Patent Publication Number: US-8111218-B2

Title: Pixel, organic light emitting display using the same, and driving method thereof

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments relate to a pixel, an organic light emitting display using the pixel, and a driving method thereof. More particularly, embodiments relate to a pixel capable of compensating for reduced luminance of an organic light emitting diode, an organic light emitting display using the pixel, and a driving method thereof. 
     2. Description of the Related Art 
     In general, flat panel displays, e.g., a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), an electroluminescent (EL) display, and so forth, may have reduced weight and volume as compared to a cathode ray tube (CRT) display. For example, the EL display, e.g., an organic light emitting display, may include a plurality of pixels, and each pixel may have an organic light emitting diode (OLED). Each OLED may include a light emitting layer emitting red (R), green (G), or blue (B) light triggered by combining of electrons and holes therein, so the pixel may emit corresponding light to form images. Such an EL display may have a rapid response time and low power consumption. 
     The conventional pixel of the EL display may be driven by a driving circuit configured to receive data and scan signals and to control light emission from its OLED with respect to the data signals. More specifically, an anode of the OLED may be coupled to the driving circuit and a first power source, and a cathode of the OLED may be coupled to a second power source. Accordingly, the OLED may generate light having a predetermined luminance with respect to current flowing therethrough, while the current may be controlled by the driving circuit according to the data signal. 
     However, the material of the light emitting layer of the conventional OLED, e.g., organic material, may deteriorate over time as a result of, e.g., contact with moisture, oxygen, and so forth, thereby reducing current/voltage characteristics of the OLED and, consequently, deteriorating luminance of the OLED. Further, each conventional OLED may deteriorate at a different rate with respect to a composition of its light emitting layer, i.e., type of material used to emit different colors of light, thereby causing non-uniform luminance. Inadequate luminance, i.e., deteriorated and/or non-uniform luminance, of the OLEDs may decrease display characteristics of the EL display device, and may reduce its lifespan and efficiency. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention are therefore directed to a pixel, an organic light emitting display including the same, and a driving method thereof, which substantially overcome one or more of the problems due to the limitations and disadvantages of the related art. 
     It is therefore a feature of an embodiment of the present invention to provide a pixel with a compensator capable of compensating for inadequate luminance of an organic light emitting diode, a display including the same, and a driving method thereof. 
     At least one of the above and other features of the present invention may be realized by providing a pixel including an organic light emitting diode, a drive transistor configured to supply an electric current to the organic light emitting diode, a pixel circuit configured to compensate a threshold voltage of the drive transistor, and a compensator for controlling the voltage of the gate electrode of the drive transistor in order to compensate a degradation of the organic light emitting diode. 
     The compensator may include a pair of transistors coupled between the voltage source and an anode electrode of the organic light emitting diode, and a feedback capacitor coupled between a common node of the pair of transistors and the gate electrode of the drive transistor. The pair of transistors may be alternately turned-on/off. A voltage of the voltage source may be higher or lower than the threshold voltage of the organic light emitting diode. 
     At least one of the above and other features of the present invention may be realized by providing an organic light emitting display, including a scan driver configured to drive scan lines, a data driver configured to drive data lines, and pixels coupled with the scan lines and the data lines. Each of the pixels may include an organic light emitting diode, a drive transistor configured to supply an electric current to the organic light emitting diode, a pixel circuit configured to compensate a threshold voltage of the drive transistor, and a compensator configured to control the voltage of the gate electrode of the drive transistor in order to compensate a degradation of the organic light emitting diode. 
