Hiroaki Iino1

1, Imaging Science and Engineering Research Center, Tokyo Institute of Technology, Yokohama, Kanagawa, Japan

Liquid crystals are very attractive materials as not only display but also organic electronics applications because the nature of liquid crystals is self-organizing nature. In organic electronics applications, it is important that the molecules must be aligned for each device, e.g., lateral current flow on the substrate is required in an organic thin film transistor (OTFT) device. In the material research in OTFTs, a lot of soluble crystalline materials are proposed, however, it is difficult to fabricate uniform thin films and to solve trade-off problem between thermal durability and solubility for crystalline thin films.
In order to solve these problems, we proposed the utilization of the liquid crystallinity in the soluble crystalline OTFT materials on the basis of our previous research for liquid crystals as a crystalline organic semiconductor [1]. Liquid crystalline materials exhibit crystal phase up to 100 oC, i.e., liquid crystal phase shows over 100 oC. We propose a new fabrication technique for uniform and molecularly flat polycrystalline thin films by utilizing liquid crystalline thin films as a precursor for polycrystalline thin films [2]. In order to improve the thermal durability of the films, we propose to utilize new liquid crystalline materials exhibiting highly ordered liquid crystal phases such as smectic E (SmE) phase, which give film stability in the thermal processes thanks to solid-like nature of the highly ordered liquid crystals [3].
In this presentation, we will show the good processability, high thermal durability of the film up to 200 oC, and high mobility over 10 cm2/Vs in polycrystalline thin films of 2-phenyl-7-octyl-beozothienobenzothiophene (Ph-BTBT-10) as representative liquid crystalline materials [4]. Ph-BTBT-10 has highly and low ordered smectic phases of SmE and SmA, respectively. The polycrystalline thin films of Ph-BTBT-10 fabricated by spin-coating at 110 oC for SmE phase were very uniform without any crystal flakes and cracks and the surface of polycrystalline thin films existed terrace structure with molecular step of ca. 3 nm that corresponds to a molecular length of Ph-BTBT-10. The polycrystalline thin film of thermal stability is much improved. The thin films unchanged after the thermal stress at 200 oC, thanks to the solid-like nature of SmE phase up to 210 oC. The FET devices after thermal stress operated, whose mobility was over 2 cm2/Vs. These results show how effective the SmE phase is to improve thermal durability of OTFT in BTBT derivatives. Polycrystalline thin films of Ph-BTBT-10 show high mobility over 10 cm2/Vs after short thermal annealing at 120 oC at crystal phase. The polycrystalline thin films changed from monolayer to bilayer crystal structures after the thermal annealing and the utilization of bilayer crystal films via monolayer crystal films is good idea to realize for fabrication of uniform thin film by solution process and high mobility of bilayer crystal structure.
[1] H. Iino, and H. Hanna, Jpn. J. Appl. Phys. 45, L867 (2006).
[2] H. Iino, and H. Hanna, Adv. Mater. 23, 1748-1751 (2011).
[3] H. Iino, T. Kobori, and J. Hanna, Jpn. J. Appl. Phys. 51, 11PD02 (2012).
[4] H. Iino, T. Usui, and J. Hanna, Nat. Commun. 6, 6828 (2015).