Effect of electrolyte concentration on electrochromic performance of sputtered tungsten oxide film: Experiments and simulation

Tungsten oxide (WO3) thin films are of critical importance in electrochromic devices as positive electrodes. However, the effect of electrolyte properties on the electrochromic properties of these films is not well ascertained. Herein, we demonstrate the effect of various LiClO4 salt concentrations in propylene carbonate (PC) on the switching speed and coloration efficiency of RF sputtered tungsten oxide film via experiments and simulation. The model developed for simulating the electrochromic performance of the WO3 thin films is based on dilute solution theory. The model is flexible enough to account for variable physical properties: lithium salt concentrations, film thickness and thickness of electrolyte diffusion layer. Relevant model parameters are obtained by fitting the model predicted cyclic voltammogram to experimental data obtained from a three-electrode cell composed of sputtered tungsten oxide, platinum, and Ag/AgCl. The switching time strongly depends on thickness of WO3 thin film and the electrolyte diffusion layer while the depth of coloration depends on concentration of the salt and operating voltage. Our simulated and experimental results provide an insight into the design of electrochromic devices with excellent switching speed and coloration efficiency.