1 4. Electrochromic Coatings for Thin Films A material is electrochromic if it has the capability to maintain reversible and persistent change in optical properties when an electrical potential is applied to it (Chatten et al. 2005). It also displays a reversible change in colour which is reliant on the combined insertion and/or extraction of ions and electrons in a material in contact with an electrolyte or ion conductor (Hjelm et al. 1996). Due to this property electrochromic materials have been of great interest in a number of applications. The first area of application for these materials was in information displays (Granqvist 1990), however was fast replaced by liquid crystal based technology. In the late 1960’s and early 1970’s the first electrochromic material found was tungsten oxide thin film. Later in the mid- 1980s (Estrada et al. 1988; Estrada et al. 1991; Svensson & Granqvist 1986; Svensson & Granqvist 1987a; Svensson & Granqvist 1987b) Figure 15: Schematic diagram of a switchable window showing the bleached and fully darkened state [diagram adapted from (Svensson & Granqvist 1984)] .
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4. Electrochromic Coatings for Thin Films
A material is electrochromic if it has the capability to maintain reversible and
persistent change in optical properties when an electrical potential is applied to it
(Chatten et al. 2005). It also displays a reversible change in colour which is reliant on
the combined insertion and/or extraction of ions and electrons in a material in contact
with an electrolyte or ion conductor (Hjelm et al. 1996). Due to this property
electrochromic materials have been of great interest in a number of applications. The
first area of application for these materials was in information displays (Granqvist
1990), however was fast replaced by liquid crystal based technology. In the late
1960’s and early 1970’s the first electrochromic material found was tungsten oxide
thin film. Later in the mid- 1980s (Estrada et al. 1988; Estrada et al. 1991; Svensson
Various techniques have been used to produce tungsten oxide thin films. These
include evaporation, sputtering and electrochemical and chemical techniques. We will
now discuss each technique in turn with reference to experiment examples.
4.5.1. Evaporation
This technique is one of the most commonly used to produce tungsten oxide thin
films. It usually involves the deposition from an electrically heated Mo boat as well as
electron beam evaporation. The main composition of the vapour corresponded to
molecules of tungsten oxide with tungsten in the +4 and +6 valance states (Azens et
al. 1995b). As well as this, thermally evaporated tungsten oxide films are found to
contain water bonding chemically in the form of hydrogen tungsten bronze. This has
been confirmed by infrared spectroscopy and XRD techniques (Agnihotry et al. 1995;
Badilescu et al. 1994). Bohnke et el (Bohnke et al. 1996) showed that the
stoichiometry of the films could be written as WOx.qH2O where x = 3.00 ± 0.03 at the
substrate and x = 3.10 ± 0.03 at the surface of the film using Rutherford
backscattering spectroscopy (RBS) and elastic recoil detection. Annealing these films
at various temperatures altered this ratio dramatically, when annealing in the range of
150 - 180°C the ratio drops steeply. Kelin and Yen (Klein & Yen 1993) found that
deposition onto a substrate at 200°C, followed by annealing at 430 °C in the presence
of oxygen, gave the monoclinic WO3 film, which became infrared reflecting after
intercalation of lithium ions. It was also found that annealing at 400 °C, 500 °C and
600 °C for one hour gave a triclinic crystalline structure (Cantalini et al. 1996). Other
evaporated films that have been reported are nano crystalline films in the presence of
argon or nitrogen followed by annealing at 400 °C in air (Ashrit et al. 1998), or in the
presence of oxygen (Lin et al. 1995).
