GB2324617A - Electrochromic devices - Google Patents

Abstract

An electrochromic device has successive layers of electrochromic (16), electrolyte (6) and counter-electrode (18) materials and the electrochromic material layer oxide is at least 600 nm thick and preferably comprises tungsten oxide. The device may be switched between coloured and bleached states by reversibly inserting mobile ions contained in electrolyte layer (6) into electrochromic layer (16). The thick electrochromic layer (16) enables substantially greater increases in solar electrochromic efficiency of the device compared to luminous electrochromic efficiency of the device.

Description

ELECTROCHROMIC DEVICES Description The invention relates to electrochromic devices as used, for example, in so-called variable transmission windows or variable reflection mirrors, and in particular to electrochromic layers 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 carried on the 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 fluorine 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.

Past electrochromic device development has tended to concentrate on the attainment of a substantial optical density change, that is, as wide a variation between bleached and coloured states as possible. However, there now appears to be scope for a device which can exhibit high differential absorption at infra-red wavelengths with only low optical density change.

Conventionally, electrochromic layers have been less than 300 nm for reasons of cost and because of production considerations. 300 nm, and thinner, electrochromic layers have been found to function satisfactorily and so there has not been any motivation for making the layers thicker than is necessary and thereby incurring increased cost and production time and effort.

The invention provides an electrochromic device comprising successive electrochromic, electrolyte and counter-electrode layers, wherein the electrochromic layer is at least 600 nm thick.

Preferably, the electrochromic layer is 1650 nm thick. Further preferably the electrochromic layer is tungsten trioxide. Alternatively, the electrochromic layer could be any electrochromic oxide such as oxides of niobium, molybdenum, titanium or cobalt.

It has been found that thicker than conventional tungsten trioxide electrochromic layers, in particular, offer reasonable increases in near infra-red absorption with a minimum decrease in visible transmission, such that a device can be used to modulate infra-red without significantly affecting visibility. This effect is observable in tungsten trioxide layers of greater than 300 nm thick but layers of less than 600 nm thick are not thought to offer a practically usable effect.

The invention further provides an electrochromic device comprising successive electrochromic, electrolyte and counter-electrode layers wherein the electrolyte layer contains mobile ions and the device can be varied between coloured and bleached states by reversibly inserting ions into the electrochromic layer and wherein the electrochromic layer is of a sufficient thickness that the solar electrochromic efficiency of the device is substantially greater than its luminous electrochromic efficiency at certain charge densities of ion insertion into the electrochromic layer.

The invention also provides an electrochromic device comprising successive electrochromic, electrolyte and counter-electrode layers, wherein the electrolyte layer contains mobile ions and the device can be varied between coloured and bleached states by reversibly inserting ions into the electrochromic layer and wherein as the mole fraction of lithium ions inserted into the electrochromic layer decreases, the difference between the solar electrochromic efficiency and the luminous electrochromic efficiency of the device continues to increase.

The invention will now be described, by way of example, with reference to the following drawings in which: Figure 1 is a table of measured values to show the comparative properties of a conventional thin tungsten trioxide layers and a thick tungsten trioxide layer for use in an electrochromic device according to the invention; Figures 2 and 3 are plots of electrochromic efficiency against wavelength for thin and thick tungsten trioxide layers respectively; Figures 4 and 5 are plots of electrochromic efficiency against mole fraction of inserted charge for thin and thick tungsten trioxide layers respectively; Figure 6 is an exploded view of an electrochromic device according to the invention; and Figure 7 is a partial transverse cross section of the device shown in Figure 6.

A number of sample-size panes of glass 25 x 50 x 3 mm (not shown) were each coated on one of their major surfaces with a layer of tungsten trioxide (WO3). Each pane was coated with a W03 layer of different thickness. The panes were previously coated on-line on the same major surface (during the glass production process) with an electrically conducting layer of fluorine doped tin oxide (SnO2:F). Reactive dc magnetron sputtering was used to set down the W03 coating on top of the SnO2.

For each W03 coated pane, transmission characteristics were measured in the range 240 - 2100 nm at various lithium insertion quantities corresponding to charge insertion densities of 0-5mC/cm~2. The lithium insertion was achieved using a standard electrochemical cell arrangement (not shown) with an electrolyte of propolene carbonate containing lithium and lithium working and counter electrodes, and by controlling the applied current. Luminous (Tium) and solar transmission (Tso,) values were calculated according to the Japanese Industrial Standard R3106-1985 for each sample at each charge insertion density using a Hitachi spectrophotometer. Results of these calculations for four of the samples measured (with W03 coatings of 90, 260, 440 and 1650 nm thick) are shown in Figure 1. It can be seen that the thicker the W03 layer, the more appreciable the differential solar absorption exhibited.

In an attempt to gain a better understanding as to why thick (1650 nm) and thin (260 nm) W03 layers possess such different characteristics, further measurements were taken over a wider range of charge densities of lithium insertion to enable comparisons to be made of electrochromic efficiencies as a function of wavelength and lithium insertion. Figures 2 and 3 are graphical representations of the results obtained for the thin and thick layers respectively.

It can be seen that with the thick layer there is a relatively low electrochromic efficiency in the visible region (400 - 750 nm) and high efficiency in the near infra-red (750 - 2100 mn) whereas with a thin layer the converse is true.

