Pixel circuit and driving method therefor, and organic light-emitting display

The present invention relates to the field of flat panel display devices and, in particular, to a pixel circuit and a method for driving it, as well as to an organic light-emitting display device.

Differing from thin-film-transistor liquid-crystal display (TFT-LCD) devices which rely on backlight systems for light emission, organic light-emitting display devices emit light by themselves and hence provide higher visibility and brightness and can be made thinner. At present, organic light-emitting display devices are praised as the next generation display devices that will replace the TFT-LCD devices.

Reference is now made to FIG. 1 which shows a circuit diagram of a pixel in an organic light-emitting display device of the prior art. As shown in FIG. 1 , each pixel in the organic light-emitting display device includes a pixel circuit 10 and an organic light-emitting diode OLED. The pixel circuit 10 is connected to a data line Dm and a scan line Sn so as to control light emission of the organic light-emitting diode OLED. The pixel circuit 10 includes a switch thin-film transistor M 1 , a drive thin-film transistor M 2 and a capacitor Cst. The switch thin-film transistor M 1 has a gate connected to a scan line Sn and a source connected to a data line Dm. The drive thin-film transistor M 2 has a gate connected to a drain of the switch thin-film transistor M 1 , a source connected to a first power source ELVDD via a first power wiring (not shown) and a drain connected to an anode of the organic light-emitting diode OLED. A cathode of the organic light-emitting diode OLED is connected to a second power source ELVSS via a second power wiring (not shown). The organic light-emitting diode OLED emits light under the effect of a current provided by the pixel circuit 10 . The capacitor Cst is connected between the gate and source of the drive thin-film transistor M 2 in order to maintain a digital signal at the gate of the switch thin-film transistor M 1 and a threshold voltage of the drive thin-film transistor M 2 over a predetermined period of time.

However, during the manufacturing process of the thin-film transistors, variations may occur in their threshold voltages. Such variations in threshold voltages of the thin-film transistor that act as driving elements may lead to the organic light-emitting diode OLED emitting light with different brightness levels in response to the digital signal which is, however, indicative of the same brightness level. This may lead to brightness non-uniformity and hence reduced display quality.

Further, the power wiring connecting the first power source ELVDD and the pixel circuits 10 have certain impedances which lead to voltage drops when currents flow in them and hence uneven positive power source voltages supplied to the pixel circuits 10 , thus further reduce brightness uniformity. Another factor that may deteriorate the problem of non-uniform brightness is light-emission efficiency degradation of the organic light-emitting diodes OLED due to their aging over time.

It is an object of the present invention to provide a pixel circuit and a method for driving it, as well as an organic light-emitting display device, in order to address the problem of non-uniform brightness arising from the use of the conventional organic light-emitting display device.

This object is attained by a pixel circuit according to the present invention, including a first thin-film transistor, a second thin-film transistor, a third thin-film transistor, a fourth thin-film transistor, a fifth thin-film transistor, a sixth thin-film transistor, a seventh thin-film transistor, a capacitor and an organic light-emitting diode, wherein a source of the sixth thin-film transistor is connected to a first power source; a drain of the sixth thin-film transistor is connected to both a drain of the first thin-film transistor and a source of the second thin-film transistor; a drain of the second thin-film transistor is connected to an anode of the organic light-emitting diode; a cathode of the organic light-emitting diode is connected to a second power source; a gate of the sixth thin-film transistor is connected to a source of the third thin-film transistor and a first terminal of the capacitor; a second terminal of the capacitor is connected to both a drain of the fourth thin-film transistor and a source of the fifth thin-film transistor; a source of the fourth thin-film transistor is connected to a data line; a drain of the fifth thin-film transistor, together with a drain of the seventh thin-film transistor, is connected to a reference power source; and a source of the seventh thin-film transistor is connected to both a source of the first thin-film transistor and a drain of the third thin-film transistor.

Optionally, in the pixel circuit, the first power source and the second power source may be configured to provide the organic light-emitting diode with power supply voltages, wherein a reference power source is configured to provide an initialization voltage to the gate and drain of the sixth thin-film transistor and the anode of the organic light-emitting diode.

