Pixel circuit and driving method thereof

A pixel circuit and a driving method thereof are provided. The pixel circuit includes first to second pixel electrodes, first to third liquid crystal capacitors, a first storage capacitor and first to third switches. The first liquid crystal capacitor and the first storage capacitor locate between the first pixel electrode and a first common voltage. The second liquid crystal capacitor locates between the first and the second pixel electrodes. The third liquid crystal capacitor locates between the second pixel electrode and the first common voltage. The first switch has ends for receiving a data voltage and a scan signal and coupled to the first pixel electrode. The second switch has ends for receiving a second common voltage and a reset signal and coupled to the first pixel electrode. The third switch has ends for receiving a reset voltage and the reset signal and coupled to the second pixel electrode.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan application serial no. 107107959, filed on Mar. 8, 2018. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a display apparatus, and particularly relates to a pixel circuit and a driving method of the pixel circuit.

2. Description of Related Art

Due to the emergence of liquid crystal display panels, the users' demands on the refreshing rate of the frames and the resolution quality of displays are becoming higher and higher. When the response speed of liquid crystal cells is not quick enough, the frame on the display panel may be blurred or not clear. Under the circumstance, the user's viewing experience may be affected.

In the known technology, since the uniform lying helix (ULH) structure liquid crystal exhibits properties such as a quick response time, a high transmittance ratio, and a low absorption rate, such structure is commonly adopted in display panels as the material for liquid crystal display panels. When driving the ULH structure liquid crystal, the designer often apply different applied electrical fields to deviate optical axes of liquid crystal molecules. However, during alternate changes between positive and negative electrical fields, some liquid crystal molecules may not be able to timely respond to the quick changes in the direction of an electrical field, and the arrangement of liquid crystal molecules may be disordered. Thus, the optical axes of liquid crystal molecules of the ULH structure liquid crystal that are driven may be deviated toward different direction and the overall transmittance ratio may be lowered. Therefore, how to reduce the lowering of the transmittance ratio in the display panel is now an issue to work on.

SUMMARY OF THE INVENTION

One or some exemplary embodiments of the invention provide a pixel circuit and a driving method of the pixel circuit. The pixel circuit and the driving method thereof are capable of resetting a uniform lying helix (ULH) structure liquid crystal in advance before the ULH structure liquid crystal is driven, and facilitating the re-arrangement of liquid crystal molecules of the ULH structure liquid crystal by applying a horizontal electrical field to the ULH structure liquid crustal, so as to reduce lowering of the transmittance ratio.

A pixel circuit according to an embodiment of the invention includes first to second pixel electrodes, first to third liquid crystal capacitors, a first storage capacitor and first to third switches. The first liquid crystal capacitor is located between the first pixel electrode and a first common voltage. The first storage crystal capacitor is located between the first pixel electrode and a first common voltage. The second liquid crystal capacitor is located between the first pixel electrode and the second pixel electrode. The third liquid crystal capacitor is located between the second pixel electrode and the first common voltage. A first switch has a first end receiving a data voltage, a control end receiving a scan signal, and a second end coupled to the first pixel electrode. A second switch has a first end receiving a second common voltage, a control end receiving a reset signal, and a second end coupled to the first pixel electrode. A third switch has a first end receiving a reset voltage, a control end receiving the reset signal, and a second end coupled to the second pixel electrode.

A driving method of a pixel circuit according to an embodiments of the invention. The pixel circuit has a first pixel electrode, a second pixel electrode, and a common electrode transmitting a first common voltage. A liquid crystal layer is disposed between the common electrode and the first pixel electrode as well as the second pixel electrode. The driving method includes the following. A second common voltage is provided to the first pixel electrode and a reset voltage is provided to the second pixel electrode during a reset period. A data voltage is provided to the first pixel electrode and the second pixel electrode is floating during a charging period. The first pixel electrode and the second pixel electrode are floating during an emitting period.

Based on the above, when the pixel circuit according to the embodiments of the invention is operated in the reset period, a horizontal electrical field formed between the first pixel electrode and the second pixel electrode may be adopted to restore liquid crystal molecules in the ULH structure liquid crystal to an initial or default state and rearrange the liquid crystal molecules of the ULH structure liquid crystal, so as to reduce lowering of the transmittance ratio.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1is a circuit diagram illustrating a pixel circuit100according to an embodiment of the invention. Referring toFIG. 1, in the embodiment, the pixel circuit100includes a first pixel electrode PX1, a second pixel electrode PX2, first to third liquid crystal capacitors C1to C3, a first storage capacitor Cst, and first to third switches M1to M3. As an example, the first to third switches M1to M3may be capacitors. However, the embodiments of the invention are not limited thereto.

