Substituting Benzodithiophene with Benzodifuran in Carboxylate-Containing Polymer for High-Performance Organic Solar Cells

Low-cost carboxylate-modified thiophene-based polymers show promising potential in organic solar cells (OSCs). Further optimizing the film morphology via simple molecular engineering to improve their power conversion efficiencies (PCEs) is significant in pursuing a cost-effective balance. Herein, we developed a new wide-bandgap polymer, TTC-F-BDF, by copolymerizing benzodifuran (BDF) with carboxylate-modified thieno[3,2-b]thiophene (TTC), which is derived from the counterpart polymer, TTC-F, which contains benzodithiophene (BDT) units. Incorporating BDF can effectively tailor molecular aggregation and packing order to optimize film morphology, thus improving charge transport, recombination, and collection processes, ultimately boosting the fill factor (FF) and PCE. The TTC-F-BDF: L8-BO-based OSCs achieved a PCE of up to 16.9% with an enhanced FF of 0.75, among the top PCE values for the carboxylate-containing copolymer-based OSCs. As a control, the TTC-F: L8-BO blends showed a PCE of 15.1%, with a moderate FF of 0.67. In Cl-BTA3 systems, TTC-F-BDF attained a higher PCE of 11.2%, compared with TTC-F (PCE = 10.0%), attributed to the improved FF (0.74 vs 0.65). Besides that, replacing BDT with a cheap BDF unit also contributes to reducing production costs. This work provides a simple and effective molecular design strategy to optimize film morphology for efficiency breakthrough in carboxylate-containing photovoltaic polymers.