Abstract:
An organic electroluminescent (EL) display device having a plurality of pixel circuits formed at crossing points of a plurality of scan lines and a plurality of data lines is provided. Each pixel circuit includes at least two driving transistors connected to a first power voltage line, the at least two driving transistors receiving a data signal through at least one of the data lines and outputting a driving current corresponding to the data signal; and an organic light emitting diode having at least two first electrodes respectively connected to the at least two driving transistors and emitting a light corresponding to the driving current. The organic light emitting diode has the at least two first electrodes and a common second electrode per pixel in order to prevent the whole pixel from not operating due to a short circuit occurring between one of the first electrodes and the second electrode.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to and the benefit of Korean Patent Application No. 10-2004-0075655, filed Sep. 21, 2004, the entire content of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an organic electroluminescent (EL) display device, and more particularly, to an organic EL display device which includes at least two driving transistors and an organic light emitting diode having at least two first electrodes and a common second electrode per pixel in order to operate the pixel due to a short circuit occurring between one of the first electrode and the second electrode  
         [0004]     2. Description of the Related Art  
         [0005]     An organic light emitting diode is an emissive device which emits fluorescent light by recombining electrons supplied from a cathode and holes supplied from an anode. An EL display device, which employs the organic light emitting diode, does not need a separate backlight, in contrast with a liquid crystal display (LCD) device, and has a wide viewing angle, fast response speed, low DC driving voltage, and light weight as compared to a passive light emitting diode. Thus, the EL display device is suitable for use as a wall-mountable display device or a portable display device.  
         [0006]     Methods of driving an organic EL display panel of an EL display device include a passive matrix driving method and an active matrix method using a thin film transistor (TFT). In the passive matrix driving method, an anode and a cathode are formed to be perpendicular to each other, and the display panel is driven by selecting a line. In the active matrix driving method, the TFT is connected to an anode, e.g., indium tin oxide (ITO), and the display panel is driven according to a voltage maintained by a capacitor connected to a gate of the TFT.  
         [0007]      FIG. 1  is a block diagram of a conventional organic EL display device.  
         [0008]     Referring to  FIG. 1 , the organic EL display device includes a display panel  10 , a data driver  20 , and a scan driver  30 .  
         [0009]     The display panel  10  includes a plurality of data lines D 1  to Dm for transmitting a data signal(s), a plurality of scan lines S 1  to Sn for transmitting a selection signal(s), which are perpendicular to the plurality of data lines D 1  to Dm, and a plurality of pixel circuits P 11  to Pnm formed at crossing points of the plurality of data lines D 1  to Dm and the plurality of scan lines S 1  to Sn.  
         [0010]     The data driver  20  outputs the data signal representing an image signal through the plurality of data lines D 1  to Dm, and the scan driver  30  sequentially outputs the selection signal to the pixel circuit P 11  to Pnm through the plurality of scan lines S 1  to Sn.  
         [0011]      FIG. 2  is a circuit diagram of one pixel of a conventional organic EL display device, i.e., one representative pixel among N×M pixel circuits in the display panel  10  of the organic EL display device of  FIG. 1 .  
         [0012]     As shown in  FIG. 2 , the pixel circuit  11  includes an organic light emitting diode OLED, a switching transistor M 1 , a driving transistor M 2 , and a capacitor Cst.  
         [0013]     The switching transistor M 1  has a gate connected to the scan line Sn and a source connected to the data line Dm, and transmits the data signal from the data line Dm to a gate of the driving transistor M 2  in response to the selection signal from the scan line Sn.  
         [0014]     The driving transistor M 2  has a source connected to a power voltage Vdd, and the gate connected to a drain of the switching transistor M 1 , and the capacitor Cst is connected between the gate of the driving transistor M 2  and the power voltage Vdd. The capacitor Cst maintains a gate-source voltage V GS  of the driving transistor M 2  for a predetermined time period.  
         [0015]     The organic light emitting diode OLED has an anode a connected to a drain of the driving transistor M 2  and a cathode b connected to a reference voltage Vss and emits light corresponding to a driving current applied through the driving transistor M 2 . Here, the reference voltage Vss connected to the cathode b of the organic light emitting diode OLED is lower than the power voltage Vdd and may be, for example, the ground voltage.  
