GB2081922A

GB2081922A - Electrochromic device

Info

Publication number: GB2081922A
Application number: GB8123684A
Authority: GB
Original assignee: National Research Development Corp UK; National Research Development Corp of India
Current assignee: National Research Development Corp UK; National Research Development Corp of India
Priority date: 1980-08-14
Filing date: 1981-08-03
Publication date: 1982-02-24
Also published as: GB2081922B

Abstract

An electrochromic device, comprises a first electrode, an electrochromic layer of a metal-sensitive compound, a layer of a solid fast ion conductor, and a second electrode. The metal-sensitive compound is substantially stoichiometric; is substantially free of water; has a mean grain size in the range 40 ANGSTROM to 250 ANGSTROM ; and has a layer thickness in the range 0.2 micron to 2 microns.

Description

SPECIFICATION Electrochromic device and method of making an electrochromic device The present invention relates to electrochromic devices, that is to say display devices which change colour on application of an electric potential, and to methods of making such electrochromic devices.

The present invention is concerned with an electrochromic device comprising a first electrode, a layer of a metal-sensitive compound in contact with the first electrode and in contact with a solid fast ion conductor in which fast ion conductor the fast ion is an ion of metal which dissolves in the metalsensitive compound to change colour thereof the fast ion conductor itself being in contact with a second electrode capable of providing ions the same as the fast ions of the conductor.

By "metal-sensitive compound" is meant a compound capable of dissolving metat atoms and which changes colour in so doing. In the electrochromic device of the kind with which the invention is concerned, the metal atoms in question are, of course, the same as those provided by discharge of the fast ions of the solid fast ion conductor.

Electrochromic devices of the kind set out above (which hereinafter will be referred to as "an electrochromic device of the kind hereinbefore defined") are described in published U.K. Patent Specifications Nos. 1540713 and 1540714, and in Pending U.K.

Patent Application No.28241/76 (U.K. Patent No. ),the general content of which has been published in published Japanese Patent Application No. 81479/77), all these patents and applications being concerned with inventions made by the inventor of the present invention.

There have also been disclosed in scientific papers published by the same inventor a number of factors concerned with the materials, dimensions, and other parameters of the substances used in electrochromic devices. These papers published by the present inventor, either alone or in combination with others, comprise ofthefollowing:- "A thin film electrochromic display based on the tungsten bronzes", Thin Solid Films 38(1976) 89100; "A solid state electrochromic cell - the RbAg4l5/WO3 system", Thin Solid Films, (1974) S45-S46; "A solid state electrochromic cells; the Na-ss- alumina/WO3 system", Thin Solid Films,40 (1977) L19-L21; "Optical and electrical properties of thin films of WO3 electrochemically coloured", Electrochimica Acta, 1977 Vol.22, pp751-759; "Atom motion in tungsten bronze thin films", Thin Solid Films, (1978) 145-150; and "Variation in the chemical potential of sodium in NaxWO3thinfilms'',Thin Solid Films,62(1979) 385-387. These papers are quoted in order to provide background information to assist those skilled in the art in carrying out the manufacture and operation of electrochromic devices in accordance with the novel features of the invention to be set out hereinafter, and also in order to set out certain known prior art features, in the case of some of the papers, as will be set out hereinafter.

The present invention relates to improvements in the type of electrochromic device set out above, and to an improved method of making the same. The invention is concerned with a number of interrelated parameters of the electrochromic device as set out above and its concerned with the particular way in which these parameters interrelate, both in the final finished and completed electrochromic device, and in the method of manufacturing the same.Thus although, as will become apparent from the following description, some of the factors concerned have been set out by the present inventor in published documents, and indeed the importance of some of these factors has been mentioned, the present invention derives from the appreciation of a particular new and non-obvious interrelationship between a number of these factors, and the optimisation of a number of preferred ranges of the parameters.

Although the known devices set out in the patents mentioned above, have been brought to experimental operational states, a number of problems have remained in the production and use of practical commercial examples, and the present invention shows how the problems involved in producing and operating such practical examples can be overcome or significantly reduced.

In particular, at least in preferred embodiments, the present invention is concerned with improved stability of electrochromic devices, both during manufacture of the devices, and during a period of subsequent use of the devices, and the invention is also particularly concerned with reducing the time taken for inducing desired colour changes in the electrochromic devices, so that when such electrochromic devices are used for display purposes, the "write" time of the device can be significantly reduced, and the device can be arranged to continue to operate at the relatively reduced "write" time over a long operational life. None of the prior publications set out above teach the particular combination and interrelationship of factors involved in the present invention which will bring about these desirable results.

According to the present invention there is provided an electrochromic device comprising a first electrode, a layer of a metal-sensitive compound in contact with the first electrode and in contact with a solid fast ion conductor in which fast ion conductor the fast ion is an ion of a metal which dissolves in the metal-sensitive compound to change the colour thereof, the fast ion conductor itself being in contact with a second electrode capable of providing ions the same as the fast ions of the conductor, in which the metal sensitive compound: (i) is substantially stoichiometric; (ii) is substantially free of water; (iii) has a mean grain size in the range 40 A to 250 A; and (iv) has a layer of thickness in the range 0.2 micron to 2 microns.

One qualitative criterion of what is meant by substantially stoichiometric is that the metalsensitive compound shall be sufficiently stoichiometric to be free of residual colouration caused by absence of atoms or molecules from the crystal structure. That is to say that in the absence of deliberate colouring of the metal-sensitive compound by insertion of metal atoms in interstiatial sites, the metal-sensitive compound is transparent or free from colour, for the thickness of the layer in use in the device, within the usual meaning of these words in the display field.

One quantitative criterion of what is meant by substantially stoichiometric is that, where the metal sensitive compound consists of a crystalline oxide thin film structure, there is less than 1015 oxygen atoms per square centimetre of film missing from the crystal structure.

