On the Electrochromism of Evaporated V2O5 Films

The electrochromism of amorphous V2O5 thin films prepared by the vacuum evaporation of V2O5 powder was studied through optical absorption, XPS and SIMS measurements. The films were colored and bleached with the aid of an electrolyte consisting of lithium perchlorate (LiCLO43H2O) in propylene carbonate (CH3CHOCOOCH2). The as-grown films were yellow, like V2O5 powder, while the colored films were greenish-gray, like VO2 powder. The XPS spectra of the O1s and V2p3/2 core levels of the as-grown films matched those of V2O5 powder, while those of the colored films were similar to those of VO2 powder. SIMS measurement revealed that Li+ ions are introduced into the films by coloration and expelled by bleaching. From these results, it is concluded that the coloration of a-V2O5 films is due to a change in the molecular state from V2O5 to VO2. This change is caused by the double injection of electrons and Li+ ions into the films.