Sunlight-driven water splitting is studied actively for production of renewable solar hydrogen [1]. Both the efficiency and the scalability of water-splitting systems are essential factors for practical utilization of renewable solar hydrogen. It is desirable to develop particulate photocatalysts and their reaction systems that efficiently split water, because particulate photocatalyst systems can be spread over wide areas by inexpensive processes potentially.

The author has studied various oxide, (oxy)nitride, and (oxy)chalcogenide photocatalysts [2]. The water splitting activity of SrTiO₃ can be improved by two orders of magnitude by doping Al. The quantum efficiency of photocatalytic overall water splitting has reached almost unity in the near UV region [3]. This is the highest reported to date and confirms that particulate photocatalysts can drive the uphill overall water splitting reaction as efficiently as the photon-to-chemical conversion process in photosynthesis. 

The author has also been developing panel reactors for large-scale applications. A prototype panel reactor containing Al-doped SrTiO₃ photocatalyst sheets splits water and releases product H₂ and O₂ gas bubbles at a rate expected at a solar-to-hydrogen energy conversion efficiency (STH) of 10% under intense UV illumination [4]. A 1-m²-sized photocatalyst panel reactor splits water under natural sunlight irradiation without a significant loss of the intrinsic activity of the photocatalyst sheets. A larger size (100 m²) solar hydrogen production system was constructed and its performance and system characteristics are currently under investigation.

To realize a sufficient STH, it is essential to develop photocatalysts active under visible light irradiation. Ta₃N₅ [5] and Y₂Ti₂O₅S₂ [6] show activity in overall water splitting via one-step excitation under visible light irradiation. Photocatalyst sheets consisting of La- and Rh-codoped SrTiO₃ and Mo-doped BiVO₄ split water into H₂ and O₂ via two-step excitation, referred to as Z-scheme, and exhibit STH exceeding 1.0% [7,8]. Some other (oxy)chalcogenides and (oxy)nitrides with longer absorption edge wavelengths are also applicable to Z-schematic photocatalyst sheets.

In my talk, the latest progress in photocatalytic materials and reactors will be presented.

[1] Hisatomi et al. Nat. Catal. 2019, 2, 387.

[2] Chen et al. Nat. Rev. Mater. 2017, 2, 17050.

[3] Takata et al. Nature, 2020, 581, 411.

[4] Goto et al. Joule 2018, 2, 509.

[5] Wang et al. Nat. Catal. 2018, 1, 756.

[6] Wang et al. Nat. Mater. 2019, 18, 827.

[7] Wang et al. Nat. Mater. 2016, 15, 611.

[8] Wang et al. J. Am. Chem. Soc. 2017, 139, 1675.

Electrosynthesis of ammonia via the electrocatalytic nitrogen reduction reaction (NRR) has recently become a topic of stirring research. The process attracts attention from both industry and academia as a possible future alternative to the classical catalytic technology and is expected to enable the renewable-powered, distributed production of “green” NH₃ – both for the chemical/fertiliser industry and as an energy carrier.

Notwithstanding a truly significant investigative effort invested worldwide, the recent progress in advancing the NRR technology remains questionable to the extent that the majority of the reports on the successful electrochemical conversion of N₂ to NH₃, in the first place those using aqueous electrolyte media, cannot be considered fully reliable. The introductory part of the talk will critically assess the state-of-the-art in the NRR field, will identify the most common missteps and will aim to provide a simple guide for undertaking reliable N₂ reduction experiments.

The second part of the talk will focus on the solutions to the fundamental problems of the NRR in aqueous media, viz. suppression of the competing dihydrogen evolution and promoting the availability of N₂, through the application of the aprotic electrolyte media. Our progress towards the direct electrocatalytic reduction of dinitrogen and the redox-mediated conversion of N₂ to ammonia under these conditions will be presented, with a specific focus on the challenges and possible pathways towards further improvements in the NH₃ electrosynthesis technology.

Rational design and comparative studies of catalysts rely on detailed information about the mechanism of catalysis that in most cases is not available. We have used time-resolved UV/VIS and mid-IR spectroscopy to resolve highly reactive catalyst intermediates on the nano- to millisecond time scale, elucidate their structures and measure their reactivity in proton- and electron transfer reactions of the mechanistic cycle [1].

The azadithiolate bridge of FeFe-hydrogenase active sites has been taken as an example design for proton relays in the second coordination sphere, facilitating protonation and deprotonation of the metal centre. We have shown that for the corresponding model complexes, the aza-group has no role as proton shuttle in the Fe^IFe⁰ state, as had been proposed [2]. Instead, the effect of aza protonation is to shift the catalyst reduction potential, surprisingly with no or only little reduction in the rate of subsequent metal protonation by external acid.

We have also been able to follow the charge transfer dynamics via mid-IR signals of the catalyst in catalyst-semiconductor systems, with both band-gap excitation and dye-sensitization. Ultra-fast (<300 fs) catalyst reduction of molecular catalysts for CO₂ or proton reduction is observed on CuInS₂ quantum dots ([3] and manuscript in progress).

While the field of sunlight-driven fuel generation has traditionally been dominated by inorganic materials, organic semiconductors are currently gaining substantial momentum for application as photocatalysts - particularly due to their much higher synthetic flexibility. For instance, their optical band gap can be tuned continuously throughout large parts of the solar spectrum by copolymerizing selected monomers in defined ratios. This tunability has sparked intense research interest in organic photocatalysts,[1] however, the fundamental understanding of photoinduced processes in these systems and the characterisation of their catalytically active sites have stayed behind the rapid development of new materials.

In this presentation, I will demonstrate how transient and operando optical spectroscopic techniques can be used to track the evolution of photogenerated reaction intermediates in polymer photocatalysts on timescales of femtoseconds to seconds after light absorption. To this end, short laser pulses are used to study these photocatalysts under transient conditions whereas long LED pulses are employed to establish operando catalytic conditions, and photogenerated reaction intermediates are then probed optically. Firstly, these techniques reveal insights into the yield of photogenerated charges, which enables an understanding of differences in hydrogen evolution activity between different materials.[2] Secondly, these techniques allow to monitor the transfer of photogenerated electrons to catalytically active sites as well as their accumulation under operando photocatalytic conditions, where differences in electron transfer time translate into different kinetic bottlenecks of the hydrogen evolution reaction for different polymers.[3] To illustrate these points, I will draw direct comparisons between nanoparticle photocatalysts made from the polymers F8BT, P3HT, and the dibenzo[b,d]thiophene sulfone homopolymer, P10, which is one of the most performant polymer photocatalysts reported to date.[2]

Artificial photosynthesis by photoelectrocatalysis is one of the most promising ways to store solar energy in the form of fuels, thus constituting a sustainable alternative to fossil fuels [1,2]. Conjugated polymers (CPs) are used as part of some photoelectrodes due to their good conductivity and the possibility to tailor their optoelectronic properties at the molecular level. Some of the most used CPs, such as PEDOT, have a linear structure; which makes them easy to process as thin films, but also unstable under UV illumination if they are in contact with water [3]. Conjugated Porous Polymers (CPP) [3-5] show higher stability due to their 3D structure. However, it is difficult to produce thin films with them by conventional methods such as drop casting or spin coating because of their morphology.

Thanks to the electropolymerization process, we are able to prepare homogeneous, transparent and light-absorbing CPP films both on conducting glass substrates and on inorganic semiconductors. One of these CPPs, IEP-19 (Imdea Energy Polymer-19), has been synthesized for the first time and it shows promising photocurrents, which are significantly higher than those of a previously known CPP with a similar structure: CPP-3TB. Moreover, hybrid photoanodes where the CPP is electropolymerized on top of the inorganic semiconductor present higher photocurrents than the semiconductors alone, showing a synergistic effect between the organic and inorganic semiconductors. These results will be explained according to the optical, photoelectrochemical and morphological properties of the photoanodes.

Multi-redox catalysis requires the transfer of more than one charge carrier and is crucial for solar energy conversion into fuels and valuable chemicals. In photo(electro)chemical systems, however, the necessary accumulation of multiple, long-lived charges is challenged by recombination with their counterparts. In this context, we have investigated charge accumulation in two model multi-redox molecular catalysts for CO₂ and proton reduction attached onto mesoporous TiO₂.[1,2] Transient absorption spectroscopy and spectroelectrochemical techniques have been employed to study the kinetics of photoinduced electron transfer from the TiO₂ to the molecular catalysts in acetonitrile, with triethanolamine as the hole scavenger. Under an applied electrochemical bias and at high light intensities, we detect charge accumulation in the millisecond timescale in the form of multi-reduced species. The redox potentials of the catalysts and the capacity of TiO₂ to accumulate electrons play an essential role in the charge accumulation process at the molecular catalyst. Recombination of reduced species with valence band holes in TiO₂ is observed to be faster than microseconds, while back electron transfer from multi-reduced species to the conduction band occurs in the millisecond timescale. Finally, under steady state irradiation conditions, we show how the redox state of the catalyst is regulated as a function of the applied bias and the excitation light intensity.[3]

Currently, water electrolysers (WE) based on acidic electrolytes, viz. proton-exchange membranes (PEM), are becoming a preferred technology for the electrolytic generation of green hydrogen fuel. One key limitation of PEM-WE is the low stability of anode catalysts, which are nowadays exclusively based on one of the rarest and most expensive metals – iridium. The less expensive transition metal oxide catalysts are most commonly less active and even more unstable. The instability problem can be overcome via integration of the catalytically active component, transition metal oxide, into a highly-conductive matrix that is thermodynamically stable under the conditions of the operating PEM-WE anode, i.e. at low pH and high temperature. The present report demonstrates the effectiveness of this approach with the antimony oxide matrix that is used to stabilise non-noble third row transition metal catalysts and ruthenium. Significantly improved stability as compared to that reported in the literature is demonstrated, in particular on a week timescale at industrially relevant temperature of 80^oC at pH 0.3. Factors affecting the activity and stability as well as possible strategies for further improvements will be also presented and discussed.

Main group sulphides such as In₂S₃, SnS₂ or ZnIn₂S₄, with bandgaps in the 2.0-2.2 eV range, can use significant amounts of visible light. So we have shown in earlier studies of their photocatalytic response, evidencing that they are active in the oxidative photodegradation of aqueous HCOOH with photons having wavelengths ≤ 650 nm. In addition, they are more photoactive in the same reaction than the typical reference compound CdS, and the first two (especially SnS₂) are also more resistant to photocorrosion in oxidative environments than the same photocatalyst CdS, as shown by the analysis of sulphide components detected in solution after prolonged photocatalysis.

More recently we have undertaken the study of their combination with enzymes for photoinduced water splitting. Thus, combining In₂S₃ with a hydrogenase enzyme allowed the photocatalytic generation of H₂ using a sacrificial agent [1]; the results suggested furthermore that the process was not limited by the transfer of electrons from the semiconductor to the enzyme. Later we have verified, for the first time ever, the photoelectrochemical generation of O₂ coupling a laccase-type enzyme with these sulphides; first with In₂S₃ [2] and later with SnS₂ [3]. It could be shown that a substantial decrease in the overvoltage needed was found in the presence of light. There were some problems with the stability of these enzymes in these O₂ generation conditions; as expected, SnS₂ could be shown to be more stable than In₂S₃.

A summary of all the mentioned results will be presented here.

The development of sustainable strategies for the production of added-value chemicals and fuels using renewable resources is particularly attractive to promote a transition towards a more sustainable energetic landscape, overcoming the dependence of fossil fuels at a global scale.^[1],[2] One of the most promising alternatives involves the use of renewable electricity (wind, solar, hydropower, etc…) to power electrochemical conversion processes, which convert abundant molecules (e.g., water, carbon dioxide, and nitrogen) into higher-value products (e.g., hydrogen, hydrocarbons, oxygenates, and ammonia). In all these processes, electrocatalytic or photoelectrocatalytic water oxidation stands out as the preferred reaction to provide the protons and electrons needed for the target reduction reactions. In this context, metal oxides are identified as excellent candidates, since these materials can fulfill most of the needed requirements, although their performance should be significantly improved for a more realistic technological assessment. Consequently, in order to boost the performance of these materials, as a preliminary step, a clear mechanistic understanding of the physical-chemical processes taking place in the bulk and at the interface with the liquid solution is essential. In the present talk, we will address different examples of the metal oxides (TiO₂, Fe₂O₃, BiVO₄...) combined with catalytic layers (Fe-Co Prussian Blue,^[3],[4] CoOx,^[5] Ag₃PO₄,^[6] NiO_x,^{[7], [8]} 2D-Sb,^[9] etc…), emphasizing the mechanistic insights leading to enhanced performance. Our studies focus on the correlation of the photoelectrochemical response of the materials with a detailed structural, optoelectronic and photoelectrochemical characterization carried out by different microscopic and spectroscopic tools.

We will present an approach to synthesizing single-phase complex metal oxides thin film photoelectrodes and discuss the challenges involved using CuBi₂O₄¹ as a model material. CuBi₂O₄ films with thickness gradients were deposited on 50x50 mm² FTO substrates by pulsed laser deposition (PLD) from a pure CuBi₂O₄ target. The relationship between the crystal structure, synthesis conditions, and properties (including photoelectrochemical performance) has been studied over a thickness range of 25 – 250 nm using combinatorial, high-throughput approaches.² A comparative study with conventional furnace annealing (FA) reveals the importance of radiative heat transfer during rapid thermal processing (RTP)³ at temperatures that exceed the normal thermal stability limit of the substrates (in this case, transparent conductive oxide films on glass). This study shows that single-phase CuBi₂O₄ is formed after RTP at 650 °C. In contrast, similar films heated up to 12 h by FA at 400 and 500 °C did not, and the films’ thicknesses had a substantial effect on the formation of phase impurities. Phase-pure CuBi₂O₄ photoelectrodes exhibit higher photocurrents, longer carrier lifetimes, higher photo-conversion efficiency, and better stability than non-pure photoelectrodes. Additionally, pure CuBi₂O₄ demonstrates typical photocurrent vs. thickness behavior of a single-photoabsorber photoelectrochemical device. However, non-pure photoelectrodes showed an unexpected photocurrent vs. thickness behavior, suggested to derive from different photoelectrochemically active impurity phases in the films.

Fe₂O₃ based photoanodes have been extensively studied in the context of solar fuels, with a variety of different strategies being proposed to mitigate their limited performance associated with short carrier lifetimes and surface recombination phenomena [1]. On the other hand, significantly less is known about other types of ferrite materials such as perovskites (general formula AFeO₃), which typically exhibit p-type conductivity. Our initial studies on phase pure LaFeO₃ and YFeO_3­ nanostructured electrodes have shown photocurrent responses towards the hydrogen evolution reaction with onset potentials (photovoltages) above 1.2 V vs RHE [2,3]. Despite the high photovoltages, the performance of these perovskites is limited by quantum yields below 1% [2-4]. In this contribution, we will examine different strategies to improve the photocurrent responses in these complex materials including surface modification and A-site substitution.

Our contribution initially focuses on pure rhombohedral LaFeO₃ featuring crystalline domains in the range of 60 nm, which are prepared by thermolysis of an ionic-liquid precursor and subsequently deposited onto FTO electrode by spin-coating [5]. Cyclic voltammetry and electrochemical impedance spectroscopy clearly show a p-type behavior with a flat band potential located at 1.44 V vs RHE. As the potential is swept into the accumulation regime, a faradaic current is observed associated with interfacial hole transfer (i.e. oxygen evolution reaction). Interestingly, a significant increase in the photocurrent responses is observed upon deposition of nanocrystalline TiO₂ layer by solution-based methods. We demonstrate that TiO₂ forms an abrupt heterojunction which act as barrier to hole-extraction, leading to photovoltages of 1.47 V vs RHE, which are among the highest reported for a single absorber. The introduction of Pt clusters further improves the quantum yield, but only in the presence of the TiO₂ overlayer. We shall also discuss the effect of introducing divalent cations such as Ca²⁺, Ba²⁺ and Sr²⁺ on the photoelectrochemical responses of LaFeO₃ [6]. These cations occupy La sites, introducing significant changes in the density of acceptor states as well as creating deep states associated which strongly accelerates the photoelectochemical oxygen reduction reaction. The complex relation between doping levels and photocurrent efficiency will be briefly discussed.

In order to obtain high solar-to-hydrogen (STH) efficiencies, a suitable semiconducting material with a band gap of 1.7 to 1.9 eV is needed as the top absorber in tandem solar water splitting devices [1]. An interesting candidate is α-SnWO₄, a ternary metal oxide with an ideal band gap of 1.9 eV. Recently it was reported that a record photocurrent density of 0.75 mA/cm² and extended stability can be achieved with NiO_x protected films prepared by pulsed laser deposition (PLD) [2]. However, the use of NiO_x protection layer limits the photovoltage as observed from the cyclic voltammetry and open circuit potential (OCP) analysis. This suggests that the interface between α-SnWO₄ and NiO_x is not ideal and understanding this interface is crucial to improve the performance further.

In this study, we present a thorough α-SnWO₄/NiO_x interface investigation by means of synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). These data are complemented with OCP analysis, density functional theory (DFT) calculations and Monte-Carlo-based photoemission peak intensity simulation. We found that the deposition of NiO_x with PLD introduces a strong upwards band bending (~400 meV) at the interface, which is favorable for charge separation. However, a significant oxidation of Sn²⁺ to Sn⁴⁺ can be simultaneously observed at the interface with increasing NiO_x layer thickness. Our photoemission spectra simulation indicates that this can be attributed to the formation of SnO₂ at the interface. A ~2 nm-thick SnO₂ layer is estimated after the deposition of 20 nm NiO_x. The implications of this SnO₂ layer to the interface junction properties and the limited photovoltage will be discussed and verified using a series of control experiments. Overall, our findings provide an important insight that future efforts need to be focused on the development of overlayers which do not oxidize the surface of α-SnWO₄, in order to further improve the performance of α-SnWO₄ photoanodes.

Artificial photosynthesis, inspired by natural photosynthesis, is considered a promising technology to store solar energy into chemical bonds (e.g., hydrogen) via (photo)electrochemical water splitting. In this process, the oxygen evolution reaction (OER) water acts as electron donor and it is considered to be the bottleneck of the process when using metal-oxide photoanodes. The efficiency of these photoanodes does not only depend on the nature of the metal oxide but also on the methodology used to synthesise them due to morphological, surface facets and doping variations, amongst others.[1] However, the mechanism of the OER on metal oxides as well as the nature of the efficiency loses remains elusive.

In this talk, I will present recent advances on the understanding of the kinetics of OER on different metal-oxide photoelectrocatalysts, focusing particularly on hematite synthesised by different deposition methods. A detailed mechanistic analysis of the OER on hematite photoanodes will be presented by the study of the water oxidation rate law under different physicochemical conditions.[2] I will also show evidence of the OER exhibiting equivalent kinetics despite the exhibited different overall performance and the correlation of lower photoanode performance with the presence of intragap states.[3]

This talk will describe our group’s recent efforts in the design and mechanistic evaluation of new molecular, macrocyclic Co(II) catalysts for aqueous H₂ generation. These catalysts were designed to incorporate redox-active bipyridine groups which are linked by nitrogen groups, both components which can participate in electron and proton transfer steps in the catalytic cycle. In comparing two molecular catalysts that differ by only one linking nitrogen, single crystal analysis reveals a profound impact on the molecular geometry, which in turn influences their relative catalytic activity. Photocatalysis experiments show that both catalysts are highly active for aqueous proton reduction at moderate pH levels, with the closed macrocycle reaching almost 2 x 10⁴ turnovers of H₂ when photo-driven by [Ru(2,2’-bipyridine)₃]²⁺ using ascorbate as an electron relay and a phosphine compound as the terminal electron donor.  Measurements of the electrocatalytic activity were used to investigate key steps in the mechanism of proton reduction, and from a detailed analysis of these experiments we propose a mechanism for catalytic proton reduction to H₂ that involves both intramolecular proton and electron transfer steps between the macrocycle ligand to the cobalt center. We will also describe how substituting pyrazine groups for the pyridyl groups in the macrocycle influence the redox properties of the macrocycle ligand, and in turn the photocatalytic activity. This work demonstrates the vital role of the second coordination sphere in the catalytic cycle, and places these relatively simple complexes on the pathway toward molecular catalysts that mimic the valuable features of enzymatic catalysis.

Carbon neutral energy sources that are scalable, deployable, and cost effective will be required at an unprecedented scale to halt irreversible climate change. To positively affect the status quo, polycrystalline, yet defective and heterogeneous, semiconductor materials are excellent candidates for targeting high efficiency, as well as low production cost, and long lifetimes of the device. However, understanding and controlling how defects, chemical heterogeneity, and microenvironments affect the efficiency and durability of integrated systems for real applications is still challenging. Yet is a necessary task to address mankind’s energy needs. This seminar will focus on the opportunities offered by the utilization of sunlight for solar fuel production. We will discuss the synthesis and the advanced characterization of integrated semiconductors and catalysts for (photo)electrocatalytic systems as they can be used under realistic operating conditions for solar fuel production.

Photoelectrochemical (PEC) water splitting is one technology to produce clean hydrogen fuel from abundant sunlight and water. To fabricate an efficient and stable photoelectrode for PEC water splitting, a multilayer structure, consisting of a protection layer (e.g., TiO₂) and a catalyst layer (e.g., Pt), is generally required. However, despite the importance of understanding the photo-physics underlying these multilayer photoelectrodes, the detailed analysis of them under operando condition is challenging due to the complexity of the device structures. In this talk, we demonstrate the versatility of the electrochemical impedance spectroscopy (EIS) method for investigating multi-layered photocathodes for PEC water splitting. By carefully analyzing the EIS data of various photocathodes with different classes of light absorbers, such as metal chalcogenide (Sb₂Se₃), metal oxide (Cu₂O), and crystalline Si, we were able to obtain information about the constituent semiconductors such as carrier lifetimes, doping densities, flat band potentials, and charge transfer rate constant under operando conditions. The EIS analysis presented in this study has made significant progress in establishing proper EIS models describing the realistic photo-physical behavior in complex multi-layered photocathodes.

Semiconductor/electrolyte interfaces attract intense interest to convert solar energy tochemicalfuels. Althoughmanyanalyticalmodelsdescribingthephotocurrent-voltage response of these devices exist, they have difficulty to reproduce full numerical simulations under small anodic bias. 

We recently derived an analytic model of a weakly absorbing n-type semiconductor/electrolyte interface with a slow rate of water oxidation reaction, fast direct recombination rate and under small anodic bias [1]. Excellent overlap of our model was demonstrated with full numerical simulations. Our model enabled us to simplify calculation of the impedance of the semiconductor/electrolyte interface derived in the work of Bertoluzzi et al[2]. The comparison of analytic and measured impedance allows to extract the reaction rate for redox reaction from the dark impedance and the direct bulk recombination constant of the semiconductor from the impedance under illumination.

This work provides easy-to-use recipe for the quantitative comparison of the recombination and reaction properties of the different semiconductor photoelectrodes by impedance spectroscopy and a starting point for development of even more general analytical models covering other recombination pathways and larger bias range in the future.

Hydrogen and other solar fuels have been highlighted as one of the future energy vectors. Having natural photosynthesis as inspiration, we can develop a device capable to split water using sunlight, obtaining oxygen and hydrogen. [1], [2] Different strategies can be used to achieve this: from separated light harvesting and catalytic systems to all integrated devices able to transform directly sunlight into fuels. These systems can be built with a variety of (photo)electrodes such as organic based material, chalcogenides or metal oxides. 

Although rapid progress is being made in the field, the efficiency of artificial systems still remains modest. Understanding of the limiting factors of these materials has allowed remarkable improvements in their performance. However, compared to of molecular systems, whose reaction mechanisms are better understood, there is still a lack of knowledge in the metal oxides mechanism of action.

In this talk I will focus on the use of the combined electrochemical and optical technique to probe the catalytic function of metal oxides for solar-to-fuel synthesis.[3-4] This technique opens a new possibility of studying multielectron reaction mechanisms on non-ideal metal oxides. I will show how using this combined technique we can elucidate the rate law of current flow in these systems and how our analysis can shed light into the reaction mechanism of photoelectrochemical solar fuels production. [5] From these studies I will discuss the key role of the catalyst through different examples. [6-8]

All this acquired knowledge will help to depict the mechanism of action of non-ideal metaloxides and so to systematically improve the next generation of photoelectrodes for water splitting.

Photoelectrochemical water splitting offers a great potential to store the intermittent solar energy as chemical fuels, i.e., hydrogen.To this end, tremendous efforts have been devoted to develop efficient light-absorbing semiconductors and electrocatalysts, which have resulted in various demonstrator devices with appreciable solar-to-hydrogen (STH) efficiencies.[1] Although it is not always discussed in these devices, efficient and safe separation of products from anode and cathode is also an essential requirement. Commercial electrolyzers employ separators, such as ion exchange membranes and diaphragms, between electrodes to avoid forming explosive product mixtures. However, implementing these separators in large scale photoelectrochemical devices may add extra complexity and potential loss. Alternatively, a hydrodynamic control can be introduced without having separators, in which products are collected from the outlet through the continuous electrolyte flow before they get mixed. The feasibility of such approach has been confirmed in previous numerical simulation reports.[2][3] However, these studies only considered dissolved gases, and gas bubbles were ignored. Also, the orientation of the device, i.e., whether the device has to be tilted from horizontal orientation for efficient solar absorption, was not considered. In tilted devices, product gas bubbles may largely influence the crossover due to the buoyancy force on the bubbles.

In this study, two-dimensional Euler-Euler multiphase fluid dynamic simulations, which calculate the volume fractions of liquid and gas phases, were introduced to investigate the crossover in a solar-driven membrane-less water splitting device. Our simulations revealed that, although overlooked in previous reports, gas bubbles contribute more to the crossover rather than dissolved gases. We also performed an extensive evaluation of various important parameters that affect the overall product crossover, e.g., the device tilt angle, bubble diameter, bubble formation efficiency, etc. For example, we found that smaller gas bubbles suppress the crossover due to the stronger momentum exchange between the liquid and gas phases. This suggests the potential benefit of using surfactants, which are known to decrease the bubble diameter, for efficient product separation in membrane-less devices. Based on our two-dimensional model, we also performed a dimensionless analysis in order to obtain a more universal picture of the membrane-less device, and we were able to present an operational design space for efficient product separation (< 1% crossover). Overall, our study highlights the critical importance in understanding and controlling the bubble formation under operating conditions to design efficient membrane-less water splitting devices.

