Electrochromism in WO_3 Evaporated Films

This investigation was made on the electrochemichromic mechanism in liquid-EC cell systems. WO_3 film were prepared by thermal evaporation in vacuum below 1×(10)^<-5> Torr on a SnO_2 : Sb coated glass substrate. The deposition rate of the films was in tee range of 250-600 Å/min.. X-ray analysis indicated the films to be amorphous. In an electrochemichromic cell containing H_2SO_4 electrolyte, the color of WO_3 film changes from trans-arent to blue by applying an electric field. The response of coloration and bleaching greatly depended upon the sheet resistance of both transparent electrodes, and the spacing between WO_3 film and counter electrode. Upon the application of 2-3 Volt across the EC cell, it was found that under the following experimental conditions the cell current shows t^<-1/2> dependence (Butler-Volmer's law) in coloring process and t^<-3/4> dependence (Faughnan's law) in the bleaching process : the thickness of films was in the range of 2000-3000 Å, and sheet resistance of transparent electrodes was below 10 ohm/□, and WO_3 film-counter electrode spacing was below 0.2mm.