Fine Molecular Tuning of Diketopyrrolopyrrole-Based Polymer Semiconductors for Efficient Charge Transport: Effects of Intramolecular Conjugation Structure

Abstract

To improve the charge carrier mobility of diketopyrrolopyrrole donor-acceptor copolymer semiconductors, the length of the donor building block is controlled using vinylene moieties, and its effects on crystalline structure and charge transport are systematically studied. We synthesize P29-DPP-TBT with two vinylene linkages between thiophene units and compare it with P29-DPP-TVT with single vinylene linkage. Density functional theory calculations predict enhanced backbone planarity of P29-DPP-TBT compared to P29-DPP-TVT, which can be related to the increased conjugation length of P29-DPP-TBT as proved by the increased free exciton bandwidth extracted from UV-vis absorption spectra and the wavenumber shift of the C-C peaks to higher values in Raman spectra. From two-dimensional grazing incident X-ray diffraction studies, it is turned out that the paracrystalline disorder is lower in P29-DPP-TBT than in P29-DPP-TVT. Near-edge X-ray absorption fine structure spectroscopy reveal that more edge-on structure of polymer backbone is formed in the case of P29-DPP-TBT. By measuring the temperature dependence of the charge carrier mobilities, it is turned out that the activation energy for charge hopping is lower for P29-DPP-TBT than for P29-DPP-TVT. Collectively, these results imply that the substitution of extended π-conjugated donor moiety of polymeric semiconductors can yield a more planar backbone structure and thus enhanced intermolecular interaction which enables more perfect crystalline structure as well as enhanced charge transport behavior. © 2017 American Chemical Society.