JP2005038634A - Current injection light-emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cheap current injection light-emitting element having high brightness, high stability, long life, and good color rendering property in spite of its very simple structure, which can be laminated with a simple method like a dip method or spin-coat method. <P>SOLUTION: The current injection light-emitting element is a light emission center of multiple elements doped with nano-particles with an inorganic fluorescent material as a matrix, and layers are formed between a light-emitting layer and a positive electrode, and between the light-emitting layer and a negative electrode which are different from each other, and the layers are laminated with a simple method like the dip method or the spin-coat method by using solution with low solubility into adjacent layers. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a current injection type light emitting device, and more particularly to a current injection type light emitting device that exhibits planar white light emission when energized.
[0002]
[Prior art]
An electroluminescent element is a light-emitting device that uses electroluminescence of a solid and fluorescent substance. At present, inorganic electroluminescent elements using inorganic materials as light emitters have been put into practical use, and backlights and flat displays for liquid crystal displays. Some applications are being developed.
[0003]
However, since the inorganic electroluminescence element has a high voltage required for light emission of 100 V or higher and is difficult to emit blue light, it is difficult to achieve full color using the three primary colors of RGB.
[0004]
On the other hand, research on electroluminescent devices using organic materials has attracted attention for a long time, and various studies have been made. However, since the luminous efficiency is very low, full-scale practical research has not progressed.
[0005]
However, in 1987, Kodak's C.I. W. Tang et al. Proposed an organic electroluminescence device having a function-separated stacked structure in which an organic material is divided into two layers, a hole transport layer and a light-emitting layer, and 1000 cd / m despite a low voltage of 10 V or less. ² It has been clarified that the above high luminance can be obtained (see Non-Patent Document 1).
[0006]
Since then, organic electroluminescence elements have attracted a great deal of attention, and research on organic electroluminescence elements having the same function-separated layered structure is actively conducted.
[0007]
However, organic electroluminescence elements using these low-molecular light-emitting materials are generally produced using a vacuum deposition apparatus, and therefore there is a demand for organic electroluminescence elements that can be produced by inexpensive methods with high equipment costs. It was.
[0008]
In response to this demand, there is an organic electroluminescence element using a polymer light emitting material. This element is described in (Non-Patent Document 2) and the like.
[0009]
These have also been energetically developed in recent years, but there is a limit to increasing the area by spin coating. A method of manufacturing using an ink jet method has also been proposed, but there have been problems with the lifetime and the uniformity of light emission during continuous light emission.
[0010]
In addition, the biggest problem when these low-molecular or high-molecular organic compounds are used for the light-emitting material is that the lifetime during driving is short. There have been many studies on this issue, but it was still a big issue. Furthermore, in order to obtain white light emission, conventionally, a blue, red, green, blue, and yellow light emitting material is mixed as a dopant in the same layer as the host material, or mixed into different layers to obtain white color. In this case as well, since deterioration due to driving of various materials is different, there is a serious drawback that chromaticity changes.
[0011]
Conventionally, inorganic phosphors have been widely used for display applications such as CRT and PDP, illumination applications, and inorganic EL. In recent years, it has become clear that the quantum yield of fluorescence improves when the phosphor is nanosized.
[0012]
An example in which an inorganic fluorescent material is applied to a light emitting material is well known as the above-mentioned inorganic EL, but there are few examples in which it is applied to a current injection type light emitting device that emits light by injecting holes and electrons. In addition, although the luminescent material using a metal complex is known widely (for example, nonpatent literature 3), these have the ligand which consists of organic materials, and are different from a pure inorganic luminescent material. .
[0013]
In addition, a wet method for synthesizing ultrafine particles of yttrium, aluminum, and garnet (YAG) has been reported, but there is no mention of their fluorescence emission, and this is applied to a current injection type luminescent material. That is not suggested. As for fine particles obtained by doping zinc into gallium nitride (GaN), there is a report on photoluminescence as a powder (Non-Patent Document 4). However, this report does not mention any application other than the use as a phosphor for field emission display, and the particle size is as large as 0.2 μm to 3 μm.
[0014]
In recent years, europium (II) compounds (EuSi ₂ ) And Si are sputtered on a silicon substrate in an argon gas atmosphere, and further annealed at 1000 ° C. to obtain EuSiO ₃ And EuSiO ₄ It is reported that an element that emits white light is obtained by sputtering an ITO thin film as a cathode on the thin film (Non-patent Document 5). However, this device has a drawback that a vacuum apparatus is required, it is difficult to increase the area, and a high temperature treatment of 1000 ° C. is required.
[0015]
As an example of a light-emitting element using an ultrafine particle fluorescent material, (Non-patent Document 6) or (Non-patent Document 7) reports the fabrication and performance of the element for ultrafine CdSe particles. A similar example is described in (Non-Patent Document 8). However, these have low luminous efficiency and have not been put to practical use. Further, since it is laminated with a polymer-type light emitting material, there is a great drawback that when the driving voltage is increased, the polymer light-emitting material emits light and the emission color changes.
[0016]
Furthermore, as an EL device using ultrafine particles (Patent Document 1), a light emitting material is composed of ultrafine particles, and a generation layer of a substance different from that of the light emitting material or a generation layer having different properties is generated on the surface of the EL device. Alternatively, an ultrafine particle phosphor is disclosed in which a light emitting mechanism is introduced at the interface between a light emitting material and a generation layer by forming a PN junction. However, only an AC drive device is shown as an application example thereof, and there is no disclosure that light is emitted by a current injection type.
[0017]
In addition, (Patent Document 2) describes that an electroluminescent device that injects holes and electrons is driven with a direct current using an ultrafine particle semiconductor of 20 nm or less as a light emitting material. It was not a satisfactory level.
[0018]
A light emitting layer is formed by applying a fluorescent substance on a flat substrate by vapor deposition, printing, or the like, and is sandwiched between electrodes and energized to obtain light emission. Inorganic and organic electroluminescent elements (hereinafter referred to as inorganic EL elements, respectively) Organic EL elements, collectively referred to as EL elements).
[0019]
Inorganic EL elements have already been put into practical use as timepiece dials, backlights for display parts of portable devices, and the like, and organic EL elements are entering the age of practical use due to recent research progress. Unlike point light sources such as light emitting diodes, these elements are expected to have unique applications because light emission is obtained in a planar shape.
[0020]
With regard to organic EL, in particular, it attracts a lot of attention because it has higher light emission efficiency and higher luminance at low DC voltage than inorganic EL. There are many researches to construct a flat display.
[0021]
As described above, the EL element is expected to be developed in the future. However, at present, there is a limit to the light emission intensity per area (hereinafter, sometimes referred to as luminance), and for example, a backlight with good visibility even outdoors. There is a problem that it is difficult to apply to lighting and lighting applications.
[0022]
In addition, when used for illumination, white light emission excellent in color rendering is required. In the current organic EL, it is necessary to use different luminescent dyes for R, G, and B. Therefore, there is a problem of preparing electrodes and a laminated state optimized for each dye. In addition, there is a problem that chromaticity fluctuates during use due to different deterioration of individual dyes.
[0023]
In order to solve such a problem, an RGB light-emitting element in which an R · G pigment is doped in polyvinyl carbazole as described in (Non-patent Document 9) has been proposed. The proposed device is an EL device that emits light after energy transfer to an R · G guest dye using polyvinylcarbazole as a host molecule for energy transfer. In this structure, if electrons and holes as carriers are injected with polyvinyl carbazole as a host molecule, then energy is transferred to the R • G dye to emit light, so it is appropriate if the doping concentration of the R · G dye is controlled. It can be expected to emit white light. However, the light emission mechanism at the light emission center is accompanied by a transition from the excited state of the organic molecule, and the stability as a practical light emitting device has been questioned.
[0024]
[Non-Patent Document 1]
Tan (C. W. Tang), V. Vanslyke, “Appl. Phys. Lett” (USA), Vol. 51, 1987, p. 913
[Non-Patent Document 2]
JH Burroughes, “Nature” (UK), 347, 1990, p. 539
[Non-Patent Document 3]
Miyata et al. (Ed., Miyata Seizou and Hari Singh Nalwa), “Organic Electroluminescence Materials and Devices”, Chapter 9
[Non-Patent Document 4]
Kanie et al., “2nd International Symposium on Blue Laser and Light Emitting Diodes” 1998, p. 552
[Non-Patent Document 5]
Bharbagara et al., "Appl. Phys. Lett" (USA), 74, 1999, p. 3203
[Non-Patent Document 6]
Alivisatos et al., “Nature” (UK), 370, 1994, p. 354
[Non-Patent Document 7]
Alivisatos et al., “Journal of Applied Physics”, Vol. 82, 1997, p. 5837
[Non-Patent Document 8]
Bawendi et al., Applied Physics Letter (Appl. Phys. Lett) "(USA), 66, 1995, p. 1316
[Patent Document 1]
Japanese Patent Publication No. 1-18117
[Patent Document 2]
US Patent No. 5537000
[Non-patent document 9]
Research group of Kido et al., Yamagata University, “The 51st Polymer Discussion (2002) Proceedings No11, 2925 page: EL characteristics of rare earth metal complex-dispersed poly (N-vinylcarbazole)”
[0025]
[Problems to be solved by the invention]
However, even when trying to obtain white light emission with a conventional EL element in this way, whether a high voltage of 100 V or higher is required, an expensive vacuum facility is required, or the durability of R, G, B dyes is respectively In contrast, there is a problem that the chromaticity of white light emission fluctuates during practical use time.
