Abstract:
A pixel circuit of an active-matrix organic light-emitting diode does not provide currents for an organic light-emitting diode during a compensation period, and provides currents, free from variation of a threshold voltage of a thin-film transistor, for the organic light-emitting diode during a data input period, so as to improve gray levels, increase contrast ratios, and decrease power consumption.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention provides a pixel circuit of an active-matrix organic light-emitting diode, and more particularly, a pixel circuit capable of compensating property variations in poly-Si TFTs. 
     2. Description of the Prior Art 
     Compared to a cathode ray tube (CRT) monitor, a flat panel display (FPD) monitor has incomparable advantages, such as low power consumption, no radiation, small volume, etc., so that the FPD monitor has become a substitute for the CRT monitor. As FPD technology advances, prices of FPD monitors are reduced, and sizes of FPD monitors are increased, which make FPD monitors more popular. Therefore, light, fine, colorful, low-power FPD monitors are expected, and a device that can combine these advantages is the Organic Light-Emitting Diode (OLED) display. 
     The OLED combines many characteristics together, such as self emission, a wide viewing angle (over 165°), short response time (about 1 μs), high brightness (100-14000 cd/m2), high luminance efficiency (16-38 Im/W), low driving voltage (3-9V DC), thin panel (2 mm), simplified manufacturing, low cost, etc., and the OLED can be applied for large-size or flexible panels. The principle of an OLED is that after conducting a bias voltage, electrons and holes are passing through a hole transport layer, an electron transport layer and then combine in an organic light emitting material to form “excitons”. Energy of the excitons is released to the ground state, and the released energy creates luminance of the OLED with colors. 
     According to different driving methods, the OLED can be divided into two kinds, and one is a passive matrix OLED, or PM-OLED, and the other is an active matrix OLED, or AM-OLED. Please refer to  FIG. 1  and  FIG. 2 .  FIG. 1  illustrates a schematic diagram of a PM-OLED of a pixel, while  FIG. 2  illustrates a schematic of an AM-OLED of a pixel. In comparison, the structure of the PM-OLED shown in  FIG. 1  is simple, so the cost is low. However, the PM-OLED must be operated under highpulse-currents to reach the brightness appropriate for human eyes. Moreover, the brightness of the PM-OLED is directly proportional to the operating current, and the higher the operating current, the lower the circuit efficiency, the life, and the resolution of the PM-OLED. As a result, the PM-OLED is usually utilized for small sized products. On the other hand, although cost and complexity of the AM-OLED are higher than the PM-OLED (but still lower than a TFT-LCD), yet each pixel can store driving signals and can be operated independently and continuously. Also, circuit efficiency of the AM-OLED is higher, so the AM-OLED is utilized for products of large size, high resolution, and high information capacity. However, there are many factors affecting performance of a large size AM-OLED panel. 
     As those skilled in the art recognize, in  FIG. 2 , a current I OLED  flowing through the OLED can be derived as: 
               I   OLED     =       1   2     ⁢     μ   ·     C   OX     ·     W   L     ·       (       V   GS     -     V   TH       )     2               
Therefore, the current I OLED  is affected by the threshold voltage V TH  of the polycrystalline silicon thin-film transistor, or poly-Si TFT, as shown in  FIG. 2 , so that the performance of pixels varies with time and can not reach uniform image. In order to improve the performance, the prior art provides various pixel circuits for compensating the variation in the poly-Si TFT.
 
