Patent Publication Number: US-8111224-B2

Title: Organic light emitting diode display and display panel and driving method thereof

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2004-0067452, filed on Aug. 26, 2004 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an organic light emitting diode display, and a display panel and driving method thereof. 
     2. Discussion of the Related Art 
     Generally, organic light emitting diode (OLED) displays are display devices that emit light by electrically exciting an organic compound. An organic light emitting diode display includes N×M organic light emitting cells (or pixels) arranged in the form of a matrix, and displays an image by driving the organic light emitting cells, using voltage or current. 
     An organic light emitting cell has a structure including an anode electrode layer (e.g., indium tin oxide: ITO), an organic thin film, and a cathode electrode layer. To achieve an improved balance between electrons and holes, and thus an enhancement in light emitting efficiency, the organic thin film has a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL). The organic thin film also includes an electron injecting layer (EIL) and a hole injecting layer (HIL). 
     Methods for driving an organic light emitting diode display include a passive matrix method and an active matrix method which uses thin film transistors (TFT) to drive organic light emitting cells of the organic light emitting diode display. In the passive matrix method, an anode and a cathode are formed crossing each other, and a line is selected in order to drive an organic light emitting cell. However, in the active matrix method, a thin film transistor is coupled to an indium tin oxide (ITO) pixel electrode of an organic light emitting cell, and the organic light emitting cell operates according to a voltage maintained by the capacitance of a capacitor coupled to a gate of the thin film transistor. The active matrix method can be further divided into a voltage programming method and a current programming method according to a signal, which is applied to program a voltage of the capacitor. 
     In a conventional organic light emitting diode display, a pixel includes a plurality of sub-pixels having respective colors in order to express various colors, and a color is expressed by a combination of the respective colors emitted from the sub-pixels. Generally, one pixel includes a sub-pixel for red (R), a sub-pixel for green (G), and a sub-pixel for blue (B), and a color is expressed by a combination of the red, green, and blue. 
     However, in order to drive the sub-pixels, the respective sub-pixels must include a driving circuit for driving an organic light emitting element (e.g., an OLED), a data line for transmitting a data signal, a scan line for transmitting a scan signal, and a power line for transmitting a power voltage. Accordingly, the organic light emitting diode display must include a large number of lines (e.g., scan and data lines) and circuits for driving the pixels. The lines are difficult to arrange in the limited display area of the conventional organic light emitting display, and an aperture ratio corresponding to an emitting pixel area of the conventional organic light emitting display can be reduced. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention provides a light emitting diode display for increasing an aperture ratio. 
     An embodiment of the present invention provides a light emitting diode display for simplifying an arrangement of lines (e.g., scan and data lines) and a configuration of elements included in a pixel. 
     An embodiment of the present invention provides a light emitting diode display for reducing a number of data lines and scan lines. 
     Additional features of the invention will be set forth in the description which follows. 
     One embodiment of the present invention provides an organic light emitting diode display including a plurality of data lines for transmitting data signals, a plurality of scan lines for transmitting selection signals, and a plurality of pixels coupled to the data lines and the scan lines. At least one of the pixels includes at least four light emitting elements for emitting a light corresponding to amount of an applied current, a light emitting element driver for receiving at least one of the data signals while a corresponding one of the selection signals is applied and for outputting a data current corresponding to the at least one of the data signals, and a switching unit for respectively transmitting the data current from the light emitting element driver to the four light emitting elements. In this embodiment, at least two light emitting elements of the four light emitting elements are formed in different places with reference to the scan lines and the data lines. 
     One embodiment of the present invention provides a display panel for an organic light emitting diode display. The display panel includes: a display area having a plurality of data lines for transmitting data signals, a plurality of selection signals for transmitting selection signals, and a plurality of pixels coupled to the data line and the scan line; a data signal driver for time-dividing at least four of the data signals and for applying the time-divided data signals to at least one of the data lines in one field; and a scan driver for sequentially applying the selection signals to the plurality of scan lines. In this embodiment, at least one of the pixel includes: at least four light emitting elements for emitting light corresponding to amount of an applied current; a light emitting element driver for receiving the time-divided data signals while the selection signals is applied, and for outputting a data current corresponding to at least one of the time-divided data signals; and a switching unit for respectively transmitting the data current to the four light emitting elements. First and second light emitting elements of the four light emitting elements are formed parallel to at least one of the scan lines, and third and fourth light emitting elements of the four light emitting elements are formed vertically with respect to the first and second light emitting elements. 