[0026]
Therefore, the present invention solves the above-described conventional problems, and has a high brightness, high stability, long life, good color rendering properties, and a simple method such as spin coating, while having an extremely simple element configuration. An object of the present invention is to provide a current injection light-emitting element that can be performed at a low cost.
[0027]
[Means for Solving the Problems]
In order to solve these problems, the current injection type light emitting device of the present invention is an emission center of one or more elements doped in inorganic fluorescent material base nanoparticles, and includes a light emitting layer and an anode, and a light emitting layer and a cathode. Different layers are provided between the layers, and a solution having low solubility in the adjacent layers is used, and the layers are stacked by a simple method such as a dip method or a spin coating method.
[0028]
With this configuration, in the present invention, by using a nanostructured inorganic phosphor and an organic-inorganic hybrid structure that protects it as a light-emitting source, it is possible to emit light with a long lifetime, high luminance, and good chromaticity stability. A possible white light-emitting element can be realized by a simple production method.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
The invention according to claim 1 is a current injection type light emitting device comprising at least an anode for injecting holes, a light emitting layer having a light emitting region, and a cathode for injecting electrons, wherein the light emitting layer has an emission center. An inorganic fluorescent material base particle having an average particle size of 100 nm or less doped with an element to be dispersed and at least one medium for dispersing the inorganic fluorescent material base particle, and ionization between the light emitting layer and the anode It is characterized by having a hole transport layer that has a larger absolute value of potential than the base particles of the inorganic fluorescent material and transports holes predominantly.
[0030]
According to the second aspect of the present invention, an absolute value of the lowest unoccupied orbital level is larger than that of the inorganic fluorescent material base particle, and an electron transport material that predominantly transports electrons is for dispersing the inorganic fluorescent material base particle. It is characterized in that it is dispersed together in a medium, or is provided as an electron transport layer alone or dispersed in a medium between a light emitting layer and a cathode.
[0031]
The invention according to claim 3 is characterized in that the absolute value of the lowest unoccupied orbital level of the hole transport layer is smaller than that of the base particle of the inorganic fluorescent material.
[0032]
The invention according to claim 4 is characterized in that the absolute value of the ionization potential of the electron transport material and the electron transport layer is larger than that of the base particle of the inorganic fluorescent material.
[0033]
The invention according to claim 5 is characterized in that there are a plurality of types of dopant elements doped in the inorganic fluorescent material base particles, and the emission wavelengths from the dopant elements at 380 to 780 nm are at least two types or more. .
[0034]
In the invention according to claim 6, when at least one of the hole transport layer and the light-emitting layer, and the light-emitting layer and the electron transport layer are stacked, the layers are stacked from a solution using a solvent having low solubility in an adjacent layer. It is characterized by things.
[0035]
The invention according to claim 7 is characterized in that, when laminating at least one of the hole transport layer and the light emitting layer, and the light emitting layer and the electron transport layer, the layers are laminated at a temperature that does not dissolve in the adjacent layers. .
[0036]
The invention according to claim 8 includes a layer for preventing movement of electrons between the light emitting layer and the anode, or a layer for preventing movement of holes between the light emitting layer and the cathode. It is characterized by.
[0037]
The invention according to claim 9 is a current injection type light emitting device comprising at least an anode for injecting holes, a light emitting layer having a light emitting region, and a cathode for injecting electrons, wherein the light emitting layer has an emission center. An II-VI group semiconductor particle having an average particle size of 100 nm or less doped with an element to become, and at least one medium for dispersing the II-VI group semiconductor particle, and the light emitting layer and the anode There is a hole transport layer in which the absolute value of the ionization potential is larger than that of the II-VI group semiconductor particles and transports holes predominantly, and the II-VI group semiconductor particles are dispersed in the solution without precipitation. It is characterized by being manufactured while maintaining the state.
[0038]
The invention according to claim 10 is a current injection type light emitting device comprising at least an anode for injecting holes, a light emitting layer having a light emitting region, and a cathode for injecting electrons, wherein the light emitting layer has an emission center. A dopant element which contains an inorganic fluorescent material base particle having an average particle size of 100 nm or less doped with an element to become and at least one medium for dispersing the inorganic fluorescent material base particle and is doped into the inorganic fluorescent material base particle There are a plurality of types, and the emission wavelength at 380 to 780 nm from the dopant element is at least two types.
[0039]
The invention according to claim 11 is characterized in that a hole transport layer which has a larger absolute value of ionization potential than that of the base particle of the inorganic fluorescent material and transports holes predominantly is provided between the light emitting layer and the anode. To do.
[0040]
In the invention according to claim 12, the absolute value of the lowest unoccupied orbital level is larger than that of the inorganic fluorescent material base particle, and an electron transport material that predominantly transports electrons is for dispersing the inorganic fluorescent material base particle. It is characterized in that it is dispersed together in a medium, or is provided as an electron transport layer alone or dispersed in a medium between a light emitting layer and a cathode.
[0041]
The invention described in claim 13 is characterized in that the absolute value of the lowest unoccupied orbital level of the hole transport layer is smaller than that of the base particle of the inorganic fluorescent material.
[0042]
The invention described in claim 14 is characterized in that the absolute value of the ionization potential of the electron transport material and the electron transport layer is larger than that of the base particle of the inorganic fluorescent material.
[0043]
In the invention according to claim 15, when at least one of the hole transport layer and the light-emitting layer, and the light-emitting layer and the electron transport layer are stacked, the layers are stacked from a solution using a solvent having low solubility in an adjacent layer. It is characterized by things.
[0044]
The invention according to claim 16 is characterized in that, when laminating at least one of the hole transport layer and the light emitting layer, and the light emitting layer and the electron transport layer, the layers are laminated at a temperature that does not dissolve in the adjacent layers. .
[0045]
The invention according to claim 17 includes a layer for preventing movement of electrons between the light emitting layer and the anode, or a layer for preventing movement of holes between the light emitting layer and the cathode. It is characterized by.
[0046]
The value obtained by subtracting the absolute value of the band gap from the absolute value of the ionization potential in the text is defined as the “absolute value of the lowest unoccupied orbital level” in the text. I think as.
[0047]
In the embodiment of the present invention, an EL element, particularly an organic EL element having an organic substance as a main constituent material of a light emitting layer will be described as a current injection type light emitting element. In addition, since there are already many books and papers on the operating principle and basic element configuration of organic EL elements as well-known techniques, only the portions necessary for explaining this embodiment are omitted here. Stay on. For basic matters, refer to, for example, “Organic Electroluminescent Materials and Devices”, Seizi Miyata and HariSing Nalwa: Golden and Breach Science Publishers.
[0048]
Hereinafter, an example of an embodiment in the present invention will be described in detail with reference to FIGS.
[0049]
(Embodiment 1)
FIG. 1 is a cross-sectional view of a main part of a current injection light-emitting device according to Embodiment 1 of the present invention. In FIG. 1, 1 is a substrate, 2 is one electrode for supplying power to the device (hereinafter referred to as the present invention). In the embodiment, it is particularly referred to as an anode), 3 is a light emitting layer, 4 is a matrix material constituting the light emitting layer 3 (hereinafter referred to as a matrix), and 5 is a particle constituting the light emitting layer 3 (hereinafter referred to as a particle). , 6 is another electrode for supplying power to the device (hereinafter referred to as a cathode in the embodiment), 7 is a hole transport layer, and 8 is an electron transport layer.
[0050]
FIG. 1 (a) shows a structure having all the above elements, and FIG. 1 (b) shows light emission composed of a substrate 1, an anode 2, a matrix 4 and particles 5 which are the minimum necessary for light emission. A structure with a layer 3 and a cathode 6 is shown. In addition, as another form, in the structure of FIG.1 (b), the structure where only any one of the hole transport layer 7 or the electron carrying layer 8 may be laminated | stacked.
