Indian Journal of Engineering & Materials SciencesVol. 1, December 1994,pp. 320-334

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Electrochromic devices: Present and forthcoming technology

S A Agnihotry & Subhas Chandra ,National Physical Laboratory, Dr K S Krishnan Road"New Delhi 110012, India

Received 20 January 1994; accepted 20 May 1994

During last two decades electrochromism in various materials has been investigated very widely.Devices based on electrochromic phenomenon have been found to be useful for a wide range of ap-plications. Electrochromic device (ECD) technology has expanded from its only application of dis-play devices in its early stage to various other applications including in automobile and building in-dustries. An ECD has undergone drastic changes in its different parts and in methods of fabricatingthem, although the basic structure has remained the same. In particular, significant advancements inthe ion conductors~the electrolytes used in ECDs have led to realization of a laminated ECD-anemerging technology of today. This review article discusses general concepts of the phenomenon ofelectrochromism, the most common electrochromic materials and basic configurational and opera- itional aspects of an ECD with special emphasis to the developmental steps of the electrolytes usedin them. The efforts towards commercial development of ECDs for various applications along with abrief analysis of some' of the opportunities and challenges faced for successful exploitation of theECD technology are also presented.

~bout mor.e than a decade back. electrochromic exhibited better durability. Attempts were alsodIsplay deVIces (ECDs) were one of the low pow- made to fabricate prototype ECDs with a varietyer technologies thought to replace: more power hun- of solid electrolytes. Improved durability was ex-gry technologies like plasma, CRT and fluorescent. hibited by a few of them but most of them facedTheir number of favourable propertiesl,2 such as some other problems.high contrast with continuous variation of trans- The seemingly unsurmountable problem of themittanl.:e, storage information without energy sup- ECD technology, especially that of longevityply, uv stability and large operatiC>n temperature which had dampened their commercialization IIrange could overcome the well known deficiencies seems to have been solved over the period of last 'it ~of liquid crystal displays (LCDs) and their pro- few years. The development of advanced design,spect for future development were thought to be high energy, rechargable batteries has led to suc-as great as those of LCDs and electroluminescent cessful preparation of a variety of polymeric elec-displays. trolytes with their characteristics suitable for

Despite the many attractive features 15 years of ECDs. Utilization of these polymeric electrolytesresearch and development by a number of acad- has helped in improving the lifetime of ECDs andemic and industrial groups have not been suffi- realization of "laminated" ECDs which is todaycient to introduce ECDs in the market place as an emerging technology.was expected. There are a variety of reasons to Another reason for the expansion of ECn tech-explain this failure, including advances in the nology in its application area... is the exploitationcompeting LCD technology. The operation of of the ability of ECDs to controllably modulatethese devices depends on ion transport, which ne- both optical and electrical properties. It modu-cessarily limits the response time to the millise- lates light not only in diffused reflectance modecond regime. Another key technological barrier to but also in specular reflectance mode and it hasa successful commercialization of ECDs was their capability to modulate luminous, thermal and so- +limited lifetime. Many innovative steps were taken lar transmittance. Due to this ECDs have generat-to address these issues and a wide variety of ECD ed considerable interest in a variety of "electro-prototypes were proposed and realised using var- optic" devices.ious EC materials as well as different assembly Some reviews on electrochromic (EC) materi-structures. The early prototype ECDs based on als, phenomenon and ECDs have appeared in theaqueous acidic electrolytes were later modified past3-9. Lately, a review on Electrochromism andwith the use of aprotic liquid electrolytes, which smart windows design 10 has also appeared. In this ,I

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-AG NJIHOTRY & CHANDRA: ElECTROCHROMIC DEVICES 321

paper, a brief review of the research on ECDs cur- This is one of the most extensively investigatedrently in "progress is presented, with special em- EC material based on which early prototypephasis to the laminated ECDs which appear of in- ECDs were prepared, using evaporated thin filmterest for the advanced technology. A brief analy- as the EC electrode. Lately, methods based onsis of some of the opportunities and challenges "Sol~gel" technique have been adopted to depositfaced, for successful exploitation of this techno- large areas19. OXides of Mo, V, Nb, and Ti belonglogy IS also presented. General concepts of the to this category.phenomenon of electrochromism along with a An interesting ex!ill1Ple of anodic ion-insertionshort overview of the most common (EC) materi- material is nickel oxide2° which is coloured in ba-als and polymeric electrolytes is also given. sic electrolytes, usually aqueous KOH solutionElectrochromism and ~Iectrochromic Materials with a reaction according to the process

E!e,ctrochromism is a multiface!ed phenomeno.n Ni(OH) +tNiOOH+H+ +e-exhibIted by a number of matenals both organIc 2and inorganic, liquids and solids. It is a reversible palegreen brownand persistent change in the optical properties of or possibly according to the processa material caused by ~ applied, ~eld or current. Ni( OH) +OH -+t NiOOH + H 0 + e-The well known matenals to exhibit EC are trans- 2 2ition metal oxides of tungsten, nickel, molybde- A reversible EC process involving Li ions innum, titanium, iridium, etc. which are ion inser- nickel oxide has also been recently observed21.

.tion materials. Such materials are of two types (i) Oxides of Ir, Rh, Co are the other candidates ofcathodically colouring-those wit1l reduced co- this category. Prussian blue (PB) Fe4 [Fe(CN)6]3' aloured state and (ii) anodically colouring-those non-oxide inorganic material is another promisingwith an oxidised coloured state. anodically colouring material studied22 and at-

These transition metal oxides are large band tempted in prototype EC devices recently. PB be-gap semiconductors and are, therefore, complete- comes blue upon oxidation, Two types of PB havely transparent in the visible region. Ion insertion been identified: 'soluble' PB-KFeFe(CN)6 and 'in-renders them colouration and their conductivity soluble' PB-Fe4[Fe(CN)6]3' both of which can beincreases by orders making them almost metallic. reversibly reduced by an injection of electrons ac-These materials are also well known to show con- companied by the intercalation of alkali metalsiderable variations in stoichiometry. Further, ions, M + , to yield transparent Everitt's salt (ES),

the~e .oxides, can. be readily, deposited int,O non- This electrochroffilc reaction for the solublestoichiometnc thin ~s whic~ show a high de- form of PB is given bygree of structural dIsorder typICal of amorphous

