FLUORESCENCE-REDDENING MEMBRANE AND RED-EMITTING DEVICE

Abstract: The present invention relates to a fluorescence-reddening membrane which comprises a light-transmittable medium and, dispersed therein, (a) a rhodamine base fluorescence pigment and (b) a fluorescence pigment which has absorptions in the blue region and induces energy transfer to and reabsorptions from the rhodamine base fluorescence pigment. According to the present invention, it is made possible to provide the fluorescence-reddening membrane capable of converting the color of light emitted by a blue-emitting organic electroluminescence device to a red light at a high conversion efficiency and also to provide an inexpensive red-emitting device capable of reducing the size and thickness of itself.

Description: TECHNICAL FIELD The present invention relates to a fluorescence-reddening membrane and a red-emitting device. More particularly, the present invention is concerned with a fluorescence-reddening membrane capable of converting the color of the light emitted by a blue-emitting organic electroluminescence ( hereinafter abbreviated to "EL") device to a red light at a high conversion efficiency as high as at least 33% and with an inexpensive red-emitting device which comprises such a fluorescence-reddening membrane and a light-emitting device, emits a red light at a high conversion efficiency, and also enables reduction in size and thickness. BACKGROUND ART An EL device which utilizes the electroluminescence is characterized by a high visual-distinguishability because of its self-emission, excellent impact resistance because of its being in the form of a complete solid device, and the like properties. Therefore, the EL device has been attracting attention for utilization as a light-emitting device in a variety of types of display apparatuses. The EL device is divided into an inorganic EL device in which an inorganic compound is used as the light-emitting material, and an organic EL device in which an organic compound is used as the light-emitting material. Of these, the organic EL device, which can decrease the applied voltage to a great extent, has been positively studied for practical application as a display device of the next generation. It is evident that multi-coloring is required for a display device in order to expand the use and application field of the above-mentioned organic EL device, as can be seen from the examples of the cathode-ray tube ( CRT ) and the liquid crystal display ( LCD ) . Several methods have heretofore been known as a method for preparing a multi-colored display apparatus by the use of an EL device, including, for example, (1) a method in which EL materials which emit light with three primary colors of red (R), green (G) and blue (B), respectively are each arranged in the form of matrix [refer to Japanese Patent Application Laid-Open Nos. 157487/1982, 147989/1983, 214593/1991; (2) a method in which an EL material which emits light with white color is combined with a color filter to take out the three primary colors of R,G,B[refer to Japanese Patent Application Laid-Open Nos. 315988 /1989, 273496/1990, 194895/1991; (3) a method in which an EL material which emits light with blue color is combined with a fluorescence conversion membrane to take out the three primary colors of R,G,B[refer to Japanese Patent Application Laid-Open No. 152897/ 1991 and the like methods. However, the above-mentioned method (1) suffers the disadvantages that the three kinds of light-emitting materials must be arranged in the form of matrix in high precision and fineness, thereby causing technological difficulty and failure in its production at a low cost, and further that the three kinds of light-emitting materials usually have each a service life frequently different from one another, whereby the chromaticity of the emitted light deviates from the normal value with the lapse of time. On the other hand, the method (2) suffers the drawback that the utilization efficiency of the EL light is lowered, that is, the conversion efficiency is lowered, since part of the output light from the EL device which emits light with white color is taken out with a color filter to utilize the light. For example, when red color is taken out by the use of a color filter from a white El color consisting simply of the three primary colors each having a same intensity, the maximum obtainable conversion efficiency is only 33%. In practical application, however, the conversion efficiency is much lower than 33% taking into consideration the emission spectrum and visibility. The method (3) is made superior to the aforesaid method (2), if the three primary colors of R,G,B are each obtained at a conversion efficiency of at least 33% in the method (3). There is well known a method in which fluorescence conversion membranes are arranged on an EL device to versatilely vary the color tone of the EL-emitting light color[ refer to Japanese Patent Application Laid-Open Nos. 18379/1988 and 152897/ 1991]. The blue color among R,G,B is emitted from the organic EL device itself, and thus may be utilized as such. In this case if the conversion efficiency is forced to be stated, it is 100%. With regard to green, a conversion efficiency of 80% is obtained by the use of coumarin 153 as is disclosed in Japanese Patent Application Laid-Open No. 152897/1991]. Nevertheless there is not yet known so far a method for converting the blue light of an EL device to red light at a conversion efficiency of at least 33%. For example, as is disclosed in Japanese Patent Publication Nos. 32879/1993 and 33514/ 1993, the light-emitting layer of the blue/green light-emitting inorganic EL device in which layer is dispersed rhodamine, that is, a red fluorescent coloring material, emits white light, thus failing to emit objective red light. likewise, white light instead of the objective red light is emitted from the blue/ green light-emitting inorganic EL device the outside of which is fitted with the fluorescence conversion membrane