Patent Application
20150002800
The present invention relates to a liquid crystal display panel.
Recently, people have proposed various measures for reducing the viewing angle dependence of the γ characteristic of a liquid crystal display panel. The “γ characteristic” is a grayscale luminance characteristic. If the γ characteristic has viewing angle dependence, the γ characteristic when the panel is viewed straight on and the γ characteristic when the panel is viewed obliquely will be different from each other, and therefore, the grayscale display state will also vary according to the viewing direction (i.e., the viewing angle).
To overcome this problem, according to a known method (see Patent Document No. 1, for example), each pixel is divided into multiple subpixels and mutually different effective voltages are applied to respective liquid crystal layers of those multiple subpixels. Such a method of dividing a single pixel into multiple subpixels is generally called a “pixel division” and various circuit structures have been developed for that purpose.
Meanwhile, Patent Document No. 2 discloses a liquid crystal display panel in which multiple regions with mutually different pretilt angles are defined in the liquid crystal layer of a pixel by using the polymer sustained alignment technology (which will be referred to herein as a “PSA technology”). According to the PSA technology, a small amount of a polymerizable material (such as a photo-polymerizable monomer) is introduced into a liquid crystal material, and the polymerizable material is irradiated with an active energy ray (which is typically an ultraviolet ray and is supposed to be an ultraviolet ray in the following description) with a predetermined voltage applied to the liquid crystal layer after the liquid crystal cell has been assembled, thereby controlling the pretilt azimuth and pretilt angle of liquid crystal molecules with the polymer thus produced. In the following description, to control the pretilt azimuth and pretilt angle of liquid crystal molecules will be sometimes referred to herein as “giving a pretilt to the liquid crystal molecules”. The alignment state of the liquid crystal molecules when the polymer is produced is sustained (i.e., memorized) even after the voltage is removed (i.e., no longer applied). In this description, the pretilt azimuth is defined by the azimuth angle within a substrate plane (e.g., when the display screen is compared to a clock face, the three o'clock direction may have an azimuth angle of zero degrees and the angle may increase counterclockwise) and the pretilt angle is defined to be the angle formed with respect to the substrate plane (which corresponds to the angle of elevation, and the pretilt angle along a normal to the substrate plane is 90 degrees). The pretilt angle in each region of the liquid crystal layer becomes the average of the pretilt angles of liquid crystal molecules which are located close to the surface of an alignment film in that region and the pretilt angles of liquid crystal molecules which are located everywhere else. In the following description, the pretilt angle to be given to those liquid crystal molecules which are located close to the surface of an alignment film will be sometimes referred to herein as the pretilt angle of that alignment film (or alignment region).
As methods for forming multiple regions with mutually different pretilt angles in each pixel, Patent Document No. 2 discloses the following two methods (1) and (2):
(1) By introducing a structure for applying mutually different voltages to the liquid crystal layer of multiple regions in a pixel, the polymerizable material included in the liquid crystal layer may be polymerized with mutually different voltages applied to those multiple regions in each pixel, thereby making the pretilt angles different from one region to another. In this case, every region of a single pixel is irradiated at the same time with an ultraviolet ray for the purpose of polymerization.
(2) Or the process step of irradiating only a particular region of a single pixel which has been selected through a photomask with an ultraviolet ray and producing a polymer in only that particular region of the pixel may be performed a number of times. In this case, when the regions to irradiate an ultraviolet ray with are changed, the voltages applied to the liquid crystal layer are also changed. According to this method (2), such a structure for applying mutually different voltages to the liquid crystal layer of multiple regions in a pixel is not needed.
Recently, as methods for giving a pretilt to liquid crystal molecules (i.e., as methods for controlling the pretilt azimuth and pretilt angle), not only the PSA technology but also methods disclosed in Patent Documents Nos. 3 to 5 are known as well. According to the methods disclosed in Patent Documents No. 3 to 5, no polymerizable material is added to the liquid crystal material and no unreacted polymerizable material will be left in the liquid crystal layer of the liquid crystal display panel, unlike a situation where the PSA technology is adopted.
Patent Documents Nos. 3 to 5 disclose a method for giving a pretilt to liquid crystal molecules using an alignment film material including a polymer material with a cross-linkable site (i.e., a cross-linkable group). Specifically, with a voltage applied to a liquid crystal layer that is interposed between two substrates, each of which is coated with such an alignment film material including a polymer material with a cross-linkable site, the alignment film material is irradiated with an ultraviolet ray, thereby causing the cross-linkable site to cross-link and forming an alignment film with a cross-linking structure. This alignment film with such a cross-linking structure functions to fix the alignment state of liquid crystal molecules which are located close to the alignment film material when the material is irradiated with the ultraviolet ray. That is why even after the voltage that has been applied to the liquid crystal layer is removed, the liquid crystal molecules that are located close to the alignment film will sustain the alignment state that has been regulated by the electric field. In this manner, the alignment film gives a pretilt to the liquid crystal molecules.
