Symposium EN06—Frontier Energy Sciences in Halide Perovskites

Lead halide perovskites are promising materials for tandem solar cell applications due to their easily tunable bandgap through cation and halide compositions. However, photo-induced halide segregation in iodide-bromide perovskite chemistries often results in iodine-rich and bromine rich domains and limits the efficiency of tandem solar cells. One approach to mitigate such segregations reported in the literature is the addition of chlorine to the cesium-formamidinium iodide-bromide perovskite compositions. However, the literature on the effects of varying degrees of chlorine incorporation in thin-film perovskite is limited. In this study, we present an experimental study of the iodine-bromine-chlorine composition space aided by density functional theory (DFT) calculations and data science methods. By loosening cesium-to-formamidinium ratio at the same time, we allow for more possible selections of triple halide compositions that could fall in the optimal range of Goldschmidt's tolerance factor and result in stable perovskite. Among these selections, we look for compositions whose band gaps are optimized for tandem applications and whose halide segregation effects are minimized. Since all possible hybrid-cation mixed halides samples at one percent increments are too numerous to fabricate, we perform a screening informed by DFT and data science methods to intelligently search the compositional space. Through iterative experimental and computational exploration, we identify triple halide perovskite compositions with optimized opto-electronic properties well suited for tandem applications. Our data science and DFT approach demonstrates an expedition to experimental work in material discovery and characterization, which could be adapted to optimizing perovskite chemistries for alternative optoelectronic applications.

The stability of methylammonium lead bromide (MAPbBr₃) single crystal was investigated when subjected to simultaneous oxygen exposure and light illumination of one solar intensity. The exposure process was monitored with X-ray photoelectron spectroscopy (XPS) with detailed control of the exposure time and pressure. It was found that the with light the oxygen degradation process of the MAPbBr₃ was substantially accelerated. Quantitative analysis showed that Carbon, Bromine and Nitrogen lost 52%, 56% and 90% of their initial concentration respectively at 10¹² Langmuir of oxygen exposure. It further showed that ~11% of the perovskite’s Pb degraded to metallic Pb at 10⁹ Langmuir, and then was oxidized with the increase of the oxygen pressure.

The antiperovskite family Mn₃AN (A = Zn, Cu, Ni, ..) exhibits negative thermal expansion, piezomagnetism, and non-collinear antiferromagnetism (AFM) due to the presence of magnetic frustration on the magnetic structure of the manganese atoms in the (111) planes [1]. Previous research on this material has been mostly focused on experimental findings, and it has dedicated primarily on understanding the magneto-structural properties of these antiperovskites. Interestingly, we found few theoretical studies devoted to elucidate the influence of the central anionic site in the Mn₃NiN antiperovskite. Therefore, our study focuses on theoretically investigating, through first-principles density functional theory (DFT) calculations, the effect of the nitrogen’s site on the structural and electronic properties of the antiperovskites Mn₃NiN. Therefore, our main analysis consists of observing changes in the electronic structure by analyzing the band structure, density of states, and the oxidation state of the atoms in the presence and absence of nitrogen considering the allowed four noncollinear antiferromagnetic orderings. Our results show a tangible modification of the electronic structure close to the Fermi energy as well as structural changes, associated to the lattice parameter, due to the incorporation of nitrogen.

[1] Y. Wang, H. Zhang, J. Zhu, X. Lü, S. Li, R. Zou, and Y. Zhao, “Antiperovskites with exceptional functionalities,” Advanced Materials, vol. 32, no. 1905007, p. 7, 2019.

The manganese-based antiperovskite nitrides family is formed by magnetic materials that exhibit non-collinear antiferromagnetism in the presence of magnetic frustration. Different reports have confirmed the existence of a strong coupling between the magnetic structure and the crystalline structure in the in members of this family, for example in Mn₃NiN. As such, this coupling has been confirmed by piezomagnetism [1], magnetovolume effects [2], and negative thermal expansion [3] studies.

So far, theoretical studies focused on clarifying this coupling and phenomenon in antiperovskites are rare in the literature. This possibly because phonon calculations, including non-collinear magnetism formalism are a challenging task in these type systems. In this research, we theoretically studied the spin-phonon coupling in Mn₃NiN through first-principle DFT calculations. We aimed to understand the relation between the behavior of the phonons and the magnetic orderings of the material, taking into consideration correlation effects. To achieve this, we computed and analyzed the phonon frequencies, eigenvectors, and phonon dispersion curves for each of the two magnetic orderings that have been experimentally confirmed in the material [4]. From these results the phonon modes involved in the coupling were determined. Our results theoretically demonstrate the dependence of phonons on magnetic ordering and correlation effects in Mn₃NiN. With this study, we expect to contribute to future research in the field of antiperovskites that display a coupling between structure and magnetism.

[1] D. Boldrin, A. P. Mihai, B. Zou, J. Zemen, R. Thompson, E. Ware, B. V. Neamtu, L. Ghivelder, B. Esser, D. W. McComb, et al., “Giant piezomagnetism in Mn3NiN,” ACS applied materials & interfaces, vol. 10, no. 22, p. 18863, 2018.

[2] K. Takenaka, M. Ichigo, T. Hamada, A. Ozawa, T. Shibayama, T. Inagaki, and K. Asano, “Magnetovolume effects in manganese nitrides with antiperovskite structure,” Science and technology of advanced materials, vol. 15, no. 1, p. 1, 2014.

[3] M. Wu, C. Wang, Y. Sun, L. Chu, J. Yan, D. Chen, Q. Huang, and J. W. Lynn, “Magnetic structure and lattice contraction in Mn3NiN,” Journal of Applied Physics, vol. 114, no. 12, p. 123902, 2013.

[4] D. Fruchart and E. F. Bertaut, “Magnetic studies of the metallic perovskite-type compounds of manganese,” Journal of the physical society of Japan, vol. 44, no. 3, p. 781, 1978.

Control and prevention of perovskite phase segregation are crucial to realizing stable and tunable multi-cation and mixed-halide perovskite optoelectronic devices. Herein we report the comprehensive analysis of band structure, compositional heterogeneity, and phase segregation in the novel alkali cesium (Cs⁺) and potassium (K⁺)-doped 3D perovskites using a suite of powerful and highly complementary techniques [such as X-ray photoemission spectroscopy (XPS), depth profiling, ultraviolet photoelectron spectroscopy (UPS) and grazing incidence wide-angle X-ray scattering (GIWAXS)]. Perovskite phase segregation information has been extracted from the optical or electrical properties using techniques such as absorption and photoluminescence spectroscopy, external quantum efficiency (EQE), transient difference absorption spectroscopy, and photovoltaic performance. However, all of these techniques provide indirect compositional information and cannot reveal the spatial distribution. We found that the incorporation of metal cations raises the conduction band minimum (CBM), facilitating electron transfer to the electron transport layer. Notably, the depth profile by XPS and GIWAXS analysis show that the formation of iodide- and bromide-rich regions are strongly correlated to the alkali metal cation concentrations: low metallic concentration (5%) can prevent phase segregation where the halides and cations are distributed evenly across the films, while high metallic cation ratio (20%) leads to halide segregation where the concentration of iodide decreases accompanied with the increase of bromide and metal cation concentration as the sampling depth increases. These differences in electronic properties, element distribution, and film morphology were reflected in solar cell performance: >20 % of power conversion efficiency (PCE) was demonstrated with low Cs⁺ incorporated perovskite, while ≈16.7 % of PCE with high Cs⁺ concentration perovskite. Similar trends were also observed in the K⁺-doped perovskite system. We believe this work will advance the perovskite community by providing a direct and straightforward route to characterize the impacts of perovskite phase segregation.

Parameter extraction based on numerical simulation is error-prone because model parameters are often correlated [1]. The accuracy of the extracted parameters can be improved by combining multiple steady-state and transient measurement techniques to reduce these correlations. This method has been successfully applied to organic solar cells [2].
Modelling perovskite solar cells, however, involves additional complexity. Characteristic features, such as IV curve hysteresis [3,4] and slow transient current rise [5,6] indicate the presence of mobile ionic charges in the perovskite layer. Simulation models including ionic charges could qualitatively reproduce these individual features [3-5]. However, the reliable determination of these ionic parameters additionally requires suitable characterization techniques.
To increase the trustability of the model, we systematically study the influence of individual model parameters on different experimental techniques. We therefore model multi-layer methylammonium lead iodide (MAPI) perovskite solar cells in a coupled opto-electrical simulation including mobile ionic charge carriers, charge trapping, Shockley-Read-Hall (SRH) recombination and doped transport layers. We find a single set of parameters to explain the combined results from various measurement techniques in DC, AC, and transient regime [6]. The applied techniques include IV curves with different scan rates, light intensity dependent open circuit voltage, impedance spectra, intensity-modulated photocurrent spectra (IMPS), transient photocurrents and transient voltage step responses. As expected, the various measurement techniques show different sensitivities to material and device parameters, which a model fit can benefit from.
Yet correlated parameters remain an issue. Namely it is difficult to distinguish between concentration and mobility of ionic charge carriers [6]. M.H. Futscher et al. [7] recently applied a measurement technique called transient ion drift to perovskite solar cells. This technique was originally developed for inorganic p-n junctions and allows to individually and quantitatively measure both concentration and mobility of slowly moving mobile charges [8]. It is unfortunately restricted to materials with low concentrations of mobile charges and comparably high doping [7-8]. Using our model we evaluate this characterization technique for devices beyond these restrictions. Specifically we investigate materials with high concentrations of mobile ions or absence of doping, as many perovskite materials meet one or both of these specifications. We further investigate the correlation between other parameters and the extracted ones.
Our consistent device simulation model allows for a better understanding of the governing physical effects and provides a powerful tool to study the influence of device parameters on the solar cell performance and efficiency. The variety of applied measurement techniques allows one to reduce the correlation between parameters. Furthermore, an in depth parameter study suggests possible paths towards an optimized device [6].

References
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[2] M.T. Neukom et al., Organic Electronics (2012), 13, 2910-2916.
[3] S. van Reenen, M. Kemerink and H.J. Snaith, J. Phys. Chem. Lett. (2015), 6, 3808 3814.
[4] P. Calado et al., Nat. Commun. (2016), 7, 13831.
[5] S.E.J. O’Kane et al., J. Mater. Chem. C (2017), 5, 452 462.
[6] M.T. Neukom et al., ACS Appl. Mater. Interfaces (2019), 11, 26, 23320-23328.
[7] M.H. Futscher et al., Mater. Horiz. (2019), 6, 1497-1503.
[8] T. Heiser and A. Mesli, Appl. Phys. A: Solids Surf. (1993), 57, 4, 325-328.

Wide-bandgap mixed-halide perovskites such as CH₃NH₃Pb(I_xBr_1-x) are of great interest because of their favorable bandgaps for use as top subcells in multijunction photovoltaic devices. Recently, we have shown through numerical simulation that the performance of single junction wide-bandgap perovskite devices strongly depends on the energetic alignment between the absorber and the charge transporting layers. Those results showed interesting behavior in the power conversion efficiency at high conduction band offset between the electron transport layer (ETL) and wide-bandgap absorber; namely, the open circuit voltage rapidly increased, but the fill factor decreased just as drastically. These results suggest the formation of a large potential barrier that impedes electron flow out of the absorber. Such a barrier was not evident at the ETL/absorber interface but may be the result of poor band alignment at the transparent conducting oxide (TCO)/ETL interface. Here, we investigate the impact of TCO band alignment with the ETL in wide-bandgap perovskite solar cells. We show that varying the conduction band position of the TCO can lead to reductions in both the recombination and fill factor loss. The reduction in recombination allows the device to maintain a high fill factor, near 90%, not previously observed for a wide range of conduction band offsets (CBOs) between the ETL and wide-bandgap absorber. Simultaneously, varying the conduction band position of the TCO preserves a high V_OC and results in a 1-2% increase in efficiency compared to our previous modeling with a standard TCO and optimized ETL/absorber CBO. These results indicate that as the bandgap of the perovskite absorber layer is increased, the choice of ETL and TCO will have a significant joint effect on performance.

Four novel polymers have been investigated as promising solution processable, dopant-free polymeric hole-transporting materials (HTMs) for efficient and stable perovskite solar cells. The poly(1-(4-hexylphenyl)-2,5-bis(5-methylthiophen-2-yl)-1H-pyrrole) p(hPhDTP), poly(1-(4-methoxyphenyl)-2,5-bis(5-methylthiophen-2-yl)-1H-pyrrole)p(mPhDTP), poly(3-hexyl-5,5′-dimethyl-2,3′-bithiophene) p(hBT) and poly(5,5′-dimethyl-2,3′-bithiophene) p(BT) HTMs have successfully been synthesized from relatively cheap raw materials with a facile synthetic route. The fabricated polymers are potentially cost-effective and exhibit favourable HOMO level with respect to the valence band of the perovskite active layer. The incorporation of the new HTMs in the perovskite solar cell architecture exhibited excellent overall power conversion efficiency (PCE) of 16.20, 14.45, 11.10 and 9.63% for P(hPhDTP), P(mPhDTP), P(hBT), and P(BT), respectively with minimal hysteresis without the use of any dopants or additives, which is higher than the 8.33% obtained using P3HT-based devices. The high efficiency can be attributed to the positive HOMO energy level and the high hole (h⁺) mobility. In addition, the long-term stability of the polymers as HTM devices were studied, where they maintained their initial efficiency for over 1200 h, whereas devices made using P3HT as HTM drops down to almost half of its value after 600 h.

Core-shell refractory plasmonic nanoparticles are used as excellent nanoantennas to improve the efficiency of lead-free perovskite solar cells (PSCs). SiO₂ is used as the shell coating due to its high refractive index and low extinction coefficient, enabling the control over the sunlight directivity. An optoelectronic model is developed using 3D finite element method (FEM) as implemented in COMSOL Multiphysics to calculate the optical and electrical parameters of plain and ZrN/SiO₂-modified PSCs. For a fair comparison, ZrN-decorated PSCs are also simulated. While the decoration with ZrN nanoparticles boosts the power conversion efficiency (PCE) of the PSC from 12.9% to 17%, the use of ZrN/SiO₂ core/shell nanoparticles shows an unprecedented enhancement in the PCE to reach 20%. The enhancement in the PCE is discussed in details.

At the device operating condition, defects such as interstitials, vacancies, and impurities at the grain boundary and surface of the photoactive layer, have a great impact on the power conversion efficiency (PCE) and device stability. To better passivate the surface and boundary defects, and further enhance the PCE and device stability, in this work, a bipolar organic material termed PFPPY is introduced as a modifier in anti-solvent. The effects of PFPPY on perovskite film quality, photovoltaic performance, and charge transfer properties have been systematically investigated. Under the optimized conditions, PFPPY treated device shows an impressive PCE of 19.62%, which is 12% higher than the reference device (17.59%), and greatly enhanced stability, maintaining 95% of its initial efficiency under RT and RH 30% condition for 650h without encapsulation.

Colloidal CsPbX₃ (X = Cl, Br, I) perovskite nanocrystals have attracted a great deal of interest due to their bright, narrow emission, high photoluminescence quantum yields, tunable band gap, and defect tolerance. Various dopants have been successfully incorporated in their structure, including notably Mn²⁺ and Yb³⁺, allowing for novel emissions properties such as broadband emission and quantum cutting. However, the structure and chemistry of dopants within the perovskite lattice are not well understood. A characteristic property of CsPbX₃ nanocrystals is their facile anion-exchange chemistry in the liquid or gas phase, which allows ready conversion between different halide structures, formation of mixed halide compositions, and access to materials that are difficult or impossible to synthesize directly. The mechanism(s) by which this anion exchange occurs are still poorly understood, as are its effects on dopant sites. Our work focuses on probing these reactions at the atomistic level to gain mechanistic insight into this important chemical transformation. We have employed X-ray total scattering measurements of the ensemble anion exchange reaction to observe structural changes during this process. These indicate a continuum shift in the structure during anion exchange, with expected expansions in the long-range order. A substantial change in the majority of atom pair positions and intensities was also observed in the Mn-doped system, while the Yb-doped system is nearly identical to the undoped structure, except for a small shift in the Pb-Cl peak. Furthermore, we are pursuing a combination of environmental TEM and HR-STEM with EDX mapping at multiple points ex-situ and in-situ in the gas phase in order to understand the mechanism of anion exchange in doped and undoped halide perovskite nanomaterials.

Metal halide perovskites possess fascinating material properties and significant technological promise. The materials consist of a nominal formula of APbX3, where A is a large cation (e.g. Cs, MA or FA), and X is a halide, however much attention has been devoted to the alloys composed from mixtures thereof, e.g. (MA/Cs/FA)Pb(Br/I)₃. Alloying at the X-site provides bandgap tunability while alloying at both sites can significantly enhance stability towards both phase change and moisture-catalyzed degredation. Recent studies have identified the phenomenon of light induced halide demixing, whereupon illumination drives the formation of pure-phase halide clusters. This entirely reversible process has been hypothesized to be due to strain induced by photoexcited polarons. The facile and reversible nature of ion mobility in this system makes it an ideal setting for studying the coupling between external perturbations and the spatial distribution of ions in mixed alloys. These experiments test whether mechanically induced strain is another such external perturbation capable of driving ion migration. To determine the extent of ion migration in mixed halide perovskites to mechanically-induced strain, polycrystalline thin films of MAPbBrI₂ were placed under deliberate tensile strain and measured via steady-state photoluminescence for changes in the spatial distribution of halides at the point of strain as a function of time.

The grain boundaries of organic-inorganic halide perovskite films not only function as defect centers, but also behave as intrinsic ion migrating channels and extrinsic moisture-induced degradation initiators, which are detrimental to the efficiency and stability of perovskite solar cells (PSCs). Here, an effective methodology for fabrication of high-quality perovskite layers with reduced non-radiative recombination, suppressed ion migration and increased moisture stability is reported for the first time, referred as the pressure-assisted solution processing (PASP). Through PASP strategy, the nucleation and growth of perovskite crystals can be controlled, leading to the micron-sized grains and microsecond-range carrier lifetimes. The resultant PSC shows champion power conversion efficiency (PCE) as high as 20.74% and a stabilized efficiency exceeding 20%, with superior long-term stability, maintaining above 90% of initial PCE even aging 60 days or continuous 1-sun illumination for 200 h in ambient environment without encapsulation. We unambiguously believe that the control of perovskite crystal nucleation and growth with high quality will be an important direction to improve the efficiency of PSCs to the theoretical limit, as well as to stabilize perovskite-based materials and devices.

Improvement of optoelectronic properties of perovskite solar cells (PSCs) can be realized by employing efficient front contact. This study aims to enhance the optical and electrical performances of PSCs by utilizing metal-oxide nanoholes at the front. Such front nanostructures significantly contribute to the broadband light-incoupling and light trapping in a solar cell, resulting in an improvement of quantum efficiency (QE) and short-circuit current density (J_SC), leading to further enhancement of energy conversion efficiency (ECE). Furthermore, the metal-oxide contact exhibits high lateral conductivity, high optical transparency, and a suitable work function, facilitating PSC’s optoelectronic performance. As most high-performance PSCs explore only the device's electrical properties, the current study reveals a detailed understanding of PSC optics, electrical characteristics, and their practical fabrication process for achieving high ECEs. The optics, optimization, and electrical effects of PSCs are investigated by advanced three-dimensional electromagnetic simulations with the combination of finite-difference time-domain (FDTD) and finite element method (FEM) techniques. The investigation implies that the nanophotonic-structured front contact allows enhancing the J_SC by 11~27% while improving the light incoupling by 10~12%, compared to that of planar configuration. Herein, the validation of numerical modeling was performed by fabricating PSC in a superstrate configuration that was optimized to reach an ECE of 17.5%, V_OC of 1.02 V, J_SC of 22.1 mA/cm², and FF of 80%. Detailed descriptions of the nanophotonic contact, device, and fabrication process are provided.

