Patent Grant
9601057
The present invention relates to the field of organic electroluminescent technologies and particularly to a pixel circuit, an organic electroluminescent display panel and a display device.
Organic Light Emitting Diode (OLED) displays have become one of focuses in the research field of flat panel displays at present, and the OLED displays have low power consumption, low production cost, self-luminescence, wide angle of view, high response speed and other advantages over liquid crystal displays. At present, the OLED display screens have come to take the place of traditional Liquid Crystal Display (LCD) screens in the display field of handsets, Personal Digital Assistants (PDAs), digital cameras and the like. Particularly, the design of pixel circuits is the core of technical matters in the OLED displays and has important research significance.
Unlike an LCD for which the brightness is controlled by a stable voltage, a current-driven OLED needs a stable current to control the emission of light. The threshold voltage Vth of a drive transistor of a pixel circuit is not uniform due to the process flow, aging of elements and other reasons, so that the current flowing through OLEDs of respective pixels varies, thus resulting in non-uniform display brightness, thereby degrading the display quality of the entire image.
For example, in an existing 2T1C pixel circuit as illustrated in
In view of this, embodiments of the present invention provide a pixel circuit, an organic electroluminescent display panel and a display device so as to improve the uniformity of image brightness in a display area of the display device.
An embodiment of the invention provides a pixel circuit, which includes: a light emitting element, a first capacitor, a reset control module, a drive control module, a compensation control module and a light emission control module, wherein:
a first signal end of the reset control module is connected with a data signal end, a second signal end of the reset control module is connected with a reset control signal end, and a third signal end of the reset control module is connected respectively with a first end of the first capacitor and a first signal end of the light emission control module; and a fourth signal end of the reset control module is connected with a first scan signal end, a fifth signal end of the reset control module is connected with a reset signal end, and a sixth signal end of the reset control module is connected respectively with a second end of the first capacitor, a first signal end of the compensation control module and a first signal end of the drive control module;
a second signal end of the drive control module is connected with a first reference signal end, and a third signal end of the drive control module is connected respectively with a second signal end of the compensation control module and a second signal end of the light emission control module; and a third signal end of the compensation control module is connected with a second scan signal end;
a third signal end of the light emission control module is connected with a light emission control signal end, a fourth signal end of the light emission control module is connected with a second reference signal end, and a fifth signal end of the light emission control module is connected with a first end of the light emitting element; and a second end of the light emitting element is connected with a third reference signal end; and
in a reset phase, the reset control module writes a reset signal transmitted from the reset signal end into the second end of the first capacitor under control of the first scan signal end.
An embodiment of the invention provides a pixel circuit, which includes: a light emitting element, a first capacitor, a drive transistor, a first switch element, a second switch element, a third switch element, a fourth switch element and a fifth switch element, wherein:
a source of the drive transistor is connected with a first reference signal end, a drain of the drive transistor is connected respectively with a signal input end of the first switch element and a signal input end of the fifth switch element, and a gate of the drive transistor is connected respectively with a second end of the first capacitor, a signal output end of the third switch element and a signal output end of the first switch element; and a control end of the first switch element is connected with a second scan signal end;
a signal input end of the second switch element is connected with a data signal end, a signal output end of the second switch element is connected respectively with a first end of the first capacitor and a signal output end of the fourth switch element, and a control end of the second switch element is connected with a reset control signal end;
a signal input end of the third switch element is connected with a reset signal end, and a control end of the third switch element is connected with a first scan signal end;
a signal input end of the fourth switch element is connected with a second reference signal end, and a control end of the fourth switch element is connected respectively with a control end of the fifth switch element and a light emission control signal end; and
a first end of the light emitting element is connected with a signal output end of the fifth switch element, and a second end of the light emitting element is connected with a third reference signal end.
An embodiment of the invention further provides an organic electroluminescent display panel which includes a plurality of the pixel circuits according to any one of the above embodiments of the invention.
An embodiment of the invention further provides a display device which includes any organic electroluminescent display panel according to the above embodiment of the invention.
Specific implementations of a pixel circuit, an organic electroluminescent display panel and a display device according to embodiments of the invention will be described below in details with reference to the drawings.
As illustrated in
A first signal end 1a of the reset control module 1 is connected with a data signal end Data, a second signal end 1b of the reset control module 1 is connected with a reset control signal end RS, and a third signal end 1c of the reset control module 1 is connected respectively with a first end m1 of the first capacitor C1 and a first signal end 4a of the light emission control module 4; and a fourth signal end 1d of the reset control module 1 is connected with a first scan signal end Scan1, a fifth signal end 1e of the reset control module 1 is connected with a reset signal end Rset, and a sixth signal end 1f of the reset control module 1 is connected respectively with a second end m2 of the first capacitor C1, a first signal end 3a of the compensation control module 3 and a first signal end 2a of the drive control module 2.
A second signal end 2b of the drive control module 2 is connected with a first reference signal end Ref1, and a third signal end 2c of the drive control module 2 is connected respectively with a second signal end 3b of the compensation control module 3 and a second signal end 4b of the light emission control module 4; and a third signal end 3c of the compensation control module 3 is connected with a second scan signal end Scan2.
A third signal end 4c of the light emission control module 4 is connected with a light emission control signal end EM, a fourth signal end 4d of the light emission control module 4 is connected with a second reference signal end Ref2, and a fifth signal end 4e of the light emission control module 4 is connected with a first end o1 of the light emitting element D1; and a second end o2 of the light emitting element D1 is connected with a third reference signal end Ref3.
In a reset phase, the reset control module 1 writes a reset signal transmitted from the reset signal end Rset into the second end m2 of the first capacitor C1 under control of the first scan signal end Scan1; in a compensation phase, the reset control module 1 writes a data signal transmitted from the data signal end Data into the first end m1 of the first capacitor C1 under control of the reset control signal end RS, and the drive control module 2 charges the first capacitor C1 through the compensation control module 3 under control of the second scan signal end Scan2; and in a light emission phase, both the light emission control module 4 and the first capacitor C1 control the drive control module 2 to drive the light emitting element D1 to emit light under control of the light emission control signal end EM.
