Abstract:
An organic light emitting display including: a driving thin film transistor (TFT) including a semiconductor layer on a substrate including a source electrode, a drain electrode, and an N-type oxide semiconductor; at least one insulating layer formed on the driving TFT; a pixel defining layer for defining a pixel region on the insulating layer; a cathode electrode coupled to a drain electrode of the driving TFT and patterned to correspond to the pixel region; an electron injection layer arranged over the entire surfaces of the pixel defining layer and the cathode electrode and formed of a material whose band gaps are 3.0 eV to 5.0 eV selected from the group consisting of an oxide, a nitride, a fluoride, and diamond on; an organic light emitting layer formed on the electron injection layer to correspond to the cathode region; and an anode electrode formed on the organic light emitting layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 2008-11024, filed Feb. 4, 2008 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Aspects of the present invention relate to an organic light emitting display, and more particularly, to an organic light emitting display including a thin film transistor (TFT) in which an N-type oxide semiconductor layer is formed. 
     2. Description of the Related Art 
     In a TFT of a conventional organic light emitting display, amorphous silicon or poly silicon is used as a semiconductor layer. However, when the semiconductor layer is formed of amorphous silicon, it is difficult to use the TFT as a driving circuit of a display panel that requires high operating speed because of low mobility in the amorphous silicon layer When the semiconductor layer is formed of poly silicon, mobility is high, but the threshold voltage is non-uniform. Thus, an additional compensating circuit is necessary. Moreover, a leakage current is generated by the TFT having a semiconductor layer of amorphous silicon or poly silicon because of the radiation of light so that the characteristics of the TFT deteriorate. 
     Therefore, oxide semiconductors are being studied to solve such problems. For example, Japanese Patent Publication No. 2004-273614 discloses a TFT that has a semiconductor layer formed of ZnO or an oxide semiconductor including ZnO. Referring to  FIG. 1 , the TFT includes a source electrode  20   a  and a drain electrode  20   b  formed on an insulating substrate S, a ZnO layer  4  provided to contact the source and drain electrodes  20   a  and  20   b , and a gate insulating layer  5  and a gate electrode  6  laminated on the ZnO layer  4 . 
     In this case, the ZnO or the oxide semiconductor including ZnO has a band gap of 3.4 eV, which is higher than light energy in the visible light region, and thus the ZnO does not absorb visible rays. Therefore, in the TFT, the leakage current caused by the absorption of visible light does not increase. However, it is generally the case that ZnO or an oxide semiconductor including ZnO exhibits N-type conductivity caused by the depletion of O 2 , whereas an organic light emitting display uses a P-type driving element. 
     In another example, Korean Patent Registration No. 635514 discloses an organic light emitting display using a common P-type driving element. Referring to  FIG. 2 , a P-type driving TFT includes a gate electrode  40 , a source electrode  50   a , a drain electrode  50   b , and a P-type semiconductor layer  60  formed on a substrate  30 . 
     In this case, the source electrode  50   a  of the driving TFT is coupled to a common power source voltage line ELVDD  96 , and the drain electrode  50   b  is coupled to the anode electrode  80  of an organic light emitting diode (OLED). An organic light emitting layer  90  is formed on the anode electrode  80 , and a cathode electrode  92  formed on the organic light emitting layer  90  is coupled to a second power source voltage line  97 , so that the organic light emitting display is driven. Reference numerals  93 ,  94 ,  98 , and  99  that are not described further herein denote an insulating layer, a pixel defining layer, a sealing material, and an encapsulating substrate. 
     In order to use a driving TFT in which an N-type oxide semiconductor layer is used for an organic light emitting display, the structure of the anode electrode of the OLED and the structure of the cathode electrode thereof should be reversed. Therefore, it is necessary to improve the structure of an organic light emitting display, and thus studies are being carried out to improve the structure of organic light emitting displays. 
     SUMMARY OF THE INVENTION 
     In order to achieve the foregoing and/or other aspects of the present invention, an organic light emitting display according to an embodiment of the present invention includes: a substrate; a driving thin film transistor (TFT) on the substrate, the TFT including a semiconductor layer formed of a source electrode, a drain electrode, and an N-type oxide semiconductor; at least one insulating layer formed on the driving TFT; a pixel defining layer for defining a pixel region on the insulating layer; a cathode electrode coupled to a drain electrode of the driving TFT and patterned to correspond to the pixel region; an electron injection layer arranged on the cathode electrode over the entire surfaces of the pixel defining layer and the cathode electrode, and formed of a material whose band gaps are 3.4 eV to 5.0 eV selected from the group consisting of an oxide, a nitride, a fluoride, and diamond;, an organic light emitting layer formed on the electron injection layer to correspond to the cathode region; and an anode electrode formed on the organic light emitting layer. 
