CA1050672A - Method of operating an electrochromic display device - Google Patents

Abstract

METHOD OF OPERATING AN ELECTROCHROMIC DISPLAY DEVICE
Abstract A method and apparatus for generating an electrochromic display with reduced write time by generating equal current pulses of such magnitude that the transition time is less than that required to deposit sufficient material to write the display in a single cycle, and distrib-uting the current pulses sequentially and cyclically to each of the selected display electrodes until the display is written. The time taken to write the display by this technique is less than that required to write the same display by applying a single write pulse.

Description

Background of the Invention Field of the Invention The present invention relates to dic:play devices and more particu-larly to a method and apparatus for generating a display by means of an electrochromic display arrangement which operates by the reversible deposition of material from an electrochromic solution to write at selected display electrodes.
Description of the Prior Art Electrochromic display devices are generally well known in the art as may be seen, for example, by reference to British Patent 1,376,799 and United States Patent 3,864,589.
Heretofore it has been the practice to generate a display by supplying continuous charge sequentially to each display electrode until ~X~7~
sufficient material has been deposited to ~orm an acceptable indication of the information to be presented. There is a limit ~o the magnitude of current that can be supplied in this Wcly since for each value of current there is a time, called the transi~ion time, at which the material has zero concentration in the neighborhood of the electrode. The magnitude of current has been chosen so that the transition time is longer than the time required to generate a display at the display electrode.
It has be~n found, in accordance with the present invention, that a simple variation of the above-described method of generating a display leads, surprisingly, to faster write time such that much speedier display generation is achieved.
Calculations by I. F. Chang and W. E. Ho~lard in the article "Performance Characteristics of Electrochromic Displays," published in the IEEE Trans. on Electron Devices, ED-22, No. 9, September 1975, pp.
749-758, show that for the material quoted only between 20 and 70 lines of display electrodes can be operated in one second, depending on ~Jhether '~
the display is transmissive or reflective. The method in accordance with the principles of the present invention improves these fi~ures more than ten fold.

Summary of the Invention It is, therefore, an object of the present invention to provide an improved display device.
It is another ob;ect of the present invention to provide an electrochromic display device with faster writing speeds.

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It i9 a further ob~ect oE the present invention to provide a display device which operates by the reversible deposition o~ material from an electrochromic solution onto selected display electrodes, and which has a plurality o~ display electrodes. The method, in accordance with the present invention, comprises genera~ing equal current pulses o~
such magnitude that the eransition time is less than that required to deposit sufficient material to provide a display. The equal current pulses are distributed sequentially and cyclically to each selected ;
display electrode until the required display is generated, whereby the time taken to generate a display by the method i5 less than that required to generate a display of the same contrast by applying a single pulse of sufficlentIy long transition time sequentially to each disylay electrode.
In accordance with tne principles of the present invention, there is provided a method of generating a tlisplay by means o~ an electro~
chromic display device which operates by ~he reversible deposition oL
material from an electrochromic solution onto selected display electrodes, and which has a plurality of display electrodes arranged, for example, in groups, such as rows. Current pulses are generated of such magnitude that the transition time is less than that required co deposit sufficient material to provide an acceptable display. Electrodes are selected, for example, from each group o~ display electrodes in accordance with the in~ormation to be displayed, and current pulses are sequentially and cyclically distributed to each group of display electrodes until the required display is generated. The time taken to generate a display by ~-the method is less than that required to generate the same display by applying a single pulse of sufficiently long transition time sequentially -to each group of display electrodes. ~ ~ ;
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The foregoing and other objects, features and advantages oE
the invention will be apparent from the following more particular description of the preferred embodiments oE the invention as illustrated in the accompanying drawings.

Brief Description of_the Drawi~
FIG. l is a graph showing the concentra~ion of electrochromic material at a display electrode.
FIG. 2 shows schematically one embodiment of the display device to which the invention can be applied.
FIG. 3 shows schematically another embodiment of the display device to which the invention can be applied.

