GB2087582A - Electrode systems for electrochromic cells - Google Patents

Abstract

In an electrochromic cell, the display electrode comprises a conductive vitreous tin oxide layer 2, which is stable, transparent and adheres well to the (glass) substrate 1, and a layer 4 of electrochromic material spaced from layer 2 by a conductive, transparent, adhesion promoting layer 3 of bismuth or antimony oxide. Preferably layer 4 is of a rare earth phthalocyanine. <IMAGE>

Description

SPECIFICATION Electrode systems for electrochromic cells This invention relates to electrode systems for electrochromic cells, especially such cells containing a diphthalocyanine as the electrochromic material.

Phthalocyanines -- also referred to as benzoporphyrins -- have been well-known for many years as powerful colouring agents useful in the preparation of dyestuffs. Most of these materials, whether mono- or diphthalocyanines, and both substituted and unsubstituted, are in the form of complexes with metals.Certain of these complexes -- especially the diphthalocyanines of tri- and tetravalent metals, principally the rare earth metals (the 1 5 elements from lanthanum to lutetium; particularly europium, terbium and lutetium), and associated metals such as yttrium -- are well-known for their electrochromic properties (a material is said to be electrochromic if it changes colour in response to changes in the magnitude or direction of the electric potential applied across it). Lutetium diphthalocyanine, a material to which a considerable amount of study has been devoted, can be persuaded, when suitably arranged in an appropriate electrolytic cell, to display the colours from violet and deep b!ue, through various shades of green, to orange and red.Materials such as this have obvious possibilities in colour display systems of many sorts, and indeed much work is being effected on the construction of suitable "electrochromic" cells for such display systems.

A typical electrochromic cell comprises, immersed in a colourless, light transparent electrolyte such as aqueous potassium chloride: a colourless, light transparent main electrode, usually in the form of a conductive layer deposited upon a colourless, light transparent nonconductive substrate, upon which is in turn deposited a thin translucent layer of the chosen phthalocyanine; a counter electrode; and, for accurate potential measurement, a reference electrode. A potential is applied between the main and counter electrodes, and within a few seconds the colour of the phthalocyanine layer (usually viewed through the substrate and main electrode) changes to that appropriate to the applied potential. Lutetium diphthalocyanine, for example, is violet/deep blue at -1.2 volts, green at 0 volts, and orange/red at +1.8 volts.

As yet, however, there are numerous problems to be overcome before colour display systems using electrochromic cells of this type become commercially acceptable. One particular problem is the relatively poor adhesion, to the conductive layer forming the main electrode, of the phthalocyanine layer; another is the relatively poor adhesion, to the non-conductive substrate, of a main electrode made from one of the available and commonly-acceptable transparent and conductive materials. Both these problems appear to stem from the nature of the electrochromic cell: even at the low operating potentials needed, and even when employing the more sophisticated electrolyte materials presently available, considerable electrolytic action seems to accur, resulting in the phthalocyanine and main electrode layers either being reduced or peeling off - or both.Both of these problems could be alleviated by the use, for the main electrode conductive layer, of a material which is intrinsically less reactive under the conditions encountered in the cell. Unfortunately, because the material must as well be both colourless and transparent, very few if any of the substances conventionally used for conductive layers of this kind are acceptable -- either they are sufficiently stable but not colourless or transparent (as is the case, for example, with thin films of gold), or they are colourless and transparent but not sufficiently stable (as is the case, for example, with indium oxide and indium/tin oxide).

The present invention seeks to provide a novel electrode system useful in electrochromic cells which is colourless and transparent, and which is relatively stable to the conditions likely to be encountered (so reducing the risk of the phthalocyanine layer peeling away).

In one aspect, therefore, this invention provides the use, in an electrochromic cell, of a main electrode system comprising a layer of vitreous tin oxide, this layer itself bearing a layer of antimony oxide or bismuth oxide upon which is deposited the chosen electrochromic material.

In another aspect, the invention provides an electrochromic cell comprising, immersed in a suitable electrolyte: a main electrode in the form of a layer of vitreous tin oxide deposited upon a suitable nonconductive substrate and bearing a layer of antimony oxide or bismuth oxide itself bearing a layer of the chosen electrochromic material; and, appropriately spaced therefrom, a counter electrode and, optionally, a reference electrode.

