Indian Journal of Pure & Applied Physics Vol. 37, April 1999, pp. 313-317

Sol-gel technology in electrochromic devices

S A Agnihotry

National Physical Laboratory, Dr K S Krishnan Road, New Delhi 110 012

R~ceived 3 February 1999

Last two decades have witnessed renewed interest in the area of electrochromic devices (ECDs) for large area applications

and also considerable advances in processing materials. Technology like sol-gel has gained a clear-cut edge over the others. The present ECD technology too seems to have taken over by the sol-gel processing. Sol-gel technology offers preparation of all components of an ECD, opening a possibility of realizing "An All Gel ECD". In sol-gel processing, precursor materials prepared by different routes have been used to deposit films either by dip or spin coating method. The present paper reviews various precursor materials and the involved sol-gel processes for preparing films of materials those can be useful for ECD fabricati on.

1 Introduction "Electrochromic devices (ECDs)" have attracted

considerable interest because of their high potential for

a variety of applications. In spite the extensive R&D efforts the ECDs still remained in the research state due

to two main reasons (I) the cost factor and (2) scaling up of the preparation of different components of ECDs.

After the identification of new technical and competitive

fields of applications where the specific advantage of

ECDs can be fully exploited the R&D efforts took a different direction. The new fields of appl ications were

those for which the response time becomes a less crucial requisite . These include Energy efficient windows or " smart windows") and automotive mirrors etc1

.

These new fields of applications demand large area

fabrication of all the components of ECDs with ease and

at b w cost. Various components of an ECD (I) the ~C

oxide elect. · dl:.':, both primary and the counter (2) the transparent conducting coatings which are used either as

the counter electrodes (3) the ion conducting electrolyte,

all can be prepared using this technology. The present

paper deals with various precursor materials, routes adopted and the advantages offered by them for coatings

in ECDs.

1.1 Electrochromic device design An ECD with its various components is a multilayer

structure backed by substrates on either side. The gen­

erally used substrates are glass plates with a transparent

and electrically conducting coating (TCC) film on them.

This is essential for applying the electrical field, which

is responsible for bringing about the desired optical changes in an ECD.

The EC oxide material is in the form of a thin film either on both or one of the TCC coated glass plates, thus forming the two electrodes of an ECD. In between the two electrodes is the electrolyte in a suitable form . In its simplest form an ECD can thus be considered to be made up of three main components such as EC oxide, electro­lyte and the transparent conducting material. ECDs for different applications can be fabricated using them in an appropriate combination.

These three components differ in their properties. The EC oxide materials those are in the form of thin porous films are mixed conductors. Their microstructure has to be conducive for easy intercalation/ deintercalation of ions. Useful EC films are often hydrous-hydrated, hy­droxylated, etc. The electrochromic efficiency and the life of the EC films are highly sensitive to the degree of hydration and hydroxylation.

Transparent conducting coatings on glass substrates are the heart of the ECDs enabling application of an electric field. The most important property for them is their electronic conductivity and the transparency in the visible region . Their ion storage capacity plays a key role in deciding the reversibility of the EC reaction when they are used combinedly as the layers for ion storage.

The third component, the electrolyte, serves to trans­port ions to and from the electrochromic film in the ECD. In order to achieve acceptable dynamics ·of an ECD the electrolyte needs to have ionic conductivity between 10-4 and 10-7 S.cm -) depending on the intended application. Its chemical and electrochemical stability

3 1 4 INDIAN J PURE & APPL PHYS, VOL 37, APRIL 1 999

within the operational windows Of temperature and volt­age to be applied to the ECD as well as its compatibil ity with its adjacent components are very important.

