Visualization of Surface Charge Carrier Diffusion Lengths in Different Perovskite Crystal Orientations Using 4D Electron Imaging

Abstract

to their outstanding optical and transport properties, including a low concentration of defect states, tunable bandgap, and ultralong charge carrier diffusion lengths. [1][2][3][4][5][6][7] Therefore, perovskite single crystals are considered to be an excellent candidate for competing with, if not replacing, polycrystalline perovskite absorber layers, which exhibit impressive solar cell device performance with a world-record photoconversion efficiency of 25.7%. [8] However, the performance of such devices has plateaued due to the large density of trap states at grain boundaries. [9,10] This obstacle can be overcome in perovskite single crystals that are free of grains, and enormous studies have been done to improve surface quality by different passivation methods for better device performance. [11,12] Hence, the correlation between the charge carrier dynamics and the surface orientation of these crystals at the femtosecond and nanometer scales needs to be fully established. Note that the charge carrier dynamics at the interface with electron and hole transporting layers is extremely difficult, if not impossible, to obtain using conventional time-resolved pump-probe techniques due to the large penetration depth Understanding charge carrier dynamics on the surface of materials at the nanometer and femtosecond scales is one of the key elements to optimizing the performance of light-conversion devices, including solar cells. Unfortunately, most of the pump-probe characterization techniques are surfaceinsensitive and obtain information from the bulk due to the large penetration depth of the pulses. However, ultrafast scanning electron microscopy (USEM) is superior in visualizing carrier dynamics at the surface with high spatialtemporal resolution. Here, the authors successfully used USEM to uncover the tremendous effect of surface orientations and termination on the charge carrier of MAPbI 3 perovskite single crystals. Time-resolved secondary electrons snapshots and density functional theory calculations clearly demonstrate that charge carrier diffusion, surface trap density, surface work function, and carrier concentration are strongly facet-dependent. The results display a diffusion length of 22 micrometers within 6.0 nanoseconds along (001) orientation. While (100) facet forms defect states that prevent carrier diffusion and shows an increase in the surface work function leading to dark contrast and fast charge carrier recombination. These findings provide a new key component to optimizing the surface of perovskites, thus paving the way for even more efficient and stable solar-cell devices based on perovskite single crystals.

Spatiotemporal Visualization of Photogenerated Carriers on an Avalanche Photodiode Surface Using Ultrafast Scanning Electron Microscopy

“…The actual contrast may be caused by the combined effects of the SE emission yield (consistent with the spot shape) and the external field on the SE collection efficiency (asymmetric distribution). In fact, several asymmetric USEM contrast images have been presented in previous reports [53,54], but the origin of the asymmetry was not discussed elaborately. The asymmetric pattern may be related to the strong SPV caused by specific materials and/or depend on the surface quality.…”