Small perturbation techniques have been widely employed as powerful tools in the understanding of the mechanisms behind the relevant chemical reactions in (photo)electrochemical (PEC) systems, as the Oxygen Evolution Reaction (OER), the Hydrogen Evolution Reaction (HER) and the CO2 Reduction Reaction (CO2 RR). This contribution is focused on the relevant mechanistic insights that can be extracted from the analysis of relevant materials for the OER, as BiVO4, TiO2 and Ni-based electrocatalysts, with Electrochemical Impedance Spectroscopy (EIS) and Intensity Modulated Photocurrent Spectroscopy (IMPS). By analyzing a specific system, an equivalent circuit model can be designed and then, different parameters, as resistance and capacitances, can be extracted, giving relevant information from the different physical processes which take place during the studied reaction, as charge extraction, injection, or recombination. The combination of small perturbation techniques with conventional electrochemical methods is crucial for decoupling the different physical processes involved during the studied reactions.  

The development of a sustainable energy economy based on renewable, carbon-neutral energy is a necessary and urgent task. Photo-electrochemical (PEC) approaches to solar fuels and materials are interesting, provided they can be efficiently, stably, scalably, and sustainably implemented.

While significant progress on the development of earth abundant materials and on high-efficiency demonstrations has been achieved, there is relatively little known about the behavior of materials and devices under more realistic, varying conditions and on the implication of degradation on the performance and lifetime.

Here, I will first discuss multi-scale numerical modeling approaches to investigate and quantify the effect of photocorrosion on the performance evolution. I will show how important the local reaction environment is for the stability of a component, and how heterogeneity in the operating variables (current density, species concentration, temperature, etc.) affect degradation. Furthermore, I will comment on the importance of device design on device stability.

Second, I will then transition towards discussing material, device and system performance as a function of varying, non-design-point operation. I will show how these dynamics can affect the performance, show how multi-physical transport can help in controlling and smoothing some of the observed variations, and generally highlight synergistic effects. I will end with providing general guidelines on operating materials, devices and systems under more realistic conditions.

In my talk I will focus on the underlying charge carrier dynamics which determine the efficiency of solar driven water splitting in metal oxide based photoelectrodes and photocatalyst  suspensions and sheets. Experimentally my talk will be based upon a range of optical absorption spectroscopies, including transient absorption and operando spectroelectrochemical analyses. I will start by considering metal oxide photoelectrodes, addressing the impact of defect / dopant sites such as oxygen vacancies in determining photoelectrode performance. I will go on to consider the kinetics of water oxidation catalysis on metal oxide photoanodes, and the potential to apply rate law analysis of these kinetics using a charge carrier density based model as an alternative to Butler-Volmer based analyses. Finally I will discuss the role of charge carrier dynamics in determining the performance of metal oxide photocatalysts, and in particular the remarkable ability of La,Rh co-doped SrTiO₃ to drive proton reduction even under positive applied potentials.

Semiconductor-based solar fuel synthesis represents an important method for direct conversion and storage of solar energy in the form of chemical energy.  The performance of such a system is highly sensitive to the nature of the semiconductor surfaces.  For instance, the photovoltage of the system is determined by the difference between the electrochemical potential of surface chemistry and the Fermi level of the semiconductor.  Due to the complexities at the solid/liquid interface, however, it is often difficult to understand the detailed surface behaviors.  Consider water oxidation as an example.  While it is known that the nominal overall 4-e, 4-H⁺ reaction features an electrochemical potential of 1.23 V vs. reversible hydrogen electrode, the process typically proceeds through multiple steps. Despite extensive studies, it remains unknown which step is the rate determining step for many heterogeneous systems.  The knowledge is particularly weak for semiconductor-based systems.  As a result, the actual electrochemical potential of the process is often difficult to define.  At the root of this challenge is the lack of knowledge on the detailed water oxidation mechanisms at the molecular level on solid-state surfaces.  In this presentation, we discuss how to address this issue using molecular catalysts.  We show that the homogeneous catalysts can work well when immobilized on the surface.  Importantly, the reaction mechanisms of these catalysts are well defined.  Facile treatments of these catalysts can readily convert them into heterogeneous catalytic centers with well-defined atomic structures.  These new catalysts provide a new platform to study photoelectrochemical water oxidation with unprecedented details.  The knowledge is expected to accelerate development of solar fuel synthesis materials.

There has been a vast improvement in photovoltaic (PV) technology in the last couple of decades; however, solar PV technology has a lot of drawbacks, including that it is intermittent, broadly dispersed, and non-transportable. Hence, cost-effective solar energy storage is critical for the widespread implementation of solar energy as the primary energy source. A novel slurry reactor design, for performing unassisted water-splitting to generate low-cost hydrogen with high solar-to-hydrogen (STH) efficiency, will be presented. The design includes employing two semiconductors in tandem, to achieve sufficient photovoltage to overcome the thermodynamic minimum of 1.23 V for water splitting at room temperature and the overpotential associated with hydrogen evolution reaction (~0.05V) and oxygen evolution reaction(~0.3V). As a proof-of-concept, tandem TiO₂/FTO/p⁺n-Si microwire photoelectrodes were fabricated demonstrating its ability to perform unassisted water splitting. These microwires were then detached from the substrate into a slurry. Ni was photoelectrodeposited at the Si base to prevent silicon oxidation, function as a hydrogen evolution catalyst, and be used as a ferromagnetic handle for magnetic alignment. Ongoing efforts to orient these microwires for minimizing spectral-mismatch and characterizing its impact on transmittance and hydrogen quantification will also be discussed.

Biomass valorization is an emerging method convert waste biomass into value-added feedstocks as an alternative to fossil-derived carbon chemicals. However, this technology has been limited by energy-intensity thermochemical reaction conditions. Alternatively, electrocatalytic biomass valorization is a sustainable and economical route that may utilize renewable electricity.

From a material design perspective, nickel and cobalt oxide derivatives have shown great potential in upgrading biomass, but the reaction still requires large overpotentials. Further, mechanistic knowledge is still lacking as the exact nature of the catalytically active sites is not fully understood. To this end, metal-organic frameworks are intriguing candidate catalyst systems as they exhibit high porosities and chemically well-defined active sites, enabling them to serve as ideal model systems for designing high efficiency catalysts. In this work, we present a MOF electrocatalyst model system featuring atomically precise Ni and Co active sites as a model system for electrochemical biomass valorization.

Synthesized via a simple solvothermal method, a MOF featuring square-planar nickel and cobalt metal ions coordinated the oxygen atoms of triphenylene li has been characterized by XRD, SEM, and TEM. Electrochemical analysis of this system reveals that the Ni and Co triphenylene MOFs surpass state-of-the-art metal oxide biomass valorization electrocatalysts in terms of onset potential and efficiency. This work opens avenues for understanding critical parameters en route to the design of next-generation biomass valorization electrocatalysts.

The interfacial charge transfer efficiency is the ratio between the charge transfer rate at the photo-electrode surface and charge carrier generation rate within the photo-electrode. It is an essential parameter not only for material performance assessment and benchmarking but also for the design and modelling of photo-electrochemical devices as it can be used to predict current densities at a given photon flux and electrode potential [1]. Many techniques for measuring interfacial charge transfer efficiencies have been proposed and further developed along the years [2-3], and the theoretical foundations have been discussed in detail for the techniques themselves and the phenomena involved.

Alas, the experimental considerations, reproducibility, accuracy, and the comparison of all these techniques have been left for consideration to each researcher, giving rise to widespread values and reported procedures that sometimes cannot be compared and reconciled with each other. The most widely reported techniques are photo-electrochemical impedance spectroscopy (PEIS), current density ratios in the presence and absence of sacrificial reagents (SR), transient photocurrents by chrono-amperometry (CA) after photon excitation, and more recently, intensity modulated photon current/voltage spectroscopy (IMPS, IMVS). From these, the charge transfer efficiency can be measured by using raw data (graphical approach), fitting to equivalent electrical circuits (EC) or by distribution of relaxation times (DRT) coupled with peak deconvolution. These techniques and data analysis approaches have their own drawbacks and impracticalities due to available hardware, quality of the data and the properties, such as nanostructure, of the photo-electrodes.

We will report the pitfalls, the accuracy and precision, and provide experimental recommendations for these techniques with a detailed comparison when employed for charge transfer efficiency determination in a stable Sn-doped hematite photo-electrode [4] for the oxygen evolution reaction. This will offer researchers in the field a pathway to select the best approach to measure this critical parameter. Findings indicate that CA offers the most reproducible values, while the use of sacrificial reagents is the least reproducible; however, these methods can only be used under specific conditions and the latter requires multiple measurements and absence of current doubling effects. IMPS coupled with EC offers reproducible and accurate values if photo-electrodes can be assessed under a wide range of light intensities and electrode potentials (rigorous approach); while PEIS coupled with DRT also offers reproducible, although underestimated, values with fewer experimental procedures and less specialised hardware.

Electrochemical conversion of abundant feedstocks to fuels and value-added chemicals is rapidly gaining significance as a promising method to harness renewable electricity. Specific reactions within this context that my research group is focused on are the reduction of CO2 and oxidation of waste biomass. Because the design of new catalytic systems is inherently linked to a precise understanding of how these reactions proceed on heterogeneous surfaces, we put considerable efforts in developing methodology for opernado probing with Raman spectroscopy CO2 reduction and biomass valorization. This talk will detail our efforts in the design, electrochemical characterization, and spectroscopic investigation of

1) Composite systems of metallic nanoparticles decorated with functional organic ligands with steer CO2 reduction reactions down a select pathway on their surface

2)  CO2 catalysis at the material-Metal Organic Framework (MOF) interface

3) Electrochemical oxidationof 5-hydroxymethylfurfural (HMF) on gold and transition metal oxide surface

In all, I show how using opernado Raman spectroscopy provides the mechanistic information on surface reaction mechanisms that enhance our understanding of functional hybrid interfaces and provides avenues for future materials design within the context of electrosynthesis of fuels and chemicals.

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

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

            Imagine an energy vector with a round-trip-efficiency > 80 % that is easily and efficiently chargeable using either sunlight or biomass. Imagine that this energy vector can produce dispatchable electricity and chemical fuels. When “solar rechargeable battery” concept was first proposed at the beginning of the 80s [1], it aimed at to play an important role on harvesting and storing sunlight energy[2].[1] However, there was no more research going on in this topic until 2013 when Yang et al. [3] published an article integrating a dye sensitized solar cell (DSSC) with a redox flow battery (RFB) for converting sunlight into storable chemical energy, displaying a solar-to-chemical energy conversion efficiency of ca. 0.09 %. In 2016, Wedge et al. [4] reported the first solar aqueous alkaline RFB using low cost, environmentally safe and stable materials: hematite photoelectrode and ferrocyanide/AQDS redox pairs. This revamped the idea of using photoelectrochemical (PEC) cells for charging two electrochemical fuels in an integrated technology, which was named solar redox flow cell (SRFC). In 2020, the world record solar-to-electrochemical energy conversion efficiency reached 20 % [5], demonstrating the potential of SRFCs over more conventional PEC technologies and over conventional PV-redox flow battery approach – Figure 1.

            This talk will present the latest developments in the field, addressing critical sustainability pathways: i) stable and efficient earth-abundant photo-absorber materials; ii) stable and high energy density redox pairs, made of earth abundant elements, matching with the energy levels of the semiconductors; and iii) optimized SRFC device architectures suitable for large-scale solar fuels production. Mimicking the nature, a SRFC device can be combined with a very recently disclosed Microbial Redox Flow Cell (MRFC) [6] to produce continuously – day and night, summer and winter – the electrochemical fuels. Finally, charged redox pairs can be used to produce dispatchable electricity but also to produce chemicals either through direct redox reactions or following an electrochemical approach.

[1] In 1991, Kumar et al. [2] asked “The entire discussion is geared towards answering a relevant question: what has gone wrong to result in the stagnation and failure in commercialization of a PEC based solar cell?”

References

1.         Licht, S., et al., A light-variation insensitive high efficiency solar cell. Nature, 1987. 326(6116): p. 863-864.

2.         Sharon, M., et al., Solar rechargeable battery—principle and materials. Electrochimica Acta, 1991. 36(7): p. 1107-1126.

3.         Liu, P., et al., A Solar Rechargeable Flow Battery Based on Photoregeneration of Two Soluble Redox Couples. ChemSusChem, 2013. 6(5): p. 802-806.

4.         Wedege, K., et al., Direct Solar Charging of an Organic–Inorganic, Stable, and Aqueous Alkaline Redox Flow Battery with a Hematite Photoanode. Angewandte Chemie International Edition, 2016. 55(25): p. 7142-7147.

5.         Li, W., et al., High-performance solar flow battery powered by a perovskite/silicon tandem solar cell. Nature Materials, 2020.

6.         Santos, M.S.S., et al., Microbially-charged electrochemical fuel for energy storage in a redox flow cell. Journal of Power Sources, 2020. 445: p. 227307.

In terms of technological readiness, the most feasible approach for solar energy utilization would be a photovoltaic (PV) panel. However, the PV is faced with challenges concerning the security of supply because of the intermittent nature of the sun. In this context, solar rechargeable redox flow battery (SRFB) technology is being in the spotlight as a mean of simultaneously storing the solar energy into chemicals, which can be readily utilized to generate electricity via reversible reactions [1,2].

However, the plain fact is that there is that most studies overlook practical challenges arising from the inherent instability and degradation of the system under the light and heat. According to our recent theoretical modeling, silicon-based solar PV system shows a severe power-loss exceeding 20% compared to the ideal case [3]. In this work, above described thermal degradation is quantified by introducing a thermo-electrochemical model, which covers both heat-transfer and electrochemical studies to minimize the gap in performances between the laboratory and practical working environments with drastic temperature change and thermal shocks.

The main focus of the study is on avoiding the thermal efficiency loss at a high temperature, which has been a critical technical barrier for the practical application. In our model system, the electrolyte flow acts as a coolant-storage multi-functional medium, and it stabilizes the operating temperature of the photo-charging system via a heat-transfer regime at solid/liquid interface between the solar-driven device and electrolyte.

Energy generation devices have been grown tremendously from fast few decades, but the system is still limited with energy storage devices. Storing energy in the batteries is not the permanent solution, nevertheless energy can’t be store longer period. Only way to store energy for longer period is storing energy in chemical bonds, such as fuels, gas or chemicals. Therefore, concept of electrochemical energy storage device has been rising recently, especially electrochemical organic synthesis, has made a footprint for the green synthesis of value-added chemicals. Impotently these electrochemical processes can be renewable.  Considering, carbon neutral industry promise – biomass has great potential for providing many flatform chemicals. The one of the elements of biomass, 5-hydroxymethylfurfural (HMF) and its oxidized form 2,5-furandicarboxylic acid (FDCA) is a monomer of biobased polymer and its properties are superior than the PET. We have investigated efficient electrochemical conversion of HMF to FDCA using abundant nickel as a catalyst. We investigated the key factors for tuning the chemical selectivity for HMF oxidation over the competing oxygen evolution reaction (OER) at the catalyst surface. We show that the selectivity for HMF oxidation is enhanced by removing trace impurities of iron species as well as adjusting the composition of the alkali hydroxide electrolyte solution. LiOH electrolyte without iron impurities is more favorable for HMF oxidation and whereas CsOH with iron species present is more active for OER and unfavorable for HMF oxidation. Under optimized condition we have achieved 98% faradaic efficiency for the production of FDCA from HMF, with iron free 1M LiOH electrolyte (pH 14). This simple approach can be used as model system for other electrochemical organic synthesis, where OER is competing process.

Redox flow batteries offer a reliable solution for future grid-scale energy storage. In comparison with mostly investigated and commercialized vanadium flow batteries, aqueous zinc-iodide flow battery is a highly promising contender due to its cost-effectiveness, environment friendliness, safety, and high energy density. However, there are still obstacles to overcome before utilizing its full potential. Therefore, various research is under investigation to improve the battery design by optimizing its components towards the development of high-performance batteries. Among those cell components, electrolyte has a major contribution in scaling up cell performance. Due to an imbalance of concentration of solutes between the anode and cathode compartments during electrochemical cycling, the differential osmotic pressure results in water migration through the ion-permeable membrane from the compartment with lower ionic strength to the compartment with higher ionic strength, which is usually overlooked by using a static positive compartment, limiting its capacity. The addition of extra solute to the electrolyte solution is a way out to resolve this issue and reach an ionically balanced situation between two compartments. In this work, we have carried out the experimental analysis of a lab-scale Zn-I flow battery as per our theoretical ionic strength calculations to reach an overall balanced system. From the theoretical calculation, it has been proven that by adding extra 2 M potassium iodide (KI) to an electrolyte of 1.5 M ZnI₂: KI, the ionic balance between the compartments could be achieved. We have performed electrochemical impedance spectroscopic (EIS) technique as a tool to study the solution resistance and ionic conductivity of the half-cell compartments. Further full cell cycling following post-mortem analysis of the half-cell electrolytes ensured no water migration between compartments during cycling, also excellent cycling efficiencies have been achieved. Overall, this mechanism shows an efficient path for the improvement in the electrolyte design in the development of high-performance Zn-I flow batteries, which enlighten a step forward to the research in next-generation aqueous flow batteries.

Keywords: Aqueous Zn-iodide flow batteries, water migration, osmotic pressure, electrochemical impedance spectroscopy

Organic-inorganic and all-inorganic lead halide perovskites (APbX₃) have made a huge step towards highly efficient emergent photovoltaic (PV) technology with already 25.2% power conversion efficiency. The high defect tolerance as well as their suitable and tunable optoelectronic properties made them an attractive alternative to the silicon and thin film PV technologies. Apart from that, perovskites show versatility in their applications fields e.g. in LEDs or photoconductors but also in energy storage such as perovskite solar cell coupled self-rechargeable batteries. However, one of the major problems encountered with Pb halide perovskites, apart from structural and chemical stability, is the toxicity associated with heavy metal lead. Potentially less toxic bismuth (Bi) halide perovskites were reported to possess promising optoelectronic properties including a high absorption coefficient and can be processed from solution using a variety of wet chemical deposition techniques and additives. In this work, different Bismuth based perovskite-like materials were screened by varying the A-site of the A₃Bi₂I₉ with organic monovalent cations like CH₃NH₃⁺ (methylammonium, MA), CH(NH₂)₂⁺ (formamidinium, FA), (CH₃)₂NH₂⁺ (dimethylammonium, DMA), (guanidinium, NH₂)₃⁺ (GA), C₅H₆N (pyridinium, Pyr), new C₄H₅N₂ (pyridazinium, Pyz) and inorganic alkali metal cations for photovoltaic application. Furthermore, Silver bismuth iodides have gained recent attention as a stable alternative to lead perovskite photovoltaics. Ag₃BiI₆ represents a rudorffite structure and can be used in the same solar cell architecture. Herein, Ag₃BiI₆ thin films were investigated experimentally and theoretically as solar cell absorbers and their degradation was attributed to ion diffusion of highly mobile AgI.

Tactile or electronic skin is needed to provide critical haptic perception to robots and amputees, as well as in wearable electronic systems that are used for health monitoring and wellness applications. Energy autonomy of skin is a critical factor in these application to enable portability and longer operation times. This lecture will present an energy autonomous electronic skin based on a novel structure, consisting of graphene based transparent tactile sensitive layer integrated on photovoltaic cells. Transparency of the touch sensitive layer allows the photovoltaic cell to effectively harvest light. The touch sensitive layer requires ultralow power (20 nW/cm²) for its operation and this leads to surplus energy generation by the photovoltaic cells underneath. The lecture will also present our advanced version of electronic skin, where no touch sensor is used and yet the innovative arrangement of solar cells allows touch sensing from large areas. Given that there is not separate touch sensor, this advanced version of electronics skin does not consume any energy for sensing operation and instead only produces energy. If this skin is present over large areas, as human skin is present over whole body, then it could generate sufficient energy to power devices such as actuators used in robotics and prosthetics. Such scenarios enabled by the energy autonomous electronics skin integrated on robotic hand will be presented in the lecture along with the tasks such as grabbing of soft objects.

The photovoltaic (PV) module directly integrated of solar conversion and electrochemical energy storage provides a new and promising insight towards solar energy utilization. Particularly, solar rechargeable redox flow batteries (SRFB) offer a cost-effective and compact solution to solve the photovoltaic intermittency issues. Despite of the large state of the art of SRFB, the actual technology is hampered by low efficiencies and “extra” potential is required for charging the battery. Among, all RFB, all vanadium is the chemistry most development and most mature, being worldwide commercialized. The vanadium redox flow battery (VRFB) presents a standard cell potential is 1.26 V, which in practise could arise up to 1.7V. This operational potential is extremely high to pair with PV commercial modules, supposing a great challenge for the solar recharging process. Herein, the design of several configurations using different materials will be presented, obtaining results quite encouraging towards the realization of the Solar-charged -VRFB (SVRFB). Two approaches have been followed: (1) inexpensive Cu(In,Ga)Se₂ modules disposed in 3 and 4 series-connected cells, allowing the full unbiased photocharge under 1 Sun illumination. The resulted SVRFB device can deliver excellent energy efficiency (77%) and solar-to-charge efficiency (7.5%) [1]; (2) Triple junction TF silicon solar cell under illumination (300 mW cm^-2), operating at 25 mA cm^-2 as bias free-photocurrent [2]. In that case, the energy density achieved values up to 54 Wh L^-1, while the solar-to-output electricity efficiency was roughly 10%. Finally, several strategies to increase the photocharge of the negative half-cell reaction in Vanadium redox flow batteries is discussed [3].

The ability to tune thin metal oxide coatings by wet-chemistry is desirable for many applications, yet it remains a key synthetic challenge. In this work, we introduce a general colloidal atomic layer deposition (c-ALD) synthesis to grow metal oxide shells (AlOx, ZnOx, TiOx) of tunable thickness (1 to 15 nm) around nanocrystalline cores of different compositions, including quantum dots (perovskites-PeQDs, CdSe, C-dots, InP).¹

We compare the c-ALD with the previously developed gas-phase ALD in film to highlight its advantages which comprise the preserved colloidal dispersability, the improved optical properties and the stability.²

Finally, we illustrate the importance of such a finely tuned metal oxide shell thickness to study nanoscale phenomena such as energy transfer between PeQDs and CdSe nanoplates, between PeQDs and metal nanoparticles and the anion exchange reaction in PeQDs.^1,3-5

Lead-halide perovskite APbX₃ (A=Cs or organic cation; X=Cl, Br, I) quantum dots (QDs) are subject of intense research due to their exceptional properties as both classical¹ and quantum light sources.^2-4 Many challenges often faced with this material class concern the long-term optical stability, a serious intrinsic issue connected with the labile and polar crystal structure of APbX₃ compounds. When conducting spectroscopy at a single particle level, due to the highly enhanced contaminants (e.g., water molecules, oxygen) over NC ratio, deterioration of NC optical properties occurs within tens of seconds, with typically used excitation power densities (1-100 W/cm²), and in ambient conditions. By using a suitable polymer matrix, these detrimental effects can be suppressed, and intrinsic exciton and multi-exciton dynamics can be explored at the single particle level. 

Here, we report a comprehensive investigation of the room temperature single QD optical properties. The results reveal the origin of the QD homogeneous PL linewidths, and the peculiar size-dependent exciton and multi-excitons recombination dynamics.  

Such findings guide the further design of robust single photon sources operating at room temperature.  

References:

[1] Akkerman et al., Genesis, challenges and opportunities for colloidal lead halide perovskite nanocrystals. Nat. Mater. 17, 394–405 (2018).

[2] Becker et al., Bright triplet excitons in caesium lead halide perovskites. Nature 553, 189–193 (2018).

[3] Rainò et al., Superfluorescence from lead halide perovskite quantum dot superlattices. Nature 563, 671–675 (2018).

[4] Utzat et al., Coherent single-photon emission from colloidal lead halide perovskite quantum dots. Science 363, 1068–1072 (2019).

[5] Rainò et al., Underestimated Effect of a Polymer Matrix on the Light Emission of Single CsPbBr₃ Nanocrystals. Nano Lett. 19, 3648-3653, (2019).

At the core of the heated debate around 0D lead bromide perovskites is the origin of the green emission, which was observed in bulk powders and single crystals, but not in nanocrystals. Early speculations of an intrinsic emission of the material gradually sedimented into two main lines of thought: some groups point at deep band levels induced by point defects, most likely Br vacancies, while others claim the green emission stems from embedded CsPbBr₃ (3D) nanocrystals or some other lower dimensional perovskite structures, whose formation within the bulk 0D material is challenging to be discerned from XRD patterns.  Although the latest research seems to lean toward the 3D contamination induced emission, no final consensus has been achieved so far.

In this work we present ab initio molecular dynamics (MD) simulations to study Cs₄PbBr₆ in a wide range of temperatures from 34 K to room temperature, and we analyze the electronic structure of the system at every timestep. We also extended the study to the 0D crystal with Br vacancies and to the 3D perovskite crystal. By comparison of the three systems, we demonstrate that point defects cannot be responsible of the green emission because we observe a fast phonon-mediated quenching mechanism of the intrinsic emission of the material already at low temperatures. This is idea is corroborated by variable temperature photoluminescence studies on 200 nm size non-emissive 0D NC, which lack the impurities present in powders.

The relaxation of above-gap (‘hot’) carriers is responsible for major efficiency losses in present-day solar cells, and involves a complex interplay between carrier-carrier and carrier-phonon coupling. Unravelling the mechanisms of cooling is therefore an essential step for both understanding and developing emerging photovoltaic materials. Perovskite nanomaterials are an exciting class of compounds because they offer facile and broad optoelectronic tunability by size, dimensionality and composition. Here, we aim to elucidate the effects of these properties on carrier cooling by employing ultrafast pump-push-probe spectroscopy. This three-pulse technique allows cooling to be isolated from a melee of other excited state processes, while also allowing independent control over the hot and cold (band-edge) carrier subpopulations. These experiments show that while carrier cooling is generally indifferent to nanocrystal size in moderately confined systems, intriguing results are obtained upon altering the shape of the nanocrystal, and are also influenced greatly by material composition.

Lead halide perovskite nanocrystals (NCs) have emerged as a potential material for LED and solar cell applications [1]. However, despite of their promising performance, the band gap of lead halide perovskite NCs remains large that limit the absorption in infrared (IR) part of spectrum [2]. In this work, we explore the possibility to harvest the IR spectrum using formamidinium lead iodine (FAPI) perovskite nanocrystals by doping of PbS NCs. As a short wave IR absorber, PbS NCs has attracted much attention yet it suffer with high dark current eventually that limit the device performance. By mixing the two compounds with an optimum ratio, it is possible to preserve most of the IR absorption while the transport driven by the wider band gap of the perovskite, this enabling a dark current reduction. In addition to understand the electronic structure of FAPI/PbS hybrid, we fabricated an FET using high capacitance solid state gating. Using this strategy, we show that the hybrid material has an n-type nature with a charge carrier mobility of 2 x 10^-3 cm² V^-1s^-1.

However, as FAPI is introduced into the PbS NCs array, the benefit of the reduced dark current is partly mitigated by a reduced IR absorption. This problem is address by introducing a plasmonic resonator. The latter relies on a grating that generate a multi passes of the light into the absorbing layer thus enhancing the IR absorption [3]. The resonant electrode enhances the light-matter coupling within the NCs film that enhance the IR absorption up to 3 times [4]. In addition, the reduction of the interelectrode spacing enable photoconduction gain leading to an improved responsivity and detectivity by two order of magnitude in comparison to pristine PbS.

