Patent Publication Number: US-11663962-B2

Title: Display panel, driving method thereof, and display device

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This is a continuation application of U.S. patent application Ser. No. 17/467,994, filed on Sep. 7, 2021, which claims priority to Chinese patent application No. 202011150068.X filed Oct. 23, 2020, the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to display technologies and, in particular, to a display panel, a driving method thereof, and a display device. 
     BACKGROUND 
     In a display panel, a pixel circuit provides a light-emitting element of the display panel with a drive current required for display and controls whether the light-emitting element enters a light-emitting stage. The pixel circuit is an indispensable element in most self-luminous display panels. 
     However, as the use time of an existing display panel increases, the internal characteristics of a drive transistor in the pixel circuit change slowly, resulting in the drift of the threshold voltage of the drive transistor, so that the comprehensive characteristics of the drive transistor are affected and then display uniformity is affected. 
     SUMMARY 
     The embodiments of the present disclosure provide a display panel, a driving method thereof, and a display device, so as to reduce a drift of a threshold voltage of an existing drive transistor. 
     The embodiments of the present disclosure provide a display panel including a pixel circuit and a light-emitting element. 
     The pixel circuit includes a drive module, a data writing module, alight emission control module, and a bias module. 
     The drive module is configured to provide the light-emitting element with a drive current and includes a drive transistor. 
     The data writing module is connected to a source of the drive transistor and configured to selectively provide the drive module with a data signal. 
     The light emission control module is configured to selectively allow the light-emitting element to enter a light-emitting stage. 
     A control terminal of the light emission control module is connected to a light emission control signal line for receiving a light emission control signal. The bias module is connected between a drain of the drive transistor and the light emission control signal line. 
     A working process of the pixel circuit includes a bias stage at which the bias module adjusts a drain potential of the drive transistor according to the light emission control signal. 
     Based on the same concept, the embodiments of the present disclosure further provide a driving method of a display panel. 
     The display panel includes a pixel circuit and a light-emitting element. 
     The pixel circuit includes a drive module, a data writing module, alight emission control module, and a bias module. 
     The drive module is configured to provide the light-emitting element with a drive current and includes a drive transistor. 
     The data writing module is connected to a source of the drive transistor and configured to selectively provide the drive module with a data signal. 
     The light emission control module is configured to selectively allow the light-emitting element to enter a light-emitting stage. 
     A control terminal of the light emission control module is connected to a light emission control signal line for receiving a light emission control signal. The bias module is connected between a drain of the drive transistor and the light emission control signal line. 
     A working process of the pixel circuit includes a bias stage at which the bias module adjusts a drain potential of the drive transistor according to the light emission control signal. 
     A driving method for at least one frame of picture of the display panel incudes steps described below. 
     In a case where a transistor in the light emission control module and the drive transistor are P-type metal-oxide-semiconductor (PMOS) transistors, at the bias stage, the light emission control signal line receives a high-level signal and the bias module increases the drain potential of the drive transistor according to the high-level signal to enable the drive transistor to enter a bias state. 
     Alternatively, in a case where a transistor in the light emission control module and the drive transistor are N-type metal-oxide-semiconductor (NMOS) transistors, at the bias stage, the light emission control signal line receives a low-level signal and the bias module decreases the drain potential of the drive transistor according to the low-level signal to enable the drive transistor to enter a bias state. 
     Based on the same concept, the embodiments of the present disclosure further provide a display device including the display panel described above. 
     In the embodiments of the present disclosure, the pixel circuit includes the bias module which is connected between the light emission control signal line and the drain of the drive transistor to adjust the drain potential of the drive transistor and improve a potential difference between a gate potential of the drive transistor and the drain potential of the drive transistor. It is known that the pixel circuit includes at least one non-bias stage. When the drive transistor generates the drive current, the gate potential of the drive transistor might be higher than the drain potential of the drive transistor, so that an I-V curve of the drive transistor deviates, resulting in a threshold voltage drift of the drive transistor. At the bias stage, the gate potential and the drain potential of the drive transistor are adjusted, so that the deviation of the I-V curve of the drive transistor at the non-bias stage can be balanced, thereby reducing the threshold voltage drift of the drive transistor and ensuring the display uniformity of the display panel. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       To illustrate the solutions in the embodiments of the present disclosure or in the related art more clearly, the drawings used in the description of the embodiments or the related art will be briefly described below. Apparently, though the drawings described below illustrate some embodiments of the present disclosure, those skilled in the art may obtain other structures and drawings according to the basic concepts of the device structures, the driving method, and the manufacturing method disclosed by various embodiments of the present disclosure, all of which should fall within the scope of the claims of the present disclosure without any doubt. 
         FIG.  1    is a schematic diagram of a first pixel circuit of a display panel according to an embodiment of the present disclosure; 
         FIG.  2    is a schematic diagram of a second pixel circuit of a display panel according to an embodiment of the present disclosure; 
         FIG.  3    is a schematic diagram of a third pixel circuit of a display panel according to an embodiment of the present disclosure; 
         FIG.  4    is a schematic diagram of a first working timing sequence of a pixel circuit; 
         FIG.  5    is a schematic diagram of a second working timing sequence of a pixel circuit; 
         FIG.  6    is a schematic diagram of a third working timing sequence of a pixel circuit; 
         FIG.  7    is a schematic diagram of a fourth working timing sequence of a pixel circuit; 
         FIG.  8    is a schematic diagram of a fourth pixel circuit of a display panel according to an embodiment of the present disclosure; 
         FIG.  9    is a schematic diagram of a fifth working timing sequence of a pixel circuit; 
         FIG.  10    is a schematic diagram of a sixth working timing sequence of a pixel circuit; 
         FIG.  11    is a schematic diagram of a fifth pixel circuit of a display panel according to an embodiment of the present disclosure; 
         FIG.  12    is a schematic diagram of a sixth pixel circuit of a display panel according to an embodiment of the present disclosure; 
         FIG.  13    is a schematic diagram of a seventh pixel circuit of a display panel according to an embodiment of the present disclosure; 
         FIG.  14    is a partial sectional view of a pixel circuit according to an embodiment of the present disclosure; 
         FIG.  15    is a top view of a pixel circuit according to an embodiment of the present disclosure; and 
         FIG.  16    is a schematic diagram of a display device according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The solutions of the present disclosure will be described clearly and completely with reference to the accompanying drawings through embodiments from which the objects, solutions, and advantages of the present disclosure will be more apparent. Apparently, the embodiments described herein are part, not all, of the embodiments of the present disclosure. All other embodiments obtained by those skilled in the art based on the basic concepts disclosed by the embodiments of the present disclosure are within the scope of the present disclosure. 
