Abstract:
An electrochromic device ( 10 ) for windows, displays, rear view mirrors, eye wear and other applications includes three layers ( 2 ), ( 4 ) and ( 5 ) of electrochromic materials and an electrolyte, with the transmittance of the layers ( 2 ) and ( 5 ) being variable by an applied electrical field, and protective layer ( 4 ) being transparent and not contributing to the coloration/bleaching of device. The protective layer ( 4 ) is made of the same electrochromic material as the electrochromic layer on the opposing side of the electrolyte.

Description:
TECHNICAL FIELD 
     The present invention relates to a laminated electrochromic device, a so-called EC device and, in particular, to such a device having a protective layer positioned on one side of the electrolyte layer. 
     STATE OF THE ART 
     Electrochromic devices can be used for modulating transmittance, reflectance, scattering, and thermal emittance by means of an applied electrical field. A standard electrochromic device employs a cathodically coloring electrochromic film (such as W oxide) and an anodically coloring (such as Ni oxide) or transparent (such as Ce oxide) counter electrode brought in contact by a solid electrolyte. Examples of such devices are displays and anti-dazzling rear-view mirrors in cars. 
     One of the most difficult problems with practical electrochromic devices is their limited long-term durability due to chemical incompatibility between the electrolyte layer and the adjoining electrochromic film and counter electrode. Thus WO 3 —the prime candidate for the electrochromic films—is stable in a moderately acidic environment whereas it is rapidly dissolved in a basic electrolyte. On the other hand, a major contender for being the counter electrode, NiO x H y , is stable in a basic environment but unstable in an acidic one. This often causes difficulties in production of such devices since the life time is not sufficient. 
     At present there is no single generally accepted concept of the best way of producing EC devices. 
     A known possible solution to the stability problem is to invoke an additional protective layer, by means of which damage of the layers can be avoided, thereby increasing the device life time. 
     It is known from U.S. Pat. No. 4,120,568 to provide a “substantially insulating dielectric layer” between the electrochromic and the electrolyte layer. The materials employed for this purpose are, for instance, SnO, SiO, SiO:Au, TiO and CrN. 
     It is also known to use non-aggressive electrolytes to obtain sufficient life times without using any protective layers. 
     There are, however, many chemically aggressive electrolytes of interest for use in EC devices due to their good electrical and optical properties, which electrolytes cannot be used in conventional EC devices. 
     DISCLOSURE OF THE INVENTION 
     The object of the present invention is to provide a solution to the chemical incompatibility problem in EC devices by means of a protective layer of well-suited material positioned on one side of the electrolyte layer, thus increasing the life time of devices. 
     According to the invention, an EC device comprises three layers of electrochromic materials, of which one acts as a protective layer. Device thereby produced use the same material for both the electrochromic or counter electrode layer and the protective layer, wherein the protective layer remains transparent and does not affect the coloration and bleaching of the device. The optical inactivity of the protective layer relies on the fundamental energetics of the multi-layer structure and is not a trivial consequence of having a thin-and hence only weakly coloring-layer. 
     The device may be operated in a light transmitting as well as a light reflecting mode. 
     According to a preferred embodiment of a device according to the invention, WO 3  and NiO x H y  are employed as the protective layers in devices with acidic and basic electrolyte, respectively. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     In order to make the present invention easy to understand and produce, it will be described with reference to the appended drawings, in which: 
     FIG.  1 ( a ) shows an embodiment of an EC device according to the invention, wherein the electrolyte is acidic. 
     FIG.  1 ( b ) shows an embodiment of an EC device according to the invention, wherein the electrolyte is basic. 
     FIG. 2 shows schematic electron density of states of the layers in an electrochromic device shown in FIG.  1 ( a ). 
     FIGS. 3 a-b  show AFM images of WO 3  films typically used for protective (a) and electrochromic (b) layers. Both films are deposited on ITO. 
     FIG. 4 shows voltammograms of WO 3  films typically used for protective (dashed line) and electrochromic (solid line) layers. Both films deposited on ITO. Data taken in a Li:PC electrolyte. 
     FIG. 5 shows voltammograms for a WO 3 /zirconium phosphate based electrolyte/WO 3 /NiO x   H   y  device, recorded after lamination (solid line) and after four months of storage (dashed line). 
     FIG. 6 shows optical transmittance in bleached and colored states for a WO 3 /zirconium phosphate based electrolyte/WO 3 /NiO x H y  device. 
     FIG. 7 shows voltammogram for a WO 3 /NiO x   H   y /aluminum oxide based electrolyte/NiO x   H   y  device. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG.  1 ( a ) shows a preferred embodiment of an electrochromic device  10  according to the invention. The electrochromic device  10  comprises an electron conducting layer  1 , an electrochromic layer  2  of cathodic electrochromic material, a layer  3  of acidic electrolyte, a protective layer  4  of cathodic electrochromic material, a counter electrode layer  5  of anodic electrochromic material, and an electron conducting layer  6 . 
     Typically, layers  2  and  4  are made of the same material, such as WO 3 , MoO 3 , TiO 2  or their mixtures. Layer  3  is, for example, a zirconium phosphate—based proton conductor, but not limited thereto. Any other suitable proton conductor can be employed. Layer  5  is made of an anodic electrochromic material, such as NiO x , NiO x H y , IrO 2 , CoO 2 , FeO 2 , MnO 2 , Cr 2 O 3 , or their mixtures. Layer  1  and  6  are typically ITO (In 2 O 3 :Sn), but not limited thereto. 
