Formation, location and beneficial role of PbI2 in lead halide perovskite solar cells

Abstract

Here we report the investigation of controlled PbI₂ secondary phase formation in CH₃NH₃PbI₃ (MAPI) photovoltaics through post-deposition thermal annealing, highlighting the beneficial role of PbI₂ on device performance. Using high-resolution transmission electron microscopy we show the location of PbI₂ within the active layer and propose a nucleation and growth mechanism. We discover that during the annealing that PbI₂ forms mainly in the grain boundary regions of the MAPI films and that at certain temperatures the PbI₂ formed can be highly beneficial to device performance – reducing current–voltage hysteresis and increasing the power conversion efficiency. Our analysis shows that the MAPI grain boundaries as susceptible areas that, under thermal loading, initiate the conversion of MAPI into PbI₂.