JP2007142011A - Organic electroluminescence element using fluorene compound and benzidine compound - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an organic electroluminescence element high in luminous efficiency at a low drive voltage and excellent in durability. <P>SOLUTION: The organic EL element including at least a hole transport layer and a luminous layer between a pair of electrodes employs a hole transport layer containing a fluorene compound having a specific aryl substituent and an aryl benzidine compound. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an organic EL element that achieves high efficiency and high durability.

  An organic EL element is a surface-emitting element in which an organic thin film is sandwiched between a pair of electrodes, and has features such as a thin and light weight, a high viewing angle, and a high-speed response, and is expected to be applied to various display elements. . An organic EL element is an element that utilizes light emitted when holes injected from an anode and electrons injected from a cathode are recombined in a light-emitting layer by applying a voltage. In 1987, C.I. W. The basic structure is a two-layer element in which a hole transport layer and a light-emitting layer are stacked as reported by Tang et al. (See, for example, Non-Patent Document 1). Thereafter, an element composed of three layers of a hole transport layer, a light emitting layer, and an electron transport layer has been proposed (see, for example, Non-Patent Document 2), and at present, a multilayer stacked element has become mainstream. A charge transport layer such as a hole transport layer or an electron transport layer facilitates charge injection into the light emitting layer and plays a role of confining charges in the light emitting layer. W. Since the report of Tang et al., Various hole transport materials and electron transport materials have been developed for the purpose of lowering the driving voltage of organic EL elements and improving the light emission efficiency.

  However, for practical use, further low voltage driving, light emission efficiency and durability improvement are required. Among them, benzidine compounds such as 4,4′-bis [N- (1-naphthyl) -N-phenylamino] biphenyl, 4,4′-bis [N- (m-tolyl) -N-phenylamino] biphenyl, etc. The organic EL device having a hole transporting layer has a high driving voltage and has a problem in thin film stability and glass transition temperature, so that it could not be said to have sufficient durability at both room temperature and high temperature.

Appl. Phys. Lett. 51.913 (1987) Jpn. J. et al. Appl. Phys. 27. L269 (1988)

  The present invention solves the above-described conventional problems, and provides a luminous efficiency at a low driving voltage using a hole transport layer composed of a fluorene compound and a benzidine compound excellent in the stability of the thin film and the hole transport property. An organic EL element having high durability is provided.

  As a result of intensive studies, the present inventors have found that an organic EL element using a hole transport layer containing two different types of hole transport materials achieves high efficiency and high durability, and completes the present invention. It came to. That is, the present invention provides an organic EL device having at least a hole transport layer and a light emitting layer between a pair of electrodes, wherein the hole transport layer is represented by the general formula (1).

（式中、Ａｒ^１〜Ａｒ^４は各々独立して置換もしくは無置換のフェニル基、１−ナフチル基、２−ナフチル基、またはビフェニル基を表す。）
で示されるフルオレン化合物、および一般式（２）

(In the formula, Ar ^{1 to} Ar ⁴ each independently represents a substituted or unsubstituted phenyl group, 1-naphthyl group, 2-naphthyl group, or biphenyl group.)
And a fluorene compound represented by the general formula (2)

（式中、Ａｒ^５，Ａｒ^６は各々独立して置換もしくは無置換のフェニル基、１−ナフチル基、２−ナフチル基、またはビフェニル基を表す。）
で示されるベンジジン化合物を含んで成ることを特徴とする有機ＥＬ素子に関するものである。

(In the formula, Ar ⁵ and Ar ⁶ each independently represent a substituted or unsubstituted phenyl group, 1-naphthyl group, 2-naphthyl group, or biphenyl group.)
It is related with the organic EL element characterized by including the benzidine compound shown by these.

  The present invention is described in detail below.

In the fluorene compound represented by the general formula (1), Ar ^{1 to} Ar ⁴ each independently represents a substituted or unsubstituted phenyl group, 1-naphthyl group, 2-naphthyl group, or biphenylyl group.