Optionally, in the pixel circuit, the gates of the second thin-film transistor and the fifth thin-film transistor may be both connected to a first scan line which is configured for control initialization and capacitor stabilization, wherein the gates of the first thin-film transistor, the third thin-film transistor and the fourth thin-film transistor are all connected to a second scan line which is configured to control writing of a data voltage and sample a threshold voltage of the sixth thin-film transistor, and the gate of the seventh thin-film transistor is connected to a third scan line which is configured to control writing of the initialization voltage.

Optionally, in the pixel circuit, a scan period for driving the pixel circuit may include a first phase, a second phase, a third phase and a fourth phase;

wherein an initialization of the gate and drain of the sixth thin-film transistor and the anode of the organic light-emitting diode is started at a beginning of the first phase; the initialization of the anode of the organic light-emitting diode is terminated at an end of the second phase; the initialization of the gate and drain of the sixth thin-film transistor is terminated at an end of the third phase; a threshold voltage of the sixth thin-film transistor is sampled in the third phase; and the sixth thin-film transistor is turned on and provides a current to the organic light-emitting diode in the fourth phase.

Optionally, in the pixel circuit, the current provided by the sixth thin-film transistor to the organic light-emitting diode may be determined by the data voltage provided by the data line and the initialization voltage provided by the reference power source, and be independent of the power supply voltages provided by the first power source and the second power source and the threshold voltage of the sixth thin-film transistor.

Optionally, the pixel circuit may further include a boost capacitor disposed between the second scan line and a connection point among the gate of the sixth thin-film transistor, the source of the third thin-film transistor and a first terminal of the capacitor.

Accordingly, the present invention also provides a method for driving the pixel circuit as defined above, including:

a scan period including a first phase, a second phase, a third phase and a fourth phase, wherein

in the first phase, a scan signal provided by the first scan line is maintained at a low level and a scan signal provided by the second scan line and a scan signal provided by the third scan line are both pulled down from a high level to the low level, leading to the first thin-film transistor, the third thin-film transistor, the fourth thin-film transistor and the seventh thin-film transistor being turned on, with the second thin-film transistor and the fifth thin-film transistor being maintained in an on state, the gate and drain of the sixth thin-film transistor and the anode of the organic light-emitting diode being initialized by an initialization voltage provided by the reference power source, and a data voltage provided by the data line being written, via the fourth thin-film transistor, to a connection point among the drain of the fourth thin-film transistor, the source of the fifth thin-film transistor and the second terminal of the capacitor;

in the second phase, the scan signal provided by the first scan line jumps from the low level to the high level and the scan signals provided by the second scan line and the third scan line are maintained at the low level, leading to the second thin-film transistor and the fifth thin-film transistor being turned off and the initialization of the anode of the organic light-emitting diode being terminated;

in the third phase, the scan signal provided by the first scan line is maintained at the high level, the scan signal provided by the second scan line is maintained at the low level and the scan signal provided by the third scan line jumps from the low level to the high level, leading to the seventh thin-film transistor being turned off, the second thin-film transistor being kept off, the initialization of the gate and drain of the sixth thin-film transistor being terminated, and the threshold voltage of the sixth thin-film transistor being sampled;

in the fourth phase, the scan signals provided by the first scan line and the third scan line are maintained at the high level and the scan signal provided by the second scan line jumps from the low level to the high level, leading to the first thin-film transistor, the third thin-film transistor and the fourth thin-film transistor being turned off, writing of the data voltage being terminated, and the sampling of the threshold voltage of the sixth thin-film transistor being completed, and following the completion of the sampling, the scan signal provided by the first scan line drops from the high level to the low level, leading to the second thin-film transistor and the fifth thin-film transistor being turned on, and the sixth thin-film transistor outputting a current via the second thin-film transistor, which drives the organic light-emitting diode to emit light.

Optionally, in the method, when the seventh thin-film transistor and the third thin-film transistor are simultaneously turned on, the gate of the sixth thin-film transistor may be initialized by the reference power source;

when the first thin-film transistor and the seventh thin-film transistor are simultaneously turned on, the drain of the sixth thin-film transistor is initialized by the reference power source;

when the first thin-film transistor, the second thin-film transistor and the seventh thin-film transistor are simultaneously turned on, the anode of the organic light-emitting diode is initialized by the reference power source.

