Investigating the effect of heteroatom substitution in 2,1,3-benzoxadiazole and 2,1,3-benzothiadiazole compounds for organic photovoltaics

Two new donor–acceptor−donor molecules, featuring either 2,1,3-benzothiadiazole (BT) or 2,1,3-benzoxadiazole (BO) and capped by benzofuranyl-thienyl groups, are presented. The structures of these novel compounds differ only in the identity of one heteroatom in the acceptor unit. Replacement of the sulfur atom in BT with oxygen results in a marked improvement in the performance of PC61BM blends in bulk heterojunction organic photovoltaic (BHJ OPV) devices: a 10-fold increase in PCE is observed when switching to the BO analogue. This is partly due to a preferable morphology in the donor-fullerene blends formed upon spin-coating from chlorinated solvents.