Stock material or miscellaneous articles – Composite – Of inorganic material
Patent
1995-01-11
1999-02-09
Yamnitzky, Marie R.
Stock material or miscellaneous articles
Composite
Of inorganic material
428704, 428917, 4284111, 428457, 313502, 313504, 313506, H05B 3312
Patent
active
058691995
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to organic electroluminescent (EL) elements.
BACKGROUND ART
It is considered that the light emission of an organic electroluminescent element is caused by a phenomenon where holes and electrons injected from electrode are recombined within a luminescent layer to generate excitons, and the excitons excite molecules of luminescent materials constituting the luminescent layer. By employing fluorescent dye as a luminescent material, there can obtain an emission spectrum, as an electroluminescence, being equivalent to photoluminescence of the dye molecules.
Tang and Vanslyke recently proposed an element comprising two layers of a hole-transport layer and an electron-transport luminescent layer which efficiently emits green light at a lower voltage of about 10 V compared with a conventional organic electroluminescent element of a single-layer 913!. This element's structure is, as shown in FIG. 19(a), composed of an anode 91, a hole-transport layer 92, an electron-transport luminescent layer 93 and a cathode 94, which are formed in this order on a glass substrate 90.
In this element, the hole-transport layer 92 not only allows holes to be injected from the anode 91 into the electron-transport luminescent layer 93, but prevents electrons injected from the cathode 94 from escaping into the anode 91 without recombining with the holes, so that the electrons are contained into the electron-transport luminescent layer 93. Thus, the electron containment effect due to the hole-transport layer 92 facilitates the recombination of the holes and the electrons compared with the conventional single-layer structure element, resulting in a substantial decrease in drive voltage.
Saito et al. showed that not only the electron-transport layer but hole-transport layer can become the luminescent layer in the two-layer 55 (1989) 1489!.
Saito et al. also proposed a three-layer structure organic electroluminescent element wherein an organic luminescent layer is interposed between a hole-transport layer and an electron-transport layer (1988) L269!.
The two-layer structure element of Saito et al. is, as shown in FIG. 19(b), composed of an anode 91, a hole-transport luminescent layer 95, an electron-transport layer 96 and a cathode 94, which are formed in this order on a glass substrate 90. In contrast to the previous element, the electron-transport layer 96 not only allow electrons to be injected from the cathode 94 into the hole-transport luminescent layer 95, but prevents the holes injected from the anode 91 from escaping into the cathode 94 avoiding the recombination with the electrons, so that the holes are contained into the hole-transport luminescent layer 95. This hole containment effect due to the electron-transport layer 96 realizes a substantial decrease in drive voltage likewise in the previous element.
The three-layer structure element of Saito et al. was attained by a further improvement in the element of Tang et al. As shown in FIG. 19(c), this element is composed of an anode 91, a hole-transport layer 92, a luminescent layer 97, an electron-transport layer 96 and a cathode 94, which are formed in this order on a glass substrate 90. The hole-transport layer 92 contains electrons into the luminescent layer 97, and the electron-transport layer 96 contains holes into the luminescent layer 97, so that the recombination of the electrons and the holes in the luminescent layer 97 is more efficient than the two-layer structure element.
Further, the electron-transport layer 96 and the hole-transport layer 92 prevent excitons generated by the above recombination from escaping into either an anode or a cathode. Therefore, the three-layer structure element of Saito et al. increases luminous efficiency furthermore.
Examples of hole-transport material which constitute the above organic electroluminescent elements are aromatic tertiary amines such as triphenylamine. Examples of electron-transport material are oxadiazoles. Examples of luminescent material are tetraphenybutadiene derivatives,
REFERENCES:
patent: 3112197 (1963-11-01), Neugebauer et al.
patent: 5093210 (1992-03-01), Ohta et al.
patent: 5130603 (1992-07-01), Tokailin et al.
patent: 5150006 (1992-09-01), Van Slyke et al.
patent: 5256945 (1993-10-01), Imai et al.
patent: 5405709 (1995-04-01), Littman et al.
Sumitomo Electric Industries Ltd.
Yamnitzky Marie R.
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