On the electrochromic properties of antimony–tin oxide thin films deposited by pulsed laser deposition

Abstract

Antimony–tin oxide (ATO) thin films were deposited using pulsed laser deposition. From a wide survey of deposition conditions (deposition atmosphere and substrate temperature), we deduced that the optimum experimental parameters to obtain electrochromic active films were a deposition temperature of 200 °C at a 10⁻² mbar oxygen pressure followed by an annealing at 550 °C, the latter being critical. Through potentiostatic tests, we showed that depending on the composition, which influences film morphology, the Sn–Sb–O films could present either a faradic or a capacitive-like behavior, associated to a color or a neutral switching over a wide range of potentials, respectively. In addition, a relationship between the film annealing temperature and its structure as well as its electrochromic properties was evidenced by means of complementary electrochemical, X-ray diffraction and electron microscopy techniques. Finally, the electrical and optical properties of the ATO films were discussed in terms of the energy band theory.

Introduction

In recent years, electrochromic devices have received considerable attention as they are able to reversibly change their optical properties when an electric current flows through the device [1], [2]. Due to this feature, they find applications in rearview mirrors [3], «smart windows» [2], display panels [4], etc.

The most extensively studied solid-state electrochromic systems are based on tungsten oxide [5], [6], [7], [8] working electrode thin films, namely a proton or lithium insertion material well known for its blue electrochromism. With respect to the counter electrodes, two options that consist in using either optically neutral (e.g., CeVO₄ [9]) or complementary colored electrodes such as IrO₂ [10] or hydrous Ni oxide [11] are being pursued. However, the latter, exclusively used in proton-functioning devices, present non negligible drawbacks such as a high cost for the first one and a poor stability in acidic electrolytes for the second one. Thus, alternative materials are required for proton-conducting counter electrodes in electrochromic windows.

Recently, a large number of studies have been devoted to low Sb-doping in ATO compounds for their transparent conducting properties [12]. Such materials, which can be grown in thin film forms either by spray pyrolysis [13], dip coating [14], [15] or sputtering [16], present a great interest with respect to antistatic coatings [17], gas sensors [18] and energy storage applications [19]. However, the electrochromic properties of ATO thin films containing high antimony content received very little attention with a few exceptions. Coleman et al. [20] demonstrated for the first time the feasibility of fast switching devices for display application made with ATO fine grains embedded in a fluoro-elastomer resin. In light of these promising results, Marcel et al. [21] studied the electrochromic properties of Sn–Sb–O powders and thin films grown by pulsed laser deposition. They pointed out that depending on the deposition conditions, ATO films could either exhibit a grey reversible coloration upon reduction in relation with a faradic behavior, or present a neutral switching over a wide range of potentials associated to a capacitive-like behavior. The present paper aims at providing more information about the influence of deposition conditions (especially post-annealing temperature and Sn–Sb–O composition changes) on the electrochemical, structural and optoelectronic properties of the resulting films in the hope of identifying the origin of their competing faradic/capacitive-like behavior.