Patent Publication Number: US-9837019-B2

Title: Pixel circuit, organic electroluminescent display panel and display device

Description:
The application is a U.S. National Phase Entry of International Application No. PCT/CN2015/078928 filed on May 14, 2015, designating the United States of America and claiming priority to Chinese Patent Application No. 201410818108.1 filed on Dec. 24, 2014. The present application claims priority to and the benefit of the above-identified applications and the above-identified applications are incorporated by reference herein in their entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to the display technical field, and in particular to a pixel circuit, an organic electroluminescent display panel and a display device. 
     BACKGROUND 
     As the development of display technology, the OLED (Organic Light Emitting Diode) has become one of the hot topics in the flat-panel display researching area. More and more Active Matrix Light emitting device (AMOLED) display panel have entered into the market. Compared with the traditional Thin Film Transistor Liquid Crystal Display (TFTLCD), the AMOLED has a faster response, higher contrast and a wider perspective. 
     Therefore, how to improve the impact of IR drop of the power supply voltage signal inputted in the pixel circuits on luminous brightness of the light emitting device OLED is an urgent problem to be solved for those skilled in the art. 
     SUMMARY 
     The embodiments of the present disclosure provide a pixel circuit, an organic electroluminescent display panel and a display device, which are used to solve the problem of IR drop of the power supply voltage signal inputted in the pixel circuits affecting luminous brightness of the light emitting device in prior art. 
     The embodiment of the present disclosure provides a pixel circuit, which comprises: a charge control module, a drive module, a reset control module, a light-emitting control module and a light emitting device, wherein 
     the control end of the charge control module is connected with the scanning signal end, the input end thereof is connected with the data signal end, and the output end thereof is connected with the first input end of the reset control module; 
     the control end of the reset control module is connected with the threshold voltage signal end, the second input end thereof is connected with the reference signal end and the input end of the drive module, respectively, the first output end thereof is connected with the control end of the drive module, and the second output end thereof is connected with the output end of the drive module and the input end of the light-emitting control module, respectively; 
     a control end of the light-emitting control module is connected with a light-emitting signal end, an output end thereof is connected with an input end of the light emitting device; and an output end of the light emitting device is connected with a low level signal end; 
     in the reset and compensation phase, the charge control module makes the data signal end and the first input end of the reset control module switch on under the control of the scan signal end; the reset control module resets the control end and the output end of the drive module and compensates reference voltage under the control of the threshold voltage signal end and the reference signal end. In the data writing phase, the charge control module writes the data signal inputted by the data signal end to the first input end of reset control module under the control of the scan signal end. In the light-emitting phase, the light-emitting control module makes the output end of the drive module and the input end of the light emitting device switch on to drive the light emitting device to emit light under the control of the light-emitting signal end. 
     The embodiment of the present disclosure provides an organic electroluminescent display panel, which comprises the pixel circuit provided by the embodiment of the present disclosure. 
     The embodiment of the present disclosure provides a display device, which comprises the organic electroluminescent display panel provided by the embodiment of the present disclosure. 
     The advantageous effects of the embodiment of the present disclosure comprise: 
     The embodiment of the present disclosure provides a pixel circuit, an organic electroluminescent display panel and a display device; in the reset and compensation phase, the charge control module in the pixel circuit makes the data signal end and the first input end of the reset control module switch on under the control of the scan signal end; the reset control module resets the control end and the output end of the drive module and compensates the reference voltage of the control end and the output end of the drive module under the control of the threshold voltage signal end and the reference signal end. In the data writing phase, the charge control module writes the data signal inputted by the data signal end to the first input end of the reset control module under the control of the scan signal end. In the light-emitting phase, the light-emitting control module makes the output end of the drive module and the input end of the light emitting device switch on under the control of the light-emitting signal end, and the light emitting device is driven to emit light by integrating the data signal inputted into the drive module with the reference voltage signal compensated to the drive module. Therefore, these realize the effect that the drive current driving the light emitting device to emit light is irrelevant to the power supply voltage signal inputted into the pixel circuits, eliminate the influence on luminous brightness of the light emitting device caused by IR drop of the power supply voltage signal inputted into the pixel circuits, and further guaranteeing the display effect of the display panel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of the structure of a well-known pixel circuit; 
