JP2002100478A - Organic electroluminescence element and its method of manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an organic electroluminescence element, capable of emitting light of high light emission 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 substrate 1. Ionization potential of the electron hole checking layer 6 is larger by 0.1 eV or higher than the ionization potential of the light-emitting layer 5; the electron transport layer 7 includes an electron transport material and an alkaline metal; and at least a part of the alkaline metal is dispersed in the electron transport layer 7 in a cation state.

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. The principle is that the electron transport layer is excited to emit light. In this basic device structure, it is conventional that holes pass through the electron transport layer and reach the cathode without recombination. This is one of the factors limiting the luminous efficiency of the device.

Conventionally, as electron transporting materials having a high electron transporting ability, aluminum complexes of 8-hydroxyquinoline and oxadiazole derivatives (Appl. Phys. Lett., Vol. 55, 1489)
P., 1989) and polymethyl methacrylate (PM
MA) etc. (Appl. Phys. Lett., 61
Vol., P. 2793, 1992), phenanthroline derivatives (JP-A-5-331459), 2-t-butyl-9,10-N, N'-dicyanoanthraquinone diimine (Phys. Stat. Sol. (A)) ,14
2, 489, 1994), but none of the materials has completely stopped the passage of holes from the hole transport layer.

As for the hole blocking layer, a tris (5,7-dichloro-8-) is provided between the light-emitting layer and the cathode as a hole-blocking layer having an ionization potential 0.1 eV or more larger than the ionization potential of the light-emitting layer. Although hydroxyquinolino) aluminum (JP-A-2-195683) is mentioned, the effect of improving the luminous efficiency was far from practical use. Silacyclopentadiene has also been proposed as a similar hole blocking layer material (Japanese Patent Application Laid-Open No. 9-87616), but the driving stability is not sufficient.

[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, a hole blocking layer, an electron transporting layer, and a cathode are formed in this order on a substrate, the ionization potential of the hole blocking layer is 0.1 eV or more than the ionization potential of the light emitting layer. An organic electroluminescent device, wherein the electron transporting layer contains an electron transporting material and an alkali metal, and at least a part of the alkali metal is dispersed in a cationic state in the electron transporting layer. The organic electroluminescent device according to the above, wherein the electron transporting material is a phenanthroline derivative or a metal complex. The organic electroluminescent device according to the above or 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
The organic electroluminescent device according to claim 1.

That is, the present inventors have conducted intensive studies for the purpose of providing an organic electroluminescent device which can emit light with high luminous efficiency and can be driven stably. As a result, a positive electrode was formed between the luminescent layer and the cathode. The inventors have found that it is preferable to provide a hole blocking layer and an electron transporting layer, and to use a specific substance for the electron transporting layer, thereby completing the present invention.

In the present invention, a compound capable of preventing holes moving from the hole transport layer from reaching the cathode and transporting electrons injected from the electron transport layer toward the light emitting layer. Forming a hole blocking layer, the hole blocking layer,
High luminous efficiency and low driving voltage are realized by combining with an electron transporting layer using a specific substance and having an extremely good electron injecting property.

The electron transporting layer is formed of a compound capable of efficiently transporting electrons injected from the cathode between the electrodes to which an electric field is applied in the direction of the light emitting layer. The electron transport layer has an effect of preventing exciton generated by recombination in the light emitting layer from being diffused and quenched by the cathode.

The electron-transporting material used for the electron-transporting layer must have high electron-injection efficiency from the cathode, high electron mobility, and efficient transport of injected electrons. As a material satisfying such conditions, a metal complex such as an aluminum complex of 8-hydroxyquinoline has been provided. However, these electron-transporting materials are generally more charged than the hole-transporting material. Low mobility.

In the present invention, the charge mobility is dramatically increased by including a cationic alkali metal together with such an electron transporting material in the electron transporting layer, thereby improving the efficiency of electron injection into the light emitting layer.

In the present invention, an excellent film quality is realized by using a phenanthroline derivative such as bathophenanthroline or a metal complex represented by an aluminum complex of 8-hydroxyquinoline as an electron transporting material of the electron transporting layer. Can be.

It is preferable to use a metal complex, a styryl compound, a triazole derivative, or a phenanthroline derivative as a material of the hole blocking layer, and the thickness of the hole blocking layer is preferably 0.5 to 50 nm.

