Spatial Distribution of Lead Iodide and Local Passivation on Organo-Lead Halide Perovskite.

We identify nanoscale spatial distribution of PbI2 on the (FAPbI3)0.85(MAPbBr3)0.15 perovskite thin film and investigate the local passivation effect using confocal based optical microscopy of steady state and time-resolved photoluminescence (PL). Different from a typical scanning electron microscope (SEM) morphology study, confocal based PL spectroscopy and microscopy allow researchers to map the morphologies of both perovskite and PbI2 grains simultaneously, by selectively detecting their characteristic fluorescent bands using band-pass filters. In this work, we compare the perovskite samples without and with excess PbI2 incorporation and unambiguously reveal PbI2 distribution for the PbI2-rich sample. In addition, using the nanoscale time-resolved PL technique we show that the PbI2-rich regions exhibit longer lifetime due to suppressed defect trapping, compared to the PbI2-poor regions. The measurement on the PbI2-rich sample indicates that the passivation effect of PbI2 in perovskite film is effective, especially in localized regions. Hence, this finding is important for further improvement of the solar cells by considering the strategy of excess PbI2 incorporation.