JPH07161476A - Light emitting element - Google Patents

Abstract

PURPOSE:To obtain a high-luminance light emitting element which can utilize the electric energy with high efficiency, by interposing a light emitting material and condensation carbon molecule, which is formed of 25 or more of carbon atoms, between a positive electrode and a negative electrode. CONSTITUTION:ITO glass is used for a positive electrode, and Ag, Au or the like is used for a negative electrode. Structure of the light emitting material is formed of positive hole transmitting layer/light emitting layer, positive hole transmitting layer/light emitting layer/electron transmitting layer, light emitting layer/electron transmitting layer, or laminated multi-layers of these combination. As the positive hole transmitting material, N,-N'-diphenyl-N, N'-di(2-methylphenyl)-4, 4'-diamine or the like is used. As the light emitting layer material, anthracene or the like is used. As the electron transmitting material, oxadiazole derivative or the like is used. As a high polymer adhesive material, solvent soluble material such as polyvinyl chloride is used, and vaporized to form the light emitting material. As a condensation carbon molecule, C60, C70 are effective, and a predetermined quantities of graphite, furalene, etc., are mixed. As the electron transmitting material, oxadiazole derivative is used. With this structure, high-luminance light emitting material is obtained, and lifetime of the element can be prolonged.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device capable of converting electric energy into light, such as a display device, a flat panel display, a backlight, an illumination, an interior, a sign,
It can be suitably used in fields such as signboards and electrophotographic machines.

[0002]

2. Description of the Related Art Many devices for converting electric energy into light have been studied and put to practical use since ancient times, and have become an indispensable technology in our daily lives. Typical light emitting elements include incandescent lamps, halogen lamps, fluorescent lamps, and light emitting diodes (L
ED), electroluminescence (EL), and the like, and are applied to many fields such as lighting, interiors, and display devices. In recent years, flat panel displays (FPDs) have attracted attention as thin displays replacing CRTs, and many technologies have been studied, and some liquid crystal displays and the like are already at a practical level. FPD
In the field, the device is thin while maintaining high display quality.
It is important to be lightweight.

To meet this requirement, the inorganic semiconductor materials such as ZnS, CaS, and SrS, which are the emission centers of M, are used.
Inorganic EL devices doped with rare earth elements such as n, Eu, Ce, Tb, and Sm are generally known. This element is a self-luminous element and has the excellent characteristics that it does not need to be illuminated by light rays from the back like a liquid crystal display and has no dependence on the viewing angle, but it requires AC driving and a driving voltage of about 100V. It is pointed out that the brightness is low, and the life is short due to deterioration of the element due to humidity. In addition, the inability to emit blue light with high brightness makes it difficult to develop as a color display.

On the other hand, in recent years, active research has been conducted on an organic laminated thin-film light emitting device in which electrons injected from a cathode and holes injected from an anode emit light when recombined in an organic phosphor sandwiched between the electrodes. It's starting to happen. This element has attracted attention because it is thin, has high-luminance light emission under a low driving voltage, and has multicolor light emission by selecting a fluorescent material.

This study was conducted by Kodak C.I. W. Tan
Since g. et al. showed that an organic laminated thin film device emits light with high brightness (Appl. Phys. Lett. 51 (12)
21, p. 913, 1987), many research institutes are investigating. A typical structure of an organic laminated thin film light emitting device presented by a research group of Kodak Company is a hole transporting diamine compound and a light emitting layer on an ITO glass substrate.
Hydroxyquinoline aluminum and M as cathode
g: Ag was sequentially provided, and a green emission of 1000 cd / m ² was possible with a driving voltage of about 10V. Some of the current organic laminated thin-film light-emitting elements have different configurations such as an element having an electron transport layer in addition to the above-mentioned element components, but basically follow the configuration of Kodak Corporation. However, the device characteristics largely depend on the material, and various substances are being studied for that reason.