     At least one of the above and other features of the present invention may be realized by providing a method for driving an organic light emitting display, including diode-connecting a drive transistor when a low scan signal is supplied to charge a storage capacitor with a voltage corresponding to a data signal and a threshold voltage of the drive transistor, maintaining one terminal of a feedback capacitor the threshold voltage of the organic light emitting diode during while the storage capacitor is charged with the voltage, another terminal of the feedback capacitor being coupled with the gate electrode of the drive transistor, and changing the one terminal of the feedback capacitor to a voltage of a voltage source after the storage capacitor is charged with the voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the attached drawings, in which: 
         FIG. 1  illustrates a schematic view of an organic light emitting display according to an embodiment of the present invention; 
         FIG. 2  illustrates a circuit diagram of an embodiment of the pixel shown in  FIG. 1 ; 
         FIG. 3  illustrates a detailed circuit diagram of the compensator shown in  FIG. 2  according to an embodiment; 
         FIG. 4  illustrates a waveform diagram for use in driving the pixel shown in  FIG. 3 ; 
         FIG. 5  illustrates a detailed circuit diagram of the compensator shown in  FIG. 2  according to an embodiment; 
         FIG. 6  illustrates a waveform diagram for use in driving the pixel shown in  FIG. 5 ; 
         FIG. 7  illustrates a detailed circuit diagram of the compensator shown in  FIG. 2  according to an embodiment; 
         FIG. 8  illustrates a waveform diagram for use in driving the pixel shown in  FIG. 7 ; 
         FIG. 9  illustrates a circuit diagram of an embodiment of the pixel shown in  FIG. 1 ; 
         FIG. 10  illustrates a waveform diagram for use in driving the pixel shown in  FIG. 9 ; and 
         FIG. 11  illustrates a graph of a simulation result of a pixel according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Korean Patent Application No. 10-2007-0020855, filed on Mar. 2, 2007, in the Korean Intellectual Property Office, and entitled: “Pixel, Organic Light Emitting Display Using the Same, and Driving Method Thereof,” is incorporated by reference herein in its entirety. 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are illustrated. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     It will also be understood that, although the terms “first,” “second,” etc., may be used herein to describe various elements, such elements should not be limited by these terms. These terms are only used to distinguish an element from other elements. Thus, a first element discussed herein could be termed a second element, etc., without departing from the teachings of example embodiments. 
     Hereinafter, embodiments according to the present invention will be described with reference to the accompanying drawings, namely,  FIG. 1  to  FIG. 11 . Here, when one element is connected to another element, one element may be not only directly connected to another element but also indirectly connected to another element via another element. Further, irrelevant elements maybe omitted for clarity. Also, like reference numerals refer to like elements throughout. 
       FIG. 1  illustrates an organic light emitting display according to an embodiment of the present invention. With reference to  FIG. 1 , the organic light emitting display according to an embodiment of the present invention may include a pixel portion  230 , a scan driver  210 , a data driver  220 , and a timing controller  250 . 
     The pixel portion  230  may include a plurality of pixels  240 , which are coupled with scan lines S 1  to Sn, first control lines CL 11  to CL 1   n , second control lines CL 21  to CL 2   n , emission control lines E 1  to En, and data lines D 1  to Dm. The scan driver  210  may drive the scan lines S 1  to Sn, first control lines CL 11  to CL 1   n,  second control lines CL 21  to CL 2   n , and the emission control lines E 1  to En. The data driver  220  may drive the data lines D 1  to Dm. The timing controller  250  may control the scan driver  210  and the data driver  220 . 
     The scan driver  210  may receive a scan driving control signal SCS from the timing controller  250 . The scan driver  210  that receives the scan driving control signal SCS may sequentially generate and provide a scan signal to the scan lines S 1  through Sn. Further, the scan driver  210  may generate a first control signal and a second control signal in response to the scan driving control signal SCS, sequentially provide the first control signal to the first control lines CL 11  to CL 1   n , and sequentially provide the second control signal to the second control lines CL 21  to CL 2   n . Moreover, the scan driver  210  may sequentially generate and provide an emission control signal to the emission control lines E 1  to En. 
     The emission control signal may have a greater width than that of the scan signal. In practice, a high emission control signal may be supplied to an i-th emission control line to overlap a low scan signal supplied to the (i−1)-th scan line and the i-th scan line. Further, a high first control signal and a low second control signal supplied from the first and second i-th control lines, respectively, may overlap a high emission control signal supplied to the i-th emission control line. 
     The data driver  220  may receive a data driving signal DCS from the timing controller  250 . When the data driver  220  receives the data driving signal DCS, the data driver  220  may generate and provide a data signal Data to the data lines D 1  through Dm. 
     The timing controller  250  may generate a data driving signal DCS and a scan driving signal SCS corresponding to synchronization signals supplied from an exterior. The data driving signal DCS generated by the timing controller  250  may be provided to the data driver  220 , and the scan driving signal SCS may be provided to the scan driver  210 . Further, the timing controller  250  may provide an externally supplied data signal Data to the data driver  220 . 
