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 Gamma, 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 Delta T. Specifically, we used 0.15 <= Gamma <= 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 similar to 300 nm-thick film prepared at Gamma = 0.90, p(tot) = 10 mTorr, and P-s = 200 W showed Delta T approximate to 65% after an initial "training" during similar to 100 voltammetric cycles; higher values of p(tot), on the other hand, yielded films whose Delta Ts degraded by similar to 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