Triiodide Formation Governs Oxidation Mechanism of Tin-Lead Perovskite Solar Cells via A-Site Choice

Mixed tin-lead (Sn-Pb) halide perovskites stand out as promising materials for next-generation photovoltaics and near-infrared optoelectronics. However, their sensitivity to oxidative degradation remains a major hurdle toward their widespread deployment. A holistic understanding of their oxidation processes considering all their constituent ions is therefore essential to stabilize these materials. Herein, we reveal that A-site cation choice plays an inconspicuous yet crucial role in determining Sn-Pb perovskite stability toward oxidation. Comparing typical A-site compositions, we show that thin films and solar cells containing cesium are more resistant to oxidative stress relative to their methylammonium analogs. We identify degradation in these compositions to be closely linked to the presence of triiodide, a harmful species evolving from native I₂ oxidants. We find that hydrogen bonding between methylammonium and I₂ promotes triiodide formation, while the strong polarizing character of cesium limits this process by capturing I₂. Inspired from these findings, we design two strategies to boost stability of sensitive methylammonium-based Sn-Pb perovskite films and devices against oxidation. Specifically, we modulate the polarizing character of surface A-sites in perovskite via CsI and RbI coatings, and we incorporate Na₂S₂O₃ as an I₂ scavenging additive. These crucial mechanistic insights will pave the way for the design of highly efficient and stable Sn-Pb perovskite optoelectronics.