An all-solid-state electrochromic (EC) device based on NiO/WO3 complementary structure and solid polyelectrolyte was manufactured for modulating the optical transmittance. The device consists of WO3 film as the main electrochromic layer, single-phase hybrid polyelectrolyte as the Li+ ion conductor layer, and NiO film as the counter electrochromic layer. Indium tin oxide- (ITO) coated glass was used as substrate and ITO films act as the transparent conductive electrodes. The effective area of the device is 5 x 5 cm(2). The device showed an optical modulation of 55% at 550 nm and achieved a coloration efficiency of 87 cm(2) C-1. The response time of the device is found to be about 10s for coloring step and 20s for bleaching step. The electrochromic mechanism in the NiO/WO3 complementary structure with Li+ ion insertion and extraction was investigated by means of cyclic voltammograms (CV) and X-ray photoelectron spectroscopy (XPS). (C) 2009 Elsevier B.V. All rights reserved.