One qualitative criterion of what is meant by substantially free of water is that the metal-sensitive compound shall be sufficiently water free to avoid molecules obstructing the conduction pathways along grain boundaries in the metal-sensitive compound, that is to say the conduction pathways between grains along which the atoms travel during diffusion of the atoms into the metal sensitive compound.

One quantitative criterion is meant by substantially free of water is that the metal sensitive compound shall have a water content of less than 1, preferably less than 0.5, mole per cent of water.

According to one qualitative criterion of the mean grain size in the metal sensitive compound, it is preferred that the mean grain size shall be small as possible consistent with the material being stable against annealing to a larger grain size, and with the material being stoichiometric. In one aspect, where the device is manufactured by evaporating or sputtering a layer of metal sensitive compound, the metal sensitive compound preferably has a grain size as small as possible consistent with being stable in use, and consistent with remaining stoichiometric during the process of depositing the layer of material.

According to a quantitative criterion of the grain size, it is preferred that the metal-sensitive compound has a mean grain size in the range 40 A to 75 . One particularly preferred mean grain size is about 50 . Another particularly preferred mean grain size is about 60 .

According to various qualitative criteria of the layer thickness of the metal-sensitive compound, it is preferred that the layer of metal-sensitive compound is sufficiently thin to be transparent; sufficiently thin to be stoichiometric (for example sufficiently thin to remain stoichiometric during the deposition of the metal-sensitive compound in making ofthe device); and sufficiently thin to avoid unwanted metal deposition in use during deliberate colouration of the metal-sensitive compound by diffusion of atoms into the compound; but that the metal-sensitive compound is sufficiently thick to be able to accept an adequate flux of diffusing atoms into individual grains during deliberate colouration of the metalsensitive compound in use.

According to quantitative criteria of the thickness of the metal-sensitive compound layer, it is preferred that the thickness of the layer is in the range 1 micron to 0.5 micron, most preferably in the range 0.75 micron to 0.5 micron. One particularly preferred value of a layer thickness is about 0.6 micron.

According to the present invention in another aspect there is also provided a method of making an electrochromic device comprising the steps, not necessarily in the order given, of depositing a layer of a metal-sensitive compound onto at least part of one side of a substrate of fast ion conductor in which fast ion conductor the fast ion is an ion of a metal which dissolves in the metal-sensitive compound to change the colour thereof, depositing a first electrode onto at least part of the metal-sensitive compound in such a manner as to avoid a direct contact between the first electrode and the fast ion conductor, and depositing onto at least part of the other side of the said substrate of fast ion conductor a second electrode capable of 20 providing ions the same as the fast ions of the fast ion conductor, in which the step of depositing the metal-sensitive compound comprises (i) depositing the metal-sensitive compound at a rate of deposition low enough to ensure that the deposited material is substantially stoichiometric;; (ii) depositing the metal-sensitive compound from a substantially water-free source of the compound and in a substantially water-free environment, (iii) depositing the metal-sensitive compound at a rate of deposition high enough to deposit the material with a mean grain size in the range 40 Angstroms to 250 Angstroms, and (iv) depositing the metal sensitive compound at a rate and for a time sufficient to deposit a layer having a thickness in the range 0.2 micron to 2 microns.

The criteria of what is meant by substantially stoichiometric may be the same criteria as have been set out hereinbefore when reference has been made to an electrochromic device according to the invention.

One criterion of what is meant by depositing from a water-free source and in a water-free environment is that the metal-sensitive compound when deposited shall be substantially free of water in accordance with one or more of the criteria set down with regard to the apparatus aspect of the invention.

One criterion of what is meant by a water-free source of the metal-sensitive compound is that the source of the metal-sensitive compound should have a water content less than 1 mole per cent of water.

One criterion of what is meant by a water-free environment is that the atmosphere in which the metal-sensitive compound is deposited has a water content less than 10-7torr.

Preferably the rate of deposition of the metalsensitive compound is in the range of 10 Angstroms to 100 Angstroms per second, most preferably a rate in the range 20 Angstroms per second to 40 Angstroms per second.

Preferably the rate and time of deposition of the metal-sensitive compound comprise a rate in the region 20 to 40 Angstroms per second carried out for a time in the range 2 to 4 minutes, preferably a rate of about 30 Angstroms per second carried for a time of about 3 minutes.

Preferably the rate and time of depositing the metal-sensitive compound, and other conditions of the method, are such that the mean grain size and the layer thickness of the metal-sensitive compound fall within one or more of the preferred ranges set out with regard to the previous aspects of the invention.

Considering now the preferred composition of an electrochromic device as hereinbefore defined, and in accordance with the invention, the metal-sensitive compound is preferably an oxide of a transmission metal, especially tungsten oxide (W03), molybdenum oxide (MoO3), or solid solutions of these and other transition metal oxides in their highest oxidation states. Such oxides are capable of dissolving metal atoms, especially monovalent atoms such as atoms of an alkali metal (for example potassium, sodium and especially lithium), copper or silver, and change colour in doing so. The most usual colour change is from colourless to blue. Other metalsensitive compounds which may be mentioned are the oxides of titanium and zirconium and suitable solid solutions thereof.The colour changes of these oxides following the dissolution of metal atoms therein is reversible and, in the electrochromic devices of the invention, reversal of the electric potential which orginally caused the colour to be produced, causes the colour to be discharged.

The use of a solid fast ion conductor as the electrolyte is an essential feature of an electrochromic device as hereinbefore defined. As has already been explained, the fast ion in the electrolyte must be one which, when discharged, forms an atom capable of disolving in the metal-sensitive compound so as to cause a change of colour therein. This means that the fast ion of the fast ion conductor is preferably an alkali metal, copper or silver ion, lithium being preferred.