     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a cross-sectional view illustrating a thin film transistor (TFT) having a semiconductor layer of ZnO according to conventional art; 
         FIG. 2  is a cross-sectional view of an organic light emitting display illustrating the cross-sections of a driving TFT and a pixel region according to conventional art; 
         FIG. 3  is a cross-sectional view of an organic light emitting display illustrating the cross-sections of a driving TFT and a pixel region according to an embodiment of the present invention; 
         FIGS. 4A and 4B  are graphs schematically illustrating the physical property values of organic light emitting displays as a function of the electron injection layer; and 
         FIG. 5  is a circuit diagram illustrating a pixel circuit of the organic light emitting display including an N-type driving TFT according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. Moreover, it is to be understood that where it is stated herein that one layer is “formed on” or “disposed on” a second layer, the first layer may be formed or disposed directly on the second layer or there may be an intervening layer between the first layer and the second layer. Further, as used herein, the term “formed on” is used with the same meaning as “located on” or “disposed on” and is not meant to be limiting regarding any particular fabrication process. 
       FIG. 3  is a cross-sectional view of an organic light emitting display  16  illustrating the cross-sections of a driving TFT  10  and a pixel region according to an embodiment of the present invention. In order to form an organic light emitting display using a driving TFT  10  in which an N-type oxide semiconductor layer is used, it is necessary to change the structure of conventional organic light emitting displays. That is, a cathode electrode is to be coupled to a drain electrode of a driving TFT  10  in order to use an N-type driving TFT  10 . 
     In a driving TFT  10  having an inverted staggered structure, a gate electrode  120  is formed on a region on a substrate  110 , and a gate insulating layer  130  is formed on the gate electrode  120  and the substrate  110 . In addition, a semiconductor layer  140  is formed to correspond to the gate electrode  120  on a region of the gate insulating layer  130 , and source and drain electrodes  150   a  and  150   b  contact different regions of the semiconductor layer  140 . 
     In this embodiment, the semiconductor layer  140  is formed of an N-type oxide semiconductor. The N-type oxide semiconductor can be formed of an oxide selected from the group consisting of ZnO, ZnGa 2 O 4 , ZnIn 2 O 4 , In 2 O 3 , ZnInGaO 4 , and ZnSnO 4 . The above materials exhibit N-type electrical characteristics because of the depletion of O 2 . 
     An insulating layer  160  is provided on a driving TFT  10  array to protect the driving TFT  10  and to separate an organic light emitting diode (OLED) formed on the insulating layer  160  from the driving TFT  10 . Here, although one insulating layer  160  is illustrated in the drawing, both an organic insulating layer and an inorganic insulating layer referred to as a planarizing layer and a passivation layer may be commonly provided. The above-described TFT  10  array refers to all of the layers for forming the TFT  10 . According to the present embodiment, the TFT  10  array refers to the gate electrode  120 , the gate insulating layer  130 , the semiconductor layer  140 , and the source and drain electrodes  150   a  and  150   b.    
     A pixel defining layer  165  is formed on the insulating layer  160  to define unit pixels, in which OLEDs are positioned and the OLEDs are formed. The OLED includes a cathode electrode  170  patterned in accordance with a pixel region, an electron injection layer  180  formed on the cathode electrode  170  and the pixel defining layer  165 , an organic light emitting layer  190  formed on the electron injection layer  180  to correspond to the cathode electrode  170 , and an anode electrode  200  formed on the organic light emitting layer  190 . 
     The cathode electrode  170  contacts with the drain electrode  150   b  of the TFT  10  through a via hole  167 , as the TFT  10  has the N-type electric characteristic. The cathode electrode  170  is patterned in accordance with the shape of the pixel region defined by the pixel defining layer through a photolithography process. The cathode electrode  170  is formed of one selected from the group consisting of indium tin oxide (ITO), Ag, and Al. In particular, ITO may be used for a transparent electrode for both-side emission. The ITO can also be doped with materials such as cesium. 
     The electron injection layer  180  is provided between the cathode electrode  170  and the organic light emitting layer  190 . The electron injection layer  180  is used for helping the band bending of the OLED and for improving the efficiency of the OLED using the tunneling effect of electrons. 
