Organic solar cells (OSCs) continue to be a promising low-cost and lead-free photovoltaic technology, which can be processed from benign solvents with efficiency over 12% along with good stability^[1]. Of critical importance to continued advances of OSCs is understanding and manipulating the composition of the amorphous mixed phase, which is governed by the thermodynamic molecular interactions^[2,3] of the polymer donor and acceptor molecules and the kinetics of the casting process. Here we highlight the significance of molecular interaction and vitrification in understanding the important aspects of morphology, performance, and stability of OSC. We present the temperature dependence of these molecular interactions as encoded by the effective temperature-dependent Flory-Huggins interaction parameter χ(T) in a model system PCDTBT:PCBM. We relate χ(T) to the device processing and performance and develop a framework that we successfully apply to 15 pairs of nonfullerene small molecule acceptor (NFA) systems^[4]. It is also shown and argued that the polymer:NFA systems with a amorphous-amorphous χ and binodal composition near the percolation threshold, during normal device operating conditions, can exhibit a stable morphology only if the crystallization of small molecule acceptor in active layer is suppressed due to a vitrified structure in the host polymer. Consequently, determining χ at processing temperature can serve as a feasible tool to predict device performance and in turn guide the choice of processing conditions where the binodal composition is close to the percolation threshold. Most significantly, our framework will pave a way to predict the morphology and stability of OSCs at actual processing and operating temperatures.

References

[1] L. Ye, Y. Xiong, Q. Zhang, S. Li, C. Wang, Z. Jiang, J. Hou, W. You, H. Ade, Adv. Mater. 2017, DOI: 10.1002/adma.201705485.

[2] M. Ghasemi, L. Ye, Q. Zhang, L. Yan, J. H. Kim, O. Awartani, W. You, A. Gadisa, H. Ade, Adv. Mater. 2017, 29, 1604603.

[3] L. Ye, B. A. Collins, X. Jiao, J. Zhao, H. Yan, H. Ade, Adv. Energy Mater. 2017, DOI: 10.1002/aenm.201703058.

[4] L. Ye, H. Hu, M. Ghasemi, T. Wang, B. A. Collins, J.-H. Kim, K. Jiang, J. Carpenter, H. Li, Z. Li, T. McAfee, J. Zhao, X. k. Chen, J. Y. L. Lai, T. Ma, J.-L. Bredas, H. Yan, H. Ade, Nat. Mater. 2017, DOI: 10.1038/s41563-017-0005-1.

nanoGe International Conference on Hybrid and Organic Photovoltaics (HOPV) started in Benidorm on year 2009. On ocasion of the 10th edition anniversary in the same setting, here I present a short account of the transformation of the scientific field, the evolution of dye-sensitised, quantum dots, and organic solar cells, and the hurrican of the perovskite soalr cells. There has been a significant change of scientific tendencies over the last years, mainly the explosion of digital communications and networking and fast publication. We comment how the conference itself reflected such tendencies.

In the second part of the talk I give a brief account of central characteristics of the solar cell device physics of the hybrid perovskite solar cells. A more complete picture of the device operation is emerging after including ionic effects in combination with electronic and optoelectronic properties. Charge accumulation at interfaces becomes manifest in terms of huge capacitance, but smart contact engineering allows one to control the surface capacitance and consequently reduce hysteresis and recombination. 

Achieving high performance that exceeds the efficiency of CIGS and CdTe, perovskite solar cell is required to ensure high durability for practical applications.¹ Although thermal stability of lead halide perovskite materials is determined by their compositions (generally limited to temperature <150^oC), stability of device is highly affected by the kind of carrier transport materials and the quality of interfaces at the perovskite junctions. Metal oxide electron transport layers (ETLs) generally have advantage in higher thermal stability than organic ETLs. We have been working with TiO₂ ETL-based multi-cation perovskite cells, which yielded efficiency over 21% by ambient air solution processes.² Intensity dependence of their Voc shows ideality factor low enough (<1.4) for the perovskite device to work as a power source of high output voltage even under weak light.² Such merit is expected to be applied to space satellite missions, which needs solar cells able to work under very weak sunlight (Mars and Jupiter). We have examined the durability of perovskite solar cells comprising thermally stable FA-based multi-cation perovskite absorber, TiO₂ ETL, and P3HT as hole transport layer. This composition exhibited thermally stability at temperature range between －80^oC and +100^oC. On exposure of the cell to high energy electron and proton beams, we found high stability and tolerance of the perovskite cells in space environment, which are superior to those of Si and GaAs solar cells.³ Focusing on the advantage of lightweight and printable thin film device, future perspectives of perovskite photovoltaic devices will be discussed.

[1] N. -G. Park, M. Gratzel, T. Miyasaka, K. Zhu, and K. Emery, Nat. Energy, 2016, 1, 16152.

[2] T. Singh, T. Miyasaka, et al, Adv. Func. Mat., 2018, DOI: 10.1002/adfm.201706287.

[3] Y. Miyazawa, T. Miyasaka, et al., submitted.

Photovoltaic devices based on hybrid metal halide perovskites are rapidly improving in power conversion efficiency. As the Shockley-Queisser limit is approached, the recombination and mobility of charge-carriers will be limited only by intrinsic properties. Yet, the mechanisms for these parameters is still poorly understood.

We show that bimolecular (band-to-band) recombination of charge-carriers in methylammonium lead triiodide perovskite can be fully explained as the inverse process of absorption [1]. The sharpening of photon, electron and hole distribution functions therefore significantly enhances bimolecular charge recombination as the temperature is lowered, mirroring trends in transient spectroscopy. We show that typical measurements of the radiative bimolecular recombination constant in CH₃NH₃PbI₃ are also strongly affected by photon reabsorption that masks a much larger intrinsic bimolecular recombination rate constant [2]. By investigating films whose thickness varies between 50 and 533 nm, we show that the bimolecular charge recombination rate indeed 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 6.8×10⁻¹⁰cm³s⁻¹ is common to all samples irrespective of film thickness [2]. Using such models, we examine the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers.

Finally, we examine the prospect of such highly performing hybrid lead iodide perovskites in solar concentrator environments [3]. We theoretically predict solar cell performance parameters as a function of solar concentration levels, based on representative assumptions of charge-carrier recombination and extraction rates in the device. We demonstrate that in the absence of degradation, perovskite solar cells can fundamentally exhibit appreciably higher energy-conversion efficiencies under solar concentration, where they are able to exceed the Shockley-Queisser limit and exhibit strongly elevated open-circuit voltages.

[1] 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).

[2] 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).

[3] Q. Lin, Z. Wang, H. J. Snaith, M. B. Johnston, and L. M. Herz,
Advanced Science 5 (2018) DOI: 10.1002/advs.201700792.

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.

Perovskite solar cells (PSCs) are promising alternatives toward clean energy because of their high-power conversion efficiency (PCE) over 22% and low materials and processing cost. However, their poor stability under operation is still limiting practical applications. In this talk, we present an innovative approach to control the surface growth of a low dimensional perovskite layer on top of a bulk three-dimensional (3D) perovskite film. This results in a structured perovskite interface where a distinct layered low dimensional perovskite is engineered on the bottom and the top of the 3D perovskite. We have investigated structural and optical properties of the stack and solar cells. When embodying the low dimensional perovskite layer, the photovoltaic cells exhibit an enhanced PCE of 20.1% on average, when compared to pristine 3D perovskite. The devices exhibit excellent stability and retain 85% of the initial PCE stressed under one sun illumination for 800 hours at 50°C in the ambient environment.

Greatcell Solar Limited (GSL) was listed on ASX as Dyesol Limited (DYE) in July 2005 and is headquartered in Queanbeyan, NSW, in Australia. The Company was founded to develop, scale-up and commercialise the rapidly emerging 3rd generation solar photovoltaic (PV) technology known as Dye Solar Cells (DSC), a technology invented by Grätzel and O’Regan at the École Polytechnique Fédérale de Lausanne in the early 1990s. Greatcell Solar is an original licensee of the EPFL core technology in perpetuity and also has an extensive patent portfolio of its own which extends across materials, processes and PV design.

DSC PV technology has lower embodied energy, is cheaper to produce and is more versatile than 1st and 2nd generation PV technology. It is particularly suited to building integrated photovoltaics (BIPV), often considered to be the end-point or ‘holy grail’ for PV development. DSC technology encountered technical challenges, though, as scientific investigation and development uncovered further potential for the 3rd generation of PV technology. Some of the materials it used were relatively expensive and the use of a liquid electrolyte raised issues as to its long-term durability. The Sun can be very destructive and long-term durability is a key attribute in successful commercialisation of PV panels.

Organometal halide perovskite semiconductors have emerged as promising candidates for optoelectronic applications including low-cost photovoltaics, and especially as wide bandgap absorbers in tandem cells. However, there remain key questions about the effects on the material properties, stability, and underlying mechanisms of alloying perovskites with Br to widen the bandgap. In this talk, we will present our work on optoelectronic properties, sub-bandgap electronic states, and cation-dependent halide demixing in Br-containing metal halide perovskites. Despite excellent intrinsic material properties revealed by power-dependent photoluminescence measurements there are indications of defect states close to midgap. Those states could impact photocarrier recombination and energy conversion efficiency in higher bandgap alloys, particularly at photovoltaic-relevant illumination densities. This talk will also address halide demixing by directly comparing the effect of the nature of the cation on phase stability under illumination. Advances in reducing halide segregation are achieved without sacrificing electronic properties and offer promise of stable and efficient top cells for future photovoltaic tandem devices.

Perovskite solar cells have shown remarkable improvement in certified power conversion efficiency (PCE) [1] of >22%. Nevertheless, many challenges regarding the stability and toxicity of the lead based perovskite material remains at the forefront of current research. The toxicity of lead which is present in a rather water-soluble form in perovskite solar cells remains an environmental concern, that is yet to be resolved. The bismuth based zero-dimensional perovskite shows a high band gap (Eg) ≈ 1.8 eV [2], [3 - 6] which makes it a suitable candidate for application in tandem solar cells. Recently, layered bismuth triiodide (BiI3) has also been used in solar cells as photoactive materials. [7-8] with the highest reported efficiency of 0.3%. [8] However, there are very few attempts to make pure bismuth triiodide based solar cells and also lack in systematic investigation on morphological tailoring, a viable route to fully utilize the potential of this material is to fine-tune the desired composition and properties for CH3NH3I_BiI3 material without any doping. In this direction, we report the vapor assisted solution process (VASP) two-step method to prepare bismuth perovskite samples at different reaction time The samples prepared at an optimum reaction time of 25 minutes exposure of MAI(v) give reproducible power conversion efficiency upto 2 %, (FF = 0.75%, Jsc = 2.9 mA/cm2, VoC = 0.91 V), highest so far reported for methyl amine based bismuth perovskite devices. This work demonstrates the efficacy of the VASP process in producing highly compact films that give improved optoelectronic performance.

(1) https://www.nrel.gov/pv/assets/images/efficiency-chart.png 

(2) A. J. Lehner, D. H. Fabini, H. A. Evans, C. A. Hébert, S. R. Smock, J. Hu, H. Wang, J. W. Zwanziger, M. L. Chabinyc and R. Seshadri, Chem. Mater. 2015, 27, 7137−7148.

(3) Dammak, H.; Yangui, A.; Triki, S.; Abid, Y.; Feki, H., J. Lumin. 2015, 161, 214−220

(4) N. Kubota, Crys. Res. Technol. 2001, 36 , 749 - 769

(5) C. Lan, Journal of alloys and compounds, 2017, 701, 834-840

(6) Y. Kim, Z. Yang, A. Jain, O. Voznyy, Gi-H. Kim, M. Liu, L. N. Quan, F. P. G.

 d. Arquer, R. Comin, J. Z. Fan and E. H. Sargent, Angew. Chem. Int. Ed., 2016, 55, 9586-9590

(7) H. Dammak, A. Yangui, S. Triki, Y. Abid, H. Feki, J. Lumin., 2015, 161, 214−220

(8) A. J. Lehner, Applied physics letters, 2015, 107, 1-4

We demonstrate that single crystals of methylammonium lead bromide (MAPbBr₃) could be grown directly on vertically aligned carbon nanotube (VACNT) forests.[1] The fast-growing MAPbBr₃ single crystals engulfed the protogenetic inclusions in the form of individual CNTs, thus resulting in a three-dimensionally enlarged photosensitive interface. Photodetector devices were obtained, detecting low light intensities (~20 nW) from UV range to 550 nm. Moreover, a photocurrent was recorded at zero external bias voltage which points to the plausible formation of a p-n junction resulting from interpenetration of MAPbBr₃ single crystals into the VACNT forest.

Moreover, bright green electroluminescence of the MAPbBr₃ single crystals, using symmetrical VACNT electrodes, was observed at room temperature for both polarities.[2] The electroluminescence spectra and light intensity was recorded from room temperature to cryogenic temperatures (20 K). The underlying mechanism behind the light emission is the well documented ion migration. In fact charged ions or vacancies inside the perovskite, drift under an external electric field accumulating at the cathode and anode, forming a p-i-n heterojunction structure. These characteristics have a strong similarity with the operational mechanism of polymer light-emitting electrochemical cells (LECs), especially the device structure and the involvement of mobile ions for efficient electroluminescence. 

This reveals that vertically aligned CNTs can be used as electrodes in operationally stable perovskite-based optoelectronic devices and can serve as a versatile platform for future selective electrode development.

[1] Andričević et al. J. Phys. Chem. C 2017, acs. jpcc.7b03421

[2] Andričević et al. - Submitted manuscript

Acknowledgements

This work was supported by the Swiss National Science Foundation (No. 513733) and the ERC advanced grant “PICOPROP” (Grant No. 670918).

Lead-based perovskite solar cells have revolutionized the field of thin-film solar cells showing certified efficiency as high as 22-23 %. Despite the high-efficiency of these lead-based perovskite solar cells, the problem associated from the toxic nature of lead has open a new research direction which focuses on lead-free perovskite materials. As an alternative, tin has been proposed to replace lead. The highest efficiency obtained with Sn only perovskite was 9 % which was based on 2D and 3D mixture of FASnI3. However, Sn-based perovskites are known to have low stability in air in which tin readily oxidizes from +2 to +4 upon exposure to air leaving oxygen vacancies which act as traps. We propose a new type of Pb-free perovskite material based on mixed metal tin and germanium. A new type of perovskite having the structure of FA0.75MA0.25Sn1-xGexI3 (abbreviated as SnGe(x)-PVK has been successfully prepared. The structure was confirmed from the XRD and XPS measurement. The electrical and optical characteristics of the perovskite materials were evaluated using PA and UV-Vis measurement showed that the materials have Eg ranging from 1.40 to 1.53 eV which confirmed the hypothesis that these materials can be used for solar cell applications. The highest performance was obtained with Ge content of x = 0.05, showing JSC 19.50 mA/cm2, VOC of 0.42 V, FF of 0.41 and efficiency of 4.48 %. The enhanced performance of this device when compared to that of SnGe(0)-PVK (3.31 %) was attributed to the surface passivation by Ge. Best performance of SnGe(0.1)-PVK solar cell was 7.5%. Additionally, the stability in air has been improved significantly with the Ge doping, retaining 80 % of its original performance, remarkable stability enhancement, compared with 10 % retention for non-doped sample (SnGe(0)-PVK). This work provides a platform for further research on lead-free mixed metal SnGe based perovskite solar cells.

In the last years organic-inorganic lead halide perovskites materials have attracted the interest of the research community thanks to their desired optoelectronic properties and due to the unprecedented scaling of their photovoltaic efficiency. However, the toxicity of the Pb atoms and the devices instability against moisture and temperature have hindered the commercialization of this technology. The substitution of the Pb²⁺ cation by a combination of a monovalent (M⁺) and a trivalent (M³⁺) cation to form double halide perovskites (A₂M⁺M³⁺X₆) has been considered an attractive alternative for the preparation of non-toxic and air stable devices. Despite the promising initial theoretical results on this class of materials, the preparation of high quality films of double perovskite is still challenging. The limited solubility of the inorganic precursors hampers the identification of a proper deposition route for the growth of thick and uniform layers suitable for photovoltaic applications. Vapour deposition techniques provide a valid alternative to overcome the limitation of the traditional wet chemistry methods, allowing the formation of a wide range of double perovskite compositions with a fine control over their thickness. In this talk we present the characterization of vapour deposited lead-free double perovskite and their implementation in photovoltaic devices.  

Concerns about the health and environmental impacts of the use of lead in commercial devices have driven research into lead-free alternatives to lead halide perovskite semiconductors.  Many of these materials which have shown promise involve bismuth halides, for example, MA₃Bi₂I₉, BiOI, and Cs₂AgBiBr₆ [1,2].  Although they have been predicted to show similar defect tolerance to lead halides, and already display advantageous stabilities and long carrier lifetimes, the power conversion efficiencies of resulting devices have lagged behind the lead halides.  The reasons for this may be a combination of the disconnected nature of the bismuth halide octahedra in the crystal structure, which limits carrier mobility, and the lower levels of absorption due to indirect bandgaps [3].    We probe the early-time excited states in many bismuth halide semiconductors using transient absorption, Raman and THz spectroscopy in order to reveal the crucial role of phonons in carrier transport and hot carrier cooling.  Our findings are consistent with previous reports of strong coupling between phonons and electronic states [4].  Balancing the positive effect of carrier-phonon coupling, combined with the localisation of electronic states, to help span the indirect bandgap with their detrimental scattering effects provides an important design criterion for efficient next-generation solar cells.

[1] Hoye, R. L. Z. et al. Adv. Mater. 1702176, (2017).

[2] Slavney, A. H., Hu, T., Lindenberg, A. M. & Karunadasa, H. I., J. Am. Chem. Soc. 3, 3–6 (2016).

[3] Xiao, Z., Meng, W., Wang, J., Mitzi, D. B. & Yan, Y., Mater. Horiz. 4, 206–216 (2016).

[4] McCall, K. M., Stoumpos, C. C., Kostina, S. S., Kanatzidis, M. G. & Wessels, B. W., Chem. Mater. 9, acs.chemmater.7b01184 (2017).

The poor opto-electronic properties often associated with tin (Sn) halide perovskites are usually ascribed to the oxidation of Sn²⁺ to Sn⁴⁺.^1,2 Due to the generation of Sn²⁺ vacancies upon oxidation that limit the electron and hole diffusion lengths to 30 nm by acting as scattering centres, it remains challenging to obtain highly efficient Sn-based perovskite solar cells in planar as well as mesoporous device architectures. Theoretical calculations corroborate that reducing the background hole concentration by suppressing oxidation could allow favourable opto-electronic properties for these materials with diffusion lengths approaching 1 micron.¹ Nevertheless, innovative approaches based on reducing agents remain rather limited as only inadequate fractions of these compounds are tolerated in the formation of the desired efficient absorbers.³ Hence, it remains particularly challenging to maintain a stoichiometric solution for the development of highly efficient Sn-based perovskite solar cells.

Having demonstrated vast potential as a narrow bandgap rear-cell in perovskite-perovskite tandems, the optimisation of Sn (and thereby Pb-Sn mixed) perovskites is of particular relevance as currently all-perovskite multi-junction solar cells are being envisioned—with reports of theoretical performances surpassing 45%.^4,5 To that end, this work focuses on the extent of oxidation present in supposedly pristine precursor solutions—an aspect currently not profoundly reported on in literature. Based on observed changes in morphology, crystallinity and optoelectronic properties of films obtained from corresponding solutions, an efficient method is discussed to estimate the early-on degradation induced at the precursor stage. The quantification can be used as an effective tool to characterise and extend the effect of reducing agents, and thereby assist in enhancing the performance of Sn-based solar cells.

Literature

1. F. Hao, C. C. Stoumpos, D. H. Cao, R. P. Chang, M. G. Kanatzidis, Lead-free solid-state organic-inorganic halide perovskite solar cells. Nature Photonics 8, 489 (2014).

2. N. K. Noel et al., Lead-free organic-inorganic tin halide perovskites for photovoltaic applications. Energy & Environmental Science 7, 3061 (2014).

3. L. Ma et al., Carrier Diffusion Lengths of over 500 nm in Lead-Free Perovskite CH3NH3SnI3 Films. Journal of the American Chemical Society 138, 14750 (2016).

4. G. E. Eperon et al., Perovskite-perovskite tandem photovoltaics with optimized bandgaps. Science 354, 6314 (2016).

5. G. E. Eperon, M. T. Hörantner, H. J. Snaith, Metal halide perovskite tandem and multiple-junction photovoltaics. Nature Reviews Chemistry 1, 0095 (11/29/online, 2017).

We report on room temperature, solution-processed transparent Ag nanowire (AgNW) electrodes with enhanced conductivity, transparency, as well as chemical and mechanical stability. AgNW films were made with fast, large area, ultrasonic spray-coating technology. Different dilution ratios of nanowires/solvent and deposition parameters (such as flow-rate and scan-rate) where tested for optimizing the sheet resistance and optical transmittance. In a fast and low temperature post-deposition plasma-process, the AgNW films where cured to reduce the nanowire contact resistance, which greatly influences the film conductivity. The process proved to be suitable for both glass and PET substrates. Nanowire films achieved comparable sheet resistance and transparency as the ITO reference electrode (>80% transparency with ~10 Ω/square.). To lower the roughness and increase the stability, AgNW films were embedded in a UV-curable polymer and then transferred onto the target substrate. The process was optimized for embedding AgNW films on glass and PET.  Improvement of the chemical stability of the embedded AgNW films was observed in first tests with methylammonium lead iodide chloride (CH3NH3PbI3-xClx) perovskite inks. In non-embedded AgNW films, despite overcoating with PEDOT:PSS degradation was observed. By embedding the AgNW films, the degradation was hampered.

Tailored monovalent cation substitution in mixed-cation hybrid perovskites enables solar cell efficiencies beyond 20% and enhanced stability. Here, we use transient absorption and photoluminescence spectroscopy to study the effect of cation substitution on the carrier recombination dynamics in Rb_x(Cs_y(MA_zFA_1-z)_1-y)_1-xPb(I_0.83Br_0.17)₃ (MA=methylammonium, FA=formamidinium) hybrid perovskite thin-films. We perform a detailed analysis of the recombination dynamics, from which we separate radiative and non-radiative recombination pathways. We find that careful tuning of the cation composition leads to a reduction in trap-assisted non-radiative recombination channels, which supports enhanced lifetimes and high luminescence yields. Unexpectedly, we further observe the reduction of a non-radiative bimolecular recombination channel, particularly upon inclusion of formamidinium. Using Raman and X-ray diffraction techniques, we study the effect of cation substitution on lattice order. We find that formamidinium inclusion, aided through the presence of Rb and Cs during fabrication, leads to a reduction of the tetragonal distortion, and an increased rigidity of the lattice. We attribute the enhanced luminescence yields to reduced defect formation through carrier trapping due to the reduced lattice disorder.

The radiative generation and recombination of charge carriers in semiconductors control both photovoltaic and LED operation. Understanding of these processes in metal-halide perovskites has advanced,^1–4 but remains incomplete.

This talk, we will discuss the current question if an indirect nature of the band structure in metal-halide perovskites affects the radiative recombination of excited states. Using ultrafast transient absorption (TA) and photoluminescence (PL) experiments, we gain insights on the formation of emissive states at early times after photoexcitation. We find that the PL signal rises over two picoseconds while initially hot photo-generated carriers cool to the band edge. This shows that PL of hot carriers is slower than that of cold carriers, as expected from strongly-allowed radiative transitions. We conclude that electrons and holes show strong overlap in momentum space, despite the potential presence of a small band offset that we model to arise from a Rashba effect. We find that photon recycling processes further affect externally measured radiative recombination rates in hybrid perovskites. Taking into account photon recycling, we connect externally measured radiative efficiencies with the actual internal values, and derive internal PLQEs exceeding 80%. We discuss how elemental composition affects the recombination processes and luminescent yields.

