EP0384829B1 - Multicolour electroluminescent flat panel display having a memory effect - Google Patents

Description

The present invention relates to a flat screen polychrome electroluminescent memory effect usable in the field of optoelectronics for the color display of complex images or for the color display of alphanumeric characters.

A display device is said to have a memory effect if its electro-optical characteristic (luminance-voltage curve) exhibits hysteresis. For the same voltage located inside the hysteresis loop, the device can thus have two stable states: off or on.

The advantages of a memory effect display are appreciable: to display a still image, it suffices to apply simultaneously and continuously to the entire screen a so-called maintenance voltage. The latter can be a sinusoidal signal or in the form of slots for example, but above all, the form and frequency of this maintenance signal can be chosen independently of the complexity of the screen, in particular the number of lines of dots. display. There is therefore in principle no limit to the complexity of a memory effect display screen. Thus, there are on the market bistable plasma screens with alternating excitation of 1200x1200 image points (pixels).

In addition, the technology of thin-film electroluminescence display with capacitive coupling (abbreviated to ACTFEL) has now reached maturity in the industry. These devices can be provided with a so-called inherent memory effect but at the cost of a significant deterioration in electro-optical performance. A more attractive method is connecting a photoconductive structure (PC) in series with an electroluminescent structure (EL) and optically coupling these two structures.

It is thus possible to produce a memory effect of the extrinsic type which is called PC-EL memory effect, the principle of which is as follows. When the device is in the off state, the photoconductive material is not very conductive and retains a significant part of the voltage V applied to the assembly. If one increases V to a value Von such that the voltage present at the terminals of the electroluminescent structure exceeds the electroluminescence threshold, the PC-EL device switches to the on state. The photoconductive material is then illuminated by the electroluminescent structure and goes into the conductive state. The voltage across its terminals drops and this results in an increase in the voltage available for the electroluminescent structure. To switch off a PC-EL device, it suffices to decrease the total voltage V to a value Voff lower than Von: this gives a luminance-voltage characteristic comprising a hysteresis.

A monochrome PC-EL structure has recently been described in document FR-A-2 574 972 and in the inventor's article entitled "Monolithic Thin-Film Photoconductor-ACEL Structure with Extrinsic Memory by Optical Coupling" and published in IEEE Transactions on Electron Devices, vol. ED-33, n ° 8, of August 1986, pages 1149-1153.

This structure is shown diagrammatically in FIG. 1. It comprises a glass substrate 10 on which are deposited an electrode 12, a first dielectric layer 14, an electroluminescent layer 16, a second dielectric layer 18, a photoconductive layer 20, a third layer dielectric 21 and finally an electrode 22. The electrodes 12 and 22 are connected to an alternating voltage source 24. In this embodiment, the layers PC and EL are thin layers, the thickness of which is of the order of a micrometer.

Such a structure is simple to produce because it does not require additional etching steps. Furthermore, the current-voltage behavior of the thin layer photoconductor in the dark is highly non-linear and reproducible. The beneficial consequences are that the electrical ignition of the device is always easy, that the hysteresis depends only slightly on the excitation frequency and that the reproducibility of the hysteresis margin from one manufacturing to another is guaranteed. .

Unfortunately, this electroluminescent structure only allows a monochrome display and there are currently no polychrome display devices using the PC-EL effect.

Indeed, the known electroluminescent devices with polychrome display are of two types.

The first solution explored intensively for obtaining polychrome screens consists in developing an electroluminescent phosphor with emission spectrum covering at least red, green and blue and called "white" phosphor, and in combining it with a mosaic of colored filters. to produce the red, green or blue emission pixels, in a manner analogous to polychrome liquid crystal screens. This solution is described in particular in the article by C. Brunel and N. Duruy, Opto, n ° 43, March-April 1988, pp. 30-35, "Color in electroluminescent flat screens" However, the luminance obtained for such polychrome screens is an order of magnitude lower than the levels required for the applications, due to the performance insufficient white phosphorus.

