Multifaceted nature of defect tolerance in halide perovskites and emerging semiconductors

Lead halide perovskites (LHPs) have shot to prominence as efficient energy-conversion materials that can be processed using cost-effective fabrication methods. A reason for their exceptional performance is their crystallographic defect tolerance, enabling long charge-carrier lifetimes despite high defect densities. Achieving defect tolerance in broader classes of materials would impact on the semiconductor industry substantially. Considerable efforts have been made to understand the origins of defect tolerance, so as to design stable and nontoxic alternatives to LHPs. However, understanding defect tolerance in LHPs is far from straightforward. This Review discusses the models proposed for defect tolerance in halide perovskites, evaluating the experimental and theoretical support for these models, as well as their limitations. We also cover attempts to apply these models to identify materials beyond LHPs that could exhibit defect tolerance. Finally, we discuss the experimental methods used to understand defects in mixed ionic–electronic conductors, as well as the important information that is necessary for a deeper understanding, in order to develop improved models that enable the design of defect-tolerant semiconductors.