Effects of ITO Electrode on Electrochromic Properties of WO3 Thin Films in Visible and Near-Infrared Region

Abstract

Indium tin oxide (ITO) thin films have been incorporated into electrochromic (EC) devices as a transparent conductive film. It is highly desirable that EC devices applied to a smart window can control both visible and infrared light, because infrared light accounts for about half of the energy of sunlight. In this study, we fabricated WO3 thin film on ITO thin films with different sheet resistances and studied their EC properties in visible and near-infrared region.ITO thin films deposited on glass were used as substrates. Tungsten oxide (WO3) thin films were deposited using RF magnetron sputtering system. Crystal structure was characterized by X-ray diffraction. Optical properties of the prepared samples were measured using UV-VIS-NIR spectrophotometer. The electrical resistance of the ITO thin films was measured by a four probe method. EC properties of the WO3 thin film were measured by cyclic voltammetry (CV) and chronoamperometry (CA) in a 0.01 M H2SO4 aqueous electrolyte.The transmittance of the high sheet-resistance ITO thin film (177 Ω/sq) decreased at a short wavelength region below 455 nm. However, it showed high transmittance above 70% in the visible and near-infrared region (455-2050 nm). In contrast, the low sheet-resistance ITO thin films showed high transmittance above 70% in a wavelength region of 335-1350 nm and its transmittance decreased above 1350 nm. The differences of the transmittance in short and long wavelength regions are considered to be caused by the Burstein-Moss effect and Drude model, respectively. Fig. 1 shows the transmittance spectra of the WO3 thin film with a thickness of 200 nm deposited on the ITO/glass substrates measured in air. Lines (1) and (2) show the transmittance spectra of colored and bleached states of WO3 thin film deposited on high sheet-resistance ITO thin film, and lines (3) and (4) show those of colored and bleached states of WO3 thin film deposited on low sheet-resistance ITO thin film. The WO3 thin film deposited on the high sheet-resistance ITO thin film showed a larger transmittance change and higher transmittance in bleached state in the near-infrared region than those of the WO3 thin film deposited on the low sheet-resistance ITO thin film. From these results, it was found that the selection of appropriate ITO electrodes is very important to obtain good EC property of WO3 thin films in visible and near-infrared region. Figure 1