JP2003073664A - Dopant for organic electroluminescent element and element using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a dopant for an organic electroluminescent element capable of enhancing emission efficiency of an element containing a host molecule and a dopant molecule in the emission layer. SOLUTION: This dopant containes an emission layer 5 of the organic electroluminescent element and its molecular structure comprises No.1 part and No.2 part as shown below and the No.1 part has a lower excitation energy then that of the No.2 part.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a dopant compound to be contained in a light emitting layer of an organic electroluminescence (EL) device together with a host compound, and an organic EL device using the same.

[0002]

2. Description of the Related Art In recent years, with the diversification of information equipment, there is an increasing need for a flat display element that consumes less power than a CRT that has been generally used in the past. Under such circumstances, research and development of a display using an organic EL element having characteristics such as high efficiency, thinness / lightness, and no dependence on viewing angle have been actively conducted.

The organic EL device injects electrons and holes into the light emitting layer from the electron injecting electrode and the hole injecting electrode, respectively, and recombines these electrons and holes at the emission center to bring the organic molecule into an excited state. It utilizes that the organic molecule emits fluorescence when it returns from the excited state to the ground state. In the organic EL element, it is possible to change the emission color when selecting a fluorescent substance which is a light emitting material, and there are increasing expectations for application to multi-color and full-color display devices.

As a method for improving the purity of light emitted from an organic EL device and the efficiency of light emission, a method of doping an organic molecule having a desired light emission wavelength and high light emission efficiency into a light emitting layer is used. In the case of this doping method, holes and electrons injected from the electrode on the host molecule are recombined to make the host molecule in an excited state, and energy is transferred to a dopant molecule having an excitation energy smaller than that energy, It emits light from the dopant molecule. Usually, a dopant molecule having a higher fluorescence quantum yield than that of the host molecule is used, so that higher luminous efficiency can be obtained.

[0005]

By the way, when the difference in excitation energy between the host molecule and the dopant molecule is large,
The above energy transfer may not be performed smoothly. For example, when a red coloring dopant DCJTB is used for Alq3 that emits green light as a typical organic EL material, the excitation energy difference between the two is large, so that light emission from Alq3 occurs in addition to light emission from DCJTB due to energy transfer. , The color purity is poor. In such cases,
By mixing the host (Alq3) and the second dopant having an intermediate excitation energy between the dopant (DCJTB), the emission efficiency can be increased. Rubrene or the like is used as such a second dopant. It is said that the second dopant has an action of mediating energy transfer and also an action of directly transferring energy to the first dopant by its own excitation by a carrier trap, and a normal doping method using no second dopant. It is possible to improve color purity and luminous efficiency more than the device obtained by the above.

However, in the co-dopant method using such a second dopant, when forming the host layer by vapor deposition, it is necessary to vapor deposit a plurality of dopants at the same time, so that the doping concentration is likely to be uneven and the efficiency is high. There is a problem that it is not possible to stably manufacture a high device. Further, in the organic EL element, further improvement in luminous efficiency is required in order to reduce power consumption.

An object of the present invention is to provide an organic EL device containing a host molecule and a dopant molecule in the light emitting layer, which is capable of enhancing the light emission efficiency, and an organic EL device using the same. is there.

[0008]

The dopant for organic EL device of the present invention is a dopant compound to be contained in the light emitting layer of the organic EL device together with the host compound, and the molecular structure thereof is the first molecular part and the second molecular part. Consists of a molecular part,
In addition, the first molecular part is characterized by having a lower excitation energy than the second molecular part.

In the conventional codopant method, as described above, two or more kinds of dopant molecules are contained in the host layer, but the distribution of the molecules becomes a random dispersion state. Therefore, the distance between the first dopant molecule and the second dopant molecule varies, and energy transfer occurs with high probability between molecules at close distances, but energy transfer occurs between molecules at distant distances. Absent. Usually, such an energy transfer mechanism is called Forster (For).
The rate constant is proportional to the 6th power of the intermolecular distance R between two molecules. Therefore, in a state where two or more kinds of dopants are randomly dispersed in the host layer, the energy transfer between many molecules is not smoothly performed.

