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Efficient molecular doping of polymeric semiconductors driven by anion exchange

Abstract

The efficiency with which polymeric semiconductors can be chemically doped—and the charge carrier densities that can thereby be achieved—is determined primarily by the electrochemical redox potential between the π-conjugated polymer and the dopant species1,2. Thus, matching the electron affinity of one with the ionization potential of the other can allow effective doping3,4. Here we describe a different process—which we term ‘anion exchange’—that might offer improved doping levels. This process is mediated by an ionic liquid solvent and can be pictured as the effective instantaneous exchange of a conventional small p-type dopant anion with a second anion provided by an ionic liquid. The introduction of optimized ionic salt (the ionic liquid solvent) into a conventional binary donor–acceptor system can overcome the redox potential limitations described by Marcus theory5, and allows an anion-exchange efficiency of nearly 100 per cent. As a result, doping levels of up to almost one charge per monomer unit can be achieved. This demonstration of increased doping levels, increased stability and excellent transport properties shows that anion-exchange doping, which can use an almost infinite selection of ionic salts, could be a powerful tool for the realization of advanced molecular electronics.

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Fig. 1: Summary of anion-exchange doping.
Fig. 2: Molecular structures and electrostatic potential maps of the cations and anions used here.
Fig. 3: Variations in anion exchange and doping concentration with the strength of ionic interactions.
Fig. 4: Highly ordered structures and coherent charge transport in doped PBTTT.

Data availability

The data that support the findings of this study are available within this Letter, its Extended Data and its Supplementary Information.

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Acknowledgements

Y.Y. was supported by a Grant-in-Aid via a Japan Society for the Promotion of Science (JSPS) Research Fellowship. S.W. acknowledges support from PRESTO-JST through the Hyper-nanospace Design Toward Innovative Functionality project (JPMJPR151E) and from the Leading Initiative for Excellent Young Researchers of JSPS. This work was also supported in part by JSPS KAKENHI grants (JP17H06123 and JP17H06200).

Author information

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Authors

Contributions

Y.Y. conceived, designed and performed the experiments and analysed the data. Y.Y. and S.W. wrote the manuscript. J. Tsurumi performed electron spin resonance analyses. M.O. assisted in performing the low-temperature magnetotransport measurements. R.F. assisted during the conductivity measurements. S.K., T.K. and T.O. contributed to interpretation of the anion-exchange phenomena. J. Takeya and S.W. supervised the work. All authors discussed the results and reviewed the manuscript.

Corresponding author

Correspondence to Shun Watanabe.

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The authors declare no competing interests.

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Peer review information Nature thanks Antonio Facchetti and Peter Ho for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Molecular structures of the compounds used here.

a, The additional anions. b, The spectator cations. Further details regarding purities and suppliers are provided in the Supplementary Information

Extended Data Fig. 2 FTIR spectroscopy to assess residual anions in PBTTT thin films.

a, FTIR spectra of PBTTT-C14 thin films: black, pristine; orange, F4TCNQ-doped; light blue, anion-exchange-doped with EMIM-TFSI; red, anion-exchange-doped with Li-TFSI; and blue, anion-exchange-doped with Li-BOB. b, Magnified FTIR spectra from 2,050 cm−1 to 2,250 cm−1. C≡N stretching (around 2,190 cm−1) appears only in the case of the F4TCNQ-doped film. c, Magnified FTIR spectra from 1,650 cm−1 to 1,950 cm−1. The doublet peak assigned to the C=O stretching mode (around 1,800 cm−1) in carbonyl groups in BOB (ref. 24) is generated only in PBTTT films anion-exchange-doped with Li-BOB. This suggests that the BOB anions were exchanged and incorporated into the PBTTT thin film.

Extended Data Fig. 3 The origin of Curie susceptibility in anion-exchange-doped PBTTT.

a, Temperature-dependent electron spin resonance (ESR) spectra obtained from PBTTT thin film anion-exchange-doped with EMIM-TFSI. The single Lorentzian ESR spectra are observed to follow the Curie law. Note that Curie susceptibility is attributed to localized spins either on F4TCNQ radical anions or on PBTTT radical cations. Hall effect measurements indicate that carriers in the highly doped PBTTT are likely to undergo delocalized transport, thus producing Pauli paramagnetic susceptibility that is negligible compared with the Curie effect. Although the g-factors of the PBTTT radical cation and F4TCNQ radical anion are identical, it is reasonable to assume that the observed Curie susceptibility originates from localized F4TCNQ radical anions. We found the experimentally determined spin concentration to be much less than the actual carrier concentration in the PBTTT thin film, as discussed in the main text. b, ESR spectrum for anion-exchange-doped PBTTT, acquired at 4.3 K with the external magnetic field perpendicular to the film plane. The result of single Lorentzian fitting is plotted as a black curve. c, The effect of temperature (T) on spin susceptibility, as determined by double integration of the ESR spectra.

Extended Data Fig. 4 Comparison of absorption spectra with and without an initiator dopant.

Optical absorption spectra of PBTTT thin films after immersion in EMIM-TFSI, with and without F4TCNQ. The spectra show that doping occurs only when F4TCNQ is dissolved in the ionic liquid. Similarly, a PBTTT thin film immersed in a solution of Li-TFSI in n-butyl acetate without F4TCNQ shows no doping. These results demonstrate that F4TCNQ is necessary to initiate the doping reaction.

