CA1191637A - Electrolytic display with a stable reference electrode system - Google Patents

Abstract

ABSTRACT

Electrolytic apparatus employing a reference electrode, e.g. an electrochromic display of the type employing potentiostatic control of write or erase operations, has at least one reference electrode on which a predetermined coating thickness of electrochromic or other electrodepositable material is maintained or replenished by intermittent connection of the electrode to a source of write current. Such a coated reference electrode has a stable potential with respect to the solution which is necessary for potentiostatic control. Two such reference electrodes may be used alternately such that one is being erased and rewritten while the other is connected as a reference.

Description

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~K9-79-015 1 ELECTROLYTIC DISPLAY I~I~H A ,~TARLE REFERENCE
ELECTRODE SYSTEM

Technical Field of the Invention The present invention relates to electrolytic apparatus employing reference electrodes and to methods of operating such apparatus.

Background of the_Invention Reference electrodes are employed in many electrochemical processes to sense the potential of a solution in an electrolytic cell. The sensed potential is often employed to control the operation of the cell po-tentiostatically.

One type of electrolytic apparatus to which the invention has specifically been applied is an electrochromic display of the kind employing an electrochromic substance in solution which is transparent while dissolved but coloured when electrodeposited upon an electrode. The coloured and transparent states form a redox pair so that the de?osi'ed coloured material can be electrolytically removed by reversing the current direction. One well known substance of this type is the 1,1' di-heptyl-4,4'-bipyridinium di-cation which is one of the class of electrochromic substances known as the viologens. Transparent in solution, it can be reduced electrochemically to the radical cation which is violet coloured. In the presence of a suitable anion such as bromide, phosphate or phosphate/hypophosphite mixtures the coloured viologen radical salt precipitates out on the cathode.

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l In order to control the write and erase operation of this type of display, it is known to provide in addition to display and counter electrodes, a reference electrode which senses the potentlal of the solution. Such a reference elec-trode can be used to control both write and erase operations depending on the particular control scheme selected.

In one known method of controlling a display, selected display electrodes are written to a predetermined contrast by employing a constant current source for a fixed period of time. Under these conditions, a fixed charge is passed and a fixed amoun-t of material is deposited. If the deposit remained absolutely stable upon the display electrodes and conditions re~ained unchanged, the written display electrodes could be erased by passing an opposite sense constant current for the same period of time. However, many electrochromi~
deposits slowly redissolve with the consequence that the elec-trode would be overerased. The forcing of a constant current after all the electrochromic material has been removed would drive the display electrodes more anodic~ Depending on the particular materials employed, this could lead to ir-reversible electrolytic damage to the display electrodes or to damage by liberation of gases within the cell.

Accordingly, the technique of potentiostatic control of erasure has been employed whereby the potential of the counter electrode is controlled with respect to the solution potential as sensed by a reference electrode in the vicinity of the display electrodes. In the method ~ost commonly em ployed, the reference electrode potential is compared with a predetermined potential corresponding to substantially com-30 plete erasure of the display electrodes and the result of the comparison used to control the potential of the counter elec-~9 79-015 3 1 trode. Erase current is thus passed through the cell until the reference electrode potential has dropped to the pre-determined level. By allowing a small safety margin, over erasure is prevented. The use of reference electrodes in this way is described in a review article entitled "Electro-chromic Displays" (New Electronics, 16 September 1975, page 66).

Another use of reference electr~des is to control the wrlte process by maintaining a threshold potential sufficient for the reduction (or oxidation) of the electrochromic sub stance. Such a use is described in UK Patent 13767~9 (Philips) and US Patent 3950077 (Jasinski, Texas Instruments).

US Patent 3950077 is primarily concerned with overcoming the alleged disadvantage of a reference electrod~ that an external potential regulating circuit is required. It proposes a non-polarizable counter electrode ~hich is a lead/lead-phos~hate half cell. The potential of such a counter electrode does not vary with respect to the solution as would a simple metallic counter electrode. Because of this the counter electrode potential accurately determines the potential at the display electrode and the need for a reerence electrode is avoided.

