JP3880967B2 - Compound for electroluminescent device - Google Patents

Description

  The present invention is a light-emitting element having a hole transport layer, a light-emitting layer, and an electron transport layer, which is widely used as various display devices, and has a low applied voltage, high luminance, and excellent stability. The present invention relates to a compound for a light emitting element (EL element).

  Since EL elements are self-luminous and brighter than a liquid crystal element and can display clearly, they have been studied by many researchers for a long time. As a light emitting element which has reached a practical level at present, there is an element using ZnS which is an inorganic phosphor. However, such an inorganic EL element has not been widely used because an applied voltage for light emission of 200 V or more is necessary.

On the other hand, a light emitting element using an organic material has been far from a practical level, but in 1987, Kodak's C.I. W. The characteristics of the multilayer structure element developed by Tang et al. They stacked a phosphor that has a stable deposited film structure and can transport electrons, and an organic material that can transport holes, and successfully injected both carriers into the phosphor to emit light. did. As a result, the light emission efficiency of the organic electroluminescence device was improved, and light emission of 1000 cd / m ² or more was obtained at a voltage of 10 V or less. Since then, many researchers have conducted research for improving the characteristics, and at present, emission characteristics of 10,000 cd / m ² or more are obtained in a short time emission. As a conventional example of this type of light emitting device, for example, there is one described in Patent Document 1.
JP-A-4-220995

  The basic light emission characteristics of such an organic light emitting element are already in a practical range, and the biggest cause that currently prevents its practical use is firstly the lack of light emission stability during driving, Insufficient storage stability. The lack of light-emission stability during driving here refers to a phenomenon in which light-emission brightness decreases when driven by applying an element current, a non-light-emitting region called a dark spot occurs, or breakdown occurs due to a short circuit of the element. In other words, the lack of storage stability refers to a phenomenon in which the light emission characteristics deteriorate even when the manufactured device is stored.

  The present inventors have studied the mechanism of deterioration in order to solve the problems relating to the light emission stability and storage stability of such EL elements. As a result, it was found that one of the major causes of characteristic deterioration is the hole transport layer. That is, hole transport materials such as (Chemical Formula 4: abbreviated TPD) and (Chemical Formula 5: abbreviated TPAC), which are generally used as a hole transport layer, are (1) crystallized by humidity, temperature, and current to form a thin film It is not uniform. (2) It has been found that the hole transport layer undergoes a change such as being decomposed by energization, and as a result, the light-emitting property is significantly deteriorated.

本発明の目的は、このような知見に基づき、発光安定性、保存安定性に優れた正孔輸送層を有する有機ＥＬ素子用化合物を提供することにある。また、それらの正孔輸送層、発光層、電子輸送層の材料として有用なテトラフェニルベンジジン化合物を提供することにある。このような正孔輸送材料の具備しなければならない条件としては、（１）優れた正孔輸送能力を持つこと、（２）熱的に安定で、ガラス状態が安定であること、（３）薄膜を形成できること、（４）電気的、化学的に安定であること、等を挙げることができる。

An object of the present invention is to provide a compound for an organic EL device having a hole transport layer excellent in light emission stability and storage stability based on such knowledge. Another object of the present invention is to provide a tetraphenylbenzidine compound useful as a material for those hole transport layer, light emitting layer, and electron transport layer. The conditions that such a hole transport material must have are: (1) excellent hole transport capability, (2) thermal stability, and stable glass state, (3) A thin film can be formed, and (4) it is electrically and chemically stable.

  In order to achieve the above-mentioned object, the present inventors prototyped an EL device composed of an ITO electrode, a hole transport layer, a light emitting layer, an electron transport layer, and a magnesium / silver electrode, and created a number of newly synthesized hole transports. The material was evaluated. As the light emitting layer, an aluminum quinoline trimer which also serves as an electron transport layer was mainly used. As a material of the hole transport layer, at least a tetraphenylbenzidine compound described by (Chemical Formula 6), or a tetraphenylbenzidine compound described by (Chemical Formula 6) and triphenylamine 3 amount described by (Chemical Formula 7) Using one of the bodies.

上記（化６）の化学式において、Ｒ１ 、Ｒ２ はターシャリーブチル基、フェニル基、低級アルキル基もしくは低級アルコキシ基を置換基として有するフェニル基、Ｒ３ は水素原子、メチル基またはメトキシ基を表す。ただし、Ｒ１＝Ｒ２＝ターシャリーブチル基ではない。

In the chemical formula of (Chemical Formula 6) , R1 and R2 each represent a tertiary butyl group, a phenyl group, 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. However, R1 = R2 = not a tertiary butyl group.

