Temperature Dependent Photoinduced Reversible Phase Separation in Mixed-Halide Perovskite

Even though tandem solar cells comprised of mixed-halide perovskites CH3NH3Pb­(I1–x Br x )3 were expected to have much higher efficiency, the observation that they undergo photoinduced phase separation/demixing put forth a limitation to their possible utility. Herein, using temperature dependent photoluminescence studies, we show that the stated photoinduced phase separation occurs only in a narrow temperature range and above a particular bromine concentration. Our observation of the disappearance of phase separation at elevated temperatures suggests the possibility that these tandem solar cells may indeed work better at elevated temperatures. Further, we provide the first experimental proof for the demixing transition temperature as predicted by Bischak et al. and also observe that demixing and remixing temperatures are pinned to crystallographic phase transition temperatures. Longer carrier lifetime of iodide-rich clusters is observed confirming the strong electron–phonon interaction (polaronic effect) which is absent in the initial mixed states.