Colloidal nanoplatelets (NPLs) have recently emerged as a novel and exciting class of materials. While several established procedures are available for highly luminescent 4.5 monolayer and thicker NPLs, with emission spanning the green and red spectral region, typical synthesis protocols to prepare blue-emitting CdSe NPLs (λ ≈ 460 nm) yields particles with large surface areas and poor photoluminescence quantum efficiency, often accompanied by a strong emission peak from intragap trap states. Further applications may therefore be hampered by their high surface-to-volume ratio.

Here we present our work on the development of a synthesis protocol that achieves improved control over the lateral size, by exploiting a series of long-chained carboxylate precursors, varying from cadmium octanoate (C₈) to cadmium stearate (C₁₈). The length of the metallic precursor is key to tune the width and aspect ratio of the final NPLs (from 3:1 to 15:1), as well as the overall reaction yield, which increases for shorter chain length. The reduced NPL lateral dimensions lead to enhanced photoluminescence quantum efficiencies, reaching up to 30%, and good colloidal stability. As the width can be tuned down to 3.7 nm, we were able to construct also a convenient sizing curve, relating the NPL absorption position and width, and careful comparison with 4.5 monolayer NPLs demonstrate that the blue-emitting NPLs are characterized by faster emission lifetimes and a higher absorption coefficient. Via a slight adjustment, we also obtained 2.5 monolayers NPLs, with near-UV emission (λ ≈ 400 nm), and a quantum efficiency up to 11%. Our results contribute to achieving stable and efficient sources for applications such as blue and UV light emitting devices or lasers, or fast quantum light sources.

Core/shell nanocrystals in which the materials change gradually from core to shell are very very promising structures to optimise the opto-electronic properties and quantum efficiencies of nanoscale semiconductors. Gradients are able to minimise crystal defects, lattice mismatch, and can be used to engineer the envelope wave function of excitons in order to suppress non-radiative Auger processes. However, due to the small size of the particles, so far no reliable method exists to quantify the extent of such a gradient.

In this work we have measured the material gradient of ZnSe/CdS core/shell nanocrystals, which were synthesised at elevated temperatures (260 and 290 °C), which controls the rate of radial ion migration [1]. We used EXAFS spectroscopy to determine the average coordination of selenium ions, which were fitted to a continuum model for the radial distribution of cations and anions [2].

It could be shown that for the 260 °C sample the data shows strong cation migration, which transports significant amounts (> 50%) of cadmium into the core, while the anion gradient is consistent with negligible ion migration beyond the interfacial monolayer. This is significant, because many shell growth protocols that are assumed to produce sharp interfaces are performed at similar temperatures. At higher temperatures of 290 °C the data deviates strongly from the model, with effectively less cation migration. This is explained by the formation of an ordered Zn_0.5Cd_0.5Se superlattice in the core in order to mitigate the lattice mismatch die to the increasing CdSe content of the core [3]. Raman spectroscopy shows selective resonant enhancement of the core LO phonon overtones, which indicates that the exciton is primarily localized in the core and at interfacial traps, and that the electronic structure flips from a type-II to a type-I system.

Hence, the combination of X-ray and Raman spectroscopy is able to identify both the chemical and electronic structure of core/shell particles and produces an accurate gradient model that can be employed in more precise and predictive structural calculations. The high-temperature product sheds light on why some highly emissive nanocrystals still blink and struggle to reach unity quantum yield [4].

Vacancy-ordered triple perovskites have recently come under the scientific spotlight as promising materials for high-performance next-generation optoelectronic technologies.[1,2] Their A3B2X9 stoichiometry facilitates the replacement of the toxic Pb2+ cation with a benign isolectronic B3+ cation (e.g. Bi3+ or Sb3+), while preserving the perovskite crystal structure. Unfortunately, however, these materials tend to exhibit large bandgaps (> 2 eV), impeding their application in many photo-catalytic/voltaic devices.[3,4]

In this work, we demonstrate a drastic shift of over 1 eV in the optical absorption onset of Cs3Bi2Br9 (from 2.58 eV to 1.39 eV), upon doping with tin. Through a combination of detailed theoretical and experimental characterisation of this novel material, we elucidate the origin of broadband absorption. Sn is found to disproportionate in the doped material, inducing a strong intervalence charge transfer (IVCT) transition, whilst preserving the structural integrity of the perovskite framework.

Our work provides valuable insight regarding the effects of mixed-valency and structure-property relationships in perovskite-inspired materials, guiding design strategies and expanding the compositional space of candidate materials. Moreover, we anticipate that this massive reduction in absorption onset could aid charge transport and/or photo-catalytic performance, opening the door to unexplored applications of this material class.

Photon upconversion, where two or more low energy photons are converted into one high energy photon, shows great potential in bioimaging, catalysis and solar energy conversion. Photon upconversion has traditionally been realized with lanthanide-doped nanoparticles, or organic dye sensitized triplet-triplet annihilation (TTA) based upconversion platforms. In recent years, QD sensitized triplet-triplet annihilation based upconversion systems have achieved impressive upconversion quantum efficiency and demonstrated many unique advantages, including high photostability, large extinction coefficient, high spectral coverage and tunability, and low singlet-triplet energy gap. In this talk, we discuss our recent work in developing and understanding QD/mediator interface for efficient QD-sensitized photon upconversion. We summarize the main results of time-resolved spectroscopic studies of various factors affecting the rate of triplet energy transfer (TET) from the QD to the surface attached mediator (TET1) and from the mediator to the emitter in solution (TET2). To identify the key design rules, we compare three PbS sensitized upconversion systems using three mediator molecules with the same tetracene triplet acceptor at different distances from the QD. Our results show that the mediator triplet state is mostly formed by direct TET from quantum dot. With increasing distance between the mediator and PbS QD, the efficiency of the TET1 from the QD to the mediator decreases due to a decrease in the rate of this triplet energy transfer step, while the efficiency of the TET2 from the mediator to emitter increases due to a reduction in the QD induced mediator triplet state decay (via the external heavy atom effect). The rate constant of TET2 is three orders of magnitude slower than the diffusion limited value. We show that the effect of QD/mediator on the total unconversion efficiency measured under CW illumination conditions can be well accounted for by the independently determined efficiencies of TET1 and TET2 steps, providing important insight on the design and rational improvement of efficient photon upconversion systems.

Singlet exciton fission is a process that occurs in select organic materials wherein a spin-singlet exciton redistributes its energy to form a spin-correlated triplet exciton pair. Incorporating singlet fission materials into light harvesting platforms offers potential to enhance their performance by 40% while singlet fission’s ability to create entangled spin states at room temperature makes it attractive for quantum computing devices. Likewise, singlet fission’s inverse process, triplet fusion, has been used to create photon upconversion systems that produce high-energy excitons from pairs of low-energy photons. Such systems can enable new near-infrared sensors and photocatalysts driven by low-energy light. Intrinsic to the design of any singlet fission or triplet fusion-based device, however, is the exchange of energy, typically in the form of a spin-triplet exciton, between an organic material from an inorganic semiconductor. Hybrid materials consisting of semiconductor quantum dots functionalized with organic molecules are a premier platform for study of this energy transfer process. The high surface to volume ratio of these materials effectively means they consist entirely of interfacial molecules and the energy level tunability of quantum dots allows exploration of how the redox properties of the interface impact energy transfer.

Here, we report results on both PbS and Si quantum dots interfaced with a range of acene and rylene energy acceptors. We find that by tuning the energy level alignment of PbS quantum dots to that of rylene acceptors, the transfer of charge carriers across the interface can be varied by an order of magnitude. Interestingly, electronic structure calculations suggest this rate variation stems from electrostatic effects that both alter interfacial energy level alignment and shift the average orientation of molecules tethered to PbS. For Si quantum dots, energy transfer to acene and rylene acceptors is decidedly slow, unfolding on nanosecond to microsecond timescales due to weak coupling. Nevertheless, this process is highly efficient due to a lack of competing deactivation pathways. Interestingly, we find subtle changes in the structure of the triplet exciton energy acceptor lead to large changes in the energy transfer rate. These rate changes are unexpected on the basis of electronic structure calculations performed on molecules tethered to Si(111) surfaces, suggesting more exotic surface structures may play a key role in facilitating triplet energy transfer from silicon to organic molecules.

Although colloidal semiconductor nanocrystals have been widely studied for three decades, the understanding of their excited-state dynamics continues to evolve. Charge-carrier trap states on nanocrystal surfaces play an essential role in processes such as electron–hole recombination and charge transfer but their dynamics are challenging to probe spectroscopically. Photogenerated holes in CdS and CdSe nanocrystals trap to the orbitals of undercoordinated S and Se atoms on the particle surface on a picosecond timescale. We recently presented evidence that trapped holes on the surfaces of CdS and CdSe nanocrystals are not stationary but instead undergo a diffusive random walk at room temperature. The initial evidence came from interpretation of electron-hole recombination dynamics in transient absorption (TA) spectroscopy data of non-uniform CdS and CdSe nanorods. More recently, temperature-dependent TA data provided insights into the mechanisms of trap-to-trap hole hopping. The experimental data, together with theoretical insights from our collaborators Joel Eaves and coworkers, builds an increasingly precise description of trapped-hole diffusion in nanocrystals. This presentation will feature our most recent progress in this area.

Cation exchange, a chemical transformation used to modify a crystal whereby a cation from solution replaces a host cation, has recently become a highly effective tool for enabling the synthesis of nanoparticles with novel chemical compositions. In particular, aliovalent doping of CdSe nanocrystals (NCs) via cation exchange of cadmium ions for silver ions has become quite popular for manipulating the optical and electronic properties of the doped NCs, such as for producing n- or p-type NCs. However, despite over a decade of study, the relationship between optical properties of the NC and the aliovalent dopants has largely gone unexplained, partially due to an inability to precisely characterize the physical properties of the doped NC.

We will discuss how electrostatic force microscopy (EFM) with single electron sensitivity can be used to determine the charges of individual, cation-doped CdSe NCs in order to investigate their net charge as a function of added cations. While there was no direct trend relating the NC charge to the relative amount of cation per NC, there was a remarkable and unexpected correlation between the average NC charge and ensemble exciton photoluminescence (PL) intensity, for all dopant cations introduced [1]. We use an effective mass theoretical model to conclude that the changes in PL intensity, as tracked also by changes in NC charge, are likely a consequence of changes in the NC radiative rate caused by symmetry breaking of the electronic states of the nominally spherical NC due to the Columbic potential introduced by ionized cations. Further, we show through energy loss spectroscopy and PL spectroscopy on individual NCs that the cation exchange process is highly heterogeneous, which has profound implications for possible future applications of doped NCs.

Delocalization of excitons within semiconductor quantum dots (QDs) into states at the interface of the inorganic core and organic ligand shell by so-called “exciton-delocalizing ligands (EDLs)” is a promising strategy to enhance coupling of QD excitons with proximate molecules, ions or other QDs. EDLs thereby enable enhanced rates of charge carrier extraction from, and transport among, QDs and dynamic colorimetric sensing. The application of reported EDLs – which bind to the QD through thiolates or dithiocarbamates – is however limited by the irreversibility of their binding and their low oxidation potentials, which lead to a high yield of photoluminescence-quenching hole trapping on the EDL. Here we discuss a new class of EDLs for QDs, 1,3-dimethyl-4,5-disubstituted imidazolylidene N-heterocyclic carbenes (NHCs), where the 4,5 substituents are either Me, H, or Cl, and 1,3-dimesitylnapthoquinimidazolylidene NHC (nqNHC). Post-synthetic ligand exchange of native oleate capping ligands for NHCs results in a bathochromic shift of the optical band gap of CdSe QDs (R = 1.17 nm) of up to 111 meV while maintaining colloidal stability of the QDs. This shift is reversible for the MeNHC-capped, HNHC-capped, and nqNHC-capped QDs upon protonation of the NHC. The magnitude of exciton delocalization induced by the NHC (after scaling for surface coverage) increases with increasing acidity of its pi-system, which depends on the substituent in the 4,5 positions of the imidazolylidene. The NHC-capped QDs maintain photoluminescence quantum yields of up to 4.2 ± 1.8 % for shifts of the optical band gap as large as 106 meV. Spectroelectrochemistry shows that the reduction of the napthoquinone moiety of QD-bound nqNHC ligandsto their radical anions results in an additional magnitude of bathochromic shift, ΔΔR, relative to the QDs capped with nqNHC ligands in their neutral state, a redox-sensitive exciton delocalizing system.

Colloidal halide perovskite nanocrystals (NCs) have the possibility of easy scale-up due to their batch synthesis and have demonstrated excellent optoelectronic properties. In particular, perovskite NCs have remarkably high photoluminescence quantum yields in solution and as thin films and impressive open circuit voltages in photovoltaic devices. Despite these promising results, little work has been done to understand the stability of CsPbI₃ NCs for optoelectronic device applications. It has been previously shown that the ligands impart tensile surface strain, which stabilizes the black three-dimensional (3D) perovskite phase against phase degradation, making CsPbI₃ NCs some of the most structurally robust inorganic halide perovskites to date. However, understanding exactly how CsPbI₃ NCs degrade under ambient conditions is critical. We demonstrate that the degradation mechanism of NCs is unique from, and 2 orders of magnitude slower than, their polycrystalline thin-film counterparts. Under specific conditions, CsPbI₃ NC films show a compositional instability instead of the phase instability seen in large grain CsPbI₃. This is mediated through reactions with superoxide and other reactive oxygen species, which are initiated from surface defect states, O₂ and light. We then use this mechanistic insight to identify multiple strategies to prolong the lifetimes of CsPbI₃ NC films, by going beyond surface strain to mitigate key surface chemistries. We demonstrate that (1) minimizing the number of surface defects (2) using an alkylammonium bromide ligand surface treatment and (3) encapsulation with an oxygen scavenging layer all increase NC film lifetimes by inhibiting various steps in the photo-oxidation degradation reaction.

Solution-processed quantum dots (QDs) are finding applications in a wide-range of technologies from displays and lighting to photovoltaics and photodetectors. Advances in real-world technologies have been enabled by an increasing ability to fine-tune opto-electronic properties with strategies including quantum confinement effects, advanced heterostructuring (band-structure engineering at the nanoscale), and chemical manipulation of interfaces and surfaces. Taken together, these strategies have yielded numerous breakthroughs and insights into key fundamental excited-state processes in semiconductor nanocrystals. In our lab, we have focused on developing heterostructures that lead to suppression of non-radiative processes, including blinking, photobleaching, and Auger recombination.[1-8] Despite realizing novel properties that support a wide range of applications,[9-13] we cannot claim to have found the perfect QD. Even the most robust nanocrystal will fail in its most fundamental of property – the ability to emit light – in the face of specific stressors, such as high photon flux, temperature, and exposure to atmospheric oxygen/water.

Previously, we developed a “single-QD stress test” that was used to evaluate degradation-photophysics in two types of ultrastable, “giant” core/thick-shell QDs (gQDs).[14] Here, I will describe the latest in our efforts to elucidate QD failure mechanisms, with the aim to pinpoint structural and chemical features leading to the “killing” of a QD.[15] Specifically, we developed a method based on solid-state spectroscopy to obtain kinetic and thermodynamic parameters of photo-thermal degradation in single QDs, systematically varying ambient temperature and photon-pump fluence. We described the resulting degradation in emission with a modified form of the Arrhenius equation and showed that this reaction proceeds via pseudo zero-order reaction kinetics by a surface-assisted process with an activation energy of 60 kJ/mol. We note that the rate of degradation is ~12 orders of magnitude slower than the rate of excitonic processes, indicating that the reaction rate is not determined by ultrafast electron/hole trapping. We further determined that full-power LED-like excitation can add 90-140 K of heat to the nanocrystals due to high excitation rates and associated nonradiative relaxation. The specific reactions that are responsible for photo-oxidative degradation in gQDs are as yet unknown. However, at least one of the primary degradation reactions is now known to be a zero-order process with a reaction activation energy that is independent of photon flux or wavelength. In the search for new robust light-emitting nanocrystals, the new analysis method will enable direct comparisons between differently engineered nanomaterials or different organic/inorganic surface treatments.

The most studied class of semiconductor nanocrystal—quasi-spherical particles known as colloidal quantum dots—is now commercially used as a fluorescent material. However, despite decades of research, state-of-the-art samples still exhibit a distribution in size and shape, reducing their performance for applications. This leads to a fundamental question: can we achieve true monodispersity in semiconductor nanocrystals via chemical synthesis?  In this talk we will discuss this issue by examining two classes of nanomaterials. First, we will consider thin rectangular particles known as semiconductor nanoplatelets (NPLs). Amazingly, NPL samples can be synthesized in which all crystallites have the same atomic-scale thickness (e.g. 4 monolayers). This uniformity in one dimension suggests that routes to monodisperse samples might exist. After describing the underlying growth mechanism for NPLs, we will then move to a much older nanomaterial—magic-sized clusters (MSCs). Such species are believed to be molecular-scale arrangements (i.e. clusters) of semiconductor atoms with a specific (“magic”) structure with enhanced stability compared to particles slightly smaller or larger. Their existence implies that MSC samples can in principle be monodisperse in size and shape. Unfortunately, despite three decades of research, the formation mechanism of MSCs remains unclear, especially considering recent experiments that track the evolution of MSCs to sizes well beyond the cluster regime. Again, we will discuss the underlying growth mechanism and its implications for nanocrystal synthesis.

Colloidal CdSe nanoplatelets (NPLs) can be made with a thickness of atomic precision in the range of about one to a few nanometers only. The lateral sizes are of the order of several to tens of nanometers. The thickness is less than the bulk exciton bohr radius and consequently leads to strong effects of spatial confinement on the internal energy of an exciton. Variation of the thickness of a NPL thus allows one to tune the photoluminescence (PL) and optical absorption spectra.

Interestingly, however, the experimental shape of PL and absorption spectra also depends on the lateral sizes of a NPL. To date, the latter has not received much attention, with the exception of a few (mainly theoretical) studies and the origin of this effect has been inconclusive.

We measured the PL and absorption spectra for a series of NPLs with different lateral sizes and find that the dependence of the optical spectra on the lateral size is fully explained by taking into account the quantum-confinement effects on the translational motion of excitons in the plane of the NPLs. The spectra of all samples considered can be reproduced very accurately by a theoretical description of exciton energies and oscillator strengths based on the quantum mechanical particle-in-a-box model and the known size-distribution of the NPLs.

Quantum confinement is certainly the most striking properties of nanocrystal at the nanoscale. In CdSe this is used to tune the energy of the first exciton from green to red enabling their use as downconverter for display. In HgTe the lack of bulk band gap and the weak conduction effective mass makes that the absorption edge can be widely tuned from UV to THz [1]. HgTe 2D nanoplatelets (NPL) are certainly the most confined form of Hgte with up to 1.5 eV of confinement energy. The sysnthesis of these NPLs has recently been reported via a two steps fabrication method including a cation exchange step from CdTe NPL [2]. Such large confinement corresponds to a wavevector range of the HgTe relation dispersion far away from the Γ point, where the latter starts being highly non-parabollic, allowing to explore a strong confinement regime. As a result, the optical properties of the NPL also gets affected. In particular, we report that the temperature dependence of the HgTe NPL optical band gap (dEG/dT<0) is opposite to the one observed in bulk and large HgTe NCs (dEG/dT>0).

To understand this trend, we systematically explore the pressure (0.-4GPa full range of existence of the zinc blende phase) temperature (10-300K) and confinement (0.25-1.5 eV) phase diagram of HgTe and correlate spectroscopic data with a 14 band kp model. The model unveils the critical role plays by the upper conduction band in the curvature of the first conduction band in the strong confinement regime.

These HgTe NPL are latter integrated into field effect transistors and photodetectors [3-4] to reveal the nature of the majority carrier and their photoconductive properties in the near and short wave infrared. 

Ref

[1] Terahertz HgTe nanocrystals: beyond confinement, N Goubet et al, .JACS 140, 5033-5036 (2018)

[5] Strongly confined HgTe 2D nanoplatelets as narrow near-infrared emitters, E Izquierdo et al, JACS 138,  10496-10501 (2016)

[3] Charge dynamics and optolectronic properties in HgTe colloidal quantum wells, C Livache et al, Nano Letters 17 (7), 4067-4074 (2017)

[4] Impact of dimensionality and confinement on the electronic properties of mercury chalcogenide nanocrystals, C Gréboval et al, Nanoscale 11 (9), 3905-3915 (2019)

Layered semiconductors attract significant attention due to their diverse physical properties controlled by their composition and the number of stacked layers, but still obtaining material in large quantity may be a challenge. Liquid exfoliated van der Waals semiconducting crystals have been recently described as the main active material in all-printed devices such as transistors or photodetectors. This technique may lower device preparation cost by accelerating production and omitting expensive methods like lithography.

Herein, large crystals of the ternary layered semiconductor - chromium thiophosphate (CrPS₄) are prepared in big amounts by a vapor transport synthesis. Optical properties are determined using photoconduction, absorption, photoreflectance, and photoacoustic spectroscopy exposing the semiconducting properties of the material. [1] A simple, versatile, one-step protocol for mechanical exfoliation onto transmission electron microscope grid is developed [1], [2] and multiple layers are characterized by advanced electron microscopy methods, including atomic resolution elemental mapping confirming the structure by directly showing the positions of the columns of different elements’ atoms. [1]

CrPS₄ is also liquid exfoliated and then obtained suspension is converted into an ink.  Finally, the CrPS₄ ink in combination with colloidal graphene is used for creating ink-jet printed photodetector. This all-printed graphene/CrPS₄/graphene heterostructure detector demonstrates specific detectivity of 8.3×10⁸ (D*). The study shows a potential application of both bulk crystal as well as individual flakes of CrPS₄ as active components in light detection, when introduced as ink printable moieties with a large benefit for manufacturing. [1]

Directing the self-assembly of colloidal nanocrystals into ordered superstructures is of fundamental and technological interest for creating designer materials that bridge multiple length scales. The assembly of polyhedral nanocrystals at the interface of two immiscible fluids presents a promising approach to create high-fidelity superlattices with exceptional translational order and enables control over the orientational order of constituent building blocks. However, the full potential of this assembly approach remains elusive since despite the ostensible simplicity of the interfacial assembly, many knowledge gaps persist concerning the nuanced physicochemical phenomena that occur during assembly.  

Using synchrotron-source grazing incidence small angle X-ray scattering (GISAXS), the fluid and particle dynamics which lead to the final highly ordered superlattices can be elucidated. In this work, we used high time resolution GISAXS to characterize the spreading and drying dynamics of PbSe nanocrystals assembling from droplet contact with the liquid substrate to the final superlattice structure. Additionally, we explain how tuning the solvent parameters, such as volatility, surface tension and polarity, determines the mesoscale morphology of 2D superlattices on ethylene glycol. Specifically, the solvent interaction with the liquid substrate and ligand shell dynamics during evaporation have significant effects on the final superlattice morphology. Improved understanding of the kinetic phenomena giving rise to superlattice topology will enable growth of high-quality superlattices with long-range order at both nano- and micro- scales.

There is an increasing interest in two-dimensional (2D) Ruddlesden-Popper perovskites for solar harvesting and light emitting applications due to their superior chemical stability as compared to bulk perovskites.[1,2] Both, purely 2D and blends of 2D/3D phases have been successfully employed in solar cells with an efficiencies of >18% and >21%, respectively.[3,4] As with earlier advances in the field of perovskites, these technological improvements are advancing at a pace that far exceeds our understanding of the physical mechanisms underlying their performance. Particularly, the reduced dimensionality in 2D perovskites results in excitonic excited states which dramatically modify the dynamics of charge collection. While the carrier dynamics in bulk systems is increasingly well understood, a detailed understanding about the spatial dynamics of the excitons in 2D perovskites is lacking.[5]

Here, we present the direct measurement of the intrinsic diffusivities and diffusion lengths of excitons in single crystalline 2D perovskites using time-resolved microscopy. Our technique allows us to follow the temporal evolution of a diffraction limited exciton population with sub-nanosecond resolution revealing the spatial and temporal exciton dynamics. We reveal two distinct temporal regimes: For early times excitons undergo unobstructed normal diffusion, while at later times exciton transport becomes subdiffusive as excitons get trapped. Using the versatility of perovskite materials, we study the influence of the organic spacer, cation and dimensionality (n = 1 and 2) on the diffusion dynamics of excitons in 2D perovskites. We find that changes in these parameters can yield diffusivities which differ in up to one order of magnitude. We show that these changes arise due to strong exciton-phonon interactions and potentially with the formation of large exciton-polarons. Our results provide insight into how excitons diffuse through 2D perovskites and yield clear design parameters for more efficient 2D perovskite solar cells and light emitting devices.[6]

Silver phenylselenolate (AgSePh) is an emerging excitonic two-dimensional semiconducting member of a hybrid metal-organic chalcogenolate family. In addition to its two-dimensional structure with high exciton binding energy, strong in-plane anisotropy, and a narrow emission spectrum at 467 nm, AgSePh does not contain any toxic element and is tolerant to both polar and non-polar solvents. AgSePh can be synthesized by a solution-phase reaction as well as a scalable vapor-phase method. Here, we show by testing 24 solvents – with different polarities, boiling points and functional groups – that complexation between silver cations and solvent molecules is the key to an increasing size of AgSePh crystals. With the introduction of amine solvents, we are able to increase the size of AgSePh crystals grown by the solution-phase biphasic reaction from ~3 µm to >200 µm and that of AgSePh thin films prepared by the vapor-phase tarnish reaction from ~200 nm to >1 µm. We also observed that the photoluminescence lifetime of AgSePh is stable after storing under ambient condition and the addition of amines boosts this lifetime from <40 ps to 200 ps. The improved syntheses reported in this work will allow easy integrations of AgSePh in both thin-film electronic and nanoelectronic applications as well as the exploration of strong excitonic anisotropy.

Abstract: Biexcitons in 2D transition metal dichalcogenide from first principle: binding energies and fine structure.

The emerging field of 2-dimensional (2D) materials keeps gaining increasing attention due to the wide range of potential applications in many domains including: optoelectronics, photovoltaic, sensing, quantum computing ...etc. Reducing the dimensionality of a system results in an enhancement of the Coulomb interaction between elementary quasiparticles (i.e. electrons and holes) as it reduces the dielectric screening. This allows for the formation of strongly bounded excitations which can be observed even at room temperature. Among these excitations, biexcitons are of special interest from both the experimental and theoretical perspectives due to its rich physics and potential applications in quantum information and lasing [1]. Understanding biexcitons would be the first step toward a clear understanding of the equilibrium dynamics of photo excited hot carriers and is also relevant in the context of exciton condensation. Moreover, the biexciton, being a complex bound state of 2 electrons and 2 holes, has a rich fine structure and many more degrees of freedom than the simple excitonic case.

