Reductive Sn²⁺ Compensator for Efficient and Stable Sn-Pb Mixed Perovskite Solar Cells

Tin-lead (Sn-Pb) mixed perovskite with a narrow bandgap is an ideal candidate for single-junction solar cells approaching the Shockley-Queisser limit. However, due to the easy oxidation of Sn²⁺, the efficiency and stability of Sn-Pb mixed perovskite solar cells (PSCs) still lag far behind that of Pb-based solar cells. Herein, highly efficient and stable FA_0.5MA_0.5Pb_0.5Sn_0.5I_0.47Br_0.03 compositional PSCs are achieved by introducing an appropriate amount of multifunctional Tin (II) oxalate (SnC₂O₄). SnC₂O₄ with compensative Sn²⁺ and reductive oxalate group C₂O₄ ^2- effectively passivates the cation and anion defects simultaneously, thereby leading to more n-type perovskite films. Benefitting from the energy level alignment and the suppression of bulk nonradiative recombination, the Sn-Pb mixed perovskite solar cell treated with SnC₂O₄ achieves a power conversion efficiency of 21.43%. More importantly, chemically reductive C₂O₄ ^2- effectively suppresses the notorious oxidation of Sn²⁺, leading to significant enhancement in stability. Particularly, it dramatically improves light stability.