A series of donor-acceptor low-bandgap conjugated polymers, i.e., PTnBT (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, absorption spectra of the polymers exhibit a noticeable red shift. Both high- and low-energy absorption bands or P'F5BT and PT6BT films locate in the visible region, which are at 468 and 662 nm for PT5BT and 494 and 657 nm for PT6BT. Consequently, their absorption spectra cover the region between 400 and 800 nm, and their optical bandgaps are 1.56 and 1.52 eV, respectively, which renders them "black" polymers. Moreover, PT5BT and PT6BT can form highly ordered thin films with field-effect mobilities up to 2.46 x 10(-2) and 1.40 x 10(-2) cm(2) V(-1) s(-1), respectively. Bulk-hetcrojunction polymer solar, cells (PSCs) fabricated with these polymers as the donor materials and 1-(3-methoxycarbonyl)propyl-l-phenvl-[6,6]-C-61 (PCBM) as the acceptor material exhibited power conversion efficiencies (PCES,) of 0.93-2.23%. PSCs based on the high-mobility "black" polymer PT6BT showed the best device performance with a PCE of 2.23%. Our results provide a rational strategy for design and synthesis of high-mobility low-bandgap conjugated polymers with broad absorption range.
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