[0051]
In the current injection type light emitting device of the present invention, holes are injected from the anode 2 into the light emitting layer 3 by passing a direct current or pulse current between the two electrodes 2 and 6, while electrons are injected from the cathode 6. It is injected into the light emitting layer 3. The conductive polymer material of the light emitting layer 3 transports holes and / or electrons, and injects holes or electrons into the inorganic phosphor that is an ultrafine particle.
[0052]
As a result, it is considered that excitons are generated in the ultrafine inorganic phosphor, and light is emitted when they are recombined, but the emission mechanism is not clear.
[0053]
As a configuration of the current injection type light emitting element, a configuration in which the cathode 6 is provided on the substrate 1 and the anode 2 is provided on the light emitting layer 3 in addition to the configuration shown in FIG. Furthermore, in order to facilitate hole injection on the substrate 1, improve the smoothness of the substrate 1, and improve the interface characteristics, it is preferable to provide the hole transport layer 7. For the hole transport layer 7, it is particularly preferable to use a material having an ionization potential value close to that between the light emitting layer 3 and the anode 2 in order to increase injection efficiency.
[0054]
An electron transport layer 8 may be provided between the light emitting layer 3 and the cathode 6. As the electron transport layer 8, Alq ₃ , Gaq ₃ , Bphen, PBD, TAZ, silole compounds and their derivatives are typical materials. Furthermore, it is also preferable to provide an electron injection layer in order to improve the efficiency of electron injection from the cathode 6.
[0055]
Here, each configuration of the current injection type light emitting device of the present invention will be described in more detail.
[0056]
First, the main function of the substrate 1 is to hold the entire element. In addition to mechanical strength, the substrate 1 is required to have properties such as flatness, insulation, blocking of the transmission of substances such as moisture and oxygen gas that cause element degradation, and when light is extracted in the direction of the substrate. Also requires transparency at the emission wavelength. As a substrate generally used as a material that satisfies these requirements, there is glass. In this embodiment mode, an example in which glass is used as a substrate will be described. Of course, the substrate in the present invention is not limited to glass, and any substrate that satisfies the above required performance may be used. Examples of materials that can be used as substrates include transparent or translucent soda lime glass, barium / strontium-containing glass, lead glass, alumina silicate glass, borosilicate glass, barium borosilicate glass, quartz glass, and other inorganic oxide glasses, Inorganic glass such as inorganic fluoride glass, or transparent or translucent polyethylene terephthalate, polycarbonate, polymethyl methacrylate, polyether sulfone, polyvinyl fluoride, polypropylene, polyethylene, polyacrylate, amorphous polyolefin, fluorine resin, etc. As a transparent or translucent As ₂ S ₃ , As ₄₀ S ₁₀ , S ₄₀ Ge ₁₀ Chalcogenoid glass such as ZnO, Nb ₂ O ₅ , Ta ₂ O ₅ , SiO, Si ₃ N ₄ , HfO ₂ In addition, a metal oxide such as TiO and a material such as a nitride can be appropriately selected and used, and a laminated substrate in which a plurality of substrate materials are laminated can also be used.
[0057]
Furthermore, an inorganic semiconductor substrate such as opaque Si (silicon), Ge (germanium), GaAs (gallium arsenide), InAs (indium arsenide), GaSb (gallium antimony), InSb (indium antimony), or opaque Fe (iron) ), Metals (alloys) such as Al (aluminum), Cu (copper), Ag (silver), metal compound substrates, etc., or polymer substrates such as opaque plastics, thermosetting resins, photocurable resins, etc. It is done. Further, a laminated substrate in which these plural substrate materials are laminated can also be used.
[0058]
A circuit composed of a transistor, a diode, a resistor, a capacitor, an inductor, and the like for driving the current injection type light emitting element may be formed on the surface or inside of the substrate 1.
[0059]
The flatness referred to here is flatness with respect to a thin film (hereinafter referred to as a thin film) of a micrometer or lower level such as an anode or a light emitting layer, and does not have a macroscopic meaning. Macroscopically, the substrate does not need to be flat, and the element may be bent or configured with a spherical surface or a cylindrical surface as the substrate. Furthermore, it can also take the structure which serves as the anode and board | substrate mentioned later, and the board | substrate may be comprised with the electroconductive substance in that case.
[0060]
Next, the anode 2 will be described. The anode 2 is an electrode for injecting holes, and it is necessary to inject holes into the light emitting layer 3 efficiently. As the anode 2, a transparent electrode can be used. The transparent electrode is a translucent conductive film, and as its material, indium tin oxide (ITO), tin oxide (SnO) ₂ ), A metal oxide such as zinc oxide (ZnO), a transparent conductive film made of a mixture such as SnO: Sb (antimony), ZnO: Al (aluminum), or Al (thickness not to impair transparency). A metal thin film such as aluminum), Cu (copper), Ti (titanium), or Ag (silver), a metal thin film such as a mixed thin film or a laminated thin film of these metals, or a conductive polymer such as polypyrrole can be used. . Further, it is possible to form a transparent electrode by laminating a plurality of these transparent electrode materials, which are formed by resistance heating vapor deposition, electron beam vapor deposition, sputtering method, electric field polymerization method, chemical polymerization method or the like. Further, it is desirable that the transparent electrode has a thickness of 1 nm or more in order to provide sufficient conductivity or to prevent uneven light emission due to unevenness on the substrate surface. In addition, it is desirable that the thickness be 500 nm or less in order to provide sufficient transparency.
[0061]
Furthermore, as the anode 2, in addition to the transparent electrode, a metal having a large work function such as Cr (chromium), Ni (nickel), Cu (copper), Sn (tin), W (tungsten), Au (gold), Alternatively, an alloy, oxide, or the like can be used, and a laminated structure of a plurality of materials using these anode materials can also be used. In addition, when a transparent electrode is not used as the anode 2, the cathode 6 may be a transparent electrode. In this case, the anode 2 is preferably formed of a material that reflects light.
[0062]
Further, an amorphous carbon film may be provided on the anode 2. In this case, both have a function as a hole injection electrode. That is, holes are injected from the anode into the light emitting layer or the hole transport layer through the amorphous carbon film. The amorphous carbon film is formed by sputtering between the anode and the light emitting layer or the hole transport layer. Examples of the carbon target by sputtering include isotropic graphite, anisotropic graphite, glassy carbon, and the like. Although not particularly limited, isotropic graphite having high purity is suitable. Specifically, when the work function of the amorphous carbon film is measured using a surface analyzer AC-1 manufactured by Riken Keiki, the work of the amorphous carbon film is shown. The function is Wc = 5.40 eV. Here, since the work function of ITO, which is generally used as an anode, is WITO = 5.05 eV, it is more efficient to inject holes into the light emitting layer or the hole transport layer using an amorphous carbon film. it can. Further, when the amorphous carbon film is formed by sputtering, reactive sputtering is performed in an atmosphere of nitrogen or a mixed gas of hydrogen and argon in order to control the electric resistance value of the amorphous carbon film. Furthermore, in the thin film formation technique by sputtering or the like, when the film thickness is 5 nm or less, the film becomes an island structure and a uniform film cannot be obtained. For this reason, when the film thickness of the amorphous carbon film is 5 nm or less, the electric resistance becomes too high. As a result, no current flows and efficient light emission cannot be obtained. Further, when the film thickness of the amorphous carbon film is 100 nm or more, the color of the film is dark and light emission from the current injection type light emitting element is not sufficiently transmitted. In this embodiment, an element using ITO as an anode will be described as an example.
[0063]
Next, as the inorganic phosphor (particle 5) contained in the light emitting layer 3, generally used inorganic phosphor materials can be widely used. In general, inorganic phosphors have specific ions or lattice defects that are emission centers dispersed in a base crystal. These emission centers exist at several atomic% or less. Examples of inorganic phosphors containing these include phosphors in which cerium is added as a luminescent center to a host known as yttrium aluminum garnet (YAG), and ZnS (as a representative substance of II-VI group semiconductors). Zinc sulfide), representative doped metal ions such as Mn (manganese), iron (Fe), Cu (copper), Ag (silver), etc., Japan Society of Applied Physics, Organic Molecules and Bioelectronics Research Group, Vol. 10, no. 3 (1999) and references cited therein can be used. In addition, ultrafine particle phosphors made of GaN microcrystals, Eu (II) xSiyOz (x, y, z can take arbitrary values), for example, EuSiO ₃ , Eu ₂ SiO ₃ , Eu ₂ SiO ₅ , Eu ₃ SiO ₅ , EuAl ₂ O ₄ , Eu ₂ Al ₂ O ₆ Eu like ²⁺ Derivatives of europium, europium metasilicate, calcium metasilicate activated with europium, strontium metasilicate and the like can be preferably used. However, it is not limited to these.