(1)        Johnston, M. B.; Herz, L. M. Hybrid Perovskites for Photovoltaics: Charge-Carrier Recombination, Diffusion, and Radiative Efficiencies. Acc. Chem. Res. 2016, 49, 146–154.

(2)        Saba, M.; Cadelano, M.; Marongiu, D.; Chen, F.; Sarritzu, V.; Sestu, N.; Figus, C.; Aresti, M.; Piras, R.; Geddo Lehmann, A.; et al. Correlated Electron–hole Plasma in Organometal Perovskites. Nat. Commun. 2014, 5, 5049.

(3)        Staub, F.; Hempel, H.; Hebig, J.-C.; Mock, J.; Paetzold, U. W.; Rau, U.; Unold, T.; Kirchartz, T. Beyond Bulk Lifetimes: Insights into Lead Halide Perovskite Films from Time-Resolved Photoluminescence. Phys. Rev. Appl. 2016, 6, 44017.

(4)        Richter, J. M.; Abdi-Jalebi, M.; Sadhanala, A.; Tabachnyk, M.; Rivett, J. P. H.; Pazos-Outón, L. M.; Gödel, K. C.; Price, M.; Deschler, F.; Friend, R. H. Enhancing Photoluminescence Yields in Lead Halide Perovskites by Photon Recycling and Light out-Coupling. Nat. Commun. 2016, 7.

In this work we present the photoluminisence of perovskite monocrystals made of methylamonium lead bromide (MAPBr) in which the bromide has been partially substituted by iodide (I) in one of the sides. In such configuration, most of the crystal has the orange color of the MAIPbBr and only one of the the sides is black as MAPI. We show that for good samples, phtoluminiscence occurs mostly in the part with MAPI. Thus most of the charges photogenerated by light in any part of the MAPBr crystal, are collected in the energetically lower states of the MAPI part of the crystal, where they recombine. This implies that charge carriers are able to diffuse lengths in the range comprised between mm and cm. Discussion will be centered in the mechanisms that lead to these large diffusion lengths that allow the concentration of recombination in the MAPI and then in the potential applications of these structures.

Trap-mediated charge recombination is the major factor limiting the efficiency of hybrid metal halide perovskite solar cells. Previous studies have shown coexistance of two crystal phases in methylammonium lead triiodide (MAPI) films at low temperatures and some evidence of charge transfer from the predominant  orthorhombic phase to small inclusions of the tetragonal phase. These inclusions act as efficient radiative recombination centers and recently a continuous wave lasing under optical pumping was demonstrated in this material. Trap-mediated recombination in such small inclusions may also be different from the bulk material but has not been studied in detail yet.  

In this work we studied charge recombination in MAPI at low temperatures using a wide range of pulsed excitation densities, including very low ones. We combined time-resolved photoluminescence with transient absorption spectroscopy to distinguish  radiative and non-radiative recombination. We observe a higher rate of radiative bimolecular recombination at early time after photoexcitation when all carriers are mobile which allows us to determine the density of mobile and trapped carriers in each crystal phase. We find that the density of trapped charges is lower in small inclusions of the tetragonal phase as compared to the dominant orthorhombic phase which leads to suppression of trap-mediated recombination at low temperature. Our results contribute to better understanding of recombination processes.

Mixed-halide perovskites have emerged as promising materials for optoelectronics due to the merit of their tunable bandgap in the entire visible region. A challenge remains however in the instability of the bandgap. This bandgap instability is attributed to phase-segregation, which strongly affects the voltage attained in mixed-halide perovskite-based solar cells and seriously restricts the applications. A comprehensive understanding toward phase-segregation is therefore highly desired to direct the further research and development for overcoming the restriction in applications of mixed-halide perovskites.

In this work, we provide an in-depth insight into this important yet unclear phenomenon with a combination of local-resolved and bulk investigations. We demonstrate phase-segregation in mixed-halide perovskite is highly topography-dependent. By using spatially-resolved photoluminescence spectroscopy, we show the gradual red-shift of the photoluminescence signal at the grain boundaries of mixed-halide perovskite during the consecutive laser illumination. Contrarily, we observe the spectrally stable emission exclusively stems from the grain centers. Such difference is further evidenced by the bulk characterizations, showing red-shift domain as a minority phase coexisting with the pristine one during illumination. By mapping the surface potential, we propose the higher concentration of positive space charge near grain boundary possibly provides the initial driving force for phase-segregation. Our work offers detailed insight into the microscopic processes occurring at the boundary of crystalline perovskite grains and will support the development of better passivation strategies, ultimately allowing to process environmentally stable perovskite films.

Due to their unique opto-electronic properties, metal halide perovskites are of great interest as solar energy material. At present, non-radiative losses prevent solar cell efficiencies to reach their theoretical maximum. In this work we suppressed the non-radiative decay in methylammonium lead iodide (MAPbI3) layers by exposing them to a light soaking treatment under ambient conditions. First of all, this treatment leads to an increase of the internal photoluminescence quantum efficiency from around 1% to 89%. Additionally, time-resolved microwave conductivity (TRMC) data demonstrate that the charge carrier lifetime increases from 6 μs to 32 μs. By applying a kinetic model to fit the TRMC traces, we find that the treatment reduces the non-radiative band-to-band recombination between electrons and holes, while the mobility and the trap density remain constant. We attribute the increase in carrier lifetimes to an increased fraction of radiative recombination, leading to enhanced recycling of carriers.* Next, these light soaking studies were extended to mixed cation, mixed halide perovskites, (FA0.79MA0.16Cs0.05) Pb (I1-xBrx)3 which are known to undergo phase separation under continuous illumination. For x < 0.5 we find that on light soaking in a nitrogen environment, the charge carrier lifetime increases, while for x > 0.5 the lifetime is reduced. Again, the charge carrier mobilities of the mixed perovskites are not affected by light soaking, which implies that the material properties of the perovskite are not influenced by light soaking. In contrast, the decay pathways have changed by light soaking. By analysing the TRMC traces, we propose that for x< 0.5 light soaking leads to a reduction of the number of shallow states. The above results help to provide a framework for which I/Br ratio the best performance and stability can be expected.

*Brenes, R.; Guo, D.; Osherov, A.; Noel, N. K.; Eames, C.; Hutter, E. M.; Pathak, S. K.; Niroui, F.; Friend, R. H.; Islam, M. S.; Snaith, H. J.; Bulović, V.; Savenije, T. J.; Stranks, S. D., Metal Halide Perovskite Polycrystalline Films Exhibiting Properties of Single Crystals. Joule 2017, 1 (1), 155-167.

Herein we propose a mechanism that provides a driving force for ion migration when an organic metal halide perovskite material is photo-excited in the presence of oxygen. By analysis of the chemical changes that occur at the semiconductor surface when the semiconductor is photo-excited under controlled atmosphere in an X-ray photoelectron spectroscopy chamber, we find direct evidence of the formation of a superficial layer of negatively charged oxygen species capable of repelling the halide anions away from the surface and towards the bulk. Not only the reported photoluminescence transient dynamics, but also the partial recovery of the initial state when photoexcitation stops, the eventual degradation after intense exposure times, and the phase segregation observed in mixed halide perovskites, can be rationalized on the basis of the proposed mechanism.

Simultaneous exposure of CH₃NH₃PbX3 perovskite films to oxygen and light with energy above the electronic bandgap yields the formation of anionic oxy-gen species that accumulate as a negatively charged layer on the semiconductor surface. These reactive species both provide the electrostatic driving force necessary to induce ion migration and, simultaneously, initiate degradation of the lattice. As a result of the repulsive (attractive) coulombic force between halide ions (halide vacancies) and surface oxygen anions, a momentary reconstruction of the imperfect bulk lattice occurs, which explains the increase of PL reported. At the same time, those superficial radical anions start degrading the semiconductor surface, a phenomenon that becomes dominant and gradually extends to the bulk of the material until it causes its decomposition. The mechanism herein proposed provides a satisfactory explanation to seemingly contradictory effects, such as the initial enhancement of the photoemission and the subsequent degradation of the samples. At the same time, it is consistent with the reported light-induced phase segregation (or Hoke effect) that occurs in mixed halides (typically bromide-iodide) perovskites.

(1) M. Anaya, J. F. Galisteo-López, M. E. Calvo, J. P. Espinós, H. Míguez, "Origin of Light Induced Ion Migration in Organic Metal Halide Perovskites in the Presence of Oxygen", submitted.

(2) J. F. Galisteo-López, Y. Li, H. Míguez, "Three-Dimensional Optical Tomography and Correlated Elemental Analysis of Hybrid Perovskite Microstructures: An Insight into Defect-Related Lattice Distortion and Photoinduced Ion Migration", J. Phys. Chem. Lett., 2016, 7, 5227.

(3) Galisteo-López, F. J.; Anaya, M.; Calvo, M. E.; Miguez, H., "Environmental effects on the photophysics of organic-inorganic halide perovskites", J. Phys. Chem. Lett., 2015, 6, 2200.

Next generation perovskite solar cells (PSCs) have set themselves apart from their dye-sensitized (DSSC) and organic (OPV) predecessors, with impressive efficiencies approaching 23%.¹ While many significant advances in PSC efficiency have come about due to hole transport material (HTM) selection, relatively little is known about charge separation at the perovskite|HTM interface. Our recent work has therefore set out to understand hole transfer as a function of key parameters: (i) the interfacial energy offset, ∆E between the HTM HOMO level and the perovskite valence band; (ii) HTM structure; (iii) the intrinsic properties of the perovskite.

This talk will detail our progress in addressing these objectives using steady-state and time-resolved photoluminescence, as well as transient absorption spectroscopy. Specifically, we will outline our observation of highly efficient (>75%), sub-nanosecond hole transfer to polymeric HTMs at remarkably low values of ∆E (<0.1eV), and suggest that further gains in V_OC could be made by incorporating HTMs with deeper-lying HOMOs into PSCs.² We will also reveal that recombination of injected holes is typically in the millisecond regime in the case of polymeric HTMs, yielding a >10⁵ ratio between interfacial charge-separation/recombination time constants. Finally, we will discuss the importance of the interplay between the structural properties of the perovskite absorber and the HTM.

[1]   National Renewable Energy Labs (NREL). Best Research Cell Efficiencies https://www.nrel.gov/pv/assets/images/efficiency-chart. png (accessed 29 Jan, 2018)

[2]   R. J. E. Westbrook et al, J. Phys. Chem. C 2018, 122, 1326−1332

Charge extraction rate in solar cells made of blends of electron donating/ accepting organic semiconductors is typically slow due to their low charge carrier mobility. This sets a limit on the active layer thickness and has hindered the industrialization of organic solar cells (OSCs). Few systems have been recently reported that exhibit exceptionally efficient charge extraction. Herein, charge transport and recombination properties of an efficient polymer (NT812):fullerene blend are investigated. This system delivers power conversion efficiency of >9% even when the junction thickness is as large as 800 nm. Experimental results indicate that this material system exhibits exceptionally low bimolecular recombination constant, 800 times smaller than the diffusion-controlled electron and hole encounter rate. Comparing theoretical results based on a recently introduced modified Shockley model for the fill factor, and experiments, clarifies that charge collection is nearly ideal in these solar cells even when the thickness is several hundreds of nanometer. This is the first realization of high-efficiency Shockley-type organic solar cells with junction thicknesses suitable for scaling up. We discuss the possible reasons behind the reduced bimolecular recombination in the exceptional BHJ systems.

References:

[1] A Armin, Z Chen, Y Jin, K Zhang, F Huang, and S Shoaee, A Shockley‐Type Polymer: Fullerene Solar Cell, Adv. Energy Mater. 2017, 1701450

[2] A Armin, J Subbiah, M Stolterfoht, S Shoaee, Z Xiao, S Lu, D J Jones, and P Meredith, Reduced Recombination in High Efficiency Molecular Nematic Liquid Crystalline: Fullerene Solar Cells, Adv. Energy Mater. 2016, 6, 1600939

[3] A Armin, J R Durrant, and S Shoaee, Interplay Between Triplet-, Singlet-Charge Transfer States and Free Charge Carriers Defining Bimolecular Recombination Rate Constant of Organic Solar Cells, J. Phys. Chem. C, 2017, 121, 13969–13976

[4] K Zhang, Z Chen, A Armin, S Dong, R Xia, H‐Lap Yip, S Shoaee, F Huang, and Y Cao, Efficient Large Area Organic Solar Cells Processed by Blade‐Coating With Single‐Component Green Solvent, Sol. RRL 2017, 1700169

Metal halide perovskites such as methylammonium lead iodide (MAPbI₃) are highly promising materials for photovoltaics. However, the relationship between the organic nature of the cation and the optoelectronic quality remains debated. In this work, we use different techniques to prepare fully inorganic black-phase CsPbI₃,¹ and compare the optoelectronic properties to their MA-based analogues.^2,3 Using the Time-Resolved Microwave Conductivity (TRMC) technique, we measure charge carrier mobilities of around 25 cm²/(Vs) in CsPbI₃ prepared via physical vapor deposition,¹ which is very comparable to the 37 cm²/(Vs) that we found in vapor-deposited MAPbI₃.² Furthermore, we observe impressively long charge carrier lifetimes exceeding 10 microseconds for these vapor-deposited CsPbI₃ films and corresponding second order recombination rate constants of 1.3 x 10^-10 cm³s^-1, which is again similar to fully optimized MAPbI₃ layers. Additionally, we find that these high quality CsPbI₃ films yield photovoltaic devices with power conversion efficiencies close to 9%.¹ Altogether, our results suggest that charge carrier mobility and lifetime are mainly dictated by the inorganic framework rather than the organic nature of the cation. However, in spite of its promising optoelectronic properties, fully inorganic CsPbI₃ perovskites suffer from inferior crystal-phase stability and thus, the presence of organic cations might still be required for production of stable, high-efficiency solar cells. On studying a number of mixed-cation perovskites, we finally find that in fact, the charge carrier mobilities and lifetimes are favorably tuned by adding controlled amounts of inorganic cations, such as Cs and Rb, to metal halide perovskites with organic cations.³

1. Hutter, E. M.; Sutton, R. J.; Chandrashekar, S.; Abdi-Jalebi, M.; Stranks, S. D.; Snaith, H. J.; Savenije, T. J. ACS Energy Lett. 2017, 2, 1901.

2. Hutter, E. M.; Hofman, J.-J.; Petrus, M. L.; Moes, M.; Abellón, R. D.; Docampo, P.; Savenije, T. J. Adv. Energy Mater. 2017, 1602349.

3. Hu, Y.; Hutter, E. M.; Rieder, P.; Grill, I.; Hanisch, J.; Aygüler, M. F.; Hufnagel, A. G.; Handloser, M.; Bein, T.; Hartschuh, A.; Tvingstedt, K.; Dyakonov, V.; Baumann, A.; Savenije, T. J.; Petrus, M. L.; Docampo, P. Adv. Energy Mater. 2018, 1703057.

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.

In this talk we discuss a couple of examples where these techniques are used to investigate perovskite solar cells:

(1) By measuring the impedance and the open-circuit photopotential at two excitation wavelengths (blue and red light), in two illumination directions (back and front), and at different temperatures we gain insight on the locus and nature of recombination in TiO₂ mesoporous-based perovskite solar cells. With this objective we study different perovskite compositions, i.e., pure MAPbI₃ and CsPbI₃, as well as mixed ion-based (FAPbI₃)_0.85(MABr₃)_0.15, and two different hole selective layers (Spiro-OMeTAD and P3HT)

(2) We again use monochromatic excitations in IS and IMPS to establish how moisture-induced degradation introduce inhomogeneities in the recombination loss at the active layer, hindered transport and charge accumulation at the interfaces.

A simple model is proposed to rationalize the small-perturbation response of the device  and its impact on efficiency-determining dynamic processes.

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 grain and grain boundaries. Although many progresses have been obtained in the quality of the film, still grain boundaries within the perovskite film in fabricated devices are present.

The effect of these grain boundaries have been investigated by many groups, we refer here to just one reference [4], but the effect of these grain boundaries to free charges and ion migration is still under debate. In  [4] it has been demonstrated as the change in average size of grains have an important impact on cell performance and ionic diffusion.

In the present work we theoretically investigate the effect of ion migration with the presence of grain boundaries. We make a multiscale simulation tool based on kinetic Monte Carlo (for ion dynamics) [5] and drift diffusion based on finite elements (for the electrical part) [6] to understand how the different ion diffusion mechanisms can impact the final cell performance.

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

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

Ultra-thin solar cells, whether organic or PVK, require photon management to improve light harvesting beyond what is afforded by a single reflection off the bottom electrode. Recently, we have demonstrated a new light trapping strategy with so-called Photonic Fiber Plates (PFP). It consists of a horizontal array of partially overlapping dielectric cylinders, coated on the under side with the solar cell itself [1].

Full-wave simulations show a dramatic difference in the distribution intensity in the PFP compared to disjoined cylinders when excited by a plane wave at normal incidence [2]. In the PFP, the light distribution is reminiscent of Whispering Gallery Modes. To elucidate the electromagnetic field dynamics, ray tracing simulations are conducted. These confirm that a fraction of the ray segments indeed follow Whispering Gallery Mode-type trajectories. However, in addition to that, the ray trajectories are found to follow chaotic paths, with sensitive dependence on initial conditions. Using ray tracing, the ergodic character of the rays is demonstrated numerically. Furthermore, the wave chaos at play in the PFP is shown to be of the intermittent type. In this presentation, we present a full characterisation of this wave chaos, including the resulting entropy production. Conterintuitively, we show that the most chaotic trajectories contribute to a lesser production of entropy than regular ones.

We show experimental results demonstrating the efficiency of the trapping mechanism and further present the case of half-PFP trapping with PVK cells, where good quantitative agreement between simulation and experiment is found. Finally, we discuss modification of the original design that are expected to further enhance the light trapping of the device.

[1] Mariano, M., Rodríguez, F. J., Romero-Gomez, P., Kozyreff, G., & Martorell, J. (2014). Light coupling into the whispering gallery modes of a fiber array thin film solar cell for fixed partial sun tracking. Scientific reports, 4, 4959.

[2] Mariano, M., Kozyreff, G., Gerling, L. G., Romero-Gomez, P., Puigdollers, J., Bravo-Abad, J., & Martorell, J. (2016). Intermittent chaos for ergodic light trapping in a photonic fiber plate. Light: Science & Applications, 5(12), e16216.

We present theoretical work connecting two models of organic solar cells (and other organic semiconductors). One of these is the Gaussian disorder model [1]. In this model, the possible states of the charge carriers are localized on randomly distributed sites. The energies of these states follow a Gaussian distribution, and jumps from site i to j follow Miller-Abrahams jump rates [2]. These rates are proportional to exp(-2 gamma d_ij) where gamma is a constant and d_ij is the distance between i and j. If the site j has a larger energy (E_j>E_i) there is also a further factor exp(-(E_j-E_i)/(k_BT)). A related model involves differential equations for the concentration of carriers involving drift, diffusion, generation, and recombination. We show how in a continuum limit the Gaussian disorder model gives rise to differential equations of this type. This approach establishes relations between the parameters of both models. It also allows to incorporate further effects into the differential equations, for instance related to the energy dependence of the concentration and to inhomogeneities in the distribution of states.

[1] H. Bässler, Phys. Status Solidi B 174, 15 (1993)

[2] A. Miller and E. Abrahams, Phys. Rev. 257 (1960)

Optoelectronic devices are playing a decisive role in nowadays societal needs in two ways: as a means to produce energy in a sustainable way (photovoltaic devices) and minimizing the energetic cost of producing light (LEDs). In either of these two complementary routes, converting electromagnetic radiation into electrical current or vice versa, the interaction of light and matter is at the heart of the process and thus maximizing it is central to design highly efficient devices.

When optimizing these systems one must consider the different processes involved in the conversion of light into current: a proper exit/entrance path of light from/to the device, optimized charge carrier transport and tailoring the optical environment of the active layer. While the former two aspects comprise the focus of many studies, the last point is commonly overlooked in the design of optoelectronic devices. Tailoring the optical environment of the active layer is a critical point in the design of any device involving the interaction of light and matter, as it strongly determines its emission and absorption properties. [[1]] Further it is becoming critical in the development of a new generation of solution processed materials from quantum dots (QD) to hybrid organic-inorganic lead-halide perovskites (HOIP) for which devices, comprising thin active layers, are rapidly approaching state of the art performances. Thus maximizing light-matter interaction within the final device is a must in order to make the most out of the active layer.

In this work we focus on the relevance of the optical design of optoelectronic devices in order to optimize their efficiency through a proper engineering of the optical environment of the active layer. From numerical simulations employing the Transfer Matrix Method and 3D Finite-Difference in the Time Domain we show [[2]] how carefully tailoring such environment, introducing small variations in the thickness of the layers of the final device, has profound implications on the way light is absorbed or emitted from real world devices. [[3],[4]]

[1] Novotny, L.; Hecht, B. Principles of Nano-Optics; Cambridge University Press, 2006.

[2] Jiménez-Solano, Galisteo-López, J.F, Míguez, H. Submitted (2018)

[3] Bernechea, M.; Miller, N. C.; Xercavins, G.; So, D.; Stavrinadis, A.; Konstantatos, G. Nat. Photonics 2016, 10, 521-526.

[4] Xiao, Z; Kerner, R. A.; Zhao, L.; Tran, N. L.; Lee, K. M.; Hoh, T-W.; Scholes, G. D.; Rand, B. P. Nat. Photonics 2017, 11, 108-116.

Pinholes are detrimental to solar cell performance and critical to many inorganic solution-processed materials. For the prominent example of polycrystalline perovskites, optimized processing routes for homogeneous layers have been reported but it is so far unclear if up-scaled wet-chemical fabrication can keep up with the high-quality morphologies achieved by spin-coating in the lab. For other new materials that are still in an early stage of technology development, such as sulfur-based absorbers, the issue of pinholes often remains unsolved. It is therefore important to develop device models that describe the impact of pinholes in the absorber layer on solar cell performance. Shunts from pinholes in the absorber layer are often thought of as an ohmic resistor. In reality a pinhole gives way to an interface between the semiconducting electron and hole transport layers which form a heterojunction with a diode-like and clearly non-linear current-voltage characteristic.

In this work, we first illustrate the fundamentally different effect of non-linear compared to linear shunts on the J-V characteristic. As an example, linear shunts will only affect the open‑circuit voltage V_oc if the fill factor FF has already decreased towards 25% while non‑linear shunts can cause a loss in V_oc when FF values are still acceptable. For typical layer stacks, we obtain the J-V characteristic of pinholes experimentally by fabricating absorber‑free devices. We then use numerical simulations where we assign the pinhole characteristic to a small area of the solar cell while the rest is assumed to be an ideal photo-generating diode. This approach allows us to investigate the impact of pinholes on device efficiency, FF and V_oc. We use a simple effective zero-dimensional parallel circuit model which is verified for a range of relevant parameters by simulations of two-dimensional diode networks. We compare different interlayer combinations that are commonly used in the context of perovskite and other solution‑processed solar cells. We find that certain combinations of electron and hole transport layers like TiO₂/spiro-OMeTAD limit the negative impact of shunts from pinholes much more than others like PEDOT:PSS/PCBM. The robustness towards shunts from pinholes should thus be considered a new design criterion for the choice of interlayers in solution-processed thin-film solar cells.

Halide perovskites have gained large interest during the last years due to their rapidly growing solar cell photoconversion efficiency. However, it is still challenging to find a perovskite compound which provides high efficiency and good stability at the same time. In this work we investigate the electronic structure of tin and lead-based halide perovskites by performing electronic structure calculations with  hybrid PBE0 functional and by taking spin-orbit coupling into the account. We find that band gap of formamidimim tin bromide is larger than that for formamidimim lead bromide, which is the opposite trend from those observed for similar pairs of halide perovskite compunds. We comment on the possible origins of such an unusual band gap inversion by comparing structures of these two compounds and effect of spin-orbit coupling. As a conclusion, we find that band gap inversion comes as the consequence of big differences in spin-orbit coupling effect, while the large cation distorts the structure and prevents higher antibonding overlap.

