Abstract:
An organic light emitting diode (OLED) with nano-dots and a fabrication method thereof are disclosed. The OLED apparatus comprises a substrate, a first electrically conductive layer, a first emission-auxiliary layer, an emissive layer, a second emission-auxiliary layer and a second electrically conductive layer. Its fabrication method is described below. Nano-dots with functional groups on the surface are incorporated into the emissive layer, the first emission-auxiliary layer or the second emission-auxiliary layer to form a layered electro-luminescent structure. By using the fabrication method, the resultant efficiency of the OLEDs can be markedly enhanced.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    (a) Field of the Invention 
         [0002]    The present invention discloses an organic light emitting diode with nano-dots and a fabrication method thereof. Nano-dots with functional groups on the surface are incorporated into an emissive layer, a first emission-auxiliary layer or a second emission-auxiliary layer to form a layered electro-luminescent structure. By using the fabrication method, the efficiency of the OLEDs can be markedly enhanced. 
         [0003]    (b) Description of the Prior Art 
         [0004]    An organic electro-luminescence display is referred to as an organic light emitting diode (OLED). C. W. Tang and S. A. VanSlyk et al. of Eastman Kodak Company used a vacuum evaporation method to make it in 1987. The hole transporting material and the electron counterpart were respectively deposited on transparent indium tin oxide (abbreviated as ITO) glass, and then a metal electrode was vapor-deposited thereon to form the self-luminescent OLED apparatus. Due to high brightness, fast response speed, light weight, compactness, true color, no difference in viewing angles, no need of liquid crystal display (LCD) type backlight plates as well as a saving in light sources and low power consumption, it has become a new generation display. 
         [0005]    Referring to  FIG. 1 , there is a cross-sectional view showing a structure of an OLED apparatus of the prior art. The structure was proposed by Steven A. Vanslyke et al. of Eastman Kodak Company in U.S. Pat. No. 5,061,569 (1991). In the invention, the OLED apparatus structure sequentially comprises, from bottom to top, a transparent substrate  11 , a transparent anode (indium tin oxide, ITO)  12 , a hole transporting layer (HTL)  13 , an organic emissive 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  1301  are injected from the anode  12  and electrons  1501  are injected from the cathode  17 . Due to the potential difference resulted from the external electrical field, the electrons  1501  and holes  1301  move in the thin film and hence recombine in the organic emissive layer  14 . A part of the energy released by the recombination of the electron and hole pairs excites the emissive molecules from a ground-state to an excited-state in the organic emissive layer  14 . As the emissive molecules fall back form the excited-state to the ground state, a certain portion of the energy is released to emit light. 
         [0006]      FIG. 2  illustrates a doped type OLED apparatus proposed by C. H. Chen et al. in Applied Physics Letters, vol. 85, p. 3301 (2004). The OLED apparatus structure sequentially comprises, from bottom to top, a transparent substrate  18 , a transparent anode  19 , a hole injection layer  20 , a hole transporting layer  21 , a dye-doped emissive layer  22 , an electron transporting layer  23 , an electron injection layer  24 , and a metal cathode  25  to emit light. 
         [0007]      FIG. 3  is also a cross-sectional view showing a structure of an OLED apparatus of the prior art, which was proposed by Raychaudhuri et al. of Eastman Kodak Company in TW Pat. No. 497283 (2002). In the invention, the OLED apparatus structure sequentially comprises, from bottom to top, a transparent substrate  26 , a transparent anode  27 , a hole injection layer  28 , a hole transporting layer  29 , an emissive layer  30 , an electron transporting layer  31 , a first buffer layer  32 , a second buffer layer  33 , and a metal cathode  34 . The first buffer layer is of alkali halide, and the second buffer layer is of phthalocyanine. When a forward bias is applied, holes and electrons can recombine and in turn emit light in the emissive layer  30 . 
         [0008]    Referring to  FIG. 4 , there is a cross-sectional view showing a structure of another OLED apparatus of the prior art. The structure was proposed by Hieronymus Andriessen et al. of AGFA Gevaert in U.S. Pat. No. 6,602,731 (2003). In the invention, the OLED apparatus structure sequentially comprises, from bottom to top, a transparent substrate  35 , a transparent anode  36 , an emissive layer  37 , and a metal cathode  38 . The emissive layer is composed of inorganic quantum dots CuS and ZnS. When a forward bias is applied, holes and electrons can recombine and hence emit light in the emissive layer  37 . 