In the above pixel circuit according to the embodiment of the invention, the compensation control module can compensate for a drift of the threshold voltage in the drive control module in the compensation phase, so in the light emission phase, an operating current at which the drive control module drives the light emitting element to emit light can be only related to the voltage of the data signal input at the data signal end and the voltage at the second reference signal end but independent of the threshold voltage in the drive control module to thereby avoid the influence of the threshold voltage on the light emitting element, so as to stabilize the operating current driving the light emitting element to emit light and improve the uniformity of image brightness in the display area of the display device.
Preferably, for the sake of a convenient implementation, in the above pixel circuit according to the embodiment of the invention, as illustrated in
A gate of the drive transistor M0 is the first signal end 2a of the drive control module 2, a source of the drive transistor M0 is the second signal end 2b of the drive control module 2, and a drain of the drive transistor M0 is the third signal end 2c of the drive control module 2.
In a particular implementation, the light emitting element D1 in the above pixel circuit according to the embodiment of the invention is generally an Organic Light Emitting Diode (OLED). The light emitting element D1 is operated to emit light for display under the action of the saturated current of the drive transistor M0.
In a particular implementation, in the above pixel circuit according to the embodiment of the invention, the drive transistor M0 driving the light emitting element to emit light is generally a P-type transistor. The threshold voltage Vth of the P-type transistor is negative, so in order to ensure the normal operation of the drive transistor M0, the voltage at the first reference signal end Ref1 needs to be a positive voltage, and the voltage at the third reference signal end Ref3 needs to be lower than the voltage at the first reference signal end Ref1. The voltage at the third reference signal end Ref3 being zero will be taken as an example for illustration throughout the following description.
In a particular implementation, in the above pixel circuit according to the embodiment of the invention, as illustrated in
A gate of the first switch transistor M1 is connected with the second scan signal end Scan2, a source of the first switch transistor M1 is connected with the drain of the drive transistor M0, and a drain of the first switch transistor Ml is connected with the second end m2 of the first capacitor C1.
Particularly in a particular implementation, the first switch transistor can be an N-type transistor or can be a P-type transistor, and the invention will not be limited in this regard. When the first switch transistor is an N-type transistor, the first switch transistor is turned on when the signal at the second scan signal end is at a high level; and when the first switch transistor is a P-type transistor, the first switch transistor is turned on when the signal at the second scan signal end is at a low level.
Particularly when the compensation control module in the above pixel circuit according to the embodiment of the invention is structured particularly as the first switch transistor, it operates under such a principle that in the compensation phase, the second scan signal end controls the first switch transistor to be turned on, and the turned-on first switch transistor changes the drive transistor into a diode, so that after turning on the diode, the voltage Vref1 at the first reference signal end charges the first capacitor until the voltage at the second end of the first capacitor is Vref1−|Vth|, to thereby achieve the storing of the threshold voltage |Vth| of the drive transistor at the gate of the drive transistor. The first switch transistor is turned off in both the reset phase and the light emission control phase.
In a particular implementation, in the above pixel circuit according to embodiments of the invention, as illustrated in
A gate of the second switch transistor M2 is connected with the reset control signal end RS, a source of the second switch transistor M2 is connected with the data signal end Data, and a drain of the second switch transistor M2 is connected with the first end m1 of the first capacitor C1.
A gate of the third switch transistor M3 is connected with the first scan signal end Scan1, a source of the third switch transistor M3 is connected with the reset signal end Rset, and a drain of the third switch transistor M3 is connected with the second end m2 of the first capacitor C1.
Particularly in a specific implementation, the second switch transistor can be an N-type transistor or can be a P-type transistor, and the invention will not be limited in this regard. When the second switch transistor is an N-type transistor, the second switch transistor is turned on when the signal at the reset control signal end is at a high level; and when the second switch transistor is a P-type transistor, the second switch transistor is turned on when the signal at the reset control signal end is at a low level.
Particularly in a specific implementation, the third switch transistor can be an N-type transistor or can be a P-type transistor, and the invention will not be limited in this regard. When the third switch transistor is an N-type transistor, the third switch transistor is turned on when the signal at the first scan signal end is at a high level; and when the third switch transistor is a P-type transistor, the third switch transistor is turned on when the signal at the first scan signal end is at a low level.
Particularly when the reset control module in the above pixel circuit according to the embodiment of the invention is structured specifically as the second switch transistor and the third switch transistor described above, it operates in such a way that in the reset phase, the first scan signal end controls the third switch transistor to be turned on, and the turned-on third switch transistor writes the reset signal Vrset transmitted from the reset signal end into the second end of the first capacitor, so that the voltage at the second end of the first capacitor is Vrset, thereby ensuring that the voltage at the gate of the drive transistor is Vrset in this phase. In this phase, the second switch transistor can be turned on or can be turned off. In the compensation phase, the reset control signal end controls the second switch transistor to be turned on, and the turned-on second switch transistor writes the data signal Vdata transmitted from the data signal end into the first end of the first capacitor, so that the voltage at the first end of the first capacitor is Vdata, and the third switch transistor is turned off in this phase. Both the second switch transistor and the third switch transistor are turned off in the light emission control phase.
Preferably in order to simplify the fabrication process, in the above pixel circuit according to the embodiment of the invention, all of the first switch transistor, the second switch transistor and the third switch transistor can be P-type transistors or can be N-type transistors, and the invention will not be limited in this regard.
Preferably in order to simplify the circuit structure, in the above pixel circuit according to the embodiment of the invention, when all of the first switch transistor, the second switch transistor and the third switch transistor are P-type transistors or N-type transistors, as illustrated in
Preferably in order to further simplify the circuit structure, in the above pixel circuit according to the embodiment of the invention, when all of the first switch transistor, the second switch transistor and the third switch transistor are P-type transistors or N-type transistors, as illustrated in
In a particular implementation, in the above pixel circuit according to the embodiment of the invention, as illustrated in
Both a gate of the fourth switch transistor M4 and a gate of the fifth switch transistor M5 are connected with the light emission control signal end EM, a source of the fourth switch transistor M4 is connected with the second reference signal end Ref2, and a drain of the fourth switch transistor M4 is connected with the first end m1 of the first capacitor C1.