The invention has, as one of its central features, the use of a first layer of vitreous tin oxide. This first layer is colourless, transparent and conductive, highly stable to the conditions normally encountered in electrochromic cells, and maintains its adhesion to the chosen nonconductive backing substrate through a large number of cycles - and is thus very satisfactory as the main electrode. However, it must be noted that if the cell is driven to a potential markedly beyond that necessary to cause the desired colour change (to and beyond +3 volts, say) even a vitreous tin oxide electrode is likely to peel away from its backing substrate, causing failure of the cell.It is not entirely clear why a vitreous tin oxide main electrode should be so much more stable than, say, a conventional indium oxide or indium/tin oxide electrode vacuum deposited or RF sputtered into place, though in the main the reason probably lies in the fact that the vitreous tin oxide has a much higher electrochemical reduction potential than do those other materials.

The vitreous tin oxide layer is a thin one, from 0.1 to 2.0 microns, especially about 1.0 micron thick. It may be formed (upon a backing substrate such as glass or quartz) by any convenient method - and a particularly preferred method commonly used for making such a layer involves a spray coating process in which a solution of a suitable tin salt (an aqueous acid solution of, for example, stannous chloride in hydrochloric acid), together with a few percent of any antimony salt (antimony trichloride or oxychloride, say) as a doping agent to improve the conductivity of the produced layer, is sprayed onto the very hot (5000 C) backing substrate. The solvent immediately evaporates, and the deposited tin salt fuses and oxidizes in an instant to give the desired vitreous tin oxide layer.By choosing the appropriate amount of dopant, and by selecting the right combination of conditions, there may be obtained a vitreous tin oxide layer of the desired thickness and a resistivity of from 5 to 20 ohms per square.

Naturally, the thus-coated substrate may be cut to whatever size is required, and the vitreous tin oxide layer may be etched to give any desired electrode -- and thus display - pattern. Standard etching techniques may be employed for this latter step, and a typical routine involves photolothographic masking, etching in 50% hydrochloric acid (with added zinc), and then cleaning and drying.

Vitreous tin oxide is a known electrode material, though it seems never to have previously been suggested for use in electrochromic cells.

As its other central feature, the invention uses a second layer of either antimony oxide or - preferably - bismuth oxide upon the vitreous tin oxide layer. This second layer is colourless, transparent, stable under the ambient conditions, and conductive. Its purpose is to improve the adhesion of the electrochromic material to the vitreous tin oxide layer; it is not clear how it does so, but there is no doubt that in cells with this second layer the electrochromic layer adheres to the main electrode far more strongly than in cells without. The adhesion layer is also a thin one, advantageously from 0.01 to 0.025 microns, especially about 0.02 microns, thick. It may be formed (upon the vitreous tin oxide layer) by any convenient method; one such method involves vacuum deposition, an appropriate coating weight being from 20 to 50 microgram per cm2.

The invention may be said to reside in the use of a bi-layer of vitreous tin oxide and either antimony oxide or bismuth oxide as the main electrode system in an electrochromic cell. The remaining components of the cell are generally conventional, and need not be discussed in any great detail. Nevertheless, the following features may be noted.

The electrochromic material is preferably a rare earth diphthalocyanine. These diphthalocyanines are believed to be of the empirical general formula (PC)2 H.M. (I) wherein M represents the rare earth metal, and "PC" represents the phthalocyanine ring system (phthalocyanine itself is shown in full in Formula II of the accompanying drawings). H - as normal -- represents hydrogen.

The structure of the compounds I is such that a single metal atom is "sandwiched" between two opposing phthalocyanine ring systems, and Formula Ill of the accompanying drawings shows a simplified, general structure for complexes of this kind (M is the metal, "Bl" represents the individual benzoisoindole ring systems making up the phthalocyanine system, and for clarity the bonds joining the two phthalocyanine systems to the metal atom have been omitted).

