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

We have investigated the electrochromic properties of amorphous granular tungsten oxide (WO _{3 + δ}) thin films with over-stoichiometric oxygen content (δ), using LiClO ₄ 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. WO _{3 + δ} films with an optimized δ value show an optical modulation of 86% at a wavelength of 630 nm and the highest coloration efficiency ever reported of ∼ 213 cm ²/C. The δ-dependent coloration mechanism is discussed using the site saturation model. It is proposed that WO _{3 + δ} films with the optimal δ value have favorable thickness and stoichiometry for the generation of Li ⁺W ⁺⁵ states.