  Point defect tolerance in materials, which extends operational lifetime, is
essential for societal sustainability, and the creation of a framework to
design such properties is a grand challenge in the material sciences. Using
three prototypical lead bromide perovskites in single crystal form and
high-resolution synchrotron-based X-ray spectroscopy, we reveal the
unexpectedly pivotal role of the A-cation in mediating the influence of
photoinduced defects. Organic A-cation hydrogen bonding facilitates chemical
flexing of the lead-bromide bond that mitigates the self-doping effect of
bromide vacancies. The contribution of partially 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
upon degradation. These findings reveal two new general design principles for
defect tolerance in materials. Our findings uncover the foundations of defect
tolerance in halide perovskites and have implications for defect calculations,
all beam-based measurements of photophysical properties and perovskite solar
cell technology.
