COMPARATIVE STUDIES OF "ALL SOL-GEL" ELECTROCHROMIC WINDOWS EMPLOYING VARIOUS COUNTER ELECTRODES

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1 COMPARATIVE STUDIES OF "ALL SOL-GEL" ELECTROCHROMIC WINDOWS EMPLOYING VARIOUS COUNTER ELECTRODES Urša Opara Krašovec, Angela Šurca Vuk, Boris Orel, National Institute of Chemistry, Hajdrihova 19, SI 1000 Ljubljana, Slovenia, Tel: +386-(0) , Fax: +386(0) , Abstract Electrochromic (EC) "smart" windows for buildings represent an effective way to modulate the intensity of incoming solar radiation making them a better option than mechanical window shutters. While it is accepted that WO 3 films represent the best option for the active electrochromic layer, the choice of the best counter electrode is still an open question. In most cases, counter electrodes with charge capacities > 20 mc/cm 2 are not sufficiently transparent in the charged state, which lowers the transparency of the electrochromic windows in the bleached state. Over the last few years, we have developed counter electrode films using sol-gel chemistry based on oxides of Ce, Fe, V and Sn. The films exhibit a high transparency in the charged state with coloration efficiencies (η) between 0.2 and 4 cm 2 /C and a transmittance in the visible region > 85%. The charge capacities of the films are from 40 mc/cm 2 (i.e. SnO 2 :Mo (1:1), Fe/V (9:1)-oxide) to 20 mc/cm 2 (Ce/V - oxide, CeVO 4 ), which means that not all of them are able to deliver sufficient charge to the active colouring layer (WO 3 ) to achieve maximum coloration. In this paper, we present the optical properties of different counter electrodes assembled in electrochromic windows. Electrochromic windows were made from sol-gel WO 3 film (150 C) acting as an active colouring film while the sol-gel organic inorganic hybrid (Li + ormolyte) is used as an ion conductor. Our results show that the electrochromic responses of our devices predicted from the charge capacities and photopic transmittances and coloration efficiencies of individual films agree well with measured values. 1.INTRODUCTION The future energy saving requirements placed on modern buildings will require that windows will have to meet higher standards, including being able to reduce heat-loss while avoiding overheating of the building and gaining solar energy while assuring comfortable daylighting. These conflicting demands can be satisfied by using switchable or smart windows, the optical properties (transmittance and reflectance) of which can be varied between low and high transmitting states. This could be done either manually or automatically by the building s own energy management system. In addition, switchable windows prevent the major visual discomfort caused by excessive glare. Various switchable smart windows exist: photochromic (transmittance change is a function of the irradiation dose), thermochromic (transmittance change responds to the temperature variation), thermotropic (transmittance decreases with increased scattering of the visible light at certain temperatures) and electrochromic (optical properties change under the action of a voltage or current pulse) [1,2]. The most appropriate configuration of an EC device for practical applications (windows) is similar to that found in a battery and consists of five active layers (Fig. 1) [2]. Sandwiched between two electronically conductive transparent electrodes coated on glass (SnO 2 /F, Pilkington K-glass) are an active electrochromic film (usually WO 3 ), a lithium or proton ionic conductor and a counter-electrode film. Under the action of a voltage pulse, the simultaneous intercalation of ions (from ionic conductor) and electrons (from transparent electrode) into