The term ‘perovskite’ denotes the crystal structures made of corner-sharing octahedra with ions placed in the octahedral interstitials with the ABX₃ formula. Particular attention has been targeted at compositions with the X-site occupied by a halide and the B-site by a metal, due to interesting functional properties, particularly those associate with photovoltaics (PV). A vast variety of elements and molecules have been incorporated into metal halide perovskite structures, the most prominent are methyl ammonium (MA⁺), and/or formamidinium (FA⁺) as A-site cations, Pb²⁺ and/or Sn²⁺ as the B-site metal, as well as I^- and Br^- on the X-site. The ability to maintain the perovskite structure, from a geometric point of view, is given by the Goldschmidt tolerance factor t_G. It takes into account the geometric constrains in the crystal lattice and is defined as the ratio of the octahedral interstitial and the A-site cation size. Compositions with non-ideal geometric conditions, such as FAPbI₃ and CsPbI₃, are prone to transition into unfavorable δ -phases. By utilizing a mixture of ions, the geometric conditions in the crystal structure can be finely tuned and thereby improve the inherent structural stability. As a consequence, the mixed perovskite composition of FA_1-xCs_xPbI₃ with optimized tolerance factors features superior structural stability compared to the pure FA- or Cs-compositions.^[1] However, previous studies showed that PV devices, featuring mixed perovskite compositions with similar tolerance factors as absorber layers, do not necessarily exhibit similar stability in structure or device performance. This strongly suggests there are further mechanisms that play a significant role in the structural and device stability in perovskite solar cells (PSCs). In this study, we explore the structure-function relationship in state-of-the-art mixed A-site halide PSCs and reveal the impact of annealing conditions of the device stability.
Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) measurements on neat FA_0.83Cs_0.17PbI₃ perovskite films that had been annealed at different temperatures revealed local variations in the perovskite composition for low annealing temperatures.^[2] Based on these findings we hypothesize that insufficient annealing leads to incomplete mixing. Local deviations from the mixed phase, either Cs-rich or FA-rich, can nucleate further phase segregation into the parent δ –phases. In sum, reaching thermal equilibrium during the film annealing is crucial for device stability.
We present a study that unambiguously correlates device performance with structural properties, revealing degradation mechanisms in FA_0.83Cs_0.17PbI₃ PSCs: Operando X-ray diffraction (XRD) allows us to probe the structure function relationship. By utilizing high-energy synchrotron X-rays, we are able to penetrate the perovskite absorber layer through the metal contact in a complete device held at max power. In order to accelerate degradation and to mimic realistic operational temperature conditions, the experiment is performed at elevated temperatures (50°C - 80°C). Using this technique, we compared devices annealed at 100°C and at 180°C. We clearly find superior performance and stability for the samples annealed at higher temperatures. Furthermore, we can identify δ –phases forming for insufficiently annealed samples, corroborating our hypothesis.
In conclusion, we present a study of state-of-the-art mixed A-site halide PSCs, directly correlate processing conditions to device stability and elucidate the underlying degradation mechanisms. Our findings help inform optimized processing protocols to lay the ground for efficient and long-term stable perovskite PV.

[1] Z. Li, M. Yang, J. S. Park, S. H. Wei, J. J. Berry, K. Zhu, Chem. Mater. 2016, 28, 284.
[2] L. T. Schelhas, Z. Li, J. A. Christians, A. Goyal, P. Kairys, S. P. Harvey, D. H. Kim, K. H. Stone, J. M. Luther, K. Zhu, V. Stevanovic, J. J. Berry, Energy Environ. Sci. 2019, 12, 1341.

Two-dimensional (2D) metal-halide perovskites have emerged as a more robust alternative to their three-dimensional counterparts. Due to quantum and dielectric confinement effects, excitons dominate the energy transport characteristics in thinnest members of the 2D perovskites family. Recently we have reported on the in-plane exciton diffusion using transient photoluminescence microscopy, where high diffusivities were found (0.2 cm²/s for PEA₂PbI₄). Using the same technique, here, we will show that this material exhibits remarkably efficient out-of-plane exciton transport (0.06 cm²/s) as well. This out-of-plane diffusivity translates to a diffusion length of exceeds 100 nm, making it relevant to device design. Moreover, our result show that the individual energy transfer steps that underly the out-of-plane transport occur on a sub-ps timescale. Such ultrafast timescales are over two orders of magnitude faster than predictions using Förster theory. We will discuss the shortcomings of Förster theory for the case of excitons in the layered perovskites. Most importantly, our results show that the excitonic energy transport is considerably less anisotropic than charge-carrier transport for 2D perovskites.


[1] Seitz, M. et al, Nat. Commun. https://doi.org/10.1038/s41467-020-15882-w
[2] Magdaleno, A. J. et al, ChemRxiv. https://doi.org/10.26434/chemrxiv.13073153.v1

Metal Halide Perovskites (MHPs) have drawn significant attention due to their outstanding optoelectronic properties, such as high absorption coefficient and long carrier lifetime. While MHPs offer industry and the academic community many exciting opportunities, consistent mechanistic understanding and consensus still lag behind more established systems. One notable property of this class of materials is their large dielectric constant, which has been observed to change significantly under illumination. [1] While the polarizability of this sytem has been invoked to explain many of its unusual properties, the behavior of the dielectric constant under illumination is still poorly understood.

In this study, we have used frequency dependent Time Resolved Microwave Conductivity (TRMC) to investigate how the dielectric constant changes under illumination as a function of time, in a range of MHPs. Both changes in photoconductance and change in dielectric constant are analyzed as a function of time, with nanosecond precision. We observe that photo-induced change in dielectric constant are roughly proportional to photo-induced carrier density but decay over different timescales, indicating that the presence of charge carriers alone is not the only factor in changes in polarizability.

[1] E. J. Juarez-Perez, R. S. Sanchez, L. Badia, G. Garcia-Belmonte, Y. S. Kang, I. Mora-Sero, and J. Bisquert, Photoinduced Giant Dielectric Constant in Lead Halide Perovskite Solar Cells, J. Phys. Chem. Lett. 5, 2390 (2014).

Unlike typical inorganic semiconductors, lead–halide perovskites exhibit significant ionic conductivity, which is believed to affect their performance and stability. Here, 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, which we then use 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 severly 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 lead–halide perovskites.

Perovskite phase cesium lead iodide CsPbI₃ has emerged as a promising all-inorganic photovoltaic (PV) absorber material for its remarkable optoelectronic properties such as a suitable band gap, high absorptivity, high charge carrier lifetime and solution processability, all desired qualities for PV cells. However, under ambient conditions, CsPbI₃ transforms into a high density, non-functional, non-perovskite yellow phase. However, perovskite-phase CsPbI₃ can be stabilized in the form of quantum dots and thin films. Proceeding with the hypothesis that this is an effect of reduced dimensionality, we have sought a robust approach for enforcing controlled nanostructure. To do so, we synthesized CsPbI₃ in the pores of various inorganic scaffolds: prefabricated anodized aluminum oxide (AAO) membranes and TiO₂ nanoparticle scaffolds. Nano-structuring provides a truncated crystal lattice, which we hypothesize weakens the impact of the long-range, compressive electrostatic forces that act upon the CsPbI₃ unit cell. Under such reduced compression, we hypothesize that the lower density, black perovskite phase, is favored. Herein is described a study of phase transition thermodynamics of CsPbI₃ for perovskite synthesized in scaffolds with pore sizes ranging from few nanometers to mesoscopic dimensions. X-ray diffraction is employed to quantify the resulting expansion of the lattice. We found a 250 C drop in the phase transition temperature and enhanced thermal and moisture stability as a consequence of nano-templating in AAO with pore sizes lower than 80nm; crystals synthesized in larger pores and mesoscopic TiO₂ showed standard phase transition temperature of 330 C, consistent with bulk CsPbI₃. This approach could potentially stabilize the functional perovskite phase of CsPbI₃ at near ambient conditions.

Hybrid perovskites are the first photovoltaic material class presenting competitive solar cell performance and, at the same time, significant mixed ionic electronic conduction at room temperature. Several studies have pointed out that the long time scale electrical response of hybrid perovskite solar cells can be related to the migration of ionic defects in the material, specifically to electrode polarization, [1] stoichiometric polarization [2] and ionic-to-electronic current amplification. [3] Yet, quantitative information on the contribution of each of these processes as a function of device properties and working conditions is still missing to date. Given that long time scale behavior has direct consequences on the photo-conversion mechanism and long time scale degradation in these materials, it is desirable to develop comprehensive device models that successfully describe both electronic and ionic phenomena. Progress in the understanding of these aspects will enable improved device design and is a most urgent challenge in the field.

In this contribution, we address such question using a combined experimental and numerical approach. First, we evaluate the contribution of different ion-related processes on the electrical response of hybrid perovskite devices. We carry out electrochemical conductivity measurements while varying the iodine partial pressure during the measurement, the device contact materials and the electrode geometry. Our results show that the behavior of bulk and interfaces are different under dark equilibrium vs out of equilibrium conditions, with consequences on the interpretation of the long time scale response. The data also highlight the need to combine multiple techniques when quantifying charge carrier conductivities in hybrid perovskite devices under light or voltage bias.

Next, we develop and validate a model that encapsulates the ionic and electronic properties of perovskite based devices. Our approach combines knowledge on the analytical description of mixed conductors and of the electronic processes determining much of the electrical behavior in solar cells under operation. First, we map the functional dependence of the recombination currents in perovskite semiconductors as a function of recombination mechanism and working conditions. By coupling electronic generation and recombination to the drift diffusion equations of electronic and ionic charge carriers, we develop a model bearing analytical consistency with the expected semiconductor and mixed conductor physics of hybrid perovskites’ bulk. We then consider different descriptions for the device interfaces, and highlight the importance of majority carriers’ behavior at establishing charge carrier equilibrium and quasi-equilibrium at these boundaries. [4] Based on these considerations, we develop equivalent circuit models that simulate impedance spectra of hybrid perovskite devices. Our numerical results give supporting evidence for our interpretation of the experimental measurements on horizontal devices under dark or under illumination.

Finally, we critically assess the potential of our model and its applicability to the experimental characterization of solar cell devices. We demonstrate the generality of the model and present a number of approximated versions, which are relevant to hybrid perovskite, but also to other traditional photovoltaic materials. The proposed approach will aid the interpretation of electrical measurements on perovskite solar cells, with also implications on other mixed conducting devices.

[1] S. Ravishankar et al. J. Phys. Chem. Lett. 8, 915 (2017).
[2] T. Y. Yang et al. Angew. Chemie - Int. Ed. 54, 7905 (2015).
[3] D. Moia et al. Energy Environ. Sci. 12, 1296 (2019).
[4] G. Y. Kim et al. Adv. Funct. Mater. 30, 1 (2020).

Mixed halide perovskites are promising materials for solar cells, light-emitting diodes and other optoelectronic devices due to their tunable bandgaps. However, under light, these systems show segregation into two phases with different halide compositions (photo-demixing). Interestingly, these two phases tend to remix back in the dark (dark-remixing)^[1]. This phase instability is undesirable when aiming for solar cells with stable output, while it opens opportunities towards new device architectures.^[2] The observed light-induced evolution of different phases involves significant ion transport. Therefore, an improved understanding of the underlying defect chemical mechanisms involved in photo-demixing (and dark remixing) can help both preventing and controlling this effect.

While several studies have characterized photo-demixing in 3D mixed halide perovskites, the role of decreasing dimensionality on this process has not yet been fully clarified. In the case of two-dimensional (2D) lead halide perovskites, increased resilience against intrinsic and extrinsic degradation routes has been attributed to the suppression of ion migration in these materials with respect to the 3D counterparts.^[3-4] It is therefore interesting to probe how the photo-demixing relates to the structure and composition of these mixed halide perovskite systems.

Here, we demonstrate that also 2D mixed halide perovskites show reversible demixing under light. The kinetics of the process is slower for the 2D systems compared with their 3D counterpart. We investigate the evolution of optical and structural properties of 2D perovskite thin films under different light bias conditions, providing information on the thermodynamics and the kinetics of the photo-demixing and dark-remixing processes. These techniques, combined with in-situ conductivity measurements, also allow us to assess the influence of light intensity, thin-film composition, substrate, and encapsulation. Lastly, we propose a model that describes photo-demixing and dark-remixing in 2D mixed lead halide perovskites based on our previous analysis of the 3D systems.^[5] Our study contributes to defining the thermodynamic picture of 2D mixed halide perovskites, which will aid the compositional engineering of optoelectronic devices where photo-demixing is controlled.

[1] Hoke ET, Slotcavage DJ, Dohner ER, Bowring AR, Karunadasa HI, McGehee MD. Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics. Chem Sci 2015, 6(1): 613-617.
[2] Mao W, Hall CR, Bernardi S, Cheng YB, Widmer-Cooper A, Smith TA, et al. Light-induced reversal of ion segregation in mixed-halide perovskites. Nat Mater 2020.
[3] Cho J, DuBose JT, Le ANT, Kamat PV. Suppressed Halide Ion Migration in 2D Lead Halide Perovskites. ACS Materials Letters 2020.
[4] Grancini G, Nazeeruddin MK. Dimensional tailoring of hybrid perovskites for photovoltaics. Nature Reviews Materials 2019, 4(1): 4-22.
[5] Kim GY, Senocrate A, Wang Y-R, Moia D, Maier J. Photo-effect on ion transport in mixed cation and halide perovskites and implications for photo demixing. Angewandte Chemie International Edition, n/a(n/a).

The present study analyses the use of poly(methyl methacrylate) (PMMA) interlayers as passivation agents between the photoactive perovskite films and the carrier transport layers in Perovskite Solar Cells (PSCs). Our study finds that the deposited PMMA acts exclusively as a very good electrical isolator. By accumulation at perovskite grain boundaries and defects, it physically prevents charge carriers from reaching these locations, rather than chemically passivating electronic trap states that may arise from surface defects. Spin-coating a very low concentration PMMA solution onto perovskite layers increases the shunt resistance and the fill factor, without increasing the series resistance, giving rise to an improvement in cell efficiency.

The characterization of the degree of passivation provided has been assessed through spatially and time-resolved photoluminescence, while a detailed atomic force microscopy analysis, complemented with ellipsometric and electrical measurements, allows distinguishing the PMMA morphology once deposited onto the perovskite film and its effect on finished devices.

This mechanism should apply to any perovskite composition or device architecture. Since the perovskite remains in immediate contact with the surrounding carrier transport layers, this method can be used in combination with other surface passivation methods, which could be applied before or after the PMMA treatment.

Hybrid perovskite solar cells (with chemical formula ABX₃) are of great interest due to the recently measured power conversion efficiency of greater than 25% (but theoretically, 33.7%). Perovskite structures are easily customisable, with a range of options for A, B and X. This enables us to both tune the electronic band gap and the stability by varying the composition. Two promising perovskites are the CH₃NH₃PbI₃ (MAPI) and CH(NH₂)₂PbBr₃ (FAPB) structures. By varying the ratio of FA and MA and doping with Br, we can potentially tune the band gap and effective masses (and hence electronic transport).

We present a theoretical investigation of the structural and electronic properties of MA_xFA_1-xPI_yBr_1-y, performed using first-principles density functional theory. Our results show that in a solid-state solution setting, decreasing fractions of y dominates the fall of cell volume which increases the band gap. We discuss why the formation energies at the most favourable ground-state hexagonal phase and the experimentally observed cubic phase vary in these compositions, including the effects of the rotation of the organic groups, affect the effective masses and, ultimately, the transport. We also explore the effects of clustering and intermixing on these structures and their stability.

In order to compromise for a more desirable bandgap for stand-alone PV and better structural stability, the hybrid perovskite constituents are then strategically layered, with n:1 ratio, into superlattice form. We explore the bulk properties and interfacial engineering to improve the material’s electronic transport.

The solar-to-power power conversion efficiencies (PCEs) of metal halide perovskites (MHP) have been improved over the last decade using a wide variety of methods, such as composition manipulation, dopant introduction, and interfacial buffers. These methods, however, have taken little regard for the electronic and interfacial effects such alterations may cause within devices under voltage bias stress -a condition required for most device operation. Here, we investigate the effect of halide and cation substitution in MHP structures [mainly, CH₃NH₃PbI_2.87Cl_0.13 and Cs_0.1(MA_0.17FA_0.83)_0.9Pb(I_0.83Br_0.17)₃] to understand current behavior while under a range of voltage bias stress in both light and dark conditions. With the use of a second device structure, without transport layers, the same voltage bias stress effects unique to the different MHP structures are observed thus confirming that the difference in the current trends are due to intrinsic behavior of the perovskite structure, rather than interfacial recombination and interactions with the transport layers. We also determine how changes in morphology and crystallite orientation in CH₃NH₃PbI_2.87Cl_0.13 and Cs_0.1(MA_0.17FA_0.83)_0.9Pb(I_0.83Br_0.17)₃ thin film absorbers can influence ion migration and therefore alter charge transport and current stability in MHP photovoltaic devices.

Concerns about the toxicity of lead-based perovskites have aroused great interest for the development of alternative lead-free perovskite-type materials. Recently, theoretical calculations predict that Pb²⁺ cations can be substituted by a combination of Cu²⁺ and Sb³⁺ cations to form a vacancy-ordered layered double perovskite structure with superior optoelectronic properties. However, accessibilities to this class of perovskite-type materials remain inadequate, hindering their practical implementations in various applications. Here, we report the first colloidal synthesis of Cs₄CuSb₂Cl₁₂ perovskite-type nanocrystals (NCs). The resulting NCs exhibit a layered double perovskite structure with ordered vacancies and a direct band gap of 1.79 eV. A composition−structure−property relationship has been established by investigating a series of Cs₄Cu_xAg_2−2xSb₂Cl₁₂ perovskite-type NCs (0 ≤ x ≤ 1). The composition induced crystal structure transformation, and thus, the electronic band gap evolution has been explored by experimental observations and further confirmed by theoretical calculations. Taking advantage of both the unique electronic structure and solution processability, we demonstrate that the Cs₄CuSb₂Cl₁₂ NCs can be solution-processed as high-speed photodetectors with ultrafast photoresponse and narrow bandwidth. We anticipate that our study will prompt future research to design and fabricate novel and high-performance lead-free perovskite-type NCs for a range of applications.

There are varied approaches toward the synthesis of three-dimensional (3D) perovskites based on methyl-ammonium-lead-iodide (CH₃NH₃PbI₃) absorbers for solar cells which have now yielded power conversion efficiencies (PCEs) approaching ~ 26%. In this work, we explored multiple synthesis routes to systematically optimize the films through a careful analysis of their optical, morphological and electrical characteristics. In the first technique, a stable Lewis base adduct solution of CH₃NH₃I-PbI₂-DMSO in dimethylformamide (DMF) is formulated, where an antisolvent of diethyl ether is gently cast on the adduct film during spin coating to reduce the rapid evaporation of DMF from the film surface. Through this careful control of the antisolvent, the growth kinetics lead to the evolution of the CH₃NH₃PbI₃ morphology from needle or rod-like toward the preferred platelet-like topology. Other commonly used anti-solvents such as toluene are limited in usage because they tend to wash off the dimethyl sulfoxide (DMSO) which contributes towards stability and reduction of pin-holes. In the second technique, CH₃NH₃I and PbI₂ in a ratio of 1:3 by weight was stirred overnight at 60°C in gamma-butyrolactone. In this technique, grain growth of 3D perovskite crystal is primarily dependent on stirring time of the precursor solution and annealing temperature. The third technique involves a two-step approach where the PbI₂ (in DMF) and CH₃NH₃I (in 2-propanol) precursors are deposited in succession on the substrate and annealed. Loading time, i.e. exposure of CH₃NH₃I to PbI₂ before spinning is initiated, influences growth of the 3D perovskite on the mesoporous titanium dioxide, where the latter serves as the electron transport layer in the solar cell stack. The loading time was optimized to obtain a cuboid morphology with large size. We can confirm that our absorber is optically active with all the three techniques used, where a sharp photoluminescence peak of high intensity is obtained at the ideal location of ~ 774 nm. The onset of absorption in the UV-VIS spectra occurs at 826 nm and increases gradually up to 490 nm. The bandgap of our film calculated using the Tauc method was found to be consistent with our PL inferred bandgap at ~ 1.6 eV. From scanning electron microscopy (SEM) imaging, a uniform sized continuous granular platelet morphology of the absorber was evident for the optimized film, with a tightly controlled roughness as well. We calculate the average grain size, and a grain size of at least 1 µm in the homogeneous and smooth films is optimal for incorporation into the solar cell stack. The X-ray diffraction (XRD) analysis shows characteristic peaks for the tetragonal perovskite structure at room temperature. Current-voltage characteristics revealed that the absorber was electrically conductive in the dark and under illumination for our photo-responsive measurements, prior to its integration into the solar cell stack for the PCE characterization studies.

