EP0209535B1 - Display device with memory effect comprising thin electroluminescent and photoconducting layers - Google Patents

Abstract

Electroluminescent display device with memory effect wherein, according to the invention, the electroluminescent layer (44) and the photoconducting layer (46) are thin layers. The assembly is interleaved between two electrode systems (42, 48) connected to a voltage source (50). Application to the making of display screens.

Description

The present invention relates to a memory effect display device comprising thin electroluminescent and photoconductive layers.

The advantage of combining the properties of electroluminescent bodies and photoconductive bodies has been seen for a long time. It lies in the possibility of obtaining a memory effect which can be explained schematically in the following manner. A display device essentially comprises an electroluminescent layer (or a stack of layers comprising an electroluminescent layer which may be called "electroluminescent structure") interposed between two electrode systems, which are connected to addressing circuits. A photoconductive layer can be arranged in series with the electroluminescent structure so as to establish, under the effect of an optical excitation, an electrical conduction between some of these electrodes. This conduction leads to the establishment of appropriate electrical potentials and the appearance of an excitation of the electroluminescent layer, which then emits radiation. This is used primarily for displaying information, but it also makes it possible to maintain the conduction of the photoconductive layer, even after cessation of optical addressing. There is therefore self-maintenance or, if you like, memory effect.

In an article entitled "The use of photo-conductive CdS: Cu-CI films in a laser-addressed electroluminescent display sereen published in the review" T _h in Solid Films 41 (1977) 151-160, G. Olive et al. have described such a device, the structure of which has been schematically reproduced in attached FIG. 1. On an insulating and transparent support 10 are deposited opaque electrodes (Au) 12 and 14, a layer 16 of photoconductive material, and a transparent electrode 18 connected to the electrode 12. An electroluminescent material 20 covers the electrode 18. An electrode transparent 22 is connected to the electrodes 12 and 18 and an opaque electrode (AI) 24 is arranged in the electroluminescent material, so that the latter is interposed between, on the one hand, this electrode 24 and, on the other hand, the two electrodes 18 and 22. A laser 26 is capable of emitting a light beam 28 which strikes the photoconductive material 16 in the zone situated between the electrodes 12 and 14.

The operation of this device is as follows. At rest, an AC voltage is applied to the electrode 24, and to the electrode 14, but the laser 26 is stopped. The photoconductive material 16 is not optically excited, it behaves like an insulator. The electrodes 14 and 12 are therefore electrically isolated from each other and the potential of the electrode 12 is floating, as is that of the electrodes 18 and 22. The electroluminescent material is not excited and therefore does not emit no light.

The excitation is controlled opfically by the laser 26. The latter emits a beam 28 which strikes the photoconductor 16 between the electrodes 12 and 14 making this zone electrically conductive. The two electrodes 12 and 14 are then connected by a conductive channel (symbolically marked by the arrow 36) and the potential of the electrodes 12, 18 and 22 is established at the value fixed by the potential applied to the electrode 14. A potential difference then appears between the electrode 24, on the one hand, and the electrodes 18 and 22, on the other hand. This causes the appearance of an electric field and the excitation of the electroluminescent material. The radiation 30 emitted by the electroluminescent material towards the front of the device allows the display of information for an observer positioned at 32. As for the rear part 34 of the emitted radiation, it excites the photoconductor and maintains its photoconduction. Therefore, the laser 26 can be put to rest without stopping the electroluminescence. We thus obtain a memory effect. The display stops as soon as the electrical excitation is removed.

A similar device has been described by A. H. Kitai et al. in an article entitled “Hysteretic Thin-Film EL Devices Utilizing Optical Coupling of EL Output to a Series Photoconductor” and published in the journal SID 84 DIGEST, 255-256.

The complexity of these devices of the prior art is largely due to the fact that the photoconduction is implemented longitudinally, or if desired, in the plane of the layers. This is highlighted in Figure 1 by arrow 36. As a result, two electrode systems are required: one (like 14) which are addressing electrodes, and the other, like 18 and 22 which are excitation electrodes.

These electrodes are not moreover of identical sizes, since their functions are different, the first being much narrower than the second, which poses problems of masks and alignment. Furthermore, in the device by G. Olive et al., The actual light-emitting structure 20 is composed of two similar light-emitting layers separated by an electrode 24 acting as an optical screen, this to isolate the photoconductive element from the ambient light incident on the device on the observer side. This principle requires a set of electrodes on four distinct levels, requiring multiple additional stages of masking, etching and deposition in the manufacturing process.

Furthermore, from document FR-A-2 335 902, an electroluminescent display device with memory effect is known which comprises a layer of photoconductive material interposed between a first and a second layer of electroluminescent materials. The first light-emitting layer has a light-emitting band which is included within the limits of the excitation band of the photoconductive layer. The second electroluminescent layer has an emission band of light which is outside these limits and which is in principle included in the visible part of the spectrum and which can be used for display purposes.