[0006] First, regarding the hole transport material, the hole transport material for organic laminated thin film light emitting device is described in "Organic EL device development strategy" (Science Forum, published in 1992) edited by the Next Generation Display Device Study Group. There is a description. According to this, as a hole transport material, a material having a high carrier transport ability is effective for improving the efficiency of light emission against power, and a suitable electron level material is effective for confining excitons in the light emitting layer and improving the efficiency of carrier injection. The choice has been shown to be important, and it is also important not to form an exciplex at the interface with the light emitting layer in order to efficiently convert electrical energy into light. Further, the film thickness and film forming ability are also important requirements in actual device fabrication. As a specific hole transport material, a hydrazone compound, a stilbene compound, a triphenylamine compound, a heterocyclic compound represented by an oxadiazole derivative or a phthalocyanine derivative, and a polycarbonate having a side chain containing the above monomer in a polymer system And styrene derivatives, polyvinylcarbazole,
Polysilane and the like are shown.

A phosphor is used as the material of the light emitting layer. The holes injected from the hole transport layer and the electrons injected from the cathode are recombined in the light emitting layer, and at this time, the phosphor is excited to emit light. Therefore, the light emitting layer material is required to have carrier transporting ability in addition to fluorescence. For example, it is known that 8-hydroxyquinoline aluminum used in the device of Tang et al. Mentioned above has an electron transporting property. Generally, when the light emitting layer has an electron transporting property, a hole transporting layer is provided between the transparent electrode (positive electrode) and the light emitting layer, and when the light emitting layer has a hole transporting property, the back electrode (negative electrode) and the light emitting layer are emitted. It is effective to provide an electron transport layer between the layers, and when a material that transports both electron and hole carriers equally is used, it is effective to sandwich the light emitting layer between the hole transport layer and the electron transport layer. It is said that. It is considered that the higher the quantum efficiency of the phosphor, the higher the intensity of light emission that can be obtained, but in reality, concentration quenching occurs and it is difficult to use itself as a light emitting material. However, Tang et al. Improve the luminous efficiency of the device by causing energy transfer from the host molecule by doping a small amount of a dye having a high fluorescence quantum efficiency such as coumarin into the 8-hydroxyquinoline aluminum luminous layer, It is known that the emission wavelength can be shifted to enable multicolor emission (J. Appl. Phys. 65.
(9) p. 3610, 1989). In addition, it has been pointed out that good film-forming property by vapor deposition and thermal and chemical stability at the time of light emission are required. A π-conjugated compound is often used as a light emitting layer material satisfying such requirements. Specifically, in addition to anthracene and pyrene, which have been known as light emitters for a long time, and the aforementioned 8-hydroxyquinoline aluminum, for example, bisstyrylanthracene derivatives, tetraphenylbutadiene derivatives,
Coumarin derivatives, oxadiazole derivatives, distyrylbenzene derivatives, pyrrolopyridine derivatives, perinone derivatives, cyclopentadiene derivatives, thiadiazolopyridine derivatives, polymer systems include polyphenylene vinylene derivatives, polyparaphenylene derivatives, and polythiophene derivatives. Therefore, it is possible to achieve multicolored light emission. Further, as a dopant added to the light emitting layer, rubrene, a quinacridone derivative, phenoxazone 6
60, DCM1, Perinone, Perylene, Coumarin 540
It is known that the luminous efficiency can be improved and the luminescence can be multicolored.

[0008] Electron transport materials have little knowledge and leave room for investigation, but oxadiazole derivatives and 8-hydroxyquinoline aluminum are known. This material has the role of improving hole blocking and electron injection efficiency, and depending on the selection of the material, a very high-performance element can be produced. The electrode also plays a major role in determining device characteristics. Since this element is a light emitting element, there is a restriction that either the cathode or the anode must be transparent. In most cases, a transparent material is used for the anode, and ITO, Nesa glass, etc. are typical, but thinly-deposited gold is also used, and any electrode that does not significantly impair light transmission can be used. However, the surface condition of the anode is required to be very clean, and it has been pointed out that, for example, cleaning the ITO glass substrate greatly affects the device characteristics.
In the case of the cathode, opaque materials such as magnesium and silver are often deposited. In addition to the above materials, aluminum, gold, indium, etc. are also known as cathode materials, but a low work function metal is used to facilitate injection of electrons, and in that sense alkali metal is effective. However, magnesium or its alloys are still often used in consideration of the stability of metals.