U.S. Pat. Nos. 4,167,309; 4,167,308; and 4,256,380 also discuss the limitations of reference electrodes in large displays.
These are essentially that, since the reference electrode cannot represent the solution potential over the whole area of the display electrodes, the large variation in potential drop through the solution bet~een different display elec-~K9-79-015 4 trodes and the counter electrode will cause uneven ~riting and erasure of the display electrodes. One of the patents, U.S.
4 1~7,308 proposes a reticulate counter electrode covering the whole area of the display which is pre-charged to stabilize its potential. One oE several ways of precharging the counter electrode is to charge it with the electrochromic substance (viologen) itself. The redox reaction of the viologen at the counter electrode then acts to stabilize its potential with respect to the solution.
Thesepatents acknowledge that the drawbacks of reference electrodes are only severe with large area dis-plays. ~ith small displays, of the order of a few centi-metres in width, there is no real alternative to the use of a reference electrode as the counter electrodes are too small ; 15 to maintain their charge for display operations and attemDts to increase their area would compromise the visibility of the ; display electrodes.

References to the actual nature of the reference electrode in the prior art are scanty. The implication is that any conductor will do. The Philips U.K. paten-t 1376799 suggests that the reference electrodes may be of the "same material as the image electrodes" or alternatively can be made from "glass, calomel or the li~en.

Disclosure of the Invention .

The suggestions as to appropriate reference electrodes in the prior art have been found to be inadequate for a prac-tical small display. ~ standard calomel reference electrode is a large and cumbersome half cell which would have to be remote from the display cell and draw fluid from it by a ~K9-79-015 5 l capillary tube. The simpler prospect of using an electrode similar to the display electrode or a simple wire ?roves un-satisfactory in that the potential Gf the electrode is un-stable with respect to the solution.

Experimental studies have demonstrated that the potential of a silver wire electrode in a viologen display varies and will drift over a period of time. Such variations can be caused by capacitive and leakage currents in the elec-trode or by impurities in the solution which react ~ith the silver electrode.

~lthough the potential of a small reference electrode can be stabilized by deposition of a sufficient charge of electrochromic material thereon, this of itself ~oes not ?ro-vide a practical reference electrode since such deposits dis-solve away.

These considerations, although particularly aDplicable to electrochromic displays, are also relevant to other electrolytic apparatus employing a reference electrode.
Accordingly at its broadest, the present invention ~rovides an electrolytic apparatus comprising:- a cell having a working electrode , a counter electrode and a reference electrode, and containing an electrolyte including a reversibly electrodepositable material in solution, and drive meansfor electrodepositing the material on or electrolytically removing the material from the working electrode, either the deposition or removal being controlled with reference to the solution potential sensed by the reference electrode, characterized in that the apparatus furthe_ comprises: first switching means for connecting the reference electrode J

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U~9-79-015 6 1 to a source of electric current to cause deposition of the material on the reference electrode, second switching means for connecting the reference electrode to the drive means to provide an indication of the solution potential, and reference control means for controllinq the first and second switching means to operate alternately so that the reference electrode has a deposit of material sufficient to stabilize its potential with respect to the solution prior to being connected to the drive mezns.

Considered from the display aspect, the present invention provides an electrochromic display comprising: a cell having a display electrode, a counter electrode and a reference electrode and containing a solution of a reversibly electrodepositable electrochromic material, and display drive means for writing said display electrode by electrodeposition of the electrochromic material thereon and erasins said electrode by electrolytic removal of the electrochromic material, either the write or erase operation being controlled with reference to the solution ~otential sensed by the reference electrode, characterized in that the display further comprises: first switching means for connecting the reference electrode to a source of electric current to cause deposition of the electrochromic material on the re'erence electrode; second switching means for connecting the reference electrode to the display drive means to provide an indication of the solution potential, and reference control means for controlling the first and second switching means to operate alternately so that the reference electrode has a deposit of electrochromic material sufficient to stabilize its potential with respect to the solution prior to being connected to the display drive means.