Photon upconversion via triplet-triplet annihilation (TTA) has promise for overcoming the Shockley–Queisser efficiency limit for single-junction solar cells by allowing the utilization of sub-bandgap photons. Recently, bulk perovskites have been employed as sensitizers in solid-state upconversion devices to circumvent poor exciton diffusion observed in previous nanocrystal-sensitized devices. However, an in-depth understanding of the underlying photophysics of perovskite-sensitized triplet generation is still lacking due to the difficulty of precisely controlling interfacial properties in fully solution-processed devices. In this study, interfacial properties of upconversion devices are adjusted by a mild surface solvent treatment, which specifically alters perovskite surface properties without perturbing the bulk, and thermal evaporation of the annihilator precludes further solvent contamination. Sequential charge transfer and interfacial trap-assisted triplet sensitization are demonstrated by comparing the upconversion performance, transient dynamics, and magnetic field dependence of the upconversion devices. Incomplete triplet conversion from transferred charges and consequent triplet-charge annihilation (TCA) are also confirmed as two limiting factors for upconversion performance. Our observations highlight the importance of controlling interfacial properties and provide guidance for further device design and optimization of upconversion devices using perovskites or other semiconductors as sensitizers.

In the case of Perovskite solar cells, the active perovskite layer acts as a crucial component in influencing the solar cell's efficiency. Crack free perovskite films with enlarged grain size are essential for a highly efficient solar cell. Triple cation perovskite mainly relies on solvent-antisolvent engineering for achieving good crystal growth and enlarged grains. In recent years many kinds of research have been done involving antisolvents in Triple cation perovskites. The Antisolvent assisted crystallization (ASAC) method is considered one such way to obtain better film quality. However, the perovskite films prepared by this ASAC method still suffers from low coverage and rapid degradation. Thus, the choice of antisolvent or mixture of antisolvents can be crucial in effectively precipitating the perovskite and demonstrating high efficiency. To achieve an ideal choice of antisolvents, we have examined a mixture consisting of Ethyl acetate (EA) with Chlorobenzene (CB) and EA with Isopropyl alcohol (IPA) in different ratios for the fabrication of the perovskite layer. In this case, EA acts as a solvent that will assist in dissolving the excess perovskite precursor due it its polar nature; and, IPA and CB will enhance crystallization by reducing the solubility of the perovskite. The devices fabricated using these mixtures of antisolvents improved efficiency by more than 13% when compared to only 8% for those prepared using only EA, CB, and IPA individually. High quality, crack free perovskite layers with enhanced grain sizes were attained from using mixtures of CB and EA, which lead to improved photovoltaic properties.

Keywords: Strain engineering, structural formulation, light-matter interaction, 2D films, phase purity, photoluminescence emission.

Two-dimensional Ruddlesden-Popper (RP) hybrid perovskites have been instrumental to the unprecedented development in the areas of energy harvesting and light emission technologies, as well as to studying cutting-edge fundamental science. RP formulations exhibit the (A')₂(A)_n–1B_nX_3n+1 structure, where A’ is the spacer cation between A-Pb-X sheets, the A cation residues between Pb-X octahedra and X is a halide. A major challenge surrounding RP hybrid perovskites with n > 1 has been poor phase control when casting the materials as films. Competing reaction pathways and stacking faults cause phase heterogeneity at the nanoscale, inducing charge carrier funneling to the lowest-bandgap phase, and poor optoelectronic control. This prohibits both fundamental film characterizations and use of RP formulations in photodetector and light-emitting devices.

Here, we experimentally identified A’ and A-site cations for which large-n RP films retain the photoluminescence emission of the single crystal formulations. Notably, the emission did not contain undesired n RP emission, nor the large 3D-like emission typical of other spin-cast RP films. We correlated changes in the proportions of different emissive features with strain and morphology, and further controlled emission using small amounts of additives. These results will help push RP-based perovskite fundamental characterization and energy-based technologies to the next level.

Despite their vertiginous growth in power conversion efficiency (PCE) terms and their ultra-competitive cost, perovskite solar cells (PSC) carry stability as their main warhorse towards commercialization. Pursuing solutions to a key problem of instability in most commonly used MAPbI₃ caused by the rotation of the complex organic molecules located in the A position (chemical formulae ABX₃), all-inorganic perovskites like CsPbI₃ and relatives are attracting considerable attention, although in comparison they present only promising results yet, being necessary a profound study of their properties to overcome new problems. Set the focus on stability, a review of the main structural, thermodynamical and mechanical properties of all-inorganic halide perovskites with general formula CsPb_1-bSn_b(I_1-xBr_x)₃ is flagged as crucial, with the goal of elucidate the connection of those properties with the stability and its dependence upon the composition of the compounds.

In this work, a DFT-based study has been performed, allowing to cover a wide range of chemical compositions and knowing in detail how the composition changes affect those different properties and consequently the stability. Firstly, stability has been assessed through a series of structural tolerance parameters, like the Goldschmidt factor, the Sun’s parameter and the intrinsic hardness. Also, thermodynamical stability in terms of formation enthalpies considering different competitor phases and compounds have been evaluated according to standard procedures. Furthermore, several mechanical properties defining response to different kind of stresses, as an important characteristic for their integration in complex multilayer devices or flexible and wearable applications, having also a big impact on their absorptivity which relies on their crystallinity and stress state, have been obtained from the elastic constants following the stress-strain methodology. All those properties will define a complete set that properly describes the different facets of the intrinsic stability of these halide perovskites and their correlation, providing a guide of how to improve that weak point through composition engineering.

Nanoplatelets (NPL) of all-inorganic perovskites are promising candidates for application in light-emitting devices due to their high exciton binding energy, their exceptionally high quantum efficiency, and their improved stability.¹ While quantum dots of this material class have been studied excessively in the weak or intermediate quantum confinement regime providing interesting information on their exciton fine structure^2,3, such signatures are expected to be strongly modified when introducing strong confinement conditions in a selected direction, as in nanoplatelets.
In this work, we examine the exciton fine structure of individual CsPbBr₃ nanoplatelets with a thickness of five monolayers at cryogenic temperatures by means of polarization-resolved micro-photoluminescence spectroscopy. We find photoluminescence signatures comprised of either two or three clearly resolvable emission lines with linewidths down to 300 μeV and a fine structure splitting of ~ 1 meV. Such splitting has already been observed in quantum dots of the same material and was attributed to intrinsic Rashba effects in the orthorhombic structure.³ The two lowest energy emissions exhibit strong linear polarization of up to > 90 % with orthogonal polarization directions, corresponding to the dominant lateral crystal axis. This correlation enables us to derive the relative orientation of the NPL under investigation relative to the observation direction and substrate orientation.
To further elucidate the exciton fine structure, we apply a rotating vector magnet field of up to 5 T to the individual NPLs. The observed emission lines split up into doublets with rising magnetic field, with an average splitting of ~ 300 µeV at 5 T. This behavior clearly indicates that all three emission lines are spin-degenerate at zero field and it yields a small effective g-factor of < 1.1 for the strongly confined perovskites. A systematic study of a variety of individual NPLs in the vector magnetic field further yields insights into the anisotropy of the exciton fine structure.

References
¹ Bertolotti et al. (2019): ACS Nano 13 (12), S. 14294. DOI: 10.1021/acsnano.9b07626.
² Pfingsten et al. (2018): Nano Lett. 18 (7), S. 4440. DOI: 10.1021/acs.nanolett.8b01523.
³ Becker et al. (2018): Nature 553 (7687), S. 189. DOI: 10.1038/nature25147.

Vacancy-ordered triple perovskites have recently come under the scientific spotlight as promising materials for high-performance next-generation optoelectronic technologies.^1–3 Their A₃B₂X₉ stoichiometry facilitates the replacement of the toxic Pb²⁺ cation with a benign isoelectronic 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. The origin of this broadband absorption is identified through a combination of detailed theoretical and experimental characterization of this novel material. Sn atoms are found to disproportionate in the doped material, inducing a strong intervalence charge transfer (IVCT) transition, whilst preserving the structural integrity of the perovskite framework. Moreover, using hybrid Density Functional Theory, including spin-orbit coupling effects, alongside the Marcus-Hush theory of IVCT behavior, we comprehensively elucidate the origin of unusual concentration dependence of absorption in the doped perovskite.
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. Furthermore, 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.

¹ Y.-T. Huang, S.R. Kavanagh, D.O. Scanlon, A. Walsh, and R.L.Z. Hoye, ArXiv:2008.08959 (2020).
² S.R. Kavanagh, Z. Li, M. Napari, R.G. Palgrave, M. Abdi-Jalebi, Z. Andaji-Garmaroudi, D.W. Davies, M. Laitinen, J. Julin, M.A. Isaacs, R.H. Friend, D.O. Scanlon, A. Walsh, and R.L.Z. Hoye, J. Mater. Chem. A 8, 21780 (2020).
³ M. Buchanan, Nature Physics 16, 996 (2020).
⁴ K.K. Bass, L. Estergreen, C.N. Savory, J. Buckeridge, D.O. Scanlon, P.I. Djurovich, S.E. Bradforth, M.E. Thompson, and B.C. Melot, Inorg. Chem. 56, 42 (2017).
⁵ R. Nie, R.R. Sumukam, S.H. Reddy, M. Banavoth, and S.I. Seok, Energy Environ. Sci. 13, 2363 (2020).

Organic-inorganic metal halide perovskites are a family of semiconductor materials with a number of unique and multifaceted optoelectronic properties, making it suitable for a wide range of applications. Despite the rapid progress of halide perovskite research in recent years, many challenges remain in understanding the fundamental properties of these materials. Among them, probing the ferroelectric response of halide perovskite-based devices has been shown to be challenging due to the soft lattice of halide perovskite and the large electronic and ionic contributions to conductivity. To date, there is limited research on ferroelectricity in metal halide perovskite thin films, and as a result, the existence and impact of these ferroelectric characteristics remain under debate. Here, we use a facile Sawyer-Tower measurement circuit with tunable stimulated frequencies to measure the ferroelectric hysteresis loop of perovskite capacitors and photodiodes at temperatures varying from 90 to 300 K. Utilizing this setup, we investigated variations in the polarization-electric field (P-E) hysteresis of different perovskite-based devices as a result of increasing and decreasing temperatures. We observed different temperature-dependent hysteresis behaviors in capacitors and photodiodes based on a prototype perovskite, methylammonium lead iodide (CH₃NH₃PbI₃). It was noted that the critical points of change in the P-E curves of these devices were observed at ~150K, which is attributed to the phase transition from the tetragonal to the orthorhombic crystal structure resulting from the temperature change. The Sawyer-Tower measurements, combined with thermal admittance spectroscopy measurements provide insights to understanding the material and device properties of halide perovskites.

This work is supported by National Science Foundation under contract number
DMR-1807818.

Lead-Free halide double perovskites with enhanced stability have gained attention as a promising environmentally friendly alternative to lead halide perovskites. Amongst different halide double perovskites, Cs₂AgBiBr₆ has shown attractive optoelectronic properties and stability, making it a promising candidate for stable high-efficiency optoelectronic devices. Motivated by a text-mining effort that not only illustrates the prevalence of powder over thin-film synthesis but also the discrepancy between the number of compositions experimentally realized and studied as compared to the many predicted compositions we present here on the intricacies of thin-film synthesis of Cs₂AgBiBr₆. Understanding the role of processing strategies on film formation pathways is of crucial importance as crystallization strongly affects the film microstructure, stability, and functionality in optoelectronic devices. Herein, using time-resolved spectroscopy, we investigate the film formation of Cs₂AgBiBr₆ in situ during blade-coating, spin coating, and the subsequent post-deposition thermal annealing under different processing conditions including antisolvent- and additive (HBr)-assisted synthesis.
We show how different processing routes affect the film formation and microstructure of Cs₂AgBiBr₆. For films prepared by spin-coating and thermal annealing, we show that dropping anti-solvents during spin coating induces immediate supersaturation and crystallization of the wet film, whereas the time point of dropping the antisolvent has implications on the film formation dynamics and the final microstructure. Furthermore, due to the limited solubility of precursor salts, the thickness of the films is constrained by the low concentration of the precursor solution. HBr additive increases the solubility of precursor salts allowing to obtain thicker and pinhole-free films. We also show how the addition of HBr induces colloid formation in solution and thus influences the crystallization pathway during thin-film processing. For films prepared by blade-coating, our findings reveal that using blade-coating not only affects the crystallization pathway but also promotes the formation of highly crystalline films with preferential orientation as compared to films prepared by spin-coating followed by thermal annealing. Our work highlights the importance of real-time investigations to provide a mechanistic understanding of film formation and therefore can inform synthetic parameter choice.

Longevity has been a long-standing challenge for perovskite photovoltaics. In general, Li-TFSI/t-BP are the state-of-the-art bi-dopants for the hole-transporting layer (HTL) in perovskite solar cells (PSCs), although such dopants significantly diminish the stability of devices. Here, we reported a novel dopant of fluorinated iron(III) porphine (Fe(III)-PP) as a potential alternative to the Li-TFSI/t-BP. The optimized PSCs exhibit a champion power conversion efficiency (PCE) of 21.734%, which is the highest efficiency among the ever reported PSCs based on novel dopant in HTL and also outperforms the standard bi-dopants Li-TFSI/t-BP (21.065%). The high migration barrier together with the hydrophobic nature of the Fe(III)-PP, which in turn suppress intrinsic ion migration and alleviate extrinsic moisture-induced degradation, resulting in prominent improvement of long-term stability of PSCs up to 1200 h under air exposure without encapsulation.

Metal Halide Perovskites (MHPs) are mixed electronic–ionic semiconductors with an extraordinary complex optoelectronic behaviour and a record efficiency surpassing 25%. Interpreting small perturbation response of perovskite solar cells (PSCs) is significantly more challenging than for most other photovoltaics. This is for a variety of reasons, of which the most significant are the mixed ionic-electronic conduction properties of metal halide perovskites and the difficulty in fabricating stable, and reproducible, devices. Experimental studies, conducted on a variety of PSCs, produce a variety of spectra shapes, with different physical interpretations.
The impedance response has commonly been analyzed in terms of sophisticated equivalent circuits that can be hard to relate to the underlying physics and which complicates the extraction of efficiency-determining parameters. By a combination of experiment and drift-diffusion (DD) modelling of the ion and charge carrier transport and recombination within the cell, the main features of common impedance spectra are well reproduced by the DD simulation. Based on this comparison, we show that the high frequency response contains all the key information relating to the steady-state performance of a PSC, focusing on the charge collection efficiency of the device. We also analyze Intensity Modulated Photocurrent Spectroscopy ( IMPS) data in MHPs based on the analysis of the internal quantum efficiency, connecting them to the charge collection obtained from Impedance, and the time signals featuring in the frequency spectra. We look at the change of each signal when optical excitation wavelength, photon flux, and temperature are varied for an archetypical methyl ammonium lead iodide solar cell. We compare the Perovskite IMPS results with relatively simpler dye-sensitized solar cells (DSC) with viscous and non-viscous electrolytes to help us to understand the origin of the three signals appearing in MHP cells and the measurement of the internal quantum efficiency.

The power conversion efficiency of mixed metal lead-tin perovskite solar
cells still lags behind those of their full-lead equivalents. One reason for
this is that Sn²⁺ is easily oxidised to Sn⁴⁺, which leads to the formation of
charge carrier trap states and to p-doping. Though several strategies have
been developed to prevent or limit this oxidation, the degradation of the
perovskite layers upon exposure to oxygen remains problematic. Moreover, the
exact influence of self-doping on the performance of lead-tin perovskites is
poorly understood. On top of that, other factors that could strongly affect
device performance, such as the influence of mobile ions, have barely been
explored for this system. Here, we take a closer look at the origin of
current losses in lead-tin perovskite solar cells and find that, beside
optical losses, the cells also suffer from significant charge collection
losses. Voltage dependent photoluminescence (PL) measurements at open-circuit
(V_OC) demonstrate a signigificant reduction in PL upon a switch from open- to
short-circuit, indicative of efficient charge extraction. However, then the
PL rises again over the course of several seconds, before stabilising at
about 6% of the PL value obtained at V_OC. The current decays on the same
timescales as the extraction becomes less efficient due to band flattening,
amounting to a loss of 1.4 mA cm^-2 under 1 sun illumination. In the
following, we show that the reduction in the charge extraction efficiency and
band flattening is linked to the movement of mobile ions in the perovskite
rather than electronic charge (doping) and we show that this effect is well
reproduced by numerical simulations. Importantly, as far as we could assess
in this research, doping in mixed lead-tin perovskites is not directly
causing current losses as the doping density is insufficient to screen the
built-in field. Finally, we generalise our findings to lead-based
perovskites, where we also find that the timescales of the band flattening
and current loss correspond to the timescales of mobiles ions. Hence, the
mechanisms appear to be generic for Pb and Pb/Sn based perovskites, which
paves the way towards understanding a key loss mechanism in perovskite cells.

Ruddlesden-Popper phase (RPP) perovskites have the general form A1_n−1A2₂B_nX_3n+1. The reduced dimensionality and structural characteristics of RPPs reportedly improve the stability of these crystals in comparison to their 3D perovskite counterparts¹. Two dimensional (2D) Ruddlesden-Popper perovskites (RPPs) of the form ABX₄ can be used as the tunable active layer in photovoltaics, as the passivating layer for 3D perovskite photovoltaics or as light emitting diodes.

Here we show a nonlinear band gap behaviour with Sn content in mixed phase 2D RPPs of the form AB1_1-xB2_xI₄. GGA level density functional theory calculations (with and without spin-orbit coupling) are employed to study the effects of the short range ordering of B1 and B2 in AB1_1-xB2_xI₄ compositions with x = 0, 0.25, 0.5, 0.75 and 1. The origins of the nonlinearity are shown to be due to the chemical behaviour of the relative energies of the B s states in the VBM composition, much like in 3D perovskites². This is also influenced by the relative X-B-X angles, where increased angles result in a higher overlap between X and B orbitals, resulting in a higher dispersion and a decreased band gap. The short range ordering of B site atoms is discussed, especially in the case of x = 0.5, where the short range ordering is most significant, the band gap energy is shown to be dependant on this. We have also highlighted the role of using SOC in calculations of 2D RPPs, discussing that SOC does not significantly affect the nonlinearity of the band gap, but strongly affects the linear term, and is also crucial for understanding other electronic properties of 2D RPPs, such as accurate effective masses. This research underlines the importance of supercell permutations as well as providing a good platform to study other 2D perovskites, such as other A sites, other mixed B sites, or mixed halide RPPs.

[1] - Spanopoulos, I.; Hadar, I.; Ke, W.; Tu, Q.; Chen, M.; Tsai, H.; He, Y.; Shekhawat, G.; Dravid, V. P.; Wasielewski, M. R., et al. Uniaxial Expansion of the 2D Ruddlesden-Popper Perovskite Family for Improved Environmental Stability. Journal of the American Chemical Society 2019, 141, 5518–5534.

[2] - Goyal, A.; McKechnie, S.; Pashov, D.; Tumas, W.; Van Schilfgaarde, M.; Stevanovic, V. Origin of Pronounced Nonlinear Band Gap Behavior in Lead–Tin Hybrid Perovskite Alloys. Chem. Mater. 2018, 30, 3920–3928.

3D bulk perovskites have shown amazing promise as photovoltaic devices, but are still curbed by various instabilities. Sn based perovskites in particular are extremely susceptible to oxidation in both air and moisture. We report on the evolution of the electronic structure with partial oxidation state of tin and iodine from ASnI₃ to a charged system of [ASnI₃]²⁺, where A = CH₃NH₃ (MA), CH(NH₂)₂ (FA) or Cs. When a simulated charge cell is tested, most of the charge is reduced in the SnI₃ unit due to the valence band edge being dominated by Sn 5s and I 5p antibonding staters, this charge reduction is accompanied by a reduction in unit cell volume in [ASnI₃]²⁺ compared to ASnI₃. Generalized gradient approximation (GGA) level density functional theory (DFT) band structure calculations with spin orbit coupling show semiconducting behaviour in ASnI₃, with metallic behaviour in [ASnI₃]²⁺. We also show a similar behaviour for APbI₃ and [APbI₃]²⁺, where the Pb atoms lose less partial charge than Sn due to the higher relative energy of the Pb 6s states compared to Sn 5s states. Phonon band structure calculations show an increase in imaginary modes in the charged system, for both Sn and Pb based systems.

With the advancement in understanding the device physics of perovskite solar cells, tremendous attention has been paid on the interface between perovskite layer and the charge transporting layer, where the significant efficiency loss and degradation occur. Surface passivation is an effective way to improve the efficiency and stability of perovskite solar cells. However, a key challenge faced by most of the passivation strategies is reducing the interface charge recombination while avoiding imposing energy barriers to charge extraction. This is usually achieved through fine control of the passivation process. In this talk, I will present a novel multi-functional semiconducting organic ammonium cationic interface modifier for post-treatment of the perovskite surface, to overcome this dilemma through molecular engineering. The conjugated organic ammonium halide salt can simultaneously facilitate charge extraction, improve energy level alignment, reduce interface recombination, and stabilize the perovskite lattice. The conjugated ligand treatment also exhibits a high degree of tolerance to the capping condition and capping layer thickness, owing to its semiconducting properties. Based on this strategy, a triple-cation mixed-halide medium-bandgap perovskite solar cell is delivered with an excellent power conversion efficiency of 22.06% (improved from 19.94%). More interesting, the ion migration and halide phase segregation in perovskite films are significantly suppressed due to the lattice anchoring effect of conjugated ligand, which lead to a long-term stability of over 1000 hours under continuous illumination. Our molecular engineering strategy provides a new practical method of interface engineering in perovskite solar cells towards improved efficiency and stability.

