The present invention relates to display technology, more particularly, to a pixel circuit for an active matrix organic light emitting diode display panel and a method for threshold voltage non-uniformity compensation associated with the pixel circuit.
Organic light emitting diode (OLED) display apparatuses are self-emissive devices, and do not require backlights. OLED display apparatuses also provide more vivid colors and a larger color gamut as compared to the conventional liquid crystal display (LCD) apparatuses. Further, OLED display apparatuses can be made more flexible, thinner, and lighter than a typical LCD apparatus.
An OLED display apparatus typically includes an anode, an organic layer including a light emitting layer, and a cathode. OLEDs can be either a bottom-emission type OLED or a top-emission type OLED. In bottom-emission type OLEDs, the light is extracted from an anode side. In bottom-emission type OLEDs, the anode is generally transparent, while a cathode is generally reflective. In a top-emission type OLED, light is extracted from a cathode side. The cathode is optically transparent, while the anode is reflective.
In an aspect, the present disclosure provides a pixel circuit in an active matrix organic light-emitting diode (AMOLED) display panel. The pixel circuit includes a first transistor comprising a bottom gate and a top gate, a drain supplied with a high-level power-supply voltage, and a source coupled to a light-emitting diode (LED). The bottom gate is provided with a first voltage signal and the source is provided with a second voltage signal in a compensation period during which a present value of a threshold voltage of the first transistor is sensed at the source and a third voltage signal is determined based on the present value of the threshold voltage. The top gate is configured to be provided with the third voltage signal in an emission period to reduce the present value of the threshold voltage.
Optionally, the LED is an organic light-emitting diode (OLED) comprising an anode coupled to the source of the first transistor and a cathode being supplied with a low-level power-supply voltage. The OLED is configured in the emission period to emit light induced by a driving current provided by the first transistor. The driving current is a turn-on current of the first transistor substantially independent of the threshold voltage.
Optionally, the pixel circuit further includes a second transistor comprising a source coupled to the bottom gate of the first transistor, a drain coupled to a data voltage port, and a gate controlled by a first control signal; a third transistor comprising a source coupled to the source of the first transistor, a drain coupled to a voltage sensing port, a gate controlled by the first control signal; a fourth transistor comprising a source coupled to the top gate of the first transistor, a drain coupled to a voltage compensation port, and a gate controlled by a second control signal; a first capacitor comprising a first electrode coupled to the bottom gate of the first transistor and a second electrode coupled to the source of the first transistor; and a second capacitor comprising a first electrode coupled to the drain of the first transistor and a second electrode coupled to the top gate of the first transistor.
Optionally, the first control signal is a high-level voltage to turn the second transistor and the third transistor on and the second control signal is a low-level voltage to keep the fourth transistor off in a reset sub-period of the compensation period. The first control signal remains to be the high-level voltage, the second control signal remains to be the low-level voltage in a charge sub-period of the compensation period subsequent to the reset sub-period.
Optionally, the data voltage port is configured to provide a first high-level voltage signal as the first voltage signal to set a high potential level at the bottom gate in the reset sub-period and the voltage sensing port is configured to provide the second voltage signal as a low-level voltage signal to set a low potential level at the source of the first transistor in the reset sub-period.
Optionally, the data voltage port is configured to provide a second high-level voltage signal as the first voltage signal in the charge sub-period. The voltage sensing port is configured to be floated by cutting off the second voltage signal in the charge sub-period. The high potential level at the bottom gate turns the first transistor on to allow the source of the first transistor is charged by the high-level power-supply voltage until a potential level of the source of the first transistor is equal to the high potential level at the bottom gate minus the present value of the threshold voltage of the first transistor.
Optionally, the voltage sensing port that is floated is used to detect the potential level at the source of the first transistor as a sensed voltage by a controller to deduce the present value of the threshold voltage based on the sensed voltage.
Optionally, the present value of the threshold voltage is used by the controller to determine the third voltage signal based on a pre-stored information about a correspondence relationship between a top-gate voltage and a threshold voltage of the first transistor. The third voltage signal is selected from a value of the top-gate voltage that corresponds to a threshold voltage having an absolute value substantially the same as the present value of the threshold voltage but with opposite sign.