A source of the fifth switch transistor M5 is connected with the drain of the drive transistor M0, and a drain of the fifth switch transistor M5 is connected with the first end of the light emitting element D1.
Preferably, in order to simplify the fabrication process, in the above pixel circuit according to the embodiment of the invention, both the fourth switch transistor and the fifth switch transistor are P-type transistors or N-type transistors, and the invention will not be limited in this regard. When both the fourth switch transistor and the fifth switch transistor are N-type transistors, the fourth switch transistor and the fifth switch transistor are turned on when the signal at the light emission control signal end is at a high level; and when both the fourth switch transistor and the fifth switch transistor are P-type transistors, the fourth switch transistor and the fifth switch transistor are turned on when the signal at the light emission control signal end is at a low level.
Particularly when the light emission control module in the above pixel circuit according to the embodiment of the invention is structured particularly as the fourth switch transistor and the fifth switch transistor described above, it operates under such a principle that in the light emission control phase, the light emission control signal end controls the fourth switch transistor and the fifth switch transistor to be turned on, and the turned-on fourth switch transistor writes the voltage Vref2 at the second reference signal end into the first end of the first capacitor, so that the voltage at the first end of the first capacitor is changed from Vdata in the compensation phase to Vref2, and due to the bootstrap function of the capacitor, the voltage at the second end of the first capacitor is changed from Vref1−|Vth| in the compensation phase to Vref1−|Vth|+Vref2−Vdata according to the principle of charge conservation of the capacitor, and in this phase, since the drive transistor operates in the saturation state, it can be seen from the current characteristic in the saturation state that the operating current IOLED flowing through the drive transistor and used to drive the light emitting element to emit light satisfies the equation of IOLED=K(Vsg−|Vth|)2=K[Vref1−(Vref1−|Vth|+Vref2−Vdata)−|Vth|]2=K(Vdata−Vref2)2, where K is a structural parameter, which is relatively stable and thus can be regarded as a constant in the same structure. As can be apparent, the operating current IOLED of the light emitting element has been independent of the threshold voltage Vth of the drive transistor but only related to the voltage Vdata of the data signal input at the data signal end and the voltage Vref2 at the second reference signal end to thereby thoroughly avoid the operating current IOLED of the light emitting element D1 from being influenced by the drift of the threshold voltage Vth of the drive transistor due to the process flow and running for a long period of time, so as to ensure the normal operation of the light emitting element D1.
Preferably in order to simplify the circuit structure, in the above pixel circuit according to the embodiment of the invention, as illustrated in
Preferably in the above pixel circuit according to the embodiment of the invention, in order to stabilize the voltage at the gate of the drive transistor, as illustrated in
A first end n1 of the second capacitor C2 is connected with the first reference signal end Ref1, and a second end n2 of the second capacitor C2 is connected with the gate of the drive transistor M0.
Particularly when the compensation control module in the above pixel circuit according to the embodiment of the invention is structured specifically as the first switch transistor and the second capacitor described above, it operates in such a way that in the compensation phase, the second scan signal end controls the first switch transistor to be turned on, and the turned-on first switch transistor changes the drive transistor into a diode, so that after turning on the diode, the voltage Vref1 at the first reference signal end charges the first capacitor and the second capacitor until the voltage at the second end of the first capacitor is Vref1−|Vth|, and at this time the voltage difference across the first capacitor is Vdata−Vref1+|Vth|, and the voltage difference across the second capacitor is |Vth|, thereby achieving the storing of the threshold voltage |Vth| of the drive transistor at the gate of the drive transistor. The first switch transistor is turned off in both the reset phase and the light emission control phase.
It shall be noted that the drive transistor and the switch transistors mentioned in the above embodiments of the invention can be Thin Film Transistors (TFTs) or can be Metal Oxide Semiconductor (MOS) field effect transistors, and the invention will not be limited in this regard. In a particular implementation, the sources and the drains of these transistors can be interchanged without being distinguished from each other. The particular embodiments are described by taking the drive transistor and the switch transistors, all of which are thin film transistors, as an example.
Moreover preferably, all of the drive transistor and the switch transistors mentioned in the above embodiments of the invention can be embodied as P-type transistors, thereby simplifying the process flow of fabricating the pixel circuit.
The operation principle of the above pixel circuit will be described below in details by taking the drive transistor and the switch transistors in the pixel circuit, all of which are P-type transistors, as an example.
An Example:
Taking the pixel circuit illustrated in
In the reset phase T1, both the signal at the reset control signal end RS and the signal at the first scan signal end Scan1 are low-level signals, and the second switch transistor M2 and the third switch transistor M3 are turned on; and both the signal at the second scan signal end Scan2 and the signal at the light emission control signal end EM are high-level signals, and the first switch transistor M1, the fourth switch transistor M4, the fifth switch transistor M5 and the drive transistor M0 are turned off. The data signal Vdata at the data signal end Data is written into the first end m1 of the first capacitor C1 through the second switch transistor M2, and the reset signal Vrset at the reset signal end Rset is written into the second end m2 of the first capacitor C1 through the third switch transistor M3, so that the voltage at the first end m1 of the first capacitor C1 is Vdata, and the voltage at the second end m2 of the first capacitor C1 is Vrset, thereby resulting in the voltage of Vrset at the gate of the drive transistor M0 in the reset phase.
