Abstract:
An organic light-emitting diode (OLED) device and a manufacturing method thereof are provided. The OLED device comprises more than one light emitting layer. The emissive zone is capable to emit red or long wavelength visible light near the cathode, and emit blue or short wavelength visible light near the anode. The device emits visible light with a lower color temperature at low voltages, and emits visible light with a higher color temperature at higher voltages. By adjusting the input voltage, the device is capable to emit white light or other color lights with desired color temperature.

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention discloses an organic light-emitting diode device and a manufacturing method thereof. By means of the design of the light emitting layer structure, the device emits visible light with a lower color temperature at low voltages, and emits visible light with a higher color temperature at high voltages. By adjusting the input voltage, the device is capable to emit white light or other color lights with desired color temperature. 
     (b) Description of the Prior Art 
     An organic electro-luminescence display or an organic EL display is also referred to as an organic light emitting diode (OLED). C. W. Tang and S. A. Vanslyke et al. of Eastman Kodak Company used a vacuum evaporation method to make it in 1987 first. The hole transporting material and electron transporting material were deposited on transparent indium tin oxide glass, and then a metal electrode was vapor-deposited thereon to form the self-luminescent OLED device. Due to high brightness, fast response speed, light weight, compactness, true color, no difference in viewing angles, no need of LCD type backlight plates as well as a saving in light sources and low power consumption, it has become a new generation display. 
     Referring to  FIG. 1 , there is a cross-sectional view showing a structure of an OLED device according to the prior art. The OLED device structure sequentially comprises, from bottom to top, a transparent substrate  11 , a transparent anode (ITO)  12 , a hole transporting layer (HTL)  13 , an organic light emitting layer (EL)  14 , an electron transporting layer (ETL)  15 , an electron injection layer (EIL)  16 , and a metal cathode  17 . When a forward bias is applied, holes  131  are injected from the anode  12  and electrons  151  are injected from the cathode  17 . Due to the potential difference incurred from the external electrical field, the electrons  151  and the holes  131  will move in the thin film and recombine with each other in the organic light emitting layer  14 . A part of the energy released by the recombination of the electron and hole pairs excites the luminescent molecules in the organic emitting layer  14  to excited-state molecules. When the excited-state molecules fall back to the ground state, a certain portion of the energy is released in a form of photons to emit light related to organic electroluminescence. 
     Referring to  FIG. 2 , there is a cross-sectional view showing a structure of another OLED device according to the prior art. The OLED device was described by C. W. Tang in U.S. Pat. No. 4,356,429 (Eastman Kodak Company, 1982). In the invention, the OLED device structure sequentially comprises, from bottom to top, a transparent substrate  21 , a transparent anode  22 , a hole injection layer  23 , a light emitting layer  24 , and a metal cathode  25 . When a forward bias is applied, holes are injected from the anode  22  and electrons are injected from the cathode  25 . Due to the potential difference incurred from the external electrical field, the electrons and holes will move in the thin film and recombine with each other in the light emitting layer  24 . A part of the energy released by the recombination of the electron and hole pairs excites the luminescent molecules in the light emitting layer  24  to excited-state molecules. When the excited-state molecules fall back to the ground state, a certain portion of the energy is released in a form of photons to emit light related to organic electroluminescence. 
     Referring to  FIG. 3 , there is also a cross-sectional view showing a structure of an OLED device of the prior art. The OLED device was proposed by C. W. Tang in U.S. Pat. No. 4,720,432 (Eastman Kodak Company, 1988). In the invention, the OLED device structure sequentially comprises, from. bottom to top, a transparent substrate  31 , a transparent anode  32 , a hole injection layer  33 , a light emitting layer  34  having an electron transporting function, and a metal cathode  35 . When a forward bias is applied, holes are injected from the anode  32  and electrons are injected from the cathode  35 . Due to the potential difference incurred from the external electrical field, electrons and holes will move in the thin film and recombine with each other in the light emitting layer  34 . A part of the energy released by the recombination of the electron and hole pairs excites the luminescent molecules in the light emitting layer  34  to excited-state molecules. When the excited-state molecules fall back to the ground state, a certain portion of the energy is released in a form of photons to emit light related to organic electroluminescence. 
