WO1998031057A1 - Thin-layered electroluminescent display device with alternating excitation and method for producing same - Google Patents

Abstract

The invention concerns an electroluminescent display device comprising an electroluminescent material consisting of a polymer deposited in a thin film (15) and at least two electrodes (12, 13) for applying an alternating electric field to said electroluminescent material, the two electrodes (12, 13) and said electroluminescent material (15) being deposited on the same face of an insulating substrate (11).

Description

 THIN FILM AND ALTERNATE DRIVE DISPLAY DEVICE AND METHOD FOR PRODUCING THE SAME

PRODUCTION

The present invention relates to an electroluminescent display device in thin layer and with alternating excitation and its production method.

Flat display screens can be used as display devices in many fields. They can be made using different techniques. We thus find liquid cry screens, plasma screens, and electroluminescent screens. The electroluminescent screens have, compared to the other types of flat screens, the advantage of implementing a very solid technique. The poin ^~ of view viewing the picture elements (or pixels) of an electroluminescent screen are sharp and the contrast is excellent with a very wide viewing angle.

Information is displayed equally, notably for automobile dashboards, indicator screens in stations and airports, displays for certain portable instruments, by the use of a colored layer serving to light mask. The display is made from lamps placed at the back of the screen. Such a structure requires complex mechanical assembly because of the fragility of the elements to be assembled and the difficulty resulting from assembly. It would therefore be advantageous to have another display device, in particular of the electroluminescent type for the qualities listed above. Figure 1 shows, schematically, an electroluminescent device according to the known art. This device comprises a transparent substrate 1, made of glass or plastic, which supports a transparent electrode 2, for example made of indium oxide. A metal electrode 3, for example made of aluminum, is placed opposite the transparent electrode 2. The space between the electrodes 2 and 3 is filled with a luminescent semiconductor material 4 which may be a polymer. The device shown operates under direct voltage, the transparent electrode 2 playing the role of anode and the electrode 3 playing the role of cathode. When the device is switched on, the indium oxide constituting the electrode 2 is positively polarized. Positive charges (holes) and negative charges (electrons) are injected into the polymer. These charges recombine to form an excited state (exciton) which returns to the ground state by emitting a photon. The color of the light emitted in the direction indicated by an arrow in Figure 1 depends on the structure of the polymer. The choice of the phosphor determines the emission wavelength. The electroluminescent polymer can be polyvinylcarbazole doped with a dye such as coumarin 515, as described in the article "Blue light-emitting diodes with doped polymers" by E. GAUTIER et al., Published in Synthetic Metals, pages 197 -200, vol. 81, 1996.

Light-emitting devices having a superposition of layers close to that of FIG. 1 and excited by an AC voltage are also known.

This device of the known art, where the electroluminescent material is incorporated between control electrodes, requires precise control of the thickness of the electroluminescent polymer. In addition, the cathode (the metal electrode) must generally be deposited under vacuum.

The present invention has been designed in particular in order to have a simple technique for producing electroluminescent display devices. This display device comprises, on an insulating substrate, at least one pair of electrodes arranged laterally, preferably interdigitated and each delimiting a chosen pattern. Preferably, at least the space between the electrodes of said pair is covered with an electroluminescent layer of organic semiconductor polymer, optionally containing a dye. An alternating voltage applied between the electrodes of the same pair creates, in the electroluminescent material, an electric field parallel to the substrate. The invention allows easy manufacture since it requires only a conductive deposit, etching of the electrodes and a deposit of electroluminescent material of non-critical thickness by screen printing or any other painting technique. The electroluminescent material is ionizable under field, which suppresses electrochemical reactions at the interfaces, cause of degradation. The invention therefore relates to an electroluminescent display device comprising an electroluminescent material consisting of a polymer deposited in a thin layer and at least two electrodes allowing the application of an alternating electric field to said electroluminescent material, characterized in that the two electrodes are arranged one next to the other, leaving a space between them, this space being occupied by said electroluminescent material.

Preferably, each electrode having the shape of a comb, the electrodes are arranged so mter-digitée, said space being constituted by the lace path defined by the teeth of the comb-shaped electrodes.

The device may include several pairs of electrodes, each pair of electrodes forming an electroluminescent display unit. In this case, each electroluminescent display unit can have a particular electroluminescent material so that the display device constitutes a polychrome screen.

The invention also relates to a method for producing a thin-film electroluminescent display device, characterized in that it comprises the following stages: electrodes intended to allow the application of an electric field to an electroluminescent material,

- The deposition, on said face of the support, of a layer of polymer constituting the electroluminescent material so that said applied electric field causes an electrolummescence effect in said electroluminescent material.

