Surface Stoichiometric Tuning for High‐Efficiency and Light Illumination Stability in Sn–Pb Perovskite Solar Cells by PbBr₂ Wet Surface Treatment

Tin–lead (Sn–Pb) mixed perovskite solar cells (PSCs) are promising as bottom subcells in all‐perovskite tandem solar cells, but the oxidation of Sn 2+ remains challenging for long‐term stability. This study reveals a compositional gradient in Sn–Pb perovskite films, where excess Sn ions accumulate at the surface, intensifying oxidation and recombination losses. To address this issue, we introduce a PbBr 2 ‐TOAB wet surface treatment strategy during the fabrication of Sn–Pb perovskite films. X‐ray photoelectron spectroscopy analysis confirms that this treatment achieves more balanced Sn:Pb stoichiometry from ~4.36:1 to ~2.74:1, ensuring improved film quality and resistance to Sn 2+ oxidation. The treatment strategy boosts the power conversion efficiency (PCE) to 21.61% (0.09 cm 2 ), with an independently certified (from the National Institute of Advanced Industrial Science and Technology) PCE of 19.12%, compared to 21.13% for control PSCs. Moreover, target larger PSCs (1 cm 2 ) achieved an impressive PCE of 20.83%. The target PSCs show enhanced stability, retaining 80% of their initial PCE after 50 h of continuous light soaking. More importantly, encapsulated target PSCs maintain 95% of their initial PCE for 300 s under continuous illumination at maximum power point tracking conditions. Time‐dependent photoluminescence examination confirms that PbBr 2 ‐TOAB treatment significantly reduces ion migration, improving stability under light illumination.