JP3194657B2 - EL device - Google Patents

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 storage stability. 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 (Chemical Formula 4: TPAC) become (1) crystallized by 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. One of the benzidine dimers described in (Chem. 5) was used as a material for the hole transport layer.

[0008]

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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.
In the chemical formula (5), X represents a substituent having the following structure described in the chemical formula (6).

[0010]

Embedded image R ₅ represents a hydrogen atom, a methyl group, a methoxy group, or a chloro atom.

[0011]

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.

[0012]

Example 1 A benzidine dimer, which is one of the hole transport materials of the present invention, is a novel compound, which is a condensation reaction of a corresponding triphenylbenzidine compound with a dihalide or a corresponding diamine. The compound can be synthesized by hydrolyzing a product obtained by a condensation reaction between an N, N'-diacetyl compound of a compound and a corresponding 4'-halogenated biphenylacetanilide compound, and then subjecting the product to a condensation reaction with a corresponding 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 typical synthesis examples are shown below.

(Synthesis Examples) Acetanilide, 20.3 g (0.15 mol) and 4,
4'-Jodobiphenyl, 73.1 g (0.18 mol), anhydrous potassium carbonate, 22.1 g (0.16 mol), copper powder, 2.16 g (0.034 mol), nilotbenzene, 35 ml were mixed. 10 at 190-205 ° C
Allowed to react for hours. The reaction product was extracted with 200 ml of toluene, the insoluble matter was separated by filtration, removed, and then concentrated to dryness. This was purified by column chromatography (carrier: silica gel, eluent: toluene / ethyl acetate = 6/1), and N- (4′-iodo-4-biphenylyl) acetanilide, 40.2 g
(64.8% yield). Melting point 135.0-13
It was 6.0 ° C.

Then, N- (4'-iodo-4-biphenylyl) acetanilide, 13.2 g (0.032 mol), diphenylamine, 6.60 g (0.039 mol), anhydrous potassium carbonate, 5.53 g (0 0.040 mol) and copper powder, 0.45 g (0.007 mol), nitrobenzene and 10 ml were mixed and reacted at 200 to 212 ° C for 15 hours. 100 ml of the reaction product in toluene
, And the insoluble matter was removed by filtration, removed, and then concentrated to an oily substance. Oily substance is isoamyl alcohol, 60ml
And 1 ml of water, 85% potassium hydroxide, 2.64
g (0.040 mol) was added and hydrolyzed at 130 ° C. After isoamyl alcohol was distilled off by steam distillation, the mixture was extracted with 250 ml of toluene, washed with water, dried and concentrated. The concentrate was purified by column chromatography (carrier: silica gel, eluent: toluene / n-hexane = 1/2).
10.5 g (72.2% yield) of N, N'-triphenylbenzidine was obtained. Melting point 167.5-168.5 °
C.

Further, 8.66 g (0.021 mol) of N, N, N'-triphenylbenzidine, 4.06 g (0.01 mol) of 4,4'-jodobiphenyl, anhydrous potassium carbonate, 2.90 g ( 0.021 mol), copper powder, 0.
32 g (0.005 mol), nitrobenzene, 10 ml
And reacted at 195 to 210 ° C. for 20 hours.
The reaction product was extracted with 140 ml of toluene, the insolubles were separated by filtration, and after concentration, 120 ml of n-hexane was added to take out crude crystals. The crude crystals are purified by column chromatography (carrier: silica gel, eluent: toluene / n-hexane = 1).
/ 2), N, N'-bis (4'-diphenylamino-4
-Biphenylyl) -N, N'-diphenylbenzidine;
4.73 g (yield: 48.5%) was obtained. Melting point is 24
2.5-243.5 ° C. The product was identified by elemental analysis and IR measurement. The elemental analysis values are as follows. Carbon: measured 88.75%, theoretical: 88.75%.
67%, hydrogen: measured 5.70%, theoretical 5.58%,
Nitrogen: measured 5.68%, theoretical 5.75%.

[0017]

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. FIG. 1 is a partial cross-sectional enlarged perspective view showing one configuration example of the organic electroluminescent element in the present embodiment. As shown in FIG. 1, an EL element includes a transparent electrode 2 on a glass substrate 1.
A hole transport layer 3, an electron transport layer and light emitting layer 4, and a Mg / Ag electrode 5
In this order. First, a sufficiently washed glass substrate (the ITO electrode was formed into a film), a hole transporting material, and an aluminum quinoline trimer purified as an electron transporting luminescent material were 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. Aluminum quinoline 3
The deposition of the monomer was performed at the same rate of 0.1 nm / sec, and the film thickness was 50 nm. 0.4 nm for Mg / Ag electrode
/ Sec at a rate of 100 nm. 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 emission 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 a study, 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 transport material according to the present invention can be used alone, but two or more types 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.

