CA1087716A

CA1087716A - Electrochromic display device

Info

Publication number: CA1087716A
Application number: CA294,071A
Authority: CA
Inventors: Donald J. Barclay; Colin L. Bird; Anthony C. Lowe; David H. Martin
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1977-04-19
Filing date: 1977-12-29
Publication date: 1980-10-14
Also published as: FR2388359A1; JPS53129998A; GB1564264A; JPS627531B2; DE2815062A1; FR2388359B1; DE2815062C2; US4167309A

Abstract

ELECTROCHROMIC DISPLAY DEVICE
Abstract In a flat panel electrochromic display, the counter electrode is in the form of a pattern of opaque conductive lines on the under-side of the front face of the panel. The electrode is precharged to a known potential by depositing electrochromic material thereon. This arrangement solves the erasure problem for matrix displays with a large number of display electrodes.

Description

1 This invention relates to an electrochromic display device.
For the purposes of this specification an electrochromic dis-play device is a device wherein the display effect is achieved as a consequence of a redox reaction caused by the passage of charge between a display electrode and a counter electrode both in con-tact with a suitable electrolyte. An example of an electrochromic display device is given in British Patent 1,376,799, granted to N.V.
Philips Slocilampenfabriken wherein the electrolyte includes heptyl viologen dications. Upon reduction at the display electrode, a pur-ple film which is insoluble in the electrolyte is deposited on thedisplay electrode. Oxidation at the display electrode returns the dications to the electrolyte. Thus, by providing a plurality of dis-play electrodes and selectively energizing them, a required display can be generated. Other materials can be used. Development has taken place of displays which employ redox reactions of inorganic materials, such as tungstic oxide, to achieve the display effect. This invention is of application to any electrochromic display.
In order to provide a practical display device it is necessary to be able accurately to erase the display. In generating the dis-play, which operation will be called writing, a certain quantity ofmaterial is deposited on the display electrode. Erasure must remove just that amount of material or it will result in the display being over driven with consequent undesirable electrochemical reactions, such as the evolution of oxygen bubbles. In a simple cell consisting of only a display electrode and a counter electrode, a third reference electrode can be used to control erasure. A coloured species is formed in the case of the heptyl viologen dication by reduction at the display electrode to the cation radical. Deposition of the species is thus equivalent to charging the electrode. The function of the re-ference electrode in erasure is to provide a means of controlling thepotential difference between the display electrode and the electrolyte.

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1 The reference electrode is positioned as close to the display elec-trode as possible without screening the display electrode, in order to minimize errors due to electrolyte resistance. It is clearly im-practicle to use this arrangement when there are a plurality of dis-play electrodes. A plurality of reference and counter electrodes would add to the cost of the display device, especially when the com-plexity of the electrical circuitry is taken into account. One pos-sibility with electrochromic displays is to have a single counter electrode with a single reference electrode located along one edge of the display device with the counter electrode located at the opposite edge of the device. The drawback to this arrangement is that the electrolyte is not of negligible resistance and so display electrodes nearer the counter electrode are preferentially erased, since current flows in the electrolyte between the display electrodes and the counter electrode. It is possible that in small display devices, for example, in single character display cells with an electrode pattern of the kind used in pocket calculators to provide a numeric display, this counter and reference electrode arrangement may operate satis-factorily. What is certain is that oppositely disposed reference and counter electrodes do not operate satisfactorily with relatively large area display devices with many display electrodes arranged in rows and columns. Because of the electrolyte resistance a transparent low-resistance counter electrode overlying all the display electrodes must be provided, so that the effective resistance between the counter electrode and any display electrode is low. Current spreading through the electrolyte is minimized and the preferential current flow is through the counter electrode. The reference electrode now no longer senses the electrolyte potential close to the display electrode but the sum of the counter electrode potential and the potential drop between the counter electrode and the electrolyte close to the reference electrode. Both these potentials are ill-defined.

1 It is the primary aim of this invent;on to overcome this con-trol problem by providing a counter electrode on the front or view-ing side of an electrochromic display device, which counter electrode is everywhere at a known potential with respect to the electrolyte.
The need for a reference electrode is avoided.
The known potential is achieved by precharging the counter elec-trode with a suitable material capable of a reversible redox reaction.
The potential at the counter electrode is thus fixed. To avoid the viewer seeing the species on the counter electrode it is made of opaque material, which, to avoid masking of the display electrodes, is in the shape of a pattern of thin lines, preferably reticulate.
Accordingly, the invention comprises an electrochromic display de-vice including a hollow panel enclosing a suitable electrolyte and having one face transparent, a counter electrode located on the inner surface of the said one face, and a plurality of display electrodes on the opposite inner face of the panel, characterized in that the counter electrode is charged to a known potential, is of opaque conductive material, and is in the shape of a pattern of lines.
With the counter electrode precharged, the potential difference be-tween the counter electrode and the electrolyte is known. If the poten-tial drop in the counter electrode is known, and this is readily found, then an edge of the counter electrode remote from its electric terminal can be used as a reference electrode. Further, and this is the pre-ferred way of using a precharged counter electrode, if the counter elec-trode is of sufficiently low resistance, the electric terminal itself becomes the reference and erasure is effected simply by connecting the display electrodes to a potential of given value relative to the counter electrode. This is a self-limiting operation similar to the discharge of a battery.
An erase operation using a reference electrode is called a potentio-static erase. The preferred method of erasure is called a voltage or tensiostatic erase.

