JP2001297883A - Organic electric-field light emission element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an organic electric-field light emission element which can be made to drive on low voltage and can maintain a luminescence characteristic stable for a long period of time. SOLUTION: In the organic electric-field light emission element which is formed by laminating of an anode, a hole transportation layer, a luminescence layer, and a negative electrode in that order e on a substrate, the hole transportation layer has four or more aromatic group ring (a heterocyclic is included) structures of 5-7 member ring, and has a fluorescence compound having an electron-receptive group.

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 thin film 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 (50-1000 Hz), 2) High drive voltage (up to 200 V), 3) Full color is difficult (especially blue), 4) Peripheral drive circuit has high cost. .

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.

[0005]

Problems to be Solved by the Invention There are two problems of the organic electroluminescent device, namely, driving stability and low driving voltage. The instability during driving includes a decrease in light emission luminance, an increase in voltage during constant current driving, and a non-light-emitting portion (dark spot).
And the like. Although there are several causes of these instabilities, the hole transporting material forming the hole transporting layer is oxidized, that is, the stability of the cation radical is repeatedly stabilized by an electrochemical method. It has been studied and it is known that many aromatic amine compounds satisfy this stability. However, a sufficient analysis has not yet been performed on the phenomenon that the luminance gradually decreases with the current emission of the element.

[0006] In the simplest two-layer device structure, anode / hole transport layer / light-emitting layer / cathode, the light emission is such that holes injected from the anode pass through the hole transport layer and are injected from the cathode. It is generally understood that electrons recombine with electrons in the light emitting layer to emit light. In a normal device, the probability of this recombination is considered to be close to 100%. However, if the charge injection is not balanced, the holes injected into the light emitting layer reach the cathode without recombination. Or the electrons injected into the light-emitting layer do not recombine and pass through the hole transport layer to reach the anode. In the former case, the luminous efficiency is reduced, and in the latter case, the element is deteriorated. In this latter process, the degradation is that the hole transport material is stable in repeated oxidation-neutralization cycles, but extremely unstable in reduction, and furthermore, indium, which is often used as an anode, is often used. -When tin oxide (abbreviated as ITO) is reduced, indium and tin components chemically change from the oxide to a metal state having a small work function, making it difficult for holes to be injected, which increases the driving voltage. Leads to. At present, improvement is desired due to the instability of the drive characteristics of the above-described elements.

[0007]

The low stability of the organic electroluminescent device at the time of driving is a serious problem for a light source such as a facsimile, a copying machine, and a backlight of a liquid crystal display. Also, it is not desirable as a display element such as a full-color flat panel display.

Accordingly, an object of the present invention is to provide an organic electroluminescent device which can be driven at a low voltage and can maintain stable light emitting characteristics for a long period of time because deterioration of the device is prevented. It is.

[0009]

Means for Solving the Problems As a result of intensive studies, the present inventors have found that the above problem can be solved by providing a region containing a fluorescent compound having an electron accepting group in the hole transport layer. They found what they could do and completed the present invention. That is, the present invention is an organic electroluminescent device in which an anode, a hole transport layer, a light emitting layer, and a cathode are sequentially laminated on a substrate, wherein the hole transport layer is a 5- to 7-membered aromatic compound. An organic electroluminescent device comprising four or more ring (including heterocyclic) structures and containing a fluorescent compound having an electron accepting group.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The organic electroluminescent device of the present invention is characterized in that the hole transport layer contains a fluorescent compound having an electron accepting group. The term "fluorescent" as used herein means that the compound emits fluorescence in a state of being diluted and dispersed to 10% or less using a solvent or a resin binder.

The term "electron-accepting" refers to a group having a Hammett constant of 0.3 or more. Since the fluorescent compound of the present invention has an electron-accepting substituent, it has an electron affinity greater than the electron affinity of the hole transport layer forming material, or the absolute value of the reduction potential is larger on the negative potential side. It has the effect of becoming. The “hole transport layer forming material” is a substance contained in the hole transport layer in an amount of more than 50% by weight, has a high hole injection efficiency from the anode, and efficiently transports the injected holes. It is a material that can be. In order to have such properties, the ionization potential is low, the transparency to visible light is high, the hole mobility is high, and the stability is further improved. Sometimes it is required that it hardly occurs. It may be a so-called low molecular compound or a high molecular compound.

When a compound having a higher electron affinity than the material for forming the hole transport layer is contained in the hole transport layer, electrons flowing from the light emitting layer toward the hole transport layer are trapped, and It is possible to prevent the forming material from being reduced. Another effect is that a hole trapping region is formed in the hole transport layer near the anode, so that holes can be easily injected from the anode.