2 Sol-gel Technique in Preparing ECD Components

Various advantages such as cost effectiveness, en­ergy saving deposition of h igh quality films on large area, easy and precise control over the m icrostructure and the water content of the depositfJd fi lms offered by sol-gel technique3 have attracted attention in the recent years in the field of ECDs. �ol-gel process involves chemical synthesis of oxides. Sols for thin film deposi­tion can be prepared from alkoxide or non-alkoxide precursors. The molecular precursors are transformed into an oxide network via inorganic polymerization reactions. In "metal alkoxide" M(OR)x precursors po­lymerization occurs via hydrolysis and condensation. Non-alkoxide precursors used are chlorides, acetates, nitrates and carbonates. Fi lms are deposited by spin­coating or dip-coating technique.

3 Transparent Conducting Coatings (TCCs)

Transparent conducting coatings are made e ither with Tin oxide (Sn02), Tin oxide doped with Antimony (Sn02 : Sb) or Indium oxide doped with tin (ln203 : Sn) commonly known as ITO. Their high transparency in the visible region and high electronic conductivity are of importance for the best performance of ECDs based on them. Additionally, their ion storage capacity is equally important when they are util ized as the counter electrodes. Ce02 is another interesting transparent ma­terial for reversible Lt intercalation/ deintercalation.

For preparation of ITO transparent conductive films by dip coating method there is only one report4 . How­ever, there are many reports for Sn02. The first success­ful preparation ofSn02 coating by dip coating technique via the esterification reaction between ethylene glycol and citric acid and a tin citrate precursor was reported by Olivi et at. These films serving as hosts for Lt intercalation! deintercalation were highly transparent and remained so irrespective of the applied voltage in the range 0.2- 1 . 1 V.'

Preparation offilms of Sn02 or its doped derivatives, with the corresponding alkoxides as the precursors have been attempted by many groups. Takahashi and Wada6 reported about the usefulness of the glycols or ethano­lamines to control the hydrolysis rate of the metal alk­oxides yielding very suitable solutions for dip coatings. According to them, these chemicals inhibit the precipi­tation of oxides on hydrolysis of the alkoxides. Their dip

coated films using the tin alkoxide-ethanolamines-H20-isopropanol solution with constant viscosity over 700 hours, yielded highly transparent (average transmittance more than 95% for undoped and higher than 80% for Sb doped) fi lms with h igh degree of uniformity. The resis­tivity of the films, with thickness above 2000 A was 0.005 n cm.

The morphology of the fi lms and the introduction of dopants for the films obtained by sol-gel dip coating technique using an alkoxide based solution has been shown to be highly dependent on the relative humidity ratio during the pulling phase. At low relative humidity (% RH) ratio value, very homogeneous and well- crys­tall ized films are obtained. With increasing RH the films appear more inhomogeneous and 70 % RH results in amorphous fi lms. For optimum Sb content nearing 1 0 % , the resistivity attained was 3 x 1 0-3 n cm .

Preparation of Sb doped and undoped Sn02 films from two alkoxides obtained directly from chlorides have been reported by man/. The general procedure to make the precursor solution was to dissolve the chlo­rides in appropriate amounts separately in absolute etha­nol followed by retluxing and mixing them in desired proportions. Chatelon et al.7 al lowed to heat the mixture unti l all the solvent was evaporated to give a powder

. which on further dissolving in alcohol formed the sol. However, in a recent report8, the mixture of the two retluxed solutions as such was used as the sol . To get reproducible properties of the fi lms it was essential to coat them in a constant viscosity and pH regimes. By varying the amount of solute added and by adding HCI, pH was control led. The sols were then aged in a humid­ity control led atmosphere to get desired viscosity.

The fi lms were prepared by dip coating techn ique using these sols followed by annealing. Among other parameters the relative humidity ratio in the dip-coating apparatus was found to be a key factor for the morphol­ogy of the fi lms as well as for the introduction of Sb doping atoms. It has been possible to obtain doped fi lms having sheet resistance of5 nlcm2 and visible transmis­sion of80 % after appropriate annealing treatment. Sn02 transparent films with high ion storage capacity for passive counter electrodes in ECDs have been reported by Krasovec et at.

