Abstract:
An organic light emitting diode comprises a cathode, an anode, an emitting layer disposed between the cathode and the anode, a hole injection layer disposed between the anode and the emitting layer, a hole transport layer disposed between the hole injection layer and the emitting layer, and a buffer layer disposed between the hole injection layer and the hole transport layer. The invention also provides a display apparatus including the organic light emitting diode.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to an organic light emitting diode, and in particular to an organic light emitting diode with a buffer layer.  
         [0003]     2. Description of the Related Art  
         [0004]     Recently, development of photoelectron devices such as organic light emitting device, organic solar energy batteries or organic thin film transistors (OTFT) is industry focus such photoelectron device provide several advantages, such as direct conversion of light into electric power without pollution and noise.  
         [0005]     In addition to solar energy batteries, organic thin film transistors can be formed on a plastic substrate to provide a flexible display due to ductility and elasticity superior to that of silicon. Conventional TFT-LCDs are formed by a process similar to the conventional semiconductor process. OTFT, however, is formed by process such as screen printing, ink-jet printing or contact printing. Polymers and amorphous molecules applied to the organic semiconductor materials of the OTFT can form the large-area semiconductor layer by spin-coating and ink-jet printing, substantially reducing the cost and processing temperature.  
         [0006]     Generally, an organic light emitting device is composed of a light emitting layer sandwiched between a pair of electrodes. When applying an electric field to the electrodes, the cathode injects holes into the lighting emitting layer and the anode injects electrons into the light emitting layer. The electrons and holes recombine in the light emitting layer to form excitons. The excitons deliver energy to the emitting molecules in the light emitting layer, which is released in the form of light. A conventional organic light emitting device comprises a hole transport layer formed on the anode, an emitting layer formed on the hole transport layer, an electron transport layer formed on the emitting layer, and a cathode formed on the electron transport layer. In addition, a conventional organic light emitting device further comprises a hole injection layer disposed between the anode and the hole transport layer to improve hole injection efficiency, and an electron injection layer disposed between the cathode and the electron transport layer to improve electron injection efficiency, thus reducing the driving voltage and increasing the recombination probability of holes and electrons. The electron injection layer of the conventional organic light emitting device, however, is costly for mass production, and therefore it is desirable to reduce the material cost thereof.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     An organic light emitting diode of the invention comprises at least a cathode and an anode, an emitting layer disposed between the cathode and the anode, a hole transport layer disposed between the hole injection layer and the emitting layer, and a buffer layer disposed between the hole injection layer and the hole transport layer.  
         [0008]     Further provided is a display device, comprising the organic light emitting diode.  
         [0009]     A detailed description is given in the following with reference to the accompanying drawing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:  
         [0011]      FIG. 1  is a cross section of a conventional organic light emitting diode; and  
         [0012]      FIG. 2  is a cross section of an organic light emitting diode according to the invention. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0013]     The invention provides an organic light emitting diode, as shown in  FIG. 2 , comprising a cathode  22  and an anode  11 , an emitting layer  16  disposed between the cathode  22  and anode  11 , a hole injection layer  120  disposed between the anode  11  and the emitting layer  16 , a hole transport layer  140  disposed between the hole injection layer  120  and the emitting layer  16 , and a buffer layer  130  disposed between the hole injection layer  120  and the hole transport layer  140 .  
         [0014]     The cathode  22  or the anode  11  is transparent, and the other may be metal such as Al, Ca, Ag, Ni, Cr, Ti, metal alloy such as Mg—Ag alloy, transparent metal oxide such as indium tin oxide (ITO), indium zinc oxide (IZO), cadmium tin oxide (CTO), metallized (AZO), zinc oxide (ZnO), indium nitride (InN), stannum dioxide (SnO 2 ) or combinations thereof. The cathode  22  and the anode  11  can be the same or different materialls.  
         [0015]     The emitting layer  16  comprises a host material and a dopant, wherein the host material comprises ADN(9,10-bis(2-naphthalenyl)anthracene) and the dopant comprises DSA(distyrylarylene), and the volume ratio of the host material to the dopant is between 50:1 and 10:1. In addition, the thickness of the emitting layer  16  is between about 30 nm and 40 nm, preferably 30 nm. The hole injection layer  120  comprises organic material, such as starburst arylamine, and p-type impurity, wherein the starburst arylamine comprises IT-NANA, 2T-NANA or m-MTDATA, and the p-type impurity comprises TCNQ, F4-TCNQ or DDQ. The volume ratio of the starburst arylamine to the p-type impurity is between about 100:1 and 100:10, and the thickness thereof is between about 15 nm and 200 nm. The hole transport layer comprises tertiary arylamine such as NPB, HT2, TPD, DPFL-NPB, DPFL-TPD, DMFL-NPB, DPML-TPD, Spiro-NPB or Spiro-TAD, and the thickness thereof is substantially between 20 nm and 40 nm, preferably 20 nm.  
         [0016]     The buffer layer  130  is formed between the hole injection layer  120  and the hole transport layer, and the thickness thereof is between about 15 nm and 200 nm, preferably 110 nm. The buffer layer  130  comprises starburst arylamine, tertiary arylamine and p-type impurities, wherein the starburst arylamine comprises IT-NANA, 2T-NANA or m-MTDATA, the tertiary arylamine comprises NPB, HT2, TPD, DPFL-NPB, DPFL-TPD, DMFL-NPB, DPML-TPD, Spiro-NPB or Spiro-TAD, and the p-type impurity comprises TCNQ, F4-TCNQ or DDQ. The volume ratio of the starburst arylamine to tertiary arylamine is between about 10:1 and 1:10, preferably 1:1, and the volume percentage of the p-type impurity in the buffer layer  130  is between about 1% and 10%. The thickness ratio of the buffer layer  130  to the hole injection layer  120  is between about 10:1 and 1:10. The electron transport layer is formed between the cathode  22  and the emitting layer  16  and the thickness thereof is between about 20 nm and 40 nm. The electron transport layer comprises Alq 3 .  
         [0017]     The organic light emitting diode of the invention further comprises an electron injection layer  20  disposed between the cathode  22  and the electron transport layer  18 . The electron injection layer  20  comprises alkali metal halide, alkaline-earth metal halide, alkali metal oxide or metal carbonate, such as LiF, CsF, NaF, CaF 2 , Li 2 O, Cs 2 O, Na 2 O, Li 2 CO 3 , Na 2 CO 3 . The disclosed chemical formula is  
                         
