Patent Application
20130021564
The present invention relates to a liquid crystal display device, and more particularly, to a vertical alignment type liquid crystal display device having a plurality of domains for orientation division in a pixel.
In recent years, thin-profile flat panel displays (FPDs) have been widely used for display devices, replacing cathode ray tubes that were traditionally used in most display devices. The FPDs use liquid crystals, light-emitting diodes (LEDs), organic electroluminescence (organic EL), or the like as display elements thereof. Among them, a display device using liquid crystals has advantages of thin-profile, light-weight, and low power consumption, and therefore, the research and development thereof have been actively pursued.
As a driving method of a liquid crystal display device (LCD), a method of using an active matrix (AM) circuit having thin-film transistors (TFTs) is used. The AM circuit is a switching circuit that controls each pixel to switch between display and non-display states. Because the AM circuit controls each pixel individually, even if the number of wiring lines in the display device is increased, each pixel can be operated reliably. Therefore, in the LCD utilizing the AM circuits, it is possible to achieve higher resolution, clearer contrast, and faster response speed.
Among LCDs utilizing the AM circuits, TN (Twisted Nematic) type LCDs are well known. In the TN type LCD, a pair of linear polarizing plates are disposed on outer surfaces of two substrates, respectively, in a crossed Nicols state. When linear polarized light enters a liquid crystal layer through one polarizing plate, the polarizing axis thereof is rotated by the optical polarity rotation and the birefringence of liquid crystal molecules, allowing the light to pass through the other polarizing plate. If a voltage is applied between a pixel electrode and an opposite electrode, the liquid crystal molecules are vertically aligned (become perpendicular) with respect to the surfaces of the two substrates. As a result, linear polarized light that entered the liquid crystal layer directly reaches the opposite side without rotating the polarizing axis thereof, and thus cannot pass through the other polarizing plate.
Because the TN type LCD described above utilizes the birefringence of the liquid crystal molecules, the viewing state varies depending on the position of a viewer relative to the alignment direction of the liquid crystal molecules. That is, the TN type LCD has a problem of a narrow viewing angle and insufficient viewing characteristics.
To solve this problem, a VA (Vertical Alignment) type LCD in which liquid crystal molecules (liquid crystal molecules having negative dielectric anisotropy) are vertically aligned relative to a substrate has been developed and put into practical use. The VA type LCD is configured to generate an oblique electric field relative to the alignment direction of the liquid crystal molecules. Therefore, when a voltage is applied between a pixel electrode and an opposite electrode, the liquid crystal molecules shift to a tilted position. If a pixel is divided into domains such that the liquid crystal molecules are tilted in a plurality of different directions in a single pixel, the viewing state becomes substantially the same regardless of viewing angle. Such an LCD in which each pixel to be driven is divided into a plurality of domains is referred to as an MVA (Multi-domain Vertical Alignment) type, and has a wider viewing angle and excellent viewing characteristics.
Patent Document 1, for example, discloses an MVA type LCD that is provided with slits. Specifically, in this LCD, by providing control electrodes, pixel electrodes formed in a TFT substrate are maintained in an electrically floating state. The pixel electrodes respectively have X-shaped slits formed therein. By controlling the orientation direction of the liquid crystal molecules through the control electrodes, viewing characteristics of four divided domains formed by the slit are compensated with each other in each pixel, achieving symmetrical and excellent viewing characteristics.
Patent Document 2 discloses an MVA type LCD in which pixel electrodes having a fishbone structure are provided between a pair of linear polarizing plates arranged in a crossed-Nicols state.
Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2001-249350 (Published on Sep. 14, 2001)
Patent Document 2: WO 2009/084162 Pamphlet (Published on Jul. 9, 2009)
As described above, in the LCD using the pixel electrode of the fishbone structure disclosed in Patent Document 2, the orientation direction of the liquid crystal molecules can be controlled to be 45° relative to the polarizing axis of the polarizing plate. However, in the configuration disclosed in Patent Document 2, the deviation of the orientation directions of the liquid crystal molecules occurs at end portions of each pixel and boundary portions between the respective domains as shown in
Further, the size of a region in which the deviation of the orientation directions of the liquid crystal molecules 4 occurs does not change regardless of pixel pitch. That is, the smaller the pixel pitch is, the larger the effect of the deviation of the orientation directions becomes.
