Symposium F.EL08—Frontiers of Halide Perovskites—Linking Fundamental Properties to Devices (Includes Symposium S.CT02—Halide Perovskites—From Lead-Free Materials to Advanced Characterization and Deposition Approaches)

Metal halide perovskite solar cell research keep progressing on various fronts. My group at OIST uses surface science and advanced characterization to obtain fundamental understanding about perovskite materials and solar cells. Surfaces and interfaces play an important role in dictating performance and stability of perovskite solar cells. In this talk, I focus on surface science understanding and microscopic insights about perovskite surfaces [1-3]. [1] R. Ohmann, Y. Li*, N.-G. Park*, Y. B. Qi* et al., J. Am. Chem. Soc. 137, 16049 (2015); [2] J. Hieulle, X. Wang, Y. Yan*, Y. B. Qi* et al., J. Am. Chem. Soc. 141, 3515 (2019); [3] C. Stecker, K. Liu, G. Wang*, Y. B. Qi* et al., ACS Nano (2019) https://pubs.acs.org/doi/10.1021/acsnano.9b06585#

The remarkable solar performance of lead halide perovskites can be attributed to their excellent physical properties that present many mysteries, challenges, as well as opportunities. Better control over the crystal growth of these fascinating materials and fabrication of heterostructures using various halide perovskites with different bandgaps would open up opportunities for exploring new properties and device applications. After a brief summary on the nanowires and nanoplates of various metal halide perovskites we have developed and their lasing properties, I will focus my discussion on the growth of heterostructures of 2D Ruddlesden-Popper (RP) layered lead iodide perovskites with defined n phases and atomically sharp interfaces both in 2D/3D perovskite heterostructures and also as multi-layered multi-colored vertical heterostructures, in which the long chain ammonium ligands serve as the barriers to prevent ion migration across the junctions. We have used these well-defined heterostuctures to study the carrier transfer mechanisms between 2D and 3D perovskites and between different RP phases. The excellent properties of these single-crystal perovskite nanostructures of diverse families of perovskite materials with different cations, anions, and dimensionality make them ideal for fundamental physical studies of carrier transport and decay mechanisms, and for enabling high performance lasers, LEDs, and other optoelectronic applications.

Halide perovskite thin films and devices exhibit a remarkable degree of heterogeneity in structure and composition at the nanoscale, varying frequently from nominal synthesis targets in ways that can be difficult to detect using ubiquitous laboratory analysis. Leveraging synchrotron-based in situ X-ray microscopies to sample composition, structure, and optoelectronic quality at the nanoscale, we reveal the dependence of nanoscale phase separation and variations in charge collection on the local chemistry of A-site perovskite alloys. At times, these stoichiometric variations arise in the as-fabricated film, revealing important chemical dependencies. However, we also find significant evolution in the microstructure of high-performance perovskites during operation and under environmental stress, leading to insights into mechanistic pathways of degradation and new avenues for stabilization in high-performance alloyed perovskites.

Halide perovskites for optoelectronics are often composed by micro- and nano-scale inhomogeneous constructs. Therefore, high spatial resolution characterization methods are required for mapping and quantifying their electrical behavior. In this talk I will present our latest developments on atomic force microscopy (AFM) methods to assess the dynamic physical and chemical processes that take once perovskite materials and photovoltaic devices are exposed to light. Briefly, we realize a 4D imaging method that enables mapping open-circuit voltage (Voc) changes with in real-time (16 seconds per scan), and at the nanoscale (< 50 nm in spatial resolution) based on illuminated Kelvin probe force microscopy (KPFM). Using this paradigm, we have demonstrated ion motion within a single nanoscale grain in MAPbI3 solar cells upon 1-sun illumination, which results in a residual Voc that lasts for several minutes even under dark conditions. For multi-cation structures, we found that Cs-based perovskites deliver fully reversible and stable nanoscale voltage response, in excellent agreement with macroscopic measurements. We correlate the Voc nanoscale maps with chemical imaging through nano-IR and discover that the local variations in voltage are related to the power conversion efficiency enhancement in KI-treated perovskite. The heterogeneity revealed in both the local electrical and chemical responses reveals that the KI additive migrates our of the perovskite films, yet surprisingly; does not affect device performance. Our functional imaging platform can be extended to other perovskite materials, including Pb-free options.

Halide perovskites are generating enormous excitement for their use in high-performance yet inexpensive optoelectronic applications. Nevertheless, a number of fundamental questions about these materials still remain and need to be answered to push devices to their theoretical performance limits. For example, we still know very little about the specific nature of the defects leading to trap states, carrier recombination pathways or anisotropies of carrier diffusion. I will present results on new techniques we are developing to address these open questions in 2D and 3D halide perovskite semiconductors. These techniques focus on understanding charge carrier behavior, including recombination, trapping and diffusion, and how these properties link to chemical and material properties. I will present high-resolution luminescence microscopy techniques employing two-photon excitation to allow us to visualize and time-resolve carrier diffusion in three-dimensions, revealing anisotropic and depth-dependent carrier diffusion properties. Furthermore, we link the local luminescence properties to high-resolution crystallographic and chemical properties using synchrotron nano-probe X-Ray beamlines and low-dose scanning electron diffraction measurements. Through these measurements, we reveal the nature of the defects associated with local non-radiative power losses and heterogeneous diffusion. Furthermore, we provide guidelines about how we can ultimately eliminate these unwanted loss pathways and homogenize carrier diffusion.

An important area of research on metal halide perovskites (MHP) concerns the presence and role of surface and interface electronic gap states introduced by defects inherent to film processing, degradation under stress (T, irradiation, environment) or chemical reaction with neighboring layers, and the restrictions that these states impose on device performance, i.e., limit in Fermi level split, or aid carrier recombination, leading to reduction of solar cell open circuit voltage. This talk reports results on the determination of the presence and density of electronic states tailing above the valence band maximum of several MHPs, e.g., MAPbI3, CsPbI3, and FA0.85MA0.15Pb(I0.85Br0.15)3. Detailed measurements of the top of the valence band of these materials are performed using high-sensitivity UPS achieved in logarithmic detection. We focus specifically on MAPbI3 films processed with different additives mixed in the precursor-DMF solution: (i) no additives; (ii) DMSO; (iii) NH4++DMA+; and (iv) N-methyl-2-pyrrolidone thione (NMPT). The latter, an S-donor solvent, was investigated for its stronger coordination to Pb2+ as compared to the O-donor solvent DMSO, the additive widely used for improving film morphology. Significant differences are revealed between films processed with DMSO and NMPT. Once carefully treated via (experimental) background and parasitic satellite line contribution subtraction, the DMSO processed film exhibits a clear density of gap states extending from the valence band maximum to the Fermi level, whereas the NMPT processed film does not. The study was then complemented by Kelvin probe-based contact potential difference measurements to address the issue of surface photovoltage (SPV) during UPS. KP-based SPV measurements on these films show a significantly larger SPV (150 mV) and EF-VBM on the DMSO-treated films than on the NMPT-treated ones (SPV= 40 mV), confirming both the presence of the gap states and their impact on the electronic structure of the MHP films. The origin and nature of these gap states are still under investigation.

Perovskite solar cells have electrified the solar cell research community with astonishing performance and surprising material properties. However, the widespread commercialization of perovskites in solar cells and other optoelectronic applications still requires a more detailed knowledge about fundamental material properties. My group is specialized in the investigation of these properties on nanometer length scales using scanning force microscopy. We recently discovered a distinct nanoscale stripe domain structure in the piezoresponse on perovskite crystals [1]. The pattern is characteristic for ferro-elastic twin domains. We are currently investigating the influence of the domain walls on the charge carrier dynamics. Next to fundamental physical properties of photovoltaic perovskites, we investigate complete solar cell devices under operation conditions. To map and follow the vertical charge distribution across the functional layers of an operating device, we developed a method to prepare smooth cuts across the device [2]. We subsequently map the potential distribution across the device using a time-resolved Kelvin probe force microscopy (tr-KPFM) [2,3]. We can simulate open- or short circuit conditions, apply an external voltage or illuminate the device. In particular, we found that thin layers of localized charge formed and stabilized at the electrode interfaces when we changed the external voltage or illuminated the device. We could show that ion migration itself is too fast to influence device hysteresis. Instead, the formation and release of ionic interface charges determine the time scales for current-voltage hysteresis in perovskite solar cells. Our results demonstrate that a precise control over the interfaces in perovskite solar cells is the key, not only for controlling and suppressing hysteresis but also for their long-term stability in perovskite solar cells. [1] J. Phys. Chem C, 120(10), 5724 (2016). [2] ACS Appl. Mat. Interfaces, 8(30), 19402 (2016). [3] Energy Environmental Science, 11, 2404 (2018).

Defects come in a several flavours: conductivity-promoting defects create free carriers that enable electronics; killer defects (deep, charged centres) trigger recombination; and charge scattering defects reduce mobility [1]. Our understanding of the defect processes in halide perovskites is gradually improving through a combination of high-resolution measurements and materials modelling [2]. However, evidence is building for the importance of point defect aggregates and structural defects in higher dimensions. I will discuss recent progress, from theory and experiment, to identify, characterise and control extended defects in metal halide perovskites. In particular, I will cover the role of grain boundaries [3] stacking faults [4]. The use of defect tolerance as a metric to develop and screen post-perovskite materials will be critically addressed. This research is part of a collaboration with the group of Sam Stranks (Cambridge), and used the UK National Supercomputer ARCHER.

[1] "Instilling defect tolerance in new compounds" Nature Mater. 16, 964 (2017); https://www.nature.com/articles/nmat4973

[2] "Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites" Nature 580, 260 (2020); https://www.nature.com/articles/s41586-020-2184-1

[3] "Accumulation of deep traps at grain boundaries in halide perovskites" ACS Energy Lett. 4, 1321 (2019); https://doi.org/10.1021/acsenergylett.9b00840

[4] "Hexagonal stacking faults act as hole blocking layers in lead halide perovskites" ACS Energy Lett. In Press (2020); https://doi.org/10.1021/acsenergylett.0c01124

The dynamic optical and electrical responses of halide perovskites are currently not fully understood, and are closely related to the irreversible changes frequently observed in devices. We determine the individual and combined effects of extrinsic (humidity and oxygen) and intrinsic (light, bias, and temperature) stressors in the physical behavior of the archetypal MAPbBr₃ and MAPbI₃, and Cs-mixed perovskites. First, a detailed comparison between MAPbBr₃ and MAPbI₃ through time-dependent voltage measurements reveals that, upon illumination, high-energy photons leads to a > 10x slower voltage decline toward equilibrium than low-energy photons in MAPbBr₃. Yet, MAPbI₃ shows wavelength-independent decay rate, resulting from ion migration. Second, through in situ photoluminescence (PL) under environmentally controlled conditions, we resolve a humidity-induced PL hysteresis. Further, we apply a machine learning algorithm to predict the luminescence response for > 12 hs. Concerning the unique behavior of multi-cation perovskites, a correlative microscopy approach is realized, combined with environmental-controlled PL measurements.

Although grain boundaries and other interfaces in photovoltaics are generally detrimental for photogenerated carrier transport, their role remains contentious in hybrid perovskite semiconductors. The uncertainty is partially due to the practical challenge that most experimental approaches are performed at the top surface, which is not only insensitive to depth-dependent inhomogeneities but also may be influenced by topographic artifacts. Accordingly, we have developed tomographic atomic force microscopy (T-AFM) for fully 3-Dimensional mapping of photogenerated carrier transport in hybrid perovskite thin films. Via progressive nanoscale planarization and photoconduction mapping during in-situ illumination in an AFM, this unambiguously reveals grain boundaries serving as highly interconnected conducting channels for carrier transport in MAPbI₃. Voltage dependent measurements reveal enhanced carrier mobilities at most grain boundaries, although careful analysis also reveals the coexistence of 5-10% insipid interfaces. Depth dependencies in photocurrent through the grains, and the grain boundaries, are also assessed. Such insights are critical for engineering hybrid perovskites to be optimized for future solar cells, photodetectors, and light sources.

CsPbI3 nanocrystals are widely reported to be more stable in the perovskite phase than thin film CsPbI3 which rapidly converts into the non-perovskite orthorhombic phase. In this work, we answer why the phase stability is enhanced at the nanoscale. In addition, contrary to many reports, we confirm that the actual perovskite phase for CsPbI3 NCs is the gamma phase, rather than the alpha cubic phase. In this work, CsPbI₃ with narrow size distributions between 5 and 15 nm were prepared to study the size-dependent properties. The size dependent structural lattice constants were characterized and found to systematically vary by 3% across the size range, with unit cell volume increasing linearly with the inverse of size to 2.1% for the smallest size. This size dependent volume change correllates to negative surface energy between -3.0 to -5.1 eV nm^-2 for ligated CsPbI₃ nanocrystals. This negative surface energy leads to strain on the nanocrystals which, when present, causes stability in the perovskite phase, whereas thin film, strain-free CsPbI₃ is only metastable in the perovskite phase. Moreover, the size-dependent bandgap is also studied and found to be best described using a nonparabolic intermediate confinement model. Using this new model, we can experimentally determine constants such as the bulk bandgap, effective mass, and exciton binding energy, and conclude that many of these main material constants vary from the bulk α-phase values. This work provides a robust route to understanding γ-phase properties of CsPbI₃.

Metal halide perovskites are highly promising semiconductors for light-emitting and photovoltaic applications. The colloidal synthesis of nanocrystals (NC) is an effective approach for obtaining nearly defect-free semiconducting materials that can be processed into inks for low cost, high performance device fabrication. However, disentangling the effects of surface ligands, morphology and boundaries on charge-carrier transport in thin films fabricated with these high-quality NCs is inherently difficult. To overcome this fundamental challenge, here we employ terahertz (THz) spectroscopy to optically probe the photoconductivity of CsPbBr3 nanocrystal films. We compare the vibrational and optoelectronic properties of the NCs with those of a corresponding bulk polycrystalline perovskite and find significant deviations. We demonstrate a dependence of the charge-carrier mobilities and recombination rates on the nanocrystal size that derives from the localized nature of charge-carriers within nanocrystals, with local mobilities dominating those associated with inter-particle transport. We further show that the colloidally synthesized NCs have distinct vibrational properties with respect to the bulk perovskite, exhibiting blue-shifted optical phonon modes with enhanced THz absorption strength that also manifest as strong modulations in the THz photoconductivity spectra. Such fundamental insights into NC versus bulk properties will guide the optimization of nanocrystalline perovskite thin films for optoelectronic applications.

Coherent x-ray diffractive imaging in Bragg geometry (BCDI) is a powerful technique to image three-dimensional morphology as well as internal deformation field distribution of nano-scaled materials [1]. Because of unique sensitivity to lattice, BCDI has been employed to map strain and defect distribution inside metal, metal oxide, and minerals with non-destructive measurements. In this talk, I will introduce current state-of-art of BCDI and the 34-ID-C beamline in the Advanced Photon Source where one can perform this experiment. This talk will also cover recent experimental results on in-situ and operando BCDI, e.g. unique lattice distortion in ZSM-5 zeolites [2], annealing effect on gold grains on gold thin films [3], oxidation induced strain and defects in magnetite [4], and defect dynamics on platinum during catalytic reaction [5]. In addition, some estimates of BCDI in the future will be discussed. [1] M. A. Pfeifer, et al., Nature 442, 63 (2006). [2] W. Cha, et al., Nat. Mater. 12, 729 (2013). [3] A. Yau, et al., Science 356, 739 (2017). [4] K. Yuan, et al., Nat. Commun. 10, 703 (2019). [5] D. Kim, et al., Nano Lett. 19, 5044 (2019).

Halide perovskites have emerged as promising materials for optoelectronics with superior photoconversion efficiency and low cost. Moreover, optical properties of halide perovskite with mixed bromide and iodide composition can be substantially tuned via light-induced ion migration. Bromide-rich and iodide-rich domains starts to form during the process, which can be initiated at low intensity of light. Phase separation is subsequently reversible under the dark. Photoluminescence emission energy redshifts as a result of phase separation, while optical constants also change. The tunability of photoluminescence emission and optical constants in a wide spectral range makes halide perovskite alloys a promising, dynamically tunable photonic materials system. Unlike conventional tunable photonic materials such as ITO, TCO, and VO2, which rely on tuning of carrier density or an abrupt structural transformation, the speed and degree of phase separation/optical property evolution of mixed halide perovskite alloys can be tuned monotonically by varying the photon intensity. Inorganic mixed halide perovskites have been widely investigated for high-efficiency solar cells, with a focus on improving stability of their light absorption by reducing ion migration. To integrate halide perovskites for tunable nanophotonics, it is essential to understand light-induced phase separation in halide perovskites such as the spacing of phase separated domains, the influence of perovskite crystallite size and orientation, etc. We have synthesized crystallographically-textured polycrystalline films of the inorganic halide perovskite alloy CsPb(BrxI1-x)3 on SrTiO3 (STO) substrates by chemical vapor transport (CVT) at 620°C (at the precursor position) with N2 as the carrier gas. These perovskite films have large single crystal domains in the [001] orientation on the order of millimeters in lateral size and are stable under ambient laboratory conditions. X-ray diffraction confirms the perovskite phase of the film and the predominantly (001) orientation. EDX mapping shows a homogeneous initial distribution of Br, I as well as Cs and Pb. Photoluminescence characterization reveals initial light emission at 2.15 eV. By shining an HgXe lamp on the film, light emission at 2.0 eV starts to occur and dominate the emission spectrum. Time dependent photoluminescence characterization at 2.0 eV shows that the forward phase separation process has a time constant of 2000s and the reverse phase separation process also has a time constant on the order of 2000s. The film remains stable under many cycles of the forward and reverse phase separation process. In-situ X-ray diffraction confirms that halide perovskite film under phase separation has diffraction shoulders corresponding to iodide-rich and bromide-rich domains, while the initial film and reverse phase separated film show the same diffraction peak. In-situ ellipsometry reveals 0.2 difference in the refractive index before and after phase separation, with 50 nm blue shift of the excitonic features of the optical constants. To understand the length scale of phase separation, we CVT synthesized CsPb(BrxI1-x)3 films with < 200nm thickness on TEM compatible Si3N4 membranes, where the film exhibit similar optical properties to those deposited on STO. We probed the perovskite film before and after light-driven phase separation with the electron beam at cryogenic temperatures. The film showed no degradation while strong cathodoluminescence and STEM EELS signals were collected with nanometer scale spatial resolution. We compare the results with optical confocal microscopy which has subwavelength resolution. Efforts on mapping the phase separation length scale with X-ray diffraction will also be discussed.

Dopants, which are often indispensable in the organic hole-transport layer (HTL) in perovskite solar cells (PSCs), significantly impact the device performance, hysteresis, and stability. The primary roles of the HTL dopants are to enhance HTL conductivity and tune the energy level of HTLs. Here, we show that the lithium dopant from HTLs can significantly dope the perovskite absorber layer through diffusion. Kelvin probe force microscopy mapping shows that lithium diffusion increases the surface potential of the perovskite film. The potential at grain boundaries showed more increase than in grain interiors, indicating inhomogeneous distribution of Li dopant in the perovskite layer. Accumulation of Li at the grain boundaries was further confirmed by lateral elemental mapping using time-of-flight secondary-ion mass spectrometry. For both p-i-n and n-i-p device structures, the diffusion and nonuniform distribution of Li+ ions are shown to influence the carrier-transport properties of perovskites and device characteristics. This work reveals the hidden doping effects of the HTL dopants on perovskite films and their impact on PSC performance. Property changes of the perovskite thin films in actual devices from their pristine states should be considered for future understanding of device physics, characterization studies, and device engineering.

Progress made to date in terms of higher efficiency and stability in the field of halide perovskites is based on increasing material complexity that is, formation of multicomponent halide perovskites comprising multiple cations and anions. Each precursor component and synthetic variable can influence the reaction pathway during the formation of halide perovskites as well as influence key properties such as morphology and radiative efficiency. Here, we present the development of a multi-modal platform that is attached to a synchrotron end station and capable of in situ monitoring the dynamic transformation processes from the chemical precursor to the final halide perovskite film. By remotely controlling the spin-coating process and precursor heating we are able to characterize perovskite formation already during spin coating and the very early stages of annealing. In two independent studies we investigate the influence of the Pb-salt on CH3NH3PbI3 formation and the Br/(I+Br) concentration on the formation pathway of mixed CH3NH3Pb(I1-xBrx)3 perovskites. We show the correlation between Pb-salt and morphology as well as Br-content and crystallization mechanisms. This multi-modal platform enables efficient screening of complex multicomponent precursors and establishing chemistry-property relationships.

We present time-domain conductivity measurements of optically excited free carriers in perovskite photovoltaics using microwave near-field microscopy. Using a pulsed excitation source that is continuously tunable through the visible spectrum we demonstrate the capability to measure local photoconductivity transients with 10 ns temporal resolution and spatial resolution as high as 50 nm. We study the effect of surface treatment on the local response of [(CsPbI3)0.05(FAPbI3)0.85(MAPbBr3)0.15] perovskite films as well as their deterioration. We find a strong dependence of the free carrier lifetimes on surface treatment, where the presence of an electron transport layer is seen to result in carrier lifetimes of less than 100 ns. Similarly, surface passivation using conformal alumina encapsulation leads to reduced carrier lifetimes compared to untreated films and imparts significant resistance to deterioration. We find that film deterioration leads to strong spatial inhomogeneities in the photoconductive response across films and results in overall longer carrier lifetimes and a redshift in the absorption onset as measured using local action spectra.

We report the in-situ charge carrier recombination dynamics in perovskite bulk and at interfaces with electrodes by employing a novel photocurrent modulation technique, which enables us to achieve micrometre spatial and sub-microsecond temporal resolution of photocurrent transients. Our capacity to identify and isolate significant loss channels for device operation, by characterizing the performance of individual electrodes and charge selective layers, has previously only been achieved via indirect techniques. We find a charge trapping length for holes in the perovskite bulk of 21 μm with an associated trapping timescale of 2 μs, demonstrating that bulk trapping is not a major limiting factor for long-distance charge transport in perovskite-based devices. We observe significant photocurrent losses on and near gold electrodes due to the formation of a reduced charge carrier mobility region near the perovskite-gold interface with an associated trapping timescale of 0.7 μs. We further determine the length and timescales of carrier transport via diffusion and photon recycling. Our ability to expand the steady-state operation of the device in space and time allows us to characterise a broad range of processes in-situ with crucial insights into device operation, and paves the way for high throughput optimisation of interfaces for high-efficiency optoelectronics.

Solar power has a key role in the crucial transition to low carbon energy generation. Perovskite solar cells, in particular, are a great photovoltaic alternative as they exhibit excellent energy harvesting abilities and do not require energy-intensive manufacturing techniques [1]. The photo-conversion efficiency of perovskite photovoltaics rapidly increased like no other contemporary photovoltaic technology and as such, there is an industry-wide gap in the fundamental understanding of the relationship between microstructure and properties [2]. In addition, poor stability against UV light, moisture and elevated temperature during scale-up of perovskite photovoltaics limits and serves as an impediment for their widespread commercial use [3]. Nanoscale structural and chemical characterization is one alternative to better relate microstructural features to their resultant properties and to further fine-tune the perovskite properties beyond the current state of the art. For this to be achieved, a reliable and effective analytical strategy is not only necessary, but also key for resolving the microstructure of perovskite semiconductors at the nanoscale.

We have developed a magnetic sector Secondary Ion Mass Spectrometer (SIMS) and coupled it to a Helium Ion Microscope (HIM) [4]. Owing to this, we combine the best of both worlds; the high lateral resolution in secondary electron (SE) imaging of the HIM (<0.5 nm with He⁺) with the high detection limit of the SIMS technique (at least in ppm for a wide variety of elements and clusters). In SIMS mode, we are able to detect elements in the full mass range (from H to U) with a lateral resolution below 20 nm. With correlative microscopy, the chemical SIMS micrographs can be co-registered with sub-nm resolution secondary electron (SE) micrographs [5].

We will show several analytical studies completed with HIM-SIMS, among them, a cross-sectional chemical map of a mixed cation perovskite device where different thin films at the nanoscale can be distinguished morphologically and chemically. HIM-SIMS analysis in positive polarity reveals a heterogeneous distribution of Cs and Pb, where specific grains are depleted of these elements. Surface analysis of a multi-cation and multi-halide perovskite in both negative and positive polarity, show a different heterogeneous elemental distribution. Here, the chemical maps of the ions of Cs, Br, I and an organic cluster display a similar distribution while Pb enrichment occurs at selected grains. HIM-SIMS is a versatile and powerful technique for the improved understanding of the impact of microstructure, such as interfaces, grain boundaries and grains, on the photovoltaic stability and performance.

The authors gratefully acknowledge support by the Swiss National Science Foundation (SNF) under grant no 200020L_172929/1, and the National Research Fund Luxembourg (FNR) through grant INTER/SNF/16/11534230

[1] A. R. Bin Mohd Yusoff, P. Gao, and M. K. Nazeeruddin, “Recent progress in organohalide lead perovskites for photovoltaic and optoelectronic applications,” Coord. Chem. Rev., vol. 373, pp. 258–294, 2018.
[2] “National Renewable Energy Laboratory (NREL) – National Center for Photovoltaics: ‘Research Cell Record Efficiency Chart.” [Online]. Available: https://www.nrel.gov/pv/%0Aassets/images/efficiency-chart.png.
[3] Y. Rong et al., “Challenges for commercializing perovskite solar cells,” Science (80-. )., vol. 361, no. 6408, 2018.
[4] T. Wirtz, O. De Castro, J.-N. Audinot, and P. Philipp, “Imaging and Analytics on the Helium Ion Microscope,” Annu. Rev. Anal. Chem., vol. 12, no. 1, pp. 523–543, 2019.
[5] F. Vollnhals et al., “Correlative Microscopy Combining Secondary Ion Mass Spectrometry and Electron Microscopy: Comparison of Intensity-Hue-Saturation and Laplacian Pyramid Methods for Image Fusion,” Anal. Chem., vol. 89, no. 20, pp. 10702–10710, 2017.

In order to further advance perovskite solar cell efficiency from the current record of 25.2%, it is important to minimize perovskite perovskite crystal defects and increase interface performance through passivation layers. 2-D perovskites have shown great potential as passivation layers but are primarily deposited by solution processing. Drawbacks of using solution processing for 2-D perovskite films include toxicity of the solvent, potential insolubility of the precursors, and lack of precise thickness control. Atomic layer deposition (ALD) could be a solution to these problems. ALD allows for the deposition of highly uniform layers of perovskite with great control over the final thickness of the film. Self-limiting in nature, ALD processes may allow for depostion of monolayers of perovskite with more control than solution processing techniques. Although an established deposition technique, ALD processes for perovskites have yet to be developed. Through this work, we consider ALD as a method of depositing 2-D perovskites as passivation layers for perovskite solar cells. These passivation layers are further incorporated into devices and their effect on solar cell efficiency is analyzed.

Organometal halide perovskites have been in the spotlight of semiconductor science over the past decade. Their exceptional electronic-optical properties and easy production from solution give them an edge over the semiconductor applications. The organic-inorganic halide perovskite solar cells (PSCs) have attracted the incredible attention of the researchers working on solar cells owing to a rapid increase in the efficiency of 25.5% since 2009^1,2.
Furthermore, tin/lead (Sn/Pb)-based perovskite offers lower toxicity compared to the lead-based perovskites³. Their bandgaps are close to 1.3-1.4 eV which is the ideal bandgap of an absorber layer in a solar cell regarding the Shockley-Queisser model⁴.
In this study, we have performed SrI₂ addition (ranging from 0.1% to 5.0% concentrations) to the (FASnI₃)_0.8(MAPbBr₃)_0.2 perovskite to improve their physical properties. Moreover, we have investigated the incorporation of SrI₂ into perovskites impacts on the morphological and optical properties of perovskite films.

[1] NREL. Best Research-Cell Efficiencies. https://www.nrel.gov/ pv/assets/pdfs/bestresearch-cell-efficiencies.20200925.pdf
[2] Correa-Baena, J.-P., et al., Science, 2017. 358(6364): p. 739-744.
[3] Tavakoli, M.M., et al., Advanced Materials, 2018. 30(11): p. 1705998.
[4] Zhou, X., et al., Advanced Materials, 2020. 32(14): p. 1908107.

The high refractive index of perovskite semiconductors at their emission wavelength (n ~ 2.3-2.6) results in a narrow emission escape cone and a large amount of photoluminescence (PL) to be trapped within perovskite thin films. Due to strong band edge absorption and a very small luminescent stokes shift, this effect leads to considerable photon reabsorption and reemission - an effect known as photon recycling (PR) [1] - as well as a concurrent red-shift of propagating PL due to emission filtering [2]. As recently highlighted, the self-absorption induced redistribution of charge carriers and outcoupling of propagated PL can lead to a significant misinterpretation of measurements of the intrinsic bimolecular recombination and charge carrier diffusion coefficients as well as of asymmetric PL spectral shapes [3, 4]. For perovskite films with low non-radiative recombination and parasitic absorption losses, PR can be of great advantage as it allows higher charge carrier densities at steady-state and thus increased device open-circuit voltage [5]. However, most current models still start from the simplified assumption that all initially trapped PL will be reabsorbed at some point without the possibility of being outcoupled, likely overestimating the effect of PR [6]. In this study, we show that in external photoluminescence quantum efficiency (PLQE) measurements employing an integrating sphere setup, the PL spectra of polycrystalline perovskite films with layer thicknesses typically employed for thin film solar cells (~150-500 nm) exhibit a notable red-shift and broadening. Such asymmetric PL spectra are often ignored or misinterpreted in the literature. We developed an all-optical model that considers both PR and outcoupling of initially trapped filtered PL due to scattering, which can reproduce spectral shapes with varying extents of red-shifts. We determined the true PL spectral shape as well as the absorption spectrum via high resolution confocal PL measurements and photothermal deflection spectroscopy respectively. Our model allows to estimate the total escape probability in addition to the initially generated internal PL intensity and thus to determine the internal PLQE of perovskite thin films. We confirm our assumptions by various control experiments and Monte Carlo simulations, benchmarking the model for perovskite thin films with different grain size, thickness and composition. Our approach serves as a new guideline for estimating these important parameters directly from measurements of the external PLQE and PL spectrum using an integrating sphere setup. [1] L. M. Pazos-Outon et al., Photon Recycling in Lead Iodide Perovskite Solar Cells. Science 2016, 351, 1430-1433. [2] B. Wenger et al., Consolidation of the Optoelectronic Properties of CH3NH3PbBr3 Perovskite Single Crystals. Nat. Commun. 2017, 8, 590. [3] T. W. Crothers et al., Photon Reabsorption Masks Intrinsic Bimolecular Charge-Carrier Recombination in CH3NH3PbI3 Perovskite. Nano Lett. 2017, 17, 5782-5789. [4] A. Bercegol et al., Quantitative Optical Assessment of Photonic and Electronic Properties in Halide Perovskite. Nat. Commun. 2019, 10 (1), 1586. [5] M. G. Abeb et al., Rigorous Wave-Optical Treatment of Photon Recycling in Thermodynamics of Photovoltaics: Perovskite Thin-Film Solar Cells. Phys. Rev. B 2018, 98, 075141. [6] J. M. Richter et al., Enhancing Photoluminescence Yields in Lead Halide Perovskites by Photon Recycling and Light Out-Coupling. Nat. Commun. 2016, 7, 13941.

The alignment of energy levels at the interface of perovskite absorber and adjacent hole- and electron-transporting material affects the formation of space-charge regions and interfacial recombination. These have consequences for charge transport in the layers and, therefore, for the efficiency of devices. We measured this alignment by the use of Kelvin-probe-force microscopy (KPFM) to gain a detailed understanding of this alignment of energy levels. This study is focused on Formamidinium tin iodide (FASnI3) as perovskite absorber. This material avoids the toxicity problems of lead and is known to yield the highest power conversion efficiencies amongst all lead free perovskite absorbers. The most common solar cell geometry for FASnI3 implements C60 as electron-transporting material, which is prepared on FASnI3 by physical vapor deposition (PVD). Intermittently to PVD, we use KPFM to simultaneously monitor morphology and work function of C60 during its growth on FASnI3. We show that a sufficiently high deposition rate is needed to avoid island formation of C60 and obtain a homogeneously covering film with constant work function. We further show that the width of the space charge layer in C60is larger than the film thickness often implemented in perovskite solar cells (20 nm - 30 nm). A band bending in the range of 300 meV is observed and consequences for prospective device operation are discussed.

Organolead halide perovskite nanocrystals (NCs) have emerged as a versatile and promising active material for many optoelectronic applications such as solar energy harvesting, light-emitting device, and photodetectors owing to their size- and composition-dependent optoelectronic properties. Although the extensive studies on their optical and electronic properties have been studied in the last several years, their low stability, structural integrity, and poor luminescence stability associated with the fast attachment-detachment dynamics of surface capping molecules during post-processing limit their practical applications in optoelectronics. Therefore, an in-depth understanding of the surface chemical processes is required. We found that the amine group (-NH3+) immobilizes Br- anion at the corner of undercoordinated PbBr64- octahedron on the MAPbBr3 surface resulting in a charge-balanced Pb-Br framework where Br/Pb ratio is slightly higher than stoichiometry, and thus provides exciton radiative recombination and high photoluminescence quantum yield. In contrast, the oleic acid is not able to coordinate with the surface moieties of the perovskite, but rather carboxyl acid groups (-COO-) etch Pb2+ into solution when only acid was used. Interestingly, a mixture of oleic acid and amine-functionalized surfactants was found to promote amine conversion to ammonium ion (-NH2 + H+ ® -NH3+), which is favorable for their localization on the surface. Our results show that the Br vacancies at the non-passivated surface result in a reduction of Pb2+ to Pb0 by trapping the electrons generated from the exciton dissociation, which provides the main pathway for exciton trapping. Acknowledgment This work is supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (No. NRF-2019R1I1A2A01060608) and by Nano-Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning. (2009-0082580).

Organic-inorganic lead halide perovskite polycrystalline thin films used as absorber layers in high efficiency solar cells have direct band gaps, high absorption coefficients, and potentially low-cost solution-based fabrication processing. However, these materials are inclined to degradation with exposure to humidity, oxygen, heat, and ultraviolet light. Formamidinium lead iodide (FAPbI3) is an attractive composition perovskite as it has a more symmetric crystal structure due to a larger cation radius for FA compared to that for methylammonium (MA) as well as an elevated decomposition temperature for improved stability. Even though FAPbI3 is among the more stable perovskite compositions, solar cells incorporating these absorbers still degrade with atmospheric exposure over time. Here we study the behavior of FAPbI3 film structure and complex optical properties under controlled temperature variations and relative humidity (RH) exposure. In situ real time spectroscopic ellipsometry (RTSE) measurements are collected over the near infrared to ultraviolet spectral range (0.75-5.9 eV) for polycrystalline FAPbI3 thin films deposited on glass substrates which are then held at different temperatures, ranging from 7 to 70oC, in a sealed nitrogen chamber with controlled 0, 26, and 85% RH introduced. From RTSE, the optical response in the form of complex dielectric function (e = e1 + ie2) spectra is deduced using a parametric model accounting for above band gap critical points, the direct band gap, an Urbach tail, and degradation induced sub-gap absorption due to defects. Variations in parameters describing spectra in e are tracked as functions of temperature, controlled RH, and the amount of time films are held under those conditions. Changes in bulk thickness, surface roughness thickness, and surface roughness relative density are also tracked with time under these conditions. For example, RTSE indicates no change in structure and minimal changes in e at 0% RH regardless of temperature. However, when FAPbI3 is exposed to 26% RH at 7oC, RTSE indicates changes in parameters describing e and structure, such as increases in film thickness with time indicative of water intercalation. To the contrary, slight film densification is observed over time when held at 26% RH and 70oC. These RTSE studies show that deviations from near room temperature with controlled relative humidity induces more changes in optical and structural properties with time than when the sample is exposed to only humidity. In all cases of RH exposure sub-gap absorption increases with time, most notably in widening of the Urbach tail obtained from the parametric model for e. To further study how other ambient conditions affect perovskite stability, RTSE is performed for FAPbI3 with exposure to controlled amounts of both oxygen and relative humidity as a function of temperature. RTSE is applied directly to measure a FAPbI3 based solar cell to track changes in the perovskite absorber over time and identify potential correlations with device performance.

Organic-inorganic lead halide perovskite-based photovoltaics (PV) exhibit high initial efficiency, can be processed at potentially low cost, and material bandgaps can be tuned by composition. Solution-processing enables fabrication of a range of different ABX3 (A: methylammonium—MA, formamidinium—FA, Cs; B: Pb, Sn; X: I, Br, Cl) perovskite compositions to meet these needs in single or tandem junction PV. Unfortunately, these perovskites may degrade upon exposure to relative humidity and oxygen in ambient air, during device processing, and overtime during device operation. Like all PV devices, performance depends on the characteristics of each material and interfaces between layers to minimize sources of optical and electronic losses. External quantum efficiency (EQE) simulations of perovskite PV assess sources of parasitic optical losses with input complex optical response of all component materials and known multiple layer device structure. From these simulations, the optical performance of devices with different structures or component materials is predicted. Comparison of simulated EQE with experimental measurement validates the optical model when simulated and experimental EQE match but also more importantly identifies electronic losses such as incomplete carrier collection when experimental EQE is less than the simulation. Obtaining the most accurate input complex optical response of perovskite films and layer thicknesses in devices is necessary to maximize the amount of information gained from EQE and its modeling. Toward this end, spectroscopic ellipsometry (SE) over the near-infrared to the ultraviolet wavelength range (0.73-5.9 eV) is used to characterize the complex optical response of solution-processed perovskite films of different compositions. Photothermal deflection spectroscopy (PDS) and unpolarized transmittance complement SE by enhanced sensitivity to low values of absorption coefficient (?) in the vicinity of the bandgap. Extension to low values of ? enables detection of exponential band tailing and Urbach Energy (EU), with narrower EU indicative of more ordered and higher electronic quality material. A parametric optical property model has been developed which includes contributions from electronic transitions above the bandgap, the direct bandgap, an exponentially decaying Urbach tail, and sub-gap absorption due to defects. Complex optical properties of perovskite films, including wide-bandgap FA1-xCsxPb(I1-yBry)3 and narrow-bandgap (FASnI3)x(MAPbI3)1-x compositions, in both as-prepared and degraded states are determined. SE data is collected for complete perovskite PV to deduce layer thicknesses and perovskite complex optical properties within the device. This information serves as direct input for EQE simulation. Comparison with perovskite PV experimental EQE enables the deduction of optical and electronic device performance losses arising from perovskite composition and device architecture, variations in near bandgap perovskite optical response resulting from complete device processing, and perovskite degradation during solar cell operation. As an example, similar simulated and experimental EQE in (FASnI3)x(MAPbI3)1-x (0.3 ? x ? 0.7) perovskite PV validates the optical model and indicates minimal losses. In another case, for FAPbI3 PV comparisons between simulation and experimental EQE indicate sources of both optical and electronic losses arising from device processing.

The electron transfer film (ETF) plays an important role in obstructing recombination of electrons and holes between the perovskite layer and fluorine-doped tin oxide in planar perovskite solar cells (PSCs). In addition, at the compact spin-coated SnO2 ETF, morphological defects, such as pinholes and cracks, due to roughness of the substrate seriously impact on the performance of PSCs. Here, we investigate the variables that can control the charge extraction using spin-coated SnO2, sputtered TiO2 and double-layered (DL) ETF composed of TiO2 on SnO2 and SnO2 on TiO2. We determined the resistance, mobility, conductivity and carrier concentration of double-layered ETF based on PSCs by a 4-probe method. The electrical properties show that the DL-ETF can effectively reduce the trap density at perovskite/ETF interface, promote the electron transfer, and suppress the interface recombination rather than SnO2 alone. The band structure and the carrier transport mechanism at perovskite/DL-ETF interface were explained by conductive atomic force microscopy, Kelvin probe force microscopy and ultraviolet photoelectron spectroscopy. Complementary composite DL-ETF has been found to eliminate severe current leakage inside the device, reducing interface defects and hysteresis. More importantly, DL-ETF accelerates at electron extraction and transport owing to the excellent mobility. Finally, DL-ETF based PSCs yield higher power conversion efficiency with improved JSC than the device with SnO2 alone. Therefore, these are beneficial for reducing the recombination of carriers in the electron traps, leading to the enhancement of device performance.

Improving the repeatability and operational stability of interfaces in perovskite devices is crucial to extend device lifetime. With each sequential step in perovskite device fabrication comes a risk of a failed device integration or an interaction that leads to longer-term device instability. Here, we present simple electrochemical techniques to evaluate fundamental properties of charge transport layers and limits of stability before completion of a full device stack, with the objective of improving batch yield, deconvoluting the troubleshooting process, and accelerating the development of new perovskite absorbers, their contact layers, and high-efficiency devices.

We focus on two exemplary measurements using easily accessible aqueous electrolytes. First, we show that an outer sphere redox couple such as the ferro/ferricyanide system allows facile evaluation of the semiconducting properties (conduction band minimum position, doping level, etc.) of SnO_x electron transport layers. The anodic and cathodic features in cyclic voltammograms of the SnO_x are correlated to final solar cell efficiency and offer an early screen of quality. Second, we show that ionic permeability/reactivity of SnO_x layers, which can lead to irreversible absorber degradation under bias, can be rapidly evaluated by potentiostatic measurements in the presence of organohalide salts. This technique reveals the influences of processing condition and deposition method on susceptibility to bias-induced degradation reactions.

These process control measurements can be readily adapted to devices of any area and are compatible with industry-scale fabrication techniques to improve process reliability and reproducibility. While our work focuses on the development and characterization of SnO_x layers, the fundamental principles can be adapted for a wide variety of charge transport layers so long as the proper electrochemical set-up is implemented, making this method a powerful tool for halide perovskite development.

Segregation of illuminated mixed-halide perovskites into iodide- and bromide-rich domains presents a critical bottleneck in the application of wide-bandgap absorbers in single- and multi-junction architectures. And while its occurrence has been well-studied in thin films, the influence on operational solar cells lacks sufficient understanding.
This work employs a multimodal characterization procedure to observe the slow progression of halide segregation in efficient solar cells prepared in the p-i-n architecture using sequentially processed perovskite absorbers. Photoluminescence spectroscopy is used to identify the stages that demixing of halide ions entails while simultaneous tracking of photovoltaic parameters allows correlating performance degradation to the migration of ionic species in each stage. A new stage of the process is thus observed upon prolonged illumination. Characterization of sub-bandgap features reveals the occurrence of photo-induced defect formation whose suppression through cationic substitution provides a strategy for stabilization of wide-bandgap compositions against halide segregation.

Partial shading of a solar module can bring a set of cells within the module to operate under reverse bias. When a solar cell is shaded in a module, the photogenerated current falls and a negative voltage (reverse bias) builds up as the other illuminated cells in series try to push current through it. Moreover, metal-halide perovskite photodetectors are regularly operated in reverse bias, and therefore it is critical to understand how these devices behave under reverse bias. Previous studies have shown that perovskite solar cells with a wide variety of compositions and contacts exhibit unusual behavior, compared to typical silicon solar cells, in reverse bias that includes both reversible performance loss and non-reversible degradation. [1, 2] Furthermore, we have shown that when a perovskite solar cell is held under reverse bias, the mobile ions move in a way such that there is favorable band energetics to allow for hole injection into the perovskite, yet the consequence of this process was unclear. [1]
Recent investigations have shown that photogenerated holes can induces electrochemical oxidation of halides into halogens. [3, 4] These halogens can leave the perovskite absorber and diffuse into contacts and cause further detrimental reactions with the electrodes. [5] To elucidate and understand the degradation mechanisms that occurs when perovskite solar cells are held under reverse bias, we have developed an advanced drift-diffusion model that incorporates an electrochemical term that can explain the short-circuit, open circuit and fill factor losses we experimentally measure. Our model conclusively shows that the injected holes can trigger the oxidation of halides to form neutral halogen interstitials. The density of halogen interstitials is much higher in reverse bias because there are hardly any electrons available to reduce the halogens back into halides. The resulting halogens act as bulk recombination centers and lead to a sharp decline in all the JV device characteristics. While the interstitial halogen density will decay when the cell is finally operated in forward bias, permanent degradation will occur if the halogen diffuses out of the perovskite layer into the transport layers or electrodes. Finally, we discuss how changing parameters such as the mobile ion density or the permeability of contacts can positively influence device performance and stability.

[1] A. R. Bowring, L. Bertoluzzi, B. C. O’Regan and M. D. McGehee, Adv. Energy Mater., 2018, 8, 1702365
[2] R. A. Z. Razera, D. A. Jacobs, F. Fu, P. Fiala, M. Dussouillez, F. Sahli, T. C. J. Yang, L. Ding, A. Walter, A. F. Feil, H. I. Boudinov, S. Nicolay, C. Ballif and Q. Jeangros, J. Mater. Chem. A, 2020, 8, 242–250.
[3] G. Y. Kim, A. Senocrate, T.-Y. Yang, G. Gregori, M. Grätzel and J. Maier, Nat. Mater., 2018, 17, 445–449.
[4] S. G. Motti, D. Meggiolaro, A. J. Barker, E. Mosconi, C. A. R. Perini, J. M. Ball, M. Gandini, M. Kim, F. De Angelis and A. Petrozza, Nat. Photonics, 2019, 13, 532–539.
[5] R. A. Kerner and B. P. Rand, ACS Appl. Energy Mater., 2019, 2, 6097–6101.

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 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 atomically sharp and flat 2D heterojunctions between two arbitrarily determined phase-pure halide perovskite single crystals. Stark differences are observed in anion diffusion across 2D halide perovskite lateral and vertical heterostructures. Halide inter-diffusion in lateral heterostructures is found to be similar to the classical Fickian diffusion featuring continuous concentration profile evolution. However, 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. 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.

Organic lead halide perovskite based solar cells (HPSCs) have achieved certified power conversion efficiency (PCE) over 25.0%. Despite the high efficiency achieved, there are still many concerns about the large-scale applicability of this solar cells because of the water soluble toxic Pb2+. The simpler approach to address this issue is to find a benign or less toxic metal to replace the lead atom in the perovskite structure, with the resulting perovskite still showing the excellent optical and electrical properties. Theoretically, tin based perovskite holds the promise to produce similar or even higher PCE compared to their Pb counterpart. However, for relatively long time the tin-based HPSCs held a PCE lower than 7% though intensive research efforts were devoted to their investigation. The facile formation of tin vacancies and easy oxidation of Sn2+ have been identified as the main reason determining the low PCE. In my presentation I will report as a small amount of 2D tin perovskite templates the growth of highly crystalline and oriented 3D FASnI3 grains (2D/3D mixtures), suppressing effectively the appearance of tin vacancies and Sn2+ oxidation. As a consequence of the reduced background charge carrier density and trap assisted charge recombination, the device showed a 50% improvement in the PCE (up to 9%) compared to that using pure 3D tin perovskite. We further succeeded reducing the defects in the 2D/3D tin perovskite films by adding ethylammonium iodide (EAI) into the corresponding perovskite precursor solution. These films display larger crystalline grains and a more compact and uniform film morphology when compared to their counterparts without EA cation. These features lead to a much lower trap density, background charge carrier density and charge recombination loss in the corresponding devices. Results, which allow hopping for future highly efficient Sn-based hybrid perovskite solar cells.

Data-sceince methods, including advanced statistics and machine learning, can help researchers diagnose, optimize, and design more efficiently. In this talk, I'll summarize the MIT PVLab's recent progress optimizing bare films and record-efficiency perovskite devices with capping layers using data-science methods. These include combining density-functional theory (DFT) and Bayesian inference to accelerate the search for stable compositions, and optimizing capping layers for combined performance and stability. Lastly, I'll highlight our progress searching for lead-free perovskite-inspired, defect-tolerant semiconductors.

In the last years, the field of solution-processed solar cells based on inorganic semiconductors has seen a great progress, showing the highest efficiencies among the emerging photovoltaic technologies. Indeed, of quantum dot solar cells have reached an efficiency over 16%, and solar cells based on perovskites have surpassed 25% efficiency. However, the future development of these cells could be limited due to the presence of highly toxic elements in their composition, more specifically lead (Pb). Efforts are thus required in searching for earth-abundant, and non-toxic materials. Inorganic colloidal nanocrystals (NCs) are attractive materials for their use as active layers in quantum dot solar cells. They are generally composed of earth-abundant elements, and they can be processed from solutions, allowing an easy and cheap fabrication of devices. Moreover, they have additional attractive properties, like high absorption coefficients, tunable bandgaps, and band energy level engineering through ligand exchange. In this context, bismuth-based colloidal NCs, appear as an attractive alternative for environmentally friendly, low cost solar cells. We have developed ways to modify the size and shape of the bismuth-based nanocrystals, and we have observed how this affects their optoelectronic properties. We have studied the modulation of Fermi level and energy bands introducing different ligands on the surface of the NCs. We have employed these NCs to fabricate solution-processed solar cells, focusing on completely non-toxic devices, achieving a certified efficiency of more than 6%, employing very thin layers (~35 nm) of AgBiS2 NCs. A summary of these results will be presented and future directions will be discussed.

