Patent Publication Number: US-7724218-B2

Title: Organic light-emitting diode display device and driving method thereof

Description:
This application claims the benefit of Korean Patent Application No. P06-0049435 filed in Korea on Jun. 01, 2006, which is hereby incorporated by reference. 
     BACKGROUND 
     1. Field 
     The present embodiments relate to an organic light-emitting diode display device and a driving method thereof. 
     2. Related Art 
     Recently, various flat panel display devices have been developed. These flat panel display devices have a reduced weight and bulk and are capable of eliminating disadvantages of a cathode ray tube. Such flat panel display devices include, for example, a liquid crystal display device (hereinafter, referred to as “LCD”), a field emission display device (hereinafter, referred to as “FED”), a plasma display panel (hereinafter, referred to as “PDP”) and an electro-luminescence display device. 
     In such flat panel display devices, the PDP has a light weight, a small bulk size and a large dimension screen because its structure and manufacturing process are simple. However, the PDP has low light-emission efficiency and large power consumption. 
     The active matrix LCD employing a thin film transistor (hereinafter, referred to as “TFT”) as a switching device has drawbacks in that it is difficult to increase the dimension screen because a semiconductor process is used. Recently, however, the LCD has an increased demand because it is mainly used for a display device of a notebook personal computer. 
     The EL device is largely classified into an inorganic EL device and an organic light-emitting diode device depending upon a material of a light-emitting layer, and is a self-luminous device. When compared with the above-mentioned display devices, the EL device has advantages of a fast response speed, large light-emission efficiency, a large brightness and a large viewing angle. 
     Referring to  FIG. 1 , the organic light-emitting diode device comprises an anode electrode made from a transparent conductive material on a glass substrate, an organic compound layer disposed on the organic light-emitting diode device, and a cathode electrode made from a conductive metal. 
     The organic compound layer is comprised of a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL and an electron injection layer. 
     If a driving voltage is applied to the anode electrode and the cathode electrode, then a hole within the hole injection layer and an electron within the electron injection layer move toward the emission layer, respectively, to excite the emission layer, so that the emission layer emits visible rays. The visible rays generated from the emission layer display a picture or a motion picture. 
     The organic light-emitting diode device has been applied to a display device of a passive matrix type or to a display of an active matrix type using a TFT as a switching element. The passive matrix type crosses the anode electrode with the cathode electrode to select a light-emitting cell in accordance with a current applied to the electrodes while the active matrix type selectively turns on an active element, for example, a TFT to select a light-emitting cell and maintains a light-emitting of the light-emitting cell using a voltage maintained at a storage capacitor. 
       FIG. 2  is a circuit diagram equivalently showing one pixel in an organic light-emitting diode display device of an active matrix type. 
     Referring to  FIG. 2 , the organic light-emitting diode display device of the active matrix type includes an organic light-emitting diode element OLED, a data line DL and a gate line GL that cross with each other, a switch TFT T 2 , a driving TFT T 1  and a storage capacitor Cst. The driving TFT T 1  and the switch TFT T 2  are implemented in a p-type MOS-FET. 
     The switch TFT T 2  is turned-on in response to a gate low-level voltage (or a scanning voltage) from the gate line GL to be electrically connected a current path between a source electrode and a drain electrode of the switch TFT T 2 . The switch TFT T 2  maintains an off-state when a voltage on the gate line GL is less than a threshold voltage (hereinafter, referred to as “Vth”) of the switch TFT T 2 , for example, a gate high-level voltage. 
     A data voltage from the data line DL is applied, via the source electrode and the drain electrode of the switch TFT T 2 , a gate electrode and a storage capacitor Cst of the driving TFT T 1  during an on-time period of the switch TFT T 2 . Alternatively, a current path between the source electrode and the drain electrode of the switch TFT T 2  is opened during an off-time period of the switch TFT T 2  to not apply the data voltage VDL to the driving TFT T 1  and the storage capacitor Cst. 
     The source electrode of the driving TFT T 1  is connected to a driving voltage line VL and one end of the storage capacitor Cst. The drain electrode of the driving TFT T 1  is connected to the anode electrode of the organic light-emitting diode display OLED. The gate electrode of the driving TFT T 1  is connected to the drain electrode of the switch TFT T 2 . Such a driving TFT T 1  adjusts a current amount between the source electrode and the drain electrode in accordance with a gate voltage supplied to the gate electrode, for example, a data voltage to have the organic light-emitting diode display OLED to be emitted at brightness corresponding to the data voltage. 
     The storage capacitor Cst stores a difference voltage between the data voltage and a high-level electric potential driving voltage VDD, which constantly maintains a voltage applied to the gate electrode of the driving TFT T 1  during one frame period. 
     The organic light-emitting diode display OLED is implemented in the structure as shown in  FIG. 1  and includes a cathode electrode connected to the drain electrode of the driving TFT T 1  and a cathode electrode supplied with a ground voltage source GND. The organic light-emitting diode display OLED is emitted by a current between a source-drain of the driving TFT T 1  defined in accordance with the gate voltage of the driving TFT T 1 . 
     The organic light-emitting diode display device as shown in  FIG. 2  determines a current flowing into the organic light-emitting diode display OLED in accordance with a characteristics of the driving TFT T 1 . Accordingly, if the characteristics of the driving TFT T 1  are uniform for each pixel, then a picture is displayed with constant brightness characteristics. The characteristics of the driving TFT T 1 , for example, a threshold voltage characteristic is different at each position in a screen of the manufactured panel. Because a high-level potential driving voltage VDD is dropped by the driving voltage line VL, brightness at each position in the screen even through the same data are supplied to the screen. 
