Patent Publication Number: US-10325553-B2

Title: Pixel circuit and method for driving a light emitting device and organic light emitting display panel

Description:
TECHNICAL FIELD 
     The present disclosure relates to a pixel circuit, a method for driving a light emitting device in the pixel circuit and an organic light emitting display panel. 
     BACKGROUND 
     With the progress of display technology, an organic light emitting display (OLED) becomes one of hot topics in the research field of flat panel display. More and more active matrix organic light emitting diode (AMOLED) display panels enter into the market. Relative to a traditional thin film transistor liquid crystal display (TFT LCD) panel, AMOLED has a faster response speed, a higher contrast and a broader view angle. 
     A general AMOLED pixel circuit is of a circuit structure comprising: a driving transistor, a switching transistor, a storage capacitor, and a light emitting device. 
     Since a threshold voltage of the driving transistor would drift with a long time operation of a display panel and OLED is current-driven, it requires a stable current to control light emitting. However, due to manufacturing process and device aging and so on, non-uniformity exists in a threshold voltage V th  of a driving transistor driving OLED, thereby resulting in that a change occurs in the current flowing through OLED of each pixel point such that display luminance is non-uniform, which influences display effect of the entire image. 
     Therefore, how to reduce the influence of change of the threshold voltage of the driving transistor in the pixel circuit on the light emitting luminance of the light emitting device so as to reduce the influence on display effect of the entire image is a problem to be solved urgently by those skilled in the art. 
     SUMMARY 
     There are provided in an embodiment of the present disclosure a pixel circuit, a method for driving a light emitting device in the pixel circuit, and an organic light emitting display panel, which are used to solve the problem existing in the prior art that a change of a threshold voltage of a driving transistor in the pixel circuit influences light emitting luminance of a light emitting device. 
     There is provided in an embodiment of the present disclosure a pixel circuit, comprising a light emitting device, and further comprising: 
     a charging module, whose input terminal is connected to a data signal terminal, and control terminal is connected to a scanning signal terminal; 
     a driving module, whose input terminal is connected to a first reference signal terminal, and control terminal is connected to an output terminal of the charging module; and 
     an initializing module, whose first input terminal is connected to an output terminal of the driving module, first output terminal is connected to an output terminal of the charging module, first control terminal is connected to an initializing signal terminal, second control terminal is connected to a control signal terminal, second output terminal is connected to an input terminal of the light emitting device, and second input terminal is connected to an output terminal of the light emitting device and a second reference signal terminal respectively; 
     wherein in an initializing phase, the initializing module is configured to initialize the driving module and the light emitting device under the control of the initializing signal terminal and the control signal terminal; in a writing phase, the charging module is configured to write a data signal inputted from the data signal terminal into the control terminal of the driving module under the control of the scanning signal terminal; and in a light emitting phase, the charging module is configured to load a first reference signal inputted from the data signal terminal to the control terminal of the driving module under the control of the scanning signal terminal, and the initializing module is configured to connect the output terminal of the driving module with the input terminal of the light emitting device under the control of the control signal terminal so that the driving module drives the light emitting device to emit light. 
     In a possible implementation, in the pixel circuit provided in the embodiment of the present disclosure, the driving module can comprise a driving transistor and a storage capacitor, wherein 
     a gate of the driving transistor is connected to the output terminal of the charging module and the first output terminal of the initializing module, a source thereof is connected to the first reference signal terminal, and a drain thereof is connected to the first input terminal of the initializing module; and 
     the storage capacitor is connected between the gate and the drain of the driving transistor. 
     In a possible implementation, in the pixel circuit provided in the embodiment of the present disclosure, the initializing module can comprise a reset unit and a control unit; wherein 
     a control terminal of the reset unit is connected to the initializing signal terminal, a first output terminal thereof is connected to the output terminal of the charging module and the gate of the driving transistor respectively, a first input terminal thereof is connected to the drain of the driving transistor and an output terminal of the control unit respectively, a second output terminal thereof is connected to an input terminal of the control terminal and the input terminal of the light emitting device, and a second input terminal thereof is connected to the output terminal of the light emitting device and the second reference signal terminal; 
     a control terminal of the control unit is connected to the control signal terminal; 
     in the initializing phase, the reset unit connects the gate and the drain of the driving transistor under the control of the initialization signal terminal, and connects the input terminal of the light emitting device with the second reference signal terminal; the control unit connects the drain of the driving transistor with the second output terminal of the reset unit under the control of the control signal terminal; and 
     in the light emitting phase, the control unit connects the drain of the driving transistor with the input terminal of the light emitting device under the control of the control signal terminal. 
