Impacts of Ion Segregation on Local Optical Properties in Mixed Halide Perovskite Films.

Despite the recent astronomical success of organic-inorganic perovskite solar cells (PSCs), the impact of microscale film inhomogeneities on device performance remains poorly understood. In this work, we study CH3NH3PbI3 perovskite films using cathodoluminescence in scanning transmission electron microscopy and show that localized regions with increased cathodoluminescence intensity correspond to iodide-enriched regions. These observations constitute direct evidence that nanoscale stoichiometric variations produce corresponding inhomogeneities in film cathodoluminescence intensity. Moreover, we observe the emergence of high-energy transitions attributed to beam induced iodide segregation, which may mirror the effects of ion migration during PSC operation. Our results demonstrate that such ion segregation can fundamentally change the local optical and microstructural properties of organic-inorganic perovskite films in the course of normal device operation and therefore address the observed complex and unpredictable behavior in PSC devices.