Optionally, in the method, in response to the scan signal provided by the second scan line, a boost capacitor raises a voltage at a connection point between the gate of the sixth thin-film transistor and the source of the third thin-film transistor as well as a first terminal of the capacitor in the fourth phase, such that a gate voltage of the sixth thin-film transistor is increased.

Accordingly, the present invention also provides an organic light-emitting display device, including the pixel circuit as described above.

In the pixel circuit and the method for driving it, as well as the organic light-emitting display device, according to the present invention, through anode initialization of the organic light-emitting diode via the first thin-film transistor, the second thin-film transistor and the seventh thin-film transistor, as well as gate and drain initialization of the sixth thin-film transistor that serves as a driving element via the first thin-film transistor, the third thin-film transistor and the seventh thin-film transistor, therefore, aging of the organic light-emitting diode and the sixth thin-film transistor can be slowed, and their service lives can be extended. In addition, because the current output by the sixth thin-film transistor that serves as the driving element is independent of its threshold voltage and impedances of power wiring, brightness non-uniformity caused by variations in thin-film transistor threshold voltages and power wiring impedances can be avoided. Therefore, the organic light-emitting display device using the pixel circuit, as well as the method for driving it can result in not only service life extension but also an improvement in display quality.

FIG. 1 shows a circuit diagram of a pixel in an organic light-emitting display device of the prior art.

FIG. 2 is a diagram of a pixel circuit in accordance with a first embodiment of the present invention.

FIG. 3 is a timing diagram illustrating a method of driving the pixel circuit in accordance with the first embodiment of the present invention.

FIG. 4 is a diagram of a pixel circuit in accordance with a second embodiment of the present invention.

Pixel circuits and driving methods thereof, as well as organic light-emitting display devices, according to the present invention, will be described below with reference to specific embodiments and the accompanying drawings. The advantages and feature of the invention will become more apparent from the following description and the appended claims. It is noted that the drawings are presented in a very simplified form not precisely drawn to scale with the only purpose of facilitating the description of the embodiments of the invention.

Reference is now made to FIG. 2 , which is a schematic illustration of a pixel circuit in accordance with a first embodiment of the present invention. As shown in FIG. 2 , the pixel circuit 20 comprises a first thin-film transistor M 1 , a second thin-film transistor M 2 , a third thin-film transistor M 3 , a fourth thin-film transistor M 4 , a fifth thin-film transistor M 5 , a sixth thin-film transistor M 6 , a seventh thin-film transistor M 7 , a capacitor C 1 and an organic light-emitting diode OLED. The source of the sixth thin-film transistor M 6 is connected to a first power source ELVDD, and the drain of the sixth thin-film transistor M 6 is connected to both the drain of the first thin-film transistor M 1 and the source of the second thin-film transistor M 2 . The drain of the second thin-film transistor M 2 is connected to an anode of the organic light-emitting diode OLED, and a cathode of the organic light-emitting diode OLED is connected to a second power source ELVSS. The gate of the sixth thin-film transistor M 6 is connected to the source of the third thin-film transistor M 3 and a first terminal of the capacitor C 1 . A second terminal of the capacitor C 1 is connected to both the drain of the fourth thin-film transistor M 4 and the source of the fifth thin-film transistor M 5 . The source of the fourth thin-film transistor M 4 is connected to a data line DATA, and the drain of the fifth thin-film transistor M 5 , together with the drain of the seventh thin-film transistor M 7 , is connected to a reference power source VREF. The source of the seventh thin-film transistor M 7 is connected to both the source of the first thin-film transistor M 1 and the drain of the third thin-film transistor M 3 .

Specifically, the pixel circuit 20 is supplied with the first power source ELVDD, the second power source ELVSS and the reference power source VREF externally (e.g., from a power supply unit) via power wiring (not shown). The first power source ELVDD and the second power source ELVSS are provided to drive the organic light-emitting diode OLED, i.e., providing the organic light-emitting diode OLED with power supply voltages, and the reference power source VREF is configured to provide an initialization voltage Vref. In general, the first power supply voltage VDD provided by the first power source ELVDD has a high level, and the second power supply voltage VSS provided by the second power source ELVSS has a low level. The initialization voltage Vref provided by the reference power source VREF is a direct current (DC) voltage having a constant value that is generally negative or close to 0 V.

