A METHOD OF DRIVING AN ELECTROCHROMIC DISPLAY DEVICE

Abstract: A method of driving an electrochromic display device having at least one counter electrode (5), at least one working electrode (3) coated with a Prussian blue film and an electrolyte (7) in contact with the working electrode and the counter electrode, comprises applying a voltage greater than (V₁-E) to the working electrode with respect to the counter electrode in order to render the working electrode in a first state or coloured state and applying a voltage less than (V₂-E) to the working electrode with respect to the counter electrode in order to render the working electrode in a second or bleached state. The voltage V₁ corresponds to the first state of the Prussian blue film, the voltage V₂ corres­ ponds to the second state of the Prussian blue film and the voltage E corresponds to the potential of the counter electrode. The counter electrode may also be coated with a Prussian blue film.

Description: ZA METHOD OF DRIVING AN EIECTROCSoMIc DISPLAY DEVICE" This invention relates to methods of driving electrochromic display devices. It is known that electrochromic display devices may be driven by applying a voltage between a counter electrode and an electrochromic electrode. However, it has not been determined the level of the voltage that should be applied to an electrochromic display device which utilises Prussian blue electrochromic material for the electrochromic electrode. Further, it has also not been determined the level of the voltage that should be applied to an electrochromic display device which uses Prussian blue as the electrochromic material for the counter electrode as well as for the electrochromic electrode. According to the present invention there is provided a method of driving an electrochromic display device having at least one counter electrode, at least one working electrode coated with a Prussian blue film and an electrolyte in contact with the working electrode and the counter electrode, the method being characterised in that a voltage greater than V1 - E is applied to the working electrode with respect to the counter electrode in order to render the working electrode in a first state and a voltage less than V2 - E is applied the working electrode with respect to the counter electrode in order to render the working electrode in a second state, the voltage V1 corresponding to said first state of the Prussian blue film, the voltage V2 corresponding to the second state of the Prussian blue film and the voltage E corresponding to the potential of the counter electrode. The counter electrode may be coated with a Prussian blue film. In one embodiment a voltage greater than V1 - E is applied to the working electrode with respect to the counter electrode in order to render the working electrode in a coloured state, and a voltage less than V2 - E2 is applied to the working electrode with respect to the counter electrode in order to render the working electrode in the bleached state, the voltage E1 being the maximum voltage of the material of the counter electrode to perform a reversible electrochemical reaction and the voltage E2 is the minimum voltage of the material of the counter electrode to perform a reversible chemical reaction. The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a cyclic voltammogram of a Prussian blue thin film; Figure 2 is a sectional view of one embodiment of an electrochromic display device; Figure 3 shows the waveform of a drive signal which is used for driving the electrochromic display device of Figure 2 by one method according to the present invention; Figure 4 is a sectional view of another embodiment of an electrochromic display device having a counter electrode of Prussian blue; Figure 5 shows the waveform of a drive signal which is used for driving the electrochromic display device of Figure 4 by a second method according to the present invention; Figure 6 is a sectional view of a further embodiment of an electrochromic display device; ; Figure 7 is a sectional view of yet another embodiment of an electrochromic display device having a counter electrode made of Prussian blue; Figure 8 shows the waveform of a drive signal which is used for driving the electrochromic display device of Figure 7 by a third method according to the present invention; Figure 9 is a sectional view of a still further embodiment of an electrochrnmic display device having a counter electrode made of Prussian blue; and Figure lo is a drive circuit for driving the electrochromic display device shown in Figure 9. Various methods according to the present invention for driving electrochromic display devices will be explained with reference to the drawings. Prussian blue changes from a coloured state to a bleached or clear state in accordance with the following electrochemical reaction: EMI3.1 (blue colour) (clear colour) In the above reaction the substance on the left hand side exhibits blue colour and the substance on the right hand side is colourless. The electrochemical reaction is a reversible oxidationreduction reaction. In a three electrode electrochromic display device having a Prussian blue coated display or working electrode, a counter electrode and a saturated calomel electrode (BYE) as a reference electrode, the Prussian blue coated working electrode becomes coloured when at about +0.6 V with respect to the reference electrode and it becomes bleached at about -0.2V. Figure 1 is a cyclic voltammogram of a Prussian blue film of a working electrode in a three electrode electrochromic display device, the three electrodes being in contact with a lMKCl electrolyte solution. The cyclic voltammogram has a peak at about +0.2V and indicates that the Prussian blue coated working