GB2126416A - Electroluminescent display devices - Google Patents

Abstract

In the manufacture of a D.C. electroluminescent display device in which a layer (14) of electroluminescent material of the kind required to be formed is sandwiched between a transparent electrode (11) on a substrate (10) and a backing electrode (15), a thin film layer (12) of resistive material, for example a metal chalcogenide, for reducing forming power levels is produced on the transparent electrode (11) prior to the deposition of the electroluminescent material (14) thereon by coating the electrode, for example by dipping or spraying, with a solution containing constituent elements of the resistive material and causing those elements to react together. The electroluminescent material (14) and backing electrode (15) are thereafter disposed on the substrate. <IMAGE>

Description

SPECIFICATION Electroluminescent display devices This invention relates to electroluminescent display devices and their methods of manufacture.

Electroluminescent display devices are typically constructed by depositing on a surface of a transparent substrate a transparent electrode such as tin oxide or indium tin oxide, the pattern of the deposited electrode serving to define regions or areas of the display which will eventually be required to emit light. The electroluminescent layer is applied to the exposed surface of the transparent electrode in the form of paint comprising an electroluminescent powder mixed with a binder. The electroluminescent layer undergoes a drying or curing process and is then covered by an electrically-conductive layer, for example aluminium, constituting the other electrode.

Finally, the device is encapsulated for protection. It is necessary for the device to undergo a forming operation at this stage in order to emit light readily. This involves applying a unidirectional voltage across the device, using the transparent electrode as anode, until the required structure is achieved, causing the resistance of the electroluminescent layer to increase, the current flow to fall and light to be emitted. Thereafter the application of a suitable, relatively low voltage across the electrodes will cause immediate emission of light.

These aspects of electroluminescent display devices have been described in numerous articies and other pubiications, including, for example, "Direct-Current Electroluminescence in Zinc Sulphide: State of the Art" in Proceedings of the IEEE, Vol. 61, No. 7, July 1973, UK Patent Specification No. 1,300,548, and "Materials control and d.c. electroluminescence in ZnS : Mn, Cu, Cl powder phosphors" in Brit J. Appl. Phys. (J. Phys.), 1969, Ser. 2, Vol. 2.

The power necessary for the forming operation can be ashigh as 5 W Cm-2 where the transparent electrode comprises tin oxide or 20 W cm-2 in the case of indium tin oxide.

The heat generated by such powers can lead to damage of the transparent electrode or electroluminescent layer and in some cases, cracking of the substrate. Because of the possibility of damage, large area direct current electroluminescent (DCEL) displays using powder phosphors such as ZnS : Mn, Cu can only be prepared satisfactorily and repeatedly if the initial forming powers are substantially reduced. Various methods have been developed to reduce the necessary forming power, one of which involves the provision of a vacuum deposited resistive, semi-insulating, interlayer between the transparent electrode and the electroluminescent layer, for example, as described in UK Patent Specification No.

1568111. Unfortunately, the vacuum deposition of this interlayer renders this particular 'passivation' technique, as it is known, unsuitable for mass production or reproducibility.

It is an object of the present invention to provide a method of making an electroluminescent display device in which the forming thereof may be achieved at relatively low power levels and which is easily reproducible and suited to mass production.

According to one aspect of the present invention there is provided a method of manufacturing an electroluminescent display device which comprises the steps of disposing a layer of resistive material on a substrate having an electrode thereon by coating the substrate with a solution containing constituent elements of the resistive material, causing those constituent elements to react so as to form the layer of resistive material on the electrode, and thereafter disposing a layer of electroluminescent material over the resistive material layer of the type which is required to undergo a forming operation in order to emit light, and disposing a second electrode on the surface of the electroluminescent layer remote from the resistive material layer.

The resistive material may comprise a semiinsulator or a semi-conductor.

The resistive material preferably comprises a metal chalcogenide.

The resistive material may comprise a metallic sulphide, for example zinc sulphide or cadmium sulphide. In this case, the solution may contain an organo-metallic material and a thio-organic material (containing sulphur), the metallic and thio components thereof being caused to react to produce the metallic sulphide. In the case of zinc sulphide for example, the organo-metallic material containing zinc may comprise zinc acetate and the thio-organic material may comprise thioacetamide. The semi-insulative layer of zinc sulphide thus formed preferably has a thickness of around 500 .

The resistive material may alternatively comprise a metallic selenide, for example, zinc selenide or cadmium selenide. In this case, the coating solution may comprise an organometallic material and an organic material containing selenium, the metallic and selenium components thereof being caused to react to form the metallic selenide. In the case of zinc selenide for example, the coating solution may comprise zinc acetate and selenourea.

The resistive material layer is preferably around 200-600 A in thickness.

The step of coating the substrate with the solution may comprise dipping the substrate into said solution to leave a thin film of the solution adhering to the substrate. Alternatively, the substrate may be coated by spraying the solution thereon or by spinning the substrate and applying the solution thereto whereby the solution is spread uniformly over the substrate.

The coating solution may also contain methanol and propan-2-ol.

