JP3529735B2 - Electroluminescent device - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to a hole transport layer, luminescent layer,
It has electron transport layer, a light-emitting element that is utilized extensively as various display devices, about the low applied voltage, high luminance, and an organic electroluminescent device excellent in stability (EL device) <br />

[0002]

2. Description of the Related Art EL devices have been researched by many researchers for a long time because they are self-luminous and thus are brighter and more vivid than liquid crystal devices. ZnS, which is an inorganic phosphor, is currently used as a light-emitting element that has reached a practical level.
There is an element using. However, since such an inorganic EL element requires an applied voltage of 200 V or more for light emission, it has not been widely used.

On the other hand, a light emitting element using an organic material is far from a practical level, but
In 987, the C.I. W. The laminated structure element developed by Tang et al. Has dramatically improved its characteristics. They succeeded in stacking a phosphor with a stable structure of vapor-deposited film that can transport electrons and an organic substance that can transport holes, and injecting both carriers into the phosphor to emit light. 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
A light emission of / m ² or more can be obtained. Since then, many researchers have conducted researches to improve the characteristics, and at present, light emission characteristics of 10,000 cd / m ² or more are obtained in short-time light emission.

The basic light-emitting characteristics of such an organic light-emitting device are already in a practical range, and the most important factor that hinders its practical use is the lack of light-emission stability during driving. Second, there is a lack of storage stability. Insufficient light emission stability at the time of driving here means that the light emission luminance is lowered when an element current is applied to drive, and a region called a dark spot which does not emit light is generated, or destruction occurs due to a short circuit of the element. That is, the lack of storage stability refers to a phenomenon in which the light emitting characteristics deteriorate even when the manufactured device is stored.

The present inventors have examined the mechanism of deterioration in order to solve the problems concerning the emission stability and storage stability of such EL devices. As a result, it was found that one of the major causes of the characteristic deterioration was the hole transport layer. That is, it is generally used as a hole transport layer ( Chemical formula 4 : abbreviation T
A hole-transporting material such as PD) or ( Chemical formula 5 : abbreviated TPAC) is (1) crystallized by humidity, temperature, and current, and the shape of the thin film becomes uneven. (..2) It was found that the hole transport layer undergoes changes such as decomposition upon application of electric current, which significantly deteriorates the light emitting property.

[Chemical 4]

[Chemical 5]

[0006]

An object of the present invention is to provide an organic EL device having a hole transport layer which is excellent in light emission stability and storage stability based on such findings . The conditions that must be provided in the hole transport material, such as this, (1) to have excellent hole transport capability,
(2) It is thermally stable and the glass state is stable,
(3) A thin film can be formed, (4) Electrical and chemical stability, etc. can be mentioned.

[0007]

In order to achieve the above object, the present inventors have made an ITO electrode, a hole transport layer, a light emitting layer,
An EL device consisting of an electron transport layer and a magnesium / silver electrode was prototyped 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 transport layer was used. As the material of the hole transport layer, at least a tetraphenylbenzidine compound described in ( Chemical Formula 6 ), or a tetraphenylbenzidine compound described in ( Chemical Formula 6 ) and triphenylamine 3 amount described in ( Chemical Formula 7 ) Used one of the body.

[0008]

[Chemical 6]

[0009] However, R1, R2 is hydrogen atom, te sheet <br/> Yaribuchiru group, a phenyl group, a phenyl group having as a substituent a lower alkyl group or lower alkoxy radical, R
3 represents a hydrogen atom, a methyl group or a methoxy group. Further, R1 and R2 are not hydrogen atoms at the same time.

[0010]

[Chemical 7]

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]

As a result of using the above hole transporting material, the present invention not only has excellent hole transporting ability but also forms a good thin film and is thermally stable. It turned out that As a result, it became clear that an EL device having excellent light emission stability and storage stability could be realized, and it could be widely used as a display device.