1~)87716 1 US Patent 3,961,842 issued to Texas Instruments Corp. describes an electrochromic display device using an electrolyte consisting of a solution of an n-heptyl viologen dication, a dihydrogen phosphate anion and potassium phosphate salt buffer and which operates by reduction to the cation radical as an insoluble film on the display electrodes.
The counter electrode is of palladium hydride which is stated to be non-polarizable over the operating range of the cell i.e. during a redox reaction, the potential of the electrode remains substantially constant.
In this situation a reference electrode is not needed. US Patent 3,961,~42 also mentions the problem, encountered during erasure, of the deposition of coloured species on the counter electrode. The electrode arrangement described therein comprises display and counter electrodes side by side with the counter electrode masked from the viewer. The US Patent does not consider the problem discussed above which makes the side-by-side positioning of display and counter electrodes not an entirely satisfactory arrangement.
The invention will further be explained by way of example with reference to the accompanying drawings, in which:
Figure 1 is a schematic cross-section through an electrochromic display device according to the invention; and Figures 2 and 3 are illustrative of different counter electrode patterns.
Referring to Figure 1, which shows a schematic cross-section of a display device according to the invention, a panel 1 consists of paral-lel sheets 2, 3, of glass or other suitable material sealed together at the periphery by sealing glass 4 or some other bonding material to enclose a hermetic space 5. Techniques for making such a panel are well-established and will not further be described. At least sheet 2 of the panel is transparent since the device is intended to be viewed from that side. Figure 1 is not to scale, the sheets 2, 3, are about UK9-77-001 - ~ -1~87~716 1 6mm thick, and are spaced apart about 2mm. Display electrodes 6 are provided on the face of sheet 3 bounding the space 5. The electrodes 6 are connected through sheet 3 to terminals 7 on the outer face of the sheet. The terminals 7 can be connected directly tO electronic switches, or, when the spacing of the display electrodes permit, con-ductors can lead from the terminals 7 to the edge of the panel for connection to drive circuitry. Details of the drive circuitry and how it is connected to the display electrodes 6 form no part of the invention. For suitable embodiments, reference is directed to our co-pending Canadian Applications 268,797 and 287,758 filed December 29, 1976 and September 29, 1977, respectively.
Our invention includes the provision of an opaque counter elec-trode 8 shaped as thin lines and located on the inner face 9 of trans-parent sheet 2. Although not shown in Figure 1, it is to be under-stood that the lines comprising the counter electrode are connected to form a single conductor. The counter electrode is deposited on the transparent substrate 2 and may be arranged in any suitable pattern.
It is most preferred that the lines form a reticulate pattern, such as the rectangular grid shown in Figure 2. Each square of the grid may bound a single display electrode 6, but satisfactory results are achieved when a plurality of display electrodes are enclosed, for example, a

2 x 2 matrix, as shown, or even as many as form a 5 x 5 matrix. The grid can be composed of rectangles or regular polyyons, rather than squares, and, if the display device is intended to display only charac-ters, a rectangle of the grid can be arranged to bound a single charac-ter position. Another possible shape is a grid-iron as shown in Figure

