Pressure‐Dependent Polymorphism and Band‐Gap Tuning of Methylammonium Lead Iodide Perovskite

Phase transformations and vibrational properties of hybrid organic-inorganic perovskite MAPbI3 bulk at high pressure

The structural stability and internal properties of hybrid organic-inorganic perovskites (HOIPs) have been widely investigated over the past few years. The interplay between organic cations and inorganic framework is one of the prominent features. Herein we report the evolution of Raman modes under pressure in the hybrid organic-inorganic perovskite MAPbI[Formula: see text] by combining the experimental approach with the first-principles calculations. A bulk MAPbI[Formula: see text] single crystal was synthesized via inverse temperature crystallization (ITC) technique and characterized by Raman spectroscopy, while the diamond anvil cells (DACs) was employed to compress the sample. The classification and behaviours of their Raman modes are presented. At ambient pressure, the vibrations of inorganic PbI[Formula: see text] octahedra and organic MA dominate at a low-frequency range (60-760 cm[Formula: see text]) and a fingerprint range (900-1500 cm[Formula: see text]), respectively. The applied pressure exhibits two significant changes in the Raman spectrum and indicates of phase transition. The results obtained from both experiment and calculations of the second phase at 3.26 GPa reveal that the internal vibration intensity of the PbI[Formula: see text] octahedra (< 110 cm[Formula: see text]) reduces as absences of MA libration (150-270 cm[Formula: see text]) and internal vibration of MA (450-750 cm[Formula: see text]). Furthermore, the hydrogen interactions around 1300 cm[Formula: see text] remain strong high pressure up to 5.34 GPa.

Stabilizing Polar Domains in MAPbBr3 via the Hydrostatic Pressure-Induced Liquid Crystal-like Transition

Pressure-induced phases of MAPbBr₃ were investigated at room temperature in the range of 0-2.8 GPa by ab initio molecular dynamics. Two structural transitions at 0.7 GPa (cubic → cubic) and 1.1 GPa (cubic → tetragonal) involved both the inorganic host (lead bromide) and the organic guest (MA). MA dipoles behave like a liquid crystal undergoing isotropic → isotropic and isotropic → oblate nematic transitions as pressure confines their orientational fluctuations to a crystal plane. Beyond 1.1 GPa, the MA ions lie alternately along two orthogonal directions in the plane forming stacks perpendicular to it. However, the molecular dipoles are statically disordered, leading to stable polar and antipolar MA domains in each stack. H-Bond interactions, which primarily mediate host-guest coupling, facilitate the static disordering of MA dipoles. Interestingly, high pressures suppress CH₃ torsional motion, emphasizing the role of C-H···Br bonds in the transitions.

Carriers, Quasi-particles, and Collective Excitations in Halide Perovskites

Halide perovskites (HPs) are potential game-changing materials for a broad spectrum of optoelectronic applications ranging from photovoltaics, light-emitting devices, lasers to radiation detectors, ferroelectrics, thermoelectrics, etc. Underpinning this spectacular expansion is their fascinating photophysics involving a complex interplay of carrier, lattice, and quasi-particle interactions spanning several temporal orders that give rise to their remarkable optical and electronic properties. Herein, we critically examine and distill their dynamical behavior, collective interactions, and underlying mechanisms in conjunction with the experimental approaches. This review aims to provide a unified photophysical picture fundamental to understanding the outstanding light-harvesting and light-emitting properties of HPs. The hotbed of carrier and quasi-particle interactions uncovered in HPs underscores the critical role of ultrafast spectroscopy and fundamental photophysics studies in advancing perovskite optoelectronics.

Proton irradiation effects on mechanochemically synthesized and flash-evaporated hybrid organic-inorganic lead halide perovskites

A hybrid organic-inorganic halide perovskite is a promising material for developing efficient solar cell devices, with potential applications in space science. In this study, we synthesized methylammonium lead iodide (MAPbI₃) perovskites via two methods: mechanochemical synthesis and flash evaporation. We irradiated these perovskites with highly energetic 10 MeV proton-beam doses of 10¹¹, 10¹², 10¹³, and 4 × 10¹³protons cm^-2and examined the proton irradiation effects on the physical properties of MAPbI₃perovskites. The physical properties of the mechanochemically synthesized MAPbI₃perovskites were not considerably affected after proton irradiation. However, the flash-evaporated MAPbI₃perovskites showed a new peak in x-ray diffraction and an increased fluorescence lifetime in time-resolved photoluminescence under high-dose conditions, indicating considerable changes in their physical properties. This difference in behavior between MAPbI₃perovskites synthesized via the abovementioned two methods may be attributed to differences in radiation hardness associated with the bonding strength of the constituents, particularly Pb-I bonds. Our study will help to understand the radiation effect of proton beams on organometallic halide perovskite materials.

Ultra-flexible and waterproof perovskite photovoltaics for washable power source applications

A washable perovskite solar cell with high efficiency (over 11%) and outstanding crumpling durability (maintaining 81.2% after 100 cycles crumpling) is demonstrated herein by combining the flexible self-encapsulation method with a waterproof glue coated substrate.

Bismuth Halide Perovskite-Like Materials: Current Opportunities and Challenges

Metal halide perovskites have recently emerged as promising photovoltaic materials for application in solar cells with high power conversion efficiencies exceeding 23 %. In the years since such high efficiencies have been attained, investigations have mainly focused on the state-of-the-art 3 D Pb-based halide perovskite materials. However, the high toxicity of Pb and intrinsic instability of the pristine perovskite materials have become great obstacles to their industrial application and commercialization. To address these serious issues, it is imperative to explore low-toxicity metal halide perovskites or their derivatives to substitute Pb-based materials for better future development. Currently, Bi-based halide perovskite-like materials are attracting increased interest as environmentally friendly alternatives for photovoltaic applications. This Concept highlights recent advances of Bi-based halide perovskite-like materials in terms of understanding and modifying their fundamental properties and related device performance, with a focus on current challenges, opportunities for future development, and diversification of device applications.

High-Pressure Band-Gap Engineering in Lead-Free Cs2 AgBiBr6 Double Perovskite

Novel inorganic lead-free double perovskites with improved stability are regarded as alternatives to state-of-art hybrid lead halide perovskites in photovoltaic devices. The recently discovered Cs₂ AgBiBr₆ double perovskite exhibits attractive optical and electronic features, making it promising for various optoelectronic applications. However, its practical performance is hampered by the large band gap. In this work, remarkable band gap narrowing of Cs₂ AgBiBr₆ is, for the first time, achieved on inorganic photovoltaic double perovskites through high pressure treatments. Moreover, the narrowed band gap is partially retainable after releasing pressure, promoting its optoelectronic applications. This work not only provides novel insights into the structure-property relationship in lead-free double perovskites, but also offers new strategies for further development of advanced perovskite devices.