EP0258130B1 - Electroluminescent flat screen - Google Patents

Description

The present invention relates to an electroluminescent flat screen. Such a screen allows the display of a large amount of graphic and / or alpha-digital information and is used as a display terminal in portable computers, in telematic terminals, such as the minitel, or as a television screen.

Conventionally, an electroluminescent screen consists of a substrate on which are stacked layers of electrically conductive materials, layers of electrical insulator and a layer of electroluminescent material, these layers being protected by a backing covering the substrate. Such an electroluminescent screen is described in particular in the article "Practical application technologies of thin-film electroluminescent panels" by Mikio Takeda et al. published in Proceedings of the SID, vol. 22-1, 1981, pp. 57 to 62.

There is shown in Figure 1 a sectional view of an electroluminescent screen according to this article.

The different active layers of the screen are deposited on a transparent glass substrate 2. A conductive layer is first deposited, for example in In₂O₃, which is then etched to form a network of transparent parallel electrodes 4. Next, a first dielectric layer 6, an electroluminescent layer 8 and a second dielectric layer 10 are deposited successively. The dielectric layers are for example of Si₃N₄ and the electroluminescent layer of ZnS: Mn.

The second series of electrodes 14 is then etched in a conductive layer deposited on the second dielectric layer. A bonding layer 12, for example made of Al₂O₃, can be placed between the layer 10 and the electrodes 14 to facilitate the bonding thereof.

Finally, the circuit is protected from mechanical attack and moisture by a glass plywood 18 fixed to the substrate 2 by a sealing bead 20, the free space between the deposited layers and the plywood 18 being previously filled with a filling material 16, such as a silicone oil.

Each intersection between an electrode 4 and an electrode 14 defines an image element constituted by the superposition of the first dielectric layer, the electroluminescent material and the second dielectric layer. The two networks of electrodes 4, 14 thus define a matrix of electroluminescent elements.

A picture element has a certain fragility. It is not uncommon in fact for an electrical breakdown to appear in an image element, which generally results in degradation of at least one of the two control electrodes associated with this image element.

It follows that the degraded electrode portion located beyond the breakdown zone is no longer supplied. The picture elements associated with this electrode portion can no longer be addressed and therefore no longer emit light.

A snap in a picture element therefore results in a display fault on a row or column portion of the display. As this defect is not tolerable, methods have been proposed to avoid degradation of the electrodes when an electrical breakdown occurs in an image element.

A first solution is proposed in the article "Thin-film electroluminescent displays produced by atomic layers" by T.Sutela published in the journal Displays, April 1984, pp. 73 to 78. This method consists in making short incisions in the electrodes , parallel to their direction, at the level of each picture element. The purpose of these incisions is to stop the spread of electrical breakdowns, on a principle analogous to that of a firebreak in a forest.

This method has the disadvantage of reducing the emissive surface because the anti-propagation incisions have a significant width, of the order of 10 to 20 microns, and they must be numerous to be generally effective.

A method for reducing the number of electrical breakdowns is also described in document GB-A-2096814. This method consists in applying a write compensation pulse before the refresh pulse and a regeneration compensation pulse after the regeneration pulse and before the next write pulse. These compensating pulses are of opposite signs to the writing and regeneration pulses and their intensity is chosen to be sufficiently low so as not to act on the picture elements.

The two known methods which have just been described aim to reduce the number of electrical breakdowns or the effect of an electrical breakdown on an electrode. However, these methods do not provide a solution once an electrode has been damaged. They therefore do not avoid a display defect on the portion of the electrode situated beyond the deteriorated part.

The object of the invention, on the contrary, is to make it possible to continue to control the display of the image elements situated beyond the deteriorated part of the electrode, that is to say to limit the defect of display with the only picture element destroyed by electrical breakdown.

To achieve this object, the invention proposes to engrave counter-electrodes on the internal face of the protective counter-plate, and to connect each counter-electrode to the two ends of a control electrode of the electroluminescent screen.

