Patent Publication Number: US-2011057207-A1

Title: White-light emitting device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 98130602, filed on Sep. 10, 2009. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is related to a light emitting device and more particularly, to a white-light emitting device. 
     2. Description of Related Art 
     In an light emitting diode (LED), group III-V elements serve as main materials of a light emitting layer, and a current is applied to the light emitting layer. Through combination of electrons and electron holes, redundant energy is released in a form of light, thus achieving a light emitting effect. In comparison with conventional light emission through heating or discharging, the light emitted from the LED is a type of cold emission; therefore, the service life of the LED is rather long, and no idling time is required. In addition, the LED has advantages of high responsive speed, small volume, little consumption of electricity, low degree of pollution (no mercury contained), great reliability, adaptation of mass production, and so on. Thus, applications of the LED are extensive. 
     Recently, light output efficiency of the LED is continuously increasing, such that conventional fluorescent lamps and incandescent bulbs tend to be gradually replaced by a white-light device using the LEDs serving as a light source of a scanner, a back light source of a liquid crystal display screen, or an illuminator, for example. For example, the white light device is produced by using a blue LED chip and yellow phosphor. The yellow inorganic phosphor can emit a yellow fluorescent light after being irradiated by blue light emitted from the blue LED chip. After the yellow fluorescent light is mixed with the original blue light emitted from the blue LED chip, a white light can be generated. 
     Conventionally, a wavelength of the light emitted from the blue LED chip is preferable 450 nm to 455 nm for facilitating the yellow phosphor having desirable light output efficiency. Accordingly, the applicable types of the blue LED chips is quite restricted. The wavelengths of the light emitted from the blue LED chips are thus restricted in a small range so that the color rendering property represented by color rendering index (CRI) of the white-light emitting device is not satisfied. 
     SUMMARY OF THE INVENTION 
     The invention provides a white-light emitting device, in which the light emitted from the LED chips has wideband wavelength to represent good color rendering property. 
     The invention is directed to a white-light emitting device including a carrier, light emitting diode (LED) chips, and a wavelength converting material. The LED chips are disposed on the carrier and electrically connected to the same. An equivalent wavelength of first light emitted from the LED chips is λ. The LED chips are divided into groups. A variation of peak wavelengths of the LED chips in one of the groups is smaller than 5 nm, and λ meets the following equation. 
     
       
         
           
             λ 
             = 
             
               
                 
                   ∑ 
                   1 
                   n 
                 
                  
                 
                   ( 
                   
                     λ 
                      
                     
                         
                     
                      
                     i 
                     × 
                     Ni 
                     × 
                     Ki 
                   
                   ) 
                 
               
               
                 
                   ∑ 
                   1 
                   n 
                 
                  
                 
                   Ni 
                   × 
                   Ki 
                 
               
             
           
         
       
     
     In the equation, n is an integer equal to or larger than 2, λi is an average peak wavelength of the LED chips in one group, Ni is a quantity of the LED chips in the one group, Ki is an average output efficiency of the LED chips in the one group, and the variation of the average peak wavelengths of the LED chips in different groups is Δλi, while 5 nm≦Δλi≦30 nm. The wavelength converting material is excited by the first light to emit a second light, and the first light and the second light are mixed to generate a white light. 
     According to an embodiment of the invention, the carrier includes a circuit board. 
     According to an embodiment of the invention, the carrier includes a lead frame. 
     According to an embodiment of the invention, the LED chips are blue LED chips, and the wavelength converting material includes a yellow phosphor. 
     According to an embodiment of the invention, the quantity of the LED chips in each group is equal to or larger than one. 
     The invention is also directed to a white-light emitting device including a carrier, a light emitting diode (LED) chip, and a wavelength converting material. The LED chip is disposed on the carrier and electrically connected to the same. An equivalent wavelength of a plurality of first light emitted from the LED chip is λ. The first light emitted from the LED chip are divided into groups. A variation of peak wavelengths of the first light in one of the groups is smaller than 5 nm, and λ meets the following equation. 
     