composed of rhodamine B[ Japanese Utility Model Application Laid-Open No.77299/1988], also from the blue / green light-emitting inorganic EL device the outside of which is fitted with the fluorescence conversion membrane composed of the pink base fluorescent coloring material (produced by Sinloihi Co. Ltd. under the trade name "FA001") Japanese Patent Application Laid-Open No. 1631519/1994]. In the case where the blue light-emitting organic EL device is fitted with the fluorescence conversion membrane composed of phenoxazone 9 and a color filter for regulating chromaticity [ Japanese Patent Application Laid-Open No. 152897/1991], there is obtained a red light having a chromaticity, x of 0.62 and y of 0.33, but the converted light thus obtained is so faint as is visible only in a bright place with an extremely low conversion efficiency. Japanese Patent Application Laid-Open No. 158091/1990 describes the fluorescent substance having a main stimulating wavelength in the range of 440 to 560 nm and a main light-emitting wavelength in the range of 510 to 650 nm. However, the above-mentioned fluorescent substance is devoid of an absorption capable of interrupting at least blue color and consequently, can emit white color only. Further, Japanese Patent Application Laid-Open No. 2205971 /1985 describes the wavelength-conversion fluorescent substance which absorbs the light having a peak wavelength in the range of 460 to 520 nm and emits light having a peak wavelength in the range of 590 to 610 nm. However, the aforesaid wavelength-conversion fluorescent substance fails to employ a fluorescent substance capable of selectively interrupting the blue color having a wavelength in the range of 460 to 520 nm and therefore, the fluorescent substance is incapable of selectively emitting red color. As described hereinbefore, since the red fluorescence pigment which is typified by rhodamine base fluorescence pigment and phenoxazone base fluorescence pigment is usually devoid of an absorption in the blue color region, the independent use of the aforesaid red fluorescence pigment in a fluorescence-reddening membrane leads to failure to sufficiently interrupt the original blue light and as a result, to selectively obtain the objective red color by reason of the mixing of the blue light with the conversion light of red color. In the case where a color filter for chromaticity regulation is superimposed on the fluorescence - reddening membrane in order to interrupt the original blue light, the red conversion efficiency is inevitably lowered. Example 14 of EP -A- 0 387 715 deals with a light-emitting device which is reported to emit a reddish orange light. The light-emitting device of example 14 comprises a electroluminescence element with, superimposed on it, two different membranes, each containing a different pigment. The document does not deal with the concept of providing a membrane comprising a combination of pigments in order to obtain the wanted shift of the emitted light from blue to red. EP -A- 0 387 715 provides a number of possible structures for the light-emitting device containing the fluorescent material. The general message is provided, that it is possible to vary the color of the resulting visible light through the appropriate combination of the emitting material and the fluorescent material. The example 14 then provides the only teaching for a fluorescent combination. The example described the preparation of the fluorescence membrane by first providing eight separate sheets of Pyridine 1 sheets with a total thickness of 600 µm and then providing two sheets of Rhodamine 6G sheets with a total thickness of 200 µm. The dispersion film of Pyridine 1 and the dispersion film of Rhodamine 6G were superimposed on the light-emitting device. This rather complicated structure was then capable to provide a light which was reddish orange in chromaticity. DISCLOSURE OF THE INVENTION It is an object of the present invention to overcome the disadvantages inherent in the prior art as described hereinbefore and at the same time, to provide an inexpensive red-emitting device which utilizes a fluorescence - reddening membrane capable of converting the color of the light emitted by a blue-emitting organic electroluminescence (EL) device to a red light at a high conversion efficiency as high as at least 33% , and enables reduction in size and thickness of itself. Under such circumstances intensive research and investigation were accumulated by the present inventors in order to achieve the above-mentioned object. As a result, it has been found that a blue color of the light emitted by an organic electroluminescence (EL) device can be converted to a red light at a high conversion efficiency as high as at least 33% by a fluorescence conversion membrane which comprises a rhodamine base fluorescence pigment and, mixed therein a specific fluorescence pigment that has absorptions in the blue region so as to sufficiently interrupt blue lights and induces effective energy transfer to or reabsorption from said rhodamine base fluorescence pigment, said resultant mixture being dispersed in a light-transmittable medium; and that the device which comprises the fluorescence -reddening membrane and a light-emitting device is capable of emitting a red light at a high conversion efficiency and reducing the size and thickness of the device itself. The present invention has been accomplished by the foregoing findings and information. That is to say, the inventors developed a fluorescence-reddening membrane which comprises a light-transmittable medium and dispersed therein, (a) a rhodamine base fluorescence pigment and (b) a fluorescence pigment that has absorptions in the blue region and induces energy transfer to or reabsorption from said rhodamine base fluorescence pigment. The present invention provides an inexpensive red light-emitting device which comprises said fluorescence-reddening membrane and a light-emitting device. It is preferable that the above-mentioned fluorescence -reddening membrane according to the present invention comprise a light-transmittable medium and, dispersed therein , a rhodamine base fluorescence pigment and a fluorescence pigment of naphthalimide base, coumarin base or the like. On the other hand, a fluorescence conversion membrane which comprises a light-transmittable medium and, dispersed therein, a rhodamine base fluorescence pigment alone, or a fluorescence pigment of naphthalimide base or coumarin base alone fails to attain a high conversion efficiency as high as at least 33% , or fails to emit a red light. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a chromaticity coordinate showing the region of each of the colors and the conversion efficiency thereof, FIG.2 is an emission spectrum of an organic EL device, FIG.3 is a transmission spectrum of a color filter for chromaticity regulation, FIG.4 is a red emission spectrum in Example 1, FIG.5 is an absorption spectrum of a fluorescence -reddening membrane in Example 1, FIG.6 is an absorption spectrum of a fluorescence -reddening membrane in Example 3, FIG.7 is an absorption spectrum of a fluorescence conversion membrane( comprising rhodamine base fluorescence pigment alone ) in Comparative Example 1, FIG.8 is an absorption spectrum corresponding to a naphthalimide base fluorescence pigment in Example 1, FIG.9 is an absorption spectrum corresponding to a coumarin base fluorescence pigment in Example 3, FIG.10 is an emission ( fluorescence ) spectrum of a naphthalimide base fluorescence pigment ( excitation at 460nm ), and FIG.11 is an emission ( fluorescence ) spectrum of a coumarin base fluorescence pigment (excitation at 460nm ). THE MOST PREFERRED EMBODIMENTS TO CARRY OUT THE INVENTION The red-emitting device according to the present invention comprises, a fluorescence-reddening membrane comprising a light-transmittable medium and, dispersed therein, (a) a rhodamine base fluorescence pigment and (b) a fluorescence pigment which has absorptions in the blue region, wherein (b) the fluorescence pigment induces energy transfer to (a) the rhodamine base fluorescence pigment or emits a fluorescence which is capable of being reabsorbed by (a) the rhodamine base fluorescence pigment; a light-emitting device; and a light-transmittable layer, which is used between the fluorescence-reddening membrane and the light-emitting device, the light-transmittable layer having a higher refractive index than air. Specifically, the fluorescence pigment which has absorptions in the blue region so that a blue light can be interrupted and at the same time, induces energy transfer to and reabsorption from the rhodamine base fluorescence pigment, preferably has absorptions in the blue region at a wavelength of 520 nm or less and an absorbance ( optical density OD ) of at least 1.0 at a wavelength of 420 to 490 nm. The aforesaid fluorescence pigment can sufficiently interrupt a blue light. In addition, the rhodamine base fluorescence pigment as the component (a) has absorptions at 450 to 610 nm. Thus the absorption region of the rhodamine base fluorescence pigment as the component(a) partially overlaps the absorption region of the fluorescence pigment as the component(b), thus enabling effective energy transfer from the component (b) to the component (a). It is also possible that the rhodamine base fluorescence pigment as the component(a) reabsorbs the fluorescence from the fluorescence pigment as the component (b). In this case the rhodamine base fluorescence pigment as the component(a) emits red fluorescence of 600 nm or more. As described hereinbefore, the fluorescence conversion membrane comprising the combination of both the fluorescence pigments dispersed in a light-transmittable medium, converts the blue light emitted by the organic EL device to a red light at a high conversion efficiency as high as 33%. Consequently, a red light emission can be assured. On the contrary, the fluorescence conversion membrane comprising the rhodamine base fluorescence pigment alone dispersed in a light-transmittable medium can not fully interrupt a blue light emitted by the organic EL device, thereby causing blue light to mix in the converted red light, and leading to failure to selectively assure a red light. In the case where a color filter for chromaticity regulation is superimposed thereon in order to interrupt the blue color, a converted red color is obtained, but the conversion efficiency is as low as less than 33% ( refer to Comparative Examples 1 & 2 ). On the other hand, the fluorescence conversion membrane comprising the naphthalimide base fluorescence pigment or the coumarin base fluorescence pigment alone dispersed in a light-transmittable medium results in failure to emit a red light, since the membrane does not contain a red base fluorescence pigment, causing it to emit a green light. The chromaticity of the red color on the CIE coordiate as used in the present invention is given in FIG.1. By the term (a) a rhodamine base fluorescence pigment as mentioned herein is meant a fluorescence pigment which contains at least one kind of rhodamine coloring matter. Specific examples thereof include rhodamine 6 G, rhodamine B, rhodamine 3 B, rhodamine 101, rhodamine 110, basic violet 11 and sulforhodamine. The ratio of said coloring matter in each of the pigments is in the range of preferably 0.1 to 10% , more preferably 1.0 to 7.0% each by weight. The ratio of said coloring matter therein of less than 0.1% by weight unfavorably leads to insufficient color emission, whereas that of more than 10% by weight unfavorably brings about deterioration of fluorescence properties due to concentration quenching by the interaction of each of the coloring matters. Examples of pigment components other than the coloring matters include polymethacrylic acid esters, polyvinyl chloride, poly(vinyl chloride/vinyl acetate) copolymer, alkyd resin, aromatic sulfonamide resins, urea resin, melamine resin and benzoguanamine resin. However, any of the foregoing coloring matters may be