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

Abstract 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 Sn 2+ , 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 0.5 MA 0.5 Pb 0.5 Sn 0.5 I 0.47 Br 0.03 compositional PSCs are achieved by introducing an appropriate amount of multifunctional Tin (II) oxalate (SnC 2 O 4 ). SnC 2 O 4 with compensative Sn 2+ and reductive oxalate group C 2 O 4 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 SnC 2 O 4 achieves a power conversion efficiency of 21.43%. More importantly, chemically reductive C 2 O 4 2− effectively suppresses the notorious oxidation of Sn 2+ , leading to significant enhancement in stability. Particularly, it dramatically improves light stability.