First principle treatment of biexcitons, on the same theoretical footing as excitons and trions, is possible thanks to the newly developed methodology of Ref. [2]which uses a hybrid approach combining configuration interaction and green function methods for the description of many-electron many-hole excitations.This methodology has been shown to give reliable results on excitons and trions [2] and it is applied here to study the binding, fine structure and non-equilibrium effects of biexcitons in 2D transition metal dichalcogenide.

In recent year, in response to the request for flexible and sustainable energy storage devices with high electrochemical performance, there has been growing interest in using paper or paper-like substrates for batteries and other energy storage devices such as environmentally friendly supercapacitors [1]. In this context, cellulose-based substrates for energy storage devices could be well-engineered, are light-weight, safe, thin and flexible[2]. We demonstrated a scalable, low cost and easy-to-process approach for the preparation of energy storage devices using large area techniques like spray and blade coating, suitable for smart electronic applications for health monitoring. Following a green strategy, all components were formulated in water-based dispersions. Symmetric paper-based supercapacitors using common copy paper and electronic paper as substrate, and Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as electrodes, are realized and investigated. The novelty of this work consists in the use of composite based on detonation nanodiamonds (DNDs) and hydroxypropyl cellulose (HPC) as solid state electrolyte and separator. We also prepared devices with solution electrolyte using the same HPC+DND composite but with the addition of sodium sulfate (Na₂SO₄). The performance obtained using solid electrolyte (HPC+DNDs) and liquid electrolyte (HPC+DNDs+Na₂SO₄) on both substrates are comparable in terms of specific capacitance: ~ 0,13 ÷ 0,52 F/g for (HPC+DNDs) and ~ 0,35 ÷ 0,82 F/g for (HPC+ND+Na₂SO₄), with power density in the range of ~19 ÷ 24 µW cm^-2[3].

Nanostructures on the base of lead, tin and copper chalcogenides with defined shape, dimensionality, faceting and surface chemistry are promising building blocks for opto-electronic devices in the near-infrared spectral range. A high degree of control has been already reached within main approaches for the dimensionality control: anisotropic growth, mesophase confined growth due to templating effect and oriented attachment. Here, we demonstrate several examples of fine-tuning of the shape and faceting of CuS, SnS and PbS quasi-two-dimensional structures with impact on electrical and optical properties. We also show synthetic details of the shape transformations combined with simulations which shed light onto the mechanism of the reached control. In case of PbS nanostripes and nanowires we show how the faceting of a nanocrystal dramatically changes its properties from semiconducting to metallic ones and analyze the reasons of the observed behavior.

References

Ramin Moayed, M. M., Kull, S., Rieckmann, A., Beck, P., Wagstaffe, M., Noei, H., ... & Klinke, C. (2020). Function Follows Form: From Semiconducting to Metallic toward Superconducting PbS Nanowires by Faceting the Crystal. Advanced Functional Materials, 30(19), 1910503.

Li, F., Moayed, M. M. R., Gerdes, F., Kull, S., Klein, E., Lesyuk, R., & Klinke, C. (2018). Colloidal tin sulfide nanosheets: formation mechanism, ligand-mediated shape tuning and photo-detection. Journal of Materials Chemistry C, 6(35), 9410-9419.

Li, F., Ramin Moayed, M. M., Klein, E., Lesyuk, R., & Klinke, C. (2019). In-plane anisotropic faceting of ultralarge and thin single-crystalline colloidal SnS nanosheets. The journal of physical chemistry letters, 10(5), 993-999.

Lesyuk, R., Klein, E., Yaremchuk, I., & Klinke, C. (2018). Copper sulfide nanosheets with shape-tunable plasmonic properties in the NIR region. Nanoscale, 10(44), 20640-20651.

The high affinity for the halides ligands with the <100> facets of the zinc blende nanoplatelets (NPLs) lead to a ligand exchange from carboxylate to halides which then partially dissolved the cadmium chalcogenides NPLs through the edges. The released monomers then recrystallized on the large top and bottom facets leading to a growth of NPLs in the thickness. CdSe NPLs with thicknesses from 3 to 9 MLs are synthesized. A direct growth is also achieved when a chalcogenide precursor is jointly introduced with a metal halide. Finally when an incomplete layer is grown, homostructures with a type I band alignement are obtained thus offering a new degree of liberty for the synthesis of structured NPLs.

Two-dimensional (2D) colloidal nanoplatelets (NPLs) are an emerging class of quantum well materials that exhibit many unique properties, including uniform quantum confinement, narrow thickness distribution, large exciton binding energy, giant oscillator strength effect, long Auger lifetime, and high photoluminescence quantum yield. These properties have led to great potentials in optoelectrical applications, such as lasing materials with a low threshold and large gain coefficient. Many of these properties are determined by the structure and dynamics of band-edge excitons in these 2D materials. Motivated by both fundamental understanding and potential applications, the properties of 2D excitons have received intense recent interests. We have carried out a series of recent studies on fundamental exciton properties in 2D NPLs, including lateral size the 2D exciton (i.e. exciton center-of-mass coherent area); exciton in-plane transport mechanism; size and thickness dependence of bi-exciton Auger recombination rate, and optical gain mechanism and threshold. In this talk I will focus on the size, thickness and material dependence of bi-exciton Auger recombination rates. We show that In CdSe NPLs, the biexciton Auger recombination lifetime does not depend linearly on its volume, deviating from the “Universal Volume” scaling law that has been reported for 0D quantum dots. Instead, the Auger lifetime scales linearly with the lateral size, and the Auger lifetime depends sensitively (nonlinearly) on the NPL thickness. In CdPbBr3 1D nanorods and 2D NPLs, the biexciton Auger lifetimes increase linearly with the rod length and NPL lateral areas, respectively, and the lifetimes are much shorter than CdSe nanocrystals with similar volume. These observations can be explained by a model in which the Auger recombination rate for 1D nanorods (NRs ) and 2D NPLs is a product of binary collision frequency in the non-quantum confined dimension, and Auger probability per collision. The former gives rise to the linear dependence on the lateral areas in 2D NPLs and rod length in 1D NRs. The Auger recombination proability per collision depends on material property and the degree of quantum confinement, which gives rise to nonlinear dependence on the thickness of NPLs and diameter of NRs, as well as material dependence of Auger lifetimes. Thus, the Auger lifetimes of 2D NPLs and 1D nanorods deviate from the volume scaling law because of the different dependences on the quantum confined and non-confined dimensions. We believe that his model is generally applicable to all 1D and 2D materials.

Two-dimensional (2D) semiconductors are of a wide interest in recent years due to their unprecedented electrical and optical characteristics. The 2D endeavor, beyond the discovery of graphene, includes the study of inorganic van der Waals (vdW) transition metal dichalcogenides and solution based-2D semiconductors. Despite the striking electronic and optical properties of the mentioned 2D materials, those are lacking long-range magnetic properties or unique magnetic textures. The current study describes the exploration of a new family of semiconductor vdW compounds that possess magnetism along with their electrical and optical properties – these compounds are transition metal phosphorous trichalcogenides (TMPTs) with a honeycomb arrangement of magnetic metal elements.  Furthermore, diamagnetic TMPTs doped with magnetic impurities will be addressed - in comparison with diluted magnetic colloidal nanoplatelets from the II-VI family.

The TMPTs have a chemical formula MPX₃ (X=S, Se) with the metals (M) from the first row of transition metals, arranged in a honeycomb array with P-P at the center of each metal hexagon. These vdW materials permit the isolation of single layers down to a molecular limit via chemical or mechanical exfoliation. The 2D limit ease the Mermin-Wagner thermal agitation restriction and therefore, support intrinsic protected long-range ferromagnetic (FM) or antiferromagnetic (AFM) order, as well as spin textures (e.g., skyrmions) that are impossible in regular 3D materials. The honeycomb arrangement renders some of the TMPTs with a valley degree of freedom, similar to that found in MoS₂ single layers. Above all, TMPT layers permit coupling of long-range magnetic ordering or/and magnetic doping with the semiconducting properties of the materials.

The lecture will include description of experimental observation exposing the magneto-optical properties of FePS₃, MnPS₃ and Mn:ZnPS₃ TMPTs, and Mn-doped II-VI nanoplatelets.  The properties were investigated using circularly polarized magneto-photoluminescence at variable temperatures and optically detected magnetic resonance spectroscopy.  The preliminary observations indicated a removal of valleys' energy degeneracy by the coupling to the AFM magnetic arrangement.  Furthermore, Mn-doped diamagnetic TMPT showed a coupling between dopant and photo-generated carriers with a behavior similar to exciton-polaron found in doped colloidal II-VI nanoplatelets.  

Overall, the TMPT and magnetically doped 2D materials open a new paradigm in science and technology, from the basic understanding of magnetism, to the discovery of a plethora of new physical phenomena, thus being a base for the development of modern memory devices, spintronics, quantum computation and information. 

Colloidal nanoplatelets (NPLs) have become a promising class of semiconductor nanocrystals (NCs) for optoelectronic applications with their distinctly different optical characteristics [1]. They exhibit narrow emission linewidth, large absorption cross-section, giant oscillator strength, and suppressed Auger recombination. However, the first examples of core/shell NPLs synthesized by using a colloidal atomic layer deposition (c-ALD) approach suffered from the low photoluminescence quantum yields (PLQY), decreased crystallinity, and limited stability.  Here, to overcome this issue, we demonstrate a high-temperature shell growth approach that enables the synthesis of NPLs with controlled shell composition [2]. Our proposed CdSe quantum wells with a graded shell, which is composed of CdS buffer interlayer and Cd_xZn_1-xS gradient shell, exhibit highly bright emission (PLQY up to 89%) in the red spectral region (634-648 nm) with a narrow emission linewidth (down to 21 nm). With the smooth confinement potential of graded shell NPLs, hence further suppressing Auger recombination, we obtained a low threshold amplified spontaneous emission (~40 µJ/cm²) under nanosecond laser excitation. We also investigated the electroluminescent performance of graded shell NPLs in solution-processed light-emitting diodes (LEDs). Our NPL-LEDs showed a very high external quantum efficiency (EQE) value of 9.92% with high brightness up to ~46000 cd/m² at 650 nm. These findings show that by carefully designing heterostructures of anisotropically shaped colloidal NPLs, we could obtain highly efficient NPLs with enhanced optical properties to realize their superior performance in optoelectronic applications, overcoming the limitations of the spherically shaped NCs.

Colloidal nanocrystal superlattices are highly ordered aggregates of particles. Crystals are highly ordered aggregates of atoms. However, nanocrystal superlattices are not conventionally considered crystals. But where does the border lie? Previously, we reported that CsPbBr₃ nanocrystal superlattices have a structural perfection comparable with that of epitaxially grown multilayers, which can be considered as full-fledged single-crystals.[1]

In this talk, we will discuss a novel approach to the characterization of periodically stacked colloidal nanocrystals, which was inspired by diffraction experiments on multilayers grown by molecular beam epitaxy.[2] Our method takes advantage of optical interference phenomena arising from the superlattice periodicity, which enrich the profile of Bragg peaks in structural information. By fitting these profiles, collected with a common lab-grade diffractometer, we can extract structural information usually requiring high-end setups such as synchrotrons. Our approach is especially suitable for bidimensional colloidal crystals like nanoplatelets and nanosheets, because they spontaneously assemble into stacked periodic structures thanks to their highly anisotropic shape. However, we expect that our approach can be also extended 2D-layered organic-inorganic materials, which are not considered superlattices but share with them the periodic alternation of different layers.

To demonstrate our approach, we analyzed nanoplatelets of CsPbBr₃ and PbS measuring with high precision thickness, interparticle distance and even distortions in their atomic lattice. In addition, we demonstrated that such nanocrystal superlattices reach stacking displacements as small as 0.3-0.5 Å. This is comparable with atomic displacement parameters found in metal-organic bulk crystals, leading to intriguing questions. For example, how different is a stacking of perovskite nanoplatelets from a bulk crystal of a hybrid Ruddlesden-Popper perovskite? Can we study nanocrystal superlattices as they were bulk crystals? In the end, are nanocrystal superlattices a new class of hybrid organic-inorganic bulk crystals?

Colloidal semiconductor nanoplatelets (NPLs) exhibit strong quantum confinement only along the vertical direction, which can be controlled with atomic precision, and have received significant attention because of their narrow emission spectra and fast fluorescence lifetimes. Here we present a synthetic approach to obtain a ternary two dimensional (2D) architecture consisting of a CdSe core, laterally encapsulated by a type-I barrier of CdS, and finally a type-II outer layer of CdTe. The introduction of CdS leads to the formation of a tunneling barrier between CdSe and CdTe, which modulates the electron-hole overlap as well as the carrier relaxation dynamics. The modulation results in a type-II emission with an extended fluorescence lifetime in addition to the emission from CdSe and CdTe. The synthesis strategies allowed us to tune the indirect and direct transition energies and intensities as a function of the barrier and crown thickness. The different emission peaks of the core/barrier/crown (CBC) heterostructure are corroborated by the photoluminescence (PL) excitation spectroscopy and single particle PL measurements. Furthermore, experimental data are also supported by k.p calculations. To summarize, we have successfully synthesized and characterized CdSe/CdS/CdTe CBC heterostructures, demonstrating that colloidal 2D nanoplatelets offer great flexibility in designing opto-electronic properties toward targeted photonic applications.

Shakeup processes are partly radiative Auger processes whereby an electron-hole pair recombines but transfers a fraction of its energy to excite a third carrier, thus reducing the energy of emitted photons. Two very recent experiments in CdSe and CdSe/CdS core/shell nanoplatelets (NPLs) have hypothesized that such processes are responsible for the multi-peaked fluorescence spectrum observed in these structures at low temperatures.[1,2] Clarifying this point is important, because it would permit defining strategies to narrow down the emission line width of colloidal NPLs, thus improving their efficiency in optical applications where color purity is desirable.

Our work provides the first theoretical description on the origin and behavior of shakeup processes in colloidal nanostructures, defines strategies to control them and assesses on the interpretation of Ref.[1,2] experiments.  The conclusions are:

(1) Shakeup processes are indeed expected in colloidal NPLs charged with trions, unlike in previous colloidal nanostructures. The magnitude of the shakeup lines is strikingly large -over one order of magnitude larger than in epitaxial quantum wells-.

(2) We show that off-centered impurities are a requirement for the processes to take place, as they are needed to break symmetry conservation rules. In doing so, we reconcile two seemingly contradictory interpretations of the asymmetric lineshape of trion emission in core/shell NPLs.[1,3].

(3) We show that the multi-peaked emission in CdSe/CdS NPLs[1] cannot be explained in terms of shakeup processes only, and propose an altermative interpretation involving emission from metastable spin triplet trion states.

The optical absorptance A of a semiconductor layer is the ratio between the absorbed and incident energy. It was shown experimentally that, after corrections due to local-field effects, the absorptance of thin InAs layers is characterized by very clear steps corresponding to nA₀, where n is an integer and A₀ is the product of pi and the fine structure constant [1]. Remarkably, the quantum of absorptance was originally found for graphene monolayers, in a wide energy region [2]. In both cases, the explanation of this observation was provided on the basis of simplified calculations applied to a two-band model. In order to go beyond these approximations, we present atomistic multi-band tight-binding calculations of the absorptance of different types of semiconductor layers. We confirm that, in absence of strong excitonic effects, A is characterized by clear steps which can be related to A₀. The cases of layers of InAs and PbSe are studied in detail, taking into account the complex band structure of these materials. In the case of InAs, remarkable agreement with experiments is found. The origin of the quantization is discussed.

In this talk I will present two routes to computationally develop new photocatalysts. In the first one, layered noble metal chalconides and pnictonides [1], which show potential to be photocatalytically active, are exfoliated, and the resulting layers are investigated with respect to their properties, most importantly their stability and performance to (photo)catalyze hydrogen and oxygen evolution reactions in dependence on the pH and other factors. We have successfully applied this strategy recently to a series of noble-metal chalcogenides [2], phosphochalcogenides [3,4] and pnictonides [5].

In the second route, photoactive molecules, for example phorphyrin derivatives [6], are incorporated into synthetic framework materials such as metal-organic frameworks (MOFs) [7], where stacking provides additional band dispersion and supports charge carrier separation [8]. A similar approach is possible for covalent-organic frameworks (COFs) [9].

We present combined experimental and theoretical studies [1-3], demonstrating that CdSe nanoplatelets are a model system to investigate the tunability of trions and excitons in laterally finite 2D semiconductors. Our results show that the trion binding energy can be tuned from 36 meV to 18 meV with lateral size and decreasing aspect ratio, while the oscillator strength ratio of trion to exciton decreases.[3] In contrast to conventional quantum dots the trion oscillator strength in a nanoplatelet at low temperature is smaller than that of the exciton. The trion and exciton Bohr radii become lateral size tunable, e.g. from ~3.5 to 4.8 nm for the trion. This lateral tunability is practically independent of the transition energy, which is determined by the strong z-confinement in the colloidal wells. We show that dielectric screening has strong impact on these properties. By theoretical modeling of transition energies, binding energies and oscillator strength of trion and exciton and comparison to experimental findings we demonstrate that these properties are lateral size and aspect ratio tunable and can be engineered by the dielectric confinement. The trion binding energy can be tuned below or above the room temperature thermal energy. This allows e.g. together with the size tunable trion to exciton oscillator strength ratio (tunable by more than a factor of three) to suppress detrimental trion emission in devices by the choice of platelet size. [3]

We further show that e.g. the size tunable Bohr radii or wavefunctions together with the appropriate matrix elements for acoustic and polar phonon coupling result in a strong tunability of e.g. the exciton mobility and linewidth.[4] At low temperature e.g. the exciton mobility and diffusion coefficient show an platelet area dependence,  resulting from the inverse area scaling of the exciton-phonon scattering rate. The exciton mobility and diffusion coefficient become size and additionally lateral aspect ratio tunable.

Our results strongly impact further studies, as the demonstrated lateral size and aspect ratio tunable trion and exciton manifold is expected to influence properties like gain mechanisms, lasing, exciton-phonon interaction and transport at low temperature, but also even at room temperature due to the high and tunable exciton and trion binding energies.

Two-dimensional fluorescent colloidal nanocrystals combine the flexibility of solution-processed nanomaterials with the advantages of a (quasi-)2D band structure that offers enhanced optical properties compared to 0D quantum dots. In this presentation, I will discuss the synthesis of a novel ternary heterostructure, composed of a CdSe core, laterally extended by a CdS tunneling barrier, and finally a CdTe crown.[1] The type-II band offset between CdSe and CdTe, in combination with the CdS barrier layer, allows to separate core and crown electron and hole wave functions, yielding an emission spectrum consisting of a long-lived indirect transition at 625 nm, as well as direct CdSe and CdTe transitions around 510 nm and 575 nm, resp. Up to 2% of the total emission can be attributed to CdSe, and we were able to demonstrate two- and even three-photon fluorescence upconversion by exciting the sample with red and near-infrared photons, to yield green emission from the CdSe core.

Colloidal 2D nanosheets and nanoplatelets with thickness- and dimensionality-dependent properties are highly interesting for innovative optoelectronics in the visible and near-infrared.

The first part of my talk is focused on our work in tailoring the synthesis and optoelectronic properties of ultrathin lead chalcogenide nanoplatelets (NPLs). E.g., we have recently shown increased exciton binding energies in thin PbS nanosheets by time-resolved THz spectroscopy.[1] Here, I will disentangle the charge-carrier dynamics of coupled states in ultrathin PbS NPLs with enhanced near-infrared emission by transient absorption spectroscopy.

In the second part of my talk, I will focus on our progress in controlling the formation of ultrathin metallic and semiconducting transition metal dichalcogenide layers by wet-chemical methods.

Our work emphasizes the excellent usability of colloidal chemistry and time-resolved spectroscopy methods for producing tailor-made 2D materials.

[1] J. Lauth*, M. Failla, E. Klein, C. Klinke, S. Kinge, L. D. A. Siebbeles, Photoexcitation of PbS Nanosheets Leads to Highly Mobile Charge Carriers and Stable Excitons, Nanoscale 2019, 11, 21569-21576.

Soluble mid-IR emitters can be used to address many issues in various research fields such as telecommunications, biosensing, gas sensing, unmanned vehicles, etc. Due to such demands for soluble mid-IR emitters, both organic and inorganic chemistry approaches have been rigorously performed. Inorganic material-based colloidal mid-IR emitters are known to be superior to organic molecules-based infrared emitter in the mid-IR range because of a lack of the intrinsic vibrational modes arising from their own structure. Colloidal quantum dots are promising materials to realize the efficient mid-IR emitter since the nanocrystal mainly comprises of inorganic constituents, and its phonon energy is a few hundred wavenumbers, which is one order of magnitude smaller than the vibrational energy of organic molecules. Furthermore, the vibrational mode of organic ligands of the nanocrystal can be suppressed by surface ligand engineering. Especially, self-doped quantum dots, in which excess carriers occupy the lowest quantized electronic state of the conduction band in steady-state, are excellent candidates for the soluble mid-IR emitter with respect to spectral line-shape and wavelength-tunability. This talk will focus on carrier recombination processes occurring in the self-doped quantum dots studied by mid-IR spectroscopy. Additionally, biologically compatible mid-IR emitting nanocrystals and several applications based on the intraband transition will be discussed as well.

There are many different synthetic methods to make infrared (IR) emitting quantum dots (QDs) such as the mercury chalcogenides. These materials have low bulk bandgaps and indeed in the case of HgTe even a zero-bandgap due to the inversion of the conduction and valence bands near the Γ point in the Brillouin zone. Shifting to the nanoparticle size scale, quantum confinement, aided by relatively large Bohr radii, can lead to very wide emission and band edge absorption tuning ranges and this has made these materials of great interest for near infrared (NIR) through to mid-infrared (MIR) photodetection applications. In this paper we describe a synthetic method based on aprotic solvent chemistry, which sits between aqueous QD synthesis and hot injection methods in other organic solvents and which is very suitable for larger scale syntheses.

In their own right, most IR fluorophores suffer from the same basic physics constraints of the interband emission process. The radiative transition rate rapidly decreases as the emission wavelength shifts further into the IR, to such an extent that non-radiative processes dominate hugely. In this range photoluminescence quantum yields (PLQYs) are invariably very much smaller those typically encountered in the visible these days, even where the underlying transition oscillator strengths might be comparable. In the short wavelength IR (SWIR) it is not uncommon to find PLQYs below 1% in the 2-3 µm range and <0.1% above 3 µm, etc. This severe decline in radiative rate can be offset to some degree by using hybrid QD materials and device structures incorporating nanophotonic or nanoplasmonic entities that can counter the slowing in the radiative rate across some spectral ranges, and we will describe some of our collaborations in these areas. In addition, we will describe other collaborative work on compact integrated optical devices that have been aided by advances in photonic and plasmonic waveguide engineering.

In this talk, I will describe recent work in our group developing an understanding of charge transport in thin films made of nanocrystal quantum dots, which will hopefully facilitate their development for use in applications requiring predictive control over both optical and electronic properties. We have studied electronic structure and the origin of electronic traps states experimentally and computationally [1,2]. We have also performed large-scale, ab initio simulations to gain insight into free carrier generation and charge hopping in strongly confined nanocrystal quantum dot-based semiconductors. We have used these findings to build a predictive model for charge transport in these systems, which we validate experimentally using time-of-flight measurements [2]. We have then applied this model to probe the impact of energetic and positional disorder in nanocrystal solids on mobility [3]. These findings, which focus on PbS quantum dots, help identify what to prioritize in terms of synthesis and fabrication in the development of nanocrystal-based devices. 

[1] “Dopants and Traps in Nanocrystal-Based Semiconductor Thin Films: Origins and Measurement of Electronic Midgap States.” S. Volk, N. Yazdani, O. Yarema, M. Yarema, and V. Wood, ACS Applied Electronic Materials 2 (2020).

[2] “Charge Transport in Semiconductors Assembled from Nanocrystal Quantum Dots” N. Yazdani, S. Andermatt, M. Yarema, V. Farto, M. H. Bani-Hashemian, S. Volk, W.M.M. Lin, O. Yarema, M. Luisier, and V. Wood. Nature Communications, 12 (2020).

[3] “Understanding the effect of positional disorder in nanocrystal quantum dot thin films on charge transport” Y. Xing et al. in preparation.

Infrared light detection enables diverse applications ranging from night vision to gas analysis. Emerging technologies such as low-cost cameras for self-driving cars require highly-sensitive, low-cost photodetectors with spectral sensitivities up to wavelengths of 10 µm. Colloidal PbS quantum dot (QD) sensitized graphene field-effect transistors may potentially lead to low cost photodetectors; however, the spectral sensitivity of these phototransistors has been limited to about 1.6 µm. Here, we present HgTe QD/graphene phototransistors with specific detectivities of 6×10⁸ Jones at a wavelength of 2.5 µm. At room temperature, the HgTe QD/graphene phototransistor does not show any photoresponse, and had to be cooled to cryogenic temperatures to exhibit detector functionality.

 The photoresponse of QD/graphene phototransistors likely depends on a Schottky-like potential barrier resulting from the band alignment of the two materials. It promotes the transfer of photo induced charge carriers from QDs to graphene. Interestingly, a charge carrier transfer at low temperatures is not frozen-out. We propose that the strength of the surface dipoles at the QD/ligand interface is temperature-dependent. This would effect the conduction- and valence band positions of the QD thin-film in respect to the vacuum level, which ultimately may alter the band alignment with graphene resulting in functional detectors. Besides extending of the spectral sensitivity, the HgTe QD/graphene photodetector exhibit high specific detectivities, in excess of 10⁸ Jones, even at kHz light modulation frequencies, making them suitable for fast video imaging.

Altogether, the simple device architecture, QD film patterning capabilities, and the extended spectral sensitivity make QD/graphene phototransistors potentially suitable for multi-color photodetector cameras.

Beyond their use as light sources for displays, nanocrystals also appear as promising candidates to design low cost infrared sensors. In such devices the carrier density is a key parameter driving the signal-to-noise ratio. The carrier density can be controlled thanks to the gate in a field effect transistor configuration. Most common gates are SiO₂ and electrolyte^[1] which are respectively limited by their low capacitance and (only) room temperature operation. Here, we explore (i) a high capacitance solid state gating from ionic glass (LaF₃) and (ii) a leakage free and low temperature operation gate based on quantum paraelectric SrTiO₃ substrate.^[2]

This high capacitance LaF₃ gate can be coupled to graphene electrodes enabling (i) IR transparency, (ii) tunable work function of the contact and (iii) propagation of the gate induced doping to the film thanks to the large quantum capacitance of graphene. We demonstrate the formation of a p-n junction improving charge extraction.^[3] The latter enable operating condition which simultaneously maximizes the response and reduces the dark current enhancing the detectivity by two orders of magnitude.