The certified efficiency of Pb-perovskite solar cells with more than 1 cm2 area is reported to be 20.9%(1). In the smaller cell, the cell efficiency with 25.2% has been reported, which is close to 22.3 % of multi Si solar cells (2). However, the use of Pb in electronic devices is limited according to the EU RoHS Directive. Thus, it is needed to develop Pb-free perovskite solar cells. Among Pb-free perovskite candidates, Sn perovskite is one of the most promising candidates for the light harvesting layer for the Pb-free perovskite, because they have perovskite structure similar to Pb-perovskite. However, the efficiency iwas not as high as those for Pb-perovslite solar cells. We have already reported that the efficiency was enhanced by Ge2+ doping to the Sn-perovskite layer, because the carrier density in the Sn-perovskite layer was reduced (3,4). Now, we discuss the further enhancement of the Sn-perovskite solar cells from the view point of the lattice strain (disordering) relaxation and the surface passivation on the perovskite slayer to show the direction of the research. We report post-treatment of Sn-perovskite layer with Lewis base (NH2-CH2-CH2-NH2) to suppress the recombination reaction in Sn-perovskite solar cells resulting in efficiencies of 10.2 %. Upon analyzing the X-ray photoelectron spectroscopy and impedance spectroscopy data, we came to the conclusion that the amine-group bonded the under-coordinated tin, passivating the dangling bonds and defects resulting in suppressed charge carrier recombination (5). This also has the effect of prolonged lifetime and higher charge carrier mobility. These findings provide the groundwork for improving the efficiency of Sn-halide PSCs to compete with that of lead-based PSCs by simple post-treatment with organic molecules (6). We and other research groups have reported that the efficiency of Sn-perovskite solar cells increases spontaneously during the storage, which we explained by the perovskite lattice strain (disordering) relaxation. This prompted us to examine the relationship between between relative lattice strain and photovoltaic performances. It is shown that the efficiency of the Sn-perovskite solar cells has good relationship with the lattice strain (disordering). By decreasing the lattice strain (disordering), Ge doped Sn-perovskite solar cells with 11.7% was prepared. The mechanism wil be discussed. 1. M.A. Green et.al., Progress in Photovoltaics, 2019;27:565-575, version 54, 2. NREL efficiency Table, https://www.nrel.gov/pv/cell-efficiency.html 3. N. Ito, S. Hayase, et al., Journal of Physical Chemistry Letters, 9, p.1682-1688, 2018. 4. C. H. Ng, S. Hayase, et.al., Nano Energy, https://doi.org/10.1016/j.nanoen.2019.01.026, 2019 5. M. A. Kumarudin, S. Hayase, et al., J. Phys. Chem., Lett., DOI:10.1021/acs jpclett.9b02024, 2019. 6. K. Nishimura, S. Hayase, et al., ACS Appl. Mater. Interfaces, https://doi.org/10.1021/acsami.9b09564 2019.

Low-dimensional halide perovskites have demonstrated promising luminescence properties for light-emitting diodes due to their high radiative recombination rate.[1] However, the lack of a complete understanding of the structure-property relationship hinders the systematic design of these materials to enhance their luminescence efficiency. Combining state-of-the-art nonadiabatic molecular dynamics and time-domain density functional theory, we explore the substantial effects of structural fluctuations on the excited state dynamics and carrier recombination in these perovskites. Here, I will demonstrate how structurally rigid perovskites experience weaker electron-phonon interactions, resulting in suppressed non-radiative carrier recombination and enhanced photoluminescence quantum yield (PLQY).[2,3] Our work revealed that stacking of the spacer cations, and halogen composition of the inorganic layers substantially tune the PLQY of these materials. Based on these understanding, we further propose a combination of suitable spacer cations and inorganic layer compositions to improve the PLQY of low-dimensional halide perovskites. References: 1. Smith et al. Chem. Rev. 2019, 119, 3104 2. Ghosh et al. Submitted, 2019 3. Leveillee et al. Under final review, Nano Lett. 2019

Vacancy-ordered triple perovskites have recently come under the scientific spotlight as promising materials for high-performance next-generation optoelectronic technologies.^1,2 Their A₃B₂X₉ stoichiometry facilitates the replacement of the toxic Pb²⁺ cation with a benign 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. Through a combination of detailed theoretical and experimental characterization of this novel material, we elucidate the origin of broadband absorption. Sn is found to disproportionate in the doped material, inducing a strong intervalence charge transfer (IVCT) transition, whilst preserving the structural integrity of the perovskite framework.
Our work provides valuable insight regarding the effects of mixed-valency and structure-property relationships in perovskite-inspired materials, guiding design strategies and expanding the compositional space of candidate materials. Moreover, we anticipate that this massive reduction in absorption onset could aid charge transport and/or photo-catalytic performance, opening the door to unexplored applications of this material class.

¹ Y.-T. Huang, S.R. Kavanagh, D.O. Scanlon, A. Walsh, and R.L.Z. Hoye, ArXiv:2008.08959 (2020).
² Z. Li, S.R. Kavanagh, M. Napari, R.G. Palgrave, M. Abdi-Jalebi, Z. Andaji-Garmaroudi, D.W. Davies, M. Laitinen, J. Julin, R.H. Friend, D.O. Scanlon, A. Walsh, and R.L.Z. Hoye, ArXiv:2007.00388 (2020).
³ 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).
⁴ R. Nie, R.R. Sumukam, S.H. Reddy, M. Banavoth, and S.I. Seok, Energy Environ. Sci. 13, 2363 (2020).

Organic-inorganic hybrid halide perovskite solar cells (HPSCs) have attracted immense attention because of excellent optoelectronic properties and record power conversion efficiency (PCE) has reached 24.2% from 3.8% within a few years. Despite the very high efficiency already attained by HPSCs (ABX3; A = MA, FA, Cs; B = Pb, Sn; X=I, Br, Cl) resulting from high absorption coefficient and electron-hole diffusion lengths; toxicity of Pb and stability of these materials are veritable issues. Replacing MA+ and FA+ with PEA+, BA+, Cs+ has shown to enhance the stability of hybrid perovskite solar cells against thermal and moisture related degradation. The superior optoelectronic properties of Pb-based halide perovskites are attributed to the inactive Pb 6s orbitals, and can be replaced by Ge2+, Sn2+, Sb3+, Bi3+, Cu2+ with inactive s orbitals. Replacement of Pb2+ with Sn2+ and Ge2+ seems to be a logical solution for addressing the toxicity issues and results in excellent optoelectronic properties such as high absorption coefficient, high hole mobility resulting in PCE of 9.6% for (GA,FA)SnI3 and 7.9% for mixed (FA,MA)GeSnI3 and 7.11% for CsSn0.5Ge0.5I3. However, a major challenge for Sn and Ge based 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 resultant short carrier diffusion lengths. The 2D structure Cs2SnI6 has received attention as a more stable alternative to CsSnI3 based halide-perovskites for photovoltaic applications due to stable Sn4+ oxidation state and the strong covalency of Sn-I bonds in the [SnI6]2- octahedral. Cs2PdBr6, Cs2TiBr6, Cs2AgBiX6, CsGe0.5Sn0.5I3, AgBiI4, (MA,Cs,Rb)3Sb2I9 have been shown to be unsuitable for PV applications due to many reasons such as bandgap > 2 eV, low minority carrier diffusion lengths, deep level defects, etc. We have reproducibly created high quality thin-films of Pb-free perovskite Cs2PtI6 via atmospheric solution process with experimentally determined bandgap of ~ 1.4 eV. Cs2PtI6 crystallizes in a cubic crystal structure with the space group with high {111} peak intensities. SEM analysis shows well-formed micron-sized crystallites with improved film density with the use of ethylene diamine treatment. These atmospherically processed films contains low oxygen levels (mostly surface CsOx) and Cl impurity from PtI4 precursors as determined by EDS, ToFSIMS and XPS. Cs2PtI6 shows enhanced stability and excellent optical absorption, hole mobility and minority carrier lifetimes. For the first time we have demonstrated a Pb-free halide perovskite material, Cs2PtI6, with high absorption coefficient compared to MAPbI3 and minority carrier lifetime > 2 ?s. The high absorption coefficient in Cs2PtI6 results from the stronger d-p transitions as compared to the p-p transitions in MAPbI3. The combination of high absorption coefficient and high minority carrier diffusion lifetime in Cs2PtI6 suggests presence of direct-indirect transition behavior similar to MAPbI3, perhaps due to high Z of Pt. Superstrate n-i-p devices with FTO/CdS/Cs2PtI6/carbon/Cu have been reported for the first time, with maximum PCE of 13.88% and low Voc deficit. Further improvement in device efficiency is envisioned with the improvement of Cs2PtI6 film quality and better selection of ETL and HTL materials. It is noted that cost of Pt-based precursors is higher compared to Pb-based precursors, however, Pt offers a model system for Pb-free perovskite materials and may offer a pathway to ultra-thin wide bandgap top cells for tandem solar cells. The key advantages of Cs2PtI6 over MAPbI3 include high stability, absorption coefficient, minority carrier lifetimes and tunable bandgap for applications beyond photovoltaics such as water splitting, photocatalytic CO2 reduction, and other optoelectronic devices.

Perovskites have emerged as low-cost, high efficiency photovoltaics with certified efficiencies of 25.2% approaching already established technologies. The perovskites used for solar cells have an ABX3 structure where the cation A is methylammonium (MA), formamidinium (FA), or cesium (Cs); the metal B is Pb or Sn; and the halide X is Cl, Br or I. Unfortunately, single-cation perovskites often suffer from phase, temperature or humidity instabilities. This is particularly noteworthy for CsPbX3 and FAPbX3 which are stable at room temperature as a photoinactive “yellow phase”instead of the more desired photoactive “black phase”that is only stable at higher temperatures. Moreover, apart from phase stability, operating perovskite solar cells (PSCs) at elevated temperatures (of 85 °C) is required for passing industrial norms. Recently, double-cation perovskites (using MA, FA or Cs, FA) were shown to have a stable “black phase”at room temperature.(1,2) These perovskites also exhibit unexpected, novel properties. For example, Cs/FA mixtures supress halide segregation enabling band gaps for perovskite/silicon or perovskite/perovskite tandems.(3) In general, adding more components increases entropy that can stabilize unstable materials (such as the “yellow phase”of FAPbI3 that can be avoided using the also unstable CsPbI3). Here, we take the mixing approach further to investigate triple cation (with Cs, MA, FA) perovskites resulting in significantly improved reproducibility and stability.(4) We then use multiple cation engineering as a strategy to integrate the seemingly too small rubidium (Rb) (that never shows a black phase as a single-cation perovskite) to study novel multication perovskites.(5) One composition containing Rb, Cs, MA and FA resulted in a stabilized efficiency of 21.6% and an electroluminescence of 3.8%. The Voc of 1.24 V at a band gap of 1.63 eV leads to a very small loss-in-potential of 0.39 V, one of the lowest measured on any PV material indicating the almost recombination-free nature of the novel compound. Polymer-coated cells maintained 95% of their initial performance at 85°C for 500 hours under full illumination and maximum power point tracking. This is a crucial step towards industrialisation of perovskite solar cells. Lastly, to explore the theme of multicomponent perovskites further, molecular cations were revaluated using a globularity factor. With this, we calculated that ethylammonium (EA) has been misclassified as too large. Using the multication strategy, we studied an EA-containing compound that yielded an open-circuit voltage of 1.59 V, one of the highest to date. Moreover, using EA, we demonstrate a continuous fine-tuning for perovskites in the "green gap" which is highly relevant for lasers and display technology. The last part elaborates on a roadmap on how to extend the multication to multicomponent engineering providing a series of new compounds that are highly relevant candidates for the coming years.(6) (1) Jeon et al. Nature (2015) (2) Lee et al. Advanced Energy Materials (2015) (3) McMeekin et al. Science (2016) (4) Saliba et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy & Environmental Science (2016) (5) Saliba et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science (2016). (6) Turren-Cruz et al. Methylammonium-free, high-performance and stable perovskite solar cells on a planar architecture Science (2018)

Currently, many different perovskite -with corner-sharing octahedra- as well as non-perovskite metal-halide solids are synthetized worldwide entailing the need for in-depth understanding of their structure/property relationships. In this regard, combining the huge accumulated knowledge over the last decades, on halide but also oxide perovskites, with modern atomic scale modeling as well as symmetry analysis has proved useful. Among others, new compositions such as A'₂A_n-1M_nX_3n+1 (where A and A' are cations, X is halide and M is metal) afford layered structures with a controlled number (n, currently ≤ 7) of octahedra in the perovskite layer. Those correspond to innate heterostructures that offer an ideal platform for fundamental understanding such as effect of quantum or dielectric confinement.^[1]

Efforts have also been successful in building new perovskite structures with cations surpassing the Goldschmidt tolerance factor. For instance, this has been recently demonstrated for layered perovskites with the use of ethylammonium, isopropylammonium or dimethylammonium as perovskitizer, having a perovskite sheet retaining its continuous corner-sharing connectivity.^[2] On another note, new three dimensional networks have been achieved by mixing larger cations (e.g., hydroxyethylammonium) with the standard ones (e.g. methylammonium, formamidinium) leading to the formation of voids in the perovskite lattice, either disordered (Hollow perovskites) or with structural signature of a periodic pattern (Deficient perovskites).^[3]

In this talk, I will discuss some of our recent theoretical results paying attention on newly discovered halide perovskite phases and opportunities to further engineer their properties in connection with their use in devices.

References including references therein: [1] C. Katan, N. Mercier, J. Even, Quantum and dielectric Confinement Effects in Lower-Dimensional Hybrid Perovskite Semiconductors, Chem. Rev. 119, 3140 (2019); [2] Y. Fu, X. Jiang, X. Li, B. Traore, I. Spanopoulos, C. Katan, J. Even, M. G. Kanatzidis, E. Harel, Cation Engineering in Two-Dimensional Ruddlesden-Popper Lead Iodide Perovskites with Mixed Large A-Site Cations in the Cages, J. Am. Chem. Soc. 142, 4008 (2020); X. Li, Y. Fu, L. Pedesseau, P. Guo, S. Cuthriell, I. Hadar, J. Even, C. Katan, C. C. Stoumpos, R. D. Schaller, E. Harel, M. G. Kanatzidis, Negative Pressure Engineering with Large Cage Cations in 2D Halide Perovskites Causes Lattice Softening, J. Am. Chem. Soc. (2020) DOI: 10.1021/jacs.0c03860; [3] A. Leblanc, N. Mercier, M. Allain, J. Dittmer, T. Pauporté, V. Fernandez, F. Boucher, M. Kepenekian, C. Katan, Enhanced Stability and Band Gap Tuning of α-[HC(NH₂)₂]PbI₃ Hybrid, ACS Appl. Mater. Interfaces, 11, 20743 (2019); C. Zheng, O. Rubel, M. Kepenekian, X. Rocquefelte, C. Katan, Electronic properties of Pb-I deficient lead halide perovskites, J. Chem. Phys. 151, 234704 (2019); C. Quarti, C. Katan and J. Even, Physical properties of bulk, defective, 2D and 0D metal halide perovskite semiconductors from a symmetry perspective, under revision.

Halide perovskites have become increasingly popular semiconductors for application in photovoltaics, LEDs, transistors and electronics. While materials like methylammonium- or Cesium- lead halide perovskites are well studied, there remain vast regions of the ABX3 chemical space that are unexplored in terms of stability, electronic structure and defect behavior. There have been several attempts to tune the properties of halide perovskites by partial substitution at the A-site by Rb, K or other large organic cations, at the B-site by Sn, Ge or other divalent cations with similar ionic radii as Pb such as Ba, and at the X site by mixing of I, Br and Cl. To discover new perovskite compositions with improved properties, materials scientists would traditionally use an Edisonian approach to synthesize and test one composition after another until a candidate is found that is stable, robust, processable, and possesses the required properties. Data-driven approaches have rendered brute-force experimentation nearly a thing of the past; today, we can leverage better-than-ever-before computing power and increasingly optimal machine learning methodologies to develop a knowledgebase of semiconductor materials that allows instant access to properties of existing compounds and efficient design of novel compositions for applications in solar cells, infrared sensors, wearable electronics and quantum information sciences. In this work, we combine first principles computations and machine learning to develop a general prediction and design framework for mixed composition halide perovskites with tailored structural, energetic, optoelectronic and defect properties. We consider a chemical space of cubic ABX3 perovskites where A is selected from the set {FA, MA, Cs, Rb, K}, B from the set {Pb, Sn, Ge, Ba, Sr, Ca}, and X from {I, Br, Cl}. Density functional theory (DFT) computations are performed to generate a dataset of the pseudo-cubic crystal structure, formation and mixing energy, band gap, dielectric constant and cation/anion vacancy formation energy of several hundred known and hypothetical perovskite compositions. This compositionally diverse perovskite dataset along with a set of unique descriptors-based on tabulated elemental properties of constituent elements at the A, B and X site in the compound-serve as input to a neural network (NN) model with multiple output neurons to simultaneously predict each of the aforementioned properties. This model forms the material à properties 'forward prediction' half of the design framework and is applicable to any amount of mixing at the A, B and X sites, which opens up immediate access to extreme levels of alloying that are expensive to model computationally. Following this, the NN model is combined with a mathematical optimization scheme employing genetic algorithms (GA), Bayesian optimization (BO) and generative adversarial networks (GAN) to efficiently design novel compositions with a set of targeted structural, energetic, electronic and defect properties, completing the properties à material 'inverse design' component. This AI-based design framework is used to quickly screen promising new perovskites which we study in greater detail using more expensive calculations at advanced levels of theory (eg. hybrid functionals that predict band gaps more accurately), and recommend such materials to experimental collaborators for synthesis, testing and characterization. With further expansions in terms of data, descriptors and learning strategies, this framework can lead to the definitive prediction and design methodology that covers the entire halide perovskite chemical universe and informs the creation of new materials that would have taken years of computation or experimentation to be discovered in the past. References 1. A. Mannodi-Kanakkithodi et al., Chemistry of Materials 31 (10), 3599-3612 (2019). 2. A. Mannodi-Kanakkithodi, et al., under review, preprint: https://arxiv.org/abs/1906.02244.

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) represent diverse quantum well heterostructures composed of alternating inorganic and organic layers. While 2D HOIPs are nominally periodic in three dimensions for X-ray scattering, the inorganic layers can orient quasi-randomly, leading to rotational stacking disorder (RSD). RSD manifests as poorly resolved, diffuse X-ray scattering along the stacking direction, limiting the structural description to an apparently disordered subcell. However, local ordering preferences can still exist between adjacent unit cells and can considerably impact the properties, particularly the electronic structure. Here, we elucidate RSD and determine the preferred local ordering in the 2D [AE2T]PbI4 HOIP (AE2T: 5,5?-bis(ethylammonium)-[2,2?-bithiophene]). We use first-principles calculations to determine energy differences between a set of systematically generated supercells with different order patterns. We show that interlayer ordering tendencies are weak, explaining the observed RSD in terms of differing in-plane rotation of PbI6 octahedra in neighboring inorganic planes. In contrast, the ordering preference within a given organic layer is strong, favoring a herringbone-type arrangement of adjacent AE2T cations. The calculated electronic level alignments of proximal organic and inorganic frontier orbitals in the valence band vary significantly with the local arrangement of AE2T cations; only the most stable AE2T configuration leads to an interfacial type-Ib band alignment consistent with observed optical properties. The present study underscores the importance of resolving local structure arrangements in 2D HOIPs for reliable structure-property prediction.

Recent computational design strategies towards designing new materials that do not contain Pb have been remarkably successful, and lead to the discovery of a series of novel compounds within the so-called halide double perovskites family. [1] Today there are five inorganic crystals that have been synthesized and characterized; Cs2BiAgCl6, Cs2BiAgBr6, Cs2SbAgCl6, Cs2SbAgBr6, and Cs2InAgCl6. Among these, Cs2BiAgBr6 has the narrower indirect band gap of 1.9 eV, and Cs2InAgCl6 is the only direct band gap semiconductor, yet with a large gap of 3.3 eV. All of them exhibit low carrier effective masses and consequently, are prominent candidates for opto-electronic applications including photovoltaics. Here, we probe oxide perovskites to look for potential promising oxide perovskites following our substitution-based computational design approach. We identify a new oxide double perovskite semiconductor: Ba2AgIO6 (BAIO), and report on its synthesis by low-temp solution process. BAIO has a band gap in the visible range (i.e. 1.9 eV), and a broad PL spectrum. Surprisingly, BAIO has an electronic band structure remarkably similar to that of our recently discovered perovskite Cs2AgInCl6. This allows us to further develop a universal analogy concept that can be used to identify links between oxide and halide perovskites. As such example, we show how BAIO and Cs2AgInCl6 are both analogs of the well-known transparent conductor BaSnO3, but the significantly lower band-gap of BAIO makes this new compound more promising for oxide-based optoelectronics and for novel monolithic halide/oxide devices.

[1] Patent WO 2017/037448 Al (2015); JPCL 7 1254 (2016); JPCL 8 772 (2017); APL 112 243901 (2018)
[2] JPCL 8 3917 (2017)
[3] Patent Application 19386013.7 (2019) ; JPCL 10 1722 (2019)

Halide-based double perovskites have gained significant attention in recent years, owing to the ability to replace the toxic Pb²⁺ cation with a pair of more benign cations (e.g. Ag⁺ and Bi³⁺) while preserving the perovskite crystal structure. However, most double perovskites, including the prototypical Cs₂AgBiBr₆, have wide bandgaps, limiting photoconversion efficiencies. Recently, it has been demonstrated that the bandgap of Cs₂AgBiBr₆ can be reduced through alloying with Sb³⁺. But the bandgap of the pure Sb-based compound (Cs₂AgSbBr₆) is also wide. A particular challenge with the Sb-Bi double perovskite alloy system is that materials with high Sb content are difficult to synthesize phase-pure due to the high formation energy of Cs₂AgSbBr₆. In this talk, we show a solution-based route we developed to achieve phase-pure Cs₂Ag(Sb_xBi_1-x)Br₆ thin films with the mixing parameter x tunable over the entire composition range. In doing so, we reveal this system to disobey Vegard’s law, exhibiting significant bandgap bowing, such that the mixed alloys (x between 0.5 and 0.9) demonstrate smaller bandgaps (down to 2.08 eV) than the pure Sb- (2.18 eV) and Bi-based (2.25 eV) double perovskites. Based on the calculated electronic band alignment, the origin of bandgap bowing in this double perovskite alloy is found to be electronic, rather than structural, in nature. The staggered bandgap alignment between Cs₂AgBiBr₆ and Cs₂AgSbBr₆, in combination with non-linear mixing of electronic states, via either short-range ordering or spin-orbit effects, produces a bandgap lower than that of either pure material. This work demonstrates a method to reduce the bandgap of Cs₂AgBiBr₆ double perovskites, such that it is better suited to photovoltaic or photocatalytic applications. More broadly, our results suggest that bandgap bowing could be found in other lead-free double perovskite alloys by pairing together materials with band positions forming a Type II heterojunction, thus offering a new route to engineer the bandgaps of these materials.

Tin-based perovskites with excellent optoelectronic properties are promising candidates for fabricating efficient lead-free perovskite solar cells (PSCs). Unfortunately, Tin-based PSCs exhibited a low efficiency, which mainly arises from serious traps-induced non-radiative recombination, leading into large energy loss with a low voltage, more seriously, Sn²⁺ is leasily oxizated to Sn⁴⁺, which leads into p-type self-doping with devices having high dark current denisty. Herein, the polymer poly(ethylene glycol) diacrylate (PEGDA) is employed as a template-agent in FASnI₃ to control nucleation and growth is reported. This FASnI₃ layer showed a compact crystal with large grain size and reduced defects. As a result, the fabricated planar heterojunction device of ITO/PEDOT:PSS/FA_xPEA_1-xSnI₃ /ICBA/BCP/Ag showed the power conversion efficiency (PCE) of 9.35% with voltage of 0.75 V. Moreover, corresponding pervoskite solar cells exhibit an enhanced stability.

Improving the heat–moisture–light stability of organic–inorganic perovskites, an outstanding semiconductor material class, is a critical challenge.[1] To identify the most stable multi-cation lead iodide perovskites containing Cs, formamidinium (FA) and methylammonium (MA), we fuse results from density functional theory (DFT) calculations and high-throughput in situ thin-film degradation test within a singular machine learning (ML) algorithm to inform the compositional optimization of Cs_xMA_yFA_1-x-yPbI₃.[2] We identify thin-film compositions centred at Cs_0.17MA_0.03FA_0.80PbI₃ that achieve a 3x delay in macroscopic degradation onset under elevated temperature, humidity and light compared with the more complex Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃. [3] We find up to 8% of MA can be incorporated into the perovskite structure before stability is significantly compromised. Cs is beneficial at low concentrations, however beyond 17% is found to contribute to reduced stability. Synchrotron-based grazing-incidence wide-angle X-ray scattering (GIWAXS) further reveals that the CsPbI₃ minority phase is both a degradation product and an accelerator for chemical decomposition in hot and humid environments, and our findings highlight this detrimental effect can be kinetically suppressed by co-optimising Cs and MA content. Our findings highlight the detrimental effect of CsPbI₃ in hot and humid environments both as a degradation product and an accelerator for further chemical decomposition even in top-performing multi-cation compositions.[4] This indicates a limitation of Cs as a perovskite-stabilizing agent, despite it has been to date one of the most widely used cations for environmental stability and performance enhancement in perovskite optoelectronics.
References
[1] M. Saliba, “Polyelemental, Multicomponent Perovskite Semiconductor Libraries through Combinatorial Screening,” Advanced Energy Materials, vol. 9, no. 25, p. 1803754, May 2019.
[2] L. T. Schelhas et al., “Insights into operational stability and processing of halide perovskite active layers,” Energy and Environmental Science, vol. 12, no. 4, pp. 1341–1348, 2019.
[3] J.-P. Correa-Baena et al., “Homogenized Halides and Alkali Cation Segregation in Alloyed Organic-inorganic Perovskites.,” Science (New York, N.Y.), vol. 363, no. 6427, pp. 627–631, Feb. 2019.
[4] M. V. Khenkin et al., “Consensus statement for stability assessment and reporting for perovskite photovoltaics based on ISOS procedures,” Nature Energy, vol. 5, no. 1, pp. 35–49, Jan. 2020.

Stability is one of the most pressing issues facing commercialization of hybrid perovskite solar cells. While there have been encouraging recent improvements in the longevity of efficient perovskite devices,^{1, 2} researchers have yet to demonstrate their ability to reach useful lifetimes on the order of decades, or even years. Key steps in reaching this goal include identifying behaviors that appear early in the life cycle of a perovskite film or device that are indicative of future failure, and developing quantitative links between these early-time features and metrics of sample lifetime. Accurate and general lifetime models that can bridge the gap between accelerated testing and normal operating conditions will enable more efficient troubleshooting of problems in state-of-the-art perovskite solar cells and hence be of great value to the photovoltaics community.
To address this challenge, we have developed techniques that monitor in situ the evolution of key optoelectronic figures of merit of perovskite thin films in response to varied environmental stresses. By combining measurements of photoluminescence, photoconductivity, and optical transmittance, we gather time-resolved information into carrier generation (quantified by quasi-Fermi level splitting/QFLS), carrier transport (quantified by mean carrier diffusion length L_D), and material integrity, respectively.³ We show that carrier transport failure is the primary mechanism curtailing photovoltaic performance of hybrid perovskites, evidenced by a rapid drop in L_D that, in most cases, occurs before the QFLS has reached a maximum. The time t_LD75 at which L_D has decreased to 75% of its starting value is therefore a useful measure of the perovskite film’s effective lifetime. Machine learning models employing a greedy feature selection algorithm predict ln(t_LD75) for MAPbI₃ to within about 12%, with a dataset spanning a wide range of environmental stress conditions. The initial time derivative of transmittance has high predictive weight in these models, illustrating the importance of the connection between the material’s initial physical degradation rate and its eventual loss of connectivity between undegraded domains within the film. As an alternative means of characterizing material integrity, we have employed dark-field optical microscopy, which is sensitive to scattered light due to roughness or secondary phases, and unlike transmittance is compatible with in situ observations of devices with a metal back contact. Heterogeneity of the dark field images of degrading perovskite films evolves in a way that is highly correlated with their transmittance, showing that this technique can be used to connect changes in solar cell operating characteristics (such as shunt resistance) with local failure of the absorber material. These early successes demonstrate that simple in situ optical techniques provide extremely useful physical information that aids the task of forecasting perovskite failure.

References:
1. Bai, S.; Da, P.; Li, C.; Wang, Z.; Yuan, Z.; Fu, F.; Kawecki, M.; Liu, X.; Sakai, N.; Wang, J. T.; Huettner, S.; Buecheler, S.; Fahlman, M.; Gao, F.; Snaith, H. J., Planar Perovskite Solar Cells with Long-Term Stability Using Ionic Liquid Additives. Nature 2019, 571 (7764), 245-250.
2. Christians, J. A.; Schulz, P.; Tinkham, J. S.; Schloemer, T. H.; Harvey, S. P.; Tremolet de Villers, B. J.; Sellinger, A.; Berry, J. J.; Luther, J. M., Tailored Interfaces of Unencapsulated Perovskite Solar Cells for >1,000 Hour Operational Stability. Nat. Energy 2018, 3 (1), 68-74.
3. Stoddard, R. J.; Dunlap-Shohl, W. A.; Qiao, H.; Meng, Y.; Kau, W. F.; Hillhouse, H. W., Forecasting the Decay of Hybrid Perovskite Performance Using Optical Transmittance or Reflected Dark-Field Imaging. ACS Energy Lett. 2020, 5 (3), 946-954.

Low-dimensional metal halides have been the focus of intense investigations in recent years following the success of the three-dimensional (3D) hybrid lead halide perovskites as optoelectronic materials for solar cells, semiconductor radiation detectors, and narrow emitters. In particular, the light emission of zero-dimensional (0D) metal halides based on the 5s² cations Sn^II and Sb^III have found utility in a variety of applications (including X-ray scintillation and remote thermometry) that are complementary to those of the 3D halide perovskites, thanks to their unusual properties including intrinsic broadband emission and highly temperature-dependent lifetimes. These properties derive from the exceptional chemistry of the 5s² lone pair, which generates the appropriate atomic environments for efficient emission as it lies on the cusp of the static 4s² lone pair and the dynamic but less pronounced 6s² lone pair. However, the descriptions and terminology of this unusual emission vary significantly, complicating efforts of various researchers to build a cohesive understanding and enable the design such emissive materials.

Here, we provide a structural overview of these materials with a focus on the dynamics driven by the stereoactivity of the 5s² lone pair to identify the structural features that enable strong emission. The chemistry of the 5s² lone pairs in various halide environments is described and compared to those of the neighboring 4s² and 6s² cations to highlight the unique combination of stereoactivity and structural flexibility of these main group metals. We relate the different theoretical models which have been able to explain the success of these bright 5s² emission centers into a cohesive framework, which is then applied to the array of compounds recently developed by our group and other researchers, demonstrating its utility and generating a holistic picture of the field from the point of view of a materials chemist. We summarize the most promising materials to identify the common trends among them, finding that Sn(II) bromides and Sb(III) chlorides lead the field with the highest efficiencies, highlighting the importance of metal-halogen overlap in the lone pair dynamics. We highlight the state-of-the-art applications which demonstrate the unique capabilities of these versatile emissive centers, describing the current progress and challenges for each application to identify promising future directions in the field. This work introduces the field of low-dimensional 5s² metal halides, pointing the way towards an understanding that will enable the design of such materials for optoelectronic devices that deliver on the promise of these applications.

“Defect tolerance” has been commonly considered as the origin of the high photovoltaic performance of halide perovskites. However, a rigorous assessment of this concept is still lacking. Based on accurate first-principles calculations, we quantitatively show that halide perovskites are not “defect tolerant”; defect-assisted nonradiative recombination is actually a key limitation of perovskite solar cells. The conventional wisdom suggests that only defects with charge-state transition levels deep in the band gap can be efficient nonradiative recombination center. We demonstrate that the presence of charge-state transition levels in the band gap is a prerequisite for being a nonradiative recombination center, but unlike in many conventional semiconductors the position of the level does not directly imply high or low capture rates because of exceptionally strong lattice coupling and anharmonicity in halide perovskites. Explicit calculations of the nonradiative capture rates are essential. We emphasize that accurate calculations, based on hybrid density functional theory and including spin-orbit coupling, are essential for obtaining reliable results. Our results show that iodine interstitials, although their transition levels are close to the band edges, can actually act as efficient nonradiative recombination centers in methylammonium-lead iodide, and are likely responsible for the experimentally observed nonradiative recombination rates. We also address the impact of impurities. The Bi impurity has been commonly believed to be an efficient recombination center. Our rigorous calculations show that while the Bi impurity does have a transition level in the band gap, it is, in fact, not a recombination center on its own. The observed impact on device performance occurs indirectly, via promoting the formation of iodine interstitials. These insights clarify misconceptions in the field, and point to fruitful directions for defect engineering toward enhanced performance.

Akin to the expansion in compositional diversity that halide double perovskites provided to three-dimensional perovskites [1-2], layered double perovskites could further expand the diversity of two-dimensional (2D) perovskites, and therefore, they could also enhance the properties or expand the possible applications of such materials [3]. Despite the great promise of halide 2D double perovskites, up to date, there are only four confirmed members of this family of materials [3-6]. Herein, we explore 90 hypothetical new members of this family of materials by a combined theoretical, computational, and experimental method. The combination of these tactics allowed us to predict several new materials, out of which we experimentally synthesized and characterized five new layered double perovskites, some of which show promising properties for their use in photovoltaics and optoelectronics. Further, our work highlights the vast diversity of compositions and therefore of applications that double-layered perovskites have yet to offer. References [1] X. Zhao, D. Yang, J. Ren, Y. Sun, Z. Xiao and L. Zhang, Joule, 2018, 2, 1-12. [2] L. Chiu et al, Nano-Micro Lett, 2019, 11:16. [3] B. Vargas, E. Ramos, E. Pérez-Gutiérrez, J.C. Alonso, D. Solis-Ibarra, J. Am. Chem. Soc, 2017, 139, 27, 9116-9119. [4] B. Vargas, R. Torres-Cadena, J. Rodríguez-Hernández, M. Gembicky, H. Xie, J. Jiménez-Mier, Y-S. Liu, E. Menéndez-Proupin, K. R. Dunbar, N. Lopez, P. Olalde-Velasco, D. Solis-Ibarra, Chem. Mater, 2018, 30, 5315-5321. [5] B. A. Connor, L. Leppert, M. D. Smith, J. B. Neaton, H. I. Karunadasa, J. Am. Chem. Soc, 2018, 140, 5235-5240. [6] C. Ortiz-Cervantes, P. Carmona-Monroy, D. Solis-Ibarra, ChemSusChem, 2019, 12, 1560-1575.

The synthesis of perovskite materials has gone beyond the traditional wet chemistry routes into other methods such as a sequential ion exchange process that can produce perovskite materials from calcite in either thin film or bulk form. The use of lead (Pb), the most successful element in terms of PSC efficiency performance, was already successfully demonstrated in an ion exchange process. In this work, tin (Sn) is chosen since it has the most potential to replace Pb since they are on the same group and for the similarity in ionic radii of their ions (Pb2+, Sn2+) at 1.19 and 1.02 ?, respectively. Methyl ammonium tin triiodide (MASnI3) has a lower bandgap (1.2 to 1.4 eV) that induces broader absorption spectrum as compared with the benchmark MAPbI3 counterpart (1.55 eV). The second alternative - strontium (Sr) has an ionic radius of 1.18 ? indicating possible ion exchange in the strontianite structure. The band gap for the Sr perovskite was estimated at 3.6 eV, considerably higher than that for the lead perovskite and therefore not suitable for direct solar cell applications but can be used as a top cell in tandem architectures. These ion exchange processes are observed using x-ray absorbance and photoelectron spectroscopies to elucidate the underlying mechanisms of the sequential ion exchange process. These will shed light into the material transformation with shape preservation from the raw shells or single crystal calcite towards the intermediate carbonate material and eventually, the Pb-free perovskite material.

The bismuth-based (oxy)iodides BiI3, BiOI and AgxBiIx+3 share similar layered crystal structures, optimal band gaps for top absorbers in tandem solar cells, and moderate growth temperatures. Similarly to halide perovskite absorbers, they contain a heavy cation with a lone pair of electrons (Bi3+) which has been proposed as one of the features enabling defect tolerance in perovskites. We have grown and characterized BiI3, BiOI, and Ag3BiI6 absorbers and solar cells in a systematic manner by employing a consistent synthesis method ((oxy)iodization of metallic precursor) and a consistent analysis routine. In this way, the individual strengths and weaknesses of the three absorbers, as well as their common challenges, can be outlined. The radiative efficiency of the three materials is within the same order of magnitude, thus indicating a similar degree of defect tolerance. Control of growth orientation should be a priority for this class of materials in view of their anisotropic properties. P-type bulk doping and selection of hole transport layers with deep valence bands could also improve performance. At the device level, we report an improved open circuit voltage of BiI3 solar cells with respect to the state of the art, and we report a proof-of-concept Ag3BiI6/silicon tandem cell. Beyond photovoltaics, the very low dark diode ideality factor in BiI3 devices and the existence of both electronic and ionic conduction in Ag3BiI6 may open up alternative unexpected applications in other fields.

Zero-dimensional (0D) inorganic and hybrid perovskites have attracted great interest for white-light emitting applications due to their broadband emissions originating from the self-trapping excitonic states. In this work, we study a series of 0D inorganic perovskites, A4PbX6 and A4SnX6 (A = K, Rb, and Cs; X = Cl, Br, and I), under the theoretical framework of one-dimensional configuration coordinate diagram using density functional theory (DFT) calculations. We demonstrate that the formation of self-trapping excitonic states of A4PbX6 and A4SnX6 is attributed to the local structural distortion of individual [PbX6]4- and [SnX6]4- octahedron (i.e., the octahedral shrinking under photoexcitation). We further predict the broadband emissions of a new family of 0D A4PbX6/A4SnX6 heterostructures, and find that among these crystal structures Rb4PbI6/Rb4SnI6 exhibits an increased photoluminescence efficiency with good stability as compared to pristine Rb4SnI6. Our findings provide a new material design strategy for boosting the self-trapping emissions with improved stability using 0D inorganic perovskite heterostructures.

As halide perovskites (PVKs) continue to garner scientific interest for their potential to enhance efficiencies of solar cells and LEDs, a push has been made towards lead-free and completely inorganic PVKs with increasingly complex stoichiometries. One example of this is the double PVK Cs₂AgBiBr₆and Sn:Cs₂AgBiBr₆. Synthesis of thin films with these complex stoichiometries is challenging due to the distinct volatility of the constituent elements, usually requiring multi-step or multi-source processes. Here we explore Pulsed Laser Deposition (PLD) as a route to grow these types of complex materials with a single source. PLD offers the advantage of being a volatility-insensitive thin film synthesis technique which we use to demonstrate single-source deposition of Cs₂AgBiBr₆ and Sn:Cs₂AgBiBr₆. For this, solid targets are made first by mechanochemical synthesis of powders, followed by a uniaxial and isostatic pressing to form the powders into dense, compact targets. X-ray diffraction (XRD) is used to verify the desired PVK phase is maintained along each step of target fabrication. Laser-ablation of the double PVK targets in vacuum leads to formation of Cs₂AgBiBr₆ and Sn:Cs₂AgBiBr₆ double PVK films after optimization of the deposition parameters. We discuss the effects of deposition pressure and laser fluence on the formation of stoichiometric and phase-pure films, as well as the critical role of the thermalization of the source elements to achieve stoichiometric transfer. In addition to the structural properties, the optoelectronic properties of the films were studied by UV-Vis-NIR spectrophotometry and time-resolved photoluminescence. Notably, >1 um thick films are deposited and all films present high structural stability over the course of 3 months, without the need for an annealing step. This work represents an important step forward in the development of controlled growth and future scalability of complex halide perovskites for efficient optoelectronic devices.

Lead halide perovskite is considered as a potential candidate for the next generation light emitting diodes (LEDs) due to its extremely narrow light emission spectrum with a typical full width half maximum (FWHM) < 20 nm. In particular, all-inorganic CsPbBr3 has demonstrated the near unity photoluminescence quantum yield (PLQY) for green light emission. A more challenging, yet technologically important, task is developing a high performance blue light emitter. By partially replacing Br of CsPbBr3 with Cl, the emission wavelength can be adjusted to the blue emission; however, CsPbBr3-xClx , suffers from ion migration and phase segregation which result in low PLQY and poor stability. Here, we present a synthesis method to produce nanostructure CsPbBr3 in the form of nanoplatelets (NPLs) that is capable of emitting blue light without the need for anion-substitution. Nanoplatelets with 2 monolayer (ML) and 3 ML of thickness yielded photoluminescence (PL) peak at 435nm with an extremely narrow FWHM of 14.2 nm and PL peak at 456nm with FWHM of 19.6nm, respectively. The luminescence of these nanoplatelets was further improved by post-synthesis treatment with tetrafluoroborate salts-almost 1150% increase in PL intensity was observed without a noticeable broadening of the PL spectrum. Details of the results and discussion of a possible mechanism behind the improvement with the post-synthesis treatment will be presented.

All inorganic halide perovskite CsPbBr3 has emerged as the next-generation material for light-emitting diodes (LEDs) due to its high color purity with a narrow spectral width (< 20 nm) and tunable band gap by adjusting the composition of halide anions, i.e. substitution of Br with I or Cl. Considering the needs for large-area scalability and fine control of the emitter thickness, vacuum-deposition will be a more commercialization-friendly technique to produce perovskite films for LEDs. Here, we prepare CsPbBr3 light emitters by thermal co-evaporation of CsBr and PbBr2 sources. The luminescence of the vacuum-deposited CsPbBr3 films was greatly improved by introducing a poly-ethylene oxide (PEO) layer underneath the CsPbBr3-almost 300-fold increase in photoluminescence quantum yield (PLQY) with the PEO layer compared to the reference sample (CsPbBr3/glass). It is believed that the interaction between PEO and Pb2+ ions leads to the passivation of non-radiative recombination defects. After the careful optimization of the PEO layer thickness and post-deposition thermal treatments of CsPbBr3, we have demonstrated PLQY of 30% from a peak at 521 nm with full-width-half-maximum of ~17nm. Furthermore, the samples exhibit excellent ambient stability. Details of the results and analysis will be discussed.

In current days, development and search of new materials for solid state thin film gas sensor to detect toxic gases, particularly using nanomaterial for their enhanced functionality, is growing rapidly as view point of both environmental and clinical application. Particularly, gas sensor with low power consumption, room temperature operation, portability and cost effectiveness is highly demandable. Perovskite halides, namely methyl ammonium lead iodide (CH3NH3PbI3) / MAPI is widely popular as photovoltaic and optoelectronic material. Recently we have found that MAPI can act also as an active material for gas sensing to trace hazards pollutants. Ammonia (NH3) not only is one of harmful pollutants in nature but also treated as a biomarker for kidney disease via exhaled breathe analysis in recent days. Thus a room temperature , cost effective, very high sensitivity NH3 gas sesor could extremely beneficial in view point of both environmental and clinical issues. The sensor using MAPI can trace ammonia gas (NH3) by a visual color change method as well as by electrical read out [1-2]. Here we have shown a MAPI based electrical sensor grown on a cheap paper as a substarte for tracing NH3 gas. The paper sensor has a very high sensitivity (98% for 10 ppm NH3) towards NH3 gas at room temperature and sub ppm (<1 ppm) detection capability (down to 10 ppb) by its noise limited detectibility [2]. This ultra high response down to ppb level can be extremely suitable for exhaled breathe analysis. Current through the sensor increases nearly by one order on exposure to only 10 ppm NH3 gas with a bias voltage of 1 V DC and hence the sensor consumes extremely low power (~ few nano watts). The paper sensor is also highly selective to NH3 gas and maintains stability over 150 days within decrease of 10% sensitivity. This could open up the possibility of perovskite halide (MAPI) as next generation solid state room temperature gas sensor for rapid and cost effective detection using paper electronics. References: [1] Maity, A; Ghosh,B; Fast Response paper based visual color change gas sensor for efficient ammonia detection at room temperature; Scientific Reports (2018), 8,16851 [2] Maity, A; Raychaudhuri, A.K.; Ghosh,B ; High sensitivity NH3 gas sensor with electrical readout made on paper with perovskite halide as sensor material; Scientific Reports (2019) ,9,7777 I

Semiconducting metal-halide perovskites present various types of chemical interactions which give them a characteristic fluctuating structure sensitive to the operating conditions of the device, to which they adjust. This makes the control of structure-properties relationship, especially at interfaces where the device realizes its function, the crucial step in order to control devices operation. In particular, given their simple processability at relatively low temperature, one can expect an intrinsic level of structural/chemical disorder of the semiconductor which results in the formation of defects. Here, first I will summarize our understanding of the nature of defects and their photo-chemistry, which leverages on the cooperative action of density functional theory investigations and accurate experimental design. Then, I will show the correlation between the nature of defects and the observed semiconductor instabilities. Instabilities are manifested as light-induced ion migration and segregation, eventually leading to material degradation under prolonged exposure to light. Understanding, controlling and eventually blocking such material instabilities are fundamental steps towards large scale exploitation of perovskite in optoelectronic devices. Finally, based on such knowledge, I will discuss different synthetic and passivation strategies which are able to stabilize the perovskite layer towards photo, thermal and electrical instabilities, leading to improved optoelectronic material quality and enhanced device reliability

The rapid development of perovskite solar cells has been based on improvements in materials and device architectures, yet further progress towards their theoretical limit will require a detailed study of the main physical processes determining the photovoltaic performance. In this talk, we combine optical and electrical spectroscopy measurements to gain insight into the main loses in perovskite solar cells. The analysis of solution-processed devices with controlled precursor concentrations is the prefect platform to develop an impedance spectroscopy equivalent circuit.[1] The model provides a tool to evaluate the processes directly affecting the device performance, including charge recombination and charge transport. In-situ absolute photoluminescence measured during cell operation enables an alternative way to probe the device recombination mechanisms. Applied to vacuum deposited perovskite solar cells, it can directly assess radiative recombination, as well as the variation of the relative radiative /non-radiative along the measured potentials. Altogether, we provide a complete view of the main loss mechanisms in perovskite solar cells, yet in a pragmatic way which allows its application as a device evaluation tool. [1] Yoo et al., Joule 3, 2535-2549

Perovskite solar cells (PSCs) have become a competitive photovoltaic (PV) technology with rapid progress of efficiencies reaching a certified 25.2% for single-junction devices. The bandgap tunability through perovskite composition engineering is attractive for developing ultrahigh-efficiency tandem solar cells, including perovskite/perovskite, perovskite/silicon, or perovskite/thin-film absorber (e.g., CIGS). Toward this end, highly efficient wide-bandgap and low-bandgap perovskites solar cells need to be development. In this talk, I will discuss our recent progress on suppressing defects in perovskites covering both the wide-bandgap (~1.7–1.8 eV) and low-bandgap (~1.2–1.3 eV) perovskite compositions. Strategies on reducing the bulk defects by utilizing 2D perovskite passivation agents (2D-PPA) will be discussed. The precursor chemistry and growth conditions affect significantly the physical and optoelectronic properties of perovskites. Surface recombination at an interface in a device is also critical to device operation. Characterization and strategies to reduce surface recombination will be presented. Applications of perovskite absorbers in highly efficient tandem devices will be discussed. Finally, the potential toxicity issue of Pb and the corresponding mitigation approaches will be discussed.