     The pixel portion  230  may be coupled to a first power source ELVDD and a second power source ELVSS, both of which may be external to the pixel portion  230 . Thus, voltages of each of the first and second power supplies ELVDD and ELVSS may be supplied to each of the pixels  240 . Accordingly, each of the pixels  240  receiving voltage from the first and second power sources (ELVDD) and (ELVSS) may generate light in accordance with the data signal Data supplied thereto. 
     The pixels  240  may compensate for degradation of organic light emitting diode (OLEDs) and threshold voltages of drive transistors included therein to generate light of desired luminance. To do this, each of the pixels  240  may include a compensator (not shown in  FIG. 1 , but discussed in detail below) for compensating the degradation of the OLEDs and the threshold voltage of the drive transistor. 
     So as to compensate the threshold voltage of the drive transistor, a pixel  240  positioned at an i-th horizontal line may be coupled to an i-th scan line Si and an (i−1)-th scan line Si−1. Thus, a zero-th scan line S 0  may be further installed preceding the first scan line S 1 . 
       FIG. 2  illustrates a circuit diagram of a pixel  240 ′ that may be used as the pixel  240  shown in  FIG. 1  according to an embodiment. For convenience of a description,  FIG. 2  illustrates the pixel  240 ′ coupled to an n-th scan line Sn and an m-th data line Dm. 
     With reference to  FIG. 2 , the pixel  240 ′ may include an OLED, a pixel circuit  244 , and a compensator  242 . The pixel circuit  244  may include first through sixth transistors M 1  to M 6  and a storage capacitor Cst. Second transistor M 2  may function as a drive transistor. The pixel circuit  244  may compensate a threshold voltage of the second transistor M 2 . The compensator  242  may compensate for degradation of the OLED. The pixel circuit  244  may control an amount of an electric current supplied to the OLED. 
     An anode electrode of the OLED may be coupled to the pixel circuit  244 , and a cathode electrode thereof may be coupled to the second power source ELVSS. The OLED may generate light having predetermined luminance corresponding to an electric current supplied from the second transistor (namely, drive transistor) M 2  via the sixth transistor M 6 . The first power source ELVDD may have a voltage higher than that of the second power source ELVSS. 
     The first transistor M 1  may be coupled to the scan line Sn and the data line Dm. The second transistor (or drive transistor) may control an amount of an electric current supplied to the OLED. The third transistor M 3  may diode-connect the second transistor M 2 . The fourth transistor M 4  may be coupled between a gate electrode of the second transistor M 2  and a voltage source Vsus. The fifth transistor M 5  may be coupled between the second transistor M 2  and the first power source ELVDD. The sixth transistor M 6  may be coupled between the second transistor M 2  and the OLED. 
     The first transistor M 1  may have a gate electrode coupled to the scan line Sn, a first electrode coupled to a data line Dm, and a second electrode coupled to a first electrode of the second transistor M 2 . When a low scan signal is supplied to the scan line Sn, the first transistor M 1  is turned-on to transfer the data signal Data supplied to the data line Dm to the first electrode of the second transistor M 2 . 
     The second transistor M 2  may have a gate electrode coupled to a first node N 1 , the first electrode coupled to the second electrode of the first transistor M 1 , and a second electrode coupled to a first electrode of the sixth transistor M 6 . The second transistor M 2  having a construction described above supplies an electric current corresponding to a voltage applied to the first node. 
     The third transistor M 3  may have a first electrode coupled to the second electrode of the second transistor M 2 , a second electrode coupled to the first node N 1 , and a gate electrode coupled to the scan line Sn. When a low scan signal is supplied to the n-th scan line Sn−1, the third transistor M 3  is turned on to diode-connect the second transistor M 2 . 
     The fourth transistor M 4  may have a first electrode coupled to the first node N 1 , a second electrode coupled to the voltage source Vsus, and a gate electrode coupled to the (n−1)-th scan line Sn−1. When a low scan signal is supplied to the (n−1)-th scan line Sn−1, the fourth transistor M 4  is turned on to initialize a voltage of the first node N 1  with a voltage of the voltage source Vsus. 
     The fifth transistor M 5  may have a first electrode coupled to the first power source ELVDD, a first electrode coupled to the first electrode of the second transistor M 2 , and a gate electrode coupled to the emission control line En. When a low emission control signal is supplied to the emission control line En, the fifth transistor M 5  is turned-on to connect the second transistor M 2  to the first power source ELVDD. 
