GB2295242A - Electrochromic display - Google Patents

Abstract

Images can be written on a continuous electrochromic layer of the Nax WO3 type. Underneath the WO3 layer is a temperature-dependent sodium conductor which is heated by individually controlled rows of parallel heating wires and, just not touching, orthogonal columns of parallel heating wires. Only where two heated wires cross does the sodium conductor become operative. The wires are addressed so as to build up the image. The sodium conductor is sodium perchlorate dissolved in polyethylene oxide loaded with titanium dioxide powder.

Description

IMAGEWISE WRITABLE ELECTROCHROMIC DISPLAY This invention relates to an imagewise writable electrochromic display, that is, an electrochromic display in which individual pixels can be addressed and written/erased.

A typical electrochromic display device is described in UK Patent GB 2081 922 B.

The device comprises the following layers (starting with the nearest to the viewer): (i) an indium-tin-oxide conductive window; (ii) tungsten trioxide (which goes from white to blue as lithium ions are incorporated); (iii) a conductor for lithium ions (e.g. Li-B-alumina); (iv) a reservoir for lithium ions (e.g. Liy 2 We3).

The device is activated by applying an appropriate potential between (i) and (iv) to move lithium ions into or out of (ii).

Like any other electrochemical cell, when charged, it seeks to discharge itself driven by the cell potential. Therefore, a matrix of rows and columns of electrochromic cells cannot be addressed by the common technique of charging row m positive and column n negative. If this were attempted, matrix element (m, n) would indeed initially react as intended because it represents the only path through which current can flow, but the electrochromic cell at (m, n) thus charged (written) will, over time, discharge through all the other cells, erasing itself and writing those other cells until they all reach a chromatic equilibrium.

There remains however a need for "line-at-a-time" addressing of matrix arrays in electrochromic display systems. Other addressing methods are already known. For example, it is taught in UK Patent GB 21 64466B to imagewise address an electrochromic cell on a tamper-resistant personal credit card, using an external card re-writer at the point of sale. The electrochromic cell in the card includes a layer which is not conductive except at high temperature, and the re-writer can not only apply charge to the electrochromic cell but can also imagewise apply heat, whereby only the heated area(s) are X Tittenlerased. This method clearly requires a separate apparatus (in this case the shopkeeper's card re-uTiter), which would be quite impracticable in most uses such as public displays.An important application of such displays could be shelf-edge price information in supermarkets, where it is desirable to be able to rewrite remote displays individually and reliably from a central point.

According to the present invention, an imagewise writable electrochromic display comprises a guest-atom-dependent electrochromic layer, a guest-atom-transmitting layer in contact therewith, a guest-atom sink/source layer in contact with the transmitting layer and means for applying a potential between the electrochromic layer and the sink/source layer, characterised in that the sink/source layer and/or the transmitting layer is inoperative at the normal temperature of the display and further characterised by rows and columns individually switchable to supply heat energy to the inoperable layers such that only in the region where a switched-on row passes a switched-on column is the thermal effect adequate to bring the sink/source layer and/or the transmitting layer into operative condition to allow the electrochromic display to be altered in that region.It is desirable for this thermal effect to be relatively strong over a small temperature rise, and, accordingly, preferably the resistivity of the transmitting layer falls by at least a factor of ten in a temperature rise of 40C deg starting at a temperature which itself is at least 40C deg above said normal temperature. A factor of 5 over a 30C deg rise starting at at least 30C deg above normal display temperature may suffice.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 illustrates schematically an array of electrochromic cells addresed by an image-writing technique which cannot work, Figure 2 shows the array of Figure 1 which has been addressed, showing why it must fail, and Figure 3 is a schematic cross-section of an imagewise writable electrochromic display according to the invention.

In Figure 1, each of an array of electrochromic cells is connected on one of its faces to one of the wires c, c + 1, c + 2 ... and is connected on its opposite face to one of the wires n, n + 1, n + 2 ... Each cell is shown, not unrealistically, as a capacitor. In Figure 2, the wire combinations (c + 1, n) and then (c + 2, n + 1) were activated. charging (writing) the correspondingly addressed electrochromic cells. However, as soon as this has been done, the written cells tend to discharge (self-erase), with Li+ ions moving as shown in the erase direction and electrons moving along the wires such as to partially write the previously non-addressed cells.

Figure 3 shows an imagewise writable electrochromic display according to the invention, which, in this example is used in shelf-edge displays in shops, but could also be used in credit cards of all kinds, and in a wide variety of other display applications.

