GB2083261A

GB2083261A - Electrochromic display device and a method of driving the same

Info

Publication number: GB2083261A
Application number: GB8125454A
Authority: GB
Original assignee: Seiko Instruments Inc
Current assignee: Seiko Instruments Inc
Priority date: 1980-08-27
Filing date: 1981-08-20
Publication date: 1982-03-17
Also published as: JPS5742091A; GB2083261B; CH651171GA3; FR2489569A1; FR2489569B1

Abstract

An electrochromic display device, e.g. in an electronic timepiece, comprises a driver (19) for transferring coloration electric charge from a first display electrode group to a second display electrode group by applying a voltage therebetween. During the memory period between display changes, transfer signal generator (18) controlled by a transfer control signal generator (17) applies an auxiliary positive signal to the bleached electrodes and a negative signal to the coloured electrodes, whereby degradation of the display (20) during a memory period is prevented. Some of the electrodes may be dummies, screened from view, to allow the total coloured area to remain constant. <IMAGE>

Description

SPECIFICATION Electrochromic display device and a method of driving the same This invention relates to electrochromic display devices and to methods of driving the same.

According to one aspect of the present invention there is provided a method of driving an electrochromic display device comprising the steps of transferring colouration electric charge from a first electrode to a second electrode by applying a voltage between said first electrode and said second electrode, and applying the positive polarity of an auxiliary signal to the first electrode and the negative polarity of the auxiliary signal to the second electrode during a display memory period.

According to another aspect of the present invention there is provided an electrochromic display device comprising drive means for transferring colouration electric charge from a first electrode to a second electrode by applying a voltage between said first electrode and said second electrode, and a transfer signal generator for applying the positive polarity of an auxiliary signal to the first electrode and the negative polarity of the auxiliary signal to the second electrode during a display memory period.

Preferably the transfer signal generator is controlled by a transfer control signal generator so that, in operation, the positive polarity of the auxiliary signal is applied to one or more electrodes in a bleached state and the negative polarity of the auxiliary signal is applied to one or more electrodes in a colouration state whereby colouration of the electrode or electrodes in the bleached state is prevented during the display memory period.

Means may be provided whereby the auxiliary signal is generated in synchronism with an electric charge transfer signal.

According to a further aspect of the present invention there is provided an electronic timepiece comprising a reference signal generator including an oscillator and a frequency divider, time counting means, a decoder for receiving the output of the time counting means, and an electrochromic display device as recited above.

Said transfer control signal generator may be connected to receive the output signal from the frequency divider.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a schematic sectional view of an electrochromic display device for illustrating a conventional method of driving an electrochromic display device using electric charge transfers; Figure 2 is a table illustrating this conventional method of electric charge transfer; Figure 3 is a block diagram of an electronic timepiece having an electrochromic display device according to the present invention; Figure 4 shows a transfer signal generator of an electrochromic display device according to the present invention; Figure 5 is a timing chart illustrating the operation of the transfer signal generator of Fig. 4; Figure 6 shows a transfer control signal generator of an electrochromic display device according to the present invention; and Figure 7 is a timing chart illustrating the operation of the transfer control signal generator of Fig. 6.

Throughout the drawings like parts have been designated by the same reference numerals.

Fig. 1 is a schematic view of an electrochromic display device and is provided for the purpose of explanation of the conventional method of driving an electrochromic display device using electric charge transfer. The electrochromic display device comprises a transparent substrate 1, transparent electrodes 2a to 2cformed on a surface of the substrate 1 by evaporating, for example, indium oxide (In203), and electrochromic layers 3ato 3c formed on the transparent electrodes by evaporation of, for example, tungsten oxide (W03) or molybdenum oxide (MoO3) and patterned to constitute display picture elements.

The transparent electrodes 2a to 2c are in contact with an electrolyte 7. A polytetrafluoroethylene porous layer 1 2 including a white powder such as titanium dioxide (TiO2) is admixed with the electrolyte 7 to form a white background against which colouration of the electrochromic layer can be seen. Although not shown in Fig. 1, the transparent electrodes 2a to 2c other than in the patterned region of the display picture elements is coated with an electrically insulating layer to prevent leakage current. The transparent electrodes 2a to 2 c and the electrochromic layers 3a to 3c constitute display electrodes A to C.

