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 liqu id crystal (NLC) cells. in a standard sandwichlike cell the nematic liquid crystal, confined between two transparent plane electrodes of purely electr onic conductors [indium tin oxide (ITO)], undergoes a molecular reorientati on under the action of an external electric field E. This electrically cont rolled birefringence (electro-optical switching) is proportional to E-2, th us polarity insensitive [L. M. Blinov and V. G. Chigrinov, Electrooptic Eff ects in Liquid Crystal Materials (Springer-Verlag, New York, 1994)]. When a thin film of tungsten trioxide is deposited by magnetron sputtering onto o ne of the transparent ITO electrodes, and a NLC cell is assembled with such asymmetry, the electro-optical response becomes polarity sensitive [G. Str angi et at., Appl. Phys. Lett. 74, 534 (1999)]. The analysis of this respon se suggests the occurrence of a reverse internal electric field, associated with the ionic diffusion process of protons always present in these sputte red WO3 films [E. Cazzanelli et at, Electrochim. Acta 44, 3101 (1999)]. By using an opportune voltage waveform it is possible to evaluate such an inte rnal field. Impedance and cyclic voltammetry measurements were carried out on these cells, comparing "as-deposited" and "annealed" tungsten trioxide e lectrodes. These studies confirm that an important ionic diffusion process is involved in the establishment of an internal electric field, which modif ies the electro-optical response of the nematic liquid crystal cell.