Even though tandem solar cells comprised of mixed-halide perovskites CH 3 NH 3 Pb(I 1−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.

Mixed halide perovskites as top absorbers with relatively high bandgaps (E g > 1.70 eV) are essential for tandem and multijunction solar cells. Light-induced phase segregation of mixed halide perovskites limits their commercialization, and it has been found that organic cation substitution in the A site can suppress photoinduced phase segregation. By substituting methylammonium (MA + ) ions into formamidinium (FA)PbBr 1.8 I 1.2 perovskites, we probed photoinduced segregation at different temperatures. In this study, we found that the segregation rate constant increased with MA + content; however, significant suppression of the rate constant was observed in the case of 10% substitution. The activation energy for photoinduced phase segregation increased (by ∼5 kJ mol −1 ) upon introduction of 10% MA + into FAPbBr 1.8 I 1.2 , reflecting an increased energetic barrier for halide segregation. The effect of A-site cation alloying against photoinduced phase segregation in active devices such as phototransistors and photovoltaic cells is also demonstrated and discussed.

Rectangular shaped, high crystalline quality, defect free and colorless 3D perovskite single crystals of CH3NH3PbCl3 were grown using the solvent evaporation method at room temperature for the first time.

Hybrid organic-inorganic metal halides of the type CHNHPbX have emerged as potential materials for photovoltaic applications. In this paper we discuss structural, electronic, and optical spectroscopy investigations performed on high quality single crystals of CHNHPbI. Our results conclusively suggest that CHNHPbI crystallizes in centrosymmetric space group and the methylammonium moiety exhibits disordered packing at room temperature. Extracted values of the exciton binding energy, the electron-phonon coupling constant, and the schematic energy level diagram constructed from the emission broadening, Raman, and photoemission spectroscopy measurements clearly show the potential of this system in photovoltaic applications.