Patent Publication Number: US-2010128021-A1

Title: Pixel and organic light emitting display device using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2008-0116855 filed on Nov. 24, 2008 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     The field relates to a pixel and an organic light emitting display device using the same, and more particularly to a pixel using an oxide transistor, and an organic light emitting display device using the same. 
     2. Description of the Related Technology 
     Various flat panel display devices having reduced weight and volume when compared to a cathode ray tube have been developed. The flat panel display devices include a liquid crystal display (LCD) device, a field emission display (FED) device, a plasma display panel (PDP) device, an organic light emitting display (OLED) device, etc. 
     The organic light emitting display has various advantages including excellent color reproducibility, slimness and the like so that its applications have largely expanded to PDAs, MP3 players, and the like, in addition to cellular phones. 
     The organic light emitting display device displays an image using organic light emitting diodes (OLED) expressing brightness of light emitted according to an amount of input current. 
       FIG. 1  is a circuit diagram of a pixel used in some organic light emitting display devices. Referring to  FIG. 1 , the pixel includes a first transistor T 1 , a second transistor T 2 , a capacitor Cst and an organic light emitting diode OLED. 
     The first transistor T 1  has a source coupled to a first power supply ELVDD, a drain coupled to an anode electrode of the organic light emitting diode OLED, and a gate coupled to a first node N 1 . 
     The second transistor T 2  has a source coupled to a data line Dm, a drain coupled to the first node N 1 , and a gate coupled to a scan line Sn. 
     The capacitor has a first electrode coupled to the first power supply ELVDD, and a second electrode coupled to the first node N 1 . 
     The organic light emitting diode OLED has an anode electrode coupled to the drain of the first transistor T 1 , and a cathode electrode coupled to a second power supply ELVSS. 
     The pixel of  FIG. 1  determines the amount of current flowing to the organic light emitting diode OLED according to a voltage difference between the source and gate of the first transistor T 1 . In other words, the amount of current flowing to the organic light emitting diode corresponding to the voltage of the first power supply ELVDD and the data signals is approximated by the following equation 1. 
     
       
         
           
             
               
                 
                   
                     I 
                     OLED 
                   
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                       2 
                     
                      
                     