In a second part we demonstrate ferroelectric gating of a HgTe NC array with a SrTiO₃ (STO) gate. The divergence of the dielectric constant of STO at low temperature enables a high capacitance gate with a thick (500 µm) insulating substrate making it leakage and breakdown free. This gate is compatible with low temperature (<100 K) operation usually required for narrow band gap NC devices. This gate is then coupled to a plasmonic resonator to obtain a broadband absorption (1.5-3 µm) of 30% of the incident light. The combination of the STO gate with the plasmonic resonator shows a detectivity that can be as high as 10¹² Jones at 30 K.

The recent progress in nanocrystal-based solar cell development is very encouraging and makes this kind of solar cell a promising candidate for next generation photovoltaics. However, understanding the factors that hinder further improvement of the solar cell performance are not trivial due to the complex interlinked parameters of the devices. Despite newly gained understanding of the underlying chemical and physical parameters, the improvements to the nanocrystal-based solar cells have been mostly trial-and-error based. In this work, we use a simulation tool based on 1D drift-diffusion to run full device simulations of simple Schottky as well as more complex heterojunction devices. By only using input parameters, which were either derived from measurements or large-scale ab initio simulations, and no additional fitted parameters, we are able to closely match the characteristics of measured devices. We use these simulations as a tool to understand the influence of interfaces, charge carrier mobility and trap-assisted recombination.

Our study demonstrates the ability to simulate nanocrystal-based solar cells, independent of device architecture and without relying on fitting. We can use this to systematically simulate improvements to devices and guide further development of nanocrystal-based solar cells.

Optical antennas have become ubiquitous tools to enhance and tailor the spontaneous emission of quantum emitters [1]. The designs rules which have been established over the years are based on the understanding that optical antennas operate through the Purcell effect—the dependence of the fluorescence rate of point-source emitters to their surrounding environment. Here, we experimentally show that this paradigm fails for ensembles of interacting emitters (such as quantum dot solids) and that a different rule governs their interactions with optical antennas—the local Kirchhoff law recently introduced by Greffet et al [2].

We discuss the specificities of this regime by considering assemblies of PbS nanocrystals in direct contact with arrays of metal nanoparticles [3-5]. We illustrate the new opportunities of these findings by showing:

1) how to overcome the limiting trade-off between high electroluminescence (which occurs for PbS nanocrystals separated by nanometre-long ligands) and high carrier mobilities (which requires PbS nanocrystal capped with much shorter molecules) [3].

2) how to turn the isotropic and unpolarised luminescence of PbS nanocrystals into vector beams and other non-trivial light streams associated with fruitful developments in fluorescence imaging, optical trapping, high-speed telecommunications and quantum technologies.

Regarding low-cost infrared photodetection, colloidal quantum dots (CQDs), thanks to their large tunability, appear to be a new interesting building-block.[1] However, due to hopping transport, the diffusion length of the carriers in CQD film is short (typically few 10-100 nm). The absorption depth of the light is much larger (few µm). As a result, there is a trade-off between transport and optical absorption: usually thin films are used then, and only few % of the incident light is absorbed.[2] Light-matter coupling based on sub-wavelength resonators are used to tackle this issue.

Our device relies on guided mode resonators (GMR) and is made of a slab of CQDs (waveguide) onto a gold grating. The latter has two roles: it focuses the light into the nanocrystal film increasing its absorption, and it plays the role of electrode. The device is designed to induce a resonance and to achieve 100% of absorption at the targeted wavelength, for one of the polarization.[3] This particular design also enables photoconductive gain to occur. Both those effects generate a boost of responsivity of few orders of magnitude. This method is versatile and can be applied at different wavelengths (1.55 µm SWIR and 2.5 µm extended SWIR) with different materials (HgTe, PbS and a mix of perovskite/PbS).[3,4]

The introduction of nano-resonators not only generates a responsivity enhancement but enable spectral shaping oh this responsivity. First, it is possible to tune the position peak (of few hundreds of nm) by changing geometrical parameters such as the period of the grating.[3] Secondly, polarized devices can be made by inducing unmatched resonances in TE and TM polarizations. Then it is possible to achieve broadband absorption by introducing multi-resonances.

Devices based on small-gap mercury chalcogenide semiconductor nanocrystal inks are demonstrating increasingly high performance short and mid-wave infrared photodetection. These systems have the potential to eliminate cryogenic cooling needs and vastly reduce device costs compared to the current single-crystal devices. To achieve this goal and develop these materials as mid-infrared lasers, more detailed understandings of the exciton and carrier dynamics are required. Here we describe mid-infrared picosecond absorption and photoluminescence studies on HgTe and HgSe nanocrystal quantum dots. Comparisons between interband and intraband transitions in intrinsic and n-type systems reveal interesting new phenomena such as slow or absent Auger relaxations in n-type systems, phonon bottlenecks, and brighter emission from intraband versus interband transitions at the same wavelength. Yet, the measured lifetimes are limited by other nonradiative processes unique to small-gap materials. Investigations of the temperature- and surface-dependence of the luminescence in novel HgX/CdX core/shell structures help unravel such mechanisms. In parallel to these fundamental spectroscopic studies, we discuss progress towards harnessing the Auger suppression in n-HgSe to achieve mid-infrared lasing in this system. The deeper understanding of nonradiative relaxation in small-gap nanocrystals afforded by these experiments provides a path towards realizing high performance infrared photodetection near room temperature and mid-infrared lasing with nanocrystal quantum dots.

Fast nonradiative relaxation in narrow gap semiconductor quantum dots (QDs) is a major bottleneck for their application in mid-infrared detection, LEDs and lasing. Nonradiative decay in the mid-IR is widely attributed to relaxation to surface vibrations via a Forster-type near-field energy transfer [1][2][3]. Given the extremely low photoluminescence quantum yields (PLQY) of mid-IR QDs ~10^-4 [3] and the long range of Forster coupling, it is necessary to grow a giant shell (>~5nm in thickness) with type-I alignment to observe a significantly slowed relaxation. Though efforts have been made in growing shells of wide-gap material on HgTe and HgSe QDs [4][5][6], limited success has been observed in growing thick shells, largely due to the poor thermal stability of the cores.

We have recently developed the synthesis of giant HgSe/CdS QDs (>15 layers) to slow the nonradiative decay. The use of single-source precursors allows the growth of thick shells at a relatively low temperature, without independent nucleation or interface alloying. The synthetic strategy provides a uniform shell coverage, along with a Cd-rich surface that is necessary for observing mid-IR intraband PL. Preliminary results on PLQY and PL lifetime measurements show that the giant HgSe/CdS QDs exhibit a nonradiative decay rate 2 orders of magnitude slower than the cores. The PLQY at 5µm is ~1%, which is 10 times brighter than previous reports of mid-IR emitting QDs. These results shed light on the nonradiative relaxation processes in HgSe-based QDs, and pave the path for developing solution-processed mid-IR LEDs and lasers.

Image sensors fabricated using colloidal quantum dot photodetectors have the potential for combining the infrared sensitivity of traditional III-V and II-IV detector materials with the resolution and scalability of CMOS image sensors. SWIR Vision Systems has developed a family of CQD-based image sensors and cameras that has begun to realize this potential.  We present our uncooled extended shortwave infrared (eSWIR) 1920 x 1080-format cameras sensitive from 300 nm to 2100 nm wavelengths and describe the performance of these cameras using the EMVA1288 testing standard. We also present our standard SWIR cameras and show a variety of use cases for this technology in industrial machine vision applications.

Metal oxide nanocrystals doped with a few percent of aliovalent dopants become electronically conducting and support strong light-matter interactions in the infrared due to localized surface plasmon resonance (LSPR). In the prototypical material tin-doped indium oxide (Sn:In₂O₃), we explored the influence of the spatial distribution of Sn dopants on optical and electronic properties. Colloidal synthesis by slow addition of precursors allows precise control over the radial distribution of Sn, which dictates the electrostatic potential landscape and, in turn, the radial density of free electrons. Sequestering dopants in either the core or shell of the nanocrystals leads to multimodal optical spectra that respond strongly to changes in the dielectric environment. In thin films of nanocrystals, electronic conductivity is greatly enhanced in shell-doped nanocrystals wherein the barrier to nanocrystal-nanocrystal electron transfer is minimized.

We studied the coupling and impact of ligands on the QD optical and electrical properties.  We demonstrated that the bandedge energies can be shifted by over 2 eV for a QD absorber with 1 eV bandgap.  We also demonstrated that the addition of ligands causes the optical absorption of the QD/ligand complex to increase due to electronic coupling between the QD and ligands.  The coupling increases for smaller ligand optical gaps.  We utilize the enhanced absorbance of the QD/ligand to construct ligand adsorption isotherms. We model these isotherms with a 2-d square lattice model, which allows us to extract differences in trends of binding free energies and nearest neighbor coupling. As expected oleate binds more strongly than any of the functionalized cinnamates, but the binding preference is mitigated by dipole-dipole interactions for both large positive and negative dipoles. We explain this trend in binding free energy as a function of dipole moment via a collective electrostatic interaction with the lattice. For cinnamic acids with electron withdrawing molecular dipoles (negative dipoles), the isotherms show behavior associated with strong nearest neighbor association that causes the ligand exchange reaction to display a phase transition from all oleate coverage to all cinnamate coverage as more cinnamate is added, with a sharpness dictated by the ligand dipole moment: more negative dipole moments leads to sharper order-disorder phase transitions than those observed with positive dipole moments, as a function of ligand addition.  Using these observations, we prepared PbS QD with Janus-shell ligands.

We developed a facile method to prepare n and p-doped PbSe QDs via a post-synthetic cation exchange technique. Quantitative XRD analysis suggests a substitutional doping mechanism, with the lattice parameters decreasing upon either Ag⁺ or In³⁺ incorporation. A significant bleach of the first excitonic transition is observed, which is coupled with the appearance of a size-dependent intraband absorption in the NIR, indicating a successful introduction of electron/hole impurities dopants. We also observe a decrease of PLQY and a faster exciton decay with higher cation incorporation. Spectroelectrochemical measurements show a characteristic n-type behavior, which agrees with the substitutional doping mechanism of In³⁺ in PbSe. We proposed a model whereby the majority of the added cations remains at the QD surface and do not interact with the PbSe QD core states.  Small amounts of excess cations diffuse into the lattice and establish equilibrium between surface-bound and lattice-incorporated cation dopants.

Optoelectronic applications in the short wave infrared (SWIR) address a number of societal and technology challenges, including safety and security, food and process quality inspection, night vision, automotive safety, biological and environmental monitoring, just to name a few. However despite the huge impact and market potential currently available technologies based on costly III-V semiconductors impose commercialization challenges. That said, CQDs offer a unique opportunity to address this in view of their unique optoelectronic properties, low cost and CMOS compatibility. In this talk I will present recent lines of activities at ICFO towards high performance light emitters in the SWIR.

I will present record quantum efficiency and power conversion efficiency LEDs based on PbS CQDs enabled by engineering the energetic landscape and density of states of the active layers towards high PLQY, ultra-low trap-state density and improved charge balance [1]. Further, band engineering at the supra-nanocrystalline level has led to droop suppression and thereby to the achievement of record EQEs of 8% at high radiance conditions and exceptional stability of devices [2]. The talk will be concluded by recent results demonstrating for the first time tunable stimulated emission across the optical telecommunication band with high modal gain in excess of 100 cm^-1 and record low threshold in the single exciton regime, despite the 8-fold degeneracy of PbS CQDs, paving the way towards infrared CQD lasing [3].

References

[1] S. Pradhan et al., High-efficiency colloidal quantum dot infrared light-emitting diodes via engineering at the supra-nanocrystalline level. Nature Nanotechnol. 14, 72-79 (2019)

[2] S. Pradhan et al., Highly efficient, Bright and Stable SWIR CQD LEDs, Adv. Fun. Mat. Accepted 2020.

[5] S. Christodoulou et al. Single-Exciton Gain and Stimulated Emission Across the Infrared Optical Telecom Band from Robust Heavily-doped PbS Colloidal Quantum Dots. arXiv preprint arXiv:1908.03796.

Colloidal nanocrystals from PbS are most prominent materials for applications in optoelectronic devices operating in near to mid-infrared. Traditionally, they are synthesized by a hot injection method based on bis(trimethylsilyl) sulfide used as sulfur precursor. More recently Hendricks et al. [1] introduced a library of substituted thio-ureas as sulfur precursors of varying reactivity that can be used for size tuning. We combined these two approaches and selected a disubstituted thiourea compound as sulfur precursor and show the growth [2] and overgrowth of colloidal nanocrystals with this precursor. The advantage of this method is, that we can obtain controlled growth over infinitely large substrates, over all the nanocrystal size range towards bulk material (Figure 1). With the thio-urea precursor homo-epitaxial growth is achieved as well as heteroepitaxial growth, certainly related to the lattice matching between the substrate and the overgrowing PbS crystal. The obtained materials are of excellent quality and based on the nanocrystals grown from the thiourea precursor, photoconducting devices are demonstrated with band gap energies reaching those of the bulk material. The nanocrystals were also applied in photovoltaic devices providing record like behavior, especially for relatively long wavelengths [3]. Thus the growth of optoelectronic structures form thiourea precursors in organic solvents represents a versatile and promising rout for the low cost fabrication of infrared-optoelectronic devices.

Since the seminal work on the solid-state perovskite solar cell (PSC) demonstrating a power conversion efficiency (PCE) of 9.7% and stability for 500 h in 2012, perovskite photovoltaics surged swiftly. As a result, a certified PCE of 25.5% was achieved in 2020. According to Web of Science, publications on PSC increase exponentially since 2012, leading to 18,000 as of September 27, 2020, which indicates that PSC is considered as a very promising photovoltaic technology. In this talk, history, progress, and perspective of PSC will be covered in terms of efficiency, stability and upscaling. High photovoltaic performance was realized by compositional engineering, device architecture and coating methodologies for the past 10 years. Toward theoretical efficiency over 30% and commercialization of long-term stable PSC, further studies on suppression of recombination and developments of scalable technologies are required. Importance of interface and grain boundary engineering is emphasized to reach the theoretical efficiency with voltage over 1.3 V and fill factor over 0.9. For commercialization, materials may be issued because the current precursor mixture is problematic due to the underlying aging effect. We developed cost-effective materials based on delta FAPbI₃ powder for reproducibly high efficiency PSC. The best PCE of 21.1% was certified using the synthesized perovskite powder. Large-area uniform perovskite coating is pre-requisite in upscaling PSC. Solvent- and/or additive-engineering-based solutions were developed for large-area perovskite films. Regarding stability, except for the encapsulation, a paradoxical approach using a hydrophilic passivation layer was developed, which demonstrated over 90% of the initial PCE after 1000 h.

In this talk I will overview the virtue of nanomaterials for solar energy conversion using as examples dye-sensitized (DSSC) and perovskite solar cells (PSC).

Some years back we introduced alkoxy functionalized donor groups as a building block in organic dyes as light harvesters for DSSC. This donor group provides a desirable 3-dimensional structure that aids in surface protection of electrons injected into the semiconductor from oxidants in the electrolyte, allowing for record-setting cobalt- and copper-based redox shuttles to be utilized more frequently. With these systems we recently set the world record efficiency for DSSC of 12.25%. DSSCs are ideally suited for ambient light and indoor applications where efficiencies up to 35% have been reached calculated with respect to the fluorescent light source.

In our work on perovskite solar cells (PSC) we have achieved efficiencies above 23% with a mixed composition of iodide/bromide and organic and inorganic cations. With the use of SnO2 compact underlayers as electron acceptor contacts we have constructed planar perovskite solar cells with a hysteresis free efficiency above 22%. Through compositional engineering, larger perovskite grains grown in a monolithic manner are observed and reproducibility and device stability are improved. With regards to lifetime testing, we have shown a promising stability at 85 oC for 500 h under full solar illumination and maximum power point tracking (95% of the initial performance was retained). Our present main directions of developing FAPbI3 perovskites and passivation interface layers will be discussed in the lecture.

Hybrid perovskites persist as one of the leading materials in photovoltaics due to remarkable solar-to-electric power conversion efficiencies.^[1-2] Their limited stability under device operation conditions, however, remains challenging.^[1-2] This stimulates the development of layered perovskite analogs based on the assemblies of organic and inorganic components featuring higher stabilities under operating conditions.^[3-7] To this end, supramolecular chemistry provides a powerful tool for controlling the properties of hybrid materials by tailoring noncovalent interactions. We demonstrate its utility through molecular modulation based on fine-tuning various noncovalent interactions (i.e. supramolecular engineering),^[7-10] such as metal coordination,^[10]   hydrogen^[6,9-10] or halogen bonding,^[8] and π-interactions,^[7] among others,^[11] in a manner that has been uniquely assessed by solid-state NMR spectroscopy.^[5-6,8-10] As a result, perovskite solar cells that exhibit superior performances can be obtained, which is accompanied by enhanced operational stabilities.^[6-10] Moreover, the underlying molecular design can be extended into creating layered perovskite architectures to enable further stability enhancements.^[3-7] This has been investigated by using a combination of techniques complemented by solid-state NMR to unravel the design principles and highlight the supramolecular approach in advancing hybrid photovoltaics.

We demonstrate the critical role of surface recombination in mixed-cation, mixed-halide perovskite, FA_0.83Cs_0.17Pb(I_0.85Br_0.15)₃. By passivating non-radiative defects with the polymerizable Lewis base (3-aminopropyl)trimethoxysilane (APTMS) we transform these thin films. We demonstrate average minority carrier lifetimes > 4 {\mu}s, nearly single exponential monomolecular PL decays, and concomitantly high external photoluminescence quantum efficiencies (>20%, corresponding to ~97% of the maximum theoretical quasi-Fermi-level splitting) at low excitation fluence. We confirm both the composition and valence band edge position of the FA_0.83Cs_0.17Pb(I_0.85Br_0.15)₃ perovskite using multi-institution, cross-validated, XPS and UPS measurements. We extend the APTMS surface passivation to higher bandgap double cation (FA,Cs) compositions (1.7 eV, 1.75 eV and 1.8 eV) as well as the widely used triple cation (FA,MA,Cs) composition and observe significant PL and PL lifetime improvements after surface passivation. Finally, we demonstrate that the average surface recombination velocity (SRV) decreases from ~1000 cm/s to ~10 cm/s post APTMS passivation for FA_0.83Cs_0.17Pb(I_0.85Br_0.15)₃. Our results demonstrate that surface-mediated recombination is the primary non-radiative loss pathway in MA-free mixed-cation mixed-halide films with a range of different bandgaps, which is a problem observed for a wide range of perovskite active layers and reactive electrical contacts. This study indicates that surface passivation and contact engineering will enable near-theoretical device efficiencies with these materials.

The bandgap tunability of mixed-halide perovskites makes them promising candidates for light emitting diodes and tandem solar cells. However, illuminating mixed-halide perovskites results in the formation of segregated phases enriched in a single-halide. This segregation occurs through ion migration, which is also observed in single-halide compositions, and whose control is thus essential to enhance the lifetime and stability of the devices. Using pressure-dependent transient absorption spectroscopy, we find that the formation rates of both iodide- and bromide-rich phases in MAPb(Br_xI_1-x)₃ reduce by two orders of magnitude on increasing the pressure to 0.3 GPa. We explain this reduction from a compression-induced increase of the activation energy for halide migration, which is supported by first-principle calculations. A similar mechanism occurs when the unit cell volume is reduced by incorporating a smaller cation. These findings reveal that stability with respect to halide segregation can be achieved either physically through compressive stress or chemically through compositional engineering.

Recent advancements in perovskite solar cell performance were achieved by stabilizing the α-phase of FAPbI3 perovskites in nip-type cell architectures, enabling power conversion efficiencies (PCEs) of 25.2%. However, these advancements could not be directly translated into similar PCE improvements in pin-type perovskite cells which limits the possibilities to create highly efficient and stable perovskite photovoltaic cells. We fabricated a high-quality double-cation perovskite (MA0.07FA0.93PbI3) with low bandgap energy (1.54 eV) using a two-step approach on a standard p-type polymer (PTAA). The fabricated neat perovskite films exhibit large grains ( 1 µm) allowing to reach external photoluminescence quantum yields (PLQYs) of 20% with an unprecedented charge carrier lifetime of over 18 µs without further passivation. The exceptional opto-electronic quality of the neat material was translated into high efficiency pin-type cells with PCEs of up to 22.5% with improved stability under illumination. The low-gap cells (1.54 eV) stand out by their exceptional fill factors of 83% due to reduced charge transport losses and high short-circuit currents ( 24 mAcm-2). Using intensity dependent QFLS measurements, we quantify an implied PCE of 28.4% in the neat material which can be realized upon minimizing interfacial recombination and a further improvement of charge extraction.

Non-radiative recombination processes are the biggest hindrance to approaching the radiative limit of the open-circuit voltage for wide-band gap perovskite solar cells.

Matched energy levels of charge transport layer are crucial to minimize non-radiative recombination pathways. By tuning the lowest-unoccupied molecular-orbital of electron transport layers via the use of different fullerenes and fullerene mixtures, we demonstrate open-circuit voltages exceeding 1.35 V in CH₃NH₃Pb(I_0.8Br_0.2)₃ device.

Further optimization of mobility in binary fullerenes electron transport layer can boost the power conversion efficiency as high as 18.6%. We note in particular that the V_oc-fill factor product is > 1.085 V, which is the highest value reported for halide perovskites with this band gap.

All-inorganic perovskite compositions are highly interesting for solar cell application due to their outstanding thermal stability and tandem-relevant band gap. Although efficiencies over 19 % have been achieved[1], all-inorganic perovskite solar cells still lag behind their organic-inorganic counterparts. These lower efficiencies are largely due to lower open-circuit voltages compared to organic-inorganic perovskites with the same band gap.

This study investigates the efficiency potential of all-inorganic perovskite layers using intensity-dependent photoluminescence (PL) measurements. This contact-less measurement of a neat absorber film or a layer stack allows for the construction of a pseudo JV curve, providing potential performance metrics such as open-circuit voltage (V_OC) and fill factor[2]. Based on realistic assumptions on photocurrent generation, an efficiency potential of the film or stack can be calculated. This is a powerful tool to compare the potential of perovskite compositions and transport layers and to identify the dominating loss mechanisms.

For this presentation, DMAI-CsPbI₃ films[1] with a band gap of 1.73 eV are fabricated and analysed. Neat films on quartz glass show a PL-derived efficiency potential of 24.2 %, a remarkable value that for the first time quantifies a potential that can be achieved with ideal transport layers. Films on top of a bilayer of compact and mesoporous TiO₂ still show an efficiency potential of 22.9 %.

In addition, CsPbI₂Br films with a band gap of 1.9 eV are fabricated and compared to CsPbI₃ films. Films on top of the self-assembled monolayer (SAM) molecule 2PACz show a very high PL quantum yield (PLQY) of above 12 %. In a previous study on organic-inorganic perovskites, only potassium-passivated triple cation perovskite films showed a higher PLQY[2]. With the measured quasi-Fermi level splitting (QFLS) of 1.54 eV, 97 % of the radiative V_OC limit was reached, which is on par with the best organic-inorganic films we measured[2]. The resulting efficiency potential of 22.8 % is surprisingly close to the efficiency potential of CsPbI₃ on TiO₂ (22.9 %), which has a much lower band gap. These findings suggest very low defect density both in the CsPbI₂Br perovskite bulk and at the interface between perovskite and contact layer.

This study shows very high efficiency potentials that can be achieved with ideal transport layers. By comparing the most relevant compositions and transport layers, this study helps to identify the most promising route for all-inorganic perovskites and the main loss-inducing interfaces.

Recently, perovskite solar cells showed power conversion efficiencies (PCE) over 25%. However, commercialization of perovskite solar cells is hampered by environmental concerns due to the toxicity of the used lead. Hence, there is a high interest to substitute the lead by less toxic elements. Tin is a promising candidate, since it has similar electronic properties as lead and so can be easily replaced in the ABX₃ perovskite structure. Tin based solar cells also showed an incredible increase in solar cell efficiency of up to 13 % in the last years.[1][2] But those solar cells still lag behind the lead based ones, mainly due to high recombination losses.[3] In order to achieve less recombination losses and thus higher power conversion efficiencies, we modified the hole transport as well as the electron transport layer in our FASnI3 perovskite solar cells (PSC).

We altered the hole transport interface by introducing nanoparticles in between the HTL and perovskite absorber. An improvement in efficiency from 3 % to 4 % could be achieved. It seems, that the nanoparticles reduce the back reflection at the hole transport-perovskite-interface and lead to a smoother perovskite layer with less pinholes, which is beneficial for further depositions from solution and the prevention of shunts.

To further minimize V_OC losses, we evaluated three different C₆₀ derivatives (PCBM, ICBA, bis-PCBM) as electron transport layer to optimize the band alignment and interface towards the perovskite absorber. By this we could reach a nearly doubling of the open circuit voltage with ICBA from 380 mV to 650 mV and 480 mV for bis-PCBM in comparison to the commonly used PCBM in our Sn-based perovskite solar cells. This leads to an improvement in efficiency from an average of 3.6 % for PCBM to 5.6 % for ICBA as ETL. Detailed analysis about this phenomenon will be presented.

To sum up by modifying the charge transport layers, we could reach nearly 6 % for FASnI₃ devices without further additives except of the generally used SnF₂.

Perovskite bulk materials and inorganic quantum dots, are the most competitive materials for the future photovoltaics. Their outstanding properties in terms of photoconversion efficiency (PCE) led to a big progress, reaching an impressive PCE of 25.1%.^[1] Formamidinium based perovskite solar cells present the maximum theoretical efficiency of the lead perovskite family. However, formamidinium perovskite exhibits significant degradation in air. ^[2] Here PbS (quantum dots and nanoplatelets) are employed to control the morphological and optoelectronic properties and a lot of efforts are dedicated in the understanding the chemical interactions and the physical process involved in the stabilization of the perovskite material. The use of PbS nanoplatelets with (100) preferential crystal orientation potentiates the effects on the crystal growth of perovskite grains and improves the stability of the material and the solar cell. A stable incident photon to current efficiency in the infrared region of the spectrum for 4 months have been obtained, one of the best stability achievement for planar perovskite solar cells. On the other hands the energetic trends revealed by our DFT calculations make clear that the origin of thermodynamically favoured black FAPbI₃ upon inclusion of PbS is due mainly to two distinct, but both needed, mechanisms: first, the structure stabilization that destabilizes the yellow phase due to its large surface energy and, second, the crucial chemical stabilization by chemical bonds between the PbS and FAPbI₃ that stabilizes the black phase.^[3]

Lead halide perovskite nanocrystals (NCs) have emerged as a potential material for LED and solar cell applications [1]. However, despite of their promising performance, the band gap of lead halide perovskite NCs remains large that limit the absorption in infrared (IR) part of spectrum [2]. In this work, we explore the possibility to harvest the IR spectrum using formamidinium lead iodine (FAPI) perovskite nanocrystals by doping of PbS NCs. As a short wave IR absorber, PbS NCs has attracted much attention yet it suffer with high dark current eventually that limit the device performance. By mixing the two compounds with an optimum ratio, it is possible to preserve most of the IR absorption while the transport driven by the wider band gap of the perovskite, this enabling a dark current reduction. In addition to understand the electronic structure of FAPI/PbS hybrid, we fabricated an FET using high capacitance solid state gating. Using this strategy, we show that the hybrid material has an n-type nature with a charge carrier mobility of 2 x 10^-3 cm² V^-1s^-1.

