Fluorine substitutions engineering of benzotriazole‐based hole transport polymers toward high‐performance CsPbI 2 Br perovskite solar cells

Developing suitable hole transport materials is of utmost importance in the quest to enhance the performance of CsPbI 2 Br perovskite solar cells (PSCs). Among the various undoped hole transport materials (HTMs), D‐π‐A type polymers incorporating benzodithiophene (BDT) as the D unit and benzotriazole (BTA) as the A unit have shown promising potential. To further optimize the energy level and enhance the hole transport ability of these HTMs, we employed a fluorine substitution strategy to synthesize P‐BTA‐2F and P‐BTA‐4F based on the polymer P‐BTA‐0F. Subsequently, we investigated the impact of varying degrees of fluorine substitution on the properties of the polymer materials and the performance of the devices. As the number of F substitutions increases, the polymer energy level of the HTM gradually shifts downward, the face‐on stacking of the HTM strengthens, the hole mobility of the HTM increases, and the rate of hole extraction and transport becomes faster. Ultimately, the CsPbI 2 Br PSCs based on the P‐BTA‐4F HTM achieve the highest power conversion efficiency (PCE) of 17.68%. Those findings demonstrate that selecting an appropriate amount of fluorine substitution is crucial for regulating the performance of polymer HTMs and improving device efficiency.