Patent Document No. 6 discloses a liquid crystal display device which divides a counter electrode that faces pixel electrodes into multiple sub-counter electrodes and which applies mutually different effective voltages to respective liquid crystal layers of multiple pixels, thereby minimizing a decrease in the aperture ratio of the display area and suppressing the whitening phenomenon efficiently. The entire disclosure of Patent Document No. 6 is hereby incorporated by reference.
If the pixel division technique disclosed in Patent Document No. 1 and other documents is adopted, a single pixel electrode is divided into multiple subpixel electrodes. That is why the larger the number of division, the lower the aperture ratio of the pixel will be, which is a problem. A similar problem will arise even when the method (1) disclosed in Patent Document No. 2 is adopted.
On the other hand, according to the method (2) disclosed in Patent Document No. 2, the process step of selectively irradiating only a particular region of a pixel with light through a photomask needs to be performed at least twice while the device is still in the form of a liquid crystal cell. Since the device still in the form of a liquid crystal cell is irradiated with light, the accuracy achieved cannot be as high as what is achieved by a so-called “contact exposure method” or “proximity exposure method”. In addition, the accuracy will further decrease due to scattering and diffusion of light to be caused by the liquid crystal cell's lines and other structures. And this problem arises every time the cell is irradiated with light.
Furthermore, if the liquid crystal display device disclosed in Patent Document No. 6 is adopted, the decrease in aperture ratio can be certainly minimized. However, if the counter voltage is changed to cope with Y shift, sometimes the transmittance may decrease accordingly.
A primary object of the present invention is to provide a liquid crystal display panel which can increase the pixel's transmittance and which has a wide viewing angle.
A liquid crystal display panel according to an embodiment of the present invention includes: a liquid crystal layer including liquid crystal molecules; first and second substrates which are arranged to face each other with the liquid crystal layer interposed between themselves; a counter electrode formed on one of the first and second substrates; a first alignment film formed on one side of the first substrate so as to face the liquid crystal layer; a second alignment film formed on one side of the second substrate so as to face the liquid crystal layer; and a pixel. In the pixel, the liquid crystal layer has a low-pretilt region and a high-pretilt region in which the liquid crystal molecules have mutually different pretilt angles. The counter electrode includes a plurality of sub-counter electrodes which are electrically independent of each other. And the plurality of sub-counter electrodes include a first sub-counter electrode associated with the low-pretilt region and a second sub-counter electrode associated with the high-pretilt region.
In one embodiment, the first alignment film has a first low-pretilt alignment region which contacts with the low-pretilt region and a first high-pretilt alignment region which contacts with the high-pretilt region. The second alignment film has a second low-pretilt alignment region which contacts with the low-pretilt region and a second high-pretilt alignment region which contacts with the high-pretilt region.
In one embodiment, the liquid crystal display panel further includes: a first alignment sustaining layer arranged closer to the liquid crystal layer than the first alignment film is; and a second alignment sustaining layer arranged closer to the liquid crystal layer than the second alignment film is. Each of the first and second alignment sustaining layers is made of a photo-polymerized compound. The first alignment sustaining layer has a first low-pretilt alignment region which contacts with the low-pretilt region and a first high-pretilt alignment region which contacts with the high-pretilt region. The second alignment sustaining layer has a second low-pretilt alignment region which contacts with the low-pretilt region and a second high-pretilt alignment region which contacts with the high-pretilt region. And the liquid crystal molecules have their pretilt angle defined by the first and second alignment sustaining layers.
In one embodiment, at least one of the first and second alignment films has a cross-linking structure, dimer or isomer formed through a photo-reaction.
In one embodiment, the first and second low-pretilt alignment regions give a pretilt angle of 84.0 degrees to less than 88.0 degrees to the liquid crystal molecules in the low-pretilt region. And the first and second high-pretilt alignment regions give a pretilt angle of 88.0 degrees to less than 90.0 degrees to the liquid crystal molecules in the high-pretilt region.
In one embodiment, the difference in pretilt angle between the high-pretilt region and the low-pretilt region is 1.0 to 3.0 degrees.
In one embodiment, in the pixel, the area of the high-pretilt region is larger than the area of the low-pretilt region.
