A medium-band-gap polymer based alkoxyl-substituted benzoxadiazole moiety for efficient polymer solar cells

A medium-band-gap donor–acceptor (D–A) polymer (PBDTS-DTBO) was designed and synthesized with alkoxyl-substituted benzo[c][1,2,5]oxadiazole (BO) as A unit and benzo[1,2-b:4,5-b']dithiophene (BDT) derivative (BDTTS) as D moiety. The resulting polymer not only possesses a high number-average molecular weight (Mn) of 53.0 kDa, but also exhibits a deep highest occupied molecular orbital (HOMO) energy level of −5.43 eV, which is in favor of a higher VOC of 0.88 V in the photovoltaic devices. Consequently, due to excellent phase separation and weak bimolecular recombination, the efficiency of the fullerene-based solar cells can reach 8.47% with a high current density (JSC) of 14.93 mA cm−2 and FF of 0.661, which is one of highest values for currently reported benzoxadiazole-based materials. In addition, considering the polymer also shows a medium optical bandgap of 1.75 eV, which can be well complementary with the classical non-fullerene acceptor ITIC. The best performance device exhibits a good efficiency of 6.45% in fullerene-free devices, which could be ascribed to the serious bimolecular recombination. This work revealed that alkoxyl-substituted benzoxadiazole could be a promising moiety to construct efficient light-harvesting polymer. [Display omitted] •Side-chain engineering promotes benzoxadiazole-based polymer with high Mn and HOMO energy level.•The efficiency of 8.47% is one highest values for BO-based solar cells.•The fullerene-based solar cells exhibit weak bimolecular recombination.