JP2002359080A - Light emitting element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light emitting element having high durability, and having high utilization efficiency of electric energy. SOLUTION: An element emits the light with the electric energy by allowing a light emitting substance to exist between an anode and a cathode. This light emitting element is characterized in that the element includes at least a light emitting material and a triplet capturing agent.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an element capable of converting electric energy into light, and relates to a display element, a flat panel display, a backlight, lighting, an interior, a sign, a sign, an electrophotographic device, an optical signal generator, and the like. The present invention relates to a light emitting element that can be used in the field of (1).

[0002]

2. Description of the Related Art In recent years, research has been actively conducted on organic laminated thin-film light-emitting devices in which electrons injected from a cathode and holes injected from an anode emit light when they recombine in an organic phosphor sandwiched between both electrodes. ing. This device has attracted attention because it is thin, emits light with high luminance under a low driving voltage, and emits multicolor light by selecting a fluorescent material.

[0003] This study was carried out by Kodak Corporation. W. Tan
g. et al. showed that the organic laminated thin film element emits light with high luminance (Appl. Phys. Lett. 51 (12)
21, p. 913, 1987), and many research institutions are conducting studies. A typical configuration of an organic laminated thin-film light emitting device presented by a research group of Kodak Company is a diamine compound having a hole transporting property on an ITO glass substrate, and a tris (8-quinolinolato) aluminum complex (Al
q ₃ ), and Mg: Ag was sequentially provided as a cathode, and green light emission of 1000 cd / m ² was possible at a driving voltage of about 10 V. The present organic laminated thin-film light-emitting device has a different configuration, such as a device provided with an electron transport layer in addition to the above-described device components, but basically follows the configuration of Kodak Company.

[0004]

However, in order to apply the organic laminated thin-film light emitting device to a light source such as a flat panel display or a backlight, it is necessary to sufficiently secure the reliability of the device. However, in the conventional organic laminated thin-film light-emitting device, since the device is inevitably deteriorated during continuous driving, the reliability cannot be said to be sufficient. There are mainly three causes of the device deterioration described below.

[0005] The first problem is that the shape of the thin film of the organic layer is deteriorated by heat generated when the device is driven. This low heat resistance is considered to be due to the low glass transition temperature (hereinafter, abbreviated as Tg) of the material, and the molecular weight of the material must be increased (see Japanese Patent Application Laid-Open No. H08-208,083).
Solutions have been attempted by making the molecular structure rigid (Japanese Patent Laid-Open No. 7-110940).

[0006] The second is deterioration of materials due to positive and negative charges flowing through the element. Alq which is a typical light emitting material
₃ , it is known that the light emission performance is reduced by positively charging, and it is studied to prevent the deterioration of Alq ₃ by adding a substance that traps a positive charge flowing into the light emitting layer. .

[0007] The third is light degradation of the light emitting material. A light-emitting material obtains energy through recombination of holes and electrons or energy transfer from another light-emitting material to form a singlet or triplet excited state, of which the singlet excited state is radiatively relaxed. Emits fluorescence. Well known dye lasers and the like (Chem. Phys. L.
ett. 277, p. 392, 1997), the light-emitting material gradually deteriorates when excitation and relaxation are repeated, and the light-emitting performance deteriorates, which causes deterioration of the element during continuous driving.

[0008] An object of the present invention is to solve such a problem and to provide a light-emitting element having high durability and high utilization efficiency of electric energy.

[0009]

That is, the present invention relates to an element which emits light by electric energy, in which a light emitting substance is present between an anode and a cathode.
A light-emitting element including a multiplet trapping agent.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, the anode is made of a conductive metal oxide such as tin oxide, indium oxide, indium tin oxide (ITO) or a metal such as gold, silver or chromium if it is transparent to extract light. Although not particularly limited, such as metals, inorganic conductive substances such as copper iodide and copper sulfide, and conductive polymers such as polythiophene, polypyrrole, and polyaniline, it is particularly preferable to use ITO glass or Nesa glass. The resistance of the transparent electrode is not limited as long as a current sufficient for light emission of the element can be supplied, but is preferably low from the viewpoint of power consumption of the element. For example, 3
Although an ITO substrate having a resistance of 00 Ω / □ or less functions as an element electrode, a substrate having a resistance of about 10 Ω / □ can be supplied at present. The thickness of the ITO can be arbitrarily selected according to the resistance value, but is usually used in a range of usually 100 to 300 nm. Further, the glass substrate is made of soda lime glass, non-alkali glass, or the like, and the thickness only needs to be sufficient to maintain the mechanical strength.
mm or more is sufficient. For the glass material,
Alkali-free glass is preferred because less ions elute from the glass is preferred, but soda-lime glass with a barrier coat such as SiO ₂ is also commercially available and can be used. The method of forming the ITO film is not particularly limited, such as an electron beam method, a sputtering method, and a chemical reaction method.