The method for producing an organic electroluminescent device according to the present invention comprises:
In the method for manufacturing an organic electroluminescent device having a film forming step of an electron transport layer, the electron transport layer, an alkali metal obtained by reducing a compound containing an alkali metal in a vacuum device,
It is characterized by being formed by co-evaporating an electron transporting material, comprising an electron transporting material and an alkali metal, and at least a part of the alkali metal is dispersed in a cationic state, An electron transporting layer having extremely excellent electron transporting properties can be efficiently formed.

[0020]

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 examples of the structure of an organic electroluminescent device according to an embodiment of the present invention.
Is a substrate, 2 is an anode, 3 is an anode buffer layer, 4 is a hole transport layer, 5 is a light emitting layer, 6 is a hole blocking layer, 7 is an electron transport layer, 8
Represents a cathode.

The substrate 1 serves as a support for the organic electroluminescent element, 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 of 70 ° C. or more is desirable, and more preferably 85 ° C. or more.

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 transporting layer 4, polyarylene ether sulfone (Polym. Adv) containing polyvinyl carbazole, polyvinyl triphenylamine (JP-A-7-53953), and tetraphenylbenzidine. Tech., Vol. 7, p. 33, 1996).

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

In the case of the coating method, one or more hole transporting materials and, if necessary, additives such as a binder resin or a coating property improving agent which do not trap holes are added and dissolved. 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 deposition 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 often 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.

As materials satisfying such conditions, 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.

Further, among the above-mentioned hole transport layer materials, an aromatic amine compound having a fluorescent property can be used as the 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 transport 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 research, 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 doping amount of the fluorescent dye with respect to the host material is preferably 10 ^-3 to 10% by weight.

A method of 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 evaporation 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 in 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.

On the light emitting layer 5, a hole blocking layer 6 is provided. 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. Further, the hole blocking layer 6 has an electron transporting layer 7.
It also has a role in preventing the alkali metal therein from diffusing into the light-emitting layer 5 to lower the luminous efficiency and stability.

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 the hole blocking material include the following distyryl arylene derivatives.

[0053]

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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.

[0055]

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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.

[0057]

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

[0059]

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

[0061]

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Of these, bathocuproin is particularly preferred.

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 alkoxy group having 1 to 6 carbon atoms such as a methoxy group, an ethoxy group and a butoxy group; a halogen atom; And the like.

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. Such a hole blocking layer 6 can be formed by the same method as that of 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 compound (JP-A-6-207169), phenanthroline derivative (JP-A-5-331459) , 2-t-butyl-
9,10-N, N'-dicyanoanthraquinone diimine and the like have been conventionally known.

Among these substances, aluminum complex of 8-hydroxyquinoline and the like are widely used because they have good sublimability and excellent film quality after vacuum deposition, but their electron mobility is positive. Compared to the hole mobility of the hole transporting material, it was lower by about two orders of magnitude and was not sufficient. Not limited to this, the electron transporting material generally has lower charge mobility than the hole transporting material.

Therefore, in the present invention, the charge mobility is improved by including a cationic alkali metal in the electron transport layer 7 together with the electron transport material. As the alkali metal, sodium, potassium, cesium, lithium, rubidium, or the like is suitably used. The content ratio of the alkali metal in the cation state in the electron transport layer 7 is determined by the molar ratio to the electron transport material of the electron transport layer 7. Electron transporting material by ratio:
The alkali metal in the cationic state is preferably 1:20 to 20: 1, more preferably 1:10 to 10: 1. If the content of the alkali metal is less than this range, the effect of the present invention (sufficient electron mobility) due to the introduction of the alkali metal into the electron transport layer cannot be sufficiently obtained. Becomes unstable, and as a result, the life of the element may be shortened.

By using a phenanthroline derivative such as bathophenanthroline or a metal complex represented by an aluminum complex of 8-hydroxyquinoline as the electron transporting material, an electron transporting layer having excellent film quality can be realized.

Specific examples of the phenanthroline derivative used as the electron transporting material include the phenanthroline derivatives exemplified as the hole blocking material. Particularly preferred is bathophenanthroline.

Specific examples of the metal complex include, in addition to the metal complexes shown below, metal complexes such as the mixed ligand complexes and binuclear metal complexes exemplified as the hole blocking material.

[0074]

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

Of these, particularly preferred examples of the electron transporting material include aluminum complexes of 8-hydroxyquinoline, aluminum complexes of 2-methyl-8-hydroxyquinoline, and aluminum complexes of 10-hydroxybenzo [h] quinoline.

The thickness of the electron transport layer 7 is usually 5 to 200
nm, preferably 10-100 nm.

