JP2001291593A - Organic electroluminescent element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an organic electroluminescent element which emits light with a high luminance at a low voltage, and which can maintain a stable luminescent property at the time of driving, and further, has a wide range of process conditions at the time of preparation. SOLUTION: This is an organic electroluminescent element obtained by an anode, sequentially laminating an organic luminous layer and a cathode in this order on a substrate, and it has a cathode interface layer containing a compound expressed in formula (1).

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 to 1000 Hz), 2) High drive voltage (up to 200 V), 3) It is difficult to achieve full color (especially blue), 4) Peripheral drive circuit costs are high. .

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 has been greatly improved as compared with the conventional EL device using a single crystal such as anthracene or the like, and the practical characteristics have been approached.

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.

There are two problems with the organic electroluminescent device, namely, improvement of driving stability and reduction of driving voltage. That is, the voltage at the time of driving the organic electroluminescent element is high, and the driving stability including heat resistance is low.
This is a serious problem as a light source such as a backlight for a copier or a liquid crystal display, and is not desirable as a display element particularly for a full-color flat panel display.

The instability during driving of the organic electroluminescent device includes a decrease in light emission luminance, an increase in voltage during constant current driving, and generation of a non-light emitting portion (dark spot). There are several sources of these instabilities, but mainly
This is considered to be caused by deterioration of the cathode material, particularly the interface in contact with the light emitting layer. In the case of organic electroluminescent devices, low work function metals such as magnesium alloys and calcium are usually used in order to easily inject electrons from the cathode to the organic layer side, but these metals are oxidized by moisture in the atmosphere. This is a major cause of instability during driving. A cathode using a low work function metal is required to lower the driving voltage of the device, but improvement is desired due to the instability described above.

[0007] A cathode made of an alloy containing 0.01 to 0.1 parts by weight of lithium metal in aluminum has been disclosed (JP-A-5-121172). However, in such a cathode, the content of lithium metal is reduced. It needs to be strictly controlled. Aluminum and lithium metal are independently controlled in a vacuum deposition method, and aluminum
Forming a lithium alloy is difficult due to the process. It is also considered that an alloy having a desired composition ratio of aluminum and lithium is prepared in the form of a pellet or a target in advance, and a cathode is formed by an electron beam evaporation method or a sputtering method. Due to the difference in vapor pressure and sputtering efficiency, the composition ratio of aluminum and lithium alloy, which are evaporation sources, fluctuates when film formation is repeated, and there is a practical problem. Further, metal lithium atoms are easily diffused and diffuse into an organic layer to extinguish light emission, and furthermore, they are very sensitive to moisture, and the requirements for element sealing are extremely severe.

Although a cathode using an alloy containing at least 6 mol% of an alkali metal element is also disclosed (Japanese Patent Laid-Open No. 4-212287), a strict protective film must be formed due to the instability of the lithium metal atoms described above. Required and diffusion instability due to lithium atoms cannot be avoided. It has been studied to insert an ultra-thin insulating layer (0.3 to 10 nm) of lithium fluoride, lithium oxide, etc. at the interface in contact with the organic light emitting layer side of the cathode (Appl. Phys. Lett., Vol. 70, 152 pages, 1997; IEE
E Trans. Electron. Devices, 44, 1245, 1997
Year), there is a problem in adhesion to an organic layer, and there is a problem of dark spot generation such as peeling of a cathode.

[0009]

As described above, the improvement in the cathode of the conventional organic electroluminescent device and the organic layer in contact therewith is insufficient in effect and very difficult to control in a process. There was a problem. In view of the above situation, the present inventor has provided an organic electroluminescent element which emits light at high luminance at a low voltage, can maintain stable light emitting characteristics even during driving, and has a wide range of process conditions during fabrication. As a result of intensive studies for the purpose of doing so, it was found that the above problem can be solved by providing a cathode interface layer made of a specific organic compound in contact with the interface of the cathode on the organic light emitting layer side. It was completed.