[0064]
The inorganic phosphor used in the present invention is so-called ultrafine particles, and the average particle size of the inorganic phosphor used in the present invention is preferably 100 nm or less, particularly preferably 50 nm or less. Most preferably, it is 30 nm or less. When the average particle size is reduced, a more uniform thin film can be obtained, a more uniform light emitting device can be obtained, and luminous efficiency is increased. Further, when the particle size is reduced, the light emission is shortened by a so-called quantum size effect.
[0065]
The particle size distribution can be used from polydisperse to monodisperse particles, but the use of particles closer to monodispersion is superior to polydisperse particles in terms of light emission uniformity and lifetime characteristics. Yes.
[0066]
The average particle size and particle size distribution of these ultrafine particles can be measured with a commercially available particle size distribution analyzer. For example, particles of 3 nm or more can be measured with a dynamic light scattering type particle size distribution measuring apparatus (LB-500 manufactured by Horiba Seisakusho). Further, those having a small particle size can be evaluated by taking a transmission electron micrograph.
[0067]
Various methods are known for producing ultrafine particles used in the present invention. For example, synthesis of YAG ultrafine particles by a conventionally known sol-gel method, spray pyrolysis method, supercritical water reaction crystallization method and coprecipitation method (Hongshi Wang et al., Material Science and Engineering, A288, p1 (2000)). There are reports such as. Also known is a method for producing ultrafine particles having a core-shell structure by forming crystals in a polymer medium (F. Caruso, Adv. Mater., Vol. 13, p14 (2001). Used in the present invention). The ultrafine particles to be produced may be produced by any method, but production by a wet method without using a vacuum apparatus is most preferable in terms of cost.
[0068]
And it is preferable to use a high molecular compound as a medium (matrix 4) contained in the light emitting layer 3 in order to disperse ultrafine particles. As the polymer compound that can be used in the present invention, any compound can be used as long as it has a higher ionization potential than the ionization potential (Ip) of the ultrafine particles that control light emission. In addition, it is preferable that the band gap of the polymer compound in which it is dispersed is larger than the band gap of the ultrafine particle phosphor, since the light emission efficiency is further improved.
[0069]
Furthermore, it is preferable to use a conductive polymer compound as the matrix 4 contained in the light emitting layer 3, and holes and electrons can be efficiently injected when the hole mobility and electron mobility of these polymer compounds are large. Therefore, it is desirable from the viewpoint of reduction of driving voltage and durability. The hole and electron mobility can be measured by the TOF method (time of flight method).
[0070]
As typical examples of the polymer compound specifically used, a π-conjugated polymer compound and a σ-conjugated polymer are used. Specific examples of π-conjugated systems include polyacetylene and derivatives thereof, polyparaphenylene and derivatives thereof, polyphenylene vinylene, poly (2,5-dialkoxy-p-phenylene vinylene, poly (2-methoxy-5- (2′-ethylhexyl) p -Phenylene vinylene and its derivatives, polythiophene, alkylthiophene and its derivatives, polypyrrole and its derivatives, polyvinyl carbazole and its derivatives, polyethylene oxide and its derivatives, polycarbonate and its derivatives, etc. The derivatives are given as specific examples, but are not limited thereto.
[0071]
These polymer compounds preferably have a glass transition temperature of 100 ° C. or higher, which leads to improvement of device stability, and those having a glass transition temperature of 120 ° C. or higher are particularly preferable in practice.
[0072]
The current injection type light emitting device of the present invention can be manufactured by a conventionally used method. That is, a spin coating method, an ink jet method, a printing method, an alternating lamination method, or the like is used. For example, there is a description in Koji Koji supervision, CMC, organic EL materials and displays (2001).
[0073]
In general, in the case of an organic EL element, it is known that an electric barrier exists between the light emitting layer 3 and the electrode when carriers are injected. The details of the barriers are too complicated to explain here, and they will be omitted here, for example, “Organic EL materials and displays (CMC Publishing)”. The barrier is simply a resistance for injecting positive and negative carriers from the electrode into the light emitting layer 3 (which may be referred to as the matrix 4). When this resistance is large, the drive voltage, which is a voltage necessary for lighting the element, naturally increases. The magnitude of the resistance is determined by the difference between the work function of the electrode and the ionization potential or electron affinity of the matrix 4 material. After all, there is an optimal combination between the material of the matrix 4 and the electrodes. In order to optimize this combination, it is important to design the element in consideration of the work function of the particles 5.
[0074]
Now, the range of the work function of the electrode that can provide an optimal combination for various matrix materials as described above is approximately 1.25 eV or more and 6.0 eV or less. In particular, when considering application to the cathode side, it is desirable that the work function of the electrode is small, but the work function of the substance cannot theoretically be zero, and there is a natural lower limit. The lower limit of the work function of a substance known as a conventional electrode material is approximately 1.25 eV. On the other hand, the larger side is limited to some extent by the physical properties of the substance used as the matrix material, and adoption of an electrode having a work function of 5.5 eV or more is not desirable in view of an increase in the barrier with the matrix 4.
[0075]
Further, the particles 5 may be a mixture of a plurality of particles having different characteristics. The term “mixture” used herein refers to, for example, mixing particulate substances having different characteristics into the matrix 4 at the same time, or mixing particles having different characteristics attached to each other, or one of the substances having different characteristics is not suitable for the other. Mix grains that are completely covered, mix grains that are flat and have different characteristics on the front and back, one is a particle and the other is a molecular species adsorbed on the surface, the crystal orientation due to crystal anisotropy It is described in a wide range of meanings such as different characteristics. By introducing the particles 5 into the matrix 4 in the state of a mixture referred to here, further improvement in device characteristics can be expected. Details will be described later.
[0076]
Another characteristic that the particles 5 should have is the size. The size of at least one of the particles 5 in the axial direction needs to be smaller than the thickness of the light emitting layer 3. If the particle 5 is larger than the light emitting layer 3, the anode 2 and the cathode 6 are short-circuited, but it does not function as a light emitting element. There are no particular limitations on the other characteristics, such as shape and constituent materials, except for the electrical conductivity and work function described above. Although the name “particle” is given, it may be flat, needle-shaped or rod-shaped. Further, it may be a region formed through a process such as phase separation in the process of forming an element without having a clear shape before the element is formed.
[0077]
In the embodiment, as an example of the particles 5, manganese, copper, or silver-doped zinc sulfide, that is, particles of ZnS: Mn, ZnS: Cu, ZnS: Ag, or a mixture thereof are used. ZnS itself is a material that is transparent in the visible range and has semiconducting properties. The band gap is larger than the band gap corresponding to the emission wavelength in the visible range, and is about 3.7 eV. This is smaller than most organic molecules, and it is convenient for carriers to move from the organic molecules that protect the particles 5 to the dopants Mn, Cu, and Ag through ZnS. Even with different dopants, the barrier for carrier injection is determined by one type of base (here, ZnS). Therefore, light emission of different colors can be obtained by carrier injection at the same energy level.
[0078]
Next, the cathode 6 is an electrode for injecting electrons, and it is necessary to efficiently inject electrons into the light emitting layer 3 (or an electron transport layer or an electron injection layer), and Al (aluminum) having a small work function, Metals such as In (indium), Mg (magnesium), Ti (titanium), Ag (silver), Ca (calcium), Sr (strontium), oxides or fluorides of these metals, alloys thereof, and laminates Etc. are generally used. In addition, by forming an ultrathin film with high light transmittance using a metal having a small work function at the interface in contact with the light emitting layer 3 (or electron transport layer or electron injection layer), and laminating a transparent electrode on top of it, It is also possible to form a transparent cathode. In particular, Mg, Mg—Ag alloy having a small work function, Al—Li alloy, Sr—Mg alloy, Al—Sr alloy, Al—Ba alloy, etc. described in JP-A-5-121172, or LiO ₂ A laminated structure such as / Al or LiF / Al is suitable as a cathode material. As a film forming method, resistance heating vapor deposition, electron beam vapor deposition, or sputtering is used.
[0079]
Note that at least one of the anode 2 and the cathode 6 may be a transparent electrode. Furthermore, both may be transparent electrodes, but in order to improve the light extraction efficiency, if one is a transparent electrode, it is preferable that the other be formed of a material that reflects light.
[0080]
Further, in order to cut off the current injection type light emitting element from the outside air and ensure long-term stability, a protective film is formed on the cathode 6 as necessary, and the laminate of the anode 2 to the cathode 6 is formed on the substrate 1. It may be protected so that it is wrapped in. That is, it is necessary for long-term reliability to provide a protective film in order to suppress oxidation of the material and reaction with moisture.