[0009]

However, the prior art has not been put to practical use as an element because of its low emission brightness against electric current and its short life. However, this problem can be treated as one problem. That is,
In the case of high-luminance light emission, if the light emission luminance with respect to the applied power is low, Joule heat is generated except for the energy that becomes light, and the life of the element is shortened. Therefore, it is not possible to obtain a light emitting element with sufficiently high brightness for given energy.
Has become a problem in the field.

It is an object of the present invention to solve the above problems and to provide an element capable of emitting light with high brightness even under a low current.

[0011]

In order to achieve the above-mentioned object, the present invention provides a "element in which a substance controlling light emission exists between a positive electrode and a negative electrode, which emits light by electric energy, wherein the element is a condensed carbon molecule. The light-emitting element is characterized by including "."

In the present invention, either the positive electrode or the negative electrode needs to be transparent in order to extract light, and a transparent material is usually used for the positive electrode. Examples of the transparent positive electrode material include tin oxide and oxide. Conductive metal oxides such as indium and indium tin oxide (ITO), or gold,
Metals such as silver and chromium, inorganic conductive substances such as copper iodide and copper sulfide, and conductive polymers such as polythiophene, polypyrrole and polyaniline are used without particular limitation. Above all, it is particularly preferable to use ITO glass or Nesa glass. The resistance of the transparent electrode is not limited as long as it can supply a sufficient current for light emission of the device, but is preferably low resistance from the viewpoint of power consumption of the device.
For example, an ITO substrate having a resistance of 300 Ω / □ or less functions as an element electrode, but since it is now possible to supply a substrate having a resistance of about 10 Ω / □, it is particularly desirable to use a low resistance product. The thickness of ITO can be arbitrarily selected according to the resistance value, but it is usually used in the range of 1000 to 3000 angstroms. Further, as the glass substrate, soda lime glass, non-alkali glass or the like is used, and since the thickness is sufficient as long as the mechanical strength is maintained, 0.7 mm or more is sufficient. Regarding the material of the glass, alkali-free glass is preferable because it is preferable that less ions are eluted from the glass.
Soda lime glass coated with a barrier coat such as iO ₂ is also commercially available and can be used. The ITO film forming method is not particularly limited, such as an electron beam method, a sputtering method and a chemical reaction method.

Since the negative electrode must efficiently supply electrons to a substance that controls light emission or a substance adjacent to the substance that controls light emission (for example, an electron transport layer), the adhesiveness and energy between the electrode and the substance adjacent to the electrode are high. It is preferable to adjust the level. Further, in order to maintain stable performance for long-term use, it is particularly desirable to use a material that is relatively stable in the atmosphere, and it is also possible to use a protective film or the like, but it is not limited to this. It is not something that will be done. Specifically, it is possible to use metals such as indium, gold, silver, aluminum, lead, magnesium, lanthanum, europium, and ytterbium, simple earths of rare earths, alkali metals, or alloys thereof. Considering the device characteristics, it is desirable to use magnesium or its alloy (for example, with silver). The electrodes are manufactured by a resistance heating method, an electron beam method, a sputtering method,
A coating method or the like is used, and the metal can be vapor-deposited alone or two or more components can be vapor-deposited simultaneously. Particularly, for forming an alloy, it is possible to easily form an alloy electrode by simultaneously vapor-depositing a plurality of metals.

The substances that control light emission are 1) hole transport layer / light emitting layer, 2) hole transport layer / light emitting layer / electron transport layer, 3) light emitting layer / electron transport layer, and 4) the above groups. It may be in a form in which the combined substances are mixed in one layer. That is, as the element structure, in addition to the multilayer laminated structure of the above 1) to 3), a single layer containing a light emitting material alone or a layer containing a light emitting material and a hole transporting material and / or an electron transporting material may be provided as in 4). Good.