Extended Data Fig. 5 Anion-exchange doping with a series of organic cations (X+).

Absorption spectra are shown following anion-exchange doping of PBTTT films with four different organic cations (with Y fixed as TFSI).

Extended Data Fig. 6 Effects of the concentrations of the initiator dopant and additional anion.

a, b, Changes in conductivity as a function of: initiator dopant (F4TCNQ) concentration (a; red); and additional anion (TFSI) concentration following anion-exchange doping (b; red). The conductivity of an F4TCNQ-doped PBTTT thin film in n-butyl acetate (nBA) is also shown as a reference (a; black). Concentration evidently has a limited effect by comparison with the salt species. The error bars in the conductivity stem from uncertainty in the thickness of PBTTT thin films, and represent one standard deviation.

Extended Data Fig. 7 Energy alignment diagram and change in Gibbs energy due to anion-exchange doping.

a, In a neutral state, the ionization potential (IP) of an organic semiconductor is equal to the edge of its semiconductor HOMO band (HOMOSC). b, In conventional molecular doping with F4TCNQ, electrons within the HOMO band of the organic semiconductor are transferred to the LUMO level (dotted line) of the F4TCNQ, such that IP is close to the LUMO level of the dopant (LUMODo). The resulting donor–acceptor association minimizes the Gibbs free energy at equilibrium (orange line) such that no further charge transfer occurs. c, In anion-exchange doping, additional energy gain reduces the Gibbs free energy of the final state (red line), thus promoting the charge-transfer reaction. In this case, the IP of the organic semiconductor exceeds the LUMO level of the dopant approximately by the energy gain resulting from anion exchange (Δex). We determined the resulting shift in IP by photoelectron yield spectroscopy to be approximately 0.2 eV (Fig. 3d).

Extended Data Fig. 8 An example of overcoming the limitation of redox potential by anion-exchange doping.

a. Ultraviolet–visible–near-infrared (UV–vis–NIR) spectra of pristine, F4TCNQ-doped, and anion-exchange-doped (with Li-TFSI) donor–acceptor copolymer thin films based on PDPP-2T-TT-OD, which has a deep HOMO level (−5.5 eV). b, Molecular structure of PDPP-2T-TT-OD. c, Energy-level alignment diagram for PDPP-2T-TT-OD, F6TCNNQ and F4TCNQ, along with the molecular structure of F6TCNNQ. Because of the deep HOMO level of PDPP-2T-TT-OD, charge transfer is not expected following conventional molecular doping with F4TCNQ. Anion-exchange doping with Li-TFSI results in bleaching of the neutral peak and the appearance of PDPP-2T-TT-OD polaron peaks, using F4TCNQ as the initiator dopant. The doping level obtained from anion-exchange doping with Li-TFSI is high compared with that reported for F6TCNNQ doping25, as determined from the intensity ratio of the neutral (815 nm) and polaron peaks (1,400 nm) (the polaron/neutral ratio is about 0.1 for F6TCNNQ molecular doping and about 0.5 for anion-exchange doping with Li-TFSI) d, UV–vis–NIR spectra of TCNQ-doped and anion-exchange-doped (with BMIM-TFSI or Li-TFSI) PBTTT thin films. A bleaching of neutral absorbance was observed only with Li-TFSI. e, Energy-level alignment diagram for PBTTT and TCNQ, along with the molecular structure of TCNQ. The LUMO level of TCNQ is too shallow (−4.5 eV) to produce ground-state charge transfer. Even so, introducing Li-TFSI (that is, anion-exchange doping) promotes efficient doping. This presumably occurs because a slight overlap of tail states between HOMO and LUMO levels could initiate charge transfer between PBTTT and TCNQ, and therefore TCNQ•− is exchanged to TFSI. Overall, control experiments show that the initiator acceptor is not necessarily a powerful acceptor, and that efficient molecular doping is driven by anion exchange.

Extended Data Fig. 9 Magnetotransport measurements of anion-exchange-doped PBTTT.

a, b, Transverse (Hall) voltage values obtained from PBTTT thin films that have been anion-exchange-doped with Li-TFSI (a) and EMIM-TFSI (b) at various temperatures. c, Effect of magnetic field, B, on the differential sheet conductivity (Δσ = σ(B) − σ(0)) at various temperatures, with B applied perpendicular to the substrate plane. See Supplementary Information for details on fitting of the magnetoconductance data.

Extended Data Fig. 10 Thermal durability of doped PBTTT thin films.

These UV–vis–NIR spectra were obtained from doped PBTTT thin films before and after annealing at the indicated temperatures. a, An F4TCNQ-doped PBTTT thin film. bi, Anion-exchange-doped films with: b, K2CO3; c, BMIM-FAP; d, BMIM-FeCl4; e, BMIM-BF4; f, BMIM-PF6; g, BMIM-TFSI; h, Li-PFSI; and i, Li-BOB. j, Ratios of conductivity before and after annealing at 120 °C and 160 °C. Details of doping conditions are provided in the Supplementary Information.

Supplementary information

Supplementary Information

This file contains Supplementary Text and Data, including Supplementary Tables 1-10, Supplementary Figures 1-10, and Supplementary References.

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Yamashita, Y., Tsurumi, J., Ohno, M. et al. Efficient molecular doping of polymeric semiconductors driven by anion exchange. Nature 572, 634–638 (2019). https://doi.org/10.1038/s41586-019-1504-9

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