[0081]
As the material of the protective film, SiON, SiO, SiN, SiO ₂ , Al ₂ O ₃ , Thin films made of inorganic oxides such as LiF, inorganic nitrides, inorganic fluorides, glass films made of inorganic oxides, inorganic nitrides, inorganic fluorides, etc., thermosetting, photocurable resins, etc. Examples thereof include a silane polymer material having a sealing effect and the like, and it is formed by vapor deposition, sputtering, or a coating method.
[0082]
Here, a method for measuring the ionization potential and the band gap will be described. The ionization potential can be estimated by irradiating the sample with ultraviolet rays and measuring the threshold excitation energy at which photoelectrons are emitted. As a simple method, measurement can be performed in the atmosphere using a commercially available device (manufactured by Riken Keiki Co., Ltd., photoelectron spectrometer AC-1). The band gap can be evaluated by measuring the spectral absorption characteristics of the sample with a spectrophotometer and determining the energy at the absorption edge as a band gap.
[0083]
The cathode 6 in the present embodiment is a deposited aluminum thin film. The cathode 6 in the present embodiment will be described as a pole that applies a negative potential to the element in a normal element driving state, like the anode 2. It is most important that the cathode 6 is a material that can apply a uniform voltage to the light emitting surface, like the anode 2. A transparent electrode such as ITO is used for light extraction when adopting a so-called top emission element configuration. In this embodiment, the anode 2 is a transparent electrode, and light extraction can be performed in the direction of the substrate. So opaque aluminum is used.
[0084]
Further, a hole transport layer 7 that serves as hole injection exists between the light emitting layer 3 and the anode 2. In order for the hole transport layer 7 to function, it is desirable that the ionization potential level of the hole transport layer 7 is located between the ionization potential levels of the anode 2 and the light emitting layer 3.
[0085]
Further, if the absolute value of the lowest unoccupied orbital level of the hole transport layer 7 is smaller than the absolute value of the lowest unoccupied orbital level of the light emitting layer 3, the function as an electron blocking layer is exhibited, and the light emitting layer 3 The efficiency of carrier recombination can be increased.
[0086]
Further, an electron transport layer 8 that serves to inject electrons exists between the light emitting layer 3 and the cathode 6. In order for the electron transport layer 8 to perform its function, it is desirable that the lowest unoccupied orbital level of the electron transport layer 8 is located between the lowest unoccupied orbital level of the anode and the light emitting layer.
[0087]
In addition, if the absolute value of the ionization potential of the electron transport layer 8 is larger than the absolute value of the ionization potential of the light emitting layer 3, the function as a hole blocking layer is exhibited, and the carrier recombination in the light emitting layer is enhanced. Efficiency can be improved.
[0088]
The hole transport layer 7, the light emitting layer 3, and the electron transport layer 8 are individually laminated from a solution by a technique such as spin coating. At this time, a solution having a low solubility in adjacent layers is used. It is desirable that the solvent is different in polarity from the adjacent layer. Alternatively, it is desirable that the temperature of the substrate and the solution when the solution is spread and laminated is cooled to a temperature at which the adjacent layer does not elute.
[0089]
If the solvent for laminating the hole transport layer 7 is polar, the solvent for laminating the light emitting layer 3 may be nonpolar, and the solvent for laminating the electron transport layer 8 may be polar. desirable. Further, if the solvent for laminating the hole transport layer 7 is nonpolar, the solvent for laminating the light emitting layer 3 is polar and the solvent for laminating the electron transport layer 8 is nonpolar. Things are desirable.
[0090]
The hole transport layer 7 and the electron transport layer 8 may be present alone, at the same time, or may have a structure in which neither is present.
[0091]
If a PEDOT-PSS aqueous solution is used as the hole transport layer 7, the light emitting layer 3 is laminated from a solution of ZnS: Mn, Cu, Ag, which is a nonpolar solvent such as toluene, and the electron transport layer 8 is highly polar. It is laminated from a bathocuproine (BCP) molecule-containing polymer solution using water or methanol as a solvent.
[0092]
The element configuration in the embodiment is as described above, but these are only described by taking out only the minimum necessary components for explaining the present invention. In order to make the device function practically, for example, it is necessary to add several components such as sealing to protect the device from deterioration due to oxygen and moisture in the air, and adopting a carrier transport layer for improving efficiency. Conceivable. These ideas are not directly included in the embodiment because they are not directly related to the essence of the present invention. Of course, it is needless to say that the device characteristics can be further improved by adding such a configuration. In this explanation, in order to make the explanation as simple as possible, it is noted that they have been completely abolished and simplified.
[0093]
Now, a simple description has been given in the prior art, but here, the deterioration of the luminance and chromaticity of the organic EL element, which the present invention intends to solve, will be described in detail.
[0094]
As described above, there is a method of improving the light emission efficiency of the light emitting layer material as one of the methods for improving the luminance of the element. This means that a material system is used in which both positive and negative carriers injected from the electrodes are recombined to release energy when the energy is released.
[0095]
When positive and negative carriers injected into the element are recombined, molecules in the material constituting the light emitting layer are excited. Excited molecules generally return to the ground state through some relaxation process, and one of the relaxation processes is relaxation by light emission. Whether a molecule is relaxed by light emission or excited when excited is highly dependent on its molecular structure. In other words, there are those that are difficult to emit light depending on the molecular species. Therefore, in order to improve the light emission efficiency of the light emitting layer, it is necessary to select and employ a molecular species that has a stronger tendency to be relaxed by light emission. The ratio of how much is converted into light when a certain amount of carriers is injected into the light emitting layer is called the quantum yield of light emission. If the quantum yield of light emission is 100%, the injected electric energy is completely converted to light.
[0096]
This conversion efficiency is one of the limits of the luminance improvement described above. That is, due to the recent progress in research, the quantum yield of light emission of materials used in the light emitting layer has reached almost 100%, which is the theoretical upper limit mentioned above. is there. Therefore, as long as such a high-performance material is used for the light-emitting layer, it can be said that improvement in luminance by improving efficiency is the limit.
[0097]
Next, another method for increasing the applied voltage will be described.
[0098]
Although it has already been described that there is a limit to improving the luminance even when the applied voltage is increased, there are two causes. This is a decrease in carrier binding efficiency and interface destruction. Each of these two causes will be described.
[0099]
First, carrier coupling efficiency. Even if the light emission quantum yield is close to 100% as described above, the source of light emission energy is positive and negative carriers to be injected. Therefore, if the amount of carriers is small, light cannot be emitted. As the number of carriers increases, the light gradually shines. In the device, when positive and negative carriers are injected from the electrode into the light emitting layer, they move toward the counter electrode by the applied electric field. At this time, the positive and negative carriers have opposite charges and move so as to pass each other. . At this time, only those that encounter oppositely charged carriers in the process of passing through the light-emitting layer can release energy by bonding, but if the applied voltage is too high, a large amount of carrier injection can be achieved. As a result, the number of carriers that reach the counter electrode without releasing energy increases. Carriers that could not be coupled to each other only passed through the element, and only electrically flowed through the resistor. In this case, energy is lost as heat. Therefore, the ratio of the injected carriers converted to light is reduced. When the voltage is further increased, the number of non-bonding carriers increases accordingly, and the light emission efficiency gradually decreases. If the voltage is increased, the number of carriers simply injected increases, so that the luminance is improved while the efficiency is low. However, it is clear that an excessive increase in voltage is not a good idea from the viewpoint of actual use and also from the viewpoint of interface breakdown described later. Thus, there is a problem in improving the luminance by increasing the applied voltage due to the decrease in carrier coupling efficiency.
[0100]
Next, the interface problem will be described. When a voltage is applied to the device, the voltage is distributed to each interface (anode-light emitting layer interface, cathode-light emitting layer interface) and the light emitting layer in the device. At this time, since the interface between the light emitting layer and the electrode is a bonding surface of different materials, there should be an undesirable adsorbed substance or a locally weakly bonded portion. In addition, there are many unstable factors such as the presence of electrical barriers and the contact between highly reactive metals and organic substances. When excessive voltage is applied, potential differences occur. Destroyed. The part once destroyed does not contribute to light emission, and the destruction region is expanded to the peripheral part by generation of an unstable substance or the like, resulting in destruction of the entire device. Since the risk of this interface breakdown increases rapidly as the applied voltage increases, excessive voltage application should be avoided. The voltage that can be applied in this way has an upper limit also from the problem of the interface, and limits the improvement in luminance due to the voltage increase.