     In a possible implementation, in the pixel circuit provided in the embodiment of the present disclosure, the reset unit can comprise a first switching transistor and a second switching transistor; wherein, 
     a gate of the first switching transistor is connected to the initializing signal terminal, a drain thereof is connected to the output terminal of the charging module and the control terminal of the driving transistor respectively, and a source thereof is connected to the drain of the driving transistor and the output terminal of the control unit respectively; and 
     a gate of the second switching transistor is connected to the initializing signal terminal, a drain thereof is connected to the input terminal of the control unit and the input terminal of the light emitting device respectively, and a source thereof is connected to the output terminal of the light emitting device and the second reference signal terminal respectively. 
     In a possible implementation, in the pixel circuit provided in the embodiment of the present disclosure , the control unit can comprise a third switching transistor; and 
     a gate of the third switching transistor is connected to the control signal terminal, a drain thereof is connected to the drain of the driving transistor and the source of the first switching transistor respectively, and a source thereof is connected to the drain of the second switching transistor and the input terminal of the light emitting device. 
     In a possible implementation, in the pixel circuit provided in the embodiment of the present disclosure, the charging module can comprise a fourth switching transistor; 
     a gate of the fourth switching transistor is connected to the scanning signal terminal, a source thereof is connected to the data signal terminal, and a drain thereof is connected to the drain of the first switching transistor and the gate of the driving transistor respectively. 
     In a possible implementation, in the pixel circuit provided in the embodiment of the present disclosure, in the writing phase, the charging module can be configured to write a threshold voltage of the driving transistor into the storage capacitor before writing the data signal into the control terminal of the driving module. 
     Further, there is provided in an embodiment of the present disclosure an organic light emitting display panel, comprising the pixel circuit provided in the embodiment of the present disclosure. 
     Further, there is provided in an embodiment of the present disclosure a method for driving a light emitting device in a pixel circuit including a charging module, a driving module, an initializing module and the light emitting device, comprising following steps: 
     in an initializing phase, initializing the driving module and the light emitting device by the initializing module under the control of an initialization signal terminal and a control signal terminal; 
     in a writing phase, writing a data signal inputted from a data signal terminal by the charging module into a control terminal of the driving module under the control of a scanning signal terminal; and 
     in a light emitting phase, loading a first reference signal inputted from the data signal terminal by the charging module to the control terminal of the driving module under the control of the scanning signal terminal, and connecting an output terminal of the driving module with an input terminal of the light emitting device by the initializing module under the control of the control signal terminal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of configuration of a general pixel circuit; 
         FIG. 2  is an operation timing diagram of the pixel circuit as shown in  FIG. 1 ; 
         FIG. 3  is a schematic diagram of configuration of a pixel circuit provided in an embodiment of the present disclosure; 
         FIG. 4  is a schematic diagram of specific configuration of a pixel circuit provided in an embodiment of the present disclosure; 
         FIG. 5  is an operation timing diagram of the pixel circuit as shown in  FIG. 4  provided in an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Specific implementations of a pixel circuit and an organic light emitting display panel provided in embodiments of the present disclosure will be described below in detail by combining with accompanying figures. 
       FIG. 1  shows circuit configuration of a general AMOLED pixel circuit. A shown in  FIG. 1 , the AMOLED pixel circuit comprises: a driving transistor M 1 , a switching transistor M 2 , a storage capacitor C, and a light emitting device OLED; wherein a gate of the driving transistor M 1  is connected to a drain of the switching transistor M 2  and one terminal of the storage capacitor C, a source thereof is connected to one terminal of the light emitting device, and a drain thereof is connected to the other terminal of the storage capacitor C and a first reference signal terminal VSS respectively; a gate of the switching transistor M 2  is connected to a scanning signal input terminal Scan, and a source thereof is connected to a data signal terminal Data; and the other terminal of the light emitting device is connected to a second reference signal terminal VDD. 
