Electrochromic device with magnetron sputtered tungsten oxide (WO3) and nafion membrane: performance with varying tungsten oxide thickness- a report

Electrochromics is the emerging technology for energy conservation and indoor climatic control through smart windows. In this study we are reporting four layer electrochromic device: ITO (400 nm)/commercially procured Nafion (183 mu m)/WO3 (44 nm to 200 nm)/ITO (400 nm). The active area (A) of the electrochromic devices are 3 cm(2). The tungsten oxide (WO3) and ITO thin films have been deposited at room temperature (300 K) by reactive DC Magnetron sputtering. The sheet resistance of ITO is 20 Omega/square. The 'as deposited' WO3 films are amorphous and have high optical transmission (75%-85%) in the visible spectrum. The optical band gap decreases with increasing thickness of WO3 thin films. The coloration efficiency (CE) of the electrochromic device increases with increasing thickness of the WO3 layer. The CE for the device with WO3 thickness 200 nm is 184 cm(2)C(-1) : the highest reported so far for a hybrid electrochromic device. The increase in the CE with thickness has been explained (for the first time) by replacing the surface charge density (Q/A) with the volume charge density (Q/A*t) in the coloration efficiency formula derived from the Beer Lambert's law.