JP3440296B2 - Electroluminescence element - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electroluminescence device, and more particularly to an electroluminescence device which can be suitably used for blue and full color flat panel displays and the like.

[0002]

2. Description of the Related Art With the recent development of full-color flat panel displays and the like, there is an increasing demand for blue electroluminescent organic electroluminescent elements (hereinafter, may be abbreviated as "EL elements"). E like this
Some organic metal complexes and π-conjugated molecules have been reported as blue light emitting materials for L devices.

For example, J. Appl. Phys. Vol. 31 (1992) p.
In p. 1812-1816, 1,3-
Bis (N, N-dimethylaminophenyl) -1,3,4
-Oxadiazole is described. Also, Chemistr
y Letters (1996) pp.47-48 describes 3- (4-biphenylyl) -4- (4-ethylphenyl) -5- (4-dimethylaminophenyl)-as a novel organic light emitting material.
The organic compound material 1,2,4-triazole is described. In addition, Appl. Phys. Lett. 67 (1995) 3853
Reports an organic compound material of 4,4′-bis (2,2′-diphenylethenyl) biphenyl as a novel organic light emitting material.

[0004]

However, these organic compound materials are poor in morphology stability and heat resistance. Therefore, under the present circumstances, it is not possible to obtain a blue light emitting material having high emission brightness and emission efficiency and excellent heat resistance and durability. For this reason, it has been impossible to obtain a blue-emitting EL element that can be practically used in the above-mentioned full-color flat panel display.

It is an object of the present invention to provide a novel EL element using a novel blue light emitting material, which has a high luminous brightness and a high luminous efficiency and is excellent in heat resistance and durability.

[0006]

An EL device of the present invention comprises a substrate, a transparent electrode, a hole transport layer, a light emitting layer and a back electrode, which are laminated in this order. . The light emitting layer is characterized by being composed of an organic compound material consisting of 5,5′-bis (dimesitylboryl) -2,2′-bithiophene.

The inventors of the present invention have earnestly studied to develop a new EL element which is excellent in light emission intensity and light emission efficiency, and is also excellent in heat resistance and durability. As a result, E comprising a substrate, a transparent electrode, a hole transport layer, a light emitting layer and a back electrode.
It has been found that in the L element, the EL element as described above can be obtained only by forming the light emitting layer from the specific organic compound material as described above.

5,5'-bis (dimesitylboryl)-
2,2′-bithiophene (hereinafter, abbreviated as “BMB-2
"T" may be referred to as "T") is a novel organic compound newly synthesized by the present inventors. At the time of synthesis, the glass transition temperature of this novel organic compound is about 107 ° C., and it has been confirmed that the amorphous state can be maintained up to a high temperature.

Therefore, the present inventors have examined how the emission brightness and the emission efficiency change when this novel organic compound is used as a light emitting layer material of an EL device. As a result, E having the above-mentioned hole transport layer
In the L element, surprisingly, about 1100 cd / m ²
It has been found that the above-mentioned high emission brightness and high emission efficiency of about 0.05 lm / W or more can be obtained. And BM
It has been found that this BMB-2T is extremely excellent as a light emitting layer material of the EL device having the above-mentioned constitution, together with high heat resistance and durability due to the high glass transition temperature which B-2T originally has. .

When BMB-2T is used as the light emitting layer material of the EL device having the above-mentioned constitution, the cause of high luminous brightness and high luminous efficiency is not clear, but BM
It is presumed that the fluorescence quantum yield of B-2T is extremely high and that it has strong blue fluorescence.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail based on the embodiments of the invention. FIG. 1 is a schematic view showing an example of the EL element structure of the present invention. Hereinafter, the present invention will be described in detail based on the EL device configuration shown in FIG. EL shown in FIG.
The element 10 includes a glass substrate 1 and ITO as a transparent electrode.
It comprises an electrode 2, a hole injection layer 3 and a hole transport layer 4. Further, it includes an exciplex formation preventing layer 5, a light emitting layer 6, and a MgAg electrode 7 as a back electrode. And these are laminated in this order.

The EL device 1 shown in FIG. 1 according to the present invention.
The light emitting layer 6 of 0 must be composed of BMB-2T as described above. BMB-2T has the structure shown below.

[0013]

[Chemical 1]

This BMB-2T is obtained as follows. That is, n-butyllithium was added to a solution obtained by dissolving 2,2′-bithiophene in a tetrahydrofuran solution, and the mixture was stirred while ice-cooling, then dimesitylboron fluoride was added and the mixture was stirred at room temperature. Then, it is obtained by purification by silica gel column chromatography and recrystallization from benzene: hexane = 1: 1.

