Abstract
A novel series of low band gap donor–acceptor copolymers derived from 4,5-bis(2-ethylhexyloxy)-benzo[2,1-b:3,4-b′]dithiophene (BDT) and 5,6-difluoro-4,7-bis(4-alkylthien-2-yl)benzo[c][1,2,5]thiadiazole bearing various alkyl side chains, such as PffBB-n (n = 10, 12, 14, and 16), were developed for high-performance bulk-heterojunction (BHJ) polymer solar cells (PSCs). PffBB-n exhibited not only strong and wide absorption but also controllable aggregation behavior in solution and thin films, in which aggregation behavior was greatly influenced by the length of alkyl side chains attached, temperature applied, and solvent used. Aggregation-induced spectral broadening further extended the absorption cut-off to ∼780 nm in thin films, leading to a narrow optical band gap of ∼1.6 eV. Because of the strong aggregation strength, PffBB-n equipped with long alkyl side chains shows enhanced ordered molecular packing and good crystallinity as revealed by X-ray diffraction studies. In addition, temperature-dependent aggr...
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