[0013]

Example 1 A tetraphenylbenzidine compound, which is one of the hole transport materials of the present invention, corresponds to a condensation reaction between a corresponding 4,4′-dihalogenated biphenyl and a corresponding diphenylamine compound, or a corresponding benzidine compound. It can be synthesized by a condensation reaction with an aryl halide. These condensation reactions are methods known as the Ullmann reaction.

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

Identification of these compounds was carried out by elemental analysis, IR
The purity was set to 99.8% or more by performing the measurement and further purifying by the recrystallization method using a solvent and the vacuum sublimation method. The purity was confirmed by TLC scanner, TG-DTA and melting point measurement. The melting point and the decomposition point serve as a measure of the thermal stability of the hole transport layer, and the glass transition point serves as a measure of the stability of the glass state. Inventors synthesized material by changing the substituents on Symbol compound in various ways. As a result, the sizes of the melting point and the decomposition point changed depending on the substituents, and in the case of some substituents, a material having a high melting point and a high decomposition point could be obtained. Below are some representative synthetic examples.

(Synthesis Reference Example 1) p-isobutylaniline, 70.0 g (0.47 mol)
Was dissolved in 126 ml of glacial acetic acid, and acetic anhydride 5 was added at 30 ° C.
9.9 g (0.58 mol) was added dropwise, and after the addition was completed, 40 °
The reaction was carried out at 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. The crystals were recrystallized with a mixed solution of 140 ml of toluene, 700 ml of n-hexane, and p-isobutylacetanilide, 6
0.4 g (yield 67.3%) was obtained. Melting point 124.5
It was ~ 125.0 ° C.

The above-obtained p-isobutylacetanilide, 17.9 g (0.094 mol), brombenzene 22.1 g (0.14 mol), anhydrous potassium carbonate, 1
6.9 g (0.12 mol), copper powder, 0.89 g (0.0
14 mol) were mixed and reacted at 168 to 217 ° C for 14 hours. The reaction product is extracted with 100 ml of toluene,
The insoluble matter was filtered off, removed, and concentrated to dryness. This was dissolved in 30 ml of isoamyl alcohol, 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 distilling off isoamyl alcohol and excess bromine by steam distillation, the mixture was extracted with 120 ml of toluene, washed with water, dried and concentrated. The concentrate is dried, N-4-isobutylphenylaniline,
17.6 g (yield 86.8%) was obtained.

Further, N-4-isobutylphenylaniline, 17.6 g (0.078 mol), 4,4'-jodobiphenyl, 12.6 g (0.031 mol), anhydrous potassium carbonate, 12.9 g (0 0.093 mol) and copper powder, 0.89 g (0.014 mol) are mixed,
The reaction was carried out at 220 ° C for 12 hours. The reaction product was extracted with 70 ml of toluene, the insoluble matter was filtered off, removed, and concentrated to give an oily substance. The obtained crude product was purified by column chromatography (carrier: silica gel, eluent: toluene / n-hexane = 1/6) and N, N′-bis (p-isobutylphenyl) -N, N′- 8.5 g (yield 45.7%) of diphenylbenzidine was obtained. Melting point is 13
The temperature 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 value 87.77%, theoretical value: 87.9
6%, hydrogen: measured value 7.43%, theoretical value 7.38%, nitrogen: measured value 4.51%, theoretical value 4,66%.

