JPH1095971A - Organic electroluminescent element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the subject element capable of exhibiting an excellent hole transport performance between a cathode and a hole transport layer and an excellent endurance, and useful as a flat panel display by providing a hole injection layer between the cathode and the hole transport layer. SOLUTION: This organic electroluminescent element holding by nipping an organic electric field light emitting layer containing a hole transport layer between a cathode and an anode, is obtained by providing a hole injection layer e.g. a layer consisting of a compound of the formula (M is a metal, preferably Ge, Sn, Pb, As and At) or a halogen salt thereof and having 0.5-10nm layer thickness. Thus, it is possible to improve the close adherence in a boundary of the cathode and the hole transport layer for improving a hole injection efficiency, and also prevent a diffusion of oxygen and an adsorbed water from the cathode into the hole transport layer on heat generation by the element, for capable of preventing a deterioration of a hole transport material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electroluminescent device including a light emitting material which emits light by current injection and an organic hole transporting material as its constituent elements, and more particularly, means for injecting holes from an anode to a hole transporting layer. The present invention relates to an organic electroluminescent device having the following characteristics.

[0002]

2. Description of the Related Art Conventionally, a cathode ray tube has been most widely used as an image display for an information communication terminal device such as a computer and a television, which has features of high luminance and good color reproducibility, but is bulky. It is heavy and has large power consumption. Therefore, there is a great demand for a lightweight, thin, and highly efficient flat panel display. Currently, the most frequently used flat panel display is an active matrix drive type liquid crystal display.However, it has a narrow viewing angle, and when using a backlight because it is not self-luminous, the power consumption of this backlight is large. There is a problem that it does not have a sufficient response to a high-definition and high-speed video signal which is expected to be put to practical use, and there are problems such as uniformity and high cost in manufacturing a large screen display. There is a possibility of light emitting diodes as a candidate for a flat panel display instead of a liquid crystal display, but it is difficult to manufacture a light emitting diode matrix on a single substrate with a large area, and it has not become a low-cost display to replace a cathode ray tube .

Recently, an organic electroluminescent device has been attracting attention as a flat panel display having a possibility of solving these problems. This is advantageous in that self-luminous light has a high response speed and has no viewing angle dependence.

An organic electroluminescent device using an organic luminescent material is one in which an organic electroluminescent layer containing an organic luminescent material is sandwiched between a translucent anode and a metal cathode. CWTang
and SAVanSlyke et al. made the organic electroluminescent layer a two-layer structure consisting of a hole transport layer and an electron transport layer, and developed a device structure that emits light when holes and electrons injected from the anode and cathode into the organic electroluminescent layer recombine. First reported (Appl. Phys.
Lett., 51 (12), 913-915 (Sept. 1987)). In this device structure, either the hole transport layer or the electron transport layer also functions as the light emitting layer. Light emission occurs in a wavelength band corresponding to the energy gap between the ground state and the excited state of the light emitting material. By thus forming the organic electroluminescent layer in a two-layer structure, a drastic reduction in driving voltage and an improvement in luminous efficiency are achieved.
Since then, research aimed at application to all solid-state flat panel displays and the like has been advanced. Various metal complexes such as a zinc complex and an aluminum complex have been proposed as light emitting materials for obtaining high luminous efficiency.

[0005] Regarding the device structure, C. Adachi, T.
Tsutsui and S. Saito et al. Described an example of a three-layer structure consisting of a hole transport layer, a light-emitting layer, and an electron transport layer. Jap. J. of App
l. Phys. 27 -2, reported in L269-L271 (1988). Further, a light emitting material is included in the electron transport layer, and a two-layer structure of an electron transport layer also serving as a light emitting layer and a hole transport layer is described in CWTang, S. et al.
J. of Appl. Phy by A. VanSlyke and CHChen et al.
s. 65 -9, 3610-3616 (1989). From these reports, the possibility of high-brightness light emission at a low voltage has been verified, and research and development of organic electroluminescent devices have been very actively performed in recent years.

However, for practical use of the organic EL device, there are some problems to be solved, such as light emission luminance and durability. In order to realize an organic electroluminescent device having high emission luminance and excellent stability over time, it is necessary to develop a durable device structure having an excellent hole transporting ability.