Halide perovskite solar cells (PSCs) have emerged as a competitive photovoltaic technology with power conversion efficiencies (PCEs) surpassing the 22 % mark. One of the main bottlenecks of the technology is their long-term stability. Understanding the different degradation mechanisms of the constituent materials, as well as interface instabilities, is of crucial importance for commercialization. Semiconductor oxides (SO) constitute a fundamental part of highly efficient photovoltaic technologies such as PSCs. Electron transport semiconductor oxides, like TiO₂, are characterized by an oxygen vacancy (O_vac)-mediated conductivity caused by a deviation in stoichiometry, the presence of impurities, or both. In oxygen-containing atmospheres, and especially under UV light, holes generated at the nonstoichiometric oxide surface react with the oxygen adsorbed at an O_vac increasing charge recombination and degradation of the solar cell. Different methods have been employed to passivate or eliminate these O_vac. For example, the application of organic interfacial modifiers with anchoring groups specifically selected to bond with oxides, or the application of less reactive SnO₂ which results in less hygroscopicity, fewer O_vac at its surface, and less UV-damage. Another possibility is the application of a coating of secondary oxides, like Al₂O_3, applied to supress surface defects, avoid interfacial recombination, and enhance device stability. A less-explored option is the application of complex oxides with singular properties, such as ferroelectric, multiferroic, magnetic, etc. In this talk, we report our most recent studies on the application of classic oxides (binary, doped, nanostructured) and complex oxide compounds (ternary, ferroelectric, etc.) as transport layers in Halide Perovskite Solar Cells. We will discuss their effect on the long-term stability of complete solar cell devices.   

[1] A. Hagfeldt, M. Lira-Cantu, Recent concepts and future opportunities for oxides in solar cells, Applied Surface Science, (2018) Submitted.

[2] A. Perez-Tomas, A. Mingorance, Y. Reyna, M. Lira-Cantu, Metal Oxides in Photovoltaics: All-Oxide, Ferroic, and Perovskite Solar Cells, in: M. Lira-Cantu (Ed.) The Future of Semiconductor Oxides in Next Generation Solar Cells, Elsevier, 2017, pp. 566.

[3] M. Lira-Cantú, Perovskite solar cells: Stability lies at interfaces, Nature Energy, 2 (2017) nenergy2017115.

[4] M. Lira-Cantu, The future of semiconductor oxides in next generation solar cells, 1st ed., Elsevier, 2017.

[5] Y. Reyna, M. Salado, S. Kazim, A. Pérez-Tomas, S. Ahmad, M. Lira-Cantu, Performance and Stability of Mixed FAPbI3(0.85)MAPbBr3(0.15) Halide Perovskite Solar Cells Under Outdoor Conditions and the Effect of Low Light Irradiation., Nano Energy, 30 (2016) 570–579.

In bulk-heterojunction organic solar cells (BHJ-OSCs), exciton dissociation and charge transport are highly sensitive to the molecular packing pattern and phase separation morphology in blend films. Efficient photovoltaic small molecules (SMs) typically possess an acceptor-donor-acceptor (A-D-A) structure that causes intrinsic anisotropy, limiting the control over molecular packing because of the lack of an effective method for modulating molecular orientation. Consequently, the performance of non-fullerene SM organic solar cells (NFSM-OSCs) is currently still lower than that of fullerene-based devices. In this report, we use a group of model compounds, named DRTB-T-CX (X=2, 4, 6 and 8), to demonstrate that adjusting the length of the end alkyl chain can be used to modify the molecular orientation. Through 2D grazing incidence wide-angle X-ray scattering (GIWAXS) characterization, we observed the transition of the molecular orientation in DRTB-T-CX films from edge-on to face-on. Meanwhile, the film comprising the compound with the preferred face-on orientation is found to have enhanced charge mobility and an increased correlation length of π-π stacking, leading to a substantial improvement in the efficiency of the NFSM-OSCs. A top-performance power conversion efficiency (PCE) of up to 11.24% is achieved with the DRTB-T-C4/IT-4F-based device, which is the best performance reported for a state-of-the-art NFSM-OSC. Remarkably, devices based on DRTB-T-C4/IT-4F with active layer thicknesses up to 300 nm can still retain a high PCE of 10% in single-junction solar cells.

The main application challenge of organic photovoltaics (OPV) is their rapid performance degradation, as compared to their inorganic counterparts. This rapid degradation is either caused by mesoscale phase segregation of donor and acceptor molecules or by compositional changes, mainly caused by uptake of oxygen and water. Although mesoscale effects on the bulk performance of OPV are well documented,¹ a detailed understanding of the effects of oxygen and water uptake in the donor and acceptor phases, and most importantly across interfaces, are lacking.

Energy Filtered Transmission Electron Microscopy (EFTEM) is a technique that enables mapping of the local chemical composition of OPV devices,² which is limited by electron beam-sample interactions, i.e., electron beam damage. The careful analysis of this beam damage enables the calculation of an analysis limit,³ which is subsequently used to enable the determination of chemical changes at the nanoscale, caused by uptake of oxygen and water during OPV degradation.

To resolve oxygen and water uptake in separate phases and across interfaces with EFTEM, a columnar model systems was employed. The phase separation of poly(3-hexylthiophene) (P3HT) and polystyrene is used to create a P3HT matrix, which is (after removing PS) filled with phenyl-C_61­-butyric acid methyl ester (PCBM) molecules to create PCBM columns in a P3HT matrix. The oxygen content throughout the three phases is subsequently quantified at different stages of degradation using EFTEM maps. EFTEM maps were acquired from samples that were either prepared in a glovebox (non-degraded reference) or prepared outside the glovebox (containing physisorbed water and oxygen) to measure oxygen and water diffusion into the OPV active layer. Furthermore, samples that were degraded in UV-light were used to measure chemical changes due to oxidation reactions. Our results indicate that physisorbed oxygen and water are mainly taken up in the PCBM phase, while UV-irradiation causes an increase in oxygen content in both the P3HT and PCBM phase, with still an excess of oxygen in the PCBM phase. In the future, we envision low-dose cryogenic EFTEM to be a critical technique in further resolving degradation issues in OPV research.

1. Jorgensen, M., Norrman, K., Krebs, F.C. Sol. Energy Mater. Sol. Cells 2008, 92, 686-714

2. Kozub, D. R., Vakhshouri, K., Orme, L. M., Wang, C., Hexemer, A., Gomez, E. D. Macromolecules 2011, 44, 5722-5726

3. Leijten, Z.J.W.A., Keizer, A.D.A., de With, G., Friedrich, H. J. Phys. Chem C 2017, 121, 10552-10561

Recently, the power conversion efficiencies (PCEs) of organic solar cells (OSCs) have been boosted to 13%,¹ which has brought OSCs close to commercialization. Currently, the most commonly used solvents for making the solutions of photoactive materials and the coating methods used in laboratories are not adaptable for future practical productions.² Therefore, taking a solution-coating method with environmentally friendly processing solvents into consideration is critical for the practical utilization of OSC technology. In this conference, we will report highly efficient blade-coated organic solar cells processed with low-toxic solvents.^3,4
References
[1] W. Zhao, S. Li, H. Yao, S. Zhang, Y. Zhang, B. Yang, J. Hou, J. Am. Chem. Soc. 2017, 139, 7148.
[2] W. Zhao, D. Qian, S. Zhang, S. Li, O. Ingan?s, F. Gao, J. Hou, Adv. Mater. 2016, 28, 4734.
[3] W. Zhao, S. Zhang, Y. Zhang, S. Li, X. Liu, J. Hou, Adv. Mater. 2018, 30, 1704837.
[4] W. Zhao, J. Hou, 2018, To be submit.

Recently, the emerging non-fullerene electron acceptor has replacing the leading role of fullerene derivatives in the field of organic solar cells (OSCs). In our work, via modulation of the intermolecular charge transfer (ICT) effect, we designed and synthesized a serial of novel small molecule acceptors, which showed very good photovoltaic performance in the OSCs. First, we weakened the ICT effect of the famous acceptor ITIC via replacing its benzene-based terminal group with its thiophene-based counterpart to synthesize the ITCC and ITCC-m, which have up-shifted the lowest unoccupied molecular orbit (LUMO) levels and large bandgaps. The OSCs based on ITCC showed increased output voltages. Then we enhanced the ICT effect of IEIC and prepared the narrowed bandgap acceptors IEICO and IEICO-4F, which demonstrated very high short-circuit current densities in the devices at low voltages losses. The designed acceptor have very good applications in the single-junction, tandem, ternary and semi-transparent OSCs. Recently, the emerging non-fullerene electron acceptor has replacing the leading role of fullerene derivatives in the field of organic solar cells (OSCs). In our work, via modulation of the intermolecular charge transfer (ICT) effect, we designed and synthesized a serial of novel small molecule acceptors, which showed very good photovoltaic performance in the OSCs. First, we weakened the ICT effect of the famous acceptor ITIC via replacing its benzene-based terminal group with its thiophene-based counterpart to synthesize the ITCC and ITCC-m, which have up-shifted the lowest unoccupied molecular orbit (LUMO) levels and large bandgaps. The OSCs based on ITCC showed increased output voltages. Then we enhanced the ICT effect of IEIC and prepared the narrowed bandgap acceptors IEICO and IEICO-4F, which demonstrated very high short-circuit current densities in the devices at low voltages losses. The designed acceptor have very good applications in the single-junction, tandem, ternary and semi-transparent OSCs.

A strategy to broaden the absorption spectra of organic solar cells is to blend multiple donor or acceptor materials into one bulk heterojunction (BHJ) resulting in D₁/D₂/A or D/A₁/A₂ devices. However, the introduction of a third material complicates the morphology formation and may degrade the charge transport.¹ A method that allows the use of conventional high-performance BHJs is by processing a D₂/A layer on top of a D₁/A blend. This cannot be realized by spin coating as the underlying layers will dissolve. In this work BHJs are processed by spontaneous spreading method which enables the fabrication of interesting device architectures. Here, a droplet of the D₂/A blend spreads on a water surface due to surface tension gradients and upon spreading the solvent evaporates. The resulting layer is transferred to a glass/ITO/ZnO/D₁/A substrate resulting in a double-layer ternary device. Solar cell characterization evidenced that D₂/A double layer devices processed by spontaneous spreading (SS) are as efficient as spin coated (SC) layers (P_Max,SC = 4.9% and P_Max,SS = 5.1% at similar active layer thickness).

In the double-layer ternary device, the complementary absorption of the two active layers results in inhomogeneous charge generation and therefore built-in field. When applying a high intensity bias source of a specific wavelength, its selective absorption in one of the two layers can modify the built-in field and collection of the charges. Drift-diffusion calculation showed that bias illumination can result in an EQE enhancement up to 23%, partly corresponding to the measured results. However, measured EQEs > 100% cannot be explained by the drift-diffusion simulations and the origin of this effect is not fully understood. It is likely related to hampered electron transport over the interface as J–V characteristics showed wavelength dependent charge transport problems. At present, this inhibits steep efficiency improvement of the ternary device (P_MAX = 5.9%) compared to the respective BHJ cells (P_{MAX, D1/A} = 3.5% and P_{MAX, D2/A} = 5.2%). Ongoing research is focused on the cause for the limiting charge transport properties and the development of new device architectures processed by spontaneous spreading.

1.             N. Gasparini et al. Nature Energy, 2016, 1, 16118

Computer simulations provide unique insight into the role of processing conditions in determining the photoactive layer morphology and ultimately the device performance of organic solar cells. They further enable us to visualize the full 3D morphology evolution, which is currently beyond the resolution of standard optical experimental techniques. We present 3D phase field simulation results of phase separation during spin-coating in solution-processed organic solar cells using high performance computing facilities. We are able to access the length and time scales associated with evaporation-induced phase separation in a ternary blend consisting of PDPP5T (diketopyrrolopyrrole-quinquethiophene) and PC₇₁BM ([6,6]-Phenyl-C71 butyric acid methyl ester) dissolved in chloroform. A finite-element based approach is used to solve the governing Cahn-Hilliard-Cook equations in a 3D domain.¹ The kinetic and thermodynamic model parameters are obtained from experiments.^2,3

Our model successfully captures the liquid-liquid phase separation in this system and the subsequent domain coarsening, leading to formation of PC₇₁BM-rich droplets suspended in a polymer matrix.⁴ The simulated morphologies agree quite well with experiments, reproducing the intriguing scaling law found between the dominant length scale and the normalized evaporation rate.^3,4 We find that thermal fluctuations play a crucial role in determining the early-stage length scales of phase separation.⁴

References

[1] O. Wodo, B. Ganapathysubramanian; Comput. Mater. Sci. 2012, 55, 113-126.

[2] S. Kouijzer, J. J Michels, M. van den Berg,  V. S. Gevaerts, M. Turbiez, M. M. Wienk, R.A. J. Janssen; J. Am. Chem. Soc. 2013, 135, 12057–12067.

[3] J.J. Van Franeker, D. Westoff, M. Turbiez, M. M. Wienk, V. Schmidt, R. A. J. Janssen; Adv. Func. Mater. 2015, 25, 855-863.

[4] V. Negi, O. Wodo, J.J. Van Franeker, R.A. J. Janssen, P.A. Bobbert; ACS Appl. Energy Mater. 2018, manuscript accepted.

The impact of device polarity on the photovoltaic performance is still under debate, although the inverted (n-i-p) device structure has become more popular than the conventional (p-i-n) configuration. It has been proposed that the use of acidic poly(ethylenedioxy­thiophene):poly(styrene sulfonate) (PEDOT:PSS) and reactive metals such as Al in the conventional device configuration cause issues regarding the device stability and performance, and that inverted devices are capable of enhancing the optical electric field and improving the interfacial contact. However, a one-to-one comparison for high-efficiency polymer solar cells has been rarely reported, which blurs our understanding of the intrinsic role of the device polarity on the power conversion efficiency (PCE).

In this contribution, we have evaluated five diketopyrrolopyrrole (DPP) based polymers with various chemical structures, including alkyl chains on the DPP units and backbones, and molecular weights, and demonstrated that the polymer solubility has a crucial role on the optimal device polarity. For polymers with good solubility, inverted devices showed a 10-25% improvement in photovoltaic performance compared to conventional one. In contrast, identical PCEs are observed for less soluble polymers, independent of device polarity. Additionally, the optimization of cosolvent and use of retroreflective foil efficiently boost the performance of DPP polymer solar cells, with PCE approaching 10%.

In organic photovoltaics (OPV) devices, the optimal photoactive layer is typically less than 100nm, with the efficiency often dropping  significantly when photoactive junction thickness increases, due to losses of both Jsc and FF. This thickness dependence is one of the main challenges for the commercialization of OPV devices which require  active layer thicknesses larger than 300nm.

When  OPV active layer thicknesses are increased  from 100nm to 300nm, the collected photocurrents typically drops 30% to 40% relative to the generated photocurrent. In this study, we demonstrate and address the importance of getting high Jsc in order to maintain the efficiency of thick devices. Studies have efficiency losses for thick OPV devices have previously primarily focused on  FF losses assigned to increased bimolecular recombination losses during collection. Herein we experimentally quantified the non-geminate recombination flux in four device series as a function of active layer thickness, and find that this recombination loss is too small at short circuit to explain the loss of J_{SC ­}for thicker devices.  Our studies rather identify that the main reason for J_SC loss in thick devices is that the device thickness becomes large than the space charge layer width, which limits the effective thickness for charge generation. We assign the origin of this space charge layer to the accumulation, under irradiation, of charge carriers in sub-bandgaptail states,. This limitation on photocurrent generation in thick devices is shown to correlate with the density of these tail states, and is shown to be distinct from space charge layer thickness limitations resulting from dopants or impurities ionization. It is also important to note that tail states are widely observed in organic semiconductors due to the disordered nature of organic material and bulk heterojunction architecture. As such the selection of organic photoactive layers with low tail state densities is shown to be a key requirement for the fabrication of thick, efficient organic solar cells.

My lecture will cover the recent emergence of a new generation of photovoltaics based on molecular sensitizers [1,2] and perovskite pigments as light harvesters. Dye sensitized solar cells have meanwhile found applications as flexible light weight power supplies for portable electronics as well as electric power producing glass panels their market being presently in the multi-megawatt range. Perovskite solar cells (PSCs) [1] directly evolved from DSCs and are attracted enormous interest due to their low cost ease of preparation and ha certified solar to electric power conversion efficiency (PCE) exceeding already the performance polycrystalline silicon solar cells. Present efforts focus on scale up of the device size [2] and achieving operational stability [3]. The high photovoltage (Voc > 1.2 V) attained with these systems renders them very attractive for the generation of fuels from sunlight, e.g. by the splitting of water into hydrogen and oxygen [4] and the cleavage of CO2 into CO and 1/2 O2.

References:

[1] B.O’Regan and M. Grätzel “A Low Cost, High Efficiency Solar Cell based on the Sensitization of Colloidal Titanium Dioxide, Nature,1991, 353, 7377-7381.

[2] M. Grätzel, Photoelectrochemical Cells,Nature 2001, 414, 332.

[3] M. Grätzel, The light and shade of perovskite solar cells., Nature Materials 2014, 13, 838-842.

[4] X. Li, D. Bi, C. Yi, J.-D. Décoppet, J. Luo, S.M. Zakeeruddin, A. Hagfeldt, M. Grätzel,

A vacuum flash–assisted solution process for high-efficiency large-area perovskite solar cells, Science, 353, 58-62 (2016)

[5] N. Arora, M.I. Dar, A. Hinderhofer, N. Pellet, F. Schreiber, S.M. Zakeeruddin, M. Grätzel, Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies greater than 20%., Science, 358, 768-771 (2017)

[6] J. Luo, J.-H. Im, M.T. Mayer, M. Schreier, Md.K. Nazeeruddin, N.-G. Park, S.D.Tilley, H.J. Fan, M. Grätzel. Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth abundant catalysts. Science 2014, 345, 1593-1596.

In a molecular photovoltaic device, charge separation and energy conversion result from the evolution of a photogenerated exciton into a charge separated state, in competition with recombination to ground. The efficiency of charge separation is a function of the molecular packing and energy level alignment near the interface, and of disorder in these properties. Understanding the effect of structure, energetics and disorder on the competition between charge separation and recombination help to identify the factors controlling device photovoltage and ultimately conversion efficiency. Here, we address the factors controlling photovoltage in molecular donor: acceptor solar cells using a combination of electrical and spectroscopic measurements and numerical models. We explore the limits to Voc using a model of non-radiative recombination, and demonstrate how choice of materials and control of processing may influence voltage losses. We use these results to consider the importance of chemical structure, the phase behaviour and microstructure of the binary system in controlling actual performance and the ultimate limitations placed on solar to electric conversion by the molecular nature of the materials. We also address methods for the experimental determination of non-radiative recombination rates.

Solution-processed bulk heterojunction (BHJ) polymer solar cells (PSCs) have exhibited great potentials for making large area and flexible solar panels through low-cost solution coating techniques. Typically, a BHJ active layer in a PSC is composed of a conjugated polymer as electron donor and an organic compound as electron acceptor. In recent years, the applications of non-fullerene-based small molecular acceptors materials have afforded great opportunities to achieve higher power conversion efficiency (PCE) in PSCs. In comparison with the PSCs based on fullerene-based acceptors like PCBM, photovoltaic performance of the fullerene-free PSCs are even more sensitive to the intrinsic properties of the polymer donors and non-fullerene acceptors in their active layers. In the past year, our group focused on the study of material design for fullerene-free PSCs and achieved a series of high performance PSCs. Based on these studies, we suggested a few feasible methods for molecular design of the donors and the acceptors and also tried to correlate the molecular energy levels, aggregation morphologies and other intrinsic properties of the active layer materials with their photovoltaic behaviors in device. Furthermore, we used the polymer donors and small molecular acceptors with the optimized chemical structures to construct a single-junction and double-junction tandem PSC with PCEs over 14%.

Thanks to the intensive research efforts of a large scientific community over the past 7 years, lead (Pb)-based hybrid perovskite solar cells have reached impressive power conversion efficiency. Against the initial criticism about their instability, also large improvements in the thermal and photo stability of this kind of solar cell were obtained by using more stable precursors, and robust hole/electron transport layers. Despite these outstanding accomplishments, the toxicity of lead causes concerns about the possible large-scale utilization of this new type of solar cell.

Among the various alternatives to lead, Sn has been recognized to have a great potential, as the Sn-based hybrid perovskites display excellent optical and electrical properties such as high absorption coefficients, very small exciton binding energies and high charge carrier mobilities. In my talk I will show that Sn-based perovskites display evidences of photoluminescence from hot-carriers with unexpectedly long lifetime. The asymmetry of the PL spectrum at the high-energy edge, is accompanied by the unusually large blue shift of the time-integrated photoluminescence with increasing the excitation power. These phenomena are associated with slow hot carrier relaxation and state-filling of band edge states.

I will further show all-tin-based hybrid perovskite solar cells with efficiencies of up to 9%. This record result is obtained with the addition of trace amount of 2D tin perovskite, which initiates the homogenous growth of highly crystalline and oriented FASnI3 grains at low temperature.

In the first part of the talk I will present recent progress in the field of short-wave infrared colloidal quantum dot solar cells, as promising solution processed platform to harness solar energy beyond Silicon´s, CIGS´ or even perovksites´ reach. I will describe the challenges associated in high performance SWIR CQD solar cells and the tools available to engineer their energy levels and trap passivation towards high performance [1]. Then I will present preliminary results on all-CQD tandem solar cells that employ a one-atom thick graphene as intermediate recombination layer eliminating the need for vacuum-based deposition of multiple metal oxide sputtered layers or use of metal nanoparticles.

In the second part of my talk I will discuss recent progress on synergistic surface and architecture engineering of CQD solar cells to minimize trap state density and reach high open circuit voltages [2, 3]. I will conclude by showing some correlations on the performance of CQD solar cells with CQD light emitting diodes that point to the fact that a CQD thin film should yield both high efficiency solar cells and light emitting diodes.

[1] Y. Bi, S. Pradhan, S. Gupta, M. Z. Akgul, A. Stavrinadis, G. Konstantatos Adv. Mater. [Online DOI: 10.1002/adma.201704928] (2018)

[2] Breaking the open-circuit voltage deficit floor in PbS quantum dot solar cells through synergistic ligand and architecture engineering S. Pradhan, A. Stavrinadis, S. Gupta, S. Christodoulou, G. Konstantatos ACS Energy Lett. 2, 1444-1449 (2017)

[3] Trap-state suppression and improved charge transport in PbS quantum dot solar cells with synergistic mixed-ligand treatments S. Pradhan, A. Stavrinadis, S. Gupta, Y. Bi, F. Di Stasio, G. Konstantatos Small 13, 1700598 (2017)

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Back-contact concepts are well established in the field of silicon solar cells, where their implementation has resulted in significant efficiency gains, compared to conventional contacting architectures. Charge collection in these devices is typically facilitated by a set of two interdigitated finger electrode arrays, co-located on the backside of the silicon wafer. Here we describe the fabrication and study of back-contact perovskite solar cells (bc-PSCs) with novel contact geometries. The main advantage of back-contact concepts is that optical transmission losses can be avoided, arising from the top charge collection electrode, which for PSCs typically is a thin conducting oxide (TCO) layer. The back-contact design furthermore provides the opportunity to study the influence of post-processing treatments on device efficiency in situ. To demonstrate this we study the evolution of bc-PSCs performance during their exposure to pyridine vapors. Furthermore we report a novel photovoltaic device concept based on a gold-perovskite-gold Schottky-junction bc-PSCs in which the work-function of the gold electrodes is controlled by the presence of self-assembled molecular monolayers (SAM). We provide evidence of the successful workfunction tuning by means of Kelvin probe microscopy while also presenting the photovoltaic performance date of these devices. We show that the presence of these SAMs can produce photovoltages of up to 600 mV and photocurrents in excess of 12 mA/cm² under simulated sunlight, despite a large center-to-center electrode spacing of 6.5 µm.  