In order to exhibit such effects, in the organic electroluminescent device of the present invention, the hole transporting layer, more precisely, the region containing the fluorescent compound of the present invention in the hole transporting layer is formed of a light emitting layer. Preferably. The electron affinity is obtained by correcting a reduction potential electrochemically measured with respect to a reference electrode. For example, when a saturated sweet potato electrode (SCE) is used as a reference electrode, electron affinity = reduction potential (vs. SCE) +4.3 eV, (“Molecular Semiconductors”, Springer-V
erlag, 1985, p. 98). Another method is to determine the ionization potential using vacuum ultraviolet photoelectron spectroscopy (UPS) or atmospheric pressure photoelectron spectroscopy (AC-1 manufactured by Riken Keiki), subtract the optical band gap from this ionization potential, Electron affinity is required.

In order for the fluorescent compound of the present invention to be thermally stable in the hole transporting layer, it is necessary that
It must be a compound containing 4 to 7 membered aromatic rings (including heterocycles). If the number is three or less, it becomes difficult for the fluorescent compounds of the present invention to aggregate or crystallize and maintain a uniform dispersion state. Since the fluorescent compound of the present invention is preferably contained in a portion of the hole transport layer that is in contact with the light emitting layer, it should not inhibit light emission in the light emitting layer. To achieve this, the maximum wavelength of light emission of the light emitting layer is
It preferably has a maximum fluorescence wavelength shorter than nm. If the light-emitting layer has a fluorescence maximum wavelength longer than that of the light-emitting layer, a part of the exciton may also excite the material on the hole transport layer side upon recombination, thereby inducing unintended light emission. In addition, when a non-fluorescent substance is contained instead of the fluorescent compound, a non-emission relaxation route is generated, which is not preferable. Therefore, it has been found that the object of the present invention can be better achieved by using a compound having a fluorescence maximum shorter than the emission maximum wavelength of the emission layer by 10 nm or more.

Preferred examples of the electron accepting group include a cyano group, a nitro group, a halogen atom and a nitrogen-containing heterocyclic group. The above-mentioned fluorescent compound preferably includes a compound having a partial structure represented by the following general formula (I).

[0016]

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Specific examples of the compound represented by the above general formula (I) are shown below. The compound contains four or more 5- to 7-membered aromatic rings (including heterocycles), and has a fluorescent and electron accepting property. The compounds are not limited to the following compounds to the extent that the requirements regarding the properties are satisfied.

[0018]

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Another preferred example of the above-mentioned fluorescent compound is a compound having a molecular structure represented by the following general formula (II).

[0020]

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In the above formula (II), Ar represents a divalent aromatic ring group, preferably a fused aromatic ring such as anthracene, naphthacene, pyrene, perylene, fluorene, phenanthrene, and X ₁ and X _{1 2} represents an electron-accepting group selected from a cyano group, a nitro group, and a halogen atom, or an aromatic ring group and a nitrogen-containing heterocyclic group having these electron-accepting groups. n and m each represent an integer from 0 to 4, and at least one of them is a value other than 0.
Specific examples of the compound represented by the general formula (II) are shown below, which include four or more 5- to 7-membered aromatic rings (including a heterocyclic ring), and requirements for fluorescence and electron acceptability. Is not limited to the following compounds.

[0022]

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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 device used in the present invention, wherein 1 is a substrate, 2 is an anode, 4 is a hole transport layer, 5 is a light emitting layer, and 6 is Each 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 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 hole transport layer 4. This anode is usually composed of metals such as aluminum, gold, silver, nickel, palladium, platinum, metal oxides such as oxides of indium and / or tin, metal halides such as copper iodide, carbon black and the like. You. Usually, the formation of the anode 2 is often performed by a sputtering method, a vacuum evaporation method, or the like. Further, the anode 2 is formed by dispersing metal fine particles such as silver, fine particles such as copper iodide, carbon black, conductive metal oxide fine particles, and the like in an appropriate binder resin solution and applying the dispersion on the substrate 1. Can also. 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 visible light transmittance is usually 60% or more, preferably 80% or more. In this case, the thickness is usually 10 to 1000 n.
m, preferably about 20 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.