Detailed Description of the Draw ~n~s An electrochromic display using a solute electrochromic material operates by the deposition of species ae a display electrode. The amount of charge (in Coulombs) required to deposit sufficient species to give an acceptable display can be calculated or determined by experiment and can, for a given electrochromic material and electrode geometry, be considered fixed. There is, howeverj a limit on the amount of charge that can be supplied, since the concentration of species at the electrode eventually reaches zero. The time taken for this to cccur is called the ~;
transition time ~
The phenomenon is illustrated in FIG. 1 which shows the concentration of species at the display electrode. Concentration is indicated as a function of distance from the electrode. Initially the concentration at the electrode is the bulk concentration C of species in the electrolyte.

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This is the condition at time t . After current has been supplied for a time t, the concentration o~ species at the electrode is reduced. It is further reduced after a time t2, until after a time T, the transition time, the concentration at the electrode reaches zero and no more species is available for deposition.
It can be shown that T~ = K ( ~ D ) ~ c/ 2 i where K is a constant for a given species and electrode, D is the diffusion coefficient, which again is a constant for a given species, C is the ~
bulk concentration of the species in the electrolyte and i is the applied -current density.
As an example, let it be assumed that the electrochromic j-material is heptyl viologen dibromide, as disclosed, fo~ example, in British Patent Specification 1,376,799.
Then K can be taken as approximately 10 coulombs and D X 10 `
cm /sec. A suitable concentration C is 0.2 X 10 3 moles/c.c. The final ~ -assumption is that the amount of charge required to achieve an acceptable ~ -display is 10 2 coulombs/cm2.
Straightforward calculation then gives ~able I which shows the transition time (Column II) for a given applied current density (Column I), the amount of charge injected in the transition time (Column III) end the number N of pulses required to inject 10 2 coulombs/cm2 (Column IV).
Table I ~ i-I II III IV
i amps/cm T secs i.T coulombs/cm N
16 X 10-6 0.16 X 10-3 60 10 4 0.~ 2 10-3 25 1 16 X 10-4 1.6 X 10-3 6 ~ ; -0.4 10 ~ X 10 3 2.5 0.1 16 X 10 2 16 X 10 3 less than 1 _; , . .
UK975-015 -5- ~ -' In order to achleve a satisfactory display with just one pulse, a current density of about 0.2 amps i5 required to be applied for jUSL under 50 X lO 3 secs. A typical display may have about 500 rows of display elements. I~ each row is addressed sequentially and suficient power applied by a single pulse to achieve the required display, the time taken to address the display is 500 X 50 X 10 3 secs., i.e., 25 secs.
The effect of applying a sequence of pulses each having a transition time such that the power injected is insufficient to provide a satisfactory display will be more fully understood from the following discussion. Suppose, for example, that a sequence of pulses each providing a current density of one amp and a length equal to the transition time is applied to each row of the display. Each row of the display is addressed consecutively and at each access of the row only one pulse is injected. In other words, the sequences are supplied in time multip~e~
to the rows of the display. Using the figures of Table I, the time ~
taken to do this is, for 500 rows, 500 X 16 X 10 4 X 6 secs., i.e., 4.8 ~ ' secs. The reason for the time saving is that during the intervals -between the pulses of a sequence, the concentration of species at the electrode recovers to reach the value of the bulk concentration. Since the rate at which deposition of species takes place is a function of the - ~ -concentration of species at the electrode, then by restarting deposition at each pulse of the sequence at the bulk concentration, a saving in the time required to deposit a certain amount of species is achieved. The time spent in waiting for the concentration to recover is used to address other rows o~ ~isplay sit~

