JP2010192719A - Organic electroluminescence element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an organic EL element adopting a substitute material for a molybdenum oxide to a charge generation layer and including a layer structure obtaining a luminescence with a higher efficiency in an MPE structure element. <P>SOLUTION: In the organic EL element, a plurality of light-emitting units 2 containing at least one layer light-emitting layer are formed between a pair of electrodes 1 and 5, and each light-emitting unit 2 is partitioned by the charge generation layer 4. In addition, the charge generation layer 4 is composed of an organic material containing a compound shown by general formula (1), and an electron injection layer 3 consisting of an alkaline metal compound is formed between the charge generation layer 4 and the light-emitting unit 2. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an organic electroluminescence element (hereinafter abbreviated as an organic EL element) having a multi-photon emission element structure.

An organic EL element is a self-luminous element that uses an organic compound as a luminescent material, and can emit light at a high speed. Therefore, it is suitable for displaying moving images, and the element structure is simple and the display panel is thinned. It has the characteristic that it is possible. Due to such excellent characteristics, organic EL elements are becoming popular in daily life as mobile phones and in-vehicle displays.
Furthermore, in recent years, it has been attracting attention as a next-generation illumination, taking advantage of the above-described thin surface light emission.

The organic EL element is required to have high efficiency and long life in order to improve practical use and spread, and a multi-photon emission (hereinafter abbreviated as MPE) element structure is proposed as one method. Has been.
Multi-photon emission is an element structure in which a plurality of light emitting units including at least one light emitting layer are stacked so as to be connected in series via a charge generation layer (CGL) (for example, a patent) Reference 1). According to such an element structure, light emission from a plurality of light emitting layers can be simultaneously obtained with the same amount of current as that of one unit element, so that the current efficiency equivalent to the number of light emitting units (hereinafter also referred to as EL-units). And external quantum efficiency can be obtained.

Conventional MPE devices generally have a layer structure such as ITO / EL-unit / Li / Al / MoO ₃ / EL-unit / Al, and molybdenum is used as part of the charge generation layer between the light emitting units. Yes.

Patent Document 2 discloses a layer containing 1,4,5,8,9,12-hexaazatriphenylenehexacarbonitrile (hereinafter abbreviated as HAT (CN) ₆ or HATCN) shown in the following (Chemical Formula 1). Although organic light-emitting elements are disclosed, they do not employ HATCN as a charge generation layer in MPE elements.

Japanese Patent No. 3933591 Japanese Patent No. 3614405

“Advanced Materials”, December 20, 2007, 2008, Vol. 20, p.324-329

  However, molybdenum oxide, which is a transition metal, has a very high melting point, which necessitates film formation at a high temperature (evaporation source temperature of 600 ° C. or higher), and adversely affects the efficiency of other organic layers such as a light-emitting layer and the process. Was giving.

  The present invention has been made to solve the above technical problem, and in the MPE element as described above, a material replacing the molybdenum oxide is used for the charge generation layer, and light emission can be obtained with higher efficiency. An object of the present invention is to provide an organic EL device having a layer structure.

  The organic EL device according to the present invention is an organic EL device having an MPE structure in which a plurality of light emitting units including at least one light emitting layer are provided between a pair of electrodes, and each of the light emitting units is partitioned by a charge generation layer. The charge generation layer is made of an organic material containing at least a compound represented by the following general formula (1), and an electron injection layer containing an alkali metal compound is provided between the charge generation layer and the light emitting unit. It is characterized by.

In the general formula (1), each R independently represents hydrogen, a halogen group, an alkyl group having 1 to 12 carbon atoms, an alkoxy group, an alkylamino group, an alkylsilyl group, an aryl group, an arylamino group, or a heterocyclic group. , A substituent selected from the group consisting of an ester group, an amide group, a nitro group and a nitrile group. Adjacent Rs may be bonded to each other to form a cyclic structure.
By using such a compound for the charge generation layer, an organic EL element that emits light with high efficiency can be configured without using molybdenum oxide.

The organic EL element according to the present invention provides an organic EL element that employs a material in place of molybdenum oxide for the charge generation layer in the MPE element structure and has a layer structure that can emit light with higher efficiency. It becomes possible to do.
Therefore, the organic electroluminescence device according to the present invention is a flat panel display for OA computers and wall-mounted televisions, which have recently been required to have better color reproducibility, as well as lighting equipment, light sources for copying machines, liquid crystal displays and instruments. Applications to light sources, display boards, and marker lamps that take advantage of the characteristics of surface light emitters, such as other types of backlight light sources, are expected.

