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Transition metal-catalysed molecular n-doping of organic semiconductors

Abstract

Chemical doping is a key process for investigating charge transport in organic semiconductors and improving certain (opto)electronic devices1,2,3,4,5,6,7,8,9. N(electron)-doping is fundamentally more challenging than p(hole)-doping and typically achieves a very low doping efficiency (η) of less than 10%1,10. An efficient molecular n-dopant should simultaneously exhibit a high reducing power and air stability for broad applicability1,5,6,9,11, which is very challenging. Here we show a general concept of catalysed n-doping of organic semiconductors using air-stable precursor-type molecular dopants. Incorporation of a transition metal (for example, Pt, Au, Pd) as vapour-deposited nanoparticles or solution-processable organometallic complexes (for example, Pd2(dba)3) catalyses the reaction, as assessed by experimental and theoretical evidence, enabling greatly increased η in a much shorter doping time and high electrical conductivities (above 100 S cm−1; ref. 12). This methodology has technological implications for realizing improved semiconductor devices and offers a broad exploration space of ternary systems comprising catalysts, molecular dopants and semiconductors, thus opening new opportunities in n-doping research and applications12, 13.

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Fig. 1: The TM catalysed n-doping concept.
Fig. 2: AuNP catalysed n-doping of PDTzTI with N-DMBI-H.
Fig. 3: The generality of metal-catalysed N-DMBI-H doping method.
Fig. 4: AuNP-catalysed N-DMBI-H doping mechanism.

Data availability

All relevant data are contained within the Article and its Supplementary Information, or are available from the corresponding authors upon request.

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Acknowledgements

H.G. and X.G. gratefully acknowledge financial support from the National Natural Science Foundation of China (51903117 and 21774055) and the Shenzhen Science and Technology Innovation Commission (JCYJ20180504165709042). A.F. acknowledges AFOSR grant FA9550-18-1-0320. S.F. and C.-Y.Y. acknowledge financial support from the Swedish Research Council (2020-03243), Olle Engkvists Stiftelse (204-0256), VINNOVA (2020-05223),Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU 2009-00971), and the European Commission through the Marie Sklodowska-Curie project HORATES (GA-955837). A.M. acknowledges CINECA award no. HP10CC5WSY 2020 under the ISCRA initiative for computational resources. H.Y.W. acknowledges financial support from the National Research Foundation (NRF) of Korea (NRF-2019R1A2C2085290). We also acknowledge technical support from SUSTech Core Research Facilities. We thank H. Li, L. Lin, Z.-Y. Ren and Y.-H. Yang for performing ESI-MS and ESR measurements. We thank L. Safaric, Q. Li and Y. Liu (Linköping University) for assistance with GC, absorption and NMR measurements.

Author information

Authors and Affiliations

Authors

Contributions

H.G., A.F. and X.G. conceived and designed the project. H.G. prepared the samples and performed the electrical, optical absorption, MIS diode, AFM and Seebeck coefficient measurements. C.-Y.Y. and S.F. conducted the doping kinetics study and H2 product analysis. X.Z. initiated the study with organometallic complex catalyst. A.M. performed the DFT calculations. K.F., Y.S., K.Y., J.C. and H.S. synthesized the organic semiconductors. Y.X. and A.F. fabricated and characterized OTFT devices. Q.L. fabricated and characterized perovskite solar cells. Z.W. and H.Y.W. conducted the GIWAXS measurement. Y.T. performed the TEM measurement. C.-Y.Y., A.M., S.F. and A.F. contributed to data analysis and scientific discussion. H.G., A.F. and X.G. wrote the manuscript. All authors discussed the experimental results and approved the manuscript.

Corresponding authors

Correspondence to Antonio Facchetti or Xugang Guo.

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Competing interests

X.G. and H.G. have filed a provisional patent application based on this work. S.F. is the chief scientific officer of n-Ink AB, a company developing organic conductive inks. A.F. is the chief technology officer of Flexterra corporation, a company developing organic semiconductors.

