High Contrast Electrochromic Tungsten Oxide Layers

Abstract

The electrochromic response time and the chemical and electrochemical stability of amorphous tungsten oxide layers have been found to be adversely affected by the degree of porosity and hydration of the oxide layers 1,2, thus complicating the preparation of stable high speed electrochromic films. Based on our investigations of the microstructure of thermally evaporated tungsten oxide we could overcome this problem by preparing high density (P = 6.4 g cm-3) tungsten oxide layers by flash evaporation or by reactive dc sputtering. In spite of the higher packing density (reduced free volume) which increased the electro-chemical stability remarkably, the electrochromic response time of thin layers could be re-duced to 50 ms to reach a contrast ratio of 1 : 2 at 633 nm, which is comparable to that of sputtered iridium oxide films. Even with 1.6 4m thick dc sputtered tungsten oxide layers a response time of 400 ms for a contrast ratio of 1 : 2 and a response time of 5 s for a contrast ratio of 1 : 1000 could be achieved.