JP2001273978A - Electric field light emitting element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an organic EL element having a positive hole transport layer which is superior in a light emission stability and a storage stability. SOLUTION: The EL element that is composed of ITO electrode, positive hole transport layer, luminous layer, electron transport layer and magnesium/ silver electrode. Numerous positive hole transport materials are synthesized anew. Aluminum quinoline 3 that also serves mainly as an electron transport layer is used as a luminous layer. As a material of the positive hole transport layer, at least either of tetraphenyl benzidine compound, or triphenyl amine trimer is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting device having at least a hole transporting layer, a light emitting layer and an electron transporting layer and widely used as various display devices. The present invention also relates to an organic electroluminescent device (EL device) having excellent stability.

[0002]

2. Description of the Related Art EL devices have been studied by many researchers for a long time because they can emit light more clearly and sharply than liquid crystal devices because of their self-emission. The light emitting element that has reached the practical level at present is ZnS which is an inorganic phosphor.
There is an element that uses. However, such inorganic EL elements have not been widely used because they require an applied voltage of 200 V or more for light emission.

On the other hand, a light emitting device using an organic material has been far from a practical level.
In 987, Kodak C.I. W. The characteristics have been drastically improved by the laminated structure element developed by Tang et al. They stacked a phosphor that can transport electrons with a stable structure of the deposited film and an organic substance that can transport holes, and succeeded in emitting light by injecting both carriers into the phosphor. did. As a result, the luminous efficiency of the organic electroluminescent device is improved, and 1000 cd at a voltage of 10 V or less.
/ M ² or more. Since then, many researchers have studied to improve the characteristics, and at present, luminescence characteristics of 10,000 cd / m ² or more are obtained by short-time light emission.

The basic light emitting characteristics of such an organic light emitting device are already in a practically usable range, and the biggest cause which hinders the practical use at present is firstly the lack of light emitting stability at the time of driving. Second, storage stability is insufficient. Insufficient light emission stability at the time of driving means a phenomenon in which the light emission luminance is reduced when driving by applying an element current, a non-light emitting area called a dark spot is generated, or destruction is caused by short circuit of the element. In other words, the lack of storage stability refers to a phenomenon in which light emission characteristics are deteriorated only by storing the manufactured device.

The present inventors have studied the mechanism of deterioration of the EL device in order to solve the problems relating to the stability of light emission and the storage stability of the EL device. As a result, it was found that one of the major causes of the property deterioration was the hole transport layer. That is, it is generally used as a hole transport layer (Chemical Formula 3: Abbreviation T)
Hole transport materials such as (PD) and (Formula 4: TPAC) are crystallized due to (1) humidity, temperature and current, and the shape of the thin film becomes non-uniform. (2) It was found that changes such as decomposition of the hole transport layer due to energization were caused, and the luminous property was significantly deteriorated.

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[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide an organic EL device having a hole transporting layer having excellent luminescence stability and storage stability based on such findings. Conditions that such a hole transport material must have are (1) excellent hole transport ability;
(2) It is thermally stable and the glass state is stable;
(3) that a thin film can be formed; and (4) that it is electrically and chemically stable.

[0007]

Means for Solving the Problems To achieve the above object, the present inventors have developed an ITO electrode, a hole transport layer, a light emitting layer,
An EL device including an electron transport layer and a magnesium / silver electrode was experimentally manufactured, and a number of newly synthesized hole transport materials were evaluated. As the light emitting layer, an aluminum quinoline trimer mainly serving also as an electron transporting layer was used. As a material of the hole transport layer, at least a tetraphenylbenzidine compound represented by (Chemical Formula 5), or a tetraphenylbenzidine compound represented by (Chemical Formula 4) and triphenylamine trimeric represented by (Chemical Formula 6) Used one of the bodies.