In the present invention, in the element structure of FIG.
A hole transport layer 4 is provided on the anode 2. Hole transport layer 4
Contains at least the aforementioned hole transport layer forming material and a fluorescent compound. It is necessary that the hole transport layer has a high hole injection efficiency from the anode and can efficiently transport the injected holes. 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, considering the application for in-vehicle display, since the element is required to have a heat resistance of 85 ° C. or higher, the material contained in the hole transport layer 4 has a glass transition temperature of 85 ° C. It is desirable that the temperature is not less than ° C.

As a material for forming the hole transport layer, for example,
Aromatic diamine containing two or more tertiary amines represented by 4′-bis [N- (1-naphthyl) -N-phenylamino] biphenyl and having two or more fused aromatic rings substituted by nitrogen atoms ( JP-A-5-234681), aromatic amine compounds having a starburst structure such as 4,4 ', 4 "-tris (1-naphthylphenylamino) triphenylamine (J. Lumin., 7
2-74, 985, 1997), aromatic amine compounds composed of tetramers of triphenylamine (Chem. Commun., 2175)
Spiro compounds such as 2,2 ', 7,7'-tetrakis- (diphenylamino) -9,9'-spirobifluorene (Synt
h. 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, as a material for forming a hole transport layer, polyarylene ether sulfone (Polym. Adv.) Containing polyvinyl carbazole, polyvinyl triphenylamine (Japanese Patent Application Laid-Open No. 7-53953), and tetraphenylbenzidine. Tech., 7, 33, 1996). The fluorescent compound of the present invention is preferably contained in a region of the hole transport layer 4 that is in contact with the light emitting layer 5. For this purpose, even when the hole transport layer 4 is uniformly contained in the entire hole transport layer 4, a region of 1 to 50% of the thickness of the hole transport layer 4 from the interface in contact with the light emitting layer 5 (layer indicated by 4 b in FIG. 2). Only may be contained. Further, the holes may be unevenly distributed in the thickness direction of the hole transport layer 4.

The content of the fluorescent compound of the present invention is in the range of 0.1 to 50% by weight in the region where the compound is contained (when contained in the whole hole transporting layer 4, the layer). Is preferred. More preferably, a concentration range of 0.3 to 30% by weight is desirable in practical characteristics. The hole transport layer 4 containing the fluorescent compound of the present invention is formed by laminating on the anode 2 by a coating method or a vacuum evaporation method.

In the case of the coating method, the material for forming the hole transport layer is 1
A coating solution is prepared by adding a kind or two or more kinds and a predetermined amount of the fluorescent compound of the present invention, if necessary, with an additive such as a binder resin or a coatability improving agent which does not trap holes, and dissolving. The hole transport layer 4 is formed by coating the anode 2 on the anode 2 by a method such as spin coating, and drying. Examples of the binder resin include polycarbonate, polyarylate, and polyester. If the amount of the binder resin is large, the hole mobility is reduced, so that a small amount is desirable, and usually 50% by weight or less is preferable.

In the case of the vacuum deposition method, the material for forming the hole transport layer and the fluorescent compound of the present invention are placed in separate crucibles installed in a vacuum vessel, and the inside of the vacuum vessel is pumped by a suitable vacuum pump.
After evacuation to about ^-4 Pa, each crucible is heated and evaporated, and the hole transport layer 4 is formed on the anode 2 on the substrate 1 placed facing the crucible by co-evaporation. The thickness of the hole transport layer 4 is usually 5 to 300 nm, preferably 10 to 10 nm.
0 nm. In order to uniformly form such a thin film, generally, a vacuum deposition method is often used.

The efficiency of hole injection is further improved,
Further, for the purpose of improving the adhesion of the entire organic layer to the anode, an anode buffer layer 3 is inserted between the hole transport layer 4 and the anode 2 (FIG. 3). The insertion of the anode buffer layer 3 has the effects 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 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. In addition, the ionization potential is low, holes can be easily injected from the anode, and the hole mobility is high.

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 transport layer. In the case of an inorganic substance, however, a sputtering method, an electron beam evaporation method, a plasma CVD method, or the like can be used.
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.
m.

The light emitting layer 5 is provided on the hole transport layer 4. The light emitting layer 5 is formed of a material that efficiently recombines electrons injected from the cathode and holes transported from the hole injection layer between the electrodes to which an electric field is applied, and emits light efficiently by the recombination. Is done. Examples of the light emitting layer material satisfying such conditions include metal complexes such as aluminum complex of 8-hydroxyquinoline (JP-A-59-194393), and
-Hydroxybenzo [h] quinoline metal complex
322362), bisstyrylbenzene derivatives (Japanese Patent Application Laid-Open Nos. 1-245087 and 2-222484), bisstyrylarylene derivatives (Japanese Patent Application Laid-Open No. 2-247278),
Metal complexes of (hydroxyphenyl) benzothiazole (JP-A-8-315983), silole derivatives and the like. These light emitting layer materials are usually laminated on the hole transport layer by a vacuum evaporation method.

