Halide Vacancies Create No Charge Traps on Lead Halide Perovskite Surfaces but Can Generate Deep Traps in the Bulk.

Metal halide perovskites (MHPs) have attracted attention because of their high optoelectronic performance that is fundamentally rooted in the unusual properties of MHP defects. By developing an ab initio-based machine-learning force field, we sample the structural dynamics of MHPs on a nanosecond time scale and show that halide vacancies create midgap trap states in the MHP bulk but not on a surface. Deep traps result from Pb-Pb dimers that can form across the vacancy in only the bulk. The required shortening of the Pb-Pb distance by nearly 3 Å is facilitated by either charge trapping or 50 ps thermal fluctuations. The large-scale structural deformations are possible because MHPs are soft. Halide vacancies on the MHP surface create no deep traps but separate electrons from holes, keeping the charges mobile. This is particularly favorable for MHP quantum dots, which do not require sophisticated surface passivation to emit light and blink less than quantum dots formed from traditional inorganic semiconductors.