Organometallic halide perovskites are promising materials for thin-film photovoltaics, with achieved efficiencies over 20% at lab-scale. For promoting the use of perovskite solar cells (PSCs) in practical applications, it is essential to achieve an upscaling route, which ensures environment-friendly processing of stable and highly efficient PSCs. Here, we report on the upscaling of sheet-to-sheet (S2S) slot-die processing of PSCs, heading towards the roll-to-roll (R2R) production. Results demonstrated that S2S processing of planar n-i-p PSCs on 152x152 mm² substrates exhibited similar stability and efficiency as small area PSCs. We also demonstrate the initial results from S2S processed p-i-n PSCs and R2R processes flexible n-i-p PSCs.

S2S slot-die coating of 152x152 mm² Glass/ITO substrates produced 64 cells with 0.16 mm² area, and single large area modules. For the latter, laser patterning of 250 µm wide interconnections was implemented. For the n-i-p structure, SnO₂-NPs, a triple-cation (Cs-MA-FA) lead perovskite, and Spiro-OMeTAD have been slot die coated as ETL, active layer, and as HTM, respectively. The control of the perovskite crystallization/drying was achieved by vacuum quenching. The small area cells processed by slot-die coating resulted in stabilized PCE of 14.5% with a yield above 95%. Laser patterned large area modules processed by the same recipe demonstrated a stabilized PCE of 13.8% on aperture area and 14.5% on active area. These efficiencies are comparable to the efficiencies obtained from spin-coated cells with 0.16 mm² area demonstrating the feasibility of upscaling.

For the p-i-n structure, NiO-NPs, triple-cation lead perovskite, and PCBM/ZnO layers have been slot-die coated as HTM, active layer and as ETL, respectively. The average PCE of 64 cells with 0.16 mm² area was 12%, with a spread of efficiencies among the cells. This observed value is lower than the PCE of spin-coated cells (14%). Further optimization on the ETL and HTM (smooth, pinhole-free layers, improved wettability during coating) needs to be done in order to improve the quality of these layers to reach higher PCE values.

For the R2R processing of flexible PSCs, the same n-i-p stack presented above (with dual-cation perovskite, Cs-FA) was processed on a 30 cm wide PET/ITO foil at a speed of 5 m/min. The control of the perovskite coating and crystallization on R2R was instrumental to fabricate flexible PSC with PCE up to 16%. This result represent an important step towards the large area processing of flexible PSCs.

Organic–inorganic hybrid perovskites are the new shooting stars on the high efficiency solar cell sky. The efficiencies of perovskite solar cells are close to those of well-established thin-film solar cells based on cadmium telluride (CdTe) or copper indium gallium diselenide (CIGS). However, in contrast to these established technologies, perovskite absorber materials can be entirely solution processed, which bears the promise of low production cost. Most research and development on scalable solution-processed perovskite photovoltaics focused so far on slot-die coating, blade coating, and spray coating. Only a few reports exist yet on non-contact inkjet printing, despite the great success of industrial inkjet printers in the fabrication of large area organic light emitting devices [1].

In this contribution, we report on our recent advances on non-contact inkjet-printed perovskite photovoltaics. Digital inkjet printing offers rapid deposition of perovskite absorber layers for large area perovskite absorber layers as well as perovskite solar cells of arbitrary shape. By adjusting the drop spacing of the inkjet printer cartridge and making use of multipass printing, we demonstrate excellent control of the perovskite layer formation and perovskite layer thickness [2,3]. Inkjet-printed perovskite solar cells based on methylammonium lead iodide as well as triple cation perovskite layers with 10 % cesium in a mixed formamidinium/methylammonium lead iodide/bromide composite are demonstrated. For the latter, initial power conversion efficiency as high as 14.0% are demonstrated. The devices also show continuous power output at constant voltage, resulting in a power conversion efficiency of 12.9 % after 5 min of MPP tracking, representing a major improvement from previously reported results. The extended study furthermore  demonstrates the improved stability of the perovskite solar cells based on triple cation perovskite absorber layers compared to perovskite solar cells base don methylammonium lead iodide.

References

[1]   Kateeva, “Kateeva webpage,” URL: “http://kateeva.com/ company/newsroom/in-the-news/”, accessed January, 2018.
[2]   F. Mathies, T. Abzieher, A. Hochstuhl, K. Glaser, A. Colsmann, U. W. Paetzold, G. Hernandez-Sosa, U. Lemmer, and A. Quintilla, J. Mater. Chem. A, vol. 4, 19207-19213, 2016.
[3]  F. Mathies, H. Eggers, B. S. Richards, G. Hernandez-Sosa, U. Lemmer, and U. W. Paetzold, in preparation.

Organic-inorganic hybrid perovskite has attracted extensive attention in recent years for its wide applications in various optoelectronic devices such as solar cells, light emitting diodes (LEDs), lasers, transistors, and photodetectors. However, it is still challenging to directly pattern perovskite thin films because perovskite is very sensitive to polar solvents and high temperature environment. In this work, we demonstrate our novel approach to fabricate high-quality perovskite grating and its potential applications in optoelectronic devices through the study of the performances of grating patterned light emitting diode.[1]

Our results show that (1) different from typical imprint method to form nanograting, we report for the first time to utilize methylamine gas (MA) to fabricate CH₃NH₃PbI₃ (MAPbI₃) periodic nanostructures via MA induced phase transition under ambient condition (the MA induced intermediate is liquid under room temperature). This approach is quite different from traditional nanoimprinting which use high pressure to press the rigid mold and completely different from most used photo-lithography or electron beam lithography technique which needs solvent to etch the sample. (2) our direct nano-patterning approach is suitable for fabrication of large-area perovskite pattern. As a proof of concept in lab scale, 15 mm*15 mm periodic nanostructures have been demonstrated. (3) Our direct nano-patterning approach can not only fabricate periodic nanostructures in different perovskite materials such as MAPbI₃ and HC(NH₂)₂PbI₃ (FAPbI₃) but also improve the crystallinity, light absorption and emission of perovskite for solar cell and LED applications. It should be noted that some works reported by others also demonstrate the fabrication of perovskite grating, however most of them lead to reduced crystallinity and PL. Consequently, our approach opens up a simple way to nano-engineering perovskite. The nano-patterned perovskite can be used in different perovskite optoelectronic devices.

Reference: [1] Adv. Funct. Mater., 2017, DOI: 10.1002/adfm.201606525.

Vacuum-deposition is one of the most technologically relevant techniques for the fabrication of perovskite solar cells. Althought it can be used to fabricate devices with power conversion efficiency close to that obtained by solution-methods, only few studies have been carried out on vapor-deposited perovskite films. Clear differences among both fabrication families, such as the perovskite grain size of the most efficient devices, raises new questions regarding the basic working mechanisms of the solar cells, which should be addressed to develop the full potential of this technology.

Here, we use the intrinsically additive nature of vapor-based processes to carry out a detailed study of the solar cell interfaces and perovskite optoelectronic properties. Kelvin probe and impedance spectroscopy analysis are used to design a band alignement adjustment, which results in CH₃NH₃PbI₃ solar cells with open circuit potential close to the thermodinamical limit and power conversion efficiency >20 %. The co-operation of multiple sources is also employed for the fabrication of multi-cations/anions perovskite compounds by thermal vacuum deposition for the first time. The limiting factors of these cells are studied through a variation of device architectures and absorber thickness.

This work presents an outlook of the main processes determining the performance of vacuum-deposited solar cells, which can be crucial to develop the ideal system based on this industrially mature technique.

Module performance ratio (MPR) describes how well a PV module performs in the real world relative to standard test conditions (1000 Wm^‑2 AM1.5, 25 °C). It is affected by factors such as intensity and angle of sunlight, spectrum, ambient temperature, wind speed and other local factors. New energy rating standards (IEC 61853) define a standardised method for a fair comparison of different modules’ expected performance in a set of predefined reference climates. In the future, it is likely that the sale value of modules will be based on energy rating instead of the traditional watt-peak power rating.

The most important factor in determining MPR is the module’s temperature coefficient. Perovskite cells are reported to have smaller temperature coefficients than silicon, and therefore have the potential to achieve favourable energy ratings.

We explain the methods used in the energy rating standards. Using a modelling approach, we explore the expected performance of single and tandem perovskite technologies under a set of reasonable assumptions about future performance trends. We reveal how perovskite modules fare relative to traditional technology and what are the key requirements for external device parameters, such as series and shunt resistance and front-back cell balance, to achieve a good rating.

Finally, we describe how performing energy rating is a challenging procedure; it requires a comprehensive set of measurements and most test-labs lack procedures and accurate calibration infrastructure specific to perovskite devices. Through a sensitivity analysis, we investigate the expected accuracy of energy rating and describe strategies to estimate the energy rating at low cost using a minimal set of measurements.

Perovskite solar cells based on an all printable mesoporous stack, made of overlapping titania, zirconia and carbon layers (C-PSC), represent a promising device architecture for both simple, low cost manufacture and outstanding stability. In this talk we will report on a breakthrough in the upscaling of this technology: a recent pilot scale trial of 20 screen printed A4 sized FTO-glass substrate perovskite modules, delivering power conversion efficiency (PCE) ranging from 3 to 7% at 1 sun on an unprecedented 198 cm2 active area.

The current fabrication process for a C-PSC is a highly manual and time consuming batch process designed on single monolithic devices. This work will introduce a number of engineering solutions to unlocking pilot scale manufacture; (i) overcoming long manufacturing times, reducing the total time from 3 hours to 30 seconds. (ii) replacing manual drop infiltration of the perovskite with a novel robotic method that enables large area, instantaneous and homogeneous deposition of perovskite droplets over the active area and finally (iii) enabling series connection and an increase in geometric fill factor of the modules using both mechanical and laser scribed interconnection processes.

The paper will also discuss the overall experience both good and bad of planning and delivering a pilot scale fabrication run of multiple perovskite modules.

Lead halide perovskites are ionic semiconductors that have recently revolutionized the photovoltaics field. Unlike in most photoactive materials, ionic conductivity plays a key role in perovskites during photovoltaic device operation. However, the physical characterization of the ionic conductivity has been rather elusive due to the highly intermixing of ionic and electronic current. 1, 2 In this work high efficiency and low defect density monocrystalline MAPbBr3 (MA=Methyl ammonium) solar cells free of hole transport layer (HTL) suppress the effect of electronic current.3 Impedance spectroscopy reveals the characteristic signature of ionic diffusion: the Warburg element and transmission line equivalent circuit in MAPbBr3 and ion accumulation at the MAPbBr3/Au interface typical for non-reactive contacts.4 Diffusion coefficients are calculated based on a good correlation between thickness of MAPbBr3 and characteristic diffusion transition frequency. In addition, a comparison of polycrystalline MAPbBr3 devices prepared either with Spiro-OMeTAD as an HTL or free of HTL allows the study of reactive external interfaces. The low frequency response in IS measurements is correlated with the chemical reactivity of moving ions with the external interfaces and diffusion into the HTL.

References

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

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

Peng, W.; Aranda, C.; Bakr, O.; Garcia-Belmonte, G.; Bisquert, J.; Guerrero, A.

Submitted.

Bisquert, J.; Fabregat-Santiago, F., Impedance Spectroscopy: A general Introduction and Application to Dye-Sensitized Solar Cells. In Dye-sensitized solar cells., Kalyanasundaram, K., Ed. CRC Press: Boca Raton, 2010.

Highly efficient solar cells using methylammonium lead iodide (MAPbI₃) have been reported by many researchers. Recently, other metal halide perovskites have been developed which promise better stability. For example replacing the organic cation methylammonium with formamidinium (FA) has been shown to improve the power conversion efficiency and stability of perovskite thin-films under certain conditions such as thermal stress [1]. Additionally, FAPbI₃ has a lower band-gap of 1.48eV, thus FAPbI₃ based solar cells have a higher short circuit current than cells based on MAPbI₃. Furthermore, mixed cation perovskites which alloy formamidinium, methylammonium and caesium have led to even greater improvements in stability and efficiency [2]. Until recently, thin films of perovskites with cations other than methylammonium have mainly be been fabricated using a solution processing route.

Here we demonstrate vacuum deposition of formamidinium lead halide absorber thin films [3]. We deposited the films by co-evaporating FAI and PbI₂. This vapour deposition method offers several advantages: the process takes place in vacuum, this reduces the influence of outside factors and increases the reproducibility; films can be vapour deposited on any substrate without the risk of solvent damage to the substrate layers; and finally, the resulting films are very uniform and pinhole-free over large areas. The good uniformity of this technique is particularly helpful for future upscaling of these solar cells. We characterised the FAPbI₃ films, using methods such as X-ray diffraction, visible- and infrared-absorption spectroscopy, scanning electron microscopy and cross-sectional transmission electron microscopy. These measurements confirm the formation of the perovskite phase of FAPbI₃.

Furthermore, we processed these co-evaporated films into planar perovskite solar cells. These solar cells reached high power conversion efficiencies, comparable to the efficiencies of MAPbI₃ control devices. As expected, the short circuit voltage was higher than for the MAPbI₃ devices while the open circuit voltage was slightly lower. The successful fabrication of vapour deposited and very uniform formamidinium lead iodide films is necessary for exact optical material characterisation. This work represents an important step towards realising large-area vapour deposited mixed cation perovskite solar cells.

References

[1] G. E. Eperon et al. Energy & Environmental Science (2014) 7, 982-988

[2] M. Saliba et al. Energy & Environmental Science 9.6 (2016) 1989-1997

[3] J. Borchert et al. ACS Energy Letters, (2017) 2, 2799-2804

Conjugated polymers based on the new chromophore indolo-naphthyridine-6,13-dione thiophene (INDT) are explored in OFET and organic photovoltaic applications. These polymers exhibit very narrow band gaps of ~1.2 eV and extremely high n-type mobility exceeding 3 cm² Vs^-1. The high n-type charge carrier mobility is correlated to remarkably high crystallinity along the polymer backbone with a correlation length in excess of 20 nm. OPV device efficiencies up to 4.1 % and charge photogeneration up to 1000 nm were demonstrated highlighting the potential of this novel chromophore in high-performance organic electronics. Although the efficiency of the devices are modest, these are some of the highest values reported for materials with such narrow band-gaps. External quantum efficiency measurements show that the majority of the photocurrent originates from the fullerene absorption; however there is also a clear and significant contribution from the polymer absorption.

These polymer blends were investigated further using ultra-fast transient absorption spectroscopy (TAS). It has been discovered that two methods of charge photogeneration are operational, depending upon whether the polymer or fullerene is excited. Fullerene excitation results in an unusually slow (nanosecond timescale) hole transfer process from the photoexcited fullerene to the polymer. Polymer excitation, in contrast, leads to standard electron transfer to the fullerene acceptor, despite the LUMO levels of the donor and acceptor being almost isoenergetic implying virtually zero driving force for charge separation. It is remarkable that such a high proportion of charge carriers are achievable through polymer excitation in ultra-low band gap polymers.

Different fullerenes were also trialed to assess the effect on charge photogeneration, using both TAS and pump-push photocurrent measurements. Interestingly, it was discovered that the INDT polymers may possibly generate an intramolecular CT state-like singlet exciton, which is only able to be efficiently separated in the presence of a fullerene with a deep enough LUMO.

A new figure of merit (FOM) as a ratio of the non-geminate recombination rate to the extraction rate of free charge carriers in bulk heterojunction (BHJ) organic solar cells (OSCs) is derived as a function of the effective carrier mobility, effective carrier concentration, active layer thickness, bimolecular recombination coefficient and internal voltage. We further derive a relation between the FOM and fill factor (FF) and find that the FF increases with a decrease in FOM, which means an increase in multiple material parameters such as effective mobility and dielectric constant, and a decrease in light intensity, active layer thickness and dimensionless reduction prefactor. This shows a strong correlation between extraction and recombination, which controls the performance of BHJ OSC and particular FF which decreases rapidly from ~0.85 to ~0.30 in MDMO-PPV:PC61BM and ~0.76 to ~0.20 in PCDTBT:PC60BM if the FOM increases from ~0.05 to ~10 in both blends. The results indicate that the influence of charge carrier extraction dominates at high FF and high power conversion efficiency (PCE) whereas the influence of charge carrier recombination dominates at low FF and low PCE, hence further improvement in the synthesis of new materials for the fabrication of BHJ OSCs is required to reduce carrier recombination as well as increase carrier extraction to the respective electrodes.

The performance of solar cells based on molecular electronic materials is limited by relatively high non-radiative voltage losses . The primary pathway for non-radiative recombination in organic donor: acceptor heterojunction devices is believed to be the decay of a charge-transfer (CT) excited state to ground via energy transfer to vibrational modes. Recently, it was shown  that the transition rate depends on the Franck-Condon factor describing the overlap of the CT and ground-state vibrational states and, therefore, on the energy of the CT state. However, experimental data do not always follow the trends suggested by the simple model. Here, we extend this recombination model to include other factors that influence the non-radiative decay rate – and therefore the open-circuit voltage – but have not yet been explored in detail. We use the extended model to understand the observed behavior of series of small molecule:fullerene blend devices, where open-circuit voltage appears insensitive to non-radiative loss. The trend could only be explained in terms of a microstructure dependent CT state oscillator strength. We present design rules for improving open-circuit voltage via control of materials parameters and propose a realistic limit to the power conversion efficiency of organic solar cells.

A novel experimental and mathematical method has been developed to directly characterise the dipole energy with a resolution up to 0.05eV by applying multiply insitu-spectroscopy. The work yields an insight into the intermediate energy states at the metal oxide/polymer layer interface and a prediction to the charge transport of the organic device.

In our work, the characterisation of chemical properties (XPS) and concentration distribution (NICISS)^[2] on a P3HT:PC₆₁BM Bulkheterojuction (BHJ) system with the high workfunction(ɸ) metal transparent oxides (MoO₃) thickness from 0.1nm to 8nm arises an observation of energy shift of polymer and heavy diffusion of MoO₃. Thus a hypothesis of dipole forming at the interface has been proposed.

Gradual changes of both WF(ɸ) and E_VB were observed when applying UPS and MIES upon the samples. Decomposition to the valence electron spectra of UPS by using Singular Value Decomposition (SVD)^[3] yields the characterisation of pristine MTO/BHJ and BHJ spectra with various energy shifts. The spectra were further processed with a deconvolution algorithm. A series of BHJ spectra with energy shift have been quantified, which forms energy ladders at the interface. We successfully reconstructed the energy levels of the MTO/polymer BHJ interface with the electronic parameters based on the evaluation of the UP spectra. The quantification of dipole energy with a range of MTO deposition thickness has been achieved and the mechanism of charge transport over the metal oxide/polymer layer interface was thus decrypted.

We also observed difference of dipole energy among MoO₃, V₂O₅ and WO₃ on BHJ. Treatments such as air exposure and annealing further alter the dipole energy, leading to a different charge transport mechanism. The performance of polymer-based devices such as OPV and OLED can thus be influenced

1. Yanting Yin, A.S., Jamie Quinton, David A. Lewis and Gunther G. Andersson, The Observation and Characterization of Dipoles forming at the MoO3-P3HT/PCBM BHJ Interface (ready for submission). 2018.

2. Andersson, G. and C. Ridings, Ion scattering studies of molecular structure at liquid surfaces with applications in industrial and biological systems. Chem Rev, 2014. 114(17): p. 8361-87.

3.Berlich, A., Y.-C. Liu, and H. Morgner, Growth of nickel nanoparticles on NiO/Ni(001): Evidence of adsorbed oxygen on metal particles by metastable induced electron spectroscopy (MIES). Surface Science, 2008. 602(24): p. 3737-374

Understanding exciton transport in conjugated polymers is a vital goal for research in optoelectronic applications. We show that we have synthesised novel polyfluorene nanofibers with controllable lengths, and that these can be connected to end blocks of different conjugated polymers. We study the exciton transfer in solution from the polyfluorene cores to the lower-energy end-blocks. We measure key parameters such as singlet exciton diffusion length and singlet exciton diffusion coefficient through ultrafast photoluminescence and absorption spectroscopies. The exciton transport is shown to be extremely efficient, made possible by the unique structural properties of the polymer nanofibers.

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 11 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 11 1 1 1 1

Time and frequency domain optoelectronic techniques such as transient photovoltage/photocurrent spectroscopy (TPV/TPC) and electrochemical impedance spectroscopy (EIS) are often used to study transport and recombination in a range of solar cell architectures. Predominantly the techniques have been used to study organic based devices such as dye-sensitized, perovskite and bulk-heterojunction organic PV devices. It is common to use either time domain or frequency domain measurements, but not necessarily the two in conjunction.

In this work, we show that a more in depth understanding of device operation can be obtained when both time domain (TPV) and frequency domain (EIS) measurements are employed together. The two types of technique are in many ways equivalent, and similar information can be obtained from both. However, there are subtle differences between the two which enable certain processes to be studied in more detail using one or the other. In this particular example the techniques have been combined to study the effect the interlayer between the active layer and metal contact has upon OPV device stability.

The devices under test were based on the non-fullerene acceptor, ITIC, and the polymer donor, PCE10. The interlayers tested included Ca, LiF, PEIE and PFN. Accelerated aging under illumination revealed significant differences in terms of stability, with the most stable device containing the PEIE interlayer and the least stable containing LiF. TPV measurements were used to study the varying levels of degradation and revealed slight differences in terms of trap formation and recombination dynamics for the devices with different interlayers. However, EIS measurements showed more pronounce differences in terms of the resistance of the different interlayers upon aging. Although EIS measurements were performed at the open-circuit potential under illumination, a small amount of current flows during the measurement which means it can be used to probe charge transport throughout the entire device and not just recombination (predominantly in the active layer) as is the case for TPV measurements. Whilst recombination behaviour can be observed using EIS measurements, it is obscured by the charge transport through the interlayer. TPV/TPC measurements are therefore more suited to studying recombination and charge accumulation in the active layer. We show that the presence of two arcs in the EIS spectra are related to recombination in the active layer and charge transport through the interlayer. The interlayer is shown to get more resistive during degradation which is responsible for the main loss in device performance.

Blends of semiconducting (SC) and ferroelectric (FE) polymers are interesting for applications such as resistive memories and organic photovoltaics (OPV). For OPV, the rationale is that the local electric field associated with the FE dipoles in a blend could aid in the dissociation of photogenerated excitons, thus improving power conversion efficiency. Unfortunately, many FE polymers either require solvents or processing steps that are incompatible with those required for SC polymers. Incorporating SC and FE components into block copolymers offers a path to facile fabrication of thin films from suitably chosen solvents. In this work, we investigate the photophysical properties of SC-FE block copolymers based on Poly-3-hexylthiophene (P3HT) and either Poly-vinylidene-difluoride (PVDF) or Poly-vinylidene-difluoride-trifluoroethylene (PVDF-TrFE).

Our results show that both copolymers exhibit similar photophysical behaviour to P3HT in solution. However, large morphological differences depending on the choice of FE building blocks, and the creation of a new ultrafast radiative decay channel are observed in solid state. Finally, after optimising film deposition via spin-coating, we present the results of fabricated solar cells utilising PVDF-TrFE as an additive to the active layer.

Semiconductor polymers have attracted considerable attention in the past few decades for their feasibility in low cost, flexible, roll-to-roll compatible electronic organic devices. To date, most of the conjugated polymers have been used as donors blended with fullerenes, which act as acceptors. However, few examples exist in the literature where polymers have been used as efficient electron acceptors.

