Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-03-26
2003-12-09
Yamnitzky, Marie (Department: 1774)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S917000, C428S212000, C313S504000, C313S506000, C257S102000, C257S103000
Reexamination Certificate
active
06660410
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an organic electroluminescence element (which may be referred to as an organic EL element hereinafter). More specifically, the present invention relates to an organic EL element using a triplet exciton of an organic luminescence material (host material).
2. Description of the Related Art
Hitherto, organic EL elements wherein an organic luminescence layer is arranged between electrodes have been eagerly researched and developed for the following reasons and the like.
(1) Since these elements are completely solid, they are easy to handle and produce.
(2) Since they can emit light by themselves, no light emitting members are necessary.
(3) Since they can be clearly watched, they are suitable for display.
(4) They permit full color display easily.
The luminescence mechanism of such organic EL elements generally makes use of a luminescence phenomenon, which is energy conversion phenomenon caused when a fluorescent molecule in a singlet excited state (which may be referred to a S1 state) in an organic luminescence medium is transited to a ground state radially.
A fluorescent molecule in a triplet excited state (which may be referred to a T1 state) in an organic luminescence medium can be supposed. However, radiative transition to a ground state is forbidden; therefore, such a molecule is gradually transited from the triplet excited state to some other state by non-radiative transition. As a result, no fluorescence is emitted but thermal energy is radiated.
Here, singlet and triplet mean multiplicity of energy decided by combination of total spin angular momentum and total orbital angular momentum of a fluorescent molecule. Specifically, a singlet excited state is defined as an energy state in the case that a single electron is transited from a ground state, where no unpaired electrons are present, to a higher energy level without changing the spin state of the electron. A triplet excited state is defined as an energy state in the case that a single electron is transited to a higher energy level while the spin state of the electron is made reverse.
Needless to say, luminescence in a triplet excited state defined as above can be observed if the luminescence is caused at a very low temperature, for example, at a liquefaction temperature of liquid nitrogen (−196° C.). However, this temperature is not a practical temperature, and the amount of the luminescence is only a little.
By the way, the total efficiency of luminescence from any conventional organic EL element is related to recombination efficiency (&phgr;rec) of injected charged carries (electrons and holes), and the probability (&phgr;rad) that generated excitons cause radiative transition. Therefore, the total efficiency (&phgr;el) of luminescence from the organic EL element can be represented by the following equation:
&phgr;
el=&phgr;rec×
0.25
&phgr;rad
The coefficient (0.25) of &phgr;rad in the equation is decided from the matter that the probability that singlet excitons are generated is regarded as ¼. Therefore, even if recombination and radiative attenuation of excitons are caused with a probability coefficient of 1, the theoretical upper limit of luminescence efficiency of the organic EL element is 25%.
As described above, in any conventional organic EL element, triplet excitons cannot be substantially used and only singlet excitons cause radiative transition. Thus, a problem that the upper limit of the luminescence efficiency is low arises.
Thus, literature 1 “Jpn. J. Appl. Phys., 38 (1999) L1502” discloses that even at room temperature, triplet excitons (triplet excited state) of an organic luminescence material (host material) are used to transfer energy from the triplet excitons to a phosphorescent dopant, so as to generate a fluorescent luminescence phenomenon. More specifically, the literature 1 reports that a fluorescent luminescence phenomenon is caused in an organic EL element comprising an organic luminescence layer composed of 4,4-N,N-dicarbazolylbiphenyl represented by the following formula (6) and an Ir complex, which is a phosphorescent dopant.
However, the half-life of the organic EL element described in the literature 1 is below 150 hours, and the usefulness of the organic EL element is insufficient.
Thus, the inventor made eager investigations. As a result, the following has been found: the glass-transition temperature of 4,4-N,N-dicarbazolylbiphenyl is as low as less than 110° C.; therefore, if the biphenyl is combined with an Ir complex, crystallization is easily caused in the organic luminescence layer comprising the combination to make the life of an organic EL element short.
Incidentally, in the present situation, a demand that the heat-resistance of organic EL elements for cars should be made higher has been increasing in light of environment inside cars in summer.
Thus, an object of the present invention is to provide an organic EL element which makes it possible to use triplet excitons of an organic luminescence material (host material) even at room temperature to emit fluorescence (including phosphorescence); has a practical life span; and has a superior heat-resistance.
SUMMARY OF THE INVENTION
[1] According to the present invention, provided is an organic EL element comprising:
an anode layer,
a cathode layer, and
an organic luminescence layer therebetween, the organic luminescence layer having a carbazole derivative with a glass-transition temperature of 110° C. or higher, and a phosphorescent dopant. Thus, the above-mentioned problems can be solved.
This organic EL element makes it possible to use the triplet exciton state of the organic luminescence material even at room temperature. Moreover, this element has a practical life, for example, a half-time of 300 hours or more, and has superior heat-resistance. Thus, this element can be sufficiently used as an organic EL element for car.
[2] In the organic EL element of the present invention, it is preferred that the carbazole derivative is at least one of compounds represented by the following general formulae (1) to (4):
wherein Ar
1
is a substituted or non-substituted aryl group having 6 to 50 nucleus carbon atoms; Ar
2
to Ar
7
are each independently a substituted or non-substituted aryl or arylene group having 6 to 50 nucleus carbon atoms; Ar
2
and Ar
3
, Ar
4
and Ar
5
, or Ar
6
and Ar
7
may be connected to each other through a single bond or through O, S or substituted or non-substituted alkylene as a connecting group; and each of repetition numbers m and n is an integer of 0 to 3,
wherein R
1
to R
6
are each independently a hydrogen or halogen atom, an alkyl, aralkyl, aryl, cycloalkyl, fluoroalkyl, amino, nitro, cyano, hydroxy, or alkoxy group; R
7
and R
8
are each independently a hydrogen atom, an alkyl, aralkyl, aryl, or cycloalkyl group; X
1
and X
2
are each independently a hydrogen atom, an alkyl, aralkyl, aryl, or cycloalkyl group; Y is a single bond, an alkyl, alkylene, cycloalkyl, aryl, or aralkyl chain; a repetition number p is an integer of 1 to 3.
wherein Ar
8
to Ar
11
are each independently an aryl group having 6 to 50 nucleus carbon atoms which may be substituted with an alkyl, alkoxy or aryl group; Ar
8
and Ar
9
, or Ar
10
and Ar
11
may be connected to each other through a single bond or through O, S or substituted or non-substituted alkylene as a connecting group; and R
9
is an alkyl or alkoxy group, or a substituted or non-substituted aryl group having 6 to 18 nucleus carbon atoms.
wherein Z is a trivalent nitrogen atom or an aromatic group; Ar
12
to Ar
14
are each independently a group represented by the following general formula (5) or an aryl group having 6 to 50 nucleus carbon atoms; and at least two of Ar
12
to Ar
14
are groups represented by the following general formula (5):
wherein R
10
to R
21
are each independently an aryl group having 6 to 50 nucleus carbon atoms which may be substituted with an alkyl, alkoxy group having 1 to 6 carbon atoms, or a pheny
Idemitsu Kosan Co. Ltd.
Parkhurst & Wendel L.L.P.
Yamnitzky Marie
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