Chloride–bromide co‐engineering enables operationally stable quasi‐2D tin perovskite solar cells under ambient conditions

Abstract

Quasi‐two‐dimensional (q‐2D) tin perovskites (TPs) are a promising solution to the inherent instability of TP solar cells (TPSCs). However, studies on their operational stability in uncontrolled ambient conditions are limited due to instabilities arising from mixed lower dimensional phases and the intrinsic rapid oxidation of TPs. Here, we demonstrate that chloride‐bromide co‐engineering effectively regulates phase distribution and suppresses halide segregation in mixed‐halide q‐2D Ruddlesden–Popper TPs. Time‐of‐flight secondary ion mass spectrometry reveals preferential chloride accumulation at the perovskite interfaces, forming hydrophobic chloride‐rich regions that can protect the absorber from environmental degradation. Structural and optical analyses show that chloride improves crystallinity, promotes favorable vertical phase ordering, and minimizes mixed‐dimensional phases. External quantum efficiency (EQE) measurements further confirm a more homogeneous band structure in chloride‐rich systems. Consequently, the optimized Br 0.35 Cl 0.65 composition exhibited enhanced photocurrent generation and remarkable operational stability, retaining 93.36% of its maximum power conversion efficiency after 12 h of continuous maximum power point tracking under ambient conditions without encapsulation. These findings highlight halide co‐engineering as an effective strategy for stable lead‐free perovskite photovoltaics.