Two new low band gap D–A structured conjugated polymers, PBDTTBI and PBDTBBT, based on 2-(4-(trifluoromethyl)phenyl)-1H-benzo[d]imidazole and benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole acceptor units with benzo[1,2-b;3,4-b′]dithiophene as a donor unit have been designed and synthesized via a Stille coupling reaction. The incorporation of the benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole unit into PBDTBBT has significantly altered the optical and electrochemical properties of the polymer. The optical band gap estimated from the onset absorption edge is ∼1.88 eV and ∼1.1 eV, respectively for PBDTTBI and PBDTBBT. It is observed that PBDTBBT exhibited a deeper HOMO energy level (−4.06 eV) with strong intramolecular charge transfer interactions. Bulk heterojunction solar cells fabricated with a configuration of ITO/PEDOT:PSS/PBDTBBT:PC₇₁BM/Al exhibited a best power conversion efficiency of 0.67%, with a short circuit current density of 4.9 mA cm⁻², an open-circuit voltage of 0.54 V and a fill factor of 25%.