In an early stage of research on an organic electroluminescent device, a TPD represented by the following formula (2) was used as a hole transporting material.
(N, N'-diphenyl-N, N'-bis (3-methylphenyl) -1,1'-bip
Henyl-4,4'-diamine has been used. However,
Since TPD has a relatively low melting point of about 170 ° C. and a glass transition point of about 60 ° C., when used as a hole transporting material for an organic electroluminescent device, reactive current consumed during light emission driving other than light emission is converted into heat. As a result, a non-emission defect may occur with an increase in the element temperature, or in a severe case, the hole transport layer may be melted to stop the light emission.

[0008]

Embedded image

To solve these inconveniences, equation (2)
The N substituent of TPD represented by N, N'-naphthyl phenyl
(US Pat. No. 5,615,569), and a bipheny group located at the center of TPD represented by the formula (2).
Compound having a naphthalene group (for example, JP-A-8-87122)
Publication), a compound having an anthracene group (for example,
8-53397) or a compound having a phenanthlene group (for example, JP-A-8-20770, JP-A-8-20771).
Publication) and the like.

[0010]

However, none of the compounds has been found to be a material having satisfactory hole transport performance. That is, in addition to the performance improvement of the hole transporting material, it is necessary to enhance the overall hole transporting capacity in consideration of the high efficiency of the hole injection at the stage of the hole injection from the anode to the hole transporting layer. The present invention has been made in view of the above technical background, and provides an organic electroluminescent device having excellent hole transport performance and durability between an anode and a hole transport layer, and aims to further improve the performance of the organic electroluminescent device. Is the subject.

[0011]

SUMMARY OF THE INVENTION The present invention proposes to achieve the above object. That is, the organic electroluminescent device of the present invention is an organic electroluminescent device having a structure in which an organic electroluminescent layer including a hole transport layer is sandwiched between an anode and a cathode, and between the anode and the hole transport layer. Further, it has a hole injection layer.

[0012] A preferred element structure of the organic electroluminescent device of the present invention is such that an organic electroluminescent layer comprising a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, and a cathode are provided on an anode. , In that order. The electron transport layer may double as the light emitting layer. Also, a fluorescent dye may be contained in the organic electroluminescent layer. By employing such a layer structure, an electroluminescent element having high luminance and excellent durability can be obtained.

The hole injection layer according to the present invention is characterized by having at least one kind of a tetraphenyl metal compound represented by the following general formula (1).

[0014]

Embedded image In the general formula (1), M represents a metal atom. Preferred metal atoms include Ge, Sn, Pb, As, At, and the like. Further, these tetraphenyl metal compounds may be salts of halogen such as chlorine and bromine.

In the present invention, the thickness of the hole injection layer is 1
It is desirable that the thickness be 0 nm or less. The lower limit of the thickness of the hole injection layer is not particularly limited as long as it is formed as a uniform continuous film, but from the viewpoint of controlling the film thickness in a film forming apparatus, the thickness is preferably one molecular layer or more, and practically 0.5 nm or more. desirable.