Preferably, the deposition of the two electrodes comprises a phase of deposition of a continuous conductive layer and a phase of etching of said continuous conductive layer to form the two electrodes.

The invention will be better understood and other advantages and features will appear on reading the description which follows, given by way of nonlimiting example, accompanied by the appended figures among which:

FIG. 1 is a schematic view in cross section of an electroluminescent display device according to the prior art, FIG. 2 is a schematic view in cross section of an electroluminescent display device according to the present invention,

FIGS. 3A to 3E are illustrative of an embodiment method according to the present invention, making it possible to obtain the electroluminescent display device represented in FIG. 2,

FIG. 4 represents an electroluminescent device according to the present invention and comprising two mter-digested electrodes, each electrode having the shape of a comb,

- Figure 5 shows a polychrome electroluminescent screen according to the present invention. The electroluminescent device of FIG. 2 represents a basic structure of the present invention. It comprises a support 11, a first electrode 12 and a second electrode 13 deposited on the support 11. The electrodes 12 and 13 are arranged one next to the other and form between them a channel 14 whose width is for example on the order of 1 µm. A layer of luminescent material 15 is then deposited on the structure by covering the electrodes 12 and 13 and the channel 14. This luminescent material is a semiconductor polymer deposited by the wet process, that is to say in a solvent, the solvent being then evaporated either naturally or forcibly, by centrifugation.

The device of Figure 2 operates on alternating current. The voltage applied between the electrodes 12 and 13 has a frequency chosen from the range from 50 to 5000 Hz. The injection voltage varies linearly with the width of the channel 14.

Under the effect of a sufficiently high electric field, induced by the voltage applied between the electrodes 12 and 13, charges of opposite signs (positive and negative) are spontaneously created in the polymer: this is ionization under the field. This electric field is of the order of 600 V / μm in the case of a polymer composition consisting of polyvinylcarbazole and coumarin 515 at a rate of 10% by mass relative to the polymer. In each of the geometric configurations, and in each particular polymer considered, it is possible to define an electric field threshold from which this ionization phenomenon appears. The electrical charges created in the polymer are distributed on both sides of the channel: the positive charges on one side and the negative charges on the other according to the sign of the applied voltage. As the device is supplied with alternating voltage, the applied voltage changes sign and the charges are then conducted, under the effect of the electric field, towards the opposite side of the channel compared to that which they occupied previously. The charges of opposite signs cross and can then recombine, as in the devices of the prior art, to form an excited state (exciton) which returns to the fundamental state by emitting a photor. The color of the emitted light depends on the polymer composition. This composition can be either a polymer in solution in which a dye is dissolved (the phosphor) or a polymer on which the dye is chemically bonded (by organic synthesis). In the case already mentioned of the polyvinylcarbazole-coumarin polymer composition 515, x vinylcarbazole links can be linked to y coumarin links, x + y being equal to 1 and y possibly ranging from 0.03 to 0.3.

The light generated by electroluminescent effect is emitted perpendicular to the support 11. If this support is transparent, light is emitted in both directions as shown by the arrows drawn in Figure 2.

The support 11 can be a glass or plastic substrate. If this support is not transparent, light is emitted only by the transparent polymer face. The polymer must be transparent to the light emitted to avoid reabsorption. For example, polyvmylcarbazole absorbs in ultraviolet. The dye dissolved or bonded to this polymer determines the color of the light emitted. Coumarin emits in blue at 490 nm, rhodamme 6G emits in orange at 55C nm and DCM emits in red at 620 nm.

As a transparent support, ordinary glasses are suitable, as well as polycarbonate. The thickness of a rigid support can be from 1 to 5 mm. Flexible supports can also be used. It may then be polycarbonate or polyethylene film of 0.1 mm. The consumption of the device is linked to the resistance and capacity characteristics of the circuit. For a 1 cm long channel, the consumption of the device is typically 10.

A typical embodiment of this circuit will now be described in relation to FIGS. 3A to 3E. The support 11 is for example an ordinary, rectangular glass plate (20 mm × 40 mm) and 1 mm thick, previously degreased and cleaned of any organic impurity in an ultrasonic bath containing a detergent. On the support 11 is deposited (see Figure 3A) a thin layer 10 of aluminum of 120 nm thick by evaporation under vacuum to a pressure of 1,33.10 ^-4 Pa (10 Torr ^_δ), has a speed of 4 nm per second. During evaporation, the support is maintained at a temperature above 100 ° C in order to ensure good contact of the aluminum on the glass by eliminating all traces of water vapor. Then deposited on the thin aluminum layer 10, a layer 20 of photosensitive positive resin in ultraviolet light. It may be a resin from the Microposit STR 1000 series from SHIPLEY, deposited by centrifugation at 1000 revolutions / minute. Its thickness is 400 nm.

