Direct Optical Characterization of Subgrain Boundaries and Carrier Diffusion Rates in Perovskites

Perovskite solar cells (PSCs) have attracted considerable attention in recent years due to their rapidly increasing power conversion efficiency (PCE), which currently exceeds 23%. An important feature in organic−inorganic halide perovskites (OIHPs) that limits PCE is their defect densities. Here we report an optical spectroscopy method for identifying subgrain special boundaries that introduce defects and limit carrier diffusion. These subgrain boundaries do not exhibit clear surface morphology features that can be recognized by scanning electron microscopy (SEM) or atomic force microscopy (AFM). Such special boundaries may explain why PSC performance has not scaled with apparent grain size. Furthermore, in order to quantitatively study the role of defects, we establish a method to estimate defect density by directly studying the spatial and temporal distribution of carriers in OIHP thin films.