In the compensation phase T2, both the signal at the reset control signal end RS and the signal at the second scan signal end Scan2 are low-level signals, and the first switch transistor M1 and the second switch transistor M2 are turned on, and at the same time the turned-on first switch transistor M1 changes the drive transistor M0 into a diode; and both the signal at the first scan signal end Scan1 and the signal at the light emission control signal end EM are high-level signals, and all of the third switch transistor M3, the fourth switch transistor M4 and the fifth switch transistor M5 are turned off. The data signal Vdata transmitted from the data signal end Data is written into the first end m1 of the first capacitor C1 so that the voltage at the first end m1 of the first capacitor C1 is Vdata; and after turning on the diode, the voltage Vref1 at the first reference signal end Ref1 charges the first capacitor C1 until the voltage at the second end m2 of the first capacitor C1 is Vref1−|Vth|. At this time the voltage difference across the first capacitor is Vdata−Vref1+|Vth|, thereby achieving the storing of the threshold voltage |Vth| of the drive transistor M0 at the gate of the drive transistor M0.
In the light emission phase T3, the signal at the light emission control signal end EM is a low-level signal, and the fourth switch transistor M4, the fifth switch transistor M5 and the drive transistor M0 are turned on; and all of the signals at the reset control signal end RS, the first scan signal end Scan1 and the second scan signal end Scan2 are high-level signals, and the first switch transistor M1, the second switch transistor M2 and the third switch transistor M3 are turned off. The voltage Vref2 at the second reference signal end Ref2 is written into the first end m1 of the first capacitor C1 so that the voltage at the first end m1 of the first capacitor C1 is changed from Vdata to Vref2, and based upon the principle of charge conservation of the capacitor, in order to ensure that the voltage difference across the first capacitor C1 is still Vdata−Vref1+|Vth|, the voltage at the second end m2 of the first capacitor C1 jumps from Vref1−|Vth| to Vref1−|Vth|+Vref2−Vdata. Since the drive transistor M0 operates in the saturation state, it can be seen from the current characteristic in the saturation state that the operating current IOLED flowing through the drive transistor M0 and used to drive the light emitting element D1 to emit light satisfies the equation of IOLED=K(Vsg−|Vth|)2=K[Vref1−(Vref1−|Vth|+Vref2−Vdata)−|Vth|]2=K(Vdata−Vref2)2, where K is a structural parameter, which is relatively stable and thus can be regarded as a constant in the same structure. As is apparent from the foregoing, the operating current IOLED of the light emitting element D1 is independent of the threshold voltage Vth of the drive transistor M0 and only related to the voltage Vdata of the signal input at the data signal end and the voltage Vref2 at the second reference signal end to thereby thoroughly prevent the operating current IOLED of the light emitting element D1 from being influenced by the drift of the threshold voltage Vth of the drive transistor due to the process flow and operating for a long period of time, so as to ensure the normal operation of the light emitting element D1.
Another Example:
Taking the pixel circuit illustrated in
In the reset phase T1, the operation principle thereof is the same as the operation principle in the reset phase in the above example. In this phase, the voltage at the first end m1 of the first capacitor C1 is Vdata, and the voltage at the second end m2 of the first capacitor C1 is Vrset, thereby resulting in the voltage of Vrset at the gate of the drive transistor M0 in the reset phase.
In the compensation phase T2, the operation principle thereof is the same as the operation principle in the compensation phase in the above example. However the second capacitor C2 is added to the compensation control module, so in this phase, the voltage Vref1 at the first reference signal end Ref1 also charges the second capacitor C2 while charging the first capacitor C1 so that the voltage difference across the first capacitor C1 is Vdata−Vref1+|Vth|, and the voltage difference across the second capacitor C2 is |Vth|, thereby achieving the storing of the threshold voltage |Vth| of the drive transistor M0 at the gate of the drive transistor M0.
In the light emission phase T3, the operation principle thereof is the same as the operation principle in the light emission phase in the above example. However the voltage at the gate of the drive transistor M0 is Vref1−|Vth|+(Vref2−Vdata)Cst1/(Cst1+Cst2) due to the coupling of the first capacitor C1 and the second capacitor C2, and the voltage at the gate of the drive transistor M0 will be relatively stable due to the addition of the second capacitor C2. Since the drive transistor M0 operates in the saturation state, it can be seen from the current characteristic in the saturation state that the operating current IOLED flowing through the drive transistor M0 and used to drive the light emitting element D1 to emit light satisfies the equation of IOLED=K(Vsg−|Vth|)2=K{Vref1−[Vref1−|Vth|+(Vref2−Vdata)Cst1/(Cst1+Cst2)]|Vth]|}2=K[(Vdata−Vref2)Cst1/(Cst1+Cst2)]2, where all of K, Cst1 and Cst2 are structural parameters, which are relatively stable and thus can be regarded as constants in the same structure. As can be apparent, the operating current IOLED of the light emitting element D1 is independent of the threshold voltage Vth of the drive transistor M0 and only related to the voltage Vdata of the signal input at the data signal end and the voltage Vref2 at the second reference signal end to thereby thoroughly prevent the operating current IOLED of the light emitting element D1 from being influenced by the drift of the threshold voltage Vth of the drive transistor due to the process flow and operating for a long period of time, so as to ensure the normal operation of the light emitting element D1.
Another Example:
Taking the pixel circuit illustrated in
In the reset phase T1, the signal at the first scan signal end Scan1 is a low-level signal, and the third switch transistor M3 is changed into a diode which is turned on; and both the signal at the second scan signal end Scan2 and the signal at the light emission control signal end EM are high-level signals, and all of the first switch transistor M1, the second switch transistor M2, the fourth switch transistor M4, the fifth switch transistor M5 and the drive switch transistor M0 are turned off. The scan signal Vscan2 at the second scan signal end Scan2 is written into the second end m2 of the first capacitor C1 through the turned-on diode so that the voltage at the second end m2 of the first capacitor C1 is Vscan2−|Vth3|, thus resulting in the voltage of Vscan2 at the gate of the drive transistor M0 in the reset phase, where Vth3 is the threshold voltage of the third switch transistor M3.
