JP3239057B2 - Organic electroluminescence device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electroluminescence device in which at least a light emitting layer using an organic material is provided between a hole injection electrode and an electron injection electrode, and in particular, efficiently emits light at a low voltage. The present invention relates to an organic electroluminescence device that can emit light stably for a long period of time.

[0002]

2. Description of the Related Art In recent years, with the diversification of information equipment and the like, the need for a flat display element which consumes less power and has a smaller space occupied area than conventional CRTs has been increased. Electroluminescent devices (hereinafter, abbreviated as EL devices) have attracted attention as one of the devices.

[0003] These EL elements are roughly classified into inorganic EL elements and organic EL elements depending on the materials used.
In a device, a high electric field is generally applied to a light emitting portion, and electrons are accelerated in the high electric field to collide with a light emitting center, thereby exciting the light emitting center to emit light. In, electrons and holes are respectively injected into the light emitting portion from the electron injection electrode and the hole injection electrode, and the thus injected electrons and holes are recombined at the emission center to excite the organic material, When this organic material returns from the excited state to the ground state, it emits fluorescence.

Here, in the inorganic EL element, since a high electric field acts as described above, the driving voltage is 1
In contrast to the necessity of a high voltage of 00 to 200 V, the above-mentioned organic EL element has an advantage that it can be driven at a low voltage of about 5 to 20 V. In such an organic EL device, a light emitting device that emits light of an appropriate color can be obtained by selecting a fluorescent substance that is a light emitting material, and it is expected that the organic EL device can be used as a full-color display device or the like. In recent years, various studies have been conducted on such organic EL devices.

[0005] An element structure of the above-mentioned organic EL element is a DH in which a hole transport layer, a light emitting layer and an electron transport layer are laminated between a hole injection electrode and an electron injection electrode.
Or a two-layer structure called an SH-A structure in which a hole transport layer and a light-emitting layer rich in electron transport properties are stacked between a hole injection electrode and an electron injection electrode. Or an SH- layer in which a light emitting layer and a electron transport layer rich in hole transport properties are laminated between a hole injection electrode and an electron injection electrode.
A two-layer structure called a B structure has been known.

As for the light emitting layer in the above-mentioned organic EL device, a light emitting layer has been developed in which a host material is doped with a small amount of dopant, and light is emitted with this dopant as a light emission center.

Here, in order to obtain a light emitting layer in which a small amount of dopant is doped into the host material, it is necessary to use a material having a good film-forming property for the host material. As a host material, a quinolinol metal complex, a benzoquinolinol metal complex, or the like has been generally used.

However, host materials having good film-forming properties as described above generally have low fluorescence quantum yields, and even in the above-described metal complexes, their fluorescence quantum yields are as low as 20% or less. Excitation of the dopant with high efficiency, it is a hindrance to obtain sufficient light emission, and when using a host material having a low fluorescence quantum yield and not so large excitation energy, the dopant Is easily lost through a non-emission process, and much of this energy is converted to heat, which causes organic E
There is a problem that the L element is deteriorated, and stable light emission cannot be performed for a long period of time, and efficient light emission cannot be obtained at a low driving voltage.

Further, in order to increase the luminous efficiency in the organic EL device, the fluorescent energy in the host material and the excitation energy in the dopant become substantially the same so that the energy can be efficiently transferred from the host material to the dopant. It is preferable to combine a host material and a dopant, but since the types of host materials are currently limited, it is difficult to use an effective dopant in combination with such a host material. There is also a problem that the luminous efficiency of the device cannot be sufficiently increased.

[0010]

SUMMARY OF THE INVENTION The present invention relates to an organic EL device.
It is an object of the present invention to solve the above-described various problems in a device, and it is possible to perform efficient light emission at a lower driving voltage as compared with a conventional organic EL device and to perform stable light emission for a long period of time. It is an object of the present invention to provide an organic electroluminescence device.

[0011]

According to the present invention,
In order to solve the above problems, in an organic electroluminescent element in which a light emitting layer using at least an organic material is provided between a hole injection electrode and an electron injection electrode, a host material in the light emitting layer And at least a first dopant having a shorter fluorescent peak wavelength than the fluorescent peak wavelength of the host material and a second dopant having a longer fluorescent peak wavelength than the fluorescent peak wavelength of the host material. Instead, the second dopan emits light.

