Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
2002-07-24
2004-08-03
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S113000
Reexamination Certificate
active
06771018
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a light-emitting device employing light emission from an organic electroluminescent device and a display device employing a plurality of such light-emitting devices by which mixture of light emitted by adjacent organic electroluminescent devices is avoided and thereby light extraction efficiency (efficiency in extracting light from the light-emitting devices to outside) is improved.
DESCRIPTION OF THE RELATED ART
An organic electroluminescent device (which will hereinafter be called “organic EL device”) is a light-emitting device which makes use of the principle that when an electric field is applied, a fluorescent material emits light in response to the charge recombination of holes injected from an anode and electrons injected from a cathode. Since a report on a low-voltage-driven organic EL device employing multilayered structure was released by C. W. Tang et al. of Eastman Kodak Co. (C. W. Tang, S. A. VanSlyke, Applied Physics Letters, Vol.51, page 913 (1987)), extensive researches have been made on organic EL devices, that is, EL devices employing organic materials.
In the above report, an organic EL device employing tris(8-quinolinol)aluminum complex for the light-emitting layer and triphenyldiamine derivative for the hole transporting layer was fabricated on a glass substrate. The multilayer structure has some advantages such as: improved hole injection to the light-emitting layer; increase of production efficiency of excitons which are generated by recombination (by blocking the paths of electrons injected from the cathode); and confinement of the excitons generated in the light-emitting layer.
As the structure of the organic EL devices, two-layer types (including a hole transporting (and injection) layer and an electron transporting light-emitting layer) and three-layer types (including a hole transporting (and injection) layer, a light-emitting layer and an electron transporting layer) are well known. In order to increase the recombination efficiency of injected holes and electrons, various improvements in the device structure or fabrication process have been introduced to such multi-layered devices.
Further, organic EL devices involve certain limitations on the probability of the creation of singlet excited states of light-emitting material molecules on carrier recombination since the carrier recombination is dependent on spin statistics, thereby the probability of light emission is necessitated to have an upper limit. The upper limit is known as approximately 25%.
Furthermore, in organic EL devices, rays of light whose outgoing angles (getting out of the light-emitting layer) are larger than a critical angle (depending on the refractive index of the light-emitting material) can not get out of the light-emitting layer due to total reflection. Therefore, when the refractive index of the light-emitting material is 1.6, only about 20% of the total light emission is available outside, and the upper limit of energy conversion efficiency becomes as low as approximately 5% taking the singlet excited state creation probability into account (Tetsuo Tsutsui “Present situation and trends in organic electroluminescence”, Display (monthly), vol.1, No.3, page 11 (September 1995). In organic EL devices, having tight limitations on the light emission probability, low light extraction efficiency (low efficiency in extracting light from the organic EL device to outside) causes fatal deterioration of the (total) luminescent efficiency.
Methods for improving the light extraction efficiency have been studied so far for light-emitting devices of similar structure such as inorganic EL devices. For example, in Japanese Patent Application Laid-Open No. SHO 63-314795, the light extraction efficiency is improved by forming or attaching light convergent optics on substrate, which is effective for devices having large light-emitting areas. However, in light-emitting devices whose pixel areas are small (such as dot matrix displays), the formation of lenses for light convergence is difficult.
In another method disclosed in Japanese Patent Application Laid-Open No. SHO 62-172691, an anti-reflection coating is formed by providing a flat layer of an intermediate refractive index between the substrate glass and the light-emitting layer, thereby the light extraction efficiency to the front is improved considerably. However, the aforementioned total reflection can not be eliminated by the method. Therefore, while being effective for inorganic EL devices (including materials with large refractive indices), the method can not effectively improve the light extraction efficiency of organic EL devices (including light-emitting materials of relatively low refractive indices).
In order to reduce the total reflection at a surface of the substrate opposite to the organic EL device, Japanese Patent Application Laid-Open No.2000-323272 disclosed a technique for providing the substrate surface with a function for diffusing light. However, the technique can not achieve a marked effect for the conventional glass substrates since the rate of total reflection of light at the interface between the organic EL device and the glass substrate is high. Further, if a light-emitting device having a plurality of organic EL devices arranged in a matrix is manufactured using such a substrate having the light-diffusing function, light emitted by an organic EL device (corresponding to a pixel area) tends to reach adjacent pixel areas, thereby the light leakage problem (light emission from pixel areas that are not supposed to emit light) occurs.
In order to resolve the light leakage problem, Japanese Patent Application Laid-Open No. HEI11-8070 disclosed a method, in which a black mask and a light diffusing layer were formed between the substrate and the organic EL devices. However, the method further deteriorates the light extraction efficiency, since part of the light emitted by the organic EL devices is absorbed by the black mask.
As described above, the prevention of light leakage and the improvement of light extraction efficiency in light-emitting devices employing organic EL devices are both still insufficient. Especially in the method of Japanese Patent Application Laid-Open No. HEI11-8070, the light extraction efficiency (main problem to be resolved) is sacrificed for the prevention of light leakage. Therefore, techniques capable of satisfying both of the requirements are being sought for, and the development of such techniques is essential for practical utilization of the organic EL devices.
SUMMARY OF THE INVENTION
It is therefore the primary object of the present invention to provide a light-emitting device and a display device of high performance by preventing the light leakage and improving the light extraction efficiency of the light-emitting device including an organic EL device.
In accordance with a first aspect of the present invention, there is provided a light-emitting device comprising an organic electroluminescent device including one or more organic thin layers at least including a light-emitting layer which are sandwiched between a transparent first electrode formed on a light-transmitting substrate and a second electrode. In the light-emitting device, the light-transmitting substrate at least includes reflection means for reflecting light emitted by the organic electroluminescent device corresponding to a pixel and thereby preventing the light from entering adjacent pixel areas, and the light-transmitting substrate has a refractive index of 1.65 or more.
In accordance with a second aspect of the present invention, in the first aspect, in a cross section taken along a plane perpendicular to both a reflecting surface of the reflection means and the light-transmitting substrate, the height h of the reflection means measured from a contacting surface of the light-transmitting substrate with the first electrode is set so as to satisfy h≧H·4t/(1+3t). Here, t=d/D, D: the distance between the centers of two reflection means surrounding the o
Ishikawa Hitoshi
Oda Atsushi
Toguchi Satoru
McGuireWoods LLP
Samsung SDI & Co., Ltd.
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