electrode completes colouration at about +0.6V and completes bleaching at about -02.V. Figure 2 shows one embodiment of an electrochromic display device. The display device has a pair of glass substrates 1 on each of which there is a transparent conductive film 2, a Prussian blue working electrode 3, a reflective plate 4, a counter electrode 5 made of tungsten oxide (W03), a spacer 6 and an electrolyte 7. The working electrode 3 is provided on the transparent conductive film 2 on one of the glass substrates 1. The counter electrode 5 is provided on the transparent conductive film 2 on the other glass substrate 1. The reflective plate 4 is provided in the space between the glass substrates 1 and the spacer 6 is sandwiched therebetween. The remainder of the space is filled with the electrolyte 7. A Prussion blue film is prepared as disclosed in U.S. Patent Application Serial No. 387 470. A 2OmM aqueous solution of Fe(C104)3 whose pH is regulated to pH2 by HC104 and a 20mM aqueous solution of Fe(CN)6 are mixed together in a volume ratio of 1:1 to form an electrolytic solution. A working electrode which is to be coated with Prussian blue and a counter electrode for the electrolytic process are immersed in the electrolytic solution. 2 An electric current having a density of 10 pA/cm is applied to the working electrode to deposit a Prussian blue film thereon. The electric charge density of the working electrode is arranged to 2 be about 9 mC/cm . The working electrode 3 is formed in this manner. Next a method according to the present invention of driving the electrochromic display device of Figure 2 will be explained. As seen from the cyclic voltammogram of Figure 1 in order to bleach a Prussian blue film a voltage less than +0.2V for instance -02.V with respect to the reference electrode should be applied thereto. In order to colour the Prussian blue film a voltage of more than +0.2V for instance +0.7V with respect to the reference electrode should be applied thereto. Now a relatively high voltage V1 which colours the Prussian- blue film is set at +0.7V with respect to the reference electrode and a relatively low voltage which bleaches the Prussian blue film is set at -0.2V with respect to the reference electrode. In the electrochromic display device of Figure 2, the electric potential E of the counter electrode is determined at +0.2V with respect to the reference electrode by actual measurement. The voltage V1 and the voltage V2 both applied to the working electrode 3 and the electric potential E of the counter electrode 5 are measured with respect to the reference electrode. In practice an electrochromic display device having a Prussian blue working electrode and a counter electrode is possible because a reference electrode is unnecessary. Accordingly in a two electrode electrochromic display device, the voltage applied to the Prussian blue working electrode is determined with respect to the counter electrode. Namely, in order to set the potential level of the working electrode 3 at V1, a voltage(Vl-E) = +0.5 volts should be applied to the working electrode 3 with respect to the counter electrode 5. In order to set the potential level of the working electrode 3 at V2, a voltage (V2-E) = -0.4 volts should be applied to the working electrode with respect to the counter electrode 5. Figure 3 shows the waveform of a drive signal applied to the electrochromic display device of Figure 2 where voltage level el is +0.5 volts, voltage level e2 is -0.4 volts, voltage level G is 0.0 volts, time interval tl is 1.5 seconds, time interval t2 is 1.0 second and time interval t3 is 1.5 seconds. Initially +0.5 volts is applied to the working electrode for 1.5 seconds, no voltage is applied for the next 1.0 second and -0.4 volts is applied to the next 1.5 seconds. During the time interval tl, the potential level of the working electrode is kept at voltage V1 and during the time interval t3, the potential level of the working electrode is kept at the voltage V2. Figure 4 is a sectional view of another electrochromic display device which consists of a glass substrate 1, a transparent conductive film 2, a Prussian blue working electrode 3, a spacer 6, an electrolyte 7, a counter substrate 8, and a Prussian blue counter electrode 9. The working electrode 3 and the counter electrode 9 are both provided on the transparent conductive film 2 which is formed on the glass substrate 1. The space defined by the spacer 6, the counter substrate 8 and the glass substrate 1 is filled with the electrolyte 7 which is in contact with the working electrode 3 and the counter electrode 9. Next, a method according to the present invention of driving the electrochromic display device of Figure 4 is explained. In this embodiment the relatively high voltage V1 which renders the working electrode coloured is set at +0.5 volts with respect to a reference electrode. The relatively low voltage V2 which renders the working electrode colourless is set at -0.2 volts with respect to the reference electrode. The electrode potential E of the counter electrode 9 is measured with respect to the reference electrode and is 0.3 volts. Therefore, in order to set the potential of the working electrode 3 at V1 volts, a voltage (V1-E) = +0.2 volts should be applied to the working electrode with respect to the counter electrode. In order to set the potential of the working electrode 3 at V2 volts, a voltage (V2-E) = 0.4 volts should be applied to the working electrode with respect to the counter electrode. Figure 5 shows a waveform of a drive signal applied to the electrochromic display device of Figure 4. Voltage level e3 is +0.2 volts, voltage level e4 is -0.4 volts, voltage level G is 0.0 volts, time interval t4 is 1.5 seconds