the cathodic electrochromic film occurs. The electrons change the optical properties of the EC film while the Li + /H + ions balance the charge difference. The process is reversed during bleaching. By changing the sign of the applied voltage within limited range allows us to control the transmittance of the device. Fig. 1 glass transparent conductive electrode counter electrode M + ionic conductor (M + =Li + or H + ) active electrode (WO 3 ) transparent conductive electrode glass (bleached state) WO 3 + xe - + xm + + E (V) - Basic design of an EC device. The transport of positive ions and electrons to obtain the coloured state of WO 3 is indicated for chosen polarisation. Our aim is to prepare an all sol-gel EC device with a transmittance in the bleached (uncharged) state above 65 % and in the coloured state below 10 %, a M + e - M x WO 3 (coloured state)

2 switching speed less than 10 minutes and a cycling stability greater than 3000 cycles [4-6]. The term all solgel means that all three internal layers, including the ionic conductor, are processed by the sol-gel route. Sol-gel processing has many advantages over traditional techniques for the preparation of advanced and functional coatings with optical, chemical, electrooptical and mechanical properties [7,8]. By using the sol-gel method, a high degree of homogeneity of the films is achieved, since the starting materials are in solution mixed on a molecular level. The simplicity of the dipcoating process, used for the deposition of films, means that a variety of dopants can be added to the initial sols in varying concentrations, yielding doped or mixed oxide films with improved electrochromic properties [9]. Also, low processing temperatures allow us to prepare organic/inorganic hybrid gels with lithium ion conductivity (ormolytes). In principle, the coloration of a device is expressed by the monochromatic absorbance ($GHILQHGE\Eq. 1 [10]. The first term represents the sum of the monochromatic absorbances of the active colouring film A EC (in our case WO 3 ) and the counter-electrode A CE in their transmitting (discharged) states. $ A EC + A CE (ECCE Eq. 1 (ECCE H[SUHVVHV WKH DELOLW\ RI WKH FRXQWHU electrode to compensate the charge inserted ( ) in the active films and depends on the monochromatic FRORUDWLRQ HIILFLHQFLHV RI WKH FRXQWHUHOHFWURGH (CE DQG WKH DFWLYH (& ILOP EC,W VKRXOG EH noted that all the quantities in Eq. 1 depend on the frequency and that the coloration efficiencies vary with the amount of charge inserted/racted in EC and CE films. According to Eq. 1, counter electrodes that exhibit a complementary electrochromic response to WO 3 film (i.e. anodic electrochromism) are the most effective for attaining a high absorbance of the device because their coloration efficiencies have an opposite sign. This means that the charge capacity of the counter-electrode does not need to match completely the charge capacity of the active EC film. Our aim is to show that a high coloration of an EC device is achievable using counter-electrodes, which conversely to certain anodic electrochromic films exhibit a low coloration efficiency ( cm 2 /C) albeit their charge capacity is comparable to WO 3 films (> 30 mc/cm 2 ). In this case the contribution from the coloration efficiency of the counter electrode (CE) is small and the absorbance of the device will depend primarily on the coloration efficiency of the active EC film and the charge ( ) which the counter-electrode is able to exchange. Such a counter-electrode is named optically passive. In this paper we focused on the assessment of photopic properties of EC devices because of their importance for assessing comfortable daylightning in buildings and not