Carrier transport properties of Calamitic liquid crystalline photoconductors

Carrier transport properties of a calamitic liquid crystalline photoconductor (Calamitic LCPC), 2-phenynaphthalene derivatives were investigated by steady state and transient photocurrent measurements. These materials exhibited calamitic mesophases such as nematic (N), smectic A (SmA), smectic B (SmB), and smectic E (SmE) phases, which depended on hydrocarbon chains attached to a core moiety of 2-phenylnaphthalene. A large photoconductive anisotropic ratio over 50 was observed under light illumination due to molecular alignment. The transient photocurrents from time-of-flight measurements exhibited non-dispersive and ambipolar carrier transits in all the mesophases, whereas only small current decays were observed in polycrystalline phase. The carrier transport in the smectic phases was electronic, while the ionic conduction was dominant in both of isotropic and nematic phases. The mobility was increased stepwise when the phase transition took place with a decrease in temperature. The mobility for smectic phases was independent of temperature and applied electric field, while those in isotropic and N phases depended on temperature positively. The highest mobility of 10(-2) cm(2)/Vs was obtained for both electrons and holes in SmE, which was four orders of magnitude faster than those of the conventional disordered photoconductors. These results indicate how effective the molecular alignment is in order to upgrade the carrier transport in spite of a small pi-conjugate core moiety of 2-phenylnaphthalene.