FR2671218A1 - Electroluminescent display device with memory and with several tints - Google Patents

Abstract

This electroluminescent display device with memory effect comprises, on an insulating substrate (10), a stack of active layers including at least one electroluminescent layer (16) and at least one photoconducting layer (20), a first (12) and a second (22) systems of orthogonal electrodes arranged on either side of the stack, an image element (I) being defined by the overlap of an electrode of the first system and of an electrode of the second system, means (23) for applying appropriate control voltages to the electrodes; it is characterised in that at least one of the layers (16) of the stack has modulation of one of its characteristics in such a way as successively to light different parts (I1, I2) of each image element as a function of the voltage applied and thus to obtain a display with several tints.

Description

ELECTROLUMINESCENT MEMORY DISPLAY DEVICE
AND HAS SEVERAL SHADES
DESCRIPTION
The present invention relates to an electroluminescent display device with an optical memory effect, making it possible to obtain a color scale while retaining the advantages of the memory effect. It finds an application in the field of optoelectronics for displaying complex images or alphanumeric characters in the same color range and more particularly in the range of gray.

 A local optical memory device has a certain number of advantages such as those described in document FR-A-2 574 1972. This local memory device comprises a photoconductive structure (PC) connected in series with a light-emitting structure (EL). both structures being optically coupled. The resulting memory effect is called the PC-EL effect.

 This type of PC-EL device has, compared to the conventional electroluminescent display, a reduction in power consumption, a decrease in current peaks, the possibility of increasing the number of points without loss of contrast thus allowing a higher resolution. high and / or larger screen size.

 In known PC-EL devices, the display is of the "all or nothing" type, which does not make it possible to obtain different levels of gray.

The operating principle of these devices is shown in Figure 1 which gives a hysteresis curve typical of the luminance-voltage response of a PC-EL memory device.

When the device is in the off state, corresponding to the luminance L2, the photoconductive material is low conductor and retains a significant portion of the voltage V applied to the assembly. If you increase for a short time
The voltage up to a value V2 such that the voltage present at the terminals of the electroluminescence structure exceeds the threshold of electroluminescent, the device
PC-EL switches to L1 on state. The photoconductive material is then illuminated by the electroluminescent structure and goes to the conductive state. The voltage at its terminals drops and this results in an increase in the voltage available to the electroluminescent structure. We then return to the voltage Vo.

 To turn off a PC-EL device, simply lower the total voltage momentarily to a value V? less than V2 then return to Vo.

 Thanks to the hysteresis of the luminance-voltage characteristic, the application of a so-called maintenance voltage V 0, intermediate between V1 and V2, makes it possible to maintain the previously displayed or extinguished state.

Although the display device has effect
Known PC-EL has many advantages, these devices do not allow the display of several shades and in particular of several gray levels.

 As a known gray-scale electroluminescent display device, mention may be made of the one described in document FR-A-2 580 848. In this device, the different gray levels are obtained by using, for each image point, electrodes of control of the electroluminescent layer formed of strips of different widths and different electroluminescent materials.

 These devices have no intrinsic memory effect and therefore do not have all the advantages of a local optical memory.

 The subject of the invention is precisely an electroluminescent display device with a memory effect which makes it easy to obtain a display with several shades and in particular with several gray levels.

 The invention relates to a memory effect electroluminescent display device comprising, on an insulating substrate, a stack of active layers comprising at least one electroluminescent layer and at least one photoconductive layer, a first and a second orthogonal electrode system disposed of both sides of the stack, an image element being defined by the overlap of an electrode of the first system and an electrode of the second system, means for applying appropriate control voltages to the electrodes, characterized in that at least one of the layers of the stack has a modulation of one of its characteristics so as to successively light up different parts of each picture element according to the applied voltage and thus obtain a display with several tints.

 By several shades, it is necessary to hear several shades of the same color such as several gray, several green, several red.

 The stack of layers may advantageously comprise an upper layer and a lower layer of dielectric, disposed on either side of the electroluminescent layer.

 According to the invention, the layer having the modulation of one of its characteristics may be the electroluminescent layer, the photoconductive layer or one of the dielectric layers disposed on either side of the electroluminescent layer.

 The modulated characteristic can between the composition and the thickness of one of these layers or the "doping" of the electroluminescent layer (concentration of phosphor ions).

 In addition, the modulation may relate to one or more active layers of the display device.

The electrode systems each consist of conductive strips parallel to each other,
The conductive strips of the first system being crossed with respect to the conductive strips of the second system. Depending on the type of operation used, in reflection or in transmission, one or two of the electrode systems must be transparent.

