Abstract:
A light-emitting device includes a photonic crystal layer formed above a light-emitting chip and covered with a phosphor layer for diffusing light emitted from the light-emitting chip. The diffused light further excites the phosphor layer to emit colored light of multiple colors. The multiple colors are then mixed to generate white light.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to light-emitting devices, and more particularly, to a light-emitting device for generating white light through a light-emitting chip. 
         [0003]    2. Description of Related Art 
         [0004]    Light-emitting diodes (LEDs) have the advantages of small size, low energy consumption, long service life, high response speed, safe low voltage operation and better directivity, and therefore are expected to be widely applied in the fields of illumination and display. In recent years, as the luminous efficiency of LEDs has continuously increased, high-power white-light LEDs have gradually replaced conventional incandescent lamps, halogen lamps and phosphor lamps so as to become a new generation of environmentally-friendly light sources, especially when they are powered by solar cells. 
         [0005]    Generally, a yellow YAG (Y 3 Al 5 O 12 :Ce and its derivatives) phosphor is added to a blue-emitting chip to fabricate a white-light LED light source. However, such a white-light LED has the following drawbacks. Firstly, blue light accounts for the majority of the emission spectrum to thereby render the color temperature high and uneven. Hence, to decrease the intensity of blue light or increase the intensity of yellow light, excitation of the yellow phosphor by blue light needs to be strengthened, thereby increasing the fabrication cost. Further, in that the produced wavelengths of the blue-emitting LED will change following a rise in temperature, it is difficult to control the color temperature of the white light source. Furthermore, the weaker emission spectrum of yellow light leads to poor color rendition. 
         [0006]    In addition, a RGB phosphor material can be added to and excited by a UV/blue-emitting LED chip so as to generate white light consisting of triple wavelengths, which has the advantage of high color rendition but low lighting efficiency. 
         [0007]    Alternately, white light can be generated through an RGB-LED chip, which has the advantages of high lighting efficiency and high color rendition. However the generation of light from the RGB-LED chip does have a disadvantage. Depending on the epitaxy materials of the crystal particles for the different colors, the voltage characteristics are different, thereby resulting in higher costs, more complicated control of the wiring design and difficulty in mixing the three color wavelengths produced. 
         [0008]    Taking into account costs and lighting efficiency, the method of forming white light by adding a yellow YAG phosphor to a blue-emitting chip is still the most efficient overall and represents the mainstream. 
         [0009]    To fabricate a conventional LED light-emitting device, an LED chip is adhered to the reflector cup of a leadframe or a surface-mount device (SMD) base through an adhesive or a silver paste. Then, the electrodes of the LED chip are electrically connected to the leadframe or the base through gold wires or aluminum wires. Thereafter, the LED chip is coated with a phosphor before being packaged by an epoxy resin. 
         [0010]      FIG. 1A  shows a conventional light-emitting device. An LED chip  11  is disposed on an UV/visible light mirror  10   a  of a cup-shaped substrate  10 . A short-wave-pass filter  13  is disposed on the top surface of the LED chip  11 . A packaging layer  12  impregnated with phosphor grains  14  fills the cup-shaped substrate  10  to thereby encapsulate the LED chip  11  and the short-wave-pass filter  13 . A glass plate  15  is disposed on the top of the substrate  10 . However, after operating for a while, the LED chip generates heat, and the heat thus generated accumulates in the phosphor layer  14  to thereby adversely affect the service life of the phosphor and lead to light degradation and color offset. 
         [0011]      FIG. 1B  shows another conventional light-emitting device, wherein an LED chip  11 ′ is disposed inside a substrate  10 ′ having a light-transmissive cup shape. A curable layer  13 ′ is formed on the substrate  10 ′ to cover the LED chip  11 ′, and a phosphor layer  14 ′ is disposed on the curable layer  13 ′. 
         [0012]    Light is emitted from the LED chip  11 ′ in all directions and therefore is not uniformly mixed with the phosphor of the phosphor layer  14 ′, thereby adding to the difficulty in controlling the color of the white light source. 
         [0013]    Such disadvantages with the prior art limit the potential of LED lighting. Therefore, it is imperative to overcome the above-described drawbacks of the prior art. 
       SUMMARY OF THE INVENTION 
       [0014]    In view of the above drawbacks of the prior art, the present invention discloses a light-emitting device, which comprises: a substrate; a light-emitting chip disposed on the substrate; a packaging layer disposed on the substrate for encapsulating the light-emitting chip; a phosphor layer disposed on the packaging layer; and a photonic crystal layer disposed on the surface of the light-emitting chip or disposed between the packaging layer and the phosphor layer, the packaging layer being sandwiched between the phosphor layer and the substrate. 