       FIG.  1    is a schematic diagram of a first pixel circuit of a display panel according to an embodiment of the present disclosure. Referring to  FIG.  1   , the display panel provided by this embodiment includes a pixel circuit  10  and a light-emitting element  20 ; where the pixel circuit  10  includes a drive module  11 , a light emission control module  12 , a bias module  13 , and a data writing module  14 ; where the drive module  11  is configured to provide the light-emitting element  20  with a drive current and includes a drive transistor T 0 ; the light emission control module  12  is configured to selectively allow the light-emitting element  20  to enter a light-emitting stage; the data writing module  14  is connected to a source of the drive transistor T 0  and configured to selectively provide the drive module  11  with a data signal Vdata; a control terminal of the light emission control module  12  is connected to a light emission control signal line for receiving a light emission control signal EM; and the bias module  13  is connected between a drain of the drive transistor T 0  and the light emission control signal line. A working process of the pixel circuit  10  includes a bias stage at which the bias module  13  adjusts a drain potential of the drive transistor T 0  according to the light emission control signal EM. 
     In this embodiment, the pixel circuit  10  includes the drive module  11 , and an output terminal of the drive module  11  (i.e., the drain) is electrically connected to the light-emitting element  20 . The drive module  11  includes the drive transistor T 0 . After the drive transistor T 0  is turned on, the drive module  11  provides the light-emitting element  20  with the drive current. The on-off of the drive transistor T 0  controls the magnitude of the drive current provided for the light-emitting element  20 . The source of the drive transistor T 0  is electrically connected to an input terminal of the drive module  11  and the drain of the drive transistor T 0  is electrically connected to the output terminal of the drive module  11 . In other embodiments, it is further optional that the drain of the drive transistor is electrically connected to the input terminal of the drive module and the source of the drive transistor is electrically connected to the output terminal of the drive module. It is understandable that the source and the drain of the transistor are not constant but will change as a drive state of the transistor changes. 
     The pixel circuit  10  includes the data writing module  14 , where a source of the data writing module  14  is connected to a data signal terminal for receiving the data signal Vdata, a drain of the data writing module  14  is connected to the source of the drive transistor T 0 , and a control terminal of the data writing module  14  is connected to a first scanning signal line for receiving a first scanning signal S 1  which controls the data writing module  14  to be turned on and off. In an embodiment, the data writing module includes a second transistor T 2 , where a source of the second transistor T 2  is connected to the data signal terminal, a drain of the second transistor T 2  is connected to the source of the drive transistor T 0 , and a gate of the second transistor T 2  is connected to the first scanning signal line. 
     The pixel circuit  10  includes the light emission control module  12 , where the control terminal of the light emission control module  12  is connected to the light emission control signal line EM. When the light emission control signal line EM outputs an effective pulse, the light emission control module  12  is turned on and drives the light-emitting element  20  to enter the light-emitting stage, and the drive current flows into the light-emitting element  20 . When the light emission control signal line EM outputs an invalid pulse, the light emission control module  12  is turned off and a path through which the drive current flows into the light-emitting element  20  is disconnected. 
     The pixel circuit  10  includes the bias module  13  which is connected between the drain of the drive transistor T 0  and the light emission control signal line EM. The bias module  13  is configured to increase or decrease the drain potential of the drive transistor T 0 . In an example in which the drive transistor T 0  is a PMOS transistor, the light emission control signal line EM receives a high-level signal and the bias module  13  increases the drain voltage of the drive transistor T 0 . In other embodiment, it is further optional that the drive transistor is an NMOS transistor, the light emission control signal line EM receives a low-level signal, and the bias module  13  decreases the drain voltage of the drive transistor T 0 . The following embodiments are described by using an example in which the drive transistor is the PMOS transistor. 
     At a non-bias stage such as the light-emitting stage of the pixel circuit, a gate potential of the drive transistor might be higher than the drain potential of the drive transistor, which will cause ion polarization inside the drive transistor in long-term use and form a built-in electric field inside the drive transistor, so that an Id-Vg curve deviates, a threshold voltage of the drive transistor continuously increases, and the drive current flowing into the light-emitting element is affected, affecting display uniformity. In this embodiment, in the working process of the pixel circuit  10 , the bias module  13  adjusts the drain voltage of the drive transistor T 0 , so as to reduce the degree of ion polarization inside the drive transistor T 0  and compensate for the threshold voltage drift of the drive transistor T 0 . 
     In the embodiments of the present disclosure, the pixel circuit includes the bias module which is connected between the light emission control signal line and the drain of the drive transistor to adjust the drain potential of the drive transistor and improve a potential difference between the gate potential of the drive transistor and the drain potential of the drive transistor. It is known that the pixel circuit includes at least one non-bias stage. When the drive transistor generates the drive current, the gate potential of the drive transistor might be higher than the drain potential of the drive transistor, so that an I-V curve of the drive transistor deviates, resulting in the threshold voltage drift of the drive transistor. At the bias stage, the gate potential and the drain potential of the drive transistor are adjusted, so that the deviation of the I-V curve of the drive transistor at the non-bias stage can be balanced, thereby reducing the threshold voltage drift of the drive transistor and ensuring the display uniformity of the display panel. 
     In an embodiment, the working process of the pixel circuit further includes at least one non-bias stage; at the bias stage, the drive transistor has a gate voltage of Vg 1 , a source voltage of Vs 1 , and a drain voltage of Vd 1 ; and at the non-bias stage, the drive transistor has a gate voltage of Vg 2 , a source voltage of Vs 2 , and a drain voltage of Vd 2 ; where |Vg 1 −Vd 1 |&lt;|Vg 2 −Vd 2 |. 
     In this case, the potential difference between the gate potential of the drive transistor T 0  and the drain potential of the drive transistor T 0  is reduced, so as to alleviate the threshold voltage drift due to the potential difference between the gate potential of the drive transistor T 0  and the drain potential of the drive transistor T 0  at the non-bias stage. 