[0008]

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Where R₁, R_TwoIs a hydrogen atom, lower alkyl
Kill group, lower alkoxy group, phenyl group, lower alkyl
Having a substituent or a lower alkoxy group as a substituent
R group, R_ThreeIs a hydrogen atom, a methyl group, a methoxy group, or
Represents a chloro atom. Also, R ₁, R_TwoAt least one of
Is normal butyl, isobutyl, secondary
Group, tert-butyl group, phenyl group, lower alkyl
Represents a phenyl group having a group or a lower alkoxy group.

[0010]

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However, R ₁ , R ₂ and R ₃ represent a hydrogen atom, a lower alkyl group or a lower alkoxy group, and R ₄ represents a hydrogen atom, a methyl group, a methoxy group or a chloro atom.

[0012]

According to the present invention, as a result of using the above hole transport materials, not only do they have excellent hole transport ability, but also they form good thin films and are thermally stable. It turned out to be. As a result, it was clarified that an EL device having excellent luminescence stability and storage stability could be realized, and could be widely used as a display device.

[0013]

Example 1 A tetraphenylbenzidine compound, which is one of the hole transporting materials of the present invention, is obtained by a condensation reaction of a corresponding 4,4'-dihalogenated biphenyl with a corresponding diphenylamine compound or a corresponding benzidine compound. Can be synthesized by a condensation reaction with an aryl halide. These condensation reactions are methods known as the Ullmann reaction.

Further, a triphenylamine trimer, which is another hole transporting material of the present invention, is obtained by a condensation reaction of a corresponding aniline compound with a corresponding 4'-halogenated biphenylacetanilide compound and hydrolysis thereof. Obtained by a condensation reaction of the resulting triamine compound and an aryl halide. These condensation reactions are methods known as the Ullmann reaction.

The identification of these compounds is based on elemental analysis, IR
The measurement was carried out, and the product was further purified by a recrystallization method using a solvent or a vacuum sublimation method, so that the purity was 99.8% or more. The purity was confirmed by TLC scanner, TG-DTA and melting point measurement. The melting point and the decomposition point are indicators of the thermal stability of the hole transport layer, and the glass transition point is an indicator of the stability of the glassy state. The inventors synthesized materials by changing the substituents of the above three kinds of compounds in various ways. As a result, the melting point and the size of the decomposition point varied depending on the substituent, and in the case of some substituents, a material having a high melting point and a high decomposition point could be obtained. The following are some representative synthetic examples.

(Synthesis Example 1) 70.0 g (0.47 mol) of p-isobutylaniline was dissolved in 126 ml of glacial acetic acid, and 59.9 g (0.58 mol) of acetic anhydride was added dropwise at 30 ° C. After completion of the dropwise addition, the mixture was reacted at 40 ° C. for 1 hour. The reaction solution was poured into 300 ml of water, and the precipitated crystals were filtered, washed with water, and dried. 140 ml of these crystals
Recrystallized with a mixed solution of
-Isobutylacetanilide, 60.4 g (yield 67.
3%). The melting point was 124.5-125.0 ° C.

The above-obtained p-isobutylacetanilide, 17.9 g (0.094 mol), bromobenzene 22.1 g (0.14 mol), anhydrous potassium carbonate,
6.9 g (0.12 mol), copper powder, 0.89 g (0.0
14 mol) and reacted at 168-217 ° C for 14 hours. The reaction product was extracted with 100 ml of toluene,
After insoluble matter was filtered off and removed, it was concentrated to dryness. This was dissolved in isoamyl alcohol, 30 ml, and water, 3.4 g, 85
% Potassium hydroxide, 11.8 g (0.18 mol) was added, and the mixture was hydrolyzed at 131 ° C. After isoamyl alcohol and excess bromobenzene were distilled off by steam distillation, the mixture was extracted with toluene (120 ml), washed with water, dried and concentrated. The concentrate is dried and N-4-isobutylphenylaniline,
17.6 g (86.8% yield) was obtained.

