Impact of Excess CH₃NH₃I on Free Carrier Dynamics in High-Performance Nonstoichiometric Perovskites

Since the discovery of organometallic trihalide perovskites, there have been tremendous efforts to exploit these hybrid materials and understand their optoelectronic properties for the development of solar cells with high power conversion efficiencies. Although the improved performance of perovskite solar cells with excess CH₃NH₃I has been reported, the dedicated research of the free charge carrier dynamics is lacking. In this study, we measured the photoluminescence (PL) intensities and lifetimes at the grains and near the grain boundaries of CH₃NH₃PbI₃ perovskite films using spatially and temporally resolved PL spectroscopy. An excess CH₃NH₃I was found to cause brighter PL intensities and longer PL lifetimes at both the grains and grain boundaries. This comparative investigation of stoichiometric and nonstoichiometric perovskite films enables us to understand the optoelectronic properties induced by excess CH₃NH₃I, opening a new way for optimization of perovskite solar cells.