     One embodiment of the present invention provides a method for driving a display panel including a plurality of data lines for transmitting data signals, a plurality of selection signals for transmitting selection signals, and a plurality of pixels coupled to the data lines and the scan lines. In this embodiment, at least one of the plurality of pixels includes at least four light emitting elements, and a field is divided into at least four subfields. In the method, the selection signals are sequentially applied to the plurality of scan lines in the respective subfields, at least one of the data signals is programmed to at least one of the plurality of data lines while a corresponding one of the selection signals is applied, and a current corresponding to at least one the data signals is sequentially transmitted to the four light emitting elements. First and second light emitting elements of the four light emitting elements are formed parallel to at least one of the scan lines, and third and fourth light emitting elements of the four light emitting elements are formed vertically with respect to the first and second light emitting elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  shows a schematic diagram for representing an organic light emitting diode display according to a first exemplary embodiment of the present invention. 
         FIG. 2  shows a schematic diagram for representing a pixel of the organic light emitting diode display of  FIG. 1 . 
         FIG. 3  shows a detailed circuit diagram for representing the pixel of  FIG. 2 . 
         FIG. 4  shows a driving timing chart of the organic light emitting diode display of  FIG. 1 . 
         FIG. 5  shows a schematic diagram for representing a pixel of an organic light emitting diode display according to a second exemplary embodiment of the present invention. 
         FIG. 6  shows a detailed circuit diagram for representing the pixel of  FIG. 5 . 
         FIG. 7  shows a driving timing chart of the organic light emitting diode display of  FIG. 5 . 
         FIG. 8  shows a detailed circuit diagram for representing a pixel of an organic light emitting diode display according to a third exemplary embodiment of the present invention. 
         FIG. 9  shows a detailed circuit diagram for representing a pixel of an organic light emitting diode display according to a fourth exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, only certain exemplary embodiments of the present invention are shown and described, simply by way of illustration. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive. There may be parts shown in the drawings, or parts not shown in the drawings, that are not discussed in the specification as they are not essential to a complete understanding of the invention. Like reference numerals designate like elements. 
     Exemplary embodiments of the present invention will now be described in detail with reference to the annexed drawings. 
       FIG. 1  shows a schematic diagram for representing an organic light emitting diode display according to a first exemplary embodiment of the present invention, and 
       FIG. 2  shows a schematic diagram for representing a pixel of the organic light emitting diode display of  FIG. 1 . 
     As shown in  FIG. 1 , the organic light emitting diode display according to the first exemplary embodiment of the present invention includes a display panel  100 , a selection signal driver  200 , an emission signal driver  300 , and a data signal driver  400 . The display panel  100  includes a plurality of selection and emission scan lines S 1  to Sn and Em 1  to Emn arranged in a row direction, a plurality of data lines D 1  to Dm arranged in a column direction, and a plurality of pixels  110 . A pixel is formed in a pixel area defined by two neighboring scan lines and two neighboring data lines. As shown in  FIG. 2 , each pixel  110  includes an organic light emitting diode (OLED) driver  111  for driving organic light emitting elements OLED_R and OLED_G. Each of the organic light emitting elements OLED_R and OLED_G emits light of a different color corresponding to an applied current. 
     The selection signal driver  200  sequentially applies a selection signal or selection signals to the plurality of selection scan lines S 1  to Sn so that a data signal or data signals can be programmed to a pixel  110  coupled to a selection scan line of the selection scan lines S 1  to Sn corresponding to the pixel  110 , and the emission signal driver  300  sequentially applies an emission control signal or emission control signals to the emission scan lines Em 1  to Emn in order to control an emission of the organic light emitting elements OLED_R and OLED_G. The data signal driver  400  applies the data signal corresponding to the pixel  110  of the selection scan line to which the selection signal is applied to the data lines D 1  to Dm when the selection signal is sequentially applied. 
