Structural Stability of Formamidinium- and Cesium-Based Halide Perovskites

The certified power conversion efficiency of state-of-the-art organic–inorganic hybrid perovskite solar cells has surpassed 25%, showing promising potential for commercialization. Compared with volatile methylammonium-based perovskites, formamidinium- and cesium-based halide perovskites have attracted attention due to their resistance to thermal degradation. However, the photoactive perovskite phases of these materials suffer from limited phase stability at room temperature and are prone to spontaneous transformation into the photoinactive perovskite phases. This Review sheds light onto the fundamental understanding of the origin of phase instability for both the intrinsic structure and the extrinsic factors. We highlight the methodologies used to suppress the undesired phase transitions of formamidinium- and cesium-based halide perovskites with an emphasis on structure–property relationships.