Understanding Photoexcited Charge Carrier Dynamics in Methylammonium Lead Iodide Perovskite Thin Films by Pump Probe Microscopy

Abstract

Metal halide perovskite-based thin films, the light harvesting layer in perovskite solar cells, have received increasing attention since 2012. Although many studies have focused on elucidating the complicated dynamics involved in the photoexcitation process of metal halide perovskite-based thin film and their impacts on charge carrier behaviors, the ultrafast photoexcited dynamics (within the first picosecond) and spatial heterogeneity of those dynamics are often overlooked or investigated separately. In this dissertation, I demonstrate the ability of pump probe microscopy (PPM) to reveal the photoexcited charge carrier dynamics in methylammonium lead iodide (MAPbI3) perovskite thin films with sub-picosecond temporal resolution and sub-micrometer spatial resolution. Spatial mapping between PPM images and fluorescence lifetime microscopy (FLIM) images of MAPBI3 perovskite thin films is achieved to bridge the different time scales of charge carrier dynamics at the same region. By spatially overlapping PPM and FLIM images with scanning electron microscopy (SEM) images, I demonstrate that the spatial heterogeneity of charge carrier dynamics is not solely dependent on the morphology of MAPBI3 perovskite thin films. Additionally, a model is introduced to decompose the pump probe and fluorescence signal and separate different charge carrier dynamics. Sub-picosecond charge carrier dynamics obtained from pump-probe signals with various probing wavelengths are used to monitor the cooling process of charge carriers and extract the time-resolved charge carrier temperature with micrometer spatial resolution. In the end, I discuss the potential of PPM in investigating degradation and different microstructures of MAPBI3 perovskite thin films.