Patent Application
20050167685
Conventional light sources, such as incandescent, halogen and fluorescent lamps, have not been significantly improved in the past twenty years. However, light emitting diode (“LEDs”) have been improved to a point with respect to operating efficiency where LEDs are now replacing the conventional light sources in traditional monochrome lighting applications, such as traffic signal lights and automotive taillights. This is due in part to the fact that LEDs have many advantages over conventional light sources. These advantages include longer operating life, lower power consumption, and smaller size.
LEDs are typically monochromatic semiconductor light sources, and are currently available in various colors from UV-blue to green, yellow and red. Due to the narrow-band emission characteristics, monochromatic LEDs cannot be directly used for “white” light applications. Rather, the output light of a monochromatic LED must be mixed with other light of one or more different wavelengths to produce white light. Two common approaches for producing white light using monochromatic LEDs include (1) packaging individual red, green and blue LEDs together so that light emitted from these LEDs are combined to produce white light and (2) introducing fluorescent material into a UV, blue or green LED so that some of the original light emitted by the semiconductor die of the LED is converted into longer wavelength light and combined with the original UV, blue or green light to produce white light.
Between these two approaches for producing white light using monochromatic LEDs, the second approach is generally preferred over the first approach. In contrast to the second approach, the first approach requires a more complex driving circuitry since the red, green and blue LEDs include semiconductor dies that have different operating voltages requirements. In addition to having different operating voltage requirements, the red, green and blue LEDs degrade differently over their operating lifetime, which makes color control over an extended period difficult using the first approach. Moreover, since only a single type of monochromatic LED is needed for the second approach, a more compact device can be made using the second approach that is simpler in construction and lower in manufacturing cost. Furthermore, the second approach may result in broader light emission, which would translate into white output light having higher color-rendering characteristics.
A concern with the second approach for producing white light is that the fluorescent material currently used to convert the original UV, blue or green light results in LEDs having less than desirable luminance efficiency and/or light output stability over time.
In view of this concern, there is a need for an LED and method for emitting white output light using a fluorescent phosphor material with high luminance efficiency and good light output stability.
A device and method for emitting output light utilizes Group IIB element Selenide-based phosphor material to convert at least some of the original light emitted from a light source of the device to a longer wavelength light to change the optical spectrum of the output light. Thus, the device and method can be used to produce white color light. The Group IIB element Selenide-based phosphor material is included in a wavelength-shifting region optically coupled to the light source, which may be a blue-green light emitting diode (LED) die.
A device for emitting output light in accordance with an embodiment of the invention includes a light source that emits first light of a first peak wavelength in the visible wavelength range and a wavelength-shifting region optically coupled to the light source to receive the first light. The wavelength-shifting region includes Group IIB element Selenide-based phosphor material having a property to convert at least some of the first light to second light of a second peak wavelength. The second light is a component of the output light.
A method for emitting output light in accordance with an embodiment of the invention includes generating first light of a first peak wavelength in the visible wavelength range, receiving the first light, including converting at least some of the first light to second light of a second peak wavelength using Group IIB element Selenide-based phosphor material, and emitting the second light as a component of the output light.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example of the principles of the invention.
With reference to
As shown in
The lamp 110 of the white phosphor-converted LED 100 is made of a transparent substance, which can be any transparent material such as clear epoxy, so that light from the LED die 102 can travel through the lamp and be emitted out of the output section 114 of the lamp. In this embodiment, the lamp 110 includes a wavelength-shifting region 116, which is also a medium for propagating light, made of a mixture of the transparent substance and fluorescent phosphor material 118 based on Group IIB element Selenide. The Group IIB element Selenide-based phosphor material 118 is used to convert some of the original light emitted by the LED die 102 to lower energy (longer wavelength) light. The Group IIB element Selenide-based phosphor material 118 absorbs some of the original light from the LED die 102, which excites the atoms of the Group IIB element Selenide-based phosphor material, and emits the longer wavelength light. The peak wavelength of the converted light is partly defined by the peak wavelength of the original light and the Group IIB element Selenide-based phosphor material 118. The unabsorbed original light from the LED die 102 and the converted light are combined to produce “white” color light, which is emitted from the light output section 114 of the lamp 110 as output light of the LED 100. In the exemplary embodiment, the Group IIB element Selenide-based phosphor material 118 has a property to convert some of the original light from the LED die 102 into light of a longer peak wavelength in the red wavelength range of the visible spectrum, which is approximately 620 nm to 800 nm.