to what ent the EC device is able to contribute to heating of the building by gaining solar energy. The reason being that we were not able to measure the monochromatic absorbances in the whole spectral region of solar spectrum due to the limited range of our spectrometer. In the first part of the paper we present the electrochromic properties of WO 3 films annealed at different temperatures. For those showing the highest coloration efficiency and fastest coloration kinetics we also determined their photopic transmittances ( ) at different states of charging ( ) and used them to assess the photopic coloration efficiency ( vis EC ) as a function of inserted charge. The second part contains information about the charge capacities and photopic transmittances ( ) of twenty-two counter-electrode films made in our laboratory [5,11-23]. For each combination of counterelectrode/wo 3 films we assessed, using Eq. 1, the electrochromic properties of model devices. Finally, we present the electrochromic properties of ten EC devices assembled from WO 3 film, organic/inorganic hybrid electrolyte and ten different counter-electrodes which have been chosen on the basis of their optical passiveness and the maximal charge capacities. 2. EXPERIMENTAL Film thickness was determined using a Profilometer Talysurf (Taylor Hobson). Electrochemical measurements were made using an EG&G PAR 273 computer controlled potenciostat driven by the 270 Electrochemical Analysis Software. A three electrode system was employed using Pt rod as a counter electrode and modified Ag/AgCl as a reference electrode for aprotic (1M LiClO 4 /propylene carbonate (PC)) electrolyte. Electrochemical testing of the EC devices was performed in a similar way as individual films with the exception that no reference electrode was used and the potential variation refers to the counter electrode. The in-situ UV-VIS spectroelectrochemical measurements were performed on a Hewlett-Packard 8452A Diode-Array spectrometer linked to the potenciostat. The transmittance spectra were measured using a three-electrode electrochemical cell with quartz windows filled with an electrolyte as the reference, while for the EC devices air was used as the reference. 7KH PRQRFKURPDWLF FRORUDWLRQ HIILFLHQF\ LV GHILQHGDVWKHFKDQJHLQRSWLFDOGHQVLW\û2' REWDLQHG at certain wavelengths divided by the inserted charge ( 7KH û2' LV D decadic logarithm of the ratio 2

3 between the monochromatic transmittance of the film in the intercalated (T ins ) and deintercalated (T ) state. The photopic transmittance ( ) of films was determined according to Eq. 2 from spectral response of films charged to different levels ( ). Eq. 2 In Eq. 2 τ(λ) represents the spectral transmittance of the sample, D λ the spectral energy distribution and V(λ) luminous efficiency of the observer. An approximation of zero reflectance was used. Photopic coloration efficiencies of EC (vis EC ) and CE (vis CE ) films were obtained after the transformation of ins and into û2' DQG GLYLGHG LW E\. These values were used to assess the optical modulation of various EC devices according to Eq. 1. EC devices were assembled with one of twentytwo different counter-electrode films using the same kind of WO 3 films. Reflectance losses of the films interface and the absorption of the ionic conductor were not taken into account. Tungsten oxide films were dip-coated [4] from peroxopolytungstic acid sols followed by annealing at 120, 150, 175, 200 C for one hour. Film thicknesses were between 290 and 310 nm/dip. Details about the electrochromic properties and synthesis of counterelectrodes could be obtained in Ref. 5, The Li + ormolyte was made using an unhydrolysed hybrid silicon precursor, modified by polypropylene glycol (4000) [24]. The precursor is hydrolysed with 0.1M HCl and LiClO 4 was added to obtain an appropriate ionic conductivity of 5x10-5 Scm -1 measured between two K-glass plates (Pilkington plc.). In such a modified ethoxysilane, the polymer units and inorganic parts are linked via urea groups [24]. The ormolyte layers were deposited using dip-coating technique on active WO 3 and counter-electrode films. Both electrodes are firmly pressed together to assure a good mechanical and electrical contact. 