         [0016]     Here, the driving current which flows through the organic light emitting diode OLED is given by the following Equation 1:  
               I   OLED     =         β   2     ⁢       (     Vgs   -   Vth     )     2       =       β   2     ⁢       (     VDD   -   Vdata   -        Vth          )     2                 Equation   ⁢           ⁢   1             
 
 where I OLED  is the driving current which flows through the organic light emitting diode OLED, Vgs is a voltage between the gate and the source of the driving transistor M 2 , Vth is a threshold voltage of the driving transistor M 2 , Vdata is a data voltage, and β is a constant. 
 
         [0017]     As can be seen in Equation 1, the driving current corresponding to the data voltage Vdata applied by the pixel circuit  11  of  FIG. 2  is supplied to the organic light emitting diode OLED, and the organic light emitting diode OLED emits light in response to the supplied current.  
         [0018]      FIG. 3  is a plan view of one pixel of a conventional organic EL display device.  
         [0019]     Referring to  FIG. 3 , the pixel includes a scan line  32  arranged in one direction, a data line  31  arranged perpendicular to the scan line  32 , and a power voltage line  37  arranged perpendicular to the scan line  32  and parallel to the data line  31 . A switching transistor  33  is connected to the data line  31  and the scan line  32 . A capacitor includes a lower capacitor electrode  35  connected to a source or drain electrode  34  of the switching transistor  33  through a contact hole, and an upper capacitor electrode  36  arranged above the lower capacitor electrode  35  to be connected to the power voltage line  37 . A gate  38  of a driving transistor  39  is connected to the lower capacitor electrode  35 , and an anode a is connected to a source or drain electrode  40  through a via hole  41 .  
         [0020]     In a unit pixel of the conventional organic light emitting diode described above, an anode a is formed on a substrate, an organic emission layer is formed on the anode a, and a cathode b is formed on the organic emission layer.  
         [0021]     Only the organic emission layer (see EL in  FIG. 4 ) is formed between the anode a and the cathode b, and an insulating layer is formed around the anode a, thereby preventing the anode a and the cathode b from being electrically connected directly without the emission layer.  
         [0022]     However, the conventional organic light emitting diode has one anode and a common cathode per unit pixel; and a relationship between the anode and the cathode may be affected by fine particles inserted between the anode and the cathode during a manufacturing process, a pattern failure of a lower layer, and/or an external pressure. For the foregoing reasons, the anode and the cathode, which should be electrically insulated from each other, may be electrically connected as shown in  FIG. 4 .  FIG. 4  is a photograph illustrating a short circuit formed between the anode and the cathode in the conventional organic EL display device. In  FIG. 4 , “a” denotes the anode, “b” denotes the cathode, and “c” denotes the fine particles.  
         [0023]     As shown in  FIG. 4 , the fine particles c exist in the insulating layer between the anode a and the cathode b, thereby causing an electrical short circuit between the anode a and the cathode b. The short circuit between the anode a and the cathode b leads to an application of a cathode voltage Vss to the anode a. As a result, a driving current (e.g., I OLED ) of a driving transistor (e.g., M 2 ) according to a data signal flows not to the organic emission layer but to the cathode b, so that light of a predetermined color is not emitted, thereby causing a dark pixel, i.e., a pixel defect. As organic EL display devices become more compact, more dark pixels resulting from short circuits between the anode and the cathode may occur. Therefore, it is desirable to resolve the foregoing problems.  
       SUMMARY OF THE INVENTION  
       [0024]     An embodiment of the present invention provides an organic EL display device, which includes at least two driving transistors and an organic light emitting diode having at least two first electrodes and a common second electrode per pixel. Accordingly, even if a short circuit occurs between one of the first electrodes and the common second electrode, the organic light emitting diode still emits light using the other first electrode, thereby eliminating effects of the short circuit.  
