Organic light emitting devices

Details Organic light emitting devices Organic light emitting devices

2001-01-26

2003-09-02

Oen, William (Department: 2855)

Electric lamp and discharge devices

With luminescent solid or liquid material

Solid-state type

Reexamination Certificate

active

06614175

ABSTRACT:

Illustrated in U.S. Pat. No. 6,392,339 on “Organic Light Emitting Devices Having Improved Efficiency and Operation Lifetime”, filed on Jul. 20, 1999 and U.S. Pat. No. 6,392,250 on “Organic Light Emitting Devices Having Improved Performance”, filed on Jun. 30, 2000, the disclosures of which are totally incorporated herein by reference, are organic light emitting devices (organic EL devices) that, for example, comprise a mixed region including a mixture of a hole transport material and an electron transport material. At least one of a hole transport material region and an electron transport material region can be formed on the mixed region. The stability of the above mentioned organic EL devices disclosed in U.S. Pat. No. 6,392,339 and U.S. Pat. No. 6,392,250 is usually reduced at temperatures above 80° C., due it is believed to a decrease in the device resistance to shorting and also since it is believed to a progressive increase in the driving voltage required to drive a certain current through the organic EL devices. As a result, the operational stability of these devices can be limited to few hundred hours or less at these high temperatures, and more specifically, at high temperatures in the range of from about 80° C. to about 100° C. Therefore, these devices are believed to be unsatisfactory in some instances, for applications in which there is desired an operational stability of the organic EL device of at least, for example, several thousand hours at temperatures of, for example, 90° C., such as, for example, in some automotive, military or other industrial applications where durability in harsh conditions is desired and/or necessary.
The appropriate components and processes of the above copending applications may be selected for embodiments of the present invention in embodiments thereof.
1. Background of the Invention
This invention relates to optoelectronic devices and, more particularly, to organic light emitting devices (organic EL devices). More specifically, the present invention relates to substantially stable organic EL devices and which devices do not in embodiments, for example, usually degrade at high temperatures, such as about 100° C., and moreover, which devices are not substantially adversely affected by high temperatures. This invention also relates in embodiments to methods for the preparation of organic light emitting devices and uses thereof.
2. Prior Art
An organic EL device can be comprised of a layer of an organic luminescent material interposed between an anode, typically comprised of a transparent conductor, such as indium tin oxide, and a cathode, typically a low work function metal such as magnesium, calcium, aluminum, or the alloys thereof with other metals. The EL device functions on the primary principle that under an electric field, positive charges (holes) and negative charges (electrons) are respectively injected from the anode and cathode into the luminescent layer and undergo recombination to form excitonic states which subsequently emit light. A number of prior art organic EL devices have been prepared from a laminate of an organic luminescent material and electrodes of opposite polarity, which devices include a single crystal material, such as single crystal anthracene, as the luminescent substance as described, for example, in U.S. Pat. No. 3,530,325. However, these devices are believed to require excitation voltages on the order of 100 volts or greater.
An organic EL device with a multilayer structure can be formed as a dual layer structure comprising one organic layer adjacent to the anode supporting hole transport, and another organic layer adjacent to the cathode supporting electron transport and acting as the organic luminescent zone of the device. Examples of these devices are disclosed in U.S. Pat. Nos. 4,356,429; 4,539,507 and 4,720,432, wherein U.S. Pat. No. 4,720,432 discloses, for example, an organic EL device comprising a dual-layer hole injecting and transporting zone, one layer being comprised of porphyrinic compounds supporting hole injection and the other layer being comprised of aromatic tertiary amine compounds supporting hole transport. Another alternate device configuration illustrated in this patent is comprised of three separate layers, a hole transport layer, a luminescent layer, and an electron transport layer, which layers are laminated in sequence and are sandwiched between an anode and a cathode. Optionally, a fluorescent dopant material can be added to the emission zone or layer whereby the recombination of charges results in the excitation of the fluorescent.
There have also been attempts to obtain electroluminescence from organic light emitting devices containing mixed layers, for example, layers in which both the hole transport material and the emitting electron transport material are mixed together in one single layer, see, for example, J. Kido et al., “Organic Electroluminescent Devices Based On Molecularly Doped Polymers,”
Appl. Phys. Lett.
61, pp. 761-763, 1992; S. Naka et al., “Organic Electroluminescent Devices Using a Mixed Single Layer,”
Jpn. J. Appl. Phys.
33, pp. L1772-L1774, 1994; W. Wen et al.,
Appl. Phys. Lett.
71, 1302 (1997); and C. Wu et al., “Efficient Organic Electroluminescent Devices Using Single-Layer Doped Polymer Thin Films with Bipolar Carrier Transport Abilities,”
IEEE Transactions on Electron Devices
44, pp. 1269-1281, 1997. In a number of such structures, the electron transport material and the emitting material are the same. However, as described in the S. Naka et al. article, these single mixed layer organic light emitting devices are generally less efficient than multi-layer organic light emitting devices. Recent EL research results indicate that those devices including only a single mixed layer of a hole transport material (composed of NBP, a naphthyl-substituted benzidine derivative) and an emitting electron transport material (composed of Alq
3
, tris(8-hydroxyquinoline) aluminum are inherently believed to be unstable. The instability of these devices is believed to be caused by the direct contact between the electron transport material in the mixed layer and the hole injecting contact comprised of indium tin oxide (ITO), which results in the formation of the unstable cationic electronic transport material, and the instability of the mixed layer/cathode interface, see H. Aziz et al.,
Science
283, 1900 (1999), the disclosure of which is totally incorporated herein by reference.
Also, there have been attempts to obtain electroluminescence from organic light emitting devices by introducing a hole transport material and an emitting electron transport material as dopants in an inert host material, as reported in the above-described article by J. Kido et al. However, such known devices have been found to be generally less efficient than conventional devices including separate layers of hole transport material and emitting electron transport material.
While recent progress in organic EL research has perhaps elevated the potential of organic EL devices, the operational stability of current available devices may still be below expectations. A number of known organic light emitting devices have relatively short operational lifetimes before their luminance drops to some percentage of its initial value. Although known methods of providing interface layers as described, for example, in S. A. Van Slyke et al., “Organic Electroluminescent Devices with Improved Stability,”
Appl. Phys. Lett.
69, pp. 2160-2162, 1996, and doping as described, for example, Y. Hamada et al., “Influence of the Emission Site on the Running Durability of Organic Electroluminescent Devices”,
Jpn. J. Appl. Phys.
34, pp. L824-L826, 1995 may perhaps increase the operational lifetime of organic light emitting devices for room temperature operation, the effectiveness of these organic light emitting devices deteriorates dramatically for high temperature device operation. In general, device lifetime is reduced by a factor of about two for each 10° C. increment in the operational temperature. Moreover, at

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