         [0009]    Also, referring to  FIG. 5 , there is a cross-sectional view showing a structure of another OLED apparatus of the prior art. The structure was proposed by Dietrich Bertram et al. in U.S. Pat. App. No. 2006/0170331 A1 (2006). In the invention, the OLED apparatus structure sequentially comprises, from bottom to top, a transparent substrate  39 , a transparent anode  40 , an emissive layer  41 , and a metal cathode  42 . The emissive layer is composed of inorganic composite quantum dots, CdSe/CdS. CdS forms the core of the quantum dot, and CdSe forms the outer shell. When a forward bias is applied, holes and electrons can recombine and hence emit light in the emissive layer  41 . 
         [0010]    Referring to  FIG. 6 , there is also a cross-sectional view showing a structure of an OLED apparatus of the prior art. The structure was proposed by Anil Raj Duggal et al. of General Electric Company in U.S. Pat. No. 6,777,724 (2004). In the invention, the OLED apparatus structure sequentially comprises, from bottom to top, a transparent substrate  43 , a transparent anode  44 , an emissive layer  45 , and a metal cathode  46 . The emissive layer comprises organic/inorganic composite quantum dots incorporated uniformly in an organic material. Each organic/inorganic composite quantum dot comprises: (Y 1-x-y Gd x Ce y )Al 5 O 12 , (Y 1-x Ge x ) 3 (Al 1-y Ga y )O 12 , (Y 1-x-y Gd x Ce y ) 3 (Al 5-z Ga z )O 12  or (Gd 1-x Ce x )Sc 2 Al 3 O 12  where 0≦x≦1, 0≦y≦1, 0≦z&lt;5 and x+y≦1. When a forward bias is applied, holes and electrons can recombine and hence emit light in the emissive layer  45 . 
         [0011]    Also, referring to  FIG. 7 , there is a cross-sectional view showing a structure of an OLED apparatus of the prior art. The structure was proposed by Rafat Ata Mustafa hikmet et al. of Koninklijke Philips Electronics in U.S. Pat. App. No. 2007/0077594 A1 (2004). In the invention, the OLED apparatus structure sequentially comprises, from bottom to top, a transparent substrate  47 , a transparent anode  48 , an emissive layer  49 , and a metal cathode  50 . The emissive layer comprises inorganic composite quantum dots incorporated uniformly in a polymer. Each inorganic composite quantum dot is composed of a II-VI group semiconductor material covering a III-V counterpart. When a forward bias is applied, holes and electrons can recombine and hence emit light in the emissive layer  49 . 
         [0012]    Referring to  FIG. 8 , there is a cross-sectional view showing a structure of another OLED apparatus. The structure was proposed by Mihri Ozkan et al. of the Regents of the University of California in U.S. Pat. No. 7,132,787 (2006). In the invention, the OLED apparatus structure sequentially comprises, from bottom to top, a transparent substrate  51 , a transparent anode  52 , a hole transporting layer  53 , an emissive layer  54 , an electron transporting layer  55 , and a metal cathode  56 . The emissive layer is composed of inorganic quantum dots CdSe to emit light. When a forward bias is applied, holes and electrons can recombine and hence emit light in the emissive layer  54 . 
         [0013]    Referring to  FIG. 9 , it is shown an OLED apparatus of the prior art. The structure was proposed by T. H. Liu et al. in TW Pat. No. 200618664 (2006). The OLED apparatus structure sequentially comprises, from bottom to top, a transparent substrate  57 , a transparent anode  58 , a hole transporting layer  59 , an emissive layer  60 , an electron transporting layer  61 , an inorganic layer  62 , and a metal cathode  63  to emit light. 
         [0014]    Referring to  FIG. 10 , there is a cross-sectional view showing a structure of another OLED apparatus of the prior art. The structure was proposed by J. H. Jou of Tsing Hua University in TW Pat. No. 200608614 (2006). In the invention, the OLED apparatus structure sequentially comprises, from bottom to top, a transparent substrate  64 , a transparent anode  65 , a hole transporting layer  66 , and an emissive layer  67  wherein the emissive layer comprises a plurality of organic/inorganic composite quantum dots incorporated in a polymer and each organic/inorganic composite quantum dot comprises: a ZnX quantum dot (X is selected from the group consisting of S, Se, Te and the combination thereof) and an organic molecule covering the surface of the quantum dot, an electron transporting layer  68 , and a metal cathode  69 . When a forward bias is applied, holes are injected from the anode  65  and electrons are injected from the cathode  69 . Due to the potential difference resulted from the external electrical field, electrons and holes inject in the thin film and further recombine in the emissive layer  67 . The quantum dots in the emissive layer can increase the carrier-recombination efficiency to emit light. 