(Device Example 1) A sufficiently cleaned ITO electrode, and a benzidine dimer compound (21) (R ₁ = H, R ₂ = H, R ₃ =
H, R ₄ = H, X = (A)), (22) (R ₁ = H, R
₂ = H, R ₃ = H, R ₄ = H, X = (B)), (23)
(R ₁ = H, R ₂ = H, R ₃ = H, R ₄ = H, R ₅ =
H, X = (C)), (24) (R ₁ = H, R ₂ = H, R
₃ = H, R ₄ = CH ₃ , R ₅ = CH ₃ , X = (C)),
(25) (R ₁ = H, R ₂ = H, R ₃ = H, R ₄ = H,
X = (D)), (26) (R ₁ = H, R ₂ = H, R ₃ =
H, R ₄ = CH ₃ , X = (D)), (27) (R ₁ =
H, R ₂ = m-OCH ₃ , R ₃ = m-OCH ₃ , R ₄ =
OCH ₃ , X = (E: bonding position is 1,4-)),
(28) (R ₁ = H, R ₂ = p-tBu, R ₃ = H, R
₄ = Cl, X = (F)), (29) (R ₁ = p-OC _{2
H 5, R 2 = H,} R 3 = H, R 4 = H, X = (G))
_{(30) (R 1 = H} , R 2 = p-nPr, R 3 = p-n
Pr, R ₄ = H, X = (H)), aluminum quinoline 3 purified as an electron transporting luminescent material
The monomer was set in a vapor deposition device. The benzidine dimer compounds (21) to (30) were converted to 50 nm at a rate of 0.1 nm / sec.
The thickness was evaporated. The film thickness was monitored using a quartz oscillator. Alkyminoline trimer deposition was also 0.1
It was performed at a speed of nm / sec, and the film thickness was 50 nm.
The Mg / Ag electrode was operated at a speed of 0.4 nm / sec, and the thickness was set to 100 nm. In each of these depositions, an EL element was manufactured continuously without breaking vacuum. Immediately after the preparation of the EL element, the electrode was taken out in dry nitrogen, and the characteristics were continuously measured. The result is shown in FIG. For example, regarding the benzidine dimer compound (21), the light emission characteristic is 3400 cd / m ² and the light emission lifetime is 7
The storage stability was 60 hours, and the storage stability was 3900 hours. The other benzidine dimer compounds (22) to (30) are also as shown in FIG.

For comparison, an EL element was manufactured under the same conditions using (Chemical Formula 3: TPD for short) and (Formula 4: 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 these results, it was found that the benzidine dimer compounds (21) to (30) according to the present invention were excellent in luminescence life and storage stability.

(Device Example 2) In the same manner as in Device Example 1, a benzidine dimer compound (23) (R ₁ = H, R ₂ = H, R ₃ =
H, R ₄ = H, R ₅ = H, X = (C)), and an EL device using a purified triazole as an electron transporting material was produced, and its characteristics were evaluated. In this case, blue light emission from a hole transporting material was confirmed because triazole has a high hole blocking property. The emission characteristics are 200 cd / m ² ,
The lifetime of the light emission was 300 hr, and the storage stability was 2700 hr. As a hole transport material for comparison,
An EL element was fabricated under the same conditions using PD) and the characteristics thereof were examined. The emission characteristics, emission lifetime characteristics, and storage stability of TPD are 100 cd / m ² , 110 Hr, and 41, respectively.
It was 0 Hr. From this, it was found that the benzidine dimer compound (23) according to the present invention was excellent in luminescence life and storage stability.

[0026]

As described above, the present invention relates to an electroluminescent device characterized by using a benzidine dimer as a material of a hole transport layer. Thus, an electroluminescent device in which luminescence stability and storage stability, which are the most serious problems of the organic electroluminescent device, can be significantly 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 benzidine dimer 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

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Mutsumi Murakami 3-10-1, Higashi-Mita, Tama-ku, Kawasaki-shi, Kanagawa Matsushita Giken Co., Ltd. (72) Inventor Taro Minamibu Tama-ku, Kawasaki-shi, Kanagawa 3-10-1, Higashi Mita Matsushita Giken Co., Ltd. (72) Inventor Hiromitsu Toyama 45 Miyukigaoka, Tsukuba, Ibaraki Prefecture Hodogaya Chemical Industry Co., Ltd.Tsukuba Research Laboratory (72) Inventor Masahiko Oshino Ibaraki 45, Miyukigaoka, Tsukuba-shi, Hokkaido Hodogaya Chemical Industry Co., Ltd. Tsukuba Research Laboratory (56) References JP-A-63-113467 (JP, A) JP-A-63-223651 (JP, A) JP-A-3-5444 (JP, A) JP-A-5-239455 (JP, A) JP-A-7-324059 (JP, A) JP-A-5-299174 (JP, A) US Patent 4,265,990 (US, A) International Publication 95 / 9147 (WO, A 1) “Organic EL Device Development Strategy”, Science Forum, (1992), pp. 73-74, 98-99, 119 Appl. Phys. Lett. , Vol. 66, No. 20,15 May 1995, pp. 2679-2681 (58) Field surveyed (Int. Cl. ⁷ , DB name) H05B 33/00-33/28 CA (STN) REGISTRY (STN)

Claims (5)

(57) [Claims] 1. An electroluminescent device using a benzidine dimer represented by the following general formula. 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. X represents a substituent having the following structure. Embedded image R ₅ represents a hydrogen atom, a methyl group, a methoxy group, or a chloro atom. 2. A thermostable electroluminescent device using the benzidine dimer described by the general formula according to claim 1. 3. 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 3. The electroluminescent device according to item 1 or 2. 4. The electroluminescent device according to claim 3, wherein the electron transport layer also functions as a light emitting layer. 5. The electroluminescent device according to claim 3, wherein the hole transport layer also serves as a light emitting layer.