Patent Publication Number: US-2011050736-A1

Title: Pixel and illuminating device thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 98129451, filed Sep. 1, 2009. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a pixel structure, more particularly, to a pixel structure having an organic light-emitting diode (OLED) and application thereof. 
     2. Description of Related Art 
     With development of electronic technology, people&#39;s demand for visual service provided by consumable electronic product becomes higher. Regardless of conventional televisions or advanced personal computers and mobile phones, etc, quality for displayed images are highly required. Presently, the most popular display panels are liquid crystal display (LCD) panels. However, although technical development of the LCD panel has a high maturity, due to inherent limitations of liquid crystal materials, development of the LCD panel is bottlenecked, for example, increasing of a response speed of the LCD is bottlenecked. Therefore, various types of display panels (for example, an organic light-emitting diode (OLED) display panel) are continually researched and developed. 
     Generally, pixels in the OLED display panel can be implemented by P-type transistor structures of 2T1C (i.e. two transistors and one capacitor). However, in such type of the pixel, a current flowing through the OLED is not only changed as a system voltage Vdd is influenced by an IR drop, but can also be different as a threshold voltage of the transistor is shifted. 
     Another type of pixel implemented by an N-type transistor structure of 2T1C is provided. However, in such type of the pixel, current flowing through the OLED is not only changed as a threshold voltage of the transistor is shifted, but is also varied as a threshold voltage of the OLED device is shifted for long operation time. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a pixel, in which brightness of an organic light-emitting diode (OLED) is not changed as a threshold voltage of a transistor used for driving the OLED is shifted. 
     The present invention is directed to an illuminating device, in which illuminating brightness of an OLED is not changed as a threshold voltage of a transistor used for driving the OLED is shifted. 
     The present invention is directed to another pixel, in which a phenomenon that brightness of an OLED of the pixel is changed as a system voltage is influenced by an IR drop can be mitigated. 
     The present invention is directed to still another pixel, in which a phenomenon that brightness of an OLED of the pixel is changed as a threshold voltage of the OLED is shifted can be mitigated. 
     The present invention provides a pixel including an OLED, a transistor, a first switch, a second switch and a capacitor. One end of the OLED is electrically connected to a first voltage. A first source/drain of the transistor is electrically connected to a first potential point. The first switch is electrically connected between a second source/drain of the transistor and a second potential point, and is controlled by a first driving signal. The second switch is electrically connected between the second source/drain of the transistor and a gate of the transistor, and is controlled by a second driving signal. The capacitor is electrically connected between the gate of the transistor and a data line. The first driving signal and the second driving signal are used to alternately enable/disable the first and the second switches, so as to drive the pixel. 
     In an embodiment of the present invention, the first driving potential point is another end of the OLED; the second potential point is a second voltage; the first driving signal is a light emitting enable signal; and the second driving signal is a compensation signal. In this case, when the compensation signal and the light-emitting enable signal simultaneously enable the second switch and the first switch, the gate of the transistor is pre-charged to a voltage closed to the second voltage. In an embodiment, a voltage of the data line is a low level voltage. 
     In an embodiment of the present invention, the first switch is a first transistor, and the second switch is a second transistor. In an embodiment, when the compensation signal and the light-emitting enable signal simultaneously enable the second transistor and the first transistor, the gate of the transistor is pre-charged to a voltage equal to the second voltage minus threshold voltages of the first and the second transistors. In an embodiment, a voltage of the data line is a low level voltage. 
     In an embodiment of the present invention, when the compensation signal enables the second switch and the light-emitting enable signal disables the first switch, the gate of the transistor is discharged to a voltage equal to a sum of a threshold voltage of the transistor and a threshold voltage of the OLED. In an embodiment, a voltage of the data line is a low level voltage. 
     In an embodiment of the present invention, when the compensation signal disables the second switch and the light-emitting enable signal enables the first switch, a voltage of the gate of the transistor is boosted to a voltage of the data line plus threshold voltages of the transistor and the OLED. 
     In an embodiment of the present invention, the transistor, the first switch and the second switch are N-type transistors. 
     In an embodiment of the present invention, the first potential point is a second voltage; the second potential point is another end of the OLED; the first driving signal is a first scan signal; and the second driving signal is a second scan signal. In this case, when the first scan signal disables the first switch and the second scan signal enables the second switch, a gate voltage of the transistor is equal to the second voltage minus a threshold voltage of the transistor. 
     In an embodiment of the present invention, when the first scan signal enables the first switch and the second scan signal disables the second switch, a gate voltage of the transistor is equal to the second voltage minus a threshold voltage of the transistor and a voltage of the data line formed when the first switch is disabled and the second switch is enabled. 
     In an embodiment of the present invention, the first scan signal is inverted to the second scan signal. In an embodiment, the transistor, the first switch and the second switch are P-type transistors. 
     In an embodiment of the present invention, the first scan signal is the same to the second scan signal. In an embodiment, the transistor and the first switch are P-type transistors, and the second switch is an N-type transistor. 
     In an embodiment of the present invention, the first voltage is a ground voltage, and the second voltage is a system voltage. 
     The present invention provides an illuminating device including at least a light-emitting unit, and the light-emitting unit includes the above pixel submitted by the present invention. 