     In the prior art, pixel circuits of the AM-OLED can be classified into: current driving, digital driving, and voltage driving pixel circuits. A current driving pixel circuit provides excellent image quality, but its panel driving speed is too slow to implement high resolution displays. A digital driving pixel circuit can reduce the poly-Si TFT threshold voltage variation sensitivity, but it needs a very fast addressing speed, so that it is not a good solution for high gray scale displays. A voltage driving pixel circuit can compensate the variation of threshold and is more attractive to integrate poly-Si TFT data drivers on a display panel. However, the prior art voltage driving pixel circuit still has some disadvantages. 
     For example, please refer to  FIG. 3 , which illustrates a prior art pixel circuit  30  of an AM-OLED. The pixel circuit  30  comprises an OLED  300 , switching transistors  302 ,  304 ,  306 , a driving transistor  308 , capacitors  310 ,  312 , scan-line signal reception ends  316 ,  318 ,  320 , and a data-line signal reception end  314 . The switching transistors  302 ,  304 ,  306 , and the driving transistor  308  are poly-Si TFTs. The scan-line signal reception ends  316  and  320  receive first scan-line signal for controlling the switching transistors  302  and  306 . The scan-line signal reception end  318  receives second scan-line signal for controlling the switching transistor  304 . The data-line signal reception end  314  receives data-line signal (V in ) for driving the driving transistor  308  to output current I OLED  to the OLED  300  and emit light at specific durations. In addition, according to characteristics of the OLED  300 , the OLED  300  can be considered to be a transistor and a capacitor as an equivalent circuit  400  shown in  FIG. 4 . The equivalent circuit  400  includes a transistor  402  and a capacitor  404 . A gate of the transistor  402  is coupled to a drain of the transistor  402 , and the capacitor  404  is coupled between the drain and a source of the transistor  402 . 
     Please refer to  FIG. 5 , which illustrates a time sequential signal waveform of the data line, the first scan line, and the second scan line. In  FIG. 5 , durations T 1 , T 2 , and T 3  are an initialization period, a compensation period, and a data-input period respectively. Referring to  FIG. 3  and  FIG. 5 , in the duration T 1 , the data-line signal are at a low voltage level, and the first scan-line signal and the second scan-line signal are at a high voltage level, so the switching transistors  302 ,  304 ,  306  are turned on. Then, electrons stored in a gate G and a source S of the driving transistor  308  flow through the switching transistors  302 ,  304 , and  306  to the data-line signal reception end  314 . Next, in the duration T 2 , the first scan-line signal stay at the high voltage level, the second scan-line signal change to the low voltage level, and the data-line signal change to the high voltage level, so the switching transistor  304  is tuned off. Then, the data-line signal is input to the gate G of the driving transistor  308  through the switching transistor  302 . Since the data-line signal is at the high voltage level (V in ) in this case, a current flow generated from the drain D to the source S of the driving transistor  308  to the OLED  300 . Meanwhile, the high-level data-line signal charges the capacitor  312 , so that the capacitor  312  stores a voltage drop ΔV: 
               Δ   ⁢           ⁢   V     =         a     1   +   a       ×     V     T   ⁢           ⁢   H   ⁢           ⁢   _   ⁢           ⁢     T   DV           -       1     1   +   a       ×     V     TH   ⁢           ⁢   _   ⁢           ⁢   OLED         +       1     1   +   a       ×     V     i   ⁢           ⁢   n                       where   ,     
     ⁢     a   =         K     T   DV         K     T   OLED             ,         
K T     DV    and K T     OLED    are conduction parameters of the driving transistor  308  and the OLED  300  respectively,
 
V TH     —     T     DV    and V TH     —     OLED  are threshold voltages of the driving transistor  308  and the OLED  300  respectively.
 
Next, in the duration T 3 , the data-line signal stay at the high voltage level, the first scan-line signal change to the low voltage level, and the second scan-line signal change to the high voltage level, so the driving transistor  308  stays on, the switching transistors  302 , and  306  are turned off, and the switching transistor  304  is turned on. Therefore, data-line signals (V in ) charge the capacitor  312  through the switching transistor  304 , and the gate voltage of the driving transistor  308  becomes V in +ΔV. If an output (source) voltage of the driving transistor  308  is V out , then a current I OLED  flowing into the OLED  300  is:
   I   OLED   =K   T     DV   ·( V   GS   −V   TH     —     T     DV   ) 2   =K   T     DV   ·( V   in   +ΔV−V   out   −V   TH     —     T     DV   ) 2    
Therefore, the current flowing into the OLED  300  is changed with the voltage drop ΔV stored in the capacitor  312 , where the voltage drop ΔV is varied with the threshold voltage. As a result, the current flowing into the OLED  300  is varied unexpectedly, causing non-uniformity of images between pixels and degradation of display quality.
 
