Two regioisomeric pi-conjugated terpolymers bearing carboxylated thiophene (CT) as electron accepting unit and thienothiophene (TT) and bithiophene (BT) as electron donating units were successfully synthesized for enhancing the performance of polymer solar cells (PSCs). Regio-regular and regio-random binary copolymers based on CT and BT were also prepared as control polymers. All polymers showed unique optical properties and crystalline behaviors. Among the PSC devices fabricated in this study, the PSC based on the regio-random terpolymer (Ran-TT) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis (4-hexylphenyl) dithieno [2,3-d:2',3'-d']-s-indaceno [1,2-b:5,6b']-dithiophene (ITIC) exhibited the highest power conversion efficiency of 5.65% with a high short circuit current density (J(sc)) of 12.63 mA cm(-2) and an open circuit voltage (V-oc) of 0.81 V. The promising PCE value is attributed to the low-lying highest occupied molecular orbital (HOMO) of Ran-TT, effective complementary absorption spectrum and favorable internal morphology of the blend film. The internal morphology of the Ran-TT was observed to be more fine phase separation than that of the regio-regular terpolymer, facilitating exciton diffusion and dissociation in the bulk heterojunction. (C) 2018 Elsevier Ltd. All rights reserved.