The electrolyte is retained between the substrate 1 and a substrate 4 by means of a spacer 6. A counter electrode 5 of, for example, gold is formed on a surface of the substrate 4 in contact with the electrolyte 7.

Referring now to the circuitry shown in Fig.

1, reference numerals 8a to 8c denote colouration switches for connecting the transparent electrodes 2a to 2 c to a negative side (cathode) of a battery 11, reference numerals 9a to 9c denote bleaching switches for connecting the transparent electrodes 2a to 2c to a positive side (anode) of the battery, and reference numeral 10 denotes a switch for injecting colouration electric charge and for connecting the counter electrode 5 with the positive side of the battery 11.

The operation of the electrochromic display device of Fig. 1 is as follows: (i) Injection of colouration electric charge.

Let it be assumed that the display electrode into which colouration electric charge is to be injected initially is the display electrode A.

When the switches 8a and 10 are closed, current flows from the counter electrode 5 to the display electrode A, whereby the electrochromic layer 3 is chemically reduced and becomes coloured. When the switches 8a and 10 are opened after the predetermined density or degree of colouration is attained, the electrochromic layer 3a maintains the reduced state and the display maintains the colouration state; this is what is called a memory state.

(ii) Electric charge transfer For transferring the colouration electric charge held by the display electrode A to, say, the display electrode C, the switches 9a and 8c are closed, the display electrode A and the positive side of the battery 11 are electrically connected, and the display electrode C and the negative side of the battery 11 are electrically connected. Thus colouration electric charge on the display electrode A passes to and is injected into the display electrode C through the electrolyte 7. As a result, the display electrode A changes from the colouration state to a bleached state, and the display electrode C becomes coloured.

This is the conventional method of electric charge transfer. When the electrochromic display is driven in practice dummy display electrodes are provided so that the number of display electrodes changing into the colouration state from the bleached state is always equal to the number of the display electrodes changing into the bleached state from the colouration state.

Fig. 2 shows the combination of electric charge transfer in the case of the display of a single numeral using seven display electrodes A to G in the shape of a Figure "8" and three dummy display electrodes 1, 2, 3. The display areas of the dummy display electrodes are designed so that the area of the dummy display electrodes 1, 2, 3 is in the ratio of 1:2:2 when the area of each display electrode A to G is 1. The dummy display electrodes are formed in the same manner and of the same material as the display electrodes A to G. The dummy display electrodes are not visible from the exterior of the electrochromic display device in which they are incorporated because they are masked by a masking plate (not shown).In Fig. 2, "0" indicates the display electrodes and the dummy display electrodes changing from the bleached state to the colouration state, "X" indicates the display electrodes and the dummy display electrodes changing from the colouration state to the bleached state, and the absence of any mark indicates the display electrodes whose display state is not changed, i.e. those which are in the memory state.

As illustrated, the memory effect of an electrochromic display device is used when it is driven in practice, the memory effect showing the following advantages: (1) The life of the electrochromic display device is prolonged, because its life is determined by the repetition cycle of colouration and bleaching.

(2) The electrochromic display device is driven with low power consumption.

The electrochromic display device requires electric charge of about 4 to 5 mC/cm2 for 1 cycle of colouration, so that the power consumption is relatively large in comparison with other types of display device such as liquid crystal display devices.

If the colouration state is held for a relatively long period of time, the colouration electric charge leaks from the display electrodes in the colouration state to the display electrodes in the bleached state, whereby the latter electrodes become gradually coloured.

The display contrast thus markedly deteriorates and in extreme cases, the colouration state is no longer visible. In the case where hours and minutes of a time indication are displayed, for example, the tens of hours indication is kept in the bleached state for 9 hours maximum and the units of hours indication is kept in the bleached state for up to 1 hour. In order to maintain the bleached state for such a long time, an insulating resistance of more than 109 53 is required to be applied across the display electrodes in the colouration state and the display electrodes in the bleached state but it is very difficult to guarantee such a high resistance in practice.