         FIG. 2  is a specific operation timing chart of the pixel circuit shown in  FIG. 1 ; 
         FIG. 3  is a schematic diagram of the pixel circuit provided by the embodiment of the present disclosure; 
         FIG. 4  is a schematic diagram of the specific structure of the pixel circuit provided by the embodiment of the present disclosure; 
         FIG. 5  is a specific operation timing chart of the pixel circuit shown in  FIG. 4 ; and 
         FIG. 6  is a schematic diagram of the structure of an organic electroluminescent display panel provided by the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Specific description of the pixel circuit, the organic electroluminescent display panel and the display device provided by the embodiment of the present disclosure are explained in more detail in relation to the drawings presented below. 
     According to a circuit structure of AMOLED pixel circuit well-known by the inventor, the circuit structure as shown in  FIG. 1  includes a first switching transistor M 1 , a second switching transistor M 2 , a third switching transistor M 3 , a fourth switching transistor M 4 , a fifth switching transistor M 5 , a storage capacitor C and a light emitting device OLED; wherein the gate of the first switching transistor M 1  is connected with the source of the second switching transistor M 2  and one end of the storage capacitor C, respectively, the source thereof is connected with the first reference signal end VDD, and the drain thereof is connected with the drain of the second switching transistor M 2  and the source of the fifth switching transistor M 5 ; the gate of the second switching transistor M 2  is connected with the scan signal end Scan; the gate of the third switching transistor M 3  is connected with the scan signal end Scan, the source of M 3  is connected with the data signal end Data, and the drain of M 3  is connected with the drain of the fourth switching transistor M 4  and the other end of the storage capacitor C; the gate of the fourth switching transistor M 4  is connected with the scan signal end Scan, and the source of M 4  is connected with the second reference signal end Vsus; the gate of the fifth switching transistor M 5  is connected with the scan signal end Scan and the drain of M 5  is connected with one end of the OLED; and the other end of the light emitting device OLED is connected with the third reference signal end VSS. 
       FIG. 2  is an operation timing chart of the pixel circuit shown in  FIG. 1 . In  FIG. 2 , it can be known that: stage t 1 , the scan signal input end Scan inputs low level signal, thus the second switching transistor M 2  and the third switching transistor M 3  are turned on and the fourth switching transistor M 4  and the fourth switching transistor M 5  are turned off; through the third switching transistor M 3  that is turned on, the signal from data signal end Data is written to the left end of the storage capacitor C, that is, the voltage of point b is Vb=Vdata; the voltage of the right end of the storage capacitor, that is, the voltage of point a is Va=VDD−Vth; wherein Vth is the threshold voltage of the first switching transistor M 1 . At the same time, because the second switching transistor M 2  is turned on, the voltage of point c equals that of point a, that is, Vc=Va=VDD−Vth; and the voltage difference between two ends of the storage capacitor C is Vb−Va=Vdata−VDD+Vth. In stage  2 , the scan signal input end Scan inputs high level signal, thus the fourth switching transistor M 4  and the fifth switching transistor M 5  are turned on and the second switching transistor M 2  and the third switching transistor M 3  are turned off; the fourth switching transistor M 4  that is turned on makes the second reference signal end Vsus and the left end of the storage capacitor C switch on, that is, point b; thus the voltage of point b is changed to Vsus and the voltage of point a, Va=Vsus−Vdata+VDD−Vth. Since the second switching transistor M 2  is in a turning-off state, the voltage of point c is maintained in VDD−Vth. At the same time, the fifth switching transistor M 5  that is turned on makes the drain of the first switching transistor M 1  and one end of the light emitting device OLED switch on, which enables the drain of the first switching transistor M 1  to output the drive current to the light emitting device OLED to drive the OLED to emit light. Based on the above analysis, it can be known that stage t 1  is the data signal writing phase and stage t 2  is the light emitting phase. In stage t 2 , the drive current that is outputted by switching transistor M 1  to drive the OLED to emit light is: 
     
