Fabrication of polycrystalline thin films of liquid crystalline materials by solution process and its application to OFE
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1091-AA11-82
Fabrication of polycrystalline thin films of liquid crystalline materials by solution process and its application to OFETs Hiroaki Iino1,2, and Jun-ichi Hanna1,2 1 Imaging Science & Engineering Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan 2 JST-CREST, 4-1-8 Hon-cho, Kawaguchi, 332-0012, Japan ABSTRACT We have fabricated polycrystalline OFETs of two different liquid crystalline materials i.e., ω,ω’-dihexylquaterthipohene (6-QTP-6) and N, N’-ditridecylperylenediimide (13-Per-13) by solution process. Liquid crystalline materials help fabricating uniform thin films on the substrate when spin-coated at their temperature range of liquid crystalline phase. The FETs fabricated with 6-QTP-6 exhibited p-channel performance and its mobility was determined to be 0.04 cm2/Vs, which was comparable to that determined by time-of-flight experiments. The FETs fabricated with 13-Per-13 exhibited n-channel performance and its FET mobility was 0.008 cm2/Vs, while the mobility was increased up to 0.11 cm2/Vs after thermal annealing of the film at a liquid crystalline temperature of 220 oC for an hour. Judging from these facts, the grain boundaries are controlled not so as to across the conduction channels formed by self-aligned π-conjugated aromatic cores in liquid crystalline molecules. We conclude that liquid crystalline material is a good candidate for quality polycrystalline thin films for OFETs. INTRODUCTION Organic polycrystalline semiconductors such as Pentacene and Oligothiophenes have been extensively studied for OFETs because of the high FET mobility up to 1 cm2/Vs by vacuum evaporation [1,2]. In materials development for OFETs, the organic semiconductors are often evaluated from the device performance in a FET configuration. This is because of not only easy FET fabrication but also no convenient technique to evaluate the intrinsic charge transport properties of the semiconductor material in its polycrystalline thin film. However, the device performance results from various properties including the charge carrier transport properties of semiconductor material and the electrical contact with the electrode material, and the electrical properties of semiconductor/gate insulator interface. As for the semiconductor material itself, it is very difficult to know its intrinsic charge carrier transport properties in the polycrystalline film because of contamination of chemical impurities during its synthesis and grain boundaries across transport channels. Even in a single crystal it is very difficult to know the intrinsic charge carrier transport because we cannot evaluate the effect of chemical impurity exactly. With this recognition, we paid attention to liquid crystalline materials, which have an anisotropic molecular shape enhanced by long alkyl chains, exhibit the self-organization in mesophase, in addition to a high solubility in common organic solvents. By the best use of step-wise self-organization from liquid crystalline phase into the crystal phase in the liquid
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