On the Electrochromism of Evaporated V₂O₅ Films

The electrochromism of amorphous V 2 O 5 thin films prepared by the vacuum evaporation of V 2 O 5 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 (LiCLO 4 ·3H 2 O) in propylene carbonate (CH 3 CHOCOOCH 2 ). The as-grown films were yellow, like V 2 O 5 powder, while the colored films were greenish-gray, like VO 2 powder. The XPS spectra of the O 1s and V 2p3/2 core levels of the as-grown films matched those of V 2 O 5 powder, while those of the colored films were similar to those of VO 2 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-V 2 O 5 films is due to a change in the molecular state from V 2 O 5 to VO 2 . This change is caused by the double injection of electrons and Li + ions into the films.