EP1456831B1 - Image display panel consisting of a matrix of electroluminescent cells with shunted memory effect - Google Patents

Description

• une couche électroluminescente 16 susceptible d'émettre de la lumière vers l'avant dudit panneau (flèches 19 d'émission de lumière),
• à l'avant de cette couche, une couche avant transparente d'électrodes 18,
• à l'arrière de cette couche, une couche photoconductrice 12, elle-même intercalée entre une couche arrière opaque d'électrodes 11 et une couche intermédiaire d'électrodes 14 au contact de la couche électroluminescente 16,
• des moyens de couplage optique entre ladite couche électroluminescente 16 et ladite couche photoconductrice 12, qui peuvent par exemple être formés par une couche de couplage spécifique 13 (comme sur la figure) ou formés dans la couche intermédiaire d'électrodes 14.

The invention relates to an image display panel formed of a matrix of electroluminescent cells, comprising, with reference to the figure 1 :

• an electroluminescent layer 16 capable of emitting light towards the front of said panel (arrows 19 of light emission),
• in front of this layer, a transparent front layer of electrodes 18,
• at the rear of this layer, a photoconductive layer 12, itself interposed between an opaque rear layer of electrodes 11 and an intermediate layer of electrodes 14 in contact with the electroluminescent layer 16,
• optical coupling means between said electroluminescent layer 16 and said photoconductive layer 12, which may for example be formed by a specific coupling layer 13 (as in the figure) or formed in the intermediate layer of electrodes 14.

The panels of this type also comprise a substrate 10, at the rear (as in the figure) or at the front of the panel, to support all the previously described layers; it is usually a glass plate or polymer material.

The photoconductive layer 12 is intended to provide the cells of the panel a memory effect which will be described later.

The electrodes of the front layer 18, the rear layer 11 and the intermediate layer 14 are adapted in a manner known per se to be able to control and maintain the emission of the cells of the panel, independently of each other; for this purpose, the electrodes of the front layer 18 are for example arranged in Y lines and the electrodes of the rear layer 11 are then arranged in X columns, generally orthogonal to the lines; the electrodes may also have the opposite configuration: front layer electrodes in columns and inline back layer electrodes; the cells of the panel are located at the intersections of the Y-line electrodes and the X column electrodes, and are therefore arranged in a matrix.

To visualize on such a panel images partitioned into a matrix of light spots, the electrodes of the different layers are fed so as to circulate an electric current through the cells of the panel corresponding to the light spots of said image; the electric current flowing between an electrode X and a Y electrode for supplying a cell positioned at the intersection of these electrodes, passes through the electroluminescent layer 16 situated at this intersection; the cell thus excited by this current then emits light 19 towards the front face of the panel; the emission of all the excited cells of the panel forms the image to be displayed.

The documents US 4035774 - IBM, US 4808880 - HUNDRED, US 6188175 B1 - CDT describe panels of this type.

The electroluminescent layer 16, when organic, generally breaks down into three sub-layers: a central electroluminescent sub-layer 160 interposed between a sub-layer 162 for transporting holes and an underlayer 161 for transporting electrons .

The electrodes of the electrode front layer 18, in contact with the underlayer 162 of hole transport, then serve as anodes; this layer of electrodes 18 must be transparent, at least partially, to allow the light emitted by the electroluminescent layer 16 to pass towards the front of the panel; the electrodes of this layer are generally themselves transparent and made of mixed tin and indium oxide ("ITO"), or conductive polymer such as polyethylenedioxythiophene ("PDOT").

The intermediate electrode layer 14 must be sufficiently transparent to allow adequate optical coupling between the electroluminescent layer 16 and the photoconductive layer 12, because this optical coupling is necessary for the operation of the panel and, in particular, for obtaining the effect memory described below.

The documents cited above also disclose configurations where, contrary to what has just been described, on the one hand, the electrodes of the intermediate layer of electrodes 14 and the underlayer 161 respectively serve the purpose. injection and transport of holes in the underlay 160, on the other hand, the electrodes of the electrode front layer 18 and the sub-layer 162 respectively serve for the injection and transport of the electrons in the electroluminescent sublayer 160.

According to another variant, the electrode front layer 18 may itself comprise several sub-layers, including an interface sub-layer with the organic electroluminescent layer 16 intended to improve the injection of holes (anode case) or electrons (cathode case).

The photoconductive layer 16 may for example be of amorphous silicon, or of cadmium sulphide.

• un élément électroluminescent E_EL englobant une zone de couche électroluminescente 16, et,
• un élément photoconducteur E_PC englobant une zone de couche photoconductrice 12 au regard de cette même zone de couche électroluminescente 16.

In display panels of this type, the role of the photoconductive layer 12 is to bring a "memory" effect to the cells of the panel; referring to the figure 2 , each cell of the panel can be represented by two elements in series:

• an electroluminescent element E _EL including an electroluminescent layer area 16, and
• a photoconductive element E _PC enclosing a photoconductive layer zone 12 with regard to this same electroluminescent layer area 16.

The memory effect obtained is based on a loop operation, as shown in FIG. figure 2 : as long as an electroluminescent element E _EL of a cell emits light 19, a part 19 'of which, by optical coupling, reaches the photoconductive element E _PC of this same cell, the switch formed by this element E _PC is closed, and as long as this switch is closed, the electroluminescent element E _EL is supplied with current between a terminal A in contact with an electrode of the front layer 18 and a terminal B in contact with an electrode of the rear layer 11; the electroluminescent element E _EL therefore emits light 19, a portion 19 'excites the photoconductive element E _PC .

This loop operation therefore relies on an adequate optical coupling between the electroluminescent layer 16 and the photoconductive layer 12; if the display panel has a specific optical coupling layer, it may be for example an opaque insulating layer pierced transparent apertures adapted and positioned in front of each electroluminescent element E _EL , that is to say of each pixel or sub-pixel of the panel; in the absence of a specific coupling layer, it is also possible to use, as coupling means, transparent openings made in the intermediate layer of electrodes 14; other optical coupling means are possible, which are known to those skilled in the art and will not be described here in detail.

This supposed memory effect is intended to facilitate the control of the pixels and sub-pixels of the panel to display images and, in particular, to be able to use a method in which, successively for each line of the panel, one goes through an addressing phase intended to turn on the cells to be lit in this line, then by a holding phase intended to keep the cells of this line in the state where the previous phase of addressing put them or left.

In practice, each line of the panel is successively scanned to put each cell of the scanned line in the desired state, on or off; after scanning a given line, all the cells of this line are maintained or fed in the same way so that only the cells put in the lit state of this line emit light while sweeping or that we address other lines; thus, preferably, during the phases of maintaining a line, the addressing phases of other lines take place.

In practice, the duration of the holding phases makes it possible to modulate the luminance of the cells of the panel and, in particular, to generate the gray levels necessary for the visualization of an image.

• lors des phases d'adressage, l'application, uniquement aux bornes A, B des cellules à allumer, d'une tension d'allumage V_a ;
• lors des phases de maintien, l'application aux bornes A, B de toutes les cellules d'une tension de maintien V_s , qui doit être suffisamment élevée pour que les cellules préalablement allumées restent allumées, et suffisamment faible pour ne pas risquer d'allumer les cellules préalablement non allumées.