     The selection and emission signal drivers  200  and  300  and the data signal driver  400  are respectively coupled to a substrate in which the display panel  100  is formed. Alternatively, the selection and emission signal drivers  200  and  300  and/or the data signal driver  400  may be directly formed on a glass substrate of the display panel  100  so that the selection and emission signal drivers  200  and  300 , and/or the data signal driver  400  may be substituted for driving circuits respectively formed on the same layers as those of the selection and emission scan lines S 1  to Sn and Em 1  and Emm, data lines D 1  to Dm, and transistors. Alternatively, the selection emission signal drivers  200  and  300  and/or data signal driver  400  may also be formed in a chip on a flexible printed circuit (FPC), a tape carried package (TCP), or a tape automatic bonding (TAB) in a state of being coupled to the display panel  100 . 
     In operation, a field is divided into two subfields in the first exemplary embodiment of the present invention, and data respectively corresponding to the organic light emitting elements OLED_R and OLED_G are programmed in the two subfields so the emission is generated. The selection signal driver  200  sequentially applies a selection signal to the selection scan lines S 1  to Sn for each subfield, and the emission signal driver  300  applies an emission control signal to the emission scan lines Em 1  to Emn so that one of the respective color organic light emitting elements OLED_R and OLED_G may be emitted in one subfield. The data signal driver  400  applies a data signal respectively corresponding to the organic light emitting elements OLED_R and OLED_G to the data lines D 1  to Dm in the two subfields. 
     An operation of the organic light emitting diode display according to the first exemplary embodiment of the present invention will be described in more detail with respect to  FIG. 3  and  FIG. 4 . 
       FIG. 3  shows a detailed circuit diagram for representing the pixel  110  of  FIG. 2 , and  FIG. 4  shows a driving timing chart of the organic light emitting diode display of  FIG. 1 . 
     In  FIG. 3 , a voltage programming method pixel  110 , which is coupled to the selection scan line S 1  and the data line D 1 , is represented, and the pixel (or pixel circuit)  110  including the organic light emitting element OLED_R for emitting a red light and the organic light emitting element OLED_G for emitting a green light are exemplified. 
     As shown in  FIG. 3 , the pixel circuit  110  according to the first exemplary embodiment of the present invention includes a driving transistor M 1 , a switching transistor M 2 , the organic light emitting elements OLED_R and OLED_G, and emission control transistors M 31  and M 32  for respectively controlling the emission of the organic light emitting elements OLED_R and OLED_G. One emission scan line Em 1  includes two emission signal lines Em 1   a  and Em 1   b . While not illustrated in  FIG. 3 , each of the other emission scan lines Em 2  to Emn also includes two emission signal lines. The emission control transistors M 31  and M 32  and the emission signal lines Em 1   a  and Em 1   b  form a switch for selectively transmitting a current of the driving transistor M 1  to the organic light emitting elements OLED_R and OLED_G. 
     The switching transistor M 2  has a gate coupled to the selection scan line S 1  and a source coupled to the data line D 1 , and transmits a data voltage from the data line D 1  in response to the selection signal from the selection scan line S 1 . The driving transistor M 1  has a source coupled to a power line for supplying a power voltage VDD and a gate coupled to a drain of the switching transistor M 2 , and a capacitor C 1  is coupled between the source and gate of the driving transistor M 1 . A drain of the driving transistor M 1  is coupled to respective sources of the emission control transistors M 31  and M 32 , and gates of the transistors M 31  and M 32  are respectively coupled to the emission signal lines Em 1   a  and Em 1   b . Drains of the emission control transistors M 31  and M 32  are respectively coupled to anodes of the organic light emitting elements OLED_R and OLED_G, and a power voltage VSS, which is less than the voltage VDD, is applied to cathodes of the organic light emitting elements OLED_R and OLED_G. A negative voltage or a ground voltage can be used for the power voltage VSS. 
     The switching transistor M 2  transmits a data voltage from the data line D 1  to the gate of the driving transistor M 1  in response to a low level selection signal from the selection scan line S 1 , and a voltage corresponding to a difference between the data voltage and the power voltage VDD transmitted to the gate of the transistor M 1  is stored by the capacitor C 1 . The emission control transistor M 31  is turned on in response to a low level emission control signal from the emission signal line Em 1   a , a current corresponding to the voltage stored by the capacitor C 1  from the driving transistor M 1  is transmitted to the organic light emitting element OLED_R, and an emission is generated by the organic light emitting element OLED_R. 
