Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Utility Patent
1998-09-14
2001-01-02
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S502000, C313S506000
Utility Patent
active
06169359
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to thin film electroluminescent phosphor material.
Thin films of rare earth doped alkaline earth sulfides, such as cerium doped strontium sulfide, have been extensively investigated for applications in full color alternating current thin film electroluminescent (ACTFEL) display devices. Such a device is disclosed by Barrow et al., U.S. Pat. No. 4,751,427, incorporated by reference herein. The emission spectrum of SrS:Ce is very broad covering both blue and green portions of the visible spectrum, i.e., 440 to 660 nm with a peak at around 500 nm. A full color ACTFEL display device can be obtained by adding a red emitting phosphor, such as CaS:Eu or one that has a red component in its emission spectrum. With such a combination of films, one can build a white light emitting phosphor stack. White phosphor structures can then be laminated with primary color filters to build a color display which is very cost effective in terms of production.
Thin film electroluminescent phosphor materials operate based on the conduction of electrons through the phosphor material as a result of a high (e.g., 1 to 5 MV/cm) electric field. The phosphor material behaves as a very wide band-gap semiconductor and charge is transported across the phosphor film at electric fields higher than a threshold value. The phosphor film is generally sandwiched on both sides by a pair of insulating materials which do not break down nor substantially conduct current at such operational electric fields. The electric field is applied using conducting electrodes deposited onto the surface of the insulating material. The resulting device is known as an alternating current thin-film electroluminescent (ACTFEL) device. The electrons that cause the light emission of ACTFEL devices are thought to originate from the regions proximate the interfaces between the phosphor material and the adjacent layers. In particular, the electrons emitted from these regions is thought to be the result of the quantum states of defects within the crystalline lattice. The brightness is related to the number of electrons that are emitted from these quantum states that move with an energy equal to or greater than that necessary to excite the luminescent atoms in the phosphor film, such as the dopants. The amount of light emitted from the phosphor (brightness) is a critical property of the electroluminescent device because brighter displays are easier to read in bright ambient light, such as sunlight.
The number of electrons which tunnel into the conduction band and contribute to the luminescence of ACTFEL phosphor material is limited by the energy depth and density of the quantum states of the defects. Unfortunately, the energy and density of these quantum states are difficult to control. Accordingly, it is difficult to control the brightness of the display.
Another important ACTFEL device characteristic is its efficiency, which is critical for battery operated applications. The efficiency of an ACTFEL device is related to the energy and density of the quantum states, which as previously mentioned are difficult to control. Accordingly, it is difficult to improve and control ACTFEL device efficiency.
It has been observed that many thin-film phosphor materials have a tendency to change in luminance after prolonged use, generally referred to as aging. More particularly, the defects at the interfaces between the phosphor material and the respective insulating material are thought to migrate towards or away from their respective interface resulting in long term changes in the electrical and optical properties of the device. It is the defects at the interfaces which are considered to be the source of the electrons for luminance of the ACTFEL device. This aging phenomena, if severe, results in an unusable display after prolonged use which is unacceptable for many applications. In addition, when a grey scale is applied to an ACTFEL device the aging changes the relative luminance of the grey scale levels relative to each other which results in an overall dimmer display where the image appears improper.
What is desired, therefore, is a phosphor material suitable for ACTFEL devices that increases the brightness of the device, increases the efficiency of the device, and decreases the effects of aging of the device.
SUMMARY OF THE INVENTION
The luminance of a phosphor suitable for an alternating current thin-film electroluminescent device is substantially improved according to the present invention by including an alkali halide. The alkali halide included within the bulk of the phosphor material results in providing a significant number of trapping states which act as donors or acceptors within the a phosphor material. The addition of the trapping states (added defects) within the bulk tend to control the electrical and optical characteristics of the phosphor material. This reduces asymmetric light output characteristics of traditional phosphor material, reduces asymmetric current movement within the phosphor material, and decreases the influence of the interfaces between the phosphor material and insulating materials resulting in decreased aging.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 4311487 (1982-01-01), Luckey et al.
patent: 5693254 (1997-12-01), Sieber et al.
patent: 5742322 (1998-04-01), Cranton et al.
patent: 5939825 (1999-08-01), Sun et al.
Davidson Mark Rogers
Holloway Paul H.
Lewis, III John S.
Sun Sey-Shing
Waldrip Karen Elizabeth
Chernoff Vilhauer McClung & Stenzel LLP
Patel Nimeshkumar D.
Planar Systems Inc.
Smith Richard J.
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