In the compensation phase T2, the signal at the second scan signal end Scan2 is a low-level signal, and the first switch transistor M1 and the second switch transistor M2 are turned on, and at the same time the turned-on first switch transistor M1 changes the drive transistor M0 into a diode; and both the signal at the first scan signal end Scan1 and the signal at the light emission control signal end EM are high-level signals, and all of the third switch transistor M3, the fourth switch transistor M4 and the fifth switch transistor M5 are turned off. The data signal Vdatatransmitted from the data signal end Data is written into the first end m1 of the first capacitor C1 so that the voltage at the first end m1 of the first capacitor C1 is Vdata; and after turning on the diode, the voltage Vref2 at the second reference signal end Ref2 charges the first capacitor C1 until the voltage at the second end m2 of the first capacitor C1 is Vref2−|Vth|. At this time the voltage difference across the first capacitor C1 is Vdata−Vref2+|Vth|, thereby achieving the storing of the threshold voltage |Vth| of the drive transistor M0 at the gate of the drive transistor M0.
In the light emission phase T3, the signal at the light emission control signal end EM is a low-level signal, and the fourth switch transistor M4, the fifth switch transistor M5 and the drive transistor M0 are turned on; and both the signal at the first scan signal end Scan1 and the signal at the second scan signal end Scan2 are high-level signals, and all of the first switch transistor M1, the second switch transistor M2 and the third switch transistor M3 are turned off. The voltage Vref2 at the second reference signal end Ref2 is written into the first end m1 of the first capacitor C1 so that the voltage at the first end m1 of the first capacitor C1 is changed from Vdata to Vref2, and based upon the principle of charge conservation of the capacitor, in order to ensure that the voltage difference across the first capacitor C1 is still Vdata−Vref2+|Vth|, the voltage at the second end m2 of the first capacitor C1 jumps from Vref2−|Vth| to Vref2−|Vth|+Vref2−Vdata. Since the drive transistor M0 operates in the saturation state, it can be seen from the current characteristic in the saturation state that the operating current IOLED flowing through the drive transistor M0 and used to drive the light emitting element D1 to emit light satisfies the equation of IOLED=K(Vsg−|Vth|)2=K[Vref2−(Vref2−|Vth|+Vref2−Vdata)−|Vth|]2=K(Vdata−Vref2)2, where K is a structural parameter, which is relatively stable and thus can be regarded as a constant in the same structure. As can be apparent, the operating current IOLED of the light emitting element D1 has been independent of the threshold voltage Vth of the drive transistor M0 but only related to the voltage Vdata of the signal input at the data signal end and the voltage Vref2 at the second reference signal end to thereby thoroughly avoid the operating current IOLED of the light emitting element D1 from being influenced by the drift of the threshold voltage Vth of the drive transistor due to the process flow and running for a long period of time, so as to ensure the normal operation of the light emitting element D1.
Another Example:
Taking the pixel circuit illustrated in
In the reset phase T1, the operation principle thereof is the same as the operation principle in the reset phase in the above example. In this phase, the voltage at the second end m2 of the first capacitor C1 is Vscan2−|Vth3|, thereby resulting in the voltage of Vrset at the gate of the drive transistor M0 in the reset phase.
In the compensation phase T2, the operation principle thereof is the same as the operation principle in the compensation phase in the above example. However the second capacitor C2 is added to the compensation control module, so in this phase, the voltage Vref2 at the second reference signal end Ref2 also charges the second capacitor C2 while charging the first capacitor C1 so that the voltage difference across the first capacitor C1 is Vdata−Vref2+|Vth|, and the voltage difference across the second capacitor C2 is |Vth|, thereby achieving the storing of the threshold voltage |Vth| of the drive transistor M0 at the gate of the drive transistor M0.
In the light emission phase T3, the operation principle thereof is the same as the operation principle in the light emission phase in the above example. However the voltage at the gate of the drive transistor M0 is Vref2−|Vth|+(Vref2−Vdata)Cst1/(Cst1+Cst2) due to the coupling of the first capacitor C1 and the second capacitor C2, and the voltage at the gate of the drive transistor M0 will be relatively stable due to the addition of the second capacitor C2. Since the drive transistor M0 operates in the saturation state, it can be seen from the current characteristic in the saturation state that the operating current IOLED flowing through the drive transistor M0 and used to drive the light emitting element D1 to emit light satisfies the equation of IOLED=K(Vsg−|Vth|)2=K{Vref2−[Vref2−|Vth|+(Vref2−Vdata)Cst1/(Cst1+Cst2)]|Vth|]}2=K[(Vdata−Vref2)Cst1/(Cst1+Cst2)]2, where all of K, Cst1 and Cst2 are structural parameters, which are relatively stable and thus can be regarded as constants in the same structure. As can be apparent, the operating current IOLED of the light emitting element D1 has been independent of the threshold voltage Vth of the drive transistor M0 but only related to the voltage Vdata of the signal input at the data signal end and the voltage Vref2 at the second reference signal end to thereby thoroughly avoid the operating current IOLED of the light emitting element D1 from being influenced by the drift of the threshold voltage Vth of the drive transistor due to the process flow and running for a long period of time, so as to ensure the normal operation of the light emitting element D1.
The operation principle has been described here only by taking the four structures of the pixel circuit according to the embodiment of the invention as examples, and the operation principle of the pixel circuit according to the embodiment of the invention in other structures is the same as that in the above examples, and a repeated description thereof will be omitted here.
Based upon the same inventive concept, an embodiment of the invention further provides a pixel circuit, as illustrated in
A source of the drive transistor M0 is connected with a first reference signal end Ref1, a drain of the drive transistor M0 is connected respectively with a signal input end 1a of the first switch element T1 and a signal input end 5a of the fifth switch element T5, and a gate of the drive transistor M0 is connected respectively with a second end m2 of the first capacitor C1, a signal output end 3b of the third switch element T3 and a signal output end 1b of the first switch element T1; and a control end 1c of the first switch element T1 is connected with a second scan signal end Scan2.