     Referring to  FIG. 4 , therein illustrated is a doped type OLED device proposed by C. W. Tang et al. in Journal of Applied Physics, vol. 65, p. 3610 (1989). The OLED device structure sequentially comprises, from bottom to top, a transparent substrate  41 , a transparent anode  42 , a hole transporting layer  43 , a single component light emitting layer  44 , a dye-doped light emitting layer  45 , a single component light emitting layer  46 , and a metal cathode  47 , which also can give organic electroluminescence. 
     Referring to  FIG. 5 , therein illustrated is a doped type OLED device proposed by C. H. Chen et al. in Applied Physics Letters, vol. 85, p. 3301 (2004). The OLED device structure sequentially comprises, from bottom to top, a transparent substrate  51 , a transparent anode  52 , a hole injection layer  53 , a hole transporting layer  54 , a dye-doped light emitting layer  55 , an electron transporting layer  56 , an electron injection layer  57 , and a metal cathode  58 , which can give organic electroluminescence. 
     An OLED can emit light of different wavelengths based on the luminescent materials used. White light can be produced by mixing complementary lights. The luminescent materials can be arranged in different layers or can be deposited in the same light emitting layer. Referring to  FIG. 6 , therein illustrated is a white light OLED device with a single light emitting layer proposed by the present inventors in Applied Physics Letters, vol. 88, p. 193501 (2006). The OLED device structure sequentially comprises, from bottom to top, a transparent substrate  61 , a transparent anode  62 , a hole transporting layer  63 , a doped type white light emitting layer  64 , an electron transporting layer  65 , an electron injection layer  66 , and a metal cathode  67 . The white light emitting layer  64  can be composed of a blue light emitting host doped with red light emitting dyes, or further, a blue light emitting host doped with green and red light emitting dyes, which emits white light related to organic electroluminescence. 
     In addition to utilizing complementary lights generated by organic electroluminescence to produce white light, organic electroluminescence and photoluminescence can be used to produce white light. Referring to  FIG. 7 , therein illustrated is a light source device combining an organic layer and a photoluminescent layer proposed by A. R. Duggal et al. in U.S. Pat. No. 6,847,162 (General Electric (GE) Company). The light source  71  sequentially comprises, from bottom to top, an OLED device  72  which emits blue light, a transparent substrate  73 , and a photoluminescent layer  74 . The photoluminescent layer  74  absorbs the blue light emitted by the OLED device  72  and emits yellow light having lower energy. The light source mixes blue light and yellow light to produce white light. 
     The color temperature of white light mixed from monochromatic lights of blue, green, red, and the like can be changed by tuning the intensity of each monochromatic light. Referring to  FIG. 8 , there is a schematic view showing a structure of a color tunable organic electroluminescent light source device proposed by A. R. Duggal et al. in U.S. Pat. No. 6,661,029 (General Electric (GE) Company). The light emitting device  81  comprises an integrated controller  82 , an red light emitting OLED  83 , an green light emitting OLED  84  and an blue light emitting OLED  85 . The multiple monochromatic light OLEDs are connected with circuits to form light emitting device sets  86 ,  87  and  88  having larger light emitting area, respectively. The device further comprises a power source  89  that is electrically connected together to the integrated controller  82  and the OLEDs  83 ,  84  and  85 . The intensity of each monochromatic light can be tuned by the integrated controller  82  to change the color temperature of the mixed white light. 
     As a result of a variety of extensive and intensive studies and discussions, the inventors herein propose an organic light-emitting diode device and a manufacturing method thereof based on their research for many years and plenty of practical experience. By means of the design of the light emitting layer structure of the device, the device is capable to emit white light or other color lights with desired color temperature without additional circuit control and only by adjusting the input voltage. The present invention has been accomplished based on these findings. 
     SUMMARY OF THE INVENTION 
     In view of the above problems, an objective of the present invention is to provide an organic light-emitting diode device and a manufacturing method thereof. The OLED device comprises more than one light emitting layer. The light emitting layer is capable to emit redder or longer wavelength visible light near a cathode, and emit bluer or shorter wavelength visible light near an anode. The device emits visible light with a lower color temperature at low voltages, and emits visible light with a higher color temperature at higher voltages. By adjusting the input voltage, the device is capable to emit white light or other color lights with desired color temperature. 