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l By providing a permanent means of depositing electro-chromic or other electrodepositable material on the reference electrode, the necessary deposit can be replenished or replaced whenever the reference is not in use.

It is normally desirable to have a continually available reference and to this end it is a preferred feature of the inventlon to provide two such reference electrodes each having a respective first and second switching means, the reference control means causing alternate operation of the second switching means so that one of the reference elec-trodes is always connected to the display drive means.

In order to ensure that the predetermined amount of electrochromic material is deposited on the reference elec-trodes it i5 preferred that the display also comprises a pair of third switching means for connecting respectively each of the reference electrodes to a source of erase current to remove any electrochromic material therefrom, the reference control means being arranged to operate the third t first and second switching means of each reference electrode in succession and so that the second switching means of either reference electrode is operated concurrently with the successive operation of the third and first switching means of the other reference electrode.

In order to facilitate the erasure of each reference electrode it is al50 a preferred feature that the potential of the source of erase current is maintained in fixed relation with that of the reference electrode currently con-nected by its second switching means to the display drive means. This can be achieved by having an offset amplifier as the source whose input is connected to the reference 3'7 ~K9-79-015 8 1 electrodes by the second s~itching means and whose output is connected to the reference electrodes by the third s~itching means.

Preferably this source of erase current is also common to the display drive means and is selectively connectible to the display electrode to erase it.

Thus the invention provides a method of operating an electrolytic apparatus comprising a cell, a working electrode, a co~nter electrode and a re~erence electrode, the cell beinq filled with an electrolyte including a reversibly electrodepositable material in solution, the method comprising the steps of electrolytically depositing said material on or removing said material from the wor~ing electrode by passing a current in an appropriate direction between the counter and ~or~ing electrode , sensing the solution potential in the neighbourhood of the working electrode by means of the reference electrode and controlling either the deposition of said material on the working electrodeor the remo~al of said material .from the working electrode by means of the sensed solution potential, the me~hod being characterized in that the sensing step comprises a discontinuous sequence of sensing operations and by the further step of passing a current through the electrolyte to cause deposition of said material onto the reference elctxode in the inter~als between the sensing operations so that the reference electrode is written with a deposit of a predetermined amount of said material which stabilizes its potential with respect to the solution.

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l From a display aspect the invention provides a method of operating such a display comprising the steps of writing or erasing the display electrode by passing current between the counter and display electrodes respectively to deposit the electrochromic material on or to remove the electrochromic material from the display electrode, sensing the solution potential in the neighbourhood of the display electrode by means of the reference electrode and controlling either the writing or erasing of the display electrode by means of the sensed solution potential, the method being characterized in that the sensing step comprise a discontinuous sequence of sensing operations and by the further step of passing a write current through the solution to cause deposition of the electrochromic material onto the reference electrode the reference electrode is written with a deposit of a pre-determined amount of electrochromic material which stabilizes its poten-tial with respect to the solution.

Brief Description of the Drawings Figure 1 shows schematically a write and an erase circuit arrangement for an electrochromic display cell, employing a reference electrode;

Figure 2 shows the variation of current with voltage in the cell of Figure 1 under different display conditions;

Figure 3 shows the variation o~ reference electrode : 25 potential with charge in the cell of Figure 1;

: Figure 4 shows schematically an electrochromic display and associated drive circuitry according to the invention, which employs dual reference electrodes;

l Figure 5 shows in detail a reference control circuit forming part of the circuitry of Figure ~; (see second sheet of drawings for Figure 5.) Figure 6 is a plan view showing the physical structure of the display of Figure 5; and Figure 7 shows a cutaway view through a portion of the display of Figure 6 to illustrate the integrated dis?lav electrode structure~

Detailed Description Before describing the detailed arrangement and control lO of reference electrodes in an electrochromic display according to the invention, a basic method for writing and erasing such displays will first be discussed in connection with Figures 1 and 2. The cell schematically illustrated in Figure 1 contains a solution of an electrochrom~c substance 15 such as viologen, a preferred form being a mi~_ure of 1,1' di-heptyl-4, 4'~ bipyridinium phospha~e and hypophosphite as described in published European patent application 0001912.