-' materials. MFeFe(CN)6+M+ +e- +tM2FeFe(CN)6Thin films of these materials can be prepared PB ES

by vari~us deposition tec~ques though ~uch of where M+ is a potassium ion, PB can be repea-,the earlier work was camed out by phYSICal va- tedly reduced to ES and reoxidized to PH. PB canpour deposition (PVD) techniques such as thermaJ also- be oxidized forming Prussian Yellow (PY).evaporation, sputtering, e-beam evaporation, ...+ -etc.11-13. However, the demand of large area coat- KFeFe(CN)6+-FeFe(~N)6+K +eings for certain applications like "smart win- Prussian YellowdOWS"14 and efforts towards making the technol- Another class of ion-insertion compounds withogy cost-effective have resulted in successful pre:- mixed ionic-electronic transport exhibiting ECparation of thin films of these materials by tech- property is that of polymers of the heterocyclicniques such as chemical vapour deposition and polyaniline type. The electrochemical pro-(CVD)15, plasma enhanced CVD (PECVD)16, dip cesses which induce changes in the electrical.andcoatingl?, etc. The EC properties of the films optical properties in these polymers are essential-however are extremely sensitive to the technique. ly oxidation or reduction reactions involving the

WO3 is the classical example of cathodic ion in- removal or addition of .7l electrons from the po-sertion material 18 and in accordance with the lymer chains followed by ion transport into or outabove general scheme the EC process can be ac- of the polymer matrix to balance the electroniccomplished by the reaction charge. Polypyrrole and polythiophene23 are theWO3+xM+ +xe- +tMxWQ3 well studied candidates of this category. The elec-pale yellow or blue trochemical process in polythiophene can be indi-

transparent cated as

'C"c -""'c

322 INDIAN J. ENG. MATER. SCI., DECEMBER 1994

I I(C4HsS)n+(ny)X- +:t[(C4HsSy+XX-)yk+ny(e-) 1 2 3 4- 5 + 2 1red blue -e He- +-Polyaniline24 offers another electrochemically -H+i H+ 11"

driven process associated with a colour change e H +ranging from transparent yellow to blue. i -H+ -H

Thus a good variety of colour contrast can be e e-attained with EC materiaJs. The choice of th~ ma- H+ e H+terial however is dictated by (i) electrochrolll1c ef-ficiency 1] (the ratio of the variation ~! opticaldensity (OP) to the injected charge q), (11) the re- .sponse time (time required to switch from one co-lour state to other), (iii) the operational life time Fig. I-Schematic of a three layered EC devic~ and the elec-(the number of possible cycles) and (iv) the o~ti- trochromic process .(I,I'-glasS plates, 2,2'-transparent con-cal memory. The EC efficiency of an EC device ductors, 3-EC fIlm (WO3), 4-HxWO3, 5-electrolyte)can be enhanced with a combination of two mate-rials ~ili "complem.entary" E~ pr.opertie~, e.g:, a lopments in EC devices have been closely follow-

,cathodically ~oloUrlng m.atenal m. conJun~tion ing those of the secondary batteries, particularlywith an ~odl~ally, colourlng matenal. Conslder- with respect to that of the electrolyte, e.g., acidicabl~ attention IS beIng focussed currently on such aqueous, aprotic non-aqueous and solid-polymeric ~,devices. ones at successive steps of the: development.

...Liquid electrolytes-The earliest versions of ECElectrochromlc D~vlces (E~.Devlces) .devices developed for display applications, elec-Obvious from. It~ defim!lon, electr~chrolll1sm trochromic displays (ECDs) consisted of the thin

cannot be acco~plished Wlth?Ut a pair of el~c- films of WO3 the most extensively studied ECtrodes ac~oss w~ch ~e field IS applied: I?sertion material. The thin films of WO3 were obtained byor extractio~ o! IOns Into the EC film I~ Induced thermal evaporation or sputtering. The electronicby the application of the ~eld and to ~rlng about conductors were transparent conducting coatingsthis an EC film ,has to be m.con!act ~th the el~c- generally made up of indium tin oxide (ITO)2S.trode on one .slde and an Ion ms~rtio.n ma~enal The electrolyte used was a liquid, the only liqui~on th~ other side., Thus an EC deVIce IS basically in the device. The developmental work at thisa multilayered deVIce that can be represented by stage was mainly for displa~ app~cations, and

EIIMtlllM21E2 since the displays were t,O ~e VIewed m the dl~se .-reflectance mode, the liquid electrolytes had m-

where E, M, I are electronic, mixed i?nic and corporated into them a high density of opticalelectronic and ionic conductors, respectively. MI scattering centers. The counter electrodes usedis the EC material and EI and E2 are transpa~ent were carbon or any other M+ reversible material.conductors backed by a surface which is normall.y Initially the liquid electrolyte frequently useda glass plate. The ionic conductor I ~y be a 1I- was the aqueous acidic one, from which protons w~requid, aqueous or non-aqueous or a solid .e~ectro- inserted into WO3 film26,27. Due to fastest movInglyte. Ideally, it should have zero. c?nductiVl~ ,for ions these proton insertion ECDs had good col-electrons and as high a conductiVlt;y as possible ouration and bleaching times, so as to be usefulfor ions. M2 is the counter electrode and is a m?- for display applications. Indeed, small display de-terial which is reversible to the ions involved m vices such as those used in wrist watches werethe reaction or an ion storage material. It could developed, However, use of liquid electrolytes de-be another EC material or the same as that of manded reliable sealing. Further, because of theMI, Fig. 1 iIIus~ates schematic of a. typic~ three high dissolution rate of WO3 films. in these elec-layered EC deVIce. When a voltage IS applied ac- ttolytes the devices could not WIthstand moreross the transparent conductors, ions are extract- than 104 colouration/bleaching cycles, with a +ed from or inserted into the EC mat~rial which change of reflectance by a factor of 1028. Somechanges its optical and electrical properties. attempts to replace aqueous acidic electrolyte by

The ~onfiguration of an EC device is thus that semisolids containing mixtures of acid and organ-of an electrochemical cell and operationally it is ic compounds like glycerine or glycol ,!"ere alsoequivalent to a battery with a visible state of made29. However, \he problem o~ corroSion of thecharge. It is due to the latter part that the deve- display electrode could not be avoided.