When applying a relatively low maintenance voltage across the first and second light-emitting layers surrounding the photoconductive layer, the electroluminescence emanating from these layers is relatively low due to the fact that it occurs a significant drop in the excitation voltage in the photoconductive layer, which has a high impedance due to the relatively low luminescence of the first electroluminescent layer. When a switching voltage is applied, there is an increase in the light emission from the first light-emitting layer, which has the effect of exciting the photoconductive layer. An electro-optical reaction is therefore obtained between the first light-emitting layer and the photoconductive layer, the drop in switching voltage which occurs in the photoconductive layer rapidly decreasing, while the voltage drop which occurs in the first and second light-emitting layer increases rapidly. When the photoconductive material is in the state in which it is fully conductive, the device has passed to a stable active state and the maintenance voltage causes the generation, by the first light-emitting layer, of an electroluminescence sufficient for the photoconductive material remains fully conductive even when the switching voltage ceases to be applied.

Although simpler than the previous ones, these devices are still complex due to the presence of two light-emitting layers and the need to adopt different optical characteristics for these layers.

The invention aims to further simplify these devices while improving their performance and their manufacturing conditions. For this purpose, the invention provides for using an electroluminescent layer (or a stack of layers comprising an electroluminescent layer) and a photoconductive layer which are all thin layers, that is to say layers whose thickness is on the order of a micron or less, practically between 0.1 and 2 microns.

The thinness of the electroluminescent layer results in an interesting advantage which is as follows. The thin electroluminescent layers deposited on a smooth and flat substrate (glass substrate for example) are themselves smooth and flat and are then the seat of an effect commonly called optical guidance. Although the levels of luminance extracted from the device are of the same order for devices based on thin layers as for devices with non-thin layers, that is to say “powder” (typically 100 to 1000 Cd / m ² to 1 kHz of excitation), the internal light fluxes are much more intense in thin film structures (typically a factor of 10) due to the optical guiding effect and the absence of optical scattering. In the stack of an electroluminescent structure and a photoconductive layer where the electroluminescent structure is a thin layer or a stack of thin layers, the light emission from the active layer is almost entirely trapped (= - 90%) and transferred in large part to the photoconductive layer, hence a markedly enhanced memory effect.

The coupling between the electroluminescent and photoconductive materials being thus clearly reinforced, the device becomes practically insensitive to ambient light. The complication of the device according to FR-A 2 335 902 becomes unnecessary.

A final advantage derived from the use of thin layers for the electroluminescent structure is that the light is not diffused by the layers and the rear photoconductive layer, of dark appearance, provides an excellent display contrast.

Another advantage to be credited to the invention is that the photoconductive layer is deposited uniformly over the entire surface of the display and absorbs most of the incident ambient light, it therefore prevents the reflection of the latter on the network d generally 48 opaque and metallic electrodes. It thus contributes to significantly enhance the contrast of the device of the invention. In the devices of the prior art, such as that of G. Olive et al., The contrast is reduced for two reasons: firstly the electroluminescent material is based on powder therefore very diffusing, then the device consists of several networks of metal electrodes 12 and 24 not masked by the photoconductive layer and which will therefore reflect the incident ambient light.

• tout d'abord, pour une couche très mince, d'épaisseur inférieure à 0,5 micron typiquement, dans l'état éteint du dispositif, la résistance d'obscurité de la couche photoconductrice est grande devant l'impédance relative à la capacité de la couche et n'a pas d'influence sur la tension aux bornes de la couche. En d'autres termes, la couche photoconductrice a un comportement électrique capacitif plutôt que résistif, celui-ci ne dépendant alors plus de la résistivité du matériau dans l'obscurité. Le dispositif pourra donc être commuté à l'état allumé par un moyen purement électrique. Mieux, la boucle d'hystérésis devient alors indépendante dé la résistivité du matériau photoconducteur dans l'obscurité. La fabrication du dispositif s'en trouve facilitée et l'obtention de l'hystérésis est beaucoup plus reproductible ;
• ensuite, pour une épaisseur de couche photoconductrice inférieure à 1 à 2 foLm, et pour une tension dans l'obscurité aux bornes de la couche photoconductrice requise de 20 à 50 V, le champ électrique dans la couche photoconductrice est alors de quelques 10⁵ V/cm. On obtient alors des effets non linéaires importants dans la conductivité du photoconducteur (comme c'est le cas notamment avec le Si amorphe dont le comportement électrique est décrit dans l'article de I. Solomon et al. « Space-charge- limited conduction for the determination of the midgap density of states in amorphous silicon : Theory and experiment », Phys. rev. B30, p. 3422 (1884)). Ces effets ressemblent à un effet avalanche (bien qu'en pratique il ne s'agisse pas d'une « avalanche » au sens strict), et l'on peut les comparer à l'effet obtenu avec 2 diodes mises tête-bêche. Plus précisément, le courant de conduction dans la couche photoconductrice, de faible valeur au-dessous d'un certain seuil en tension, augmente brutalement au-dessus de ce seuil. L'intérêt pratique est que la tension aux bomes de la couche photoconductrice dans l'obscurité est en fait la tension de seuil de l'« avalanche » qui s'avère être beaucoup moins dépendante des paramètres de fabrication que la résistivité à faible champ électrique (cas par exemple des couches photoconductrices épaisses) et donc être particulièrement reproductible: Un grand avantage commun aux deux cas précédents est que, la couche photoconductrice étant très résistive, on évite un couplage parasite entre éléments d'image voisins par conduction planaire dans la couche photoconductrice, même à des résolutions élevées (typiquement jusqu'à 10 points/mm).