In the embodiment, the first liquid crystal capacitor C1and the first storage capacitor Cst are located between the first pixel electrode PX1and a first common voltage Vcom1. The second liquid crystal capacitor C2is located between the first pixel electrode PX1and the second pixel electrode PX2. The third liquid crystal capacitor C3is located between the second pixel electrode PX2and the first common voltage Vcom1.

A drain (corresponding to the first end) of the first switch M1receives a data voltage Vdata, a gate (corresponding to the control end) of the first switch M1receives a scan signal Scan, and a source (corresponding to the second end) of the first switch M1is coupled to the first pixel electrode PX1. A drain (corresponding to the first end) of the second switch M2receives a second common voltage Vcom2, a gate (corresponding to the control end) of the second switch M2receives a reset signal Reset, and a source (corresponding to the second end) of the second switch M2is coupled to the first pixel electrode PX1. A drain (corresponding to the first end) of the third switch M3receives a reset voltage Vreset, a gate (corresponding to the control end) of the third switch M3receives the reset signal Reset, and a source (corresponding to the second end) of the third switch M3is coupled to the second pixel electrode PX2.

Based on the above, the pixel circuit100of the embodiment may control whether the first switch M1is turned on or off by using the scan signal Scan, so as to control whether the data voltage Vdata is written into the pixel circuit100. In addition, whether the second switch M2and the third switch M3are turned on or off may be controlled by using the reset signal Reset, so as to control whether the second common voltage Vcom2and the reset voltage Vreset are respectively provided to the first pixel electrode PX1and the second pixel electrode PX2.

Specifically, the first pixel electrode PX1in the embodiment may be a sheet electrode (not shown), and the second pixel electrode PX2may be a patterned electrode (not shown). In addition, the patterned electrode may exhibit a pattern of a comb-like structure. However, the invention is not limited thereto. The first pixel electrode PX1as a sheet electrode and the second pixel electrode PX2as a patterned electrode may be overlapped with respect to each other without electrical contact. In addition, electrical fields of the first pixel electrode PX1and the second pixel electrode PX2are formed through a hollow (or gap) portion of the second pixel electrode PX2. Accordingly, a horizontal electrical field may be generated in the liquid crystal based on a voltage difference between the first pixel electrode PX1and the second pixel electrode PX2, so as to facilitate re-ordering of liquid crystal molecules.

In the embodiment of the invention, the scan signal Scan may be transmitted via one of a plurality of gate lines in a display panel (not shown), for example. In addition, the data voltage Vdata may be transmitted via one of a plurality of data lines in the display panel (not shown). Moreover, a plurality of pixels of the display panel (not shown) are in an array arrangement and are respectively arranged at intersections of the data lines and the gate lines. Accordingly, a pixel circuit (e.g., the pixel circuit100) may be controlled via the corresponding gate lines and data lines to carry out circuit operations.

In the embodiment, the pixels of the display panel (not shown) may be construed with reference to the pixel circuit100. For example, the pixel circuit100may control whether the first switch M1is turned on or off by using the scan signal Scan. When the first switch M1is turned on, the pixel circuit100may provide the data voltage Vdata to the first pixel PX1, and the storage capacitor Cst may store the data voltage Vdata.

FIG. 2is a schematic waveform diagram illustrating a pixel circuit according to an embodiment of the invention.FIGS. 3A to 3Care schematic diagrams illustrating liquid crystal states during a reset period, a charging period, and an emitting period of a pixel circuit according to an embodiment of the invention. Referring toFIG. 2, in the embodiment, a frame period TFR of the pixel circuit100may be divided into a reset period Tr, a charging period Tch, and an emitting period Te. In addition, the reset period Tr, the charging period Tch, and the emitting period Te are not overlapped with each other, and the charging period Tch is arranged between the reset period Tr and the emitting period Te. For example, in the frame period TFR, the reset period Tr and the charging period Tch of the pixel circuit100may be considered as a period when the pixel circuit100writes data, and the emitting period Te of the pixel circuit100may be considered as a display time period of the pixel circuit100.

Referring toFIGS. 1 and 3A, in the embodiment, the second pixel electrode PX2may be located between a common electrode Pcom transmitting the first common voltage Vcom1and the first pixel electrode PX1. In addition, a liquid crystal layer LCX is disposed between the common electrode Pcom and the first pixel electrode PX1as well as the second pixel electrode PX2. Moreover, a material of the liquid crystal layer LCX may include a uniform lying helix (ULH) structure liquid crystal. However, the embodiments of the invention are not limited thereto.

Besides, the first to third liquid crystal capacitors C1to C3of the embodiment may be considered as equivalent capacitors formed in the ULH structure liquid crystal. Besides, based on different circuit designs, the first common voltage Vcom1of the embodiment may be a direct current (DC) common voltage or an alternating current (AC) common voltage. In the embodiment, the first common voltage Vcom1is an AC common voltage, for example.