         [0025]     In an exemplary embodiment of the present invention, an organic EL display device includes an organic electroluminescent (EL) display device having a plurality of pixel circuits formed at crossing points of a plurality of scan lines and a plurality of data lines, each of the pixel circuits including: at least two driving transistors connected to a first power voltage line, the at least two driving transistors receiving a data signal through at least one of the data lines and outputting a driving current corresponding to the data signal; and an organic light emitting diode having at least two first electrodes respectively connected to the at least two driving transistors and emitting a light corresponding to the driving current.  
         [0026]     In another exemplary embodiment of the present invention, an organic EL display device includes an organic electroluminescent (EL) display device having a plurality of pixel circuits formed at crossing points of a plurality of scan lines and a plurality of data lines, each of the pixel circuits including: a switching transistor having a gate connected to at least one of the scan lines and a source connected to at least one of the data lines, the switching transistor transmitting a data signal from the at least one of the data lines; a capacitor having a first capacitor electrode connected to a drain of the switching transistor and a second capacitor electrode connected to a positive power voltage line, the capacitor storing the data signal during one frame; at least two driving transistors having sources commonly connected to the positive power voltage line and gates commonly connected to the drain of the switching transistor, the at least two driving transistors outputting a driving current corresponding to the data signal; and an organic light emitting diode having at least two first electrodes respectively connected to drains of the at least two driving transistors and emitting a light corresponding to the driving current.  
         [0027]     In yet another exemplary embodiment of the present invention, an organic EL display device includes a plurality of pixel circuits formed at crossing points of a plurality of scan lines and a plurality of data lines, each pixel circuit including: a switching transistor having a gate connected to at least one of the scan lines and a source connected to at least one of the data lines, the switching transistor transmitting a data signal from the at least one of the data lines; a capacitor having a first capacitor electrode connected to a drain of the switching transistor and a second capacitor electrode connected to a negative power voltage line, the capacitor storing the data signal during one frame; at least two driving transistors having sources commonly connected to the negative power voltage line and gates commonly connected to the drain of the switching transistor, the at least two driving transistors outputting a driving current corresponding to the data signal; and an organic light emitting diode having at least two first electrodes respectively connected to drains of the at least two driving transistors and emitting a light corresponding to the driving current.  
         [0028]     In still another exemplary embodiment of the present invention, an organic EL display device includes: at least two thin film transistors having respective semiconductor layers, gate electrodes, source electrodes, and drain electrodes arranged on a substrate; at least two first electrodes respectively connected to the source electrodes or the drain electrodes of the at least two thin film transistors; an organic emission layer formed on the at least two first electrodes; and a second electrode formed on the entire surface of the organic emission layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]     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 present invention.  
         [0030]      FIG. 1  is a block diagram of a conventional organic EL display device;  
         [0031]      FIG. 2  is a circuit diagram of one pixel of a conventional organic EL display device;  
         [0032]      FIG. 3  is a plan view of one pixel of a conventional organic EL display device;  
         [0033]      FIG. 4  is a photograph showing a short circuit between an anode and a cathode of a conventional organic EL display device;  
         [0034]      FIG. 5  is a circuit diagram of one pixel according to an embodiment of the present invention;  
         [0035]      FIG. 6  is a circuit diagram of one pixel having an NMOS-type driving transistor according to another embodiment of the present invention;  
         [0036]      FIG. 7  is a plan view of one pixel of an organic EL display device according to an embodiment of the present invention; and  
         [0037]      FIG. 8  is a cross-sectional view taken along line I-I′ of  FIG. 7 . 
     
    
     DETAILED DESCRIPTION  
       [0038]     The present invention will now be described with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown.  
         [0039]      FIG. 5  is a circuit diagram of one pixel according to an embodiment of the present invention, i.e., one representative pixel among N×M pixels, which is connected to a data line Dm and a scan line Sn.  
         [0040]     Referring to  FIG. 5 , the pixel circuit includes a switching transistor M 1 ″, a first driving transistor M 2 ″, a second driving transistor M 3 , a capacitor Cst″, and an organic light emitting diode OLED″ having first and second anodes a 1  and a 2 . The first and second driving transistors M 2 ″ and M 3  are each comprised of a PMOS transistor.  