       
         
           
             λ 
             = 
             
               
                 
                   ∑ 
                   1 
                   n 
                 
                  
                 
                   ( 
                   
                     λ 
                      
                     
                         
                     
                      
                     i 
                     × 
                     Ri 
                   
                   ) 
                 
               
               
                 
                   ∑ 
                   1 
                   n 
                 
                  
                 Ri 
               
             
           
         
       
     
     In the equation, n is an integer equal to or larger than 2, λi is an average peak wavelength of the first light in one group, Ri is an average output efficiency of the first light in the one group, and the variation of the average peak wavelengths of the first light in different groups is Δλi, while 5 nm≦Δλi≦30 nm. The wavelength converting material is excited by the first light to emit a second light, and the first light and the second light are mixed to generate a white light. 
     According to an embodiment of the invention, the carrier includes a circuit board. 
     According to an embodiment of the invention, the carrier includes a lead frame. 
     According to an embodiment of the invention, the LED chip is a blue LED chip, and the wavelength converting material includes a yellow phosphor. 
     In view of the above, a plurality of LED chips are used in the invention to generate a mixed light having an equivalent wavelength. The mixed light efficiently excites the wavelength converting material, and is helpful for improving the color rendering property of the white-light emitting device. In addition, a single LED chips emitting light in an equivalent wavelength is also used in the invention for improving the color rendering property of the white-light emitting device. 
     In order to make the aforementioned and other features and advantages of the invention more comprehensible, embodiments accompanying figures are described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1A  illustrates a schematic view of a white-light emitting device according to an embodiment of the invention. 
         FIG. 1B  illustrates a schematic view of a white-light emitting device according to another embodiment of the invention. 
         FIG. 2  illustrates relationships between the wavelength and the output efficiency of the white-light emitting device according to variation of the average peak wavelengths in different groups. 
         FIG. 3  illustrates relationships between variation of the average peak wavelengths in different groups and the normalized output efficiency of the white-light emitting device. 
         FIG. 4  illustrates a schematic view of an LED chip of a white-light emitting device according to another embodiment of the invention. 
         FIG. 5  illustrates a schematic view of another LED chip of a white-light emitting device according to further another embodiment of the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1A  illustrates a schematic view of a white-light emitting device according to an embodiment of the invention. Referring to  FIG. 1A , a white-light emitting device  100 A includes a carrier  110 , LED chips  120   a ˜ 120   e , and a wavelength converting material  130 . The LED chips  120   a ˜ 120   e  are disposed on the carrier  110  and electrically connected to the same. An equivalent wavelength of first light L 1  emitted from the LED chips  120   a ˜ 120   e  is λ. The wavelength converting material  130  is excited by the first light L 1  to emit a second light L 2 , and the first light L 1  and the second light L 2  are mixed to generate a white light. In the present embodiment, the wavelength converting material  130  is, for example, a phosphor. 
     Specifically, the LED chips  120   a ˜ 120   e  have slightly different characteristics, in particular, the peak wavelengths of the light emitted from the LED chips  120   a ˜ 120   e  are different. The LED chips  120   a ˜ 120   e  are divided into groups, and a variation of peak wavelengths of the LED chips  120   a ˜ 120   e  in one of the groups is smaller than 5 nm. The peak wavelength is defined as the wavelength of the strongest light emitted from one of the LED chips  120   a ˜ 120   e.    
     In addition, λ meets an equation: 
     
       
         
           
             λ 
             = 
             
               
                 
                   ∑ 
                   1 
                   n 
                 
                  
                 
                   ( 
                   
                     λ 
                      
                     
                         
                     
                      
                     i 
                     × 
                     Ni 
                     × 
                     Ki 
                   
                   ) 
                 
               
               
                 
                   ∑ 
                   1 
                   n 
                 
                  
                 
                   Ni 
                   × 
                   Ki 
                 
               
             
           
         
       
     
