WO2009027160A1

WO2009027160A1 - Electrochromic optical device showing reduced switching times

Info

Publication number: WO2009027160A1
Application number: PCT/EP2008/059782
Authority: WO
Inventors: Edy Widjaja; Peter Persoone
Original assignee: Nv Bekaert Sa
Priority date: 2007-08-27
Filing date: 2008-07-25
Publication date: 2009-03-05

Abstract

An optical device (200) based on an electrochromic layered stack (210,310) is disclosed. The optical device comprises infrared absorbing material (220) that is by preferencebased on nanoparticles (222) embedded in a matrix (224). The matrix can be a polymer film, the substrate (230) itself or one or more layers of the electrochromic layered stack. The infrared absorbing material is in thermal contact with the electrochromic layered stack. The optical device solves the problem occurring at low temperatures of having too long switching times between the bleached and darkened state of the electrochromic stack. By introducing the infrared absorbing material the switching times can be reduced by 17 up to 54 % when starting from the bleached state, depending on the incandescent solar irradiation and the outdoor temperature.

Description

ELECTROCHROMIC OPTICAL DEVICE SHOWING REDUCED SWITCHING TIMES.

Description

Technical Field

[0001] The invention relates to the field of electrochromic devices as used in electrically switchable glazing for human habitats (for e.g. architectural, automotive or marine application, in cars buses, trains, houses and the like) or for dedicated eye protection (such as for goggles, visors and other similar appliances).

Background Art

[0002] Electrically switchable electrochromic devices have great potential for controlling ambient light conditions in living areas or as an adapting protection aid for the eye. Such switchable devices are described in their rudimentary versions in e.g. Solar Energy Materials page 17(1989) September 1 of 2 or US 4832463.

[0003] In general electrochromic devices comprise a layered stack of a transparent first electroconductive layer, an electrochromic layer, an ion conductor, an optional counter electrode and a second transparent electroconductive layer. The modulation of light is based on the intercalation under an electric field of a small cation (usually a small ion such as a proton or a lithium ion although some materials can intercalate a sodium or even a potassium ion) into an amorphous or crystalline network of the electrochromic layer. The intercalation results in a change of electronic properties of the network entailing an increased absorption of light and hence a reduced transmission of the light through the device. The function of the ion conductor is to separate the ions from the electrons. The counter electrode collects the ions that have crossed the ion conductor and serves as a reservoir. When the material of the counter electrode is such that it bleaches when ions are absorbed and darkens when ions are extracted, it can further increase the overall ratio between dark and clear state of the device (complementary counter electrode). [0004] The materials of which the various layers are made can differ greatly.

While for the electrochromic electrode a solid state material like tungsten oxide is many times preferred, the ion conductor can be a liquid electrolyte or an organic solid or gel electrolyte or an inorganic solid electrolyte. The counter electrode can be a polymeric layer or a solid state layer.

[0005] Such an electrochromic device is usually deposited on a substrate made of glass or made of an organic, highly transparent material such as PMMA. The substrate can be stiff or can be flexible depending on its thickness and its material properties. The electrochromic device can also be laminated between two transparent sheets.

[0006] One of the problems that hampers the further introduction of electrochromic devices is the switching time. Indeed, the rate limiting mobile species in the device are cations that have to diffuse under the influence of an electric field out of the electrochromic electrode into the ion conductor to the counter electrode upon bleaching (and back on darkening). The time it takes the cations to cross the device depends on several factors such as the delay in build up of electric field (due to the resistance and capacitance of the thin film device), the strength of the electric field, the thickness of the layers, the cation species mobility, but above all it depends on the temperature of the device. As the transport mode of the ions is based on diffusion, the process is highly temperature activated.