The hole injecting layer is formed of a hole transporting substance alone or a hole transporting substance and a polymer binder, and the hole transporting substance is N, N'-diphenyl-N, N '.
-Triphenylamines such as di (3-methylphenyl) -4,4'-diamine, tertiary amines such as N-isopropylcarbazole, pyrazoline derivatives, stilbene compounds, hydrazone compounds, oxadiazole derivatives and phthalocyanines Heterocyclic compounds represented by derivatives,
In the polymer system, polycarbonate having a side chain of the above monomer, styrene derivative, polyvinylcarbazole, polysilane and the like are preferable, but not particularly limited.

The material for the light emitting layer is mainly anthracene and pyrene which have been known as light emitters for a long time, and the above-mentioned 8-hydroxyquinoline aluminum. Oxadiazole derivative,
As the distyrylbenzene derivative, the pyrrolopyridine derivative, the perinone derivative, the cyclopentadiene derivative, the oxadiazole derivative, the thiadiazolopyridine derivative, and the polymer system, a polyphenylene vinylene derivative, a polyparaphenylene derivative, and a polythiophene derivative can be used. Further, as the dopant added to the light emitting layer,
The aforementioned rubrene, quinacridone derivative, phenoxazone 660, DCM1, perinone, perylene, coumarin 54
0 or the like can be used as it is.

As the electron-transporting substance, it is necessary to efficiently transport the electrons from the cathode between the electrodes to which an electric field is applied, and the electron injection efficiency is high, and it is desirable that the injected electrons are efficiently transported. . For that purpose, it is required that the substance has a high electron affinity, a high electron mobility, an excellent stability, and an impurity that becomes a trap is hard to be generated during production and use. As substances satisfying such conditions, oxadiazole derivatives and 8
-Hydroxyquinoline aluminum and the like are not particularly limited.

The above materials used for the hole transporting layer, the light emitting layer and the electron transporting layer can be used to form each layer independently, and as the polymer binder, polyvinyl chloride, polycarbonate, polystyrene and poly (N- Vinylcarbazole), polymethylmethacrylate, polybutylmethacrylate, polyester, polysulfone, polyphenylene oxide, polybutadiene, hydrocarbon resin, ketone resin, phenoxy resin, polysulfone, polyamide,
Solvent-soluble resins such as ethyl cellulose, vinyl acetate, ABS resin, polyurethane resin, phenol resin, xylene resin, petroleum resin, urea resin, melamine resin, unsaturated polyester resin, alkyd resin, epoxy resin,
It is also possible to use it by dispersing it in a curable resin such as a silicone resin.

The method of forming the substance that controls the light emission is not particularly limited, such as resistance heating vapor deposition, electron beam vapor deposition, sputtering, molecular lamination method and coating method, but usually resistance heating vapor deposition and electron beam vapor deposition are the characteristics. It is preferable from the aspect. The thickness of the layer cannot be limited because it depends on the resistance value of the substance that controls light emission, but empirically it is 100 to 100.
Choose from between 00 Angstroms. For example, polyvinylcarbazole is used for the hole transport layer and 8-
When hydroxyquinoline aluminum is used, the thickness of each layer is such that the thickness of polyvinylcarbazole is 100 to 7
00 angstrom is preferable, 200-500 angstrom is more preferable, and the film thickness of 8-hydroxyquinoline aluminum is preferably 200-2000 angstrom, more preferably 500-1200 angstrom.

