Preparation of a novel organic semiconductor composite consisting of a liquid crystalline semiconductor and crosslinked polymer and characterization of its charge carrier transport properties

We have fabricated a novel organic semiconductor composite consisting of a liquid crystalline semiconductor, i.e., 6-(4'-octylphenyl)-2-dodecyloxynaphthalene (8-PNP-O12), and a crosslinked polymer derived from 1,6- hexanediol diacrylate (HDA) by photopolymerization; its charge carrier transport properties were studied by the time-of-flight technique. The charge carrier transport in the resulting composites depended on polymerization conditions and on the nature of the charge. In the composite polymerized in the isotropic phase, the hole mobility was reduced by one order of magnitude when the HDA content exceeded more than 20 wt%; in a composite polymerized in the smectic mesophases, however, the hole mobility was unchanged irrespective of the monomer concentration up to 20 wt% compared with that of the pure 8-PNP-O12. On the other hand, negative charge carrier transport was significantly affected by photopolymerization even at a concentration of HDA of 5 wt%, irrespective of the mesophase in which the polymerization was carried out: the carrier mobility was significantly reduced and depended on the temperature, suggesting that chemical impurities formed in photopolymerization affect negative charge carrier transport. We discuss here the interesting charge carrier transport properties in relation to the micro-phase separated structures that characterize the liquid crystal and polymer composite.