Highly Efficient Tin Perovskite Solar Cells via Suppressing Superoxide Generation

Power conversion efficiency (PCE) and stability of tin perovskite solar cells (TPSCs) are major concerns in developing lead-free photovoltaics. Photovoltaic performance of TPSCs often suffers from the oxidation of Sn²⁺, organic degradation, and ion migration, which inevitably cause plenty of trap states and render inferior device parameters. Herein, a natural ascorbic acid is first introduced for high-performance TPSCs as a multifunctional reductant to suppress the oxidation of Sn²⁺ and regulate trap states accordingly. Interestingly, it is found that the ascorbic acid reduces Sn⁴⁺ to Sn²⁺ by C-C double bonds and forms a complex with Cs_0.05FA_0.95SnI₃ perovskites via strong hydrogen bonding interactions. By virtue of theoretical calculations, the mechanism of the ascorbic acid role is further clarified. Apart from effective passivation and suppressing trap density, a superoxide interaction between perovskite and ascorbic acid is proposed. The existence of ascorbic acid successfully improved the energy barrier for O₂⁻ generation. As a result, a significantly improved PCE from 8.95% to 13.32% is achieved for Cs_0.05FA_0.95SnI₃ TPSCs with 0.5% ascorbic acid incorporation under AM 1.5 G illumination. In addition, our devices maintain 90% value of initial PCE after 500 h storage.