Referring now to Fig. 3 there is shown an electronic timepiece having an electrochromic display device according to the present invention. A quartz crystal oscillator 1 3 generates a time reference signal which is received by a frequency divider 14 and generates time reference signals.The time reference signals of predetermined frequency are fed to a counter 1 5. The output of the counter 1 5 is fed to a decoder 1 6 and a transfer control signal generator 1 7. A transfer signal generator 18 receives the output signals from the decoder 1 6 and the transfer control signal generator 1 7 and generates electric charge transfer signals which are supplied to a driver 1 9. An electrochromic display device 20 according to the present invention is driven by the output signals from the driver 1 9. Auxiliary signals to be described in more detail hereinafter applied to the display electrode or electrodes in the bleached state during a memory period are generated by the transfer signal generator 1 8 in response to the output of the transfer control signal generator 1 7.

The transfer signal generator 1 8 and the driver 1 9 are shown in greater detail in Fig.

4. Fig. 4 shows the transfer signal generator for one display electrode and it will be appre ciated that a similar transfer signal generator is required for each display electrode and each dummy display electrode.

In the transfer signal generator a data input terminal D of a D-type flip-flop 21 is connected to receive a decode signal De from the decoder 1 6 and a clock terminal C is connected to a clock terminal C 1 of the frequency divider 1 4. A Q output terminal of the D-type flip-flop 22 is connected to one input of a two-input NOR gate 24. The other input of the NOR gate 24 is connected to receive a control signal S of the transfer control signal generator 17, and the output terminal of the NOR gate 24 is connected to one input of a two-input NOR gate 25. The other input of the NOR gate 25 is connected to an output terminal of an inverter whose input terminal is connected to the output terminal of the decoder 16. The output of the NOR gate 25 is connected to a point N.One input of a twoinput NAND gate 26 is connected to the output terminal of the inverter 22 and the other input terminal of the NAND gate 26 is connected to an output terminal of an inverter 23 whose input terminal is connected to receive the control signal S of the transfer signal generator 1 7. The output of the NAND gate 26 is connected to a point P.

The driver comprises a P channel MOSFET (hereinafter referred to as a PMOS) 27 and an N channel MOSFET (hereinafter referred to as an NMOS) 28. The source of the PMOS 27 is connected to the positive side of the battery 11, the gate is connected to the point P and the drain is connected to the display electrode of the electrochromic display device. The source of the NMOS 28 is connected to the negative side of the battery 11, the gate is connected to the point N and the drain is connected to the drain of the PMOS 27.

The operation of the transfer signal generator of Fig. 4 will be illustrated in conjunction with the timing chart shown in Fig. 5. Fig. 5 shows, by way of example, signal waveforms for the display of numerals "0", "1", "2" and "3" of the display electrode G for the inputs of minutes of a time indication. The explanation will be given according to timings I) to (Vll).

(I) Bleaching by electric charge transfer.

When bleaching is instructed by changing the decode signal De to LOW level, the display electrode G is connected to the positive side of the battery 11, whereby the display electrode G goes to the bleached state. The electric charge transfer timing is controlled by the control signal S.

(II) Bleaching memory.

The display electrode is floated for about 1 minute until the next transfer timing after the electric charge transfer is over, and the bleached state is held by the memory effect.

(III) Bleaching by auxiliary signals.

Auxiliary signals are applied in synchronism with the timing of the electric charge transfer.

In conventional electric charge transfer, the display is performed by the memory effect in this period since the display is not changed (the display electrode G is in the bleached state although the display changes from "0" to "1"). Since the auxiliary signals are at a HIGH level, the display electrodes in the bleached state are connected to the positive side of the battery. As a result, electric charge leaks during the memory period, and few colouration electric charges injected into the display electrode G in the colouration state are injected into the display electrodes to be newly coloured, mixed with net transferred electric charge. The auxiliary signals are applied every 1 minute to the tens of hours indication and the units of hours indication in which the bleached memory period is relatively long, whereby the colouration caused by electric charge leakage is prevented.

(IV) Bleaching memory.

The same as (II).

(V) Colouration by electric charge transfer.

When colouration is instructed by the decode signal De changing to HIGH level, the display electrode G is connected to the negative side of the battery 11, and electric charge is injected from those display electrodes which are to be in the bleached state whereby the display electrode G goes into the colouration state. The transfer timing for colouration is controlled by the control signal S, and needless to say, it is equal to the transfer timing for bleaching.

(IV) Normalisation of the colouration density.

