JP3596084B2 - Electroluminescent device - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to an electroluminescence (electroluminescence) element applied to a display device such as a color display, and more particularly to an element that generates blue electroluminescence using an organic material as a light emitting layer.
[0002]
[Prior art]
In recent years, attention has been paid to electroluminescence using organic materials. Already in 1966, blue light emission from anthracene crystals was reported. At this time, there was no surface that was more excellent in light emission characteristics than the electroluminescence of the inorganic material because the driving voltage was still high (100 volts or more), the luminance was low, and the efficiency was poor.
[0003]
However, in 1987, Dr. Tan and Vanslike of Eastman Kodak Company of the United States announced an injection-type electroluminescent light emitting device using an organic material. The structure is composed of a metal electrode / a light emitting layer (n-type) / a hole injection layer (p-type) / a transparent electrode. The light emitting layer is an aluminum quinoline complex, and the hole injection layer is a diamine. It is considered that electrons are injected from the metal electrode and holes are injected from the diamine layer, and light is emitted after exciton is formed at the boundary between the aluminum quinoline complex layer and the diamine layer (emission of many electroluminescences). The mechanism has not been fully elucidated). This device emits green light, is driven at a DC voltage of 10 volts, has a luminance of 1000 candelas / square meter, and has a luminous efficiency of 1.5 lumens / watt.
[0004]
Since the announcement of Kodak, research on organic material electroluminescence has been actively conducted in various fields. Aiming at application to color displays, the goal was to develop organic materials that emit light in each of the three primary colors of RGB, to improve light emission stability and luminance, and to develop a thin device structure that can be driven at low voltage and its manufacturing method. Research is being carried out.
[0005]
[Problems to be solved by the invention]
In order to realize a color display using organic material electroluminescence, an element that emits light in three primary colors of RGB is indispensable. However, at the present stage, although the green light emitting element has reached the practical level, the red and blue elements have characteristics far from the practical level.
[0006]
In particular, with regard to blue electroluminescence, it is difficult to obtain even stable light emission due to heat generated from a thermal relaxation process accompanying light emission, singlet oxygen, and the like. When an organic material having a high recrystallization property is used for the light emitting layer, when solidified, the emission wavelength becomes longer due to the formation of a dimer or the like, so that blue light emission cannot be realized, and even if light is emitted, the light is immediately extinguished. Phenomenon is seen.
[0007]
Coumarin-based laser dyes are known as organic materials that exhibit excellent electroluminescence properties. This is a material having a large fluorescence yield, and there are research reports of using it as a doping material for improving the color purity of green light emission or as a doping material for red light emission. However, there is no report that stable blue light emission was realized using a coumarin-based laser dye. Rather, coumarin-based laser dyes are not considered suitable as blue electroluminescent materials.
[0008]
For example, Coumarin 450 and the like have a maximum fluorescence wavelength near 446 nm and correspond to chromaticity of blue of RGB primary colors. Although a coumarin-based laser dye that emits short-wavelength fluorescence as described above exists, it generally has high crystallinity when used alone. For example, when a light-emitting layer is formed by forming a vapor-deposited film using coumarin 450 alone, the vapor-deposited film is recrystallized and does not become a desirable amorphous thin film. Then, even if the light emission is slightly blue, the life is remarkably short, the luminance is low and it is not stable. Alternatively, a result was obtained in which a dimer was formed and no blue light was emitted.
[0009]
The present invention has been made in view of the above-mentioned conventional problems, and has as its object to realize an electroluminescent device that emits blue light with good characteristics using a known coumarin-based laser dye.
[0010]
[Means for Solving the Problems]
The electroluminescent device of the present invention is a device having a laminated structure of an organic material including a light emitting layer formed of a thin film of an organic material, wherein the light emitting layer is made of an amorphous material composed of a mixture of a plurality of different types of coumarin-based laser dyes. It is desirable that the luminescent layer of the light-emitting layer contains coumarin 450 and that the luminescent layer be used as the luminescent center.
[0011]
In the electroluminescent device of the next invention, the light-emitting layer is an amorphous thin film in which coumarin 450 serving as a light-emission center is dispersed in an appropriate host material as a guest material, and its fluorescence center wavelength is equal to or less than the fluorescence wavelength of coumarin 450. It is characterized by being. Here, as the host material , a laser dye whose fluorescent center is on the shorter wavelength side than the fluorescent wavelength of coumarin 450 , or an organic material having an electron transporting property or a hole transporting property is used.