     In a same manner as above, the emission control transistor M 32  is turned on in response to a low level emission control signal from the emission signal line Em 1   b , the current corresponding to the voltage stored by the capacitor C 1  from the driving transistor M 1  is transmitted to the organic light emitting element OLED_G, and an emission is generated by the organic light emitting element OLED_G. 
     In the first exemplary embodiment, the two emission control signals respectively applied to the two emission signal lines (e.g., the emission signal lines Em 1   a  and Em 1   b ) respectively have low levels which are not overlapped in a field so that the pixels of the first exemplary embodiment may respectively express different colors with respect to each other. 
     A method for driving the organic light emitting diode display of  FIG. 1  will be described with reference to  FIG. 4 . As shown in  FIG. 4 , a field  1 F has two subfields  1 SF and  2 SF, and a signal for operating the organic light emitting elements OLED_R and OLED_G of the respective pixels is applied in the subfields  1 SF and  2 SF. Periods of the subfields  1 SF and  2 SF correspond to each other in  FIG. 4 . 
     A data voltage R corresponding to the organic light emitting element OLED_R of a first row pixel is applied to the data lines D 1  to Dm when a low level selection signal is applied to a first row selection scan line S 1  in the subfield  1 SF. 
     A low level emission control signal is applied to a first row emission signal line Em 1   a , the data voltage R is applied to the capacitor (e.g., the capacitor C 1 ) through the switching transistor M 2  of each pixel in the first row, and a voltage corresponding to the data voltage R is charged into the capacitor C 1 . The emission control transistor M 31  of the first row pixel is turned on, a current corresponding to a gate-source voltage stored in the capacitor C 1  is transmitted to the red organic light emitting element OLED_R from the driving transistor M 1 , and an emission is generated. 
     A data voltage R corresponding to a red light of a second row pixel is applied to the data lines D 1  to Dm when a low level selection signal is applied to a second row selection scan line S 2 . The low level emission control signal is applied to the second row emission signal line Em 2   a . The current corresponding to the data voltage R from the data lines D 1  to Dm is supplied to the red organic light emitting element OLED_R of the second row pixel, and an emission is generated. 
     A data voltage R is sequentially applied from a third row pixel to an (n−1) th  row pixel, and the red organic light emitting element OLED_R from the third row pixel to the (n−1) th  row pixel is emitted. Lastly, a data voltage R corresponding to a red light of an n th  row pixel is applied to the data lines D 1  to Dm and the low level emission control signal is applied to the n th  row emission control signal line Emna when the low level selection signal is applied to the n th  row selection signal line Sn. The current corresponding to the data lines D 1  to Dm is supplied to the red organic light emitting element OLED_R of the n th  row pixel, and an emission is generated. 
     As described, the data voltage R corresponding to the red light is applied to the respective pixels  110  formed in the display panel  100  in the subfield  1 SF. The emission control signal applied to the emission signal lines Em 1   a  to Emna is maintained at the low level for a predetermined time, and the organic light emitting element OLED_R coupled to the emission control transistor M 31  to which the corresponding emission control signal is applied is continuously emitted while the emission control signal is at the low level. That is, in each pixel  110 , the red organic light emitting element OLED_R is emitted with a brightness corresponding to the data voltage R applied for a period corresponding to the subfield  1 SF. 
     In the next subfield  2 SF, a low level selection signal is sequentially applied from the first to n th  row selection scan lines S 1  to Sn in a manner that is substantially the same as the application of the low level selection signal of the subfield  1 SF, and a data voltage G corresponding to a green light of the corresponding row pixel is applied to the data lines D 1  to Dm when the selection signal is applied to the respective selection scan lines S 1  to Sn. The low level emission control signal is sequentially applied to the emission signal lines Em 1   b  to Emnb as the low level selection signal is sequentially applied to the selection scan lines S 1  to Sn. A current corresponding to the applied data voltage is then transmitted to the red organic light emitting element OLED_G through the emission control transistor M 32 , and an emission is generated. 
     In the subfield  2 SF, the emission control signal applied to the emission signal lines Em 1   b  to Emnb is also maintained at the low level for a predetermined period, and the green organic light emitting element OLED_G coupled to the emission control transistor M 32  to which the corresponding emission control signal is applied is continuously emitted while the emission control signal is at the low level. In  FIG. 4 , the predetermined period corresponds to the subfield  2 SF. That is, the green organic light emitting element OLED_G is emitted with a brightness corresponding to the data voltage G applied for a period corresponding to the subfield  2 SF in the respective pixels. 