The thinness of the photoconductive layer results in two properties:

• first of all, for a very thin layer, of thickness less than 0.5 micron typically, in the off state of the device, the dark resistance of the photoconductive layer is large compared to the impedance relative to the capacity of layer and has no influence on the voltage across the layer. In other words, the photoconductive layer has a capacitive rather than a resistive electrical behavior, the latter then no longer depending on the resistivity of the material in the dark. The device can therefore be switched to the on state by purely electrical means. Better still, the hysteresis loop then becomes independent of the resistivity of the photoconductive material in the dark. The manufacture of the device is thereby facilitated and obtaining the hysteresis is much more reproducible;
• then, for a thickness of photoconductive layer less than 1 to 2 foLm, and for a voltage in the dark across the required photoconductive layer of 20 to 50 V, the electric field in the photoconductive layer is then some 10 ⁵ V / cm. Significant nonlinear effects are then obtained in the conductivity of the photoconductor (as is particularly the case with Si amorphous whose electrical behavior is described in the article by I. Solomon et al. "Space-charge- limited conduction for the determination of the midgap density of states in amorphous silicon: Theory and experiment", Phys. rev. B30, p. 3422 (1884)). These effects resemble an avalanche effect (although in practice it is not an “avalanche” in the strict sense), and we can compare them to the effect obtained with 2 diodes placed head to tail. More precisely, the conduction current in the photoconductive layer, of low value below a certain voltage threshold, increases abruptly above this threshold. The practical interest is that the voltage at the terminals of the photoconductive layer in the dark is in fact the threshold voltage of the "avalanche" which turns out to be much less dependent on the manufacturing parameters than the resistivity at low electric field. (case for example of thick photoconductive layers) and therefore to be particularly reproducible: A great advantage common to the two preceding cases is that, the photoconductive layer being very resistive, one avoids a parasitic coupling between neighboring picture elements by planar conduction in the layer photoconductive, even at high resolutions (typically up to 10 points / mm).

As for the electrode systems used, they can be of various types depending on the application envisaged. In the case of a matrix display screen, the electrode systems consist of two families of conductive strips, the strips of one of the systems being crossed relative to the strips of the other system. The volume delimited by each intersection between an electrode of one system and an electrode of the other constitutes a picture element. An image can then be displayed on such a matrix screen by exciting a certain number of these image elements. A well-known method of displaying for a matrix screen is the "one line at a time" technique by which the lines (one of the two electrode systems) are excited or addressed one after the other sequentially; the columns (the other electrode system) are addressed simultaneously at the same time.

In such a variant, the optical excitation which renders the photoconductive material is obtained by the light emitted by the electroluminescent layer itself under the effect of an electrical excitation temporarily exceeding a certain threshold, the addressing of the device thus being entirely electric.

But according to another variant, the device can comprise a specific optical addressing device capable of causing the conduction of certain areas of the photoconductive layer.

This optical device can be a laser, an optical pencil or any other light source.

According to yet another variant, the addressing means may be an electron beam. The device in question will be close to that described in the article entitled “Device Characterization of an Electron-Beam-Switched Thin-Film ZnS: Mn Electroluminescent Faceplate published by O. Sahni et al. in "IEEE Transactions on Electron Devices", vol. ED 28, n ° 10, June 81, page 708. The addressing means then consists of a single electroluminescent element covering the entire rear surface (electron gun side) of the front face of a cathode ray tube and fed independently of the gun . The contribution of the invention lies in the addition of a photoconductive layer with the same surface as the light-emitting layer or layers and interposed between the latter and the rear electrode made of AI.

