A soluble high molecular weight copolymer of Benzo[1,2-b:4,5-b′] dithiophene and benzoxadiazole for efficient organic photovoltaics

The synthesis and characterization of a soluble high molecular weight copolymer based on 4,8-bis(1-pentylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene and 2,1,3-benzoxadiazole is presented. High efficiency organic photovoltaic (OPV) devices comprised of this polymer and phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) were fabricated by additive processing with 1-chloronapthalene (CN). When the active layer is cast from pristine chlorobenzene (CB), power conversion efficiencies (PCEs) average 1.41%. Our best condition - using 2% chloronapthalene as a solvent additive in CB - results in an average PCE of 5.65%, with a champion efficiency of 6.05%. The use of the bulky side chain, 1-pentylhexyloxy at the 4 and 8 positions of benzo[1,2-b:4,5-b′]dithiophene allows for the synthesis of a high molecular weight, soluble copolymer with 2,1,3-benzoxadiazole. The high molecular weight and solubility allows for the preparation of organic solar cells with power conversion efficiencies near 6% when additive processing is employed.