The fast ion conductor preferably has a resistivity less than 1 x 106 ohm-cm, most preferably less than 1 x 104 ohm-cm, and should be a material which conducts electricity only by ionic conduction of ions of the metal to be inserted in the metal sensitive compound. For convenience of use, the fast ion conductor preferably is of a material which may be deposited, cast or otherwise shaped into sheets or plates. It should be a material which will not react disadvantageously with or in response to the environment to which may be exposed, (as is similarly the case for all the components), or it may be protectable (as may all the components of the device) from an environment with which it may be undesirably reactive, for example by encapsulation, to prevent chemical reaction, or byfilitration to prevent or reduce, say, exposure to radiation, e.g.

sunlight, where this is undesirable.

Examples of suitable fast ion conductors are given in UK Patent No. 1540713, and examples of silvercontaining fast ion conductors are the complex halides, particularly iodides, of silver of alkali metal or quaternary ammonium ions.

Other fast ion conductors which may be used are copper-containing fast ion conductors, which are also described in UK Patent No. 1540713.

However, in use of the present invention, the fast ion conductor is preferred to be an alkali metal containing, fast ion conductor such as sodium alumina, lithium alumina and potassium (3- alumina. These are mixed oxides of Na2O, Li2O or K20 and A1203 of variable composition. Typically, they contain from 5 to 11 aluminium oxide molecules per molecule of alkali metal oxide. A particularly preferred fast ion conductor is lithium-(3-alumina.

Other fast ion conductors are described as "ionconducting crystals" in US Patent 3971624 (German Patent Application No.2433044) and may also be employed in the device of this invention.

The fast ion conductor will preferably be white, or may be colourless, or it may be otherwise coloured, in which case the colour will be selected to provide an appropriate contrast with the metal-sensitive compound in at least one of its coloured states.

When the fast ion conductor is transparent it is often advantageous to employ a backing, distant from the metal-sensitive compound, capable of providing an appropriate contrast with the metal-sensitive compound. Such a backing may be the second electrode.

Said second electrode may be silvered and give a mirror-like contrast.

Thus in accordance with one preferred form, it may be arranged that the layer of solid fast ion conductor is opaque so as to provide a background to deliberate colouration of the layer of metalsensitive compound when viewed with the solid fast ion conductor to the rear of the layer of metalsensitive compound.

In accordance with an alternative form it may be arranged that one of the electrodes is optically reflecting, and the layer of metal-sensitive compound, the solid fast ion conductor and the other electrode, are optically transparent in the absence of deliberately induced colouration during operation of the device, and the arrangement is such that when viewed with the optically reflecting electrode at the rear of the device a mirror effect is obtained in the absence of deliberate colouration.

In the electrochromic cells of the present invention, the application of a potential across a solid electrolyte formed of one of the aforesaid fast ion conductors causes the fast ion to be discharged and the resulting metal atoms to become dissolved in the metal sensitive compound. It is this process which causes a colour to develop in the metal-sensitive compound. Similarly, reversal of the potential causes metal atoms in the metal-sensitive compound to migrate as ions to the fast ion conductor. In order that equilibrium in the fast ion conductor shall be maintained, the electrode in contact with the fast ion conductor that is referred to above as the second electrode, must be capable of providing and accepting the fast ions. Where the fast ion is silver or copper, the second electrode may itself be of silver or copper. Moreover, since the amount of the metal atoms involved in the operation of the electrochromix cells of the present invention is usually very small, these electrodes can themselves be very small without there being any danger of any inadequate supply of the metal ions, although the dimensions may vary between very wide limits and the minimum area that may be employed in any particular instance may be determined easily by simple trial.

Where the fast ion conductor contains, as the fast ions, alkali metal such as sodium, lithium or potassium ions, it is usually inconvenient to make the second electrode of the alkali metal, because of the well known reactivityofthese elements in the free metallic state. It is however, possible to make a second electrode of an appropriate bronze, e.g. a tungsten bronze of general formula MXWO3 where M is for example lithium, sodium or potassium and x preferably has a value between 0.05 to 0.5. Such material is capable of losing metal as the corresponding metal ion to the fast ion conductor. Ferrites comprising appropriate ions also may be employed as the second electrode for example a lithium ferrite may be used.Other suitable materials, alloys or compounds may be used in a simiiar manner and many are described for example in US Patent 3971624 as being suitable for the non-polarisable electrode described in that patent.

For ease of observation of the colour changes which take place it is desirable for at least one of the electrodes to be transparent to visible light. This may be achieved for example by using a suitable transparent electrode material, e.g. and indium-tin oxide as the first electrode in contact with the metal-sensitive compound.

It will be appreciated that the electrochromic device is made entirely of solid materials, which is an important practical advantage and makes for good strength and stability in use. Moreover, it can be constructed in a form suitable for rapid response to relatively small applied potentials of the order of 1 volt.

Considering the various items of prior art listed above, in relation to the features of the invention set out above, it is firstly to be noted that the importance of the stoichiometric nature of the metal-sensitive compound and the water-free requirement for the compound has not previously been appreciated. In previously made examples of electrochromic cells, the metal-sensitive material has been made to be transparent, for example as in U.K. Patent No, 1540713, but this has been achieved for films of relatively large grain size.

The techniques of manufacture described in this patent would result in a mean grain size of the order of 50 Anstrom units. The requirements involve in producing films of smaller grain size, whilst maintaining the stoichiometric nature of the material sufficient to keep the material optically clear, has not previously been appreciated.

Similarly, it is believed that in experimental work carried out previously there have on occasion been achieved electrochromic devices with a relatively small mean grain size, but the advantage to be had from such a grain size, but the advantage to be had from such a grain size in terms of diffusion of ions into the metal-sensitive host material has been negated because the manufacturing techniques involved have involved the presence of water which has produced blocking of the conduction pathways along the grain boundaries.

