ITO-free silver-doped DMD structures: HiPIMS transparent-conductive nano-composite coatings for electrochromic applications

Abstract

Nanocomposites of dielectric/metal/dielectric (DMD) materials that are both optically transparent and electrically conductive are of great interest for an array of modern optoelectronic applications, including electrochromic and energy storage devices. Here we report for the first time the creation of ITO-free DMD nanocomposite structures that are composed of only three layers: (Ag-doped WO3)/(Ag)/(WO3). We employed high power impulse magnetron sputtering (HiPIMS) as a new technology to deposit the internal WO3 layer and the external Ag/WO3 nanocomposite. High rates of silver ionization in the HiPIMS process and energetic arrival of silver ions on the negatively biased DMD structure enabled their penetration into the external tungsten oxide layer, forming a nanocomposite structure in a single-step process. We provided evidence that this unique structure pushes the performance of transparent-conductive DMD materials beyond their intrinsic limits. To optimize the surface plasmon resonance effect and thus the electrochromic performance of the structures, we changed the size of silver nanoclusters through vacuum post-annealing in the same sputtering chamber at varied temperatures. The optimized Ag-WO3/Ag/WO3 structure was transparent (average transmittance = 75.89 ± 0.05% in the wavelength range of 300–700 nm) and electrically conductive (sheet resistance = 23.6 ± 0.8 Ω/□). Moreover, it showed favorable electrochromic characteristics with high coloration efficiency (32.1 cm2 C−1), fast switching speed, and excellent long-term stability for at least 2000 cycles. The mechanisms of improvement in the electrochromic performance of such bifunctional structures underpinned by surface plasmon resonance, as modulated by the size and average distance of silver nanoclusters, are elucidated. The versatile new approach using HiPIMS for the fabrication of transparent-conductive nanocomposite coatings on ITO-free substrates holds great potential to manufacture next-generation optoelectronic materials such as electrochromic devices.