As shown in FIG. 2 , the source of the sixth thin-film transistor M 6 is connected to the first power source ELVDD, and the drain of the sixth thin-film transistor M 6 is connected to the anode of the organic light-emitting diode OLED via the second thin-film transistor M 2 . The cathode of the organic light-emitting diode OLED is connected to the second power source ELVSS. The sixth thin-film transistor M 6 acts as a drive transistor to provide the organic light-emitting diode OLED with a current, and the organic light-emitting diode OLED emits light in response to this current.

With continued reference to FIG. 2 , the drain of the fifth thin-film transistor M 5 and the drain of the seventh thin-film transistor M 7 are both connected to the reference power source VREF. The source of the fifth thin-film transistor M 5 is connected to a first node N 1 , and the gate of the fifth thin-film transistor M 5 is connected to a first scan line S 1 , so that the fifth thin-film transistor M 5 can respond to a scan signal provided by the first scan line S 1 to provide the initialization voltage Vref from the reference power source VREF to the first node N 1 . The source of the seventh thin-film transistor M 7 is connected to a third node N 3 and the gate of the seventh thin-film transistor M 7 is connected to a third scan line S 3 , so that the seventh thin-film transistor M 7 can respond to a scan signal provided by the third scan line S 3 to provide the initialization voltage Vref from the reference power source VREF to the third node N 3 . The source of the third thin-film transistor M 3 is connected to a second node N 2 and the gate of the third thin-film transistor M 3 is connected to a second scan line S 2 , so that the third thin-film transistor M 3 can respond to a scan signal provided by the second scan line S 2 to provide a voltage at the third node N 3 to the second node N 2 . The gate of the first thin-film transistor M 1 is connected to the second scan line S 2 and the gate of the second thin-film transistor M 2 is connected to the first scan line S 1 , so that the first thin-film transistor M 1 and the second thin-film transistor M 2 can respond to the scan signals provided by the second scan line S 2 and the first scan line S 1 , respectively, to provide the voltage at the third node N 3 to the anode of the organic light-emitting diode OLED.

As shown in FIG. 2 , when the fifth thin-film transistor M 5 is turned on, the initialization voltage Vref provided by the reference power source VREF is applied to the first node N 1 . When the seventh thin-film transistor M 7 is turned on, the initialization voltage Vref provided by the reference power source VREF is applied to the third node N 3 . When the seventh thin-film transistor M 7 , the third thin-film transistor M 3 and the first thin-film transistor M 1 are simultaneously turned on, the initialization voltage Vref provided by the reference power source VREF to the third node N 3 is applied to the second node N 2 and the drain of the sixth thin-film transistor M 6 , thereby initializing the gate and drain of the drive transistor M 6 . When the seventh thin-film transistor M 7 , the first thin-film transistor M 1 and the second thin-film transistor M 2 are simultaneously turned on, the initialization voltage Vref provided by the reference power source VREF is applied to the anode of the organic light-emitting diode OLED, thereby initializing the anode of the organic light-emitting diode OLED.

With continued reference to FIG. 2 , the source of the fourth thin-film transistor M 4 is connected to the data line DATA on which a data voltage Vdata output by a drive chip (not shown) is transmitted. The drain of the fourth thin-film transistor M 4 is connected to both the second terminal of the capacitor C 1 and the source of the fifth thin-film transistor M 5 , and the gate of the fourth thin-film transistor M 4 is connected to the second scan line S 2 , so that the fourth thin-film transistor M 4 can respond to the scan signal provided by the second scan line S 2 to provide the data voltage Vdata transmitted on the data line DATA to the first node N 1 .

The fourth thin-film transistor M 4 is turned on or off under the effect of the scan signal provided by the second scan line S 2 , and when the fourth thin-film transistor M 4 is turned on, the data line DATA and the first node N 1 are electrically connected to each other, thereby providing the data voltage Vdata from the data line DATA to the first node N 1 .

The capacitor C 1 is connected between the first node N 1 and the second node N 2 , in order to control the voltage at the first node N 1 such that it corresponds to an amount of voltage change at the second node N 2 . That is, the difference between the voltages at the second node N 2 and the first node N 1 will be charged to the capacitor C 1 . With the charging being completed, the capacitor C 1 maintains this voltage difference.

