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

► We report on electrochromic characteristics in WO3 + δ films. ► Electrochromic properties of WO3 + δ films depend on different oxygen contents. ► Coloration efficiency of 213 cm2/C is obtained in a WO3 + δ film with an optimized δ. ► The electrochromic mechanism is presented.