       
         
           
             
               I 
               OLED 
             
             = 
             
               
                 
                   β 
                   2 
                 
                 ⁢ 
                 
                   
                     ( 
                     
                       Vgs 
                       - 
                       Vth 
                     
                     ) 
                   
                   2 
                 
               
               = 
               
                 
                   
                     β 
                     2 
                   
                   ⁢ 
                   
                     
                       ( 
                       
                         Va 
                         - 
                         Vc 
                         - 
                         Vth 
                       
                       ) 
                     
                     2 
                   
                 
                 = 
                 
                   
                     β 
                     2 
                   
                   ⁢ 
                   
                     
                       ( 
                       
                         Vsus 
                         - 
                         Vdata 
                         - 
                         Vth 
                       
                       ) 
                     
                     2 
                   
                 
               
             
           
         
       
     
     Wherein, Vgs is the voltage difference between the gate and the source of the first switching transistor M 1 , and β is the parameter relating to technological parameters and geometrical size of the first switching transistor M 1 . From the above derivation formula, it can be seen that the drive current driving the light emitting device OLED to emit light is relevant to the voltage signal inputted by the second reference signal end Vsus. In the detailed embodiment, the voltage signal inputted by the second reference signal end Vsus may be the voltage signal of the first reference signal end VDD, that is, the power supply voltage signal. However, when the power network that provides power supply voltage signal for the pixel circuits transmits power supply voltage signal from the signal source to its connected pixel circuits respectively through the power signal line, voltage drop (IR drop) occurs. The IR drop could influence the luminous brightness of the light emitting device OLED, and then impact the display effect of the display panel. 
     As shown in  FIG. 3 , the embodiment of the present disclosure provides a pixel circuit, which comprises: a charge control module  01 , a drive module  02 , a reset control module  03 , a light-emitting control module  04  and a light emitting device  05 ; wherein the control end of the charge control module  01  is connected with the scan signal end Gate, the input end thereof is connected with the data signal end Data and the output end thereof is connected with the first input end of the reset control module  03 ; 
     the control end of the reset control module  03  is connected with the threshold voltage signal end VTH, the second input end thereof is connected with the reference signal end VDD and the input end of the drive module  02 , respectively, the first output end thereof is connected with the control end of the drive module  02 , and the second output end thereof is connected with the output end of the drive module  02  and the input end of the light-emitting control module  04 , respectively; 
     the control end of the light-emitting control module  04  is connected with the light-emitting signal end EM, the output thereof is connected with the input end of the light emitting device  05 , and the output end of the light emitting device  05  is connected with the low level signal end VSS; 
     in the reset and compensation phase, the charge control module  01  makes the data signal end Data and the first input end of the reset control module  03  switch on under the control of the scan signal end Gate, and the reset control module  03  resets the control end and the output end of the drive module  02  and compensates the reference voltage under the control of the threshold voltage signal end VTH and the reference signal end VDD; in the data writing phase, the charge control module  01  writes the data signal inputted by the data signal end Data to the first input end of reset control module  03  under the control of the scan signal end Gate; in the light-emitting phase, the light-emitting control module  04  makes the output end of the drive module  02  and the input end of the light emitting device  05  switch on under the control of the light-emitting signal end EM to drive the light emitting device  05  to emit light. 