In this embodiment, the pixel circuit 20 is a 7T1C circuit including the seven thin-film transistors and the capacitor. The pixel circuit 20 is connected to the three scan lines. In this embodiment, the gates of the second thin-film transistor M 2 and the fifth thin-film transistor M 5 are both connected to the first scan line S 1 which is configured for initialization control and capacitor stabilization. The gates of the first thin-film transistor M 1 , the third thin-film transistor M 3 and the fourth thin-film transistor M 4 are all connected to the second scan line S 2 which is configured to control writing of the data voltage Vdata and sample the threshold voltage of the drive transistor. The gate of the seventh thin-film transistor M 7 is connected to the third scan line S 3 which is configured to control writing of the initialization voltage Vref.

The gate of the sixth thin-film transistor M 6 can be initialized when the initialization voltage Vref provided by the reference power source VREF is applied to the gate of the sixth thin-film transistor M 6 via the seventh thin-film transistor M 7 and the third thin-film transistor M 3 . The drain of the sixth thin-film transistor M 6 can be initialized when the initialization voltage Vref provided by the reference power source VREF is applied to the drain of the sixth thin-film transistor M 6 via the seventh thin-film transistor M 7 and the first thin-film transistor M 1 . The anode of the organic light-emitting diode OLED can be initialized when the initialization voltage Vref provided by the reference power source VREF is applied to the anode of the organic light-emitting diode OLED via the seventh thin-film transistor M 7 , the first thin-film transistor M 1 and the second thin-film transistor M 2 . In this way, the service lives of the organic light-emitting diode OLED and the drive thin-film transistor M 6 can be extended.

In addition, the current provided by the sixth thin-film transistor M 6 to the organic light-emitting diode OLED is determined by the data voltage Vdata provided by the data line DATA and the initialization voltage Vref provided by the reference power source VERF and is independent of the power supply voltages provided by the first power source ELVDD and the second power source ELVSS and of the threshold voltage of the sixth thin-film transistor M 6 . Therefore, use of the pixel circuit 20 can avoid non-uniform brightness caused by variations in thin-film transistor threshold voltages and differences in power wiring impedances and hence increase display quality.

Accordingly, the present invention also provides a method for driving the pixel circuit. With combined reference to FIGS. 2 and 3 , the method includes:

a scan period including a first phase T 1 , a second phase T 2 , a third phase T 3 and a fourth phase T 4 , wherein

in the first phase T 1 , the scan signal provided by the first scan line S 1 is maintained at the low level and the scan signals provided by the second scan line S 2 and the third scan line S 3 are both pulled down from the high level to the low level, leading to the first thin-film transistor M 1 , the third thin-film transistor M 3 , the fourth thin-film transistor M 4 and the seventh thin-film transistor M 7 being turned on, the second thin-film transistor M 2 and the fifth thin-film transistor M 5 being kept on, the gate and drain of the sixth thin-film transistor M 6 and the anode of the organic light-emitting diode OLED being initialized by the initialization voltage Vref provided by the reference power source VREF, and the data voltage Vdata provided by the data line DATA being written, via the fourth thin-film transistor M 4 , to the connection point N 1 between the drain of the fourth thin-film transistor M 4 and the source of the fifth thin-film transistor M 5 as well as the second terminal of the capacitor C 1 ;

in the second phase T 2 , the scan signal provided by the first scan line S 1 jumps from the low level to the high level and the scan signals provided by the second scan line S 2 and the third scan line S 3 are maintained at the low level, leading to the second thin-film transistor M 2 and the fifth thin-film transistor M 5 being turned off and the initialization of the anode of the organic light-emitting diode OLED being terminated;

in the third phase T 3 , the scan signal provided by the first scan line S 1 is maintained at the high level, the scan signal provided by the second scan line S 2 is maintained at the low level and the scan signal provided by the third scan line S 3 jumps from the low level to the high level, leading to the seventh thin-film transistor M 7 being turned off, the second thin-film transistor M 2 and the fifth thin-film transistor M 5 being kept off, the initialization of the gate and drain of the sixth thin-film transistor M 6 being terminated, and the threshold voltage of the sixth thin-film transistor M 6 being sampled;

in the fourth phase T 4 , the scan signals provided by the first scan line S 1 and the third scan line S 3 are maintained at the high level and the scan signal provided by the second scan line S 2 jumps from the low level to the high level, leading to the first thin-film transistor M 1 , the third thin-film transistor M 3 and the fourth thin-film transistor M 4 being turned off, writing of the data voltage Vdata being terminated, and the sampling of the threshold voltage of the sixth thin-film transistor M 6 being completed; and following the completion of the sampling, the scan signal provided by the first scan line S 1 drops from the high level to the low level, leading to the second thin-film transistor M 2 and the fifth thin-film transistor M 5 being turned on, and the sixth thin-film transistor M 6 outputting a current via the second thin-film transistor M 2 , which drives the organic light-emitting diode OLED to emit light.