and time interval t5 is 1.5 seconds. During the time interval t4, +0.2 volts is applied to the working electrode 3 with respect to the counter electrode 9 and during the time interval t5 -0.4 volts is applied to the working electrode 3 with respect to the counter electrode 9. The electric potential of the working electrode is kept at V1 during the time interval t4 and is kept at V2 during the time interval t5. The drive voltage which is applied between the working electrode 3 and the counter electrode 9 is clearly determined by evaluating the voltage V1 corresponding to the coloured state of the working electrode, the potential level V2 corresponding to the bleached state of the working electrode and the potential level of the counter electrode, these being measured with respect to a reference electrode. Thus an electrochromic display device utilising Prussian blue as the electrochromic material is stably driven by this method according to the present invention. Figure 6 is a sectional view of another embodiment of an electrochromic display device having a pair of glass substrates 1, a transparent conductive film 2, a Prussian blue working electrode 3, a reflective plate 4, a counter electrode 5 made of tungsten oxide (WO3), a spacer 6 and an electrolyte 7. The working electrode 3 is provided on the transparent conductive film 2 on one of the glass substrates 1. The counter electrode 5 is provided on the transparent conductive film 2 on the other glass substrate 1. The space between the pair of glass substrates 1 and the spacer 6 contains the electrolyte 7 and the reflective plate 4. A method of driving the electrochromic display device of Figure 6 will now be explained. The electrode potential of the Prussian blue electrode material in the coloured state is the voltage V1 = +0.6 volts and the electrode potential of the Prussian blue electrode material in the bleached state is the voltage V2 = 10.2 volts. In the previous embodiments, the counter electrode potential E is stable and does not vary. However, depending upon the method of driving the electrochromic display device, in some cases the counter electrode potential E can vary. In the electrochromic display device of Figure 6 the maximum potential E1 of the counter electrode which enables tungsten oxide to undergo a reversible electrochemical reaction is +0.3 volts. The minimum counter electrode potential E2 which enables tungsten oxide to undergo a reversible electrochemical reaction is -0.4 volts. Consequently, in order to render the working electrode 3 coloured a voltage more than (V1-E1) = +0.3 volts is applied to the working electorde with respect to-the counter electrode 5. In order to render the working electrode colourless, a voltage less than (V2-E2)= +0.2 volts should be applied to the working electrode with respect to the counter electrode. In this embodiment +0.8 volts, which is greater than (Vl-El)= +0.3 volts is applied to the working electrode so as to render it coloured and -0.1 volts which is less than (V2-E2)= +0.2 volts is applied to the working electrode so as to render it colourless. Figure 7 is a sectional view of another embodiment of an electrochromic display device consisting of a glass substrate 1, a transparent conductive film 2, a Prussian blue working electrode 3, a spacer 6, an electrolyte 7, a coUnter substrate 8 and a Prussian blue counter electrode 9. The working electrode 3 and the counter electrode 9 are provided on the transparent conductive film 2 on the glass substrate 1. The space defined by the spacer 6, the glass substrate 1 and the counter substrate 8 is filled with the electrolyte 7 which is in contact with both the working electrode 3 and the counter electrode 9. A method according to the present invention of driving the electrochromic display device of Figure 7 will now be described with reference to Figure 2. The working electrode has a value of the voltage V1 of +0.6 volts and a value of the voltage V2 of -0.2 volts. The maximum counter electrode potential E1 which enables Prussian blue electrochromic material to perform a reversible electrochemical reaction is +0.3 volts. The minimum counter electrode potential E2 which enables Prussian blue electrochromic material to perform a reversible electrochemical reaction is -0.2 volts. Consequently in order to render the working electrode 3 coloured a voltage greater than (ViE1)=0.7 volts should be applied to the working electrode with respect to the counter electrode. In order to render the working electrode 3 colourless a voltage less than (V2-E2)=0.0-volts should be applied to the working electrode with respect to the counter electrode. Thus in this embodiment an electrochromic display device +0.2 volts which is greater than -0.7 volts is applied to the working electrode 3 in the coloured state with respect to the counter electrode 9 and -0.6 volts which is smaller than 0.0 volts is applied to the working electrode in the bleached state with respect to the counter electrode. Figure 8 shows the waveform of a drive signal applied to the electrochromic display device of Figure 7 in a method according to the present invention. A voltage level e5 is +0.2 volts, a voltage level e6 is -0.7 volts, a voltage level G is 0 volts, time interval t6 is 5 seconds and time interval t7 is 5 seconds. By driving the electrochromic display device of Figure 7 by this 6 method it can undergo 10 cycles. Figure 9 shows yet another embodiment of an electrochromic display device consisting of a pair of glass substrates 1, a transparent conductive film 2, a Prussian blue working electrode .3, a reflective plate 4, a spacer 6, an electrolyte 7 and a Prussian blue counter electrode 9. The working electrode 3 is provided on the transparent