     In a conventional OLED, the electron injection layer is formed under the cathode electrode. However, according to this embodiment of the present invention, the electron injection layer  180  is formed on the cathode electrode  170  that contacts with the drain electrode  150   b  since the TFT  10  for driving the OLED has N-type electric characteristics. 
       FIGS. 4A and 4B  are graphs schematically illustrating the physical property values of organic light emitting displays as a function of the electron injection layer.  FIG. 4A  illustrates a correlation between the current density of the organic light emitting display and the brightness of the light being emitted.  FIG. 4B  illustrates a correlation between the driving voltage and the current density of the organic light emitting display. In  FIGS. 4A and 4B , sample A does not have an electron injection layer, sample B has an electron injection layer of LiF, and sample C has an electron injection layer of MgO. As illustrated in  FIGS. 4A and 4B , when an electron injection layer is used, the brightness of the organic light emitting display increases for the same current density, whereas the driving voltage thereof is reduced. 
     According to an embodiment of the present invention, the electron injection layer  180  can be formed of a material whose band gap is 3.4 eV to 5.0 eV, one selected from the group consisting of an oxide, a nitride, a fluoride, and diamond. For example, the electron injection layer  180  can be formed of MgO, Al 2 O 3 , SiO 2 , Si 3 N 4 , or LiF. To reduce the thickness of the organic light emitting display, the thickness of the electron injection layer  180  can be 1 nm to 2 nm. 
     Furthermore, in order to help the transmission of electrons, depending on the materials of the cathode electrode  170  and the organic light emitting layer  190 , an electron transport layer may further be formed on the electron injection layer  180 . Also, a hole injection layer, a hole transport layer, or both of them can be further included to help hole injection and transmission between the organic light emitting layer  190  and the anode electrode  200 , depending on the materials of the organic light emitting layer and the anode electrode. 
     Hereinafter, the pixel circuit of the organic light emitting display including the N-type driving TFT according to an embodiment of the present invention will be described with reference to, which is a circuit diagram illustrating the pixel circuit of the organic light emitting display including an N-type driving TFT according to an embodiment of the present invention. As illustrated in  FIG. 5 , the pixel circuit of the organic light emitting display includes a switching TFT M 1 , a driving TFT M 2 , a capacitor Cst, and an organic light emitting diode (OLED). 
     In this embodiment, the switching TFT M 1  and the driving TFT M 2  are formed of an N-type driving TFT. However, the present invention is not limited to this embodiment. That is, the switching TFT M 1  can be either an N-type or P-type semiconductor. However, as in this embodiment, when the driving TFT M 2  is an N-type driving TFT, the switching TFT M 1  can also be an N-type driving TFT to facilitate manufacturing. The gate electrode of the switching TFT M 1  is coupled to a scan line Sn to transmit a data voltage from a data line Dm coupled to a drain electrode to the gate of the driving TFT M 2  in response to a selection signal from the scan line. The source electrode of the driving TFT M 2  is coupled to a reference voltage ELVss, the gate electrode of the driving TFT M 2  is coupled to the source electrode of the switching TFT M 1 , and the drain electrode of the driving TFT M 2  is coupled to the cathode electrode of the OLED. The capacitor Cst is commonly coupled to the gate electrode of the driving TFT M 2  and the source electrode of the driving TFT M 2  to maintain a gate-source voltage VGS within a uniform range. In the OLED, the anode electrode is commonly coupled to a power source voltage ELVdd, and the cathode electrode is coupled to the drain of the driving TFT M 2 . 
     The operation of the pixel circuit according to this embodiment of the present invention will be described as follows. When the switching TFT M 1  is turned on by the selection signal applied to the gate of the switching TFT M 1  through the scan line, the data signal transmitted through the data line Dm is transmitted to the capacitor Cst so that the data signal is stored. Then, the data signal stored in the capacitor Cst is transmitted to the driving TFT M 2 . Therefore, the driving TFT M 2  supplies driving current to the OLED through the cathode electrode in response to the applied data signal so that light is emitted. 
     While the present invention has been described in connection with certain example embodiments, various modifications can be performed within the spirit and scope of the invention. For example, the driving TFT has the inverted staggered structure in the above-described embodiment, but may be formed of an N-type oxide semiconductor in a top gate structure, a bottom gate structure, or a coplanar structure. In addition, in the above embodiment, a compensation circuit such as an additional threshold voltage can be further included in the pixel circuit of the organic light emitting display. 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.