Electrochromic characteristics of radio frequency plasma sputtered WO3 thin films onto flexible polyethylene terephthalate substrates.

In electrochromic materials optical features change when voltage is applied across them. The optical features should be reversible; i.e. the original state should be recoverable if the polarity of the voltage is changed. These features make electrochromic materials of substantial interest for several applications such as smart windows, wearable displays, mirrors and television screens. An electrochromic device (ECD) behaves like a thin film battery that changes optical properties with oxidation reactions. An ECD has at least one electrochromic layer as well as anode, cathode and electrolyte components.

The most widely used cathodic electrochromic material is tungsten oxide (WO3) because of its advantages such as high color efficiency, high contrast, non-toxic nature, high cyclic stability and comparatively low cost. Tungsten oxide thin films have been prepared via a variety of different processes including physical deposition such as sputtering and evaporation or by techniques based on chemical processes occurring either in the gas phase, such as chemical vapor deposition, or in solution including electrodeposition, anodization, sol-gel deposition and spray pyrolysis. Both amorphous and crystalline WO3 thin films have been used for electrochromic applications. In the basics of the magnetron sputtering process, a target plate is bombarded with energetic ions generated in glow discharge plasma. The bombardment process causes the removal of target atoms, which may then condense on a substrate as a thin film. This method has an important impact in application areas consisting of corrosionresistant coatings and coating with specific optical or electrical properties. Also, magnetron sputter technology permits the minimization of impurities in the films; moreover, the film thickness can easily be controlled. However, the sputtering process is limited by low ionization efficiencies in the plasma.

The purpose of this study was to investigate the influence not only of the thickness of WO3 films but also the kind and concentration of electrolyte on the electrochromic properties of flexible ECDs. Both flexible conducting ITO layer and electrochromic WO3 layer were obtained using the radio frequency (rf) magnetron sputtering method. WO3 thin films were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The electrochromic properties of the films were comprehensively investigated via cyclic voltammetry, chronoamperometry, and chronocoulometry methods under the influence of electrolyte concentration. ECDs were tested via electrochemical and transmittance measurements in order to evaluate the electrochromic performance levels.