As shown in FIG. 3B, the resin layer 20 is exposed by a light beam delivered by an argon laser of 10 μ power at 364 nm before the objective of the focusing microscope on the resin layer. The focal spot is 500 nm in diameter and the laser beam scans the surface of the resin layer at a speed of 300 μm / second. The scanning determines the shape of the etched part 21 of the resin which will provide a channel in the present case, but which could provide other shapes such as a comb as will be seen below. The insolated resin is developed for 30 seconds in a bath of Microposit MF-319 (SHIPLEY). The structure shown in FIG. 3C is obtained, where the resin layer 20 has an etched zone 22. In the next step, the part of the aluminum layer not covered with resin is then chemically attacked in phosphoric acid to 50 ° C for 90 seconds. At the end of this step, the initial aluminum layer is transformed into two electrodes 12 and 13 arranged side by side and separated by the channel 14 of 1.2 μm in width as can be seen in FIG. 3D.

The excess resin is then dissolved in ordinary acetone. The electroluminescent material can then be deposited. For this, a drop of polymeric composition, comprising polyvmylcarbazole at 35 g / 1 in chlorobenzene and coumarin 515 at a rate of 10% by mass relative to the polymer, is deposited by centrifugation at 1000 rpm on channel 14. This formulation leads to a polymer thickness of 250 nm (see Figure 3E). The electrical contacts with the electrodes can be obtained by means of a conductive resin (silver paste). In this device according to the present invention, no upper metal counter-electrode is used as for the prior art (the electrode 3 of FIG. 1). This electrode, obtained by evaporation under vacuum, is always difficult to produce In order to produce a display screen of "macroscopic" dimensions (several cm ² for example), it is advisable to give the electrodes the shape of combs and to arrange them so as to nest them one inside the other, each tooth of a comb-shaped electrode being between two consecutive teeth of the other comb-shaped electrode. This is what is shown in Figure 4 which is a top view of such a device. This figure shows a support 31 supporting two electrodes 32 and 33 in the form of a comb. Each tooth 34 of the electrode 32 (except the last) is between two consecutive teeth 35 of the electrode 33. Conversely, each tooth 35 of the electrode 33 (except the last) is between two consecutive teeth 34 of the electrode 32. The length of the channel 36 between the electrodes 32 and 33 is thus considerably increased since this channel is now formed by the yaw path defined by the interdigitated electrodes 32 and 33. The electrodes 32 and 33 are advantageously extended through contact zones 38 and 39, respectively, to allow the application of an electric field. The distance between two adjacent teeth 34 and 35 is close to 1 μm. The electroluminescent polymer 37 is simply painted over the combs.

Once the metal electrodes have been engraved according to the chosen pattern, the polymer can be painted without special precautions on the electrodes. The thickness of the electroluminescent polymer is indifferent as soon as it exceeds the thickness of the electrode, which is always the case by screen printing. It is also possible to deposit a drop of polymer in solution, the volatile solvent of the solution evaporating spontaneously. Etching of electrodes by UV micro-lithography through a mask is also possible (see the book "Positive photoresist, materials and processes" by DEFOREST, McGra-Hill, 1983). It eliminates the need for laser and produces more complex patterns. This technique is applicable for supports of dimensions less than 30 cm.

It may be wise to use, on the same display device, several different polymeric compositions allowing the emission of lights of different colors. This is shown in Figure 5 which is a top view of a polychrome electroluminescent screen. This screen comprises four electroluminescent units 41, 42, 43 and 44 of the type of FIG. 4 with inter-digested comb electrodes formed on the support 40. For example, the unit 41 can emit a green color, the unit 42 a blue color, unit 43 a red color and unit 44 a yellow color. Any soluble dye having a high photoluminescence yield can enter the polymer composition. As a source of dyes which can be used, mention may be made of the catalog of EXCITON laser dyes, Dayton, Ohio 45437, United States.

Each light-emitting unit 41, 42, 43 and 44 in FIG. 5 can have one of its electrodes, respectively 411, 421, 431 and 441, connected in common to a general ground electrode 401. The application of a appropriate voltage on each of the other electrodes 412, 422, 432, 442 allows the emission of a light of particular color.