     According to the pixel of the present invention and the driving method thereof, the brightness of the OLED in the pixel and the threshold voltage of the transistor used for driving the OLED are irrelevant, so that the brightness of the OLED is not changed as the threshold voltage of the transistor is shifted. Moreover, regarding the OLED, a phenomenon that the brightness of the OLED is changed as the threshold voltage thereof is shifted and a phenomenon that the brightness of the OLED is changed as the system voltage is influenced by the IR drop can be mitigated. In addition, the pixel of the present invention can also serve as a light-emitting unit for organic light-emitting illumination applications, and with functions of compensating inconsistency of the threshold voltage of the conventional transistor and attenuation of the OLED as the using time thereof increases, according to this patent, not only theses shortages can be compensated, but also illuminating brightness of the illuminating device can be adjusted according to different voltages. 
     In order to make the aforementioned and other features and advantages of the present invention comprehensible, several exemplary embodiments accompanied with figures are described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1A  is an equivalent circuit diagram of a pixel according to a first embodiment of the present invention. 
         FIG. 1B  is a drive timing diagram of a pixel according to a first embodiment of the present invention. 
         FIG. 1C  is an equivalent circuit diagram of another pixel according to a first embodiment of the present invention. 
         FIG. 2A  is a characteristic curve diagram of a current flowing through an OLED and a data voltage transmitted by a data line according to a first embodiment of the present invention. 
         FIG. 2B  is an error rate diagram illustrated according to  FIG. 2A . 
         FIG. 3  is a characteristic curve diagram of a current flowing through an OLED and a system voltage according to a first embodiment of the present invention. 
         FIG. 4  is a flowchart illustrating a driving method of a pixel according to a first embodiment of the present invention. 
         FIG. 5A  is an equivalent circuit diagram of a pixel according to a second embodiment of the present invention. 
         FIG. 5B  is a drive timing diagram of a pixel according to a second embodiment of the present invention. 
         FIG. 5C  is a diagram illustrating a simulation result of voltage variation at an anode of an OLED according to a second embodiment of the present invention. 
         FIG. 6A  is a characteristic curve diagram of currents flowing through OLEDs and a data voltage transmitted by a data line according to a second embodiment of the present invention. 
         FIG. 6B  is another characteristic curve diagram of currents flowing through OLEDs and a data voltage transmitted by a data line according to a second embodiment of the present invention. 
         FIG. 7  is a flowchart illustrating a driving method of a pixel according to a second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     An embodiment is provided below for describing a pixel of the present invention, so as to fully convey the concept of the present invention to those skilled in the art. 
     Referring to  FIG. 1A ,  FIG. 1A  is an equivalent circuit diagram of a pixel according to a first embodiment of the present invention. The pixel  100  of the present embodiment includes an organic light-emitting diode (OLED) D 1 , a transistor DTFT 1 , a switch SW 1 , a switch SW 2  and a capacitor C st1 . In the present embodiment, the transistor DTFT 1  is, for example, a P-type transistor, and the switches SW 1  and SW 2  are formed by transistors connected as switches. 
     A cathode of the OLED D 1  is electrically connected to a first voltage (which is a ground voltage GND in the present embodiment). A first source/drain of the transistor DTFT 1  is electrically connected to a first potential point (for example, a second voltage, which is a system voltage Vdd in the present embodiment). The switch SW 1  is electrically connected between a second source/drain of the transistor DTFT 1  and a second potential point (for example, an anode of the OLED D 1 ), wherein the switch SW 1  is controlled by a first driving signal, for example, a scan signal SCAN 1 . The switch SW 2  is electrically connected between the second source/drain of the transistor DTFT 1  and a gate of the transistor DTFT 1 , and is controlled by a second driving signal, for example, a scan signal SCAN 2 . The capacitor C st1  is electrically connected between the gate of the transistor DTFT 1  and a data line DT. 
     Referring to  FIG. 1B  for operation of the pixel  100  of the first embodiment, and  FIG. 1B  is a drive timing diagram of the pixel according to the first embodiment of the present invention. It should be noticed that operation of the switch SW 1  is inversed to that of the switch SW 2 , wherein the switches SW 1  and SW 2  are, for example, formed by P-type thin-film transistors. The switches SW 1  and SW 2  are enabled when control signals thereof have a low level, and are disabled when the control signals have a high level. 
     In the present embodiment, a driving time zone is mainly divided into two periods, and during a period T 1 , the scan signal SCAN 1  has the high level and the scan signal SCAN 2  has the low level, so that the switch SW 1  is disabled and the switch SW 2  is enabled, and the gate and the second source/drain of the transistor DTFT 1  are mutually coupled to form a diode connection. Therefore, a path for the system voltage Vdd transmitting a current to the OLED D 1  is blocked since the switch SW 1  is disabled, and the gate of the transistor DTFT 1  is charged until a voltage thereof is equal to the system voltage Vdd minus a threshold voltage V TH, DTFT1  of the transistor DTFT 1  since the switch SW 2  is enabled, i.e. Vdd−|V TH, DTFT1 |. Meanwhile, the data line DT transmits data, so that the capacitor C st1  stores a data voltage V data  transmitted by the data line DT, and has a voltage difference of Vdd−|V TH, DTFT1 |−V data . 
     Then, during a second period T 2 , the scan signal SCAN 1  is transited to the low level, and the scan signal SCAN 2  is transited to the high level, so that the switch SW 1  is enabled and the switch SW 2  is disabled. Therefore, the system voltage Vdd can drive the transistor DTFT 1  to transmit a current to the OLED D 1  through the switch SW 1 , so as to light the OLED D 1 . Meanwhile, the data voltage transmitted by the data line DT is decreased to 0V (volt), so that a gate voltage of the transistor DTFT 1  is influenced by a capacitance effect of the capacitor C st1 , and is pulled down to a voltage equal to the system voltage Vdd minus the threshold voltage V TH, DTFT1  of the transistor DTFT 1  and the voltage V data  of the data line, i.e. Vdd−|V TH, DTFT1 |−V data . 
     Since the transistor DTFT 1  is maintained to a saturation area, a current I D1  flowing through the OLED D 1  is calculated according to a following equation: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           I 
                           
