title: 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.
abstract: 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.
authors.affiliation: King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia.
authors.forename: Clément
authors.initials: C
authors.lastname: Cabanetos
authors.forename: Abdulrahman
authors.initials: A
authors.lastname: El Labban
authors.forename: Jonathan A
authors.initials: JA
authors.lastname: Bartelt
authors.forename: Jessica D
authors.initials: JD
authors.lastname: Douglas
authors.forename: William R
authors.initials: WR
authors.lastname: Mateker
authors.forename: Jean M J
authors.initials: JM
authors.lastname: Fréchet
authors.forename: Michael D
authors.initials: MD
authors.lastname: McGehee
authors.forename: Pierre M
authors.initials: PM
authors.lastname: Beaujuge