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Polarons in Conjugated Polymers

  • Christoph Cobet
  • Jacek Gasiorowski
  • Dominik Farka
  • Philipp Stadler
Chapter
Part of the Springer Series in Surface Sciences book series (SSSUR, volume 52)

Abstract

Conjugated polymers and polymer blends are key components in the development of organic electronics and (photo-) electrocatalysis. In particular, the possibility to produce organic but highly conducting films make these compounds very attractive. Therefore, enormous effort was put in the understanding and improvement of the electrical conductivity of polymer films. Conjugated polymers in their pristine form are mostly insulating or rarely semiconducting. The alternating single and double bonds in each \(\pi \)-conjugated polymer chain give rise to the formation of a band gap; the HOMO-LUMO gap. Semiconducting or conducting properties are obtained for example by optical, chemical, or electrochemical doping. The doping can be permanent as in the case of the polymer blends like PEDOT:PSS or short term. In both cases, the injected charge carriers commonly self-localize due to the strong electron-phonon interaction which yields in the formation of new quasi-particles called polarons. As a result, characteristic sub-band gap excitations emerge in optical measurements which extend from UV to the medium infrared spectral range. Optical methods in general, and spectroscopic ellipsometry in particular, are thus apparent characterization methods in scientific investigations as well as candidates to solve in-line monitoring and control issues. In the following section, we will briefly review the basic concepts of polymer “doping”, the formation of polarons and the origin of sub-band gap excitations. In a survey of methods we will shortly discuss ATR-FTIR and transmission/reflection spectroscopy results. A specific attention will be drawn on the in-situ spectroelectrochemical characterization, since electrochemical doping provides control on the doping level and allows e.g. a quantification of exchanged charges. In-situ ellipsometry could be used to monitor respective changes in the polymer optoelectronic properties. We will not aim for an overview about known types of conducting polymers in general or state of the art developments in organic electronics. The focus is a discussion of the physics of UV-VIS-MIR polaronic and electronic excitations as well as state-of-the-art ellipsometric characterization.

Notes

Acknowledgements

The authors would like to thank Kurt Hingerl, Niyazi S. Sariciftci, Helmut Neugebauer, and Reghu Menon for their valuable comments and enlightening discussions. Furthermore, we acknowledge manifold contributions of Achim W. Hassel, Günther Knör, Jan Philipp Kollender, Andrei I. Mardare, Kerstin Oppelt, Thomas Plach, Stefanie Schlager, Matthew S. White, Karin Wiesauer, and Cigdem Yumusak for the results presented here.

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Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Center of Surface and Nanoanalytics (ZONA)Johannes Kepler University LinzLinzAustria
  2. 2.Linz Institute of Organic Solar Cells (LIOS)Johannes Kepler University LinzLinzAustria
  3. 3.EV Group E.Thallner GmbHSt. Florian am InnAustria

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