Low band gap conjugated polymers consisting of alternating dodecyl thieno[3,4-b]thiophene-2-carboxylate and one or two thiophene rings: Synthesis and photovoltaic property

In order to develop conjugated polymers with low band gaps and deep HOMO levels, copolymers consisting of alternating dodecyl thieno[3,4-b]thiophene-2-carboxylate and one (DTT-T₁) or two thiophene rings (DTT-T₂) were synthesized. The estimated optical band gap and HOMO level of DTT-T₁ and DTT-T₂ were ∼1.58/1.61 and ∼-5.15/-5.20 eV, respectively, indicating that the polymers have relatively low band gaps and deep HOMO levels, as compared to many other reported polymers. Photovoltaic devices were fabricated using DTT-T₂ and a fullerene derivative (PCBM), and whose power conversion efficiency was 0.21% under the illumination of AM 1.5 (100 mW/cm²). The low conversion efficiency of the devices was attributed to the inefficiency of exciton formation due to the low absorption coefficient and self-quenching of the polymer as well as the un-optimized device conditions.