Method of driving thin film EL panel for aging

Abstract: An aging drive method for a thin film EL panel comprises performing a preparatory step of short-circuit­ ing all transparent electrodes by a first connecting line, short-circuiting every other metal electrodes by a second connecting line and short-circuiting the other metal electrodes by a third connecting line, and thereafter repeatedly executing four fields periodically for a specified period of time to thereby cause all picture elements of the panel to luminesce for aging. Each of the four fields comprises a first step of applying a first voltage across the first and second connecting lines and across the first and third connecting lines to charge all the picture elements, and a second step of applying a second voltage across the second and third connecting lines while holding the transparent electrodes in a float­ ing state to cause luminescence of the picture elements of the metal electrodes short-circuited by the second or third connecting line with the voltage resulting from the charge on the picture elements stored in the first step and with the second voltage. The four fields are different from one another in the combination of the polarities of the first and second voltages.

Description: The present invention relates to an aging drive method for thin film electroluminescent (EL) panels which comprise a group of transparent electrodes, a group of metal electrodes disposed thereover and extending in a direction intersecting the group of transparent electrodes, and an EL emitting layer interposed between the two groups of electrodes to provide picture elements at the respective intersections. The method is especially useful for thin film EL panels of large area. Thin film EL panels are generally aged for a specified period of time following the preparation of the thin film by applying an alternating voltage to the picture elements positioned at the intersections of a group of transparent electrodes with a group of metal electrodes. For example, to stabilize the variations in the luminescence brightness, etc. which occur with time and to reject the faulty device due to an initial malfunction. As disclosed in U.S. Patent No. 4,412,155 granted to the present applicant and in U.S. Patent Application Ser. No. 06/401,385 filed on July 23, 1982 by the present applicant (US-A-4 818 913 published on April 4, 1989), the EL panel is aged by applying alternating voltage pulses across the group of transparent electrodes which are all short-circuited, and the group of metal electrodes which are all short-circuited. This causes all the picture elements to luminesce simultaneously, and this procedure is repeated for a specified period of time. However, when the above aging method is used for a thin film EL panel having a large area, the waveform of the alternating voltage pulses applied to the picture element involves a time lag needed for the voltage to reach a definite level. This is known as waveform rounding, and is due to a time constant which is dependent on the resistance of the transparent electrode and the capacitance of the picture element. The method is therefore unable to age all the picture elements uniformly. This problem can be overcome by dividing the metal electrodes, which are arranged in parallel, into a group of odd-numbered electrodes and a group of even-numbered electrodes, and applying a voltage across the two groups. This method is free from the influence of the time constant due to the resistance of the transparent electrodes since the picture elements of the group of odd-numbered metal electrodes are connected in series with the picture elements of the other group through the transparent electrodes. Nevertheless, if a dielectric breakdown occurs in a small number of picture elements in this method, a voltage drop at the faulty picture elements is added to the other faultless picture elements. This results in a large voltage pulse greater than the voltage pulse needed for aging being applied to the faultless picture elements thereby inducing further dielectric breakdown. This is because the group of faulty picture elements is connected in series with the group of faultless picture elements. The present invention provides an aging drive method for a thin film EL panel comprising a first group of electrodes, a second group of electrodes disposed thereover and extending in a direction intersecting the first group of electrodes, and an EL emitting layer interposed between the two groups of electrodes to provide picture elements at the respective intersections, which comprises performing a preparatory step of short-circuiting all the first group of electrodes by a first conductor, characterised by short-circuiting some of the second group of electrodes by a second conductor and short-circuiting the remaining of the second group of electrodes by a third conductor such that each second group electrode short-circuited by the second conductor has a second group electrode short-circuited by the third conductor immediately adjacent to it, and thereafter repeatedly performing four main steps periodically for a specified period of time thereby causing all the picture elements to luminesce, each of the four main steps comprising in combination a first step of applying a first voltage across the first conductor and the second conductor and across the first conductor and the third