3. A lateral edge 10 is not essential. While two rows of display elec-trodes are shown between the lines 11, more or less than two rows can be arranged therebetween. The pattern of the counter electrode need not consist of straight lines. For example, the lines 11 in Figure 3 can be zig-zag or curved. Much depends on the visual effect produced by 1~)87716 1 the particular choice of electrochromic material, and on the nature of the display - alphanumeric, graphic or image.
The counter electrode may be deposited on the transparent plate 2 by evaporation. This may give rise to a highly reflective surface visible through the plate 2. Accordingly, it is preferred that the inner surface of the plate 2 is modified such that the metal interface becomes a good diffuse reflector. Electrical connection to the counter electrode is effected by means of a conventional edge connection arrange-ment.
We now consider the structure of the counter electrode, as distinct from its shape. It will be understood that any of the shapes described can be combined with any of the structures.
One way of charging the counter electrode is to trap material in close proximity to a conductor so that the counter electrode consists of a pattern of conductors as described above coated with a charged material. In a first structural embodiment, the conductors are of platinum coated with porous anodised aluminum (A1203). In the pores is trapped a suitable redox couple such as FeII/FeIII. This is done by steeping the anodised coating in a solution of FeII/FeIII and sub-sequently boiling to close the pores. After incorporation in the dis-play device, during a write/erase operation the electrochemical reaction at the counter electrode is oxidation/reduction of iron species, with the result that the counter electrode potential will vary during these operations by only a few millivolts.
Another embodiment of the counter electrode uses a polymeric film which is coated on the counter electrode using any suitable technique such as photolithography. The electrolyte is heptyl viologen dibromide so the preferred conductor material is platinum. Quaternisation of the pyridine residues in poly-(2-vinylpyridine) produces a quaternary ammonium residue which is coated on the counter electrode. Quaternary ammonium bromide salts form charge-transfer complexes with elemental 10877~6 1 bromine. The polymer layer is charged with elemental bromine. This has the advantageous effect of enhancing the conductivity of the poly-mer film as well as charging the counter electrode to within a pre-determined potential range.
In a third embodiment the counter electrode is treated as symmet-rical with the display electrode so that erasure of the display elec-trode is accompanied by writing at the counter electrode and vice-versa.
The effect is as if the erase operation transferred coloured species from the display electrode to the counter electrode. During an initial write and erase sequence the counter electrode is charged through the intervention of a suitable agent. The coloured species is not visible because of the opacity of the counter electrode, and, subsequently, voltage erase can be successfully used. By way of non-restrictive ex-ample, we consider an implementation of this embodiment using heptyl viologen dications as the active display material. As agent a small amount of sodium bisulphite, for example O.OlM, is used. During a first write cycle, heptyl viologen cation radicals are deposited on the display electrodes, while at the counter electrode irreversible oxi-dation of bisulphite ions occurs. During the subsequent erase cycle the reoxidation of the display electrode deposit is accompanied by de-position of heptyl viologen cation radicals on the counter electrode which thus becomes charged. During subsequent write cycles, reduction of heptyl viologen occurs at the display electrodes and oxidation at the counter electrode. The opposite reactions occur during subsequent erase cycles.
The product of oxidation of the bisulphite ion has not been identi-fied but is thought to be dithionate (S206--), see M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions (Pergamon Press).
An additional advantage of the use of sodium bisulphite is its property of oxygen scavenging, since it reacts with free oxygen to form sodium sulphate. Free oxygen, which may diffuse into the display from 1~87716 1 the atmosphere, is deleterious to operation of an electrochromic dis-play using heptyl viologen.
To summarize, in order to enable accurate erasure of an electro-chromic display, the invention proposes the use of an opaque counter electrode in the shape of a pattern of lines, preferably recticulate, on the underside of the face of the display device through which the display is viewed. The counter electrode is brought to a predetermined potential either by trapping a suitable material capable of a reversible redox reaction at the counter electrode or by initially charging the counter electrode with heptyl viologen cation radical. This enables voltage erase to be used.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrochromic display device, including a hollow panel en-closing a suitable electrolyte and having one face transparent, a counter electrode located on the inner surface of the said one face, and a plurality of display electrodes on the opposite inner face of the panel, characterized in that the counter electrode is charged to a known potential range, is of opaque conductive material and is in the shape of a pattern of lines.

2. A device as claimed in claim 1, wherein the pattern of lines is reticulate.

3. A device as claimed in claim 2, wherein the pattern of lines is a rectangular grid.

4. A device as claimed in claim 2 or claim 3, wherein each mesh of the reticulate pattern overlies a plurality of display electrodes.

5. A device as claimed in claim 1, wherein the pattern of lines is in grid-iron form.

6. A device as claimed in claim 1, wherein the lines of the pattern are not straight.

7. A device as claimed in claim 1 wherein the charging of the counter electrode is effected by trapping a material which produces a known potential at the counter electrode.

8. A device as claimed in claim 7, wherein the material is trapped within the pores of an anodized Al2O3 layer.

9. A device as claimed in claim 8, wherein the material is a redox couple.

10. A device as claimed in claim 9, wherein the couple is FeIII/FeII.

11. A device as claimed in claim 1 wherein the charging is effected by charging a polymeric charge transfer material coated on the counter electrode.

12. A device as claimed in claim 11, wherein the charge transfer material is poly-(2-vinylpyridine).

13. A device as claimed in claim 11 or claim 12, wherein the electro-lyte includes as active material heptyl viologen dibromide, and the charge transfer material is charged with bromine.

14. A device as claimed in claim 1, wherein the electrolyte contains heptyl viologen dications as active materials, and charging is ac-complished by causing deposition of heptyl viologen cation radicals on the counter electrode.

15. A device as claimed in claim 14, wherein the charging is accom-plished by the use of sodium bisulphite.