Displacement of polarons by vibrational modes in doped conjugated polymers

M. Anderson, C. Ramanan, C. Fontanesi, A. Frick, S. Surana, D. Cheyns, M. Furno, T. Keller, S. Allard, U. Scherf, D. Beljonne, G. D'Avino, E. Von Hauff, E. Da Como

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Abstract

Organic pi-conjugated polymers are deemed to be soft materials with strong electron-phonon coupling, which results in the formation of polarons, i.e., charge carriers dressed by self-localized distortion of the nuclei. Universal signatures for polarons are optical resonances below the band gap and intense vibrational modes (IVMs), both found in the infrared (IR) spectral region. Here, we study p-doped conjugated homo- and copolymers by combining first-principles modelling and optical spectroscopy from the far-IR to the visible. Polaronic IVMs are found to feature absorption intensities comparable to purely electronic transitions and, most remarkably, show only loose resemblance to the Raman or IR-active modes of the neutral polymer. The IVM frequency is dramatically scaled down (up to 50%) compared to the backbone carbon-stretching modes in the pristine polymers. The very large intensity of IVMs is associated with displacement of the excess positive charge along the backbone driven by specific vibrational modes. We propose a quantitative picture for the identification of these polaron shifting modes that solely based on structural information, directly correlates with their IR intensity. This finding finally discloses the elusive microscopic mechanism behind the huge IR intensity of IVMs in doped polymeric semiconductors.

Original languageEnglish
Article number055604
JournalPhysical Review Materials
Volume1
Issue number5
DOIs
Publication statusPublished - 30 Oct 2017

Funding

We wish to thank W. A. Lambson, R. Cousins, and P. Reddish for technical support. This project has received funding from the European Union Horizon 2020 research and innovation programme under Grant agreement No. 646176 (EXTMOS). Work in Amsterdam is supported by the NWO under the ECHO grant “Good vibrations in organic semiconductors: a new approach to correlate molecular dynamics with carrier.” Research in Wuppertal is supported by BMBF funding, under the project EPOS 03EK3529K. We thank the Royal Society for the Wolfson Lab Refurbishment Grant at University of Bath and the Belgian National Research Fund (FNRS/FRS) at University of Mons. D.B. is a FNRS research director.

ASJC Scopus subject areas

  • General Materials Science
  • Physics and Astronomy (miscellaneous)

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