Similarly, in the technology disclosed in Patent Document 1 described above, it is difficult to make the liquid crystal molecules aligned orderly along the X-shaped slit in the proper orientation directions. As a result, the deviation of the orientation directions of the liquid crystal molecules occurs, causing an imbalance between the vertical orientation and the horizontal orientation of the liquid crystal molecules in each pixel. That is, in the technology disclosed in Patent Document 1, the balance of the gamma characteristics is worsened, resulting in the degradation of display quality of the LCD.
In the technology disclosed in Patent Document 1, linear polarizing plates cannot be used as polarizing plates, which worsens the viewing characteristics of the LCD. In this case, retardation plates need to be provided. Also, because it is necessary to form the control electrodes below the pixel electrodes, the structure of the LCD becomes more complex. Further, because the pixel electrodes are in a floating state, a problem of burn-in may be caused by residual electric charges.
The present invention was made in view of the above-mentioned problems, and aims at providing an LCD that can achieve excellent balance of gamma characteristics of the LCD and that thereby has high display quality.
In order to solve the above-mentioned problems, a liquid crystal display device according to the present invention is a vertical alignment type that has a plurality of pixels and a pair of polarizing plates that are disposed such that transmission axes thereof are orthogonal to each other, including: a pixel electrode; an opposite electrode facing the pixel electrode; and a liquid crystal layer disposed between the pixel electrode and the opposite electrode in each of the plurality of pixels, wherein the pixel electrode that is divided into a plurality of domains includes a frame portion along an entire inner circumference of the pixel and a plurality of fine electrode portions each having one end connected to the frame portion and another end separated therefrom, the plurality of fine electrode portions being extended toward an inside of the frame portion, wherein, in each of the domains, the plurality of fine electrode portions provided in the domain are extended in the same direction, and an extending direction thereof differs from an extending direction of the plurality of fine electrode portions provided in another domain, and wherein each of the fine electrode portions is extended in a direction that forms a 45-degree angle with respective extending directions of the transmission axes, and is extended so as to approach the respective extending directions of the transmission axes.
According to this configuration, in each domain of the pixel electrode, the plurality of fine electrode portions that are extended in a direction that forms a 45-degree angle with extending directions of the transmission axes of the polarizing plates are formed. Further, the plurality of fine electrode portions provided in one domain are extended in the same direction, and the extending direction differs from an extending direction of the plurality of fine electrode portions provided in another domain. In this configuration, when a voltage is applied to the liquid crystal layer, the liquid crystal molecules are oriented along the fine electrode portions. Also, by the effect of the frame portion along the entire inner circumference of the pixel, the liquid crystal molecules tilt from the center of the pixel electrode toward the outer circumference of the opposite electrode. That is, the liquid crystal molecules are oriented in the direction that forms a 45-degree angle with the extending directions of the transmission axes of the polarizing plates, while tilting from the center of the pixel electrode toward the outer circumference of the opposite electrode.
The other ends of the fine electrode portions are separated from each other. That is, in the center portion of the pixel electrode, a slit is formed. This makes it possible to prevent the orientation directors of the liquid crystal molecules from being increased in one direction in each pixel, and by the effects of the frame portion and the fine electrode portions of the pixel electrode, the orientation directors of the liquid crystal molecules can be evenly distributed. As a result, the deviation of the orientation directions of the liquid crystal molecules in each pixel can be prevented, which allows for a good balance of the orientation directions of the liquid crystal molecules. This can prevent an imbalance of gamma characteristics resulting from the deviation of the orientation directions of the liquid crystal molecules, and thus, it becomes possible to prevent the display gray scale conditions from varying depending on the viewing angle, thereby further improving the display quality of the display surface.
Further, in the above-mentioned configuration, because light passing through the liquid crystal layer is emitted in a plurality of different directions, the substantially same view can be achieved regardless of viewing angle relative to the display surface.
Additional objects, features, and effects of the present invention shall be readily understood from the descriptions that follow. Advantages of the present invention shall become apparent by the following descriptions with reference to the appended drawings.
