Patent Application
20060006396
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 one or more fluorescent phosphor materials with high luminance efficiency and good light output stability.
A device and method for emitting output light utilizes orange/red light emitting ZnSe0.5S0.5:Cu,Cl phosphor material and green light emitting BaSrGa4S7:Eu phosphor material to convert some of the original light emitted from a light source of the device to a longer wavelength light in order to change the optical spectrum of the output light. The device and method can be used to produce white light using the light source, which may be a blue light emitting diode (LED) die. The orange/red light emitting ZnSe0.5S0.5:Cu,Cl phosphor material and green light emitting BaSrGa4S7:Eu phosphor material are included in a wavelength-shifting region optically coupled to the light source.
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 blue wavelength range and a wavelength-shifting region optically coupled to the light source to receive the first light. The wavelength-shifting region includes ZnSe0.5S0.5:Cu,Cl phosphor material having a property to convert some of the first light to second light of a second peak wavelength in the orange/red wavelength range. The wavelength-shifting region further includes BaSrGa4S7:Eu phosphor material having a property to convert some of the first light to third light of a third peak wavelength in the green wavelength range. The first light, the second light and the third light are components 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 blue wavelength range, receiving the first light, including converting some of the first light to second light of a second peak wavelength in the orange/red wavelength range using ZnSe0.5S0.5:Cu,Cl phosphor material and converting some of the first light to third light of a third peak wavelength in the green wavelength range using BaSrGa4S7:Eu phosphor material, and emitting the first light, the second light and the third light as components 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 two types of fluorescent phosphor materials, orange/red light emitting ZnSe0.5S0.5:Cu,Cl phosphor 118 and green light emitting BaSrGa4S7:Eu phosphor 119. The ZnSe0.5S0.5:Cu,Cl phosphor material 118 and the BaSrGa4S7:Eu phosphor material 119 are used to convert some of the original light emitted by the LED die 102 to lower energy (longer wavelength) light. The ZnSe0.5S0.5:Cu,Cl phosphor material 118 absorbs some of the original light of a first peak wavelength from the LED die 102, which excites the atoms of the ZnSe0.5S0.5:Cu,Cl phosphor material, and emits longer wavelength light of a second peak wavelength in the orange/red wavelength range of the visible spectrum, which is approximately 610 nm to 650 nm. Similarly, the BaSrGa4S7:Eu phosphor material 119 absorbs some of the original light from the LED die 102, which excites the atoms of the BaSrGa4S7:Eu phosphor material, and emits longer wavelength light of a third peak wavelength in the green wavelength range of the visible spectrum, which is approximately 520 nm to 540 nm. The second and third peak wavelengths of the converted light are partly defined by the peak wavelength of the original light, and the ZnSe0.5S0.5:Cu,Cl phosphor material 118 and the BaSrGa4S7:Eu phosphor material 119. 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.
The ZnSe0.5S0.5:Cu,Cl phosphor 118 can be synthesized by various techniques. One technique involves dry-milling a 1:1 molar ratio of undoped ZnSe and ZnS materials into fine powders or crystals, which may be less than 5 μm. A small amount of CuCl2 dopants is then added to de-ionized water or a solution from the alcohol family, such as methanol, and ball-milled with the undoped ZnSe0.5S0.5 powders. The amount of CuCl2 dopants added to the solution can be anywhere between a minimal amount (few parts per million) to approximately four percent of the total weight of ZnSe0.5S0.5 material and CuCl2 dopants. 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 ZnSe0.5S0.5:Cu,Cl phosphor powders with desired particle size distribution, which may be in the micron range.
The BaSrGa4S7:Eu phosphor 119 can also be synthesized by various techniques. One technique involves using BaS, SrS and Ga2S3 as precursors. The precursors are ball-milled in de-ionized water or a solution from the alcohol family, such as methanol, along with a small amount of Eu dopant, fluxes (Cl and F) and excess sulfur. The amount of Eu dopant added to the solution can be anywhere between a minimal amount to approximately ten percent of the total weight of all ingredients. The doped material is then dried and subsequently milled to produce fine particles. The milled particles are then loaded into a crucible, such as a quartz crucible, and sintered in an inert atmosphere at around eight hundred degrees Celsius (800° C.) for one to two hours. The sintered materials can then be sieved, if necessary, to produce BaSrGa4S7:Eu phosphor powders with desired particle size distribution, which may be in the micron range.
Following the completion of the ZnSe0.5S0.5:Cu,Cl and BaSrGa4S7:Eu synthesis processes, the ZnSe0.5S0.5:Cu,Cl and BaSrGa4S7:Eu 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 ratio between the two different types of phosphor powders can be adjusted to produce different color characteristics for the white phosphor-converted LED 100. As an example, the ratio between the ZnSe0.5S0.5:Cu,Cl phosphor powers and the BaSrGa4S7:Eu phosphor powders may be [1:7], respectively. The remaining part of the lamp 110 can be formed by depositing the transparent substance without the ZnSe0.5S0.5:Cu,Cl and BaSrGa4S7:Eu phosphor powders to produce the 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 the ZnSe0.5S0.5:Cu,Cl and BaSrGa4S7:Eu phosphor materials 118 and 119 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 device in accordance with the invention.
Turning now to
A method for producing 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. The scope of the invention is to be defined by the claims appended hereto and their equivalents.
