Three D-A alternating copolymers P1-3 with 3,7-linked 2,8-bis(alkoxy)dibenzothiophene as the donor unit and benzothiadiazole (P1 and P2) or 3,4-bis(octyloxy)benzothiadiazole (P3) as the acceptor unit have been designed and synthesized. P1-3 show two broad absorption peaks in the visible region, and the internal charge transfer (ICT) absorptions at about 530 nm in solutions and 560 nm in films of P3 are much stronger than that of P1 and P2. All the polymers show narrow band gaps below 2.0 eV and the low-lying HOMO energy levels of approximately -5.30 eV. The hole mobilities of polymer films spin-cast from 1,2-dichlorobenzene (DCB) solutions are 3.0 X 10(-4), 2.7 X 10(-4), and 2.3 X 10(-3) cm(2) V-1 s(-1) for P1, P2, and P3, respectively. Under simulated solar illumination of AM 1.5G (100 mW/cm(2)), a PCE of 4.48% with a V-oc of 0.83 V, a J(sc) of 9.30 mA/cm(2), and an FF of 0.58 has been achieved for PSCs with the P3:PC71BM blend (1:3, by weight) as the active layer in simply processed devices, whereas after the optimization, PCEs of only 1.02% and 1.71% have been obtained for P1- and P2-based devices, respectively. This is the first report on dibenzothiophene-based conjugated polymers used for high efficiency polymer solar cells, demonstrating that photovoltaic performance can be improved by fine-tuning the conjugated polymer structure.