ただし、Ｒ１ 、Ｒ２ 、Ｒ３ は水素原子、低級アルキル基、または低級アルコキシ基、Ｒ４ は水素原子、メチル基、メトキシ基、またはクロル原子を表す。

However, R1, R2, R3 represents 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.

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

  As described above, the present invention is an electroluminescent device characterized by using a tetraphenylbenzidine compound as a material for the hole transport layer. By using the material of the present invention, conventional organic electroluminescence can be obtained. It is possible to realize an electroluminescent device that is remarkably improved in light emission stability and storage stability, which are the biggest problems of the device.

  Examples of the compound for electroluminescence device of the present invention will be described with reference to examples.

  The tetraphenylbenzidine compound which is one of the hole transport materials of the present invention comprises a condensation reaction of a corresponding 4,4′-dihalogenated biphenyl and a corresponding diphenylamine compound, or a corresponding benzidine compound and a corresponding aryl halide, It can be synthesized by the condensation reaction. These condensation reactions are known as Ullmann reactions.

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

  These compounds were identified by elemental analysis and IR measurement, and further purified by recrystallization using a solvent and vacuum sublimation to a purity of 99.8% or higher. The purity was confirmed by TLC scanner, TG-DTA, and melting point measurement. The melting point and decomposition point are a measure of the thermal stability of the hole transport layer, and the glass transition point is a measure of the stability of the glass state. The inventors synthesized materials by changing the substituents of the above 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 several substituents, a material having a high melting point and decomposition point could be obtained. The following are some representative synthesis examples.

(Synthesis Reference 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. Reacted for hours. The reaction solution was poured into 300 ml of water, and the precipitated crystals were filtered, washed with water and dried. This crystal was recrystallized with a mixed solution of 140 ml of toluene and 700 ml of n-hexane to obtain 60.4 g (yield 67.3%) of p-isobutylacetanilide. The melting point was 124.5-125.0 ° C.

  P-Isobutylacetanilide obtained above, 17.9 g (0.094 mol) and 22.1 g (0.14 mol) bromobenzene, anhydrous potassium carbonate, 16.9 g (0.12 mol), copper Powder and 0.89 g (0.014 mol) were mixed and reacted at 168 to 217 ° C. for 14 hours. The reaction product was extracted with 100 ml of toluene, insoluble matter was filtered off, removed, and concentrated to dryness. This was dissolved in 30 ml of isoamyl alcohol, added with water, 3.4 g, 85% potassium hydroxide, 11.8 g (0.18 mol), and hydrolyzed at 131 ° C. Isoamyl alcohol and excess bromobenzene were distilled off by steam distillation, then extracted with 120 ml of toluene, washed with water, dried and concentrated. The concentrate was dried to obtain 17.6 g (yield 86.8%) of N-4-isobutylphenylaniline.

  Furthermore, N-4-isobutylphenyl aniline, 17.6 g (0.078 mol), 4,4′-Jodobiphenyl, 12.6 g (0.031 mol), anhydrous potassium carbonate, 12.9 g (0.093 mol) ) And copper powder, 0.89 g (0.014 mol) were mixed and reacted at 190 to 220 ° C. for 12 hours. The reaction product was extracted with toluene and 70 ml, and insoluble matter was filtered off, removed and concentrated to an oily product. 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. The melting point was 133.8 to 135.3 ° C. The product was identified by elemental analysis and IR measurement. Elemental analysis values are as follows. Carbon: measured value 87.77%, theoretical value: 87.96%, 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′-diiodobiphenyl, 85.3 g (0.21 mol), anhydrous potassium carbonate, 24.9 g (0.18 mol), copper powder, 2. 48 g (0.039 mol), nitrobenzene, and 40 ml were mixed and reacted at 190 to 205 ° C. for 10 hours. The reaction product was extracted with 200 ml of toluene, insoluble matter was filtered off, removed, and concentrated to dryness. This was purified by column chromatography (carrier: silica gel, eluent: toluene / n-hexane = 1/6), N- (4′-iodo-4-biphenylyl) acetanilide, 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 (0.050 mol) ) And copper powder, 0.64 g (0.010 mol), nitrobenzene, and 10 ml were 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 an oily product. The oily substance was dissolved in 50 ml of isoamyl alcohol, added with 1 ml of water, 2.64 g (0.040 mol) of 85% potassium hydroxide, and hydrolyzed at 130 ° C. After isoamyl alcohol was distilled off by steam distillation, the product 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 = 3/1), N, N′-bis (4′-phenylamino-4-biphenylyl) aniline, 8.85 g (Yield 76.3%) was obtained.

