Mitigating Mobile‐Ion‐Induced Instabilities and Performance Losses in 2D Passivated Perovskite Solar Cells

Abstract Bulky ammonium salt‐based passivation is an effective strategy for enhancing the performance and stability of perovskite solar cells (PSCs). Especially, phenethylammonium iodide (PEAI) is known to greatly improve open‐circuit voltage (V_OC) and fill factor (FF). Despite these benefits, PEAI passivation leads to substantial short‐circuit current density (J_SC) losses and rapid degradation under operational conditions. In this work, it is revealed that the J_SC loss as well as the accelerated degradation in PEAI‐passivated devices is caused by an increased mobile ion density. To mitigate this performance and stability‐limiting mechanism, ultrathin layers of ammonium benzenesulfonate (ABS) and/or ethylenediammonium diiodide (EDAI₂) salts are then introduced between the PEAI and the perovskite, which stabilize the 2D perovskite layer and impede diffusion even under upon prolonged illumination. This leads to a reduced mobile ion density both in fresh devices and in the long term, lowering losses J_SC, and thus enables power conversion efficiencies of ≈25% with enhanced stability. Overall, this study not only addresses the limitations of PEAI‐based 2D passivation but also paves the way for understanding 2D‐induced ionic J_SC losses.