[0007] While the switching time of the device is less than 6 to 7 minutes for temperatures above 10°C they can rise above 30 minutes for temperatures between -3°C to -1 °C (see 'A design guide for early-market electrochromic windows', prepared for 'California Energy Commission' in the framework of a 'Public Interest Energy Research', May 2006). Consider for example an office in a tower building where employees enter after a frosty but clear night. When the employees want to dim the glare of the low standing early morning sun by means of the electrochromic switchable windows, it will take an unacceptably long time before enough darkening has occurred. Clearly a solution is needed for this problem. [0008] One known solution to the problem has been laid open in US 6856444. The system described determines the temperature of the electrochromic device through a low frequency resistivity measurement: the resistivity of the device gives an indication of its temperature. The voltage applied for changing the colouring state of the device is made dependent from the sensed temperature: at lower temperature a higher voltage can be applied without damaging the electrochromic stack. This higher voltage - and thus electric field - results in a shortening of the switching time.

[0009] A further elaboration has been described in US 7133181 wherein the device is heated through application of an alternating current. The duty cycle of the alternating current is steered such that not only heating is obtained but also a controlled bleaching or darkening, wherein also account is taken of the temperature of the device so as to minimise the switching time. Higher voltages are used in this method (12 to 24 V), but this does not harm the device as they are applied for a very short time.

[0010] Although probably adequate, these methods to reduce the switching time do require additional steering electronics. The inventors therefore sought other and simpler solutions to reduce the long switching times at low temperatures.

Disclosure of Invention

[0011] It is a first object of the invention to solve the switching time problem of electrochromic optical devices. It is a further object of the invention to provide an optical device with an acceptable switching time in a broad temperature range. It is another object of the invention to realise this improved switching time without an additional energy input than that is available from the ambient environment. It is also an object of the invention to do this by simple means.

[0012] According a first aspect of the invention, an optical device is presented

(FIGURE 2, 200). A window or a sunroof of a car can be considered as an Optical device' as well as an insulated glass unit (IGU) or a skylight in a building. Other examples are a visor that is integral with a helmet or a set of goggles. These are a limited number of examples and the list is by no means exhaustive nor should be considered delimitative. [0013] An essential part of the optical device is an electrochromic layered stack (see FIGURE 2, 210), that generally comprises an ion conductor (212) sandwiched between two electrochromic active layers (214, 214') that on their turn are sandwiched between two transparent, electrically conductive layers (216, 216'). By preference the electrochromic devices are of the 'all- solid state type' that are sometimes also called 'ceramic' electrochromic devices.

[0014] The transparent, electrically conductive layers (216, 216) are by preference made of a TCO (Transparant Conductive Oxide) such as indium tin oxide (ITO), or aluminium zinc oxide (ZOA) and variations thereon such as fluoride doped tin oxide (FTO). Also stacks comprising a thin metallic interlayer such ITO/Ag/ITO can also be used as contacting layer.

[0015] Basically two types of electrochromic materials exist: there is the cathodically colouring type of material that darkens when the mobile cation enters the network and there is the anodically colouring type of material that darkens when the mobile cation is driven out of the network.

[0016] Many inorganic metal oxides such as TiO2, Nb2θs, MOO3, Ta2θs, and WO3 exhibit cathodic electrochromism. Other inorganic metal oxides such as Cr2θ3, Mn2θ3, CO2O3, NiO, Rh2θ3, or Ir2θ3 exhibit anodic electrochromism. Other materials like V2O5 - although they can intercalate small cations - do not change colour (although it has an intrinsic yellowish colour). Also ternary oxide compounds may show enhanced electrochromism such as W_xMθ(i_-X)θ3 or W5(i_-X)V6xOi5.

[0017] The layers comprising such oxides may be deposited by the known techniques such as chemical vapour deposition (starting from e.g. gaseous metallo organic compounds) or by physical vapour deposition (either within or without a reactive atmosphere, by evaporation or by sputtering with our without plasma activation by, for example, radiofrequent excitation) or by wet chemical methods (e.g. sol-gel).

[0018] Preferably a cathodic and anodic colouring electrochemical layer are combined (214, 214') separated by an ion conductor. Such an arrangement has the advantage that when cations drift away from the anodic colouring layer to the cathodic colouring layer under the influence of an applied electrical field, both the layers colour and the absorption of the two is multiplied. Upon reversing the polarity, the cations are expelled from the cathodic colouring material, making it bleach, while the anodic colouring layer absorbs the cations, thereby becoming more transparent.

