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Direct Spectroscopic Observation of Hole Polaron in Lead Halide Perovskites.

Intrinsic photophysical origin of lead halide perovskites (LHPs) succeeding in optolectronic applications still remains hotly debated. Here, by using the ultrafast X-ray transient absorption spectroscopy, we successfully tracked the fate of photogenerated charge carriers at room temperature within the thin films of two classic LHPs, namely MAPbBr3 (MA: CH3NH3) and FAPbBr3 (FA: CH(NH2)2). We clearly observed in both thin films that the hole polaron is formed by localizing the photogenerated hole at Br 4p orbital and concurrently distorting the local structure surrounding Br atom after the photoexcitation. Furthermore, the bigger cation FA in the cavity of [PbBr6]4- octahedral framework induces larger hole polaron effect due to its p orbital hybridization into valence and conduction bands, correlating with the slower charge carrier recombination dynamics. Our direct experimental observation of the localized hole polaron in perovskites should advance the fundamental comprehension of charge carrier behavior within LHPs and their related devices.