The electron transporting layer 7 containing such an electron transporting material and a cationic alkali metal is formed on the light emitting layer 5 by coating or vacuum deposition in the same manner as the hole transporting layer 4. It is formed. Usually, a vacuum evaporation method is used.

In the case of the vacuum evaporation method, the alkali metal is usually deposited using an alkali metal dispenser in which nichrome is filled with a compound containing an alkali metal such as an alkali metal chromate and a reducing agent. By heating the dispenser in a vacuum vessel, an alkali metal compound such as an alkali metal chromate is reduced and the alkali metal is evaporated. The above metal complex to be co-evaporated with the alkali metal is put into a crucible placed in a vacuum vessel, and the inside of the vacuum vessel is evacuated to about 10 ^-6 Torr by an appropriate vacuum pump, and then each crucible and dispenser are simultaneously heated. To evaporate,
An electron transport layer 7 is formed on a substrate placed facing the crucible and dispenser.

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 alkali metal 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 low work function metal, 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.

Also, 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 present invention, by providing an electron transporting layer containing an alkali metal having high electron transporting ability, the device is excellent in stability and electrons are efficiently injected, and as a result, holes are generated in the light emitting layer. The recombination of electrons and electrons effectively occurs, and an organic electroluminescent device having high luminous efficiency is provided. In particular, by using a specific electron transporting material, the film quality of the electron transporting layer can be improved, and the electron injection efficiency can be further increased. Further, by introducing the hole blocking layer, it is possible to prevent the alkali metal in the electron transporting layer from diffusing into the light emitting layer, and to obtain a device having higher luminous efficiency and higher stability.

[0088]

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 Geomatics Co .; electron beam film-formed product; sheet resistance: 15 Ω) was formed by using ordinary photolithography and hydrochloric acid etching. 2m
The anode 2 was formed by patterning into an m-width stripe. The patterned ITO substrate is cleaned by ultrasonic cleaning with acetone, water cleaning with pure water, and ultrasonic cleaning with isopropyl alcohol, and then dried by nitrogen blowing.
Finally, ultraviolet ozone cleaning was performed, and the apparatus was installed in a vacuum evaporation apparatus. After rough exhaust of the above apparatus is performed by an oil rotary pump, an oil diffusion pump equipped with a liquid nitrogen trap is used until the degree of vacuum in the apparatus becomes 2 × 10 ⁻⁶ Torr (about 2.7 × 10 ⁻⁴ Pa) or less. Exhausted.

As a material of the anode buffer layer 3, copper phthalocyanine (HI-1) having the following structural formula was used, and a molybdenum boat was used to deposit at a deposition rate of 0.1 nm / sec and a degree of vacuum of 1.4 × 10 ^−6.
At Torr (about 1.9 × 10 ⁻⁴ Pa), the anode buffer layer 3 was formed on the anode 2 with a thickness of 10 nm.

[0092]

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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. At this time, the temperature of the crucible was controlled in the range of 255 to 270 ° C. The degree of vacuum during deposition is 8.0 × 10 ^-7 Torr (about 1.1 ×
At a ^{pressure of} 10 ⁻⁴ Pa), a hole transport layer 4 having a thickness of 60 nm was formed at a deposition rate of 0.2 nm / sec.

[0094]

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Next, as materials for the light emitting layer 5, an 8-hydroxyquinoline complex of aluminum (EM-1) having the following structural formula and a benzopyran derivative of an orange fluorescent dye (DYE-1) having the following structural formula are used. Were co-evaporated on the hole transport layer 4 at a ratio of 100: 1.
The temperature of the crucible at this time is 265-270 ° C and 315-320, respectively.
The temperature was controlled in the range of ° C. Vacuum degree during deposition is 8.0 × 10 ^-7 Torr
(About 1.1 × 10 ⁻⁴ Pa), the deposition rates of EM-1 and DYE-1 were 0.1 nm / sec, and the total film thickness was 30 nm.

[0096]

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[0097]

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Next, using a silanol aluminum 8-hydroxyquinoline complex (HB-1) having 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 227 ° C, and the degree of vacuum is 0.8 × 10 ^-7 Torr (about
A hole blocking layer 6 having a thickness of 10 nm was formed at a rate of 1.1 × 10 ⁻⁴ Pa) and a deposition rate of 0.2 nm / sec.

[0099]

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The substrate temperature during vacuum deposition of the hole blocking layer 6 from the anode buffer layer 3 was kept at room temperature.

