JP2000286055A - El element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an element of low driving voltage and high luminous efficiency by comprising a hole injection layer between an EL layer and an anode, and including a material of less than a predetermined value of the ionization potential and a material of more than a predetermined value of ionization potential in the hole injection layer. SOLUTION: A hole injection layer includes a material with a ionization potential of 5.50 eV or below and a material with an ionization potential of 5.60 eV or above. The material with an ionization potential of 5.50 eV or below is a hole transporting material (for example, oxadiazol, oxazol, triazol and the like), and a material with an ionization potential of 5.60 eV or above is a charge generating material (for example, a-Se-based material, a-Si-based material, polyvinyl carbazol, poly-N-ethylene unsaturated group substituted- phenothiazines and the like), and the injection barrier of the hole from the electrode can be lowered by also using a photoconductive material, and utilizing the light emitted from the EL layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an EL device (electroluminescence device) used for a display or the like.

[0002]

2. Description of the Related Art The structure of an EL element is such that a light emitting layer containing at least an organic fluorescent substance and a charge transporting agent, and a hole transporting layer and an electron transporting layer, etc., are formed between two opposing electrodes in a single layer. Alternatively, it has a multilayer structure. When a voltage is applied between the two electrodes, holes and electrons are injected into the light emitting layer from the anode and the cathode, respectively, and are recombined to excite the fluorescent substance, thereby deactivating the exciton. The EL element emits light due to the fluorescence.

[0003] The organic EL element has a major feature that it can emit light with a simple element structure, and that a light-weight and low-cost display can be easily manufactured. In recent years, application to the display has been actively studied. . In particular, displays for displaying moving images have been actively developed.

In such an organic EL device, there is known a technique of providing a hole injection layer for improving hole injection efficiency from an electrode in order to increase luminous efficiency. In this case, generally, as the material used for the hole injection layer, a material having an ionization potential larger than the work function of the electrode and smaller than the ionization potential of the material used for the hole transport layer which is a part of the EL layer is selected. Thereby, the injection barrier from the electrode to the organic layer is relaxed, and the hole injection efficiency is increased.
However, in this technique, the hole injection barrier between the electrode / hole transport layer is only divided into two interfaces of the electrode / hole injection layer and the hole injection layer / hole transport layer. Since the barrier that must be overcome before holes injected from the hole are injected into the hole transport layer is virtually unchanged,
The improvement in injection efficiency was insufficient.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide an EL device having a lower driving voltage and a higher luminous efficiency as compared with a conventional EL device by reducing the injection barrier of holes from the electrodes. Is to provide.

[0006]

Means for Solving the Problems The present inventor has proposed that a combination of a compound having an ionization potential of 5.50 eV or less and a compound having an ionization potential of 5.60 eV or more is added to the hole injection layer so that holes can be removed from the electrode. The present inventors have found that the barrier for injecting holes into the transport layer is remarkably reduced, and the luminous efficiency of the EL layer can be improved, thereby completing the present invention.

Therefore, an EL device according to the present invention is an EL device having an anode, a cathode, and an EL layer between the anode and the cathode, wherein the EL device is provided between the EL layer and the anode. And a hole injection layer, wherein the hole injection layer contains at least a substance having an ionization potential of 5.50 eV or less and a substance having an ionization potential of 5.60 eV or more. It is.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.

[0009] Structure Figure 1 of the EL element, showing a cross-sectional view of one embodiment of a device of the EL device of the present invention. In the figure, 1 is a glass substrate, 2 is an anode electrode, 3 is a hole injection layer, 4 is an organic EL layer, and 2 'is a cathode electrode.

The organic EL layer 4 shown in FIG. 1 may be a single layer, or may have a multilayer structure by combining a hole transport layer, a light emitting layer, an electron injection layer and the like.

Hole injection layer The hole injection layer is provided between the anode electrode and the EL layer, and is a layer that promotes injection of holes from the electrode. The hole injection layer of the present invention has an ionization potential of 5.50 e.
V or less and the ionization potential is 5.60 eV
It contains the above substances, and preferably these are a hole transport material and a charge generation material, respectively. In the hole injection layer of the present invention, if necessary, a binder resin, an electron-accepting substance,
A sensitizing dye for shifting the photoconductive spectral sensitivity, a light stabilizer, an antioxidant, and the like may be mixed. Further, it is preferable that the hole injection layer has photoconductivity.