dispersed alone in a light-transmittable medium within the foregoing proportion. Examples of the chemical constitutions of the rhodamine coloring matters are given in the following. Any of the rhodamine coloring matters may be used alone or in combination with at least one other, and the mixture of basic violet 11 and rhodamine 6 G is preferable. On the other hand, by the term a naphthalimide base fluorescence pigment as mentioned herein is meant a fluorescence pigment which contains at least one kind of naphthalimide coloring matter. Specific examples thereof include solvent yellow 44 and solvent yellow 116. The ratio of said coloring matter in each of the pigments is in the range of preferably 0.1 to 10%, more preferably 1.0 to 7.0% each by weight. The ratio of said coloring matter therein of less than 0.1% by weight unfavorably leads to insufficient color emission, whereas that of more than 10% by weight unfavorably brings about deterioration of fluorescence properties due to concentration quenching by the interaction of each of the coloring matters. Examples of pigment components other than the coloring matters include polymethacrylic acid esters, polyvinyl chloride, poly(vinyl chloride/vinyl acetate) copolymer, alkyd resin, aromatic sulfonamide resins, urea resin, melamine resin and benzoguanamine resin. However, any of the foregoing coloring matters may be dispersed alone in a light-transmittable medium. Examples of the chemical constitutions of the naphthalimide coloring matters are given in the following. Any of the naphthalimide coloring matters may be used alone or in combination with at least one other, and the mixture of solvent yellow 116 and solvent yellow 44 is preferable. In addition, by the term a coumarin base fluorescence pigment as mentioned herein is meant a fluorescence pigment which contains at least one kind of the coumarin coloring matter. Specific examples thereof include coumarin 153, coumarin 6, coumarin 7, coumarin 30 and basic yellow 51. The ratio of the coumarin coloring matter in each of the pigment is in the range of preferably 0.1 to 10% , more preferably 1.0 to 7.0% each by weight. The ratio of the coloring matter therein of less than 0.1% by weight unfavorably leads to insufficient color emission, whereas that of more than 10% by weight unfavorably brings about deterioration of fluorescence properties due to concentration quenching by the interaction of each of the coloring matters. Examples of pigment components other than the coloring matters include polymethacrylic acid esters, polyvinyl chloride, poly(vinyl chloride/vinyl acetate) copolymer, alkyd resin, aromatic sulfonamide resins, urea resin, melamine resin and benzoguanamine resin. However, any of the foregoing coloring matters may be dispersed alone in a light-transmittable medium. Examples of the chemical constitutions of the coumarin coloring matters are given in the following. Any of the above-described coumarin coloring matters may be used alone or in combination with at least one other, among which are preferable coumarin 6 and coumarin 7 each having a high absorption coefficient. The light-transmittable medium to be used for the fluorescence-reddening membrane needs only to have light-transmittability and membrane formability without specific limitation, and is exemplified for use by high molecular compounds, inorganic glass and a medium for printing. Examples of the high molecular compounds include polyvinylpyrrolidone, polymethyl methacrylate, polymethyl acrylate, polystyrene, polycarbonate, polyvinyl acetate, polyvinyl chloride, polybutene, polyethylene glycol, a copolymer thereof, and further, photosensitive resins such as photoresists, thermosetting resins such as epoxy resins. Moreover, a pigment component other than the above-exemplified coloring matters may be used as a light-transmittable medium. Examples of the inorganic glass include borate glass and silica glass. Any of the above-exemplified light-transmittable media may be used alone or in combination with at least one other species. Of these is preferable a medium for printing, especially a light-transmittable medium for printing in which polyvinyl chloride resin or polyester resin is dissolved in a solvent. The fluorescence-reddening membrane used in the red-emitting device according to the present invention comprises, as principal components, the component(a), the fluorescence pigment as the component (b) and the light-transmittable medium. The fluorescence pigment as the component(b) is preferably contained in the aforesaid fluorescence-reddening membrane in an amount of 5 to 80% by weight. The amount thereof less than 5% by weight leads to insufficiency in coloring matter density required to assure the desired red light and to the necessity for extremely thickening the membrane ( as thick as 100µm or more), whereby uniform membrane thickness is made difficult to assure; whereas the amount thereof more than 80% by weight brings about poor membrane-formability as well as mechanically brittle membrane. From the viewpoints of red color purity and membrane formability, the above-mentioned fluorescence pigment is dispersed in the light-transmittable medium preferably in an amount in the range of 10 to 60% by weight. The ratio of the (a) rhodamine base fluorescence pigment to the (b) naphthalimide-base or coumarin-base fluorescence pigment, each being used in the invention depends upon the species of the fluorescence pigments, and it is preferably in the range of 20:1 to 1:20 by weight. In the case where the ratio by weight of the (a) rhodamine base fluorescence pigment exceeds the aforestated range, the amount of the (b) naphthalimide-base or coumarin - base fluorescence pigment decreases, whereby the blue light emitted by the organic EL device is made impossible to be sufficiently interrupted and accordingly the desired red light is made unobtainable. Specifically, when the optical density ( OD ) at a wavelength in the range of 420 to 490 nm, that is, the blue light region, is