                             D 
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                                   DTFT 
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                                 ( 
                                 
                                   
                                     V 
                                     
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                                   DTFT 
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     Where I D1  is the current flowing through the OLED D 1  (i.e. I OLED1 ), V GS, DTFT1  is a gate-source voltage of the transistor DTFT 1 , and K DTFT1  is a current constant of the transistor DTFT 1 . Moreover, the equation (1) can be further modified to be an equation (2): 
     
       
         
           
             
               
                 
                   
                     I 
                     
                       D 
                        
                       
                           
                       
                        
                       1 
                     
                   
                   = 
                   
                     
                       1 
                       2 
                     
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                       2 
                     
                   
                 
               
               
                 
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     According to the equations (1) and (2), it is known that the gate-source voltage V GS, DTFT1  of the transistor DTFT 1  does not contain a parameter of the system voltage Vdd, so that a situation that the current I D1  (i.e. I OLED1 ) flowing through the OLED D 1  is changed as the system voltage Vdd is influenced by an IR drop is naturally avoided. Moreover, the current I D1  (i.e. I OLED1 ) neither contains a parameter of the threshold voltage V TH, DTFT1  of the transistor DTFT 1 , so that the current I D1  is not changed as the threshold voltage V TH, DTFT1  of the transistor DTFT 1  is shifted. In other words, brightness of the OLED D 1  lighted by the pixel  100  of the present embodiment does not relate to the threshold voltage V TH, DTFT1  of the transistor DTFT 1  and the system voltage Vdd. 
     It should be noticed that the transistor DTFT 1 , the switches SW 1  and SW 2  used by the pixel  100  are P-type transistors. However, to achieve the aforementioned functions, in another embodiment, the switch SW 2  can also be implemented by a N-type transistor connected as a switch as that shown in a pixel  200  of  FIG. 1C . Operation of the pixel  200  of  FIG. 1C  is substantially the same to that of the pixel  100 . In detail, in the pixel  200 , the operation of the switch SW 2  is inversed to that of the switch SW 1 , and when the scan signals SCAN 1  and SCAN 2  are the same, operations of the switch SW 1  formed by the P-type transistor and the switch SW 2  formed by the N-type transistor are inversed. In an actual application, a same scan line can be used to transmit the same scan signal (for example, the scan signal SCAN 1 ) to the switches SW 1  and SW 2 , so as to enable or disable the switch SW 1  when the control signal has the low level or the high level, and enable or disable the switch SW 2  when the control signal has the high level or the low level. 
     Simulation results of the pixel  200  are provided below with reference of figures to further describe the aforementioned deduction. In the following embodiment, the system voltage Vdd is set to 9.5V, and the data voltage V data  transmitted by the data line DT is between 1V and 3.5V. Moreover, a channel width-to-length ratio (W/L) of the transistor DTFT 1  is 10 um/4 um, and the threshold voltage V TH, DTFT1  of the transistor DTFT 1  is 1V. 
     Referring to  FIG. 2A ,  FIG. 2A  is a characteristic curve diagram of the current flowing through the OLED and the data voltage transmitted by the data line according to the first embodiment of the present invention. Wherein, a curve  310 + represents a waveform of the current I OLED1  of the OLED D 1  when the threshold voltage V TH, DTFT1  of the transistor DTFT 1  is shifted for +0.33V, a curve  310  represents a waveform of the current I OLED1  of the OLED D 1  when the threshold voltage V TH, DTFT1  of the transistor DTFT 1  is not shifted, and a curve  310 − represents a waveform of the current I OLED1  of the OLED D 1  when the threshold voltage V TH, DTFT1  of the transistor DTFT 1  is shifted for −0.33V. 
     According to  FIG. 2A , the curves  310 ,  310 + and  310 − are very close. In other words, even if the threshold voltage V TH, DTFT1  of the transistor DTFT 1  is shifted for ±0.33V, the current I OLED1  of the OLED D 1  is almost not influenced by the shifting of the threshold voltage V TH, DTFT1 , and approximately relates to the data voltage V data  transmitted by the data line DT. 
     Further, an error rate ER of the current I OLED1  of the OLED D 1  generated when the threshold voltage V TH, DTFT1  of the transistor DTFT 1  is shifted can be calculated according to an equation (3): 
     
