Tailoring WO3 thin films for smart windows: Role of Ar/O2 ratio in reactive sputtering on film quality and electrochromism

Tungsten oxide (WO₃) thin films were deposited on fluorine-doped tin oxide (FTO) and Corning glass (CG) substrates using direct current magnetron sputtering (DCMS) at room temperature. The films were synthesized under varying argon-to-oxygen (Ar:O₂) gas flow ratios of 1:1, 2:1, 3:1, 4:1, 5:1, and 6:1 to investigate the influence of sputtering atmosphere on their structural, optical, and electrochromic properties. X-ray diffraction (XRD) analysis confirmed that all films exhibited an amorphous structure, favorable for electrochromic behavior due to improved ion mobility. UV–Vis spectroscopy revealed that optical transmittance varied with gas ratio, peaking at 91 % for the film deposited at an Ar:O₂ ratio of 4:1. This suggests enhanced film uniformity and reduced light scattering or absorption at this ratio. Electrochemical analysis showed that the 4:1 ratio also yielded the best electrochromic performance. At an operating voltage window of ±0.4 V, the film demonstrated a high coloration efficiency (CE) of 42 cm²/C. Chronoamperometry measurements indicated fast switching behavior with a coloration time of 3.71 s and a bleaching time of 1.71 s. These results emphasize the critical role of gas flow ratio in tailoring the functional characteristics of WO₃ films. The optimal 4:1 Ar:O₂ ratio offers a balanced combination of high optical transparency, fast response time, and efficient charge usage, making these films highly suitable for electrochromic applications such as smart windows and low-power display technologies.