  Further, N, N′-bis (4′-phenylamino-4-biphenylyl) aniline, 8.70 g (0.015 mol), iodobenzene, 6.74 g (0.033 mol), anhydrous potassium carbonate, 4.56 g (0.33 mol), copper powder, 0.48 g (0.0075 mol), nitrobenzene and 10 ml were mixed and reacted at 195 to 205 ° C. for 16 hours. The reaction product was extracted with 100 ml of toluene, insoluble matter was filtered off and concentrated, and n-hexane was added to take out crude crystals. The crude crystals were purified by column chromatography to obtain 5.50 g (yield: 50.1%) of N, N′-bis (4′-diphenylamino-4-biphenylyl) aniline. A clear melting point was not observed. The product was identified by elemental analysis and IR measurement. 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%.

  Next, these were actually evaluated as EL elements, and the light emission characteristics, stability of the light emission characteristics, and storage stability of the elements were examined. As shown in FIG. 1, the EL element includes a hole transport layer 3, an electron transport layer / light emitting layer 4, and an Mg / Ag electrode 5 on a glass substrate 1 on which an ITO electrode is previously formed as a transparent electrode 2. It produced by vapor-depositing in order. First, a sufficiently cleaned glass substrate (ITO electrode was already formed), a hole transport material, and an aluminum quinoline trimer purified as an electron transport luminescent material were set in a vapor deposition apparatus. A hole transport layer was deposited at a rate of 0.1 nm / second, and samples with different thicknesses were prepared to determine the thickness at which optimum light emission was obtained. Although the film thickness differs depending on the material, the optimum film thickness was between 40 and 60 nm. The film thickness was monitored with a crystal resonator. The aluminum quinoline trimer was similarly deposited at a rate of 0.1 nm / second, and the film thickness was 50 nm. The Mg / Ag electrode was run at a speed of 0.4 nm / second, and its thickness was 100 nm. These vapor depositions were continuously performed without breaking the vacuum. Immediately after the device was fabricated, the electrode was taken out in dry nitrogen, and then the characteristics were measured.

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

  For the evaluation of the hole transport material of the present invention, an aluminum quinoline trimer was used as the electron transport layer / light emitting layer 4, but of course, in the present invention, various rare earth complexes, oxazole derivatives, polyparaffins are used as the material of the light emitting layer. Various materials such as phenylene vinylene can be used. Further, by adding a dopant such as quinacridone or coumarin to the light emitting layer, a higher performance EL can be manufactured. Furthermore, it can also be set as the electroluminescent element which consists of three layers, an electron carrying layer, a light emitting layer, and a positive hole transport layer. Moreover, a hole transport layer can also be used as a light emitting layer by combining the hole transport material of this invention and the suitable electron transport material.

  As a result of such studies, it has been found that when the hole transport material has a melting point of 130 ° C. or higher and a decomposition point of 300 ° C. or higher, excellent light emission stability and storage stability can be obtained. Therefore, the substituent of the above compound is not limited to the substituent of the present invention, and any substituent having the above melting point and decomposition point can be used.

  Moreover, although the hole transport material by this invention can also be used independently, two or more types can be formed into a film by co-evaporation etc. and can be 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 such as TPAC and TPD. When two or more kinds are simultaneously vapor-deposited, the effect of making it difficult to cause crystallization is often exhibited.

(Element Reference Example 1)
Thoroughly cleaned ITO electrode, tetraphenylbenzidine compound (1) (R1 = pnBu, R2 = H, R3 = H, mp = 132.9 ° C.) as a hole transporting material, electron transporting light emitting material The aluminum quinoline trimer purified as follows was set in a vapor deposition apparatus. Compound (1) was deposited in a thickness of 50 nm at a rate of 0.1 nm / second. The film thickness was monitored with a crystal resonator. Similarly, the deposition of alkinoline was performed at a rate of 0.1 nm / second, and the film thickness was 50 nm. The Mg / Ag electrode was run at a speed of 0.4 nm / second, and its thickness was 100 nm. These vapor depositions were continuously performed without breaking the vacuum. Immediately after the device was fabricated, the electrode was taken out in dry nitrogen, and then the characteristics were measured. The light emission characteristics were 2500 cd / m ² , the light emission lifetime was 620 Hr, and the storage stability was 2200 Hr.