The thickness of the light emitting layer 6 is not particularly limited as long as the object of the present invention can be achieved. However, the thickness of the light emitting layer 6 is preferably 200 Å or more, and more preferably 300 Å or more. Similarly, the thickness of the light emitting layer 6 is preferably 700 Å or less, and more preferably 500 Å or less. By fixing the thickness of the light emitting layer 6 within the above range, it is possible to obtain high light emission brightness under low voltage application.

The EL device 10 shown in FIG. 1 has an exciplex formation preventing layer 5 between the hole transport layer 4 and the light emitting layer 6. When the light emitting layer 6 made of BMB-2T of the present invention is formed adjacent to the hole transport layer 4, an exciplex is formed at these interfaces. Then, the blue light emitted from the light emitting layer 6 in the EL element 10 of the present invention is mixed with the yellow light having a long wavelength based on the exciplex. Therefore, the EL element 10 as a whole emits bluish white light.

On the other hand, as shown in FIG. 1, the exciplex formation preventing layer 5 is provided between the hole transport layer 4 and the light emitting layer 6.
The formation of the above can effectively prevent the generation of the exciplex. As a result, the ratio of yellow light emission is reduced, and an EL element which emits light closer to blue light can be obtained.

The material that can be used for the exciplex formation preventing layer 5 is not particularly limited,
It is preferably composed of an organic compound material of 1,3,5-tris (biphenyl-4-yl) benzene (hereinafter, may be abbreviated as “TBB”). This organic compound material is a novel organic compound newly produced by the present inventors and has the following structure.

[0019]

This organic compound has a high ionization potential and a large HOMO-LUMO energy gap. Therefore, when it is used as the exciplex formation preventing layer as described above, an excellent effect is exhibited.

TBB is obtained as follows. That is, potassium pyrosulfate and concentrated sulfuric acid are added to a solution obtained by dissolving 4-acetylbiphenyl in a nitrobenzene solution, and the mixture is heated and stirred. Then, it is obtained by purification by alumina column chromatography and recrystallization from benzene: hexane.

The thickness of the exciplex formation preventing layer 5 is not particularly limited as long as it can prevent the generation of the exciplex and reduce the proportion of white light emitted from the exciplex. However, the upper limit of the thickness of the exciplex formation preventing layer 5 is preferably 70Å, more preferably 60Å. The lower limit of the thickness of the exciplex formation preventing layer 5 is 40.
It is preferably Å, more preferably 50 Å. This makes it possible to more effectively prevent the generation of exciplex and control the light emitting region of the light emitting layer 6.

Further, the EL element 10 of the present invention shown in FIG.
Is the hole injection layer 3 between the hole transport layer 4 and the glass substrate 1.
have. As a result, the number of holes injected from the ITO electrode 2 into the light emitting layer 6 is increased, and the light emission luminance and light emission efficiency of the light emitting layer 6 are further improved.

The material that can be used for the hole injection layer 3 is not particularly limited as long as the object of the present invention can be achieved. However, it is preferable that the organic compound material is 4,4 ′, 4 ″ -tris (3-methylphenylphenylamino) triphenylamine (hereinafter, may be abbreviated as “m-MTDATA”). Such an organic compound has the following structure.

[0025]

M-MTDATA is obtained as follows. That is, 4 ′, 4 ″, 4 ′ ″-triiodotriphenylamine, (3-methylphenyl) phenylamine, potassium hydroxide, and copper powder were dissolved in decalin, and then under a nitrogen atmosphere. Heat and stir at. Then, it is obtained by purification by silica gel column chromatography and recrystallization from tetrahydrofuran: ethanol = 5: 1.

Regarding the thickness of the hole injection layer 3, the light emitting layer 6 is also included.
There is no particular limitation as long as it is possible to increase the amount of holes injected into the light emitting layer and improve the light emission luminance and the light emission efficiency of the light emitting layer 6. However, the upper limit of the thickness of the hole injection layer 3 is preferably 500 Å, more preferably 300 Å
Is preferred. The lower limit of the thickness of the hole injection layer 3 is preferably 100Å, more preferably 200Å.
Is preferred. By setting the thickness of the hole injection layer 3 within the above range, the hole injection amount and the electron injection amount into the light emitting layer 6 are balanced, and high emission brightness and high emission efficiency can be obtained.

The light emitting layer 6 and the like as shown in FIG. 1 are formed on the glass substrate by means of vacuum vapor deposition or the like.

The hole transport layer 4 may be, for example, commercially available N, N'-bis (3-methylphenyl) -N, N'-.
Diphenyl- (1,1'-biphenyl) -4,4'-diamine (hereinafter sometimes abbreviated as "TPD") can be used. This TPD has the following structure.

[0030]

As the substrate, a PET film or the like can be used in addition to the glass substrate shown in FIG.
Also for the transparent electrode, diamond or the like can be used in addition to the ITO electrode. Further, as the back electrode, Ca, Al or the like can be used in addition to the MgAg electrode.