4 EIectrochromic (EC) Coatings

There have been many successful attempts to deposit various materials that show cathodic or anodic electro­chromism . These include W03, MoO), Nb20s, Ti01, NiO and Ce02. Some binary materials l ike WO,-Mo03,

,

AGNIHOTRY; ELECTROCHROMIC DEVICES 3 1 5

W03-Ti02 and others with better properties have also been reported.

4.1 Tungsten oxide (W03) W03, the most widely investigated EC material, has

been deposited using various precursor materials. One of the earliest efforts1o has been through the colloidal tungstic acid solutions, obtained by ion exchange method. It consists of passing a tungstate (sodium, po­tassium or ammonium) solution of known molarity through a column containing a proton exchange resin. As a result of the exchange of cations by protons a clear yellow coloured tungstic acid solution-sol is obtained which is used to deposit the fi lms before it gets con­verted into a gel .

The major advantage of this method is the formation ofW03 at room temperature. Since there are no decom­position products that are formed, thick fi lms without stress can be produced. The stability of the sols and the adhesion of the coatings to the substrates can be im­proved with appropriate additives and solvents. Our preliminary studies I I of the amorphous fi lms prepared by this method have exhibited good electrochromic properties.

Tungsten alkoxide based solutions have also been used' 2. '3 as precursors for W03 coating. This is a classi­cal route for making sol-gel fi lms, but tungsten alkox­ides are expensive. Most EC applications are for large areas and the cost factor is very important. Secondly, in a few attempts made, it was found that the crack free uniform coatings could only be made when the layer thickness for each dipping process was less that 500 A. Multiple coatings, increasing the cost could attain h igher thickness required in ECDs. Thus, the viabi l ity of this route for device fabrication is not good.

In another method based on chloro-alkoxides, tung­sten chloride is reacted with anhydrous alcohol 14 . The resulting material is diluted with more alcohol to yield dipVing solution. Low cost of the starting material and better uniformity of the films are the main advantages of this method. Chloro-alkoxides have also been used to make solutions so that W03 films can be deposited by spray pyrolysis.

Another route yield ing good qual ity W03 films is based on peroxometal l ic acids 1 S. 16 . In th is method, tung­sten or tungsten chloride powder is digested by cold aqueous solution of hydrogen peroxide. The product, peroxotungstic acid, is isolated and then dissolved in a polar solvent such as alcohol or water. The fi lms are then prepared by spinning or by dipping the substrates into

these solutions and firing them to decompose the acid into tungsten oxide. This is also an inexpensive route and the decomposition of the acid takes place at low temperatures ( 1 00 to 200 0C)1 7 to give coatings. The most important advantage of this route is the microstruc­ture of the films that is conducive for fast ionic transport.

4.2 Vanadium oxide The most common oxide of vanadium that has been

used for EC devices is V20s . Although, the change in the optical spectrum between the oxidized and the re­duced state by ion intercalation is not as large as W03, it has been shown to have good ion storage capacity and reversibility when used with Lt ions. The reversibil ity of V 205 in the crystalline state is superior to that in the amorphous state. When used in conjunction with W03, these coatings impart a yellow colouration to the devices in the bleached state, reducing the maximum amount of l ight transmission.

Sol-gel vanadium oxide has beer deposited onto in­dium tin oxide coated glass substrate by making a col­loid of the oxide from alkoxyvandate18. Alkoxyhalides ofvanadium have also been used for EC coatings. In this method, vanadium trifluoride oxide, vanadium oxychlo­ride or vanadium tribromide are reacted with an anhy­drous alcohol. The resulting compound is used to make the dip solution. The fluorinated precursor gave the largest optical modulation in the coating.

L ivage el al. 1 9 synthesized V20S ge l s from a VO(OAm')3 precursor. This alkoxyvandate when hyro­Iyzed with an excess of water gave a red colloidal solution within a few m inutes. This when deposited onto ITO electrodes by spin coating, formed pale yellow transparent layers. Drying them under ambient condi­tions gave rise to V20S. 1 .8 H20, xerogel about 0.5 Ilm in thickness.