The cathode is not particularly limited as long as it can efficiently inject electrons into the organic material layer.
Gold, silver, copper, iron, tin, zinc, aluminum, indium, chromium, lithium, sodium, potassium, calcium, magnesium, etc., and lithium, sodium for increasing the electron injection efficiency and improving the device characteristics , Potassium, calcium, magnesium or alloys containing these low work function metals are effective. But,
These low work function metals are generally unstable in the air in many cases. For example, an organic layer is doped with a very small amount of lithium or magnesium (1 nm or less as indicated by a film thickness gauge by vacuum deposition) to have high stability. Although a method using an electrode can be mentioned as a preferable example, it is not particularly limited because an inorganic salt such as lithium fluoride can be used. In addition, platinum, gold,
Lamination of metals such as silver, copper, iron, tin, aluminum, and indium, or alloys using these metals, and inorganic substances such as silica, titania, and silicon nitride, polyvinyl alcohol, vinyl chloride, and hydrocarbon polymers. Are preferred examples. The method for producing these electrodes is not particularly limited, as long as electrical conduction such as resistance heating, electron beam, sputtering, ion plating, and coating can be achieved.

SUMMARY OF THE INVENTION It is an object of the present invention to prevent the deterioration of the light emitting element due to the light deterioration of the light emitting material when the light emitting element is continuously driven. It is well known from laser dyes and the like that the excited triplet state of a light-emitting material is related to the photodegradation of the light-emitting material. In a light-emitting element that emits light by an electric field, a light-emitting material is excited by recombination energy of holes and electrons flowing through the element, and the excited state is a singlet and a triplet.
The ratio of the multiplet becomes 1: 3. Therefore, the triplet excited state is more present in the electric field excited light emitting element than in the case of light excitation such as laser. Therefore, reducing the triplet excited state of the light-emitting material is one of the most effective methods for preventing the deterioration of the light-emitting material.

The triplet excited state of the light emitting material can be reduced by coexisting a substance that efficiently receives energy from the triplet excited state of the light emitting material, ie, a triplet trapping agent. This is achieved by triplet energy transfer from the luminescent material to the triplet scavenger. In triplet energy transfer, when the triplet energy of the energy donor is larger than the triplet energy of the energy acceptor, efficient energy transfer is performed. Therefore, it is desirable that the relationship of T ≧ T ₀ be established between the triplet energy T of the light emitting material as the energy donor and the triplet energy T ₀ of the triplet trapping agent as the energy acceptor. . For better energy transfer, it is more desirable that the luminescent material and the triplet trap are in the same layer. Specific triplet scavengers are not particularly limited, but include aromatic hydrocarbons such as naphthalene, anthracene, and pentacene; N, N '.
-Diphenyl-N, N'-di (3-methylphenyl)-
4,4′-diphenyl-1,1′-diamine, N, N ′
Triphenylamines such as -dinaphthyl-N, N'-diphenyl-4,4'-diphenyl-1,1'-diamine, bis (N-allylcarbazole) or bis (N-
Alkylcarbazoles), pyrazoline derivatives, stilbene compounds, hydrazone compounds, oxadiazole derivatives and phthalocyanine derivatives, heterocyclic compounds represented by porphyrin derivatives, quinolinol derivative metal complexes represented by 8-hydroxyquinoline aluminum, tropolone metals Complex, flavonol metal complex, perylene derivative, perinone derivative, naphthalene, coumarin derivative,
Oxadiazole derivatives, aldazine derivatives, bisstyryl derivatives, pyrazine derivatives, phenanthroline derivatives and the like can be mentioned. These triplet scavengers may contain only one kind or two or more kinds. In addition, these 3
The singlet trapping agent may emit light or may not emit light, but must receive a triplet energy from the light-emitting material and thus need to be a compound different from the light-emitting material.

The luminescent substance in the present invention includes 1) a hole transport layer / a light emitting layer, 2) a hole transport layer / a light emitting layer / an electron transport layer,
3) light emitting layer / electron transport layer, 4) hole transport layer / light emitting layer / hole blocking layer, 5) hole transport layer / light emitting layer / hole blocking layer / electron transport layer, 6) light emitting layer / hole Blocking layer / electron transport layer and
7) Any of the forms in which the above combined substances are mixed in a single layer may be used. That is, as the element configuration, the above 1) to
In addition to the multi-layer structure of 6), only a single layer of a luminescent material or a layer containing a luminescent material and a hole transporting material or an electron transporting material as in 7) may be provided. Further, the luminescent substance in the present invention corresponds to both a substance which emits light by itself and a substance which assists the light emission, and refers to a compound, a layer, or the like involved in light emission. The triplet trapping agent in the present invention may be contained in any of the above luminescent substances, and may be contained in one or more layers. Further, it may be contained in the whole layer or a part of the layer. Further, the triplet trapping agent in the present invention may be used as a mixture with a hole-transporting substance, a luminescent material, an electron-transporting material, a hole-blocking substance, or the like as described below, or may be chemically bonded. You can do it.