In one embodiment, the Group IIB element Selenide-based phosphor material 118 included in the wavelength-shifting region 116 of the lamp 110 is phosphor made of Zinc Selenide (ZnSe) activated by one or more suitable dopants, such as Copper (Cu), Chlorine (Cl), Fluorine (F), Bromine (Br), Silver (Ag) and rare earth elements. In an exemplary embodiment, the Group IIB element Selenide-based phosphor material 118 is phosphor made of ZnSe activated by Cu, i.e., ZnSe:Cu. Unlike conventional fluorescent phosphor materials that are used for producing white color light using LEDs, such as those based on alumina, oxide, sulfide, phosphate and halophosphate, ZnSe:Cu phosphor is highly efficient with respect to the wavelength-shifting conversion of light emitted from an LED die. This is due to the fact that most conventional fluorescent phosphor materials have a large bandgap, which prevents the phosphor materials from efficiently absorbing and converting light, e.g., blue-green light, to longer wavelength light. In contrast, the ZnSe:Cu phosphor has a lower bandgap, which equates to a higher efficiency with respect to wavelength-shifting conversion via fluorescence.
The ZnSe-based phosphor is the preferred Group IIB element Selenide-based phosphor material 118 for the wavelength-shifting region 116 of the lamp 110. However, the Group IIB element Selenide-based phosphor material 118 of the wavelength-shifting region 116 may be phosphor made of Cadmium Selenide (CdSe) activated by one or more suitable dopants, such as Cu, Cl, F, Br, Ag and rare earth elements. Alternatively, the Group IIB element Selenide-based phosphor material 118 of the wavelength-shifting region 116 may include a combination of ZnSe and CdSe activated by one or more suitable dopants.
The preferred ZnSe:Cu phosphor can be synthesized by various techniques. One technique involves dry-milling a predefined amount of undoped ZnSe material into fine powders or crystals, which may be less than 5 gm. A small amount of Cu dopant is then added to a solution from the alcohol family, such as methanol, and ball-milled with the undoped ZnSe powders. The amount of Cu dopant added to the solution can be anywhere between a minimal amount to approximately six percent of the total weight of ZnSe material and Cu dopant. The doped material is then oven-dried at around one hundred degrees Celsius (100° C.), and the resulting cake is dry-milled again to produce small particles. The milled material is loaded into a crucible, such as a quartz crucible, and sintered in an inert atmosphere at around one thousand degrees Celsius (1,000° C.) for one to two hours. The sintered materials can then be sieved, if necessary, to produce ZnSe:Cu phosphor powders with desired particle size distribution, which may be in the micron range.
The ZnSe:Cu phosphor powders may be further processed to produce phosphor particles with a silica coating. Silica coating on phosphor particles reduces clustering or agglomeration of phosphor particles when the phosphor particles are mixed with a transparent substance to form a wavelength-shifting region in an LED, such as the wavelength-shifting region 116 of the lamp 110. Clustering or agglomeration of phosphor particles can result in an LED that produces output light having a non-uniform color distribution.
In order to apply a silica coating to the ZnSe:Cu phosphor particles, the sieved materials are subjected to an annealing process to anneal the phosphor particles and to remove contaminants. Next, the phosphor particles are mixed with silica powders, and then the mixture is heated in a furnace at approximately 200 degrees Celsius. The applied heat forms a thin silica coating on the phosphor particles. The amount of silica on the phosphor particles is approximately 1% with respect to the phosphor particles. The resulting ZnSe:Cu phosphor particles with silica coating may have a particle size of less than or equal to thirty (30) microns.
Following the completion of the synthesis process, the ZnSe:Cu phosphor powders can be mixed with the same transparent substance of the lamp 110, e.g., epoxy, and deposited around the LED die 102 to form the wavelength-shifting region 116 of the lamp. The remaining part of the lamp 110 can be formed by depositing the transparent substance without the ZnSe:Cu phosphor powders to produce the white phosphor-converted LED 100. Although the wavelength-shifting region 116 of the lamp 110 is shown in
In
In an alternative embodiment, the leadframe of a white phosphor-converted LED on which the LED die is positioned may include a reflector cup, as illustrated in
The different lamp configurations described above can be applied other types of LEDs, such as surface-mounted LEDs, to produce other types of white phosphor-converted LEDs with Group IIB element Selenide-based phosphor material in accordance with the invention. In addition, these different lamp configurations may be applied to other types of light emitting devices, such as semiconductor lasing devices, to produce other types of light emitting devices in accordance with the invention. In these light emitting devices, the light source can be any light source other than an LED die, such as a laser diode.
Turning now to
A method for producing white output light in accordance with an embodiment of the invention is described with reference to
Although specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. Furthermore, the invention is not limited to devices and methods for producing white output lights. The invention also includes devices and methods for producing other types of output light. As an example, the Group IIB element Selenide-based phosphor material in accordance with the invention may be used in a light emitting device where virtually all of the original light generated by a light source is converted to light of different wavelength, in which case the color of the output light may not be white. The scope of the invention is to be defined by the claims appended hereto and their equivalents.
This application is a continuation-in-part of U.S. application Ser. No. 10/761,762, filed Jan. 21, 2004, for which priority is claimed. The entire prior application is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10761762 | Jan 2004 | US |
| Child | 10920791 | Aug 2004 | US |