Within the cell are shown three electrodes, a disolav electrode 10, a counter electrode 11 and a reference 20 electrode 12. In practice, the display electrode 10 s one of a nwnber of display electrodes which are selected as plcture elements (pels) i.n accordance with the infornation to be displayed. However, only one such electrode is shown for ease of explanation. A preferred form of displa~ electrode 25 for a viologen electrochr~mic system is a rough plated silver electrode. The rough silver acts as a diffuser of light and has a matt white appearance when unwrittenO The rough surface also has electrochemical advantages as exolalned in `f''` ' ~, . . _ ..

l published European patent application 0004548. The preferred counter electrode for this type of system is a platinum black plated foil at one side of the display cell.

The reference electrode 12 is an electrical conductor which is assumed to have a stable potential with respect to the solution. Preferably the reference electrode will be of the same material as the display electrodes, in this case silver.

The con-trast achievable with such a display is pro-portional to the electric char~e passed. To ensure uni-formity of colour, a constant current writing method is em-ployed. Thus to write display electrode 10, the counter elec-trode 11 is connected to a source of potential ~V and a constant current source 13 is connected to the floating dis-play electrode lO via switch 14 for a predetermined time period. The passage of a predetermined amount of charge results in the reduction of a corresponding amount of viologen to its coloured radical cation state at the elec-trode 10. The reduced radical cation combines with the anions in the solution and precipitates on the display elec-trode. For an adequate contrast, with the viologen system described, a charge of about 2 mC cm Z is needed. The reference electrode plays no part in the write operatlon and is disconnected by a switch 15.

With reference to Figure 2, conditions at the display electrode during the write process are represented by a point 20 on the common portion of two cur~es 21 ~nd 22 in the lower left hand quadrant. The potential of the display electrode is essentially determined by the value chosen for the constant current from source 13. When the write step has l finished and the current source 13 is disconnected the potential of the display electrode will rise to a rest potential arbitrarily shown as zero on the curve. This is the potential of a disconnected viologen coated silver elec-trode in a viologen solution. ~n electrode so written willremain coated with viologen for some time in the absence of an externally applied potential. This is the so called "memory" effect of this type of electrochromic display.

Considering now the erase process, the upper portion of curve 21 (Figure 2) shows the variation of current through a cell such as that of Figure 1 for a written display electrode which is being driven anodic. As long as the electrode remains coated with viologen, the erase current rises expo-nentially in similar fashion to the write process. The curve 21 corresponds to the o~idation of viologen radical cation back to the dication whlch redissolves in the solution. This process is terminated by the removal of all the viologen from the electrode.
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By way of comparison, if an unwritten electrode is driven anodic, curve 22 shows that no significant current flows until a poten-tial VT is exceeded after which current increases rapidly. This increase in current cor~esponds to unwanted side reactions of the display electrode. In the case of a viologen on silver system, VT is about 550 mV and the side reaction is the irreversible anodisation of the silver to a black for-m. If the erase process is controlled potentiostatically the display electrode potential can be limited to a value VE which, while corresponding to complete erasure, stops short of the side reaction threshold VT.

1 Potentiostatic erasure is effected in the cell of Figure 1 by closing a switch 15 to connect the output of an offset buffer 16, which is a high input impedance amplifier, to the electrode lO. The input to the amplifier 16 is the solution potential sensed by the reference electrode 12. The offset V of the amplifier is made equal to the potential difference VE. Without drawing current from the reference electrode, the amplifier 16 supplies erase current to display electrode 10 until its potential reaches the output potential VERAsE of the offset amplifier which is VE with respect to the reference electrode.