As for the photoconductive material, its absorption spectrum must be adapted to the emission spectrum of the electroluminescent element to ensure the latter maximum sensitivity to this electroluminescent emission. It can be constituted by the bodies already used in this kind of device: CdS, CdSe, or CdS-CdSe, or even CdS: Cu, CI. Thus, with CdS-CdSe, the inventor was able to obtain switching times of the order of a millisecond with electrical addressing; A. H. Kitai et al. reported an electrical switching time of 20 ms.

However, according to the invention, it is also possible to use another body, the photoconductivity of which has already been recognized but which has never been used in memory electroluminescent devices, probably for this reason that this photoconductivity is much lower than that of previous bodies, which made it unusable in the devices of the prior art, for which the conduction is in the plane of the layers. It is hydrogenated amorphous silicon (a-Si: H). The interest of this material lies in its rapid response and its ability to resist high electric fields, as shown by intensive studies carried out on this material for the production of solar cells and that of thin-film transistors. Recent work by the inventor shows that the switching time could be reduced to 0.1 ms. This reduction in switching time is very important because it means that, for a display screen with 250 lines energized sequentially, it will take 250 x 0.1 = 25 ms to address a screen, which becomes compatible with the rates of the systems. video, which was not the case with the devices of the prior art.

Maintenance of the on state for a switched display point must be carried out alternately with the maintenance voltage. It can be obtained in two non-mutually exclusive ways. If the decay time of the light-emitting doping center is sufficiently slow to allow recovery of the light pulses from one alternation to the other of the maintenance voltage, the photoconductive layer will still be subjected to the luminescence tail of the previous light impression on the new alternation or electrical pulse and the device will remain in the on state. If the decay time of the transmitting center is too short or the frequency of the maintenance voltage too low, it will then be necessary to choose a photoconductive material with a sufficiently slow response time to allow maintenance of the switched on state of the device.

It can therefore be seen that, by choosing the photoconductive material and the suitable production conditions, it will be possible to produce devices with fast switching times making it possible to operate at the video rate on a large number of lines or, on the contrary, devices operating at low maintenance frequency and therefore with low electrical consumption. Another material that could also be used advantageously is zinc monoxide (ZnO).

• la figure 1, déjà décrite, représente un dispositif selon l'art antérieur,
• la figure 2 représente, en coupe le long d'une électrode ligne, un dispositif de l'invention dans un mode de réalisation où l'adressage est tout électrique,
• la figure 3 est un schéma qui montre l'équivalent électrique d'un point d'affichage,
• la figure 4 est une courbe montrant comment la luminance d'un point d'affichage change en fonction de la tension électrique appliquée,
• la figure 5 montre un autre mode de réalisation où l'adressage est optique,
• la figure 6 représente, en coupe le long d'une électrode ligne un dispositif de l'invention dans un mode de réalisation inversé par rapport à celui de la figure 2,
• la figure 7 montre trois variantes de réalisation d'un empilement couche électroluminescente- couches diélectriques-couche photoconductrice.

In any case, the characteristics of the invention will appear better after the description which follows, of exemplary embodiments given by way of explanation and in no way limiting. This description refers to attached drawings in which:

• FIG. 1, already described, represents a device according to the prior art,
• FIG. 2 represents, in section along a line electrode, a device of the invention in an embodiment where the addressing is entirely electrical,
• FIG. 3 is a diagram which shows the electrical equivalent of a display point,
• FIG. 4 is a curve showing how the luminance of a display point changes as a function of the applied electrical voltage,
• FIG. 5 shows another embodiment where the addressing is optical,
• FIG. 6 represents, in section along a line electrode, a device of the invention in an embodiment reversed with respect to that of FIG. 2,
• FIG. 7 shows three alternative embodiments of a stack of light-emitting layer-dielectric layers-photoconductive layer.

In those of the figures which represent the device of the invention in cross section, the layers are not drawn to scale and this for clarity. It suffices to indicate again that the photoconductive layer and the electroluminescent layer as well as any other layers of the electroluminescent structure generally have a thickness of the order of a micron (practically between 0.1 and 2 microns). As for the electrodes 42, they are conventionally produced by depositing a layer of indium tin oxide (“ITO”) typically 0.2 microns thick. The insulating substrate can be glass, for example 7059 glass from the Coming brand or ordinary “soda-lime” glass. The electrodes 48 can be opaque and produced by deposition of aluminum for example or transparent and produced by deposition of ITO for example.

The device represented in FIG. 2 comprises a transparent substrate 40, transparent in-line electrodes 42 (the cut shown is supposed to be made along one of these lines), a thin light-emitting layer 44, a thin photoconductive layer 46 and column electrodes 48. The electroluminescence layer can be replaced by a stack of layers comprising an electroluminescent layer. The other layers can be dielectric layers for an electroluminescent structure of the thin film type with alternating excitation or a resistive protective layer for a thin film structure with unidirectional excitation. The electrode systems in rows and columns are permanently connected to an alternating voltage generator 50, the applied voltage is called the maintenance voltage. Furthermore, the line electrodes 42 are connected to a line addressing circuit 52L and the column electrodes 48 to a column addressing circuit 52C. These circuits can be placed in parallel with the generator 50 as in FIG. 3 or in series. The observation is preferably carried out through the substrate 42, at 53.

