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Anharmonic crystal structure dynamics have been observed in halide perovskites (APbX3, A = CH3NH3, Cs / X = I, Br) on picosecond timescales. Here, we report that the soft nature of the perovskite crystal lattice gives rise to dynamic fluctuations in the electronic properties of excited states. We use linear polarization selective transient absorption spectroscopy (LP-TA) to investigate how such crystal dynamics affect the electronic states occupied by photoexcited carriers in hybrid metal halide perovskite thin films (CH3NH3PbX3, X = I,Br) and nanocrystals (CsPbI3) at room temperature. This method is sensitive to the coupling between the optical polarization vector of the absorbed light and the transition dipole matrix (TDM) element of the electronic states, which allows us to probe optical anisotropies in the excited state population.
Optical alignment upon linearly-polarized excitation occurs in a range of semiconductors, with a variety of underlying causes. In GaAs, the dependence of the optical TDM on the angular momentum of the electronic wave functions imprints a short-lived anisotropic carrier momentum distribution on the excited state population. This is lost through femtosecond carrier-carrier scattering. By contrast, optical alignment in molecular materials, with more localised excitonic states, may arise from an alignment of TDM with physical structure. Loss of polarization memory in this case arises from physical reorientation of the photoexcited molecule or diffusion of the excited state to regions with different dipole matrix orientation. We show that hybrid perovskites lie between these two extremes. Their soft structural nature allows dynamic symmetry breaking of the delocalised electronic states, preserving optical alignment over picoseconds, which is far beyond the timescale of momentum-scattering events. We also find that optical alignment is lost on the timescales of local structural reorientation rather than diffusion. We condiser this reorientation to occur in fluctuating polar nano-regions of the lead-halide lattice. We suggest that these electronically distinct regions represent the fundamental units of perovskite electronic structure, rather than crystal domains or individual nanocrystals. The electronically soft nature of halide perovskite crystals gives rise to behaviour not observed either in classical organic or inorganic semiconductors, which has far-reaching implications for the understanding and application of this important class of materials.
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