The mechanism by which the hole injection barrier is lowered is not clear, but when a compound having an ionization potential of 5.50 eV or less and a compound having an ionization potential of 5.60 eV or more are mixed in the hole injection layer, these compounds are obtained. It is considered that electronic interaction occurs between the compounds and the hole injection barrier is reduced. Such interactions include:
For example, a decrease in conductivity due to the formation of a charge transfer complex or the like, or a decrease in interface barrier due to trapping of space charges near the electrode interface can be considered.

Although the substance having an ionization potential of 5.50 eV or less used for the hole injection layer of the present invention is not particularly limited, it is preferably a hole transporting material. Specifically, for example, oxadiazole, oxazole, triazole, thiazole, triphenylmethane, styryl, pyrazoline, hydrazone,
Aromatic amine, carbazole, polyvinyl carbazole, stilbene, enamine, azine, triphenylamine, butadiene, polycyclic aromatic compound,
Examples include substances such as stilbene dimers.

The π-conjugated polymers among the hole transporting agents include polyacetylene, polydiacetylene, poly (P-phenylene), poly (P-phenylene sulfide), poly (P-phenylene oxide) and poly (P-phenylene oxide). 1,6
-Heptadiyne), poly (P-phenylenevinylene),
Poly (2,5 thienylene), poly (2,5-pyrrole), poly (m-phenylene sulfide), poly (4
4'-biphenylene) and the like.

[0015] Among the hole transferring material, a charge transfer polymer complex, polystyrene · AgCl0 _4, polyvinyl naphthalene · TCNE, polyvinylnaphthalene · P-
CA, polyphenylnaphthalene / DDQ, polyvinyl mesitylene / TCNE, polynaphthacetylene / TCNE,
Polyvinyl anthracene / Br ₂ , polyvinyl anthracene / I ₂ , polyvinyl anthracene / TNB, polydimethylaminostyrene / CA, polyvinyl imidazole / CQ, poly P-phenylene I ₂ / poly-1-vinyl pyridine / I ₂ , Poly-4-vinylpyridine.I ₂ , poly-
P-1-phenylene I ₂ , polyvinyl pyridium T
CNQ and the like. Examples of the low-molecular charge transfer complex include TCNQ-TTF, and examples of the metal complex polymer include polycopper phthalocyanine.

As the charge transporting substance, a material having a small ionization potential is preferable, and a butadiene type, an enamine type, a hydrazone type and a triphenylamine type are particularly preferable.

The substance having an ionization potential of 5.60 eV or more used for the hole injection layer of the present invention is not particularly limited, but is preferably a charge generation material.
Specifically, a-Se-based materials, a-Si-based materials, polyvinyl carbazole (PVK), poly-N-ethylenically unsaturated group-substituted phenothiazines, polyvinyl pyrene,
Pyrylium-based dyes, thiapyrylium-based dyes, azurenium-based dyes, azurenium salts, cyanine-based dyes, squarylium-based dyes, phthalocyanine pigments, perylene pigments,
Pyranthrone pigments, polycyclic quinone pigments, indigo pigments, quinacridone pigments, pyrrole pigments, azo pigments,
Ansanthrone-based pigments and the like can be mentioned. The pigment-based charge generation material can be used as a dispersion film with a resin.
In particular, an azo pigment charge generator having a large ionization potential is preferable.

[0018] More preferably, by using a photoconductive material as a charge generating material having an ionization potential of 5.50 eV or more, the light emitted from the EL layer can be used to further prevent the injection of holes from the electrodes. Can be reduced. Furthermore, when the hole injection layer has photoconductivity, the spectral sensitivity can be shifted by adding a sensitizing dye.

The sensitizing dyes that can be used in the hole injection layer of the present invention include Victoria blue, crystal violet, brilliant green, malachite green, rhodamine, methylene blue, pyrylium salts, benzopyrylium salts, trimethine benzopyri. Examples thereof include a lithium salt and a triallylcarbonium salt.

The thickness of the hole injection layer is 0.001 to 10 μm.
m, preferably 0.01 to 1 μm. If the film thickness is smaller than this, it is difficult to form the film, and if the film thickness is larger, injection of holes from the electrode does not occur.

When a charge generating material and a hole transporting material are mixed in the hole injecting layer, the amount of the charge generating material is 0.1 to 1 mol of the charge generating material.
0.1 to 1000 mol, preferably 0.1 to 10 mol
Mix well. When a binder resin is mixed, the amount is 0.1 to 10 parts by weight, preferably 0.2 to 1 part by weight, per 1 part by weight of the charge generating agent.