Due to the low intrinsic hole mobility caused by orthogonal conformation of two fluorene units in Spiro-OMeTAD which is a classic hole-transporting material (HTM) in perovskite solar cells (PSCs), Spiro-OMeTAD based PSCs generally can only obtain high performances through a sophisticated doping process with dopants/additives which adds to the cost and complicacy of device fabrication, but also adversely affects the stability of PSC device. Herein, a novel dispiro-based HTM, WH-1, is designed by cleverly replacing the central carbon atom of Spiro-OMeTAD with cyclohexane, and the spatial configuration of HTM is changed from vertical orthogonality of the two fluorene units to parallel arrangement, which is beneficial to the formation of homogeneous and compact HTM film on the surface of perovskite film, improvement of intermolecular electronic coupling and intrinsic hole mobility. WH-1 is obtained in a two-step facile synthesis with a high yield from commercially available materials. WH-1 is used in PSCs as dopant-free HTM, which is the first time that the dispiro-based molecule is applied as dopant-free HTM, and power conversion efficiency (PCE) of 19.57% is obtained, rivaling Li-TFSI/t-BP doped Spiro-OMeTAD in PCE (20.29%), and showing obvious superior long-term stability.

Liquid crystal displays (LCDs) are widely used to obtain high resolution full color images in many electronic devices. Unfortunately, such devices are far from being energetically efficient. In fact, in current portable electronics or large TV monitor screens based on LCDs, a considerable amount of light emitted by the light-emitting diodes does not reach the eye of the user but instead it is lost as heat somewhere in the device.
Here, we propose a novel light guiding system which we name half-Cylinder Photonic Plate (h-CPP). The h-CPP is based on a periodic array of intercalated half-cylinders where light propagates chaotically and transversally to the cylinders, exiting the array from the top surface with a diffused light intensity pattern. By applying inverse electromagnetic design to modify the optical properties of such periodic array surface, we are able to induce a lossless polarization selectivity which can be made suitable for LCD applications. Furthermore, the design of the h-CPP allows to collect the non-transmitted light, unusable for the display and otherwise wasted, by two perovskite solar cells. Thereby, this light can be recycled back to electricity, considerably increasing the overall device efficiency. Based on such h-CPP, we will present a combination of numerical and experimental results demonstrating the polarization as well as light recycling capacities of our novel design.

Hybrid organic-inorganic perovskites (HOIPs) have been introduced as promising semiconductor materials for fabrication of photovoltaic (PV) devices about a decade ago [1]. Since then the photoconversion efficiencies of record HOIP PV devices have exceeded 25 % [2], which approaches efficiencies of traditional single crystal Si solar cells. Nevertheless, theoretical calculations predict that there is potential to boost the efficiency of HOIP PV devices even further, to more than 30 % [3]. However, to do so we need to first overcome limitations that prevent these materials from reaching their theoretical efficiencies.

One of the main performance-limiting factors for HOIP PVs has been attributed to the presence of non-radiative losses [4]. These non-radiative losses have been shown to be inhomogeneously distributed at the nanoscale, implying local carrier trapping. Therefore, understanding the nature of these non-radiative losses and their detrimental impact on PV device performance, requires use of techniques having high degrees of spatial and temporal resolution. Recently, using a custom-designed time-resolved photoemission electron microscope (TR-PEEM), we have directly imaged nanoscale distribution of defects that induce non-radiative losses in HOIP thin films, identified their formation sites with respect to surface microstructure, and revealed their hole-trapping nature [5].

Here, we further elucidate the nature of nanoscale defects in triple cation mixed halide perovskite thin films that have been fabricated by solution-processing methods. We employ the TR-PEEM technique to explore the nanoscale electronic properties of these defects, including their spatial distribution, energy-level position within the band gap, and their charge trapping dynamics. We correlate these defects to non-radiative recombination sites, and to the surface morphology. We find the presence of multiple types of defects, the formation of which is related to the growth stoichiometry. We reveal that these nanoscale defects have inhomogeneities in their charge trapping dynamics. Our study provides insight into the relationship between composition of HOIP thin films and formation of nanoscale performance-limiting defects.

References: [1] Kojima et al., J. Am. Chem. Soc., 131, 17, 6050 – 6051 (2009); [2] Best Research-Cell Efficiency Chart, nrel.gov (2020); [3] Park and Segawa, ACS Photonics 5, 8, 2970 - 2977 (2018); [4] Stranks, ACS Energy Lett., 2, 7, 1515 - 1525 (2017); [5] Doherty, Winchester et al., Nature, 580, 360 - 366 (2020).

Since the discovery of anomalous polarization phenomena in solar cells based on organic-inorganic hybrid perovskites, much attention has been devoted to the mixed ionic-electronic conducting properties of these materials.^{[1, 2]} Strikingly, investigation of mixed conduction in methylammonium lead iodide (MAPbI₃) under light showed – along with the expected increase in electronic conductivity – huge enhancement of ionic conductivity (photo-ionic effect).^[3] The photo-induced ion conductivity is assumed to be the consequence of hole localization that neutralizes iodide locally; the so-formed neutral iodine atoms can occupy interstitial sites where they are further stabilized by the polarizable environment in MAPbI₃.

As mixed-cation/anion perovskites show the greatest potential for achieving high-efficiency solar cells, understanding the role of cations and anions on this“photo-ionic effect” is of utmost importance. Here, we quantitatively deconvolute ionic and electronic transport properties in which the A-site cation or the anion is partially or fully substituted. Specifically, the electronic and ionic conductivities of MAPbBr₃, CsPbI₃, CsPbBr₃, as well as FAPbI₃ -MAPbI₃ and MAPbBr₃- MAPbI₃ mixtures are investigated.^[4] As far as the photo-ionic effect is concerned, we find that A-site substitution was of only small influence compared with anion substitution. By substituting I by Br, a significant lowering of the photo-ionic effect was observed. This corroborates our hole trapping mechanism for the photo-induced ion transport owing to the fact that Br is less polarizable than the counterpart of I. Our electrical characterization performed on mixed halide perovskite MAPb(I_1-xBr_x)₃ thin films as a function of x confirms the previously reported phase instability under light of these materials in the intermediate composition range. On the basis of these observations, we draw a possible interpretation explaining the photo-demixing in mixed halide perovskites where the formation of neutral iodine interstitial defects stabilizes the coexistence of I-rich and Br-rich domains. This factor may represent an important contribution to the driving force for phase segregation and emphasizes the importance of the coupling between electronic and ionic defects in the phase behavior of halide perovskites.

[1] Yang TY, Gregori G, Pellet N, Gratzel M, Maier J. The Significance of Ion Conduction in a Hybrid Organic-Inorganic Lead-Iodide-Based Perovskite Photosensitizer. Angew Chem Int Ed Engl 2015, 54(27): 7905-7910.

[2] Senocrate A, Moudrakovski I, Kim GY, Yang TY, Gregori G, Gratzel M, et al. The Nature of Ion Conduction in Methylammonium Lead Iodide: A Multimethod Approach. Angew Chem Int Ed 2017, 56(27): 7755-7759.

[3] Kim GY, Senocrate A, Yang T-Y, Gregori G, Grätzel M, Maier J. Large tunable photo effect on ion conduction in halide perovskites and implications for photodecomposition. Nature materials 2018, 17(5): 445-449.

[4] Kim GY, Senocrate A, Wang Y-R, Moia D, Maier J. Photo-effect on ion transport in mixed cation and halide perovskites and implications for photo demixing. Angewandte Chemie International Edition, n/a(n/a).

Mixed electronic-ionic conductors such as hybrid perovskite materials exhibit the complexity in conduction. A deeper understanding of both ionic conductivity and electronic conductivity is important. Herein, the role of A-site cation and X-site halide ion was investigated by employing Photoelectrochemical Impedance Spectroscopy technique on perovskite-electrolyte interface-based devices in controlling the charge and ionic conductivity of the perovskite material. It was observed that ionic conductivity of the perovskite material can be easily tuned either by changing the A-site cation (MA+/FA+) or by changing the X-site halide ion (I-/Br-). Significantly different (opposite) photo-induced ionic conductivity in different cation or different halide-based perovskites was noted. Therefore mixed-cation or mixed-halide perovskite can be utilized to reduce photo-induced ion conductivity of the material. Furthermore, very fine-tuning is also possible by modulating the externally applied voltage. The effect of ion accumulation and migration on the charge storage and transport property of the device is analyzed using vacancy hopping and jump relaxation model. Ionic conductivity spectra revealed Jonscher’s law dependence at mid- and low- frequency regime with a constant plateau at high-frequencies. It was concluded that the interplay of ion migration and accumulation dictates the resulting conduction and storage property of the complete device structure. These perovskite/electrolyte-based devices, therefore, can be promising candidates for electrolyte gated perovskite field-effect transistors where switchable ion conductivity can be achieved either by external applied electric field or photo-excitation.

Halide perovskites perform remarkably in optoelectronic devices^1,2. This class of materials exhibits a considerable defect tolerance, with long charge carrier lifetimes in spite of large concentrations of defects, which has been related to exotic phenomena such as polarons, Rashba effect and photon recycling^3,4. However, their performance is still not properly understood given they exhibit compositional and structural heterogeneity in addition to large concentrations of deep charge carrier traps in well localized spatial clusters^5,6. In this talk, we resolve this paradox revealing that compositional disorder of the perovskite can induce carrier funneling away from deep trap states, improving performance⁷.

We use a series of multimodal microscopy techniques based on synchrotron nanoprobe and optical spectroscopic techniques, including hyperspectral and transient absorption microscopy. We map the local chemical and structural heterogeneity of device relevant FA0.79MA0.16Cs0.05Pb(I0.83Br0.17)3 (FA=formamidinium, MA=methylammonium) perovskite samples on the nanoscale using nano X-ray fluorescence and diffraction. We spatially correlate these measurements with local values of quasi-Fermi level splitting and Urbach energy, which are important device metrics, as well as ultrafast carrier dynamics from transient absorption microscopy. We show that local quasi-Fermi level hotspots are caused by carriers funneling onto local regions with low electronic disorder. Importantly, this funneling mechanism collects charges over micrometers and thus sequesters carriers away from trap clusters associated with higher electronic disorder. The visualization of this nanoscale landscape provides an explanation for the purported defect tolerance in these materials and may open an avenue for a new class of intrinsically disordered but high performing materials.


1. Kim, Y.-H. et al. Comprehensive defect suppression in perovskite nanocrystals for high-efficiency light-emitting diodes. Nat. Photonics, doi:10.1038/s41566-020-00732-4 (2021).
2. Al-Ashouri, A. et al. Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction. Science 370, 1300, doi:10.1126/science.abd4016 (2020).
3. Frohna, K. et al. Inversion Symmetry and Bulk Rashba Effect in Methylammonium Lead Iodide Perovskite Single Crystals. Nat. Commun. 9, 1829 (2018).
4. deQuilettes, D. W., Frohna K. et al. Charge-Carrier Recombination in Halide Perovskites. Chemical Reviews, doi:10.1021/acs.chemrev.9b00169 (2019).
5. Doherty, T. A. S. et al. Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites. Nature 580, 360-366, doi:10.1038/s41586-020-2184-1 (2020).
6. Tennyson, E. M., Doherty, T. A. S. & Stranks, S. D. Heterogeneity at multiple length scales in halide perovskite semiconductors. Nat. Rev. Mater., doi:10.1038/s41578-019-0125-0 (2019).
7. Frohna, K., Anaya, M. et al. Spatial Chemical Heterogeneity Circumvents Power Losses from Local Electronic Disorder in High-Performance Halide Perovskite Solar Cells. Submitted (2021).

Perovskite solar cells possess a revolutionary potential for space applications. The thin-film solar cells can be processed onto thin polymer foils that enable an exceptional specific power, i.e. obtainable electric power per mass, being superior to their inorganic counterparts. However, research towards space applications was mainly restricted to terrestrial conditions so far. We report the first demonstration of operation of perovskite solar cells in space conditions [1]. A stabilized payload orientation allowed us to obtain current-voltage characteristics under stable irradiance conditions in low Earth orbit altitudes. By tracking the evolution of short-circuit currents we can identify phases of direct solar irradiation and weak diffuse-light irradiation arising from the Earth’s surface. During phases of strong solar irradiance, both perovskite solar cell types (planar and mesoscopic nip-type) exceeded power densities (power per area) of 14 mW cm^-2. Thus, the different solar cells reached their performance expectations exposed to space conditions. Also during a phase of being turned away from the sun, the solar cells still generated power, just collecting faint scattered light from the Earth. Interestingly, the power densities reached levels as anticipated for deep-space irradiance conditions. These results highlight both the suitability for near-Earth applications as well as the potential for deep space missions for perovskite cells.

[1] L. Reb et al., Joule 4,1880-1892 (2020), https://doi.org/10.1016/j.joule.2020.07.004.

Wide-bandgap mixed-halide perovskites such as CH₃NH₃Pb(I_xBr_1-x) are of great interest because of their favorable bandgaps for use as top subcells in multijunction photovoltaic devices. Recently, we have shown through numerical simulation that the performance of single junction wide-bandgap perovskite devices strongly depends on the energetic alignment between the absorber and the charge transporting layers. Those results showed interesting behavior in the power conversion efficiency at high conduction band offset between the electron transport layer (ETL) and wide-bandgap absorber; namely, the open circuit voltage rapidly increased, but the fill factor decreased just as drastically. These results suggest the formation of a large potential barrier that impedes electron flow out of the absorber. Such a barrier was not evident at the ETL/absorber interface but may be the result of poor band alignment at the transparent conducting oxide (TCO)/ETL interface. Here, we investigate the impact of TCO band alignment with the ETL in wide-bandgap perovskite solar cells. We show that varying the conduction band position of the TCO can lead to reductions in both the recombination and fill factor loss. The reduction in recombination allows the device to maintain a high fill factor, near 90%, not previously observed for a wide range of conduction band offsets (CBOs) between the ETL and wide-bandgap absorber. Simultaneously, varying the conduction band position of the TCO preserves a high V_OC and results in a 1-2% increase in efficiency compared to our previous modeling with a standard TCO and optimized ETL/absorber CBO. These results indicate that as the bandgap of the perovskite absorber layer is increased, the choice of ETL and TCO will have a significant joint effect on performance.

Lead halide perovskites are promising materials for tandem solar cell applications due to their easily tunable bandgap through cation and halide compositions. However, photo-induced halide segregation in iodide-bromide perovskite chemistries often results in iodine-rich and bromine rich domains and limits the efficiency of tandem solar cells. One approach to mitigate such segregations reported in the literature is the addition of chlorine to the cesium-formamidinium iodide-bromide perovskite compositions. However, the literature on the effects of varying degrees of chlorine incorporation in thin-film perovskite is limited. In this study, we present an experimental study of the iodine-bromine-chlorine composition space aided by density functional theory (DFT) calculations and data science methods. By loosening cesium-to-formamidinium ratio at the same time, we allow for more possible selections of triple halide compositions that could fall in the optimal range of Goldschmidt's tolerance factor and result in stable perovskite. Among these selections, we look for compositions whose band gaps are optimized for tandem applications and whose halide segregation effects are minimized. Since all possible hybrid-cation mixed halides samples at one percent increments are too numerous to fabricate, we perform a screening informed by DFT and data science methods to intelligently search the compositional space. Through iterative experimental and computational exploration, we identify triple halide perovskite compositions with optimized opto-electronic properties well suited for tandem applications. Our data science and DFT approach demonstrates an expedition to experimental work in material discovery and characterization, which could be adapted to optimizing perovskite chemistries for alternative optoelectronic applications.

The stability of methylammonium lead bromide (MAPbBr₃) single crystal was investigated when subjected to simultaneous oxygen exposure and light illumination of one solar intensity. The exposure process was monitored with X-ray photoelectron spectroscopy (XPS) with detailed control of the exposure time and pressure. It was found that the with light the oxygen degradation process of the MAPbBr₃ was substantially accelerated. Quantitative analysis showed that Carbon, Bromine and Nitrogen lost 52%, 56% and 90% of their initial concentration respectively at 10¹² Langmuir of oxygen exposure. It further showed that ~11% of the perovskite’s Pb degraded to metallic Pb at 10⁹ Langmuir, and then was oxidized with the increase of the oxygen pressure.

Control and prevention of perovskite phase segregation are crucial to realizing stable and tunable multi-cation and mixed-halide perovskite optoelectronic devices. Herein we report the comprehensive analysis of band structure, compositional heterogeneity, and phase segregation in the novel alkali cesium (Cs⁺) and potassium (K⁺)-doped 3D perovskites using a suite of powerful and highly complementary techniques [such as X-ray photoemission spectroscopy (XPS), depth profiling, ultraviolet photoelectron spectroscopy (UPS) and grazing incidence wide-angle X-ray scattering (GIWAXS)]. Perovskite phase segregation information has been extracted from the optical or electrical properties using techniques such as absorption and photoluminescence spectroscopy, external quantum efficiency (EQE), transient difference absorption spectroscopy, and photovoltaic performance. However, all of these techniques provide indirect compositional information and cannot reveal the spatial distribution. We found that the incorporation of metal cations raises the conduction band minimum (CBM), facilitating electron transfer to the electron transport layer. Notably, the depth profile by XPS and GIWAXS analysis show that the formation of iodide- and bromide-rich regions are strongly correlated to the alkali metal cation concentrations: low metallic concentration (5%) can prevent phase segregation where the halides and cations are distributed evenly across the films, while high metallic cation ratio (20%) leads to halide segregation where the concentration of iodide decreases accompanied with the increase of bromide and metal cation concentration as the sampling depth increases. These differences in electronic properties, element distribution, and film morphology were reflected in solar cell performance: >20 % of power conversion efficiency (PCE) was demonstrated with low Cs⁺ incorporated perovskite, while ≈16.7 % of PCE with high Cs⁺ concentration perovskite. Similar trends were also observed in the K⁺-doped perovskite system. We believe this work will advance the perovskite community by providing a direct and straightforward route to characterize the impacts of perovskite phase segregation.

Parameter extraction based on numerical simulation is error-prone because model parameters are often correlated [1]. The accuracy of the extracted parameters can be improved by combining multiple steady-state and transient measurement techniques to reduce these correlations. This method has been successfully applied to organic solar cells [2].
Modelling perovskite solar cells, however, involves additional complexity. Characteristic features, such as IV curve hysteresis [3,4] and slow transient current rise [5,6] indicate the presence of mobile ionic charges in the perovskite layer. Simulation models including ionic charges could qualitatively reproduce these individual features [3-5]. However, the reliable determination of these ionic parameters additionally requires suitable characterization techniques.
To increase the trustability of the model, we systematically study the influence of individual model parameters on different experimental techniques. We therefore model multi-layer methylammonium lead iodide (MAPI) perovskite solar cells in a coupled opto-electrical simulation including mobile ionic charge carriers, charge trapping, Shockley-Read-Hall (SRH) recombination and doped transport layers. We find a single set of parameters to explain the combined results from various measurement techniques in DC, AC, and transient regime [6]. The applied techniques include IV curves with different scan rates, light intensity dependent open circuit voltage, impedance spectra, intensity-modulated photocurrent spectra (IMPS), transient photocurrents and transient voltage step responses. As expected, the various measurement techniques show different sensitivities to material and device parameters, which a model fit can benefit from.
Yet correlated parameters remain an issue. Namely it is difficult to distinguish between concentration and mobility of ionic charge carriers [6]. M.H. Futscher et al. [7] recently applied a measurement technique called transient ion drift to perovskite solar cells. This technique was originally developed for inorganic p-n junctions and allows to individually and quantitatively measure both concentration and mobility of slowly moving mobile charges [8]. It is unfortunately restricted to materials with low concentrations of mobile charges and comparably high doping [7-8]. Using our model we evaluate this characterization technique for devices beyond these restrictions. Specifically we investigate materials with high concentrations of mobile ions or absence of doping, as many perovskite materials meet one or both of these specifications. We further investigate the correlation between other parameters and the extracted ones.
Our consistent device simulation model allows for a better understanding of the governing physical effects and provides a powerful tool to study the influence of device parameters on the solar cell performance and efficiency. The variety of applied measurement techniques allows one to reduce the correlation between parameters. Furthermore, an in depth parameter study suggests possible paths towards an optimized device [6].