Further, 17.6 g (0.078 mol) of N-4-isobutylphenylaniline, 12.6 g (0.031 mol) of 4,4'-jodobiphenyl, anhydrous potassium carbonate, 12.9 g (0 0.093 mol) and 0.89 g (0.014 mol) of copper powder were mixed.
The reaction was performed at 220 ° C. for 12 hours. The reaction product was extracted with toluene (70 ml), and the insolubles were removed by filtration, removed, and then concentrated to an oil. The obtained crude product was purified by column chromatography (carrier: silica gel, eluent: toluene / n-hexane = 1/6) to give N, N′-bis (p-isobutylphenyl) -N, N′-. 8.5 g (yield 45.7%) of diphenylbenzidine was obtained. Melting point 13
It was 3.8 to 135.3 ° C. The product was identified by elemental analysis and IR measurement. The elemental analysis values are as follows. Carbon: measured 87.77%, theoretical: 87.9
6%, hydrogen: measured value 7.43%, theoretical value 7.38%, nitrogen: measured value 4.51%, theoretical value 4,66%.

(Synthesis Example 2) Acetanilide, 23.
0 g (0.17 mol) and 4,4′-jodobiphenyl, 85.3 g (0.21 mol), anhydrous potassium carbonate,
24.9 g (0.18 mol), copper powder, 2.48 g (0.
039 mol), 40 ml of nitrobenzene, and 1
The reaction was performed at 90 to 205 ° C for 10 hours. The reaction product was extracted with 200 ml of toluene, and the insoluble matter was removed by filtration.
Concentrated to dryness. This was purified by column chromatography (carrier: silica gel, eluent: toluene / n-hexane = 1/6), and N- (4′-iodo-4-biphenylyl) was obtained.
Acetanilide was obtained in an amount of 45.5 g (yield 64.8%). The melting point was 135.0-136.0 ° C.

Subsequently, N- (4'-iodo-4-biphenylyl) acetanilide, 18.2 g (0.044 mol), aniline, 1.84 g (0.020 mol), anhydrous potassium carbonate, 6.91 g (0 0.050 mol) and copper powder, 0.64 g (0.010 mol), nitrobenzene,
10 ml were mixed and reacted at 190 to 215 ° C. for 15 hours. The reaction product was extracted with 100 ml of toluene, and the insolubles were removed by filtration, removed, and then concentrated to an oil. The oily substance was dissolved in 50 ml of isoamyl alcohol, 1 ml of water, 85% potassium hydroxide, 2.64 g (0.04
0 mol) and hydrolyzed at 130 ° C. After isoamyl alcohol is distilled off by steam distillation, toluene 250m
The extract was washed with water, dried and concentrated. The concentrate is purified by column chromatography (carrier: silica gel, eluent:
Toluene / n-hexane = 3/1) and N, N′-bis (4′-phenylamino-4-biphenylyl) aniline (8.85 g, yield 76.3%) were obtained.

Further, N, N'-bis (4'-phenylamino-4-biphenylyl) aniline, 8.70 g (0.
015 mol), iodobenzene, 6.74 g (0.03 g)
3 mol), 4.56 g (0.33 mol) of anhydrous potassium carbonate, 0.48 g (0.0075 mol) of copper powder, 10 ml of nitrobenzene, and 1 ml at 195 to 205 ° C.
The reaction was performed for 6 hours. The reaction product was extracted with 100 ml of toluene, insolubles were filtered off, concentrated, and n-hexane was added to take out crude crystals. The crude crystals were purified by column chromatography, and N, N'-bis (4'-diphenylamino-4-
Biphenylyl) aniline, 5.50 g (Yield: 50.1)
%). No clear melting point was found. Elemental analysis,
The product was identified by IR measurement. The elemental analysis values are as follows. Carbon: measured 88.80%, theoretical: 88.61%, hydrogen: measured 5.77%, theoretical 5.65%, nitrogen: measured 5.62%, theoretical 5.74
%.

