JP3398548B2 - Organic electroluminescent device - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an organic electroluminescent device. More specifically, the present invention relates to an organic electroluminescent device used for a light source, a display and the like.

[0002]

2. Description of the Related Art Conventionally, ZnS: has been used as an electroluminescent device.
Inorganic electroluminescent devices using materials such as Mn and SrS: Ce as light emitting layers have been put into practical use. However, these inorganic electroluminescent devices have a problem that a driving voltage as high as 200 V is required and that a light emitting layer having a blue color and high brightness that can be put to practical use is not obtained.

Various efforts have been made to solve such problems, but in recent years, electroluminescent devices using organic materials have been attracting attention. In particular, the organic electroluminescent device having a two-layer structure in which a hole injecting and transporting layer and a light emitting layer are laminated is
It is possible to easily obtain blue, green, and red emission colors with low driving voltage and high brightness. For example, Appl.Phys.Le
tt.51 (12), pp.913-915 (1987), JP-A-59-19439
No. 3 publication and Pioneer Technical Report No. 9 pages 16 to 23, aromatic compounds used as electrophotographic photoconductors in the hole injecting and transporting layer, having quinolyl aluminum complex (hereinafter abbreviated as Alq ₃ ) which is a green phosphor in the light emitting layer. An organic electroluminescent device having a two-layer laminated structure using a diamine compound has been announced. In this device, green light emission of about 1000 cd / m ² was obtained by applying a DC voltage of 10 V or less.

[0004]

However, such an organic electroluminescent device generally has a problem that it has a short life and poor reliability. This is N, N'- which is an aromatic diamine compound mainly used as a hole injecting and transporting layer.
It is believed that this is due to the instability of bis (3-methylphenyl)-(1,1'-biphenyl) -4,4'-diamine (hereinafter abbreviated as TPD). For example, on page 19 of the 11th EL subcommittee material (94.05.28) of the 125th Committee on Photoelectric Interconversion of the Japan Society for the Promotion of Science, TPD changes from the surface layer due to crystallization immediately after vapor deposition, and It is described that when the device is used as a hole injecting and transporting layer, the TPD film is destabilized due to heat generated by light emission, and the light emission efficiency is reduced.

[0005]

In order to obtain an organic electroluminescent device having a long life and high reliability, the inventors of the present invention have
We searched for a material that does not easily crystallize. As a guideline at that time, a material having a high melting point or a material having a high glass transition point (a temperature at which an amorphous substance is crystallized and measured by a differential scanning calorimetry (DSC) or the like). Was considered appropriate. This is based on, for example, the report of the 38th Joint Lecture Meeting on Applied Physics, 28p-Q-9. In other words, in this report, a substance with a high melting point is preferable in order to avoid melting when the device generates heat due to light emission.
Further, it is described that the temperature of the device rises above the glass transition point of TPD and TPD is crystallized and deteriorated.

That is, the inventors of the present invention have conducted various studies as a hole injecting / transporting material in place of TPD, and as a result of various studies, a material which is less likely to be crystallized and is less likely to be thermally decomposed by heat generation.
It was found that the N, N ′ type di-biphenylamino compound represented by the following formula (I) has a high melting point, a high glass transition point, and a high mobility, and at the same time, the compound is used as a hole injecting / transporting material. It was found that an organic electroluminescent device having a high brightness and a long life can be obtained when used as the above, and the present invention was completed. Furthermore, by using a specific oxazole compound represented by the following formulas (IV) and (V) in the light emitting layer, an exciplex phenomenon (complex formation in an excited state: light emitted from the light emitting layer is in the long wavelength region side or It has also been found that pure blue light emission can be realized by suppressing the phenomenon that the color development of the luminescent material itself is inhibited) by shifting to the short wavelength region side.

Thus, according to the invention, an anode on a substrate,
A hole injecting and transporting layer, a light emitting layer, and a cathode are laminated, and the hole injecting and transporting layer is represented by the formula (I).