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Peer review information Nature thanks Peter Ho, Karl Leo and Shun Watanabe for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 TEM images of thermally evaporated AuNP films on TEM grids coated with a thin carbon film with the nanoparticle size (diameter d) distribution as a function of the nominal film thickness.

ae, The nominal AuNP film thickness is 0.1 nm (a), 0.2 nm (b), 0.4 nm (c), 0.8 nm (d) and 1.5 nm (e). The TEM images are shown in both high magnification (left column; scale bar, 10 nm) and lower magnification (middle column; scale bar, 50 nm). The AuNP size distribution probability plots in the right column are all from statistical analysis of more than 150 nanoparticles, with their average sizes and standard deviations shown inside.

Extended Data Fig. 2 Electrical conductivity measurements for blend doped PDTzTI films on glass and PET substrates with different electrical contact materials.

ac, Representative IV characteristics of 60mol% N-DMBI-H blend doped PDTzTI films on glass substrates as plotted in log scale (a) and linear scale (b, c), respectively. When using AuNPs, there is a significant charge injection barrier between the Al electrode and the highly conductive PDTzTI film as shown by the non-linear IV characteristics in panel b. Nonetheless, the σ of the Al device, estimated from the currents at |V| = 5 V, is 3.6 ± 0.3 S cm−1, which is in the same order of magnitude of that of the Au device (14.1 ± 0.7 S cm−1). c, Without using AuNP catalyst, the device current is orders of magnitude lower than that of the device with AuNP, with a σ = (2.6 ± 0.2) × 10−4 S cm−1 for the Al device and (4.6 ± 1.2) × 10−4 S cm−1 for the Au device. In all these experiments, the electrodes were fabricated by thermal evaporation using a shadow mask except for the control Au device (without AuNPs), which was prepared by lift-off photolithography to avoid that residual Au clusters catalyse the reaction as we show clearly in Supplementary Fig. 8. d, Representative IV characteristics of 60mol% N-DMBI-H plus AuNPs blend doped PDTzTI films on glass (blue line) and plastic (PET, red line) substrates. In all these experiments d = 1.4 nm, Tann = 120 °C, tann = 10 s when using AuNP catalyst or tann = 10 min when not using catalyst, demonstrating that the catalysed n-doping method is fully compatible with PET substrates. All contact geometries are 100 μm (channel length) and 2 mm (channel width).

Extended Data Fig. 3 AuNP catalysed N-DMBI-H doping efficiency estimation from electrical conductivity.

af, N-DMBI-H doping efficiency (η) versus dopant molar ratio in blend-doped PDTzTI (a), PDTzTIT-2F (b), PDTzTIT (c), PBTzI (d), N2200 (e), and f-BTI2TEG-FT (f) films calculated using the measured electrical conductivity and various electron mobility values. The red curves originate using the measured OTFT mobility (μe,OTFT values from our published results32,37,58 or Supplementary Table 6 for f-BTI2TEG-FT). The plots show that the μe,OTFT is a reasonable assumption for the actual carrier mobility of doped films (μe,doped) at high charge density for PDTzTI, PDTzTIT-2F, PDTzTIT and PBTzI (Extended Data Fig. 3a–d and Supplementary Table 3). The estimated high η is in good agreement with our experimental observations of strong film colour change in their UV–vis–NIR spectra and high electrical conductivity (for example, Fig. 2b, Supplementary Figs. 9 and 10, Table 1). Obviously, the μe,doped at different dopant loading must afford a η < 100%, to be realistic. μe,doped is likely to be charge density dependent and also changes with the N-DMBI-H loading and variation of film morphology, and also may differ significantly from the μe,OTFT as exemplified by N2200 (e) and f-BTI2TEG-FT (f) analysis. For N2200, the calculated ηmax using the μe,OTFT is <0.1%, which does not agree with the experimental ESR results (>40%) and the strong variation of the optical absorption. Thus, the actual μe,doped must be <<μe,OTFT and possibly <0.001 cm2 V−1 s−1 when examining the η-dopant molar ratio graph in panel e. For the polymer f-BTI2TEG-FT, a ηmax >3,000% is calculated using the μe,OTFT, which is unrealistic. Thus, the actual μe,doped is >>μe,OTFT and possibly around 1 cm2 V−1 s−1 at high charge densities, as estimated from the graph in panel f.