       FIG. 3  shows a vertical strip phenomenon of a screen generated at the same gray scale data by a voltage drop defined by a threshold voltage deviation of the driving TFT T 1  and the driving voltage line VL at the organic light-emitting diode display device of the active matrix type. 
     For example, as shown in  FIG. 4 , because a power of laser is instabilized in accordance with in length of time when an amorphous silicon a-Si formed on a TFT substrate of the organic light-emitting diode display device is crystallized in a poly silicon p-Si at a laser crystallization process, the semiconductor characteristics of the TFT substrate are uninform. Because a membranous of a silicon thin film generated at a border between portions irradiated at different time, the scanning and the laser irradiation are performed for the surface of the substrate at a regular interval, the semiconductor characteristics of the TFT substrate are uniform. When the semiconductor characteristics of the TFT substrate generates a deviation depending upon a position, a stripe phenomenon is generated as shown in  FIG. 3  and brightness is not uniformly generated at the same gray scale data. 
     SUMMARY 
     The present embodiments may obviate one or more of the limitations of the related art. For example, in one embodiment, an organic light-emitting diode display device is adaptive to minimize a voltage drop by a driving voltage supply line and an adverse effect by a threshold voltage change of a thin film transistor to uniform display brightness. 
     In a first embodiment, the organic light-emitting diode display device includes a driving voltage source generating a driving voltage. A reference voltage source generates a reference voltage. A reference current source generates a reference current. A storage capacitor is connected between a first node and a second node. An organic light-emitting diode device is connected between a third node and a ground voltage source. A first scanning signal is supplied to a first scan line. A second scanning signal is supplied to a second scan line. The second scanning signal has an inverse-phase against the first scanning signal. A data line crosses the first and second scan lines, and to which a data voltage is supplied. 
     A first switch element maintains an off-state during a first period, and then supplies the reference voltage to the first node in response to the first scanning signal, during a second period. A second switch element supplies the data voltage to the first node in response to the second scanning signal, during the first period, and then maintaining an off-state during the second period. A third switch element adjusts a current which is supplied to the organic light-emitting diode device in accordance with a voltage of the second node. A fourth switch element supplies the reference current to the second node in response to the second scanning signal, during the first period, and then maintains an off-state, during the second period. A fifth switch element forms a current path between the second node and the third node in response to the second scanning signal, during the first period, and then maintains an off-state, during the second period. A sixth switch element cuts-off a current flowing into the organic light-emitting diode device via the third node, during the first period, and then forms a current path between the third node and the organic light-emitting diode device in response to any one of the first scanning signal and a voltage of the second node. 
     An organic light-emitting diode display device according to a second embodiment includes a driving voltage source that generates a driving voltage. A reference voltage source generates a reference voltage. A reference current source generates a reference current. A storage capacitor connected between a first node and a second node. An organic light-emitting diode device is connected between a third node and a ground voltage source. A scanning signal is supplied to a scan line. A data voltage is supplied to a data line that crosses the first and second scan lines. 
     A first switch element maintains an off-state in response to a first voltage of the scanning signal, during a first period, and then supplies the reference voltage to the first node in response to a second voltage of the scanning signal, during a second period. A second switch element supplies the data voltage to the first node in response to a first voltage of the scanning signal, during the first period, and then maintains an off-state, during the second period. A third switch element adjusts a current which is supplied to the organic light-emitting diode device in accordance with a voltage of the second node. A fourth switch element supplies the reference current to the second node in response to a first voltage of the scanning signal, during the first period, and then maintains an off-state, during the second period. A fifth switch element forms a current path between the second node and the third node in response to a first voltage of the scanning signal, during the first period, and then maintains an off-state, during the second period. A sixth switch element cuts-off a current flowing into the organic light-emitting diode device via the third node, during the first period, and then forms a current path between the third node and the organic light-emitting diode device in response to any one of a voltage of the second node and a second voltage of the scanning signal, during the second period. 
     A method of driving an organic light-emitting diode display device according to the first embodiment, including a plurality of data lines and data lines that cross with each other, a storage capacitor connected between a first node and a second node and an organic light-emitting diode element connected to a third node and a ground voltage source. The method comprising generating a driving voltage, a reference voltage, and a reference current; supplying a first scanning signal to a first scan line and, at the same time, supplying a second scanning signal having an inverse-phase against the first scanning signal to a second scan line; supplying gate voltages to the data lines; during a first period when the first scanning signal maintains a first logic voltage and the second scanning signal maintains a second logic voltage, turning-off a first switch element to which the reference voltage is supplied and connected to the first node and a sixth switch element connected between the third node and the organic light-emitting diode element, turning-on a second switch element to which the