Examples of white phosphorus and their insufficient performance are given in the article SID 88 Digest, p. 293-296 by Shosaku Tanaka et al., "Bright white-light electroluminescent devices with new phosphor thin films based on SrS".

Document FR-A-2362552 also discloses a color screen resulting from the association of a screen emitting in white using a three-color electroluminescent mixture, and red, green and blue chromatic filters.

The second solution consists in using a first substrate comprising EL layers which is made transparent or semi-transparent by an appropriate choice of rear electrodes. To this structure, a second so-called "inverted" substrate is associated, equipped with layers E1 and transparent rear electrodes. The first structure is monochrome or bichrome, and the second structure is monochrome and is complementary to the first. A two-color or three-color display device is thus obtained. This solution is described in the article by Brunel and Duruy above and in the article by Christopher N. King et al., "Full-color 320x240 TFEL display panel", p. 14-17, Eurodisplay, London 15-17 September 1987.

This structure is relatively complex. In addition, the luminance is low for the intended applications and the voltages and electric currents used are relatively high.

In addition, the use of a monochrome display device of the PC-EL type under intense ambient lighting can cause significant degradation of the PC-EL hysteresis. Indeed, the illumination by an intense external source of the photoconductive layer can cause a reduction in the voltage across the latter and therefore a lowering of the ignition voltage. In practice, this leads to an accidental ignition of certain pixels which are normally extinct.

The subject of the invention is therefore a flat polychrome electroluminescent display screen with memory effect, in particular making it possible to remedy these drawbacks.

The flat polychrome display screen according to the invention comprises, on an insulating substrate defining one of the faces of the screen, a single electroluminescent layer and at least one photoconductive layer, these layers being stacked one on the other. , the assembly of these two layers being interposed between a first system of transparent electrodes and a second system of electrodes, connected to electrical means to excite certain zones of the light-emitting layer, and is characterized in that the light-emitting layer is consisting of a white phosphor and in that at least two series of colored filters are interposed between the light-emitting layer and the observer.

By white phosphorus is meant an electroluminescent material emitting at least in blue, red and green.

The polychrome screen of the invention, thanks to the combination of white phosphorus and one or more photoconductive layers, has a high luminance. The PC-EL memory effect makes it possible to increase the excitation frequency of white phosphorus, regardless of the complexity of the screen, for example from 60 Hz to 1 kHz. With the white phosphors of the state of the art (see article above from Shosaku Tanaka), we can then reach 120 Cd / m² for the luminance of the white after filtering (1 kHz), instead of 9 Cd / m² at 60 Hz for a structure without a PC layer and comprising a white phosphorus and colored filters (see article by Brunel and Duruy). The screen of the invention is then compatible with all the applications envisaged.

Furthermore, for each pixel, only a small part of the energy radiated by the electroluminescent layer is used for display due to filtering (<30%) but the entire EL emission spectrum and all the emitted energy is exploitable for effect PC-EL. It is therefore preferable to choose a PC layer with a broad sensitivity spectrum to maximize the PC-EL effect.

The filters of the invention have not only the known role of "coloring" the emission of each pixel but also the advantage of significantly reducing the light intensity of the ambient lighting incident on the PC layer and therefore of avoiding 'accidental lighting of some pixels which are normally off; the hysteresis is then practically insensitive to any ambient light.

The most used photoconductive materials for PC-EL structures are CdS _x Se _1-x , a-Si _1-x C _x : H with x between 0 and 1, CdS, CdSe and a-Si: H.

These materials have narrow sensitivity spectra. Also, the association or stacking of two (or more) photoconductive materials of different composition makes it possible to obtain a photoconductive structure with a broad sensitivity spectrum.

Although the use of a photoconductive structure with a broad sensitivity spectrum is preferred in order to ensure maximum overlap of this sensitivity spectrum with the emission spectrum of white phosphorus, it is possible to use a single photoconductive material with narrow sensitivity spectrum. In this case, the photoconductive material must be chosen so that its sensitivity spectrum is located in the range of wavelengths where the light emitting emission is the most intense, compared to ambient lighting.