In one embodiment, in the pixel, the area of the high-pretilt region is approximately twice as large as the area of the low-pretilt region.
In one embodiment, the liquid crystal display panel has a gap between the first and second sub-counter electrodes, and the gap overlaps with either a non-transparent metal layer or a boundary region between the low-pretilt region and high-pretilt region of the liquid crystal layer.
An embodiment of the present invention provides a liquid crystal display panel which can increase the pixel's transmittance and which has a wide viewing angle.
Hereinafter, a liquid crystal display panel and method for fabricating the panel according to an embodiment of the present invention will be described with reference to the accompanying drawings. In the following description, a vertical alignment liquid crystal display panel operating in normally black mode using liquid crystal molecules with negative dielectric anisotropy will be described as an example. However, this is just an example and the present invention has various other embodiments.
a) and 1(b) are schematic cross-sectional views illustrating liquid crystal display panels 100A and 100B according to an embodiment of the present invention. Specifically,
FIG. 2A(a) is a schematic plan view illustrating the pixel electrode 14 of one pixel that the liquid crystal display panels 100A and 100B each have. FIG. 2A(b) is a schematic plan view illustrating the counter electrode 24 of the one pixel that the liquid crystal display panels 100A and 100B each have. FIG. 2B(a) is a schematic plan view illustrating the pixel electrode 14 of another pixel, and FIG. 2B(b) is a schematic plan view illustrating the counter electrode 24 of that another pixel.
The liquid crystal display panel 100A shown in
In this embodiment, the first and second alignment films 16a and 26a are vertical alignment films. The liquid crystal display panel 100A further includes a first alignment sustaining layer 17a arranged closer to the liquid crystal layer 30A than the first alignment film 16a is and a second alignment sustaining layer 27a arranged closer to the liquid crystal layer 30A than the second alignment film 26a is. The first and second alignment sustaining layers 17a and 27a are each made of a polymerized compound which has been produced through a photo reaction. Although a specific method of forming the first and second alignment sustaining layers 17a and 27a according to this embodiment will be described later, the methods disclosed in Patent Documents Nos. 4 and 5 may be adopted, for example. The entire disclosures of Patent Documents Nos. 4 and 5 are hereby incorporated by reference.
The first substrate 10A (which is a TFT substrate in this example) includes a transparent substrate 12 (which may be a glass substrate), pixel electrodes 14, and TFTs and various lines (including gate bus lines, source bus lines, and storage capacitor (CS) bus lines) which are not shown in
In each pixel of the liquid crystal display panel 100A, the liquid crystal layer 30A has a low-pretilt region R1a (in which the liquid crystal molecules 32 have a pretilt angle Θ1) and a high-pretilt region R2a (in which the liquid crystal molecules 32 have a pretilt angle Θ2) where the liquid crystal molecules 32 have mutually different pretilt angles. In
The first alignment sustaining layer 17a has a first low-pretilt alignment region 17a1 (which has a pretilt angle Θa1 and) which contacts with the low-pretilt region R1a and a first high-pretilt alignment region 17a2 (which has a pretilt angle Θa2 and) which contacts with the high-pretilt region R2a. The second alignment sustaining layer 27a has a second low-pretilt alignment region 27a1 (which has a pretilt angle Θb1 and) which contacts with the low-pretilt region R1a and a second high-pretilt alignment region 27a2 (which has a pretilt angle Θb2 and) which contacts with the high-pretilt region R2a. The pretilt angles Θa2 and Θb2 of the first and second high-pretilt alignment regions 17a2 and 27a2 are less than 90.0 degrees. The pretilt angles (Θa1, Θb1 of the low-pretilt alignment regions are smaller than the pretilt angles (Θa2, Θb2) of the high-pretilt alignment regions.
The pretilt angle Θ1 of the liquid crystal molecules 32 in the low-pretilt region R1a is determined by the pretilt angle Θa1 in the first low-pretilt alignment region 17a1 of the first alignment sustaining layer 17a and the pretilt angle Θb1 in the second low-pretilt alignment region 27a1 of the second alignment sustaining layer 27a, and Θ1 satisfies Θ2≧Θa1 and Θ1≧Θb1. In the same way, the pretilt angle Θ2 of the liquid crystal molecules 32 in the high-pretilt region R2a is determined by the pretilt angle Θa2 in the first high-pretilt alignment region 17a2 of the first alignment sustaining layer 17a and the pretilt angle Θb2 in the second high-pretilt alignment region 27a2 of the second alignment sustaining layer 27a, and Θ2 satisfies Θ2≧Θa2 and Θ2≧Θb2. In this example, Θa1<Θa2 and Θb1<Θb2 are satisfied, and therefore, Θ1<Θ2 is realized. It should be noted that Θa1 and Θb1 may be equal to each other or different from each other. Likewise, Θa2 and Θb2 may also be equal to each other or different from each other.