Optionally, the first control signal is a high-level voltage to turn on the second transistor to allow the first voltage signal as a data signal to be applied from the data voltage port to the bottom gate and turn on the third transistor to allow the second voltage signal as a low-level voltage signal to be applied from the voltage sensing port to the source of the first transistor in the emission period. The second control signal is a high-level voltage to turn on the fourth transistor to allow the third voltage signal to be applied via the voltage compensation port to the top gate, thereby resulting in a changed value of threshold voltage to be substantially zero. A turn-on current of the first transistor is provided to the LED as a light-emitting driving current substantially independent of the changed value of threshold voltage.
Optionally, the turn-on current through the first transistor is substantially independent of the low-level power-supply voltage supplied to the cathode of the LED.
Optionally, the pixel circuit is one of a plurality of pixel circuits in the AMOLED display panel. The correspondence relationship between a top-gate voltage and a threshold voltage of the first transistor of each one of the plurality of pixel circuits is stored in the controller which is configured to sense a present value of the threshold voltage from a corresponding voltage sensing port of each of the plurality of pixel circuits and provide a corresponding third voltage signal to a corresponding voltage compensation port of the each of the plurality of pixel circuits based on the present value of the threshold voltage sensed by the controller.
Optionally, the compensation period is followed by a holding period before the emission period starts, during the holding period the first voltage signal and the second voltage signal are provided with low-level voltages.
In another aspect, the present disclosure provides an active matrix organic light emitting diode (AMOLED) display panel comprising a matrix of pixel circuits. Each pixel circuit in the matrix includes a first transistor comprising a bottom gate and a top gate, a drain supplied with a high-level power-supply voltage, and a source coupled to a light emitting diode (LED). The bottom gate is provided with a first voltage signal and the source is provided with a second voltage signal in a compensation period during which a present value of a threshold voltage of the first transistor is sensed at the source and a third voltage signal is determined based on the present value of the threshold voltage. The top gate is configured to be provided with the third voltage signal in an emission period to reduce the present value of the threshold voltage. The LED is an organic light-emitting diode comprising an anode coupled to the source of the first transistor and a cathode being supplied with a low-level power-supply voltage, the LED being configured in the emission period to emit light induced by a driving current provided by the first transistor that is a turn-on current substantially independent of the threshold voltage.
Optionally, each pixel circuit in the matrix further includes a second transistor comprising a source coupled to the bottom gate of the first transistor, a drain coupled to a data voltage port, and a gate controlled by a first control signal; a third transistor comprising a source coupled to the source of the first transistor, a drain coupled to a voltage sensing port, a gate controlled by the first control signal; a fourth transistor comprising a source coupled to the top gate of the first transistor, a drain coupled to a voltage compensation port, and a gate controlled by a second control signal; a first capacitor comprising a first electrode coupled to the bottom gate of the first transistor and a second electrode coupled to the source of the first transistor; and a second capacitor comprising a first electrode coupled to the drain of the first transistor and a second electrode coupled to the top gate of the first transistor.
Optionally, each of pixel circuits receives the first voltage signal from the data voltage port and the second voltage signal from the voltage sensing port in the compensation period to allow the present value of the threshold voltage of the first transistor to be deduced from a sense voltage detected via the voltage sensing port by a controller to determine a corresponding value for the third voltage signal to be applied to the voltage compensation port in the emission period.
Optionally, the controller is configured to pre-store a correspondence relationship between a top-gate voltage and a threshold voltage of the first transistor of each pixel circuit in the matrix and to determine the third voltage signal individually for each pixel circuit in the compensation period based on the present value of the threshold voltage deduced individually for each pixel circuit.
Optionally, the controller is further configured to apply the third voltage signal in the emission period to the top gate of the first transistor via the corresponding voltage compensation port of a corresponding pixel circuit to change the threshold voltage of the first transistor of the corresponding pixel circuit to substantially zero.