Examples of the substituted or unsubstituted phenyl group represented by Ar ^{1 to} Ar ⁴ include a phenyl group, a 4-methylphenyl group, a 3-methylphenyl group, a 2-methylphenyl group, a 4-ethylphenyl group, and a 3-ethylphenyl group. 2-ethylphenyl group, 4-n-propylphenyl group, 4-n-butylphenyl group, 4-isobutylphenyl group, 4-tert-butylphenyl group, 4-cyclopentylphenyl group, 4-cyclohexylphenyl group, 4 -Methoxyphenyl group, 4-ethoxyphenyl group, 4-isopropoxyphenyl group, 4-n-butoxyphenyl group, 4-sec-butoxyphenyl group, 4-tert-butoxyphenyl group, 4-trifluoromethylphenyl group, A 4-fluorophenyl group etc. can be illustrated.

Examples of the substituted or unsubstituted 1-naphthyl group represented by Ar ^{1 to} Ar ⁴ include 1-naphthyl group, 2-methyl-1-naphthyl group, 2-ethyl-1-naphthyl group, 2-butyl-1-naphthyl group. Examples thereof include a group, 4-methyl-1-naphthyl group, 4-ethyl-1-naphthyl group, 4-butyl-1-naphthyl group and the like.

Examples of the substituted or unsubstituted 2-naphthyl group represented by Ar ^{1 to} Ar ⁴ include a 2-naphthyl group, a 6-methoxy-2-naphthyl group, a 6-ethoxy-2-naphthyl group, and 6-n-butoxy-2. Examples include -naphthyl group, 6-n-hexyloxy-2-naphthyl group, 7-methoxy-2-naphthyl group, 7-n-butoxy-2-naphthyl group, and the like.

Examples of the substituted or unsubstituted biphenylyl group represented by Ar ^{1 to} Ar ⁴ include a 4-biphenylyl group, a 4′-methyl-4-biphenylyl group, a 4′-ethyl-4-biphenylyl group, and a 4′-butyl-4- Biphenylyl group, 4'-methoxy-4-biphenylyl group, 4'-ethoxy-4-biphenylyl group, 4'-butoxy-4-biphenylyl group, 3'-phenyl-4-biphenylyl group, 3'-tolyl-4- Biphenylyl group, 4′-phenyl-4-biphenylyl group, 4′-tolyl-4-biphenylyl group, 3-biphenylyl group, 4′-methyl-3-biphenylyl group, 4′-ethyl-3-biphenylyl group, 4 ′ -Butyl-3-biphenylyl group, 4'-methoxy-3-biphenylyl group, 4'-ethoxy-3-biphenylyl group, 4'-butoxy-3-biphenylyl group, 4'-phenyl-3 It can be exemplified biphenylyl group, 4'-tolyl-3-biphenylyl group.

In the fluorene compound represented by the general formula (1), at least one of Ar ¹ and Ar ² is a 1-naphthyl group because of its high glass transition temperature, excellent thin film stability, and easy availability. A 2-naphthyl group and a 4-biphenylyl group, and at least one of Ar ³ and Ar ⁴ is preferably a 1-naphthyl group, a 2-naphthyl group, or a 4-biphenylyl group. However, when both Ar ¹ and Ar ² are 4-biphenylyl groups, Ar ³ and Ar ⁴ may be substituted or unsubstituted phenyl groups from the viewpoint of ensuring a sufficient glass transition temperature.

In the benzidine compound represented by the general formula (2), Ar ⁵ and Ar ⁶ each independently represent a substituted or unsubstituted phenyl group, 1-naphthyl group, 2-naphthyl group, or biphenylyl group.

