An investigation was conducted on electrochromic properties of WO3-z (tungsten oxide) thin films sputtered on 60 Omega/square flexible PET (polyethylene terephthalate)/ITO (indium tin oxide) substrates at various oxygen flow rates. A metallic W target, sputtered by radio frequency (r.f.) power with argon gases and reacted with oxygen gases at room temperature (23 degrees C). was proven to provide extraordinary electrochromic properties. Cyclic voltammetry measurement demonstrated that only low driving voltages from -1 V to 1 V were needed to provide Li+ ion insertion and extraction to and from a working electrode. The flexible PET/ITO/WO3-z film was mounted on the working electrode and dipped in a 0.1 M LiClO4-PC (propylene carbonate) electrolyte. The electrochromic properties of flexible PET/ITO/WO3-z were investigated by cyclic potentiostatic switching and ultraviolet (UV)-visible spectroscopy. An oxygen flow rate of 0.71 sccm with a low O/W-ratio of 2.916, an oxygen deficiency z of 0.084 and an insertion coefficient x of 0.150 can offer an excellent light modulation of up to 73.1% of transmittance variation, optical density change of 0.89 and color efficiency of 62.4 cm(2)/C at a wavelength of 650 nm in 14 s of Li+ insertion and extraction. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.