Adjustable spectrum photoconductive materials such as CdS _x Se _1-x and a-Si _1-x C _x : H are quite suitable in this case.

For further information on the manufacture and on the properties of hydrogenated and carbonated amorphous silicon, reference may be made to document FR-A-2 105 777 filed in the name of the inventor.

This material is preferably deposited by the low-power plasma-assisted chemical vapor deposition (PECVD) technique (around 0.1 W / cm²). For further details on the a-Si _1-x C _x : H deposition method, reference may be made to the article by MP Schmidt et al., Philosophical Magazine B, 1985, vol. 51, n ° 6, p. 581-589, "Influence of carbon incorporation in amorphous hydrogenated silicon".

For further details on the sensitivity spectra of CdS _x Se _{1-x materials} , reference may be made to the document by Robert et al., Journal of Applied Physics, vol. 48, n ° 7, July 1977, p. 3162-3164, "II-VI solid-solution films by spray pyrolysis".

Preferably, a-Si _1-x C _x : H is used with 0 ≦ x ≦ 1 and for example 0 ≦ x ≦ 0.5. Indeed, this photoconductive material has a number of advantages. In particular, it exhibits a drop in sensitivity on the long wavelength side (that is to say on the low energy side) corresponding to a drop in optical absorption (optical band gap). (Remember that (nm) = 1240 / E (eV)).

A characteristic of the photoconductivity spectrum of this material is the energy E₀₄ (in eV) for which the absorption coefficient is 10⁴cm⁻¹. This energy E₀₄ can be adjusted by varying the carbon content x, that is to say, the methane content C in the methane-silane gas mixture used for the manufacture of this photoconductive material, in other words C = [ CH₄] / [CH₄ + SiH₄].

On the side of short wavelengths (high energies), the sensitivity of the photoconductive material also drops because the radiation is absorbed in all the first layers of the photoconductive layer and photoconduction, sought in the direction normal to the plane of the layers (electrical excitation is prevented because the core of the photoconductive material is not exposed to excitation radiation.

The photosensitivity spectrum resulting from a-Si _1-x C _x : H, for a layer with a thickness of 1 micrometer, is a wide peak whose width at mid-height is approximately 50 nanometers and whose maximum is E₀₄. The width at half height corresponds to the distance separating the low and high cutoff thresholds from the PC material.

The white phosphors which can be used in the invention are those given in the article by Shosaku Tanaka cited above and in the article by Yoshio Abe "Multi-color electroluminescent devices utilizing SrS: Pr, Ce phosphor layers and color filters" to be published in the "Proceedings of the 4th International Workshop on Electroluminescence, Tottori 1988".

Preferably, the following two white phosphors are used because of their increased performance:
SrS: Ce, K, Eu and SrS: Pr, Ce.

The use of white phosphorus SrS: Ce, K, Eu is suggested in the article entitled "Bright white light electroluminescent devices with new phosphor thin films based on SrS" by S. Tanaka, published in SID International Symposium, Digest of Technical Papers , May 1988, pages 293-296.

The colored filters which can be used in the invention must have their transmission spectrum and their coloring spectrum adapted to the emission spectrum of the white phosphorus chosen to obtain the purest red, green and blue components possible.

The colored filters can be interference filters. These filters make it possible to obtain low pass, high pass and band pass spectra with arbitrary cut-off wavelengths. In addition, they exhibit a brutal spectral transition from the state passing to blocking as well as a great chemical and thermal stability. On the other hand, these filters are often expensive. Also, when possible, colored glasses or organic filters are used instead.

Organic filters are in particular those used for polychrome liquid crystal screens such as polymer (or gelatin) layers loaded with dyes or organic pigments; polyimide layers with dyes; organic pigments or dyes evaporated under vacuum: perylene (red), lead phthalocyanine (blue), phthalocyanine copper (green), quinacridone (magenta), isoindolinone (yellow); electroplated pigments.

