characteristization of sputter deposited wo3 and ceo tio2 thin films for electrochromic application

Ž.Thin Solid Films3922001134᎐141Characterization of sputter-deposited WO and CeO᎐TiO32y x2thinfilms for electrochromic applicationsN.Janke U,A.Bieberle,R.Weißmann()Department of Materials Science Glass and Ceramics,Uni¨ersity of Erlangen᎐Nuremberg,GermanyReceived11December2000;received in revised form13March2001;accepted14March2001AbstractDC magnetron sputter-deposited WO and CeO᎐TiO thinfilms are investigated for electrochromic applications.Emphasis32y x2lies both on material characterization and electrochemical measurements.Various surface analytical techniques are used to obtain detailed information on the relation between thinfilm properties and charge density as the main electrochemical parameter.Parameters in electrochemical testing,such as the applied voltage,the amount of water in the liquid electrolyte and conditions of sample storage,are varied to clarify their influence on experimental results.A comparison of the charge density measured electrochemically and that calculated from wet chemical analysis of etched samples reveals a significant share of proton intercalation,which falsifies the measured values.Furthermore,this paper contributes to discussions of the intercalationŽ.mechanism in CeO᎐TiO with regard to material properties,particularly the binding state XPS and thinfilm density 2y x2Ž.GI-XRS.ᮊ2001Elsevier Science B.V.All rights reserved.Keywords:Electrochromism;Cerium᎐titanium oxide;Structural properties;Tungsten oxide1.IntroductionElectrochromic materials change their opticalproperties when ions are intercalated or deintercalateddue to an applied voltage,and electrons are injected ordeinjected correspondingly.As the ionic transport,andthus the change in optical properties,is reversible,electrochromic thinfilms can be used for devices withvariable transmission.Applications include dazzle-free w x w x car mirrors1,and glazings for automotive2orarchitectural applications with an integrated,variablesun-protection system,usually called‘smart windows’w x3᎐8.Fundamental information on electrochromism andw x electrochromic devices is given in9᎐11.Our paper describes the material characterizationand electrochemical measurements of electrochromic U Corresponding author.thinfilms for architectural glazings on the basis ofŽ.tungsten oxide WO.The coloring agent WO is33 separated from the non-coloring ion storage layer, composed of CeO᎐TiO,with a Li q-conducting2y x2polymeric electrolyte.A non-coloring ion storage layerhas the great advantage that it can be combined withWO,as well as with all other electrochromic materials 3which darken by reduction.The electricalfield,which is the driving force for the intercalation and deintercalation of ions,is established by applying a voltage to the transparent electrodes Ž.TCO,transparent conductive oxide,which are located beyond the electrochromic and ion storage layer,re-spectively.The set-up and device characteristics arew xdescribed in more detail in12.w xBaudry13first proposed CeO᎐TiO as a non-col-22oring,thinfilm ion-storage layer.Much work has been carried out since then to deposit thesefilms by the sol᎐gel route with appreciable intercalation character-w xistics14᎐20.The intercalation mechanism,as well as0040-6090r01r$-see front matterᮊ2001Elsevier Science B.V.All rights reserved.Ž.PII:S0040-60900100898-7()N.Janke et al.r Thin Solid Films 3922001134᎐141135the influence of material properties on electrochemical behavior,is not yet well understood.Keomany et al.´w x 18proposed that the mixed oxide layer consists of an amorphous TiO matrix,which embeds nanocrystals of 2CeO .As the diffusion coefficient of Li q is three 2orders of magnitude larger in TiO compared to CeO ,22TiO is believed to act as a non-intercalating transport 2phase.The nanocrystals of CeO are assumed to be the 2pinpoints for Li q ion storage.The model of CeO nanocrystals in an amorphous 2TiO matrix is in accordance with the results of the 2above-mentioned publications on sol ᎐gel-prepared CeO ᎐TiO but there is no evidence for an entire 22crystallization of CeO ,i.e.it must be assumed that the 2amorphous matrix consists of both TiO and CeO .22Considering diffusion in the disordered and ordered state,we conclude that intercalation takes place