conductor to charge all the picture elements, and a second step of applying a second voltage across the second conductor and the third conductor while holding the first group of electrodes in a floating state to cause luminescence of the picture elements of the second group of electrodes short-circuited by the second conductor or the third conductor by a combination of the voltage resulting from the charge on the picture elements stored in the first step and with the second voltage, the four main steps being different from one another in the combination of the polarities of the first and second voltages. With the aging drive method of the invention described above, the picture elements of the second electrodes short-circuited by the second conductor or the third conductor are caused to luminesce by the voltage resulting from the charge accumulated on the picture elements in the first step, so that the amount of current through the transparent electrodes is smaller than in the prior art thereby reducing the influence of the time constant. This renders the present method usable for thin film EL panels of large capacity. Moreover, the first and second voltages can be lower than conventionally, with the result that even if luminescent picture elements undergo a minute dielectric breakdown to cause a marked voltage drop, an abnormal voltage will not be applied to the other picture elements. Thus, no dielectric breakdown will be induced in the other picture elements. Fig. 1 is a diagram showing a circuit construction embodying the invention; Fig. 2 is a timing chart of the embodiment of Fig. 1; Figs 3 and 4 are diagrams showing equivalent circuits of the embodiment of the invention; Fig. 5 is a diagram illustrating the structure of an example of thin film EL panel to which the invention is applied; and Fig. 6 is a diagram showing how electrodes are connected for practicing the aging drive method of the invention. The first conductor, the second conductor and the third conductor to be used in the aging drive method of the present invention are known connecting lines of low resistance which are capable of electrically connecting the transparent electrodes and metal electrodes. The method of the invention consists essentially of a preparatory step, and main steps which are performed after the preparatory step. The preparatory step is an electrode connecting process required for applying voltage to the transparent electrodes and the metal electrodes. In the main steps, voltage is actually applied to the transparent electrodes and the metal electrodes. Since the voltage application condition differs from step to step, these main steps will be referred to as "fields" in the following description of embodiments. The present invention will be described below in detail with reference to the embodiments shown in the drawings. Fig. 5 is a perspective view partly broken away and showing a thin film EL display panel of double insulation film structure to which the aging drive method of the invention is applied. With reference to Fig. 5, the panel comprises a multiplicity of transparent strip electrodes 2, for example, of Indium Tin Oxide (ITO) arranged in parallel as a group and formed on a glass substrate 1. The panel also comprises a dielectric layer 3 as of Si₃N₄, and EL emitting layer 4 of ZnS doped with Mn or like active agent, and a dielectric layer 5 as of Si₃N₄. These layers are formed over the group of transparent electrodes, 2 for example, by vacuum evaporation or sputtering as a three-layer structure. The panel further comprises a group of metal electrodes 6 of A1 or like metal formed on the dielectric layer 5 and extending in a direction intersecting the transparent electrodes 2 at right angles therewith. The panel is equivalent to a capacitance device. When a specified alternating voltage is applied across a desired transparent electrode and a desired metal electrode, a minute portion of the EL layer held between the two electrodes at their intersection luminesces, thus providing a picture element for displaying characters, symbols, patterns or the like. Fig. 6 is a plan view showing how the electrodes are connected according to the aging drive method of the invention. With reference to the drawing, the electrodes are connected in the following manner as a preparatory step according to the invention. The transparent electrodes 2, 2, ... are all short-circuited by a connecting line Y. The metal electrodes 6 are divided into two groups, i.e., odd-numbered electrodes 6A, 6A, ... and even-numbered electrodes 6B, 6B, .... The odd-numbered metal electrodes 6A, 6A, ... are all short-circuited by a connecting line XA, and the even-numbered metal electrodes 6B, 6B,... are all short-circuited by a connecting line XB. Fig. 1 shows the construction of an aging drive cicuit for performing the main steps of the present method. With reference to the drawing, indicated at ELA are the picture elements of the EL panel provided by the odd-numbered metal electrodes 6A, 6A,... and the transparent electrodes 2,2,... and at ELB the picture elements of the panel provided by the even-numbered metal electrodes 6B, 6B... and the transparent electrodes 2, 2,.... The circuit has switching transistors TR1 to TR6 and diodes D1 to D6. According to the present embodiment, the aging drive method comprises a first to a fourth field. In the first field, a first voltage VD of positive