EL Layer The EL (electroluminescence) layer may be a single layer composed of only a light emitting layer, or may be a multilayer structure by combining a hole transport layer, an electron transport layer, and the like. These layers can be doped with an appropriate material for the purpose of adjusting the emission wavelength, improving the emission efficiency, and the like. The material used for each layer of the EL layer can be an inorganic material or an organic material, but typically, the light emitting layer is an organic layer. As a forming method, a film may be formed by a vacuum film forming method such as vapor deposition or sputtering, or may be formed by applying a coating solution.

(Light Emitting Layer) In the present invention, the light emitting layer can have both a light emitting function and an electron transporting function by, for example, an aluminum quinolium complex, but is not particularly limited as long as it is a material that emits fluorescence. Examples of the luminescent material used include the following.

<Dye System> Cyclopentadiene derivative, tetraphenylbutadiene derivative, triphenylamine derivative, oxadiazole derivative, pyrazoloquinoline derivative, distyrylbenzene derivative, distyrylarylene derivative, silole derivative, thiophene ring compound, pyridine ring Compounds, perinone derivatives, perylene derivatives, oligothiophene derivatives, trifmanylamine derivatives, oxadiazole dimers, pyrazoline dimers.

<Metal complex type> Aluminum quinolinol complex,
Benzoquinolinol beryllium complex, benzoxazole zinc complex, benzothiazole zinc complex, azomethyl zinc complex, porphyrin zinc complex, europium complex,
Al, Zn, Be, etc., or Tb, E
A metal complex having a rare earth metal such as u or Dy and having an oxadiazole, thiadiazole, phenylpyridine, phenylbenzimidazole, quinoline structure or the like as a ligand.

<Polymer system> Polyfluorene derivatives such as polyparaphenylenevinylene derivatives, polythiophene derivatives, polyparaphenylene derivatives, polysilane derivatives, polyacetylene derivatives, and polyvinylcarbazole.

<Doping Material> Doping can be performed in the light emitting layer for the purpose of improving the light emission efficiency, changing the light emission wavelength, and the like. As this doping material, for example,
Examples include perylene derivatives, coumarin derivatives, rubrene derivatives, quinacridone derivatives, squarium derivatives, porphyrin derivatives, styryl dyes, tetracene derivatives, pyrazoline derivatives, decacyclene, and phenoxazone.

(Charge Transport Layer) The charge transport layer, which is a preferred component of the EL device, includes a hole transport layer and / or an electron transport layer. These are described, for example, in Japanese Patent Application No. 9-15 / 1997.
As described in Japanese Patent No. 5284, there is no particular limitation as long as it is generally used for EL devices.

(Charge Injection Layer) The charge injection layer, which is a component of the EL device, includes a hole injection layer and / or an electron injection layer. Among them, the hole injection layer is an essential layer characteristic of the present invention as described above as another item.
The electron injection layer is a layer that is preferably used. For example, as described in Japanese Patent Application No. 9-155284,
There is no particular limitation as long as it is generally used for an L element.

Insulating Layer The insulating layer preferably used in the present invention is provided to avoid conduction between pixels when the EL is patterned. Examples of a material for forming the insulating layer include insulating polymers such as polyimide.

The electrode electrode layer, either the anode electrode and the cathode electrode, transparent or translucent, as the anode electrode, a large conductive material work function so as to facilitate holes injected are preferred, Preferably, it is 4.60 eV or more. Conversely, as the cathode electrode, a conductive material having a large work function is preferable so that electrons can be easily injected. Further, a plurality of materials may be mixed. The resistance of each electrode is preferably as low as possible. In general, a metal material is used, but an organic substance or an inorganic compound may be used.

Generally, the electrode layer is formed by vapor deposition, sputtering, CV
The film is formed in a vacuum such as D, but a conductive polymer, a conductive organic compound, or the like may be dissolved or dispersed in water or a solvent, and the film may be formed by coating.

As a material of the anode electrode, for example, IT
O, indium oxide, gold, polyaniline, and tin oxide are listed, and among them, ITO which is transparent and has a large work function is preferable. As a material of the cathode electrode, for example, a magnesium alloy (MgAg or the like), an aluminum alloy (AlL
i, AlCa, AlMg, etc.) and metallic calcium.

Cathode Buffer Material and Electron Injection Layer The cathode buffer material and the electron injection layer that can be preferably used in the present invention are not particularly limited as long as they are materials that promote the injection of electrons from the cathode. Examples of such a material include aluminum lithium, lithium fluoride, strontium, magnesium oxide, magnesium fluoride, strontium fluoride, calcium fluoride, barium fluoride, aluminum oxide, strontium oxide, calcium, polymethyl methacrylate, and polystyrene sulfone. Acid sodium.

