High-Performance Tin–Lead Mixed-Perovskite Solar Cells with Vertical Compositional Gradient

Sn–Pb mixed perovskites with bandgaps in the range of 1.1–1.4 eV are ideal candidates for single-junction solar cells to approach the Shockley–Queisser limit. However, the efficiency and stability of Sn–Pb mixed-perovskite solar cells (PSCs) still lag far behind those of Pb-based counterparts due to the easy oxidation of Sn²⁺. Here, a reducing agent 4-hydrazinobenzoic acid is introduced as an additive along with SnF₂ to suppress the oxidation of Sn²⁺. Meanwhile, a vertical Pb/Sn compositional gradient is formed spontaneously after an antisolvent treatment due to different solubility and crystallization kinetics of Sn- and Pb-based perovskites and it can be finely tuned by controlling the antisolvent temperature. Because the band structure of a perovskite is dependent on its composition, graded vertical heterojunctions are constructed in the perovskite films with a compositional gradient, which can enhance photocarrier separation and suppress carrier recombination in the resultant PSCs. Under optimal fabrication conditions, the Sn–Pb mixed PSCs show power conversion efficiency up to 22% along with excellent stability during light soaking.