Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
2001-03-14
2003-08-05
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C257S103000
Reexamination Certificate
active
06603140
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an organic electroluminescent (EL) device and more particularly, to a device capable of emitting light from a thin film of an organic compound upon application of electric field.
BACKGROUND ART
Organic EL devices are light emitting devices comprising a thin film containing a fluorescent organic compound interleaved between a cathode and an anode. Electrons and holes are injected into the thin film where they are recombined to create excitons. Light is emitted by utilizing luminescence (phosphorescence or fluorescence) upon deactivation of excitons.
The organic EL devices are characterized by plane light emission at a high luminance of about 100 to 100,000 cd/m
2
with a low voltage of about 10 volts and light emission in a spectrum from blue to red color by a simple choice of the type of fluorescent material.
The organic EL devices, however, are undesirably short in emission life, less durable on storage and less reliable because of the following factors.
(1) Physical Changes of Organic Compounds
Growth of crystal domains renders the interface non-uniform, which causes deterioration of electric charge injection ability, short-circuiting and dielectric breakdown of the device. Particularly when a low molecular weight compound having a molecular weight of less than 500 is used, crystal grains develop and grow, substantially detracting from film quality. Even when the interface with ITO is rough, significant development and growth of crystal grains occur to lower luminous efficiency and allow current leakage, ceasing to emit light. Dark spots which are local non-emitting areas are also formed.
(2) Oxidation and Stripping of the Cathode
Although metals having a low work function such as Na, Mg, Li, Ca, K, and Al are used as the cathode in order to facilitate electron injection, these metals are reactive with oxygen and moisture in air. As a result, the cathode can be stripped from the organic compound layer, prohibiting electric charge injection. Particularly when a polymer or the like is applied as by spin coating, the residual solvent and decomposed products resulting from film formation promote oxidative reaction of the electrodes which can be stripped to create local dark spots.
(3) Low Luminous Efficiency and Increased Heat Build-up
Since electric current is conducted across an organic compound, the organic compound must be placed under an electric field of high strength and cannot help heating. The heat causes melting, crystallization or decomposition of the organic compound, leading to deterioration or failure of the device.
(4) Photochemical and Electrochemical Changes of Organic Compound Layers
Coumarin compounds were proposed as the fluorescent material for organic EL devices (see JP-A 264692/1988, 191694/1990, 792/1991, 202356/1993, 9952/1994, and 240243/1994). The coumarin compounds are used in the light emitting layer alone or as a guest compound or dopant in admixture with host compounds such as tris(8-quinolinolato)-aluminum. Such organic EL devices have combined with the light emitting layer a hole injecting layer, a hole transporting layer or a hole injecting and transporting layer which uses tetraphenyldiamine derivatives based on a 1,1′-biphenyl-4,4′-diamine skeleton and having phenyl or substituted phenyl groups attached to the two nitrogen atoms of the diamine, for example, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine. These organic EL devices, however, are unsatisfactory in emission life and reliability with respect to heat resistance. When these compounds are used as a host, high luminance devices are not available.
To meet the demand for organic EL devices of the multi-color light emission type, multilayer white light emitting organic EL devices were proposed (Yoshiharu Sato, Shingaku Giho, OME94-78 (1995-03)). The light emitting layer used therein is a lamination of a blue light emitting layer using a zinc oxazole complex, a green light emitting layer using tris(8-quinolinolato)aluminum, and a red light emitting layer of tris(8-quinolinolato)aluminum doped with a red fluorescent dye (P-660, DCM1).
The red light emitting layer is doped with a luminescent species to enable red light emission as mentioned above while the other layers are subject to no doping. For the green and blue light emitting layers, a choice is made such that light emission is possible with host materials alone. The choice of material and the freedom of adjustment of emission color are severely constrained.
In general, the emission color of an organic EL device is changed by adding a trace amount of a luminescent species, that is, doping. This is due to the advantage that the luminescent species can be readily changed by changing the type of dopant. Accordingly, multi-color light emission is possible in principle by doping a plurality of luminescent species. If a single host is evenly doped with all such luminescent species, however, only one of the luminescent species doped would contribute to light emission or some of the luminescent species dopes would not contribute to light emission. In summary, even when a single host is doped with a mixture of dopants, it is difficult for all the dopants to contribute to light emission. This is because of the tendency that energy is transferred to only a particular luminescent species.
For this and other reasons, there are known until now no examples of doping two or more luminescent species so that stable light emission may be derived from them.
In general, the luminance half-life of organic EL devices is in a trade-off to the luminescence intensity. It was reported (Tetsuo Tsutsui, Applied Physics, vol. 66, No. 2 (1997)) that the life can be prolonged by doping tris(8-quinolinolato)aluminum or N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine with rubrene. A device having an initial luminance of about 500 cd/m
2
and a luminance half-life of about 3,500 hours was available. The emission color of this device is, however, limited to yellow (in proximity to 560 nm). A longer life is desired.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide an organic EL device using a photoelectric functional material experiencing minimal physical changes, photochemical changes or electrochemical changes and capable of light emission of plural colors at a high luminous efficiency in a highly reliable manner. Another object is especially to provide a high luminance light emitting device using an organic thin film formed from a high molecular weight compound by evaporation, the device being highly reliable in that a rise of drive voltage, a drop of luminance, current leakage, and the appearance and development of local dark spots during operation of the device are restrained. A further object is to provide an organic EL device adapted for multi-color light emission and capable of adjustment of an emission spectrum. A still further object is to provide an organic EL device featuring a high luminance and a long lifetime.
These and other objects are attained by the present invention which is defined below as (1) to (18).
(1) An organic electroluminescent device comprising
a light emitting layer containing a coumarin derivative of the following formula (I), and
a hole injecting and/or transporting layer containing a tetraaryldiamine derivative of the following formula (II),
wherein each of R
1
, R
2
, and R
3
, which may be identical or different, is a hydrogen atom, cyano, carboxyl, alkyl, aryl, acyl, ester or heterocyclic group, or R
1
to R
3
, taken together, may form a ring; each of R
4
and R
7
is a hydrogen atom, alkyl or aryl group; each of R
5
and R
6
is an alkyl or aryl group; or R
4
and R
5
, R
5
and R
6
, and R
6
and R
7
, taken together, may form a ring, and
wherein each of Ar
1
, Ar
2
, Ar
3
, and Ar
4
is an aryl group, at least one of Ar
1
to Ar
4
is a polycyclic aryl group derived from a fused ring or ring cluster having at least two benzene rings; each of R
11
Inoue Tetsushi
Kobori Isamu
Nakaya Kenji
Ohisa Kazutoshi
Crane Sara
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
TDK Corporation
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