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). 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 by 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 (Japanese Patent Laid-Open No. 4-335087) and a quinacridone derivative (Japanese Patent Laid-Open No.
No. 70773), a fused polycyclic aromatic ring such as perylene (Japanese Patent Application Laid-Open No. 5-198377), and the like.
By doping at 0.1 to 10% by weight, the light emitting 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, quinacridone derivative, or perylene 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.

Examples of the polymer-based light emitting layer materials include poly (p-phenylenevinylene) and poly [2-methoxy-5] described above.
-(2-ethylhexyloxy) -1,4-phenylenevinylene], a polymer material such as poly (3-alkylthiophene), a system in which a luminescent material and an electron transfer material are mixed with a polymer such as polyvinylcarbazole, and the like. Can be These materials are coated on the hole injection layer by a method such as spin coating or dip coating similarly to the hole injection layer to be thinned. The thickness of the light emitting layer 5 is usually 10 to 200 nm, preferably 10 to 200 nm. 30 ~
100 nm.

In order to further improve the luminous efficiency of the organic electroluminescent device, an electron transport layer can be further laminated on the light emitting layer 4. The compound used in the electron transport 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 transporting material include aluminum complexes of 8-hydroxyquinoline and oxadiazole derivatives (App
l. Phys. Lett., 55, 1489, 1989) and a system in which they are dispersed in a resin such as polymethyl methacrylate (PMMA), phenanthroline derivatives (JP-A-5-331459), 2-t- Butyl-9,10-N, N'-dicyanoanthraquinone diimine, n-type hydrogenated amorphous silicon carbide, n-type zinc sulfide, n-type zinc selenide, and the like. The thickness of the electron transporting layer is usually 5 to 200 nm, preferably 10 to 100 nm.

The cathode 6 plays a role of injecting electrons into the light emitting layer 5. As the material used for the cathode, the material used for the anode 2 can be used. However, for efficient electron injection, a metal having a low work function is preferable. Tin, magnesium, indium, calcium, aluminum , Silver or other suitable metals or alloys thereof. 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, inserting an ultra-thin insulating film (0.1 to 5 nm) of LiF, MgF ₂ , Li ₂ O or the like at the interface between the cathode and the light emitting layer or the electron transporting layer is also an effective method for improving the efficiency of the device. (Appl. Phys. Lett., Vol. 70, p. 152, 1997; JP-A-10-74586; IEEE Trans. Electron. Devices,
44, 1245, 1997). The thickness of the cathode 7 is usually
Is the same as

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. In addition, it is also possible to stack the cathode 6, the light emitting layer 5, the hole transporting layer 4, and the anode 2 in this order on the substrate, that is, at least one of them is transparent as described above. It is also possible to provide the organic electroluminescent device of the present invention between two substrates having a high density.
Similarly, it is also possible to laminate in a structure opposite to the above-mentioned respective layer constitutions shown in FIGS.

[0042]

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. Reference Example 1 A glass substrate was ultrasonically washed with acetone, washed with pure water, ultrasonically washed with isopropyl alcohol, dried with dry nitrogen, and dried.
After performing the V / ozone cleaning, the apparatus was set in a vacuum evaporation apparatus and evacuated using an oil diffusion pump until the degree of vacuum in the apparatus became 2 × 10 ⁻⁶ Torr or less.

As a material for forming a hole transport layer, 4,4′-bis [N
-(1-Naphthyl) -N-phenylamino] biphenyl (H
1)

[0044]

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Was put in a ceramic crucible, and 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 260 to 280 ° C. The degree of vacuum during the deposition was 2.0 × 10 ⁻⁶ Torr (about 2.7 × 10 ⁻⁴ Pa), and a uniform and transparent film having a thickness of 101 nm was obtained at a deposition rate of 0.35 nm / sec. The ionization potential of this thin film sample was measured using an ultraviolet electron analyzer (AC-1) manufactured by Riken Keiki Co., Ltd., and showed a value of 5.25 eV. The energy gap determined from the absorption edge of the absorption spectrum of this deposited film was 2.95 eV. Therefore, the electron affinity of this hole transport layer forming material is
It became 2.30 eV.