A signal input end 2a of the second switch element T2 is connected with a data signal end Data, a signal output end 2b of the second switch element T2 is connected respectively with a first end m1 of the first capacitor C1 and a signal output end 4b of the fourth switch element T4, and a control end 2c of the second switch element T2 is connected with a reset control signal end RS.
A signal input end 3a of the third switch element T3 is connected with a reset signal end Rset, and a control end 3c of the third switch element T3 is connected with a first scan signal end Scan1.
A signal input end 4a of the fourth switch element T4 is connected with a second reference signal end Ref2, and a control end 4c of the fourth switch element T4 is connected respectively with a control end 5c of the fifth switch element T5 and a light emission control signal end EM.
A first end o1 of the light emitting element D1 is connected with a signal output end 5b of the fifth switch element T5, and a second end o2 of the light emitting element D1 is connected with a third reference signal end Ref3.
In the above pixel circuit according to the embodiment of the invention, the signal input end of the third switch element is connected with the drain of the drive transistor, and the signal output end of the third switch element is connected with the gate of the drive transistor and the first capacitor, so a drift of the threshold voltage in the drive transistor can be compensated for by the third switch element and the first capacitor, so that an operating current at which the drive transistor drives the light emitting element to emit light can be only related to the voltage of the data signal input at the data signal end and the voltage at the second reference signal end but independent of the threshold voltage of the drive transistor to thereby avoid the influence of the threshold voltage on the light emitting element, so as to stabilize the operating current driving the light emitting element to emit light and improve the uniformity of image brightness in the display area of the display device.
The operation principle of the above pixel circuit according to the embodiment of the invention will be described below briefly.
Particularly the above pixel circuit according to the embodiment of the invention operates in three phases which are a reset phase, a compensation phase and a light emission phase respectively.
In the reset phase, the third switch element writes a reset signal transmitted from the reset signal end into the second end of the first capacitor under control of the first scan signal end. That is, in this phase, the first scan signal end controls the third switch element to be turned on, and the turned-on third switch element writes the reset signal Vrset transmitted from the reset signal end into the second end of the first capacitor, so that the voltage at the second end of the first capacitor is Vrset to thereby ensure that the voltage at the gate of the drive transistor is Vrset in this phase.
In the compensation phase, the second switch element writes a data signal transmitted from the data signal end into the first end of the first capacitor under control of the reset control signal end, and the drive transistor charges the first capacitor through the first switch element under control of the second scan signal end. That is, in this phase, the second scan signal end controls the first switch element to be turned on, and the turned-on first switch element changes the drive transistor into a diode, so that after turning on the diode, the voltage Vref1 at the first reference signal end charges the first capacitor until the voltage at the second end of the first capacitor is Vref1−|Vth|, thereby achieving the storing of the threshold voltage |Vth| of the drive transistor at the gate of the drive transistor.
In the light emission phase, all of the fourth switch element, the fifth switch element and the first capacitor control the drive transistor to drive the light emitting element to emit light under control of the light emission control signal end. That is, in this phase, the light emission control signal end controls the fourth switch element and the fifth switch element to be turned on, and the turned-on fourth switch element writes the voltage Vref2 at the second reference signal end into the first end of the first capacitor and makes the drive transistor operate in the saturation state, and the turned-on fifth switch element connects the drain of the drive transistor with the light emitting device to thereby drive the light emitting device to emit light.
In the above pixel circuit according to the embodiment of the invention, the operating current at which the drive transistor drives the light emitting element to emit light can be only related to the voltage of the data signal input at the data signal end and the voltage at the second reference signal end but independent of the threshold voltage of the drive transistor to thereby avoid the influence of the threshold voltage on the light emitting element, that is, an image at the same brightness can be obtained when the same data signal and the same second reference signal are loaded to different pixel units, to thereby improve the uniformity of the image brightness in the display area of the display device.
In a particular implementation, the light emitting element D1 in the above pixel circuit according to the embodiment of the invention is generally an Organic Light Emitting Diode (OLED). The light emitting element D1 is operated to emit light for display under the action of the saturated current of the drive transistor M0.
In a particular implementation, in the above pixel circuit according to the embodiment of the invention, the drive transistor M0 driving the light emitting element to emit light is generally a P-type transistor. The threshold voltage Vth of the P-type transistor is negative, so in order to ensure the normal operation of the drive transistor M0, the voltage at the first reference signal end Ref1 needs to be a positive voltage, and the voltage at the third reference signal end Ref3 needs to be lower than the voltage at the first reference signal end Ref1. The voltage at the third reference signal end Ref3 being zero will be taken as an example for illustration throughout the following description.
Particularly in a particular implementation, in the above pixel circuit according to the embodiment of the invention, all of the first switch element, the second switch element, the third switch element, the fourth switch element, and the fifth switch element are switch transistors.
It shall be noted that the drive transistor and the switch transistors described in the above embodiments of the invention can be Thin Film Transistors (TFTs) or Metal Oxide Semiconductor (MOS) field effect transistors, and the invention will not be limited in this regard. In a particular implementation, the sources and the drains of these transistors can be interchanged without being distinguished from each other. The particular embodiments are described by taking the drive transistor and the switch transistors, all of which are thin film transistors, as an example.
Particularly in a particular implementation, in the above pixel circuit according to the embodiment of the invention, the sources of the switch transistors are generally the signal input ends of the switch elements, the drains of the switch transistors are generally the signal output ends of the switch elements, and the gates of the switch transistors are generally the control ends of the switch elements.
In a particular implementation, in the above pixel circuit according to the embodiment of the invention, the switch transistor used as the first switch element, the second switch element, the third switch element, the fourth switch element or the fifth switch element can be an N-type transistor or a P-type transistor, and the invention will not be limited in this regard. When the switch transistor is an N-type transistor, the switch transistor is turned on when the signal received at the gate of the switch transistor is at a high level; and when the switch transistor is a P-type transistor, the switch transistor is turned on when the signal received at the gate of the switch transistor is at a low level.