data voltage is supplied and connected to the first node, a fourth switch element to which the reference current is supplied and connected to the second node, and a fifth switch element connected between the second node and the third node, respectively, to charge the data voltage into the first node, connecting the second node to the third node to supply the driving voltage, and operating a third switch connected to the third node as a diode to drive the organic light-emitting diode element into a diode; and during a second period when the first scanning signal maintains a second logic voltage and the second scanning signal maintains a first logic voltage, turning-off the first and sixth switch elements, turning-on the second, the fourth and the fifth switch elements to cut-off the data voltage to be supplied to the first node and the reference current supplied to the second node, and charging the first node and the second node using the reference voltage to allow a current to be flowed into the organic light-emitting diode element via the third and sixth switch elements. 
     A method of driving an organic light-emitting diode display device according to the second embodiment, including a plurality of data lines and data lines that cross with each other, a storage capacitor connected between a first node and a second node and an organic light-emitting diode element connected to a third node and a ground voltage source. The method including generating a driving voltage, a reference voltage, and a reference current; sequentially supplying scanning signals to the scan lines; supplying data voltages to the data lines; during a first period when the scanning signal maintains an active logic voltage, turning-off a first switch element to which the reference voltage is supplied, and connected to the first node, turning-on a second switch element to which the data voltage is supplied and connected to the first node, a fourth switch element to which the reference current is supplied and connected to the second node, and a fifth switch element connected between the second node and the third node, respectively, to connect the second node to third node thereby charging the data voltage into the first node, and to connect the second node to the third node thereby supplying the driving voltage, operating a third switch element connected to the third node as a forward-vias diode to drive the organic light-emitting diode element, and operating a sixth switch element connected between the third node and the organic light-emitting diode element as a reverse-vias diode; and during a second period when the scanning signal maintains an inactive logic voltage, turning-on the first switch element and turning-off the second, the fourth and the fifth switch elements to cut off the data voltage supplied to the first node and cutting-off the reference current supplied to the second node, and charging the first node and the second node using the reference voltage to flow into the organic light-emitting diode element via the third and sixth switch element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing a structure of a related art organic light-emitting diode display device; 
         FIG. 2  is a circuit diagram showing one pixel in an organic light-emitting diode display device of a related art active matrix type; 
         FIG. 3  is a diagram showing a vertical strip phenomenon of a display picture generated in accordance with a characteristics deviation of a thin film transistor according to the related art; 
         FIG. 4  is a diagram schematically showing a laser crystallization process converting an amorphous silicon into a poly silicon according to the related art; 
         FIG. 5  is a block diagram showing an organic light-emitting diode display device according to a first embodiment; 
         FIG. 6  is a waveform diagram showing an output waveform of the drivers shown in  FIG. 5 ; 
         FIG. 7  is a circuit diagram showing a pixel according to  FIG. 5 ; 
         FIG. 8  is a circuit diagram showing a pixel according to  FIG. 5 ; 
         FIG. 9  is a block diagram showing an organic light-emitting diode display device; 
         FIG. 10  is a waveform diagram showing an output waveform of drivers according to  FIG. 9 ; 
         FIG. 11  is a circuit diagram showing a pixel according to  FIG. 9 ; and 
         FIG. 12  is a circuit diagram showing a pixel according to  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION 
     In a first embodiment, as shown in  FIG. 5  to  FIG. 8 , an organic light-emitting diode display device includes a display panel  50  provided m×n pixels  54 , a data driver  52  supplying a data voltage to data lines DL 1  to DLm, a scan driver  53  sequentially supplying an inverse-phase scanning pulse pair to m scan electrode pairs (E 1  to En and S 1  to Sn) and a timing controller  51  controlling the drivers  52  and  53 . 
     In the display panel  50 , pixels  54  are formed at pixel areas defined by an intersection of n first and second scan lines (E 1  to En and S 1  to Sn) and m data lines DL 1  to DLm. Signal lines supplying a reference voltage Vref of a constant-voltage, a reference current Iref of a constant-current and a high-level electric potential driving voltage VDD to the pixels  54  are formed at the display panel  50 . 
     The data driver  52  converts a digital video data RGB from the timing controller  51  into an analog gamma compensation voltage. The data driver  52  supplies an analog gamma compensation voltage as a data voltage Vdata to the data lines DL 1  to DLm in response to a control signal DDC from the timing controller  51  during the aligned programming period PP before an organic light-emitting diode element OLED of each pixel  54 . 
     The scan driver  53  sequentially supplies first scanning pulses EM 1  to EMn of a high-level voltage in response to a control signal SDC from the timing controller  51  to the first scan lines E 1  to En and generates second scanning pulses SCAN 1  to SCANn in an inverse-phase against the first scanning pulses EM 1  to EMn at the same time, and sequentially supplies the second scanning pulses SCAN 1  to SCANn in such a manner to be synchronized with the first scanning pulses EM 1  to EMn to the second scan lines S 1  to Sn. 
     The timing controller  51  supplies a digital video data RGB to the data driver  52  and generates a control signal DDC and GDC controlling an operation timing of the scan driver  53  and the data driver  52  using, for example, a vertical/horizontal synchronizing signal and a clock signal. 
     A constant-voltage source supplies the reference voltage Vref and a high-level electric potential driving voltage VDD. A constant-current source supplies the reference current Iref to the display panel  50 . 