     Accordingly, to achieve the above objective, an organic light-emitting diode device of the present invention comprises a substrate, a first conductive layer, a hole transporting layer, a first light emitting layer, a second light emitting layer, an electron transporting layer, an electron injection layer, and a second conductive layer. The first light emitting layer is capable to emit bluer or shorter wavelength visible light, and the second light emitting layer is capable to emit redder or longer wavelength visible light. 
     Accordingly, to achieve the above objective, a manufacturing method of an organic light-emitting diode device of the present invention comprises:
     a) providing a substrate;   b) forming a first conductive layer on the substrate;   c) forming a light emitting layer on the first conductive layer; and   d) forming a second conductive layer on the light emitting layer;
 
wherein the light emitting layer is capable to emit bluer or shorter wavelength visible light near the first conductive layer anode (anode), and emit redder or longer wavelength visible light near the second conductive layer (cathode).
   

     In order that the technical features and effects of the present invention may be further understood and appreciated, the preferred embodiments are described below in detail with reference to the related drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects, features and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying related drawings according to exemplary preferred embodiments of an organic light-emitting diode device and a manufacturing method thereof of the present invention. 
         FIG. 1  is a cross-sectional view showing a structure of an OLED device according to the prior art; 
         FIG. 2  is a cross-sectional view showing a structure of another OLED device according to the prior art; 
         FIG. 3  is a cross-sectional view showing a structure of an OLED device of the prior art; 
         FIG. 4  is a cross-sectional view showing a structure of another OLED device of the prior art; 
         FIG. 5  is a cross-sectional view showing a structure of another OLED device of the prior art; 
         FIG. 6  is a cross-sectional view showing a structure of an OLED device of the prior art; 
         FIG. 7  is a cross-sectional view showing a structure of a light source device combining organic electroluminescence and photoluminescence of the prior art; 
         FIG. 8  is a schematic view showing a structure of a color tunable organic electroluminescent light source device of the prior art; 
         FIG. 9  is a cross-sectional view showing a structure of an OLED device according to a preferred embodiment of the present invention; 
         FIG. 10  is a cross-sectional view showing a structure of another OLED device according to another preferred embodiment of the present invention; 
         FIG. 11  is a flow chart of a manufacturing method of an OLED device according to a preferred embodiment of the present invention; 
         FIG. 12  is a diagram showing the luminance and color temperatures of an OLED device varying with voltages according to a preferred embodiment of the present invention; and 
         FIG. 13  is a diagram showing the luminance and color temperatures of an OLED device varying with voltages according to another preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 9 , there is a cross-sectional view showing a structure of an OLED device according to a preferred embodiment of the present invention. In this embodiment, the OLED device structure sequentially comprises, from bottom to top, a substrate  91 , a first conductive layer  92 , a hole transporting layer  93 , a first light emitting layer  94 , a first electron transporting and hole blocking layer  95 , a second light emitting layer  96 , a second electron transporting and hole blocking layer  97 , an electron injection layer  98 , and a second conductive layer  99 . The first conductive layer  92  is deposited on the substrate  91 . The hole transporting layer  93  is deposited on the first conductive layer  92 . The first light emitting layer  94  is deposited on the hole transporting layer  93 . The first electron transporting and hole blocking layer  95  is deposited on the first light emitting layer  94 . The second light emitting layer  96  is deposited on the first electron transporting and hole blocking layer  95 , and the second light emitting layer  96  is further provided thereon with a second electron transporting and hole blocking layer  97 . The electron injection layer  98  is deposited on the second electron transporting and hole blocking layer  97 , and the second conductive layer  99  is deposited on the electron injection layer  98 . 
     As described above, the first emissive zone is capable to emit bluer or shorter wavelength visible light, and the second emissive zone is capable to emit redder or longer wavelength visible light. 