         [0015]    Referring to  FIG. 11 , there is a cross-sectional view showing a structure of another OLED apparatus of the prior art. The structure was proposed by J. H. Jou of Tsing Hua University in TW Pat. App. No. 096120455 (2007). In the invention, the OLED apparatus structure sequentially comprises, from bottom to top, a transparent substrate  70 , a transparent anode  71 , a hole transporting layer  72 , an emissive layer  73 , an electron transporting layer  74 , and a metal cathode  75 . The hole transporting layer comprises poly(ethylenedioxythiophene): poly(styrene sulfonic acid) (PEDOT: PSS) doped with nano dots. The nano dot is synthesized by a sol-gel method and its chemical formula is M x O y  where M is metal (titanium (Ti), zinc (Zn), silver (Ag), copper (Cu), nickel (Ni), tin (Sn), iron (Fe)) and inorganic metalloid (silicon (Si)), and O is an oxygen atom. By using the above fabrication method, the resultant efficiency of the OLED can be markedly enhanced. 
         [0016]    As a result of a variety of extensive and intensive studies and discussions, the inventors herein propose an enhanced high efficiency organic light emitting diode with nano-dots synthesized by a sol-gel method and a fabrication method thereof based on their research for many years and plenty of practical experience, thereby accomplishing the foregoing expectations. 
       SUMMARY OF THE INVENTION 
       [0017]    In view of the above problems, the present invention discloses an organic light emitting diode with nano-dots and a fabrication method thereof. The OLED apparatus comprises a substrate, a first electrically conductive layer, a first emission-auxiliary layer, an emissive layer, a second emission-auxiliary layer and a second electrically conductive layer. Its fabrication method is described below. Nano-dots with functional groups on the surface are incorporated into the emissive layer, the first emission-auxiliary layer or the second emission-auxiliary layer to form a layered electro-luminescent structure. By using the fabrication method, the resultant efficiency of the OLEDs can be markedly enhanced. 
         [0018]    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 
         [0019]    The objects, features and advantages of the invention will become more apparent from the following detailed description of the exemplary preferred embodiments of an organic light emitting diode with nano-dots and a fabrication method thereof according to the present invention with reference to the related drawings. 
           [0020]      FIG. 1  is a cross-sectional view showing a structure of an OLED apparatus according to the prior art. 
           [0021]      FIG. 2  is a cross-sectional view showing a structure of another OLED apparatus according to the prior art. 
           [0022]      FIG. 3  is a cross-sectional view showing a structure of an OLED apparatus of the prior art. 
           [0023]      FIG. 4  is a cross-sectional view showing a structure of another OLED apparatus of the prior art. 
           [0024]      FIG. 5  is a cross-sectional view showing a structure of another OLED apparatus of the prior art. 
           [0025]      FIG. 6  is a cross-sectional view showing a structure of an OLED apparatus of the prior art. 
           [0026]      FIG. 7  is a cross-sectional view showing a structure of an OLED apparatus of the prior art. 
           [0027]      FIG. 8  is a cross-sectional view showing a structure of another OLED apparatus of the prior art. 
           [0028]      FIG. 9  is a cross-sectional view showing a structure of an OLED apparatus of the prior art. 
           [0029]      FIG. 10  is a cross-sectional view showing a structure of another OLED apparatus of the prior art. 
           [0030]      FIG. 11  is a cross-sectional view showing a structure of another OLED apparatus of the prior art. 
           [0031]      FIG. 12  is a cross-sectional view showing a structure and a schematic view showing the energy levels of an OLED apparatus according to the present invention. 
           [0032]      FIG. 13  is a flow chart of a fabrication method of an OLED apparatus according to the present invention. 
           [0033]      FIG. 14  is a cross-sectional view showing a structure and a schematic view showing the energy levels of an OLED apparatus according to a preferred embodiment of the present invention. 
           [0034]      FIG. 15  is a schematic view showing the energy levels of an OLED apparatus according to a preferred embodiment of the present invention. 
           [0035]      FIG. 16  is a cross-sectional view showing a structure and a schematic view showing the energy levels of another OLED apparatus according to a preferred embodiment of the present invention. 
           [0036]      FIG. 17  is an energy level diagram of another OLED apparatus according to a preferred embodiment of the present invention. 
           [0037]      FIG. 18  is a cross-sectional view showing a structure and a schematic view showing the energy levels of an OLED apparatus according to an embodiment of the prior art. 
