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

A novel D-π-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 (Egopt) 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 Voc of 0.94 V without annealing and any additives. To the best of our knowledge, this is among the highest Voc 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.