Abstract:
The present invention relates to monochromatic organic light emitting devices. The organic light emitting device includes a substrate, an anode, a cathode and an organic electroluminescent medium disposed between the anode and the cathode, wherein the organic electroluminescent medium includes compound monochromatic luminescent layer; and the compound monochromatic luminescent layer includes host A doped with monochromatic dopant and host B doped with monochromatic dopant, wherein the host A is consisted of two kinds of materials with different transporting characteristics, one is hole-transporting material, and the other is electron-transporting material. In addition, the present invention further relates to white organic light emitting devices, wherein the organic electroluminescent medium is consisted of at least one compound monochromatic luminescent layer, which includes host A doped with monochromatic dopant and host B doped with monochromatic dopant. The present invention provides a design to improve the lifetime of the organic light emitting device markedly.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to organic light emitting devices, and more particularly, to monochromatic organic light emitting devices for improving the lifetime markedly, and further relates to white organic light emitting devices for improving the lifetime markedly. 
       BACKGROUND OF THE INVENTION 
       [0002]    Organic light emitting devices have attracted a lot of interests because of the advantages such as thin, large-plane, solid-state and flexible, wherein white organic light emitting devices have become focus due to demonstrated applications in the solid-state light source or as backlight for liquid crystal displays. 
         [0003]    Bernanose. A et al. have began the studies on organic light emitting devices (hereinafter, organic light emitting device is often abbreviated as “OLED”) since 1950s, and the initial material was anthracene, because the thickness was so large and resulted in that the drive voltage was too high. Until the year 1987, a low-weight molecule OLED with the structure of ITO/Diamine/Alq 3 /Mg:Ag was reported by C. W. Tang and Vanslyke of Eastman Kodak Co. in USA, and the luminance of the device was up to 1000 cd/m 2  at the drive voltage 10V at that time, and the external quantum efficiency was 1.0%. The studies on the electroluminescent devices have attracted a lot of interests of scientists, which indicated the probabilities of applications in displays. And then the studies and the industrializations of OLEDs as prelude are opened. 
         [0004]    The high efficiency, high luminescent intensity and color stability of OLEDs are particularly significant to their industrializations. Recent years, phosphorescent materials are introduced to OLEDs which result in that the triplet and singlet excitons are fully used, and so that the luminescent intensity and efficiency are improved markedly. However, the lifetime of blue devices and blue emitting layer of devices is the key problem combining the characteristics of OLEDs, and ways of developing long-lifetime blue materials and optimizing the structures of OLEDs are used to improve the lifetime. At the structure optimization aspect, the Chinese patent document 200510007765.9 and 200510007786.0 of sanyo disclosed a device where the lifetime was improved through introducing two kinds of dopants, and the Chinese patent document 01120883.X of Eastman Kodak disclosed a device where the lifetime was as well improved by introducing the first dopant which could accept electron-hole energy from host and the second dopant which could accept energy from hole of host. The doped luminescent layers above are all doped in single luminescent layer, and the disadvantage is the low efficiency. 
       SUMMARY OF THE INVENTION 
       [0005]    It is an object of the embodiments to provide a monochromatic organic light emitting device with improved lifetime. 
         [0006]    It is a further object of the embodiments to provide a white organic light emitting device with improved lifetime. 
         [0007]    These objects are achieved by the following methods: 
         [0008]    In one aspect, a monochromatic organic light emitting device, comprising: 
         [0009]    a) a substrate; 
         [0010]    b) an anode; 
         [0011]    c) a cathode; 
         [0012]    d) organic electroluminescent medium disposed between the anode and cathode; 
         [0013]    wherein the organic electroluminescent medium comprises a compound monochromatic luminescent layer including host A doped with monochromatic dopant and host B doped with monochromatic dopant, 
         [0014]    wherein the host A includes two kinds of materials with different transporting characteristics, one is hole-transporting material, and the other is electron-transporting material. 
         [0015]    Wherein the compound monochromatic luminescent layer includes either of blue emitting layer, green emitting layer, red emitting layer, or yellow emitting layer. 
         [0016]    Wherein the hole-transporting material includes triarylamine, carbazole derivatives and pyrazolin derivatives. 
         [0017]    Wherein the chemical structure of the hole-transporting material is represented by the following formula {circle around (1)}-{circle around (3)}: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0018]    Wherein the electron-transporting material includes anthracene, oxadiazole derivative, metal chelates and conjugated polycyclic aromatic compounds. 
         [0019]    Wherein the chemical structure of the electron-transporting material is represented by the following formula {circle around (4)}-{circle around (7)}: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0020]    Wherein one of the host A and the dopant thereof are the same as the host B and the dopant thereof. 
         [0021]    Wherein the host A can be a single material with both the hole-transporting characteristic and the electron-transporting characteristic, such as CBP, and the chemical structure of CBP as follows: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0022]    Wherein the host A doped with monochromatic dopant could be adulterated with auxiliary materials, and the auxiliary material could be another monochromatic dopant. 
         [0023]    Wherein the monochromatic luminescent layer is blue emitting layer, and the chemical structure of the blue dopant is represented by the following formula {circle around (8)}-(13): 
         [0000]    
       
                 
         
             
             
         
       
       
                 
         
             
             
         
       
     