Examples of the substituted or unsubstituted phenyl group represented by Ar ⁵ and Ar ⁶ include a phenyl group, a 4-methylphenyl group, a 3-methylphenyl group, a 2-methylphenyl group, a 4-ethylphenyl group, and a 3-ethylphenyl group. 2-ethylphenyl group, 4-n-propylphenyl group, 4-n-butylphenyl group, 4-isobutylphenyl group, 4-tert-butylphenyl group, 4-cyclopentylphenyl group, 4-cyclohexylphenyl group, 4 -Methoxyphenyl group, 4-ethoxyphenyl group, 4-isopropoxyphenyl group, 4-n-butoxyphenyl group, 4-sec-butoxyphenyl group, 4-tert-butoxyphenyl group, 4-trifluoromethylphenyl group, A 4-fluorophenyl group etc. can be illustrated.

Examples of the substituted or unsubstituted 1-naphthyl group represented by Ar ⁵ or Ar ⁶ include 1-naphthyl group, 2-methyl-1-naphthyl group, 2-ethyl-1-naphthyl group, and 2-butyl-1-naphthyl group. Examples thereof include a group, 4-methyl-1-naphthyl group, 4-ethyl-1-naphthyl group, 4-butyl-1-naphthyl group and the like.

Examples of the substituted or unsubstituted 2-naphthyl group represented by Ar ⁵ or Ar ⁶ include a 2-naphthyl group, a 6-methoxy-2-naphthyl group, a 6-ethoxy-2-naphthyl group, and 6-n-butoxy-2. Examples include -naphthyl group, 6-n-hexyloxy-2-naphthyl group, 7-methoxy-2-naphthyl group, 7-n-butoxy-2-naphthyl group, and the like.

Examples of the substituted or unsubstituted biphenylyl group represented by Ar ⁵ and Ar ⁶ include a 4-biphenylyl group, a 4′-methyl-4-biphenylyl group, a 4′-ethyl-4-biphenylyl group, and a 4′-butyl-4- Biphenylyl group, 4'-methoxy-4-biphenylyl group, 4'-ethoxy-4-biphenylyl group, 4'-butoxy-4-biphenylyl group, 3'-phenyl-4-biphenylyl group, 3'-tolyl-4- Biphenylyl group, 4′-phenyl-4-biphenylyl group, 4′-tolyl-4-biphenylyl group, 3-biphenylyl group, 4′-methyl-3-biphenylyl group, 4′-ethyl-3-biphenylyl group, 4 ′ -Butyl-3-biphenylyl group, 4'-methoxy-3-biphenylyl group, 4'-ethoxy-3-biphenylyl group, 4'-butoxy-3-biphenylyl group, 4'-phenyl-3 It can be exemplified biphenylyl group, 4'-tolyl-3-biphenylyl group.

  As the benzidine compound represented by the general formula (2), 4,4′-bis [N- (1-naphthyl) -N-phenylamino] biphenyl is widely used as a general-purpose hole transport material, and is inexpensive. It is preferable in the production of an organic EL device because it is easily available.

  In the case of forming a hole transport layer comprising the fluorene compound represented by the general formula (1) and the benzidine compound represented by the general formula (2) in the organic EL device, the organic EL element is represented by the general formula (1). The fluorene compound and the benzidine compound represented by the general formula (2) are sequentially laminated, and the fluorene compound represented by the general formula (1) and the benzidine compound represented by the general formula (2) are co-deposited and mixed. The method of forming the layer can be selected.

  The thickness of the hole transport layer containing the fluorene compound represented by the general formula (1) and the benzidine compound represented by the general formula (2) is not particularly limited, but is usually 1 nm to 1 μm, preferably 5 It is -500 nm, More preferably, it is 10-100 nm.

  Further, when the fluorene compound represented by the general formula (1) and the benzidine compound represented by the general formula (2) are stacked as the hole transport layer, the fluorene represented by the general formula (1) having a small ionization potential. A configuration in which the compound is provided on the anode side and the benzidine compound represented by the general formula (2) having a large ionization potential is provided on the light emitting layer side is preferable. Examples of a method for forming the hole transport layer include a vacuum deposition method, a spin method, and the like. Known methods such as a coating method and a casting method can be applied. Moreover, as a film thickness of the benzidine compound layer shown by General formula (2), 5-50% is preferable with respect to the film thickness of a positive hole transport layer, and 5-25% is more preferable.

