Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
2002-11-26
2004-05-11
Wille, Douglas A (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C313S504000
Reexamination Certificate
active
06734457
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an organic light emitting device constituted by an anode, an organic compound film capable of emitting light under the action of an electric field, and a cathode. In particular, the present invention relates to an organic light emitting device using a light emitting material which emits light in a triplet exited state.
2. Description of the Related Art
An organic light emitting device is a device designed by utilizing a phenomenon in which electrons and holes are caused to flow into an organic compound film through two electrodes by application of a voltage to cause emission of light from molecules in an excited state (excited molecules) formed by recombination of the electrons and holes.
Emission of light from an organic compound is a conversion into light of energy released when excited molecules are formed and then deactivated into the ground state. Deactivation processes causing such emission of light are broadly divided into two kinds: a process in which deactivation proceeds via a state in which excited molecules are singlet excited molecules (in which fluorescence is caused), and a process in which excited molecules are triplet excited molecules. Deactivation processes via the triplet excited molecule state include an emission process in which phosphorescence is caused and a triplet—triplet extinction process. However, there are basically only a small number of organic materials capable of changing in accordance with the phosphorescent deactivation process at room temperature. (In ordinary cases, thermal deactivation different from deactivation with emission of light occurs.) The majority of organic compounds used in organic light emitting devices are materials which emit light by fluorescence via the singlet excited molecule state, and many organic light emitting devices use fluorescence.
Organic light emitting devices using such organic compounds capable of emitting light by fluorescence are based on the two-layer structure which was reported by C. W. Tang et al. in 1987 (Reference 1: C. W. Tang and S. A. Vanslyke, “Organic electroluminescent diodes”, Applied Physics Letters, Vol. 51, No. 12, 913-915 (1987)), and in which an organic compound film formed of layers of two or more organic compounds and having a total thickness of about 100 nm is interposed between electrodes. Adachi et al. thereafter proposed a three-layer structure in 1988 (Reference 2: Chihaya ADACHI, Shozuo TOKITO, Tetsuo TSUTSUI and Shogo SAITO, “Electroluminescence in Organic Films with Three-Layered Structure”, Japanese Journal of Applied Physics, Vol. 27, No. 2, L269-L271 (1988)). Multilayer device structures based on applications of these layered structures are being presently used.
Devices in such multilayer structures are characterized by “layer function separation”, which refers to the method of separately assigning functions to layers, instead of making one organic compound have various functions. For example, a device of two-layer structure uses a hole transporting layer having the function of transporting positive holes, and a light emitting and electron transporting layer having the function of transporting electrons and the function of emitting light. Also, a device of three-layer structure uses a hole transporting layer having only the function of transporting positive holes, an electron transporting layer having only the function of transporting electrons, and a light-emitting layer which is capable of emitting light, and which is formed between the two transporting layers. Such a layer function separation method has the advantage of increasing the degree of molecular design freedom of organic compounds used in an organic light emitting device.
For example, a number of characteristics, such as improved facility with which either of electrons and holes are injected, the function of transporting both the carriers, and high fluorescent quantum yield, are required of a device of single-layer structure. In contrast, in the case of a device of two-layer structure or the like using an electron transporting and light emitting layer, an organic compound to which positive holes can be easily injected may be used as a material for a hole transporting layer, and an organic compound to which electrons can be easily injected and which have high fluorescent quantum yield may be used as a material for an electron transporting layer. Thus, requirements of one layer are reduced and the facility with which the material is selected is improved.
In the case of a device of three-layer structure, a “light emitting layer” is further provided to enable separation between the electron transporting function and the light emitting function. Moreover, a material in which a fluorescent pigment (guest) of high quantum yield such as a laser pigment is dispersed in a solid medium (host) material can be used for the light emitting layer to improve the fluorescent quantum yield of the light emitting layer. Thus, not only the effect of largely improving the quantum yield of the organic light emitting device but also the effect of freely controlling the emission wavelength through the selection of fluorescent pigments to be used can be obtained (Reference 3: C. W. Tang, S. A. Vanslyke and C. H. Chen, “Electroluminescence of doped organic thin films”, Journal of Applied Physics, Vol. 65, 3610-3616 (1989)). A device in which such a pigment (guest) is dispersed in a host material is called a doped device.
Another advantage of the multilayer structure is a “carrier confinement effect”. For example, in the case of the two-layer structure described in Reference 1, positive holes are injected from the anode into the hole transporting layer, electrons are injected from the cathode into the electron transporting layer, and the holes and electrons move toward the interface between the hole transporting layer and the electron transporting layer. Thereafter, while holes are injected into the electron transporting layer because of a small ionization potential difference between the hole transporting layer and the electron transporting layer, electrons are blocked by the hole transporting layer to be confined in the electron transporting layer without being injected into the hole transporting layer because the electrical affinity of the hole transporting layer is low and because the difference between the electrical affinities of the hole transporting layer and the electron transporting layer is excessively large. Consequently, both the density of holes and the density of electrons in the electron transporting layer are increased to achieve efficient carrier recombination.
As an example of a material that is effective in exhibiting the carrier confinement effect, there is a material having an extremely large ionization potential. It is difficult to inject holes into the material having a large ionization potential, so that such a material is widely used as a material capable of blocking holes (hole blocking material). For example, in the case where the hole transporting layer composed of an aromatic diamine compound and the electron transporting layer composed of tris(8-quinolinolato)-aluminum (hereinafter referred to as “Alq”) are laminated as reported in Reference 1, if a voltage is applied to the device, Alq in the electron transporting layer emits light. However, by inserting the hole blocking material between the two layers of the device, holes are confined in the hole transporting layer, so that light can be emitted from the hole transporting layer side as well.
As described above, layers having various functions (hole transporting layer, hole blocking layer, electron transporting layer, electron injection layer, etc.) are provided to improve the efficiency and to enable control of the color of emitted light, etc. Thus, multilayer structures have been established as the basic structure for current organic light emitting devices.
Under the above-described circumstances, in 1998, S. R. Forrest et al. made public a doped device in which a triplet
Tokuda Atsushi
Tsutsui Tetsuo
Yamazaki Hiroko
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Semiconductor Energy Laboratory Co,. Ltd.
Wille Douglas A
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