A new D-A copolymer, poly(thieno[3,2-b]thiophene-alt-bithiazole) (PTTBTz), based on thieno[3,2-b]thiophene donor unit and bithiazole acceptor unit, was synthesized by the Pd-catalyzed Stile-coupling reaction for the application as donor material in polymer solar cells (PSCs). PTTBTz film possesses high thermal stability with 5% weight-loss temperature at 450 degrees C, a lower-lying HOMO energy level at -5.20 eV, a higher hole mobility of 6.45 x 10(-3) cm(2)/(V s), and a crystalline structure. Photovoltaic performance of the polymer was investigated with [6,6]-phenyl-C-71-butyric acid methyl ester (PC70BM) or indene-C-60 bisadduct (ICBA) as acceptor and with 3% DIO additive. The power conversion efficiency (PCE) of the PSC based on PTTBTz:ICBA (1:1 w/w) reached 5.35% with a high V-oc of 1.03 V, a J(sc) of 8.55 mA/cm(2), and an FF of 0.608, whereas the PCE of the PSC based on PTTBTz:PC70BM (1:1 w/w) was 4.57% with a V-oc of 0.82 V, a J(sc) of 9.89 mA/cm(2), and an FF of 0.563, under the illumination of AM1.5, 100 mW/cm(2). PTTBTz is one of the D-A copolymers that shows better photovoltaic performance with ICBA as acceptor than PC70BM. PTTBTz/ICBA could be a promising front active layer for high-efficiency tandem PSC because of its high V-oc.