Electrochromic technology has been widely applied due to its advantages of energy conservation, environmental friendliness, intelligence, and controllability, among which the industrialization of WO₃-based electrochromic devices is most mature. However, in a conventional electrochromic device consisting of upper transparent electrode ITO/counter-electrode/electrolyte/WO₃/lower transparent electrode ITO, the existence of counter-electrode layers not only reduces the transmittance of the device in the bleached state but also affects its cyclic stability due to incomplete matching of the electrochemical properties between the counter-electrode layers and electrochromic layers. Though the structures of electrochromic devices have been continuously optimized, the existing counter-electrode in WO₃-based electrochromic devices cannot be solved. Therefore, this paper first prepares WO₃ films by magnetron sputtering and then fabricates a brand-new curtain-like electrochromic device without a counter-electrode layer based on the "current-driven model". Highly controllable color changes are achieved in the curtain-like device, and influence of the counter-electrode on device performance is eliminated because the counter-electrode is no longer required. This research can provide new pathways for structure innovation of electrochromic devices.

WO₃ films are fabricated by magnetron sputtering of reactive radio frequency (RF) using a W target (purity of 99.99%), an Ar flow rate of 12 sccm (1 sccm=1 m³/min), and an O₂ flow rate of 4.8 sccm. The substrates are clean ITO glasses with the sheet resistance of 8 Ω/sq. The employed RF power is 80 W, and the substrate heating temperature is 200 ℃. WO₃ films at different thicknesses are obtained through different sputtering durations. Meanwhile, a 1 mol/L Li ion electrolyte is prepared by dissolving LiClO₄ in polycarbonate (PC) solution. Finally, the curtain-like WO₃-based electrochromic devices are fabricated by sealing the ITO glass, electrolyte, and WO₃ film with the UV sealant. Scanned electron microscopy images of the WO₃ films are taken on a Sigma 500 instrument (Zeiss) at 10.0 kV. Phase structures of the films and ITO substrate are examined by X-ray diffraction (XRD) analysis through Cu Ka radiation (Philips X' Pert diffractometer), and X-ray photoelectron spectra (XPS) are recorded by a Thermo Fisher Scientific ESCALAB 250Xi XPS system. Finally, cyclic voltammetry (CV) tests are carried out at the voltage range of -0.8-0.8 V at a scan rate of 100 mV/s on the electrochemical workstation (CHI760E), and transmittance spectra are recorded via the UV-Vis spectrophotometer (Hitachi F-4600, Japan).

Firstly, amorphous WO₃ films with the thickness of 800 nm are prepared (Fig. 1) and are preferred for ion injection and extraction. The optical modulation rate at wavelength of 550 nm is as high as 78%, and decay rate of the storage charge density is only 3.5% after 1000 CV cycles, which is far better than reported (Fig. 2). Then, curtain-like WO₃-based electrochromic devices are proposed to switch between being colored and bleached without a counter-electrode under the control of a flowing current in the bottom ITO layer (Fig. 3). Bezel between the storage area and the window area of the curtain-like device is designed by introducing an artificial fissure (Fig. 4). Additionally, effect of the WO₃ film thickness on the response time, recovery time and cyclic performances of the device is investigated, and the results show that the best overall performances are realized at film thickness of 800 nm (Fig. 5). The size of the storage area is also explored, and when it is similar with that of the window area, the cyclic performances could be ensured (Fig. 6). Finally, the curtain-like WO₃-based electrochromic device fabricated using the above-mentioned parameters shows a higher transmittance than conventional structured ones, with excellent memory effects (Fig. 7).

This paper prepares high-performance WO₃ films by magnetron sputtering, whose modulation rate at a wavelength of 550 nm is as high as 78%, and decay rate of the storage charge density is only 3.5% after 1000 CV cycles. Then a curtain-like electrochromic device without a counter-electrode layer is designed based on the current-driven model, where the bezel, thickness of WO₃ films, and size of the ion storage area are systematically optimized. Results show that an artificially-introduced fissure in the WO₃ film can simultaneously increase the response time and modulation rate of the device, and facilitates the design of the working area bezel. The overall performances of the device are best when the WO₃ film is at a thickness of 800 nm. The cycling life of the device can be guaranteed when the ion storage area and the working area are similar. Finally, with the employment of optimal designing parameters, the fabricated curtain-like WO₃-based electrochromic device shows excellent performances, and the transparent transmittance at a wavelength of 550 nm is 76%, which is 9 percentage points higher than that of the conventional device with an 80 nm TiO₂ film as the counter-electrode. Moreover, the device also shows an excellent memory effect with the colored transmittance at 20%, only increasing by 7.5 percentage points after being placed for 10 days. These characteristics result in the great application advantages of curtain-like devices in information encryption and famous painting protection. Compared with traditional structures, the curtain-like electrochromic device features a simple structure and highly controllable color-changing patterns, which can provide guidance for the structure innovation of electrochromic devices.