[0035]

EXAMPLE In order to investigate the effect of injecting holes from the anode in the hole injection layer, an element composed of an ITO electrode / a hole injection layer / a hole transport layer / a gold electrode (a part of the EL element of the present invention) ) And the IV characteristics were examined. Hereinafter, description will be made with reference examples and reference comparative examples.

(Measurement of ionization potential and work function) The work function of the electrode, the ionization potential of the charge generation material and the ionization potential of the hole transport material were measured using a surface analyzer (AC-1: manufactured by Riken Keiki) under atmospheric photoelectron analysis. It was measured. The irradiation light amount was measured at 10 to 1000 nW. The structures of the photoconductive materials (charge generation materials) CG1 to CG10 are shown below, and the ionization potential is shown in Table 1. The structures of the hole transport materials CT1 to CT21 are shown below, and the ionization potential is shown in Table 2. Further, Table 3 shows the work functions of the metal materials (electrode materials).

[0037]

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[0038]

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[0039]

[Table 1]

[0040]

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[0041]

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[0042]

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[0043]

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[0044]

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[0045]

[Table 2]

[0046]

[Table 3] (Preparation of sample) As a coating liquid for the hole injection layer, 3 parts by weight of a photoconductive material (charge generation material), 1 part by weight of polyvinyl formal resin, 98 parts by weight of 1,4-dioxane and cyclohexanone shown in Table 1 were used. 98 parts by weight were mixed and sufficiently kneaded by a mixer to obtain a pigment dispersion. The number of moles of the hole transport material shown in Table 2 equal to the number of moles of the charge generation material was 1,4.
-Dissolved in a mixed solvent of 98 parts by weight of dichiosan and 98 parts by weight of cyclohexanone, or an appropriate solvent, and mixed with the above pigment dispersion such that 1 mol of the hole transport material per 1 mol of the charge generation material.

A coating solution for the hole injection layer was applied on the washed electrode substrate using a spinner so that the dry film thickness became 1 μm, and dried in a clean oven at 80 ° C. for 30 minutes. Then, as an upper electrode, a gold film having a size of 4 mm square and a film thickness of 50 nm was formed by vacuum evaporation.

(Measurement of hole injection current by IV measurement)
FIG. 2 shows a current measuring device for measuring the measurement sample.
In the figure, 21 is a glass substrate, 22 is an ITO electrode, 22 'is a gold electrode, 23 is a hole injection layer, 25 is a hole transport layer, 2
Reference numeral 6 denotes a source meter (Type 2400: manufactured by Keyence Corporation), and reference numeral 27 denotes a personal computer. In this current measuring device, the current was measured by applying a DC voltage of 20 V between both electrodes, with the ITO electrode of the measurement sample being + and the gold electrode being-.

(Reference Example 1) FIG.
G2 shows the values of the injection current observed using CT1 to CT21 having different ionization potentials as the hole transport material. Further, the case where the hole transport material is not mixed is also shown.

As a result, the hole injection current increased when the ionization potential of the hole transport material was 5.50 eV or less. This result was the same even when the mixing ratio of CG2 and the hole transport material was changed. When the hole transport material was not mixed, the injection current density was 10 ⁻⁷ A / cm ² or less, which was the same as the case where the hole transport material of 5.50 eV or more was combined.

Reference Example 2 FIG. 4 shows C as a hole transport material.
Using T6, the values of injection current observed using CG1 to CG10 having different ionization potentials as charge generation materials are shown. Further, the case where the charge generation material is not mixed is also shown.

Thus, when the ionization potential of the charge generation material was 5.60 eV or more, the hole injection current increased.
The same result was obtained even when the mixing ratio of CT6 and the hole transport material was changed. When the charge generation material is not mixed, the injection current density becomes 10 ⁻⁷ A / cm ² or less.
This was the same as the case where the charge generation material of 60 eV or less was combined.

(Reference Example 3) FIG.
In the hole injection layer in which G2 and CT6 were combined as the hole transport material, the value of the injection current observed was shown with respect to the work function of the electrode when the electrode was changed. When the work function of the electrode was 4.62 eV or more, the hole injection current increased.

Hereinafter, examples of the EL device of the present invention will be further described.

Example 1 A hole injection layer was formed on an ITO electrode by combining each of the hole transport materials CT1 to CT21 with each of the charge generation materials CG1 to CG10. A film was formed in the same manner as in Reference Example except that the hole injection layer was formed to have a thickness of 50 nm.
1 [1,1'-biphenyl1] -4,4'-dia
min) was deposited at a rate of 5 nm / sec at a degree of vacuum of 1 × 10 ⁻⁶ torr and laminated to a thickness of 60 nm.

