JP2002100479A - Organic electroluminescence element and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an organic electroluminescence element, capable of emitting the light with high light-emitting efficiency with stable drivability. SOLUTION: This organic electroluminescence element is formed by arranging an anode 2, a light-emitting layer 5, an electron hole checking layer 6, an electron transport layer 7 and a cathode 8, in this order on a base board 1. The electron transport layer 7 includes a host material, having molecular weight of not less than 400 and a dopant material having molecular weight less than 400, and electron mobility of the dopant material is higher than the electron mobility of the host material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electroluminescent device, and more particularly, to a thin film device which emits light by applying an electric field to a light emitting layer made of an organic compound.

[0002]

2. Description of the Related Art Conventionally, as a thin film type electroluminescent (EL) element, Zn, which is a group II-VI compound semiconductor of an inorganic material, is used.
In general, S, CaS, SrS, and the like are doped with Mn or a rare earth element (Eu, Ce, Tb, Sm, or the like) which is a luminescence center. However, EL devices manufactured from the above inorganic materials include: 1) AC drive is required (50-1000Hz), 2) High drive voltage (about 200V), 3) Full color is difficult (especially blue), 4) Peripheral drive circuit is expensive. Have.

However, in recent years, in order to improve the above problems,
Development of EL devices using organic thin films has been started. In particular, in order to enhance the luminous efficiency, the type of the electrode was optimized for the purpose of improving the efficiency of carrier injection from the electrode, and a hole transport layer composed of an aromatic diamine and a luminescent layer composed of an aluminum complex of 8-hydroxyquinoline were used. Of an organic electroluminescent device equipped with an organic layer (Appl. Phys. Lett., Vol. 51,
913, 1987), the luminous efficiency is greatly improved as compared with the conventional EL device using a single crystal such as anthracene. For example, doping a fluorescent dye for laser such as coumarin using an aluminum complex of 8-hydroxyquinoline as a host material (J. Appl. Phy.
s., Vol. 65, p. 3610, 1989), the improvement of the luminous efficiency, the conversion of the luminous wavelength, and the like are also performed, and the characteristics are approaching the practical characteristics.

In addition to the electroluminescent device using a low molecular material as described above, poly (p-phenylenevinylene) and poly [2-methoxy-5- (2-ethylhexyloxy)- Development of electroluminescent devices using polymer materials such as 1,4-phenylenevinylene] and poly (3-alkylthiophene), and devices in which a low molecular light emitting material and an electron transfer material are mixed with a polymer such as polyvinyl carbazole Is also being developed.

[0005] One of the major problems in applying such an organic electroluminescent device to the field of flat panel displays is to improve the luminous efficiency. In particular, low power consumption is an important point in application to a display element of a portable device. In application to a small character display element, a simple matrix driving method is mainly used.
In this method, it is necessary to illuminate the element with a high duty ratio at a high luminance in a very short time, so that there is a problem that the voltage is increased and the power emission efficiency is reduced.

[0006] The organic electroluminescent devices reported so far basically emit light by a combination of a hole transport layer and an electron transport layer. That is, the holes injected from the anode move through the hole transport layer, and recombine near the interface between the two layers with the electrons injected from the cathode and move through the electron transport layer. In principle, light is emitted by exciting the electron transport layer.

Conventionally, as an electron transporting material having a high electron transporting ability, an aluminum complex of 8-hydroxyquinoline has been reported. However, these are substances having good sublimability and excellent film quality after vacuum deposition. On the other hand, the electron mobility was lower than the hole mobility of a general hole transporting substance by about two orders of magnitude and was not sufficient.

Low-molecular-weight compounds having a molecular weight of less than 400, such as oxadiazole derivatives, quinoxaline derivatives, and phenanthroline derivatives, exhibit high electron mobility, but have low Tg and are easily crystallized. Was difficult to use alone.

[0009]

As described above, in order to improve the luminous efficiency of the organic electroluminescent device, it is necessary to efficiently transport electrons injected from the cathode to the luminescent layer and to pass through the luminescent layer. It is necessary to completely block holes and reduce the amount of current that does not contribute to recombination, and further improvement studies are desired for device structures and materials for producing devices with high luminous efficiency and stability. I was

The present invention has been made in view of the above-mentioned conventional situation, and can emit light with high luminous efficiency.
It is an object of the present invention to provide an organic electroluminescent device that can be driven stably.

[0011]

The gist of the present invention resides in the following organic electroluminescent devices. In an organic electroluminescent device in which an anode, a light emitting layer, an electron transporting layer, and a cathode are formed in this order on a substrate, the electron transporting layer comprises a host material having a molecular weight of 400 or more and a molecular weight of 400 or more.
An organic electroluminescent device comprising: a dopant material having an electron mobility of less than that of the host material. 4. The organic electroluminescent device according to the item 1, wherein the dopant material of the electron transport layer is selected from oxadiazole derivatives, quinoxaline derivatives, and phenanthroline derivatives. 3. The organic electroluminescent device according to claim 1, wherein the host material of the electron transport layer is a metal complex. Having a hole blocking layer between the light emitting layer and the electron transporting layer,
The organic electroluminescence device according to the above-mentioned, wherein the ionization potential of the hole blocking layer is 0.1 eV or more larger than the ionization potential of the light emitting layer. The organic electroluminescent device according to the above, wherein the hole blocking layer contains a metal complex, a styryl compound, a triazole derivative, or a phenanthroline derivative. The thickness of the hole blocking layer is in the range of 0.5 to 50 nm
3. The organic electroluminescent device according to claim 1.

That is, the present inventors have made the following studies with the aim of providing an organic electroluminescent device that can emit light with high luminous efficiency and can be driven stably.

First, a metal complex typified by an aluminum complex of 8-hydroxyquinoline is used as a host material for the electron transport layer, and an electron having a molecular weight of less than 400 is selected from among oxadiazole derivatives, quinoxaline derivatives, and phenanthroline derivatives. By using a transportable compound as a dopant material and using them together, it has become possible to achieve both high electron mobility and excellent film quality.