Initially the current is high, as shown at point 23 of curve 21 and remains at this level until the viologen is almost removed. The difference between the potential of the electrode 10 at point 23 and the target erase potential VE
is accounted for by the I.R drop in the cell. Point 23 and a point 24 corresponding to complete erasure lie on a load line 25 which the potential o elec~rode 10 follows as the last viologen is removed and the current in the cell falls.

The implementation of potentiostatic erasure by means of an offset buffer connected to the display rather than to the counter electrode, although different from that shown in the New Electronics article is electrically equivalent and has advantages in connection with the invention which will become apparent.

Successful repeated employment of potentiostatic erasure as described above depends on the potential of the reference electrode being stable at all times with respect to the solution~ It is found however that the potential of a silver wire reference electrode in a viologen solution does not ;3V~

~9-79-015 14 l remain stable but tends to drift. Variations of potential may be caused by capacitative and leakage currents in the solution charging the reference electrode to a different potential or by the effect of deposited impurities on the surface of the reference electrode.

The potential of a silver wire electrode also increases if viologen is deposited thereon in the same manner as on a display electrode. The reduction and deposition of viologen is equivalent to electrically charging the electrode and a typical curve showing varia-tion of potential with charge is shown in Figure 3. The unpredictable variations in potential were found to occur only in the lower portion of the curve.
At hi~her charge levels, above 20~C cm a, the potential is stable and corresponds to the potential of the viologen redox reaction rather than that of the silver itself. Such a coated wire can therefore function as a reference electrode providing that the coating which redissolves fairly rapidly in the order of ~ minute can be maintained or replenished.

In Figure 4, there is shown an electrochromic display, operating according to the basic principles of Figure 1 and Figure 2, but which is provided with dual reference elec-trodes 30 and 31. These electrodes alternately cycle bet~een "reference" and "refresh" modes under control of a reference control circuit 32. The control circuit 32 ensures that one of these two electrodes is always in the reference mode in which it is coated with sufficient viologen to stabillze its potential as described in connection with Figure 3. While one of the electrodes 30 and 31 is in the reference mode the other is being erased and rewritten. The erasure of the elec-trode to be refreshed is desirable so that the amount oviologen subsequently rewritten can be accurately ~K9-79-015 15 l controlled. The detailed operation of the reference control circuit 32 will be described shortly in connection with Figure 5 after first describing further the display and display drive circuit of Figure 4.

The display comprises a sealed cell 33 containing an aqueous solution of a mixture of 1,1' di-heptyl-4, 4'- bipyridinium phosphate and hypophosphite. Within the cell, in addition to the reference electrodes 30 and 31, is a platinum black counter electrode 34, connected to a source of potential Vc, and an array of identical matt silver display electrodes 35, each constituting one picture element or "pel". For ease of illustration only sixteen pels are shown, arranged in a four by four array. In practice a much larger number would be employed.

The pels 35 are formed over a corresponding array of field effect transistors 36 and each pel is electrically connected to the drain of an associated FET 36 by via metal-lurgy. The FET's are themselves formed on a silicon sub-strate and overlaid with inorganic and organic passivation layers.

The write and erase operations of the display cell 33 are controlled by associated display drive circuits in response to externally supplied control signals. The write operation is a constant current process and the erase operation is potentiostatic, as shown in principle in Figures 1 and 2, but with the difference that a large number of display electrodes are involved.

Each of the pels 35 may be individually selected for writing by means of its associated FET 36 which behaves as a i3;7 ~K9-79-015 16 l switch. The individual pels are identified by means of row and column data loaded into shift registers 38 and 39. The row and column shift registers control associated row and column drivers 40 and 41 which activate selected row and column lines 42 and 43 to the gates and sources respectively of the F~T matri~. Thus if a row line 42 is activated, that row of FET's connect the pels 35 of the row to any write or erase currents flowing on column lines 43.

