Electrical and electro-optical investigations of liquid crystal cells containing WO3 thin films

An interesting application of the fast ion transport properties of tungsten trioxide is presented, when it is inserted as an electrode in nematic liquid crystal (NLC) cells. In a standard sandwichlike cell the nematic liquid crystal, confined between two transparent plane electrodes of purely electronic conductors [indium tin oxide (ITO)], undergoes a molecular reorientation under the action of an external electric field E. This electrically controlled birefringence (electro-optical switching) is proportional to E2, thus polarity insensitive [L. M. Blinov and V. G. Chigrinov, Electrooptic Effects in Liquid Crystal Materials (Springer-Verlag, New York, 1994)]. When a thin film of tungsten trioxide is deposited by magnetron sputtering onto one of the transparent ITO electrodes, and a NLC cell is assembled with such asymmetry, the electro-optical response becomes polarity sensitive [G. Strangi et al., Appl. Phys. Lett. 74, 534 (1999)]. The analysis of this response suggests the occurrence of a reverse internal electric field, associated with the ionic diffusion process of protons always present in these sputtered WO3 films [E. Cazzanelli et al., Electrochim. Acta 44, 3101 (1999)]. By using an opportune voltage waveform it is possible to evaluate such an internal field. Impedance and cyclic voltammetry measurements were carried out on these cells, comparing "as-deposited" and "annealed" tungsten trioxide electrodes. These studies confirm that an important ionic diffusion process is involved in the establishment of an internal electric field, which modifies the electro-optical response of the nematic liquid crystal cell.