It can also be seen that at low charge densities of lithium insertion, the thick layer presents an absorption band in the infra-red region which is relatively narrow with a high peak height. On the other hand, the equivalent characteristic in the thin layer is broad and relatively flat.

An alternative way of viewing the electrochromic efficiencies is to plot them with respect to the mole fraction, Lix, inserted. Figures 4 and 5 are plots of integrated luminous and solar efficiencies for the thick and thin layers respectively as a function of the X value in LiXWO3 using the formula x=lgEl Fpt where q is the charge inserted, M is the molecular weight of tungsten trioxide, F is Faraday's constant, p is the density and t is the thickness of the layer.

It can be seen that at high Lix values both layers have similar efficiencies, but at low Lix values the solar efficiency of the thick film continues to increase.

Having established the characteristics of a thick W03 layer, an electrochromic device was made up with such a layer. With reference to Figures 6 and 7, the device indicated generally at 1 has first and second sheets of glass 2,4 each 100 mmx 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 is coated on its inner face 8,10 with an electrically conductive film 12,14 of SnO2:F. The SnO2 is applied on-line, during the glass production process. Applied over the top of the SnO2 film 12 is an electrochromic layer 16 of W03, and applied over the top of the SnO2 film 14 is a cerium titanium counter-electrode layer 18. Both the W03 and the cerium titanium layers 16,18 are applied by reactive dc magnetron sputtering. The W03 layer is 1650 nm thick. Also applied over each SnO2 film 12,14, along one vertical edge, is an elongate electrical contact, commonly known as a bus bar 20,22. These are in the form of copper strips stuck on to the SnO2 films 12,14 with conductive adhesive. Power supply wires 24,26 are connected to each of the bus bars 20,22.

The device 1 is put together as a cast-in-place laminate, using a known technique. First of all the two sheets 2,4 are formed into a cell by bonding them together (the electrochromic and counter-electrode layers 16,18 innermost) with double sided acrylic tape (not shown) between the margins of the two sheets 2,4. Liquid electrolyte, previously degassed by stirring under vacuum, is poured into the cell. The electrolyte interlayer 6 is then cured and the cell is sealed with an epoxy resin (not shown). The device 1 is preconditioned by cyclically driving it between voltages of +3V for gradually increasing periods of time.

The device 1 is driven by applying a constant current of 10 mA (which equates to 2 2 approximately 150 CLAcm for a device active area of 65 cm2) through the tungsten trioxide layer 16, the electrolyte layer 6 and the cerium titanium oxide layer 18, via the power supply wires 24,26 and the SnO2 films 12,14. As a protective measure, the applied voltage is never allowed to exceed +3V. Applying a negative voltage to the tungsten trioxide layer 16, so as to generate a current flowing in a first direction, causes lithium ions from the electrolyte layer 6 to be inserted into the tungsten trioxide layer 16, which produces a visible blue colouration.

Applying a positive voltage has the opposite effect, generating a current flowing in a second, opposite direction, and the device 1 is bleached towards its colourless state.

In order to utilise such a device in a situation where the reduction of total solar heat transmission is required, it would be installed as the outer pane in a double glazed unit and the inner pane would be an infra red reflecting glass so that infia red radiation which was absorbed by the device and re-radiated inwards could be reflected back outside.

PILKINGTON K GLASS (trade mark) available in the UK from Pilkington United Kingdom Limited is an example of a suitable infra-red reflecting glass. Were the thick W03 layer to be utilised in a solid state electrochromic device, the infra red reflecting capability could be supplied by the indium tin oxide coating which conventionally makes up such a device rather than by a separate coating on a separate pane.

Claims (8)

1. An electrochromic device comprising successive electrochromic, electrolyte and counter-electrode layers, wherein the electrochromic layer is at least 600 nm thick.

2. An electrochromic device according to claim 1 wherein the electrochromic layer is 1650 thick.

3. An electrochromic device according to claim 1 or claim 2 wherein the electrochromic layer is tungsten trioxide.

4. An electrochromic device comprising successive electrochromic, electrolyte and counter-electrode layers wherein the electrolyte layer contains mobile ions and the device can be varied between coloured and bleached states by reversibly inserting the ions into the electrochromic layer wherein the electrochromic layer is of a sufficient thickness that the solar electrochromic efficiency of the device is substantially greater than the luminous electrochromic efficiency of the device at certain charge densities of ion insertion into the electrochromic layer.

5. An electrochromic device according to claim 4 wherein the solar electrochromic efficiency of the device is substantially greater than the luminous electrochromic efficiency of the device at relatively low charge densities of ion insertion into the electrochromic layer.

6. An electrochromic device comprising successive electrochromic, electrolyte and counter-electrode layers wherein the electrolyte layer contains mobile ions and the device can be varied between coloured and bleached states by reversibly inserting the ions into the electrochromic layer and wherein as the mole fraction of ions inserted into the electrochromic layer decreases, the difference between the solar electrochromic effficiency and the luminous electrochromic efficiency of the device continues to increase.

7. An electrochromic device according to any of claims 4 to 6 wherein the ions are lithium.

8. An electrochromic device substantially as herein described with reference to figure 6 and 7 of the drawings.