Patent Application
20060113918
The present invention is about driving an amorphous silicon thin-film transistor, more particularly driving an organic light emitting diode (OLED) display, to enhance the stability of the threshold voltage (Vth) as a function of time on the amorphous silicon thin-film transistors.
There are two ways to drive an organic light emitting display (OLED): one is passive matrix driving and the other is active matrix driving. In an active matrix device, a good service life and high resolution can be achieved without being driven to an extremely high brightness. Therefore, OLED combined with thin-film transistor (TFT) to realize the active matrix technology conforms to the present market requirements for fluidity of images as well as higher and higher resolution in display panels that fully demonstrate the superior properties of OLED. As a result of the continuous improvements in light emitting efficiency on OLED materials, using amorphous silicon thin-film transistor (a-Si TFT) as the driving platform for OLED devices is no longer infeasible. As a result of the maturity of manufacturing processes and equipments in a-Si TFT, a lower manufacturing cost can be achieved which greatly lower the over-all cost of the active matrix OLED.
Although a-Si TFT has absolute advantage of lower cost, there are still technical issues needed to improve if a-Si TFT is to be used to drive OLED. Two major goals must be achieved. The first goal is to improve the stability of the a-Si TFT device, and the second is to increase the driving capability of current in the a-Si TFT device.
The fundamental working principle is as follows: Through controlling the signal of scan line 17 to trigger transistor 11 ON, which then input the voltage signal representing gray scale data of image into storage capacitance 13 to control the gate of transistor 12. The current is flowing through the transistor 12, which can be varied by changing the gate voltage Vgs, of transistor 12. Naturally, in order to make transistor 12 produce a driving current, the Vgs value in transistor 12 must be greater than its threshold voltage Vth.
Conventional scanning structure employs a continuous scanning mode, beginning with the first line on the (n)th-frame, and consecutively scan to the last line of the frame, immediately followed by the first line on the (n+1)th-frame, and consecutively scan to the last line of the (n+1)th frame, as shown in
The conventional scan mode stated above, when applied to OLED structures driven by a-Si TFT, will produce a continuous positive Vgs voltage on transistor 12. A continuous positive Vgs bias, called Positive Stress, it will rapidly degrade the a-Si TFT devices on transistor 12. Also, the threshold voltage, Vth, on transistor 12 will increase with time instead of maintaining at the original level which will incur a “Positive Shift” as shown in
Therefore, the positive shift as a result of instability in threshold voltage, Vth, brings about two problems: The first is that the original brightness of OLED can not be maintained as a result of the decrease in output current on transistor 12, with time. The second problem is that the degree of degradation on transistor 12 in the sub-pixel varies with time. Because the difference in positive stress on transistor 12 of each sub-pixel will bring about a difference in brightness on the sub-pixel of the display panel, resulting in so called “Temporal Non-Uniformity”.
To solve the weaknesses mentioned above, the U.S. Pat. No. 6,677,713 “Driving Circuit and Method for Light Emitting Device” proposed 3T1C driving circuitry as shown in
Therefore the working principle is that when even number of continuous primary scanning pulse S1 trigger transistor 21 On, allows the data voltage in data line 26 corresponding to a frame of image to input to node B, toggles the driving transistor 22 On, And proceeds a time-interval, TON, of image display; when even number of continuous secondary scanning pulse S2 triggers transistor 25 On, allows a closure voltage Vref2 into node. B, and toggles transistor 22 Off, and proceeds a time-interval TOFF of image off. The relationship between scan line and time in driving structure of the image frame is shown in
The U.S. Pat. No. 6,677,713, as compared to the conventional technology, uses an amorphous silicon secondary transistor 25 to recover the threshold voltage Vth of driving transistor 22 to its initial value, and prevents Vth from increasing beyond its original value, and from the degradation of driving transistor 22 with time, so the problem of difference in brightness of each sub-pixel on the display panel can be resolved.
However in the patent, an amorphous silicon transistor and a secondary scan line 28 have to be added to each sub-pixel to process settings of the negative driving bias. In other words, a set of scan driver need to be added to the system which will increase the complexity in manufacturing and, with the additional driving circuitry, substantially increase its cost.
Therefore this invention proposes an innovated way to improve the stability of a driving device for organic electric-excited light emitting transistor driven by amorphous silicon thin film transistor, the main purpose is to eliminate the non-uniformity of the threshold voltage Vth on thin film transistor, and extend life of the active matrix display panels.