In particular, in the first phase T 1 , following the scan signals provided by the second scan line S 2 and the third scan line S 3 dropping from the high level to the low level, the first thin-film transistor M 1 , third thin-film transistor M 3 , fourth thin-film transistor M 4 and seventh thin-film transistor M 7 are turned on from cut off mode. Additionally, as the scan signal provided by the first scan line S 1 is maintained at the low level, the second thin-film transistor M 2 and the fifth thin-film transistor M 5 are kept on. As a result, the initialization voltage Vref provided by the reference power source VREF is supplied, via the fifth thin-film transistor M 5 , to the connection point (first node N 1 ) between the drain of the fourth thin-film transistor M 4 and the source of the fifth thin-film transistor M 5 as well as the other terminal of the capacitor C 1 .

At the same time, the initialization voltage Vref provided by the reference power source VREF is supplied to each of: the connection point (third node N 3 ) between the source of the first thin-film transistor M 1 and the drain of the third thin-film transistor M 3 via the seventh thin-film transistor M 7 ; the gate of the sixth thin-film transistor M 6 via the third thin-film transistor M 3 , thereby initializing the gate of the sixth thin-film transistor M 6 ; the drain of the sixth thin-film transistor M 6 via the first thin-film transistor M 1 , thereby initializing the drain of the sixth thin-film transistor M 6 ; and the anode of the organic light-emitting diode OLED via the first thin-film transistor M 1 and the second thin-film transistor M 2 , thereby initializing the anode of the organic lighting emitting diode OLED. In this way, the aging of the organic light-emitting diode OLED and the drive thin-film transistor M 6 is slowed, and their service lives are extended.

In this process, since the fourth thin-film transistor M 4 is on, the data voltage Vdata provided by the data line DATA is written to the first node N 1 via the fourth thin-film transistor M 4 . As can be known from the above description, a summed voltage of the data voltage Vdata and the initialization voltage Vref, i.e., Vdata+Vref, is supplied to the first node N 1 .

In the second phase T 2 , following the scan signal provided by the first scan line S 1 jumping from the low level to the high level, the second thin-film transistor M 2 and fifth thin-film transistor M 5 are turned off, making the reference power source VREF unable to provide the initialization voltage Vref to the anode of the organic light-emitting diode OLED via the second thin-film transistor M 2 . The initialization of the anode of the organic light-emitting diode OLED is therefore terminated.

In this process, the initialization of the first node N 1 by the reference power source VREF is stopped. Meanwhile, as the fourth thin-film transistor M 4 is turned on, only the data voltage Vdata is provided to the first node N 1 via the data line DATA.

In the third phase T 3 , following the scan signal provided by the third scan line S 3 jumping from the low level to the high level, the seventh thin-film transistor M 7 is turned off and therefore stops providing the initialization voltage Vref provided by the reference power source VREF to the third node N 3 between the source of the first thin-film transistor M 1 and the drain of the third thin-film transistor M 3 . This makes the reference power source VREF unable to provide the initialization voltage Vref to the gate and drain of the sixth thin-film transistor M 6 via the first thin-film transistor M 1 , the third thin-film transistor M 3 and the seventh thin-film transistor M 7 . The initialization of the gate and drain of the sixth thin-film transistor M 6 is therefore stopped. Meanwhile, as the scan signal provided by the second scan line S 2 is maintained at the low level, the first power supply voltage VDD is transmitted from the first power source ELVDD to the source of the sixth thin-film transistor M 6 , and enables sampling of the threshold voltage of the sixth thin-film transistor M 6 and charging of the capacitor C 1 until the voltage at the second node N 2 , i.e., the gate voltage of the sixth thin-film transistor M 6 , reaches VDD−Vth, where Vth is an absolute value of the threshold voltage of the sixth thin-film transistor M 6 .

