Backbone Conformation Tuning Of Carboxylate-Functionalized Wide Band Gap Polymers For Efficient Non-Fullerene Organic Solar Cells

Two carboxylate-functionalized wide band gap polymers, 2TC-TT-BDTFT and 2T-TTC-BDTFT, which feature a fluorinated benzodithiophene (BDTFT)-alt-2,5- di(thiophen-2-yl)thieno [3,2-1)]-thiophene (2T-TT) backbone having different carboxylate attaching positions, were designed and synthesized. By variation of the substitution position of carboxylate groups on the 2T-TT unit, the backbone conformation of the designed building blocks 2TC-TT and 2T-TTC and their corresponding donor acceptor polymers was fine-tuned as demonstrated by single crystal study and DFT calculation, thus yielding a large device performance difference in organic solar cells. As a result of the relatively higher planarity of the 2T-TTC unit in which the two carboxylate groups were attached on the inner thieno[3,2-b]thiophene moiety, the 2T-TTC-BDTFT polymer exhibited a red-shifted UV-vis absorption, stronger aggregation, and improved charge transport property than its polymer analogue 2TC-TT-BDTFT, in which the two outer thiophene rings were functionalized with carboxylate groups. Benefiting from the improved exciton dissociation and charge collection efficiency, better film morphology, and higher photoresponse, non-fullerene organic solar cells based on 2T-TTC-BDTFT:m-ITIC achieved a power conversion efficiency (PCE) of 11.15% with a fill factor (FF) of similar to 70%, while the 2TC-TT-BDTFT:m-ITIC cells showed a relatively lower PCE of 9.65% and FF of 59.31%. The much higher FF of 2T-TTC-BDTFT-based solar cells reflects the great merit of the carboxylation on thienothiophene moiety rather than the outer thiophene counterpart. Therefore, the modulation of the carboxylate position on polymer backbones is an efficient strategy to tune the backbone conformation, interchain packing, film morphology, and the resulting optical, electrical, and photovoltaic properties. Moreover, both the 2T-TTC-BDTFT:m-ITIC and 2TC-TT-BDTFT:m-ITIC solar cells showed excellent stability during annealing and long-term storage. These results demonstrate that carboxylate-functionalized 2T-TTC and 2TC-TT have great potentials as a weak electron-accepting building block for wide band gap polymers for high-performance non-fullerene organic solar cells, and the carboxylate position on the polymer backbones is critical for performance improvement of organic photovoltaic devices.