Ion‐Diffusion Management Enables All‐Interface Defect Passivation of Perovskite Solar Cells

Abstract Perovskite solar cells (PSCs) have demonstrated over 25% power conversion efficiency (PCE) via efficient surface passivation. Unfortunately, state‐of‐the‐art perovskite post‐treatment strategies can solely heal the top interface defects. Herein, an ion‐diffusion management strategy is proposed to concurrently modulate the top interfaces, buried interfaces, and bulk interfaces (i.e., grain boundaries) of perovskite film, enabling all‐interface defect passivation. Specifically, this method is enabled by applying double interactive salts of octylammonium iodide (OAI) and guanidinium chloride (GACl) onto the 3D perovskite surface. It is revealed that the hydrogen‐bonding interaction between OA⁺ and GA⁺ decelerates the OA⁺ diffusion and therefore forms a dimensionally broadened 2D capping layer. Additionally, the diffusion of GA⁺ and Cl⁻ determines the composition of the bulk and buried interface of PSCs. As a result, n–inter–i–inter–p, i.e., five‐layer structured PSCs can be obtained with a champion PCE of 25.43% (certified 24.4%). This approach also enables the substantially improved operational stability of perovskite solar cells.