In the publication mentioned above, in Thin Solid Films, (1977) L19-L21, mention is made that a major cause of device failure in electrochromic devices as hereinbefore defined is the presence of pot-holes on a alumina substrate on which tungsten trioxide is deposited. As will described hereinafter, it is important that the direct contact is avoided between the electrodes of electrochromic device and the fast ion conductor, to avoid plating out of the metal which is intended to be dissolved in the metal-sensitive material.

This paper discusses a number of other fabrication techniques relevant to the method of the present invention.

In the publication mentioned above in Electrochimica Acta 1977, Vol. 22, pp.75, there is discussion of the possible role of grain boundaries in diffusion of metal atoms into a metal-sensitive device, and reference is made to possible rapid diffusion down the grain boundaries, followed by slower diffusion into the crystallite.

In the publication mentioned above in Thin Solid Films, (1978) 145-150, mention is made of the considerable water content of tungsten trioxide as ordinarily supplied, and general precautions necessary for preparing water-free tungsten trioxide, but no teaching is given as to the reason why water-free tungsten trioxide is required.

In the disclosure of the paper mentioned above in Thin Solid Films, 50(1978)145-150, there is some discussion of the effect of the texture of a tungsten trioxide film on the diffusion of metal atoms into such a metal-sensitive compound, but this academic discussion has not been translated into an effective proposal for an electrochromic device of the nature defined in accordance with the present invention. In particular there is no teaching in the paper mentioned of the interrelationship between a low mean grain size, and the manufacturing techniques, and other manufacturing techniques, and other parameters, of practical electrochromic device.

With regard to the feature of the invention concerning the thickness of the layer of optically responding metal-sensitive compound, the criteria for the thickness set out above are based on a realisation of the relationship between thickness and diffusion rates into the metal-sensitive compound quite different from the supposed relationship which has governed proposals in the prior art. All publications concerning electrochromic devices of the kind set out above have emphasised the requirement for a very thin film of metal-sensitive compound. For example in the Pending U.K. Patent Application No. 28241/76, the thickness of the metal-sensitive compoud is required to be 1 micron or less, conveniently of from 1 to 0.01 of a micron and preferably a thickness between 0.5 and 0.05 of a micron. A thickness between 0.5 and 0.2 of a micron is particularly preferred.In general, in prior artwork it has been assumed that the rate of diffussion of the metal atoms into the host metal-sensitive compound, has been increased by making the layer of metal-sensitive compound as thin as possible consistent with practical fabrication techniques and device structure. However, in accordance with the present invention it has been discovered that the optimum thickness is not simply to be as thin as possible, but is dependent upon a number of interrelated factors. An imporant one of these factors is that the diffusion rate of metal atoms into the host metal-sensitive material is not always increased by reducing the layer thickness.It has been established that the diffusion coefficient is a function of the concentration gradient of the atoms in the grains of the metal-sensitive compound, and that, within limits, the thicker the film of meta I-sensitive compound, the faster the rate of diffusion of the atoms into the material. It will appreciated that in terms of a practical display device, this means that, within limits, the thicker the layer of metal-sensitive compound, the faster the "write" time for producing deliberate colouration of the electrochromic device.

It is believed that an explanation of this phenomena is concerned with the two part diffusion of atoms into metal-sensitive material. The atoms moving down the grain boundaries do so very quickly, whilst diffusion into individual grains is slow. Some of the atoms diffuse into a grain and there then occurs a large concentration gradient in the grain, as there are no metal atoms in the centre of the grain and there is a big concentration of atoms around the periphery. This produces an electric field within each grain the effect of which is that the value of the diffusion coefficient itself depends upon the concentration gradient. In normal circumstances the value of flux in a diffusion of material depends upon the concentration gradient, but the coefficient of diffusion is generally a constant.In the present case, with metal-sensitive materials such as set out above, the diffusion "constant" is not in fact constant but is itself dependent upon the concentration gradient.

Thus within certain limits, if the electrochromic device is desired to operate more quickly, it is necessary for the metal-sensitive film to be made thicker rather than thinner.

Thus the present invention provides criteria for reducing the "write" time of an electrochromic device by requiring an optimum thickness of the metal-sensitive compound layer having regard to the other factors mentioned above such as mean grain size and stoichiometric nature of the material.

Considering in more detail the relationship between grain size and fabrication techniques, it is known of course that where a layer of metalsensitive material is deposited for example by evaporation, the faster the evaporation rate, the smaller the grain size. In accordance with the present invention the material should be evaporated quickly to achieve small grain size, but within certain constraints. One constraint is that evaporation should not be so rapid that the material becomes non-stoichiometric, which would show up in colour in the material.

Considering the example of tungsten trioxide, where the material is pure and stoichiometric the formula can be written as W03. Where the material is non-stoichiometric, the formula may be written e.g. as W029. This indicates in such a case that the ratio of oxygen to tungsten is not the normal amount required by the standard chemical formulation demanded by the simple theories of valency. This occurs for example where too rapid evaporation technique is attempted. When the tungsten trioxide source is heated up, if it is heated too fiercely some oxygen may escape, producing non-stoichiometric WO3. Thus some of the oxygen will be lost out of the system through the vacuum pump, so that when evaporation commences there will be deposited an excess of tungsten because some of the oxygen has been lost in the vacuum system.In effect this gives the normal crystal structure of W03 but with some oxygen atoms missing from the structure. Thus this effect is a limitation on the rate and extent to which one can evaporate tungsten trioxide in the attempt to obtain small grain.