The condensed carbon molecule preferably has 25 or more carbon atoms, and examples thereof include graphite, diamond, fullerenes, and vapor-deposited carbon. In particular, this fullerene is known to exhibit photoconductivity when used in combination with polyvinylcarbazole (Nat).
ure VOL356, p. 585 (1992)), it is possible to significantly improve device characteristics by using this mixture for the light emitting device. The most effective case is when it is used as a hole transport material, but it can also be used in other layers and is not particularly limited. Fullerenes are C28, C32, C50, C60, C70 ...
・ Various forms such as C540, but among them C6
0 and C70 are effective. These substances can be used alone or in a mixture. Since it is desirable that the material has absorption within the emission wavelength of the device in order to exhibit high performance, a fullerene or a sensitizer having absorption in the visible light region can be added. The mixing ratio can be arbitrarily selected from 0.1% by weight to 10000% by weight with respect to polyvinylcarbazole, but considering the thin film forming ability, it is preferably between 0.2 and 10% by weight. Further, these fullerenes can be used as a mixture with the hole transport material, the light emitting material, and the electron transport material by a coating method in a solution or a co-evaporation method. Further, it can be used alone as a hole transport material or an electron transport material.

Similar to fullerene, other condensed carbon molecules can be formed into a thin film by a method such as resistance vapor deposition, electron beam or sputtering, but can be formed by CVD or a gas phase reaction method. . And
Similar to fullerenes, it can be mixed with a hole transporting material, a light emitting material, or an electron transporting material by a method of coating in solution or a co-evaporation method, and as a hole transporting material or an electron transporting material. It can also be used alone.

The electrical energy in the present invention mainly refers to a direct current, but it is also possible to use a pulse current or an alternating current. The current value and voltage value are not particularly limited,
Considering the power consumption and life of the device, it can be arbitrarily selected so that the maximum brightness can be obtained with energy as low as possible.

[0023]

The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited to these examples.

Example 1 A glass substrate (15 Ω / □) on which an ITO transparent conductive film was deposited in a thickness of 1500 Å was cut into a predetermined size, and after etching, cleaning was performed. A solution was prepared by adding 1.5% by weight of fullerene (C ₆₀ / C ₇₀ = 92/8) to PVCz in a 0.85% by weight solution of polyvinylcarbazole (PVCz) in dichloroethane. The ITO substrate was vertically dipped in this solution and dip-coated at a pulling rate of 50 mm / min. This was placed in a vacuum vapor deposition apparatus and evacuated until the degree of vacuum inside the apparatus became 5 × 10 ⁻⁶ Torr or less. Aluminum 8-hydroxyquinoline aluminum was deposited to a thickness of 1000 Å. Next, magnesium (500 Å) and silver (1500 Å) are vapor-deposited to form 5 × 5 mm.
A corner element was made. The maximum brightness of this light emitting device is 14
It was 860 cd / m ² (13.8 V, 100 mA).

Example 2 The maximum luminance of the device obtained in exactly the same manner as in Example 1 except that the addition amount of fullerene was 10% by weight with respect to PVCz was 10670 cd / m ² (16.2 V, 1).
It was 00 mA).

Comparative Example 1 The maximum luminance of the device obtained in exactly the same manner as in Example 1 except that fullerene was not added was 9980c.
It was d / m ² (13.3 V, 110 mA).

Example 3 The maximum luminance of the device obtained in exactly the same manner as in Example 1 except that 8-hydroxyquinoline aluminum aluminum was vapor-deposited to a thickness of 800 Å was 7
It was 740 cd / m ² (16.4 V, 100 mA).

Comparative Example 2 The maximum luminance of the device obtained in exactly the same manner as in Example 3 except that fullerene was not added was 5010c.
It was d / m ² (19.0 V, 80 mA).

[0029]

INDUSTRIAL APPLICABILITY The present invention can provide a high-brightness light emitting device with high utilization efficiency of electric energy, and can contribute to prolonging the life of the device.

Claims (4)

[Claims] 1. A light emitting device, wherein a substance controlling light emission is present between a positive electrode and a negative electrode, and the device emits light by electric energy, wherein the device contains condensed carbon molecules. 2. The light emitting device according to claim 1, wherein the condensed carbon molecule is composed of 25 or more carbon atoms. 3. A light emitting device according to claim 1 or 2, wherein the condensed carbon molecule is used as a hole and / or electron transport material. 4. The light emitting device according to claim 1, wherein the condensed carbon molecule has a condensed structure of a 6-membered ring and / or a 5-membered ring carbon skeleton.