in the amorphous rather than in the crystalline part of CeO .2Our examination of the material properties and in-tercalation characteristics of DC magnetron-sputtered CeO ᎐TiO thin films contributes to a better under-2-x 2standing of ion conduction and storage mechanisms in CeO ᎐TiO films for electrochromic applications.2-x 2The electrochromic thin films were characterized with high-resolution SEM,grazing-incidence diffrac-Ž.tometry and reflectometry GI-XRD,GI-XRS ,X-ray Ž.photoelectron spectroscopy XPS ,and etching experi-ments combined with wet chemical analysis.Electrochemical testing of electrochromic coatings carried out by intercalation and deintercalation experi-ments in a liquid LiClO r propylene carbonate electro-4lyte requires careful attention to experimental parame-ters.The water content depends on the purity of the chemicals,as well as on adsorption of humidity from the atmosphere,and strongly influences the intercala-tion characteristics.Furthermore,electrochemical properties depend on constant voltage and storage conditions of the coated glass.In order to obtain elec-trochemical characteristics that allow the comparison of different electrochromic materials,it is necessary to define a limit for the water content and to control experimental parameters and sample storage condi-tions.2.Experimental 2.1.Film preparationThe electrochromic films were prepared on a produc-tion line by high-rate DC magnetron sputter deposition Ž.from metallic targets.Both indium ᎐tin oxide ITO and Ž.fluorine-doped tin oxide FTO on soda lime glass with a sheet resistance of R s 20⍀I y 1were used as I substrates.The film thickness of the WO and 3CeO ᎐TiO samples investigated is given in Table 1.2y x 2Table 1Film thickness of the samples investigated Ž.Sample Substrate EC coating d nm A ITO WO 2203B ITO WO 2603C ITO CeO ᎐TiO 2402y x 2D FTO CeO ᎐TiO 2302y x 2DFTOCeO ᎐TiO 1852y x 22.2.Analytical methodsIntercalation and deintercalation characteristics and cyclic voltammetry were performed in a liquid electro-lyte,consisting of 1M water-free LiClO dissolved in 4Ž.propylene carbonate PC,99%purity,Merck .The experimental set-up consisted of a three-electrode con-w x figuration,which is described elsewhere 10.The size of the immersed part of the samples,which acts as working electrode,was 70cm 2.The counter electrode was a carbon plate with a hole for transmitting light.It was fixed parallel to the sample in a Teflon holder.A Žcommercially available glass electrode InLab 301,.Mettler-Toledo was used as reference electrode.The potential between the working and counter electrodes was kept constant in the switching experiments,and was swept in cyclic voltammetry using a potentiostat Ž.Potentioscan Wenking POS 73.The current was mea-sured as a function of time while the voltage was switched between "1V every 4min.The charge Žy 2.density Q mC cm was determined by integration of each switching cycle.In situ transmission measurements were carried out with visible light,which was filtered through a colored glass slide with a sharp maximum in transmission at a wavelength of 520nm.Before passing the sample,the beam was split into two beams of 50%intensity each to generate a reference beam.The intensity of both beams was measured by photocells and was used to calculate Ž.the change in transmission ⌬T %during cycling.In cyclic voltammetry measurements,the voltage was scanned at 10mV s y 1,i.e.one cycle lasts for 400s.The electron binding states of W,Ce and Ti were Ž.examined by X-ray photoelectron spectroscopy XPS using a PHI ESCA System 5300.Monochromatic AlK ␣Ž.radiation 1486.6eV was used for stimulation,as un-filtered MgK ␣caused X-ray-induced damage of the binding state.Structural characterization of the electrochromic coatings was performed by grazing-incidence diffrac-Ž.tommetry GI-XRD with a SIEMENS D 500.Film density and roughness were determined by grazing-inci-Ž.dence X-ray reflectivity spectroscopy GI-XRS using a SIEMENS D 5000reflectometer.Ž.Ž.The molar ratio n Ce r n Ti of the CeO ᎐TiO 2y x 2ion-storage layer was determined by wet chemical anal-()N.Janke et al.r Thin Solid Films 3922001134᎐141136Ž.Fig.1.Structure of CeO ᎐TiO SEM,magnification =20000.2y x 2ysis.The coatings were dissolved in strong acid and the solutions were analyzed by inductively coupled plasma Ž.ICP .The etchant for WO and CeO ᎐TiO thin 32y x 2films was 10wt.