In the organic electroluminescent device of the present invention, an extremely thin hole injecting layer is newly provided between the anode and the hole transporting layer, so that the interface between the anode made of a transparent conductive material such as ITO and the hole transporting layer is provided. And the hole injection efficiency is improved. The hole injection layer also functions as a barrier layer for preventing diffusion of oxygen or adsorbed water from a transparent conductive material such as ITO into the hole transport layer at the time of heat generation of the element, thereby preventing deterioration of the hole transport material. Further, although this is an estimation stage, a tetraphenyl metal compound layer having an energy level intermediate between the energy level of a metal oxide such as ITO and the energy level of a hole transport material that is an organic substance is inserted. This is thought to contribute to the improvement of the hole injection efficiency also from this aspect. in this way,
Since the hole injection layer controls the state of the interface between the anode and the hole transport layer, a thin film having a thickness of 10 nm or less is sufficient. Since the hole transport efficiency is improved by these comprehensive actions, an organic electroluminescent device having high luminance and excellent durability can be manufactured. Further, this tetraphenyl metal compound is sublimable, and it is possible to easily form a dense film having a controlled film thickness by a means such as a molecular beam evaporation method.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail with reference to the drawings. First, FIG. 1 shows a device configuration in which the organic electroluminescent device of the present invention is applied to an EL device.
A description will be given with reference to schematic cross-sectional views shown in (a) to (d).
Among them, FIG. 1A shows the device configuration of a transmission type organic electroluminescent device, and FIG. 1B shows the device configuration of a reflection type organic electroluminescent device. FIGS. 1C and 1D are enlarged cross-sectional views of the organic electroluminescent layer 3. In any of the element structures, reference numeral 1 denotes a substrate made of a transparent material such as glass or plastics, or another appropriate material. When the organic electroluminescent element is used in combination with another display element, a driving circuit, or the like, the substrate 1 can be shared. Sign 2
Denotes an anode, and in addition to ITO (Indium Tin Oxide) and SnO ₂ , Sb-containing SnO ₂ , Al-containing ZnO or Au
It is made of a transparent conductive material such as a thin film. Also polythiophene,
A conductive polymer thin film such as polypyrrole may be used. The electric resistance of the anode is preferably low in order to reduce power consumption and heat generation of the element. The method for forming the anode is not particularly limited, and a vacuum deposition method using an electron beam or the like, a sputtering method, a CVD method, or the like can be appropriately employed.

Reference numeral 11 denotes a characteristic portion of the organic electroluminescent device of the present invention, which is a hole injection layer inserted between the anode 2 and the hole transport layer 6. The hole injection layer 11 is composed of one or a mixture of the tetraphenyl metal compounds represented by the above-mentioned general formula (1), and is formed by a vacuum deposition method using resistance heating, an ion plating method, a molecular beam deposition method, or the like. In addition to film forming methods using vacuum techniques such as molecular beam epitaxy, chemical modification, spin coating, or LB (L
angmuir-Brodjet method or the like can be used. Since the hole injection layer 11 is an extremely thin film having a thickness of 10 nm or less, it is desirable to adopt a film formation method having good film thickness controllability.

Reference numeral 3 denotes an organic electroluminescent layer, the structure of which will be described later.

Reference numeral 4 denotes a cathode, and as an electrode material, for example, an active metal having a low work function such as Li, Mg or Ca;
An alloy with Ag, Al, In or the like or a laminated structure can be adopted. The film formation method is not particularly limited, and a vacuum evaporation method such as resistance heating, an ion plating method, a sputtering method, or the like can be employed. In the case of the transmission type organic electroluminescent device shown in FIG. 1A, by adjusting the thickness of the cathode 4, a light transmittance suitable for the intended use can be obtained. To complement the conductivity of the cathode 4, an additional IT
A transparent conductive film 4a such as O or SnO ₂ may be laminated and used. Reference numeral 5 denotes a protective layer, and any material can be adopted as long as the material satisfies airtightness, regardless of an organic material such as plastics or an inorganic material such as SiO ₂ .

As the basic structure of the organic electroluminescent layer 3, any conventionally proposed structure can be adopted as long as it can obtain organic electroluminescence. That is, as shown in FIG. 1C, in addition to the three-layer structure in which the anode 2 / hole transport layer 6 / light emitting layer 7 / electron transport layer 8 / cathode 4 are laminated in this order, the hole transport layer 6 and the electron transport layer In the case where any one of the light-emitting layers 8 has a light-emitting property, the light-emitting layer 7 is also used as these layers, and as shown in FIG.
It is also possible to have a two-layer structure of / electron transport layer 8 / cathode 4.

The hole transport layer 6 is made of a hole transport material alone,
Alternatively, it is formed by uniformly dispersing a hole transport material in a matrix such as an organic polymer. Examples of the hole transport material include TPD shown in the formula (2), a compound in which the biphenyl skeleton is substituted with a condensed ring, a tertiary amine such as N-isopropylcarbazole, a pyrazoline derivative, a stilbene compound, and a hydrazone compound. In the case of a heterocyclic compound represented by an oxadiazole derivative or a phthalocyanine derivative, or a polymer, a polycarbonate derivative, a polystyrene derivative, polyvinyl carbazole, or polysilane having these monomers in a side chain can be preferably used, but is not particularly limited.