As will be described later by way of specific examples, the first and second low-pretilt alignment regions 17a1 and 27a1 suitably give a pretilt angle Θ1 of 84.0 degrees to less than 88.0 degrees to the liquid crystal molecules 32 in the low-pretilt region R1a, and the first and second high-pretilt alignment regions 17a2 and 27a2 suitably give a pretilt angle Θ2 of 88.0 degrees to less than 90.0 degrees to the liquid crystal molecules 32 in the high-pretilt region R2a. In a normally black mode vertical alignment liquid crystal display panel, if the pretilt angle Θ1 in the low-pretilt region R1a were less than 84.0 degrees, the black luminance would increase so much that the contrast ratio would decrease. Also, if the pretilt angle Θ2 in the high-pretilt region R2a reached 90.0 degrees, the pretilt of the liquid crystal molecules 32 would lose stability and have their alignment disturbed, thus resulting in lower response speed (from black to white). Furthermore, to improve the γ characteristic effectively, the difference between the pretilt angle Θ1 in the low-pretilt region R1a of the liquid crystal layer 30A and the pretilt angle Θ2 in the high-pretilt region R2a within a pixel is suitably equal to or greater than 1.0 degree. The pretilt angles Θ1 and Θ2 of the low-pretilt region R1a and the high-pretilt region R2a are suitably set so as to satisfy these conditions.
The liquid crystal display panel 100B shown in
In each pixel of the liquid crystal display panel 100B, the liquid crystal layer 30B has a low-pretilt region R1b (in which the liquid crystal molecules 32 have a pretilt angle Θ1) and a high-pretilt region R2b (in which the liquid crystal molecules 32 have a pretilt angle Θ2) where the liquid crystal molecules 32 have mutually different pretilt angles. The first alignment film 16b has a first low-pretilt alignment region 16b1 (which has a pretilt angle Θa1 and) which contacts with the low-pretilt region R1b and a first high-pretilt alignment region 16b2 (which has a pretilt angle Θa2 and) which contacts with the high-pretilt region R2b. Just like the first alignment film 16b, the second alignment film 26b also has a second low-pretilt alignment region 26b1 (which has a pretilt angle Θb1 and) which contacts with the low-pretilt region R1b and a second high-pretilt alignment region 26b2 (which has a pretilt angle Θb2 and) which contacts with the high-pretilt region R2b.
The pretilt angle Θ1 of the liquid crystal molecules 32 in the low-pretilt region R1b is determined by the pretilt angle Θa1 in the first low-pretilt alignment region 16b1 of the first alignment film 16b and the pretilt angle Θb1 in the second low-pretilt alignment region 26b1 of the second alignment film 26b, and Θ1 satisfies Θ1≧Θa1 and Θ1≦Θb1. In the same way, the pretilt angle Θ2 of the liquid crystal molecules 32 in the high-pretilt region R2b is determined by the pretilt angle Θa2 in the first high-pretilt alignment region 16b2 of the first alignment film 16b and the pretilt angle Θb2 in the second high-pretilt alignment region 26b2 of the second alignment film 26b, and Θ2 satisfies Θ2≧Θa2 and Θ2≧Θb2. In this example, Θa1<Θa2 and Θb1<Θb2 are satisfied, and therefore, Θ1<Θ2 is realized. It should be noted that Θa1 and Θb1 may be equal to each other or different from each other. Likewise, Θa2 and Θb2 may also be equal to each other or different from each other.
The first and second low-pretilt alignment regions 16b1 and 26b1 suitably give a pretilt angle Θ1 of 84.0 degrees to less than 88.0 degrees to the liquid crystal molecules 32 in the low-pretilt region R1b, and the first and second high-pretilt alignment regions 16b2 and 26b2 suitably give a pretilt angle Θ2 of 88.0 degrees to less than 90.0 degrees to the liquid crystal molecules 32 in the high-pretilt region R2b.
In the liquid crystal display panels 100A and 100B of this embodiment, the liquid crystal molecules 32 in the low-pretilt region R1a or R1b may have a pretilt angle of 87.0 degrees (i.e., Θa1=Θb1=87.0 degrees), the liquid crystal molecules 32 in the high-pretilt region R2a or R2b may have a pretilt angle of 89.5 degrees (i.e., Θa2=Θb2=89.5 degrees), and their difference may be 2.5 degrees, for example.
