High performance liquid crystalline organic field effect transistor materials: How does the liquid crystallinity solve problems in conventional soluble OFET materials?
Materials research for OFETs have been focused on exploring a soluble organic semiconductor exhibiting high mobility over 1 cm2/Vs, and several new materials satisfying the high mobility were found, which include triisopropylsilylacethylenylpentacene (TIPS-pentacene) [1], and benzothienobenzothiophenes (BTBT) [3]. However, these materials still remains problems to be solved for practical applications: one is an inhomogeneous texture of the films that results from recrystallization of OFET materials during solvent evaporation and affects reliability of the FET performance and process reliability: the other is thermal stability of the films required for additional thermal process such as wiring and passivation in device fabrication and up to 150oC or higher.
Both of the problems can be solved by utilizing the liquid crystallinity that often appears in soluble OFET materials chemically modified with long alkyl chains: the uniform and molecularly flat surface morphology are achieved by utilizing a liquid crystalline film as a precursor for polycrystalline films [4]; the poor thermal stability can be solved by utilizing highly ordered smectic phases that exhibit solid-like nature.
We have designed and synthesized BTBT derivatives that exhibit a highly ordered smectic mesophase of SmE phase, and fabricated OFETs with their polycrystalline thin films prepared by spin-coating their solutions at a liquid crystalline temperature.
The films exhibited a uniform texture observed by a polarized optical microscope and molecular steps by AFM, and high thermal stability up to 200oC in terms of de-wetting on the substrate. As for the device performance, the OFETs exhibited a high mobility up to 20 cm2/Vs and high thermal durability.
We conclude that utilizing the liquid crystallinity in OFET materials is the key for high device performance and process reliability toward practical applications of OFETs.
[1] J. E. Anthony, D. I. Eaton, and S. R. Perkin, Org. Lett., 4, 15 (2002).
[2] H. Ebata, T. Izawa, E. Miyazaki, K. Takimiya, M. Ikeda, H. Kuwabara, and T. J. Yui, J. Am. Chem. Soc. , 129, 15732 (2007).
[3] H. Iino, J. Hanna, Adv. Mat., DOI: 10.1002/adma.201004474 (2011).
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