The step of causing components of said solution to react to form the resistive material may comprise a baking operation. This baking may be at a temperature of 80-500"C and preferably around 350"C.

The method may also include the step of air drying the solution-coated substrate prior to the baking operation.

Direct current electroluminescent display devices and their method of manufacture in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawing which shows a schematic sectional side view of an electroluminescent device.

Referring to Fig. 1, the direct current electroluminescent display device includes a transparent substrate 10 of glass or a polymeric material on one surface of which is provided by any convenient means, such as spraying, evaporation, sputtering or chemical vapour deposition, a transparent electrically-conductive layer 11 to form the positive electrode for the device. The layer 11 may be of tin oxide, indium oxide, indium tin oxide, titanium dioxide, cadmium stannate or any other suitable material. The layer 11 is around 3000-4000"A in thickness.

In order to permit the forming process to be achieved with reduced electric power, a layer 1 2 of resistive zinc sulphide material is provided on the exposed surface of the electrode layer 11 whose thickness is around 500'A.

The method by which this layer is deposited will be described in greater detail hereinafter.

Alternative semi-insulative materials, such as cadmium sulphide, or semi-conductive materials may be used to constitute the layer 1 2 instead.

An electroluminescent layer 14 comprising a powder phosphor mixture of around 20 microns (200,000 A) in thickness is disposed over the layer 1 2. The phosphor material may typically be particles of zinc sulphide doped with manganese individually coated with copper (ZnS; Mn, Cu), and mixed with an appropriate binder. Details of the phosphor materials, their preparation methods for their deposition are given in the articles referred to previously.

Finally, a second electrode 1 5 of, for example, evaporated aluminium or copper is disposed over the exposed surface of the electroluminescent layer 14.

The layers 11 to 1 5 are preferably encapsulated for protection.

The layer 1 2 is deposited on the electrode layer 11 by coating the electrode layer 11 with a soltuion containing an organo-metallic material and a thio-organo material and thereafter causing the metallic/thio components to react to produce a metallic sulphide layer.

More especially, in the case where the layer 1 2 is to comprise zinc sulphide, the coating operation involves coating, for example by dipping, the substrate 10 with the electrode layer 11 thereon in a solution containing 0.05M of zinc acetate and 0.05M of thiocetamide in a mixture of metanol and propan2-ol alcohols which promote uniform coating.

The dipped substrate with a thin film of the coating solution adhering to its electrode surface, the other surfaces having been wiped clean, is then dried in air at room temperature for approximately 5 minutes to allow the alcoholic components of the solution to evaporate. Following evaporation in this manner, a thin layer of zinc acetate/ thiocetamide com plex remains on the electrode layer 11.

The so-coated substrate and electrode combination is then placed in an oven and baked in order to cause the zinc and sulphur components of the complex to react and produce a transparent, semi-insulative layer of zinc sulphide. The time taken for such conversion is dependent upon the temperature of the oven.

It has been found that around 10 minutes baking at approximately 350ec is particularly preferred although any temperature from 80 to 500"C, with the baking time being increased or decreased accordingly, may be used instead.

The baking operation serves to drive off the residual volatile ingredients of the zinc acetate and thiocetamide complex. Thereafter, the substrate 10 with the layers 11 and 1 2 is washed in de-ionised water and then alcohol to remove any remaining impurities.

It has been found that the transparent layer 12 of zinc sulphide produced by one dipping operation in the aforementioned manner is around 500"A in thickness and very adherent.

If desired, the layer 12 may comprise cadmium sulphide by using cadmium acetate rather than zinc acetate in the dipping -solution.

Other suitably resistive metal chalcogenides may be used to constitute the resistive layer 1 2 using a similar method, the coating solution containing organic materials carrying the metallic and chalcogenic constituent elements for the metal chalcogenide material which, following coating of the substrate with the solution are caused to react to produce the metal chalcogenide layer 12. For example the layer 1 2 may comprise a metallic selenide such as zinc selenide or cadmium selenide, the former being formed by employing a coating solution containing a mixture of zinc acetate and selenourea.

Instead of using a dipping operation, the coating of the substrate may alternatively be achieved by spraying the solution uniformly over the exposed surface of the electrode of the substrate or by spinning the substrate and dripping the solution thereon whereby the solution spreads evenly over the surface of the substrate.

Successive layers of the coating may be built up to increase the overall thickness of the resistive material layer 1 2 formed thereby if desired by repeating the dipping, spraying or spinning operation. The overall thickness of the layer 12 may be around 200-600 .

The electroluminescent display device fabricated in accordance with the above method needs to undergo a forming operation before it can emit light. As previously mentioned this is accomplished by applying a unidirectional voltage across the electrode layers 11 and 1 5, using the electrode layer 11 as anode. The heat and electric field produced by this voltage cause the copper in the electroluminescent layer 14 to migrate away from the layer 1 2 with a consequential increase in the resistance and light output of the electroluminescent layer 14.

