Polarization Dependence of Water Adsorption to CH3NH3PbI3 (001) Surfaces

Zero Threshold for Water Adsorption on MAPbBr3

Hybrid organic-inorganic perovskites (HOIPs) have shown great promise in a wide range of optoelectronic applications. However, this performance is inhibited by the sensitivity of HOIPs to various environmental factors, particularly high levels of relative humidity. This study uses X-ray photoelectron spectroscopy (XPS) to determine that there is essentially no threshold to water adsorption on the in situ cleaved MAPbBr3 (001) single crystal surface. Using scanning tunneling microscopy (STM), it shows that the initial surface restructuring upon exposure to water vapor occurs in isolated regions, which grow in area with increasing exposure, providing insight into the initial degradation mechanism of HOIPs. The electronic structure evolution of the surface was also monitored via ultraviolet photoemission spectroscopy (UPS), evidencing an increased bandgap state density following water vapor exposure, which is attributed to surface defect formation due to lattice swelling. This study will help to inform the surface engineering and designs of future perovskite-based optoelectronic devices.

A moderate intensity ligand works best: a theoretical study on passivation effects of pyridine-based molecules for perovskite solar cells

Improving battery stability while maintaining high photoelectric conversion efficiency remains the bottleneck in the current development of perovskite solar cells (PSCs). Three π-conjugated pyridine-based molecules, pyridine (Py), bipyridine (Bpy), and terpyridine (Tpy), were adopted to passivate the PSCs in recent experiments (J. Chen, S.-G. Kim, X. Ren, H. S. Jung and N.-G. Park, J. Mater. Chem. A, 2019, 7, 4977-4987; J. Zhang, J. Duan, Q. Zhang, Q. Guo, F. Yan, X. Yang, Y. Duan and Q. Tang, Chem. Eng. J., 2022, 431, 134230), in which Bpy works best in terms of photovoltaic properties and moisture tolerance. In this work, based on density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations, we demonstrate that Bpy displays the least structural fluctuation when adsorbed on the perovskite surface, enlarges the bandgap suppressing electron-hole recombination, and exhibits remarkable shielding effects against moisture. The appropriate anchoring ability of Bpy retains robust binding strength and preferable charge transfer ability compared to Py at the interfaces between the passivation molecules (PMs) and MAPbI₃. In contrast, although Tpy possesses the strongest charge-transfer capability, it introduces midgap states owing to intense electronegativity, providing additional pathways for nonradiative charge relaxation. Besides, Tpy triggers rapid diffusions of water and larger atomic fluctuations, destroying the structures of the perovskite through the removal of lead atoms. Our computational results not only rationalize the experimental observations but also provide valuable guidance at the atomic level to design novel PMs that endow PSCs with outstanding photovoltaic performance as well as stability against moisture.

A Review on Halide Perovskite-Based Photocatalysts: Key Factors and Challenges

A growing number of research articles have been published on the use of halide perovskite materials for photocatalytic reactions. These articles extend these materials' great success from solar cells to photocatalytic technologies such as hydrogen production, CO₂ reduction, dye degradation, and organic synthesis. In the present review article, we first describe the background theory of photocatalysis, followed by a description on the properties of halide perovskites and their development for photocatalysis. We highlight key intrinsic factors influencing their photocatalytic performance, such as stability, electronic band structure, and sorption properties. We also discuss and shed light on key considerations and challenges for their development in photocatalysis, such as those related to reaction conditions, reactor design, presence of degradable organic species, and characterization, especially for CO₂ photocatalytic reduction. This review on halide perovskite photocatalysts will provide a better understanding for their rational design and development and contribute to their scientific and technological adoption in the wide field of photocatalytic solar devices.

Stability of Tin- versus Lead-Halide Perovskites: Ab Initio Molecular Dynamics Simulations of Perovskite/Water Interfaces

Tin-halide perovskites (THPs) have emerged as promising lead-free perovskites for photovoltaics and photocatalysis applications but still fall short in terms of stability and efficiency with respect to their lead-based counterpart. A detailed understanding of the degradation mechanism of THPs in a water environment is missing. This Letter presents ab initio molecular dynamics (AIMD) simulations to unravel atomistic details of THP/water interfaces comparing methylammonium tin iodide, MASnI₃, with the lead-based MAPbI₃. Our results reveal facile solvation of surface tin-iodine bonds in MASnI₃, while MAPbI₃ remains more robust to degradation despite a larger amount of adsorbed water molecules. Additional AIMD simulations on dimethylammonium tin bromide, DMASnBr₃, investigate the origins of their unprecedented water stability. Our results indicate the presence of amorphous surface layers of hydrated zero-dimensional SnBr₃ complexes which may protect the inner structure from degradation and explain their success as photocatalysts. We believe that the atomistic details of the mechanisms affecting THP (in-)stability may inspire new strategies to stabilize THPs.