%NH OH in water and concentrated 4Ž.hydrochloric acid HCl ,respectively.Wet chemical analysis was also used to determine the amount of Li q in intercalated and deintercalated samples.The concentration of Li q per area is con-verted into charge density and compared with the re-sults of the electrochemical measurements.3.Material characterizationHigh-resolution SEM pictures of the sample surfaces Ž.show a characteristic structure Fig.1,as already de-w x scribed in other publications 19,21,22.Both CeO ᎐TiO and WO coatings reveal similar struc-2y x 23tures,which were proven to be a replication of the surface of the TCO material.The granules have a diameter of approximately 150nm.X-Ray diffraction measurements did not reveal the presence of any crystalline phases in the samples.The ŽWO and CeO ᎐TiO coatings are amorphous Fig.32y x 2.2.The oxidation state of as-deposited and deinterca-6q Ž.lated tungsten oxide is W according to XPS Fig.3.Intercalation of Li q reduces W 6q to W 5q .W 4q ,which is believed to be responsible for irreversible intercala-w x tion 23,could not be identified.Ž.The ion-storage layer Fig.4consists of substoichio-metric CeO with mixed oxidation states of Ce 3q and 2y x Ce 4q .Titanium is totally oxidized to Ti 4q during the deposition process,due to the high oxidation potential of cerium oxide.Intercalation leads to the reduction of cerium oxide.TiO remains unchanged,although it is 2known to be electrochromic in certain chemical and w x structural states 24.The intercalation reaction can be written in a simplified formas:Ž.Fig.2.Diffractograms of WO and CeO ᎐TiO GI-XRD .32y x 2y Li q q y e y q CeO -TiO m Li CeO -TiO 2y x 2y 2y x 2Table 2summarizes the density of samples A ᎐E,derived from GI-XRS measurements.The density of the WO coatings is approximately 80%of the density 3w Ž.y 3of crystalline tungsten oxide ␳WO s 7.16g cm 3w xx 25.The values for CeO ᎐TiO vary between 3.32y x 2and 5.7g cm y 3.Ž.Ž.The molar ratios n Ce r n Ti of the samples are 2.3Ž.Ž.Ž.C ,1.9D and 1.7E ,according to wet chemical analysis.The corresponding compositions are 31%Ž.TiO ᎐69%CeO ᎐TiO C ,34%TiO ᎐66%22y x 22Ž.CeO ᎐TiO D and 37%TiO ᎐63%CeO ᎐TiO 2y x 222y x 2Ž.E .The samples are composed of approximately one-third TiO and two-thirds substoichiometric CeO .224.Electrochemical measurements4.1.Intercalation and deintercalation characteristics 4.1.1.Influence of the ¨oltage appliedThe voltage applied between the sample andtheŽ.Fig.3.Binding state of W in WO XPS .3()N.Janke et al.r Thin Solid Films 3922001134᎐141137Fig.4.Binding state of Ce and Ti in CeO ᎐TiO ,respectively 2y x 2Ž.XPS .reference electrode was varied between 0.6and 1.15V.Voltages higher than 1.15V are not recommended,as the liquid electrolyte contains a certain amount of water,which decomposes at a voltage of approximately 1.2V.Gas may develop and isolate the electrochemi-cally active layer from the liquid electrolyte.Fig.5illustrates an almost linear increase in charge density with increasing voltage.The driving force for intercalation in the case of elevated voltage results in higher charge density.The decrease in charge density within the first 55cycles is due to irreversible interca-lated Li ions.Wet chemical analyses confirmed that the amount of intercalated Li ions is much larger at 1.15V than at lower voltages.The irreversible intercalation may be due to the formation of W 4q ,which cannot be w x reoxidized easily 23.Hence,Li ions have to stay in the material in order to assure charge neutrality.The dif-fusion paths become blocked.Detaching of the WO layers during the first 303cycles was not observed by wet chemical analysis.The concentration of tungsten was found to be constant in the etchants.4.1.2.Influence of the amount of water in the electrolyte It is well known that the electrolyte,propylene car-w x bonate with LiClO ,is a very hydroscopic liquid 26.In 4order to characterize the impact of absorbed water onthe electrochemical behavior of the electrochromic andTable 2Ž.Density of WO and CeO ᎐TiO GI-XRS 32y x 2y 3Ž.Sample ␳g cm A 5.80B 5.65C 5.70D 4.40E3.30Fig.5.Influence of the voltage applied on the charge density of WO .3ion-storage samples,the charge density was determined using electrolytes with different amounts of water.The amount