Synthesis, characterization and photovoltaic properties of random poly(arylene-vinylene)s containing benzothiadiazole

New random poly(arylene-vinylene)s obtained by combining different amounts of benzo[2,1,3]thiadiazole units with 9,9-dialkylfluorene and/or 1,4-dialkoxybenzene building blocks were synthesized by the Suzuki-Heck polymerization and characterized for use in bulk hetero-junction solar cells. Their optical, electrochemical, morphological and photovoltaic features were investigated. Notwithstanding the relatively low weight-average molecular weights of the obtained polymers (7000–13000 Da), they formed good quality films by spin-coating. UV–Vis measurements permitted the evaluation of their band gap (1.77–2.12 eV), enabling them to harvest a broad portion of the solar spectrum from 350 nm to 650–700 nm. An electrochemical study revealed that the copolymers are endowed with HOMO/LUMO energy levels suitable for both an efficient electron transfer and a high open circuit voltage (Voc) for devices embodying the polymer/PCBM blends. This investigation pinpoints the important role of the copolymer composition (in terms of molar ratio of the monomeric units) on the performance of the donors in BHJs. In fact, in disagreement with the presumed Voc and current densities, the terpolymer poly[1,4-bis(2-ethylhexyloxy)-2,5-phenylene-vinylene-co-9,9-bis(2-ethylhexyl)-2,7-fluorenylene-vinylene-co-4,7-benzo[2,1,3]thiadiazolylene-vinylene] showed the best performance of the copolymer series, with a PCE of 0.4% and a Voc of 0.76 V, probably due to the favorable phase separation in the blend and consequently a better exciton dissociation.