         [0041]     The switching transistor M 1 ″ has a gate connected to the scan line Sn and transmits a data voltage from the data line Dm connected to its source to gates of the first and second driving transistors M 2 ″ and M 3  in response to a selection signal from the scan line Sn.  
         [0042]     The first driving transistor M 2 ″ has a source connected to a positive (or first) power voltage line of a power voltage Vdd, the gate connected to the drain of the switching transistor M 1 , and a drain connected to the first anode a 1  of the organic light emitting diode OLED″.  
         [0043]     The second driving transistor M 3  has a source connected to a positive (or first) power voltage line of the power voltage Vdd, the gate connected to the drain of the switching transistor M 1 , and a drain connected to the second anode a 2  of the organic light emitting diode OLED″.  
         [0044]     In  FIG. 5 , the gates of the first and second driving transistors M 2 ″ and M 3  are commonly connected, and the sources of the first and secondary driving transistors M 2 ″ and M 3  are commonly connected.  
         [0045]     The capacitor Cst is connected between the gates and sources of the first and second driving transistors M 2 ″ and M 3  to maintain a gate-source voltage VGS during one frame.  
         [0046]     The organic light emitting diode OLED″ includes a cathode b which is commonly connected to a negative (or second) power voltage line of a reference voltage Vss, and an anode comprised of the first and second anodes a 1  and a 2 . The first anode a 1  is connected to the drain of the first driving transistor M 2 ″ and the second anode a 2  is connected to the drain of the second driving transistor M 3 . Thus, the organic light emitting diode OLED″ emits light corresponding to driving currents supplied from the first and second driving transistors M 2 ″ and M 3 .  
         [0047]     An operation of the pixel circuit of  FIG. 5  is explained below. When the switching transistor M 1 ″ is turned on in response to a selection signal applied to the gate of the switching transistor M 1  through the scan line Sn, a data signal transmitted through the data line Dm is transmitted to a first (or lower) electrode of the capacitor Cst″, so that a voltage corresponding to a difference between the positive power voltage Vdd and the data signal is stored. Thereafter, the voltage stored in the capacitor Cst is transmitted to the first and second driving transistors M 2 ″ and M 3 . The first and second driving transistors M 2 ″ and M 3  transmit a driving current corresponding to Equation 1 to the organic light emitting diode OLED″ through the first and second anodes a 1  and a 2 , in response to the data signal, so that the organic light emitting diode OLED″ emits light.  
         [0048]     As described above, if a short circuit between one of the first and second anodes a 1  and a 2 , e.g., the second anode a 2  and the cathode b occurs due to an external pressure and/or fine particles leading to a pixel defect, the driving current flowing through the second driving transistor M 3  connected to the second anode a 2  flows to the cathode b, so that the organic light emitting diode OLED″ corresponding to the second anode a 2  does not emit light.  
         [0049]     However, the organic light emitting diode OLED″ corresponding to the first anode a 1  connected to the first driving transistor M 2 ″ receives the driving current normally to emit light.  
         [0050]     In such an instance, the brightness of the pixel is reduced as compared to a state before the short circuit between the second anode a 2  and the cathode b occurs, but a dark pixel is not generated, and thus the pixel defect is not noticeable.  
         [0051]     In  FIG. 5 , the first and second driving transistors M 2 ″ and M 3  are each comprised of a PMOS transistor, but in alternative embodiments the first and second driving transistors may be each comprised of an NMOS transistor.  
         [0052]      FIG. 6  is a circuit diagram of one pixel according to another embodiment of the present invention in which a driving transistor is comprised of an NMOS transistor.  
         [0053]     As shown in  FIG. 6 , the pixel circuit includes a switching transistor M 1 ′, a first driving transistor M 2 ′, a second driving transistor M 3 ′, a capacitor Cst′, and an organic light emitting diode OLED′ having first and second cathodes a 1 ′ and a 2 ′. The first and second driving transistors M 2 ′ and M 3 ′ are each comprised of an NMOS transistor.  
         [0054]     The switching transistor M 1 ′ has a gate connected to the scan line Sn and transmits a data voltage from a data line Dm connected to its source to gates of the first and second driving transistors M 2 ′ and M 3 ′ in response to a selection signal from a scan line Sn.  