     In one embodiment, as shown in  FIG. 7  and  FIG. 8 , each of the pixels  54  includes the organic light-emitting diode element OLED, six TFTs and one storage capacitor. 
       FIG. 7  shows a first embodiment of the pixels  54  at the organic light-emitting diode display device. 
     In one embodiment, as shown in  FIG. 7 , the first TFT M 1  is maintained at an off-state by the first scanning pulses EM 1  to EMn supplied from the first scan lines E 1  to En during the programming period PP while forms a current path between the reference voltage source Vref and an a-node during a light-emitting period EP. A gate electrode of the first TFT M 1  is connected to the first scan lines E 1  to En, and a source electrode of the first TFT M 1  is connected to the reference voltage source Vref. A drain electrode of the first TFT M 1  is connected to the a-node. 
     The second TFT M 2  is turned-on by the second scanning pulses SCAN 1  to SCANn supplied from the second scan lines S 1  to Sn to connect a current path between the data line DL 1  to DLm and the a-node and to be charged the data voltage Vdata into the storage capacitor Cs during the programming period PP while cuts-off a current path between the data line DL 1  to DLm and the a-node during the light-emitting period EP. A gate electrode of the second TFT M 2  is connected to the second scan lines S 1  to Sn, and a source electrode of the second TFT M 2  is connected to the data line DL 1  to DLm. A drain electrode of the second TFT M 2  is connected to the a-node. 
     The third TFT M 3  is a driving TFT and turned-on in response to a gate voltage, for example, a b-node voltage to connect a current path between a high-level electric potential driving voltage VDD and a c-node during the programming period PP and the light-emitting period EP. A gate electrode of the third TFT M 3  is connected to the b-node, and a source electrode of-the third TFT M 3  is connected to a high-level electric potential driving voltage VDD. A drain electrode of the third TFT M 3  is connected to the c-node. 
     The fourth TFT M 4  is turned-on by the second scanning pulses SCAN 1  to SCANn supplied from the second scan lines S 1  to Sn to connect a current path between the b-node and the constant-current source Iref during the programming period PP while cuts-off a current path between the b-node and the constant-current source Iref during the light-emitting period EP. A gate electrode of the fourth TFT M 4  is connected to the second scan lines S 1  to Sn, and a source electrode of the fourth TFT M 4  is connected to the b-node. A drain electrode of the fourth TFT M 4  is connected to the constant-current source Iref. 
     The fifth TFT M 5 , similar to the fourth TFT M 4 , is turned-on by the second scanning pulses SCAN 1  to SCANn supplied from the second scan lines S 1  to Sn to connect a current path between the b-node and the c-node during the programming period PP while cuts-off a current path between the b-node and the c-node during the light-emitting period EP. A gate electrode of the fifth TFT M 5  is connected to the second scan lines S 1  to Sn, and a source electrode of the fifth TFT M 5  is connected to the c-node. A drain electrode of the fourth TFT M 4  is connected to the b-node. 
     The sixth TFT M 6  is maintained at an off-state by the first scanning pulses EM 1  to EMn supplied from the first scan lines E 1  to En during the programming period PP while forms a current path between the c-node and the organic light-emitting diode element OLED during the light-emitting period EP. A gate electrode of the sixth TFT M 6  is connected to the first scan lines E 1  to En, and a source electrode of the sixth TFT M 6  is connected to the c-node. A drain electrode of the sixth TFT M 6  is connected to an anode electrode of the organic light-emitting diode element OLED. 
     The storage capacitor Cs charges a threshold voltages component and a high-level electric potential driving voltage VDD during the programming period PP, and maintains the charged voltage during the light-emitting period EP. 
     The organic light-emitting diode element OLED has the same structure as  FIG. 1 , and is emitted by a current IOLED flowing via the third TFT M 3  and the sixth TFT M 6  as shown in a dotted line of  FIG. 7  during the light-emitting period EP. 
     The first TFT M 1  charges a reference voltage Vref into one electrode of the storage capacitor Cs, and charges a driving voltage which information of has a threshold voltage of the third TFT M 3  and a high-level electric potential driving voltage VDD information into the other electrode of the storage capacitor Cs and a gate electrode of the third TFT M 3  using the reference voltage Vref during the programming period PP. 
     The second, the fourth and the fifth TFT M 2 , M 4  and MS charges a data voltage Vdata into one electrode of the storage capacitor Cs, and charges a threshold voltage of the third TFT M 3  into the other electrode of the storage capacitor Cs using a reference current Iref to carry out a scanning of a data voltage Vdata and a sampling operation of a threshold voltage during the programming period PP. 
     In one embodiment, the first scanning pulses EM 1  to EMn is maintained at a high-level voltage to turn-off the first and sixth TFT M 1  and M 6 , and the second scanning pulses SCAN 1  to SCANn are maintained at a low-level voltage to turn-on the second, the fourth and the sixth TFT M 2 , M 4  and MS during the programming period PP. A data voltage Vdata from the data line DL 1  to DLm is charged, via the second TFT M 2 , into one electrode of the storage capacitor Cs connected to the a-node. A gate voltage lower than a source voltage of the third TFT M 3  is charged into the other electrode of the storage capacitor Cs connected to the b-node. The difference voltage between the gate voltage and the source voltage of the third TFT M 3  is equal or larger than the threshold voltage of the third TFT M 3 . At the same time, for example, the third TFT M 3  is connected as a diode element because the fifth TFT M 5  is turned-on. Accordingly, a reference current Iref flows into a high-level electric potential driving voltage VDD source, the third TFT M 3 , the fifth TFT M 5 , the fourth TFT M 4  and the constant-current source Iref, sequentially, by the third TFT M 3  operated by a diode during the programming period PP as shown in a solid line of  FIG. 7 . An a-node voltage Va between a drain electrode of the first TFT M 1  and the storage capacitor Cs and a b-node voltage Vb between the storage capacitor Cs and a gate electrode of the third TFT M 3  are defined by the following Equation 1 and Equation 2, respectively.
 