The implementation of such a control method of the cells of the panel generally goes through:

• during the addressing phases, the application, only to the terminals A, B of the cells to be lit, of an ignition voltage V _a ;
• during the holding phases, the application at the terminals A, B of all the cells of a holding voltage V _s , which must be sufficiently high for that the previously lit cells remain lit, and low enough not to risk turning on previously unlighted cells.

The addressing phase is therefore a selective phase; on the contrary, the holding phase is not selective, which makes it possible to apply the same voltage to all the cells and considerably simplifies control of the panel.

• un élément inorganique électroluminescent Zel et un élément photoconducteur LPC branchés en série comme dans les panneaux de visualisation du type précité,
• en outre, un élément photoconducteur d'effacement, référencé EPC dans ce document, branché en parallèle sur ledit élément électroluminescent.

The document "IBM Technical Disclosure Bulletin", Vol.24, No. 5, pp.2307-2310, entitled "Erasable memory storage display" , describes a display panel whose each cell comprises:

• an electroluminescent inorganic element Zel and a photoconductive element LPC connected in series, as in the display panels of the aforementioned type,
• in addition, an erasure photoconductive element, referenced EPC in this document, connected in parallel to said electroluminescent element.

The photoconductive erasing element in parallel with the electroluminescent element has a resistance varying between a low R-ON value when it is excited by an erase illumination and a low R-OFF value when it is not enlightened; according to this document, this erasure photoconductor element is used to turn off the corresponding cells that would be switched on and in the holding phase; the control method of the panel thus comprises cell erasure phases, during which these cells are illuminated by erasure illumination.

During an erasure phase, which usually ends a holding phase, it is obviously necessary for each cell that is lit in the ON state to be erased, and whose erasure photoconductive element is excited, the resistance R-ON is lower than the resistance R _ON-EL that the electroluminescent element E _{EL has} in the lit state, so that it can be considered that the intensity of the electric current passing through this cell still in the ON state essentially passes through the erasure photoconductive element, and not by the electroluminescent element E _EL , since it is precisely a question of extinguishing it.

Apart from the erasing phases, the erasure photoconductive elements have a resistance R-OFF and the elements E electroluminescent _EL panel are either in the off state and exhibit resistance R _OFF-EL, either the on state and exhibit a resistance R _ON-EL; nothing is said in this document about the value of R-OFF compared to the value of R _OFF-EL , so that the person skilled in the art can not learn anything about the effective and efficient shunt function that would have or not the photoconductive erasure elements in the non-excited state with respect to the electroluminescent elements in the off state.

Thus, this document is limited to describing means capable of effectively shunting electroluminescent elements in the on state to erase them, while the invention, as will be seen below, proposes, for a very different purpose, means for shunting the electroluminescent elements in the off state.

The memory effect will now be described more precisely when a control method of this type is applied to a memory-effect electroluminescent panel of the type just described, in the case where the zones of the layer of intermediate electrodes 14 specific to each electroluminescent element E _EL are electrically insulated from each other, so that the electrical potential at the common point C of the electroluminescent element E _EL and the photoconductive element E _PC is floating.

Still referring to the figure 2 , the display panel forms a set of cells C _{n, p} capable of emitting light and fed by lines of electrodes Y _n , Y _{n + 1} of the front layer 18 connected to points A corresponding to a terminal electroluminescent element E _EL and the electrode columns X _p , X _{p + 1} of the rear layer 11 connected to points B corresponding to a photoconductive element terminal E _PC .

• pour une cellule C_n,p , une séquence d'adressage de cette ligne au temps t₁, avec allumage de cette cellule qui reste allumée pour t>t₁,
• pour une cellule de la ligne suivante C_n+1,p , une séquence d'adressage de cette ligne au temps t₂, sans allumage de celle cellule qui reste éteinte pour t>t₂.

The figure 3 illustrates, according to this classical control mode:

• for a cell C _{n, p} , an addressing sequence of this line at time t ₁ , with ignition of this cell which remains on for t> t ₁ ,
• for a cell of the following line C _{n + 1, p} , an addressing sequence of this line at time t ₂ , without switching on that cell which remains off for t> t ₂ .

The three chronograms Y _n , Y _{n + 1} , X _p indicate the voltages applied to the row electrodes Y _n , Y _{n + 1} and the column electrode X _p to obtain these sequences.

The bottom of the figure 3 indicates the potential values at terminals A, B ( figure 2 ) cells C _{n, p} , C _{n + 1, p} and the state turned on ("ON") or off ("OFF") of these cells.

• un potentiel V_a à une cellule à l'état OFF, cette cellule bascule à l'état ON;
• un potentiel V_s ou (V_s-V_off) à une cellule à l'état ON, cette cellule reste à l'état ON ;
• un potentiel (V_a-V_off) ou V_s à une cellule à l'état OFF, cette cellule reste à l'état OFF ;

To obtain the ON or OFF state indicated at the bottom of this figure, it is therefore necessary that, by applying to the terminals A, B of a cell as represented in FIG. figure 2 :

• a potential V _a to a cell in the OFF state, this cell switches to the ON state;
• a potential V _s or (V _s -V _off ) to a cell in the ON state, this cell remains in the ON state;
• a potential (V _a -V _off) or V _s to a cell in the OFF state, this cell remains in the OFF state;

• à la tension seuil V_S.EL aux bornes AC de la diode électroluminescente E_EL de la cellule (figure 2), en deçà laquelle cette diode s'éteint et au delà de laquelle elle s'allume ; la caractéristique typique d'une telle diode E_EL est représentée à la figure 5 (intensité lumineuse émise -en lumen - en fonction de la tension appliquée - en Volt) ;
• à la tension V_T aux bornes AB d'une cellule au delà de laquelle une cellule éteinte à l'état OFF s'allume et passe à l'état ON.

The figure 4 resumes these different potential values by locating them in relation to:

• at the threshold voltage V _S.EL at the terminals AC of the electroluminescent diode E _EL of the cell ( figure 2 ), below which this diode is extinguished and beyond which it lights up; the typical characteristic of such a diode E _EL is shown in FIG. figure 5 (luminous intensity emitted - in lumen - according to the applied voltage - in Volt);
• at the voltage V _T at the terminals AB of a cell beyond which a cell switched off in the OFF state turns on and goes to the ON state.

To obtain the desired memory effect, the value of the voltage V _{off that} can be applied to the column electrodes such as X _p must be chosen so that the voltage Va-Voff applied across a cell is not sufficient. to turn it on, so that Va-Voff <V _T and that the voltage Vs-Voff does not affect the on or off state of the cell, so that V _s.EL <Vs-Voff.

As illustrated by figure 4 , for a good operation of the panel, it is therefore appropriate for a cell C _{n, p} to which a voltage Va> V _T has been applied. continues to emit a significant amount of light even if the voltage applied to its terminals decreases to the value V _s -V _off , which remains higher than V _S.EL ; for this type of operation, it is necessary for the cell, that is to say that the electroluminescent element E _EL and the photoconductive element E _PC connected in series have a high hysteresis.

• la tension V Eel aux bornes AC de l'élément électroluminescent de la cellule ;
• la tension V Epc aux bornes CB de l'élément photoconducteur de la cellule ;
• l'intensité I du courant circulant dans cette cellule.