In the organic EL device dopant of the present invention, the first dopant molecule and the second dopant molecule are chemically bound to each other, so that the action of the first dopant molecule can be achieved within one dopant molecule. The illustrated part and the part showing the action of the second dopant molecule are provided. By combining the first molecule part and the second molecule part into one compound, the first molecule part and the second molecule part always exist in a close distance, and thus energy transfer It occurs with an extremely high probability.

Energy transfer from the second molecular part to the first molecular part because the first molecular part having a lower excitation energy than the second molecular part is chemically bound to the second molecular part. Are likely to occur very efficiently. Further, by setting the excitation energy of the second molecular part lower than the excitation energy of the host compound, the energy transfer from the host compound is transferred via the second molecular part to the first molecular part. To be done efficiently. As a result, higher luminous efficiency can be obtained as compared with the conventional co-dopant method in which two types of dopant molecules are separately used.

The dopant molecule of the present invention comprises a second molecular part which mediates energy transfer or is excited by a carrier trap, and a first molecular part which actually emits light. Since these molecular parts exert their functions cooperatively and effectively, the dopant of the present invention is
It can be called, so to speak, a molecular synchronized dopant (molecular synchronized type dopant).

Since the dopant of the present invention is a molecular synchro-type dopant composed of a first molecular part and a second molecular part, the deposition control becomes easier and stable as compared with the case where a plurality of conventional dopants are used. It is possible to manufacture a highly efficient device.

In the present invention, the first molecular part and the second molecular part
It is preferable that they are bound to the molecular part by a carbon-carbon bond. In particular, it is preferable that they are bound by a carbon-carbon single bond.

The organic EL device of the present invention is characterized by containing the above-mentioned dopant compound of the present invention in the light emitting layer. In the organic EL device of the present invention, the excitation energy of the second molecular part of the dopant compound is preferably set lower than the excitation energy of the host compound contained in the light emitting layer.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION In the conventional codopant method, the first dopant (DCJTB) shown in (Chemical Formula 2) and the second dopant shown in (Chemical Formula 3) are used for the host (Alq3) shown in (Chemical Formula 1).
Dopant (rubrene) is used.

[0017]

[Chemical 1]

[0018]

[Chemical 2]

[0019]

[Chemical 3]

On the other hand, in the present invention, instead of the first dopant and the second dopant, a molecular synchro-type dopant as shown in Chemical formula 4 is used.

[0021]

[Chemical 4]

In the above dopant compound, the first molecular part has a DCJTB structure and the second molecular part has a rubrene structure. The first molecular part and the second molecular part are linked by a carbon-carbon bond.

When the dopant of the present invention is used, the excitation energy is transferred from Alq3, which is the host, to the rubrene part, which is the second molecular part, or the rubrene part itself is excited by carrier trapping in the rubrene part. Then, with a high probability, energy is transferred to the first molecular part, DCJTB part, and light is emitted. Since the second molecular part and the first molecular part are linked by a carbon chain, energy transfer from the second molecular part occurs with a high probability, which results in high luminous efficiency.

The dopant of the present invention shown in (Chemical Formula 4) is obtained by the following Steps 1 to 19 shown in (Chemical Formula 5) to (Chemical Formula 11).
Can be produced by the reaction. That is, step 1
Through step 7, an intermediate of (7) is synthesized, and this is reacted with the compound of (11) synthesized in steps 8 to 11 to give a second intermediate (12) as shown in step 12. To synthesize. This (1
From the intermediate of 2), as shown in steps 13 and 14,
The third intermediate of (14) is synthesized. The intermediate of this (14)
By reacting with the compound of (18) synthesized in Step 15 to Step 18, as shown in Step 19, DCJTB-RB which is the dopant of the present invention is synthesized.

[0025]

[Chemical 5]

[0026]

[Chemical 6]

[0027]

[Chemical 7]

[0028]

[Chemical 8]

[0029]

[Chemical 9]

[0030]

[Chemical 10]

[0031]

[Chemical 11]

As a dopant of another embodiment according to the present invention, the compound shown in the following (Chemical Formula 12) may be mentioned.