Va=Vdata   [Equation 1]
 
 Vb=VDD−|V   T′ |  [Equation 2]
 
     herein, ‘Vdata’ represents a data voltage in Equation 1, and ‘V T′ ’ in Equation 2 is defined by the following Equation 3. 
     
       
         
           
             
               
                 
                   
                      
                     
                       V 
                       
                         T 
                         ′ 
                       
                     
                      
                   
                   = 
                   
                     
                        
                       Vth 
                        
                     
                     + 
                     
                       
                         
                           2 
                           ⁢ 
                           LIref 
                         
                         
                           
                             k 
                             ′ 
                           
                           ⁢ 
                           W 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     herein, ‘Vth’ represents a threshold voltage of the third TFT M 3 , ‘k’ represents a constant defined by mobility and a parasitic capacitance of the third TFT M 3 , ‘L’ represents a channel length of the third TFT M 3  and ‘W’ represents a channel width of the third TFT M 3 , respectively. 
     A reference current Iref in Equation 3 is defined by Equation 4. 
     
       
         
           
             
               
                 
                   Iref 
                   = 
                   
                     
                       
                         k 
                         ′ 
                       
                       2 
                     
                     ⁢ 
                     
                       W 
                       L 
                     
                     ⁢ 
                     
                       
                         ( 
                         
                           
                              
                             
                               V 
                               
                                 T 
                                 ′ 
                               
                             
                              
                           
                           - 
                           
                              
                             Vth 
                              
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     herein, a reference current Iref represents a current sensing a threshold voltage VTH of the third TFT M 3  and a programming period sensing a threshold voltage VTH of the third TFT M 3  is reduced as the current value is higher, but a power consumption can be increased that much. Accordingly, a reference current Iref is experimentally determined in consideration of a panel characteristics, a driving time and a power consumption. For example, a reference current Iref can be differentiated depending upon a semiconductor characteristics of the TFT provided with a panel, a driving frequency standard and a requirement of a power consumption, etc. 
     The first scanning pulses EM 1  to EMn are inversed into a low-level voltage to turn-on the first and sixth TFT M 1  and M 6 , and the second scanning pulses SCAN 1  to SCANn are inversed into a high-level voltage to turn-off the second, the fourth and the fifth TFT M 2 , M 4  and MS during the light-emitting period EP. Accordingly, a data voltage Vdata and a reference current Iref supplied to the pixel  54  are cut-off, and the reference voltage Vref is charged, via the first TFT M 1 , into one electrode of the storage capacitor Cs connected to the a-node. In this embodiment, the other electrode of the storage capacitor Cs connected to the b-node is bootstrapped by a reference voltage Vref to change a charge electric potential. Accordingly, the third TFT M 3  emits a light in accordance with a voltage of the changed b-node. The organic light-emitting diode element OLED is emitted by a reference current Iref flowing into a high-level electric potential driving voltage VDD source, the third TFT M 3 , the sixth TFT M 6 , the organic light-emitting diode element OLED and the ground voltage source GND, sequentially, during the light-emitting period EP as shown in a dotted line of  FIG. 7 . An a-node voltage Va and a b-node voltage Vb are defined by the following Equation 5 and Equation 6, respectively, and a current IOLED flowing into the organic light-emitting diode element OLED is defined by Equation 7 during the light-emitting period EP.
 
