WO2010106933A1

WO2010106933A1 - Inorganic phosphor particle and dispersion-type electroluminescence device using the same

Info

Publication number: WO2010106933A1
Application number: PCT/JP2010/053743
Authority: WO
Inventors: Masashi Shirata
Original assignee: Fujifilm Corporation
Priority date: 2009-03-17
Filing date: 2010-03-02
Publication date: 2010-09-23
Also published as: JP2010215787A; TW201038717A

Abstract

Inorganic phosphor particles for dispersion-type electroluminescence, each of the inorganic phosphor particles containing: zinc sulfide, wherein the inorganic phosphor particles have an average particle size of 1 mm or more and less than 20 mm and a variation coefficient of 3% or more and less than 40% with respect to particle sizes, and electroluminescence from the inorganic phosphor particles has emission maximum in a wavelength range of from 480 nm to 520 nm.

Description

DESCRIPTION

INORGANIC PHOSPHOR PARTICLE AND DISPERSION-TYPE ELECTROLUMINESCENCE DEVICE USING THE SAME

Technical Field

The present invention relates to high-luminance, long-life electroluminescence (EL) powder particles, and further to a dispersion-type EL device (hereinafter abbreviated as "EL device" too) that has a light-emitting layer formed by dispersion-coating the EL powder particles.

Background Art

EL phosphor are voltage-excited phosphor, and dispersion-type EL devices, namely light-emitting devices each of which is formed by sandwiching power of such a phosphor between electrodes, and thin-film EL devices are known. The dispersion- type EL devices are generally configured so as to have a structure that phosphor powder dispersed in a binder with a high dielectric constant is sandwiched between two electrodes at least one of which is transparent, and they emit light through application of an alternating electric field between the two electrodes. The light- emitting devices using EL phosphor powder can be made with thicknesses of several millimeters or below, and have many advantages that they are surface illuminants, liberate little heat, have high luminous efficiency and so on. Therefore, they are expected to have uses e.g. as road signs, a wide variety of interior and exterior illuminations, light sources for flat-panel displays such as liquid-crystal displays and illuminating light sources for large-size advertisements. i However, the light-emitting devices made by using phosphor powder have drawbacks of being low in emission intensity and short in emission lifetime as compared to light-emitting devices based on other principles, and this is a reason why attempts have been made to introduce various improvements.

Further, as a method for enhancement of luminescence intensity, the method of using phosphor particles of small sizes is known (JP-A-2002-235080 or JP-A- 2004-265866, the term "JP-A" as used herein refers to an "unexamined published Japanese patent application"). An EL device formed using phosphor particles of small sizes allows the phosphor particles in its light-emitting layer to increase in number per unit volume; as a result, luminance of the EL device can be enhanced. However, the EL devices using phosphor particles of small sizes have a drawback of hastening degradation in luminance of the EL device.

In addition, JP-A-2006-63317 discloses the electroluminescence phosphor including particles which have an average size in a range of 0.5 μm to 20 μm and at least 10 stacking fault layers at intervals of at most 5 nm, and besides, which contain copper as their luminescence centers and further contain gold, cesium, bismuth or so on.

Summary of Invention

Up to now, however, there has been no example of reports on luminescence wavelengths and life spans of dispersion-type EL devices.

Therefore, the invention aims to provide inorganic phosphor particles for use in dispersion-type EL devices, which have a specified average particle size and a specified variation coefficient with respect to particle sizes, produce luminescence at a specific wavelength, and moreover excel in luminescence intensity and have an improved luminescence life, and further to provide a dispersion-type EL device using such inorganic phosphor particles.

As a result of our intensive studies, it has been found that, when an inorganic phosphor having an average particle size of 1 μm or more and less than 20 μm and a variation coefficient of 3% or more and less than 40% with respect to its particle sizes and showing its emission maximum in a wavelength range of 480 nm to 520 nm is produced and introduced into a dispersion-type electroluminescence device, both enhancement of luminescence intensity and extension of luminescence lifetime can be achieved, thereby making the invention.

(1) Inorganic phosphor particles for dispersion-type electroluminescence, each of the inorganic phosphor particles containing: zinc sulfide, wherein the inorganic phosphor particles have an average particle size of 1 μm or more and less than 20 μm and a variation coefficient of 3% or more and less than 40% with respect to particle sizes, and electroluminescence from the inorganic phosphor particles has emission maximum in a wavelength range of from 480 nm to 520 nm.

(2) The inorganic phosphor particles as described in (1), each of the inorganic phosphor particles further containing:

Cu as an activator in a concentration of from 0.10 to 0.16 mol% per mol of Zn.

(3) The inorganic phosphor particles as described in (1) or (2), wherein at least 30% by number of the inorganic phosphor particles are particles each having at least 10 stacking fault planes at intervals of at most 5 nm.

(4) The inorganic phosphor particles as described in any of (1) to (3), each of the inorganic phosphor particles further containing: a co-activator selected from the group consisting of Cl, Br and I.

(5) A dispersion-type electroluminescence device using the inorganic phosphor particles as described in any of (1) to (4).

(6) The dispersion-type electroluminescence device as described in (5), further containing a red conversion material at any location within layers of the dispersion-type electroluminescence device.

(7) The dispersion-type electroluminescence device as described in (5), further containing red light-emitting phosphor particles aside from the inorganic phosphor particles at any location within layers of the dispersion-type electroluminescence device.

Additionally, the term "phosphor particles" as used in the invention means particles that produce luminescence through application of a voltage.

