WO1999008153A1

WO1999008153A1 - Electrochromic devices

Info

Publication number: WO1999008153A1
Application number: PCT/GB1998/002380
Authority: WO
Inventors: John Robert Siddle
Original assignee: Pilkington Plc
Priority date: 1997-08-07
Filing date: 1998-08-06
Publication date: 1999-02-18
Also published as: GB9716619D0

Abstract

An electrochromic device has successive layers of electrochromic, electrolyte and counter-electrode materials (16, 6, 18). The electrochromic material (16) comprises an oxide of a mixture including tungsten and zirconium wherein the proportion of zirconium is about 15 atomic percent. Increasing the proportion of zirconium in the mixture has the effect of decreasing the electrochromic efficiency of the device, and the proportion of zirconium is preferably increased to a level at which the effect on the electrochromic efficiency results in the device being substantially colour neutral.

Description

ELECTROCHROMIC DEVICES

The invention relates to electrochromic devices as used, for example, in so-called variable transmission windows or variable reflection mirrors, and in particular to an electrochromic material for such devices.

Electrochromic devices are known to have successive layers of electrochromic, electrolyte and counter-electrode materials. The device may have first and second laminar substrates each covered on one side with an electrically conducting film, the layers interposed between the two substrates and the film covered sides innermost. Alternatively, the device may have one laminar substrate covered on one side with an electrically conducting film with the layers being carried on this film covered side and a further electrically conducting film applied over the exposed layer. The most common substrate material is glass, but plastics materials, like acrylic, may also be used.

By way of example, the electrically conductive films may be indium doped tin oxide, the electrochromic material may be tungsten trioxide, the counter-electrode material may be cerium titanium oxide and the electrolyte material may be a suitable ion conducting polymer to which lithium perchlorate has been added.

A tungsten trioxide/cerium titanium oxide device can be changed between bleached and coloured states by altering the applied electrical potential, that is, the potential applied via the electrically conductive films (acting as electrodes) across the electrochromic, electrolyte and counter-electrode layers. The polarity of the potential dictates the direction of transfer of cations (provided by the lithium perchlorate) through the electrolyte material, between the electrochromic and the counter-electrode materials. The cation transfer is reversible. When reduced, or in other words with lithium cations inserted, tungsten trioxide is coloured blue, whereas, when oxidised (when cations are de-inserted), it is virtually colourless. Conversely, cerium titanium is chosen as a counter-electrode material because it is virtually colourless when either reduced or oxidised, or at least any colouring on reduction is indiscernible.

Other electrochromic/counter-electrode material combinations may work in reverse, with the electrochromic layer colouring on oxidation, and different combinations can produce different colours and degrees of colour change. There are also devices wherein a single layer acts as both the counter-electrode and the electrically conducting film. Furthermore, there are devices, such as those available from the Gentex company, which have a single material which functions as the electrochromic, counter-electrode and electrolyte layers.

The changeability of an electrochromic device lends itself to use in, amongst other applications, a window where variable transmission characteristics are required. These are seen as being of particular use in integrated energy management systems for buildings; one idea being to modulate the solar gain of the building to maximise energy benefits. For instance, by colouring the window during the hottest part of a summer day, the amount of solar radiation entering a building can be minimised, and on dull winter days the window can be bleached so as to make best use of the available natural light.

There is also a requirement for windows which have variable transmission characteristics and colour neutrality, that is to say, they are virtually colourless whether "coloured" ( in a state of relatively low solar transmission) or "bleached" (in a state of relatively high solar transmission). The demand for such products is coming from, for instance, architects who consider it undesirable to have windows repeatedly cycling from one colour to another and possible colour variations across a building facade as a result of adjacent windows colouring according to different sensed conditions.

JP-A-59121317 discloses an electrochromic device in which zirconium oxide is added to the tungsten trioxide electrochromic layer so as to bring about a shift in the absorption spectrum of the device and to increase visible optical density.

The invention provides an electrochromic device comprising successive layers of electrochromic, electrolyte and counter-electrode materials, wherein the electrochromic material comprises an oxide of a mixture including tungsten and zirconium. Preferably the proportion of zirconium in the oxide is about 15 atomic percent.

Contrary to suggestions in the prior art, it has been found that the addition of zirconium to tungsten trioxide electrochromic layers decreases visible optical density and thereby provides the opportunity of having a "colour neutral" electrochromic device with variable solar transmission.

The invention also provides the use of an oxide of a mixture including tungsten and zirconium as the electrochromic material in an electrochromic device, wherein the proportion of zirconium in the oxide is about 15 atomic percent. The invention further provides an electrochromic device comprising successive layers of electrochromic, electrolyte and counter-electrode materials, wherein the electrochromic material comprises an oxide of a mixture including tungsten and zirconium and wherein increasing the proportion of zirconium in the oxide has the effect of decreasing the electrochromic efficiency of the device.

The invention will now be described, by way of example, with reference to the following drawings in which:

Figure 1 is a perspective view of a glass substrate coated with layers of indium doped tin oxide and tungsten zirconium oxide;

Figure 2 is a graph showing the transmission properties of substrates of the type shown in figure 1 with different compositions of tungsten zirconium oxide layer;

Figure 3 is a table of comparative properties for substrates of the type shown in figure 1 with different compositions of tungsten zirconium oxide layer; and

Figures 4 and 5 are an exploded view and a partial cross-sectional view respectively of an electrochromic device according to the invention.