Va=Vref   [Equation 5]
 
 Vb=VDD−|V   T′   |+V ref− V data   [Equation 6]
 
     herein, a reference voltage Vref represents a voltage maintaining one voltage of the storage capacitor Cs during the light-emitting period EP and is defined by a arbitrary constant-voltage determined from a value of a data voltage and a reference current Iref. 
     
       
         
           
             
               
                 
                     
                   
                     
                       
                         
                           
                             I 
                             OLED 
                           
                           = 
                             
                           ⁢ 
                           
                             
                               
                                 k 
                                 ′ 
                               
                               2 
                             
                             ⁢ 
                             
                               W 
                               L 
                             
                             ⁢ 
                             
                               
                                 ( 
                                 
                                   VDD 
                                   - 
                                   
                                     ( 
                                     
                                       VDD 
                                       - 
                                       
                                          
                                         
                                           V 
                                           
                                             T 
                                             ′ 
                                           
                                         
                                          
                                       
                                       + 
                                       Vref 
                                       - 
                                       Vdata 
                                     
                                     ) 
                                   
                                   - 
                                   
                                      
                                     Vth 
                                      
                                   
                                 
                                 ) 
                               
                               2 
                             
                           
                         
                       
                     
                     
                       
                         
                           = 
                             
                           ⁢ 
                           
                             
                               
                                 k 
                                 ′ 
                               
                               2 
                             
                             ⁢ 
                             
                               W 
                               L 
                             
                             ⁢ 
                             
                               
                                 ( 
                                 
                                   Vdata 
                                   - 
                                   Vref 
                                   + 
                                   
                                     
                                       
                                         2 
                                         ⁢ 
                                         LIref 
                                       
                                       
                                         
                                           k 
                                           ′ 
                                         
                                         ⁢ 
                                         W 
                                       
                                     
                                   
                                 
                                 ) 
                               
                               2 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   ] 
                 
               
             
           
         
       
     
