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

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 ptot, and sputtering power Ps—on the EC performance with foci on electrochemical cycling durability and optical modulation range ΔT. Specifically, we used 0.15 ≤ Γ ≤ 0.90, 5 ≤ ptot ≤ 30 mTorr, and 200 ≤ Ps ≤ 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, ptot = 10 mTorr, and Ps = 200 W showed ΔT ≈ 65% after an initial "training" during ~100 voltammetric cycles; higher values of ptot, on the other hand, yielded films whose ΔTs degraded by ~10% during the cycling, and a lower value of ptot 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.