According to the invention, all known electrode systems for display can be used. In particular, one of the electrode systems can consist of point electrodes and the other system consists of a common electrode. Advantageously, the electrode systems each consist of conductive strips parallel to each other, the conductive strips of the first system being crossed relative to the conductive strips of the second system.

In addition, the device of the invention can operate in reflection or in transmission. Depending on the type of operation used, one or two of the electrode systems may be transparent.

Other characteristics and advantages of the invention will emerge more clearly from the description which follows, given by way of nonlimiting illustration, with reference to the appended FIGS. 2 to 5, FIG. 1 having already been described.

FIG. 2 schematically represents an embodiment of the display device according to the invention.

FIG. 3 gives the appearance of the sensitivity and emission spectra which the photoconductive and electroluminescent layers must have, as well as the transmission spectrum of the filters of the device of FIG. 2.

Figures 4 and 5 show alternative embodiments of the device according to the invention.

In Figure 2, the device according to the invention comprises a first electrode system consisting of conductive strips 30, parallel to each other. These conductive strips 30 are generally reflective and made of aluminum. These electrodes 30 are arranged on a photoconductive layer 32 in a-Si _1-x C _x : H, with 0 ≦ x ≦ 1, 1 micrometer thick covering an electroluminescent structure consisting of a single emitting layer 34, as shown in the Figure 2, or associated with one or more dielectric layers, as shown in Figure 1 or in document FR-A-2 574 972.

The electroluminescent material is in particular one of those mentioned above; its thickness is between 0.5 and 2 micrometers (typically 0.7 m). The dielectric layers 14, 18, 21 possibly associated with the material El can be made of one of the materials chosen from Si₃N₄, SiO₂, SiO _x N _y , Ta₂O₅ and have a thickness of 200 nm.

In order to simplify the drawings and the corresponding description, the rest of the text will relate only to an electroluminescent layer 34 alone.

On the electroluminescent layer 34, there is the second electrode system 36 consisting of conductive strips parallel to each other and made of a transparent ITO material for example, the electrodes 36 being arranged pependicularly to the electrodes 30.

The second system of electrodes 36 is supported by an insulating substrate 38 generally made of glass, provided on its internal face with three series 40, 41, 42 of colored filters respectively red, green and blue. The observation of the display is made by the rear face of the device, that is to say on the side of the substrate 38. Similarly, the ambient lighting strikes the device on the side of the substrate (white lamp 43 for example) .

The filters 40, 41, 42 of the device of the invention allow filtering of the light intensity of the ambient lighting (lamp 43 for example) while coloring the electroluminescent emission of layer 34.

These filters are for example in the form of strips parallel to each other and to one of the electrode systems 30 or 36, the red filters 40, green 41 and blue 42 being alternated.

The device according to the invention functions essentially like the polychrome devices of the prior art and in particular by using peripheral control circuits 45 of the kind used in flat liquid crystal screens; these circuits deliver appropriate alternating signals and are connected to electrodes 36 and 30; the oscillation frequency of the control signals is 1 kHz for example, the 0-peak amplitude is 150 to 300 volts (typically 130 volts).

In part a of FIG. 3, the emission spectrum 44 of ambient light and the emission spectrum 46 of a white phosphorus are shown. On part b of FIG. 3, the transmission spectrum of the filters R (red), green V and blue B is shown. On part c of FIG. 3, the sensitivity spectrum of a broadband photoconductive material (PC) and on part d the sensitivity spectrum of a narrow spectrum photoconductive material.

These spectra give the variations of the light intensity I as a function of the wavelength, the light intensity being given in arbitrary unit and the wavelength in nanometer.

According to the invention, the red R, green V and blue B transmission spectra of the color filters are contained in the emission spectrum of white phosphorus.

In FIG. 3b, the high cut-off frequencies λ _B of the blue filter have been symbolized above which light (ambient + that emitted by white phosphorus) is filtered and below which light is transmitted; the low cut-off frequency λ _V1 of the green filter below which the light is blocked; the high cut-off frequency λ _V2 of the green filter above which the light is blocked and the low cut-off frequency λ _R of the red filter below which the light is blocked. These cut-off wavelengths correspond to 50% of the transmitted light intensity.