[0010]

That is, the gist of the present invention is to provide an organic electroluminescent device in which an anode, an organic light emitting layer, and a cathode are sequentially laminated on a substrate, wherein the cathode has an organic light emitting layer side. And an organic electroluminescent device having a cathode interface layer containing a compound represented by the following general formula (I) in contact with the interface of

[0011]

Embedded image

(Wherein R ^{1 to} R ⁸ each independently represent a hydrogen atom, a halogen atom, an alkyl group, an aralkyl group, an alkenyl group, a cyano group, an amino group, an acyl group, an alkoxycarbonyl group, a carboxyl group, Represents an alkoxy group, a haloalkyl group, a hydroxyl group, an aromatic hydrocarbon ring group which may have a substituent or an aromatic heterocyclic group which may have a substituent, and R ^{1 to} R ⁴ , R ⁵ to R ⁸ may form a ring with adjacent substituents, X is an oxygen atom,
Represents a sulfur atom or an NR ⁹ group, R ⁹ represents a hydrogen atom, an alkyl group, an aromatic hydrocarbon ring group which may have a substituent, and M represents an alkali metal atom. )

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The organic electroluminescent device of the present invention is characterized by having a cathode interface layer containing the alkali metal complex represented by the above general formula (I). The above compounds have excellent stability when thinned, and greatly contribute to stabilization of the device.
In the above general formula, R ^{1 to} R ⁸ are preferably each independently a hydrogen atom; a halogen atom such as a chlorine atom, a bromine atom and an iodine atom; and a carbon atom having 1 to 6 carbon atoms such as a methyl group and an ethyl group. Alkyl group; aralkyl group such as benzyl group and phenethyl group; alkenyl group having 2 to 6 carbon atoms such as vinyl group; cyano group; amino group; dialkylamino group such as dimethylamino group and diethylamino group; diphenylamino group; Acyl groups such as methoxycarbonyl group,
C2-C6 alkoxycarbonyl groups such as ethoxycarbonyl groups; carboxyl groups; C1-C6 alkoxy groups such as methoxy groups and ethoxy groups; haloalkyl groups such as trifluoromethyl groups; hydroxyl groups; phenyl groups and naphthyl groups And aromatic heterocyclic groups such as pyridyl group, quinolyl group, thienyl group, carbazolyl group, indolyl group and furyl group.

Substituents on these aromatic hydrocarbon groups or aromatic heterocyclic groups include alkyl groups having 1 to 6 carbon atoms such as methyl group and ethyl group; lower alkoxy groups such as methoxy group; phenoxy groups and the like. Aryloxy groups such as benzyloxy group; aryl groups such as phenyl group and naphthyl group; substituted amino groups such as dimethylamino group.

Particularly preferably, R ^{1 to} R ⁸ include a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, an aryl group such as a phenyl group and a naphthyl group. Note that R ^{1 to} R ⁴ or R ^{5 to} R ⁸ may form a ring with adjacent substituents. In this case, the formed ring includes a benzene ring and a naphthalene ring.

X represents an oxygen atom, a sulfur atom, or an NR ⁹ group, and R ⁹ is preferably a hydrogen atom; an alkyl group having 1 to 6 carbon atoms such as a methyl group or an ethyl group; And an aromatic hydrocarbon ring group such as a phenyl group, a naphthyl group, and a biphenyl group. In this case, examples of the substituent include an alkyl group having 1 to 6 carbon atoms and an alkoxy group having 1 to 6 carbon atoms. M represents an alkali metal atom.

Preferred specific examples of the compound represented by the general formula (I) are shown in Table 1 below, but are not limited thereto.