The display comprises, in order, a first substrate 1 being 100 mm x 50 mm of glass sheet or ceramic such as aluminium oxide or plastic such as a polyethylene terephthalate.

Lines 2, e.g. evaporated or sputter deposited, of a metal such as chromium, or an alloy of e.g. nickel-iron or nickel-chromium, form a first set (horizontal) of electrically resistive heating elements 1.5 mm wide and 100 nm thick, on which set is sputtered an electrically insulating layer 3 such as silicon dioxide to a thickness of 100 nm. On this, there is evaporated or sputter deposited a second (vertical) set of electrically resistive metal lines 4 for heating running at right angles to the first set of lines 2 and of about the same thickness and width. Then comes an electrically insulating layer 5 the same as 3. A layer of electrical conductor 6 is then laid down by any one of several methods, vacuum deposition, chemical vapour deposition or electroless plating. The material of the layer 6 might be copper, or silver or nickel or indium tin oxide, all about 100 nm thick.Over this comes a thick layer 7 of sodium vanadium bronze, made into a printing ink and applied by screen painter (or doctor blade) and given a low temperature heat treatment to remove carrier material. Alternatively vacuum deposited V2O5 can be used with either simultaneous deposition of sodium or subsequent insertion of sodium up to, typically, a formula value of nap 5 V205. Other oxide bronze guest/host combinations would prove equally applicable, e.g. host of MoO3, WO3, TiO2, Nb203 or IrO2 and guest atoms of H.

Li, Na, K, Cs, Rb or Mg. A layer 8 is next provided of polymer electrolyte for the same cation as the guest atom in the oxide bronze, in this example sodium, and having a conductivity which increases strongly with temperature. The electrolyte might be sodium perchlorate in polyethylene oxide polymer in a typical molecular ratio of one to sixteen.

The polymer would be heavily loaded (10% by volume) with a white opaque 5-micronrange powder, such as TiO2, giving optical isolation between 3 and 7 and increasing the electrical resistivity of the electrolyte. Such a polymer has an electrical resistivity at 30"C of about 1010 ohm cm, at 80C of about 5 x 106 ohm cm, and at 1200C of about 5 x 105 ohm cm. Other salts and polymer hosts are possible, for example sodium borate, estimate or trifluoromethane sulphonate in n-butyl acrylate, polypropylene oxide or other polymer.Contributing to a large change in conductivity between 80"C and 1200C are cations which are relatively immobile at lower temperatures, hence H and Li guest atoms are possible, Na is suitable and K and especially Mg, Cs and Rb are preferred. The layer 8 is applied by printing it with a solvent carrier and then evaporating the carrier leaving behind the unplasticised polymer electrolyte. Next is a tungsten trioxide film 9 deposited on a transparent conducting coating 10 which in turn was deposited upon a viewing window in the form of a glass or transparent plastic substrate 11. The tungsten trioxide film 9 is the operative electrochromic layer, colouring and uncolouring (as viewed through the transparent substrate 11) when guest cations are inserted and withdrawn.While the layer 7 will also usually be electrochromic, this is an accident of chemistry and of no consequence to the user of the display since the layer 7 is hidden from view by the opaque powder in the layer 8.

This upper set 9, 10, 11 is sandwiched on to the lower set 8 to 1 to make a complete structure. The periphery of the structure is sealed with a water resistant polymer, e.g. a butyl such as used in making glass double glazing units. The overlap, around the edges, of the various layers is arranged so that there will be access to the resistive metal heater lines and to the electrodes ofthe electrochromic materials, these all being electrically isolated from each other.

The device, which is intended for normal display use at room temperature, functions as follows. A resistive "horizontal" metal line 2 is activated (e.g. on a preprogrammed signal from some central location) by passing a current down the line 2 for t seconds of sufficient strength to raise the temperature of the polymer electrolyte immediately above it to a temperature T1 ,max and to a temperature 2 T, for a period of time t1 where t1 will be less than t. At the same time, on this preprogrammed signal, a selection of the whole set of orthogonal "vertical" resistive metal lines 4 is addressed, which means that where a pixel is to be written a current is passed along the line, and where no writing of a pixel is required no current is passed.At the cross over of two current carrying lines the pixel area will rise in temperature to a value T2,max in the time t and to a temperature 2 T2 in the time t1. A suitable choice of electrical addressing will ensure that T2 2 T1 ,max and that t1 is typically half t. In the time interval t1, while the pixel is at temperature 2 T2 the electrochromic cell is switched to writing for a period t2, where t2 < tl < t. It is in this period, t2, that the line of pixels is written as dictated by the information placed on the second set of metal lines.Typically T1,max = 800C, T1 = 65"C, T2,max = 140"C, T2 = 120 C: t is 1 second, tl is 0.8 sec and t2 is 0.5 sec.