     In short, during the compensation period, the prior art pixel circuit  30  provides an unnecessary current to the OLED  300 , and during the data-input period, the current flowing into the OLED  300  is affected by the threshold voltage, causing a bad gray level, a low contrast, and an increasing power consumption of the display panel. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary objective of the claimed invention to provide a pixel circuit of an active-matrix organic light-emitting diode. 
     The present invention discloses a pixel circuit of an active-matrix organic light-emitting diode. The pixel circuit comprises a first switching transistor, a second switching transistor, a third switching transistor, a driving transistor, a first capacitor, a second capacitor, and a fourth switching transistor. The first switching transistor comprises a first electrode coupled to a data line, a second electrode coupled to a first scan line, and a third electrode. The second switching transistor comprises a first electrode coupled to the data line, a second electrode coupled to a second scan line, and a third electrode. The third switching transistor comprises a first electrode coupled to the third electrode of the second switching transistor, a second electrode coupled to the first scan line, and a third electrode. The driving transistor comprises a first electrode coupled to a first voltage, a second electrode coupled to the third electrode of the first switching transistor, and a third electrode coupled to third electrode of the third switching transistor. The first capacitor comprises one end coupled to the first electrode of the driving transistor and the third electrode of the second switching transistor, and the other end coupled to the first electrode of the third switching transistor. The second capacitor comprises one end coupled to the third electrode of the first switching transistor and the second electrode of the driving transistor, and the other end coupled to the third electrode of the second switching transistor and the first electrode of the third switching transistor. The fourth switching transistor comprises a first electrode coupled to the third electrode of the third switching transistor and the third electrode of the driving transistor, a second electrode coupled to the first scan line, and a third electrode coupled to an organic light-emitting diode. 
     The present invention further discloses a method for driving the above-mentioned pixel circuit. The method comprises during an initialization period, adjusting voltage levels of the first scan line and the second scan line to a first voltage, and adjusting a voltage level of the data line to a second voltage; during a compensation period, adjusting the voltage levels of the data line and the first scan line to the first voltage, and adjusting the voltage level of the second scan line to the second voltage; and during a data-input period, adjusting the voltage levels of the data line and the second scan line to the first voltage, and adjusting the voltage level of the first scan line to the second voltage. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic diagram of a prior art PM-OLED pixel circuit. 
         FIG. 2  illustrates a schematic of a prior art AM-OLED pixel circuit. 
         FIG. 3  illustrates a schematic of a prior art AMOLED pixel circuit. 
         FIG. 4  illustrates an equivalent circuit of an OLED. 
         FIG. 5  illustrates a time sequential signal waveform of a data line, a first scan line, and a second scan line in  FIG. 3  and  FIG. 6 . 
         FIG. 6  illustrates a schematic diagram of a pixel circuit of an AM-OLED in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Please refer to  FIG. 6 , which illustrates a schematic diagram of a pixel circuit  60  of an AM-OLED in accordance with the present invention. The pixel circuit  60  comprises an OLED  600 , switching transistors  601 ,  602 ,  603 ,  604 , a driving transistor  608 , capacitors  610 ,  612 , scan-line signal reception ends  616 ,  618 ,  620 ,  622 , and a data-line signal reception end  614 . The switching transistors  601 ,  602 ,  603 ,  604 , and the driving transistor  608  are poly-Si TFTs. Notice that a polarity of the switching transistor  604  is opposite to polarities of the switching transistors  601 ,  602 ,  603 , and the driving transistor  608  (in an embodiment, the switching transistors  601 ,  602 ,  603 , and the driving transistor  608  are n-type, while the switching transistor  604  is p-type). The capacitor  610  sustains a gate voltage of the driving transistor  608  against leakage currents. The capacitor  612  stores a threshold voltage of the driving transistor  608  (which will be detailed). The scan-line signal reception ends  616 ,  620 , and  622  receive a first scan-line signal for controlling the switching transistors  601 ,  603 , and  604 . The scan-line signal reception end  618  receives a second scan-line signal for controlling the switching transistor  602 . The data-line signal reception end  614  receive data-line signal (V in ) for driving the driving transistor  608  to output current I OLED  to the OLED  600  at specific durations. 
     The pixel circuit  60  is operated according to the time sequential signal waveform shown in  FIG. 5 . Referring to  FIG. 6  and  FIG. 5 , in the duration T 1 , the data-line signal are at a low voltage level, and the first scan-line signal and the second scan-line signal are at a high voltage level, so the switching transistors  601 ,  602 , and  603  are turned on, and the switching transistor  604  is turned off. Then, electrons stored in a gate G and a source S of the driving transistor  608  flow through the switching transistors  601 ,  602 , and  603  to the data-line signal reception end  614 . Next, in the duration T 2 , the first scan-line signal stay at the high voltage level, the second scan-line signal change to the low voltage level, and the data-line signal change to the high voltage level, so the switching transistors  601 ,  603  stay on, and the switching transistor  602  is turned off. Then, the data-line signal input to the gate G of the driving transistor  608  through the switching transistor  601 , so as to drive the driving transistor  608  and charge the capacitor  612 . Meanwhile, since the switching transistor  604  is still off, a source current of the driving transistor  608  does not flow into the OLED  600 , but flows into the capacitors  610  and  612  through the switching transistor  603 . As a result, the capacitor  612  stores a voltage drop ΔV equaling to a threshold voltage of the driving transistor  608 . That is,
 
ΔV=V TH     —     T     DV    
 
where, V T     —     T     DV    is the threshold voltage of the driving transistor  608 . Therefore, during the compensation period, the present invention pixel circuit  60  does not output current to the OLED  600 .
 
     Next, in the duration T 3 , the data-line signal stay at the high voltage level, the first scan-line signal change to the low voltage level, and the second scan-line signal change to the high voltage level, so the switching transistors  601  and  603  are turned off, the switching transistors  602  and  604  are turned on. Then, data-line signal (V in ) charge the capacitor  612  through the switching transistor  602 , and a gate voltage V G  of the driving transistor  608  becomes:
 
 V   G   =V   in   +V   TH     —     T     DV    
 
If an output (source) voltage of the driving transistor  608  is V out , then a current I OLED  flowing into the OLED  600  is:
 
 I   OLED   =K   T     DV   ·( V   GS   −V   TH     —     T     DV   ) 2   =K   T     DV   ·( V   in   +V   TH     —     T     DV     −V   out   −V   TH     —     T     DV   ) 2   =K   T     DV   ·( V   in   −V   out ) 2 .
 
Therefore, the current flowing into the OLED  600  is not affected by the threshold voltage of the driving transistor  608 , so as to improve a gray level, increase a contrast ratio, and decrease power consumption.
 
     In comparison, during the compensation period, the prior art pixel circuit provides an unnecessary current to the OLED, and during the data-input period, the current flowing into the OLED is affected by the threshold voltage. On the other hand, during the compensation period, the present invention pixel circuit does not provide current to the OLED, and during the data-input period, the current flowing into the OLED is not affected by the threshold voltage, so as to improve a gray level, increase a contrast ratio, and decrease power consumption. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.