Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-09-25
2003-06-03
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S917000, C313S504000, C257S088000
Reexamination Certificate
active
06572987
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a light-emitting device which uses, as a source of light, an element (hereinafter referred to as “organic EL element”) comprising a layer (hereinafter referred to as “organic EL film”) containing an organic compound capable of obtaining luminescence (electroluminescence, hereinafter referred to as “EL”) that takes place upon the application of an electric field, an anode layer and a cathode layer. The EL in the organic compound can be divided into the one that emits light (fluorescent light) when a singlet excited state returns back to a ground state and the one that emits light (phosphorescent light) when a triplet excited state returns back to the ground state. This invention is particularly concerned with a light-emitting device using an organic compound capable of generating phosphorescent light as EL. In this specification, the light-emitting device stands for a picture display device or a light-emitting device using an organic EL element as a light-emitting element. A module in which a TAB (tape automated bonding) tape or a TCP (tape carrier package) is mounted on the organic EL element, a module in which a printed wiring board is provided at an end of the TAB tape or the TCP and a module in which an IC (integrated circuit) is mounted on the organic EL element by the COG (chip on glass) system all pertain to the light-emitting devices.
2. Prior Art
The organic EL element is the one that emits light upon the application of an electric field, and is drawing attention as a flat panel display element of the next generation owing to its properties such as reduced weight, operation on a low DC voltage and high-speed response. Besides, the organic EL element emits light by itself offering a wide visual angle, from which it is expected that the organic EL element can be effectively utilized as a display screen for portable devices.
It is said that the organic EL element has a light emitting mechanism in which the electrons injected through a cathode recombine with the holes injected through an anode to form molecules in an excited state (hereinafter referred to as “molecular exciters”), and energy is released when the molecular exciters return back to the ground state to emit light. The excited state can take the form of a singlet state (S*) and a triplet state (T*), and it has been considered that the statistic ratio of formation is S*:T*=1:3 (literature 1: Tetsuo Tsutsui, “Academy of Applied Physics, Organic Molecules/Division of Bioelectronics/Text of Third Lecture”, p. 31, 1993).
When a general organic compound is maintained at room temperature, however, emission of light (phosphorescent light) from the triplet excited state (T*) is not observed. This also holds even for the organic EL element and, usually, the emission of light (fluorescent light) from the singlet excited state (S*) only is observed. Therefore, the theoretical limit of internal quantum efficiency (ratio of photons that are generated to the carriers that are injected) of the organic EL element has been considered to be 25% based on a ground that S*:T*=1:3.
Light that is emitted is not all released to the outside of the light-emitting device, and part of light is not recovered due to the materials (organic EL film, electrodes) constituting the organic EL element and due to refractive index specific to the substrate material. The ratio of the emitted light taken out of the light-emitting device is called light recovery efficiency. When the organic EL element is provided on the glass substrate, it has been said that the recovery efficiency is about 20%.
Because of the above reasons, even if the injected carriers have all formed exciters, it has been said that the theoretical limit of the ratio (hereinafter referred to as “external quantum efficiency”) of the photons that can be finally taken out of the light-emitting device to the number of the injected carriers, is 25%×20%=5%. That is, even if the carriers are all recombined, only 5% of them is recovered as light.
In recent years, however, there have been successively announced organic EL elements capable of converting energy (hereinafter referred to as “triplet excited energy”) released at the time when the triplet excited state returns back to the ground state into light, and their high light-emitting efficiencies are now drawing attention (literature 2: D. F. O'Brien, M. A. Baldo, M. E. Thompson and S. R. Forrest, “Improved Energy Transfer in Electrophosphorescent Devices”, Applied Physics Letters, Vol. 74, No. 3, 442-444, 1999)(literature 3: Tetsuo Tsutsui, Moon-Jae Yang, Masayuki Yahiro, Kenji Nakamura, Teruichi Watanabe, Taishi Tsuji, Yoshinori Fukuda, Takeo Wakimoto and Satoshi Miyaguchi, “High Quantum Efficiency in Organic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive Center”, Japanese Journal of Applied Physics, Vol. 38, L1502-L1504, 1999).
The literature 2 uses a metal complex with platinum as a central metal (hereinafter referred to as “platinum complex”) and the literature 3 uses a metal complex with iridium as a central metal (hereinafter referred to as “iridium complex”). It can be said that either metal complex has a feature of introducing an element in the third transition system as a central metal. Some of them easily surpass the above-mentioned theoretical limit of 5% of the external quantum efficiency.
By alternatingly laminating a layer of the iridium complex and a layer of DCM2 which is a known fluorescent coloring matter, further, the triplet excited energy formed by the iridium complex can be migrated into the DCM2 contributing to emitting light from the DCM2 (literature 4: M. A. Baldo, M. E. Thompson and S. R. Forrest, “High-Efficiency Fluorescent Organic Light-Emitting Devices using a Phosphorescent Sensitizer”, Nature (London), Vol. 403, 750-753, 2000). Emission of light from DCM2 is the emission of light (fluorescent light) from the singlet excited state, and offers an advantage in that the triplet excited energy efficiently generated from the iridium complex can be utilized as the singlet excited energy of DCM2 of other molecules.
As described in literatures 2 to 4, the organic EL element capable of converting the triplet excited energy into light, makes it possible to accomplish an external quantum efficiency higher than convention alone. The luminous intensity increases with an increase in the external quantum efficiency. It is therefore considered that the organic EL element capable of converting the triplet excited energy into light will occupy an increasing weight in the future development as means for accomplishing the emission of light of high brightness and high light emission efficiency.
However, platinum and iridium are both so-called noble metals. Therefore, the platinum complex and the iridium complex using them are expensive and, it is expected that they pose barrier against lowering the cost in the future. In addition, if it is taken into consideration the effect of the metal complex containing heavy metals upon the human body, it is desired to use safer and easily disposable material.
The color of light emitted by the iridium complex is green, i.e., has a wavelength located in the middle of the visible light region. There has not been reported light of any other color emitted by the metal complex using iridium. Further, when the platinum complex is used as a dopant, light that is emitted exhibits red color of a relatively good purity. When the concentration is low, however, the host material, too, shines causing the color purity to be deteriorated. When the concentration is high, however, the light-emitting efficiency decreases due to the concentration quenching.
Namely, highly efficient emission of light of red and blue colors of high purities is not obtained from the organic EL elements capable of converting the triplet excited energy into light. From the standpoint of fabricating a flat panel display of full colors by emitting light of red, green and blue colors, therefore, light of r
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Kelly Cynthia H.
Semiconductor Energy Laboratory Co,. Ltd.
Thompson Camie
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