[0019] Preferably WO3 - either pure or doped with vanadium or molybdenum - is used as an cathodic, electrochromic electrode and NiO - preferably doped with tungsten or vanadium - is used as an anodic, electrochromic counterelectrode.

[0020] The electrochromic electrode and counter electrode are separated by an ion conductor that must conduct ions well, but not electrons. The materials used for the ion conductor are therefore materials with a network lattice that allows the passage of the small cations. Examples are Siθ2, Zrθ2, MgF, AI2O3 or Ta2θs (although the latter is also listed as an electrochromic material, it can also be used for the ion conductor, when used with e.g. WO3 that has a lower bandgap). More preferred - in case lithium is the mobile ion species - is to use lithium containing salts such lithium phosphate, lithium phosphorus oxynitride, lithium niobate, lithium silicate, lithium aluminum silicate, lithium silicon oxynitride, and lithium silicon phosphorus oxynitride, lithium aluminum fluoride, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt oxide, lithium vanadium oxide, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt vanadium oxide, lithium titanium oxide, lithium silicon tin oxynitride or combinations thereof.

[0021] Alternatively - although less preferred - the ion conductor can be polymeric based. Usually the polymer comprises a network former and a lithium salt such as lithium perchlorate (LiCIO₄) or lithium tetrafluoroborate (LiBF₄). As network former a vast variety of polymers have proven useful ranging from polymers based on polyethylene (poly ethylene oxide (PEO)), polycarbonates (PC), acrylates (such as methyl methacrylate (PMMA), hydroxyethyl methacrylate (HEMA) and neopentyl glycol demethylacrylate (NPG)) or silanes to name just a few and without being exhaustive. Polymeric based ion conductors tend to be more vulnerable to higher temperatures.

[0022] Deposition of the ion conductor can be performed by the same conventional techniques as used for the deposition of the electrochromic layers such as - in case of inorganic ion conductor materials - chemical vapour deposition (starting from e.g. gaseous metallo organic compounds) or by physical vapour deposition (either within or without a reactive atmosphere, by evaporation or by sputtering with our without plasma activation by, for example, radiofrequent excitation) or - in case of organic as well as inorganic materials - by wet chemical methods (e.g. sol-gel). Alternatively - in the case of organic ion conductor materials - the ion conductor can be laminated between two half cells comprising a conductive transparent electrode and a first and second electrochromic material wherein the electrochromic materials are complementary and face each other. Again organic ion conductor materials are less preferred as they tend to suffer more of high temperatures.

[0023] Finally, depending on the type of materials used, it may be necessary to introduce an amount of the small mobile cation. Preferably this cation is lithium. It can be introduced by separate sputtering or evaporation, by non- stoichiometric sputtering of one of the compounds of the stack, by electrochemical deposition or by any other known method in the art.

[0024] The optical device further comprises an infrared absorbing material 222. Such a material transforms the incandescent infrared radiation into heat. Infrared radiation is broadly understood to be electromagnetic waves having a wavelength between 750 nm and 1 mm. For the purpose of this application the range will be limited from 750 to 2500 nm, a range that will be called Near Infrared (NIR). However, it is hereby acknowledged that also visible light will heat up the infrared absorbing material, but of course to a much lesser extent. It all depends on what one considers still acceptable for visual light transmission of the infrared absorbing material. The incandescent infrared radiation will mainly come from the sun or from heat radiating bodies in the surroundings. [0025] Finally the optical device must comprise a substrate 230 to carry both the infrared absorbing materials and the electrochromic stack. Such a substrate must provide sufficient mechanical strength to carry the electrochromic stack and the infrared absorbing material. One immediately thinks of a flat glass pane, but other materials such as polycarbonate (PC) or poly methyl methacramate (PMMA), poly ethylene terephthalate (PET) as well as different shapes (curved sideviews, goggles, sunroof) are also possible as a substrate.

[0026] A first infrared absorbing material that can be used are metal organic complexes that are mixed into the matrix. Examples are e.g. given in WO 2001 005894.