As in the organic EL device of the present invention,
When a first dopant having a shorter fluorescent peak wavelength and a second dopant having a longer fluorescent peak wavelength are contained in the host material in the light emitting layer, light emission is performed in the first dopant having a shorter fluorescent peak wavelength. Instead, efficient light emission is performed in the second dopant having a long fluorescence peak wavelength.

Here, the reason why efficient light emission is performed by the second dopant as described above is not clear. However, according to experimental considerations of the present inventors, it is found that holes injected from the hole injection electrode and electrons injected from the hole injection electrode. Excitation energy when electrons injected from the electrode are recombined in the light-emitting layer is transmitted from the host material to the first dopant, and the second dopant having a longer fluorescence peak wavelength is transmitted from the first dopant.
The second dopant is efficiently excited by being transmitted to the dopant, and the first dopant having a shorter fluorescence peak wavelength and having a higher excitation energy than the second dopant is present around the second dopant. This is probably because the excitation energy of the dopant was less deactivated during the non-emission process.

Since the second dopant emits light efficiently as described above, high-luminance light emission can be obtained at a low driving voltage, and stable light emission can be obtained for a long period of time. There is no need to select the host material and the dopant so that the fluorescence energy of the host material and the excitation energy of the dopant are substantially the same, and the range of selection of the host material and the dopant is widened.

In order to further enhance the luminous efficiency of the second dopant, it is preferable to select and use a material having a high fluorescence quantum yield not only for the second dopant but also for the first dopant.

In the organic EL device according to the present invention, a material having a large work function such as gold or ITO (indium-tin oxide) is used as the hole injection electrode, while the electron injection electrode is not used. It is preferable to use an electrode material having a small work function, such as magnesium or the like. In order to extract EL light generated in this organic EL element, at least one of the electrodes needs to be transparent. Use ITO having a large work function.

Further, the element structure of the organic EL element in the present invention may be any of the above-mentioned DH structure, SH-A structure and SH-B structure.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an organic EL device according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings, and a comparative example will be described. The organic EL device in this embodiment efficiently emits light at a low voltage. To reveal.

(Example 1) Organic E in Example 1
As shown in FIG. 1, the L element is made of ITO on a glass substrate 1.
A hole injecting electrode 2 having a thickness of 1000 ° and a hole transport layer 3 having a thickness of 400 ° which is composed of a triphenylamine derivative (m-MTDATA) shown in Chemical Formula 1 below. With respect to a host material comprising polyvinyl carbazole represented by the following chemical formula 2, rubrene represented by the following chemical formula 3 is 5% by weight as a second dopant having a long fluorescent peak wavelength, and the following chemical formula is used as a first dopant having a short fluorescent peak wavelength. The diamine derivative (TPD) shown in 4 is 5
Light-emitting layer 4 doped to 400% in thickness by doping by weight%
And a 10-benzo [h] quinolinol-beryllium complex (BeBq ₂ ) shown in Chemical Formula 5 below and having a film thickness of 4
It has a DH structure in which an electron transport layer 5 having a thickness of 00 ° and an electron injection electrode 6 made of a magnesium-indium alloy and having a thickness of 2000 ° are sequentially laminated.

[0020]

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[0021]

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[0022]

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[0023]

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[0024]

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In manufacturing the organic EL device of the first embodiment, first, the hole injection electrode 2 was made of ITO.
Is washed with a neutral detergent and then washed in acetone for 20 minutes and in ethanol for 20 minutes.
Ultrasonic cleaning was performed for each minute. Then, the glass substrate 1 was placed in boiling ethanol for about 1 minute, taken out, and immediately blow-dried. afterwards,
On the hole injection electrode 2 described above, the m-M
A hole transport layer 3 is formed by vacuum-depositing TDATA,
Furthermore, the polyvinyl carbazole shown in Chemical formula 2 is doped with 5% by weight of rubrene shown in Chemical formula 3 and the TPD shown in Chemical formula 4, respectively, so that polyvinyl carbazole, rubrene and TPD are added to the above-mentioned hole. The light-emitting layer 4 was formed by co-evaporation on the transport layer 3, and then BeBq ₂ shown in Chemical Formula 5 was vacuum-deposited on the light-emitting layer 4 to form the electron transport layer 5. Each of these vapor depositions was performed by a resistance heating vapor deposition method, and the degree of vacuum was 1 × 10 ⁻⁵ Torr.
r, the substrate temperature was 20 ° C., and the deposition rate was 2 ° / sec. Then, an electron injection electrode 6 made of a magnesium-indium alloy was formed on the electron transport layer 5.

