Tin-Free Synthesis of a Ternary Random Copolymer for BHJ Solar Cells: Direct (Hetero)arylation versus Stille Polymerization

In a recent report (Eur. J. Org. Chem. 2014, 30, 6583) we emphasized the importance of addressing research efforts in OPV-active materials toward synthetic processes scalable up to industrial production. In this context, palladium-catalyzed direct (hetero)arylation polymerization (DHAP) can be a suitable approach to reduce the number of reaction steps and to avoid the use of toxic reagents in the synthesis of donor polymers. Random donor-acceptor copolymers have been shown to be promising materials for bulk heterojunction (BHJ) solar cells with high efficiencies and increased thermal stability. We report here the synthesis by DHAP of a ternary double-acceptor/donor random copolymer including benzo[c][1,2,5]thiadiazole and benzo[d][1,2,3]triazole as the accepting units and benzo[1,2-b;4,5-b?]dithiophene as the donor moiety. The results are discussed in comparison with the synthesis of the same polymer via the Stille polymerization. The coupling products formed in the early stage of the polycondensation have been isolated and characterized by NMR spectroscopy to gain insight into the regiochemistry of DHAP. The polymers synthesized have been tested in BHJ solar cells with PC71BM as the electron acceptor material. Power conversion efficiencies (up to 2.8%) are comparable or lower (depending on the processing conditions) than those of the same polymer synthesized via the Stille coupling reaction; however, the DHAP protocol is more convenient in terms of synthetic complexity.