Abstract
The present work is aimed to a deeper understanding of the polarity sensitive electro-optical response observed in a liquid crystal cell with asymmetric insertion of a thin film of tungsten trioxide, deposited by r.f. sputtering on a transparent ITO substrate. After preliminary structural characterization of the films, the electro-optical response of the liquid crystal cells containing a WO3 layer has been analyzed. The voltage thresholds of the Fredericksz transition are increased or decreased for the anodic or the cathodic charge of the tungsten trioxide film, respectively. In such a way a polarity-sensitive electro-optical response is generated, with the same frequency of the applied field. Impedance and cyclic voltammetry measurements have been performed on liquid crystal cells having WO3 films inserted as electrode, for as-deposited films as well as for annealed films at increasing temperatures. A strong decrease of the ionic current is observed in the cell where the film underwent the highest temperature treatment, above 300 °C. All the measurements suggest that a charge redistribution occurs for the hydrogen ions, always present in the films, during the intercalation-deintercalation processes induced by the applied voltage, and it is responsible for the changes of the effective electric field acting on the liquid crystal layer.
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