As shown in FIGS. 2A(a) and 2A(b), each of the liquid crystal display panels 100A and 100B has three subpixel electrodes and three sub-counter electrodes within a single pixel. The pixel electrode 14 shown in FIG. 2A(a) includes three subpixel electrodes 14S within a single pixel. The counter electrode 24 shown in FIG. 2A(b) includes a sub-counter electrode 24a and two sub-counter electrodes 24b within a single pixel. In the liquid crystal display panels 100A and 100B, each pixel electrode may have the following dimensions:
Size of the pixel electrode: 66 μm×198 μm,
Electrode line width L of its branch portions: 3 μm,
Slit width S: 3 μm,
Size of the subpixel electrodes 14S: 66 μm×62 μm,
Gap 14e between the subpixel electrodes: 6 μm, and
Gap 24e between the sub-counter electrodes: 6 μm
For example, one low-pretilt region R1a or R1b and two high-pretilt regions R2a or R2b may be defined within a single pixel. The sub-counter electrode 24a is arranged so as to face the low-pretilt region R1a or R1b, and the two sub-counter electrodes 24b are arranged so as to face the two high-pretilt regions R2a or R2b. Also, when viewed along a normal to the liquid crystal display panel 100A or 100B, the low-pretilt region R1a or R1b is arranged between the two high-pretilt regions R2a or R2b.
Hereinafter, the subpixel electrodes will be described with reference to FIG. 2A(a), in which when the display screen is compared to a clock face, the three o'clock direction is supposed to have an azimuth angle of zero degrees and the angle is supposed to increase counterclockwise.
The three subpixel electrodes 14S shown in FIG. 2A(a) are arranged so as to face the sub-counter electrodes 24a, 24b and are electrically connected together. Also, each of these three subpixel electrodes 14S may define four liquid crystal domains with mutually different alignment azimuths.
Each of these subpixel electrodes 14S includes a trunk portion 14a2 which runs in the azimuth angle 0 degree-180 degree direction, another trunk portion 14a1 which runs in the azimuth angle 90 degree-270 degree direction, a plurality of branch portions 14b1 (which will be referred to herein as “first branch portions” and) which runs in the azimuth angle 45 degree-225 degree direction (which will be referred to herein as a “first direction”) and a plurality of branch portions 14b2 (which will be referred to herein as “second branch portions” and) which runs in the azimuth angle 135 degree-315 degree direction (which will be referred to herein as a “second direction”).
The trunk portion 14a2 includes a trunk portion 14a2a which runs in the azimuth angle 0 degree direction from the intersection between the trunk portions 14a2 and 14a1 around the center of the subpixel electrode 14S and another trunk portion 14a2b which runs in the azimuth angle 180 degree direction from that intersection. The trunk portion 14a1 includes a trunk portion 14a1a which runs in the azimuth angle 90 degree direction from that intersection and another trunk portion 14a1b which runs in the azimuth angle 270 degree direction from that intersection.
The branch portions 14b1 include a plurality of branch portions 14b1a which runs in the azimuth angle 45 degree direction from the trunk portion 14a1a or 14a2a and a plurality of branch portions 14b1b which runs in the azimuth angle 225 degree direction from the trunk portion 14a1b or 14a2b. The branch portions 14b2 include a plurality of branch portions 14b2b which runs in the azimuth angle 135 degree direction from the trunk portion 14a1a or 14a2b and a plurality of branch portions 14b2a which runs in the azimuth angle 315 degree direction from the trunk portion 14a2a or 14a1b.
The respective sub-counter electrodes 24a and 24b are electrically independent of each other and are arranged with gaps 24e left between them. The two sub-counter electrodes 24b are arranged in association with the high-pretilt regions and the sub-counter electrode 24a located between the two sub-counter electrodes 24b is arranged in association with the low-pretilt region. The gaps 24e suitably overlap with a non-transparent metal layer (such as a gate bus line, a source bus line or a storage capacitor line) or the boundary region between the low-pretilt region R1a and the high-pretilt region R2a of the liquid crystal layer 30A, 30B. If such gaps 24e are left, then the effect of checking a decrease in aperture ratio can be achieved.
