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.

The impact of the bulky‐cation‐modified interfaces on halide perovskite solar cell stability is underexplored. In this work, the thermal instability of the bulky‐cation interface layers used in the state‐of‐the‐art solar cells is demonstrated. X‐ray photoelectron spectroscopy and synchrotron‐based grazing‐incidence X‐ray scattering measurements reveal significant changes in the chemical composition and structure at the surface of these films that occur under thermal stress. The changes impact charge‐carrier dynamics and device operation, as shown in transient photoluminescence, excitation correlation spectroscopy, and solar cells. The type of cation used for surface treatment affects the extent of these changes, where long carbon chains provide more stable interfaces. These results highlight that prolonged annealing of the treated interfaces is critical to enable reliable reporting of performances and to drive the selection of different bulky cations.

The fundamental structure–property relationships in the CsyFA1−yPb(BrxI1−x)3 compositional space are reported.