Thin film electroluminescent device

Abstract

 A thin film EL device which performs electrolumines­cence in response to the application of an electric field, comprising a light-emitting layer at least one side of which is covered with an insulating layer, and a pair of electrodes sandwiching said light-emitting layer, with at least one of said electrode being transparent, in which the insulating layer is made of a thin film of an organic dielectric material which may contain a fine powder of an inorganic insulating material.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention:

[0001] The present invention relates to a thin film electro­luminescent (EL) device which is provided with a light-­emitting layer and insulating layers so that it performs electroluminescence in response to the application of an electric field. More particularly, it is concerned with the insulating layer of the device.

2. Description of the Prior Art:

[0002] Since the development of the thin film EL device which emits a bright light in response to the application of an AC electric field to the emitting layer of metal sulfide doped with an element for luminescent centers, various investigations have been made on the structure of the device. The conventional known thin film EL device has the basic structure as shown in Fig. 2.

[0003] It is constructed on a transparent substrate 1 such as a glass plate. On the substrate are arranged a plurality of long, narrow transparent electrodes 2 parallel to one another. On the electrodes 2 is formed a first insulating layer 3 which is typically composed of an SiO₂ layer and an Si₃N₄ layer laminated on top of the other in the order mentioned. On the first insulating layer 3 is formed a light-emitting layer 4 of ZnS doped with an active substance. The light-emitting layer 4 is covered with a second insulating layer 5 which is typically composed of an Si₃N₄ film and an Al₂O₃ film laminated on top of the other in the order mentioned. (Thus the light-emitting layer 4 and the insulating layers 3 and 5 sandwiching it form a three-layer structure.) On the second insulating layer 5 are arranged a plurality of long, narrow back electrodes 6 of Al or the like in the direction perpendicular to the aforesaid transparent electrodes 2. The transparent elec­trodes 2 and the back electrodes 6 are connected to an AC source 7 which drives the thin film EL device.

[0004] The conventional thin film EL device constructed as mentioned above has a disadvantage that it needs many steps and a long time for production and hence it is high in cost. The complex production steps and high cost are mainly attributable to the first and second insulating layers of laminated structure which take a long time when formed by sputtering (Sputtering is a common process used to form the insulating layers.).

[0005] With the foregoing in mind, the present inventors carried out a series of researches on the materials and processes for producing the aforesaid insulating layers. As the result, it was found that the insulating layers can be made in a short time in a simple manner from an organic dielectric material and that the resulting thin film EL device emits as bright a light the conventional one provided with inorganic insulating layers. The present invention was completed on the basis of this finding.

[0006] Some of the organic dielectric materials used for the insulating layers in the present invention have conventionally been in use as a binder for the luminescent material of the EL element of luminescent material-dispersed type which is free of the insulating layers. However, it has not been known that they can be used for the insulating layer of the thin film EL device of laminated structure as in the present invention.

SUMMARY OF THE INVENTION

[0007] According to the present invention, it provides a thin film EL device comprising a light-emitting layer, at least one side of which is covered with an insulating layer, and a pair of electrodes sandwiching said light-emitting layer, with at least one of said electrodes being transparent, in which the insulating layer is made of a thin film of an organic dielectric material which may contain a fine powder of an inorganic insulating material.

[0008] The thin film El device of the present invention can be produced by a simple process in a short time which results in a low production cost because the insulating layer is formed from an organic dielectric material by coating, spraying, or screen printing which is easy to carry out. In addition, the insulating layer of an organic dielectric material pre­vents the propagation of possible dielectric breakdown because it is softer than that of an inorganic dielectric material. In other words, it is of self-healing type.

[0009] The thin film EL device of the present invention is com­parable to the conventional one in performance. An additional advantage of the thin film EL device is that it can be driven at a reduced voltage if the thin film of an organic dielectric material is incorporated with a fine powder of an inorganic insulating material having a high dielectric constant.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] 

Fig. 1(a) and Fig. 1(b) are schematics showing the thin film EL device pertaining to the present invention.

Fig. 2 is a schematic showing the conventional thin film EL device.

Fig. 3 is a voltage-brightness curve describing the characteristic properties of the thin film EL device as shown in Fig. 1(a).

Figs. 4(a) to 4(d) are schematics showing other thin film EL devices pertaining to the present invention.

Fig. 5 is a voltage-brightness curve describing the characteristic properties of the thin film EL device as shown in Fig. 1(b).

Fig. 6 is a schematic showing the second insulating layer used in the thin film EL device as shown in Fig. 1(b).

