It is commonly assumed that charge-carrier transport in doped π-conjugated polymers is dominated by one type of charge carrier, either holes or electrons, as determined by the chemistry of the dopant. Here, through Seebeck coefficient and Hall effect measurements, we show that mobile electrons contribute substantially to charge-carrier transport in π-conjugated polymers that are heavily p-doped with strong electron acceptors. Specifically, the Seebeck coefficient of several p-doped polymers changes sign from positive to negative as the concentration of the oxidizing agents FeCl3 or NOBF4 increase, and Hall effect measurements for the same p-doped polymers reveal that electrons become the dominant delocalized charge carriers. Ultraviolet and inverse photoelectron spectroscopy measurements show that doping with oxidizing agents results in elimination of the transport gap at high doping concentrations. This approach of heavy p-type doping is demonstrated to provide a promising route to high-performance n-type organic thermoelectric materials.

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Published in Nature Materials, v. 2021.

Copyright © 2021, The Author(s), under exclusive licence to Springer Nature Limited

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K.R.G., Z.L., T.L., A.M.B. and A. Abtahi. acknowledge the donors of the American Chemical Society Petroleum Research Fund for partial support of this research (grant no. 57619-DNI10). K.R.G., A. Abtahi., K.N.B. and C.R. acknowledge support from the National Science Foundation (DMR-1905734). U.S.R. and C.R. acknowledge partial support from the Office of Naval Research Young Investigator Program (N00014-18-1-2448). J.L.H. and A. Ansary. were supported through the United States Department of Energy (0000223282) for performance of the low-temperature electrical conductivity measurements. Supercomputing resources on the Lipscomb High-Performance Supercomputing Cluster were provided by the Information Technology Services and the Center for Computational Sciences at the University of Kentucky. V.P. and H.H.C. acknowledge support from the National Science Foundation (ECCS-1806363). H.H.C. acknowledges partial support from the Center for Advanced Soft Electronics at Pohang University, which is funded by the Ministry of Science, ICT and Future Planning of the Republic of Korea as a Global Frontier Project (CASE-2011-0031628). J.M. and X.L. appreciate the support from the National Science Foundation (CAREER award no. 1653909).

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Source data for Figs. 2–5 are provided with this paper. Additional data are available from the corresponding author upon reasonable request.

Available for download on Sunday, July 04, 2021