The operation of this device can be explained using FIGS. 3 and 4. On the first, we see the equivalent electrical diagram of a display point, that is to say of the parallelepiped volume between an electrode row and a column electrode. The photoconductive layer 46 is electrically equivalent to a variable resistance R46 and to a fixed capacitance C46. The light-emitting layer 44 is equivalent to a variable resistance R44 and to a fixed capacitor C44. An additional capacitance C44 ′ represents the contribution of one or more dielectric layers generally deposited on and / or before the electroluminescent layer (as will be seen below with reference to FIG. 7).

The graph in FIG. 4 shows the variation of the luminance L emitted by a display point as a function of the voltage V applied between the electrodes which frame this point. Luminescence does not appear until this voltage has reached a value V1 which corresponds to a certain threshold of electric field necessary for obtaining the phenomenon of electroluminescence. From this value, the excited point emits light. The rear part of the light radiation emitted by the layer 44 strikes the photoconductor 46 which, insulating as it was (resistance R46 strong), becomes conductive (resistance R46 weak). Almost all of the voltage is then applied to the electroluminescent layer 44 and the electric field applied to this layer increases suddenly. The voltage can therefore be reduced without stopping the electroluminescence. This will only disappear when the field has dropped below the threshold value, which corresponds to a voltage V2 lower than V1. If the voltage applied to the electrodes is equal to a value V3 between V1 and V2, the display will be maintained. It is the generator 50 which delivers this voltage V3 permanently applied to the electrodes. The role of the addressing circuits 52L and 52C is to bring, for a short time, to the point that we want to excite, an increase in voltage, of amplitude equal to or greater than V1-V3. To extinguish a luminescent point, it suffices to apply an erase pulse which brings the voltage below V2 for a short time.

The generator 50 can be a sinusoidal voltage generator. However, rectangular or pulse signal generators are also suitable.

The device which has just been described has the particularity of being solely electrical addressing. However, optical addressing devices also fall within the scope of the invention. Such a device is shown in the figures. As shown, it always includes a substrate 40, row electrodes 42, a thin light-emitting layer 44, a thin photoconductive layer 46, column electrodes 48 and a generator 50, but the addressing means here consists of a laser 54 and a deflection device 56. The latter can be produced using a galvanometric mirror or a bundle of fibers. The optical addressing means can also be an optical pencil. The light beam 58 can be directed onto any one of the display points defined by the overlap of two electrodes of the systems 42 and 48. The optical excitation of one of the points makes the layer 46 conductive in this area, which causes the fall of the equivalent resistance R46. The voltage of the source 50 being always equal to V3, the electroluminescent material is excited by a field whose value exceeds the electroluminescence threshold, which causes the emission of electroluminescence and the switching of the point in the on state. For all the other points, the voltage V3 will be insufficient to cause electroluminescence. The total image being displayed, it can be deleted by opening a switch 51, which stops the maintenance excitation.

The device shown in FIG. 6 is similar to that of FIG. 2 except that the thin light-emitting layer or the stack of thin layers comprising an light-emitting layer 44 is located on the thin photoconductive layer 46 and that the column electrodes 48 are transparent. necessarily. The observation is preferably carried out through the electrodes 48 at 53b: Such a structure may be necessary if, for example, the conditions under which the thin photoconductive layer is deposited are such as to degrade the characteristics and the layer (s) composing the electroluminescent element, it is then preferable to deposit these second.

• 
  Si φ_PC est l'éclairement de la couche photoconductrice induit par l'émission φ_Z de ZnS, on a :
  
• La seule quantité aisément mesurable est L ; on déduit de (1) et (2) :
  
• Dans le cas précis envisagé, on obtient :

The purpose of FIG. 7 is to show that, in practice, the electroluminescent and photoconductive layers can be associated with dielectric layers. In addition to the electroluminescent layer 61 is surrounded by dielectric layers 62 and 63, the photoconductive layer 64 being deposited on the upper dielectric layer 63. These layers have substantially different refractive indices in practice, causing significant optical guidance effects. These effects can be specified on a concrete case. The electroluminescence layer 61 can be of ZnS of index n _z = 2.3 approximately. The dielectric layers 62 and 63 can be of Y ₂₀₃ of index n = 1.9 approximately. The photoconductive layer 64 can be a-Si: H of index 3.4 approximately. The index of the glass substrate and of the transparent electrodes is typically 1.5. The application of the law of refraction (conservation of the product n. Sin 8 from one medium to another) gives the following results. If φ _Z is the luminous flux emitted inside the ZnS and L the luminance measured in the air normally at the plane of the substrate on the observer side, we calculate that:

• 
  If φ _PC is the illumination of the photoconductive layer induced by the emission φ _Z of ZnS, we have:
  
• The only easily measurable quantity is L; we deduce from (1) and (2):
  
• In the specific case considered, we obtain:

In a structure of the type of the invention and assuming complete absorption of the light incident on the photoconductive layer by the latter, L will have a typical value of 300 Cdlm ² at an excitation frequency of 1 kHz for ZnS : Mn. The linearity φ _PC received by the photoconductive layer will therefore be - 7,300 lux. The typical ambient lighting of an indoor workstation is around 400 lux, which is much lower than φ _PC ·

The optical screen described in the article by G. Olive et al. cited above and source of technological complications is therefore no longer necessary with the device of the invention. This is due to the excellent optical coupling between the light-emitting layer and the photoconductive layer and also due to the intense light flux emitted in the ZnS: Mn in a thin layer. The optical coupling between electroluminescent layer and photoconductive layer can be further improved by choosing dielectrics with high refractive index such as for example Ta ₂ O ₅ (n ≃ 2.1) or ferroelectric materials such as PbTi0 ₃ (n - 2 , 7).

et

Returning to FIG. 7b, in b, the electroluminescent layers 61 and photoconductive layers 64 are in contact with each other but the assembly is protected by a lower dielectric layer 62 and a upper dielectric layer 65. In this case, the optical coupling between the light-emitting layer and the photoconductive layer is maximum: the entire flux radiated by the light-emitting layer and not extracted from the structure in air is recovered by the photoconductive layer. We calculate that:

and

In the example of an electroluminescent layer in ZnS and dielectric layers in Y ₂ 0 ₃ , we obtain:

A luminance L of 300 Cdlm ² gives an illumination φ _PC of the photoconductive layer of approximately 18,000 lux. The use of such a device without an optical screen and outdoors (illumination less than 10,000 Lux) then becomes possible.

avec k = 2 π/λ, où k est le nombre d'ondes, λ la longueur d'onde d'émission, n l'indice de la couche 65 et n_eff l'indice effectif de la cavité formée par la couche électroluminescente et les couches 65 et 62: n_eff ≃ n_Z· L'axe des x est normal au plan des couches et a pour origine l'interface entre la couche électroluminescente et la couche 65. Pour une couche électroluminescente en ZnS et une couche 65 en Yz03, on calcule :

In c, the device shown is obtained from the device presented in b by inserting an additional dielectric layer 65. This type of structure has several advantages. First of all, it is known that multilayer dielectrics have electrical and protective properties under strong field superior to those of a simple dielectric layer. Furthermore, the electrical properties of the electroluminescent structure (threshold voltage, threshold stiffness) are very sensitive to the nature and quality of the interfaces between the electroluminescent layer proper and the neighboring layers. The dielectric of layer 65 will be chosen so as to optimize its interface with the electroluminescent layer. We can also choose a dielectric with a high refractive index to get as close as possible to the optimal optical coupling according to the well-known principle of anti-reflection layers. Another original and simpler means to achieve to obtain the maximum optical coupling is to deposit a layer 65 thin enough to allow this coupling by evanescent waves. Thus, with a light wave emitted in the electroluminescent layer and having an angle of incidence with the interface between the electroluminescent layer and the layer 65 greater than the critical angle defined by the law of refraction, an evanescent wave is associated in layer 65 whose wave function is characterized by:

with k = 2 π / λ, where k is the number of waves, λ the emission wavelength, n the index of the layer 65 and n _eff the effective index of the cavity formed by the light-emitting layer and layers 65 and 62: n _eff ≃ n _Z · The x-axis is normal to the plane of the layers and originates from the interface between the electroluminescent layer and layer 65. For a ZnS electroluminescent layer and a layer 65 in Yz03, we calculate:

with i = 500 A.

If therefore a layer 65 in Y ₂ 0 ₃ of thickness 0.05 microns approximately is chosen, a fraction 1 / e ² of the light power incident on the interface between the electroluminescent layer and the layer 65 is transmitted to each reflection at the photoconductive layer. There is thus an almost integral transfer of the light wave from the light-emitting layer to the photoconductive layer after a few reflections, this despite the relatively low index of the layer 65 in Y ₂ 0 ₃ .