     The first electrode of the sixth transistor M 6  may be coupled to the second electrode of the second transistor M 2 , a second electrode coupled to the OLED, and a gate electrode of the sixth transistor M 6  may be coupled to the emission control line En. When a low emission control signal is supplied to the emission control line En, the sixth transistor M 6  is turned-on to connect the second transistor M 2  with the OLED. 
     The compensator  242  may control a voltage in the gate electrode of the second transistor M 2 , namely, a voltage of the first node N 1 , corresponding to a degradation of the OLED. Accordingly, the compensator  242  may be coupled with the voltage source Vsus, the first control line CL 1   n , and the second control line CL 2   n . The compensator  242  may control the voltage of the first node N 1  corresponding to the degradation of the OLED. The voltage of the voltage source Vsus may be set to a voltage lower than a voltage Voled of the OLED. The voltage Voled of the OLED may be set to a voltage applied to the OLED, e.g., a threshold voltage of the OLED. The voltage Voled of the OLED may change in accordance with degradation of the OLED. In practice, as the OLED degrades, the threshold voltage of the OLED is increased. 
       FIG. 3  illustrates a circuit view of a pixel  240   a  including a compensator  242   a  in accordance with an embodiment for use as the pixel  240 ′ shown in  FIG. 2 . 
     With reference to  FIG. 3 , the compensator  242   a  may include a seventh transistor M 7 , an eighth transistor M 8 , and a feedback capacitor Cfb. The seventh transistor M 7  and the eighth transistor M 8  may be coupled between the voltage source Vsus and the anode electrode of the OLED. The feedback capacitor Cfb may be coupled between the first node N 1  and a second node N 2 , which is a node common to the seventh transistor M 7  and the eighth transistor M 8 . 
     The seventh transistor M 7  may be coupled between the second node N 2  and the OLED. The seventh transistor M 7  may be controlled by a second control signal supplied to the second control line CL 2   n . For example, when a low second control signal is supplied to the seventh transistor M 7 , the seventh transistor M 7  is turned-on. Otherwise, the seventh transistor M 7  is turned-off. 
     The eighth transistor M 8  may be coupled between the second node N 2  and the voltage source Vsus. The eighth transistor M 8  may be controlled by a first control signal supplied to the first control line CL 21 . For example, when a low first control signal is supplied to the eighth transistor M 8 , the eighth transistor M 8  is turned-on. Otherwise, the eighth transistor M 8  is turned-off. 
     The seventh transistor M 7  and the eighth transistor M 8  may be alternately turned-on/off. The feedback capacitor Cfb may transfer a voltage drop of the second node N 2  to the first node N 1 . 
       FIG. 4  illustrates a waveform diagram for driving the pixel  240   a  shown in  FIG. 3 . 
     With reference to  FIG. 3  and  FIG. 4 , when a low scan signal is supplied to the (n−1)-th scan line Sn−1, the fourth transistor M 4  is turned-on. When the fourth transistor M 4  is turned-on, a voltage of the voltage source Vsus is supplied to the first node N 1 . That is, while a low scan signal is supplied to the (n−1)-th scan line Sn−1, a voltage of the first node N 1  is initialized with a voltage of the voltage source Vsus. The voltage of the voltage source Vsus may be set to a value lower than that of the data signal Data. 
     When a high emission control signal is supplied to the emission control En, the fifth transistor M 5  and the sixth transistor M 6  are turned-off. When a high first control signal is supplied to the first control line CL 1   n , the eighth transistor M 8  is turned-off. When a low second control signal is supplied to the second control line CL 2   n , the seventh transistor M 7  is turned-on. When the seventh transistor M 7  is turned-on, the voltage Voled of the OLED is supplied to the second node N 2 . When the sixth transistor M 6  is turned-off, the voltage Voled of the OLED is set to a threshold voltage of the OLED. 
     When a low scan signal is supplied to the n-th scan line Sn, the first transistor M 1  and the third transistor M 3  are turned-on. When the third transistor M 3  is turned-on, the second transistor M 2  is diode-connected. When the first transistor M 1  is turned-on, the data signal Data supplied to the data line Dm is provided to the first electrode of the second transistor M 2  through the first transistor M 1 . When a voltage of the first node N 1  is set to be lower than that of the data signal Data, the data signal Data is supplied to the first node N 1  through the second transistor M 2  and the third transistor M 3 . Since the data signal Data is supplied to the first node N 1  through the diode-connected second transistor M 2 , the storage capacitor Cst is charged with a voltage corresponding to the data signal Data and a threshold voltage of the second transistor M 2 . 
