Electrochromism in WO_3 evaporated films

The performance characteristics of EC display devices were investigated. The devices used in this study were EC cells having a SnO_2 : Sb-WO_3-liquid electrolyte-SnO_2 : Sb structure. The WO_3 films were prepared on SnO_2 : Sb coated glass substrate by thermal evaporation at pressure below 1.5×(10)^<-5> Torr. The thickness of the WO_3 films were in the range of 2000-3900A. The liquid electrolytes used were H_2SO_4,Li_2SO_4,Na_2SO_4 and K_2SO_4. For any liquid electrolyte, a bule color appeared for EC devices with transparent electrode negatively biased. The optical transmission spectrum of the WO_3 films showed a peak at around 3800A and a broad bottom peaked around 9000A. Coloration current response of the liquid EC cells showed a t^<-1/2> dependence of current, and bleaching current response of the EC cells did a t^<-3/4> dependence of current under the condition of dc applied voltage below 2 V across the EC cells. Under the dynamic condition, corrosion of the counter electrode were with the forward biased (bleaching) greated than with the reverse biased (coloration).