Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2002-03-27
2004-09-07
Yamnitzky, Marie (Department: 1774)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S917000, C313S504000, C257S102000
Reexamination Certificate
active
06787249
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an organic light emitting element having an anode, a cathode, and a layer containing an organic compound in which light emission can be obtained by applying an electric field (hereafter referred to as an “organic compound layer”). In general, light emitted when returning to a base state from a singlet excitation state, and light emitted when returning to a base state from a triplet excitation state exist as organic compound light emissions generated by the application of an electric field. In particular, the present invention relates to organic light emitting elements using organic compounds in which light emission is capable of being generated from a triplet excitation state. Note that the term light emitting device in this specification indicates image display devices and light emitting devices using organic light emitting elements as light emitting elements. Further, modules in which a connector, for example an isotropic conductive film (flexible printed circuit, FPC), a TAB (tape automated bonding) tape, or a TCP (tape carrier package) is attached to organic light emitting elements are all included in the category of light emitting devices, as are modules in which a printed wiring board is provided in an end portion of TAB tape or TCP, and modules in which an IC (integrated circuit) is directly mounted to light emitting elements by a COG (chip on glass) method.
2. Description of the Related Art
Organic light emitting elements are elements which emit light by the application of an electric field. The light emitting mechanism is one in which electrons injected from a cathode recombine within an organic compound layer with holes injected from an anode, forming excited state molecules (hereafter referred to as “molecular excitons”), by the application of a voltage to the organic compound layer sandwiched between the electrodes. Energy is released when the molecular excitons return to a base state, emitting light.
The organic compound layer is normally formed by a thin film having a thickness less than 1 &mgr;m for these types of organic light emitting elements. Further, organic light emitting elements are self light emitting elements in which light is emitted by the organic compound layers, and therefore a back light like that used in a conventional liquid crystal display is not necessary. Consequently, the ability to manufacture light emitting elements that are extremely thin with light weight is a big advantage.
Furthermore, the period of time from the injection of a carrier until recombination occurs in an organic compound layer having a thickness on the order of 100 to 200 nm, for example, is on the order of several tens of nanoseconds when considering the carrier mobility of the organic compound layer. Even when including a period required for a process from when the carrier recombines until light is emitted, the light emission can be performed within order of microsecond. The light emitting elements therefore have a fast response speed.
In addition, drive using a direct current voltage is possible because the organic light emitting elements are carrier injecting light emitting elements, and therefore it is difficult for noise to develop. With regard to driving voltage, it has been reported (reference 1: Tang, C. W., and VanSlyke, S. A., “Organic Electroluminescent Diodes”, Applied Physics Letters, Vol. 51, No. 12, pp. 913-5 (1987)) that a sufficient brightness of 100 cd/m
2
at 5.5 V was achieved by first taking an extremely thin film of an organic compound layer with a uniform film thickness on the order of 100 nm, selecting an electrode material so as to make the carrier injection barrier with respect to the organic compound layer smaller, and in addition, introducing a hetero structure (two layer structure).
Organic light emitting elements are therefore under the spotlight as next generation flat panel display elements due to their thin size, light weight, high speed response, and driving at D.C. voltage and low voltage. Further, light emitting elements are self light emitting and have a wide angular field of view, and therefore their visibility is comparatively good and they are considered to be effective as elements used in the display screens of portable devices.
It has already been stated that emitted light in organic light emitting elements is a phenomenon in which light is emitted when molecular excitons return to a base state, and it is possible for singlet excitation state (S*) and triplet excitation state (T*) molecular excitons to exist as molecular exciton types formed by organic compounds. Further, the statistical generation ratio in organic light emitting elements is considered to be S*:T*=1:3 (reference 2: Shirato, J., “Monthly Display Supplement, From Organic EL Display Fundamentals to the Latest Information” (TechnoTimes Corp.), p. 28-29).
However, for general organic compounds at room temperature, the emission of light from the triplet excitation state (T*) is not observed, and normally only light emitted from the singlet excitation state (S*) can be observed. The base state of organic compounds is a singlet base state (S
0
), and therefore transitions from T* to S
0
(phosphorescence process) become prohibited transitions to a considerable degree and transitions from S* to S
0
(fluorescence process) become allowed transitions.
In other words, normally only the singlet excitation state (S*) contributes to light emission, and this is the same for organic light emitting elements. However, the theoretical limit of the internal quantum efficiency in organic light emitting elements (the proportion of photons generated with respect to the carrier injected) is 25% based on the fact that S*:T*=1:3.
Furthermore, the light emitted is not all emitted to the outside. A portion of the light cannot be extracted due to the component of the organic light emitting element (organic compound layer materials, electrode materials) and the index of refraction indigenous to the substrate material. The ratio of light extracted to the outside with respect to the light emitted is referred to as a light extraction efficiency. The light extraction efficiency is thought to be on the order of 20% for organic light emitting elements having a glass substrate.
For the above reasons, even if all of the injected carrier are able to form molecular excitons, the theoretical limit of the final ratio of photons extracted to the outside with respect to the number of injected carriers (hereafter referred to as an “external quantum efficiency”) is 25%×20%=5%. That is, even if all of the carrier recombines, only 5% is extracted as light.
However, in recent years organic light emitting elements capable of converting energy emitted when returning to a base state from a triplet excitation state (T*) (hereinafter referred to as “triplet excitation energy”) into an emission light have been announced one after another, their high light emission efficiency grabbing attention. (Reference 3: 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)). (Reference 4: Tetsuo TSUTSUI, Moon-Jae YANG, Masayuki YAHIRO, Kenji NAKAMURA, Teruichi WATANABE, Taishi TSUJI, Yoshinori FUKUDA, Takeo MAKIMOTO 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, pp. L1502-L1504 (1999)).
An organometallic complex having platinum as a central metal (hereafter referred to as a “platinum complex”) is used in reference 3, while an organometallic complex having iridium as a central metal (hereafter referred to as an “iridium complex”) is used in reference 4. Both organometallic complexes are characterized by their introduction of a tertiary transition element as a metal center. Of course, materials do exist that exceed the theoretical limiting value of 5% for the e
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Semiconductor Energy Laboratory Co,. Ltd.
Yamnitzky Marie
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