The row drive circuit 40 comprises a string of transistor pairs, such as enhancement mode device 44 and depletion mode device 45, each associated with one stage of the shift register 38. These two devices for~ a line driving inverter which isolates the shift register circuit from the loading of the row select line.

The column driver 41 is slightly more complicated in that it has to provide both erase and write current to the lines 43. Selection of a line 43 for either operation is by means of a transistor switch 46 in accordance with the contents of an associated shift register stage.

The write operation is selected by switching a reference current I(Sw) to the write line 47. Transistor 4~ controls the gate voltage of a number of coupled transistors 49 such that they act as current sources of magnitude equal to the reference current, one for each column, equivalent to the sources 13 of Figure 1. Thus, if a selection transistor 46 is on, a constant current, I , will be drawn from the associated column line 43. The writing process is such that the display is written one row at a time so that only a single FET 36 in any column can be on at any given time.

3~7 ., l The potentiostatic erase process is also controlled by the row and column drivers and can be a block operation. In other words all pels, both written and unwritten, in an area to be erased are selected by loading the row select and column select shift registers 3~ and 39 with the appropriate data pattern. The erase operation is selected by the appli-cation of an externally generated ERASE signal to line 50.
If an ENABLE ERASE signal has been generated by control circuit 32 an A~D gate 51 raises a line 52 to switch on a string of transistors 53. These transistors, when switched on, connect the potentiostatic erase voltage VERASE applied on a line 54 to all the selected column lines 43 via tran-sistors 46. The potentiostatic erase voltage is generated from the solution potential sensed by reference electrodes 30 and 31 by an offset amplifier in reference control circuit 32 similar to amplifier 16 of Figure 1. ~ecause the potentio-static erase process is self limiting, no damage results from the selection and connection of unwritten as well as written pels to the erase potential.

The reference control circuit 32 of Flqure ~ is shown in detail in Figure 5. Essentially the two reference electrodes 30 and 31 are connected to terminals 60 and 61 in the control circuit and the erase potential, VE~sE, required on line 54 of the column drive is provided at output 62 of the control circuit.

The reference control circuit is driven by a timing circuit which produces a number of control signals W1, W2, R1, E1 and E2. These determine whether the individual reference electrodes 30 and 31 are in reference or refresh modes and, if in refresh mode, whether they are ~elng erased or rewritten. The timing circuit comprises an oscillator 64 UK9-7~-015 18 ~- whose output pulses are counted by a counter 65. The counter output is decoded by a decoder 66 which sequentially activates lines 67-70 to produce the control signals Wl, E2, W2 and E1 in that order. The signal R1 is produced by a flip flop 71 which is set and reset by signals E1 and E2. The counter is reset to zero each time the display is powered up by a POWER ON RESET signal at terminal 72.

The cGntrol slgnals are applied to a number of analog switches 73, 74 and 75 which are responsive to the signals to make the appropriate connection~. At the heart of the circuit, the switch 75 in response to reference control signal R1 determines which of the reference electrodes is in "reference" mode and connects that electrode to the positive input of a high input impedance and negative feedback ampli-fier 76. The output of the amplifier is the offset erase potential VERAsE at terminal 62 and is determined by the solution potential sensed by the reference elec~.rode currently connected and a built in offset voltage.

To produce the desired offset, a transistor 77 draws a constant current through an emitter resistor from the feed-back loop of the amplifier. A capacitor 78 smooths any transients at the amplifier input when the switch 75 alter-nates the reference electrodes.

The offset amplifier 76 is employed not only for display erasure but also for reference electrode erasure. Erasure of -the reference electrode in refresh mode is carried out potentiostatically with reference to the solution sensed by the reference electrode in reference mode. The erase potential VERAsE at the amplifier output is connected by switch 7~ in response to either E1 or E2 to the respective electrode to be erased.