(Synthesis Reference Example 2) Acetanilide, 23.0 g (0.17 mol) and 4,
4'-yaw de biphenyl, 85.3 g (0.21 mol), anhydrous potassium carbonate, 24.9 g (0.18 mol), copper powder, 2.48 g (0.039 mol), nitrobenzene, mixing 40ml 10 at 190-205 ° C
Reacted for hours. The reaction product was extracted with 200 ml of toluene, the insoluble matter was filtered off, removed and then concentrated to dryness. This was purified by column chromatography (carrier: silica gel, eluent: toluene / n-hexane = 1/6) and N- (4'-
Iodo-4-biphenylyl) acetanilide, 45.5
g (yield 64.8%) was obtained. Melting point 135.0-13
It was 6.0 ° 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 was mixed and reacted at 190 to 215 ° C for 15 hours. The reaction product was extracted with 100 ml of toluene, the insoluble matter was filtered off, removed, and concentrated to give an oily substance. The oily substance was dissolved in 50 ml of isoamyl alcohol, and 1 ml of water, 85% potassium hydroxide, 2.64 g (0.04
0 mol) was added, and the mixture was hydrolyzed at 130 ° C. After distilling off isoamyl alcohol by steam distillation, toluene 250m
It was extracted with 1, washed with water, dried and concentrated. The concentrate was 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
3 mol), anhydrous potassium carbonate, 4.56 g (0.33 mol), copper powder, 0.48 g (0.0075 mol), nitrobenzene, and 10 ml are mixed, and 1 at 195 to 205 ° C
The reaction was carried out for 6 hours. The reaction product was extracted with 100 ml of toluene, the insoluble matter was filtered off, and after concentration, 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
%) Was obtained. No clear melting point was seen. Elemental analysis,
The product was identified by IR measurement. The elemental analysis values are as follows. Carbon: measured value 88.80%, theoretical value: 88.61%, hydrogen: measured value 5.77%, theoretical value 5.65%, nitrogen: measured value 5.62%, theoretical value 5.74
%.

[0022]

[Embodiment 2] Next, these were actually evaluated as EL devices, and the emission characteristics of the devices, the stability of the emission characteristics, and the storage stability were examined. As shown in FIG. 1, the EL device comprises a hole transport layer 3, an electron transport layer / light emitting layer 4, and a Mg / Ag electrode 5 on which an ITO electrode is preliminarily formed as a transparent electrode 2 on a glass substrate 1. It vapor-deposited in order and produced. First,
A glass substrate that has been thoroughly washed (ITO electrodes have already been formed into a film),
The purified aluminum quinoline trimer as a hole transport material and an electron transport light emitting material was set in a vapor deposition apparatus. A hole transport layer was vapor-deposited at a rate of 0.1 nm / sec, samples having different film thicknesses were prepared, and the thickness at which optimum light emission was obtained was determined. Although the film thickness depends on the material, the optimum film thickness was between 40 and 60 nm. The film thickness was monitored by a crystal oscillator. The vapor deposition of aluminum quinoline trimer is also 0.1 nm /
The film thickness was set to 50 nm at a speed of 2 seconds. Mg /
The Ag electrode was formed at a speed of 0.4 nm / sec and its thickness was 100 nm. All of these vapor depositions were continuously performed without breaking the vacuum. Immediately after the device was manufactured, the electrode was taken out in dry nitrogen and the characteristics were continuously measured.

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

In order to evaluate the hole transporting material of the present invention, aluminum quinoline trimer was used as the electron transporting layer / light emitting layer 4. Of course, in the present invention, various rare earth complexes and oxazole derivatives are used as the material of the light emitting layer. Various materials such as polyparaphenylene vinylene can be used. 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 having three layers of an electron transport layer, a light emitting layer, and a hole transport layer can be used. Further, by combining the hole transport material of the present invention with a suitable electron transport material, the hole transport layer can be used as a light emitting layer.

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

The hole transporting material according to the present invention can be used alone, but two or more kinds can be formed into a film by co-evaporation or the like and used in a mixed state. Furthermore, the hole transport material of the present invention can be used by co-evaporation with conventional hole transport materials TPAC and TPD. When two or more kinds are simultaneously vapor-deposited and used, the effect of making it difficult to cause crystallization is often exhibited.