[0008]

[Chemical 7]

(In the formula, R ₁ is a hydrogen atom or a methyl group.
R ₂ is a hydrogen atom or a methyl group, and n is 1 or
2, R ₃ is a hydrogen atom, methoxy group, p-position is a methyl group
It is a phenyl group which may be replaced . The organic electroluminescent element characterized by containing the aromatic diamino compound shown by these is provided.

Further, according to the present invention, there is also provided a five-layer organic electroluminescent device having an electron injecting and transporting layer formed between a light emitting layer and a cathode.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The anode, the hole injecting and transporting layer, the light emitting layer and the cathode in the organic electroluminescent device of the present invention are preferably formed in this order on a substrate. Further, an electron injecting and transporting layer may be optionally formed between the light emitting layer and the cathode. The substrate is not particularly limited, and various materials such as glass, ceramics and hard plastics can be used, and a transparent substrate is preferable.

As the electrode material for forming the anode and the cathode, for example, metals such as gold, silver, copper, aluminum, indium, magnesium and alloys thereof, and I
Examples thereof include metal oxides such as TO and SnO ₂ . In the compound of formula (I) contained in the hole injecting and transporting layer in the organic electroluminescent device of the present invention, R ₁ Is hydrogen
Or a methyl group, and R ₂ is a hydrogen atom or a methyl group.
, N is 1 or 2, R ₃ is a hydrogen atom, a methoxy group, p
A phenyl group which may be substituted with a methyl group at the position
It

[0013]

[0014]

[0015]

[0016]

[0017]

[0018]

[0019]

Specific examples of the compound represented by formula (I) include compounds represented by the following structural formulas. The exemplified compounds
Other than (10), (17) and (19) are reference compounds.
It

(Exemplary aromatic diamino compound)

[0022]

[Chemical 10]

[0023]

[Chemical 11]

[0024]

[Chemical 12]

[0025]

[Chemical 13]

Of these aromatic diamino compounds, the exemplified compounds ( 10 ), ( 17), (19) and the like are preferable compounds in terms of luminance, thermal stability and synthesis. Most of these exemplified compounds are novel compounds. However,
(1) (2) (11) (15) etc. are Beilstein der Chem
It is described in ie E II Vol13 99P.

The above compound can be easily synthesized by the same method as in the above literature or according to the method. For example, the exemplified compound (10) is specifically synthesized by the following procedure.

[0028]

[Chemical 14]

Each of the above steps 1 to 3 will be described. First, terphenyl was dissolved in glacial acetic acid, concentrated sulfuric acid was gradually added thereto, and the mixture was reacted at about 60 ° C. for 5 hours, and then excess saline was added to obtain terphenyl sulfonic acid sodium salt. Next, 100 to 1 of 4-aminobiphenyl is used in a solventless system.
A small excess of metallic sodium is gradually added at 30 ° C. to obtain a mud-like aromatic amine sodium mixed melt.

Next, the sulfonate obtained above and the sodium mixed melt obtained above were mixed at a molar ratio of 1: 8 to 10 to 20
By reacting at 0 ° C for about 24 hours, N, N "-
(P-Biphenyl) -4,4 "-diaminoterphenyl is obtained. In this step, a solvent such as nitrobenzene or dichlorobenzene may be used. This step is described in JP-A-60-97942. It was done with reference to.

Next, the synthesis (Synthesi)
s) Exemplified compound (10) is obtained by reacting according to the synthetic method (one type of so-called Ullmann reaction) described on pages 383 to 385 (1987). Further, for the purification of Exemplified Compound (10), since there are few solvents having excellent solubility, it was first recrystallized from dimethylformamide and then purified by sublimation.

The hole injecting and transporting layer in the present invention may be formed of one layer or a laminated structure of two or more layers. In this case, a layer containing the aromatic diamino compound of formula (I) may be laminated on the upper layer or the lower layer of the laminated structure of one or more layers not containing the aromatic diamino compound of formula (I). preferable. When the hole injecting and transporting layer is composed of one layer containing the compound of the formula (I), its film thickness is
It is about 20 to 3000Å, preferably about 50 to 500Å. In addition to the layer containing the compound of formula (I), when one or more layers not containing the compound of formula (I) are laminated, the compound of formula (I) is contained. The layer thickness is about 20-3000Å, preferably about 50-50.
It is 0Å.