The electrochemical insertion ofLt ions was revers­ible within the voltage range + 1 .and - 2 V in cyclic voltammetry. Two peaks for reduction and oxidation, sim i lar to those observed for crystall ine V 205 were shown. Lt insertion occurring at wel l- defined sites was proposed. On application of voltage of± 2V, the colour changes from pale yellow to green in 1 second, was observed. Energy consumption of 1 0 mCI cm2 for an optical density of 0.5 was reported.

Nagase el al.20 prepared V205 films by mixing V205 powder with benzyl alcohol and isobutanol. The result­ing suspension was heated at 1 1 0°C for 4 hours equipped with a condenser. Transparent organic Vanadium solu­tion was obtained after an insoluble residue was re-

316 INDIAN J PURE & APPL PHYS, VOL 37, APRIL 1999

moved offby centrifuging. A small portion of it was spin coated on ITO coated glass plate followed by drying at room temperature. The films, heat treated at 400°C, showed clear two-step electrochromism, deep blue +7

green +7 yellow, with its response time for obtaining ~OD = 0.5 being 2 sec. The yellow and green colours showed good memory effects for more than 20 hours, while the blue one was considerably degraded within 10 hr.

Precursor solutions were obtained21 by dissolving vanadium tri ( isopropoxide) YO (OCJ H7h in iso­propanol cata lyzed with acetic ac id. Air humidity hy­drolys is with stirring was allowed to take place. On further diluting this with isopropanol, a clear orange so lution was obtained after stirring for 2 hr. Films made by sp in coating method retained their amorphous nature up to 350°C, whereas, at higher temperatures slight crysta ll inity was evident from the XRD investigations. XPS studies revealed the stoichiometry to be Y205. Good e lectrochemical cycling reversibility was shown by these fi lms in 1 M LiCl04/ propylene carbonate solu­tion . Colour change in vis ible transmi ttance was shown by 2100 A thick fi lms. XPS spectra showed reduction of y 5

+ to y4+ in coloured state w ith injected Lt. Thus, these fi lms were shown to be useful for transparent counter electrodes in .£CDs.

Our sp in coated ye llow crystalline films using vana­dium isopropoxide solution with post deposition anneal­in g t reatm e nt a t 67 3 K, s howed reversible electrochromism and stabi lity in an organic LiCl0 4 elec­trolyt ic solution22

. Their observed ion storage capaci ty with very little optical changes in the wavelength region 400 nm-900 nm was best suited as passive counter electrodes in a WOJ based transmissive ECDs, useful for smart wi ndow applications.

5 Gel Electrolytes There have been ver.y few reports of making the

electrolyte layer by so l-ge l techn ique. In the firs t sol-gel ECD, a gel ion conductor based on Ti0 2-glycerol hybrid was used along with sol- gel derived Ce02ITi02 and WOJ fil ms2J . In an All Gel ECD reported by Ozer el al.24

a highly viscous gel e lectrolyte, based on Polyvinyl­butyral (PY8), a Li salt and a plasticizing solvent was deposited onto e lectrochrom ic layer by spinning. ECDs prepared with such gels having ionic conductivity near­ing to that of liquid electro lytes and the best adhesion, have very good performance characteristics.

Recently, All Sol-Gel ECDs have been reported by Orel el aP. The ionically conductive inorganic-organic

hybrid electrolyte reported by them is a novel example of organically modified electrolyte or ormolyte. These authors report stable operation of ECDs under properly chosen potential range.

7 Conclusion It is evident from the above few examples that sol-gel

technique can be used in fabricating all the components of ECDs with good electrochromic efficiency and will lead to future generation of " All Sol-Gel ECDs".

Acknowledgements Thanks are due to Prof A K Raychaudhuri for his keen

interest in this work. The financial support from Depart­ment of Science & Technology, Government ofindia is gratefully acknowledged.

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