By doping an electron transporting compound having a molecular weight of less than 400, the electron mobility of the electron transporting layer is improved, but of the holes injected from the anode,
No remarkable effect was obtained as to the effect of preventing what reaches the cathode without recombination with electrons.

Therefore, the present inventors have further studied and found that a hole blocking layer having an ionization potential of 0.1 eV or more larger than the ionization potential of the light emitting layer is provided so as to be in contact with the interface of the light emitting layer on the cathode side. By providing an electron transport layer containing two or more of the above-described electron transport materials on the cathode side of the hole blocking layer, high electron transport properties can be obtained.
The present inventors have found that it is more preferable because both excellent film quality can be achieved and the luminous efficiency can be improved by reducing the amount of current that does not contribute to recombination.

In the present invention, the material of the hole blocking layer is a metal complex, a styryl compound, a triazole derivative,
Alternatively, a phenanthroline derivative is preferably used, and the thickness of the hole blocking layer is preferably 0.5 to 50 nm.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the organic electroluminescent device of the present invention will be described in detail with reference to the drawings.

FIGS. 1 and 2 are cross-sectional views schematically showing an example of the structure of an organic electroluminescent device according to an embodiment of the present invention, wherein 1 is a substrate, 2 is an anode, 3 is an anode buffer layer, and 4 is A hole transport layer, 5 is a light emitting layer, 6 is a hole blocking layer, 7 is an electron transport layer, and 8 is a cathode.

The substrate 1 serves as a support for the organic electroluminescent device, and is made of a quartz or glass plate, a metal plate or a metal foil, a plastic film or a sheet, or the like. Particularly, a glass plate or a plate of a transparent synthetic resin such as polyester, polymethacrylate, polycarbonate, and polysulfone is preferable. When using a synthetic resin substrate, it is necessary to pay attention to gas barrier properties. If the gas barrier property of the substrate is too small, the organic electroluminescent device may be deteriorated by the outside air passing through the substrate, which is not preferable. For this reason, a method of providing a dense silicon oxide film or the like on at least one surface of the synthetic resin substrate to secure gas barrier properties is also a preferable method.

The anode 2 is provided on the substrate 1, and the anode 2 plays a role of injecting holes into the hole transport layer 4. This anode is usually made of a metal such as aluminum, gold, silver, nickel, palladium, platinum, a metal oxide such as an oxide of indium and / or tin, a metal halide such as copper iodide, carbon black, or poly. (3-methylthiophene), conductive polymers such as polypyrrole and polyaniline. Usually, the formation of the anode 2 is often performed by a sputtering method, a vacuum evaporation method, or the like. In the case of fine metal particles such as silver, fine particles such as copper iodide, carbon black, conductive metal oxide fine particles, or conductive polymer fine powder, they are dispersed in a suitable binder resin solution and Anode 2 by applying to
Can also be formed. Further, in the case of a conductive polymer, a thin film can be formed directly on the substrate 1 by electrolytic polymerization, or the conductive polymer can be applied on the substrate 1 to form the anode 2 (Appl. Phys. Lett. , 60, 2711, 1992
Year). The anode 2 can be formed by laminating different materials. The thickness of the anode 2 depends on the required transparency. When transparency is required, it is desirable that the transmittance of visible light is usually 60% or more, preferably 80% or more. In this case, the thickness of the anode 2 is usually 5 to 1000%.
nm, preferably about 10 to 500 nm. If opaque, the anode 2 may be the same as the substrate 1. Further, it is also possible to laminate a different conductive material on the anode 2.

On the anode 2, a hole transport layer 4 is provided. The conditions required for the material of the hole transport layer 4 include high hole injection efficiency from the anode and efficient transport of the injected holes. Therefore, the ionization potential is small, the transparency to visible light is high, the hole mobility is large, the stability is further improved, and impurities serving as traps are hardly generated at the time of manufacture or use. Required. Further, it is necessary that a substance that quenches light emitted from the light-emitting layer is not contained. In addition to the above general requirements, when considering applications for in-vehicle display, the element is required to have further heat resistance. Therefore, a material having a Tg value of 70 ° C. or more is desirable, and a material having a Tg of 85 ° C. or more is more desirable.

Examples of such a hole transport material include 1,1-bis (4-di-p-tolylaminophenyl) cyclohexane and 4,4'-bis [N- (1-naphthyl) -N-
Two or more 3 represented by phenylamino] biphenyl
Aromatic diamines containing a secondary amine and having two or more condensed aromatic rings substituted by nitrogen atoms (JP-A-5-234681);
Aromatic amine compounds having a starburst structure such as 4 ', 4 "-tris (1-naphthylphenylamino) triphenylamine (J. Lumin., 72-74, 985, 1997); Aromatic amine compounds composed of tetramers (Ch
em. Commun., p. 2175, 1996), a derivative of triphenylbenzene and an aromatic triamine having a starburst structure (U.S. Pat. No. 4,923,774), N, N'-diphenyl-N, N'- Bis (3-methylphenyl) biphenyl-4,4'-diamine and the like, compounds in which a pyrenyl group is substituted with a plurality of aromatic diamino groups, aromatic diamines having a styryl structure (JP-A-4-290851),
Those in which an aromatic tertiary amine unit is linked by a thiophene group (JP-A-4-304466), those in which a starburst-type aromatic triamine (JP-A-4-308688) is linked, and those in which a tertiary amine is linked by a fluorene group ( JP-A-5-25473), triamine compounds (JP-A-5-239455), bisdipyridylaminobiphenyl, N, N, N-triphenylamine derivatives (JP-A-6-1972), and phenoxazine structures. Aromatic diamine (JP-A-7-138562) and diaminophenylphenanthridine derivative (JP-A-7-2
No. 52474), silazane compounds (U.S. Pat.
950,950), silanamine derivatives (Japanese Unexamined Patent Publication No.
49079), phosphamine derivatives (Japanese Unexamined Patent Publication No.
-25659), spiro compounds such as 2,2 ', 7,7'-tetrakis- (diphenylamino) -9,9'-spirobifluorene (Synth. Metals, vol. 91, p. 209, 1997), etc. Is mentioned. These compounds may be used alone or as a mixture of two or more as necessary.