Reference Example 2 The fluorescent compounds (1-1) and (2-1) of the present invention were dissolved in a mixed solvent of acetonitrile and THF (volume ratio: 1: 1).
It was dissolved at a concentration of 0.02 mol / liter, and the reduction potential was measured by cyclic voltammetry using tetrabutylammonium perchlorate and Bu4NClO4 as a supporting electrolyte and silver as a reference electrode. The oxidation potential of ferrocene is measured as an internal standard, and the reduction potential of each fluorescent compound is determined.
It was -0.52V and -1.75V when calculated with respect to the SCE standard. Therefore, the electron affinities were 3.78 eV and 2.55 eV, respectively.

Reference Example 3 The fluorescent compounds (1-1) and (2-1) of the present invention were
When diluted with 5-dichloromethane as a solvent at a concentration of 5 × 10 ⁻⁶ and excited at the absorption maximum wavelength, they were 466 nm and 432 nm, respectively.
It showed strong fluorescence at nm. Example 1 An organic electroluminescent device having a structure shown in FIG. 2 was produced by the following method. In this device, the region 4a that does not contain the fluorescent compound of the present invention and the region 4b that contains the compound are combined,
It corresponds to a hole transport layer.

A transparent conductive film of indium tin oxide (ITO) having a thickness of 120 nm was deposited on a glass substrate (manufactured by Geomatic Corporation; an electron beam film-formed product; sheet resistance of 15Ω) using a conventional photolithography technique and hydrochloric acid etching. Two
An anode was formed by patterning into a stripe having a width of mm.
The patterned ITO substrate was cleaned in the order of ultrasonic cleaning with acetone, water cleaning with pure water, and ultrasonic cleaning with isopropyl alcohol, dried with nitrogen blow, and finally cleaned with ultraviolet and ozone.

Next, the cleaned substrate was set 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. The 4,4′-bis [N- (1-naphthyl) -N-phenylamino] biphenyl used in Reference Example 1, which was placed in a ceramic crucible placed in the above apparatus, was heated to 230 ° C. for vapor deposition. went. 2.3x10 ^-6 Torr vacuum during deposition
(About 3.1 × 10 ⁻⁴ Pa), the deposition rate was 0.3 nm / sec, and a film having a thickness of 40 nm was formed on the anode 2 to complete the hole transport layer 4a.

Next, the fluorescent compound (1-1) of the present invention, which is provided in a ceramic crucible different from the hole transport layer forming material, is simultaneously heated and co-deposited to form the hole transport layer 4b.
Was formed with a thickness of 20 nm. At this time, the content of the compound (1-1) was 2.3% by weight in the hole transport layer 4b. Subsequently, as a material of the light emitting layer 5, an 8-hydroxyquinoline complex of aluminum represented by the following structural formula, Al (C ₉ H ₆ NO) ₃ ,

[0051]

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Was deposited in the same manner as in the hole transport layer.
At this time, the crucible temperature of the 8-hydroxyquinoline complex of aluminum was controlled in the range of 265 to 275 ° C., the degree of vacuum at the time of deposition was 2.1 × 10 ⁻⁶ Torr (about 2.8 × 10 ⁻⁴ Pa), and the deposition rate was 0.3 n.
At m / s, the thickness of the deposited luminescent layer was 75 nm. The substrate temperature during vacuum deposition of the hole transport layers 4a and 4b and the light emitting layer 5 was kept at room temperature.

Here, the element on which the light emitting layer 5 was deposited was once taken out of the vacuum deposition apparatus into the atmosphere, and a 2 mm wide stripe-shaped shadow mask was used as a cathode deposition mask, and the ITO stripe of the anode 2 was used. In close contact with the element so that it is perpendicular to the device, it is installed in another vacuum deposition apparatus and the degree of vacuum in the apparatus is 2x10 ^-6 Torr (about 2.7x
Evacuation was performed until the pressure became 10 ^-4 Pa) or less. First, as a cathode 7, 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 7.0 × 10 ⁻⁶ Torr (about
9.3 × 10 ⁻⁴ Pa) and a film thickness of 1.5 nm was formed on the light emitting layer 5. Next, the aluminum was similarly heated by a molybdenum boat, and the deposition rate was 0.5 nm / sec and the degree of vacuum was 1 × 10 ⁻⁵ Torr (approximately
An aluminum layer having a thickness of 40 nm was formed at 1.3 × 10 ⁻³ Pa).
In addition, copper is added on top to increase the conductivity of the cathode,
Similarly, heating with a molybdenum boat, deposition rate 0.3n
The cathode 6 was completed by forming a copper layer having a thickness of 40 nm at m / sec and a degree of vacuum of 1 × 10 ⁻⁵ Torr (about 1.3 × 10 ⁻³ Pa). The substrate temperature during the deposition of the three-layer cathode 6 was kept at room temperature.