In this way, the control electrodes are supplied by their two ends. Thus, when a cut appears on a control electrode, following the electrical breakdown of a picture element, the portion of the electrode situated beyond the cut continues to be supplied.

All image elements except the one where appeared on electrical breakdown, are then supplied. The display defect thus remains limited to a single image element, which is practically not detectable by an observer.

Specifically, the subject of the invention is an electroluminescent flat screen according to claim 1. It applies to a screen comprising a transparent substrate, a first family of parallel line electrodes etched on said substrate, said electrodes being transparent, a layer of an electroluminescent material interposed between two layers of dielectric, a second family of parallel column electrodes etched on said dielectric layer, the two families of electrodes being crossed and defining in the electroluminescent layer a set of optical emitters arranged in a matrix , and a protective backing plate sealed on said substrate. According to claim 1, said flat screen is characterized in that the counterplate carries, on its inner face, at least one counter electrode and in that means are provided so that each counter electrode is electrically connected to the two ends of the same electrode of the first or second family of electrodes.

According to a particular embodiment, the protective counterplate carries, on its inner face, a family of parallel counter-electrodes partially covered with a layer of electrical insulator, each counter-electrode being connected to the two ends of an electrode of the first family of electrodes, and being isolated from the electrodes of the second electrode family by said layer of electrical insulator.

According to another preferred embodiment, the plywood carries, on its inner face, a family of parallel counter-electrodes, each counter-electrode being electrically connected to an electrode of the second family of electrodes.

According to another advantageous embodiment, the plywood carries, on its inner face, a first family of counter-electrodes, a layer of electrical insulator and a second family of counter-electrodes, the first and second families of counter electrodes being crossed, each counter electrode of the first family of counter electrodes being electrically connected to the two ends of an electrode of the first family of electrodes and each counter electrode of the second family of counter electrodes being electrically connected to the two ends of an electrode of the second family of electrodes.

Preferably, the electrical contact between an electrode and a counter-electrode is obtained, at each end of said electrode, by a conductive pad. This can for example be carried out using a conductive ink deposited by screen printing.

The invention according to independent claim 8 can also be implemented in the case of a double screen comprising two substrates assembled with each other, each substrate carrying a first family of parallel electrodes, a set of layers constituting the electroluminescent elements, and a second family of parallel electrodes, the electrodes of the first and second families being crossed, the assembly being carried out so as to electrically connect the two ends of an electrode of the second family of electrodes of a substrate with the two ends of an electrode of the second family of electrodes of the other substrate.

the invention according to independent claim 9 can also be advantageously used in large electroluminescent screens in which the column electrodes are formed by two half-columns and the row electrodes are paired.

A screen with a slightly different structure but based on the same principle of dividing each column into two half-columns is written in the article "A large-area electroluminescent display", RT Flegal et al., SID 85 Digest, pp. 213-214.

• la figure 1, déjà décrite, est une vue en coupe d'un écran électroluminescent de type connu,
• la figure 2 représente un mode de réalisation d'un écran électroluminescent selon l'invention, dans lequel la contreplaque porte une famille de contre-électrodes,
• la figure 3 est une vue en coupe de l'écran électroluminescent de la figure 2 au moment de l'assemblage du substrat et de la contreplaque,
• la figure 4 illustre une disposition avantageuse des plots conducteurs, pour éviter un contact électrique entre deux électrodes voisines,
• la figure 5 illustre un écran électroluminescent suivant un second mode de réalisation de l'invention, dans lequel la contreplaque porte deux familles de contre-électrodes croisées,
• la figure 6 illustre un mode de réalisation d'un écran électroluminescent selon l'invention, comprenant deux matrices d'éléments d'image électroluminescents, et
• la figure 7a illustre la structure d'un écran électroluminescent dont les lignes sont codées par paire et la figure 7b illustre ledit écran muni d'une contreplaque suivant l'invention.