                       
                         ( 
                         
                           
                             V 
                             GS 
                           
                           - 
                           
                             V 
                             th 
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     Herein, I OLED  represents the amount of current flowing to the organic light emitting diode OLED, V GS  represents a voltage difference between the source and gate of the first transistor T 1 , and V th  represents a threshold voltage of the first transistor T 1 . 
     Therefore, the current flowing to the organic light emitting diode OLED is affected by the threshold voltage of the first transistor T 1 . 
     If transistor T 1  is an oxide transistor and the gate of the transistor T 1  is maintained with a higher voltage than the source thereof, the transistor is deteriorated such that the threshold voltage increases. 
     Such a deterioration phenomenon causes unexpected current to flow to the organic light emitting diode OLED, and accordingly the organic light emitting diode emits light such that afterimages are formed. Such a deterioration phenomenon becomes greater as the voltage of the gate of the transistor is greater and the stress time is longer. 
     With the problems as described above, it is problematic to form the pixel of the organic light emitting display device using the oxide transistor. 
     SUMMARY OF THE INVENTION 
     One aspect is a pixel including an organic light emitting diode configured to emit light according to an amount of a driving current, a first transistor; including a first electrode coupled to a first power supply, a second electrode coupled to the organic light emitting diode, and a gate coupled to a first node, where the first transistor conducts the driving current according to a voltage of the first node, a second transistor configured to transmit data signals to the first node according to a first scan signal, a third transistor configured to apply a discharge voltage to the first node according to a third scan signal, and a capacitor configured to maintain the voltage of the first node. 
     Another aspect is an organic light emitting display device including a pixel unit configured to display an image in response to data signals and scan signals, a data driver configured to generate the data signals, and a scan driver configured to generate the scan signals, where the pixel unit includes a plurality of pixels, each pixel including an organic light emitting diode configured to emit light according to an amount of a driving current, a first transistor, including a first electrode coupled to a first power supply, a second electrode coupled to the organic light emitting diode, and a gate coupled to a first node, where the first transistor conducts the driving current according to a voltage of the first node, a second transistor configured to transmit data signals to the first node according to a first scan signal, a third transistor configured to apply a discharge voltage to the first node according to a third scan signal, and a capacitor configured to maintain the voltage of the first node. 
     Another aspect is a method of driving an organic light emitting display device, which displays an image by controlling amounts of currents flowing from a plurality of transistors to a plurality of organic light emitting diodes, the method including applying a data signal to the gate of one of the transistors according to a first scan signal, applying a discharge voltage to the gate of the one transistor according to a second scan signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, together with the specification, illustrate exemplary embodiments, and, together with the description, serve to explain various inventive aspects. 
         FIG. 1  is a circuit diagram of a pixel used in an organic light emitting display device; 
         FIG. 2  is a block diagram of an organic light emitting display device; 
         FIG. 3  is a circuit diagram of one embodiment of a pixel for the organic light emitting display device of  FIG. 2 ; 
         FIG. 4  is a waveform view of signals input to the pixel of  FIG. 3 ; 
         FIG. 5  is a circuit diagram of an embodiment of a pixel for the organic light emitting display device of  FIG. 2 ; 
         FIG. 6  is a waveform view of signals input to the pixel of  FIG. 5 ; 
         FIG. 7  is a circuit diagram of an embodiment of a pixel for the organic light emitting display device of  FIG. 2 ; and 
         FIG. 8  is a circuit diagram of another embodiment of a pixel for the organic light emitting display device of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS 
     Some embodiments provide a pixel using an oxide transistor, and an organic light emitting display device using the same. With the pixel and the organic light emitting display device, the deterioration of the transistor is prevented by discharging the transistor, making it possible to implement the pixel using the oxide transistor. 
     Hereinafter, certain exemplary embodiments will be described with reference to the accompanying drawings. Here, when a first element is describes as being coupled to a second element, the first element may be directly coupled to the second element or may be indirectly coupled to the second element via a third element. Further, some of the elements that are not essential to the complete understanding of the invention may be omitted for clarity. Also, like reference numerals generally refer to like elements throughout. 
       FIG. 2  is a structure view of an organic light emitting display device, which includes a pixel unit  100 , a data driver  200 , and a scan driver  300 . 
     The pixel unit  100  is arranged with a plurality of pixels  101 , wherein each pixel  101  includes an organic light emitting diode (not shown) configured to emit light according to the flow of current therethrough. In addition, the pixel unit  100  has n scan lines S 1 , S 2 , . . . Sn- 1 , and Sn formed in a row direction and configured to transmit scan signals, and m data lines D 1 , D 2 , Dm- 1 , and Dm formed in a column direction and configured to transmit data signals are arranged. 