     As described and/or shown, in accordance with a method for driving an organic light emitting diode display according to the first exemplary embodiment of the present invention, one field is divided into two subfields which are sequentially driven. An organic light emitting element of one color is emitted in one pixel for each subfield, and organic light emitting elements of two colors are sequentially emitted through the two subfields. 
     According to the first exemplary embodiment of the present invention, light emitting elements emitting various colors are operated in one pixel by using common driving and switching transistors and a capacitor, and therefore a configuration of elements used in a pixel and lines (e.g., scan and data lines) for transmitting a current, a voltage, and/or a signal are simplified. 
     While it has been described in  FIG. 4  that an organic light emitting diode display can be operated using a single scan method and/or a progressive scan method, the present invention would not be limited to the above, and the present invention can include various other scan methods such as a dual scan method or an interlaced scan method. 
     While a pixel circuit of a voltage programming method using a switching transistor and a driving transistor has been described in the first exemplary embodiment of the present invention, the present invention can include a voltage programming method using a transistor for compensating a threshold voltage of the driving transistor or a transistor for compensating a voltage reduction. 
       FIG. 5  schematically shows a pixel of an organic light emitting diode display according to a second exemplary embodiment of the present invention. 
     A pixel of the organic light emitting diode display according to the second exemplary embodiment of the present invention substantially corresponds to the pixel circuit according to the first exemplary embodiment of the present invention except that one pixel operates four organic light emitting elements formed in two rows. 
     In detail, an OLED driver  111  operates (or drives) two organic light emitting elements OLED_R and OLED_G formed in a first row and two organic light emitting elements OLED_R and OLED_G formed in a second row. At this time, a field is divided into four subfields to be used, and each of the organic light emitting elements OLED_R and OLED_G of the first and second rows is sequentially emitted in the respective subfields. 
     In the second exemplary embodiment of the present invention, four organic light emitting elements OLED_R and OLED_G vertically and horizontally neighboring with each other are operated by one OLED driver  111 , and therefore organic light emitting elements in two rows are operated by one selection scan line S 1 , and organic light emitting elements in two columns are operated by one data line D 1 . Accordingly, the number of selection scan lines and data lines formed in a display panel is reduced to half as compared with a display panel having an OLED driver that drives only two organic light emitting elements, and aperture ratio is increased. 
     In addition, the internal configuration of the selection signal and data signal drivers (e.g., the selection signal and data signal drivers  200  and  400 ) for driving the scan lines S 1  to Sn and the data lines D 1  to Dm is simplified, the area that each driver occupies is reduced when the driver is formed on the display panel, and therefore a dead space (non-emission area) is reduced. 
       FIG. 6  shows a detailed circuit diagram for representing the pixel of  FIG. 5 . In  FIG. 6 , three pixels  110   a  to  110   c  formed in a pixel area defined by three data lines D 1  to D 3  and the selection signal S 1  are illustrated for exemplary purposes, and the invention is not thereby limited. 
     Hereinafter, a configuration and operation of the pixel circuit according to the second exemplary embodiment of the present invention will be described with reference to  FIG. 6 . It will be described focusing on a pixel  110   a  formed in a pixel area defined by the data line D 1  and the selection scan line S 1  among the three pixels  110   a  to  110   c , and parts that are substantially the same as the parts described in the first exemplary embodiment of the present invention will not be described again. 
     According to the second exemplary embodiment of the present invention, the OLED driver  111  includes a driving transistor M 11 , a switching transistor M 12 , a capacitor C 11 , and four emission control transistors M 13   a , M 13   b , M 13   c , and M 13   d.    
     The emission control transistors M 13   a  and M 13   b  transmit a current to two organic light emitting elements OLED_R 1  and OLED_G 1  formed in the first column, gates of the emission control transistors M 13   a  and M 13   b  are respectively coupled to the emission signal lines Em 1   a  and Em 1   b , sources of the emission control transistors M 13   a  and M 13   b  are coupled to a drain of the driving transistor M 11 , and drains of the emission control transistors M 13   a  and M 13   b  are coupled to anodes of the organic light emitting elements OLED_R 1  and OLED_G 1 . 