Quantitative Predictions of Moisture-Driven Photoemission Dynamics in Metal Halide Perovskites via Machine Learning

Metal halide perovskite (MHP) photovoltaics may become a viable alternative to standard Si-based technologies, but the current lack of long-term stability precludes their commercial adoption. Exposure to standard operational stressors (light, temperature, bias, oxygen, and water) often instigate optical and electronic dynamics, calling for a systematic investigation into MHP photophysical processes and the development of quantitative models for their prediction. We resolve the moisture-driven light emission dynamics for both methylammonium lead tribromide and triiodide thin films as a function of relative humidity (rH). With the humidity and photoluminescence time series, we train recurrent neural networks and establish their ability to quantitatively predict the path of future light emission with 18% error over 4 h. Together, our in situ rH-PL measurements and machine learning forecasting models provide a framework for the rational design of future stable perovskite devices and, thus, a faster transition toward commercial applications.

Toward stable lead halide perovskite solar cells: A knob on the A/X sites components

Hybrid lead halide ABX3 perovskite solar cells (PSCs) have emerged as a strong competitor to the traditional solar cells with a certified power conversion efficiency beyond 25% and other remarkable features such as light weight, solution processability, and low manufacturing cost. Further development on the efficiency and stability brings forth increasing attention in the component regulation, such as partial or entire substitution of A/B/X sites by alternative elements with similar size. However, the relationships between composition, property, and performance are poorly understood. Here, the instability of PSCs from the photon-, moisture-, thermal-, and mechanical-induced degradation was first summarized and discussed. In addition, the component regulation from the A/X sites is highlighted from the aspects of band level alignment, charge-carrier dynamics, ion migration, crystallization behavior, residual strain, stoichiometry, and dimensionality control. Finally, the perspectives and future outlooks are highlighted to guide the rational design and practical application of PSCs.

Atomic Level Insights into Metal Halide Perovskite Materials by Scanning Tunneling Microscopy and Spectroscopy

Metal halide perovskite materials (MHPMs) have attracted significant attention because of their superior optoelectronic properties and versatile applications. The power conversion efficiency of MHPM solar cells (PSCs) has skyrocketed to 25.5 %. Although the performance of PSCs is already competitive, several important challenges still need to be solved to realize commercial applications. A thorough understanding of surface atomic structures and structure-property relationships is at the heart of these remaining issues. Scanning tunneling microscopy (STM) and spectroscopy (STS) can be used to characterize the surface properties of MHPMs, which can offer crucial insights into MHPMs at the atomic scale. This Review summarizes recent progress in STM and STS studies on MHPMs, with a focus on the surface properties. We provide understanding from the comparative perspective of several different MHPMs. We also highlight a series of novel phenomena observed by STM and STS. Finally, we outline a few research topics of primary importance for future studies.

Polarization-Controlled Surface Defect Formation in a Hybrid Perovskite

Hybrid perovskites have two properties that are absent in traditional inorganic photovoltaic materials, namely, polarization and mobile ionic defects, the interaction between which may introduce new features into the materials. By using the first-principles calculations, we find that the formation energies of the vacancy defects at a tetragonal MAPbI₃(110) surface are highly related to the surface polarization. The positive total polarization and local polarization of MA facilitate the formation of surface MA vacancies, whereas the negative total polarization and local polarization of MAI are favorable for the formation of surface iodine vacancies. The phenomena can be explained quantitatively on the basis of the two kinds of Coulomb interactions between the charged defect and the polarization-induced electrostatic field. The comprehensive insights into the interaction between the polarization and the ionic defects in hybrid perovskites can provide a new avenue for defect control for high-performance perovskite solar cells via surface polarization.