         [0024]    Wherein the organic electroluminescent medium includes one or more of hole-injection layer, hole-transporting layer, electron-injection layer and electron-transporting layer. 
         [0025]    In another aspect, a white organic light emitting device, comprising: 
         [0026]    a) a substrate; 
         [0027]    b) an anode; 
         [0028]    c) a cathode; 
         [0029]    d) organic electroluminescent medium disposed between the anode and cathode; 
         [0030]    wherein the organic electroluminescent medium at least comprises one compound monochromatic light emitting layer including host A doped with monochromatic dopant and host B doped with monochromatic dopant. 
         [0031]    wherein the host A includes two kinds of materials with different transporting characteristics, one is hole-transporting material, and the other is electron-transporting material. 
         [0032]    Wherein the compound monochromatic luminescent layer includes either of the blue emitting layer, green emitting layer, red emitting layer, or yellow emitting layer. 
         [0033]    Wherein the hole-transporting material includes triarylamine, carbazole derivatives and pyrazolin derivatives. 
         [0034]    Wherein the chemical structure of the hole-transporting material is represented by the following formula {circle around (1)}-{circle around (3)}: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0035]    Wherein the electron-transporting material includes anthracene, oxadiazole derivative, metal chelates and conjugated polycyclic aromatic compounds. 
         [0036]    Wherein the chemical structure of the electron-transporting material is represented by the following formula {circle around (4)}-{circle around (7)}: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0037]    Wherein one of the host A and the dopant thereof are the same as the host B and the dopant thereof. 
         [0038]    Wherein the host A can be a single material with both the hole-transporting characteristic and the electron-transporting characteristic, such as CBP, the chemical structure of CBP in (14). 
         [0039]    Wherein the organic electroluminescent medium includes compound blue emitting layer, green emitting layer and red emitting layer. 
         [0040]    Wherein the organic electroluminescent medium includes compound blue emitting layer and yellow emitting layer. 
         [0041]    Wherein the organic electroluminescent medium includes compound green emitting layer, blue emitting layer and red emitting layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]      FIG. 1  shows a cross-sectional representation of one embodiment of the structure of the blue organic light emitting device, comprising: 
           [0043]      01  substrate,  02  anode,  03  cathode,  04  hole-injection layer,  05  hole-transporting layer,  06  blue emitting layer  1  (with host A),  07  blue emitting layer  2  (with host B),  08  electron-transporting layer,  09  luminescent layer 
           [0044]      FIG. 2  is a cross-sectional representation of one embodiment of the structure of the green organic light emitting device, comprising: 
           [0045]      01  substrate,  02  anode,  03  cathode,  04  hole-injection layer,  05  hole-transporting layer,  06  green emitting layer  1  (with host A),  07  green emitting layer  2  (with host B),  08  electron-transporting layer,  09  luminescent layer 
           [0046]      FIG. 3  is a cross-sectional representation of one embodiment of the structure of the white organic light emitting device with two luminescent centers, comprising: 
           [0047]      01  substrate,  02  anode,  03  cathode,  04  hole-injection layer,  05  hole-transporting layer,  12  yellow emitting layer,  06  blue emitting layer  1 ,  07  blue emitting layer  2 ,  08  electron-transporting layer,  09  luminescent layer 
           [0048]      FIG. 4  is a cross-sectional representation of one embodiment of the structure of the white organic light emitting device with three luminescent centers, comprising: 
           [0049]      01  substrate,  02  anode,  03  cathode,  04  hole-injection layer,  05  hole-transporting layer,  11  green emitting layer,  06  blue emitting layer  1 ,  07  blue emitting layer  2 ,  13  red emitting layer,  08  electron-transporting layer,  09  luminescent layer 
           [0050]      FIG. 5  is a graph, showing the lifetime comparison of different devices of example 1 
           [0051]      FIG. 6  is a graph, showing the lifetime comparison of different devices of example 3 
           [0052]      FIG. 7  is a graph, showing the lifetime comparison of different devices of example 4 
           [0053]      FIG. 8  is a graph, showing the lifetime comparison of different devices of example 5 
           [0054]      FIG. 9  is a graph, showing the lifetime comparison of different devices of example 7 
           [0055]      FIG. 10  is a graph, showing the lifetime comparison of different devices of example 9 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0056]    The structure of the organic light emitting device in the present invention is shown in  FIG. 1 , wherein 
         [0057]      01  is substrate which can be a glass substrate or a flexible substrate made of polyethylene terephthalate or polyimide material. 
         [0058]      02  is anode which can be inorganic material or organic conducting polymer. The inorganic material is commonly made of metal oxide such as such as indium-tin oxide (ITO), zinc oxide and tin-zinc oxide or some metals with high work function such as aurum (Au), copper and argentums (Ag), and a preferable anode is ITO film. The preferable organic conducting polymer anode is polyethylene dioxythiophene (PEDOTPSS for short) or PANI films. 
         [0059]      03  is cathode which can be a metal material with low work function selected from lithium (Li), magnesium (Mg), calcium (Ca), Strontium (Sr), aluminum (Al), indium (In) and their alloy with copper, aurum (Au) or argentums (Ag). The cathode can also be metal and metal fluoride alternately, and a preferable cathode layer is LiF and Al. 
         [0060]      04  is hole-injection layer (unnecessary). The host material can be CuPc, and the inorganic material can be halide and oxide of bismuth. 
         [0061]      05  is hole-transporting layer. The host material can be aromatic amine and graft polymer. A preferable material is NPB. The inorganic material can be halide or oxide of bismuth. 
         [0062]      09  is the luminescent layer selected from low molecular weight compounds generally. The luminescent layer can be fluorescent materials such as Alq 3 , Gaq 3 , Al (Saph-q) or Ga (Saph-q), which is doped with either of fused-ring aromatic compounds (such as rubrene), coumarin (such as DMQA, C545T) or di-pyran (such as DCJTB, DCM), and the concentration is in the range of from 0.01 wt % to 20 wt %. In addition, the luminescent material can also be carbazole derivatives such as CBP, PVK, which is doped with phosphorescent material, such as Ir(ppy) 3 , Ir(ppy) 2 (acac) or PtOEP. 
         [0063]      08  is electron-transporting layer, commonly selected from low molecular weight electron-transporting materials containing organic metal complex (such as Alq 3 , Gaq 3 , Al (Saph-q), BAlq or Ga (Saph-q)), fused-ring aromatic compounds (such as pentacene, perylene) or o-phenanthroline (such as Bphen, BCP). 
         [0064]    Next described are some examples and figs aimed to explain the technique scheme of the present invention, wherein the examples are only used to understand this invention well, but not limited to this invention. 
       EXAMPLE 1 
       [0065]    A blue light emitting device is reported in Example 1 and the device structure is shown in  FIG. 1 , wherein the luminescent layer  09  is compound blue emitting layer containing two layers  06  and  07 . The blue emitting layer  1 ( 06 ) contains host A doped with blue dopant where the host A is formed by two kinds of materials with different transporting characteristics, one of which is electron-transporting material represented by the following formula 4 (BH1 for short): 
         [0000]    
       
