Pixel circuit and driving method thereof, display device

The present application is the U.S. national phase entry of PCT/CN2015/086863 with an International filing date of Aug. 13, 2015, which claims the benefit of Chinese Application No. 201510152814.1, filed Apr. 1, 2015, the entire disclosures of which are incorporated herein by reference.

The present invention relates to the field of display technology, particularly to a pixel circuit and a driving method thereof, as well as a display device.

The organic light emitting diode (OLED) is one of the hotspots in the research field of panel display nowadays. As a current mode light emitting device, it has been more and more applied in high performance displays in virtue of its characteristics of self-luminous, quick response, wide visual angle and being capable of being fabricated on a flexible substrate. At present, in displays such as the mobile phone, PDA, digital camera, the OLED has begun to replace the conventional liquid crystal display (LCD). The design of the pixel driving circuit is the core technology of the OLED display, and has important research significance. Different from the TFT (thin film transistor)-LCD which makes use of stable voltages to control the brightness, the OLED belongs to current driving, which needs stable currents to control lighting. Due to the process procedure and the device aging, in an original 2T1C driving circuit (including two thin film transistors and a capacitance), threshold voltages of respective pixel points for driving the TFT are non-uniform. Thus it results in variation of the current that flows through the OLED of each pixel point, which renders the display brightness non-uniform, thereby influencing the display effect of the whole image.

An object of the present invention is to provide a pixel circuit so as to avoid non-uniform display brightness caused by drift of the turn-on threshold value of the drive transistor.

A first aspect of the present invention provides a pixel circuit, comprising a drive transistor, a first energy storage element and a driving module, and having a reset voltage input terminal, a data voltage input terminal, a working voltage input terminal and a plurality of control signal input terminals; wherein,

a source of the drive transistor is connected with a first end of the first energy storage element;

the driving module is connected with the first end of the first energy storage element, a second end of the first energy storage element, a gate of the drive transistor, the reset voltage input terminal, the data voltage input terminal and the working voltage input terminal, and connects the plurality of control signal input terminals; the driving module has a first node, when the first node and the second end of the first energy storage element are both floated, a voltage difference between the first node and the first end of the first energy storage element keeps constant;

the driving module conducts the working voltage input terminal with the first end of the first energy storage element when a first control signal input terminal is inputted with an effective level; conducts the first node with the reset voltage input terminal when a second control signal input terminal is inputted with an effective level; conducts the data voltage input terminal with the gate of the drive transistor and the second end of the first energy storage element when a third control signal input terminal is inputted with an effective level; and conducts the first node with the gate of the drive transistor when a fourth control signal input terminal is inputted with an effective level.

According to an embodiment, the driving module comprises:

a first switch transistor connected between the working voltage input terminal and the first end of the first energy storage element, a gate of the first switch transistor being connected with the first control signal input terminal;

a second switch transistor connected between the reset voltage input terminal and the first node, a gate of the second switch transistor being connected with the second control signal input terminal;

a third switch transistor connected between the data voltage input terminal and the gate of the drive transistor;

a fifth switch transistor connected between the second end of the first energy storage element and the gate of the drive transistor;

a fourth switch transistor connected between the first node and the gate of the drive transistor, a gate of the fourth switch transistor being connected with the fourth control signal input terminal; and

a second energy storage element, a first end of the second energy storage element being connected with the second end of the first energy storage element, a second end of the second energy storage element being connected with the first node.

According to an embodiment, gates of the third switch transistor and the fifth switch transistor are both connected with the third control signal input terminal, and have a same effective level.

According to an embodiment, the respective switch transistors are all P-type transistors.

According to an embodiment, the effective level of the fourth switch transistor is opposite to the effective level of the third switch transistor and the fifth switch transistor, the fourth control signal input terminal and the third control signal input terminal can be the same input terminal.

According to an embodiment, the first switch transistor and the fourth switch transistor are both P-type transistors; the second switch transistor, the third switch transistor and the fifth switch transistor are all N-type transistors.

According to an embodiment, the first energy storage element and/or the second energy storage element are capacitance.

According to an embodiment, the drive transistor is a P-type transistor.

A second aspect of the present invention provides a method for driving any of the above pixel circuits, comprising a reset phase, a compensation phase and a lighting phase:

in the reset phase, the first control signal input terminal, the second control signal input terminal and the third control signal input terminal are all inputted with a corresponding effective level; the fourth control signal input terminal is inputted with a corresponding ineffective level;

in the compensation phase, the third control signal input terminal is inputted with a corresponding effective level; the first control signal input terminal, the second control signal input terminal and the fourth control signal input terminal are inputted with a corresponding ineffective level;

in the lighting phase, the first control signal input terminal and the fourth control signal input terminal are inputted with a corresponding effective level; the second control signal input terminal and the third control signal input terminal are inputted with a corresponding ineffective level.

