Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
2000-05-03
2003-08-19
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S509000, C313S504000, C313S506000
Reexamination Certificate
active
06608439
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to color organic light emitting diode (OLED) video displays. In particular, the present invention is directed to color conversion elements incorporating semiconductor nanocrystals which are dispersed in a transparent binding material. Fabricated independently from the OLED layers, the color conversion elements absorb light that is emitted from the OLED layers at a first wavelength and re-emit this light at a different wavelength. The color conversion elements of the present invention may be employed in either down-emitting or up-emitting color OLED display devices.
BACKGROUND OF THE INVENTION
Organic light emitting diodes (“OLEDs”) have been known for approximately two decades. All OLEDs work on the same general principles. One or more layers of semiconducting organic material are sandwiched between two electrodes. An electric current is applied to the device, causing negatively charged electrons to move into the organic material(s) from the cathode. Positive charges, typically referred to as holes, move in from the anode. The positive and negative charges meet in the center layers (i.e., the organic material), recombine, and produce photons. The wavelength of the photons—and consequently the color of the emitted light—depends on the electronic properties of the organic material in which the photons are generated.
In a typical OLED, at least one of the electrodes is transparent. The cathode may be constructed of a low work function material. The holes may be injected from a high workfunction anode material into the organic material. Typically, the devices operate with a DC bias of from 2 to 30 volts. The films may be formed by evaporation, spin casting, self-assembly or other appropriate film-forming techniques. Thicknesses typically range from a few mono layers to about 2,000 Angstroms.
In a typical passive matrix-addressed OLED display numerous OLEDs are formed on a single substrate and arranged in groups in a regular grid pattern. Several OLED groups forming a column of the grid may share a common cathode, or cathode line. Several OLED groups forming a row of the grid may share a common anode, or anode line. The individual OLEDs in a given group emit light when their cathode line and anode line are activated at the same time.
An OLED may be designed to be viewed either from the “top” —the face opposite the foundational substrate—or from the “bottom” i.e., through the substrate, from the face opposite the light emitting layer. Whether the OLED is designed to emit light through the top or the bottom, the respective structure between the viewer and the light emitting material needs to be sufficiently transparent, or at least semi-transparent, to the emitted light. In many applications it is advantageous to employ an OLED display having topside light output. This permits the display to be built on top of a silicon driver chip for active matrix addressing.
The color of light emitted from the OLED display device can be controlled by the selection of the organic material. Specifically, the precise color of light emitted by a particular structure can be controlled both by selection of the organic material as well as by selection of luminescent impurities or dopants, added to the organic materials. By changing the kinds of organic solids making up the light-emitting layer, many different colors of light may be emitted, ranging from deep blue to red.
The color of light emitted from an OLED display device may be affected not only by the source material and/or doping of the light emitting layer, but also by color filters and color converters or color changing films that are formed above the OLED pixels or light emitting layers.
OLEDs have a number of beneficial characteristics. These include a low activation voltage (about 5 volts), fast response when formed with a thin light-emitting layer, and high brightness in proportion to the injected electric current. OLEDs are currently the subject of aggressive investigative efforts.
Although substantial progress has been made in the development of OLEDs to date, additional challenges remain. For example, there are drawbacks to the various existing approaches to the fabrication of the components that generate colored light in OLED displays. One approach provides a self-emissive pixelated display with RGB subpixels placed next to each other. This approach, in principle, allows the best possible performance because no light is lost through filter absorption or color conversion. It requires, however, precise shadow mask fabrication and alignment in the process of vacuum deposition for displays using low-molecular-weight material. Such precision in the fabrication of shadow masks is technologically difficult for miniature, high-resolution displays with pixel sizes in the range of several microns.
In a second design approach for making color OLEDs, pixels emitting white light are combined with precisely aligned color filter elements. The white light is changed to the color of the particular color filter. The color filters can be inefficient, however, because the filters inevitably absorb some light.
A third approach aligns photoluminescing color conversion elements with pixels emitting near ultraviolet or blue light.
The latter two approaches are technologically feasible given the present state of the art because all pixels emit the same color and the filter media can be patterned and aligned to the OLED pixels. When the relevant layers have high quantum efficiency of photoluminescence and internal losses are minimized, the third, or color conversion, technique provides better efficiency.
But even the color conversion technique has drawbacks. Most materials used for color conversion have broad emission photoluminescence spectra that require additional optical filters for spectra correction. These additional optical filters on top of the color conversion materials introduce additional loss of intensity.
In the present invention, Applicant presents an effective new design for color conversion through fabrication of appropriate, inorganic-based elements that provide narrow photoluminescence emission bands upon optical stimulation by a higher photon energy source. Semiconductor nanocrystals are known to have narrow and tunable emission bands which are determined very specifically by their size, yet are not dependent on the details of the near ultraviolet or blue excitation spectrum. This permits all three color conversion elements (red, green and blue) to be pumped by a single excitation source, for example, the organic electroluminescence display matrix element. Semiconductor nanocrystals (e.g., passivated CdSe) are widely tunable in the size range of approximately 10 to 200 Angstroms, which covers optical conversion through the visible spectrum, and can be controllably fabricated with narrow size distributions from scalable colloidal precipitation and other techniques known in the prior art.
In the present invention, the layer or layers containing the semiconductor nanocrystals are fabricated independently from the OLED layers. Stable films of semiconductor nanocrystals can be patterned using standard photolithographic techniques unlike the OLED layers which are sensitive to humidity and other environmental variations.
Previous approaches for color conversion have focused specifically on the use of organic dye molecular systems such that the red, green and blue color converters may require completely different synthetic routes, thus increasing manufacturing complexity. For example, U.S. Pat. No. 5,126,214 to Tokailin et al. discloses the use of an organic electroluminescent matrix element, together with a fluorescent material part that corresponds functionally to the color conversion element of the present invention. The fluorescent material part emits a fluorescence in a visible light range from red to blue. Tokailin et al. disclose the use of fluorescent dye materials to provide color conversion. In contrast, the present invention uses semiconductor nanocrystals for color conversion.
Similarly,
Campos Richard A.
Sokolik Igor
Bazerman & Drangel PC
eMagin Corporation
Patel Nimeshkumar D.
Santiago Mariceli
LandOfFree
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