The use of colored filters with distinct transmission spectra with a small overlap area, that is to say corresponding to λ _B <λ _V1 <λ _V2 <λ _R , makes it possible to filter part of the ambient light thus rendering the hysteresis of the luminance-voltage curve of the PC-El structure practically insensitive to ambient lighting.

The photoconductive material can be a photoconductive material with a broad sensitivity spectrum (FIG. 3c) which allows maximum overlap with the emission spectrum of white phosphorus. This corresponds to a low cutoff wavelength of the photoconductor λ₁ close to that λ blanc of the white phosphorus and to a high cutoff wavelength λ₃ of the photoconductor close to that λ₄ of the white phosphorus. λ₀₄ corresponds to the maximum sensitivity wavelength of the photoconductive material.

The photoconductive material can also be a material with a narrow sensitivity spectrum (FIG. 3d), this spectrum then being located in a region where the light intensity of the electroluminescent emission is higher than that of ambient light; the PC spectrum can be located in blue as symbolized by curve 48 or in deep red, as symbolized by curve 50. The wavelengths of low and high cutoffs and maximum sensitivity are respectively λ′₁, λ′₀₄, λ′₂ and λ˝₁, λ˝₀₄, λ˝₂ for curves 48 and 50. In particular, λ′₂ is chosen less than λ _B and conversely λ˝₁ is chosen to be greater than λ _R.

The different layers constituting the display screen of the invention can be arranged in different ways as shown in Figures 4 and 5. The only requirement is that the filters 40, 41, 42 are arranged between the observer and the electroluminescent layer 34.

Also, as shown in Figure 4, it is possible to reverse the position of the filters and electrodes 36 relative to Figure 2; the colored filters are placed between the second series of electrodes 36 and the electroluminescent structure 34. In this embodiment, the filters can be deposited by electrodeposition; they then take the form of strips parallel to the electrodes 36. In order to better see this arrangement, the directions of the electrodes 30 and 36 of FIG. 4 have been reversed with respect to FIG. 2.

Compared to the embodiment of FIG. 2, it is also possible to reverse the position of the glass substrate 38 with the filters. However, the corresponding screen is subject to parallax effects unless the substrate is thin, that is to say of the order of 0.1 mm.

It is also possible, as shown in FIG. 5, to reverse the location of the two electrode systems. In this case, the observation is made from the front face of the display screen. In this embodiment, there are, from top to bottom, the color filters 40, 41, 42, the transparent electrodes 36, the electroluminescent structure 34, a first layer photoconductive 32a and a second photoconductive layer 32b, the reflecting electrodes 30 and finally the glass substrate 38. Again, the filters can be deposited by electrodeposition.

The use of the two photoconductive layers 32a, 32b makes it possible to obtain a photoconductive structure with a wide sensitivity band. Of course, this stack of PC layers can be used in the other embodiments of FIGS. 2 and 4.

For observation from the front, it is also possible to reverse the arrangements of the color filters 40, 41, 42 and the electrodes 36.

It is also possible to use only two sets of colored filters, green and red for example. This gives a two-color screen and not a three-color screen.

Various embodiments of the screen according to the invention are given below. In these examples, the electroluminescent material is a-Si _1-x C _x : H, with 0 ≦ x ≦ 1.

Example 1

In this example, a single layer of photoconductive material having a narrow sensitivity spectrum (FIG. 3d, curve 48), located in blue, is used.

Color filters are interference filters; the blue filter has a high cut-off wavelength λ _B = 500 nm, the red filter has a low cut-off wavelength λ _R = 600 nm and the green filter has low cut-off wavelengths λ _V1 and high λ _V2 of 500 and 600 nm respectively.

The photoconductive material a-Si _1-x C _x : H of 1 ”m thickness has a wavelength of maximum sensitivity λ′₀₄ <480 nm (that is to say <λ _B ) which corresponds at E′₀₄ ≧ 2.58 eV and therefore at a methane concentration C ≧ 0.85 and therefore at x ≧ 0.22.