[0018]

[Table 1]

In the tables, R ^{1 to} R ⁸ , which are not particularly indicated, and “-” represent a hydrogen atom. Hereinafter, the organic electroluminescent device of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view schematically showing an example of the structure of a general organic electroluminescent element used in the present invention.
2 represents an anode, 3 represents an organic light emitting layer, 4 represents a cathode interface layer, and 5 represents 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 low, the organic air-emitting device may be deteriorated by outside air passing through the substrate, which is not preferable. Therefore, a method of providing a dense silicon oxide film or the like on one or both sides of the synthetic resin substrate to ensure gas barrier properties is also a preferable method.

An anode 2 is provided on the substrate 1, and the anode 2 plays a role of injecting holes into the organic light emitting layer. The anode is usually made of a metal such as aluminum, gold, silver, nickel, palladium and platinum; a metal oxide such as an oxide of indium and / or tin; a metal halide such as copper iodide; carbon black; (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 particles of metal such as silver, fine particles of copper iodide or the like, carbon black, fine particles of conductive metal oxide, fine particles of conductive polymer, etc., they are dispersed in an appropriate binder resin solution and To form the anode 2. 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, 1
992). 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, the transmittance of visible light is usually 60% or more, preferably 80% or more. In this case, the thickness is usually 5 to 1000 nm, preferably 10 to 10 nm. It is about 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.

An organic light emitting layer 3 is provided on the anode 2. The organic light emitting layer 3 efficiently transports the holes injected from the anode 2 and the electrons injected from the cathode 4 and recombine between the electrodes to which an electric field is applied, and emits light efficiently by the recombination. Formed from material. Examples of the single-layer type organic light-emitting layer 3 include poly (p-phenylenevinylene) (Nature, 347, 539, 1990, etc.) and poly [2-
Methoxy-5- (2-ethylhexyloxy) -1,4-phenylenevinylene] (Appl. Phys. Lett., 58, 1982, 1991)
And others), poly (3-alkylthiophene) (Jpn. J. App
l. Phys., 30, L1938, 1991, etc.), or a system in which a luminescent material and an electron transfer material are mixed with a polymer such as polyvinyl carbazole (Applied Physics, 61, 1044, 19)
1992).

Usually, in order to improve the luminous efficiency of the organic light-emitting layer 3, as shown in FIG. 2, the hole-transporting layer 3b and the electron-transporting layer 3c are often divided into function-separated elements (Appl. Phys. .
Lett., 51, 913, 1987). Which layer and which region of the hole transport layer 3b and the electron transport layer 3c emit light depends on how the hole transport material in the hole transport layer 3b and the electron transport material in the electron transport layer 3c are. It depends on which material is selected or on the combination of both materials.
Hereinafter, in the case of such a function separation type element, the hole transport layer 3
Of the layer b and the electron transport layer 3c, a layer including a light emitting region may be referred to as a “light emitting layer”.

In the above function-separated element, the material of the hole transport layer 3b is a material having a high hole injection efficiency from the anode 2 and capable of efficiently transporting the injected holes. It is necessary. For that purpose, it is required that the ionization potential is small, the hole mobility is large, the stability is further excellent, and impurities serving as traps are hardly generated during production or use.

As such a hole transporting material, for example, 4,4′-bis [N- (1-naphthyl) -N-phenylamino]
Containing two or more tertiary amines represented by biphenyl;
Aromatic diamines in which at least two condensed aromatic rings are substituted with nitrogen atoms (JP-A-5-234681), 4,4 ', 4 "-tris (1-
Aromatic amine compounds having a starburst structure such as naphthylphenylamino) triphenylamine (J. Lumi
n., 72-74, 985, 1997), an aromatic amine compound composed of a tetramer of triphenylamine (Chem. Commun., 21).
75, 1996), spiro compounds such as 2,2 ', 7,7'-tetrakis- (diphenylamino) -9,9'-spirobifluorene (Syn
th. Metals, vol. 91, p. 209, 1997).
These compounds may be used alone, or may be used as a mixture as necessary.