[0012]
[Action]
According to the present invention, an amorphous thin film containing a coumarin-based laser dye can be formed as a light emitting layer, and efficient blue light emission can be stably obtained from the light emitting layer.
[0013]
【Example】
First, an element structure of an electroluminescent element according to the present invention will be briefly described with reference to FIG. As shown in FIG. 1, an anode electrode 2 as a transparent electrode is formed on a glass substrate 1 as a transparent substrate, and a hole transport layer 3, a light emitting layer 4, an electron transport layer 5, and a back electrode are sequentially formed thereon. The cathode electrode 6 is formed by lamination. Details of each layer will be described in the following examples.
[0014]
[Example 1]
An ITO (film thickness: about 100 nm) ITO substrate serving as the anode electrode 2 is provided on a 30 mm × 30 mm glass substrate 1, and a light emitting element for masking a region other than a 2 mm × 2 mm light emitting region by SiO ₂ evaporation is provided on this substrate. Is prepared. Next, TPD (N, N'-diphenyl-N, N'-di (3-methylphenyl) 4,4'-diaminobiphenyl) having the structural formula shown in FIG. After evaporating about 50 nm (evaporation rate: 2 to 4 ° / sec) and further evaporating the light emitting layer 4 to about 50 nm (evaporation rate: 2 to 4 ° / sec), t-butyl of the structural formula shown in FIG. -PBD was deposited by about 50 nm (deposition rate: 2 to 4 ° / sec). After that, Al was vapor-deposited as a cathode electrode 6 at a rate of about 2 kÅ (a vapor deposition rate of 11 to 13Å / sec) to produce a light-emitting device.
[0015]
The light-emitting layer 4 is formed by arranging coumarins 440, 445, and 450 (the structural formulas are shown in FIGS. 2, 3, and 4) in the same weight ratio on the same Ta vapor deposition board and vapor-depositing the same. A light-emitting layer was obtained. The sublimation points of the coumarins 440, 445 and 450 are almost the same temperature, and co-evaporation can be performed using the same board. This method is a meaningful method in an actual mass production process.
[0016]
When the characteristics of the organic electroluminescent blue light emitting device thus manufactured were measured, the maximum emission wavelength was 447 nm, and it was clear from the shape of the spectrum that coumarin 450 was the emission center. Further, a luminance of about 300 cd / m ² was obtained for an applied voltage of 15 V.
[0017]
[Example 2]
An ITO (film thickness: about 100 nm) ITO substrate serving as an anode electrode 2 is provided on a 30 mm × 30 mm glass substrate 1, and a substrate other than a 2 mm × 2 mm light emitting region is masked on this substrate by SiO ₂ vapor deposition. Create a cell. Next, TPD (N, N'-diphenyl-N, N'-di (3-methylphenyl) 4,4'-diaminobiphenyl) is deposited as a hole transport layer 3 by vacuum deposition at about 50 nm under vacuum (deposition rate). After the light emitting layer 4 is vapor-deposited to a thickness of about 50 nm (evaporation rate: 2 to 4 ° / sec), an aluminum quinolinol complex is used as the electron transport layer 5 in the form of A1q3 (tris (8 -Quinolinol) aluminum) was deposited at about 20 nm (deposition rate: 2 to 4 ° / sec). Thereafter, A1 was vapor-deposited as the cathode electrode 6 at a rate of about 2 kÅ (a deposition rate of 11 to 13Å / sec) to produce a light emitting device.
[0018]
The light emitting layer 4 was doped with coumarin 450 as a guest material using the laser dye QUI as a host material. By arranging and vapor-depositing each on a different Ta vapor deposition board, an amorphous light-emitting layer could be obtained. At the time of vapor deposition, adjustment was performed so that the respective vapor deposition rates became 2 to 4 ° / sec, and the light emitting layer 4 was produced.
[0019]
When the characteristics of the light-emitting device thus manufactured were measured, it was found that the maximum emission wavelength was 446 nm, and that the coumarin 450 was the emission center from the shape of the spectrum. Further, a luminance of about 280 cd / m ² was obtained with respect to an applied voltage of 14 V.