 In order to have a very inhomogeneous applied field, notably resulting in successive ignitions of different parts of the picture element, it is also possible to change the shape of the electrodes. For example, electrodes formed of bands of different widths may be used, so that the regions near the thin bands, corresponding to a higher electric field, will light up before the regions near the wider bands. It is also possible to use electrodes having pointed portions so that the "peak" effect gives higher field regions than others.

 According to the invention, it is possible to obtain more than two stable luminance states and thus create a gray scale. By dividing the image element into as many parts as desired, it is possible to obtain as many gray levels as desired. Better still, by producing in each image element, a property gradient of the modulated active layer, it is possible to vary the gray levels in a continuous manner.

As a photoconductive material that may be used in the invention, mention may be made of CdS Se or
x 1-x hydrogen and carbon amorphous silicon of formula a-Si C: H, with x ranging from 0 to 1. These materials have
1-xx the advantage of presenting an optical spectrum of adjustable sensitivity.

 Their composition can therefore be adapted according to the chosen electroluminescent material. In addition, these materials can be used when the modulation relates to the composition of the photoconductive layer.

Preferably, a-Si C: H is used with
1-xx 0 <x <0.5.

The electroluminescent materials that can be used in the invention can be broadband and spectrum-determined materials such as for example
2+
ZnS: Mn of relatively narrow emission band and
2+ located in yellow and orange; the CaS: Eu to
2+ red dominance; the SrS: Eu dominant going from
3+ red to orange; CaS: This predominantly from
3+ green to orange; SrS: This predominantly from blue to green.

 These materials are in particular used when the modulated characteristic is the thickness of one of the active layers of the stack or when it concerns the composition of the photoconductive material.

When the modulation concerns the composition of the electroluminescent layer, broadband electroluminescent materials can be used for
Which emission spectrum can be modified as
2+ for example Ca Sr S: Eu with x ranging from 0 to 1, La
x 1-x dominant for x = 1 being red and for x = O,
3+ orange; Ca Sr S: Ce with x ranging from 1 to 8, x = 1
x 1-x corresponding to a green dominant and x = O to a blue dominant.

It is also possible to mix two phosphor activators in a matrix to adapt the broad emission band of the electroluminescent material; the spectrum obtained is then a combination of the elementary spectra of the two activators; examples are:
2+ 3+ 2+ 3+ 3+ 3+
SrS: Eu, Ce; CaS: Eu, Ce; SrS: This, Pr
It is also possible to use materials with several narrow bands or rays, for example
3+
ZnS: Sm predominantly red; ZnS: Tb at a dominant
3+ green and a dominant green-blue; ZnS: Tm predominantly blue and near infrared (780 nm); 3+
SrS: Pr with two dominant ones, one in the red, one in the blue-green. One can also use alloys such 3+ 3+ 3+ as Zn Sr S: Tb; Zn Ca S: Tb; Sr Ca S: Tb
x 1-xx 1-xx 1-x with x ranging from 0 to 1

 The modulation may furthermore relate to the phosphor ion content of the electroluminescent layer.

 By playing on the nature of the phosphor ions and on their contents, we actually play on the composition of the electroluminescent layer.

The dielectric layers are for example in
SiO N with Ocx <2 and O <y <4/3 when modulation
xy concerns the composition of one of the dielectric layers.

Other characteristics and advantages of the invention will emerge more clearly from the following description, given by way of illustration and without limitation, with reference to the appended drawings, in which:
FIG. 1, already described, schematically represents the luminance-voltage curve of a PC-EL display device according to the prior art,
FIG. 2 diagrammatically represents an embodiment of the display device according to the invention,
FIG. 3 gives the luminance and voltage curve (LV) of the device of FIG. 2,
FIG. 4 diagrammatically represents another embodiment of the device according to the invention,
FIG. 5 gives the luminance and voltage curve (LV) of the device of FIG. 4,
FIG. 6 illustrates the method for manufacturing the thickness modulation of the upper dielectric layer of the device of FIG. 4,
FIG. 7 gives the luminance and voltage curve (LV) of another embodiment of the device of the invention,
FIG. 8 diagrammatically represents the embodiment of the device of the invention whose operation is that of FIG. 7, and
FIG. 9 illustrates the method for manufacturing the thickness modulation of the upper dielectric layer of the device of FIG. 8.

 In Figure 2, there is shown a first embodiment of the device according to the invention. This device comprises a glass substrate 10 on which are deposited in order, a first electrode system consisting of conductive strips 12 parallel to each other, a first dielectric layer 14, an electroluminescent layer 16, a second dielectric layer 18, a photoconductive layer 20 and a second electrode system 22 consisting of conductive strips 22 parallel to each other and perpendicular to the conductive strips 12.

