Probing Inter- and Intra-chain Excitonic Coupling in Crystalline Polythiophene Nanofibers and Nanoparticles
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Probing Inter- and Intra-chain Excitonic Coupling in Crystalline Polythiophene Nanofibers and Nanoparticles
Mina Baghgar,* Joelle A. Labastide,* Aidan McKenna, Adam J. Wise, and Michael D. Barnes** Dept. of Chemistry, Department of Physics University of Massachusetts-Amherst Amherst MA 01003 *These authors contributed equally to this work ** Corresponding author ([email protected]) ABSTRACT We summarize some recent new results probing inter- and intra-chain coupling in aggregated P3HT in isolated nanoparticles and nanofibers. Time-resolved photoluminescence studies show interesting correlations between amplitude and decay constant for different decay components that are tied to both polymer regio-regularity and nanoparticle processing conditions. In the frequency domain, we observe distinct signatures of both H- and J-aggregate type exciton coupling, manifested as different vibronic progressions with different electronic origins, linewidths, and Huang-Rhys factors. We show how the extent of this H/J composite coupling can be tuned to a certain extent by changes in molecular parameters (polymer molecular weight and regioregularity) and by solvent processing conditions. Finally we discuss recent results of near-field optical absorption probes of nanoparticles and nanofibers where optical contrast is afforded by the different absorption cross-section (at 532 nm) for aggregated vs. unaggregated P3HT. INTRODUCTION Over the past several years, an enormous international research effort has focused on the search for high-efficiency organic-based solar energy devices.(1-6) Amenable to solution-processing techniques (e.g. roll-to-roll, or ink-jet printing processing),(7, 8) as well as an enormous available structural parameter space, semiconducting polymers and polymer blends hold the potential for realizing this goal. However, unlike their ‘hard’ semiconductor analogs, the ‘soft’ nature of organic semiconductors - referring specifically to the response of the nuclear framework upon exciton generation- conspires to significantly limit charge mobility in these materials, and hence overall device efficiency.(9, 10) An issue of perhaps equal importance is the limitation on mobility imposed by morphological heterogeneities in polymer-blend thin films which has prompted a great deal of recent research aimed at controlled morphologies for long-range transport of charges. (11-16) In this paper, we discuss our recent efforts in elucidating structural details in nanostructured polythiophene (2D nanofibers, and quasi-1D nanoparticles) and the connection with fundamental processes behind charge generation and separation which ultimately govern power conversion efficiencies.
In crystalline P3HT nanostructures, polymer chains form cofacial pi-‐stacks, which form electronic pathways for hole-‐transport along the crystal growth axis. In crystalline aggregates, charge transport relies on the strength and spatial extent of the electronic coupling between chains in a pi-‐stack
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