     In the pixel circuit provided by the embodiment of the invention, in the reset and compensation phase, the charge control module  01  makes the data signal end Data and the first input end of the reset control module  03  switch on under the control of the scan signal end Gate, data signal inputted by the data signal end Data is inputted into the first input end of the reset control module  03 , the reset control module  03  resets the control end and the output end of the drive module  02  and compensates the reference voltage under the control of the threshold voltage signal end VTH and the reference signal end VDD; that is, in the reset phase, low level Low is inputted into the reference signal end VDD, and the reset control module  03  resets the control end and the output end of the drive module  02 ; and in the compensation phase, high level ELVDD is inputted into the reference signal end VDD and the reset control module  03  charges the control end and the output end of the drive module  02  to implement the compensation; in the data writing phase, the charge control module  01  writes the data signal inputted by the data signal end Data to the first input end of the reset control module  03  under the control of the scan signal end Gate and then the reset control module  03  writes data signal to the control end of the drive module  02 ; in the light-emitting phase, the light-emitting control module  04  makes the output end of the drive module  02  and the input end of the light emitting device  05  switch on under the control of the light-emitting signal end EM, the light emitting device  05  is driven to emit light after eliminating the power supply voltage signal by integrating the data signal written to the drive module  02  with the reference voltage signal compensated to the drive module  02 . Therefore, these realizes the effect that the drive current driving the light emitting device to emit light is irrelevant to the power supply voltage signal, eliminate the influence on luminous brightness of the light emitting device caused by IR drop of the power supply voltage signal inputted into the pixel circuits, and further guaranteeing the display effect of the display panel. 
     When the invention is carried out in detail, in the pixel circuit provided by the the embodiment of the present disclosure, the reset control module  03  as shown in  FIG. 4  may comprises: the first capacitor C 1 , the second capacitor C 2 , and the first switching transistor T 1 ; wherein the gate of the first switching transistor T 1  is connected with the threshold voltage signal end VTH, the source thereof is connected with the control end of the drive module  02 , and the drain thereof is connected with the output end of the drive module  02  and the input end of the light-emitting control module  04 , respectively; the first capacitor C 1  is connected between the output end of the charge control module  01  and the control end of the drive module  02 ; the second capacitor C 2  is connected between the reference signal end VDD and the control end of the drive module  02 . In the reset phase, the first switching transistor T 1  is turned on under the control of the threshold voltage signal end VTH, the first switching transistor T 1  that is turned on makes the control end and the output end of the drive module  02  switch on, low level Low is inputted into the reference signal end, and the process of resetting the control end and the output end of the drive module  02  are performed; in the compensation phase, high level ELVDD is inputted into the reference signal end VDD, the first switching transistor T 1  is still in a turning-on state, the first switching transistor T 1  that is turned on makes the control end and the output end of the drive module  02  switch on, and the reference voltage of the control end and input end of the drive module  02  is compensated. 
     When the invention is carried out in detail, in the pixel circuit provided by the the embodiment of the present disclosure, the first switching transistor T 1  as shown in  FIG. 4  is a P-type transistor; when low level signal is inputted into the threshold voltage signal end VTH, the first switching transistor T 1  is in a turning-on state, the first switching transistor T 1  that is turned on makes the control end and the output end of the drive module  02  switch on. 
     When the invention is carried out in detail, in the pixel circuit provided by the embodiment of the present disclosure, the charge control module  01  as shown in  FIG. 4  may comprises: the second switching transistor T 2 , the gate of the second switching transistor T 2  is connected with the scan signal end Gate, the source thereof is connected with the data signal end Data, and the drain thereof is connected with the first input end of the reset control module  03 . In the reset and compensation phase, the second switching transistor T 2  is turned on under the control of the scan signal end Gate, the second switching transistor T 2  that is turned on makes the data signal end Data and the first input end of the reset control module  03  switch on and then writes the data signal inputted by the data signal end Data to the first input end of the reset control module  03 ; in the data writing phase, the second switching transistor T 2  is still in the turning-on state, the second switching transistor T 2  that is turned on writes the reference voltage signal Vref inputted by the data signal end Data to the first input end of the reset control module  03 . 
     When the invention is carried out in detail, in the pixel circuit provided by the embodiment of the present disclosure, the second switching transistor T 2  as shown in  FIG. 4  is a P-type transistor; when low level signal is inputted into the scan signal end Gate, the second switching transistor T 2  is in a turning-on state, the second switching transistor T 2  that is turned on makes the data signal end Data and the first input end of the reset control module  03  switch on. 
