GB2062329A

GB2062329A - Electrochromic Display Device

Info

Publication number: GB2062329A
Application number: GB8027288A
Authority: GB
Original assignee: Seiko Instruments Inc
Current assignee: Seiko Instruments Inc
Priority date: 1979-10-15
Filing date: 1980-08-21
Publication date: 1981-05-20
Also published as: FR2467454A1; JPS5657096A; DE3037267A1

Abstract

An electrochromic display device comprises a transparent substrate (1) on which are formed a number of display electrodes (A to G) each of which comprises an electrically conductive portion (2a to 2g) and an electrochromic portion (3a to 3g) in electrolyte 7 preferably containing opaque white powder. A second substrate (4) has thereon a counter electrode (5). Dummy electrodes (1D to 3D) are formed on the second substrate (or the first substrate). To change the indication, charge is transferred between selected display electrodes 3 via switches 8a-g, 9a-9g, using the dummy electrodes connected via switches 81D-83D, 91D-93D to balance the total area. To refresh the display, compensating for lost charge, voltage is applied for controlled time between counter electrode 5 and those dummy electrodes previously coloured. <IMAGE>

Description

SPECIFICATION Electrochromic Display Device This invention relates to electrochromic display devices.

According to the present invention there is provided an electrochromic display device comprising: a transparent first substrate on which is formed a plurality of display electrodes each of which comprises an electrically conductive portion and an electrochromic portion; a second substrate on which a counter electrode is formed; a plurality of dummy electrodes on the surface of one substrate; and voltage means for selectively applying the voltage between the counter electrode and at least one dummy electrode The electrochromic display device preferably includes an electrolyte containing an opaque powder disposed between the substrates.

The voltage means may be arranged to apply a voltage of not more than 1.5 volts between the counter electrode and at least one dummy electrode.

The voltage means may be arranged to apply a positive voltage to the counter electrode and a negative voltage to at least one dummy electrode.

The voltage means may be arranged to apply a voltage between the counter electrode and both at least one dummy electrode and at least one display electrode. In one embodiment the dummy electrodes are provided in the second substrate.

In another embodiment the electrochromic display device includes mask means on the first substrate to obscure the display produced by the dummy electrodes provided on the first substrate.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a schematic diagram of an electrochromic display device to illustrate the fundamental principle of electric charge transfer; Figure 2 is a table showing the operation of an electrochromic display device according to the present invention; Figure 3 illustrates a circuit diagram of one embodiment of an electrochromic display device according to the present invention; Figure 4 illustrates another embodiment of an electrochromic display device according to the present invention; Figure 5 is a graph illustrating the relationship between charge injection time and temperature for an electrochromic display device according to the present invention; and Figure 6 illustrates another embodiment of an electrochromic display device according to the present invention.

Figure 1 illustrates schematically an electrochromic display device to aid understanding of electric charge transfer.

Transparent electrodes 2a to 2c are formed by evaporating, for example, indium oxide (In203) on a surface of a transparent substrate 1. The electrodes 2a to 2c are in contact with an electrolyte 7. Films 3a to 3c of electrochromic material are formed by evaporating, for example, tungsten oxide (W03) or molybdenum oxide (MoO3) to produce a plurality of display elements.

Though not shown in Figure 1, the portions of the transparent electrodes other than in the area of the display elements are covered with an insulating film (not shown) to prevent current leakage. The transparent electrodes 2a to 2c and the respective electrochromic films 3a to 3c will be referred to herein as display electrodes A to C.

The electrolyte 7 is sealed by a spacer 6 between the substrate 1 and the substrate 4 on which a counter electrode 5 made of gold is provided. The electrolyte 7 is mixed with a opaque powder such as titanium dioxide (TiO2) to form a white background to the display produced by the electrochromic display device.

Colouration switches 8a to 8c respectively connect each of the transparent electrodes 2a to 2c to the negative side of a battery 11. Bleaching switches 9a to 9c respectively connect each of the transparent electrodes 2a to 2c to the positive side of the battery 11. A switch 10 for injecting colouration electric charge connects the counter electrode 5 to the positive side of the battery 11.

The operation of the electrochromic display device of Figure 1 will now be described.

Colouration electric charge is initially injected into the electrochromic display device. It will be assumed that the display electrode to which colouration electric charge is injected initially is the display electrode A. When the switches 8a, 10 are closed, current flows from the counter electrode 5 to the display electrode A and the electrochromic film 3a is reduced or deoxidised and becomes coloured. After the electrochromic film 3a has a predetermined degree of colouration, the switches 8a, 10 are opened. The electrochromic film 3a maintains the deoxidised state i.e. the colouration state of the display electrode A is memorised.

Referring now to electric charge transfer, the colouration electric charge on the display electrode A is transmitted to, say, the display electrode C by closing the switches 9a, 8c and connecting the positive side of the battery to the display electrode A and the negative side of the battery to the display electrode C. At this time, the colouration electric charge from the display electrode A is transferred through the electrolyte 7 and is injected into the display electrode C. As a result, the display electrode A changes from a coloured state to a bleached state and the display electrode C changes from the bleached state to the coloured state.