                 
         
             
             
         
       
     
         [0066]    The other is hole-transporting material represented by the following formula {circle around (1)} (NPB for short): 
         [0000]    
       
                 
         
             
             
         
       
     
         [0067]    The formula of the blue dopant is {circle around (8)} (BD1 for short): 
         [0000]    
       
                 
         
             
             
         
       
     
         [0068]    The blue emitting layer  2 ( 07 ) is formed by a host B and a blue dopant, where the host B is electron-transporting material and the blue dopant is BD1, and the device has the following device structure: 
         [0000]      ITO/NPB/BH1:NPB:BD1/BH1:BD1/Alq 3 /LiF/Al   (1)
 
         [0069]    The device of device structure (1) is fabricated by the following procedures successively: 
         [0070]    1) A transparent glass substrate is cleaned ultrasonically with boiling scour water and deionized (DI) water. Then the substrate is dried under infra-red lamp. An anode material is deposited on the cleaned glass as an anode layer and the thickness of it is 180 nm. 
         [0071]    2) The cleaned anode film-coated glass substrate is put in the vacuum of about 1×10 −5  Pa, then a NPB film is vapor-deposited as a hole-transporting layer on the anode layer. The deposition rate is about 0.1 nm/s and the resulting NPB layer thickness is about 20 nm 
         [0072]    3) The blue emitting layer  1  is vapor-deposited on the hole-transporting layer through the method of evaporating three materials at the same time. The weight ratio of NPB and BD1 to BH1 is respectively 20% and 3% and the thickness of this layer is 10 nm 
         [0073]    4) The blue emitting layer  2  is subsequently vapor-deposited on the blue emitting layer  1  through the method of evaporating two materials at the same time. The deposition rate of BH1 is 0.2 nm/s and the weight ratio of BD1 to BH1 is 3%, and the thickness of this layer is 20 nm 5) An Alq 3  film is subsequently vapor-deposited on the blue emitting layer  2  as electron-transporting layer. The deposition rate is 0.2 nm/s and the thickness of it is 50 nm. 
         [0074]    6) Finally, a LiF layer and an Al layer, in sequence, are vapor-deposited as cathode on the above layers. The deposition rate of LiF is 0.01˜0.02 nm/s, and the layer thickness is 0.7 nm The deposition rate of Al is 2.0 nm/s, and the thickness of it is 150 nm 
       COMPARATIVE EXAMPLE 1 
       [0075]    The device has the following device structure: 
         [0000]      IOT/NPB/BH1:BD1/Alq 3 /LiF/Al   (2)
 
         [0076]    The device of device structure (2) is fabricated by the following procedures successively: 
         [0077]    The device of device structure (2) is fabricated with the same procedures as above described towards the device structure (1), except for cancelling the blue light emitting layer  1 . 
       COMPARATIVE EXAMPLE 2 
       [0078]    The device has the following device structure: 
         [0000]      ITO/NPB /BH1:NPB:BD1/Alq 3 /LiF/Al (3) 
         [0079]    The device of device structure (3) is fabricated by the following procedures successively: 
         [0080]    The device of device structure (3) is fabricated with the same procedures as above described towards the device structure (1), except for cancelling the blue light emitting layer  2 . 
         [0081]    The following Table 1 exhibits the characteristics of these devices of Example 1 and Comparative example 1 and 2, and the corresponding graphs are shown in  FIG. 5 . 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Lifetime at 
                   
               
               
                   
                   
                 the 
               
               
                   
                   
                 same 
               
               
                   
                   
                 initial 
               
               
                   
                 Device structure of 
                 luminance 
                 Efficiency 
               
               
                 Devices 
                 the luminescent layer 
                 (h) 
                 (cd/A) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 1 
                 BH1:20%NPB:5%BD1(10 nm)/ 
                 428 
                 5.9 
               
               
                   
                 BH1:5%BD1(15 nm) 
               
               
                 Comparative 
                 BH1:20%NPB:5%BD1(25 nm) 
                 132 
                 4.5 
               
               
                 example 1 
               
               
                 Comparative 
                 BH1:5%BD1(25 nm) 
                 77 
                 6.1 
               
               
                 example 2 
               
               
                   
               
             
          
         
       
     
         [0082]    It can be seen from Table 1 and  FIG. 5  that the device lifetime of Example 1 is improved markedly compared to that of Comparative example 1 wherein the device has only blue luminescent layer  1  with host A, and Comparative example 2 wherein the device has only blue luminescent layer  2  with host B. Additionally, the efficiency of Example 1 is not decreased so much as higher than the devices with only blue luminescent layer  1  or  2 . 
       EXAMPLE 2 
       [0083]    The device has the following device structure: 
         [0000]      ITO/NPB/BH1:NPB(X %):BD1(Ynm)/BH1:BD1/Alq 3 /LiF/Al   (4)
 
