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

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.