It is the schematic sectional drawing which showed typically the layer structure of the organic EL element which concerns on this invention. It is the graph showing the relationship between the current density of each element in an Example, and current efficiency. It is the graph showing the relationship between the current density of each element in an Example, and external quantum efficiency.

Hereinafter, the present invention will be described in more detail with reference to the drawings.
In FIG. 1, the outline | summary of the layer structure of the organic EL element which concerns on this invention is shown.
As shown in FIG. 1, the organic EL device according to the present invention includes a plurality of light emitting units 2 including at least one light emitting layer between a pair of electrodes of an anode 1 and a cathode 5. This is an organic EL element having an MPE element structure partitioned by the charge generation layer 4. The charge generation layer 4 is made of an organic material including at least the compound represented by the general formula (1), and the electron injection layer 3 includes an alkali metal compound between the charge generation layer 4 and the light emitting unit 2. Is provided.
With such a configuration, the compound represented by the general formula (1) contained in at least a part of the charge generation layer is a hole transporting material (for example, N, N′-diphenyl) of an adjacent light emitting unit. -N, N′-bis (1-naphthyl) -1,1′-biphenyl-4,4′-diamine (hereinafter abbreviated as α-NPD)) and the like function as an excellent charge generation layer. Furthermore, by using an alkali metal compound (for example, LiF / Al) having a function as an electron injection layer as a layer adjacent to the other light emitting unit, a conventional charge using molybdenum oxide or the like can be obtained by combining both. It is possible to obtain light emission with high efficiency equal to or higher than that when the generation layer is used.
Further, even with an MPE element structure having a charge generation layer, it is possible to manufacture an element by a low temperature process, and to reduce damage to the organic layer during the process.

  In the organic EL device according to the present invention, the compound used in the charge generation layer partitioning each light emitting unit is 1, 4, 5, 8, 9, 12-hexaaza as represented by the general formula (1). A compound having triphenylene (hereinafter abbreviated as HAT) as a mother skeleton, and in the general formula (1), each R independently represents hydrogen, a halogen group, an alkyl group having 1 to 12 carbon atoms, an alkoxy group, The substituent is selected from the group consisting of an alkylamino group, an alkylsilyl group, an aryl group, an arylamino group, a heterocyclic group, an ester group, an amide group, a nitro group, and a nitrile group. Further, adjacent Rs may be bonded to each other to form a cyclic structure.

Among the above substituents, the halogen group means any of fluorine, chlorine, bromine and iodine.
The alkyl group refers to, for example, a saturated aliphatic hydrocarbon group such as a methyl group, an ethyl group, a propyl group, or a butyl group. The alkyl group has 1 to 12 carbon atoms and may be linear or branched. May be.
The alkoxy group represents, for example, a saturated aliphatic hydrocarbon group via an ether bond such as a methoxy group, and may be linear or branched.
The alkylamino group represents, for example, an aliphatic hydrocarbon group via a nitrogen atom such as a dimethylamino group or a diethylamino group, and may be linear or branched.
The alkylsilyl group refers to a silicon compound group such as a trimethylsilyl group.
The aryl group represents an aromatic hydrocarbon group such as a phenyl group, a naphthyl group, and an anthranyl group, and may be unsubstituted or substituted.
The arylamino group represents an aromatic hydrocarbon group via a nitrogen atom such as a diphenylamino group or a ditolylamino group, and may be unsubstituted or substituted.
The heterocyclic group represents a group containing any one of nitrogen, oxygen and oxygen as a ring constituent element in addition to carbon. For example, triazine, oxazole, oxadiazole, thiazole, thiadiazole, furan, furazane, thiophene, pyran, pyrrole, pyrazole, imidazole, imidazolidine, imidazoline, coumarone, chromene, indole, indoline, isocoumarone, cinnoline, quinazoline, quinoxaline, phthalazine Phenothiazine, acridine, phenanthridine, quinoline, isoquinoline, naphthyridine, pyridine, pyrimidine, triazole, carbazole and the like, which may be unsubstituted or substituted.

  Among the compounds represented by the general formula (1), 1,4,5,8,9,12-hexaazatriphenylenehexacarbonitrile (HATCN) represented by the above (Chemical Formula 1) in which R is a nitrile group is particularly preferable. It can be used suitably.

The alkali metal compound used for the electron injection layer is preferably at least one metal compound selected from the group of alkali metal halides and alkaline earth metal halides.
Examples of the alkali metal halide include lithium fluoride, sodium fluoride, potassium fluoride, cesium fluoride, and lithium chloride.
Examples of the alkaline earth metal halide include magnesium fluoride, calcium fluoride, barium fluoride, and strontium fluoride.