For these devices according to the present invention, the choice of the material of the electrodes is not very critical since the injection of current does not actually take place from the electrode to the polymer. In fact, the charges are injected by ionization in the field. As the electrode material, any stable metal can be chosen

(gold, silver, copper, aluminum) as well as any doped semiconductor with low resistivity (indium oxide, silicon, polyaniline, polypyrrole or polythiophene). The choice of the semiconductor polymer is less critical than in the devices of the prior art. Its electron carrier character (n) or holes (p) is immaterial. The semiconductor polymer must be a good insulator and must have good mobility (n or p). The polyvinylcarbazole used in electrophotography meets these criteria. It is also possible to use a polyphenylenevinylene or polythiophene derivative or any other semiconductor polymer such as those described in "Handbook of conducting polymers" by Skothei, published by Marcel Dekker in 1986.

The brightness of the devices obtained is important. It can reach ÏO ⁵ cd / m ² in blue light and 10 times more in green light. On a comparable surface, it may be a gain greater than 10 ⁴ compared to the solutions of the prior art, of the type described in FIG. 1. The reason for this gain is attributable to the good mobility of the unipolar carriers

(p) in polyvinylcarbazole as well as the good injection balance of carriers of the two signs.

The stability of the device proposed by the invention is greatly increased compared to the solutions of the known art. The service life can be increased by 3 orders of magnitude. The carrier injection equilibrium is in fact achieved by ionization in the field, which reduces the losses by the Joule effect. In addition, no permanent current flows between the electrodes and the polymer, which eliminates the electro-chemical reactions occurring at the interfaces and which represent the first cause of degradation of the electroluminescent devices of the known art as evidenced by the article " Electrode interface effects on indium-tin-oxide polymer / metal light emitting diodes "by E. GAUTIER et al., Published in the journal Appl. Phys. Lett. 69 (8) of August 19, 1996, pages 1071-1073.

These electroluminescent display devices can be perfectly integrated into autonomous micro-systems having a display function (in mobile telephony for example). They can be used in automobile dashboards and to make large displays for signaling in communication nodes (highways, airports, stations).

Claims

1. Electroluminescent display device comprising an electroluminescent material consisting of a polymer deposited in a thin layer (15; 37) and at least two electrodes (12, 13; 32, 33) allowing the application of an alternating electric field to said electroluminescent material, characterized in that the two electrodes (12, 13; 32, 33) are arranged one beside the other, leaving a space between them, this space being occupied by said electroluminescent material (15; 37 ). 2. electroluminescent display device according to claim 1, characterized in that the two electrodes (12, 13; 32, 33) allow the application of an alternating electric field of a determined value so as to reveal a phenomenon ionization under field in said electroluminescent material. 3. electroluminescent display device according to one of claims 1 or 2, characterized in that each electrode (32,33) having the shape of a comb, the electrodes are arranged in a mter-digital manner, said space being constituted by the lace-up path defined by the teeth (34, 35) of the comb-shaped electrodes. 4. electroluminescent display device according to any one of claims 1 to 3, characterized in that it comprises several pairs of electrodes, each pair of electrodes forming an electroluminescent display unit (41,42,43, 44). 5. electroluminescent display device according to claim 4, characterized in that each electroluminescent display unit (41,42,43,44) has a particular electroluminescent material so that the display device constitutes a polychrome screen . 6. electroluminescent display device according to any one of claims 1 to 5, characterized in that the electrodes (12, 13; 32, 33) are made of a material chosen from metallic materials or doped semiconductor materials with low resistivity. 7. Electroluminescent display device according to any one of claims 1 to 6, characterized in that the polymer constituting said electroluminescent material (15; 37) is chosen from polyvinylcarbazole, polyphenylenevinylene and polythiophene. 8. An electroluminescent display device according to any one of claims 1 to 7, characterized in that the polymer constituting said electroluminescent material (15; 37) contains a dye chosen from coumarin, rhodamine and DCM. 9. electroluminescent display device according to any one of claims 1 to 8, characterized in that the electrodes and the electroluminescent material are arranged on a support (11; 31; 40) made of a material chosen from glass, polycarbonate or polyethylene. 10. Method for producing an electroluminescent display device in a thin layer, characterized in that it comprises the following steps: - depositing, on one face of a support (11) at least two electrodes (12, 13) intended to allow the application of an electric field to an electroluminescent material, - depositing, on said face of the support (11), a layer of polymer constituting the electroluminescent material (15) so that said applied electric field causes an effect