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AGNiHOTRY & CHANDRA: ELECTROCHROMIC DEVICES 323

The prototype ECDs extensively developed reflective and transmissive EC devices were fabri-next utilised a non-aqueous eiectrolyte of the kind cated.LiCIO4 + propylene carbonate (PC)3O.31. The oper- Bulk electrolytes-Preliminary solid state elec-ational temperature range and the chemical stabi- trolytes incorporated into ECDs were mostly pro-lity of the EC electrode were thereby improved ton conductors of the type H3PO4 (WO3h2.29H2Obut the response time became longer. The dura- (phosphotungstic acid)39, zrO (H2PO4).7H2O (zir-bility of such devices was more than 107 cycles32. conium phosphate)4O, Sn(HPO4).H2O4}.42,Techniques such as obliq\Je deposition32.33, were H3OUO2PO4.3H2O (hydrogen uranyl phosphate-developed to enhance the response time and HUP )43 and some of the hydrated antimony basedmake the ECDs as fast as those with acidic aque- oxides44. Since all these were in the bulk form theous electrolytes. Additional modifications in the EC devices fabricated were in the reflectingdriving mechanism introduced34, could lead ECDs mode. Although the durability of these devicesto commercial availability with prototypes of was about 106 cycles, some of them showed de-clocks, watches, information and automobile dis- gradation due to irreversible reactions at the in-.plays by the year 198535. In order to make the terface between WO3 and the electrolyte and in adevice response time independent of the total few others corrosion of WO3 electrodes wasnumber of segments being addressed, an address found to be as rapid as in aqueous acidic electro-system with transferred charge between simul- lytes possibly due to unbound water present intaneously colouring and bleaching was employ- them.~ ed36. Various lithium reversible counter electrodes Aprouc ion conductors based on Na + ions

were evaluated in terms of their ability to main- such as Na20.11Al2O3 (Na+-p-Alumina)45 andtain an approximate constant electrode potential Nal+xZr2SixP3-xO12 (Nasicon)46 both in the cer-during colour/bleach cycling of the devices. amic form have also been reported. For EC de-

EC d .. th nfi U.vices based on the former, both colouration andeVlces W1 co gura on ..

..bleaching times were temperature dependent and

ITO/w°3/LICIO4 + PC/LIy WO3/ITO to achieve a good cycle stability had to be operat-showed colouration time of about. 0.5 s to a level ed above 70°C, Whereas the ECDs based on the8 to 10 mC/cm2 leading of OD of about 0.9 or latter had limited response time due to high inter-contrast ratio of 8:1 at 633 nm in a diffused ref- face resistance between WO3 and Nasicon andlectance mode with cell voltage of -1.5 V (ref. were found to be unstable.37). Thin film electrolytes-An interesting opera-

The problem of dissolution of WO3 films de- tional characteristic was exhibited by all so~dgrading the devices could not be overcome fully state ECDs using thin films of dielectric materials

,j even in the Li based non aqueous electrolytes. such as SiO47, MgF248-51, Lip49,52, Cr2Q353,54,WO3 films also dissolved in these electrolytes Ta20555-57, etc. These EC devices iexcept thethough at much a slower rate. Another disadvan- ones with Cr203' did not show any coldtirationtage of Li ions was that th--ey failed to erase com- when operated in vacuum or when the ambientpletely after several months of cycling. Depending relative humidity was below a certain critical le-on the degree of hydration of the WO3 film an ag- vel. They showed no colouration below a "thresh-ing effect has also been reported for such old voltage", unlike the other ECDs, which corre-systems38. sponded to the dissociation potential of H2O.

Both these observations confirmed the role of ad-Solid electr~lytes-, The best solution to the sorbed water in them. The reported cycling dura-

problems of dissolutIon of WO3 films in liquid bility of these devices was less by two orders as~lectrolyte~ and the requirement of hermetic seal- compared to ECDs known earlier. In terms ofmg necessItated by these electrolytes was the use optical density and stability only a few ECDs ofof s,olid state electrolytes. Fabrication of such EC this type gave satisfactory results. ECDs based ondeVIces would be much easier too. Much of the Cr203 were observed to maintain their water and.dev~lop,mental

work was then directed towards operate even in vacuum. For a reflectance changefabncauon of '~ solid-state EC devices". Both of 50% such ECDs could run for more than 106.proton and lithium conducting materials either in ~ycles53.the. b~ form or thin film form were attempted, A thin film of the Ag+ fast ion conductorwhich m~luded phosphates and related col1l- Rb~1.5 was also employed as the solid electro-pounds ,W1th unbound water, ceramics, dielectrics. lyte along With the metallic Ag counter electrode58.Depending on the nature of the electrolyte both EC devices in reflection mode were fabricated

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J24 INDIAN J. ENG. MATER. SCI., DECEMBER 1994j

by sequentially evaporating W03, Li3N and Ni on used as electrolytes in EC devices earlier. Theseto InZ03 coated glass plates by Miyamura et al.59, new electrolytes combine the advantage of polym-Bias voltage of -I V was sufficient for colouration. er materials, e.g., plasticity, light weight, easy pro-However, reverse voltage did not erase the co- cessing, etc. with conductivity performance ap- -tlour. Thin films of LiAlF460, Li doped MgFz61, proaching that of a liquid electrolyte. Their un-Liz W046Z and LiNb0363 have also been attempt- ique property of forming thin films which cannoted. be attained by hard inorganic solid electrolytes.

Most of the solid electrolytes discussed above have bee~ exploited to bridge the gap between li-were much superior with regard to the stability quid electrolytes and hard solid inorganic electro-factor but posed problems of other kind. The lytes. The formation of favourable interfaces be-problem unique to EC devices based on dielectric tween electrode materials and solid electrolytesfiltns as electrolytes was that of depositing a frequently encounter difficulties in all solid statetransparent conductor on EC film without affect- EC devices due to change in volume of the elec-ing the stoichiometry and hence the EC activity of trode material in the devices in operation. Thethe deposited film. Additionally in these electro- elastic properties of polymers solve this problem.ly{~s water played a major role and reliability thus Thes.e electrolytes have opened up a possibility ofdepended on the sealing. Solid bulk electrolytes realization of a new concept. revolutionary designsoffered a very high interface resistance and many for EC devices. Indeed the developmental stepsothers were with very low ionic conductivitY. for EC devices based on polymeric electrolytesNone of the solid electrolytes offered ionic con- have very closely followed the advancement of iductivity sufficiently high to make the device as these materials developed for the secondary bat-fast as that based on liquid electrolytes. The ionic teries.conductivities of aqueous acidic and aprotic non- The earliest W03 based ECDs with "ion con-aqueous -liquid electrolytes were nearing 3 x 10-1 taining polymers" or "polyelectrolytes" as semisol-and 3 x 10-3 Scm-1 respectively, while the latter id electrolytes were reported by Randin64.65. Po-also offered a wider operational temperature lyelectrolytes have wide potential window of re-range for the device. In- contrast Li solid conduc- dox stability arising from hydrogen bonding be-tors LizO, LiNb03, LiAlSi04 had the ionic con- tween the OH group and the sulphonate group:ductivity in the range 10-6-10-7 Scm-l. The wa- But the presence of OH group is detrimental toter sensitive dielectrics Taz05:3.92 HzO, the electrochemical stability. The EC devicesSbz05;4HzO, HUOZPO4:4HzO however showed it were fabricated using organic polymers containingin the range 1-4 x 10 -3 Scm -I. the sulphonic acid group. Best results were ob-