       
         
           
             
               
                 
                   ER 
                   = 
                   
                     
                       
                         
                           I 
                           
                             OLED 
                              
                             
                                 
                             
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                                   TH 
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                                     DTFT 
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                                 ± 
                                 0.33 
                               
                                
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                       - 
                       
                         
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                           OLED 
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                           1 
                         
                       
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                   3 
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     Where I OLED1 (ΔV TH, DTFT1 =÷0.33V) represents the current I OLED1  of the OLED D 1  when the threshold voltage V TH, DTFT1  of the transistor DTFT 1  is shifted for ±0.33V, and I OLED1 (ΔV TH, DTFT1 =0V) represents the current I OLED1  of the OLED D 1  when the threshold voltage V TH, DTFT1  of the transistor DTFT 1  is not shifted. 
       FIG. 2B  is deduced according to  FIG. 2A  and the equation (3), wherein two close curves  320 + and  320 − respectively represent the error rate ER of the current I OLED1  of the OLED D 1  in the pixel  200  that is generated when the threshold voltage V TH, DTFT1  of the transistor DTFT 1  is shifted for +0.33V and −0.33V. Moreover, in view of specific data, the error rate ER of the current I OLED1  of the OLED D 1  in the pixel  200  that is generated when the threshold voltage V TH, DTFT1  of the transistor DTFT 1  is shifted for ±0.33V is less than 1.5%. The calculated error rate is tiny, which means a relationship between the brightness of the OLED D 1  lighted by the pixel  200  and the threshold voltage V TH, DTFT1  of the transistor DTFT 1  is tiny, which can be neglected. 
     On the other hand, assuming the system voltage Vdd has a voltage drop of 1V due to an influence of the IR drop, influences of the system voltage Vdd for a current I OLED     —     A  of an OLED in a conventional pixel using a P-type transistor structure of 2T1C (i.e. two transistors and one capacitor) and the current I OLED1  of the OLED D 1  in the pixel  200  of the present embodiment are respectively simulated as curves  410   a  and  410  of  FIG. 3 . According to  FIG. 3 , it is known that the currents I OLED     —     A  of the OLED in the conventional pixel that are influenced by the system voltage Vdd may have a great difference, though the currents I OLED1  of the OLED D 1  in the pixel  200  that are influenced by the system voltage Vdd may have a little difference. Therefore, a phenomenon that the current of the OLED in the conventional pixel is changed as the system voltage is influenced by the IR drop can be effectively mitigated. 
     According to the above descriptions, the present invention further provides a driving method of the pixels  100  and  200  for those with ordinary skill in the art. 
     Referring to  FIG. 1A  and  FIG. 4 ,  FIG. 4  is a flowchart illustrating a driving method of the pixel (for example, the pixel  100  or  200 ) according to the first embodiment of the present invention. First, in step S 501 , during a first period, the switch SW 1  (a first switch) is disabled and the switch SW 2  (a second switch) is enabled, so that a gate voltage of the transistor DTFT 1  is equal to the second voltage (which is the system voltage Vdd in the present embodiment) minus the threshold voltage of the transistor DTFT 1 . Next, in step S 503 , during a second period, the switch SW 1  (the first switch) is enabled and the switch SW 2  (the second switch) is disabled, so as to light the OLED D 1 . Particularly, the brightness of the lighted OLED D 1  is not influenced by the threshold voltage of the transistor DTFT 1  and the system voltage Vdd, so that images displayed by a display panel having the pixels  100  and  200  can be more even. Moreover, other details of the driving method are contained in the above embodiment, and therefore detailed descriptions thereof are not repeated. 
     Second Embodiment 
     Another embodiment of the pixel of the present invention is provided below, and the concept of the present embodiment is similar to that of the first embodiment, though a main difference there between is that the transistor and the two switches used by the pixel of the present embodiment are all N-type transistors, as that shown in  FIG. 5A . The pixel  600  of the present embodiment includes an OLED D 2 , an N-type transistor DTFT 2 , a switch SW 3  formed by an N-type transistor, a switch SW 4  formed by another N-type transistor, and a capacitor C. It should be noticed that the same or like reference numerals in the present embodiment and the first embodiment denote the same or like elements, and descriptions thereof are not repeated. 