[0032]

EXAMPLES Hereinafter, specific examples of the EL device of the present invention will be shown by examples. Example In this example, an EL device 10 as shown in FIG. 1 was produced. A commercially available glass substrate 1 on which the ITO electrode 2 is formed is used.
-250 Å hole injection layer 3 made of MTDATA
Was formed so that a part thereof was in contact with the surface of the glass substrate 1.

Then, a hole transport layer 4 made of TPD was formed on the hole injection layer 3 by vacuum vapor deposition to have a thickness of 200Å. Then, an exciplex formation preventing layer made of TBB was formed in a thickness of 50Å by vacuum evaporation. Then, the light emitting layer 6 made of BMB-2T is formed to a thickness of 30 by vacuum vapor deposition.
After being formed to 0Å, the MgAg electrode 7 was formed to a thickness of 1000Å by vacuum evaporation.

ITO of EL element 10 thus obtained
A voltage of 8 V was applied between the electrode 2 and the MgAg electrode 7, and the emission spectrum was measured at room temperature and atmospheric pressure. As a result, a spectrum as shown in FIG. 2 was obtained. As is clear from the emission spectrum shown in FIG. 2, the EL element obtained in this example has a peak at around 446 nm and emits blue light. Further, when the voltage-current characteristics and the voltage-luminance characteristics of the EL element in this example were examined, a graph as shown in FIG. 3 was obtained. As apparent from FIG. 3, showing a light emitting voltage application of about 4 volts, the maximum luminance was 1100 cd / ^{m 2} at applied voltage of about 16 volts, luminous efficiency at 300 cd / ^{m 2} light emission 0.05Lm / It turned out to be W.

From the above-mentioned examples, it is understood that the EL device of the present invention shows values substantially equivalent to the high emission luminance and the emission efficiency which have been conventionally obtained. Then, based on the high glass transition temperature of BMB-2T forming the light emitting layer 6, the amorphous state can be maintained up to an extremely high temperature.
Therefore, it can be seen that the heat resistance and the durability are also excellent.

The present invention has been described in detail based on the embodiments of the invention with reference to specific examples. However, the present invention is not limited to the above contents and does not depart from the scope of the present invention. , All modifications and changes are possible.

[0037]

According to the present invention, it is possible to obtain an EL element which emits blue light and is excellent in light emission brightness and light emission efficiency. Further, since it has high heat resistance and high durability due to the organic compound material used for the light emitting layer, even when the present EL device is used for a full-color flat panel display, for example, the emission brightness and the emission efficiency are high. Does not decrease. Therefore, it can be sufficiently used as a light emitting element in various devices.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an example of an EL device configuration of the present invention.

FIG. 2 is a graph showing an example of an emission spectrum of the EL device of the present invention.

FIG. 3 is a graph showing voltage-current characteristics and voltage-luminance characteristics of the EL element of the present invention.

[Explanation of symbols]

1 glass substrate 2 ITO electrode 3 Hole injection layer 4 Hole transport layer 5 Exciplex formation prevention layer 6 light emitting layer 7 MgAg electrode 10 EL element

Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) C09K 11/06 H05B 33/14 H05B 33/22 CA (STN) REGISTRY (STN)

Claims (9)

(57) [Claims] 1. An electroluminescent device comprising a substrate, a transparent electrode, a hole transport layer, a light emitting layer, and a back electrode, which are laminated in this order, wherein the light emitting layer comprises: 5'-bis (dimesitylboryl)-
An electroluminescent device comprising an organic compound material of 2,2′-bithiophene. 2. The thickness of the light emitting layer is 300 to 500Å
The electroluminescent element according to claim 1, wherein 3. An exciplex formation preventing layer is provided between the hole transport layer and the light emitting layer,
The electroluminescent element according to claim 1. 4. The exciplex formation preventing layer,
The electroluminescent device according to claim 3, wherein the electroluminescent device is made of an organic compound material of 1,3,5-tris (biphenyl-4-yl) benzene. 5. The electroluminescence device according to claim 3, wherein the exciplex formation preventing layer has a thickness of 40 to 70Å. 6. The hole injecting layer is formed between the transparent electrode and the hole transporting layer so that at least a part of the hole injecting layer is in contact with the main surface of the substrate.
5. The electroluminescent element according to any one of 5 to 5. 7. The hole injection layer according to claim 6, wherein the hole injection layer is made of an organic compound material of 4,4 ′, 4 ″ -tris (3-methylphenylphenylamino) triphenylamine. Electroluminescent device. 8. The thickness of a portion of the hole injection layer sandwiched between the transparent electrode and the hole transport layer is 100 to 50.
It is 0Å, The electroluminescent element of Claim 6 or 7 characterized by the above-mentioned. 9. 5,5′-Bis (dimesitylboryl)-
An organic compound material for a light emitting layer of an electroluminescence device, which is 2,2′-bithiophene.