                         
                         
                         
 
       COMPARATIVE EXAMPLE  
       [0018]     As shown in  FIG. 1 , a glass substrate  10  with ITO film formed thereon was provided, and then cleaned by cleaning agent, propyl alcohol, ethanol or ultrasonic, and dried by argon and treated with ozone. 2T-NATA and F4-TCNQ was deposited on the glass substrate  10  under 10 −4  Pa by co-evaporation deposition to a thickness of about 150 nm as a hole injection layer  12 , with volume ratio thereof about 100:6. NPB (4,4′-bis[N-(naphthyl)-N-phenyl-amino]biphenyl) was deposited on the hole injection layer  12  by evaporation deposition to a thickness of about 20 nm as a hole transport layer  14 . ADN (9,10-bis(2-naphthalenyl)anthracene)and DSA(distyrylarylene) were deposited on the hole transport layer  14  by co-evaporation deposition to a thickness of about 30 nm as a light emitting layer  16 , with volume ratio thereof about 100:2.5. Alq 3  (tris(8-hydroxyquinoline)aluminum(III)) was deposited on the light emitting layer  16  by evaporation deposition to a thickness of about 30 nm as an electron transport layer  18 . LiF was deposited on the electron transport layer  18  to a thickness of about 1 nm as electron injection layer  20 . Al was then deposited on the electron injection layer as a cathode, and packaged to be a light emitting diode.  
       Example 1-2  
       [0019]     As shown in  FIG. 2 , a glass substrate  10  with ITO film  11  formed thereon was provided, and cleaned by cleaning agent, propyl alcohol, ethanol or ultrasonic, and dried by argon and treated with ozone. In example 1 and example 2 of the invention, 2T:NATA and F4-TCNQ were deposited on the glass substrate  10  under 10 Pa by co-evaporation deposition to a thickness of about 20 nm and 40 nm respectively as a hole injection layer  120 , with volume ratio thereof about 100:6. 2T-NATA, NPB (4,4′-bis[N-(naphthyl)-N-phenyl-amino]biphenyl) and F4-TCNQ were deposited on the hole injection layer  120  by co-evaporation deposition to a thickness of about 130 nm and 110 nm respectively as a buffer layer  130 , with volume ratio of 2T-NATA to NPB about 1:1. NPB (4,4′-bis[N-(naphthyl)-N-phenyl-amino]biphenyl) was deposited on the buffer layer  130  to a thickness of 20 nm as a hole transport layer  140 . ADN(9,10-bis(2-naphthalenyl)anthracene) and DSA(distyrylarylene) were deposited on the hole transport layer  140  by co-evaporation deposition to a thickness of about 30 nm as a light emitting layer  160 , with volume ratio thereof about 100:2.5. Alq 3  (tris(8-hydroxyquinoline)aluminum(III)) was deposited on the light emitting layer  16  by evaporation deposition to a thickness of about 30 nm as a electron transport layer  18 . LiF was deposited on the electron transport layer  18  to a thickness of about 1 nm as electron injection layer  20 . Al was then deposited on the electron injection layer as a cathode, and packaged to be a light emitting diode.  
         [0020]     Table 1 shows variation in operational voltage and brightness with thickness of the buffer layer  130  in examples 1-2 and the comparative example, wherein x is the thickness of the hole injection layer and y is the thickness of the buffer layer. Operational voltage of the organic light emitting diode in the comparative example is about 6.2V. As the buffer was formed between the hole injection layer and the hole transport layer, the operational voltage decreased to 5.7V. When buffer layer thickness increased to 130 nm and hole injection layer thickness decreased to 20 nm, the operational voltage remained about 5.7V and brightness did not change with the variation in thickness. Accordingly, the buffer layer reduced the amount of hole injection layer and operational voltage thereof.  
                                                                           TABLE 1                                       thickness(nm)   operational                example   X   Y   voltage(V)   brightness(cd/m 2 )                    1   20   130   5.7   1000       2   40   110   5.7   1000       comparative   150   0   6.2   1000                  
 
         [0021]     Table 2 shows variation in operational voltage and brightness with doping amount of p-type impurity (F4-TCNQ) in the buffer layer. The difference between the examples 3-5 and example 1 is the doping amount of p-type impurity. According to Table 2, the operational voltage of the organic light emitting diode obviously decreased with the doping amount of the p-type impurity increasing. As the doping amount of the p-type impurity increased over 10% the operational voltage remained the same. Accordingly, the preferred doping amount of p-type impurity is between 1% and 10%.  
                                                         TABLE 2                                       doping ratio(%)   operational               example   z   voltage(V)   brightness(cd/m 2 )                                        3   2   6.0   1000           1   6   5.7   1000           4   12   5.4   1000           5   16   5.4   1000                      
 
         [0022]     Finally, while the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.