of absorbed water was measured by Karl-Fisher titration,as well as UV-VIS-NIR spectro-scopy.Less than 0.5%of water was detected directly after preparation of the liquid electrolyte.Within the first 2weeks,the electrolyte absorbed approximately Ž4᎐5%water the electrolyte stayed in air and was .stirred each day before the measurement .An additio-nal amount of 5%water was deliberately added on the 15th day.Figs.6and 7illustrate the influence of increasing water content on the charge density of WO and 3CeO ᎐TiO ,respectively.The charge density de-2y x 2creases with increasing water content,whereas the curvature of the graphs does not change significantly with increasing water content within the first 13days.The charge density decreases linearly with increasing water content.As soon as an extra amount of water is added,the charge density of WO decreases straight to 3Ž.zero Fig.6.Wet chemical analyses indicated no de-tachment of the electrochromic material;thus,de-crease may be due to the formation of crystalline hydrates,which hinder intercalation and deintercala-w x tion processes due to blocking effects 26᎐28.The degradation of CeO ᎐TiO is similar to that 2y x 2of WO within the first 13days.However,the decrease 3in the charge density at high water content,i.e.after 15days storage and an addition of 5%water,is less Ž.pronounced in the case of CeO ᎐TiO Fig.7com-2y x 2Ž.pared to WO Fig.6.Consequently,the CeO ᎐TiO 32y x 2ion-storage layer has a higher stability in water com-pared to the WO electrochromic layer.34.1.3.Influence of sample storage conditionsThe surrounding atmosphere attacks the elec-trochromic and the ion-storage layers.In the literature,()N.Janke et al.r Thin Solid Films 3922001134᎐141138Fig.6.Influence of the amount of water in the electrolyte on the charge density of WO .3two steps for the degradation of WO are distinguished 3w x 9,27:1.Water is adsorbed on the surface and in the poresof the sample directly after the samples leave the coating chamber and get in contact with air.Equi-librium is reached after approximately 24h.2.After storage in air for a longer time,water ischemisorbed in the layer.The degradation in-creases continually with time and is enhanced at elevated temperature and humidity.At a tempera-ture higher than 25ЊC and a humidity of more than 50%,a dense amorphous layer develops.The degradation within the first 24h was not ex-amined closely,as the physically adsorbed water was already in equilibrium at the time of the experiments.The influence of different environmental parameters on the degradation characteristics of WO is illustrated 3Ž.in Fig.8.High humidity 90%decreases the charge density of WO up to 10mC cm y 2.3Fig.7.Influence of the amount of water in the electrolyte on the charge density of CeO ᎐TiO .2y x2Fig.8.Influence of sample storage on the charge density of WO .3Conditions of climatic board are 90%humidity,30ЊC.The curvatures of the graphs are not identical under different storage conditions.The charge density of the samples stored in the desiccator decreases continually within the first five cycles,whereas the samples stored in wet and warm conditions run through a local maxi-mum.This may be due to a change in the number of protons in the coatings.At higher humidity,the layers adsorb additional protons.The amount of protons,and thus the process of intercalation,changes.After some cycles,the system reaches equilibrium and the charge density decreases in the same way as for the samples stored in the desiccator.In addition,CeO ᎐TiO samples degrade in humid 2y x 2atmosphere.The charge density decreases almost by the same order of magnitude.4.1.4.Intercalation of protonsThe charge density was determined by electrochemi-cal measurements in the liquid electrolyte,as well as by wet chemical analysis.Electrochemical measurements consider the total amount of positive charge inter-and deintercalated into the electrochromic and ion-storage layers,respectively.The positive charge consists of Li ions from LiClO ,as well as H q ions from the ab-4sorbed water.The charge density interpreted as the number of intercalated Li q ions is therefore false,due to an unknown amount of protons intercalated in the same cycles.Wet chemical analysis,which identifies only the amount of Li q ions,is therefore necessary in order to increase the value and reliability of the elec-trochemical measurements.The amount of Li ions determined by wet chemical analysis can be converted into charge density.The irreversible charge density signifies the amount of Li q that is not deintercalated.Figs.9and 10illustrate the charge density of WO 3and CeO ᎐TiO -coated glass,respectively.The fol-2y x 2lowing abbreviations are used:()N.Janke et al.r Thin Solid Films 3922001134᎐141139Fig.9.Charge density of WO for inter-and deintercalation due to 3electrochemical measurements and wet chemical analysis.