Description of Embodiments

Detail descriptions of the invention are presented below. <Phosphor Particles>

In the invention, the inorganic phosphor particles showing their emission maxima in a wavelength range of 480 μm to 520 μm are zinc sulfide particles, and their average size is 1 μm or more and less than 20 μm and the variation coefficient of particle sizes is 3% or more and less than 40%.

The phosphor particles in the invention can be formed by a firing method (solid-phase method) widely used in this field. For instance, zinc sulfide is made into fine-particle powder ranging in particle size from 10 nm to 50 nm (usually referred as "crude powder"), and this is used as primary particles, i.e. a base material. Zinc sulfide can form crystals on two crystal systems, a hexagonal system which is stable at high temperatures and a cubic system which is stable at low temperatures. The zinc sulfide used in the invention may be crystals on either of those systems or a mixture of crystals on both of those systems. These crystals, together with flux in addition to impurities referred to as activators and co-activators, are placed in a crucible and fired over a time period from 30 minutes to 10 hours at high temperatures ranging from 900°C to 1,300⁰C, whereby intermediate phosphor particles are prepared. The suitable firing temperature for forming the present phosphor particles having the above-specified average size and low variation coefficient with respect to particle sizes is from 95O⁰C to 1,250⁰C, preferably from 1,000⁰C to 1,200⁰C. And the suitable firing time is from 30 minutes to 6 hours, preferably from 1 hour to 4 hours. The suitable proportion of the flux used is 20 mass% or higher, preferably 30 mass% or higher, far preferably 40 mass% or higher. The proportion (mass%) of flux used is given by the expression, proportion (mass%) of flux = {mass of flux/(mass of primary particles of raw-material phosphor + mass of flux)} x 100. As is the case in the copper-activated zinc sulfide phosphor described hereinafter, when copper as an activator is previously mixed in crude powder, the activator copper is integral with the raw-material powder of phosphor. In this case, the mass of the raw-material powder of phosphor, inclusive of copper, is adopted as the measured value. There may be cases where the mass of flux measured at room temperature is different from that measured at the firing temperature. For instance, while barium chloride is in a state of BaCl₂2H₂O at room temperature, at the firing temperature it is supposed that barium chloride looses water of hydration and comes to have a chemical composition Of BaCl₂. Herein, the proportion of flux is therefore calculated on the basis of the mass of flux in a state of stability at room temperature. And subsequently the thus prepared intermediate phosphor powder is subjected to second firing. Heating (annealing) in the second firing is carried out at a temperature of 500⁰C to 800°C that is lower than the heating in the first firing for 30 minutes to 3 hours that is shorter than the heating in the first firing. By this heating, it is possible to allow the activator to deposit with a concentration on stacking faults described hereinafter.

Next the intermediate phosphor is etched with an acid, such as hydrochloric acid, to eliminate metal oxides adhering to its surface, and further washed with KCN or the like to eliminate the activator adhering to the surface. Then, the thus treated phosphor undergoes drying to give an electroluminescence phosphor.

In the foregoing manner, particles having an average size in a range of 1 μm or more and less than 20 μm and a variation coefficient of 3% or more and less than 40% with respect to particle sizes can be obtained.

For average size and size variation coefficient measurements on the present phosphor particles, methods based on laser scattering, as applied in Laser Diffraction/Scattering Particle Size Distribution Analysis System LA-920 made by HORIBA, Ltd., can be used. Herein, the term "average size of particles" refers to the median particle diameter.

As the activator used in the invention, Cu is suitable. As to Cu sources, there is no restriction, and copper sulfate, copper sulfide, copper chloride, copper nitrate and the like can be used. The amount of copper added, though not particularly limited, is from IxIO^"5 to 5xlO^"2 mole, preferably from IxIO^"4 to IxIO^"2 mole, far preferably from 5XlO^"4 to 8χlO^"3 mole, per mole of base material (zinc sulfide).

When Cu is used as an activator, the suitable Cu concentration in the present inorganic phosphor particles is from 0.10 to 0.16 mol% per mol of Zn. The Cu concentrations of 0.16 mol% per mol of Zn or below are preferred because they don't cause a problem that Cu added in excess of the amount required for doping the particles separates out on the particle surface and therewith the particle surface is stained. And the Cu concentrations of 0.10 mol% per mol of Zn or above are also preferred because they allow formation of luminescence centers in sufficient quantity. The Cu content is preferably from 0.11 to 0.15 mol% per mol of Zn, far preferably from 0.120 to 0.140 mol% per mol of Zn.

The Cu content can be adjusted by controlling the amount of a Cu source mixed in the crude powder.

Quantitative determination of the Cu content can be made by dissolving phosphor particles in hydrochloric acid or nitric acid or, in some cases, aqua regia and performing analysis on the resulting solution by ICP spectrometry (Inductively Coupled Plasma emission spectrometry).

As a co-activator used in the invention, at least one element chosen from halogens F, Cl and Br is suitable. Of these halogens, Cl and Br are preferred. The amount of a co-activator used is not particularly limited and, when the co-activator is F, Cl or Br, renewed addition thereof is unnecessary because the flux functions also as a supplier of the co-activator.

The phosphor material in the invention is zinc sulfide. It is known that, when the activator used is Cu, the Cu in the zinc sulfide phosphor generally acts as an acceptor and pairs up with a co-activator which assumes the role of a donor, thereby forming the green luminescence center designated as G-Cu. There is the emission maximum of G-Cu in a wavelength range of 480 to 520 run. Likewise, the blue luminescence center designated as B-Cu and formed of Cu situated between lattices of zinc sulfide has its emission maximum in a wavelength range of 440 to 470 nm. In other words, Cu forms two kinds of luminescence centers in zinc sulfide. In the case of forming G-Cu, Cu occupies the lattice position of Zn in zinc sulfide, while the co-activator such as F, Cl or Br occupies the lattice position of S. In the case of B-Cu, the lattice position of Zn becomes a point defect since Cu is present between lattices in zinc sulfide.