In this process, as the second thin-film transistor M 2 is off, the electrical connection between the sixth thin-film transistor M 6 serving as a drive transistor and the organic light-emitting diode OLED is blocked, and the organic light-emitting diode OLED hence does not emit light.

In the fourth phase T 4 , following the scan signal provided by the second scan line S 2 jumping from the low level to the high level, the first thin-film transistor M 1 , the third thin-film transistor M 3 and the fourth thin-film transistor M 4 are turned off, leading to the writing of the data voltage Vdata and the charging of the capacitor C 1 being stopped. As a result, the sampling of the threshold voltage of the sixth thin-film transistor M 6 is completed.

In this process, since the fourth thin-film transistor M 4 is turned off, writing of the data voltage Vdata provided by the data line DATA to the first node N 1 is stopped, and the voltage at the first node N 1 is therefore equal to the data voltage Vdata.

Subsequently, the data voltage Vdata provided by the data line DATA drops from the high level to the low level, the drive chip outputs digital signals for the next row of pixels. Meanwhile, as the scan signal provided by the first scan line S 1 also drops from the high level to the low level, the second thin-film transistor M 2 and the fifth thin-film transistor M 5 are turned on, leading to the initialization voltage Vref provided by the reference power source VREF being supplied to the first node N 1 via the fifth thin-film transistor M 5 and the sixth thin-film transistor M 6 being turned on and outputting a current via the second thin-film transistor M 2 . As the voltage of the capacitor C 1 does not change abruptly, the voltage at the second node N 2 (i.e., the gate voltage Vg 6 of the sixth thin-film transistor M 6 ) varies with the voltage at the first node N 1 .

As described above, the voltage at the first node N 1 changes from Vdata to Vref, i.e., by Vdata−Vref. Therefore, the gate voltage Vg 6 of the sixth thin-film transistor M 6 is given as:

Vg 6 =VDD−Vth −( V data− V ref)  Eqn. 1,

where, Vth is the absolute value of the threshold voltage of the sixth thin-film transistor M 6 , VDD is the first power supply voltage provided by the first power source ELVDD, Vdata is the data voltage provided by the data line DATA, and Vref is the initialization voltage provided by the reference power source VREF.

As the source voltage of the sixth thin-film transistor M 6 is equal to the first power supply voltage VDD provided by the first power source ELVDD, the gate-source voltage Vsg 6 of the sixth thin-film transistor M 6 , i.e., a voltage difference between the gate and source of the sixth thin-film transistor M 6 , is:

Vsg 6 =VDD− ( VDD−Vth− ( V data− V ref))  Eqn. 2,

From Eqns. 1 and 2, we can obtain:

Vsg 6 −Vth=V data− V ref  Eqn. 3.

The organic light-emitting diode OLED emits light in proportion to the current Ion flowing therein, which is give by:

Ion= K ×( Vsg 6 −Vth ) 2   Eqn. 4,

where, K is the product of the electron mobility, aspect ratio and capacitance per unit area of the thin-film transistor.

From Eqns. 3 and 4, the following equation can be obtained:

Ion= K ×( V data− V ref) 2 .

As indicated by this equation, the current flowing in the organic light-emitting diode OLED is independent of the power supply voltages and the threshold voltage of the sixth thin-film transistor M 6 , and is related only to the data voltage Vdata, the initialization voltage Vref and the constant K. Therefore, even if there were variations in the threshold voltages of the sixth thin-film transistors M 6 and an impact of power wiring impedances on the power supply voltages actually acting on the pixel circuits, the currents Ion in the organic light-emitting diodes OLED would not be affected at all. Thus, the problem of non-uniform brightness arising from threshold voltage variations and power wiring impedances can be overcome by use of the pixel circuit 20 and the method for driving it. At the same time, the service lives of the organic light-emitting diodes OLED and the sixth thin-film transistors M 6 that serve as drive transistors can also be extended.