[0027] Preferably the infrared absorption is achieved by means of small particles or small crystals 222 that are dispersed in a matrix. The size of such particles is about 1 to 500 nm, or more preferably between 50 to 200 nanometer, or most preferred between 75 to 125 nm. In what follows particles or crystals of this size will be called nanoparticles. Macroscopic metallic particles are known to reflect infrared radiation (and much other electromagnetic waves) efficiently - and hence do not transform the radiation into heat - due to the presence of extended electron waves. By artificially limiting the size over which the electron waves can spread simply by reducing the size over which the metallic particle extends, one can reduce the reflectivity of the particle and obtain an absorptive selective optical filter when incorporating such particles in an insulating film (normally only noble metals can be used for this as they do not oxidize and remain in metallic form). As the surface plasmon frequency - corresponding to the electromagnetic wave frequency that shows resonant absorption with the free electrons - is proportional to the root of the free electron density, the absorption of noble metal particles is still very much in the visible part of the electromagnetic spectrum (giving the film a pronounced colour). By using nanoparticles with a reduced free electron density (notably semiconductors, or even dielectrics), the absorptive plasmon frequency can be shifted to the near infrared and the visual colour distortion can be reduced, resulting in an improved visual light transmission (VLT).

[0028] The matrix can be a polymeric film 224. The matrix can also be the substrate 230. The matrix can also be one or more layers of the electrochromic stack 210. The dispersion is such that the nanoparticles are physically separated from each other in the matrix. The carrying matrix is preferably loaded with about 0.5 to 10 grams of nanoparticles per square meter of surface area. More preferred is when it is loaded with 1 to 5 grams per square meter. Too high loading may lead to formation of conglomerates of nanoparticles, while a too low concentration will not exhibit any positive effects.

[0029] The beneficial effects of the invention are obtained when the infrared absorbing material is in thermal contact with the electrochromic stack. The incandescent infrared radiation is transferred into heat by the infrared absorbing material which is then used to warm the electrochromic stack. Nanoparticles are generally introduced into optical films to make them absorb specific spectral bands. The use of nanoparticles in order to heat- up films is not widely accepted. Next to that, it should not be forgotten that the visual parts of the spectrum can also contribute considerably to the heating of the optical device especially when the device is in the coloured state of the electrochromic device. A 'bootstrap' effect occurs when the film starts to colour, inducing an increased absorption which leads to increased heating and hence faster colouring.

[0030] Nowadays a whole family of novel nanoparticles - all based on ceramic type materials - have become available for infrared absorption purposes for window films: a. Nanoparticles of semiconductive oxides such as indium oxide (In2θ3), tin doped indium oxide (Sn:ln2θ3), tin oxide (Snθ2), Fluorine doped tin oxide (SnO2:F), antimony doped tin oxide (Sb:Snθ2), zinc oxide (ZnO), indium doped zinc oxide (ln:ZnO), aluminium doped zinc oxide (AkZnO), iron oxide (Fβ2θ3), rutheniumoxide (RUO2). b. Nanoparticles of semiconductive nitrides and suicides such as titaniumnitride (TiN), tantalum nitride (TasNs), titanium suicide (TiSi) or molybdenum suicide (Mθ2Si3) (as per US 6060154). c. There are the rare-earth borides as disclosed in US 6277187. The metal hexaboride nanoparticles have MeBθ as a general formula wherein 'Me' is a metal out of group of the lanthanides (i.e. elements with atomic number from 58 to 71) but excluding promethium (Pm, element 61), some transition group metals such as yttrium (Y), lanthanium (La), titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chrome (Cr), molybdenum (Mo), tungsten (W), some alkali earth metals as calcium (Ca) and strontium (Sr). LaBθ ('labsix' as it is called) is the most preferred in this respect (US 6277187). d. The nanoparticles of 'c' can be treated in a dispersion medium with an organic or inorganic alkoxide comprising the metal 'Me', or silicon or zirconium, or aluminium or titanium in the molecular structure to improve the binding with the polymeric film (US 6060154) e. Or the nanoparticles of 'c' can be treated with as surface treatment agent containing silicon that is selected from the group of silazane type or chlorosilane type treatment agents, or a treatment with a surface agent based on the alkoxides under 'd.' above to protect the surface of the nanoparticles against moistening (US 2005/0161642). f. Another useful class of nanoparticles are the tungsten oxides and composite tungsten oxides (US 2006/0008640). Particularly preferred are off-stoichiometric W_yO_z particles with 2<^z/_y<3. Also such particles wherein part or all of the tungsten atoms are replaced with any other element X that can be an alkali, alkaline earth, transition, or poor metal or non-metals out of the group of H, He, B, F, P, S, Se, Br, Te have advantageous properties. Most preferred is that X is out of the group IMB, IVB, VB or IVA out of the periodic table of elements. The stoichiometry is than X_xWyOz with 0.001 <^x/_y<1 and 2<^z/_y<3. Mixtures of several types of nanoparticles in the matrix are not excluded and in some cases an enhancing effect of combining two types has been observed (US 6277187). Also other nanoparticles can be added in order to correct the colour of the resulting films.