The intrinsic ionic defects, specifically halide ion vacancies, often dictate the mobility of halide species within the perovskite film during the operation of solar cells. Of particular interest is the halide ion mobility in metal halide perovskites, which plays an important role in determining the performance of perovskite solar cells. Photoinduced phase segregation seen in mixed halide perovskite films under steady state irradiation offers a convenient way to visualize halide ion segregation. Interestingly, upon storage in dark, the process is reversed and the original mixed halide composition gets restored. Whereas entropy of mixing explain the thermally activated mixing of halide ions to yield mixed halide perovskite, the opposite trend observed during photoirradiation remains an intriguing phenomenon. The threshold energy of incident light to observe halide segregation increases with increasing temperature. The diffusion of these halide species, which is tracked through changes in the absorption spectra at different temperatures, offers a direct measurement of thermally activated halide diffusion in perovskite films. The increase in the rate constant of halide diffusion with increasing temperature (from 5×10^-5 s^-1 at 23 °C to 2.2×10^-3 s^-1 at 140 °C) follows an Arrhenius relationship with activation energy of 51 kJ/mole. The thermally activated halide exchange shows the challenges of employing layers of different metal halide perovskites in stable tandem solar cells

Defect tolerance has emerged as a central theme for the success of halide perovskites as photovoltaics. Some current estimates put the number of mobile ionic defects in common halide perovskites above 1017 cm-3, yet the halide perovskite band structure and resulting shallow nature of the hypothesized dominant defects limits their ability to act as recombination centers in these materials. However, this defect tolerance does not imply that these ionic defects have no impact on device performance. Current-voltage hysteresis, reverse-bias breakdown, photo-brightening, photodegradation, thermal stability, and other behaviors related to perovskite performance have all been attributed to the appreciably ionic transport in perovskites made possible by this large defect population. In this work I will present recent advances in both the development of experimental methods for analyzing the formation and behavior of mobile ions in halide perovskites, and our understanding of the effects of these mobile ions on device performance. I will discuss methods to investigate defect formation and transport, including our results using in situ photoluminescence and X-ray diffraction techniques to track ion movement and correlate that with the optoelectronic properties of methylammonium perovskites. Additionally, I will share our work looking at the impacts of mobile ions on device performance, particularly with regards to limits on the open-circuit voltage of perovskite solar cells. Finally, I will highlight the unique roles that contacts and surfaces can play in optimizing the performance and stability of perovskite solar cells in light of these mobile ionic defects.

Solar cells, light-emitting diodes and photodetectors are amongst the promising examples of the use of hybrid organic-inorganic perovskite semiconductors in optoelectronic applications. The surface of these materials plays a crucial role in the performance and stability of such devices as the presence of surface trap states strongly affects the recombination rate of excited charge carriers. It has previously been reported that molecular ligands, such as benzylamine, are capable of significantly reducing the amount of surface trap states in thin films. In this work, we aim to clarify the mechanisms that govern the surface passivation of hybrid perovskites by benzylamine. We use freshly cleaved single crystal to investigate the influence of benzylamine on a well-defined perovskite surface. We demonstrate that benzylamine permanently passivates surface trap states in these single crystals which results in enhanced photoluminescence and charge carrier lifetimes. Additionally, we show that, when exposed to benzylamine, the methylammonium cations are replaced by benzylammonium, thereby creating a thermodynamically more stable two-dimensional (2D) perovskite (BA)2PbBr4 on the surface of the three-dimensional crystal. The formation of this 2D perovskite phase drives a highly anisotropic etching of the underlying crystal surface, with the {100} planes being most prone to etching. Initially, isolated square etching pits appear spread over the surface. As time elapses, these etching pits broaden and eventually merge to give rise to large flat terraces that are oriented normally to the cleaving plane. These results highlight that even a simple passivation procedure can have a marked effect on the morphology and composition of the perovskite material. A thorough understanding of the mechanisms governing the surface passivation is thus crucially important to optimize and design novel passivation schemes, with the ultimate goal of further advancing the efficiency of optoelectronic devices based on hybrid perovskites.

Using Environmental X-ray Photoelectron Spectroscopy (EXPS), we evaluate the solid and gas phase products from degradation of metal halide perovskites (MHPs) during interaction with water and light. The stability of novel MHPs is traditionally assessed under accelerated stress conditions in complete photovoltaic devices but the associated trial and error approach provides little insight into the chemical origin of device or material failure. Alternatively, EXPS addresses MHP instability head on, testing resilience against water and other stimuli monitored by surface chemistry and electronic structure. With the environmental chamber of the EXPS we control sample temperature, atmosphere, and illumination. A quadrupole mass spectrometer allows identification and quantification of gaseous products. We report time-resolved chemical information about interface reactions, changes to surface bonding, activation energies, and chemical pathways to determine degradation pathways in situ. Using the multi-messenger EXPS approach, we observe significant chemical changes under accelerated stress factor degradation tests within one working day and compare these results to MHPs degraded under standard water and light stress conditions. These results provide insight into previous studies describing the high defect tolerance of MHPs in juxtaposition to their instability under operating conditions after certain chemical thresholds are met. (Snaith, 2018; Steirer et al., 2016). Snaith, H.J., 2018. Present status and future prospects of perovskite photovoltaics. Nat. Mater. 17, 372-376. https://doi.org/10.1038/s41563-018-0071-z.

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

Two-dimensional halide perovskites are exciting new semiconductors that show great promising in low cost and high-performance optoelectronics devices including solar cells, LEDs, photodetectors, transistors, etc. In the first part of this talk, I will present a molecular approach to the synthesis of high-quality organic-inorganic hybrid perovskite quantum wells through incorporating widely tunable organic semiconducting building blocks as the surface capping ligands. By introducing sterically tailored groups into the molecular motif, the strong self-aggregation of the conjugated organic molecules can be suppressed, and single crystalline organic-perovskite hybrid quantum wells and superlattices can be easily obtained via one-step solution-processing. Energy transfer and charge transfer between adjacent organic and inorganic layers are extremely fast and efficient, owing to the atomically-flat interface and ultra-small interlayer distance. In the second part, I will present the synthesis and characterization of a stable lead-free Sn (II)-based two-dimensional perovskite featuring a p-conjugated oligothiophene ligand. The conjugated ligands facilitate formation of micrometer-size large grains, improve charge injections, and stabilize the inorganic perovskite layers. Thin film field-effect transistors based on (4Tm)2SnI4 exhibit enhanced hole mobility up to 2.3 cm2V-1s-1, and dramatically improved stability over the previous benchmark material (PEA)2SnI4. Stabilization mechanisms were investigated via single crystal structure analysis as well as density functional theory calculations. It was found that the large conjugated organic layers not only serve as thick and dense barriers for moisture and oxygen, but also increase the crystal formation energy via strong intermolecular interactions.

Metal halides offer unique structural engineerability of practically useful optical functionalities. In particular, low-dimensional compounds often exhibit broadband emission from highly localized, self-trapped excitions. These compounds exhibit strong temperature dependence of the photoluminescence lifetime. Using a range of tin based halides, [C(NH2)3]2SnBr4 to Cs4SnBr6 and (C4N2H14I)4SnI6, we show the radiative-lifetime-based thermal imaging [1]. The fluorescence lifetimes are governed by the phonon-assisted de-trapping of self-trapped excitons, and their values can be varied over several orders of magnitude by adjusting the temperature (up to 20 ns °C-1). The sensitive range conveniently spans up to 100 °C. The resulting thermometric precision is as high as 0.013 °C, using conventional TSCPC fluorescence lifetime imaging (FLI). We also showcase a great utility of the time-of-flight technology (ToF-FLI) for cost-effective fluorescence lifetime imaging. In our ongoing work, we find that analogous Sb(III) compounds as thermoluminophores are much more environmentally stable than easily oxidizable Sn(II) compounds and can be used to increase spatial resolution [2]. We also present unicolor security tags composed of lifetime-tunable lead-halide perovskite nanocrystals that can be deciphered with both existing time-correlated TCSPC-FLI microscopy and a novel ToF-FLI prototype. In particular, we partially substituted formamidinium for ethylenediammonium {en} to generate “hollow” {en}FAPbBr3 NCs, which exhibit same PL spectrum but 3-fold longer fluorescence lifetimes [3].


References
[1] S. Yakunin et al. Nature Materials, 2019, 18, 846-852
[2] V. Morad et al. submitted
[2] S. Yakunin et al. submitted

Halide perovskite have revolutionized not just photovoltaic field but the complete optoelectronic devices spectra for their outstanding properties as low-non radiative recombination, high absorption coefficient, good transport properties, facile material synthesis and huge versatility and tunability. When halide perovskites become nano the properties of their nanoparticles are also exceptional with impressive photoluminescence quantum yields (PLQY) close or even as high as 100%. However, the long term stability of perovskite nanoparticles is the Achilles’ heel of these materials. In this presentation we analyze as with an appropriated synthesis routes, affecting the nanoparticle capping or the perovskite composition, is possible to produce ultra-stable halide perovskite nanoparticles for more than 15 months.
On the other hand the interaction of halide perovskite and colloidal semiconductor quantum dots can produce interesting synergistic interactions. We show that the interaction of PbS quantum dots and nanoplatelets can produce the stabilization of FAPbI₃ perovskite black phase and also the increase of the efficiency, stability and reproducibility of the photovoltaic devices prepared with of FAPbI₃ with embedded QDs. The use of PbS nanoplatelets also improve the long term stability of CsFAPbI₃ perovskite. 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. This result points the interest of Perovskite-Quantum Dot Nanocomposites, for further development of advanced optoelectronic devices.

The past few years have seen a significant improvement in the efficiency of organometal halide perovskite-based light emitting diodes (PeLEDs). However, the poor operation stability of the devices still hinders the commercialization of this technology for practical applications. In this talk I will introduce our recent efforts in understanding the degradation mechanisms of PeLEDs and developing material strategies to improve the device stability. Firstly, using time-of-flight secondary ion mass spectrometry we find the degradation of FAPbI₃- based PeLEDs during operation is directly associated with ion migration and incorporation of binary alkali cations, i.e., Cs⁺ and Rb⁺, in FAPbI₃ could suppress ion migration and significantly enhance the lifetime of PeLEDs. Combining experimental and theoretical approaches, we further reveal that Cs⁺ and Rb⁺ ions stabilize the perovskite films by locating at different lattice positions, with Cs⁺ ions present relatively uniformly throughout the bulk perovskite while Rb⁺ ions preferentially on the surface and grain boundaries, both leading to strong Coulomb interactions between the cations and the inorganic framework and consequently greatly enhanced device stability. Secondly, we introduced electroabsorption spectroscopy (EA) to study the degradation process of PeLEDs during operation and directly evaluate the stability of each functional layer (i.e., charge transporting layers and light-emitting layer) by monitoring their unique optical signatures. The EA measurements unambiguously reveal that the degradation of the PeLEDs occurs predominantly in the perovskite layer. With finite element method-based device modeling, we further point out that the degradation is likely to initiate from the interface between the perovskite and hole transporting layers and that the interface defects could further accelerate the degradation. Inspired by these observations, a surface treatment step is introduced to passivate the perovskite surface with phenylalkylammonium iodide. After molecular optimization, the PeLEDs exhibit an external quantum efficiency of 17.5% and a long T50 lifetime of over125 hours under a high biasing current density of 100 mA cm².

Perovskite light emitting diodes (PeLEDs) have drawn considerable attention for their favorable optoelectronic properties. Perovskite light emitting electrochemical cells (PeLECs) - devices that utilize mobile ions - have recently been reported but have yet to reach the performance of the best PeLEDs. We leveraged a poly(ethylene oxide) electrolyte and a lithium salt in CsPbBr3 thin films to produce PeLECs of improved brightness and efficiency. In particular, we found that a single layer PeLEC from CsPbBr3:PEO:LiPF6 with 0.5% wt. LiPF6 produced bright (~15000 cd/m2) electroluminescence of improved efficiency (3.0 Lm/W). To understand this improved performance among PeLECs, we characterized these perovskite thin films with photoluminescence (PL) spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). These studies revealed that this optimal LiPF6 concentration improves electrical double layer formation, reduces the occurrence of voids, charge traps, and pinholes, and increases grain size and packing density.

Solution processed organic-inorganic metal halide perovskites with the chemical formula ABX₃ have emerged as next generation light emitting materials. Recently, ion migration has been demonstrated to significantly influence the performance of metal halide perovskite devices. The interplay between electronic and ionic currents provides a new pathway to utilize metal halide perovskites as emissive materials in light emitting electrochemical cells (PeLEC). Light emitting electrochemical cells (LECs) utilize ionic motion to generate electrical double layers for high carrier injection and potential doping effects to achieve efficient operation in single layer devices. Recently, our group demonstrated bright and effectual PeLECs with simple single layer architectures, leveraging a polyelectrolyte and a lithium ionic additive (LiPF₆) to achieve devices with a high maximum luminance of >10000 cd m⁻² and operation exceeding 100 hours over 800 cd m⁻². To understand this improved performance among PeLECs, we characterized these perovskite thin films with photoluminescence (PL) spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). These studies revealed that this optimal LiPF₆ concentration improves electrical double layer formation, reduces the occurrence of voids, charge traps, and pinholes, and increases grain size and packing density. Electrochemical impedance spectroscopy with equivalent circuit modeling revealed that electrical double layer widths are minimized with an optimal LiPF₆ concentration and inversely correlate with efficient performance. These results demonstrate that an optimal LiPF₆ concentration improves stability and efficiency through improved double layer formation and retention of perovskite structure.

Due to exceptional color purity and good thermal stability, all-inorganic halide perovskite (CsPbBr3) has been attracting interest as the next-generation materials for light-emitting diodes (LEDs). However, its low exciton binding energy is one of the factors limiting luminescence efficiency and spatial confinement of excitons is a promising way to resolve this issue. Here, we present two strategies of spatially confining excitons in vacuum-deposited Cs-Pb-Br light emitters. In the first part, we have modulated growth kinetics unique to thermal evaporation to form fine-grain-structured CsPbBr3 polycrystalline films. By adjust the deposition rate, nucleation rate was controlled, which then influenced the finalgrain structure. With this strategy, we demonstrate a green light-emitting (at 524 nm) LED with a maximum current efficiency of 1.07 cd/A and an extremely narrow electroluminescence spectrum of 18 nm—the results that highlight the potential of vacuum-processed CsPbBr3 films for high efficiency LEDs. In the second part, we have controlled dimensionality of by adjusting the flux ratio of the two source materials, CsPb and PbBr2, to deposit Cs-Pb-Br films. Under the excess flux of CsPb the so-called zero-dimensional Cs4PbBr6 formed, producing strong green emission with high photoluminescence quantum yield close to 90%. The green emission from Cs4PbBr6 is unusual considering its large bandgap of 3.9 eV. Through in-depth transmission electron microscopy study and optical measurement such as photothermal deflection spectroscopy and photoluminescence contour map, we identified the inclusion of trace amount of CsPbBr3 "impurities" as the source of the green emission. We have achieved a 60-fold improvement in external quantum efficiency from an LED with a CsPbBr3-embedded Cs4PbBr6 light emitter compared to the control device with CsPbBr3. Details of the materials characterization and device analysis will be discussed.

Halide perovskite materials have been actively studied to chase other suitable applications with unveiled characteristics. Piezoelectricity-related applications using the halide perovskites are another area that may be suitable with unique potentials. However, the studies on piezoelectricity of the halide perovskites are very limited probably due to the difficulty in fabricating quality thin films or bulk specimens although the possession of ferroelectricity of the materials has been theoretically estimated with calculated polarization values in literature. Here, we report the high piezoelectricity of representative inorganic halides, CsPbBr3, which was optimized with poling process, and their energy harvesting characteristics in the form of thin film. This is the very first report of revealing effective piezoelectric coefficient for any inorganic halide perovskite and demonstrating piezoelectric energy harvesting performance using inorganic halides. So far, the piezoelectric coefficients of 6-25 pm/V were reported only for the methylammonium lead halides (MAPbX3) or formamidinium lead halides (FAPbX3)-based hybrid perovskite thin films. Our highest value of ~40.3 pm/V for the optimized CsPbBr3 with film thickness and poling field was chased to understand origin of the high piezoelectricity via structural estimation associated with the changes in bonding angles. As expected from the high piezoelectricity, the electromechanical energy-harvesting performance was very competitive, such as 16.4 V output voltage and 604 nA output current for a larger size sample of 5 cm2. The values obtained for the inorganic CsPbBr3 thin films are the highest ones compared to any reported values for inorganic-organic hybrid halide perovskite thin films.

In recent years, lead halide perovskites have been found to be interesting candidates for various applications such as solar cells and light-emitting diodes, due to their excellent optical and electronic properties(1, 2). However, a deeper understanding of the intrinsic charge transport in thin films of these materials is currently lacking and it is essential for the bottom-up design of opto-electronic devices(2). Here, we fabricated three-dimensional (3D) perovskite thin film field-effect transistors (FETs), and employed them to investigate the temperature-dependent charge transport. We explored several state-of-the-art 3D perovskite compositions starting with methylammonium lead iodide, followed by numerous triple cation and Rb-incorporated perovskite compositions(3). A significant issue with these materials is the migration of ions and this results in partial screening of the applied gate field which gives a lower field-effect mobility than expected in these FETs(4). Initial trials resulted in FETs that did not display any modulation of the drain current with the gate voltage due to ion migration. This problem was suppressed through extensive optimization of both the deposition methods and the compositions using Design of Experiments (DOE) principles. The transfer characteristics of optimized FET devices show that these devices exhibit n-type behavior and low hysteresis even at room temperature. One of the triple cation compositions shows the best performance and it exhibits a good saturation electron field-effect mobility and an on/off ratio of ∼10⁴ at room temperature. Our results bring us closer to achieving the high field-effect mobility values predicted for lead halide perovskites and in understanding the charge transport mechanism in thin films of these materials.

References
1. Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The emergence of perovskite solar cells. Nature Photonics 2014, 8, 506-514.
2. J. Wang et al., Investigation of electrode electrochemical reactions in CH₃NH₃PbBr₃ perovskite single-crystal field-effect transistors. Advanced Materials 2019, 31, 1902618.
3. M. Saliba et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science 2016, 354, 206-209.
4. S. P. Senanayak et al., Understanding charge transport in lead iodide perovskite thin-film field-effect transistors. Science Advances 2017, 3, e1601935.

A new approach for fabricating high-quality ternary photoelectrodes such as CuBi2O4 will be presented. Pulsed laser deposition (PLD)1 is used to deposit the binary oxides Bi2O3 and CuO on FTO substrates, and rapid thermal processing (RTP)2 is used to achieve an efficient solid-state reaction between the two oxide films. It is shown that when Bi2O3 is deposited first, the thin film photoelectrodes are comprised of single-phase, compact, crystalline, and well-defined CuBi2O4 particles. A comparative study with conventional furnace heating reveals the significance of radiative heating in the processing of complex metal oxides photoelectrodes. Additionally, the much shorter heating times enable the use of FTO substrates at temperatures up to 650 °C, also resulting in a low thermal budget (the product of process temperature and processing duration at elevated temperatures). In a comparative study with CuBi2O4 photoelectrodes produced by spray-pyrolysis, a benchmark synthesis technique of photoactive materials3 and complex metal-oxide photoelectrodes,4 the CuBi2O4 layer fabricated by PLD + RTP photoelectrodes featured superior electronic properties, which were sustained even in cases of a secondary oxide co-existence in the film by deliberately altering the Bi:Cu stoichiometry ratio during the PLD stage. The photoelectrodes have shown an efficient charge separation, low tail energies, and band-gap energy virtually identical to that of a CuBi2O4 single crystal, shown by modulated surface photovoltage analysis5 and confocal Raman microscopy. Furthermore, bare photoelectrodes, without the addition of electron scavengers, protection layers, or catalysts, exhibited exceptional photoelectrochemical stability, substantially improving the prospects of implementing CuBi2O4 photoelectrodes in photoelectrochemical devices towards high solar-to-hydrogen efficiency. References: (1) Christen, H. M.; Eres, G. Recent Advances in Pulsed-Laser Deposition of Complex Oxides. J. Phys. Condens. Matter 2008, 20 (26), 264005. (2) Fiory, A. T. Rapid Thermal Processing for Silicon Nanoelectronics Applications. J. Miner. Met. Mater. Soc. 2005, 57 (6), 21-26. (3) Hu, J.; Gouda, L.; Kama, A.; Tirosh, S.; Gottesman, R. Radiative Recombination Changes under Light-Soaking in CsPbBr3 Films on TiO2 and Insulating Glass Contacts: Interface vs. Bulk Effects. ACS Appl. Energy Mater. 2019, 2, 3013-3016. (4) Song, A.; Plate, P.; Chemseddine, A.; Wang, F.; Abdi, F.; Wollgarten, M.; van de Krol, R.; Berglund, S. Cu:NiO as a Hole-Selective Back Contact to Improve the Photoelectrochemical Performance of CuBi2O4 Thin Film Photocathodes. J. Mater. Chem. A 2019, 7, 9183-9194. (5) Chen, R.; Fan, F.; Dittrich, T.; Li, C. Imaging Photogenerated Charge Carriers on Surfaces and Interfaces of Photocatalysts with Surface Photovoltage Microscopy. Chem. Soc. Rev. 2018, 47 (22), 8238-8262.

Functional materials combining the optoelectronic functionalities of semiconductors with control of the spin degree of freedom are highly sought after for the advancement of quantum technology devices and provide exciting avenues for polarized light-emission. Previous work towards this goal introduced small amounts of magnetic elements into crystalline semiconductor, e.g. through vacuum-based deposition, to obtain dilute magnetic semiconductors (DMS).

In my talk, I will present our efforts on gaining control over spin dynamics and spin interactions through compositional and structural tuning in solution-processable hybrid perovskite semiconductors. We aim to exploit the exceptional optoelectronic properties of these hybrid perovskites, together with their tolerance in the electronic states to dopants and defects, to make advances towards high-performance DMS. I will further discuss how we employ ultrafast optical spectroscopy to gain insights into fundamental excitation and spin dynamics in our novel materials, highlighting the potential of magneto-optic control.

I will present first results on chemical doping of layered Ruddlesden-Popper hybrid perovskites with transition metals. I will report how optical excitations interact with elemental spins in these materials, also on ultrafast timescales, to achieve control over exciton emission through magnetic proximity effects. Harnessing spin-orbit coupling effects, we achieved circular polarization up to 15% in the excitonic photoluminescence at low temperatures and high fields, which keeps a strong magnetic field dependence below 1 Tesla. Our finding constitutes the first example of optical polarization control in magnetically-doped hybrid perovskites and will stimulate research on this highly-tuneable material platform that promises tailored interactions between magnetic moments and electronic states.

Two-dimensional Ruddlesden-Popper phase hybrid metal halide perovskites have been investigated for use in a range of optoelectronic devices including photovoltaic cells. Their main appeal for photovoltaics is improved aqueous stability, which is enabled by their incorporation of organic cations which are more hydrophobic than those usually used in 3D perovskites. However, quantum and dielectric confinement effects increase the exciton binding energies in these materials to as much as 100s of meV. These increased binding energies correspond to an increase in exciton population at ambient temperatures which can greatly alter the optoelectronic properties. Optical Pump/THz-probe (OPTP) spectroscopy measurements have revealed decreased effective charge-carrier mobilities and significantly shortened charge-carrier lifetimes for 2D perovskites (1). In hybrid 2D-3D materials, incorporating small amounts of the 2D material can improve optoelectronic properties (2), suggesting a balance between excitons and free charges is achievable by tuning the composition. OPTP and other ultrafast spectroscopic techniques are further used to elucidate the interplay between free charges and excitons in these materials and comment on the limits these properties impose on charge transport in devices.

1. R. L. Milot et al., "Charge-Carrier Dynamics in 2D Hybrid Metal–Halide Perovskites," Nano Lett. 16(11), 7001-7007 (2016).
2. L. R. V. Buizza et al., "Charge-Carrier Dynamics, Mobilities, and Diffusion Lengths of 2D–3D Hybrid Butylammonium–Cesium–Formamidinium Lead Halide Perovskites," Adv. Funct. Mater., 1902656 (2019).

There remains debate on the nature of electronic states in hybrid metal-halide perovskite thin films, and their impact on charge-carrier dynamics, due to the unique dependence of its band structure on lattice deformations. Here, we present our unique combination of single- and two-photon transient absorption and photoluminescence spectroscopy experiments to map bright and dark transitions in the bulk and at the surface of methylammonium lead bromide thin films. Supported by first-principles calculations, we report a peculiar alignment of energy levels throughout the film, which efficiently funnels carriers away from trap-rich surface states and into the bulk, where an efficient and strong radiative recombination takes place. We show that, for operational carrier densities, these effects lead to high emission rates from heavily occupied bright states within Rashba split bands. Our findings unify the debate on the indirect-direct nature of the hybrid perovskites and have crucial relevance for charge extraction and injection processes at perovskite interfaces, which need to be optimized for maximum device efficiencies.

Over the last years we have witnessed a tremendous interest into hybrid organic-inorganic perovskite solar cells (PSCs) research. However, the physical and chemical phenomena occurring in different regions in the film (electrode interface, air interface, grain boundaries) and time scales under different stresses (light, oxygen, moisture, temperature, potential) have been identified to limit the attainable device performance. Especially, the chemistry at the interfaces critically affects the fundamental optoelectronic properties of the perovskite-based absorber as well as the working principles of the corresponding solar cells.
Here, we employed advanced and complementary characterisation techniques, namely multidimensional absolute photoluminescence (PL) and X-Ray Photoelectron Spectroscopy (XPS), to correlate optoelectronic and chemical properties. On the one side, through hyperspectral imaging (HI) and time-resolved fluorescence imaging (TR-FLIM) we investigated intrinsic material and carrier transport properties with quantitative indicators such as Quasi Fermi Level Splitting (QFLS), carrier mobility and recombination rates. On the other side, complementary XPS analyses were implemented on the same batch of samples to precisely determine chemical changes to the surface but also in the bulk, as investigated by XPS depth profiling, to correlate the modification of optoelectronic properties with changes in the composition of the perovskite layer.
We have investigated the behaviour of a triple cation mixed halide perovskite thin film deposited on a glass/FTO/c-TiO₂/m-TiO₂ stack. A series of mixed halide multi-cations perovskite thin films were light soaked for 10 minutes, 1 hour, 5 hours and 15 hours in air. We found that light exposure for a short time (10 minutes or 1 hour) results in a net enhancement of the optoelectronic properties of the material. Indeed, the QFLS of the perovskite thin films increases from 1.31 to 1.36 eV, remaining the same for longer exposure times. Time-resolved PL measurements confirm that the passivated recombination centers are located at the front interface (perovskite/air). As such, both steady-state and transient measurements evidence a surface passivation effect. For longer light soaking times (5 and 15 hours), persistent changes to the perovskite layer composition are observed. The modifications of the chemical environments suggest the formation of a stable PbO_x passivation layer starting from 5 hours of continuous light soaking in air atmosphere. The formation of this oxidized passivation layer goes along with sample degradation. Starting from 5 hours light soaking a phase segregation appears leading to a degradation of the transport properties of the material. XPS depth profiles indicate that the main responsible of this sample degradation is the oxygen diffusion through the perovskite active layer.

2D/3D heterojunction perovskite solar cells have demonstrated superior efficiency and stability compared to their fully 3D counterparts. These interfaces are formed by exposing the 3D perovskite surface to ligands that induce the formation of 2D layers, though this transformation is not yet understood. We used in situ Grazing-Incidence Wide-Angle X-Ray Scattering (GIWAXS) to identify intermediate states in the transformation from 3D to 2D. For (MAPbBr₃)_0.05(FAPbI₃)_0.95 perovskites treated with a dilute ligand solution, we observe the initial formation of n = 3 and n = 2 quantum wells before the final formation of n = 1 wells, suggesting a gradual reduction in dimensionality rather than immediate conversion to 2D perovskite monolayers. For MAPbI₃, we instead observe intermediate PbI₂ sheets that eventually turn into ligated perovskite quantum wells. A molecular picture of these transformations will help guide the design of superior 2D/3D interfaces for perovskite solar cell devices.

CsPbI3 perovskite nanocrystals boost their promising applications in high efficiency solar cells, LEDs, and photodetectors because of low-cost, and solution-processing. However, for these optoelectronic applications, the fundamental question of carrier transport mechanism remain unclear, in particular, in-situ devices.
In this report, we use the ultrafast photocurrent spectroscopy with sub-20 ps time resolution to elucidate the nature of carrier transport. In early time before 200 ps, the ultrafast photocurrent dependence with temperature clearly demonstrates a phonon-scattering mechanism, wherein the photocurrent increase with decreasing temperature. This unusual transport mechanism, which is absent in the disordered system of nanocrystal arrays, is consistent with the high carrier mobility ~ 10 cm2/Vs, and long carrier travelling length ~ 100 nm. After 200 ps, the carriers carry out hopping transport mechanism, which is manifested by the defect scattering effect.

The re-absorption of emitted photons within a material, photon recycling, has received increased attention in recent years following its observation in halide perovskite thin films and light-emitting diodes^1,2. It has been shown to lower the effective bimolecular recombination rate and thus increase excitation densities within a material³. However, the relationship of photon recycling to controllable device parameters is poorly understood, as is photon recycling's relative importance in different optoelectronic devices.

Here we present a general framework to quantify photon recycling in any material or device. Using experimental parameters, we apply our model to idealized methylammonium lead iodide (MAPbI₃) solar cells and cesium lead iodide (CsPbI₃) light-emitting diodes (LEDs) at operating voltages. Specifically, we explore the relationship of photon recycling to controllable parameters: thickness; charge-trapping rate; non-ideal interface transmission; and absorptance. We show that in MAPbI₃ solar cells the number of photon recycling events per absorbed photon at maximum power point is 0.4 in the (optimal) case of no charge trapping, due to most charges being extracted to the external circuit before they can radiatively recombine. Conversely, in a CsPbI₃ LED there can be up to 8 photon recycling events per injected charge in this optimal case (for emission into a full 2π hemisphere and applied voltage giving emission of ~1000 lm m^-2). In both device types we demonstrate that photon recycling and device performance (efficiency or amount of emitted light for solar cells and LEDs respectively) improve with thicker cells, those with less charge-trapping and improved back reflection. However, while device performance improves with better front transmission and light scattering, the degree of photon recycling rapidly reduces. For example, for a solar cell with a high degree of light scattering (to improve absorptance), there is less than 0.1 photon recycling event per initially absorbed photon. Similarly, in LEDs, the number of photon recycling events reduces to less than 1 when the emitter has a rough surface, but there is a significant increase in the quantity of light emitted from the system due to fewer trapped photon modes.

Our experimentally parameterized simulations reveal that photon recycling is significantly more important in LEDs than solar cells at operating voltages. Furthermore, we show that maximizing absorption and emission processes remains paramount for optimizing devices, even if this is at the expense of photon recycling. We therefore demonstrate that photon recycling cannot always be seen as a beneficial process in perovskite solar cells and LEDs.

References

1 C. Cho, B. Zhao, G. D. Tainter, J. Y. Lee, R. H. Friend, D. Di, F. Deschler and N. C. Greenham, Nat. Commun., 2020, 11, 1–8.
2 L. M. Pazos-Outon, M. Szumilo, R. Lamboll, J. M. Richter, M. Crespo-Quesada, M. Abdi-Jalebi, H. J. Beeson, M. Vrućinić, M. Alsari, H. J. Snaith, B. Ehrler, R. H. Friend and F. Deschler, Science (80-. )., 2016, 351, 1430–1433.
3 T. W. Crothers, R. L. Milot, J. B. Patel, E. S. Parrott, J. Schlipf, P. Müller-Buschbaum, M. B. Johnston and L. M. Herz, Nano Lett., 2017, 17, 5782–5789.

Halide perovskite nanocrystals (NCs) hold great promise for optoelectronic applications because of their large absorption cross-sections and high photoluminescence quantum yields. Moreover, by varying either their halide composition or their size/dimensionality, the absorption onsets and emission maxima can be tuned throughout the entire visible spectrum. Commercialization is currently impeded however due to several issues. Susceptibility to environmentally-induced degradation necessitates the use of stabilizing and encapsulation, which hinders charge injection and halide ion diffusion impedes the realization of heterostructures.
In this presentation, we look at nonradiative energy transfer (FRET) in mixed halide perovskite nanosystems, for enhancing the stability and integration into optoelectronic devices and novel architectures. Halide perovskite nanoplatelets (NPls) enable a precise tuning of absorption and emission by controlling the number of layers in the NPls. We look at energy transfer between CsPbBr₃ NPls of different thicknesses and analyze efficiency and transfer times.^[1] Using this, we then look into realizing multi-layer heterojunctions comprising different NC thicknesses for concentrating charge carriers or promoting charge carrier collection. We also show how highly stable polymer micelle-encapsulated NCs can be integrated into light-emitting devices through energy transfer.^[2] We specifically investigate how the polymer shell and NC size affect the transfer efficiencies and device performance. These results open up pathways for realizing new device architectures and enhance the stability of the perovskite NCs, both important for pushing perovskite NCs from a mainly scientific state toward commercialization.

[1] Singldinger, A.; Gramlich, M.; Gruber, C.; Lampe, C.; Urban, A. S. Nonradiative Energy Transfer between Thickness-Controlled Halide Perovskite Nanoplatelets. ACS Energy Lett. 2020, 5, 1380-1385.
[2] Hintermayr, V. A.; Lampe, C.; Low, M.; Roemer, J.; Vanderlinden, W.; Gramlich, M.; Bohm, A. X.; Sattler, C.; Nickel, B.; Lohmuller, T.; et al. Polymer Nanoreactors Shield Perovskite Nanocrystals from Degradation. Nano Lett. 2019, 19, 4928-4933.

Halide perovskite (HaP) materials have attracted substantial renewed research interest for over a decade, largely due to initially dramatic gains in photovoltaic power conversion efficiencies demonstrated by lead HaP-based solar cells ^[1]. Efforts to commercialize HaP-based solar cells, light-emitting diodes (LED’s) and other opto-electronic devices are currently underway around the globe. The prototypical hybrid organic-inorganic HaP is methylammonium lead tri-iodide (MAPbI₃ or MAPI) which features an organic cation (methylammonium or MA) located at the A-site embedded within a lead-iodide (B-X) inorganic sub-lattice. Since the early demonstrations of MAPI solar cells, device applications of HaPs have transitioned towards the use of mixed-cation (with/without MA plus other A-site cations) formulations due to concerns about the thermal and chemical stability of MA in device-relevant, ambient conditions ^[2]. The role of MA in the HaP matrix was considered by some to be structural only, which indirectly influences the optical and electrical properties of MAPI ^[3,4]. Density-functional theory (DFT) calculations of the electronic structure supported that view ^[5,6]. Selective thermal excitation of the MA did not alter the optical properties of MAPI^[7]. Mechanical nano-indentation studies of APbBr₃ crystals showed that the type of A-site cation, be it organic or inorganic, has little effect on the chemical bonding in HaPs ^[8].
In spite of the considerable evidence for the passive opto-electronic role and thermal instability of MA, the highest-performing HaP solar cells typically incorporated some fraction of MA in the absorber layer ^[9]. Hot fluorescence was observed from APbBr₃ crystals with an MA or FA, but not cesium, cation ^[10]. In this work, we demonstrate that MA is an active participant in the nitrogen 1s (N1s) core-excited electronic structure of MAPI. In doing so, we reveal the existence of additional mechanisms: (i) ultrafast <7 fs charge transfer between molecular unoccupied states in the MA and MAPI conduction band states and (ii) MA carrier lifetime > 7 fs. Using Resonant Photoelectron Spectroscopy (RPES), which involves resonantly exciting the nitrogen in MA with soft X-rays and measuring the N1s decay spectra (KVV Auger and autoionization), we convert MA into a core-excited microscopic probe of the electronic coupling to its environment. The N1s decay spectra corresponding to N1s-to-2p* resonant excitations to bound states below the ionization threshold show evidence of spectator and participator autoionization and normal Auger decay. We find MA to be quasi-localized electronically, meaning its electronic coupling to the Pb-I sub-lattice is intermediate between weak and strong coupling, since a fraction of the resonantly-excited N1s core electrons de-localize into the environment surrounding the MA with a timescale that is comparable to the N1s core hole lifetime (~7 fs). To access the intrinsic materials physics of MAPI, the experiments were performed with in-vacuum cleaved single crystals at a low photon flux synchrotron beamline. Beam-induced sample damage was judiciously checked with XPS and was found to be negligible.
The presence of MA-to-(Pb-I) charge transfer in the N1s core-excited state reveals that the MA cation is not chemically and electronically inert, and suggests that bi-directional MA-(Pb,I) charge transfer may be possible in valence-excited states, which is relevant for the operation of MA-based HaP solar cells, LED’s, and other opto-electronic devices.
[1] NREL, 2020.
[2] M. Saliba et al., Science 2016, 354, 206.
[3] J. M. Frost et al., Nano Lett. 2014, 14, 2584.
[4] D. A. Egger et al., Adv. Mater. 2018, 30, 1800691.
[5] B. Philippe et al., JPCC 2017, 121, 26655.
[6] J. Endres et al., JPCL. 2016, 7, 2722.
[7] P. Guo et al., Nat. Commun. 2019, 10, 482.
[8] Y. Rakita, D. Cahen, G. Hodes, 2018.
[9] Q. Jiang et al., Nat. Photonics 2019, 13, 460.
[10] H. Zhu et al., Science 2016, 353, 1409.

The exploration of lead-free perovskite-inspired semiconductors for optoelectronic applications has captured ever-growing attention, driven by their promise to replicate the success of the lead-based counterparts, but without the toxicity concerns associated with the latter.¹ In the search for lead-free perovskite-inspired semiconductors, emphasis has been placed on the identification of materials with a defect-tolerant character, as this has been associated with a high optoelectronic potential.² While computational studies have been of fundamental importance in screening lead-free perovskite-inspired semiconductors for defect tolerance, the detailed experimental assessment of the defect tolerance in such materials is lacking in the field. This has been particularly limiting in regard to the experimental identification of the most promising lead-free perovskite-inspired semiconductors. In fact, at an experimental level, investigators have most often relied on phenomenological findings to probe the defect tolerance of such materials. For instance, device performance boosts achieved through specialized processing strategies have often been linked to defect passivation, while photoluminescence lifetimes have been regarded as the default indicator of defect tolerance.^3,4

Recognizing that an experimental quantitative assessment of the defect parameters (volumetric densities, characteristic energies, and capture cross sections) is key to developing a detailed understanding of defect tolerance in perovskite-inspired semiconductors, in this study we have explored the applicability of deep-level transient spectroscopy methods to reliably characterize the defect properties of a range of lead-free perovskite-inspired compounds. In particular, we have focused on a number of antimony- and bismuth-based perovskite-inspired materials, as they have been generally associated with a defect-tolerant character.² Herein, we firstly examine the applicability of conventional deep-level transient spectroscopy (DLTS) for their experimental defect characterization. We then discuss current-based deep-level spectroscopy variants, of which we show the particular configurations that we have found to be widely applicable to the defect characterization of lead-free perovskite-inspired compounds. Finally, by comparing with the reported approaches for the quantitative defect characterization of lead-halide perovskites, we discuss the defect tolerance of the antimony- and bismuth-based perovskite-inspired materials we have characterized (e.g., A₃Sb₂I₉ absorbers with A being a monovalent cation, bismuth oxyiodide,⁵ silver iodobismuthates). This study thus contributes to identifying a viable strategy for the experimental screening of defect tolerance in lead-free perovskite-inspired materials, potentially enabling the experimentally-grounded identification of the most promising compositions towards high-performance low-toxicity optoelectronics.

1 A. Abate, Joule, 2017, 1, 659–664.
2 R. E. Brandt, J. R. Poindexter, P. Gorai, R. C. Kurchin, R. L. Z. Hoye, L. Nienhaus, M. W. B. Wilson, J. A. Polizzotti, R. Sereika, R. Zaltauskas, L. C. Lee, J. L. MacManus-Driscoll, M. Bawendi, V. Stevanović and T. Buonassisi, Chem. Mater., 2017, 29, 4667–4674.
3 B. Yang, J. Chen, F. Hong, X. Mao, K. Zheng, S. Yang, Y. Li, T. Pullerits, W. Deng and K. Han, Angew. Chemie Int. Ed., 2017, 56, 12471–12475.
4 J. Pal, S. Manna, A. Mondal, S. Das, K. V. Adarsh and A. Nag, Angew. Chemie Int. Ed., 2017, 56, 14187–14191.
5 T. N. Huq, L. C. Lee, L. Eyre, W. Li, R. A. Jagt, C. Kim, S. Fearn, V. Pecunia, F. Deschler, J. L. MacManus-Driscoll and R. L. Z. Hoye, Adv. Funct. Mater., 2020, 1909983.

Despite the rapid development of halide perovskite with power conversion efficiency over 25% to date, there are many unresolved issues such as the identification and characterization of defects affecting device performance. This dearth of knowledge is inhibiting the commercialization of HAP solar cells. The migration of ionic species in HAPs influences the degradation of perovskite solar cells and their long-term reliability and robustness. In this work, we investigated ion migration in crystalline methylammonium lead bromide C/MAPbBr₃/Au solar cells in the dark using the hp DC Voltage source 4140B picoAmmeter in the temperature range 255 K < T < 320 K and various bias pulse sequences. Arrhenius plots constructed from the data were used to determine ion migration activation energies for various ion species. Atypical temperature dependency of the current transients was observed: fast transients at low temperature and slower ones at elevated temperatures. In addition, characteristic features typical of ion migration such as scan rate and bias sensitivity with hysteresis accompanying temperature cycling were also observed. The ion transport mechanisms in the C/MAPbBr₃/Au perovskite crystals agree with observations in the MAPbI₃ analogue. We conclude that the fast exponential corresponds to the migration of the negatively charged halide species, Br, with activation energy of 0.41 eV; and, the slower exponential is due to the positively charged lead (Pb²⁺) species with activation energy of 2.82 – 2.87 eV.

The use of excess lead iodide (PbI₂) during the synthesis and processing of lead halide perovskites has been exploited as a way to improve performance of perovskite solar cells [1,2]. On the other hand, it has also been proposed that excess PbI₂ can increase the trap-state density [3] and enhance the thermal decomposition to PbI₂ [4]. Although the presence of PbI₂ in perovskite films plays an important role, the current understanding of how PbI₂ phase impurities form interfaces with halide perovskites and affect their electronic properties is poor. Here, we model CsPbI₃-PbI₂ superlattices and identify their interfacial geometry, thermodynamic stability, and electronic structure. Our first-principles density functional theory (DFT) calculations reveal that PbI₂ present in CsPbI₃ can induce electron trap state at the interface between these two phases. However, the electron trapping tendency is sensitive to the atomic nature of the phase boundary, which can explain the variation reported in different investigations.

[1] C. Roldán-Carmona, P. Gratia, I. Zimmermann, G. Grancini, P. Gao, M. Graetzel, and M. K. Nazeeruddin, Energy Environ. Sci. 8, 3550 (2015); https://doi.org/10.1039/C5EE02555A
[2] Y. C. Kim, N. J. Jeon, J. H. Noh, W. S. Yang, J. Seo, J. S. Yun, A. Ho-Baillie, S. Huang, M. A. Green, J. Seidel, T. K. Ahn, and S. I. Seok, Adv. Energy Mater. 6, 1502104 (2016); https://doi.org/10.1002/aenm.201502104
[3] H.-Y. Wang, M.-Y. Hao, J. Han, M. Yu, Y. Qin, P. Zhang, Z.-X. Guo ,X.-C. Ai, and J.-P. Zhang, Chem. Eur. J. 23, 3986 (2017); https://doi.org/10.1002/chem.201605668
[4] T. Matsui, T. Yamamoto, T. Nishihara, R. Morisawa, T. Yokoyama, T. Sekiguchi, and T. Negami, Adv. Mater. 31, 1806823 (2019); https://doi.org/10.1002/adma.201806823

It is now well established that metal halide perovskites are mixed ionic-electronic conductors. The halide ions are highly mobile at room temperature [1]. Ion migration is speculated to be the origin, or an important contributing factor, for many unusual phenomena in photovoltaic devices, such as current-voltage hysteresis, and photoinduced phase separation and transformations [2,3]. Illumination of a photovoltaic device will result in electical and thermal changes. Absorption of light can access metastable charge states of defects due to the trapping of electrons or holes and result in hot carriers with excess energy, which cool to the lattice temperature by phonon emission that could enhance the rate of ion migration [4]. Thus, these ionic migration processes must be supressed in such device structures. The rate of halide ion jumps in halide perovskites depends not only on the activation energy but also on the pre-exponential factor of diffusion. Here, we report the microscopic mechanisms for iodine ion diffusion mediated by vacancies from first-principles density functional theory (DFT) calculations. We quantify the potential energy landscsape for chemical transport including the activation energies of ions migration and the migration frequency at the room temperature.

[1] Ionic transport in hybrid lead iodide perovskite solar cells. Nature commun. 6, 7495 (2016) ; https://doi.org/10.1038/ncomms8497
[2] Anomalous hysteresis in perovskite solar cells. J. Phys. Chem. Lett. 5, 1511 (2014) ; https://doi.org/10.1021/jz500113x
[3] Reversible photo-induced trap formation in mixedhalide hybrid perovskites for photovoltaics. Chem. Sci. 6, 613 (2015) ; https://doi.org/10.1039/c4sc03141e
[4] Taking control of ion transport in halide perovskite solar cells. ACS Energy Lett. 3, 1983 (2018); https://doi.org/10.1021/acsenergylett.8b00764

The unprecedented rise in power conversion efficiency of solar cells based on metal halide perovskites (MHPs) has led to an enormously growing research effort over the last 6 years. In this paper, we report how the electrodeless transient microwave photoconductivity technique (TRMC) can reveal detailed information, regarding the interesting photo-physical properties of MHPs. First, we recorded two photon absorption (2PA) spectra of MHP thin films over a wavelength regime between 0.5E_g and E_g, from which the 2PA coefficients have been calculated. A two-step upward trend is observed in the 2PA coefficient spectrum for CH₃NH₃PbI₃. Using a parameterized tight binding model, spectra were calculated by integration over all Brillouin zones showing compelling similarity with experimental results. We conclude that the second upward trend in the 2PA spectrum originates from additional optical transitions to the heavy and light electron bands formed by the strong spin-orbit coupling.
Next, we performed TRMC measurements using optical excitation above and below the band-gap on pristine MAPbI₃, MAPbI₃/C60, and MAPbI₃/Spiro-OMeTAD thin films. In pristine MAPbI₃ we have observed a band-like charge transport for photon energies down to 1.40eV, from which we calculated an Urbach energy of approximately 16 meV, indicative of a relatively low structural disorder. Our TRMC measurements on the bilayers revealed that electrons in defect states close to the VB can be excited by photons with an energy as low as 1.30 eV. Their excitation leads to free electrons in the CB that undergo charge injection into the C60 yielding long-lived charge separation. In contrast, excitation of MAPbI₃/Spiro-OMeTAD at 1.3 eV shows a rapidly decaying signal implying that holes are localized and cannot be transferred over the interface to the Spiro-OMeTAD.

The coupling of electrons with lattice vibrations is thought to play a crucial role in determining the efficiency of optoelectronic processes in metal-halide perovskites. Herein, we report the first comparison of the low-frequency Raman (LFR) spectra of thermodynamically-stable, inactive δ-CsPbI₃ and metastable, active γ-CsPbI₃ in the range of 5 – 150 cm^-1 [1]. While previous reports have suffered from material degradation during measurements, our successful recording of these spectra was enabled by a combination of nanoconfinement to stabilize the respective CsPbI₃ phases against temperature- and humidity-induced polymorph transitions,[2] as well as the use of a below-bandgap 976-nm wavelength laser to avoid laser-induced degradation. Temperature-dependent LFR spectra confirmed that γ-CsPbI₃ could be stabilized to temperatures as low as 77 K upon nanoconfinement. The main vibrational modes at these low frequencies were determined via density functional theory calculations to primarily involve bending and stretching of Pb-I bonds. We further explored the compositional dependence of the γ phase phonon modes via systematic substitution of Cs cations with methylammonium cations (Cs_yMA_y-1PbI₃) or I anions with Br anions (CsPbI_xBr_3-x). The three main peaks exhibited in γ-CsPbI₃ were found to be present in all variations of the composition, with peak locations shifting to lower frequencies with expanding lattice dimensions. These results indicate that the phonon modes in these MHPs depend primarily on the structure of the metal-halide framework, not the specific nature of the A-site cations and halide anions.


[1] Yang, Y., Robbins, J., Ezeonu, L., Ma, Y., Sparta, N., Kong, X., Strauf, S., Podkolzin, S., and Lee, S. “Probing Lattice Vibrations of Stabilized CsPbI₃ Polymorphs via Low-Frequency Raman Spectroscopy” Journal of Materials Chemistry C (2020): Available at http://pubs.rsc.org/en/Content/ArticleLanding/2020/TC/D0TC02123G.
[2] Kong, X., Shayan, K., Hua, S., Strauf, S., and Lee, S. S. “Complete Suppression of Detrimental Polymorph Transitions in All-Inorganic Perovskites via Nanoconfinement” ACS Applied Energy Materials 2, (2019): 2948–2955.