A review of experimental and computational attempts to remedy stability issues of perovskite solar cells

Photovoltaic technology using perovskite solar cells has emerged as a potential solution in the photovoltaic makings for cost-effective manufacturing solutions deposition/coating solar cells. The hybrid perovskite-based materials possess a unique blend from low bulk snare concentrations, ambipolar, broad optical absorption properties, extended charge carrier diffusion, and charge transport/collection properties, making them favourable for solar cell applications. However, perovskite solar cells devices suffer from the effects of natural instability, leading to their rapid degradation while bared to water, oxygen, as well as ultraviolet rays, are irradiated and in case of high temperatures. It is essential to shield the perovskite film from damage, extend lifetime, and make it suitable for device fabrications. This paper focuses on various device strategies and computational attempts to address perovskite-based solar cells' environmental stability issues.

Competing Dissolution Pathways and Ligand Passivation-Enhanced Interfacial Stability of Hybrid Perovskites with Liquid Water

Material instability issues, especially moisture degradation in ambient operating environments, limit the practical application of hybrid perovskite in photovoltaic and light-emitting devices. Very recent experiments demonstrate that ligand passivation can effectively improve the surface moisture tolerance of hybrid perovskites. In this work, the interfacial stability of as-synthesized pristine and alkylammonium-passivated methylammonium lead iodide (MAPbI₃) with liquid water is systematically investigated using molecular dynamics simulations and reaction kinetics models. Interestingly, the more hydrophilic [PbI₂]⁰ surface is more stable than the less hydrophilic [MAI]⁰ surface because of the higher polarity of the former surface. Linear alkylammoniums significantly stabilize the [MAI]⁰ surface with highly reduced (by 1-2 orders of magnitude) dissociation rates of both MA⁺ and ligands themselves, while branched ligands, surprisingly, lead to higher dissociation rates as the surface coverage increases. Such anomalous behavior is attributed to the aggregation-assisted dissolution of surfactant-like ligands as micelles during the degradation process. Short-chain linear alkylammonium at the full surface coverage is found to be the optimal ligand to stabilize the [MAI]⁰ surface. This work not only provides fundamental insights into the ionic dissolution pathways and mechanisms of hybrid perovskites in water but also inspires the design of highly stable hybrid perovskites with ligand passivation layers. The computational framework developed here is also transferrable to the investigation of surface passivation chemistry for weak ionic materials in general.

Organic Cations Protect Methylammonium Lead Iodide Perovskites against Small Exciton-Polaron Formation

Working organic-inorganic lead halide perovskite-based devices are notoriously sensitive to surface and interface effects. Using a combination of density functional theory (DFT) and time-dependent DFT methods, we report a comprehensive study of the changes (with respect to the bulk) in geometric and electronic structures going on at the (001) surface of a (tetragonal phase) methylammonium lead iodide perovskite slab, in the dark and upon photoexcitation. The formation of a hydrogen bonding pattern between the -NH₃ groups of the organic cations and the iodine atoms of the outer inorganic layout is found to critically contribute to the relative thermodynamic stability of slabs with varying surface compositions and terminations. Most importantly, our results show that the hydrogen bond locking effects induced by the MA groups tend to protect the external two-dimensional lattice against large local structural deformations, i.e., the formation of a small exciton-polaron, at variance with purely inorganic lead halide perovskites.

Microscopic investigations on the surface-state dependent moisture stability of a hybrid perovskite

Hybrid organic-inorganic perovskite (HOIP) materials have caught significant attention in photovoltaics and photoelectronics for their outstanding photovoltaic properties. However, their instability to various environment, such as illumination, temperature, moisture and oxygen, hinders their way to commercialization. To figure out the interaction mechanism between H₂O and CH₃NH₃PbI₃ (MAPbI₃), extensive theoretical studies have been carried out; however, the experimental results are insufficient and inconsistent. Here, we systematically investigate and compare the influence of H₂O on MAPbI₃ perovskite films with or without DMF) post-annealing in dark or light condition. The interaction between H₂O and the surface of pristine MAPbI₃ leads to the fusion of grain boundaries thus grain growth into micron level in short-time moisture exposure. While the penetration of H₂O into MAPbI₃ results in swelled crystalline whisker, cracking into smaller grains in long-time exposure upon the release of H₂O. However, no degradation occurs in dark condition. As the DMF post-annealing treatment changes the surface states of MAPbI₃, the interactions between the external H₂O and internal MAPbI₃ significantly varies from the pristine MAPbI₃. Three different surface states with different topographies have influence on the interaction process and mechanism with H₂O, leading to different decomposition rates, the striped surface that is the most rough among the three and experiencing the minimum change in surface potential with exposure to 80% humidity decomposes into PbI₂ fastest. However, the addition of light will once again affect the aforementioned process. It is found that even ambient light could severely speed up the moisture-induced decomposition of MAPbI₃, while the N,N-dimethylformamide (DMF) post-annealing treatment significantly improves the stability of MAPbI₃ films upon exposure to humidity and illumination, benefiting from the MAI-deficient thus H₂O resistant surface.

