Highly Air-Stable Tin-Based Perovskite Solar Cells through Grain-Surface Protection by Gallic Acid

Maintaining the stability of tin halide perovskites is a major challenge in developing lead-free perovskite solar cells (PSCs). Adding extra SnX₂ (X = F, Cl, or Br) in the precursor solution to inhibit Sn²⁺ oxidation is an essential strategy to improve device efficiency and stability. However, SnX₂ on the surface of perovskite grains tends to prohibit charge transfer across perovskite films. Here, we report a coadditive engineering approach by introducing antioxidant gallic acid (GA) together with SnCl₂ to improve the performance of tin-based PSCs. The SnCl₂-GA complex can not only protect the perovskite grains but also more effectively conduct electrons across it, leading to highly stable and efficient PSCs. The unencapsulated devices can maintain ∼80% of their initial efficiency after 1000 h of storage in ambient air with a relative humidity of 20%, which is the best air stability achieved in tin-based PSCs to date.