In yet another aspect, the present disclosure provides a display apparatus including an AMOLED display panel described herein and a controller coupled to the AMOLED display panel and configured to pre-store a correspondence relationship between a top-gate voltage and a threshold voltage of the first transistor of each pixel circuit in the matrix. The controller is further configured to determine the third voltage signal individually for each pixel circuit in the compensation period based on the present value of the threshold voltage deduced individually for each pixel circuit. The controller also is configured to apply the third voltage signal in the emission period to the top gate of the first transistor via the corresponding voltage compensation port of a corresponding pixel circuit to reduce the threshold voltage of the first transistor of each pixel circuit.
In still another aspect, the present disclosure provides a method of compensating a threshold voltage of a driving transistor of a pixel circuit of an AMOLED display panel. The method includes providing a dual-gate transistor as the driving transistor in the pixel circuit. The dual-gate transistor includes a bottom gate and a top gate. The method further includes providing a first voltage signal to the bottom gate and a second voltage signal to the source in a compensation period to sense a present value of a threshold voltage of the driving transistor. Additionally, the method includes determining a third voltage signal based on the present value of the threshold voltage. Furthermore, the method includes applying the third voltage signal to the top gate in an emission period of the operation timing to change the present value of the threshold voltage to proximately zero.
Optionally, the method of providing the first voltage signal to the bottom gate and the second voltage signal to the source in the compensation period includes providing a first high-level voltage signal as the first voltage signal to the data voltage port and providing a low-level voltage signal as the second voltage signal to the voltage sensing port in a reset sub-period of the compensation period, during which the first control signal is a high-level voltage to turn the second transistor and the third transistor on and the second control signal is a low-level voltage to turn the fourth transistor off.
Optionally, the method of providing the first voltage signal to the bottom gate and the second voltage signal to the source in the compensation period further includes providing a second high-level voltage signal as the first voltage signal to the data voltage port and leaving the voltage sensing port to be floated in a charge sub-period of the compensation period, during which the first control signal remains the high-level voltage and the second control signal remains the low-level voltage to allow charging of the source of the dual-gate transistor to reach a potential level equal to that of the second high-level voltage signal minus the present value of the threshold voltage of the dual-gate transistor so that a driving chip can deduce the present value of the threshold voltage by sensing the potential level at the source of the dual-gate transistor via the voltage sensing port.
Optionally, the method of determining the third voltage signal includes selecting a top-gate voltage of the dual-gate transistor that corresponds to a threshold voltage the same as the present value but with an opposite sign based on a correspondence relationship between the top-gate voltage and the threshold voltage of the dual-gate transistor pre-stored in the driving chip.
Optionally, the method of applying the third voltage signal to the top gate in an emission period comprises applying the third voltage signal to the voltage compensation port in the emission period during which each of the first control signal and the second control signal is a high-level voltage to turn the second transistor, the third transistor, and the fourth transistor on, the first voltage signal is provided as a data signal to the data voltage port and the second voltage signal is provided as a low-level voltage signal to the voltage sensing port. The third voltage signal is passed to the top gate of the dual-gate transistor to reduce the threshold voltage and a turn-on current of the driving transistor is induced by high-potential level of the data signal and provided as a driving current to cause the LED to emit light. The turn-on current is substantially independent of the threshold voltage of the dual-gate transistor.
The following drawings are merely examples for illustrative purposes according to various disclosed embodiments and are not intended to limit the scope of the present disclosure.
The disclosure will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of some embodiments are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed.
Typical AMOLED display panels use thin-film transistor (TFT) to construct the pixel circuits to provide driving current for the organic light-emitting diodes (OLED). The TFTs in the pixel circuits usually are low-temperature poly-silicon thin-film transistors (LTPS TFT) or oxide thin-film transistor (Oxide TFT). Both LTPS TFT and Oxide TFT have a higher mobility and more stable characteristics compared to the amorphous-Si TFT, and thus is more suitable to be used in an AMOLED display. However, due to the limitation of the crystallization process, LTPS TFTs, which are manufactured on a large glass substrate, have non-uniformity in electrical parameters such as threshold voltage, mobility, etc., and such non-uniformity may result in variances of current and luminance of OLED which can be perceived by human eyes, i.e., Mura phenomenon. Oxide TFTs can be made with much better uniformity on large area substrate. But after long-time operation driven by voltages and under high temperature, the threshold voltages of the Oxide TFTs drift. In a large area display panel, different TFTs at different locations have different drifts of threshold voltages due to variations of a displayed image at different pixels, causing variations in display intensities. Because of this type of variations is related to a previously displayed image, it results in an image blur phenomenon.
