Dependence of coloration efficiency of tungsten oxide films deposited by reactive sputtering on film thickness

抄録

INTRODUCTION

Tungsten trioxide (WO₃), a typical electrochromic (EC) material, has been actively studied for functional window applications because of its reversible color change [1]. We have studied the properties of relatively thicker WO₃ films (500~1500 nm) prepared by reactive magnetron sputtering to achieve superior light shielding performance [2]. In the previous study, we optimized the cyclic voltammetry (CV) conditions, such as voltage range and scan rate for films of 1000 nm thickness [3]. In this study, under the optimized CV conditions, we investigated the cyclic characteristics of WO₃ on coloration and bleaching up to 50 cycles. It was found that the coloration efficiency (CE) strongly depended on the WO₃ film thickness at the same CV condition.

EXPERIMENT

WO₃ thin film was deposited by reactive DC magnetron sputtering. ITO or quartz glass substrates were used. The deposition conditions were as follows: DC power was 50 W, Ar flow rate and pressure were 10.0 sccm and 3.0 Pa, and oxygen flow rate was 2.0 sccm. Three different thicknesses of WO₃ films (500, 1000, and 1500 nm) were prepared by adjusting the deposition time. CV measurements were performed in a tripolar cell with Pt as the reference and counter electrodes and WO₃/ITO glass as the working electrode. The electrolyte was a solution of 1 M LiClO₄ in propylene carbonate (50 mL). The voltage scan range was set to -1.2~+1.6 V, at a rate of 10 mV/s. 50 cycles were performed, with coloration and bleaching as one cycle. Transmission spectra were measured for the as-deposited state and at colored and bleached states at the initial and every 10 cycles from 10 to 50.

RESULT AND DISCUSSION

Figure 1 shows the coloration efficiency dependence on the number of cycles for each film thickness. The CEs of the samples of 1000 nm and 1500 nm thicknesses were about the same, while the 500 nm sample showed higher CE than the others. This indicates that lithium cations may be incorporated differently in the 500 nm sample than in the 1000 and 1500 nm samples. Additional measurements for 700 nm will be made to investigate the cation behavior in more detail.

REFERENCES

[1] Granqvist, Thin Solid Films 564, 1 (2014).

[2] Mian, et al., Physica Status Solidi (a) 219, 2100646 (2021).

[3] Hosaka et al., IVC22, Mon-PO1A-18.