Referring toFIGS. 1, 2, and 3A, specifically, when the pixel circuit100is operated in the reset period Tr, the scan signal Scan may be set to be disabled (e.g., at a low voltage level). Accordingly, the first switch M1may be turned off. Under the circumstance, the pixel circuit100is unable to provide the data voltage Vdata to the first pixel electrode PX1. Besides, in the reset period Tr, the reset signal Reset may be enabled (e.g., at a high voltage level). Accordingly, the second switch M2and the third switch M3may be turned on. Under the circumstance, the first common voltage Vcom1and a second common voltage Vcom2may be switched from a high common voltage VCH to a low common voltage VCL. In addition, the pixel circuit100may provide the second common voltage Vcom2to the first pixel electrode PX1, so that the first pixel electrode PX1is provided with the low common voltage VCL. Moreover, the pixel circuit100may also provide the reset voltage Vreset to the second pixel electrode PX2, so that the second pixel electrode PX2may have the reset voltage Vreset. In the embodiment, a waveform of the second common voltage Vcom2may be the same as a waveform of the first common voltage Vcom, and the reset voltage Vreset is different from the low common voltage VCL. However, the embodiments of the invention are not limited thereto.

Besides, when the pixel circuit100is operated in the reset period Tr, since the first pixel electrode PX1receives the low common voltage VCL from the second common voltage Vcom2, and the second pixel electrode PX2receives the reset voltage Vreset, the second pixel electrode PX2may generate an electrical field EF1toward a direction of the first pixel electrode PX1on the second liquid crystal capacitor C2. In other words, a horizontal electrical field is formed between the first pixel electrode PX1and the second pixel electrode PX2. Moreover, the horizontal electrical field generated between the first pixel electrode PX1and the second pixel electrode PX2is adopted to restore the arrangement of liquid crystal molecules in the ULH structure liquid crystal of the display panel to a default or initial state.

Referring toFIGS. 1, 2, and 3B, specifically, when the pixel circuit100is operated in the charging period Tch, the scan signal Scan may be set to be enabled (e.g., at a high voltage level). Accordingly, the first switch M1may be turned on. Under the circumstance, the pixel circuit100may provide the data voltage Vdata to the first pixel electrode PX1, so that the storage capacitor Cst may store the data voltage Vdata. Besides, during the charging period Tch, the reset signal Reset may be set to be disabled (e.g., at a low voltage level). Accordingly, the second switch M2and the third switch M3may be turned off, so that the reset voltage Vreset is unable to be provided to the second pixel electrode PX2. Under the circumstance, the first common voltage Vcom1and the second common voltage Vcom2remain at the low common voltage VCL and the second pixel electrode PX2may be floating.

Besides, when the pixel circuit100is operated in the charging period Tch, since the first pixel electrode PX1has the received data voltage Vdata, and the second pixel electrode PX2is in a floating state, the first pixel electrode PX1may generate an electrical field EF2toward a direction of the common electrode Pcom of the first common voltage Vcom1on the first liquid crystal capacitor C1. In other words, a vertical electrical field is formed between the first pixel electrode PX1and the first common voltage Vcom1. Moreover, the vertical electrical field generated between the first pixel electrode PX1and the first common voltage Vcom1may be adopted to rotate optical axes of the liquid crystal molecules in the ULH structure liquid crystal of the display panel, so that the liquid crystal molecules may form bright/dark grayscale levels.

Referring toFIGS. 1, 2, and 3C, specifically, when the pixel circuit100is operated in the emitting period Te, the scan signal Scan may be set to be disabled (e.g., at a low voltage level). Accordingly, the first switch M1may be turned off. Under the circumstance, the pixel circuit100is unable to provide the data voltage Vdata to the first pixel electrode PX1and the first pixel electrode PX1may be floating. In addition, during the emitting period Te, the reset signal Reset may be set to be disabled (e.g., at a low voltage level). Accordingly, the second switch M2and the third switch M3are turned off, so that the reset voltage Vreset is unable to be provided to the second pixel electrode PX2and the second pixel electrode PX2may remain floating. Under the circumstance, the first common voltage Vcom1and the second common voltage Vcom2may remain at the low common voltage VCL.

Besides, when the pixel circuit100is operated in the emitting period Te, since the first pixel electrode PX1still keeps the received data voltage Vdata, and the first pixel electrode PX1and the second pixel electrode PX2remain floating, the ULH structure liquid crystal in the pixel circuit100may still be driven, and the pixel circuit100may display a desired grayscale level based on the data voltage Vdata.

InFIGS. 3A to 3C, the first pixel electrode PX1may be formed on a substrate SB1. In addition, a protective layer BP and the second pixel electrode PX2are sequentially formed on the first pixel electrode PX1. Besides, the common electrode Pcom may be formed below a substrate SB2. Nevertheless,FIGS. 3A to 3Cmerely serve as schematic views of the liquid crystal states of the embodiment. Other components may be further disposed between the respective layers of components. For the ease of illustrations,FIGS. 3A to 3Cmerely illustrate the necessary components of the embodiment of the invention, and the invention is not limited thereto.