The electrodes 22 are generally reflective and made of aluminum; they are arranged on a photoconductive layer made of a-Si 1-x C: H with
x 0 <x <0.5 of about 1 micrometer thick. The dielectric layers 14 and 18 have a thickness of about 200 nm and are made of one of the materials selected for example from Si N, SiO 2, SiO N with
xy O <x <2 and O <y <4/3.

 The second electrode system consists of a transparent material and in particular ITO (indium and tin oxide).

The electroluminescent layer 16 is
2+ ZnS: Mn
An electrical control circuit 23 connected to the electrode systems 12 and 22 allows the application of appropriate control voltages and a lamp 25 placed opposite the substrate represents ambient illumination.

 According to the invention, the layer 16 in this embodiment comprises a thickness modulation. In particular for each image point I, defined by the intersection of an electrode 12 and an electrode 22, the layer 16 has two thicknesses e1 and e2 with e1 <e2.

The layer 16 has a square-wave modulation, the recessed portions and the overhanging portions being, for example, of the same width. In particular, the thickness e1 is equal to 400 nm and the thickness e2 is equal to 800 nm.

 Due to the crenellated shape of the layer 16, the upper layers 18 and 20, of constant thickness, also have a crenellated form.

 Thus, each image point is divided into two parts II and I2 respectively corresponding to the two thicknesses e1 and e2 of the electroluminescent layer 16, which can be controlled independently.

 Indeed, for each image element, the application of an increasing voltage makes it possible to turn on successively the two parts I1 and I2 of the image point. This system makes it possible to obtain two lit states, or two gray levels, and a state that is off, stable after addressing.

 In FIG. 2, the hatched portion 2 times of the layer 16, to the left of the figure, indicates a half-lit image point and the twice hatched portion of your layer 16, on the right of the figure, represents a point image completely lit.

 FIG. 3 shows the luminance-voltage curve (L-V) of the device of FIG. 2.

For a hold voltage VO, the picture element can have the three luminance states L1 (off), L2 and L3 (on) with L1 <L2 <L3. Starting, for example, from the off state L1, the application for a short time of an intensity ignition signal V2> VO and return to VO maintains the element in the state of luminance L2 because only Part II of the picture element has been excited (V2 is the ignition voltage of half-point I1). For a stronger applied addressing voltage, of intensity V3> V2 for a short time, the two parts II and I2 of the display element are excited and after returning to VO, the equilibrium luminance
L3 is higher (V3 is the ignition voltage of
I1 + I2). The extinction is obtained by applying a voltage V1 <VO for a short time then return to VO.

 In this embodiment, the ignition voltage has been varied to obtain two gray levels.

 According to the invention, it is also possible to vary the extinction voltage of the device of the invention and to modulate the luminance in the state illuminated during the extinction phase. This is achieved using the device shown in FIG. 4.

 This device is distinguished from the device of FIG. 2 only by the fact that the electroluminescent layer 16a is here of constant thickness of approximately 800 nm and that it is the upper insulating layer 18a which comprises a modulation of thickness.

Here, the layer 18a comprises a square-wave modulation and therefore comprises at each image point I a thickness E1 and a thickness E2 with E1 <E2, for example Ex = 150 nm and E2 = 300 nm. The other layers and their composition are identical to those of the device of FIG.

 The part I1 corresponds to the thickness E1 and the part 12 corresponds to the thickness E2 of the dielectric layer 18a.

 FIG. 5 shows the luminance-voltage curve (L-V) of the device of FIG. 4.

In this figure, the voltages satisfy the following conditions: V1 <V2 <VO <V3 <V4, VO being the holding voltage.

 Starting from the lit state L3 (completely lit point), the application for a short time of a voltage V2 then return to VO allows the maintenance of the element (part II) in the luminance state L2 (half - lit) and the application of a voltage V1 for a short time then return to VO allows the maintenance of the element in the state off L1.

 FIG. 6 shows the method for obtaining the thickness modulation layer 18a of FIG. 4. First, the layer 18a is deposited on the electroluminescent layer 16a over a thickness of homogeneous equal to the maximum thickness E2 and then covering the whole of a layer of photoresist 24. The latter is insolated in places where the thickness of the layer 18a must be reduced and then revealed.

 Partial chemical etching 26 of the layer 18a makes it possible to remove a portion of the material from the layer 18a, with the consequence of reducing the thickness of this layer in the exposed areas.

The chemical attack is carried out until
The desired thickness E1 for the layer 18a. Finally, the resin layer is completely removed. The deposition of the upper layers is then done according to the prior art.