There is thus produced a region of the electroluminescent layer 14, generally indicated at 16, immediately adjacent and extending parallel to the layer 1 2 which is of high resistivity and in which electroluminescence is induced to produce light emission. The region 1 6 is typically around one micron 10000 A) in thickness. Thereafter, the device can be operated under pulsed DC voltage or AC voltage drive conditions.

It has been found that an electroluminescent display device having a 500"A thick interlayer 1 2 of zinc sulphide produced by the aforementioned method requires only around 10-2 W Cm-2 forming power, this being a considerable reduction the levels of power needed for electroluminescent devices not having such an interlayer as mentioned previously.

The described coating process by which the layer 1 2 is deposited on the electrode layer 11 ;its particularly beneficial in that it readily lends itself to mass production and reproducibility in constrast with the vacuum deposition techniques employed heretofore.

It is envisage that other phosphor materials may be employed, as described in the publica tion-entitled "DCEL Dot-Matrix Displays in a Range of Colours" by Vecht el al in S.l.D.

(Society for Information Displays) 1 980 page 110, such as Calcium Sulphide doped with Enropium (CaS : Eu), Calcium Sulphide doped with Cerium (CuS : Ce), Strontium Sulphide doped with Cerium (SrS : Ce), and Strontium Sulphide doped with Manganese (SrS : Mn).

Claims (30)

1. A method of manufacturing an electroluminescent display device which comprises the steps of disposing a layer of resistive material on a substrate having an electrode thereon by coating the substrate with a solution containing constituent elements of the resistive material, causing those constituent elements to react so as to form the layer of resistive material on the electrode, and thereafter disposing a layer of electroluminescent material over the resistive material layer of the type which is required to undergo a forming operation in order to emit light, and disposing a second electrode on the surface of the electroluminescent layer remote from the resistive material layer.

2. A method according to Claim 1, wherein said resistive material comprises a semi-insulator.

3. A method according to Claim 1, wherein said resistive material comprises a semi-conductor.

4. A method according to Claim 1, wherein said resistive material comprises a metal chalcogenide.

5. A method according to Claim 4, wherein said resistive material comprises zinc sulphide.

6. A method according to Claim 4, wherein said resistive material comprises cadmium sulphide.

7. A method according to Claim 4, wherein said resistive material comprises zinc selenide.

8. A method according to Claim 4, wherein said resistive material comprises cadmium selenide.

9. A method according to Claim 4, wherein the coating solution contains organic materials carrying the metallic and chalcogenic constituent elements for the metal chalcogenide.

10. A method according to Claim 9, wherein said metal chalcogenide comprises a metallic sulphide.

11. A method according to Claim 10, wherein said coating solution contains an organo-metallic material and a thio-organic material, and wherein the metallic and thio components thereof are caused to react to produce said metallic sulphide.

1 2. A method according to Claim 11, wherein said metallic sulphide comprises zinc sulphide.

1 3. A method according to Claim 12, wherein said organo-metallic material comprises zinc acetate and said thio-organo material comprises thioacetamide.

1 4. A method according to Claim 11, wherein said metallic sulphide comprises cadmium sulphide.

1 5. A method according to Claim 14, wherein said organo-metallic material comprises cadmium acetate and said thio-organo material comprises thioacetamide.

16. A method according to Claim 9, wherein said metal chalcogenide comprises a metallic selenide.

1 7. A method according to Claim 16, wherein said coating solution contains an organo-metallic material and an organic material containing selenium, and wherein the metal and selenium components thereof are caused to react to form said metallic selenide.

1 8. A method according to Claim 17, wherein said metallic selenide comprises zinc selenide and wherein said coating solution contains zinc acetate and selenoura.

1 9. A method according to any one of the preceding claims, wherein the step of coating the substrate with the solution comprises dipping the substrate into said solution to leave a thin film of the solution adhering to the substrate.

20. A method according to any one of Claims 1 to 18, wherein the step of coating the substrate with said solution comprises spraying the solution onto the substrate.

21. A method according to any one of Claims 1 to 18, wherein the step of coating the substrate with said solution comprises spinning the substrate and applying the solution to the spinning substrate whereby the solution is spread uniformly over the substrate.

22. A method according to any one of the preceding claims, wherein said coating solution includes alcohol.

23. A method according to Claim 22, wherein said coating solution includes a mixture of methanol and propan-2-ol.

24. A method according to any one of the preceding claims, wherein the step of causing said constituent elements of the coating solution to react to form the resistive material layer comprises a baking operation.

25. A method according to Claim 24, wherein said baking is at around 80 to 500etc.

26. A method according'to Claim 24 or 25, further including the step of air drying the solution-coated substrate prior to the baking operation.

27. A method according to any one of the preceding claims, wherein said resistive material layer is around 200 to 600 A in thickness.

28. A method according to Claim 27 and Claim 1 2 or 13, wherein said zinc sulphide resistive layer has a thickness of around 500 A.

29. A method according to any one of the preceding claims, further comprising the step of forming the electroluminescent material by applying a unidirectional voltage across the said electrodes.

30. A method of manufacturing an electroluminescent display device substantially as hereinbefore described with reference to the accompanying drawing.