       FIG. 2  is an operating timing diagram of the pixel circuit as shown in  FIG. 1 . It can be known from  FIG. 2  that, during a period of time t 1 , a high level signal is inputted to the scanning signal input terminal Scan, and thus the switching transistor M 2  is turned on, now a signal of the data signal terminal Data is written into the storage capacitor C and the gate of the driving transistor M 1 , so that the driving transistor M 1  is turned on and the light emitting device starts to operate and emit light; during a period of time t 2 , a low level signal is inputted to the scanning signal input terminal Scan, and thus the switching transistor M 2  is turned off, now due to discharging effect of the storage capacitor C, the gate of the driving transistor M 1  would be maintained in a high level state, the driving transistor M 1  is turned on continuously, and OLED would operate and emit light continuously. 
       FIG. 3  shows a schematic diagram of configuration of a pixel circuit provided in an embodiment of the present disclosure. As shown in  FIG. 3 , the pixel circuit comprises: a charging module  01 , a driving module  02 , an initializing module  03 , and a light emitting device  04 . 
     In the circuit as shown in  FIG. 3 , an input terminal of the charging module  01  is connected to the data signal terminal Data, a control terminal thereof is connected to the scanning signal terminal Scan, and an output terminal thereof is connected to a control terminal of the driving module  02  and a first output terminal of the initializing module  03 ; 
     an input terminal of the driving module  02  is connected to a first reference signal terminal Ref 1 , and an output terminal thereof is connected to a first input terminal of the initializing module  03 ; 
     a first control terminal of the initializing, module  03  is connected to an initializing signal terminal Init, a second control terminal thereof is connected to a control signal terminal Ctrl, a second output terminal is connected to an input terminal of the light emitting device  04 , and a second input terminal is connected to an output terminal of the light emitting device  04  and the second reference signal terminal Ref 2  respectively. 
     In an initializing phase, the initializing module  03  initializes the driving module  02  and the light emitting device  04  under the control of the initializing signal terminal Init and the control signal terminal Ctrl; in a writing phase, the charging module  01  writes a data signal inputted from the data signal terminal Data into the control terminal of the driving module  02  under the control of the scanning signal terminal Scan; and in a light emitting phase, the charging module  01  loads the first reference signal inputted from the data signal terminal Data to the control terminal of the driving module  02  under the control of the scanning signal terminal Scan, and the initializing module  03  connects the output terminal of the driving module  02  with the input terminal of the light emitting device  04  under the control of the control signal terminal Ctrl so that the driving module  02  drives the light emitting device  04  to emit light. 
     In the pixel circuit provided in the embodiment of the present disclosure, the initializing module  03  is added. In the initializing phase, the initializing module  03  initializes the driving module  02  and the light emitting device  04  under the control of the initializing signal terminal Init and the control signal terminal Ctrl; in the writing phase, the charging module  01  writes the data signal inputted from the data signal terminal Data into the control terminal of the driving module  02  under the control of the scanning signal terminal Scan; and in the light emitting phase, the charging module  01  loads the first reference signal inputted from the data signal terminal Data to the control terminal of the driving module  02  under the control of the scanning signal terminal Scan, and the initializing module  03  connects the output terminal of the driving module  02  with the input terminal of the light emitting device  04  under the control of the control signal terminal Ctrl so that the driving module  02  drives the light emitting device  04  to emit light, thereby realizing the function of normal light emitting of the light emitting device  04 . As compared with the pixel circuit in the prior art, the pixel circuit provided in the embodiment of the present disclosure is capable of initializing the driving module  02  and the light emitting device  04  in the initializing phase, which eliminates the influence of a voltage difference in a previous phase on a subsequent phase, providing a compensation voltage and a driving voltage for the driving module  02  in the writing phase, which avoids the influence of change of the threshold voltage on the light emitting luminance of the light emitting device  04 , and raises uniformity of the light emitting luminance of the light emitting device  04 , so that the quality of the display picture is ensured. 
       FIG. 4  shows a schematic diagram of exemplary configuration of a pixel circuit provided in an embodiment of the present disclosure. As shown in  FIG. 4 , in the pixel circuit provided in the embodiment of the present disclosure, the driving module  02  can comprise a driving transistor D 1  and a storage capacitor C 1 . 