According to the present invention, the slit is formed in the center portion of the pixel electrode, and therefore, the orientation directors of the liquid crystal molecules are evenly distributed by the effects of the frame portion and the fine electrode portions. Therefore, deviation of the orientation directions of the liquid crystal molecules in each pixel is prevented, thereby improving the balance between the orientation directions of the liquid crystal molecules. This can prevent an imbalance of gamma characteristics resulting from the deviation of the orientation directions of the liquid crystal molecules, and thus, it becomes possible to prevent the display gray scale conditions from varying depending on the viewing direction, thereby further improving the display quality of the display surface. Further, according to the present invention, because light passing through the liquid crystal layer is emitted in a plurality of different directions, the substantially same view can be achieved regardless of viewing angle relative to the display surface.
a) is a graph showing gamma characteristics in the vertical direction of a conventional pixel.
a) is a graph showing gamma characteristics in the vertical direction of a pixel according to one embodiment of the present invention.
(Overview of Liquid Crystal Display Device)
One embodiment of the present invention will be explained with reference to figures. First, an overview of a liquid crystal display device (LCD) according to the present embodiment will be explained.
The LCD of this embodiment is a VA (Vertical Alignment) type LCD in which liquid crystal molecules having negative dielectric anisotropy (ε<0) are aligned vertically to a substrate. The LCD of this embodiment is constituted of a backlight unit and a liquid crystal display element unit. A planar light source device is provided as the backlight unit, and a liquid crystal panel is provided as the liquid crystal display element unit. The liquid crystal panel includes a TFT substrate having thin-film transistors (TFTs), pixel electrodes, and the like, corresponding to respective pixels, and an opposite substrate having a color filter, an opposite electrode, and the like. A liquid crystal layer is sealed between the two substrates. Linear polarizing plates are respectively disposed on an outer side of the TFT substrate (side opposite to the liquid crystal layer) and on an outer side of the opposite substrate (side opposite to the liquid crystal layer), and the two linear polarizing plates are arranged in a crossed Nicols state.
In the TFT substrate, scanning lines (gate bus lines) and signal lines (source bus lines) are formed, and pixel electrodes are formed on the scanning lines and the signal lines through an insulating film. A region enclosed by two adjacent scanning lines and two adjacent signal lines forms a single pixel. The pixel of the LCD according to this embodiment will be explained in detail with reference to
As shown in
The pixel electrode 1 has a frame portion 6 “along the entire inner circumference” of the pixel 10. “Along the entire inner circumference” means that the frame portion 6 is formed inside of the pixel 10 along a border between the inside and the outside of the pixel 10. In other words, the frame portion 6 is formed inside of the pixel 10 along the four sides of the pixel 10. In the frame portion 6, fine electrodes 7a to 7d (fine electrode portions) each having one end connected to the frame portion 6 and the other end separated therefrom. The fine electrodes 7a to 7d are extended toward inside of the frame portion 6. Specifically, in the domain 5a of the pixel electrode 1, a plurality of fine electrodes 7a that make a 45-degree angle with the frame portion 6 (direction from the domain 5a toward the domain 5c) are formed. In the domain 5b of the pixel electrode 1, a plurality of fine electrodes 7b that make a 45-degree angle with the frame portion 6 (direction from the domain 5b toward the domain 5d) are formed. Similarly, in the domain 5c of the pixel electrode 1, a plurality of fine electrodes 7c that make a 45-degree angle with the frame portion 6 (direction from the domain 5c toward the domain 5a) are formed. In the domain 5d of the pixel electrode 1, a plurality of fine electrodes 7d that make a 45-degree angle with the frame portion 6 (direction from the domain 5d toward the domain 5b) are formed. That is, the plurality of fine electrodes 7a to 7d are formed so as to make a 45-degree angle with the extending directions of the polarizing axes of the linear polarizing plates, respectively. The pixel electrode 1 does not have anything formed therein other than the frame portion 6 and the fine electrodes 7a to 7d. That is, the center portion of the pixel electrode 1 has an opening, forming a slit 8.