made to less than 1.0, the blue light leaks remarkably, thereby making the desired red light difficult to obtain. On the other hand,in the case where the ratio by weight of the (a) rhodamine base fluorescence pigment is made lower than the aforestated range, the energy transfer from the (b) fluorescence pigment to the (a) rhodamine base fluorescence pigment, or reabsorption from the pigment(a) decreases, thereby making the desired red light difficult to obtain in high efficiency. Therefore, from the aspect of the red light purity and the conversion efficiency, the ratio by weight of the (a) rhodamine base fluorescence pigment to the (b) fluorescence pigment which has absorptions in the blue region and thus interrupts blue light is more preferably in the range of 10:1 to 1: 10. The process for producing the fluorescence-reddening membrane used in the red-emitting device according to the present invention is not specifically limited, but can be selected for use from a variety of processes. For example, the objective fluorescence-reddening membrane is obtained by a process in which the (a) rhodamine base fluorescence pigment and the (b) naphthalimide-base or coumarin-base fluorescence pigment are mixed with and dispersed in the light-transmittable medium, and the mixture thus produced is formed into a membrane by any of the methods including casting, spin coating, printing, bar coating, extrusion molding, roll molding, pressing, spraying and roll coating. An organic solvent, when used for the membrane forming, is exemplified by dichloromethane; 1,2-dichloroethane; chloroform; acetone; cyclohexanone; toluene; benzene; xylene; N,N-dimethylformamide; dimethylsulfoxide; 1,2-dimethoxyethane; diethylene glycol dimethyl ether; N-methylpyrrolidone; ethylene glycol monomethyl ether( methyl Cellosolve ); ethylene glycol monoethyl ether ( ethyl Cellosolve ); ethylene glycol monoethyl ether acetate( ethyl Cellosolve acetate ). Any of these solvents may be used alone or in combination with at least one other. In the case of casting, for example, a fluorescence-reddening membrane in the form of thin film can be produced by dissolving the fluorescence pigments and the light-transmittable medium in a suitable solvent selected from among the aforestated solvents, and gradually adding dropwise the solution thus obtained onto a substrate such as a glass substrate to evaporate the solvent. Aside from the foregoing, the red-emitting device according to the present invention comprises a light emitting device. A blue light-emitting organic EL device is preferably usable for said light emitting device, and can be prepared by any of the methods disclosed in Japanese Patent Application Laid-Open Nos. 47890/1991, 231970/1991, 17765/1993, 135878/1993, 140145/1993, 247458/1993, 247459/1993, 100857/1994, 132080/1994. For example, the blue light-emitting organic EL device can be fabricated by a method wherein a transparent supporting substrate composed of a glass substrate on which a membrane of indium / tin oxide( hereinafter abbreviated to"ITO") is formed as an electrode is subjected to vacuum deposition successively by the use of 4,4'-bis[ N-phenyl-N-(3-methylphenyl) amino] biphenyl ( TPD ); 4,4'-bis(2,2-diphenyl- vinyl)-biphenyl ( DPVBi ); tris(8-quinolinol)aluminum ( Alq ) ; and magnesium / silver electrode to form a multi-layer structure (refer to Preparation Example 1). Since the light-emitting device and the fluorescence-reddening membrane are brought into tight contact with each other in the preparation of the red-emitting device according to the present invention, there is used, between each of the membranes, a material having a high refractive index as compared with air such as the above-described light-transmittable medium in order to prevent light scattering and also enhance the conversion efficiency. For the sake of simplicity, it is possible to insert an inert liquid such as a fluorohydrocarbon therebetween( refer to Examples 1& 2 ). It is a further aspect of the present invention to provide a fluorescence-reddening membrane which comprises a light-transmittable medium dispersed therein (a) a rhodamine base fluorescence pigment and (b) a fluorescence pigment which has absorptions in the blue region, wherein (b) the fluorescence pigment induces energy transfer to (a) the rhodamine base fluorescence pigment or emits a fluorescence which is capable of being reabsorbed by (a) the rhodamine base fluorescence pigment, and (b) the fluorescence pigment has absorptions in the blue region at a wavelength of 520 nm or less and an optical density OD of 1.0 or more at a wavelength in the range of 420 to 490 nm. For the further details of the fluorescence-reddening membrane according to the present invention we refer to the general description provided hereinbefore in connection with the description of the red-emitting device according to the invention. In the following, the present invention will be described in more detail with reference to working examples. Preparation Example 1 ( preparation of blue-emitting organic EL device) A transparent supporting substrate was prepared by forming a membrane of an ITO electrode in a thickness of 100 nm on a glass substrate sized 25 mm× 75 mm× 1.1 mm, and was subjected to ultrasonic cleaning by the use of isopropyl alcohol for 5 minutes, to cleaning with pure water for 5 minutes, and further to ultrasonic cleaning by the use of isopropyl alcohol for 5 minutes. Thereafter, the resultant transparent supporting substrate was fixed to a substrate holder of a vacuum deposition apparatus. In a molybdenum - made boat for resistance heating was placed 200 mg of 4,4'-bis[ N-phenyl-N- (3-methylphenyl) amino] biphenyl ( TPD ). Further, in another molybdenum-made boat for resistance heating was placed 200 mg of 4,4'-bis(2,2-diphenylvinyl)biphenyl ( DPVBi ); and also 200 mg of tris (8-quinolinol) - aluminum ( Alq ). Then, the inside of a vacuum chamber was depressurized to 1× 10 -4 Pa. The boat containing the TPD was heated to 215 to 220°C