The typical characteristic of a photoconductive element E _PC of a cell C _{n, p} of the panel is represented in FIG. figure 6 (electrical intensity - in Ampere - depending on the illumination - in lumen - when this E _PC element is subjected to a voltage of 10 V); given the characteristics already mentioned ( figure 5 ) of the electroluminescent element E _EL , it is now possible to represent the overall current-voltage characteristics of all these elements E _EL and E _PC in series forming a cell C _{n, p} of the panel: see FIG. figure 7 , which illustrates, when applying a voltage increasing from 0 to 20 V and then decreasing from 20 to 0 V at the AB terminals of a cell:

• the voltage V Eel at the AC terminals of the electroluminescent element of the cell;
• the voltage V Epc at the terminals CB of the photoconductive element of the cell;
• the intensity I of the current flowing in this cell.

It is found that during a growth cycle up to ignition (high intensity) and then decreasing until extinction, the evolution of the intensity I of the current in this cell shows no hysteresis, which shows that there is in fact no holding zone (see figure 4 ) voltage values in which, the cell having been previously lit, it remains lit; therefore, the memory effect described above is not obtained.

The object of the invention is to overcome the absence or insufficiency of memory effect.

• un réseau avant d'électrodes et un réseau arrière d'électrodes, les électrodes du réseau avant croisant les électrodes du réseau arrière au niveau de chacune desdites cellules,
• au moins une couche électroluminescente formant, pour chaque cellule, au moins un élément électroluminescent,
• une couche photoconductrice pour obtenir ledit effet mémoire, formant, pour chaque cellule, un élément photoconducteur,

l'au moins un élément électroluminescent et l'élément photoconducteur de chaque cellule étant reliés électriquement en série et les deux bornes extrêmes de ladite série étant reliées l'une à une électrode dudit réseau avant et l'autre à une électrode dudit réseau arrière,

• des moyens de couplage optique, au niveau de chaque cellule, entre au moins une couche électroluminescente du panneau et ladite couche photoconductrice ,

caractérisé en ce qu'il comprend, pour chaque cellule, un élément de shunt disposé en parallèle de l'au moins un élément électroluminescent de la dite cellule et dont la résistance ne dépend pas de l'éclairement.For this purpose, the subject of the invention is an image display panel comprising a matrix of memory-effect electroluminescent cells capable of emitting light towards the front of said panel, comprising:

• a front electrode array and a back electrode array, the front array electrodes intersecting the rear array electrodes at each of said cells,
• at least one electroluminescent layer forming, for each cell, at least one electroluminescent element,
• a photoconductive layer for obtaining said memory effect, forming, for each cell, a photoconductive element,

the at least one electroluminescent element and the photoconductive element of each cell being electrically connected in series and the two end terminals of said series being connected to an electrode of said front network and the other to an electrode of said rear network,

• optical coupling means, at each cell, between at least one electroluminescent layer of the panel and said photoconductive layer,

characterized in that it comprises, for each cell, a shunt element disposed in parallel with the at least one electroluminescent element of said cell and whose resistance does not depend on the illumination.

Since the resistance of the shunt elements does not depend on the illumination, the use as shunts of erasing photoconductive elements as described in the document "IBM Technical Disclosure Bulletin," Vol.24, No. 5, pp.2307-2310 previously cited is completely excluded; here is meant here by shunt element a conventional resistance made with a non-photoconductive material and having a resistance that does not vary substantially depending on the illumination.

Preferably, the electroluminescent layer (s) of the panel are organic.

The invention also applies to panels of the same type as those described in the document US 4035774 IBM previously cited which comprise a rear electroluminescent layer for emitting light adapted to the activation or excitation of the photoconductive cells and a front electroluminescent layer for emitting the light necessary for the visualization of the images; the photoconductive layer is interposed between the two electroluminescent layers and is optically coupled only or mainly with the rear electroluminescent layer; each cell here comprises two electroluminescent elements, one rear, the other front, and a photoconductor element inserted; the extreme terminals of the series formed by these three elements are connected to a rear electrode, the other to a front electrode.

• un réseau avant d'électrodes et un réseau arrière d'électrodes, les électrodes du réseau avant croisant les électrodes du réseau arrière au niveau de chacune desdites cellules,
• une couche organique électroluminescente formant, pour chaque cellule, un élément électroluminescent dont une borne est reliée à une électrode dudit réseau avant,
• une couche photoconductrice pour obtenir ledit effet mémoire, formant, pour chaque cellule, un élément photoconducteur dont une borne est reliée à une électrode dudit réseau arrière,
• des moyens pour relier électriquement au même potentiel, au niveau de chaque cellule, l'autre borne de l'élément électroluminescent et l'autre borne de l'élément photoconducteur,
• des moyens de couplage optique entre ledit élément électroluminescent de chaque cellule et ledit élément photoconducteur de cette même cellule,

caractérisé en ce qu'il comprend, pour chaque cellule, un élément de shunt disposé en parallèle de l'élément électroluminescent de la dite cellule et dont la résistance ne dépend pas de l'éclairement.In the case, the most frequent, where the panel comprises only a single electroluminescent organic layer, the invention relates to an image display panel comprising a matrix of memory-effect electroluminescent cells capable of emitting light. forward light of said panel, comprising:

• a front electrode array and a back electrode array, the front array electrodes intersecting the rear array electrodes at each of said cells,
• an organic electroluminescent layer forming, for each cell, an electroluminescent element, a terminal of which is connected to an electrode of said front network,
• a photoconductive layer for obtaining said memory effect, forming, for each cell, a photoconductive element, a terminal of which is connected to an electrode of said rear network,
• means for electrically connecting the other terminal of the electroluminescent element and the other terminal of the photoconductive element to the same potential at each cell,
• optical coupling means between said electroluminescent element of each cell and said photoconductive element of said same cell,

characterized in that it comprises, for each cell, a shunt element disposed in parallel with the electroluminescent element of said cell and whose resistance does not depend on the illumination.

According to this most common embodiment of the invention, the equivalent electrical diagram of any cell of the panel is shown in FIG. figure 9 ; the references E _PC , E _EL refer to the photoconductive element and to the electroluminescent element of this cell, respectively, as in FIG. figure 2 previously described; according to the invention, this cell comprises in in addition to a shunt element E _S.EL , of constant resistance independent of illumination R _S.EL , connected in parallel to the electroluminescent element E _EL .

We will now determine what value it is important to give the resistance R _S.EL of the shunt element E _S.EL to get the best of the invention.

In the first place, it is evident that the resistance R _{S.EL must} be greater than the resistance R _ON-EL exhibited by the electroluminescent element E _EL in the on state, so that it can be considered that, when the cell is in the ON state, the intensity of the electric current flowing through it essentially passes through the electroluminescent element E _EL ; R is therefore preferably R _SEL > R _ON-EL ; the ohmic losses in the shunt element are thus limited when the cells are switched on; to further limit losses, it is preferable that R _S.EL > 2 x R _ON-EL .

It should be noted that this characteristic further distinguishes the shunt element according to the invention from the erasure photoconductive element of the panel described in the document "IBM Technical Disclosure Bulletin," Vol.24, No. 5, pp.2307-2310 previously cited; indeed, since the resistance R _S.EL of this shunt element is greater than the internal resistance R _ON-EL that the electroluminescent element E _{EL has} in the lit state, it is in no way susceptible to shunt effectively the corresponding electroluminescent element E _EL when lit; it should be noted that, in the opposite case, the shunt element according to the invention would extinguish or erase the corresponding electroluminescent element, which would be absolutely contrary to the aim pursued by the invention.

