Novel conjugated copolymers consisting of bis(2-octyldodecyloxy)benzo[1,2-b:3,4-b]dithiophene (BDT) and N-alkyl-2,2'-bithiophene-3,3'-dicarboximide (BTI) with thiophene and bithiophene linkages have been synthesized and evaluated in bulk heterojunction solar cell. BDT, BTI and thiophene (or bithiophene) units were incorporated using Stille polymerization to generate poly(5-dodecyl-3-(2-thienyl)-4H-dithieno[3,2-c:2,3-e]azepine-4,6(5H)-dione-co-4,8-di(2-octyldodecyloxy)benzo[1,2-b;3,4-b]dithiophene) (P1, P2 and P3) and poly(5-dodecyl-3-di(thien-2-yl)-4H-dithieno[3,2-c:2,3-e]azepine-4,6(5H)-dione-co-4,8-di(2-octyldodecyloxy)benzo[1,2-b:3,4-b]dithiophene) (P4, P5 and P6). The BTI unit, as the electron deficient moeity, and the BDT-thiophene (or bithiophene) unit, as the electron rich moeity, have been applied for the efficient intramolecular charge transfer. The introduction of thiophene or bithiophene unit improved power conversion efficiency (PCE) of the OPVs. The best device with P2:PC61BM (1:1) showed a open circuit voltage (V-OC) of 0.88 V, a short circuit current (J(SC)) of 7.36 mA/cm(2), and fill factor (FF) of 0.63, which yield PCE of 4.17%. (C) 2015 Elsevier B.V. All rights reserved.