         [0015]    In the above-described light-emitting device, the light-emitting chip is a light-emitting diode (LED). Preferably, the light-emitting chip is a blue-emitting LED, and the phosphor layer comprises red and green phosphors and a protective material encapsulating the phosphors. 
         [0016]    The packaging layer is made of an epoxy resin or a silicone resin, and the profile of the surface of the package layer is planar or curved. The photonic crystal layer comprises a plurality of nanoparticles. 
         [0017]    The present invention further discloses a method for fabricating a light-emitting device, which comprises: disposing at least a light-emitting chip on a substrate; forming a packaging layer on the substrate to encapsulate the light-emitting chip; and forming a photonic crystal layer and a phosphor layer on the packaging layer such that the photonic crystal layer is sandwiched between the packaging layer and the phosphor layer. 
         [0018]    Therein, the step of forming the photonic crystal layer and the phosphor layer further comprises providing a thin film comprising the phosphor layer and the photonic crystal layer, and covering the surface of the packaging layer with the thin film such that the photonic crystal layer is sandwiched between the packaging layer and the phosphor layer. In other embodiments, the photonic crystal layer is formed on the packaging layer and the phosphor layer is further formed on the photonic crystal layer such that the photonic crystal layer is sandwiched between the packaging layer and the phosphor layer. 
         [0019]    The present invention further discloses another method for fabricating a light-emitting device, which comprises: disposing at least a light-emitting chip on a substrate; forming a photonic crystal layer on the surface of the light-emitting chip; forming a packaging layer on the substrate to encapsulate the light-emitting chip and the photonic crystal layer; and forming a phosphor layer on the packaging layer. 
         [0020]    In the above-described two methods, the light-emitting chip is a light-emitting diode (LED). Preferably, the light-emitting chip is a blue-emitting LED, and the phosphor layer comprises red and green phosphors and a protective material encapsulating the phosphors. The packaging layer is made of an epoxy resin or a silicone resin, and the profile of the surface of the package layer is planar or curved. The photonic crystal layer comprises a plurality of nanoparticles and is formed by imprinting or E-beam lithography (EBL). 
         [0021]    Therefore, the present invention uses the photonic crystal layer to disrupt the total reflection characteristic of the packaging layer such that light emitted from the LED chip can be diffused and emitted out of the packaging layer instead of being limited inside the packaging layer due to the total reflection, thereby efficiently improving the light extracting efficiency and overcoming the conventional drawback of low lighting efficiency. 
         [0022]    Further, the phosphor layer is disposed outside the packaging layer so as to not be directly affected by heat generated by the LED chip, thereby reducing damage to the phosphor layer due to heat exposure, extending the service life of the phosphor layer and allowing sufficient excitation by the LED chip and good light mixing. As such, highly efficient light output is achieved and the drawbacks of light degradation and color offset are overcome. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0023]      FIGS. 1A and 1B  are cross-sectional side views of conventional light-emitting devices; 
           [0024]      FIGS. 2A to 2D  are cross-sectional side views showing a method for fabricating a light-emitting device according to a first embodiment of the present invention; 
           [0025]    FIG.  2 D′ is a cross-sectional side view showing another embodiment of the step of  FIG. 2D , and FIG.  2 D″ is a cross-sectional side view showing another embodiment of the substrate structure of the light-emitting device; 
           [0026]      FIGS. 3A to 3D  are cross-sectional side views showing a method for fabricating a light-emitting device according to a second embodiment of the present invention; and 
           [0027]    FIG.  3 D′ is a cross-sectional side view showing another embodiment of the packaging layer of the light-emitting device. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0028]    The following illustrative embodiments are provided to illustrate the disclosure of the present invention, these and other advantages and effects being apparent to those skilled in the art after reading the specification. 
       First Embodiment 
       [0029]    The present invention relates to an LED light source technique that combines a photonic crystal technique with a conventional packaging technique.  FIGS. 2A to 2D  show a method for fabricating a light-emitting device according to a first embodiment of the present invention. 
         [0030]    As shown in  FIG. 2A , at least a light-emitting chip  21  such as a light-emitting diode (LED) is disposed on a substrate  20  such as a circuit board, a leadframe or a reflector cup. The first embodiment is illustrated with a circuit board that serves as the substrate  20 . In the first embodiment, the light-emitting chip  21  is a blue-emitting LED, and the light-emitting chip  21  is electrically connected to the substrate  20  through conductive wires  210 . But it should be noted that the electrical connecting method is not limited to wire-bonding. Many other techniques already known in the art can be used. Since the electrical connecting method is not the characteristic of the present invention, detailed description thereof is omitted herein. 