Thus, there was a need for solid electrolytes, tained for poly-AMPS (poly- 2acrylamido-for improvement of stability-both chemical and 2methyl-propane sulphonic acid). The conductiv- ~electrochemical. Such electrolytes should also giV{ ity of these semisolid electrolytes was in the rangeuniform contact with the EC layer and the coun- 10-z-10-1 Scm-1 and was sensitive to water con-ter electrode with minimum interface resistance. tent. A trade off was necessary between W03 sta-They should have high ionic conductivity and bility and polymer conductivity. Prototype ECDsnegligible electronic conductivity. Further, if they based on this electrolyte showed good perform-are to be used in reflective devices, pigmentation ance characteristics.should be possible to provide an opaque diffuse The EC devices using the above electrolyte butbacKground. An electrolyte with such properties in the form of a "sheet" in combination with awould playa major role in determining the colou- carbon paper counter electrode exhibited cycleration efficiency, speed and power requirement lives greater than 107 cycles with a switching timeapart from the stability. These are the important of 0.9 s and 60% contrast level at operationalparameters deciding the viability of EC techno- voltage less than 1 Y;6. The moisture content inlogy. For the obvious reason that no single elec- these electrolytes was responsible for the breakdowntrolyte at this developmental stage could offer all at elevated temperatures and hermetic sealing wasor many of these requirements the commercial needed for continued high temperature exposure. +development of EC devices slowed down. W03, W03 + MoO3 and hexagonal crystalline

Polymeric electrolytes-The best compromise KxWQ3~7,68 based transmissive EC devices pre-between hard solid electrolytes and liquid electro- pared, incorporatin.8 It203 as the complementarylytes with limited stability was thought to be the EC electrode and poly-AMPS as an electrolyteelectrolytes based on polymers-a class of elec- showed a high degree of optical modulation.trolytes substantially different than the material~ Solvent swollen polyelectrolytes also show ex-

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AGNIHOTRY & CHANDRA: ELECfROCHROMIC DEVICES 325

cellent protonic conductivity. Water insoluble but and K + as the conductivity responsible cationswater containing perfluoro-sulphonic acid (Naf- have also been stUdied widely Na + containing

" ion) membrane reaches a conductivity of 0.01 salts such as NaI, NaSCN and K + containing saltsScm-l. However, EC devices based on Nafion65 like KI, KSCN, KCF3SO378-80, formed complexes'posed .contact problems at the solid electrode-sol- with PEa. Their conductivities depended uponid membiane interface. Also the pigmentability the stoichiometries and they do not show highwas ~ot easy. conductivities at RT. AmongstK + containing salts

.Proton conducting polymeric materials with a KCF3SO3 exhibited conductivity in the range 10-4variety of unhydrous polymers such as PEO69, Scm-1 and below, almost two orders higher thanpolyvinylpropylene (PVP)70.71, PEr2.73, Nylon 669, that attainable with KSCN. An all solid stateacrylamide69 with orthophosphoric acid and transmissiv~ EC deyice with WO3 and PB as theH2SO4 have also been' reported. Films prepared two complementary EC electrodes, incorporatingfrom the solutions in common solvents and dried PEO+ KCF3SO3 based sol~ent free electrolyte -

at ambient or moderate temperatures at a pres- showed excellent colouration efficiency (Tj) of 0.14sure of 10- 3 Torr showed conductivity in the cm2 mC-l, almost equal to the sUm of Tj's of WO3

range 10-3-10-6 Scm-1. In spite of good conduc- and PB. The transmittance could be controlledtivity exhibited by PVA+H3PO4 the response time between 14 and 56%8°. The excellent memory ef-achieved by the EC device was in excess of 1.5 s. fect of more than two weeks was attributed to an

~ All the EC devices based on these electrolytes electronically good insulating character of. theshowed degradation when wet and so mainten- electrolyte.ance of rigorously unhydrous conditions was ne- Among Li T conducting. PEa + salt complexescessary. Secondly, some of the polymers showed ECD related work is mainly reported forconductivity variations with temperatUre affecting PEa + LiCIO 4 electrolytes81- 84. The dependencethe operatiQn of the device. of conductivity on temperatUre as commonly ob-

Solvent free electrolytes have been obtained in served for all PEa based electrolytes and non-the form of films in another class of polymeric adequate conductivity at RT for the device opera-electrolytes based on the "polymer-salt-complex" tion are the main disadvantages of this electrolyte.or "salt-in-polymer" complexes. The basic princi- Recently, "smart windows" with PEa + LiCF3SO3pIe of this class of electrolytes is that a salt is dis- complex films have been reported85. Such ECsolved in the solvating polymer matrix through di- window (ECW) with CeO2-TiO2 as the ion sto-rect interactiort of the cation and electron pair rage layer exhibited good optical (40% variation).borne by a heteroatom, generally 0 or N yielding and lifetime behaviour but the response time ,was

;.. a conductive solid solution. In this class of elec- about 25 s which was attributed to decreased Li +trolytes PEO-salt-complexes have been stUdied chemical diffusion coefficient.much more extensively than any other complex, Replacing the ion-storage electrode by V2O584probably owing to their ready availability at var- resulted in an EC device with transmittance mod-ious molecular weights and their attractive me- ulation between 41 and 13% at x= 0.633 om withchanical properties as also superior complexing a time constant of 10 s for colouration and 2 sability. for bleaching at RT. The presence of solid/solid

Both proton and alkali ion conducting PEa interface in this case modified the charge transferbased electrolytes have been examined for their process whereas the kinetics were the same as.ionic

conductivity, operational temperatUre win- with a liquid electrolyte. Residual colouration indow and other characteristics from the point of the bleached state, the main drawback has beenview of their application into various devices. ascribed to the poor electronic conductivity of theComplexes formed between a variety of salts with fully oxidized V2O5 films.NH: ions as proton conducting species, such as In t.he range of temperatUre and composition ofNH4CIO474.75, NH4r6, NH4SO3CF3 and interest for different applications the PEa based

-~ NH4SCN77 have been reported. These complexes electrolytes are binary systems, not single phaseare crystalline at room temperature (RT) with the ' homogeneous materials. Ion transport in them oc-

ionic conductivity around 10-5 Scm-1. The curs primarily in amorphous domains and somovement of anions I-, CIO4 has also been their partial crysta1linity~at operational tempera-shown to contribute to the conduction process re- tUre is an unwelcome complication with regard tosuIting in lower transference number for protons conductivity. The higher glass transition tempera-(~),e.g.,0.74inNH4IandO.85inNH4CIO4.. tUre (J;) of PEa, which further gets raised with

Alkali metal iQ~-?EO based syst~ms- with Na + the addition of a salt results in better conductivity

326 INDIAN J. ENG. MATER. SCI., DECEMBER 1994

at higher temepratures. The RT conductivity is in- more important the memory. A wide variety ofsufficient for an EC device to give faster reponse. prototype ECDs were proposed and realized us-The dependence of conductivity on temperature ing various EC materials and different assembly ~affecting the response times of an EC device is structures. However, most commonly used ECmost disadvantageous. Polyethyleneimine (PEl) is material was WO3. Fig. 2 illustrates schematica close analogue of PEa and shares with it the configuration of an ECD. Operation in diffusecomplications of multiphase behaviour, however, reflection mode is made possible by incorporatingsome ECD related work based on PEl has also scattering centers into the electrolyte. The counter .been reported. electrode can simply be any material assuring.