However, as FAPI is introduced into the PbS NCs array, the benefit of the reduced dark current is partly mitigated by a reduced IR absorption. This problem is address by introducing a plasmonic resonator. The latter relies on a grating that generate a multi passes of the light into the absorbing layer thus enhancing the IR absorption [3]. The resonant electrode enhances the light-matter coupling within the NCs film that enhance the IR absorption up to 3 times [4]. In addition, the reduction of the interelectrode spacing enable photoconduction gain leading to an improved responsivity and detectivity by two order of magnitude in comparison to pristine PbS.

Perovskite solar cells have improved drastically over the past decade, overcoming hurdles of temperature- and water-induced instability to achieve efficient, stable devices. Three-dimensional (3D) perovskites have excellent properties including high charge-carrier lifetimes and mobilities, strong absorption and good crystallinity – ideal for photovoltaic devices. However, 3D perovskite materials struggle especially with moisture-induced degradation¹. The addition of large, hydrophobic organic cations can lead to the formation of two-dimensional (2D) perovskite structures. Devices made with 2D perovskites show much greater stability, but with far lower power conversion efficiencies than their 3D cousins². Materials combining 2D and 3D structures have thus recently become one the most promising candidates for use in solar cells^3,4. In order to fully understand the optoelectronic properties of these 2D-3D hybrid systems we look at BA_x(FA_0.83Cs_0.17)_1-xPb(I_0.6Br_0.4)₃ across the composition range 0 ≤ x ≤ 80 %⁵. We find that small amounts of butylammonium (BA) help to improve crystallinity and passivate grain boundaries, thus reducing monomolecular charge-carrier recombination, and boost charge-carrier mobilities. Excessive amounts of BA lead to poor crystallinity and inhomogeneous films forming, greatly reducing charge-carrier transport capabilities. For low amounts of BA the benevolent effects of reduced recombination and enhanced mobilities lead to outstanding diffusion lengths.

Perovskites offer exciting opportunities to realize efficient multijunction photovoltaic devices. This requires high-V_OC and often Br-rich perovskites, which currently suffer from halide segregation. Here, we study triple-cation perovskite cells over a wide bandgap range (∼1.5−1.9 eV). While all wide-gap cells (≥1.69 eV) experience rapid phase segregation under illumination, the electroluminescence spectra are less affected by this process. The measurements reveal a low radiative efficiency of the mixed halide phase which explains the V_OC losses with increasing Br content. Photoluminescence measurements on nonsegregated partial cell stacks demonstrate that both transport layers (PTAA and C₆₀) induce significant nonradiative interfacial recombination, especially in Br-rich (>30%) samples. Therefore, the presence of the segregated iodide-rich domains is not directly responsible for the V_OC losses. Moreover, LiF can only improve the V_OC of cells that are primarily limited by the n-interface (≤1.75 eV), resulting in 20% efficient 1.7 eV bandgap cells. However, a simultaneous optimization of the p-interface is necessary to further advance larger bandgap (≥1.75 eV) pin-type cells.

Segregation of illuminated mixed-halide perovskites into iodide- and bromide-rich domains presents a critical bottleneck in the application of wide-bandgap absorbers in single- and multi-junction architectures. And while its occurrence has been well-studied in thin films, the influence on operational solar cells lacks sufficient understanding.

This work employs a multimodal characterization procedure to observe the slow progression of halide segregation in efficient solar cells prepared in the p-i-n architecture using sequentially processed perovskite absorbers. Photoluminescence spectroscopy is used to identify the stages that demixing of halide ions entails while simultaneous tracking of photovoltaic parameters allows correlating performance degradation to the migration of ionic species in each stage. A new stage of the process is thus observed upon prolonged illumination. Characterization of sub-bandgap features reveals the occurrence of photo-induced defect formation whose suppression through cationic substitution provides a strategy for stabilization of wide-bandgap compositions against halide segregation.

Perovskite solar cells have been attracting the scientific world attention since 2009 due to outstanding absorption and charge transport properties. The possible ion migration, several ion types content and non-mono crystal structure make them really intriguing as well as challenging system to investigate by optical techniques. Electrons in the perovskite system, after light absorption, are promoted from the valence to the conduction band. First few hundreds of femtosecond after absorption are governed by cooling of hot carriers. When the process is finished, sharp absorption bleach due to the band filling phenomena occurs which decay correlates with photoluminescence kinetics and represents the excited carrier lifetime [1,2]. That decay proceeds by several paths such as the recombination (first-, second- and third-order) and charge injection to an electron transporting material (ETM) or a hole transporting material (HTM).        

We focused on a triple cation perovskite FA_0.76MA_0.19Cs_0.05(I_0.81Br_0.19)₃ sandwiched between a spiro-OMeTAD (HTM) and mesoporous TiO₂ (ETM) layers prepared under drybox (w/o oxygen and water) or ambient (in the presence of oxygen and ambient room humidity) conditions [3]. We performed femtosecond to nanosecond transient absorption as well as picosecond to nanosecond time-resolved emission studies of the prepared cells. We probed the cells from different sides to obtain information about the charge dynamics at ETM or HTM interfaces. The investigation was also supported by the electrochemical impedance and x-ray diffraction measurements.

The morphological studies indicate that the content of unreacted PbI₂ phase in the perovskite structure is much higher near the interface with titania than near the interface with spiro-OMeTAD. The stationary emission spectra and transient bleach peaks of perovskites show additional long-wavelength features close to the titania side. Time-resolved techniques reveal further differences in charge dynamics at both interfaces. The population decay is significantly faster at the ETM side than that at the HTM side for the cells prepared under ambient conditions, and the hole injection is faster for the solar cells with higher photocurrent in the cells prepared under drybox conditions. The charge recombination loss on the millisecond time scale is found to be slower at the interface with titania than with spiro-OMeTAD. The ideality factor of the cells is found to increase with increasing DMSO content in the precursor solution indicating a change in recombination mechanism from bulk to surface recombination [3].

Unlike typical inorganic semiconductors, lead–halide perovskites (LHPs) exhibit significant ionic conductivity, which is believed to affect their performance and stability. Motivated by a recent experimental study that suggested pressure as a means to control ionic conductivity in CsPbBr₃ [1], we present a detailed theoretical study of the atomic scale effects of pressure on anion migration in the low temperature orthorhombic Pnma phase of CsPbBr₃. Using nudged elastic band calculations based on density functional theory, we compute all symmetrically inequivalent activation barriers for anion migration to their closest neighbours, as a function of hydrostatic pressure in the range 0.0–2.0 GPa. We then use those values as parameters in a kinetic model which allows us to connect the atomic scale calculations to the macroscopic anion mobility tensor as a function of applied pressure.

We find that the mobility is enhanced by pressure in the plane spanned by the [100] and [001] lattice directions, while along the [010] direction it is diminished, leading to an effective 3D-to-2D transition of the mobility at elevated pressures. This can be explained by the fact that a network of only a few symmetrically inequivalent paths dominates the mobility at elevated pressures. Our results demonstrate the significant influence of pressure on both the rate and direction of anion migration in CsPbBr₃, which we consider likely to hold for other LHPs.

Lead halide perovskites are becoming subject of intense studies as they are reaching outstanding solar performance figures while they are rather easy to prepare, the main issue with them being their fragility. Our group has carried out theoretical calculations on them using hybrid functionals [1], has dealt with MD studies on them [2] and discussed the effects of ferroelectric domains on the diffusion of electrons and holes [3]. We have in addition tried their substitution with transition metals, trying to obtain the so-called in-gap band structure which has shown promise of enhancing PV solar efficiencies [4].

One case that has led to interesting results is that of partial substitution of Pb²⁺ by Cr²⁺, which has given, using GW-type calculations including spin-orbit effects, a band structure with the desired characteristics: a moderately narrow band, partially occupied and separated enough from both conduction and valence bands [5].

Then we tried the synthesis of such material, achieving the preparation of a perovskite MAPb_1-xCr_xBr_3-2xCl_2x (x=0.25 and 0.5) using mechano-chemical synthesis methods [6]. Its diffractogram agrees with that expected for a perovskite disordered in both halide and metal atoms, with nanocrystal sizes in the 30-50 nm range. Both magnetic measurements and XANES spectra indicate the almost exclusive presence of Cr²⁺ ions as Pb²⁺ substituents, with a magnetic transition to an antiferromagnetic state below 40 K which is confirmed by differential specific heat measurements. Most importantly, UV-Vis-NIR diffuse reflectance spectra show a feature at ca. 1.6 eV of photon energy, absent in the perovskite not substituted with Cr, which is accompanied also by a weaker feature at 0.75 eV. These latter results indicate that the desired in-gap band structure has been achieved, especially since the sum of the energies of these two latter features coincides with the overall bandgap of the perovskite. As far as we know, this is the first time that an isovalent substitution of Pb by Cr in a lead halide perovskite has been achieved.

Transient photoluminescence (tr-PL) measurements are among the most popular characterization techniques to monitor the charge-carrier dynamics and investigate recombination losses in halide perovskite layers and layer stacks.^[1-3] In particular, it is imperative to better understand and characterize interfacial recombination losses that often limit device performance in finished perovskite solar cells. However, interpretation of PL transients on multilayer samples including interfaces is a complex endeavour due to the superposition of various transient effects that modulate the charge-carrier concentration in the perovskite layer and thereby the measured PL. These effects include bulk and interfacial recombination, charge transfer to electron or hole transport layers and capacitive charging or discharging.^[4-6] The combination of these effects leads to substantial deviations from an exponential decay but is rather difficult to describe analytically. Hence, the state-of-the-art of interpretation of tr-PL decays of layer stacks or even full devices is currently still at an early stage. Data interpretation is often restricted to fitting one or several exponential functions to the decay to extract a “lifetime”. This approach causes a loss of information and impedes fully understanding and using the information contained in PL transients.

Here we combine numerical simulations with Sentauraus TCAD and experiments done over ~7 orders of magnitude in dynamic range on a variety of different sample geometries from perovskite films on glass to full devices to present an improved understanding of this method. We propose a presentation of the differential decay time of the tr-PL decay that follows from taking the derivative of the photoluminescence at every time.^[7] Plotting this differential decay time as a function of the time-dependent quasi-Fermi level splitting enables us to distinguish between the different contributions of radiative and non-radiative recombination as well as charge extraction and capacitive effects to the decay.

The excellent optoelectronic properties of metal halide perovskites (MHPs) have attracted extensive scientific interests and boosted their application in optoelectronic devices. Despite their attractive optoelectronic properties, their poor stability under ambient conditions remains the major challenge, hindering their large-scale practical applications. In particular, some MHPs undergo spontaneous phase transitions from perovskites to non-perovskites. Compositional engineering via mixing cations or anions has been widely reported to be effective in suppressing such unwanted phase transition. However, the atomistic and electronic origins of the stabilization effect remain unexplored. Here, by combining Density Functional Theory (DFT) calculations and Crystal Orbital Hamilton Population (COHP) analysis, we provide insights for the undesired phase transition of pristine perovskites (FAPbI₃, CsPbI₃, and CsSnI₃) and reveal the mechanisms of the improved phase stability of the mixed compounds (Cs_xFA_1-xPbI₃, CsSn_yPb_1-yI₃, and CsSn(Br_zI_1-z)₃). We identify that the phase transition is correlated with the relative strength of the M-X bonds as well as that of the hydrogen bonds (for hybrid compositions) in perovskite and non-perovskite phases. The phase transition can be suppressed by mixing ions, giving rise to either increased bond strength for the perovskite or decreased bond strength in their non-perovskite counterparts. Our results present a comprehensive understanding of the mechanisms for the phase instability of metal halide perovskites and provide design rules for engineering phase-stable perovskite compositions.

I will describe our use of multiscale modelling techniques to explore electron and ion dynamics in perovskite materials. My talk will focus firstly on mesoscale modelling of carrier dynamics in halide perovskites using ensemble Monte Carlo methods adapted from inorganic device simulation. I will explain how we can identity the effects of large polaron formation in MAPbI₃ on carrier scattering and mobilities, and explore the evolution of hot carrier distribution functions due to carrier-carrier scattering and its effects on carrier cooling. Secondly, I will describe our studies of the influence of hydrostatic pressure on ion migration in CsPbBr₃, using a transition matrix based microkinetic model parametrised from the results of DFT calculations. The phase diagram of CsPbBr₃ closely resembles the hybrid counterparts, making it a good proxy for studying ion mobility in other halide perovskites.

One of the most common methods for the deposition of perovskite layers in photovoltaic devices is the antisolvent engineering method. In this method, during the spin-coating of the perovskite precursor solution, an antisolvent is dripped onto the sample, triggering the removal of the host solvent and the crystallization of the perovskite layer. A few selected antisolvents have emerged as the most successful in the deposition of high quality perovskite layers, however, they do not seem to share any common properties with both polar and nonpolar, high and low boiling point solvents employed for device fabrication.

In this talk, I will introduce a general method that allows the fabrication of highly efficient perovskite solar cells by any antisolvent. Through a detailed study of perovskite films and devices fabricated by 14 different antisolvents, we identify the two key antisolvent properties that influence the deposition procedure that would lead to the formation of high quality perovskite films. When taking these properties into account, each antisolvent can be utilized to produce high performance devices with efficiencies up to 22%.

Solar energy can lead a “paradigm shift” in the energy sector with a new low-cost, efficient, and stable technology. Nowadays, three-dimensional (3D) methylammonium lead iodide perovskite solar cells are undoubtedly leading the photovoltaic scene with their power conversion efficiency (PCE) >25%, holding the promise to be the near future solution to harness solar energy [1]. Tuning the material composition, i.e. by cations and anions substitution, and functionalization of the device interfaces have been the successful routes for a real breakthrough in the device performances [2]. However, poor device stability and still lack of knowledge on device physics substantially hamper their take-off. Here, I will show a new concept by using a different class of perovskites, arranging into a two-dimensional (2D) structure, i.e. resembling natural quantum wells. 2D perovskites have demonstrated high stability, far above their 3D counterparts [3]. However, their narrow band gap limits their light-harvesting ability, compromising their photovoltaic action. Combining 2D and 3D into a new hybrid 2D/3D heterostructure will be here presented as a new way to boost device efficiency and stability, together. The 2D/3D composite self-assembles into an exceptional gradually

organized interface with tunable structure and physics. To exploit new synergistic function, interface physics, which ultimately dictate the device performances, is explored, with a special focus on charge transfer dynamics, as well as long term processing happening during aging. As shown in Fig.1, when 2D perovskite is used on top of the 3D, an improved stability is demonstrated. 2D perovskite acts as a sheath to physically protect the 3D underneath. In concomitance, we discovered that the stable 2D perovskite can block ion movement, improving the interface stability on a slow time scale. The joint effect leads to PCE=20% which is kept stable for 1000 h [3,4]. Incorporating the hybrid interfaces into working solar cells is here demonstrated as an interesting route to advance in the solar cell technology bringing a new fundamental understanding of the interface physics at multi-dimensional perovskite junction. The knowledge derived is essential for a deeper understanding of the material properties and for guiding a rational device design, even beyond photovoltaics.

The surge of perovskite solar cell device performance has been facilitated through breakthroughs in process engineering. Most process optimization has been carried out for spin-coating, which is the most common deposition technique used in metal-halide perovskite solar cell fabrication in labs around the world. Through a series of experiments using a small-footprint optical monitoring setup [1], we were able to gain insight into common perovskite precursor solutions and deposition strategies. For the most commonly used deposition method in perovskite solar cell manufacturing, spin-coating, we were able to distinguish different regimes of spin-coating from optical signatures. Process parameters like the spin-coating speed and precursor solution concentration critically determine wet film thinning and perovskite crystallization. We studied the effect of anti-solvent drip timing on the formation mechanism of metal-halide perovskite semiconductors for two different standard precursor solutions: MAPbI₃ and (Cs,MA,FA)Pb(Br,I)₃.[2] Our results enabled us to rationalize the much better reproducibility of (Cs,MA,FA)Pb(Br,I)₃ perovskite precursor solutions demonstrating that the halide ratio critically affects the fraction of solvate intermediate phase formed during crystallization. Other process parameters rarely considered, such as the timing of moving samples to a hotplate after spin-coating, were found to have a substantial influence on device performance as the compositional homogeneity is strongly affected.[3] Our in-depth insights into spin-coating of metal-halide perovskites prove to be of high value when translating processing strategies to scalable manufacturing methods such as slot-die coating and inkjet printing.

The interest in perovskite photovoltaics has significantly increased over the last few years. High power conversion efficiencies (PCE) and low-cost manufacturing make perovskite PVs a very promising candidate for future applications. Despite the very advantageous features of perovskite materials, several issues still need to be solved before the commercialization of perovskite solar cells (PSCs). The main challenges of bringing perovskite technologies to the market are (i) scaling-up of the cells and modules dimension, (i) usage of lead in the perovskite solar panels, and (iii) stability of the PSC modules. These three challenging topics will be discussed in the presentation, and possible solutions to overcome these issues will be proposed. The implementation of the proposed measures will help to demonstrate the feasibility of high-volume production. It is an important milestone towards the industrial manufacturing of perovskite photovoltaics and their future commercialization.

The talk will review consensus procedures for planning, conducting, and reporting stability testing of perovskite solar cells (PSC) recently formulated by a broad research community [1]. In particular, the suggested protocols highlight the importance of testing for: (1) Redistribution of charged species upon application of electric fields [2]; (2) distinguishing between degradation induced by various stress factors, (3) reversible degradation with qualitatively different recovery dynamics [3,4]. The recommended protocols are not meant to replace existing qualification standards, but rather to contribute to developing an understanding of PSC degradation mechanisms. Acceptance of these protocols and sharing the suggested datasets will facilitate inter-laboratory coordination and assist in the accumulation of PSC stability data acquired under well-defined and comparable conditions. This would allow the application of advanced approaches to analyzing large data sets, such as machine learning methods, and accelerate the development of stable PSC devices.

References

M. V. Khenkin, E. A. Katz, A. Abate, G. Bardizza, J. J. Berry, C. J. Brabec, F. Brunetti, V. Bulović, Q. Burlingame, A. Di Carlo, R. Cheacharoen, Y.-B. Cheng, A. Colsmann, S. Cros, K. Domanski, M. Dusza, C. J. Fell, S. R. Forrest, Y. Galagan, D. Di Girolamo, M. Grätzel, A. Hagfeldt, E. von Hauff, H. Hoppe, J. Kettle, H. Köbler, M. S. Leite, S. (Frank) Liu, Y.-Lin Loo, J. M. Luther, C.-Q. Ma, M. Madsen,  M. Manceau, M. Matheron, M. McGehee, R. Meitzner, M. K. Nazeeruddin, A. F. Nogueira, Ç. Odabaşı, A. Osherov, N.-G. Park, M. O. Reese, F. De Rossi, M. Saliba, U. S. Schubert, H. J. Snaith, S. D. Stranks, W. Tress, P. A. Troshin, V. Turkovic, S. Veenstra, I. Visoly-Fisher, A. Walsh, T. Watson, H. Xie, R. Yıldırım, S. M. Zakeeruddin, K. Zhu and M.  Lira-Cantu. Consensus on ISOS Protocols for Stability Assessment and Reporting for Perovskite Photovoltaics. Submitted.

M. V. Khenkin, K.M. Anoop, E. A. Katz and I. Visoly-Fisher. Bias Dependent Degradation of Various Solar Cells: Lessons for Stability of Perovskite Photovoltaics. Energy & Environmental Science, v. 12, No. 2, p. 550-558 (2019).

M. V. Khenkin, K.M. Anoop, I. Visoly-Fisher, S. Kolusheva, Y. Galagan, F. Di Giacomo, O. Vukovic, B. R. Patild, G. Sherafatipourd, V. Turkovic, H.-G. Rubahnd, M. Madsen, A. Mazanik and E. A. Katz. Dynamics of photoinduced degradation of perovskite photovoltaics: from reversible to irreversible processes. ACS Applied Energy Materials, 1, 799-806 (2018).

M. V. Khenkin, K. M. Anoop, I. Visoly-Fisher, Y. Galagan, F. Di Giacomo, B. R. Patil, G. Sherafatipour, V. Turkovic, H.-G. Rubahn, M. Madsen, T. Merckx, G. Uytterhoeven, J. P. A. Bastos, T. Aernouts, F. Brunetti, M. Lira-Cantu and E. A. Katz. Reconsidering Figures of Merit for the Performance and Stability of Perovskite Photovoltaics. Energy & Environmental Science, 11, 739-743 (2018).

We address a controversy surrounding the luminescence properties of low-dimensional halide perovskites and clarify that an often-observed broad luminescence arises from defect states instead of commonly invoked self-trapped excitons.

Whereas initial research into two-dimensional perovskites was predominantly driven by efforts to employ their narrow emission linewidth for LEDs or to boost the stability of their three-dimensional counterparts in photovoltaics, the latest hotly examined observation is the presence of broad emission bands. This broad emission has the potential for direct white light generation and significant research is currently conducted to find and optimise compounds for this purpose. Crucially, these efforts commonly base on the assumption that the origin of this luminescence is a so-called self-trapped exciton. Whilst this concept is elegant and theoretical calculations have offered some support, experimental evidence for this interpretation is so far scarce.

We therefore studied single-crystals of two-dimensional lead iodide perovskites through a variety of spectroscopic techniques and prove that the broad emission is in fact due to defect states in the bulk of the material. We study two compounds with different A-site cation and further vary the halide to underline the universality of our findings and meticulously exclude all other origins of broad emission bands that have hitherto been proposed.

Our work sheds light on the role of electron-phonon interactions and defect states in lead-based perovskites as well as an improved understanding of the luminescence properties of these compounds.

Perovskite nanostructures have been engineered for LEDs, lasers and photodetectors[1], their reduced dimensionality resulting in quantum confinement of charge carriers which yields dramatically different optoelectronic properties, including enhanced photoluminescence quantum yield[2] and lower thresholds for amplified spontaneous emission[3]. Although the creation of such perovskite nanostructures has clear advantages, it often relies on challenging top-down fabrication methods. It would therefore be highly advantageous if instead nanoscale domains were found to form intrinsically through self assembly in the perovskite.

In this study[4], I report the discovery of intrinsically-occurring nanostructures in FAPbI₃, which exhibit quantum confinement effects manifested as an oscillatory absorption feature above the band gap. These features are present at room temperature but sharpen and become more apparent as the temperature is lowered towards 4 K. I demonstrate that the energetic spacings and temperature-dependence of the peaks vary in a manner consistent with quantum confinement intrinsically associated with the lattice of the material. I suggest the origin of this confinement to be nanodomains with an extent of approximately 10-20 nm. This interpretation is supported by correlating absorption spectra against ab initio calculations based on the bandstructure of FAPbI₃ in the presence of infinite barriers, and simulations for superlattices with moderate barrier heights. I further explore ferroelectricity/ferroelasticity and delta-phase twin boundaries as two possible causes of these domains. Altogether, such absorption peaks present a novel and intriguing quantum electronic phenomenon in a nominally bulk semiconductor, offering intrinsic nanoscale optoelectronic properties without necessitating cumbersome additional processing steps.

The dynamic response of metal halide perovskite devices shows a variety of physical responses that need to be understood and classified for enhancing the performance and stability and for identifying new physical behaviours that may lead to developing new applications. These responses are the outcome of complex interactions of electronic and ionic carriers in the bulk and at interfaces. Based on a systematic application of frequency modulated techniques and time transient techniques to the analysis of kinetic phenomena, we present a picture of the dominant effects governing the kinetic behaviour of halide perovskite devices. First with impedance spectroscopy we provide an interpretation of capacitances as a function of frequency both in dark and under light, and we discuss the meaning of resistances and how they are primarily related to the operation of contacts in many cases. Working in samples with lateral contacts, we can identify the effect of ionic drift on changes of photoluminescence, by the creation of recombination centers in defects of the structure. We also address new methods of characterization of the optical response by means of light modulated spectroscopy. The IMPS is able to provide important influence on the measured photocurrent. We apply the dynamic picture to the characterization of perovskite memristors. A memristor is a device that has different metastable states at a voltage V. It has a resistance that depends on the history of the system, and the states can be switched by applied voltage. It is simpler than a transistor in that the control occurs by 2 contacts. As a summary we suggest an interpretation of the effects of charge accumulation, transport, and recombination, how these effects influence the current-voltage characteristics and time transient properties, and we suggest a classification of the time scales for ionic/electronic phenomena in the perovskite solar cells.

Current-voltage measurement and impedance spectroscopy (IS) are two closely related techniques. In this talk, we first show how different external contacts in Perovskite solar cells (PSCs) change the extracted-current transport resistance and the corresponding effects in the IS and the jV curve.[1] Hysteresis is the difference between the jV scan measured form short circuit to open circuit conditions (forward scan) and the opposite direction scan (reverse scan). Normal hysteresis, improved FF and V_oc in the reverse scan compared to the forward scan, is commonly observed in PSCs. This hysteresis has been related to the low-frequency capacitance in the IS response.[2, 3] Inverted hysteresis, which improves FF and V_oc in the forward scan, as well as negative capacitance at the IS low-frequency domain, are also familiar features in PSC, but their origin is still under discussion. In this talk, we show the emergence of these responses in two separate experiments employing different PSCs formulations. By mean of the Surface Polarization Model[4, 5] we expose that these features have a time constant associated with ions/vacancies kinetics interaction with the surface. These interactions increase the interfacial recombination, reducing the recombination resistance obtained by the IS measurements and provoking a flattening of the j-V curve.[6]

Outstanding efficiency (up to 25.2% on glass[1], 19.5% on flexible substrates[2]), low cost, and processability from solution are key advantages of perovskite solar cells (PSC). This technology is rising as a promising candidate for building-integrated photovoltaics, space and automotive applications, and even foldable and lightweight devices for consumer electronics when deployed on flexible substrates[3].

Efforts have been made to up-scale the fabrication of PSC to produce large solar modules[4]: well-known techniques such as screen printing, blade- and slot-die coating, inkjet printing have been adapted and tested as possible manufacturing processes. We have focused on an automated spray coating technique[5], starting from the first fundamental layer for the standard n-i-p structure, the electron transport layer (ETL).

Herein, we thus present the development of uniform large-area (> 120 cm2) compact films of tin oxide nanoparticles (SnO2-NP) on rigid glass/ITO substrates via spray deposition, and we show their subsequent application as ETL in PSCs.