Metal diphthalocyanines are presently prepared by a process in which an organic derivative of the metal (an acetate, for example) is reacted, at about 3000C in a sealed tube, with an appropriate phthalonitrile. The metal M may be any appropriate rare earth metal. For the formation of a useful electrochromic diphthalocyanine, however, it is most conveniently gadolinium (Gd), terbium (Tb), dysprosium (Ds), holmium (Ho), erbium (Er), thulium (Tm), and lutetium (Lu). To a considerable extent the choice made depends upon the exact properties -- thus, colour -- required, and upon cost. The phthalonitrile is most preferably phthalonitrile itself (the presence of substituent groups causes as yet unpredictable changes in the electrochromic effects of the resulting phthalocyanine).

The required layer of rare eath diphthalocyanine may be formed upon the bi-layer main electrode system of the invention in the conventional way - thus, by vacuum sublimation/deposition (10-5 mm Hg at about 5000C) -- to a thickness of from 0.1 to 0.25 microns (a coating weight of from 20 to 50 micrograms per cm2).

The counter-electrode is placed in the cell suitably spaced from the main electrode, and normally it will consist of a conductive layer similar in shape and size to the main electrode and arranged a few hundreds of microns away in parallel and in register therewith. Where the cell is to be viewed from the main electrode side and by reflected light (light passing into the cell from the viewing side and being reflected back through the cell from the far side by a suitable reflective surface), the counter-electrode may very preferably double as the reflective surface, and be a shiny (though non-specular) conductive layer such as is formed by vapour deposition of, for example, metallic aluminium, indium or silver upon a ground glass plate. However, if the counter-electrode is not also the reflective surface, and instead there is used an independent reflective surface placed on the main electrode side of the counter-electrode (this reflective surface being, for example, a layer of white paper or titanium dioxide), then the counter-electrode can take any available form - thus, a metal, for instance aluminium, indium, silver, gold or platinum, on a glass backing or in plate or foil sheet form.

Where, alternatively, the cell is to be viewed from the main electrode side but by transmitted light, or from the counter-electrode side and by reflected light, then the counter-electrode may be any suitable colourless and transparent (or effectively transparent) conductive layer -- another vitreous tin oxide layer, for example, or a nickel or platinum wire mesh.

Electrochromic cells are usually (though not necessarily) viewed from the side of the main electrode, either by reflected light or by transmitted light originating at or beyond the counter-electrode side. As explained above, the reflective surface may be a shiny (though nonspecular) surface (which layer may, because of its conductive nature, very conveniently double as the cell counter-electrode), or it may be a white surface, such as is formed by a titanium oxide or barium sulphate layer on glass, or by a sheet of porous white polypropylene or white filter paper, or by an opal white glass itself. White surfaces such as these can, if they are thin enough to be translucent, also be used in devices viewed by transmitted light.

It is useful, though not essential, for an electrochromic cell to incorporate a reference electrode (a silver wire coated with silver chloride, for example) generating a standard potential against which can be measured the potential actually existing across the main and counter-electrode (which may not necessarily be exactly the same as the applied potential).

The electrolyte employed may be any suitable electrolyte, and may be in liquid or "solid" form, either by itself or absorbed onto some appropriate porous material that may also act as a cell spacer.

Examples of such suitable electrolytes are aqueous solutions of mineral acid salts, for instance 1 to 5% w/v aqueous potassium chloride or sodium sulphate, or ionic organic materials, for instance tetraethyl ammonium p-toluene sulphonate or POLYBRENE (hexamethrine bromide - a polymeric quaternary ammonium compound).

The mechanical components of the cell - the end faces and the spacing and sealing systems -- can be any appropriate. Normally, the end faces will be glass or quartz plates (conveniently bearing on their inner faces the main and counter-electrodes), and will be spaced and sealed together either by a plastics (viz., MYLAR) gasket and epoxy resin, by a thermoplastic gasket, or by a glass frit. In the process of actually constructing the cell, the electrolyte may, as usually, be inserted into the assembled cell through a filling hole which is thereafter sealed off.

