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2D coherent charge transport in highly ordered conducting polymers doped by solid state diffusion

Abstract

Doping is one of the most important methods to control charge carrier concentration in semiconductors. Ideally, the introduction of dopants should not perturb the ordered microstructure of the semiconducting host. In some systems, such as modulation-doped inorganic semiconductors or molecular charge transfer crystals, this can be achieved by spatially separating the dopants from the charge transport pathways. However, in conducting polymers, dopants tend to be randomly distributed within the conjugated polymer, and as a result the transport properties are strongly affected by the resulting structural and electronic disorder. Here, we show that in the highly ordered lamellar microstructure of a regioregular thiophene-based conjugated polymer, a small-molecule p-type dopant can be incorporated by solid state diffusion into the layers of solubilizing side chains without disrupting the conjugated layers. In contrast to more disordered systems, this allows us to observe coherent, free-electron-like charge transport properties, including a nearly ideal Hall effect in a wide temperature range, a positive magnetoconductance due to weak localization and the Pauli paramagnetic spin susceptibility.

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Figure 1: Highly ordered microstructure of doped PBTTT/F4-TCNQ.
Figure 2: Delocalized charge transport in doped PBTTT/F4-TCNQ.
Figure 3: Weak localization in the magnetoconductance of doped PBTTT/F4-TCNQ.
Figure 4: Comparison with the transport properties of disordered PEDOT:PSS.

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Acknowledgements

The research leading to these results has received funding from an ERC Synergy Grant SC2 (No. 610115) supported by the European Research Council. K.K. thanks the Samsung Scholarship Foundation for financial support. S.W. thanks H. Matsui and J. Takeya of University of Tokyo for stimulating discussions, and is supported by Research Fellowships of Japan Society for the Promotion of Science for Young Scientists, and JST PRESTO. K.B. acknowledges funding by the German Research Foundation (BR 4869/1-1). S.-i.K. and H.T. acknowledge funding from the Japan Society for the Promotion of Science (No. 25287073). The authors thank D. Venkateshvaran and A. Sadhanala of the University of Cambridge for help with the measurements. Part of this work is based on research conducted at the Cornell High Energy Synchrotron Source (CHESS). CHESS is supported by the NSF & NIH/NIGMS via NSF award DMR-1332208. We thank D.-M. Smilgies, X. Sheng and J. Dolan for their help during the D1 experiment at CHESS. We thank A. Hexemer, R. Pandolfi and C. Zhu for supporting the data evaluation, who are supported by the US Department of Energy under Contract No. DE-AC02-05CH1123, the ECA award Program and the LBNL LDRD “TReXS”. The authors thank Professor Klaus Müllen of Max Planck Institute for Polymer Research for providing CDT-BTZ.

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K.K. and S.W. conceived, designed, performed the experiments, fabricated all the samples, analysed the data and wrote the manuscript with significant input from H.S. All X-ray measurements were done by K.B. and A.S. A.B. performed XPS measurements. I.N. measured the temperature-dependent UV–Vis optical absorption spectrum of PBTTT/F4-TCNQ and M.N. performed FTIR measurements (Supplementary information). ESR measurements were performed by H.T. and S.-i.K. (for PBTTT/F4-TCNQ), and by D.M. and K.M. (for PEDOT/PSS). Z.F. and M.H. synthesized and purified the PBTTT. S.W. and H.S. supervised this work. All authors discussed the results and reviewed the manuscript.

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Correspondence to Henning Sirringhaus.

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Kang, K., Watanabe, S., Broch, K. et al. 2D coherent charge transport in highly ordered conducting polymers doped by solid state diffusion. Nature Mater 15, 896–902 (2016). https://doi.org/10.1038/nmat4634

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