On the other hand, it is known that zinc sulfide is apt to develop point defects because it generally becomes rid of part of sulfur or zinc during the firing, which affects its performance as a luminescent material. The invention has found that Cu and its co-activator can efficiently make up for point defects and avoid occurrence of point defects by forming G-Cu efficiently without forming B-Cu and controlling the luminescence wavelengths to the 480- to 520-nm range; as a result, high intensity and long lifetime of luminescence can be achieved.

When the average particle size is in a range of 1 to 20 μm in particular, such values are small as compared to a range of 20 to 30 μm in which general EL phosphor have their average particle sizes, and they bring about large surface areas and high susceptibility to defect formation. Therefore, it has been required to form G-Cu with higher efficiency. The invention makes it possible to inhibit point defects from developing by adjusting the firing conditions and Cu content as appropriate.

In the invention, it is preferable that the intermediate phosphor particles prepared by the firing is further subjected to washing with ion exchanged water for elimination of redundant activator, co-activator and flux contained therein.

In the interior of the intermediate phosphor particles prepared by firing, there are stacking fault planes (twin crystal structures) which develop spontaneously. Further application of an impactive force having its magnitude in a certain range to the particles can substantially increase the density of the stacking faults without destroying the particles. As hitherto known methods for applying an impactive force, there are a method of bringing the intermediate phosphor particles into contact with one another and mixing them together, a method of mixing and mingling the phosphor particles with spherical powder like alumina (ball mill), a method of accelerating the particles and making them collide with one another, and so on. In zinc sulfide in particular, there are two crystal systems, a cubic system and a hexagonal system. In the former system, close-packed atomic planes ((111) planes) take on a three-layer structure built up in the sequence A B C A B C - - ; while in the latter system, close-packed atomic planes, which are perpendicular to the c-axis, take on a two-layer structure built up in the sequence A B A B - - . On this account, when an impact is given to zinc sulfide crystals by means of a ball mill or the like, slippage of close-packed atomic planes is caused in the cubic system and C-planes drop out, whereby the crystals partly take on a hexagonal structure of the sequence A B A B; as a result, there occurs edge dislocation, and twinning may also occur by a reversal in the sequence of planes A and B. Since impurities in crystals are generally concentrated onto areas of lattice defects, an activator such as copper sulfide is deposited on stacking faults when made to diffuse into zinc sulfide having stacking faults by application of heat. Because the interface between the activator-deposited area and zinc sulfide as the matrix becomes the center of electroluminescence phosphor, it is preferable that at least 30% by number of the present phosphor particles are particles each of which includes at least 10 stacking fault planes at intervals of at most 5 nm. It is far preferred that the particles each including at least 10 stacking fault planes at intervals of at most 5 nm make up at least 50%, especially at least 80%, of the present particles.

Additionally, there is no particular restriction as to the crystal structure of the phosphor particles. In the zinc sulfide particles, the blende structure (cubic crystal structure) and the Wurtzite structure (hexagonal crystal structure) may be present in any proportions. Detailed descriptions of general stacking faults can be found in B. Henderson, Lattice Defects (Koshi Kekkan, Japanese version translated by Doyama Masao and published by MARUZEN Co., Ltd.), Chapters 1 and 7. In the case of zinc sulfide, the stacking faults are described in Andrew C. Wright & Ian V.F. Viney, Philosohical Mag., B, 2001, Vol. 81, No. 3, pp. 279-297.

The stacking faults are evaluated by observation of multilayer structure developing on particle sides (particle surfaces) when phosphor particles are etched by an acid such as hydrochloric acid.

As other methods for forming phosphor, vapor-phase methods including a laser abrasion method, a CVD method, a plasma method and a combination of a sputtering method, a resistance heating method, an electron-beam method or the like with a vacuum evaporation on running oil substrate technique, liquid-phase methods including a double decomposition method, a method of utilizing pyro lysis reaction of precursors, an inverted micelle method, combinations of these methods with high- temperature firing, a freeze drying method and the like, a urea fusion method, a spray pyrolysis method, and so on can be used too.

In addition, it is preferable that the present phosphor particles contain at least one kind of metal element which belongs to the secondary transition elements from the Group 6 to the Group 10 of the periodic table. Of these elements, molybdenum, platinum and iridium are preferred over the others. The amount of such a metal contained in zinc sulfide is preferably from IxIO^"7 mole to IxIO^"3 mole, far preferably from Ix 10^"6 mole to 5XlO^"4 mole, per mole of zinc sulfide. It is preferable that such a metal is incorporated into zinc sulfide particles by adding it to deionized water together with fine powder of zinc sulfide and a specified amount of copper sulfate and making them into slurry, and subjecting the slurry to thorough mixing, then drying, and further firing together with a co-activator and flux. Alternatively, it is also preferable that incorporation of such a metal into zinc sulfide particles is performed by firing the particles together with a co-activator and flux which is mixed in advance with a complex powder containing the metal. In either case, although any compound containing a metal element to be incorporated can be used as a source compound for metal doping, a complex containing the metal or metal ion coordinating with oxygen or nitrogen atoms is preferably used. The ligands thereof may be any compounds, whether inorganic or organic. Such a complex allows evermore increases in luminance and longevity.