In summary, the document "IBM Technical Disclosure Bulletin," Vol.24, No. 5, pp.2307-2310 previously cited discloses means for shunting the electroluminescent elements in the on state, while the invention provides means for shunting the electroluminescent elements in the off state.

In the second place, the resistance R _S.EL should be lower, preferably much lower, than the internal resistance R _OFF-EL that the electroluminescent element E _{EL has} in the off state, so that the it can be considered that, when the cell is in OFF OFF state, the intensity of the electric current flowing through it essentially passes through the shunt element E _S.EL ; we therefore R _SEL < R _OFF-EL , preferably R _SEL <½ R _OFF-EL ; in other words, the shunt element according to the invention is "on" when the electroluminescent element E _EL is in the off state, while the erasure photoconductive element described in the document "IBM - Technical Disclosure Bulletin" previously cited is adapted to be capable of becoming "on" when the electroluminescent element E _EL is in the on state.

Note that we generally have R _OFF-EL > R _ON-EL , which allows to advantageously combine the two conditions stated above: R _S.EL > R _ON-EL and R _S.EL <R _OFF-EL .

$${\mathrm{V}}_{\mathrm{Eel}}\mathrm{=}{\mathrm{V}}_{\mathrm{S}\mathrm{.}\mathrm{EL}}\mathrm{-}\mathrm{\epsilon ʹ}\mathrm{=}{\mathrm{R}}_{\mathrm{S}\mathrm{.}\mathrm{EL}}\mathrm{x\; I}$$

Let R _{OFF-PC be} the resistance of the photoconductive element E _PC to the non-excited state OFF; under the control conditions of a panel previously described with reference to figures 3 and 4 or, according to the definition already given, V _T the voltage at the terminals AB of this cell beyond which this cell switched off (in the OFF state) lights up and goes to the ON state; then, for a voltage V _T - ε very slightly less than the ignition voltage V _T (ε very small), the voltage V _Eel across the electroluminescent element E _EL is very close to the threshold voltage previously defined V _S.EL , so that: V _Eel = V _S.EL - ε '(ε' very small); if V _PC is the voltage across the photoconductive element E _PC , then V _T - ε = V _PC + V _S.EL - ε '; on the other hand, if I is the intensity of the current flowing through the cell, if we consider that all this current passes through the shunt element E _S.EL and not by the electroluminescent element E _EL because the cell is extinguished, we have: $${\mathrm{V}}_{\mathrm{T}}\mathrm{-}\mathrm{\epsilon }\mathrm{=}{\mathrm{V}}_{\mathrm{PC}}\mathrm{+}{\mathrm{V}}_{\mathrm{S}\mathrm{.}\mathrm{EL}}\mathrm{-}\mathrm{\epsilon \text{'}}\mathrm{=}\left({\mathrm{R}}_{\mathrm{OFF}\mathrm{-}\mathrm{PC}}\mathrm{+}{\mathrm{R}}_{\mathrm{S}\mathrm{.}\mathrm{EL}}\right)\mathrm{x\; I}$$

$${\mathrm{V}}_{\mathrm{Eel}}\mathrm{=}{\mathrm{V}}_{\mathrm{S}\mathrm{.}\mathrm{EL}}\mathrm{-}\mathrm{\epsilon \text{'}}\mathrm{=}{\mathrm{R}}_{\mathrm{S}\mathrm{.}\mathrm{EL}}\mathrm{x\; I}$$

From these two equations, one deduces: V _T - ε = (1+ R _OFF / R _S.EL ) (V _S.EL - ε '), or, simplifying: V _T = (1 + R _OFF-PC / R _SEL ) V _SEL or (V _T / V _SEL ) = (1 + R _OFF-PC / R _SEL ).

Considering the diagram of the control voltages of the panel of the figure 4 the width of the "holding zone" corresponds to V _T -V _S.EL ; in practice, to benefit from a "holding zone" large enough to be able to easily control the display panel, it is appropriate that the difference V _T -V _S.EL is greater than or equal to 8 or 9 Volt; in the case where, for example, the trigger threshold voltage of the light emitting diode is V _S.EL = 9 V, it is therefore appropriate that (V _T / V _S.EL ) ≥ 2, that is to say (R _OFF-PC / R _S.EL ) ≥ 1 or R _S.EL ≤ R _OFF-PC ; in order to limit losses, the light-emitting diode technique for visualization of images is oriented towards lowering the triggering threshold voltages below the value of 9 Volt, which implies that for the width of the "holding zone" remains greater than 8 or 9 volts, the ratio (V _T / V _S.EL ) is strictly greater than 2, even equal to or greater than 3 and the ratio (R _OFF-PC / R _S.EL ) is strictly greater than 1, or even greater than or equal to 2.

Thus, preferably, for each cell of the panel according to the invention, the resistance R _S.EL of the shunt element E _S.EL of the electroluminescent element E _EL of this cell is less than or equal to the resistance R _{OFF -PC} of the corresponding photoconductive element E _PC when it is not in the excited state and is lower than the resistance R _OFF-EL of the corresponding electroluminescent element E _EL when it is off, which generally supposes than R _OFF-EL > R _OFF-PC .

Preferably, the resistance R _S.EL of the shunt element E _S.EL of the electroluminescent element E _EL of this cell is strictly less than the resistance R _OFF-PC of the corresponding photoconductive element E _PC when is not in the excited state, or even less than or equal to half of this resistance.

Thanks to the shunt element E _S.EL of the electroluminescent element according to the invention, it can be seen, as is illustrated in more detail in the example below, that the panel is now equipped with a memory effect. actually exploitable by a conventional control method as previously described, and that the evolution of the intensity I of the current in each cell of the panel shows a hysteresis and a holding zone (see figure 4 and 10 ) of voltage values in which, the cell having been previously lit, it remains on.

According to another advantageous embodiment of the invention, the panel according to the invention also comprises, for each cell, a shunt element disposed in parallel with the photoconductive element of said cell.

It is thus possible to significantly reduce the energy consumption of the panel; in addition, this additional shunt facilitates the de-excitation of the elements photoconductors and advantageously makes it possible to reduce the switching times of the cells of the panel.

The equivalent electrical diagram of any cell of the panel according to this other advantageous embodiment of the invention is shown in FIG. figure 15 ; the references E _PC , E _EL refer respectively to the photoconductive element and to the electroluminescent element of this cell; this cell comprises here not only a shunt element E _S.EL , resistance R _S.EL , connected in parallel to the electroluminescent element E _EL , but also a shunt element E _S.PC , resistance R _S.PC , connected in parallel to the photoconductive element E _PC.