     Referring to  FIG. 10 , there is a cross-sectional view showing a structure of an OLED device according to another preferred embodiment of the present invention. In this embodiment, the OLED device structure sequentially comprises, from bottom to top, a substrate  101 , a first conductive layer  102 , a hole transporting layer  103 , a first light emitting layer  104 , a second light emitting layer  105 , a first electron transporting and hole blocking layer  106 , a third light emitting layer  107 , a second electron transporting and hole blocking layer  108 , an electron injection layer  109 , and a second conductive layer  1010 . The first conductive layer  102  is deposited on the substrate  101 . The hole transporting layer  103  is deposited on the first conductive layer  102 . The first light emitting layer  104  is deposited on the hole transporting layer  103 . The second light emitting layer  105  is deposited on the first light emitting layer  104 . The first electron transporting and hole blocking layer  106  is deposited on the second light emitting layer  105 . The third light emitting layer  107  is deposited on the first electron transporting and hole blocking layer  106 , and the third light emitting layer  107  is further provided thereon with a second electron transporting and hole blocking layer  108 . The electron injection layer  109  is deposited on the second electron transporting and hole blocking layer  108 , and the second conductive layer  1010  is deposited on the electron injection layer  109 . 
     As described above, the first emissive zone is capable to emit blue visible light, and the second emissive zone is capable to emit green visible light, and the third emissive zone is capable to emit red visible light. 
     Moreover, the light emitting layer further comprises more than one fluorescent or phosphorescent luminescent material as the materials of the light emitting layer, or there is/are provided a single organic material or multiple combinations of organic materials as a host material that is mixed with the fluorescent or phosphorescent luminescent material. The light emitting layer further incorporates one or multiple combinations of a carrier transporting material, a carrier injection material, a carrier blocking material or a functional auxiliary material such that the light emitting layer has functionalities. The light emitting layer emits light represented by x coordinate in a range from 0.25 to 0.55 and y coordinate in a range from 0.25 to 0.55 based on a CIE color system. The light emitting layer has a light source that exhibits a color-rendering index greater than  70 . The hole transporting layers  93 ,  103  can generally be made of hole transporting materials, such as poly(3,4-ethylene-dioxythiophene)-poly-(styrenesulfonate) (PEDOT:PSS), N,N′-bis-(1-naphthy)-N,N′-biphenyl-1,1′-biphenyl-4,4′-diamine (NPB), or the like. The electron transporting and hole blocking layers  95 ,  97 ,  106  and  108  can generally be made of materials having an electron transporting and hole blocking function, such as 1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene(TPBi), tris(8-hydroxyquinoline)aluminum (Alq 3 ), or the like. The electron injection layers  98 ,  109  can generally be made of electron injection materials like lithium fluoride (LiF), etc. The second conductive layers  99 ,  1010  can generally be made of conductive materials like Al, etc. The substrates  91 ,  101  can generally be glass substrates, plastic substrates or metal substrates. The first conductive layers  92 ,  102  can generally be indium tin oxide (ITO) layers or indium zinc oxide (IZO) layers. 
     Referring to  FIG. 11 , there is a flow chart of a manufacturing method of an OLED device according to a preferred embodiment of the present invention. The method comprises the following steps: 
     Step S 111 : providing a substrate; 
     Step S 112 : forming a first conductive layer on the substrate; 
     Step S 113 : forming a hole transporting layer on the first conductive layer; 
     Step S 114 : forming a first light emitting layer on the hole transporting layer; 
     Step S 115 : forming a first electron transporting and hole blocking layer on the first light emitting layer; 
     Step S 116 : forming a second light emitting layer on the first electron transporting and hole blocking layer; 
     Step S 117 : forming a second electron transporting and hole blocking layer on the second light emitting layer; 
     Step S 118 : forming an electron injection layer on the second electron transporting and hole blocking layer; and 
     Step S 119 : forming a second conductive layer on the electron injection layer. 
     The first emissive zone is capable to emit bluer or shorter wavelength visible light, and the second emissive zone is capable to emit redder or longer wavelength visible light. The light emitting layer further comprises more than one fluorescent or phosphorescent luminescent material as the materials of the light emitting layer, or there is/are provided a single organic material or multiple combinations of organic materials as a host material that is mixed with the fluorescent or phosphorescent luminescent material. The light emitting layer further incorporates one or multiple combinations of a carrier transporting material, a carrier injection material, a carrier blocking material or a functional auxiliary material such that the light emitting layer has functionalities. The light emitting layer emits light represented by x coordinate in a range from 0.25 to 0.55 and y coordinate in a range from 0.25 to 0.55 based on a CIE color system. The light emitting layer has a light source that exhibits a color-rendering index greater than  70 . The hole transporting layer can generally be made of a hole transporting material, such as PEDOT:PSS, NPB, or the like. The electron transporting and hole blocking layers can generally be made of materials having an electron transporting and hole blocking function, such as TPBi, Alq 3 , or the like. The electron injection layer can generally be made of an electron injection material like LiF, etc. The second conductive layer can generally be made of a conductive material like Al, etc. The substrate can be a glass substrate, plastic substrate or metal substrate. The first conductive layer can generally be made of ITO or IZO. 
