Transient Sub-Band-Gap States at Grain Boundaries of CH₃NH₃PbI₃ Perovskite Act as Fast Temperature Relaxation Centers

Extensive spectroscopic studies have been performed on grain boundaries (GBs) of thin-film metal halide perovskites, which inevitably form with current fabrication methods, but direct, on-site determination of the transition energy and dynamics of the associated defect states and their impact on local carrier behaviors have remained elusive. Here, scanning electron microscopy (SEM) correlated to transient absorption microscopy (TAM) on CH₃NH₃PbI₃ perovskite particles is used to identify a defect state ∼60 meV into the band gap at GBs, which accelerates carrier cooling and act as additional energy acceptors. An in-depth statistical analysis performed on a large data set (806 distinct spatial locations) reveals that the shallow defect state, generally considered to be benign, plays a significant role in accelerating carrier cooling, which is detrimental to hot carrier solar cells.