Composition dictates octahedral tilt and photostability in halide perovskites

Abstract Halide perovskites are excellent candidate materials for use in solar cell, LED, and detector devices, in part because their composition can be tuned to achieve ideal optoelectronic properties. Empirical efficiency optimisation has led the field towards compositions rich in FA (formamidinium) on the A‐site and I on the X‐site, with additional small amounts of MA (methylammonium) or Cs A‐site cations and Br X‐site anions. However, it is not clear how and why the specific compositions of alloyed, i.e., mixed component, halide perovskites relate to photo‐stability of the materials. Here, we combine synchrotron grazing incidence wide‐angle x‐ray scattering, photoluminescence, high‐resolution scanning electron diffraction measurements and theoretical modelling to reveal the links between material structure and photostability. Namely, we find that increased octahedral titling leads to improved photo‐stability that is correlated with lower densities of performance‐harming hexagonal polytype impurities. Our results uncover the structural signatures underpinning photo‐stability and can therefore be used to make targeted changes to halide perovskites, bettering the commercial prospects of technologies based on these materials. This article is protected by copyright. All rights reserved