Photochemical Precharging of Tungsten Trioxide for Enhanced Transmittance Modulation in Flexible Electrochromic Devices

UV irradiation is used to precharge sputtered tungsten trioxide (WO3) on polyethylene terephthalate enhancing the photochromic response with organic solvents. A comparison between the optical and electrochemical properties of photochemically and electrochemically charged WO3 results in a correlation of the transmittance to the respective charge density. This allows for a precise control of the charge density in the precharging process monitored by UV-Vis spectroscopy. A proof-of-concept flexible electrochromic device combining precharged WO3 (charge density: 20 mC cm-2) and Prussian blue (15 mC cm-2) exhibits a superior change in the visible light transmittance (τv) from 8% (dark) to 79% (bleached) and a coloration efficiency of 139 cm2 C-1 at 716 nm.