Energy Level Modulation of HOMO, LUMO, and Band-Gap in Conjugated Polymers for Organic Photovoltaic Applications

Authors

  • Bong-Gi Kim,

    1. Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
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  • Xiao Ma,

    1. Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
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  • Chelsea Chen,

    1. Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
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  • Yutaka Ie,

    1. The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
    2. JST-PREST, Kitakyushu Science and Research Park, 2-5 Hibikino, Wakamatsu-ku, Kitakyushu city 808-0135, Japan
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  • Elizabeth W. Coir,

    1. Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
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  • Hossein Hashemi,

    1. Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
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  • Yoshio Aso,

    1. The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
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  • Peter F. Green,

    1. Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
    2. Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
    3. Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
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  • John Kieffer,

    1. Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
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  • Jinsang Kim

    Corresponding author
    1. Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
    2. Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
    3. Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
    • Macromolecular Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA.
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Abstract

To devise a reliable strategy for achieving specific HOMO and LUMO energy level modulation via alternating donor-acceptor monomer units, we investigate a series of conjugated polymers (CPs) in which the electron withdrawing power of the acceptor group is varied, while maintaining the same donor group and the same conjugated chain conformation. Through experiment and DFT calculations, good correlation is identified between the withdrawing strength of the acceptor group, the HOMO and LUMO levels, and the degree of orbital localization, which allows reliable design principles for CPs. Increasing the acceptor strength results in an enhanced charge transfer upon combination with a donor monomer and a more pronounced decrease of the LUMO level. Moreover, while HOMO states remain delocalized along the polymer chain, LUMO states are strongly localized at specific bonds within the acceptor group. The degree of LUMO localization increases with increasing polymer length, which results in a further drop of the LUMO level and converges to its final value when the number of repeat units reaches the characteristic conjugation length. Based on these insights we designed PBT8PT, which exhibits 6.78% power conversion efficiency after device optimization via the additive assisted annealing, demonstrating the effectiveness of our predictive design approach.

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