[0020]
[Example 3]
An ITO (film thickness: about 100 nm) ITO substrate as the anode electrode 2 is provided on a 30 mm × 30 mm glass substrate 1, and an element other than a 2 mm × 2 mm light emitting region is masked on this substrate by SiO ₂ evaporation. Create a cell. Next, TPD (N, N'-diphenyl-N, N'-di (3-methylphenyl) 4,4'-diaminobiphenyl) is deposited as a hole transport layer 3 by vacuum deposition at about 50 nm under vacuum (deposition rate). After evaporating the light emitting layer 4 to about 50 nm (evaporation rate: 2 to 4 ° / sec), the electron transport layer 5 is coated with t-butyl 1-PBD at about 50 nm (evaporation rate: 2 to 4 ° / sec). Evaporated. Thereafter, A1 was vapor-deposited as the cathode electrode 6 at a rate of about 2 kÅ (a deposition rate of 11 to 13Å / sec) to produce a light-emitting device.
[0021]
The light-emitting layer 4 was doped with coumarin 450 as a guest material using Zn (oxz) 2 having a structural formula shown in FIG. 8 as a zinc complex as a host material. By arranging and vapor-depositing each on a different Ta vapor deposition board, an amorphous light-emitting layer could be obtained. At the time of vapor deposition, adjustment was performed so that the respective vapor deposition rates became 2 to 4 ° / sec, and the light emitting layer 4 was produced.
[0022]
When the characteristics of the organic electroluminescent blue light emitting device thus manufactured were measured, the maximum emission wavelength was 446 nm, and it was clear from the shape of the spectrum that coumarin 450 was the emission center. Further, a luminance of about 400 cd / m ² was obtained with respect to an applied voltage of 14 V.
[0023]
(Other embodiments)
In each of the above embodiments, the materials of the anode electrode 2, the electron transport layer 5, the hole transport layer 3, and the cathode electrode 6 are not limited to the above. For example, if the hole transport layer 3 is used, a benzidine derivative, a styrylamine derivative, A hole-transporting organic substance such as a triphenylmethane derivative or a hydrazone derivative may be used. Similarly, an electron transporting organic substance such as a perylene derivative, a coumarin derivative, a bisstyryl derivative, or a pyrazine derivative may be used for the electron transport layer 5. As for the material of the cathode electrode 6, in order to inject electrons efficiently, it is preferable to use a metal having a small work function from the vacuum level of the electrode material. For example, In, Mg, Ag, Ca, Ba, Li, etc. May be used singly or as an alloy with another metal with increased stability. In the present invention, ITO, which is a transparent electrode, is used for the anode electrode 6 in order to take out the organic electroluminescent blue light emission from the side of the anode electrode 6, but of course, in order to efficiently inject holes, the vacuum level of the anode electrode material is used. Alternatively, an electrode having a large work function from, for example, Au, SnO ₂ + Sb, or ZnO + A1 may be used.
[0024]
【The invention's effect】
As described above, according to the present invention, even if a coumarin-based laser dye, which was originally considered to have a high crystallinity and to be formed into a dimer or the like by being thinned, and to have a long emission wavelength, was used, the blue color was obtained. It became clear that light emission was possible.
[0025]
As a result, it is possible to obtain blue light emission even using commonly known coumarin-based short-wavelength laser dyes, etc., and to shorten the time and the potential for material development. It is considered possible to show.
[Brief description of the drawings]
FIG. 1 is a schematic view showing a structure of a light emitting element to which the present invention is applied.
FIG. 2 is a structural formula of Coumarin 450.
FIG. 3 is a structural formula of coumarin 445.
FIG. 4 is a structural formula of coumarin 440.
FIG. 5 is a structural formula of A1q3.
FIG. 6 is a structural formula of TPD.
FIG. 7 is a structural formula of t-Bu-PBD.
FIG. 8
Structural formula of Zn (oxz) 2.
[Explanation of symbols]
1: transparent substrate 2: transparent electrode (anode electrode)
3 Hole transport layer 4 Light emitting layer 5 Electron transport layer 6 Back electrode (cathode electrode)

Claims (2)

An electroluminescent element having a laminated structure of an organic substance including a light emitting layer formed of a thin film of an organic substance,
The light-emitting layer is an amorphous thin film composed of only a plurality of different types of coumarin-based laser dyes, and the light-emitting layer contains coumarin 450 (coumarin 2) represented by the following structural formula;

An electroluminescent device characterized in that this is the emission center. 2. The electroluminescent device according to claim 1, wherein the laminated structure of the organic substance includes, on a transparent substrate, respective layers of a transparent electrode, a hole transport layer, an electron transport layer, and a back electrode in addition to the light emitting layer. .

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