JP2673261B2 - Organic electroluminescent device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an organic electroluminescent device,
More specifically, the present invention relates to an element that emits light in response to an electric signal by using an organic substance as a light emitting substance.

In particular, the present invention provides efficient light emission even at a low voltage,
The present invention relates to an electroluminescence device having sufficient brightness.

[Conventional technology]

The organic electroluminescent element is configured such that an organic luminous body is sandwiched between opposed electrodes, and electrons are injected from one electrode and holes are injected from the other electrode.
It emits light when the injected electrons and holes recombine in the light emitting layer.

A single crystal substance such as single crystal anthracene was used as a light-emitting body in such an element, but the single crystal substance is expensive to manufacture and has many problems in terms of mechanical strength. Further, it is not easy to reduce the thickness, and the light emission is weak with a single crystal of about 1 mm, and a driving voltage of 100 V or more is often required, which is not practical.

Therefore, for example, an attempt to obtain a film of anthracene having a thickness of 1 μm or less is performed by a vapor deposition method [Thin Solid Films 94, 171 pages 1982] or a Langmuir-Projet method [Thin Solid Films]. Films (Thin Solid Films) Vol. 99, p.

However, in order to obtain sufficient performance, it is necessary to form a thin film with a thickness of several thousand angstroms under strictly controlled film forming conditions.Also, although the light emitting layer is formed as a highly accurate thin film, it can be used as a carrier. The density of certain holes or electrons is very low, and the excitation efficiency of functional molecules due to carrier migration and recombination is low, so efficient light emission cannot be obtained, and in particular, it is satisfactory in terms of power consumption and brightness. The current situation is not.

Furthermore, it is possible to obtain a high luminous efficiency by providing a hole injection layer between the anode and the light emitting layer and increasing the density of holes which are carriers, and it is disclosed in JP-A-57-51781.
It is known from the publication 9-194393.

However, these use an electron-transporting compound as a light-emitting material, which requires a substance having both high luminous efficiency and high electron-transporting properties. However, such a substance having sufficiently satisfactory properties has not been found, and thus, at present, satisfactory performance in terms of luminance and power consumption has not been obtained.

Therefore, an attempt has been made to provide an electron transport layer between the light emitting layer and the cathode in order to separate the functions of light emission and electron injection / transport, and to prevent pinholes caused by using a microcrystalline light emitting layer. . [Japanese Journal of Applied Physics (Japanese Journal
of Applied Physics) Vol. 27, No. 2, L269, 1988, and Vol. 27, No. 4, L713, 1988, issued]
Although a slight effect can be seen in the reduction of the driving voltage, no improvement in the luminous efficiency with respect to the current density is seen, and therefore, in the current situation, satisfactory performance is not obtained in terms of power consumption.

[Problems to be solved by the invention]

The present invention solves these problems and provides a highly efficient electroluminescent element.

That is, an object of the present invention is to provide an organic electroluminescent device which has good luminous efficiency even at a low voltage and a low current density, can obtain sufficiently high luminance, is inexpensive and easy to manufacture.

[Means for solving the problem]

The present inventors diligently studied an organic electroluminescent device and an electron transfer compound in which a hole injecting and transporting layer, a light emitting layer, and a cathode are sequentially provided on an anode, and as a result, a hole injecting and transporting layer and an emission are sequentially formed on the anode. Layer, in an electroluminescent element provided with a cathode, by providing a hole blocking layer having a first oxidation potential 0.1 V or more larger than the first oxidation potential of the light emitting layer between the light emitting layer and the cathode, The inventors have found that the above object can be achieved and completed the present invention.

 Hereinafter, the present invention will be described in detail.

The present invention relates to an organic electroluminescent device having a hole injecting and transporting layer, a light emitting layer, and a cathode in this order on an anode,
Between the light emitting layer and the cathode, 0.1 V is higher than the first oxidation potential of the light emitting layer.
It is based on the finding that high luminous efficiency and sufficient brightness can be obtained by providing the hole blocking layer having a large first oxidation potential.

That is, it is not necessary for the substance used for the light-emitting layer to have both properties of high light emission efficiency and high electron transfer property as in the conventional case. Can be combined with a substance having a high electron transfer property, and higher luminous efficiency in electroluminescence can be achieved. Furthermore, the first oxidation potential of the hole blocking layer is higher than the first oxidation potential of the light emitting layer.
When it is higher than 0.1 V, particularly high luminous efficiency can be obtained.