In addition to the above compounds, as a material of the hole transport layer 4, polyarylene ether sulfone (Polym. Adv) containing polyvinyl carbazole, polyvinyl triphenylamine (JP-A-7-39553), and tetraphenylbenzidine is used. Tech., Vol. 7, p. 33, 1996).

The hole transport layer 4 is formed by laminating the above-described hole transport material on the anode 2 by a coating method or a vacuum evaporation method.

In the case of the coating method, one or more hole transport materials and, if necessary, additives such as a binder resin or a coating property improving agent which do not trap holes are added, dissolved and coated. A solution is prepared, applied on the anode 2 by a method such as spin coating, and dried to form a hole transport layer. Examples of the binder resin include polycarbonate, polyarylate, and polyester. If the amount of the binder resin added is large, the hole mobility is reduced. Therefore, it is desirable that the amount of the binder resin is small.

In the case of the vacuum evaporation method, the hole transporting material is put into a crucible placed in a vacuum vessel, and the inside of the vacuum vessel is evacuated to about 10 ^-4 Pa by a suitable vacuum pump, and then the crucible is heated. Then, the hole transport material is evaporated, and the hole transport layer 4 is formed on the substrate 1 on which the anode is formed, which is placed facing the crucible.
Is formed.

The thickness of the hole transport layer 4 is usually 10 to 300 n.
m, preferably 30-100 nm. In order to uniformly form such a thin film, generally, a vacuum deposition method is often used.

In the present invention, for the purpose of further improving the efficiency of hole injection and improving the adhesion of the whole organic layer to the anode 2, as shown in FIG. The anode buffer layer 3 may be inserted between them. The insertion of the anode buffer layer 3 has the effect of reducing the initial drive voltage of the device and suppressing the voltage rise when the device is continuously driven with a constant current. The conditions required for the material used for the anode buffer layer are that a uniform thin film can be formed with good contact with the anode and that it is thermally stable, that is, the melting point and the glass transition temperature are high.
300 ° C or higher and glass transition temperature of 100 ° C or higher are required. In addition, the ionization potential is low, holes can be easily injected from the anode 2, and the hole mobility is high.

For this purpose, porphyrin derivatives, phthalocyanine compounds (JP-A-63-295695), hydrazone compounds, alkoxy-substituted aromatic diamine derivatives, p- (9-anthryl) -N, N -Ji-
p-tolylaniline, polythienylenevinylene and poly-
p-phenylenevinylene, polyaniline (Appl. Phys.
Lett., 64, 1245, 1994), polythiophene (Opti
Cal Materials, vol. 9, p. 125, 1998), organic compounds such as star-bust type aromatic triamines (JP-A-4-308688), and sputtered carbon films (Synth. Met., 91).
Vol. 73, 1997) and metal oxides such as vanadium oxide, ruthenium oxide and molybdenum oxide (J. Phys. D,
29, 2750, 1996).

As a compound frequently used as a material of the anode buffer layer 3, a porphyrin compound or a phthalocyanine compound is exemplified. These compounds may have a central metal or may be non-metallic. Specific examples of preferred such compounds include the following compounds. Porphine 5,10,15,20-tetraphenyl-21H, 23
H-porphine 5,10,15,20-tetraphenyl-21H, 23
H-porphine cobalt (II) 5,10,15,20-tetraphenyl-21H, 23
H-porphine copper (II) 5,10,15,20-tetraphenyl-21H, 23
H-porphine zinc (II) 5,10,15,20-tetraphenyl-21H, 23
H-porphine vanadium (IV) oxide 5,10,15,20-tetra (4-pyridyl) -21
H, 23H-porphine 29H, 31H-phthalocyanine copper (II) phthalocyanine zinc (II) phthalocyanine titanium phthalocyanine oxide magnesium phthalocyanine lead phthalocyanine copper (II) 4,4 ′, 4 ″, 4 ′ ″-tetraaza-29
H, 31H-phthalocyanine

The anode buffer layer 3 can be formed as a thin film in the same manner as the hole transport layer 4. However, in the case of an inorganic substance, a sputtering method, an electron beam evaporation method, or a plasma CVD method is used.

The thickness of the anode buffer layer 3 formed as described above is usually 3 to 200 nm, preferably 10 to 100 nm.

The light emitting layer 5 is provided on the hole transport layer 4. The light emitting layer 5 is a recombination between the holes injected from the anode 2 and moving through the hole transport layer 4 and the electrons injected from the cathode 8 and moving through the electron transport layer 7 between the electrodes to which the electric field is applied. Formed from a fluorescent compound that is excited by and emits strong light.

The fluorescent compound used in the light emitting layer 5 is a compound having a stable thin film shape, exhibiting a high fluorescence yield in a solid state, and capable of efficiently transporting holes and / or electrons. It is necessary. Further, it is required that the compound be electrochemically and chemically stable, and hardly generate impurities serving as traps at the time of production or use.

Materials satisfying such conditions include 8
Metal complexes such as aluminum complexes of -hydroxyquinoline (JP-A-59-194393);
[h] quinoline metal complex (JP-A-6-322362),
Bisstyrylbenzene derivatives (Japanese Patent Application Laid-Open Nos. 1-245087 and 2-222484), bisstyrylarylene derivatives (Japanese Patent Application Laid-Open No. 2-247278), and metal complexes of (2-hydroxyphenyl) benzothiazole (Japanese Patent Application Laid-open No. −315983
Publication), silole derivatives and the like. These light emitting layer materials are usually laminated on the hole transport layer by a vacuum evaporation method.