Suppressed phase transition of a Rb/K incorporated inorganic perovskite with a water-repelling surface

Inorganic cesium lead halide (CsPbI3) is a promising candidate for next-generation photovoltaic devices, but photoactive α-phase CsPbI3 can rapidly transform to non-photoactive yellow δ-CsPbI3 in a humid atmosphere. Here, we report that partial substitution of cesium by the potassium or rubidium element can effectively improve the phase stability against moisture by forming a water-repelling surface layer with Rb/K segregation. Using density functional theory, we found that the water-induced polarization, which triggers the PbI62- octahedron distortion and accelerates the phase transition, can be effectively alleviated by incorporating Rb/K elements. Further exploration of transition states suggests that Rb/K doped surface layers result in a higher activation barrier for water penetration. The electronic structure analysis further reveals that the barrier enhancement originates from the absence of the participation of inner 5p electrons in Rb/K-H2O binding, which induces a much lower energy barrier in pristine CsPbI3. Based on these improvements, the doped perovskites remained in the major α-phase after direct exposure to ambient air (RH ∼ 30%) for 5 hours, while pristine CsPbI3 showed an irreversible degradation. With the clarified mechanism of enhanced phase stability of Rb/K incorporation, we suggest such a doping method as a promising strategy to be widely applied in the field of photovoltaic devices.

Crystalline Clear or Not: Beneficial and Harmful Effects of Water in Perovskite Solar Cells

Clarification of how water affects the photovoltaic performance of perovskite solar cells is one of the major challenges to successfully develop a large-scale low-cost fabrication process. Many authors have reported beneficial effects of moisture during the fabrication of perovskite solar cells (PSCs), such as enhanced crystallinity, photoluminescence and photovoltage. However, the highest power conversion efficiency reported until this date was obtained under completely dry atmosphere conditions, avoiding the presence of water during perovskite formulation and preserving the damage caused by moisture exposure with encapsulation techniques. This apparent contradiction makes patent the necessity of an extensive clarification to establish the conditions in which water represents a beneficial or harmful factor in the development of high efficiency and stable perovskite devices. In this review, we summarized the effects of water, both as an additive into the perovskite formulation as an additive and as moisture exposure during fabrication. We discuss in depth the structural and chemical effects, analysing also the photovoltaic consequences during operation conditions. As a final input, we remark a useful method to perform high efficiency PSCs under different lab ambient conditions and highlight the latest advances in hydrophobic devices and encapsulation techniques.

Role of Water in Suppressing Recombination Pathways in CH3NH3PbI3 Perovskite Solar Cells

Moisture degradation of halide perovskites is the Achilles heel of perovskite solar cells. A surprising revelation in 2014 about the beneficial effects of controlled humidity in enhancing device efficiencies overthrew established paradigms on perovskite solar cell fabrication. Despite the extensive studies on water additives in perovskite solar cell processing that followed, detailed understanding of the role of water from the photophysical perspective remains lacking; specifically, the interplay between the induced morphological effects and the intrinsic recombination pathways. Through ultrafast optical spectroscopy, we show that both the monomolecular and bimolecular recombination rate constants decrease by approximately 1 order with the addition of an optimal 1% H₂O by volume in CH₃NH₃PbI₃ as compared to the reference (without the H₂O additive). Correspondingly, the trap density reduces from 4.8 × 10¹⁷ cm^-3 (reference) to 3.2 × 10¹⁷ cm^-3 with 1% H₂O. We obtained an efficiency of 12.3% for the champion inverted CH₃NH₃PbI₃ perovskite solar cell (1% H₂O additive) as compared to the 10% efficiency for the reference cell. Increasing the H₂O content further is deleterious for the device. Trace amounts of H₂O afford the benefits of surface trap passivation and suppression of trap-mediated recombination, whereas higher concentrations result in a preferential dissolution of methylammonium iodide during fabrication that increases the trap density (MA vacancies). Importantly, our study reveals the effects of trace H₂O additives on the photophysical properties of CH₃NH₃PbI₃ films.