[0033]

[Chemical 12]

In this dopant compound, the first molecular part has a DCJTB structure as in the case of the dopant compound shown in Chemical formula 4. The second molecular part is naphthyl rubrene (NR) shown in (Chemical Formula 13) below.
It has the structure of.

[0035]

[Chemical 13]

(Chemical formula 14) to (Chemical formula 16) show dopant compounds of still another embodiment according to the present invention.

[0037]

[Chemical 14]

In the chemical formula 14, the first molecular part is DC
It is the structure of JTB, the second molecular part is the structure of rubrene, and is similar to (Chemical formula 4), but the position where the first molecular part and the second molecular part are bonded is Different from the compound shown.

[0039]

[Chemical 15]

In the dopant shown in Chemical formula 15, the first molecular part has a DCJTB structure and the second molecular part has a diphenylnaphthacene structure.

[0041]

[Chemical 16]

In the dopant shown in Chemical formula 16, the first molecular part has a structure of a perylene derivative, and the second molecular part is
The molecular part of has the structure of rubrene.

[0043]

EXAMPLES The present invention will be described below with reference to specific examples, but the present invention is not limited to the following examples.

Example 1 An organic EL device having the structure shown in FIG. 1 was produced. As shown in FIG. 1, a thin film of an organic compound is sequentially deposited on a positive electrode 2 made of ITO formed on a glass substrate 1 by a vacuum deposition method to form a hole injection layer 3,
The hole transport layer 4, the light emitting layer 5, and the electron transport layer 6 were formed. A cathode 7 made of Mg was formed on the electron transport layer 6 by vacuum deposition.

The hole injection layer 3 was formed by depositing 2TNATA having the structure shown in (Chemical Formula 17) to a film thickness of 200 Å. The hole transport layer 4 was formed by depositing α-NPB having a structure shown in (Chemical Formula 18) to a film thickness of 300Å. The electron transport layer 6 was formed by depositing Alq3 to a film thickness of 200Å. The light emitting layer 5 was formed by depositing a host and a dopant described below to have a film thickness of 200Å. The cathode 7 was formed by depositing Mg to a film thickness of 2000Å.

[0046]

[Chemical 17]

[0047]

[Chemical 18]

In the light emitting layer 5, Alq is used as a host.
3 was used, and the molecular synchronization type dopant shown in (Chemical Formula 4) was used as the dopant. The concentration of the dopant was adjusted to 7% by weight with respect to the host. The molecular synchro-type dopant used here is, as shown in (Chemical Formula 4), rubrene whose excitation energy corresponds to the yellow wavelength region is the second molecular part, and DCJTB whose excitation energy corresponds to the red wavelength region is the first. It is a dopant molecule that is a molecular part of. After the excitation energy is transferred from the host Alq3 to the second molecular part, the rubren part, or the rubrene itself is excited by the carrier trap in the rubren part, the energy is highly likely to be transferred to the first molecular part, the DCJTB part. It moves and emits red light. Therefore, it is possible to emit light with high luminous efficiency.

(Comparative Example 1) Instead of using the molecular synchro-type dopant of the present invention shown in Chemical formula 4, the host A
DCJTB as the first dopant is added to 3 lq3.
An organic EL device was produced in the same manner as in Example 1 except that the light emitting layer was prepared such that the weight percent and rubrene as the second dopant were 4 weight percent.

[Evaluation of Luminous Properties of Organic EL Element] With respect to the organic EL elements of Example 1 and Comparative Example 1 obtained as described above, a current density of 10 mA / cm ² was applied to the organic EL elements to obtain luminous efficiency. The chromaticity was measured. The measurement results are shown in Table 1.

[0051]

[Table 1]

As is clear from Table 1, the device of Example 1 has a luminous efficiency 60% higher than that of the device of Comparative Example 1 and has a better chromaticity. In addition, as a comparative example, organic E was prepared by changing the concentrations of rubrene and DCJTB variously.
An L element was produced, but the organic EL elements of the examples had a luminous efficiency of 50% as compared with all the organic EL elements of the comparative examples.
It was above the above and the chromaticity was also good. The concentration% of rubrene: DCJTB in each comparative example is specifically 2%: 1.5.
%, 2%: 3%, 2%: 4.5%, 2%: 6%, 4%:
1.5%, 4%: 4.5%, 4%: 6%, 6%: 3%,
6%: 4.5%, and 6%: 6%.