           [0038]      FIG. 19  is an energy level diagram of an OLED apparatus according to an embodiment of the prior art. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]    Referring to  FIG. 12 , there is a cross-sectional view showing a structure of an OLED according to a preferred embodiment of the present invention. The OLED structure sequentially comprises, from bottom to top, a substrate  76 , a first electrically conductive layer  77 , a first emission-auxiliary layer  78  doped with nano-dots, a dye-doped light emissive layer  79 , a second emission-auxiliary layer  80  and a second electrically conductive layer  81 . The first electrically conductive layer  77  is deposited on the substrate  76 . The first emission-auxiliary layer  78  doped with nano-dots is deposited on the first electrically conductive layer  77 . The emissive layer  79  is deposited on the first emission-auxiliary layer  78  doped with nano-dots. The second emission-auxiliary  80  is deposited on the emissive layer  79 , and the second electrically conductive layer  81  is deposited on the second emission-auxiliary layer  80 . 
         [0040]    As described above, the dye-doped emissive layer  79  comprises a host material and more than one guest material which can be a fluorescent or phosphorescent emitter. Moreover, the first emission-auxiliary layer  78  doped with nano-dots is a composite of a hole transporting material, poly(ethylenedioxythiophene): poly(styrene sulfonic acid) (PEDOT: PSS), and nano-dots with functional groups on its surface (polymeric nano-dots). The chemical formula of the nano-dots is M x O y R z  where M is a metal, transition metal, metalloid or metal alloy, O is an oxygen atom and R is an organic group. The metal is selected from the group consisting of aluminum (Al), tin (Sn), magnesium (Mg) and calcium (Ca). The transition metal is selected from the group consisting of titanium (Ti), manganese (Mn), zinc (Zn), gold (Au), silver (Ag), copper (Cu), nickel (Ni) and iron (Fe). The metalloid is silicon (Si). The organic group is selected from the group consisting of amino, alkyl, alkenyl and hydroxyl. In addition, the surface charges of the nano-dots measured by means of an electrophoresis light scattering method are from +1 to +200 mV or from −1 to −200 mV. The doping weight percentage of the nano-dots is from 0.1 to 15 wt %, and their particle diameters are in the range of 1 to 30 nm. The second emission-auxiliary layer  80  comprises an electron transporting material and an electron injection material. The electron transporting material can be 1,3,5-tris(N-phenyl-benzimidazol-2-yl)benzene (TPBi), tris(8-hydroxyquinoline) aluminum (Alq 3 ), or the like, and the electron injection material can be lithium fluoride (LiF), cesium fluoride (CsF), or the like. The second electrically conductive layer  81  can generally be made of electrically conductive materials like aluminum (Al), silver (Ag), etc. The substrate  76  can generally be a glass substrate, a plastic substrate or a metal substrate. The first electrically conductive layer  77  can generally be an indium tin oxide (ITO) layer or an indium zinc oxide (IZO) layer. 
         [0041]    Referring to  FIG. 13 , there is a flow chart of a fabrication method of an OLED according to a preferred embodiment of the present invention. The method comprises the following steps: 
         [0042]    Step S 82 : providing a substrate; 
         [0043]    Step S 83 : forming a first electrically conductive layer on the substrate; 
         [0044]    Step S 84 : forming a first emission-auxiliary layer doped with nano-dots on the first electrically conductive layer; 
         [0045]    Step S 85 : forming a dye-doped emissive layer on the first emission-auxiliary layer doped with the nano-dots; 
         [0046]    Step S 86 : forming a second emission-auxiliary layer on the emissive layer; 
         [0047]    Step S 87 : forming a second electrically conductive layer on the second emission-auxiliary layer; 
         [0048]    The composition of the emissive layer comprises a host material and more than one guest material, comprising a fluorescent emissive material or phosphorescent emissive material. The first emission-auxiliary layer doped with the nano-dots is a composite of a hole transporting material, poly(ethylenedioxythiophene): poly(styrene sulfonic acid) (PEDOT: PSS), and nano-dots with functional groups on its surface (polymeric nano-dots). The chemical formula of the nano-dots is M x O y R z  where M is a metal, transition metal, metalloid or metal alloy, O is an oxygen atom and R is an organic group. The metal is selected from the group consisting of aluminum (Al), tin (Sn), magnesium (Mg) and calcium (Ca). The transition metal is selected from the group consisting of titanium (Ti), manganese (Mn), zinc (Zn), gold (Au), silver (Ag), copper (Cu), nickel (Ni) and iron (Fe). The metalloid is silicon (Si). The organic group is selected from the group consisting of amino, alkyl, alkenyl and hydroxyl. In addition, the surface charges of the nano-dots measured by means of an electrophoresis light scattering method are from +1 to +200 mV or from −1 to −200 mV. The doping concentration of the nano-dots is from 0.1 to 15 wt %, and their particle diameters are in the range of 1 to 30 nm. The second emission-auxiliary layer comprises an electron transporting material and an electron injection material. The electron transporting material can be TPBi and Alq 3 , etc., and the electron injection material can be LiF, CsF, or the like. The second electrically conductive layer can generally be made of electrically conductive materials like Al, Ca and Ag, etc. The substrate can generally be a glass substrate, a plastic substrate or a metal substrate. 