Attempts towards increasing the operational ~lectrochemical balance.temperature window and the conductivity which Seiko Instrument Corporation, a leader in flatwas realized to be dependent upon the amor- panel display researc;h and instrumentation deve-phous domain of the complex investigations were loped ECU watches (Fig. 3) based on liquidcarried out with polymers having low glass tran- aprotic electrolyte in 1981 and about 4000sition temperature and the ones with branched watches were put on the market in 1982 but theirstructure. A very promising linear polyether that manufacture has since been suspended89.9O, Withdoes not have the problem of crystallinity consists the realization that the cost of the LCDs wasof medium chain polypropylene glycols (PPG) dropping sharply.linked by methylene groups. Possibility of opera- In 1985 I. I I ECD .alltion of EC devices at RT consisting of comple- .' re anve y .arg~ s came muse, .mentary EC electrodes WO3 and V2Os with PPG- of which were public dIsplays. ECI.> .p.anels of

PMMA-LiCIO486 and PPG-PMMA-LiSO3CF387 5 x 7 dots and 15 x 16. dots .and mulndigIt 7 seg:

have been shown recently. ment panels wer: available m the ~ket. AsahiTh ealiz n. ths t high d n' .ty . I Glass Company developed these WIth necessary

era on a con uc VI m po ym- .91er electrolytes is dependent on local thermal mo- ~pr~veme~tS; to over~ome th~ p.roblem of ~ul-tion of polymer segments led to the exploration of nplexmg .dnVIng and usmg speCl.alized ICs.. BIPO-comb polymers as hosts for polymer electrolytes lar transIstors were used as dnvers. In. FIgS 4aformation. Polysiloxane is the best example of this and 4b are ~hown an ECD module and It~ struc-type wi~h a flexible backbone and. a sho~ chain ~~e respectively, as developed by Asahi Glass Ipolar oligomer capable of complexmg alkali metal M.

hi EI . Ind d Hi hi M i

salt~. A new type of PEO-polysiloxane hybrid 9~tsus ta ectnc ustry an tac .ax- !

(PEOS) in comb-mattori- willi LiCIO4 swollen with ,,:ell also ~anufactured ECDs as p~acncal de-PC reached ionic conductivity of 10-3 Scm-'. VIces for us~g.as stock exchange dIsplays ~dTable 1 shows ionic conductivity values for differ- overcurrent mdicators around the same penod ,.ent electrolytes useful in ECDs. A transparent (~983-1985). A prototype ECD devel°l:'ed.by. Na-solid state EC device composed of PB, WO3 and tIonal .PhY.SICal Laboratory (NPL), IndIa, IS illus-PEaS show~ response time slowed down by a trated m FIg. 5.factor of 1.5-2 compared with their liquid con- Application areas of ECDs were expected totaining counterparts88. Table 2 gives performance expand gradually in future and together with theircharacteristics of some WO3 based ECDs. increased application their production cost was

expected to decrease gradually. But as pointedApplications of EC Devices and Development of out earlier speed and life of ECDs were the twoEC Technology parameters which hampered the commercializa- .

The most common and convenient use of EC tion of ECD technology. The present develop-materials is in the realization of devices for con- ment of All-Solid-State EC. devices have tremen-trol of light modulation. Extensive efforts have dously improved their life but the speed of thebeen made in fabricating a large number of elec- device still remains a parameter not allowingtro optic devices. ECDs to compete with the other display technol- -+

ogies, particularly for applications such as watchesElectrochromic display devices (ECDs) and computers demanding a very fast switching

In the past, attempts were mainly directed to fa- time. But they are ideal for display applicationsbricate ECDs which basically operate in diffuse with large area and high contrast and probablyI reflectance mQde offering many superior propert- where frequent changes in information and so fastI ies such as high contrast, no angular dependency, response times are not required such as the publicI dynamic variation of density of colouration and displays used at airports, railway platforms, etc.

~ -~"~, .,

AGNIHOTRY & CHANDRA: ElECTROCHROMIC DEVICES 327

Table I-Ionic conductivity of various electrolytes for ECDs

Electrolyte Conducting 0, Scm -1 Remarks

Liquids ions

AqueousIMHzSO.j (H+) 3xl0-1.

IMLiCIO.j (Li+) 7><-10-2IMLiOH (Li+) I.S x 10-1

IMKOH (K+) 2xl0-1

Non-aqueousIMLiCIO4inPC (Li+) 3xl0-3"SolidsBulkH3PO4(WO3h2.29H2O (H+) 1 X 10-1

H3OUO2PO4.3H2O (H+) S X 10-3

HUO2PO4.4H2O (H+) 4x 10-3

ZrO(H2PO4h.3.6H2O (H+) 2xl0-3 tH+~1

Na20.11Al2O3 (Na+) 10-2 temperature dependentNal +.zr2SixP3-xOI2 (Na+) 10-3 "

»Sn(HPO4h.H2O (H+) 10-5

RbA~15 (Ag+) 2.7xl0-1 Unstable due to moisture attackThin filmsLiAlF4 (Li+) 10-4

Li2O:LiNbO3,LiAISO4 (Li+) 10-6 to 10-7SiO H+

MgF2 H+

.Cr203 H+." Ta20S.3.92H2O H+ 1 x 10-3 water sensitive

Sb2O5"4H2O H+ 1 X 10-3

PolymericPoly-AMPS H+ 1 x 10-2 to 10-1 water sensitive

PVP H+ 10-8

PEa H+ 10-7.PPG+PMMA H+ 10-5

PEO+H3PO4 H+ 10-5 (2S'C)

PVP+H3PO4 H+ Sxl0-5PEl + H2SO4 H+ 4 X 10-5

PEl + H}PO.j H+ S x 10-6

Polymer Salt ComplexesSalt in Polymer 1: > > RT

gPEO+LiI(O/M=8) Li+ 10-4 (SS'C), 10-5 (3S'C)PEO + LiCIO4 (O/M=8) Li+ 10-4 (SO'C), 10-5 (31'C)

PEa -t NaI (O/M= 10) Na+ iO-4 (SO'C), 10-5 (4S'C)

PEO + NH4X (X=CIO4, I,CF3SO3, H+ 10-5 (RT)

SCN)PEO + KCF3SO3 K+ 10-4 (70'C), 10-5 (40'C)PEa, polysiloxane hybrid + LiCIO4 Li + 10-3

.PPQ+LiI(O/M=8). Li+ 10-4 (8S'C), 10-5 (6S'C)PVB+LiCI Li+ 3XI0-3PVB + Lil Li+ 1.6xl0-3

PMMA+LiCIO4 Li+ 10-3-10-4 Tg= -90'C«3S Wt%ofPMMA) Rubbery at RT

Polymer in SaltPPQ(10) + [SOLii + 30LiOAC+ 20LiCIO4] Li+ 1 x 10-~ Tg<RT

(90) Rubbery at RT

-"- , .

328 INDIAN J. ENG. MATER: SCI., DECEMBER 1994

Table 2-Performance characteristics of some WO., based ECDs with different electrolytes

Sr. Electrolyte Counter Performance Characteristics ReferenceNo. Electrode

I H2SO4 + water WO., 104 c/b cycles, change of reflectance by a factor 28of 10 ..

2 LiClO4 + propylene carbonate LiyWO.1 to:=0.5s 378-IOmC/cm2~OD = 0.9 at A = 633 nm

v= 1.5 V 'Cc~3 H.1PO4(W°.1)2.29H2O H. W°.1 V= I V 39:

phosphotungstic acid (PWA) When in contact with H x W°.1' PWA gets re-

duced, Rapid degradation4 Zr(H2PO4)2.1.6H2Ozirconium H..WO3 V< I V 39,

phosphate (ZP) Loss of contrast under open-circuit condition, jWO3 film dissolves (at the rate 10 nm/day)in the electrolyte

5 Sn(HPO4)2.H2O W°.1 durability> 106 41(SPA) VO2 c/b cy.:les ~

6 Nal +xZr2Si,P.1-.0r2 Nasicon Na"I'W°.1 Write/erase times are limited due to large 46interface resistance across electrolyte/counterelectrode

7 Na-p-Alumina Na. WO3 to: and ~ are temperature dependent and good 45cycle stability is achieved only above 70.C

8 Cr203 Au c/b cycles> 5 x 106, bleaching accompanied 53with a decrease in current efficiency. Reflect-ance change of 50%

9 MgF2.H2O Au 104 c/b cycles 14~T=22%

10 Ta20s:H2O Iroxide Switching time 0.1 s 102~ T= 60%> 106 c/b cycles

11 poly-AMPS Carbon 107 c/b cycles, 0.9 s, 60% contrast level, 64,65V< I V .»

12 poly (AMPS, PEO) Ir oxide 2 x 10s cycles 67

13 PEO+KCF3SO3 PB prussian blue 1J= 140cm2/C,~T=42% 80

14 (PEO)8+UClO4 NiO to:=~=IOs 83103 cycles, irreversible reaction at the interfacebetween EC layer and electrolyte

15 PEO + UCF3SO3 V2Os t" = 10 s, ~ = 2 s 84

~T= 28% at A=633 om

16 PEO + UCF3SO3 CeO2- TiO2 porous, ~ T= 40% 85-slightly crystallized t= 25 s

17 PPG+PMMA+UClO4 V2Os ~T=52%atA=633om 87

18 PEDS (PEO-poly siloxane PB Slow response 88hybrid)

19 PVA+H3PO4+KH2PO4 PB V=0.5V, 971J = 127.7 cm2/C, A = 690 om

1J=138cm2/C,A=850om ..degradation after a few cycles, associated with

the electrolyte

20 OMPE (Oxymethylene polyoxyethy-lene) PB 1.3 x 103 c/b cycles 98+ UClO4 6:1 1J= 102 cm2/C 1st cycle

1J= 67cm2/C387thcycleLack of K + ions available to the PB film

.." -

AGNIHOTRY & CHANDRA: ELECTROCHROMIC DEVICES 329

Long lived ECDs have now become a technical Attempts to manufacture ECDs for such applic-possi1?ility, Some of the recent rcports have also ations are currently in progress and practical de-raised the hopes of having multicoloured ECDs, vices may be in use in large number in the near

future,6 I Electrochromic windows (ECWs)

Another promising and exciting application ofEC materials not demanding a fast response timeis in "large area" transmissive EC devices, the so

4 called "Smart Windows", used for regulation ofincident solar energy and for the improvement ofthe energy efficiency of buildings, vehicles, air-crafts and ships, The dynamic control of incomingradiation through window offers many advantages,Smart windows can balance the need for air con-ditioning and tl:lereby lessen the burdens from at-

Fig. 2-Schematic of an ECD (i,i'-glass plates, 2-transparent mospheric pollution,conductor, 3-counter electrode, 4-EC film, 5-electrolyte with ECWs differ basically from ECDs in that in

scattering centers, 6-sealants), ECWs the entire window is in the optical path)- which requires a transparent electrolyte and a

w:

Fig, 5-A prototype ECD panel developed by National Phy-Fig. 3-ECD watch developed by Seiko Instruments Corpor- sical Laboratory (NPL), India,

ation, Japan,PC bo.rd

\ EI.'lic ~cer~' \...L- '\~.. -.°;: --~ -\- Fr.,ne

:,!" .II' .',; ,'.'., :: ;:1,~.;".':~:~ ..~ c .,)~ J;:~~;:: '

f.i;~jHrtI !.::iLH~~;; Ei :[:. 1'1.1.1(:.., iLE } .". -.:,~ ;..:/:~~:~::o, -':~"'~:'-:""[ I-""" ::.:.::".1t~F:.JF;,f 11,...1 T I 1~lr! t.I_IHR... 111111111 I---...,.." ' 0

EII.tic CO""

ECO

r:8 ~.C.r

i Fig. 4-(a) ECD module developed by Asahi Glass Co Japan

& (b) it's structure

330 INDIAN J. ENG. MATER. SCI., DECEMBER 1994

Fig. 6-Schematic of an ECW with complementary EC films(I,l'-glass plates, 2,2'-transparent conductors, 3,4-comple-

mentary EC films, 5-transparent electrolyte, 6-sealants)

Fig. 7-Prototypes of ECWs developed by Asahi Glass Com-pany, Japan. View from the Seto Bridge Museum revealing anunderwater scene through ECWs. Some panels are in the

bleached state and some in the coloured state

counter electrode which is either optically passive(i.e., colorless in both oxidized and reduced states)or electrochromic in a complementary way withrespect to the primary EC electrode, i.e., if theprimary EC electrode colours cathodically the CEshould colour anodically. A schematic of an ECWwith complementary EC films is illustrated in Fig.Q.

Presently, ECW assemblies have been investi-gated extensively. Combinations of EC materialscomplementary to each other and modifications

.of various components to increase the efficiencyof ECWs with minimum power requirements areunder rigorous studies. Energy requirements aslow as 1 kWh per year for a square meter win-dow area have been reported.

lOOr-----------,

30

90

80

70Bleached

; 60uc£.~ 50

I-E!40

20

10

300 400 500 600 700 800

Wavelength. nm

Fig. ~-Spectral response of a prototype ECW in colouredand bleached states

Active ECW programmes are being conductedat major glass manufacturers worldwide. Proto-type ECWs manufactured by Asahi Glass Com-pany in Japan 92-94 using polymeric electrolytescan modulate transmission between 80 and 10%.ECWs of areas 16 and 1200 em? when operatedat same voltage show transmission modulation of30% respectively in 5 and 120 s. Two hundredprototypes of these windows measuring 40 x 40ern? have been installed in the Seto Bridge Mu-seum, Kojima (Fig. 7). Another fifty windowshave been installed in the Daiwa House'". Theseare one of the largest installations of ECWs.

Complementary ECWs based on W03 with PBas the auxiliary electrode developed by CentralGlass Company Japan'" were used by Nissan as"sunroofs" modulating transmission from 60 to5% and exhibiting excellent memory characteris-tics. General Motors Research Laboratory has re-ported'" WO/PB complementary EC cells withpolymeric electrolytes for window applications.Degradation observed in the EC cell withPVA+ H3P04 + KI:I2P04 as the electrolyte was im-proved by using Oxymethylene polyoxyethylene(OMPE) + LiCIO 498. Optical response of a proto-type W03-polymer electrolyte-polymer ion sto-rage device is illustrated in Fig. 8.

Saint Gobin Vitrage in France" have installedW03 based ECWs on a very large scale (Fig. 9).The preferred electrolyte is H3P04 + Polyoxyethy-lene (POE) in 50:50 acetonitrile and tetrahydro-furan. Operation within three volt stability limit ofthe electrolyte could vary the luminous trans mis-

AGNIHOTRY & CHANDRA: ELECTROCHROMIC DEVICES 331

Fig. 9-ECWs installed by Saint Gobin Vitrage, France

2 4 5 6 l'

Fig. to-Schematic of a diffusion controlled ECM (I, I -glassplates, 2-EC film, 3-Reflector electrode, 4-electrolyte, 5-Res-

ervoir, o-electrode)

sion from 78 to 32%.PPG, Industries in USN°O also have W03 ba-

sed ECWs with Poly-AMPS based electrolyteand copper mesh as counter electrodes. Whenpowered at a current density of about 0.12 mNern", in two minutes transmission can be changedbetween 70 and 20%.

Electrochrornic Mirrors (ECMs)ECMs are the devices in which efficient reflec-

tion modulation is offered by an EC film over ametal reflector. ECMs can be either diffusioncontrolled or field controlled. In diffusion con-trolled ECM (Fig. 10) the optically active EC filmis positioned in front of a thin film electrochemi-cal cell and is in contact with its front electrodewhich functions also as the reflector of the device.hydrogen atoms formed at this front electrode dif-fuse through reflector electrode into the EC layerand colour it. Thus, colouration and bleaching areachieved purely chemically. A field controlledECM (Fig. 11) is a totally symmetric system aboutthe ..central reflector and thus gives same times forcolouration and bleaching, which are achievedelectrochemically. All solid state ECMs meetingthe general performance, safety and reliabilityspecifications desirable for the rear view mirrorsin automobiles can be fabricated without capital

2 5 4' 2' l'e

04j----i~-t-H+

Fig. II-Schematic of a field controlled ECM (I,I'-glassplates, 2,2'-electrodes, 3-EC film, 4,4'-electrolyte, 5-reflector)

tOOr------------------------,•••80ue2u.!!~III

a:::

Bleached+t.35V

400 500 600 700Wavelength. nm

Fig. 12-Spectral response of a field controlled all solid stateECM in coloured and bleached states

intensive and technically sophisticated vacuumdeposition techniques. Probably for this reasonECMs as rearview mirrors are the first high vo-lume applications for large area chromogenics.

Following the automatic ECMs for rearview, in-troduced in late 1980's by Ford Motor Companyand General Motors Corporation 101all-solid-stateECMs were introduced in Japan by Nissan Motoraround 1988. These complementary systems withW03 and iridium oxide/tin oxide as the EC elec-trodes had a thin film of Ta20s as the solid elec-trolyte. These ECMs could dim to 15% reflectionin the coloured state from 50% reflection in thebleached state in about 1-4 s and showed cyclelife of 107 cycles. Both, interior flat EC rearviewmirrors and outside convex rearview mirrors wereavailable 102.

A more efficient reflection modulation has beenreported on the basis of symmetric design with acentral metal (platinum or palladium) reflector,with good cycle reversibility'P''.

Schott Glass CO.104in Germany developed a se-ven layered ECM with W03 as the primary ECelectrode in combination with iridium oxidewhich were used by Nikon and Ichikoh in Japanand EIC labs in USA. Donelly':", OCU, Toyotahave all utilized the ECM technology to come outwith commercial products.

332 INDIAN J. ENG. MATER. SCI., DECEMBER 1994

GATE0 AI (100oA) SOURCE OF oH+

-1O0oA) Cr203(1600A)

-/~~~~~~~= ~

T ~~TCTIVE LAYERWO3(3000")

NiELECTRODES

SUBSTRATE OXIDIZED Si WAFER

Fig. 14-Schematic cross section of a three terminal WO..lmeffilstor device

sistance of the device can be tailored and stabi-lized over a wide dynamic range and the pro-gramming speed can be modulated by the controlof voltage. The device stability over severalmonths is achievable. Schematic cross section of athree terminal WO3 memistor device is seen inFig. 14. '*

This property of EC devices to tailor a resis-tance value within a wide range can be of signifi-cant importance for many other applications.