     Also, the pixel unit  100  is driven by receiving first power (not shown) and second power (not shown) having a lower voltage level than the first power. The pixel units  100  emit light by allowing a current to flow in their organic light emitting diodes according to the scan signals, the data signals, the first power, and the second power, thereby displaying an image. 
     Also, pixel unit  100  includes wires (not shown) for applying a negative supply capable of discharging charges in each pixel  101 . 
     In some embodiments, the data driver  200 , which is a means generating data signals, generates data signals using image signals having red, blue and green components. The data driver  200  applies the generated data signals to the data lines D 1 , D 2 , . . . Dm- 1 , and Dm of the pixel unit  100 . 
     In some embodiments, the scan driver  300 , which is a means generating scan signals, is coupled to scan lines S 1 , S 2 , . . . Sn- 1 , and Sn to transfer scan signals to a specific row of the pixel unit  100 . The data signals output from the data driver  200  are transmitted to the pixel  101  with the scan signals so that a voltage corresponding to a data signal is transferred to the pixel  101 . 
       FIG. 3  is a circuit diagram of one embodiment of a pixel for use in the organic light emitting display device of  FIG. 2 . 
     Referring to  FIG. 3 , the pixel includes a first transistor M 11 , a second transistor M 21 , a third transistor M 31 , a capacitor Cst and an organic light emitting diode OLED. Also, in this embodiment, each transistor is implemented as an NMOS oxide transistor. In other embodiments corresponding PMOS transistors are used. 
     The first transistor M 11  allows a driving current to flow from a first power supply ELVDD to a second power supply ELVSS, wherein the amount of the driving current is determined according to the voltage applied to the gate of the first transistor M 11 . The first transistor M 11  has a source coupled to the first power supply ELVDD, a drain coupled to an anode electrode of the organic light emitting diode OLED, and a gate coupled to a first node N 1 . 
     The second transistor M 21  allows data signals to be selectively transferred to the gate of the first transistor M 11 . The second transistor M 21  has a source coupled to a data line Dm, a drain coupled to the first node N 1 , and a gate coupled to a first scan line Sn. 
     The third transistor M 31  is used to discharge the gate of the first transistor M 11 . The third transistor M 31  has a source coupled to a second scan line Sn- 2 , a drain coupled to the first node N 1 , and a gate coupled to a third scan line Sn- 1 . 
     The capacitor Cst 1  allows the gate voltage of the first transistor M 11  to be maintained. The capacitor Cst 1  has a first electrode coupled to the first power supply ELVDD, and a second electrode coupled to the first node N 1 . 
     The organic light emitting diode OLED emits light as a result of receiving a driving current. The organic light emitting diode OLED has an anode electrode coupled to the drain of the first transistor M 11 , and a cathode electrode coupled to the second power supply ELVSS. 
       FIG. 4  is a waveform view of signals input to the pixel of  FIG. 3 . 
     Referring to  FIG. 4 , when the second previous scan signal Sn- 2  changes from a high state to a low state, and the previous scan signal Sn- 1  changes from a low state to a high state, the third transistor M 31  is turned on. As a result, the voltage V N1  of first node N 1  receives the scan signal Sn- 2 , which is in a low state. Therefore, the charge at the gate of the first transistor M 11  is removed by the second scan signal Sn- 2 . In other words, the interface trap charge is reduced. Through the operation as described above, it is possible to suppress the phenomenon that the threshold voltage of the first transistor M 11  is increased. 
     When the previous scan signal Sn- 1  changes from a high state to a low state, and the first scan signal sn transferred through the current scan line Sn changes from a low state to a high state, the second transistor M 21  is turned on, and the third transistor M 31  is turned off. Therefore, the data signal flows to the first node N 1  through the data line Dm, and the voltage V N1  of the first node N 1  corresponds to the voltage of data signal. The first node N 1  maintains the voltage of the data signal with the capacitor Cst. 
     Therefore, current corresponding to the current of data signal flows from the source to the drain of the first transistor M 11  and to the organic light emitting diode OLED, such that the pixel emits light. 
     As shown in  FIG. 4 , the duration of the light emitting period is longer than the duration of the discharging period. 
       FIG. 5  is a circuit diagram of another embodiment of a pixel for use in the organic light emitting display device of  FIG. 2 . Referring to  FIG. 5 , the pixel includes a first transistor M 12 , a second transistor M 22 , a third transistor M 32 , a capacitor Cst 2  and an organic light emitting diode OLED. Each transistor is implemented as an NMOS oxide transistor, however, in other embodiments PMOS devices may alternatively be used. 
     The first transistor M 12  allows a driving current to flow from a first power supply ELVDD to a second power supply ELVSS, wherein the amount of the driving current corresponds to the voltage applied to the gate of the first transistor M 12 . The first transistor M 12  has a source coupled to the first power supply ELVDD, a drain coupled to an anode electrode of the organic light emitting diode OLED, and a gate coupled to a first node N 1 . 
     The second transistor M 22  allows data signals to be selectively transferred to the gate of the first transistor. The second transistor M 22  has a source coupled to a data line Dm, a drain coupled to the first node N 1 , and a gate coupled to a first scan line Sn. 
     The third transistor M 32  is used to discharge the gate of the first transistor M 12 . The third transistor M 32  has a source coupled to an eighth previous scan line Sn- 8 , a drain coupled to the first node N 1 , and a gate coupled to a seventh previous scan line Sn- 7 . 
     The capacitor Cst 2  allows the gate voltage of the first transistor M 12  to be maintained. The capacitor Cst 2  has a first electrode coupled to the first power supply ELVDD, and a second electrode coupled to the first node N 1 . 