     Layer  1  is deposited on a supporting layer, such as a glass layer,  7 . Layer  6  is deposited on a supporting layer, such as a glass layer,  8 . 
     The thickness of the layers  2  and  5  are such that the required optical modulation of the device is obtained, typically the thickness is between 0.1 and 1 micrometer. The thickness of layer  4  is typically above 0.1 micrometer, this layer is always transparent and does not contribute to the optical coloration. 
     FIG.  1 ( b ) shows another preferred embodiment of an electrochromic device according to the invention, comprising basic electrolyte. Layer  1  is an electron conducting layer. Counter electrode layer  2  is made of anodic electrochromic material, such as NiO x , NiO x H y , IrO 2 , CoO 2 , FeO 2 , MnO 2 , Cr 2 O 3 , or their mixture. Layer  3  is basic electrolyte. Protective layer  4  is of anodic electrochromic material. Electrochromic layer  5  is of cathodic electrochromic material, such as WO 3 , MoO 3 , TiO 2  or their mixture. Layer  6  is an electron conductor. Layers  1  and  6  are typically ITO (In 2 O 3 :Sn), but not limited thereto. 
     The thickness of the layers  2  and  5  are such that the required optical modulation of the device is obtained, typically between 0.1 and 1 micrometer. The thickness of layer  4  is above 0.1 micrometer; this layer is always transparent and does not contribute to the optical coloration. 
     Optical inactivity of the protective layer in such device can be explained by arguments similar to the ones used below for device with acidic electrolyte. 
     FIG. 2 gives a schematic presentation of electron density of states for the multilayer structure shown in FIG.  1 ( a ), with WO 3  used for the electrochromic and protective layers, and NiO x H y  used for the counter electrode. In the bleached state, the Fermi energy E F bl  for NiO x H y  is expected to lie slightly above the valence band edge. For W oxide, E F bl  lies in the band gap separating a valence band dominated by O2p states from a conduction band dominated by W5d states. 
     When a coloration voltage U is applied between the NiO x H and WO 3  films, their Fermi levels E F col  are separated. Electrons enter the W5d states where they cause polaron absorption provided that the W oxide film is heavily disordered. A corresponding charge is subtracted from the top of the valence band of the Ni oxide film hereby rendering this material absorbing by a mechanism that appears to be non-polaronic but is not known in detail. Thus the initially transparent device turns absorbing by a combination of anodic electrochromism in NiO x H y  and cathodic electrochromism in WO 3 . The applied coloration potential does not induce any changes in the band structure of the WO 3  film on the NiO x H y  side, which explains the optical inactivity of the protective layer in an EC device according to the invention. 
     The relative energies of the density of states shown in FIG. 2 can be inferred by additional arguments. Thus if an absorbing device combining NiO x H y  and WO 3  is shorted it turns transparent, implying that—as expected—the bottom of the conduction band of WO 3  lies at a higher energy than the top of the valence band of NiO x H y . 
     The WO 3  films used for protective layers have been characterized in terms of AFM and cyclic voltammetry in 1M Li:PC, in both cases in comparison to typical WO 3  films used for EC layers. 
     The AFM images in FIGS. 3 a  and  3   b  show that the protective layer is more compact than the electrochromic one, the corresponding RMS roughness values are 1.5 and 4, respectively. 
     Voltammograms taken in 1M Li:PC for typical WO 3  films used for electrochromic and protective layers—both deposited on ITO—are shown in FIG.  4 . Considerably lower current at the same potential scan speed can be assigned to slower ion transport through the protective layer film compared to the electrochromic WO 3  film, which in turn is due to the higher density of the protective layer film. 
     FIG. 5 shows voltammograms for a WO 3 /zirconium posphate based electrolyte/WO 3 /NiO x H y  device, recorded after lamination and after 4 months of storage. In a similar device without protective layer, the lifetime of the NiO x H y  film is of the order of a few seconds. There are no features in voltammograms that can be clearly assigned to any effects introduced by the presence of the protective layer. 
     FIG. 6 shows optical transmittance in bleached and colored states for a WO 3 /zirconium posphate based electrolyte/WO 3 /NiO x H y  device. Coloration/bleaching was accomplished at 1.9/−1V for three minutes. It can be seen that there is no absorption introduced by the WO 3  protective layer in the bleached state of device. 
     FIG. 7 shows a voltammogram for a WO 3 /NiO x H y /aluminium oxide based electrolyte/NiO x H y  device. The electrolyte in this device was not transparent, making the transmittance measurements impossible. Visual observations showed, however, that the coloration/bleaching of device on either side was not affected by the NiO x H y  protective layer. Again, voltammogram of such device does not contain any features introduced by the protective layer. 
     Deposition techniques for electrochromic and counter electrode films and device lamination are described in A. Azens, L. Kullman, G. Vaivars, H. Nordborg, C. G. Granqvist, Solid State Ionics, in press. The protective layers were deposited by means of DC magnetron sputtering, but are not limited thereto. The sputter-deposited WO 3  films used for protective layers were deposited at ˜15 times lower sputter-gas pressure than the electrochromic ones to obtain layers with higher density. A good device performance was obtained with 0.3-0.5 micrometer thick protective layer films. 
     Contact means, such as leads, are connected to the layers  1  and  6 . 
     Suitable electrolytes can be selected from the group of proton conductors, such as zirconium posphate—based composites.