The phosphor particles may have non-luminescent shell layers on their respective surfaces. Details of such a non-luminescent shell layer are the same as those disclosed e.g. in JP-A-2005-283911, paragraphs [0028] to [0033]. <EL Device>

The dispersion-type electroluminescence device using the present inorganic phosphor particles (hereinafter abbreviated as the present EL device or the like too) is described below.

The dispersion-type EL device using the present inorganic phosphor particles has at least one light-emitting layer containing the present inorganic phosphor particles between a pair of opposed electrodes e.g. one of which is a transparent electrode. Between the light-emitting layer and an electrode, dielectric layers such as an insulating layer and a cut-off layer are preferably disposed in order to prevent an electrical breakdown of the EL device and concentrate a stable electric field onto the light-emitting layer. <Light-emitting layer>

When an EL device is made by using the present phosphor particles, the particles are dispersed into an organic dispersion medium, and the resulting dispersion liquid is coated and formed into a light-emitting layer.

As the organic dispersion medium, though an organic polymer material or an organic solvent having a high boiling temperature is usable, an organic binder mainly composed of an organic polymer material is preferably used.

The organic binder is preferably a material having a high dielectric constant, with examples including fluorine-containing polymer compounds (e.g. polymers containing polymerization units derived from fluorinated ethylene and trifluorochloroethylene), polysaccharides having cyanoethylated hydroxyl groups (e.g. cyanoethyl pullulan, cyanoethyl cellulose), polyvinyl alcohol (e.g. cyanoethylated polyvinyl alcohol), phenolic resins, polyethylene, polypropylene, polystyrene resins, silicone resins, epoxy resins, vinylidene fluoride resins and the like. And it is preferable that the organic binder contains all or part of those resins. In addition, the dielectric constant of the organic binder can also be adjusted by mixing with fine particles having a high dielectric constant, such as BaTiO₃ or SrTiO₃ particles, in an appropriate amount.

As a method of dispersing the phosphor particles into the binder, the method of using a homogenizer, a planetary mill, a roll mill, an ultrasonic dispersing machine or the like can be adopted.

As to the mixing proportion between such a binder and the phosphor particles, the content of the phosphor particles in the light-emitting layer is adjusted to a range of preferably 30 mass% to 90 mass%, far preferably 60 mass% to 85 mass%, with respect to the total solids in the light-emitting layer. By this adjustment, the light-emitting layer formed can have a smooth surface.

It is especially preferred that a macromolecular compound having cyanoethylated hydroxyl groups be used in such an amount as to constitute at least 20% by mass ratio, preferably at least 50% by mass ratio, of the organic dispersion medium of the overall light-emitting layer.

The thickness of the light-emitting layer thus prepared is preferably from 20 μm to less than 80 μm, far preferably from 25 μm to less than 75 μm. The thicknesses of 20 μm or above are preferable because the surface of the light-emitting layer can have satisfactory smoothness, and the thicknesses less than 80 μm are also preferable because an electrical field can be applied effectively to the phosphor particles. In a special case where the cut-off layer described below is provided, designing the light-emitting layer to be thick in addition to the insulating layer described below to be thin is preferable because it allows recovery from lowering of initial luminance and ensures sufficient durability effect. In order to achieve better initial luminance, the thickness of the light-emitting layer is preferably adjusted to 70 μm or below. <Cut-off Layer>

The present EL device may have a cut-off layer between a transparent electrode and the light-emitting layer. Details of such a cut-off layer are the same as those disclosed e.g. in JP-A-2007- 12466, paragraphs [0013] to [0020]. <Insulating Layer>

The insulating layer in the present EL device can use any materials so long as they are high in dielectric constant and insulation strength and have high breakdown voltages. The materials can be chosen from metal oxides or nitrides. For example, BaTiO₃, KNbO₃, LiNbO₃, LiTaO₃, Ta₂O₃, BaTa₂O₆, Y₂O₃, Al₂O₃, AlON and the like can be used. These compounds may be provided in a state of uniform film, or they may be used as film containing an organic binder and having a particulate structure. For example, the film made up Of BaTiO₃ fine particles and BaTiO₃ sol as described in Mat. Res. Bull., vol. 36, p. 1065 can be used. It is advisable for the film to have a thickness from 10 μm to less than 35 μm. And the film thickness is preferably from 12 μm to less than 33 μm, far preferably from 15 μm to less than 31 μm. Too thin thicknesses are undesirable because they tend to cause electrical breakdown, while too thick thicknesses are also undesirable because voltage applied to the light-emitting layer becomes low and substantially reduces luminescence efficiency.

Examples of an organic binder usable in the insulating layer include polymers relatively high in dielectric constant, such as cyanoethyl pullulan, cyanoethylated polyvinyl alcohol and cyanoethyl cellulose resins, polyethylene, polypropylene, polystyrene resins, silicone resins, epoxy resins, vinylidene fluoride resins and the like. It is also possible to control the dielectric constant by mixing those resins with the right amount of fine particles having a high dielectric constant, such as BaTiO₃ and SrTiO₃. As a method of dispersing such particles into the binder resin, the method of using a homogenizer, a planetary mill, a roll mill, an ultrasonic dispersing machine or the like can be adopted. <Red Color Material>

For the present electroluminescence device, it is advantageous to produce white luminescence from the viewpoint of extending the range of uses to which the device is put. In order to produce white luminescence, a red light-emitting material is used in addition to zinc sulfide particles producing luminescence which shows its emission maximum in a wavelength range of 480 to 520 nm. The red light-emitting material may be an organic material capable of absorbing the luminescence of zinc sulfide and converting to red color, or it may be an inorganic material capable of showing red electroluminescence. As the former, organic fluorescent dyes or fluorescent pigments in particular can be used to advantage, and they may be dispersed into the light-emitting layer or the insulating layer, or placed between the light-emitting layer and a transparent electrode or on the side facing the light- emitting layer across the transparent electrode. The latter can be incorporated into the light-emitting layer in common with the zinc sulfide phosphor particles or, in addition to the layer containing zinc sulfide phosphor particles, a red inorganic phosphor material layer can be introduced between the transparent electrode and the insulating layer.