4.5.2. Sputtering
There are two main types of sputtering techniques, direct current (DC) and radio
frequency (RF) power sputtering. DC power sputtering is used in order to deposit
films from metallic i.e. conducting targets. RC power sputtering is used for deposition
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from oxidic i.e. non-conducting targets. Both these depositions are done in the
presence of a reactive gas (Granqvist 1990). Reactive DC magnetron sputtering has
been used in large scale manufacturing with deposition rates in range of 2-3 nms-1
(Göttsche et al. 1993). Witham et al (Witham et al. 1993) used the DC magnetron
sputtering technique to produce films using substrate temperatures between 70°C and
100°C. Raman spectra indicated the presence of O-WO and W=O bonds in a growth
of the mean crystallite site in films deposited onto the substrate at temperatures up to
200 °C (Kubo & Nishikitani 1998). Between temperatures of 300 - 350 °C it was
found that crystallisation of the films occurs (Georg et al. 1998; Hale et al. 1998;
Wang & Bell 1996) and the addition of dopents to the tungsten oxide such as titanium
usually stabilised the disordered (amorphous) structure to higher temperature
(Göttsche et al. 1993). The work of Nanba et al (Nanba et al. 1994) have also shown
that films prepared by RF sputtering have density close to that of the bulk and unlike
film prepared by evaporation techniques, no water was present in the films. By the
combination of vibrational spectroscopy and X-ray diffraction showed that these films
consisted of 3, 4 and 6 membered rings of corner sharing WO6 octahedra as in the
hexagonal WO3 crystals. However it was also shown that the formation of a
hexagonal structure depended on the oxygen pressure in the sputtering plasma, p0.
High p0 leads to the stabilization of a tetragonal phase consisting of four-membered
rings. High temperatures of 600 ˚C and above were shown to lead to crystalline
structures (LeGore et al. 1997). It has also been shown that crystal structures are also
affected by film thickness (Taylor & Patterson 1994). Figure 18 outlines the
sputtering technique.
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Figure 18: Schematic of Sputter Deposition for coating surfaces of glass transported as indicated by the horizontal arrows. Diagram adapted from (Granqvist 2003).
4.5.3. Electrochemical and Chemical Techniques
There are three main electrochemical and chemical techniques that have been used to
deposit tungsten oxide film, electrodeposition, anodization and chemical vapour
deposition (CVD). The Sol-Gel method has also been used but has received less
attention. Several investigations of electrodeposition from solutions of tungsten power
dissolve in aqueous H2O2 have been carried out (Jelle et al. 1998) in order to make
mixed oxide with Mo, Ni, Co, Cr, Fe, Ru and Zn (Monk & Chester 1993; Pennisi &
Simone 1995). As well as this tungsten oxide and phosphomolybdic acid (Pan & Lee
1996), polyaniline- polyvinyl alcohol on tungsten oxide films (Mitsuyuki 1994),
polyaniline and polyvinyl sulfate (Ogura et al. 1998) were grown. Electrodeposition
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tungsten oxide films also make use of Na2WO2.2H2O aqueous electrolytes. X-ray
photoelectron spectra have also shown that these films contain tungsten in valence
states such as +4, +5 and +6 (Yao et al. 1996).
The next technique to consider is anodization in electrolytes of H3PO4, H2SO4, HClO4
or methanesulfonic acid (Goossens & Macdonald 1993; Kim et al. 1995; Kim et al.
1996). In these experiments growth conditions are the focus instead of ensuring the
film properties (Granqvist 2000).
Finally chemical vapour deposition (CVD) and spray pyrolysis are two other
techniques which are used to make tungsten oxide thin films. Previous CVD methods
have used precursors such as W(CO)6, WF6, W(OC2H5)n, where n=5 or 6, and
organometallic tungsten compounds (Ashraf et al. 2008). Atmospheric pressure
chemical vapour deposition (APCVD), shown by Blackman et al (Blackman & Parkin
2005), uses the reaction between WCl6 and various oxygen containing solvents to
produce tungsten oxide. W(CO)6 was also been used as a precursor in APCVD in the
absence of oxygen. This produces tungsten metal which has been contaminated with
carbon, and is known as ‘reflective tungsten’. In the presence of oxygen however,
tungsten oxide is deposited. Another form of CVD which has been carried out is low
pressure chemical vapour deposition (LPCVD). Reaction between W(CO6) and
oxygen using this method resulted in the deposition of W18O49 films, known as ‘black
tungsten’, which is annealed in the presence of oxygen at 500-600 °C oxidized to give
monoclinic WO3. The most recent form of CVD to be used has been aerosol assisted
chemical vapour deposition (AACVD) (Ashraf et al. 2007). Reactions of W(CO)6 in
acetone, methanol, acetonitrile and a 50:50 mixture of acetone and toluene were
carried out.. These reactions resulted in the deposition of blue partially reduced WO3-x
(where x= 0.02- 0.1) films. However, films deposited using only toluene contained a
mixture of tungsten metal and W3O. All the films were annealed to yellow randomly
orientated crystalline monoclinic WO3 (Ashraf et al. 2007). AACVD involves the use
of liquid-gas aerosols to transport soluble precursors to a headed substrate. The use of
aerosols eliminates the need to use only volatile precursors which allow for a greater
range of precursors which can be used in the CVD process (R. Binions 2008).
