Thin film electroluminescent device

Abstract: A thin film EL device which performs electrolumines­ cence 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: The present invention relates to a thin film electro-luminescent (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: 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 sulphide doped with an element for luminescent centres, 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. 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. 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, a process commonly used to form the insulating layers. 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 of cyanoethyl cellulose, and that the resulting thin film EL device emits as bright a light as the conventional one provided with inorganic insulating layers. The present invention was completed on the basis of this finding. Such 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; see, for example, DE-A-2 120 926. These materials have also been used conventionally as a binder for a high-permittivity powder such as TiO₂ or BaTiO₃ in an insulating layer ; see, for example, US-A-4 417 174. However, it has not been known that they can be used separately for the insulating layer of the thin film EL device of laminated structure as in the present invention. SUMMARY OF THE INVENTION According to the present invention, a thin film EL device comprises a light-emitting layer both sides of which are 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 at least one of said insulating layers is made solely of a thin film of an organic dielectric material of a cyanoethyl cellulose. 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, an insulating layer of such 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. The thin film EL device of the present invention is comparable to the conventional one in performance. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic 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. Figs. 4(a) and 4(b) are schematics showing other thin film EL devices pertaining to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 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 one another. A typical example of the lamination structure is shown below. Electrode / insulating layer / light-emitting layer / insulating layer / electrode / substrate (double-insulating layer structure). The double-insulating layer structure is preferable because it is most effective to protect the device from dielectric breakdown. According to the present invention, at least one of the insulating layers is composed of a thin film of an organic dielectric material of a cyanoethyl cellulose. 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 of a cyanoethyl cellulose. The organic dielectric material of a cyanoethyl cellulose 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 suitably has a thickness in the range of 0.1 to 10 µm, preferably 0.1 to 1 µm. 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. The light-emitting layer of the present invention is suitably made of a known metal sulphide (such as ZnS, CdS, CaS, SrS, and BaS) or metal selenide (such as ZnSe and CaSe). It preferably has 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 centres. The light-emitting layer may be formed by vacuum deposition, sputtering, or CVD process. 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. Examples The preferred embodiments of the invention will be described in more detail with reference to the drawings. In the drawings, the same reference numbers designate the same parts as in the conventional device 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. The following explanation is directed to an example embodying a thin film EL device of double-insulating layer structure in which the second insulating layer is made of a cyanoethyl cellulose as the organic dielectric material. The thin film EL device is shown in Fig. 1, 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 133.3 Nm⁻² (i.e. about 1 Torr) for the effective removal of the organic solvent. 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. The thin film EL device prepared as mentioned above has the voltage-brightness characteristics ( l ₁) as shown in Fig. 3. There is also shown for comparison the voltage-brightness characteristics ( l ₂) 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. In the above-mentioned example, the organic dielectric material of a cyanoethyl cellulose 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, as shown in Figs. 4(a) and 4(b), respectively, in which the thin film of the organic dielectric material is indicated by a reference numeral 8.

Claims: A thin film EL device comprising a light-emitting layer (4) both sides of which are covered with an insulating layer (3,8), and a pair of electrodes (2,6) sandwiching said light-emitting layer, with at least one (2) of said electrodes being transparent, in which at least one (8) of said insulating layers is made solely of a thin film of an organic dielectric material of a cyanoethyl cellulose. The device of claim 1 in which said insulating layer (8) has a thickness of 0.1 to 10 µm. The device of claim 1 in which said insulating layer (8) has a thickness of 0.1 to 1 µm. The device of claim 1 in which one (3) of said insulating layers is made of an inorganic insulating material. The device of claim 1 which is formed on a transparent substrate (1). The device of claim 1 in which the emitting layer (4) is one made of a metal sulphide or a metal selenide. The device of claim 1 in which the emitting layer (4) has a thickness of 4000 to 10000 Å.

Inventors: MIKAMI AKIYOSHI, NAKAJIMA SHIGEO, NAKAYA HIROAKI, YAMASHITA TAKUO, YOSHIDA MASARU, MIKAMI, AKIYOSHI, NAKAJIMA, SHIGEO, NAKAYA, HIROAKI, YAMASHITA, TAKUO, YOSHIDA, MASARU
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1989
Main classification: H05B33
IPC codes: H05B33