     When the invention is carried out in detail, in the pixel circuit provided by the embodiment of the present disclosure, the drive module  02  as shown in  FIG. 4  may comprises: the drive transistor D 1 ; the gate of the drive transistor D 1  is connected with the first output end of the reset control module  03 , the source thereof is connected with the reference signal end VDD, the drain thereof is connected with the second output end of the reset control module  03  and the input end of the light-emitting control module  04 , respectively; and the drive transistor D 1  is a P-type transistor. In the reset phase, low level signal Low is inputted in the reference signal end which resets the gate of the drive transistor D 1  to ensure that the drive transistor D 1  is in the turning-on state in the compensation phase to implement the compensation process. 
     When the invention is carried out in detail, in the pixel circuit provided by the embodiment of the present disclosure, the light-emitting control module  04  as shown in  FIG. 4  may comprises: the third switching transistor T 3 ; the gate of the third switching transistor T 3  is connected with the light-emitting signal end EM, the source thereof is connected with the output end of the drive module  02  and the second output end of the reset control module  03 , respectively, and the drain thereof is connected with the input end of the light emitting device  05 . In the light-emitting phase, the third switching transistor T 3  is turned on under the control of the light-emitting signal end EM, the third switching transistor T 3  that is turned on makes the output end of the drive module  02  and the input end of the light emitting device  05  switch on, which enables the drive module  02  to drive the light emitting device  05  to realize the normal luminescence function in the light-emitting phase. 
     When the invention is carried out in detail, in the pixel circuit provided by the embodiment of the present disclosure, the third switching transistor T 3  as shown in  FIG. 4  is a P-type transistor; when low level signal is inputted in the light-emitting signal end EM, the third switching transistor T 3  is in a turning-on state, the third switching transistor T 3  that is turned on makes the output end of the drive module  02  and the input end of the light emitting device  05  switch on. 
     It shall be noted that the switching transistor and the drive transistor mentioned in the above the embodiment of the present disclosure might be Thin Film Transistor (TFT) but also be Metal Oxide Semiconductor (MOS), and there is no limitation thereto. In the specific embodiment, the source and the drain of these transistors can be interchanged without particular distinction. 
     The operation process of the pixel circuit provided by the embodiments of the present invention is described in detail in relation to the pixel circuit and the timing of operation provided by the embodiment of the present disclosure below. According to the pixel circuit shown in  FIG. 4  and the input and output timing chart of  FIG. 4  shown in  FIG. 5 , the operation process of the pixel circuit provided by the embodiments of the present invention is described. Particularly, four stages t 1 -t 4  in the input and output timing chart shown in  FIG. 5  are selected. In the description below, 1 expresses high level signal and 0 expresses low level signal. 
     In t 1  stage, VDD=low, VTH=0, Gate=0, Data=DATA, and EM=1. Due to VTH=0 and Gate=0, the first switching transistor T 1  and the second switching transistor T 2  are turned on; due to EM=1, the third switching transistor T 3  is turned off. The first switching transistor T 1  that is turned on makes the gate and the drain of the drive transistor D 1  switch on, and the second switching transistor T 2  that is turned on makes the data signal end Data and one end of the first capacitor C 1  switch on; at this time, low level signal Low is inputted in the reference signal end VDD, thus the voltage of the gate and the drain of the drive transistor D 1  are both low−Vth, wherein Vth is the threshold voltage of the drive transistor D 1 , the voltage difference between two ends of the first capacitor C 1  is DATA−LOW+Vth, and the voltage difference between two ends of the second capacitor C 2  is Vth, t 1  stage is the reset phase. 
     In t 2  stage, VDD=ELVDDE, VTH=0, Gate=0, Data=DATA, and EM=1. Due to VTH=0 and Gate=0, the first switching transistor T 1  and the second switching transistor T 2  are turned on; due to EM=1, the third switching transistor T 3  is turned off. The first switching transistor T 1  that is turned on makes the gate and the drain of the drive transistor D 1  switch on, and the second switching transistor T 2  that is turned on makes the data signal end Data and one end of the first capacitor C 1  switch on; at this time, high level signal ELVDD is inputted into the reference signal end VDD, thus the gate of the drive transistor D 1  is charged through the drive transistor D 1  and the first switching transistor T 1 ; when the gate of the drive transistor D 1  is charged to ELVDD−Vth, the charge to the drive transistor D 1  stops and concludes. The voltage of the drain of the drive transistor D 1  is also ELVDD−Vth, then the voltage difference between two ends of the first capacitor C 1  is DATA−ELVDD+Vth and the voltage difference between two ends of the second capacitor C 2  is Vth. Stage t 2  is the phase of compensation. 