3. RESULTS AND DISCUSSION 3.1.WO 3 films 780nm λ λ = 380nm Tvis = 780nm D τ ( λ) V ( λ) λ λ λ = 380nm D V ( λ) λ The electrochromic modulation of the WO 3 films heated at different temperatures as a function of inserted charge ( ) was obtained from the measured spectral transmittance of films charged galvanostatically with a constant current (± 50 $LQWKHWKUHHHOHFWURGHFHOO7KH PRQRFKURPDWLF FRORUDWLRQ HIILFLHQFLHV ZHUH calculated for different wavelengths. The monochromatic coloration efficiencies of the WO 3 films vary with the curing temperature (Fig. 2) and show different values between 400 and 1100 nm. The films heat-treated at 175 C have a maximum monochromatic HIILFLHQF\ but the bleaching of the films at constant potential (+1.5V vs. Ag/AgCl) is slow (> 30 s). In addition, a fully bleached state is achieved at higher potentials (+1.5 V vs. Ag/AgCl) while films annealed at lower temperatures are bleached at 1.0 V vs. Ag/AgCl. A WO 3 film heat-treated for 1 hour at 150 C is used as the active layer in our devices. The reason for choosing this film is its good electrochemical stability and fast colouring/bleaching kinetics together with high PRQRFKURPDWLF DQG vis values (Figs. 2 and 3). Chronopotenciostatic colouring at -1.0 V vs. Ag/AgCl and bleaching at +1.0 V vs. Ag/AgCl of the film is completed in less than 20 s. η (cm 2 /C) λ = 1000 nm λ = 820 nm λ = 634 nm λ = 480 nm Temperature ( C) Fig. 2 0RQRFKURPDWLF FRORUDWLRQ HIILFLHQFLHV )) of the WO 3 films heat-treated at different temperatures (120, 150, 175 and 200 C) obtained for different wavelengths after galvanostatic (I = ± 50 A) insertion of 20 mc/cm 2. η cm 2 /C λ = 1000 nm λ = 820 nm λ = 634 nm λ = 480 nm Fig. 3 Monochromatic coloration efficiency of the WO 3 films heat-treated at 150 C as a function of inserted charge ( ) obtained for different wavelengths. 3

4 7KH PRQRFKURPDWLF YDOXHV RI :2 3 films shown in Fig. 2 also depend on the amount of inserted charge. This is shown in Fig. 3 for a WO 3 film cured at ƒ& )RU H[DPSOH WKH PD[LPDO PRQRFKURPDWLF value is 22 cm 2 &DW nm and only slightly varies with the amount of inserted charge (Fig. 3). 1, Fig. 4 Photopic transmittances ( ) (columns) with corresponding coloration efficiencies (vis) ( ) of WO 3 films (150 C) as a function of inserted charge ( ). $GLIIHUHQWGHSHQGHQFHRIvs.4LVQRWHGDW nm revealing a maximum coloration efficiency up to 180 cm 2 /C with small inserted charges (2.5 mc/cm 2 ). The exact reason for the observed differences is not fully understood albeit we assume that it originates from the different mechanisms of coloration i.e. the interband transition and the polaron hopping. Irrespective of this, WO 3 films need only a small amount of charge to reduce the monochromatic transmittance in the spectral region where the polaron absorption is intense while a much higher charging is necessary to colour the films in the visible region of the solar spectrum. As expected (Fig. 4) the photopic coloration efficiencies (vis) exhibits a maximal value at 20 mc/cm 2 and not at small values of inserted charge typical for polaron absorption. Photopic vis values obtained in this way are used to calculate the photopic