In addition, among the above-described hole transport layer materials, an aromatic amine compound having a fluorescent property can also be used as a light emitting layer material.

The thickness of the light emitting layer 5 is usually 3 to 200 nm, preferably 5 to 100 nm.

The light emitting layer 5 can be formed in the same manner as the hole transporting layer 4, but usually a vacuum evaporation method is used.

For the purpose of improving the luminous efficiency of the device and changing the luminescent color, for example, doping a fluorescent dye for laser such as coumarin using an aluminum complex of 8-hydroxyquinoline as a host material (J. Appl. Phys. , 65
Vol. 3610, 1989). This doping method can be applied to the light emitting layer 5, and as the doping material, various fluorescent dyes other than coumarin can be used. Fluorescent dyes that emit blue light include perylene, pyrene,
And anthracene and derivatives thereof. Green fluorescent dyes include quinacridone derivatives, coumarin derivatives and the like. As the yellow fluorescent dye, rubrene,
Perimidone derivatives and the like. Examples of the red fluorescent dye include benzopyran derivatives such as DCM, rhodamine derivatives, benzothioxanthene derivatives, and azabenzothioxanthenes.

In addition to the above-described fluorescent dyes for doping, depending on the host material, laser studies, Vol. 8, p. 694, p.
958 (1980); Vol. 9, pp. 85 (1981) can be used as a doping material for the light emitting layer.

The amount of the fluorescent material to be doped with respect to the host material is preferably 10 ^-3 to 10% by weight.

A method for doping the above-mentioned fluorescent dye into the host material of the light emitting layer 5 will be described below.

In the case of coating, the host material, a fluorescent dye for doping, and if necessary, additives such as a binder resin which does not act as an electron trap or a quencher of light emission, and a coating property improving agent such as a leveling agent are added. A coating solution is prepared by dissolving the solution, and the solution is applied onto the hole transport layer 4 by a method such as spin coating, and dried to form the light emitting layer 5. As the binder resin, polycarbonate, polyarylate,
Polyester and the like. If the amount of the binder resin is large, the hole / electron mobility is reduced. Therefore, a small amount is desirable, and 50% by weight or less is preferable.

In the case of the vacuum deposition method, the host material is placed in a crucible placed in a vacuum vessel, the fluorescent dye to be doped is placed in another crucible, and the inside of the vacuum vessel is placed at 10 ^-6 Torr by a suitable vacuum pump. After evacuating to a degree, each crucible is simultaneously heated and evaporated to form a layer on the substrate placed facing the crucible. As another method, a mixture of the above materials at a predetermined ratio may be put in the same crucible and evaporated.

When the above-mentioned fluorescent dye as a dopant is doped into the light emitting layer, it is uniformly doped in the thickness direction of the light emitting layer 5, but may have a concentration distribution in the film thickness direction. For example, doping may be performed only in the vicinity of the interface with the hole transport layer 4 or, conversely, may be performed only in the vicinity of the interface of the hole blocking layer 6 described later.

The hole blocking layer 6 is provided on the light emitting layer 5. The hole blocking layer 6 functions to prevent holes moving from the hole transport layer 4 from reaching the cathode 8 and to efficiently transfer electrons injected from the electron transport layer 7 toward the light emitting layer 5. This layer is formed of a compound that can be transported and has an ionization potential 0.1 eV higher than the ionization potential of the light emitting layer 5. The hole blocking layer 6 is used to trap exciton generated by recombination in the light emitting layer 5 in the light emitting layer.
It is necessary to have a wider band gap than the light emitting layer material. The band gap in this case is determined from the difference between the oxidation potential and the reduction potential determined electrochemically or from the light absorption edge. The hole blocking layer 6 has a function of confining both charge carriers and excitons in the light emitting layer 5 to improve the light emission efficiency.

Materials for the hole blocking layer satisfying such conditions include metal complexes such as mixed ligand complexes and binuclear metal complexes;
Styryl compounds such as distyrylbiphenyl derivatives (JP-A-11-242996), 3- (4-biphenylyl) -4-phenyl-5- (4-tert-butylphenyl) -1,2,
Triazole derivatives such as 4-triazole (Japanese Unexamined Patent Publication No.
No. 41759) and phenanthroline derivatives such as bathocuproin (Japanese Patent Application Laid-Open No. 10-79297).