Here, the respective fluorescence peak wavelengths and bands of the above-mentioned polyvinyl carbazole used as the host material, the above-mentioned TPD used as the first dopant, and the above-mentioned rubrene used as the second dopant in the above-mentioned light emitting layer 4 are shown. The gap was as shown in Table 1 below, and the fluorescence peak wavelength was longer in the order of the diamine derivative used as the first dopant, polyvinylcarbazole used as the host material, and rubrene used as the second dopant.

[0027]

[Table 1]

(Comparative Example 1) Organic E in Comparative Example 1
The L element also has a DH structure similar to that of the organic EL element of Example 1 described above. In Comparative Example 1, polyvinyl carbazole shown in Chemical formula 2 was used as the host material in the light emitting layer 4, and this host The material was doped with only 5% by weight of rubrene shown in Chemical Formula 3 as a dopant.
An organic EL device was obtained in the same manner as in the above case.

(Example 2) Organic E in Example 2
As shown in FIG. 2, an L element is formed on a glass substrate 1 by ITO.
A transparent hole injecting electrode 2 having a thickness of 1000 °, a hole transport layer 3 having a thickness of 400 ° made of a diamine derivative represented by Chemical Formula 4, and zinc-azomethine represented by Chemical Formula 6 below. With respect to the host material composed of the complex, 5 wt% of a europium complex [Eu (TTA) ₃ phen] shown in the following chemical formula 7 as a second dopant having a long fluorescent peak wavelength and the following as a first dopant having a short fluorescent peak wavelength are shown below. The oxadiazole derivative (OXD-
7) A light emitting layer 4 doped with 5% by weight to a thickness of 600 ° and an electron injection electrode 6 made of a magnesium-indium alloy and having a thickness of 2000 ° are sequentially laminated. It has an A structure.

[0030]

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[0031]

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[0032]

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Here, each of the zinc-azomethine complex used as the host material in the light emitting layer 4, the OXD-7 used as the first dopant, and the europium complex used as the second dopant was used. The fluorescence peak wavelength and band gap are as shown in Table 2 below, and the fluorescence peak wavelengths of OXD-7 used as the first dopant, zinc-azomethine complex used as the host material, and europium complex used as the second dopant were used. It was getting longer in order.

[0034]

[Table 2]

Comparative Example 2 Organic E in Comparative Example 2
The L element also has the same SH-type as the organic EL element of Example 2 above.
In Comparative Example 2, the zinc-azomethine complex shown in Chemical Formula 6 was used as the host material in the light-emitting layer 4 and the dopant was used in this Comparative Example 2 as a dopant. 5 europium complex alone
An organic EL device was obtained in the same manner as in Example 2 except for doping by weight%.

(Example 3) Organic E in Example 3
The L element also has the same SH-type as the organic EL element of Example 2 above.
In Example 3, the host material in the light-emitting layer 4 was Tris (8
-Quinolinol) aluminum complex (Alq ₃ ), with respect to this host material, 5% by weight of a coumarin derivative represented by the following formula 10 as a second dopant having a long fluorescence peak wavelength and a first dopant having a short fluorescence peak wavelength as a second dopant. An organic EL device was produced in the same manner as in Example 2 except that the anthracene shown in Chemical Formula 11 below was doped at 5% by weight.

[0037]

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[0038]

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[0039]

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Here, the respective fluorescence peak wavelengths of the above-mentioned Alq ₃ used as the host material, the above-mentioned anthracene used as the first dopant, and the above-mentioned coumarin derivative used as the second dopant in the above-mentioned light-emitting layer 4 were obtained. The band gap is as shown in Table 2 below, and the fluorescence peak wavelength was 1st.
The anthracene used for the dopant, the Alq ₃ used for the host material, and the coumarin derivative used for the second dopant increased in this order.

