Crystalline WO3 Films Research Articles

Near-infrared reflectance modulation with electrochromic crystalline WO3 films deposited on ambient temperature glass substrates by an oxygen ion-assisted technique

Electrochromic, crystalline WO3 films have been deposited on glass substrates at ambient temperature by an oxygen-ion-assisted technique using oxygen ion energies ≥300 eV and oxygen ion to vapor molecule (WO3) ratios, γ≥2.5. After lithiation, the resulting LixWO3 films exhibited >50% reflectivity in the near infrared, and the reflectivity dispersion was fit by a Drude free-electron model, yielding the Drude parameters: plasma energy, Ep=3.3 eV; and the loss (damping) parameter, EΓ=1.0 eV. (The bound electron permittivity, εb, was fixed at 4.0.) These values are comparable to those obtained with WO3 films rf sputter deposited onto substrates at temperatures >420 °C. During the ion-assisted deposition the substrate temperature reached approximately 90 °C, caused primarily by radiation from the WO3 evaporant source. This indicates that economical low-temperature substrates, such as plastics, could be used. These results suggest that practical electrochromic smart windows for energy-efficient buildings might be produced using ion-assisted deposition techniques.

Estimation of the composition parameter of electrochemically colored amorphous hydrogen tungsten oxide films

The electrical and optical steady state observed in electrochemical coloration has been studied using asymmetric cells consisting of evaporated amorphous tungsten oxide films with 350–6000 Å thickness. The counter electrode used is indium wire, steel wire, or antimony-tin oxide film, and the electrolyte is a 1-N H2SO4 aqueous solution containing 10 vol % glycerol. The current and optical transmittance of the cells decrease with increasing time during coloration, and simultaneously reach a steady state. The optical density (λ=0.5 μm) in the steady state is proportional to the thickness of the tungsten oxide film, and the absorption coefficient at λ=0.5 μm of the colored oxide film in the state is approximately 9.0×104 cm−1. The effective charges which contribute to the coloration of films calculated from the charge injected until the electro-optical steady state were found to be 1.03–1.20×103 C/cm3. Assuming that the evaporated tungsten oxide films used have a distorted ReO3 structure, and that a hydrogen tungsten bronze HxWO3 is formed by coloration, the composition parameter x calculated from the average value of the effective charge, is 0.36, which is comparable with that of hydrogen tungsten bronze H0.33WO3 obtained for the colored crystalline WO3 films.

Preparation and properties of the dc reactively sputtered tungsten oxide films

Preparation and properties of the dc reactively sputtered tungsten oxide films have been investigated in relation to the oxygen concentration in sputtering atmosphere. The films with 1500–20 700 Å thickness were deposited on the glass substrates maintained at 200 °C by dc reactive sputtering from a metallic tungsten target under a constant operating pressure of 6.5×10−2 Torr in Ar-1%–30% O2 gas mixture. The films formed in an Ar-3%–20% O2 gas mixture are crystalline WO3, and have an electrical resistivity of 107–1011 Ω cm which is dependent on the oxygen concentration of the sputtering atmosphere. These films have a spectral transmittance above 80% in the visible and near-infrared regions, and optical band gap of the films ranges from 3.15 to 2.98 eV, depending on the oxygen concentration. Densities of the film deposited in Ar-3% O2 and in Ar-20% O2 gas mixtures are 5.85 g/cm3 and 6.65 g/cm3, respectively. Electrochemichromic properties of the transparent-crystalline WO3 films were studied using asymmetric cells, and were found to be dependent on the crystal orientation of the films. The films with the orientation of WO3 (020) and WO3 (021) formed in an Ar-3%–6% O2 gas mixture have a very good electrochromic property, and the cells composed of the crystalline WO3 films have a higher coloration rate than the cells composed of the vacuum-evaporated amorphous film. The films with the orientation of WO3 (001) and WO3 (021) formed in an Ar-8%–20% O2 gas mixture were found to have a poor electrochromic property.

Modulated transmittance and reflectance in crystalline electrochromic WO3 films: Theoretical limits

A quantitative theory for the optical properties of crystalline electrochromic materials is introduced. It is based on the double injection model and includes ionized impurity scattering of free electrons. Solar and luminous properties are evaluated versus electron density for doped WO3 films. Our results point to the possibility of creating efficient ‘‘smart windows’’ with dynamic control of the inflow of radiant energy.