     The emission control transistors M 13   c  and M 13   d  transmit a current to two organic light emitting elements OLED_R 3  and OLED_G 3  formed in the second column, gates of the emission control transistors M 13   c  and M 13   d  are respectively coupled to the emission signal lines Em 1   c  and Em 1   d , sources of the emission control transistors M 13   c  and M 13   d  are coupled to the drain of the driving transistor M 11 , and drains of the emission control transistors M 13   c  and M 13   d  are coupled to anodes of the organic light emitting elements OLED_R 3  and OLED_G 3 . 
     When the pixel is formed as above and a low level emission control signal is sequentially applied to the emission signal lines Em 1   a  to Em 1   d  in the four subfields, the current of the driving transistor M 11  is transmitted to the organic light emitting elements OLED_R 1 , OLED_G 1 , OLED_R 3 , and OLED_G 3  through the emission control transistors M 13   a  to M 13   d , and an emission is generated. 
     Further, in the second exemplary embodiment of the present invention, organic light emitting elements emitting red, green, and blue lights are repeatedly formed in a horizontal direction, one OLED driver  111  operates organic light emitting elements horizontally neighboring each other, and therefore one OLED driver  111  operates organic light emitting elements (e.g., organic light emitting element OLED_R 1  and OLED_G 1 ), which emit different colors with respect to each other. 
     In more detail and referring also to  FIG. 7 , when one subfield  1 F is divided into first to fourth subfields  1 SF,  2 SF,  3 SF, and  4 SF to be used, in the first subfield  1 SF, a data voltage corresponding to the organic light emitting element OLED_R 1  for emitting a red light is applied to the data line D 1 , a low level emission control signal is applied to the emission signal line Em 1   a , and a current of the driving transistor M 11  flows to the organic light emitting element OLED_R 1 . In the second subfield  2 SF, a data voltage corresponding to the organic light emitting element OLED_G 1  for emitting a green light is applied to the data line D 1 , a low level emission control signal is applied to the emission signal line Em 1   b , and a current of the transistor M 11  flows to the organic light emitting element OLED_G 1 . In the third subfield  3 SF, a data voltage corresponding to the organic light emitting element OLED_R 3  for emitting a red light is applied to the data line D 1 , a low level emission control signal is applied to the emission signal line Em 1   c , and a current of the driving transistor M 11  flows to the organic light emitting element OLED_R 3 . In the fourth subfield  4 SF, a data voltage corresponding to the organic light emitting element OLED_G 3  for emitting a green light is applied to the data line D 1 , the low level emission control signal is applied to the emission signal line Em 1   d , and a current of the driving transistor M 11  flows to the organic light emitting element OLED_G 3 . 
     As described and/or shown, four organic light emitting elements are operated by one driver in the second exemplary embodiment because one field is divided into four subfields and four organic light emitting elements are sequentially operated in the respective subfields. 
     However, in the second exemplary embodiment of the present invention, it is difficult to control the white balance of red, green, and blue images by controlling characteristics of the driving transistor when one driver operates organic light emitting elements which emit different colors with respect to each other. 
     Therefore, in a third exemplary embodiment of the present invention in order to enhance the second exemplary embodiment of the present invention, a driver is allowed to operate organic light emitting elements that emit the same colors. 
     A pixel of an organic light emitting diode display according to a third exemplary embodiment of the present invention will be described with reference to  FIG. 8 . 
       FIG. 8  shows a circuit diagram for representing the pixel of the organic light emitting diode display according to the third exemplary embodiment of the present invention. 
     According to the third exemplary embodiment of the present invention, respective pixels  110   a ′ to  110   c ′ include an OLED driver and four organic light emitting elements, and data signals corresponding to red, green, and blue are applied to data lines D 1  to D 3 . 
     The OLED driver of the pixel  110   a ′ is coupled to the data line D 1 , and applies a current corresponding to a red light to organic light emitting elements OLED_R 1 , OLED_R 2 , OLED_R 3 , and OLED_R 4  through emission control transistors M 13   a , M 23   b , M 13   c , and M 23   d . A drain of driving transistor M 11  is coupled to the emission control transistors M 13   a , M 23   b , M 13   c , and M 23   d , and a current of the driving transistor M 11  is transmitted to the organic light emitting elements OLED_R 1 , OLED_R 2 , OLED_R 3 , and OLED_R 4  in response to an emission control signal applied to a gate of each of the emission control transistors M 13   a , M 23   b , M 13   c , and M 23   d.    
