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Doping of molecular semiconductors through proton-coupled electron transfer


The chemical doping of molecular semiconductors is based on electron-transfer reactions between the semiconductor and dopant molecules; here, the redox potential of the dopant is key to control the Fermi level of the semiconductor1,2. The tunability and reproducibility of chemical doping are limited by the availability of dopant materials and the effects of impurities such as water. Here we focused on proton-coupled electron-transfer (PCET) reactions, which are widely used in biochemical processes3,4; their redox potentials depend on an easily handled parameter, that is, proton activity. We immersed p-type organic semiconductor thin films in aqueous solutions with PCET-based redox pairs and hydrophobic molecular ions. Synergistic reactions of PCET and ion intercalation resulted in efficient chemical doping of crystalline organic semiconductor thin films under ambient conditions. In accordance with the Nernst equation, the Fermi levels of the semiconductors were controlled reproducibly with a high degree of precision—a thermal energy of about 25 millielectronvolts at room temperature and over a few hundred millielectronvolts around the band edge. A reference-electrode-free, resistive pH sensor based on this method is also proposed. A connection between semiconductor doping and proton activity, a widely used parameter in chemical and biochemical processes, may help create a platform for ambient semiconductor processes and biomolecular electronics.

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Fig. 1: Confirmation of chemical doping in aqueous solutions.
Fig. 2: pH dependence of doping levels in PBTTT thin films.
Fig. 3: Elemental analysis of thin films doped at various pH values.
Fig. 4: Repeatable control of doping levels through PCET.
Fig. 5: High doping levels achieved in aqueous doping solutions.

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Data availability

The data supporting the plots within this study are available from Zenodo at


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M.I. was supported by JST, the establishment of university fellowships towards the creation of science technology innovation (grant number JPMJFS2144). This work was supported in part by JSPS KAKENHI grants (numbers JP20K15358, JP20H00392, JP22H02160 and JP22H04959). This work was supported in part by JST, CREST grant number JPMJCR21O3, Japan.

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Authors and Affiliations



M.I. and Y.Y. conceived, designed and performed the experiments, and analysed the data. M.I. and Y.Y. wrote the paper. S.W., K.A. and J.T. supervised the work. All authors discussed the results and reviewed the paper.

Corresponding author

Correspondence to Yu Yamashita.

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

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Extended data figures and tables

Extended Data Fig. 1 Effects of dissolved oxygen on the p-type doping of PBTTT thin films.

a, Conductivities of PBTTT thin films immersed in aqueous solutions with LiTFSI at pH 1-4. Solution pH was adjusted by H2SO4 and KOH. The error bars indicate uncertainties in the conductivity mainly stemming from uncertainties in the thickness of the PBTTT thin films and represent 1 s.d. among four individual samples. The markers indicate average values. b, UV-vis-NIR spectra of PBTTT thin films immersed in aqueous solutions with LiTFSI at pH 1. The red and blue lines refer to samples immersed without and with Ar bubbling, respectively. The plots suggest that dissolved oxygen promotes p-type doping at lower pH by the uncontrolled way in contrast to BQ/HQ.

Extended Data Fig. 2 Measurements of electrode potentials during doping processes.

a, A setup for electrode potential measurements. Ag/AgCl and PBTTT electrodes were immersed in aqueous solutions with BQ/HQ and LiTFSI. The PBTTT electrode was fabricated by forming a PBTTT thin film on a Au-coated glass substrate. b, Doping measurement setup based on the in situ monitoring of the Fermi level of PBTTT. The doping level of the OSCs (not necessarily PBTTT), which varies depending on the redox potential of PCET-type agents, can be monitored using potential measurements and precisely controlled by the addition of an acid or base.

Extended Data Fig. 3 Maintained crystalline structures of PBTTT thin films in repeated oxidation and reduction processes.

a, b, Grazing-incidence wide-angle X-ray scattering images of a PBTTT thin film repeatedly immersed in aqueous solutions at pH 2 (a) and 4 (b), resulting in the oxidation and reduction, respectively, of the PBTTT film. The numbers above the images correspond to the cycle number shown in Fig. 4a in the main text. The similar diffraction patterns of (a) and (b) show no significant change in crystalline structure by successive immersion processes. c, d, XRD spectra of a PBTTT thin film repeatedly immersed in aqueous solutions at pH 2 and 4, whose conditions are same as (a) and (b), in the out-of-plane (c) and in-plane (d) directions.

Extended Data Fig. 4 Effective p-type doping over a wide range of pH values.

a-d, UV-vis-NIR spectra and the chemical structure of poly(3-hexylthiophene-2,5-diyl) (P3HT) (a–c) and conductivities of P3HT thin films immersed in aqueous solutions with BQ/HQ and LiNFSI at pH 1-7 (see Supplementary Information for details on the solution preparation). The error bars represent uncertainties in the conductivity due to uncertainties in the thickness of the P3HT thin films and represent one standard deviation. e, f, Summary of the d-spacing values calculated from the XRD results in the direction of out-of-plane (c) and in-plane (d). The error bars indicate uncertainties in the d-spacings due to compound errors resulting from the propagation of uncertainties in the fitting of the diffraction peaks and represent one standard deviation. All results show a trend similar to that of PBTTT (Fig. 2 in the main text), thus suggesting that our doping method is versatile and allows for the selection of materials to meet the desired purpose.

Extended Data Fig. 5 Evaluation of doping levels based on elemental analysis.

F/C atomic ratio of samples doped under various conditions (same conditions as that of Fig. 5 in the main text). The error bars represent uncertainties in the atomic ratio due to uncertainties in the fitting and represent one standard deviation. The horizontal dotted line corresponds to the doped state with one hole and one anion for the each monomer unit of polymer, which is calculated based on the atomic composition of PBTTT and dopant anions.

Supplementary information

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Supplementary Methods and Discussion, including Supplementary Tables 1–4 and Figs 1–8.

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Ishii, M., Yamashita, Y., Watanabe, S. et al. Doping of molecular semiconductors through proton-coupled electron transfer. Nature 622, 285–291 (2023).

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