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

Bulky ammonium salt-based passivation has proven to be an effective strategy for enhancing the performance and stability of perovskite solar cells (PSCs). Especially, phenethylammonium iodide (PEAI), has been successfully used to increase the open-circuit voltage (VOC) and fill factor (FF) by passivating surface defects and reducing non-radiative recombination at the perovskite/C60 interface. Despite these benefits, PEAI passivation has been associated with substantial short-circuit current density (JSC) losses and rapid degradation under operational conditions. In this work, we thoroughly investigate the origin of this JSC loss using a combination of scan-rate-dependent fast-hysteresis, bias�assisted charge extraction (BACE), and voltage-dependent photoluminescence (PL) measurements. We find that the JSC loss as well as the accelerated degradation observed in PEAI-passivated devices is correlated to an increase in the mobile ion density.To mitigate this performance and stability limiting mechanism we then introduce ultrathin layers of ammonium benzenesulfonate (ABS) and/or ethylenediammonium diiodide (EDAI2) salts between the PEAI and the perovskite. This strategy effectively reduced the density and impact of PEAI-induced mobile ions, thereby lowering ionic JSC losses. Champion devices with EDAI2/PEAI bilayers then reach power conversion efficiencies of ~25 %. Using grazing incidence wide-angle X-ray scattering (GIWAXS), we demonstrated that ABS and EDAI2 interlayers stabilize the 2D perovskite layer, impeding the diffusion of this layer into the 3D perovskite upon prolonged illumination,reducing the mobile ion density in the long term.This enabled a 42% increase in stability compared to PEAI-only devices under prolonged illumination at 40°C.This study not only addresses the limitations of PEAI-based 2D passivation but also paves the way for an understanding of a 2D-induced ionic JSC loss for future advancements in high-performance and stable PSCs.