Understanding structures and properties of phosphorene/perovskite heterojunction toward perovskite solar cell applications

Two-dimensional black phosphorus (phosphorene) has drawn much attention in recent years due to its excellent electronic and optical properties. In this manuscript, we employ ab initio calculations to investigate the structural origin of the phosphorene/perovskite heterostructure. The calculations suggest that the chemical stability and the mechanical stability depend on the surface terminations, and the mechanical stability of the phosphorene/perovskite heterojunction should be further improved. The weak interactions between the P atoms in the phosphorene and the under-coordinated Pb atoms at the perovskite surfaces, as well as the weak interfacial charge transfer characters, are proposed to be mainly responsible for the moderate heterostructure stability. Suggestions to improve the stability of the heterojunction are provided. This study helps the fundamental understanding of the interaction between the phosphorene and the halide perovskite materials, and could provide a foundation for the better understanding of the low-dimensional materials in perovskite-based optoelectronic devices.

Halide Perovskites: Is It All about the Interfaces?

Design and modification of interfaces, always a critical issue for semiconductor devices, has become a primary tool to harness the full potential of halide perovskite (HaP)-based optoelectronics, including photovoltaics and light-emitting diodes. In particular, the outstanding improvements in HaP solar cell performance and stability can be primarily ascribed to a careful choice of the interfacial layout in the layer stack. In this review, we describe the unique challenges and opportunities of these approaches (section 1). For this purpose, we first elucidate the basic physical and chemical properties of the exposed HaP thin film and crystal surfaces, including topics such as surface termination, surface reactivity, and electronic structure (section 2). This is followed by discussing experimental results on the energetic alignment processes at the interfaces between the HaP and transport and buffer layers. This section includes understandings reached as well as commonly proposed and applied models, especially the often-questionable validity of vacuum level alignment, the importance of interface dipoles, and band bending as the result of interface formation (section 3). We follow this by elaborating on the impact of the interface formation on device performance, considering effects such as chemical reactions and surface passivation on interface energetics and stability. On the basis of these concepts, we propose a roadmap for the next steps in interfacial design for HaP semiconductors (section 4), emphasizing the importance of achieving control over the interface energetics and chemistry (i.e., reactivity) to allow predictive power for tailored interface optimization.

Fractional deviations in precursor stoichiometry dictate the properties, performance and stability of perovskite photovoltaic devices

The last five years have witnessed remarkable progress in the field of lead halide perovskite materials and devices. Examining the existing body of literature reveals staggering inconsistencies in the reported results among different research groups with a particularly wide spread in the photovoltaic performance and stability of devices. In this work we demonstrate that fractional, quite possibly unintentional, deviations in the precursor solution stoichiometry can cause significant changes in the properties of the perovskite layer as well as in the performance and stability of perovskite photovoltaic devices. We show that while the absorbance and morphology of the layers remain largely unaffected, the surface composition and energetics, crystallinity, emission efficiency, energetic disorder and storage stability are all very sensitive to the precise stoichiometry of the precursor solution. Our results elucidate the origin of the irreproducibility and inconsistencies of reported results among different groups as well as the wide spread in device performance even within individual studies. Finally, we propose a simple experimental method to identify the exact stoichiometry of the perovskite layer that researchers can employ to confirm their experiments are performed consistently without unintentional variations in precursor stoichiometry.