         [0084]    The device of device structure (4) is fabricated by the following procedures successively: 
         [0085]    The device of device structure (4) is fabricated with the same procedures as above described towards the device structure (1), except for the weight ratio of NPB and BD1 to BH1 and the total thickness of the blue light emitting layer  1 , wherein the weight ratio of NPB to BH1 is X % and the layer thickness is Ynm. 
         [0086]    The following Table 2 exhibits the characteristics of these devices with different weight ratio and thickness of Example 2. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                 Lifetime 
                 Efficiency 
               
               
                 Device structure of the luminescent layer 
                 Xwt % 
                 Y (nm) 
                 (h) 
                 (cd/A) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 BH1:NPB(20%):BD1(3%)(10 nm)/BH1:BD1 
                 20 
                 10 
                 500 
                 6 
               
               
                 BH1:NPB(40%):BD1(3%)(10 nm)/BH1:BD1 
                 40 
                 10 
                 200 
                 5.6 
               
               
                 BH1:NPB(60%):BD1(3%)(10 nm)/BH1:BD1 
                 60 
                 10 
                 200 
                 4 
               
               
                 BH1:NPB(20%):BD1(3%)(20 nm)/BH1:BD1 
                 20 
                 20 
                 250 
                 5 
               
               
                 BH1:NPB(40%):BD1(3%)(20 nm)/BH1:BD1 
                 40 
                 20 
                 200 
                 4.5 
               
               
                 BH1:NPB(60%):BD1(3%)(20 nm)/BH1:BD1 
                 60 
                 20 
                 150 
                 3.5 
               
               
                   
               
             
          
         
       
     
         [0087]    It can be seen from Table 2, the longest lifetime is obtained at the following condition: the weight ratio of NPB and BD1 to BH1 is 20% and 3% respectively, and the layer thickness of the blue light emitting layer  1  is 10 nm 
         [0088]    The marked lifetime improvement of the device having compound blue luminescent layer according to the above examples and comparative examples, may be attributed to the following reasons: 
         [0089]    The first, effectively widening the blue emission zone can frequently prolong the lifetime of devices. Commonly, there is energy barrier, carriers mainly accumulated at the interface of HTL/BH1:BD (hole-transporting layer represented by HTL, blue host represented by BH1, blue dopant represented by BD), and uncommonly, a compound blue luminescent layer is introduced in the present invention, wherein the blue luminescent layer  1  can transport holes and electrons to the interface of BH 1 :BH 2 :BD/BH 1 :BD, due to the host A of blue luminescent layer  1  containing not only hole-transporting material but also electron-transporting material (hole-transporting layer represented by HTL, blue host with electron-transporting characteristics represented by BH1, blue host with hole-transporting characteristics represented by BH2, and blue dopant represented by BD), so the recombination zone was extended to two interfaces of HTL/BH 1 :BH 2 :BD and BH 1 :BH 2 :BD/BH 1 :BD, and the broadening of the emission zone resulted in the device having the longest lifetime. Meanwhile, the concentration of the dopant of blue light emitting layer  1  is high enough for forming continuous energy level, which effectively increase the carriers transporting, and thus, improving the device lifetime and efficiency. 
         [0090]    Secondly, if the unrecombined holes enter the electron transporting layer Alq 3 , they will form the unstable Alq 3  cationic species that will decrease the stability of the device. But in the present invention, the blue luminescent layer  2  is inserted between the Alq 3  layer and the blue luminescent layer  1  and the holes could be blocked and consumed by recombination in the blue light emitting layer  2 . There are fewer holes to inject into Alq 3 , which prevents the formation of Alq3 + , so the stability and efficiency was enhanced. 
         [0091]    Thirdly, comparing to comparative example 1 or 2 only with either blue light emitting layer  1  or blue light emitting layer  2 , the blue emission of the device of example 1 came from the compound blue emitting structure which consisted of the blue luminescent layer  1  and  2 , thus both of that were complementary to each other during the course of device decay which prolonged the device lifetime. 
         [0092]    Finally, the introducing of the host A caused the enhancement of vitrification temperature. Such as NPB, its vitrification temperature is low generally, however, that was enhanced because of the doping with other materials in the blue luminescent layer  1 , which improved the heat stability of the whole device. 
       EXAMPLE 3 
       [0093]    Another blue light emitting device using different blue material is reported in Example 3, and the device structure, same as Example 1, is showed in  FIG. 1 . The devices also employ compound luminescent layer, wherein the electron-transporting material of blue luminescent layer  1  makes use of BAlq, and the hole-transporting material makes use of NPB, while blue dopant is TBPe. The BAlq and TBPe are also used in blue luminescent layer  2  as host and dopant respectively. The device has the following device structure: 
         [0000]      ITO/NPB/BAlq:NPB:TBPe/BAlq:TBPe/Alq 3 /LiF/Al   (5)
 
         [0094]    The device of device structure (5) is fabricated by the procedure similar to Example 1 
       COMPARATIVE EXAMPLE 3 
       [0095]    The device has the following device structure: 
         [0000]      ITO/NPB/BAlq:TBPe/Alq 3 /LiF/Al   (6)
 
         [0096]    The device of device structure (6) is fabricated by the following procedures successively: 
         [0097]    The device of device structure (6) is fabricated with the same procedures as above described towards the device structure (5), except for cancelling the blue light emitting layer  1 . 
       COMPARATIVE EXAMPLE 4 
       [0098]    The device has the following device structure: 
         [0000]      ITO/NPB/BAlq:NPB:TBPe/Alq 3 /LiF/Al   (7)
 