     When a high scan signal is supplied to the n-th scan line Sn, the first transistor M 1  and the third transistor M 3  are turned-off. When a high second control signal is supplied, the seventh transistor M 7  is turned-off. Accordingly, the OLED is electrically isolated from the second node N 2 . Consequently, the second node N 2  maintains the threshold voltage of the OLED. When supply of the high emission control signal stops, i.e., the emission control signal transitions low, the fifth transistor M 5  and the sixth transistor M 6  are turned-on. 
     When the fifth transistor M 5  and the sixth transistor M 6  are turned-on, the first power source ELVDD, the second transistor M 2 , and the OLED are electrically connected to each other. Accordingly, the second transistor M 2  supplies an electric current corresponding to a voltage applied to the first node N 1  to the OLED. 
     When a low first control signal is supplied, the eighth transistor M 8  is turned-on. When the eighth transistor M 8  is turned-on, a voltage of the second node N 2  decreases to a voltage of the voltage source Vsus. At this time, the gate voltage of the second transistor M 2 , i.e., a voltage of the first node N 1 , also decreases corresponding to a voltage decrease of the second node N 2 . Further, the second transistor M 2  supplies an electric current corresponding to the dropped voltage to the OLED. 
     As time goes by, the OLED may degrade. As the OLED degrades, a voltage applied to the OLED increases. Accordingly, as the OLED degrades, a voltage drop, i.e., the difference between Vsus and Voled, at the second node N 2  increases. In other words, as the OLED degrades, the voltage Voled of the OLED supplied to the second node N 2  increases. Accordingly, the voltage drop at the second node N 2  increases when the OLED degrades. 
     When the voltage drop at the second node N 2  increases, a voltage drop at the first node N 1  increases. Accordingly, an amount of an electric current supplied to the OLED from the second transistor M 2  increases for the same data signal Data. Thus, in embodiments, as the OLED degrades, the electric current supplied to the OLED from the second transistor M 2  increases. Accordingly, luminance deterioration due to degradation of the OLED may be compensated. Further, embodiments may control a duration of supply of the electric current from the second transistor M 2  corresponding to the first node to the OLED, allowing a degree of compensation according to the degradation of the OLED to be controlled. 
     In other words, while a high first control signal is supplied to the first control line CL 1   n , the degradation of the OLED is not compensated. When a low first control signal is supplied to the first control line CL 1   n  is supplied, the degradation of the OLED is compensated. Thus, in accordance with an embodiment, luminance of the OLED may be controlled by controlling the first control signal supplied to the first control line CL 1   n . In other words, by supplying low first control signal for a longer time, the luminance of the OLED may be increased. 
       FIG. 5  illustrates a pixel  240   b  including a compensator  242   b  for use as the pixel  240 ′ shown in  FIG. 2 . A description of elements of the compensator  242   b  shown in  FIG. 5  that are the same as the embodiment shown in  FIG. 3  will be omitted. 
     With reference to  FIG. 5 , the compensator  242   b  may include the seventh transistor M 7 , the eighth transistor M 8 , and the feedback capacitor Cfb. The seventh transistor M 7  and the eighth transistor M 8  may be coupled between the voltage source Vsus and the anode electrode of the OLED. The feedback capacitor Cfb may be coupled between the first node N 1  and the second node N 2 . 
     The seventh transistor M 7  may be coupled between the second node N 2  and the OLED. The seventh transistor M 7  may be controlled by the second control signal supplied to the second control line CL 2   n . For example, when a low second control signal is supplied, the seventh transistor M 7  is turned-on. Otherwise, the seventh transistor M 7  is turned-off. 
     The eighth transistor M 8  may be coupled between the second node N 2  and the voltage source Vsus. The eighth transistor M 8  may be controlled by the emission control signal supplied to the emission control line En. For example, when a low emission control signal is supplied, the eighth transistor M 8  is turned-on. Otherwise, the eighth transistor M 8  is turned-off. 
     The compensator  242   b  may have substantially the same functions and construction as the compensator  242   a , except the eighth transistor M 8  is coupled to the emission control line En. Accordingly, in the pixel  240   b , the first control line CL 1   n  may be removed. 