Herein, we present a transient absorption spectroscopy (TAS) study of two different cross-conjugated polymers based on Tyrian Purple that can be used both as electron acceptor (blended with P3HT) and donor (blended with PCBM). These two polymers have been demonstrated to generate free charges when deposited either with P3HT or with PCBM. Moreover, similar P3HT cation TAS signal amplitudes to P3HT:PCBM reference were obtained when blending these polymers with P3HT. Therefore, a similar amount of P3HT cations were generated blending P3HT with either these cross-conjugated polymers or with PCBM. In addition, transient species obtained showed long lifetimes when the cross-conjugated polymer were used as acceptor and donor. The fitting to these decays to a power law equation also showed than in both cases the charges were localised in very deep traps, which justify the long lifetimes. This is in accordance with the reported calculated structures for the HOMO and LUMO, where a strong HOMO localisation was shown. All these characteristics provides a promising direction for the development of cross-conjugated polymers for potential use in photovoltaic devices.

We address the issue of short exciton diffusion length by focussing on radiative, or fluorescence, losses. These losses sensitively depend on the cell design [1] because of the strong confinement that is characteristic of organic solar cells. Therefore, we discuss the cell performance as a function of the cell geometrical parameters and of the internal luminescence quantum efficiency (ILQE), which is the fraction of the exciton decay that is intrinsically due to fluorescence.

In order to determine the optimal design, we have established a generalisation of the Shockley-Queisser theory for organic solar cells [2,3] taking into account the diffusive exciton transport and microcavity effects both for exciton radiation and sunlight injection. Hence, given the ILQE, we are able to determine the buffer layers thicknesses in order to increase the exciton lifetime and, hence, the exciton diffusion length.

Managing fluorescence losses may significantly improve the cell performance.  This is what we demonstrate using realistic material parameters inspired from literature data for which we obtain an increase of power conversion efficiency from 11.3% to 12.7% as the ILQE goes from zero to one. Conversely, to ignore the dependence of the exciton radiative decay on the environment may lead to suboptimal design and to unnecessary low cell efficiency. We illustrated this latter situation with experimental material data. Our results invite us to rediscuss the statement "good solar cells should also be good emitters".

Finally, we observe that there is a qualitative change as soon as the bulk radiative decay rate is no zero (ILQE>0). This is due to the quenching effect experienced by radiative excitons in the vicinity of a dissipative medium. This shows that not to consider exciton radiative decay (ILQE=0) is an inaccurate modelling assumption.

References:

[1] G. Kozyreff et al. Opt. Express, 21:A336-A354, 2013.

[2] B. Godefroid and G. Kozyreff. Phys. Rev. Applied, 8:034024, 2017,

[3] Uwe Rau et al. Phys. Rev. Applied, 7:044016, 2017.

Although photocathodes that are based on Si, GaInP₂, GaP and copper indium gallium sulphide/selenide (CIGS) exhibit high solar-to-hydrogen conversion efficiencies, they either contain rare elements or require high cost processing techniques. In order to compete with crystalline silicon-based photovoltaic-coupled electrolysis for widespread solar hydrogen generation, new materials are required that are simultaneously high efficiency, Earth-abundant, and stable in an aqueous electrolyte, while being fabricated at low cost.

In this presentation, I will discuss our work with highly active photocathodes based on Sb₂Se₃ that feature abundant MoS_x catalyst. Photocurrents up to ~16 mA cm^-2 have been obtained at 0 V vs RHE in 1 M H₂SO₄ under simulated one sun AM 1.5 G irradiation. These photocathodes show high incident photon to current conversion efficiencies over the entire visible spectrum, and until the band edge of Sb₂Se₃ (1.2 eV). The small decrease in photocurrent density with time (20 h) was found to result from degradation of the amorphous catalyst, while the Sb₂Se₃ absorber resisted photocorrosion in the strongly acidic media. The simple and low cost fabrication method–combined with the high performance and stability of the photocathode–makes Sb₂Se₃/MoS_x a strong candidate for large scale solar hydrogen production. Additionally, some other materials will be discussed, such as CuO and Cu₂O-based photocathodes.

Halide Perovskite has excited the solar cell community by the quick rise of photoconversion efficiency to certified values close to 23%. Such outstanding behavior is based on, among other, in a low non-radiative recombination, making this family of materials attractive not just for photovoltaic devices but for other optolectronic devices as LEDs and lasers. Since the seminal works of Pérez-Prieto and Kovalenko, and coworkers, it can be observed even a further enhancement of the optical properties of this materials with photoluminescence quantum yields (PLQY) close to 100%, with nanoparticles prepared with easy procedures and without any core/shell structure. In thi presentation I will shown the recent results of my group on the preparation, study and characterization of halide perovskite nanoparticles with partial substitution of Pb by Sn, where Sn²⁺ cation is stabilized obtaining stable nanoparticles with outstanding optical properties. I will show the use of these halide perovskite nanocrystals in solar cells and LED devices, studying the contact effect on cell performance. Nanocrystals have been also electrochemically analyzed opening also a field for their use in alternative applications.

All-inorganic colloidal caesium lead halide perovskite quantum dots (QDs) have recently emerged as promising material for optoelectronics (solar cells, light emitting diodes, lasers, et al.).[1] CsPbBr₃ QDs have also been demonstrated as stable two-photon-pumped lasing medium. However, the details of the photophysics under two-photon absorption (2PA) are yet to be investigated. CsPbBr₃ QDs show red-shifted photoluminescence (PL) emission and increased PL lifetime under 2PA compared with PL emission under one-photon absorption (1PA). These differences have not been thoroughly addressed by previous reports, although here are various speculations about possible explanations of the different PL emission and decay in case of one and two photon exciotation, such as re-absorption, alternative electronic relaxation pathways or size inhomogeneity.

Herein, we used ultra-fast spectroscopies to study the photophysical process of 2PA in CsPbBr₃ QDs. We found that, the 2PA cross-section follows a power law dependence on QD sizes with exponent 3.3±0.2. [2] The empirically obtained power-law dependence and wavelength dependence allow us to assign the two-photon excited final state to an exciton state. [3] The quadratic dependence of the two-photon excited PL intensities on excitation intensities indicates that the 2PA process is mediated by a virtual state. The 2PA cross-section shows a stronger size-dependence compared to the 1PA cross-section. Therefore, the larger QDs can be preferably excited under 2PA. We conclude that the size inhomogeneity is the reason for the red-shifted PL spectrum and the relatively slower PL decay with two-photon excitation. Furthermore, we simulated PL emission with 2PA and 1PA based on size-dependent PL emission and the NCs size distribution from TEM. The simulations agree well with the experimental results giving support to our conclusion that the differences between the excited state dynamics after 2PA and 1PA excitation in CsPbBr₃ QDs are due to the different size selection by the excitation under these two conditions. [4] Such size selection can be a general feature of the perovskite QDs and may be tuned via QDs size distribution to influence their performance within QD- based nonlinear optical materials and devices.

References

[1] M. V. Kovalenko, L. Protesescu, M. I. Bodnarchuk, Science 2017, 358, 745-750.

[2] J. Chen, K. Žídek, P. Chábera, et al., J. Phys. Chem. Lett., 2017, 8, 2316–2321

[3] S. C. Pu, M. J. Yang, C. C. Hsu, et al., Small, 2006, 2, 1308-1313

[4] J. Chen, P. Chábera, T. Pascher, et al., J. Phys. Chem. Lett., 2017, 8, 5119–5124.

Organo-metal halide perovskites (OMHPs) have demonstrated semiconducting properties of remarkably high quality. However, for further progress it is necessary to understand and control non-radiative decay processes, which depend strongly on fabrication and storage conditions. Here, we study non-radiative decay in methyl-ammonium lead iodide nanocrystals by employing methods of single molecule spectroscopy in a temperature range between room temperature and 77 K. Due to the small size of the nanocrystals (50-100 nm), the properties of individual non-radiative centers become apparent.

Looking at individual nanocrystals, we find highly diverse PL enhancement behavior upon cooling and only the ensemble-averaged trend reproduces a slope, which can be described by an Arrhenius-law, as often reported for OMHP films in the literature. From this we infer that the rate of non-radiative decay in OMHPs depends strongly on the local concentration of non-radiative centers, whereas the common practice of relating the PL intensity solely to the ratio between excitons and free charges seems to be oversimplified.

A certain fraction of very efficient luminescence quenchers (‘super traps’ [1,2]) leads to PL intensity fluctuations, also referred to as ‘blinking’, because of their ability to switch between a passive and an active state on time scales up to several seconds, while another fraction of quenchers is considered as persistent in time. Interestingly, blinking of the nanocrystals reduces remarkably upon cooling due to reduction of the relative switching amplitudes and decreasing switching rates. Thus, we assume that the PL enhancement has actually distinct origins: Reaching the quenchers at low temperatures via diffusion and/or capture of charges by non-radiative centers may be inhibited by thermal barriers, and also switching of the quencher from passive to the active state seems to require a certain amount of energy. We employ a simple model to simulate PL blinking in order to estimate the underlying switching times, which cannot be directly accessed in the experimental blinking transients, because they typically contain contributions of several super traps.

Overall, our study reveals, for the first time, a microscopic view on the luminescence quenching processes and the distinct phenomena contributing to the PL enhancement upon cooling. Moreover, the temperature-dependent study of PL blinking provides mechanistic insight into the switching behavior of super traps and will hopefully help to unravel their chemical nature in future work.

[1] Y. Tian et al., Nano Lett. 2015, 15, 1603

[2] A. Merdasa et al., ACS Nano 2017, 11, 5391

Organometal halide perovskites have attracted widespread attention as the most favorable prospective material for photovoltaic technology because
of their high photoinduced charge separation and carrier transport perfor- mance. However, the microstructural aspects within the organometal halide perovskite are still unknown, even though it belongs to a crystal system.

Here direct observation of the microstructure of the thin lm organometal halide perovskite using transmission electron microscopy is reported. Unlike previous reports claiming each phase of the organometal halide perovskite solely exists at a given temperature range, it is identi ed that the tetragonal and cubic phases coexist at room temperature, and it is con rmed that superlattices composed of a mixture of tetragonal and cubic phases are self- organized without a compositional change. The organometal halide perov- skite self-adjusts the con guration of phases and automatically organizes a buffer layer at boundaries by introducing a superlattice. This report shows the fundamental crystallographic information for the organometal halide perov- skite and demonstrates new possibilities as promising materials for various applications.

ABX₃ perovskite family mobilizes a strong interest in the photovoltaics field where optical properties plays an essential role. The ease to play from the solution side with the perovskite precursors or their integration with mesostructures brings a unique opportunity for tailor these properties which are meaningful for a myriad of optoelectronic design. (1)  ABX₃ perovskites can be also processed as colloidal suspension of nanocrystals in order to enhance their optoelectronic performance. However, many additional steps are needed to obtain stable nanocrystals layers.

In this work, we combine the solution process with the mesostructured templating to develop a method that allows us to synthesize, in a facile way, ABX₃ perovskite nanocrystals within the voids of inorganic porous structures.(2) These nanocrystals display features distinctive of quantum confinement, supported by an exhaustive structural and optical characterization. The synthetic approach consists in the use of the pores of optical quality inorganic films as nanoreactors to synthesize MAPbX₃ crystallites with narrow size distribution and average radius comprised between 1 nm and 4 nm. Nanocrystal size can be tuned by the template of choice or by the concentration of the precursors. That provides a material with a tunable electronic bandgap, which may be of interest for the development of tandem solar cells based on MAPbI3, which present superior photovoltaic performance than those formed by Br or Cl. Besides, the careful infiltration of the hybrid ABX₃ – MOx network with an elastomer allows the lift of the whole ensemble form the rigid substrate leading to a flexible and high emissive film (QY>50%) that can found applications in emission devices.(3) These results open interesting possibilities for the application of MAPbX₃ perovskite nanocrystals in optoelectronics as they show they can be synthesized within optical quality films and, due to confinement effects, become stable visible light emitting materials at the desired frequency range.

(1) M.E. Calvo J. Mater. Chem. A 2017, 5, 20561

(2) M. Anaya, A. Rubino, T.C. Rojas, J.F. Galisteo-López, M.E. Calvo, H. Míguez Adv. Opt. Mater. 2017, 5, 1601087

(3) A. Rubino et al. to be published 2018

Colloidal cesium lead halide quantum dot gives impact to perovskite research field with outstanding photophysical properties. Scientific approaches from both quantum dot and perovskite perspectives attracts energy-related researchers to study fundamental properties and to apply them in optoelectronic devices. Recent advances opened application of the CsPbX₃ quantum dots to photovoltaic devices with more than 12 % photoconversion efficiency, but its photo-induced charge transfer processes in devices is needed to be studied more. Herein we studied the light-induced charge generation and flow processes in the CsPbX₃ quantum dots with electron/hole transfer medium for photovoltaic device/LED applications. Interestingly, we found that the CsPbX₃ quantum dots based device can work as both solar cell and LEDs at the same moment. Interestingly, with different deposition condition or different composition, the charge generation and collection from CsPbX₃ can vary and so their performances can be enhanced as well. Through electrochemical impedance spectroscopic characterizations, we also tracked the light-driven processes.

Hybrid organic-inorganic and fully inorganic lead halide perovskite materials have the potential to revolutionize not only the photovoltaics industry but also lighting technologies. In particular, their nanostructures have energized the lighting communities’ efforts to create devices from earth abundant and inexpensive constituents due to their narrow size distributions as well as their narrow emission line widths, and high photoluminescence quantum yields (PLQY) of over 90%. Recently, fully inorganic perovskite nanocrystals (NCs), such as CsPbX3 (X = Cl, Br, I) NCs, have become particularly interesting for lighting applications due to their higher stability over hybrid organic-inorganic ones and efficient synthesis methods.
Here, we have synthesized fully inorganic CsPbX3 (X = Cl, Br, I) NCs using a hot injection synthesis route. We have characterized their structural, morphological and photophysical properties in detail. We have found that the purification of NCs is very important for the film formation, where we have observed rods and inhomogeneous coverage if no further purification steps were employed after the synthesis. We have implemented these NCs with high PLQYs into light emitting electrochemical cells (LECs), achieving a brightness of 8 cd/m2 for the NCs based on a mixture of bromide and iodide halides at low driving currents. Overall, we believe that fully inorganic perovskite NCs are promising components of lighting devices thanks to their outstanding performance.

PbS colloidal quantum dots (CQDs) have broad and strong light absorption spectrum covering the ultraviolent-visible-near infrared region, allowing use of very thin CQD solid films for CQD solar cells with high power conversion efficiency (PCE). Lightweight, flexible and semitransparent solar cells are very interesting for applications for example in spacecraft, aircraft and personal pack load. We have prepared an ultra-flexible and lightweight CQD solar cell constructed with a solution-processed Ag nanowire network with good mechanical properties as the front transparent and conductive electrode on a polyethylene naphthalate substrate with a thickness of only 1.3 µm. The thickness of the full CQD solar cell is less than 2 µm, and ~10 % PCE with a weight of 6.5 g m-2 is achieved. The flexible solar cell shows good mechanical properties that maintains high level photovoltaic performance under extreme deformation and highly durable repeated compression-stretching deformation. We have also prepared semitransparent CQD solar cells with a combination of high light transmission and high solar cell efficiency, which also can be interesting in many applications, for example windows in buildings. In the presentation we also show how the surface defects of the CQDs can be minimized using different surface shells of the CQDs, yielding highly efficient solar cells.

The development of robust and inexpensive semiconducting materials that operate at high efficiency are needed to make the direct solar-to-fuel energy conversion by photoelectrochemical cells economically viable. In this presentation our laboratory’s progress in the development new light absorbing materials and co-catalysts will be discussed along with the application toward overall solar water splitting tandem cells for H₂ production. Specifically, this talk will highlight recent results with the ternary oxides (CuFeO₂ and ZnFe₂O₄) 2D transition metal dichalcogenides, and organic (π-conjugated) semiconductors as solution-processed photoelectrodes.

With respect to ternary oxides, in our recent work [1,2] we demonstrate state-of-the-art photocurrent with optimized nanostructuring and address interfacial recombination by the electrochemical characterization of the surface states and attached co-catalysts.

In addition, we report an advance in the performance of solution processed two-dimensional (2-D) WSe₂ for high-efficiency solar water reduction by gaining insight into charge transport and recombination by varying the 2D flake size[3]and passivating defect sites[4].

Finally, with respect to π-conjugated organic semiconductors, in our recent work [5] we demonstrate a π-conjugated organic semiconductor for the sustained direct solar water oxidation reaction. Aspects of catalysis and charge-carrier separation/transport are discussed.

[1] Prevot, M. S.; Li, Y.; Guijarro, N.; Sivula, K. J. Mater. Chem. A 2016, 4, 3018-3026.

[2] Guijarro, N.; Bornoz, P.; Prevot, M.; Yu, X.; Zhu, X.; Johnson, M.; Jeanbourquin, X.; Le Formal, F.; Sivula, K., Sustainable Energy Fuels 2018, 2, 103-117.

[3] Yu, X.; Sivula, K., Chem. Mater. 2017, 29, 6863-6875.

[4] Yu, X.; Guijarro, N.; Johnson, M.; Sivula, K. Nano Lett. 2018, 18, 215-222.

[5] Bornoz, P.; Prévot, M. S.; Yu, X.; Guijarro, N.; Sivula, K. J. Am. Chem. Soc. 2015, 137, 15338.

Understanding and optimizing charge-transfer reactions in dye-sensitized solar cells under operational conditions is of crucial importance for further improvements in efficiency of these devices. In recent work we demonstrated that tandem redox mediator electrolytes, where tris(p-anisyl)amine (TPAA) is added in the standard cobalt tris(bipyridine) electrolyte, lead to higher efficiencies through an efficient cascade of electron transfer reactions.¹ Here, we explore the effects of fine-tuning the length of the alkoxyl chain of TPAA intermediates on solar cell performance and charge transfer kinetics. By combining the D-A-π-A organic dye AQ310 and a series of tandem electrolytes, efficiencies ranging from 9.7 % to 11 % were obtained, corresponding to an up to 50 % improvement compared to the same systems with bare cobalt tris(bipyridine) electrolyte. Notably, high open-circuit voltages of more than 1 V are obtained. Detailed charge transfer studies reveal significantly accelerated dye regeneration rate by the redox intermediate and slowed-down recombination kinetics, with a clear dependency on the length of alkoxyl chain. These results highlight the structural importance of redox intermediate for optimized charge-transfer in sensitized-semiconductor/electrolyte interface and pave the way to the further improvements in dye-sensitized solar cells.

Ref.  1: Hao, Y.; Yang, W.; Zhang, L.; Jiang, R.; Mijangos, E.; Saygili, Y.; Hammarström, L.; Hagfeldt, A.; Boschloo, G.  Nature Commun. 2016, 7, Art. No. 13934.

Redox mediators in dye sensitized solar cells (DSCs) or hole transport materials (HTMs) in solid state DSCs (ssDSCs) play a major role determining the photocurrent and the photovoltage. The driving force for dye regeneration with the redox mediator should be sufficiently low to provide high photovoltages. With the introduction of new copper complexes as promising redox mediators or HTMs in DSCs both criteria are satisfied to enhance power conversion efficiencies and stability. The high photovoltages of over 1.0 V were achieved by the series of copper complex based redox mediators without compromising photocurrent densities. The solar-to-electrical power conversion efficiencies for [Cu(tmby)₂]^2+/1+, [Cu(dmby)₂]^2+/1+ and [Cu(dmp)₂]^2+/1+ based electrolytes were 10.3%, 10.0% and 10.3%, respectively, using the organic Y123 dye under AM1.5G illumination.¹ Solar cells that operate efficiently under indoor lighting are of great practical interest as they can serve as electric power sources for portable electronics and devices for wireless sensor networks or the Internet of Things. Our photosystem combines two judiciously designed sensitizers, coded D35 and XY1, with the copper complex Cu(II/I)(tmby) as a redox shuttle (tmby, 4,4′,6,6′-tetramethyl-2,2′-bipyridine), and features a high open-circuit photovoltage of 1.1 V. The DSC achieves an external quantum efficiency for photocurrent generation that exceeds 90% across the whole visible domain from 400 to 650 nm, and achieves power outputs of 15.6 and 88.5 μW cm^–2 at 200 and 1,000 lux, respectively, under illumination from a model Osram 930 warm-white fluorescent light tube. This translates into a PCE of 28.9%.^2,3

1) Saygili, Y.; Söderberg, M.; Pellet, N.; Giordano, F.; Cao, Y.; Munoz-Garcia, A. B.; Zakeeruddin, S. M.; Vlachopoulos, N.; Pavone, M.; Boschloo, G.; Kavan, L.; Moser, J.-E.; Grätzel, M.; Hagfeldt, A.; Freitag, M. J. Am. Chem. Soc. 2016.

2) Freitag, M.; Daniel, Q.; Pazoki, M.; Sveinbjornsson, K.; Zhang, J.; Sun, L.; Hagfeldt, A.; Boschloo, G. Energy Environ. Sci., 2015, 8, 2634–2637.

3) Freitag, M.; Teuscher, D. J.; Saygili, Y.; Zhang, D. X.; Giordano, D. F.; Liska, D. P.; Hua, P. J.; Zakeeruddin, S. M.; Moser, J.-E.; Grätzel, M.; Hagfeldt, A. Dye-Sensitized Solar Cell for Efficient Power Generation under Ambient Lighting. Nature Photonics, 2017,  11, 372–378.

To further improve the energy conversion efficiency of solar cells, one of the most efficient strategy is constructing hybrid or tandem structures by using absorbers with different absorption bands.

We have developed a novel hybrid multilayered structure with different dyes for Dye-sensitized solar cell (DSC). The panchromatic response, superposition of J_SC and conversion efficiency were achieved. The efficiency of 11.05% was improved by 37.6%, 21.0%, and 12.1% compared with tandem DSC, co-sensitized DSC and layered adsorption methods under similar conditions. We also designed several kinds of tandem DSCs with the superposed J_SC and conversion efficiency. The Near-IR dyes with strong and broad Near-IR absorptions were developed for hybrid/tandem structures. Furthermore, we also made our efforts on green synthesis of new ionic liquid materials for high-efficient DSCs and perovskite solar cells (PSCs).

References

[1] B. O’Regan, M. Grätzel, Nature, 1991, 353:737.

[2] Q. Miao, L. Wu, J. Cui, M. Huang, T. Ma, Adv. Mater., 2011, 23:2764-2768.  

[3] Q. Miao, S. Zhang, J. Mater. Chem. A, 2017, 5:14630-14638.       

[4] Q. Miao, S. Zhang, ACS Appl. Mater. Interfaces, 2017, 9(50):44199-44213.

[5] Q. Miao, S. Zhang, et al., Chem. Commun., 2013, 49, 6980-6982.

[6] S. Zhang, J. Sun, X. Zhang, J. Xin, Q. Miao, J. Wang, Chem. Soc. Rev., 2014, 43, 7838-7869.

[7] Q. Miao, S. Zhang, Adv. Func. Mater., 2017, Under Review.     

[8] Q. Miao, J. Gao, Z. Wang, H. Yu, Y. Luo, T. Ma, Inorg.Chim. Acta, 2011,376: 619-627.

[9] Q. Miao, M. Wu, W. Guo, T. Ma, Front.Optoelectron., 2011, 4(1): 103-107.

[10] W. Guo, Q. Miao, G. Xin, L. Wu, T. Ma, Key Eng. Mater., 2011, 451:21-27.

[11] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050-6051.

[12] H. Kim, C. Lee, J. Im, K. Lee, T. Moehl, A. Marchioro, S. Moon, R. Humphry-Baker, J. Yum, J. E. Moser, M. Grätzel, N. Park, Sci. Rep., 2012, 2:591.

[13] M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. Snaith, Science, 2012, 338:643.

Cu(I)/(II) oxazoline-bipyridine complexes as highly-stable tetradendate redox mediators

Hannes Michaels, Marina Freitag, Gerrit Boschloo
Department of Chemistry, Ångström Laboratory, Uppsala University, Sweden

In the past years, several metal complexes redox electrolytes for dye-sensitized solar cells based on Cu as well as Co have been reported.^1,2 These complexes, however, are often subject to conformational change upon charging/discharging.

