Optical properties of Li + doped electrochromic WO 3 thin films

Abstract

As component parts of electrochromic devices, WO 3 layers are understood to be chiefly responsible for the colour change of the former when cationic ions are inserted into their crystalline lattice. In seeking to develop a suitable all-solid electrochromic device, the light ions have to be inserted into the structure in one step of a single fabrication process (a one-run process). In this paper, Li + ions were incorporated into the WO 3 layer from the gas phase during the deposition process. Electrochromic WO 3 thin films were deposited on fused silica and silicon substrates by physical vapour deposition techniques. The substrate temperature during the WO 3 deposition process was kept at 150°C and the oxygen pressure at 5×10 –2Pa. In order to improve the performance of this one-run doping method, a series of samples, with Li to W atomic ratio 0 (as-deposited) and 0.4, was prepared to study the effect of Li insertion on the optical properties of the WO 3 thin films. Optical characterisation in the ultra violet–visible and infrared ranges was carried out using ultra violet–visible transmittance, ultra violet–visible multichannel spectroscopic ellipsometry and FTIR spectroscopy. The optical properties were clearly dependent on the level of Li. The evolution of the optical properties in a sample, with a Li–W ratio of 0.4, was also analysed after exposure to atmosphere. These results represent an initial step towards achieving a better understanding of problems related to the doping of electrochromic layers with lithium ions, including irreversibility, stoichiometry effects, the correlation between optical and electrical properties and optical absorption modelling.