Electrochromic display device

An electrochromic display device is operated by the selective application of electrical signals to first and second electrode matrix addressing means, the potential of the signals being sufficient to produce local color changes in an electrochromic material. Each local color change may be brought about by a short `expose` pulse with a potential above a threshold for coloration, but with insufficient charge to cause coloration, followed by a longer `develop` pulse which is below the threshold potential. The first and second matrix addressing means respectively provide these pulses. The electrodes for the two sets of pulses needed to produce an image are multiplexed to substantially reduce the number of driving circuits required.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a display device of the kind in which electrical 
signals are selectively applied to a material to cause local changes in an 
optical characteristic of the material. 
The invention is particularly concerned with such a display device in which 
the electrical signals are applied in matrix fashion to a suitable 
material. A material in which visible images may be formed by 
chemically-produced colour changes as a result of the selective use of 
electrical potentials, is hereinafter called an electrochromic material. 
With many electrochromic materials, the images are reversible, that is, 
they can be erased by applying a reverse potential to the imaging 
material. 
2. Description of the Prior Art 
In one kind of matrix electrode configuration the electrodes used to 
selectively apply potentials across the material are in the form of two 
spaced sets of parallel conductive strips which overlie one another in an 
orthogonal matrix configuration. By applying potentials to the appropriate 
strip of each set, it is possible to address any point in the material 
defined by an intersection of two strips. By making each applied potential 
less than the coloration threshold potential for the material but greater 
than one half of the threshold potential, it is possible to cause 
coloration only at the selected intersection. 
This configuration suffers from the disadvantage that in order to pass 
sufficient charge through the electrochromic material to cause coloration 
in a usefully short time it is necessary to pass a large current pulse. 
Although this can be achieved, the necessary circuitry is expensive and 
difficulty may be experienced in fabricating electrodes of sufficiently 
high conductivity to cope with such currents. 
In our copending U.S. Pat. No. 4,175,836, issued Nov. 29, 1979, there is 
described and claimed a method and apparatus for causing coloration of an 
electrochromic material which includes passing through the material an 
electrical pulse of the polarity which tends to cause coloration and at a 
potential above the threshold potential for coloration of the material, 
followed by a DC potential of the same polarity but of magnitude less than 
the threshold potential. The pulse may contain considerably less than the 
total electrical charge required to cause coloration, and this gives rise 
to the advantage that it is possible to manufacture the necessary 
electronics much more cheaply than if the full charge required to cause 
coloration were applied to the matrix. 
Although the invention of the above mention patent specification makes 
possible a reduction in the cost of a matrix-addressed display device, it 
is still necessary to have a large number of separate matrix-addressing 
lines. Thus in, for example, a 100.times.100 matrix, 200 lines are needed. 
In Strom U.S. Pat. No. 3906451 there is disclosed an apparatus and method 
for erasing selected gas discharge cells by the use of non-coincident 
pulses. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a display device in 
which individual points in the display are energised by successive 
electrical signals, the electrodes being multiplexed to substantially 
reduce the number of driving circuits required. 
It is a further object of the invention to provide such a display device 
utilising an electrochromic material. 
It is another object of the invention to provide such a display using a 
matrix addressing system for energising the individual display points. 
According to the present invention, there is provided a display device 
including apparatus for electrically addressing a material in matrix 
fashion wherein each individual point in the matrix is addressable to 
produce a local change in an optical characteristic of the material by the 
application at that point of two electrical signals in succession, the 
apparatus including: 
first and second matrix addressing means for sequentially applying first 
and second electrical signals to the matrix, the combination of which is 
sufficient to cause a local change, 
the first matrix addressing means for selectively applying the first 
signals to groups of rows and columns of the matrix defining submatrices, 
the second matrix addressing means for selectively applying the second 
signals to individual rows and columns in the defined submatrices to 
select at least one of the individual cross points therein and producing a 
local change thereat. 
The foregoing and other objects, features and advantages of the invention 
will be apparent from the following more particular description of a 
preferred embodiment of the invention, as illustrated in the accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The apparatus of the invention is particularly useful for providing a 
visible display with an electrochromic material, which material, as 
mentioned in the introductory part of the specification, is one which 
forms a visible image when electrical potentials are selectively applied 
across it. In practice, in order to make an image visible, it is only 
necessary for the material to change colour; thus, if working with a white 
background, a material is particularly suitable if it is white or 
transparent in one state, but changes to some other colour, preferably 
contrasting with white, in its other state. This electrochromic material 
may either be of an inorganic solid, for example a transition metal 
compound such as tungsten oxide, or an organic liquid or solid such as one 
of the viologen derivatives. Examples of transition metal compounds used 
as electrochromic materials may be found in UK Pat. Specification No. 
1186541. Examples of viologen derivatives used as electrochromic materials 
may be found in UK Patent Specifications Nos. 1314049 and 1407133, as well 
as in in UK Patent Specifications 1302000 and 1376799. Particularly 
suitable compounds are N(p-cyanophenyl) substituted derivatives of 
bi-cyclic compounds having two conjugated nitrogen-containing aromatic 
rings. 
As described in the above mentioned patent specifications, there are many 
derivatives of the bipyridyl group which exhibit colour changes in 
response to electric current flow. The N(p-cyanophenyl) compound and 
especially N,N'di(p-cyanophenyl)-4,4' bipyridylium dichloride, is 
particularly useful in that it is reversibly electrolytically reduced on 
passage of a current in the appropriate direction to provide an radical 
which is coloured, usually green, while the parent compound is colourless 
or pale yellow. Furthermore, in its reduced state, the material is almost 
completely insoluble, so that it stays on or adjacent one of the 
electrodes, without displaying the tendency of some of the viologens to 
redissolve in the absence of a reducing current. Thus with the preferred 
compound, an image, once formed, tends to be stable even in the absence of 
any current, but will nevertheless disappear entirely under reverse 
current flow. It has also been found desirable to include with the 
electrochromic material a second reversibly oxidizable material, 
preferably ferrous ammonium sulphate. This provides a ferrous 
.revreaction. ferric ion combination in a particularly suitable form, 
improving the speed of bleaching on reversing the potential. It does, 
however, cause a certain amount of deterioration in the memory (ie the 
stability of the image in the absence of an electric current). This memory 
effect can be restored by the addition of, for example, an organic acid 
such as tartaric acid. Further examples of such additives are given in 
German OLS No. 2511314. 
The examples of viologen-type electrochromic materials discussed above are 
normally used in a liquid form. In an alternative configuration, they may 
be used in solid form, typically as a layer of polymeric based material. 
The electrochromic material is found to have a relatively sharp coloration 
threshold voltage, below which no coloration occurs. This threshold effect 
means that display devices using the electrochromic material may be matrix 
addressed, for example as described in the introductory part of the 
specification. As an alternative to the DC method of causing coloration of 
an electrochromic material, it is found that significant coloration can 
also be caused by the passage of short duration pulses of current, 
provided that for a given degree of coloration, the total charge passed 
must be the same as in the D C case. Thus, typically between one and ten 
millicoulombs cm.sup.-2 need to be passed to produce significant 
coloration, and the pulse duration may be as short as 200 microseconds. 
Referring now to FIG. 1, it has been shown that it is possible to cause 
coloration in an electrochromic material by first passing a short duration 
pulse 1, which does not contain the total charge required for colouring, 
but which is above threshold voltage, through the cell. This short 
duration pulse will be called the `expose` pulse. By subsequently applying 
a longer duration pulse 2 of a DC voltage below the amplitude of the 
threshold voltage across the cell it is possible to cause coloration. This 
longer duration pulse will be called the `develop` pulse. In this way, it 
is possible to employ a short duration expose pulse of relatively low 
current, which permits significantly simpler circuits to be used to 
provide the pulses and permits the use of electrodes which do not need to 
be of such high conductivity as when single pulses are used. Neither the 
expose pulse nor the develop pulse alone will produce any visible 
coloration, but when both are applied, the characteristic colour will 
develop over several tens of milliseconds. Development will take place 
provided the develop pulse is applied within about ten seconds of the 
expose pulse. The charge flowing from the expose pulse source under these 
conditions can be as low as twenty microcoulombs cm.sup.-2, passed in 
twenty microseconds. The expose and develop pulses are conveniently 
applied in succession through respective diodes 3 and 4 to a single 
`point` 5 of an electrochromic material, to cause coloration at that 
point. 
Referring now to FIG. 2, there is shown a simplified device in accordance 
with the invention. Lines X.sub.1 to X.sub.8 (the columns) and Y.sub.1 and 
Y.sub.8 (the rows) define a matrix of points at their intersections. The 
matrix is divided into submatrices by connections 10 and 11 enabling the 
application of electrical signals to groups of adjacent rows and columns. 
In the simple case illustrated, the groups are groups of two, and 
electrical signals may be applied by way of lines X.sub.12, X.sub.34, 
X.sub.56, X.sub.78 and Y.sub.12, Y.sub.34, Y.sub.56, Y.sub.78. Line 
X.sub.12 applies an electrical signal, through a diode 12 in each of the 
lines, to lines X.sub.1 and X.sub.2. The remaining lines, X.sub.34 etc, 
and Y.sub.12 etc, are similarly connected to lines X.sub.3 and X.sub.4 
etc, and to lines Y.sub.1 and Y.sub.2 etc. Thus in order to select 
sub-matrix 13, which contains four points at the intersections of lines 
X.sub.3, X.sub.4, Y.sub. 5 and Y.sub.6, it is necessary to apply signals 
to lines X.sub.34 and Y.sub.56. 
