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Control of aggregate formation in poly(3-hexylthiophene) by solvent, molecular weight, and synthetic method

Authors

  • Christina Scharsich,

    1. Organic Semiconductors, Experimental Physics II, Department of Physics and Bayreuth Institute of Macromolecular Science (BIMF), University of Bayreuth, Bayreuth 95440, Germany
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  • Ruth H. Lohwasser,

    1. Applied Functional Polymers, Macromolecular Chemistry I, Department of Chemistry, University of Bayreuth, Bayreuth 95440, Germany
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  • Michael Sommer,

    1. Applied Functional Polymers, Macromolecular Chemistry I, Department of Chemistry, University of Bayreuth, Bayreuth 95440, Germany
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  • Udom Asawapirom,

    1. Macromolecular Chemistry Group, Bergische Universität Wuppertal, Wuppertal 42097, Germany
    Current affiliation:
    1. National Nanotechnology Center (NSTDA), 130 Thailand Science Park, Phahonyothin Rd., Klong 1 Klong Luang, Pathumthani 12120, Thailand
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  • Ullrich Scherf,

    1. Macromolecular Chemistry Group, Bergische Universität Wuppertal, Wuppertal 42097, Germany
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  • Mukundan Thelakkat,

    1. Applied Functional Polymers, Macromolecular Chemistry I, Department of Chemistry, University of Bayreuth, Bayreuth 95440, Germany
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  • Dieter Neher,

    1. Soft Matter Physics, Institute of Physics and Astronomy, University of Potsdam, Potsdam 14476, Germany
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  • Anna Köhler

    Corresponding author
    1. Organic Semiconductors, Experimental Physics II, Department of Physics and Bayreuth Institute of Macromolecular Science (BIMF), University of Bayreuth, Bayreuth 95440, Germany
    • Organic Semiconductors, Experimental Physics II, Department of Physics, University of Bayreuth, Bayreuth 95440, Germany
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Abstract

Aggregate formation in poly(3-hexylthiophene) depends on molecular weight, solvent, and synthetic method. The interplay of these parameters thus largely controls device performance. In order to obtain a quantitative understanding on how these factors control the resulting electronic properties of P3HT, we measured absorption in solution and in thin films as well as the resulting field effect mobility in transistors. By a detailed analysis of the absorption spectra, we deduce the fraction of aggregates formed, the excitonic coupling within the aggregates, and the conjugation length within the aggregates, all as a function of solvent quality for molecular weights from 5 to 19 kDa. From this, we infer in which structure the aggregated chains pack. Although the 5 kDa samples form straight chains, the 11 and 19 kDa chains are kinked or folded, with conjugation lengths that increase as the solvent quality reduces. There is a maximum fraction of aggregated chains (about 55 ± 5%) that can be obtained, even for poor solvent quality. We show that inducing aggregation in solution leads to control of aggregate properties in thin films. As expected, the field-effect mobility correlates with the propensity to aggregation. Correspondingly, we find that a well-defined synthetic approach, tailored to give a narrow molecular weight distribution, is needed to obtain high field effect mobilities of up to 0.01 cm2/Vs for low molecular weight samples (=11 kDa), while the influence of synthetic method is negligible for samples of higher molecular weight, if low molecular weight fractions are removed by extraction. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

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