Large polarons in lead halide perovskites.

Lead halide perovskites show marked defect tolerance responsible for their excellent optoelectronic properties. These properties might be explained by the formation of large polarons, but how they are formed and whether organic cations are essential remain open questions. We provide a direct time domain view of large polaron formation in single-crystal lead bromide perovskites CHNHPbBr and CsPbBr. We found that large polaron forms predominantly from the deformation of the PbBr frameworks, irrespective of the cation type. The difference lies in the polaron formation time, which, in CHNHPbBr (0.3 ps), is less than half of that in CsPbBr (0.7 ps). First-principles calculations confirm large polaron formation, identify the Pb-Br-Pb deformation modes as responsible, and explain quantitatively the rate difference between CHNHPbBr and CsPbBr. The findings reveal the general advantage of the soft [PbX] sublattice in charge carrier protection and suggest that there is likely no mechanistic limitations in using all-inorganic or mixed-cation lead halide perovskites to overcome instability problems and to tune the balance between charge carrier protection and mobility.