electrochromic response of the devices using Eq η vis (cm 2 /C) 3.2 Counter electrodes Photopic transmittances of twenty-two different counter-electrode films in the intercalated (T ins vis ) and deintercalated (T vis ) states were determined from the measured spectral transmittances of the films in both states [5,11-23] using Eq.2. The T ins vis values are between 0.54 (V:Ti = 1:1; 400 C) and 0.92 (SnO 2 ) (Fig. 5) and are much higher compared to a WO 3 film (T ins vis =0.096) (Figs. 4 and 5). The values of these types of counter-electrode films are above 0.75, which confirms the optical passiveness of these types of counterelectrode. Among the investigated counter-electrode films few meet the high charge-capacity vs. small photopictransmittance variation criteria for an optically passive counter-electrode. A typical example is the undoped SnO 2 [21] counter-electrode, which exhibits the highest T ins vis, but its charge capacity is only 2 mc/cm 2. Such a counter-electrode is not effective and brings about a very small optical modulation of the EC device (Fig. 6). ins A low charge capacity combined with a high is also characteristic for other single oxide films, like CeO 2, TiO 2, Fe 2 O 3 [19] and for V 2 O 5 when cycled in the safe potential range (4.8 to 2.9 V vs. Li) [11,12]. Higher charge capacity can be obtained for V 2 O 5 at lower cathodic potentials (1.8 V vs. Li), however, the cycling stability of the films when charged/discharged between 4.6 V and 1.8 V vs. Li (unsafe range [11,12]) is low and leads to the degradation of the film. Following our goal, to use counter-electrode exhibiting a small optical modulation with charging and high charge capacity, mixed-oxide films were examined. Fig. 5 shows that the smallest difference between in the charged (T ins vis ) and discharged (T vis ) states i.e is obtained for crystalline CeVO 4 films [13-15], which have a charge capacity up to 20 mc/cm 2. Higher charge capicities (24.2 to 29.7 mc/cm 2 ) characterise V:Ti = 1:1 and V:Ti:Zr = 1:1:0.2 films [16], however, the ins differences between and are increased to and 0.118, respectively. Other promising compounds with a high charge capacity are mixed crystalline or amorphous Fe/V-oxide films (Fe:V = 1:1 and 1:2) [17,18] despite having lower T ins vis values (between 0.66 and 0.69) than CeVO 4 films. Besides mixed Ce/V-, Ti/V- and Fe/V-oxide films, Sn/Mo-oxide films [22,23] also fulfil the chargecapacity/photopic-transmittance criteria for an optically passive counter-electrode. Films with an excess of SnO 2 (Sn/Mo = 9:1, 2:1) have charge densities up to 20 mc/cm 2 and T ins vis above The latter value decreases to for Sn/Mo = 1:1 films, the charge capacity of which (-52.2 mc/cm 2 ) is the only one that exceeds the charge capacity of a WO 3 electrochromic film (-42.2 mc/cm 2 ). Further studies will be needed to determine the stability and optical properties of this promising counterelectrode. 4

5 7YLV Fig EC devices :2 92VDIH 92XQVDIH &H2 &H9 &H92 &H9 97Lƒ& 97Lƒ& 97L=Uƒ& 97L=Uƒ& 97L&Hƒ& 97L&Hƒ& )H2ƒ& 6Q2 6Q26E 6Q26E0R Photopic transmittances ( ) values of various counter-electrode films in charged ( ins -dark columns) and discharged ( -white columns) states obtained after inserting the maximal possible charge ( ) which is typical for certain electrode [5,11-23]. Initially, we determined electrochromic properties of model EC devices - consisting of twenty-two counterelectrode/wo 3 films by calculating their photopic transmittances in the bleached (T ins vis ) and coloured (T vis ) states using the Eq. 1 and 2. The T ins vis and values in Fig. 6 show the maximal attainable colouring/bleaching changes of certain WO 3 /counterelectrode combinations to reveal