Top Electrode-Free Electrochromic Device with Ultra-Flexibility and Wide Color Gamut Based on Porous Resonant Cavity

Recently, various types of resonant cavities are widely fused with inorganic electrochromic material WO₃ to compensate for its monotonous color modulation range. However, combining stabilized flexibility and vivid color performance in electrochromic devices is limited by the inherent brittleness and constrained transparency of commonly used top transparent electrodes, such as indium tin oxide (ITO). Herein, a novel top electrode-free structure of Cr/WO₃/Au metal-dielectric-metal (MDM) resonant cavity type flexible multicolor electrochromic device based on inner-layer porous Nylon 66 substrate is reported. By implementing a UV-cured gel electrolyte and PET thermoplastic sealing process, the electrochromic device exhibits ultra-flexibility, retaining 92.9% of its initial performance after 900 bending cycles at a radius of 4 mm, a wide color gamut spanning hues from violet to red, 40.59% maximum reflectance, and fast response times (t_colored/t_bleached = 4.5 s/4.8 s). The porous MDM cavity generates a sharper scattered multi-beam interference resonance and provides a multi-path transport channel for ions, while the bottom porous Au reflection layer serves as the conductive electrode thus replacing the top transparent electrodes in traditional sandwich-like electrochromic device structure. Moreover, a large-area multipixel electrochromic array is further proposed to display different patterns, which demonstrates potential applications in future wearable electronic labels and smart camouflage.