  In semiconductors, exciton or charge carrier diffusivity is typically
described as an inherent material property. Here, we show that the transport of
excitons (i.e., bound electron-hole pairs) in CsPbBr3 perovskite nanocrystals
(NCs) depends markedly on how recently those NCs were occupied by a previous
exciton. Using fluence- and repetition-rate-dependent transient
photoluminescence microscopy, we visualize the effect of excitation frequency
on exciton transport in CsPbBr3 NC solids. Surprisingly, we observe a striking
dependence of the apparent exciton diffusivity on excitation laser power that
does not arise from nonlinear exciton-exciton interactions nor from thermal
heating of the sample. We interpret our observations with a model in which
excitons cause NCs to undergo a transition to a metastable configuration that
admits faster exciton transport by roughly an order of magnitude. This
metastable configuration persists for ~microseconds at room temperature, and
does not depend on the identity of surface ligands or presence of an oxide
shell, suggesting that it is an intrinsic response of the perovskite lattice to
electronic excitation. The exciton diffusivity observed here (>0.15 cm2/s) is
considerably higher than that observed in other NC systems on similar
timescales, revealing unusually strong excitonic coupling in a NC material. The
finding of a persistent enhancement in excitonic coupling between NCs may help
explain other extraordinary photophysical behaviors observed in CsPbBr3 NC
arrays, such as superfluorescence. Additionally, faster exciton diffusivity
under higher photoexcitation intensity is likely to provide practical insights
for optoelectronic device engineering.
