Buried SnI₂ Induces Gradient Heterojunctions in Sn–Pb Perovskite Solar Cells

Tin–lead (Sn–Pb) mixed perovskite solar cells (PSCs) offer the potential for higher power conversion efficiency (PCE) than their pure lead counterparts. However, the lack of a well-defined Sn/Pb compositional profile results in disordered internal electric fields, limiting carrier separation. Here, we introduce a SnI2 predeposition strategy that induces a vertical Sn/Pb composition gradient within the perovskite film. This gradient forms a continuous heterojunction, establishing a built-in electric field that enhances carrier separation and directional extraction. As a result, the optimized devices achieve a PCE of 23.2% along with improved stability, retaining 89.6% of their initial efficiency after 1032 h of storage in nitrogen. This work demonstrates a compositional and interfacial engineering approach for advancing the efficiency and durability of Sn–Pb mixed PSCs.