The light emitting material used for the light emitting layer is Tris- (8-hydroxyquinoline) -alumini represented by the following formula (3).
um (hereinafter abbreviated as Alq), anthracene, pyrene, bisstyrylanthracene derivative, tetraphenylbutadiene derivative, coumarin derivative, oxadiazole derivative, distyrylbenzene derivative, pyrrolopyridine derivative, perinone derivative, cyclopentadiene derivative , An oxadiazole derivative, a thiadiazolopyridine derivative, and a polymer system such as a polyphenylenevinylene derivative, a polyparaphenylene derivative, and a polythiophene derivative, but are not limited thereto.
Further, rubrene, a quinacridone derivative, DCM, perinone, perylene, coumarin, or the like may be used as a dopant added to the light emitting layer.

[0024]

Embedded image

The electron transporting material used for the electron transporting layer may be a substance having a high electron injection efficiency and a high electron transporting efficiency. For this purpose, the electron transporting material has a high electron affinity and a high electron mobility and a high stability. It is desirable that the material does not generate impurities during light emission. Examples of such a material include Alq represented by the above-described formula (3), but other materials may be used.

Such various organic light emitting materials are formed by sequentially laminating the materials themselves, but may be dispersed and laminated in a polymer polymer and sandwiched between an anode and a cathode. Examples of the polymer include, but are not limited to, polyvinyl chloride, polycarbonate, and polystyrene. The method for forming each of these layers contributing to light emission is preferably a dry film forming method such as a vapor deposition method using resistance heating or an electron beam, an ion plating method, a molecular beam vapor deposition method, a molecular beam epitaxy method, and a sputtering method. It is also possible to use a wet film forming method such as a chemical modification method, a spin coating method, and an LB method.

In order to improve the charge transport performance of holes or electrons, one or both of the hole transport layer 6 and the electron transport layer 8 have a structure in which a plurality of types of materials are laminated.
Alternatively, a structure in which a plurality of types of materials are mixed may be used.
Further, in order to improve the light emitting performance, any one or more of the hole transport layer 6, the light emitting layer 7, and the electron transport layer 8 may contain a fluorescent material. The fluorescent material is not particularly limited, and examples thereof include quinacridone and DCM (4-dicyanomethylene-6- (p-dimethylaminostyryl) -2-methyl-4H-pyran) represented by the following structural formula (4). Is done. In these cases,
In order to further improve the luminous efficiency, a thin film for controlling the transport of holes or electrons can be included in the layer structure.

[0028]

Embedded image

In the EL device shown in FIG.
By applying a DC voltage between the anode 2 and the cathode 4,
The holes injected from the anode 2 pass through the hole transport layer 6,
In addition, electrons injected from the cathode 4 pass through the electron transport layer 8,
Each reaches the light emitting layer 7. As a result, electron / hole recombination occurs in the light emitting layer 7 to generate singlet excitons, and the singlet excitons emit light of a predetermined wavelength. In the case of the layer configuration in which the light emitting layer shown in FIG. 1D is omitted, light of a predetermined wavelength is generated from the interface between the hole transport layer 6 and the electron transport layer 8. These light emissions are observed from the substrate 1 side. In the case of the transmission type EL element shown in FIG. 1A, light emission is also observed from the protective layer 5 side.

The current applied to the organic electroluminescent device is usually a direct current, but a pulse current or an alternating current may be used. The current value and the voltage value are not particularly limited as long as they are within a range that does not cause element destruction. However, in consideration of power consumption and life of the organic electroluminescent element, it is desirable to emit light efficiently with as little electric energy as possible.

A specific example in which the light emitting device of the present invention is applied to an actual organic EL device is shown in a schematic perspective view of FIG. E in FIG.
The L element is obtained by disposing a stacked body including the hole transport layer 6 and at least one of the light emitting layer 7 and the electron transport layer 8 between the cathode 4 and the anode 2. Cathode 4 and anode 2
Are patterned so as to be striped and orthogonal to each other in a matrix, and are configured to apply signal voltages in a time series by the control circuits 9 and 10 having a built-in shift register, and emit light at the crossing positions. . The EL element having such a configuration can be used not only as a display for characters and symbols, but also as an image reproducing device.
Also, it is possible to configure a multi-color or full-color all-solid-state flat panel display by arranging stripe patterns of the cathode 4 and the anode 2 for each color of red (R), green (G), and blue (B). Become.