[0056]

Embedded image Subsequently, on this hole transport layer, as a light emitting layer, tris (8 quinolinolate) aluminum having the following structure was added in an amount of 1 × 10 5
Vapor deposition at a rate of 5 nm / sec at a vacuum of ^-6 torr, 60
It was laminated to a thickness of nm.

[0057]

Embedded image Further, MgAg was formed on the light emitting layer as a cathode electrode.
(Mg: Ag = 10: 1) at a speed of 5 nm / sec at a vacuum degree of 1 × 10 ⁻⁶ torr and an area of 0.16 cm ^2.
The layers were laminated at a thickness of 0 nm to obtain an EL device of the present invention.

(Evaluation of Light Emission Characteristics) In order to evaluate the light emission characteristics of the obtained EL device, a light emission measuring device shown in FIG. 6 was constructed. In the figure, 61 is a glass substrate, 62 is an anode electrode, 63 is a conductive memory layer, 65 is a hole transport layer,
66 is a light emitting layer, 62 'is a cathode electrode, 67 is a source meter (2400, manufactured by Keithley), 69 is a luminance meter (BM-8, manufactured by Minolta), and 68 is a personal computer. In this measurement device, a DC voltage was applied between the two electrodes, with the anode electrode of each of the organic EL devices prepared above as + and the cathode electrode as −, and the emission luminance at this time was measured with a luminance meter.

Table 4 shows that each of the charge generation materials CG1 to CG10 was used as a hole injection layer for the hole transport layer CT.
1 to CT21, 5
The emission characteristics when a DC voltage of V was applied were shown. Here, A those 1000 cd / m ² or more light emission was confirmed, B what emission of 500~1000cd / m ² could be confirmed, those emission of 100~500cd / m ² could be confirmed C, D in which light emission of less than 100 cd / m ² was confirmed was designated as D. The device without the hole injection layer emitted light of 1000 cd / m ² or less, and was B.

[0060]

[Table 4]

[0061]

According to the present invention, it is possible to provide an EL device capable of obtaining a higher luminance with a lower driving voltage than a conventional EL device.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of an example of the EL device of the present invention.

FIG. 2 is an explanatory diagram of a current measuring device using a reference example of the present invention.

FIG. 3 shows that CT1 having different ionization potentials is used as a hole transporting material, using CG2 as a charge generating material.
It is a graph which shows the value of injection current observed using CT21.

FIG. 4 shows that CT6 has different ionization potentials using CT6 as a hole transport material and charge generation materials.
It is a graph which shows the injection current value observed using CG10.

FIG. 5 is a graph showing an observed injection current value with respect to a work function of an electrode.

FIG. 6 is an explanatory diagram of a luminescence measuring device used in an example of the EL element of the present invention.

DESCRIPTION OF SYMBOLS 1 Glass substrate 2 Anode electrode 2 'Cathode electrode 3 Hole injection layer 4 Organic EL layer 21 Glass substrate 22 ITO electrode 22' Gold electrode 23 Hole injection layer 25 Hole transport layer 26 Source meter (2400 27 PC 61 Glass substrate 62 Anode electrode 62 'Cathode electrode 63 Conductive memory layer 65 Hole transport layer 66 Light emitting layer 67 Source meter (Keisley 2400) 68 Personal computer 69 Luminance meter (Minolta BM) -8)

Claims (7)

[Claims] 1. An EL device comprising an anode, a cathode, and an EL layer between the anode and the cathode, wherein a hole injection layer is provided between the EL layer and the anode. The hole injection layer has an ionization potential of 5.50 e.
V or less and the ionization potential is 5.60 eV
Characterized by containing at least the above substances,
EL element. 2. The EL device according to claim 1, wherein said anode has a work function of 4.60 eV or more. 3. The EL device according to claim 1, wherein the substance having an ionization potential of 5.50 eV or less is a hole transporting material, and the substance having an ionization potential of 5.60 eV or more is a charge generating material. . 4. The EL device according to claim 1, wherein said EL layer is a single layer. 5. The EL layer according to claim 1, wherein the EL layer has a light emitting layer and at least one or more layers selected from a charge transport layer, a charge injection layer, a hole transport layer, and an insulating layer.
The EL device according to any one of claims 1 to 3, wherein 6. The EL device according to claim 1, wherein said hole injection layer has photoconductivity. 7. The method according to claim 7, wherein the hole injection layer comprises:
The EL device according to claim 3, wherein a hole transporting material having an ionization potential of 5.50 eV or less and a charge generating material having an ionization potential of 5.60 eV or more are dispersed.