Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
1999-07-21
2001-10-30
Lee, Eddie (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C257S087000, C257S089000, C257S102000, C257S103000, C438S099000
Reexamination Certificate
active
06310360
ABSTRACT:
I. FIELD OF INVENTION
The present invention is directed to organic light emitting devices (OLEDs) comprised of emissive layers that contain an organic compound functioning as an emitter and a separate intersystem crossing (“ISC”) entity which operates to enhance the efficiency of the emission.
II. BACKGROUND OF THE INVENTION
II. A. General Background
Organic light emitting devices (OLEDs) are comprised of several organic layers in which one of the layers is comprised of an organic material that can be made to electroluminesce by applying a voltage across the device, C. W. Tang et al., Appl. Phys. Lett. 1987, 51, 913. Certain OLEDs have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays (S. R. Forrest, P. E. Burrows and M. E. Thompson, Laser Focus World, Feb. 1995). Since many of the thin organic films used in such devices are transparent in the visible spectral region, they allow for the realization of a completely new type of display pixel in which red (R), green (G), and blue (B) emitting OLEDs are placed in a vertically stacked geometry to provide a simple fabrication process, a small R-G-B pixel size, and a large fill factor, International Patent Application No. PCT/US95/15790.
A transparent OLED (TOLED), which represents a significant step toward realizing high resolution, independently addressable stacked R-G-B pixels, was reported in International Patent Application No. PCT/US97/02681 in which the TOLED had greater than 71% transparency when turned off and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode and a Mg—Ag-ITO electrode layer for electron-injection. A device was disclosed in which the ITO side of the Mg—Ag-ITO layer was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED. Each layer in the stacked OLED (SOLED) was independently addressable and emitted its own characteristic color. This colored emission could be transmitted through the adjacently stacked, transparent, independently addressable, organic layer or layers, the transparent contacts and the glass substrate, thus allowing the device to emit any color that could be produced by varying the relative output of the red and blue color-emitting layers.
PCT/US95/15790 application disclosed an integrated SOLED for which both intensity and color could be independently varied and controlled with external power supplies in a color tunable display device. The PCT/US95/15790 application, thus, illustrates a principle for achieving integrated, full color pixels that provide high image resolution, which is made possible by the compact pixel size. Furthermore, relatively low cost fabrication techniques, as compared with prior art methods, may be utilized for making such devices.
II.B. Background of emission
II.B.1. Basics
II.B.1.a. Singlet and Triplet Excitons
Because light is generated in organic materials from the decay of molecular excited states or excitons, understanding their properties and interactions is crucial to the design of efficient light emitting devices currently of significant interest due to their potential uses in displays, lasers, and other illumination applications. For example, if the symmetry of an exciton is different from that of the ground state, then the radiative relaxation of the exciton is disallowed and luminescence will be slow and inefficient. Because the ground state is usually anti-symmetric under exchange of spins of electrons comprising the exciton, the decay of a symmetric exciton breaks symmetry. Such excitons are known as triplets, the term reflecting the degeneracy of the state. For every three triplet excitons that are formed by electrical excitation in an OLED, only one symmetric state (or singlet) exciton is created. (M. A. Baldo, D. F. O'Brien, M. E. Thompson and S. R. Forrest, Very high-efficiency green organic light-emitting devices based on electrophosphorescence, Applied Physics Letters, 1999, 75, 4-6.) Luminescence from a symmetry-disallowed process is known as phosphorescence. Characteristically, phosphorescence may persist for up to several seconds afier excitation due to the low probability of the transition. In contrast, fluorescence originates in the rapid decay of a singlet exciton. Since this process occurs between states of like symmetry, it may be very efficient.
Many organic materials exhibit fluorescence from singlet excitons. However, only a very few have been identified which are also capable of efficient room temperature phosphorescence from triplets. Thus, in most fluorescent dyes, the energy contained in the triplet states is wasted. However, if the triplet excited state is perturbed, for example, through spin-orbit coupling (typically introduced by the presence of a heavy metal atom), then efficient phosphoresence is more likely. In this case, the triplet exciton assumes some singlet character and it has a higher probability of radiative decay to the ground state. Indeed, phosphorescent dyes with these properties have demonstrated high efficiency electroluminescence.
Only a few organic materials have been identified which show efficient room temperature phosphorescence from triplets. In contrast, many fluorescent dyes are known (C. H. Chen, J. Shi, and C. W. Tang, “Recent developments in molecular organic electroluminescent materials,” Macromolecular Symposia, 1997, 125, 1-48; U. Brackmann, Lambdachrome Laser Dyes (Lambda Physik, Gottingen, 1997) and fluorescent efficiencies in solution approaching 100% are not uncommon. (C. H. Chen, 1997, op. cit.) Fluorescence is also not affected by triplet-triplet annihilation, which degrades phosphorescent emission at high excitation densities. (M. A. Baldo, et al., “High efficiency phosphorescent emission from organic electroluminescent devices,” Nature, 1998, 395, 151-154; M. A. Baldo, M. E. Thompson, and S. R. Forrest, “An analytic model of triplet-triplet annihilation in electrophosphorescent devices,” 1999). Consequently, fluorescent materials are suited to many electroluminescent applications, particularly passive matrix displays.
II.B.1.b. Overview of invention relative to basics
This invention pertains to the use of intersystem crossing agents to enhance emission efficiency in organic light emitting devices. An intersystem crossing agent, or molecule, is one which can undergo intersystem crossing, which involves the transfer of population between states of different spin multiplicity. Lists of known intersystem crossing agents, or molecules, are given in A. Gilbert and J. Baggott, Essentials of Molecular Photochemistry, Blackwells Scientific, 1991.
In one embodiment of the present invention, we focus on a way to use an intersystem crossing agent to increase efficiency in a system with a fluorescent emitter. Therein, we describe a technique whereby triplets formed in the host material are not wasted, but instead are transferred to the singlet excited state of a fluorescent dye. In this way, all excited states are employed and the overall efficiency of fluorescence increased by a factor of four. In this embodiment, the ISC agent traps the energy of excitons and transfers the energy to the fluorescent emitter by a Forster energy transfer. The energy transfer process desired is:
3
D*+
1
A→
1
D+
1
A* (Eq. 1)
Here, D and A represent a donor molecule and a fluorescent acceptor, respectively. The superscripts 3 and 1 denote the triplet and singlet states, respectively, and the asterisk indicates the excited state.
In a second embodiment of the present invention, we focus on a way to use an intersystem crossing agent to increase efficiency in a system with a phosphorescent emitter. Therein, we describe a technique whereby the ISC agent is responsible for converting all of the excitons from a host material into their triplet states and then t
Baldo Marc A.
Forrest Stephen R.
Thompson Mark E.
Kenyon & Kenyon
Lee Eddie
Richards N. Drew
The Trustees of Princeton University
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