Ž.Q :charge density intercalation ;measurement in rev the liquid electrolyteŽ.Q :charge density deintercalation ;wet chemical irr analysisŽ.Q :charge density intercalation ;wet chemical rev q irr analysisFor both materials,it was found that the charge density determined by wet chemical analysis Q is rev q irr lower than that calculated from electrochemical mea-surements combined with wet chemical analysis after Ž.deintercalation Q q Q .This is due to the fact that rev irr the charge density Q q Q includes the intercala-rev irr q q Žtion of Li as well as H ions electrochemical mea-.q surement ,whereas only Li intercalation is con-Ž.sidered in case of wet chemical analysis Q .The rev q irr difference between Q q Q and Q is therefore rev irr rev q irr mainly due to H ion intercalation,which is not con-sidered with the measurement in the liquid electrolyte.Ž.In case of CeO -TiO Fig.10,the reversible part 2y x 2of the charge density determined by electrochemical Ž.measurement Q is even higher than the reversible rev and irreversible charge density determined by wet Ž.chemical analysis Q .Equal amounts of Li andrev qirr Fig.10.Charge density of CeO ᎐TiO for inter-and deintercala-2y x 2tion due to electrochemical measurements and wet chemicalanalysis.Ž.Fig.11.Charge density of WO sample A,B and CeO ᎐TiO 32y x 2Ž.samples C,D,E .H ions are intercalated in CeO -TiO .It was found 2y x 2that the amount of intercalated protons in CeO -TiO 2y x 2and in WO is almost the same.However,the interca-3lation of Li is much easier in the electrochromic WO 3layer.Thus,it can be concluded that the microstruc-tures of the two materials are different:CeO -TiO 2y x 2seems to have a structure consisting of very small micro-channels.Small protons can easily intercalate into this structure,whereas the intercalation of larger Li ions is hindered.The number of intercalated Li ions is therefore much lower in CeO -TiO compared to 2y x 2WO .34.1.5.Charge density and change in transmissionThe charge density of both WO samples is nearly 3identically in the first 30cycles,as can be seen from Fig.11.The change in transmission is higher in sample Ž.A Fig.12.Charge density in the 15th intercalation cycle Q 15and the change in transmission ⌬T can be seen in 15Table3.Ž.Fig.12.Change of transmission samples A,B .()N.Janke et al.r Thin Solid Films 3922001134᎐141140Table 3Charge density and change of transmission Sample Q ⌬T 1515y 2Ž.Ž.mC cm %A 35.764.2B 34.841.3C 2.6᎐D 16.4᎐E21.9᎐Both WO samples A and B have charge density of 3approximately 35mC cm y 2,whereas the change in transmission is higher in sample A.There is no signifi-cant change in optical properties detectable in CeO -2y x TiO .2The charge density,however,exhibits a broad dis-Ž.tribution in the case of CeO -TiO Fig.11,Table 3.2y x 2Sample C shows a rather weak tendency for intercala-tion,whereas samples D and especially E reveal charge density which is high enough to be used as ion-storage material.There is a close connection between interca-lated charge and density,as can be concluded from comparison with Table 2.The relationship between density and electrochemical properties is already well w x known from tungsten oxide 9.4.2.Cyclic ¨oltammetryŽ.