         [0049]    Referring to Table 1, it is a comparative table showing the power efficiency of exemplary examples according to the present invention and the comparative example as follows. 
       EXAMPLE 1 
       [0050]    Example 1 is an OLED apparatus made according to the present invention. With reference to the apparatus structure shown in  FIG. 14  and the energy level diagram shown in  FIG. 15 , its fabrication method is described below. The device was fabricated by a solution process using an ITO coated glass substrate. The substrate  88  is cleaned in ultrasonic baths of detergent, de-ionized water, acetone and isopropyl alcohol in turn, and then treated with the boiling hydrogen peroxide. The resulted substrate is purged with nitrogen and then placed into a nitrogen glove box for the solution process. 
         [0051]    The first step is to spin coat a 35 nm first emission-auxiliary layer  90  on the pre-cleaned first electrically conductive layer  89  under nitrogen. The first emission-auxiliary layer  90  is composed of PEDOT: PSS doped with nano-dots which possesses positive surface-charge. The second step is to deposit a 35 nm blue emissive layer  91  via wet-process. A 32 nm electron transporting layer of TPBi is then deposited at 2×10 −5  torr. Finally, a 0.7 nm second emission-auxiliary layer  92  of lithium fluoride and a 150 nm aluminum layer  93  are sequentially deposited on the ITO transparent conductive glass by thermal evaporation. 
         [0052]    10 nm of nano-dots possessing positive surface-charge is used to incorporate into aqueous PEDOT: PSS in the first emission-auxiliary layer. In the emissive layer, toluene is used to be the solvent, and the host material of 4,4′-bis(carbazol-9-yl) biphenyl (CBP) doped with 16 wt % blue emitter of bis(3,5-difluoro-2-(2-pyridyl)-phenyl-(2-carboxypyridyl) iridium (III) (FIrpic) is used to prepare the emissive solution. 
         [0053]    The first emission-auxiliary layer doped with the nano-dots possessing positive surface-charge can effectively block holes and increase the electron/hole-injection balance and recombination efficiency, thereby markedly enhancing the efficiency of the OLED. The resultant power efficiency at 100 cd/m 2  was increased from 18 to 37 lm/W, an increase of 205. The blue OLED exhibits CIE color coordinates of (0.18, 0.35). 
       EXAMPLE 2 
       [0054]    Example 2 is an OLED apparatus made according to the present invention. With reference to the apparatus structure shown in  FIG. 16  and the schematic energy level diagram shown in  FIG. 17 , 10 nm of nano-dots possessing negative surface-charge is incorporated into aqueous PEDOT: PSS in an appropriate concentration to form an emission-auxiliary material  96 . 
         [0055]    The first emission-auxiliary layer suitably doped with the nano-dots possessing positive surface-charge can effectively trap holes and increase the electron/hole-injection balance and recombination efficiency, thereby markedly enhancing the efficiency of the OLED. The resultant power efficiency at 100 cd/m 2  was increased from 18 to 31 lm/W, an increase of 172. The blue OLED exhibits CIE color coordinates of (0.18, 0.34). 
       Comparative Example 
       [0056]    Comparative Example is an OLED apparatus made according to the prior art. The apparatus structure is as shown in  FIG. 18 . The material of the first emission-auxiliary layer  102  of the OLED structure is PEDOT: PSS. The schematic energy level diagram is given for reference in  FIG. 19 . In comparison with the OLED in Example 1 made according to the present invention, the OLED made in Comparative Example has unimproved electron/hole-injection balance and recombination efficiency such that the efficiency is significantly reduced, as shown as respective power efficiencies in Table 1. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Power Efficiency 
                 CIE Chromaticity 
                   
               
               
                   
                 (lm/W) 
                 Coordinates 
                 Remark 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 1 
                 37 
                 (0.18, 0.35) 
                 present 
               
               
                   
                   
                   
                 invention 
               
               
                 Example 2 
                 31 
                 (0.18, 0.34) 
                 present 
               
               
                   
                   
                   
                 invention 
               
               
                 Comparative 
                 18 
                 (0.18, 0.34) 
                 prior art 
               
               
                 Example