In practice this is observed, in that whilst evaporating, it is possible to monitor the oxygen pressure in the gas system. Evaporation is arranged to take place at the beginning of the evaporation cycle while the oxygen pressure is rising. When the oxygen pressure turns the peak and begins to fall, the evaporation technique is terminated. If more deposition is required, a fresh source is provided and the evaporation cycle started again. A typical evaporation rate might be 30 Angstroms per second. This typically gives a mean grain size of 50 Angstroms. By the term mean grain size is meant the reciprocal of the number of grains in a unit length.For example the mean grain size is measured in practice by taking a magnified photograph by electron microscope of the material, drawing a straight line across a random selection of grain shapes, and counting the number of grains intersected by the straight line along a chosen unit length. One conception of the idea of mean grain size is that, if the grains were all to be spherical, and were to be placed side by side along a straight line, the mean grain size would correspond to the diameter of a spherical grain. The grains can be observed under high energy electron microscope, for example a million volt electron microscope.

Typically when depositing by evaporation, the sample is taken and placed in a silica glass tube with a resistance heater around the outside to raise its temperature so that the vapour pressure increases and eventually the material evaporates. This is done inside a vacuum system and the material deposits on any cold surface.

If the rate of evaporation or sputtering is too fast, although the material deposited may be stoichiometric for the first deposited layers, it will not remain stoichiometric for the thickness required.

This factor has not been so significant in prior art systems, because it was desired to keep the films as thin as possibe, but in accordance with the present invention (where the importance of a relatively thick film has been realised in order to reduce diffusion times in deliberate colouration of the metal-sensitive material), it has been found important to maintain the deposition of stoichiometric material over the whole of the evaporation time so that the relatively thick layer deposited is stoichiometric throughout.

Thus to summarisethe importance of the rate of evaporation, it is desirable to evaporate quickly so as to obtain a small grain size, but not to evaporate too quickly so as to produce non-stoichiometric layers (over the relatively greater thickness required by the present invention) which will produce unwanted colouration ofthe metal-sensitive material.

It will be appreciated that other techniques for the preparation of the various components of devices of the invention, alternative to those described above, will be available to the skilled man. Such techniques include sputtering (including reactive sputtering involving simultaneous chemical reaction, e.g. sputtering tungsten in oxygen), chemical vapour deposition from unstable compounds, and chemical precipitation as well as evaporation referred to above.

Selection of the preparative technique employed will be made in the light of the chemical nature of the component to be produced and the physical form required.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a cross-section through an electrochromic device embodying the invention; Figure 2(a) to 2(h) illustrate diagrammatically eight stages in the making of an electrochromic device according to the invention; Figure 3 shows a cross-section through part of an electrochromic device according to the present invention, and illustrates the end result of steps (a) to (f) shown diagrammatically in Figure 2; Figure 4 shows a cross-section through an alternative embodiment of an electrochromic device embodying the invention, in which one electrode is optically refelcting to give a mirror-effect.

Figure 1 shows a cross-section through an electrochromic device according to the present invention, although it is to be appreciated that the drawings of Figure 1, and of the other Figures, are not to scale and in particular the thickness of layers shown does not necessarily show any scale proportion between the various layers.

Referring to Figure 1, the device indicated generally at 11 is enclosed in a transparent box 12 for example of synthetic plastics material, and the active layers of the device 11 are encapsulated in silicon oxide shown at the outer layers 13 and 14. The viewing side of the device is the upper side Figure 1, and considering the layers of the device from the top to bottom, the first layer under the encapsulation layer 13 is a layer of indium tin oxide 15 which constitutes a first electrode. Beneath this is a layer 16 of a colour-responding metal-sensitive compound which in the example shown is a tungsten trioxide.

Next below this is a layer 17 of a fast ion conductor which in the example shown is a lithium alumina which may be represented in a preferred form by the formula Li-ss-alumina. Next below this is a layer 18 of tungsten trioxide containing lithium atoms, which constitutes a second electrode, and which in a preferred form may be represented by the formula LiXWO3 where xis preferably 0.2. The upper layer of indium tin oxide 15 has a metal contact 19 and the second electrode 18 is connected to a second contact 20 which is shown emerging from the casing 12, but is connected to the layer 18 at a position outside the section shown in Figure 1.

Referring now to Figure 2 there is illustrated diagrammatically a series of steps in one preferred method of fabricating a device according to the invention and generally as shown in Figure 1. It is to be noted that the method of attaching the first electrode 15 to be described with reference to Figure 2 does not necessarily correspond to the section shown in Figure 1, but the relationship between Figures 1 and 2 is generally similar and like reference numerals will be used for like elements. However the particular method of attaching a contact 19 to the indium tin oxide layer 15 produces a slightly different result from that shown in Figure 1, and is illustrated in detail in Figure 3.

Referring to Figure 2(a) the device is built up on the ceramic fast ion conductor 17 which forfabrica- tion purposes constitutes the substrate. Firstly the ceramic substrate 17 has its surfaces highly polished, and a water-free polycrystalline oxide bronze film 16 is deposited by evaporation on to the dry alumina 17 through a mask which gives the seven section pattern shown in Figure 2(a). Next as shown in Figure 2(b) there is deposited exactly on to the tungsten oxide layer 16 a layer 15 of indium tin oxide by way of a masking technique, taking care, as shown in Figure 3, that the indium tin oxide layer 15 does not come in contact with the alumina layer 17.

Next as shown in Figure 2(c) a layer 1 3A of silicon dioxide is laid over the side of the device, with the right-hand side of the device masked off, in a position such that the silicon oxide 13A provides a "stand-off" overlapping the edge of the layers 15 and 16. Next as shown in Figure 2(d) two contacts 1 9A and 1 9B are deposited over the stand-off silicon oxide layer 13A so as to make contact with the two left-hand segments of the indium-tin oxide layer 15, but to ensure that the indium-tin oxide of the contacts 1 9A and 1 9B does not come in contact with the alumina layer 17.

Next the steps of Figures 2(c) and 2(d) are repeated in different configurations around the seven segments of the layers 15 and 16 so as to provide seven contacts 19 as shown in Figure 2(b), the stand-off layers 13 being omitted for simplicity. Finally an encapsulation layer 13 of silicon oxide is deposited over a central square of the upper surface of the device, as shown in Figure 2(f) which leaves the contacts 19 available at the sides of the device.