(Element Reference Example 1) A thoroughly washed ITO electrode and a tetraphenylbenzidine compound (1) as a hole transport material (R1 = pnB)
u, R2 = H, R3 = H, mp = 132.9 ° C),
The purified aluminum quinoline trimer as an electron-transporting luminescent material was set in the vapor deposition apparatus. Compound (1) was vapor-deposited with a thickness of 50 nm at a rate of 0.1 nm / sec. The film thickness was monitored by a crystal oscillator. Alkyminoline is also vapor-deposited at a rate of 0.1 nm / sec and its film thickness is 50
nm. The Mg / Ag electrode was formed at a speed of 0.4 nm / sec and its thickness was 100 nm. All of these vapor depositions were continuously performed without breaking the vacuum. Immediately after the device was manufactured, the electrode was taken out in dry nitrogen and the characteristics were continuously measured. Luminous property is 2500 cd / m
² , emission life is 620Hr, storage stability is 2200H
It was r.

( Comparative Example ) As a hole transport material for comparison (Chemical 4: TPD),
(Chemical formula 5: Abbreviation TPAC) was used to fabricate an EL element under the same conditions, and its characteristics were examined. The emission characteristics, emission lifetime, and storage stability of TPD are 2200 cd / m, respectively.
² , 220 Hr and 460 Hr. On the other hand, TPAC
In terms of luminescence, longevity of luminescence, and storage stability,
It was 500 cd / m ² , 280 Hr, and 560 Hr .

[0029] (devices Example 1) element respectively in Reference Example 1 and the same method, tetraphenyl benzidine compound (4) (R1 = tBu, R2 = H,
R3 = H), (5) (R1 = tBu, R2 = tB
u, R3 = H), (6) (R1 = C6 H5, R2 =
H, R3 = H), (7) (R1 = C6 H5, R2 =
C6 H5, R3 = H), (8) (R1 = C6 H5
, R2 = C6H5, R3 = CH3), (9) (R
1 = p-CH3-C6H4, R2 = H, R3 = O
CH3), (10) (R1 = p-CH3 -C6 H4
, R2 = p-CH3-C6H4, R3 = H) was used as a hole transport material, and the characteristics thereof were evaluated. ( Element reference example 2 ) Tetraphenyl was prepared in the same manner as in element reference example 1.
Benzidine compound (・ ・ 2) (R1 = IBu, R2 =
H, R3 = H), (3) (R1 = IBu, R2 =
H, R3 = CH3 ) Used as a hole transport material
A device was prepared and its characteristics were evaluated. The result is shown in FIG. The substitution positions of R1 and R2 in the tetraphenylbenzidine compounds (2) to (10) all represent p-position. Therefore, the tetraphenylbenzidine compounds ( 4 ) to (10) according to the present invention have
It was found that the storage stability was excellent.

Element Reference Example 3 Triphenylamine Trimer Compound Usable as Hole Transport Material
Examples of the product include the following compounds. Triphenylamine trimer compound (11) (R1 = H, R2 = H, R
3 = H, R4 = H), (12) (R1 = H, R2 =
H, R3 = H, R4 = CH3), (13) (R1 =
tBu, R2 = p-CH3, R3 = p-CH3, R
4 = H), (14) (R1 = H, R2 = H, R3 =
H, R4 = OCH3), (15) (R1 = H, R2
= M-CH3, R3 = m-CH3, R4 = H),
(16) (R1 = H, R2 = p-OCH3, R3 =
p-OCH3, R4 = H), (17) (R1 = p-
CH3, R2 = H, R3 = H, R4 = CH3.
), (18) (R1 = p-CH3, R2 = p-iB
u, R3 = p-iBu, R4 = H), (19) (R1
= P-nBu, R2 = m-CH3, R3 = H, R4
= Cl) (20) (R1 = p-OC2 H5, R2
= P-CH3, R3 = p-CH3, R4 = H ).