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1 Table I:[ shows the time required to generate a display in a device of 500 rows of display sites (Column III), using N pulses (Column II) of current density i (Column I). The time is calculated by multi-plying the transition time by N times the number of rows in the device.
Table II
I II III
i N secs 0.48 4 25 1.25 l 6 4.8 Clearly, the method of the invention requires drive circuitry capable of handling high current values, and there is a design choice as to the most appropriate speed of operation of the device against the cost of providing such high current circuits. `~ ~
In addition to the time saving in writing, it should be noted ; -that a further advantage of the invention is that the display becomes visible to the user simultaneously over the whole display area instead of line by line as heretofore was the case. ;
Two examples of electrochromic display device to which the invention may be applied will now be given. Both involve use of constant ~ ~;
current drivers, but it should be understood that the invention is also applicable to other methods of activating a display, such as potentio-static or constant voltage drive. In both cases 9 the electrochromic material used is heptyl viologen dibromide.

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FIG. ~ shows an electrochromic display device of the ~ind described in U.S. Patent 3,864,589 wherein each display site i3 at the crosspoint of orthogonal electrodes. A hvllow flat panel 1 contains heptyl viologen dibromide in aqueous solution. On the upper inner face of the panel is deposited a set of parallel transparent vertical electrodes

2, for example, of tin oxide. On the lower inner face of the panel is deposited another set of parallel electrodes 3 which are arranged or-thogonally to the electrodes 2. Each crossover of the electrodes 2 and

3 defines a display site. When current of the appropriate polarity passes between the electrodes 2 and 3 defining a display site, a mauve deposit appears at one of the electrodes where it is overlapped by the other electro~e. Typically, it is preferable that the lower electrode receive the deposit. The electrodes 2 are connected to a potential source V through respective transistors 4. For simplicitv, the transistors

4 are shown as field effect transistors although they may be bipolar.
The conductivity condition of each transistor is controlled by rl respective bistable circuit 5, of conventional construction, in such a way as to provide a constant current to each electrode 2. ~3 indicated schematically `in FIG. 2, when a bistable circuit 5 is in the "1" state, the associated transistor 4 is rendered conductive, and when it is in the "0" state, the associated transistor 4 is nonconductive. Taken together, the bistable circuits 5 comprise an input register whereby the required `~
display of a single row is defined.
The horizontal electrodes 3, as shown in FIG. 2, are connected - through respective transistors 6 to a pulse generator 7. The gates of the transistors 6 are connected to the respective outputs of a distributor 8 which is also driven by pulse generator 7. The generator 7 consists ~975-015 -8-~ t7 ~ ~
of an adjustable oscillator and a pulse shaper both of which elements are of conventional construction. The di.stributor 8 is ring counter, also of conventional construction, with output lines 9 respectively connected to the gate electrode of each transistor stage. When the count reaches a stage, the line 9 is energized in the usual manner.
In operation. as the row electrodes 3 are successively addressed in cyclic fashion as hereinabove described, the bistable devices 5 of those column positions at which displa~J is required are set to the "1"
state. The signals necessary to do this are transferred in known manner ;
from a binary store and are preferably synchronized by pulse generator - 7. The information transfer forms no part of the invention and can be accomplished in any convenient manner. ;
An alternative form of disp.lay device is shown in FIG. 3. The .;
device 10 does not differ from that shown in FIG. 2 in the manner of ~ ~
control, and corresponding elements have the same reference numerals as : ;
in FIG. 2. The difference is to be found in the electrode arrangement.
In particular, the upper electrode 11 is a sheet of transparent conductive material, such as tin oxide, located on the upp~r inner face of the panel, whereas the lower electrodes are conductive pads 12 on the lower inner face of the panel. Each pad 12 is connected through the lower face of the panel to respective transistors 13, which are sho~n as a field-effect transistors but which may be bipolar transistors. The gate electrodes of all transistors 13 connected to pads 12 belonging to the same row are connected to a common row line 14 and the row lines 14 are connected to respective transistors 6. Transistors 6 are connected to pulse generator 7 and distributor 8 as described with re~erence to FIG.