     A cathode of the OLED D 2  is electrically connected to a first voltage (which is the ground voltage GND in the present embodiment), and a first source/drain of the transistor DTFT 2  is electrically connected to a first potential point, for example, an anode of the OLED D 2 . The switch SW 3  is electrically connected between a second source/drain of the transistor DTFT 2  and a second potential point (for example, a second voltage, which is the system voltage Vdd in the present embodiment), and is controlled by a first driving signal, for example, a light-emitting enable signal EM. The switch SW 4  is electrically connected between the second source/drain of the transistor DTFT 2  and a gate of the transistor DTFT 2 , and is controlled by a second driving signal, for example, a compensation signal SLT. The capacitor C st2  is electrically connected between the gate of the transistor DTFT 2  and a data line DT. 
     Referring to  FIG. 5B  for operation of the pixel  600  of the present embodiment, and  FIG. 5B  is a drive timing diagram of the pixel according to the second embodiment of the present invention. Wherein, the switches SW 3  and SW 4  are enabled when control signals thereof have a high level, and are disabled when the control signals have a low level. 
     In the present embodiment, a driving time zone is mainly divided into three periods, and during a period T 3 , the light-emitting enable signal EM and the compensation signal SLT all have the high level, so that the switches SW 3  and SW 4  are simultaneously enabled. Meanwhile, a voltage of the data line DT is set to a low level voltage (for example, 0V, so that a left end of the capacitor C st2  is regarded to be electrically connected to the ground voltage). Therefore, the system voltage Vdd can pre-charge the gate of the transistor DTFT 2  through the switches SW 3  and SW 4 . 
     It should be noticed that the gate of the transistor DTFT 2  is approximately pre-charged to the system voltage Vdd during the period T 3 , though considering threshold voltages V TH, TFT3  and V TH, TFT4  of the switches SW 3  and SW 4  formed by the N-type transistors, the pre-charged gate voltage of the transistor DTFT 2  is substantially equal to the system voltage Vdd minus the two threshold voltages V TH, TFT3  and V TH, TFT4 , i.e. Vdd−V TH, TFT3 −V TH, TFT4 . However, since Vdd−V TH, TFT3 −V TH, TFT4  is very closed to the system voltage Vdd, in the present embodiment, the transistor DTFT 2  is regarded to be pre-charged to the system voltage Vdd during the period T 3 . 
     Then, after pre-charging of the transistor DTFT 2  is completed, during a period T 4 , the light-emitting enable signal EM is transited to the low level, the compensation signal SLT is maintained to the high level, and the voltage of the data line DT is still set to the low level voltage (for example, 0V). Now, the switch SW 3  is disabled, so that the pre-charging path is blocked. On the other hand, the switch SW 4  is maintained enabled, so that the gate and the second source/drain of the transistor DTFT 2  are mutually coupled to form a diode connection. Therefore, the gate of the transistor DTFT 2  that is originally charged to the system voltage Vdd can be discharged to the capacitor C st2  through the switch SW 4 . Wherein, the gate of the transistor DTFT 2  is discharge until a voltage thereof is equal to a threshold voltage V TH, DTFT2  of the transistor DTFT 2  plus a threshold voltage V TH, OLED2  of the OLED D 2 , i.e. V TH, DTFT2 +V TH, OLED2 , and the capacitor C st2  can store this voltage. 
     Next, during a period T 5 , the light-emitting enable signal EM is transited to the high level, and the compensation signal SLT is transited to the low level, so that the switch SW 3  is enabled and the switch SW 4  is disabled. Now, the system voltage Vdd drives the transistor DTFT 2  to transmit a current to the OLED D 2  to light the OLED D 2 . Meanwhile, the data line DT transmits data to the capacitor C st2 . Therefore, according to a boost effect, the capacitor C st2  can boost the gate voltage of the transistor DTFT 2  from an original voltage equal to a sum of the threshold voltages of the transistor DTFT 2  and the OLED D 2  (V TH, DTFT2 +V TH, OLED2 ) to the data voltage V data  transmitted by the data line DT plus the sum of the threshold voltages of the transistor DTFT 2  and the OLED D 2  (V TH, DTFT2 +V TH, OLED2 ), i.e. V data +V TH, DTFT2 +V TH, OLED2 . 
     Since the transistor DTFT 2  is maintained in the saturation area, a current I D2  flowing through the OLED D 2  is calculated according to a following equation: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           I 
                           
