The Many Faces of Mixed Ion Perovskites: Unraveling and Understanding the Crystallization Process

Understanding the crystallization process of organic–inorganic halide perovskites is of paramount importance for fabrication of reproducible and efficient perovskite solar cells. We report for the first time on the discovery and interplay of ubiquitous hexagonal polytypes (6H and 4H) during the crystallization process of mixed ion perovskite, namely (FAPbI₃)_x(MAPbBr₃)_1–x. These polytypes, the first reported 3D hexagonal lead-halide-based perovskites, orchestrate a perovskite crystallization sequence revealed as 2H (delta phase)-4H-6H-3R­(3C), commonly found among inorganic transition metal oxide perovskites under extreme conditions. We show that the chemical pressure arising from the incorporation of >3% Cs⁺ cations into the lattice successfully inhibits the formation of these environmentally sensitive polytypes, elucidating the origin of the widely reported improved device stability and reproducibility of Cs⁺-containing mixed ion perovskites.