Various embodiments of the invention are now described, though only by way of illustration, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic cross-sectional view of part of a first electrochromic cell according to the invention; Figure 2 is a diagrammatic cross-sectional view of part of a second electrochromic cell according to the invention; and Figure 3 is a diagrammatic cross-sectional view of part of a third electrochromic cell according to the invention.

Whe-e possible, similar parts have been given the same reference numeral in all of the Figures.

The Figures are not to scale.

Figure 1 shows a cross-section of one form of electrochromic cell according to the invention. A glass plate (1) coated with a 1.0 micron conductive layer (2) of vitreous tin oxide upon which is a 0.02 micron adhesion layer (3) of bismuth oxide (this bi-layer is the main electrode system) carries a 0.2 micron electrochromic layer (4). Separated by a 0.5 mm thick layer (5) of electrolyte is the counter electrode (6) which takes the form of a 10 micron metal film deposited on a ground glass plate (7).The device is viewed from the left (in the direction of the arrow), illuminated by reflected light passing into the cell from the left, through the colourless transparent glass front plate 1 , the colourless, transparent vitreous tin oxide layer 2, the colourless transparent bismuth oxide layer 3, the coloured translucent electrochromic layer 4 and the colourless transparent electrolyte 5, being non-specularly reflected off the rough, metal-plated surface of the ground glass back plate 7, and passing back out through the cell via the electrolyte 5, the electrochromic layer 4, the bismuth oxide layer 3, the vitreous tin oxide layer 2 and the glass front plate 1.Depending upon the potential applied across the main electrode (the vitreous tin oxide layer 3) and the counter electrode (the metal film 6), so the electrochromic layer 4 adopts a corresponding colour, and the cell as viewed appears to have that colour.

Another form of electrochromic cell is shown in Figure 2. Here, the electrolyte (5) is absorbed in a porous spacing layer (5A) which also forms a white reflecting layer for the display. The counter electrode (6) consists of an opaque metal plate.

A third form of electrochromic cell is illustrated in Figure 3. In this cell the counter electrode (6) consists of an effectively translucent open mesh of fine wire, and for a back reflector a separate nonspecular layer (8) of titanium dioxide has been placed on the outside surface of the back plate 7.

Test Results A series of cells, some having the inventive bilayer of vitreous tin oxide and antimony oxide or bismuth oxide, others not, was subjected to the Test Procedure described hereinafter.

The Cells For the purpose of this Test, simple cells were constructed using a 2 cm2 sheet of glass coated on one face with a layer approximately 1 micron thick of the chosen electrode material (with or without a vacuum deposited adhesion layer approximately 0.02 micron thick of bismuth oxide) which itself carried a vacuum deposited coating approximately 0.1 5 micron thick of lutetium diphthalocyanine. This electrochromic electrode system was placed in a beaker together with a silver plate counter electrode spaced a few centimeters away, and the beaker was then filled with 5% w/v aqueous potassium chloride solution.

Such a Test Cell is presently considered sufficiently reliable to demonstrate the invention's advantages as compared with the prior art.

A series of such cells was constructed, using different electrode materials, as follows:- a) 10 cells were made with a commercial electrode which is either indium oxide electrode or indium oxide/tin oxide, and without any adhesion layer.

b) 5 cells were made with the same commercial electrode as in a), but with a bismuth oxide adhesion layer.

c) 5 cells were made with a vitreous tin oxide electrode (as the invention) but without an adhesion layer.

d) 1 5 cells were made fully in accordance with the invention -that is, with a vitreous tin'dioxide electrode with a bismuth oxide adhesion layer.

The manner of construction was as follows:- Stage 1: Formation of a bismuth oxide adhesion layer.

A commercially-avaiiable electrode-material- glass sheet substrate (bearing a coating of the chosen electrode material about 1 micron thick) was placed in a vacuum deposition apparatus about 10 cms above a graphite crucible containing approximately 0.5 g bismuth oxide. The system was evacuated down to 10-5 mm Hg, and the crucible heated by a focused 3 KV electron beam (current about 10 mA). Condensation of the bismuth oxide onto the substrate was monitored by observing the frequency shift (about 900 Hz) of a crystal monitor placed near the substrate, this having first been calibrated against samples measured directly using an interference microscope.

