Electron-Volt Fluctuation of Defect Levels in Metal Halide Perovskites on a 100 ps Time Scale

Metal halide perovskites (MHPs) have gained considerable attention due to their excellent optoelectronic performance, which is often attributed to unusual defect properties. We demonstrate that midgap defect levels can exhibit very large and slow energy fluctuations associated with anharmonic acoustic motions. Therefore, care should be taken classifying MHP defects as deep or shallow, since shallow defects may become deep and vice versa. As a consequence, charges from deep levels can escape into bands, and light absorption can be extended to longer wavelengths, improving material performance. The phenomenon, demonstrated with iodine vacancy in CH3NH3PbI3 using a machine learning force field, can be expected for a variety of defects and dopants in many MHPs and other soft inorganic semiconductors. Since large-scale anharmonic motions can be precursors to chemical decomposition, a known problem with MHPs, we propose that materials that are stiffer than MHPs but softer than traditional inorganic semiconductors, such as Si and TiO2, may simultaneously exhibit excellent performance and stability.

Automated summary

Metal halide perovskites (MHPs) are attracting attention for their excellent optoelectronic performance, which is influenced by the large and slow energy fluctuations of midgap defect levels associated with anharmonic acoustic motions. Careful classification of MHP defects is necessary, as shallow defects may become deep and vice versa, leading to improved material performance. This phenomenon, demonstrated with iodine vacancy in CH3NH3PbI3, is expected in various MHPs and other soft inorganic semiconductors. Materials that are stiffer than MHPs but softer than traditional inorganic semiconductors may offer excellent performance and stability.