Achieving Seasonal Reproducibility in Wide‐Bandgap Sn‐Based Perovskite Solar Cells via Proton‐Locking Interface Engineering

ABSTRACT The advancement of wide‐bandgap (WBG) Sn‐perovskite devices is substantially hindered by seasonal instability and limited reproducibility, primarily due to moisture‐induced over‐doping of PEDOT:PSS and Sn ²⁺ oxidation. Here, we introduce a hydrophobic proton‐locking interface engineering strategy by incorporating a novel S‐benzyl‐L‐cysteine (SBLC) molecule into PEDOT:PSS. The hydrophobic benzyl backbone, amine groups, and significant dipole moment of SBLC facilitate strong coordination with Sn ²⁺ , effectively preventing moisture ingress and stabilizing buried interfacial energetics. This multifunctional modulation suppresses defects and promotes uniform crystallization across varying humidity and seasonal conditions. The optimized Target device, based on a WBG Sn‐perovskite composition of PEA _0.10 FA _0.75 EA _0.15 SnI _2.15 Br _0.85 , achieves a power conversion efficiency of 11.50% and retains >80% of its average performance across 279 devices fabricated monthly over 11 months, thereby establishing the first seasonal reproducibility benchmark. Furthermore, this approach exhibits increased stability of various stress conditions and enables a record efficiency of 17.40% in all‐perovskite tandem devices featuring a WBG Sn‐perovskite. These findings provide a scalable pathway toward reproducible, tandem‐compatible, lead‐free photovoltaics.