                             D 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                         = 
                         
                           
                             1 
                             2 
                           
                            
                           
                             
                               
                                 K 
                                 
                                   DTFT 
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                               
                                
                               
                                 ( 
                                 
                                   
                                     V 
                                     
                                       GS 
                                       , 
                                       
                                         DTFT 
                                          
                                         
                                             
                                         
                                          
                                         2 
                                       
                                     
                                   
                                   - 
                                   
                                     V 
                                     
                                       TH 
                                       , 
                                       
                                         DFT 
                                          
                                         
                                             
                                         
                                          
                                         2 
                                       
                                     
                                   
                                 
                                 ) 
                               
                             
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             1 
                             2 
                           
                            
                           
                             
                               
                                 K 
                                 
                                   DTFT 
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                               
                                
                               
                                 ( 
                                 
                                   
                                     ( 
                                     
                                       
                                         
                                           
                                             
                                               V 
                                               data 
                                             
                                             + 
                                             
                                               V 
                                               
                                                 TH 
                                                 , 
                                                 
                                                   DTFT 
                                                    
                                                   
                                                       
                                                   
                                                    
                                                   2 
                                                 
                                               
                                             
                                             + 
                                           
                                         
                                       
                                       
                                         
                                           
                                             
                                               V 
                                               
                                                 TH 
                                                 , 
                                                 
                                                   OLED 
                                                    
                                                   
                                                       
                                                   
                                                    
                                                   2 
                                                 
                                               
                                             
                                             - 
                                             
                                               V 
                                               
                                                 TH 
                                                 , 
                                                 
                                                   OLED 
                                                    
                                                   
                                                       
                                                   
                                                    
                                                   2 
                                                 
                                               
                                             
                                           
                                         
                                       
                                     
                                     ) 
                                   
                                   - 
                                   
                                     V 
                                     
                                       TH 
                                       , 
                                       
                                         DTFT 
                                          
                                         
                                             
                                         
                                          
                                         2 
                                       
                                     
                                   
                                 
                                 ) 
                               
                             
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Where I D2  is the current flowing through the OLED D 2  (i.e. I OLED2 ), V GS, DTFT2  is a gate-source voltage of the transistor DTFT 2 , and K DTFT2  is a current constant of the transistor DTFT 2 . Moreover, the equation (4) can be further modified to be an equation (5): 
     
       
         
           
             
               
                 
                   
                     I 
                     
                       D 
                        
                       
                           
                       
                        
                       2 
                     
                   
                   = 
                   
                     
                       1 
                       2 
                     
                      
                     
                       
                         
                           K 
                           
                             DTFT 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                          
                         
                           ( 
                           
                             V 
                             data 
                           
                           ) 
                         
