Electrochromic devices (ECDs) based on the resonant cavity structure can convert subtle changes in the refractive index of the electrochromic thin film into color changes with high contrast, based on the principle of optical interference, thus enabling the regulation of multiple colors. They exhibit great potential in the fields of dynamic color display, wearable electronics, and anti-counterfeiting. However, at present, the vast majority of research is based on liquid or gel electrolyte systems, which have defects such as liquid leakage and poor cycling stability in practical applications. These drawbacks severely limit their future development. In contrast, all-solid-state ECDs are free from these concerns. They have good cycling stability and the potential for industrial production, allowing for large-scale manufacturing. Nevertheless, due to the multi-layer thin-film stacking structure of all-solid-state ECDs and considering the ion transport efficiency and electrochromic performance, it is difficult to construct a resonant cavity structure to achieve multiple color changes. If multi-color regulation can be realized in all-solid-state ECDs, the application scope of colored ECDs will be further expanded, and they will have broader application prospects in fields such as color display, wearable electronics, and anti-counterfeiting.

In this study, without altering the structure of traditional all-solid-state ECDs, a dielectric-metal-dielectric (DMD) composite electrode was fabricated using the magnetron sputtering method. The effects of three different metal materials (Au, Ag, and Cu) and the thickness of the indium tin oxide (ITO) dielectric layer on the DMD structure were investigated respectively. An in-situ reflection spectrum of the samples in the range of 350 to 750 nm was characterized using a Hitachi UV-4100 ultraviolet/visible/near-infrared spectrometer at a scanning speed of 1200 nm·min⁻¹ to select the optimal metal material. By using an optical simulation software and the optical constants of ITO and Ag, the human eye’s photopic luminous efficiency integral value (R) was calculated to further optimize the thickness of each film layer in the DMD structure, aiming to optimize the experimental spectrum and achieve a variety of gorgeous structural colors. Subsequently, all-solid-state ECDs were prepared. The cross-sectional morphology of the samples was characterized using a field-emission scanning electron microscope (model: FEI Magellan 400), and a line-scan energy-dispersive X-ray spectroscopy (EDS) was performed to identify the distribution of characteristic elements in each film layer, so as to optimize the thin-film preparation process.

By utilizing the DMD composite electrode structure, a resonant cavity was constructed inside the electrode. Based on the principle of optical interference, a variety of static structural colors were successfully achieved. Moreover, before and after the color change of WO₃, multiple color changes were realized (Fig. 1). Silver (Ag) was selected as the optimal metal layer, and it was found that when the thickness of the Ag layer was optimized to 10 nm, the DMD structure could produce significant optical changes, enabling the dynamic modulation of various visible lights. Based on the simulation of the R value, by adjusting the thickness of the ITO layer and the multi-layer DMD structure, seven static structural colors with high brightness and high saturation were successfully obtained [Fig. 6(a)]. Additionally, an all-solid-state ECD with a red structural color was fabricated. Under the action of an applied voltage, a reversible conversion from red to deep red was successfully achieved (Fig. 7).

In this study, the difficulty in constructing a resonant cavity within the all-solid-state electrochromic system was overcome. Without altering the structure of traditional all-solid-state ECDs, a DMD composite electrode structure was designed and fabricated. A systematic exploration was carried out on the collaborative regulation mechanism of three different metal materials, namely Au, Ag, and Cu, and the effect of thickness of the indium tin oxide (ITO) dielectric layer on the interfacial light transmission behavior was studied. Various DMD composite electrode structures were prepared using the magnetron sputtering method, and with the effect of WO₃, the regulation of multiple color changes was achieved. Both experimental results and optical simulations demonstrate that when the thickness of the Ag layer is 10 nm, it forms the best match with the dielectric constant gradient of ITO, resulting in the strongest light interference effect, which enables the preparation of various structural colors. Meanwhile, by precisely controlling the thickness gradient of the ITO layer in the DMD structure, seven different static reflective structural colors in the CIE 1931 chromaticity diagram were realized within the all-solid-state system. Furthermore, by combining with the electrochromic ability of the WO₃ thin film, an all-solid-state ECD with a reversible conversion from red to deep red was successfully fabricated, achieving a reflection modulation amplitude of up to 50% in the visible light band. It has promising application prospects in fields such as color display, wearable electronics, and anti-counterfeiting.