Availability of Liquid Crystalline Materials for Organic Polycrystalline Semiconductor Thin Films
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0965-S08-10
Availability of Liquid Crystalline Materials for Organic Polycrystalline Semiconductor Thin Films Hiroaki Iino1,2 and Jun-ichi Hanna1,2 1 Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, J1-2, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan 2 Japan Science and Technology Agency (JST), Core Research for Evolutional Science and Technology (CREST), 4-1-8 Hon-cho, Kawaguchi, 332-0012, Japan ABSTRACT We have investigated the charge carrier transport properties of polycrystalline films fabricated with discotic and calamitic liquid crystals. The polycrystalline films of these liquid crystals exhibiting particular textures under a polarized microscope, i.e., hexagonal and co-circular patterns for discotic and calamitic liquid crystals, respectively, which indicate that the molecular alignment in the liquid crystalline phase is well preserved exhibit clear hole transport along with molecular stacks even in a 15 µm thick film, judging from transient photocurrents by time of flight method. The hole mobilities are over 0.1 cm2/Vs at room temperature. These results indicate formation of fewer grain boundaries across conduction channels. On the other hand, the carrier transport properties in the polycrystalline films of non-liquid crystalline molecules with the same molecular shape exhibit no particular patterns as described and photocurrents are annihilated in the bulk as a function of time, indicating many deep trap sites in grain boundaries. The present results indicate that the liquid crystalline molecule is a good candidate for preparing quality organic polycrystalline semiconductor thin films in opto-electronic applications. INTRODUCTION Organic polycrystalline thin films of π-conjugated aromatic molecules such as pentacene and oligothiophenes attract high attention as a material for organic field effect transistors because of their high mobility of 0.1 - 1 cm2/Vs[1], which is comparable to that of hydrogenated amorphous silicon thin films. In organic polycrystalline thin films, however, grain boundaries remain problematic: grain boundaries across the applied electric field always deteriorate the charge carrier transport[2]. In order to solve this problem, one of the best methods is to use organic single crystals. However, it is very difficult to make uniform single crystals in large area. For better charge carrier transport in a polycrystalline film, it is essential to suppress the formation of grain boundaries across the electric field applied in a given device (Figs. 1 (a) and (b)), rather than to reduce the total number of isotropic grain boundaries in the film (Fig. 1 (c)). In fact, from the viewpoint of controlling molecular orientation during crystal growth, rod-like molecules having a linearly π-conjugated aromatic nucleus such as pentacene and oligothiophenes facilitate orientation perpendicular to the substrate surface, promoting layer-by-layer crystal growth[3,4], while fullerene (C60), which has an isotropic molecular shape, does not show layer-by-layer growth even on
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