In the organic electroluminescent device of the present invention, a light emitting layer is formed on the hole injecting and transporting layer. This light emitting layer may be formed of, for example, Alq ₃ , Beq ₂ (quinolyl beryllium complex), or the like. Further, it is preferable that this layer is doped with an aromatic diamino compound represented by the above formula (I) . Compound is doped into the light - emitting layer is, N, N, N ', N'-tetrakis (P- biphenyl) - in the case of a benzidine compound is more preferably doped at least 1% in the emission layer. This makes it possible to obtain an organic electroluminescent device having better light emission efficiency. The thickness of the light emitting layer is about 0.01 to
1 μm is preferable. If the film thickness is too large, the flow of current will be poor, and it will be difficult to drive at a low voltage, which is not preferable. On the other hand, if it is too thin, it becomes difficult to obtain sufficient brightness, which is not preferable.

In the present invention, the light emitting layer may contain an oxazole compound represented by the formula (IV).

[0035]

[Chemical 15]

(Where Ar 'is

[0037]

[Chemical 16]

-Ar "-, -CH = CH-Ar" -CH
= A _{CH-, R 4, R 5,} R 6, and R ₇ is a hydrogen atom, a halogen atom, a lower alkyl group, a lower alkoxy group, a cyano group, also a phenyl group include a trifluoromethyl group and a substituted group, Alternatively, it is a naphthalene ring formed by combining R ₅ and R ₆ or R ₆ and R ₇ , m and l are integers from 1 to 4, k is an integer from 0 to 2, and A
r ″ is an arylene group having 6 to 10 carbon atoms.) Further, an oxazole compound represented by the following formula (V) may be contained in the light emitting layer.

[0039]

[Chemical 17]

(In the formula, Ar ′ has the same meaning as Ar ′ in the formula (IV), and R ₈ and R ₉ are a hydrogen atom, a lower alkyl group, a phenyl group which may have a substituent or a heterocyclic group. ) These oxazole compounds are particularly effective when forming a light emitting layer of a blue light emitting device, and an organic electroluminescent device with less exciplex phenomenon can be obtained. In this case, as the hole injecting and transporting layer, other compounds of the above formula (I) can be used, but it is preferable to use the compound of the formula (I).

In the oxazole compound represented by the general formulas (IV) and (V), "the carbon number is 6 to 1 in Ar".
“The arylene group of 0” is a p-phenylene group, m-
Phenylene group, P, P'-biphenylene group, 1,4-naphthylene group, P, P'-diphenylmethane group or 2,7
-Phenanthrylene group and the like. Further, examples of the “lower alkyl group” include a methyl group, an ethyl group and the like.

On the other hand, examples of the "lower alkoxy group" include methoxy group and ethoxy group. Further, examples of the “halogen atom” include chlorine atom, bromine atom and the like. Further, examples of the substituent of the phenyl group include the above lower alkyl group and lower alkoxy group.

It is preferable that l and m are 1, and k is 0 or 1. Even when an oxazole compound is used for the light emitting layer, the same tendency as described above is exhibited depending on the film thickness, so that the film thickness is preferably substantially the same as the above film thickness. As the oxazole compound represented by the formula (IV) and the formula (V), a compound represented by the following structural formula is preferable.

(Exemplified Oxazole Compound)

[0045]

[Chemical 18]

[0046]

[Chemical 19]

[0047]

[Chemical 20]

In the organic electroluminescent element of the present invention, an electron injecting and transporting layer may be further laminated on the light emitting layer (between the light emitting layer and a cathode described later). This can promote injection and movement of electrons into the light emitting layer. As a material for forming the electron injecting and transporting layer, a compound having a high glass transition point and hardly causing crystallization is preferable, and a compound which can be generally used as an electron transporting compound in the field of electrophotographic photosensitive material is mentioned. Specifically, trinitroflorenone, 1-cyano-1-phenyl-2- (p-trifluoromethyl) ethylene, quinacridone, 2,7-dinitro-9- (4'-trifluoromethylbenzylidene) fluorene ( The following structural formula) is mentioned.

