Thickness dependence of electro-optical properties of WO3 films as an electrochromic functional material for energy-efficient applications

WO3 films of different thicknesses (100, 200, and 300nm) were prepared by two vacuum physical vapor deposition (PVD) methods (thermal evaporation and an electron gun). Electrophysical measurements (cyclic voltammetry and chronoamperometry) were carried out using a potentiostat in an electrochromic cell (liquid device) that consists of a substrate coated with double layers of F: SnO2 and WO3 served as working electrodes. X-ray diffraction showed an amorphous structure for all the deposited WO3 thin films. While the transmission in the bleached state does not change and it is close to the as-deposited state, it is slightly less at larger thickness. The cyclic voltammogram was more open (more current flow) for thicker films due to the less dense structure with many channels. Also, the coloration efficiency was higher at low thickness. The response time was of the order of seconds and decreased for coloration, while it increased for the bleaching process as the film thickness increases. According to the proposed figure of merit, the electrochromic performance deteriorates as the WO3 film thickness decreases. However, the thickness is more effective in the case of an electron gun. Generally, continuous films but thinner than 200nm, are recommended for deposition methods.