Specific examples of the mixed ligand complex include bis (2-methyl-8-quinolinolato) (phenolato) aluminum, bis (2-methyl-8-quinolinolato) (ortho-cresolato) aluminum, Methyl-
8-quinolinolato) (meta-cresolato) aluminum, bis (2-methyl-8-quinolinolato) (para-cresolato) aluminum, bis (2-methyl-8-quinolinolato) (ortho-phenylphenolato) aluminum, bis (2 -Methyl-8-quinolinolato) (meta-phenylphenolato) aluminum, bis (2-methyl-
8-quinolinolato) (para-phenylphenolato) aluminum, bis (2-methyl-8-quinolinolato)
(2,3-dimethylphenolato) aluminum, bis (2-methyl-8-quinolinolato) (2,6-dimethylphenolato) aluminum, bis (2-methyl-8-quinolinolato) (3,4-dimethylphenolato) ) Aluminum, bis (2-methyl-8-quinolinolato) (3,5
-Dimethylphenolato) aluminum, bis (2-methyl-8-quinolinolato) (3,5-di-tert-butylphenolato) aluminum, bis (2-methyl-8-
Quinolinolato) (2,6-diphenylphenolato) aluminum, bis (2-methyl-8-quinolinolato)
(2,4,6-triphenylphenolato) aluminum, bis (2-methyl-8-quinolinolato) (2,4
6-trimethylphenolato) aluminum, bis (2-
Methyl-8-quinolinolato) (2,3,6-trimethylphenolato) aluminum, bis (2-methyl-8-quinolinolato) (2,3,5,6-tetramethylphenolato) aluminum, bis (2-methyl -8-quinolinolato) (1-naphtolat) aluminum, bis (2-methyl-8-quinolinolato) (2-naphtolato) aluminum, bis (2-methyl-8-quinolinolato) (triphenylsilanolato) aluminum, bis (2 -Methyl-8
-Quinolinolato) (triphenylgermanolato) aluminum, bis (2-methyl-8-quinolinolato) (tris (4,4-biphenyl) silanolato) aluminum,
Bis (2,4-dimethyl-8-quinolinolato) (ortho-phenylphenolato) aluminum, bis (2,4-
Dimethyl-8-quinolinolato) (para-phenylphenolato) aluminum, bis (2,4-dimethyl-8-quinolinolato) (meta-phenylphenolato) aluminum, bis (2,4-dimethyl-8-quinolinolato)
(3,5-dimethylphenolato) aluminum, bis (2,4-dimethyl-8-quinolinolato) (3,5-di-tert-butylphenolato) aluminum, bis (2-methyl-4-ethyl-8-) Quinolinolato) (para-cresolato) aluminum, bis (2-methyl-4-)
Methoxy-8-quinolinolato) (para-phenylphenolato) aluminum, bis (2-methyl-5-cyano-)
8-quinolinolato) (ortho-cresolato) aluminum, bis (2-methyl-6-trifluoromethyl-8-
Quinolinolato) (2-naphtolat) aluminum, bis (2-methyl-8-quinolinolato) (phenolato) gallium, bis (2-methyl-8-quinolinolato) (ortho-cresolato) gallium, bis (2-methyl-8-quinolinolato) ) (Para-phenylphenolatogallium, bis (2-methyl-8-quinolinolato) (1-naphthrat)
Gallium, bis (2-methyl-8-quinolinolato) (2
-Naphthrat) gallium, bis (2-methyl-8-quinolinolato) (triphenylsilanolato) gallium, bis (2-methyl-8-quinolinolato) (tris (4,4-
Biphenyl) silanolato) gallium, and the like.
Particularly preferred is bis (2-methyl-8-quinolinolato) (triphenylsilanolato) aluminum.

As a specific example of the binuclear metal complex, bis (2
-Methyl-8-quinolato) aluminum-μ-oxo-
Bis- (2-methyl-8-quinolinolato) aluminum,
Bis (2,4-dimethyl-8-quinolato) aluminum-μ-oxo-bis- (2,4-dimethyl-8-quinolylato) aluminum, bis (4-ethyl-2-methyl-
8-quinolinolato) aluminum-μ-oxo-bis-
(4-ethyl-2-methyl-8-quinolinolato) aluminum, bis (2-methyl-4-methoxyquinolinolato) aluminum-μ-oxo-bis- (2-methyl-
4-methoxyquinolinolato) aluminum, bis (5-
Cyano-2-methyl-8-quinolinolato) aluminum-μ-oxo-bis (5-cyano-2-methyl-8-quinolinolato) aluminum, bis (5-chloro-2-methyl-8-quinolinolato) aluminum-μ- Oxo
Bis- (5-chloro-2-methyl-8-quinolinolato)
Aluminum, bis (2-methyl-5-trifluoromethyl-8-quinolinolato) aluminum-μ-oxo-
Bis- (2-methyl-5-trifluoromethyl-8-quinolinolato) aluminum; Particularly preferably, bis (2-methyl-8-quinolato) aluminum-μ-oxo-bis- (2-methyl-8-quinolate)
Aluminum.

Specific examples of the styryl compound used as a hole blocking material include the following distyryl arylene derivatives.

[0051]

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As the triazole derivative, a compound having at least one 1,2,4-triazole ring residue represented by the following structural formula can be used.

[0053]

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Specific examples of the compound having at least one 1,2,4-triazole ring residue represented by the above structural formula are shown below.

[0055]

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The phenanthroline derivative is a compound having at least one phenanthroline ring represented by the following structural formula.

[0057]

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Specific examples of the compound having at least one phenanthroline ring represented by the above structural formula are shown below.

[0059]

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Of these, particularly preferred is bathocuproin.

Although the above-mentioned exemplary compounds of the distyrylarylene derivative, the triazole derivative and the phenanthroline derivative are not described in the structural formula, the aromatic ring in each compound may be further substituted. Examples of the substituent include an alkyl group having 1 to 6 carbon atoms such as a methyl group, an ethyl group and a butyl group; an alkoxide group having 1 to 6 carbon atoms such as a methoxy group, an ethoxy group and a butoxy group; a halogen atom; Is mentioned.

These compounds may be used alone or as a mixture of two or more as necessary.

The thickness of the hole blocking layer 6 is usually 0.3 to 100
nm, preferably 0.5 to 50 nm.

The hole blocking layer 6 can be formed by the same method as the hole transport layer 4, but usually, a vacuum evaporation method is used.

For the purpose of further improving the luminous efficiency of the device, an electron transport layer 7 is provided on the hole blocking layer 6. The electron transport layer 7 is formed of a compound capable of efficiently transporting electrons injected from the cathode 8 between the electrodes to which an electric field is applied in the direction of the light emitting layer 5. The electron transport layer 7 has an effect of preventing excitons generated by recombination in the light emitting layer 5 from being diffused and quenched by the cathode 8.

The electron transporting compound used for the electron transporting layer 7 has a high electron injection efficiency from the cathode 8 and
It is necessary that the compound has high electron mobility and can efficiently transport injected electrons.