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     According to the equations (4) and (5), it is known that the gate-source voltage V GS, DTFT2  of the transistor DTFT 2  does not contain a parameter of the threshold voltage V TH, OLED2  of the OLED D 2 , and the current I D2  (i.e. I OLED2 ) flowing through the OLED D 2  does not contain the parameter of the threshold voltage V TH, OLED2  of the OLED D 2 . Moreover, the current I D2  (i.e. I OLED2 ) neither contains the parameter of the threshold voltage V TH, DTFT2  of the transistor DTFT 2 . In other words, brightness of the OLED D 2  lighted by the pixel  600  of the present embodiment does not relate to the threshold voltages V TH, OLED2  of the OLED D 2  and V TH, DTFT2  of the transistor DTFT 2 . 
     Simulation results of the pixel  600  are provided below with reference of figures to further describe the aforementioned deduction. In the following embodiment, the system voltage Vdd is set to 10V, and a voltage value of the light-emitting enable signal EM or the compensation signal SLT having the high level is 15V. Moreover, a channel width-to-length ratio of the transistor DTFT 2  is 20 um/2 um. 
     Referring to  FIG. 5C ,  FIG. 5C  is a diagram illustrating a simulation result of voltage variation at the anode of the OLED according to the second embodiment of the present invention. Wherein, the data voltage V data  transmitted by the data line DT in the pixel  600  is, for example, 3V to obtain three curves of  710 +,  710 − and  710 . The curve  710 + represents a voltage waveform of the anode of the OLED D 2  when the threshold voltage V TH, DTFT2  of the transistor DTFT 2  is shifted for +0.33V, the curve  710  represents a voltage waveform of the anode of the OLED D 2  when the threshold voltage V TH, DTFT2  of the transistor DTFT 2  is not shifted, and a curve  710 − represents a voltage waveform of the anode of the OLED D 2  when the threshold voltage V TH, DTFT2  of the transistor DTFT 2  is shifted for −0.33V. 
     According to  FIG. 5C , the curves  710 ,  710 + and  710 − are very close, and during the period T 5  (i.e. a lighting period of the OLED D 2 ), the anode voltage of the OLED D 2  is approximately between 3.086V and 3.090V. In other words, even if the threshold voltage V TH, DTFT2  of the transistor DTFT 2  is shifted for ±0.33V, the anode voltage of the OLED D 2  is almost not influenced by the shifting of the threshold voltage V TH, DTFT2 . 
     Further, an error rate ER of the anode voltage of the OLED D 2  generated when the threshold voltage V TH, DTFT2  of the transistor DTFT 2  is shifted can be calculated according to an equation (6): 
     
       
         
           
             
               
                 
                   ER 
                   = 
                   
                     
                       
                         
                           V 
                           
                             OLED 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                          
                         
                           ( 
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               
                                 V 
                                 
                                   TH 
                                   , 
                                   
                                     DTFT 
                                      
                                     
                                         
                                     
                                      
                                     2 
                                   
                                 
                               
                             
                             = 
                             
                               
                                 ± 
                                 0.33 
                               
                                
                               V 
                             
                           
                           ) 
                         
                       
                       - 
                       
                         
                           V 
                           
                             OLED 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                          
                         
                           ( 
                           
                             
                               Δ 
                                
                               
                                   
                               
                                
                               
                                 V 
                                 
                                   TH 
                                   , 
                                   
                                     DTFT 
                                      
                                     
                                         
                                     
                                      
                                     2 
                                   
                                 
                               
                             
                             = 
                             
                               0 
                                
                               V 
                             
                           
                           ) 
                         
                       
                     
                     
                       
                         V 
                         
                           OLED 
                            
                           
                               
                           
                            
                           2 
                         
                       
                        
                       
                         ( 
                         
                           
                             Δ 
                              
                             
                                 
                             
                              