Extended Data Fig. 4 2D-GIWAXS images of N-DMBI-H + AuNP doped organic semiconductor films.

The semiconductor films were spin-cast from their pristine or blend solutions with N-DMBI-H in CHCl3 (5 mg ml−1 for PDTzTI and N2200, 20 mg ml−1 for PDI-C6C7), then annealed at Tann = 120 °C, tann = 10 s. In all these experiments, AuNP d = 1.4 nm and the substrate is silicon. The images clearly show that, as the N-DMBI-H loading increases, the doped film crystallinity gradually decreases, eventually to a very low degree of crystallinity and near amorphous structure for all these organic semiconductor films at 100mol% N-DMBI-H loading.

Extended Data Fig. 5 Explored AuNP catalysed N-DMBI-H doping mechanisms.

All the investigated routes share the same first step, namely the hydride transfer from N-DMBI-H to AuNP surface. Depending on the possible active-doping-species, we propose four different reaction routes after the initial hydride transfer step. In route a, Aux–H directly acts as the active-doping-species and forms Aux–H after electron transfer to PDI, then, the H on AuNP surface combines and releases H2 gas to regenerate clean AuNP surface for the next catalytic cycle. In route b, H is released from Aux–H to form Aux•− as the active-doping-species, then H combines to give H2 gas, while Aux•− transfers the electron to the PDI and goes to the next catalytic cycle. In route c, H is transferred from Aux–H to PDI and forms hydrogenated PDI anion (denoted as PDI–H) as the active-doping-species, which transfers electron to another PDI molecule and forms PDI–H, then two PDI–H combine each other to give H2 gas while PDI is regenerated for the next doping reaction cycle. Finally, in route d, H is released from Aux–H and serves as the active-doping-species, the clean AuNP surface goes to next catalytic cycle, while H transfers the electron to the PDI and forms H which combines to give H2 gas. For all the reaction routes, the final reaction products are N-DMBI+, PDI•− and H2 gas, AuNP only serves as the reaction catalyst. PDI-C6C7 molecule is modelled by a simpler PDI in which the 1-hexylheptyl side chain is replaced by a methyl group.

Extended Data Fig. 6 Gibbs free energy profile (kcal mol−1) of the investigated nanoparticle catalysed N-DMBI-H doping mechanisms.

PDI-C6C7 molecule is modelled by a simpler PDI in which the 1-hexylheptyl side chain is replaced by a methyl group. AuNP is modelled by the Au (111) surface. Solvation effects are taken into account for all the molecular species. Route a is most energetically favourable with its Gibbs free energy diagram also given in Fig. 4f of the main text.

Extended Data Fig. 7 Application of catalysed n-doping of organic polymers to n-type organic thermoelectronic devices.