     In this case, a gate of the driving transistor D 1  is connected to the output terminal of the charging module  01  and the first output terminal of the initializing module  03 , a source thereof is connected to the first reference signal terminal Ref 1 , and a drain thereof is connected to the first input terminal of the initializing module  03 ; and the storage capacitor C 1  is connected between the gate and the drain of the driving transistor D 1 . 
     Specifically, in the pixel circuit provided in the embodiment of the present disclosure, the driving transistor D 1  can be an N type transistor or a P type transistor, which is not limited thereto. In the initializing phase, the initializing module  03  connects the gate of the driving transistor D 1  with the drain thereof under the control of the initializing signal terminal Init and the control signal terminal Ctrl, and also connects the drain of the driving transistor D 1  with the second reference signal terminal Ref 2 , so that the net charge across the storage capacitor C 1  is zero and at the same time the two terminals of the light emitting device  04  are shorted, so that the light emitting device  04  is in a non light emitting state, which eliminates the influence of the voltage difference of the storage capacitor C 1  in the previous phase on the subsequent phase and meanwhile provides time for the process of restoring the threshold voltage of the driving transistor D 1 . 
     In the pixel circuit provided in the embodiment of the present disclosure, in order to avoid that the data signal written in the writing phase is insufficient to turn on the driving transistor D 1 , in the writing phase, the charging module  01  can further be configured to write the threshold voltage of the driving transistor D 1  into the storage capacitor C 1  before writing the data signal into the control terminal of the driving module  02 , that is, in the writing phase, the charging module  01  writes a signal inputted from the data signal terminal Data and being greater than the gate turn-on voltage of the driving transistor D 1  into one terminal of the storage capacitor C 1 , i.e., the gate of the driving transistor D 1 . Now, the gate of the driving transistor D 1  is turned on to charge the other terminal of the storage capacitor C 1  until the voltage difference across the storage capacitor C 1  is the threshold voltage Vth of the driving transistor D 1 . Then, the threshold voltage of the driving transistor D 1  is stored in the storage capacitor C 1 , and thus the threshold voltage of the driving transistor D 1  is offset. 
     Exemplarily, in the pixel circuit provided in the embodiment of the present disclosure, as shown in  FIG. 4 , the initializing module  04  can comprise a reset unit  031  and a control unit  032 . 
     In this case, a control terminal of the reset unit  031  is connected to the initializing signal terminal Init, a first output terminal thereof is connected to the output terminal of the charging module  01  and the gate of the driving transistor D 1  respectively, a first input terminal thereof is connected to the drain of the driving transistor D 1  and an output terminal of the control unit  032  respectively, a second output terminal thereof is connected to an input terminal of the control terminal  032  and the input terminal of the light emitting device  04 , and a second input terminal thereof is connected to the output terminal of the light emitting device  04  and the second reference signal terminal Ref 2 ; a control terminal of the control unit  032  is connected to the control signal terminal Ctrl. 
     In the initializing phase, the reset unit  031  connects the gate of the driving transistor D 1  with the drain thereof under the control of the initialization signal terminal Init, and connects the input terminal of the light emitting device  04  with the second reference signal terminal Ref 2 ; the control unit  032  connects the drain of the driving transistor D 1  with the second output terminal of the reset unit  031  under the control of the control signal terminal Ctrl, so that the net charge across the two terminals of the storage capacitor C 1  is zero, i.e., eliminating the voltage difference of the storage capacitor C 1  in the precious phase on the subsequent phase and meanwhile providing time for the process of restoring the threshold voltage of the driving transistor D 1 ; in the light emitting phase, the control unit  032  connects the drain of the driving transistor D 1  with the input terminal of the light emitting device  04  under the control of the control signal terminal Ctrl, so that the driving transistor D 1  drives the light emitting device  04  to emit light normally under the control of the driving voltage. 
     Exemplarily, in the pixel circuit provided in the embodiment of the present disclosure, as shown in  FIG. 4 , the reset unit  031  can comprise a first switching transistor T 1  and a second switching transistor T 2 . 