As described above, an organic electroluminescent device having a light emitting area of 2 mm × 2 mm was obtained. Table 1 shows the light 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 / cm ^2.
The slope of the current density characteristic, - the value of m ^{2, and} the luminance / current intensity
The voltage shows a value at 100 cd / m ² . The maximum wavelength of the emission spectrum of the device was 535 nm.

FIG. 4 is a graph showing the voltage-luminance characteristics of this device. Low voltage has been achieved. Heat resistance test of the element
FIG. 5 is a graph showing the results of evaluation based on a decrease in luminance when driven at a high current density of 250 mA / cm ² . Example 2 A device was produced in the same manner as in Example 1 except that (2-1) was used as the fluorescent compound of the present invention contained in the hole transport layer 4b. The results of evaluating the characteristics of the device in the same manner as in Example 1 are shown in Table 1, FIG. 4, and FIG. The maximum wavelength of the emission spectrum of this device was 534 nm.

Comparative Example 1 A device was produced in the same manner as in Example 1, except that the hole transport layer did not contain the fluorescent compound of the present invention. The emission characteristics of this device are shown in Table 1, FIGS. 4 and 5. Examples 1 and 2
The driving voltage was higher and the luminance deteriorated faster than in the device of No. 1 above.

[0057]

[Table 1]

[0058]

According to the organic electroluminescent device of the present invention, a device which can be driven at a low voltage and maintains stable light emission for a long period of time can be obtained since it has a hole transport layer containing a specific fluorescent compound. be able to. Accordingly, the organic electroluminescent device according to the present invention can be used as a flat panel display (for example, for an OA computer or a wall-mounted television) or a light source utilizing the characteristics of a surface light emitter (for example, a light source of a copier, a backlight of a liquid crystal display or an instrument). It can be applied to light sources, display boards, and marker lights, and its technical value is particularly large.

[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 another example of the organic electroluminescent device.

FIG. 4 is a graph showing voltage-luminance characteristics in Examples 1, 2 and Comparative Example 1.

FIG. 5 shows driving time in Examples 1 and 2 and Comparative Example 1.
4 is a graph showing luminance characteristics.

[Explanation of symbols]

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

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C09K 11/06 690 C09K 11/06 690 H05B 33/14 H05B 33/14 B (72) Inventor Makoto Fugino 1000Kamoshida-cho, Aoba-ku, Yokohama-shi, Kanagawa-ken, Yokohama Research Laboratory, Mitsubishi Chemical Co., Ltd. AB04 AB06 AB11 BA06 CA01 CB01 DA01 DB03 EA02 EB00

Claims (7)

[Claims] An organic electroluminescent device comprising an anode, a hole transport layer, a light emitting layer, and a cathode sequentially laminated on a substrate,
The organic electric field, wherein the hole transport layer has a fluorescent compound having four or more aromatic rings (including heterocycles) of 5 to 7-membered rings and having an electron accepting group. Light emitting element. 2. The organic electroluminescent device according to claim 1, wherein a region containing the fluorescent compound having an electron accepting group in the hole transport layer is in contact with the light emitting layer. 3. The fluorescent compound having an electron-accepting group, wherein the fluorescent maximum wavelength is shorter than the fluorescent maximum wavelength of the compound forming the light emitting layer by 10 nm or more.
The organic electroluminescent device according to claim 1. 4. The organic electric field according to claim 1, wherein the electron affinity of the fluorescent compound having an electron accepting group is higher than the electron affinity of the hole transport layer forming material. Light emitting element. 5. The method according to claim 1, wherein the thickness of the region containing the fluorescent compound having an electron accepting group in the hole transport layer is in the range of 1 to 50% of the thickness of the entire hole transport layer. The organic electroluminescent device according to claim 1. 6. The compound according to claim 1, wherein the content of the fluorescent compound having an electron-accepting group in the hole transport layer is 0.1 to 50% by weight in a region containing the compound. An organic electroluminescent device according to any of the above items. 7. The method according to claim 1, wherein the electron accepting group is a cyano group, a nitro group,
The organic electroluminescent device according to any one of claims 1 to 6, wherein the organic electroluminescent device is a halogen atom or a nitrogen-containing heterocyclic group.