     In t 3  stage, VDD=ELVDD, VTH=1, Gate=0, Data=Vref, and EM=1. Due to Gate=0, the second switching transistor T 2  is turned on; due to VTH=1 and EM=1, the first switching transistor T 1  and the third switching transistor T 3  are turned off. The second switching transistor T 2  that is turned on makes the data signal end Data and one end of the first capacitor C 1  switch on; at this time, the reference signal end VDD inputs high level signal ELVDD and data signal inputted by the data signal end Data is Vref, thus the voltage of the gate of the drive transistor D 1  is ELVDD−Vth−DATA+Vref while the voltage of the drive transistor D 1  is maintained in the voltage ELVDD−Vth at the former phase. Stage t 3  is the data writing phase. 
     In t 4  stage, VDD=ELVDD, VTH=1, Gate=1, Data=0, and EM=0. Due to EM=0, the third switching transistor T 3  is turned on; due to VTH=1 and Gate=1, the first switching transistor T 1  and the second switching transistor T 2  are turned off. The third switching transistor T 3  that is turned on makes the drain of the drive transistor D 1  and the input end of the light emitting device  05  switch on, which enables the drive transistor D 1  to drive the light emitting device  05  to implement normal luminescence function. Stage t 4  is the light-emitting phase. 
     In t 4  stage, the voltage of the gate of the drive transistor D 1  is ELVDD−Vth−DATA+Vref, the voltage of the drain thereof is ELVDD−Vth, and the drive current driving the light emitting device  05  to emit light is I=K(Vgs−Vth) 2 =K(Vref−DATA−Vth) 2 ; wherein K is the parameter in relation to technological parameters and geometrical size of the drive transistor D 1 , and Vgs is the voltage difference between the gate and the source of the drive transistor D 1 . From the above analysis, it can be known that the on-state current of the light emitting device  05  is indeed irrelevant to the power supply signal of the reference voltage end VDD so that the influence on luminous brightness of the light emitting device  05  caused by the IR drop of the power supply voltage signal inputted into the pixel circuits is eliminated. 
     Based on the same inventive concept, the embodiment of the present disclosure provides an organic electroluminescent display panel, which comprises the above pixel circuits provided by the embodiment of the present disclosure. Since the theory of using the organic electroluminescent display panel to solve the problem is similar to that of the pixel circuit, for the implementation of the organic electroluminescent display panel, see the implementation of the pixel circuit. The repetitious details need not be given here. 
     When the invention is carried out in detail, in the organic electroluminescent display panel provided by the embodiment of the present disclosure, as shown in  FIG. 6 , a gate drive module may provide the corresponding control signal for the reference signal end VDD, scan signal end Gate, threshold voltage signal end VTH and light-emitting signal end EM in the pixel circuit, respectively and a source drive module provides data signal DATA and Vref for the data signal end Data at different stages. 
     Based on the same inventive concept, the embodiment of the present disclosure provides a display device which can be applied to mobile phones, tablet PCs, televisions, monitors, notebook computers, digital photo frame, navigation or any products or components with display function. Since the theory of using the display device to solve the problem is similar to that of the organic electroluminescent display panel, for the implementation of the display device, see the implementation of the organic electroluminescent display panel. The repetitious details need not be given here. 
     The embodiment of the present disclosure provides a pixel circuit, an organic electroluminescent display panel and a display device; in the reset and compensation phase, the charge control module in the pixel circuit makes the data signal end and the first input end of the reset control module switch on under the control of the scan signal end, and the reset control module resets the control end and the output end of the drive module and compensates the reference voltage under the control of the threshold voltage signal end and the reference signal end; in the data writing phase, the charge control module writes the data signal inputted by the data signal end to the first input end of the reset control module under the control of the scan signal end; in the light-emitting phase, the light-emitting control module makes the output end of the drive module and the input end of the light emitting device switch on under the control of the light-emitting signal end, and finally drives the light emitting device to emit light by integrating the data signal inputted into the drive module with the reference voltage signal compensated into the drive module. Then, these realize the effect that the drive current driving the light emitting device to emit light is irrelevant to the power supply voltage signal, eliminate the influence on luminous brightness of the light emitting device caused by IR drop of the power supply voltage signal inputted into the pixel circuits, and further guaranteeing the display effect of the display panel. 
     Obviously, those skilled in the art may make any possible changes and modifications without departing from the spirit and essence of the invention. And such changes and modifications are also considered as being within the scope of the invention if they belong to the scope of the claims and equal technology of the invention. 
     The present application claims priority to Chinese Patent Application No. 201410818108.1 filed on Dec. 24, 2014, the contents of which are hereby incorporated by reference in its entirety as part of the disclosure of the present application.