Another purpose is to achieve the same result as in U.S. Pat. No. 6,677,713 without additional transistors or scan lines. That is, this invention involves a simpler system, which implies a lower cost for the manufacturers employing it.
To achieve the objectives mentioned above, this invention propose a driving scheme, the circuitry of which involves a driving transistor with its drain connected to power supply Vdd, its source connected to the anode of a light emitting diode. The cathode of light emitting diode is then connected to a comparatively fixed low potential Vss. A scan transistor, with its gate connected to the scan line, its source connected to data line and the drain connected to the gate of a driving transistor and an end of a storage capacitance. The other end of the storage capacitance is connected to a resetting signal line, which provides a resetting signal Vcom of high potential V1 and low potential V2 time pulses.
According to the resetting signal Vcom time pulse a low potential V2 input to the storage capacitance toggles the gate of transistor to negative potential and temporarily prevent the organic light emitting diode (OLED) from emitting light, whereas a high potential V1 input to the storage capacitance toggles the gate of transistor to positive potential and trigger the organic light emitting diode (OLED) to emit light. That is, the positive or negative bias driven by driving transistor in each sub-pixel on display panel can be controlled through a single resetting signal voltage Vcom.
The foregoing, as well as additional objects, features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.
The driving circuitry for each sub-pixel in this invention and the schematic diagram of the connection as well as control of each sub-pixel on display panel are shown in
The driving structure of this invention and the corresponding time sequence of control signal are shown in
When the resetting signal Vcom is at high potential level V1, the scanning signal Vscan on scan line 37 will trigger scanning transistor T1, and send the data signal Vdata representing gray scale data on data line 36, into an end of storage capacitance C. This can be used to control the gate (G) of driving transistor T2, which incurs different Vgs voltages at different gate voltages Vg, and produces different driving current. Now the Vgs potential on driving transistor T2 is positive (Vg is greater than Vs), which implies all transistors T2 in sub-pixels on the display panel are at positive stress (Ps).
When the resetting signal Vcom at high potential V1 is decreased to the low potential V2, the gate voltage Vg on transistor T2 will drop from Vdata to [Vdata−(V1−V2)], decreased by a level of (V1−V2), since the storage capacitance maintains the potential difference across both ends. Through proper choice of V1 and V2 voltages (for example a V1 of 20 volts and a V2 of −10 volts), the gate voltage Vg on transistor T2 becomes negative, therefore no current is output to the organic light emitting diode 34, and the source voltage Vs of driving transistor T2 will be at closure voltage, Voled/off, of the organic light emitting diode 34 (if Vss is zero). At the same time, Vgs value on transistor T2 will be a negative value [Vdata−(V1−V2)−Voled/off] (Vg is lower than Vs, as shown in
As compared with the traditional driving scheme in which Vgs voltage in driving transistor 12 is constantly maintained at positive stress and produce a phenomenon called “positive shift”. In this invention, the Vgs voltage in driving transistor T2 is under alternating positive and negative stresses which lowers the degradation rate of a-Si TFT devices, inhibits positive shift as a result of critical potential Vth on driving transistor, and increases the stability of a-Si TFT device as shown in
In summary, the improvement of driving structure to enhance the stability of organic electric-excited light emitting display device driven by amorphous silicon thin film transistor has the following advantages:
Therefore the difference in driving structure between present invention and the U.S. Pat. No. 6,677,713 is that in this proposed technology, after the data of each scan line in the (n)th image frame on the panel is written, each scan line holds a different period of time before entering negative stress, hence the driving transistors of each sub-pixel on the display device are negative stressed at the same time. However in the U.S. Pat. No. 6,677,713, after the data of each scan line in the Nth image frame on the panel is written, each scan line holds the same period of time before entering negative stress, hence the driving transistors of each sub-pixel on the display device are negatively stressed consecutively rather than simultaneously.
Although there is a difference in driving structure, both technologies provide the same effect to the vision, and both utilize the phenomenon of persistence of vision. The eye will not perceive the flickering of an image with frequency higher than 60 Hz. This invention shares the same objectives and effects the U.S. Pat. No. 6,677,713 provides, but with a decreased complexity of system and lower cost for driving circuitry.