Organic–inorganic halide perovskite nanocrystals (PNCs) are gaining increasing attention in contemporary research due to their promising optoelectronic performance. Photoexcitation of these PNCs with an ultrashort laser pulse can produce coherent phonons along the lattice displacement coordinate, leading to lattice vibrations. The halide ions of organic-inorganic hybrid perovskites can strongly influence the interaction between the central organic moiety and the inorganic metal halide octahedral units and thus their lattice vibrations. Here, we employ femtosecond pump–probe spectroscopy to initiate the photophysics by the formation of coherent phonons and to observe the subsequent coherent vibrational dynamics of formamidinium lead halide (FAPbX₃, X=Br, I) PNCs. We find that the FAPbX₃ PNCs generate halide-dependent coherent vibronic wave packets upon non-resonant excitation, and the dominant contributions are attributed to the Pb–X bending and stretching modes of PbX₆^4- octahedral units of the lattice. More importantly, for the first time, we observe higher harmonic vibrational modes in FAPbI₃ PNCs, which points to a more anharmonic potential energy surface in the case of FAPbI₃ as compared to FAPbBr₃ PNCs. This is likely due to the weaker interaction between the central FA moiety (which sits in a larger octahedral interstitial site) and the inorganic cage for FAPbI₃ PNCs, and thus the PbI₆^4- unit can vibrate more freely. This weakening reveals the intrinsic anharmonicity in the Pb-I framework, and thus facilitating the energy transfer into overtone and combination bands. Furthermore, our control experiment with MAPbBr₃ reveals the energy transfer between framework phonons due to the intrinsic anharmonicity of the lead-halide framework is indeed influenced by the interaction between the framework and the organic molecules, and not only by the halide nature. The insights interestingly not only unravel the underlying reason for the halide-dependent stability of these materials but also shed light on their charge-carrier mobility and polaronic properties.

Methylammonium Lead Bromide (MAPbBr3) is the most investigated 3D Perovskite for visible LED applications. It is characterized by a much higher exciton binding energy compared to its Iodide equivalent (MAPbI3). This leads to complex excited-state phenomena, with free-carriers, excitons and defect states contributing to the photoluminescence (PL) properties. We have used a variety of spectroscopic techniques to study photoexcited MAPbBr3 films, including time-resolve PL measurements, femtosecond transient absorption (TA) spectroscopy and optical-probe-terahertz-pump (THz) experiments. The co-existence of free carriers and excitons is demonstrated at low excitation densities in as-cast polycrystalline MAPbBr3 (crystal size of the order of few ?m). The excitation wavelength and density are then varied in order to access different free-carrier to exciton ratios in order to evaluate their recombination dynamics and THz conductivities. Finally, we show how reducing the grain size of the Perovskite to the micro- and nanoscale (by using different additives during solution-processing) increases the PL quantum yield, which we explain by the presence of a bright exclusively exitonic population even at low excitation densities, together with reduced surface trapping thanks to passivation by the additives.

Due to a larger organic cation, formamidinium lead triiodide (FAPbI3) has a narrower, more optimal band gap than MAPbI3, which extends the photocurrent generation onset into the near-infrared. FAPbI3 is also more resistant to heat stress than MAPbI3 while retaining a comparable charge-carrier mobility[1] long charge-carrier diffusion length[2] and impressive solar cell efficiencies as high as 16%[3]. The current most efficient single-junction perovskite photovoltaics are based on FAPbI3, albeit with chemical substitution to enhance the stability of the perovskite phase. Improving our understanding of the fundamental optoelectronic properties of FAPbI3 is therefore essential to improving the performance of the semiconductor devices based on it. However, despite its remarkable promise, the photophysics of FAPbI3 remains under-studied compared to MAPbI3 [4]. In this study, I report on novel and intriguing emission and absorption processes in FAPbI3. First, we investigated [5] the influence of sub-bandgap trap states on charge-carrier recombination through an analysis of the low-temperature photoluminescence (PL) of FAPbI3. We observed a power-law time dependence in the emission intensity and an additional low-energy emission peak that exhibits an anomalous relative Stokes shift. Using a rate-equation model and a Monte Carlo simulation, we revealed that both phenomena arise from an exponential trap-density tail with characteristic energy scale of ?3 meV. Since charge-carrier recombination from sites deep within the tail causes emission with energy downshifted by up to several tens of meV, such phenomena may in part be responsible for Voc losses commonly observed in these materials. Secondly, we investigated the temperature-dependent absorption of FAPbI3, supported by first-principles modelling. Altogether, we elucidate how the polar FA cation affects the optoelectronic properties of FAPbI3. [1] W. Rehman, R. L. Milot, G. E. Eperon, C. Wehrenfennig, J. L. Boland, H. J. Snaith, M. B. Johnston, L. M. Herz, Adv. Mater. 2015, 27, 7938. [2] A. A. Zhumekenov, M. I. Saidaminov, M. A. Haque, E. Alarousu, S. P. Sarmah, B. Murali, I. Dursun, X.-H. Miao, A. L. Abdelhady, T. Wu, O. F. Mohammed, O. M. Bakr, ACS Energy Lett. 2016, 32. [3] J. W. Lee, D. J. Seol, A. N. Cho, N. G. Park, Adv. Mater. 2014, 26, 4991. [4] C. L. Davies, J. Borchert, R. L. Milot, H. Kraus, M. B. Johnston, L. M. Herz, J. Phys. Chem. Lett. 2018, 9, 4502. [5] A. D. Wright, R. L. Milot, G. E. Eperon, H. J. Snaith, M. B. Johnston, L. M. Herz, Adv. Funct. Mater. 2017, 27, 1700860.

Reports of slow charge-carrier cooling in hybrid metal halide perovskites have prompted hopes of achieving higher photovoltaic cell voltages through hot-carrier extraction. However, observations of long-lived hot charge carriers even at low photoexcitation densities and an orders-of-magnitude spread in reported cooling times have been challenging to explain. In this work we present ultrafast time-resolved photoluminescence measurements on formamidinium tin triiodide, showing fast initial cooling over tens of picoseconds and demonstrating that a perceived secondary regime of slower cooling instead derives from electronic relaxation, state-filling, and recombination in the presence of energetic disorder. We identify limitations of some widely used approaches to determine charge-carrier temperature and make use of an improved model which accounts for the full photoluminescence line shape. The adoption of this model offers a path to more accurate determination of charge-carrier temperatures, and ready comparison between perovskite compositions to assess the true potential for hot-carrier solar cells. Further, we do not find any persistent polarization anisotropy in FASnI3 within 270 fs after excitation, indicating that excited carriers rapidly lose both polarization memory and excess energy through interactions with the perovskite lattice. References Kimberley J. Savill, Matthew T. Klug, Rebecca L. Milot, Henry J. Snaith, and Laura M. Herz, Charge-Carrier Cooling and Polarization Memory Loss in Formamidinium Tin Triiodide. J. Phys. Chem. Lett. 2019, 10 (20), 6038-6047

We report on the origin of the multiple excitonic states reported previously by us [1,2] in two-dimensional hybrid perovskites, by systematic analysis of the optical observables with the variation of the metal cation from lead to tin. We particulary discuss the temperature dependence of the exciton recombination dynamics obtained via time-resolved photo-luminescence (PL) and transient absorption spectroscopies. We quantitatively analyze the monotonous increase in the exciton lifetime as well as PL quantum yield with increasing temperature in the range of 5 K — 290 K. We consider such a behaviour as an evidence for the presence of dark excitonic states within the finestructure that are de-populated at higher temperatures with an activation energy of ~8-10 meV. We discuss the role of the phonons in the exciton inter-conversion dynamics through non-adiabatic exciton-phonon couplings [3]. [1] A. R. Srimath Kandada and C. Silva, Perspective: Exciton polarons in two-dimensional hybrid metal-halide perovskites, arXiv:1908.03909[cond-mat.mtrl-sci] (2019). [2] F. Thouin, D. Valverde-Chavez, C. Quarti, D. Cortecchia, I. Bargigia, D. Beljonne, A. Petrozza, C. Silva and A. R. Srimath Kandada, Phonon coherences reveal the polaronic character of the excitons in two-dimensional lead-halide perovskites, Nature Materials, 18, 349-356 (2019). [3] F. Thouin, A. R. Srimath Kandada, D. Valverde-Chavez, D. Cortecchia, I. Bargigia, A. Petrozza, X. Yang, E. R. Bittner and C. Silva, Electron-phonon couplings inherent in polarons drive exciton dynamics in two-dimensional metal-halide perovskites, Chem. Mater., 31, 7085-7091(2019).

Lead-halide perovskite solar cells have improved dramatically over the past decade, increasing in efficiency and overcoming hurdles of temperature- and humidity-induced instability [1]. Materials that combine the high charge-carrier lifetimes and mobilities, strong absorption and good crystallinity of 3D perovskites with the hydrophobic properties of 2D perovskites have become leading candidates for use in solar cells [2]. In order to fully understand the optoelectronic properties of these 2D-3D hybrid systems, we investigate the hybrid perovskite BAx(FA0.83Cs0.17)1-xPb(I0.6Br0.4)3 across the compositional range 0 - x - 0.8. Small amounts of butylammonium (BA) are found to help improve crystallinity and passivate grain boundaries, reducing trap-mediated recombination and enhancing charge-carrier mobilities. However, over-hybridisation leads to poor crystallinity and inhomogeneous film formation, greatly reducing charge-carrier mobilites. We find long charge-carrier diffusion lengths up to 7.7 -m at low BA content of x = 0.167, paving the way for highly efficient and stable perovskite solar cells based on 2D-3D hybrid materials [3]. Separately, lead-free all-inorganic materials have become of interest in order to address issues surrounding the stability of lead-halide perovskites, and concerns around the toxicity of PbI2 [4]. Here we investigate a novel silver-bismuth material that shows ultrafast charge-carrier dynamics, observed via optical-pump terahertz-probe spectroscopy. A wide-ranging spectroscopic and temperature-based investigation of this material points towards localisation effects and strong charge-lattice interactions, whilst still showing good charge-carrier transport properties, lending promise for potential applications of this material to photovoltaics. References: [1] McMeekin, D.P., Sadoughi, G., Rehman, W., Eperon, G.E., Saliba, M., Hörantner, M.T., Haghighirad, A., Sakai, N., Korte, L., Rech, B. and Johnston, M.B., 2016. A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells. Science, 351(6269), 151-155. [2] Wang, Z., Lin, Q., Chmiel, F.P., Sakai, N., Herz, L.M. and Snaith, H.J., 2017. Efficient ambient-air-stable solar cells with 2D-3D heterostructured butylammonium-caesium-formamidinium lead halide perovskites. Nature Energy, 2(9), 17135. [3] Buizza, L.R., Crothers, T., Wang, Z., Patel, J.B., Milot, R., Snaith, H., Johnston, M. and Herz, L., 2019. Charge-carrier dynamics, mobilities and diffusion lengths of 2D-3D hybrid butylammonium-caesium-formamidinium lead halide perovskites. Advanced Functional Materials, 29, 1902656. [4] Volonakis, G., Filip, M.R., Haghighirad, A.A., Sakai, N., Wenger, B., Snaith, H.J. and Giustino, F., 2016. Lead-free halide double perovskites via heterovalent substitution of noble metals. The journal of physical chemistry letters, 7(7), 1254-1259.

Functional materials with photochromism behaviors are highly attractive for various potential applications, such as sensors, displays or lighting devices, clothing, decoration, memory, and commercial consumer products (e.g. photochromic cars, sunglases, T-shirts, shoes, and umbrella), where a reversible color change occurs with changes in light intensity. Photochromic properties are commonly observed in large molecular organic compounds or polymer materials with several states of different colors. However, most of the materials exhibit photochromic behavior with various states in the solution state of that material, limiting the potential and extensive applications. Light-induced phase segregation with photoluminescence (PL) peak shift in mix-halide perovskites owing to reversible energy bandgap tunability of halide-rich domains under illuminations allows to obtain photochromic behaviors in solid-state materials beyond organic polymers. Inspired by this phenomenon, our color imaging experiment demonstrates a nice photochromic behavior of such mixed-halide perovskites upon dark and light excitation, while the color emitted by the crystals changes from red to yellow and vice versa with the intensity of excitation light source. Interestingly, this photochromic light-emitting characteristic can be well-matched with the laser light-induced reversible PL. Unlike the photochromic property commonly observed in solution-state polymer materials, solid-state perovskite materials with photochromic behavior might open wide applications in consumer products and electronic devices. In the next part, we report on a reversible PL peak in iodide-based organic-inorganic lead halide perovskite materials under a two-photon absorption (TPA) process, while tuning the excitation wavelength. This phenomenon occurs when the incoming femtosecond (fs) laser photon energy is higher than threshold energy. Intriguingly, two shorter wavelength peaks are visible and become prominent when the excitation photon energy is being tuned in the high energy spectrum, while laser power is remained constant. The same phenomenon of reversible PL peak is also observed in various iodine-based organic-inorganic halides as well as all-inorganic perovskite single crystals and polycrystals. We attribute to the reversible PL peak phenomenon to the photoinduced structural deformation and the associated change in the optical bandgap of iodide perovskites under the (fs) laser excitation. Our findings will introduce a new degree of freedom in future research as well as adding new functionalities to optoelectronic applications in these emerging perovskite materials.

Lead-iodide based perovskite semiconductors are emerging as promising light absorbers for solution-processed thin-film photovoltaic applications. In addition, these materials are also intensively studied for light-emitting diodes, photodetectors, and lasers. Recently, hybrid organic-inorganic perovskite (HOIP) semiconductors are being considered as candidates for spintronic applications due to a large spin-orbit coupling (SOC) and controllable Rashba-splitting. Here we discuss the spin-coherence lifetimes in single crystals of 2D Ruddlesden-Popper perovskites PEA2PbI4(MAPbI3)n?1(PEA, phenethylammonium; MA, methylammonium; n = 1, 2, 3, 4). Room temperature spin-coherence times show a nonmonotonic layer thickness dependence with an increasing spin-coherence lifetime with increasing layer thickness from n = 1 to 4, followed by a decrease in lifetime from n = 4 to ?.Our results are consistent with two contributions; Rashba-splitting increases the spin-coherence lifetime going from the n = ? to the layered systems, whilephonon-scattering which increases for smaller layers decreases the spin-coherence lifetime.The unique tunability of HOIPs offers an unprecedented opportunity to directly incorporate chiral organic molecules in the production of a 2D-layered hybrid perovskite thin film and/or crystal that induces chirality into the inorganic sub-lattice bandedge states. Thus, there is an interesting opportunity to explore how chiral molecules embedded into the multilayers provide spin-control within these hybrid semiconductors. Here, we focus on controlling spin-transport within 2D-layerd chiral Pb-I perovskite systems. We demonstrate spin-polarization of charge-transport through the chiral perovskites by showing that the injected current is preferential to one of the spin-states and depends upon the handedness of the incorporated chiral organic molecules; thus, the hybrid chiral system acts as a spin-filter.

We quantify the role of many-body elastic scattering effects on exciton dephasing rates in two-dimensional hybrid metal-halide perovskites by means of nonlinear coherent excitation spectroscopy at a temperature of 5 K. We find that the exciton-density dependence of excitation-induced dephasing (EID) is two to three orders of magnitude lower than in other atomic monolayer semiconductors such as transition metal dichalchogenides. We find that, EID is different for the multiple excitons evident in the excitation line shape, as is their phonon-mediated temperature dependence. We ascribe these observations to screening effects due to polaronic dressingby the lattice. We further examine the evolution of the two-dimensional coherent exciton lineshape at 5 K with population waiting time. We find that the 2D coherent lineshape evolves with over time windows in which the population itself is static, in a manner that indicates time-dependent EID. Specifically, we find that the dephasing slows down with population waiting time. This spectral evolution invokes time-varying inter-exciton coherences, and we discuss the possible role of dark exciton states in these quantum dynamics.

Solid-state nanocrystal-based upconversion (UC) systems have disadvantages with respect to poor exciton diffusion lengths, strong singlet reabsorption and the low resulting achievable UC efficiencies. To circumvent these issues, we employ bulk lead halide perovskite (LHPs) thin films as triplet sensitizers for near-infrared-to-visible UC. The sensitized UC process is based on triplet-triplet annihilation (TTA) in the organic semiconductor rubrene. Conservative estimates result in reported UC efficiencies upwards of 3%.1However, to further boost the UC efficiency, it is required to understand and mitigate the effects of shallow trap states in the LHP sensitizer. Here, we investigate the underlying mechanism of energy transfer at the LHP/rubrene interface, and the effect of the LHP perovskite film thickness on the UC dynamics and efficiency threshold. In the LHP/rubrene bilayer device, we observe two opposing effects caused by interactions between the LHP and rubrene: i) passivation of surface defects by rubrene, which results in a longer native photoluminescence (PL) lifetime, and ii) quenching of the PL lifetime due to energy transfer to the triplet state of rubrene. Our observations indicate that non-radiative trap filling in the LHP film and charge transfer to rubrene are likely competitive quenching pathways. As a result, the threshold Ithat which the UC process becomes efficient, observed as a slope change in the power-dependent upconverted PL, shifts to lower values with increasing film thickness. In contrast to previous reported systems, the slope change is directly dependent on the underlying LHP dynamics:i) below Ith, we find a slope twice the value of the LHP PL, and ii) above Ith, the upconverted PL follows the same power-law as the underlying recombination of electrons and holes in the LHP thin films.2,3However, two unusual effects are also observed in the system: i) two distinct rise times in the upconverted emission, indicating two distinct triplet sensitization rates. ii) A reversible photobleach of the upconverted emission, which cannot be correlated to a physical degradation of the system. By manipulating the triplet population, we are able to show that a higher triplet population does not necessarily result in a higher UC yield for the LHP/rubrene system demonstrated here, rather can be detrimental to the efficiency due to strong singlet reabsorption.3 (1) Nienhaus, L.; Correa-Baena, J.-P.; Wieghold, S.; Einzinger, M.; Lin, T.-A.; Shulenberger, K. E.; Klein, N. D.; Wu, M.; Bulovi?, V.; Buonassisi, T.; et al. Triplet-Sensitization by Lead Halide Perovskite Thin Films for Near-Infrared-to-Visible Upconversion. ACS Energy Lett. 2019, 4, 888-895. (2) Wieghold, S.; Bieber, A. S.; VanOrman, Z. A.; Daley, L.; Leger, M.; Correa-Baena, J.-P.; Nienhaus, L. Triplet Sensitization by Lead Halide Perovskite Thin Films for Efficient Solid-State Photon Upconversion at Subsolar Fluxes. Matter 2019, 1, 705-719. (3) Wieghold, S.; Bieber, A. S.; VanOrman, Z. A.; Nienhaus, L. Influence of Triplet Diffusion on Lead Halide Perovskite-Sensitized Solid-State Upconversion. J. Phys. Chem. Lett. 2019,10, 3806-3811.

2D perovskites is a fascinating class of compounds where the 3D perovskite lattice is cut and spatially confined in two dimensions and an “inert” layer of intercalated organic ions is inserted between the layers. The 2D perovskites layers can be formed by the introduction of bulky organic cations that tend to interdigitate between the perovskite sheets. They exhibit a large compositional and structural phase space, which has led to novel and exciting physical properties. The 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. Because these materials have had a strong impact in how record breaking and stable solar cells are constructed, it is critical to understand the thermodynamic and chemical limitations of the maximum thickness that can be sandwiched between the organic bilayers while retaining the structural integrity of the 2D perovskite. What are the limits within which the 2D architecture that define the thickness of the inorganic slabs before the system diverts into the familiar 3D structure? To answer this fundamental question, we employed 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 application of these low dimensional semiconductors in stable photovoltaics will be discussed.

Organic-inorganic hybrid perovskite solar cells (PSCs) have gained tremendous attention as materials for photovoltaics due to their high efficiencies and their compatibility with low-cost low-temperature fabrication methods (such as solution processing). Perovskite film formation is complex, involving the formation of intermediates and/or metastable phases that strongly affect the final perovskite film microstructure. Therefore, understanding the mechanism of perovskite formation and the crystallization pathways is key for facile control of perovskite formation. We are using time-resolved x-ray scattering to investigate the perovskite formation of MAPbI₃-based perovskites and mixed cation (Cs, FA)PbI₃ perovskites in-situ during spin coating and the subsequent post-deposition treatments, where FA is formamidinium and MA is methylammonium.
For Cs0.15FA0.85PbI3 we study: i) the crystallization pathways during spin coating; ii) the subsequent post-deposition thermal annealing, and iii) crystallization during blade-coating. Our findings reveal that the formation of a non-perovskite FAPbI3 phase during spin coating is initially dominant regardless of the processing and that the processing treatments has a significant impact on the as-cast film structure and affects the phase evolution during subsequent thermal treatment. We show that blade-coating can be used to overcome the non-perovskite phase formation via solvothermal direct crystallization of perovskite phase. This work show how real-time investigation of perovskite formation can help to establish processing-microstructure-functionality relationships. Our work highlights the importance of real-time investigation of perovskite film formation which can aid in establishing processing-microstructure-functionality relationships and help to provide a fundamental understanding of the mechanisms of perovskite formation.

Many common crystal structures can be described by a single (or very few) repeated structural motifs (“monomorphous structures”) such as octahedron in cubic halide perovskites. Interestingly, recent accumulated evidence suggests that electronic structure calculations based on such macroscopically averaged monomorphous cubic (Pm-3m) halide perovskites obtained from x-ray diffraction, show intriguing deviations from experiment. These include systematically too small band gaps, dielectric constants dominated by the electronic, negative mixing enthalpy of alloys, and significant deviations from the measured pair distribution function. We show here that a minimization of the systems T = 0 internal energy via density functional theory reveals a distribution of different low-symmetry local motifs, including tilting, rotations, and B-atom displacements (“polymorphous networks”). This is found only if one allows for larger-than-minimal cell size that does not geometrically exclude low symmetry motifs. As the (super) cell size increases, the energy is lowered relative to the monomorphous cell, and stabilizes after ∼32 formula units (160 atoms) are included. Being a result of nonthermal energy minimization in the internal energy without entropy, this correlated set of displacements must represent the intrinsic geometry preferred by the underlying chemical bonding (lone pair bonding), and as such has a different origin than the normal, dynamic thermal disorder modeled by molecular dynamics. Indeed, the polymorphous network, not the monomorphous ansatz, is the kernel structure from which high temperature thermal agitation develops. The emerging physical picture is that the polymorphous network has an average structure with high symmetry, yet the local structural motifs have low symmetries. We find that, compared with monomorphous counterparts, the polymorphous networks have significantly lower predicted total energies, larger band gaps, and ionic dominated dielectric constants, and agree much more closely with the observed pair distribution functions. An analogous polymorphous situation is found in the paraelectric phase of a few cubic oxide perovskites where local polarization takes the role of local displacements in halide perovskites, and in the paramagnetic phases of a few 3d oxides where the local spin configuration takes that role. See PHYSICAL REVIEW B 101, 155137 (2020).

In metal halide perovskite solar cells, electron transport layers (ETLs) such as TiO₂ dictate the overall photovoltaic performance. However, the same electron capture property of ETL indirectly impacts halide ion mobility and overall phase stability, as is evident from the TiO₂-assisted halide ion segregation in mixed halide perovskite (MHP) films under pulsed laser excitation (387 nm, 500 Hz). This segregation is only observed when deposited on an ETL such as TiO₂ but not on insulating ZrO₂ substrate. Injection of electrons from excited MHP into the ETL (k_et = 10¹¹ s^–1) followed by scavenging of electrons by O₂ causes hole accumulation in the MHP film. Localization of holes on the iodide site in the MHP induces instability causing iodide from the lattice to move away toward grain boundaries. Suppression of segregation occurs when holes are extracted by a hole transport layer (spiro-OMeTAD) deposited on the MHP, thus avoiding hole build-up. These results provide further insight into the role of holes in the phase segregation of MHPs and hole mobility in perovskite solar cells.

Two-dimensional halide perovskites have properties that are markedly different than those of the corresponding 3D perovskites, for instance a considerably larger band gap and much stronger interactions between electrons and hole in the inorganic part of the material, i.e. a much large exciton binding energy. The exciton binding energy and other opto-electronic properties can be tuned by modifications in the composition of the inorganic layer, or by changing the layer thickness, for instance in Ruddlesden-Popper structures, as we hav shown recently.

Until now, most of the large organic cations used in 2D or quasi-2D perovskite materials only act as a spacer, defining the dimensionality of the system. Their HOMO-LUMO gap is generally very large and the electronic properties of the resulting materials are fully determined by the properties of the inorganic layers. Currently, is an increased interest in the introduction of specific functionality in the organic component of the material.

In this work, we aim to introduce such additional functionality in the organic part. An example of such functionality is the introduction of electron donors or acceptors resulting in enhanced charge separation. We have explored the introduction of functional organic chromophores theoretically by density functional theory calculations and show that it is possible to introduce conjugated molecules that have a significant effect on the electronic structure. Strong electron acceptors or donors lead to conduction band or valence band edges that are localized on the organic part of the materials. This could lead to enhanced charge separation.

In a subsequent step, several 2D perovskites with functional organic components have been synthesized. These systems include colloidal perovskite platelets with strongly electron accepting perylenebisimids on the surface, solid 2D materials with electron donating conjugated molecules such as benzothieno[3,2-b][1]benzothiophene (BTBT) and pyrene, and materials that include two-component charge transfer complexes (pyrene:TCNQ) between the inorganic layers. Using a combination of time resolved fluorescence spectroscopy and ultrafast transient absorption we show that introduction of such functional components can directly affect the excited state behavior, in several cases leading to charge separation between the inorganic perovskite layer and the organic layer. In a next step, we show that this can indeed lead to the formation of mobile charges in the perovskite layer with a much higher yield and a much longer lifetime than in the analogous materials without functionality in the organic component. This shows that careful engineering of the organic part in 2D perovskite materials shows a lot of promise for the programmed design of functionality in these materials.

The soaring power conversion efficiency (PCE) of solution process-based organic-inorganic hybrid perovskites solar cells (PSCs) assures their membership in the competitive photovoltaic technologies. However, stability, toxicity and scalability are the three grand challenges that still impedes the market adoption of PSCs. Careful fundamental studies on how their fascinating optoelectronic properties is related to their structures may provide leads to these challenges and also spin off new applications. In this talk, I will present our recent study on the dopants, isotope and pressure effects on hybrid perovskite materials (HPMs). We discovered an interesting bipolar electron injection in LiCl-doped MAPbI3-based UV detectors that respond to the energy of UV photons in distinguished photocurrents polarity. We also investigated how the underline interaction between the mass of the cationic rotors and the inorganic lattice in HPMs by isotope effect. In-situ synchrotron X-ray and time-resolved photoluminescence revealed abnormal optoelectronic properties of HPMs by modulating their bond angles and bond lengths using high pressure techniques. Moreover, I will also discuss our new technology to sequestrate lead leakage on perovskite solar cells that can absorb >96% leaked Pb ions under severe device damge while without sacrificing the PCE under normal operational conditions.

Organic-inorganic halide perovskites feature excellent optoelectronic properties but poor chemical stability. While passivating perovskite grain boundary (GB) by polymers shows prospects on long-term performance of perovskite solar cells (PSCs), its detailed impact on the ion migration phenomenon, which largely deteriorates the PSC stability, remains less probed. Here, we introduce a new polar polymer, polycaprolactone (PCL), to passivate GBs of methylammonium lead triiodide (MAPbI₃) perovskites with only 1 – 2 polymer monolayers via direct backbone attachment. The PSCs with passivated MAPbI₃, using classic but less stable Spiro-OMeTAD (2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene) hole transport layer (HTL), exhibit improved power conversion efficiencies up to 20.1 %, with 90 % of the initial PCE being preserved after 400-h ambient storage, and 80 % even after 100-h, 85 C aging. The improved PSC stability indicates critical roles of PCL GB passivation in retarding moisture-induced decomposition and suppressing ion migration within the perovskite. Time-of-flight secondary ion mass spectrometry reveals that I^- ions can actively migrate into electrode, HTL, and their interface in non-passivated PSCs, even without externally applied electric field, while such migrations are significantly mitigated in PCL-passivated PSCs. This effective GB passivation by PCL suggests an important potential of polymer additives towards the development of stable high-performance PSCs.

One of the key features of halide perovskite photovoltaics (PV) is the absorber’s ability to accommodate various ions of different sizes in the crystal lattice. Consequently, halide perovskite solar cells yield the highest efficiency and stability when absorbers comprised of a mixture of ions are utilized. However, some compositions such as FAPbI₃ (FA = formamidinium) or the fully inorganic composition CsPbI₃ exhibit non-ideal geometric conditions and are prone to form non-perovskite δ-phases that are detrimental for their application in PV. Alloying compositions allows for a fine tuning of the geometric conditions and therefore the inherent structural stability.^[1] Nevertheless, any chemical heterogeneity in the film can instigate phase segregation of the material into pure phases which as noted prefer their non-perovskite δ-phases and consequently lead to degradation of the device performance. In this study, we present X-ray scattering-based characterization technique as a novel approach of probing the nanoscale heterogeneity.
Spatially resolved analyses have repeatedly revealed microscopic heterogeneity in halide perovskite thin films, including compositional heterogeneity. Evidence has been found that decomposition upon exposure to the aforementioned stress factors is triggered/facilitated by local compositional variation from the averaged mixed phase.^[2] Consequently, a lack of homogeneity even on the nanoscale may eventually lead to decomposition and device degradation. Characterizing the chemical heterogeneity in these delicate materials on small length scales, however, has proven challenging and we herein present a novel approach of probing the nanoscale heterogeneity.
Analyzing the structural change across phase transitions allows us to probe the nanoscale heterogeneity in perovskite thin films. More precisely, we hypothesize that the cubic to tetragonal phase transition width is a measure of the chemical heterogeneity in the mixed perovskite composition. To analyze the perovskite phase transition from the tetragonal to the cubic phase at higher temperatures, temperature-dependent in situ wide angle X-ray scattering is utilized. Whereas the transition is abrupt for pure MAPbI3 (MA = methyl ammonium), the transition is stretched over a temperature range for mixed compositions.^[3] We hypothesize that this is due to heterogeneity in the probed area. Local variations in the chemical composition can exhibit slightly varying transition temperatures from the completely mixed phase and thereby lead to the observed “smearing” of the phase transition. To test our hypothesis, we prepare a set of FA_0.83Cs_0.17PbI₃ perovskite thin films, choosing a variation of annealing conditions. The annealing temperature has previously been shown to impact the compositional heterogeneity.
Here, we report a systematic study of the influence of annealing conditions on the phase transition in state-of-the-art perovskites, indicating a correlation between the thermodynamic limit of mixing and the processing conditions. If the slope in the tilt angle over temperature is an indicator for the nanoscale heterogeneity, we can expect this to be related to the device stability and therefore be of high relevance for future processing protocols.

[1] Z. Li et al., Chem. Mater. 2016, 28, 284.
[2] L. T. Schelhas et al., Energy Environ. Sci. 2019, 12, 1341.
[3] J. Barrier et al., submitted n.d.

Transmission electron microscopy (TEM) has significant potential to advance fundamental understanding of hybrid organic-inorganic perovskite (HOIP) behavior via high resolution characterization of structure and morphology, chemical analysis, beam-induced current mapping, or even ultrafast structural dynamics at high spatial resolution. However, the organic components and van der Waals bonding critical to the hybrid nature of the material renders HOIPs susceptible to degradation under various environmental conditions, including electron beam irradiation. One promising technique to mitigate this damage is through temporally modulated electron beams, which deliver electron doses with prescribed regularity in place of a thermionic beam in which electron emission is randomly distributed in time. In the present work, the temporally-modulated electron beam is created by training a femtosecond pulsed laser on the source material in the TEM gun region. Importantly, the range of pulsed-electron beam parameters that can be covered in this way is directly dependent upon the pulsed-laser properties, amounting to fine control over both the specimen relaxation time between packets and the instantaneous dose rate. This enables exploration of a host of factors, from spatially-localized specimen heating to complex cumulative, molecular to nanoscale energy-deposition effects. Here, we use a systematic approach to demonstrate the ways in which pulsed beams consisting of discrete packets of photoelectrons delivered to the specimen at precise times can be used to reduce damage to polycrystalline methyl-ammonium lead iodide (MAPbI₃) specimens, demonstrated quantitatively by using Bragg-peak-intensity attenuation to track damage as a function of total dose and dose rate. We also discuss insights garnered from these experiments into damage mechanisms in HOIPs, and report real-space imaging studies on the material using temporal modulation techniques. Because of the two-fold aspect of this work, in which damage is both mitigated and studied, it is of both fundamental and practical relevance.

Mechanical stability of hybrid organic-inorganic perovskites (HOIPs) is essential to achieve long-term durable HOIP-based devices. While HOIPs in two-dimensional (2D) form offer numerous options in the structure and composition to tune their mechanical properties, little is known about the structure/mechanical-properties relationship in this family of materials. Here we investigated a series of 2D lead-halide HOIPs by nano-indentation to explore the impact of critical factors controlling the properties of both the organic and inorganic layers on the materials’ out-of-plane mechanical performance. We find that the presence of soft organic layer and weak Van der Waals (VdW) interfaces renders 2D Ruddlesden-Popper (RP) HOIPs more compliant than their 3D counterparts. For 2D RP lead iodide HOIP crystals with n-alkylammonium as spacer molecules, shorter alkyl chain and higher n number (i.e., thicker PbI₆^4- inorganic slab between organic layers) result in higher Young’s Moduli and hardness values, which gradually converge to those of 3D HOIPs. We further discover that the lead-halide bond in the inorganic framework can significantly influence the mechanical properties of 2D RP HOIPs with n = 1. Like 3D HOIPs, stronger lead-halide bond strength leads to higher Young’s modulus in these 2D HOIPs, i.e. E_Cl>E_Br>E_I, while the hardness of 2D RP HOIPs follows a trend different from that in 3D HOIPs (i.e., H_Br>H_Cl>H_I for 2D HOIPs), probably due to the combined effects from the Pb-X bond strength and inorganic framework structural change (e.g., symmetry and distortion). Moreover, we uncover that the interface between the organic layers in 2D HOIPs can be an effective route to engineer the materials’ mechanical properties. Replacing the weak CH₃-CH₃ VdW forces by covalent bonds or phenyl-phenyl interactions in the interface can lead to a much stiffer and harder 2D HOIPs. Finally, we show that the mechanical performance of 2D HOIPs with linear aliphatic diammonium spacer molecules is affected by the two basic structural parameters, i.e., the thicknesses of the organic and inorganic layers. Thinner organic layer and thicker inorganic layer can result in 2D HOIPs with larger elastic modulus and hardness values, similar to what is found in 2D RP HOIPs. 2D HOIPs exhibit hybrid mechanical features of those from metals, ceramics and polymers, which are tunable over a wide range through the vast compositional choices. Our results offer intriguing insights into the structure-property relationship of 2D HOIPs from a mechanical perspective, providing guidelines and inspirations to achieve materials design with required mechanical properties for applications.

Organic-inorganic lead halide perovskites with ABX₃ structure, established as rapidly emergent energy materials for optoelectronic applications, such as photovoltaics (PV) and light emitting devices (LED). The power conversion efficiencies (PCE) of perovskite solar cells (PSC) reached threshold values beyond 25% competing with silicon counterparts. The versatility of compositions and structures lead to various application forms such as thin films for PSCs or nanocrystals for LEDs or quantum dot solar cells. To obtain the perovskite semi-conductor in preferred form, solution-based synthesis is done using a variety of lead precursors, solvents and additives. These perovskite inks dictate the crystallization and grain morphology of thin films or nanocrystals, however, the nucleation dynamics in solution are not well understood yet. For this purpose, perovskite precursor solutions were investigated via solution nuclear magnetic resonance (NMR) spectroscopy. The Pb²⁺ coordination behavior in perovskite solution was tracked very extensively for the first time by ²⁰⁷Pb NMR spectroscopy and chemical shifts in the range of -500 to 2500 ppm were obtained in deuterated DMF and DMSO solutions. Since the signals obtained by NMR spectroscopy are the direct response from the local electronic structure of the ²⁰⁷Pb-nuclei, any changes in the structural environment are reflected as chemical shift δ²⁰⁷Pb. The lead iodide species in solution are part of an equilibrium, which was investigated via detecting the chemical shift δ²⁰⁷Pb upon variation of accessible solution parameters, such as molarity, temperature, iodide amount and coordinating solvent. In DMSO, 2D and 3D structures of connected lead iodide octahedra are dominant, whereas structures of lower dimensionality are mainly found in DMF solutions. Therefore, ²⁰⁷Pb NMR spectroscopy acts as a valuable probe for studies on the solution species and dynamic processes, that are involved in the formation of the perovskite crystals.

Ion diffusion has been demonstrated to seriously affect the optoelectronic properties of Halide Perovskites (HaPs). Till now the fastest diffusion has been attributed to the movement of the halide anions, largely neglecting the contribution of mobile protons, on the basis of computed estimates of their density. In this talk, I will show the proofs of the process of proton diffusion inside HaPs following deuterium-hydrogen exchange and migration in MAPbI₃, MAPbBr₃ and FAPbBr₃ single crystals through D/H NMR quantification, Raman spectroscopy and Elastic Recoil Detection Analysis, challenging the original assumption of halide-dominated diffusion. These results are confirmed by impedance spectroscopy, where MAPbBr₃-based solar cells respond at very different voltage frequencies compared to CsPbBr₃-based ones. I will show that water plays a key role in allowing the migration of protons as we do not detect deuteration in its absence. The water contribution is modeled to explain and forecast its effect as a function of its concentration in the perovskite structure. These findings are of great importance as they evidence how unexpected, water-dependent proton diffusion can be at the basis of the ~7 orders of magnitude spread of diffusion (attributed to I^- and Br^-) coefficient values, reported in the literature. The reported enhancement of the optoelectronic properties of HaP when exposed to small amounts of water, may be related to our finding.

Perovskite solar cells have gained prominence for their high efficiency and ease of fabrication, yet are hampered by instability. While ion mixing improves device efficiency and stability in general, recent work is demonstrating that incorporation of ions of particularly small or large size (via applying additives in the fabrication process) enhances both further. However there is a lack of understanding of the mysterious mechanisms through which these ions modify material properties (either in the bulk or grain boundaries) or alter the crystallization pathways. In this talk, I will outline recent advances in material characterization (from both experiments and atomistic simulations using DFT) of the impact of several additives (organic salts (PEAI, BAI, GAI) or inorganic salts (alkali halide)) and investigate how they modify the microstructure of the perovskites. I will also discuss the proposed mechanisms and their prospects in the evolution of efficiency and long term stability perovskite solar cells.

Multicomponent perovskite solar cells have reached the recent efficiency breakthrough of 25.2%, 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 relative humidity and time to drop the antisolvent during the preparation of perovskite films and 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].

An overview of Hunt Perovskite Technologies, LLC (HPT) metal halide perovskite (MHP) PV research program will be presented. Founded in 2013, HPT has focused their research on stabilizing the MHP material through better chemistry, as highlighted by stabilizing the pure alpha-phase FAPbI₃ MHP material in 2015. In this materials study, we have come to appreciate the unique defect structure of the MHP material by way of its ionic composition. Descriptions of MHP macro and molecular defect structure will be discussed, as well as their effect on the material’s durability and PV device performance.

With its rapidly increased cell efficiency to as high as over 25% and incredible suitability to process, equipment and operation, the perovskite is truly a rising star for next generation solar cells. In fact, its cell efficiency has surpassed all other thin film solar cells. We have studied a few different deposition technologies, including dry processing such as vacuum-based alternating precursor deposition, partial co-evaporation of inorganic components; dry atmospheric deposition by using direct contact reactive process; and wet solution processing such as small area spin-coating, large-area blade coating and high-throughput slot-die coating. At present, the wet solution deposition processes are well-advanced with significantly higher efficiencies over 23%. In comparison, the vacuum processes are under development with best cell efficiency still lower than 22%. This article is intended to compare and opt for the best one with most promises for future largescale processing by screening these technologies developed in research laboratories. With mature industrial equipment and engineering processes, we improved the vacuum deposited perovskite cell efficiency to 21.32%, highest for this type to our knowledge. In addition, the films produced using dry deposition seem to show better stability against high humidity, temperature and light-soaking.

Highly efficient perovskite solar cells (PSCs) with power conversion efficiencies (PCEs) > 25% are fabricated with formamidinium (FA)-based lead triiodide (FAPbI₃) perovskites. In general, mixed cations and anions containing FA, methylammonium (MA), Cs, I and Br ions are used to stabilize the black α-phase against d-phase of the pristine FAPbI₃. Disadvantageously, the additives such as MA, Cs and Br to FAPbI₃ widen its bandgap and reduce the thermal stability due to the lower thermal decomposition of MA to FA. Therefore, in order to further increase PCE through the enhanced photocurrent density due to the increased light harvesting, stabilizing the a-FAPbI₃ without blue-shift of the bandgap is needed. In addition, structural dimensions, distortion, and dielectric properties can vary significantly depending on the ionic radius and type of the A-site cation. In this presentation, I would like to introduce the impact of cationic management on the efficiency and long-term stability of perovskite solar cells.

In this talk, I will present our recent progress in understanding the fundamental properties of metal halide perovskites and the related device developments. First, the nature of defects induced charge traps and doping in perovskite materials will be discussed, and their chemical origin will be analyzed. A recent development in characterizing the spatial and energetic distributions of defects will be presented, which revealed the significant difference in trap density between single crystals and polycrystalline films, but similarity in distribution of the traps. Second, the healing of defects and tuning the doping in perovskites by multiple processes will be presented that boost the efficiency and stability of perovskite solar cells and modules. TFinally, a new charge recombination layer for all perovskite tandem solar cells will be presented which not only simply the fabrication process, but also significantly enhance tandem solar cell efficiency and stability.

Lead halide perovskite is a new type of semiconductor optoelectronic material, which owns large absorption coefficient, long diffusion length, which make it as an excellent photovoltaic material. Perovskite solar cells with different bandgaps of absorber need to be studied for making efficient single or tandem solar cells. In this talk, we will show: 1) By contact engineering, for the bandgap around 1.5 eV, we have achieved close to 25% efficiency, 2) By the composition engineering, we have pushed the PCE of inorganic perovskite CsPbI₃ based solar cells to 20% efficiency, 3) According to the defect engineering, over 20% efficiency of the perovskite solar cells have been demonstrated for the perovskite bandgap around 1.35 eV.

Perovskite quantum dots (QDs) as a new type of colloidal nanocrystals have gained significant attention for both fundamental research and commercial applications owing to their appealing optoelectronic properties and excellent chemical processability.¹ For their wide range of potential applications, synthesizing colloidal QDs with high crystal quality is of crucial importance. However, like most common QD systems, those reported perovskite QDs still suffer from a certain density of trapping defects, giving rise to detrimental non-radiative recombination centers and thus low photoluminescence quantum yield (PL QY). Halide vacancy in perovskites has been widely accepted to be the main factor preventing near-unity PL QY by creating uncoordinated Pb octahedra and the subsequent localized deep trap states.² Very recently, we have succeeded in synthesis of phase-stable and less-defect perovskite QDs, including FAPbI₃ QDs, CsPbI₃ QDs and Sn-Pb alloyed QDs.^3-6 We have demonstrated that a high room-temperature PL QY of up to 100% can be obtained in FAPbI₃ and CsPbI₃ perovskite QDs, signifying the achievement of almost complete elimination of the trapping defects. This is realized by the modulation of halide precursor involved in the synthesis, in which trioctylphosphine (TOP) is used to dissolve halide salts. For Sn-Pb alloyed QDs, although the alloyed QDs possess much improved structural stability than that of the pure Pb- and Sn-based one, they still suffer from an extremely low PL QY of ~0.3%, indicative of the severe charge trapping in these Sn-containing perovskites. Such low PL QY is believed to result from the widely known facile oxidation of Sn²⁺ to Sn⁴⁺. To address this issue, we have demonstrated very recently that the deep-level trap states in Sn-based perovskites can be well suppressed by Na⁺ doping, which gives rise to a dramatically improved PL QY of ~28% without alerting their favorable electronic structure. X-ray photoelectron spectroscopy (XPS) studies suggest the formation of a stronger chemical interaction between I^- and Sn²⁺ ions upon Na doping. On the other hand, though the reported TOP-based route has demonstrated efficacy in producing CsSn_xPb_1-xI₃ QDs (x = 0~0.6) with superior structural stability, such synthetic protocol can only find success in making these I-related QDs, preparing those pure Cl- and Br-based analogues remains challenging because of the insufficient solubility of PbBr₂ and PbCl₂ in TOP. Therefore, by taking advantage of the strong coordinating ability of trioctylphosphine oxide (TOPO), we have recently reported the first synthesis of ASn_xPb_1-xX₃ QDs (A = Cs, FA, MA) with excellent tunability in both Sn/Pb ratio and a full range of halide covering. The ability to generate all possible, unique combinations has offered great material versatility for the perovskite QD family. Further, by employing ultrafast spectroscopic techniques, we have resolved the fundamental photoexcited charge carrier dynamics in these alloyed QDs, which shall pave the way for future study for enhancing their usefulness in optoelectronic applications.^7-9 We expect the successful synthesis of the “ideal” perovskite QDs will exert profound influence on their applications to both QD-based light-harvesting and -emitting devices in the near future.

1. Swarnkar, A. et al., Science 2016, Vol. 354, pp. 92-95.
2. Alivisatos et al., J. Am. Chem. Soc. 2018, Vol. 140, pp. 17760.
3. F. Liu, Q. Shen et al., ACS Nano, Vol. 11, No. 10, pp. 10373-10383, 2017.
4. F. Liu, Q. Shen et al., J. Am. Chem. Soc., Vol. 139, No. 46, pp. 16708-16719, 2017.
5. F. Liu, Q. Shen et al., J. Phys. Chem. Lett., Vol. 9, No. 2, pp. 294-297, 2018.
6. F. Liu, Q. Shen et al., Chem. Mater., Vol. 31, No. 3, pp. 798-807, 2019.
7. F. Liu, Q. Shen et al., Angew. Chem. Int. Ed., Vol. 59, pp. 1-5, 2020.
8. F. Liu, Q. Shen et al., Chem. Mater., Vol. 32, No. 3, pp. 1089-1100, 2020.
9. C. Ding, Q. Shen et al., Nano Energy 2020, 67, 104267.

Solar cells utilising a metal–halide perovskite (MHP) as the active layer boast maximum power conversion efficiencies rivalling those of commercial silicon based cells, despite the low temperature, solution-based processing methods used in their fabrication. However, those fabrication methods lead to structural and stoichiometric heterogeneity on multiple length scales, with high concentrations of mobile ionic defects, which are believed to underlie undesirable device characteristics such as current–voltage hysteresis and performance degradation.
To further development of perovskite based solar cells, an increased understanding of the complex and often intertwined phenomena underlying both their high performance and the degradation of this performance is essential. The insights and predictions provided by the application of multiscale modelling techniques to the study of phenomena occurring at multiple length- and time-scales continue to play a critical role in this effort.
In this talk I will focus on our use of atomic, mesoscale and continuum modelling techniques to explore aspects of electron and ion dynamics in MHPs and devices based on them. Applications include the effects of pressure and grain boundaries on ion motion; polaronic and hot carrier dynamics; and the influence of the electron and hole transporting layers on current–voltage hysteresis in perovskite based solar cells.

Over the last ten years, the halide perovskites have grown from an academic curiosity into one of the hottest topics in renewable energy research, and more than 15000 papers on perovskite solar cell have now been published. The sheer amount of papers makes it increasingly difficult to get a good overview of what has been going on in the field, and we firmly believe that much could be gained if the data in all those papers could be stored in one place where it would be easily accessible by the research community. Such a collection of data would lead to a better overview, better experimental design, as well as new insights that would be hard to get from data scattered over thousands of articles. It thus represents a way to accelerate the pace of development by applying scientific openness. In this project, we have with the help of a large group of volunteers manually extracted all extractable device data found in the over 15000 perovskite papers so far published. Around the time of presentation this data will be made available in the form of a web application together with interactive graphics enabling anyone to explore and learn from the completeness of the published perovskite device data. We will in this presentation describe what we have created, what can be conclude from the accumulated data, as well as the potential future benefits for the perovskite community if this approach to data sharing gets universally implemented.
Over the last ten years, the halide perovskites have grown from an academic curiosity into one of the hottest topics in renewable energy research, and more than 15000 papers on perovskite solar cell have now been published. The sheer amount of papers makes it increasingly difficult to get a good overview of what has been going on in the field, and we firmly believe that much could be gained if the data in all those papers could be stored in one place where it would be easily accessible by the research community. Such a collection of data would lead to a better overview, better experimental design, as well as new insights that would be hard to get from data scattered over thousands of articles. It thus represents a way to accelerate the pace of development by applying scientific openness. In this project, we have with the help of a large group of volunteers manually extracted all extractable device data found in the over 15000 perovskite papers so far published. Around the time of presentation this data will be made available in the form of a web application together with interactive graphics enabling anyone to explore and learn from the completeness of the published perovskite device data. We will in this presentation describe what we have created, what can be conclude from the accumulated data, as well as the potential future benefits for the perovskite community if this approach to data sharing gets universally implemented.