  As a specific element configuration of the organic EL element according to the present invention, for example, when there are two light emitting units, ITO / EL-unit / LiF / Al / HAT / EL-unit / Al, three light emitting units are provided. In the above case, a layer structure such as ITO / EL-unit / LiF / Al / HAT / EL-unit /... / LiF / Al / HAT / EL-unit / Al can be used.

  The layer structure of each light-emitting unit itself is not particularly limited, but includes at least one light-emitting layer, and other than that, the layer structure includes a hole transport layer and an electron transport layer. Moreover, it can also be set as the well-known layer structure also including a positive hole injection layer, a positive hole transport light emitting layer, an electron injection layer, an electron transport light emitting layer, etc. For example, one light emitting unit can have a layer configuration such as a hole injection transport layer / light emission layer / electron injection transport layer.

Materials used for the light-emitting layer, hole injection layer, hole transport layer, hole transport light-emitting layer, electron injection layer, electron transport layer, and electron transport light-emitting layer are not particularly limited. It can be appropriately selected from those used, and may be either low molecular or high molecular.
Note that, from the viewpoint of process efficiency, it is preferable to select a material that can be formed by a low-temperature process.

Each of the above layers is formed by a dry method such as a vacuum deposition method or a sputtering method, an inkjet method, a casting method, a dip coating method, a bar coating method, a blade coating method, a roll coating method, a gravure coating method, a flexographic printing method, or a spray coating. It can carry out by wet methods, such as a method. Preferably, the film is formed by vacuum deposition.
Further, the film thickness of each layer is appropriately determined depending on the situation in consideration of adaptability between the layers and the required total layer thickness, and it is usually preferably in the range of 5 nm to 5 μm. .

The electrode of the organic EL device according to the present invention is preferably one in which a transparent conductive thin film is formed on a transparent substrate.
The substrate serves as a support for the organic electroluminescence element, and when the substrate side is a light emitting surface, it is preferable to use a transparent substrate having translucency in visible light. The light transmittance is preferably 80% or more, and preferably 85% or more. More preferably, it is 90% or more.
As the transparent substrate, generally, glass substrates such as optical glass such as BK7, BaK1, and F2, quartz glass, alkali-free glass, borosilicate glass, and aluminosilicate glass, acrylic resin such as PMMA, polycarbonate, polyether sulfonate, polystyrene Polymer substrates such as polyolefins, epoxy resins, polyesters such as polyethylene terephthalate are used.
The thickness of the substrate is usually about 0.1 to 10 mm, but preferably 0.3 to 5 mm in view of mechanical strength, weight, etc., and 0.5 to 2 mm. More preferably.

An anode is usually formed on the substrate. The anode is composed of a metal, alloy, conductive compound or the like having a high work function (4 eV or more), and is preferably formed as a transparent electrode on the transparent substrate.
For the transparent electrode, metal oxides such as indium tin oxide (ITO), indium zinc oxide, and zinc oxide are generally used. In particular, ITO is preferably used from the viewpoint of transparency and conductivity.
The film thickness of the transparent electrode is preferably 80 to 400 nm, and more preferably 100 to 200 nm, in order to ensure transparency and conductivity.
The anode is usually formed by a sputtering method, a vacuum deposition method or the like, and is preferably formed as a transparent conductive thin film.

On the other hand, the cathode facing the anode is made of a metal, alloy, or conductive compound having a small work function (4 eV or less). For example, aluminum, an aluminum-lithium alloy, a magnesium-silver alloy, lithium fluoride, and the like can be given. A single layer may be used, or a multilayer may be formed by combining materials having different work functions.
The film thickness of the cathode is preferably 10 to 500 nm, and more preferably 50 to 200 nm.

EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example, this invention is not restrict | limited by the following Example.
[Comparative Example 1]
(Production of 1 unit element)
As an electron transport material, bis 4,6- [3,5- (di-4-pyridyl) phenyl] -2-methylpyrimidine (hereinafter abbreviated as B4PYMPM) shown in the following (Chemical Formula 3) is used. One unit element having a layer configuration of TAPC (70 nm) / TCTA: Ir (ppy) ₃ (5 nm) / CBP: Ir (ppy) ₃ (5 nm) / B4PYMPM (70 nm) / LiF (1 nm) / Al (100 nm) This was prepared for comparison.
The structural formulas of TAPC (Chemical Formula 4), TCTA (Chemical Formula 5), and Ir (ppy) ₃ (Chemical Formula 6) are shown below.

