Electrochromic tungsten oxide films prepared by sputtering: Optimizing cycling durability by judicious choice of deposition parameters

Highlights: • Thin films of W oxide were prepared by reactive DC magnetron sputtering. • The role of critical deposition parameters on the electrochromic performance with foci on electrochemical cycling durability and optical modulation range were investigated. • A judicious choice of deposition parameters could yield films with superior cycling durability. • This work delineates a pathway towards WO₃ films with excellent durability of the electrochromic properties. -- Abstract: Thin films of W oxide were prepared by reactive DC magnetron sputtering (5 cm-diameter W target), and their electrochromic (EC) properties were investigated in an electrolyte of LiClO₄ in propylene carbonate. The purpose of the study was to elucidate the role of critical deposition parameters—oxygen/argon gas flow ratio for the sputter plasma Γ, total pressure in the sputter plasma p_tot, and sputtering power P_s—on the EC performance with foci on electrochemical cycling durability and optical modulation range ΔT. Specifically, we used 0.15 ≤ Γ ≤ 0.90, 5 ≤ p_tot ≤ 30 mTorr, and 200 ≤ P_s ≤ 400 W and studied cycling durability for up to 500 voltammetric cycles in the range 2.0–4.0 V vs. Li/Li⁺ together with optical properties at a wavelength of 528 nm. Most significantly, we discovered that a judicious choice of deposition parameters could yield films with superior cycling durability. Thus a ~300 nm-thick film prepared at Γ = 0.90, p_tot = 10 mTorr, and P_s = 200 W showed ΔT ≈ 65% after an initial "training" during ~100 voltammetric cycles; higher values of p_tot, on the other hand, yielded films whose ΔTs degraded by ~10% during the cycling, and a lower value of p_tot led to dark films with only marginal electrochromism. Hence our work delineates a pathway towards W oxide films with excellent durability of the EC properties.