In addition to the above compounds, examples of hole transport materials include polyvinyl carbazole, polyvinyl triphenylamine (Japanese Patent Application Laid-Open No. 7-53953), and polyarylene ether sulfone containing tetraphenylbenzidine (Polym. Adv. Tech.). , 7, 33, 1996). The hole transport layer 3b is formed by laminating the above 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 transporting materials and, if necessary, an additive such as a binder resin or a coating improver which does not trap holes are added,
The solution is dissolved to prepare a coating solution, applied to the anode 2 by a method such as spin coating, and dried to form the hole transport layer 3b. Examples of the binder resin include polycarbonate, polyarylate, and polyester. Since the binder resin reduces the hole mobility if the amount added is large,
A smaller amount is desirable, and usually 50% by weight or less is preferred.

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 to form a hole transport layer 3b on the anode 2 on the substrate 1 placed facing the crucible. The thickness of the hole transport layer 3b is usually 10 to 300 nm, preferably 30 to 100 nm. In order to uniformly form such a thin film, generally, a vacuum deposition method is often used. An electron transport layer 3c is provided on the hole transport layer 3b. The electron transport layer 3c is formed of a material that can efficiently transport electrons from the cathode in the direction of the hole transport layer 3b between electrodes to which an electric field is applied.

The electron transporting material used for the electron transporting layer 3c needs to be a compound having a high electron injection efficiency from the cathode 4 and capable of transporting the injected electrons efficiently. For this purpose, it is required that the material has a high electron affinity, a high electron mobility, a high stability, and an impurity which becomes a trap and hardly occurs during 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 (JP-A-1-245087 and JP-A-2-222484) and the like. In general, the electron transport layer 3c using these compounds can simultaneously play a role of transporting electrons and a role of emitting light when holes and electrons recombine.

When the hole transporting layer 3b has a light emitting function, the electron transporting layer 3c may only play the role of transporting electrons. The thickness of the electron transport layer 3c is usually 10 to 200 n
m, preferably 30-100 nm. The electron transporting layer can be formed in the same manner as the hole transporting layer, but usually, a vacuum evaporation method is used.

For the purpose of improving the luminous efficiency of the device and changing the luminescent color, a hole transporting material or an electron transporting material in the layer serving as the luminescent layer may be used as a host material and doped with a fluorescent dye. 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, 361)
0, 1989). The advantages of this method are: 1) improved luminous efficiency by high-efficiency fluorescent dye, 2) variable emission wavelength by selecting fluorescent dye, 3) fluorescent dye which causes concentration quenching can be used, 4) fluorescent light with poor thin film property Dyes can also be used.

For the purpose of improving the driving life of the device, it is effective to dope a fluorescent dye using the light emitting layer material as a host material. For example, using a metal complex such as an aluminum complex of 8-hydroxyquinoline as a host material, a naphthacene derivative represented by rubrene (Jpn.
J. Appl. Phys., 7A, L824, 1995), quinacridone derivative (Appl. Phys. Lett., 70, 1665, 1997)
By doping 0.1 to 10% by weight of the host material, the emission characteristics of the device, particularly the driving stability, can be greatly improved. As a method of doping the fluorescent material such as the naphthacene derivative and the quinacridone derivative into the light emitting layer host material, there are a method of co-evaporation and a method of previously mixing an evaporation source at a predetermined concentration.

The region to which the fluorescent dye is doped may be the whole or a part of the hole transport layer 3b and / or the electron transport layer 3c, and may be uniform in the thickness direction of each layer. It may be doped or 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 transporting layer 3b in the electron transporting layer 3c, or conversely, may be performed in the vicinity of the interface on the cathode side. The amount of the fluorescent dye to be doped is preferably usually 10 ⁻³ to 10% by weight based on the host material.

The cathode interface layer 4 is laminated on the organic light emitting layer 3 (the electron transport layer 3c in the case of a function separation type element). The present invention is characterized in that a layer containing the organic compound represented by the general formula (I) is used as the cathode interface layer. The thickness of the cathode interface layer is preferably from 0.3 to 10 nm, particularly preferably from 0.5 to 5 nm. In order to form such a thin film, a vacuum deposition method is generally used.

