A novel donor–acceptor alternating copolymer based on angular-shaped benzo[2,1-b:3,4-b′]diselenophene for bulk heterojunction solar cells

A novel angular shaped monomeric donor of 4,5-bis(2-ethylhexyloxy)benzo[2,1-b:3,4-b′]diselenophene (BDSe) was synthesized and exploited as the “donor” moiety to copolymerize with 4,7-bis(4-(2-ethylhexyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) for constructing the “donor–acceptor” (D–A) polymer PBDSe-DTBT. The selenium-substituted polymer displayed both a low optical band gap of 1.71 eV and a deep HOMO level of −5.4 eV as well as excellent solubility and thermal stability. With 1% DIO as the film processing additive, bulk heterojunction solar cell based on the PBDSe-DTBT:[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) blend provided a promising average power efficiency (PCE) of 5.6%, with an open circuit voltage (Voc) of 0.80 V, a short circuit current (Jsc) of 12.30 mA cm−2 and a fill factor (FF) of 0.57. Compared to devices processed without additives, 1% DIO additive processed BHJ devices showed enhanced absorption coefficient, better molecular packing, suppressed bimolecular recombination as well as a more balanced hole and electron transport, leading to significant improvement of Jsc and FF.