     In addition, in some implementations of this embodiment, (Vg 1 −Vs 1 )×(Vg 2 −Vs 2 )&lt;0 or (Vg 1 −Vd 1 )×(Vg 2 −Vd 2 )&lt;0. 
     In the working process of the pixel circuit, the gate voltage and the drain voltage of the drive transistor satisfy that (Vg 1 −Vd 1 )×(Vg 2 −Vd 2 )&lt;0. At the non-bias stage, the gate voltage of the drive transistor is higher than the drain voltage of the drive transistor in the pixel circuit, that is, Vg 2 &gt;Vd 2 , and then Vg 2 −Vd 2 &gt;0. At the bias stage, the data signal is written to the drain of the drive transistor, so that the gate voltage of the drive transistor is lower than the drain voltage of the drive transistor, that is, Vg 1 &lt;Vd 2 , and then Vg 1 −Vd 1 &lt;0. Then, (Vg 1 −Vd 1 )× (Vg 2 −Vd 2 )&lt;0. 
     In other embodiments, in the working process of the pixel circuit, the gate voltage and the source voltage of the drive transistor satisfy that (Vg 1 −Vs 1 )×(Vg 2 −Vs 2 )&lt;0 if the source and the drain of the drive transistor are interchanged. At the non-bias stage, the gate voltage of the drive transistor is higher than the source voltage of the drive transistor in the pixel circuit, that is, Vg 2 &gt;Vs 2 , and then Vg 2 −Vs 2 &gt;0. At the bias stage, the data signal is written to the source of the drive transistor, so that the gate voltage of the drive transistor is lower than the source voltage of the drive transistor, that is, Vg 1 &lt;Vs 2 , and then Vg 1 −Vs 1 &lt;0. Then, (Vg 1 −Vs 1 )× (Vg 2 −Vs 2 )&lt;0. 
     In addition, in this embodiment, since the non-bias stage such as the light-emitting stage of the display panel is relatively long, it may be set that Vd 1 −Vg 1 &gt;Vg 2 −Vd 2 &gt;0 to fully balance, at the bias stage, the threshold voltage drift at the non-bias stage and avoid taking so long a time at the bias stage. In this way, Vd 1  is much higher than Vg 1  at the bias stage so that the expected bias effect can be achieved as soon as possible at the bias stage. In other embodiments, if the source and the drain of the drive transistor are interchanged, it may be set that Vs 1 −Vg 1 &gt;Vg 2 −Vs 2 &gt;0, which depends on a specific circuit. 
     In an embodiment, in other implementations of this embodiment, the bias stage has a duration of t 1  and the non-bias stage has a duration of t 2 , where (|Vg 1 −Vs 1 |−|Vg 2 −Vs 2 |)× (t 1 −t 2 )&lt;0 or (|Vg 1 −Vd 1 |−|Vg 2 −Vd 2 |)× (t 1 −t 2 )&lt;0. 
     In this embodiment, at the bias stage, the drain voltage of the drive transistor is made higher than the gate voltage of the drive transistor, that is, Vg 1 −Vd 1 &lt;0. At the non-bias stage, the gate voltage of the drive transistor is higher than the drain voltage of the drive transistor, that is, Vg 2 −Vd 2 &gt;0. When the drive transistor is biased, in response to a relatively large bias voltage, bias time may be appropriately reduced, and in response to a relatively small bias voltage, the bias time may be appropriately prolonged. 
     Based on this, that |Vg 1 −Vd 1 |−|Vg 2 −Vd 2 |&gt;0 indicates a relatively large bias voltage and the duration of the bias stage may be appropriately reduced, that is, t 1 &lt;t 2 , so as to reduce the deviation between threshold voltages at the bias stage and the non-bias stage. That |Vg 1 −Vd 1 |−|Vg 2 −Vd 2 |&lt;0 indicates a relatively small bias voltage and the duration of the bias stage may be appropriately prolonged, that is, t 1 &gt;t 2 , so as to reduce the deviation between threshold voltages at the bias stage and the non-bias stage. 
     In other embodiments, if the source and the drain of the drive transistor are interchanged, the gate and the drain of the drive transistor at the bias stage and the non-bias stage satisfy that (|Vg 1 −Vs 1 |−|Vg 2 −Vs 2 |)× (t 1 −t 2 )&lt;0, so as to reduce the threshold voltage deviation at the non-bias stage. 
     It is to be noted that the bias stage and the non-bias stage in the above implementations, especially those involving a duration comparison, generally refer to a continuous bias stage and a continuous non-bias stage to be compared. 
     In an embodiment, the non-bias stage is the light-emitting stage of the display panel. Exemplarily, at the light-emitting stage, the drive transistor T 0  has a source voltage of 4.6 V, a gate voltage of 3 V, and a drain voltage of 1 V, and the gate voltage of the drive transistor is higher than the drain voltage of the drive transistor. At the bias stage, the drive transistor is biased to compensate for the threshold voltage drift of the drive transistor at the light-emitting stage. 
     In this embodiment, the transistor in the light emission control module  12  and the drive transistor T 0  are the same type of transistors. If they are both the PMOS transistors, at the bias stage, the light emission control signal line receives the high-level signal and the bias module  13  increases the drain voltage of the drive transistor T 0  according to the high-level signal. Alternatively, if the transistor in the light emission control module  12  and the drive transistor T 0  are both the NMOS transistors, at the bias stage, the light emission control signal line receives the low-level signal and the bias module decreases the drain voltage of the drive transistor T 0  according to the low-level signal. The bias module  13  adjusts the drain potential of the drive transistor T 0  according to the light emission control signal EM. 
     In an embodiment, as shown in  FIG.  1   , the pixel circuit  10  further includes a compensation module  15 , where the compensation module  15  is connected between the gate of the drive transistor T 0  and the drain of the drive transistor T 0  and configured to compensate a threshold voltage of the drive transistor T 0 ; and a control terminal of the compensation module  15  is connected to a second scanning signal line for receiving a second scanning signal S 2  which controls the compensation module  15  to be turned on or off. At the bias stage, the compensation module  15  remains off. Since the potential difference between the gate potential and the drain potential of the drive transistor T 0  is adjusted at the bias stage and the compensation module  15  is connected between the gate and the drain of the drive transistor T 0 , if the compensation module  15  is turned on, the gate potential will be basically equal to the drain potential. Therefore, at the bias stage, the compensation module  15  remains off. In an embodiment, the compensation module  15  includes a third transistor T 3 , where a source of the third transistor T 3  is connected to the drain of the drive transistor T 0 , a drain of the third transistor T 3  is connected to the gate of the drive transistor T 0 , and a gate of the third transistor T 3  is connected to the second scanning signal line for receiving the second scanning signal S 2 . 