     The OLED driver of the pixel  110   b ′ is coupled to the data line D 2 , and applies a current corresponding to a green light to organic light emitting elements OLED_G 1 , OLED_G 2 , OLED_G 3 , and OLED_G 4  through emission control transistors M 13   b , M 33   a , M 13   d , and M 33   c . That is, a drain of driving transistor M 21  is coupled to the emission control transistors M 13   b , M 33   a , M 13   d , and M 33   c , and a current of the driving transistor M 21  is transmitted to the organic light emitting elements OLED_G 1 , OLED_G 2 , OLED_G 3 , and OLED_G 4  in response to an emission control signal applied to a gate of each of the emission control transistors M 13   b , M 33   a , M 13   d , and M 33   c.    
     In a like manner, the OLED driver of the pixel  110   c ′ is coupled to the data line D 3 , and applies a current corresponding to a blue light to organic light emitting elements OLED_B 1 , OLED_B 2 , OLED_B 3 , and OLED_B 4  through emission control transistors M 23   a , M 33   b , M 23   c , and M 33   d . That is, a drain of driving transistor M 31  is coupled to the emission control transistors M 23   a , M 33   b , M 23   c , and M 33   d , and a current of the driving transistor M 31  is transmitted to the organic light emitting elements OLED_B 1 , OLED_B 2 , OLED_B 3 , and OLED_B 4  in response to an emission control signal applied to a gate of each of the emission control transistors M 23   a , M 33   b , M 23   c , and M 33   d.    
     As a result, the data voltage corresponding to one color is applied to one data line in one field, and one driving transistor transmits a current corresponding to the data voltage to the organic light emitting elements emitting the same colors. 
     In the third exemplary embodiment, a white balance of a display panel is controlled because a current flowing to same color organic light emitting elements is controlled when a driving transistor in one pixel has a controlled channel-width-and-length ratio of the driving transistor in one field. That is, the channel-width-and-length ratios of the transistors M 11  to M 31  are established to be different from each other in  FIG. 8 , to thereby control the different amounts of currents respectively flowing to the red, green, and blue organic light emitting elements derived by different data voltages to have substantially corresponding levels with respect to each other. 
     The current flowing to the organic light emitting element is, however, affected by a threshold voltage of the driving transistor when the pixel circuit is formed as in the third exemplary embodiment of the present invention. Therefore it will be difficult to get high gray scales when variation of the threshold voltages between film transistors exist due to irregularity in a producing process. 
     A compensation circuit for compensating the threshold voltage is provided in a fourth exemplary embodiment of the present invention, and therefore the current flowing to the organic light emitting element is not affected by the threshold voltage of the driving transistor. 
       FIG. 9  shows a circuit diagram for representing a pixel of an organic light emitting diode display according to a fourth exemplary embodiment of the present invention. 
     The pixel circuit according to the fourth exemplary embodiment of the present invention is substantially the same as the third exemplary embodiment of the present invention except that an OLED driver further includes two additional transistors for compensating variation of a threshold voltage of a driving transistor and an additional capacitor. 
     Hereinafter, the pixel circuit according to the fourth exemplary embodiment of the present invention is shown to include pixels  110   a ″,  110   b ″,  110   c ″ but will be described focusing on the pixel  110   a ″, and parts that are substantially the same as the parts described in the third exemplary embodiment of the present invention will be omitted. A selection scan line for transmitting a present selection signal will be referred to as “a present scan line,” and a selection scan line for transmitting a selection signal before the present selection signal is transmitted will be referred to as “a previous scan line.” 
     In  FIG. 9 , a capacitor C 12  is coupled between a gate of a transistor M 11  and a capacitor C 11 . A transistor M 14  is coupled between the gate and a drain of the transistor M 11 , and diode connects the transistor M 11  in response to a selection signal from a previous scan line Sn- 1 . A transistor M 15  is coupled between a power line for supplying a power voltage VDD and an electrode of the capacitor C 12  and the capacitor C 11 , and applies a power voltage VDD to the electrode of the capacitor C 12  in response to the selection signal from the previous scan line Sn- 1 . 
     In operation, when a low level voltage is applied to the previous scan line Sn- 1 , the transistor M 14  is turned on, the transistor M 11  is diode-connected, the transistor M 15  is turned on, and the threshold voltage of the transistor M 11  is stored by the capacitor C 12 . 