Materials satisfying such conditions include 8
Metal complexes such as aluminum complexes of -hydroxyquinoline (JP-A-59-194393);
[h] metal complex of quinoline, oxadiazole derivative, distyrylbiphenyl derivative, silole derivative, 3- or 5-
Hydroxyflavone metal complex, benzoxazole metal complex, benzothiazole metal complex, trisbenzimidazolylbenzene (US Pat. No. 5,645,948), quinoxaline derivative (US Pat. No. 5,077,142, JP-A-6-2071)
No. 69), phenanthroline derivatives (JP-A-5-3314)
No. 59), 2-t-butyl-9,10-N, N'-dicyanoanthraquinonediimine and the like have been conventionally known.

Among these substances, low-molecular-weight compounds having a molecular weight of less than 400, such as oxadiazole derivatives, quinoxaline derivatives, and phenanthroline derivatives, exhibit high electron mobility, but have low Tg and are easily crystallized.
It was difficult to use alone. Further, a metal complex such as an aluminum complex of 8-hydroxyquinoline is a substance having good sublimability and excellent film quality after vacuum deposition, and its electron mobility is compared with the hole mobility of the hole transporting substance. And 2
The order of magnitude was not enough.

In the present invention, the electron transport layer has a molecular weight of 40.
A metal complex having a molecular weight of 400 to 800 represented by an aluminum complex of 8-hydroxyquinoline as a host material, and a low molecular weight compound having a molecular weight of less than 400, preferably 130 to 399 as a dopant material. By using both, it is possible to achieve both high electron mobility and excellent film quality.

In the present invention, if the proportion of the dopant material doped into the electron transporting layer is small, high electron mobility cannot be obtained, and if too large, the film quality deteriorates. Is a molar ratio of host material: dopant material = 99: 1 to 50: 5.
It is preferably set to 0.

Specific examples of the metal complexes having a molecular weight (Mol. Wt.) Of 400 or more include the following metal complexes, and metal complexes such as mixed ligand complexes and binuclear metal complexes exemplified in the hole blocking layer. Is mentioned.

[0072]

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The compounds represented by the above structural formulas are also
As in the distyrylarylene derivative and the like described above as the hole blocking layer material, the aromatic ring in each compound may be further substituted, and the substituent may be the same group as in the distyrarylene derivative or the like. No.

Among these, an aluminum complex of 8-hydroxyquinoline is particularly preferable as the electron transporting layer.
Aluminum complexes of -methyl-8-hydroxyquinoline and aluminum complexes of 10-hydroxybenzo [h] quinoline.

As the dopant material, a substance selected from oxadiazole derivatives, quinoxaline derivatives, and phenanthroline derivatives is preferable. Specifically, for example, as the oxadiazole derivative, a compound having at least one 2,4-oxadiazole ring residue represented by the following structural formula can be used.

[0076]

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Specific examples of compounds having a molecular weight (Mol. Wt.) Of less than 400 among compounds having at least one 2,4-oxadiazole ring residue represented by the above structural formula are shown below.

[0078]

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Molecular weight (Mol. Wt.) Of quinoxaline derivative
The following are specific examples of those having a value of less than 400.

[0080]

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Specific examples of the phenanthroline derivative include those having a molecular weight (Mo
l. Wt.) is less than 400.

[0082]

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Note that the electron mobility of the host material and the dopant material of the electron transporting layer is Time-of-flight.
It is measured using the t (TOF) method. The TOF method irradiates the sample surface with pulse light while applying an electric field to the measurement sample,
This is a method of measuring the charge carrier mobility by measuring a transient photocurrent generated by the generated charge carriers moving in the thin film. At this time, the absolute value of the mobility differs depending on the strength of the applied electric field and the state of the sample (whether it is composed of a single compound or dispersed in a binder resin, etc.), but the electron of the host material and the dopant material is different. It suffices that the relative magnitude relationship of the mobility satisfies the relationship of the present invention.

The thickness of the electron transport layer 6 is usually 5 to 200
nm, preferably 10-100 nm.

The electron transport layer 7 is formed by laminating on the light emitting layer 5 by a coating method or a vacuum evaporation method in the same manner as the light emitting layer 5. Usually, a vacuum evaporation method is used.

In the case of the vacuum deposition method, the metal complex as the host material is put in a crucible placed in a vacuum vessel and oxadiazole as a dopant material to be co-deposited in the same manner as when the light emitting layer 5 is deposited. A substance selected from a derivative, a quinoxaline derivative, and a phenanthroline derivative is placed in another crucible, and the inside of the vacuum vessel is 10 ^-6 Torr by an appropriate vacuum pump.
After evacuating to the extent, each crucible is heated and evaporated at the same time to form the electron transport layer 7 on the substrate placed facing the crucible. As another method, a mixture of the above materials at a predetermined ratio in advance may be evaporated using the same crucible.

At this time, co-evaporation is performed uniformly in the thickness direction of the electron transport layer 7, but there may be a concentration distribution of the dopant material in the thickness direction.

The cathode 8 plays a role of injecting electrons into the electron transport layer 7. As the material used for the cathode 8, the material used for the anode 2 can be used, but for efficient electron injection, a metal having a low work function is preferable, and tin, magnesium, indium, calcium, A suitable metal such as aluminum or silver or an alloy thereof is used. Specific examples include a low work function alloy electrode such as a magnesium-silver alloy, a magnesium-indium alloy, and an aluminum-lithium alloy. Further, LiF,
Inserting an ultra-thin insulating film (0.1 to 5 nm) such as MgF ₂ or Li ₂ O is also an effective method for improving the efficiency of the device (Appl.
Phys. Lett., 70, 152, 1997; JP-A-10-74586
Issue; IEEETrans. Electron. Devices, 44, 1245
P. 1997).

The thickness of the cathode 8 is usually the same as that of the anode 2.

In order to protect the cathode made of a metal having a low work function, further laminating a metal layer having a high work function and being stable to the atmosphere increases the stability of the device. For this purpose, metals such as aluminum, silver, copper, nickel, chromium, gold, platinum and the like are used.