[0101]
Next, the problem of chromaticity will be described. When white light is emitted in an organic EL element, a method of combining different luminescent dyes of RGB or a method of emitting blue light having the shortest wavelength as EL and using it as an excitation light source to extract green light emission and red light emission as fluorescence, etc. Is mentioned. In any case, the lifetime of the emission intensity of each color of RGB is different, and the deterioration of the dye having the shortest lifetime determines the chromaticity lifetime as white light emission. In the organic EL element, each RGB color alone has a guaranteed lifetime of 10,000 hours. However, even if any one of the RGB colors is attenuated by 1/16 gradation, a change in white chromaticity can be visually confirmed. For this reason, the lifetime of the chromaticity of white light emission is substantially several hundred to 1,000 hours.
[0102]
Hereinafter, a manufacturing method, a structure, and an operation of a current injection type light emitting device in the present invention will be described in detail.
[0103]
A manufacturing procedure of the element is shown in FIG. FIG. 2 is a diagram for explaining a manufacturing procedure of the current injection type light emitting element according to Embodiment 1 of the present invention.
[0104]
The particles 5 used in the device fabrication of the embodiment are a mixture of ZnS: Mn, ZnS: Cu, ZnS: Ag, and the shape thereof is almost spherical and the particle size is about 10 to 20 nm. In practice, the particle size is not limited in practice, but the thickness of the light-emitting element in the technical field to which the present invention is applied is usually several hundred nm to several microns, which impairs the generality of the description. Therefore, the particle size of 10 nm is selected.
[0105]
Here, FIG. 3 is an energy diagram of the current injection type light emitting device in the first exemplary embodiment of the present invention. The unit of the numbers in FIG. 3 is eV. The upper numerical values of PEDOT, ZnS, and BCP indicate the lowest unoccupied orbital level, and the lower numerical values indicate the ionization potential level. The specifically manufactured element will be described later, and first, the principle of the present invention will be described.
[0106]
A case where a voltage is applied to the element of the form of FIG. 1 will be described. For the subsequent device operation, some assumptions are made to simplify the explanation. That is, an appropriate voltage is applied to the device, an appropriate electrode is used for the matrix 4, and all injected carriers are combined in the light emitting layer 3, and the energy released thereby is All shall be converted to light. This assumption only simplifies the explanation and does not have any influence on the essential content of the present invention.
[0107]
Now, holes are injected into the light emitting layer 3 from the anode 2 through the hole transport layer 7, and electrons are injected into the light emitting layer 3 from the cathode 6 through the electron transport layer 8. While positive and negative carriers hop on the molecules constituting the light emitting layer 3, positive charges follow an electric field generated by a potential gradient in the light emitting layer, and negative charges move in the opposite direction. Then, the positive and negative charges encountered in the light emitting layer 3 are annihilated and transfer energy to the molecule, and the molecule is excited. Excited molecules relax while emitting light. Thus, light is emitted from the element. At this time, the voltage applied to the element is distributed in the light emitting layer 3 as shown in FIG. A potential gradient as shown in FIG. 4 is generated in the light emitting layer 3. This gradient moves the injected carrier. In order to simplify the potential gradient in the light emitting layer 3, the hole transport layer 7 and the electron transport layer 8 are omitted in FIG. FIG. 4 is a diagram for explaining the electric field distribution in the current injection type light emitting device according to the first embodiment of the present invention.
[0108]
Attention is now directed to the energy level diagram of FIG. 3 in more detail. Holes are injected from the anode 2 to the light emitting layer 3 through the hole transport layer 7. In order to perform this efficiently, it is desirable that the ionization potential level of the hole transport layer 7 is located between the level of the anode 2 and the ionization potential level of the light emitting layer 3. Further, electrons are injected from the cathode 6 to the light emitting layer 3 through the electron transport layer 8. In order to perform this efficiently, the lowest unoccupied orbital level of the electron transport layer 8 is preferably located between the level of the cathode 6 and the lowest unoccupied orbital level of the light emitting layer 3. At this time, the pair of electrons and holes recombined in the light emitting layer 3 can convert the energy into light emission, but the energy of the electrons and holes that reach the counter electrode without recombination can only be a thermal energy. Cannot convert. Since bathocproin (BCP), which is the electron transport layer in FIG. 3, has an absolute value of an ionization potential level larger than that of the light emitting layer, it cannot block the holes toward the cathode 6 without being recombined in the light emitting layer 3. Can do. Due to this hole blocking effect, the frequency of recombination of electron-hole pairs in the light emitting layer is increased, and more efficient light emission can be realized.
[0109]
Further, polyvinylcarbazole (PVK) or the like can be used as an alternative to PEDOT which is the hole transport layer 7. Since PVK has an absolute value of the lowest unoccupied orbital level smaller than that of ZnS of the light emitting layer 3, electrons directed to the anode 2 can be blocked without recombination within the light emitting layer 3.
[0110]
By laminating using a solution having low solubility in an adjacent layer, the carrier block layer can be laminated on the light emitting layer 3 with a clear interface without damaging the previously laminated layer. As a result, the recombination frequency of electron-hole pairs in the light emitting layer can be increased and the light emission efficiency can be increased.
[0111]
Next, the emission spectrum and chromaticity in FIG. 5 will be described. FIG. 5 is a graph showing an emission spectrum of the current injection type light emitting device in the first exemplary embodiment of the present invention. An example of the particles 5 in the light emitting layer 3 is ZnS. When ZnS is used as a base material and the type of dopant metal is different, the emission color is different. In doped phosphors, electron and hole pairs are injected into ZnS through ZnS conductors and ionization potential levels, respectively. When this electron-hole pair recombination is performed in the dopant metal in the ZnS crystal, light emission having a wavelength corresponding to the band gap energy of the dopant is generated. Therefore, Mn dopant produces 580 nm, orange, Cu dopant produces 530 nm green, and Ag dopant produces 460 nm blue. By appropriately setting the concentrations of these three kinds of dopants, the respective RGB emission colors are made appropriate. Moreover, the energy barrier at the time of light emission of each color of RGB is uniformly caused by the ZnS matrix. For this reason, there is no adverse effect that only one color is rapidly deteriorated and the chromaticity of white has a short lifetime.
[0112]
In the present embodiment, description has been made using ZnS: Mn as particles, but of course the particles 5 to be mixed are not limited to ZnS: Mn. For example, it is II-VI group semiconductor particles including CdSe and ZnSe, and the dopant metal atom is not limited to Mn, Ag, Cu but Sb, Eu, etc., and any element that emits light in the range of 380 to 780 nm. Good. Thus, the particle 5 may be anything as long as it is an inorganic fluorescent material base particle having an average particle size of 100 nm or less doped with an element serving as an emission center.
[0113]
Next, characteristics of elements actually manufactured through the above-described procedure will be described.
[0114]
FIG. 6 is a graph showing the relationship between the applied voltage and current density of the current injection type light emitting device according to Embodiment 1 of the present invention, and FIG. 7 is a graph of the current injection type light emitting device according to Embodiment 1 of the present invention. It is a graph showing the relationship between current density and light emission luminance. It can be seen from FIG. 6 that when a forward voltage is applied to the element, a threshold value is obtained from around 5 to 6 V and current starts to flow. Further, FIG. 7 shows that the current density and luminance have a first order correlation after the current starts to flow.
[0115]
(Embodiment 2)
FIG. 8 is a cross-sectional view of a main part of the current injection type light emitting device according to the second embodiment of the present invention, and 4 is a matrix material (hereinafter referred to as a matrix) constituting the light emitting layer 3.
[0116]
The second embodiment conforms to the first embodiment except that the matrix 4 of the light emitting layer 3 has a structure in which a hole transport material, an electron transport material, a hole block material, and an electron block material are mixed. Has structure and properties.
[0117]
In FIG. 8A, the hole transport material in which the ionization potential level is located between the ionization potential levels of the anode 2 and the light emitting layer 3 is mixed and dispersed in the matrix 4. In addition, if the absolute value of the lowest unoccupied orbital level of the hole transport material is smaller than the absolute value of the lowest unoccupied orbital level of the light emitting layer 3, the function as an electron blocking material is expressed, and the inside of the light emitting layer 3 The carrier recombination can be made highly efficient.
[0118]
Alternatively, the electron transport material where the lowest unoccupied orbital level is located between the lowest unoccupied orbital level of the cathode 6 and the light emitting layer 3 is mixed and dispersed in the matrix 4. In addition, if the absolute value of the ionization potential level of the electron transport material is larger than the absolute value of the ionization potential level of the light emitting layer 3, the function as a hole blocking material is exhibited, and the carrier relocation in the light emitting layer 3 is performed. Bonding can be made highly efficient.
[0119]
The hole transport material, the electron transport material, the hole block material, and the electron block material that are mixed and dispersed in the matrix 4 may be used singly or in combination.