     The organic light emitting diode OLED emits light in response to receiving a driving current. The organic light emitting diode OLED has an anode electrode coupled to the drain of the first transistor M 12 , and a cathode electrode coupled to the second power supply ELVSS. 
       FIG. 6  is a waveform view of signals input to the pixel of  FIG. 5 . 
     Referring to  FIG. 6 , when the eighth previous scan signal sn- 8  changes from a high state to a low state, and the seventh previous scan signals sn- 7  changes from a low state to a high state, the third transistor M 32  is turned on. As a result, the voltage V N1  of first node N 1  becomes low. Therefore, the charge at the gate of the first transistor M 12  is removed by the eighth previous scan signal Sn- 8 . Consequently, the interface trap charge is reduced. Through the operation as described above, it is possible to suppress the phenomenon that the threshold voltage of the first transistor M 12  increases with age. 
     When the seventh previous scan signal Sn- 7  changes from a high state to a low state and the current scan signal Sn changes from a low state to a high state, the second transistor M 22  is turned on and the third transistor M 32  is turned off. Therefore, the data signal is provided to the first node N 1  through the data line Dm. At this time, the first node N 1  maintains the voltage of data signal with the capacitor Cst 2 . 
     As a result, current corresponding to the data signal flows from the source to the drain of the first transistor M 12  and to the organic light emitting diode OLED. 
     As shown in  FIG. 6 , the duration of the light emitting period is similar to the duration of the discharging period. 
     Comparison of the embodiments of  FIGS. 3 and 5  shows that the relative durations of the light emitting and discharging periods is determined by which scan signals are used to discharge the gate of the first transistor. Accordingly, the scan signals to use can be selected to achieve a desired relative duration. 
       FIG. 7  is a circuit diagram of an embodiment of a pixel for use in the organic light emitting display device of  FIG. 2 . Referring to  FIG. 7 , the pixel includes a first transistor M 13 , a second transistor M 23 , a third transistor M 33 , a capacitor Cst 3  and an organic light emitting diode OLED. Also, in this embodiment, each transistor is implemented as an NMOS oxide transistor, however PMOS transistors can also be used. 
     The first transistor M 13  allows a driving current to flow from a first power supply ELVDD to a second power supply ELVSS, wherein the amount of driving current corresponds to the voltage applied to the gate of the first transistor M 13 . The first transistor M 13  has a source coupled to the first power supply ELVDD, a drain coupled to an anode electrode of the organic light emitting diode OLED, and a gate coupled to a first node N 1 . 
     The second transistor M 23  allows data signals to be transferred selectively to the gate of the first transistor. The second transistor M 23  has a source coupled to a data line Dm, a drain coupled to the first node N 1 , and a gate coupled to a first scan line Sn. 
     The third transistor M 33  is used to discharge the gate of the first transistor M 13 . The third transistor M 33  has a source coupled to a negative supply voltage Vneg, a drain coupled to the first node N 1 , and a gate coupled to the previous scan line Sn- 1 . According to the voltage at the previous scan line Sn- 1 , negative voltage Vneg is selectively transferred to the first node N 1  to discharge the first node N 1  and the capacitor Cst 3 . 
     The capacitor Cst 3  allows the gate voltage of the first transistor M 13  to be maintained. The capacitor Cst 3  has a first electrode coupled to the first power supply ELVDD, and a second electrode coupled to the first node N 1 . 
     The organic light emitting diode OLED emits light in response to receiving a driving current. The organic light emitting diode OLED has an anode electrode coupled to the drain of the first transistor M 13 , and a cathode electrode coupled to the second power supply ELVSS. 
       FIG. 8  is a circuit diagram of another embodiment of a pixel for use in the organic light emitting display device of  FIG. 2 . Referring to  FIG. 8 , the pixel includes a first transistor M 14 , a second transistor M 24 , a third transistor M 34 , a capacitor Cst 4  and an organic light emitting diode OLED. Also, each transistor is implemented as an NMOS oxide transistor, although PMOS devices may alternatively be used. 
     The first transistor M 14  allows a driving current to flow from a first power supply ELVDD to a second power supply ELVSS, wherein the amount of the driving current corresponds to the voltage applied between the gate and source of the first transistor M 14 . The first transistor M 14  has a source coupled to the first power supply ELVDD, a drain coupled to an anode electrode of the organic light emitting diode OLED, and a gate coupled to a first node N 1 . 
     The second transistor M 24  allows data signals to be selectively transferred to the gate of the first transistor  14 . The second transistor M 24  has a source coupled to a data line Dm, a drain coupled to the first node N 1 , and a gate coupled to a first scan line Sn. 
     The third transistor M 34  is used to discharge the first node N 1 . The third transistor M 34  has a source coupled to a negative voltage Vneg, a drain coupled to the first node N 1 , and a gate coupled to a seventh previous scan line Sn- 7 . Accordingly, the negative voltage Vneg is transferred to the pixel according to the voltage of the seventh previous scan line Sn- 7 . 
     The capacitor Cst 4  allows the gate voltage of the first transistor M 13  to be maintained. The capacitor Cst 4  has a first electrode coupled to the first power supply ELVDD, and a second electrode coupled to the first node N 1 . 
     The organic light emitting diode OLED emits light in response to receiving the driving current. The organic light emitting diode OLED has an anode electrode coupled to the drain of the first transistor M 14 , and a cathode electrode coupled to the second power supply ELVSS. 
     While certain embodiments have been described, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements.