Effect of Substrate Temperature on the Electrochromic Properties of Cobalt Hydroxide Thin Films Prepared by Reactive Spu
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Effect of Substrate Temperature on the Electrochromic Properties of Cobalt Hydroxide Thin Films Prepared by Reactive Sputtering KyoungMoo Lee, Yoshio Abe, Midori Kawamura, and Hidenobu Itoh Department of Materials Science and Engineering, Kitami Institute of Technology, 165 Koen-cho, Kitami, 090-8507, Japan ABSTRACT Cobalt hydroxide thin films with a thickness of 100 nm were deposited onto glass, Si and indium tin oxide (ITO)-coated glass substrates by reactively sputtering a Co target in H2O gas. The substrate temperature was varied from -20 to +200°C. The EC performance of the films was investigated in 0.1 M KOH aqueous solution. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy of the samples indicated that Co3O4 films were formed at substrate temperatures above 100°C, and amorphous CoOOH films were deposited in the range from 10 to -20°C. A large change in transmittance of approximately 26% and high EC coloration efficiency of 47 cm2/C were obtained at a wavelength of 600 nm for the CoOOH thin film deposited at -20°C. The good EC performance of the CoOOH films is attributed to the low film density and amorphous structure. INTRODUCTION Electrochromic (EC) devices change their optical properties (darken and lighten) with the application of a low reversible voltage. An EC device generally consists of an ion-conducting layer sandwiched between an EC layer, an ion storage layer, and two transparent conductive layers. WO3, MoO3, and V2O5 are typical EC materials that are colored by reduction processes, and Ni oxide and Co oxide are used as ion storage and second EC layers, which are colored by oxidation. The EC materials are colored and bleached by the injection and ejection of electrons and ions [1]. Such EC devices have been developed for applications, such as displays, anti-glare automobile rearview mirrors, “smart” energy-efficient windows, and sunroofs [1-3]. The electrochromism of Co(OH)2 is similar to that described for Ni(OH)2, where the oxidized form (CoOOH) produces a dark brown coloration while the reduced form (Co(OH)2) is transparent [4-5]. Although, Co(OH)2 thin films have been prepared by wet-processing techniques, such as electrodeposition [6-7] and chemical bath deposition [8], applications of dry-processing technique, such as sputtering, are scarce. In our laboratory, Ueta et al. [9] prepared NiOOH thin films by reactive sputtering using O2 + H2O mixed gas, and obtained an
EC coloration efficiency of 29 cm2/C in 1 M KOH aqueous electrolyte solution. In the present study, we investigate the RF sputter deposition of CoOOH thin films in H2O gas, and examine the structure and electrochemical properties of the films. The EC properties of CoOOH thin films are discussed with an aim to clarify the applicability of the sputtering technique. EXPERIMENT Thin films with a thickness of 100 nm were prepared by RF magnetron sputtering. A 50 mm disk Co target (99.9% purity) was sputtered in H2O gas. The total sputtering gas pressure and sputtering power were maintained at 6.7 Pa and 50 W,
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