Octahedral units in halide perovskites

Metal halide perovskites, with an ABX₃ crystal structure, possess excellent photophysical properties for (opto)electronic applications including photovoltaics, light-emitting diodes, photodetectors and transistors. To pave the pathway towards commercial applications, enormous efforts have been made to obtain high-performance perovskite-based devices. The octahedral unit is considered to be the fundamental and functional unit of halide perovskite materials, consisting of a central B cation surrounded by six X anions, with typical dimensions of 5-6 Å. One promising pathway towards stable and high-performance perovskite devices is to manipulate this octahedral unit. Although rational engineering of octahedra has been used in oxide perovskites to unlock various functionalities, analogous targeted work has been relatively underexplored in halide perovskites. Here, we present fundamental concepts regarding octahedral configurations in metal halide perovskites and its effects on crystal structures, photophysical properties and device performance. We outline techniques that can be used for characterizing octahedral units and summarize different approaches to rationally manipulate these units. Finally, we discuss the challenges and outlook for future research to align octahedral units with device investigations.