Publication date: 17th February 2025
Mixed tin–lead (Sn–Pb) halide perovskites have emerged as promising candidates for next-generation photovoltaic technologies and near-infrared optoelectronic applications, owing to their narrow bandgaps and superior optoelectronic characteristics. Nevertheless, their vulnerability to oxidative degradation poses a significant challenge to their commercial viability. In this investigation, we demonstrate that the selection of the A-site cation critically influences the oxidation stability of Sn–Pb perovskites. Through a comparative analysis of perovskite thin films and solar cells utilizing different A-site cations—methylammonium (MA⁺), formamidinium (FA⁺), and cesium (Cs⁺)—we establish that Cs-containing perovskites exhibit markedly improved resistance to oxidative stress relative to MA-based counterparts.
Our study uncovers that degradation in MA-rich perovskites is predominantly driven by the formation of triiodide (I₃⁻), a potent oxidizing agent derived from native iodine (I₂) species. The hydrogen bonding interactions between MA⁺ cations and I₂ facilitate the generation of I₃⁻, thereby accelerating the oxidation of Sn(II) to Sn(IV) and consequent perovskite degradation. In stark contrast, Cs⁺ cations, with their strong polarizing capabilities, effectively sequester I₂, thereby inhibiting I₃⁻ formation and enhancing oxidative stability.
Leveraging these mechanistic insights, we propose two strategic approaches to bolster the stability of MA-based Sn–Pb perovskites against oxidation. Firstly, we enhance the polarizing environment of surface A-site cations through the application of CsI and RbI coatings, which effectively reduce triiodide formation and mitigate oxidative pathways. Secondly, the incorporation of sodium thiosulfate (Na₂S₂O₃) as an I₂ scavenger within the perovskite matrix significantly suppresses the deleterious effects of native I₂.
Our findings underscore the critical importance of A-site cation selection and surface engineering in managing oxidative degradation mechanisms in Sn–Pb perovskites. These advancements pave the way for the development of highly efficient and durable Sn–Pb perovskite-based solar cells and optoelectronic devices, facilitating their transition toward practical and long-term applications.
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