Light-tunable three-phase coexistence in mixed halide perovskites

Mixed iodine-bromine perovskites used in solar cells undergo below a critical temperature an intrinsic demixing into phases with different compositions. In addition, under illumination they show nucleation of iodine-rich phase. We predict from thermodynamic considerations that mixed like $\mathrm{MAPb}{({\mathrm{I}}_{1\ensuremath{-}x}{\mathrm{Br}}_{x})}_{3}$ the interplay these effects can lead to coexistence bromine-rich, iodine-rich, and nearly iodine-pure nucleated This three-phase occurs region composition-temperature phase diagram near point for demixing. investigate hysteresis evolution this when or intensity are cycled. Depending on particular way is established, nuclei should form either only both bromine-rich phases. Experimental verification fundamentally novel type light-tunable be possible by combination absorption photoluminescence experiments.