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Carrier Transport Properties in Crosslinked Polymer / Liquid Crystalline Semiconductor Composite

Naoki Yoshimoto, Masahiro Funahashi and Jun-ichi Hanna1 Imaging Science and Engineering Laboratory, Tokyo Institute of Technology 4259 Nagatsuta, Midori-ku, Yokohama 226-8503 JAPAN

ABSTRACT

A novel composite system containing of liquid crystalline semiconductor and crosslinked polymer exhibited high hole mobilities with comparison of that of pure liquid crystalline molecules. The carrier transport properties were influenced by the polymerized conditions. The mobilities were independent on temperature and electric field in the case of mesophases polymerized samples. These carrier transport properties were essentially different of those of well-known molecularly doped polymers.

INTRODUCTION

The electrical conduction in liquid crystals had been believed to be ionic for a long he ling time since it called attention in the late 1960s. However, recently it was discovered that some types of liquid crystals, i.e., discotic[1] and smectic liquid crystals[2-4] having aromatic π-conjugate moiety as a core part, exhibit very fast electronic conduction characterized by high mobility over 10-3cm2/Vs. This mobility is 2~3 order higher than that of the conventional amorphous organic semiconductors practically used in the photoreceptors for photocopiers and laser printer, and more recently in light emitting diodes. Thus, the liquid crystals are now being recognized as a new class of high quality organic semiconductors. In the applications described above, the materials are prepared in either homogeneous thin films such as molecularly doped polymers with small molecular weight semiconductors and their vacuum evaporated films or heterogeneous films consisting of organic semiconducting pigment and binder polymers[5,6]. These are subjected to the solubility of the organic semiconductors in binder polymers. The liquid crystalline materials, however, can be prepared in CC5.29.1

the different films from the conventional homogeneous or heterogeneous films, due to their self-organization and microscopic phase separation. Typical examples are polymer dispersed liquid crystals (PDLCs) and polymer stabilized liquid crystals (PSLCs), which are hybrid systems comprised of liquid crystal (LC) and polymer networks[7,8]. These materials systems provide us not only with self-standing films containing liquid crystals but also with unique properties which can not be realized in the conventional films. Therefore, a new materials system hybridized the liquid crystalline semiconductor with network polymer is quite interesting to be investigated not only for the self-standing films but also their electrical properties. Thus, we prepared a novel hybrid system comprised of a smectic liquid crystalline semiconductor and a photo-crosslinked polymer and characterized its carrier transport properties by time-of-flight (TOF) technique. We discovered that carrier transport properties in liquid crystalline semiconductor / crosslinked polymer composite depend on t