[0120]
In FIG. 8B, the electron transport material in which the lowest unoccupied orbital level is located between the lowest unoccupied orbital level of the cathode 6 and the light emitting layer 3 is mixed and dispersed in the matrix 4. . Further, if the absolute value of the ionization potential level of the electron transport material is larger than the absolute value of the ionization potential level of the light emitting layer, the function as a hole blocking material is exhibited and carrier recombination in the light emitting layer 3 is performed. Can be made highly efficient.
[0121]
The electron transport material and the hole blocking material mixed and dispersed in the matrix 4 may be used alone or in a mixture of various materials.
[0122]
Moreover, the structure provided with the positive hole transport layer 7 as shown to the form of FIG.8 (b) may be sufficient. A structure without the hole transport layer 7 may be employed.
If the structure includes the hole transport layer 7, as described in the first embodiment, the hole transport layer 7 and the light emitting layer 3 are individually laminated from a solution by a technique such as spin coating. At that time, it is desirable to use a solvent having a low solubility in the adjacent layer, that is, a solvent having a polarity different from that of the adjacent layer. Alternatively, it is desirable that the temperature of the substrate and the solution when the solution is spread and laminated is cooled to a temperature at which the adjacent layer does not elute.
[0123]
If the solvent for laminating the hole transport layer 7 is polar, the solvent for laminating the light emitting layer 3 is preferably nonpolar. If the solvent for laminating the hole transport layer 7 is nonpolar, it is desirable that the solvent for laminating the light emitting layer 3 is polar.
[0124]
In FIG. 8C, the hole transport material in which the ionization potential level is located between the ionization potential levels of the anode 2 and the light emitting layer 3 is mixed and dispersed in the matrix 4. In addition, if the absolute value of the lowest unoccupied orbital level of the hole transport material is smaller than the absolute value of the lowest unoccupied orbital level of the light emitting layer 3, the function as an electron blocking material is expressed, and the inside of the light emitting layer 3 The carrier recombination can be made highly efficient.
[0125]
The hole transport material and the electron block material mixed and dispersed in the matrix 4 may be used singly or in combination.
[0126]
Moreover, the structure provided with the electron carrying layer 8 as shown in the form of FIG.8 (c) may be sufficient. A structure without the electron transport layer 8 may be used.
[0127]
If the structure is provided with the electron transport layer 8, as described in the first embodiment, the light emitting layer 3 and the electron transport layer 8 are individually laminated from a solution by a technique such as spin coating. In particular, it is desirable to use a solvent having a low solubility in the adjacent layer, that is, a solvent having a polarity different from that of the adjacent layer. Alternatively, it is desirable that the temperature of the substrate 1 and the solution when the solution is developed and laminated is cooled to a temperature at which the adjacent layer does not elute.
[0128]
If the solvent for laminating the light emitting layer 3 is polar, it is desirable that the solvent for laminating the electron transport layer 8 is nonpolar. If the solvent for laminating the light emitting layer 3 is nonpolar, it is desirable that the solvent for laminating the electron transport layer 8 is polar.
[0129]
(Embodiment 3)
FIG. 9 is a cross-sectional view of a main part of the current injection light-emitting device according to Embodiment 3 of the present invention. In FIG. 9, 9 is an electron block layer and 10 is a hole block layer.
[0130]
As the electron blocking layer 9, it is desirable to use a conductive polymer having a small absolute value of the lowest unoccupied orbital level, such as polypyridine or polyaniline. Since the absolute value of the conduction band level of the electron blocking layer 9 is significantly smaller than the absolute value of the lowest unoccupied orbital level of the phosphor matrix in the light emitting layer 3, it does not contribute to light emission in the light emitting layer 3. This prevents electrons from reaching the anode 2 that is the counter electrode, increases the probability of existence in the light emitting layer 3, promotes recombination of electron-hole pairs, and increases the efficiency of light emission.
[0131]
As the hole blocking layer 10, it is desirable to use a conductive polymer having a large absolute value of ionization potential level such as polyparaphenylene sulfide or polymetaphenylene. The absolute value of the lowest unoccupied orbital level of the hole blocking layer 10 is significantly larger than the absolute value of the lowest unoccupied orbital level of the phosphor matrix in the light emitting layer 3, thereby contributing to light emission in the light emitting layer 3. Without obstructing the hole which tries to reach the cathode 6 which is the counter electrode, the existence probability in the light emitting layer 3 is increased, the recombination of the electron hole pair is further promoted, and the light emission is made highly efficient.
[0132]
The electron blocking layer 9 and the hole blocking layer 10 may have a structure provided alone or a structure provided with both.
[0133]
In the third embodiment, an electron blocking layer 9 for preventing the movement of electrons is provided between the light emitting layer 3 and the anode 2, or the movement of holes is performed between the light emitting layer 3 and the cathode 6. Except for the structure including the hole blocking layer 10 for preventing the above, the structure and characteristics according to the first embodiment are provided.
[0134]
【Example】
(Example 1)
<Synthesis of ZnS: Mn, Cu, Ag ultrafine particle phosphor and its solution>
The synthesis was basically performed according to the literature by Isobe et al. (See Journal of Luminescence, 93, (2001) p. 1-8).
[0135]
That is, an aqueous solution in which a predetermined amount of zinc acetate, manganese acetate, copper acetate, and silver acetate (manufactured by Wako Pure Chemical Industries, Ltd.) is mixed with an aqueous solution of sodium disulfide is converted into sodium bis (2-ethylhexyl) sulfosuccinate (AOT reverse micelles). ) Each was mixed into an isooctane solution and ultrasonically dispersed. The two reverse micelle solutions were mixed while gradually stirring. Stirring was performed with a magnetic stirrer. Thereafter, an excess of trioctylphosphine (TOPO) was added to dissociate the reverse micelle structure of AOT to obtain a TOPO-protected ZnS nanocrystal solution. Solvent extraction was performed in a toluene / water two-phase system, and AOT was extracted into the aqueous phase to obtain a toluene solution of TOPO-protected ZnS nanocrystals.
[0136]
<Production of element>
A PEDOT / PSS aqueous solution was spin-coated in a nitrogen atmosphere on a sufficiently cleaned glass substrate with ITO (transmittance 85% or more) to obtain a thin film having a thickness of 50 nm. The obtained thin film was dried in nitrogen at 110 ° C. for 10 minutes.
[0137]
Further, a toluene solution of the above-described TOPO-protected ZnS nanocrystals was spin-coated in a nitrogen atmosphere on the obtained thin film to laminate a thin film having a thickness of 50 nm. The obtained thin film was dried in nitrogen at 100 ° C. for 1 hour. Next, an aqueous solution of bathocuproine polymer was spin-coated under a nitrogen atmosphere, and the thin film having a thickness of 50 nm was laminated. This was also dried in nitrogen at 120 ° C. for 1 hour.
[0138]
The obtained element was set in a vacuum deposition apparatus, and 10 nm of lithium fluoride was deposited as an electron injection layer. Subsequently, 1000 nm of aluminum was deposited as a cathode. Thereafter, the device was moved to a nitrogen-substituted glove box, a UV curable resin for organic EL sealing (30Y296G manufactured by ThreeBond) was applied to the periphery of the glass substrate, and the glass plate was adhered from the cathode side for sealing.
[0139]
The ionization potential of PEDOT / PSS was 5.5 eV, and the band gap estimated from the absorption edge was 2.6 eV.
[0140]
<Evaluation of device>
The completed light-emitting element was driven by direct current using ITO as an anode and Al as a cathode. The emission start voltage was 7V. The luminance when a voltage of 10V is applied is 100 cd / m ² Met. The luminance was measured with a BM-8 luminescent meter manufactured by Topcon. Further, the emission spectrum was measured with a multi-channel analyzer manufactured by Hamamatsu Photonics, and it was confirmed that the emission peak was in the vicinity of 450, 530 and 580 nm. The emission color was white.
[0141]
Next, this element is applied to an initial luminance of 100 cd / m. ² A continuous lighting test was conducted at The luminance after 1000 hours is 95 cd / m ² Met. From this result, it can be seen that the lifetime of the device of Example 1 of the present invention is extremely excellent.
[0142]
(Example 2)
<Synthesis of ZnS: Mn, Cu, Ag ultrafine particle phosphor and its solution>
In the toluene solution of TOPO-protected ZnS nanocrystals synthesized in Example 1, the mercaptoethanol-protected ZnS nanocrystal aqueous solution in which the protecting group was replaced with mercaptoethanol and the solvent was replaced with water was obtained.
[0143]
<Production of element>
A toluene solution of polyvinylcarbazole (PVK) was spin-coated in a nitrogen atmosphere on a sufficiently cleaned glass substrate with ITO (transmittance of 85% or more) to obtain a thin film having a thickness of 50 nm. The obtained thin film was dried in nitrogen at 110 ° C. for 10 minutes.
