The opto-electrical, charge transport and photovoltaic properties of the highly efficient large band gap polymer, P(BDT-TDPPDT), containing electron rich 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b'] dithiophene (BDT) and a weak electron accepting 2,5-dioctyl-4,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole1,3-(2H,5H)-dione (TDPPDT) units were modulated by the incorporation of a relatively strong electron deficient thieno[3,4-b] thiophene (TT) unit on its backbone. A new random copolymer (RP2) was prepared by polymerizing the BDT derivative with TDPPDT and TT derivatives at a 2:1:1 ratio. The estimated optical band gap (E-g) and highest occupied/lowest unoccupied molecular orbital (HOMO/LUMO) energy levels of RP2 were 1.70 eV and -5.30 eV/-3.60 eV, respectively. The determined hole mobility of RP2 was 4.0 x 10(-4) cm(2)V(-1)s(-1). The polymer solar cells (PSCs) prepared with a simple device configuration of ITO/PEDOT:PSS/RP2:PC70BM/Al offered a maximum power conversion efficiency (PCE) of 5.12% with an open circuit voltage (V-oc) of 0.75 V, a current density (J(sc)) of 12.99 mA/cm(2), and a fill factor (FF) of 53%. The incorporation of a TT unit on the P(BDT-TDDPT) backbone was found to reduce the band gap by 0.41 eV and increase slightly the hole mobility of the resulting copolymer, RP2. Consequently, the PSC prepared from RP2 showed the enhanced photocurrent compared with that of the PSC prepared from P(BDT-TDPPDT). (C) 2015 Elsevier Ltd. All rights reserved.