Thermodynamics and the Intrinsic Stability of Lead Halide Perovskites CH3NH3PbX3

The role of thermodynamics in assessing the intrinsic instability of the CH₃NH₃PbX₃ perovskites (X = Cl,Br,I) is outlined on the basis of the available experimental information. Possible decomposition/degradation pathways driven by the inherent instability of the material are considered. The decomposition to precursors CH₃NH₃X(s) and PbX₂( s) is first analyzed, pointing out the importance of both the enthalpic and the entropic factor, the latter playing a stabilizing role making the stability higher than often asserted. For CH₃NH₃PbI₃, the disagreement between the available calorimetric results makes the stability prediction uncertain. Subsequently, the gas-releasing decomposition paths are discussed, with emphasis on the discrepant results presently available, probably reflecting the predominance of thermodynamic or kinetic control. The competition between the formation of NH₃(g) + CH₃X(g), CH₃NH₂(g) + HX(g) or CH₃NH₃X(g) is analyzed, in comparison with the thermal decomposition of methylammonium halides. In view of the scarce and inconclusive thermodynamic studies to-date available, the need for further experimental data is emphasized.

Aqueous Synthesis of Methylammonium Lead Halide Perovskite Nanocrystals

Methylammonium lead halide perovskite nanocrystals offer attractive optoelectronic properties but suffer from fast degradation in the presence of water. In contradiction to this observation, we demonstrate the possibility of a direct aqueous synthesis of CH₃ NH₃ PbX₃ (X=Br or Cl/Br) nanocrystals through the reaction between the lead halide complex and methylamine when the pH is maintained in the range of 0-5. Under these synthetic conditions, the positively charged surface of the perovskite nanocrystals and the proper ionic balance help to prevent their decomposition in water. Additional surface capping with organic amine ligands further improves the photoluminescence quantum yield of the perovskite nanocrystals to values close to 40 %, ensures their stability under ambient conditions for several months, and their photoluminescence performance under continuous 0.1 W mm^-2 405 nm light irradiation for over 250 hours.

Critical Role of Water in Defect Aggregation and Chemical Degradation of Perovskite Solar Cells

The chemical stability of methylammonium lead iodide (MAPbI₃) under humid conditions remains the primary challenge facing halide perovskite solar cells. We investigate defect processes in the water-intercalated iodide perovskite (MAPbI₃_H₂O) and monohydrated phase (MAPbI₃·H₂O) within a first-principles thermodynamic framework. We consider the formation energies of isolated and aggregated vacancy defects with different charge states under I-rich and I-poor conditions. It is found that a PbI₂ (partial Schottky) vacancy complex can be formed readily, while the MAI vacancy complex is difficult to form in the hydrous compounds. Vacancies in the hydrous phases create deep charge transition levels, indicating the degradation of the lead halide perovskite upon exposure to moisture. Electronic structure analysis supports a mechanism of water-mediated vacancy pair formation.

First-principles investigation of the Lewis acid-base adduct formation at the methylammonium lead iodide surface

We have here performed a campaign of ab initio calculations focusing on the anchoring mechanism and adduct formation of some Lewis bases, both aliphatic and aromatic, on a PbI2-rich flat (001) methylammonium lead iodide (MAPI) surface. Our goal is to provide theoretical support to the recently reported experimental techniques of MAPI surface passivation via Lewis acid-base neutralization and similarly of MAI·PbI2·(Lewis base) adduct formation. We tested several X-donor bases (X = :N, :O, :S), paying attention to the thermodynamic stability of the final MAPI·base adducts and to their electronic properties. Factors that impact on the passivation mechanism are the directionality of the Lewis base lone pair and its enhanced/reduced overlap with MAPI Pb p orbitals, the dipole moment of the base and, similarly, the electronegativity of the X donor atom. Also non-covalent interactions, both at the surface side (intra, MAPI) and at the very interface (inter, MAPI·Lewis base), seem to contribute to the stability of the final adducts. Here we show that the thermodynamic stability does not necessarily correspond to the most effective base → acid dative bond formation. Starting from a low coverage (12.5% of the undercoordinated Pb atoms available at the surface are passivated) this paper paves the way towards the study of cooperative and steric effects among Lewis bases at higher coverages representing, to the best of our knowledge, one of the very first studies focusing on the molecular anchoring on the surfaces of this very important class of materials.

In Situ Monitoring the Uptake of Moisture into Hybrid Perovskite Thin Films

Solution-processed hybrid perovskites are of great interest for use in photovoltaics. However, polycrystalline perovskite thin films show strong degradation in humid atmospheres, which poses an important challenge for large-scale market introduction. With in situ grazing incidence neutron scattering (GISANS) we analyzed water content, degradation products, and morphological changes during prolonged exposure to several humidity levels. In high humidity, the formation of metastable hydrate phases is accompanied by domain swelling, which transforms the faceted crystals to a round-washed, pebble-like form. The films incorporate much more water than is integrated into the hydrates, with smaller crystals being more affected, making the degradation strongly dependent on film morphology. Even at low humidity, water is adsorbed on the crystal surfaces without the formation of crystalline degradation products. Thus, although production in an ambient atmosphere is of interest for industrial production it might lead to long-term degradation without appropriate countermeasures like postproduction drying below 30% RH.