• 1. Dans les dispositifs antérieurs à mémoire inhérente, la largeur de la couche d'hystérésis n'est pas facilement ajustée. En outre, l'effet d'hystérésis disparaît progressivement après un fonctionnement prolongé. Dans le cas de la cellule de l'invention, le photoconducteur est seul responsable de l'hystérésis (cf. courbe de la figure 4) de sorte que ses propriétés peuvent être optimisées séparément de la couche électroluminescente.
• 2. L'effet mémoire inhérent n'est obtenu qu'aux fortes concentrations de manganèse (supérieures à 1 % en poids typiquement). Mais ces concentrations sont situées au-delà de la valeur optimale pour laquelle le rendement d'électroluminescence est le plus grand ; ce rendement tombe alors au tiers ou même jusqu'au dixième de sa valeur maximale. La combinaison matériau électroluminescent-matériau photoconducteur selon l'invention, en séparant les fonctions luminescence et mémorisation, permet d'adopter pour le manganèse l'optimum de concentration en ce qui concerne le rendement de luminescence. Un gain d'un ordre de grandeur est ainsi possible sur la luminance du dispositif.
• 3. Avec l'invention, l'effet mémoire peut être obtenu même si le dopant de la couche électroluminescente n'est pas le manganèse ; ainsi d'autres couleurs que le jaune (qui correspond à Mn) sont possibles. On sait que l'effet mémoire permet d'exciter l'élément électroluminescent avec une tension d'entretien de fréquence nettement supérieure à la fréquence de rafraîchissement d'un écran électroluminescent sans mémoire : classiquement 1 kHz à comparer à 100 Hz pour le deuxième cas. Ce gain d'un ordre de grandeur en fréquence et donc en luminance sera d'autant plus apprécié que les rendements électroluminescents des corps émettant dans d'autres couleurs que le jaune sont beaucoup plus faibles.
• 4. Pour des raisons qui ne sont pas encore parfaitement élucidées, certaines technologies ne permettent pas d'obtenir un effet mémoire propre à la couche électroluminescente. Ainsi, il n'a jamais été rapporté à la connaissance de l'inventeur d'effet mémoire inhérent au ZnS : Mn quand celui-ci est déposé par évaporation par effet Joule, par pulvérisation cathodique ou encore par la méthode « Atomic Layer Epitaxy développée par Lohja (Finlande). L'invention permet de pallier cet inconvénient en apportant un moyen spécifique d'introduction de cet effet

. The device of the invention offers another advantage in the field of display. It makes it possible, in fact, to take better advantage of the memory effect that is encountered in certain electroluminescent devices. It is known, in fact, that certain electroluminescent materials containing manganese have a memory effect (regardless of the presence of any photoconductive material). This effect has been described in an article entitled “Character Display using Thin-Film EL Panel with Inherent Memory published by Chuji Suzuki et al. in SID 76 Digest, p. 50-51. However, this memory effect is much more difficult to control than that which is used in the present invention, for the following reasons:

• 1. In prior devices with inherent memory, the width of the hysteresis layer is not easily adjusted. In addition, the hysteresis effect gradually disappears after prolonged operation. In the case of the cell of the invention, the photoconductor is solely responsible for the hysteresis (cf. curve of FIG. 4) so that its properties can be optimized separately from the electroluminescent layer.
• 2. The inherent memory effect is only obtained at high manganese concentrations (typically more than 1% by weight). However, these concentrations are situated beyond the optimum value for which the electroluminescence yield is the greatest; this yield then falls to a third or even to a tenth of its maximum value. The combination of electroluminescent material and photoconductive material according to the invention, by separating the luminescence and memorization functions, makes it possible to adopt for manganese the optimum concentration as regards the luminescence yield. A gain of an order of magnitude is thus possible on the luminance of the device.
• 3. With the invention, the memory effect can be obtained even if the dopant of the light-emitting layer is not manganese; thus other colors than yellow (which corresponds to Mn) are possible. We know that the memory effect makes it possible to excite the electroluminescent element with a frequency maintenance voltage markedly higher than the refresh rate of an electroluminescent screen without memory: conventionally 1 kHz to compare with 100 Hz for the second case . This gain of an order of magnitude in frequency and therefore in luminance will be all the more appreciated as the electroluminescent yields of the bodies emitting in colors other than yellow are much lower.
• 4. For reasons which are not yet fully understood, certain technologies do not make it possible to obtain a memory effect specific to the electroluminescent layer. Thus, it has never been reported to the knowledge of the inventor of the memory effect inherent in ZnS: Mn when it is deposited by evaporation by Joule effect, by sputtering or even by the “Atomic Layer Epitaxy developed method by Lohja (Finland). The invention overcomes this drawback by providing a specific means of introducing this effect.