Our work investigated the effect of the spray deposition parameters (such as gas pressure, nozzle aperture, spray deposition velocity, flow rate, spray distance and spray cycle times) on the morphology and electro-optical properties of the SnO2 films, as well as the influence of the substrate temperature: given a set of deposition parameters, we found out that SnO2 layers sprayed at a lower temperature (25-30 °C) performed better as ETL in PSCs than those produced at higher temperatures (60-120°C), opening up to the application to flexible substrates.

Furthermore, PSCs endowed with SnO2 films, sprayed at low-temperature, performed as good as those with standard spin-coated films, delivering up to 16.8% efficiency under 1 sun illumination and demonstrating that automated spray coating at low temperature can be an effective strategy for PSC technology transfer from lab to industry to manufacture large-area devices, even on flexible substrates.

While solution-processable metal halide perovskites have sparked a major interest in (opto)electronic applications, light emitting diodes (PeLEDs) from metal perovskites have not been utilized by inkjet-printing [1]. Our work represents the first demonstration of inkjet-printed PeLEDs by utilizing a modified poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) templating layer. By adding potassium chloride (KCl) to the commonly used hole injection material PEDOT:PSS underneath the inkjet-printed perovskite layer, we significantly influence the crystallization behavior and later device performance of the PeLEDs.

The so-called “salty” PEDOT:PSS acts as a seeding template to induce crystal nucleation of the metal halide perovskites inkjet-printed on top, while having only a minor effect on the electrical properties of PEDOT:PSS. Together with the perovskite polymer composite ink, optimized for inkjet printing, this templating approach eliminates the conventional antisolvent treatment required to induce favorable perovskite nucleation.

PeLEDs utilizing the KCl-induced templating effect in a planar PEDOT:PSS/MAPbBr₃:PEG architecture show greatly improved performance, mainly due to improved crystallization dynamic in contrast to PeLEDs containing pure PEDOT:PSS. Specifically, KCl-modified PEDOT:PSS contact layers enabled the realization of inkjet-printed PeLEDs with a 30-fold increased luminance at the same operating voltage. Impressively, the ink-jet printed PeLEDs shown in this paper have comparable performance parameters to spin-coated reference devices and pave the way to cost-effective, scalable, and printable PeLEDs [2].

Hybrid organic-inorganic metal-halide perovskite solar cells (PSCs) have achieved a tremendous rise in power conversion efficiency (PCE) from 3.8 %[1] to an impressive level of 25.2 %[2]. However, a reliable transfer of solution processing from spin coating to scalable printing techniques and a homogeneous deposition on large substrate sizes is challenging.

Typically, Poly(triaryl amine) (PTAA) is replacing the widely used Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)[3,4] as hole transport layer (HTL) in planar inverted (p-i-n) PSCs because of its efficient carrier transport properties.[5-7] PSCs with PTAA mostly benefit from boosted open circuit voltage[5,8,9] due to a proper energy level alignment.[10]

However, PTAA is a non-polar polymer with a low surface energy and thus is highly hydrophobic, which leads to severe dewetting of the subsequently deposited polar perovskite precursor solutions.[11-13]

Based on our recent publication on a universal nanoparticle (NP) wetting agent for perovskite precursor solutions on non-wetting materials deposited via spin coating[14], we here show the utilization of blade coated non-conductive silicon oxide (SiO₂) NP dispersions to enable the deposition of a homogeneous perovskite layer on the highly hydrophobic HTL. The NPs enhance the HTL surface energy, thus, wetting and homogeneous spreading of the precursor solution is strongly improved so that pinholes in the perovskite layer and thereby short-circuited devices are avoided. In this work, we demonstrate the transfer of this wetting concept to scalable gas stream-assisted blade coating and solution-processed PSCs and modules in the inverted device architecture with PTAA as HTL on large-area substrates for the first time. In order to prevent void formation at the HTL interface of gas stream-assisted blade coated perovskite layers, the effect of blending small amounts of lead chloride (PbCl₂) in the perovskite precursor solution is investigated, which also improves reproducibility and device performance. Following these optimizations, blade coated PSCs with 0.24 cm² active area achieve up to 17.9 % PCE. Furthermore, to prove scalability, we show enlarged substrates of up to 9×9 cm² and analyze the homogeneity of the perovskite layer in blade coating direction. Moreover, by implementing the blade coated NP wetting agent, we fabricate large-area modules with a maximum PCE of 9.3 % on a 49.60 cm² aperture area. This represents a further important step bringing solution-processed inverted PSCs closer to application.

Mixed halide perovskites (MHPs) under photoirradiation phase segregate into I- and Br-rich domains and, if in contact with a solvent, the film will then expel iodine into solution. Hole trapping at the I-site in MHPs dictates the iodine migration. We have now succeeded in modulating the iodide expulsion process in MHPs through externally applied electrochemical bias. At anodic potentials, electron extraction at the electrode interface becomes more efficient, leading to build-up of holes within the MHP film. This in turn favors phase segregation and increases the rate iodine expulsion. Conversely, at cathodic bias we facilitate electron-hole recombination within the MHP film and slow down iodine expulsion. The tuning of EFermi through external bias modulates charge extraction at the perovskite electrode interface and indirectly controls the build-up of holes, which in turn induces iodine expulsion. Suppressing iodine migration in perovskite solar cell is important for attaining greater stability of perovskite solar cells since they operate under internal bias.

Heavy water or deuterium oxide (D₂O) comprises of deuterium, a hydrogen isotope twice the mass of hydrogen. In contrast to the report of shorter charge carrier lifetimes and lower/invariant efficiencies on deuteration of perovskite, we herein uncover the unexpected effect of D2O as solvent additive to enhance the power conversion efficiency and stability in solar cell devices. Here, we demonstrate the PCE increment of triple-A cation (cesium (Cs)/methylammonium (MA)/formaminidium (FA)) perovskite solar cells from approximately 19.2% (reference) to ~21 % (using 1 vol% D₂O) with higher stability in comparison with 1% H₂O (by vol) additive. The in-depth investigation using ultrafast optical spectroscopy divulge the suppression of trap states from 2.5 x 10¹⁷ cm^-3 to 0.7 x 10¹⁷ cm^-3 and increase of PL lifetime from 35 nm to 70 nm. Fourier transform infrared spectroscopy and solid-state nuclear magnetic resonance (NMR) spectroscopy validates N-H₂ group as the preferential isotope exchange site and induced alteration of the FA to MA ratio as a result of perovskite deuteration. Theoretical simulations using first-principles density functional shows a decrease in PbI₆ phonon frequencies in the deuterated perovskite lattice which stabilizes the PbI₆ structures and weakens the electron-LO phonon (Fröhlich) coupling. Herein, our findings of selective isotope exchange in perovskite opens the opportunities for tuning perovskite optoelectronic properties.

Abstract

Organic-inorganic halide perovskites are promising as the light absorber of solar cells because of their efficient solar power conversion. Power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) have already reached a very high level of up to 25.2 %, which is comparable to silicon solar cell technology. Most of high-performing PSCs reported to date contain a small molecular hole transport layer (HTL) material of 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD). An issue frequently occurring in spiro-OMeTAD-based PSCs is quick performance degradation at high temperature. In this study, we discover that post-doping of the spiro-OMeTAD layer by iodine released from the perovskite layer is one possible mechanism of the high-temperature PSC degradation. The iodine doping leads to the highest occupied molecular orbital level of the spiro-OMeTAD layer becoming deeper and, therefore, induces the formation of an energy barrier for hole extraction from the perovskite layer. We demonstrate that it is possible to suppress the high-temperature degradation by employing an iodine-blocking layer or an iodine-free perovskite in PSCs. These findings will guide the way for the realization of thermally stable perovskite optoelectronic devices in the future.

Room temperature ion migration under an external electric field is being increasingly accepted as one of the major origins of the commonly observed current-voltage hysteresis in solar cells and light-emitting diodes, fabricated using three-dimensional (3D) hybrid lead halide perovskites. ^[1] This mixed ionic-electronic nature of perovskite and other perovskite-related materials can now be taken advantage of in resistance-switching memory devices whose performance, in terms of the ON/OFF ratio, depends on the efficiency of the vacancy/ion migration process. However, in the halide perovskites field, a direct link between the average/local structure and the preferred ion migration hopping pathway has yet to be established. In our study, we combined the study of average/local structural characterization and detailed electrical measurements to shine a light on the interrelationships between the structure and efficiency of ion migration in layered methylammonium copper halide materials (MA₂CuX₄). ^[2] We adopted the solvent acidolysis crystallization technique to grow various halide-deficient single crystals and with the help of synchrotron X-ray powder diffraction (XRPD) and pair distribution function (PDF) analyses, we identified the halogen vacancy site in the copper halide octahedra, the octahedra tilting, and the thermal vibrations of the atoms around their average positions. We correlated the variations in these parameters to the hysteresis observed in the current-voltage curves and subsequently to the ON/OFF ratios of proof-of-concept memory devices fabricated using inert Pt electrodes. Furthermore, our best ON/OFF ratio of 10 from our Pb-free devices compares well to the results obtained from two-dimensional Pb-based devices utilizing inert electrodes. Our experimental results made on single crystalline samples highlights the need to study detailed structural factors that could affect ionic migration in perovskite and perovskite-related compounds, which is important for the performance of memristor and optoelectronic devices.

Thanks to their high absorption coefficient and ideal band-gap [1], halide perovskite materials are good candidates for the next generation of solar cells with an impressive certified power conversion efficiency of 25.2 %. Defect-induced trap states are believed to be partially responsible for the instability of perovskite materials through their formation, passivation and subsequent degradation of the material [2]. Extracting information related to these trap states such as their concentration and the trapping rate is thus essential in order to compare samples and devices to help drive the improvement of perovskite solar cells. Time-resolved photoluminescence (TRPL) is a powerful tool to investigate excited charge carrier (electrons and holes) recombinations in semiconductors and molecular systems. However, due to the non-excitonic nature of excited charge carriers in lead halide perovskite materials coupled with the presence of localised trap states in their band-gap, the TRPL of these materials is complicated to interpret. Here we discuss two models used in the literature to simulate charge carrier recombinations and TRPL in perovskite materials. These models consider the bimolecular nature of direct electron-hole recombination but differ in their treatment of trap-mediated recombinations; one describing trapping as a monomolecular process [3] whereas the other considers it to be a bimolecular process between free carriers and the available trap states [4]. The dependency on the excitation fluence of each model is discussed as well as the importance to allow complete recombination of all excited charge carriers between two consecutive excitation pulses. Finally, we compare these models to the commonly used bi-exponential model.

Within the past few years, metal halide perovskites have been attracting significant interest due to their and their versatile use in a wide range of applications. These materials have been used in lasers, photodetectors, and most commonly, in photovoltaic devices and light emitting diodes. Despite the cheap and simple fabrication methods by which these materials are deposited, the resulting perovskite films are effectively high-quality semiconductors, and the power conversion efficiencies of lead halide perovskite solar cells are now exceeding certified values of 23%. However, perovskite-based devices are yet to achieve their full potential. One of the major hindrances to achieving this is an incomplete understanding of perovskite surfaces and interfaces. Deficiencies at these interfaces may be responsible for the largest losses in perovskite-based optoelectronic devices; limiting charge extraction, increasing non-radiative recombination rates and leading to hysteresis, and significantly increasing the voltage loss in perovskite photovoltaics. Herein, I will present interface modification strategies to mitigate these deficiencies. Charge-transfer dopants are used to dope the perovskite at the interface, resulting in the formation of narrow homojunctions. These homojunctions result in reduced interfacial recombination, suppressed hysteresis and improved device performance, yielding steady-state device efficiencies of over 21%. I will also explore the use fluoride-containing ionic liquids at the metal-oxide perovskite interface and show that not only do they affect the work function of the metal oxide, but also interact strongly with the perovskite, significantly improving the quality of the perovskite film. The utility of these defect mitigation strategies can readily be applied beyond perovskite PV and is likely to also improve the performance of a range of other perovskite-based optoelectronic devices

Thin-film perovskite solar cells (PSCs), whose record efficiency has rocketed from under 4% to over 25% (comparable to silicon solar cells) in just ten years, offer unprecedented promise of low-cost, high-efficiency renewable electricity generation. Organic-inorganic halide perovskite (OIHP) materials at the heart of PSCs have unique crystal structures, which entail rotating organic cations inside inorganic cages, imparting them with desirable optical and electronic properties. To exploit these properties for PSCs application, the reliable deposition of high-quality OIHP thin films over large areas is critically important. The microstructures and grain-boundary networks in the resulting polycrystalline OIHP thin films are equally important as they control the PSC performance and stability. Fundamental phenomena pertaining to synthesis, crystallization, coarsening, microstructural evolution, and grain-boundary functionalization involved in the processing of OIHP thin films for PSCs will be discussed with specific examples. In addition, the unique mechanical behavior of OIHPs, and its implication on the reliability of PSCs, will be discussed. The overall goal of our research is to have deterministic control over the scalable processing of tailored OIHP thin films with desired compositions, phases, microstructures, and grain-boundary networks for efficient, stable, and reliable PSCs of the future.

Opto-electronic devices based on all-inorganic perovskite systems are an energy-efficient source of lighting due to their high photoluminescence quantum yield (QY). However, dominant surface trapping continues to plague the field, despite their high defect tolerance, as evidenced by the several fold improvements in the external quantum efficiency of perovskite nanocrystals (NCs) upon appropriate surface passivation or physical confinement between high bandgap materials. Here, we introduce the concept of drip-feeding of photo-excited electrons from an impurity-induced spin-forbidden state to address this major shortcoming.  An increased and delayed (about several milliseconds) excitonic QY, and density functional theory establish the electron back-transfer signifying efficient recombination.  We term this electron back-transfer from Mn²⁺ to the host conduction band in this prototypical example of Mn-doped CsPbX₃ (X = Cl, Br) NCs through vibrational coupling as Vibrationally Assisted Delayed Fluorescence (VADF).

References:

1. Pradeep K. R., Debdipto Acharya, Priyanka Jain, Kushagra Gahlot, Anur Yadav, Andrea Camellini, Margherita Zavelani-     Rossi, Giulio Cerullo, Chandrabhas Narayana, Shobhana Narasimhan, and Ranjani Viswanatha, Harvesting Delayed Fluorescence in Perovskite Nanocrystals Using Spin-Forbidden Mn d States, ACS Energy Letters 2020 5 (2), 353-359

Mixed halide perovskites (MHP) have been highlighted as promissory materials in optoelectronics, due to improved light harvesting, photocarrier generation, and the ease for tuning their optical properties, specially their band gap.[1] This feature has open the door to analogous solar driven process as photocatalysis for carrying out the photodegradation of recalcitrant organic compounds more efficiently.[2] Nonetheless, the photocatalytic (PC) activity of MHP mainly depends on the surface chemical environment formed during their synthesis. This correlation has not been studied yet. In this work, we deduced the nature and the role of surface chemical states of MHP nanocrystals (NC) synthesized by hot-injection (H-I) and anion-exchange (A-E) methods, on their PC performance for the oxidation of β‑naphthol as a model system. We identified iodide vacancies as the main surface chemical states that promote the formation of highly reactive superoxide ions. These species define the PC activity of A‑E-MHP. Conversely, the PC performance of H-I-MHP is dictated by an adequate balance between band gap and highly oxidizing valence band. In this context, MHP can be considered as good photocatalysts for efficient environmental remediation.

The application of Lead Halide Perovskites (LHP) nanomaterials in many technological fields can take advantage of the physico-chemical properties typical of highly anisotropic morphologies, which can be introduced in nanocrystals by extreme reduction of their thickness [1, 2]. Overcoming critical issues of stability for extremely downsized nanocrystals [3], quasi-2D highly stable CsPbBr₃ nanoplatelets (NPLs) have been synthetized, which exhibit large exciton binding energies and intense blue emission, with PL maximum tunable through fast post-synthetic anionic-exchange reactions. In addition, due to the large surface area, NPLs show a clear tendency to self-assemble, making them particularly promising for LED applications [4]. A precise and accurate atomistic description of LHP structures is of high relevance for formulating reliable considerations over their exceptional properties. An in-depth structural and morphological characterization of CsPbBr₃ NPLs has been carried out by means of a combination of Transmission Electron Microscopy (TEM) imaging and Wide Angle X-ray Total Scattering (WAXTS) techniques. The analysis of high resolution WAXTS data, collected with synchrotron source on colloidal suspensions, was based on the construction of atomistic models of NPLs and the application of the Debye Scattering Equation (DSE) [5]. This advanced approach of structural analysis addresses limitations of traditional X-ray diffraction techniques when applied to complex nanomaterials [6]. In the CsPbBr₃ NPLs case, it enabled to elucidate multiple structural and morphological aspects. Through the analysis, robust information on the NPLs thickness of six PbBr₆ octahedra monolayers was obtained. Moreover, the orthorhombic crystal structure and its specific relative orientation with respect to the NPLs facets were determined. In particular, the results indicate that the NPLs most extended surfaces expose octahedral equatorial bromides, whereas the axial bromides run parallel to the crystallographic b axis that is the most expanded. The overall surface composition remains defective in Br and Cs [7].

Perovskite CsPb(Br_xI_1-x)₃ nanocrystals are attractive building blocks for light-emitting assemblies for a few reasons. First, modern syntheses enable preparation of size and shape-pure CsPb(Br_xI_1-x)₃ nanocrystal samples with tunable composition and optical properties. Second, microscopic assemblies of these nanocrystals can be readily grown by low-cost fabrication methods, and their structure examined by standard x-ray diffraction. Third, the strong light absorption and peculiar emission characteristics of these nanocrystals create the possibility of collective emission phenomena. A practical application of such assemblies, for example, as coherent light sources embedded in a circuit, requires a stable emission under optical or electric stimuli and environmental conditions. Dynamic surface passivation of nanocrystals and halide photochemistry are two factors that alter their light emission in counterintuitive ways. In our contribution, we will discuss these barriers to the stable optical performance of CsPb(Br_xI_1-x)₃ nanocrystal assemblies and ways to overcome them.

Connecting nanocrystals with removing interface ligand barriers is one of the key steps for efficient carrier transportation in opto-electronic device fabrication. Typically, ion migration for crystal deformation or connection with another nanocrystals needs a solvent as medium. However, in contrary, this has been observed for CsPbBr₃ perovskite nanocrystals in film where nanocrystals were swelled to get wider and fused with adjacent nanocrystals in self-assembly in film with solvent evaporation. Depending on precursor composition and exposed facets, again these connections could be programmed for tuning their connecting directions leading to different shapes. Aging further on solid substrate, these were also turned to continuous film of nanostructures eliminating all inter-particles gaps on the film. This transformation could be ceased at any point of time, simply by heating or adding sufficient ligands. Analysis suggested that these unique and controlled connections were only observed with polyhedron shaped nanostructures with certain compositions and not with traditionally cubes. Details of these transformations, the change in optical properties, modulations of shapes, importance of solvent evaporation and the impact of different shaped nanostructures in this process would be discussed and presented in the presentation.

Halide perovskite semiconductors can merge the highly efficient operational principles of conventional inorganic semiconductors with the low‑temperature solution processability of emerging organic and hybrid materials, offering a promising route towards cheaply generating electricity as well as light. Following a surge of interest in this class of materials, research on halide perovskite nanocrystals (NCs) as well has gathered momentum in the last years. While most of the emphasis has been put on CsPbX₃ perovskite NCs, more recently the so-called double perovskite NCs, having chemical formula A⁺₂B⁺B³⁺X₆, have been identified as possible alternative materials, together with various other metal halides structures and compositions, often doped with various other elements. This talk will also discuss the research efforts of our group on these materials. We will highlight how for example halide double perovskite NCs are much less surface tolerant than the corresponding Pb-based perovskite NCs and that alternative surface passivation strategies need be devised in order to further optimize their optical performance. Other topics that will be covered are the role of surface ligands on stabilizing the NCs, including those with alloy compositions, and the synthesis of heterostructures in which one domain is a halide perovskite and the other domain is another material.

Quasi-two-dimensional (2D) semiconductor nano-platelets (NPs) manifest strong quantum confinement with exceptional optical characteristics of narrow photoluminescence peaks with energies tunable by thickness with monolayer precision.[1] We employed scanning tunneling spectroscopy (STS) in conjunction with optical measurements to probe the thickness-dependent band gap and 2D density of excited states in a series of CdSe nanoplatelets.[2] The STS fundamental band gaps are larger than the optical gaps as expected from the contributions of exciton binding in the absorption, as confirmed by theoretical calculations.  Strikingly, the energy difference between the heavy-hole and light-hole levels in the tunneling spectra are significantly larger than the corresponding values extracted from the absorption spectra. We have shown that this difference is mainly connected with enhancing of dielectric confinement of light holes relative to the heavy  hole observed in STS measurement and an increase of light  hole exciton binding energy relative to  heavy hole  exciton binding energy observed in absorption spectra.[2]    The dielectric confinement  increases also  the distance   between first and second 2D sub-bands of  electrons and holes  in perovskite NPs.    As a result the sum of the distance between first and second electron and hole subbands measured by   STS  should be  larger than distance between first and second absorption maxima. 

1. S. Ithurria, M. D. Tessier, B. Mahler, R. P. S. M. Lobo, B. Dubertret, and Al. L. Efros, A. L. Nat. Mater. 2011, 10, 936−941.

2. B. Ji, E. Rabani, Al. L. Efros, R. Vaxenburg, O. Ashkenazi, D. Azulay, U. Banin, and O. Milo ACS Nano, 2020, 14, 8257-8265.

Interest in perovskite (ABX₃) LEDs has exploded over the past several years due to their strong light-emitting properties, tunable emission, and facile fabrication. High performance red and green devices have been demonstrated by several groups, with external quantum efficiencies (EQEs) over 20%, matching OLED performance. Blue LEDs, however, have lagging significantly behind. Here, we identify and rectify the two crucial issues holding these materials back: the low internal photoluminescence yield and the LED device structure itself.

First, we show that NiOx, a common hole transport layer in these materials, induces defects in the nanocrystals, reducing emission by an order of magnitude and introducing rapid decay channels. To fix this issue, we design a new hole transport layer based on a combination of a traditional hole transport layer and perfluorinated ionomer, allowing the nanocrystals to emit with their native efficiency and lifetime. We then show that this translates directly to devices, increasing the EQE from 0.03% to 0.50%. Further, we show that the benefits apply to devices across the visible spectrum, with efficient blue, sky-blue, and green devices.

Next, we demonstrate that a large portion of the lost efficiency is due to inherent emission losses in the blue perovskite nanocrystals, as they demonstrate a thin film photoluminescence quantum yield of less that 10%. This can be rectified, however, by taking advantage of a surprising effect. When doping the nanocrystals with Mn, which introduces an energetic loss pathway to an orange emissive state, we counterintuitively observe an increase in perovskite emission. By carefully tuning the amount of Mn in the nanocrystal, we can increase the quantum yield over 3x. This translates directly to device performance, as doped devices reach a maximum EQE of 2.1%. The combination of the device and dopant improvements increases performance by over 60x, showing that blue perovskite materials can be competitive with their red and green cousins. Finally, we use this efficient blue emitter to build an all-perovskite white LED, which has important applications in lighting.

We will conclude the talk by demonstrating that Mn is not alone as an effective dopant: several other atomic dopants across the periodic table provide similar benefit. We will present the similarities and differences amongst these dopant conditions and show how we can use them to push device performance even further.

Colloidal metal halide perovskite nanocrystals (NCs) with chiral ligands are outstanding candidates as a circularly polarized luminescence (CPL) light source due to many advantages such as high photoluminescence quantum efficiency, large spin–orbit coupling, and extensive tunability via composition and choice of organic ligands. However, achieving pronounced and controllable polarized light emission remains challenging. Here, we develop strategies to achieve high CPL responses from colloidal formamidinium lead bromide (FAPbBr₃) NCs at room temperature using chiral surface ligands. First, we show that replacing a portion of typical ligands (oleylamine) with short chiral ligands ((R)-2-octylamine) during FAPbBr₃ NC synthesis results in small and monodisperse NCs that yield high CPL with average luminescence dissymmetry g-factor, g_lum = 6.8 × 10^–2. To the best of our knowledge, this is the highest among reported perovskite materials at room temperature to date and represents around 10-fold improvement over the previously reported colloidal CsPbCl_xBr_yI_3-x-y NCs. In order to incorporate NCs into any optoelectronic or spintronic application, the NCs necessitate purification, which removes a substantial amount of the chiral ligands and extinguishes the CPL signals. To circumvent this issue, we also developed a postsynthetic ligand treatment using a different chiral ligand, (R-/S-)methylbenzylammonium bromide, which also induces a CPL with an average glum = ±1.18 × 10^–2. This postsynthetic method is also amenable for long-range charge transport since methylbenzylammonium is quite compact in relation to other surface ligands. Our demonstrations of high CPL and glum from both as-synthesized and purified perovskite NCs at room temperature suggest a route to demonstrate colloidal NC-based spintronics.

We discuss the discovery and recent developments of colloidal lead halide perovskite nanocrystals (LHP NCs_, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) [1-5].  LHP NCs exhibit spectrally narrow (<100 meV) fluorescence, originating form bright triplet excitons [6], and tunable over the entire visible spectral region of 400-800 nm. Cs- and FA-based perovskite NCs are promising for LCD displays, for light-emitting diodes and as precursors/inks for perovskite solar cells.  Perovskite NCs also readily form long-range ordered superlattices, which exhibit accelerated coherent emission (superfluorescence) [7]. Unique structure engineerability of perovskites allows for (nearly) independent tuning of the emission color and radiative rates, which can be used in printable unicolor security tags [8].

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

2. L. Protesescu et al. J. Am. Chem. Soc. 2016, 138, 14202–14205

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

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

5. Q.A. Akkerman et al. Nature Materials 2018, 17, 394–405

6. M. A. Becker et al, Nature 2018, 553, 189-193

7. G. Raino et al. Nature 2018, 563, 671–675

8. S. Yakunin et al. in revision

Metal halide perovskite materials have emerged as a promising light emitter with various advantages including high color purity, easy color tunability, high charge-carrier mobility, solution processability, and low material cost. However, perovskite light-emitting diodes (PeLEDs) showed low electroluminescence (EL) efficiency at room temperature because of its intrinsically low exciton binding energy. Here, we present high-efficiency PeLEDs using various strategies to overcome the EL efficiency limitations. We suggest that the efficiency in PeLEDs can be increased by realizing core/shell structured perovskites, which can decrease the grain size and passivate the surface traps of perovskite grains. By introducing the organic-shielded nanograin engineering method, organic conducting materials could surround the perovskite grains in form of a core/shell structure to maximize the EL efficiency (current efficiency = 87.35 cd/A).[1] Also, new strategies to improve efficiency and operational stability of PeLEDs were applied by assembling 2D perovskites as shells for 3D bulk perovskites. Realization of the 3D/2D core/shell structure could successfully suppress the ion migration in perovskite materials, extending the operational lifetime ~15 times and extremely suppressing abnormal luminance overshoot in PeLEDs.[2]

Colloidal metal halide perovskite nanocrystals (NCs) are characterized by a large surface-to-volume ratio that renders them extremely sensible to surface processes. Passivating ligands, employed to stabilize NCs in organic solvents, play a pivotal role in influencing the structure and the optoelectronic properties of these materials. Despite major progresses attained in the last years to model the surface of NCs, there are still several key questions to be answered on the nature of the NC-ligand interactions and how trap states, which are deleterious to optical efficiency, develop on the surface.

