Organic electroluminescent device

Details Organic electroluminescent device Organic electroluminescent device

2001-02-08

2002-12-03

Lambkin, Deborah C. (Department: 1626)

Organic compounds -- part of the class 532-570 series

Organic compounds

Heterocyclic carbon compounds containing a hetero ring...

C549S031000, C549S433000, C585S667000

Reexamination Certificate

active

06489489

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an organic electroluminescent (EL) device, and more precisely, to a blue-emitting, organic EL device having a long life and having high luminous efficiency and good thermal stability.
BACKGROUND OF THE INVENTION
As being self-luminescent, EL devices have high visibility. In addition, they have high impact resistance as being completely solid devices. Therefore, the use of EL devices in various displays as light emitters is being widely noticed.
EL devices are grouped into inorganic EL devices in which are used inorganic compounds as light-emitting materials, and organic EL devices in which are used light-emitting organic compounds. Of those, organic EL devices have been being much studied and expected as light emitters in the coming generations, since they require a greatly reduced level of voltage, they can be easily small-sized, they consume small electric power, they can emit light in a mode of plane emission, and they can easily emit three primary colors.
Known are various structures of organic EL devices having a basic constitution of positive electrode/organic light-emitting layer
egative electrode and optionally provided with a hole injection and transportation layer and an electron injection layer, such as positive electrode/hole transportation layer/organic light-emitting layer
egative electrode, and positive electrode/hole transportation layer/organic light-emitting layer/electron injection layer
egative electrode, etc.
For such organic EL devices, for example, metal complexes of phenolato-substituted 8-hydroxyquinolines have been disclosed as blue-emitting materials (see Japanese Patent Application Laid-Open No. 5-198378). However, these are problematic in that their luminous efficiency is low to be at most 0.2 lumen(lm)/W or so. The reason is because the fluorescence quantum efficiency of the host compounds is low to be at most 0.07 or so. Doping of fluorescent substances into the host compounds is effective in prolonging the life of the doped host compounds, but is not in improving their luminous efficiency.
As luminous materials (host materials) for organic EL devices capable of emitting blue at high efficiency, distyrylarylene compounds have been disclosed (see Japanese Patent Application Laid-Open No. 2-247278). In addition, it has been disclosed that the doping of organic host compounds with fluorescent substances produces the improvement in the luminous efficiency of the doped organic host compounds while prolonging their life (see International Patent Application Laid-Open No. 94/06157).
Many of those blue-emitting compounds generally have a low glass transition temperature (Tg), since &pgr;-electrons are distributed in narrow regions therein and since they have a low molecular weight. Therefore, organic EL devices comprising those compounds are problematic in that their thermal stability is poor. Organic EL devices for outdoor applications or applications in vehicles require high-temperature storage stability generally at 75° C. or so. However, conventional organic EL devices are problematic in those applications in that, when they are kept at high temperatures of 75° C. or so, the color to be emitted by them varies and their luminous efficiency is lowered. For these reasons, the applications of organic EL devices are inevitably limited.
Given that situation, various studies have been made in order to improve the thermal stability of organic EL devices. One example is to modify luminous materials to have dimer or oligomer structures, thereby making the materials have an elevated glass transition temperature. For this, referred to is Japanese Patent Application Laid-Open No. 8-12600, which discloses compounds (phenylanthracene derivatives) having a glass transition temperature of 181° C. In this publication, they tried to improve the efficiency of their devices and to prolong the life thereof by mixing the hole transportation layer and the light-emitting layer. However, the devices disclosed have a luminous efficiency of at most 0.6 lm/W, which is lower than 1 lm/W, and the capacity of the devices is not satisfactory.
As has been mentioned hereinabove, no blue-emitting, organic EL devices having a long life, high efficiency and good thermal stability, which are all indispensable in their practical use, is unknown.
SUMMARY OF THE INVENTION
Given this situation, the object of the invention is to provide a practical, blue-emitting, organic EL device having a long life and having high luminous efficiency and good thermal stability.
We, the present inventors have assiduously studied in order to obtain an organic EL device having those favorable properties, and, as a result, have found that an organic EL device, in which the organic blue-emitting layer comprises an organic host compound having a specific fluorescence quantum efficiency and a fluorescent substance, and the organic host compound and the fluorescent substance are selected such that the device retains a monomeric blue-emitting ability, and in which the organic compound layers constituting the device have a specific glass transition temperature, meets the requirements. The invention has been attained on the basis of these findings.
Specifically, the invention provides an organic EL device comprising organic compound layers, of which at least one is an organic blue-emitting layer, as sandwiched between a pair of electrodes, which is characterized in that (1) the organic blue-emitting layer comprises an organic host compound having a fluorescence quantum efficiency of not smaller than 0.3 in a solid state, and a fluorescent substance, and the organic host compound and the fluorescent substance are selected such that the device retains a monomeric blue-emitting ability, and (2) all the organic compound layers have a glass transition temperature of not lower than 75° C., while the organic compound layers adjacent to the organic blue-emitting layer have a glass transition temperature of not lower than 105° C.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
The organic EL device of the invention has an organic blue-emitting layer comprising an organic host compound and a fluorescent substance.
The organic host compound which is one component constituting the organic blue-emitting layer is not specifically defined, provided that its function is such that holes and electrons are injected thereinto and are transported therethrough to be recombined together to give off fluorescence, that its fluorescence quantum efficiency is not smaller than 0.3, and that it forms, along with the fluorescent substance, the organic blue-emitting layer having a glass transition temperature of not lower than 75° C. Therefore, various compounds are employable herein.
For example, the organic host compound may be selected from distyrylarylene derivatives of a general formula (I):
In formula (I), k, m and n each are 0 or 1, and (k+m+n)≧1. In the formula, R
1
to R
12
each independently represent a hydrogen atom, an alkyl group having from 1 to 6 carbon atoms, an alkoxy group having from 1 to 6 carbon atoms, an aryloxy group having from 6 to 18 carbon atoms, an aryl group having from 6 to 20 carbon atoms, an amino group, an alkylamino group, an arylamino group, a cyano group, a nitro group, a hydroxyl group, a halogen atom, or a group of:
The alkyl group having from 1 to 6 carbon atoms includes, for example, a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, an i-butyl group, a sec-butyl group, a t-butyl group, an i-pentyl group, a t-pentyl group, a neopentyl group, an n-hexyl group, and an i-hexyl group. The alkoxy group having from 1 to 6 carbon atoms includes, for example, a methoxy group, an ethoxy group, an n-propoxy group, an i-propoxy group, an n-butyloxy group, an i-butyloxy group, a sec-butyloxy group, an i-pentyloxy group, a t-pentyloxy group, and an n-hexyloxy group. The aryloxy group having from 6 to 18 carbon atoms includes, for example, a phenoxy group, and a naphthyloxy group. The aryl group having from

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