A furan-bridged D-pi-A copolymer with deep HOMO level: synthesis and application in polymer solar cells

A novel D-pi-A copolymer, PFBT-BDT, based on furan-containing benzothiadiazole (FBT) and benzodithiophene (BDT) was designed and synthesized by Pd-catalyzed Stille-coupling method. The copolymer showed good solubility and film-forming ability combining with good thermal stability. PFBT-BDT exhibited a broad absorption from 300 to 630 nm with an absorption peak centered at 522 nm. The optical band gap (E-g(opt)) determined from the onset of absorption of the polymer film was 1.96 eV. The LUMO and HOMO energy levels of the polymer were estimated to be -3.48 eV and -5.44 eV, respectively. The polymer solar cell fabricated from the blend of the polymer as donor and PC71BM as acceptor exhibited a moderate power conversion efficiency of 2.81% with a high V-oc of 0.94 V without annealing and any additives. To the best of our knowledge, this is among the highest V-oc values for PSCs based on benzodithiophene derivatives. This work demonstrates that the replacement of thiophene moieties in conjugated polymers with more electron-withdrawing furan moieties is able to significantly lower the HOMO energy levels, and therefore, increase the open circuit voltage of solar cells.