Here, the element on which the vapor deposition up to the hole blocking layer 6 was performed was once taken out of the vacuum vapor deposition apparatus into the atmosphere, and a stripe-shaped shadow mask having a width of 2 mm was used as a vapor deposition mask. The stripe is closely attached to the element so as to be perpendicular to the stripe, and placed in another vacuum deposition apparatus, and the degree of vacuum in the apparatus is set to 2 × 10 ^-6 Torr (about
Evacuation was performed until the pressure became 2.7 × 10 ⁻⁴ Pa) or less.

Next, a phenanthroline derivative (ET-1) of the following structural formula and sodium were similarly applied on the hole blocking layer 6 as materials for the electron transporting layer 7 in a ratio of 50:50.
Co-evaporation was performed at a molar ratio of. For the deposition of sodium, a sodium dispenser having sodium chromate (manufactured by SAES Getters) was used. At this time, the temperature of the crucible was controlled in the range of 240 to 280 ° C., and the current of the dispenser was controlled in the range of 4 to 5 A. The degree of vacuum during deposition is 1.6 × 10 ^-6 Torr (about 2.1 ×
10 ⁻⁴ Pa), the deposition rate of the phenanthroline derivative (ET-1) and sodium was 0.05 nm / sec, and the total film thickness was 35 nm.

[0103]

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Subsequently, using a molybdenum boat as the cathode 8, aluminum was deposited at a deposition rate of 0.6 nm / sec.
Evaporation was performed at 1.0 × 10 ⁻⁵ Torr (about 1.3 × 10 ⁻³ Pa), and an aluminum layer having a thickness of 80 nm was formed on the electron transport layer 7. The substrate temperature during the deposition of the electron transport layer 7 and the 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 .

The maximum wavelength of the emission spectrum of the device is 590 nm.
And was identified as being from a fluorescent dye (DYE-1).

This device did not show a remarkable rise in driving voltage even after storage for a long period of time and did not 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.

When the state of sodium in the electron transport layer was confirmed by X-ray spectroscopy, most of the co-evaporated sodium was in a cationic state.

Comparative Example 1 An element was prepared in the same manner as in Example 1, except that the hole blocking layer was not provided and the thickness of the electron transport layer was changed to 45 nm. Table 1 shows the emission characteristics of this device. The maximum wavelength of the emission spectrum of the device is 59
2 nm and was identified as being from a fluorescent dye (DYE-1). Although this device emitted orange light, its luminous efficiency was greatly reduced as compared with Example 1.

Comparative Example 2 As the material of the electron transporting layer, only 8-hydroxyquinoline complex of aluminum (EM-1) was used.
Magnesium fluoride (MgF ₂ : 1.5 nm), aluminum (4
A device was manufactured in the same manner as in Example 1, except that a three-layer cathode obtained by sequentially vacuum-depositing 0 nm) and silver (40 nm) was used. The emission characteristics of this device are shown in Table 1. The maximum wavelength of the emission spectrum of the device was 593 nm, and it was identified as being derived from the fluorescent dye (DYE-1). Although this device emitted orange light, its luminous efficiency was greatly reduced as compared with Example 1.

[0111]

[Table 1]

[0112]

As described above in detail, according to the organic electroluminescent device of the present invention, since it has the electron transporting layer exhibiting high electron transporting properties, a device capable of emitting light with high efficiency can be obtained.
Further, by providing the hole blocking layer, the movement of holes to the cathode is prevented, the charge injection balance is improved, and stable light emission can be obtained only from the light emitting layer.

Accordingly, 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 an outdoor display), a multi-color display, 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 (5)

[Claims] 1. An anode, a light emitting layer, a hole blocking layer,
In an organic electroluminescent device in which an electron transport layer and a cathode are formed in this order, the ionization potential of the hole blocking layer is 0.1 eV or more larger than the ionization potential of the light emitting layer, and the electron transport layer is formed of an electron transport material and an alkali metal. And at least a part of the alkali metal is dispersed in a cation state in the electron transport layer. 2. The organic electroluminescent device according to claim 1, wherein the electron transporting material is a phenanthroline derivative or a metal complex. 3. 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. 4. 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. 5. A method for manufacturing an organic electroluminescent device having a step of forming an electron transport layer, wherein the electron transport layer comprises: an alkali metal obtained by reducing a compound containing an alkali metal in a vacuum device; A method for manufacturing an organic electroluminescent device, wherein the organic electroluminescent device is formed by co-evaporating a conductive material.