References
[1] M.T. Neukom et al., Sol. Energy (2011), 85, 1250.
[2] M.T. Neukom et al., Organic Electronics (2012), 13, 2910-2916.
[3] S. van Reenen, M. Kemerink and H.J. Snaith, J. Phys. Chem. Lett. (2015), 6, 3808 3814.
[4] P. Calado et al., Nat. Commun. (2016), 7, 13831.
[5] S.E.J. O’Kane et al., J. Mater. Chem. C (2017), 5, 452 462.
[6] M.T. Neukom et al., ACS Appl. Mater. Interfaces (2019), 11, 26, 23320-23328.
[7] M.H. Futscher et al., Mater. Horiz. (2019), 6, 1497-1503.
[8] T. Heiser and A. Mesli, Appl. Phys. A: Solids Surf. (1993), 57, 4, 325-328.

Colloidal CsPbX₃ (X = Cl, Br, I) perovskite nanocrystals have attracted a great deal of interest due to their bright, narrow emission, high photoluminescence quantum yields, tunable band gap, and defect tolerance. Various dopants have been successfully incorporated in their structure, including notably Mn²⁺ and Yb³⁺, allowing for novel emissions properties such as broadband emission and quantum cutting. However, the structure and chemistry of dopants within the perovskite lattice are not well understood. A characteristic property of CsPbX₃ nanocrystals is their facile anion-exchange chemistry in the liquid or gas phase, which allows ready conversion between different halide structures, formation of mixed halide compositions, and access to materials that are difficult or impossible to synthesize directly. The mechanism(s) by which this anion exchange occurs are still poorly understood, as are its effects on dopant sites. Our work focuses on probing these reactions at the atomistic level to gain mechanistic insight into this important chemical transformation. We have employed X-ray total scattering measurements of the ensemble anion exchange reaction to observe structural changes during this process. These indicate a continuum shift in the structure during anion exchange, with expected expansions in the long-range order. A substantial change in the majority of atom pair positions and intensities was also observed in the Mn-doped system, while the Yb-doped system is nearly identical to the undoped structure, except for a small shift in the Pb-Cl peak. Furthermore, we are pursuing a combination of environmental TEM and HR-STEM with EDX mapping at multiple points ex-situ and in-situ in the gas phase in order to understand the mechanism of anion exchange in doped and undoped halide perovskite nanomaterials.

Metal halide perovskites possess fascinating material properties and significant technological promise. The materials consist of a nominal formula of APbX3, where A is a large cation (e.g. Cs, MA or FA), and X is a halide, however much attention has been devoted to the alloys composed from mixtures thereof, e.g. (MA/Cs/FA)Pb(Br/I)₃. Alloying at the X-site provides bandgap tunability while alloying at both sites can significantly enhance stability towards both phase change and moisture-catalyzed degredation. Recent studies have identified the phenomenon of light induced halide demixing, whereupon illumination drives the formation of pure-phase halide clusters. This entirely reversible process has been hypothesized to be due to strain induced by photoexcited polarons. The facile and reversible nature of ion mobility in this system makes it an ideal setting for studying the coupling between external perturbations and the spatial distribution of ions in mixed alloys. These experiments test whether mechanically induced strain is another such external perturbation capable of driving ion migration. To determine the extent of ion migration in mixed halide perovskites to mechanically-induced strain, polycrystalline thin films of MAPbBrI₂ were placed under deliberate tensile strain and measured via steady-state photoluminescence for changes in the spatial distribution of halides at the point of strain as a function of time.

The presence of an organic or inorganic A-site cation has been associated with subtle differences in the electronic structures of lead bromide perovskite compounds. Through a bromine- and lead-selective core level spectroscopic study of three archetypal hybrid-organic and all-inorganic APbBr₃ compounds, including the first high-resolution hard X-ray absorption spectroscopy measurements on lead halide perovskite single crystals, we find the A-site cation has a substantial effect on the unoccupied states and a relatively smaller effect on the occupied states. We examine the conduction band states with Br K-edge X-ray absorption spectroscopy, finding Pb-Br states near the conduction band edge and higher-lying (A cation)-Br states. We find a linear correlation between relative Goldschmidt tolerance factors and relative conduction band widths. Through a combination of non-resonant X-ray emission spectroscopy and hard X-ray photoelectron spectroscopy, we find that the occupied states of the three compounds differ primarily at the valence band edge. We directly access lead-bromide bonding ionicity via X-ray emission spectroscopy and account for relative shifts in the energy levels with the chemical shift. Our findings have implications for interfacial energy level alignment in perovskite devices, the defect tolerance hypothesis, slow hot carrier cooling and other intriguing effects in this class of materials.

Application of semiconductor thin film materials in photovoltaic devices requires optimized optoelectronic properties and thus understanding of the underlying physical mechanisms and limits governing carrier transport and recombination. Although material parameters such as doping density, recombination lifetimes and mobilities belong to the most basic properties eventually defining the solar cell operation, they can be quite elusive or ambiguous in their characterization. Here, chalcogenides (e.g. chalcopyrite and kesterite) and halide perovskites share some similarities, but also a number of differences. Considerable discussion prevails about the role of interface versus bulk recombination in devices, the values of carrier mobilities, doping levels and minority carrier lifetimes. Combining different characterization techniques such as time-resolved luminescence, photoluminescence quantum yield as well as pump-probe terahertz spectroscopy we attempt to draw a relatively consistent picture of material properties and device function, exposing current limits and paths to improvement.

The fabrication of cost-effective multi-junction (MJ) device architectures is presently considered the strategy to increase the Power Conversion Efficiency (PCE) of the solar cells in order to lower the levelized cost of energy (LCOE), decreasing therefore the price of an installed photovoltaic (PV) system. The outstanding advancement of metal halide perovskite absorber (PSC) technologies in the last years motivates the PV community to investigate tandem hybrid architectures, consisting of a perovskite based top cell with a commercial bottom PV device such as crystalline silicon (c-Si) or copper indium gallium selenide (CIGS). The combination with the latter, in particular, offers unique advantages: i) compatibility with lightweight flexible plastic or metal foils as substrates, in view of a roll to roll (R2R) manufacturing line, further reducing the manufacturing and the installation costs, as well as the environmental footprint of the KWh generated; ii) large scale integration of PV devices in, amongst others, buildings (BIPV and vehicles (VIPV).
This contribution will provide an overview of the activities within the Solliance consortium on 4 terminal tandem and preliminary work on 2 terminal architectures, on rigid, as well as flexible substrates. In order to increase the overall PCE of the tandem hybrid configuration, the optimization of the sub-cells’ performance with respect to their spectral responses is required. Several combinations of suitable complementary band gaps of the two absorbers are explored, as well as the optimization of the near infrared transparency of the top perovskite cell by tuning the free carrier absorption of the TCO adopted. Furthermore, in the case of the 2-terminal architecture, the development of an efficient interconnected layer among the two sub-cells is utmost crucial to provide the monolithic series interconnection and, at the same time, to minimize the parasitic absorption.
Optimized TCO, transport layers and anti-reflective properties lead to a near-infrared transmittance of about 90% (an average taken from the wavelength range of 700-800 nm to 1200 nm) on glass or plastic based top PSC optimizing therefore the response of the filtered CIGS. As a result, coupling a semitransparent glass-based PSC and a flexible CIGS we calculate a 4T tandem PCE of 25.2% on small area devices (<0.1cm²). When using a flexible semi-transparent perovskite solar cell, a record-breaking power conversion efficiency of 23.0% is calculated. We also evaluated the PSC-CIGS combination on a larger area (> 80 cm²), currently adopting a no-optimized (in the NIR region) large area perovskite module. We calculated already a 1.9% absolute increase in the efficiency with respect to the single CIGS device, with a further improvement that can be achieved by improving the light management and fine tuning the perovskite band gap. Whereas the 4T architecture offers already a significant increment in performance with respect to the single junction devices, the development of the 2T tandem PSC-CIGS exhibits more challenges. As observed by the first results, the efficiency of the flexible 2T tandem PSC-CIGS, still lags behind the power conversion efficiency of the single CIGS. New upscalable perovskite formulations as well as the optimization of the recombination layer will also be addressed as crucial topics to boost the overall performance.

Great progress in perovskite photovoltaics has been enabled by the excellent intrinsic material properties of lead-halide perovskites. These relatively newly-available materials are already opening paths to higher performance in mass-market PV technologies, including tandem PV modules. Nonetheless, reaching competitive levelized energy costs will hinge on achieving comparable robustness to existing silicon-based technology. This is a high and rising benchmark, which has not yet been approached by halide perovskite technologies. This makes it less likely that investors will support the upscaling and deployment of perovskite-based solar technology.

Chalcogenide perovskites, in which halide anions are replaced by chalcogenides (O, S, Se) could offer a new path forward. Examples include BaZrS₃ and SrHfS₃, among several others. The solar cell-relevant properties of these materials are projected to be comparable to halide perovskites. More critically though, their chemistry provides them with excellent stability under conditions for which halide perovskites would rapidly degrade, e.g. in water, humid air or at high temperatures.

The major challenge with chalcogenide perovskites has been to synthesise them in thin film form with good quality. Therefore, despite impressive measurements from bulk materials, it has not been possible to validate their application in thin film solar cells. There are several reasons for the synthetic difficulties, connecting to intrinsic material properties as well as to the use of inappropriate synthesis methods. To date, we still have incomplete understanding of the growth processes of these materials.

In this presentation, we briefly summarise the potential of the chalcogenide perovskites based on prior literature. Then, our own progress toward thin films suitable for solar-cell integration will be presented. A number of sputter-based growth routes to BaZrS₃ have been investigated and evaluated using combinatorial approaches. Materials have been characterised using XRD, STEM-EDS, SEM/TEM imaging, XAFS, Photoluminescence mapping and Ion-beam techniques including RBS and ERDA, to construct a picture of the growth sequence and final compositional, microstructural and optoelectronic characteristics of the films. This is a means to establish the first “cookbook” for chalcogenide perovskite films suitable for solar cells, i.e. the optimal growth conditions for high quality material. Film formation will be discussed in the context of thermodynamic and kinetic aspects, applicable to any type of growth method. The aim of this talk is to promote and support research into these highly interesting materials in other labs, and stimulate collaboration on this exciting topic.

Spiro-OMeTAD, CuSCN and PTAA are attractive hole transport materials (HTM) in conventional n-i-p architectures for perovskite cells¹. In this work, we combine these three HTMs on semi-transparent (ST) solar cells with different sputtered electrodes made of In₂O₃-SnO₂ (ITO) or In₂O₃-ZnO (IZO) to study their impact on the optical and electrical properties of the cells.

First, thanks to an optical model based on the Transfer Matrix Method and careful material characterizations^2,3, a good agreement has been obtained between the transmission, reflection and absorption spectra of the complete structure of the ST cell with Spiro and ITO. This model allows to simulate the efficiency of a tandem cell with a silicon bottom cell, to identify optical losses and to optimize the thickness and properties of the each layer. It is shown that the Spiro and ITO layers have a non-negligible parasitic infrared absorption in the IR, resulting in a lower efficiency for the filtered silicon cell. The gain in efficiency enabled by the use of PTAA, CuSCN and IZO, which are almost transparent in the IR region, is assessed.

Second, triple cation perovskite solar cells with these three HTMs are synthetized and characterized in detail using scanning electron microscopy, ellipsometry spectroscopy, glow discharge emission optical spectroscopy, X-ray diffraction and transmission spectroscopy. Regarding the electrical properties, the Spiro is a layer known for its sensitivity, and the poor reproducibility obtained on ST cells is often associated with degradation due to energetic sputtering⁴. Investigations using CuSCN as HTM showed a slightly better reproducibility than Spiro but strong light soaking effects also appeared, and the resulting efficiencies are low. The replacement of the Spiro by PTAA has improved the reproducibility of cells, while maintaining good performances. A 17.4% efficiency perovskite semi-transparent cell with PTAA was obtained and which is significantly above our previous results with Spiro (16.4%)⁵.

After electrode sputtering deposition, a characteristic S-shape appears in IV characteristics. By regularly following the cells over time while keeping them under vacuum atmosphere and in the dark, IV curves recovered until the S-shape disappeared completely. For example, a cell with Spiro and ITO have 8.7% efficiency and 43% Fill Factor (FF) the first day and reach 16.8% with FF=70% after 66 days. When the Spiro is replaced by PTAA, the S-shape is less pronounced and the recovery time markedly reduced to 15 days. These effects are not visible on cells with CuSCN.

Lee et al.⁶ obtained a similar S-shaped curve on perovskite cell with a Spiro/silver electrode, with a short recovery time of about one day. A local reaction between the dopant Li-SIF and the silver could cause the deoxygenation of the Spiro at the surface, which would create a temporary electrical barrier. It is interesting to note that this dopant is common to PTAA and Spiro but not to CuSCN. Preliminary results obtained by time resolved photoluminescence (TRPL) shows a decrease of the PL signal over time. Experiments are underway to determine if the cause of this decrease is due to improved extraction at the HTM/TCO barrier as suggested by Lee et al.⁶, or if it comes from a reduction in radiative recombination related to defect healing. We will discuss the reaction mechanisms for these three HTM.

[1] Z. Shariatinia, Renew. Sustain. Energy Rev., 119, 109608, 2020
[2] E. Raoult et al., 36th Eur. Photovolt. Sol. Energy Conf. Exhib. 757, 2019
[3] E. Raoult et al., SPIE: Physics, Simulation, and Photonic Engineering of Photovoltaic Devices IX, 2020
[4] H. Kanda et al., J. Phys. Chem. C, 120, 50, 28441, 2016
[5] F. J. Ramos et al., Sci. Rep., 8, 1, 2018
[6] D. G. Lee et al.,, ACS Appl. Mater. Interfaces, 11, 51, 48497, 2019

Tandem photovoltaic (PV) devices offer great advantages over individual photovoltaic cells including the potential to reach power conversion efficiencies (PCEs) greater than that of the composing individual cells. The current record PCE for a PV tandem cell is over 29%, set by a perovskite/Si tandem. Perovskite solar cells (PSCs) are ideal for tandem devices because they have proven to be highly efficient and the bandgap can be tuned. However, PSCs are extremely sensitive to air and light. Another great candidate for tandem devices are chalcopyrite thin-film solar cells. Chalcopyrite cells also have tunable bandgaps and high efficiencies. The manufacturing process for chalcopyrite cells includes parameters that would degrade the performance of the bottom cell in a monolithic tandem device and the architecture of the cell prohibits a sufficient amount of energy below the bandgap from reaching the bottom cell. Thus, we present a strategy for fabricating a semi-monolithic chalcopyrite/perovskite tandem device by employing a transparent polymer impregnated with conductive particles (TPCP).
The TPCP was designed for electrically coupling solar cells into tandem devices. The TPCP can be employed in three ways: (i) fully-bonded, (ii) semi-bonded , and (iii) freestanding. When fully-bonded, the TPCP permanently bonds two surface allowing for passage of light and electrical current. TPCPs have been used to fabricate a CuGaSe₂/Si tandem device with an open circuit voltage of 1.04 V and PCE of 2.45%. Fully-bonded TPCPs were used to exfoliate the CuGaSe₂ cell at the MoSe₂ layer with an FTO-coated glass handle. The exfoliated CuGaSe₂ cell was then fully bonded with the TPCP to a Si bottom cell. The semi-bonded TPCP passes light and electrical current while also providing the protective features of a polymer over a surface. The semi-bonded TPCP has successfully been employed as an encapsulation technique for preserving PSCs. The TPCP encapsulation did not affect the performance of the PSC. An encapsulated PSC exposed to air and light for 14 days did not show any visible signs of degradation. The freestanding TPCP enables the coupling of PV cells without permanently bonding the cells. Ultraviolet-visible spectroscopy measured over 90% transmittance across the visible spectrum. The resistance of the TPCP measured by linear sweep voltammetry yielded a series resistance to the order of 10^-1 Ω●cm².
Based on the successful fabrication of the CuGaSe₂/Si tandem device and encapsulation of the PSC, we will investigate the fabrication of a chalcopyrite/perovskite tandem device composing of a wide bandgap chalcopyrite top cell and a narrow bandgap PSC. Additionally, the application of the freestanding TPCP to fabricate a tunable tandem device with exchangeable cells will be attempted.

The first part of the presentation summarizes parts of our work on bulk ion conductivity in halide perovskites. It discusses contributions from halide ions and methylammonium cations, and their relevance for polarization phenomena and stability [1]. The finding that light enhances the ion conductivity in the iodides [2] but not in the bromides, offers a simple explanation for the light induced demixing [3]. How far this surprising light effect on ion conduction has the potential to lead to novel “opto-ionic” devices is discussed [4].
The second part of the presentation refers to interfaces. It is shown that mobility of ions and the significant fraction of ionic charge carriers also gives rise to ionic built-in potentials, a point that had not been addressed in the photovoltaic literature and might lead to a paradigm change in the understanding of photoactive interfaces [5].

[1] A. Senocrate and J. Maier, J. Am. Chem. Soc. 141 (2019) 8382
[2] G.Y. Kim et al., Nat. Mater. 17 (2018) 445
[3] G.Y. Kim et al., Photo-effect on ion conduction in mixed cation and halide perovskites and implications for photo de-mixing, Angew. Chem, to be published
[4] A. Senocrate et al., Helv. Chim. Acta 103 (2020) e2000073
[5] G.Y. Kim et al., Adv. Funct. Mater. 30 (2020) 2002426

Trapping of holes at iodide sites in mixed halide perovskite, MHP (MAPbBr_1.5I_1.5 ) films cause iodide ions to migrate toward grain boundaries, thus inducing the formation of iodide rich phases and bromide rich phases. When mixed halide perovskite films are in contact with solution, the migration of iodine extends beyond phase segregation as it gets expelled into solution. The selective removal of iodine from MHP upon continued irradiation transforms the perovskite film into a bromide-rich perovskite film. Substituting A-site cation of MHP with cesium (Cs) slows down iodide expulsion due to increased thermodynamic stabilization of MHP lattices. Similar lattice stabilization has also been observed in Cl-alloyed perovskites. Furthermore, photoinduced iodide expulsion process in MHPs can be modulated through externally applied electrochemical bias. At anodic potentials, electron extraction at TiO₂/MHP interfaces becomes more efficient, leading to hole build-up within MHP films. This improved charge separation, in turn, favors iodine migration as evident from the increased apparent rate constant of iodine expulsion (k_expulsion = 0.0030 s^-1). Conversely, at cathodic bias (-0.3 V vs. Ag/AgCl potential) electron-hole recombination is facilitated within MHP films, slowing down iodine expulsion by an order of magnitude (k_expulsion = 0.00018 s^-1). The tuning of the E_Fermi level through external bias modulates electron extraction at the TiO₂/MHP interface indirectly controls the build-up of holes, which ultimately induces iodine migration/expulsion. Suppressing iodine migration in perovskite solar cells is important for attaining greater stability since they operate under internal electrical bias.

Mixed halide perovskites (MHPs) under photoirradiation phase segregate into iodide- and bromide-rich domains. When in contact with a solvent, the perovskite film expels iodine into solution. The trapping of holes at iodide sites dictates the overall iodide migration process. 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 (k_expulsion = 0.0030 s^-1 at 0.5 V vs. Ag/AgCl). Conversely, at cathodic bias we facilitate electron-hole recombination within the MHP film and slow down iodine expulsion (k_expulsion = 0.00018 s^-1 at -0.3 V vs. Ag/AgCl). The tuning of E_Fermi 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.

Tunability of optoelectronic properties of lead halide perovskites achieved through halide mixing can potentially enable their multiple applications e.g. in tandem solar cells and light-emitting diodes. However, mixed halide perovskites are unstable under illumination due to their segregation into Br-rich and I-rich phases, which negatively affects the performance characteristics and operational stability of devices. Research efforts over the past years provided a substantial understanding of the factors influencing light-induced halide phase segregation. While several mechanisms were proposed, none of them could account for all available experimental data; and hence the origin of the effect is still under active debate. Herein, we thoroughly investigated the photodegradation of CsPbI₂Br and Cs_1.2PbI₂Br_1.2 using a set of complementary techniques. In-situ atomic force microscopy provided a spectacular visualization of the real-time halide phase segregation dynamics demonstrating that iodoplumbate is selectively expelled from the mixed halide perovskite grains and nucleates as a separate I-rich phase at the grain boundaries. We propose a mechanism based on the reversible Pb²⁺/Pb⁰ and I^-/I₃^- redox (photo)chemistry, which explains our experimental findings and other previously reported results. Furthermore, it sheds new insights on the underlying mechanisms of multiple phenomena related to light- or electric field-induced degradation of various lead halide perovskites.
More details can be found in our recently accepted paper: L. A. Frolova et al., Reversible Pb²⁺/Pb⁰ and I^-/I^3- redox chemistry drives the light-induced phase segregation in all-inorganic mixed halide perovskites. Adv. Energy Mater. 2021, DOI: 10.1002/aenm.202002934.

