Mechanism of Crystallization and Implications for Charge Transport in Poly(3‐ethylhexylthiophene) Thin Films

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  • Duc T. Duong
    Department of Materials Science and Engineering Stanford University Stanford California 94305 USA
  • Victor Ho
    Department of Chemical and Biomolecular Engineering University of California, Berkeley Materials Science Division Lawrence Berkeley National Laboratory Berkeley California 94720 USA
  • Zhengrong Shang
    Department of Materials Science and Engineering Stanford University Stanford California 94305 USA
  • Sonya Mollinger
    Department of Materials Science and Engineering Stanford University Stanford California 94305 USA
  • Stefan C.B. Mannsfeld
    Stanford Synchrotron Radiation Lightsource SLAC National Accelerator Laboratory Menlo Park California 94025 USA
  • Javier Dacuña
    Department of Electrical Engineering Stanford University Stanford California 94305 USA
  • Michael F. Toney
    Stanford Synchrotron Radiation Lightsource SLAC National Accelerator Laboratory Menlo Park California 94025 USA
  • Rachel Segalman
    Department of Chemical and Biomolecular Engineering University of California, Berkeley Materials Science Division Lawrence Berkeley National Laboratory Berkeley California 94720 USA
  • Alberto Salleo
    Department of Materials Science and Engineering Stanford University Stanford California 94305 USA

Description

<jats:p>In this work, crystallization kinetics and aggregate growth of poly(3‐ethylhexylthiophene) (P3EHT) thin films are studied as a function of film thickness. X‐ray diffraction and optical absorption show that individual aggregates and crystallites grow anisotropically and mostly along only two packing directions: the alkyl stacking and the polymer chain backbone direction. Further, it is also determined that crystallization kinetics is limited by the reorganization of polymer chains and depends strongly on the film thickness and average molecular weight. Time‐dependent, field‐effect hole mobilities in thin films reveal a percolation threshold for both low and high molecular weight P3EHT. Structural analysis reveals that charge percolation requires bridged aggregates separated by a distance of ≈2–3 nm, which is on the order of the polymer persistence length. These results thus highlight the importance of tie molecules and inter‐aggregate distance in supporting charge percolation in semiconducting polymer thin films. The study as a whole also demonstrates that P3EHT is an ideal model system for polythiophenes and should prove to be useful for future investigations into crystallization kinetics.</jats:p>

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