General design principles for defect tolerance in materials from the study of lead bromide perovskites

The ability to design defect tolerant materials for societal sustainability is a grand challenge in the physical and material sciences.  Our detailed study of the origins of defect tolerance in lead halide perovskites yields two new general design principles for defect tolerance in all materials.  Using three prototypical lead bromide perovskites in single crystal form and synchrotron-based high-resolution X-ray spectroscopy, we reveal the pivotal role of the A-cation in mediating the influence of photoinduced defects.  A-cation hydrogen bonding facilitates chemical flexing of the lead-bromide bond that mitigates bromide vacancy self-n-doping.  The contribution of semi-ionic lead-bromide bonding to the electronic band edges, where the bonding becomes more ionic upon the formation of defects, mitigates re-hybridization of the electronic structure.  Our findings clarify the underpinnings of defect tolerance in halide perovskites and have implications for defect calculations, beam-based measurements of photophysical properties and long lifetime perovskite solar cell technology.