Two-color electrochromic devices using a tungsten oxide and nickel oxide double layer

Abstract Tungsten oxide (WO 3 ) and nickel oxide (NiO) are typical inorganic electrochromic (EC) materials. The color of WO 3 changes from transparent to blue by reduction, whereas that of NiO changes from transparent to brown by oxidation. This study fabricated EC devices with an indium tin oxide (ITO)/WO 3 /NiO/ITO/Nafion/ITO structure. The WO 3 /NiO double layer and Nafion were used as the EC layer and electrolyte, respectively. The color of the device changed from blue to transparent and then to brown by varying the applied potential from −2.5 to +1.0 V and then to +2.0 V. The WO 3 and NiO films were reduced at −2.5 V; the WO 3 film was oxidized at +1.0 V, and both were oxidized at +2.0 V. The transmittance in the visible and near-infrared regions changed independently. Hence, the devices are considered applicable for dual-band EC smart windows.