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)].

2

nickel oxide thin films were also found to display electrochromic properties. Other

such materials, which also exhibit electrochromic properties include MoO3, IrO2.O3,

Rh2O3, Co2O3, V2O5, and oxides based on Ti and Nb (Deb 1973). After further

research in the 1980’s (Svensson & Granqvist 1984) a number of inorganic and

organic electrochromic materials were used to develop films with varying optical

properties for windows application. This created the concept of ‘smart windows’

(Svensson & Granqvist 1985), which could be used in order to regulate the energy

efficiency within buildings due to integral development of a dynamic adjustable

optical shutter.

Particular areas of interest for the use of these windows have not only been for energy

saving, but colour rending (Soule et al. 1995), visual quality (Moeck et al. 1998),

directional optical performance (van Nijnatten & Spee 1997) and durability

(Granqvist 2000) of these windows have also been studied.

4.1. Electrochromic Technology

A standard electrochromic device consists of five superimposed layers placed on a

substrate (Avendaño et al. 2003a) or put in between two substrates, made of glass

(Azens et al. 2002) or flexible polymer foils (Granqvist 1990). The possibility of the

use of textile material (Beaupre et al. 2006) has also been studied. The five layers of

the device used are split into the following components (Svensson & Granqvist 1984):

two conductor layers which insert charge, and active electrochromic film layer, a

central pure ion conductor (i.e. an electrolyte) that may be either organic or inorganic

and finally an ion storage media with electrochromic properties opposite to the first

electrochemical film. An illustrated example of a general electrochemical device is

shown in Figure 16. This layering arrangement gives rise to a reversible chemical

reaction which can rotate between the electrochromic material and the ion storage

medium (Svensson & Granqvist 1984). Simultaneous to this electrons or holes and

protons and ions can be injected into the device in order to change their optical

adsorption.

3

Figure 16: A basis design of an electrochromic device [diagram adapted from (Granqvist 1990)].

In the past several years a more specific design of a centrally positioned ion conductor

joining a cathodically colouring electrochromic tungsten oxide base films and an

anodically colouring nickel oxide base film has been of most interest (Bowman &

Gregg 1998; Gregg & Bowman 1997). The three layers in the centre are positioned in

between transparent conducting oxides (TCOs) such as indium tin oxide. If a voltage

of 1 volt is applied to the device in between the TCOs, the ions then move between

the ion storage media and the electrochemical film, changing the optical properties of

the film. If the voltage is reversed or short- circuited the films original optical

properties are restored (Granqvist 1990). The electrochromic devices can also be

non-scattering (Lindstrom et al. 1997; Ronnow et al. 1996). Angular selectivity is also

possible if the film deposition is done with an oblique angle of the deposition flux

4

(Bellac et al. 1995). In order to reduce the reactivity between the electrolyte and

neighbouring oxide layers additional layers can be added (Azens et al. 2001; Yoo et

al. 2006). As well as this five layer device, four layer devices utilising antimony

doped tin oxide (ATO) as a conductive layer may be possible (Coleman et al. 1999;

Smith 2004). There are conflicting reports of ATO based four layers devices. In one

study thin films of ATO were shown to exhibit only weak electrochromism (Orel et

al. 1994) although other studies on supported ATO powders show a high level of

colouration and have been suggested for use in printed displays (Coleman et al. 1999).

Other more simple arrangements such as ‘monolithic’ types may also be used

(Thangadurai & Weppner 2002; Thangadurai & Weppner 2004).

4.2. Recent Discoveries

Of all the films that have been made the most extensively used and researched are

tungsten oxide thin films, WO3 (Blackman & Parkin 2005; Deepa et al. 2006; Deepa

et al. 2005a; Deepa et al. 2005b). Other films based on WO3 with inorganic and

organic dopants have also been widely researched, these include: WO3- Fe2O3

(Azimirad et al. 2006), WO3- TiO2 (Patil et al. 2005b), WO3- Ta2O5

(Shim et al.

2006), WO3: P (Avellaneda & Bulhões 2006a), WO3- MoO3 (Gesheva et al. 2006),

multilayer of WO3- chitosan (Huguenin et al. 2005a) and WO3- TiO2- chitosan

(Huguenin et al. 2005b), nanocomposites of WO3-Au (Park 2005) and WO3- Pt (Park

et al. 2006)], and WO3 doped with tris(2, 2’- bipyridine) ruthenium(II) (Sone et al.

