Characterizing excited state transport and charge carrier dynamics in lead halide perovskites

Abstract

Understanding fundamental processes which drive the behavior of photoexcited charge carriers is essential to the development of novel semiconducting materials. The studies presented in this work combine ultrafast microscopy with a novel data analysis technique to provide an in-depth characterization of the excited state transport and recombination dynamics which occur in a series of lead halide perovskites. An investigation of the impact halide composition has on recombination dynamics in CsPbI 2Br revealed that trap-mediated recombination dominates at low fluences, with Auger recombination becoming increasingly important as the excitation density increases. Additionally, the average diffusivity measured for CsPbI 2Br (DA = 0.27 cm2/s) is nearly 10x lower than that observed in MAPbI 3. Further, it was determined that the dielectric constants relevant to photoexcited charge carriers in CsPbBr3 and MAPbBr3 perovskites (11.5 and 13, respectively) are intermediate between the high and low frequency limits, and that halide chemistry plays an integral role in determining the screening ability of lead halide perovskites. By correlating charge carrier diffusivities to locally measured crystal quality, it was found that solution processing methods can cause subtle lattice defects which act to impede transport and risk going undetected by bulk measurement techniques. Finally, to improve upon the traditional method for extracting diffusivities from transport measurements, which relies on perfectly Gaussian point spread functions, a new method was developed which instead relies on a numerical convolution of the actual point spread function with the diffusion equation. Compared to the traditional Gaussian method, the numerical convolution method proved to more accurately determine the diffusion coefficient, especially in the case of an anomalous point spread function.