The electroluminescent material is SrS: Ce, K, Eu or SrS: Pr, Ce with a thickness of 1 ”m.

Example 2

It differs from Example 1 by the use of a photoconductive material having a narrow sensitivity spectrum located in deep red.

This material a-Si _1-x C _x : H has a wavelength of maximum sensitivity λ˝₀₄> 625 nm, that is to say> λ _R , which corresponds to E˝₀₄ ≦ 2.0 eV and therefore at a concentration C ≦ 0.30 and at x ≦ 0.03.

Example 3

In this example, we use a photoconductive structure composed of two superimposed PC layers and of different composition (Figure 5), thus resulting in a PC structure with a broad sensitivity spectrum (Figure 3c).

The first photoconductive material (32a) has a wavelength λ₀₄₁ of 600 nm, which corresponds to E₀₄₁ = 2.07 eV and therefore to C = 0.40 and x = 0.04.

The second photoconductive material (32b) has a wavelength λ₀₄₂ of 500 nm, which corresponds to E₀₄₂ = 2.48 eV and therefore to C = 0.80 and x = 0.20.

In the embodiments represented in FIGS. 2 and 4, the colored filters based on gelatin or on polymer conventionally used are to be discarded since these filters are deposited before the electroluminescent and photoconductive materials, during the manufacture of the screen , and therefore that they undergo restrictive thermal cycles, typically from 150 to 200 ° C; these filters only support temperatures <100 ° C.

Claims (12)

1. Flat polychromatic electroluminescent display screen with a PC-EL memory effect comprising on an insulating substrate (38) defining one of the faces of the screen, a single electroluminescent layer (16, 34) and at least one photoconductive layer (20, 32, 32a, 32b), said layers being stacked on top of one another, the assembly of said two layers being interposed between a first transparent electrode system and a second electrode system connected to the electrical means (45) in order to excite certain zones of the electroluminescent layer, characterized in that the electroluminescent layer (34) is constituted by a white phosphor and that at least two series of coloured filters (40-42) are interposed between the electroluminescent layer (34) and the observer, said filters ensuring the polychromatic display and serving as a screen making it possible to reduce the light intensity of the incident ambient illumination on the photoconductive layer and preventing the accidental lighting up of certain normally extinguished pixels.
2. Flat screen according to claim 1, characterized in that the coloured filters (40-42) are positioned between the insulating substrate (38) and the first electrode system (36) facing said substrate, which is then transparent.
3. Flat screen according to claim 1, characterized in that the filters (40-42) are located on the first electrode system (36) (Fig. 5) and constitute the other face of the screen.
4. Flat screen according to any one of the claims 1 to 3, characterized in that the electroluminescent layer (16) is placed between a first (14) and a second (18) dielectric layer.
5. Flat screen according to any one of the claims 1 to 4, characterized in that a dielectric layer (21) is located between the PC layer (20) and the facing electrode system (30).
6. Flat screen according to any one of the claims 1 to 5, characterized in that the electrode systems (30, 36) are in each case constituted by parallel conductive strips, the conductive strips of the first system crossing the conductive strips of the second.
7. Flat screen according to claim 6, characterized in that it comprises three series of filters, respectively blue, red and green, formed from strips parallel to the conductive strips of the first (36) or second (30) electrode systems.
8. Flat screen according to any one of the claims 1 to 7, characterized in that the photoconductive layer (32a, 32b, 32) is of carbonated or hydrogenated amorphous silicon of formula a-Si_1-xC_x:H with 0< x < 1.
9. Flat screen according to any one of the claims 1 to 8, characterized in that the white phosphor is chosen from among SrS:Ce,K,Eu and SrS:Pr,Ce.
10. Flat screen according to any one of the claims 1 to 9, characterized in that it comprises several stacked photoconductive layers (32a, 32b).
11. Flat screen according to claim 1, characterized in that the filters (40-42) are electrodeposited on the first electrode system (36).
12. Flat screen according to any one of the claims 1 to 11, characterized in that the second electrode system (30) is reflecting.

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