Perovskite/Si tandem technology has been highlighted as the next technology to achieve ultrahigh efficiency, however progress has been deterred due to difficulties in optimizing the unstable and low-efficiency wide bandgap perovskite (~1.7 electron volts). Here, we have developed a wide bandgap perovskite that simultaneously improves the electrical properties and enhances the passivation effect using a novel strategy that involves tuning the anion of 2D additives (PEAI_xSCN_1-x).

We have observed the formation of defects in 2D phase via high-resolution TEM analysis and revealed that anion-engineering of 2D-forming additives leads to improved crystallinity and enhanced charge transport properties (higher mobility, longer carrier mobility, and uniform current flow through grains).

Additionally, we demonstrated a 20.7% efficient wide bandgap perovskite solar cell along with excellent long-term stability under continuous light illumination (80% of the initial efficiency was maintained after 1000 hours of illumination). This is the record efficiency among the reported wide bandgap perovskite solar cells. Using this anion-engineered wide bandgap perovskite cell as a top cell, we fabricated a monolithic 2-terminal perovskite/Si tandem cell with 26.7% efficiency (26.2% certified). Notably, the perovskite top cell in our champion tandem device contributes to ~64% of the total open-circuit voltage, which is the largest contribution from any perovskite top cell among the reported perovskite/Si tandem cells that identified relative contributions of each cell.

This work will be essential for future research on perovskite passivation using 2D additives and present a huge impact for the development of perovskite/Si tandem cells with efficiencies over 30%.

Hybrid perovskites materials have been skyrocketing the field of new generation photovoltaics and immediately expanding into advanced optoelectronics. Among the tremendous efforts in commercialization for perovskite solar cell (PSC) technology, the flexible Lead-free PSCs has drawn a great attention from the public to meet with the increasing demands for portable and wearable electronic devices. However, the fracture behavior in the prepared perovskite thin film will typically occur within the grain boundaries, which will retard the PCE performance and negatively affect the mechanical stability during the bending test. Here, we functionalized the volatile and brittle grain boundaries by incorporating low-dimensional (LD) phase. The LD phase was in-situ grown within the grain boundaries. Such in-situ grown LD phase not only enhanced the stability by passivating the grain boundaries defects, but also avoided the crack within the grain boundaries by linking with the neighbored grains. As a result, the prepared flexible lead-free PSCs showed remarkable endurance to mechanical cyclic-bending fatigue, with a PCE retention of 80% after 2,000 cycles.

Power conversion efficiencies (PCEs) of perovskite photovoltaics now exceed 25%. Perovskite solar cells hold great promise because of their potential low cost, tunable optoelectronic properties, and the possibility of improving their stability. For their commercial success, it is important to keep improving both their performance characteristics and stability. In this paper, a combined experimental and theoretical technique is used to study the effects of pressure on efficiencies of formamidinium-based perovskite solar cells. The interfacial surface contacts in the multilayered structures are predicted before fabricating the perovskite solar cells. A range of pressures is then applied to heal both pre-existing cracks within layered films and along the multilayered interfaces. The results show that the PCEs of the perovskite solar cells increased from ~18% to ~ 23% for applied pressures between ~ 0 – 6 MPa. A decrease in the PCEs is observed for pressure applied beyond 6 MPa. The implications of the results are discussed for lamination and roll-to-roll fabrication of perovskite solar cells.

Single-crystal materials are playing a dominant role in shaping established solar cell technologies such as Silicon and GaAs. With their low concentrations of non-radiative carrier recombination centers, single-crystals have led the way in terms of efficiency and are on of the most promising paths forward to approaching the theoretical Shockley-Queisser (SQ) limit for solar cells. In theory, lead halide perovskite single-crystals, with their orders of magnitude lower defect density, longer carrier diffusion lengths, and superior light absorption compared to their polycrystalline counterparts, should offer a chance for perovskite solar cell (PSC) technology to overcome the limitations of polycrystalline thin films and get as close as practical to the SQ theoretical limit. Unfortunately, due to challenges in crystal growth and device integration conditions, the pace of progress (In comparison to polycrystalline PSCs) has been very slow in single-crystal devices. In a recent study, we demonstrated the potential of SC-PSCs by realizing ~21.1% efficient devices based on ∼20 μm-thick methylammonium lead iodide monocrystalline absorber films. The preferred method, thus far, to grow MAPbI₃ single-crystal films for PSCs involves solution-processing at temperatures exceeding 120 °C, which adversely affects the film's crystalline quality, especially at the surface, primarily due to the loss of methylammonium iodide. Here, we devised an approach that enables the thin film MAPbI₃ crystal growth on a charge carrier transport layer at lower temperatures (<90 °C), and, thereby, limiting the escape of methylammonium iodide (MAI) from the crystal structure. We showed that this strategy extends the radiative charge carrier lifetime due to a significant reduction in non-radiative recombination channels. SC-PSCs fabricated with this method exhibit an enhanced V_OC of up to 1.15 V (Control 1.08 V) and a champion PCE of 21.93%. With no grain-boundaries at play, our work indicates the importance of the surface in advancing perovskite photovoltaic technology and provides an approach to grow perovskite single-crystal films under more favorable conditions for better optoelectronic qualities. We believe that our findings will be of wide interest to the perovskite materials and solar cell communities.

Although known for many years, organic-inorganic hybrid and related inorganic halide-based perovskites have received extraordinary attention recently, in part due to the unique physical properties and chemical diversity offered by these structures, which make them outstanding candidates for applications in photovoltaic and other semiconductor devices (especially for systems based on Pb, Sn and Ge). Perovskite structures consist of networks of corner-sharing metal halide octahedra that can extend in three or lower dimensions, providing opportunity to probe how dimensionality of the inorganic framework impacts semiconducting character. Further, incorporation of more complex organic cations within lower-dimensional perovskites enables combining outstanding inorganic-derived semiconducting character with organic-derived properties (e.g., luminescence, chirality), a feature extending well-beyond what is attainable with more established inorganic semiconductors (e.g., Si or GaAs) and underlying the creation of multifunctional organic-inorganic compounds and electronics [1]. This talk will emphasize chemical/structural versatility [2] and implications for semiconductor design (both with regards to properties and processing), as well as new characterization approaches for elucidating materials properties. Examples of topics that may be covered include custom-designed organic cations that template or stabilize the formation of difficult-to-realize lead-free perovskite structures [3], merging computational/experimental approaches to predict/verify optoelectronic character and guide design of complex organic-inorganic hybrids [4] and the use of a new carrier-resolved photo-Hall (CRPH) technique [5] to explore the impact of additive engineering on majority/minority carrier properties in perovskite films. Outstanding functionality and versatile processing provide two pillars for future application and study of this exciting and evolving materials family. References: [1] D. B. Mitzi, K. Chondroudis, C. R. Kagan, IBM J. Res. & Dev. 45, 29 (2001). [2] B. Saparov, D. B. Mitzi, Chem. Rev. 116, 4558 (2016). [3] M. K. Jana, S. M. Janke, D. J. Dirkes, S. Dovletgeldi, C. Liu, X. Qin, K. Gundogdu, W. You, V. Blum, D. B. Mitzi, J. Am. Chem. Soc. 141, 7955 (2019). [4] W. A. Dunlap-Shohl, E. T. Barraza, A. Barrette, S. Dovletgeldi, G. Findik, D. J. Dirkes, C. Liu, M. K. Jana, V. Blum, W. You, K. Gundogdu, A. D. Stiff-Roberts, D. B. Mitzi, Mater. Horiz. 6, 1707 (2019). [5] 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, https://doi.org/10.1038/s41586-019-1632-2 (2019).

Atomic layer deposition (ALD) is a sequential thin film deposition technique with great potential for increasing the performance of high-efficiency perovskite solar cells (PSCs) through advantages such as high uniformity and thickness control. Although ALD is limited to inorganic precursors, molecular layer deposition (MLD) allows for the use of organic precursors and can be used in conjunction with ALD to result in high-quality hybrid organic—inorganic films. Organic—inorganic perovskite materials for PSCs are typically solution processed; this is undesirable when compared to vapor processes due to the use of toxic solvents which require careful control of the atmosphere, making it difficult to manufacture. This research focuses on the design of a process for depositing hybrid organic—inorganic perovskite films through a combined ALD/MLD process. There are currently no processes for depositing perovskite materials by ALD/MLD without some degree of solution processing. Since sequential deposition methods allow for precise thickness control, it would be possible to then deposit both methylammonium lead iodide and 2-D perovskites. 2-D perovskites have shown the potential to be effective surface passivators for perovskite solar cells. As passivation layers, 2-D perovskites improve long-term stability, decrease interfacial defects and charge recombination, and increase efficiency by enhanced charge extraction from the bulk perovskite.

Employing scalable deposition techniques adaptable to complex multi-cation halide perovskites is a significant step towards the commercialization of perovskite optoelectronics. The hybrid vapor-solution method has been shown to render uniform and conformal perovskite layers on textured surfaces such as random pyramids of Si substrates ¹,². During that method, a seed layer containing Cs/Pb iodide is deposited by co-evaporating CsI and PbI₂ in a vacuum chamber followed by transformation into a perovskite layer by spin-coating an isopropanol solution of formamidinium/methylammonium-halide (FA/MA-halide). In this study, the deposition rate of CsI with respect to PbI₂ in terms of crystallinity and device performance was investigated. A CsI:PbI₂ rate ratio of 1:10 was found optimal. Later, the concentrations of FAI, MABr, and MACl were systematically optimized to deposit triple halide multi-cation perovskite solar cells. This is the first time a triple-halide multi-cation perovskite layer is fabricated using the hybrid vapor-solution method. A comparison between perovskites with double and triple halides showed a superior solar cell performance of Br-free triple cation perovskite compared to the other species. This is attributed to the wider absorption range of that perovskite with narrower bandgap. The champion devices showed efficiencies of 18.1% in the n-i-p and 17.6% in the p-i-n structure. The presented analysis exhibits good control on the halide composition during the hybrid vapor-solution method.

References
(1) Sahli, F.; Werner, J.; Kamino, B. A.; Bräuninger, M.; Monnard, R.; Paviet-Salomon, B.; Barraud, L.; Ding, L.; Diaz Leon, J. J.; Sacchetto, D.; Cattaneo, G.; Despeisse, M.; Boccard, M.; Nicolay, S.; Jeangros, Q.; Niesen, B.; Ballif, C. Fully Textured Monolithic Perovskite/Silicon Tandem Solar Cells with 25.2% Power Conversion Efficiency. Nat. Mater. 2018, 17 (9), 820–826. https://doi.org/10.1038/s41563-018-0115-4.
(2) Schulze, P. S. C.; Wienands, K.; Bett, A. J.; Rafizadeh, S.; Mundt, L. E.; Cojocaru, L.; Hermle, M.; Glunz, S. W.; Hillebrecht, H.; Goldschmidt, J. C. Perovskite Hybrid Evaporation/ Spin Coating Method: From Band Gap Tuning to Thin Film Deposition on Textures. Thin Solid Films 2020, 704, 137970. https://doi.org/10.1016/j.tsf.2020.137970.

Organic-inorganic halide perovskites (OHPs) are widely studied due to their excellent optoelectronic properties for solar cell and light-emitting diode applications. Conventional solution-processing techniques such as spin-coating method are commonly used in lab-scale device production but it is essential to explore other deposition techniques compatible for large-scale production. To overcome this, single-source flash evaporation technique, in which a single source of materials of interest is rapidly heated to be deposited in a few seconds, is one of the candidate techniques for large-scale thin film deposition of OHPs. [1.2] In this work, we investigated the reliability and controllability of the single-source flash evaporation technique for methylammonium lead iodide (MAPbI3) perovskite. In-depth statistical analysis was employed to demonstrate that the MAPbI3 films prepared via the flash evaporation have an ultrasmooth surface and uniform thickness throughout the 4-inch wafer scale. We also show that the thickness and grain size of the MAPbI3 film can be controlled by adjusting the amount of the source and number of deposition steps. In addition, we demonstrate that the reliability of the technique has a direct impact on the device characteristics of the fabricated photodetector devices via statistical analysis.

References
[1] Longo, G., Gil-Escrig, L., Degen, M., Sessolo, M., Bolink, H. Chemical Communications, 51, 7376 (2015).
[2] Wei, H., Ma, H., Tai, M., Wei, Y., Li, D., Zhao, X., Lin, H., Fan, S., Jiang, K. RSC Advances, 7, 34795 (2017).

The currently prevailing technique to produce high quality perovskite films is one-step spin-coating with antisolvent dripping. However, this method has disadvantages when considering mass production due to large waste of precursor solution and poor coverage over a large substrate. Solution-shearing, a type of blade coating methods, is better suited for large scale production and has been applied to printing of halide perovskite. However, devices fabricated by spin-coating generally outperforms those by solution-shearing, which suggests that the quality of spin-coated films is better than solution-sheared films. Herein, we report on a simple modification of a shearing blade that enabled us to access a wider property-map of the coated perovskite films, thereby improving device performance compared to a conventional glass blade. More specifically, the key was functionalization of the blade's surface into non-wetting. The non-wetting blade formed a thicker meniscus because a lower surface energy of the blade applied a weaker shearing force to the precursor solution compared to a wetting blade. The thicker meniscus yielded not only a thicker film but also a larger average domain size because of the reduced nucleation rate. Carrier lifetime, measured by time resolved photoluminescence, of the perovskite film with the non-wetting blade was twice as long as that with the conventional blade, suggesting much improved film quality with the surface modification of the blade. These structural and optoelectronic changes led to an improved power conversion efficiency of 19.0% while that of the reference device was 17.7%. Additionally, we have successfully coated a uniform perovskite film over 70 cm2 using the non-wetting blade demonstrating large-scalability of solution shearing.

The emergence of hybrid organic-inorganic perovskites (HOIPs) has been an exciting development in semiconductor research. Due to their cost-effective processing, high charge carrier mobilities, and band gap tunability, these materials have been incorporated in a wide array of applications such as solar cells, light emitting diodes, and photodetectors. HOIPs have been processed using a wide variety of methods: solution deposition, vacuum deposition, and even from a melt. Here, we report the first example of metal halide perovskite that has been deposited by laser printing on a flexible substrate. Laser printing is a solvent-free manufacturing method that is scalable and high-throughput, and that can be easily adopted and modified for flexible substrates of any shape and size. To formulate the toner, the CH3NH3PbI3 powder was first obtained using standard procedures. This powder was combined with carnauba wax, hexamethyldisilazane (HMDS) treated fumed silica, and a charge control agent, to create a toner that was then printed onto polyethylene terephthalate (PET) using an office laser printer. Scanning electron microscope (SEM) electron dispersive X-ray spectroscopy (EDS) revealed that a vertical phase-separation of the toner components occurred in the printed film, with the perovskite layer segregated in the middle. Atomic force microscope (AFM) measurements were used to evaluate the morphology of the films as a function of processing conditions. Electrical measurements were performed in air and dark and an average resistivity of 1.6*107 ?m was obtained, a value comparable with that of spin-coated films. Space-charge limited current (SCLC) mobilities were estimated to be 20 cm2/Vs. The current-voltage characteristics exhibit no hysteresis and the properties are constant over a large period of time, due to the fact that the non-perovskite toner components encapsulate the perovskite layer. Our work introduces an exciting new avenue for the fabrication of stable perovskite devices that is scalable and devoid of hazardous solvents.

Since their emergence in 2009 perovskites solar cells have reached a remarkable efficiency of 25.2 % [1]. Metal-halide perovskites exhibit low Shockley-Read-Hall recombination rates, high absorption coefficients across much of the solar spectrum as well as high charge-carrier diffusion lengths and mobilities [2]. These properties are ideal for a wide range of applications, including solar cells, transistors, light-emitting devices and lasers. A particularly simple and popular design for perovskite solar cells is the planar architecture, which was first fabricated using co-evaporation of methylammonium lead iodide (MAI) and lead iodide (PbI2) [3]. Co-evaporation makes it possible to deposit perovskite thin-films that are smooth, very planar and pin-hole-free over large areas [4]. This makes evaporated thin-films advantageous for the fabrication of perovskite solar cells and for detailed studies of the optoelectronic properties of the material. Furthermore, no solvents are required which makes this method fully additive and prevents damage to underlying layers. This is important both for the manufacturing of tandem solar cells and the upscaling of perovskite solar cells. While this method was initially mainly used for the deposition of MAPbI3, it has recently been extended to other perovskite materials including formamidinium lead iodide (FAPbI3) [4], triple-cation perovskites [5] and perovskites incorporating tin [6]. Despite this progress, several hurdles remain on the way to large-area co-evaporated perovskite solar cells. One issue are challenges with the evaporation of MAI [7]. Especially the rate control with conventional quartz micro balances (QMBs) does not work reliably for this material. Furthermore, it has been reported that impurities in MAI influence the film morphology and efficiency of solution-processed perovskite solar cells [8]. Therefore it is important to study the evaporation mechanics of MAI and the influence of impurities in-depth, which we set out to do in this contribution [9]. We compared two different MAIs, an impure one, and a highly purified MAI. A wide range of techniques was employed to characterise the precursors and deposited films, including nuclear magnetic resonance (NMR), infra red spectroscopy and mass spectroscopy. We were able to identify impurities in MAI and track their presence over the course of an evaporation. Furthermore, we characterised the purity of commercially available MAI and found drastic differences between manufacturers. Finally we fabricated solar cells with MAI of different purity and characterised their performance. We found that impurities critically influence the process control during the co-evaporation of perovskite thin-films. Based on this finding we give recommendations to improve the control over the evaporation and therefore the reproducibility of the process. These insights pave the way towards the upscaling of co-evaporated perovskite solar cells. [1] https://www.nrel.gov/pv/cell-efficiency.html [2] MB Johnston, LM Herz (2016) Acc. Chem. Res., 49: [3] Liu, M., Johnston, M. B., & Snaith, H. J. (2013). Nature, 501(7467), [4] Borchert, J., Milot, R. L., Patel, J. B., Davies, C. L., Wright, A. D., Martínez Maestro, L., Johnston, M. B. (2017). ACS En. Lett., 2(12), [5] Gil-Escrig, L., Momblona, C., La-Placa, M.-G., Boix, P. P., Sessolo, M., & Bolink, H. J. (2018). Adv. En. Mat., 8(14), [6] Ball, J., Buizza, L., Sansom, H., Farrar, M., Klug, M., Borchert, J., Patel, J.B., Herz, L.M., Johnston, M.B. and Snaith, H.J., 2019. ACS En. Lett. (2019) 4, [7] Borchert, J., Boht, H., Fränzel, W., Csuk, R., Scheer, R. and Pistor, P., (2015). J. of Mat. Chem. A, 3(39), [8] Levchuk, I., Hou, Y., Gruber, M., Brandl, M., Herre, P., Tang, X., Hoegl, F., Batentschuk, M., Osvet, A., Hock, R. and Peukert, W., 2016 Adv. Mat. Int., 3(22), [9] Borchert, J., Levchuk, I., Snoek, L.C., Rothmann, M.U., Haver, R., Snaith, H.J., Brabec, C.J., Herz, L.M. and Johnston, M.B., 2019. ACS AMI, 11(32)

Perovskite Solar Cells (PSC) have achieved up to 25.2% efficiency in the lab-scale by solution processing. However, one of the barriers towards their commercialization is scalability. Achieving this is difficult because of the various layers that are commonly deposited through spin coating, a solution process that is undesirable in industry. The use of toxic solvents presents another challenge to fabricate perovskite modules. To overcome the limitations of solution processing, thermal evaporation has been explored as a robust deposition method that can be upscaled. Thermal evaporation provides three main advantages. First, it can provide scalability. Second, it allows better control and homogeneity of the thickness of the films. And third, it allows for the fabrication of graded compositions throughout the thin film. However, thermal evaporation of perovskites has been difficult to achieve due to the challenges associated with measuring accurately the deposition rate of organic cations such as methylammonium iodide. Here, we systematically study different approaches to coevaporate perovskites with various stoichiometries. We correlate film stoichiometry to the deposition temperature/rate of the organic precursors through XRD and XPS measurements, demonstrating good control and reproducibility of the films over a 10 cm2 area. The films with varying stoichiometries were incorporated into solar devices where it was observed an improvement of efficiency in large areas with respect to the conventional deposition techniques.

Despite the exceptional increase in the power conversion efficiency of perovskite solar cells (PSCs) over the past decade, the feasibility of commercial systems depends upon achieving stable device performance over thousands of hours. While the crystalline structure of three-dimensional (3D) metal halide perovskites is responsible for unique semiconductor properties advantageous to photovoltaics, such as strong absorption and a tunable bandgap1, this structure also leads to aggressive degradation by humidity, light, and oxygen that prevent the required stability. Fortunately, two-dimensional (2D) perovskites have been shown to have improved stability due to the hydrophobic nature of these materials.2,3 Thus, work is being done to incorporate 2D perovskites as passivating layers for the photovoltaic active regions in 3D metal halide PSCs. While quasi-2D perovskite structures are widely studied, experimental investigations of 2D-on-3D interfaces have been limited due to processing challenges.4 The purpose of this work is to investigate how resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE) can overcome these processing challenges to create high-quality 2D-on-3D hybrid organic-inorganic perovskite heterostructures. Eventually, such structures could be used as passivation layers in PSCs. RIR-MAPLE is a vapor deposition technique that uses a low-power, Er:YAG (2.94 ?m) laser to evaporate a matrix solvent in a frozen target, which gently ejects the precursor material onto the substrate without evaporating the precursor itself.5,6 In this work, 2D-on-3D heterostructure thin films of (PEA)2PbBr4 (PEA = phenethylammonium, C8H12N) on MAPbI4 (MA = methylammonium, CH3NH3) are deposited by RIR-MAPLE. Growth conditions and in-situ annealing protocols at the interface of the two materials are investigated to achieve a heterostructure with little to no compositional mixing between the layers. X-ray diffraction spectroscopy and time-of-flight secondary ion mass spectrometry (TOF-SIMS) are performed to confirm that the films are compositionally distinct across the interface. UV-Vis absorbance spectroscopy of the 2D-on-3D heterostructures is conducted to determine the optical properties of the films. Furthermore, efficient charge transport in the vertical direction is influenced by the quantum well orientation of the 2D perovskite layer. Wells oriented perpendicular to the 3D surface will assist in vertical transport, and wells that are parallel to the 3D surface will impede vertical transport. Spin-coated PEA-based 2D perovskites have been shown to have parallel orientation with respect to the substrate, while preliminary work has demonstrated that these materials have a random orientation of crystalline grains when deposited by RIR-MAPLE. To explore charge transport in the 2D-on-3D heterostructure, current measurements through an anode-heterostructure-cathode configuration are conducted to focus on the transport through the interface itself. In this way, additional variables related to full device architectures are minimized. Separate 2D and 3D hybrid perovskite films, as well as spin-coated heterostructures, are characterized as references for comparison to the RIR-MAPLE-deposited heterostructures. A. D. S.-R. and N. E. W. acknowledge research supported as part of the Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE), an Energy Frontier Research Center funded by the U. S. Department of Energy, Office of Science, Basic Energy Sciences (BES) (MAPLE film deposition of 2D hybrid perovskites). References 1. Mitzi, D. B., J. Chem. Soc. Dalt. Trans. 1-12 (2001). 2. Milot, R. L. et al., Nano Lett. 16, 7001-7007 (2016). 3. Grancini, G. & Nazeeruddin, M. K., Nat. Rev. Mater. 4, 4-22 (2019). 4. Grancini, G. et al., Nat. Commun. 8, 15684 (2017). 5. Barraza, E. T. et al., J. Electron. Mat. 47, 917-926 (2018). 6. Stiff-Roberts, A. D. & Wright, N. in Low-Dimensional Materials and Devices. 11085, 26 (SPIE, 2019).

Optimizing hybrid perovskite devices has been a central research topic in the last few years. This usually is performed through a thorough band alignment engineering of the hole and electron extraction layers with respect to the absorber. This led to the emergence of an uncountable number of organic, inorganic and hybrid layers, usually suitable for specific absorbers, which makes it hard to repeat their improving effects on slightly different absorbers. This comes from the fact that surfaces and therefore interfaces are complex media that are still not well understood but are the major starting points for the fabrication of efficient working solar cells. Density of surface states, surface workfunction homogeneity, grains and grain boundaries properties, optoelectronic properties at the surface are the most important characteristics to determine in order to boost the power conversion efficiency. Kelvin Probe Force Microscopy (KPFM) is a suitable tool to access most of these properties but the measurements outcomes are delicate to interpret, especially for perovskites, essentially because they are often performed under ambient conditions. In this contribution we present our results obtained on methylammonium lead iodide perovskites (MAPI), grown by co-evaporation, where the methylammonium iodide (MAI) concentration is changed by varying the pressure inside the physical vapor deposition (PVD) chamber. We focus our work on the analysis of the surface of our samples without exposing them to air to minimize their contamination to moisture and oxygen. For that, we used Frequency Modulation KPFM (FM-KPFM) under ultra-high vacuum (UHV), photoluminescence (PL), time resolved-PL (TRPL), structural and compositional techniques to understand the impact of MAI concentration in MAPI on the surface. In a first part, we show the impact of varying the MAI pressure inside the PVD by looking at two different pressures. We compare the surface of these two samples by first using KPFM under UHV, which gives the impression that different compounds are deposited on the surface of the samples. For a better interpretation of our results, we additionally use X-ray diffraction (XRD) and Energy Dispersive Spectroscopy (EDS) which enables us to create a link between I/Pb ratio and the transition from a cubic to tetragonal structure. In a second part, we discuss the effect that annealing the samples under inert gas has on the surface workfunction. We see that depending on the initial MAI concentration, the outcome of annealing at several temperatures is different. We again use EDS and XRD for a better understanding but we also perform PL and TRPL. We start to illuminate our sample to access surface photovoltage values and we see how the difference in MAI concentration initially and through the annealing steps influence the ionic mobility at the surface, where different ionic species migrate depending on the absorber's state. Finally, we introduce PL and TRPL to potentially link minority carrier lifetime, PL bandgap and surface photovoltage and we perform a thickness study with KPFM to define an energy band diagram and summarize the key effects of varying the MAI concentration on the surface and why it is important to clearly identify the state of the surface before optimizing the device via band alignment engineering.

To go beyond the present 25.2 % solar cell efficiency record, halide perovskite point defects should be either reduced or passivated. Device interfaces need to be designed to minimize recombination and boost charge carrier extraction. 2D perovskites are effective interface passivation layers which, included in solar cell device structures, enabled the highest published efficiencies. However, 2D perovskites are mostly processed via solution, which limits the control over the deposited film resulting in losses that manifest as increased series resistance or recombination. The requirement of solvent orthogonality for the 3D and 2D perovskite deposition limits the choice of possible material compositions, whereas the solution deposition of the 3D perovskite limits the use of the 2D passivation to the top 3D surface. Vapor deposition methods are established in manufacturing and enable us to overcome these limitations. Here, we demonstrate a vapor deposition route for 2D passivation of a 3D perovskite, which enables a down to monolayer control and conformal film uniformity. The quality of the 2D passivation layer is probed combining time resolved photoluminescence with spatially resolved hyperspectral luminescence imaging and X-ray beam induced current. Films are then incorporated in complete solar cell device structures demonstrating enhanced performances with respect to the solution processed counterparts. In addition, benefiting from a full vapor deposition of the 2D-3D perovskite stack, a bottom interface 2D passivation is demonstrated.

Lead halide perovskites have shown outstanding properties when applied in optoelectronic devices, such as solar cells and light-emitting diodes.[1,2] Nevertheless, the presence of highly toxic lead together with their low environmental stability might become a bottleneck for their commercialization. Herein we present different inorganic halides based on non-toxic metals with potential application in optoelectronics. Among these, Cu(I)-based fully inorganic ternary metal halides are particularly promising, especially for applications in light-emitting diodes (LEDs). Blue, highly luminescent materials (required for the fabrication of white light-emitting diodes) are commonly more challenging to obtain than red or green emitters, due to the increased probability of non-radiative recombination. We report the synthesis of two different series of inorganic cesium copper halides, with the general formulas Cs3Cu2X5 and CsCu2X3 (X = Cl, Br, I, and mixtures thereof). Cs3Cu2I5 has been identified as the most promising material, as it maintains blue luminescence with photoluminescence quantum yield exceeding 40% even after being exposed to air for several days. Importantly, we employed a dry mechanochemical synthesis which avoids the use of solvents and high temperatures and allows the preparation of phase-pure compounds.[3] Homogeneous Cs3Cu2I5 thin-films were also fabricated by single-source vacuum deposition (SSVD) employing the pre-synthesized luminescent compounds. The emission spectrum and crystallinity is maintained in thin films, even without any further post-deposition treatment.[4] Using similar synthesis and thin-film processing techniques, we investigate several cesium bismuth halides and their potential use in thin-film solar cells. Finally, we present the synthesis and characterization of an unexplored family of materials, called anti-perovskites, which are based on silver chalcogenides compounds. These materials have potential applications in IR photodiodes and tandem solar cells, thanks to their narrow bandgap < 1 eV. References: [1]. Leijtens, T., Bush, K. A., Prasanna, R., y McGehee, M. D. "Opportunities and challenges for tandem solar cells using metal halide perovskite semiconductors". Nat. Energy, 3, 828-838 (2018). [2]. Xing, J. et al. "Color-stable highly luminescent sky-blue perovskite light-emitting diodes". Nat. Commun. 9, 1-8 (2018). [3]. Palazon, F., El Ajjouri, Y., y Bolink, H. J. "Making by Grinding: Mechanochemistry Boosts the Development of Halide Perovskites and Other Multinary Metal Halides". Adv. Energy Mater. 1902499 (2019). [4]. Sebastia-Luna, P., Navarro-Alapont, J., Sessolo, M., Palazon, F., y Bolink, H. J. "Solvent-Free Synthesis and Thin-Film Deposition of Cesium Copper Halides with Bright Blue Photoluminescence". Submitted

Multijunction all-perovskite solar cells offer a route towards efficiencies of III-V materials at low cost by combining the advantages of low thermalization loss in multijunction architectures with the beneficial properties of perovskites - namely low processing cost, high throughput fabrication, and compatibility with flexible substrates. Here, I will discuss why flexible all-perovskite tandems are a very attractive iteration of the perovskite technology. I will focus on what is needed to make a good perovskite-perovskite tandem on a flexible substrate, and the advances that we have made towards this goal.
I will cover advances including a) the development and understanding of wide bandgap materials that leverage a novel bandgap-tuning strategy (without increasing bromine, hence avoiding significant phase segregation) to reach the ideal bandgaps for all-perovskite tandems, resulting in efficient devices that are stable to the combined stresses of heat and light; b) advances that we have made in interlayer design, working towards more ideal material properties for reducing shunt resistance; and c) the integration of these approaches to develop >23% efficient monolithic all-perovskite tandems on rigid substrates and 21% efficient devices on flexible plastic substrates.

Perovskite solar absorbers have the potential to disrupt the photovoltaic industry predominantly with their simultaneous promise of high efficiency and low cost. Whilst these two properties ensure a steady stream of research and development opportunities, commercial scaling requires assuring that the devices are sufficiently characterized to be confident of their long-term performance. Oxford PV has been addressing the challenges associated with commercially scaling its perovskite on silicon tandem solar cell design. Key practices that have been adopted during the development of highly efficient, reliable, and cost-competitive solar cells are the use of rigorous characterization tools and techniques, and the development of necessary models. This talk will highlight some of the challenges and considerations associated with bringing perovskite on silicon tandem solar cells from a laboratory prototype to a commercial product ready for manufacturing and how characterization has shaped and continues to inform that process.

Single-junction crystalline silicon solar cells dominate the photovoltaics market today, but their power conversion efficiencies (PCEs) are approaching the practical limit for this technology. The emergence of hybrid organic-inorganic metal halide perovskite semiconductors over the past decade has opened a window of opportunity to advance the performance of the market-dominating photovoltaic technologies through the development of tandem solar cells. These tandem solar cells promise high PCEs (>33%). To reach this landmark, a number of scientific and technological challenges related to high-efficiency perovskite-based tandem solar cell architectures, stable and high-quality perovskite semiconductor materials, and light management concepts need to be addressed. In this presentation, our progress on passivation strategies and novel interlayers to reduce the voltage losses in wide-bandgap and low-bandgap perovskite solar cells (PSCs), and in turn to increase the PCE of perovskite-based tandem photovoltaics, will be presented.
The search for efficient passivation interlayers for wide-bandgap perovskite top solar cells in n-i-p architecture directed our research in the past towards solution-processed 2D/3D perovskite heterostructures. Making use of these heterostructures for interface passivation, wide-bandgap PSCs (E_g ~1.72 eV) with record open-circuit voltage for this bandgap and very high PCEs of up to 19.4% were demonstrated [1]. The reduced voltage deficit is a result of the decreased non-radiative recombination losses at the perovskite/hole-transport layer (HTL) interface. The solution-processed 2D/3D perovskite heterostructure is compatible with 3D perovskite absorber layers of a wide range of bandgaps (1.65 eV ≤ E_g ≤ 1.85 eV) [2]. Using this strategy in 4-terminal perovskite/silicon and perovskite/CIGS tandem solar cells, high PCEs > 27% [3], and > 25% [2] were demonstrated, respectively, thereby significantly exceeding the record PCESs reported for the respective single-junction solar cell technologies.
Beyond these past results, at the MRS fall meeting we will further present our latest work on novel interlayers and passivation strategies to minimize the voltage loss in wide-bandgap (1.65 eV ≤ E_g ≤ 1.75 eV) as well as low-bandgap (E_g ~ 1.26 eV) perovskite solar cells in p-i-n architecture. The p-i-n architecture is a key to realize high-performance 2-terminal perovskite/silicon as well as perovskite/perovskite tandem solar cells. Moreover, we will present a novel route to deposit uniform ultra-thin polymer passivation layers onto perovskite thin films by chemical vapor deposition (CVD) [4]. The CVD polymerization promises reliable, scalable, and homogeneous deposition for future large-area processing of perovskite-based tandem solar cells.

References:
[1] Gharibzadeh, S. et al. Record Open-Circuit Voltage Wide-Bandgap Perovskite Solar Cells Utilizing 2D/3D Perovskite Heterostructure. Adv. Energy Mater. 9, 1–10 (2019).
[2] Gharibzadeh, S.; Hossain I. M. et al. 2D/3D Heterostructure for Semitransparent Perovskite Solar Cells with Engineered Bandgap Enables Efficiencies Exceeding 25% in Four-Terminal Tandems with Silicon and CIGS. Adv. Funct. Mater. 30, (2020).
[3] Duong, T. et al. High Efficiency Perovskite-Silicon Tandem Solar Cells: Effect of Surface Coating versus Bulk Incorporation of 2D Perovskite. Adv. Energy Mater. 10, 1903553 (2020).
[4] Malekshahi Byranvand M.; Behboodi-Sadabad F. et al. Chemical Vapor Deposited Polymer Layer for Efficient Passivation of Planar Perovskite Solar Cells. submitted

Integrating metal halide perovskite top cells with crystalline silicon (c Si) bottom cells into monolithic tandem devices has recently attracted increased attention due to the high efficiency potential of these cell architectures. To further increase the tandem device performance to a level well above the best single junctions, optical and electrical optimizations as well as a detailed device understanding for such architectures need to be developed [1]. Here we present our recent results on such monolithic tandem cells in the common 2-terminal (2T) as well as in a novel 3-terminal (3T) contacting scheme.
In the 2T design, we recently demonstrated a certified power conversion efficiency (PCE) of 29.15% [2,3]. I will discuss some key elements to this result: using a silicon heterojunction cell electron contact on the rear side, enabling the integration of a perovskite top cell in the p-i-n configuration; employing self-assembled monolayers (SAMs) as hole selective contacts for further reduction of non-radiative recombination losses, etc. With the latter, efficiencies of up to ~ 21 % were demonstrated for p-i-n perovskite single junctions [3].
In 3T tandems, one of the three contacts is shared by both subcells, allowing for the individual operation of both subcells in a monolithic device with a single substrate, thus combining the characteristic advantages of 2T and 4-terminal (4T) devices: high combined photogeneration currents as in state-of-the-art 2T tandems, and ideally independent electrical operation similar to 4T tandems, making the cell more robust against varying spectra. I will discuss our recent proof-of-concept study [4], demonstrating a 3T perovskite/silicon tandem cell by manufacturing an n-i-p perovskite top cell on an interdigitated back contact (IBC) silicon heterojunction solar cell. In our proof-of-concept device, the combined PCE of ~ 17% is still limited both optically and electrically, and it will be discussed how optimizations of the cell stack can bring this architecture to a PCE level comparable to 2T tandems. Furthermore, I will demonstrate that the subcells can indeed be operated largely independently at their individual maximum power points, allowing to extract the maximum power output for varying spectral conditions similar to 4T cells. A minimized series resistance at the shared electron contact on the cell’s rear side is identified as the crucial parameter for independent operation of the subcells, and I will discuss pathways to approach the simulated efficiency potential by an optimized device design.

References:
[1] Jost, Kegelmann, Korte, Albrecht, Adv. Energy Materials, 2020, doi: 10.1002/aenm.201904102
[2] NREL Best Research-Cell Efficiency Chart, https://www.nrel.gov/pv/cell-efficiency.html
[3] Köhnen, Jost, Stannowski, Albrecht et al., Sustainable Energy and Fuels, doi: 10.1039/C9SE00120D
[3] Al-Ashouri, Magomedov, Korte, Albrecht et al., En. Env. Sci, 2019, doi: 10.1039/C9EE02268F
[4] Tockhorn, Wagner, Korte et al., ACS Appl. En. Mat., 2020, doi: 10.1021/acsaem.9b01800

Among all commercialization routes for perovskite photovoltaics (PV), the perovskite/Si tandem is of particular interest as it capitalizes the mature silicon industry with decades of experience. Here we report low-temperature solution process for efficient two-terminal perovskite/Si tandem solar cells, wherein electrical and optical properties of the tunneling junction are carefully modulated. Moreover, we improve materials stability by introducing Eu²⁺/Eu³⁺ “redox shuttle” to suppress the Pb⁰/I⁰ impurities which inevitably occurred during device operation. In addition, we carefully analyze the cost of tandem devices in terms of levelized cost of energy (LCOE) by adopting the “bottom-up cost model”. It reveals perovskite PV exhibit low materials cost, which reduces the LCOE substantially in tandem devices. We conclude that further efforts are required to simultaneously improve the efficiency and lifetime of perovskite modules, which ultimately endows the competitiveness of perovskite tandem solar cells.

Photovoltaic (PV) devices based on metal halide perovskite (MHP) absorbers have reached outstanding performance over the past few years, surpassing power conversion efficiency of over 25% for lab cells and with large area devices in excess of 16%. For the solar application stability, the most demanding requirement to assess for PV, remains the outstanding issue. The problem of stability motivates basic science driven work on MHP based PV at NREL. Material and device insight can enable MHP PV stability along with the associated opportunities to further improve efficiency, scalability and realize unique device architectures. This talk will highlight work at NREL to develop understanding of defects and interfaces in MHPs and what these experiments indicate regarding the requirements to enhance stability of HPSCs. Discussion will focus on efforts to more carefully understand the implications of processing on a range of defects and interfaces to provide a foundation for enabling the technological requirements for PV and other devices. Ongoing efforts make, measure and ultimately control defects via material formation and processing in the context of high-volume manufacturing will also be touched upon. Hypothesized links between device architecture choices, questions regarding materials integration and coupling of formation along with resulting defects to stability will be discussed. More importantly approaches to move from hypothesis to insight using different experimental approaches will be presented. Work at NREL along these lines to provide critical insight regarding interface formation and passivation that appears to have direct implications for performance and its evolution over time in the resulting devices will be presented. New approaches to understanding the critical physical, chemical and electronic properties to provide predictive insight will also be discussed.

Surfaces and charge-extracting contacts remain major sources of non-radiative recombination loss; limiting the performance of perovskite PV compared to theoretical limits. We investigate mixed-cation mixed-halide perovskites and demonstrate that, with chemical surface passivation, they can achieve >4 μs minority carrier lifetimes, and >20% external PL quantum yields (PLQY), and nearly single-exponential decay kinetics. This high PLQY observed corresponds to ~97% of the Shockley-Queisser theoretical quasi-fermi level splitting, a first for MA-free mixed-cation mixed-halide compositions. The high PLQY and long PL lifetimes allow us to measure average surface recombination velocities (SRV) <10cm/s, with a champion low of ~1cm/s (a conservative estimate). We show how such improvements in SRV should lead to an absolute improvement in PCE of >4% even with the best of contact alignments. We investigate different MA-free high bandgap compositions, including bandgaps relevant for tandem applications (1.7-1.8eV), and demonstrate performance improvements across all these compositions with our surface passivation strategy. Lastly, using a combination of surface characterization techniques, we show Lewis base interactions of the passivating agent with the perovskite surfaces.

In solar cells, perovskite films are typically sandwiched in between charge selective layers, either organic or inorganic semiconductors, as well as their combinations. Electron/hole selectivity results from appropriate alignment of the electronic transport levels of these layers with the electronic bands of the perovskite. While transport layers should efficiently extract the photogenerated charge carriers, they should also not quench the perovskite luminescence to avoid a reduction of the quasi-Fermi level splitting and hence the maximum attainable photovoltage. Finally, ohmic charge injection/extraction and a high built-in potential are desirable to maximize fill factor both in single junction and tandem perovskite solar cells. We will review these design rules using fully vacuum deposited solar cells as a platform, as every layer can be fine-tuned or exchanged without substantially alter the device fabrication. The focus will be on wide bandgap, mixed halide perovskite solar cells, which suffer from a higher voltage deficit as compared to narrow bandgap analogous. We will highlight the importance of using non-quenching, energetically-matched hole and electron transport layers in order to reduce non-radiative charge recombination losses. Novel strategies and multilayers structures (including inorganic, organic and ionic compounds) to improve the device functioning will be presented. The implication for the perovskite compositional stability as well as for the device lifetime will also be discussed.

Within a decade, there has been a surge of interest into halide perovskite semiconductors. With tuneable bandgaps, versatile and facile deposition processes and excellent optoelectronic properties, these materials have found applications in photovoltaics (PV), broad-spectrum photodetectors, light-emitting diodes and lasers. While research into using these materials for emission and X-ray detection applications is now beginning to surge; currently, halide perovskites are most well known for their remarkable PV performance, with perovskite solar cells achieving certified power conversion efficiencies exceeding 25% in a remarkably short developmental timescale. Despite the truly impressive device performance, these materials have not yet reached their full potential. The major obstacle to this is an incomplete understanding of crystallisation processes, as well as defects which exist on the surfaces and interfaces of perovskite thin-films. Deficiencies at these interfaces, likely formed during the crystallisation process, are responsible for the major losses in perovskite-based optoelectronic devices; limiting charge extraction, increasing non-radiative recombination, contributing to hysteresis and increasing the voltage-loss of perovskite photovoltaics. Herein, I will present various effective strategies to reduce the defect densities of perovskite films and thus improve the performance of perovskite optoelectronic devices. First, I will discuss how manipulating precursor solutions can have significant impact on the quality of the perovskite films, through enabling reduced defect density. Consequently, this is one approach which can be used to boost device performance. Next, I will focus on a strategy in which we extrinsically modify the bottom interface of perovskite solar cells such that the crystallization of the perovskite yields films with fewer defects. Specifically, we show that ionic liquids can have a dramatic effect on the interfacial trap density in a perovskite solar cell. And finally, I will illustrate an approach in which we manipulate the charge-carrier density in the perovskite thin-film after its fully crystallized. We utilise molecular dopants to significantly alter the interfacial energetics in order to suppress non-radiative recombination losses. As a result, we observe a boost in both performance and stability. Importantly, these interface modification strategies can be used in tandem, resulting in further improvements in device efficiency. The utility of these defect mitigation strategies can readily be applied beyond perovskite PV and is likely to also improve the performance of a range of other perovskite-based optoelectronic devices.

Transparent electrodes are essential components of several kinds of high-efficiency solar cells. The photovoltaic devices working in the broadband spectra, from the ultraviolet (UV) to the near-infrared (NIR), require minimized parasitic absorption and high conductivity for these electrodes. Transparent conductive oxide (TCO) thin films are widely accepted choices for such applications. Among various successful TCOs, recently, Zr-doped indium oxide (IZrO) has been demonstrated as a broadband transparent and conductive material suitable for silicon heterojunction (SHJ) and silicon/perovskite tandem solar cells.^1,2 The main advantage of IZrO is combining specifications of wide bandgap, high carrier density, and high electron mobility in the same material.
Magnetron sputtering is by far the most established technique to fabricate TCO layers. During sputtering target material is removed in a ballistic process that sensitively depends on many interconnected processing parameters. This makes a stoichiometric transfer of target material challenging, while high kinetic energies of the arriving species may damage sensitive functional layers beneath. Conversely, in Pulsed Laser Deposition (PLD), the energy source for material ablation is physically decoupled from the vacuum equipment. This favors the stoichiometric transfer of the target material as well as provides a larger choice for deposition pressures and thermalizing the arriving species.
Despite these advantages, PLD is still considered an exotic fabrication method within the photovoltaic community due to the common concerns about the scalability of the technique reasoned by the small sample size for uniform coating and low deposition rates. In fact, significant progress in the upscaling of PLD³ has already allowed fabrication of high-quality films by on 200 mm circular wafers and fabrication of superconducting tape with deposition rates > 750 nm/min⁴, which is considerably high deposition rate.
In our work, we introduce wafer-scale (4”) room-temperature fabrication of high-mobility IZrO by PLD with deposition rates on par (>3 nm/min) with lab-scale RF sputtering equipment. We studied the optoelectronic properties and microstructure of the films and made a comparison with IZrO films grown by RF sputtering, both fabricated from an IZrO solid target with the same composition.
X-ray diffraction (XRD) reveals that as-deposited IZrO films are amorphous with small embedded nanocrystals. Their orientation is preserved as the preferred one after solid-phase crystallization at 200 °C. IZrO films exhibit excellent optoelectrical properties combining low absorptance (<10%) in the measured range (300-1500 nm) with high mobility (>60 cm²/Vs) leading to impressive sheet resistivity (R_sh) of <20 Ohm/sq for a 100-nm PLD grown film.
When applied as a front electrode in solar cells, IZrO enables high short circuit currents and the possibility to reduce the amount of indium by reducing the thickness of the films compared to standard ITO, which is critical considering the large scale utilization of such materials. On the other hand, higher deposition pressures during PLD fabrication offer a promising way to mitigate the sputter-induced damage for the deposition of rear transparent electrodes on top of sensitive functional layers as in the perovskite solar cells, which opens new opportunities for the development of high-efficiency tandem solar cells.

References
1. M. Morales-Masis et al., IEEE J. Photovoltaics, vol. 8, no. 5, p. 1202–1207, 2018.
2. E. Aydin et al., Advanced Functional Materials, vol. 29, no. 25, p. 1901741, 2019.
3. J. A. Greer, Journal of Physics D: Applied Physics, vol. 47, no. 3, p. 034005, 2013.
4. S. Lee et. al, Superconductor Science and Technology, vol. 27, no. 4, p. 044022, 2014.

