21.41%‐Efficiency CsPbI3 Perovskite Solar Cells Enabled by an Effective Redox Strategy with 4‐Fluorobenzothiohydrazide in Precursor Solution

Abstract To simultaneously stabilize cesium lead triiodide (CsPbI 3 ) precursor solution and passivate the defects in CsPbI 3 film is greatly significant for achieving highly stable and efficient CsPbI 3 perovskite solar cells (PSCs). Herein, an effective redox 4‐fluorobenzothiohydrazide (FBTH) is developed to stabilize the precursor solution and passivate iodine/lead‐related defects for high‐quality CsPbI 3 film. The comprehensive research confirms that 1) a new compound FBTH‐I is obtained from an effective redox interaction between FBTH and molecular iodine (I 2 ) in perovskite precursor solution, which can effectively impede the formation of I 2 molecule and restrain I − migration in perovskite film by forming N–H···I bond; 2) FBTH‐I can also passivate Pb‐related defects via forming S···Pb interaction. Consequently, the CsPbI 3 PSC based on FBTH‐treated precursor solution exhibits a fascinating power conversion efficiency (PCE) of 21.41%, which is one of the highest PCE values among the reported pure CsPbI 3 PSCs so far, and an outstanding stability against the harsh conditions, such as thermal annealing and continuous light‐illumination.