2006). Other metal oxide electrochromic films that have been studied are MoO3 (Sian

& Reddy 2005), IrO2 films (Patil et al. 2005a), such as IrO2- MoO3 (Patil et al. 2006),

IrO2- Ta2O5 (Backholm et al. 2006)], TiO2 (Lee 2005) films, including TiO2-V2O5

(Ivanova & Harizanova 2005). V2O5 (Seman et al. 2005) and V2O5- Ta (Avellaneda &

Bulhões 2006b) films have also been studies, as well as NiO (Bouessay et al. 2005)

based films such as NiO-TiO2 (Al-Kahlout et al. 2006), NiO-Ta2O5 (Ahn et al. 2005)],

cobalt and tantalum nickel oxide based films and polyethylene oxide doped NiO

(Zhou et al. 2006

) and Nb2O5 (Bueno et al. 2005) films. The addition of fluorines has also shown some

advantage in electrochemical films (Azens et al. 1995a). Fully transparent counter

passive electrodes can also be given by CeO2 in electrochromic devices, such as CeO2

and CeO2- TiO2 (Verma et al. 2005), CeO2- SiO2 (Berton et al. 2003), CeO2- TiO2-

5

ZrO2 (Avellaneda et al. 2005), CeO2-ZrO2 (Veszelei et al. 1998a) and CeO2- HfO2

(Veszelei et al. 1998b). V2O5 (Rauh & Cogan 1993), CeO2 (Seike & Nagai 1991) and

Ir2O3 (Demiryont 1992) have also being suggested as possibilities. Titanium dioxide

has received much attention as such counter electrodes show high storage capacities

and colouring efficiencies (Shinde et al. 2007; Wang & Hu 1999; Yonghong et al.

1997).

Electrochromism has been exhibited in electrochromic oxides which belong to two

major groups: cathodic electrochromic materials and anodic electrochromic materials

(Granqvist 1990). Those oxides like W, Mo, Ti and Nb that change colour under

insertion of electrons are cathodic electrochromic materials. Those oxides based on Ir,

Ni and inorganic non-oxide compounds with the general formula Mk[M’(CN)6]l

(where M and M’ are transition metals ions with different valencies) undergo colour

change under extraction of electrons are anodic electrochromic materials. The most

promising are electrochromic materials based on the tungsten and nickel oxides, WO3

and NiO.

4.3. NiO Thin Films

The electronic structure of a molecule is very important in understanding its

electromagnetic behaviour. However, even after extensive research of the detailed

electronic structure of nickel oxide it is still not accurately known and has led to a

lack of understanding of electrochromism in nickel oxide. Even so NiO thin films

show electrochromic properties such as a highly bleached transmittance state which

makes them potentially useful for application of ‘smart windows’. Electrochromism

in NiO thin films can be describe by the following equation:

NiOAxhxANiOx

!+++" (5)

where A- = F-, CN-, OH-/ H. Nickel-based oxide thin films have anodic

electrochromic properties (Avendaño et al. 2003b), and undergo a colour change from

transparent to brown upon charge extraction. This is an advantage when nickel oxides

are used in electrochromic devices with blue tungsten oxide, because NiO absorbs

6

mainly in the blue region of the visible spectrum whereas WO3 absorbs mainly in the

red region, hence resulting in a neutral grey colour in the dark state.

However, a problem with NiO films still occurs, it has been found that the residual

optical absorption in the 400 < λ < 500 nm wavelength range prohibits a fully

transparent state. Numerous DC and RF reactive magnetron co-sputtering experiments

have been carried out to deposit nickel-based oxides from targets of magnesium,

aluminium, and silicon, vanadium, zirconium, niobium, silver, and tantalum. It was

found that when the above dopents were added to the nickel oxides, the films showed

an enhanced short-wavelength transmittance in the bleached state (Avendaño et al.

2003a; Avendaño et al. 2003b; Avendaño et al. 2004; Azens et al. 2002). However for

admixtures of vanadium and silver there was no improvement in the optical

transmittance shown. These results are summarised in Figure 17.

In a first principle study of the quasiparticle excitation spectrum of nickel oxide (Li et

al. 2005), it was found that a feature in the band structure could explain both an

absorption edge of 3.1eV and an energy gap at 4.3eV. This was then confirmed by

further experimental studies (Powell & Spicer 1970; Scheidt et al. 1981). It has also

been investigated recently that oxygen defects induce an in-gap state, observed at

about 0.5eV below the Fermi level (EF) (Nakajima et al. 2005). On the whole as

deposited films can be characterised by low density, small grain size and oxygen

excess. The bulk is always oxygen deficient in comparison with the surface grain.