[0041]

[Table 3]

Comparative Example 3 Organic E in Comparative Example 3
The L element also has the same SH-type as the organic EL element of Example 3 above.
In Comparative Example 3, Alq ₃ shown in Chemical Formula 9 above was used as the host material in the light emitting layer 4, and the coumarin shown in Chemical Formula 10 above was used as a dopant for this host material. An organic EL device was obtained in the same manner as in Example 3 except that only 5% by weight of the derivative was doped.

Next, the above Examples 1 to 3 and Comparative Examples 1 to 3
Of the organic EL element, a voltage of + was applied to the hole injection electrode 2 and a voltage of − was applied to the electron injection electrode 6, respectively. The maximum luminance in each organic EL element and the luminance when a current of 10 mA was flowed per 1 cm ² were obtained. (Luminance-current efficiency) and the voltage (light emission starting voltage) required to obtain a luminance of 1 cd / m ² in each organic EL element were examined. The results are shown in Table 4 below. When the organic EL elements of Examples 1 to 3 were caused to emit light as described above, the organic EL element of Example 1 emitted yellow light by rubrene having an emission peak wavelength of 560 nm in Example 2. In the organic EL device of Example 3, red emission was caused by the europium complex having an emission peak wavelength of 614 nm, and in the organic EL device of Example 3, the emission peak wavelength was 532 n.
Green luminescence was obtained by the coumarin derivative which became m.

[0044]

[Table 4]

As is apparent from the results, the host material in the light emitting layer was doped with two kinds of dopants as described above, and the host material in the light emitting layer was doped only with one kind of dopant. When compared with those of the comparative example, in the corresponding organic EL devices of Example 1 and Comparative Example 1, Example 2 and Comparative Example 2, and Example 3 and Comparative Example 3,
In each case, the organic EL element of the example has a higher maximum luminance, a higher luminance at 10 mA / cm ² indicating luminance-current efficiency, a lower light emission starting voltage,
High-luminance and efficient light emission can be performed, and light emission can be performed at a low driving voltage.

[0046]

As described above in detail, in the organic EL device according to the present invention, the host material in the light emitting layer using the organic material includes at least the first dopant having a shorter fluorescence peak wavelength than the host material, Since the second dopant having a long peak wavelength is contained, efficient light emission is performed in the second dopant without light emission of the host material and the first dopant,
Light emission can be performed at a low driving voltage, efficient light emission with high luminance can be obtained, and stable light emission can be performed for a long period of time.

[Brief description of the drawings]

FIG. 1 shows DH in Example 1 and Comparative Example 1 of the present invention.
It is the schematic which showed the state of the organic EL element which became the structure.

FIG. 2 is a schematic diagram showing a state of an organic EL element having an SH-A structure in Examples 2 and 3 and Comparative Examples 2 and 3 of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 glass substrate 2 hole injection electrode 3 hole transport layer 4 light emitting layer 5 electron transport layer 6 electron injection electrode

Continuation of the front page (56) References JP-A-9-63770 (JP, A) Junji Kido, "Light Emission Mechanism and Material Design in Organic EL Devices", Denki Kagaku and Industrial Physical Chemistry, The Electrochemical Society of Japan, October 5, 1995, Vol. 63, No. 10, p. 901-905 (58) Fields investigated (Int. Cl. ⁷ , DB name) H05B 33/00-33/28 JICST file (JOIS )

Claims (1)

(57) [Claims] 1. An organic electroluminescent device comprising at least a light emitting layer using an organic material provided between a hole injection electrode and an electron injection electrode, wherein at least the host material is used with respect to the host material in the light emitting layer. No. 1 whose fluorescence peak wavelength is shorter than the fluorescence peak wavelength of
A host material containing a dopant and a second dopant having a longer fluorescence peak wavelength than the fluorescence peak wavelength of the host material;
Without the light emission of the first dopant and the second dopant
An organic electroluminescence device characterized by emitting light .