It is well known that the electrochemical first oxidation potential of an organic compound is primarily correlated with the highest occupied electron energy level (ie, ionization potential) in the ground state of the compound. Therefore, if the ionization potential of the hole blocking layer is larger than that of the light emitting layer, the hole blocking layer does not contribute to light emission if the hole blocking layer is not provided or if the difference in ionization potential is small even if the hole blocking layer is provided. It is considered that the holes passing through the inside can be confined in the light emitting layer by the hole blocking layer and can contribute to light emission, and high light emission efficiency can be achieved.

As the substance used for the hole blocking layer of the present invention, any compound can be used as long as the first oxidation potential thereof is higher than the first oxidation potential of the light emitting layer by 0.1 V or more. Those that hinder the injection of electrons into the layer are not preferable, therefore, the potential transfer compound is preferable, and any electron transfer compound such as an organic compound or a metal complex can be used.

The light-emitting substance used in the light-emitting layer may be any light-emitting substance such as an organic or metal complex having a high emission quantum efficiency. In the present invention, the hole blocking layer and the light-emitting layer are the first and second layers. Proper combination according to the monoxide potential is important.

The first oxidation potential of each compound was 0.1 mol / tetra-
Using n-butylammonium perchlorate as a supporting electrolyte, a silver-silver chloride electrode as a reference electrode and a platinum electrode as a counter electrode at 25 ° C. in a 1 × 10 ⁻⁴ to 1 × 10 ⁻⁶ mol acetonitrile solution of the compound. A bolographic analyzer P-1100 using a glassy carbon rotating electrode
(Manufactured by Yanagimoto Seisakusho Co., Ltd.) to obtain a voltage-current curve, and the half-wave potential is determined from this curve. For compounds that do not dissolve in acetonitrile, dimethylformamide or dimethylsulfoxide can be used as a solvent, if necessary.

And, as compared with the first oxidation potential of the compound used for the light emitting layer,
High electroluminescence emission efficiency can be obtained by selecting and using a compound having a large first oxidation potential of 0.1 V or more for the hole blocking layer. When the difference between the first oxidation potentials is less than 0.1 V, it does not contribute to light emission and the effect of confining holes that pass through the light emitting layer into the light emitting layer is not sufficient, and high light emission efficiency cannot be obtained.

Preferred examples of the electron-transporting compound used in the hole blocking layer include the following compounds. Various dyes and pigments such as triphenylmethane, diphenylmethane, xanthene, acridine, azine, thiazine, thiazole, oxazine and azo having an amino group or a derivative thereof, indanthrene dyes such as flavantrone, perinone pigments and perylene pigments. , Cyanine dyes, electron acceptors such as 2,4,7-trinitrofluorenone, tetracyanoquinodimethane, and tetracyanoethylene, metal or metal-free phthalocyanines with electron-withdrawing substituents on the ring, pyridyl on the ring Group, quinolyl group, quinoxalyl group, and the like, 8-
Examples thereof include metal complexes of hydroxyquinoline and its derivatives.

The light emitting layer is used as a single substance of a light emitting substance having a high light emitting quantum efficiency or in a form in which these are dissolved and dispersed in a binder resin, and examples thereof include the following compounds. Naphthalene derivative, anthracene derivative, polymethine-based, okiazine-based, xanthene-based, coumarin-based, cyanine-based dyes, aromatic amines, aromatic imines,
Examples thereof include oxazole derivatives, oxadiazole derivatives, thiazole derivatives, metal complexes of 3-hydroxyquinoline and its derivatives, ruthenium complexes, rare earth complexes and their derivatives.

In order to produce the organic electroluminescence device of the present invention, an anode, a hole injecting and transporting layer, a light emitting layer, a hole blocking layer and a cathode may be provided in this order, or an anode, a hole blocking layer, a light emitting layer,
The hole injecting and transporting layer and the anode may be provided in this order.

As the anode, a transparent or opaque conductive material formed on an insulating support is used. When the cathode is opaque, the anode needs to be transparent. Preferred examples are conductive oxides such as tin oxide, indium oxide and indium tin oxide (ITO), metals such as gold, silver and chromium, inorganic conductive substances such as copper iodide, orithiophene, polypyrrole and polyaniline. And the like.

Preferred as the cathode, for example, indium, silver,
Translucent or opaque electrodes formed from tin, aluminum, lead, magnesium, and the like.

The hole blocking layer of the present invention may be formed by vapor-depositing an electron-transporting compound, or may be formed by coating with or without a binder as required.

As the binder, ordinary polymers can be used, for example, polystyrene, polymethyl methacrylate,
Examples include polyacrylonitrile, polyester, polycarbonate, polysulfone, and polyphenylene oxide. The amount of the binder used in this case is not particularly limited, but is preferably 100 parts by weight or less with respect to 1 part by weight of the electron transfer compound. The hole blocking layer does not necessarily have to be a single layer and may be laminated in two or more layers if necessary, but the thickness is preferably 50 Å or more and 1 μm or less.

