Abstract
Lattice compression through hydrostatic pressure has emerged as an effective means of tuning the structural and optoelectronic properties of hybrid halide perovskites. In addition to external pressure, the local strain present in solution-processed thin films also causes significant heterogeneity in their photophysical properties. However, an atomistic understanding of structural changes of hybrid perovskites under pressure and their effects on the electronic landscape is required. Here, we use high level ab initio simulation techniques to explore the effect of lattice compression on the formamidinium (FA) lead iodide compound, FA1–xCsxPbI3 (x = 0, 0.25). We show that, in response to applied pressure, the Pb–I bonds shorten, the PbI6 octahedra tilt anisotropically, and the rotational dynamics of the FA+ molecular cation are partially suppressed. Because of these structural distortions, the compressed perovskites exhibit band gaps that are narrower (red-shifted) and indirect with spin-split band edges. Furthermore, the shallow defect levels of intrinsic iodide defects transform to deep-level states with lattice compression. This work highlights the use of hydrostatic pressure as a powerful tool for systematically modifying the photovoltaic performance of halide perovskites.
Highlights
Organic−inorganic perovskite solar cells have shown an unprecedented increase in power conversion efficiency, exceeding 23% over the past few years.[1−9] The most common three-dimensional lead iodide perovskites have the composition APbI3, where A is a monovalent cation (e.g., methylammonium CH3NH3+ (MA+), formamidinium CH(NH2)2+ (FA+), or cesium Cs+), which fills the cuboctahedral cage formed by corner-sharing PbI6 octahedra
The application of hydrostatic pressure has emerged as an effective means of tuning the structural phases and photovoltaic properties of hybrid perovskites.[17−23] Under pressure, the crystal structure of these materials evolves through several structural phases that are inaccessible by temperature variation
Our recent studies have focused on defect migration, compositional engineering, and environmental stability of hybrid halide perovskites.[30−39] Here we investigate the structural, optoelectronic, and defect properties of FAPbI3 and the mixed-cation FA0.75Cs0.25PbI3 under hydrostatic pressure using ab initio simulation methods
Summary
Organic−inorganic perovskite solar cells have shown an unprecedented increase in power conversion efficiency, exceeding 23% over the past few years.[1−9] The most common three-dimensional lead iodide perovskites have the composition APbI3, where A is a monovalent cation (e.g., methylammonium CH3NH3+ (MA+), formamidinium CH(NH2)2+ (FA+), or cesium Cs+), which fills the cuboctahedral cage formed by corner-sharing PbI6 octahedra. FA+ cations in FAPbI3 increasing its resistance stabilizes the perovskite phase, to chemical degradation.[11,13−16]. Because of their enhanced structural stability, as well as improved photovoltaic properties, these mixed-cation perovskites are currently the most promising materials for perovskite solar cells. The optimized band gap, increased charge-carrier lifetimes, reduced trap-state densities, and tuned carrier conductivities collectively result in improved photovoltaic performance in compressed halide perovskites.[20−22] Solution-processed polycrystalline thin films of halide perovskites often experience significant local strain.[24,25] The experimentally observed spatial heterogeneity of the optoelectronic and photoluminescence properties of these films are speculated to be closely connected to the local strain in the perovskites.[26−29] It is clear, that an indepth atomistic understanding of the structural and optoelectronic properties of lead halide perovskites under pressure has yet to be established
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