         [0031]    As shown in  FIG. 2B , a packaging layer  22  made of a transparent adhesive material is formed on the substrate  20  to encapsulate the light-emitting chip  21  and the conductive wires  210 . The packaging layer  22  may be made of an epoxy resin or a silicone resin. In the present embodiment, the profile of the surface of the packaging layer  22  is planar. In other embodiments, the profile of the surface of the packaging layer  22  can be curved as needed. 
         [0032]    As shown in  FIG. 2C , a photonic crystal layer  23  is formed on the packaging layer  22 , the photonic crystal layer  23  comprising a plurality of nanoparticles regularly arranged in two layers. The photonic crystal layer  23  is made of the material of which an auto-focus layer disclosed in Taiwan Patent Publication No. 200835100 is made. In the present embodiment, the photonic crystal layer  23  is formed on the packaging layer  22  by E-beam lithography (EBL). Further, the photonic crystal layer  23  can be arranged according to the requirements of the light-emitting surface. 
         [0033]    The photonic crystal layer  23  is disposed on the surface of the packaging layer  22  to disrupt the total reflection characteristic of the packaging layer  22  so as to allow light emitted from the LED chip to be diffused and emitted out of the packaging layer  22  instead of being confined to the packaging layer  22  due to the total reflection, thereby efficiently improving the light extracting efficiency. 
         [0034]    As shown in  FIG. 2D , a phosphor layer  24  is formed on the photonic crystal layer  23 . The phosphor layer  24  comprises red and green light-producing phosphors  24   a . A transparent protective material  24   b  encapsulates the phosphors  24   a.    
         [0035]    Blue light emitted from the blue-emitting chip  21  travels in a single direction instead of various directions upon penetrating the photonic crystal layer  23 , as shown in  FIG. 2D , so as for the blue light to uniformly excite the red and green phosphors of the phosphor layer  24  for emitting red light and green light, in addition to the blue light that passes unchanged. Then, the red, green and blue lights are sufficiently mixed to generate white light. 
         [0036]    The present invention generates a white light source by adding red and green phosphors  24   a  to a blue-emitting chip  21 . Therefore, the present invention has the advantage of high color rendition, which enables the color temperature of the white light source to be easily controlled. Further, the light extracting efficiency is improved through the photonic crystal layer  23 , thereby overcoming the low light efficiency of the prior art. 
         [0037]    Alternatively, as shown in FIG.  2 D′, the photonic crystal layer  23  is formed on the phosphor layer  24  so as to form a thin film  25  comprising the photonic crystal layer  23  and the phosphor layer  24 , and then the thin film  25  is imprinted to cover the surface of the packaging layer  22 . As a result, the photonic crystal layer  23  is sandwiched between the packaging layer  22  and the phosphor layer  24 . 
         [0038]    Intended to integrate the technology related to the phosphor layer  24  with the technology related to the photonic crystal layer  23 , the present invention uses a photonic crystal layer  23  to disrupt the total reflection of the packaging layer so as to improve the light-extracting efficiency. Further, the phosphor layer  24  of the present invention is disposed on the photonic crystal layer  23  instead of directly encapsulating an LED chip as in the prior art. Thus, the photonic crystal layer  23  serves as an intermediate buffer between the phosphors  24   a  and the light-emitting chip  21  so as to prevent heat generated by the light-emitting chip  21  from directly affecting the phosphors  24   a , thereby preventing damage which might otherwise occur to the phosphors  24   a  due to the heat, extending the service life of the phosphors  24   a  and allowing sufficient excitation with plenty of light from the light-emitting chip  21  and light mixture. As such, a highly efficient light output is achieved. 
         [0039]    The present invention further provides a light-emitting device, comprising: a substrate  20 ; a light-emitting LED chip  21  disposed on the substrate  20 ; a packaging layer  22  disposed on the substrate  20   a  for encapsulating the light-emitting chip  21 , wherein the packaging layer  22  may be made of an epoxy resin or a silicone resin, and the profile surface of the packaging layer  22  is a planar surface or a curved surface; a phosphor layer  24  disposed on the packaging layer  22 ; and a photonic crystal layer  23  disposed between the packaging layer  22  and the phosphor layer  24  and comprising a plurality of nanoparticles. 
         [0040]    The phosphor layer  24  comprises red and green phosphors  24   a . A transparent protective material  24   b  encapsulates the phosphors  24   a.    