         [0099]    The device of device structure (7) is fabricated by the following procedures successively: 
         [0100]    The device of device structure (7) is fabricated with the same procedures as above described towards the device structure (5), except for cancelling the blue light emitting layer  2 . 
         [0101]    The following Table 3 exhibits the characteristics of these devices of Example 3 and Comparative example 3 and 4, and the corresponding graphs are shown in  FIG. 6 . 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 Lifetime 
                   
               
               
                   
                   
                 at the 
               
               
                   
                   
                 same 
               
               
                   
                   
                 initial 
               
               
                   
                 Device structure of 
                 luminance 
                 Efficiency 
               
               
                 Devices 
                 the luminescent layer 
                 (h) 
                 (cd/A) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 3 
                 BAlq:20%NPB:3%TBPe(10 nm)/ 
                 125 
                 4.3 
               
               
                   
                 BAlq:3%TBPe(15 nm) 
               
               
                 Comparative 
                 BAlq:3%TBPe(25 nm) 
                 16.3 
                 4.4 
               
               
                 example 3 
               
               
                 Comparative 
                 BAlq:20%NPB:3%TBPe(25 nm) 
                 35 
                 2.3 
               
               
                 example 4 
               
               
                   
               
             
          
         
       
     
         [0102]    It can be seen from Table 3 and  FIG. 6  that the device lifetime of example 3 is improved markedly compared to that of Comparative example 3 and 4. Additionally, the efficiency of Example 3 is not decreased. 
       EXAMPLE 4 
       [0103]    A green light emitting device is reported in Example 4 and the device structure is shown in  FIG. 2 , wherein the luminescent layer  09  is compound green emitting layer containing two layers: layer  10  and layer  11 . The green emitting layer  1 ( 10 ) contains host A doped with green dopant where the host A is formed by two kinds of materials with different transporting characteristics, one of which is electron-transporting material BAlq, the other is hole-transporting material NPB, and the green dopant is C545T. The green emitting layer  2 ( 11 ) is formed by a host B and a green dopant, where the host B is electron-transporting material BAlq and the green dopant is C545T, and the device has the following device structure: 
         [0000]      ITO/NPB/BAlq:NPB:C545T/BAlq:C545T/Alq 3 /LiF/Al   (8)
 
         [0104]    The device of device structure (8) is fabricated by the procedure similar to Example 1. 
       COMPARATIVE EXAMPLE 5 
       [0105]    The device has the following device structure: 
         [0000]      ITO/NPB/BAlq:C545T/Alq 3 /LiF/Al   (9)
 
         [0106]    The device of device structure (9) is fabricated by the following procedures successively: 
         [0107]    The device of device structure (9) is fabricated with the same procedures as above described towards the device structure (8), except for cancelling the green emitting layer  1 . 
       COMPARATIVE EXAMPLE 6 
       [0108]    The device has the following device structure: 
         [0000]      ITO/NPB/BAlq:NPB:C545T/Alq 3 /LiF/Al   (10)
 
         [0109]    The device of device structure (10) is fabricated by the following procedures successively: 
         [0110]    The device of device structure (10) is fabricated with the same procedures as above described towards the device structure (8), except for cancelling the green emitting layer  2 . 
         [0111]    The following Table 4 exhibits the characteristics of these devices of Example 4 and Comparative example 5 and 6, and the corresponding graphs are shown in  FIG. 7 . 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                 Lifetime 
                   
               
               
                   
                   
                 at 
               
               
                   
                   
                 the same 
               
               
                   
                   
                 initial 
               
               
                   
                 Device structure of 
                 luminance 
                 Efficiency 
               
               
                 Device 
                 the luminescent layer 
                 (h) 
                 (cd/A) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 4 
                 BAlq:20%NPB:2%C545T(10 nm)/ 
                 895 
                 10.1 
               
               
                   
                 BAlq:2%C545T(15 nm) 
               
               
                 Comparative 
                 BAlq:2%C545T(25 nm) 
                 118 
                 10.5 
               
               
                 example 5 
               
               
                 Comparative 
                 BAlq:20%NPB:2%C545T(25 nm) 
                 252 
                 8.1 
               
               
                 example 6 
               
               
                   
               
             
          
         
       
     
         [0112]    It can be seen from Table 4 and  FIG. 7  that the device lifetime of Example 4 is improved markedly compared to that of Comparative example 5 and 6. Additionally, the efficiency of Example 4 is not decreased. 
       EXAMPLE 5 
       [0113]    A white organic light emitting device with two luminescent centers is reported in Example 5 and the device structure is shown in  FIG. 3 , wherein the luminescent layer  09  includes yellow emitting layer  12  and compound blue emitting layer containing two layers: blue emitting layer  1 ( 06 ) and blue emitting layer  2 ( 07 ). The yellow emitting layer  12  is formed by hole-transporting material NPB and yellow dopant (such as rubrene). The blue emitting layer  2 ( 07 ) includes electron-transporting material BH1 and blue dopant BD1, and the blue emitting layer  1 ( 06 ) includes host A doped with green dopant where the host A is formed by two kinds of materials with different transporting characteristics, one of which is electron-transporting material BH1, the other is hole-transporting material NPB. The preferable device has the following device structure: 
         [0000]      ITO/NPB/NPB:rubrene/BH1:NPB:BD1/BH1:BD1/Alq 3 /LiF/Al   (11)
 
