Impact of Local Heterogeneities on Transient Absorption Microscopy of Spatiotemporal Charge Carrier Transport

Transient absorption microscopy (TAM) offers high spatial and temporal resolution which can capture excited state dynamics in heterogeneous functional materials. We show that charge carrier diffusion in three dimensions has to be considered when interpreting TAM data of heterogeneous thin films to obtain robust results. We present a method for analysing TAM data by simulating three-dimensional charge carrier transport and predicting the resulting diffraction patterns. We apply this method to experimental data of excited state population dynamics in methylammonium lead halide perovskite thin films. The results display a multitude of kinetics that would be easily misinterpreted as multiple species or physico-chemical mechanisms if the effect of local morphology is neglected. Our numerical simulations show that thickness heterogeneities of a sample lead to concentration gradients in the local excited state concentrations which strongly affect the transport of charge carriers and light diffraction. Thus, our framework provides a solution for including effects of local heterogeneities when evaluating charge carrier kinetics in TAM, for obtaining clean local excited state population dynamics. Further, we show that data analysis based on our simulations can be used to measure diffusion rates in heterogeneous samples that are smaller than the detection limit in homogeneous samples.