Nickel atoms in the valence states of +2 and +3 ions are always available, however as

colouration occurs this leads to the increase of Ni3+ ions. The colouration process was

found to be a surface phenomenon, with it most likely to occur in the outer grain

(Granqvist 1990).

7

Figure 17: Absorption spectra of electrochromic nickel-oxide-based films in their

bleached states, where X is Mg, Al, Si, V, Zr, Nb, Ag or Ta and NiXO indicate that X

is present in the oxide but the quantity is not specified [diagram adapted from (Avendaño et al. 2004).

4.4. WO3 Thin Films

Tungsten oxide thin films have been investigated extensively for their electrochromic

properties and it is the most widely used electrochromic material (Niklasson et al.

2004). Electrochromism in WO3 thin films can be describe by the following

equation:

33WOAxexAWOx

!++"+ (6)

8

where A+ = H+, Li+, Na+ or Ag+.

Intervalence charge transfer (IVCT) (Chatten et al. 2005) and small polaron

absorption (Schirmer et al. 1977) theories are widely used to describe chromic

properties within thin film. IVCT theory includes the formation of a W5+ from the

localisation of excess electrons on an available W6+ sites. When a photon is absorbed,

this results in the transportation of an electron in one W5+ site to a neighbouring W6+

site. Colouration which results from electrochromism occurs due to the presence of

colour centres. In this theory, the colour centres are located in several places: the

excess electron can be localised (small polaron), delocalised on a few ions (large

polaron) or infinity delocalisation (conduction electron). In the small polaron model

the excess and associated lattice distortion can be described as a polaron that moves

from one site to the next:

phononABBA EWWWWh ++!++++++

)(6

)(5

)(6

)(5" (7)

A slight variation to this model has been suggested in which the host contains W4+

and W6+ states whilst the necessary W5+ for colouration is formed by the addition of

electrons to W6+ ions (Granqvist 1993; Lee et al. 1999a; Lee et al. 1999b). The nature

of a polaron inWO3 depends on both the structure and distribution of the oxygen

vacancies in other words, the amount of defects within the structure (Niklasson et al.

2004). The atomic structure of WO3 was found to consist of corner sharing octahedra

of varying distortion. A number of distortions are possible with an increase in

temperature. As the temperature varies from -140 to +830 °C the structure of WO3

transforms from monoclinic to orthorhombic to hexagonal (Azimirad et al. 2006;

Chatten et al. 2005).

Oxygen vacancies have been considered as colour centres as this is a successful way

of adding electrons into the structure and producing W5+ ions. Experimental evidence

has shown that oxygen vacancy is important for electrochromic properties (Avendaño

et al. 2004), because WO3-y films (where y is the amount of oxygen deficiency) show

different electrochromic properties depending on the level of oxygen deficiency

(Zhang et al. 1997). If y > 0.5, then the films are metallic and conductive, if y= 0.3-

9

0.5 then the films are blue and conductive and finally if y < 0.3 then the films are

transparent and resistive.

Amorphous WO3 films have been commonly shown to have cathodic electrochromic

properties. Upon the insertion of small ions such as H+, Li+ and Na+ the optical

properties of these coatings switch from a transparent state to a blue coloured one (de

Wijs & de Groot 1999; Niklasson et al. 2004). It has also been shown that colouration

is due to simultaneous intercalation and insertion of electrons resulting in a strong

broad absorption band at about 1.2 eV. The removal of oxygen can induce large

structural relaxations and the formation of (W-W) 10+ complexes readily occurs

(Granqvist 1990). The valance band largely consists of oxygen 2p orbitals while the

conduction band occurs mainly from tungsten 5d orbitals. Therefore the optical

absorption of electrochromic materials is best described as due to the transitions

between occupied and empty localized conduction band states. In contrast crystalline

tungsten oxide changes from a transparent state to a near-infrared absorbing one.