to deposit the TPD onto the substrate at a deposition rate of 0.1 to 0.3 nm / second to form a membrane in a film thickness of 60 nm as a hole injection layer, while the substrate maintained room temperature. In addition, without taking the resultant hole injection layer out of the vacuum chamber, the DPVBi was deposited onto the substrate at a boat temperature of 250°C at a deposition rate of 0.1 to 0.2 nm / second to form a membrane in a film thickness of 40 nm as an light-emitting layer. Subsequently, the Alq was further deposited onto the substrate at a boat temperature of 250°C at a deposition rate of 0.1 to 0.3 nm / second to form a membrane in a film thickness of 20 nm as an electron-transporting layer. The substrate thus treated was taken out of the vacuum chamber, equipped with a stainless-steel made mask on the side of the electron-transporting layer, and again fixed to the substrate holder. Then 0.5 g of silver wire was put in a tungsten - made basket; 1.g of magnesium ribbon was put in a boat made of molybdenum; the inside of the vacuum chamber was depressurized to 1× 10 -4 Pa; and silver ( deposition rate of 0.1 nm/sec ) and magnesium deposition rate of 0.8 nm/sec ) were simultaneously deposited to form a cathode membrane, thereby preparing a blue-emitting organic EL device. The emission spectrum of the resultant organic EL device is given in FIG.2. Preparation Example 2 ( Preparation of color filter for chromaticity regulation ) A color filter for chromaticity regulation was prepared by subjecting a red color resist( produced by Fuji Hunt Electronics Technology Co.,Ltd. under the trade name CR-2000 ) to spin coating on a glass substrate, drying it at 80°C in an oven and curing at 200°C in the oven. The resultant membrane had a thickness of 2.3 µm as measured with a surface roughness meter. The transmission spectrum of the membrane is given in FIG.3. Example 1 A bar coated membrane to be used as a fluorescence-reddening membrane was prepared by dissolving 0.12 g of a naphthalimide base fluorescence pigment containing benzoguanamine resin, 2% by weight based on the same of solvent yellow 116 and 6% by weight based on the same of solvent yellow 44; and 0.2 g of a rhodamine base fluorescence pigment containing benzoguanamine resin, 2% by weight based on the same of basic violet 11 and 2% by weight based on the same of rhodamine 6G, in 2 g of an ink ( solid content of 40% by weight ) in which polyvinyl chloride resin ( molecular weight of 20,000 ) as a printing medium was dissolved in cyclohexanone. The resultant membrane had a thickness of 36 µm as measured with a micrometer. Subsequently, the blue-emitting organic EL device ( chromaticity x of 0.16 and y of 0.15, blue ) which had been obtained in Preparation Example 1 was allowed to emit light under the conditions including a voltage of 7V and a current density of 4.2 mA /cm 2 , and was brought into tight contact with the fluorescence-reddening membrane thus prepared, while a fluorohydrocarbon liquid ( produced by 3 M Co., Ltd. under the trade name FC-70 ) was inserted therebetween so as to form a red-emitting deveice. A measurement was made of the brightness of the output light by means of a brightness meter( produced by Minolta Co., Ltd. under the trade name CS-100 ). As a result, the blue light prior to the superimposition of the fluorescence-reddening membrane had a brightness of 100 cd/m 2 , whereas the red light after the superimposition thereof having a chromaticity x of 0.59 and y of 0.34 was obtained with a brightness of 34 cd/m 2 and with a conversion efficiency of 34%. The emission spectrum of the red light thus obtained is given in FIG.4. Moreover, the absorption spectrum of the fluorescence -reddening membrane thus prepared is given in FIG.5. It can be seen that in the absorption spectrum corresponding to the naphthalimide base fluorescence pigment ( FIG.8 ), said pigment has absorptions in the blue region, that is, at 520 nm and less and also has an optical density OD of 1.0 or more at a wavelength in the range of 420 to 490 nm, thereby sufficiently interrupting the blue light from the organic-EL device. It is further understood that effective energy transfer is made to the rhodamine base fluorescence pigment by partially overlapping the absorption spectrum corresponding thereto ( FIG.7), or the rhodamine base fluorescence pigment is allowed to reabsorb the light emitted by the naphthalimide base fluorescence pigment ( FIG, 10 ). Example 2 A bar coated membrane to be used as a fluorescence-reddening membrane was prepared by dissolving 1.3 g of a rhodamine base fluorescence pigment containing benzoguanamine resin, 1% by weight based on the same of basic violet 11 and 1% by weight based on the same of basic red 1; 20 mg of solvent yellow 116; and 20mg of solvent yellow 44, in 2.3 g of an ink ( solid content of 40% by weight ) in which polyvinyl chloride resin( molecular weight of 20,000 ) as a printing medium was dissolved in cyclohexanone. The resultant membrane had a thickness of 60 µm as measured with a micrometer. Subsequently, the blue-emitting organic EL device ( chromaticity x of 0.16 and y of 0.15, blue ) which had been prepared in Preparation Example 1 was allowed to emit light under the conditions including a voltage of 7V and a current density of 4.2 mA /cm 2 , and was brought into tight contact with the fluorescence-reddening membrane thus prepared, while a fluorohydrocarbon liquid( FC-70 ) was inserted therebetween so as to form a red-emitting device. A measurement was made of the brightness of the output light by means of a brightness meter( produced by Minolta Co., Ltd . under the trade name CS-100 ). As a result, the blue light prior to the superimposition of the fluorescence-reddening membrane had a brightness of 100 cd/m 2 , whereas the red light after the superimposition thereof having a chromaticity x of 0.60 and y of 0.33 was obtained with a brightness of 33 cd/m 2 and with a conversion efficiency of 33% The absorption spectrum of the