The characteristics and advantages of the invention will emerge more clearly from the description which follows, given by way of illustration but not limitation, with reference to the appended drawings, in which:

• FIG. 1, already described, is a sectional view of an electroluminescent screen of known type,
• FIG. 2 represents an embodiment of an electroluminescent screen according to the invention, in which the counterplate carries a family of counter-electrodes,
• FIG. 3 is a sectional view of the electroluminescent screen of FIG. 2 at the time of the assembly of the substrate and the plywood,
• FIG. 4 illustrates an advantageous arrangement of the conductive pads, to avoid electrical contact between two neighboring electrodes,
• FIG. 5 illustrates an electroluminescent screen according to a second embodiment of the invention, in which the plywood carries two families of crossed counter-electrodes,
• FIG. 6 illustrates an embodiment of an electroluminescent screen according to the invention, comprising two matrices of electroluminescent picture elements, and
• FIG. 7a illustrates the structure of an electroluminescent screen whose lines are coded in pairs and FIG. 7b illustrates said screen provided with a backing plate according to the invention.

A first embodiment of an electroluminescent screen according to the invention will be described with reference to FIGS. 2 and 3.

The electroluminescent screen comprises a transparent substrate 2 on which the matrix of electroluminescent elements is produced, and a backing plate 18. These two elements are intended to be assembled later.

The matrix of electroluminescent elements is produced on the substrate 2 in a conventional manner, for example as described with reference to FIG. 1. A first conductive layer is deposited and then etched so as to form a first family of parallel electrodes 4. These electrodes are transparent and are made, for example, of indium tin oxide.

These electrodes are covered with a first layer of dielectric 6, a layer of electroluminescent material 8 and a second layer of dielectric 10 (FIG. 3). All of these three layers are represented by a single layer 22 in FIG. 2. The dielectric layers are for example made of Si₃N₄ and the electroluminescent layer made of ZnS: Mn.

This layer 22 is covered with a conductive layer in which is engraved a second family of electrodes 14. These electrodes are not necessarily transparent and are made for example of aluminum. Between the layer 22 and the electrodes 14, an Al₂O₃ bonding sublayer can be deposited to allow better contact between the electrodes 14 and the layer 22. In the embodiment of FIG. 2, the control circuits 24 and 26 of the first and second families of electrodes are also produced on the substrate 2.

Conventionally, a plywood 18 is added to provide mechanical protection and protection against moisture of the layer 22. The fixing between the substrate 2 and the plywood 18 is done by a sealing bead 20.

In the embodiment shown, this sealing bead is produced on the backing plate 18. However, it can also be produced on the substrate 2.

According to the invention, the plywood 18 carries at least one etched counter-electrode intended to be connected to the two ends of an electrode of the substrate 2. In the embodiment shown, the plywood 18 carries, by way of example, a family of parallel counter-electrodes 28. These are made, for example, of aluminum.

These counter-electrodes 28 can be etched in a layer deposited directly on the plywood 18. It is also possible to deposit beforehand on the plywood 18, an Al₂O₃ bonding sub-layer or a SiO₂ layer constituting a chemical barrier.

The counter electrodes 28 are intended to be connected to the electrodes 14 etched on the substrate 2. This connection is made by a set of conductive pads 30 produced on the counter electrodes 28.

For the production of these conductive pads, a conductive ink deposited by screen printing can be used. This technique has the advantage of being easy to implement and of being inexpensive.

One can for example use an ink of the epoxy resin type with two polymerization states, such as the ink EPO-TEK WE-12. In this case, the ink deposited is first dried at 90 ° C. for 90 minutes to obtain a first prepolymerized state which allows storage for several months at room temperature.

The sealing bead 20 can also be produced by means of an ink deposited by screen printing such as the EPO-TEK H78 ink. This ink is dried in the same way as the ink used for producing the conductive pads 30.

The assembly of the substrate 2 and the plywood 18 is done for example in a glove box under a flow of dry gas. The precise positioning of the plywood 18 relative to the substrate 2 is obtained by bringing two marks 32 and 36 on the substrate 2 into coincidence with marks 34 and 38 respectively on the plywood 18.