In this study, we investigate a new tetradendate Cu(I)/(II) oxazoline-bipyridine complex. The connection of all four coordinating nitrogen atoms into one ligand molecule minimizes coordinational reorganization energies upon oxidation state change and leads to extraordinary stability of these complexes. We successfully prepare dye-sensitized solar cells using the Y123 dye as photoabsorber with conversion efficiencies exceeding 8% at 1000 W m^‑2 AM1.5G.

(1)       Freitag, M.; Giordano, F.; Yang, W.; Pazoki, M.; Hao, Y.; Zietz, B.; Grätzel, M.; Hagfeldt, A.; Boschloo, G. J. Phys. Chem. C 2016, 120 (18), 9595–9603.

(2)       Saygili, Y.; Söderberg, M.; Pellet, N.; Giordano, F.; Cao, Y.; Munoz-García, A. B.; Zakeeruddin, S. M.; Vlachopoulos, N.; Pavone, M.; Boschloo, G.; Kavan, L.; Moser, J. E.; Grätzel, M.; Hagfeldt, A.; Freitag, M. J. Am. Chem. Soc. 2016, 138 (45), 15087–15096.

Molecular hydrogen produced via solar energy is emerging as a prominent way to convert and store the conspicuous, yet intermittent, amount of energy that the Sun daily irradiates on Earth. Hybrid Organic photoelectrochemical (HOPEC) water splitting is gaining momentum in this field, benefiting of the organic semiconductors properties over their inorganic counterpart. Their low cost, stability and ease of large-area production help overcoming the limitations of standard photoelectrochemical water splitting. The potential of hybrid organic systems has been proven by our previous works¹. Indeed, excellent photocurrent performances² or extended operational lifetime³ have been obtained through careful optimization of device architecture.

Our latest advancements aim to the application of this technology and to the validation of materials which can reduce the fabrication costs and ease the scalability. Innovative photocathodes architectures employing a small molecule-based hole selective layer (HSL) and exploiting the properties of advanced organic semiconductors in the bulk-heterojunction (BHJ) are herein presented. Selective layers play a relevant role in exploiting the performances of a given donor-acceptor blend. Given its suitable energetic alignment and excellent hole mobility, copper phthalocyanine (CuPC) fits as a promising candidate for this application⁴. Electrochemical tests were carried out to assess the stability of this material, which was then successfully employed as hole selective contact in combination with the P3HT:PCBM BHJ.

Furthermore, our team investigated the photoelectrochemical behaviour of materials which are currently obtaining excellent results in the world of organic photovoltaic (OPV). Among them we can list the high-performance photoabsorbers PCE11 and PCDTBT, and the non-fullerene acceptors IDTBR and IDFBR, which were found to be responsible of a sharp increase in the open circuit voltage in OPV devices⁵. A careful electrochemical characterization is performed on each material in half-devices configuration and, finally, in an optimized photocathode architecture.

To test our devices in a complete water-splitting system we finally coupled our state-of-the-art photocathode² with a high-performing perovskite⁶ in a tandem configuration. The resulting HOPEC-PV system successfully performs the full water splitting reaction without the application of any external bias with a photocurrent density well above the 1mA/cm² threshold.

With this contribution we want to stress the potential of this field, encouraging the research of dedicated materials for this peculiar application.

1.Fumagalli,F. J.MaterChem.A,4, 2178(2016)

2.Rojas,H. EnergyEnviron.Sci.,9, 3710-3723(2016)

3.Mezzetti,A. FaradayDiscuss.,198, 433-448(2017)

4.Thalluri,G.K.V.V. Dalt.Trans. 41, 11419(2012)

5.Baran,D. EnergyEnviron.Sci. 9, 3783–3793(2016)

6.Tao,C. Adv.Mater. 29, 1–7(2017)

Solar-driven hydrogen production via water splitting is a promising technology for a future solar fuel economy. The development of efficient, stable, economical and abundant p-type semiconductor materials is essential, and one of the principal challenges is to improve the solar-to-hydrogen conversion efficiency, which is generally limited by either a low light harvesting efficiency (high band gap) or an unfavorable kinetic balance of slow charge transfer to the solution and fast recombination. In this work, combinatorial chemistry has been employed to select a Cu-Bi ternary oxide that could be a suitable option for a solar water splitting system. CuBi₂O₄ photoelectrodes have been deposited by inkjet printing and the photoelectrochemical properties have been studied in detail using intensity-modulated photocurrent spectroscopy (IMPS). In the current –potential curves, a steady-state photocurrent corresponding to water reduction at p-type CuBi₂O₄ is observed, with an onset at 0.75 V vs RHE; the maximum current density obtained for a 280 nm film at 0.2 V vs. RHE was 0.12 mA cm^-2. In the hydrogen photogeneration potential range, IMPS illustrates that the recombination rate constant is larger than that corresponding to electron transfer to the solution, resulting in a relative transfer efficiency between 0.2 - 0.4, explaining the relatively low photocurrent. The results illustrate the promise of this p-type oxide for application in a tandem solar water splitting device, and strategies to improve the efficiency are discussed. IMPS analysis illustrate that at sufficiently positive applied potential (> 0.8 V vs RHE), the CuBi₂O₄ response is characteristic of an n-type semiconductor showing a photo-oxidation process, however, the rate constant for hole transfer to the solution is small resulting in a negligible steady state anodic photocurrent.

Overcoming the limitations of redox mediator mass transport and electron back recombination, within the mesoporous TiO2 photoelectrode, remains a fundamental challenge for ongoing dye-sensitised solar cell (DSSC) research. This is even more critical for the engineering of commercially viable and long-term stable DSSCs, which inevitably requires more robust and viscous electrolytes that tend to suffer from significant mass transport issues. Our work provides new insights into the recombination kinetics inside the TiO2 photoelectrode, and presents useful strategies for mitigating electron recombination. We developed a simple yet powerful method to investigate the spatial recombination kinetics across the cross-section of the TiO2 photoelectrode, by simply measuring photocurrent transients as a function of the photogeneration profile. Essentially, non-uniform light absorption across the photoelectrode is exploited in order to study the relative spatial photogeneration profile of oxidised redox mediators inside the TiO2. Using this approach, we show that accumulation of the redox mediator can gradually occur inside the mesoporous photoelectrode, which can result in non-linear recombination kinetics and J-V hysteresis. The novel findings in our work should be broadly relevant to ongoing DSSC and related photoelectrochemical solar cell research.

Dye-sensitized solar cells (DSCs) offer an attractive option as a third-generation photovoltaic technology due to their high-power conversion efficiency (PCE) and potential for low-cost power production as well as indoor (diffuse light) applications. To date, PCEs over 12% have been achieved by using organic donor-(π-linker)-acceptor (D-π-A) type of dyes as photosensitizers and Co(II/III) complexes as redox shuttles in liquid electrolytes. However, liquid electrolyte-based DSCs represent a risk of leakage problems associated with the volatile nature of a liquid solvent, which significantly impedes this type of solar cell for large-scale commercial applications. In order to resolve this issue, solid-state p-type organic hole-transport materials (HTMs), such as 2,2’,7,7’-tetrakis-(N,N-di-p-methoxyphenyl-amine)9,9’ spiro-bifluorene (Spiro-OMeTAD), was introduced as hole conductor to replace the liquid electrolyte in all-solid-state dye-sensitized solar cells (sDSC) by Bach et al. in 1998. Up to now, a record efficiency of 7.7% have been reported by using the organic dye LEG4, characterized by a high molar extinction coefficient, as the absorber and 1,1,2,2-tetracholoroethane (TeCA) doped Spiro-OMeTAD as the HTM. However, the efficiencies of all sDSCs are still far below the PCEs of liquid electrolyte-based DSCs. To the main reason is that the active nanoporous TiO2 layer of an all sDSC has been limited to less than 2.5 μm, resulting in incomplete light harvesting and consequently a low photocurrent of the devices. Therefore, the development of novel photosensitizers with very high molar extinction coefficients and broad absorption spectra to enhance the light harvesting efficiency rendering high PCEs in all sDSCs is a main target for improvement. In this talk, I will introduce our recent progress on the development of new organic photosensitizers for high performance all sDSCs.

The replacement of fossil fuels by a clean and renewable energy source is one of the mostn urgent and challenging issues our society is facing today, which is why intense research is devoted to this topic recently. Nature has been using sunlight as the primary energy input to oxidize water and generate carbohydrates (a solar fuel) for over a billion years. Inspired, but not constrained, by nature, artificial systems [1] can be designed to capture light and oxidize water and reduce protons or other organic compounds to generate useful chemical fuels. In this context this contribution will present how molecular water oxidation catalysts can be anchored on solid supports to generate powerful hybrid electro- and photo-anodes for water splitting. [2]

[1] Berardi, S.; Drouet, S.; Francàs, L.; Gimbert-Suriñach, C.; Guttentag, M.; Richmond, C.; Stoll, T.; Llobet, A. Chem. Soc. Rev., 2014, 43, 7501-7519.

[2] (a) Matheu, R.; Ertem, M.Z.; Benet-Buchholz, J.; Coronado, E.; Batista, V. S.; Sala, X.; Llobet, A. J. Am. Chem. Soc. 2015, 137, 10786-10795. (b) Creus, J.; Matheu, R.; Peñafiel, I.; Moonshiram, D.; Blondeau, P.; Benet-Buchholz, J.; García-Antón, J.; Sala, X.; Godard, C.; Llobet, A. Angew. Chem. Int. Ed. 2016, 55, 15382-15386. (c) Matheu, R.; Moreno-Hernández, I.A.; Sala, X.; Gray, H.B.; Brunschwig, B.S.; Llobet, A; Lewis, N.S. J. Am. Chem. Soc. 2017, 139, 11345-11348

Electronic processes at the hetero-interfaces between electron donating and electron accepting molecules determine the photocurrent, photovoltage and ultimately, the power conversion efficiency of organic solar cells. While high incident-photon-to-extracted-charge conversion yields of over 85%, and absorbed photon-to-extracted-charge conversion yields of 90-100% have been achieved, the difference between the optical gap of main absorber and open-circuit voltage (Voc) is much larger than for inorganic and perovskite based solar cells. The main improvements of the Voc of organic solar cells have so far been made by tailoring donor-acceptor interfacial energetics, taking advantage of well-known principles of molecular design. Nevertheless, for most material systems we consistently find a large, almost constant difference (~0.6 eV) between eVoc and the energy of the intermolecular charge transfer (CT) state, ECT. Added to this, electron transfer losses are usually larger than 0.1 eV, resulting in overall voltage losses, often much larger than 0.7 eV. In this contribution, we will discuss the influence of molecular properties, such as the electronic coupling between electron donor and acceptor, the molecular reorganization energy as well as non-radiative triplet states on the free carrier recombination and the Voc. Furthermore, we show that, by reducing the physical interfacial area available for free charge carrier recombination, and by changes in the standard organic photovoltaic device architecture, it is possible to reduce the overall voltage losses to values below 0.6 eV. However, we further find that a fundamental lower limit of these voltage losses is the result of coupling of the CT state to high frequency molecular vibrations of the ground state. This has important implications for the efficiency upper limits of organic photovoltaics.

Organic solar cell (OSC) technology has attracted much attention due to its promise as low-cost conversion of solar energy. Despite recent progress, several limitations are holding back OSC development. For instance, best-efficiency OSCs are mostly based on relatively thin (100 nm) active layers. Thick-film OSCs generally exhibit lower fill factors and efficiencies compared to the best thin-film OSCs. Here we report multiple cases of high-performance thick-film (300 nm) OSCs (efficiencies up to 11.7%, fill factors up to 77%). Our simple temperature dependent aggregation control and materials design rules allowed us to develop, within a short time, over twenty polymer:fullerene combinations, all of which yielded higher efficiency than previous state of art devices (~10%). The common structural feature of the three new donor polymers, the 2-octyldodecyl (2OD) alkyl chains sitting on quaterthiophene, causes a temperature-dependent aggregation behavior that allows for the processing of the polymer solutions at moderately elevated temperature, and more importantly, controlled aggregation and strong crystallization of the polymer during the film cooling and drying process. This results in a well-controlled and near-ideal polymer:fullerene morphology (containing highly crystalline, preferentially orientated, yet small polymer domains) that is controlled by polymer aggregation during warm casting and thus insensitive to the choice of fullerenes.

This approach can be applied to non-fullerene OSCs. The energy loss from the optical bandgap (Egap) to the open-circuit voltage (Voc) of a solar cell is a simple measure of its effectiveness in generating voltage. For best-efficiency (>10%) organic solar cells (OSCs), the energy loss is typically in the range of 0.85-0.9 eV, while the loss is only 0.4-0.55 eV for other more efficient solar cell systems. High energy loss is one key factor that limits the performance of OSCs. In this paper, we report efficient (9.5%) OSCs with a high Voc of 1.11 V, despite a relatively narrow optical bandgap of 1.66 eV for the absorber. Importantly, the high efficiency and low energy loss were achieved without using the conventional fullerene acceptors, which have dominated OSCs for nearly two decades. The origin of the small energy loss of our non-fullerene OSCs can be attributed to two factors. First, our OSC exhibits a high electroluminescence (EL) quantum efficiency that is comparable to those of inorganic solar cells and that helps to reduce non-radiative recombination loss.

The efficiency of photovoltaic devices could be improved beyond the Shockley-Queisser limit if it were possible to convert higher-energy photons in the solar spectrum into two bandgap-energy photons. The process of singlet exciton fission in organic semiconductors is a promising route to achieve this, but the challenge is to achieve photon emission from both of the triplet excitons generated following photon absorption. Triplet energy transfer into emissive lead sulfide nanoparticles has been demonstrated, with the potential to achieve a “photon multiplier film” than could be applied to the front surface of a silicon solar cell. I will present recent progress in this area, aiming to achieve highly luminescent nanoparticles whilst still allowing triplet excitons to tunnel easily onto the particles from a surrounding organic singlet fission material.  I will also present new results studying luminescence under high magnetic fields to understand the interactions between triplet excitons generated by singlet fission.

Semiconducting metal-halide perovskites present various types of chemical interactions which give them a characteristic fluctuating structure sensitive to the operating conditions of the device, to which they adjust. This makes the control of structure-properties relationship, especially at interfaces where the device realizes its function, the crucial step in order to control devices operation. In particular, given their simple processability at relatively low temperature, one can expect an intrinsic level of structural/chemical disorder of the semiconductor which results in the formation of defects.

Here, first I will present our results on the role of structural and point defects in determining the nature and dynamic of photo-carriers in metal-halide perovskites. Then, I will discuss our understanding of key parameters which must be taken into consideration in order to evaluate the suscettibility of the perovkite crystals (2D and 3D) to the formation of defects, allowing one to proceed through a predictive synthetic procedure. Finally, I will show the correlation between the presence/formation of defects and the observed semiconductor instabilities/degradation.

An outstanding property of lead-halide perovskites is the incredibly low band-gap to open circuit voltage loss, which is optimized devices is close to the thermodynamic limit (1.6 eV gap; 1.3 V voltage, loss ~0.3 V). The high open circuit voltage is extremely attractive for both photovoltaics and water splitting, since it requires no more than two series connected cells to operate water electrolysis. These observations suggest an apparently low density of traps in MAPbI3, contrary to the expectedly high defect density of a low-temperature, solution-processed material, suggesting that metal halide perovskites are inherently defect tolerant due to dominant defects introducing only shallow traps in the material band-gap.[1] A mechanism for the protection of charge carriers implying large polarons has also been invoked to explain the long carrier lifetimes and diffusion lengths favoring efficient carrier collection at selective contacts, leading to power conversion efficiencies competing with established thin film photovoltaics. [2]

We present on overview of first-principles computational analyses devoted to understanding the outstanding optoelectronic properties of lead-halide perovskites. We show that despite the fairly high defect density due to lead and MA-related defects, only less abundant iodine defects introduce deep electron and hole traps in MAPbI3.[3, 4] The peculiar iodine redox chemistry leads, however, to kinetic deactivation of filled electron traps, leaving short-lived hole traps as potentially harmful defects. Hole traps can be eventually converted to electron traps under mild oxidizing conditions, clarifying the defect tolerance. A polaronic mechanism, triggered by a photoinduced structural deformation, is presented which is also responsible for the reduced electron/hole recombination observed in lead-halide perovskites. [2,5]

The two ingredients, intimately related to the constituting lead-halide chemistry, represent the key to the success of perovskite-based PV and can represent the basis for development of new materials with similar target characteristics, possibly avoiding the environmental risks posed by lead.

References

1) W. J. Yin et al. Appl. Phys. Lett. 2014, 104, 063903.

2) K. Miyata et al. Sci. Adv. 2017, 3, e1701217.

3) D. Meggiolaro et al. ACS Energy Lett., 2017, 2, 2794.

4) E. Mosconi et al. Energy Environ. Sci. 2016, 9, 3180.

5) F. Ambrosio et al. Energy Environ. Sci., 2018, DOI: 10.1039/C7EE01981E.

We invite you to know a little bit more about the next HOPV Conference edition that will be held in Rome, Italy on May 2019.

Prof De Angelis, as organizer, will introduce the main topics and invited speakers already confirmed and will encourage all the assistant to join us again for a new amazing edition in Rome.

You can check all the information on coming edition at the Conference website and you can also contact the Conference Secretariat at hopv19@nanoge.org for further information.

We look very much forward to seeing you in Italy in a year time!

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Organic-inorganic perovskites are quickly overrunning research activities in new materials for cost-effective and high-efficiency photovoltaic technologies.  Since the first demonstration from Kojima and co-workers in 2009, several perovskite-based solar cells have been reported and certified with rapidly improving power conversion efficiency.  Recent reports demonstrate that perovskites can compete with the most efficient inorganic materials, while they still allow processing from solution as a potential advantage to deliver a cost-effective solar technology.

Compare to the impressive progress in power conversion efficiency, stability studies are rather poor and often controversial.  An intrinsic complication comes from the fact that the stability of perovskite solar cells is strongly affected by any small difference in the device architecture, preparation procedure, materials composition and testing procedure.

In the present talk, we will focus on the stability of perovskite solar cells in working condition.  We will discuss a measuring protocol to extract reliable and reproducible ageing data.  We will present new materials and preparation procedures, which improve the device lifetime without giving up on high power conversion efficiency.

Oxygen is an extremely common contaminant in perovskite solar cells (PSCs), which can be introduced in the halide perovskite layer both during synthesis and/or during device operation through simple exposure to air. The effect of oxygen has been reported to be beneficial for the optical properties of halide perovskites,[1, 2] but also strongly detrimental for device performance and stability.[3-5] Given the ubiquity of O₂ and its significant effects on halide perovskites and related devices, it is of utmost importance to study its interaction with these materials. In this contribution, we investigate the consequences of oxygen exposure on the stability and transport properties of halide perovskites, focusing on methylammonium lead iodide. This material is thermodynamically expected to be unstable against O₂, and severe degradation is indeed experimentally observed, but solely under illumination.  In the dark, a sluggish surface reaction kinetics keeps the material metastable, as revealed by ¹⁸O tracer diffusion experiments. Light removes these kinetic hindrances, resulting in an accelerated O₂ incorporation that ultimately leads to degradation. Remarkably, this accelerated incorporation (in conditions that precede degradation) can also greatly alter the electronic and ionic transport properties of the material in a way that is analogous to acceptor doping. Since state-of-the-art solar cell devices use perovskite formulations containing multiple A-site cations and halide ions, as a final step we analyze the impact of cation and anion mixing on the O₂-degradation kinetics.

References:

[1] Y. Tian, M. Peter, E. Unger et al., Phys. Chem. Chem. Phys. 2015, 17, 24978.

[2] J. F. Galisteo-López, M. Anaya, M. E. Calvo et al., J. Phys. Chem. Lett. 2015, 2200.

[3] N. Aristidou, I. Sanchez-Molina, T. Chotchuangchutchaval et al., Angew. Chemie 2015, 54, 8208.

[4] N. Aristidou, C. Eames, I. Sanchez-Molina et al., Nat. Commun. 2017, 8, 1–40.

[5] A. J. Pearson, G. E. Eperon, P. E. Hopkinson et al., Adv. Energy Mater. 2016, 6, 1600014.

Organometal halide perovskites are a class of photovoltaic (PV) materials which have fascinated PV community for demonstrating outstandingly high power conversion efficiencies (PCE) comparable to established PV technologies. Despite their high PCEs, perovskite solar cells (PSCs) are yet not a marketable product. This is due to their low stability towards the key stress factors of solar modules in outdoor installations like moisture, oxygen, and light exposure; especially the ultraviolet (UV) light. Although there is consensus on the general problem of low stability, yet the existing studies on photostability of PSCs are not conclusive. Neither the origin of UV degradation is fully understood, nor the harmful UV spectral range and intensities are well-known.

In this work, we systematically study the impact of different UV spectra, in order to shade light on the inconclusive literature and define the harmful UV spectrum. For this purpose, we consider two UV spectra: (i) 310-316 nm (ii) 360-380 nm and stress various device architectures based on spin coated methylammonium lead iodide (CH₃NH₃PbI₃) as absorber layer with various electron transport layers (ETLs). We show that only the deep UV wavelengths (310-316 nm) are responsible for degradation in PSCs and this finding is consistent for all four ETLs (i.e. (i) SnO₂ (ii) compact-TiO₂ (iii) electron-beam TiO₂ and (iv) nanoparticles-TiO₂) used in this study. This instability is triggered as perovskite absorber material undergoes decomposition after absorbing UV photons, resulting in diminished photocurrent and the low stabilized PCEs. This observation is independent of the intensity of UV radiation as low intensities (10%) showed same trend in the degradation behaviour. Thus, we assume that it occurs independent of the magnitude of UV light intensity.

Different remedies are proposed to prevent UV triggered degradation of PSCs. One strategy is to cut-off the UV wavelengths which are primarily responsible for degradation. However, this comes at a cost of fractional loss in photocurrent. The other scheme is to use luminescent downshifting (LDS) layers to regain about half of the lost photocurrent as well in addition of blocking the harmful UV content. Potential loss in short-circuit current density (J_SC) by eliminating harmful UV photons and amount of recovered J_SC in result of downshifting is also presented in this work.

Synoptically, the deep UV photons degrade the PSCs irrespective of whichever ETL and light intensity we use.

Organic-inorganic metal halide hybrid perovskites in solar cells, or simply “perovskite solar cells (PeSCs)” are poised to revolutionize the photovoltaics industry. Record power conversion efficiency (above 22% in the laboratory) achieved by exploiting the key processing merits of perovskites – their inherent solution-processability and low-temperature sintering requirements – promises a disruptive breakthrough by lowering the cost of energy output. The challenge now lies in maintaining the high efficiencies achieved in the laboratory while advancing the core processing merits of perovskites through an industry-relevant scalable manufacturing method and environment. Most laboratory cells are processed in an inert atmosphere and make the use of spin-coating methods for the deposition of the numerous functional layers comprised in the solar cell. Both practices represent cost bottlenecks to delivering the goal of cheap power: spin coater is not scalable to a continuous processing scheme while maintaining inert environment in industrial-scale manufacturing would be costly as well as limiting to operational-freedom. Fast and continuous manufacturing making use of scalable printing and coating methods in an ambient environment combined with low temperature drying steps epitomizes the ideal low-cost high-throughput production method for harnessing perovskites’ full potential. Depending on the intended application, both sheet-to-sheet fabrication on glass and roll-to-roll fabrication on flexible substrates can be envisioned. A planar device configuration in which the perovskite film is deposited in a one-step process in combination with the use of an anti-solvent or gas-assisted quenching remains the simplest and a cost-effective method. These methods are arguably the most commonly used approaches to generate uniform, pinhole-free perovskite films in the laboratory. In this talk, we will present our experience in translating gas and antisolvent-assisted perovskite film formations to industry-relevant scalable fabrication methods for the fabrication of one-step planar devices. Both laboratory batch-to-batch processing on glass and roll-to-roll on flexible substrate with the use of scalable coating methods under ambient conditions will be discussed.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a frequently used as hole transport layer in planar p-i-n perovskite solar cells. We show that processing of a metal halide perovskite layer on top of PEDOT:PSS via spin coating of a precursor solution chemically reduces the oxidation state of PEDOT:PSS. The partial reduction of PEDOT:PSS from the highly oxidized bipolaron state to the polaron state, reduces the work function of the PEDOT:PSS whereby the work function becomes equal to the ionization potential. This reduction in the work function of the PEDOT:PSS also reduces the work function of the perovskite layer that is positioned on top of it. As a consequence, the solar cells display inferior performance with a reduced open-circuit voltage and a reduced short-circuit current density. Additionally, the increase of current density with light intensity becomes more sublinear. The reduced PEDOT:PSS can be (partially) re-oxidized even by short (8 min.) exposure to oxygen during thermal annealing of the PEDOT:PSS/perovskite layer stak, restoring its functionality in the solar cell. Therefore, annealing the PEDOT:PSS/Perovskite stack in the presence of oxygen results in solar cells with increased open-circuit voltage, short-circuit current density and high efficiency. Additionally, we show that the bulk properties of the perovskite layer do not change by annealing in different atmospheres.