Next there is applied to the rear surface of the substrate of alumina 17 a layer 18 of tungsten bronze containing lithium to a proportion represented by the formula Lio.2WO3, and a conducting coating such as indium-tin oxide 15 (or chromium metal to a thickness of about 0.1 micron), and this followed by an encapsulation layer deposited over the whole of the bottom of the device at 14 as shown in Figure 2(h). These last two layers are not shown in detail in Figure 3.

There will now be described various practical details which need to be taken into account in preparing preferred devices according to the invention.

In the examples illustrated, the substrate 17 is the mechanical support for the display and provides the background "colour", which is preferably white.

Typical substrate materials are Li2O:8At203 called Li-ss-alumina; LiNaO:8At203 called mixed lithium sodium alumina and Na-ss-alumina; and "Nasicon", Na3Zr2PSi202. All these are ceramic materials which for thicknesses greater than about 0.25 mm appear white. Below this thickness the translucence of the substrate may become unacceptable in that the back electrode can be seen through the front of the cell. The substrate is preferably highly densified, typically 99.5% and of small grain size, typically 2 micron. The substrate is highly polished front and back to a roughness less than 0.5 micron. The surface is cleaned and dried at high temperatures (200"C) in dry gas or vacuum.

The "stand off" layers 13A may for example be MgF2 (evaporated) or SiO2 (sputtered). The technology of depositing such thin transparent electrical insulating films is well known.

There are a number of techniques that can be used in order to obtain the required water-free W03 for the metal-sensitive layer. A first method consists of sintering WO3 close to its melting temperature so that large grains are obtained (e.g. 1/10 mm diameter).

This large grain material is evaporated to give a water-free deposit. Another acceptable film preparation is reactive sputtering of tungsten in 02. Film thickness is controllable, but the important parameter of mean grain size is a more complicated matter. As has been explained the smaller the mean grain size, the faster will be the ultimate speed of a display element. Evaporation at a rate of hundreds of A/sec may give a mean grain size of about 50 .

Sputtering at about 10 A/sec may give a mean grain size of about 100 A.

As has been described, a material parameter of great importance is the diffusion coefficient of metal atoms into the host material (and its temperature coefficient).

Examples of diffusion coefficients at limitingly low concentration gradients are: Na in W03 at 25"C; 6 x 10-2 cm2/sec: Li in W03 at 0 C; 5 x 10-17 cm2/sec.

The transparent conducting coating 15 to form the electrodes may be 0.8 ln203/0.2 Sn02 (called l.T.O.) which can be d.c. or r.f. sputtered onto W03 at room temperature giving a sheet resistance of about 100 ohm per square. It is important that suitable conditions of sputter rate and pressure of 02/Ar sputtering gas be maintained in order that the conducting film remains of high transparency. The idea of these conditions being important is well known to those knowledgeable in the art. But it must be recalled that about 200"C is the maximum temperature to which for example the W03 films may be raised without failure.

In the operation of devices embodying the invention certain aspects of device addressing need to be considered. For atom insertion systems of the kind discussed above it is normally necessary to have between 1 and 8 millicoulombs per square centimeter of charge to colour the cells. Not only is this range of charge required, but the host solid must be able to accept this charge in a prescribed time without any deleterious side effects. It has been determined that the only significant harmful effect is electroplating the metal rather than dissolving it in the host. To avoid this a limit should be placed on the amount of metal insertion. For example where the metal-sensitive compound is represented by LixWO3, the value of x preferably should not exceed 0.25, thereby giving a highly acceptable safety factor, and ensuring no post-writing colour density change.

Constant voltage addressing, which is the more or less practical approach, gives high current densities at the start of a writing pulse. This can give rise to difficulties and it is suggested that the initial portion of the writing voltage pulse be made to rise in an exponential way over about 1/10 of the full pulse width.

Another factor which should be noted is that it is detrimental to the functioning of an electrochromic display device if it is "over-bleached". "Overbleaching" can result by the application of a positive potential to the working electrode for too long a time so that the resulting metal free WO3 is highly resistive. This can be avoided by incorporating a background of permanently embedded metal in the W03 which confers upon it a background conductivits which will prevent this "over-bleaching".Thus incorporation of aluminium metal in the W03 evaporation source will result in the formation of a dilute A4XWO3. The total of A( should not exceed 5 x 10'5 cm-2 and may be as low as 5 x 1013cm-2 with a preferred value 1 x 1015 cm-2.

There will now be described an example of a particular fabrication of a device embodying the invention.

Example 1 A substrate consisting of a 2 inch x 2 inch piece of 0.75 mm thickness lithium alumina of conductivity about 2000 ohms cm at room temperature is mounted in a vacuum chamber. The lithium (3- alumina can be represented by the formula Liss- alumina. A nearby water-free source of tungsten oxide W03 is heated by a resistance heater to a temperature of about 1350"C in a vacuum chamber which is maintained water-free by cryogenic pumping using liquid N2.

The tungsten oxide is deposited through a mask on to the lithium sodium alumina for a time of three minutes, at a deposition rate of 30 Angstroms per second. The mean grain size of the deposited tungsten oxide is 50 Angstroms and the thickness of the layer deposited is approximately 0.5 micron. The source of tungsten oxide and the vacuum in which the evaporation takes place are maintained sufficiently water-free for the water content of the deposited tungsten oxide to be less than 0.5 mole per cent. The oxygen pressure in the evaporation chamber is monitored during the evaporation, and evaporation is terminated before any significant drop in oxygen pressure is detected, below the maximum oxygen pressure detected.The evaporation is carried out so that the deposited layer of tungsten oxide remains stoichiometric throughout its depth to a degree such that less than 1015 oxygen atoms per sq. cm. are missing from the deposited layer. Next there is deposited on the tungsten bronze layer an electrode of indium-tin oxide applied by sputtering from a target consisting of a compaction of an intimate mixture of tin and indium oxides Sn02 and ln203 in the ratio 88:12. The indium tin oxide layer is optically transparent, about 0.75 microns thick and having a resistivity of 100 ohms per sq. cm.