Preferably in order to simplify the process flow of fabricating the pixel circuit, in the above pixel circuit according to the embodiment of the invention, as illustrated in
Preferably in order to simplify the circuit structure, in the above pixel circuit according to the embodiment of the invention, as illustrated in
Preferably in order to simplify the circuit structure, in the above pixel circuit according to the embodiment of the invention, as illustrated in
Preferably in order to further simplify the circuit structure, in the above pixel circuit according to the embodiment of the invention, as illustrated in
Preferably in order to simplify the fabrication process, in the above pixel circuit according to the embodiment of the invention, both the fourth switch element and the fifth switch element are P-type transistors or N-type transistors, and the invention will not be limited in this regard. Preferably in order to simplify the circuit structure, in the above pixel circuit according to the embodiment of the invention, as illustrated in
Preferably in the above pixel circuit according to the embodiment of the invention, in order to stabilize the voltage at the gate of the drive transistor, as illustrated in
A first end n1 of the second capacitor C2 is connected with the first reference signal end Ref1, and a second end n2 of the second capacitor C2 is connected with the gate of the drive transistor M0.
Preferably all of the drive transistor and the switch transistors used as the switch elements mentioned in the above pixel circuit according to the embodiment of the invention can be embodied as P-type transistors to thereby simplify the process flow of fabricating the pixel circuit. Of course, in a particular implementation, in the above pixel circuit according to the embodiment of the invention, the drive transistor is embodied as a P-type transistor, and all of the switch transistors used as the switch elements can be embodied as N-type transistors.
The operation principle of the above pixel circuit will be described below in details by taking the drive transistor which is a P-type transistor, and the switch transistors, all of which are N-type transistors, in the pixel circuit, as an example.
Another Example:
Taking the pixel circuit illustrated in
In the reset phase T1, both the signal at the reset control signal end RS and the signal at the first scan signal end Scan1 are high-level signals, and the second switch element T2 and the third switch element T3 are turned on; and both the signal at the second scan signal end Scan2 and the signal at the light emission control signal end EM are low-level signals, and the first switch element T1, the fourth switch element T4, the fifth switch element T5 and the drive transistor M0 are turned off. The data signal Vdata at the data signal end Data is written into the first end of the first capacitor C1 through the second switch element T2, and the reset signal Vrset at the reset signal end Rset is written into the second end of the first capacitor C1 through the third switch element T3, so that the voltage at the first end of the first capacitor C1 is Vdata, and the voltage at the second end of the first capacitor C1 is Vrset, thereby resulting in the voltage of Vrset at the gate of the drive transistor M0 in the reset phase.
In the compensation phase T2, both the signal at the reset control signal end RS and the signal at the second scan signal end Scan2 are high-level signals, and the first switch element T1 and the second switch element T2 are turned on, and at the same time the turned-on first switch element T1 changes the drive transistor M0 into a diode; and both the signal at the first scan signal end Scan1 and the signal at the light emission control signal end EM are low-level signals, and all of the third switch element T3, the fourth switch element T4 and the fifth switch element T5 are turned off. The data signal Vdata transmitted from the data signal end Data is written into the first end m1 of the first capacitor C1 so that the voltage at the first end m1 of the first capacitor C1 is Vdata; and after turning on the diode, the voltage Vref1 at the first reference signal end Ref1 charges the first capacitor C1 until the voltage at the second end m2 of the first capacitor C1 is Vref1−|Vth|. At this time the voltage difference across the first capacitor C1 is Vdata−Vref1+|Vth|, thereby achieving the storing of the threshold voltage |Vth| of the drive transistor M0 at the gate of the drive transistor M0.
In the light emission phase T3, the signal at the light emission control signal end EM is a high-level signal, and the fourth switch element T4, the fifth switch element T5 and the drive transistor M0 are turned on; and all of the signals at the reset control signal end RS, the first scan signal end Scan1 and the second scan signal end Scan2 are low-level signals, and the first switch element T1, the second switch element T2 and the third switch element T3 are turned off. The voltage Vref2 at the second reference signal end Ref2 is written into the first end m1 of the first capacitor C1 so that the voltage at the first end m1 of the first capacitor C1 is changed from Vdata to Vref2, and based upon the principle of charge conservation of the capacitor, in order to ensure that the voltage difference across the first capacitor C1 is still Vdata−|Vref1+|Vth|, the voltage at the second end m2 of the first capacitor C1 jumps from Vref1−|Vth| to Vref1−|Vth|+Vref2−Vdata. Since the drive transistor M0 operates in the saturation state, it can be seen from the current characteristic in the saturation state that the operating current IOLEDflowing through the drive transistor M0 and used to drive the light emitting element D1 to emit light satisfies the equation of IOLED=K(Vsg−|Vth|)2=K[Vref1−(Vref1|Vth|+Vref2−Vdata)−|Vth|]2=K(Vdata−Vref2)2, where K is a structural parameter, which is relatively stable and thus can be regarded as a constant in the same structure. As can be apparent, the operating current IOLED of the light emitting element D1 has been independent of the threshold voltage Vth of the drive transistor M0 but only related to the voltage Vdata of the signal input at the data signal end and the voltage Vref2 at the second reference signal end to thereby thoroughly avoid the operating current IOLED of the light emitting element D1 from being influenced by the drift of the threshold voltage Vth of the drive transistor due to the process flow and running for a long period of time, so as to ensure the normal operation of the light emitting element D1.
Another Example:
Taking the pixel circuit illustrated in
In the reset phase T1, the operation principle thereof is the same as the operation principle in the reset phase in the above example. In this phase, the voltage at the first end m1 of the first capacitor C1 is Vdata, and the voltage at the second end m2 of the first capacitor C1 is Vrset, thereby resulting in the voltage of Vrset at the gate of the drive transistor M0 in the reset phase.
