Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
1999-12-22
2004-08-24
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S503000, C313S506000, C428S690000
Reexamination Certificate
active
06781305
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an organic electroluminescent device utilizing an electroluminescent phenomenon produced in organic substances, more particularly to a device which is configured to interpose an organic luminescent medium-containing layer between a positive electrode and a negative electrode and designed to emit light when an electric field is applied thereto.
2. Description of Related Art
An organic electroluminescent (may be hereinafter referred to as EL) is formed of a thin film containing an organic fluorescent material interposed between positive and negative electrodes, and is such designed that a hole and an electron are injected into the thin film where they recombine to create an electron excited state, such as an exiton. As this excited state is deactivated, light emission occurs (by fluorescence, phosphorescence, delayed fluorescence, luminescent phenomena accompanying transport of energy, or the like). The organic EL device emits light utilizing this mechanism.
Characteristically, the organic EL device is capable of planar light emission with a high level of luminance ranging from 100 to 10,000 cd/m
2
when the applied voltage is about 10 volts. Also, different emission hues, from blue to red, can be obtained by selecting the type of the organic fluorescent material used in the organic EL device.
The improvement in emission efficiency of the organic EL device can be achieved by increasing the efficiency of electron injection, and the use of low work function metals or their alloys for negative electrode materials have been attempted to increase the electron injection efficiency. In U.S. Pat. No. 4,885,211 and Japanese Patent Laying-Open No. Hei 2-15595, for example, a negative electrode material is disclosed which contains at least 50 atomic % of Mg and at least 0.1 atomic % of metal having a work function of at least 4.0 eV. Japanese Patent Laying-Open No. Hei 8-209120 discloses the use for a negative electrode of an alloy formed of 0.005-10% by mass of an alkaline metal and a second metal. Japanese Patent Laying-Open No. Hei 9-232079 discloses a negative electrode material formed of an alloy which contains, by a total amount, 0.5-5 atomic % of an alkaline metal or an alkaline earth metal having a work function of up to 2.9 eV. Japanese Patent Laying-Open No. Hei 10-12381 discloses the use of a ternary alloy for a negative electrode material, which contains 1-30 atomic % of a metal having a work function of at least 4.0 eV, 0.002-2.0 atomic % of Li, and the balance of Mg.
However, the conventional techniques such as described in the above-cited references use negative electrode materials containing extremely lower work function metals, i.e., metals having higher tendencies to release electrons. When exposed to moisture or oxygen present in the air, such materials readily undergo oxidation to result in the accelerated deterioration of the negative electrodes. This has led to such problems as luminance reduction, build-up of operating voltage, formation and expansion of nonradiative regions called “dark spots”.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an organic electroluminescent device which, due to its utilization of a specific negative electrode material, can exhibit high levels of emission efficiency and emissive luminance and suppress a luminance drop during a long-term operation.
In accordance with a first aspect of the present invention, an organic electroluminescent device has a luminescent material-containing layer interposed between a positive electrode and a negative electrode, and is designed to supply an electrical energy to the luminescent material that emits light upon receipt of the energy. The negative electrode characteristically contains at least one element, “f”, selected from elements having electronegativity values greater than that of calcium (Fauling electronegativity value=1.0) and equal to or less than that of vanadium (Pauling electronegativity value=1.6), and at least one element, “p”, selected from elements having electronegativity values equal to or greater than that of aluminum (Pauling electronegativity value=1.5).
Examples of useful “f” elements include Be (1.5), Ti (1.5), V (1.6), Cr (1.6), Mn (1.5), Zr (1.4), Nb (1.6), La (1.1), C (1.1-1.2), Pr (1.1-1.2), Nd (1.1-1.2), Sm (1.1-1,2), Gd (1.1-1.2), Tb (1.1-1.2), Dy (1.1-1.2), Ho (1.1-1.2), Er (1.1-1.2), Tm (1.1-1.2), Lu (1.1-1.3), Hf (1.3) and Ta (1.5), wherein numerical values given in parentheses represent publicly available Pauling electronegativity values.
Examples of useful “p” elements include H (2.1), B (2.0), C (2.5), N (3.0), O (3.5), F (4.0), Al (1.5), Si (1.8), P (2.1), S (2.5), Cl (3.0), Ga (1.6), Ge (1.8), As (2.0), Se (2.4), Br (2.8), In (1.7), Sb (1.9), Te (2.1), I (2.5), Ti (1.8), Zn (1.6), Cd (1.7) and Hg (1.9), wherein numerical values given in parentheses represent publicly available Pauling electronegativity values.
In accordance with a second aspect of the present invention, an organic electroluminescent device has a luminescent material-containing layer interposed between a positive electrode and a negative electrode, and is designed to supply an electrical energy to the luminescent material that emits light upon receipt of the energy. The negative electrode characteristically contains at least one element, “f”, selected from elements having electronegativity values greater than that of calcium (Pauling electronegativity value=1.0) and equal to or less than that of vanadium (Pauling electronegativity value=1.6), at least one element, “p”, selected from elements having electronegativity values equal to or greater than that of aluminum (Pauling electronegativity value=1.5), and at least one element, “d”, selected from elements having electronegativity values equal to or greater than any of those of iron (Pauling electronegativity value=1.6), cobalt (Pauling electronegativity value=1.6) and nickel (Pauling electronegativity value=1.6) and equal to or less than that of gold (Pauling electronegativity value=2.4), wherein the “d” element selected is the element that is excluded from the selection of the “f” or “p” element.
Examples of useful “d” elements include Re (1.9), Fe (1.8), Ru (2.2), Os (2.2), Co (1.8), Rh (2.2>, Ir (2.2), Ni (1.8), Pd (2.2), Pt (2.2), Cu (1.9), Au (2.4), Hg (1.9), Tl (1.8), Si (1.8), Ge (1.8), P (2.1), As (2.0), Sb (1.9), Se (2.4) and Te (2.1), wherein numerical values given in parentheses indicate publicly available Pauling electronegativity values.
In a third aspect of the present invention, the element, “p”, as used in the aforementioned first and second aspects, is selected from elements having electronegativity values equal to or greater than that of aluminum (Pauling electronegativity value=1.5), less than that of carbon (Pauling electronegativity value=2.5), and less than that of iodine (Pauling electronegativity value=2.5).
It is preferably that those elements, “f”, “p” and “d”, are selected from different groups in the periodic table, respectively. A preferred element content of the negative electrode material is in the range of 0.1-10% by mass (more preferably in the range of 0.3-3% by mass) for the “f” element, in the range of 0.1-99.5% by mass for the “p” element, and in the range of 0-99.8% by mass for the “d” element. When the three elements, “f”, “p” and “d”, are all contained in the negative electrode material, it is preferred that a sum of the “p” and “d” element contents is not below 90% by mass.
In a fourth aspect of the present invention, the luminescent material-containing layer, as used in the first through third aspects, contains at least a host as a principal constituent and a fluorescent dopant. A ratio in molar mass of the dopant molecule to the host molecule (dopant/host) is generally in the range of 0.344-2.90, preferably in the range of 0.441-2.26.
In a fifth aspect of the present invention, the “f” is at least one ele
Fasse W. F.
Fasse W. G.
Patel Ashok
Santiago Mariceli
Sanyo Electric Co,. Ltd.
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