Finally, in the case of the inks mentioned for producing the conductive pads and the sealing bead, the polymerization of these inks is obtained by heating for 15 minutes at 150 ° C.

The heating for the polymerization of the inks can be localized, if the other elements of the screen are not provided to withstand such a temperature.

Although Figures 2 and 3 show the conductive pads and the sealing bead on the plywood 18, it is obvious that these elements can also be deposited on the substrate 2 or can be distributed on the substrate 2 and on plywood 18.

During the assembly of the substrate 2 and the plywood 18, the conductive pads 30 are crushed between a counter electrode 28 and an electrode 14 of the second family of electrodes of the substrate 2. This crushing of the pads can cause electrical contact. between two adjacent electrodes or two counter electrodes.

To avoid this drawback, the conductive pads can be arranged as shown in FIG. 4, where the conductive pads of the consecutive counter-electrodes 28 are offset relative to each other.

The addition of counter-electrodes on the counter-plate 18, according to the invention, makes it possible to supply each electrode 14 by its two ends. This allows in the event of a local break in an electrode 14, for example as a result of an electrical breakdown, to continue to supply the portion of the electrode situated beyond the break.

In addition, the fact of supplying each electrode 14 by these two ends makes it possible to reduce the effects of time constant, particularly sensitive for large screens, which are caused by the resistance of the electrodes. Similarly, the supply of an electrode by its two ends has the result of reducing the current peaks, which makes it possible to increase the lifetime of the electroluminescent screen.

In the embodiment of FIGS. 2 and 3 the plywood 18 carries a family of counter-electrodes 28 intended to be connected to the electrodes 14 of the substrate 2. These electrodes constituting the upper layer of the substrate 2, contact can take place directly between these and the counter-electrodes 28.

A variant of this embodiment consists in replacing the counter-electrodes 28 by another family of electrodes intended to be connected to the electrodes 4 of the substrate 2. In this case, the counter-electrodes must be covered with a layer of electrical insulator to avoid contact with the electrodes 14 during assembly between the substrate 2 and the plywood 18.

The fact of supplying the two ends of the electrodes 4 has the same advantages as the fact of supplying the two ends of the electrodes 14, as described with reference to FIGS. 2 and 3, namely a reduction in the effects of constant of time and a reduction in current peaks. The supply of the electrodes 4 by their two ends is particularly advantageous because these electrodes have a small thickness - they must be transparent and not create any step effect - and therefore have an electrical resistance high.

Another variant consists in making simultaneously on the plywood 18 two families of crossed counter-electrodes intended to be each connected to one of the two families of electrodes of the substrate 2. Such an embodiment is shown in FIG. 5.

In this figure, the elements identical to those of Figure 2 have the same references. The matrix of electroluminescent elements is produced on the substrate 2 using a first family of parallel electrodes 4, a layer 22 composed of a first layer of dielectric, a layer of electroluminescent material and d a second layer of dielectric, and a second family of parallel electrodes 14, the electrodes of the two families being crossed.

In accordance with the invention, counter-electrodes are etched on the counterplate 18. More precisely, in the embodiment of FIG. 5 the counterplate 18 comprises a first family of parallel counter-electrodes 42 intended to be connected to the electrodes 4 of the substrate 2, an electrical insulating layer 44 produced for example Al₂O₃, Y₂O₃, Ta₂O₅ or others (in particular a screen-printing insulating paste).

A second family of parallel counter-electrodes 28 is then etched in a conductive layer deposited on the layer 44. These counter-electrodes 28 are intended to be electrically connected to the electrodes 14 of the substrate 2.

The layer 44 makes it possible to electrically isolate the two families of counter-electrodes etched on the counterplate 18 the deposition of each conductive layer in which the counter-electrodes are etched can be preceded by the deposition of a bonding sub-layer, for example in Al₂O₃, or of a layer constituting a chemical barrier, for example in SiO₂, Si₃N₄ or other.

On each counter electrode 28, 42 are made two conductive pads 30, 46 for connecting each of said counter electrodes to the two ends of the corresponding electrode of the substrate 2.