fluorescence-reddening membrane thus prepared was the same as that in FIG.5. Example 3 A bar coated membrane to be used as a fluorescence-reddening membrane was prepared by dissolving 0.12 g of a coumarin base fluorescence pigment containing benzoguanamine resin and 5% by weight based on the same of coumarin 6 ; and 0.2 g of a rhodamine base fluorescence pigment same as that used in Example 1, in 2 g of an ink ( solid content of 40% by weight ) in which polyester resin( molecular weight of 40,000 ) as a printing medium was dissolved in ethyl cellosolve acetate. The resultant membrane had a thickness of 35 µm as measured with a micrometer. Subsequently, the blue-emitting organic EL device ( chromaticity x of 0.16 and y of 0.15, blue ) which had been obtained in Preparation Example 1 was allowed to emit light under the conditions including a voltage of 7V and a current density of 4.2 mA /cm 2 , and was brought into tight contact with the fluorescence-reddening membrane thus prepared, while a fluorohydrocarbon liquid( FC-70 )was inserted therebetween so as to form a red-emitting deveice. A measurement was made of the brightness of the output light by means of a brightness meter( produced by Minolta Co., Ltd. under the trade name CS-100 ). As a result, the blue light prior to the superimposition of the fluorescence-reddening membrane had a brightness of 100 cd/m 2 , whereas the red light after the superimposition thereof having a chromaticity x of 0.60 and y of 0.32 was obtained with a brightness of 33 cd/m 2 and with a conversion efficiency of 33%. Moreover, the absorption spectrum of the fluorescence -reddening membrane thus prepared is given in FIG.6. It can be seen that in the absorption spectrum corresponding to the coumarin base fluorescence pigment ( FIG.9 ), said pigment has absorptions in the blue region, that is, at a wavelength of 520 nm and less and also has an optical density OD of 1.0 or more at a wavelength in the range of 420 to 490 nm, thereby sufficiently interrupting the blue light from the organic EL device. It is further understood that effective energy transfer is made to the rhodamine base fluorescence pigment by partially overlapping the absorption spectrum corresponding thereto ( FIG.7), or the rhodamine base fluorescence pigment is allowed to reabsorb the light emitted by the coumarin base fluorescence pigment( FIG.11 ). Example 4 A bar coated membrane to be used as a fluorescence-reddening membrane was prepared by dissolving 1.3 g of the rhodamine base fluorescence pigment same as that used in Example 2 and 30 mg of coumarin 7, in 2.3 g of an ink ( solid content of 40% by weight ) ) in which polyester resin ( molecular weight of 40,000 ) as a printing medium was dissolved in ethyl cellosolve acetate. The resultant membrane had a thickness of 45 µm as measured with a micrometer. Subsequently, the blue-emitting organic EL device ( chromaticity x of 0.16 and y of 0.15, blue ) which had been prepared in Preparation Example 1 was allowed to emit light under the conditions including a voltage of 7V and a current density of 4.2 mA /cm 2 , and was brought into tight contact with the fluorescence-reddening membrane thus prepared, while a fluorohydrocarbon( FC-70 ) liquid was inserted therebetween so as to form a red-emitting deveice. A measurement was made of the brightness of the output light by means of a brightness meter( produced by Minolta Co., Ltd . under the trade name CS-100 ). As a result, the blue light prior to the superimposition of the fluorescence-reddening membrane had a brightness of 100 cd/m 2 , whereas the red light after the superimposition thereof having a chromaticity x of 0.59 and y of 0.33 was obtained with a brightness of 33 cd/m 2 and with a conversion efficiency of 33%. Moreover, the absorption spectrum of the fluorescence -reddening membrane thus prepared was same as that in FIG.6. Comparative Example 1 A bar coated membrane to be used as a fluorescence-reddening membrane was prepared by dissolving 0.2 g of the rhodamine base fluorescence pigment same as that used in Example 1, in 2.0 g of an ink ( solid content of 40% by weight ) in which polyvinyl chloride resin( molecular weight of 20,000 ) as a printing medium was dissolved in cyclohexanone. The resultant membrane had a thickness of 40 µm as measured with a micrometer. Subsequently, the blue-emitting organic EL device ( chromaticity x of 0.16 and y of 0.15, blue which had been prepared in Preparation Example 1 was allowed to emit light under the conditions including a voltage of 7V and a current density of 4.2 mA /cm 2 , and was brought into tight contact with the fluorescence-reddening membrane thus prepared, while a fluorohydrocarbon liquid( FC-70 ) was inserted therebetween so as to form a red-emitting device. A measurement was made of the brightness of the output light by means of a brightness meter( produced by Minolta Co., Ltd . under the trade name CS-100 ). As a result, the blue light prior to the superimposition of the fluorescence-reddening membrane had a brightness of 100 cd/m 2 , whereas a light after the superimposition thereof having a chromaticity x of 0.56 and y of 0.23 other than the red light was obtained with a low brightness of only 10 cd/m 2 and with a low conversion efficiency of only 10% Moreover, the absorption spectrum of the fluorescence -reddening membrane thus prepared is given in FIG.7. It can be seen therefrom that the absorption spectrum is devoid of absorptions in the blue region, that is, devoid of optical density OD of 1.0 or more at a wavelength in the range of 420 to 490 nm, thereby failing to sufficiently interrupt the blue light from the organic El device. It is further understood that effective energy transfer is never made to the rhodamine base fluorescence pigment from the blue light of the organic EL device, and that the fluorescence pigment is not allowed to reabsorb the emitted light, thus causing failure to generate red light and bringing about a low conversion efficiency. Comparative Example 2 A cast membrane