The cathode 5 plays a role of injecting electrons into the organic light emitting layer 3 via the cathode interface layer 4. In the present invention, the metal used as the cathode includes aluminum, indium, magnesium, calcium, zinc, vanadium, chromium, tin, and copper, and particularly preferably, aluminum. A reaction occurs during deposition between the cathode metal and the organic compound constituting the cathode interface layer, and as a result, a reaction product that facilitates electron injection is formed at the cathode interface. Further, the organic compound represented by the general formula (I) has an effect of lowering the vacuum level and lowering the LUMO level of the light emitting layer, and as a result, has a function of lowering the electron injection barrier and lowering the driving voltage.

The thickness of the cathode 4 is usually the same as that of the anode 2. For the purpose of protecting the cathode, it is preferable to further form a protective layer made of a metal material having a high work function and being stable to the atmosphere, for increasing the stability of the device. Aluminum, copper, chromium, gold and silver metals are used for this purpose. The metal material used for the protective layer is a cathode 4
The material may be the same as or different from the metal material used for the metal.

The organic electroluminescent device of the present invention may have a layer between any layers other than the cathode-cathode interface layer, in addition to the layers described above.

For example, in order to further improve the efficiency of hole injection and to improve the adhesion of the whole organic layer to the anode, an anode buffer layer 3a is provided between the hole transport layer 3b and the anode 2.
May be inserted (FIG. 3). The insertion of the anode buffer layer 3a has the effect of lowering 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 the contact with the anode can be made well, a uniform thin film can be formed, and it is thermally stable, that is, the melting point and the glass transition temperature are high, and the melting point is 300 ° C or more. A glass transition temperature of 100 ° C or higher is required. Furthermore, the ionization potential is low and the hole injection from the anode is easy,
The hole mobility is large.

For this purpose, a thalocyanine compound such as copper phthalocyanine (JP-A-63-295695) and polyaniline (Appl. Phys. Lett., Vol. 64, 1245) have been used so far.
P., 1994), polythiophene (Optical Materials, 9)
Vol. 125, 1998), sputtered carbon films (Synth. Met., Vol. 91, p. 73, 1997), and metal oxides such as vanadium oxide, ruthenium oxide and molybdenum oxide. (J.Phys. D, 29, 2750, 1996)
Have been reported.

In the case of the anode buffer layer, a thin film can be formed in the same manner as in the case of the hole transporting layer. In the case of an inorganic substance, however, a sputtering method, an electron beam evaporation method, or a plasma CVD method may be used.
Method is used. The thickness of the anode buffer layer 3a formed as described above is usually 3 to 100 nm, preferably 10 to 50 nm.
It is.

As a method for further improving the luminous efficiency of the organic electroluminescent device, an electron injection layer can be further laminated on the organic light emitting layer 3. The compound used for the electron injection layer is required to be capable of easily injecting electrons from the cathode and having a higher electron transport ability. Examples of such an electron transport material include 8-hydroxyquinoline aluminum complexes and oxadiazole derivatives (Appl. Phys. Lett., Vol. 55, p. 1489, 1989, etc.), which have already been cited as a material for the light-emitting layer, and poly (ethylene). Methyl methacrylate (PMMA)
(Appl. Phys. Lett., 61, 279)
P. 3, 1992), phenanthroline derivatives (JP-A-5-3
No. 31459). The thickness of the electron injection layer is
Usually, it is 5 to 200 nm, preferably 10 to 100 nm.

Incidentally, it is also possible to stack the cathode 5, the cathode interface layer 4, the organic luminescent layer 3, and the anode 2 in this order on the substrate, that is, as described above, at least one of them. It is also possible to provide the organic electroluminescent device of the present invention between two substrates having high transparency. Similarly, it is also possible to laminate in a structure opposite to the above-mentioned respective layer constitutions shown in FIGS.