With reference to figure 1, five substrates 100, each 50 x 50 mm, were coated by reactive dc magnetron sputtering on one major face with an electrically conductive 3000 nm thick layer 200 (exaggerated in the figure for ease of understanding) of indium doped tin oxide (ITO). An electrochromic layer of tungsten zirconium oxide 300 was applied to each substrate 100, over the top of the respective ITO layer 200, also by reactive dc magnetron sputtering. Each substrate 100 was sputtered with a different composition of tungsten zirconium oxide. The atomic weight percent of the zirconium in each was altered by varying the relative proportions of tungsten and zirconium used as the targets in the sputtering apparatus. In practice, the proportion of zirconium was raised by increasing the number of small squares of zirconium (each 400 mm ) that were placed on the tungsten target (circular, 150 mm in diameter). Using Auger and ED AX spectroscopy, the actual ratio of zirconium to tungsten in the layers 300 applied to each of the substrates 10 was estimated to be 1.7, 9.8, 10.9, 12.2, 15 atomic percent zirconium respectively. Each of the layers was in the region of 250 nm thick. As a control, a sixth substrate 100 with an ITO layer 200 had a layer of tungsten trioxide applied to it.

The electrochromic behaviour of the tungsten zirconium oxide layer 300 on each substrate 100 was measured using an electrochemical cell (not shown). Each substrate formed one of the electrodes in the cell which had a lithium perchlorate/propylene carbonate electrolyte and lithium counter and reference electrodes. Each tungsten zirconium oxide layer 300 was subjected to cyclic voltammetry and its current behaviour was recorded. In addition, spectral measurements were made using a Hitachi U4000 spectrophotometer.

For each tungsten zirconium oxide layer 300, electrochromic modulation was measured across the visible/near infra-red region for four of the substrates (not the 9.8 atomic % Zr) with lOmCcm^" of charge (lithium) inserted into the tungsten zirconium oxide layer. The same measurements were also taken for the tungsten trioxide coated substrate with the same charge insertion density and in the bleached state. The results are shown graphically in figure 2, and it can be seen that the effect of the zirconium is to decrease the electrochromic efficiency (effectively, the difference in transmission between the uncharged and charged states) of the tungsten trioxide across the full width of the wavelengths measured, eventually ending up with a relatively flat transmission characteristic at high zirconium content.

Also measured, in a standard manner using Window 4.1 software, were the integrated visible and solar transmission properties, T_vis and T_sol, and the L, a* and b* colour coordinates. These measurements were taken for all of the substrates including the one with the tungsten trioxide coating. Figure 3 lists the results of these measurements, and it can be seen that the tungsten zirconium oxide layer containing 15 atomic percent zirconium shows only a slight variation in its colour co-ordinates a* and b* between the "as produced" state and levels of charge insertion, namely 20 mC/cm^" , which bring about significant changes in the visible and luminous transmission properties of the layer.

An electrochromic device was made as shown in figures 4 and 5. The device indicated generally at 1 had first and second sheets of glass 2,4 each 100 mm x 100 mm, separated by a 1 mm thick translucent interlayer of polymer electrolyte 6, the composition of which is disclosed in PCT/EP95/01861. Each of the sheets 2,4 was coated by reactive dc magnetron sputtering on its inner face 8,10 with an electrically conductive film 12,14 of ITO. Applied over the top of the ITO film 12, also by reactive dc magnetron sputtering, was an electrochromic layer 16 of tungsten zirconium oxide with 15 atomic percent zirconium. Applied over the top of the ITO film 14, again by reactive dc magnetron sputtering, was a counter-electrode layer 18 of cerium titanium oxide. Also applied over each ITO film 12,14, along one vertical edge, was an elongate electrical contact, commonly known as a bus bar 20,22. These were in the form of copper strips stuck on to the ITO films 12, 14 with conductive adhesive. Power supply wires 24,26 were connected to each of the bus bars 20,22.

Each device 1 was put together as a cast-in-place laminate, using a known technique. First of all the two sheets 2,4 were formed into a cell by bonding them together (the electrochromic and counter-electrode layers 16,18 innermost) with double side acrylic tape (not shown) between the margins of the two sheets 2,4. Liquid electrolyte, previously degassed by stirring under vacuum, was poured into the cell. The electrolyte interlayer 6 was then cured and the cell was sealed with an epoxy resin (not shown).

The device was preconditioned in a known manner by repeatedly cycling it between positive and negative applied voltage limits. The voltage was applied across the electrochromic, electrolyte and counter-electrode layers 16,6,18 through the electrically conductive films 12,14. Applying a negative voltage to the tungsten zirconium oxide layer 16, so as to generate a current flowing in a first direction, caused lithium ions from the electrolyte layer 6 to be inserted into the tungsten zirconium oxide layer 16, which produced a slight grey colouration. Applying a positive voltage had the opposite effect, generating a current flowing in a second, opposite, direction, and the device is bleached towards its neutral colour state.

Claims

1. An electrochromic device comprising successive layers of electrochromic, electrolyte and counter-electrode materials, wherein the electrochromic material comprises an oxide of a mixture including tungsten and zirconium.

2. An electrochromic device according to claim 1 wherein the proportion of zirconium in the oxide is about 15 atomic percent.

3. The use of an oxide of a mixture including tungsten and zirconium as the electrochromic material in an electrochromic device, wherein the proportion of zirconium in the oxide is about 15 atomic percent.

4. An electrochromic device comprising successive layers of electrochromic, electrolyte and counter-electrode materials, wherein the electrochromic material comprises an oxide of a mixture including tungsten and zirconium and wherein increasing the proportion of zirconium in the oxide has the effect of decreasing the electrochromic efficiency of the device.

5. An electrochromic device according to claim 4 wherein the proportion of zirconium is increased to a level at which the effect on the electrochromic efficiency results in the device being substantially colour neutral.

6. An electrochromic device according to claim 5 wherein the proportion of zirconium is increased up to about 15 atomic percent.