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

Abstract

To study the impact of introducing fluorine atoms onto the conjugated phenyl side chains of benzo[1,2-b:4,5-b']dithiophene (BDT)-based copolymers, three novel donor-π-acceptor (D-π-A) alternative polymers PE40, PE42, and PE44 were designed and synthesized. The phenyl-substituted-BDT, thieno[3,2-b]thiophene, and benzo[d][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 PE40, the polymers PE42 and PE44 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 PE40:Y6 device yielded a power conversion efficiency (PCE) of up to 7.07% with a short-circuit (JSC) of 21.36 mA cm-2, an open-circuVOC) of 0.65 V, and a fill factor (FF) of 0.51, and PE42:Y6 exhibited a better PCE of 10.11% (JSC = 23.25 mA cm-2, VOC = 0.74 V, and FF = 0.59), while PE44:Y6 exhibited the best PCE of 13.62% (JSC = 25.29 mA cm-2, VOC = 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 PE44: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 VOC, JSC, and FF, and finally improve the performance of PSCs.