     As shown in the above Equation 7, in the organic light-emitting diode display device. The equation defines a current IOLED that flows into the organic light-emitting diode element during the light-emitting period EP not includes an item of a high-level electric potential driving voltage VDD and a threshold voltage Vth of the third TFT M 3 . For example, a current IOLED flowing into the organic light-emitting diode element during the light-emitting period EP is never affected by a high-level electric potential driving voltage VDD and a threshold voltage Vth of the TFT. 
       FIG. 8  shows a second embodiment of the pixels  54  at the organic light-emitting diode display device. 
     In one embodiment, as shown in  FIG. 8 , each of the pixels  54  includes the first to sixth TFT M 1  to M 6 , the storage capacitor Cs and the organic light-emitting diode element OLED. The TFTs M 1  to M 6  are implemented in a p-type MOS-FET. Since the first to fifth TFT M 1  to M 5 , the storage capacitor Cs and the organic light-emitting diode element OLED are identical to those described in the embodiment of the above-mentioned  FIG. 6 , a detailed explanation as to it will be omitted. 
     The third TFT M 3  is operated by a diode to flow a reference current Iref during the programming period PP like the above-mentioned embodiment. 
     The sixth TFT M 6  is connected to a backward diode by the fifth TFT M 5  turned-on during the programming period PP to cut-off a current IOLED supplied to the organic light-emitting diode element OLED while forms a current path between the c-node and the organic light-emitting diode element OLED during the light-emitting period EP to supply a current IOLED to the organic light-emitting diode element OLED. A gate electrode of the sixth TFT M 6  is connected to the b-node. A source electrode of the sixth TFT M 6  is connected to the c-node, and a drain electrode of the sixth TFT M 6  is connected to an anode electrode of the organic light-emitting diode element OLED. 
     Such a pixel  54  shown in  FIG. 8  is almost equally operated in comparison to the above-mentioned embodiment of  FIG. 6 . 
     The first TFT M 1  is turned-off by the first scanning pulse EM 1  to EMn while the second, the fourth and the fifth TFT M 2 , M 4  and M 5  are turned-on by the second scanning pulse SCAN 1  to SCANn during the programming period PP. For example, at the same time, the third TFT M 3  is operated as a forward diode by the turned-on fifth TFT M 5  to flow a reference current Iref. The sixth TFT M 6  is operated as a backward diode to cut-off a current supplied to the organic light-emitting diode element OLED. A data voltage Vdata is charged into the a-node and a threshold voltage of the third TFT M 3  is sampled into the b-node during the programming period PP. 
     A voltage of the first scanning pulse EM 1  to EMn is inversed to turn-off the second, the fourth and the fifth TFT M 2 , M 4  and M 5  and to turn-on the first TFT M 1  during the light-emitting period EP. The third and sixth TFT M 3  and M 6  supplies a current IOLED not affected by a high-level electric potential driving voltage VDD and a threshold voltage Vth to the organic light-emitting diode element OLED during the light-emitting period EP. 
       FIG. 9  to  FIG. 12  show an embodiment of an organic light-emitting diode display device that is adaptive for applying in a CMOS (Complementary Metal Oxide Semiconductor) process which forms a N-type MOS-FET and a P-type MOS-FET on the same substrate at the same time. 
     Referring to  FIG. 9  to  FIG. 12 , the organic light-emitting diode display device according to the first embodiment includes a display panel  90  provided m×n pixels  94 , a data driver  92  supplying a data voltage to data lines DL 1  to DLm, a scan driver  93  sequentially supplying an scanning pulse of a low-level voltage to n scan electrode S 1  to Sn and a timing controller  91  controlling the drivers  92  and  93 . 
     In the display panel  90 , pixels  94  is formed at pixel areas defined by an intersection of the scan lines S 1  to Sn and the data lines DL 1  to DLm. Signal lines supply a reference voltage Vref of a constant-voltage, a reference current Iref of a constant-current and a high-level electric potential driving voltage VDD to the pixels  94  are formed at the display panel  90 . The scan lines E 1  to En supplying scanning signals EM 1  to EMn of a high-level voltage are removed at the display panel  90  in  FIG. 9  in comparison to the display panel  50  in  FIG. 5  to reduce the number of a signal line and to further simplify a panel structure. In the display panel in  FIG. 5 , the TFTs are comprised of only the P-type MOS-FETs at a pixel array area while in the display panel in  FIG. 9 , the TFTs are comprised of the P-type MOS-FETs and the N-type MOS-FETs at a pixel array area. 
     The data driver  92  is essentially the same as the data driver  52  in  FIG. 5 . 
     The scan driver  53  sequentially supplies scanning pulses SCAN 1  to SCANn of a low-level voltage to the scan lines S 1  to Sn in response to a control signal SDC from the timing controller  51  as shown in  FIG. 10 . 
     In one embodiment, the timing controller  91  supplies a digital video data RGB to the data driver  92  and generates a control signal DDC and GDC controlling an operation timing of the scan driver  93  and the data driver  92  using, for example, a vertical/horizontal synchronizing signal and a clock signal. 
     Alternatively, a constant-voltage source supplying the reference voltage Vref and a high-level electric potential driving voltage VDD and a positive voltage source supplying the reference current Iref are connected to the display panel  90 . 
     In one embodiment, each of the pixels  94  includes six TFTs M 1  to M 6 , the storage capacitor and the organic light-emitting diode element OLED shown in  FIG. 11  and  FIG. 12 . 
       FIG. 11  shows the first embodiment of the pixels  94  at the organic light-emitting diode display device shown in  FIG. 9 . Since the second to fifth TFT M 2  to M 5 , the storage capacitor Cs and the organic light-emitting diode element OLED in  FIG. 11  are identical to those described in the embodiment of the above-mentioned  FIG. 7  and  FIG. 8 , a detailed explanation as to it will be omitted. 
     In one embodiment, as shown in  FIG. 11 , each of the pixels  94  includes the first TFT M 1  comprised of the N-type MOS-FET, the second to sixth TFT M 2  to M 6  comprised of the P-type MOS-FET, the storage capacitor Cs and the organic light-emitting diode element OLED. 