     In this case, a gate of the first switching transistor T 1  is connected to the initializing signal terminal Init, a drain thereof is connected to the output terminal of the charging module  01  and the control terminal of the driving transistor D 1  respectively, and a source thereof is connected to the drain of the driving transistor D 1  and the output terminal of the control unit  032  respectively; and a gate of the second switching transistor T 2  is connected to the initializing signal terminal Init, a drain thereof is connected to the input terminal of the control unit  032  and the input terminal of the light emitting device  04  respectively, and a source thereof is connected to the output terminal of the light emitting device  04  and the second reference signal terminal Ref 2  respectively. 
     Specifically, in the pixel circuit provided in the embodiment of the present disclosure, the first switching transistor T 1  and the second switching transistor T 2  can be N type transistors simultaneously or be P type transistors simultaneously, which is not limited thereto. In the initializing phase, the first switching transistor T 1  and the second switching transistor T 2  are turned on under the control of the initializing signal terminal Init, the turned on first switching transistor T 1  turns on the gate and drain of the driving transistor DI, and the turned on second switching transistor T 2  shorts the two terminals of the light emitting device  04 , so that the light emitting device is in the non light emitting state. 
     Exemplarily, in the pixel circuit provided in the embodiment of the present disclosure, as shown in  FIG. 4 , the control unit  032  can comprise a third switching transistor T 3 . A gate of the third switching transistor T 3  is connected to the control signal terminal Ctrl, a drain thereof is connected to the drain of the driving transistor D 1  and the source of the first switching transistor T 1  respectively, and a source thereof is connected to the drain of the second switching transistor T 2  and the input terminal of the light emitting device  04 . 
     Specifically, in the pixel circuit provided in the embodiment of the present disclosure, the third switching transistor T 3  can be the N type transistor or be the P type transistor, which is not limited thereto. In the initializing phase, the third switching transistor T 3  is turned on under the control of the control signal terminal Ctrl, and the turned on third switching transistor T 3  connects the drain of the driving transistor D 1  with the input terminal of the light emitting device  04 , and then the drain of the driving transistor D 1  is connected with the second reference signal terminal Ref 2  through the second switching transistor T 2 ; in the light emitting phase, the third switching transistor T 3  is turned on under the control of the control signal terminal Ctrl, and the turned on third switching transistor T 3  connects the drain of the driving transistor D 1  with the input terminal of the light emitting device  04 , so that the driving transistor D 1  drives the light emitting device  04  to emit light. 
     Exemplarily, in the pixel circuit provided in the embodiment of the present disclosure, as shown in  FIG. 4 , the charging module  01  can comprise a fourth switching transistor T 4 . A gate of the fourth switching transistor T 4  is connected to the scanning signal terminal Scan, a source thereof is connected to the data signal terminal Data, and a drain thereof is connected to the drain of the first switching transistor T 1  and the gate of the driving transistor D 1  respectively. 
     Specifically, in the pixel circuit provided in the embodiment of the present disclosure, the fourth switching transistor T 4  can be the N type transistor or be the P type transistor, which is not limited thereto. In the writing phase, the fourth switching transistor T 4  is turned on under the control of the scanning signal terminal Scan, the turned on fourth switching transistor T 4  connects the data signal terminal Data with the gate of the driving transistor D 1 , writes the data signal of the data signal terminal Data into the gate of the driving transistor D 1 , and at the same time charges the storage capacitor C 1 , i.e., storing the threshold voltage of the driving transistor into the storage capacitor C 1 ; in the light emitting phase, also, the turned on fourth switching transistor T 4  connects the data signal terminal Data with the gate of the driving transistor D 1 , so that the first reference signal VDD inputted from the data signal terminal Data is loaded to the gate of the driving transistor DI as a driving voltage of the driving transistor D 1  to drive the light emitting device  04  to emit light. 
     It should be noted that the switching transistors and the driving transistor mentioned in the embodiments of the present disclosure can be thin film transistors (TFT), or can be a metal oxide semiconductors (MOS), which is not limited thereto. In a specific implementation, sources and drains of these transistors can be exchanged with each other and no specific distinction is made to these sources and drains. The specific embodiments are described by taking the thin film transistor as an example. 