  When a mixed layer is formed by co-evaporating the fluorene compound represented by the general formula (1) and the benzidine compound represented by the general formula (2) as the hole transport layer, the general formula ( It is preferable to contain 1 to 50% by weight of the benzidine compound represented by 2).

  The hole transport layer in the present invention does not prevent inclusion of components other than the general formulas (1) and (2) as long as the effects of the present invention are not impaired.

  The organic EL device in the present invention is not particularly limited, and is composed of a substrate, an anode, a hole transport layer, a light emitting layer, an electron transport layer, and a cathode, and the hole transport layer is represented by the general formula (1). It is a layer comprising a fluorene compound and a benzidine compound represented by the general formula (2). Further, if necessary, a hole injection layer may be provided between the anode and the hole transport layer, or an electron injection layer may be provided between the electron transport layer and the cathode. Further, a new layer can be provided, or any of the layers can be omitted, and the electron transport layer and the electron injection layer, and the light emitting layer and the electron injection and transport layer can be combined into one layer.

  The substrate of the organic EL device of the present invention can be used as long as it is transparent and has a smooth surface, and glass, plastic and the like can be used. Further, the thickness of the substrate can be arbitrarily selected as long as the element can be supported.

  For the anode of the organic EL device of the present invention, for example, indium tin oxide (ITO), tin oxide, gold, silver, copper, chromium, or the like can be used. As a method for forming the anode, for example, a known method such as a vacuum deposition method or a sputtering method can be applied, and the film thickness is not particularly limited, but is usually 10 nm to 1 μm, preferably 10 to 500 nm. is there.

  For the hole injection layer of the organic EL device of the present invention, a known material having a smaller ionization potential than the fluorene compound represented by the general formula (1) and having a function of injecting holes from the anode can be used. Examples thereof include phthalocyanine derivatives, triarylamine derivatives, polyaniline derivatives, polythiophene derivatives, and the like. As a method for forming the hole injection layer, for example, a known method such as a vacuum deposition method, a spin coating method, or a casting method can be applied, and the film thickness is not particularly limited, but is usually 1 nm to 1 μm. Yes, preferably 1 to 100 nm.

  For the light-emitting layer of the organic EL device of the present invention, a known light-emitting material conventionally used in organic EL devices can be used, electrons and holes can be injected, and fluorescence and / or phosphorescence can be used. If it is a substance which has this, there will be no restriction | limiting in particular. Examples of luminescent materials include oxadiazole derivatives, triarylamine derivatives, styrylbenzene derivatives, coumarin derivatives, acridone derivatives, quinacridone derivatives, diketopyrrolopyrrole derivatives, oxazone derivatives, pyran derivatives, condensed aromatic compounds and derivatives thereof, Examples include aluminum complexes of 8-quinolinol derivatives and rhenium complexes, iridium complexes, platinum complexes, and europium complexes having chlorine, phenylpyridine, benzoquinoline, quinolinol, bipyridyl, phenanthroline, acetylacetone, and the like as ligands. As a method for forming the light emitting layer, for example, a known method such as a vacuum deposition method, a spin coating method, or a casting method can be applied, and the light emitting layer may be formed of only the light emitting material, May be doped with a light emitting material.

  For the electron transport layer of the organic EL device of the present invention, a known material having a function of receiving electrons injected from the cathode and transporting them to the light emitting layer can be used. For example, triazole derivatives, oxazole derivatives, oxazis Examples include azole derivatives, imidazole derivatives, aluminum complexes of 8-quinolinol derivatives, and organic silane derivatives. As a method for forming the electron transport layer, for example, a known method such as a vacuum deposition method, a spin coating method, or a casting method can be applied, and the film thickness is not particularly limited, but is usually 1 nm to 1 μm. The thickness is preferably 1 to 100 nm. In addition, the electron transport layer may contain a reducing dopant such as an alkali metal or an alkaline earth metal.