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Spray pyrolysis (Figure 19) can be considered as a type of AACVD method as the
spray droplets are most likely to evaporate before coming into contact with the
substrate. Monoclinic WO3 has been obtained using H2WO2 in aqueous ammonia
sprayed onto the substrate at 150 °C followed by annealing at 400 °C (Arakaki et al.
1995). Crystalline structures can be formed by spray depositions at higher substrate
temperature (Kikuchi et al. 1993; Patil & Patil 1994).
Sol-Gel has also been used to produce thin films of WO3 (Orel et al. 1999). A variety
of metal alkoxides precursors have been used including tungstic acid (Xu & Chen
polytungstate (Oi et al. 1992), peroxopolytungstic acids (Kiminori et al. 1991),
tungsten hexaethoxide (Unuma et al. 1986) and tungsten oxychloride (Judeinstein &
Livage 1991; Livage 1992). Further to this, Cronin et al. (Cronin et al. 1993) have
developed a new Sol by reacting metallic tungsten with a mixture of hydrogen
peroxide and acetic or propionic acid. The resulting tungsten peroxy acid was
esetrified to produce a peroxyester derivative. Electrochromically active coatings
were formed after removing the volatile organics at temperatures as low as 100 ºC.
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Figure 19: Schematic of Spray Pyrolysis methodology for coating surfaces of glass transported as indicated by the horizontal arrows. Diagram adapted from (Granqvist 2003).
4.5.4. Comparison of Deposition Techniques
Each coating methodology has various advantages and disadvantages, though all are
capable of producing the desired material. CVD is used extensively in the glazing
industry as it can be easily incorporated onto a float glass line and can take place at
atmospheric pressure, as such it can be a very cheap way to produce added value
products. This methodology also works well at the high temperature of the float bath
(typically 660 ºC). Conventional CVD processes are not suitable for lower
temperature deposition, which limits the choice of substrate; however variants such as
atomic layer deposition (ALD) and flame assisted CVD (FACVD) show some
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promise in this area. Film growth in CVD is quick, growth rates can be relatively
high, in the order of 100 nm.min-1. However, finding suitable precursors to use at
high temperatures can be a problem and controlling film composition as the glass
cools can be difficult. CVD systems may produce significant levels of undesirable
gaseous byproducts such as hydrogen chloride (Choy 2003).
PVD methods such as sputtering have some advantages over CVD. Growth rates are
considerably slower, typically 1 nm.min-1, this allows for more efficient use of
precursors and ultra thin films are easy to produce. PVD processes may operate at
lower temperatures, making them suitable for use with a variety of substrates. There
are no issues with precursor selection as no chemical reaction takes place; the main
issue in this instance is target purity. Multilayer systems are easy to produce by
incorporating a variety of targets within the system. PVD methods are offline
processes which occur under vacuum, as such these are time consuming and may be
expensive due to the costs of vacuum systems (Ohring 1992).
Sol-Gel is an offline process; additionally the time taken to age the Sol is critical in
forming the desired product, adding an extra time period to the process. Composition
is easy to control, however it can be difficult to control film thickness over a large
substrate area. This can lead to high levels of wastage, making the process less
efficient. It can also be difficult to create multi-layer products using Sol-Gel (Ohring
1992).
At the current time PVD processes are the preferred method of producing
electrochromic thin films (Lampert 1998), due to the high level of control and relative
ease of producing a multi-layer system. CVD techniques currently find use in the
production of Low-E coatings and transparent conducting oxides. However, variants
such as AACVD, ALD and FACVD are developing continuously and may provide
some competition in the longer term.
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