[0032] A first simple and therefore preferred embodiment is when the nanoparticles are dispersed in a polymeric film. The material of the polymeric film can be an inorganic or an organic binder or a resin. An example of an inorganic binder is a silicate binder. Organic binders can be based on polymers such as the families of the polyesters, polyacryls, polyvinyls or polyurethanes. An adhesive layer can also be considered as polymeric film. Indeed, nanoparticles let themselves easily mix into known adhesives such as poly vinyl acetate (PVA) or poly vinyl butyral (PVB) that are used for laminating film to glass or glass to glass.

[0033] One way of obtaining an intimate thermal contact between the infrared absorbing material and the electrochromic stack is through the polymeric film that on the one hand is in thermal contact with the infrared absorbing material and on the other hand with the substrate. The substrate is heated which on its turn heats the electrochromic stack. I.e. the substrate is then positioned between the polymeric film and the electrochromic stack. By preference the polymeric film is oriented towards the incandescent infrared radiation (see FIGURE 3a).

[0034] Another way of heating the electrochromic stack is to bring the polymeric film in direct contact with the electrochromic stack. Either the polymeric film is then situated between the electrochromic stack and the substrate or the electrochromic stack is situated between the polymeric film and the substrate. Those arrangements wherein the infrared radiation hits the polymer film first and then the electrochromic stack are most preferred as some of the layers in the electrochromic stack may reflect the infrared radiation. An example is shown in FIGURE 3b.

[0035] Another preferred embodiment of heating the electrochromic stack is to eliminate the polymer film and have the infrared absorbing material incorporated into the substrate as per FIGURE 3c. This can be done by using a glass-ceramic substrate. An example of an alkali aluminosilicate glass with dispersed 5 to 20 nm ZnO nanocrystals in it is described in Transparent glass-ceramics based on ZnO crystals' (L. R. Pinckney, Physics and Chemistry of Gasses: European Journal of Glass Science and Technology, Part B, Vol. 47, Nr. 2, april 2006) and also in US 2004/0142809. By adding the correct dopants, a high absorption in the infrared can be obtained. The substrate can also be an organic transparent material such as PC, PMMA, PET wherein nanoparticles are dispersed.

[0036] A further and most preferred embodiment is when nanoparticles are incorporated in one or more layers of the electrochromic stack (schematically represented in FIGURE 3 d). In this embodiment, the infrared absorbing material is in very intimate contact with the electrochromic stack. For example when the electrochromic electrode is deposited through a sol-gel process, the nanoparticles can be intermixed with the gel. The nanoparticles are in many cases compatibly with the electrochromic compound (note e.g. that W_yO_z was also mentioned as a possible infrared absorbent, as well as an electrochromic compound). Nanoparticles can also be dispersed in the ion conducting layer when that layer is polymer based, and the nanoparticles are mixed in before curing the layer. Nanoparticles can also be intermixed with the material of the counterelectrode. An example is W:NiO powder (a cathodic colouring counterelectrode material) that is mixed with nanoparticles of LaBθ in Poly(3,4-ethylenedioxythiophene) or PEDOT. The latter is a transparent conductor that can also be used as conductive layer. However, one must be careful that the infrared radiation can reach the nanoparticles i.e. the transparent conductive electrode directed towards the incandescent radiation must be infrared transmittent.