(Comparative example)
For comparison, EL elements were fabricated under the same conditions using (Chemical Formula 4: abbreviated TPD) and (Chemical Formula 5: abbreviated TPAC) as hole transport materials, and the characteristics thereof were examined. The emission characteristics, emission lifetime, and storage stability of TPD were 2200 cd / m ² , 220 Hr, and 460 Hr, respectively. On the other hand, the light emission property, light emission lifetime, and storage stability in TPAC were 2500 cd / m ² , 280Hr, and 560Hr, respectively.

(Element Example 1)
Tetraphenylbenzidine compound (5) (R1 = tBu, R2 = tBu, R3 = H), (7) (R1 = C6H5, R2 = C6H5, R3 = H) in the same manner as in Device Reference Example 1, respectively. (8) (R1 = C6H5, R2 = C6H5, R3 = CH3), (10) (R1 = p-CH3-C6H4, R2 = p-CH3-C6H4, R3 = H) An EL element used as a material was produced and its characteristics were evaluated.

(Element Reference Example 2)
Tetraphenylbenzidine compound (2) (R1 = iBu, R2 = H, R3 = H), (3) (R1 = iBu, R2 = H, R3 = CH3), 4) (R1 = tBu, R2 = H, R3 = H), (6) (R1 = C6H5, R2 = H, R3 = H), (9) (R1 = p-CH3-C6H4, R2 = H) , R3 = OCH3) was prepared as a hole transport material, and its characteristics were evaluated. The result is shown in FIG. In addition, all substitution positions of R1 and R2 in the tetraphenylbenzidine compounds (2) to (10) are p-positions. From this, it was found that the tetraphenylbenzidine compounds (..5), (7), (8), and (10) according to the present invention are excellent in light emission life and storage stability.

(Element Reference Example 3)
Examples of the triphenylamine trimer compound that can be used for the hole transport material include the following compounds. Triphenylamine trimer compound (11) (R1 = H, R2 = H, R3 = H, R4 = H), (12) (R1 = H, R2 = H, R3 = H, R4 = CH3), ( 13) (R1 = tBu, R2 = p-CH3, R3 = p-CH3, R4 = 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-iBu, R3 = p-iBu, R4 = H), (19) ( R1 = p-nBu, R2 = m-CH3, R3 = H, R4 = Cl) (20) (R1 = -OC2 H5, R2 = p-CH3, R3 = p-CH3, R4 = H).

(Element Reference Example 4)
Triphenylamine trimer compound (11) (R1 = H, R2 = H, R3 = H, R4 = H) and tetraphenylbenzidine compound (4) (R1 = p) in the same manner as in Device Reference Example 1, respectively. -TBu, R2 = H, R3 = H) were co-evaporated to produce an EL device used as a hole transporting material, and its characteristics were evaluated. The light emission characteristics were 3300 cd / m ² , the light emission lifetime was 720 Hr, and the storage stability was 2900 Hr.

  The present invention provides a tetraphenylbenzidine compound suitable as a material for a hole transport layer and a light emitting layer of an organic electroluminescent device. By using the material of the present invention, most of the conventional organic electroluminescent devices are provided. It is possible to realize an electroluminescent device that is greatly improved in light emission stability and storage stability, which has been a major problem.

The partial cross-sectional enlarged perspective view which shows the structure of the electroluminescent element in one Example of this invention. List of characteristics of electroluminescent device using a tetraphenylbenzidine 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

Claims (7)

A tetraphenylbenzidine compound described by the following general formula.

In the above chemical formula , R1 and R2 represent a tertiary 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. However, R1 = R2 = not a tertiary butyl group. A compound for an electroluminescent device, which is a tetraphenylbenzidine compound described by the following general formula.

In the above chemical formula , R1 and R2 represent a tertiary 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. However, R1 = R2 = not a tertiary butyl group. A compound for a thermostable electroluminescent element, which is a tetraphenylbenzidine compound described by the following general formula.

In the above chemical formula , R 1 and R 2 represent a tertiary butyl group, a phenyl group, a phenyl group having a lower alkyl group or a lower alkoxy group as a substituent, and R 3 represents a hydrogen atom, a methyl group or a methoxy group. However, R1 = R2 = not a tertiary butyl group. The compound for an electroluminescent element according to claim 2, wherein the compound for an electroluminescent element is used for a hole transport layer. The compound for heat-stable electroluminescent elements according to claim 3, wherein the compound for thermostable electroluminescent elements is used for a hole transport layer. The compound for an electroluminescent element according to claim 2, wherein the compound for an electroluminescent element is used for a light emitting layer. The compound for heat-stable electroluminescent elements according to claim 3, wherein the compound for thermostable electroluminescent elements is used for a light-emitting layer.

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