     When the low level voltage is applied to the present scan line Sn, the transistor M 12  is turned on and a data voltage Vdata is charged into the capacitor C 1 . The threshold voltage Vth of the transistor M 11  is stored by the capacitor C 12 , and therefore a voltage corresponding to a sum of the threshold voltage Vth of the transistor M 11  and the data voltage Vdata is applied to the gate of the transistor M 11 . 
     A current as given in Equation 1 is transmitted to the organic light emitting element and an emission is generated when the low level voltage is applied to one of emission scan lines Emna to Emnd, and emission control transistors M 13   a , M 23   b , M 13   c , and M 23   d  are turned on. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           I 
                           OLED 
                         
                         = 
                         
                           
                             β 
                             2 
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 Vgs 
                                 - 
                                 Vth 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             β 
                             2 
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 
                                   ( 
                                   
                                     Vdata 
                                     + 
                                     Vth 
                                     - 
                                     VDD 
                                   
                                   ) 
                                 
                                 - 
                                 Vth 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             β 
                             2 
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 VDD 
                                 - 
                                 Vdata 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     where I OLED  denotes a current flowing to the organic light emitting element, Vgs denotes a voltage between the source and gate of the transistor M 11 , Vth denotes a threshold voltage of the transistor M 11 , Vdata denotes a data voltage, and β is a constant. 
     Accordingly, the current flowing to the organic light emitting elements OLED_R 1 , OLED_R 2 , OLED_R 3 , and OLED_R 4  is not affected by the threshold voltage of the transistor M 11 , and an image with desired gray scales is represented. 
     When the selection signal is applied to the previous scan line Sn- 1  and a voltage corresponding to the threshold voltage of the transistor M 11  is stored by the capacitor C 12  in the pixel  110   a ″, the voltage stored to the capacitor C 12  is affected by a voltage of the drain electrode of the driving transistor M 11 . At this time, the voltage of the drain electrode is affected by the current flowing through the transistor M 11  in a previous subfield. 
     In the fourth exemplary embodiment of the present invention, the driving transistor M 11  outputs a current corresponding to a red light in the previous subfield and the present subfield, and therefore a voltage for compensating a variation of the threshold voltage of the transistor M 11  is stored by the capacitor C 12  under the same condition as the present subfield. Therefore, the variation of the threshold voltage at the driving transistor M 11  is effectively compensated because the voltage corresponding to the threshold voltage is charged in the previous and present subfields under the same condition even though a parasitic capacitance exists at the drain electrode of the driving transistor M 11  and a different voltage from the threshold voltage of the driving transistor M 11  is charged. 
     While the present invention has been described in connection with certain exemplary embodiments, it is to be understood by those skilled in the art that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications included within the spirit and scope of the appended claims and equivalents thereof. 
     While it has been illustrated that one driving transistor is coupled to four organic light emitting elements through emission control transistors in  FIG. 5  to  FIG. 9 , one driving transistor may be established to operate other numbers of organic light emitting elements. That is, one driving transistor may operate red, green, and blue organic light emitting elements respectively formed in two columns when the pixel is formed as in  FIG. 5  and  FIG. 6 , and one driving transistor may operate three organic light emitting elements which emit the same colors as each other and are respectively formed in two columns when the pixel is formed as in  FIG. 8  and  FIG. 9 . One driving transistor may also be established to operate organic light emitting elements formed in more than three rows according to certain exemplary embodiments. 
     While it has been described that the driving transistor is a P-channel transistor, an N-channel transistor may be used according to certain exemplary embodiments. In addition, besides a MOS transistor, the driving transistor may be realized using another active element for controlling a current transmitted to a third electrode corresponding to a voltage applied between a first electrode and a second electrode. 
     As described, the present invention provides a light emitting diode display in which an aperture ratio is increased by using one driver to operate a plurality of organic light emitting elements. 
     The present invention also provides a light emitting diode display for simplifying an arrangement of lines (e.g., scan and data lines) and a configuration of elements in a pixel. 
     Further, internal configurations of a data signal driver and a scan driver (e.g., a selection signal driver and/or an emission signal driver) may be simplified by reducing the number of data lines and scan lines formed in a display panel, and the dead space (non-emission area) may be reduced by reducing an area needed for the drivers in the display panel.