Phenanthrodithiophene (PDT)–Difluorobenzothiadiazole (DFBT) Copolymers: Effect on Molecular Orientation and Solar Cell Performance of Alkyl Substitution onto a PDT Core

Synthesis, characterization, and solar cell application of three 4,7-dialkylated phenanthro[1,2-b:8,7-b′]dithiophene (PDT)–difluorobenzothiadiazole (DFBT copolymers (P1–P3) with different linear alkyl side chains to improve solubility, molecular weight, and molecular orientation are described. The utilization of Ir-catalyzed direct borylation and sequential functionalization can selectively afford the target 4,7-dialkylated PDT as the monomers. Migita–Kosugi–Stille coupling in the presence of CuI can accelerate polymerization to afford high-molecular-weight polymers along with their improved solubility. The effect of alkyl substitution at the 4,7-positions on the electronic structure of PDT–DFBT copolymers is negligible. By installation of additional alkyl chains at the 4,7-positions of PDT, the synthesized polymers P1–P3 have lower intermolecular interaction than that of nonalkylated P0, but they still maintained aggregation behavior in solution. In addition, they formed a favorable face-on orientation w...