a, Illustration of an n-type thermoelectric device where this method can be used to generate an n-doped organic semiconductor film. b, N-type thermoelectric performance of an off-centre spin-casted, sequentially doped f-BTI2TEG-FT+PBTI blend films, using AuNP catalyst. Recently, side-chain engineering of conjugated polymers with hydrophilic groups59 has shown improved n-dopability and conductivity with N-DMBI-H due to enhanced dopant/semiconductor miscibility. For example, by simply replacing the hydrophobic alkyl chain (2-octyldodecyl) in f-BTI2-FT51 with triethylene glycol (TEG)-based chain in f-BTI2TEG-FT (Supplementary scheme 1, Supplementary Figs. 3441), σ of uncatalysed N-DMBI-H doping was found to increase from (8.9 ± 0.5)  × 10−3 S cm−1 to 1.4 ± 0.1 S cm−1, respectively. However, N-DMBI-H+AuNP doping of f-BTI2TEG-FT can achieve a σ of 25.1 ± 0.6 S cm−1 (blend doping), 38.4 ± 2.2 S cm−1 (sequential doping, on-centre), and 74.3 ± 4.6 S cm−1 (sequential doping, off-centre) (Table 1, Supplementary Figs. 42, 43), despite a low transistor mobility of 0.018 cm2 V−1 s−1 (Supplementary Fig. 44, Supplementary Table 6). Impressively, an even higher σ of 104.0 ± 7.9 S cm−1 (maximum 116.3 S cm−1) can be obtained by adding a small amount (15wt%) of the high mobility PBTI polymer50 (Mn = 35.5 kDa) to f-BTI2TEG-FT, which serves as high-mobility pathways between doped f-BTI2TEG-FT domains for improved charge transport42. Thus, based on the latter blend, we fabricated an organic thermoelectric device which showed a remarkable power factor (PF) of 65.7 ± 5.5 μW m−1 K−2 with a Seebeck coefficient of −79.5 ± 2.8 μV K−1 (Extended Data Fig. 7b, Supplementary Fig. 45). The conductivity and power factor values are among the highest to date for solution-processed molecular n-doped conjugated polymers12,46. Error bars represent the standard deviations from their mean values.

Extended Data Fig. 8 Application of catalysed n-doping of organic polymers to organic thin-film transistor (OTFT) devices.

a, Chemical structure of the polymer N2200-EG7 (P) and the PDIR-CN2 semiconductor (OSC) used for OTFT devices. b, Top-gate bottom-contact OTFT structure with Au source/drain/gate electrodes and using Au contacts to catalyse the n-doping of the NDI polymer P with N-DMBI-H (blend method). ce, Corresponding transfer plots (c), mobility evolutions (d) and low-drain voltage output plots (e) of the indicated devices. f, Top-gate bottom-contact OTFT structure with Ag source/drain/gate electrodes using Pd2(dba)3 to catalyse the n-doping of the NDI polymer P with N-DMBI-H (blend method). gi, Corresponding transfer plots (g), mobility evolutions (h) and low-drain voltage output plots (i) of the indicated devices.

Extended Data Fig. 9 Perovskite solar cells fabricated with undoped/doped polymer films as the electron transporting layer (ETL).

a, Perovskite cell structure, chemical structures of the hole transporting layer (HTL) and ETL polymers used in this study. b, J–V plots of the best device using PDTzTIT polymer as ETL. c, J–V plots of the best device using PDTzTIT-2F polymer as ETL.

Supplementary information

Supplementary Information

This file contains Supplementary Figs. 1–46, Supplementary Tables 1–7, Supplementary scheme 1 and Supplementary references.

Supplementary Video 1

Video showing that fast N-DMBI-H doping reaction happens in the presence of AuNP. The PDI-C6C7 + N-DMBI-H (10 eq.) blend solutions in CH2Cl2 were drop-casted in ambient air at 25 °C onto bare glass substrate (left) or glass with pre-deposited AuNP (right, nominal evaporation thickness 1.2 nm, estimated NP size d ≈ 6.4 nm). An immediate colour change was observed when the blend solution contacted AuNP, while there was no obvious change without AuNP, thus clearly demonstrating the high catalytic activity of AuNP which significantly enhances the N-DMBI-H doping reaction.

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Guo, H., Yang, CY., Zhang, X. et al. Transition metal-catalysed molecular n-doping of organic semiconductors. Nature 599, 67–73 (2021). https://doi.org/10.1038/s41586-021-03942-0

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  • DOI: https://doi.org/10.1038/s41586-021-03942-0

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