[0037] According a second aspect of the invention, a method to improve the switching speed of an electrochromic device is revealed by bringing a electrochromic stack into thermal contact with an infrared absorbing material, wherein the infrared absorbing material converts infrared radiation into heat that on its turn lowers the switching speed of the electrochromic stack Brief Description of Figures in the Drawings

[0038] FIGURE 1 (a) shows the temporal behaviour of the transmission of a generic electrochromic device at a first temperature tempi and (b) at a second temperature temp2.

[0039] FIGURE 2 shows the different layers in the optical device.

[0040] FIGURE 3 (a), (b), (c) and (d) show different arrangements how the films can be stacked.

[0041] FIGURE 4 shows the influence of the temperature on the switching time of the electrochromic stack.

Mode(s) for Carrying Out the Invention

[0042] FIGURE 1 shows the time dependency of a generic electrochromic device. The system will overtime reach an asymptotic transmission T_maχ when kept in the bleaching polarity and Tmm when kept under the darkening polarity. In principle the values of T_maχ and Tmm will not depend on temperature. However, the time that is needed to reach these values will greatly vary with temperature. FIGURE 1a shows the time dependency of the transmission of the generic device at a first temperature tempi, while FIGURE 1 b shows the same at a second temperature temp2 that is significantly lower than tempi. 102, 102' shows the decay upon darkening the device, 104, 104' upon reversing the polarity. In order to quantify these switching rates one can define a decay time tio^decay as that time wherein the transmission equals T_max/10 when starting from the fully bleached state. Mutatis mutandis one can define the time tio^rιse as that time wherein the transmission rises from Tmm to Tmm-10.

[0043] FIGURE 4 shows the temperature dependence of the decay time tio^decay out of the Tmax fully bleached state wherein in abscissa the temperature in °C is represented and in ordinate the tio^decay time in seconds. The curve is based on measurements performed on an insulated glass unit (IGU) with an electrochromic layered stack situated on an inside surface of one of the glass panes. The stack consisted of five layers: a fluorinated indium tin oxide (Snθ2:F) as a first transparent electroconductive layer, sputter deposited tungsten oxide (WO3) as an electrochromic layer, a silicate based ion conductor, a tungsten doped nickeloxide (W:NiO) layer as a counterelectrode and a tin doped indium oxide (ITO) as second transparent conductive layer. When the temperature is lower than -10°C, it takes more than 2000 seconds (more than half an hour) to obtain a contrast ratio of 10 from the original transmission. Due to the Arrhenius type of behaviour, this time is extremely temperature sensitive. The following time dependency could be derived for temperatures below 40°C (above that temperature, other delay factors come into play):

t₁₀ ^decay ₌ 7.7·10-¹⁰·exp(7531/T)

wherein T represents the absolute temperature.

[0044] When now an infrared absorbing film such as Ultra Performance 75 available from Bekaert Speciality Films (UP75) is laminated to the glass substrate an influence on the temperature of the optical device can be expected. UP75 is a multilayer construction containing an IR absorbing coating filled with ATO and ITO nanoparticles. UP75 exhibits a visible light transmission of 75% and a total solar energy absorptance of 46%.

[0045] The influence of the film has been calculated for the case wherein the UP75 film is laminated to glass pane at the side opposite to that of the electrochromic stack (the configuration of FIGURE 3a). This calculation has been done with the Windowδ programme that is available from the Lawrence Berkeley National Laboratory. The software works according the principles described in the ISO 9050 and ISO 15099 international standards. The input for it was based on the actual transmission, absorption and reflection date of the various layers. The results are summarised in TABLE 1.

[0046] TABLE 1 shows for three outdoor temperatures -10, 0 and +10°C and four levels of total solar irradiance: 300, 500, 750 and 1000 W/m² the influence on the temperature of the optical device (see PART I of TABLE 1). The level of incandescent infrared radiation very much depends on the weather, the incident angle of the sun on the optical device, the wind conditions and so on, but due to the infrared absorbing material any condition will help to reduce the switching times. At the most optimal conditions, the solar spectrum radiates ab. 1000 W/m² (Air mass 1.5 solar radiation (AM 1.5)).