     In an embodiment,  FIG.  2    is a schematic diagram of a second pixel circuit of a display panel according to an embodiment of the present disclosure and  FIG.  3    is a schematic diagram of a third pixel circuit of a display panel according to an embodiment of the present disclosure. In this embodiment, referring to  FIGS.  2  and  3   , the pixel circuit  10  further includes a reset module  16 . In an embodiment, as shown in  FIG.  2   , the reset module  16  is connected between a reset signal terminal and the drain of the drive transistor T 0  and configured to provide the gate of the drive transistor T 0  with a reset signal Vref. Alternatively, in an embodiment, as shown in  FIG.  3   , the reset module  16  is connected between the reset signal terminal and the gate of the drive transistor T 0  and configured to provide the gate of the drive transistor T 0  with the reset signal Vref. 
     The pixel circuit shown in  FIG.  2    is used as an example here. Reference is made to  FIGS.  4  to  6   , where  FIG.  4    is a schematic diagram of a first working timing sequence of the pixel circuit,  FIG.  5    is a schematic diagram of a second working timing sequence of the pixel circuit, and  FIG.  6    is a schematic diagram of a third working timing sequence of the pixel circuit. It is to be noted that the terms such as “first” present here and below are merely intended to distinguish different schematic diagrams and should not be construed as a sequence of the schematic diagrams. In addition, optionally, the third transistor T 3  and a fifth transistor T 5  are NMOS transistors and the other transistors are PMOS transistors, where the NMOS transistors may be oxide semiconductor transistors. 
     In an embodiment, as shown in  FIGS.  4  to  6   , within one frame of picture of the display panel, the working process of the pixel circuit includes a pre-stage and the light-emitting stage; where within at least one frame of picture, the pre-stage of the pixel circuit includes the bias stage. In this embodiment, the pre-stage includes the bias stage and an intermediate stage; where at the bias stage, the compensation module  15  is turned off; at the intermediate stage, the compensation module  15  is turned on. As shown in  FIG.  4   , the bias stage precedes the intermediate stage. Alternatively, as shown in  FIG.  5   , the bias stage is after the intermediate stage. Moreover, as shown in  FIG.  6   , when the pre-stage includes at least two bias stages, the intermediate stage may exist between any adjacent two bias stages. This will be further described in detail hereinafter. 
     In this embodiment, a data writing period of the display panel includes S frames of a refresh picture which includes a data writing frame and a retention frame, where S&gt;0; the data writing frame includes a data writing stage; and the retention frame includes no data writing stage. 
     In this embodiment, referring to  FIGS.  2  and  4   , the data writing frame includes the bias stage; where the intermediate stage includes a reset stage and the data writing stage in sequence; at the reset stage, the reset module  16  and the compensation module  15  are turned on and the gate of the drive transistor T 0  receives the reset signal to be reset; and at the data writing stage, the data writing module  14 , the drive module  11 , and the compensation module  15  are all turned on and the data signal is written to the gate of the drive transistor T 0 . Since the data writing frame includes the data writing stage and the gate of the drive transistor T 0  needs to be reset before the data writing stage, the pre-stage of the data writing frame needs to include the reset stage and the data writing stage. At other stages of the pre-stage, the compensation module may remain off, and the drain potential of the drive transistor T 0  is increased under the control of the light emission control signal EM and the bias module  13 . 
     In an embodiment, in the case where the data writing frame includes the bias stage, the reset stage may be further included before the bias stage, and then the bias stage is entered. Since the potential difference between the gate and the drain of the drive transistor T 0  is adjusted at the bias stage, the reset stage is performed before the bias stage. For example, if the drive transistor is the PMOS transistor, the gate of the drive transistor may be provided with the low-level signal at the reset stage to be reset; then, at the bias stage, the compensation module is turned off and the drain potential of the drive transistor is increased under the action of the light emission control signal EM and the bias module  13 . The above process adjusts both the gate potential and the drain potential of the drive transistor, thereby improving a bias effect. 
       FIG.  7    is a schematic diagram of a fourth working timing sequence of the pixel circuit. Referring to  FIG.  7   , the retention frame includes the bias stage and the intermediate stage includes the reset stage. At the reset stage, the reset module  16  and the compensation module  15  are turned on and the gate of the drive transistor T 0  receives the reset signal Vref to be reset. The retention frame includes no data writing stage. Therefore, if the pre-stage of the retention frame includes the bias stage and the retention frame further includes the reset stage in conjunction with the pixel circuit in  FIG.  2   , the intermediate stage includes the reset stage. In an embodiment, the reset stage may be performed before the bias stage or after the bias stage. When the pre-stage includes at least two bias stages, the reset stage may also be performed between any adjacent two intermediate stages. In an embodiment, the reset stage precedes the bias stage. Since the potential difference between the gate and the drain of the drive transistor T 0  is adjusted at the bias stage, the reset stage is performed before the bias stage. For example, if the drive transistor is the PMOS transistor, the gate of the drive transistor may be provided with the low-level signal at the reset stage to be reset; then, at the bias stage, the compensation module is turned off and the drain potential of the drive transistor is increased under the action of the light emission control signal EM and the bias module  13  The above process adjusts both the gate potential and the drain potential of the drive transistor, thereby improving the bias effect. 
     In an embodiment, the data writing period of the display panel includes the S frames of the refresh picture which includes the data writing frame and the retention frame, where S&gt;0; the data writing frame includes the data writing stage; and the retention frame includes no data writing stage. 
     The pixel circuit shown in  FIG.  3    is used as an example.  FIG.  8    is a schematic diagram of a fourth pixel circuit of a display panel according to an embodiment of the present disclosure. Referring to  FIG.  8   , in an embodiment, the third transistor T 3  and the fifth transistor T 5  are the NMOS transistors and the other transistors are the PMOS transistors, where the NMOS transistors may be the oxide semiconductor transistors. 
