A new acceptor unit containing dimethyl-2H-benzimidazole was synthesized and utilized for the synthesis of a conjugated polymer containing electron donor–acceptor pairs for OPV devices. The dimethyl-2H-benzimidazole unit was designed to act as a substitute for the BT unit of PCDTBT. This novel dimethyl-2H-benzimidazole unit has two methyl groups which can supply higher solubility than those of the BT series. A thin film of PCDTMBI, containing the dimethyl-2H-benzimidazole unit, shows two broad absorption bands with maxima at 400 and 636 nm and an absorption onset of 756 nm, corresponding to a band gap of 1.64 eV. The oxidation onset of PCDTMBI was estimated to be 0.67 V, which corresponds to a HOMO energy level of −5.47 eV. The LUMO energy level of the polymer was thus determined to be −3.82 eV. A device with a PCDTMBI:PC₇₁BM blend had a V_OC value of 0.65 V, a J_SC value of 10.0 mA cm⁻², and a FF of 0.48, leading to an efficiency of 3.12%. The enhanced efficiency of PCDTMBI was caused by the higher IPCE value between 400 and 700 nm and high mobility (2.2 × 10⁻³ cm² V⁻¹ s⁻¹).

The effects of substitute side chain (alkyl or alkoxy) on optical and electrical properties of low-band-gap conjugated copolymers were investigated using poly(2,7-dihydroindeno[2,1-a]indene-co-4,7-di-2-thienyl-2,1,3-benzothiadiazole), poly(2,7-dihydroindeno[2,1-a]indene-co-4,7-bis(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole) (PININE-DHTBT), and poly(2,7-dihydroindeno[2,1-a]indene-co-4,7-bis[3-(hexyloxy)-2-thienyl]-2,1,3-benzothiadiazole) (PININE-DHOTBT). Alkyl introduced PININE-DHTBT exhibits blueshifted absorption spectrum, while alkoxy introduced PININE-DHOTBT exhibits redshifted absorption spectrum. Because of steric hindrance of alkyl or alkoxy side chain, highest occupied molecular orbitals of PININE-DHTBT and PININE-DHOTBT highly localized on donor segments, thereby decreasing hole mobilities of PININE-DHTBT and PININE-DHOTBT. Consequently, despite the spectral advantage of PININE-DHOTBT for photovoltaic cells, the actual solar cell property of PININE-DHOTBT was not enhanced.