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The source electrodes of all transistors 13 connected to pads 12 belonging to the same column in FIG. 3 are connected to respective column conductors 15. Bistable circuits 5 are arranged to activate respective column conductors 15 when the bistable circuits are in the "1" staee in such a way as to provide a c:onstant current to the column conductors 15. The electrode 11 is connected to a potential source V.
It will readily be seen that the device lO of PIG. 3 is directly equivalen~
to device 1 of FIG, 2 and operates in the same way, the only difference~
is that the display sites, instead of being defined by the crossovers of orthogonal electrodes, are defined by the pads 12.
There has been described a method and apparatus for providing a display by ~means of an electrochromic display operation having a liquid electrolyte. Display sites are sequentially and cyclically supplied with equal current pulses of such magnitude that the transition time is less than that required for a current of that magnitude to generate a display. The supply of pulses continues until the requlred display is achieved.
It is clear that the effectivensss of the invention depends on the number of display sites and thus the recovery time of the solution.
Many factors affect this: the bulk concentration, the solute, and the nature of the electrochromic material, for example. It can, however, be said that even with only ten display sites or rows of display sites, an improvement over the k~own technique of using a single pulse to generate a display can be expected. Since recovery time is empiric, no minimum number of display sites to which the invention can be applied can be stated.

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It should be understood thar ideal pulse generation and distribution have been assumed. The values given are realtive and indicate the proportional improvement to be expected in accordance with the principles of the present invention. Absolute timing values can be expected to be a few percent higher in each case.
While the invention has been particularly shown and described wi~h reference to preferred embodiments thereof~ it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the inven~ion.

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Claims (8)

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

1. A method of generating a display by means of an electrochromic display device which operates by the reversible deposition of material from an electrochromic solution onto selected display electrodes, and which has a plurality of display electrodes, the improvement comprising the steps of;
generating equal current pulses of such magnitude that the transi-tion time is less than that required to deposit sufficient material to provide a display; and distributing said current pulses sequentially and cyclically to each selected display electrode until the required display is generated, whereby the time taken to generate a display by such method is less than that required to generate the same display by applying a single pulse of sufficiently long transition time sequentially to each display electrode.

2. A method of generating a display by means of an electrochromic display device which operates by the reversible deposition of material from an electrochromic solution onto selected display electrodes, and which has a plurality of display electrodes arranged in groups, the method comprising the steps of;
generating equal current pulses of such magnitude that the transition time is less than that required to deposit sufficient material to provide a display;
selecting from each group of display electrodes those electrodes at which a display is required; and distributing said current pulses sequentially and cyclically to the selected ones of each group of display electrodes until the required display is generated, whereby the time taken to generate a display is less than that required to generate the same display by applying a single pulse of sufficiently long transition time sequentially to each group of display electrodes.

3. The method as set forth in claim 2 wherein said electro-chromic solution is heptyl viologen dibromide in aqueous solution.

4. The method as set forth in claim 3 wherein said plurality of display electrodes are arranged in sets of horizontal and vertical electrodes having crossover points which define the display sites of the display device.

5. A method of generating a display by means of an electro-chromic display device which operates by the reversible deposition of material from an electrochromic solution onto selected display electrodes of a plurality of display electrodes, the improvement comprising;
generating current pulses of such magnitude that the transition time for said material is less than the time required to deposit suffi-cient material to provide a display;
selecting from said plurality of display electrodes those electrodes at which a display is to be written; and distributing said current pulses sequentially and cyclically to the selected ones of said display electrodes until the required display is written, whereby the time taken to generate a display is less than that required to generate the same display by applying a single pulse of sufficiently long transition time sequentially to each group of display electrodes.

6. The method as set forth in claim 5 wherein said electro-chromic solution is heptyl viologen dibromide in aqueous solution.

7. The method as set forth in claim: 6 wherein said current pulses have a current density between approximately 0.1 and 10 amps/cm2.

8. The method as set forth in claim 5 wherein said step of selecting comprises selecting by a matrix address arrangement.