reSeArCH Letter
electrical conductivity and B is the magnetic flux density) at various
temperatures. The positive magnetoconductance displayed by the
doped PBTTT thin film is attributed to weak localization^11 ,^21 , as both
the magnitude and the curvature of the data can be fit using the well-
established Hikami–Larkin–Nagaoka weak localization model^22. This
requires only the characteristic magnetic field at which the matrix ele-
ment responsible for backscattering loses its phase as the fitting param-
eter. This fitting allows the phase-coherent length, λφ, to be determined
at various temperatures (Fig. 4f; see also Supplementary Information
section 1.7). Figure 4f compares the phase-coherent lengths of three
doped PBTTT thin films (conventional F4TCNQ doping^11 , orange;
anion-exchange doping with EMIM-TFSI, light blue, and with Li-TFSI,
red) and demonstrates that the phase-coherent length becomes longer
as the doping level increases.
This approach to doping substantially increases the doping level,
which in turn promotes two-dimensional, coherent carrier transport,
and also imparts excellent thermal stability. Because the additional ani-
ons replace the initial dopant in the film and remain within the polymer
network, they should have an effect on the physicochemical proper-
ties of the material^9 ,^23. Thus, we also assessed the thermal stability and
durability of doped PBTTT thin films. To do so, we evaluated the ther-
mal durability of the doped thin films through conductivity measure-
ments, based on the proportion of the original conductivity retained
after annealing in an argon-purged glove box for 10 min at 120 °C or
160 °C (Extended Data Fig. 10 and Supplementary Information section
1.8). The conductivity of an F4TCNQ-doped thin film was reduced by
three orders of magnitude after heating at 160 °C, but this change was
dramatically suppressed in the case of doping with hydrophobic closed-
shell anions. We note that further improvements to thermal durability
could therefore be achieved by tuning the physicochemical properties
of the additional anion.
Our results show that the molecular doping of polymeric semicon-
ductors via anion exchange increases both the doping level and the
thermal durability of the polymer thin film. This process uses ionic
interactions to build new host–guest structures and increase the dop-
ing levels, thus overcoming the limitations based on the redox poten-
tial, and could potentially be extended to electron doping—that is, to
cation-exchange doping. This technique suggests opportunities for
the storage, transport and conversion of functional molecules within
solid-state conjugated materials. The remarkably high doping concen-
trations and enhanced conductivity demonstrated here should also
improve our understanding of charge-transport physics, as the half-
filled state in highly crystalline polymeric semiconductors is likely to
trigger an electronic phase transition.Data availability
The data that support the findings of this study are available within this Letter, its
Extended Data and its Supplementary Information.Online content
Any methods, additional references, Nature Research reporting summaries, source
data, extended data, supplementary information, acknowledgements, peer review
information; details of author contributions and competing interests; and statements of
data and code availability are available at https://doi.org/10.1038/s41586-019-1504-9.Received: 4 December 2018; Accepted: 20 June 2019;
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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 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.Competing interests The authors declare no competing interests.Additional information
Extended data is available for this paper at https://doi.org/10.1038/s41586-
019-1504-9.
Supplementary information is available for this paper at https://doi.org/
10.1038/s41586-019-1504-9.
Reprints and permissions information is available at http://www.nature.com/
reprints.
Correspondence and requests for materials should be addressed to S.W.
Peer review information Nature thanks Antonio Facchetti and Peter Ho for their
contribution to the peer review of this work.
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