained data enables to establish correlations between composition, crystallinity, morphology, thickness, and photocurrent density. Several promising compositions were identified using the combinatorial approach. Furthermore, we demonstrate the combinatorial glancing angle sputter deposition (GLAD) approach for the fabrication of thin film materials libraries consisting of columnar nanostructures.

The emission of fully inorganic cesium lead halide (CsPbX₃, where X = I,Br,Cl) perovskite-type nanocrystals is tunable over a wide energy range with ultrahigh photoluminescence quantum yields of up to 90%^[1] and exhibits narrow emission lines. Due to their facile solution processability and their potential for high-efficiency photovoltaics and light sources they have gained enormous interest.

Experiments on single perovskite quantum dots reveal a unique energetic level structure with a lowest bright triplet state^[2], thus enabling photon emission rates ~20 and ~1000 times higher compared to any other conventional semiconductor nanocrystals at room and cryogenic temperatures, respectively. We investigate the nature of this exceptionally fast photon emission by temperature dependent quantum yield measurements. Furthermore we discriminate it from composition dependent “A-type” blinking behaviour in intensity-decay time correlation measurements and demonstrate stable, narrowband emission, with suppressed blinking and small spectral diffusion^[3] for single CsPbBr₂Cl nanocrystals. By means of polarization dependent high resolution spectroscopy, the complex nature of the exciton fine structure splitting and charged exciton emission has been characterized.

Based on these measurements, supported by effective-mass models and group theory calculations, we conclude that the triplet exciton state is responsible for the extraordinary photon emission properties of lead halide perovskites. Our results can assist to identify other semiconductors that exhibit bright triplet excitons, with potential implications for improved optoelectronic devices.

References:

[1] Protesescu et al., Nano Lett. 15, 3692–3696 (2015)

[2] Becker et al., Nature, 553, 187-193 (2018)

[3] Rainò et al., ACS Nano 10, 2485–2490 (2016)

Two-dimensional (2D) materials have received much attention in the past years for a wide variety of photonic applications due to their pronounced excitonic features leading to unique properties in terms of light emission. However, only a few studies focus on the use of these materials for light amplification or net optical gain development and the ensuing high carrier density photo-physics. The beneficial nature of the strong excitonic effects on optical gain remain hence unquantified and , despite the large binding energies, it remains unclear what the involvement of is at the concomitant high carrier densities. Here, we use colloidal 2D CdSe nanoplatelets as a model system and show, using a quantitative and combinatory approach to ultrafast spectroscopy, that several distinct and carrier density-dependent optical gain regimes exist for these materials. At low density, optical gain is found to originate from excitonic molecules delivering large material gains up to 20.000 cm^-1, yet with an Auger limited lifetime of few hundred picoseconds. At increasing pair density, we observe a surprising transition to a combined regime of blue-shifted and disruptively large optical gain, combined with the typical exciton mediated gain. We show that this peculiar situation originates from a carrier cooling bottleneck at high density. Surprisingly, the insulating (multi-)exciton gas is found to co-exist with the conductive phase in a density regime nearly one order of magnitude beyond the expected Mott transition.  The ensuing exciton ground state absorption even counter-acts the development of net optical gain in certain spectral regions. Our results shed a new light on the disruptive photo-physics of high binding energy excitons in strongly excited 2D materials and pave the way for the development of more efficient broadband optical gain media and/or high density excitonic devices such as polariton lasers.

Colloidal lead halide perovskite nanocrystals (APbX_3, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) emerge as promising materials for optoelectronic applications such as in television displays, light-emitting devices, and solar cells. The sponaneous and stimulated emission spectra of these NCs are readily tunable over the entire visible spectral region of 410-700 nm [1-2]. The photoluminescence of these NCs is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color standard, and high quantum yields of up to 100%. Cs_1-xFA_xPbI₃ and FAPbI₃ reach the near-infrared wavelengths of 800 nm [3].  A particularly difficult challenge lies in warranting the practical utility of such semiconductor NCs in the red and infrared spectral regions. A promising approach lies in the formation of multinary compositions such as Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs. We show that droplet-based microfluidics can successfully guide the synthesis of such complex compositions [4]. We could fine-tune the photoluminescence maxima of such multinary NCs between 700 and 800 nm, minimize their emission linewidths (to below 40nm), and maximize their photoluminescence quantum efficiencies (up to 89%) and phase/chemical stabilities. Most importantly, we demonstrate the excellent transference of reaction parameters from microfluidics to a conventional flask-based environment, thereby enabling up-scaling and further implementation in optoelectronic devices. As an example, Cs_xFA_1–xPb(Br_1–yI_y)₃ NCs with an emission maximum at 735 nm were integrated into light-emitting diodes, exhibiting high external quantum efficiency of 5.9% and very narrow electroluminescence spectral bandwidth of 27 nm.

The processing and optoelectronic applications of perovskite NCs are, however, hampered by the loss of colloidal stability and structural integrity due to the facile desorption of surface capping molecules during isolation and purification. To address this issue, we have developed a new ligand capping strategy utilizing common and inexpensive long-chain zwitterionic molecules, resulting in much improved chemical durability [5].

             Perovskite NCs also readily form long-range ordered asssemblies known as superlattices. These assemblies exhibit accelerated coherent emission (superfluorescence), not observed before in semiconductor nanocrystal superlattices [6].