Solid state devices based on halide perovskites are now competing with other well-established semiconductors like silicon and gallium arsenide. However, the intrinsic instability of three-dimensional (3D) perovskites poses a great challenge in their widespread commercialization. The soft crystal lattice of hybrid halide perovskites facilitates anionic diffusion which impacts material stability, optoelectronic properties, and solid state device performance. Two-dimensional (2D) halide perovskites with bulky organic barriers have been used for improving the extrinsic stability as well as suppressing intrinsic anionic diffusion. With this strategy, devices with enhanced stability and reduced hysteresis have been achieved. Nevertheless, a fundamental understanding of the role of compositional tuning, especially the impact of organic cations, in inhibiting anionic diffusion across the perovskite-ligand interface is missing. Through this study, we demonstrate a quantitative investigation of the anionic inter-diffusion in flat 2D vertical heterojunctions between two arbitrarily determined phase-pure halide perovskite single crystals. Contrary to their 3D counterparts, anion diffusion across 2D halide perovskite vertical heterostructures does not follow the classical Fickian diffusion featuring continuous concentration profile evolution. Instead, the assembled vertical heterostructures show a “quantized” layer-by-layer diffusion behavior governed by a local free energy minimum and ion-blocking effects of the organic cations. We found that for a given organic cation, 2D perovskites (n = 1) are better inhibitors of halide diffusion compared to quasi 2D (n > 1) and 3D perovskites. Surprisingly, halide inter-diffusion coefficients for 2D perovskites capped with short aliphatic organic cations (e.g. butylammonium) are only 1~2 order of magnitudes smaller than that of 3D perovskites, suggesting the ineffectiveness of these cations in blocking the anionic migration. Furthermore, we found that bulkier and more rigid π-conjugated organic cations inhibit halide inter-diffusion much more effectively (D < 10^-20 m²/s) compared to aliphatic cations. These results offer significant insights into the mechanism of anionic diffusion in 2D perovskites and provide a new materials platform for heterostructure assembly and device integration.

Slow hot carrier cooling in halide perovskites holds the key to the development of perovskite hot carrier (HC) and multiple exciton generation (MEG) solar cells that could potentially overcome the Shockley-Queisser limit. In this talk, I will focus on the new photophysical insights from our recent works on the hot carrier phenomena in halide perovskites.

Solution-processable halide perovskite semiconducting materials have unique optoelectronic properties for applications in perovskite solar cells and photodetectors (PDs). The discrete perovskite PDs have a broadband photoresponse, which is sensitive over the visible wavelength range, while the discrete polymer PDs exhibit a broadband photoresponsivity with extended absorption in the near-infrared (NIR) wavelength range, e.g., from visible light to the NIR wavelength of >1100 nm. The visible-blind NIR photodetection with these broadband PDs requires the use of the filters or the specially designed photonic structures for attaining the photodetection over a well-defined wavelength range. However, the use of the external filters adds additional cost and reduces the overall photosensitivity of the PDs.

Different narrowband photodetection approaches have been attempted, e.g., incorporating a microcavity structure and using charge collection narrowing (CCN) effect in the PDs. In the CCN-type PDs, a relatively thick photoactive layer, e.g., > 2500 nm thick photoactive layer, is adopted to reduce the collection efficiency of the charge carriers generated by the visible light absorbed near the upper region of a thick photoactive layer. The use of a thick active photoactive layer in a CCN-type PD faces some technical challenges: (1) The responsivity of the CCN-type PDs is limited as a large amount of the incident light is attenuated by the thick photoactive layer. (2) Perovskite PDs with a thick photoactive layer often associates with a slower response speed due to the decrease in its carrier transit time limited cut-off frequency. (3) The photodetection spectrum in the CCN-type PDs is limited by the absorption edge of the photoactive layer. This talk discusses a novel hybrid PD with a heterostructure perovskite/polymer photoactive layer for alleviating the challenges: (1) to achieve high hole transport efficiency by incorporating a thinner perovskite, (2) to realize narrowband NIR detection through buildup of the space charges at the perovskite/polymer interface, and (3) to improve the device design freedom by incorporating different combinations of perovskite charge transporting layer and the NIR absorbing layer in the perovskite/polymer structure. The hybrid PDs with a thinner photoactive layer results in an obvious increase in the responsivity and response speed. The hybrid PDs thus demonstrated have a narrowband NIR detection and a -3 dB cut-off frequency of 300 kHz, offering an exciting option for a plethora of applications in bio-imaging, environmental detection, and security monitoring.

Designed to outperform conventional computers when performing machine-learning tasks, neuromorphic computation is the principle whereby certain aspects of the human brain are replicated in hardware. While great progress has been made in this field in recent years, almost all input signals provided to neuromorphic processors are still designed for traditional (von Neumann) computer architectures. For example, in a CCD detector an array of pixels is sampled at fixed intervals in time.

In this work we have taken inspiration from the human retina and demonstrated an event-driven sensor which pre-processes optical signals by design. Using a metal halide perovskite thin film as one dielectric layer of a bilayer capacitor, we demonstrate a device which changes its capacitance under illumination. When in series with a resistor, and a constant bias is applied across this device, the voltage dropped across the resistor will spike temporarily as the capacitor (dis)changes, before returning to its equilibrium value. The result is a sensor which spikes in response to changes in illumination, but otherwise outputs zero voltage. A simple model was developed to parameterize and simulate these sensors.

It is hoped that this work represents the first step towards a paradigm shift for the design of sensing systems for neuromorphic computation, and artificial intelligence in general. When combined with similar spiking sensors that respond to other stimuli (e.g. sound, vibration, temperature, etc..), the academic and commercial communities should be able to realize complex sensing systems for a broad range of applications in artificial neural networks. Such a sensor is not only optimized for use with spiking neuromorphic processors but anticipated to have broad appeal from fields such as LIDAR (light detection and ranging), autonomous vehicles, facial recognition, navigation, and robotics.

Halide perovskites (HPs) has been conceived as resistive switching (RS) memory devices; this is due to their low operation voltage, high on/off ratio, and tunable band gap. However, the HP-based RS memory devices have several challenges owing to the short endurance of 350 cycles, data retention of 4×10³ s and stability of perovskite film.^1,2 Herein, 2D/3D halide perovskite heterointerface was made by a low-temperature all-solution process. The 2D/3D halide perovskite RS memory devices exhibits excellent performance with an endurance of 2700 cycles, a high on/off ratio of 10⁶, and data retention of 10⁴ s. The 2D halide perovskite layer could disturb the Ag ion migration into the 3D halide perovskite layer by calculated thermally-assisted ion-hopping activation energy and the results of the time-of-flight secondary ion mass spectroscopy. Moreover, the 2D halide perovskite layer controls the rupture of the Ag conductive filament. Therefore, the 2D halide perovskite layer enhances the RS memory properties by controlling both Ag migration and filament rupture. This study provides another strategy for improving memory properties of HPs based resistive switching memory devices.

Reference
1. E. Yoo, M. Lyu, J.-H. Yun, C. Kang, Y. Choi and L. Wang: Bifunctional resistive switching behavior in an organolead halide perovskite based Ag/CH3NH3PbI3−xClx/FTO structure. Journal of Materials Chemistry C 4, 7824 (2016).
2. J. Choi, S. Park, J. Lee, K. Hong, D.H. Kim, C.W. Moon, G.D. Park, J. Suh, J. Hwang, S.Y. Kim, H.S. Jung, N.G. Park, S. Han, K.T. Nam and H.W. Jang: Organolead Halide Perovskites for Low Operating Voltage Multilevel Resistive Switching. Advanced Materials 28, 6562 (2016).

Aromatic diammonium cation, x-(aminomethyl)pyridinium (AMPY, x = 3 or 4) can form 2D Dion-Jacobson lead iodide perovskite, which adopt the general formula of (xAMPY)(MA)_n-1Pb_nI_3n+1 (MA = methylammonium, n = 1−4). By modifying the position of the -CH₂NH₃⁺ group from 4AMPY to 3AMPY, the stacking of the inorganic layers changes from exactly eclipsed to slightly offset. The perovskite octahedra tilts are also different between the two series, with the 3AMPY series exhibiting smaller bandgaps than the 4AMPY series. Compared to the aliphatic cation of the same size (AMP = (aminomethyl)piperidinium), the aromatic spacers increase the rigidity of the cation, reduce the interlayer spacing and decrease the dielectric mismatch between inorganic layer and the organic spacer, showing the indirect but powerful influence of the organic cations on the structure and consequently on the optical properties of the perovskite materials. Preliminary solar cell devices based on the n = 4 perovskites as absorbers of both series exhibit promising performances. More interestingly, when using 1:4 ratio of AMPY:PbO as starting materials, totally different structures with 3D connection motif form instead. However, because of the steric requirement of the Goldschmidt tolerance factor rule, it is impossible for (xAMPY)M₂I₆ to form proper perovskite structures. Instead, a combination of corner-sharing and edge-sharing connectivity is adopted in these compounds leading to the new 3D structures. Devices of (3AMPY)Pb₂I₆ crystals exhibit clear photoresponse under ambient light without applied bias, reflecting a high carrier mobility (μ) and long carrier lifetime (τ) and great potential for photo and X-ray detection applications.

The champion efficiency of perovskite solar cells is over than 25%, which is great potential for practical application. Most of the high performance perovskite solar cells are based on the one step deposition with solvent engineering. Two-step deposition is easily controlled, and could be good for deposition of uniform and large area devices, while the efficiency of the device using two-step method is lag behind the one step. Here, we rationally designed the two-step method, and pushed the perovskite solar cells performance close to 25%.

Minimizing energy loss is a key aspect to transcend the current limitations on the performance of perovskite photovoltaics (PVSC). Our recent study combined composition and interface engineering to dramatically improve the efficiency and long-term operational stability of the PVSC. Moreover, a very innovative approach has also been developed to combine both efficient charge extraction and the ability to trap decomposed Pb-containing perovskites into one material to address the most challenging environmental contamination issue for the commercialization of PVSCs.

This talk will be divided into two parts: (1) Progress of achieving the highest efficiency (23.4%) in inverted perovskite solar cells (PVSCs) through smart engineering of the interfaces; (2) Demonstration of the simultaneous achievement of high PCE (>22%), excellent operational stability (85°C at MPP for 1000h under 1 Sun illumination), and the ability to capture degraded Pb-containing by-products by employing 2D metal-organic frameworks (2D-MOF) as a charge extraction layer in PVSCs.

Molecular design for improving and modifying the quality of the interfacial structures involved in carrier transports is becoming the major challenge towards high performance and stable perovskite solar cells (PSCs). Various approaches of interfacial engineering have been attempted in the past decade by using modulator molecules and mixing 2D and 3D structures to perovskites, as overviewed by our group as a comprehensive review.¹ These efforts lead to enhance the stability of the organic—inorganic hybrid perovskite devices, which have currently achieved a record efficiency of 25.5%. However, organic cations in hybrid perovskites (methylammonium, etc.) and use of diffusible ionic dopants in hole transport materials (HTMs) are responsible for low stability of perovskites at high temperatures (>120^oC). In this respect, use of all-inorganic perovskite materials and dopant-free HTMs will be a main direction of perovskite photovoltaics.² CsPbX₃(X=Br, I) is the widely studied candidate of the all-inorganic materials although the stability of its photo-active black phase depends on the halide composition and CsPbI₃ lacks in high stability. We have conducted the device fabrication using CsPbI₂Br as the perovskite absorber. The CsPbI₂Br film was made into junction with a solution-processed film of dopant-free HTMs which are copolymer materials and thermally stable up to 300^oC. The quality of interfacial structure at the CsPbI₂Br and HTM junction was improved by inserting an ultra-thin layer (<3 nm) of amorphous SnOx. The all-inorganic and dopant-free PSCs thus fabricated yielded a power conversion efficiency exceeding 15%. Further we could enhance the open-circuit voltage (Voc) of this PSC up to 1.42V as the highest Voc ever achieved with a single perevskite cell.³ In preparation of all-inorganic perovskites, a big challenge should also be directed to development of lead-free perovskite materials for environmental safety in practical applications.²
All-inorganic dopant-free PSCs are expected to have robust stability against high temperatures. Among extensive applications, use of thermally robust PSCs in space environments is promising because the device is exposed to temperatures in the range of —100 to +100^oC. Further, thin perovskite photovoltaic films demonstrate high stability and tolerance against exposure to high energy particle irradiations (proton and electron beams).^4,5 In the near future, perovskite photovoltaics will undergo more research for space applications.

References:
1. A. K. Jena, A. Kulkarni, and T. Miyasaka, Chem. Rev. 2019, 119, 3036–3103.
2. T. Miyasaka, A. Kulkarni, G. M. Kim, S. Oez and A. K. Jena, Adv. Energy Mat., 2019, 1902500.
3. Z. Guo, T. Miyasaka, et al. J. Am. Chem. Soc. 2020, 142, 21, 9725–9734.
4. Y. Miyazawa, M. Ikegami, H.-W. Chen, T. Ohshima, M. Imaizumi, K. Hirose, and T. Miyasaka, iScience 2018, 2, 148-155.
5. S. Kanaya, G. M. Kim, M. Ikegami, T. Miyasaka, K. Suzuki, Y. Miyazawa, H. Toyota, K. Osonoe, T. Yamamoto and K. Hirose, J. Phys. Chem. Lett., 2019, 10, 6990-6995.

Over the past decade heavy investment in understanding perovskite solar cells have led to high power conversion efficiency (PCE) regular (n-i-p) structure devices fabricated from low cost, scalable methods. In recent years, interest in the inverted structure (p-i-n) perovskite solar cells due to suppressed hysteresis effects, lower temperature processing, flexibility, and, most importantly, applications in tandem structures. However, the device performance of inverted structure devices lags behind, likely due to added recombination at one or both of the perovskite/charge transport layer (CTL) interfaces. Here, we investigate passivation of both of these interfaces to reduce the interfacial recombination in the inverted planar perovskite solar cell. For this study we use fully solution processed Poly[bis(4-phenyl) (2,5,6-trimethylphenyl) amine (PTAA) hole transport layer (HTL) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM)/Bathocuproine (BCP) electron transport layer (ETL). For the absorber perovskite material, we use a triple cation and mixed halide material with composition Cs_0.05(FA_0.83MA_0.17)_0.95PbI_0.83Br_0.17 and 1.62 eV bandgap. For the passivation layer we use alumina (Al₂O₃) deposited via atomic layer deposition. We fabricated devices with and without alumina at the HTL/perovskite, perovskite/ETL and both CTL/perovskite interfaces. The unpassivated devices show Voc of 1.08 V, fill factor (FF) of 72.1% and PCE of 15.4%. The Voc does not change when the alumina is included at the HTL/perovskite interface, but the FF improves to 74.8%, leading to a PCE of 15.7%. When less than 1nm of alumina is added at only the perovskite/ETL interface, the Voc does not change, but the FF and PCE increase to 75.8% and 16.2%, respectively; however, when the alumina thickness is 1-1.6 nm, the V_OC, FF and PCE increase to 1.10 V, 76.0% and 16.5%, respectively. When 1-1.6 nm of alumina is included at both CTL/perovskite interfaces, the performance increases further, with champion devices exhibiting PCEs of 18.3% with Voc of 1.11 V and FF of 76.8%. In addition, the passivated devices show less hysteresis than unpassivated devices. These results demonstrate that the interface between the CTL and perovskite affects the devices performance, and that thin layers of inorganic materials can be used to reduce the interfacial recombination. In addition, incorporation of an inorganic layer at both perovskite interfaces may lead to an encapsulation-like effect and reduce degradation of the devices through several mechanisms, including limiting water ingress. The device performance is being monitored to investigate if this is the case. The results show a promising next step toward developing high PCE and stable inverted perovskite solar cells via interface passivation.

Organic-inorganic halide perovskites (OIHPs) has emerged as the most promising light-absorber materials in the photovoltaic community due to their near-ideal bandgaps. However, the low formation energies of OIHPs render them unstable. While significant progress has been made in improving the stability of OIHPs, perovskite solar cells (PSCs) will also need to be mechanically reliable if they are to service satisfactorily for years. In this context, we study the fracture behavior of PSCs by measuring their cohesion energies (Gc) using double cantilever beam method and report a novel approach to strategically enhance the interfacial adhesion and performance of PSCs by interface modification. The reliability was greatly improved after interfacial toughening. This work points to a new route for designing mechanically robust PSCs with long-term durability.

Introduction
Tin-lead (Sn-Pb) perovskite solar cells (PSCs) are now getting great attention due to the rapid increase in photoconversion efficiency (PCE) more than 20%, which brings them near to their Pb counterparts [1]. They possess the bandgap of 1.2-1.3 eV, which is according to Shockley-Queisser (SQ) limit can give higher PCE than Pb-PSCs where bandgap lies in between 1.45-1.55 eV. Also, according to SQ limit the Voc of 0.9-1 eV is possible for the solar absorbing materials with a bandgap of 1.2-1.3 eV [2]. In our previous work, we have reported a PCE of 20.4% using Cs_0.025FA_0.475MA_0.5Sn_0.5Pb_0.5I₃ (1.27 eV) perovskite as the absorber layer in PSCs with a Voc of 0.81 V [3]. In the work, we revealed that lattice strain relaxation is one of the important properties to look for cation mixing at A position of ABX₃ (where, A is monovalent cation, B is bivalent cation and X is halide anion) perovskite. Therefore, following the same strategy, in the present research, we added a small fraction of Br anion with I anion at X position that led to further decrease in lattice strain and reduction of urbach energy that resulted into increase in Voc. In addition to this, we used Lewis base surface passivation that solved the two major problems of Sn-Pb perovskite, first is the reduction in formation of amount of Sn⁴⁺ and second is neutralization of positive surface of perovskite due to I anion vacancy. Further, it is shown that lewis base post treatment of tin-lead perovskite led to the evolution of n-type surface that resulted in increased electron diffusion length and charge collection in the device.
Results and discussion
The perovskite used in the study is Cs_0.025FA_0.475MA_0.5Sn_0.5Pb_0.5I_3-xBr_x (where x=0, 0.025, 0.05, 0.1). Four devices with different concentration of Br anion are prepared such as Br 0%, Br 2.5%, Br 5% and Br 10%. Device with Br2.5% showed a best PCE of 21.10% with a FF of 0.81, Voc of 0.84V and Jsc of 31.22 mA/cm². The change PCEs with Br concentration is well supported by UV, PL, SEM, XRD, XPS analysis of perovskite films. Finally, PSC with 22.64% (highest so far in Sn-Pb) after optimization will be reported.
References
K. Xiao and H. Tan et al., Nature Energy, 2020, 5, 870-880.
W. Shockley, H. J. Queisser, J. Appl. Phys. 1961, 32, 510.
G. Kapil and S. Hayase et al., ACS Energy Letters, 2019, 4, 1991-1998.