Then upon ion/electron insertion where the electrons are in the conduction band the

state changes to a reflecting state (Goldner et al. 1983). The optical properties of

crystalline tungsten can be described in terms of the small polaron theory (Goldner et

al. 1985), however this is less successful when describing the properties of the

amorphous material, even though it has been used widely in the advancement of

‘smart window’ technology. Quantum chemical studies (Broclawik et al. 2006) have

been carried out in order to investigate the optical absorption mechanism in

amorphous tungsten. Such investigations have shown that optical absorption reduces

monotomically (sometimes becoming negative) with the estimated level of

intercalation (Denesuk & Uhlmann 1996), due to numerous factors such as side

reactions and localised delocalised electron transitions. In order to account for this,

the ‘site-saturation’ model has been proposed in which the transitions between three

kinds of states relating to W4+, W5+, and W6+, has been studied in order to gain

further knowledge into the electrochromic absorption in amorphous tungsten oxides

(Berggren & Niklasson 2006; Denesuk & Uhlmann 1996).

10

4.5. Thin Film Preparation of WO3 Films

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

11

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.

12

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

13

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).

14

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

1988), sodium tungstate (Judeinstein & Livage 1989), ammonium peroxo-

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.

15

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

16

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.

17

References for section 4. Agnihotry, S. A., Ramchandran, R. R. & Chandra, S. 1995 Pre-existence of HxWO3

in e-beam deposited WO3 films. Solar Energy Materials and Solar Cells 36, 289.

Ahn, H.-J., Shim, H.-S., Kim, Y.-S., Kim, C.-Y. & Seong, T.-Y. 2005 Synthesis and characterization of NiO-Ta2O5 nanocomposite electrode for electrochromic devices. Electrochemistry Communications 7, 567.

Al-Kahlout, A., Pawlicka, A. & Aegerter, M. 2006 Brown coloring electrochromic devices based on NiO-TiO2 layers. Solar Energy Materials and Solar Cells 90, 3583.

Arakaki, J., Reyes, R., Horn, M. & Estrada, W. 1995 Electrochromism in NiOx and WOx obtained by spray pyrolysis. Solar Energy Materials and Solar Cells 37, 33.

Ashraf, S., Blackman, C. S., Naisbitt, S. C. & Parkin, I. P. 2008 The gas-sensing properties of WO3-x thin films deposited via the atmospheric pressure chemical vapour deposition (APCVD) of WCl6 with ethanol. Measurement Science and Technology 19, 025203.

Ashraf, S., Blackman, C. S., Palgrave, R. G., Naisbitt, S. C. & Parkin, I. P. 2007 Aerosol assisted chemical vapour deposition of WO3 thin films from tungsten hexacarbonyl and their gas sensing properties. Journal of Materials Chemistry 17, 3708.

Ashrit, P. V., Bader, G. & Truong, V.-V. 1998 Electrochromic properties of nanocrystalline tungsten oxide thin films. Thin Solid Films 320, 324.

Avellaneda, C. O. & Bulhões, L. O. S. 2006a Kinetics and thermodynamic behavior of WO3 and WO3:P thin films. Solar Energy Materials and Solar Cells 90, 395.

Avellaneda, C. O. & Bulhões, L. O. S. 2006b Optical and electrochemical properties of V2O5:Ta Sol-Gel thin films. Solar Energy Materials and Solar Cells 90, 444.

Avellaneda, C. O., Bulhões, L. O. S. & Pawlicka, A. 2005 The CeO2-TiO2-ZrO2 sol-gel film: a counter-electrode for electrochromic devices. Thin Solid Films 471, 100.

Avendaño, E., Azens, A., Isidorsson, J., Karmhag, R., Niklasson, G. A. & Granqvist, C. G. 2003a Optimized nickel-oxide-based electrochromic thin films. Solid State Ionics 165, 169.

Avendaño, E., Azens, A., Niklasson, G. A. & Granqvist, C. G. 2003b Nickel-oxide-based electrochromic films with optimized optical properties. Journal of Solid State Electrochemistry 8, 37.

Avendaño, E., Azens, A., Niklasson, G. A. & Granqvist, C. G. 2004 Electrochromism in nickel oxide films containing Mg, Al, Si, V, Zr, Nb, Ag, or Ta. Solar Energy Materials and Solar Cells 84, 337.

Azens, A., Granqvist, C. G., Pentjuss, E., Gabrusenoks, J. & Barczynska, J. 1995a Electrochromism of fluorinated and electron-bombarded tungsten oxide films. Journal of Applied Physics 78, 1968.

Azens, A., Isidorsson, J., Karmhag, R. & Granqvist, C. G. 2002 Highly transparent Ni-Mg and Ni-V-Mg oxide films for electrochromic applications. Thin Solid Films 422, 1.

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