Towards understanding the doping mechanism of organic semiconductors by Lewis acids

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Abstract

Precise doping of organic semiconductors allows control over the conductivity of these materials, an essential parameter in electronic applications. Although Lewis acids have recently shown promise as dopants for solution-processed polymers, their doping mechanism is not yet fully understood. In this study, we found that B(C6F5)3 is a superior dopant to the other Lewis acids investigated (BF3, BBr3 and AlCl3). Experiments indicate that Lewis acid–base adduct formation with polymers inhibits the doping process. Electron–nuclear double-resonance and nuclear magnetic resonance experiments, together with density functional theory, show that p-type doping occurs by generation of a water–Lewis acid complex with substantial Brønsted acidity, followed by protonation of the polymer backbone and electron transfer from a neutral chain segment to a positively charged, protonated one. This study provides insight into a potential path for protonic acid doping and shows how trace levels of water can transform Lewis acids into powerful Brønsted acids.

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Fig. 1: Chemical structures of the polymers studied, and the various molecules used to dope them.
Fig. 2: Electrical characteristics of PCPDTPT and PCPDTBT with varying amounts of dopant.
Fig. 3: Thin-film properties of PCPDTPT and PCPDTBT with Lewis acid BCF.
Fig. 4: Proposed doping mechanism of PCPDTBT by BCF.
Fig. 5: Experimental evidence for the proposed doping mechanism of PCPDTBT by BCF.
Fig. 6: Lewis acid doping of a polymer lacking Lewis basic nitrogen atoms.

Data availability

The main data supporting the findings of this study are available within the Article and Supplementary Information files. Additional data are available from the corresponding authors on request.

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Acknowledgements

This work was supported by the Department of Energy under Award No. DE-SC0017659. D.X.C. was supported by the National Science Foundation Graduate Research Fellowship Program under grant no. 1650114. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. D. Leifert was supported by the Alexander von Humboldt Foundation (Feodor Lynen Return Fellowship). XPS, UPS and EPR were obtained at a facility supported by the MRSEC programme of the NSF foundation (no. DMR-1121053). We acknowledge support from the Center for Scientific Computing from the CNSI for the DFT calculations (no. NSF CNS-1725797). This research used beamline 7.3.3 of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. T.L. and K.R.G. contributed the UPS and IPES measurements under work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences and the EPSCoR programme, under award no. DE-SC0018208. A.E.M. and N.K. acknowledge support by the DFG (FoMEDOS—Projektnummer 286798544). V.V.B. would like to thank J. Vollbrecht for help in processing data, and A. Lill for preparation of substrates with interdigitated contacts.

Author information

B.Y. wrote the paper, proposed the mechanism, took all NMR, EPR and ENDOR measurements and performed the DFT calculations. B.Y. helped with absorption spectroscopy measurements. D.X.C. performed XPS and UPS measurements and helped write the manuscript. V.V.B. performed the electrical measurements and helped with writing. D. Leifert synthesized PCPDTPT and helped with absorption spectroscopy. M.W. synthesized PhF2,5. A.D. helped with absorption spectroscopy and GIWAXS. M.S. collected GIWAXS. A.E.M. and D. Lungwitz helped with XPS. T.L. and K.R.G. contributed IPES and UPS. P.J.S. performed the AFM measurements. N.K supervised XPS and helped with the writing. G.C.B. and T.-Q.N. supervised the project and helped with the writing.

Correspondence to Guillermo C. Bazan or Thuc-Quyen Nguyen.

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Supplementary Information Additional data from electrical measurements, method for determining free charge carrier density, spectroscopic data, AFM images and computational results.

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Supplementary Information

Supplementary Notes, Figs. 1–42, Tables 1–4 and refs. 1–27

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Yurash, B., Cao, D.X., Brus, V.V. et al. Towards understanding the doping mechanism of organic semiconductors by Lewis acids. Nat. Mater. 18, 1327–1334 (2019) doi:10.1038/s41563-019-0479-0

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