[0044]

EXAMPLES Next, the present invention will be described more specifically with reference to examples, but the present invention is not limited to the description of the following examples unless it exceeds the gist. Example 1 An organic electroluminescent device having a structure shown in FIG. 3 was produced 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 of 15Ω) was etched using a conventional photolithography technique and hydrochloric acid etching. 2
The anode 2 was formed by patterning into a stripe having a width of mm. 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 the rough exhaust of the above device is performed by an oil rotary pump, exhaust is performed using an oil diffusion pump equipped with a liquid nitrogen trap until the degree of vacuum in the device becomes 2 × 10 ⁻⁶ Torr (about 2,7 × 10 ⁻⁴ Pa) or less. did.

Copper phthalocyanine shown below in a molybdenum boat placed in the above apparatus (crystal form is β-form)

[0047]

Embedded image

Was heated to perform vapor deposition. Vacuum 2x10 ^-6 To
Vapor deposition was performed at rr (approximately 2.7 × 10 ⁻⁴ Pa) and a vapor deposition time of 1 minute to obtain a 10 nm-thick anode buffer layer 3a. Next, placed in a ceramic crucible placed in the device, shown below,
4'-bis [N- (1-naphthyl) -N-phenylamino] biphenyl

[0049]

Embedded image

The evaporation was performed by heating with a tantalum wire heater around the crucible. The temperature of the crucible at this time is 280 ~ 2
The temperature was controlled in the range of 70 ° C. 1.5x10 ^-6 Torr vacuum during deposition
(Approximately 2.0 × 10 ⁻⁴ Pa) and a hole transport layer 3b having a thickness of 60 nm was obtained in a deposition time of 3 minutes. Subsequently, as a material of the electron transporting layer 3c having a light emitting function, an 8-hydroxyquinoline complex of aluminum represented by the following structural formula, Al (C ₉ H ₆ NO) ₃

[0051]

Embedded image

Was deposited in the same manner as the hole transport layer 3b. At this time, the crucible temperature of the aluminum 8-hydroxyquinoline complex was controlled in the range of 290 to 300 ° C., the degree of vacuum at the time of vapor deposition was 1.6 × 10 ⁻⁶ Torr (about 2.1 × 10 ⁻⁴ Pa), and the vapor deposition time was 3 minutes. The thickness of the deposited electron transport layer 3c was 75 nm.
The above-described anode buffer layer 3a, hole transport layer 3b, and electron transport layer
The substrate temperature during vacuum deposition of 3c was kept at room temperature.

Here, the element on which the vapor deposition up to the electron transport layer 3c has been performed is once taken out of the vacuum vapor deposition apparatus into the atmosphere, and a 2 mm wide stripe-shaped shadow mask is used as a cathode vapor deposition mask. Adhere to the element so as to be orthogonal to the stripe, and install it in another vacuum evaporation apparatus and set the degree of vacuum in the apparatus to 2x10 ^-6 Torr in the same way as for the organic layer.
(About 2.7 × 10 ⁻⁴ Pa) or less.

Subsequently, on the electron transport layer 3c, the compound (16) shown in Table 1 was similarly applied using a ceramic crucible.
By heating to 350 ° C., the cathode interface layer 4 having a thickness of 0.8 nm was formed in a deposition time of 5 minutes. The degree of vacuum during deposition is 3.5 × 10 ^-6 Torr (about 4.7 × 10 ^-4
Pa). Subsequently, aluminum was formed as the cathode 5 on the cathode interface layer 4 at a deposition rate of 0.4 nm / sec with a thickness of 80 nm. The degree of vacuum at the time of vapor deposition was 1.0 × 10 ⁻⁵ Torr (about 1.3 × 10 ⁻³ Pa). The substrate temperature during the deposition of the cathode interface layer and the cathode was kept at room temperature.