conductive film on one of the glass substrates 1. The counter electrode 9 is provided on a transparent conductive film on the other glass substrate 1. The space defined by the pair of glass substrates 1 and the spacer 6 includes the reflective glass plate 4 and is filled with the electrolyte 7, the working electrode 3 and the counter electrode 9 being in contact with the electrolyte. A method according to the present invention for driving the electrochromic display device of Figure 9 will now be described. As explained in relation to the electrochromic display device of Figure 7, the value of the voltage V1 of the working electrode is +0.6 volts and the value of the voltage V2 of the working electrode is -0.2 volts. The value E1 of the electric potential of the counter electrode is +1.3 volts and the value of the electric potential E2 of the counter electrode is -0.2 volts. Consequently in order to render the working electrode 3 coloured a voltage greater than (V1-E1) =0.7 volts with respect to the counter electrode should be applied to the working electrode. In order to render the working electrode 3 colourless a voltage less than (V2-E2)=O.O volts with respect to the counter electrode should be applied to the working electrode. Thus -0.2 volts, which is greater than -0.7 volts, is applied to the working electrode 3 in the coloured state and -0.1 volts which is less than 0.0 volts is applied to the working electrode with respect to the counter electrode in the bleached state. Figure 10 shows a drive circuit for driving an electrochromic display device by a method according to the present invention. A constant voltage power supply 10 has an output of 0.2 volts. A constant voltage power supply 11 has an output of 1.0 volts. A constant voltage power supply 12 has an output of 5.0 volts. A relay switch 13 is operated at 5 volts. The drive circuit has a switch 14, an output terminal 15 connected to a Prussian blue working electrode of an electrochromic display device, and an output terminal 16 connected to a Prussian blue counter electrode. The positive side of the constant voltage power supply 10 and the positive side of the constant voltage power supply 11 are connected together and to the output terminal 16. The negative side of the constant voltage power supply 10 is connected to one input terminal of the relay switch 13. The e negative side of the constant voltage power supply 11 is connected to the other input terminal of the relay switch 13. One side of a coil of the relay switch 13 is connected to one terminal of the switch 14. The other side of the coil of the relay switch 13 is connected to the positive side of the constant voltage power supply 12. The other terminal of the switch 14 is connected to the negative side of the constant voltage power supply 12. Depending upon the ON or OFF condition of the switch 14, the relay switch 13 is controlled so that the output terminal 15 alternatively is at a voltage of -0.2 volts or -1.0 volts with respect to the output terminal 16.

Claims: CLAIMS 1. A method of driving an electrochromic display device having at least one counter electrode (5), at least one working electrode (3) coated with a Prussian blue film and an electrolyte (7) in contact with the working electrode and the counter electrode, the method being characterised in that a voltage greater than (V1 - E) is applied to the working electrode (3) with respect to the counter electrode (5) in order to render the working electrode in a first state and a voltage less than (V2 - E) is applied to the working electrode with respect to the counter electrode in order to render the working electrode in a second state, the voltage V1 corresponding to said first state of the Prussian blue film, the voltage V2 corresponding to the second state of the Prussian blue film and the voltage E corresponding to the potential of the counter electrode (5). 2. A method as claimed in claim 1 characterised in that the counter electrode (5) is coated with a Prussian blue film. 3. A method as claimed in claim 1 or 2 characterised in that a voltage greater than (V1 - E1) is applied to the working electrode with respect to the counter electrode in order to render the working electrode in a coloured state, and a voltage less than (V2 - E2) is applied to the working electrode (3) with respect to the counter electrode (5) in order to render the working electrode in the bleached state, the voltage E1 being the maximum voltage of the material of the counter electrode (5) to perform a reversible electrochemical reaction and the voltage E2 is the minimum voltage of the material of the counter electrode to perform a reversible chemical reaction. 4. A drive method of an electrochromic display device having at least one counter electrode, at least one electrochromic electrode coated with Prussian blue thin film and an electrolyte being in contact with both of said counter electrode and electrochromic electrode characterised in that a voltage more than V1-E is applied to said electrochromic electrode with respect to said counter electrode in order to render said electrochromic electrode the first state and a voltage less than V2-E is applied to said electrochromic electrode with respect to said counter electrode in order to render said electrochromic electrode the second state wherein V1 is high electrode potential corresponding to said first state of said Prussian blue electrochromic material, V2 is low electrode potential corresponding to said second state of said Prussian blue electrochromic material and E is the counter electrode potential.

Inventors: SAKUHARA, TOSHIHIKO, SHIGENO, MASATSUGU
Applicants: SEIKO INSTRUMENTS INC.
Application year: 1983
Main classification: G02F1
IPC codes: G02F1, G09G3