On the Electrochromism of Evaporated V2O5 Films

Abstract

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 (LiCLO₄·3H₂O) 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 O_1s and V_2p3/2 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.