Next, a hole injection / transport layer is provided on the anode or the light emitting layer. In this case, the hole injecting and transporting layer is a layer that facilitates injection of holes from the anode and further transports the injected holes to the light emitting layer, and a hole transporting compound can be used in the layer. However, it is desirable that the light is transmissive to the light generated in the light emitting layer.

Furthermore, in order to obtain an optimum organic electroluminescence device, it is necessary to appropriately adjust the energy levels (ionization potential, electron affinity, etc.) of the hole injecting and transporting layer and the light emitting layer.

The hole-transporting compound is an electron-donating compound and is used alone or in the form of being dissolved or dispersed in a binder resin.

Preferred examples include the following compounds. Polyvinyl carbazole, 2,6-
Polyesters obtained from dimethoxy-9,10-dihydroxyanthracene and dicarboxylic acids, anthracene derivatives such as 2,6,9,10-tetraisopropoxyanthracene; 2,5-bis (4-diethylaminophenyl) -1,3 ,Four
-Oxadiazoles such as oxadiazole; N, N'-
Triphenylamine derivatives such as diphenyl-N, N '-(3-methylphenyl) 1,1'-diphenyl-4,4'-diamine; 1-phenyl-3- (p-diethylaminostyryl) -5- (p -Diethylaminophenyl) -2-pyrazolin and other pyrazoline derivatives; 4- (diethylamino)
Styryl compounds such as styryl-2-anthracene; p-
Hydrazone compounds such as diethylaminobenzaldehyde- (diphenylhydrazone); stilbene compounds; metal or metal-free phthalocyanines; porphyrin compounds.

Examples of the binder resin include polyvinyl chloride, polycarbonate, polystyrene, polyester, polysulfone, polyphenylene oxide, polyurethane, and epoxy resin.

The hole injection transport layer does not necessarily have to be a single layer,
If necessary, two or more layers may be laminated. The thickness is preferably as thin as not causing pinholes, and is usually 1 μm or less.

(Examples) Hereinafter, the present invention will be described in more detail with reference to Examples, but this does not limit the scope of the present invention.

Example 1 N, N'-diphenyl-N, N '-(3-methylphenyl) 1,1'-diphenyl-4,4 as a hole injecting and transporting layer was formed on ITO glass (manufactured by HOYA Corporation). The diamine was heated to 150 ° C. under a vacuum of 3 × 10 ^-6 torr and evaporated to a thickness of 750Å.

Next, tris (8-hydroxyquinolino) aluminum (first oxidation potential 1.14V) was used as a light emitting layer in an amount of 2.3 × 10 ^−6.
It was heated to 208 ° C. with a vacuum degree of torr and evaporated to a thickness of 425 Å. Then, tris (5,7-
Dichloro-8-hydroxyquinolino) aluminum (first oxidation potential 1.35V) at a vacuum degree of 2.5 × 10 ^-6 torr 250
It was heated to ℃ and evaporated to a thickness of 400Å. Next, metal indium was deposited thereon as a negative electrode through a shadow mask in an area of 0.1 cm ² to define the area of the device.

When a direct current voltage was applied to the device thus produced with the ITO electrode as an anode, a green light of 520 nm was emitted. The brightness was 120 cd / m ² at 30 V and 14 mA / cm ² .

Example 2 Tris (5,7-dibromo-8-hydroxyquinolino) aluminum (first oxidation potential 1.42V) as a hole blocking layer
Is heated to 268 ° C at a vacuum degree of 2.0 × 10 ^-6 torr and 420 Å
A device was prepared in the same manner as in Example 1 except that the device was vapor-deposited to a thickness of 1.

This device emitted green light with a brightness of 125 cd / m ² at 28 V and 13 mA / cm ² .

Comparative Example 1 In Example 1, the hole blocking layer was not provided, and 830 tris (8-hydroxyquinolino) aluminum, which is a light emitting layer, was used.
A device was prepared in the same manner as in Example 1 except that vapor deposition was performed with a thickness of Å.

To make this device emit light with a brightness of 120 cd / m ² , 29
V, 33 mA / cm ² conditions were required.

Comparative Example 2 Perylene-3,4,9,10-tetracarboxylic acid-bis- (2 ', 6'-diisopropylanilide) as a hole blocking layer
(Manufactured by BASF) (first oxidation potential 0.87 V) was heated to 270 ° C. at a vacuum degree of 1.2 × 10 ⁻⁶ Torr and vapor-deposited to a thickness of 500 Å, and used in the same manner as in Example 1. A device was created.