The Impact of Atmosphere on the Local Luminescence Properties of Metal Halide Perovskite Grains

Metal halide perovskites are exceptional candidates for inexpensive yet high-performing optoelectronic devices. Nevertheless, polycrystalline perovskite films are still limited by nonradiative losses due to charge carrier trap states that can be affected by illumination. Here, in situ microphotoluminescence measurements are used to elucidate the impact of light-soaking individual methylammonium lead iodide grains in high-quality polycrystalline films while immersing them with different atmospheric environments. It is shown that emission from each grain depends sensitively on both the environment and the nature of the specific grain, i.e., whether it shows good (bright grain) or poor (dark grain) luminescence properties. It is found that the dark grains show substantial rises in emission, while the bright grain emission is steady when illuminated in the presence of oxygen and/or water molecules. The results are explained using density functional theory calculations, which reveal strong adsorption energies of the molecules to the perovskite surfaces. It is also found that oxygen molecules bind particularly strongly to surface iodide vacancies which, in the presence of photoexcited electrons, lead to efficient passivation of the carrier trap states that arise from these vacancies. The work reveals a unique insight into the nature of nonradiative decay and the impact of atmospheric passivation on the microscale properties of perovskite films.

JDFTx: software for joint density-functional theory

Density-functional theory (DFT) has revolutionized computational prediction of atomic-scale properties from first principles in physics, chemistry and materials science. Continuing development of new methods is necessary for accurate predictions of new classes of materials and properties, and for connecting to nano- and mesoscale properties using coarse-grained theories. JDFTx is a fully-featured open-source electronic DFT software designed specifically to facilitate rapid development of new theories, models and algorithms. Using an algebraic formulation as an abstraction layer, compact C++11 code automatically performs well on diverse hardware including GPUs (Graphics Processing Units). This code hosts the development of joint density-functional theory (JDFT) that combines electronic DFT with classical DFT and continuum models of liquids for first-principles calculations of solvated and electrochemical systems. In addition, the modular nature of the code makes it easy to extend and interface with, facilitating the development of multi-scale toolkits that connect to ab initio calculations, e.g. photo-excited carrier dynamics combining electron and phonon calculations with electromagnetic simulations.

Collective Molecular Mechanisms in the CH3NH3PbI3 Dissolution by Liquid Water

The origin of the dissolution of methylammonium lead trihalide (MAPI) crystals in liquid water is clarified by finite-temperature molecular dynamics by developing a MYP-based force field (MYP1) for water-MAPI systems. A thermally activated process is found with an energy barrier of 0.36 eV consisting of a layer-by-layer degradation with generation of inorganic PbI₂ films and solvation of MA and I ions. We rationalize the effect of water on MAPI by identifying a transition from a reversible absorption and diffusion in the presence of vapor to the irreversible destruction of the crystal lattice in liquid due to a cooperative action of water molecules. A strong water-MAPI interaction is found with a binding energy of 0.41 eV/H₂O and wetting energy of 0.23 N/m. The water vapor absorption is energetically favored (0.29 eV/H₂O), and the infiltrated molecules can migrate within the crystal with a diffusion coefficient D = 1.7 × 10^-8 cm²/s and activation energy of 0.28 eV.

Moderate Humidity Delays Electron-Hole Recombination in Hybrid Organic-Inorganic Perovskites: Time-Domain Ab Initio Simulations Rationalize Experiments

Experiments show both positive and negative changes in performance of hybrid organic-inorganic perovskite solar cells upon exposure to moisture. Ab initio nonadiabatic molecular dynamics reveals the influence of humidity on nonradiative electron-hole recombination. In small amounts, water molecules perturb perovskite surface and localize photoexcited electron close to the surface. Importantly, deep electron traps are avoided. The electron-hole overlap decreases, and the excited state lifetime increases. In large amounts, water forms stable hydrogen-bonded networks, has a higher barrier to enter perovskite, and produces little impact on charge localization. At the same time, by contributing high frequency polar vibrations, water molecules increase nonadiabatic coupling and accelerate recombination. In general, short coherence between electron and hole benefits photovoltaic response of the perovskites. The calculated recombination time scales show excellent agreement with experiment. The time-domain atomistic simulations reveal the microscopic effects of humidity on perovskite excited-state lifetimes and rationalize the conflicting experimental observations.

