Development of electrochromic smart windows by sol-gel techniques

Abstract

A novel nanocomposite lithium ion-conducting electrolyte has been developed, based on organically modified silanes, which is suitable for application in a sol-gel electrochromic system. The system developed consists of FTO-coated (Fluorine doped Tin Oxide) glass coated with tungsten oxide, WO3, at one side of the device as the electrochromic layer, with a cerium oxide-titanium oxide layer, CeO2-TiO2, acting as ion-storage layer or counter electrode. The adhesive properties of the electrolyte enabled the manufacture of electrochromic devices in a laminated structure: glassFTOWO3 anocomp.elect.CeO2-TiO2FTOglass. The conductivity of the nanocomposite electrolyte system varies between 10-4 and 10-5 Scm-1 at 25°C depending on the exact composition. The temperature dependence of the conductivity exhibits typical Vogel-Tamman-Fulcher (VTF) behaviour. The thickness of the electrolyte between the two halves of the device could be adjusted by the use of a spacer technique in the range 10 - 150 µm. Optoelectrochemical measurements were conducted on electrochromic devices to study the kinetics of colouration and bleaching as a function of the number of switching cycles. At present, cells are constructed in two formats:10 X 15 cm2 and 35 X 35 cm2. Switching times under one minute were achieved for the smaller format with a corresponding optical modulation between 75% to 20% (at λ = 0.633 µm). In the case of the larger format the switching time increases to several minutes due to the increase in geometric area.