Based on the above, in the embodiment of the invention, when the pixel circuit100is operated in the reset period Tr, the reset signal Reset may be enabled (e.g., at a high voltage level) to turn on the second switch M2and the third switch M3in advance before the liquid crystal molecules are driven. Accordingly, the pixel circuit100may provide the second common voltage Vcom2to the first pixel electrode PX1, so that the first pixel electrode PX1may have the low common voltage VCL. Moreover, the pixel circuit100may also provide the reset voltage Vreset to the second pixel electrode PX2, so that the second pixel electrode PX2may have the reset voltage Vreset. Under the circumstance, a horizontal electrical field may be formed between the first pixel electrode PX1and the second pixel electrode PX2and the re-arrangement of the liquid crystal molecules in the display panel may be facilitated. Accordingly, during the process of alternately switching between positive and negative electrical fields, the influence of the directions of the electrical fields on the optical axes of some liquid crystal molecules, which may lead to a disordered arrangement of liquid crystal molecules and a lower transmittance ratio, may be reduced.

FIG. 4is a schematic cross-sectional view illustrating a display panel according to an embodiment of the invention. The display panel includes an active array substrate410, a liquid crystal layer420, and a color filter substrate430. On the substrate SB1of the active array substrate410, gates G1and G2and an electrode E1are firstly formed, and then gate insulating layers GI1and GI2are sequentially formed. On the gate insulating layer GI2, channel layers CH1and CH2and an electrode E2are formed. The electrodes E1and E2are adopted to form a capacitor CX, such as the storage capacitor Cst shown inFIG. 1. Moreover, etch stop layers ES1and ES2, sources S1and S2, and drains D1and D2are formed on the channel layers CH1and CH2. The gate G1, the channel layer CH1, the etch stop layer ES1, the source S1, and the drain D1form a transistor T1, and the gate G2, the channel layer CH2, the etch stop layer ES2, the source S2, and the drain D2form a transistor T2.

On the electrode E2, the etch stop layers ES1and ES2, the sources S1and S2, and the drains D1and D2, a protective layer BP1and an insulating layer PL are sequentially formed. Then, the first pixel electrode PX1is formed on the insulating layer PL, and the first pixel electrode PX1contacts the source S2through vias of the protective layer BP1and the insulating layer PL. On the first pixel electrode PX1, a protective layer BP2and the second pixel electrode PX2are sequentially formed. In addition, the second pixel electrode PX2contacts the source S1through vias of the protective layer BP1and the insulating layer PL. Then, a protective layer BP3is formed on the second pixel electrode PX2to form the active array substrate410.

Besides, a black matrix BM1is formed on the substrate SB2of the color filter substrate430. Then, a coating layer OC1is formed. On the coating layer OC1, the common electrode Pcom and a passivation layer PV1are sequentially formed. Accordingly, the color filter substrate430is completed. The usage “on . . . ” is used with reference to the orientation in the manufacturing process, instead of the orientation in the drawings. Then, the active array substrate410and the color filter substrate430are assembled to each other, and liquid crystal is filled to form the liquid crystal layer420. Accordingly, the display panel is completed.

FIG. 5is a flowchart illustrating a driving method of a pixel circuit according to an embodiment of the invention. Referring toFIGS. 1, 2, and 5, at Step S510, when the pixel circuit100is operated in the reset period Tr, the pixel circuit100may provide the second common voltage Vcom2to the first pixel electrode PX1, and the pixel circuit100may also provide the reset voltage Vreset to the second pixel electrode PX2. At Step S520, when the pixel circuit100is operated in the charging period Tch, the pixel circuit100may provide the data voltage Vdata to the first pixel electrode, and the second pixel electrode PX2may be floating. At Step S530, when the pixel circuit100is operated in the emitting period Te, the first pixel electrode PX1and the second pixel electrode PX2may be floating. Details for implementing the respective steps are already described in the foregoing embodiments and examples, and thus will not be repeated herein.

In view of the foregoing, according to the pixel circuit and the driving method of the pixel circuit according to the embodiments of the invention, the horizontal electrical field generated between the first pixel electrode and the second pixel electrode may be adopted to restore the arrangement of the liquid crystal molecules in the ULH structure liquid crystal of the display panel to the default or initial state. Accordingly, the re-arrangement of the liquid crystal molecules may be facilitated. In addition, the influence of the directions of the electrical fields on the optical axes of some liquid crystal molecules during the process of alternately switching between positive and negative electrical fields, which may lead to a disordered arrangement of liquid crystal molecules and a lower transmittance ratio, may be reduced.