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 CH₃NH₃Pb(Br_xI_1-x)₃ MHPs as a function of excitation intensity and nominal halide ratio by in situ photoluminescence micro-spectroscopy. We encapsulate the MHPs with a layer of polystyrene polymer film to isolate them from the effects of the immediate atmosphere. Under this passivated condition, the phase segregation of the MHPs is very different from those without polymer passivation reported in the literature. 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 observe 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 as early as milliseconds after illumination.