(Example 2) As the molecular synchro-type dopant, the dopant shown in (Chemical Formula 12) was used as the host A.
An organic EL device was produced in the same manner as in Example 1 except that the light emitting layer 5 was formed by using it at a concentration of 7% by weight with respect to 1q3.

The molecular synchro-type dopant shown in (Chemical Formula 12) has a structure of DCJTB in which naphthyl rubrene (NR) whose excitation energy corresponds to the green wavelength region is the second molecular part and whose excitation energy corresponds to the red wavelength region. It is a dopant molecule used as the first molecular part. After the excitation energy is transferred from the host Alq3 to the second molecular part NR part or the NR itself is excited by the carrier trap in the NR part, there is a high probability that the first
Energy is transferred to the DCJTB part, which is the molecular part of, and red light is emitted. Therefore, it is possible to emit light with high luminous efficiency.

(Comparative Example 2) As a dopant of the light emitting layer 5, light was emitted so that DCJTB as a first dopant was 3% by weight and NR as a second dopant was 4% by weight in place of the above molecular synchro type dopant. An organic EL device was produced in the same manner as in Example 2 except that the layers were formed.

[Evaluation of Light-Emitting Properties of Organic EL Element] In the organic EL elements of Example 2 and Comparative Example 2 described above, 10 mA / cm
^A current was passed at a current density of ² and the luminous efficiency and chromaticity were measured. The measurement results are shown in Table 2.

[0057]

[Table 2]

As is clear from Table 2, the luminous efficiency of the organic EL device of Example 2 according to the present invention is as follows.
The luminous efficiency is 78% higher than that of the L element,
Better chromaticity is obtained.

As a comparative example, NR and DCJTB
The organic EL device was manufactured by changing the dopant concentration of the above variously, and the luminous efficiency and the chromaticity were evaluated. It was found that the luminous efficiency of the organic EL device of Example 2 was 60% or more higher than that of all the light emitting devices of Comparative Examples, and the chromaticity was good. The dopant concentration of NR: DCJTB in each comparative example is 2%: 1.5%, 2%: 3% by weight%.
2%: 4.5%, 2%: 6%, 2%: 1.5%, 4%:
4.5%, 4%: 6%, 6%: 3%, 6%: 4.5%,
And 6%: 6% respectively.

Although the present invention has been described with reference to the concrete embodiments, the present invention is not limited to the above embodiments, and various modifications can be made. For example, it can be applied to light emitting elements of colors other than the red light emitting element,
The first molecular part and the second molecular part are not limited to the above structure.

If the carbon chain connecting the first molecular part and the second molecular part is too long, the transfer of excitation energy between the molecular parts becomes difficult to occur. It is preferable that the carbon chain has a number of about 1 to 5.

Further, the dopant of the present invention comprises the first molecular part and the second molecular part, but may further comprise the third molecular part.

[0063]

EFFECTS OF THE INVENTION According to the present invention, in an organic EL device containing a host molecule and a dopant molecule in a light emitting layer, the light emitting efficiency can be increased and the chromaticity can be improved.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing the structure of an organic EL device of one embodiment according to the present invention.

[Explanation of symbols]

1 ... Glass substrate 2 ... Anode 3 ... Hole injection layer 4 ... Hall transport layer 5 ... Light emitting layer 6 ... Electron transport layer 7 ... Cathode

Claims (4)

[Claims] 1. A dopant compound contained in a light emitting layer of an organic EL device together with a host compound, the molecular structure of which is composed of a first molecular part and a second molecular part, and the first molecular part is a first molecular part. A dopant for an organic EL device, which is a molecular part having an excitation energy lower than that of the second molecular part. 2. The dopant for an organic EL device according to claim 1, wherein the first molecular part and the second molecular part are bonded by a carbon-carbon bond. 3. An organic EL device containing the dopant compound according to claim 1 or 2 in a light emitting layer. 4. The organic compound according to claim 3, wherein the light emitting layer contains a host compound, and the excitation energy of the second molecular part of the dopant compound is lower than the excitation energy of the host compound. EL element.