                             
                               V 
                               
                                 TH 
                                 , 
                                 
                                   DTFT 
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                               
                             
                           
                           = 
                           
                             0 
                              
                             V 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     Where V OLED2 (ΔV TH, DTFT2 =±0.33V) represents the anode voltage V OLED2  of the OLED D 2  when the threshold voltage V TH, DTFT2  of the transistor DTFT 2  is shifted for ±0.33V, and V OLED2 (ΔV TH, DTFT2 =0V) represents the anode voltage V OLED2  of the OLED D 2  when the threshold voltage V TH, DTFT2  of the transistor DTFT 2  is not shifted. 
     According to  FIG. 5C  and the equation (6), it can be calculated that the error rate generated when the threshold voltage V TH, DTFT2  is shifted for +0.33V is 0.021%, and the error rate generated when the threshold voltage V TH, DTFT2  is shifted for −0.33V is 0.081%, i.e. the error rate is between 0.021% and 0.081%. The calculated error rate is tiny, which means a relationship between the brightness of the OLED D 2  lighted by the pixel  600  and the threshold voltage V TH, DTFT2  of the transistor DTFT 2  is tiny, which can be neglected. 
     Referring to  FIG. 6A ,  FIG. 6A  is a characteristic curve diagram of the currents I OLED2  and I OLED     —     B  flowing through the OLED D 2  and the OLED of the conventional pixel applying the N-type transistor structure of 2T1C, and the data voltage V data  transmitted by the data line DT according to the second embodiment of the present invention. Wherein, curves  812 −,  812  and  812 + respectively represent a waveform of the current I OLED2  of the OLED D 2  when the threshold voltage V TH, DTFT2  of the transistor DTFT 2  in the pixel  600  is shifted for −0.33V, not shifted and shifted for +0.33V, and curves  812   a −,  812   a  and  812   a + respectively represent a waveform of the current I OLED     —     B  of the OLED in the conventional pixel when the threshold voltage of the transistor is shifted for −0.33V, not shifted and shifted for +0.33V. 
     According to  FIG. 6A , the three separated curves  812   a −,  812   a  and  812   a + denote that the OLED in the conventional pixel that is influenced by the shifting of the threshold voltage of the transistor can generate the currents I OLED     —     B  with great differences. However, the curves  812 −,  812  and  812 + are almost overlapped, which means shifting of the threshold voltage V TH, DTFT2  of the transistor DTFT 2  hardly influences the current I OLED2  of the OLED D 2 . 
     According to another aspect, as shown in  FIG. 6B ,  FIG. 6B  is another characteristic curve diagram of the currents I OLED2  and I OLED     —     B  flowing through the OLED D 2  and the OLED of the conventional pixel applying the N-type transistor structure of 2T1C, and the data voltage V data  transmitted by the data line DT according to the second embodiment of the present invention. Wherein, curves  814  and  814 + respectively represent a waveform of the current I OLED2  of the OLED D 2  when the threshold voltage V TH, OLED2  of the OLED D 2  in the pixel  600  is not shifted and is shifted for +0.33V, and curves  814   b  and  814   ba + respectively represent a waveform of the current I OLED     —     B  of the OLED when the threshold voltage of the OLED in the conventional pixel is not shifted and is shifted for +0.33V. 
     Accordingly, the curves  814   b  and  814   b + denote that the OLED in the conventional pixel that is influenced by the shifting of the threshold voltage of the transistor can generate the currents I OLED     —     B  with great differences. However, the curves  814  and  814 + are almost the same, so that shifting of the threshold voltage V TH, OLED2  of the OLED D 2  hardly influences the current I OLED2  of the OLED D 2 . 
     According to the above descriptions, the present invention further provides a driving method of the pixel  600  for those with ordinary skill in the art. 
     Referring to  FIG. 5A  and  FIG. 7 ,  FIG. 7  is a flowchart illustrating a driving method of the pixel  600  according to the second embodiment of the present invention. First, in step S 901 , during a first period, the switch SW 3  (the first switch) and the switch SW 4  (the second switch) are enabled, so that the gate of the transistor DTFT 2  is pre-charged. Next, in step S 903 , during a second period, the switch SW 3  (the first switch) is disabled and the switch SW 4  (the second switch) is enabled, so that the gate of the transistor DTFT 2  is discharged to a voltage equal to a sum of the threshold voltages of the transistor DTFT 2  and the OLED D 2 . Next, in step S 905 , during a third period, the switch SW 3  (the first switch) is enabled and the switch SW 4  (the second switch) is disabled, so as to light the OLED D 2 . Particularly, the brightness of the lighted OLED D 2  is not influenced by the threshold voltages of the transistor DTFT 2  and the OLED D 2 , so that a display panel having the pixels  600  may have a better display quality. Moreover, other details of the driving method are contained in the above embodiment, and therefore detailed descriptions thereof are not repeated. 
     It should be noticed that the pixels  100 ,  200  and  600  can also be applied for organic light-emitting illumination applications. To be specific, each of the pixels  100 ,  200  and  600  can be used as a light-emitting unit (not shown) and applied to an illuminating device (not shown), wherein an illuminating brightness of the illuminating device can be adjusted by adjusting a voltage value. Therefore, problems of inconsistency of the threshold voltage of the conventional transistor and attenuation of a function of the OLED as the using time thereof increases can be resolved, so that illuminating quality of the illuminating device can be improved. 
     It should be noticed that in the above embodiments, the OLED is used for descriptions, though the present invention is not limited thereto, and a polymer light emitting diode (polymer LED, PLED) can also be used in the aforementioned pixels. 
     In summary, according to the driving method of the present invention and an ingenious arrangement of the components in the pixel of the present invention, the brightness of the OLED in the pixel can be irrelevant to the threshold voltage of the transistor used for driving the OLED, so that the brightness of the OLED is not changed even if the threshold voltage of the transistor is shifted. Moreover, a phenomenon that the brightness of the OLED is changed as the threshold voltage thereof is shifted and a phenomenon that the brightness of the OLED is changed as the system voltage is influenced by the IR drop can be effectively mitigated, so that the display panel having the pixel of the present invention may have a better display quality. In addition, the pixel of the present invention can also be applied for organic light-emitting illumination applications, and with functions of compensating inconsistency of the threshold voltage of the conventional transistor and attenuation of the OLED as the using time thereof increases, according to this patent, not only theses shortages can be compensated, but also illuminating brightness of the illuminating device can be adjusted according to different voltages. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.