In addition to the division of the matrix into submatrices, it is also 
divided into sets of corresponding points within each sub-matrix. To do 
this, line X.sub.A is connected, through a diode 14 in each line, to lines 
X.sub.1, X.sub.3, X.sub.5 and X.sub.7. In other words, line X.sub.A is 
connected to the left hand column of each sub-matrix. Similarly X.sub.B is 
connected (via diodes) to the right hand column of each sub-matrix, and 
Y.sub.A and Y.sub.B are connected respectively (again via diodes) to the 
upper and lower rows of the submatrices. By applying signals to X.sub.A 
and Y.sub.A, it is possible to select the top left hand points of all the 
sub-matrices. 
Thus, if it were desired to address the top left hand point of sub-matrix 
13 (ie the point defined by the intersection of lines X.sub.3 and 
Y.sub.5), this would be done by applying an expose pulse to lines X.sub.34 
and y.sub.56, followed by a develop pulse to lines X.sub.A and Y.sub.A. 
In a practical situation, many points throughout the matrix need to be 
addressed to build up an image of graphic information. FIG. 3 shows the 
kind of pattern of pulses which might be applied in such a situation for 
the simple matrix system of FIG. 2. 
The upper portion of FIG. 3 illustrates examples of pulses which might be 
applied as expose pulses on lines X.sub.12, X.sub.34, Y.sub.12, and 
Y.sub.34. This covers just four of the sub-matrices of FIG. 2 and it will 
be understood that the remaining submatrices are defined by adding lines 
X.sub.56, X.sub.78, Y.sub.56 and Y.sub.78 to those depicted in FIG. 3. The 
lower portion of FIG. 3 depicts the develop pulses applied on lines 
X.sub.A, X.sub.B, Y.sub.A and Y.sub.B. 
In the first time period t.sub.1, a succession of expose pulses are applied 
by way of the relevant ones of lines X.sub.12, etc, Y.sub.12, etc, so as 
to successively apply expose pulses to all the sub-matrices in which it is 
desired to address the top left hand point. In the second time period, 
t.sub.2, the top left hand points of all those sub-matrices which were 
selected by the expose pulses in the first time period, t.sub.1 are 
developed by the application of develop pulses by way of lines X.sub.A and 
Y.sub.A. 
In a similar fashion, expose pulses for all those sub-matrices in which it 
is desired to address the bottom left hand point are generated in the 
third time period t.sub.3, and are followed in the fourth time period 
t.sub.4 by the develop pulses for the bottom left hand points, which are 
applied by way of lines X.sub.A and Y.sub.B. Exactly similar 
considerations apply for the remaining four time periods shown in FIG. 3, 
and which deal successively with the top right (X.sub.B, Y.sub.A) and 
bottom right (X.sub.B, Y.sub.B) points of the sub-matrices. 
Provided the delay between the develop pulse of one addressing operation 
and the expose pulse of the next is adequate, the previously addressed 
points in each submatrix will not be further developed. 
Although the foregoing description is concerned with the case where the 
expose pulses are applied to sub-matrices followed by the application of 
develop pulses to corresponding individual points within each of the 
sub-matrices, it is possible to interchange the points to which the pulses 
are applied. In other words, the expose pulses may be applied first to the 
corresponding individual points, followed by the application of develop 
pulses to the selected sub-matrices. 
In practice, a 100.times.100 array of points might have 10 lines like 
X.sub.12, 10 like Y.sub.12, 10 like Y.sub.A and 10 like X.sub.A, thus 
being driven by forty lines instead of the two hundred required by simple 
matrix addressing. If rapid selection is required, then unequal numbers of 
expose and develop lines would be used as to develop operation is 
relatively slow. 
In certain circumstances, especially when only alphanumeric information is 
to be reproduced, it may be convenient to make each sub-matrix suitable 
for reproducing a single character. One typical character generating 
matrix is the 5.times.7 matrix, and the matrix of the present apparatus 
may be divided into a set of 5.times.7 sub-matrices. The `expose` pulses 
then pick out only those sub-matrices in which a particular point is to be 
addressed, followed by `develop` pulses which develop that point in each 
of the exposed sub-matrices. This procedure is repeated 34 more times, 
thereby covering all 35 points in the sub-matrices and completing the 
image. 
While the invention has been particularly described and shown with 
reference to a preferred embodiment thereof, it will be understood by 
those skilled in the art that variations and modifications may be made 
without departing from the spirit and scope of the invention.