A specific method for manufacturing the element is as follows.
First, a glass substrate on which a patterned transparent conductive film (ITO) is formed to a thickness of 130 nm is subjected to ultrasonic cleaning with pure water and a surfactant, flowing water cleaning with pure water, and 1: 1 of pure water and isopropyl alcohol. Washing was performed in the order of ultrasonic washing with the mixed solution and boiling washing with isopropyl alcohol. The substrate was slowly pulled up from the boiling isopropyl alcohol, dried in isopropyl alcohol vapor, and finally subjected to ultraviolet ozone cleaning.
This substrate is used as the anode 1, placed in a vacuum chamber, evacuated to 1 × 10 ⁻⁶ Torr, and each molybdenum boat filled with a vapor deposition material and a predetermined pattern are formed in the vacuum chamber. A vapor deposition mask was installed, and the boat was energized and heated to evaporate the vapor deposition material, so that the layers were sequentially formed to have the above layer structure.

Finally, while maintaining the vacuum chamber in a vacuum, the mask was exchanged and a mask for cathode deposition was installed to form a two-layer cathode composed of a lithium fluoride (LiF) layer and an aluminum (Al) layer.
Then, the vacuum chamber is returned to atmospheric pressure, the substrate on which each layer is deposited as described above is taken out, transferred to a nitrogen-substituted glove box, sealed with another glass plate using a UV curable resin, and one unit element is Obtained.

[Example 1]
(Production of 2-unit MPE element)
The part obtained by removing the electrode from the one unit element is one light emitting unit, and the charge generation layer (CGL) is composed of HATCN and the first layer of TAPC in the light emitting unit. Units are stacked, ITO (130 nm) / EL-unit (150 nm) / LiF (1 nm) / Al (1 nm) / HATCN (5 nm) / EL-unit (150 nm) / LiF (1 nm) / Al (100 nm) layer An MPE element having two light emitting units having the configuration was produced.
The film formation of each layer was sequentially performed in the same manner as in the production of the one-unit element.

[Example 2]
(Production of 3 unit MPE element)
In the same manner as in Example 1, ITO (130 nm) / EL-unit (150 nm) / LiF (1 nm) / Al (1 nm) / HATCN (5 nm) / EL-unit (150 nm) / LiF (1 nm) / Al (1 nm) ) / HATCN (5 nm) / EL-unit (150 nm) / LiF (1 nm) / MP (Equipment) having three light emitting units having a layer structure of Al (100 nm) was produced.

(Element evaluation)
Each of the devices prepared above was subjected to green phosphorescence emission when a DC voltage of 10 V was applied.
Moreover, the current efficiency and the external quantum efficiency were determined for each element.
FIG. 2 is a graph showing the relationship between the current density and the current efficiency of 1 unit element (Comparative Example 1), 2 unit MPE element (Example 1), and 3 unit MPE element (Example 2).
FIG. 3 is a graph showing the relationship between the current density of each element and the external quantum efficiency.

For one unit element (Comparative Example 1), a current efficiency of 80 cd / A and an external quantum efficiency of 22.5% were obtained at 100 cd / m ² .
The 2-unit MPE element (Example 1) has a current efficiency of 100 cd / m ² when considering the film thickness so that the light emission from Ir (ppy) ₃ in each light-emitting unit is intensified by optical interference. The light emission efficiency was 170 cd / A, the external quantum efficiency was 46%, and the light emission efficiency was twice or more that of the one unit element.
Further, the 3-unit MPE element (Example 2) also has a thickness of 100 cd / m ² when considering the film thickness so that the light emission from Ir (ppy) ₃ in each light-emitting unit is intensified by optical interference. The current efficiency was 250 cd / A, the external quantum efficiency was 64%, and the light emission efficiency about three times that of the one unit element was obtained.

DESCRIPTION OF SYMBOLS 1 Anode 2 Light emitting unit 3 Electron injection layer 4 Charge generation layer 5 Cathode

Claims (1)

A plurality of light emitting units each including at least one light emitting layer between a pair of electrodes, wherein each light emitting unit is an organic electroluminescence element having a multi-photon emission structure partitioned by a charge generating layer, wherein the charge generating layer Is composed of an organic material containing at least a compound represented by the following general formula (1), and an electron injection layer containing an alkali metal compound is provided between the charge generation layer and the light emitting unit. Organic electroluminescence device.

(In formula (1), each R is independently hydrogen, halogen group, alkyl group having 1 to 12 carbon atoms, alkoxy group, alkylamino group, alkylsilyl group, aryl group, arylamino group, heterocyclic group, (This is a substituent selected from the group consisting of an ester group, an amide group, a nitro group and a nitrile group. Adjacent Rs may be bonded to each other to form a cyclic structure.)

Images