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Site-specific chemical doping reveals electron atmospheres at the surfaces of organic semiconductor crystals

Abstract

Chemical doping controls the electronic properties of organic semiconductors, but so far, doping protocols and mechanisms are less developed than in conventional semiconductors. Here we describe a unique, site-specific, n-type surface doping mechanism for single crystals of two benchmark organic semiconductors that produces dramatic improvement in electron transport and provides unprecedented evidence for doping-induced space charge. The surface doping chemistry specifically targets crystallographic step edges, which are known electron traps, simultaneously passivating the traps and releasing itinerant electrons. The effect on electron transport is profound: field-effect electron mobility increases by as much as a factor of ten, and its temperature-dependent behaviour switches from thermally activated to band-like. Our findings suggest new site-specific strategies to dope organic semiconductors that differ from the conventional redox chemistry of randomly distributed substitutional impurities. Critically, they also verify the presence of doping-induced electron atmospheres, confirming long-standing expectations for organic systems from conventional solid-state theory.

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Fig. 1: Chemical doping of Cl2-NDI and PDIF-CN2 single crystals by exposure to N-silane vapour.
Fig. 2: Impact of doping on n-type single crystal FET characteristics.
Fig. 3: Impact of doping on the temperature dependence of electron transport.
Fig. 4: AFM height and SKPM potential images of doped crystals.
Fig. 5: Spectroscopic evidence for doping and scheme of the dopant-induced space charge distribution.

Data availability

The experimental data that support the findings of this study are available online at https://doi.org/10.6084/m9.figshare.13602551.

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Acknowledgements

This work was supported primarily by the MRSEC programme of the National Science Foundation (NSF) under grant no. DMR-2011401 (T.H.). C.D.F. also acknowledges partial support from grant no. NSF DMR-1806419 and the University of Minnesota (P.P.R. and Y.W.). Parts of this work were carried out in the College of Science and Engineering Characterization Facility, University of Minnesota, which has received capital equipment funding from the NSF through the UMN MRSEC programme under award no. DMR-2011401, and in the Minnesota Nano Center, which is supported by NSF through the National Nano Coordinated Infrastructure Network, under award no. ECCS-2025124. Parts of this work were also carried out at the Center for Nanosystems Chemistry at the Universität Würzburg, which has received funds from the Bavarian State Ministry of Science and Arts in the framework of the research programme ‘Solar Technologies Go Hybrid’ (F.W.). Some of the SKPM measurements were performed at Shandong University, supported by the National Natural Science Foundation of China grant no. 62074093 (T.H.). T.H. also acknowledges support from the Qilu Young Scholars Programme of Shandong University.

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C.D.F. designed and guided the research programme. T.H. grew single crystals, fabricated the devices and performed measurements and analysis. M.S., R.R. and F.W. synthesized the Cl2-NDI materials, performed UV-vis near infrared spectroelectrochemistry and contributed to the scientific discussion of the results. P.P.R. and Y.W. provided theoretical support and contributed to the interpretation of the results. T.H. and C.D.F. wrote the paper with input from all authors.

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Correspondence to Tao He or C. Daniel Frisbie.

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Peer review information Nature Materials thanks Sasha Sadewasser and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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He, T., Stolte, M., Wang, Y. et al. Site-specific chemical doping reveals electron atmospheres at the surfaces of organic semiconductor crystals. Nat. Mater. (2021). https://doi.org/10.1038/s41563-021-01079-z

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