Electrochromism and photochromism in MoO3films

Electrochromic devices for use as alphanumeric displays have been the subject of recent interest. Their optical absorption is changed when current passes through them or if exposed to uv light. Electrochromic materials are generally transition metal compounds such as WO3and MoO3. Some people have suggested coloration occurs by electrochemical redox. Others have proposed that coloration results from electron filling of deep traps incorporated during film deposition. We studied electrical and (uv) photo-coloration of MoO3and found that both electrical and photo-coloration are accompanied by a chemical phase change. Based on this, together with electrical charging/discharging characteristics, the conclusion is reached that both photo-and electro-chromism in such materials is chemical in nature. The significance of this, regarding electrochromic devices is discussed.