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l Once a reference electrode has been erased, the appro-priate signal W1 or W2 causes that electrode to be rewritten by closing one contact of switch 73. Closure of switch 73 connects the refererlce electrode to a constant current source formed by two transistors 79 and 80 and their associated emitter resistors. The transistor 80 provides the reference constant current for both transistors 79 and 77.

The current source 79 is connected for the duration of signal Wl or W2 so that sufficient viologen is deposited to produce a reference of stable potential. As soon as the reference electrode has been written the switch 75 is operated to connect it as reference input to the offset buffer. This alternate cycling between reference and refresh modes continues while the display is powered and is com-pletely asynchronous with the normal write and erase operations of the display. The cycle time is of the order of 10 seconds which is sufficient to allow 20~Ccm 2 to be written and erased in this particular system. The variant of potentiostatic erasure employed, in which the working rather than the counter electrode is manipulated permits a reference electrode to be erased even though the dis~lay is being written and vice versa. The reference and refresh operatlons are not affected by and do not themselves affect the concurrent display operation.

When the display is first powered up the state of the reference electrodes is unknown. It is very likely that neither is written, since any viologen left from the last operational cycle of the display will have redissolved into the solution. Consequently, neither of the electrodes 30 and 31 i5 a reliable reference and the first operation of the new reference control cycle is to wri~e electrode 30 in response l to signal Wl. As soon as electrode 30 has been written, the signal E2 is produced to attempt to erase reference electrode 31, and siynal Rl simultaneously connects electrode 30 as reference. Signal E2 is used to set the flip flop 63 which was reset when terminal 72 received the "power-on reset"
signal. The setting of flip~flop 63 produces the ENABLE
ERASE signal for AND circuit 51 (Figure 4). In this manner, display erasure is initially inhibited until one reference electrode has been written~

The control and operation of a display cell employing dual reference electrodes has been described in detail in connection with Figures 4 and 5. The physical structure of this cell and of the electrodes will now be descrlbed in further detail in connection with Figures 6 and 7.

Figure 6 shows a display cell in which an array 90 of matt silver display electrodes corresponding to electrodes 35 o~ ~igure 4 is formed over an array of FET's integrated on a silicon wafer 91. The display electrodes 90 are sealed ~ithin a rectangular frame 92 and cover 93 made of trans-2arent acrylic material. Filler tubes 9~ ?assing through onewall of the frame enable the cell to be filled wi th an aqueous solution of a mi~ture of 1,1' di-heptyl-4, 4'-bipyridinium phosphate and hypophosphite and subsequently sealed. The counter electrode is an L section platinum foil strip 95 on which platinum black has been deposited and is located along one edge of the display cell. A pair of reference electrodes 96 and 97, equivalent to electrodes 30 and 31 of Figure 4, are located adjacent the opposite edge of the display and consist of fine silver rods about lmm in diameter. These rods pass through and are sealed into the frame 92 of the cell.

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~K9-79-015 21 l The wafer 91 and display cell are mounted on a copper block heat sink (not visible) which itself is mounted on a printed circuit board 98 carrying circuitry (not shown).
Wires to pads 99 on the printed circuit board connect the S reference electrodes -to external circultry on the board including the reference control circuit of Figure 5. The counter electrode 95 is similarly connected by wire to a pad on the circuit board.

The silicon wafer 91 includes not only the FET matrix L0 for switching the pels but also the row and column select and drive circuits shown in Figure d, External connection to these circui~s is made by way of pads 103 on the periphery of the wafer outside the frame. Fine wires connect these pads over an insulating sleeve 101 on the edge of the wafer to a complementary array of pads 102 on the printed circuit board.

The actual structure o the display electrodes forming array 90 and their connection to the underlying .~ET matrix in wafer 91 is shown on an exaggerated scale in Figure 7. Hatch lines have not been used in this schematic sectional view in the interests of clarity.