         [0114]    The white organic light emitting device of device structure (11) is fabricated by the following procedures successively: 
         [0115]    1) A transparent glass substrate is cleaned ultrasonically with boiling scour water and deionized (DI) water. Then the substrate is dried under infra-red lamp. An anode material is deposited on the cleaned glass as an anode layer and the thickness of it is 180 nm 2) The cleaned anode film-coated glass substrate is put in the vacuum of about 1×10 −5  Pa, then a NPB film is vapor-deposited as a hole-transporting layer on the anode layer. The deposition rate is about 0.1 nm/s and the resulting NPB layer thickness is about 20 nm And then a yellow emitting film is vapor-deposited on the hole-transporting layer through the method of evaporating two materials at the same time. The deposition rate of NPB is 0.2 nm/s and the weight ratio of rubrene to NPB is 2 wt %, and the thickness of this layer is 15 nm 
         [0116]    3) The blue emitting layer  1  is vapor-deposited on the yellow emitting layer through the method of evaporating three materials at the same time. The weight ratio of NPB and BD1 to BH1 is respectively 20% and 3% and the thickness of this layer is 10 nm. 
         [0117]    4) The blue emitting layer  2  is subsequently vapor-deposited on the blue emitting layer  1  through the method of evaporating two materials at the same time. The deposition rate of BH1 is 0.2 nm/s and the weight ratio of BD1 to BH1 is 3%, and the thickness of this layer is 20 nm 
         [0118]    5) An Alq 3  film is subsequently vapor-deposited on the second blue emitting layer as electron-transporting layer. The deposition rate is 0.2 nm/s and the thickness of it is 50 nm 
         [0119]    6) Finally, a LiF layer and an Al layer, in sequence, are vapor-deposited as cathode on the above layers. The deposition rate of LiF is 0.01˜0.02 nm/s, and the layer thickness is 0.7 nm The deposition rate of Al is 2.0 nm/s, and the thickness of it is 150 nm 
       COMPARATIVE EXAMPLE 7 
       [0120]    The device has the following device structure: 
         [0000]      ITO/NPB/NPB:rubrene/BH1:BD1/Alq 3 /LiF/Al   (12)
 
         [0121]    The white organic light emitting device of device structure (12) is fabricated by the following procedures successively: 
         [0122]    The device of device structure (12) is fabricated with the same procedures as above described towards the device structure (11), except for cancelling the green emitting layer  1 . 
       COMPARATIVE EXAMPLE 8 
       [0123]    The device has the following device structure: 
         [0000]      ITO/NPB/NPB:rubrene/BH1:NPB:BD1/Alq 3 /LiF/Al   (13)
 
         [0124]    The white organic light emitting device of device structure (13) is fabricated by the following procedures successively: 
         [0125]    The device of device structure (13) is fabricated with the same procedures as above described towards the device structure (11), except for cancelling the green emitting layer  2 . 
         [0126]    The following Table 5 exhibits the characteristics of these devices of Example 5 and Comparative example 7 and 8, and the corresponding graphs are shown in  FIG. 8 . 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                 Lifetime 
                 Efficiency 
                   
               
               
                 Device 
                 Device structure of the luminescent layer 
                 (h) 
                 (cd/A) 
                 Color 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 5 
                 NPB:rubrene/BH1:20%NPB:3%BD1(10 nm)/ 
                 312 
                 10 
                 White 
               
               
                   
                 BH1:3%BD1(20 nm) 
               
               
                 Comparative 
                 NPB:rubrene/BH1:3%BD1(20 nm) 
                 66 
                 10.3 
                 White 
               
               
                 example 7 
                   
                   
                   
                 on the 
               
               
                   
                   
                   
                   
                 yellow 
               
               
                   
                   
                   
                   
                 side 
               
               
                 Comparative 
                 NPB:rubrene/BH1:20%NPB:3%BD1(20 nm) 
                 142 
                 8.3 
                 White 
               
               
                 example 8 
               
               
                   
               
             
          
         
       
     
       EXAMPLE 6 
       [0127]    The device has the following device structure: 
         [0000]      ITO/NPB/NPB:DCM/BAlq:NPB(X %):TBPe (Ynm)/BAlq:TBPe/Alq 3 /LiF/Al   (14)
 
         [0128]    The white organic light emitting device of device structure (14) is fabricated by the following procedures successively: 
         [0129]    The device of device structure (14) is fabricated with the same procedures as above described towards Example 5, except for different materials&#39; weight ratio and total thickness, wherein the weight ratio of NPB and TBPe to BAlq is X % and 3% respectively, and the layer thickness is Ynm. 
         [0130]    The following Table 6 exhibits the characteristics of these devices with different weight ratio and thickness of Example 6. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                 Device structure of the compound blue 
                   
                   
                 Lifetime 
                 Efficiency 
                   
               
               
                 emitting layer 
                 Xwt % 
                 Y (nm) 
                 (h) 
                 (cd/A) 
                 Color 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 BAlq:NPB(20%):TBPe(3%)(10 nm)/BAlq:TBPe 
                 20 
                 10 
                 803 
                 8 
                 White 
               
               
                 BAlq:NPB(40%):TBPe(3%)(10 nm)/BAlq:TBPe 
                 40 
                 10 
                 507 
                 7.5 
                 White 
               
               
                 BAlq:NPB(60%):TBPe(3%)(10 nm)/BAlq:TBPe 
                 60 
                 10 
                 411 
                 6 
                 White 
               
               
                   
                   
                   
                   
                   
                 on the 
               
               
                   
                   
                   
                   
                   
                 blue 
               
               
                   
                   
                   
                   
                   
                 side 
               
               
                 BAlq:NPB(20%):TBPe(3%)(20 nm)/BAlq:TBPe 
                 20 
                 20 
                 453 
                 7 
                 White 
               
               
                 BAlq:NPB(40%):TBPe(3%)(20 nm)/BAlq:TBPe 
                 40 
                 20 
                 387 
                 6.5 
                 White 
               
               
                 BAlq:NPB(60%):TBPe(3%)(20 nm)/BAlq:TBPe 
                 60 
                 20 
                 344 
                 5.5 
                 White 
               
               
                   
                   
                   
                   
                   
                 on the 
               
               
                   
                   
                   
