High Fill Factor CsPbI₂Br Perovskite Solar Cells Via Crystallization Management

Abstract Inorganic CsPbI₂Br perovskite has a substantial potential for triple‐junction tandem solar cells as a top subcell, however it exhibits relative instability in the air compared with organic‐inorganic perovskites as well as significantly lower efficiency than the theoretical efficiency limit. To further enhance the air‐stability and efficiency of CsPbI₂Br‐based perovskite solar cells (PSCs), it is vitally crucial to improve the crystallinity and passivate the defects within films that accelerate the phase transformation to the photo‐inactive phase in the air. Here, it is reported that crystallization management via incorporating sodium formate (NaFo) in a CsPbI₂Br perovskite solution effectively leads to enlarged grain size and the reduced trap density. The Na⁺ cation and HOOC⁻ anion produce a synergistic effect for engineering the defects by acting as cation and pseudo‐halide anion passivators, respectively. As a result, the NaFo‐incorporating device shows an improved power conversion efficiency (PCE) of 17.7% with a fill factor (FF) of 84.5%. To the best of the authors' knowledge, this progressive FF value is the highest value among CsPbI₂Br‐based PSCs reported thus far. In addition, the NaFo‐incorporated device shows improved air stability compared to the control device, retaining over 95% of its initial PCE for 1000 hours under 10% relative humidity at room temperature without any encapsulation.