If a voltage is applied to between the pixel electrode 14 and the counter electrode 24 that are arranged to face each other with the liquid crystal layer interposed between them, an electric field generated in the liquid crystal layer produces four kinds of liquid crystal domains, in which the directors point mutually different azimuths. In this case, the azimuth pointed by the directors in each liquid crystal domain is parallel to any of the branch portions 14b1a, 14b1b, 14b2a and 14b2b. If the TFT substrate is arranged to face down and the counter substrate is arranged to face the viewer, the directors in the domain produced by the branch portions 14b1a point the azimuth angle 225 degree direction. Likewise, the directors in the domain produced by the branch portions 14b2b point the azimuth angle 315 degree direction. The directors in the domain produced by the branch portions 14b1b point the azimuth angle 45 degree direction. And the directors in the domain produced by the branch portions 14b2a point the azimuth angle 135 degree direction. Of two polarizers which are arranged as crossed Nicols, one polarizer has its polarization axis (i.e., transmission axis) defined in the horizontal direction (i.e., 0 degree-180 degree direction) and the other polarizer has its polarization axis defined in the vertical direction (i.e., 90 degree-270 degree direction). And the directors in the four kinds of liquid crystal domains point azimuths which are bisectors between the two orthogonal polarization axes. As a result, a wide viewing angle is realized.
In each subpixel electrode 14S, the branch portions 14b1 and 14b2 have the same width L and the interval between two arbitrary adjacent branch portions 14b1 (which will be referred to herein as a “slit width S”) is the same as the interval between two arbitrary adjacent branch portions 14b2 (which will also be referred to herein as a “slit width S”). The branch portions 14b1 and 14b2 suitably have an electrode line width L of 1 μm to 5 μm and a slit width S of 1 μm to 5 μm, too.
Two adjacent subpixel electrodes 14S are electrically connected together with a linking portion 14d extended from the trunk portion 14a1. Between those subpixel electrodes 14S, there is only a gap 14e but the linking portion 14d.
Furthermore, as also can be seen from the subpixel electrodes 24a and 24b shown in FIG. 2A(b), the combined area of the high-pretilt regions R2a or R2b suitably accounts for a larger percentage of a single pixel than the area of the low-pretilt region R1a or R1b does. For example, the combined area of the high-pretilt regions R2a or R2b is suitably twice as large as the area of the low-pretilt region R1a or R1b. If such a relation is satisfied, the effect of further reducing the viewing angle dependence of the γ characteristic can be achieved.
In this example, an electrode pattern with a fishbone structure comprised of the trunk portions 14a1 and 14a2 and a plurality of branch portions 14b1 and 14b2 as shown in FIG. 2A(a) is used as an exemplary pixel electrode structure. However, as long as a pixel electrode can be divided into multiple subpixel electrodes, the fishbone structure does not always have to be used. Alternatively, the pixel electrode 14 may also have a solid structure in which the subpixel electrodes 14S have no trunk portions or branch portions as shown in FIG. 2B(a). In that case, the liquid crystal molecules in each pixel will be aligned radially around an alignment control structure 24f (which is typically a rivet made of a photosensitive resin or a no electrode (hole) region) arranged on each sub-counter electrode 14S so as to face the center of its associated subpixel electrode 14S as shown in 2B(b). In other words, in that case, there will be an infinitely divided structure around the alignment control structure 24f, i.e., an infinite number of domains in which the directors point mutually different alignment azimuths, and the alignment azimuths pointed by those directors will change continuously around the alignment control structure 24f. Such an alignment is sometimes called a “continuous pinwheel alignment”. Also, when a voltage is applied to the liquid crystal layer, an oblique electric field will be generated in the vicinity of the gaps 14e between the subpixel electrodes 14S and such an infinitely divided structure will be formed around the alignment control structure 24f under the influence of that electric field.
Now take a look at
Hereinafter, it will be described how to fabricate the liquid crystal display panel 100A shown in
A liquid crystal cell with a liquid crystal layer including a liquid crystal material (which may be a negative liquid crystal material produced by Merck & Co., Inc.) and 0.25 wt % of photo-polymerizable monomer (which may be an acrylic monomer produced by Merck & Co., Inc.) is provided between the first and second substrates 10A and 20A. A vertical alignment film (which may be a vertical alignment film produced by JSR Corporation and which corresponds to the alignment films 16a and 26a) is formed on each of the first and second substrates 10A and 20A (which may be a TFT substrate and a color filter substrate, respectively), and a liquid crystal layer is formed so as to contact with each of those vertical alignment films.
Next, with a voltage of 2.5 V (which may be an AC voltage at a frequency of 60 Hz) applied to the sub-counter electrode 24b associated with a region to be the high-pretilt region R2a, a voltage of 20 V (which may be an AC voltage at a frequency of 60 Hz) applied to the sub-counter electrode 24a associated with a region to be the low-pretilt region R1a, a voltage of 10 V (which may be a DC voltage) applied to a terminal corresponding to the gate terminal of a TFT, and a voltage of 0 V applied to a terminal corresponding to the source electrode and a terminal corresponding to the storage capacitor electrode, the entire surface of the liquid crystal cell was irradiated with an ultraviolet ray from over the first substrate 10A under such a condition that the exposure would be 5 J/cm2 at a wavelength of 365 nm.
