A molecular design principle of lyotropic liquid-crystalline conjugated polymers with directed alignment capability for plastic electronics

Abstract

Conjugated polymers with a one-dimensional p-orbital overlap exhibit optoelectronic anisotropy. Their unique anisotropic properties can be fully realized in device applications only when the conjugated chains are aligned. Here, we report a molecular design principle of conjugated polymers to achieve concentration-regulated chain planarization, self-assembly, liquid-crystal-like good mobility and non-interdigitated side chains. As a consequence of these intra- and intermolecular attributes, chain alignment along an applied flow field occurs. This liquid-crystalline conjugated polymer was realized by incorporating intramolecular sulphur–fluorine interactions and bulky side chains linked to a tetrahedral carbon having a large form factor. By optimizing the polymer concentration and the flow field, we could achieve a high dichroic ratio of 16.67 in emission from conducting conjugated polymer films. Two-dimensional grazing-incidence X-ray diffraction was performed to analyse a well-defined conjugated polymer alignment. Thin-film transistors built on highly aligned conjugated polymer films showed more than three orders of magnitude faster carrier mobility along the conjugated polymer alignment direction than the perpendicular direction.

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Figure 1: The chemical structures of the designed CPs, their absorption spectra (in solution and as films) and the suggested mechanism for directed CP alignment.
Figure 2: The grooves generated from the flow of CP1 solution and their morphology analysis.
Figure 3: GIXRD with CP1.
Figure 4: Directed alignment and its optical/electrical anisotropic properties.

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Acknowledgements

The authors thank E-H. Sohn at Seoul National University (SNU) for the 2D GIXRD measurement and J. Kieffer and X. Ma for the calculation of the polymer conformation. This work was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences, as part of the Center for Solar and Thermal Energy Conversion, an Energy Frontier Research Center (DE-SC0000957). E.J.J. was partly supported by the WCU (World Class University) programme through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R31-2008-000-10075-0) and S.S. was supported by an NSF CAREER Award (DMR 0644864).

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J.K. designed and directed the project. B-G.K. and E.J.J. synthesized the series of polymers. B-G.K. prepared and analysed the aligned CP films, B-G.K., J.W.C. and B.K. fabricated the organic thin-film transistors, and S.S. performed the X-ray measurements. B-G.K. and J.K. wrote the manuscript, and all other authors had input.

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Correspondence to Jinsang Kim.

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The authors declare no competing financial interests.

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Kim, BG., Jeong, E., Chung, J. et al. A molecular design principle of lyotropic liquid-crystalline conjugated polymers with directed alignment capability for plastic electronics. Nature Mater 12, 659–664 (2013). https://doi.org/10.1038/nmat3595

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