Fermi Level Shift Induced by Defect-Photocarrier Interaction at Mixed- Cation Halide Perovskite Surface

In recent years, organometal halide perovskites have garnered tremendous attention for photovoltaic and other optoelectronic applications. However, because these materials inherently have a high concentration of point defects, they usually are not stable under device operation conditions. We investigated the interaction between defects and photocarriers by purposely increasing the concentration of defects in MAPbI₃ films. In films with excess iodide vacancies (V_I), exposure to light induces a significant Fermi level shift (E_f ) of up to 0.7 eV. Moreover, halide vacancies can trap photocarriers for hours, even after light exposure ends. We attribute these observations to the capturing of photoexcited electrons by V_I at the perovskite surface resulting in lattice deformation that stabilizes the captured electron. This process is akin to polaron formation at a defect site. We further investigated the defect-photocarrier interaction in mixed cation halide perovskites with cations like formamidinium (FA) and cesium (Cs) that have different radii. We found that the light-induced shift in the Fermi level in FA_0.2MA_0.8PbI₃ is significantly larger than that in Cs_0.1MA_0.9PbI₃. The larger E_f shift by the incorporation of large ions (FA⁺) suggests that a larger lattice can facilitate the observed defect-photocarrier interaction.