Dithienocarbazole- and benzothiadiazole-based donor-acceptor conjugated polymers for bulk heterojunction polymer solar cells

Donor-acceptor (D-A)-conjugated polymers P(BT-C1) and P(BT-C2), with dithieno[2,3-b;7,6-b]carbazole (C1) or dithieno[ 3,2-b;6,7-b]carbazole (C2) as D-unit and benzothiadiazole (BT) as A-unit, were synthesized. The optical bandgaps of the polymers are similar (1.84 and 1.88 eV, respectively). The structures of donor units noticeably influence the energy levels and backbone curvature of the polymers. P(BT-C1) shows a large backbone curvature; its highest occupied molecular orbital (HOMO) energy level is -5.18 eV, whereas P(BT-C2) displays a pseudo-straight backbone and has a HOMO energy level of -5.37 eV. The hole mobilities of the polymers without thermal annealing are 1.9x10(-3) and 2.7x10(-3) cm(2)V(-1)s(-1) for P(BT-C1) and P(BT-C2), respectively, as measured by organic thin-film transistors (OTFTs). Polymer solar cells using P(BT-C1) and P(BT-C2) as the donor and phenyl-C-71-butyric acid methyl ester (PC71BM) as the acceptor were fabricated. Power conversion efficiencies (PCEs) of 4.9% and 5.0% were achieved for P(BT-C1) and P(BT-C2), respectively. The devices based on P(BT-C2) exhibited a higher V (oc) due to the deeper HOMO level of the polymer, which led to a slightly higher PCE.