Patent Publication Number: US-2006006396-A1

Title: Phosphor mixture of organge/red ZnSe0.5S0.5:Cu,Cl and green BaSrGa4S7:Eu for white phosphor-converted led

Description:
BACKGROUND OF THE INVENTION  
      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.  
     SUMMARY OF THE INVENTION  
      A device and method for emitting output light utilizes orange/red light emitting ZnSe 0.5 S 0.5 :Cu,Cl phosphor material and green light emitting BaSrGa 4 S 7 :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 ZnSe 0.5 S 0.5 :Cu,Cl phosphor material and green light emitting BaSrGa 4 S 7 :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 ZnSe 0.5 S 0.5 :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 BaSrGa 4 S 7 :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 ZnSe 0.5 S 0.5 :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 BaSrGa 4 S 7 :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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagram of a white phosphor-converted LED in accordance with an embodiment of the invention.  
       FIGS. 2A, 2B  and  2 C are diagrams of white phosphor-converted LEDs with alternative lamp configurations in accordance with an embodiment of the invention.  
       FIGS. 3A, 3B ,  3 C and  3 D are diagrams of white phosphor-converted LEDs with a leadframe having a reflector cup in accordance with an alternative embodiment of the invention.  
       FIG. 4  shows the optical spectrum of a white phosphor-converted LED in accordance with an embodiment of the invention.  
       FIG. 5  is a flow diagram of a method for emitting output light in accordance with an embodiment of the invention.  
    
