Fluorine substitutions engineering of benzotriazole-based hole transport polymers toward high-performance CsPbI2Br perovskite solar cells

Developing suitable hole transport materials is of utmost importance in the quest to enhance the performance of CsPbI₂Br 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 CsPbI₂Br 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.