Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
2002-10-09
2004-09-07
Jackson, Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C257S098000, C438S029000
Reexamination Certificate
active
06787796
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of Korean Patent Application No. 2002-10466 filed on Feb. 27, 2002, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) display device, and more particularly, to an organic electroluminescent display device having an improved light extraction efficiency of light generated by an organic layer.
2. Description of the Related Art
Generally, organic electroluminescent display devices are spontaneous emission display devices that emit light by electrically exciting fluorescent organic compounds. The organic electroluminescent display devices are expected to be applied to next generation display devices that can overcome shortcomings of liquid crystal display devices, with characteristics including a low driving voltage, enabling the production of thin and small display panels having a wide viewing angle, and a high response speed.
An organic electroluminescent display device using a layered structure has been developed by the Eastman Kodak Company, and has been commercialized as a green-emitting display device having an improved lifetime by the Pioneer Electronic Corporation. Recently, many kinds of new materials having various molecular structures that are advantageous in organic materials, and organic electroluminescent (EL) display devices for use as color display devices having excellent characteristics including a lower DC drive voltage, small and thin structure, and spontaneous emission, have been the subjects of active research.
An organic electroluminescent (EL) display device includes organic layers having a predetermined pattern on a glass or a transparent insulating substrate, and electrode layers formed on and under the organic layers. The organic layers are made of organic compounds. Usable materials of the organic compounds include copper phthalocyanine (CuPc), N,N-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPD), and tris(8-hydroxyquinoline) aluminium (Alq3).
In the organic EL device having the above-described configuration, if a positive voltage and a negative voltage are applied to the electrodes, holes injected from the electrode to which the positive voltage is applied move to an emitter layer via a hole transport layer, and electrons from a corresponding one of the electrodes are injected into the emitter layer via an electron transport layer. The electrons and holes are recombined at the emitter layer to generate excitons. The generated excitons de-excite from an excited state to a base state so as to have fluorescent molecules of the emitter layer emit light, thereby forming an image.
The light efficiency of the organic EL device driven as described above is divided into an internal efficiency, and an external efficiency or an extraction (taking-out) efficiency. The internal efficiency depends on the photoelectric conversion efficiency of an organic luminescent material. The external efficiency depends on the refractive index of each organic layer constituting the organic EL device. In other words, in the organic EL device, light having an outgoing angle larger than a critical angle cannot be taken out because the light is reflected at an interface between the substrate and the electrode layer, or between the organic layer and the electrode layer.
FIG. 1
shows a conventional organic EL device, where light emitted from an organic layer
10
is transmitted to a transparent glass substrate
11
from an interface between the glass substrate
11
and an indium tin oxide (ITO) electrode
12
. The light transmitting efficiency is represented by the following formula:
½(N
out
/N
in
)
2
where N is a refractive index.
Based on the above formula, the light extraction efficiency for each color of the conventional organic EL device is shown in the Table 1 below.
TABLE 1
Blue organic layer
Green organic layer
Red organic layer
Wavelength (nm)
450
530
620
Refractive index of ITO
2.01
1.93
1.76
electrode (n)
Refractive index of
1.525
1.52
1.515
glass substrate
Light extraction
29%
34%
37%
efficiency
As shown in the Table 1, as much light as 60% or more becomes extinct in the organic EL device due to a difference in refractive index between the ITO electrode
12
and the glass substrate
11
.
An example of an organic EL device which prevents a reduction in light extraction efficiency is disclosed in Japanese Patent Publication No. 63-314795. The organic EL device includes a substrate having an ability to collect light, similarly to a projecting lens. However, the formation of such a projecting lens having the ability to collect light on a substrate is hard to accomplish in a device having a small pixel area, due to luminescence by an organic layer.
On the other hand, Japanese Patent Publication No. 62-172691 discloses an organic EL device comprising a first dielectric layer interposed between a transparent electrode layer and a light-emitting layer, and a second dielectric layer having a refractive index between the refractive index of a substrate glass and a refractive index of the light-emitting layer formed between the transparent electrode and the light-emitting layer.
Furthermore, Japanese Patent Publication No.1-220934 discloses an organic EL device having a lower electrode, an insulation layer, a light-emitting layer and an upper electrode formed on a substrate, and having a mirror which reflects light onto one side surface of the light-emitting layer.
However, since the film thickness of the light-emitting layer of the organic EL device is very small, the formation of a reflection mirror having a tapered surface on the side surface of the organic EL device is difficult using the currently available technique, and a high cost is required to perform the above formation.
The Japanese Patent Publication No. 11-283751 discloses an organic EL device characterized by making a diffraction grating or a zone plate into a component in an organic electroluminescence element which has one or more organic layer between an anode plate and a cathode.
In such an organic EL device, since irregularities must be formed on a surface of a substrate or a fine electrode pattern layer, or a separate diffraction grating must be provided, the fabricating process thereof becomes complex, making it difficult to attain an efficient productivity. Also, formation of an organic layer on the irregularities makes roughness severer due to the irregularities, thereby resulting in deterioration of durability and reliability of the organic EL device.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an organic EL device which can enhance the luminance of an image by reducing an internal light loss, so as to increase a light-emitting efficiency, and a method of manufacturing the same.
It is another object of the present invention to provide an organic EL device which can reduce a light loss using a light scattering effect at an interface between a layer having a low refractive index and a layer having a high refractive index, and a method of manufacturing the same.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above and other objects of the present invention, there is provided an organic EL device comprising a substrate layer, a first electrode layer formed on the substrate layer, an organic layer formed on the first electrode layer, and a second electrode layer formed on the organic layer, and a light loss preventing layer having areas with different refractive indices, formed between ones of the substrate, first electrode, organic, and second electrode layers having a large difference in refractive indices.
The light loss preventing layer may be formed between the substrate layer and the first electrode layer. Adjacent one of the a
Do Young-rag
Kim Yoon-chang
Park Jin-woo
Song Young-woo
Jackson Jerome
Samsung SDI & Co., Ltd.
Staas & Halsey , LLP
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