The influence of solid-state microstructure on the origin and yield of long-lived photogenerated charge in neat semiconducting polymers

Authors

  • Obadiah G. Reid,

    1. Chemical and Materials Science Center, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401
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  • Jennifer A. Nekuda Malik,

    1. Department of Materials and Centre for Plastic Electronics, Imperial College London, London, United Kingdom
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  • Gianluca Latini,

    1. Center for Biomolecular Nanotechnologies @UNILE Istituto Italiano di Tecnologia, Via Barsanti, 73010 Arnesano (LE), Italy
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  • Smita Dayal,

    1. Chemical and Materials Science Center, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401
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  • Nikos Kopidakis,

    1. Chemical and Materials Science Center, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401
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  • Carlos Silva,

    1. Department of Physics, Université de Montréal, Montréal, Quebec H3C 3J7, Canada
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  • Natalie Stingelin,

    Corresponding author
    1. Department of Materials and Centre for Plastic Electronics, Imperial College London, London, United Kingdom
    2. Department of Materials, Swiss Federal Institute of Technology (ETH) Zürich, Zürich, Switzerland
    • Department of Materials and Centre for Plastic Electronics, Imperial College London, London, United Kingdom
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  • Garry Rumbles

    Corresponding author
    1. Chemical and Materials Science Center, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401
    2. Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, Colorado 80309
    • Chemical and Materials Science Center, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401
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Abstract

The influence of solid-state microstructure on the optoelectronic properties of conjugated polymers is widely recognized, but still poorly understood. Here, we show how the microstructure of conjugated polymers controls the yield and decay dynamics of long-lived photogenerated charge in neat films. Poly(3-hexylthiophene) was used as a model system. By varying the molecular weight, we drive a transition in the polymer microstructure from nonentangled, chain-extended, paraffinic-like to entangled, semicrystalline (MW = 5.5–347 kg/mol). The molecular weight range at which this transition occurs (MW = 40–50 kg/mol) can be deduced from the drastic change in elongation at break found in tensile tests. Linear absorption measurements of free-exciton bandwidth and time-resolved microwave conductivity (TRMC) measurements of transient photoconductance track the concomitant evolution in optoelectronic properties of the polymer as a function of MW. TRMC measurements show that the yield of free photogenerated charge increases with increasing molecular weight in the paraffinic regime and saturates at the transition into the entangled, semicrystalline regime. This transition in carrier yield correlates with a sharp transition in free-exciton bandwidth and decay dynamics at a similar molecular weight. We propose that the transition in microstructure controls the yield and decay dynamics of long-lived photogenerated charge. The evolution of a semicrystalline structure with well-defined interfaces between amorphous and crystalline domains of the polymer is required for spatial separation of the electron and hole. This structural characteristic not only largely controls the yield of free charges, but also serves as a recombination center, where mobile holes encounter a bath of dark electrons resident in the amorphous phase and recombine with quasi first-order kinetics. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

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