which devices are most appropriate for assuring comfortable daylightning and the highest possible shading to avoid excessive glare. As expected, the device with a Sn/Mo (1:1) oxide counter-electrode film produces the best shading properties, that is the smallest T ins vis value (0.04). The combination of a WO 3 film with either Ce/V-, V/Ti-, V/Ti/Zr-, V/Ti/Ce- or Sn/Mo- (9:1 and 2:1) oxide counter-electrodes show a similar photopic transmittance in the charged state (0.2 < T ins vis < 0.3). This is because the films have similar charge capacities of ~ 20 mc/cm 2 (Fig. 5), which means that the same amount of charge can be shutteled between the counter-electrode and WO 3 film. Among them, devices combining WO 3 and Ce/V-oxide 4LQV>P&FP@ films assure high daylightning exhibiting > 0.85, while V/Ti-oxide films have values between 0.74 and 0.76 and Fe/V (1:1 and 1:2)- and Sn:Sb-oxide films [22,23] have even lower T vis values (0.64 to 0.69). In Fig. 6 we represent the maximal possible colouring/bleaching changes achievable for certain combination of the active and counter-electrode films yet the photopic transmittances of the devices at intermediate values of inserted charge have not been determined. Namely in Eq. 1, vis of counter-electrode (vis CE ) and WO 3 (vis EC ) films were taken as fixed values despite that, as shown for WO 3 films (Fig.4), it varies with inserted charge. Accordingly, we made ten EC devices assembled from ten different counter-electrode films using the same type of WO 3 film (150 C, thickness is 300 nm See Sec. 3.1) and glued together with a Li + ion conductor (see Experimental). We measured their electrochromic response as a function of inserted charge and compared this to the predicted values (Fig. 6). The following counter-electrodes were used: Ce/V- oxide (i), V 2 O 5 (ii) and Fe/V-oxide (iii) films. Different amount of charge was inserted by applying constant current (± 0.5 ma) and the corresponding values calculated using Eq. 2. 5

6 7YLV 92VDIH 92XQVDIH &H2 &H9 &H92 &H9 97Lƒ& 97Lƒ& 97L=Uƒ& 97L=Uƒ& 97L&Hƒ& 97L&Hƒ& )H2ƒ& 6Q2 6Q26E 6Q26E0R Fig. 6 Photopic transmittance ( ) of model devices consisting of various counter-electrode/wo 3 films combinations: dark columns coloured states ( ins ), white column bleached state ( ) and - the maximal charge capacity ( ) of counter electrodes. (i) The first group includes mixed Ce/V oxide (Ce/V (2:1) oxide (60 nm), Ce/V (1:2) oxide (100 nm), CeVO 4 (65 nm) and Li 0.3 CeVO 4 (50 nm) films. All these counter-electrodes are considered as being the most optically passive (0.05 < η CE < 1.4 cm 2 /C) exhibiting charge capacities up to 20 mc/cm 2 (Fig. 5). Measurements show that the photopic transmittances of the devices in the bleached state (T vis ) are higher than 0.65 (Fig. 7-10). For example, the crystalline CeVO 4 films have the highest of the devices in the bleached state (i.e. 0.72) proving that a smart device employing this type of counter-electrode would assure ins comfortable daylightning of the building interior. decreases to 0.15 when WO 3 films are charged to maximum i.e. 20 mc/cm 2 for CeVO 4 counterelectrode, which is insufficient to fully colour the WO 3 film making the device inadequate for privacy control. Ce/V (2:1) oxide 4LQV>P&FP@ Fig. 7 Photopic transmitance ( ) of WO 3 Li + ormolyte Ce/V (2:1) oxide device as a function of and predicted ( - fixed vis EC at 20 mc/cm 2 and - vis EC taken from Fig. 4) values. 6