     In an embodiment, as shown in  FIG.  8   , the data writing frame includes the bias stage and the intermediate stage includes the data writing stage; and at the data writing stage, the data writing module  14 , the drive module  11 , and the compensation module  15  are all turned on and the data signal Vdata is written to the gate of the drive transistor T 0 . In the pixel circuit shown in  FIG.  3   , since the reset module  16  is connected to the gate of the drive transistor, the compensation module  15  does not need to be turned on at the reset stage. Therefore, at the reset stage, the light emission control signal EM and the bias module  13  can also control the drain potential of the drive transistor T 0 , that is, the bias stage and the reset stage can be performed at the same time. Therefore, in this embodiment, the intermediate stage may include merely the data writing stage. As described above, if the pre-stage further includes the reset stage, the bias stage at least partially overlaps the reset stage. 
     In this embodiment, a duration of the intermediate stage is less than a duration of the bias stage for the following reason: as described in the preceding embodiments, the intermediate stage mainly includes the reset stage or the data writing stage at which a related signal is written to the node, so the reset stage and the data writing stage need not be too long; as described above, the bias stage is performed to offset the threshold voltage drift of the drive transistor at the non-bias stage such as the light-emitting stage which generally takes a long time, so the bias stage also takes a certain time to fully achieve the bias effect. Therefore, the duration of the intermediate stage may be less than the duration of the bias stage. 
     In addition, in this embodiment, the pre-stage includes N bias stages, where N≥1.  FIGS.  6  and  8    illustrate a case where the bias stage includes two bias stages. In other embodiments, three or more bias stages may be included. As shown in  FIGS.  6  and  8   , the bias stage includes a first bias stage and a second bias stage; and the pre-stage includes the first bias stage, the intermediate stage, and the second bias stage in sequence. In an embodiment, the duration of the intermediate stage is less than a duration of the first bias stage and a duration of the second bias stage. As described above, signals are written to the related node at the intermediate stage and the threshold voltage drift of the drive transistor at the non-bias stage is offset at the bias stage. Therefore, generally, the duration of the intermediate stage may be set to be less than the duration of the first bias stage and the duration of the second bias stage, so as to fully achieve the bias effect. 
     In addition, in some implementations, the duration of the first bias stage is equal to the duration of the second bias stage. In other implementations, the duration of one of the first bias stage and the second bias stage is at least greater than the duration of the other one of the first bias stage and the second bias stage. In this case, the bias stage with the greater duration may be selected as a primary bias stage and the other bias stage is an auxiliary bias stage. The primary bias stage is the main bias stage. However, to prevent the bias effect of the primary bias stage from being insufficient, the auxiliary bias stage may be performed to supplement the bias effect. In some cases, the duration of the first bias stage is greater than that of the second bias stage. In other cases, it is further optional that the duration of the first bias stage is less than that of the second bias stage. 
     In the case where the pre-stage includes the N bias stages, where N≥1, in an embodiment, any two bias stages in the pre-stage may have different durations. For example, the duration of the first bias stage is greater than the durations of the other bias stages, which may be understood as follows: the first bias stage is the primary bias stage and performed mainly to offset the threshold voltage drift at the non-bias stage. However, to prevent the bias effect of the first bias stage from being insufficient, the other auxiliary bias stages may be set to fully supplement the bias effect. On this basis, it may be set that in the pre-stage, the durations of the bias stages sequentially decrease, so that the insufficient bias effect of the former bias stage may be supplemented by the later bias stage. Based on the same concept, it may be set reversely, for example, the duration of the last bias stage is greater than the durations of the other bias stages. In particular, the durations of the bias stages in the pre-stage increase sequentially. The bias effect can be gradually achieved through the bias stages whose durations gradually increase. In addition, based on the preceding concepts, it may be further set that the duration of a certain intermediate bias stage is greater than the duration of the first bias stage and the duration of the second bias stage, that is, the last bias stages are used as auxiliary bias stages and one intermediate bias stage is used as the primary bias stage. 
     The preceding embodiments and those illustrated in  FIGS.  4  to  8    are all for the case where the pre-stage includes the intermediate stage. In other implementations of this embodiment, the pre-stage may not include the intermediate stage. 
     With reference to the pixel circuit in  FIG.  3   ,  FIG.  9    is a schematic diagram of a fifth working timing sequence of the pixel circuit. Referring to  FIG.  9    which illustrates the working timing sequence of the retention frame, the retention frame includes the bias stage and the pre-stage further includes the reset stage. At the reset stage, the gate of the drive transistor T 0  receives the reset signal Vref to be reset, where the reset stage at least partially overlaps the bias stage. The retention frame does not need the data writing stage. In the pixel circuit shown in  FIG.  3   , since the reset module  16  is connected to the gate of the drive transistor T 0 , the compensation module  15  does not need to be turned on at the reset stage, so that the reset stage is performed simultaneously with at least part of the bias stage. Such a setting can adjust the gate potential of the drive transistor T 0  on the one hand and adjust the drain potential of the drive transistor T 0  on the other hand, so as to simultaneously adjust the gate potential and the drain potential, reduce the potential difference between the gate potential and the drain potential, and improve the bias effect. 
     In addition, with reference to the pixel circuit in  FIG.  3   ,  FIG.  10    is a schematic diagram of a sixth working timing sequence of the pixel circuit. Referring to  FIG.  10    which illustrates the working timing sequence of the retention frame, the retention frame includes the bias stage and the pre-stage of the retention frame is the bias stage. The retention frame includes no data writing stage. Therefore, if the retention frame does not include the reset stage, the compensation module  15  does not need to be turned on during the retention frame and the light emission control signal EM and the bias module  13  can control the drain potential of the drive transistor T 0  throughout the pre-stage, so the pre-stage is the bias stage. 
     In this embodiment, as shown in  FIGS.  1  to  3   , the light emission control module  12  includes a first light emission control module  12   a  and a second light emission control module  12   b ; the first light emission control module  12   a  is connected between a first power signal terminal and the source of the drive transistor T 0  and configured to selectively provide the drive module  11  with a first power signal PVDD; and the second light emission control module  12   b  is connected between the drain of the drive transistor T 0  and the light-emitting element  20  and configured to selectively allow the drive current to flow into the light-emitting element  20 . 