DETAILED DESCRIPTION OF THE INVENTION

[0011] The thin film EL device pertaining to the present invention is usually formed by lamination on a transparent substrate made of glass or plastics. It is made up of several layers laminated on top of another. Typical exam­ples of the lamination structure are shown below.

(1) Electrode / insulating layer / light-emitting layer /­insulating layer / electrode / substrate (double-­insulating layer structure).

(2) Electrode / insulating layer / light-emitting layer /­electrode / substrate.

(3) Electrode / light-emitting layer / insulating layer /­electrode / substrate.

[0012] The first one (double-insulating layer structure) is preferable because it is most effective to protect the device from dielectric breakdown.

[0013] According to the present invention, the insulating layer is composed of a thin film of an organic dielectric material. In the device of the aforesaid double-insulating layer struc­ture, at least one of the insulating layers is composed of a thin film of an organic dielectric material. In such a case, it is preferable to combine the first insulating layer (adjacent to the substrate) made of an inorganic insulating material (metal oxide or nitride such as SiO₂, Si₃N₄, TiO₂, Ta₂O₅, Al₂O₃ or a combination thereof) and the second insulating layer made of the organic dielectric material.

[0014] The organic dielectric material is selected from a variety of organic polymers which have low moisture absorption, high transparency, good moldability, and chemical stability. Examples of the organic polymer include a polyvinyl chloride, polybinylidene chloride, polystyrene, polyethylene, acrylic, eopxy, polyimide or cellulosic resin. Preferable among them is one which has a dielectric constant higher than 8 and a low di­electric loss. For this reason, the most desirable one is a cellulosic resin, especially cyano-lower-alkyl (C₁₋₅) cellulose such as cyanoethyl cellulose. The organic dielectric material mentioned above can be made into a thin film easily by coating, spraying, or screen printing after dissolution in a solvent. Heat treatment may be used to form the thin film, as the case may be. The thin film have suitably a thickness in the range of 0.1 to 10 µm, preferably 0.1 to 1 µm.

[0015] The insulating layer can be formed in an extremely simple manner without vacuum or with a low vacuum, in a very short time. This leads to a great reduction in production cost as compared with the conventional device.

[0016] The light-emitting layer of the present invention is suitably made of a known metal sulfide (such as ZnS, CdS, CaS, SrS, and BaS) or metal selenide (such as ZnSe and CaSe). It preferably have a thickness in the range of 4,000 to 10,000 Å. The light-emitting layer may contain Mn or a rare earth element as luminescent centers. The light-emitting layer may be formed by vacuum deposition, sputtering, or CVD process.

[0017] The thin film EL device of the present invention has a pair of electrodes, at least one of which is trans­parent. An example of the transparent electrode is the ITO electrode. Usually, it is placed next to the trans­parent substrate. The other electrode does not always need to be transparent. It may be a thin film of Al, Cu, Au, or the like. The electrodes have a desired pattern formed by mask depositing or etching.

[0018] In the case where the aforesaid organic dielectric material is insufficient to meet the requirement of di­electric constant, it may be used in combination with a fine powder of an inorganic insulating material having a high dielectric constant. Specific examples of the powder include a powder of lead titanate (PbTiO₃), barium titanate (BaTiO₃), strontium titanate (SrTiO₃), lead titanate zirco­nate (PZT), lanthanum lead titanate zirconate (PLZT), or the like. The powder is suitably added in such an amount that the resulting dielectric material has a dielectric constant of 8 or above. Usually, the amount is suitably 10 to 50 times that of weight of the organic dielectric material.

[0019] The powder should have a particle diameter which is fairly small relative to the thickness (0.1 to 10 µm) of the insulating layer. The desired particle diameter ranges from 0.01 to 0.1 µm. This is because large powder particles make the insulating layer heterogeneous micro­scopically and hence make uneven the electric field which is applied to the light-emitting layer to drive the EL device, with the result that the voltage to start the light emitting varies from one place to another.

[0020] The insulating layer containing the aforesaid powder should be used as the second insulating layer in the device of double-insulating layer structure, because it might be less transparent than that without the powder.

Examples

[0021] The invention will be described in more detail with reference to the drawings. In the drawings, same refer­ence numbers designate same parts in the conventional device as shown in Fig. 2. The fundamental structure of the device in the example is not explained because it is the same as the conventional one.

[0022] The following explanation directs to the example em­bodying a thin film EL device of double-insulating layer structure in which the second insulating layer is made of cyanoethyl cellulose as the organic dielectric material.

