The charge-transport properties of ω,ω′-alkylalkoxylbiphenyls, one of the simplest calamitic (rodlike) liquid crystals exhibiting smectic mesophases, were investigated using the time-of-flight experiments. A mobility of ∼10−3 cm2∕V s was determined for holes in both smectic B (SmB) and smectic E (SmE) phases. This high electronic mobility, comparable to the highest mobilities in amorphous organic semiconductors, indicates that the self-organization of such simple molecules including small aromatic compounds is quite effective in enhancing the charge transport in organic materials. The mobilities of both smectic mesophases, however, depend on temperatures, which is not the case for other typical smectic liquid crystals reported previously. Interestingly, the mobility in the SmE phase exhibits a Poole–Frenkel type of field dependence. Detailed experimental studies have shown that this field dependence is not due to the trapping effect of carriers caused by chemical contaminants or structural defects such as domain boundaries and dislocation, but arises from the widely distributed density of states with a distribution width of 0.11–0.13 eV for the SmE phase, as estimated based on a two-dimensional Gaussian disorder model. The phase-specific carrier transport in biphenyls may provide us with unique insights leading to a more complete understanding of the charge carrier transport in the mesophase materials.
A series of 4-alkyl-4′-alkoxylbiphenyls, which are one of the representative calamitic liquid crystals having a small aromatic core moiety, has been synthesized and their charge carrier transport properties are characterized in order to examine how small the core size can be for a high mobility. The biphenyls exhibit both smectic B and E phases and the hole transport characterized by a high mobility of 10−3 cm2 V−1 s−1 in the smectic mesophases. The mobility depends on temperature in both smectic phases, and furthermore it depends on the electric field in the smectic E phase. The fact that such a small molecule of biphenyl exhibits the mobility comparable to those of triphenylenes and TPD (N,N′-diphenyl-N,N′-bis(3-methylphenyl)[1,1′-biphenyl]-4,4′-diamine) gives an insight into the molecular design of high mobility liquid crystalline materials.
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