[0047] Four different starting situations (PART Il of the table) must be considered: The optical device without an infrared absorbing layer

A. The electrochromic device (ECD) is in the bleached state at the beginning of the swithching

B. The electrochromic device is in the darkened state at the start The optical device with an infrared absorbing layer attached to it

C. The optical device is in the bleached state at the start

D. The optical device is initially in the darkened state

The initial state of the electrochromic layer stack is of course also very much important: when initially the stack is in the darkened state, it will absorb much more incandescent infrared radiation than when being in the clear state. The indoor temperature (in the room, the car the bus,...) was set at 5 degrees higher than the outdoor temperature.

[0048] For cases 'A' and 'C no account has been taken of the effect that when the electrochromic device starts from the clear state, the increased absorption by the infrared absorbing material will lead to faster darkening, that on itself will lead to an increased absorption of infrared and visual radiation by the electrochromic layer. However, this secondary effect will only enhance the switching speed, so the estimated times will be better than in table 1 (bootstrap effect).

[0049] PART Il of TABLE 1 shows the calculated temperatures reached by the optical device under the various conditions when a steady state temperature has been reached. The influence of the infrared absorbing material is of course most prominent when the optical device starts from a clear state, as in the darkened state the addition absorption contribution of the infrared absorbing material is small, (see Δ(T))

[0050] PART III of TABLE 1 shows the influence of the addition of the infrared absorbing material on the switching time of the device. Even at the worst conditions (-10°C outside temperature and only 300 W/m² solar irradiance) the presence of the infrared material already gives an improvement of 21 % in switching time: from 1103 seconds to 870 seconds. The percentage improvement is higher when the irradiance improves. When the outside temperature raises, the percentage improvement becomes generally less.

[0051] When the optical device starts from the darkened state, the improvement is much less, but then the switching time is already low due to the infrared and visible light absorption of the electrochromic device.

[0052] The other configurations (FIGURE 3b, 3c, 3d) will yield equal or better results for like conditions as the thermal contact is in those cases generally better.

[0053] Tabel 1

Claims

1. An optical device comprising an all solid state electrochromic layered stack, a substrate and an infrared absorbing material, said electrochromic layered stack for electrically switching said optical device between a transparent and a dark state, said infrared absorbing material being in thermal contact with said electrochromic layered stack, said substrate for carrying said layered stack and said infrared absorbing material characterised in that, said infrared absorbing material comprises nanoparticles for converting said infrared radiation into heat so as to reduce the switching time of said electrochromic layered stack through the temperature increase caused by the heat generated by the infrared absorbing material.

2. The optical device of claim 1 wherein said infrared absorbing material comprises nanoparticles of the ceramic type.

3. The optical device according to claim 2 wherein said nanoparticles are selected from the group consisting of indium oxide, tin oxide, zinc oxide, aluminium zinc oxide, tungsten oxide, composite tungsten oxide, indium tin oxide, antimony tin oxide, rare earth metal hexaborides, transition metal hexaborides or combinations thereof.

4. The optical device according to any one of claim 2 to 3 wherein said nanoparticles have a diameter ranging between 1 and 500 nm.

5. The optical device according to any one of claim 2 to 4 wherein said nanoparticles are dispersed in a concentration of 0.5 to 10 grams per square meter of surface area of the optical device.

6. The optical device according to any one claim 2 to 5 wherein infrared absorbing material is incorporated in a polymeric film that is carried by said substrate.

7. The optical device according to claim 6 wherein said thermal contact between said electrochromic layered stack and said infrared absorbing material is through said substrate and said polymeric film.

8. The optical device according to claim 6 wherein said thermal contact between said electrochromic layered stack and said infrared absorbing material is through said polymeric film.

9. The optical device according to any one claim 2 to 5 wherein infrared absorbing material is incorporated in said substrate such that said thermal contact between said infrared absorbing material and said electrochromic layered stack is through said substrate.

10. The optical device according to any one claim 2 to 5 wherein infrared absorbing material is incorporated in said electrochromic layered stack.

11. A method to reduce the switching time of an electrochromic device by bringing said electrochromic device in thermal contact with an infrared absorbing material that converts infrared radiation into heat.