New Aspects of Organic Electric Materials in Calamitic Liquid Crystalline Photoconductors
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RODUCTION Organic photoconductors have been utilized in photoreceptors for xerographic copiers and laser printers [1], and more recently in active components of organic electroluminescent devices [2]. These applications require large-area unifo rmity in thin layer, so that the materials practically used are amorphous and prepared either by polymerization, vacuum evaporation, or molecular doping into polymer films , of photoconductive small molecules . However, the electrical properties of the resulting amorphous films are degraded significantly compared with their own molecular crystals : the carrier mobility is decreased down to 10-6~10-5 cm2/Vs and depend on both electric field and temperature. In addition, carrier traps play serious role for determining the response time, which are caused by impurities and carrier-dipole interactions in the amorphous films . These limit their practical applications. The considerable improvement of mobility has been achieved up to 10-3cm2/Vs by utilizing less polar polymer matrices for the molecularly doped polymers in order to reduce the carrier-dipole interaction [3]. However, there still remains a serious problem of the mobility, i.e., its field and temperature dependence. In order to relax the present limitation of device applications with organic photoconductors , a new material exhibiting a high mobility independent of electric field
and temperature and few deep defect densities has to be realized. From this viewpoint, we had paid our attention to the potential in liquid crystalline materials that exhibit liquid -like fluidity and crystal-like molecular alignment at the same time, because these properties are a good basis of quality electrical properties and large-area uniformity in practical applications. We found that the calamitic, i.e., rod-like liquid crystals exhibit the fast electronic carrier transport [4,5,6,7], whose ele ctrical conduction has been thought to be ionic due to ionic impurities and/or their own ionized species for a long time since the first report of Heilmeier [8]. 2-phenylnaphthalene derivatives are typical examples, whose smectic mesophases exhibit unique features in carrier transport, i.e., a fast hole and electron mobility up to 10-2 cm2/Vs and its independence of temperature and electric field. Interestingly, the carrier transport vanishes when the mesophases transform into the crystalline phase irrespective of any smectic phases. This is probably attributed to the carrier trapping at the deep defects created at the grain boundary, where impurities piles up and defective molecular alignment. Thus, we have been interested in how the defects in the bulk and at the domain boundary affect the carrier transport in mesophases . In previous work related to measurements of transient photocurrents in the cells with different thickness and domain sizes , we found that there is no deterioration of carrier mobility in all the liquid crystalline mesophases even in the case of carrier transport distance, i.e. cell gap exceeding 100 µm. This means that there
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