As described above, an organic electroluminescent device having a size of 2 mm × 2 mm was obtained. After the device was taken out of the cathode vapor deposition apparatus, a forward voltage was applied between the anode and the cathode in the atmosphere, and the light emission characteristics were measured. Table 2 shows the light emission characteristics of this device. In Table 2, the emission luminance is 250m
A / cm ² at current density, luminous efficiency at 100 cd / m ² , luminance / current is the slope of the luminance-current density characteristic, and voltage is
The values at 100 cd / m ² are shown. Also, the voltage of this element-
The luminance characteristics are shown in the graph of FIG. In this device, high luminance of 1000 to 10,000 cd / m ² was obtained at a low voltage of 10 V or less.

FIG. 5 is a graph showing the result of a heat resistance test of the device, which was evaluated based on a decrease in luminance when driven at a high current density of 250 mA / cm ² . Extremely stable characteristics with little decrease in luminance were obtained. Comparative Example 1 An element was produced in the same manner as in Example 1 except that the cathode interface layer was not provided. The emission characteristics of this device obtained according to Example 1 are shown in Table 2 and the graph of FIG. 4, and the driving characteristics at a high current density are shown in FIG. As compared with the device having the cathode interface layer, the voltage of the device was increased, and the durability was extremely low.

[0057]

[Table 2]

[0058]

According to the cathode of the organic electroluminescent device of the present invention, it is possible to emit light with high luminance and high efficiency at a low voltage, and it is stable even when driven at a high current density. An element with little deterioration can be obtained. Therefore, the organic electroluminescent device according to the present invention can be used as a light source (for example, a light source of a copier, a light source of a copier,
It can be applied to liquid crystal displays and backlight sources for instruments, display panels, and sign lights, and its technical value is great.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing an example of an organic electroluminescent device.

FIG. 2 is a schematic sectional view showing another example of the organic electroluminescent device.

FIG. 3 is a schematic sectional view showing still another example of the organic electroluminescent element.

FIG. 4 is a graph showing voltage-luminance characteristics in Example 1 and Comparative Example 1.

FIG. 5 is a graph showing the results of a heat resistance test of the devices in Example 1 and Comparative Example 1.

[Explanation of symbols]

 Reference Signs List 1 substrate 2 anode 3 organic light emitting layer 3a anode buffer layer 3b hole transport layer 3c electron transport layer 4 cathode interface layer 5 cathode

Claims (3)

[Claims] 1. An organic electroluminescent device comprising a substrate, on which an anode, an organic light-emitting layer, and a cathode are sequentially laminated, wherein the cathode is in contact with an interface on the organic light-emitting layer side, and has the following general formula (I):
An organic electroluminescent device having a cathode interface layer containing a compound represented by the formula: Embedded image (Wherein, R ^{1 to} R ⁸ each independently represent a hydrogen atom, a halogen atom, an alkyl group, an aralkyl group, an alkenyl group,
Cyano group, amino group, acyl group, alkoxycarbonyl group, carboxyl group, alkoxy group, haloalkyl group,
Represents a hydroxyl group, an aromatic hydrocarbon ring group which may have a substituent or an aromatic heterocyclic group which may have a substituent, and R ^{1 to} R ⁴ and R ^{5 to} R ⁸ are each adjacent The substituents may form a ring. X is an oxygen atom, a sulfur atom,
Or an NR ⁹ group, wherein R ⁹ is a hydrogen atom, an alkyl group,
Alternatively, it represents an aromatic hydrocarbon ring group which may have a substituent, and M represents an alkali metal atom. ) 2. The organic electroluminescent device according to claim 1, wherein the thickness of the cathode interface layer is in the range of 0.3 to 10 nm. 3. The organic electroluminescent device according to claim 1, wherein the metal forming the cathode is selected from aluminum, indium, magnesium, calcium, zinc, vanadium, chromium, and tin.