Care is taken to avoid any indium-tin oxide from having direct contact with the alumina layer, and this is achieved by depositing "stand-off" layers of silicon oxide of thickness 0.15 micron in the manner generally desribed with reference to Figure 2 above.

The substrate is held at room temperature throughout.

The reverse side of the substrate of lithium alumina is coated with a layer of tungsten bronze containing lithium atoms to a degree represented by the formula Na02WO3 by a two source evaporation technique in which both lithium and tungsten trioxide are evaporated together on to the substrate. The evaporation is carried out at the same rate and for the same time as for the upper layer of WO3. An electrode contact of indium-tin oxide is then deposited on to the lithium tungsten bronze, and the back of the device is covered by an encapsulating layer of silicon oxide to a layer of about 0.2 microns.

Throughoutthefabrication care is necessary, by normally available techniques, to maintain the materials and operating conditions free of water. Also care is necessary to avoid direct contact of any of the indium-tin oxide layers with the alumina substrate.

For this reason both surfaces of the substrate are highly polished before deposition of tungsten bronze thereon, since any irregularities in the surface of the alumina substrate might allow contact of the indium-tin oxide layers through the tungsten bronze. Any such direct contact of indium-tin oxide with the alumina substrate gives the possibility of discharge of lithium at the interface, which eventually would lead to device failure.

There will now be described with reference to Figure 4 a modification of the embodiment of an electrochromic device shown in Figure 1. The modification lies in that the layer of fast ion conductor is made to be transparent (instead of opaque as in Figure 1) and the second electrode is made to be optically reflective, so as to give a mirror-effect when viewed with the second electrode to the rear of the device.

Referring to Figure 4, the device 111 is made by depositing the appropriate layers on a glass substrate 110, using techniques, and materials generally similar to those previously described. The glass substrate 110 is preferably 1 to 3 mm thick and the first layer deposited is a layer 115 of indium tin oxide, of dimensions and qualities such as describdd hereinbefore, which constitutes the first electrode of the device. Next there are deposited various stand off layers 1 13A of Si02 in the manner described before, for the purpose of preventing contact at edges between the layer 115 of l.T.O. and subsequent layers of material. The SiO2 layer 1 13A is preferably of thickness 0.1 to 0.5 micron, most preferably 0.2 micron.

Next there is deposited a layer 116 of WO3 (or MoO3 or mixed oxides), again of thickness and quality as has been described, to form a layer of metal-sensitive compound. Next there is deposited a layer 117 of a solid fast ion conductor, for example a Li fast ion conductor such as Li3N. Preferably the layer 117 is 0.1 to 3.0 micron thick, more preferably 0.2 to 1 micron, and most preferably 0.5 micron.

Next there is deposited an optically reflecting layer 118 forming a second electrode and acting as a source and sink of the metal ions to be inserted in the fast ion conductor 117. For example, the optically reflecting layer 118 may be aluminium containing 31/2 atomic per cent dissolved Li. The layer may be 0.3 to 3 micron thick, more preferably 0.5 to 2 micron, and most preferably 0.8 micron.

Finally the device is covered by a backing layer 113 which may conveniently be an encapsulation layer of SiO2 as has been described hereinbefore. Alternativelythe layer 113 may be of Cr metal of thickness 0.1 to 0.5 micron, preferably 0.2 to 0.4 micron, and most preferably 0.25 micron. Further alternatively the layer 113 may be Al203 deposited upon the outside of an Al electrode 118, the Al203 having been produced by oxidation of the Al, the Al203 being a coherent (non porous) oxide film.

The operation of the device shown in Figure 4 is generally the same as that shown in Figure 1, except for the mirror effect already mentioned. Various patterns, e.g. a 7-segment display, can be built up using metal masks to delineate, or it is possible to use photolithographictechniques, provided care is taken to avoid the use of water. The second, or counter, electrode 118 is a reflecting surface and the solid electrolyte 117 is sufficiently thin that a little light is absorbed (i.e. less than 50% absorbed, preferably less than 10% absorbed). The device is observed through the glass 110, and the contrast then is that in the unwritten state there is observed a mirror like reflection, and in the written state the WO3 layer 116 with metal atoms inserted is blue, so that there is observed a blue hue whose depth depends upon the amount of inserted metal atoms.

Claims

1. An electrochromic device comprising a first electrode, a layer of a metal-sensitive compound in contact with the first electrode and in contact with a solid fast ion conductor in which fast ion conductor the fast ion is an ion of a metal which dissolves in the metal-sensitive compound to change the colour thereof, the fast ion conductor itself being in contact j with a second electrode capable of providing ions the same as the fast ions of the conductor, in which the metal sensitive compound: (i) is substantially stoichiometric; (ii) is substantially free of water; (iii) has a mean grain size in the range 40 A two 250 A; and (iv) has a layerofthickness in the range 0.2 micron to 2 microns.

2. A device according to Claim 1 in which the metal-sensitive compound is sufficiently stoichiometric to be free of residual colouration caused by absence of atoms or molecules from the crystal structure.

3. A device according to Claim 1 or 2 in which the metal-sensitive compound consists of a crystalline oxide thin film structure, and there are less than 1015 oxygen atoms per square centimetre of film missing from the crystal structure.

4. A device according to any preceding claim in which the metal-sensitive compound is sufficiently water free to avoid molecules obstructing the conduction pathways along grain boundaries in the metal-sensitive compound.