Hereinafter, the organic electroluminescent device of the present invention will be described.
A detailed description will be given while appropriately adding comparative examples.

Example 1 In this example, an organic electroluminescent device was manufactured by employing tetraphenylgermanium (TPGe) among the tetraphenylmetal compounds of the general formula (1) as a material for a hole injection layer.

An anode made of ITO having a thickness of 100 nm was formed on one surface of a 30 mm × 3
A 0 mm glass substrate was set. As a vapor deposition mask, a plurality of metal masks each having a unit opening of 2.0 mm × 2.0 mm are arranged close to the substrate, and TPGe is, for example, 1 nm under an ultra-high vacuum of 10 ⁻⁷ Pa or less by a molecular beam vapor deposition method. To form a hole injection layer. The film formation rate was set to 0.1 by a film thickness monitor using a quartz oscillator.
nm / sec or less. The reason why the molecular beam evaporation method was used to form the hole injection layer is that the thickness control in the formation of an ultrathin film is easier than in the case of the ordinary resistance heating evaporation method or the like. Next, the substrate on which the hole injection layer was formed was transported to a resistance heating type vacuum evaporation apparatus, and 25 cm above the evaporation source.
Setting. As a vapor deposition mask, a metal mask having a plurality of 2.0 mm × 2.0 mm unit openings is also arranged close to the substrate, and 10 μm is formed by a vacuum vapor deposition method.
A TPD represented by the above formula (2) was formed as a hole transport layer to a thickness of, for example, 50 nm under a vacuum of ^-4 Pa or less. The deposition rate was 2 nm / sec. Further, as a material which also functions as a light emitting layer and an electron transport layer, the above-mentioned formula (3)
Was deposited in contact with the hole transport layer. A
The layer thickness of lq is, for example, 50 nm, and the deposition rate is 2 nm.
/ Sec. As a cathode material, a laminated film of Mg and Ag is adopted, and the deposition rate is also 4 nm / s by vapor deposition.
ec is, for example, 50 nm (Mg) and 150 nm
(Ag) to form a basic form of the organic electroluminescent device according to Example 1.

Evaluation of Light Emitting Characteristics Light emitting characteristics were evaluated by applying a forward bias DC voltage under a nitrogen atmosphere to the organic electroluminescent device of Example 1 thus manufactured. It emitted a green light, which was found by spectrometry to have a peak at 540 nm in FIG. For the spectroscopic measurement, a spectroscope using a photodiode array made by Otsuka Electronics as a detector was used. The spectrum in FIG. 3 matched the emission spectrum of Alq, and it was confirmed that the emission of the EL element was due to Alq. When the applied voltage was gradually increased and the luminance was measured by a luminance meter, a luminance of 1000 cd / m ² was obtained at an applied voltage of 9 V. After manufacturing this organic electroluminescent device, it was left indoors for one month, but no device deterioration was observed. The applied voltage 1
When 0 V was continuously applied and continuous light emission was performed to forcibly deteriorate the device, the device life until the light emission completely disappeared was 10 days.

Example 2 In this example, tetraphenyltin (TPS) was used instead of TPGe used as the hole injection layer material in the previous example 1.
Except that n) was adopted, an organic electroluminescent device was manufactured in accordance with Example 1 with respect to the layer configuration and the film forming method of each organic electroluminescent layer.

The organic electroluminescent device of the second embodiment is also the same as that of the first embodiment.
And emitted the same green light emission. As a result of the spectroscopic measurement, the spectrum was consistent with the spectrum of the organic electroluminescent device of Example 1, and it was confirmed that the spectrum was due to the emission of Alq.
When the applied voltage was gradually increased and the luminance was measured by a luminance meter, a luminance of 1500 cd / m ² was obtained at an applied voltage of 10 V. After producing this organic electroluminescent device, 1
Although left for a month, no element deterioration was observed.