         [0055]     The first driving transistor M 2 ′ has a source connected to a negative (or first) power voltage line of a reference voltage Vss, the gate connected to a drain of the switching transistor M 1 ′, and a drain connected to the first cathode a 1 ′ of the organic light emitting diode OLED′.  
         [0056]     The second driving transistor M 3 ′ has a source connected to a negative (or first) power voltage line of the reference voltage Vss, the gate connected to the drain of the switching transistor M 1 ′, and a drain connected to the second cathode a 2 ′ of the organic light emitting diode OLED′.  
         [0057]     In  FIG. 6 , the gates of the first and second driving transistors M 2 ′ and M 3 ′ are commonly connected, and the sources of the first and second driving transistors M 2 ′ and M 3 ′ are commonly connected.  
         [0058]     The capacitor Cst′ is connected between the gates of the first and second driving transistors M 2 ′ and M 3 ′ and the sources of the first and second driving transistors M 2 ′ and M 3 ′ to maintain a gate-source voltage V GS  during one frame.  
         [0059]     The organic light emitting diode OLED′ includes an anode b′ which is commonly connected to a positive (or second) power voltage line of a power voltage Vdd, and a cathode comprised of the first and second cathodes a 1 ′ and a 2 ′. The first cathode a 1 ′ is connected to the drain of the first driving transistor M 2 ′ and the second cathode a 2 ′ is connected to the drain of the second driving transistor M 3 ′ 
         [0060]     Those skilled in the art will understand an operation of the pixel circuit of  FIG. 6  with reference to the above description on the operation of the pixel circuit of  FIG. 5 , and thus a detailed description of the operation of the pixel circuit of  FIG. 6  is omitted. A structure of the pixel circuit comprised of a PMOS type driving transistor will be explained below.  
         [0061]      FIG. 7  is a plan view of one pixel of an organic EL display device according to an embodiment of the present invention.  
         [0062]     Referring to  FIG. 7 , the pixel includes a scan line  132  arranged in one direction, a data line  131  arranged perpendicular to the scan line  132 , and a positive power voltage line  137  arranged perpendicular to the scan line  132  and parallel to the data line  131 .  
         [0063]     A switching transistor  133  is connected to the scan line  132  and the data line  131 . A capacitor includes a lower capacitor electrode  135  connected to a source or drain electrode  134  of the switching transistor  133  through a contact hole, and an upper capacitor electrode  136  arranged above the lower capacitor electrode  135  to be connected to the positive power voltage line  137 .  
         [0064]     A first driving transistor  140  has a gate  141  connected to the lower capacitor electrode  135  and a source  142  connected to the positive power voltage line  137 . A second driving transistor  150  has a gate  151  connected to the lower capacitor electrode  135  and a source  152  connected to the positive power voltage line  137 .  
         [0065]     An organic light emitting diode OLED″ includes an anode a comprised of first and second anodes a 1  and a 2 , an organic emission layer formed on the first and second anodes a 1  and a 2 , and a common cathode formed on the organic emission layer. Here, the first anode a 1  is connected to a source or drain electrode  143  of the first driving transistor  140  through a via hole  144 , and the second anode a 2  is connected to a source or drain electrode  153  of the second driving transistor  150  through a via hole  154 . In one embodiment, the first and second anodes a 1  and a 2  are formed to have the same area.  
         [0066]     In  FIG. 7 , if a short circuit occurs between the second anode a 2  and the cathode b due to an external pressure and/or fine particles, the driving current does not flow to the organic emission layer from the second driving transistor  150  but flows into the cathode b, thereby light is not emitted. But, since the driving current from the first driving transistor  140  flows to the organic emission layer through the first anode a 1 , light is emitted. An area corresponding to the second anode a 2  does not emit light, but an area corresponding to the first anode a 1  emits light, and thus while the brightness of the pixel is reduced, a dark pixel does not occur, and the pixel defect is unnoticeable.  
         [0067]      FIG. 8  is a cross-sectional view taken along line I-I′ of  FIG. 7 .  
