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Phase Segregation and Photothermal Remixing of Mixed-Halide Lead Perovskites.

Mixed-halide lead perovskites (MHPs) are promising materials for photovoltaics and optoelectronics due to their highly tunable bandgaps. However, they phase segregate under continuous illumination and/or electric field, whose mechanism is still under debate. Herein, we systematically measure the phase segregation behavior of polymer encapsulated CH3NH3Pb(BrxI1-x)3 MHPs as a function of excitation intensity and nominal halide ratio by in situ photoluminescence micro-spectroscopy and observed surprising phase dynamics at the beginning of the illumination. The initial phase segregation to I-rich and Br-rich phase is observed followed by the formation of a new mixed-halide phase within several seconds that has not been reported before. We propose that the photothermal effect is amplified at the small-size I-rich domains which significantly changes the local phase-segregation in the otherwise uniform film within milliseconds after illumination.