(Orientation of Liquid Crystal Molecules 4)
By using the above-mentioned pixel electrode 1, viewing characteristics of an LCD can be improved as described in detail with reference to
As shown in
As described above, the liquid crystal molecules 4 are oriented in the directions that respectively form a 45-degree angle with the polarizing axes of the polarizing plates, and therefore, when linear polarized light enters the liquid crystal layer through one linear polarizing plate, the polarizing axis thereof is rotated by the optical polarity rotation and the birefringence of the liquid crystal molecules 4, allowing the light to pass through the other linear polarizing plate. Further, by dividing the pixel 10 into the four domains 5a to 5d, the liquid crystal molecules 4 are oriented in different directions in the respective domains 5a to 5d, thereby allowing the liquid crystal molecules 4 to be oriented in a plurality of different directions in the single pixel 10. This way, light passing through the liquid crystal layer is emitted in the plurality of different directions, and therefore, it becomes possible to achieve the substantially same view regardless of viewing angle relative to the display surface. Thus, with the above-mentioned configuration, excellent viewing characteristics can be achieved.
When no voltage is applied between the two substrates, the liquid crystal molecules 4 in the liquid crystal layer are vertically aligned to the surfaces of the two substrates. Therefore, linear polarized light that entered the liquid crystal layer reaches the opposite side without rotating the polarizing axis thereof, and thus cannot pass through the other linear polarizing plate.
As described above, in the LCD according to this embodiment, when no voltage is applied, the liquid crystal molecules 4 are vertically aligned to the substrate surfaces, and because the liquid crystal layer thereby becomes non-birefringent, the LCD performs a black display. When a voltage is applied between the substrates, and the liquid crystal molecules are tilted in the directions that respectively form a 45-degree angle with the polarizing axes of the polarizing plates, the LCD performs a white display. If the liquid crystal molecules 4 are tilted in a direction that is parallel with or orthogonal to the polarizing axes upon voltage application, the liquid crystal layer would not become birefringent to the linear polarized light, resulting in a black display. For this reason, it is necessary to control the tilt direction of the liquid crystal molecules 4. According to this embodiment, by using the pixel electrode 1 constituted of the frame portion 6 along the entire inner circumference of the pixel electrode 1 and the plurality of fine electrodes 7a to 7d that respectively form a 45-degree angle with the polarizing axes of the polarizing plates, it becomes possible to make the liquid crystal molecules 4 oriented in the four directions that respectively form a 45-degree angle with the polarizing axes of the polarizing plates with high degree of accuracy.
In the present embodiment, it is not necessary to provide an additional electrode or the like below the pixel electrode 1, for example, to control the orientation direction of the liquid crystal molecules 4, and as a result, the number of constituting members of the pixel 10 can be reduced. When the additional electrode or the like is provided in the pixel 10, it makes it difficult to make the liquid crystal molecules 4 oriented in desired directions. In the present embodiment, it is not necessary to provide an additional electrode or the like below the pixel electrode 1, for example, to control the orientation direction of the liquid crystal molecules 4, and as a result, the number of constituting components of the pixel 10 can be reduced.
In the present embodiment, the pixel electrode 1 is divided into the four domains 5a to 5d, but the present invention is not necessarily limited to this, and as long as a disclination line does not appear with respect to the polarizing axes of the linear polarizing plates, the pixel electrode 1 may be divided into any number of domains. The disclination line is a region where the orientation of the liquid crystal molecules 4 is discontinued, resulting in brightness reduction.
(Gamma Characteristics of Pixel 10)
In an LCD using the pixel electrode disclosed in Patent Document 1 or the pixel electrode of the fishbone structure disclosed in Patent Document 2, for example, upon voltage application, the liquid crystal molecules are tilted from the outer circumference of the pixel electrode toward the center of the opposite electrode. This is because an oblique electric field from the outer circumference of the pixel electrode toward the center of the opposite electrode is generated in the liquid crystal layer by the pixel electrode and the opposite electrode. In such a configuration, deviation of the orientation directions of the liquid crystal molecules occurs at the end portions of each pixel and boundary portions between the respective domains. Specifically, the orientation directors of the liquid crystal molecules in the vertical direction are increased in each pixel. As a result, an imbalance between the vertical orientation and the horizontal orientation of the liquid crystal molecules in each pixel occurs. This causes a balance between gamma characteristics in the vertical direction and gamma characteristics in the horizontal direction to be worsened, resulting in degradation of display quality of the LCD.
