Title: Real-space imaging of charge carrier diffusion lengths in perovskites single crystal

Abstract

Understanding the charge carrier dynamics that occur on the surface of photoactive materials at the nanometer and femtosecond scales is one of the keys to optimizing the performance of a variety of light-conversion devices, including solar cells. Unfortunately, most of the pump-probe characterization techniques available are surface-insensitive and obtain information mainly from the bulk due to the large penetration depth of the excitation and probe pulses. One technique, however, is unique. Ultrafast scanning electron microscopy (USEM) is superior in visualizing carrier dynamics at the surface of materials with high spatial temporal resolution (i.e., nanometer (nm) and femtosecond (fs) scales, respectively). Here, we successfully used USEM, for the first time, to uncover the tremendous effect of different surface orientations and termination of perovskite single crystals on the charge carrier behaviour of MAPbI3 perovskites, the most common and efficient absorber layer in perovskite solar cells. Time-resolved snapshots of secondary electrons (SEs) and density functional theory (DFT) calculations clearly demonstrate that charge carrier diffusion, surface carrier concentration, and surface trap density are strongly facet dependent. For instance, the results indicate that charge carriers along the (001) crystal orientation displayed a maximum diffused area of 22 micrometers within 6.0 nanoseconds. In contrast, the (100) facet formed defect states and showed a large surface work function that completely prevented charge carrier diffusion and dark contrast formation. Our findings provide the key to further optimizing the surface of perovskite crystals, thus paving the way for even more efficient solar-cell devices based on perovskite single crystals.