Preparation conditions of sputtered electrochromic WO3 films and their infrared absorption spectra

Tungsten oxide films were prepared by rf sputtering in an argon-oxygen atmosphere from W and WO₃ targets. To bring about reversible electrochromic (EC) characteristics, as-deposited films require an aging process (i.e. cycles of injection/ejection of charge carriers). The infrared absorption band at around 3300 cm⁻¹ increases during the aging process, and it is assigned as OH stretching vibrations of absorbed water.By coloration after aging, the 3300 cm⁻¹ band decreases, and a new band appears at 2400 cm ⁻¹. The latter band is considered to be to the stretching mode of radicals incorporated in the WO₃ matrix. At low coloration levels, the 2400 cm⁻¹ band increases slightly with injected charge, and a coloration mechanism other than the usual double injection model may be considered.The coloration efficiency depends on the preparation conditions. Its maximum value is the same for films prepared from W and WO₃ targets, and is 60 cm²/C at a wavelength of 600 nm. When a tungsten target is used, the substrate temperature is low and the deposition rate is high compared with a WO₃ target.