These conductive pads can be produced by means of a conductive ink deposited by screen printing. The same technique can be used to produce the sealing bead 20 from a non-electrically conductive ink.

The control circuits 24 and 26 of the families of electrodes 4 and 14 can be produced independently on the substrate 2 or on the plywood 18. The production on the plywood 18 may have an advantage for the functional test before assembly. In fact, in this embodiment, the electroluminescence and addressing functions of the electroluminescent elements are respectively produced on the substrate 2 and on the plywood 18, and can therefore be tested separately.

The invention can also be implemented for the production of an electroluminescent screen comprising two layers of electroluminescent material of different colors. Such an electroluminescent screen is shown in FIG. 6.

In this embodiment, the conventional plywood used to protect the matrix of electroluminescent elements produced on the substrate is replaced here by a second substrate itself carrying a matrix of electroluminescent elements.

The two substrates 2A, 2B have a conventional structure, and identical, as regards the matrix of electroluminescent elements. They each comprise a first family of electrodes 4A, 4B parallel, a layer 22A, 22B composed of two dielectric layers between which is disposed a layer of electroluminescent material, and a second family of electrodes 14A, 14B parallel, the first and second families being crossed.

The electroluminescent materials of the two substrates are different. Furthermore, the electrodes 14A, 14B are transparent as well as, for example, the electrodes 4A if the viewing is done through the substrate 2A.

Conductive pads 30A, 30B are arranged at the two ends of each of the electrodes 14A, 14B of the second families of electrodes of each substrate.

The electroluminescent screen is completed by a control circuit 24A of the first family of electrodes 4A of the substrate 2A, a control circuit 24B of the first family of electrodes 4B of the substrate 2B, and a control circuit 26 for controlling the second families of electrodes 14A, 14B of the substrates 2A, 2B.

The control circuit 24 can be produced on any one of the two substrates. In the embodiment of FIG. 6, it is produced on the substrate 2A and is connected directly to the electrodes 14A of the second family of electrodes of this substrate. It is also connected to the electrodes 14B of the second family of electrodes of the substrate 2B via the conductive pads 30A, 30B.

The electroluminescent screen finally comprises a passivation layer (not shown) to avoid electrical contact between the electrodes 4A and the electrodes 4B and possibly, the circuits 24A and 24B if these are opposite, and a sealing bead 22 to protect the electroluminescent matrices from ambient humidity.

The invention can also be used advantageously in large light-emitting screens in which the column electrodes are formed by two half-columns and the row electrodes are paired.

Such a screen is shown in Figure 7a.

A screen with a slightly different structure but based on the same principle of dividing each column into two half-columns is written in the article "A large-area electroluminescent display", RT Flegal et al., SID 85 Digest, pp. 213-214.

In this screen, each column electrode is formed by an upper half-column 48 and a lower half-column 50. The upper half-columns are addressed by a circuit 52 and the lower half-columns by a control circuit 54.

The line electrodes are divided equally into a subset of upper lines and a subset of lower lines, each line electrode of one subset being connected to a line electrode of the other subset.

By way of example, in FIG. 7a, the row electrode of rank i, 1iN, where N is the number of rows of a subset, is connected to the row electrode N-i + 1 of the other subset. The line electrodes are therefore controlled in pairs, by a control circuit 56.

The connection between a line electrode of one sub-assembly and the corresponding line electrode of the other sub-assembly, which is shown diagrammatically in FIG. 7a, can be carried out in practice by means of etched counter electrodes, according to the invention, on the counterplate covering the matrix of electroluminescent elements.

Such counter electrodes are shown in Figure 7b. These counter electrodes 58 are in the form of U nested one inside the other so as to connect each of the two ends of a line electrode of a subset to one of the ends of the line electrode associated with the other subset.

In this way, each line electrode is electrically supplied by its two ends.

An advantage provided by the proposed arrangement over the arrangement described in the article by R.T. Flegal is the saving in surface area of the substrate carrying the electroluminescent structure, since the connections between the lines of different sub-assemblies are made on the plywood.

