Lithium vanadium bronze thin films for electrochromic applications

Abstract

Thin films of fine grained polycrystalline stoichiometric vanadium pentoxide (85% of bulk density) have been prepared by vacuum evaporation. These films have been made into lithium vanadium bronze, LixV2O5, by inserting lithium, either electrochemically or chemically. In addition, lithium vanadium bronze thin films have been prepared by co-evaporation of Li and V2O5. The optical properties, absorption and refractive index, have been measured from 2000 to about 200 nm. Strong absorption arises from indirect transitions across the main gap and is ≈2.2 eV for x=0. The variation of the energy gap up to an x value of 2 has been obtained from the absorption data and the importance of irreversible phase changes noted. High x value bronze is useful as a counter electrode material in glazings, having an energy gap of about 3 eV. There is a considerable, technologically significant, band tail in the absorption spectrum thought to arise from polaronic-type states, perhaps modified by the presence of guest species ions. The optical behavior upon lithium electrochemical insertion has also been examined. It is found that nonstoichiometry gives rise to an overall reduction in optical change per guest atom inserted. These effects are fast compared with electrochemical insertion times.