Electrochromic responses of low-temperature-annealed tungsten oxide thin films in contact with a liquid and a polymeric gel electrolyte

Thin films of tungsten trioxide (WO3) for electrochromic application were synthesized by potentiostatic method by using a peroxytungstic acid as a solution precursor. The morphology of the films with and without postthermal annealing was analyzed by atomic force microscopy. When they were in contact with the liquid electrolyte (LiI in propylene carbonate, PC) and under alternatively applied negative (−1.5 V) and positive (+1.0 V) potentials, the transient optical transmittance modulations at wavelength of 650 nm of the as-deposited and 60 °C annealed WO3 samples were higher than that of 100 °C annealed WO3 films, and the switching times between the colored and bleached states were related to the surface morphology of the WO3 films. In polymeric gel electrolyte (LiI and polymethyl methacrylate in PC) devices, longer time was required for complete coloration as well as bleaching process compared with the liquid one. A parametric analysis was made for each of the transient optical transmittance curves of WO3-based electrochromic devices to extract the values of the response time in coloration (reduction) and bleaching (oxidation) processes. It concludes that the coloration process was determined by the exchange of current density at the electrolyte–WO3 interface and a possible inhomogeneous interfacial potential for ion intercalation retards the effective coloration time. The bleaching process seems to be controlled by the space charge-limited lithium ion diffusion in WO3 electrode and the ionic conductivity of the electrolyte as well.