2D-layered perovskites form assembled natural quantum well structures, where the perovskite layer is separated by large organic cations. They have been known for decades, but they are attracting growing interest, for their strong photoluminescence properties, their chemical versatility and their low sensitivity to external degradation mechanisms due to UV light and moisture. In particular, the chemical versatility of the organic part is a decisive advantage compared to their 3D counterparts.
Energy matching and band alignment engineering between the organic/inorganic parts can either be done by tuning the halide composition, by modifying the number of haloplumbate layers or by adjusting the HOMO/LUMO gap and/or levels of the organic spacer, that is to say by incorporating optically active molecules in the perovskite structure. In this case, instead of playing a simple passive barrier role, the organic part can be used to engineer the optoelectronic properties to these materials. Depending on the optical matching, defined for hybrid perovskites as the spectral overlap between the perovskite and organic absorbers optical transitions, along with the type I/II of interface, charge and/or energy transfers can be induced between the inorganic and organic moieties
We have synthesised two novel layered perovskites containing optically active s-tetrazine (R-C₂N₄-R) as sole included organic spacer. Thanks to the various energy resonances between the inorganic and the organic component, both at the level of the single particle electronic states and at the level of many-body exciton states, this system represents the ideal test case to discuss in the detail charge and energy transfer processes at the type II interface. Furthermore, the incorporation of this novel chromophore as spacer is based on a new design concept, which exploits heterocycles with large fraction of nitrogen, rather than extending the size of carbon-based pi-core. We further performed extensive optical characterization supported by computational studies, in order to gain a complete picture of all potential charge and energy transfer processes ongoing in this complex perovskite-luminophore system. More specifically, both excited states as well as band alignment were studied to rationalize the complete quenching of the perovskites light-emission. We conclude that several decay channels may coexist between the s-tetrazine and the perovskite frame, and highlight the necessity of depicting these materials at the level of both excitonic and monoelectronic states.
Reference : F. Lédée et al, Materials Horizons 2021, https://doi.org/10.1039/D0MH01904F

Fully inorganic cesium lead halide nanocrystals exhibit giant oscillator strength and long dephasing times at cryogenic temperature, making them ideally suited to explore settings that make use of their exceptionally strong intrinsic light-matter coupling. Through drying-mediated self-assembly of the colloidal perovskite nanocrystals, we can realize various superlattice geometries.
In these assemblies, we observe coherent, cooperative emission so-called superfluorescence. There the quantum dots spontaneously synchronize, thereby forming effectively a giant dipole, and emit an intense burst of light. We will discuss the characteristic signatures of excitation-dependent decay acceleration, superlinear burst intensity and Burnham-Ciao ringing within the different assemblies. Furthermore, we will compare the behavior in these assemblies to giant nanocrystals.
Our experiments demonstrate that in contrast to many other quantum dot systems, cesium lead halide nanocrystals can be made with sufficiently small energy spread to efficiently couple many of them and explore collective optical properties. Superlattices of these nanocrystals are a versatile tool that opens a new door to investigate these effects.

The 2D halide perovskites layers can be formed by the introduction of bulky organic cations that tend to interdigitate between the perovskite sheets. It is a multi-member family of materials with variable thickness of the inorganic slabs presents natural quantum wells (QW) type structures with increased Coulombic shielding, lowering the exciton binding energy and increasing the effective charge carrier mobility. These materials are enabling higher solar cell stabilities and record-breaking efficiencies. It is important to understand the thermodynamic and chemical limitations of their structures and how they vary from one 2D member to the next. We employ a combination of synthetic chemistry, calorimetry and crystallography to obtain and identify the thickest known 2D halide perovskite compound characterized to date and we use thermodynamics analysis to assess the enthalpy of formation for these compounds. The current application of these low dimensional semiconductors in stable photovoltaics will be discussed.

Halide perovskites provide a flexible platform for tuning optoelectronic properties through changes in composition, connectivity, and dimensionality. I will present our recent studies on further increasing the accessible electronic structures of this versatile family of materials by synthesizing mixed-metal perovskites and expanded analogs of halide perovskites. Through both stoichiometric and substoichiometric metal substitutions we can synthesize a variety of halide perovskites, including those with unprecedented lattices and unusual electronic structures. I will also discuss our recent efforts on expanding the cavities in typical 3D perovskites to incorporate more complex organic ions, including aromatic molecules with low-lying states that form the conduction band and organic radicals that add spins to the lattice.

Hybrid organo-metal halide perovskite solar cells (PSCs) are promising candidates for next generation photovoltaic device primarily due to their high efficiency, printability and low cost. PSCs have exhibited externally verified power conversion efficiencies (PCE) exceeding 25% outclass from 3.8% in 2009, which have encouraged recent efforts on scalable coating technique in PSCs towards manufacturing. However, devices fabricated by scalable techniques are still lagged the state-of-the-art spin coated devices because the power conversion efficiency (PCE) is highly dependent on the morphology and crystallization kinetics under a controlled environment, and delicate solvent system engineering.

In this talk, we will present the recent works using in-situ technique to guide the development of high performance PSCs using blade coating technology.
(a). Via a laminar air-knife assisted room temperature meniscus coating approach to control the drying kinetics during the solidification process, we recently manufacturing friendly, antisolvent-free room-temperature coating of hysteresis-free PSCs with power conversion efficiency (PCE) of 20.26% for 0.06 cm2 and 18.76% for 1 cm2 devices. Moreover, this approach offers a solid model platform for in-situ UV-vis and microscope investigation of the perovskite film drying kinetics.
(b) One step further, based on the widest studied champion perovskite solution system DMF-DMSO, we report air-knife assisted fabrication of perovskite solar cell in AMBIENT condition at high relative humidity of 55±5%. In-depth in-situ time-resolved UV-vis spectrometry is carried out to investigate the impact of solvent removal and crystallization rate, which are proven to be a critical factor on influencing the crystallization kinetics and morphology due to moisture attack. The UV-vis spectrometry also enables accurate determination of wet precursor film thickness – a key parameter for fabrication. Anti-solvent free, high humidity ambient coating of hysteresis-free PSCs with PCE of 21.1% and 18.0% are demonstrated for 0.06 cm² and 1 cm² devices, respectively. These PSCs coated in high humidity ambient conditions also show comparable stability with those made in N2 glovebox.
(c) The room temperature meniscus coating of high-quality perovskite films incorporated with a multifunctional sulfobetaine based zwitterionic surfactant. Systematic in-situ studies uncover the perovskite crystallization pathway and emphasize the surfactant's synergistic role in film construction, crystallization kinetics modulation, defect passivation, and moisture barrier protection. This strategy is applicable across perovskite compositions and device architectures with the enhanced power conversion efficiencies up to 22%. In addition, this approach significantly improves the stability of perovskite films and devices under different aging conditions.

REFERENCES
[1] H Hu, Z Ren, PWK Fong, M Qin, D Liu, D Lei, X Lu, G Li*. Room Temperature Meniscus Coating of> 20% Perovskite Solar Cells: A Film Formation Mechanism Investigation. Advanced Functional Materials, 1900092 (2019).
[2] PWK Fong, Gang Li. Under review.
[3] K. Liu et al. Zwitterionic Surfactant Assisted Room Temperature Coating of Efficient Perovskite Solar Cells. 2020. https://doi.org/10.1016/j.joule.2020.09.011

The interaction of halide perovskite and colloidal semiconductor nanostructures (quantum dots or nanoplatelets) can produce interesting synergistic interactions. We show that the interaction of PbS quantum dots and nanoplatelets can produce the stabilization of FAPbI₃ and FACsPbI₃ perovskite black phase and also the increase of the efficiency, stability and reproducibility of the photovoltaic devices prepared with these halide perovskites. Incorporation of PbS QDs allows the dramatic decrease of the annealing temperature for the formation of black FAPbI₃ phase perovskite thin film, from the 170C required without QDs to 85C when QDs are present. In addition, stability of these systems including embedded nanostructures is extended not just for samples prepared in the glove box but fabricated in ambient conditions. These result points the interest of Perovskite-Quantum Dot Nanocomposites, for further development of advanced optoelectronic devices.
In addition, a new methodology to analyze halide perovskite outdoor will be described. We will show that this methodology complement the analysis obtained with the figure of merit T₈₀, parameter that refers to the time at which the device reaches 80% of its initial rated power, providing further physical insight into the recombination mechanism dominating the performance, improving the understanding of the degradation processes and device characterization.

Engineering two-/three- dimensional (2D/3D) perovskite solar cells is nowadays a popular strategy for efficient and stable perovskite solar cells ^1-3.
However, the exact function of the 2D/3D interface in controlling the long-term device behavior and the interface physics therein are still obscure.
Here I will discuss the 2D functions which can simultaneously act as surface passivant, electron blocking layer, a sheath to physically protect the 3D underneath, but also impact on the ion movement and charge accumulation. We found a peculiar dynamical structural mutation happening at the 2D/3D interface: the small cations in the 3D cage move towards the 2D layer, which acts as an ion scavenger. If structurally stable, the 2D physically blocks the ion movement at the interface boosting the device stability. Otherwise, the 2D embeds them, dynamically self-transforming into a quasi-2D structure. ²
In concomitance, we discovered that the stable 2D perovskite can block ion movement, improving the interface stability on a slow time scale. ^2,4
The judicious choice of the 2D constituents is decisive to control the 2D/3D kinetics and improve the device lifetime, but also can impact on the interface energetics, which can vary and influence the interface processes and ultimately device open circuit voltage. This knowledge turns fundamental for device design, opening a new avenue for perovskite interface optimization.
References
[1] J.-P. Correa-Baena et al., Science 358, 739–744 (2017).
[2] A. Sutanto et al. J. Mater. Chem. A 8, 2343-2348 (2020).
[3] V. Queloz et al. J. Phys. Chem. Lett. 10, 19, 5713-5720 (2019).
[4] A. Sutanto et al. Nano Lett 20, 3992-3998 (2020)
Acknowledgements I acknowledge the “HY-NANO” project that has received funding from the European Research Council (ERC) Starting Grant 2018 under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 802862).

We report on the scalable open-air fabrication of large-area perovskite solar modules comprising several key advances in the field including scalable large-area spray deposition, new scribing techniques for monolithic integration, advanced optoelectronic characterization using photoluminescence, and a demonstration of reproducibility and high-throughput manufacturability across multiple module batches and open-air process environments. The Rapid Spray Plasma Processing (RSPP) method enables crystallization of perovskite films in ambient with linear speeds of 12 m/min without a post-anneal. RSPP is a moisture-resistant process with better device performance, higher luminescent yield over the largest reported field of view in literature, and carrier lifetimes > 10X than spin coated perovskite to achieve the highest device performances for open-air perovskite deposition. Manufacturability and reproducibility of RSPP is shown using monolithic integration of series-connected modules with 17 subcells on active areas of 5.9 cm² with a rapid fiber laser scribing method. This low-cost, 1064 nm laser scribing process using a unique indirect ablation mechanism has not been demonstrated in literature and is fully compatible with all scribing processes for monolithic integration. A stable cell and module power output of 18.0% and 15.5%, respectively, was achieved with a subcell V_oc > 1.06 V that is reproducible and significantly outperforms spin-coated modules. A comprehensive and complete technoeconomic model for the full 9-step in-line perovskite manufacturing process starting with the glass substrate and ending with the junction box of the encapsulated module is used to calculate the module manufacturing cost, to estimate the balance of system costs and to determine a levelized cost of energy for a range of module efficiencies and lifetimes. The analysis provides novel insights and guidelines for necessary tool speeds, efficiencies, and lifetimes to enable the viability of perovskite module manufacturing for utility-scale energy generation.

Two-dimensional (2D) hybrid halide perovskites with the formula (A')₂(A)_n-1Pb_nI_3n+1 have remarkable stability and promising efficiency in photovoltaic and optoelectronic devices, yet fundamental understanding of film formation, key to optimizing these devices, is lacking. In particular, understanding the processes that lead to the perpendicular orientation of the layers in relation to the substrate, essential for charge carrier extract, and the phase distribution, which has been shown to have a beneficial carrier cascade effect, is needed. Here, in-situ grazing-incidence wide-angle X-ray scattering (GIWAXS) is used to monitor film formation during spin-coating.[1] This elucidates the general film formation mechanism of 2D halide perovskites during one-step spin-coating. There are three stages of film formation: sol-gel, oriented 3D, and 2D. Three precursor phases form during the sol-gel stage and transform to perovskite, first giving a highly oriented 3D-like phase at the air/liquid interface followed by subsequent nucleations forming slightly less oriented 2D perovskite. Furthermore, heating before crystallization leads to fewer nucleations and faster removal of the precursors, improving orientation. This outlines the primary causes of phase distribution and perpendicular orientation in 2D perovskite films and paves the way for rationally designed film fabrication techniques.

References

[1] Hoffman, J. M.; Strzalka, J.; Flanders, N. C.; Hadar, I.; Cuthriell, S. A.; Zhang, Q.; Schaller, R. D.; Dichtel, W. R.; Chen, L. X.; Kanatzidis, M. G. Adv. Mater., 2020, 32, 2002812.

The difficulty in fabricating lightweight, stable, and efficient photovoltaic devices is compounded when in done in situ during space missions. Popular deposition techniques like spin coating and drop casting require gravity, making them useless in orbit. To overcome this problem, we have developed a thin film preparation technique utilizing electrohydrodynamic deposition (EHD). This technique entails drawing bi-solvent perovskite precursor solution from a source needle to substrate via an applied electric field. The difference in vapor pressure between the solvents drives surface coverage and film formation in a phenomenon known as Marangoni flow. Thus, EHD is not only a versatile and scalable method, but uniquely suited to our goal of developing stable and high-efficiency thin film devices without assistance from a gravitational field.

In addition to the deposition technique, other factors, including the chemical composition of the absorbing layer, selection of charge transport layers, and thickness of each layer, are among the dozens of choices that must be made when making a photovoltaic device. To best direct our research, metrics such as power conversion efficiency (PCE), annealing temperature, and long-term stability were extracted from literature published during between 2012 and the present. These variables were then analyzed to find useful correlations and compositions among high-performing cells.

A key trend among devices is the moderate correlation between number of cations in a cell and its efficiency. While this may not be surprising, we found that the efficiency of methylammonium/formamidinium (MA/FA) lead perovskite cells was comparable to, or better than, cesium/MA/FA (Cs/MA/FA) cells. Since Cs greatly complicates fabrication and is generally added for increased stability (unnecessary for cells in space), we choose to focus on MA, followed by MA/FA films. Another influential finding was that cells with high PCE used tin oxide as their electron transport layer (ETL), which will also be incorporated in our devices. Other key findings include interesting trends regarding the relative roles of open circuit voltage versus short circuit current, and the halide composition of the perovskite.
This talk will focus on the EHD process, the data analytics, and our results in thin film fabrication.
*This work was supported by NASA grant no. NNX15AQ01

Chalcogenide CdTe and Cu(In,Ga)(S,Se)₂ (CIGS) and halide-perovskite-related (e.g. CH₃NH₃PbI₃) semiconductors represent the fastest growing commercial and the most vigorously studied thin-film photovoltaic (PV) technologies, respectively. Alternatives to these semiconductors are also being sought to mitigate toxicity and elemental abundance concerns, for improved compatibility with pervasive scaling of the technologies. Inherent to these pursuits, is the need to understand materials and device limitations, especially as they relate to defects. In this talk, we will consider additive engineering and doping in CH₃NH₃PbI₃-related semiconductors [1,2], as well as proposed defect-resistant Cu₂Ba(Sn,Ge)(S,Se)₄ materials [3,4], and employ a range of characterization techniques, including carrier-resolved photoHall, conventional Hall effect, terahertz spectroscopy and other time-resolved spectroscopies, to understand associated properties and performance-limiting or -enhancing mechanisms.

References:
[1] O. Gunawan, S. R. Pae, D. M. Bishop, Y. Virgus, J. H. Noh, N. J. Jeon, Y. S. Lee, X. Shao,
T. Todorov, D. B. Mitzi, B. Shin, Nature 575, 151 (2019).
[2] J. Euvrard, O. Gunawan, D. B. Mitzi, Adv. Energy Mater. 9, 1902706 (2019).
[3] D. Shin, B. Saparov, D. B. Mitzi, Adv. Energy Mater. 7, 1602366 (2017).
[4] G. C. Wessler, T. Zhu, J.-P. Sun, A. Harrell, W. P. Huhn, V. Blum, D. B. Mitzi, Chem. Mater. 30, 6566 (2018).

The solar cells based on the emerging organic-inorganic hybrid halide perovskite is progressing rapidly in the last decade, but the commercialization of the perovskite solar cells still faces significant challenges. The major issues are the poor long-term stability and toxicity. In light of this, materials design and screening of novel stable perovskites is becoming an important research direction. In contrast to conventional trail-and-error processes, materials genome techniques and high-throughput calculations have played important roles in this area and accelerated materials discovery. In this talk, I will present a review that summarizes our recent progress in this field, mainly focusing on four classes of perovskites including AM²⁺X₃ halide single perovskites, AM⁴⁺Y₃ chalcogenide single perovskites, A₂M⁺M³⁺X₆ halide double perovskites, and A₂M³⁺M⁵⁺Y₆ chalcogenide double perovskites. The stability, electronic, optical, and defect properties of these materials in terms of their applications as solar cell absorbers are discussed.

Majority and minority carrier properties such as type, density and mobility represent fundamental yet difficult to access parameters governing semiconductor device performance, most notably solar cells. Obtaining this information simultaneously under light illumination would unlock many critical parameters such as recombination lifetime, recombination coefficient, and diffusion length, however this goal has remained elusive. We demonstrate here a new carrier-resolved photo-Hall technique that rests on a new equation relating hole-electron mobility difference (△μ), Hall coefficient (H), and conductivity (σ), and a rotating parallel dipole line ac-field Hall system with Fourier/lock-in detection for clean Hall signal measurement (Nature 575, 151, 2019). We successfully apply this technique to a world-record-quality perovskite film and map the results against varying light intensities, demonstrating unprecedented simultaneous access to the above-mentioned parameters.

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.

PV-powered vehicle application are very attractive for reducing CO₂ emission and creation of new market. Development of high-efficiency (> 30%) and low-cost solar cell modules is necessary [1]. The II-VI compound, chalcopyrite and kesterite based tandem solar cells are thought to be one of the most promising PV devices because of high-efficiency and low-cost potential. However, efficiencies of II-VI compound, chalcopyrite and kesterite based tandem solar cells (27.2% with III-V/CIGS 3-junction, 24.2% with perovskite/CIGS 2-junction, 16.8% with CdZnTe/Si 2-junction, 15.3% with CdTe/CISe 2-junction, 8.5% with CGSe/CIGSe 2-junction, 3.5% with CZTS/Si 2-junction tandem solar cells) are lower compared to 37.9% with III-V 3-junction and 35.9% with III-V/Si 3-junction tandem solar cells. Therefore, it is necessary to clarify and reduce several losses of tandem solar cells. This paper presents high efficiency potential of II-VI compound, chalcopyrite and kesterite based tandem solar cells analyzed by using our analytical procedure [2] and discusses about non-radiative recombination losses in those tandem solar cells.
One of problems to attain the higher efficiency II-VI compound, chalcopyrite and kesterite based tandem solar cells is to reduce non-radiative recombination loss. The open-circuit voltage V_oc drop compared to bandgap energy (E_g/q - V_oc) is dependent upon non-radiative voltage loss (V_{oc, nrad}) that is expressed by external radiative efficiency (ERE). Open-circuit voltage is expressed by

where the second term shows non-radiative voltage loss, is radiative open-circuit voltage and 0.23V was used as △ (= E_g/q - ) in this study.
Correlation between V_oc values for II-VI compound, chalcopyrite and kesterite based tandem solar cells reported in the references and ERE values estimated by eq. (1) suggests that II-VI compound, chalcopyrite and kesterite based tandem solar cells have larger non-radiative loss and further improvements in efficiency are thought to be possible by improving external radiative efficiency (ERE). The ERE values estimated in this study are 0.00116% for 27.2% III-V/CIGS 3-junction, 0.024% for 24.2% perovskite/CIGS 2-junction, 0.00162% for 16.8% CdZnTe/Si 2-junction, 1.17x10^-5% for 15.3% CdTe/CISe 2-junction, 5.87x10^-8% for 8.5% CGSe/CIGSe 2-junction, and 8.05x10^-10% for 3.5% CZTS/Si 2-junction tandem solar cells compared to 2.2% for 37.9% III-V 3-junction and 1.62% for 35.9% III-V/Si 3-junction tandem solar cells. According to our analysis [3], the 2-junction and 3-junction tandem solar cells have potential efficiencies of more than 36% and 42%, respectively.
Several non-radiative recombination losses such as bulk recombination, interface recombination and so forth in tandem solar cells are discussed in this paper. In addition, resistance loss of tandem solar cells including inter-connection is discussed in this study.

References
[1] M. Yamaguchi et al., Prog. Photovolt. (2020) https://doi.org/10.1002/pip.3343.
[2] M. Yamaguchi et al., J. Mater. Res. 32, 3445 (2017).
[3] M. Yamaguchi et al., J. Phys. D. Appl. Phys. 51, 133002 (2018).

Organic-inorganic hybrid halide perovskite solar cells (HPSCs) have attracted immense attention because of excellent optoelectronic properties and a record power conversion efficiency (PCE) that has surpassed 25.5% within a few years. HPSCs are competitive with conventional thin-film and c-Si photovoltaic technologies because of their high efficiency, tunable bandgap, high absorption coefficient, and easy and low-cost fabrication. However, despite the very high efficiency already attained by (FA,MA,Cs)Pb(I,Br)₃ single-junction solar cells, the toxicity of Pb and stability of these materials are challenges to commercialization of this technology. Many Pb-free technologies have been developed such as GeI₂ doped Sn based perovskites with promising PCE > 13%. However, a major challenge for Pb-free perovskites is poor power conversion efficiencies as divalent Sn²⁺ and Ge²⁺ tend to oxidize into 4+ oxidation state, generating excessive defects (Sn and Ge vacancies), and result in short carrier diffusion lengths.