[0022]

Example 2 Next, these were actually evaluated as EL devices, and the light emission characteristics, stability of the light emission characteristics, and storage stability of the devices were examined. As shown in FIG. 1, the EL element has a structure in which an ITO electrode is previously formed as a transparent electrode 2 on a glass substrate 1, and a hole transport layer 3, an electron transport layer / light emitting layer 4, and a Mg / Ag electrode 5 are formed on the glass substrate 1. It was produced by vapor deposition in order. First,
A sufficiently cleaned glass substrate (the ITO electrode has been formed),
An aluminum quinoline trimer purified as a hole transporting material and an electron transporting luminescent material was set in a vapor deposition apparatus. A hole transport layer was deposited at a rate of 0.1 nm / sec, and a sample having a different film thickness was prepared to determine a thickness at which optimal light emission was obtained. Although the film thickness varies depending on the material, the optimum film thickness was between 40 and 60 nm. The film thickness was monitored using a quartz oscillator. The deposition of aluminum quinoline trimer is also 0.1 nm /
The film thickness was 50 nm. Mg /
The Ag electrode was formed at a speed of 0.4 nm / sec, and the thickness was set to 100 nm. All of these depositions were performed continuously without breaking vacuum. Immediately after the device was manufactured, the electrodes were taken out in dry nitrogen, and the characteristics were measured.

The light emitting characteristics of the obtained device are 100 mA / c.
It was defined as the emission luminance when a current of m ² was applied. In addition, the stability of light emission was measured by applying a current for obtaining light emission of 200 cd / m ² continuously, and measuring a change in light emission luminance at that time. The lifetime of light emission was defined as the time required for the luminance to decrease to 100 cd / m ² . The storage stability was defined as the time required for the device to stand for a certain period of time in dry air at room temperature, applying a current of 20 mA / cm ² until the luminance became half of the initial light emission characteristics.

In order to evaluate the hole transporting material of the present invention, an aluminum quinoline trimer was used as the electron transporting layer and the light emitting layer 4. Of course, in the present invention, various rare earth complexes and oxazole derivatives were used as the material of the light emitting layer. And various materials such as polyparaphenylene vinylene. Also,
By adding a dopant such as quinacridone or coumarin to the light-emitting layer, a higher-performance EL can be manufactured. Furthermore, an electroluminescent device including three layers of an electron transporting layer, a light emitting layer, and a hole transporting layer can be provided. Further, by combining the hole transporting material of the present invention with a suitable electron transporting material, the hole transporting layer can be used as a light emitting layer.

As a result of such examination, it was found that the hole transport material was 1
It was found that when the resin had a melting point of 30 ° C. or more and a decomposition point of 300 ° C. or more, excellent luminescence stability and storage stability were obtained. Therefore, the substituent of the above compound is not limited to the substituent of the present invention, and any compound having the above melting point and decomposition point can be used.

The hole transporting material according to the present invention can be used alone, but two or more kinds can be used in a mixed state by forming a film by co-evaporation or the like. Further, the hole transport material of the present invention can be used by co-evaporation with a conventional hole transport material such as TPAC or TPD. The simultaneous deposition of two or more types often exhibits an effect of making crystallization difficult.

(Element Example 1) A thoroughly washed ITO electrode, a tetraphenylbenzidine compound (1) (R ₁ = pn-Bu, R ₂ = H, R ₃ = H, m as a hole transport material)
(p = 132.9 ° C), and a purified aluminum quinoline trimer as an electron transporting luminescent material was set in a vapor deposition apparatus. 0.
Compound (1) was deposited at a rate of 1 nm / sec to a thickness of 50 nm. The film thickness was monitored using a quartz oscillator.
Alkyminoline was also deposited at a rate of 0.1 nm / sec, and the film thickness was 50 nm. The Mg / Ag electrode is operated at a rate of 0.4 nm / sec and the thickness is set to 100 nm.
And All of these depositions were performed continuously without breaking vacuum. Immediately after the fabrication of the device, the electrode was taken out in dry nitrogen, and the characteristics were subsequently measured. The light emission characteristics were 2500 cd / m ² , the light emission lifetime was 620 Hr, and the storage stability was 2200 Hr.