  As the electron injection layer of the organic EL device of the present invention, a known material having a function of preventing current leakage and improving electron injection can be used. For example, an alkali metal halide and an alkaline earth metal halogen can be used. A compound. As a method for forming the electron injection layer, for example, a known method such as a vacuum deposition method or a sputtering method can be applied, and the film thickness is not particularly limited, but is usually 0.01 to 100 nm, preferably Is 0.1 to 50 nm.

  For the cathode of the organic EL device of the present invention, for example, alkali metal or alkaline earth metal can be used, and these can also be used in combination with other metals. As a method for forming the cathode, for example, a known method such as a vacuum deposition method or a sputtering method can be applied, and the film thickness is not particularly limited, but is usually 10 nm to 1 μm, preferably 10 to 500 nm. is there.

  According to the organic EL device of the present invention, a positive electrode comprising at least a fluorene compound represented by the general formula (1) and a benzidine compound represented by the general formula (2) between the hole injection layer and the light emitting layer. By using the hole transport layer, it is possible to provide an organic EL element having a high driving efficiency with a low driving voltage and excellent durability.

  EXAMPLES Hereinafter, although this invention is demonstrated further in detail based on an Example, this invention is not limited by these Examples.

  In addition, the drive voltage and light emission luminance measurement of the organic EL element were performed using a luminance meter LUMINANCE METER (BM-9) manufactured by TOPCON.

Example 1
The glass substrate on which the ITO transparent electrode (anode) having a thickness of 200 nm was laminated was subjected to ultrasonic cleaning with acetone and pure water and boiling cleaning with isopropyl alcohol. Furthermore, ultraviolet ozone cleaning was performed, and after evacuation with a vacuum pump until it became 1 × 10 ⁻⁴ Pa after installation in a vacuum deposition apparatus. First, copper phthalocyanine was deposited on the ITO transparent electrode at a deposition rate of 0.05 nm / second to form a 25 nm hole injection layer. Subsequently, 2,7-bis [N- (1-naphthyl) -N-phenylamino] -9,9-bis (1,1′-biphenyl-4-yl) fluorene was deposited at a deposition rate of 0.08 nm / sec to 40 nm. Further, 4,4′-bis [N- (1-naphthyl) -N-phenylamino] biphenyl was deposited at a deposition rate of 0.08 nm / second to 5 nm to form a hole transport layer. Subsequently, tris (8-quinolinolato) aluminum was vapor-deposited at a deposition rate of 0.1 nm / second to 60 nm to form a light-emitting layer, and then lithium fluoride was deposited as an electron injection layer at a deposition rate of 0.01 nm / second to 0.5 nm. Aluminum was deposited as a cathode at a thickness of 100 nm at a deposition rate of 0.25 nm / second to produce an organic EL device.

When the drive voltage and the light emission luminance when a current of 7.5 mA / cm ² was applied to the manufactured element were measured, the drive voltage was 4.8 V and the light emission luminance was 380 cd / m ² . Further, when continuously driven at 20 mA / cm ² under a nitrogen atmosphere, the luminance reduction rate at 500 hours was within 10%.

Example 2
2,7-bis [N, N instead of 2,7-bis [N- (1-naphthyl) -N-phenylamino] -9,9-bis (1,1′-biphenyl-4-yl) fluorene -Bis (1,1'-biphenyl-4-yl) amino] -9,9-diphenylfluorene was deposited in an organic EL device in the same manner as in Example 1 except that 40 nm was deposited at a deposition rate of 0.08 nm / sec. Produced.

When the drive voltage and the light emission luminance when a current of 7.5 mA / cm ² was applied to the manufactured element were measured, the drive voltage was 4.9 V and the light emission luminance was 370 cd / m ² . Further, when continuously driven at 20 mA / cm ² under a nitrogen atmosphere, the luminance reduction rate at 500 hours was within 10%.

Example 3
Instead of 4,4′-bis [N- (1-naphthyl) -N-phenylamino] biphenyl, 4,4′-bis [N- (m-tolyl) -N-phenylamino] biphenyl was deposited at a deposition rate of 0. An organic EL device was produced in the same manner as in Example 1 except that 5 nm was deposited at 08 nm / second.

