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n-type charge transport in heavily p-doped polymers

Abstract

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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Fig. 1: Conjugated polymer structures and acronyms.
Fig. 2: Effect of varying doping ratios on α, σ and power factor.
Fig. 3: Effect of varying FeCl3 doping ratios on UPS and IPES spectra.
Fig. 4: A.c. Hall effect measurements of conjugated polymer films that are heavily doped with FeCl3.
Fig. 5: Quantitative spin concentrations, density of states schematics and charge-carrier transport schematics in conjugated polymers as a function of FeCl3 or NOBF4 doping ratio.

Data availability

Source data for Figs. 25 are provided with this paper. Additional data are available from the corresponding author upon reasonable request.

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Acknowledgements

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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Contributions

K.R.G. and Z.L. proposed the ideas, designed the experiments and prepared the manuscript. K.R.G supervised the project. Z.L. carried out the Seebeck coefficient and electrical conductivity measurements, and prepared samples for UV–vis–NIR, UPS, IPES and Hall effect measurements. Z.L., T.L. and A.M.B. performed the UPS and IPES measurements. H.H.C. and V.P. carried out Hall effect experiments and helped in interpreting the Hall effect data. X.L. and J.M. synthesized DPP-containing polymers. X.L. and T.L. measured the UV–vis–NIR absorbance spectra. U.S.R. and C.R. performed the DFT calculations. Z.L., J.A.H., T.L. and K.N.B. prepared samples for EPR or measured EPR spectra. D.R.S., J.L.H., A. Ansary and Z.L. performed the temperature-dependent electrical conductivity measurement. A. Abtahi measured temperature-dependent Seebeck coefficients, and helped with room-temperature Seebeck coefficient and electrical conductivity measurements. All authors analysed data and helped with the writing of the manuscript.

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Correspondence to Kenneth R. Graham.

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Peer review information Nature Materials thanks Xavier Crispin, Masakazu Nakamura and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–15, Tables 1–5 and Discussion 1–6.

Source data

Source Data Fig. 2

Seebeck and electrical conductivity data presented in Fig. 2

Source Data Fig. 3

UPS and IPES data presented in Fig. 3

Source Data Fig. 4

Data from Hall effect measurements presented in Fig. 4

Source Data Fig. 5

EPR data presented in Fig. 5

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Liang, Z., Choi, H.H., Luo, X. et al. n-type charge transport in heavily p-doped polymers. Nat. Mater. 20, 518–524 (2021). https://doi.org/10.1038/s41563-020-00859-3

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