A series of donor− acceptor low-bandgap conjugated polymers, ie, PT n BT (n= 2− 6), composed of alternating oligothiophene (OTh) and 2, 1, 3-benzothiadiazole (BT) units were synthesized by Stille cross-coupling polymerization. The number of thiophene rings in OTh units, that is n, was tuned from 2 to 6. All these polymers display two absorption bands in both solutions and films with absorption maxima depending on n. From solution to film, ...