A leap forward in solving the above issues is to analyze the surface using first principle simulations, such as Density Functional Theory (DFT). Until now some of the major drawbacks of this approach have been: (i) the size of the system that can be handled that in the best cases is restrained to a few hundredths atoms (i.e. a small sized NC surrounded by short ligands), and (ii) the description of static properties with the absence of dynamic effects.

Here, I will present a tool to automatically parametrize the force-field of nanoscale semiconductor crystallites, then we show the first multiscale modeling of real sized CsPbX₃ NCs (X = Cl, Br, I) passivated with oleate and quaternary ammonium ligands with a simulation box containing one million of atoms including the solvent. Molecular dynamics simulations, carried out up to the nanosecond timescale, provide crucial insights on the surface dynamics, and the role of the ligands in influencing the properties of these materials.  

The halide perovskites have received a humongous attention in the last decade due to their unique opto-electronic properties and extensive compositional tunability. At the focus of interest are the three-dimensional (3D) structures with the generic chemical formula ABX3 (A- counter ion, B-metal, X-halide), as well as emerging two-dimensional Ruddlesden-Popper halide perovskites with a chemical formula L2An-1BnX3n+1 (L-site: van der Waals (vdW) ligands between the perovskite layers). The 2D halide perovskites possess a quantum well structure with a sandwich configuration, where inorganic perovskites are stacked between organic spacer ligands. While the electronic and optical properties of the mentioned halide perovskites have been meticulously studied in recent years, the era of magnetism was explored to a lesser extent.

This work focuses on a thorough investigation of intrinsic phenomena leading to effective magnetic fields, including spin-orbit coupling, breaking of inversion of symmetry, nuclear field and intentional doping or alloying. The material platform is based on the 3D and 2D of lead-halide perovskite with/without the incorporation of magnetic impurities (e.g., transition metal or lanthanide cations). The concentration of the foreign cations varies from a doping to an alloying level. The physical phenomena are explored using polarized magneto-photoluminescence and optically detected magnetic resonance spectroscopy. A few specific examples will be discussed: (a) Rashba and intrinsic nuclear magnetic fields in 3D structures; (b) The added value of magnetic impurities, with the example of Ni- doping in 3D perovskite and Gd-doping in 2D structures, either as bulk single crystals or as nanostructures.

Lead halide perovskites have emerged as promising new semiconductor materials for high-efficiency photovoltaics, light-emitting applications and quantum optical technologies. Their luminescence properties are governed by the formation and radiative recombination of bound electron-hole pairs known as excitons, whose bright or dark character of the ground state remains debated [1, 2].

Spectroscopically resolved emission from single lead halide perovskite nanocrystals at cryogenic temperatures provides unique insight into physical processes occurring within these materials. The emission spectra collapse to narrow lines revealing a rich spectroscopic landscape and unexpected properties, completely hidden at the ensemble level and in bulk materials.

In this talk, I will discuss how magneto-photoluminescence spectroscopy provides a direct spectroscopic signature of the dark exciton emission of single lead halide perovskite nanocrystals [3]. The dark singlet is located several millielectronvolts below the bright triplet, in fair agreement with an estimation of the electron-hole exchange interaction. Nevertheless, these perovskites display an intense luminescence because of an extremely reduced bright-to-dark phonon-assisted relaxation [4]. Resonant photoluminescence excitation spectroscopy allows the determination of the optical coherence lifetimes in these nanocrystals and to assess their suitability as sources of indistinguishable single photons [5]. Memories in the Photoluminescence Intermittency of Single Nanocrystals are observed [6].

References:

[1] M. Fu, P. Tamarat, J. Even, A. L. Rogach, and B. Lounis, “Neutral and Charged Exciton Fine Structure in Single Lead Halide Perovskite Nanocrystals Revealed by Magneto-optical Spectroscopy,” Nano Lett., vol. 17, no. 5, pp. 2895–2901, Apr. 2017.

[2] G. Nedelcu, A. Shabaev, T. Stöferle, R. F. Mahrt, M. V. Kovalenko, D. J. Norris, G. Rainò, and A. L. Efros, “Bright triplet excitons in caesium lead halide perovskites,” Nature, vol. 553, no. 7687, pp. 189–193, Jan. 2018.

[3] P. Tamarat, M. I. Bodnarchuk, J.-B. Trebbia, R. Erni, M. V. Kovalenko, J. Even, and B. Lounis, “The ground exciton state of formamidinium lead bromide perovskite nanocrystals is a singlet dark state,” Nat. Mater., pp. 1–9, May 2019.

[4] P. Tamarat, J.-B. Trebbia, M. I. Bodnarchuk, M. V. Kovalenko, J. Even, and B. Lounis, “Unraveling exciton-phonon coupling in individual FAPbI3 nanocrystals emitting near-infrared single photons.,” Nat. Commun., vol. 9, no. 1, p. 3318, Aug. 2018.

[5] P. Tamarat et al., submitted (2020)

[6] L. Hou, C. Zhao, X. Yuan, J. Zhao, F. Krieg, P. Tamarat, M. V. Kovalenko, C. Guo, B. Lounis,"Memories in the Photoluminescence Intermittency of Single Cesium Lead Bromide Nanocrystals" Nanoscale 12 (2020) 6795-6802

In the last few years, fully inorganic cesium lead halide nanocrystals have shown to exhibit extraordinary optical properties. We have found that their unprecedently fast radiative decay at cryogenic temperature is a direct consequence from a unique bright triplet exciton state with giant oscillator strength that leads to exceptionally strong intrinsic light-matter coupling. At the same time, their fluorescence intermittency is comparably very low for some halide compositions, which is important to make high-quality photon sources. Yet, detailed intermittency studies in single CsPbBr₃ nanocrystals that are underpinned with Monte Carlo simulations allow us to shed light on the exciton and charge dynamics and processes.

When assembling the colloidal perovskite nanocrystals into superlattices we can exploit their high oscillator strength to achieve coherent, collective emission from such an ensemble. In this so-called superfluorescence the quantum dots spontaneously synchronize by interaction via the vacuum modes of the electromagnetic field. We will also discuss our progress towards incorporating perovskite quantum dots into optical microcavities in order to form exciton-polaritons in the strong light-matter interaction regime that have the potential to create coherent macroscopic quantum fluids.

Radiographical imaging with X-rays, gamma-rays, and thermal neutrons (~25 meV) has developed into a crucial tool for medical imaging and security applications over the past century. Fast neutron (> 1 MeV) imaging is a rising technique which benefits from the high penetration power of fast neutrons, ideal for imaging large-scale objects such as construction beams and as-built plane turbines. However, widespread application of fast neutron imaging is hindered by inefficient detection of fast neutrons. The leading material in the field consists of microscale ZnS:Cu embedded in hydrogen-dense polypropylene (PP), with indirect detection of fast neutrons through the detection of recoil protons generated by fast neutrons scattering off hydrogen. However, such detectors exhibit drawbacks such as long-lived afterglows (order of minutes), light scattering at the plastic-phosphor interface, and high gamma-ray absorption and sensitivity. Hence, alternative solutions are needed to improve the performance of fast neutron detectors. Meanwhile, the advent of semiconductor nanocrystals (NCs) has ushered in a golden age for nanoscale emissive materials, with the defect-tolerant halide perovskites receiving significant attention in the past five years.

Here, we demonstrate the efficacy of colloidal perovskite nanocrystals in hydrogen-dense solvents as scintillators for fast neutron imaging. Light yield, spatial resolution, and neutron-vs.-gamma sensitivity of several compositions are compared, including both chalcogenide (CdSe and CuInS₂)-based and perovskite-based NCs (FAPbBr₃, CsPbBr₃, and CsPbBrCl₂:Mn). FAPbBr₃ NCs exhibit the brightest total light output at 19.3% of the commercial ZnS:Cu(PP) standard. Colloidal NCs uniformly showed less sensitivity to gamma radiation than ZnS:Cu, with the ratio of detected neutrons to gamma-rays ranging from 2.2 in CsPbBrCl₂:Mn NCs to 4.1 for CsPbBr₃ NCs, compared to 1.0-1.1 for ZnS:Cu(PP). For example, 79% of the FAPbBr₃ light yield results from neutron-induced radioluminescence and hence the neutron-specific light yield of FAPbBr₃ is 30.4% of that of ZnS:Cu(PP), despite the tenfold higher phosphor load of ZnS:Cu(PP) relative to the perovskite NCs. Metal blocks with sharp edges used to estimate the spatial resolution reveal that the high Stokes shift CsPbBrCl₂:Mn NCs offer the best spatial resolution at ~2.6 mm, while that of FAPbBr₃ NCs is ~5.2 mm due to greater reabsorption and re-emission. Importantly, all NCs showed no evidence for afterglow on the order of seconds. Concentration and thickness-dependent measurements highlight the importance of high concentrations and reducing self-absorption, yielding design principles for perovskite NC-based scintillators to enable effective fast neutron imaging.

The doping of colloidal halide perovskite nanocrystals (PNCs) with manganese cations (Mn²⁺) has recently enabled enhanced stability, novel optical properties and featured charge carrier dynamics in PNCs. However, the influence of Mn-doping on the synthetic routes and the band structures of the host PNCs has still not been clearly elucidated. Herein, we demonstrate that Mn-doping promotes a facile, less toxic, and less corrosive path toward the synthesis of all-inorganic bismuth-based PNCs (Cs₃Bi₂I₉) by effectively suppressing the CsI by-product of the Cs₃BiI₆ intermediate decomposition reaction. Furthermore, the energy levels of the as-formed Cs₃Bi₂I₉ PNCs can be tuned upon different Mn-doping amounts. Our theoretical and experimental results show that the valence band maximum level of the host Cs₃Bi₂I₉ PNCs is deepened with an increased Mn-doping amount up to 5% of Bi content. This results in a higher open-circuit voltage of the corresponding PNCs-based solar cells compared to those employing the undoped Cs₃Bi₂I₉. Our photophysical studies also demonstrate that the excitonic lifetime of the host Cs₃Bi₂I₉ PNCs is prolonged with the increased Mn-doping amount, mainly due to the hindering of back energy transfer from doped Mn²⁺ to the excited state of the host PNCs. This work opens new insights on Mn-doping’s role in the synthetic route and optoelectronic properties of lead-free halide PNCs for further unexplored applications.

Advances in automation and data analytics can aid exploration of the complex chemistry of nanoparticles. Lead halide perovskite colloidal nanocrystals provide an interesting proving ground: there are reports of many different phases and transformations, which has made it hard to form a coherent conceptual framework for their controlled formation through traditional methods. In this work, we systematically explore the portion of Cs–Pb–Br synthesis space in which many optically distinguishable species are formed using high-throughput robotic synthesis to understand their formation reactions. We deploy an automated method that allows us to determine the relative amount of absorbance that can be attributed to each species in order to create maps of the synthetic space. These in turn facilitate improved understanding of the interplay between kinetic and thermodynamic factors that underlie which combination of species are likely to be prevalent under a given set of conditions. Based on these maps, we test potential transformation routes between perovskite nanocrystals of different shapes and phases. We find that shape is determined kinetically, but many reactions between different phases show equilibrium behavior. We demonstrate a dynamic equilibrium between complexes, monolayers, and nanocrystals of lead bromide, with substantial impact on the reaction outcomes. This allows us to construct a chemical reaction network that qualitatively explains our results as well as previous reports and can serve as a guide for those seeking to prepare a particular composition and shape.

CsPbBr₃ nanocrystals (NCs) have attracted much attention over the past five years due to their exceptional optoelectronic properties and potential applications in devices such as light-emitting diodes (LEDs), lasers, and single-photon emitters. However, their fundamental photophysical properties, especially at low temperatures, are still under active debate. To date, almost all of the reports have used photoluminescence (PL) alone to infer the lattice dynamics in these materials. Here, we measure both the temperature-dependent (35 K - 300 K) absorption and PL spectra of zwitterionic ligand-capped CsPbBr₃ NCs with four different edge lengths (L = 4.9 - 13.2 nm). The excitonic transitions observed in the absorption spectra can be explained with an effective mass model considering the quasicubic NC shape and non-parabolicity of the electronic bands. We observe a temperature-dependent Stokes shift; while the trend is similar to the Stokes shift observed in both MAPbBr₃ and CsPbBr₃ single crystals, it does not approach zero at cryogenic temperatures, pointing to an additional contribution intrinsically present in the NCs. Surprisingly, the effective dielectric constant determined from the best fit model parameters is independent of temperature, contrary to the previous report that the change in dielectric constant leads to the Stokes shift temperature dependence. Overall, our study sheds light on the fundamental lattice dynamics in these materials, and can potentially be used to guide future material optimization for device applications.

In the context of perovskite solar cells (PSC), we have discovered an unprecedented family of lead and iodide deficient 3D hybrid perovskites (d-HP family), owning a general formulation (A’)_3.48x(A)_1-2.48x[Pb_1-xI_3-x], where A= MA⁺ , FA⁺, and A’= X-(CH₂)₂NH₃⁺ (X= OH (HEA⁺), SH (TEA⁺) or CN (CNEA⁺).[1] Such (A’,A)_1+x[Pb_1-xI_3-x] perovskites result from the substitution of x (PbI)⁺ units by x organic cations, while keeping a 3D network of corner-sharing octahedra. In the structure, Pb²⁺ and I^- vacancies are ordered leading to an unusual tetragonal unit cell (14/14/6). DFT investigations show that a direct band gap is kept in these materials while Eg values increases as x is increasing. Low-temperature magneto-optical spectroscopy of crystals of d-HP with varying concentration of (PbI) vacancies in high magnetic fields up to 68 T has revealed the coexistence of 3D-like optical transitions at low energy (1.54 eV) and lower dimensional (LD) transitions at high energy (1.9 eV- 2.4 eV), the intensity variation between the LD and 3D-like transitions being related to the concentration of (PbI) vacancies. The reduced dimensionality associated with the high energy transitions is also supported by the observed smaller diamagnetic shift and charge carrier lifetimes.[2]

These d-HP can be prepared as pure crystallized powder phases as well as thin films in the 0<x£0.20 range. The more interesting phases are those based on FA⁺, called d-α-FAPI-H and d-α-FAPI-T (H: HEA, T: TEA). In contrast with α-FAPI, the synthesis of these d-α-FAPI phases are carried out at room temperature. Moreover, we show that the stability of the α phase at room temperature and under ambient condition (thin films, case of d-α-FAPI-Tx materials) is exceptionally improved compared to both α-FAPI and α-Cs_0.15FA_0.85PbI₃ reference materials.[3]

The discovery of spectacular photovoltaic properties of organic-inorganic hybrid perovskites has actuated intensive studies on the whole family of halide perovskites. These materials are characterized by remarkable diversity of optoelectronic parameters and have a great potential for numerous technological application. The crystal structures of halide perovskites of a general formulae ABX₃ (A is a monovalent organic/inorganic cation, B is a divalent metal, and X = Cl, Br or I) are formed from the three-dimensional network of corner-sharing BX₆ octahedrons with the cations A situated within the cages. The bond lengths and the angles between the bonds in the perovskite framework can be modified by high pressure without any chemical interference. Therefore, the lattice compression is a clean way of tuning electronic structure and properties responsible for photovoltaic performance. The high-pressure single-crystal X-ray diffraction experiments is an excellent tool to obtain precise information on the symmetry of phases and their stability regions, and on the changes in the bond lengths and angles. These structural parameters, determined as a function of pressure, provide experimental basis for correlation with optoelectronic properties of materials, as well as for theoretical modelling. In response to external pressure the perovskite framework is modified through the contraction of B-X bonds or/and bending of B-X-B bridges between the BX6 octahedrons. Numerous high-pressure studies have evidenced that the shortening of bonds B-X narrows the bandgap, whereas it is widened by the B-X-B angle bending. The resultant bandgap modification depends on the contributions of these competitive pressure effects [1,2]. Our diffraction studies supported by optical observations provided also a new insight into the transformations of the perovskite structures. The occurrence of slow-kinetics transformations and the coexistence of phases are discussed in the context of the possible origins of numerous considerable discrepancies between the high-pressure studies reported in the literature.

Lead-free perovskites are receiving ever increasing attention after inspiring success of lead-based halide perovskites, mostly due to atmospheric instability and lead toxicity associated with the latter. Despite significant progress in homovalent and heterovalent substitution of Pb with non-toxic elements, stable lead-free perovskites with an ideal bandgap (1.2-1.4 eV) for photovoltaics are still missing. In this work, we report organic-inorganic gold halide double perovskites ((CH3NH3)2Au2X6, X = Br, I) which shows ideal bandgap for photovolltaics. In contrast to other double perovskites, two different oxidation states (+1 and +3 for perovskite structure) of Au is stacked alternatively to form a halogen-bridged perovskite structure. These compounds are solution processable and show bandgap tunability by halide exchange. Density functional theory calculations confirm the direct nature of bandgaps of the compounds with small effective mass for excellent charge transport. In addition, the Au-halide perovskites show high chemical stability, low trap density, and reasonable photoresponse. These combined properties demonstrate that Au-based halide perovskites can be a promising group of compounds for optoelectronic applications.

Mixed-valent metal-halide semiconductors containing ns² lone pairs exhibit intense absorption bands with unusual colors, while zero-dimensional (0D) ns²-based metal-chlorides are colorless but have recently demonstrated unique properties that are promising for optoelectronic applications such as broadband lighting, thermometry, and radiation detection. Here, we solvothermally synthesize a new family of mixed-valent alkali pnictogen halides with composition Rb₂₃Bi^III_xSb^III_7-xSb^V₂Cl₅₄ (0 ≤ x ≤ 7). The deep red Rb₂₃Sb^III₇Sb^V₂Cl₅₄ crystallizes in an orthorhombic space group (Cmcm) with a unique, layered 0D structure driven by the repulsion of the 5s² lone pairs of the Sb^IIICl₆ octahedra. In contrast to the prototypical Cs₄Sb^IIISb^VCl₁₂ tetragonal structure with its highly symmetric octahedral environments, here the Sb^IIICl₆ octahedra exhibit trigonal or disphenoidal distortion, yielding layers with distinct orientations of the octahedra as these asymmetric units pack around the Sb^VCl₆ octahedra. This complex phase is likely the true structure of a previously reported monoclinic red “Rb_2.67SbCl₆” phase, the structure of which was not determined. Partially or fully substituting Sb^III with isoelectronic Bi^III yields the series Rb₂₃Bi^III_xSb^III_7-xSb^V₂Cl₅₄ (0 < x ≤ 7), which exhibit a similar layered 0D structure but with rotational disorder that yields a trigonal crystal system with a polar space group (R32). Second harmonic generation of 532 nm light from a 1064 nm laser using Rb₂₃Bi^III₇Sb^V₂Cl₅₄ powder confirms the non-centrosymmetry of this space group. As with the prototypical mixed-valent pnictogen halides, the visible absorption bands of the Rb₂₃Bi^III_xSb^III_7-xSb^V₂Cl₅₄ family are the result of intervalent Sb^III-Sb^V and mixed-valent Bi^III-Sb^V charge transfer bands (CTB), with a blueshift of the absorption edge as Bi^III substitution increases. No PL is observed from this family of semiconductors, but a crystal of Rb₂₃Bi^III₇Sb^V₂Cl₅₄ exhibits a high resistivity of 1.0 x 10¹⁰ Ωcm and X-ray photoconductivity with a promising μτ product of 8.0 x 10^-5 cm² s^-1 V^-1. The unique 0D layered structures of the Rb₂₃Bi^III_xSb^III_7-xSb^V₂Cl₅₄ family highlight the versatility of the ns² lone pair in semiconducting metal-halides, pointing the way towards new functional 0D metal-halide compounds.

2D layered hybrid perovskites (2D HOIPs) are currently in the spotlight for applications such as solar cells, light-emitting diodes, transistors and photodetectors. The structural freedom of 2D HOIPs allows for the incorporation of organic cations that can potentially possess properties contributing to the performance of the hybrid as a whole. In this talk, the incorporation of a benzothieno[3,2-b]benzothiophene (BTBT) alkylammonium cation into the organic layer of a 2D HOIP will be discussed. Obtaining highly crystalline films of a 2D HOIP containing such a BTBT cation proved to be challenging. We hypothesized that the limited mobility of this bulky and rigid organic cation hinders the formation of an ordered structure during film formation. In order to provide enhanced mobility to the components during film formation, we employed a combination of a solvent vapour atmosphere with thermal annealing. Films obtained using this solvent vapour annealing approach possess significantly enhanced absorption, emission and crystallinity compared to films obtained using regular thermal annealing. The photoconductivity of the films was determined using time-resolved microwave conductivity (TRMC) as well as in a device. In both cases, the solvent vapour annealed films show a markedly higher photoconductivity than the films obtained using the regular thermal annealing approach. The formation and degradation of the 2D layered perovskite films was studied in detail using in-situ absorption spectroscopy and X-ray diffraction.

The optical and light emission properties of tin and lead halide perovskites are exceptional because of the robust room temperature performance, broad wavelength tunability, high efficiency and good quenching-resistance to defects. These highly desirable attributes promise to transform current light emitting devices, phosphors and lasers. One disadvantage in most of these materials is the sensitivity to moisture. Here we report a new air-stable one-dimensional (1D) hybrid lead-free halide material (DAO)Sn₂I₆ (DAO: 1,8-octyldiammonium) that is resistant to water for more than 15h. The material exhibits a sharp optical absorption edge at 2.70 eV and a strong broad orange light emission centered at 634 nm, with a full width at half maximum (FWHM) of 142 nm (0.44 eV).  The emission has a long photoluminescence (PL) lifetime of 582 ns, while the intensity is constant over a very broad temperature range (145-415 K) with a photoluminescence quantum yield (PLQY) of at least 20.3% at RT. Above 415 K the material undergoes a structural phase transition from monoclinic (C2/c) to orthorhombic (Ibam) accompanied by a red shift in the bandgap and a quench in the photoluminescence emission. Thin films of the compound readily fabricated from solutions exhibit the same optical properties, but with improved PLQY of 36%, for a 60 nm thick film, among the highest reported for lead-free low-dimensional 2D and 1D perovskites and metal halides. These results demonstrate the versatile nature of hybrid halide materials, laying the path for the design of next generation of water stable lead-free semiconductors.  

Over the last seven years we have witnessed the rise of lead-halide perovskites for optoelectronic applications such as photovoltaics, sensors and light-emitting diodes. Similarly, oxide perovskites have a much longer history and are pivotal in many technological applications. Yet, a rational connection between these two important classes of materials is missing.  In this talk, we will employ a computational design strategy to explore this missing link and demonstrate that for each halide perovskite there are several lookalike oxide perovskites with similar optoelectronic properties. We will begin by showcasing recent efforts towards new materials that are alternatives to tradional lead-halide perovskites, for which computational design approaches from first-principles have been extensively successful and revealed a series of new compounds within the so-called halide double perovskites family. Among these, Cs₂BiAgBr₆ has the narrower indirect band gap of 1.9 eV, and Cs₂InAgCl₆ is the only direct band gap semiconductor, yet with a large gap of 3.3 eV [1-3]. All of them exhibit low carrier effective masses and consequently, are prominent candidates for a range of opto-electronic applications such as photovoltaics, light-emitting devices, sensors, and photo-catalysts. Here, we will outline the computational design strategy that lead to the synthesis of these compounds, and particularly focus on the insights we can get from first-principles calculations in order to facilitate the synthesis, improve their opto-electronic properties and the in-silico identification of compounds with properties that are similar to the lead-halide perovskites. This rational design approach allows us to further develop a universal analogy concept that can be used to identify analogs between oxide and halide perovskites. Our new concept of analogs led us to identify a new oxide double perovskite semiconductor, Ba₂AgIO₆, which exhibits an electronic band structure remarkably similar to that of our recently discovered halide double perovskite Cs₂AgInCl₆, but with a band gap in the visible range at 1.9 eV. We report of the successful synthesis of Ba₂AgIO₆ by solution process and we perform crystallographic and optical characterization. We show that Ba₂AgIO₆ and Cs₂AgInCl₆ are both analogs of the well-known transparent conductor BaSnO₃, but the significantly lower band-gap of Ba₂AgIO₆ makes this new compound much more promising for oxide-based optoelectronics and for novel monolithic halide/oxide devices [4].

Lead halide perovskites face a significant challenge of instability: organic cations are too volatile, while inorganic Cs is too small to meet the cage size tolerance, and as a result, perovskite converts into a more stable yellow phase. Finding a way to either stabilize the Pb perovskite or completely eliminate the Pb is a high-reward but very challenging problem.

I will discuss our efforts in applying machine learning both in experimental and theoretical search for new photovoltaic perovskites.

In particular, exploring computationally dopant combinations that could stabilize current state-of-the-art Pb perovskite and then employing high-throughput synthesis procedures to improve the precursors' solubility and dopability of the perovskite.

An alternative approach is to devise Pb-free perovskites. Existing databases contain several hundred materials with appropriate bandgap, but selecting ones that are easily synthesizable and containing little electronic defects require improvements to existing machine learning models.

To further improve the powder conversion efficiency of halide perovskite solar cells (PSCs), it is highly preferred to understand the electrical properties of the perovskite absorbers, since the PCE is determined by the electrical properties of PSCs, such as defect activation energy and density, carrier concentration, and dielectric constant. Capacitance–based techniques, such as thermal admittance spectroscopy (TAS) and capacitance–voltage (C–V), have been the choice of method for measuring electrical properties of semiconductor devices and have played important roles in the development of thin-film solar cell technologies. These techniques have been used to measure the electrical properties of PSCs such as defect activation energy and density, carrier concentration, and dielectric constant, which provide key information for evaluating the device performance. We show that these techniques may not be used to reliably analyze the properties of defects in the perovskite layer or at its interface, since the hole-transport layers (HTLs) can introduce high-frequency capacitance signature due to the response of charge carriers in HTLs. For HTL-free PSCs, the high-frequency capacitance can be considered as the geometric capacitance for analyzing the dielectric constant of the perovskite layer, since there is no trapping and de-trapping of charge carriers in the perovskite layer. We further find that the low-frequency capacitance signature can be used to calculate the activation energy of the ionic conductivity of the perovskite layer, but the overlapping effects with charge transport materials must be avoided.