The organic electroluminescent device shown in FIGS. 1 and 2 is an example of the embodiment of the present invention, and the present invention is not limited to the one shown in the drawings unless it exceeds the gist. . For example, it is also possible to stack the cathode 8, the electron transport layer 7, the hole blocking layer 6, the light emitting layer 5, the hole transport layer 4, and the anode 2 in this order on the substrate, that is, on the substrate. As described above, the organic electroluminescent device of the present invention can be provided between two substrates, at least one of which has high transparency. Similarly, in FIG. 2, it is also possible to stack the layers in a structure opposite to the illustrated layer configuration.

Further, between the anode 2 and the light emitting layer 5, the light emitting layer 5
Between the electron transport layer 7 and the electron transport layer 7, and between the electron transport layer 7 and the cathode 8, there are provided any layers other than the above-described anode buffer layer 3, hole transport layer 4, and hole blocking layer 6. Good.

The organic electroluminescent device of the present invention has the following features.
The present invention can be applied to any one of an organic electroluminescent device, a device having a structure arranged in an array, and a structure in which an anode and a cathode are arranged in an XY matrix.

According to the organic electroluminescent device of the present invention, the electron transport layer contains a low molecular weight compound such as an oxadiazole derivative, a quinoxaline derivative, or a phenanthroline derivative having a high electron transport ability and a metal complex having a good film quality. In addition, current is injected efficiently with excellent stability of the device, and as a result, recombination of holes and electrons effectively occurs in the light emitting layer, so that a device with high luminous efficiency can be obtained. Further, when a hole blocking layer is provided, light emission from only the light emitting layer is obtained,
A light-emitting element with high color purity can be obtained.

[0095]

EXAMPLES The present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to the following examples unless it exceeds the gist.

Example 1 An organic electroluminescent device having the structure shown in FIG. 2 was manufactured by the following method.

An indium tin oxide (ITO) transparent conductive film deposited on a glass substrate 1 with a thickness of 120 nm (manufactured by Geomatic Corporation; electron beam film-formed product; sheet resistance: 15 Ω) was formed by using a usual photolithography technique and hydrochloric acid etching. 2m
The anode 2 was formed by patterning into an m-width stripe. The patterned ITO substrate is washed with water with ultrapure water, ultrasonic cleaning with a surfactant, ultrasonic cleaning with ultrapure water, and water with ultrapure water in that order, and then dried with nitrogen blow. The substrate was subjected to ultraviolet ozone cleaning and set in a vacuum evaporation apparatus. After the rough evacuation of the above apparatus was performed by an oil rotary pump, the degree of vacuum in the apparatus was 1.0 × 10 ⁻⁶ Torr (about 1.3
The pressure was evacuated using an oil diffusion pump equipped with a liquid nitrogen trap until the pressure became 10 × 10 ⁻⁴ Pa or less.

As a material for the anode buffer layer 3, copper phthalocyanine (HI-1) having the following structural formula was used, and a molybdenum boat was used at a deposition rate of 0.14 nm / sec and a degree of vacuum of 8.0 × 10 8
^{At -7} Torr (about 1.1 × 10 ^-4 Pa), the anode buffer layer 3 is
A film was formed on the anode 2 with the following film thickness.

[0099]

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Next, as a material of the hole transport layer 4, 4,4′-bis [N- (1-naphthyl) -N-phenylamino] biphenyl (HT-1) having the following structural formula was added to a ceramic crucible. It was then heated by a tantalum wire heater around the crucible to perform vapor deposition. The temperature of the crucible at this time was controlled in the range of 259 to 277 ° C. The degree of vacuum during deposition is 7.0 × 10 ^-7 Torr (about 9.3 ×
At a pressure of 10 ⁻⁵ Pa), a hole transport layer 4 having a thickness of 60 nm was formed at a deposition rate of 0.22 nm / sec.

[0101]

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Next, as materials for the light emitting layer 5, an 8-hydroxyquinoline complex of aluminum (EM-1) of the following structural formula and a benzopyran derivative of an orange fluorescent dye (DYE-1) of the following structural formula are used. Was co-evaporated on the hole transport layer 4 at a ratio of EM-1: DYE-1 = 100: 1. The temperature of the crucible at this time was controlled in the range of 285 to 294 ° C and 314 to 317 ° C, respectively. The degree of vacuum at the time of vapor deposition was 7.0 × 10 ⁻⁷ Torr (about 9.3 × 10 ⁻⁵ Pa).
And DYE-1 have a deposition rate of 0.13 nm / sec and a total film thickness of 30 n.
m.

[0103]

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[0104]

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Next, using a silanol aluminum 8-hydroxyquinoline complex (HB-1) of the following structural formula as a material for the hole blocking layer 6, vapor deposition was performed on the light emitting layer 5. The temperature of the crucible when forming the hole blocking layer 6 is
The temperature is controlled in the range of 224 to 235 ° C, and the degree of vacuum is 6.0 × 10 ^-7 Torr (about
8.0 × 10 ⁻⁴ Pa) and a 10 nm-thick hole blocking layer 6 were formed at a deposition rate of 0.11 nm / sec.

[0106]

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Subsequently, as the material of the electron transport layer 7, the above-mentioned aluminum 8-hydroxyquinoline complex (EM-
1) and the oxadiazole derivative (ET-1) shown below
In the same manner as above on the hole blocking layer 6 (EM-1):
Co-evaporation was performed at a ratio of (ET-1) = 95: 5 (molar ratio). The temperature of the crucible at this time was 298-308 ° C and 11
The temperature was controlled in the range of 8 to 121 ° C. The degree of vacuum during evaporation is 6.0 × 10
The electron transport layer 7 having a film thickness of 35 nm was formed at ^-7 Torr (about 8.0 × 10 ^-5 Pa), at a deposition rate of EM-1 and ET-1 of 0.13 nm / sec.

[0108]

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The electron mobility of the host material EM-1 and the dopant material ET-1 measured by the TOF method satisfies the conditions of the present invention.

