Ionic transport phenomena in organic semiconductor materials underpin emerging technologies ranging from bioelectronics to energy storage. The performance of these systems is affected by an interplay of film morphology, ionic transport and electronic transport that is unique to organic semiconductors yet poorly understood. Using in situ electrochemical strain microscopy (ESM), we demonstrate that we can directly probe local variations in ion transport in polymer devices by measuring subnanometre volumetric expansion due to ion uptake following electrochemical oxidation of the semiconductor. The ESM data show that poly(3-hexylthiophene) electrochemical devices exhibit voltage-dependent heterogeneous swelling consistent with device operation and electrochromism. Our data show that polymer semiconductors can simultaneously exhibit field-effect and electrochemical operation regimes, with the operation modality and its distribution varying locally as a function of nanoscale film morphology, ion concentration and potential. Importantly, we provide a direct test of structure–function relationships by correlating strain heterogeneity with local stiffness maps. These data indicate that nanoscale variations in ion uptake are associated with local changes in polymer packing that may impede ion transport to different extents within the same macroscopic film and can inform future materials optimization.
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This paper is based primarily on work supported by the National Science Foundation, NSF DMR-1607242. We gratefully acknowledge graduate fellowship support for L.Q.F. from the University of Washington Clean Energy Institute, as well as support from the Washington Research Foundation and Alvin L. and Verla R. Kwiram endowed fund. J.O. and C.K.L. acknowledge support from the University of Washington Clean Energy Institute, as well as support from the National Science Foundation under NSF DMR-1533372 and 1629369. The authors thank P. A. Cox, D. Moerman, K. Corp and L. Bradshaw for experimental assistance, as well as S. Holliday and T. Martin for helpful discussions. Part of this work was conducted at the Molecular Analysis Facility, a National Nanotechnology Coordinated Infrastructure site at the University of Washington that is supported in part by the National Science Foundation (grant ECC-1542101), the University of Washington, the Molecular Engineering & Sciences Institute, the Clean Energy Institute and the National Institutes of Health.
The authors declare no competing financial interests.
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Giridharagopal, R., Flagg, L., Harrison, J. et al. Electrochemical strain microscopy probes morphology-induced variations in ion uptake and performance in organic electrochemical transistors. Nature Mater 16, 737–742 (2017). https://doi.org/10.1038/nmat4918
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