As the phosphor particles emitting light in red preferably used in the invention, similarly to the inorganic phosphor particles as the present zinc sulfide phosphor material, the base material thereof is selected from the group consisting of compounds containing one or plural elements belonging to the Group II elements and one or plural elements belonging to the Group VI elements of the periodic table, and compounds containing one or plural elements belonging to the Group III elements and one or plural elements belonging to the Group V elements of the periodic table. These elements are chosen arbitrarily according to the wavelength region of luminescence required. The activator, though not particularly restricted, is chosen from transition metals such as Cu and Mn, and the co-activator is chosen from the Group VII elements such as F, Cl, Br and I or the Group III elements such as Al, Ga and In. It is also preferable that those base materials are further doped with rare earth elements such as Ce, Eu and Sm. To be more specific, the material of phosphor particles capable of producing red luminescence is ZnS:Cu,In, CaS:Eu,Ce or the like. Red conversion organic materials are particular described in detail below.

When the present electroluminescence device is designed to show white luminescence, the wavelength of red luminescence is preferably adjusted to fall in a range of 590 nm to 650 nm. In order to achieve red-luminescence wavelengths included in such a range, a red conversion material may be incorporated into the light-emitting layer or introduced between the light-emitting layer and the transparent electrode or introduced to the side facing the light-emitting layer across the transparent electrode. However, it is best to incorporate the conversion material into the insulating layer. And it is much preferred that the insulating layer in the device is divided into two or more sections and part of the sections is made into a layer containing the red conversion material, though it is also preferable that a material for conversion to a red color is incorporated in throughout the insulating layer in the present electroluminescence device. The layer containing a red conversion material is preferably situated between an insulating layer containing no red conversion material and the light-emitting layer, and it is also advantageous for the insulating layer containing a red conversion material to be placed so that two insulating layers containing no red conversion material straddle the insulating layer containing a red conversion material.

In the case of placing a layer containing a red conversion material between an insulating layer containing no red conversion material and the light-emitting layer, the thickness of the layer containing a material for conversion to a red color is preferably from 1 μm to 20 μm, far preferably from 3 μm to 17 μm. In the insulating layer to which a red conversion material is added, the concentration of the red conversion material is preferably from 1 mass% to 20 mass%, far preferably from 3 mass% to 15 mass%, based on the dielectric particles, typified by BaTiO₃. In the case of placing a layer containing a red conversion material so that two insulating layers containing no red conversion material straddle the insulating layer containing a material for conversion to a red color, the thickness of the layer containing a red conversion material is preferably from 1 μm to 20 μm, far preferably from 3 μm to 10 μm. In the insulating layer to which a red conversion material is added, the concentration of the red conversion material is preferably from 1 mass% to 30 mass%, far preferably from 3 mass% to 20 mass%, based on the dielectric particles. In the case where two insulating layers containing no red conversion material straddle the layer containing a red conversion material, it is also preferable that the layer containing a red conversion material is formed into a dielectric particles-free layer which contains only a binder having a high dielectric constant and the red conversion material.

The luminescence wavelength in a case where the material used here for conversion to a red color is in a powdery state is preferably from 590 run to 750 run, far preferably from 600 run to 650 run, especially preferably from 605 run to 630 nm. The red luminescence wavelength in another case where the red conversion material is added to an electroluminescence device and the resulting device produces electroluminescence is preferably from 590 nm to 650 nm as mentioned above, far preferably from 595 nm to 630 nm, especially preferably from 600 nm to 620 nm.

Suitable examples of a binder in the layer containing a red conversion material include polymers relatively high in dielectric constant, such as cyanoethyl pullulan, cyanoethylated polyvinyl alcohol and cyanoethyl cellulose, polyethylene, polypropylene, polystyrene resins, silicone resins, epoxy resins and vinylidene fluoride resins.

As in common with the foregoing, fluorescent pigments and fluorescent dyes can be preferably used in the invention. Suitable examples of compounds forming the luminescence centers of those pigments and dyes include compounds having as their respective skeletons rhodamine, lactone, xanthene, quinoline, benzothiazole, triethylindoline, perylene, triphennine and dicyanomethylene. Besides these compounds, cyanine dyes, azo dyes, polyphenylenevinylene polymers, disilaneoligothienylene polymers, ruthenium complexes, europium complexes and erbium complexes can be used to advantage. These compounds may be used alone or as combinations of two or more different kinds. Additionally, these compounds may be used after they are dispersed into polymers or the like. As a fluorescent pigment having its fluorescent maximum in the foregoing wavelength range, SEL- 1003, a product of SINLOIHI Co., Ltd., can be used.

And wavelengths of maximum light emissions from those fluorescent pigments and dyes can be adjusted to the range as specified above by use of filters such as a band reflection filters. <Transparent Electrode>

A transparent electrode can be prepared by making a transparent conductive material, such as indium tin oxide (ITO), tin oxide, antimony-doped tin oxide, zinc- doped tin oxide or zinc oxide, adhere evenly to a glass substrate as a matter of course or a transparent film such as a polyethylene terephthalate or triacetyl cellulose base and forming the material into film by means of an evaporation method, a coating method, a printing method or so on.