polarity lower than luminescence start voltage is applied across the odd-numbered metal electrodes 6A, 6A,.. and the transparent electrodes 2, 2,... and across the even-numbered metal electrodes 6B, 6B,... and the transparent electrodes 2, 2,.... Subsequently, a second voltage VD is applied across the odd-numbered metal electrodes 6A, 6A,... and the even-numbered metal electrodes 6B, 6B,... while holding the transparent electrodes 2, 2,... in floating state to cause the picture elements ELA to luminesce by the application of voltage of positive polarity to the transparent electrodes. In the second field, the first voltage VD of positive polarity is applied across the odd-numbered metal electrodes 6A, 6A,... and the transparent electrodes 2, 2... and across the even-numbered metal electrodes 6B, 6B,... and the transparent electrodes 2, 2,.... Subsequently, the second voltage VD is applied across the even-numbered metal electrodes 6B, 6B,... and the odd-numbered metal electrodes 6A, 6A,... while holding the transparent electrodes 2, 2... in floating state to cause the picture elements ELB to luminesce by the application of voltage of positive polarity to the transparent electrodes. In the third field, the first and second voltages are opposite in polarity to those in the first field to cause the picture elements ELA to luminesce by the application of voltage of negative polarity. In the fourth field, the first and second voltages are opposite in polarity to those in the second field to cause the picture elements ELB to luminesce by the application of voltage of negative polarity. These four fields are repeated periodically for a specified period of time. The operation of the circuit in these fields will be described below. Fig. 2 shows the timing chart of the switching transistors TR1 to TR6 and the waveforms of voltages applied to the picture elements ELA of the odd-numbered metal electrodes and the picture elements ELB of the even-numbered metal electrodes. First Field First, the switching transistors TR6 and TR3 are brought into conduction, and the switching transistor TR1 is then brought into conduction, whereby charge C·VD is stored on the picture elements ELA and the picture elements ELB. Fig. 3 shows a circuit equivalent to the drive circuit at this time. The transistor TR1 is thus turned on slightly after the transistor TR3 to diminish the voltage drop due to the current through the transparent electrodes. Next, the transistor TR6 and the transistor TR3 are brought out of conduction, and the transistor TR4 is brought into conduction to reduce the voltage on the metal electrodes for the picture elements ELB to 0 V. Consequently, owing to the capacitive coupling between the picture elements ELA and the picture elements ELB, the potential on the transparent electrodes becomes -α·VD, so that a voltage of (1 + α)·VD is applied to the picture elements ELA. Since this voltage is not lower than the luminescence threshold voltage, the picture elements ELA luminesce. On the other hand, the voltage applied to the picture elements ELB is α ·VD and is lower than the luminescence threshold value, so that the picture elements ELB do not luminesce. Fig. 4 shows a circuit equivalent to the drive circuit at this time. The value α is dependent on the magnitude of voltage VD and is determined by the following calculation. With reference to Fig. 3, the charges QA, QB on the respective picture elements ELA, ELB are QA = QB = C·VD (1) wherein C is the capacitance of the picture elements ELA, ELB not luminescing. Similarly, with reference to Fig. 4, the charges Q′A, Q′B on the picture elements ELA, ELB are Q′A = C′·{VD - (-α·VD)} = (1 + α)·C′·VD (2) Q′B = C ·{0 - (-α·VD)} = α ·C·VD (3) wherein C′ is the capacitance of the picture element ELA when it is luminescent. The amount of charge transferred from the picture elements ELA, ELB in Fig. 3 to the picture elements ELA, ELB in Fig. 4 is given by ΔQ = Q′A - QA = -(Q′B - QB) (4) since the picture element ELA and the picture elements ELB are opposite in the polarity of transfer charge but equal in the amount of thereof. Substitution of Equation (4) in Equations (1) to (3) gives (1 + α)·C′ - C = -(α·C - C) Accordingly, α is given by α = (2C - C′)/(C + C′) When the voltage VD is too low to cause luminescence, α is 0.5, but is smaller than 0.5 in luminescent state. When the luminescence threshold voltage of the picture element is assumed to be Vth, the picture element luminesces if the voltage VD is at least (2/3)·Vth. If the picture element ELA, for example, undergoes a minute dielectric breakdown to result in an abrupt voltage drop inthis case, a voltage exceeding ±VD is not applied to the picture element ELB since the voltage across the picture elements ELA and ELB is VD. Consequently there is not likelihood that an dielectric breakdown will be induced in the picture element ELB. In the second step, i.e in the state of Fig. 4, the transistor TR6 is out of conduction, so that the luminescence current does not flow through the transparent electrodes and is therefore free of the influence of the electrode resistance R. Accordingly, even if used for EL panels of large area, the present method is free of the objection that the waveform of the applied voltage will be altered by the time constant. Second Field First, the transistors TR6 and TR1 are brought into conduction, and the transistor TR3 is then brought into