In each case, the bismuth oxide layer formed was about 0.02 micron thick.

Stage 2: Formation of a lutetium diphthalocyanine electrochromic layer.

The chosen electrode-material-coated glass substrate (with or without a bismuth oxide adhesion layer) was placed in a vacuum deposition apparatus about 10 cm above a quartz pot surrounded by a graphite resistance heater and containing about 0.05 g of lutetium diphthalocyanine. The system was evacuated to 10~5 mm Hg, and the pot warmed slowly to about 45O0C, at which point the lutetium diphthalocyanine volatilized, condensing out on the cooler substrate above. The thickness of the condensed layer was monitored by observing the frequency shift (about 700 Hz for a 0.2 micron thick layer) of a crystal monitor placed alongside the substrate and first calibrated against a standard layer measured directly using an interference microscope.

The Test The Test employed was as follows:- Each cell was connected to a potentiostat, and the applied voltage on the main electrode cycled between +1.5 v and -0.8 v. For each cycle the applied voltage was held at each end figure for 5 seconds and then turned off, the cell being left for 30 seconds before application of the voltage of opposite sign. Thus, in one cycle the main electrode was sequentially held at + 1.5 v for 5 seconds, at O v for 30 seconds, at -0.8 v for 5 seconds, and finally at O v for 30 seconds. During each cycle the cell was examined by transmitted white light for signs of failure (shown by the appearance of white spots within the nominally coloured area).

The Results The results obtained are shown in the following Table.

TABLE Cell under test Number of cycles (a-d above) to failure a) (Prior Art) < 5 b) < 10 c) < 20 d) (Invention) > 100 It is clear from these figures that the use of a vitreous tin oxide/bismuth oxide main electrode system in accordance with the invention vastly improves the life of electrochromic cells. It should be noted, incidentally, that at least one inventive cell (d-type) had still not failed after 250 cycles.

Claims (7)

1. As the main electrode system in an electrochromic cell, a layer of vitreous tin oxide itself bearing a layer of antimony oxide or bismuth oxide upon which is deposited a chosen electrochromic material.

2. An electrode system as claimed in claim 1, wherein the vitreous tin oxide layer is from 0.1 to 2.0 microns thick, with a resistivity of from 5 to 20 ohms per square.

3. An electrode system as claimed in either of the preceding claims, wherein the antimony oxide or bismuth oxide adhesion layer is from 0.01 to 0.025 microns thick.

4. An electrode system as claimed in any of the preceding claims and substantially as described hereinfore.

5. An electrochromic cell comprising, immersed in a suitable electrolyte: a main electrode as claimed in any of the preceding claims, and thus in the form of a layer of vitreous tin oxide deposited upon a suitable non conductive substrate and bearing a layer of antimony oxide or bismuth oxide itself bearing a layer of the chosen electrochromic material; and, appropriately spaced therefrom, a counter electrode.

6. An electrochromic cell as claimed in claim 5, wherein the electrochromic material is a rare earth diphthalocyanine.

7. An electrochromic cell as claimed in claim 6, wherein the rare earth diphthalocyanine is lutetium diphthalocyanine.

7. An electrochromic cell as claimed in claim 6, wherein the rare earth diphthalocyanine is lutetium or holmium diphthalocyanine.

8. An electrochromic cell as claimed in any of claims 5 to 7, wherein the layer of electrochromic material is from 0.1 to 0.25 micron thick.

9. An electrochromic cell as claimed in any of claims 5 to 8, wherein the counter-electrode doubles as the reflective surface, enabling the cell to be viewed from the main electrode side and by reflected light, and is a shiny (though nonspecular) conductive layer.

10. An electrochromic cell as claimed in any of claims 5 to 9, wherein there is incorporated a reference electrode.

11. An electrochromic cell as claimed in any of claims 5 to 10, wherein the electrolyte is 1 to 5% w/v aqueous potassium chloride or sodium sulphate

12. An electrochromic cell as claimed in any of claims 5 to 11 and substantially as described hereinbefore.

New claims or amendments to claims filed on 6 Owt 1981 Superseded claim 7 New or amended claim: