Enhancement in oscillator strength of color centers in electrochromic thin films deposited from WO2 powder

Abstract

Color centers in amorphous films evaporated from WO₂ powder and WO₃ powder were investigated. It was found that the device using ‘‘WO₂’’ film, which has a WO₃‐like structure, exhibits a high‐speed response and a high optical density, which is attributed to the enhancement in oscillator strength of the color centers in ‘‘WO₂’’ films. The use of a configuration coordinate model revealed that the enhancement in the oscillator strength is caused by the increase in the degree of extension of the polaron wave function in the ‘‘WO₂’’ film. The absorption band of the color center in ‘‘WO₂’’ exhibits a low‐energy shift as compared with that in WO₃, which supports the increase in the degree of the small polaron extension in ‘‘WO₂.’’ We also studied the electrical conductivity, density, ir absorption spectra, and voltammograms of the ‘‘WO₂’’ and WO₃ films. These results suggest that the increase in the degree of the extension of the polaron wave function is caused by some change of the lattice condition which is introduced by a lack of oxygen in the ‘‘WO₂’’ film.