Let R _{OFF-PC be} the resistance of the photoconductive element E _PC to the non-excited state OFF; the resistance R _S.PC must be chosen much lower than the internal resistance R _OFF-PC that the photoconductive element E _{PC has} in the off state, so that it can be considered that, when the cell is at when the state is OFF, the intensity of the electric current flowing through it essentially passes through the shunt element E _S.PC ; therefore, we have R _SPSP <R _OFF-PC , preferably R _SPS <R _OFF-PC .

$${\mathrm{V}}_{\mathrm{Eel}}\mathrm{=}{\mathrm{V}}_{\mathrm{S}\mathrm{.}\mathrm{EL}}\mathrm{-}\mathrm{\epsilon ʹ}\mathrm{=}{\mathrm{R}}_{\mathrm{S}\mathrm{.}\mathrm{EL}}\mathrm{x\; I}$$

In the control conditions of a panel (previously described with reference to figures 3 and 4 ), or, according to the definition already given, V _T the terminal voltage AB of this cell beyond which this cell off (in the OFF state) turns on and goes to the ON state; then, for a voltage V _T - ε very slightly lower than the ignition voltage V _T (ε very small), the voltage V _Eel across the electroluminescent element E _EL is very close to the threshold voltage previously defined V _S.EL , so that: V _Eel = V _S.EL - ε '(ε' very small); if V _Epc is the voltage across the photoconductive element E _PC , then V _T - ε = V _Epc + V _S.EL - ε '; on the other hand, if I is the intensity of the current flowing through the cell, if we consider that all this current passes through the shunt elements E _S.PC and E _S.EL , and not by the photoconductive element E _PC and the electroluminescent element E _EL because the cell is off, we have: $${\mathrm{V}}_{\mathrm{T}}\mathrm{-}\mathrm{\epsilon }\mathrm{=}{\mathrm{V}}_{\mathrm{Epc}}\mathrm{+}{\mathrm{V}}_{\mathrm{S}\mathrm{.}\mathrm{EL}}\mathrm{-}\mathrm{\epsilon \text{'}}\mathrm{=}\left({\mathrm{R}}_{\mathrm{S}\mathrm{.}\mathrm{PC}}\mathrm{+}{\mathrm{R}}_{\mathrm{S}\mathrm{.}\mathrm{EL}}\right)\mathrm{x\; I}$$

$${\mathrm{V}}_{\mathrm{Eel}}\mathrm{=}{\mathrm{V}}_{\mathrm{S}\mathrm{.}\mathrm{EL}}\mathrm{-}\mathrm{\epsilon \text{'}}\mathrm{=}{\mathrm{R}}_{\mathrm{S}\mathrm{.}\mathrm{EL}}\mathrm{x\; I}$$

From these two equations, one deduces: V _T - ε = (1 + R _S.PC / R _S.EL ) (V _S.EL - ε '), or, by simplifying: V _T = (1 + R _{S. PC} / R _SEL ) V _SEL or (V _T / V _SEL ) = (1 + R _SSP / R _SEL ).

Considering the diagram of the control voltages of the panel of the figure 4 the width of the "holding zone" corresponds to V _T -V _S.EL ; in practice, to benefit from a "holding zone" large enough to be able to easily control the display panel, it is appropriate that the difference V _T -V _S.EL is greater than or equal to 8 or 9 Volt; in the case where, for example, the trigger threshold voltage of the light emitting diode is V _S.EL = 9 V, it is therefore appropriate that (V _T / V _S.EL ) ≥ 2, that is to say (R _S.PC / R _S.EL ) ≥ 1 or R _S.EL ≤ R _S.PC ; in order to limit losses, the light-emitting diode technique for visualization of images is oriented towards lowering the triggering threshold voltages below the value of 9 Volt, which implies that for the width of the "holding zone" remains greater than 8 or 9 volts, the ratio (V _T / V _S.EL ) is strictly greater than 2, even equal to or greater than 3 and the ratio (R _S.PC / R _S.EL ) is strictly greater than 1, or even greater than or equal to 2.

Thus, preferably, for each cell of the panel according to the invention, the resistance R _S.PC of the shunt element E _S.PC of the photoconductive element E _PC of this cell is greater than or equal to the resistance R _{S Of} the shunt element E _S.EL of the electroluminescent element E _EL of this same cell.

Preferably, there is _R.sub.PCS / _R.sup.SEL ≥ 2, and even more preferably _R.S.PC / _R.sup.SEL ≥ 3.

Preferably, the panel according to the invention comprises, at each cell, a conductive element at each interface between the at least one electroluminescent layer and the photoconductive layer for electrically connecting in series the corresponding electroluminescent and photoconductive elements and the conductive elements. different cells are electrically isolated from each other.

Preferably, the conductive elements between the same electroluminescent layer and the same photoconductive layer form a single conducting layer which is of course discontinuous so that the conductive elements of the different cells are electrically isolated from each other; in the case of a panel of the type described in the document US 4035774 already cited comprising two electroluminescent layers, so there are two interface conductive layers.

In the more frequent case of a single electroluminescent layer panel, each shunt element of the electroluminescent element is connected to the same electrode of the front network and to the same conductive element of the intermediate layer as the electroluminescent element E _EL qu he shunts; where appropriate, each shunt element of the photoconductive element is connected to the same electrode of the back network and to the same conductive element of the intermediate layer as the photoconductive element E _PC that it shunts; shunt element means any shunting means: several examples will be given later.

Advantageously, the panel according to the invention comprises means for controlling the cells for displaying images, suitable for implementing a method in which, successively for each line of cells of the panel, a selective addressing phase is used. intended to turn on the cells to be lit in this line, then by a non-selective holding phase intended to keep the cells of this line in the state where the previous phase of addressing put them or left.

• la figure 1 est un schéma en coupe d'une cellule d'un panneau électroluminescent à couche photoconductrice de l'art antérieur,
• la figure 2 illustre le schéma équivalent électrique de la cellule de la figure 1,
• la figure 3 donne trois chronogrammes des tensions appliquées à deux électrodes de ligne et à une électrode de colonne d'un panneau matriciel électroluminescent à effet mémoire, lorsqu'on utilise un procédé classique de commande de panneau adapté pour tirer parti de l'effet mémoire des cellules de ce panneau,
• la figure 4 illustre le positionnement des différentes tensions appliquées aux électrodes d'un panneau lors de l'application d'un procédé de commande de la figure 3,
• les figures 5 et 6 représentent les caractéristiques typiques respectivement d'un élément électroluminescent E_EL et d'un élément photoconducteur E_PC d'une cellule d'un panneau telle que représentée aux figures 1 et 2 ;
• la figure 7 illustre, selon l'art antérieur, la répartition des tensions V_E-el et V_E-pc respectivement aux bornes de l'élément électroluminescent E_EL et de l'élément photoconducteur E_PC d'une cellule d'un panneau telle que représentée aux figures 1 et 2, lorsqu'on applique aux bornes AB de cette cellule un cycle de tension croissante (0 à 20 V), puis décroissante (20 à 0 V) ; cette figure illustre également l'évolution de l'intensité du courant circulant dans cette cellule ;
• la figure 8 est un schéma en coupe d'une cellule d'un panneau électroluminescent à couche photoconductrice selon un mode de réalisation de l'invention,
• la figure 9 illustre le schéma équivalent électrique de la cellule de la figure 8,
• la figure 10 illustre, selon l'invention, la répartition des tensions V_E-el et V_E-pc respectivement aux bornes de l'élément électroluminescent E_EL et de l'élément photoconducteur E_PC d'une cellule d'un panneau telle que représentée aux figures 8 et 9, lorsqu'on applique aux bornes AB de cette cellule un cycle de tension croissante (0 à 20 V), puis décroissante (20 à 0 V) ; cette figure illustre également l'évolution de l'intensité du courant circulant dans cette cellule ;
• les figures 11 et 12 sont des coupes d'un premier mode de réalisation d'un panneau selon l'invention, respectivement selon la direction des électrodes lignes et selon la direction des électrodes colonnes, destinées à illustrer un procédé de fabrication de ce panneau ;
• les figures 13 et 14 sont des coupes d'un second mode de réalisation d'un panneau selon l'invention, respectivement selon la direction des électrodes lignes et selon la direction des électrodes colonnes, destinées à illustrer une variante du procédé de fabrication de ce panneau illustré aux figures 11 et 12.
• la figure 15 illustre le schéma équivalent électrique d'une cellule selon un autre mode de réalisation avantageux de l'invention.