In a large size display application, there is a certain resistance in the power cord of the backboard, and all of pixels are provided with driving current by the positive power supply (ARVDD) of the backboard, so the supply voltage in the area near the location of the power supply ARVDD is higher than that in the area located far from the location of the power supply ARVDD, and such phenomenon is called IR Drop. As the current of OLED depends on the voltage of ARVDD, IR Drop also results in variances of current in different areas, and Mura phenomenon in turn occurs in display.
Moreover, there is also the non-uniformity in electrical parameters due to the non-evenness of the film thickness generated when OLED device is evaporated. For forming pixel circuits with either amorphous-Si or oxide based N-type TFT, a storage capacitor is used to be coupled between a gate electrode of a driving TFT and an anode of OLED. When data voltage signal is transmitted to the gate electrode, the actual gate-to-source voltage Vgs applied on the driving TFT is different if the anode voltage of OLED of each pixel circuit is different. This causes different driving currents in different OLED which in turn cause different display intensities among different pixels.
Voltage programming pixel-driving method for AMOLED pixel circuit is commonly used which is similar to traditional AMLCD pixel-driving method. A driving chip (integrated circuit) provides a gray-scale voltage signal which can be transformed to a gray-scale current signal of the driving TFT within the pixel circuit to drive the OLED light emission to achieve gray-scale intensity. This pixel-driving method has been widely used because of attributes such as fast driving speed, simple structure, and suitability for large size panel, etc.
where mn is carrier mobility, Cox is gate oxide layer capacitance, W/L is width to length ratio of the driving transistor, Vdata is data signal voltage. VOLED is OLED working voltage shared by all pixel circuits. Vthn is a threshold voltage of the driving transistor which has a positive value for an enhanced type TFT and a negative value for depletion type TFT. Based on the above expression of the driving current associated with the 2T1C pixel circuit, the driving current may be different in different pixel circuit if the threshold voltage Vthn is different. As the threshold voltage of the driving transistor associated with the pixel circuit drifts with time, it causes different driving current to change with time, which in turn causes image blur phenomenon. Therefore, the 2T1C pixel circuit needs to add extra TFTs and capacitors to design a circuit with a compensation function for compensating the TFT non-uniformity and OLED non-uniformity.
Because of non-uniformity of TFT threshold voltages and OLED devices, pixel circuits of AMOLED display panels need to implement a compensation mechanism in one way or another to correct either the Mura phenomenon or Blur phenomenon, especially for large sized display panel. A conventional pixel circuit with 3T1C structure for compensating TFT threshold voltage drift includes a driving transistor T1, a switching transistor T2, and a sensing transistor T3, one storage capacitor Cst, a first power line for supplying a high-potential voltage VDD, a second power line for supplying a low-potential voltage VSS (lower than the high-potential voltage VDD), a reference line for supplying a reference voltage Vsense which is lower than the high-potential voltage VDD and higher than the low-potential voltage VSS. The switching transistor T2 is controlled by a gate-driving signal Vdata applied to the gate node, and is electrically connected between a node N1 of the driving transistor T1 and a data line. The storage capacitor Cst, connected between the node N1 and a node N2, serves to maintain a predetermined voltage for a one-frame time. The sensing transistor T3 is controlled by the gate-driving signal Vdata applied to the gate node to apply a reference voltage Vsense supplied through the reference voltage line to the second node N2 (e.g. the source node of the driving transistor T1) and also allow a driving chip connected to the reference voltage line to sense the voltage at the node N2. Based on this circuit structure, a sensing driving operation of the AMOLED pixel circuit is performed in three periods of time: a sensing period, a compensation period, and an emission period to achieve a compensation of the threshold voltage of the driving transistor so that the driving current of OLED device is substantially independent of the threshold voltage. However, compensation of the threshold voltage using the sensing driving operation based on the above 3T1C pixel circuit is limited by a certain range of the threshold voltage. If the drift of the threshold voltage becomes too large during working process of the AMOLED display panel, the value of threshold voltage may surpass the certain range so that the drift of the threshold voltage may not be fully compensated. In other words, the compensation accuracy for some pixel circuits will be lowered, leading to poor effect on correcting non-uniformities in TFT threshold voltages of large AMOLED display panel.