The hole transporting layer is formed by a hole transporting substance alone or by laminating and mixing two or more kinds of substances or by a mixture of a hole transporting substance and a polymer binder. N, N'-diphenyl-N, N'-di (3-methylphenyl) -4,4'-diphenyl-1,1'-diamine, N, N'-dinaphthyl-N, N'-diphenyl-
Triphenylamines such as 4,4′-diphenyl-1,1′-diamine, bis (N-allylcarbazole) or bis (N-alkylcarbazole), pyrazoline derivatives, stilbene compounds, hydrazone compounds, oxadi Heterocyclic compounds represented by azole derivatives, phthalocyanine derivatives, porphyrin derivatives, and polymer-based polycarbonates and styrene derivatives having the above monomers in the side chain, polyvinylcarbazole, polysilane, and the like are preferable, but a thin film necessary for device fabrication is formed. However, the compound is not particularly limited as long as it can inject holes from the anode and can transport holes.

The light emitting layer is formed of a light emitting material (host material, dopant material), which may be a mixture of a host material and a dopant material, or a single host material. The host and dopant materials are
Each type may be one type, a plurality of combinations, or any combination. The dopant material may be included in the entire host material, may be partially included, or may be included therein. Even if the dopant material is laminated,
They may be dispersed or any of them.

The light emitting material can be selected from various materials according to a desired light emitting color. In order to obtain high-intensity light emission, there is no particular limitation, but naphthalene, anthracene, phenanthrene, pyrene, triphenylene, perylene, traxene, fluorene, indene, 9,9'- Aromatic hydrocarbon compounds such as spirobifluorene and derivatives thereof, furan, pyrrole, thiophene, silole, 9-
Silafluorene, 9,9′-spirobisilafluorene,
Aromatic heterocyclic compounds such as benzothiophene, benzofuran, indole, dibenzothiophene, dibenzofuran, imidazopyridine, phenanthroline, pyrazine, naphthyridine, quinoxaline, pyrrolopyridine, thioxanthene and derivatives thereof, and quinolinols such as tris (8-quinolinolato) aluminum complex Metal complex, bipyridine metal complex, rhodamine metal complex, azomethine metal complex, distyrylbenzene derivative, tetraphenylbutadiene derivative, stilbene derivative, aldazine derivative, coumarin derivative, phthalimide derivative, naphthalimide derivative, perinone derivative, pyrrolopyrrole derivative, cyclopentadiene Derivatives, acridone derivatives, imidazole, thiazole, thiadiazole, carbazole, oxazole, oxadia Lumpur, imidazole, azole derivatives and metal complexes thereof, pyrromethene derivatives and triazole, merocyanine derivatives, and porphyrin derivatives can be suitably used for a high fluorescence quantum yield.

Since the concentration quenching phenomenon occurs when the doping amount is too large, the doping amount is preferably used at 10% by weight or less, more preferably at 2% by weight or less based on the host substance. As a doping method, it can be formed by a co-evaporation method with a host material, but it may be mixed with the host material in advance and then evaporated at the same time. In addition, the dopant material may be included in the entire host material, partially, or may be included. The dopant material may be stacked, dispersed, or the like.

As the electron transporting material in the present invention,
It is necessary to efficiently transport electrons from the cathode between the electrodes to which an electric field is applied, and it is desirable that the electron injection efficiency is high and the injected electrons are transported efficiently. For this purpose, it is required that the material has a high electron affinity, a high electron mobility, a high stability, and a small amount of impurities serving as traps during production and use. As a substance satisfying such conditions, Tris (8
Quinolinol derivative metal complex represented by -quinolinolato) aluminum complex, tropolone metal complex, flavonol metal complex, perylene derivative, perinone derivative, naphthalene, coumarin derivative, oxadiazole derivative, aldazine derivative, bisstyryl derivative, pyrazine derivative, phenanthroline derivative, Examples include, but are not particularly limited to, silole derivatives, quinoxaline derivatives, quinoline derivatives, and phosphorus oxide compounds. These electron transporting materials may be used alone or may be laminated or mixed with different electron transporting materials.