       FIG. 6  illustrates a waveform diagram for use in driving the pixel  240   b  shown in  FIG. 5 . 
     With reference to  FIG. 5  and  FIG. 6 , a low scan signal supplied to an (n−1)-th scan line Sn−1 turns-on the fourth transistor M 4 . When the fourth transistor M 4  is turned-on, a voltage of the voltage source Vsus is supplied to the first node N 1 . Accordingly, the first node N 1  is initialized with a voltage of the voltage source Vsus. 
     When a high emission control signal is supplied to the emission control line En, the fifth transistor M 5 , the sixth transistor M 6 , and the eighth transistor M 8  are turned-off. When a low second control signal is supplied to the second control line CL 2   n , the seventh transistor M 7  is turned-on. When the seventh transistor M 7  is turned-on, the voltage Voled of the OLED is supplied to the second node N 2 . 
     When a low scan signal is supplied to the n-th scan line Sn, the first transistor M 1  and the third transistor M 3  are turned-on. When the third transistor M 3  is turned-on, the second transistor M 2  is diode-connected. When the first transistor M 1  and the third transistor M 3  are turned-on, the data signal Data supplied to the data line Dm is provided to the first node N 1 . At this time, the storage capacitor Cst is charged with a voltage corresponding to the data signal Data and a threshold voltage of the second transistor M 2 . 
     When a high scan signal is supplied to the n-th scan line Sn, the first transistor M 1  and the third transistor M 3  are turned-off. When a high second control signal is supplied, the seventh transistor M 7  is turned-off. When a low emission control signal is supplied, the fifth transistor M 5 , the sixth transistor M 6 , and the eighth transistor M 8  are turned-on. When the eighth transistor M 8  is turned-on, a voltage of the second node N 2  drops from a voltage of the OLED to a voltage of the voltage source Vsus. A voltage of the first node N 1  also drops corresponding to a voltage drop of the second node N 2 . Since the voltage drop in the first node N 1  corresponds to a degradation degree of the OLED, the degradation of the OLED may be compensated. 
     Meanwhile, because the fifth transistor M 5  and the sixth transistor M 6  are turned-on, the second transistor M 2  controls an amount of an electric current supplied to the OLED corresponding to a voltage applied to the first node N 1 . The OLED generates light of predetermined luminance corresponding to the electric current supplied from the second transistor M 2 . 
       FIG. 7  illustrates a pixel  240   c  having a compensator  242   c  for use as the pixel  240 ′ shown in  FIG. 2 . A description of the elements of the compensator  242   c  shown in  FIG. 7  that are the same as that of the compensator  242   a  shown in  FIG. 3  will not be repeated. 
     With reference to  FIG. 7 , the compensator  242   c  may include a seventh transistor M 7 ′, the eighth transistor M 8 , and the feedback capacitor Cfb. The seventh transistor M 7 ′ and the eighth transistor M 8  may be coupled between the voltage source Vsus and the anode electrode of the OLED. The feedback capacitor Cfb may be coupled between the first node N 1  and the second node N 2 . 
     The seventh transistor M 7 ′ may be coupled between the second node N 2  and the OLED. The seventh transistor M 7 ′ may be controlled by an emission control signal supplied to the emission control line En. For example, when a high emission control signal is supplied, the seventh transistor is turned-on. Otherwise, the seventh transistor M 7 ′ is turned-off. The seventh transistor M 7 ′ may have a conductivity type different from that of the transistors M 1  to M 6 , e.g., may be an NMOS transistor. 
     The eighth transistor M 8  may be coupled between the second node N 2  and the voltage source Vsus. The eighth transistor M 8  may be controlled by the emission control signal supplied to the emission control line En. For example, when a high emission control signal is supplied, the eighth transistor M 8  is turned-off. Otherwise, the eighth transistor M 8  is turned-on. The eighth transistor M 8  may have the same conductivity type than that of the transistors M 1  to M 6 , e.g., may be a PMOS transistor. 
     Thus, the compensator  242   c  may have substantially the same functions and construction as those the compensator  242   a , except that the seventh transistor M 7 ′ and the eighth transistor M 8  may have different conductivity types, and the seventh transistor M 7 ′ and the eighth transistor M 8  are coupled to the emission control line En. Accordingly, in the pixel  242   c , the first control line CL 1   n  and the second control line CL 2   n  may be omitted. 