The theoretical principle of electric charge transfer is as described above but in practice it is not always the case that the number of the display electrodes to be changed from the coloured state to the bleached state is equal to the number of display electrodes to be changed from the bleached state to the coloured state. In practice, therefore, in order to equalise the number of display electrodes to be changed from the coloured state to the bleached state and the number of display electrodes to be changed from the bleached state to the coloured state it has been found by the present inventors necessary to provide dummy display electrodes.

Figure 2 illustrates an electric charge transfer sequence for displaying numerals using seven display electrodes a to g arranged in a figure 8 configuration. Three dummy electrodes 1, 2, 3 are provided. Assuming that the area of each of the display electrodes a to g is one unit, the dummy electrodes 1, 2, 3 have areas in the ratio of 1:2:2.

The dummy electrodes are of the same construction as the display electrodes.

In Figure 2 a display electrode to be changed to the coloured state is indicated by the symbol "0", a display electrode to be changed to the bleached state is indicated by the symbol "X" and a display electrode whose state is not to be changed has no signal.

As shown in Figure 2, the number of display electrodes to be changed to the coloured state is equal to the number of display electrodes to be changed to the bleached state and as a result electric charge transfer can be perfectly carried out.

Figure 3 illustrates one embodiment of an electrochromic display device according to the present invention. Like parts in Figures 1 and 3 have been designated by the same reference numerals. A plurality (only one shown) of dummy display electrodes D are provided. The dummy electrode D consists of a transparent electrode 2D formed by evaporation on the surface of the substrate 4 in contact with the electrolyte 7 and an electrochromic film 3D formed by evaporation on the transparent electrode 3D. A colouration switch 8D is arranged so as to connect the transparent electrode 2D to the negative side of the battery 11. A bleaching switch 9D is arranged so as to connect the transparent electrode 2D to the positive side of the battery.

In the electrochromic display device shown in Figure 3 the opaque powder, e.g. titanium dioxide, mixed with the electrolyte 7 obscures the dummy electrode D and so any display produced is not visible.

Electric charge transfer in an electrochromic display device has the disadvantage that the degree of colouration deteriorates with increase in the number of electric charge transfers between display electrodes since a slight loss of charge occurs during each transfer. From experiment, it is found that the degree of colouration in the coloured state is noticeably reduced when the number of transfers exceed 1000. Therefore, a difference in the degree of colouration appears between groups of display electrodes involved in a relatively large number of charge transfers and groups of display electrodes involved in a relatively small number of charge transfers. Thus colouration of the display is not uniform and gradually deteriorates as a whole.

Figure 4 illustrates another embodiment of an electrochromic display device according to the present invention. This does not suffer from this latter problem. Again, like parts in Figures 1, 3 and 4 have been designated by the same reference numerals. Display electrodes A to G are formed by evaporating electrochromic films 3a to 3g respectively on to transparent electrodes 2a to 2g. Each transparent electrode 2a to 2g is connected to the negative side of the battery 1 11 through a respective switch 8a to 8g and to the positive side of the battery 11 to a respective switch 9a to 9g.

Dummy display electrodes 1 D to 3D are formed by evaporating electrochromic films 31 D to 33D on to respective transparent electrodes 21 D to 23D. Each transparent electrode 21 D to 23D is connected to the negative side of the battery 11 to respective switches 81 D to 83D and to the positive side of the battery through respective switches 91 D to 93 D.

It will be assumed that the duration of a refresh operation is the time from just after the display is changed to show the numeral "1" to just before the display is changed to show the numeral "2".

A refresh operation is effected in the electrochromic display device of Figure 4 by the following steps: 1. After the display is changed to show the numeral "1" the dummy electrodes 2D, 3D are in the coloured state.

2. The switch 10 and switches 82D, 83D are closed and the other switches shown in Figure 4 are opened. At this time, electric charge is injected only to the dummy electrodes 2D, 3D from the counter electrode 5.

3. After a predetermined injection time has elapsed, the switch 10 and the switches 82D, 83D are opened. At the time, all display electrodes A to G and the dummy electrodes 1 D to 3D are in the memorized condition.

4. The switches 8a, 8d, 8e. 89 and the switches 9c, 91 D, 92D are closed to change the display to show the numeral "2".

5. In accordance with the electric charge transfer sequence shown in Figure 2, the switches are opened or closed in turn to change periodically the display.

The degree of colouration will always be maintained at a predetermined level by carrying out steps 1 to 5 described above. This is for the following reasons.

The amount of electric charge injected from the counter electrode to to the dummy electrodes 2D, 3D in the refresh operation (step 2) is determined by the time for the switch 10 and the switches 82D. 83D are closed (that is the injection time) so as to be equal to the amount of electric charge which is lost by all the display electrodes (the display electrodes A to G and the dummy electrodes 1 D to 3D).

Therefore, the total amount of electric charge for colouration stored in the electrochromic display device is always constant when the refresh operation is completed.