5. A device according to any preceding claim in which the metal sensitive compound has a water content of less than 1 mole per cent of water.

6. A device according to Claim 5 in which the metal-sensitive compound has a water content of less than 0.5 mole per cent of water.

7. A device according to any preceding claim in which the mean grain size is as small as possible consistent with the material being stable against annealing to a larger grain size, and with the material being stoichiometric.

8. A device according to any preceding claim in which the metal-sensitive compound has a mean grain size in the range 40 to 100 .

9. A device according to Claim 8 in which the mean grain size is in the range 45 to 75 .

10. A device according to any preceding claim in which the layer of metal-sensitive compound is sufficiently thin to be transparent; sufficiently thin to be stoichiometric; and sufficiently thin to avoid unwanted metal deposition in use during deliberate colouration of the metal-sensitive compound by diffusion of atoms into the compound; but is sufficiently thick to be able to accept in adequate flux of diffusing atoms into individual grains during deliberate colouration of the metal-sensitive compound in use.

11. A device according to any preceding claim in which the thickness of the layer of metal-sensitive compound is in the range 1 micron to 0.5 micron.

12. A device according to Claim 4 in which the layer thickness is in the range 0.75 micron to 0.5 micron.

13. A device according to any preceding claim in which the metal-sensitive compound comprises an oxide or oxides of a transition metal or metals.

14. A device according to any preceding claim in which the metal-sensitive compound comprises tungsten oxide (we3), or molybdenum oxide (MoO3), or a solid solution of these or other transition metal oxides in their highest oxidation states.

15. A device acording to any preceding claim in which the fast ion of the fast ion conductor comprises an alkali metal, or copper, or silver.

16. A device according to Claim 15 in which the fast ion conductor is an alkali-metal-containing fast ion conductor selected from the group comprising sodium-ss-alumina, lithium-ss-alumina and potas sium-p-alumina.

17. A device according to any preceding claim in which the fast ion conductor has a resitivity less than 1 x 106 ohm-cm.

18. A device according to Claim 17 in which the said resistivity is less than 1 x 104 ohm-cm.

19. A device according to any preceding claim in which the layer of solid fast ion conductor is opaque so as to provide a background to deliberate colouration of the layer of metal sensistive compound when viewed with the solid fast ion conductor to the rear of the layer of metal-sensitive compound.

20. A device according to any of Claims 1 to 19 in which one of the electrodes is optically reflecting, and the layer of metal-sensitive compound, the solid fast ion conductor and the other electrode, are optically transparent in the absence of deliberately induced colouration during operation of the device, and the arrangement is such that when viewed with the optically reflecting electrode at the rear of the device a mirror effect is obtained in the absence of deliberate colouration.

21. A method of making an electrochromic device comprising the steps, not necessarily in the order given, of depositing a layer of a metalsensitive compound onto at least part of one side of a substrate of fast ion conductor in which fast ion conductor the fast ion is an ion of a metal which dissolves in the metal-sensitive compound to change the colour thereof, depositing a first electrode onto at least part of the metal-sensitive compound in such a manner as to avoid a direct contact between the first electrode and the fast ion conductor, and depositing onto at least part of the other side of the said substrate of fast ion conductor a second electrode capable of providing ions the same as the first ions of the fast ion conductor, in which the step of depositing the metal-sensitive compound comprises:: (i) depositing the metal-sensitive compound at a rate of deposition low enough to ensure that the deposited material is substantially stoichiometric; (ii) depositing the metal-sensitive compound from a substantially water-free source of the compound and in a substantially water-free environment; (iii) depositing the metal-sensitive compound at a rate of deposition high enough to deposit the material with a mean grain size in the range 40 Angstroms to 250 Angstroms, and; (iv) depositing the metal-sensitive compound at a rate and for a time sufficient to deposit a layer having a thickness in the range 0.2 micron to 2 microns.

22. A method according to Claim 21 in which the source of the metal-sensitive compound has a water content less than 1 mole per cent of water.

23. A method according to claim 21 or 22 in which the atmosphere in which the metal-sensitive compound is deposited has a water content less than 10-7 torr.

24. A method according to any of Claims 21 to 23 in which the rate of deposition of the metal-sensitive compound is in the range of 10 Angstroms to 100 Angstroms per second.

25. A method according to Claim 24 in which the said rate is a rate in the range 20 Angstroms per second to 40 Angstroms per second.

26. A method according to any of Claims 21 to 25 in which the rate and time of deposition of the metal-sensitive compound comprise a rate in the region 20 to 40 Angstroms per second carried out for a time in the range 2 to 4 minutes.

27. A method according to Claim 26 in which the rate and time comprise a rate of about 30 Angstroms per second carried out for a time of about 3 minutes.

28. A method according to any of Claims 21 to 27 in which the metal-sensitive compound comprises an oxide or oxides of a transition metal or metals.

29. Method according to any of Claims 21 to 28 in which the metal-sensitive compound comprises tungsten oxide (we3), or molybdenum oxide (MoO3), our a solid solution of these or other transition metal oxides in their highest oxidation states.

30. A method according to any of Claims 21 to 29 in which the fast ion of the fast ion conductor comprises an alkali metal, or copper, or silver.

31. A method according to Claim 30 in which the fast ion conductor is an alkali-metal-containing fast ion conductor selected from the group comprising sodium-ss-alumina, lithium-ss-alumina and potas sium-(3-alumina.

32. A method of making an electrochromic device substantially as set out in Example 1 as hereinbefore described.

33. An electrochromic device when produced by a method according to any of Claims 21 to 32.

34. An electrochromic device substantially as hereinbefore described with reference to Figure 1,or Figures 1,2 and 3, or Figure 4, of the accompanying drawings.

35. A method of making an electrochromic device substantially as herein before described with reference to to Figure 1, or Figures 1,2 and 3, or Figure 4, of the accompanying drawings.