A third aspect of the present invention provides a display device comprising any of the above pixel circuits.

The pixel circuit provided by the present invention can prevent the driving current flowing through the electroluminescent unit from being influenced by the turn-on threshold value of the corresponding drive transistor, thereby solving the problem of non-uniform display brightness caused by drift of the turn-on threshold value of the drive transistor thoroughly.

FIG. 1 is a structural schematic view of a pixel circuit provided by an embodiment of the present invention;

FIG. 2 is a circuit schematic view of a driving module of the embodiment in FIG. 1 ;

FIG. 3 is a circuit structural view of a pixel circuit provided by an embodiment of the present invention;

FIG. 4 is a timing diagram of a control signal of a method for driving the pixel circuit in FIG. 3 ;

FIG. 5 a to FIG. 5 c are schematic views of current flow direction and node voltage value of the pixel circuit in FIG. 3 under different timing sequences.

In order to make the objects, the technical solutions and the advantages of the embodiments of the present invention clearer, next, the technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the drawings in the embodiments of the present invention. Apparently, the embodiments described are only a part rather than all of the embodiments of the present invention. Based on the embodiments of the present invention, all the other embodiments obtained by the ordinary skilled person in the art on the premise of not paying any creative work belong to the protection scope of the present invention.

The pixel circuit provided by an embodiment of the present invention is as shown in FIG. 1 , the pixel circuit can comprise: a drive transistor DT, a first energy storage element C 1 , an electroluminescent element OLED and a driving module. The pixel circuit has a reset voltage input terminal Reset, a data voltage input terminal Data, a working voltage input terminal Vdd and four control signal input terminals S 1 , S 2 , S 3 , S 4 . Preferably, the drive transistor can be a P-type transistor.

A source of the drive transistor DT is connected with a first end a of the first energy storage element C 1 .

The drive module is connected with the first end a of the first energy storage element C 1 , a second end b of the first energy storage element C 1 , a gate g of the drive transistor DT, the reset voltage input terminal Reset, the data voltage input terminal Data and the working voltage input terminal Vdd, and connects the four control signal input terminals S 1 , S 2 , S 3 , S 4 . The driving module has a first node inside (not shown in the figure), when the first node and the second end b of the first energy storage element C 1 are both floated, a voltage difference between the first node and the first end a of the first energy storage element C 1 keeps constant.

The driving module conducts the working voltage input terminal Vdd with the first end a of the first energy storage element C 1 when a first control signal input terminal S 1 is inputted with an effective level; conducts the first node with the reset voltage input terminal Reset when a second control signal input terminal S 2 is inputted with an effective level; conducts the data voltage input terminal Data with the gate of the drive transistor DT and the second end b of the first energy storage element C 1 when a third control signal input terminal S 3 is inputted with an effective level; and conducts the first node with the gate of the drive transistor when a fourth control signal input terminal S 4 is inputted with an effective level.

The pixel circuit provided by the present invention can make the voltage of the control end of the driving module to contain a component of the turn-on threshold value of the drive transistor. Thus in the display process, the contained turn-on threshold value component can counteract the turn-on threshold value of the drive transistor. In this way, the driving current flowing through the electroluminescent unit can be prevented from being influenced by the turn-on threshold value of the corresponding drive transistor, thereby solving the problem of non-uniform display brightness caused by drift of the turn-on threshold value of the drive transistor thoroughly.

The effective level here should be understood as a level that can make the driving module to turn on the corresponding function. For example, for the first control signal input terminal S 1 , its corresponding effective level should be understood as a level that can conduct the working voltage input terminal with the first end of the first energy storage element. Correspondingly, the effective level to which the second control signal input terminal S 2 corresponds should be understood as a level that can conduct the first node with the reset voltage input terminal, and so on. For different control signal input terminals, their corresponding effective levels are likely to be different.

In specific implementation, as shown in FIG. 2 , the driving module can comprise: five switch transistors T 1 , T 2 , T 3 , T 4 , T 5 and a second energy storage element C 2 . A source of the first switch transistor T 1 is connected with the working voltage input terminal Vdd, a drain of the first switch transistor T 1 is connected with the first end a of the first energy storage element C 1 , a gate of the first switch transistor T 1 is connected with the first control signal input terminal S 1 . A drain of the second switch transistor T 2 is connected with the reset voltage input terminal Reset, a source of the second switch transistor T 2 is connected with the first node (denoted as N 1 in the figure), a gate of the second switch transistor T 2 is connected with the second control signal input terminal S 2 .