Accordingly, the present invention provides, inter alia, a pixel circuit, an AMOLED display panel and a display apparatus having the same, and a pixel-driving method thereof that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.
In one aspect, the present disclosure provides a pixel circuit of an AMOLED display panel that is capable of controlling the drift of threshold voltage of the driving transistor. Both the drift direction and drift value can be controlled so that the non-uniformity issue due to large drift of threshold voltage of the driving transistor can be substantially eliminated.
In some embodiments, the driving transistor T1 provided with a dual-gate transistor, the switching transistor T2, and the sensing transistor T3 plus the first capacitor C1 as a part of the present pixel circuit provide a function of compensating a drift of the threshold voltage of the driving transistor T1 during normal work condition of the driving transistor by providing a driving current for the OLED in an emission period to be substantially independent of the threshold voltage. The bottom gate BG of the dual-gate transistor is controlled by the switching transistor T2. The top gate TG of the dual-gate transistor is controlled by the controlling transistor T4 to tune its potential level so that the threshold voltage of the driving transistor T1 can be controlled. In particular, both an absolute value and a sign of the threshold voltage can be controlled since by applying different top-gate voltages to the top gate of the dual-gate transistor the threshold voltage thereof can be effectively changed from a positive value to a negative value or vice versa (as shown in an example in
In some embodiments, the first capacitor C1 is directly coupled between the bottom gate BG (i.e., the node N1) and the source of driving transistor T1 (i.e., the node N2) as a storage capacitor to provide sufficient capacitance for stabilizing a potential level difference Vgs between the gate and the source of the driving transistor T1. In some embodiments, the second capacitor C2 is directly coupled between the drain and the top gate TG of the driving transistor T1 to provide a sufficient capacitance for stabilizing a potential level at the top gate TG after it is charged from the voltage compensation port by the third voltage signal Vtg.
The operation of the pixel circuit can be executed for at least one cycle per pixel (of driving the OLED of the pixel to emit light). The compensation period of each cycle includes a reset sub-period followed by a charge sub-period. In the reset sub-period, the first control signal G1 is a high-level voltage sufficient to turn on the second transistor T2 and also turn on the third transistor T3. The second control signal G2 is a low-level voltage to turn the fourth transistor T4 off. The first voltage signal Vdata is provided, by the controller, as a first high-level voltage signal VGM supplied to the data voltage port. The second transistor T2 is turned on to pass the first high-level voltage signal to the node N1 which is the bottom gate BG of the driving transistor T1. Optionally, the first high-level voltage signal can be sufficiently high to turn the driving transistor T1 on. At the reset sub-period, the second voltage signal Vsense is provided, also by the controller, as a low-level voltage signal Vrefl to the voltage sensing port and passed to the node N2 which is the source of the driving transistor T1 as the third transistor T3 is turned on. In the reset sub-period, the third voltage signal Vtg is set off and the fourth transistor is turned off by the second control signal G2 set at the low-level voltage. The potential levels at both sides of the first capacitor C1 are set and prepared for the next charge sub-period. The potential level VOLED at the anode of the OLED is low so that no light is emitted.