For comparison, an EL device was manufactured under the same conditions using (Chemical Formula 3: TPD for short) and (Chemical formula: TPAC for short) as the hole transport material, and the characteristics thereof were examined. The light emission characteristics, lifetime of light emission, and storage stability of TPD are 220
0 cd / m ² , 220 Hr, and 460 Hr. On the other hand, the light emission property, light emission life property, and storage stability of TPAC are 2500 cd / m ² , 280 Hr, and 560 H, respectively.
r. From this, it was found that the tetraphenylbenzidine compound (1) in this example was excellent in luminescence life and storage stability.

(Embodiment 2) Same as in Embodiment 1
Respectively, using a tetraphenylbenzidine compound (2)
(R₁= IBu, R_Two= H, R_Three= H), (3) (R₁
= IBu, R_Two= H, R_Three= CH_Three), (4) (R₁=
tBu, R_Two= H, R_Three= H), (5) (R₁= TB
u, R_Two= TBu, R_Three= H), (6) (R1 = C₆H
_Five, R_Two= H, R _Three= H), (7) (R₁= C₆H_Five,
R_Two= C₆H_Five, R_Three= H), (8) (R ₁= C
₆H_Five, R_Two= C₆H_Five, R_Three= CH_Three), (9) (R
₁= P-CH_Three-C₆H_Four, R_Two= H, R_Three= OC
H_Three), (10) (R₁= P-CH_Three-C₆H ₆, R_Two
= P-CH_Three-C₆H_Four, R_Three= H) as the hole transport material
The used EL element was manufactured and its characteristics were evaluated. So
FIG. 2 shows the results. In addition, the above tetraphenylbenzene
R of gin compounds (2) to (10)₁And R_TwoSubstitution position
All positions indicate the p-position. From this, the technology according to the present invention is used.
Traphenylbenzidine compounds (2) to (10) are
It was found that it had excellent light life and storage stability.

(Embodiment 3) Same as in Embodiment 1
The triphenylamine trimer compound (1
1) (R₁= H, R_Two= H, R_Three= H, R_Four= H),
(12) (R₁= H, R_Two= H, R_Three= H, R_Four= CH
_Three), (13) (R₁= TBu, R_Two= P-CH_Three, R
_Three= P-CH_Three, R_Four= H), (14) (R₁= H, R
_Two= H, R_Three= H, R_Four= OCH_Three), (15) (R₁
= H, R_Two= M-CH_Three, R_Three= M-CH_Three, R_Four=
H), (16) (R₁= H, R_Two= P-OCH_Three, R_Three
= P-OCH_Three, R_Four= H), (17) (R₁= P-C
H_Three, R_Two= H, R_Three= H, R_Four= CH _Three), (1)
8) (R₁= P-CH_Three, R_Two= P-iBu, R_Three= P
-IBu, R_Four= H), (19) (R₁= PnBu,
R_Two= M-CH_Three, R_Three= H, R_Four= Cl) (20)
(R₁= P-OC_TwoH_Five, R_Two= P-CH_Three, R_Three= P
-CH _Three, R_Four= H) using EL as a hole transport material
A device was manufactured and its characteristics were evaluated. Fig. 3 shows the results.
Show. This indicates that triphenylamine 3 according to the present invention
Dimer compounds (11) to (20) have a luminescent lifetime and storage stability
It turned out that it was excellent.

(Embodiment 4) The triphenylamine trimer compound (1) was prepared in the same manner as in Embodiment 1.
1) (R ₁ = H, R ₂ = H, R ₃ = H, R ₄ = H) and a tetraphenylbenzidine compound (4) (R ₁ = p-tB
u, R ₂ = H, R ₃ = H) were co-evaporated to produce an EL device used as a hole transport material, and its characteristics were evaluated. The light emission characteristic is 3300 cd / m ² , and the light emission life is 720 H
r, storage stability was 2900 Hr. From the results, the triphenylamine trimer compound (11) according to the present invention was obtained.
It was found that the hole transport layer formed by co-evaporation of the compound and the tetraphenylbenzidine compound (4) was excellent in light emission life and storage stability.