to be used as a fluorescence-reddening membrane was prepared by dissolving 4.2 mg of rhodamine B and 1.8 g of polyvinyl pyrrolidinone( PVP, molecular weight of 360,000, approx. ) in dichloromethane. The resultant membrane had a thickness of 50 µm as measured with a micrometer. Subsequently, the blue-emitting organic EL device ( chromaticity x of 0.16 and y of 0.15, blue ) which had been obtained in Preparation Example 1 was allowed to emit light under the conditions including a voltage of 7V and a current density of 4.2 mA /cm 2 . The blue-emitting organic EL device, the fluorescence-reddening membrane thus prepared and the membrane of the color filter for chromaticity regulation which had been obtained in Preparation Example 2 were brought into tight contact with one another in this order, while a fluorohydrocarbon liquid ( FC-70 )was inserted therebetween so as to form a red-emitting deveice. A measurement was made of the brightness of the output light by means of a brightness meter( produced by Minolta Co., Ltd. under the trade name CS-100). As a result, the blue light prior to the superimposition of the fluorescence-reddening membrane and the membrane of the color filter for chromaticity regulation had a brightness of 100 cd/m 2 , whereas a red light after the superimposition thereof having a chromaticity x of 0.56 and y of 0.28 was obtained with a low brightness of only 5 cd/m 2 and with a low conversion efficiency of only 5%. It can be seen therefrom that, although the red light was obtained by the location of the color filter interrupting the blue color, effective energy transfer is never made to the rhodamine base fluorescence pigment from the blue light of the organic EL device, and that the fluorescence pigment is not allowed to reabsorb the emitted light, thus bringing about a low conversion efficiency. As a conclusion, the fluorescence-reddening membrane according to the present invention can convert the light emitted by a blue-emitting organic EL device at a high conversion efficiency as high as 33% or more, and besides the red-emitting device which comprises the fluorescence-reddening membrane and a light-emitting device can emit a red light at a high conversion efficiency, curtail its production cost, and reduce the size and thickness of itself. INDUSTRIAL APPLICABILITY The red-emitting device according to the present invention is favorably used for a back light of office automation ( OA ) equipment, a clock and watch, a back light of each of various displays and a self-emitting multi-color or full-color display.

Claims: A red-emitting device which comprises, (1) a fluorescence-reddening membrane comprising a light-transmittable medium and, dispersed therein, (a) a rhodamine base fluorescence pigment and (b) a fluorescence pigment which has absorptions in the blue region, wherein (b) the fluorescence pigment induces energy transfer to (a) the rhodamine base fluorescence pigment or emits a fluorescence which is capable of being reabsorbed by (a) the rhodamine base fluorescence pigment; (2) a light-emitting device; and (3) a light-transmittable layer, which is used between the fluorescence-reddening membrane and the light-emitting device, the light-transmittable layer having a higher refractive index than air. The red-emitting device according to Claim 1, wherein (b) the fluorescent pigment dispersed in the light-transmittable medium is a naphthalimide base fluorescence pigment. The red-emitting device according to Claim 1, wherein (b) the fluorescent pigment dispersed in the light-transmittable medium is a coumarin base fluorescence pigment. The red-emitting device according to Claim 2 or 3, wherein (a) the rhodamine base fluorescence pigment comprises a mixture of basic violet 1 and rhodamine 6G. The red-emitting device according to Claim 2, wherein the naphthalimide base fluorescence pigment comprises a mixture of solvent yellow 116 and solvent yellow 44. The red-emitting device according to Claim 3, wherein the coumarin base fluorescence pigment comprises coumarin 6 or coumarin 7. The red-emitting device according to any one of Claims 1 to 3, wherein the light-transmittable medium is a printing medium. A fluorescence-reddening membrane which comprises a light-transmittable medium dispersed therein (a) a rhodamine base fluorescence pigment and (b) a fluorescence pigment which has absorptions In the blue region, wherein (b) the fluorescence pigment induces energy transfer to (a) the rhodamine base fluorescence pigment or emits a fluorescence which is capable of being reabsorbed by (a) the rhodamine base fluorescence pigment, and (b) the fluorescence pigment has absorptions in the blue region at a wavelength of 520 nm or less and an optical density OD of 1.0 or more at a wavelength in the range of 420 to 490 nm. The fluorescence-reddening membrane according to Claim 8, wherein (b) the fluorescent pigment dispersed in the light-transmittable medium is a naphthalimide base fluorescence pigment. The fluorescence-reddening membrane according to Claim 8, wherein (b) the fluorescent pigment dispersed in the light-transmittable medium is a coumarin base fluorescence pigment. The fluorescence-reddening membrane according to Claim 9 or 10, wherein (a) the rhodamine base fluorescence pigment comprises a mixture of basic violet 11 and rhodamine 6G. The fluorescence-reddening membrane according to Claim 9, wherein the naphthalimide base fluorescence pigment comprises a mixture of solvent yellow 116 and solvent yellow 44. The fluorescence-reddening membrane according to Claim 10, wherein the coumarin base fluorescence pigment comprises coumarin 6 or coumarin 7. The fluorescence-reddening membrane according to any one of Claims 8 to 10, wherein the light transmittable medium is a printing medium.

Inventors: EIDA MITSURU, IKEDA HIDETSUGU, TSUCHIYA JUN, EIDA, MITSURU, IKEDA, HIDETSUGU, TSUCHIYA, JUN
Applicants: IDEMITSU KOSAN CO, IDEMITSU KOSAN COMPANY LIMITED
Application year: 1996
Main classification: C09K11
IPC codes: F21V9, C09K11