From the anode buffer layer 3 to the electron transport layer 7
Was kept at room temperature until vacuum deposition.

Here, the element on which the vapor deposition up to the electron transporting layer 7 was performed was once taken out of the vacuum vapor deposition apparatus into the atmosphere, and a 2 mm wide stripe-shaped shadow mask was used as a cathode vapor deposition mask. Adhere to the element so as to be perpendicular to the stripe, and install it in another vacuum evaporation apparatus, and as in the case of the organic layer, the degree of vacuum in the apparatus is 2 × 10 ^-6 Torr
(About 2.7 × 10 ⁻⁴ Pa) or less. As the cathode 8, first, magnesium fluoride (MgF ₂ ) was deposited using a molybdenum boat at a deposition rate of 0.1 nm / sec and a degree of vacuum of 6.0 × 10 ^−6.
Torr (about 8.0 × 10 ^-4 Pa), electron transport layer 7 with a thickness of 1.5 nm
Was formed on the substrate. Next, the aluminum was similarly heated by a molybdenum boat, and the deposition rate was 0.4 nm / sec.
An aluminum layer having a thickness of 40 nm was formed at 1.0 × 10 ⁻⁵ Torr (about 1.3 × 10 ⁻³ Pa). Further, copper is further heated thereon using a molybdenum boat in order to increase the conductivity of the cathode, at a deposition rate of 0.5 nm / sec and a degree of vacuum of 8.0 × 10 ⁻⁶ Torr.
(About 1.1 × 10 ⁻³ Pa) to form a copper layer having a thickness of 40 nm and form a cathode 8
Was completed. The substrate temperature during vapor deposition of the above three-layer cathode 8 was kept at room temperature.

As described above, an organic electroluminescent device having a light emitting area of 2 mm × 2 mm was manufactured. Table 1 shows the emission characteristics of this device. In Table 1, the light emission luminance is a value at a current density of 250 mA / cm ² , and the light emission efficiency is 100 cd / m ^2.
, The luminance / current shows the slope of the luminance-current density characteristic, and the voltage shows the value at 100 cd / m ² . Table 1 shows the peak maximum wavelength of the EL spectrum and the CIE chromaticity coordinate values (JIS Z870).
1) is also described.

[0113] Even after storage for a long period of time, no remarkable rise in driving voltage was observed, and there was no decrease in luminous efficiency or luminance, and stable storage stability of the device was obtained. In addition, temperature 60 ℃,
Even after standing for 96 hours at a humidity of 90%, the change in the light emission characteristics was not a problem in practical use.

Comparative Example 1 An oxadiazole derivative (E
A device was produced in the same manner as in Example 1 except that only the 8-hydroxyquinoline complex of aluminum (EM-1) was used without using T-1). Table 1 shows the emission characteristics of this device. The device emitted orange light, but the luminous efficiency was greatly reduced.

Example 2 An element was fabricated in the same manner as in Example 1 except that a phenanthroline derivative (ET-2) shown below was used in place of the oxadiazole derivative (ET-1) as a dopant material for the electron transport layer. Produced. For ET-2, the electron mobility measured by the method described above was EM-
Greater than 1. Table 1 shows the emission characteristics of this device. In this device, the luminous efficiency was greatly improved as compared with Comparative Example 2.

[0116]

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Example 3 As a doping material for the light emitting layer, a coumarin derivative (DYE-2) of a yellow fluorescent dye shown below was used in place of the benzopyran derivative of an orange fluorescent dye (DYE-1), and a host material and EM-1 were used. A device was prepared in the same manner as in Example 1, except that co-evaporation was performed at a ratio of: DYE-2 = 100: 1. Table 1 shows the emission characteristics of this device. In this device, very high luminous efficiency was obtained.

[0118]

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[0119]

[Table 1]

[0120]

As described in detail above, according to the organic electroluminescent device of the present invention, since the electron transport layer having a good film quality and high electron transportability is provided, the charge injection balance is good and high efficiency light emission is possible. A stable element can be obtained.

Therefore, the organic electroluminescent device according to the present invention takes advantage of the features of a flat panel display (for example, an OA computer, a wall-mounted television, a vehicle-mounted or outdoor display device), a multi-color display device, or a surface light-emitting device. Light sources (e.g., light sources for copiers, backlight sources for liquid crystal displays and instruments), display boards, and marker lights can be considered. Its technical value is great.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view illustrating an embodiment of an organic electroluminescent device of the present invention.

FIG. 2 is a schematic cross-sectional view showing another embodiment of the organic electroluminescent device of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Anode 3 Anode buffer layer 4 Hole transport layer 5 Light emitting layer 6 Hole blocking layer 7 Electron transport layer 8 Cathode

Claims (6)

[Claims] 1. An organic electroluminescent device in which an anode, a light emitting layer, an electron transport layer and a cathode are formed in this order on a substrate, wherein the electron transport layer comprises a host material having a molecular weight of 400 or more and a dopant having a molecular weight of less than 400. And an electron mobility of the dopant material is higher than an electron mobility of the host material. 2. The method according to claim 1, wherein the dopant material of the electron transport layer is at least one member selected from the group consisting of oxadiazole derivatives, quinoxaline derivatives, and phenanthroline derivatives. Organic electroluminescent device. 3. The organic electroluminescent device according to claim 1, wherein the host material of the electron transport layer is a metal complex. 4. The method according to claim 1, further comprising a hole blocking layer between the light emitting layer and the electron transporting layer, wherein an ionization potential of the hole blocking layer is 0.1 eV or more larger than an ionization potential of the light emitting layer. 4. The organic electroluminescent device according to any one of items 1 to 3. 5. The organic electroluminescent device according to claim 1, wherein the hole blocking layer contains a metal complex, a styryl compound, a triazole derivative or a phenanthroline derivative. 6. The organic electroluminescent device according to claim 1, wherein the thickness of the hole blocking layer is in the range of 0.5 to 50 nm.