L. Protesescu et al. Nano Letters 2015, 15, 3692–3696

M. V. Kovalenko et al. Science 2017, 358, 745-750

L. Protesescu et al. ACS Nano 2017, 11, 3119–3134

I. Lignos et al. ACS Nano 2018, DOI: 10.1021/acsnano.8b01122

F. Krieg et al. ACS Energy Letters 2018, 3, 641–646.

Raino, M. Becker, M. Bodnarchuk et al. 2018, submitted

Perovskite solar cells have electrified the solar cell research community with astonishing performance and surprising material properties. Very efficient (>20 %) devices with perovskite layers of low defect density can be prepared by cheap and simple solution based processes at moderate temperatures (<150°C). For commercializing this technology, a stable and reliable operation has to be ensured. In perovskite solar cells, however, the output power is strongly influenced by the history of the device in terms of bias voltage (causing hysteresis) or illumination (known as light soaking effect). The underlying process is assumed to be the slow migration of ionic charges within the perovskite layer.

In my presentation I will demonstrate how scanning probe microscopy can help understanding these processes. Using Kelvin probe force microscopy, we were able to follow the vertical charge distribution in the active perovskite layer of an operating device. We observed the formation of a localized interfacial charge at the anode interface, which screened most of the electric field in the cell. The formation of this charge happened within 10 ms after applying a forward voltage to the device. After switching off the forward voltage, however, these interfacial charges were stable for over 500 ms and created a reverse electric field in the cell. This reverse electric field directly explains higher photocurrents during reverse bias scans by electric field-assisted charge carrier extraction. We thereby show that instead of the slow migration of mobile ions, the formation and the release of interfacial charges is the dominating factor for current-voltage hysteresis.

Recent research and progress in organic photovoltaic (OPV) repeatedly insist on the importance of the molecular organization of the compounds forming the active bulk-heterojunction (BHJ) blends. The morphology of the blend has been to tremendously affect both the charge transfer at the donor-acceptor interface and the carrier transport to the electrodes. And still, for each material combination, much remains to be understood to fully assess its specific and ultimate morphology. For this purpose, high resolution characterization methods are of primary interest to locally depict tthe photovoltaic process. Conductive Atomic Force Microscopy (C-AFM) and Kelvin Probe Force Microscopy (KPFM) have already proven to be of significant help to yield nanoscale two-dimensional mapping of electrical properties. C-AFM and related PeakForce TUNA emerged as powerful technique to electrically evidence phase separation in blends. An additional key feature lies in local I-V curve providing useful information about the charge transport mechanisms within the materials forming the blends. Quantitative measurements leading to local determination of hole mobility have already been reported. It appears that upon illumination the technique has shown to be sensitive to photocurrent. With photoconductive-AFM (pc-AFM), a dedicated external calibrated module has been recently introduced allowing full quantitative mapping of photovoltaic mechanisms.

In this lecture, we will carefully analyze, as a model sample, BHJ made of poly(3-hexylthiophene) (P3HT) and fullerene derivative (PCBM). While photocurrent is determined at 0 V DC bias, additional parameters including the open-voltage, the fill factor and the resistances can be obtained spanning the DC bias between the probe and the sample back electrode.(KPFM is also used in this work to delineate phase separation and potential variations at interfaces. Upon illumination, photovoltage can also be evidenced. With the additional external calibrated illumination module, mapping of photovoltage in BHJ blends can be obtained, opening the doors of local characterization of charge transfer at donor-acceptor interfaces, where crucial processes are occurring in photovoltaic devices.

In fine, we will also address the photovoltaic properties of  other systems such hybrid TiO₂ nanopillars : conjugated polymers.

Tin(II)sulfide nanosheets with lateral sizes in the range of several micrometers and thicknesses of only tens of nanometers are synthesized by injection of S-oleylamine into a hot solution of oleylamine, oleic acid, HMDS and tin chloride. We will present structural data to follow the nanosheets growth and show how additional octadecene can influence the lateral growth direction to form either rectangular or hexagonal nanosheets.

In the second part we focus on the electrical properties of individual hexagonal SnS-nanosheets. Here we present data from combined Kelvin Probe Force Microscopy (KPFM) with simultaneous Scanning Photocurrent Measurements (SPCM) to attribute the electro-optical properties to mutual interaction between locally generated charge carriers and electric potentials. For example the occurrence of zero-bias photocurrent can directly be attributed to changes of the band bending due to the optically generated charge carriers.

The surface ligation of semiconductor nanocrystals determines their optical and transport properties, and purposeful control of surface chemistry is under active investigation.  Colloidal 2D CdSe nanoribbons or quantum belts provide an ideal system for the study of rapid, reversible, and complete exchange of surface ligation at room temperature.  L-type primary-amine ligation is readily replaced by Z-type Lewis acid ligation of type MX₂ (M = Zn, Cd; X = carboxylate or halide).  Re-exposure to primary amines removes the Z-type ligands and restores L-type ligation.  Primary-amine ligation is readily converted to bound-ion-pair X-type ligation upon exposure of the quantum belts to acids HX (X = halide, carboxylate, or nitrate).  Salts of type R₄NX or NaX (X = carboxylate or halide) are unable to displace L-type ligation, but readily substitute for Z-type ligation, forming bound-ion-pair X-type ligation.  This bound-ion-pair X-type ligation is readily exchanged to L-type ligation upon exposure to primary amines.  The surface exchanges are characterized by a variety of means, and the absorption and emission spectra of the quantum belts are particularly sensitive to surface ligation.  Such surface exchanges promise to optimize the properties of the 2D nanocrystals.