Organic el material

Details Organic el material Organic el material

2002-02-21

2003-06-24

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Plural power supplies

Plural cathode and/or anode load device

C315S169100, C345S076000, C345S077000

Reexamination Certificate

active

06583583

ABSTRACT:

FIELD OF TECHNOLOGY
This invention relates to novel tris(8-quinolinolato)aluminum which is useful as a constituent of organic EL materials gaining importance in the manufacture of flat panel displays and to a process for refining said compound.
BACKGROUND TECHNOLOGY
An organic electroluminescent device (hereinafter referred to as an organic EL device) is constructed of layers of organic compounds comprising a layer of a luminescent material and a pair of electrodes holding the layers of organic compounds in between and, concretely, the basic construction is anode/organic luminescent layer/cathode and modifications of this construction by suitable addition of a hole injecting layer and an electron injecting layer are known. As for a material useful for an organic luminescent layer (hereinafter referred to as an organic EL material), a representative among known examples is tris(8-quinolinolato)aluminum (hereinafter referred to as Alq3) reported in Appl. Phys. Lett., 51, 913 (1987) by C. V. Tang, S. A. VanSlyke and others of Eastman Kodak Company. Substituted 8-quinolinol-aluminum complexes and 8-quinolinol-oxoaluminum complexes are also known. Since the compound in question is a complex, it is sometimes called tris(8-quinolinolato)aluminum complex or simply complex.
Intensive studies in recent years have raised the practical status of organic EL devices and, in the meanwhile, it has been suspected that the purity of organic compounds constituting the luminescent layer and the impurities contained in such organic compounds affect the basic properties such as luminous intensity and luminous efficiency and the life of organic EL devices. As for Alq3, for example, the essential quality requirements that Alq3 must satisfy are described in Japanese patent number 2902745 and 2823352, and a process for manufacturing Alq3 is described in Japanese patent publication JP11-171801A.
It is publicly known that Alq3 is a mixture of two isomers, facial and meridional, and a variety of analytical data are reported: for example, identification of two isomers by
1
H-AMR spectrometry in Anal. Chem., 1968, 40(13), 1945-51; mass spectra in Talanta, 1967(14), 1213-20; and infrared absorption spectra in Acta Chem. Scand., 1968, 22(4), 1067-75. However, virtually no report has been published on how the isomers affect the performance of organic EL devices on a quantitative basis.
Not much has been clarified either about the effects of trace impurities, crystal structure and steric structure; for example, it is not clear whether the positive presence of trace impurities is beneficial or not and how much of what impurities if beneficial or what crystal structure is preferable. This state of affairs is essentially due to Alq3 being highly pure and the relationship between high-purity Alq3 and the mechanism of light emission being unclear.
A refining procedure based on sublimation is generally adopted to raise the purity of Alq3 to a level suitable for use as an organic EL material. It is questionable, however, whether or not the conventional process for refining by sublimation with the use of an electric furnace and a glass apparatus is able to raise the purity of Alq3 to a satisfactory level for use as an organic EL material. A number of points still remain unclarified regarding the practically important relationship between, the performance of an organic EL device and factors such as the purity of organic EL materials and the kind and quantity of impurities and some of the points conceivably originate in the aforementioned refining technique. Constructional improvement of EL devices is said to have enhanced their basic performance to a level adequate for practical use today and, with the aim of commercial manufacture of Alq3 in view, there is also a demand for development of a technique for refining Alq3 by sublimation suitable for practical use. Moreover, development of analytical and manufacturing techniques for Alq3 as an organic EL material has become an important theme.
An object of this invention is to provide Alq3 which performs excellently as a material for an organic EL device and to provide a process for preparing said Alq3.
DISCLOSURE OF THE INVENTION
This invention relates to an organic EL material comprising tris(8-quinolinolato)aluminum complex which is characterized by generating heat of 2 J/g or less in the range of 350-400° C. and absorbing heat of 70-120 J/g in the range of 400-450° C. with the maximum of an endothermic peak being located near 420° C. on analysis by differential scanning calorimetry in a stream of nitrogen. This invention also relates to said organic EL material comprising tris(8-quinolinolato)aluminum complex which shows no peak at 418±2 cm
−1
and a peak at 423±2 cm
−1
in the infrared absorption spectrum measured by FT-IR
Furthermore, this invention relates to a process for refining tris(8-quinolinolato)aluminum for an organic EL material which comprises refining raw material tris(8-quinolinolato)aluminum by sublimation at a reduced pressure of 10 Torr or less while keeping the temperature in the sublimation zone above 300° C. and below 420° C. and the temperature in the principal zone of the product recovery zone above 100° C. and below 250° C. Still more, this invention relates to a process for refining said tris(8-quinolinolato)aluminum for an organic EL material wherein the apparatus for refining by sublimation comprises the sublimation zone and the recovery zone including the product recovery zone and at least the temperature in the principal zone of the product recovery zone is controlled within ±10° C. of the set temperature.
Also, in an organic EL device comprising layers of glass substrate/transparent electrode/hole transporting layer/luminescent layer/electrode or at least layers of glass substrate/transparent electrode/hole injecting layer/hole transporting layer/luminescent layer/electron transporting layer/electrode, this invention relates to an organic EL device wherein Alq3 to be incorporated in the luminescent layer and/or the electron transporting layer is said organic EL material or Alq3 that has been refined by said process for refining tris(8-quinolinolato)aluminum for an organic EL material.
The present inventors have conducted a variety of chemical and physical analyses to understand the quality of Alq3 which exerts a marked influence on the performance of an organic EL device and found that, apart from the aforementioned spectral data (IR,
1
H-NMR and so on) reported so far, the results of thermal analysis are related to the performance of an organic EL device. The present inventors analyzed both Alq3 refined conventionally by sublimation (hereinafter referred to as conventional substance) and Alq3 refined by sublimation with the use of an original apparatus of their own development (hereinafter referred to as novel substance) by spectrometric techniques such as IR,
1
H-NMR, mass, UV and fluorescence and by differential thermal analysis and differential scanning calorimetry and investigated the correlation of the analytical results with the basic performance of an organic EL device comprising ITO/hole transporting layer (TPD)/electron transporting layer (Alq3)/Al.Li electrode.
Comparison of the FT-IR (Fourier transform infrared spectrometry) spectra of the conventional and novel substances showed a difference on the order of 3-5 cm
−1
at some of the absorption peaks. It is said in FT-IR spectrometry that a difference greater than the resolving power of the apparatus (it is set at 2 cm
−1
here) is significant. However, it is fairly difficult to explain quantitatively slight structural changes (intramolecular and intermolecular) of the complex on the basis of the aforementioned differences in the FT-IR spectra. One reason for it is the extreme difficulty in preparing or isolating only one of the two isomers. Moreover, no well-defined differences were recognized between the conventional and novel substances in the spectra determined by UV, fluorescence, mass and
1
H-NMR (400 MHz).
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