The hole blocking layer is formed by laminating and mixing two or more kinds of hole blocking substances alone,
Examples of the hole blocking substance include phenanthroline derivatives, silole derivatives, quinolinol derivative metal complexes, oxadiazole derivatives, oxazole derivatives, quinoline derivatives,
A phosphorus oxide compound or the like is preferable, but is not particularly limited as long as it is a compound that can prevent holes from flowing out of the device from the cathode side and lowering luminous efficiency.

The above hole transport layer, light emitting layer, electron transport layer,
The material used for the hole blocking layer can form each layer alone, but as a polymer binder, polyvinyl chloride, polycarbonate, polystyrene, poly (N-vinylcarbazole), polymethyl methacrylate, polybutyl methacrylate, Polyester, polysulfone, polyphenylene oxide, polybutadiene, hydrocarbon resin,
Solvent-soluble resins such as ketone resin, phenoxy resin, polysulfone, polyamide, ethyl cellulose, vinyl acetate, ABS resin, polyurethane resin, phenol resin, xylene resin, petroleum resin, urea resin, melamine resin, unsaturated polyester resin, alkyd resin, It is also possible to use it by dispersing it in a curable resin such as an epoxy resin or a silicone resin.

The method for forming the luminescent material is not particularly limited, such as resistance heating evaporation, electron beam evaporation, sputtering, molecular lamination, and coating. However, resistance heating evaporation and electron beam evaporation are usually used in terms of characteristics. preferable. The thickness of the layer depends on the resistance of the luminescent material and cannot be limited, but is selected from the range of 1 to 1000 nm.

Further, in order to prevent deterioration of the organic layers and electrodes of the device, it is preferable to form a sealing layer on the device. For this purpose, a method in which the light emitting region is sealed with a sealing plate and an adhesive, and the inside of the sealed space is evacuated, or a method in which a low humidity inert gas or oil is filled, is used. Not something.

The electric energy mainly refers to a direct current, but it is also possible to use a pulse current or an alternating current.
The current value and the voltage value are not particularly limited. However, in consideration of the power consumption and life of the device, it is necessary to obtain the maximum luminance with the lowest possible energy.

[0025]

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 (available from Asahi Glass Co., Ltd., 15Ω / □, electron beam deposited) on which an ITO transparent conductive film was deposited to a thickness of 150 nm was cut into a size of 30 × 40 mm and etched. The obtained substrate was subjected to ultrasonic cleaning with acetone and "Semicocline 56" for 15 minutes each, and then with ultrapure water. Subsequently, the substrate was subjected to ultrasonic cleaning with isopropyl alcohol for 15 minutes, immersed in hot methanol for 15 minutes, and dried. This substrate was subjected to UV-ozone treatment for 1 hour immediately before producing the element, and was placed in a vacuum evaporation apparatus, and the degree of vacuum in the apparatus was 5 × 10 ⁻⁵ Pa.
Evacuation was performed until the following. First, 4,4′-bis (N- (m-tolyl) is used as a hole transport material by a resistance heating method.
(-N-phenylamino) biphenyl was deposited to a thickness of 50 nm. Next, tris (8-quinolinolate) is used as a light emitting material.
Aluminum (III), n-propyl-1,8-naphthalimide is deposited as a triplet trapping agent in a thickness of 45 nm, and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline is deposited as an electron transporting material in a thickness of 5 nm. The thickness was laminated. Next, lithium was doped to a thickness of 0.5 nm, and aluminum was vapor-deposited to a thickness of 200 nm to form a cathode, thereby producing a 5 × 5 mm square device. The film thickness referred to here is a value indicated by a crystal oscillation type film thickness monitor. When the light emitting device was driven at a constant current of 5 mA, the luminance did not decrease to half even after 500 hours.

Comparative Example 1 A device was prepared in the same manner as in Example 1 except that no triplet supplement was added. When the device was driven at a constant current of 5 mA, the luminance was reduced by half in 100 hours.

[0028]

As described above, by allowing the triplet trapping agent to coexist with the light emitting material, the deterioration of the light emitting material can be prevented.
The durability of the element has been improved. That is, the present invention can provide a light-emitting element having excellent durability and high use efficiency of electric energy.

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Claims (4)

[Claims] 1. A light-emitting element in which a light-emitting substance is present between an anode and a cathode and emits light by electric energy, wherein the element contains at least a light-emitting material and a triplet trapping agent. 2. The light emission according to claim 1, wherein the relationship of T ≧ T ₀ is established between the triplet energy T of the luminescent material and the triplet energy T ₀ of the triplet trapping agent. element. 3. The light emitting device according to claim 1, wherein the light emitting material and the triplet trapping agent are present in the same organic layer. 4. The light emitting device according to claim 3, wherein the light emitting material is a light emitting layer.