       FIG. 8  illustrates a waveform diagram for use in driving the pixel  240   c  shown in  FIG. 7 . 
     With reference to  FIG. 7  and  FIG. 8 , when a low scan signal is supplied to an (n−1)-th scan line Sn−1, the fourth transistor M 4  is turned-on. When the fourth transistor M 4  is turned-on, a voltage of the voltage source Vsus is supplied to the first node N 1 . Accordingly, the first node N 1  is initialized with a voltage of the voltage source Vsus. 
     When a high emission control signal is supplied to the emission control En, the fifth transistor M 5 , the sixth transistor M 6 , and the eighth transistor M 8  are turned-off, whereas the seventh transistor M 7 ′ is turned-on. When the seventh transistor M 7 ′ is turned-on, a voltage of the OLED is supplied to the second node N 2 . 
     During supply of the high emission control signal to the emission control line En, a low scan signal is supplied to the n-th scan line Sn to turn-on the first transistor M 1  and the third transistor M 3 . When the third transistor M 3  is turned-on, the second transistor M 2  is diode-connected. When the first transistor M 1  and the third transistor M 3  are turned-on, the data signal Data supplied to the data line Dm is provided to the first node N 1 . At this time, the storage capacitor Cst is charged with a voltage corresponding to the data signal and a threshold voltage of the second transistor M 2 . 
     Next, a high scan signal and a low emission control signal may be sequentially supplied. When a high scan signal is supplied, the first transistor M 1  and the third transistor M 3  are turned-off. When a low emission control signal is supplied, the fifth transistor M 5 , the sixth transistor M 6 , and the eighth transistor M 8  are turned-on, but the seventh transistor M 7 ′ is turned-off. When the eighth transistor M 8  is turned-on, a voltage of the second node N 2  drops from a voltage of the OLED to a voltage of the voltage source Vsus. At this time, a voltage of the first node N 1  also drops corresponding to a voltage drop of the second node N 2 . Since a voltage drop in the first node N 1  corresponds to a degradation degree of the OLED, the degradation of the OLED may be compensated. 
     Since the fifth transistor M 5  and the sixth transistor M 6  are turned-on, the second transistor M 2  controls an amount of an electric current supplied to the OLED corresponding to a voltage applied to the first node N 1 . The OLED generates light of predetermined luminance corresponding to the electric current supplied from the second transistor M 2 . 
       FIG. 9  illustrates a circuit diagram of a pixel  240 ″ for use as the pixel  240  shown in  FIG. 1 . Construction of the pixel  240 ″ shown in  FIG. 9  that is the same as the pixel  240 ′ shown in  FIG. 2  will not be described. With reference to  FIG. 9 , the pixel  240 ″ may include a compensator  243  and a pixel circuit  245 . 
     The pixel circuit  245  may include first to sixth transistors M 1  to M 6 . The third transistor M 3  may be coupled between the gate electrode and the second electrode of the second transistor M 2 , and may diode-connect the second transistor M 2 . When a low second control signal is supplied to a second control line CL 2   n,  the third transistor M 3  is turned-on. Otherwise, the third transistor M 3  is turned-off. 
     The sixth transistor M 6  may be coupled between the second transistor M 2  and the OLED. When a high emission control signal is supplied to an (n+1)-th emission control line En+1, the sixth transistor M 6  is turned-off. Otherwise, the sixth transistor M 6  is turned-on. 
     The compensator  243  may include the seventh transistor M 7  and the eighth transistor M 8 . The seventh transistor M 7  may be coupled with the second node N 2  and the OLED. When a low second control signal is supplied to a second control line CL 2   n , the seventh transistor M 7  is turned-on. Otherwise, the seventh transistor M 7  is turned-off. The eighth transistor M 8  may be coupled between the second node N 2  and the voltage source Vsus. When a high emission control signal is supplied to the emission control line En, the eighth transistor M 8  is turned-off. Otherwise, the eighth transistor M 8  is turned-on. 
     Furthermore, in another embodiment of the present invention, a voltage of the voltage source Vsus may be set to be higher or lower than a voltage of the OLED. A detailed description thereof will be provided below. 
       FIG. 10  illustrates a waveform diagram for use in driving the pixel  240 ″ shown in  FIG. 9 . 
