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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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.
Chiang, C. K. et al. Electrical conductivity in doped polyacetylene. Phys. Rev. Lett. 39, 1098–1101 (1977).
MacDiarmid, A. G. & Heeger, A. J. Organic metals and semiconductors: the chemistry of polyacetylene, (CH)x, and its derivatives. Synth. Met. 1, 101–118 (1980).
McQuillan, B., Street, G. B. & Clarke, T. C. The reaction of hf with polyacetylene. J. Electron. Mater. 11, 471–490 (1982).
Han, C. C. & Elsenbaumer, R. L. Protonic acids: generally applicable dopants for conducting polymers. Synth. Met. 30, 123–131 (1989).
Mai, C.-K. et al. Facile doping of anionic narrow-band-gap conjugated polyelectrolytes during dialysis. Angew. Chem. Int. Ed. 52, 12874–12878 (2013).
Patil, A. O. et al. Self-doped conducting polymers. Synth. Met. 20, 151–159 (1987).
Chayer, M., Faïd, K. & Leclerc, M. Highly conducting water-soluble polythiophene derivatives. Chem. Mater. 9, 2902–2905 (1997).
Blochwitz, J., Pfeiffer, M., Fritz, T. & Leo, K. Low voltage organic light emitting diodes featuring doped phthalocyanine as hole transport material. Appl. Phys. Lett. 73, 729–731 (1998).
Salzmann, I., Heimel, G., Oehzelt, M., Winkler, S. & Koch, N. Molecular electrical doping of organic semiconductors: fundamental mechanisms and emerging dopant design rules. Acc. Chem. Res. 49, 370–378 (2016).
Walzer, K., Maennig, B., Pfeiffer, M. & Leo, K. Highly efficient organic devices based on electrically doped transport layers. Chem. Rev. 107, 1233–1271 (2007).
Yim, K.-H. et al. Controlling electrical properties of conjugated polymers via a solution-based p-type doping. Adv. Mater. 20, 3319–3324 (2008).
Sivaramakrishnan, S. et al. Solution-processed conjugated polymer organic p-i-n light-emitting diodes with high built-in potential by solution- and solid-state doping. Appl. Phys. Lett. 95, 213303 (2009).
Gao, W. & Kahn, A. Controlled P-doping of zinc phthalocyanine by coevaporation with tetrafluorotetracyanoquinodimethane: a direct and inverse photoemission study. Appl. Phys. Lett. 79, 4040–4042 (2001).
Zhang, Y., de Boer, B. & Blom, P. W. M. Controllable molecular doping and charge transport in solution-processed polymer semiconducting layers. Adv. Funct. Mater. 19, 1901–1905 (2009).
Gao, J. et al. The effect of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane charge transfer dopants on the conformation and aggregation of poly(3-hexylthiophene). J. Mater. Chem. C 1, 5638–5646 (2013).
E. Jacobs, I. et al. Comparison of solution-mixed and sequentially processed P3HT:F4TCNQ films: effect of doping-induced aggregation on film morphology. J. Mater. Chem. C 4, 3454–3466 (2016).
Scholes, D. T. et al. Overcoming film quality issues for conjugated polymers doped with F4TCNQ by solution sequential processing: hall effect, structural, and optical measurements. J. Phys. Chem. Lett. 6, 4786–4793 (2015).
Zalar, P. et al. Increased mobility induced by addition of a Lewis acid to a Lewis basic conjugated polymer. Adv. Mater. 26, 724–727 (2014).
Panidi, Julianna et al. Remarkable enhancement of the hole mobility in several organic small‐molecules, polymers, and small‐molecule:polymer blend transistors by simple admixing of the Lewis acid P‐dopant B(C6F5)3. Adv. Sci. 5, 1700290 (2017).
Pingel, P. et al. P-type doping of poly(3-hexylthiophene) with the strong Lewis acid tris(pentafluorophenyl)borane. Adv. Electron. Mater. 2, 1600204 (2016).
Han, Y. et al. Doping of large ionization potential indenopyrazine polymers via Lewis acid complexation with tris(pentafluorophenyl)borane: a simple method for improving the performance of organic thin-film transistors. Chem. Mater. 28, 8016–8024 (2016).
Welch, G. C., Coffin, R., Peet, J. & Bazan, G. C. Band gap control in conjugated oligomers via Lewis acids. J. Am. Chem. Soc. 131, 10802–10803 (2009).
Welch, G. C. & Bazan, G. C. Lewis acid adducts of narrow band gap conjugated polymers. J. Am. Chem. Soc. 133, 4632–4644 (2011).
Zalar, P., Henson, Z. B., Welch, G. C., Bazan, G. C. & Nguyen, T.-Q. Color tuning in polymer light-emitting diodes with Lewis acids. Angew. Chem. 124, 7613–7616 (2012).
Randell, N. M., Fransishyn, K. M. & Kelly, T. L. Lewis acid–base chemistry of 7-azaisoindigo-based organic semiconductors. ACS Appl. Mater. Interfaces 9, 24788–24796 (2017).
Liang, Z., Boland, M. J., Butrouna, K., Strachan, D. R. & Graham, K. R. Increased power factors of organic–inorganic nanocomposite thermoelectric materials and the role of energy filtering. J. Mater. Chem. A 5, 15891–15900 (2017).
Dai, A. et al. Enhanced charge-carrier injection and collection via lamination of doped polymer layers p-doped with a solution-processible molybdenum complex. Adv. Funct. Mater. 24, 2197–2204 (2014).
Ratcliff, E. L., Lee, P. A. & Armstrong, N. R. Work function control of hole-selective polymer/ITO anode contacts: an electrochemical doping study. J. Mater. Chem. 20, 2672–2679 (2010).
Li, J. et al. Introducing solubility control for improved organic P-type dopants. Chem. Mater. 27, 5765–5774 (2015).
J. Lawrence, E., S. Oganesyan, V., G. Wildgoose, G. & E. Ashley, A. Exploring the fate of the tris(pentafluorophenyl)borane radical anion in weakly coordinating solvents. Dalton Trans. 42, 782–789 (2013).
Kwaan, R. J., Harlan, C. J. & Norton, J. R. Generation and characterization of the tris(pentafluorophenyl)borane radical anion. Organometallics 20, 3818–3820 (2001).
Piers, W. E. The chemistry of perfluoraryl boranes. Adv. Organomet. Chem. 52, 1–76 (2005).
Siedle, A. R., Newmark, R. A., Lamanna, W. M. & Huffman, J. C. Structure of a zirconoxyborane having a zirconium-fluorine-carbon bridge. Organometallics 12, 1491–1492 (1993).
Danopoulos, A. A. et al. Equilibria in the B(C6F5)3–H2O system: synthesis and crystal structures of H2O·B(C6F5)3 and the anions [HOB(C6F5)3]– and [(F5C6)3B(µ-OH)B(C6F5)3]–. Chem. Commun. 0, 2529–2560 (1998).
Kalamarides, H. A., Iyer, S., Lipian, J., Rhodes, L. F. & Day, C. Pentafluoroaryl transfer from tris(pentafluorophenyl)boron hydrate to nickel. Synthesis and X-ray crystal structure of (PPh2CH2C(O)Ph)Ni(C6F5)2. Organometallics 19, 3983–3990 (2000).
Beringhelli, T., Maggioni, D. & D’Alfonso, G. 1H and 19F NMR investigation of the reaction of B(C6F5)3 with water in toluene solution. Organometallics 20, 4927–4938 (2001).
Guidotti, S. et al. Synthesis and reactivity of (C6F5)3B−N-heterocycle complexes. 1. Generation of highly acidic Sp3 carbons in pyrroles and indoles. J. Org. Chem. 68, 5445–5465 (2003).
Focante, F., Mercandelli, P., Sironi, A. & Resconi, L. Complexes of tris(pentafluorophenyl)boron with nitrogen-containing compounds: synthesis, reactivity and metallocene activation. Coord. Chem. Rev. 250, 170–188 (2006).
Tang, T., Lin, T., Wang, F. & He, C. A new aspect of cyclopentadithiophene based polymers: narrow band gap polymers upon protonation. Chem. Commun. 51, 13229–13232 (2015).
Niklas, J. et al. Highly-efficient charge separation and polaron delocalization in polymer–fullerene bulk-heterojunctions: a comparative multi-frequency EPR and DFT study. Phys. Chem. Chem. Phys. 15, 9562–9574 (2013).
Wang, M. et al. Hole mobility and electron injection properties of D-A conjugated copolymers with fluorinated phenylene acceptor units. Adv. Mater. 29, 1603830 (2017).
Wamser, C. A. Equilibria in the system boron trifluoride—Water at 25°. J. Am. Chem. Soc. 73, 409–416 (1951).
Skinner, H. A. & Smith, N. B. The heat of hydrolysis of boron tribromide. Trans. Faraday Soc. 51, 19–22 (1955).
Bottero, J. Y., Cases, J. M., Fiessinger, F. & Poirier, J. E. Studies of hydrolyzed aluminum chloride solutions. 1. Nature of aluminum species and composition of aqueous solutions. J. Phys. Chem. 84, 2933–2939 (1980).
E. Piers, W. & Chivers, T. Pentafluorophenylboranes: from obscurity to applications. Chem. Soc. Rev. 26, 345–354 (1997).
Beckett, M. A., Brassington, D. S., Coles, S. J. & Hursthouse, M. B. Lewis acidity of tris(pentafluorophenyl)borane: crystal and molecular structure of B(C6F5)3·OPEt3. Inorg. Chem. Commun. 3, 530–533 (2000).
Lee, J. et al. A planar cyclopentadithiophene–benzothiadiazole-based copolymer with Sp2-hybridized bis(alkylsulfanyl)methylene substituents for organic thermoelectric devices. Macromolecules 51, 3360–3368 (2018).
Yan, H. et al. Lewis acid doping induced synergistic effects on electronic and morphological structure for donor and acceptor in polymer solar cells. Adv. Energy Mater. 8, 1703672 (2018).
Poverenov, E., Zamoshchik, N., Patra, A., Ridelman, Y. & Bendikov, M. Unusual doping of donor–acceptor-type conjugated polymers using lewis acids. J. Am. Chem. Soc. 136, 5138–5149 (2014).
Ying, L. et al. Regioregular pyridal[2,1,3]thiadiazole π-conjugated copolymers. J. Am. Chem. Soc. 133, 18538–18541 (2011).
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.
The authors declare no competing interests.
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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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
Nature Materials (2019)