     Referring to  FIG. 12 , there is a diagram showing the luminance and color temperatures varying with voltages according to an preferred embodiment of the present invention. 
     Referring to  FIG. 13 , there is a diagram showing the luminance and color temperatures varying with voltages according to another preferred embodiment of the present invention. 
     EXAMPLE 1 
     Example 1 is an OLED device made according to the present invention. With reference to the device structure shown in  FIG. 9 , a glass substrate  91  coated with ITO as a transparent conductive anode  92  was subjected to ultrasonic vibration cleaning sequentially with a detergent, deionized water, acetone and isopropyl alcohol, and then placed in boiling hydrogen peroxide for surface treatment, followed by drying the surface with a nitrogen stream. Under the nitrogen atmosphere, PEDOT:PSS was spin coated to form a 35 nm thick hole transporting layer  93 . Next, the substrate was moved into a vacuum chamber, and the vacuum chamber was exhausted to a pressure of 10 −5  Torr. A 10 nm thick first light emitting layer  94 , a 3 nm thick first electron transporting and hole blocking layer (TPBi)  95 , a 5 nm thick second light emitting layer  96 , a 35 nm thick second electron transporting and hole blocking layer (TPBi)  97 , a 0.7 nm thick LiF electron injection layer  98 , and a 150 nm thick aluminum electrode  99  were sequentially deposited by a heating vapor deposition method. The composition of the first light emitting layer  94  comprises a DPASN blue light emitting material that can emit blue visible light during electroluminescence. The composition of the second light emitting layer  96  comprises DPASN doped with 1 wt % of a red light emitting material DCJTB that can emit red visible light during electroluminescence. The OLED device emits light having a color temperature of 2200K at a voltage of 4V, and emits light having a color temperature of 5800K at a voltage of 12V. The variation of luminance and color temperatures with voltages is shown  FIG. 12 . 
     EXAMPLE 2 
     Example 2 is another OLED device made according to the present invention. With reference to the device structure shown in  FIG. 10 , a glass substrate  101  coated with ITO as a transparent conductive anode  102  was subjected to ultrasonic vibration cleaning sequentially with a detergent, deionized water, acetone and isopropyl alcohol, and then placed in boiling hydrogen peroxide for surface treatment, followed by drying the surface with a nitrogen stream. Under the nitrogen atmosphere, PEDOT:PSS was spin coated to form a 35 nm thick hole transporting layer  103 . Next, the substrate was moved into a vacuum chamber, and the vacuum chamber was exhausted to a pressure of 10 −5  Torr. A 10 nm thick first light emitting layer  104 , a 2 nm thick second light emitting layer  105 , a 3 nm thick first electron transporting and hole blocking layer (TPBi)  106 , a 5 nm thick third light emitting layer  107 , a 35 nm thick second electron transporting and hole blocking layer (TPBi)  108 , is a 0.7 nm thick LiF electron injection layer  109 , and a 150 nm thick aluminum electrode  1010  were sequentially deposited by a heating vapor deposition method. The composition of the first light emitting layer  104  comprises a DPASN blue light emitting material that can emit blue visible light during electroluminescence. The composition of the second light emitting layer  105  comprises DPASN doped with 0.1 wt % of C545T that can emit green visible light during electroluminescence. The composition of the third light emitting layer  107  comprises DPASN doped with 1 wt % of a red light emitting material DCJTB that can emit red visible light during electroluminescence. The OLED device emits light having a color temperature of 2200K at a voltage of 4V, and emits light having a color temperature of 9000K at a voltage of 11V. The variation of luminance and color temperatures with voltages is shown  FIG. 13 .