Light-Induced Formation of Pb3+ Paramagnetic Species in Lead Halide Perovskites

Hybrid halide perovskites are soft materials processed at room temperature, revolutionary players in the photovoltaic field. Nowadays, investigation of the nature and role of defects is seen as one of the key challenges toward full comprehension of their behavior and achievement of high device stability under working conditions. We reveal the reversible generation, under illumination, of paramagnetic Pb3+ defects in CH3NH3PbI3, synthesized in ambient conditions, induced by the presence of Pb-O defects in the perovskite structure that may trap photogenerated holes, possibly mediated by the concomitant oxidation and migration of ions. According to the mechanism that we hypothesize, one charge is trapped for each paramagnetic center generated; thus, it does not contribute to the photocurrent, potentially limiting the solar cell performance. Our study, based on combined experimental/theoretical approach, reveals the dynamic evolution of the perovskite characteristics under illumination that needs to be considered when investigating the material physical-chemical properties.