Gradual Fluorination on the Phenyl Side Chains for Benzodithiophene-Based Linear Polymers to Improve the Photovoltaic Performance.

To study the impact of introducing fluorine atoms onto the conjugated phenyl side chains of benzo[1,2-:4,5-']dithiophene (BDT)-based copolymers, three novel donor-π-acceptor (D-π-A) alternative polymers , , and were designed and synthesized. The phenyl-substituted-BDT, thieno[3,2-]thiophene, and benzo[][1,2,3]triazole (BTA) served as the donor, π-bridge, and acceptor units, respectively, to enable linear polymer backbones. When introducing two or four fluorine atoms into the phenyl side units of , the polymers and demonstrate a gradual decrease of energy levels and an increase of crystallinity in the pristine and blend films. It was noted that the increase in fluorine atoms gradually improved the performance parameters of polymer solar cells (PSCs) with Y6 as the acceptor. The :Y6 device yielded a power conversion efficiency (PCE) of up to 7.07% with a short-circuit () of 21.36 mA cm, an open-circu) of 0.65 V, and a fill factor (FF) of 0.51, and :Y6 exhibited a better PCE of 10.11% ( = 23.25 mA cm, = 0.74 V, and FF = 0.59), while :Y6 exhibited the best PCE of 13.62% ( = 25.29 mA cm, = 0.82 V, and FF = 0.66). The suitable energy offsets between the donor and the acceptor, high and balanced charge-carrier mobility, and the optimal morphology of the blend film contributed to the high performance of :Y6 combination. Our results demonstrate that introducing more fluorine atoms onto the phenyl side units of BDT is a prospective approach to break the trade-offs between , , and FF, and finally improve the performance of PSCs.