Other features and advantages of the invention will appear in the description of a preferred embodiment, given in a non-limiting manner and with reference to the appended drawings, in which:

• the figure 1 is a sectional diagram of a cell of a prior art photoconductive layer electroluminescent panel,
• the figure 2 illustrates the electrical equivalent diagram of the cell of the figure 1 ,
• the figure 3 gives three timing diagrams of the voltages applied to two line electrodes and a column electrode of a memory effect electroluminescent matrix panel, when using a conventional panel control method adapted to take advantage of the memory effect of the memory cells. this panel,
• the figure 4 illustrates the positioning of the different voltages applied to the electrodes of a panel during the application of a control method of the figure 3 ,
• the figures 5 and 6 represent the typical characteristics respectively of an electroluminescent element E _EL and a photoconductive element E _PC of a cell of a panel as represented in FIGS. Figures 1 and 2 ;
• the figure 7 illustrates, according to the prior art, the distribution of the voltages V _E-el and V _E-pc respectively at the terminals of the electroluminescent element E _EL and the photoconductive element E _PC of a cell of a panel as shown to the Figures 1 and 2 when applying to the AB terminals of this cell an increasing voltage cycle (0 to 20 V), then decreasing (20 to 0 V); this figure also illustrates the evolution of the intensity of the current flowing in this cell;
• the figure 8 is a sectional diagram of a cell of a photoconductive layer electroluminescent panel according to one embodiment of the invention,
• the figure 9 illustrates the electrical equivalent diagram of the cell of the figure 8 ,
• the figure 10 illustrates, according to the invention, the distribution of the voltages V _E-el and V _E-pc respectively across the electroluminescent element E _EL and the photoconductive element E _PC of a cell of a panel as shown in FIGS. Figures 8 and 9 when applying to the AB terminals of this cell an increasing voltage cycle (0 to 20 V), then decreasing (20 to 0 V); this figure also illustrates the evolution of the intensity of the current flowing in this cell;
• the Figures 11 and 12 are sections of a first embodiment of a panel according to the invention, respectively in the direction of the row electrodes and in the direction of the column electrodes, intended to illustrate a method of manufacturing this panel;
• the Figures 13 and 14 are sections of a second embodiment of a panel according to the invention, respectively in the direction of the electrodes lines and in the direction of the column electrodes, intended to illustrate a variant of the manufacturing process of this panel illustrated in FIGS. Figures 11 and 12 .
• the figure 15 illustrates the electrical equivalent diagram of a cell according to another advantageous embodiment of the invention.

The figures representing chronograms do not take into account a scale of values in order to better reveal certain details that would not be clearly visible if the proportions had been respected.

In order to simplify the description and to show the differences and advantages that the invention presents with respect to the prior art, identical references are used for the elements that provide the same functions.

We will now describe a panel according to a general embodiment of the invention, that is to say having shunt elements only electroluminescent elements; a method of manufacturing this panel will also be described.

• des barrières 20 entourant la zone de couche électroluminescente 16 et la zone de couche intermédiaire d'électrodes 14 de cette cellule, dont la base s'appuie sur la couche photoconductrice 12, et dont le sommet parvient au moins à hauteur de la couche avant transparente d'électrodes 18 ;
• un couche de shunt 21 appliquée sur le versant de ces barrières de manière à mettre en contact électrique la couche photoconductrice 12 et l'électrode transparente de la couche 18 ; cette couche de shunt 21 forme l'élément de shunt E_S.EL selon l'invention ; la résistance R_S.EL de cet élément de shunt E_S.EL est proportionnelle à la largeur de la couche 21 (qui s'étend dans le sens de la hauteur des barrières) et inversement proportionnelle à son épaisseur ; le dimensionnement de cette couche de shunt, notamment son épaisseur, le matériau de cette couche de shunt 21 sont choisis de manière à ce que, au niveau de chaque cellule, la résistance R_S.EL de cet élément de shunt E_S.EL qu'il forme soit :
• d'une part inférieure ou égale à la résistance R_OFF-PC de l'élément photoconducteur E_PC correspondant à la zone de couche électroluminescente 16 de cette cellule, lorsqu'elle n'est pas à l'état excité ;
• d'autre part, inférieure à la résistance R_OFF-EL de l'élément électroluminescent E_EL qu'il shunte, correspondant à la zone de couche photoconductrice12 de cette cellule, lorsqu'elle n'est pas à l'état excité ;

With reference to the figure 8 each cell of the panel according to the invention comprises, in addition to the elements of the panel already described with reference to the figure 1 which present here the same references:

• barriers 20 surrounding the electroluminescent layer area 16 and the electrode intermediate layer area 14 of this cell, the base of which rests on the photoconductive layer 12, and whose apex reaches at least at a height of the transparent front layer electrodes 18;
• a shunt layer 21 applied on the slope of these barriers so as to electrically contact the photoconductive layer 12 and the transparent electrode of the layer 18; this shunt layer 21 forms the shunt element E _S.EL according to the invention; the resistance R _S.EL of this shunt element E _S.EL is proportional to the width of the layer 21 (which extends in the direction of the height of the barriers) and inversely proportional to its thickness; the dimensioning of this shunt layer, in particular its thickness, the material of this shunt layer 21 are chosen so that, at each cell, the resistance R _S.EL of this shunt element E _S.EL qu it forms either:
• on the one hand less than or equal to the resistance R _OFF-PC of the photoconductive element E _PC corresponding to the electroluminescent layer area 16 of this cell, when it is not in the excited state;
• on the other hand, lower than the resistance R _OFF-EL of the electroluminescent element E _{EL which} it shunts, corresponding to the photoconductive layer zone 12 of this cell, when it is not in the excited state;

Finally, the material of this shunt layer 21 is not photoconductive so that the resistance of the corresponding shunt elements does not depend on the illumination.

• les éléments de shunt E_S.EL sont isolés les uns des autres,
• les zones de la couche d'électrodes intermédiaires 14, propres à chaque cellule, sont isolées électriquement les unes des autres, de sorte que le potentiel électrique au point commun de l'élément électroluminescent E_EL et de l'élément photoconducteur E_PC de cette cellule est flottant.

The barriers 20 then form a two-dimensional network for delimiting the cells of the panel; the dimensioning of these barriers, in particular their height, the material of these barriers are chosen so that, at each cell, the electrical resistance of these barriers, measured between their base and their vertex, is much greater than that of R _S.EL of the shunt element E _S.EL of this cell; thus, these barriers electrically isolate the panel cells from one another; so,

• the shunt elements E _S.EL are isolated from each other,
• the zones of the layer of intermediate electrodes 14, specific to each cell, are electrically insulated from each other, so that the electric potential at the common point of the electroluminescent element E _EL and the photoconductive element E _PC of this cell is floating.