[0023] The thin film EL device is shown in Fig. 1(a), in which there is shown the second insulating layer 8 made of an organic dielectric material. The second insulating layer in this example has a thickness of about 0.2 µm, although the thickness may range from 0.1 to 10 µm. It is prepared by applying a solution of cyanoethyl cellulose (in an organic polar solvent such as dimethyl formamide, N-methylpyrrolidone, and nitromethane) onto the light-­emitting layer 4 by coating, spraying, or screen printing, which is followed by heating at about 100°C in an oven for the removal of the organic solvent. The heating may be carried out under a vacuum of about 1 Torr for the effec­tive removal of the organic solvent.

[0024] The light-emitting layer 4 is formed by vacuum depo­sition from ZnS doped with 0.5 wt% of Mn. It has a thick­ness of 7000 to 8000 Å. The first insulating layer 3 is composed of a 300 Å thick layer of SiO₂ and a 2000 Å thick layer of Si₃N₄. The transparent electrode 2 is an ITO film, and the back electrode 6 is an Al film.

[0025] The thin film EL device prepared as mentioned above has the voltage-brightness characteristics (1₁) as shown in Fig. 3. There is also shown for comparison the voltage brightness characteristics (1₂) of the conventional EL device. It is noted from Fig. 3 that the thin film EL device having the second insulating layer 8 exhibits almost the same voltage-brightness characteristics as the conventional EL device. In addition, there is no differ­ence between them in dielectric strength and stability.

[0026] In another embodiment, the second insulating layer 8 was formed from polyimide resin ("PIX-1400" made by Hitachi Kasei Kogyo Co., Ltd. in Japan) in place of cyanoethyl cellulose as the organic dielectric material. The polyimide was applied by spinner coating, followed by heating at 350°C. The thickness of the polyimide layer was about 2000 Å (0.2 µm). The resulting thin film EL device has the voltage-brightness characteristics (1₃) as shown in Fig. 3.

[0027] In the above-mentioned example, the organic dielec­tric material is used for the second insulating layer; however, it may also be used for the first insulating layer or both of the first and second insulating layers.

[0028] In the above-mentioned example, the organic dielec­tric material is used for the thin film EL device of double-insulating layer structure; however, it may also be used for those thin film EL devices of such structure that the insulating layer is on only one side of the light emitting layer. Some examples of them are shown in Figs. 4(a) to 4(d), in which the thin film of the organic dielectric material is indicated by a reference numeral 8.

[0029] In the second example of the present invention, the thin film EL device of double-insulating layer structure has the second insulating layer made of cyanoethyl cellu­lose (as the organic dielectric material) incorporated with a powder of barium titanate (BaTiO₃) having a particle diameter of about 0.1 µm (as the insulating material having a high dielectric constant). The thin film EL device has the structure as shown in Fig. 1(b). It has the second insulating layer 8, which is formed from a mixture of an organic dielectric material and a powder of an insulating material having a high dielectric constant, said powder having a particle diameter smaller than 1 µm. The mixture is prepared by dissolving cyanoethyl cellulose in an organic solvent and mixing the solution with a powder of BaTiO₃ having a particle diameter smaller than 1 µm using a ball mill. The resulting pasty liquid is applied onto the light-emitting layer 4 by coating, spraying, or screen printing, which is followed by heating at about 100°C in an oven for the removal of the solvent. The heating may be carried out under a vacuum of about 1 Torr for the effective removal of the organic solvent. The thus formed second insulating layer 8 has a thickness of about 10 µm and contains about 50 times (by weight) as much BaTiO₃ as cyanoethyl cellulose. Incidentally, the dielectric constant of the second insulating layer 8 may be adjusted by changing the mixing ratio of cyanoethyl cellulose and BaTiO₃ powder.

[0030] The thin film EL device prepared as mentioned above has the voltage-brightness characteristics (1₄) as shown in Fig. 5. There is also shown for comparison the voltage-­brightness characteristics (1₅) of the conventional EL device. It is noted from Fig. 3 that the thin film EL device having the second insulating layer 8 exhibits almost the same voltage-brightness characteristics as the conventional EL device.

