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Linear Side Chains in Benzo[1,2‑b:4,5‑b′]dithiophene–Thieno[3,4‑c]pyrrole-4,6-dione Polymers Direct Self-Assembly and Solar Cell Performance

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journal contribution
posted on 19.02.2016, 15:38 by Clément Cabanetos, Abdulrahman El Labban, Jonathan A. Bartelt, Jessica D. Douglas, William R. Mateker, Jean M. J. Fréchet, Michael D. McGehee, Pierre M. Beaujuge
While varying the size and branching of solubilizing side chains in π-conjugated polymers impacts their self-assembling properties in thin-film devices, these structural changes remain difficult to anticipate. This report emphasizes the determining role that linear side-chain substituents play in poly­(benzo­[1,2-b:4,5-b′]­dithiophene–thieno­[3,4-c]­pyrrole-4,6-dione) (PBDTTPD) polymers for bulk heterojunction (BHJ) solar cell applications. We show that replacing branched side chains by linear ones in the BDT motifs induces a critical change in polymer self-assembly and backbone orientation in thin films that correlates with a dramatic drop in solar cell efficiency. In contrast, we show that for polymers with branched alkyl-substituted BDT motifs, controlling the number of aliphatic carbons in the linear N-alkyl-substituted TPD motifs is a major contributor to improved material performance. With this approach, PBDTTPD polymers were found to reach power conversion efficiencies of 8.5% and open-circuit voltages of 0.97 V in BHJ devices with PC71BM, making PBDTTPD one of the best polymer donors for use in the high-band-gap cell of tandem solar cells.