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.

甲基铵碘化铅（MAPbI ₃）钙钛矿因其高效，低成本的太阳能电池吸引了相当大的兴趣，但目前因其不良的环境和热稳定性而受到困扰。为了帮助开发强大的设备，我们在这里研究MAPbI ₃微孔板的微观降解途径。使用原位透射电子显微镜观察热降解过程，我们发现在85°C的适度加热下，晶体结构从四方MAPbI ₃逐渐演化。到具有固定晶体学方向的三角形碘化铅层状晶体。我们的固态微动弹性带计算结果证实，表面引发的逐层降解路径展现出最低的能垒，用于晶体跃迁。我们从实验和理论上进一步表明，钙钛矿与氮化硼薄片的包封抑制了表面降解，极大地提高了其热稳定性。这些研究为钙钛矿的热稳定性提供了机械方面的见解，从而提出了改进稳定性的新设计。