Referring to
In some embodiments, the controller is configured to be a driving chip disposed along with the AMOLED display panel. Whenever an AMOLED display panel finishes its process of laying out all those thin-film transistors (TFTs) on a glass substrate to form a matrix of pixel circuits, each of the TFTs is subjected to multiple IV tests. At least for each driving transistor, which is a dual-gate transistor having a top gate and a bottom gate configured as shown in
Referring to
Referring to
In another embodiment, since the driving current is only depended on the voltage levels at the bottom gate BG and the source N2 respectively set to be the Vdata from the first voltage signal and the Vrefl from the second voltage signal, which are completely independent of the low-level power-supply voltage VSS supplied to the cathode of the OLED device, the driving current then is also substantially free of impact of any variation in the low-level power-supply voltage VSS. Therefore, the pixel circuit of
Referring to
In another aspect, the present disclosure provides an AMOLED display panel having a matrix of pixel circuits with each pixel circuit being configured the same way as shown in
Optionally, the controller is configured to pre-store a correspondence relationship (e.g., a one-to-one correspondence relationship) between a top-gate voltage and a threshold voltage of the first transistor of each pixel circuit in the matrix and to determine the third voltage signal individually in the compensation period based on the present value of the threshold voltage deduced individually for each pixel circuit.
Optionally, the controller is further configured to apply the third voltage signal in the emission period to the top gate of the first transistor via the corresponding voltage compensation port of a corresponding pixel circuit to reduce the threshold voltage of the first transistor of each pixel circuit to substantially zero.
In another aspect, the present disclosure provides a display apparatus including an AMOLED display panel described herein and a controller coupled to the AMOLED display panel and configured to pre-store a correspondence relationship (e.g., a one-to-one correspondence relationship) between a top-gate voltage and a threshold voltage of the first transistor of each pixel circuit in the matrix. The controller is further configured to determine the third voltage signal individually in the compensation period based on the present value of the threshold voltage deduced individually for each pixel circuit. The controller is additionally configured to apply the third voltage signal in the emission period to the top gate of the first transistor via the corresponding voltage compensation port of a corresponding pixel circuit to reduce the threshold voltage of the first transistor of each pixel circuit to substantially zero. Examples of appropriate display apparatuses include, but are not limited to, an electronic paper, a mobile phone, a tablet computer, a television, a monitor, a notebook computer, a digital album, a GPS, etc.
In yet another aspect, the present disclosure provides a method of compensating threshold voltage of driving transistor of a pixel circuit of an AMOLED display panel. In some embodiments, the method includes providing a dual-gate transistor as the driving transistor in the pixel circuit. The dual-gate transistor has a bottom gate and a top gate.
Optionally, the method includes providing the dual-gate transistor to form a pixel circuit described herein.
Optionally, the method is executed according to a timing diagram described herein. Referring to
Referring to
Referring to
Referring to
The foregoing description of the embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form or to exemplary embodiments disclosed. Accordingly, the foregoing description should be regarded as illustrative rather than restrictive. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiments are chosen and described in order to explain the principles of the invention and its best mode practical application, thereby to enable persons skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use or implementation contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents in which all terms are meant in their broadest reasonable sense unless otherwise indicated. Therefore, the term “the invention”, “the present invention” or the like does not necessarily limit the claim scope to a specific embodiment, and the reference to exemplary embodiments of the invention does not imply a limitation on the invention, and no such limitation is to be inferred. The invention is limited only by the spirit and scope of the appended claims. Moreover, these claims may refer to use “first”, “second”, etc. following with noun or element. Such terms should be understood as a nomenclature and should not be construed as giving the limitation on the number of the elements modified by such nomenclature unless specific number has been given. Any advantages and benefits described may not apply to all embodiments of the invention. It should be appreciated that variations may be made in the embodiments described by persons skilled in the art without departing from the scope of the present invention as defined by the following claims. Moreover, no element and component in the present disclosure is intended to be dedicated to the public regardless of whether the element or component is explicitly recited in the following claims.
Filing Document | Filing Date | Country | Kind |
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PCT/CN2017/095577 | 8/2/2017 | WO | 00 |
Publishing Document | Publishing Date | Country | Kind |
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WO2019/023962 | 2/7/2019 | WO | A |
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