     In an embodiment, the first light emission control module  12   a  includes a fourth transistor T 4 , where a source of the fourth transistor T 4  is connected to the first power signal terminal, a drain of the fourth transistor T 4  is connected to the source of the drive transistor T 0 , and a gate of the fourth transistor T 4  is connected to a light emission control signal terminal. The second light emission control module  12   b  includes a first transistor T 1 , where a source of the first transistor T 1  is connected to the drain of the drive transistor T 0 , a drain of the first transistor T 1  is connected to the light-emitting element  20 , and a gate of the first transistor T 1  is connected to the light emission control signal terminal. 
     In this embodiment, as shown in  FIGS.  1  to  3   , a control terminal of the first light emission control module  12   a  and a control terminal of the second light emission control module  12   b  are connected to the same light emission control signal line. This is applicable to the case where the first light emission control module  12   a  and the second light emission control module  12   b  may be simultaneously turned on and off. 
     In addition, in this embodiment, referring to  FIG.  11    which is a schematic diagram of a fifth pixel circuit of a display panel according to an embodiment of the present disclosure, the control terminal of the first light emission control module  12   a  is connected to a first light emission control signal line for receiving a first light emission control signal EM 1 ; and the control terminal of the second light emission control module  12   b  is connected to a second light emission control signal line for receiving a second light emission control signal EM 2 . The bias module  13  may be connected to the first light emission control signal line or the second light emission control signal line. When the bias module  13  is connected to the second light emission control signal line, since the first light emission control module  12   a  is connected to the first power signal terminal and the source of the drive transistor T 0  and the second light emission control module  12   b  is connected between the drain of the drive transistor T 0  and the light-emitting element  20 , generally, to sufficiently make the drain of the drive transistor T 0  disconnected from the light-emitting element  20  and ensure that the light-emitting element  20  does not emit light at a non-light-emitting stage, the second light emission control module  12   b  remains off at the non-light-emitting stage. If the first transistor T 1  is the PMOS transistor, the second light emission control signal EM 2  remains to be the high-level signal at the pre-stage. Therefore, it is set that the bias module is connected to the second light emission control signal line, which can ensure that at the pre-stage, the bias stage has a relatively great duration, facilitating the improvement of the bias effect. 
     As shown in  FIG.  11   , in this embodiment, the bias module  13  includes a first capacitor C 1 , where a first plate of the first capacitor C 1  is connected to the drain of the drive transistor T 0  and a second plate of the first capacitor C 1  is connected to the light emission control signal line. At the bias stage, the first capacitor C 1  increases or decreases the drain voltage of the drive transistor T 0  according to the light emission control signal EM 2  on the light emission control signal line. Since the capacitor has the function of being charged and discharged, the capacitor is set, so that the drain voltage of the drive transistor T 0  can be controlled by the light emission control signal EM 2 . Meanwhile, no additional signal needs to be applied to control the capacitor. Therefore, the bias module  13  is configured to be the first capacitor C 1 , which can simplify the working process of the circuit. 
     In addition, in this embodiment, the pixel circuit further includes a second capacitor C 2 , where the second capacitor C 2  includes a third plate connected to the first power signal terminal and a fourth plate connected to the gate of the drive transistor T 0  and is configured to store the data signal Vdata transmitted to the gate of the drive transistor T 0 . In this embodiment, a capacitance value of the first capacitor C 1  may be greater than or equal to a capacitance value of the second capacitor C 2 . In some embodiments, the capacitance value of the first capacitor C 1  is smaller than the capacitance value C 2  of the second capacitor. Since the function of the second capacitor is to store the data signal Vdata written to the gate of the drive transistor T 0  and the data signal Vdata written to the gate of the drive transistor T 0  is one factor for determining the drive current generated by the drive transistor T 0  at the light-emitting stage, the capacitor with a strong storage capacity is required to fully store the signal for the drive transistor T 0  at the data writing stage. However, currently the bias stage is performed to adjust the potential difference between the gate potential and the drain potential of the drive transistor T 0 . Therefore, from the perspective of accurate data storage, a storage capacity of the second capacitor is required more greatly than a storage capacity of the first capacitor. Therefore, in this embodiment, it is set that the capacitance value of the first capacitor C 1  is smaller than the capacitance value of the second capacitor C 2 . 
     Further, the capacitance value of the first capacitor C 1  and the capacitance value of the second capacitor C 2  satisfy that C 2 ×⅛≤C 1 ≤C 2 ×¼. The inventor of the present application has found that when C 2 ×⅛≤C 1 ≤C 2 ×¼, the capacitance value of the first capacitor C 1  can meet the requirements of the bias stage and the problem that too large a capacitance value of the first capacitor C 1  results in an increase in load of the pixel circuit which affects the signal transmission of the light emission control signal line can be avoided. 
     Referring to  FIG.  12    which is a schematic diagram of a sixth pixel circuit of a display panel according to an embodiment of the present disclosure, in this embodiment, the bias stage further includes a gating module  18 , where the gating module  18  is connected between the light emission control signal line and the first capacitor C 1  and configured to selectively allow the light emission control signal EM to control the drain potential of the drive transistor T 0 ; and the gating module  18  includes a first bias transistor T 8 , where a source of the first bias transistor T 8  is connected to the light emission control signal line, a drain of the first bias transistor T 8  is connected to the first capacitor C 1 , and a gate of the first bias transistor T 8  is connected to a first bias signal line for receiving a first bias signal ST 1 . When merely the first capacitor C 1  is included between the light emission control signal line and the drain of the drive transistor T 0 , the start and end of the bias stage cannot be controlled at any time. As long as the compensation module  15  is turned off at the pre-stage, the bias stage is entered. In some cases, to better control the start and end of the bias stage, the gating module  18  is provided so that the start and end of the bias stage can be controlled by the first bias signal ST 1 . 
     Referring to  FIG.  13    which is a schematic diagram of a seventh pixel circuit of a display panel according to an embodiment of the present disclosure, optionally, in this embodiment, the bias module  13  further includes a second bias transistor T 9 , where a source of the second bias transistor T 9  is connected to the light emission control signal line, a drain of the second bias transistor T 9  is connected to the drain of the drive transistor T 0 , and a gate of the second bias transistor T 9  is connected to a second bias control signal line for receiving a second bias control signal ST 2 . At the bias stage, the second bias transistor T 9  is turned on and the light emission control signal EM is transmitted to the drain of the drive transistor T 0 . In this case, the second bias transistor T 9  is set and may be turned on at the beginning of the bias stage and turned off at the end of the bias stage, thereby controlling the start and end of the bias stage. 
