Liquid Crystalline Semiconductors Materials, properties and applicat
This is an exciting stage in the development of organic electronics. It is no longer an area of purely academic interest as increasingly real applications are being developed, some of which are beginning to come on-stream. Areas that have already been com
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Charge Carrier Transport in Liquid Crystalline Semiconductors Jun-Ichi Hanna
2.1 Historical Studies of the Electrical Properties of Liquid Crystals It is only in relatively recent years, compared with the discovery of liquid crystals, that the electrical properties of liquid crystals have attracted significant scientific interest. The first attention to them was paid in the late 1960s by two groups at almost the same time, but from different points of view. One was Kusabayashi and Labes [1], who measured the electrical conductivity in various liquid crystals including nematic, cholesteric and smectic liquid crystals. They studied the conductivity change in liquid crystals accompanying phase transitions from the point of view of extending understanding of the electrical properties of molecular crystals. Their findings, however, did not arouse further scientific attention at the time. The other early study was by Heilmeier’s group [2], who invented a liquid crystal display (LCD) in the so-called dynamic scattering mode. The principle of this liquid crystal display was based on light scattering caused by dynamic motion of aligned liquid crystalline molecules, when ions drift in a liquid crystal cell at a given bias voltage. They studied ion transport in the nematic phase of a p-azoxyanisole derivative and determined its mobility to be 105 cm2 V1 s1 from transient currents induced by a step-voltage technique [3]. This LCD invention attracted a lot of attention and the charge carrier transport properties in various liquid crystals, especially nematic liquid crystals, were investigated in the 1970s [4, 5]. All the results reported at that time indicated that the conduction in liquid crystals was governed by conduction of ions, whose typical mobility was 105 –106 cm2 V1 s1 . It led to a general understanding that charge carrier transport in liquid crystals, taking into account
J.-I. Hanna () Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, J1-2 Nagatsuta Midori-ku, Yokohama 226-8503, Japan e-mail: [email protected] R.J. Bushby et al. (eds.), Liquid Crystalline Semiconductors, Springer Series in Materials Science 169, DOI 10.1007/978-90-481-2873-0 2, © Springer ScienceCBusiness Media Dordrecht 2013
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that discotic liquid crystals had not been discovered at that time, was governed by ionic conduction. It is quite understandable, because the liquid-like nature of liquid crystalline materials favors this idea. In the 1980s, after the discovery of discotic liquid crystals in 1977 [6], because of the more solid-like nature of discotic liquid crystals, the focus of significant interest in the quest for electronic conduction in liquid crystals shifted from calamitic liquid crystals to the discotic ones with columnar mesophases. In 1988, Boden et al. at Leeds University studied ac-conductivity in hexakis-hexyloxytriphenylene (HAT6) chemically doped with AlCl3 . They demonstrated one-dimensional hole conduction along the columns through the anisotropic ac-conductivity as
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