As shown in
As shown in
As described above, by the deviation of the orientation directions of the liquid crystal molecules, the balance between the gamma characteristics in the vertical direction and the gamma characteristics in the horizontal direction is worsened. Therefore, when the display surface of the LCD is viewed diagonally, the gray scale display conditions vary depending on the viewing angle, resulting in degradation of display quality of the screen. The size of a region where the deviation of the orientation directions of the liquid crystal molecules occurs does not change regardless of pixel pitch. That is, the smaller the pixel pitch is, the larger the effect of the deviation of the orientation directions becomes.
On the other hand, as described above, the pixel electrode 1 according to this embodiment is configured such that the liquid crystal molecules 4 are tilted from the center of the pixel electrode 1 toward the outer circumference of the opposite electrode 9. Further, the pixel 10 is divided into the four domains 5a to 5d such that the liquid crystal molecules 4 are oriented in a plurality of different directions within the single pixel 10. This way, it becomes possible to make the liquid crystal molecules 4 oriented in the four directions that respectively form a 45-degree angle with the polarizing axes of the polarizing plates with high degree of accuracy. Such a configuration is shown in
As shown in
The gamma characteristics in this case is shown in
As shown in
Also, as shown in
As described above, according to this embodiment, the balance between the gamma characteristics in the vertical direction and the gamma characteristics in the horizontal direction is improved, thereby improving the viewing characteristics in the horizontal direction. This makes it possible to prevent the display gray scale conditions from varying depending on the viewing direction, resulting in further improvement of the display quality of the display surface.
In this embodiment, the slit 8 is formed in the center portion of the pixel 10, and because of the orientation direction of the liquid crystal molecules 4, a light-shielding state is achieved at the boundary portions between the respective domains 5a to 5d. Specifically, as described above, because the liquid crystal molecules 4′ in the slit 8 line up perpendicularly to the pixel electrode 1 (substrate plane), i.e., the liquid crystal molecules 4′ are not oriented in any other directions but the perpendicular direction to the pixel electrode 1, a substantial light-shielding state is achieved. In the pixel electrode of the conventional fishbone structure shown in
The present invention is not limited to the above-mentioned embodiment, and various modifications can be made without departing from the scope defined by the claims. That is, embodiments that can be obtained by combining techniques that have been appropriately modified without departing from the scope defined by the claims are also included in the technological scope of the present invention.
<Summary of Embodiment>
As described above, in the liquid crystal display device according to the present invention, an extending direction of each of the transmittance axes is parallel with or orthogonal to an extending direction of one side of the pixel.
In the liquid crystal display device according to the present invention, the extending direction of each of the transmittance axes is a direction that forms a 45-degree angle with an extending direction of one side of the pixel.
According to this configuration, linear polarized light that entered the liquid crystal layer through one polarizing plate rotates the polarizing axis thereof by the optical polarity rotation and the birefringence of the liquid crystal molecules, and can thereby pass through the other polarizing plate. As a result, excellent viewing characteristics can be obtained.
In the liquid crystal display device according to the present invention, the pixel electrode is divided into four domains.
According to this configuration, the liquid crystal molecules are oriented in four directions that respectively form a 45-degree angle with the respective extending directions of the respective transmittance axes of the polarizing plate. This makes it possible to improve the balance between the gamma characteristics in the vertical direction and the gamma characteristics in the horizontal direction, thereby creating little difference in the gamma characteristics between the vertical view and the horizontal view. As a result, the display gray scale conditions can be prevented from varying depending on the viewing direction, thereby further improving the display quality of the display surface.
In the liquid crystal display device according to the present invention, the pixel electrode is electrically connected to a thin-film transistor.
According to this configuration, by directly connecting the thin-film transistor to the pixel electrode, a loss in a voltage applied to the pixel electrode can be reduced.
The specific embodiments and examples provided in the detailed description of the present invention section are merely for illustrating the technical contents of the present invention. The present invention shall not be narrowly interpreted by being limited to such specific examples. Various changes can be made within the spirit of the present invention and the scope as defined by the appended claims.
The present invention can be suitably used for a liquid crystal display device that requires high display quality.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-072915 | Mar 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2010/070918 | 11/24/2010 | WO | 00 | 9/25/2012 |