In the compensation phase T2, the operation principle thereof is the same as the operation principle in the compensation phase in the above example. However the second capacitor C2 is added to the compensation control module, so in this phase, the voltage Vref1 at the first reference signal end Ref1 also charges the second capacitor C2 while charging the first capacitor C1 so that the voltage difference across the first capacitor C1 is Vdata−Vref1+|Vth|, and the voltage difference across the second capacitor C2 is |Vth|, thereby achieving the storing of the threshold voltage |Vth| of the drive transistor M0 at the gate of the drive transistor M0.
In the light emission phase T3, the operation principle thereof is the same as the operation principle in the light emission phase in the above example. However the voltage at the gate of the drive transistor M0 is Vref1−|Vth|+(Vref2−Vdata)Cst1/(Cst1+Cst2) due to the coupling of the first capacitor C1 and the second capacitor C2, and the voltage at the gate of the drive transistor M0 will be relatively stable due to the addition of the second capacitor C2. Since the drive transistor M0 operates in the saturation state, it can be seen from the current characteristic in the saturation state that the operating current IOLED flowing through the drive transistor M0 and used to drive the light emitting element D1 to emit light satisfies the equation of IOLED=K(Vsg−|Vth|)2=K{Vref1−[Vref1−|Vth|+(Vref2−Vdata)Cst1/(Cst1+Cst2)]−|Vth|]}2=K[(Vdata−Vref2)Cst1/(Cst1+Cst2)]2, where all of K, Cst1 and Cst2 are structural parameters, which are relatively stable and thus can be regarded as constants in the same structure. As can be apparent, the operating current IOLED of the light emitting element D1 has been independent of the threshold voltage Vth of the drive transistor M0 but only related to the voltage Vdata of the signal input at the data signal end and the voltage Vref2 at the second reference signal end to thereby thoroughly avoid the operating current IOLED of the light emitting element D1 from being influenced by the drift of the threshold voltage Vth of the drive transistor due to the process flow and running for a long period of time, so as to ensure the normal operation of the light emitting element D1.
The operation principle has been described here only by taking the two structures of the pixel circuit according to the embodiment of the invention as examples, and the operation principle of the pixel circuit according to the embodiment of the invention in other structures is the same as that in the above examples, and a repeated description thereof will be omitted here.
Based upon the same inventive concept, an embodiment of the invention further provides an organic electroluminescent display panel which includes a plurality of the pixel circuits according to any one of the above embodiments of the invention. Since the organic electroluminescent display panel addresses the problem under a similar principle to the pixel circuit described above, for an implementation of the organic electroluminescent display panel, reference can be made to the implementation of the pixel circuit, and a repeated description thereof will be omitted here.
Preferably in order to simplify the circuit structure, in the above organic electroluminescent display panel according to the embodiment of the invention, as illustrated in
Preferably in order to simplify the circuit structure, in the above organic electroluminescent display panel according to the embodiment of the invention, when the pixel circuit is structurally embodied particularly as the above five switch transistors and one drive transistor according to the above embodiment of the invention, the reset signal end of each pixel circuit in the other rows than the first row in the organic electroluminescent display panel can also be connected with the drain of the fifth switch transistor or the signal output end of the fifth switch element in the previous pixel circuit.
Based upon the same inventive concept, an embodiment of the invention further provides a display device which includes the above organic electroluminescent display panel according to the embodiment of the invention, and the display device can be a display, a handset, a TV set, a notebook computer, an all-in-one machine and the like.It should be understood by those ordinarily skilled in the art that all the other components indispensable to the display device are included, so a repeated description thereof will be omitted here, and the invention will not be limited in this regard.
The embodiments of the invention provide a pixel circuit, an organic electroluminescent display panel and a display device. The pixel circuit includes: a light emitting element, a first capacitor, a reset control module, a drive control module, a compensation control module and a light emission control module. In a reset phase, the reset control module writes a reset signal transmitted from the reset signal end into the second end of the first capacitor under control of the first scan signal end. In a compensation phase, the reset control module writes a data signal transmitted from the data signal end into the first end of the first capacitor under control of the reset control signal end, and the drive control module charges the first capacitor through the compensation control module under control of the second scan signal end; and in a light emission phase, both the light emission control module and the first capacitor control the drive control module to drive the light emitting element to emit light under control of the light emission control signal end. The compensation control module can compensate for the drift of the threshold voltage in the drive control module in the compensation phase, so in the light emission phase, the operating current at which the drive control module drives the light emitting element to emit light can only be related to the voltage of the data signal input at the data signal end and the voltage at the second reference signal end but independent of the threshold voltage in the drive control module to thereby avoid the influence of the threshold voltage on the light emitting element, so as to stabilize the operating current driving the light emitting element to emit light and improve the uniformity of image brightness in the display area of the display device.
Evidently those skilled in the art can make various modifications and variations to the invention without departing from the spirit and scope of the invention. Thus the invention is also intended to encompass these modifications and variations thereto as long as the modifications and variations come into the scope of the appended claims and their equivalents.
This application is a continuation of U.S. patent application Ser. No. 14/463,511 which claims the benefit of priority to Chinese Patent Application No. 201410253894.5, filed with the Chinese Patent Office on Jun. 9, 2014 and entitled “PIXEL CIRCUIT, ORGANIC ELECTROLUMINESCE DISPLAY PANEL AND DISPLAY DEVICE”, the content of which is incorporated herein by reference in its entirety.
| Number | Name | Date | Kind |
|---|---|---|---|
| 8780102 | Han | Jul 2014 | B2 |
| 20050243036 | Ikeda | Nov 2005 | A1 |
| 20080158205 | Park | Jul 2008 | A1 |
| 20090167648 | Jeon | Jul 2009 | A1 |
| 20110090213 | Han | Apr 2011 | A1 |
| 20110134100 | Chung | Jun 2011 | A1 |
| 20130194248 | Kim | Aug 2013 | A1 |
| 20140333515 | In | Nov 2014 | A1 |
| 20150002557 | Ishii | Jan 2015 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20160275866 A1 | Sep 2016 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14463511 | Aug 2014 | US |
| Child | 15167863 | US |
