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-al-lcylthien-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 cutoff to similar to 780 nm in thin films, leading to a narrow optical band gap of similar to 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 aggregation of the PffBB-n:PC71BM blend was investigated and optimized in the PSC fabrication. PSCs fabricated with the PffBB-n:PC71BM blend, conducted at 80 degrees C optimized coating temperature, showed relatively high power conversion efficiency (PCE) ranging from 8.22 to 9.93%. The well-ordered BHJ film morphology of the PffBB-14:PC71BM blend led to superior balanced charge carrier mobility, good exciton dissociation, and the least recombination loss and hence PffBB-14-based PSC reached the highest photovoltaic performance with a PCE of 9.93%, a V-oc of 0.92 V, a J(sc) of 16.77 mA cm(-2) and, a FF of 64.36%. Our results demonstrated that the synergetic effect of alkyl side chain modification and processing temperature modulation to control aggregation provides practical and powerful tools to optimize the absorption broadening and optoelectronic properties of an active layer in a BHJ PSC, thus enhancing its ultimate performance.