The underlying silicon substrate 91 has formed on it by conventional @ET technology the array of FET's 36 and selection lines 42 and 43 illustrated in Flgure 4. One dif-fusion region 109 constituting a drain of one of the FET's is shown connected to a portion of an overlying aluminium selection line 110 through an opening in a layer 111 of thermal silicon dioxide. The aluminium lines and the under-lying silicon are covered in conventional fashion by an inor-ganic passivation layer 112 of silicon dioxide which has via holes through it to provide a path for connection to the 3 ~

~K9-79-015 22 l drains of the FET's. A triple layer of chromiumtgold/chromium is evaporated over the silicon dioxide layer 112 and etched through a mask into discrete regions as shown. Some of these regions 113 provide an electrically conductive path through the via to the FET. Other regions L14 serve as light barriers beneath the inter pel gaps to prevent the generation of stray currents by photoconduction in the substrate 91.

An organic passivation layer of polyimide is next deposited over the triple layer regions and selectively etched to expose the via metallurgy. The top layer of chromium in the vias is etched away to leave a clean bare gold surface.

Next a silver layer 116 is evaporated over the entire array area and makes elec-trical contact with the gold in the vias. Further silver is then electroplated through a resist pattern onto the evaporated silver to define the display elec-trodes 117. Fin~ ly, gaps 118 between the display electrodes are opened by removinq the resist and etching away the under-lying evaporated silver back to the polyimide.

Although in the display described, silver rods or wireshave been employed as reference electrodes, it should be realised that other metals could be employed~ Also, the reference electrodes could be deposited on the same substrate as the display electrodes providing they were not in the field of view.

Although two reference electrodes operating alternately are preferred, a single reference electrode could be employed if it could be replenished during display operations, such as constant current writing, for which no reference is needed.

:1319 a ~ ~ 4='1 ~K9-79-015 23 1 Although the display described in Figures 6 and 7 employs an array of identical picture elements, the invention is in no sense restricted to this type of display. The elec-trodes could instead be in the form of characters as for a watch or calculator display. Furthermore although a display integrated on silicon together with associated circuitry has been described, the principles of the invention are equally applicable to a simpler display employing discrete wholly external circuits and another substrate, such as glassr for supporting the electrodes.

Finally, although the invention has been specifically described in terms of an electrochromic display, the invention is applicable to any electrolytic apparatus employing a reference electrode e.g. a plating apparatus which is potentiostatically controlled.

Claims (5)

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

1. An apparatus for use with a source of electric current and an electrochromic cell having a working electrode, a first reference electrode for sensing the potential of the cell and drive means for depositing material on and removing material from the working electrode, said apparatus comprising first switching means for connecting the first reference electrode to a first source of electric current to cause deposition of material on the first reference electrode, second switching means for connecting the first reference electrode to the drive means to provide an indication of the cell potential, a second reference electrode in said cell, said second electrode adapted to serve as an alternate for the first reference electrode in sensing the potential of the cell, said second electrode adapted to serve as a control during the potentiostatic erasure of the first reference electrode, third switching means for connecting said second reference electrode to a first source of electric current to cause deposition of material on the second reference electrode, fourth switching means for connecting said second reference electrode to the drive means to provide an indication of the cell potential, and reference means connected to said first and second switching means for controlling the deposition of material on the first reference electrode, said reference control means connected to said third and fourth switching means for controlling the deposition of material on said second reference electrode wherein said reference means causes alternate operation between said second and said fourth switching means so that only one of the reference electrodes is always connected to the drive means.

2. An apparatus as described in claim 1 wherein said reference control means includes a clock pulse source, a counter and a decode circuit.

3. An apparatus as described in claim 1 including a fifth switching means for connecting one of said reference electrodes to a second source of electric current and to cause the removal of material on said reference electrode.

4. An apparatus as described in claim 3 wherein said second source of current is an offset amplifier.

5. An apparatus as described in claim 3 wherein said second source of current is selectively connectable to the working electrode.