JP2005101004A - Organic electroluminescent element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an organic electroluminescent element capable of allowing efficient luminescence by a driving voltage lower than that of a conventional organic EL element, and allowing stable luminescence for a long period of time. <P>SOLUTION: This organic electroluminescent element is formed by providing a luminescent layer 4 using at least an organic material between a hole injection electrode 2 and an electron injection electrode 6. A host material in the luminescent layer is made to contain a first dopant and a second dopant, and the second dopant is made to cause luminescence without being attended by luminescence of the first dopant. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an organic electroluminescent device in which a light emitting layer using at least an organic material is provided between a hole injection electrode and an electron injection electrode, and in particular, can emit light efficiently at a low voltage, and can be used for a long time. The present invention relates to an organic electroluminescence element that can emit light stably.

  In recent years, with the diversification of information equipment and the like, the need for a flat display element that consumes less power and occupies a smaller area than a CRT that has been conventionally used has increased. A luminescence element (hereinafter abbreviated as an EL element) has attracted attention.

  This EL element is roughly classified into an inorganic EL element and an organic EL element depending on the material used. In general, in an inorganic EL element, a high electric field is applied to the light emitting portion, and electrons are accelerated in this high electric field to emit light. In the organic EL element, electrons and holes are injected into the light emitting portion from the electron injection electrode and the hole injection electrode, respectively. Thus, the injected electrons and holes are recombined at the emission center to excite the organic material, and emit fluorescence when the organic material returns from the excited state to the ground state.

  Here, the inorganic EL element requires a high voltage of 100 to 200 V as a driving voltage for applying a high electric field as described above, whereas the organic EL element has a voltage of about 5 to 20 V. There is an advantage that it can be driven at a low voltage. In addition, in such an organic EL element, there is an expectation that a light-emitting element that emits light in an appropriate color can be obtained by selecting a fluorescent substance that is a light-emitting material, and can be used as a full-color display device. In recent years, various studies have been conducted on such organic EL elements.

  The element structure of the organic EL element is a three-layer structure called a DH structure in which a hole transport layer, a light emitting layer, and an electron transport layer are stacked between a hole injection electrode and an electron injection electrode. Or a two-layer structure called 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 a hole injection electrode and an electron injection 2. Description of the Related Art A two-layer structure called an SH-B structure in which a light-emitting layer and an electron transport layer rich in hole transport properties are stacked between electrodes is known.

  Moreover, about the light emitting layer in said organic EL element, what doped the trace amount dopant with respect to the host material, and light-emitted using this dopant as a light emission center was developed (refer patent document 1).

  Here, in order to obtain a light emitting layer in which a small amount of dopant is doped with respect to the host material as described above, it is necessary to use a material having a good film forming property for the host material. In general, quinolinol metal complexes, benzoquinolinol metal complexes, and the like have 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 metal complexes, the fluorescence quantum yields are as low as 20% or less. In the case of using a host material with a low fluorescence quantum yield and a low excitation energy, the excitation energy of the dopant is excited by exciting the dopant with high efficiency and obtaining sufficient light emission. Can easily be lost through a non-emission process, and much of this energy is converted into heat, which can degrade the organic EL device, making it impossible to emit light stably over a long period of time, or obtaining efficient light emission with a low driving voltage. There was a problem that I could not.

Furthermore, in order to increase the light emission efficiency in the organic EL element, the host material in which the fluorescence energy in the host material and the excitation energy in the dopant are substantially the same so that the energy is efficiently transferred from the host material to the dopant. However, it is difficult to use effective dopants in combination with such host materials because the types of host materials are currently limited. There was also a problem that the efficiency could not be increased sufficiently.
Japanese Unexamined Patent Publication No. 63-264692

  An object of the present invention is to solve various problems as described above in an organic EL element, and can emit light efficiently with a lower driving voltage than a conventional organic EL element. An object of the present invention is to provide an organic electroluminescence device capable of performing stable light emission.

  In the present invention, in order to solve the above-described problems, in an organic electroluminescence device in which a light emitting layer using at least an organic material is provided between a hole injection electrode and an electron injection electrode, The host material contains the first dopant and the second dopant, and emits the second dopant without causing the first dopant to emit light.

  Furthermore, in this invention, the diamine derivative of the first dopant has a fluorescence peak wavelength shorter than the fluorescence peak wavelength of the host material, and the rubrene of the second dopant has a fluorescence peak wavelength longer than the fluorescence peak wavelength of the host material. It is.

  Like the organic EL element in this invention, since the host material in the light emitting layer contains a first dopant made of a diamine derivative and a second dopant made of rubrene, efficient light emission is performed. It is. Further, since a diamine derivative having a fluorescence peak wavelength shorter than that of the host material is used as the first dopant and rubrene is used as a second dopant having a fluorescence peak wavelength longer than that of the host material, the diamine derivative of the first dopant emits light. Instead, efficient light emission is performed in the second dopant rubrene.

  And since the 2nd dopant light-emits efficiently as mentioned above, while being able to obtain high-intensity light emission with a low drive voltage, stable light emission can be obtained over a long period of time.