     In the equation, n is an integer equal to or larger than 2, λi is an average peak wavelength of the LED chips  120   a ˜ 120   e  in one group, Ni is a quantity of the LED chips  120   a ˜ 120   e  in the one group, and Ki is an average output efficiency of the LED chips  120   a ˜ 120   e  in the one group. Herein, a variation of the average peak wavelengths of the LED chips  120   a ˜ 120   e  in different groups is Δλi, while 5 nm≦Δλi≦30 nm. In specific, the variation of the average peak wavelengths, that is Δλi, is obtained by subtracting any two average peak wavelengths λi. 
     In an embodiment, the carrier  110  includes a circuit board or a lead frame. Generally, the circuit boards has a variety of types, such as a glass epoxy resin circuit board (FR-4 and FR-5), a metal core printed circuit board (MCPCB), a ceramic circuit board, or the like. Hence, this embodiment does not restrict the types of the circuit boards to be used. 
     For emitting a white light, the LED chips  120   a ˜ 120   e  can be blue LED chips, and the wavelength converting material  130  comprises a yellow phosphor. In the present embodiment, the quantity of the LED chips  120   a ˜ 120   e  in each group is equal to or larger than one. That is to say, the white-light emitting device  100 A is constituted of at least two or more chips. In fact, five LED chips  120   a ˜ 120   e  are used in the present embodiment, but larger quantity or smaller quantity of the LED chips can be used to form the white-light emitting device  100 A in other embodiments.  FIG. 1B  illustrates a schematic view of a white-light emitting device according to another embodiment of the invention. It is exemplified that three LED chips  120   a ˜ 120   c  are disposed inside the white-light emitting device  100 B as shown in  FIG. 1B . 
       FIG. 2  illustrates relationships between the wavelength and the output efficiency of the white-light emitting device according to variation of the average peak wavelengths in different groups. Referring to  FIG. 2 , according to the simulation results, the larger the variation of the average peak wavelengths is, the wider the wavelength of the light emitted from the white-light emitting device ranges. Consequently, the white-light emitting device has superior color rendering property. 
     In other words, different groups of LED chips  120   a ˜ 120   e  are conducive to improve the color rendering property of white-light emitting device  100 A. In addition, the simulation result represents that the color rendering index of a conventional white-light emitting device is 62.7, and the color rendering index of the white-light emitting device designed based on the concept of the present embodiment is 68.6 under the same color temperature such as 6,000 K. Accordingly, the white-light emitting device of the present invention can improve the display quality of a display when the white-light emitting device is applied to the display. 
       FIG. 3  illustrates relationships between variation of the average peak wavelengths in different groups and the normalized output efficiency of the white-light emitting device. According to  FIG. 3 , if the variation of the average peak wavelengths is large, the normalized output efficiency of the white-light emitting device may be reduced. Therefore, the variation of the average peak wavelengths in the present embodiment is from 5 nm to 30 nm so that the white-light emitting device  100 A or  100 B has both good color rendering property and good output efficiency. 
     Several values are provided hereinafter to elaborate the invention which should not be construed as limited to the values set forth herein. Referring to  FIG. 1  continuously, the peak wavelengths of the LED chips  120   a ˜ 120   e  can respectively be 445 nm, 447 nm, 449 nm, 455 nm, and 457 nm. At the same time, the output efficiencies of the LED chips  120   a ˜ 120   e  can be 2.5, 2.4, 2.6, 2.9, and 3.1, respectively. Under such configuration, the LED chips  120   a ,  120   b , and  120   c  can be demarcated as a first group, and the LED chips  120   d  and  120   e  are demarcated as a second group. Herein, the average peak wavelengths λ 1  of the LED chips  120   a ˜ 120   c  in the first group is 447 nm, and the average output efficiency K 1  thereof is 2.5. The average peak wavelengths λ 2  of the LED chips  120   d  and  120   e  in the second group is 456 nm, and the average output efficiency K 2  thereof is 3.0. In specific, the variation Δλi of the average peak wavelengths between the two groups is the difference of 447 nm and 456 nm, that is 9 nm. 
     Hence, the equivalent wavelength of the first light L 1  is: 
     
       
         
           
             λ 
             = 
             
               
                 
                   
                     ( 
                     
                       447 
                       × 
                       3 
                       × 
                       2.5 
                     
                     ) 
                   