Alternatively, a multilayer structure formed by sandwiching a thin film of silver between layers of high refractive indexes may be used. In addition, conductive polymers including conjugated polymers such as polyaniline and polypyrrole can be used to advantage. Descriptions of these transparent conductive materials can be found e.g., in Denjiha Shield Zairyo no Genjo to Shorai, published by Toray Research Center, and JP-A-9-147639.

As the transparent electrode, it is also advantageous to use a transparent conductive sheet having a current-carrying property improved by forming a conductive surface provided with a uniform reticulate-, comb- or grid-type fine-line structural member made of a metal and/or an alloy on a transparent conductive sheet, which is formed by depositing the transparent conductive material on the transparent film and forming the deposit into film, or on a conductive polymer film.

When the transparent conductive sheet or conductive polymer film is used in combination with those fine lines, copper, silver, nickel and aluminum are preferably used as materials for fine lines of metal or alloy. Depending on the intended purposes, however, the transparent conductive materials as recited above may be used instead of metals or alloys. And they are preferably materials high in electrical conductivity and thermal conductivity. The fine lines may have any width, but the width thereof is preferably between about 0.1 μm and 1,000 μm. Moreover, the fine lines are aligned preferably with 50-μm to 5-cm pitches, particularly preferably with 100-μm to 1 -cm pitches.

The height (thickness) of the fine-line structural member is preferably from 0.1 μm to 10 μm, particularly preferably from 0.5 μm to 5 μm. Although either of the fine-line structural member and the transparent conductive film may be the top, the smoothness (unevenness) of the resultant conductive surface is preferably 5 μm or below. From the viewpoint of close contact, the smoothness is preferably from 0.01 μm to 5 μm, particularly preferably from 0.05 μm to 3 μm.

Here the smoothness (unevenness) of a conductive surface refers to the mean amplitude of asperities on a 5 square millimeters of conductive surface, as measured by a three-dimensional roughness meter (e.g. SURFCOM 575A-3DF, made by TOKYO SEIMITSU Co., Ltd.). As to microscopic asperities beyond the resolving power of the meter used, the smoothness is evaluated by measurements with a scanning tunneling microscope (STM) or an electron microscope.

As to the width-height-pitch relationship of fine lines, the suitable width of fine lines, though may be chosen according to the intended purpose, is typically from 1/10000 to 1/10 the fine-line pitch.

The same goes for the height of fine lines, and the range of 1/100 to 10 times the width of fine lines is suitable as the height range of the fine lines.

The surface resistivity of a transparent electrode usable in the invention is preferably from 0.1 Ω/D to 100 Ω/D, far preferably from 1 Ω/D to 80 Ω/D, as determined in conformance with the measuring method described in JIS K6911.

When the transparent electrode is provided with a fine-line structural member made of a metal and/or alloy, it is appropriate to control the reduction in light transmittance. Therefore, it is preferred that the light transmittance of 90% or above be achieved by adjusting the pitch, width and height of fine lines to the foregoing ranges.

In the invention, it is appropriate that the transparent electrode have a transmittance of 70% or above, preferably 80% or above, especially preferably 90% or above, as measured by light with a wavelength of 550 nm.

For the purposes of enhancing the luminance and ensuring white luminescence, it is appropriate that the transparent electrode allow at least 80%, preferably at least 90%, of light with wavelengths in a region of 420 nm to 650 nm to pass through it. And from the viewpoint of ensuring white luminescence, it is preferred that the transparent electrode allow at least 80% of light with wavelengths in a region of 380 nm to 680 nm. The light transmittance of a transparent electrode can be measured with a spectrophotometer. <Back Electrode>

For a back electrode on the side of which light is not taken out, any of materials having conductivity can be used. The material can be chosen appropriately from metals, such as gold, silver, platinum, copper, iron and aluminum, or graphite with reference to the form of the device to be made, the temperature during the making process and so on. Alternatively, any transparent electrodes, limited only by conductivity, e.g. ITO, may be used as the back electrode. Moreover, from the viewpoint of enhancing the durability, it is important for the back electrode to have high thermal conductivity, and the thermal conductivity of the back electrode is preferably 2.0 W/cm-deg or above, especially 2.5 W/cm-deg or above.

In addition, for the purpose of ensuring high degrees of thermal radiation and current passage on the periphery of the EL device, it is also favorable to use a metal sheet or metal mesh as the back electrode. <Manufacturing Method>

The present EL device is not particularly restricted as to its manufacturing method, and the methods as described in detail in JP-A-2007- 12466, paragraphs [0046] to [0049], can be adopted as appropriate. <Sealing>

At the final stage, the present dispersion-type EL device is preferably processed with sealing film so as to have immunity to moisture and oxygen from the external environment. Details of the sealing are the same as those described in JP-A- 2007-12466, paragraphs [0050] to [0055]. The invention is especially effective in applications for which the EL device is used in a state of producing luminescence of high intensity (e.g. 600 cd/m² or above). More specifically, the invention is effective in the cases of using the EL device under a driving condition that a voltage from 100V to 500 V is applied between the transparent electrode and the back electrode or under a condition that driving is performed using an alternator of frequencies ranging from 800 Hz to 4,000 KHz.

Examples

Examples of the present dispersion-type EL device are illustrated below, but the present dispersion-type EL device should not be construed as being limited to these examples. EXAMPLE 1

On a 70 μm-thick aluminum electrode (back electrode), the layers shown below, the first layer (30 μm in thickness) and the second layer (55 μm in thickness), were formed in order of mention by application of coating solutions for forming the individual layers, and further a polyethylene terephthalate film (75 μm in thickness) on which indium-tin oxide was deposited and formed into a 40 μm-thick transparent electrode by sputtering was laid so that the transparent electrode side thereof (conductive surface side) faced the aluminum electrode side, and besides, the transparent electrode and the second layer, a phosphor particles-containing layer (light-emitting layer), were adjacent to each other, and then they were subjected to pressure bonding with a 19O⁰C heat roller under atmosphere of nitrogen.

