Electrochromic devices (ECDs) change their transmittance by applying an electrical voltage or current. As smart windows, they are a low-energy dimmable solution for the reduction of glare and the regulation of incident sunlight in buildings. For the large-scale production of ECDs based on tungsten trioxide (WO₃) and Prussian blue (PB), a pretreatment applicable in a high-throughput roll-to-roll process is desirable. A WO₃/PB ECD without any pretreatment or formation cannot reach the maximum possible transmittance change when both electrodes are in the same redox state after cell assembly. To achieve the necessary charge balance, we report a contact-free precharging method for WO₃ based on its photochromism in comparison to the state-of-the-art electrochemical reduction.

It has been reported that small organic molecules intensify the photochromic coloration of WO₃ under UV irradiation. In this study, the enhancement of the photochromic effect of WO₃ by low-hazardous organic solvents is used as a precharging method for sputtered WO₃ on indium tin oxide coated polyethylene terephthalate (PET-ITO) films. By comparing photochemically precharged WO₃ with electrochemically reduced WO₃, a correlation between the visible light transmittance and the inserted charge density is obtained. This allows a precise control of the charge density in the precharging process monitored by UV-Vis spectroscopy. A proof-of-concept WO₃/PB ECD exhibits a high change in the visible light transmittance (τv) from 8% (dark) to 79% (bleached) and a coloration efficiency of approx. 139 cm²/C at 716 nm. Since this precharging method does not require direct contact with the WO₃, it is potentially suitable for a high-throughput and cost-efficient roll-to-roll production process. The precharging method demonstrated here might be transferrable to WO₃ on glass-based substrates.