Cs₂PtI₆ is a high absorption coefficient and optimal bandgap (E_g = 1.4 eV) material, which has shown promising power conversion efficiency of > 12%[1]. The material has shown high absorption coefficient, carrier lifetime > 2 μs and stability to heat and light soaking stress. ITO/SnO₂/Cs₂PtI₆/PCBM/Carbon devices have also shown to be stable in air without encapsulation for 60 days[2]. Hamdan and Chandiran have demonstrated Cs₂PtI₆ photoanode to be stable under AM1.5G illumination and extreme pH for over 12 hours, making the material promising for photoelectrochemical devices[3]. Although Pt offers an ideal model system, the search for a low-cost, stable, and high performance Pb-free halide perovskite continues. Replacement of Pt with Ni for A₂BX₆ is expected to reduce the bandgap with the reduction in cation size[4]. Here, we explore the effect of Pt precursor, solvents, and Pt-Ni solid solution on optical bandgap, absorption coefficient, structure, and morphology of Cs₂PtI₆ films.

Cs₂PtI₆ films processed with PtI₄ precursor in a solvent mixture of DMF:DMSO[5] results in crystalline films with an optical bandgap of ~ 1.4 eV. PtI₂ precursor processed with a solvent mixture of DMF:GBL also results in structure and optical bandgap albeit with higher film porosity and preferred crystallographic orientation. Both these processes result in low oxygen content in the films as confirmed by EDS and XPS. On the other hand, films processed with PtI₂ precursor in DMF:DMSO solvent mixture results in oxygen containing films with a bandgap measured as 2.2 eV using UV-Vis-NIR and photoluminescence spectroscopy. Intermixing of B cation with a 50 at% of Pt and 50 at% of Ni results in an optical bandgap of 1.48 eV with the crystal structure of Cs₂PtI₆. Detailed composition analysis shows oxygen is incorporated at anion site resulting in a mixed halide-chalcogenide phase with a tunable bandgap with oxygen concentration. Cs₂(Pt_xNi_1-x)(I_yO_z)₆ offers an interesting material with tunable bandgap for optoelectronic and sensor applications.


[1] Schwartz, D.; et al. Phys. Status Solidi Rapid Res. Lett. 2020, 14, 2000182.
[2] Yang, S.; et al. ACS Appl. Mater. Interfaces 2020, 12, 40, 44700–44709.
[3] Hamdan, M.; Chandiran, A. K. Angew. Chem., Int. Ed. 2020, 59 (37), 16033– 16038.
[4] Cai, Y.; et al. Chem. Mater. 2017, 29, 7740– 7749.
[5] Abbreviations: DMF = Dimethylformamide, DMSO = Dimethyl sulfoxide, GBL = γ-Butyrolactone, EDS = Energy-dispersive X-ray spectroscopy, XPS = X-ray photoelectron spectroscopy, UV-Vis-NIR = Ultraviolet-Visible-near-Infrared

Recently organic–inorganic halide perovskite solar cell have great attention for its high performance together with easy production and wide variety of the process & flexibility of substrate materials. The power conversion efficiency has already reached over 25% in 2020 much beyond another solar cells such as CIGS or amorphous Si. The further performance still looks promising toward the Shockley–Queisser limit at around 30%. For that purpose, physical chemistry understanding based on the crystallography must be essential to design the good light harvesting, good charge separation and good charge transfer.

Recently we reported the scientific revelation that the crystal phase of thin film CH3NH3PbI3 consists of the mixture of tetragonal phase and cubic phase. Moreover bold zebra patten with d-spacing with 14.2Å (2θ=6.22° for CuKα) was clearly observed by high resolution TEM with FIB processing that consists with tetragonal and cubic phase superlattice. To make more high performance, here the crystal phase control with liquid nitrogen quenching just after the hot plate heating was newly examined. The resulting cell performance was impressive, about 2% more than that without Liq. N2 treatment. For such treatment the detail about the TEM observation will also be discussed.

Metal halide perovskites (MHPs) have significant potential to revolutionize optoelectronics yielding the next generation of highly efficient, low cost devices including solar cells, light-emitting diodes, and photodetectors. However, a new materials engineering approach is required to realize the full potential of these compounds, since MHPs have a broad range of compositions and crystal structures that yield a multitude of diverse and yet unexplored property combinations. Currently, the primary concern at the forefront of scientific and application-driven research is the stability of these materials in the pure or device-integrated form.
An automated experimental workflow based on guided combinatorial synthesis and rapid throughput characterization has been developed to explore properties and stability of metal halide perovskite systems in ambient conditions [1]. We also develop a machine learning-based workflow to quantify the evolution of each system as a function of composition based on overall changes in photoluminescence spectra, as well as specific peak positions and intensities. We will discuss the stability dependence on composition and extremely non-uniform behavior within the composition space. The interpolative regression analysis of photoluminescent properties helps to distinguish mixtures that form solid solutions from those that segregate into multiple materials in ambient conditions. In addition, the analysis of bandgap energy provides key information about the optimal compositions for certain applications. This workflow is a necessary step to guide the material synthesis and to gain an insight into the intrinsic materials stability by studying the complex phase diagram of mixed perovskites. The workflow allows not only to explore large composition spaces, but also identify stability behavior. Furthermore, it is universal, simple and can be applied to other perovskite systems and solution-processable materials and can be implemented at low cost as compared to systems reported previously, a necessary condition of rapid adoption of these approaches. To further explore the materials level stability, we have studied several compositions of MHPs using time of flight secondary ion mass spectroscopy (ToF-SIMS) under operation condition in a laterally designed electrode configuration [2,3]. To interpret the multidimensional data obtained from such experiments, we develop a machine learning workflow combining the Hough transform and non-negative matrix factorization and non-negative tensor decomposition, to avoid this limitation and extract salient features of associated chemical changes and to separate the light- and voltage-dependent dynamics. Combining these in-situ characterizations and this machine learning workflow provides comprehensive information on the chemical nature of moving species, ion accumulation, and interfacial electrochemical reactions in metal halide perovskites devices, feeding back into automated discovery process.
References:
[1] Higgins, K., Valleti, S. M., Ziatdinov, M., Kalinin, S. V., Ahmadi M. (2020) ACS Energy Letters, 5, 3426-3436
[2] Higgins, K., Lorenz, M., Ziatdinov, M., Vasudevan, R. K., Ievlev, A. V., Lukosi, E. D., Ovchinnikova, O. S., Kalinin, S. V., Ahmadi, M. Advanced Functional Materials (2020) 30, 2001995.
[3] Liu, Y., Borodinov, N., Lorenz, M., Ahmadi, M., Kalinin, S. V., Ievlev, A. V., Ovchinnikova, O. S. Advanced Science (2020) 7, 2001176.

Material synthesis is a very important aspect in human kinds endeavor to discover and create new materials for energy applications. In order to make materials with desired functions we need to know how these functions relate to structure, synthetic variables, arrangement of atoms and molecules and how functions evolve during synthesis.

In this regard, the field of halide perovskites moved towards more and more complex compositions enabling improved device performance and stability. Most of the improvements however, are chemical intuition driven or achieved through empirical optimization of processing conditions. Establishing the relationships between synthesis condition and film properties will enable active control of synthesis parameters and increase reproducibility. The formation of halide perovskites from colloidal precursors including the initial stages of formation and the physicochemical evolution of properties via polydisperse nanocrystal nucleation and solvent-complexation will be discussed. We identified for example how the precursor and antisolvent influence the crystallization pathway, how morphology can be templated, and how additives can aid room temperature processing. By correlating diffraction and photoluminescence (PL) measurements, it will be demonstrated how in situ PL can reveal subtle changes throughout all synthesis steps including the onset of film decomposition.

Multicomponent perovskite solar cells have reached the recent efficiency breakthrough of 25.5%, higher than silicon polycrystalline photovoltaics. Such fantastic result was only possible due to a precise control and engineering of the morphology, interfaces and the use of multiple cations in perovskite A-site, as Rb, Cs, MA (methylamonnium) and FA (formamidinium). For tandem perovskite solar cells, a mixture of different anions, as Br and I is also desired to adjust the band gap. Such cocktail of different cations and anions influences the formation of intermediates, new phases, favours halide homogenization, etc; so that at the end, the efficiency of the device is closely related to not only the optical quality of the film (e.g. crystallinity), but morphology and composition.
In this presentation, we will summarize important results using in situ experiments to probe perovskite formation (2D and 3D), stability and composition. We employed time-resolved grazing incidence wide angle x-ray scattering (GIWAXS), small angle x-ray scattering (SAXS) and high-resolution XRD taken at the Brazilian Synchrotron National Laboratory and SSRL-Stanford.
In situ GIWAXS experiments allowed us to understand the influence of the solvents, time to initiate the crystallization, humidity in the preparation of perovskite films and to get important information about final composition and morphology [1]. It is well known that a 2D layer on the top of a 3D bulk perovskite improves stability and performance. In situ GIWAXS revealed us that during thermal annealing the 2D layer transforms itself into a disorder layer, improving hole transfer and stability [2]. This technique was also employed to identify the first intermediates formed during the degradation of different Cs and Br perovskite compositions under ambient conditions [3].
In situ SAXS is another powerful technique to follow the first stages of the 2D perosvkite’s formation. Our results suggest that the formation of the individual slabs in BA₂[FAPbI₃]PbI₄ is quite fast (within the first 10 s) and, then, these slabs self-assemble into bulk crystallites during the next 40 minutes [4].
Another topic of interest is the photoinduced phase segregation in FACs-based perovskite films. We investigated this phenomenon by photoluminescence (PL) studies and in situ high-resolution XRD (dark and under illumination). We found out that cubic-tetragonal transition is able to stabilize the samples with higher Br and Cs content, however this is not true for the films with 17% Br. Thus, other mechanisms must be operating and ruling phase segregation [5].
[1] R. Szostak,et. al, “Revealing the perovskite film formation using the gas quenching method by in situ GIWAXS: morphology, properties and device performance”, Advanced Functional Materials, in press, 2020 (DOI: 10.1002/adfm.202007473)
[2] A. Sutanto, R. Szostak, et. al., “In Situ Analysis Reveals the Role of 2D Perovskite in Preventing Thermal-Induced Degradation in 2D/3D Perovskite Interfaces", Nano Letters, 20(5) 3992-3998 (2020)
[3] P. E. Marchezi, et. al., , “Degradation mechanisms in mixed-cation and mixed-halide Cs_xFA_1-xPb(Br_yI_1-y)₃ perovskite films under ambient conditions” J. Mater. Chem. A, 9, 9302-9312 (2020)
[4] R. F. Moral, et. al. “Synthesis of Polycrystalline Ruddlesden-Popper Organic Lead Halides and Their Growth Dynamics”, Chemistry of Materials, 31 (22) (2019), 9472-9479
[5] R.E Beal, A. F. Nogueira, et. al., “Structural Origins of Light-Induced Phase Segregation in Organic-Inorganic Halide Perovskite Photovoltaic Materials” Matter, v.2, Issue 1, (2020)

A complete understanding of the influence of the individual and combined effects of extrinsic (humidity and oxygen) and intrinsic (light, bias, and temperature) stressors on halide perovskite materials is key for the ultimate development of stable optoelectronic devices. Here, we present a suite of optical and electrical complementary tools that provide a detailed description of the dynamic responses within these materials. We unfold the impact of distinct humidity levels on charge carrier radiative recombination in CsxFA1−xPb(IyBr1−y)3 perovskites through in situ PL, where we temporally and spectrally measure light emission within loops of critical relative humidity (rH) levels. Our results demonstrate that the Cs/Br ratio strongly affects the spectral stability of light emission hysteresis, as well as its extent. The photo-emission dynamics in metal halide perovskites with both I and Br is also interrogated by environmental PL, where we find that the presence of Br suppresses hysteresis. We realize a machine learning (ML) approach based on supervised learning combined with environmental PL measurements and determine the (ir)reversible changes that takes place in MAPbBr₃ and MAPbI₃ model systems, and in state-of-the-art multication compositions such as Cs_0.05FA_0.79MA_0.16Pb(I_0.83Br_0.17)₃ and (Cs_0.07Rb_0.03FA_0.76MA_0.14Pb(I_0.85Br_0.15)₃. We further resolve transient processes such as ion motion by spatially resolving the local photovoltage and photocurrent at the nanoscale. Here, we quantify how the addition of Rb reduces the inactivity of the perovskites’ grains. Combined, the macro- and nanoscale environmental measurements performed provide a comprehensive platform for tracking, in real-time, the relevant changes that can lead to degradation. Moreover, they represent a reliable diagnosis tool that can be expanded to any perovskite chemical composition.

Combining perovskites with well-established photovoltaic materials such as silicon is an attractive option for producing cheap, high efficiency and high voltage solar cells. Perovskite/silicon tandems have the potential for further progress in increasing the efficiency to 30% and beyond, and we discuss some of the key ways forward to achieving this goal. We demonstrate a 4-terminal tandem perovskite/silicon configuration in which the efficiency is as high as 27.7% through a passivation approach using 2D perovskites. We also demonstrate efficiency above 21% and fill factor of 83% for a 1cm² single junction perovskite cell using a nanotextured electrode transport layer. Perovskite/silicon tandems integrated with an appropriate catalyst are also a promising way to achieve high efficiency solar hydrogen generation. Using this approach we demonstrate a water-splitting system with a solar to hydrogen efficiency over 17%.

Perovskite/Silicon tandem solar cells (PSTSCs) have potentially remarkable energy conversion efficiency at a reasonable cost, which can significantly boost the future PV landscape. Recently, a newly certification efficiency of 29.15% had been reported, which further demonstrated the potential commercialization of this devices. Despite these advances, stability and large area are the barriers which hinder the industrialization of the tandem devices. And, these issues are closely related to perovskite top cells. Therefore, eliminating the instability problem of perovskite materials and further increasing the efficiency of large scale perovskite devices which will promote the application of perovskite/Silicon tandem cells in the real world condition. In a word, there are many opportunities for perovskite devices to accelerate the development of PSTSCs.

I will discuss advances in perovskite tandems, both silicon:perovskite and perovskite:perovskite. I will discuss challenges in achieving the union of operating stability and high performance (especially high Voc) in both the small-bandgap and the large-bandgap cells making up the tandem stack.

Solar cells have presented significant improvements since their first introduction, and every day new developments are still being reported for different solar cell technologies.¹ Conversion efficiency improvements slow down as they become closer to their practical limits and it becomes a more challenging task for researchers to further boost the conversion efficiency. Researchers are continuously searching for alternative routes to surpass fundamental limits. One of the most promising routes is using tandem solar cell structures, which has a long proven success.²
Tandem solar cells enable better utilization of available illumination spectrum which leads to a higher conversion efficiency mainly by better spectral matching. However, some major design challenges are waiting to be addressed such as obtaining maximum absorption in absorbers of both subcells while having electrically advantageous design thicknesses in a material-independent way. Perovskite solar cells have attracted a lot of researchers since their introduction due to their excellent optoelectronic properties. The vast number of researches yields a very broad range of functional materials such as electron/hole transport layers, and transparent conductive electrodes (TCE); as well as various perovskites with different optical properties and bandgaps. Hence, optical design guidelines of perovskite solar cells should be carried out in a way to account for the optical properties of different materials. Therefore, previously we have documented optical design guidelines in a material independent approach for the perovskite solar cells in their standalone configurations.³
In this work, first, bandgaps of perovskite absorbers in both subcells are varied to find optimum pairs that yield the highest power output for both connection configurations, four- and two terminal. Subsequently, optimum optical design parameters for each layer in the tandem structure for a broad range of refractive index of conceptualized functional layers are computed. This expands the applicability of this approach to provide design guidelines for the already documented and future materials. Optimizations are carried out with the integration of machine learning algorithms due to the vastness of the parameter sets. Finally, optical loss mechanisms are quantized and alternative ways of addressing them are discussed.

1 Green MA, Dunlop ED, Hohl Ebinger J, Yoshita M, Kopidakis N, Hao X (2020) Solar cell efficiency tables (version 56). Progress in Photovoltaics: Research and Applications 28:629–638. https://doi.org/10.1002/pip.3303
2 Graydon O (2009) Solar success for Sharp. Nature Photonics 3:684–684. https://doi.org/10.1038/nphoton.2009.224.
3 Koç M, Soltanpoor W, Bektas G, Bolink HJ, Yerci S (2019) Guideline for Optical Optimization of Planar Perovskite Solar Cells. Advanced Optical Materials 7:1900944. https://doi.org/10.1002/adom.201900944

When exposed to elevated temperatures, metal halide perovskites suffer from decomposition to PbI₂, volatilization of the organic component from the crystal structure, and even metal diffusion of the top contact through the top carrier selective contact to the perovskite. Current methods to evaluate perovskite solar cell thermal stability involve fabrication of full cells and repeated testing over 1000+ hours, requiring considerable manual labor, equipment, and resources.

In our work, we employ a custom-built photo enclosure for high-resolution and real-time optical monitoring of large-area module test architectures during thermal aging for over 1000+ hours. The photo enclosure consists of an optical camera, adjustable light source, heated stage, and standardized color panel that act to normalize the RGB values of the illuminated samples between batches.^[1] We analyze the color of the perovskite layer and establish threshold color intensity values corresponding to critical color changes in the film signaling an appreciable quantity of PbI₂ formation. These color changes indicate the presence of thermal degradation without the need for confirmation through extensive characterization methods. We map the entire module surface for color changes to determine both local and regional degradation modes to identify trends in spatial non-uniformities of modules. From the optical degradation profile of the module, we subsequently apply localized characterization techniques to highlight different features or high-interest areas without analyzing the full sample area. Furthermore, we employ our optical monitoring system to a wide range of strategies designed to mitigate thermal degradation, including incorporation of perovskite additives, perovskite surface functionalization, and hybrid organic-inorganic transport layers. The ability to rapidly evaluate thermal stability without the need for complex characterization equipment and time and resources for fabrication and testing of full devices streamlines the design iteration process towards achieving fully stable perovskite solar modules.

[1] Sun, Shijing; Tiihonen, Armi; et al. (2020): A Physical Data Fusion Approach to Optimize Compositional Stability of Halide Perovskites. ChemRxiv. Preprint. https://doi.org/10.26434/chemrxiv.12601997.v2

Perovskite-based semiconductors have considerable potential to become an important technology for the fabrication of efficient thin-film photovoltaics (PV). Owing to the ease of fabrication by various wet-chemical deposition techniques, outstanding optoelectronic properties as well as simple bandgap tunability, the international research community puts great effort into the technology’s development, resulting in small-area perovskite solar cells exceeding 25% power conversion efficiency (PCE) within only a decade of research. However, perovskite-based PV still faces several challenges, such as low operational stability and the need for upscaling of the fabrication processes to application relevant device areas. In this context, literature reports a clear trend of significant PCE losses during upscaling of device area, as many established deposition processes become increasingly complex to control over larger areas.
In this work, we present for the first time the combination of an all-evaporated layer stack sequence with all-laser-scribed module interconnections as a promising method to close the gap between small-area record solar cells and large-area solar modules. By bringing together the ease of upscaling deposition area by vacuum-based techniques and the high-quality, high-throughput laser scribing method for fabrication of interconnections, a synergy employable as future industrial approach for high-quality module fabrication with excellent reproducibility and production yield is formed. First, an optimization of the laser scribing lines required for efficient serial interconnections is performed, eliminating reported problems such as incomplete ablation, redeposition of ablation debris and flake formation. Based on this, all-evaporated small area solar cells with an aperture area of 0.1 cm² are scaled up by a factor of up to 500 to a total aperture area of 51 cm², with geometric fill factors of up to 96% and most importantly without significant upscaling losses. An in-depth analysis of the fabricated devices utilizing current-density-voltage-characteristics, laser-beam-induced current mapping and photoluminescence measurements reveals excellent layer homogeneity and interconnection quality. The fabricated solar modules reach PCEs of up to 16.5% for aperture areas above 51 cm² (active area PCE of 17.5% with small-area solar cells achieving a PCE of 18.0%), being a world record for fully-evaporated perovskite solar modules. The difference between small- and large-area devices corresponds to an exceptionally low upscaling loss close to 0.5%_abs per magnitude of upscaled area, which significantly surpasses not only the upscaling efficiency of other solution-based upscaling methods of perovskite PV but also that of other thin-film technologies such as CIGS. Finally, the investigations are used to comprehensively assess the employed upscaling strategy, identifying further potential for improvement with the ultimate objective of approaching the industrial application of the perovskite PV technology.