Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
2000-06-30
2002-05-21
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C257S144000
Reexamination Certificate
active
06392250
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optoelectronic devices and, more particularly, to organic light emitting devices. This invention also relates to methods of forming the organic light emitting devices.
2. Discussion of the Related Art
Tang and Van Slyke reported efficient electroluminescence from a bilayer organic device in 1987. C. W. Tang and S. A. Van Slyke, “Organic Electroluminescent Diodes,”
Appl. Phys. Lett
. 51, pp. 913-915, 1987. Since that time, organic light emitting devices (OLEDs) have attracted great attention because of their potential toward the fabrication of large-area displays. See, J. R. Sheats et al, “Organic Electroluminescent Devices,”
Science
273, pp. 884-888, 1996; J. Salbeck, “Electroluminescence with Organic Compounds,”
Ber. Bunsenges. Phys. Chem
. 100, pp. 1667-1677, 1996; and Z. Shen et al., “Three-Color, Tunable, Organic Light-Emitting Devices,”
Science
276, pp. 2009-2011, 1997.
To achieve efficient electroluminescence organic light emitting devices can include separate layers of a hole transport material (HTM) and an emitting electron transport material (ETM). The structure of such a conventional bilayer organic light emitting device
10
is shown in FIG.
1
. The organic light emitting device
10
includes a substrate
12
composed of, for example, glass; an anode
14
on the substrate
12
and typically composed of a transparent conductor, for example, indium tin oxide (ITO); a hole transport material layer
16
on the anode
14
; an electron transport material layer
18
on the hole transport material layer
16
; and a cathode
20
on the electron transport layer
18
and typically composed of a low work function metal or metal alloy. During operation, an applied electric field causes positive charges (holes) and negative charges (electrons) to be respectively injected from the anode
14
and the cathode
20
to recombine and thereby produce light emission.
Also in organic light emitting devices, the separate hole transport and electron transport layers can be doped with organic dyes in order to enhance the efficiency, obtain a different emission color, or to improve the stability of the organic light emitting devices, reference 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, and J. Shi et al., “Doped Organic Electroluminescent Devices with Improved Stability,”
Appl.Phys. Lett
. 70, pp. 1665-1667, 1997.
There have also been attempts to obtain electroluminescence from organic light emitting devices containing mixed layers, that is for example, layers in which both the hole transport material and the emitting electron transport material are mixed together in one single layer, reference 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 many 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.
Moreover, the above-described references have not addressed the stability of these single mixed layer organic light emitting device structures. Research by the present inventors on organic light emitting devices structures including only a single mixed layer of a hole transport material (composed of NBP, a naphtyl-substituted benzidine derivative) and an emitting electron transport material (composed of Alq
3
, tris (8-hydroxyquinoline) aluminum) have revealed that these known single mixed layer organic light emitting device structures are inherently 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, as well as to the instability of the mixed layer/cathode interface, reference H. Aziz et al., “Degradation Mechanism of Small Molecule-Based Organic Light Emitting Devices,”
Science
283, pp. 1900-1902, 1999, the disclosure of which is totally incorporated herein by reference in its entirety.
There have also been attempts to obtain electroluminescence from organic light emitting devices by introducing hole transport materials and emitting electron transport materials 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.
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 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, in the above-described publication by Y. Hamada et al., have increased the operational lifetime of organic light emitting devices for room temperature operation, the effectiveness of the known organic light emitting devices deteriorates dramatically for high temperature device operation, as the existing methods used to extend the device lifetimes lose their effectiveness at higher temperatures. In general, device lifetime is reduced by a factor of about two for each 10° C. increment in the operational temperature. As a result, the operational lifetime of known organic light emitting devices at a normal display luminance level of about 100 cd/m
2
is limited to about a hundred hours or less at temperatures in the range of 60-80° C., reference the above-described article by J. R. Sheats et al. and also S. Tokito et al., “High-Temperature Operation of an Electroluminescent Device Fabricated Using a Novel Triphenlamine Derivative,”
Appl. Phys. Lett
. 69, 878 (1996). These operational device lifetimes are unsatisfactory for use of the organic light emitting devices at these high temperatures, where an operational device lifetime in the order of several thousand hours is generally needed for various potential applications of organic light emitting devices.
SUMMARY OF THE INVENTION
U.S. patent application Ser. No. 09/357,551, filed on Jul. 20, 1999, and incorporated herein by reference in its entirety, describes organic light emitting devices (OLEDs) that 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 is formed on the mixed region. The organic light emitting devices have enhanced efficiencies and operational lifetimes and can provide operational stability at high temperature device operation conditions. Thus, these organic light emitting devices overcome the above-described disadvantages of known organic light emitting devices.
Other electroluminescent (EL) devices are described in U.S. Pat. Nos. 5,942,340; 5,952,115; 5,932,363; 5,925,472 and 5,891,587, which are each incorporated herein by reference in their entirety. U.S. Pat. No. 5,925,472 describes organic light emitting devices with blue luminescent materials comprised of metal chelates of oxadiazole compounds. These devices may provide a greenish blue color emission.
The organic light emittin
Aziz Hany M.
Hu Nan-Xing
Popovic Zoran D.
Le Bau T
Palazzo Eugene O.
Xerox Corporation
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