Lead halide perovskites have developed from the initially used methylammonium lead iodide (MAPbI₃) to structures containing anion (e.g. Br^- and I^-) and cation (e.g. formamidinium (FA), MA, Cs⁺ and Rb⁺) mixtures. Many aspects related to these materials still need to be understood including the materials’ photo-stability. Processes such as ion migration are likely to play a role in the photon to electron conversion mechanism and for the long-term stability. It is therefore of high importance to understand how the exact composition of the perovskite influences these properties.

We have recently developed a photoelectron spectroscopy-based methodology in which the chemical changes at the surface of the perovskite can be studied under visible light illumination [1]. These measurements are carried out at the LowDose PES beamline at the synchrotron Bessy II, where a X-ray low photon flux is combined with a highly efficient spectrometer. This enables us to study the electronic structure of the perovskite surface without any X-ray degradation. In our first study, we demonstrated that ion movement of halides can be observed through intensity changes of the halides at the surface for an MAFA-based perovskite during visible laser illumination [1]. Furthermore, we observed the reduction of a fraction of Pb²⁺ at the perovskite surface to Pb⁰, which was partially reversible in the dark.

We now have used the same methodology to study photo-induced processes in perovskites with different compositions including different I/Br ratios and Cs ions. This presentation will summarise the results of this study. We show that the presence of Cs ions can lead to a strong suppression of halide ion migration at high I/Br ratios, while other aspects of the photo-stability of the perovskite can be enhanced independent of I/Br ratio. Our results give therefore an indication of the mechanism with which Cs ions enhance the stability of perovskite solar cells.

[1] Ute B. Cappel, et al. ACS Appl. Mater. Interfaces 9, 34970-34978 (2017).

Perovskite solar cells based on a triple mesoporous stack (titania scaffold, insulating layer, carbon-based electrode), represent likely the most cost-efficient option amongst the possible architecture/materials combinations for this technology [1]. This device structure is endowed with appealing features for up-scaling and commercialization: all layers are printable [2], large modules have been already demonstrated [3], the perovskite solution can be infiltrated throughout the stack via scalable processes, e.g. ink jet printing [4] and robotic mesh [5], over 1000 hours stability has been reported both indoor (1 sun, AM1.5) [2] and outdoor [6], when 5-AVAI (5-ammonium valeric acid iodide) is added to the perovskite solution.

We report the outputs of an inter-lab stability experiment on our AVA-MAPI carbon cells - which involved several partners, within the StableNextSol COST action’s frame, and the application of the ISOS protocols - highlighting both the promising and the disappointing results [7]:

ISOS-D1, D2: over 1000 hours stability;

ISOS-O1 (Barcelona and Malta): 30 days stability;

ISOS-L1, L2 at open circuit: over 50% performance drop in less than 10 hours;

ISOS-L1 at maximum power point: drop to 80% of initial efficiency value (T₈₀) after 79 hours.

Aiming to unfold the dynamics behind the poor stability under light, we investigated the composition of the precursors solution, ie PbI₂ to organic halides ratio and AVAI to MAI ratio, and how even slightly changes can affect the solution stability, the infiltration through the stack, the crystallisation of the perovskite within the pores, the formation of PbI₂ even in un-aged cells, which, on one side, can improve the devices’ performance, acting as a passivation layer, while, on the other side, can reduce the light stability in the presence of oxygen.

References

1            H. Chen, et al., Adv. Mat., 2017, 29 (24), 1603994.

2            A. Mei, et al., Science, 2014, 345 (6194), 295–298.

3            A. Priyadarshi, et al., Energy Environ. Sci., 2016, 9, 3687–3692.

4            S. G. Hashmi, et al., Adv. Mater. Technol., 2016, 2(1), 1600183.

5            S. Meroni, et al., Science and Technol. of Adv. Mat., 2018, 19(1), 1-9.

6            X. Li, et al., Energy Technol., 2015, 3, 551–555.

7            F. De Rossi, et al., manuscript in preparation.

Among the recent developments in perovskite solar cells, one thing that has been promient and promising is simultaneous improvement in efficiency and stability of the cells, which has been accomplished by essentially improving the structural (intrinsic) stability of the perovskites through mixing different cations and/or anions in the perovskite structure. However, the overall stability of cells depend on other layers like hole transport layer, electron transport layer and their interfaces with the perovsktie. In fact, in a recent study on thermal stability of regular MAPbI₃ solar cells, we found that spiro-OMeTAD plays a notorious role in performance deterioration of the cells at elevated temperature (60, 80, 100 and 120 ^oC). It seems that, not the degradation of MAPbI₃ (trace of PbI₂) but some physical/chemical alteration at the perovskite/spiro-OMeTAD interface is the main reason for performance degradation in the MAPbI₃ cells, because performance decreased only when the MAPbI₃ films with spiro-OMeTAD on its top were heated at different temepratures while it remained unchanged when the MAPbI₃ films wihtout the spiro-OMeTAD were heated at same conditions (slight degradation to PbI₂).  Having found this, we then explored the effects of surface modification of MAPbI₃ with cation precursors like FAI, CsI, MAI, MABr etc.  on thermal stability of the MAPbI₃ cells. As a result of such surface modification, thermal stability of the cells improved significantly. For instance, cells made with FAI-treated MAPbI₃ films showed much less degradation in performance as comapred to pure MAPbI₃ and MAI-treated MAPbI₃ cells when they were heated at 80, 100 and 120 ^oC for 1 h. Indeed, MAI-treated MAPbI₃ cells were found to be worse in comparison to regular MAPbI₃ cells, which implies certain involvement of MA⁺ ions in altering the interface between MAPbI₃ and spiro-OMeTAD. In addtion, effect of exposure of the perovskite films surface to different environments like O₂, N₂ and vacuum on performace and stability was investigated. The results also show reasonbale influence of such post-treatment of MAPbI₃ surface on performance and long term stability of the cells.

Lead halide based perovskites have emerged as promising active materials for photovoltaic cells. Enormous efforts have been devoted to device fabrication and optimization leading to power conversion efficiencies exceeding 22%, which gives perovskite solar cells the competitive advantage over many other well-known solar technologies. Despite superb efficiencies achieved in laboratory-scale devices, it was soon recognized that long-term stability was rapidly compromised under ambient conditions and such instability could jeopardize the future of perovskite solar cells. ^[1-3]

In the present communication, main intrinsic stability problems related to the photochemistry of hybrid perovskite are addressed and current strategies to overcome these problems are analyzed.

References

[1] L. K. Ono, E. J. Juarez-Perez, Y. B. Qi, ACS Applied Materials & Interfaces 2017, 10;

[2] S. Wang, Y. Jiang, E. J. Juarez-Perez, L. K. Ono, Y. B. Qi, Nat. Energy 2016, 2, 16195;

[3] E. J. Juarez-Perez, Z. Hawash, S. R. Raga, L. K. Ono, Y. B. Qi, Energy Environ. Sci. 2016, 9, 3406.

Organic–inorganic lead halide perovskites have shown photovoltaic performances above 20% in a range of solar cell architectures while offering simple and low-cost processability. Despite the multiple ionic compositions that have been reported so far, the presence of organic constituents is an essential element in all of the high-efficiency formulations, with the methylammonium and formamidinium cations being the sole efficient options available to date. In this study, we demonstrate improved material stability after the incorporation of a large organic cation, guanidinium, into the MAPbI₃ crystal structure, which delivers average power conversion efficiencies over 19%, and stabilized performance for 1,000 h under continuous light illumination, a fundamental step within the perovskite field. We have extensively characterized these mixed cation perovskites defining the maximum percentage of guanidinium that can be incorporated into the MAPbI₃ perovskite. Interestingly, up to a 25% of methylammonium is substituted by the large guanidinium cation thanks to the formation of three hydrogen bonds per organic cation.

Layered low-dimensional perovskite structures employing bulky organic ammonium cations have shown significant improvement on stability but poorer performance generally compared to their three-dimensional (3D) counterparts. In this study, we report on a mixed passivation (MP) treatment that uses a mixture of bulky organic ammonium iodide (iso-butylammonium iodide, iBAI) and formammidinium iodide (FAI), enhancing both power conversion efficiency and stability. Through a combination of inactivation of the interfacial trap sites, characterized by photoluminescence measurement, and formation of an interfacial energetic barrier by which ionic transport is reduced, demonstrated by Kelvin Probe Force Microscopy, MP treatment of the perovskite/hole transport layer interface significantly suppresses photo-current hysteresis. Using this MP treatment, our champion mixed halide perovskite cell achieved a reverse scan and stabilized power conversion efficiency of 21.7%. Without encapsulation, the devices showed excellent moisture-stability, sustaining over 87% of the original performance after 38 days storage in ambient environment under 75 ±20% relative humidity. This work shows that FAI/iBAI, is a new and promising material combination for passivating perovskite/selective-contact interfaces.

The Internet of Things (IoT) is the network of physical objects—devices, vehicles, buildings and other items—embedded with electronics, software, sensors, and network connectivity that enables these objects to collect and exchange data. The number of applications in the fields of industrial and environmental monitoring, energy management, building and home automation is growing exponentially. The powering of all these objects is then a major concern, their autonomy is a requirement. The solutions to get these objects autonomous is closely linked to the energy harvesting from the surroundings.

Photovoltaic is one of this energy harvesting method from light. Moreover, the Perovskite based PV is an emerging technology with the promise of very efficient devices with high performances well over 20%, already demonstrated for small area lab-scale devices (ca 10 mm² or below). But yet, a number of challenges are still to be met to ensure a bright industrial future for Perovskite Solar Cells (PSCs): the development of large scale efficient modules with industrial processing route, and good stability.

The first focus of this work is the development from Perovskite cells of NIP planar architectures (surface of 0.30 cm²) to efficient solar modules with a processing route compatible with large scale production. The different layers of charge transport materials or Perovskite active materials are deposited by wet process on the whole surface of 5x5 cm² glass substrates. Those layers and the metal electrode are structured with a picosecond laser to obtain modules with performances > 10%. The laser ablation is used to structure the different layers and to create the crucial step of serial association of the multiple cells and allows high Geometrical Fill Factor > 90%.

In a second part, we will present the performances of PSCs and modules in indoor environments, typically in the range of 200 to 1000 lux with artificial lightings. Their behavior will be studied with different lighting sources and with the variable illumination measurement method (VIM irradiance around 0,001 W/m² to more than 5 suns). The results will be discussed in function of the lighting conditions (irradiance and lighting source) and the impact of the passage from cells to module will be studied.

Preliminary results obtained on PSCs of 0.30 cm² under artificial light at 200 lux (Neon tube) in air without encapsulation are very promising with power densities over 40 microW.cm^-², exceeding the performances of amorphous silicon cells, recognized technology for indoor applications.  

The introduction of cesium (Cs) and/or rubidium (Rb) cations to the FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃ perovskite has recently shown to result in remarkable enhancements in solar cell performance. However, the origin of these improvements has not been fully understood yet. In order to elucidate the impact of the inorganic cation additives on the trap landscape and charge transport properties within perovskite solar cells, Time-of-Flight (ToF), Time-Resolved Microwave Conductivity (TRMC), and Thermally Stimulated Current (TSC) measurements were performed.^[1] By combining these complementary experimental techniques we can assess both local features within the perovskite crystals and macroscopic properties of perovskite thin films and full devices. Most importantly, our results show that Cs-incorporation significantly reduces the trap density in the perovskite layer and removes deep trap states. This is in good agreement with the observed improvements in V_oc and fill factor of Cs-containing devices. In comparison, Rb-addition results in an increased charge carrier mobility, which is accompanied by a minor increase in device efficiency and reduced current-voltage hysteresis. We found indications for an inhomogeneous distribution of Rb within the perovskite layer and therefore hypothesize that the effect of Rb is mainly present in surface passivation. By mixing Cs and Rb, the advantages of both inorganic cations can be found in the resulting state-of-the-art quadruple cation perovskite (Cs–Rb–FA–MA) devices, showing the lowest trap density, the highest charge mobility and the most stable power output. Our in-depth study provides insights into the role of these additives in multiple-cation perovskite solar cells, which are essential for the future optimization of high-performance devices.

[1]        Hu, Y.; Hutter, E. M.; Rieder, P.; Grill, I.; Hanisch, J.; Aygüler, M. F.; Hufnagel, A. G.; Handloser, M.; Bein, T.; Hartschuh, A.; Tvingstedt, K.; Dyakonov, V.; Baumann, A.; Savenije, T. J.; Petrus, M. L.; Docampo, P. Adv. Energy Mater. 2018, in press.

In this talk a common mechanism underlying of hybrid perovskite nanowire formation will be discussed in detail [1]. The central role of the solvatomorph phase as the intermediate phase in crystallization will be highlighted. Next, our latest findings on the guided growth of perovskite nanowires by ‘solvatomorph-graphoepitaxy’ will be presented [2]. This method turned out to be a fairly simple approach to overcome the spatially random surface nucleation. The process allows the synthesis of extremely long (centimeters) and thin (a few nanometers) nanowires with a morphology defined by the shape of nanostructured open fluidic channels. This method might allow the integration of perovskites into advanced CMOS technologies.

CH₃NH₃PbI₃ nanowires in association with carbon nanostructures (carbon nanotubes and graphene) make outstanding composites with rapid and strong photoresponse. They can serve as conducting electrodes, or as central components of detectors. Performance of several miniature photo-field effect transistor devices based on these composite structures will be demonstrated.

Solvatomorph-graphoepitaxy method could open up an entirely new spectrum of architectural designs of organometal-halide-perovskite-based heterojunctions -and tandem solar cells, LEDs, photodetectors and new type of magneto-optical data storage devices [5].

References :

[1] Horváth et al. Nano Letters, 2014, 14 (12), 6761–6766

[2] Spina et al. Scientific Reports, 2016, 6

[3] Spina et al. Small, 2015, 11, 4824-4828

[4] Spina et al. Nanoscale, 2016, 8, 4888

[5] Náfrádi et al. Nature Communications 7, 13406

Acknowledgement:

This work was supported by the ERC Advanced Grant (PICOPROP#670918) and the Swiss National Foundation (No.513733).

High band gap semiconducting metal oxide films have been widely used as charge extraction layers in organic solar cells over two decades. However, these oxides suffer from high resistivity and can therefore only be included as very thin layers in the solar cells. In order to increase layer thickness and thus processing robustness without sacrifying performance, increase in conductivity can be reached by creating metal vacancies and doping with some elements. ^[1] Recent works were dedicated to doping of n-type metal oxides such as ZnO demonstrating that doping leads not only to robust processing, but also to improved air processibility ^[2], device performance as well as color tuning of the solar cells ^[3].

Here, we focus on the development of doped p-type metal oxide semiconductor nanocrystals for solution-processing of hole transporting layers applied in both normal and inverted device structures. Amongst the large variety of metal oxides used, we focus on doped NiO_x and WO_x using different dopants (Li, Cu, and Sn). We applied these materials to high efficiency polymer solar cells using both fullerene and non-fullerene acceptors. The impact of the doping on the performance, air and thick layer processing will be discussed.

  1. Matsubara, K., Huang, S., Iwamoto, M., Pan, W. Nanoscale 6, 688–692, (2014).

  2.  Prosa et al., ACS Appl. Mater. Interfaces 8, 1635–1643, (2016).

  3.  Gaceur et al., Adv. Funct. Mater. 26, 243–253, (2016).

The highest efficiency Perovskite Solar Cells (PSC) often contain two [1], three [2] or even four [3] cations. As well a higher efficiency, mixed cation devices show higher reproducibility and stability [2]. However, the reasons behind the improvements are still being investigated and are not clearly understood. Here we present the results of a systematic study looking at nine different sized cations, from ammonium (ionic radius 146 pm) to guanidinium (ionic radius 278 pm).

5% of each cation was substituted into methyl ammonium lead iodide (MAPI) and the degree of substitution was confirmed by NMR. We chose 5% substitution as all the materials showed the same crystal structure, the same band gap, and no 2D perovskite phases were formed. This allowed us to make a direct comparison between our results and understand the large changes in material and device properties introduced by some of the cations. Three of the partially substituted MAPI powders were studied by muon spectroscopy (μ-SR). We recently showed that μ-SR is able to detect both cation dynamics and iodide diffusion in perovskite materials [4] and we have extended the technique to multi-cation systems. We also prepared inverted cells containing the nine different perovskite materials and compared them using a range of characterisation techniques, including impedance spectroscopy. The results are striking, confirming that all of the cations have an effect at just 5% substitution, while some of the cations substantially change the fundamental properties of the perovskite itself.  

[1] Nature Energy, 2017, 2, 972–979

[2] Energy Environ. Sci., 2016, 9, 1989-1997

[3] Energy Environ. Sci., 2017,10, 2509-2515

[4] https://arxiv.org/abs/1801.03845

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.

This work is based on the development and characterization of perovskite solar modules. In particular, we report on the state of art of the PV performance for modules on a substrate area of 10x10cm². The main topic is the optimization of the perovskite and the ETL/HTL layers deposited by solution processing. Morphology and thickness of the layers and its interfaces are challenging issues to reach high PV performance. The improvement of photovoltaic parameters on substrate areas equal to 10x10 cm² is helpful to evaluate the scaling up of this technology. We performed different experiments in order to improve V_OC and FF of the PK modules. In particular, the design of an optimized layout for PK modules is one of the more important action done to reach our goal. Firstly, we fabricated modules with non-optimized layout formed on 10 cells with 7mm width. The results show an efficiency of 10.9% on as substrate area of 10x10cm². It can be seen that the main factors limiting the efficiency are both low FF and low J_sc values, equal to 58.8% and 16.7mA/cm², respectively. In order to improve these crucial parameters, two kinds of optimization steps were realized regarding the PK layer and the module layout. The light absorption of the PK layer was increased by varying the spin-coating program during the PK deposition. Furthermore, an optimized layout was designed in order to reduce the impact of the ohmic losses due to the FTO substrate. The new layout is formed by 15 series connected cells with a cell width of 4.5mm. An optimized condition for P1-P2-P3 laser ablations was used to pattern the module. The results show an active area (47.2 cm²) efficiency of 15% for a module fabricated with planar architecture on a SnO₂ ETL and substrate area of 10x10cm². Furthermore, a Maximum Power Point Tracking was used to assess the stabilized efficiency of the modules which results to be equal to 12% after 300s.

Perovskite based solar cells, mostly employ solution processed perovskite layers. Evaporated methylammonium lead iodide perovskite layers have also been reported and been employed in solar cells. Our group has developed several perovskite based solar cells, using vacuum based perovskite preparation methods. These metal oxide free p-i-n type perovskite cells exhibit high power-conversion efficiencies. We have extended this work to fully evaporated perovskite devices reaching power conversion efficiencies as high as 20 % in a planar single junction device and similar performance in tandem devices.  Avenues to further increase the device performance by using multiple cation perovskite prepared via sublimation will also be presented.

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Semi-transparent perovskite solar cells (STPSCs) are highly attractive to improve the efficiency of commercial silicon photovoltaic modules in tandem architectures. However, the further commercial development of STPSCs is requires competitive stability and high IR transparency requirements. Here we report the fabrication of highly efficient and stable STPSCs containing a spatial atomic layer deposited (S-ALD) ZnO layer. The results show the key role of the S-ALD ZnO layer. Firstly, the compact S-ALD ZnO layer acts as a buffer layer to prevent sputter damage to underlying layers during the sputter deposition of the ITO layer. Secondly, the compact layer improves the stability of perovskite solar cells. We made a comparison between triple-cation perovskite (Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_2.7Br_0.3)) and dual-cation perovskite (Cs_0.15FA_0.85Pb(I_2.75Br_0.25)) cells. The triple-cation STPSCs exhibit a stabilized output efficiency of 17.1% (comparing with 16.6% stabilized output efficiency for the dual-cation STPSCs). First results indicate, cells based on the dual-cation perovskite display better thermal and light soaking stability. Encapsulated dual-cation STPSCs retain 95% of their initial stabilized power output after 2000h aging at 85°C. Interestingly, using highly transparent hole and electron metal oxide extraction layers and optimized MgF₂ anti-reflection coating (ARC), STPSCs show comparable efficiency by illumination from the both rear and front ITO sides. Most importantly, using stable and optically optimized STPSCs, we present high-efficiency perovskite/c-Si 4-Terminal tandem cells. STPSCs are modified for tandem application by limiting the ITO NIR absorption and reducing the reflection losses using light management. Regarding the ITO improvement, the free carrier density is reduced to minimize the NIR absorption and the mobility is increased to retain sufficient conductivity. The STPSCs with optimized IR transparency (average transmittance of about 93% in the wavelength range of 800 nm to 1200 nm) were coupled with commercial IBC bottom cell (9.7% efficiency when it is filtered by ST-PSCs) leading to a tandem cell efficiency of 26.1%. The 4T tandem cell efficiency is obtained with the tandem measurement procedure also used by other authors [1].

[1]. J. Werner, L. Barraud, A. Walter, M. Bräuninger, F. Sahli, D. Sacchetto, N. Tétreault, B. Paviet-Salomon, S. Moon, C. Allebé, M. Despeisse, S. Nicolay, S. De Wolf, B. Niesen and C. Ballif, "Efficient Near-Infrared-Transparent Perovskite Solar Cells Enabling Direct Comparison of 4-Terminal and Monolithic Perovskite/Silicon Tandem Cells", ACS Energy Letters, vol. 1, pp. 474, 2016.

Reducing Fresnel reflection losses with hole transporting layer that function as optical couplers in 4T tandems; Based on the understanding of Fresnel reflection in a full device stack, we demonstrate that parasitical energy losses can be minimized using wide-band gap semiconductors with beneficial refractive index as selective contacts. Furtheremore, and as opposed to previous reports, were the improvement of efficiency is done through energy-intensive deposition of both top and bottom electrodes, here, we demonstrate that solution-processed silver nanowires, a model composite for percolation- based electrodes, allows controlling the overall optical and electrical losses with outstanding precision by a simple spray coating process. As a result we present a solution-processed semitransparent perovskite-based solar cell with 17.1% efficiency showing 84.8% overall optical transmission averaged in the relevant range 800 nm to 1100 nm. Our devices allowed the elaboration of 4-terminal silicon/perovskite tandem devices with 26.7% efficiency.

Laminated 2T tandems, an alternative approach: We explore a multipurpose interconnection layer for the fabrication of monolithic perovskite/silicon tandem solar cells with high power conversion efficiency. The interconnection of independently processed silicon and perovskite sub-cells could be a simple add-on lamination step, alleviating the common fabrication complexity of perovskite/silicon tandem devices. We envision that this lamination concept can be extrapolated for the pairing of multiple photovoltaic technologies, creating a universal platform that facilitates mass production of tandem devices with high power conversion efficiency.