    
     DETAILED DESCRIPTION  
      With reference to  FIG. 1 , a white phosphor-converted light emitting diode (LED)  100  in accordance with an embodiment of the invention is shown. The LED  100  is designed to produce “white” color output light with high luminance efficiency and good light output stability. The white output light is produced by converting some of the original light generated by the LED  100  into longer wavelength light using orange/red light emitting ZnSe 0.5 S 0.5 :Cu,Cl phosphor material and green emitting BaSrGa 4 S 7 :Eu phosphor material.  
      As shown in  FIG. 1 , the white phosphor-converted LED  100  is a leadframe-mounted LED. The LED  100  includes an LED die  102 , leadframes  104  and  106 , a wire  108  and a lamp  110 . The LED die  102  is a semiconductor chip that generates light of a particular peak wavelength. In an exemplary embodiment, the LED die  102  is designed to generate light having a peak wavelength in the blue wavelength range of the visible spectrum, which is approximately 420 nm to 490 nm. The LED die  102  is situated on the leadframe  104  and is electrically connected to the other leadframe  106  via the wire  108 . The leadframes  104  and  106  provide the electrical power needed to drive the LED die  102 . The LED die  102  is encapsulated in the lamp  110 , which is a medium for the propagation of light from the LED die  102 . The lamp  110  includes a main section  112  and an output section  114 . In this embodiment, the output section  114  of the lamp  110  is dome-shaped to function as a lens. Thus, the light emitted from the LED  100  as output light is focused by the dome-shaped output section  114  of the lamp  110 . However, in other embodiments, the output section  114  of the lamp  100  may be horizontally planar.  
      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 ZnSe 0.5 S 0.5 :Cu,Cl phosphor  118  and green light emitting BaSrGa 4 S 7 :Eu phosphor  119 . The ZnSe 0.5 S 0.5 :Cu,Cl phosphor material  118  and the BaSrGa 4 S 7 :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 ZnSe 0.5 S 0.5 :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 ZnSe 0.5 S 0.5 :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 BaSrGa 4 S 7 :Eu phosphor material  119  absorbs some of the original light from the LED die  102 , which excites the atoms of the BaSrGa 4 S 7 :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 ZnSe 0.5 S 0.5 :Cu,Cl phosphor material  118  and the BaSrGa 4 S 7 :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 ZnSe 0.5 S 0.5 :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 CuCl 2  dopants is then added to de-ionized water or a solution from the alcohol family, such as methanol, and ball-milled with the undoped ZnSe 0.5 S 0.5  powders. The amount of CuCl 2  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 ZnSe 0.5 S 0.5  material and CuCl 2  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 ZnSe 0.5 S 0.5 :Cu,Cl phosphor powders with desired particle size distribution, which may be in the micron range.  
      The BaSrGa 4 S 7 :Eu phosphor  119  can also be synthesized by various techniques. One technique involves using BaS, SrS and Ga 2 S 3  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 BaSrGa 4 S 7 :Eu phosphor powders with desired particle size distribution, which may be in the micron range.  
      Following the completion of the ZnSe 0.5 S 0.5 :Cu,Cl and BaSrGa 4 S 7 :Eu synthesis processes, the ZnSe 0.5 S 0.5 :Cu,Cl and BaSrGa 4 S 7 :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 ZnSe 0.5 S 0.5 :Cu,Cl phosphor powers and the BaSrGa 4 S 7 :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 ZnSe 0.5 S 0.5 :Cu,Cl and BaSrGa 4 S 7 :Eu phosphor powders to produce the LED  100 . Although the wavelength-shifting region  116  of the lamp  110  is shown in  FIG. 1  as being rectangular in shape, the wavelength-shifting region may be configured in other shapes, such as a hemisphere, as shown in  FIG. 3A . Furthermore, in other embodiments, the wavelength-shifting region  116  may not be physically coupled to the LED die  102 . Thus, in these embodiments, the wavelength-shifting region  116  may be positioned elsewhere within the lamp  110 .  
      In  FIGS. 2A, 2B  and  2 C, white phosphor-converted LEDs  200 A,  200 B and  200 C with alternative lamp configurations in accordance with an embodiment of the invention are shown. The white phosphor-converted LED  200 A of  FIG. 2A  includes a lamp  210 A in which the entire lamp is a wavelength-shifting region. Thus, in this configuration, the entire lamp  210 A is made of the mixture of the transparent substance, and the ZnSe 0.5 S 0.5 :Cu,Cl and BaSrGa 4 S 7 :Eu phosphor materials  118  and  119 . The white phosphor-converted LED  200 B of  FIG. 2B  includes a lamp  210 B in which a wavelength-shifting region  216 B is located at the outer surface of the lamp. Thus, in this configuration, the region of the lamp  210 B without the ZnSe 0.5 S 0.5 :Cu,Cl and BaSrGa 4 S 7 :Eu phosphor materials  118  and  119  is first formed over the LED die  102  and then the mixture of the transparent substance and the phosphor materials is deposited over this region to form the wavelength-shifting region  216 B of the lamp. The white phosphor-converted LED  200 C of  FIG. 2C  includes a lamp  210 C in which a wavelength-shifting region  216 C is a thin layer of the mixture of the transparent substance and the ZnSe 0.5 S 0.5 :Cu,Cl and BaSrGa 4 S 7 :Eu phosphor materials  118  and  119  coated over the LED die  102 . Thus, in this configuration, the LED die  102  is first coated or covered with the mixture of the transparent substance and the ZnSe 0.5 S 0.5 :Cu,Cl and BaSrGa 4 S 7 :Eu phosphor materials  118  and  119  to form the wavelength-shifting region  216 C and then the remaining part of the lamp  210 C can be formed by depositing the transparent substance without the phosphor materials over the wavelength-shifting region. As an example, the thickness of the wavelength-shifting region  216 C of the LED  200 C can be between ten (10) and sixty (60) microns, depending on the color of the light generated by the LED die  102 .  
      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  FIGS. 3A, 3B ,  3 C and  3 D.  FIGS. 3A-3D  show white phosphor-converted LEDs  300 A,  300 B,  300 C and  300 D with different lamp configurations that include a leadframe  320  having a reflector cup  322 . The reflector cup  322  provides a depressed region for the LED die  102  to be positioned so that some of the light generated by the LED die is reflected away from the leadframe  320  to be emitted from the respective LED as useful output light.  
      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 ZnSe 0.5 S 0.5 :Cu,Cl and BaSrGa 4 S 7 :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  FIG. 4 , the optical spectrum  424  of a phosphor-converted LED with a blue (440-480 nm) LED die in accordance with an embodiment of the invention is shown. The wavelength-shifting region for this LED was formed with sixty-five percent (65%) of ZnSe 0.5 S 0.5 :Cu,Cl and BaSrGa 4 S 7 :Eu phosphors relative to epoxy. The percentage amount or loading content of ZnSe 0.5 S 0.5 :Cu,Cl and BaSrGa 4 S 7 :Eu phosphors included in the wavelength-shifting region of the LED can be varied according to phosphor efficiency. As the phosphor efficiency is increased, e.g., by changing the amount of dopant(s), the loading content of the ZnSe 0.5 S 0.5 :Cu,Cl and BaSrGa 4 S 7 :Eu phosphors may be reduced. The optical spectrum  424  includes a first peak wavelength  426  at around  460  nm, which corresponds to the peak wavelength of the light emitted from the blue LED die. The optical spectrum  424  also includes a second peak wavelength  428  at around 540 nm, which is the peak wavelength of the light converted by the BaSrGa 4 S 7 :Eu phosphor in the wavelength-shifting region of the LED, and a third peak wavelength  430  at around 625 nm, which is the peak wavelength of the light converted by the ZnSe 0.5 S 0.5 :Cu,Cl phosphor in the wavelength-shifting regions of the LED.  
      A method for producing output light in accordance with an embodiment of the invention is described with reference to  FIG. 5 . At block  502 , first light of a first peak wavelength in the blue wavelength range is generated. The first light may be generated by an LED die. Next, at block  504 , the first light is received and some of the first light is converted to second light of a second peak wavelength in the orange/red wavelength range using the ZnSe 0.5 S 0.5 :Cu,Cl phosphor material. Furthermore, at block  504 , some of the first light is converted to third light of a third peak wavelength in the green wavelength range using BaSrGa 4 S 7 :Eu phosphor material. Next, at block  506 , the first light, the second light and the third light are emitted as components of the output light.  
      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.