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

Presented is a new method to achieve the polarity-dependent optical response in a nematic liquid crystal (NLC) cell. It consists of the asymmetrical insertion of mixed conductor film (WO3) as electrode. Focus is made on the NLC behavior strongly influenced by the electrochemical process, which occurs at the different interfaces. The basis of the asymmetrical electro-optic response must be searched in the ionic diffusion process, which takes place in the WO3 electrode during the anodic polarization. Under the action of a low-frequency external electric field, the protons always present in these films, migrate towards the oxide-NLC interface, giving rise to an interface formation which counteracts the threshold phenomena of the molecular reorientation. The electrical measurements carried out on the cells with 'as-deposited' WO3 electrodes pointed out a remarkable diffusive process, which vanishes for the thermally annealed WO3 films. The interesting electrical and optical behavior of the presented device fits well within a simple model of a double layer formation at the WO3-NLC interface depending on the polarity of the external perturbation.