Synthesis and characterization of low bandgap conjugated poly[(3-alkylylthiophene-2,5-diyl)-alt-(3-cyanothiophene-2,5-diyl)] by direct heteroarylation polymerization

A series of novel conjugated copolymers of poly[(3-alkylthiophene-2,5-diyl)-alt- (3-cyanothiophene-2,5-diyl)]s, (PACTs), containing electron-releasing 3-alkylyl thiophene-2,5-diyl (AT) and electron-withdrawing dibromo 3-cyanothiophene- 2, 5-diyl (CT) were synthesized by the polycondensation of a direct heteroarylation reaction. The regular alteration of the electron donor and electron acceptor unit in a polymer backbone led to a decrease in the energy bandgap of conjugated poly(3-alkylylthiophene-2,5-diyl)s (P3HT). To accelerate the polycondensation reaction, microwave-assisted heating was used to reduce the reaction time and increase the molecular weight of the polymers. Owing to the strong intermolecular interactions between the neighboring 3-cyanothiophene-2,5-diyl units in the polymer backbone, the lamellar and edge-on structures of the polymer chains were induced on the substrate. The photovoltaic performance of these conjugated copolymers was also evaluated by preparing bulk heterojunction solar cell devices of ITO/PEDOT: PSS/PACT:PC₇₁BM/Al.