Metal halide perovskite solar cells have largely fulfilled their potential of combining III-V-like optoelectronic properties with low-cost, solution-processing deposition techniques, achieving single-junction power conversion efficiencies (PCEs) >25%. A semiconducting nickel oxide (NiO_x) thin film is a highly desirable wide bandgap hole harvesting contact for perovskite solar cells and organic solar cells because it is low-cost, stable, and readily scalable.(1)Perovskite solar cells with NiO_x HTLs have delivered some of the best reported stability results under a variety of stressors, including the 85°C/85% relative humidity damp heat test, temperature cycling from -40°C to 85°C, and extended operational stability at elevated illumination and temperature.(2, 3) However, perovskite solar cells made with NiO_x HTLs typically result in lower open-circuit voltages (V_OC) compared to ultra-thin, undoped organic hole transport layers, which are more expensive and present problems for both scalability and stability. Enhancing the V_OC and increasing efficiency of NiO_x-based devices will directly result in a decreased photovoltaic module cost ($/W).
Here, we identify a surface-assisted electron transfer-proton transfer (ET-PT) mechanism at the NiO_x/perovskite interface leading to barriers for hole harvesting and device V_OC loss. Specifically, under-coordinated metal cation sites (Ni^≥3+) act both as a Lewis acid/oxidant towards dissociated lead halide precursors (i.e. I^-) and as a Brønsted base (proton-acceptor) with protonated amine precursors, forming A-site deficient – and therefore PbI_2-xBr_x-rich – perovskite at the perovskite/NiO_x interface. This defective perovskite region both increases recombination at the interface and acts as a hole extraction barrier, resulting in device V_OC loss.
We demonstrate the elimination of the extraction barrier via a simple, easily scaled approach: titration of surface oxide proton-acceptor sites by incorporating 1-5 mol% excess A-site cation to the perovskite precursor solution. This amount is sufficient to counteract the loss of organic cation, preventing formation of a PbI_2-xBr_x layer at the interface, ultimately leading to a V_OC increase >200 mV. We demonstrate that this mechanism applies to all forms of NiO_x, including thin films created from sol-gel precursors, nanoparticles, atomic layer deposition (ALD), and radio frequency (RF) sputtering, and is generalizable across multiple perovskite active layers, including state-of-the-art single junction and tandem relevant compositions. A p-i-n perovskite solar cell with 19.7% PCE at maximum power point (MPP) and V_OC up to 1.15 V is achieved. Devices maintain >90% of their initial performance after 2000 hours of thermal stability testing at 85 °C or >80% of their initial performance after 400 hours of accelerated operational stability testing under illumination and 55 °C in nitrogen. Our results extend the generality of acid/base-coupled oxidation-reduction reactions as the driving force for interface equilibration at metal oxide/perovskite interfaces, suggesting that metal halide perovskites have similar reactions with many types of metal oxide layers.

1. M. D. Irwin, D. B. Buchholz, A. W. Hains, R. P. H. Chang, T. J. Marks, p-Type semiconducting nickel oxide as an efficiency-enhancing anode interfacial layer in polymer bulk-heterojunction solar cells. Proc. Natl. Acad. Sci. 105, 2783–2787 (2008).
2. C. C. Boyd, R. Cheacharoen, T. Leijtens, M. D. McGehee, Understanding Degradation Mechanisms and Improving Stability of Perovskite Photovoltaics. Chem. Rev. 119, 3418–3451 (2019).
3. J. Xu, C. C. Boyd, Z. J. Yu, A. F. Palmstrom, D. J. Witter, B. W. Larson, R. M. France, J. Werner, S. P. Harvey, E. J. Wolf, W. Weigand, S. Manzoor, M. F. A. M. van Hest, J. J. Berry, J. M. Luther, Z. C. Holman, M. D. McGehee, Triple-halide wide-band gap perovskites with suppressed phase segregation for efficient tandems. Science. 367, 1097–1104 (2020).

Perovskite solar cells (PSCs) have potential to deliver TW-scale energy via low-cost manufacturing. However, scaling is limited by slow, high temperature annealing of the inorganic transport layers and insufficient understanding of how scalable processing impacts interfacial device physics. This study addresses the NiO, a leading hole transport layer (HTL) for both high-stability single junction architectures and tandem cells. We present a method for scaling ultrathin NiO HTLs by pairing high-speed (60 m/min) flexographic printing with a rapid annealing Cu:NiO sol-gel ink. By engineering precursor rheology for rapid film-leveling, large area (> 50 cm²) NiO HTLs were flexographically printed with high uniformity and photovoltaic performance matching that of spin coated films. Doping and the combustion precursor design enables 30X faster annealing while reducing intrinsic variability of work function and conductivity originating from NiOH surface species.

Integrating these printed transport layers in planar inverted PSCs allows rapid fabrication of high-efficiency (PCE > 16 %) Cs_xFA_1-xPbI_1-yBr_y solar cells with low hysteresis and open circuit voltages (V_OC) of 1.11 V. XPS studies of the transition from Ni-OH to Ni-O bonding for low-temperature annealed NiO clarify the balance required for both effective carrier collection and high photostability. Paired with variable illumination intensity measurements and detailed transient photovoltage characterization, these measurements provide an improved understanding of the intrinsic defects present at the NiO-perovskite interface and the impact of scalable synthesis of interfacial charge transport layers.

Vacuum deposition is a key technique in the field of perovskite photovoltaics due to the advantages it presents with respect to the classical deposition methods such as solution processed perovskites. The lack of solvents and the need of annealing makes it a good candidate for industrialization and upscaling as it has already been demonstrated in the OLED field.
Despite its advantages, tuning the interfaces is essential to reach the high power conversion efficiencies found in solution processed perovskite solar cells. Metal oxides are an interesting choice for extraction layers due to their improved stability and reduced parasitic absorption. In this work we present our recent advances in using metal oxides as extraction layers optimising the interfaces in order to reach record open circuit voltages in CH3NH3PbI3 perovskite and using MoO3 as extraction layer by molecular passivation in both PiN and NiP structure improving the stability and opening new possibilities for tandem applications. We also show how we implemented this stable pinhole-free bottom layers in large area devices and mini modules, which is very promising for future industrial purposes.

Halide double perovskites provide a flexible platform for manipulating electronic structure through synthetic design. I will present some recent studies from our group, where we have attempted to increase the compositional and structural complexity of halide perovskites through chemical substitution. In our quest to understand the electronic structure of double perovskites, we developed a simple model that allows us to predict the bandgap symmetry based on chemical composition. Our work constructs an intuitive picture of how the molecular orbitals of metal-hexahalide coordination complexes give rise to electronic bands in the extended perovskite lattice and describes how these molecular orbitals vary across the bands to give rise to the overall band structure. Although the effects of quantum confinement of Pb and Sn perovskites have been well studied, 2D analogs of 3D double perovskites are notably lacking. I will present a series of 2D double perovskites and discuss the optical and electronic consequences of dimensional reduction of the 3D double perovskite lattice to 2D derivatives.

Halide Perovskites exhibit outstanding light-harvesting and emissive properties. Presently, record power conversion efficiencies (PCE) >25% in solar cells and external quantum efficiencies (EQE) >20% in light-emitting devices have been demonstrated. The perovskite field has since expanded well beyond photovoltaics and LEDs into other optoelectronic domains of lasers, x-ray detectors, memristors etc. Emerging frontier areas include multiphoton, multi-exciton generation and hot carrier applications. In this talk, I will focus on the hot carrier phenomena in halide perovskites and our recent work in this area.

Low-dimensional perovskites are rapidly emerging due to their distinctive emission properties. Here, first I will present the fundamental properties of pristine 2D perovskites, such as the nature of the primary photo-excitations, their relaxation through electronic states and their diffusion across thin films. Such properties will be correlated and modulated by the structural characteristics of the 2D perovskites. Then, I will discuss the possibility of perovskite doping by controlled addition of impurity ions which may enable new electronic, magnetic, and optical performances. I will explore new doping strategies based on transition and rare earth metals for tuning of the emission wavelength (from UV to NIR, further extending the typical range covered with perovskites) and luminescent enhancement. The suitability of doping for the realization of 4 level perovskite lasers will be assessed. Doping with heterovalent ions will be tested to selectively increase p and n type conductivity of the material. The addition of alkaline-earth metals as B-site dopants will be tested to improve the material stability

In this report, the photoluminescent properties of a lead-free double perovskite Cs₂NaInCl₆ doped with Sb³⁺ are explored. The host crystal structure is a cubic double perovskite with Fm-3m symmetry, a = 10.53344(4) Å, and rock salt ordering of Na⁺ and In³⁺. It is a wide bandgap compound (Eg ≈ 5.1 eV), and substitution with Sb³⁺ leads to strong absorption in the UV due to localized 5s² → 5s¹5p¹ transitions on Sb³⁺ centers. Radiative relaxation back to the 5s² ground state, via a ³P₁ → ¹S₀ transition, leads to intense blue luminescence, centered at 445 nm, with a photoluminescent quantum yield of 79%. The Stokes shift of 0.94 eV is roughly 33% smaller than it is in the related vacancy ordered double perovskite Cs₂SnCl₆. The reduction in Stokes shift is likely due to a change in coordination number of Sb³⁺ from 6-coordinate in Cs₂NaInCl₆ to 5-coordinate in Cs₂SnCl₆. In addition to the high quantum yield, Cs₂NaInCl₆:Sb³⁺ exhibits excellent air/moisture stability and can be prepared from solution; characteristics make it a promising blue phosphor for applications involving near-UV excitation.

Two-dimensional (2D) halide perovskites are a class of emitter materials that are enjoying growing attention recently. The materials are related to 3D halide perovskites but either part or all of the small A-site cations are replaced by large organic molecules with an amonium binding moiety, for example butylamonium or phenyl-ethylamonium. This (generally) leads to the formation of a layered structure where a layer of metal-halide octahedra is formed and the large organic cations separate the layers from each other. Mixing of large and small A-site cations can lead to quasi-2D multi-layer materials where multiple layers of metal halide octahedra are separated by the organic molecules, for instance forming Ruddlesden-Popper or Dion-Jacobson structures. The emissive properties of these materials are intricately linked to the structural dynamics and formation of possible defects in the inorganic layers.

In this work we explore how modifications of the large organic amonium ion can affect the structural dynamics of the inorganic layer, combining molecular dynamics simulations with time-resolved spectroscopy. Using time-resolved fluorescence and ultrafast transient absorption spectroscopy we have studied the photophsyical behavior of 2D perovskites with a range of different organic cations inorder to gain insight in the relation between the structure and the exciyted state dynamics, These measurements indicate marked differences that can not solely be attributed to changes in the average structure of the materials.

To explain these differences we have performed classical molecular dynamics simulations for a variety of structures, for instance aliphatic alkyl chains, amonium ions containing aromatic moieties such as phenyls or perylene, but also di-functional ions containing two binding sites that can connect to an inorganic pervskite layer on both sides. The results indicate that there are marked differences between the different structures in terms of their structural rigidity. Multi-layer Ruddlesden-Popper structures are generally more rigid than single layers, however, the rigidity of the single layer structures can be strongly enhanced by engineering of the interactions between organic cations, for instance by introduction of large conjugated ions. In addition, di-functional ligands resulting in Dion-Jacobson like structures also lead to more rigid structures. Comparison with the experimental results allows us to derive some design rules that indicate how engineering of the organic part of the structure can help in improving the the emissive properties of the materials that is mostly due to the inorganic layer structure.

Mixed cation and anion APbX₃-type lead halide perovskites [A⁺=Cs⁺, CH₃NH₃⁺ [MA⁺], or (NH₂)₂CH⁺ [FA⁺]; X^-=Br^- or I^-] have received extensive attention for use as low cost, efficient light harvesters in solar cells. Issues regarding long-term active layer stability, however, remain unresolved. Electric field-induced ion migration in perovskite solar cells is of particular concern because it results in current-voltage (J-V) hysteresis, metastable power conversion efficiencies and accelerated active layer degradation. Facile ion migration within the perovskite lattice stems from relatively fragile ionic bonding between heavy, low-valence ions. This results in small activation barriers and correspondingly moderate diffusion coefficients for halide anions and A-site cations. The mobile nature of A⁺ cations and X^- anions is thus an instability of all metal halide perovskites. While numerous recent studies have focused on halide migration and segregation, significantly less work has gone into investigating cation migration in metal halide perovskites. This is especially true of double- and triple-cation FA_xMA_yCs_1-x-yPbX₃ alloys.
Here, we demonstrate that bias-induced cation migration is mitigated in mixed α-phase/δ_ortho-CsPbI₃ triple-cation FA_xMA_yCs_1-x-yPbI₃ films (referred as α/δ-FA_0.33MA_0.33Cs_0.33PbI₃). We verify this by comparing cation movement in lateral devices and working solar cells made of pure α-phase double-/triple-cation perovskite phase systems (FA_0.85Cs_0.09PbI₃ and FA_0.76MA_0.15Cs_0.09PbI₃) as well as α/δ-FA_0.33MA_0.33Cs_0.33PbI₃. The effects of bias-induced A⁺ migration on perovskite stability and device performance are assessed via infrared photothermal heterodyne imaging (IR-PHI), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and solar cell operational stability testing. Enhanced α/δ-FA_0.33MA_0.33Cs_0.33PbI₃ cation stability likely stems from “post-perovskite” δ_ortho-CsPbI₃ inclusions within parent thin films that act as barriers through which cations cannot easily migrate. This resistance to cation migration translates into improved device operational stability of α/δ-FA_0.33MA_0.33Cs_0.33PbI₃ lateral devices and solar cells relative to complementary, triple-cation, α-phase FA_0.76MA_0.15Cs_0.09PbI₃ devices. This study therefore provides new insights into cation migration in working devices and outlines a materials design strategy for stabilizing metal halide perovskites against bias-induced cation migration.

Organic-inorganic halide perovskite solar cells (PSCs) have been considered a viable member of next generation photovoltaics, which can address the scalability changes with a low-cost solution process. The energy lose in the hybrid system has been approved to be principally caused by defects/traps at grain boundaries and surfaces as well as point defects such as interstitial defects or vacancies in the perovskite compounds crystal lattice. Because of the presence of these defects, both anions (I) and cations (MA⁺) can migrate through the organic layer which provides a degradation pathway for perovskite materials even in those well-encapsulated devices: the halide ions can react with metal electrodes such as Ag, forming insulating components (AgI). In addition to the corrosion of electrode, halide ions migrating into organic hole transport layer（HTL）which is deleterious to transport properties, thus retarding device efficiency, especially under solar cell working conditions. Inspired by the quasi-2D-perovskite which shows a great stability in photovoltaic performance, here, the hexagonal boron nitride (h-BN) was incorporated into the perovskite absorber as an ionic diffusion barrier. The lamellar structure of h-BN consists of a lattice similar to graphite which can be subjected to exfoliation leading to few-layers nanosheets. The exfoliated 2D-layered materials present high surface area and quantum confinement, and also high chemical stability. The integration of this low-cost, lower dimensional material enables a better device stability as the result from the efficient suppression on the ion-migration induced degradation. With the aid of time-of-flight secondary ion mass spectroscopy (TOF-SIMS) and energy dispersive x-ray spectroscopy (EDX). The h-BN/ perovskites has an initial PCE over 20% that shows no degradation exhibit in over 72 h and retained 80% of the original PCE after 800hr, whereas the PCE of pristine MAPI devices diminishes by more than 30% within the first 24 hr.

Instability under illumination in hybrid organic-inorganic perovskite solar cells represents a major roadblock to the development and commercialization of this thin-film technology. Recently, it has been shown that varying the nature of the organic cation in the perovskite structure represents a potential pathway to tuning stability of materials under illumination. However, many potential mixed-cation perovskite compositions remain unexplored, and, much is still unknown about the degradation kinetics in mixed-cation thin-films. In this work, we successfully fabricated thin-films of substituted methylammonium lead triiodide (MAPI), where we substituted the methylammonium ion with varying percentages of three larger organic cations: imidazolium, dimethylammonium, and guanidinium. We achieved substitution of these large cations for compositions of up to 20-30% while maintaining the structure and optoelectronic properties of the films. We studied the degradation kinetics of the mixed-cation films under illumination using powder x-ray diffraction. We observed that the degradation is autocatalytic. Furthermore, we calculated rate coefficients for the degradation process and observed that the degradation was slowed in films which contained ≤20% of any of the larger cations compared to pristine MAPI films. A maximum decrease of 62% in the degradation rate coefficient was observed in the case of 5% guanidinium substitution. Ultimately, this work provides key insight into the degradation kinetics of mixed-cation perovskite films under illumination and provides a strategy for further stabilization of perovskite thin-films for improved photovoltaic performance.

Perovskite solar cells (PSCs) research has been projected over the past decade due to its promising photovoltaic properties, increasing efficiency and low cost. With a need to eliminate toxic lead from prevailing halide perovskite solar cells, several low-toxicity cations have been proposed, among them Sn(II) is considered as one of the most suitable replacements due to its similar electronic configuration and ionic radius to Pb(II). But the Sn-based perovskites are still plagued with low efficiency, oxidation and phase instability. Here we demonstrate the use of atmospheric solution-processed lead-free all inorganic molecular iodosalt compound cesium tin iodide (Cs₂SnI₆) as the light absorber in PSCs. The presence of excess SnF₂ in precursor solution assists phase separation. Due to the good binding affinity to SnF₂, pyrazine is expected to reduce Sn vacancy thus restricts phase separation. The effect of GuaSCN results in increasing grain size and improving film morphology thus improves the structural and optoelectronic properties of low bandgap perovskites. The present work aims at analyzing the effect of pyrazine and guanidinium thiocyanate (GuaSCN) in oxidation reduction and fabrication of low bandgap (1.3 eV) Cs₂SnI₆ films when used separately as a mediator during film fabrication in GBL. Initial results suggest both pyrazine and guanidinium thiocyanate give absorption onset at higher optical wavelength (>1000 nm) and low bandgap (~1.2-1.3 eV). Films in addition of Pyrazine both with and without SnF₂ (0 mol% and 40 mol%) shows mostly cubic Cs₂SnI₆ diffraction patterns in XRD analysis which implies that the effect of SnF₂ is not considerable in this case. The SCN ions increases grain size and produces cubic Cs₂SnI₆ crystals. Heating at higher temperature leads to samples with lower bandgap. Both the absorption vs. wavelength and tauc plot analyses show bandgap at around 1.25 eV. Most of the samples after dark testing for over 100 hours at 65 °C retain their properties and show similar absorption edge and bandgap patterns. Samples having exposure to light at 65 °C show higher bandgap over 2 eV. In this study, the effect of these additives in Cs(Pb,Sn)I₃ perovskite with a Sn-Pb ratio of 60:40 is also included. Primary results show separately use of both pyrazine and GuaSCN in precursor solution in a mixture of GBL and DMF result in low bandgap perovskites. Both Cs(Pb,Sn)I₃ and Cs₂SnI₆ without these additives tend to have higher bandgap at around 2.3 eV when deposited on glass substrates. Further analysis of the effect of Pyrazine and GuaSCN additives in the stability, morphology and lifetime of Cs(Pb,Sn)I₃ and Cs₂SnI₆ perovskite films will be continued in this study.

The exploration of lead-free perovskite-inspired semiconductors for optoelectronic applications (e.g., single- and multi-junction solar cells, photocathodes for water splitting, photodetectors) has captured ever-growing attention, driven by their promise to replicate the success of the Pb-based counterparts without the toxicity concerns associated with the latter. In particular, an emerging approach to Pb replacement involves Sb, whose attractiveness resides in its highly-polarizable Sb³⁺ cations with filled 5s orbitals, a stable oxidation state, and an ionic radius comparable to Pb²⁺. A number of antimony-halide perovskite derivatives with a formula A₃Sb₂X₉ (A⁺: monovalent cation; X^-: halide anion) have thus been explored. Notwithstanding the rise in their reported performance, A-Sb-X absorbers are yet to deliver photoconversion efficiencies comparable to the Pb-based counterparts. Importantly, this has been linked to the zero-dimensional (0D) structure of most A-Sb-X embodiments, as the more attractive two-dimensional (2D) embodiments have been demonstrated only in very few instances, due to constraints on composition or processing conditions.

With the aim of contributing to the realization of the full potential of antimony-halide perovskite derivatives, we have investigated two such systems featuring a desirable 2D structure, exploring the impact of dimensionality, microstructure, and crystallographic orientation on their photoconversion behavior. Herein, we firstly focus on Rb₃Sb₂I₉, the only ternary A-Sb-I with a direct bandgap, and also having the smallest exciton binding energy of all reported A-Sb-I embodiments.¹ In order to assess the role of microstructure on the photoconversion efficiency of Rb₃Sb₂I₉, we have developed two strategies for its microstructural enhancement, which involve the control of the supersaturation level during the annealing step, or a high-temperature treatment in SbI₃ vapor. Both strategies deliver a dramatic improvement in the film morphology and microstructure.² Within sandwich-type planar devices, the resultant high-quality films enable a substantial improvement in external quantum efficiency (EQE), reaching up to 65.4% at short circuit, i.e., the highest to date for perovskite-inspired antimony-/bismuth-halide A₃M₂X₉ (M: Sb³⁺/Bi³⁺). Importantly, this is achieved without the aid of a mesoporous architecture for charge extraction, thereby revealing the inherent capability and potential of Rb₃Sb₂I₉ for efficient photoconversion.

We additionally discuss the impact of dimensionality on the photoconversion capabilities of the Cs₃Sb₂X₉ system. Departing from the dedicated high-temperature (≥ 230 °C) processing protocols pursued in the prior literature, we demonstrate that halide mixing enables the 0D-to-2D structural conversion of the Cs₃Sb₂X₉ system at temperatures < 150 °C.³ The resultant 2D absorbers manifest a considerable photoconversion efficiency boost with respect to the 0D counterpart, attaining a peak EQE of 62.5%, which is at the state-of-the-art for all antimony-/bismuth-halide perovskite derivatives.

Through photocurrent-power characterization and Hall effect measurements, we finally reveal that the EQE boost achieved with both 2D Rb₃Sb₂I₉ and Cs₃Sb₂X₉ arises from their improved charge carrier transport, which we relate to the microstructural enhancement, dimensionality tuning, and crystallographic orientation. Such performance boost and mechanistic insight constitute an important step in realizing the full potential of lead-free antimony-halide perovskite derivatives and related bismuth-based homologs for their application in optoelectronics and photovoltaics.

1 J. P. Correa-Baena et al., Chem. Mater., 2018, 30, 3734–3742.
2 F. Li, Y. Wang, K. Xia, R. L. Z. Hoye and V. Pecunia, J. Mater. Chem. A, 2020, 8, 4396–4406.
3 Y. Peng et al., Appl. Mater. Today, 2020, 19, 100637.

It is common practice in the lead halide perovskite solar cell field to add a small molar excess of lead iodide (PbI₂) to the precursor solution in order to increase device performance. However, recent reports have shown that an excess of PbI₂ can accelerate performance loss. In addition, PbI₂ is photoactive (bandgap ~2.3 eV), which may lead to parasitic absorption losses in a solar cell.

We use XRD and SEM to show that incorporation of PbI₂ into the perovskite film is not straightforward and can have very different effects, depending on how and where PbI₂ is present. At low concentrations, PbI₂ facilitates charge extraction and improves device performance. For higher concentrations, PbI₂ influences how perovskite crystallises and a thick PbI₂ layer can be formed that impedes charge extraction. We show that devices using an optimized quantity of excess PbI₂ exhibit better device performance as compared to stoichiometric devices, both initially and for the duration of a stability test under operating conditions, primarily by enhancing charge extraction. However, PbI₂ is gradually degraded during device operation, largely negating the beneficial effect on charge extraction and introducing trap states that are detrimental for device performance. We find that although excess PbI2 provides a good template for enhanced performance as compared to stoichiometric devices, it can not meet stability requirements needed for commercialization of perovskite solar cells. The community must continue to seek other additives or synthesis routes that fulfill the same beneficial role as excess PbI₂, but which do not suffer from light induced degradation.

Despite the tremendous interest in halide perovskite solar cells, the structural reasons that cause the all-inorganic perovskite CsPbI₃ to be unstable at room temperature remain mysterious especially since many tolerance factor-based approaches predict CsPbI₃ should be stable as a perovskite. Here single-crystal X-ray diffraction and X-ray pair distribution function (PDF) measurements characterize bulk perovskite CsPbI₃ from 100 to 295 K to elucidate its thermodynamic instability. While Cs occupies a single site from 100 to 150 K, it splits between two sites from 175 to 295 K with the second site having a lower effective coordination number, which along with other structural parameters suggests that Cs rattles in its coordination polyhedron. PDF measurements reveal that on the length scale of the unit cell, the Pb-I octahedra concurrently become greatly distorted, with one of the I-Pb-I angles approaching 82° compared to the ideal 90°. The rattling of Cs, low number of Cs-I contacts, and high degree of octahedral distortion cause the instability of perovskite-phase CsPbI_3. These results reveal the limitations of tolerance factors in predicting perovskite stability and provide detailed structural information that suggests methods to engineer stable CsPbI₃-based solar cells.

Halide perovskites have emerged as attractive materials for electronic and optoelectronic applications because of their unique properties and low-temperature processing. However, the mechanical aspects of the halide thin films have been very limitedly reported although strong bending fracture behavior is required for the flexible system. Here, we suggest a very promising processing way to enhance the bending fracture behavior of the flexible halide thin films with fundamental definitions of the mechanical parameters of the halide materials that have been also unavailable thus far. The processing is based on the assignments of in-situ compressive or tensile stress during the film deposition with a controllable manner. As a result, a substantial improvement of the fracture resistance against the bending operation was observed for the compressively stressed halide thin films. For example as a result of bending fracture evaluation, cesium lead bromide (CsPbBr₃) and methylammonium lead iodide (MAPbI₃) thin films prepared on PEDOT:PSS-coated polyethersulfone (PES) substrates demonstrated high fracture mode I toughness of 1.18 and 1.66 MPa m^0.5 for the in-situ compressive strain of -1.09%, which correspond to the enhancements by ~97 and ~100 %, respectively, compared to those for the reference samples. The structural origins of the mechanical enhancements are elucidated with experimental evidences on the variations in lattice distortion and bonding length.

Hybrid organic-inorganic perovskites (HOIPs) are promising semiconductors with long carrier lifetimes, synthetic tunability, remarkable defect tolerance and simple processing. Recently, 2D analogues have been made by adding insulating cations (e.g. butylammonium) to separate the perovskite structure into sheets. These 2D materials can be formed either in thin films, large single crystals or as dispersed nanoplatelets of only a few PbX6 octahedra thick. The number of PbX6 octahedra confined between insulating layers (i.e. the ‘n-value’) can be tuned by varying precursor stoichiometry, enabling discrete tunability of the bandgap through dielectric and quantum confinement. Controlling the structure of layered hybrid metal halide perovskites, such as the Ruddlesden-Popper (R-P) phases, is challenging because of their tendency to form mixtures of varying composition. Colloidal growth techniques can form dispersions of particles with properties that match the bulk properties of layered R-P phases, but controlling the composition of these particles remains challenging.

In this talk, we explore the microstructure of particles of R-P phases of methylammonium lead iodide prepared by antisolvent precipitation from ternary mixtures of different alkylammonium cations, where one cation is able to form perovskite phases (CH₃NH₃⁺) and the other two promote layered structures as spacers (e.g. C₄H₉NH₃⁺ and C₁₂H₂₅NH₃⁺). We illustrate how competition between cations that act as spacers between layers, or as grain-terminating ligands, determines the colloidal particle size of R-P crystallites that form in solution. Optical measurements reveal that quantum well dimensions can be tuned by engineering the ternary cation composition. Transmission synchrotron wide-angle X-ray scattering and small angle neutron scattering reveal changes in the structure of colloids in solvent and after deposition into thin films. In particular, we find that spacers can alloy between R-P layers if they share common steric arrangements, but tend to segregate into polydisperse R-P phases if they do not mix. This study provides a framework to compare the microstructure of colloidal layered perovskites and suggests clear avenues to control phase and colloidal morphology.

Emerging perovskite solar cells (PSCs) are not only required to be highly efficient, but also durable. The mechanical stability of PSCs also requires good adhesion and contacts across their interfaces. These are studied using atomic force microscopy (AFM) and fracture mechanics approaches. AFM pull-off forces are measured for bi-material interfaces that are present in model perovskite solar cells. These are then incorporated into adhesion models for the estimation of adhesion energies and mode I interfacial fracture energies. The mode mixity dependence of interfacial fracture toughness is also studied for interfaces between the perovskite and electron transport layers in PSCs. This is done using Brazil Disk specimens that are used to study the robustness of the interfaces across a range of mode mixities between pure mode I and pure mode II. The underlying crack/microstructure interactions and fracture modes are used to provide insights into the underlying crack-tip shielding due to crack bridging and microcracking. These are modeled using zone and row models that are used to predict the mode-mixity dependence of fracture toughness. The implications of the results are also discussed for the determination of crack path selection criteria in perovskite solar cells.

Organic-inorganic hybrid perovskite compounds are currently the archetypal materials for high performance photovoltaic (PV) and optoelectronic devices. However, the remaining bottlenecks preventing their large scale production are their environmental/thermal instability and lead toxicity. Herein, we demonstrate a novel approach to synthesize single-phase electrospun Cs₂SnI_xCl_6-x double perovskites with varying halide content (I, Cl or mixed I/Cl) as active materials for potential application in perovskite solar cells (PSCs). The x-ray photoelectron spectroscopy and Raman spectroscopy analyses indicated the in-situ formation of graphene oxide (GO) during the annealing process. The GO layer was found to enhance the optical properties and thermal stability of the fabricated perovskites even at high Cl content. Moreover, the presence of GO as an insulating layer significantly decreases the bandgap energy of the resulting perovskites. The perovskites with a mix of iodide and chloride ions showed significantly improved optical properties with higher photoluminescence (PL) intensity than that of pure chloride or iodide counterparts. Moreover, the compound with low chloride content showed superior thermal stability to those reported in the literature. Therefore, the application of the electrospinning technique is a useful strategy to in-situ incorporate GO in a lead-free perovskite matrix for potential photovoltaic and optoelectronic applications.

Metallic halide organic–inorganic hybrid perovskites with excellent optoelectrical properties have potential applications in optoelectronic devices and systems. In this work, we demonstrate the feasibility of preparing CH₃NH₃PbBr₃ single crystal thin films on poly-TPD hole transport layer by controlling the heating rates. The crystallinity and trap state density of the CH₃NH₃PbBr₃ thin films are characterized by X-ray diffraction and photoluminescence and time-resolved photoluminescence, respectively. The crystallinity with the peak intensity of (100) plane of the CH₃NH₃PbBr₃ thin films prepared at 2 ^oC/h is nine times of that prepared at 10 ^oC/h. The CH₃NH₃PbBr₃ thin films prepared at 2 ^oC/h exhibit a carrier lifetime of 21 ns, which is much larger than 0.6 ns for the CH₃NH₃PbBr₃ thin films prepared at 10 ^oC/h, and an excellent structural stability. Acknowledgements: KW is grateful for the support from National Key Research and Development Program (No.2017YFE0120400) and National Natural Science Foundation of China (No.61875082). FY is grateful for the support from National Science Foundation (CMMI-1854554)

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.

Two-dimensional (2D) hybrid organic-inorganic halide perovskites are a rapidly emerging class of semiconductors whose structural tunability and optoelectronic properties have led to the successful fabrication of solar cell devices with high power conversion efficiency. The features of the 2D perovskite structure that are tuned are the organic “spacer” cation and the number of the inorganic layers (n). There is a significant interest to target higher n-member compounds for light harvesting applications, due to their attractive photophysical properties such as low bandgap value and high carrier mobility. Herein, we present a comprehensive study on exploring the limit of layer thickness of bulk 2D lead halide perovskites through structural and thermodynamic study. Phase-pure synthesis of a new n=6 perovskite compound is reported with the characterization of the single crystal structure and optical properties of the material. Thorough calorimetry studies examine the thermodynamic stability of 2D lead iodide perovskites with layer thickness n=2-6. The favorable enthalpy of formation n=6 crystals shows that higher n-layer thickness is dependent on specific structural aspects. Additionally, the systematic investigation of the variances on the structural parameters and optical properties between different structure types of 2D perovskites, offers guiding principles for the targeted design and synthesis of 2D perovksites for efficient solar cell fabrication. Lastly, the stability studies of thin films demonstrate consistent observations with the thermochemical findings, further shedding light on the behavior of high n-layer perovskite materials.

Lead halide perovskites are one of the most exciting and heavily researched classes of optoelectronic materials due to their unique ability to form high quality crystals with tunable band gaps in the visible and near infrared using simple solution precipitation reactions. This facile crystallization is driven by their ionic nature; just as with other salts, it is challenging to form amorphous halide perovskites, particularly in thin-film form where they can most easily be studied. Herein, we show that rapid desolvation promoted by addition of acetate precursors is a general method for making amorphous lead halide perovskite films with a wide variety of compositions, including those using all the common cations (cesium, methylammonium, formamidinium) and anions (bromide, iodide). We show that by controlling the amount of acetate, it is possible to tune from fully crystalline to fully amorphous films, with an interesting intermediate state consisting of crystalline islands embedded in an amorphous matrix. The amorphous lead halide perovskite has a large and tunable optical band gap and improves the optical properties of incorporated crystalline perovskite, opening up the intriguing possibility of using amorphous perovskite as a passivating window layer, as is currently done in record efficiency silicon solar cells.

Organic-inorganic halide perovskite (OIHP) thin films have attracted a great deal of interest as photoactive materials in various optoelectronic devices due to their unprecedented photophysical properties, but their long-term reliability is likely to be limited by their poor mechanical properties. In this context, we explain different possible modes of fracture in OIHP based devices, and for the first time demonstrate facile healing of through-thickness cracks in OIHP thin films which can otherwise be detrimental to the device properties. This is achieved through application of either a moderate compressive stress at room-temperature or a simple heat treatment at moderate temperatures. Based on the findings from a series of ex situ residual stress measurements, crack-healing process is found to be time-dependent, which indicates that facile mass-transport in OIHPs plays a key role in this phenomenon. The mechanisms governing crack-healing process are explained based on the fundamentals of brittle fracture. This discovery has broad implications for the prevention and/or restoration of the overall performance, environmental stability, and mechanical reliability of OIHP based devices like solar cells which are poised for commercialization.

Hybrid halide lead perovskites are promising materials for photovoltaic applications. Nonetheless, these materials are highly sensitive and degrade in various environmental conditions. Among these, moisture is a prime cause of material degradation, representing a threat to the commercialization of perovskite-based materials. Hence, it is essential to study how water interacts with a perovskite thin film, what factors trigger instability in the material, and loss in solar cell performances. In order to understand these degradation mechanisms, we expose Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃ perovskite thin films with different precursor stoichiometries (organic to inorganic ratios) to a moisture treatment with high relative humidity. We monitor the evolution of perovskite crystalline structure, and we correlate it to changes in performances. Characterization with SEM images show morphological changes in the grain size, and XRD patterns evidence structural features that are furthered explained and understood by grazing incidence wide angle X-ray scattering (GIWAXS). Remarkably, we observe improved photovoltaic efficiency for treated films with an excess organic component, correlated with vertical orientation along the (001) plane. In addition, synchrotron-based X-ray fluorescence and X-ray beam induced current (XRF-XBIC) is used to correlate the elemental distribution with the current output of the devices.

Materials research is generating a wealth of data across a vast community. Specifically, the volume of available data on hybrid organic-inorganic perovskites (HOIPs) and related growth in this area is now immense. Keeping track of data of different origins, sample types or levels of theory, with a diverse set of different relevant observables and discoveries, is a challenging task at best. We here present an open database, "HybriD³" (Design, discovery and dissemination (D³) of data related to hybrid materials, https://materials.hybrid3.duke.edu), aiming to collect, curate, and make available materials data related to HOIP. The database is designed to provide a broad set of data, i.e., experimental and computational, related to in principle any materials property of relevance to the community: structure, optical or electronic properties, and more. A key goal is to provide the ability to curate data, that is, identify property information closest to the actual properties of a real material prepared in a specific way (bulk crystalline, powder, thin film, nanocrystalline, ...). Importantly, the database is open to the community and designed to accept community input. While the "HybriD³" database is focused on a particular materials class, the problem of making materials data of all kinds available in a structured, reproducible way is general. The software underlying the HybriD³ database is thus available as a separate open-source project "MatD³" (https://github.com/HybriD3-database/MatD3). We also describe this software stack, which can enable materials data at any scale, from small workgroups via focused projects all the way to large and general, open and reproducible materials data collections.

As halide perovskites have made their significant impact in the fields of photovoltaics, light emitters and other optical devices, research interest has been reignited in families of lead-free 2D, 1D and 0D perovskite-derived materials that, while poorer photovoltaics, can access some of the strong emissive properties of the halide perovskites, while also demonstrating emergent optoelectronic properties of their own.[1] The layered Cs₃Pn₂X₉ (Pn = Sb, Bi; X = Cl, Br, I) family demonstrates structured excitonic emission at room temperature[2,3] while the K₂PtCl₆-structured ‘vacancy-ordered double perovskites’ such as M₂SnI₆ (M=Rb, Sn) have shown both evidence of defect tolerance similar to that seen in the lead halide perovskites as well as strong exciton-phonon coupling.[4,5] As such, the exploration of perovskite-inspired lower dimensionality compounds represents an exciting direction in the research of optical materials.

In this study, inspired by our experimental collaborators’ colloidal synthesis of nanocrystals of Cs₂ZrCl6 and Cs₂ZrBr₆, which also belong to the K₂PtCl₆ structure type and demonstrate broad white photoluminescence when formed into thin films, we perform hybrid Density Functional Theory (DFT) calculations in order to predict and rationalize the observed optical and electronic behavior of the Cs₂ZrX₆ materials family, producing good agreement to experiment. By calculating the thermodynamic and luminescent properties of the intrinsic defects within the system, as well as excitonic properties within the QSGW method and the Bethe-Salpeter equation, we attempt to accurately predict the origin of such promising emissive properties. Finally, we present direct comparisons to the analogous M₂SnX₆ family and draw conclusions on the feasibility of extending the field of lead-free perovskite-inspired emitters, and whether such optimal optical properties are available beyond the post-transition metals.

[1] Xiao, Z.; Song, Z.; Yan, Y. From Lead Halide Perovskites to Lead-Free Metal Halide Perovskites and Perovskite Derivatives. Adv. Mater. 2019, 31, 1803792
[2] Bass, K.K.; Estergreen, L.; Savory, C.N.; Buckeridge, J. et al. Vibronic Structure in Room Temperature Photoluminescence of the Halide Perovskite Cs3Bi2Br9. Inorg. Chem. 2017, 56, 42
[3] McCall, K.M.; Stoumpos, C.C.; Kostina, S.S.; Kanatzidis, M.G. and Wessels, B.W. Strong Electron-Phonon Coupling and Self-Trapped Excitons in the Defect Halide Perovskites A3M2I9 (A=Cs,Rb; M=Bi,Sb). Chem. Mater. 2017, 29, 4129
[4] Maughan, A.E.; Ganose, A.M.; Almaker, M.A.; Scanlon, D.O. and Neilson, J.R. Tolerance Factor and Cooperative Tilting Effects in Vacancy-Ordered Double Perovskite Halides. Chem. Mater. 2018, 30, 3909
[5] Maughan, A.E.; Ganose, A.M.; Bordelon, M.M.; Miller, E.M. et al. Defect Tolerance to Intolerance in the Vacancy-Ordered Double Perovskite Semiconductors Cs2SnI6 and Cs2TeI6. J. Am. Chem. Soc. 2016, 138, 8453

The certified power conversion efficiency (PCE) of state-of-the-art organic-inorganic metal halide perovskite solar cells (PSCs) has surpassed 25%, coming after single crystalline silicon solar cells. The most widely studied methylammonium (MA) lead iodide undergoes a structural phase transition and degradation due to the volatile MA cation, detrimental to the long-term stability of the devices. Formamidinium (FA), used as the majority cation in most of PCE >20% PSCs, is thermally more stable and has an optimal red-shifted bandgap. However, FA with the relatively large radius induces titled PbI₆ octahedra in FAPbI₃, leading to the photoinactive yellow δ-phase at room temperature. Herein, iodide and FA in FAPbI₃ were gradually replaced by bromide and cesium (Cs) with smaller radius to suppress the yellow δ-phase and systematically explore the compositional space via regulating the tolerance factor, revealing the crucial link between the crystal structure and optoelectronic properties of the mixed-cation (FA/Cs) lead mixed-halide (I/Br) perovskite. An empirical expression for FA/Cs-Pb-I/Br bandgap as a function of composition was determined, which contributes to tailored bandgap suitable for tandem solar cells. In addition, the FA/Cs-Pb-I/Br films were probed by advanced synchrotron X-ray characterization to understand halide phase segregation, which has been found to be detrimental for solar cell performance.

Cesium titanium bromide, Cs₂TiBr₆, has attracted research interest due to its ~1.8 eV band gap, which makes it desirable for forming tandem junctions with silicon. However, the instability of large-grain Cs₂TiBr₆ in ambient atmosphere prompts further fundamental understanding of its surface chemistry and oxidation. To ascertain stability of the material in ambient atmosphere and establish band edge positions of the material we conducted controlled air exposure experiments and UHV cleaving of nascent material. X-ray photoelectron spectroscopy (XPS) established correct synthesis, effects of atmospheric exposure, and characterization of UHV cleaved samples. Ultraviolet photoelectron spectroscopy (UPS) established band edge positions of Cs₂TiBr₆ with and without surface metal oxide species present. Powder X-ray diffraction (pXRD) quantified the effects of atmospheric exposure over time. The presence of metal oxides on Cs₂TiBr₆ surfaces shifted band alignments towards TiO₂ while UHV cleaved, oxide-free surfaces demonstrated an intrinsically n-type semiconductor. These results elucidate the effects of surface oxides on Cs₂TiBr₆ surfaces, and future routes through which interfacial reactivity may be used to improve atmospheric stability.

In recent years, the 3D/low-dimensional (LD) perovskite heterostructure consisting of LD perovskite called ‘capping layer’, deposited atop of 3D perovskite in a device [1], has emerged as a solution for improving perovskite solar cells’ (PSCs’) stability and interfacial passivation. However, the capping layer materials and processing conditions have been optimized for single-junction PSCs, and not for multi-junction PSCs. To apply the capping layer in multi-junction PSCs, research efforts are required to understand and optimize the bandgap-tailored capping layer material. We propose a detailed study exploring at least 10 different types of LD perovskite with various halides (I, Br, and Cl) and thicknesses, and CH₃NH₃Pb(I-Br)₃ absorber mixtures with increasing bandgap from 1.59 to 2.27 eV [2]. The samples are degraded in a well-controlled environmental chamber, under high humidity (85% RH), high heat (85°C), and low illumination (0.17 sun). The same set of samples are screened using photoconductivity, as a proxy for its interfacial passivation property. We also utilize decision tree-based machine learning models and Shapley values [3] to help determine which molecular properties in the capping layers significantly affect the absorber mixtures’ stability and conductivity. Our results indicate that capping layers needs to be tailored based on the perovskite absorbers’ composition, bandgap, and organic cation types to achieve high stability. Furthermore, by comparing the optimum bandgap-tailored capping layers through X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and ultraviolet photoelectron spectroscopy (UPS), we can understand how the film stability is affected by inorganic and organic bonds on the surface of the capping layers, as well as how interfacial passivation is affected by band alignment. Our study suggests that capping layer tuning is a promising approach to increase the stability of perovskite-based tandem solar cells.

References
[1] W. Huang, T. Bu, F. Huang, and Y. B. Cheng, “Stabilizing High Efficiency Perovskite Solar Cells with 3D-2D Heterostructures,” Joule, vol. 4, no. 5, pp. 975–979, May 2020, doi: 10.1016/j.joule.2020.04.009.
[2] L. Atourki et al., “Role of the chemical substitution on the structural and luminescence properties of the mixed halide perovskite thin MAPbI 3-X Br x (0 ≤ x ≤ 1) films,” Appl. Surf. Sci., vol. 371, pp. 112–117, May 2016, doi: 10.1016/j.apsusc.2016.02.207.
[3] N. T. P. Hartono et al., “How machine learning can help select capping layers to suppress perovskite degradation,” ChemRxiv, pp. 1–47, Jan. 2020, doi: 10.26434/CHEMRXIV.11766528.V1.

Double perovskites have emerged as promising candidate materials for high-performance next-generation optoelectronic technologies, owing to the ability to replace the toxic Pb²⁺ cation with a pair of more benign cations (e.g. Ag⁺ and Bi³⁺), while preserving the perovskite crystal structure.¹ However, most double perovskites, including the prototypical Cs₂AgBiBr₆, have prohibitively wide bandgaps, limiting photoconversion and photocatalytic efficiencies.² Recently, it has been demonstrated that the bandgap of Cs₂AgBiBr₆ can be reduced through alloying with Sb³⁺,³ yet the bandgap of the pure Sb-based double perovskite (Cs₂AgSbBr₆) remains excessively wide for photo-voltaic/catalytic applications. In this work, we develop a solution-based route to synthesize phase-pure Cs₂Ag(Sb_xBi_1-x)Br₆ thin films, with the mixing parameter x tunable over the entire composition range. In doing so, we observe this system to disobey Vegard’s law, exhibiting significant bandgap bowing, such that the mixed alloy (x = 0.9) demonstrates the smallest bandgap (2.08 eV, compared to 2.18 eV and 2.25 eV for the pure Sb- and Bi-based double perovskites, respectively). We investigate the possible mechanisms for this nonlinear bandgap variation through relativistic hybrid Density Functional Theory (DFT) calculations, combined with in-depth measurements of the composition, phase and grain structure to yield detailed understanding of the underlying physical origins. The energetic alignment of electron states within these materials is accurately calculated, including consideration of deformation potentials to adequately account for the spurious supercell effects associated with such calculations.^4,5 Our work reveals pathways to bandgap engineering in double perovskite alloys, such that it is better suited to photovoltaic or photocatalytic applications, which may be generally applicable to other mixed-metal double perovskite systems.



¹ C.N. Savory, A. Walsh, and D.O. Scanlon, ACS Energy Lett. 1, 949 (2016).
² A.H. Slavney, T. Hu, A.M. Lindenberg, and H.I. Karunadasa, Journal of the American Chemical Society 138, 2138 (2016).
³ G. García-Espejo, D. Rodríguez-Padrón, R. Luque, L. Camacho, and G. de Miguel, Nanoscale 11, 16650 (2019).
⁴ K.T. Butler, C.H. Hendon, and A. Walsh, Journal of the American Chemical Society 136, 2703 (2014).
⁵ S.-H. Wei, S.B. Zhang, and A. Zunger, Journal of Applied Physics 87, 1304 (2000).

Lead-halide perovskites have emerged as a promising class of semiconductors; however they suffer from issues related to lead-toxicity and instability. We report results of a first-principles-based design of heavy-metal-based oxynitrides as alternatives to lead-halide perovskites. We have used density-functional-theory calculations to search a vast composition space of ABO₂N and ABON₂ compounds, where B is a p-block cation, and A is an alkaline, alkali-earth, rare-earth or transition metal cation, and identify 10 new ABO₂N oxynitride semiconductors that we expect to be formable. Specifically, we discover a new family of ferroelectric semiconductors with ASnO₂N stoichiometry (A = Y, Eu, La, In, Sc) in the LuMnO₃-type structure, which combine the strong bonding of metal oxides with the low carrier effective mass and small, tunable band gaps of the lead-halide perovskites. These tin oxynitrides have predicted direct band gaps ranging from 1.6 – 3.3 eV, and a sizeable electric polarization up to 17 μC/cm², which is predicted to be switchable by an external electric field through a non-polar phase. With their unique combination of polarization, low carrier effective mass and band gaps spanning the entire visible spectrum, we expect ASnO₂N ferroelectric semiconductors will find useful applications as photovoltaics, photocatalysts, and for optoelectronics.
Acknowledgments: This work was supported by the National Science Foundation through grant number DMR-1806147.

Perovskite solar cells have emerged as a key photovoltaic technology given the remarkable rise in their efficiency in the past decade. Long non-radiative lifetimes as a result of slow defect-mediated recombination in perovskite materials are key in achieving high open-circuit voltages necessary for high efficiency solar cells.
The quantitative impact of bulk or interface defects on recombination is often estimated by defect statistics based on the model of Shockley, Read and Hall. However, defect statistics depend on defect capture coefficients to calculate defect occupancy and the recombination rate through the defect. Due to the lack of enough experimental data on capture coefficients of novel materials or, alternatively, models to calculate trap energy dependent capture coefficients it is frequently assumed that capture coefficients are symmetric, i.e, k_n = k_p for all defect levels within the bandgap in a device. However, equal or at least similar values of the capture coefficients are not necessarily a typical or even a likely scenario.
To go beyond the state-of-the-art device simulation in photovoltaics involving constant capture coefficients, we present a generalized microscopic model based on the Harmonic oscillator approximation to calculate defect capture coefficients as a function of the host material and the energetic position of the defects. Then we use this capture coefficients to classify the “deep” defects inside a device not merely based on the energetic position of the device but also as a function of the quantitative impact it has on the device. Furthermore, we discuss certain photovoltaic device design principles that can substantially reduce recombination through the dominant recombination centers.