A source of the third switch transistor T 3 is connected with the data voltage input terminal Data, a drain of the third switch transistor T 3 is connected with the gate g of the drive transistor DT. A source of the fifth switch transistor T 5 is connected with the second end b of the first energy storage element C 1 , a drain of the fifth switch transistor T 5 is connected with the gate g of the drive transistor DT. The gates of the third switch transistor T 3 and the fifth switch transistor T 5 are both connected with the third control signal input terminal S 3 , and have the same effective level.

A drain of the fourth switch transistor T 4 is connected with the first node N 1 , a source of the fourth switch transistor T 4 is connected with the gate g of the drive transistor DT, a gate of the fourth switch transistor T 4 is connected with the fourth control signal input terminal S 4 .

A first end of the second energy storage element C 2 is connected with the second end b of the first energy storage element, a second end of the second energy storage element C 2 is connected with the first node Ni.

Since the gates of the third switch transistor T 3 and the fifth switch transistor T 5 are connected with the same control signal input terminal, the third switch transistor T 3 and the fifth switch transistor T 5 can be controlled to turn on and turn off through a same signal line. In this way, the use of the signal line can be reduced. It is not difficult to understand that in actual applications, the third switch transistor T 3 and the fifth switch transistor T 5 can also be connected with different control signal lines. Correspondingly, the effective levels of the third switch transistor T 3 and the fifth switch transistor T 5 also do not have to be consistent.

In specific implementation, the effect level of the fourth switch transistor T 4 can be opposite to the effective level of the third switch transistor T 3 and the fifth switch transistor T 5 , the fourth control signal input terminal S 4 and the third control signal input terminal S 3 can be the same input terminal. In other words, if the third switch transistor T 3 and the fifth switch transistor T 5 are P-type transistors, the fourth switch transistor T 4 will be an N-type transistor. In this way, the number of the signal lines used can be reduced, thereby reducing the area occupied by the pixel circuit.

The first switch transistor T 1 and the fourth switch transistor T 4 can be both P-type transistors. The second switch transistor T 2 , the third switch transistor T 3 and the fifth switch transistor T 5 can be all N-type transistors. The advantage of doing so is that the effective levels (pulse levels) of respective signals for driving the pixel circuit are all high levels. Since the structure of the shift register for outputting a pulse signal of a high level is much simpler than the structure of the shift register for outputting a pulse signal of a low level, the pixel circuit provided by the embodiment of the present invention can reduce design complexity of the driving circuit.

Alternatively, in specific implementation, all the above respective switch transistors can also be P-type transistor. The advantage of doing so is that they can be fabricated using the same process, thereby reducing complexity in fabricating the corresponding display device.

The reset voltage input terminal Reset here can be a ground terminal.

In specific implementation, the first energy storage element C 1 and/or the second energy storage element C 2 here can be both capacitance. Certainly, in actual applications, other elements with the storage function can also be used based on the design requirement.

In FIG. 1 , the anode of the electroluminescent element OLED is connected with the drain of the drive transistor, the cathode is connected to the ground terminal GND. Certainly, in actual applications, the cathode can also be connected with other low voltage input terminals, such as VSS etc.

The present invention further provides a driving method of a pixel circuit, which can be used for driving any of the above pixel circuits, the method comprising a reset phase, a compensation phase and a lighting phase.

In the reset phase, the first control signal input terminal, the second control signal input terminal and the third control signal input terminal are all inputted with a corresponding effective level; the fourth control signal input terminal is inputted with a corresponding ineffective level.

In the compensation phase, the third control signal input terminal is inputted with a corresponding effective level; the first control signal input terminal, the second control signal input terminal and the fourth control signal input terminal are inputted with a corresponding ineffective level.

In the lighting phase, the first control signal input terminal and the fourth control signal input terminal are inputted with a corresponding effective level; the second control signal input terminal and the third control signal input terminal are inputted with a corresponding ineffective level.

Through the driving method provided by the present invention, the driving current flowing through the electroluminescent unit can be prevented from being influenced by the turn-on threshold value of the corresponding drive transistor, thereby solving the problem of non-uniform display brightness caused by drift of the turn-on threshold value of the drive transistor thoroughly.