To have writing at the pixel and not along the whole line, an activation energy in this example of geater than 0.75 eV is advisable. In addition, writing should only just occur at 1200C. With these conditions, writing would definitely not occur at 80oC. The conditions imply a diffusion coefficient at 120 C of D = L2/t, where L is the film thickness and t is the writing time for a pixel.Typically L = 3 x 10-5 cm and t = 0.5 sec, so that D = 1.8 x 10-9 cm2/sec. (This assumes a slab model for diffusion, which is operationally useful.) Such a high temperature diffusion coefficient can be obtained in two ways, either by selecting a suitable (rather sessile) guest atom, e.g. potassium in W03, or by depositing large grained active material (typical grain size is 100 A): the effective diffision coefficient is sensitive to the mean grain size of the host material. Sodium into W03 of grain size about 600A would give such an effective diffusion coefficient. A combination of these two ways can be employed.After 1 second the next horizontal line 2 and the next selection of vertical lines 4 receives current, activating the desired pixels, and so forth until a complete display is written, this having been done a-line-at-a-time.

The high temperature-dependence of the electrolyte resistivity (layer 8) will ensure that unwritten pixels when at room temperature, approximately 30"C, will not be written (coloured) by discharge of already written pixels. Thus the electrolyte resistance at 30"C is about 108 ohms/cm2, which at one volt potential will take 105 seconds (- 1 day) to pass 1 milli-coulomb per cm2. Such devices would require rewriting only after several days.

Alternatively it would be possible to make use of the temperature dependent diffusion of e.g. potassium or magnesium tungsten bronzes or sodium titanium bronzes.

rather than use the temperature-dependent properties of the electrolyte.

The matter of self discharge has been discussed above and illustrated in Figure 2.

The thermal matrix was invented to overcome the stated problem. But the difference in cell potential between a written and an unwritten pixel is only about 100 mV to 200 mV. The writing potential is about 1500 mV. Thus if an array of pixels can be written in sH seconds.

where s is the number of lines to be written and H is the time to write each line. and the significant discharge time is longer than sH seconds we can heat the entire set of lines all at the same time (sH seconds) without fear of discharge. That is, writing and discharge only occur at the elevated temperature. In this case each pixel would be electrically isolated as shown in Figure 2. The self-discharge current shown to flow in that diagram would not flow for sufficient time to give a significant self-discharge. Thus the invention extends to a matrix-addressed array of isolated pixels which are only writable at high temperature (e.g. 120 C) and which do not significantly discharge in the period that the complete matrix is hot. The materials proposed above would suffice. However there is a limit to the number of lines which are multiplexible on this strategy. The experimentally observed ratio in resistance R between 150 mV and 1500 mV for a WO3 host material is about 100:1 so that if we have a pixel written to c milli Coulombs of charge per cm2 1 and allow f milli Coulombs of charge per cm2 of discharge (where c > f), then we may address s lines where s = R x f/c. If R = 100 and f/c = 0.1 then s = 10. This would therefore be sufficient for one set of 5 x 7 characters or even 7 x 9 characters.

Claims (4)

1. An electrochromic display, comprising a guest-atom-dependent electrochromic layer, a guest-atom-transmitting layer in contact therewith, a guest-atom sink/source layer in contact with the transmitting layer and means for applying a potential between the electrochromic layer and the sink/source layer, characterised in that the sink/source layer and/or the transmitting layer is inoperative at the normal temperature of the display and further characterised by rows and columns individually switchable to supply heat energy to the inoperable layers such that only in the region where a switched-on row passes a switched-on column is the thermal effect adequate to bring the sink/source layer and/or the transmitting layer into operative condition to allow the electrochromic display to be altered in that region.

2. An electrochromic display according to Claim 1, wherein the guest atom is Mg, K, Cs or Rb.

3. An electrochromic display according to Claim 1 or 2, wherein the resistivity of the transmitting layer falls by at least a factor often in a temperature rise of 40C deg starting at a temperature which itself is at least 40C deg above said normal temperature..

4. An electrochromic display according to any preceding claim, wherein the rows and columns are resistive heating wires, the rows being electrically insulated from the columns.