[0049]

[Chemical 21]

This electron injecting and transporting layer can be formed on the light emitting layer by vapor deposition. When vapor-deposited on the light emitting layer, the film thickness is about 10 to 1000 nm, preferably about 20 to 200 nm. The organic electroluminescent device of the present invention is usually prepared by laminating an anode on a substrate, laminating a hole injection layer on the anode, laminating a luminescent layer thereon, and optionally forming a luminescent layer on the luminescent layer. And a cathode is laminated via an electron injecting and transporting layer. As a method for forming each of these layers, a known film forming method such as a vacuum vapor deposition method, a spin coating method, a casting method and a dip coating method can be applied.

According to the organic electroluminescent device of the present invention, since the hole injecting and transporting layer is composed of a specific organic compound having a high glass transition point and hardly causing crystallization, an organic electroluminescent device having a long life can be obtained. To be Furthermore, a pure blue light emitting device can be obtained by using a specific oxazole compound as a light emitting material for forming a light emitting layer.

The present invention will be described in detail below with reference to examples. Examples 1 to 3 (Reference Example) and Comparative Example 1 As shown in FIG. 1, 150 nm of ITO, which is a transparent electrode, was formed as an anode 2a on a transparent glass substrate 1 having a thickness of 1 mm. Further, an aromatic diamino compound shown in Table 1 was formed to a thickness of 75 nm as a hole injecting and transporting layer 3 thereon by a vacuum vapor deposition method. Next, on the hole injecting and transporting layer 3, Alq ₃ having a thickness of 60 nm was formed as the light emitting layer 4 by a vacuum vapor deposition method. Further, Mg: Ag was formed by co-evaporation as the cathode 2b by a vacuum vapor deposition method.

At this time, as a comparative example, JP-A-59-19 was used.
An element using the TPD described in Japanese Patent No. 4393 as a hole injecting and transporting layer was similarly prepared.

[0054]

[Chemical formula 22]

The current value and the maximum brightness value were measured when a voltage of 15 V was applied between the anode 2a and the cathode 2b of the organic electroluminescent device thus prepared. In addition, as the life (half-life), a luminance of 200 cd / m ² to 100 cd / m
^The time required to drop to ² was also measured. The measurement was performed in Ar gas at room temperature (20 ° C.).

[0056]

[Table 1]

From Table 1, the light emitting device using the aromatic diamino compound of the present invention in the hole injecting and transporting layer is superior to TPD in both the maximum brightness and the life.
In particular, compounds 11 and 13 are 3-4 times as compared to TPD.
It was found to have twice the performance. Such factors include high hole transport efficiency of the polybiphenyl structure in the compound of the present invention and high glass transition temperature of the compound.

For example, when the hole transport efficiency and the glass transition point of the compound 11 and TPD are compared, the results shown in Table 2 are obtained.

[0059]

[Table 2]

Examples 4 to 7 (Examples 4 and 5 are reference examples) Among the oxazole compounds, Compound 9 and Compound 19 were used as the light emitting material for forming the light emitting layer 4, and aromatic was used as the hole injecting and transporting layer. Compound 1 of the diamino compounds
Four types of organic electroluminescent devices were prepared by using 1 and Compound 17 in the same manner as in Examples 1 to 3 above. A voltage of 15 V was applied to the light emitting device thus produced, and the maximum brightness and brightness at that time were 200 cd / m ² to 100 c.
The life represented by the time required to decrease to d / m ² and the maximum luminance wavelength were measured. The results are shown in Table 3.