         [0041]    Further, as shown in FIG.  2 D″, a substrate  20 ′ is provided, which is a reflector cup with its opening facing upwards. At least a light-emitting chip  21  is disposed in the opening of the substrate  20 ′. The packaging layer  22  fills the opening to thereby encapsulate the light-emitting chip  21  so as to protect the light-emitting chip  21  against external erosion. Another purpose of the packaging layer  22  is to allow the reflector cup to collect light emitted from interface surfaces and side surfaces of the light-emitting chip  21 . A photonic crystal layer  23 ′ comprising a plurality of nanoparticles and a phosphor layer  24  comprising red and green phosphors are sequentially formed on the packaging layer  22  so as to form a light-emitting device. 
       Second Embodiment 
       [0042]      FIGS. 3A to 3D  show a method for fabricating a light-emitting device according to a second embodiment of the present invention. The second embodiment is mostly similar to the first embodiment, but the second embodiment differs from the first embodiment in terms of the position of the photonic crystal layer. Therefore, description of similar parts of the first and second embodiments is omitted herein, and the differences between the first and second embodiments are detailed as follows. 
         [0043]    As shown in  FIG. 3A , at least a light-emitting chip  31  is disposed on a substrate  30 . 
         [0044]    As shown in  FIG. 3B , a photonic crystal layer  33  is formed on the surface of the light-emitting chip  31 . The photonic crystal layer  33  comprises a plurality of nanoparticles arranged in one layer. The photonic crystal layer  33  is formed on the light-emitting chip  31  by imprinting or E-beam lithography (EBL). 
         [0045]    As shown in  FIG. 3C , a packaging layer  32  is formed on the substrate  30  to encapsulate the light-emitting chip  31  and the photonic crystal layer  33 . The photonic crystal layer  33  is disposed on the surface of the light-emitting chip  31  to prevent total reflection of light from taking place at the interface of the light-emitting chip  31  in the packaging layer  32 , thereby allowing the light emitted from the light-emitting chip  31  to be diffused and emitted out of the packaging layer  32  and accordingly increasing the light extracting efficiency. 
         [0046]    As shown in  FIG. 3D , a phosphor layer  34  is formed on the packaging layer  32 . Further, as shown in FIG.  3 D′, in other embodiments, the profile of the surface of the packaging layer  32 ′ can be curved such as hemispherical, and the phosphor layer  34 ′ is formed in a shape corresponding to the profile of the packaging layer  32 ′. 
         [0047]    Further, the photonic crystal layer  33  can be arranged according to the shape of the light-emitting surface of the light-emitting chip  31 , as shown in FIG.  3 D′. 
         [0048]    The present invention further provides a light-emitting device, comprising: a substrate  30 ; a light-emitting LED chip  31  disposed on the substrate  30 ; a packaging layer  32  disposed on the substrate  30  for encapsulating the light-emitting chip  31 , wherein the packaging layer  32  is made of an epoxy resin or a silicone resin and the profile of the surface of the packaging layer  32  is planar or curved; a photonic crystal layer  33  disposed on the surface of the light-emitting chip  31  and comprising a plurality of nanoparticles; and a phosphor layer  34  disposed on the packaging layer  32 . 
         [0049]    The phosphor layer  34  comprises red and green phosphors  34   a . A protective material  34   b  encapsulates the phosphors  34   a.    
         [0050]    The phosphor layers  24 ,  34  are disposed to the outside of the packaging layers  22 ,  32  so as not to be in direct contact with the light-emitting chips  21 ,  31 , thereby efficiently preventing heat generated by the light-emitting chips  21 ,  31  from adversely affecting the service life of the phosphors  24   a ,  34   a  of the phosphor layers  24 ,  34  and avoiding the conventional drawbacks of light degradation and color offset. 
         [0051]    Further, the present invention uses the photonic crystal layers  23 ,  33  to collect and guide light such that light emitted from the light-emitting chips  21 ,  31  can uniformly enter the phosphor layers  24 ,  34  and sufficiently mix with the phosphors  24   a ,  34   a , thereby efficiently generating white light with good color. 
         [0052]    Therefore, the present invention uses the photonic crystal layer to disrupt the total reflection characteristic of the packaging layer such that light emitted from the LED chip can be diffused and emitted out of the packaging layer instead of being confined to the packaging layer due to the total reflection, thereby efficiently improving the light extracting efficiency. 
         [0053]    Further, the phosphor layer is disposed outside the packaging layer so as to not be directly affected by heat generated by the LED chip, thereby reducing damage which might otherwise occur to the phosphor layer due to the heat, extending the service life of the phosphor layer and allowing sufficient excitation with plenty of light from the LED chip and good light mixing. As such, highly efficient light output is achieved. 
         [0054]    The above-described descriptions of the detailed embodiments are intended to illustrate the preferred implementation according to the present invention but are not intended to limit the scope of the present invention. Accordingly, many modifications and variations can be made to the embodiments by persons skilled in the art should and yet still fall within the scope of present invention defined by the appended claims.