Reference is now made to FIG. 4 , which is a diagram of a pixel circuit in accordance with a second embodiment of the present invention. As shown in FIG. 4 , the pixel circuit 30 comprises a first thin-film transistor M 1 , a second thin-film transistor M 2 , a third thin-film transistor M 3 , a fourth thin-film transistor M 4 , a fifth thin-film transistor M 5 , a sixth thin-film transistor M 6 , a seventh thin-film transistor M 7 , a capacitor C 1 and an organic light-emitting diode OLED. A source of the sixth thin-film transistor M 6 is connected to a first power source ELVDD, and a drain of the sixth thin-film transistor M 6 is connected to both a drain of the first thin-film transistor M 1 and a source of the second thin-film transistor M 2 . A drain of the second thin-film transistor M 2 is connected to an anode of the organic light-emitting diode OLED, and a cathode of the organic light-emitting diode OLED is connected to a second power source ELVSS. A gate of the sixth thin-film transistor M 6 is connected to a source of the third thin-film transistor M 3 and a first terminal of the capacitor C 1 . A second terminal of the capacitor C 1 is connected to both a drain of the fourth thin-film transistor M 4 and a source of the fifth thin-film transistor M 5 . A source of the fourth thin-film transistor M 4 is connected to a data line DATA, and a drain of the fifth thin-film transistor M 5 , together with a drain of the seventh thin-film transistor M 7 , is connected to a reference power source VREF. A source of the seventh thin-film transistor M 7 is connected to both a source of the first thin-film transistor M 1 and a drain of the third thin-film transistor M 3 .

Specifically, the pixel circuit 30 possesses all the features of the pixel circuit 20 of Embodiment 1, and this embodiment differs from Embodiment 1 in that a boost capacitor C 2 is further disposed between a second node N 2 and a second scan line S 2 , which is configured to raise the voltage at the second node N 2 .

With combined reference to FIGS. 3 and 4 , in the fourth phase T 4 , when a scan signal provided by the second scan line S 2 jumps from the low level to the high level, the boost capacitor C 2 pulls up the voltage at the second node N 2 , so as to raise the voltage at the second node N 2 , i.e., the gate voltage Vg 6 of the sixth thin-film transistor M 6 , according to the amount of change in the scan signal provided by the second scan line S 2 and a ratio of a capacitance of the boost capacitor C 2 to the sum of a capacitance of the capacitor C 1 and the capacitance of the boost capacitor C 2 , i.e., {C 2 /(C 1 +C 2 )}, such that current leakage in the sixth thin-film transistor M 6 is reduced and an improvement in display contrast can be obtained.

In this embodiment, the scan signals provided by the first scan line S 1 , the second scan line S 2 and the third scan line S 3 evolve in the same time sequence as those provided by the first scan line S 1 , the second scan line S 2 and the third scan line S 3 of Embodiment 1, which will not be described in duplicate again. Regarding to the details, reference can be made to the description of Embodiment 1 with respect to the first to fourth phases T 1 -T 4 in the method for driving the pixel circuit.

It is noted that the embodiments disclosed herein are described in a progressive manner in which each embodiment is described with the emphasis on its differences from other embodiments, and reference can be made between different embodiments for the same features. In addition, in the disclosed embodiments, as the pixel circuits correspond to the methods for driving them, they are described in a simpler way, and reference can be made to the description of the methods for the corresponding features of the pixel circuits.

Accordingly, the present invention also provides organic light-emitting display devices comprising the pixel circuits as defined above.

Conclusively, in the pixel circuits and the methods for driving them, as well as the organic light-emitting display devices, according to the present invention, through anode initialization of the organic light-emitting diode via the first thin-film transistor, the second thin-film transistor and the seventh thin-film transistor, as well as gate and drain initialization of the sixth thin-film transistor that serves as a driving element via the first thin-film transistor, the third thin-film transistor and the seventh thin-film transistor, aging of the organic light-emitting diode and the sixth thin-film transistor can be slowed, and their service lives can be extended. In addition, because the current output by the sixth thin-film transistor is independent of its threshold voltage and power wiring impedances, the problem of brightness non-uniformity caused by variations in thin-film transistor threshold voltages and power wiring impedances can be addressed. Further, an improvement in display contrast can be obtained by increasing the gate voltage of the sixth thin-film transistor by the boost capacitor and thereby reducing current leakage therein. Thus, use of the pixel circuits and the methods for driving them for the organic light-emitting display devices can result in not only service life extension but also an improvement in display quality.

The foregoing description is merely preferred embodiments of the present invention and does not limit the scope of the invention in any way. All changes and modifications made by those of ordinary skill in the art concerning the foregoing disclosure fall within the scope of the appended claims.