                   
                   
                 blue 
               
               
                   
                   
                   
                   
                   
                 side 
               
               
                   
               
             
          
         
       
     
         [0131]    It can be seen from Table 6 that the longest lifetime is obtained at the following condition: the weight ratio of NPB and TPBe to BAlq is 20% and 3% respectively, and the layer thickness of the blue light emitting layer  1  is 10 nm 
       EXAMPLE 7 
       [0132]    Another white organic light emitting device with three luminescent centers is reported in Example 7 and the device structure is shown in  FIG. 4 , wherein the luminescent layer  09  includes green emitting layer  11 , compound blue luminescent layer containing the blue emitting layer  1 ( 06 ) and the blue emitting layer  2 ( 07 ), and the red emitting layer  13 . wherein the green emitting layer  11  includes NPB as host and Ir(ppy) 3  as green dopant; the blue emitting layer  2 ( 07 ) includes electron-transporting material BAlq as host and TBPe as blue dopant; the emitting layer  1 ( 06 ) includes host A doped with blue dopant and the host A is formed by two kinds of materials with different transporting characteristics; the red emitting layer includes Alq 3  as host and DCJTB or Ir(piq) 2 (acac) as red dopant. The preferable device has the following device structure: 
         [0000]      ITO/NPB/NPB:C545T/BAlq:NPB:TBPe/BAlq:TBPe/Alq 3 :Ir(piq) 2 (acac)/Alq 3 /LiF/Al   (15)
 
         [0133]    The white organic light emitting device of device structure (15) is fabricated by the following procedures successively: 
         [0134]    1) A transparent glass substrate is cleaned ultrasonically with boiling scour water and deionized (DI) water. Then the substrate is dried under infra-red lamp. An anode material is deposited on the cleaned glass as an anode layer and the thickness of it is 180 nm 
         [0135]    2) The cleaned anode film-coated glass substrate is put in the vacuum of about 1×10 5  Pa, then a NPB film is vapor-deposited as a hole-transporting layer on the anode layer. The deposition rate is about 0.1 nm/s and the resulting NPB layer thickness is about 20 nm And then a green emitting film is vapor-deposited on the hole-transporting layer through the method of evaporating two materials at the same time. The deposition rate of NPB is 0.2 nm/s and the weight ratio of C545T to NPB is 2 wt %, and the thickness of this layer is 15 nm 
         [0136]    3) The blue emitting layer  1  is vapor-deposited on the green emitting layer through the method of evaporating three materials at the same time. The weight ratio of NPB and TBPe to BAlq is respectively 20% and 3% and the thickness of this layer is 20 nm 
         [0137]    4) The blue emitting layer  2  is subsequently vapor-deposited on the blue emitting layer  2  through the method of evaporating two materials at the same time. The weight ratio of TBPe to BAlq is 3%, and the thickness of this layer is 20 nm 
         [0138]    5) A red emitting film is vapor-deposited on the blue emitting layer  2  through the method of evaporating two materials at the same time. The weight ratio of Ir(piq) 2 (acac) to Alq 3  is 5%, and the thickness of this layer is 10 nm 
         [0139]    6) An Alq 3  film is subsequently vapor-deposited on the red emitting layer as electron-transporting layer. The deposition rate is 0.2 nm/s and the thickness of it is 50 nm 
         [0140]    7) Finally, a LiF layer and an Al layer, in sequence, are vapor-deposited as cathode on the above layers. The deposition rate of LiF is 0.01˜0.02 nm/s, and the layer thickness is 0.7 nm The deposition rate of Al is 2.0 nm/s, and the thickness of it is 150 nm 
       COMPARATIVE EXAMPLE 9 
       [0141]    The device has the following device structure: 
         [0000]      ITO/NPB/NPB:C545T/BAlq:TBPe/Alq 3 :Ir(piq) 2 (acac)/Alq 3 /LiF/Al   (16)
 
         [0142]    The white organic light emitting device of device structure (16) is fabricated by the following procedures successively: 
         [0143]    The device of device structure (16) is fabricated with the same procedures as above described towards device (15), except for cancelling the blue emitting layer  1 . 
       COMPARATIVE EXAMPLE 10 
       [0144]    The device has the following device structure: 
         [0000]      ITO/NPB/NPB:C545T/BAlq:NPB:TBPe/Alq 3 :Ir(piq) 2 (acac)/Alq 3 /LiF/Al   (17)
 
         [0145]    The device of device structure (17) is fabricated with the same procedures as above described towards device (15), except for cancelling the blue emitting layer  2 . 
         [0146]    The following Table 7 exhibits the characteristics of these devices of Example 7 and Comparative example 9 and 10, and the corresponding graphs are shown in  FIG. 9 . 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 7 
               
               
                   
               
               
                   
                   
                 Lifetime 
                 Efficiency 
                   
               
               
                 Device 
                 Device structure of the luminescent layer 
                 (h) 
                 (cd/A) 
                 Color 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 7 
                 NPB:C545T/BAlq:20%NPB:3%TBPe(20 nm)/ 
                 248 
                 14.8 
                 White 
               
               
                   
                 BAlq:3%TBPe(20 nm)/Alq 3 :Ir(piq) 2 (acac) 
               
               
                 Comparative 
                 NPB:C545T/BAlq:3%TBPe(20 nm)/ 
                 72 
                 15 
                 White 
               
               
                 example 9 
                 Alq 3 :Ir(piq) 2 (acac) 
               
               
                 Comparative 
                 NPB:C545T/BAlq:20%NPB:3%TBPe(20 nm)/ 
                 128 
                 11 
                 White 
               
               
                 example 
                 Alq 3 :Ir(piq) 2 (acac) 
                   
                   
                 on the 
               
               
                 10 
                   
                   
                   
                 red side 
               
               
                   