Further, on the obtained thin film, the above-described mercaptoethanol-protected ZnS nanocrystal aqueous solution was spin-coated in a nitrogen atmosphere to laminate a thin film having a thickness of 50 nm. The obtained thin film was dried in nitrogen at 100 ° C. for 1 hour. Next, a toluene solution of bathocuproine in polymethylmethacrylate was spin-coated under a nitrogen atmosphere to stack the thin films having a thickness of 50 nm. This was also dried in nitrogen at 120 ° C. for 1 hour.
[0144]
The obtained element was set in a vacuum deposition apparatus, and 10 nm of lithium fluoride was deposited as an electron injection layer. Subsequently, 1000 nm of aluminum was deposited as a cathode. Thereafter, the device was moved to a nitrogen-substituted glove box, a UV curable resin for organic EL sealing (30Y296G manufactured by ThreeBond) was applied to the periphery of the glass substrate, and the glass plate was adhered from the cathode side for sealing.
[0145]
The absolute value of the ionization potential of PVK was 5.8 eV, and the band gap estimated from the absorption edge was 3.5 eV.
[0146]
<Evaluation of device>
The completed light-emitting element was driven by direct current using ITO as an anode and Al as a cathode. The emission start voltage was 7V. The luminance when a voltage of 10V is applied is 100 cd / m ² Met. The luminance was measured with a BM-8 luminescent meter manufactured by Topcon. Further, the emission spectrum was measured with a multi-channel analyzer manufactured by Hamamatsu Photonics, and it was confirmed that the emission peak was in the vicinity of 450, 530 and 580 nm. The emission color was white.
[0147]
Next, this element was used for initial luminance of 100 cd / m ² A continuous lighting test was conducted at The luminance after 1000 hours is 90 cd / m ² Met. From this result, it can be seen that the lifetime of the device of Example 2 of the present invention is extremely excellent.
[0148]
【The invention's effect】
As described above, according to the present invention, a white planar light emitting element having high brightness, high stability, long life, and good color rendering is realized while having an extremely simple element configuration. For example, the device can be manufactured by a simple method such as spin coating as shown in the embodiment. In this case, the process is simple and an expensive vacuum device is not used frequently. Is possible. The element provided by the present invention can be expected to have a wide range of applications including a fixed light source such as indoor lighting as an unprecedented white planar high-intensity light emitting device.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a main part of a current injection type light emitting device in Embodiment 1 of the present invention.
FIGS. 2A and 2B illustrate a manufacturing procedure of a current injection light-emitting element according to Embodiment 1 of the present invention. FIGS.
FIG. 3 is a diagram showing an energy diagram of the current injection type light emitting device in the first embodiment of the present invention.
FIG. 4 is a diagram for explaining the electric field distribution of the current injection light-emitting element in the first embodiment of the present invention.
FIG. 5 is a graph showing an emission spectrum of the current injection light-emitting element according to Embodiment 1 of the present invention.
FIG. 6 is a graph showing the relationship between the applied voltage and the current density of the current injection light-emitting element in Embodiment 1 of the present invention.
FIG. 7 is a graph showing the relationship between the current density and the light emission luminance of the current injection light emitting device in the first embodiment of the present invention.
FIG. 8 is a cross-sectional view of a main part of a current injection type light emitting device in a second embodiment of the present invention.
FIG. 9 is a cross-sectional view of a main part of a current injection type light emitting device in a third embodiment of the present invention.
[Explanation of symbols]
1 Substrate
2 Anode
3 Light emitting layer
4 Matrix
5 particles
6 Cathode
7 Hole transport layer
8 Electron transport layer
9 Electronic block layer
10 Hole blocking layer

Claims (17)

A current injection type light emitting device comprising at least an anode for injecting holes, a light emitting layer having a light emitting region, and a cathode for injecting electrons,
The light emitting layer contains an inorganic fluorescent material base particle having an average particle size of 100 nm or less doped with an element serving as an emission center, and at least one medium for dispersing the inorganic fluorescent material base particle, and the light emission A current injection type light emitting device comprising a hole transport layer that has a larger absolute value of ionization potential than the base particles of the inorganic fluorescent material and transports holes predominantly between the layer and the anode. An absolute value of the lowest unoccupied orbital level is larger than that of the inorganic fluorescent material base particle, and an electron transporting substance that predominantly transports electrons is dispersed together in a medium for dispersing the inorganic fluorescent material base particle, 2. The current injection type light emitting device according to claim 1, wherein the current injection type light emitting device is provided between the light emitting layer and the cathode alone or as an electron transport layer dispersed in a medium. 3. The current injection type light emitting device according to claim 1, wherein the absolute value of the lowest unoccupied orbital level of the hole transport layer is smaller than that of the base particle of the inorganic fluorescent material. . 4. The current injection type light emitting device according to claim 1, wherein an absolute value of an ionization potential of the electron transport material and the electron transport layer is larger than that of the base particle of the inorganic fluorescent material. . The number of types of dopant elements doped in the inorganic fluorescent material base particles is plural, and emission wavelengths at 380 to 780 nm from the dopant elements are at least two types or more. The current injection type light emitting device according to any one of the above. When laminating at least one of the hole transport layer and the light emitting layer and the light emitting layer and the electron transport layer, the hole transport layer and the light emitting layer are laminated from a solution using a solvent having low solubility in an adjacent layer. The current injection type light emitting device according to any one of claims 1 to 5. 7. When laminating at least one of the hole transport layer and the light-emitting layer, and the light-emitting layer and the electron transport layer, the hole transport layer and the light-emitting layer are laminated at a temperature that does not dissolve in an adjacent layer. The current injection type light emitting device according to any one of the above. A layer for preventing movement of electrons is provided between the light emitting layer and the anode, or a layer for preventing movement of holes is provided between the light emitting layer and the cathode. The current injection type light emitting device according to any one of 1 to 7. A current injection type light emitting device comprising at least an anode for injecting holes, a light emitting layer having a light emitting region, and a cathode for injecting electrons, wherein the light emitting layer is an average particle doped with an element serving as an emission center II-VI group semiconductor particles having a size of 100 nm or less and at least one medium for dispersing the II-VI group semiconductor particles, and an ionization potential between the light emitting layer and the anode It is provided with a hole transport layer that transports holes smaller than II-VI group semiconductor particles, and the II-VI group semiconductor particles are produced while being dispersed in a solution without precipitation. A current injection type light emitting device characterized by the above. A current injection type light emitting device comprising at least an anode for injecting holes, a light emitting layer having a light emitting region, and a cathode for injecting electrons,
The light emitting layer contains inorganic fluorescent material base particles having an average particle size of 100 nm or less doped with an element serving as a light emission center, and at least one medium for dispersing the inorganic fluorescent material base particles, and the inorganic fluorescent material A current injection type light emitting device characterized in that there are a plurality of types of dopant elements doped in the material base particles, and at least two types of emission wavelengths at 380 to 780 nm from the dopant elements. The positive hole transport layer which has an absolute value of ionization potential larger than the inorganic fluorescent material base particle and transports holes predominantly is provided between the light emitting layer and the anode. Current injection type light emitting device. An absolute value of the lowest unoccupied orbital level is larger than that of the inorganic fluorescent material base particle, and an electron transporting substance that predominantly transports electrons is dispersed together in a medium for dispersing the inorganic fluorescent material base particle, 12. The current injection type light emitting device according to claim 10, wherein the current injection type light emitting device is provided between the light emitting layer and the cathode alone or as an electron transport layer dispersed in a medium. . 13. The current injection type light emitting device according to claim 10, wherein the absolute value of the lowest unoccupied orbit of the hole transport layer is smaller than that of the inorganic fluorescent material base particle. The current injection type light emitting device according to any one of claims 10 to 13, wherein absolute values of ionization potentials of the electron transport material and the electron transport layer are larger than those of the inorganic fluorescent material base particles. . When laminating at least one of the hole transport layer and the light emitting layer and the light emitting layer and the electron transport layer, the hole transport layer and the light emitting layer are laminated from a solution using a solvent having low solubility in an adjacent layer. The current injection type light emitting device according to any one of claims 10 to 14. 16. When laminating at least one of the hole transport layer and the light emitting layer, and the light emitting layer and the electron transport layer, the hole transport layer and the light emitting layer are laminated at a temperature that does not dissolve in an adjacent layer. The current injection type light emitting device according to any one of the above. A layer for preventing movement of electrons is provided between the light emitting layer and the anode, or a layer for preventing movement of holes is provided between the light emitting layer and the cathode. The current injection type light emitting device according to any one of 10 to 16.

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