     In an embodiment, as shown in  FIGS.  1  to  3  and  11  to  13   , in this embodiment, the pixel circuit further includes an initialization module  17 , where the initialization module  17  is connected between an initialization signal terminal and the light-emitting element  20  and configured to selectively provide the light-emitting element  20  with an initialization signal Vini; and a control terminal of the initialization module  17  is connected to a fourth scanning signal line for receiving a fourth scanning signal S 4 . 
     In an embodiment, the initialization module  17  includes a seventh transistor T 7 , where a source of the seventh transistor T 7  is connected to the initialization signal terminal, a drain of the seventh transistor T 7  is connected to the light-emitting element  20 , and a gate of the seventh transistor T 7  is connected to the fourth scanning signal line. 
     When the initialization module  17  is turned on, the pixel circuit  10  enters an initialization stage. In this embodiment, the bias stage does not overlap the initialization stage. In some embodiments, the bias stage may partially overlap the initialization stage. At the bias stage, the display panel is required not to emit light, but the transistor might have a certain leakage current. Therefore, if the light-emitting element  20  receives no initialization signal Vini, the light-emitting element  20  may be at the risk of emitting light at the bias stage. Therefore, at the bias stage, the light-emitting element  20  is initialized, which can further ensure that the light-emitting element does not emit light. Further, the initialization stage may end earlier than the bias stage, simultaneously with the bias stage, or later than the bias stage. These solutions are all applicable. A flexible design is allowed according to the specific circuit. 
     In this embodiment, it may be set that a first bias control signal ST 1  and the fourth scanning signal S 4  are the same signal, or the second bias control signal ST 2  and the fourth scanning signal S 4  are the same signal. In this embodiment, the fourth scanning signal S 4  controls the start and end of the initialization stage. As described above, the initialization stage may be performed at the bias stage, that is, the fourth scanning signal S 4  is reused as the first bias control signal ST 1  or the second bias control signal ST 2 , which can avoid the problem where too many drive signals are introduced into the display panel, which results in a larger working load of the display panel and an increase in the frame of the display panel. 
     In the present application, in an embodiment, part of T 0 , T 1 , T 2 , T 3 , T 4 , T 5 , and T 6  may be PMOS transistors with polysilicon as an active layer and part of T 0 , T 1 , T 2 , T 3 , T 4 , T 5 , and T 6  may be NMOS transistors with an oxide semiconductor as the active layer. For example, T 3  and T 5  are NMOS transistors and the other transistors are PMOS transistors. It is understandable that the effective pulse of a scanning signal for the NMOS transistor is at a high level and the effective pulse of a scanning signal for the PMOS transistor is at a low level. It is to be noted that the pixel circuits shown in  FIGS.  1  to  13    are merely examples and not to limit the structure of the pixel circuit in the embodiments of the present disclosure. 
     In an embodiment, a width-to-length ratio of a channel region of the NMOS transistor is greater than that of a channel region of the PMOS transistor since the NMOS transistor mainly functions as a switch transistor and requires an ability of a quick response in the present application, and the transistor with the greater width-to-length ratio has the shorter channel region which helps to improve the responsive ability of the transistor. 
     In addition, in the present application, four scanning signals, S 1 , S 2 , S 3 , and S 4 , may be different. In some particular cases, for example, if the timing sequence meets a certain condition, at least two of the four signals, S 1 , S 2 , S 3 , and S 4 , may be the same signal. For example, when T 5  and T 7  are the same type of transistors such as PMOS transistors or NMOS transistors, S 3  and S 4  may be the same signal. In another example, when T 3  and T 7  are the same type of transistors such as PMOS transistors or NMOS transistors, S 2  and S 4  may be the same signal. A specific situation depends on a specific circuit structure and a timing sequence and is not particularly limited in this embodiment. 
     Based on the same concept, the embodiments of the present disclosure further provide a driving method of a display panel. In conjunction with  FIG.  1   , the display panel includes a pixel circuit  10  and a light-emitting element  20 ; where the pixel circuit  10  includes a drive module  11 , a data writing module  14 , a light emission control module  12 , and a bias module  13 ; where the drive module  11  is configured to provide the light-emitting element  20  with a drive current and includes a drive transistor T 0 ; the data writing module  14  is connected to a source of the drive transistor T 0  and configured to selectively provide the drive module  11  with a data signal Vdata; the light emission control module  12  is configured to selectively allow the light-emitting element  20  to enter a light-emitting stage; a control terminal of the light emission control module  12  is connected to a light emission control signal line for receiving a light emission control signal EM; and the bias module  13  is connected between a drain of the drive transistor T 0  and the light emission control signal line. A working process of the pixel circuit includes a bias stage at which the bias module  13  adjusts a drain potential of the drive transistor T 0  in response to the light emission control signal EM. 
     A driving method for at least one frame of picture of the display panel includes steps described below. 
     In the case where a transistor in the light emission control module  12  and the drive transistor T 0  are PMOS transistors, at the bias stage, the light emission control signal line receives a high-level signal and the bias module  13  increases a drain voltage of the drive transistor T 0  according to the high-level signal to enable the drive transistor T 0  to enter a bias state. Alternatively, in the case where the transistor in the light emission control module  12  and the drive transistor T 0  are NMOS transistors, at the bias stage, the light emission control signal line receives a low-level signal and the bias module  13  decreases the drain voltage of the drive transistor T 0  according to the low-level signal to enable the drive transistor T 0  to enter the bias state. 
     In other implementations of this embodiment, the driving method may include the driving method used in the working process of the pixel circuit in any one of the embodiments described below. The same content will not be repeated in this embodiment, but it should be considered that all the driving methods described above fall within the scope of the driving method provided in this embodiment. 
       FIG.  14    is a partial sectional view of a pixel circuit according to an embodiment of the present disclosure.  FIG.  15    is a top view of a pixel circuit according to an embodiment of the present disclosure. Referring to  FIGS.  14  and  15   , the pixel circuit includes two types of transistors including a transistor Tm and a transistor Tn, where a gate of the transistor Tm is disposed in a first metal layer M 1 , a source and a drain of the transistor Tm are disposed in a fourth metal layer M 4 , and the transistor Tm includes a first active layer w 1  disposed between the first metal layer M 1  and a base substrate; the transistor Tn includes a first gate and a second gate, where the first gate is disposed in a second metal layer M 2  and a second gate is disposed in a third metal layer M 3 ; the transistor Tn includes a second active layer w 2  disposed between the second metal layer M 2  and the third metal layer M 3 ; and a source and a drain of the transistor Tn are disposed in the fourth metal layer M 4 . The transistor Tm may be a low-temperature polysilicon transistor and the transistor Tn may be an oxide semiconductor transistor. 