When the drive voltage and the light emission luminance when a current of 7.5 mA / cm ² was applied to the manufactured element were measured, the drive voltage was 4.9 V and the light emission luminance was 383 cd / m ² . Further, when continuously driven at 20 mA / cm ² under a nitrogen atmosphere, the luminance reduction rate at 500 hours was within 10%.

Example 4
Instead of 4,4′-bis [N- (1-naphthyl) -N-phenylamino] biphenyl, 4,4′-bis [N- (m-tolyl) -N-phenylamino] biphenyl was deposited at a deposition rate of 0. An organic EL device was produced in the same manner as in Example 2 except that 5 nm was deposited at 08 nm / second.

When the drive voltage and the light emission luminance when a current of 7.5 mA / cm ² was applied to the manufactured element were measured, the drive voltage was 4.8 V and the light emission luminance was 368 cd / m ² . Further, when continuously driven at 20 mA / cm ² under a nitrogen atmosphere, the luminance reduction rate at 500 hours was within 10%.

Comparative Example 1
The organic layer was formed in the same manner as in Example 1 except that 4,4′-bis [N- (1-naphthyl) -N-phenylamino] biphenyl was deposited as a hole transporting layer at a deposition rate of 0.08 nm / sec. An EL element was produced.

When the drive voltage and the light emission luminance were measured when a current of 7.5 mA / cm ² was applied to the manufactured element, the drive voltage was 5.3 V and the light emission luminance was 254 cd / m ² . In addition, when continuously driven at 20 mA / cm ² in a nitrogen atmosphere, the luminance reduction rate at 500 hours was 30%.

Comparative Example 2
The organic layer was formed in the same manner as in Example 1 except that 4,4′-bis [N- (m-tolyl) -N-phenylamino] biphenyl was deposited as a hole transport layer at 45 nm with a deposition rate of 0.08 nm / sec. An EL element was produced.

When the drive voltage and the light emission luminance were measured when a current of 7.5 mA / cm ² was applied to the manufactured element, the drive voltage was 5.2 V and the light emission luminance was 248 cd / m ² . Further, when continuously driven at 20 mA / cm ² in a nitrogen atmosphere, the luminance reduction rate in 500 hours was 50%.

Claims (6)

In the organic EL device having at least a hole transport layer and a light emitting layer between a pair of electrodes, the hole transport layer has the general formula (1).

(In the formula, Ar ^{1 to} Ar ⁴ each independently represents a substituted or unsubstituted phenyl group, 1-naphthyl group, 2-naphthyl group, or biphenyl group.)
And a fluorene compound represented by the general formula (2)

(In the formula, Ar ⁵ and Ar ⁶ each independently represent a substituted or unsubstituted phenyl group, 1-naphthyl group, 2-naphthyl group, or biphenyl group.)
An organic EL device comprising a benzidine compound represented by the formula: The hole transport layer is composed of at least two layers of a fluorene compound layer represented by the general formula (1) and a benzidine compound layer represented by the general formula (2), and the fluorene compound layer represented by the general formula (1) is an anode. The organic EL element according to claim 1, wherein the organic EL element is provided on the light emitting layer side and provided on the light emitting layer side. The organic EL element according to claim 2, wherein the film thickness of the benzidine compound layer represented by the general formula (2) is 5 to 50% with respect to the film thickness of the hole transport layer. 2. The organic material according to claim 1, wherein the hole transport layer comprises a mixed layer in which at least a fluorene compound represented by the general formula (1) and a benzidine compound represented by the general formula (2) are co-deposited. EL element. The organic EL device according to claim 4, wherein the hole transport layer contains 1 to 50% by weight of a benzidine compound represented by the general formula (2). 6. The organic EL according to claim 1, wherein the benzidine compound represented by the general formula (2) is 4,4′-bis [N- (1-naphthyl) -N-phenylamino] biphenyl. element.