Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1999-03-31
2003-01-21
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S917000, C313S504000, C313S506000
Reexamination Certificate
active
06509109
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) device. More precisely, it relates to an organic EL device suitable to light sources for displays, printer heads and others for domestic and industrial use.
2. Description of the Related Art
Reference 1, JP-A-4-297076 discloses one example of conventional organic EL devices. As in
FIG. 10
, the organic EL device
60
disclosed by Reference 1 comprises an organic laminate layer of three organic films
52
,
54
and
56
as sandwiched between a cathode layer
58
and an anode layer
50
of a transparent electrode. Of the three organic films, the first organic film
52
that is adjacent to the cathode layer
58
is doped with a donor impurity, while the second organic film
54
that is adjacent to the anode layer
50
is doped with an acceptor impurity. As the acceptor impurity, used is any of CN-substituted compounds and quinone compounds (e.g., chloranil). The third organic film
56
sandwiched between the first organic film
52
and the second organic film
54
is a light emission layer. Carriers are confined in the light emission layer
56
by the adjacent first and second organic films
52
and
54
. Accordingly, the organic EL device
60
could produce high luminance (that is, its luminous efficiency is high) even at low driving voltage.
Reference 2, “Digest of Technical Papers, SID'97, p. 775, 1997” discloses another example of conventional organic EL devices. In the organic EL device disclosed by Reference 2, the electron transportation layer is made of a material that comprises tris(8-hydroxyquinoline)Al (Alq complex) with Li added thereto.
In the organic EL device disclosed by Reference 1, CN-substituted compounds and quinone compounds used as the acceptor impurity have good electron transportation capabilities but their acceptor capabilities are strong. Concretely, their electron affinity is on the level of at least 3.7 eV and is high. Therefore, the acceptor impurity of those compounds often reacts with compounds that constitute the light emission region, thereby forming charge-transfer complexes or exciplexes. For these reasons, the organic EL device faces the problems of luminance depression and short lifetime.
In addition, in the organic EL device disclosed by Reference 1, the difference in the electron affinity between the donor impurity-doped, first organic layer and the light emission region is at least 0.5 eV and is large. Therefore, in this, a blocking contact is formed at the interface between the first organic layer and the light emission region. In that condition, the electron injection from the first organic layer to the light emission region is often unsatisfactory, and, as a result, the organic EL device shall face an additional problem of further reduction in its luminous efficiency.
On the other hand, Alq complexes in the organic EL device disclosed by Reference 2 also contain nitrogen atoms. In this, the electron transportation layer that comprises such an Alq complex and a Li compound has good electron transportation capabilities, but easily forms charge-transfer complexes or exciplexes. In addition, the organic EL device often requires high driving voltage. Therefore, like that disclosed by Reference 1, the organic EL device disclosed by Reference 1 also faces the problems of luminance depression and short lifetime.
SUMMARY OF THE INVENTION
Given that situation, we, the present inventors assiduously studied the problems noted above, and, as a result, have found that, when a nitrogen-free aromatic compound is used in the electron injection region, or when a nitrogen-containing aromatic compound, if used in that region, is combined with a specific reducing dopant, then the driving voltage for the organic EL device can be reduced and the luminance of the device can be increased and, in addition, the lifetime of the device can be prolonged. Based on these findings, we have completed the present invention. Specifically, the object of the invention is to provide an organic EL device that produces high luminance even at low driving voltage and has a long lifetime.
According to the first aspect of the invention, there is provided an organic EL device comprising at least an anode layer, a light emission region, an electron injection region and a cathode layer as laminated in that order, wherein the electron injection region contains a nitrogen-free aromatic compound and a reducing dopant and the electron affinity of the electron injection region is controlled to fall between 1.8 and 3.6 eV.
As containing a nitrogen-free aromatic compound, the electron injection region has good electron injection capabilities, and, in addition, this is prevented from reacting with the material that constitutes the light emission region adjacent thereto. Specifically, the nitrogen-free aromatic is composed of carbon and hydrogen, or of carbon, hydrogen and oxygen, and does not have any nitrogen-containing groups such as nitrogen-containing aromatic groups and electron-attractive groups (e.g., —CN, —NO
2
, amido, imido). Therefore, in the constitution of the organic EL device of the invention, charge-transfer complexes or exciplexes with low luminous efficiency are well prevented from being formed in the interface between the electron injection region and the light emission region.
In addition, since the electron injection region contains a reducing dopant along with a nitrogen-free aromatic compound, the aromatic skeleton of the nitrogen-free aromatic compound in the region is well reduced to be in an anionic state. Accordingly, in the organic EL device of the invention, charge-transfer complexes or exciplexes with low luminous efficiency are much more prevented from being formed, whereby the luminance of the device is increased and the lifetime thereof is prolonged.
Moreover, since its electron affinity is suitably controlled, the electron injection region shall have more improved electron injection capabilities, and, in addition, charge-transfer complexes or exciplexes are well prevented from being formed at the interface between the electron injection region and the light emission region. Further, the blocking contact is prevented from being formed at the interface between the electron injection region and the light emission region. With that constitution, therefore, the luminance of the device is increased and the lifetime thereof is prolonged.
In the organic EL device of the first aspect of the invention, it is desirable that the electron injection region has a glass transition point of not lower than 100° C.
The organic EL device in which the electron injection region has a glass transition point of not lower than 100° C. could have high heat resistance. For example, the device is resistant to heat at a temperature of 85° C. or higher. In the device with that constitution, the electron injection region is protected from being broken within a short period of time by the Joule s heat to be generated through current injection from the current injection layer to the light emission region while the device is driven to emit light. As a result, the device could have a prolonged lifetime.
In the organic EL device of the first aspect of the invention, it is also desirable that the aromatic compound contains a residue of at least one aromatic ring selected from the group consisting of anthracene, fluorene, perylene, pyrene, phenanthrene, chrysene, tetracene, rubrene, terphenylene, quaterphenylene, sexiphenylene and triphenylene.
In the organic EL device of the first aspect of the invention, it is still desirable that the aromatic compound contains a residue of at least one aromatic ring selected from the group consisting of styryl-substituted aromatic rings, distyryl-substituted aromatic rings and tris-styryl-substituted aromatic rings.
In the organic EL device of the first aspect of the invention, it is still desirable that the reducing dopant is at least one substance selected from the group consisting of alkali metals, alkaline earth metals, rare eart
Fukuoka Kenichi
Hosokawa Chishio
Nakamura Hiroaki
Tokailin Hiroshi
Garrett Dawn
Idemitsu Kosan Co. Ltd.
Kelly Cynthia H.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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