Enhancing the electrochromic switching speed of WO3 thin films via gold coating

Abstract

In this study, we investigated the electrochromic properties of WO3 thin films after the introduction of ultrathin as-deposited Au layers and nanoparticles formed by post-annealing treatment. The Au films deposited at room temperature with a thickness of 3 nm formed dense and flat layers, and the films agglomerated into spherical nanoparticles with increasing annealing temperature. In the case of thicker Au films, the surface roughness increased and nanoparticles were formed upon annealing, and the particle diameters increased proportionally to the film thickness. Both as-deposited 3 and 5 nm thick Au films, as well as annealed Au nanoparticles exhibited pronounced surface plasmon resonance peaks. In contrast, for 10 nm thick Au films, the surface plasmon resonance peak appeared only after nanoparticle formation induced by annealing. The single-layer WO3 film exhibited a transmittance modulation of 70.2 % at 630 nm; when a 3 nm Au layer was introduced, the modulation decreased to 59.6 %. However, the bleaching and coloration times were significantly accelerated from 3.8 s and 15.0 s to 1.2 s and 10.2 s, respectively. Most notably, after 1,000th cycles, the 3 nm Au coated WO3 thin film retained 47.9 % of its initial modulation, which represents a substantial improvement over the single-layer WO3 that retained just 7.7 %. In contrast, increasing the Au layer thickness beyond 3 nm degraded the electrochromic performance of the WO3 film. Our results demonstrate that carefully controlled Au nanostructuring is a promising route for optimizing the switching dynamics of WO3-based electrochromic devices.