Electronic materials in perovskite and related structure types are a rich field of study because they combine characteristics of ionic and covalent bonding in crystal structures that are tolerant of distortions and chemical substitution. Tremendous efforts are now underway to understand the structure-property relationships that underpin the photovoltaic performance of lead halide perovskites (LHPs), expanding on and linking decades of research on photovoltaic absorbers, on the one hand, and complex oxide physics on the other. Now we extend this spirit of interdisciplinary, fundamental inquiry to a much less-studied class of materials, the chalcogenide perovskites. Recent works have shown that chalcogenide perovskites have outstanding properties for optoelectronic applications, including: slow minority carrier recombination, strong light absorption, excellent environmental stability, and dopability. Efforts are just now beginning to develop a fundamental understanding of these phenomena and to exploit these exciting materials for applications.

We present a diverse set of studies of the atomistic origin of structure-property relationships in chalcogenide perovskites. We combine impedance spectroscopy, X-ray absorption spectroscopy (XAS), electronic transport, and first-principles theory to understand how atomic structure and particular chemical bonds support outstanding semiconducting properties. Our impedance spectroscopy measurements on single crystals reveal that chalcogenide perovskites are highly-polarizable semiconductors, with stronger dielectric response than any other family of semiconductors with band gap in the VIS-NIR, exceeding even LHPs. Our theoretical calculations agree quantitatively with our measurements, and illustrate the particular polar phonon modes responsible for the large dielectric response. Our XAS studies combining experiment and theory illustrate the importance of directional bonds in determining the near-band edge electronic structure (including the band gap), especially for layered variants in the Ruddlesden-Popper structure type. This electronic anisotropy is further illustrated by electronic transport measurements made on oriented single crystals.

Our results illustrate, in a fundamental and detailed level, the ways in which chalcogenide perovskites differ from their oxide and halide cousins. We show that the structure-property relationships follow from covalent metal-chalcogen bonds and strong electron-phonon coupling. We conclude that chalcogenide perovskites are highly promising for optoelectronic and energy-conversion technologies.

[1] Jaramillo, R. & Ravichandran, J. In praise and in search of highly-polarizable semiconductors: Technological promise and discovery strategies. APL Materials 7, 100902 (2019).

Different additive materials are widely used to improve the power conversion efficiency (PCE) of organic-inorganic halide perovskite solar cells. In this work, we report a novel organic additive material C₆H₅NHNH₃I (Phenylhydrazinium Iodide) which is very effective to enhance the PCE of single cation-single halide based Methylammonium Lead Iodide perovskite solar cells (MLIPSC). In addition to that, we have found the additive is also effective for other types of perovskite composition such as double cation-double halide perovskite solar cells. C₆H₅NHNH₃I promotes perovskite crystal growth and improves the light absorption capability of the active layer. Furthermore, because of reduced defects and charge traps in the additive treated perovskite film, charge carrier lifetime becomes almost double from 1.4 µs to 2.5 µs which ensures minimum recombination of the photogenerated electrons and holes. Besides, approximately 10% increased external quantum efficiency (EQE) and 14% improved built in potential (V_bi) of the perovskite solar cells indicate superior device performance. Eventually, accelerated short circuit current density and open circuit voltage of the C₆H₅NHNH₃I treated perovskite solar cells contribute to nearly 20% increment of PCE in MLIPSC.

Significant achievements were made in the past few years with perovskite material in optoelectronic applications. One of the advantages of the halide perovskite in photo detection applications is that the band gap of the material can be tuned to cover a wide range of wavelengths by halide mixing. In this paper, the research is focused on the fabrication and characterization of interdigital photodetectors made with CsPbI₂Br. The photodetector consists of two interdigitate chromium electrodes deposited on glass substrates with the perovskite filling the spacing between the electrodes. The interdigital pattern of the device was fabricated using photolithography method to deposit 80 nm of chromium. To study the effect of the electrode spacing on the device performance, four types of photodetectors with four different spacing; 50, 20, 10, and 5µm were fabricated. The perovskite was synthesized using one step deposition method with 0.6 molar concentration of CsPbI₂Br precursor. The material was characterized by obtaining the absorbance, the photoluminescence, and the X-ray diffraction spectra. One single peak in the PL spectrum was observed at 648 nm indicating a band gap 1.91 eV. The current voltage curves of the photodetectors were obtained under dark and light conditions using keithley 4200 semiconductor characterization system and Newport solar simulator. The spectral response of the device was obtained suing Bruker ifs 125hr spectrometer. The device with 10 µm spacing showed ~370 on/off ratio at 5 V bias under 100 mW/cm² light intensity. Under the same conditions, the device with 5 µm showed an increase in the on/off ratio to 6800. The detectivities of the 10 and 5 µm devices were calculate to be 4.9 × 10⁹, and 2.46 × 10¹⁰ Jones respectively.

Grain boundary (GB) passivation by polymer additives has been shown effective in improving the performance of solution processed hybrid perovskite solar cells (PSCs). However, the critical stability issue associated with detrimental ion migration in PSCs remains less explored by polymer additives. Here, we show that a new linear polar polymer, polycaprolactone (PCL), can passivate GBs of methylammonium lead triiodie (MAPbI₃) perovskites via direct backbone attachment, enabling significantly enhanced environmental stability of n-i-p type PSC. The champion PSC with optimal PCL concentration shows an improved power conversion efficiency (PCE) of 20.1 %. 90% of initial PCE was preserved after 400 h ambient storage. After 100 h thermal aging (85 ^○C), 80% of the original PCE has also been preserved. The tested PSCs use less stable Spiro-OMeTAD (2,2′,7,7′-tetrakis[N,N-di(4-emethoxyphenyl)amino]-9,9′-spirobifluorene) as hole transport layer (HTL), and, thus, the improved stability indicates critical roles of PCL in retarding moisture ingression as well as suppressing ion migration. Detailed structures of PCL GB passivation is visualized directly via lateral scanning force and transmission electron microscopies. Secondary ion mass spectrometry and ion conduction measurements confirm the PCL GB passivation significantly retards I^- ion migration in MAPbI₃ PSCs, while without PCL, I^- ions are actively accumulating in the organic HTL and at the interface between electrodes and the HTL even without externally applied electric field. We envision this passivation strategy by PCL GB functionalization can help the development of environmentally stable, high-performance PSCs.(This work was supported by the Morin Foundation Trust and the NSF, Inspire program #1344267)

In order to design more efficient and durable perovskite solar cells (PSCs), the effects of annealing temperature on interdiffusion of the constituent elements in layered formamidinium-based PSCs were studied. First, the PSCs were fabricated by spin-coating lead (II) iodide solution onto the mesoporous electron transporting layer (ETL) followed by a mixture of formamidinium ammonium iodide, with small proportion methylammonium bromide and methylammonium chloride. The effect of varying annealing temperature of the perovskite films is then studied to gain insight into the changes that occur on the perovskite film and the overall effects of interdiffusion of constituent elements on photoconversion efficiencies of the PSCs. The results show there is diffusion of tin from fluorine-doped tin oxide to the mesoporous ETL due to sintering. The level of tin in the mesoporous layer decreases with increased annealing temperatures of the perovskite films. The quality of perovskite films is also improved with better uniformly distributed grain sizes for annealing temperatures between room temperature and 130 ^oC compared to higher annealing temperatures of 140 ^oC and 150 ^oC. The overall effects of the interdiffusion of layer constituents on photoconversion efficiencies are then discussed for the design of PSCs.

Highly efficient and stable inverted, i.e. p-i-n, perovskite solar cells (PSCs) are fabricated by employing a novel hole transport bilayer. Compared to devices with single PTAA interlayer, which have an average power conversion efficiency (PCE) of 18.9%, an enhanced average PCE of 20.2% and a maximum PCE of 21.16% in the bilayer-based devices are observed. The XRD and AFM studies show an improved crystallinity and larger grain size of perovskite films on the bilayer surface. The ultraviolet photoelectron spectroscopy and Mott-Schottky analysis suggest an increased built-in potential with an enhanced upward band bending at the bilayer/perovskite interface. Compared to devices with single PTAA, CuI and PEDOT:PSS interlayer, the bilayer-based devices show significant suppression of J-V hysteresis and stable current output at maximum power point. The bilayer-based devices also demonstrate a remarkable long-term stability, as tested in inert atmosphere.

Here, we discuss how density functional theory (DFT)-based methods can help us in designing strategies to mitigate the stability and efficiency challenges for hybrid organic-inorganic perovskite (HOIP) systems. On one front, we use first-principles theory to rationalize the elastic properties of layered hybrid perovskites, correlating such properties with interlayer organic-organic interactions and also the HOIP’s structural properties e.g., its dimensionality or intrinsic interlayer defects and voids. [1] On a different but complementary front, we have studied fundamental chemistry behind the water degradation of 3D hybrid perovskites and have related this to the chemical properties of the organic cations. [2] We are now expanding this work to study water interactions in layered quasi-2D systems, unraveling the fundamental role of dimensionality in water stability of perovskites. Beyond the realm of bulk-like perovskites, we also study the determining role of organic cations’ chemistry on surface stability and defect formation for HOIP nanoparticles. The role of organic cations goes further than directly influencing the inorganic backbone structure, and for some specific functionalized organic cations and through intricate hydrogen-bonding networks, the organic-organic interactions can become important at dictating the electronic and structural properties of layered perovskites. Having a multipronged approach in which we identify descriptors for elastic and electronic properties of perovskite, as well as their stability, allows us to rationalize and help improve experimentally-relevant pathways to create more robust HOIP-based devices capable of performing in real-world PV or LED applications. Few such possibilities which are discussed here include: surface passivation with mixed short organic cations, supramolecular interactions between long cations, secondary organic functionalities, and dimensionality engineering.

[1] Reyes-Martinez, Marcos A., Peng Tan, Arvin Kakekhani, Sayan Banerjee, Ayan A. Zhumekenov, Wei Peng, Osman M. Bakr, Andrew M. Rappe, and Yueh-Lin Loo. "Unraveling the Elastic Properties of (Quasi) Two-Dimensional Hybrid Perovskites: A Joint Experimental and Theoretical Study." ACS Applied Materials & Interfaces 12, no. 15 (2020): 17881-17892.

[2] Kakekhani, Arvin, Radhika N. Katti, and Andrew M. Rappe. "Water in hybrid perovskites: Bulk MAPbI3 degradation via super-hydrous state." APL Materials 7, no. 4 (2019): 041112.

Organic-Inorganic Perovskite solar cells has a great advantage to reach the radiative limit for PCE enhancement. The single junction devices are restricted to attain the radiative limit because of the energy losses. Multi-structure architectures like tandem, heterostructure are adapted to recover the thermodynamic energy loss. The variable bandgap of perovskite is an advantage, which varies from 1.5eV~1.8eV providing a complete solar spectrum coverage. Stacking of high-bandgap perovskite with low bandgap perovskites is a good approach for encountering the energy losses. The perovskite heterostructure is fabricated by sequential deposition of quantum dots and thermally evaporated p-n junctions. Till now, there is no demonstration of any solution process able method in perovskites for stacking perovskite absorbers because of the dissolution of perovskite precursors by the subsequent layers. Therefore, we demonstrated a PDMS printing technique for stacking perovskite layers inspired by multi-stacked organic solar cells. The printing technique is assisted with modification of surface energy of PDMS by grafting process, which facilitates printing of high crystalline pinhole free perovskite layers. On proving ground, we also stacked perovskite layers to form a perovskite heterostructures. We demonstrated the printing of multi-layer perovskite layers to encounter the thermodynamic losses. To understand the advantage of heterostructure for encountering the energy losses, a constant thickness of single layer and heterostructure was maintained for compared the photovoltaic performance. We achieved a photovoltaic performance of ~5%, ~9% for MaPbI3 and MaPbBrI2 layer and ~10% for MAPbBrI2/MAPbI3 heterostructure. This gave us an indication that the heterostructure was able to suppress some non-radiative recombination to overcome the energy losses, which can be a great application for stacking perovskite layers to use in different optoelectronic applications.

Organic-inorganic halide perovskites (OIHPs) have emerged as the most promising light-absorber materials in thin-film photovoltaic (PVs). 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 have studied the fracture behavior of PSCs by measuring their cohesion energies using double cantilever beam method and report a novel approach to strategically enhance the interfacial adhesion and performance of PSCs by interface modification. This work points to a new route for designing mechanically robust PSCs with long-term durability.

Recently, metal-halide perovskite materials are considered as a promising candidate for lasing applications since perovskites may overcome typical limitations showed in organic gain media and the “green-gap” in inorganic semiconductors. While lasing has been demonstrated based on perovskite materials with both 3D and low-dimensionalities, quasi-2D perovskites are a better gain media due to its unique properties such as large binding energy and self-assembled quantum wells. It has been shown that efficient energy funneling from low-dimensional domains to high-dimensional domains will not only enhance the radiative recombination but also facilitate population inversion. However, the trend of how the dimensionality of quasi-2D perovskites affects amplified spontaneous emission (ASE) threshold is still controversial. Here, we investigate ASE of all-inorganic quasi-2D perovskite thin films with different dimensionalities, and try to understand the role of dimensionality in both materials properties and charge carrier dynamics.
In this work, we formed quasi-2D perovskite thin films based on the all-inorganic 3D cesium lead halide perovskite framework due to the improved thermal and optical stability. The dimensionality of quasi-2D perovskites is tuned by varying the concentration of the organic spacer phenethylamine (PEA). In our work, the large organic cation PEA acts as a nucleation center facilitating the crystallization process of perovskites. Via grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements and scanning electronic microscopy, we found that the addition of PEA also enhances the quality of the thin films by controlling the domain size and morphology. As a result, the PLQY increases with the PEA concentration. However, the ASE threshold is not correlated with the PLQY and it reaches to the lowest value (7 µJ/cm²) at a moderate PEA concentration of 40%. To study the correlation between PEA concentration and ASE threshold, we used transient photoluminescence and transient absorption spectroscopy to study the photophysics of these films. Our results showed that with increasing PEA concentration to 40%, the non-radiative recombination is significantly reduced due to defect passivation, leading to the increasing PLQY and decreasing of ASE threshold. However, the existing of the low-dimensional domains became significant with a higher PEA concentration (60%), which acted as long-lived trap states for excitons and limited the population inversion process. Finally, by using the optimized PEA ratio, the ASE emission was observed over the broad visible range from red to blue, demonstrating its wide wavelength-tunability.

Despite the power conversion efficiency (PCE) of the perovskite solar cells soars more than 25 % in less than ten years, their limited stability remains one of the crucial hurdles on the path to commercialization. To improve the device stability, various trap state passivation strategies targeting the bulk perovskite, perovskite surface, and device interfaces have been employed. However, knowledge of the temporal, spatial, and energetic correlation of the defect/trap states is the foundation for fundamental and technical understanding of the impact of the trap/defect states, manifested by the device stability and PCE optimization.
In this report, we directly observe the ultrafast dynamics of trapped carriers in organic lead halide perovskite MAPbI₃ thin film by ultrafast photocurrent spectroscopy (UPCS). Under low carrier density, the trapped carriers undergo a multiple trapping and release (MTR) transportmechanism among the shallow trap states and deep trap state, demonstrated by a time-dependent activation energy. Above the trap filling region, in addition to the MTR transport mechanism, the trap-assist Auger recombination occurs, which is demonstrated by the laser intensity dependent decay time. Our results elucidated the trapped carrier dynamics from sub-25 ps to couple ns in the solution-processed film with mobility of ~ 1 cm²/Vs.

Optoelectronic properties of 2D metal halide perovskites are dominated by the transitions and interactions between excitons. Here, we present a study on the spin relaxation mechanisms for these excitonic states and demonstrate how these mechanisms can be altered. Our analysis utilizes ultrafast spectroscopy methods including Faraday rotation and circularly polarised transient absorption spectroscopy.

Organometal halide perovskites are promising materials for optoelectronic applications due to excellent light absorption, charge transport and emissive properties, coupled with potentially low-cost scalable fabrication [1][2]. Perovskite solar cells (PSCs), as well as other devices, currently suffer from moisture and thermal instability of the perovskite semiconductors [3]. There are several complimentary methodologies of increasing the stability of PSCs, and amongst these the formation of hybrid two/three dimensional Ruddleson Popper perovskites is one of the most promising [4]. Mixed dimension perovskites (MDPs) can be formed by the incorporation of long alkyl-chain cations into the crystal structure leading to the formation of a layered crystal structure [5]. The individual layers are separated by covalently linked cations, with a 3D intra layer geometry. The optical, crystalline, electrical and magnetic properties of 3D perovskites can be readily tuned by varying the stoichiometric concentration and chain length of the dopant, leading to a considerable scope for application dependent material engineering. However, exploring this large yet versatile parameter space of dopants requires an understanding of the underlying excitonic processes and morphological dependencies.

In this work, the formation of 2D/3D mixed alkylammonium lead iodide perovskites using Butylammonium (BAI) and Octylammonium (OAI) iodides as additives in methyl-ammonium lead iodide perovskites is demonstrated. A solvent annealing method is employed to improve film morphology. The effect of incorporating these cations on the optical properties of the films is investigated using micro-photoluminescence and absorption studies, and a left shift in band-gap with increasing 2D character is illustrated, in accordance with literature[6]. BAI and OAI demonstrate very different spectral properties, with much larger spectral shifts in the longer chain derivitive, hinting towards a strong effect of the alkyl chain. We further utilise multi-gaussian fitting to reveal insights into edge-states as well as disorder in MDPs, which can have imporant consequences for photovoltaic and light-emitting device applications.

[1] N. Elumalai, M. Mahmud, D. Wang, and A. Uddin, “Perovskite Solar Cells: Progress and Advancements,” Energies, vol. 9, no. 11, p. 861, 2016, doi: 10.3390/en9110861.
[2] Z. Li et al., “Scalable fabrication of perovskite solar cells,” Nat. Rev. Mater., vol. 3, pp. 1–20, 2018, doi: 10.1038/natrevmats.2018.17.
[3] N. Grossiord, J. M. Kroon, R. Andriessen, and P. W. M. Blom, “Degradation mechanisms in organic photovoltaic devices,” Org. Electron., vol. 13, no. 3, pp. 432–456, 2012, doi: 10.1016/j.orgel.2011.11.027.
[4] P. Gao, A. R. Bin Mohd Yusoff, and M. K. Nazeeruddin, “Dimensionality engineering of hybrid halide perovskite light absorbers,” Nat. Commun., vol. 9, no. 1, pp. 1–14, 2018, doi: 10.1038/s41467-018-07382-9.
[5] R. L. Milot et al., “Charge-Carrier Dynamics in 2D Hybrid Metal-Halide Perovskites,” Nano Lett., vol. 16, no. 11, pp. 7001–7007, 2016, doi: 10.1021/acs.nanolett.6b03114.
[6] D. H. Cao, C. C. Stoumpos, O. K. Farha, J. T. Hupp, and M. G. Kanatzidis, “2D Homologous Perovskites as Light-Absorbing Materials for Solar Cell Applications,” J. Am. Chem. Soc., vol. 137, no. 24, pp. 7843–7850, 2015, doi: 10.1021/jacs.5b03796.

Metal halide perovskites have emerged as promising materials for optoelectronic applications like solar cells or light-emitting diodes (LEDs). Here, we investigate mixed-halide perovskites and control the cation alloying to enhance optoelectronic performance through alteration of the charge carrier dynamics. In contrast to single-halide perovskites, we find high luminescence yields for photoexcited carrier densities far below solar illumination conditions. We employ time-resolved spectroscopy and show that the charge carrier recombination regime changes from second to effectively first order within the first tens of nanoseconds after excitation. Supported by microscale mapping of the optical bandgap, electrically gated transport measurements and first-principles calculations, we demonstrate that spatially varying energetic disorder in the electronic states causes local charge accumulation, creating p- and n-type photodoped regions, which unearths a strategy for efficient light emission at low charge-injection in solar cells. [1]

[1] Feldmann, Macpherson et al. Nature Photonics 14, 123–128 (2020).

Halide perovskite materials have promising performance characteristics for low-cost optoelectronic applications. Photovoltaic devices fabricated from perovskite absorbers have reached power conversion efficiencies above 25 per cent in single-junction devices and 28 per cent in tandem devices. Though widely considered defect tolerant materials, perovskites still exhibit a sizeable density of deep sub-gap non-radiative trap states, which create local variations in photoluminescence (1, 2) that fundamentally limit device performance. These trap states have also been associated with light-induced halide segregation in mixed halide perovskite compositions (3) and local strain (4), both of which can detrimentally impact device stability (5). Understanding the nature of these traps will be critical to ultimately eliminate losses and yield devices operating at their theoretical performance limits with optimal stability. We recently revealed that the majority of these trap states in state-of-the-art (Cs_0.05FA_0.78MA_0.17)Pb(I_0.83Br_0.17)₃ perovskites form in nanoscale clusters at grain boundaries of compositionally and structurally distinct entities [Nature, 2020].

In this talk we detail our low-dose, high-resolution and multimodal approach to link photophysical properties to nanoscale structural and compositional heterogeneities. By combining scanning electron microscopy techniques with spatially resolved photoemission measurements, we compare and contrast observed structural defects in (Cs_0.05FA_0.78MA_0.17)Pb(I_0.83Br_0.17)₃ and (Cs_0.05FA_0.78MA_0.17)PbI₃ halide perovskite thin films and discuss how they ultimately limit device performance. In so doing, we reveal the nature and origin of the “inhomogenous” perovskite grains that have been directly linked to non-radiative recombination and were previously unidentified (1). Finally, by combining scanning electron diffraction measurements with environmental chambers, we perform in-situ experiments to explore the local nanoscale structural modifications that occur in perovskite materials under real world operating conditions and how these structural modfications are linked to device degradation.

References
1. T. A. S. Doherty, A. J. Winchester, S. Macpherson, D. N. Johnstone, V. Pareek, E. M. Tennyson, S. Kosar, F. U. Kosasih, M. Anaya, M. Abdi-Jalebi, Z. Andaji-Garmaroudi, E. L. Wong, J. Madéo, Y.-H. Chiang, J.-S. Park, Y.-K. Jung, C. E. Petoukhoff, G. Divitini, M. K. L. Man, C. Ducati, A. Walsh, P. A. Midgley, K. M. Dani, S. D. Stranks, Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites. Nature. 580, 360–366 (2020).
2. D. W. de Quilettes, S. M. Vorpahl, S. D. Stranks, H. Nagaoka, G. E. Eperon, M. E. Ziffer, H. J. Snaith, D. S. Ginger, Impact of microstructure on local carrier lifetime in perovskite solar cells. Science. 348, 683–686 (2015).
3. A. J. Knight, A. D. Wright, J. B. Patel, D. P. McMeekin, H. J. Snaith, M. B. Johnston, L. M. Herz, Electronic Traps and Phase Segregation in Lead Mixed-Halide Perovskite. ACS Energy Lett. 4, 75–84 (2019).
4. T. W. Jones, A. Osherov, M. Alsari, M. Sponseller, B. C. Duck, Y.-K. Jung, C. Settens, F. Niroui, R. Brenes, C. V. Stan, Y. Li, M. Abdi-Jalebi, N. Tamura, J. E. Macdonald, M. Burghammer, R. H. Friend, V. Bulović, A. Walsh, G. J. Wilson, S. Lilliu, S. D. Stranks, Lattice strain causes non-radiative losses in halide perovskites. Energy Environ. Sci. 12, 596–606 (2019).
5. J. Zhao, Y. Deng, H. Wei, X. Zheng, Z. Yu, Y. Shao, J. E. Shield, J. Huang, Strained hybrid perovskite thin films and their impact on the intrinsic stability of perovskite solar cells. Science Advances. 3, eaao5616 (2017).

Metal halide perovskites are remarkable thin film semiconductors with numerous favorable properties that are optimal for photovoltaic and light emission applications¹. These characteristics include high absorption coefficients, long charge carrier lifetimes and a tunable bandgap.

We have previously shown that the luminescence efficiency and charge carrier recombination behavior in mixed halide perovskites is significantly altered by the cation alloy employed during fabrication via facile solution processing². The addition of alkali metal cations provides passivation of sub-bandgap defect states and also stabilizes an energetic landscape that facilitates photodoped carrier accumulation, leading to enhanced optoelectronic performance.

Our current work aims to further unveil the influence of microscopic compositional variations on local photoinduced carrier accumulation in (CsMAFA)PbI_xBr_3-x perovskite films. We combine Femtosecond Transient Absorption Microscopy with nanoscale X-ray Fluorescence and Atomic Force Microscopy to track carrier diffusion and recombination across a heterogenous chemical and morphological landscape. We find that the complex interplay of local carrier recombination and diffusion will have important ramifications for solar cell and LED device performance.

[1] Stranks, S. D. & Snaith, H. Nature Nanotech 10, 391-402 (2015)
[2] Feldmann, S., Macpherson S. et al. Nature Photonics 14, 123-128 (2020)

The highest solar cell efficiencies in halide perovskite devices are achieved using compositional derivatives of tetragonal methylammonium lead iodide (MAPbI₃). Commercialization of flexible perovskite devices will require a thorough understanding of how applied strain modifies the perovskite structure and defect-related properties. A complication of studying strain behavior in tetragonal MAPbI₃ is that it is ferroelastic, meaning that it spontaneously forms multiple domains, also called “twins”. These twins exhibit identical crystal structure but different orientation, are inherently strained, and are separated by domain walls. The relative proportions of different twins can be semi-reversibly changed (“domain switching”) upon application of stress, via a process called ferroelastic hysteresis. Separate studies have shown that applying strain to MAPbI₃ films modifies defect-related properties such as the degradation rate, the activation energy of ion migration and the photoluminescence lifetime. However, potential correlations between changes to these properties in films and ferroelasticity have proven difficult to establish, as a detailed structural description of the ferroelastic behavior of MAPbI₃ under strain is missing.

In this work, we characterized in detail the ferroelastic hysteresis loop of MAPbI₃. Films of MAPbI₃ were cast on polymer substrates. We used Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS) to first identify twin type and to assess the impact of the polymer substrate on the twins formed. Changes in the populations of different ferroelastic twins, in the strain magnitudes of different lattice planes, and in minimum domain sizes were then characterized for increasingly large applied strains. We thus identified approximate locations for the coercive stress (onset of domain switching) and saturation (all possible domain switching has occurred) on the stress-strain curve of MAPbI₃, as well as transformation strain magnitudes imparted by the hysteresis. Changes in the minimum domain sizes enabled deduction of evolution of the number of domain walls. The concentration of domain walls in films was found to closely correlate with previously-reported changes in the activation energy of ion migration, photoluminescence lifetime and degradation rate of MAPbI₃ films upon application of strain. The understanding gained by these results will aid design of more stable MAPbI₃ flexible devices with better-controlled ionic and electronic properties.

Metal halide perovskite quantum wells (QWs) have been used to fabricate efficient optoelectronic devices, and exhibit stability superior to that of their bulk 3D counterparts. The perovskite QWs are tuned in synthesis so that they possess different bandgaps and exciton binding energies owing to variable quantum confinement as a function of QW thickness. Accordingly, the device performance of these materials depends on the efficiency of various interwell carrier dynamical processes, principally exciton and charge transfer. I will discuss the use of transient absorption and ultrafast two-dimensional electronic spectroscopy to probe interwell exciton transfer on timescales of 100s of femtoseconds, and show that interwell charge transfer occurs on timescales of 10s to 100s of picoseconds. These results, in addition to ultraviolet photoelectron spectroscopy experiments, are used to reconcile conflicting observations of type-I and type-II band alignment amongst perovskite QWs.

A low recombination rate of photogenerated carriers in methylammonium lead iodide (MAPbI₃) enhancing the carrier lifetime and thereby diffusion length is believed to be occurred due to Rashba band splitting ^[1] which arises due to a large spin-orbit coupling (SOC) arising out of high-Z elements (lead and iodine) in the compound along with a lack of centrosymmetry which appears due to a tilted PbI₆ octahedra and rotating electric dipoles of the organic ions leading to an effective magnetic field and thereby lifting the Kramer’s spin-degeneracy of the electrons.^[2] Regarding the spin-rotation or spin-texture (χ) of the two split-levels, two schools of thought persist amongst theoreticians: (1) spin-forbidden transition model and (2) spin-allowed transition model. In the spin-forbidden transition model, after spin-split, the CB and VB closer to the Fermi energy possess opposite spin-textures. A recombination between electrons in CB-minimum and holes in VB-maximum is hence a spin-forbidden process leading to a low recombination rate and thereby an enhanced carrier lifetime and a longer diffusion length.^[1] On the other hand, such possibility has been overruled in the spin-allowed transition model as CB and VB possess a similar type of spin-texture closer to the Fermi energy where the recombination of photo-excited carriers is apparently possible due to spin-allowed nature of the transition.^[3] In this regard, we have reported an experimental observation of Rashba splitting in MAPbI₃ through spin-polarized scanning tunneling spectroscopy to probe the split bands.^[4] dI/dV spectra recorded at many different points of the film allowed us to spot both the Rashba split-levels and also to deliberate on their spin-textures. We observe that the bands split in such a manner that the conduction and valence bands closer to the Fermi energy have the same type of spin-textures (a spin-allowed transition model). Still a low recombination rate of photogenerated carriers in MAPbI₃ has been analyzed by considering Wannier-type excitons, a molecular nature of spin-domains as observed from the dI/dV images, and therefore a spin-forbidden nature of inter-domain transition.
Reference
1. Zheng, F.; Tan, L. Z.; Liu, S.; Rappe, A. M., Rashba Spin–Orbit Coupling Enhanced Carrier Lifetime in CH₃NH₃PbI₃. Nano Lett. 2015, 15 (12), 7794-7800.
2. Kepenekian, M.; Even, J., Rashba and Dresselhaus Couplings in Halide Perovskites: Accomplishments and Opportunities for Spintronics and Spin–Orbitronics. The Journal of Physical Chemistry Letters 2017, 8 (14), 3362-3370.
3. Zhang, X.; Shen, J.-X.; Van de Walle, C. G., Three-Dimensional Spin Texture in Hybrid Perovskites and Its Impact on Optical Transitions. The Journal of Physical Chemistry Letters 2018, 9 (11), 2903-2908.
4. Maiti, A.; Khatun, S.; Pal, A. J., Rashba Band Splitting in CH₃NH₃PbI₃: An Insight from Spin-Polarized Scanning Tunneling Spectroscopy. Nano Lett. 2020, 20 (1), 292-299.

Being one of the most promising photovoltaic materials, the metal halide perovskites have attracted plenty of scientific effort in improving their opotoelectronic properties and long-term stability. Both properties are highly sensitive to the nature and the concentration of the intrinsic defects in the perovskites. The first-principles calculation is very useful to predict the thermodynamics relevant to the formation of the defects and their consequences in the optoelectronic properties and plays an essential role in complementing experiments in understanding the defect chemistry and physics. However, due to the intrinsic complexity of the halide perovskites, i. e. hybrid composition and covalent-ionic bonds, diverse results were reported using different functionals within the density functional theory framework. Here, using the typical perovskite, MAPbI₃, as an example, we show a comprehensive comparison of a wide range of functionals including GGAs, meta-GGA (SCAN), and combination of them with atom-pairwise (DFT-D3) and non-local (rev-vdW-DF2 and rVV10) van der Waals (vdW) correction. Our results indicate meta-GGA (SCAN) predicts consistently more accurate geometries and energies than GGAs and the vdW correction is essential for describing the vdW bonds associated with the organic cations in the hybrid perovskites. Based on the above, we suggest the SCAN+rVV10 is the most promising functional and is expected to improve on the accuracy for other halide perovskites in this large family of materials.

Metal halide perovskite solar cells (PSC) are a high-efficiency, low-cost alternative to Si photovoltaics. However, various environmental stressors (light, humidity, temperature, bias, and oxygen) initiate degradation in perovskite devices, which limits device lifetimes and impedes commercialization. Discovering materials and fabrication parameters that provide a stable performance is therefore vital. Machine learning (ML) accelerates design at every level of the PSC development process while offering novel physical insight. We present a framework for applying ML to (A) perovskite compositional screening, (B) material synthesis and stability analysis, and (C) full device development and testing. Key parameters such as crystal lattice parameters, quasi-fermi-level splitting, and power conversion efficiency (PCE) are extracted from experimental data, including X-ray diffraction (XRD), photoluminescence (PL), and current-voltage (I-V) characteristics. Traditionally, these data lead to a time-consuming trial-and-error process that informs optimal perovskite compositions, fabrication processes, and device operating conditions. ML models expedite this process and, once trained, use easily acquired experimental data to immediately predict figures of merit or future material/device performance (including forecasting time-resolved behavior). These predictions then guide further experimental investigation. Our generalized roadmap involves (1) identifying the material question of interest, (2) obtaining data for model training, (3) data pre-processing, (4) feature engineering, and (5) model optimization and testing. We demonstrate this process from start-to-finish using several baseline models trained on currently available data from the perovskite community, noting potentials for fine-tuning and extension. As one example, we use past time-series PL and relative humidity (rH) data to forecast humidity-dependent light emission for methylammonium lead triiodide (MAPI), methylammonium lead tribromide, and triple-cation (formamidinium-methylammonium-cesium) perovskite thin films. For this task, we implement an echo state network (ESN), a type of recurrent neural network (RNN). The optimized model predicts PL output with <15% normalized root mean square error (NRMSE) for all compositions over 12+ hours. We train an additional ESN to forecast power output from a PSC with a triple-cation absorber, using light intensity and temperature data as inputs, and obtain an NRMSE of only 3.1%. In addition to time-series prediction, ML models can extrapolate from ground truth experimental data to novel conditions. This reduces laboratory time drastically and eliminates the need for exhaustive trial-and-error work. For example, we train a long short-term memory (LSTM) model on temperature-dependent aging data from a MAPI PSC. We generate degradation traces at unseen temperatures with <3% NRMSE. Finally, we show image-based machine learning, using a convolutional neural network (CNN) to extract hidden trends from dark-field images of spiro-OMeTAD (a common hole-transport layer in PSC devices) and predict film conductance. Although the images have no visual trends discernable by the human eye, the model achieves an NRMSE of 19%. These examples illustrate the versatility of ML as a tool in PSC research, and our workflow is easily generalizable to other facets of development. We propose integrating projects from all three levels of PSC design (composition, material, and device) into a streamlined ML pipeline that includes materials discovery, fabrication, characterization, and stability testing. This pipeline takes advantage of our current technological capabilities in ML, data science, and autonomous experimentation and presents a rational, accelerated pathway to PSC commercialization.

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

Metal halide perovskites have shown a promising future for application in field-effect transistors, light-emitting diodes, sensors, solar cells, and photodetectors due to their excellent optic-electrical properties. Among the excellent properties of perovskites, for example, strong absorption, the long diffusion length of photo-generated charge carriers and low binding energies make it possible to realize next-generation solar cells with 25.2% power conversion efficiency (PCE) in a decade. At the same time, perovskite solar cells (PSCs) have shown a powerful photovoltaic application for replacing silicon solar cells not only due to high PCE but also because of its low processing costs. Despite such advantages, the application of PSCs is currently limited by combining high performance and operational stability, because the PCE of PSCs can degrade due to the presence of temperature, light, humidity, and oxygen. So far, the degradation research on PSCs is carried out without having an established standard protocol. The microscopic degradation mechanisms are not clear under operating conditions upon suggested inert atmospheres based on the International Summit on Organic Photovoltaic Stability protocols. Therefore, it is necessary to establish an improved standard protocol for the long-term degradation of PSCs.

In this respect, we investigate the degradation processes of PSCs under both, AM 1.5G and different atmosphere conditions (vacuum and nitrogen) with in-operando grazing incidence wide-angle X-ray scattering (GIWAXS) and grazing incidence small-angle X-ray scattering (GISAXS). With these approaches, we find that mixed cations lead mixed halides lattice exhibit the effect of photostriction when being operated in different atmospheres. In addition, the nature of photostriction can help the phase segregation of halide perovskites. Pressure conditions can suppress this photostriction behavior. Theoretical calculations based on density functional theory confirm that lattice photostriction behavior can suppress phase segregation induced by thermodynamic driving forces. A standard protocol is suggested based on our findings.

Perovskite solar cells (PSC) are believed to be the core of next-generation solar cells due to their rapidly increasing efficiency and popularity as an alternative energy source to fossil fuels. However, the interfacial contact between the electron transport layer (ETL) and the perovskite layer in the cell often results in problems of hysteresis. This research investigates different ETL materials to remedy hysteresis in organic-inorganic CH3NH3PbI3 (MAPbI3) PSCs. Compact TiO2 (c-TiO2), SnO2 (0.1M and 0.2M), and mesoporous TiO2 (m-TiO2) (1:4 and 1:6 w:w) were deposited as the chosen ETL materials via spin-casting and followed by high-temperature annealing. The PSC devices were built using FTO glass, MAPbI3, and gold electrodes (80nm).
Scanning electron microscopy (SEM) was utilized to take cross-sectional images of the solar cell after depositing the perovskite layer. Higher values in the precursor ETL solution concentration correlated with increased ETL thickness. Qualitative analysis of the c-TiO2/m-TiO2 bilayer SEM image demonstrated that the perovskite layer also effectively diffused into the porous m-TiO2, which increased the interfacial contact between the perovskite and the ETL. Although the control c-TiO2 had a power conversion efficiency (PCE) of 14.5%, the sample had a hysteresis index of 0.400. In comparison, the 34mM TiO2 sample had a PCE of 11.9% with a more desired hysteresis index of 0.034. The m-TiO2 bilayer sample, albeit having a PCE of 6.8%, had a hysteresis index of 0.121, suggesting that the mesoporous layer aided in reducing the hysteresis index of the original c-TiO2 ETL.
We gratefully acknowledge support from the Louis Morin Charitable Trust.

Ultralong micro-belts of MAPbI3 show bright and stable fluorescence in solution. Luminescent perovskites have been synthesized through centrifugation of a solution-processed with a ligand-assisted reprecipitation technique (LARP). All electron spectroscopic analysis provided evidence for the stacking of nanocrystals as bundles of self-assembled micro-belts. Under an applied bias, migration of free charge carriers through the long axis is feasible. Thus, long micro-belts are promising channel materials for two-electrode nanodevices for good electrical conductivity.

The moisture-induced instability of metal halide perovskites is one of the major challenges for perovskite devices. Although compositional engineering has been widely employed to improve the overall stability of perovskites, its effect on the moisture-induced instability received little attention. Here, we systematically study the interaction of water with the surfaces of primary perovskites, AMX₃ (A⁺ = MA⁺, FA⁺, Cs⁺; M²⁺ = Pb²⁺, Sn²⁺; X^- = I^-, Br^-), by using Density Functional Theory (DFT) calculations and comprehensive chemical bonding analysis. We reveal that the hydrophilic group NH₃⁺ of MA⁺ cation may be the cause for instability issues. We find that the adsorption of water on FAPbI₃ and CsPbI₃ are much weaker than on MAPbI₃ due to the less polarity of FA⁺ and Cs⁺. When exchanging M²⁺ cations, water adsorption on MASnI₃ is also less energetically favorable than on MAPbI₃ because of the weaker ionic interaction of H₂O-MASnI₃. When exchanging X^- anion, water adsorption on MAPbBr₃ is slightly weaker than on MAPbI₃ due to the slightly weaker covalent interaction of H₂O-MAPbBr₃. Our results present a comprehensive understanding of the compositional effect on the interactions of water with perovskites and provide rational design strategies to improve their stability against moisture via compositional engineering.

In the dark and at room temperature most mixed halide perovskites are thermodynamically stable. Under illumination, however, funneling of photo-excited carriers to a phase with a lower band gap than the parent phase leads to halide demixing. This is problematic for the application of mixed halide perovskites in solar cells. Here, we present a unified thermodynamic theory for this light-induced phase separation and apply it to five mixed iodine-bromine perovskites. Under illumination, the spinodals separating the metastable and unstable regions in the composition-temperature phase diagrams show an upward shift in temperature, while the binodals separating the stable and metastable regions acquire an additional branch signaling the nucleation of a low-band gap iodine-rich phase. We find that the threshold illumination intensity is mainly governed by the band gap difference of the parent and iodine-rich phase. Partial replacement of organic cations by cesium reduces this difference and therefore has a stabilizing effect.

Two-dimensional (2D) halide perovskites are promising materials for environmentally stable next-generation optoelectronic device applications. However, there is very little atomistic understanding of the charge carrier dynamics at ambient conditions for these materials, limiting the possibilities to tune their optoelectronic performances through compositional engineering routes. Here, by combining nonadiabatic molecular dynamics with time-domain density functional theory methods at room temperature, we study the dominant non-radiative carrier recombination and dephasing processes in monolayered lead halide perovskites. Our systematic study demonstrates that performance-limiting nonradiative carrier recombination processes greatly depend on the electron-phonon interactions induced by structural fluctuations and instantaneous charge localization in these materials. The stiffer interlayer packing in presence of selectively chosen spacer cations (benzene ring or cyclic dication based), which separates the lead iodide slabs, reduces the thermal fluctuations in these 2D-perovskites to a greater extent.^[1-2] These reduce the inelastic electron-phonon scattering and enhance the photogenerated charge carrier lifetime in layered perovskites making them suitable for various optoelectronic devices. The computational insights gained from these studies allow us to outline a set of robust design principles for 2D halide perovskites to strategically tune their optoelectronic properties.
Reference:
1. Ghosh et al. J. Phys. Chem. Lett, 2020, 11, 2955
2. Ghosh et al. J. Mater. Chem. A, 2020, Just Accepted (DOI: 10.1039/D0TA07205B)

The compounds belonging to the family of perovskites show a great variety of mechanical, magnetic and optical properties, and for these reasons are nowadays key materials for many technologies, including piezoelectrics, photovoltaic solar cells and photocatalytic systems.
Among perovskites, the most studied crystal structure ABX₃ for photocatalysis and photovoltaic applications is composed of an organic/inorganic compounds made of: monovalent cation A = methylammonium (MA: CH₃NH₃⁺); formamidinium (FA: CH(NH₂)₂⁺); a divalent metal cation B = Pb²⁺, Sn²⁺; and an halide anion X = I⁻; Br⁻; Cl⁻. These hybrid lead-based materials have excellent properties and contain elements that are both cheap and abundant. However, they tend to partially decompose when exposed to air, water, ultra-violet light, or heat. For these reasons, several substitutions have been attempted to improve the properties and performances of these materials. One route to improve the stability of these structures is to explore the all-inorganic perovskite systems and thus finding alternatives to replace organic components. Particularly, our attention is devoted to the all-inorganic perovskites, i.e. CsPbBr₃ and CsPb₂Br₅, which do not have any labile or expensive components and show remarkable stability. Furthermore, unlike the hybrid structures, the entire fabrication process of all-inorganic analogous materials can be operated in ambient atmosphere without humidity control systems. Additionally, CsPbX₃ (X = Cl, Br, and I) perovskite family shows high performances for photocatalytic and photo-electrochemical degradation processes of different organic dye compounds, and are also exceptional candidates as photocatalysts for organic reactions. Among the different strategy developed for their synthesis, the surfactants-based solution route is widely applied, which also allows to control the stability and the growth kinetics according to the surface chemical phenomenon and the solubility equilibrium.
Herein we report a novel synthetic strategy, which represents a facile, one-step, low-temperature and surfactant free approach to the synthesis of Cs-Pb-Br microcrystals, using Cs and Pb β-diketonate metallorganic complexes. The present precipitation approach represents an innovative and interesting strategy of synthesis, which avoids the use of surfactant species and toxic solvents, and, taking the advantages of the β-diketonate metalorganic Cs and Pb precursors reactivity, it allows, reproducibly and selectively, the synthesis of CsPbBr₃ and CsPb₂Br₅ microcrystals. Structural, morphological and compositional analyses of the final products, addressed through X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and Energy Dispersive X-ray (EDX) analysis, show the formation of pure and stable CsPbBr₃ or CsPb₂Br₅ systems characterized by squared microcrystals, with a grain size up to 3μm. The stability of the two different phases has been tested as a function of heating treatment and UV exposure as well. Moreover, as a proof-of-concept, the photocatalytic properties of degradation processes have been tested using organic dye solutions, showing high degradation yield up to 70% under UV lamp (λ=360 nm).

Lead-based halide perovskites (CsPbX₃, X=Cl, Br, I) are well-known for their attractive properties which make them suitable materials for many optoelectronic devices. But the toxicity of lead (Pb) undoubtedly hinders the development and commercialization of these devices. Therefore, Pb-free, environmental friendly all-inorganic halide perovskites are now the subject of enormous interest to the scientific community. It has been suggested that antimony (Sb) can be a possible replacement of Pb (Cs₃Sb₂X₉) in these inorganic halide perovskites. Until now Sb-based all-inorganic perovskites have been synthesized by solution processes but properties are still inferior to Pb-based materials. However, due to the low melting point of Sb halides, synthesis of high quality Cs₃Sb₂X₉ using chemical vapor process is still a challenge. In this work, we present a report on the synthesis and properties of Pb-free all-inorganic Cs₃Sb₂Br₉ perovskite microplates by a two-step chemical vapor deposition (CVD) approach. In step-I, CsBr powder was vaporized at 530°C at a pressure of 4.8 Torr under 50 sccm of Ar flow for 10 minutes in a single-zone CVD furnace. In step-II, due to low melting point, SbBr₃ powder was vaporized at 90°C under 15 sccm of Ar flow while the pressure and holding time were remained same as step-I. Substrates were placed at 22 cm downstream from the center. Finally Cs₃Sb₂Br₉ microplates with average size 20 μm were obtained after annealing the as-grown materials at 140 °C for 25 minutes. XRD analysis of the microplates shows the pure phase crystalline nature of the Cs₃Sb₂Br₉ perovskite structure. The optical band gap was measured by spectroscopic ellipsometry to be 2.6 eV. The UV-vis absorption spectrum revealed the exciton absorption peak at around 2.82 eV. Simple photoconductive devices fabricated using these microplates exhibit current on/off ratio of 10² upon 450 nm photon irradiation with a fast response of rise and decay time down to 79 ms and 28 ms, respectively. All these results clearly suggest the technological potential of inorganic lead-free Cs₃Sb₂Br₉ perovskite microplates for future optoelectronic devices.

Abstract: We present an interfacial engineering method to fabricate an efficient and thermally stable MAPbI₃ based perovskite solar cell with a double decked hole (H) and electron (E) extraction layers (EL) in an inverted (p-i-n) architecture. The double HEL film is fabricated by thermal evaporation of MoO₃ and spin-coating of poly(3,4-ethylene dioxy-thiophene)–poly(styrene sulfonate) (PEDOT:PSS). The double EEL is formed by spin-coating of phenyl-C61-butyric acid methyl ester (PC₆₁BM) and bathocuproine (BCP). The role of BCP as a buffer layer is quite established in the community to prevent the formation of metal induced charge state at the perovskite- EEL interface. Despite of presence of BCP interlayer, the thermal stability of p-i-n architecture still suffers. We observed that the underneath MoO₃ layer provides an improved morphology for perovskite film and reduces the shunt path. However, the BCP buffer interlayer not only passivates the defects at the perovskite- EEL interface and but also helps to plenaries the thin-films for better contact with Ag. Thus, double EEL and HEL facilitate better charge collection at the interfaces and shows improved power conversion efficiency (PCE) of ~15%. Besides the improved PCE, double EEL and HEL based structures also provide an approach to have better reproducibility and high thermal stability of perovskite solar cells. We also address the issue of area dependent efficiency and explore the reasons behind the increased efficiency of smaller area devices. We believe reproducibility, stability and area dependence are the most important aspects for current perovskite PV research and thus this study will provide an important insight for further development.

Keywords: MAPbI₃, double extraction layer, thermal stability, solar cells, BCP, MoO₃

References:

1. S. Singh, R. Shourie, and D. Kabra, Efficient and thermally stable CH3NH3PbI3 based perovskite solar cells with double electron and hole extraction layers. J. Phys. D: Appl. Phys. 2019, 52, 255106.
2. Y. Zhao, A. M. Nardes and K. Zhu, Effective hole extraction using MoOx-Al contact in perovskite CH3NH3PbI3 solar Cells. Appl. Phys. Lett. 2014, 104 213906.