According to a variant of the invention not shown, the shunt layer has discontinuities on the periphery of the barriers of a cell, so that, for example, only the barriers on one side of each cell are covered with this layer. shunt; on the other hand, it is obviously essential that this shunt layer 21 electrically contacts the photoconductive layer 12 and the transparent electrode of the layer 18.

According to a variant not shown, this electrical contact can be provided indirectly via the electrodes of the intermediate layer 14.

• un élément électroluminescent E_EL englobant une zone de couche électroluminescente 16, et,
• en série avec l'élément électroluminescent E_EL, un élément photoconducteur E_PC englobant une zone de couche photoconductrice 12 au regard de cette même zone de couche électroluminescente 16.
• en parallèle avec l'élément électroluminescent E_EL, un élément de shunt E_S.EL, formé par la couche de shunt 21 de cette cellule.

Referring to the figure 9 , each cell of the panel can be represented by the following elements:

• an electroluminescent element E _EL including an electroluminescent layer area 16, and
• in series with the electroluminescent element E _EL , a photoconductive element E _PC including a photoconductive layer zone 12 with respect to this same electroluminescent layer area 16.
• in parallel with the electroluminescent element E _EL , a shunt element E _S.EL , formed by the shunt layer 21 of this cell.

• la tension V Eel aux bornes AC de l'élément électroluminescent E_EL de la cellule et de l'élément de shunt E_S.EL ;
• la tension V Epc aux bornes CB de l'élément photoconducteur E_PC de la cellule ;
• l'intensité I du courant circulant dans l'élément électroluminescent E_EL .

On the basis of the typical electrical characteristics previously described with reference to figures 5 and 6 of the electroluminescent element E _EL and the photoconductive element E _PC , and by choosing R _S.EL = 25 kΩ approximately equal to 1/4 R _OFF-PC (with R _OFF-PC = 100 kΩ approximately), one examines the global current-voltage characteristics of this cell according to the invention: see figure 10 , which illustrates, when applying a voltage increasing from 0 to 20 V and then decreasing from 20 to 0 V at the AB terminals of a cell:

• the voltage V Eel at the AC terminals of the electroluminescent element E _EL of the cell and the shunt element E _S.EL ;
• the voltage V Epc at the terminals CB of the photoconductive element E _PC of the cell;
• the intensity I of the current flowing in the electroluminescent element E _EL .

It is found that during a growth cycle up to ignition (high intensity) and then decreasing to extinction, the change in intensity I of the current in this cell shows a significant hysteresis, thanks to the addition of the shunt element E _S.EL according to the invention.

It is then possible to use, for the control of the cells of the panel and the visualization of images, a method in which, successively for each line of the panel, there is a selective phase of addressing intended to light the cells to be lit. in this line, then by a non-selective holding phase intended to keep the cells of this line in the state where the previous phase of addressing put them or left.

• il suffit de choisir Va (tension d'allumage de la cellule) supérieur ou égal à la tension V_T ; la tension V_T est celle qui, appliquée aux bornes d'une cellule éteinte à l'état OFF, provoque son allumage et son passage à l'état ON ; la valeur de V_T est reportée à la figure 10 ;
• il suffit de choisir V_S (tension de maintien de la cellule) et V_off tels que la valeur (V_S -V_off) soit supérieure ou égale à la tension V_S.EL ; la tension V_S.EL est celle qui appliquée aux bornes d'un élément électroluminescent E_EL, provoque son allumage (V>V_S.EL) ou son extinction (V<V_S.EL) ; la valeur de V_S.EL est également reportée sur la figure 10.

By repeating the previous definitions of Va, V _S , V _off with reference to figures 3 and 4 , to apply this control method:

• it suffices to choose Va (ignition voltage of the cell) greater than or equal to the voltage V _T ; the voltage V _T is that which, applied to the terminals of an off cell in the OFF state, causes its ignition and its transition to the ON state; the value of V _T is carried over to the figure 10 ;
• it suffices to choose V _S (cell holding voltage) and V _off such that the value (V _S -V _off ) is greater than or equal to the voltage V _S.EL ; the voltage V _S.EL is that applied across the electroluminescent element E _EL , causes its ignition (V> V _S.EL ) or its extinction (V <V _S.EL ); the value of V _S.EL is also reported on the figure 10 .

As explained previously, there is furthermore V _T = (1+ R _OFF-PC / R _S.EL ) V _S.EL.

Unlike the prior art, it is found that there is a holding zone (see figure 4 and 10 ) voltage values in which, the panel cell having been previously lit, it remains lit; thanks to the shunt element E _S.EL specific to the invention, we thus obtain the previously described memory effect for all the cells of the panel.

In order to manufacture the electroluminescent display panels according to the invention, conventional layer deposition and etching methods are used for those skilled in the art of this type of panel; a method of manufacturing such a panel will now be described with reference to Figures 11 and 12 which are sections of the panel respectively in the direction of the line electrodes and in the direction of the column electrodes.

On a substrate 10 formed for example by a glass plate, a homogeneous aluminum layer is deposited by cathodic sputtering or by vacuum evaporation ("PVD"), then the layer obtained is etched so as to form a parallel electrode array. or column electrodes X _p , X _{p + 1} : the opaque back layer of electrodes 11 is thus obtained.

On this layer of column electrodes 11, a homogeneous layer of photoconductive material 12 is then deposited: for example amorphous silicon by plasma-enhanced chemical vapor deposition ("PECVD" or Plasma Enhanced Chemical Vapor Deposition). , or one organic photoconductive material by chemical vapor deposition ("CVD") or spin coating ("spin-coating" in English).

The optical coupling layer 13 is then applied, comprising, for each future electroluminescent cell C _{n, p} , a coupling element 25 formed of an opaque aluminum layer portion pierced at its center with an opening 26 intended to leave passing light to the photoconductive layer 12 is carried out by depositing a homogeneous layer of aluminum 25 and then etching optical coupling openings 26, positioned in the center of the future cells of the panel and etching areas defining the future barriers 20 intended to partition the panel into cells.

Subsequently, by sputtering under vacuum, a thin and conductive layer 14 of mixed tin-indium oxide ("ITO") is used, intended to form intermediate connection electrodes between the photoconductive elements of the photoconductive layer. and the electroluminescent elements of each cell. This layer is then etched, always to define the areas on which the barriers 20 will be placed.

Then the two-dimensional network of barriers 20 for partitioning the panel into electroluminescent cells C _{n, p} and electrically isolating the shunt elements E _S.EL of each cell is formed: for this purpose, a homogeneous layer of organic spin-coating resin, then this layer is etched to form the two-dimensional barrier network 20.

Then the material for "shunt" according to the invention is deposited in a homogeneous solid layer over the entire active surface of the panel; this layer matches the reliefs that the surface of the panel presents at this stage of the process; the shunt elements E _S.EL according to the invention are then obtained by full-plate anisotropic etching so as to leave a shunt layer of thickness equal to the initial thickness of the deposit only on the walls of the barriers 20; by locating in the figure, the etching therefore operates only in the vertical direction and removes only the horizontal parts of the shunt layer; the shunt layer 21 and the shunt elements E _S.EL according to the invention are then obtained for each cell; for example the material of "Shunting" may be titanium nitride (TiN) obtained by chemical vapor deposition ("CVD"); the anisotropic etching can be done in a "high density" plasma etching chamber using a suitable chemistry known in itself. For a cell of 500x500 μm ² , it would take between 2 nm and 100 nm of titanium nitride (TiN - material whose resistivity is adjustable from 2.10 ^-4 Ω.cm to 10 ^-2 Ω.cm) to obtain a shunt resistance R _SEL around 5kΩ, capable of bringing the operation in bi-stable mode memory effect according to the invention.