     The pixel circuit includes a first capacitor C 1  and a second capacitor C 2 , where the first capacitor C 1  includes a first plate C 11  and a second plate C 12 , and the second capacitor C 2  includes a third plate C 23  and a fourth plate C 24 . The first plate and the second plate are disposed in any two of the first active layer w 1 , the first metal layer M 1 , the second metal layer M 2 , the second active layer w 2 , the third metal layer M 3 , and the fourth metal layer. The third plate and the fourth plate are disposed in any two of the first active layer w 1 , the first metal layer M 1 , the second metal layer M 2 , the second active layer w 2 , the third metal layer M 3 , and the fourth metal layer M 4 . 
     In some cases, the first plate and the third plate are disposed in the same layer, and the second plate and the fourth plate are disposed in the same layer. In this case, an area of the first plate is smaller than that of the third plate, and an area of the second plate is smaller than that of the fourth plate, so that a capacitance value of the first capacitor C 1  is smaller than that of the second capacitor C 2 . 
     In some cases, the first plate and the third plate are disposed in the same layer, and the second plate and the fourth plate are disposed in different layers. In an embodiment, a distance between the first plate and the second plate is greater than a distance between the third plate and the fourth plate, so that the capacitance value of the first capacitor C 1  can be smaller than that of the second capacitor C 2 . In this case, in an embodiment, the first plate and the third plate are disposed in the first metal layer M 1 , the fourth plate is disposed in the second metal layer M 2 , and the second plate is disposed in the second active layer or the third metal layer M 3  or the fourth metal layer M 4 . 
     In some cases, the first plate, the second plate, the third plate, and the fourth plate are disposed in different layers, and a specific position of each plate is in any one of the first active layer w 1 , the first metal layer M 1 , the second metal layer M 2 , the second active layer w 2 , the third metal layer M 3 , and the fourth metal layer M 4 , which is all within the scope of this case. 
     In an embodiment, a first insulating layer is included between the first plate and the second plate, and a second insulating layer is included between the third plate and the fourth plate, where a dielectric constant of the first insulating layer is smaller than that of the second insulating layer, so that the capacitance value of the first capacitor C 1  is smaller than that of the second capacitor C 2 . In addition, in an embodiment, when the drive transistor is the PMOS transistor, the transistor Tm may be the drive transistor. In this case, the hydrogen content of the second insulating layer is greater than the hydrogen content of the first insulating layer. In this embodiment, the second capacitor C 2  is a storage capacitor in the pixel circuit, the second capacitor C 2  generally overlaps the drive transistor in a direction perpendicular to a surface of the display panel, and the drive transistor has a top-gate structure. Therefore, the second capacitor C 2  is generally disposed on a side of the first active layer w 1  facing away from the base substrate. In particular, the third plate C 23  of the second capacitor C 2  may be reused as the gate of the transistor Tm and the fourth plate may be disposed in the second metal layer M 2  and overlap the gate of the transistor Tm. In this case, the drive transistor is the PMOS transistor and the low-temperature polysilicon transistor. An active layer in the low-temperature polysilicon transistor needs to be hydrogenated, resulting in the higher hydrogen content in its surrounding layer. Therefore, in this embodiment, the hydrogen content of the second insulating layer is greater than the hydrogen content of the first insulating layer. 
     In an embodiment, the oxygen content of the first insulating layer is greater than the oxygen content of the second insulating layer. Since the capacitance value of the first capacitor C 1  is smaller than that of the second capacitor C 2 , in some cases, a thickness of the first insulating layer is greater than that of the second insulating layer. Therefore, at least one of the first plate or the second plate of the first capacitor C 1  is closer to an active layer in the transistor Tn (that is, the active layer in the oxide semiconductor transistor) than the third plate and the fourth plate. To ensure the normal function of the oxide semiconductor transistor, a layer surrounding an oxide semiconductor active layer has relatively low hydrogen content and relatively high oxygen content. Therefore, in this case, the oxygen content of the first insulating layer is greater than the oxygen content of the second insulating layer. 
     In the embodiments of the present disclosure, the pixel circuit includes the bias module which is connected between the light emission control signal line and the drain of the drive transistor to adjust the drain potential of the drive transistor and improve a potential difference between a gate potential of the drive transistor and the drain potential of the drive transistor. It is known that the pixel circuit includes at least one non-bias stage. When the drive transistor generates the drive current, the gate potential of the drive transistor might be higher than the drain potential of the drive transistor, so that an I-V curve of the drive transistor deviates, resulting in a threshold voltage drift of the drive transistor. At the bias stage, the gate potential and the drain potential of the drive transistor are adjusted, so that the deviation of the I-V curve of the drive transistor at the non-bias stage can be balanced, thereby reducing the threshold voltage drift of the drive transistor and ensuring the display uniformity of the display panel. 
     Based on the same concept, the embodiments of the present disclosure further provide a display device including the display panel according to any one of the embodiments described above. In an embodiment, the display panel is an organic light-emitting display panel or a micro light-emitting diode (LED) display panel. 
     Referring to  FIG.  16    which is a schematic diagram of a display device according to an embodiment of the present disclosure, in an embodiment, the display device is applied to an electronic device  200  such as a smart phone and a tablet computer. It is understandable that the above-mentioned embodiments merely provide some examples of the structure of the pixel circuit and the driving method of the pixel circuit. The display panel further includes other structures, which will not be repeated here. 
     It is to be noted that the above are merely some embodiments of the present disclosure and the technical principles used therein. It is to be understood by those skilled in the art that the present disclosure is not limited to the embodiments described herein. Those skilled in the art can make various apparent modifications, adaptations, combinations, and substitutions without departing from the scope of the present disclosure. Therefore, though the present disclosure has been described in detail through the embodiments described above, the present disclosure is not limited to the embodiments described above and may include other equivalent embodiments without departing from the concept of the present disclosure. The scope of the present disclosure is determined by the scope of the appended claims.