Revisiting the Iodine Vacancy Surface Defects to Rationalize Passivation Strategies in Perovskite Solar Cells

Current knowledge on the nature of surface iodine vacancies (V_I), which are important for the photovoltaic performance and stability of perovskite solar cells, is debatable. We investigated V_I on a stable MAI-terminated CH₃NH₃PbI₃ (MAPbI₃) surface. First-principles calculations indicated the sensitivity of the atomic structure of surface V_I to the charge states and locations on the surface layer. V_I in the outermost layer are benign; however, those near the surface can be detrimental. Illumination can promote the diffusion of V_I from the outermost layer into the bulk, making them detrimental. There are two mechanisms for the surface passivation of V_I: (i) passivation in the second layer to eliminate deep-state V_I and (ii) passivation in the outermost layer to inhibit V_I diffusion upon illumination (working condition of solar cells). This work rationalizes contradictory reports on the surface properties of halide perovskites and proposes insights into their surface passivation to fabricate high-performing solar cells.