     In one embodiment, the first TFT M 1  is maintained at an off-state by the scanning pulses SCAN 1  to SCANn supplied from the scan lines S 1  to Sn to a low-level voltage during the programming period PP while turned-on by a high-level voltage supplied from the scan lines S 1  to Sn to form a current path between the reference voltage source Vref and an a-node during the light-emitting period EP. Accordingly, the first TFT M 1  is comprised of the N-type MOS-FET, a gate electrode of the first TFT M 1  is connected to the scan lines S 1  to Sn, and a drain electrode of the first TFT M 1  is connected to the reference voltage source Vref. A source electrode of the first TFT M 1  is connected to the a-node. 
     In one embodiment, the sixth TFT M 6  is connected to an backward diode by the turned-on fifth TFT M 5  to cut-off a current IOLED supplied to the organic light-emitting diode element OLED during the programming period PP while it forms a current path between the c-node and the organic light-emitting diode element OLED to supply a current IOLED to the organic light-emitting diode element OLED during the light-emitting period EP. A gate electrode of the sixth TFT M 6  is connected to the b-node, and a source electrode of the sixth TFT M 6  is connected to the c-node. A drain electrode of the sixth TFT M 6  is connected to an anode electrode of the organic light-emitting diode element OLED. 
     In one embodiment, as shown in  FIG. 11 , a pixel  94  is almost equally operated in comparison to the above-mentioned embodiments. 
     If the scanning pulses SCAN 1  to SCANn of a low-level voltage are generated, then the first TFT M 1  is turned-off while the second, the fourth and the fifth TFT M 2 , M 4  and M 5  are turned-on during the programming period PP. At the same time, for example, the third TFT M 3  is operated as a forward diode by the turned-on fifth TFT M 5  to flow a reference current Iref and the sixth TFT M 6  is operated as a backward diode to cut-off a current supplied to the organic light-emitting diode element OLED. A data voltage Vdata is charged into the a-node and a threshold voltage of the third TFT M 3  is sampling into the b-node during the programming period PP. 
     A voltage of the scan lines S 1  to Sn is risen to a high-level voltage to turn-off the second, the fourth and the fifth TFT M 2 , M 4  and M 5  and to turn-on the first TFT M 1  during the light-emitting period EP. The third TFT M 3  supplies a current IOLED which a gate voltage of the sixth TFT M 6  is bootstrapped by the storage capacitor Cs to be not affected by a high-level electric potential driving voltage VDD and a threshold voltage Vth to the organic light-emitting diode element OLED during the light-emitting period EP. 
     In one embodiment, as shown in  FIG. 12 , each of the pixels  94  includes the first and sixth TFT M 1  and M 6  comprised of the N-type MOS-FET, the second to fifth TFT M 2  to M 5  comprised of the P-type MOS-FET, the storage capacitor Cs and the organic light-emitting diode element OLED. 
     The first TFT M 1  is substantially the same as that shown in  FIG. 11  with a view of a function and a connection relationship. 
     The sixth TFT M 6  is turned-off by the scanning pulses SCAN 1  to SCANn supplied from the scan lines S 1  to Sn to a low-level voltage to cut-off a current IOLED supplied to the organic light-emitting diode element during the programming period PP while turned-on by a high-level voltage on the scan lines S to Sn to form a current path between the c-node and the organic light-emitting diode element, and to supply a current IOLED to the organic light-emitting diode element OLED during the light-emitting period EP. Accordingly, the sixth TFT M 6  is comprised of the N-type MOS-FET, and a gate electrode of the sixth TFT M 6  is connected to the b-node. A drain electrode of the sixth TFT M 6  is connected to the c-node, and a source electrode of the sixth TFT M 6  is connected to an anode electrode of the organic light-emitting diode element OLED. 
     Such a pixel  94  shown-in  FIG. 12  is almost equally operated in comparison to the above-mentioned embodiments. 
     If the scanning pulses SCAN 1  to SCANn of a low-level voltage are generated, then the first and sixth TFT M 1  and M 6  are turned-off while the second, the fourth and the fifth TFT M 2 , M 4  and M 5  are turned-on during the programming period PP. The third TFT M 3  is operated as a forward diode by the turned-on fifth TFT M 5  to flow a reference current Iref and the sixth TFT M 6  cuts-off a current supplied to the organic light-emitting diode element OLED. A data voltage Vdata is charged into the a-node and a threshold voltage of the third TFT M 3  is sampling into the b-node during the programming period PP. A voltage of the scan lines S 1  to Sn is risen to a high-level voltage to turn-off the second, the fourth and the fifth TFT M 2 , M 4  and M 5  and to turn-on the first and sixth TFT M 1  and M 6  during the light-emitting period EP. A gate voltage of the third TFT M 3  is bootstrapped by the storage capacitor Cs to be supplied a current IOLED not affected by a high-level electric potential driving voltage VDD and a threshold voltage Vth to the organic light-emitting diode element OLED during the light-emitting period EP. 
     The switch elements of  FIG. 7  and  FIG. 8  are comprised of the P-type MOS-FET, but the switches also can be comprised of the N-type MOS-FET. If the switch elements of  FIG. 7  and  FIG. 8  are comprised of the N-type MOS-FET, then a logic value or a polarity of a voltage of the scanning pulses shown in  FIG. 6  are inversed. Likewise, a type of switch elements of  FIG. 11  and  FIG. 12  is changed and a logic value of a scanning pulse or a polarity can be changed. 
     An organic light-emitting diode display device and a driving method thereof minimizes a voltage drop by a driving voltage supply line and an adverse effect by a threshold voltage change of a thin film transistor using six switch elements and one storage capacitor to uniform display brightness. 
     Although the present invention has been explained by the embodiments shown in the drawings described above, it should be understood to the ordinary skilled person in the art that the invention is not limited to the embodiments, but rather that various changes or modifications thereof are possible without departing from the spirit of the invention. Accordingly, the scope of the invention shall be determined only by the appended claims and their equivalents.