Ž.Both WO Fig.13and CeO -TiO Fig.14thin 32y x 2films show the characteristic smooth voltammograms w x for the amorphous state 19,26,29.Phase transitions during intercalation and deintercalation cannot be de-tected.During cycling of WO ,the time for deintercalation 3decreases.In contrast,the CeO -TiO ion-storage 2y x 2layer intercalates and deintercalates more slowly in the first 10cycles.The diffusion coefficient of Li q in the elec-trochromic coatings can be derived from the w x Randles ᎐Sevcik equation 10:Fig.13.Cyclic voltammetry of WO .3Fig.14.Cyclic voltammetry of CeO ᎐TiO .2y x 21r 2nF1r 21r 2I s 0.4463nFCD ␯p ž/RTwhere I is the peak current density in A cm y 2,n is p the number of electrons involved in the redox reaction,F is the Faraday constant,R is the gas constant,T is the temperature,C is the concentration of Li q in the liquid electrolyte in mol cm y 3,␯is the potential sweep rate in V s y 1,and D is the diffusion coefficient in cm 2s y 1.The diffusion coefficients for Li q are:D s 6.1=10y 11cm 2s y 1in WO ;andLi 3D s 1.1=10y 11cm 2s y 1in CeO ᎐TiO .Li 2y x 2This result is in accordance with the greater charge density of tungsten oxide.The diffusion coefficient for w x WO fits well to values published in 9,where D f 3Li 10y 11cm 2s y 1is given for sputtered tungsten oxide coatings.5.ConclusionsWO and CeO ᎐TiO thin films were deposited by 32y x 2DC magnetron sputtering on TCO-coated flat glass.In electrochemical measurements in a LiClO r propylene 4carbonate electrolyte,the samples achieve charge den-sity up to 36and 22mC cm y 2for WO and 3CeO ᎐TiO ,respectively.The diffusion coefficient 2y x 2of Li q in both materials derived from cyclovoltamme-try is of the order of D f 10y 11cm 2s y 1.Li To summarize our results concerning experimental details,careful attention needs to be paid to the condi-tions for sample storage,as well as to the water content of the hydroscopic electrolyte.Humid atmosphere en-hances degradation of electrochromic coatings during storage,due to saturation with water.Higher water content forces degradation,even in the first 30cycles.It is therefore critical to compare measurements when the water content of the liquid electrolyte is not con-()N.Janke et al.r Thin Solid Films3922001134᎐141141trolled.Tungsten oxide is more sensitive thanCeO᎐TiO to degradation due to water.2y x2The voltage has to be applied carefully,as a0.15-V increase in voltage resulted in an increase in charge density of10mC cm y2.Electrochemical insertion considers the total amount of intercalated charge.Therefore,it is necessary to determine the share of protons during intercalation, comparing the charge density gained conventionally and from etching experiments.In the case of tungsten oxide,the charge density is mainly due to Li q interca-lation.However,in case of the CeO᎐TiO ion-stor-2y x2age layer,proton intercalation plays a major role.The as-depositedfilms are amorphous according to GI-XRD and cyclovoltammetry.The relative density ofthe WOfilms,determined from GI-XRS,is approxi-3mately80%of the density of crystalline tungsten oxide.CeO᎐TiOfilms require a low density in order to 2y x2achieve appreciable intercalation characteristics. According to XPS,tungsten is bound as W6q inWO.The mixed-oxide ion-storage layer consists of 3cerium in two valences,Ce3q and Ce4q,and titanium as Ti4q.The chemistry can be described asCeO᎐TiO,with x indicating the substoichiometric 2y x2cerium oxide.Intercalation of Li ions causes reduction of W6q to W5q.W5q can be easily reoxidized by deintercalation.In case of the ion-storage layer,cerium oxide CeO2y x is shown by XPS measurements to be the active part in inter-and deintercalation processes.A redox reaction between Ce3q and Ce4q appears,as indicated earlier w xin14.TiO does not participate in the intercalation 2process,although it is well known as an electrochromic oxide.In accordance with the above-mentioned model forw x q intercalation in CeO᎐TiO18,we conclude that Li22diffusion occurs through an amorphous phase.As the samples tested are totally amorphous,a crystalline phase is definitely not necessary for the intercalation of ions.As the charge density depends on the thin-film density,we conclude that a lower density is accom-panied by a more disordered structure,which makes diffusion easier.The nano-crystals which appear in sol᎐gelfilms may nevertheless force intercalation insol᎐gel-prepared CeO᎐TiO,as their grain boun-22daries surely will act as fast diffusion paths for Li qions.Intercalation in CeO crystals is not likely to play2a major role,as the diffusion kinetics are weak in the bulk material.AcknowledgementsWe would like to express our thanks to Dr Becker ofŽ.Pilkington Flabeg Fuerth and Dr Wittkopf of Pilking-Ž.ton Flachglas Gelsenkirchen for their cooperation throughout this research.The help of Dr H.Gobel,¨Ž. 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