Next, a specific circuit structure will be combined to explain the driving method and the working principle of the pixel circuit provided by the embodiment of the present invention. As shown in FIG. 3 , which is a possible circuit schematic view of the pixel circuit, comprising five switch transistors T 1 -T 5 , a P-type drive transistor DT, an electroluminescent element OLED, a capacitance C 1 and a capacitance C 2 . The switch transistors T 1 , T 2 , T 3 , T 4 and T 5 constitute a driving module. The switch transistors T 1 and T 4 can be both P-type transistors, the turn-on level is a low level. The switch transistors T 2 , T 3 and T 5 can all be N-type transistors, the turn-on level is a high level. The pixel circuit further has a reset voltage input terminal Reset, a working voltage input terminal Vdd, a data voltage input terminal Data, a ground terminal GND and control signal input terminal S 1 , S 2 , S 3 . The source of the switch transistor T 1 is connected with the working voltage input terminal Vdd, the drain of the switch transistor T 1 is connected with the first end a of the capacitance C 1 , the gate of the switch transistor T 1 is connected with the control signal input terminal S 1 . The source of the switch transistor T 2 is connected with the first node N 1 , the drain of the switch transistor T 2 is connected with the ground terminal GND, the gate of the switch transistor T 2 is connected with the control signal input terminal S 1 . The gates of the switch transistors T 3 , T 4 and T 5 are all connected with the third control signal input terminal S 3 . The source of the switch transistor T 3 is connected with the data voltage input terminal Data, the drain of the switch transistor T 3 is connected with the gate g of the drive transistor DT. The source of the switch transistor T 4 is connected with the first node N 1 , the drain of the switch transistor T 4 is connected with the gate g of the drive transistor DT. The source of the switch transistor T 5 is connected with the gate g of the drive transistor DT, the drain of the switch transistor T 5 is connected with the second end b of the capacitance C 1 . The first end of the capacitance C 2 is connected with the second end b of the capacitance C 1 , the second end of the capacitance C 2 is connected with the first node Ni. The anode of the OLED is connected with the drain of the drive transistor DT, the cathode is connected with the ground terminal GND.

FIG. 4 is a timing diagram of key signals in a driving method for driving the pixel circuit as shown in FIG. 3 , comprising a reset phase D 1 , a compensation phase D 2 and a lighting phase D 3 .

In the reset phase D 1 , the control signal input terminal S 1 is applied with a low level, the control signal input terminals S 2 and S 3 are applied with a high level, the data voltage input terminal Data is applied with a data voltage Vdata. The switch transistors T 1 , T 2 , T 3 and T 5 are turned on, the switch transistor T 4 is turned off The current flow direction can make reference to FIG. 5 a . The voltage at end a of the capacitance C 1 is set as Vdd, the voltage at end b of the capacitance C 1 is set as Vdata, the voltage at the node N 1 is set as 0, the voltage across the capacitance C 2 are Vdata, the voltage at the first end a of the capacitance C 1 is Vdd.

In the compensation phase D 2 , the control signal input terminals Si and S 3 are applied with a high level, the control signal input terminal S 2 is applied with a low level. The switch transistors T 3 and T 5 are turned on, the switch transistors T 1 , T 2 and T 4 are turned off. As shown in FIG. 5 b , the voltage at the second end b of the capacitance C 1 is still Vdata, the voltage at the first end a of the capacitance C 1 begins to decline, the capacitance C 1 begins to discharge through the drive transistor DT, the voltage at the end a of the capacitance C 1 declines to be Vdata+|Vth| finally (wherein Vth is the threshold voltage of the drive transistor DT). The voltage across the capacitance C 2 are still Vdata. Therefore, the series total voltage of the capacitances C 1 and C 2 , i.e., the voltage difference between the node N 1 and the first end a of the capacitance C 1 is Vdata+|Vth|.

In the lighting phase D 3 , the control signal input terminals S 1 , S 2 and S 3 are all applied with a low level. The switch transistors T 1 and T 4 are turned on, the switch transistors T 2 , T 3 and T 5 are turned off The voltage at the first end a of the capacitance C 1 is Vdd, the voltage at the node N 1 jumps, which is jumped to be Vdd−(Vdata+|Vth|). Since the node N 1 is conducted with the gate g of the drive transistor DT, the voltage at the gate g of the drive transistor DT is also Vdd−(Vdata+|Vth|).

Here, the current flowing through the OLED is:

From the formula it can be seen that the current flowing through the OLED is only influenced by self-factors such as the carrier mobility μ of the drive transistor, the capacitance C OX of the gate oxide layer energy storage element, the size of the device itself (width W, length L). The current is controlled through the data voltage Vdata, and is unrelated to the threshold voltage Vth of the drive transistor and the power supply voltage Vdd. In this way, the problem of drift of the turn-on threshold value Vth caused by the process procedure and long time operation of the drive transistor DT will be solved thoroughly, the influence to the I OLED will be eliminated, thereby ensuring normal operation of the electroluminescent element OLED.

The present invention further provides a display device comprising any of the above pixel circuits.

The display device can be any product or component having the display function such as: electronic paper, a mobile phone, a panel computer, a television, a display, a laptop, a digital photo frame, a navigator.

What are stated above are only specific implementing modes of the present invention, however, the protection scope of the present invention is not limited to this. Any variations or replacements that can be easily conceived by the skilled person familiar with the present technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scopes of the claims attached.