     There will be described below in detail operating processes of the pixel circuit provided in the embodiments of the present disclosure in connection with the pixel circuits provided in the embodiments of the present disclosure as well as operation timings.  FIG. 5  schematically shows an operation timing diagram of the pixel circuit provided in the embodiment of the present disclosure as shown in  FIG. 4 . The operating processes of the pixel circuits provided in the embodiments of the present disclosure are described by using the pixel circuit designed with the N type transistors as shown in  FIG. 4  we well as the input and output timing diagrams of  FIG. 4  as shown in  FIG. 5 . Specifically, four periods of time t 1 -t 4  in the input and output timing diagrams as shown in  FIG. 5  are selected. In the following description, 1 represents a high level signal, and 0 represents a low level signal. 
     During the period of time t 1 , Init=1, Ctrl=1, Scan=0, Ref 1 =1, and Ref 2 =0. Since Init=1 and Ctrl=1, the first switching transistor T 1 , the second switching transistor T 2  and the third switching transistors T 3  are turned on. Since Scan=0, the fourth switching transistor T 4  is turned off. The turned on first switching transistor T 1  connects the gate and drain of the driving transistor D 1 , the turned on third switching transistor T 3  connects the drain of the driving transistor D 1  with the input terminal of the light emitting device  04 , and the turned on second switching transistor T 2  shorts the two terminals of the light emitting  04 , so that the light emitting device  04  is in the non light emitting state, and at the same time, the drain of the driving transistor D 1  is connected with the second reference signal terminal Ref 2 , and the net charge across the storage capacitor C 1  is zero clearing. The t 1  phase is the initializing phase. 
     During the period of time t 2 , Init=0, Ctrl=0, Scan=1, Ref 1 =1, and Ref 2 32 0. Since Init=0 and Ctrl=0, the first switching transistor T 1 , the second switching transistor T 2  and the third switching transistors T 3  are turned off, and the light emitting device  04  is still in the non light emitting state; since Scan=1, the fourth switching transistor T 4  is turned on, the turned on fourth switching transistor T 4  connects the data signal terminal Data with the gate of the driving transistor D 1 , and the data signal inputted from the data signal terminal Data is written into one terminal of the storage capacitor C 1 . Now, the data signal inputted from the data signal terminal Data is greater than a voltage signal for turning on the gate of the driving transistor D 1 , for example, a half of the first reference signal VDD. Since the data signal written into one terminal of the storage capacitor C 1 , i.e., the gate of the driving transistor D 1 , is VDD/2, the gate of the driving transistor D 1  is turned on to charge the other terminal of the storage capacitor C 1 , through the adjusting of the turning-on of the driving transistor DI, until the voltage of the other terminal of the storage capacitor C 1  is VDD/2-Vth, where Vth is the threshold voltage of the driving transistor D 1 . At this time, the driving transistor DI is in a critical turn-on state, and the threshold voltage Vth is stored in the storage capacitor C 1 . The t 2  phase is the compensating phase. 
     During the period of time t 3 , Init=0, Ctrl=0, Scan=1, Ref 1 =1, and Ref 2 32 0. Since Init=0 and Ctrl=0, the first switching transistor T 1 , the second switching transistor T 2  and the third switching transistors T 3  are turned off, and the light emitting device  04  is still in the non light emitting state; since Scan=1, the fourth switching transistor T 4  is turned on, the turned on fourth switching transistor T 4  connects the data signal terminal Data with the gate of the driving transistor D 1 , and the data signal Vdata_n inputted from the data signal terminal Data is written into one terminal of the storage capacitor C 1 , i.e., the gate of the driving transistor D 1 . Now, the other terminal of the storage capacitor C 1  is Vdata_n-Vth, the driving transistor D 1  is still in the critical turning-on state, and meanwhile the threshold voltage Vth of the driving transistor D 1  is stored in the storage capacitor C 1  continuously. The phase t 3  is the writing phase of the data signal. 
     During the period of time t 4 , Ctrl=1, Scan=1, Ref 1 =1, and Ref 2 32 0. Since Init=0, the first switching transistor T 1  and the second switching transistor T 2  are turned off; since Ctrl=1, the third switching transistor T 3  is turned on; since Scan=1, the fourth switching transistor T 4  is turned on, and the turned on fourth switching transistor T 4  connects the data signal terminal Data with the gate of the driving transistor D 1 . Now, the data signal inputted from the data signal terminal Data is the first reference signal VDD, and is written into the gate of the driving transistor D 1  through the turned on fourth switching transistor T 4  to control the driving transistor DI turned on and drive the light emitting device  04  to emit light. The phase t 4  is the light emitting phase. 
