Layer-by-Layer Degradation of Methylammonium Lead Tri-iodide Perovskite Microplates

The methylammonium lead iodide (MAPbI₃) perovskite has attracted considerable interest for its high-efficiency, low-cost solar cells, but is currently plagued by its poor environmental and thermal stability. To aid the development of robust devices, we investigate here the microscopic degradation pathways of MAPbI₃ microplates. Using in situ transmission electron microscopy to follow the thermal degradation process, we find that under moderate heating at 85°C the crystalline structure shows a gradual evolution from tetragonal MAPbI₃ to trigonal lead iodide layered crystals with a fixed crystallographic direction. Our solid-state nudged elastic band calculations confirm that the surface-initiated layer-by-layer degradation path exhibits the lowest energy barrier for crystal transition. We further show experimentally and theoretically that encapsulation of the perovskites with boron nitride flakes suppresses the surface degradation, greatly improving its thermal stability. These studies provide mechanistic insight into the thermal stability of perovskites that suggests new designs for improved stability. The organolead halide perovskites have shown extraordinary potential for low-cost solution-processable photovoltaic devices, with the power conversion efficiency rapidly soaring beyond 20%. However, the perovskite devices today are generally plagued by their poor environmental and thermal instability, and the atomistic mechanism for the thermal degradation remains largely unexplored. Here we report a systematic in situ transmission electron microscopy investigation of perovskite structure transition at 85°C. Along with theoretical calculations, we discover a surface reaction initiated layer-by-layer degradation pathway during the MAPbI₃ perovskite thermal degradation. Based on this finding, we developed a strategy using few-layer hBN encapsulation to restrain the surface reaction and thus considerably improve the thermal stability. This work reveals the atomistic pathway of perovskite structure evolution under heating and suggests a potential strategy for creating perovskite devices with improved stability. In situ TEM study reveals the atomistic structural evolution in perovskite material under moderate heating, for the first time revealing a surface-initiated layer-by-layer degradation model. Along with theoretical calculations, the study offers important insight regarding structural evolution and suggests effective strategies to improve the stability of perovskite materials.