conduction, whereby charge C·VD is stored on the picture elements ELA and ELB. Next, the transistors TR6 and TR1 are brought out of conduction, and the transistor TR2 is brought into conduction to reduce the voltage on the metal electrodes for the picture elements ELA to 0 V. Consequently, owing to the capacitive coupling between the picture elements ELA and the picture elements ELB, the potential on the transparent electrodes becomes -α·VD, with the result that a voltage of (1 + α)·VD is applied to the picture elements ELB. Since this voltage is not lower than the luminescence threshold voltage, the picture elements ELB luminesce. On the other hand, the voltage applied to the picture elements ELA is α·VD and is lower than the luminescence threshold value, so that the picture elements ELA do not luminesce. Third Field First, the transistors TR5 and TR4 are brought into conduction, and the transistor TR2 is then brought into conduction, whereby charge -C·VD is stored on the picture elements ELA and ELB. Next, the transistors TR5 and TR4 are brought out of conduction, and the transistor TR3 is brought into conduction to raise the voltage on the metal electrodes for the picture elements ELB to VD. Consequently, owing to the capacitive coupling between the picture elements ELA and the picture elements ELB, the potential on the transparent electrodes becomes (1 + α)·VD, with the result that a voltage of -(1 + α)·VD is applied to the picture elements ELA, causing these elements to luminesce. On the other hand, the voltage applied to the picture elements ELB is -α·VD and does not cause luminescence of these elements ELB. Fourth Field First, the transistors TR5 and TR2 are brought into conduction, and the transistor TR4 is then brought into conduction, whereby charge -C·VD is stored on the picture elements ELA and ELB. Next, the transistors TR5 and TR2 are brought out of conduction, and the transistor TR1 is brought into conduction to raise the voltage on the metal electrodes for the picture elements ELA to VD. Consequently, owing to the capacitive coupling between the picture elements ELA and the picture elements ELB, the potential on the transparent electrodes becomes (1 + α)·VD, with the result that a voltage of -(1 + α)·VD is applied to the picture elements ELB, causing these elements to luminesce. On the other hand, the voltage applied to the picture elements ELA is -α·VD and does not cause luminescence of these elements ELA. The four fields of the embodiment described above which are repeated periodically for a specified period of time are the first field wherein voltage of positive polarity is applied to the transparent electrodes to cause the luminescence of the picture elements ELA, the second field wherein voltage of positive polarity is applied to the transparent electrodes to cause the luminescence of the picture elements ELB, the third field wherein the application of voltage of negative polarity causes the luminescence of the picture elements ELA, and the fourth field wherein the application of voltage of negative polarity effects the luminescence of the picture elements ELB, whereas the combination of these four fields can be different. For example, the first field by be followed by the second, fourth and third fields in this order, or by the third, second and fourth fields, or by the third, fourth and second fields, or by the fourth, second and third fields, or by the fourth, third and second fields in the order mentioned. Thus, the four fields which differ from one another in the combination of the polarities of the first voltage VD and the second voltage VD are executed repeatedly for a specified period of time. With the aging drive method of the invention for thin film EL panels, current flows through the transparent electrodes for charging the picture elements, but the luminescence current for causing the luminescence of the picture elements flows from metal electrodes to metal electrodes through the transparent electrodes, so that the amount of current through the transparent electrodes can be much smaller than in the conventional aging drive method wherein voltage is applied across the transparent electrodes and the metal electrodes. As a result, the EL panel can be driven for aging with reduced variations in the drive current due to the influence of the transparent electrode resistance and with diminished waveform rounding of the applied current due to the influence of the time constant. This assures an improved aging efficiency and makes the present method usable for aging EL display panels of large area. Further since the charge on the nonluminescent picture elements is utilized for the application of voltage to the picture elements to be luminesced, the voltage to be applied from an external source can be lower than the voltage actually applied for the luminescence of picture elements. Accordingly, even if some luminescent picture elements undergo a minute dielectric breakdown to result in an abrupt voltage drop, no abnormal voltage will be applied to the other picture elements, which can therefore be protected from an induced dielectric breakdown. Because the aging drive method of the invention can be practiced with a diminished influence of the electrode resistance without inducing an dielectric breakdown by a circuit of simple construction, the method is useful for apparatus for aging of large sized EL panels for mass-production.