Hybrid Organic-Inorganic Perovskites on the Move

Hybrid organic-inorganic perovskites (HOIPs) are crystals with the structural formula ABX3, where A, B, and X are organic and inorganic ions, respectively. While known for several decades, HOIPs have only in recent years emerged as extremely promising semiconducting materials for solar energy applications. In particular, power-conversion efficiencies of HOIP-based solar cells have improved at a record speed and, after only little more than 6 years of photovoltaics research, surpassed the 20% threshold, which is an outstanding result for a solution-processable material. It is thus of fundamental importance to reveal physical and chemical phenomena that contribute to, or limit, these impressive photovoltaic efficiencies. To understand charge-transport and light-absorption properties of semiconducting materials, one often invokes a lattice of ions displaced from their static positions only by harmonic vibrations. However, a preponderance of recent studies suggests that this picture is not sufficient for HOIPs, where a variety of structurally dynamic effects, beyond small harmonic vibrations, arises already at room temperature. In this Account, we focus on these effects. First, we review structure and bonding in HOIPs and relate them to the promising charge-transport and absorption properties of these materials, in terms of favorable electronic properties. We point out that HOIPs are much "softer" mechanically, compared to other efficient solar-cell materials, and that this can result in large ionic displacements at room temperature. We therefore focus next on dynamic structural effects in HOIPs, going beyond a static band-structure picture. Specifically, we discuss pertinent experimental and theoretical findings as to phase-transition behavior and molecular/octahedral rearrangements. We then discuss atomic diffusion phenomena in HOIPs, with an emphasis on the migration of intrinsic and extrinsic ionic species. From this combined perspective, HOIPs appear as highly dynamic materials, in which structural fluctuations and long-range ionic motion have an unusually strong impact on charge-transport and optical properties. We highlight the potential implications of these effects for several intriguing phenomenological observations, ranging from scattering mechanisms and lifetimes of charge carriers to light-induced structural effects and ionic conduction.

Ab initio static and dynamic study of CH3NH3PbI3 degradation in the presence of water, hydroxyl radicals, and hydroxide ions

Density functional theory calculations are combined with the ab initio molecular dynamics and nudged elastic band techniques to unveil the microscopic details of CH₃NH₃PbI₃ degradation in the presence of water, hydroxyl radicals, and hydroxide ions.

The effect of moisture on the structures and properties of lead halide perovskites: a first-principles theoretical investigation

The degradation mechanism of perovskite materials when exposed to moisture and sunlight has been fully explored.

Preferential (100)-oriented CH3NH3PbI3 perovskite film formation by flash drying and elucidation of formation mechanism

It was found that the amount of thermal energy delivered during annealing and the amount of residual solvent remaining after spin coating play critical roles in determining the growth properties of (100)-oriented perovskite films.

Polarization-driven catalysis via ferroelectric oxide surfaces

Ferroelectric polarization can tune the surface chemistry: enhancing technologically important catalytic reactions such as NO_x direct decomposition and SO₂ oxidation.

Material Innovation in Advancing Organometal Halide Perovskite Functionality

Organometal halide perovskites (OMHPs) have garnered much attention recently for their unprecedented rate of increasing power conversion efficiency (PCE), positioning them as a promising basis for the next-generation photovoltaic devices. However, the gap between the rapid increasing PCE and the incomplete understanding of the structure-property-performance relationship prevents the realization of the true potential of OMHPs. This Perspective aims to provide a concise overview of the current status of OMHP research, highlighting the unique properties of OMHPs that are critical for solar applications but still not adequately explained. Stability and performance challenges of OMHP solar cells are discussed, calling upon combined experimental and theoretical efforts to address these challenges for pioneering commercialization of OMHP solar cells. Various material innovation strategies for improving the performance and stability of OMHPs are surveyed, showing that the OMHP architecture can serve as a promising and robust platform for the design and optimization of materials with desired functionalities.