7 CeVO 4 Ce/V (1:2) oxide Fig. 8 Photopic transmitance ( ) of WO 3 Li + ormolyte CeVO 4 device as a function of charge inserted into WO 3 layer: measured (columns) and predicted (see Fig. 7) values. Fig. 10 Photopic transmitance ( ) of WO 3 Li + ormolyte Ce/V (1:2)-oxide EC device as a function of and predicted (see Fig. 7) values. Li 0.3 CeVO 4 Fig. 9 Photopic transmitance ( ) of a WO 3 Li + ormolyte Li 0.3 CeVO 4 device as a function of charge inserted into WO 3 layer. (ii) The second group of devices was constructed from 60 nm thick V 2 O 5 counter-electrode films (Figs. 11 and 12). Even though the V 2 O 5 electrodes (Sec. 3.2) could be charged up to 35 mc/cm 2 (unsafe range - [11,12]) when charged in the safe potential range the charge capacity is too small (15 mc/cm 2 ) to colour the WO 3 film to its full ent. As expected, WO 3 Li + ormolyte V 2 O 5 devices exhibit different coloration (Figs. 11 and 12) when charged in the safe and unsafe potential ranges. For example, insertion of 15 mc/cm 2 (safe range) change from 0.60 to 0.22, while charging to 35 mc/cm 2 (unsafe range) results in a ins higher coloration of the device ( = 0.05). However, consecutive charging to 35mC/cm 2 leads to the degradation of the electrolyte, resulting in the formation of cracks and pinholes. (iii) The last group of devices includes Fe/V oxide (Fe/V= 1:1 oxide (50 nm), Fe/V=1:2 oxide (70 nm) and Fe/V=1:9 oxide (100 nm)) counter-electrode films. The main advantage of these counter-electrodes over V 2 O 5 is that they retain a stable electrochemical response when charged up to 35 mc/cm 2 (Figs ). The most appropriate optical modulation (T vis = 0.62 and T ins vis = 0.04) is found for the WO 3 Li + ormolyte Fe/V (1:2) oxide device (Fig. 14). Albeit the electrochemical stability of these films tested in the three-electrode cell is high (> 2000 cycles) the long-term electrochromic stability of these device is yet to be ascertained. From an optical point of view the main disadvantage of these devices is that the in the bleached state (T vis ) does 7

8 not exceed 0.62 and is even 0.53 in the case of Fe/V (1:9)-oxide counter-electrode. The low transmittance attained with Fe/V (1:9)-oxide makes this type of device practical for use in EC displays, for privacy control, and EC mirror since high photopic transmittance in the bleached state is not essential. V 2 O 5 (safe) Fe/V-oxide (1:1), 400 C Fig. 11 Photopic transmitance ( ) of WO 3 Li + ormolyte V 2 O 5 (safe region) device as a function of and predicted (see fig. 7) values. Fig. 13 Photopic transmitance ( ) of WO 3 Li + ormolyte Fe/V (1:1) oxide device as a function of and predicted (see fig. 7) values. V 2 O 5 (unsafe) Fe/V-oxide (1:2), 400 C Fig. 12 Photopic transmitance ( ) of WO 3 Li + ormolyte V 2 O 5 (unsafe region) device as a function of and predicted (see fig. 7) values. Fig. 14 Photopic transmitance ( ) of WO 3 Li + ormolyte Fe/V (1:2) oxide device as a function of and predicted (see fig. 7) values. 8

9 Fe/V-oxide (1:9), 300 C 1,0 bleached state Transmittance 20 mc/cm 2 Fig. 15 Photopic transmitance ( ) of WO 3 Li + ormolyte Fe/V (1:9)-oxide (300 C) device as a function of charge inserted into WO 3 layer: measured (columns) and predicted (see fig. 7) values. coloured state λ (nm) 1,0 Fig. 17 Transmittance spectra of WO 3 Li + ormolyte CeVO 4 EC device (full line) and WO 3 films (dotted line) in coloured and bleached states. Transmittance 35 mc/cm 2 bleached state Finally, we present typical spectral responses of WO 3 Li + ormolyte Fe/V(1:2)-oxide (Fig. 16) and WO 3 Li + ormolyte CeVO 4 (Fig. 17) devices in coloured and bleached states to demonstrate the optical passiveness of the counter electrodes. Transmittance spectra of a WO 3 film charged/discharged to ±35 mc/cm 2 (Fig. 16) and to ±20 mc/cm 2 (Fig. 17) show that the transmittance spectra of EC devices in the coloured state resemble the spectral transmittance of a WO 3 film in the charged state [1-5]. coloured state λ (nm) Fig. 16 Transmittance spectra of WO 3 Li + ormolyte Fe/V (1:2)-oxide (full line) and WO 3 films (dotted line) in coloured and bleached states. 