Competing photo-induced trap formation and healing in lead halide perovskites

Metal halide perovskites have been successfully applied as optically active layers in photovoltaics and in various optoelectronic devices. Long-term reliability must however be assured. Instabilities are manifested as light-induced ion migration and segregation, which can eventually lead to material degradation. Discordant reports have reported a beneficial role of ion migration under illumination, leading to defect healing. By combining  ab initio simulations with photoluminescence measurements under controlled conditions, we demonstrate that photo-instabilities are related to light-induced formation and annihilation of defects acting as carrier trap states. We show that these phenomena coexist and compete. In particular, long-living carrier traps related to halide defects trigger photo-induced material transformations, which drive both processes.  On short scales, defect annihilation can prevail over defect formation, which occurs on longer scales. Nevertheless, defect formation can be controlled by blocking under-coordinated surface sites, which act as a defect reservoir. By a specific surface passivation strategy we are thus able to stabilize the perovskite layer towards photo-induced instabilities, leading to improved optoelectronic material quality and enhanced photo-stability in a working solar cell.