                   + 
                   
                     ( 
                     
                       456 
                       × 
                       2 
                       × 
                       3.0 
                     
                     ) 
                   
                 
                 
                   
                     ( 
                     
                       3 
                       × 
                       2.5 
                     
                     ) 
                   
                   + 
                   
                     ( 
                     
                       2 
                       × 
                       3.0 
                     
                     ) 
                   
                 
               
               = 
               
                 451 
                  
                 
                     
                 
                  
                 nm 
               
             
           
         
       
     
     In other words, it is not required that the LED chips  120   a ˜ 120   e  used in the present embodiment have the wavelength of 450 nm to 455 nm to obtain the first light L 1  having the equivalent wavelength λ of 450 nm to 455 nm. Therefore, many types of LED chips can be used as the LED chips  120   a ˜ 120   e  of the present embodiment so that the usage of types of the LED chips  120   a ˜ 120   e  applied in the white-light emitting device  100 A is much flexible. According to  FIG. 2  and  FIG. 3 , the white-light emitting device  100 A has good color rendering property and good output efficiency based on the design. Furthermore, the much flexible usage of types of the LED chips  120   a ˜ 120   e  applied in the white-light emitting device  100 A is apt to reduce the manufacturing cost. 
     Except the foregoing embodiments, the invention further comprises the following embodiment, wherein the white-light emitting device has only one chip.  FIG. 4  illustrates a schematic view of an LED chip of a white-light emitting device according to another embodiment of the invention. Referring to  FIG. 4 , the LED chip  220  can be applied to the white-light emitting device  100 A of  FIG. 1A  to replace the LED chips  120   a ˜ 120   e . Specifically, the LED chips  220  has a plurality of light emitting layer such as a first light emitting layer  220   a  and the second light emitting  220   b . In addition, the LED chip  220  has electrodes  222  and  224 . The present embodiment takes the chip having two light emitting layers as an example. In other embodiments, the light emitting layers can be more than two. For example,  FIG. 5  illustrates a schematic view of another LED chip of a white-light emitting device according to further another embodiment of the invention. The LED chip  520  illustrated in  FIG. 5  has three light emitting layers  220   a - 220   c , for example. 
     Specifically, in the LED chips  220 , the first light emitting layer  220   a  and the second light emitting layer  220   b  are suitable for emitting the first light La and the first light Lb, respectively. The first light La and Lb emitted from the LED chip  220  are divided into groups. A variation of peak wavelengths of the first light La and Lb in one of the groups is smaller than 5 nm. In addition, the equivalent wavelength of the first light La and Lb is 2 which meets an equation: 
     
       
         
           
             λ 
             = 
             
               
                 
                   ∑ 
                   1 
                   n 
                 
                  
                 
                   ( 
                   
                     λ 
                      
                     
                         
                     
                      
                     i 
                     × 
                     Ri 
                   
                   ) 
                 
               
               
                 
                   ∑ 
                   1 
                   n 
                 
                  
                 Ri 
               
             
           
         
       
     
     In the equation, n is an integer equal to or larger than 2, λi is an average peak wavelength of the first light La and Lb in one group, Ri is an average output efficiency of the first light La and Lb in the one group, and the variation of the average peak wavelengths of the first light La and Lb in different groups is Δλi, while 5 nm≦Δλi≦30 nm. 
     In view of the foregoing embodiment, it is helpful for improving the color rendering property of the white-light emitting device by mixing different groups of the light to accomplish the needed wavelength. In addition, the variation of the average peak wavelengths of the first light La and Lb in one of the groups is located in a limited range, therefore, the output efficiency of the LED chip  220  still maintains in an ideal state. 
     In light of the foregoing, the white-light emitting device has good color rendering property by using a plurality of LED chips or disposing a plurality of light emitting layers in a single LED chip. The invention provides a white-light emitting device, in which the usage of the types of the LED chips is much flexible to facilitate mass production. 
     Although the invention has been described with reference to the above embodiments, it will be apparent to one of the ordinary skill in the art that modifications to the described embodiment may be made without departing from the spirit of the invention. Accordingly, the scope of the invention will be defined by the attached claims not by the above detailed descriptions.