Embodiment 3 In this embodiment, tetraphenyl lead (TPPb) is used instead of TPGe used as the hole injection layer material in the previous embodiment 1.
Except for adopting the above, an organic electroluminescent device was produced in accordance with the same manner as in Example 1 except for the layer configuration and the film forming method of each organic electroluminescent layer.

The organic electroluminescent device of the third embodiment is the same as that of the first embodiment.
And emitted the same green light emission. As a result of the spectroscopic measurement, the spectrum was consistent with the spectrum of the organic electroluminescent device of Example 1, and it was confirmed that the spectrum was due to the emission of Alq.
When the applied voltage was gradually increased and the luminance was measured by a luminance meter, a luminance of 1200 cd / m ² was obtained at an applied voltage of 10 V. After producing this organic electroluminescent device, 1
Although left for a month, no element deterioration was observed.

COMPARATIVE EXAMPLE For comparison, the layer structure of each organic electroluminescent layer and the film forming method were the same as those in Example 1 except that a hole injection layer of a tetraphenyl metal compound was not formed between the anode and the hole transport layer. An organic electroluminescent device was produced according to the standard.

The organic electroluminescent device of the comparative example also emitted green light similar to that of Example 1. As a result of the spectroscopic measurement, the spectrum matched the spectrum of the organic electroluminescent device of Example 1,
It was confirmed that this was due to the emission of Alq. However, the power consumption for obtaining the same brightness as the organic electroluminescent device of Example 1 was increased by about 32%, and the device life was also reduced from 10 days of the organic electroluminescent device of Example 1 to 2 days.

Although the organic electroluminescent device of the present invention has been described in detail, the present invention is not limited to these embodiments. For example, in addition to TPGe, TPSn, and TPPb described in the examples as the tetraphenyl metal compound used as the hole injection layer material, various metal compounds and their halogen salts may be used. As the hole transporting material of the hole transporting layer or the electron transporting material of the electron transporting layer, other than the compounds of the examples, other conventionally known hole transporting materials or electron transporting materials may be used. In addition, as for each layer configuration and electrode structure of the organic electroluminescent layer, any of conventionally known structures can be adopted.

[0043]

As apparent from the above description, according to the present invention, the hole transport performance of the organic electroluminescent device is improved, and a stable organic electroluminescent device can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing an element configuration in which an organic electroluminescent element is applied to an EL element.

FIG. 2 is a schematic perspective view showing a device configuration in which an organic electroluminescent device is applied to an actual EL device.

FIG. 3 is an emission spectrum of the organic electroluminescent device of Example 1.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Substrate, 2 ... Anode, 3 ... Organic electroluminescent layer, 4 ... Cathode,
4a: transparent conductive film, 5: protective layer, 6: hole transport layer, 7
... Emission layer, 8 ... Electron transport layer, 9,10 ... Control circuit, 11
… Hole injection layer

Claims (7)

[Claims] 1. An organic electroluminescent device having a structure in which an organic electroluminescent layer including a hole transport layer is sandwiched between an anode and a cathode, further comprising a hole injection layer between the anode and the hole transport layer. An organic electroluminescent device comprising: 2. An organic electroluminescent layer comprising a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, and a cathode, which are sequentially stacked on the anode in this order. The organic electroluminescent device according to claim 1, wherein: 3. An organic electroluminescent layer comprising a hole injection layer, a hole transport layer, an electron transport layer, and a cathode, which are sequentially stacked on the anode in this order. Item 2. An organic electroluminescent device according to Item 1. 4. The hole injection layer according to the following general formula (1)
The organic electroluminescent device according to claim 1, comprising at least one kind of a tetraphenyl metal compound represented by the following formula: Embedded image (In the formula, M represents a metal atom.) 5. The organic electroluminescent device according to claim 1, wherein the thickness of the hole injection layer is 10 nm or less. 6. The organic electroluminescent device according to claim 1, wherein the organic electroluminescent layer further contains a fluorescent dye. 7. The organic electroluminescent device according to claim 1, wherein said organic electroluminescent device is an electroluminescent device.
The organic electroluminescent device according to claim 1.