(Device Example 2 ) In the same manner as in Device Reference Example 1, triphenylamine trimer compound (11) (R1 = H, R2 = H, R) was used.
3 = H, R4 = H) and the tetraphenylbenzidine compound (4) (R1 = p-tBu, R2 = H, R3 =
H) was co-evaporated to prepare an EL device used as a hole transport material, and its characteristics were evaluated. Luminous properties are 3300 cd
/ M ² , emission life is 720 hours, storage stability is 290
It was 0 Hr. From the results, it was found that the hole transport layer formed by co-evaporation of the triphenylamine trimer compound (11) and the tetraphenylbenzidine compound (4) according to the present invention has excellent emission life and storage stability. It was

[0032]

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

[Brief description of drawings]

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

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

Continued front page    (72) Inventor Masao Fukuyama               3-10 Higashisanda, Tama-ku, Kawasaki City, Kanagawa Prefecture               No. 1 Matsushita Giken Co., Ltd. (72) Inventor Mutsumi Murakami               3-10 Higashisanda, Tama-ku, Kawasaki City, Kanagawa Prefecture               No. 1 Matsushita Giken Co., Ltd. (72) Inventor Taro Nanbu               3-10 Higashisanda, Tama-ku, Kawasaki City, Kanagawa Prefecture               No. 1 Matsushita Giken Co., Ltd. (72) Inventor Hiromitsu Toyama               Hodogaya, 45 Miyukigaoka, Tsukuba City, Ibaraki Prefecture               Chemical Industry Co., Ltd. Tsukuba Research Center (72) Inventor Masahiko Oshino               Hodogaya, 45 Miyukigaoka, Tsukuba City, Ibaraki Prefecture               Chemical Industry Co., Ltd. Tsukuba Research Center                (56) Reference JP-A-4-220995 (JP, A)                 JP 63-295695 (JP, A)                 JP-A-4-161480 (JP, A)                 JP-A-5-239455 (JP, A)                 JP-A-2-277071 (JP, A)                 Japanese Patent Laid-Open No. 5-165239 (JP, A)                 JP-A-4-181258 (JP, A)

Claims (6)

(57) [Claims] 1. A film grown by an evaporation method using a tetraphenylbenzidine compound represented by the following general formula:
An electroluminescent device having a hole transport layer. [Chemical 1] However, R1 and R2 represent a hydrogen atom, a tert-butyl group, a phenyl group, a phenyl group having a lower alkyl group or a lower alkoxy group as a substituent, and R3 represents a hydrogen atom, a methyl group or a methoxy group. Further, R1 and R2 are not hydrogen atoms at the same time. 2. Using the tetraphenyl benzidine compounds described by the following formula, a film grown by evaporation positive
A thermostable electroluminescent device having a hole transport layer . [Chemical 2] However, R1 and R2 represent a hydrogen atom, a tert-butyl group, a phenyl group, a phenyl group having a lower alkyl group or a lower alkoxy group as a substituent, and R3 represents a hydrogen atom, a methyl group or a methoxy group. Further, R1 and R2 are not hydrogen atoms at the same time. 3. A electrodes, a hole transport layer, luminescent layer, claim 1 comprising an electron transport layer and the electrode, characterized in that it comprises a film obtained by growing by vapor deposition as a hole transporting layer Well
Electroluminescent device of the other two described. 4. An electrode, a hole-transporting layer, a light-emitting layer, an electron-transporting layer and an electrode, wherein the hole-transporting layer is at least two kinds selected from the tetraphenylbenzidine compounds according to claim 1 . Material containing compound, or claim
A vapor deposition method using one or more compounds selected from the tetraphenylbenzidine compounds described in 1 and one or more compounds selected from the triphenylamine trimer described by the following general formula. electroluminescent device according to claim 1, characterized by using the grown film by. [Chemical 3] However, R1, R2, and R3 represent a hydrogen atom, a lower alkyl group, or a lower alkoxy group, and R4 represents a hydrogen atom, a methyl group, a methoxy group, or a chloro atom. 5. The electroluminescent device according to claim 3, wherein the electron transport layer also serves as a light emitting layer. 6. The electroluminescent device according to claim 3, wherein the hole transport layer also serves as a light emitting layer.