Colorful and tunable full-visible-spectrum electrochromic displays activated by flexible optical resonance

Electrochromic devices (ECDs) that realize active transmittance or reflectance modulation under reversible electrical stimuli have triggered intense research interest and consequently plenty of practical applications. However, the majority of inorganic electrochromic materials have limited color switching versatility. To solve the drawbacks, we introduce a WO₃-based ECD integrated with a thin-film resonance cavity composed of reflection (metallic tungsten, W), dielectric (WO₃), and air layers, which endows the device with outstanding color modulability across the full visible spectrum region. By adjusting the thickness of WO₃ (113 - 236 nm) film and its refractive index (n) and extinction coefficient (k) through applying electrical bias ranging from -1.5 to 0.5 V, the ECDs exhibit abundant vivid colors, including yellow, green, blue, and magenta, etc. The strategy of thin-film resonance cavity brings improved electrochromic properties in terms of fast response (3.3 s for coloration and 2.9 s for bleaching), high coloration efficiency (75.3 cm² C^-1), long chemical stability (1000 cycles retaining 81% of optical modulation), and remarkable spectrum peak migration (∼181 nm). More importantly, a polyethylene terephthalate (PET) substrate was introduced in the devices to demonstrate the suitability for flexible ECDs, which can accelerate the development of flexible colorful displays in the future.