  In order to further increase the light emission efficiency in the diamine derivative of the first dopant, rubrene having a high fluorescence quantum yield is used not only for the first dopant but also for the second dopant.

  In the organic EL device of 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 magnesium or the like is used as the electron injection electrode. It is preferable to use an electrode material having a small work function. In order to extract the EL light generated in the organic EL element, it is necessary to make at least one of the electrodes transparent. Generally, the hole injection electrode has a transparent work function. A large material such as ITO is used.

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

  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 is made to contain the first dopant made of the diamine derivative and the second dopant made of rubrene. Therefore, efficient light emission can be performed with the second dopant having a long fluorescence peak wavelength, light can be emitted with a low driving voltage, high luminance and efficient light emission can be obtained, and stable light emission over a long period of time. Can be done.

  Hereinafter, an organic EL device according to an embodiment of the present invention will be specifically described with reference to the accompanying drawings, and a comparative example will be given to clarify that the organic EL device in this embodiment emits light efficiently at a low voltage. .

(Example 1)
As shown in FIG. 1, the organic EL device in Example 1 includes a transparent hole injection electrode 2 made of ITO on a glass substrate 1 and having a film thickness of 1000 mm, and triphenyl shown in Chemical Formula 1 below. As a second dopant having a long fluorescence peak wavelength, a hole transport layer 3 composed of an amine derivative (m-MTDATA) and having a film thickness of 400 mm and a host material composed of polyvinylcarbazole shown in the following chemical formula 2 A light-emitting layer 4 doped with 5% by weight of diamine derivative (TPD) shown in chemical formula 4 below as a first dopant having a short fluorescence peak wavelength of 5% by weight of rubrene shown in chemical formula 3 of FIG. An electron transport layer 5 composed of 10-benzo [h] quinolinol-beryllium complex (BeBq2) shown in the following chemical formula 5 and having a thickness of 400 mm, magnesium And an electron injection electrode 6 where the film thickness is composed of indium alloy becomes 2000Å is turned DH structure laminated sequentially.

  In manufacturing the organic EL device of Example 1, the glass substrate 1 on which the hole injection electrode 2 was formed with ITO was first washed with a neutral detergent, and then this was washed in acetone for 20 minutes in ethanol. For 20 minutes. And after putting this glass substrate 1 in the boiled ethanol for about 1 minute and taking this out, air drying was performed immediately. Thereafter, m-MTDATA shown in Chemical Formula 1 is vacuum-deposited on the hole injection electrode 2 to form a hole transport layer 3, and the polyvinyl carbazole shown in Chemical Formula 2 is shown in Chemical Formula 3. Then, polyvinyl carbazole, rubrene, and TPD were co-evaporated on the hole transport layer 3 to form a light emitting layer 4 so that 5% by weight of each of rubrene and TPD shown in Chemical Formula 4 was doped. The electron transport layer 5 was formed by vacuum-depositing BeBq2 shown in Chemical Formula 5 on the light emitting layer 4. These vapor depositions were performed by resistance heating vapor deposition under the conditions of a vacuum degree of 1 × 10 −5 Torr, a substrate temperature of 20 ° C., and a vapor deposition rate of 2 Å / 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 band gaps of the polyvinyl carbazole used as the host material in the light emitting layer 4, the TPD used as the first dopant, and the rubrene used as the second dopant are as follows. Table 1 shows that the fluorescence peak wavelength was longer in the order of the diamine derivative used as the first dopant, the polyvinyl carbazole used as the host material, and the rubrene used as the second dopant.

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

  Next, the organic EL elements of Example 1 and Comparative Example 1 were used, and a positive voltage was applied to the hole injection electrode 2 and a negative voltage was applied to the electron injection electrode 6, respectively. The luminance (luminance-current efficiency) when a current of 10 mA was applied to the pixel and the voltage (light emission starting voltage) required to obtain a luminance of 1 cd / m @ 2 for each organic EL element were examined. The results are shown in Table 2 below. It was shown to. In addition, when each organic EL element of Example 1 was made to emit light as mentioned above, in the organic EL element in Example 1, yellow light emission by rubrene whose emission peak wavelength was 560 nm was obtained.

  As is clear from this result, the host material in the light emitting layer is that of each example in which two kinds of dopants are doped as described above, and the comparative example in which only one kind of dopant is doped. In each of the corresponding organic EL elements of Example 1 and Comparative Example 1, the organic EL element of the example has a higher maximum luminance and 10 mA / cm @ 2 indicating luminance-current efficiency. The luminance of the light source is also high, and the light emission start voltage is also low, so that high luminance and efficient light emission can be performed and light can be emitted with a low driving voltage.

It is the schematic which showed the state of the organic EL element which became DH structure in Example 1 and Comparative Example 1 of this invention. It is the schematic which showed the state of the organic EL element which became the SH-A structure in the Example and comparative example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Glass substrate 2 Hole injection electrode 3 Hole transport layer 4 Light emitting layer 5 Electron transport layer 6 Electron injection electrode

Claims (1)

  In the 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 first dopant, a second dopant, and a host material in the light emitting layer And an organic electroluminescence element characterized by causing the second dopant to emit light without causing the first dopant to emit light.

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