               
             
          
         
       
     
         [0147]    It can be seen from Table 7 and  FIG. 9  that the device lifetime of Example 7 is improved markedly compared to that of Comparative example 9 and 10. Additionally, the device efficiency of Example 7 is not decreased. 
       EXAMPLE 8 
       [0148]    A white organic light emitting device with two luminescent centers is reported in Example 8, wherein the luminescent layer includes yellow emitting layer, the blue emitting layer  1  and the blue emitting layer  2 , wherein the blue emitting layer  1  contains a host B with both the hole-transporting characteristic and the electron-transporting characteristic. 
         [0149]    The preferable device has the following device structure: 
         [0000]      ITO/NPB/NPB:rubrene/CBP:TBPe/BAlq:TBPe/Alq 3 /LiF/Al   (18)
 
         [0150]    The device of device structure (18) is fabricated with the same procedures as above described towards Example 5, except for changing the blue emitting layer  1  which contains a host B CBP. The deposition rate of CBP is 0.1 nm/s and the weight ratio of TBPe is 3%. The thickness of the blue emitting layer  1  and the blue emitting layer  2  is 10 nm and 20 nm respectively. 
         [0151]    Meanwhile, the following comparative devices are fabricated with the same procedures: 
         [0000]      ITO/NPB/NPB:rubrene/BAlq:TBPe/Alq 3 /LiF/Al   (19)
 
         [0000]      ITO/NPB/NPB:rubrene/CBP:TBPe/Alq 3 /LiF/Al   (20)
 
         [0152]    The following Table 8 exhibits the characteristics of these devices. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 8 
               
               
                   
               
               
                 Device structure of 
                 Lifetime 
                 Efficiency 
                   
               
               
                 the luminescent layer 
                 (h) 
                 (cd/A) 
                 color 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 NPB:rubrene/CBP:3%TBPe(10 nm)/ 
                 775 
                 9 
                 White 
               
               
                 BAlq:3%TBPe(20 nm) 
               
               
                 NPB:rubrene/CBP:3%TBPe(20 nm)/ 
                 601 
                 7.9 
                 White 
               
               
                 BAlq:3%TBPe(20 nm) 
               
               
                 NPB:rubrene/BAlq:3%TBPe(20 nm) 
                 497 
                 9 
                 White 
               
               
                 NPB:rubrene/CBP:3%TBPe(10 nm) 
                 452 
                 7.5 
                 White 
               
               
                   
                   
                   
                 on the 
               
               
                   
                   
                   
                 yellow 
               
               
                   
                   
                   
                 side 
               
               
                   
               
             
          
         
       
     
         [0153]    It can be seen from Table 8 that the lifetime of the device using a host B with both the hole-transporting characteristic and the electron-transporting characteristic in the blue emitting layer  1  and blue emitting layer  2  is improved markedly compared to that of comparative devices, meanwhile, the device efficiency of Example 8 is not decreased. The longest lifetime is obtained at the following condition: the weight ratio of TBPe is 3%, and the layer thickness of the blue light emitting layer  1  is 10 nm 
         [0154]    Additionally, the dopant of the blue emitting layer can also be either BCzVBi, BCzVB, DPAVBi, DPAVB, BDAVBi or N-BDAVBi. 
       EXAMPLE 9 
       [0155]    A white organic light emitting device including a compound blue emitting layer is reported in Example 9, wherein the blue emitting layer  1  and blue emitting layer  2  both contain a yellow dye. The blue emitting layer  1  contains host A and a blue dopant, and the host A includes an electron-transporting material and a hole-transporting material. The blue emitting layer  2  contains an electron-transporting material and a blue dopant. 
         [0156]    The preferable device has the following device structure: 
         [0000]      ITO/NPB/BH1:NPB:BD1:rubrene/BH1:BD1:rubrene/Alq 3 /LiF/Al   (21)
 
         [0157]    The white organic light emitting device of device structure (21) is fabricated by the following procedures successively: 
         [0158]    The device above is fabricated with the same procedures as above described towards Examples, and the differences are the blue emitting layer  1  through the method of evaporating four materials at the same time and the thickness of it is 10 nm. The blue emitting layer  2  through the method of evaporating three materials at the same time and the thickness of it is 15 nm 
         [0159]    Meanwhile, the following comparative devices are fabricated with the same procedures: 
         [0000]      ITO/NPB/BH1:BD1:rubrene/Alq 3 /LiF/Al   (22)
 
         [0000]      ITO/NPB/BH1:NPB:BD1:rubrene/Alq 3 /LiF/Al   (23)
 
         [0160]    The following Table 9 exhibits the characteristics of these devices of Example 9 and Comparative examples, and the corresponding graphs are shown in  FIG. 10 . 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 9 
               
               
                   
               
               
                   
                   
                 Lifetime at 
                   
                   
               
               
                   
                   
                 the 
               
               
                   
                   
                 same 
               
               
                   
                   
                 initial 
               
               
                   
                   
                 luminance 
                 Efficiency 
               
               
                 Device 
                 Device structure of the luminescent layer 
                 (h) 
                 (cd/A) 
                 Color 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 9 
                 BH1:20%NPB:5%BD1:0.5%rubrene(10 nm)/ 
                 1400 
                 11.3 
                 White 
               
               
                   
                 BH1:5%BD1:0.5%rubrene(15 nm) 
               
               
                 Comparative 
                 BH1:5%BD1:1%rubrene(25 nm) 
                 663 
                 11.8 
                 White 
               
               
                 example 
               
               
                 11 
               
               
                 Comparative 
                 BH1:20%NPB:5%BD1:1%rubrene(25 nm) 
                 834 
                 9.6 
                 White 
               
               
                 example 
               
               
                 12