This is a process that uses the PSA technology (and will be sometimes referred to herein as a “PSA process”). According to the conventional PSA technology, the counter electrode never has a plurality of sub-counter electrodes or the PSA process is never carried out with multiple different voltages applied within a single pixel, and therefore, the liquid crystal molecules 32 will have a uniform pretilt angle within a single pixel after subjected to the PSA process.
On the other hand, according to an embodiment of the present invention, a plurality of sub-counter electrodes 24a, 24b are provided within a single pixel, and the PSA process is carried out with at least two different voltages applied within a single pixel, and therefore, there can be multiple groups of liquid crystal molecules 32 with mutually different pretilt angles within a single pixel. For example, after being subjected to the PSA process, their pretilt angle will be 89.5 degrees in the high-pretilt region R2a and will be 87.0 degrees in the low-pretilt region R1a and the difference in pretilt angle between those regions within a single pixel will be 2.5 degrees.
Now take a look at
Hereinafter, it will be described how to fabricate the liquid crystal display panel 100B shown in
A liquid crystal cell with a liquid crystal layer including a liquid crystal material (which may be a negative liquid crystal material produced by Merck & Co., Inc.) is provided between the first and second substrates 10B and 20B. A photosensitive vertical alignment film (which may be a photosensitive vertical alignment film produced by JSR Corporation and which corresponds to the alignment films 16b and 26b) is formed on each of the first and second substrates 10B and 20B (which may be a TFT substrate and a color filter substrate, respectively), and a liquid crystal layer is formed so as to contact with each of those photosensitive vertical alignment films.
Next, with a voltage of 2.5 V (which may be an AC voltage at a frequency of 60 Hz) applied to the sub-counter electrode 24b associated with a region to be the high-pretilt region R2b, a voltage of 20 V (which may be an AC voltage at a frequency of 60 Hz) applied to the sub-counter electrode 24a associated with a region to be the low-pretilt region R1b, a voltage of 10 V (which may be a DC voltage) applied to a terminal corresponding to the gate terminal of a TFT, and a voltage of 0 V applied to a terminal corresponding to the source electrode and a terminal corresponding to the storage capacitor electrode, the entire surface of the liquid crystal cell was irradiated with an ultraviolet ray from over the first substrate 10B under such a condition that the exposure would be 5 J/cm2 at a wavelength of 365 nm.
This is a process of giving a pretilt to liquid crystal molecules using a cross-linking structure, dimer or isomer 35 formed through a photo-reaction of the photosensitive vertical alignment film. According to the conventional technology using a photosensitive vertical alignment film, the counter electrode never has a plurality of sub-counter electrodes or the process of giving a pretilt to liquid crystal molecules is never carried out with multiple different voltages applied within a single pixel, and therefore, the liquid crystal molecules 32 will have a uniform pretilt angle within a single pixel after subjected to that process.
On the other hand, according to an embodiment of the present invention, a plurality of sub-counter electrodes 24a, 24b are provided within a single pixel, and the process of giving a pretilt to liquid crystal molecules is carried out with at least two different voltages applied within a single pixel, and therefore, there can be multiple groups of liquid crystal molecules 32 with mutually different pretilt angles within a single pixel. For example, after being subjected to that process, their pretilt angle will be 89.5 degrees in the high-pretilt region R2b and will be 87.0 degrees in the low-pretilt region R1b and the difference in pretilt angle between those regions within a single pixel will be 2.5 degrees.
a) shows the respective (front) V-T (voltage-transmittance) characteristics of a liquid crystal display panel in which “high-pretilt+low-pretilt” regions are defined within a pixel by the method shown in
As a comparative example,
a) shows the (front and oblique) γ characteristics of a liquid crystal display panel as a specific example of the present invention, and
As can be seen from
Meanwhile, as can be seen from
Unlike the liquid crystal display device disclosed in Patent Document No. 6, the liquid crystal display panels 100A and 100B can exhibit the characteristics shown in
As can be seen from
In the liquid crystal display panel in which the entire pixel is defined to be either a high-pretilt region or a low-pretilt region, there is a significant variation in transmission intensity (i.e., the γ shift) at an oblique viewing angle, and the degree of the γ shift hardly changes according to the pretilt angle. The reason is that the γ characteristic is not set by the voltage but by the grayscale based on the transmission intensity (relative intensity). That is to say, even if the V-T characteristic is shifted by changing the pretilt angle of the entire pixel, the grayscale is set based on the transmission intensity (that is a relative intensity). That is why even if the voltage value associated with each grayscale changes, the γ characteristic hardly changes, and therefore, the γ shift hardly changes, either.