Claims: An aging drive method for a thin film electroluminescent panel comprising a first group of electrodes (2), a second group of electrodes (6) disposed thereover and extending in a direction intersecting the first group of electrodes (2), and an electroluminescent emitting layer (4) interposed between the two groups of electrodes (2,6) to provide picture elements (ELA, ELB) at the respective intersections, which comprises performing a preparatory step of short-circuiting all the first group of electrodes (2) by a first conductor (Y), characterised by short-circuiting some of the second group of electrodes (6) by a second conductor (XA) and short-circuiting the remaining of the second group of electrodes (6) by a third conductor (XB) such that each second group electrode (6) short-circuited by the second conductor (XA) has a second group electrode (6) short-circuited by the third conductor (XB) immediately adjacent to it, and thereafter repeatedly performing four main steps periodically for a specified period of time thereby causing all the picture elements (ELA, ELB) to luminesce, each of the four main steps comprising in combination a first step of applying a first voltage across the first conductor (Y) and the second conductor (XA) and across the first conductor (Y) and the third conductor (XB) to charge all the picture elements (ELA, ELB), and a second step of applying a second voltage across the second conductor (XA) and the third conductor (XB) while holding the first group of electrodes (2) in a floating state to cause luminescence of the picture elements (ELA, ELB) of the second group of electrodes short-circuited by the second conductor (XA) or the third conductor (XB) by a combination of the voltage resulting from the charge on the picture elements (ELA, ELB) stored in the first step and with the second voltage, the four main steps being different from one another in the combination of the polarities of the first and second voltages. A method as claimed in claim 1 wherein the four main steps are a first main step wherein the first and second voltages are positive, a second main step wherein the first voltage is positive and the second voltage is negative, a third main step wherein the first and second voltages are negative, and a fourth main step wherein the first voltage is negative and the second voltage is positive. A method as defined in claim 1 wherein the four main steps are a first main step wherein the first and second voltages are positive, a second main step wherein the first voltage is positive and the second voltage is negative, a third main step wherein the first voltage is negative and the second voltage is positive, and a fourth main step wherein the first and second voltages are negative. A method as defined in claim 1 wherein the four main steps are a first main step wherein the first and second voltages are positive, a second main step wherein the first and second voltages are negative, a third main step wherein the first voltage is positive and the second voltage is negative, and a fourth main step wherein the first voltage is negative and second voltage is positive. A method as defined in claim 1 wherein the four main steps are a first main step wherein the first and second voltages are positive, a second main step wherein the first and second voltages are negative, a third main step wherein the first voltage is negative and the second voltage is positive, and a fourth main step wherein the first voltage is positive and the second voltage is negative. A method as defined in claim 1 wherein the four main steps are a first main step wherein the first and second voltages are positive, a second main step wherein the first voltage is negative and the second voltage is positive, a third main step wherein the first voltage is positive and the second voltage is negative, and a fourth main step wherein the first and second voltages are negative. A method as defined in claim 1 wherein the four main steps are a first main step wherein the first and second voltages are positive, a second main step wherein the first voltage is negative and the second voltage is positive, a third main step wherein the first and second voltages are negative, and a fourth main step wherein the first voltage is positive and the second voltage is negative. A method as defined in claim 1 wherein the first and second voltages are equal in magnitude. A method as defined in claim 1 wherein the magnitude of the first and second voltages are not higher than the luminescence threshold voltage of the picture elements (ELA, ELB). A method as defined in claim 1 wherein the magnitude of the first and second voltages are not smaller than 2/3 of the luminescence threshold voltage of the picture elements (ELA, ELB).

Inventors: ISAKA KINICHI, KISHISHITA HIROSHI, OHBA TOSHIHIRO, SHIMOYAMA HIROYUKI, UEDE HISASHI, ISAKA, KINICHI, KISHISHITA, HIROSHI, OHBA, TOSHIHIRO, SHIMOYAMA, HIROYUKI, UEDE, HISASHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1988
Main classification: G01R31
IPC codes: G09G3, H05B33