The amounts of additives in each layer as shown below are expressed in mass per square meter of the EL device. Each layer was formed by adjusting the viscosity of each coating solution by addition of dimethylformamide, applying the resultant coating solution to form a coating, and then drying the coating at 110⁰C for 10 hours. First Layer: Insulating Layer (free of red conversion material) Cyanoethyl pullulan 14.0 g

Cyanoethylated polyvinyl alcohol 10.0 g

Barium titanate particles

(average sphere-equivalent diameter: 0.05 μm) 100.0 g

Second Layer: Light-emitting layer

Cyanoethyl pullulan 18.0 g

Cyanoethylated polyvinyl alcohol 12.0 g

Phosphor particles 120.O g

Methods of making varieties of phosphor particles and characteristics of the phosphor particles made by each method are described below.

Water was added to 150 g of fine powder ZnS (purity 99.999%, produced by Furuuchi Chemical Corporation) and stirred to form slurry, and thereto an aqueous solution containing 0.538 g of CuSO₄5H₂O was added to prepare ZnS crude powder (average particle size: 100 nm) partly replaced with Cu (1.4χlO^"3 mole of Cu per mole of ZnS). A 25.0 g portion of the crude powder prepared was mixed with 4.0 g of S powder, and thereto 4.2 g of BaCl₂2H₂O, 11.2 g of MgCl₂6H₂O and 9.0 g of SrCl₂6H₂O were further added as flux. The resultant mixture was fired at 1 ,200⁰C for 4 hours in atmosphere of Ar to prepare a phosphor intermediate. The phosphor intermediate was washed 5 times with ion exchanged water, and dried. The thus washed intermediate was pulverized for 40 minutes with a ball mill, and then annealed at 700⁰C for 6 hours. The phosphor particles thus obtained were cleaned with a 10% aqueous solution of KCN heated to 70⁰C in order to eliminate redundant copper (copper sulfide) on the particle surface, and then washed 5 times with water. Thus, phosphor particles A were obtained.

Phosphor particles B were made in the same manner as the phosphor particles A were made, except that the amount of the flux added was changed to the following: 2.1 g of BaCl₂2H₂O, 4.25 g of MgCl₂ OH₂O and 1.0 g of SrCl₂ OH₂O.

Phosphor particles C were made in the same manner as the phosphor particles A were made, except that the amount of CuSO₄5H₂O was changed to 0.231 g-

Phosphor particles D were made in the same manner as the phosphor particles B were made, except that the amount of CuSO₄5H₂O was changed to 0.384 g-

Phosphor particles E were made in the same manner as the phosphor particles A were made, except that the amount of CuSO₄5H₂O was changed to 0.614 g-

Phosphor particles F were made in the same manner as the phosphor particles A were made, except that the amount of CuSO₄5H₂O was changed to 0.481 g-

Phosphor particles G were made in the same manner as the phosphor particles A were made, except that the amount of CuSO₄5H₂O was changed to 0.384 g-

Phosphor particles H were made in the same manner as the phosphor particles A were made, except that the amount of CuSO₄5H₂O was changed to 0.652 g- The average particle size, coefficient of variation in particle sizes and Cu content of the phosphor particles of each description are shown in Table 1. Additionally, the Cu content was measured by ICP emission spectroscopy of a solution prepared by dissolving phosphor particles in aqua regia, and expressed as a percentage on Zn. And the average particle size and the coefficient of variation in particle sizes were measured with Laser Diffraction/Scattering Particle Size Distribution Analysis System LA-920 made by HORIBA, Ltd.

Further, coatings were made using the phosphor particles of all descriptions, respectively. Each coating and a film provided with a transparent electrode were bonded together under pressure, then electrode terminals (60 μm-thick aluminum plates) were wired to an aluminum electrode and the transparent electrode, respectively, and further these members were sandwiched in between moisture-proof films, GX film produced by TOPPAN PRINTING CO., LTD., and subjected to sealing by thermocompression bonding under vacuum degassing. Thus, each EL device was made. The wavelength of emission maximum each EL device showed when driven at settings of 120V and 1.4kHz is also listed in Table 1.

Table 1

The phosphor particles used in each of the EL devices 101 and 105 to 108 were small in both average size and coefficient of size variation, and the emission maximum of each EL device was within the range specified by the invention. In contrast to these devices, the average particle size and the coefficient of particle size variation in the EL device 102, the wavelength of emission maximum shown by the EL device 103, and not only the average particle size and the coefficient of particle size variation in the EL device 104 but also the wavelength of emission maximum shown by the EL device 104 were all outside the scope of the invention. Furthermore, relative values of initial luminance value of each device with respect to the luminance value of EL device 101 driven by 120 V and 1.4 kHz being 100, and luminance value of each device after continuously driving for 150 hours on a condition where the driving voltage of each device is adjusted by fine control so as to obtain the same initial luminance value and the initial luminance value is taken as 100. The results are shown in Table 2.

Table 2

The EL devices 101 and 105 to 108 were high in not only initial luminance values but also luminance values after 150-hour continued driving, compared with the other devices. This is because, since the initial luminance of the other devices was low, high driving voltages were required for the other devices to achieve the same luminance as the EL device 101 delivered; as a result, excessive load was imposed on the particles. Conversely, the EL device 101 can achieve the needed luminance under a less load; as a result, degradation of the device can be retarded. EXAMPLE 2

Phosphor particles I were made in the same manner as adopted in the making of the phosphor particles A, except that the pulverization with the ball mill was carried out for 120 minutes.

