A non-fused ring building block of an electron-rich quinoid structure, 2,5-thienoquinodimethane, has been synthesized and used in the synthesis of novel donor (D)–acceptor (A) type low bandgap polymers for the first time. Namely, 2,5-thienoquinodimethane with 4-(tert-butyl)phenyl or 4-(octyloxy)phenyl side chain as a solubilizing group was copolymerized with an electron-deficient diketopyrrolopyrrole subunit (PQD1 and PQD2, respectively). These polymer films exhibited broad and intense absorption bands in the region of 400–1000 nm. Photovoltaic devices with active layers consisting of PQD1 or PQD2 with [6,6]-phenyl-C71-butyric acid methyl ester (PCBM) revealed a broad photoresponse range covering from 400 to 1000 nm, whereas the power conversion efficiencies (η) were found to be moderate (1.44% for PQD1 and 0.96% for PQD2) under the illumination of AM 1.5G, 100 mW cm−2. The superior η value of the PQD1:PCBM-based device relative to the PQD2:PCBM-based device can be attributed to the more favorable phase separation nanostructure in the active layer as well as the higher crystallinity of PQD1 than PQD2. These results provide valuable, basic guidelines for rational designs of quinoidal heterole-based low bandgap polymers for high performance organic solar cells.
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