Control of Multi-Fluorination Number and Position in D-π-A Type Polymers and Their Impact on High-Voltage Organic...

Exploring the structure-performance relationship of high-voltage organic solar cells (OSCs) is significant for pushing material design and promoting photovoltaic performance. Herein, we chose a D-π-A type polymer composed of 4,8-bis(thiophene-2-yl)-benzo[1,2-b:4,5-b']dithiophene (BDT-T) and benzotriazole (BTA) units as the benchmark to investigate the effect of the fluorination number and position of the polymers on the device performance of the high-voltage OSCs, with a benzotriazole-based small molecule (BTA3) as the acceptor. F00, F20, and F40 are the polymers with progressively increasing F atoms on the D units, while F02, F22, and F42 are the polymers with further attachment of F atoms to the BTA units based on the above three polymers. Fluorination positively affects the molecular planarity, dipole moment, and molecular aggregations. Our results show that V_OC increases with the number of fluorine atoms, and fluorination on the D units has a greater effect on V_OC than on the A unit. F42 with six fluorine atom substitutions achieves the highest V_OC (1.23 V). When four F atoms are located on the D units, the short-circuit current (J_SC) and fill factor (FF) plummet, and before that, they remain almost constant. The drop in J_SC and FF in F40- and F42-based devices may be attributed to inefficient charge transfer and severe charge recombination. The F22:BTA3 system achieves the highest power conversion efficiency of 9.5% with a V_OC of 1.20 V due to the excellent balance between the photovoltaic parameters. Our study provides insights for the future application of fluorination strategies in molecular design for high-voltage organic photovoltaics.