     Referring to  FIG. 9  and  FIG. 10 , first, a high emission control signal is supplied to the emission control line En and a low second control signal is supplied to the second control line CL 2   n . When a high emission control signal is supplied, the fifth transistor M 5  and the eighth transistor M 8  are turned-off. When a low second control signal is supplied, the third transistor M 3  and the seventh transistor M 7  are turned-on. 
     When the third transistor M 3  is turned-on, the first node N 1  is electrically connected to the second power source ELVSS through the third transistor M 3 , the sixth transistor M 6 , and the OLED. In this case, the first node N 1  is initialized with a voltage of the second power source ELVSS. In practice, the first node N 1  is initialized with a voltage slightly greater than a voltage of the second power source ELVSS. Since the fifth transistor M 5  is turned-off, the OLED generates weak light that does not influence an image to be displayed. 
     Then, a high emission control signal is supplied to the (n+1)-th control line En+1, and a low scan signal is supplied to the scan line Sn. When a high emission control signal is supplied to the (n+1)-th control line En+1, the sixth transistor M 6  is turned-off. At this time, since the seventh transistor M 7  remains turned-on, the second node N 2  is set to a threshold voltage of the OLED. 
     When the low scan signal is supplied to the scan line Sn, the first transistor M 1  is turned-on. When the first transistor M 1  is turned-on, the data signal Data supplied to the data line Dm is provided to the first node N 1 . At this time, the storage capacitor Cst is charged with an electric current corresponding to the data signal and the threshold voltage of the OLED. 
     After the storage capacitor Cst is charged with a predetermined voltage, the emission control signal transitions low and the second control signal transitions high. When a high scan signal is supplied to the scan line Sn, the first transistor M 1  is turned-off. When supply of the second control signal stops, the third transistor M 3  and the seventh transistor M 7  are turned-off. 
     When a low emission control signal is supplied to the emission control line En stops, the eighth transistor M 8  is turned-on. When the eighth transistor M 8  is turned-on, a voltage of the second node N 2  decreases or increases to a voltage of the voltage source Vsus. 
     As described above, when the voltage of the voltage source Vsus is less than the threshold voltage of the OLED, the degradation of the OLED may be compensated. Alternatively, when the voltage of the voltage source Vsus is greater than the threshold voltage of the OLED, the degradation of the OLED may be compensated. 
     For example, when the voltage of the voltage source Vsus is set to 5V, and an initial threshold voltage of the OLED is 1V, a voltage rise of a voltage in the second node N 2  is 4V. The voltage of the first node N 1  also increases by 4V. When the OLED degrades, e.g., to a threshold voltage of 2V, the voltage rise of the second node N 2  is 3V, i.e., the voltage rise decreases. The voltage rise of the first node N 1  also corresponds to the voltage rise of the second node N 2 . Thus, as the OLED degrades, the voltage rise of the first node N 1  decreases. Accordingly, as the OLED degrades, more electric current may be supplied to the OLED. 
     After a voltage of the first node N 1  is increased or reduced in accordance with a voltage of the voltage source Vsus, a low emission control signal is supplied to the (n+1)-th emission control line En+1 to turn-on the sixth transistor M 6 . Accordingly, the second transistor M 2  supplies an electric current corresponding to a voltage applied to the first node N 1  to the OLED. 
       FIG. 11  illustrates a comparison of a pixel without a compensation circuit and with a compensation circuit according to embodiments. In  FIG. 11 , 6TFT indicates the pixel  240 ′ shown in  FIG. 2  without the compensator  242 , 8TFT indicates the pixel  240   a  shown in  FIG. 4 , and 7TFT indicates the pixel  240 ″ shown in  FIG. 9 . In  FIG. 11 , a Y-axis indicates a percentage deviation of an electric current flowing to the OLED and an X-axis indicates a change of a threshold voltage corresponding to a degradation of the OLED. 
     With reference to  FIG. 11 , when the pixel  240 ′ does not include the compensator  242 , electric current flowing to the OLED as the OLED degrades is decreased. However, according to embodiments, electric current flowing to the OLED increases as the OLED degrades. 
     As described above, in the pixel, the organic light emitting display, and a driving method thereof, a voltage of the gate electrode in a drive transistor may be controlled corresponding to the degradation of an OLED is degraded, thereby compensating the degradation of the OLED. Furthermore, since embodiments may compensate a threshold voltage of the drive transistor, images having adequate luminance may be displayed regardless of a deviation of the threshold voltage. 
     Exemplary embodiments of the present invention have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims.