Influence of Dopant Concentration on the Electrochromic Properties of Tungsten Oxide Thin Films

Abstract

In order to achieve neutral coloration suitable for smart window applications WO₃ thin films modified by V₂O₅ doping were deposited using RF magnetron sputtering method with 100 W RF power at room temperature (RT). The influence of dopant concentration on the structural and electrochromic performance have been investigated. Amorphous structure was revealed by the surface characteristics facilitates electrochromic process. Uniformly distributed channels between the aggregated grains provide shorter path ways to enhance coloration efficiency. PL emission presented higher photonic efficiency for 2% of V₂O₅ doped WO₃ film. Reversible color change between transparent oxidized and deep blue colored reduced state depicted faster ion intercalation / deintercalation kinetics and faster switching responses with the bleaching time of 3.3 s and coloration time of 4.1 s. Better reversibility of 59.10 %, larger photopic contrast ratio of 2.29 and greater coloration efficiency of 66.75 cm²C⁻¹ at 630 nm unveiled by 2% of V₂O₅ doping, makes the film promising for practical applications.

Introduction

Global warming requires good upcoming energy saving mechanism for windows in modern buildings. These windows should reduce heat-loss, avoid overheating and provide comfortable day lighting. Switchable smart windows can be used to meet out these conflicting difficulties [1]. Transition metal oxides establish a stimulating group of semiconducting materials due to their technological applications in the field of display devices, switchable mirrors and energy efficient smart windows, sun roofs rear and side view mirrors [2], [3], [4]. The most widely studied oxide is tungsten oxide [5]. Its advantages are high coloration efficiency, relatively low price, non-toxic nature, the dark blue color of its absorbing state which is acceptable for most applications and huge variety of deposition methods for thin films production [6].

Amorphous WO₃ is an excellent material for electrochromic applications. When ions are intercalated, charge compensating electrons enter the localized states and modify the electronic structure of WO₃. The Fermi level is moved upwards, and the excess electrons fill the t_2g band of WO₃. This strappingly revises the optical properties from transparent to deep blue color. Investigations are carried out in doping of metal ions in amorphous tungsten oxide to enhance its electrochromic, gas sensing and electrochemical properties [7], [8].

Though tungsten oxide is the extensively studied auspicious electrochromic (EC) material for smart window applications, commercial electrochromic devices require improvement in their EC properties since the bright blue color of WO₃ films in the reduced state is not favorable as bronze or pale yellow colors for most building applications. It has been proved that better electrochromic performance can be achieved by the addition of dopants to WO₃ [9].

Vanadium pentoxide (V₂O₅) on the other hand has both electrochromic and photocatalytic properties and its proper mixture composition with WO₃ would exhibit a more neutral color than the simple WO₃ films [10]. V₂O₅ mixed WO₃ films are more neutral in color than pure WO₃ films [11]. According to the literature, pulsed laser deposited double layer of WO₃/V₂O₅ (Ti) thin films exhibit neutral brownish blue electrochromic color suitable for window applications [12]. It is also reported that color neutrality can be obtained by vanadium doping in tungsten oxide. Optical absorption in these vanadium doped films is due to polaronic transition V⁴⁺ → V⁵⁺ and W⁵⁺ → W⁶⁺ in WO₃ [13]. The interaction between WO₃ and V₂O₅ is unique because of the similarities of ionic radii and the structure in their highest oxidation state [14].

Quantitative parameters of the EC properties of WO₃ thin films such as coloration efficiency, optical density, reversibility, response time, stability and kinetics of coloration – bleaching mechanism strongly depend on the structural, morphological and compositional characteristics which in turn depend on the deposition techniques and deposition parameters [15], [16]. Composite WO₃-V₂O₅ films have been prepared by various techniques such as electron – beam evaporation [17], reactive DC sputtering [11], [13], pulsed laser deposition [9] and chemical deposition like sol–gel [3]. Among the various thin film deposition techniques, RF magnetron sputtering has the advantage of being used for deposition of uniform films on larger area substrates as well as the flexibility of changing the deposition conditions [16].

The aim of the present work is to improve the performance of WO₃ thin films in general and EC properties in particular, by doping it with V₂O₅ using RF magnetron sputtering.