4. CONCLUSIONS Among the counter-electrodes we tested, CeVO 4 and Li 0.3 CeVO 4 EC devices have the highest photopic transmittances in the bleached state ( Thus EC devices using these counter-electrodes are suitable for making smart windows. Unfortunately, in the coloured state, the maximal remains above The reason why the T ins vis (coloured state) of a CeVO 4 device is higher than that of the WO 3 film is because of the insufficient charge capacity of the counter-electrode (< 20 mc/cm 2 ). Accordingly, better shading is obtained with a Fe/V (1:2)-oxide device (T ins vis = 0.04) but its capacity to assure adequate daylightning ( = 0.62) is inferior to a CeVO 4 device. 9

10 All the devices tested exhibit a smaller transmittance compared to a counter-electrode/wo 3 film combination. This is due to the absorptance of the electrolyte and the reflection losses of the film stack. Also, it is possible to obtain acceptable shading ( ~ 0.05) if the counter-electrode exhibits a charge capacity > 30 mc/cm 2. It is clear that mixed vanadium oxide films are the most promising counter-electrodes because they combine electrochemical stability with high charge capacity. This still should be tested for Snoxide based counter-electrodes. REFERENCES [1] C. G. Granqvist, Chromogenic Materials for Transmittance Control of Large-Area Windows, Workshop on Materials Science and Physics of Non-Conventional Energy Sources, ICTP, Trieste, September [2] C. G. Granqvist: Handbook of Inorganic Electrochromic Materials, Elsevier Science, Amsterdam, [3] P. Judeinstein, J. Livage, A. Zarudiansky and R. Rose, Solid State Ionics, 28-30, 1722 (1988). [4] B. Orel, U. Opara Krašovec, U. /DYUHQþLþ Štangar, P. Judeinstein, J. Sol-Gel Science and Technology 11, 87 (1998). [5] B. Orel, U. Opara Krašovec, M. 0DþHN ) Švegl, U. /DYUHQþLþŠtangar, Sol. Energy Mater. & Sol. Cells 56, 343 (1999). [6] B. Orel, A. Šurca, U. Opara Krašovec, Acta Chim. Slov. 45, 487 (1998). [7] C. J. Brinker, G. W. Scherer: Sol-Gel Science, Academic Press, Boston, [8] J. Livage, M. Hanry, C. Sanchez, Prog. Solid St. Chem. 18, 259 (1988). [9] M. A. Aegerter: Sol-Gel Chromogenic Materials and Devices in Structure and Bonding, Springer-Verlag, Berlin, Vol. 85, 1996, pp [10] R. D. Rauh, Electrochim. Acta 44, 3165 (1999). [11] A. Šurca, B. Orel, G. 'UDåLþ, B. Pihlar, J. Electrochem. Soc. 146, 232 (1999). [12] A. Šurca, B. Orel, Electrochim. Acta 44, 3051 (1999). [13] U. Opara Krašovec, B. Orel, R. Reisfeld, Electrochem. Solid State Lett. 1, 104 (1998). [14] U. Opara Krašovec, B. Orel, A. Šurca, N. Bukovec, R. Reisfeld, Solid State Ionics 118, 195 (1999). [15] G. Picardi, F. Varsano, F. Decker, U. Opara Krašovec, A. Šurca, B. Orel, Electrochim. Acta 44, 3157 (1999). [16] A. Šurca, B. Orel, S. %HQþLþ % Pihlar, Electrochim. Acta 44, 3075 (1999). [17] S. %HQþLþ % Orel, A. Šurca, U. /DYUHQþLþ Štangar, Sol. Energy (in press). [18] A. Šurca, B. Orel, U. Opara Krašovec, U. /DYUHQþLþ Štangar, J. Electrochem. Soc. (in press). [19] B. Orel, M. 0DþHN)Švegl, K. Kalcher, Thin Solid Films 246, 131 (1994). [20] B. Orel, U. /DYUHQþLþŠtangar, Z. Crnjak Orel, P. Bukovec, M. Kosec, J. Non-Cryst. Solids 167, 272 (1994). [21] B. Orel, U. /DYUHQþLþ Štangar, K. Kalcher, J. Electrochem. Soc. 141, L127 (1994). [22] U. Opara Krašovec, B. Orel, S. +RþHYDU, 0XãHYLþ - Electrochem. Soc. 144, 3398 (1997). [23] B. Orel, U. /DYUHQþLþ Štangar, U. Opara, M. *DEHUãþHN.Kalcher, J. Mater. Chem. 5, 617 (1995). [24] N. Grošelj, M. *DEHUãþHN 8Opara Krašovec, B. Orel, G. 'UDåLþ 3 Judeinstein, Solid State Ionics 125, 125 (1999). 10