After this, in step 4, since the electric charge on the dummy electrode 2D is transferred to the display electrodes A, D, E, G together with the electric charge stored on the dummy electrode 1 D and the display electrode C in the refresh operation, the electric charge injected during the refresh operation is distributed to the display electrodes A, D, E, G. Furthermore, the electric charge injected to the dummy electrode 3D is also transferred and distributed to the dummy electrode 1 D and the display electrode E when the display shows the numeral "6".

The injected electric charge distributed during the display of numerals "2" and "6" is distributed to other display electrodes every time the display is changed in accordance with the sequence of electric charge transfer shown in Figure 2. The amount of electric charge stored in each display electrode is equalized and the refresh operation is carried out to produce a constant density of colouration.

Figure 5 shows the relationship between the injection time and temperature where the applied voltage is 1.5 volts and the total area of the dummy electrodes is equal to the sum of the areas of the display electrodes A to G. In, for example, a digital electronic timepiece having an electrochromic display device according to the present invention, four digits are needed to display hours and minutes and thus nine dummy electrodes are required (or dummy electrodes with an area corresponding to the area of 1 7 display electrodes).

From Figure 5 it will be appreciated that the injection time is constant within the temperature range from 0CC to 500C. Thus the refresh operation is independent of temperature in this range and temperature compensation is not required.

Figure 6 illustrates another embodiment of an electrochromic display device according to the present invention. A plurality of dummy display electrodes 1 D, 2D are arranged on the same surface of the substrate 1 as the display electrodes A to F and a masking plate 1 5 is arranged on the opposite surface of the substrate 1. The masking plate 1 5 is placed above the dummy electrodes 1 D, 2D so as to cover any display produced thereby. The switches are not shown in Figure 6 for clarity but may be as in Figure 4.

The refresh operation in Figure 6 is carried out between the dummy electrodes and the counter electrode 5 in a similar way to that described above in relation to Figure 4.

In the above description, the electrolyte 7 is iiquid but a solid electrolyte may be used.

Moreover both the group of display electrodes in the coloured state and the group of dummy electrodes in the coloured state may be used in the refresh operation although the electrochromic display device described above uses nine dummy electrodes. Furthermore the number of the electrodes which are used for the refresh operation may be arbitrarily set below the maximum number of display electrodes in the coloured state.

The electrochromic display devices according to the present invention and described above have the following features: (1) Since the charge injection time required for compensating the degree of colouration is approximately constant within the temperate range OOC to 500C, the electrochromic display device will have a constant degree of colouration without the need to provide temperature compensation.

(2) Since a plurality of dummy electrodes are used for compensating the degree of colouration, no new arrangement of the display electrodes for the refresh operation is required.

(3) The momentary increase in the degree of colouration during the refresh operation is not visible and display quality is never reduced because the dummy electrodes are hidden.

Claims

1. An electrochromic display device comprising: a transparent first substrate on which is formed a plurality of display electrodes each of which comprises an electrically conductive portion and an electrochromic portion; a second substrate on which a counter electrode is formed; a plurality of dummy electrodes on the surface of one substrate; and voltage means for selectively applying the voltage between the counter electrode and at least one dummy electrode.

2. An electrochromic display device as claimed in claim 1 including an electrolyte containing an opaque powder disposed between the substrates.

3. An electrochromic display device as claimed in claim 1 or 2 in which the voltage means is arranged to apply a voltage of not more than 1.5 volts between the counter electrode and at least one dummy electrode.

4. An electrochromic display device as claimed in any preceding claim in which the voltage means is arranged to apply a positive voltage to the counter electrode and a negative voltage to at least one dummy electrode.

5. An electrochromic display device as claimed in any preceding claim in which the voltage means is arranged to apply a voltage between the counter electrode and both at least one dummy electrode and at least one display electrode.

6. An electrochromic display device as claimed in any preceding claim in which the dummy electrodes are provided in the second substrate.

7. An electrochromic display device as claimed in any of claims 1 to 5 including mask means on the first substrate to obscure the display produced by the dummy electrodes provided on the first substrate.

8. An electrochromic display device substantially as herein described with reference to and as shown in the accompanying drawings.

9. An electrochromic display device comprising two substrates, an electrolyte and white powder between the two substrates, a display electrode group having film of electrochromic material, the display electrode group being formed on a surface of a first substrate being in contact with the electrolyte, a counter electrode formed on a surface of one substrate of the two substates being in contact with the electrolyte, a plurality of display dummy electrode formed on a surface of one substrate of the two substrates being in contact with the electrolyte and means for applying the voltage between the counter electrode and at least one electrode of the display dummy electrodes.

10. An electrochromic display device comprising two substrates, an electrolyte and white powder between the two substrates, a display electrode group having film of electrochromic material, the display electrode group being formed on a surface of a first substrate being in contact with the electrolyte, a counter electrode formed on a surface of one substrate of the two substrates being in contact with the electrolyte, a plurality of display dummy electrodes formed on a surface of one substrate of the two substrates being in contact with the electrolyte and means for applying the voltage between the counter electrode and both at least one electrode of the display dummy electrodes and at least one electrode of the display electrode group.