Multi-junction device architectures represent a promising strategy to further advance the efficiency of organic solar cells. For solution-processed organic solar cells, tandem and triple junction cells have been reported in the past. Here we demonstrate a first case of a quadruple-junction polymer solar cell, featuring four different and complementary band gap absorber layers that absorb light up to 1150 nm. The quadruple junction cell is fabricated using a combination of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate and ZnO as interconnection layer in a n-i-p (inverted) configuration and reaches a power conversion efficiency of about 7.5% with an open-circuit voltage of 2.46 V. The thicknesses of the individual absorber layers were optimized using optical modeling and the experimental determination of the internal quantum efficiency (IQE) of the absorber layers for different thicknesses. Measuring the external quantum efficiency (EQE) of the quadruple cells has been accomplished using a protocol which makes use of bias light of different wavelengths, involving optical modeling and correcting for the build-up electric field. The short-circuit current density determined from integration of the EQE with the AM1.5G solar spectrum is consistent with the one measured in simulated solar light. The results are further validated by comparing the EQE of the quadruple cell to the calculated fraction of absorbed photons from optical modeling and the IQE of the absorber layers, obtaining a good agreement. The efficiency of the quadruple-junction polymer cells appears to be limited by bimolecular recombination in the active layers, which prevents the use of thick (>200 nm) layers.

The outstanding progress of hybrid organic-inorganic Perovskite Solar Cells (PSC) have made them one of the most promising photovoltaic technologies currently. In particular, their excellent electro-optical properties such as wide tunable bandgap and high diffusion lengths, in combination with a great variety of possibilities for their fabrication have converted them in interesting candidates for top cells in perovskite/silicon tandems.

In this work, the fabrication of highly efficient semitransparent PSCs with a sputtered-ITO back contact is reported. It is worthy to remark that the MoO_x layer commonly reported between Spiro-OMeTAD and ITO to avoid degradation was found unnecessary to ensure good quality and reproducibility of the system. Therefore, the architecture here proposed was ITO/Spiro-OMeTAD/(Cs,MA,FA)Pb(Br,I)₃/mp-TiO₂/bl-TiO₂/FTO/Glass. After a careful optimization of perovskite thickness and ITO deposition conditions an overall 17% efficiency in small area (J_SC=20.34 mA cm^-2, V_OC=1095 mV, FF=76.2%) and 16.7% for 0.64cm² (J_SC=21.53mA cm^-2, V_OC=1065mV, FF=73.0%) were achieved.

Using time-resolved photoluminescence imaging (TRPL) on both p-type and n-type sides of the semitransparent PSC (i.e. illuminating through ITO and FTO respectively), the principal physical properties linked to recombination and diffusion processes were studied. Hence, carrier mobility in the absorber layer and the quality of the perovskite/TiO₂ interface were identified as the most limiting parameters for overall performance, confirming MoO_x film were not critical for a semitransparent PSC with high efficiency.

After that, a conventional commercially-available aluminum back surface field (Al-BSF) silicon solar cell 19.5% efficient (J_SC=38.46mA cm^-2, V_OC=639mV, FF=79.4%) was reduced to adapt with the size of our bigger semitransparent PSC (18.5% efficiency after cutting). For 4-terminal (4T) tandem assembly, our champion semitransparent PSC was employed as top cell and the reduced Si cell as bottom one, employing an optical coupling by dripping a liquid between both. Under these conditions, the bottom silicon solar cell showed a 5.7% behavior (J_SC=12.75mA cm^-2, V_OC=585mV, FF=77.0%). Consequently, a 22.4% efficiency can be asserted for the 4T perovskite/silicon tandem construction, which corresponds with a 3% improvement over the reference efficiency of the commercial Si solar cell (a 4% when if the cut one is considered).

One promising strategy for surpassing the practical efficiency limit of crystalline silicon solar cells (c-Si) is to add a wide band-gap top cell to absorb high-energy photons with less thermalization loss. Perovskite solar cells (PSC) are ideal candidates for the role of top cell to a c-Si bottom cell due to factors such as their sharp optical absorption edge and highly tunable bandgap. The architecture of these tandem solar cells can be 4-terminal mechanically stacked or 2-terminal monolithic. We demonstrate a versatile low-temperature fabrication method for the PSC, which combines evaporation and spin coating. This method is compatible with significant surface roughness of the bottom substrate or cell of a monolithic tandem. In addition, the composition of the perovskite layer can be varied independently between different A-site cations (methylammonium, formamidinium, cesium, and various large organic cations) and halogen anions (chloride, bromide, iodide). Through such chemical engineering, we show perovskite materials with band gap values between 1.5 and 1.8 eV, along with a modified layer structure through the incorporation of large organic cations such as guanidinium, imidazolium, benzylammonium, and phenethylammonium. When employing an optimized cesium formamidinium lead mixed iodide/bromide composition as a top cell, we demonstrate a current matched monolithic tandem solar cell with a total current over 40 mA cm^-2. This material and device development is supported by extensive characterization, which includes ellipsometry and other optical spectroscopy methods, as well as X-ray diffraction, atomic force microscopy, and various electron microscopy techniques.

Optical design of perovskite solar cells (PSCs) has been demonstrated to be very relevant to push the performance of this emerging technology.^{[1, 2, 3]}

Recently, the research interest has put under the spotlight ABX₃ perovskites for their application in multifunction devices that can surpass the Shockley-Queisser (SQ) limit.^[4,5] In our talk we will discuss about the possibilities that an optical model opens to improve the efficiency of perovskite-based tandem solar cells. We make use of semi-analytical models based on the transfer matrix to describe light distribution within the device. We will show how design provides a straightforward path towards architectures in which current matching is attained, eventually preventing the field from time-consumable trial and error experiments. Then, we will identify which are the effects that the different components have in the electro-optical performance of the device. Based on this, we will conclude with a roadmap towards the achievement of optically optimised tandem solar cells, where parasitic absorption and reflectance losses are minimized. We merge our optical model with a genetic algorithm to fully optimise a FA_0.83Cs_0.17PbI_1.8Br_1.2/MAPb_0.15Sn_0.85I₃ tandem device that outperforms >30% efficiency. This kind of approach promises to be of great importance for the immediate future of this rapidly growing field.

[1] M. Anaya et al., Journal of Physical Chemistry Letters (2015), 6, 48-53.
[2] W. Zhang & M. Anaya et al., Nano Letters (2015), 15, 1698-1702.
[3] J.-P. Correa-Baena & M. Anaya et al. Advanced Materials (2016), 28, 5031–5037.

[4] M. Anaya & J.-P. Correa-Baena et al., Journal of Materials Chemistry A (2016), 4, 11214–11221.

[5] M. Anaya et al., Joule (2017), 4, 769-793.

With Perovskites Solar cells matching that of crystalline silicon the rush to commercialisation is even more paramount than ever before. One area of interest is for Perovskites to be used in window and Tandem applications. However conventional Perovskites use a solid, non-transparent back contact, this is problematic if the application range is to diversify. Here we present a transparent back contact deposited via solution processed silver Nano-wires with efficiencies of 11% and an average transmission of 2.2% over (400-750nm) in a conventional mesoporous architecture using a triple cation based perovskite (FTO/cTiO₂/mesoporousTiO₂/Perovskite/Spiro-OMeTAD/ Silver Nano-wires). Thinning the Perovskite absorbing material allowed us to increase average transmission to over 14.5% (over 400-750nm) with a tradeoff in reducing PCE to around 3.6%, a result that is competitive with current alternative PV technologies currently on the market for semi-transparent PV windows. Various silver Nano-wire materials and carbon nanotube composites were trialled and optimised for this work in order to give the best compromise between optical and device performances. These were then characterised further using atomic force microscopy to measure Nano-wire/ nanotube dimensions and dispersions. Lastly, conductivity measurements (sheet resistances) were taken using a Jandel 4 point probe and z conductivity measured using conductive atomic force microscopy. Optical modelling was also carried out to probe layer thickness and optical constants while CIE measurements were taken in order to assess and quantify colour and clarity of the resulting layers.

With demonstrated power conversion efficiencies close to 23%, perovskite-based photovoltaics is already able to compete with established technologies like silicon, CdTe and CIGS. However, next to high efficiencies, the potential low-cost fabrication of devices with sufficient stability under real-world conditions is of key importance for the future economic prospects of the perovskite technology.

In this contribution, we report on a novel inexpensive architecture for efficient and highly reproducible, all-evaporated perovskite solar cells. Our evaporated CH₃NH₃PbI₃ absorber is sandwiched in a p-i-n structure between inexpensive and highly stable nickel oxide as hole transport material and a combination of C₆₀ and bathocuproine (BCP) as electron hole transport material. In contrast to that, most of the approaches in the community employ highly expensive hole transport materials like Spiro-MeOTAD or PTAA with prices up to 1,000,000 $/kg, which would hamper the commercialization of the technology. Most importantly, the common organic hole transport material Spiro-MeOTAD shows low stability at elevated temperatures above 60 °C, making it an unsuitable choice for applications under typical outdoor conditions. By replacing the unfavorable Spiro-MeOTAD by electron-beam deposited nickel oxide and the gold back electrode by copper, we reduce the cost of materials on the lab-scale to one third of the price of common stacks (e.g., ITO/TiO₂/CH₃NH₃PbI₃/Spiro-MeOTAD/Au). At the same time, power conversion efficiencies of the devices reach stabilized values above 14% without hysteresis. Moreover, high thermal stability of the employed transport materials is demonstrated in extremely stable devices even at 80 °C, which is a typical operating temperature for solar modules under real-world situations as well as standard test condition in established performance tests. In contrast, our reference all-solution-based devices with Spiro-MeOTAD degrade fast to about 80% of their initial values under the same conditions.

Towards an industrialization of the perovskite technology, a highly controllable deposition and an easy upscaling is needed. Our all-evaporated approach is able to meet these criteria. Even on small lab-scale areas, 30% lower cell-to-cell variations in comparison to common spin-coating approaches are achieved. In terms of upscaling, homogenous and reproducible depositions up to areas of 8x8 cm² are demonstrated and investigated by light beam induced current mapping. Finally, as an inverted architecture with the anode deposited on top of the substrate the discussed layer stack is a promising candidate for two-terminal tandem cells and modules on top of CIGS or p-type silicon.

Perovskite solar cells demonstrate an enormous potential for next generation photovoltaics because of their ease and low cost of fabrication in combination with their excellent light-harvesting properties. These devices usually consist of ca. 300-500 nm thick layer of an organometallic halide perovskite, sandwiched between two charge-transporting layers. It is well documented that charge generation in these perovskites is fast and efficient while recombination is slow, meaning that the device properties are largely limited by processes at the internal interfaces or within the charge-transporting layers.

Here, we present the results of a detailed study on hybrid perovskite solar cells comprising all-organic charge transport layers. We show the energetics of the organic semiconductor affects the device performance through the rate of interfacial charge recombination, and how these recombination processes are severely reduced through the proper choice of the organic material.^[1] We also highlight the role of the charge transport material in determining the speed of charge extraction and with that the fill factor of the device.^[2,3] Through proper design of all layers and interfaces, stable 1 cm² – sized perovskite solar cells with record fill factors (> 81%) and high open circuit voltages (1.17 V), approaching a power conversion efficiency of 20 %, could be realized.^[4]

^[1] C.M. Wolff, F. Zu, A. Paulke, L.P. Toro, N. Koch, and D. Neher, Adv. Mater. 1700159 (2017).

^[2] M. Stolterfoht, C.M. Wolff, Y. Amir, A. Paulke, L. Perdigon-Toro, P. Caprioglio, and D. Neher, Energy Environ. Sci. 10, 1530 (2017).

^[3] J.A. Love, M. Feuerstein, C.M. Wolff, A. Facchetti, and D. Neher, ACS Appl. Mater. &Interf. online DOI 10.1021/acsami.7b10361 (2017).

^[4] M. Stolterfoht, C.M. Wolff, S. Zhang, J.A.M. Prieto, C.J. Hages, Th. Unold, S. Albrecht, P.L. Burn, P. Meredith, D. Neher, submitted

Using two different techniques, Fourier-transform photocurrent spectroscopy (FTPS) and photoluminescence (PL), we have probed the temperature dependence of methylammonium lead iodide (MALI) absorption spectra to be able to distinguish the static and dynamic part of Urbach energy.  We extract the Urbach energy as the reciprocal value of the slope of the absorption at the band edge plotted in logarithmic scale. Its value depends on the material disorder and generally correlates well with the loss in the open-circuit voltage (V_OC) of optimized cells, compared to their bandgap, for a given photovoltaic technology [1].  When cooling perovskites, we find a strong decrease in their Urbach energy and a slow decrease of their optical band gap energy. We observe that all absorption curves measured at different temperatures intersect at one point called the Urbach focus, which was in our case at 1.52 eV, which represents a general lower limit for the optical band gap in MALI. Fitting Urbach energies from PL measurements we obtain phonon energies of 135 ± 20 cm^-1 [2], which implicates that the dynamic disorder of MALI is given by cage vibrations. Finally, we will show and discuss that the density of active static defects in perovskites is very low in comparison to other materials used for solar cells, especially for a solution processed one. These results will help establish practical efficiency limits of perovskite solar cells, as compared to the Shockley-Queisser limit.

[1] S. De Wolf et al.: J. Phys. Chem. Lett. 5 (2014) 1035.

[2] M. Ledinský et al.: J. Phys. Chem. Lett. 6 (2015) 401.

So far, little attention has been paid to doping of hybrid perovskites and hence little is known not only about the doping concentration but also the type of dopants, whether it is unintentional doping, e. g. due to non-stoichiometric composition and defects or due to substitutional doping, the inclusion of foreign atoms. A detailed knowledge on the type of doping in perovskite solar cells is crucial to understand the device behavior, particularly the more this material class develops towards commercialization. Currently, numerous approaches are being pursued to generate intentionally doped hybrid perovskite semiconducting materials. Capacitance-voltage measurements (CV) by means of Mott-Schottky (MS) analysis represents a well-established method to determine doping concentration as well as the built-in potential in semiconductors. It is based on analyzing the variation of the depletion layer thickness, and the corresponding change of capacitance with applied electric field. In perovskite solar cells, however, the so-called MS-plot is rarely linear which makes a reliable determination of the parameters difficult.

In this work, we present a modified CV measurement scheme to determine the doping concentration and profile in different hybrid perovskite solar cells based on MS analysis. In detail, we determined the doping concentration in planar-type p-i-n perovskite solar cells based on methylammonium lead iodide (MAPI) as well as formamidinium lead iodide (FAPI). In the MAPI device, we found an inhomogeneous doping profile across the perovskite absorber with concentrations in the range from 10¹⁶ – 10¹⁷ cm^-3 instead of a constant doping concentration as observed in the FAPI device. We relate this to the presence of mobile dopants. Finally, we present temperature dependent impedance measurements, which support the picture of mobile dopants.

The lifetime of photogenerated charge carriers is one of the most important parameters in solar cells, as it rules the recombination rate that defines the open circuit voltage and the required minimum extraction time. It is therefore also one of the most discussed factors in all photovoltaic research fields. Electrical characterization methods such as transient photovoltage (TPV), open circuit voltage decay (OCVD) and impedance spectroscopy are often employed to determine charge carrier lifetimes. These methods were however initially developed for thick solar cells with an indirect bandgap and are therefore not always directly transferable to many of the new generation thin film photovoltaic devices. All thin film solar cells such as those presented on this conference (dye sensitized, organics, PbS quantum dots as well as metal halide perovskites) are all necessarily relying on materials with a high absorption coefficient. A high absorption coefficient is from reasons of reciprocity relations however always linked with a high radiative rate constant and therefore an inherent very short charge carrier lifetime. A careful consideration of what is actually being measured via electrical means is therefore imperative, and also the incentive of this contribution. We will here evaluate both perovskite and organic solar cells and compare them to an ideal thick silicon diode. We first show that the problem of lifetime determination via electrical means arises from that the relaxation of spatially separated charges interferes substantially with the general lifetime assignment. We will finally provide a simple analytical expression outlining under what conditions relevant steady state bulk recombination lifetimes are electrically accessible in solar cells.

Impedance spectroscopy is a powerful technique in characterization of solar cells, in particular perovskite solar cells (PSCs). Small amplitude perturbation is used in order to obtain linearized cell response in frequency domain. The linearization allows to model impedance response with linear circuit elements, which can be associated with real physical parameters of the solar cell by using an equivalent circuit. A variety of equivalent circuits was used by many research groups in order to describe impedance spectra of PSCs and there is no general agreement on which equivalent circuit is preferable for the modelling of PSCs impedance spectra. In this work a range of equivalent circuits was analyzed both analytically and numerically for two and three-component impedance spectra of PSCs and full analytical and numerical equivalence was found for Voight, matryoshka and hybrid Voight-matryoshka circuits. Therefore, any one of those could be chosen, depending on the physical model of the device operation considered by a researcher. In addition, an empirical analysis which does not involve any equivalent circuit was applied and the parameters obtained using this analysis were compared to those obtained from the equivalent circuits mentioned above. This empirical analysis includes the calculation of the distribution of relaxation times (DRT). DRT analysis could be especially useful in case of poorly resolved impedance arcs. Good agreement was found between the parameters of the empirical analysis and the ones received using equivalent circuits. In contrast, equivalent circuits of the Maxwell type have a more complex relation to the parameters obtained empirically. Impedance spectra containing inductive feature (called either “loop” or “tail”) are also discussed in terms of negative resistive and capacitive elements. This feature is the signature of an additional process present in the system and very often it evolves into a regular impedance arc in the series of impedance measurements for the same perovskite cell. We show that inductive feature appearing in an impedance spectrum could be described only with Voight element without utilization of any inductive element. A general protocol for the analysis of impedance spectra of PSCs is proposed.

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. In order to improve these devices and begin to challenge perovskite and 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 triple mesoscopic 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. A range of anomalous behaviours have been observed in these cells in response to illumination. They undergo a slow (several minutes) 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. Both of these processes are reversible, although in the case of the light soaking effect the deterioration in performance is almost instantaneous upon turning off the light (i.e. there is no residual effect when the light is turned back on).

A range of complimentary optoelectronic techniques have been employed in order to study these effects, including transient photovoltage (TPV), differential capacitance and impedance spectroscopy. 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. This behaviour suggests the presence of high rates of interfacial recombination at the TiO₂ surface. In these mesoporous based carbon cells the phenomena is observed at room temperature and is very slow to disappear under continued bias light illumination. In the previous case of 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. For the planar devices this effect was only observed at low temperature (at which the ionic motion was sufficiently slowed) and even then disappeared much quicker than in the mesoporous carbon devices at room temperature. Knowledge of this process has allowed us to assess the impact of different treatments and processing conditions on interfacial recombination, which has helped to guide the improvement in device efficiency.

Non-radiative losses limit attainable open-circuit voltage (V_oc) in perovskite solar cells (PSCs) and hinder the breaking of current power conversion efficiency (PCE) record. A deeper understanding of the existing non-radiative pathways in PSCs requires an in-situ optoelectronic characterization of working devices. Based on differential charging and transient photovoltage decay, we investigated the evolution of V_oc by measuring the density of charge carriers and their overall recombination lifetime in working devices under various illumination intensities. The mechanism of hole transport materials (HTMs) affecting V_oc is studied in inverted CH₃NH₃PbI₃ (MAPI) solar cells based on PEDOT:PSS, PTAA and PTPD. V_oc shows little correlation with the HOMO energy of HTMs, as PEDOT:PSS demonstrates the deepest HOMO level but the lowest V_oc and thus PCE (0.86 V, 12.9%) compared with PTAA (1.07V, 17.0%) and PTPD (1.09V, 18.5%) cell. Such a large loss in V_oc is otherwise associated with extraction of photo-generated holes from perovskite, leading to electron-hole recombination primarily occurring at PEDOT:PSS/perovskite heterojunction. In contrast hole extraction by PTAA and PTPD is less efficient and electrons recombine with holes mainly in bulk perovskite. Recombination at HTL/perovskite interfaces reduces the effective electronic band gap of the cell compared with recombination occurring in bulk. It is observed that density of photo-generated charges is comparable in the two devices but V_oc loss is mainly due to reduced potential energy for given charge density. The overall charge recombination lifetime in the complete cell is only slightly increased by the interfacial recombination, which is not the main cause of V_oc loss. In addition, extraction of holes is due to more p-type nature (higher work function) and higher conductivity of PEDOT:PSS compared with PTAA/PTPD, which causes stronger band bending at heterojunction but is not regarded as beneficial in PSCs. The results we showed suggest that charge extraction by the transport layers is detrimental to device performance. Employing charge transport layer with relatively low intrinsic carrier density enhances bulk recombination of electrons and holes, which should be a viable strategy to achieve high V_oc in PSCs.

The performance of perovskite solar cells (PSCs) has skyrocketed in recent years, but the devices still suffer from unreliable performance in the form of anomalous hysteresis. Although this concern has been linked to several processes such as ion migration in perovskite layer, trapping electronic carriers at the interfaces and ferroelectric polarization, none of these reasons fully explain the hysteresis in solar cells. On the other hand, recent studies showed that the combination of ion migration and charge recombination is required to achieve hysteresis in current-voltage characteristics. Therefore, Fermi level alignment between charge extraction layers and perovskite is important to prevent charge accumulation, causing charge recombination.
Here, we tune the Fermi level alignment between the electron transport layer (ETL) consisting of atomic layer deposited tin oxide, SnOX, and the perovskite absorber, Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3, by annealing of the ETL and highlight that this parameter is interlinked with current-voltage hysteresis in PSCs. Furthermore, thermally stimulated current (TSC) measurements reveal that the depth of trap states of the ETL correlates with Fermi level positions extracted from ultra-violet photoelectron spectroscopy (UPS), ultimately linking it to the energy difference between the Fermi level and conduction band maximum. In the presence of deep trap states, charge accumulation at the interface is promoted, causing charge recombination which increases the hysteresis of the solar cells. Thus, we believe that our work will shed light on the serious impediment of PSCs.
 

Within the last years, the so-called mixed cation and halide ‘FAMA’ perovskite has replaced the archetype methylammonium lead iodide as the mainstay in the field of perovskite photovoltaics. Thereby, methylammonium (MA) and bromide is used to stabilize the crystal structure of formamidinium (FA) lead iodide perovskite. Recently, M. Saliba et al. further improved the stability of these state-of-the-art FAMA perovskite solar cells by introducing Cesium as well as Rubidium to the system, boosting the device power conversion efficiency up to 21.6%.  [1]

In this work, we reveal the impact of Cs and/or Rb on the electronic properties of the FAMA perovskite crystal lattice by directly investigating the energetic landscape of electronic trap states via thermally stimulated current (TSC) spectroscopy on triple as well as quadruple cation perovskite solar cells. [2] In TSC, the device is cooled down in the dark well below the activation energy of possible trap states (here: T=30K). Subsequently, trap states are filled with charge carriers created by illumination. After a dwell time allowing charge carriers to relax into the trap states, the solar cell is heated up to 300K with a constant heat ramp, while the current is precisely measured. This current is attributed to charge carriers being released from previously occupied traps in the semiconductor, allowing to draw conclusions about their density and energetic distribution. For the study, the perovskite layer was altered between FAMA, FAMA with 5% of Cs, FAMA with 5% of Rb and FAMA with 5% of Cs as well as Rb in planar type solar cells with a layer sequence of FTO/SnO₂/perovskite/Spiro-OMeTAD/Au (from bottom to top). We found that while Rb had no significant influence on the trap landscape of FAMA, a controlled addition of Cs effectively lowers the trap density in FAMA-based devices, which can be directly linked to the higher performance observed in Cs containing perovskite solar cells

References

[1]       M. Saliba, T. Matsui, K. Domanski et al., Science (2016), 354, 206-209

[2]       Y. Hu, E.M. Hutter, P. Rieder et al., Adv. Energy Mater. 2018, 1703057