Claims (9)

1. Flat electroluminescent screen comprising a transparent substrate (2) on which are successively deposited a first group of parallel electrodes (4), said electrodes being transparent, a layer (8) of an electroluminescent material inserted between two dieletric layers (6, 10), a second group of parallel electrodes (14), the two groups of electrodes intersecting and defining in the electroluminescent layer a plurality of matrix-arranged optical emitters, said flat screen also having a control circuit (24) for the electrodes (4) and a control circuit (26) for the electrodes (14) and a protective counter-plate sealed on said substrate by a sealing band (20), characterized in that the counter-plate carries on its inner face at least one counter-electrode (28, 42) and in that means (30) are provided so that each counter-electrode is electrically connected to the two ends of the same electrode of the first or second groups of electrodes.
2. Flat electroluminescent screen according to claim 1, characterized in that the protective counter-plate (18) carries on its inner face a group of parallel counter-electrodes (42) partly covered by an electrically insulating layer (44), each counter-electrode being connected to the two ends of an electrode (4) of the first group of electrodes and is insulated from the electrodes (14) of the second group of electrodes by said electrically insulating layer.
3. Flat electroluminescent screen according to claim 1, characterized in that the counter-plate (18) carries on its inner face a group of parallel counter-electrodes (28), each counter-electrode being electrically connected to an electrode (14) of the second group of electrodes.
4. Flat electroluminescent screen according to claim 1, characterized in that the counter-plate (18) carries on its inner face a first group of counter-electrodes (42), an electrically insulating layer (44) and a second group of counter-electrodes (28), the first and second groups of counter-electrodes intersecting, each counter-electrode (42) of the first group of counter-electrodes being electrically connected to the two ends of an electrode (4) of the first group of electrodes and each counter-electrode (28) of the second group of counter-electrodes is electrically connected to the two ends of an electrode (14) of the second group of electrodes.
5. Flat electroluminescent screen according to any one of the claims 1 to 4, characterized in that the electrical contact between an electrode and a counter-electrode is obtained, at each end of said electrode, by a conductive stud (30).
6. Flat electroluminescent screen according to claim 5, characterized in that each conductive stud (30) is constituted by a conductive ink deposited by screen process printing.
7. Flat electroluminescent screen according to claims 5 or 6, characterized in that the conductive studs of two adjacent electrodes are reciprocally displaced.
8. Flat electroluminescent screen, characterized in that it comprises two substrates (2A, 2B) assembled with one another, each substrate successively carrying a first group of parallel electrodes (4A, 4B), a plurality of layers (22A, 22B) constituting the electroluminescent elements and a second group of parallel electrodes (14A, 14B), the electrodes of the first and second groups intersecting, the assembly of the substrates being realized in such a way as to electrically connect the two ends of an electrode (14A) of the second group of electrodes of a substrate to the two ends of an electrode (14B) of the second group of electrodes of the other substrate (2B), said screen also comprising a control circuit (24A) for controlling the electrodes of the first group of electrodes of a substrate (2A), a control circuit (24B) for controlling the electrodes of the first group of electrodes of the other substrate (2B) and a control circuit (26) for controlling the electrodes of the second groups of electrodes of the two substrates.
9. Flat electroluminescent screen comprising a transparent substrate on which are successively deposited a group of parallel row electrodes constituted by two row electrode subassemblies, said electrodes being transparent and each row electrode of a subassembly being paired with a row electrode of the other subassembly, an electroluminescent material layer inserted between two dielectric layers and a group of parallel column electrodes, each column being formed by two half-columns (48, 50), each half-column intersecting a row subassembly, said flat screen also comprising a row control circuit (56) and two half-column control circuits (52, 54), said screen also comprising a protective counter-plate sealed on said substrate by a sealing band (20), characterized in that the counter-plate carries on its inner face a group of counter-electrodes and in that means are provided for electrically connecting via a counter-electrode each end of a row electrode to one end of the row electrode with which it is paired.

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