The effect of dielectric coatings on charge carrier behavior in liquid crystal cells

Thermal depolarization current (TDC) and dielectric measurements of samples, M-LC-M, M-SiO2-LC-SiO2-M, and M-PI-LC-PI-M [LC = Liquid Crystal, p-C5H11C(O)OC6H4C(O)OC6H4CN-p, PI = polyimide, M = Cr and Al] have been carried out in order to investigate the behavior of charge carriers and the effects of insulating films used as blocking electrodes in liquid crystal cells. Electrode polarizations due to mobile ions contained in LC have been observed for the samples with bare and SiO2-covered electrodes. The blocking ability of the evaporated SiO2 layers exists under ac fields, but decreases with applied dc voltage. The SiO2 has been also indicated to trap the ions formed near the electrode. On the other hand, an interfacial polarization between spin-coated PI and LC has been observed for the samples having polyimide films on the electrodes. The polyimide layers have shown the blocking ability even under dc fields. The behavior of the TDC spectra will also be discussed in detail.