[0032]

As described above, the present invention relates to an electroluminescent device characterized by using a tetraphenylbenzidine compound and a triphenylamine trimer as the material of the hole transport layer. By using a material, it is possible to realize an electroluminescent device in which luminescence stability and storage stability, which are the most serious problems of the conventional organic electroluminescent device, are remarkably improved.

[Brief description of the drawings]

FIG. 1 is an enlarged partial cross-sectional perspective view showing a configuration of an electroluminescent device according to an embodiment of the present invention.

FIG. 2 is a list showing characteristics of an electroluminescent device using a tetraphenylbenzidine compound as a hole transport layer in one embodiment of the present invention.

FIG. 3 is a list showing characteristics of an electroluminescent device using a triphenylamine trimer compound as a hole transport layer in one embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Glass substrate 2 Transparent electrode 3 Hole transport layer 4 Electron transport layer and light emitting layer 5 Mg / Ag electrode

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification FI FI Theme Court (Reference) (72) Inventor Masao Fukuyama 3-10-1 Higashi-Mita, Tama-ku, Kawasaki City, Kanagawa Prefecture Matsushita Giken Co., Ltd. ( 72) Inventor Mutsumi Murakami Matsushita Giken Co., Ltd. 3-10-1, Higashi Mita, Tama-ku, Kawasaki City, Kanagawa Prefecture (72) Inventor Taro Nanbu 3-chome 1-1, Higashi Mita Tama-ku, Kawasaki City, Kanagawa Prefecture Matsushita Giken Co., Ltd. (72) Inventor Hiromitsu Toyama 45 Miyukigaoka, Tsukuba City, Ibaraki Prefecture Inside the Hodogaya Chemical Industry Co., Ltd.Tsukuba Research Laboratory (72) Inventor Masahiko Oshino 45 Miyukigaoka Tsukuba City, Ibaraki Prefecture Hodogaya Chemical Industry Co., Ltd. Tsukuba Research Institute

Claims (5)

[Claims] 1. An electroluminescent device having a film grown by a vapor deposition method using a tetraphenylbenzidine compound represented by the following general formula. Embedded image However, R ₁ and R ₂ are a hydrogen atom, a lower alkyl group, a lower alkoxy group, a phenyl group, a phenyl group having a lower alkyl group or a lower alkoxy group as a substituent, and R ₃ is a hydrogen atom, a methyl group, a methoxy group, or Represents a chloro atom. At least one of R ₁ and R ₂ represents a phenyl group having a normal butyl group, isobutyl group, secondary butyl group, tert-butyl group, phenyl group, lower alkyl group or lower alkoxy group. 2. The method according to claim 1, further comprising an electrode, a hole transport layer, a light emitting layer, an electron transport layer, and an electrode, wherein at least the hole transport layer is provided with a film grown by an evaporation method. Item 2. An electroluminescent device according to item 1. 3. An electrode, a hole transport layer, a light emitting layer, an electron transport layer and an electrode, wherein the hole transport layer is at least two kinds selected from the tetraphenylbenzidine compound according to claim 1. A material containing a compound, or one or more selected from the tetraphenylbenzidine compounds of claim 1 and one or more selected from tetraphenylamine trimers represented by the following general formula: An electroluminescent device characterized by using a film grown by a vapor deposition method using a material containing the above compound. Embedded image Here, R ₁ , R ₂ and R ₃ represent a hydrogen atom, a lower alkyl group or a lower alkoxy group, and R ₄ represents a hydrogen atom, a methyl group, a methoxy group or a chloro atom. 4. The electroluminescent device according to claim 2, wherein the electron transport layer also functions as a light emitting layer. 5. The electroluminescent device according to claim 2, wherein the hole transport layer also functions as a light emitting layer.