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 μm)/WO₃ (44 nm to 200 nm)/ITO (400 nm). The active area (A) of the electrochromic devices are 3 cm². 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 Ω/◻. 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 cm² 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. (paper)