Metal halide perovskites (MHPs) for perovskite solar cells (PSCs) have gained great attention in just few years due to their rapid increase of photoconversion records of efficiency. In addition, MHPs exhibit a competitive fabrication cost together with a simple route to synthetize. Due to these facts, PSCs are recognized by scientists as a promising technology with enormous potential in the energy market. However, it is well known that many factors cause the degradation of PSCs, thus, industrial applications are critically hampered by instability issues.
Nowadays, experimental and theoretical researchers are investigating many possible alternatives to increase the stability of PSCs. In this context, computational techniques are being extremely useful to understand the properties of MHPs at microscopic level that cause instability of the material. Density functional theory (DFT) calculations are the key method for studying material properties, but the high computational cost limits the study to small systems and short time-scales. Classical simulations seem to be an option to overcome these limitations, but they suffer from other drawbacks such as classical simulations cannot describe electrons and chemical reactions. The study of many applications of MHPs requires the simultaneously modeling of electrons and ions in relatively large systems, sometimes not computationally affordable by the standard techniques. Therefore an intermediate approach between DFT and classical simulations is often desired. In this regard, semi-empirical Quantum Mechanics methods, such as density functional tight binding (DFTB), combine the functionalities of describing both electrons and ions. Traditional DFTB methods are based on a simplification of the Kohn-Sham DFT total energy, using pre-computed interactions of element pairs, considerably reducing the computational cost. However, this parameterization lacks of transferability and is limited to a number of elements. A new extended tight binding method called GFN1-xTB has been recently developed to cover all the elements of the periodic table. This method was designed for the fast calculation of diverse physicochemical properties of systems of a few thousand atoms and has the advantageous of maintaining high accuracy and a limited number of parameters which can be refined for the study of a given application.
The performance of GFN1-xTB is benchmarked for a set of MHPs and compared with density functional theory calculations. We study organic and inorganic MHPs with formula ABX₃, with A = MA, FA, and Cs; B = Pb and Sn; X = I, Br, and Cl, being a total of 18 MHPs with cubic, tetragonal, and orthorhombic forms. We included the computation of structural, vibrational, and optoelectronic properties of these semiconductors. This work aims to provide a general overview of the performance of GFN1-xTB for the fast computation of the main features of MHPs. Our results indicate that this method can describe the targeted features of the MHPs, but performs worse for geometry relaxation calculations. We also identified the drawbacks of the method for particular compositions of MHPs, which hamper its use for solar cell applications.

While much attention has been paid to halide perovskites based on Pb and Sn cations on the B-site, Ge compounds offer a distinct combination of structure and properties. All three cations feature the same s² lone pair configuration; however, the stereochemical activity varies in each case. We performed a computational analysis via density-functional-theory and Berry phase calculations of the CsGeX₃ (X = Cl, Br, I) series. Our motivation is to clarify the existence and magnitude of ferroelectricity and piezoelectricity, which has important technological consequences such as polar nanodomains that can reduce the recombination of light-excited electron-hole pairs and extend the carrier lifetime [2]. The explanation for the origin of spontaneous polarization in Ge halide perovskites in terms of an on-site second-order Jahn-Teller effect has not been hitherto investigated [3]. In this work, we investigate the role of halide anions in spontaneous polarization within the modern first-principles theory of polarization. Our calculation results show that interaction between Ge lone-pair electrons and halogen p-orbital mainly determines the depth of the ferroelectric double well and the spontaneous polarization strength with Cl > Br > I. Further exploration of lone pair systems offers an exciting pathway to design novel photoelectric devices [4,5].

[1] Lead-free germanium iodide perovskite materials for photovoltaic applications. J. Mater. Chem. A. 3, 23829-23832 (2015); https://doi.org/10.1039/C5TA05741H

[2] Atomistic origins of high-performance in hybrid halide perovskite solar cells. Nano Lett. 14, 2584–2590 (2014); https://doi.org/10.1021/nl500390f

[3] Resolving the Physical Origin of Octahedral Tilting in Halide Perovskites. Chem. Mater. 28, 4259–4266 (2016); https://doi.org/10.1021/acs.chemmater.6b00968

[4] Stereochemistry of post-transition metal oxides: revision of the classical lone pair model. Chem. Soc. Rev. 40, 4455-4463 (2011); https://doi.org/10.1039/C1CS15098G

[5] The underappreciated lone pair in halide perovskites underpins their unusual properties. MRS Bulletin. 45, 467-477 (2020); https://doi.org/10.1557/mrs.2020.142

Metal halide perovskites have been the focus of many computational studies over the past few years. The bulk of these investigations were done using methods based on quantum mechanics (QM). However, the computational cost of QM methods is high, severely limiting the length and time scales of the systems that can be investigated. Molecular dynamics (MD) simulations have been successfully applied to simulate larger material systems and at longer time scales by making use of classical force fields (CFF). Nonetheless, the predefined connectivity in CFF makes them unsuited for the simulation of chemical reactions. A reactive force field (ReaxFF) is an extension to CFF by including a dynamic bond order that is calculated from the interatomic distances and allows for the simulation of bond breaking and formation. In this presentation, I will outline our progress in the creation of the first ReaxFF for halide perovskites, CsPbI₃.

Perovskite solar cells have electrified the solar cell research community with astonishing performance and surprising material properties. However, the widespread commercialization of perovskites in solar cells and other optoelectronic applications still requires a more detailed knowledge about fundamental material properties. My group is specialized in the investigation of these properties on nanometer length scales using scanning force microscopy. We recently discovered a distinct nanoscale stripe domain structure in the piezoresponse on perovskite crystals [1]. The pattern is characteristic for ferro-elastic twin domains. We are currently investigating the influence of the domain walls on the charge carrier dynamics. Next to fundamental physical properties of photovoltaic perovskites, we investigate complete solar cell devices under operation conditions. To map and follow the vertical charge distribution across the functional layers of an operating device, we developed a method to prepare smooth cuts across the device [2]. We subsequently map the potential distribution across the device using a time-resolved Kelvin probe force microscopy (tr-KPFM) [2,3]. We can simulate open- or short circuit conditions, apply an external voltage or illuminate the device. In particular, we found that thin layers of localized charge formed and stabilized at the electrode interfaces when we changed the external voltage or illuminated the device. We could show that ion migration itself is too fast to influence device hysteresis. Instead, the formation and release of ionic interface charges determine the time scales for current-voltage hysteresis in perovskite solar cells. Our results demonstrate that a precise control over the interfaces in perovskite solar cells is the key, not only for controlling and suppressing hysteresis but also for their long-term stability in perovskite solar cells. [1] J. Phys. Chem C, 120(10), 5724 (2016). [2] ACS Appl. Mat. Interfaces, 8(30), 19402 (2016). [3] Energy Environmental Science, 11, 2404 (2018).

Defects come in a several flavours: conductivity-promoting defects create free carriers that enable electronics; killer defects (deep, charged centres) trigger recombination; and charge scattering defects reduce mobility [1]. Our understanding of the defect processes in halide perovskites is gradually improving through a combination of high-resolution measurements and materials modelling [2]. However, evidence is building for the importance of point defect aggregates and structural defects in higher dimensions. I will discuss recent progress, from theory and experiment, to identify, characterise and control extended defects in metal halide perovskites. In particular, I will cover the role of grain boundaries [3] stacking faults [4]. The use of defect tolerance as a metric to develop and screen post-perovskite materials will be critically addressed. This research is part of a collaboration with the group of Sam Stranks (Cambridge), and used the UK National Supercomputer ARCHER.

[1] "Instilling defect tolerance in new compounds" Nature Mater. 16, 964 (2017); https://www.nature.com/articles/nmat4973

[2] "Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites" Nature 580, 260 (2020); https://www.nature.com/articles/s41586-020-2184-1

[3] "Accumulation of deep traps at grain boundaries in halide perovskites" ACS Energy Lett. 4, 1321 (2019); https://doi.org/10.1021/acsenergylett.9b00840

[4] "Hexagonal stacking faults act as hole blocking layers in lead halide perovskites" ACS Energy Lett. In Press (2020); https://doi.org/10.1021/acsenergylett.0c01124

The dynamic optical and electrical responses of halide perovskites are currently not fully understood, and are closely related to the irreversible changes frequently observed in devices. We determine the individual and combined effects of extrinsic (humidity and oxygen) and intrinsic (light, bias, and temperature) stressors in the physical behavior of the archetypal MAPbBr₃ and MAPbI₃, and Cs-mixed perovskites. First, a detailed comparison between MAPbBr₃ and MAPbI₃ through time-dependent voltage measurements reveals that, upon illumination, high-energy photons leads to a > 10x slower voltage decline toward equilibrium than low-energy photons in MAPbBr₃. Yet, MAPbI₃ shows wavelength-independent decay rate, resulting from ion migration. Second, through in situ photoluminescence (PL) under environmentally controlled conditions, we resolve a humidity-induced PL hysteresis. Further, we apply a machine learning algorithm to predict the luminescence response for > 12 hs. Concerning the unique behavior of multi-cation perovskites, a correlative microscopy approach is realized, combined with environmental-controlled PL measurements.

Although grain boundaries and other interfaces in photovoltaics are generally detrimental for photogenerated carrier transport, their role remains contentious in hybrid perovskite semiconductors. The uncertainty is partially due to the practical challenge that most experimental approaches are performed at the top surface, which is not only insensitive to depth-dependent inhomogeneities but also may be influenced by topographic artifacts. Accordingly, we have developed tomographic atomic force microscopy (T-AFM) for fully 3-Dimensional mapping of photogenerated carrier transport in hybrid perovskite thin films. Via progressive nanoscale planarization and photoconduction mapping during in-situ illumination in an AFM, this unambiguously reveals grain boundaries serving as highly interconnected conducting channels for carrier transport in MAPbI₃. Voltage dependent measurements reveal enhanced carrier mobilities at most grain boundaries, although careful analysis also reveals the coexistence of 5-10% insipid interfaces. Depth dependencies in photocurrent through the grains, and the grain boundaries, are also assessed. Such insights are critical for engineering hybrid perovskites to be optimized for future solar cells, photodetectors, and light sources.

Halide perovskite thin films and devices exhibit a remarkable degree of heterogeneity in structure and composition at the nanoscale, varying frequently from nominal synthesis targets in ways that can be difficult to detect using ubiquitous laboratory analysis. Leveraging synchrotron-based in situ X-ray microscopies to sample composition, structure, and optoelectronic quality at the nanoscale, we reveal the dependence of nanoscale phase separation and variations in charge collection on the local chemistry of A-site perovskite alloys. At times, these stoichiometric variations arise in the as-fabricated film, revealing important chemical dependencies. However, we also find significant evolution in the microstructure of high-performance perovskites during operation and under environmental stress, leading to insights into mechanistic pathways of degradation and new avenues for stabilization in high-performance alloyed perovskites.

The remarkable solar performance of lead halide perovskites can be attributed to their excellent physical properties that present many mysteries, challenges, as well as opportunities. Better control over the crystal growth of these fascinating materials and fabrication of heterostructures using various halide perovskites with different bandgaps would open up opportunities for exploring new properties and device applications. After a brief summary on the nanowires and nanoplates of various metal halide perovskites we have developed and their lasing properties, I will focus my discussion on the growth of heterostructures of 2D Ruddlesden-Popper (RP) layered lead iodide perovskites with defined n phases and atomically sharp interfaces both in 2D/3D perovskite heterostructures and also as multi-layered multi-colored vertical heterostructures, in which the long chain ammonium ligands serve as the barriers to prevent ion migration across the junctions. We have used these well-defined heterostuctures to study the carrier transfer mechanisms between 2D and 3D perovskites and between different RP phases. The excellent properties of these single-crystal perovskite nanostructures of diverse families of perovskite materials with different cations, anions, and dimensionality make them ideal for fundamental physical studies of carrier transport and decay mechanisms, and for enabling high performance lasers, LEDs, and other optoelectronic applications.

We quantify the role of many-body elastic scattering effects on exciton dephasing rates in two-dimensional hybrid metal-halide perovskites by means of nonlinear coherent excitation spectroscopy at a temperature of 5 K. We find that the exciton-density dependence of excitation-induced dephasing (EID) is two to three orders of magnitude lower than in other atomic monolayer semiconductors such as transition metal dichalchogenides. We find that, EID is different for the multiple excitons evident in the excitation line shape, as is their phonon-mediated temperature dependence. We ascribe these observations to screening effects due to polaronic dressingby the lattice. We further examine the evolution of the two-dimensional coherent exciton lineshape at 5 K with population waiting time. We find that the 2D coherent lineshape evolves with over time windows in which the population itself is static, in a manner that indicates time-dependent EID. Specifically, we find that the dephasing slows down with population waiting time. This spectral evolution invokes time-varying inter-exciton coherences, and we discuss the possible role of dark exciton states in these quantum dynamics.

Lead-iodide based perovskite semiconductors are emerging as promising light absorbers for solution-processed thin-film photovoltaic applications. In addition, these materials are also intensively studied for light-emitting diodes, photodetectors, and lasers. Recently, hybrid organic-inorganic perovskite (HOIP) semiconductors are being considered as candidates for spintronic applications due to a large spin-orbit coupling (SOC) and controllable Rashba-splitting. Here we discuss the spin-coherence lifetimes in single crystals of 2D Ruddlesden-Popper perovskites PEA2PbI4(MAPbI3)n?1(PEA, phenethylammonium; MA, methylammonium; n = 1, 2, 3, 4). Room temperature spin-coherence times show a nonmonotonic layer thickness dependence with an increasing spin-coherence lifetime with increasing layer thickness from n = 1 to 4, followed by a decrease in lifetime from n = 4 to ?.Our results are consistent with two contributions; Rashba-splitting increases the spin-coherence lifetime going from the n = ? to the layered systems, whilephonon-scattering which increases for smaller layers decreases the spin-coherence lifetime.The unique tunability of HOIPs offers an unprecedented opportunity to directly incorporate chiral organic molecules in the production of a 2D-layered hybrid perovskite thin film and/or crystal that induces chirality into the inorganic sub-lattice bandedge states. Thus, there is an interesting opportunity to explore how chiral molecules embedded into the multilayers provide spin-control within these hybrid semiconductors. Here, we focus on controlling spin-transport within 2D-layerd chiral Pb-I perovskite systems. We demonstrate spin-polarization of charge-transport through the chiral perovskites by showing that the injected current is preferential to one of the spin-states and depends upon the handedness of the incorporated chiral organic molecules; thus, the hybrid chiral system acts as a spin-filter.

Solid-state nanocrystal-based upconversion (UC) systems have disadvantages with respect to poor exciton diffusion lengths, strong singlet reabsorption and the low resulting achievable UC efficiencies. To circumvent these issues, we employ bulk lead halide perovskite (LHPs) thin films as triplet sensitizers for near-infrared-to-visible UC. The sensitized UC process is based on triplet-triplet annihilation (TTA) in the organic semiconductor rubrene. Conservative estimates result in reported UC efficiencies upwards of 3%.1However, to further boost the UC efficiency, it is required to understand and mitigate the effects of shallow trap states in the LHP sensitizer. Here, we investigate the underlying mechanism of energy transfer at the LHP/rubrene interface, and the effect of the LHP perovskite film thickness on the UC dynamics and efficiency threshold. In the LHP/rubrene bilayer device, we observe two opposing effects caused by interactions between the LHP and rubrene: i) passivation of surface defects by rubrene, which results in a longer native photoluminescence (PL) lifetime, and ii) quenching of the PL lifetime due to energy transfer to the triplet state of rubrene. Our observations indicate that non-radiative trap filling in the LHP film and charge transfer to rubrene are likely competitive quenching pathways. As a result, the threshold Ithat which the UC process becomes efficient, observed as a slope change in the power-dependent upconverted PL, shifts to lower values with increasing film thickness. In contrast to previous reported systems, the slope change is directly dependent on the underlying LHP dynamics:i) below Ith, we find a slope twice the value of the LHP PL, and ii) above Ith, the upconverted PL follows the same power-law as the underlying recombination of electrons and holes in the LHP thin films.2,3However, two unusual effects are also observed in the system: i) two distinct rise times in the upconverted emission, indicating two distinct triplet sensitization rates. ii) A reversible photobleach of the upconverted emission, which cannot be correlated to a physical degradation of the system. By manipulating the triplet population, we are able to show that a higher triplet population does not necessarily result in a higher UC yield for the LHP/rubrene system demonstrated here, rather can be detrimental to the efficiency due to strong singlet reabsorption.3 (1) Nienhaus, L.; Correa-Baena, J.-P.; Wieghold, S.; Einzinger, M.; Lin, T.-A.; Shulenberger, K. E.; Klein, N. D.; Wu, M.; Bulovi?, V.; Buonassisi, T.; et al. Triplet-Sensitization by Lead Halide Perovskite Thin Films for Near-Infrared-to-Visible Upconversion. ACS Energy Lett. 2019, 4, 888-895. (2) Wieghold, S.; Bieber, A. S.; VanOrman, Z. A.; Daley, L.; Leger, M.; Correa-Baena, J.-P.; Nienhaus, L. Triplet Sensitization by Lead Halide Perovskite Thin Films for Efficient Solid-State Photon Upconversion at Subsolar Fluxes. Matter 2019, 1, 705-719. (3) Wieghold, S.; Bieber, A. S.; VanOrman, Z. A.; Nienhaus, L. Influence of Triplet Diffusion on Lead Halide Perovskite-Sensitized Solid-State Upconversion. J. Phys. Chem. Lett. 2019,10, 3806-3811.

Methylammonium Lead Bromide (MAPbBr3) is the most investigated 3D Perovskite for visible LED applications. It is characterized by a much higher exciton binding energy compared to its Iodide equivalent (MAPbI3). This leads to complex excited-state phenomena, with free-carriers, excitons and defect states contributing to the photoluminescence (PL) properties. We have used a variety of spectroscopic techniques to study photoexcited MAPbBr3 films, including time-resolve PL measurements, femtosecond transient absorption (TA) spectroscopy and optical-probe-terahertz-pump (THz) experiments. The co-existence of free carriers and excitons is demonstrated at low excitation densities in as-cast polycrystalline MAPbBr3 (crystal size of the order of few ?m). The excitation wavelength and density are then varied in order to access different free-carrier to exciton ratios in order to evaluate their recombination dynamics and THz conductivities. Finally, we show how reducing the grain size of the Perovskite to the micro- and nanoscale (by using different additives during solution-processing) increases the PL quantum yield, which we explain by the presence of a bright exclusively exitonic population even at low excitation densities, together with reduced surface trapping thanks to passivation by the additives.

Functional materials combining the optoelectronic functionalities of semiconductors with control of the spin degree of freedom are highly sought after for the advancement of quantum technology devices and provide exciting avenues for polarized light-emission. Previous work towards this goal introduced small amounts of magnetic elements into crystalline semiconductor, e.g. through vacuum-based deposition, to obtain dilute magnetic semiconductors (DMS).

In my talk, I will present our efforts on gaining control over spin dynamics and spin interactions through compositional and structural tuning in solution-processable hybrid perovskite semiconductors. We aim to exploit the exceptional optoelectronic properties of these hybrid perovskites, together with their tolerance in the electronic states to dopants and defects, to make advances towards high-performance DMS. I will further discuss how we employ ultrafast optical spectroscopy to gain insights into fundamental excitation and spin dynamics in our novel materials, highlighting the potential of magneto-optic control.

I will present first results on chemical doping of layered Ruddlesden-Popper hybrid perovskites with transition metals. I will report how optical excitations interact with elemental spins in these materials, also on ultrafast timescales, to achieve control over exciton emission through magnetic proximity effects. Harnessing spin-orbit coupling effects, we achieved circular polarization up to 15% in the excitonic photoluminescence at low temperatures and high fields, which keeps a strong magnetic field dependence below 1 Tesla. Our finding constitutes the first example of optical polarization control in magnetically-doped hybrid perovskites and will stimulate research on this highly-tuneable material platform that promises tailored interactions between magnetic moments and electronic states.

Two-dimensional Ruddlesden-Popper phase hybrid metal halide perovskites have been investigated for use in a range of optoelectronic devices including photovoltaic cells. Their main appeal for photovoltaics is improved aqueous stability, which is enabled by their incorporation of organic cations which are more hydrophobic than those usually used in 3D perovskites. However, quantum and dielectric confinement effects increase the exciton binding energies in these materials to as much as 100s of meV. These increased binding energies correspond to an increase in exciton population at ambient temperatures which can greatly alter the optoelectronic properties. Optical Pump/THz-probe (OPTP) spectroscopy measurements have revealed decreased effective charge-carrier mobilities and significantly shortened charge-carrier lifetimes for 2D perovskites (1). In hybrid 2D-3D materials, incorporating small amounts of the 2D material can improve optoelectronic properties (2), suggesting a balance between excitons and free charges is achievable by tuning the composition. OPTP and other ultrafast spectroscopic techniques are further used to elucidate the interplay between free charges and excitons in these materials and comment on the limits these properties impose on charge transport in devices.

1. R. L. Milot et al., "Charge-Carrier Dynamics in 2D Hybrid Metal–Halide Perovskites," Nano Lett. 16(11), 7001-7007 (2016).
2. L. R. V. Buizza et al., "Charge-Carrier Dynamics, Mobilities, and Diffusion Lengths of 2D–3D Hybrid Butylammonium–Cesium–Formamidinium Lead Halide Perovskites," Adv. Funct. Mater., 1902656 (2019).

Currently, many different perovskite -with corner-sharing octahedra- as well as non-perovskite metal-halide solids are synthetized worldwide entailing the need for in-depth understanding of their structure/property relationships. In this regard, combining the huge accumulated knowledge over the last decades, on halide but also oxide perovskites, with modern atomic scale modeling as well as symmetry analysis has proved useful. Among others, new compositions such as A'₂A_n-1M_nX_3n+1 (where A and A' are cations, X is halide and M is metal) afford layered structures with a controlled number (n, currently ≤ 7) of octahedra in the perovskite layer. Those correspond to innate heterostructures that offer an ideal platform for fundamental understanding such as effect of quantum or dielectric confinement.^[1]

Efforts have also been successful in building new perovskite structures with cations surpassing the Goldschmidt tolerance factor. For instance, this has been recently demonstrated for layered perovskites with the use of ethylammonium, isopropylammonium or dimethylammonium as perovskitizer, having a perovskite sheet retaining its continuous corner-sharing connectivity.^[2] On another note, new three dimensional networks have been achieved by mixing larger cations (e.g., hydroxyethylammonium) with the standard ones (e.g. methylammonium, formamidinium) leading to the formation of voids in the perovskite lattice, either disordered (Hollow perovskites) or with structural signature of a periodic pattern (Deficient perovskites).^[3]

In this talk, I will discuss some of our recent theoretical results paying attention on newly discovered halide perovskite phases and opportunities to further engineer their properties in connection with their use in devices.

References including references therein: [1] C. Katan, N. Mercier, J. Even, Quantum and dielectric Confinement Effects in Lower-Dimensional Hybrid Perovskite Semiconductors, Chem. Rev. 119, 3140 (2019); [2] Y. Fu, X. Jiang, X. Li, B. Traore, I. Spanopoulos, C. Katan, J. Even, M. G. Kanatzidis, E. Harel, Cation Engineering in Two-Dimensional Ruddlesden-Popper Lead Iodide Perovskites with Mixed Large A-Site Cations in the Cages, J. Am. Chem. Soc. 142, 4008 (2020); X. Li, Y. Fu, L. Pedesseau, P. Guo, S. Cuthriell, I. Hadar, J. Even, C. Katan, C. C. Stoumpos, R. D. Schaller, E. Harel, M. G. Kanatzidis, Negative Pressure Engineering with Large Cage Cations in 2D Halide Perovskites Causes Lattice Softening, J. Am. Chem. Soc. (2020) DOI: 10.1021/jacs.0c03860; [3] A. Leblanc, N. Mercier, M. Allain, J. Dittmer, T. Pauporté, V. Fernandez, F. Boucher, M. Kepenekian, C. Katan, Enhanced Stability and Band Gap Tuning of α-[HC(NH₂)₂]PbI₃ Hybrid, ACS Appl. Mater. Interfaces, 11, 20743 (2019); C. Zheng, O. Rubel, M. Kepenekian, X. Rocquefelte, C. Katan, Electronic properties of Pb-I deficient lead halide perovskites, J. Chem. Phys. 151, 234704 (2019); C. Quarti, C. Katan and J. Even, Physical properties of bulk, defective, 2D and 0D metal halide perovskite semiconductors from a symmetry perspective, under revision.

Perovskites have emerged as low-cost, high efficiency photovoltaics with certified efficiencies of 25.2% approaching already established technologies. The perovskites used for solar cells have an ABX3 structure where the cation A is methylammonium (MA), formamidinium (FA), or cesium (Cs); the metal B is Pb or Sn; and the halide X is Cl, Br or I. Unfortunately, single-cation perovskites often suffer from phase, temperature or humidity instabilities. This is particularly noteworthy for CsPbX3 and FAPbX3 which are stable at room temperature as a photoinactive “yellow phase”instead of the more desired photoactive “black phase”that is only stable at higher temperatures. Moreover, apart from phase stability, operating perovskite solar cells (PSCs) at elevated temperatures (of 85 °C) is required for passing industrial norms. Recently, double-cation perovskites (using MA, FA or Cs, FA) were shown to have a stable “black phase”at room temperature.(1,2) These perovskites also exhibit unexpected, novel properties. For example, Cs/FA mixtures supress halide segregation enabling band gaps for perovskite/silicon or perovskite/perovskite tandems.(3) In general, adding more components increases entropy that can stabilize unstable materials (such as the “yellow phase”of FAPbI3 that can be avoided using the also unstable CsPbI3). Here, we take the mixing approach further to investigate triple cation (with Cs, MA, FA) perovskites resulting in significantly improved reproducibility and stability.(4) We then use multiple cation engineering as a strategy to integrate the seemingly too small rubidium (Rb) (that never shows a black phase as a single-cation perovskite) to study novel multication perovskites.(5) One composition containing Rb, Cs, MA and FA resulted in a stabilized efficiency of 21.6% and an electroluminescence of 3.8%. The Voc of 1.24 V at a band gap of 1.63 eV leads to a very small loss-in-potential of 0.39 V, one of the lowest measured on any PV material indicating the almost recombination-free nature of the novel compound. Polymer-coated cells maintained 95% of their initial performance at 85°C for 500 hours under full illumination and maximum power point tracking. This is a crucial step towards industrialisation of perovskite solar cells. Lastly, to explore the theme of multicomponent perovskites further, molecular cations were revaluated using a globularity factor. With this, we calculated that ethylammonium (EA) has been misclassified as too large. Using the multication strategy, we studied an EA-containing compound that yielded an open-circuit voltage of 1.59 V, one of the highest to date. Moreover, using EA, we demonstrate a continuous fine-tuning for perovskites in the "green gap" which is highly relevant for lasers and display technology. The last part elaborates on a roadmap on how to extend the multication to multicomponent engineering providing a series of new compounds that are highly relevant candidates for the coming years.(6) (1) Jeon et al. Nature (2015) (2) Lee et al. Advanced Energy Materials (2015) (3) McMeekin et al. Science (2016) (4) Saliba et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy & Environmental Science (2016) (5) Saliba et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science (2016). (6) Turren-Cruz et al. Methylammonium-free, high-performance and stable perovskite solar cells on a planar architecture Science (2018)

Organic-inorganic hybrid perovskite solar cells (PSCs) have gained tremendous attention as materials for photovoltaics due to their high efficiencies and their compatibility with low-cost low-temperature fabrication methods (such as solution processing). Perovskite film formation is complex, involving the formation of intermediates and/or metastable phases that strongly affect the final perovskite film microstructure. Therefore, understanding the mechanism of perovskite formation and the crystallization pathways is key for facile control of perovskite formation. We are using time-resolved x-ray scattering to investigate the perovskite formation of MAPbI₃-based perovskites and mixed cation (Cs, FA)PbI₃ perovskites in-situ during spin coating and the subsequent post-deposition treatments, where FA is formamidinium and MA is methylammonium.
For Cs0.15FA0.85PbI3 we study: i) the crystallization pathways during spin coating; ii) the subsequent post-deposition thermal annealing, and iii) crystallization during blade-coating. Our findings reveal that the formation of a non-perovskite FAPbI3 phase during spin coating is initially dominant regardless of the processing and that the processing treatments has a significant impact on the as-cast film structure and affects the phase evolution during subsequent thermal treatment. We show that blade-coating can be used to overcome the non-perovskite phase formation via solvothermal direct crystallization of perovskite phase. This work show how real-time investigation of perovskite formation can help to establish processing-microstructure-functionality relationships. Our work highlights the importance of real-time investigation of perovskite film formation which can aid in establishing processing-microstructure-functionality relationships and help to provide a fundamental understanding of the mechanisms of perovskite formation.

2D perovskites is a fascinating class of compounds where the 3D perovskite lattice is cut and spatially confined in two dimensions and an “inert” layer of intercalated organic ions is inserted between the layers. The 2D perovskites layers can be formed by the introduction of bulky organic cations that tend to interdigitate between the perovskite sheets. They exhibit a large compositional and structural phase space, which has led to novel and exciting physical properties. The 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. Because these materials have had a strong impact in how record breaking and stable solar cells are constructed, it is critical to understand the thermodynamic and chemical limitations of the maximum thickness that can be sandwiched between the organic bilayers while retaining the structural integrity of the 2D perovskite. What are the limits within which the 2D architecture that define the thickness of the inorganic slabs before the system diverts into the familiar 3D structure? To answer this fundamental question, we employed 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 application of these low dimensional semiconductors in stable photovoltaics will be discussed.

Many common crystal structures can be described by a single (or very few) repeated structural motifs (“monomorphous structures”) such as octahedron in cubic halide perovskites. Interestingly, recent accumulated evidence suggests that electronic structure calculations based on such macroscopically averaged monomorphous cubic (Pm-3m) halide perovskites obtained from x-ray diffraction, show intriguing deviations from experiment. These include systematically too small band gaps, dielectric constants dominated by the electronic, negative mixing enthalpy of alloys, and significant deviations from the measured pair distribution function. We show here that a minimization of the systems T = 0 internal energy via density functional theory reveals a distribution of different low-symmetry local motifs, including tilting, rotations, and B-atom displacements (“polymorphous networks”). This is found only if one allows for larger-than-minimal cell size that does not geometrically exclude low symmetry motifs. As the (super) cell size increases, the energy is lowered relative to the monomorphous cell, and stabilizes after ∼32 formula units (160 atoms) are included. Being a result of nonthermal energy minimization in the internal energy without entropy, this correlated set of displacements must represent the intrinsic geometry preferred by the underlying chemical bonding (lone pair bonding), and as such has a different origin than the normal, dynamic thermal disorder modeled by molecular dynamics. Indeed, the polymorphous network, not the monomorphous ansatz, is the kernel structure from which high temperature thermal agitation develops. The emerging physical picture is that the polymorphous network has an average structure with high symmetry, yet the local structural motifs have low symmetries. We find that, compared with monomorphous counterparts, the polymorphous networks have significantly lower predicted total energies, larger band gaps, and ionic dominated dielectric constants, and agree much more closely with the observed pair distribution functions. An analogous polymorphous situation is found in the paraelectric phase of a few cubic oxide perovskites where local polarization takes the role of local displacements in halide perovskites, and in the paramagnetic phases of a few 3d oxides where the local spin configuration takes that role. See PHYSICAL REVIEW B 101, 155137 (2020).

Multicomponent perovskite solar cells have reached the recent efficiency breakthrough of 25.2%, 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 relative humidity and time to drop the antisolvent during the preparation of perovskite films and 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].

With its rapidly increased cell efficiency to as high as over 25% and incredible suitability to process, equipment and operation, the perovskite is truly a rising star for next generation solar cells. In fact, its cell efficiency has surpassed all other thin film solar cells. We have studied a few different deposition technologies, including dry processing such as vacuum-based alternating precursor deposition, partial co-evaporation of inorganic components; dry atmospheric deposition by using direct contact reactive process; and wet solution processing such as small area spin-coating, large-area blade coating and high-throughput slot-die coating. At present, the wet solution deposition processes are well-advanced with significantly higher efficiencies over 23%. In comparison, the vacuum processes are under development with best cell efficiency still lower than 22%. This article is intended to compare and opt for the best one with most promises for future largescale processing by screening these technologies developed in research laboratories. With mature industrial equipment and engineering processes, we improved the vacuum deposited perovskite cell efficiency to 21.32%, highest for this type to our knowledge. In addition, the films produced using dry deposition seem to show better stability against high humidity, temperature and light-soaking.

Highly efficient perovskite solar cells (PSCs) with power conversion efficiencies (PCEs) > 25% are fabricated with formamidinium (FA)-based lead triiodide (FAPbI₃) perovskites. In general, mixed cations and anions containing FA, methylammonium (MA), Cs, I and Br ions are used to stabilize the black α-phase against d-phase of the pristine FAPbI₃. Disadvantageously, the additives such as MA, Cs and Br to FAPbI₃ widen its bandgap and reduce the thermal stability due to the lower thermal decomposition of MA to FA. Therefore, in order to further increase PCE through the enhanced photocurrent density due to the increased light harvesting, stabilizing the a-FAPbI₃ without blue-shift of the bandgap is needed. In addition, structural dimensions, distortion, and dielectric properties can vary significantly depending on the ionic radius and type of the A-site cation. In this presentation, I would like to introduce the impact of cationic management on the efficiency and long-term stability of perovskite solar cells.

In this talk, I will present our recent progress in understanding the fundamental properties of metal halide perovskites and the related device developments. First, the nature of defects induced charge traps and doping in perovskite materials will be discussed, and their chemical origin will be analyzed. A recent development in characterizing the spatial and energetic distributions of defects will be presented, which revealed the significant difference in trap density between single crystals and polycrystalline films, but similarity in distribution of the traps. Second, the healing of defects and tuning the doping in perovskites by multiple processes will be presented that boost the efficiency and stability of perovskite solar cells and modules. TFinally, a new charge recombination layer for all perovskite tandem solar cells will be presented which not only simply the fabrication process, but also significantly enhance tandem solar cell efficiency and stability.

An overview of Hunt Perovskite Technologies, LLC (HPT) metal halide perovskite (MHP) PV research program will be presented. Founded in 2013, HPT has focused their research on stabilizing the MHP material through better chemistry, as highlighted by stabilizing the pure alpha-phase FAPbI₃ MHP material in 2015. In this materials study, we have come to appreciate the unique defect structure of the MHP material by way of its ionic composition. Descriptions of MHP macro and molecular defect structure will be discussed, as well as their effect on the material’s durability and PV device performance.

Although known for many years, organic-inorganic hybrid and related inorganic halide-based perovskites have received extraordinary attention recently, in part due to the unique physical properties and chemical diversity offered by these structures, which make them outstanding candidates for applications in photovoltaic and other semiconductor devices (especially for systems based on Pb, Sn and Ge). Perovskite structures consist of networks of corner-sharing metal halide octahedra that can extend in three or lower dimensions, providing opportunity to probe how dimensionality of the inorganic framework impacts semiconducting character. Further, incorporation of more complex organic cations within lower-dimensional perovskites enables combining outstanding inorganic-derived semiconducting character with organic-derived properties (e.g., luminescence, chirality), a feature extending well-beyond what is attainable with more established inorganic semiconductors (e.g., Si or GaAs) and underlying the creation of multifunctional organic-inorganic compounds and electronics [1]. This talk will emphasize chemical/structural versatility [2] and implications for semiconductor design (both with regards to properties and processing), as well as new characterization approaches for elucidating materials properties. Examples of topics that may be covered include custom-designed organic cations that template or stabilize the formation of difficult-to-realize lead-free perovskite structures [3], merging computational/experimental approaches to predict/verify optoelectronic character and guide design of complex organic-inorganic hybrids [4] and the use of a new carrier-resolved photo-Hall (CRPH) technique [5] to explore the impact of additive engineering on majority/minority carrier properties in perovskite films. Outstanding functionality and versatile processing provide two pillars for future application and study of this exciting and evolving materials family. References: [1] D. B. Mitzi, K. Chondroudis, C. R. Kagan, IBM J. Res. & Dev. 45, 29 (2001). [2] B. Saparov, D. B. Mitzi, Chem. Rev. 116, 4558 (2016). [3] M. K. Jana, S. M. Janke, D. J. Dirkes, S. Dovletgeldi, C. Liu, X. Qin, K. Gundogdu, W. You, V. Blum, D. B. Mitzi, J. Am. Chem. Soc. 141, 7955 (2019). [4] W. A. Dunlap-Shohl, E. T. Barraza, A. Barrette, S. Dovletgeldi, G. Findik, D. J. Dirkes, C. Liu, M. K. Jana, V. Blum, W. You, K. Gundogdu, A. D. Stiff-Roberts, D. B. Mitzi, Mater. Horiz. 6, 1707 (2019). [5] 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, https://doi.org/10.1038/s41586-019-1632-2 (2019).

Perovskite quantum dots (QDs) as a new type of colloidal nanocrystals have gained significant attention for both fundamental research and commercial applications owing to their appealing optoelectronic properties and excellent chemical processability.¹ For their wide range of potential applications, synthesizing colloidal QDs with high crystal quality is of crucial importance. However, like most common QD systems, those reported perovskite QDs still suffer from a certain density of trapping defects, giving rise to detrimental non-radiative recombination centers and thus low photoluminescence quantum yield (PL QY). Halide vacancy in perovskites has been widely accepted to be the main factor preventing near-unity PL QY by creating uncoordinated Pb octahedra and the subsequent localized deep trap states.² Very recently, we have succeeded in synthesis of phase-stable and less-defect perovskite QDs, including FAPbI₃ QDs, CsPbI₃ QDs and Sn-Pb alloyed QDs.^3-6 We have demonstrated that a high room-temperature PL QY of up to 100% can be obtained in FAPbI₃ and CsPbI₃ perovskite QDs, signifying the achievement of almost complete elimination of the trapping defects. This is realized by the modulation of halide precursor involved in the synthesis, in which trioctylphosphine (TOP) is used to dissolve halide salts. For Sn-Pb alloyed QDs, although the alloyed QDs possess much improved structural stability than that of the pure Pb- and Sn-based one, they still suffer from an extremely low PL QY of ~0.3%, indicative of the severe charge trapping in these Sn-containing perovskites. Such low PL QY is believed to result from the widely known facile oxidation of Sn²⁺ to Sn⁴⁺. To address this issue, we have demonstrated very recently that the deep-level trap states in Sn-based perovskites can be well suppressed by Na⁺ doping, which gives rise to a dramatically improved PL QY of ~28% without alerting their favorable electronic structure. X-ray photoelectron spectroscopy (XPS) studies suggest the formation of a stronger chemical interaction between I^- and Sn²⁺ ions upon Na doping. On the other hand, though the reported TOP-based route has demonstrated efficacy in producing CsSn_xPb_1-xI₃ QDs (x = 0~0.6) with superior structural stability, such synthetic protocol can only find success in making these I-related QDs, preparing those pure Cl- and Br-based analogues remains challenging because of the insufficient solubility of PbBr₂ and PbCl₂ in TOP. Therefore, by taking advantage of the strong coordinating ability of trioctylphosphine oxide (TOPO), we have recently reported the first synthesis of ASn_xPb_1-xX₃ QDs (A = Cs, FA, MA) with excellent tunability in both Sn/Pb ratio and a full range of halide covering. The ability to generate all possible, unique combinations has offered great material versatility for the perovskite QD family. Further, by employing ultrafast spectroscopic techniques, we have resolved the fundamental photoexcited charge carrier dynamics in these alloyed QDs, which shall pave the way for future study for enhancing their usefulness in optoelectronic applications.^7-9 We expect the successful synthesis of the “ideal” perovskite QDs will exert profound influence on their applications to both QD-based light-harvesting and -emitting devices in the near future.

1. Swarnkar, A. et al., Science 2016, Vol. 354, pp. 92-95.
2. Alivisatos et al., J. Am. Chem. Soc. 2018, Vol. 140, pp. 17760.
3. F. Liu, Q. Shen et al., ACS Nano, Vol. 11, No. 10, pp. 10373-10383, 2017.
4. F. Liu, Q. Shen et al., J. Am. Chem. Soc., Vol. 139, No. 46, pp. 16708-16719, 2017.
5. F. Liu, Q. Shen et al., J. Phys. Chem. Lett., Vol. 9, No. 2, pp. 294-297, 2018.
6. F. Liu, Q. Shen et al., Chem. Mater., Vol. 31, No. 3, pp. 798-807, 2019.
7. F. Liu, Q. Shen et al., Angew. Chem. Int. Ed., Vol. 59, pp. 1-5, 2020.
8. F. Liu, Q. Shen et al., Chem. Mater., Vol. 32, No. 3, pp. 1089-1100, 2020.
9. C. Ding, Q. Shen et al., Nano Energy 2020, 67, 104267.

Two-dimensional halide perovskites are exciting new semiconductors that show great promising in low cost and high-performance optoelectronics devices including solar cells, LEDs, photodetectors, transistors, etc. In the first part of this talk, I will present a molecular approach to the synthesis of high-quality organic-inorganic hybrid perovskite quantum wells through incorporating widely tunable organic semiconducting building blocks as the surface capping ligands. By introducing sterically tailored groups into the molecular motif, the strong self-aggregation of the conjugated organic molecules can be suppressed, and single crystalline organic-perovskite hybrid quantum wells and superlattices can be easily obtained via one-step solution-processing. Energy transfer and charge transfer between adjacent organic and inorganic layers are extremely fast and efficient, owing to the atomically-flat interface and ultra-small interlayer distance. In the second part, I will present the synthesis and characterization of a stable lead-free Sn (II)-based two-dimensional perovskite featuring a p-conjugated oligothiophene ligand. The conjugated ligands facilitate formation of micrometer-size large grains, improve charge injections, and stabilize the inorganic perovskite layers. Thin film field-effect transistors based on (4Tm)2SnI4 exhibit enhanced hole mobility up to 2.3 cm2V-1s-1, and dramatically improved stability over the previous benchmark material (PEA)2SnI4. Stabilization mechanisms were investigated via single crystal structure analysis as well as density functional theory calculations. It was found that the large conjugated organic layers not only serve as thick and dense barriers for moisture and oxygen, but also increase the crystal formation energy via strong intermolecular interactions.

In the last years, the field of solution-processed solar cells based on inorganic semiconductors has seen a great progress, showing the highest efficiencies among the emerging photovoltaic technologies. Indeed, of quantum dot solar cells have reached an efficiency over 16%, and solar cells based on perovskites have surpassed 25% efficiency. However, the future development of these cells could be limited due to the presence of highly toxic elements in their composition, more specifically lead (Pb). Efforts are thus required in searching for earth-abundant, and non-toxic materials. Inorganic colloidal nanocrystals (NCs) are attractive materials for their use as active layers in quantum dot solar cells. They are generally composed of earth-abundant elements, and they can be processed from solutions, allowing an easy and cheap fabrication of devices. Moreover, they have additional attractive properties, like high absorption coefficients, tunable bandgaps, and band energy level engineering through ligand exchange. In this context, bismuth-based colloidal NCs, appear as an attractive alternative for environmentally friendly, low cost solar cells. We have developed ways to modify the size and shape of the bismuth-based nanocrystals, and we have observed how this affects their optoelectronic properties. We have studied the modulation of Fermi level and energy bands introducing different ligands on the surface of the NCs. We have employed these NCs to fabricate solution-processed solar cells, focusing on completely non-toxic devices, achieving a certified efficiency of more than 6%, employing very thin layers (~35 nm) of AgBiS2 NCs. A summary of these results will be presented and future directions will be discussed.

Organic lead halide perovskite based solar cells (HPSCs) have achieved certified power conversion efficiency (PCE) over 25.0%. Despite the high efficiency achieved, there are still many concerns about the large-scale applicability of this solar cells because of the water soluble toxic Pb2+. The simpler approach to address this issue is to find a benign or less toxic metal to replace the lead atom in the perovskite structure, with the resulting perovskite still showing the excellent optical and electrical properties. Theoretically, tin based perovskite holds the promise to produce similar or even higher PCE compared to their Pb counterpart. However, for relatively long time the tin-based HPSCs held a PCE lower than 7% though intensive research efforts were devoted to their investigation. The facile formation of tin vacancies and easy oxidation of Sn2+ have been identified as the main reason determining the low PCE. In my presentation I will report as a small amount of 2D tin perovskite templates the growth of highly crystalline and oriented 3D FASnI3 grains (2D/3D mixtures), suppressing effectively the appearance of tin vacancies and Sn2+ oxidation. As a consequence of the reduced background charge carrier density and trap assisted charge recombination, the device showed a 50% improvement in the PCE (up to 9%) compared to that using pure 3D tin perovskite. We further succeeded reducing the defects in the 2D/3D tin perovskite films by adding ethylammonium iodide (EAI) into the corresponding perovskite precursor solution. These films display larger crystalline grains and a more compact and uniform film morphology when compared to their counterparts without EA cation. These features lead to a much lower trap density, background charge carrier density and charge recombination loss in the corresponding devices. Results, which allow hopping for future highly efficient Sn-based hybrid perovskite solar cells.

Data-sceince methods, including advanced statistics and machine learning, can help researchers diagnose, optimize, and design more efficiently. In this talk, I'll summarize the MIT PVLab's recent progress optimizing bare films and record-efficiency perovskite devices with capping layers using data-science methods. These include combining density-functional theory (DFT) and Bayesian inference to accelerate the search for stable compositions, and optimizing capping layers for combined performance and stability. Lastly, I'll highlight our progress searching for lead-free perovskite-inspired, defect-tolerant semiconductors.

Defect tolerance has emerged as a central theme for the success of halide perovskites as photovoltaics. Some current estimates put the number of mobile ionic defects in common halide perovskites above 1017 cm-3, yet the halide perovskite band structure and resulting shallow nature of the hypothesized dominant defects limits their ability to act as recombination centers in these materials. However, this defect tolerance does not imply that these ionic defects have no impact on device performance. Current-voltage hysteresis, reverse-bias breakdown, photo-brightening, photodegradation, thermal stability, and other behaviors related to perovskite performance have all been attributed to the appreciably ionic transport in perovskites made possible by this large defect population. In this work I will present recent advances in both the development of experimental methods for analyzing the formation and behavior of mobile ions in halide perovskites, and our understanding of the effects of these mobile ions on device performance. I will discuss methods to investigate defect formation and transport, including our results using in situ photoluminescence and X-ray diffraction techniques to track ion movement and correlate that with the optoelectronic properties of methylammonium perovskites. Additionally, I will share our work looking at the impacts of mobile ions on device performance, particularly with regards to limits on the open-circuit voltage of perovskite solar cells. Finally, I will highlight the unique roles that contacts and surfaces can play in optimizing the performance and stability of perovskite solar cells in light of these mobile ionic defects.

The certified efficiency of Pb-perovskite solar cells with more than 1 cm2 area is reported to be 20.9%(1). In the smaller cell, the cell efficiency with 25.2% has been reported, which is close to 22.3 % of multi Si solar cells (2). However, the use of Pb in electronic devices is limited according to the EU RoHS Directive. Thus, it is needed to develop Pb-free perovskite solar cells. Among Pb-free perovskite candidates, Sn perovskite is one of the most promising candidates for the light harvesting layer for the Pb-free perovskite, because they have perovskite structure similar to Pb-perovskite. However, the efficiency iwas not as high as those for Pb-perovslite solar cells. We have already reported that the efficiency was enhanced by Ge2+ doping to the Sn-perovskite layer, because the carrier density in the Sn-perovskite layer was reduced (3,4). Now, we discuss the further enhancement of the Sn-perovskite solar cells from the view point of the lattice strain (disordering) relaxation and the surface passivation on the perovskite slayer to show the direction of the research. We report post-treatment of Sn-perovskite layer with Lewis base (NH2-CH2-CH2-NH2) to suppress the recombination reaction in Sn-perovskite solar cells resulting in efficiencies of 10.2 %. Upon analyzing the X-ray photoelectron spectroscopy and impedance spectroscopy data, we came to the conclusion that the amine-group bonded the under-coordinated tin, passivating the dangling bonds and defects resulting in suppressed charge carrier recombination (5). This also has the effect of prolonged lifetime and higher charge carrier mobility. These findings provide the groundwork for improving the efficiency of Sn-halide PSCs to compete with that of lead-based PSCs by simple post-treatment with organic molecules (6). We and other research groups have reported that the efficiency of Sn-perovskite solar cells increases spontaneously during the storage, which we explained by the perovskite lattice strain (disordering) relaxation. This prompted us to examine the relationship between between relative lattice strain and photovoltaic performances. It is shown that the efficiency of the Sn-perovskite solar cells has good relationship with the lattice strain (disordering). By decreasing the lattice strain (disordering), Ge doped Sn-perovskite solar cells with 11.7% was prepared. The mechanism wil be discussed. 1. M.A. Green et.al., Progress in Photovoltaics, 2019;27:565-575, version 54, 2. NREL efficiency Table, https://www.nrel.gov/pv/cell-efficiency.html 3. N. Ito, S. Hayase, et al., Journal of Physical Chemistry Letters, 9, p.1682-1688, 2018. 4. C. H. Ng, S. Hayase, et.al., Nano Energy, https://doi.org/10.1016/j.nanoen.2019.01.026, 2019 5. M. A. Kumarudin, S. Hayase, et al., J. Phys. Chem., Lett., DOI:10.1021/acs jpclett.9b02024, 2019. 6. K. Nishimura, S. Hayase, et al., ACS Appl. Mater. Interfaces, https://doi.org/10.1021/acsami.9b09564 2019.