High Fill Factor CsPbI₂Br Perovskite Solar Cells Via Crystallization Management

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.