Referring to the figure 12 perpendicular to the column electrodes X _p , X _{p + 1} and between the future cells, it is then possible to mount on the barriers 20 an array of separators 20 'perpendicular to the column electrodes X _p , X _{p + 1} : for this purpose, first depositing a homogeneous layer of organic barrier resin by spin coating in the English language, then etching this layer so as to form the separator network 20 '. ; the height of the separators, ie the thickness of the layer deposited, must be much greater than the thickness of the layers still to be deposited in the subsequent phases of the process, as illustrated in FIG. figure 12 .

Between the barriers 20 coated with the shunt layer 21 according to the invention, the organic layers 161, 160, 162 intended to form the electroluminescent elements E _EL of the electroluminescent layer 16 are then deposited; these organic layers 161, 160, 162 are known per se and are not described here in detail; other variants may be envisaged without departing from the invention, in particular the use of inorganic electroluminescent materials.

Between the over-raised barriers 20 'perpendicular to the column electrodes X _p , X _{p + 1} , the transparent conductive layer 18 is then deposited so as to form electrode lines Y _n , Y _{n + 1} : preferably this layer includes the cathode and a layer of ITO. It is necessary that the deposition conditions are such that the slice of the shunt elements E _S.EL of each cell is covered by this transparent layer 18. An image display panel according to the invention is thus obtained.

With reference to Figures 13 and 14 an alternative manufacturing method of the panel according to the invention will now be described; the process remains the same as the previously described method, with the difference that we will use the surface layer of the slopes of the barriers 20 as shunting element E _S.EL according to the invention, instead of the shunt layer 21; for this purpose, we will boost the barriers on the surface to make the surface layer more conductive; this method is advantageous because it makes it possible to avoid depositing a specific shunt layer; Given the usual dimensions of the barriers (of the order of 1 μm thick for 40 μm wide), the leakage generated by the surface doping of the barriers will be sufficient to obtain the desired shunt effect between the electrodes at the terminals of the elements. electroluminescent E _EL within each cell; since the conductive doping of the barriers is only superficial, the same electrical insulation as before is maintained between the cells of the panel.

According to a third variant, the shunt function according to the invention is provided by a doping of the electroluminescent organic multilayer 16 adapted to create parallel channels of non-recombinant transport of charges through this layer.

The skilled person will deduce directly from the above detailed description and his general knowledge the elements necessary for the realization of a panel according to a preferred embodiment of the invention, that is to say comprising shunt elements both at the level of the electroluminescent elements and the photoconductive elements, on the basis of the general description of this embodiment made at the head of this document.

The present invention is applicable to any type of electroluminescent matrix panels, whether using organic electroluminescent materials or inorganic electroluminescent materials.

Claims (12)

1. An image display panel comprising a matrix of electroluminescent cells (1) with memory effect that are capable of emitting light toward the front of said panel, comprising:

   - a front array (18) of electrodes and a rear array (11) of electrodes, the electrodes of the front array crossing the electrodes of the rear array at each of said cells (1),

   - at least one electroluminescent layer (16) forming, for each cell (1), at least one electroluminescent element (E_EL),

   - a photoconductive layer (12) for obtaining said memory effect, forming, for each cell (1), a photoconductive element (E_PC);

   the at least one electroluminescent element (E_EL) and the photoconductive element (E_PC) of each cell being electrically connected in series and the two outermost terminals of said series being connected, in the case of one of them to an electrode of said front array (18) and in the case of the other to an electrode of said rear array (11),

   - means for optical coupling, at each cell, between the at least one electroluminescent layer (16) of the panel and said photoconductive layer (12), said panel comprising, for each cell (1), a shunt element (E_S.EL) (21) placed in parallel with at least one electroluminescent element (E_EL) of said cell, characterized in that the resistance of said shunt element does not depend on the illumination.
2. The panel as claimed in claim 1, characterized in that, for each cell, the resistance (R_S.EL) of the shunt element (E_S.EL) of at least one electroluminescent element (E_EL) of this cell is greater than the resistance (R_ON-EL) that the electroluminescent element (E_EL) has in the on state.
3. The panel as claimed in either of claims 1 and 2, characterized in that at least one electroluminescent layer (16) is organic.
4. The panel as claimed in any one of claims 1 to 3, characterized in that, for each cell, the resistance (R_S.EL) of the shunt element (E_S.EL) of at least one electroluminescent element (E_EL) of this cell is less than or equal to the resistance (R_OFF-PC) of the corresponding photoconductive element (E_PC) when it is not in the excited state and is less than the resistance (R_OFF-EL) of at least one corresponding electroluminescent element (E_EL) when it is off.
5. The panel as claimed in claim 4, characterized in that the resistance (R_S.EL) of the shunt element (E_S.EL) of at least one electroluminescent element (E_EL) of this cell is strictly less than the resistance (R_OFF-PC) of the corresponding photoconductive element (E_PC) when it is not in the excited state.
6. The panel as claimed in claim 5, characterized in that the resistance (R_S.EL) of the shunt element (E_S.EL) of at least one electroluminescent element (E_EL) of this cell is less than or equal to one half of the resistance (R_OFF-PC) of the corresponding photoconductive element (E_PC) when it is not in the excited state.
7. The panel as claimed in any one of the preceding claims, characterized in that it also includes, for each cell (1), a shunt element (E_S.PC) (22) placed in parallel with the photoconductive element (E_PC) of said cell.
8. The panel as claimed in claim 7, characterized in that, for each cell, the resistance (R_S.PC) of the shunt element (E_S.PC) of the photoconductive element (E_PC) of this cell:

   - is less than or equal to the resistance (R_OFF-PC) of this photoconductive element (E_PC) when it is not in the excited state; and

   - is greater than or equal to the resistance (R_S.EL) of the shunt element (E_S.EL) of at least one electroluminescent element (E_EL) of this same cell.
9. The panel as claimed in claim 8, characterized in that, for each cell, R_S.PC/R_S.EL ≥ 2, R_S.PC being the resistance of the shunt element of the photoconductive element of this cell and R_S.EL being the resistance of the shunt element of the at least one electroluminescent element of this same cell.
10. The panel as claimed in claim 9, characterized in that, for each cell, R_S.PC/R_S.EL ≥ 3.
11. The panel as claimed in any one of the preceding claims, characterized in that it includes, within each cell, a conductive element at each interface between at least one electroluminescent layer and the photoconductive layer in order for the corresponding electroluminescent and photoconductive elements to be electrically connected in series and in that said conductive elements of various cells (1, 1') are electrically isolated from one another.
12. The panel as claimed in any one of the preceding claims, characterized in that it includes means for driving the cells for image display, these being designed to implement a procedure in which, successively for each row of cells of the panel, a selective address phase, designed to turn on the cells to be turned on in this row, is followed by a non-selective sustain phase, designed to keep the cells of this row in the state in which they had been put or left during the preceding address phase.

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