[0061]

[Table 3]

From Table 3, in the light emitting device using the aromatic diamino compound of the present invention for the hole injecting and transporting layer, the TP described in Comparative Example 1 was obtained in both maximum brightness and lifetime.
It was found to be superior to D. Example 8 (Reference Example) In the light emitting device of Example 2, as the light emitting layer 4, 1 part of the aromatic diamino compound 8 was added to 10 parts of Alq _{3 by a} vacuum deposition method.
An organic electroluminescent device was produced in the same manner as in Example 2 except that the light emitting layer was formed in a partially mixed system. As a result of measuring the maximum brightness and the life in the same manner as in Example 2, the maximum brightness was 6900 cd / m ^2, which was not significantly changed, but the life was 870 (hours), which was about 10% better. It is considered that the cause of this is that the crystallization of Alq ₃ was prevented by the addition of the aromatic diamino compound and that the aromatic diamino compound itself also had considerably high light emission characteristics.

[0063]

According to the organic electroluminescence device of the present invention, the hole injecting and transporting layer has a high hole transporting efficiency and is made of a specific organic compound having a high glass transition point and hardly causing crystallization. Therefore, it is possible to obtain an organic electroluminescence device having high brightness and a long life. Further, since the specific oxazole compound is used as the light emitting material for forming the light emitting layer, it is possible to obtain a high-luminance, long-life organic electroluminescence device having a blue light emitting property.

Therefore, the organic electroluminescence device of the present invention can be applied as a light source which requires long-term reliability, and can be applied to displays and the like, and has a high industrial value.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a main part showing an embodiment of an organic electroluminescent device of the present invention.

[Explanation of symbols]

1 substrate 2a anode 3 Hole injection transport layer 4 Light emitting layer 2b cathode

Continuation of the front page (56) Reference JP 10-60425 (JP, A) JP 9-194441 (JP, A) JP 9-148072 (JP, A) JP 8-259934 (JP , A) JP 8-55974 (JP, A) JP 8-53397 (JP, A) JP 8-48656 (JP, A) JP 7-188130 (JP, A) JP 7-310071 (JP, A) JP-A-6-228550 (JP, A) JP-A-1173034 (JP, A) International Publication 95/9147 (WO, A1) (58) Fields investigated (Int.Cl) . ^7, DB name) H05B 33/22 H05B 33/14 C09K 11/06 G03G 5/06 CA (STN) REGISTRY (STN)

Claims (6)

(57) [Claims] 1. An anode, a hole injecting and transporting layer, a light emitting layer and a cathode are laminated on a substrate, and the hole injecting and transporting layer is represented by the formula (I): (In the formula, R ₁ is a hydrogen atom or a methyl group, and R ₂ is water.
Is an elementary atom or a methyl group, n is 1 or 2, and R ₃ is hydrogen
Even if the atom, methoxy group or p-position is replaced by a methyl group
Good phenyl group . ) An organic electroluminescent device comprising an aromatic diamino compound represented by 2. The organic electroluminescent device according to claim 1, wherein an electron injecting and transporting layer is formed between the light emitting layer and the cathode. 3. The organic electroluminescent device according to claim 1, wherein the light emitting layer contains a compound of formula (I). 4. A compound of formula (I) is represented by: The organic electroluminescent device according to any one of claims 1 to 3, which is selected from the aromatic diamino compounds represented by. 5. The light-emitting layer has the formula (IV): (In the formula, Ar ′ is , -Ar "-or -CH = CH-Ar" -CH = CH-
And R ₄ , R ₅ , R ₆ and R ₇ are a hydrogen atom, a halogen atom, a lower alkyl group, a lower alkoxy group, a cyano group, a trifluoromethyl group or a phenyl group which may have a substituent, or R ₅ and R ₆ or a naphthalene ring formed by combining R ₆ and R ₇ , m and l are integers from 1 to 4, k is an integer from 0 to 2, and Ar ″ is the number of carbon atoms. 6-10 is an arylene group.) the organic electroluminescent device according to any one of claims 1-4 containing oxazole compounds represented by. 6. The light-emitting layer has the formula (V): (In the formula, Ar 'has the same meaning as Ar' in formula (IV), and R '
₈ and R ₉ are a hydrogen atom, a lower alkyl group, a phenyl group which may have a substituent or a heterocyclic group. Claim 1-5 or 1 containing oxazole compounds represented by)
The organic electroluminescent device as described in 1.