Optically addressable EC image recording de-vices-Construction of such devices based on anorganometallic material Poly Re(Co)3 (Vbpy) (CI)has also been reported 107. The key to operationof such devices is EC. bistability, i.e., the abilityfor two colour states to exist (and coexist) over acommon range of electrochemical potential. Thebistability effect is achieved chemically by coup-ling electron transfer to follow up steps which

Fig. I3-Commercially available ECM in three different render the transfer energetically irreversible. Gen-stages of_reflection modulation eration of EC images in yellow on green back- :x

ground which are stable for hours was shown toFig. 12 illustrates spectral response of a typical be a .p~ssibi.lity. ~though ~e.lif~ of the device

field controlled ECM in its fully coloured and was liffilted It proVIded an Indication of the levelbleached states and a commercially available of EC versatility and sophistication one may hopeECM in its bleached state and two different to achieve by utilizing metallopolymers in colourstages of reflection modulation is shown in Fig. generation scheme.13. Applications based on combination of EC tech-

nology with other technologies9-High resolutionOther applications electrophotographic devices- These involve a

Memistors for neural networks-Changes in re- combination of photoconducting and electroch-sist~ce by about four orders of magnitude fol- romic la.yers that utilize the threshold voltage for

loWIng colouration and bleaching in an E<;:: device colouranon... ihas been efficiently used to fabricate solid state Electron beam lithography-Use IS made of the I~ film memistors 106, which are potentially use- differential characteristics of the coloured and un- Iful ~ electronic neural networks for adaptive coloured EC films. t

learnIng and optimization applications. A WO3 Photochemical energy conversion and storage.

~ased n~n-volatile, ~lectrically programmable var- Opportunities and Challenges for ECDlab Ie re~lstance deVIce as analog synaptic memory Technologyconnection has. been reported. A hy~roscopic thin Electrochromic phenomenon thus provides uni-film of Cr203 IS the source of H+ Ions. The re- que opportunities for the fabrication of a multi-

AGi!'lI[JHOTRY & CHANDRA: ElECTROCHROMIC DEVICES 333

tude of technologically important devices. Feasi- 5 Lampert C M, Solar Energy Mater, 11 (1984) 1.bility to achieve a very fast response time is far 6 Agnihotry S A, Saini K K & Chandra S, Indian J Pure

~ from reality since the functionability of the device Appl Phys, 7.4 (1986) 19,34.th I t d .n tran ort 7 Lampert C M & Granqvist C G, Large area chromogen-depends upon bo e ec ron an 10 .sp. ic.l: Materials and devices for transmittance contro/, SPIE

However, feasibility of preparing EC deVices of Oct Eng Press, Bellingham, USA), 1990."large area" as demanded by majority of the ap- 8 Scrosati B, in Solid state ionics, materials and applic-plications described above has already been at~ons, edited by Chowd,ari .B V R, Chandra S, Singh S &proved. The newer techniques developed to de- Snvastava P C (World Scientific Company), 1992, 326.. EC film I ..I:"' '

fi tl 9 Deb S K, Solar Energy Mater Solar Cells, 25 (1992~pOSIt s o~er arge area ~ler Slgm can .y 327.

from the conventIonal PVD techniques used earh- 10 Granqvist C G, Solid State Ionics, 53-56 (1992)479,er. The new techniques not only allow large area 11 NagaiJ&KamimoriT,JpnJAppIPhys,23(1984)734.deposition of uniform coatings of EC materials 12 Gre~n M, Thin Solit! ~ilms, 50 (1978) 145,but they are also cost effective. With the newer 13 A~hot.ry ~ A, SaInI K K, Saxena T K & Chandra S,

, ., I EC Thm Solid Films, 14 (1986) 183.econolll1C techniques commg up to arge area 14 Svensson J S E M & Granqvist C G Solar energy Matet:

coatings today's prototype ECWs and ECDs are 12 (1985) 391. ' ,

surely to turn into commercial products put into 15 DavazzogIoU D, Leveque G & Donnadieu A, Solar En-use worldwide. For ECMs requiring modest cycle ergyMater,17(1988)379.durability ready built market is available and it is 16 Tracy C E & Benson D K, J Vac Sci Techno/, 4 (1986)

, d ' h th " 2377.~ expected ~o expan Wit e ever mcreasmg num- 17 Agrawal A, Cronin J P & Zhang R, Solar Energy Mat,

berQfvehicles. 31(1993)9.EC technology thus has high potential in large 18 DebS K, PhilosMag, 27(1973)801,

number of applications. Today EC based techno- 19 Chemseddine A, Moineau R & Livage J, Solid State Ion-logy.is available to match applications in automo- ics,9-10(1983)357.." ..,.

Th 20 Conell R S, Corrigan D A & Powell B R, Solar EnergybIle, ?uil~mg and dIsplay deVices .mdustnes. e Mater Solar Cells, 25 (1992) 301,

combmatIon of many presently available and vers- 21 Passerini S, Scrosatti B & Gorenstein A J, J Electrochematile EC materials.as also the new ones being re- Soc, 137 (1990) 3297.ported recentlylOS,109 with ionic conducting polym- 22 Itaya K, Ataka T & Toshima S, J Am Chern Soc, 104. b h b ,all t '

btdt (1982)4267enc mem ranes as su stan~ y con n u e 0 23 Leventis N & Chung Y C, J Mater Chern, 2 (1992) 289.the progress of advanced lammated structures of 24 Leventis N & Chung Y C J E-lectrochem Soc, 137EC devices. Latest methods like "dry lithiation" in (1990) 3321. 'EC materials 110 development of rubbery solid po- 25 Agnihotry S A, Saini K K, Saxena T K, NagpaI K C &

lymeric electrolytes with high ionic ambient con- Subhas Chandra,! Phys 1): Appl Phys, 18 (1985) 2087.7t ductivitylll are definite to lead to EC devices with 26 Faugnnan B W, Crandall R S & Heyman P M, RCA

h " W 'th th Rev, 36 (1975) 176,better performance c aractenstics. 1 e pos- 27 GigliaRD,SIDSympProc, 6 (1975) 52, ¥sibility of depositing EC materials on plastic sub- 28 RandinJ P,J Electron Mater, 7 (1978) 47.strates, future generation EC devices can be real- 29 Shizukuishi M, Kaga E, Shimizu I, Kokado H & Inoueized as low cost attractive products direct to a E,JpnJAppJPhys,20(1981)581,

k 30 MoritaH & WashidaH, OyoButari, 51 (1982)4819.large consumer mar et. 31 Ando E, Kawakami K, Matsuhiro K & Masuda Y, Di9

Acknowledgements pla~,J~tl985).The authors wish to thank Prof. E S Rajagopal Di- 32 Miy'!shi T & Iwasa K, SID SY'!'P Proc, 11 (1980) 126.

'. .' .33 Agnihotry S A, Bawa S S,Buadar A M, Sharma C P &rector,. Na~onal PhYSICal Labo~atory for his keen m- Subhas Chandra, Proc SPI~ 4228 (1983) 45.terest m this work. The finanCIal support from De- 34 Lorteije J H J, Sonderd Nachricht Z Bd, 28 (1975) 196.partment of Electronics, India is gratefullyacknowl- 35 Wada T, SID (Japan) Symp, Tokyo, Japan 1980.edged. 36 Kaneko N, Tabata J & Miyoshi, SID Digest, (1981) 74.

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