         [0068]     Referring to  FIG. 8 , a buffer layer  205  is arranged on a substrate  200 , and first and second semiconductor layers  210  and  220  are formed on the buffer layer  205 . The buffer layer  205  is optional, but is included in one embodiment because it prevents impurities from invading the device from the substrate  200 . The buffer layer  205  may be formed of silicon nitride (SiN x ), silicon oxide SiO 2 , and/or silicon oxynitride (SiO x N y ). The first semiconductor layer  210  is formed of an amorphous silicon layer and/or a crystalline silicon layer and includes source and drain regions  210   a  and  210   b  and a channel region  210   c.  The second semiconductor layer  220  is formed of an amorphous silicon layer and/or a crystalline silicon layer and includes source and drain regions  220   a  and  220   b  and a channel region  220   c.  A gate insulating layer  230  and gate electrodes  215  and  225  are formed above the substrate having the first and second semiconductor layers  210  and  220 , and an interlayer insulator  240  is formed over the substrate  200  having the gate electrodes  215  and  225 , and source and drain electrodes  217   a  and  217   b  and  227   a  and  227   b  are formed to be connected to the source and drain regions  210   a  and  210   b  and  220   a  and  220   b  of the first and second semiconductor layers  210  and  220 , respectively.  
         [0069]     A passivation layer  250  is formed above the substrate  200  having the source and drain electrodes  217   a  and  217   b  and  227   a  and  227   b  to protect the lower layers from moisture and impurities and/or during an etching process. The passivation layer  250  is formed of SiO 2 , SiN x , and/or a stacked layer of SiO 2 /SiN x .  
         [0070]     A planarization layer  260  may be formed on the passivation layer  250 . First and second anodes  270  and  280  are formed on the planarization layer  260 . The first and second anodes  270  and  280  are made of a transparent material such as indium tin oxide (ITO) and/or indium zinc oxide (IZO); and/or a compound or stacked layer including a reflective layer made of a material having high reflectivity, such as aluminum (Al), an aluminum alloy, silver (Ag), and/or a silver alloy, and a transparent material layer made of indium tin oxide (ITO) and/or indium zinc oxide (IZO).  
         [0071]     The first anode  270  is electrically connected to one of the source and drain electrodes  217   a  and  217   b,  e.g., the drain electrode  217   b,  through a via hole  262  formed in the planarization layer  260  and the passivation layer  250 , and the second anode  280  is electrically connected to one of the source and drain electrodes  227   a  and  227   b,  e.g., the drain electrode  227   b,  through a via hole  263  formed in the planarization layer  260  and the passivation layer  250 .  
         [0072]     A pixel defining layer  285  is formed above the substrate having the first and second anodes  270  and  280 . The pixel defining layer  285  is etched to have two opening portions which expose the first and second anodes  270  and  280 , respectively.  
         [0073]     An organic emission layer  290  is formed on the first and second anodes  270  and  280 , and a common cathode  295  is formed over the entire surface of the substrate to cover the organic emission layer  290 .  
         [0074]     As described above, in an organic EL display device of the present invention, even if a short circuit occurs between one of at least two anodes and a cathode, and a portion of an organic light emitting diode corresponding to the short circuit anode does not emit light, the other portion(s) of the organic light emitting diode corresponding to the rest of the anodes emits light normally, and a pixel defect due to the short circuit is unnoticeable.  
         [0075]     In FIGS.  5  to  8 , the pixel circuit includes the two driving transistors and the organic light emitting diode which is comprised of the two anodes and the common cathode, but the pixel circuit can be configured to have more than two, i.e., n driving transistors, and/or the organic light emitting diode can be configured to have n anodes and a common cathode. That is, according to the present invention, the number of driving transistors and anodes per pixel is at least two.  
         [0076]     As described above, according to the present invention, in an organic EL display device having at least two driving transistors and an organic light emitting diode having at least two first electrodes and a common second electrode per pixel, even if a short circuit occurs between one of the first electrodes and the second electrode, since the organic light emitting diode corresponding to the other first electrode(s) emits light, the resulting pixel darkening effect is unnoticeable.  
         [0077]     Although the present invention has been described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that a variety of modifications and variations may be made to the present invention without departing from the spirit or scope of the present invention defined in the appended claims and their equivalents.