A new series of conjugated copolymers (PBDT-TPD, PBDT-Th-TPD, PBDT-TT-TPD) containing donor-acceptor (D − A) structure electron-rich benzo[1,2-b:4,5-b ′ ]dithiophene (BDT) units with branched alkyl thiophene side chains and electron-deficient 5-(2-octyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD) units was designed and synthesized. To tune the optical and electrochemical properties of the copolymers, the conjugation length of the copolymers was extended by introducing -conjugated spacers such as thiophene and thieno[3,2-b]thiophene units. It was observed that PBDT-TPD showed broader absorption spectra in the longer wavelength region and the absorption maximum was red-shifted compared to that of PBDT-Th-TPD, PBDT-TT-TPD. Stokes shifts were calculated to be 52 nm for PBDT-TPD, 153 nm for PBDT-Th-TPD and 146 nm for PBDT-TT-TPD. Further, PBDT-TPD exhibited a deeper highest occupied molecular orbital energy level of −5.53 eV as calculated by cyclic voltammetry. Bulk heterojunction solar cells fabricated using PBDT-TPD as donor material exhibited a power conversion efficiency of 1.92%.