Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
1999-02-01
2001-11-13
Font, Frank G. (Department: 2877)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S503000
Reexamination Certificate
active
06316874
ABSTRACT:
BACKGROUND OF THE INVENTION TECHNICAL ART
The present invention relates generally to an organic EL (electroluminescent) device, and more specifically to an inorganic/organic junction structure used for a device comprising an organic compound thin film which emits light at an applied electric field.
BACKGROUND ART
In general, an organic EL device is basically built up of an ITO or other transparent electrode formed on a glass substrate, an organic amine base hole transporting layer laminated on the transparent electrode, an organic light emitting layer formed of a material having electronic conductivity and giving out strong light emission, for instance, an Alq
3
material, and an electrode provided on the organic light emitting layer and formed of a material having a low work function, for instance, an MgAg material.
As reported so far in the art, the device has a structure wherein one or plural organic compound layers are interleaved between a hole injecting electrode and an electron injecting electrode. The organic compound layer has a double- or triple-layer structure.
Examples of the double-layer structure are a structure wherein a hole transporting layer and a light emitting layer are formed between the hole injecting electrode and the electron injecting electrode, and a structure wherein a light emitting layer and an electron transporting layer are formed between the hole injecting electrode and the electron injecting electrode. In an exemplary triple-layer structure, a hole transporting layer, a light emitting layer and an electron transporting layer are provided between the hole injecting electrode and the electron injecting electrode. A single-layer structure wherein a single layer has all functions, too, is reported in conjunction with a polymer or mixture system.
Typical structures of the organic EL device are shown in
FIGS. 3 and 4
.
In
FIG. 3
, a hole transporting layer
14
and a light emitting layer
15
, each made of an organic compound, are formed between a hole injecting electrode
12
provided on a substrate
11
and an electron injecting electrode
13
. In this case, the light emitting layer
15
also functions as an electron transporting layer.
In
FIG. 4
, a hole transporting layer
14
, a light emitting layer
15
and an electron transporting layer
16
, each made of an organic compound, are formed between a hole injecting electrode
12
provided on a substrate
11
and an electron injecting electrode
13
.
A problem common to these organic EL devices is reliability. In principle, an organic EL device comprises a hole injecting electrode and an electron injecting electrode and requires an organic layer for efficient injection and transportation of holes and electrons from between these electrodes. However, these materials are sensitive to damage during device fabrication, and offer a problem in conjunction with an affinity for electrodes. Another problem is that the deterioration of an organic thin film is much severer than that of an LED or LD.
Most organic materials are relatively expensive. Otherwise stated, there is a great merit in providing low-cost organic EL device products by substituting a partial constitution film with an inexpensive inorganic material.
Furthermore, the development of a device having higher light emission efficiency, lower driving voltage and lesser power consumption than ever before is strongly desired.
To provide a solution to such problems, methods of taking advantage of merits of both an organic material and an inorganic semiconductor material have been envisaged. That is, an organic/inorganic semiconductor junction structure wherein an organic hole transporting layer is substituted by an inorganic p-type semiconductor has been contemplated. Such contemplation has been investigated in Japanese Patent No. 2636341, and JP-A's 2-139893, 2-207488 and 6-119973. However, it is still difficult to obtain an organic EL device superior to prior art organic ELs in terms of emission performance and basic device reliability.
SUMMARY OF THE INVENTION
One object of the invention is to provide an organic EL device which possesses the merits of both an organic material and an inorganic material, has an extended life, an improved light emission efficiency, a low driving voltage and high luminance, and is fabricated at low cost.
Another object of the invention is to provide an organic EL device having practically great value in high-performance flat color display applications.
The above object is achieved by the inventions defined below.
(1) An organic EL device comprising a substrate, a hole injecting electrode and a cathode formed on said substrate, and at least one organic layer located between these electrodes, wherein:
an inorganic insulating electron injecting layer is located between said cathode and said organic layer,
said inorganic insulating electron injecting layer comprising:
as an oxide having a low work function at least one oxide having a work function of lower than 2.5 eV, and
as an oxide having a high work function at least one oxide having a work function of 2.5 to 4 eV.
(2) The organic EL device according to (1), wherein said oxide having a low work function is at least one oxide selected from the group consisting of lithium oxide, sodium oxide, potassium oxide, rubidium oxide, cesium oxide, calcium oxide, strontium oxide, barium oxide, and yttrium oxide.
(3) The organic EL device according to (1), wherein said oxide having a high work function is at least one oxide selected from the group consisting of beryllium oxide, magnesium oxide, titanium oxide, iron oxide, zirconium oxide, hafnium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, ytterbium oxide, lutetium oxide, and thorium oxide.
(4) The organic EL device according to (1), wherein said inorganic insulating electron injecting layer contains 5 to 50 mol % of said oxide having a high work function.
(5) The organic EL device according to (1), wherein a work function difference between said oxide having a low work function and said oxide having a high work function is 0.5 to 2.5 eV.
(6) The organic EL device according to (1), wherein said inorganic insulating electron injecting layer has a band gap of at least 4 eV.
(7) The organic EL device according to (1), wherein said inorganic insulating electron injecting layer has a structure where said oxide having a low work function is present in a large amount on an organic layer side and said oxide having a high work function is present in a large amount on a cathode side.
(8) The organic EL device according to (1), wherein said cathode comprises aluminum as a main component
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Arai Michio
Kobori Isamu
Mitsuhashi Etsuo
Font Frank G.
Lee Andrew H.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
TDK Corporation
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