The presence of various types of chemical interactions in metal‐halide perovskite semiconductors gives them a characteristic “soft” fluctuating structure, prone to a wide set of defects. Understanding of the nature of defects and their photochemistry is summarized, which leverages the cooperative action of density functional theory investigations and accurate experimental design. This knowledge is used to describe how defect activity determines the macroscopic properties of the material and related devices. Finally, a discussion of the open questions provides a path towards achieving an educated prediction of device operation, necessary to engineer reliable devices.

By correlating photovoltaic and material properties with metal content, we identify compositional ranges of low and high optoelectronic quality in (FA0.83Cs0.17)(Pb1−ySny)I3 perovskites.