[0031] The BaTiO₃ powder is required to have a particle diam­eter smaller than 1 µm. If the particle diameter is large relative to the thickness of the second insulating layer 8, the large particles make the second insulating layer 8 heterogeneous microscopically as shown in Fig. 6. In Fig. 6, a BaTiO₃ particle is indicated by 11 and cyanoethyl cellulose, by 12. It should be noted that the thin film at the cross-section 13 is composed mainly of BaTiO₃ particles, whereas the thin film at the cross section 14 is composed of BaTiO₃ particles and cyanoethyl cellulose in about equal quantities. The same is true of the distribu­tion pattern in the horizontal direction. The uneven distribution causes the second insulating layer 8 to vary in dielectric constant from place to place. The uneven distribution also causes the film thickness to fluctuate microscopically as shown in Fig. 6. These variation and fluctuation make uneven the electric field which is applied to the light-emitting layer 4 to drive the EL device, with the result that the voltage to start the light emitting varies from one place to another or the brightness greatly changes with time.

[0032] In the case where the second insulating layer 8 is incorporated with a BaTiO₃ powder having an average particle diameter of 10 µm, the thin film EL device exhibits the voltage brightness characteristic (1₆) as shown in Fig. 6. It is noted that the brightness is decreased and the driving voltage is increased. In addi­tion, this thin film EL device greatly deteriorates with time in brightness.

[0033] The above-mentioned explanation demonstrates that when the second insulating layer is made of a mixture com­posed of cyanoethyl cellulose and a BaTiO₃ powder having a particle diameter smaller than 1 µm, the resulting thin film EL device exhibits almost the same characteristics as the conventional one.

[0034] Incidentally, cyanoethyl cellulose as the organic dielectric material may be replaced by a synthetic resin such as vinyl resin, polystyrene, polyethylene, acrylic resin, epoxy resin, and polyimide resin, and BaTiO₃ as the insulating material having a high dielectric constant may be replaced by PbTiO₃, SrTiO₃, PZT, PLZT, or the like. They produce the same effect as mentioned above. The organic dielectric material which is mixed with the insulat­ing material having a high dielectric constant in powder form having a particle diameter smaller than 1 µm, may be used as the second insulating layer 8, or the first insulating layer or both of the first and second insulat­ing layers. The application onto the second insulating layer is preferable.

[0035] The thin film EL device of the present invention is not limited to that of double-insulating layer structure; however, it also embraces the one having the insulating layer on only one side of the light-emitting layer. Such a thin film EL device has the same structure as shown in Fig. 4.

[0036] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention.

[0037] There are described above novel features which the skilled man will appreciate give rise to advantages. These are each independent aspects of the invention to be covered by the present application, irrespective of whether or not they are included within the scope of the following claims.

Claims

1. A thin film EL device comprising a light-emitting layer at least one side of which is covered with an insulating layer, and a pair of electrodes sandwiching said light-emitting layer, with at least one of said electrodes being transparent, in which the insulating layer is made of a thin film of an organic dielectric material which may contain a fine powder of an inorganic insulating material.

2. The device of claim 1 in which the organic dielectric material is a polyvinyl chloride, polyvinlyidene chloride, polystyrene, polyethylene, acrylic, epoxy, polyimide or cellulosic resin.

3. The device of claim 1 in which the organic dielectric material has a dielectric constant of 8 or more.

4. The device of claim 1 in which the organic dielectric material is a cellulosic resin.

5. The device of claim 4 in which the cellulosic resin is a cyano-lower alkyl cellulose.

6. The device of claim 1 in which the insulating layer has a thickness of 0.1 to 10 µm.

7. The device of claim 1 in which the insulating layer has a thickness of 0.1 to 1 µm.

8. The device of claim 1 in which the inorganic insulat­ing material is selected from the group consisting of lead titanate, barium titanate, strontium titanate, lead tita­nate zirconate and lanthanium lead titanate zirconate.

9. The device of claim 1 in which the fine powder has a particle diameter of 0.01 to 0.1 µm.

10. The device of claim 1 in which both sides of the light-emitting layer are covered with the insulating layer.

11. The device of claim 1 in which one side of the light­emitting layer is covered with the insulating layer, and another side is covered with an insulating layer of an inorganic insulating material.

12. The device of claim 1 which is formed on a transparent substrate.

13. The device of claim 1 in which the emitting layer is one made of a metal sulfide or a metal selenide.

14. The device of claim l in which the emitting layer has a thickness of 4000 to 10000 Å.

15. A thin film EL device in which a light-emitting layer (4) is disposed between opposed electrodes (2, 6) and in which to at least one side of the light-emitting layer between said layer (4) and the respective electrode (2, 6) there is an insulating layer (3, 8) comprising organic dielectric material.

Drawing