Effects of oxygen stoichiometry on electrochromic properties in amorphous tungsten oxide films

We have investigated the electrochromic properties of amorphous granular tungsten oxide (WO3+δ) thin films with over-stoichiometric oxygen content (δ), using LiClO4 with propylene carbonate as an electrolyte. Different optical and electrochromic characteristics are observed with increasing δ. All the devices are electrochemically stable for more than 5000 color/bleach cycles without apparent degradation, and they have a faster response to coloration than to bleaching. WO3+δ films with an optimized δ value show an optical modulation of 86% at a wavelength of 630nm and the highest coloration efficiency ever reported of ~213cm2/C. The δ-dependent coloration mechanism is discussed using the site saturation model. It is proposed that WO3+δ films with the optimal δ value have favorable thickness and stoichiometry for the generation of Li+W+5 states. ► We report on electrochromic characteristics in WO3+δ films. ► Electrochromic properties of WO3+δ films depend on different oxygen contents. ► Coloration efficiency of 213cm2/C is obtained in a WO3+δ film with an optimized δ. ► The electrochromic mechanism is presented.