     At the moment of the driving transistor D 1  being turned on, the voltage at the other terminal of the storage capacitor C 1  is Vdata n-Vth, and thus the turn-on current of the light emitting device  04  is: I=K (Vgs−Vth) 2  =K (VDD−(Vdata_n−Vth)−Vth) 2  =K (VDD−Vdata_n) 2 , where K is a constant related with process parameters and geometric dimensions of the driving transistor D 1 , Vgs is a voltage difference between the gate and source of the driving transistor D 1 . It can be known from the above analysis that the turn-on current of the light emitting device  04  is indeed unrelated with the threshold voltage of the driving transistor D 1 , so that the influence of the change of the threshold voltage of the driving transistor D 1  on the light emitting luminance of the light emitting device  04  is eliminated, and the uniformity of the light emitting luminance of the light emitting device  04  better is enhanced well. 
     During the subsequent periods of time, the respective control signals are the same as the control signal during the period of time t 4 , and thus the light emitting state of the light emitting device  04  is maintained until the high level is again inputted to the initializing signal terminal Init in a period of time. 
     There is further provided in an embodiment of the present disclosure a method for driving a light emitting device in a pixel circuit. The pixel circuit includes a charging module, a driving module, an initializing module and the light emitting device. the method comprises following steps: in an initializing phase, initializing the driving module and the light emitting device by the initializing module under the control of an initialization signal terminal and a control signal terminal; in a writing phase, writing a data signal inputted from a data signal terminal by the charging module into a control terminal of the driving module under the control of a scanning signal terminal; and in a light emitting phase, loading a first reference signal inputted from the data signal terminal by the charging module to the control terminal of the driving module under the control of the scanning signal terminal, and connecting an output terminal of the driving module with an input terminal of the light emitting device by the initializing module under the control of the control signal terminal, so that the driving module drives the light emitting device to emit light 
     Based on the same inventive concept, there is provided in an embodiment of the present disclosure an organic light emitting display panel, comprising the pixel circuits provided in the embodiments of the present disclosure. The organic light emitting display panel can be applicable to any product or components having a display function, such as a mobile phone, a tablet computer, a TV set, a display, a notebook computer, a digital photo frame, and a navigator and so on. Since the principle of solving the problem by the organic light emitting display panel is similar to that of the pixel circuit, the implementation of the organic light emitting display panel can be referred to the implementation of the pixel circuit. The repeated description is not provided herein. 
     There are provided in the embodiments of the present disclosure the pixel circuit, the method for driving the light emitting device and the organic light emitting display panel. The initializing module is added to the pixel circuit. In the initializing phase, the initializing module initializes the driving module and the light emitting device under the control of the initializing signal terminal and the control signal terminal; in the writing phase, the charging module writes the data signal inputted from the data signal terminal into the control terminal of the driving module under the control of the scanning signal terminal; and in the light emitting phase, the charging module loads the first reference signal inputted from the data signal terminal to the control terminal of the driving module under the control of the scanning signal terminal, and the initializing module connects the output terminal of the driving module with the input terminal of the light emitting device under the control of the control signal terminal so that the driving module drives the light emitting device to emit light, thereby realizing the function of normal light emitting of the light emitting device. Compared with the pixel circuit in the prior art, the pixel circuits provided in the embodiments of the present disclosure are capable of initializing the driving module and the light emitting device in the initializing phase, which eliminates the influence of the voltage difference in a previous phase on a subsequent phase, providing an compensating voltage and a driving voltage for the driving module in the writing phase, which avoids the influence of change of the threshold voltage on the light emitting luminance of the light emitting device, and raises uniformity of the light emitting luminance of the light emitting device, so that the quality of the display picture is ensured. 
     Obviously, those skilled in the art can make various alternations and modifications to the present disclosure without departing from the spirit and scope of the present disclosure. If these alternations and modifications of the present disclosure belong to the scope of the claims of the present disclosure as well as their equivalent technology, then the present disclosure intends to comprise these alternations and modifications. 
     The present application claims the priority of a Chinese patent application No. 201410503599.0 filed on Sep. 25, 2014. Herein, the content disclosed by the Chinese patent application is incorporated in full by reference as a part of the present disclosure.