The thus obtained phosphor particles were etched with 3N HCl, and the multilayer structures developing on particle sides (particle surfaces) were observed by use of an electron microscope under an acceleration voltage condition of 200 kV. As a result, it was found that the proportion by number of particles each of which includes a part having at least 10 stacking faults planes at intervals of at most 5 run was 64% in the case of the phosphor particles I as against 41% in the case of the phosphor particles A.

When an EL device 201 made by using the phosphor particles I and carrying out the same method as in Example 1 was evaluated by the same method as adopted in Example 1, the EL device 201 showed almost the same wavelength of emission maximum as the EL device 101 showed. On the other hand, the voltage required for the EL device 201 to achieve the luminance equivalent to that of the EL device 101 was 112V and the luminance value after the EL device 201 underwent 150-hour continued driving was 89. Thus, reduction in voltage and extension of lifetime were ascertained. EXAMPLE 3

Dry powder prepared from 25 g of zinc sulfide (ZnS) powder particles by adding thereto copper sulfate pentahydrate in a concentration of 1.3x10^"3 mole/mole with respect to zinc and iridium chloride in an amount of 2XlO^"4 mole/mole with respect to of zinc, aluminum nitrate as a co-activator in a concentration of 3x10^"3 mole/mole with respect to zinc, ammonium chloride (NH₃Cl) powder as flux in an appropriate amount and magnesium oxide powder in an amount of 10 mass% based on the phosphor powder were placed in an alumina crucible and fired at l,150°C for 2 hours. Thereafter, the temperature of the substance in the crucible was lowered. After 5 g of the fired particles and 20 g of 1-mm alumina balls were charged into a 15-mm-φ glass jar and subjected to 60-minute ball milling at a rotation speed of 10 rpm, an intermediate phosphor particles were separated from the alumina balls by means of a 100-mesh sieve. To the thus separated intermediate phosphor particles, 5 g of zinc oxide and 0.25 g of sulfur were further added, thereby making dry powder. The dry powder thus made was charged into the alumina crucible once again, and fired at 700°C for 6 hours. The fired particles were pulverized again, dispersed into water warmed to 40⁰C, and made to precipitate. After removal of the supernatant liquor, the precipitated particles was washed, and then mixed with a 10 mass% aqueous solution of hydrochloric acid, subjected to successive dispersion, sedimentation and supernatant removal, whereby unnecessary salts were eliminated, and dried. Further, a 10 mass% aqueous solution of KCN was heated to 70⁰C and used for eliminating oxides including ZnO present on the particle surface. Furthermore, the particles were etched with 6N hydrochloric acid, and thereby the surface layer thereof was removed in an amount accounting for 10 mass% of the particles in their entirety. Thus, phosphor particles J were obtained.

In addition, phosphor particles K were made in the same manner as the phosphor particles J were made, except that aluminum nitrate was not added but, in place thereof, NaCl and MgCl₂ were added as a co-activator (flux) in equal amounts of 12 mass% based on the ZnS powder used as the raw material.

The average particle sizes, coefficients of particle size variations, Cu contents and stacking faults of the phosphor particles J and the phosphor particles K, respectively, are shown in Table 3. Additionally, the stacking faults were viewed as the percentage by number of particles each of which includes a part having at least 10 stacking fault planes at intervals of at most 5 nm.

Table 3

In order to make evaluations by the same method as in Example 1 , EL devices 301 and 302 were made using the phosphor particles J and the phosphor particles K, respectively.

As in the case of Example 1, emission maximum, relative values of initial luminance, with the EL device 301 being taken as 100, and relative luminance values after the 150-hour continued driving, which the EL devices 301 and 302 showed respectively, are listed in Table 4. Table 4

Thus, it has been shown that the use of Cl, instead of Al, as a co-activator can enhance initial luminance and retard degradation.

Industrial Applicability

The present inorganic phosphor particles and the dispersion-type electroluminescence device using these inorganic phosphor particles ensure compatibility between high intensity and long lifetime of luminescence. In addition, when the present device is designed to produce white luminescence in combination with a red color material, it can show excellent color reproducibility (color rendering).

This application is based on Japanese patent application JP 2009-064410, filed on March 17, 2009, the entire content of which is hereby incorporated by reference, the same as if set forth at length.

Claims

1. Inorganic phosphor particles for dispersion-type electroluminescence, each of the inorganic phosphor particles comprising: zinc sulfide, wherein the inorganic phosphor particles have an average particle size of 1 μm or more and less than 20 μm and a variation coefficient of 3% or more and less than 40% with respect to particle sizes, and electroluminescence from the inorganic phosphor particles has emission maximum in a wavelength range of from 480 nm to 520 nm.

2. The inorganic phosphor particles according to claim 1, each of the inorganic phosphor particles further comprising:

Cu as an activator in a concentration of from 0.10 to 0.16 mol% per mol of Zn.

3. The inorganic phosphor particles according to claim 1 or 2, wherein at least 30% by number of the inorganic phosphor particles are particles each having at least 10 stacking fault planes at intervals of at most 5 nm.

4. The inorganic phosphor particles according to any of claims 1 to 3, each of the inorganic phosphor particles further comprising: a co-activator selected from the group consisting of Cl, Br and I.

5. A dispersion-type electroluminescence device using the inorganic phosphor particles according to any of claims 1 to 4.