The display electrodes and the dummy display electrodes in the colouration state are connected to the negative side of the battery 11 until the next display change after electric charge transfer is over. This state is equivalent to the state of the colouration electrodes being shorted and connected to one another in parallel. The electric charge on the display electrodes in the colouration state is redistributed so that non-uniformity in the electric charge density among the display electrodes is completely eliminated. On the other hand, the display electrodes and the dummy display electrodes in the bleached state are floated as in (II).

(VII) Colouration memory.

The display electrode G is kept in the colouration state although it is in an electric charge transfer period, so that the display state is maintained floating by the memory effect.

Fig. 6 shows the transfer signal generator 1 7 in greater detail and comprising an inverter 29, an AND gate 30 and a flip-flop 31 and Fig. 7 is a timing chart illustrating the operation of the transfer signal generator.

While the display electrode G for the units of minutes indication has been used as an example above, it is to be appreciated that the other display electrodes and the dummy display electrodes are operated in the same way.

While the auxiliary signals are applied to the display electrodes and the dummy electrodes in the bleached state at intervals of 1 minute synchronised with electric charge transfer timing in a bleaching memory period in the above operation, the same effect is achieved by applying the auxiliary signals at intervals of several minutes or several tens of minutes.

Further, it is effective to apply the auxiliary signals at timing other than the electric charge transfer timing in the bleaching memory period.

The electrochromic display device according to the present invention and illustrated above is such that colouration of the display electrodes in a bleached state during the memory period is prevented by applying the auxiliary signals to the display electrodes in a bleached memory state at predetermined times, whereby the electrochromic display device has excellent display quality and can be driven with relatively low power consumption.

Claims

1. A method of driving an electrochromic display device comprising the steps of transferring colouration electric charge from a first electrode to a second electrode by applying a voltage between said first electrode and said second electrode, and applying the positive polarity of an auxiliary signal to the first electrode and the negative polarity of the auxiliary signal to the second electrode during a display memory period.

2. An electrochromic display device comprising driver means for transferring colouration electric charge from a first electrode to a second electrode by applying a voltage between said first electrode and said second electrode, and a transfer signal generator for applying the positive polarity of an auxiliary signal to the first electrode and the negative polarity of the auxiliary signal to the second electrode during a display memory period.

3. An electrochromic display device as claimed in claim 2 in which the transfer signal generator is controlled by a transfer control signal generator so that, in operation, the positive polarity of the auxiliary signal is applied to one or more electrodes in a bleached state and the negative polarity of the auxiliary signal is applied to one or more electrodes in a colouration state whereby colouration of the electrode or electrodes in the bleached state is prevented during the display memory period.

4. An electrochromic display device as claimed in claim 2 or 3 in which means are provided whereby the auxiliary signal is generated in synchronism with an electric charge transfer signal.

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

6. An electronic timepiece comprising a reference signal generator including an oscillator and a frequency divider, time counting means, a decoder for receiving the output of the time counting means, and an electrochromic display device as claimed in any of claims 2 to 5.

7. An electronic timepiece as claimed in claim 6 when dependent upon claim 3 in which said transfer control signal generator is connected to receive the output signal from the frequency divider.

8. A method of driving an electrochromic display device substantially as herein described with reference to and as shown in the accompanying drawings.

9. A driving method for an electrochromic display device comprising the steps of transferring colouration electric charges from first electrode to second electrode by applying a voltage between said first electrode and said second electrode and applying the positive polarity of an auxiliary signal to said first electrode and the negative polarity thereof to said second electrode in a display memory period.

10. An electrochromic display device in which colouration electric charges in coloured electrodes and coloured dummy electrodes are transferred to bleached electrodes and bleached dummy electrodes by applying a voltage between the coloured electrode group and the bleached electrode group by a driver, wherein the positive polarity of the signal from a transferring signal generator connected to said driver and controlled by a transferring control signal generator is applied to the bleached electrode group and the negative polarity thereof is applied to the coloured electrode group whereby the colouration into the bleached electrode group is prevented in a display memory period.

11. In an electronic timepiece having a reference signal generator including an oscillator and a frequency divider, a time counting means, a decoder for receiving the output of said time counting means, a driver and an electrochromic display device, the improvement comprising a transferring control signal generator for receiving the output of said frequency divider and controlling an auxiliary signal for applying between the bleached electrode group of said electrochromic display device and the coloured electrode group thereof and a transferring signal generator among said decoder, said driver and said transferring control signal generator.