To make this device emit light with a brightness of 120 cd / m ² , 32 V, 4
The condition of 5 mA / cm ² was required.

As described above, in Examples 1 and 2, the luminous efficiency with respect to the current density was remarkably improved as compared with Comparative Examples 1 and 2.

Example 3 2x 1-phenyl-3- (p-diethylaminostyryl) -5- (p-diethylaminophenyl) -2-pyrazoline as a hole injecting and transporting layer on ITO glass (manufactured by HOYA Corporation). It was vapor-deposited to a thickness of 500Å at a vacuum degree of 10 ^-6 torr. Then, as a light emitting layer, 2- [2- (4-dimethylaminophenyl) -vinyl] naphthothiazole (first oxidation potential: 0.667 V) was added at 600 Å at a vacuum degree of 1.8 × 10 ^-6 torr.
Deposited to a thickness of. Then as a hole blocking layer on it Was vapor-deposited at a vacuum degree of 3.2 × 10 ^-6 Torr to a thickness of 480 Å.
Then, indium was deposited as a cathode in the same manner as in Example 1 to prepare a device.

This device emitted bluish-white light of 30 cd / m ² at 16 V and 85 mA / cm ² .

Comparative Example 3 As the hole blocking layer, A device was prepared in the same manner as in Example 3 except that the film was vapor-deposited to a thickness of 380Å at a vacuum degree of 2.8 × 10 ^-6 torr.

This device showed only an emission of 15 cd / m ² at 38 V and 120 mA / cm ² .

Example 4 4- (diethylamino) styryl-2-anthracene was used as a hole injecting and transporting layer at a vacuum degree of 2 × 10 ⁻⁶ Torr.
It was vapor-deposited to a thickness of 0Å.

Next, as a light emitting layer, 2,5-bis [5'-t-butylbenzoxazolyl (2 ')] thiophene (first oxidation potential 0.732V) was added at a vacuum degree of 7.8 × 10 ^-7 Torr to a thickness of 700Å. It was vapor-deposited.

Furthermore, an azo pigment is formed on this as a hole blocking layer.
CI Pigment Red 112 (CI12370) (first oxidation potential 1.30 V) was vapor-deposited to a thickness of 520 Å at a vacuum degree of 4.2 × 10 ^-6 torr, and indium was used as a cathode on it in the same manner as in Example 1. Was vapor-deposited to prepare an element.

This device emitted 25 cd / m ² of 445 nm light at 38 V and 85 mA / cm ² .

As a comparative example, an element was prepared in the same manner as in Example 4 except that the hole blocking layer was not provided. However, this element did not emit light at all even when a voltage was applied, and finally caused dielectric breakdown and the element was broken. It was

Example 5 N, N'-diphenyl-N, N '-(3-methylphenyl) 1,1'-diphenyl-4,4' as a hole injecting and transporting layer was formed on ITO glass (manufactured by HOYA Corporation). -Diamine 3 x 10 ^-6
It was heated to 150 ° C with a vacuum degree of torr and evaporated to a thickness of 750Å. On this, as a light emitting layer, 1,6-dianinoanthracene (first oxidation potential 0.785 V) was vapor-deposited at a vacuum degree of 3.3 × 10 ⁻⁶ Torr to a thickness of 650 Å.

Furthermore, flavantron (CI70600: Ind Yellow G; manufactured by BASF) as a hole blocking layer is formed on top of this (first oxidation potential).
0.920V) was deposited at a vacuum degree of 3.2 × 10 ^-6 Torr to a thickness of 400Å. Then, indium was deposited as a cathode in the same manner as in Example 1 to prepare a device.

This device emitted yellow light at a luminance of 50 cd / m ² at 38 V and 42 mA / cm ² .

〔The invention's effect〕

According to the present invention, an organic electroluminescent device comprising an anode, a hole injection / transport layer, a light emitting layer, and a cathode,
Between the light emitting layer and the cathode, 0.1 V is higher than the first oxidation potential of the light emitting layer.
By providing the hole blocking layer having a large first oxidation potential as described above, it is possible to obtain an organic electroluminescence device which has good luminous efficiency and sufficient brightness, is inexpensive, and is easily manufactured.

Claims (1)

(57) [Claims] 1. An organic electroluminescent device comprising a positive electrode, a hole injecting and transporting layer, a light emitting layer, and a cathode, which are transparent on at least one of these electrodes, and emits light between the light emitting layer and the cathode. An organic electroluminescent device comprising a hole blocking layer having a first oxidation potential greater than the first oxidation potential of the layer by 0.1 V or more.