Another advantage of the invention must be emphasized again. If the photoconductive material used is sufficiently resistant to high electric fields, a thinner photoconductive layer 46, for example less than 1 micron, is deposited and a large capacitive coupling takes place between the electrode on the photoconductive layer and the light-emitting structure (coupling represented by capacitor C46 in Figure 3). This coupling allows the device to be switched on even in total darkness, where the resistivity of the photoconductor is very high. The value of such a capacity depends essentially on the thickness of the photoconductive layer and on the permittivity of the material. However, these quantities are perfectly uniform over the entire surface of a device and reproducible from one device to another. On the contrary, in a device of the prior art like that of FIG. 1, it is much more difficult to ensure uniformity over a large area and the reproducibility of the resistivity of the photoconductor in the dark; in this case, there is a very large dispersion in the control voltages. This is the reason why we use it most often. optical addressing means (laser 26 of FIG. 1). In the invention, on the contrary, an all-electric addressing screen is perfectly reliable.

The photoconductive element can have a photoresistor behavior in the sense that, at a given level of lighting, it will behave like a resistor - R 46 in FIG. 3 - and that its resistance R 46 will depend on the level of lighting only and no terminal voltage. It is recognized that it is difficult to ensure excellent reproducibility of the resistivity of a photoconductor in the dark, the underlying mechanisms being generally poorly understood and unwanted impurities of a poorly known nature which can, for example, modify this resistivity. Without capacitive coupling added to the resistor R 46 of the photoconductive element, the ignition of the device in the dark will be very difficult, the ignition voltage V ₁ will have to be very high in certain cases, this voltage being found entirely at the terminals of the electroluminescent element after triggering of the latter, it may even lead to the destruction of the latter. The voltage V ₁ will also be very sensitive to stray light such as ambient lighting

An original solution proposed in the invention is to use a photoconductor having a photodiode behavior. Such behavior can be achieved by adapting the process for producing the photoconductive layer to make it very resistive in the dark and by applying high fields thereto. Thus, a device comprising an electroluminescent structure and a photoconductive layer in a-Si: H of type N ^{+ -} IN ⁺ (I: intrinsic) and tested by the inventor showed properties similar to those of FIG. 4, the voltage “avalanche” (V ₁ ) at the terminals of the photoconductive layer in the dark being about 20 V for 2 μm of thickness of the photoconductive layer and corresponding to a field of the order of 10 ⁵ V / cm . This field value is characteristic of the material and is reproducible from one sample to another. An electroluminescent-photoconductive device integrating a photoconductive element of photodiode type can be schematized like the figure 3, but with a photoconductive element equivalent to 2 head-to-tail diodes of variable characteristics according to the lighting, in the case - not restrictive - where the electroluminescent element is of the alternating excitation type. The width of the hysteresis V ₁ -V ₂ (Figure 4) is at most equal to _V1 and it is reproducible. The conduction of the photoconductor in V ₁ is linked to mechanisms which resemble the avalanche phenomenon, which are not directly linked to the photoconductivity of the material. The voltage V ₁ is therefore not sensitive to low levels of illumination. In practice, the voltage to be maintained across the photoconductor before the device is triggered is of the order of 20 to 50 V. Furthermore, the electrical protection layers of the electroluminescent element like the layers dielectrics for the electroluminescent type with alternating excitation thin layers and like the resistive layer for the electroluminescent type with unidirectional excitation will effectively protect the photoconductive layer itself. Thus, we can consider as a first approximation that to obtain the current necessary for the ignition of the electroluminescent element, if the device is not switched on, it will be necessary to apply a voltage _V1 across the terminals of the photoconductive element; if the device is already switched on, a lower voltage v ₂ will suffice. It is thus shown that the hysteresis width V ₁ -V ₂ is equal to _Vi -v _s and that the photoconductive element is protected by the electroluminescent element, the latter acting as a current limiter, this protection being distributed over the entire surface of the photoconductor in the case of the invention.

Claims (7)

1. Electroluminescent display means with a memory effect comprising, on an insulating support (40), an electroluminescent film (44) and a photoconductive film (46) stacked on one another, the assembly of said two films being placed between two systems of electrodes (42, 48), the latter being connected to an electric power source (50) permitting the excitation of certain zones of the electroluminescent film, characterized in that the electroluminescent film (44) and the photoconductive film (46) are thin films having a thickness of approximately 1 micron.

2. Means according to claim 1, characterized in that the electroluminescent film is a stack of a thin electroluminescent film and thin dielectric films.

3. Means according to claim 1, characterized in that the photoconductive film is placed between the electroluminescent film and a dielectric film.

4. Means according to claim 1, characterized in that a thin dielectric film is placed between the photoconductive film and the electroluminescent film.

5. Means according to claim 1, characterized in that the optical excitation initially making the photoconductive material conductive is obtained by the light emitted by the thin electroluminescent film, which is itself under the effect of an adequate electrical excitation applied to the two systems of electrodes, so that the addressing of the means is all electric (52C, 52L).

6. Means according to claim 1, characterized in that the photoconductive material is hydrogenated amorphous silicon.

7. Means according to claim 1, characterized in that the photoconductive film is CdS, CdSe or ZnO.

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