 The invention is not limited to obtaining two levels of gray and by dividing the image element into more than two parts, it is possible to obtain as many gray levels as desired.

This is shown in FIG. 7, where the luminance-voltage hysteresis curve is such that, following the write voltage V2, V3 or V4, etc., a luminance L2 is obtained after returning to the holding voltage VO.
L3, L4 which increases with the pulse of the write voltage, which is exactly compatible with the general standards of the video display. The application of a voltage V1 makes it possible to obtain the off state
L1. Here, the voltages satisfy the following conditions Y1 <VO <V2 <V3 <V4.

 This can be achieved by continuously modulating, as shown in FIG. 8, the thickness of the upper insulating layer 18b. In this embodiment, the thickness of the second insulator layer 18b varies continuously between two extreme values I1 and I2, typically from 100 nm to 300 nm, for each image point I.

 The method for obtaining such a layer 18b is illustrated in FIG. 9. In particular, the layer 18b is deposited by evaporation 28 through a mechanical mask 30 placed at a certain distance (typically 1mm) from the substrate.

 A device similar to that of FIG. 2, but where the thickness of the photoconductive layer 20 and not that of the electroluminescent layer is modulated, will have the same coaport as that described in FIG. 3. This layer 20 is a layer of silicon alloy and hydrogenated amorphous carbon, α-Si C: H with 0 <x <0.5.

1-xx
The voltage at the terminals of the photoconductive layer obeys the laws of "space-charge limited conduction" as described in the article OPTOELECTRONICS-DEVICES and TECHNOLOGIES, "Electroluminescent memory display using amorphous silicon-carbon alloys" by I. SOLOMON and P. THIOULOUSE, pp. 295-316 of December 1989, vol. 4, n02. The injection of electrons into the PC layer causes:
the trapping of a part of these electrons in states of the forbidden band (space charge);
the creation of a potential barrier opposing a new injection;
the need to exceed a voltage threshold in order to be able to inject new electrons, this threshold voltage corresponds to the voltage at the terminals of the PC layer in a PC-EL device in operation.

 If the PC layer is exposed to light, a portion of the trapped electrons are released by optical excitation, causing a lowering of the potential barrier and a decrease in the voltage across the PC layer.

In these conditions :
- the voltage across the PC layer in the dark, and therefore the ignition voltage of the PC-EL device (on ignition, the device is initially in the off state) can be modulated by changing the thickness of the PC layer, the voltage across the PC layer increasing with the thickness;
the voltage at the terminals of the PC layer under illumination, and consequently the extinction voltage of the PC-EL device can be modulated by modifying the carbon content of the PC layer: by increasing the carbon content, the sensitivity is decreased from the layer to the light and the voltage is increased at the terminals of the PC layer under illumination.

Thus, the thickness of the PC layer plays mainly on the voltage at the terminals of the layer
PC in the dark and little on its voltage under illumination. Therefore, by modulating the thickness of the PC layer, the ignition voltage V2 is modulated and the extinction voltage V1 is practically not adjusted.

The method of carrying out the thickness modulation described in FIG. 6 remains valid for the PC layer.

Claims (7)

 1. A memory effect electroluminescent display device comprising, on an insulating substrate (10), a stack of active layers comprising at least one electroluminescent layer (16, 16a) and at least one photoconductive layer (20), a first (12) and a second (22) orthogonal electrode system disposed on either side of the stack, an image element (I) being defined by the overlap of an electrode of the first system and an electrode of the second system means (23) for applying suitable control voltages to the electrodes, characterized in that at least one layer (16, 18a, 18b) of the stack has a modulation of one of its characteristics. in order to successively light different parts (11, 12) of each picture element according to the applied voltage and thus obtain a multi-tone display.  2. Device according to claim 1, characterized in that the modulated characteristic is the thickness or the composition.  3. Device according to claim 1 or 2, characterized in that the electroluminescent layer is interposed between a lower layer (14) and a top layer (18, 18a, 18b) of dielectric.  4. Device according to any one of claims 1 to 3, characterized in that the layer having a modulation of one of its characteristics is the electroluminescent layer (16).  5. Device according to claim 3 or 4, characterized in that the layer having a modulation of one of its characteristics is one of the dielectric layers (18a, 18b).  6. Device according to any one of claims 3 to 5, characterized in that the layer having a modulation of one of its characteristics is the upper layer (18a, 18b) of dielectric.  7. Device according to any one of claims 1 to 6, characterized in that the modulation of the characteristic consists of a gradual variation (18b) of said characteristic, from one side to the other of the modulated layer.