On the other hand, as can be seen from
The liquid crystal display panel of the specific example of the present invention can reduce the γ shift in the low-grayscale range, in particular. If the γ shift increased in the low-grayscale range (i.e., if the transmission intensity increased at an oblique viewing angle), then the display quality would be affected more significantly than in a situation where the γ shift increases in the high-grayscale range. In a mode with a high contrast ratio such as a VA (vertical alignment) mode, in particular, if the shift in the low-grayscale range were significant, then a whitening phenomenon would be produced and the display panel would give an impression as if the panel had a low contrast ratio, and would have a decreased product value. In the liquid crystal display panels 100A and 100B according to an embodiment of the present invention, the γ shift in the low-grayscale range is reduced to achieve significant improvement effects and to check a decrease in transmittance.
In the liquid crystal display device 100A described above, the pretilt angle of the high-pretilt region R2a is set to be 89.5 degrees and the pretilt angle of the low-pretilt region R1a is set to be 87.0 degrees. However, if the voltage applied to the sub-counter electrodes during the PSA process and the exposure of an ultraviolet ray are changed, the pretilt angles of the respective regions can be changed. The following Table 1 shows a relation between the PSA process condition and the pretilt angle. In Table 1, the high-pretilt region and the low-pretilt region sometimes have the same pretilt angle. However, this setting was adopted to find the relation between the pretilt angle and the luminance in the black display mode (black luminance) or the relative contrast ratio, and a common voltage was applied to the sub-counter electrodes of the respective regions.
The pretilt angle tends to increase roughly proportionally to the (AC) voltage applied to the liquid crystal layer and the exposure of the ultraviolet ray (UV ray exposure). If the entire surface is exposed, the pretilt angle can be controlled by setting the voltages applied to the respective regions to be arbitrary ones.
Also, even if a photosensitive group is included in the alignment film and if pretilt is given by applying a voltage to the liquid crystal layer and exposing it to an ultraviolet ray after the liquid crystal material has been injected as in the liquid crystal display panel 100B, the pretilt angle also tends to increase proportionally to the voltage applied to the liquid crystal layer and the exposure of the ultraviolet ray. That is why if the entire surface is exposed, the pretilt angle can be controlled by setting the voltages applied to the respective regions to be arbitrary ones.
As can be seen from
The following Table 2 shows the relation between the pretilt angle and the optical property values. The data shown in the following Table 2 includes the results of measurement obtained from a liquid crystal display panel in which a low-pretilt region R1a and a high-pretilt region R2a are present within a single pixel as in the liquid crystal display panel 100A:
As can be seen from Table 2, if the average pretilt angle within a single pixel is equal to or greater than 87.0 degrees, the increase in black luminance ratio and the decrease in relative contrast ratio can be limited to 20% or less. It can also be seen that if the average pretilt angle within a single pixel is equal to or smaller than 86.0 degrees, the black luminance ratio and the relative contrast ratio start to steeply rise and fall, respectively.
That is to say, to check the decrease in relative contrast ratio, the average pretilt angle within a single pixel is suitably set to be equal to or greater than 87.0 degrees. If the high-pretilt region R2a and the low-pretilt region R1a have a ratio of two to one, the high-pretilt region R2a suitably has a pretilt angle of 88.0 degrees to less than 90.0 degrees and the low-pretilt region R1a suitably has a pretilt angle of 84.0 degrees to less than 88.0 degrees. In this case, the pretilt angle of the high-pretilt region R2a is supposed to be less than 90.0 degrees, because if the high-pretilt region R2a had a pretilt angle of 90.0 degrees or more, the liquid crystal molecules 32 would have their alignment azimuth unfixed (i.e., their alignment would lose stability) and sometimes would cause some roughness and a significant delay in response.
Next, the effect of improving the γ characteristic will be described with reference to
L1: Θ1=87.0°, Θ2=90.0°, Θ2−Θ1=3.0
L2: Θ1=87.0°, Θ2=89.5°, Θ2−Θ1=2.5
L3: Θ1=87.0°, Θ2=89.0°, Θ2−Θ1=2.0
L4: Θ1=88.0°, Θ2=89.0°, Θ2−Θ1=1.0
L5: Θ1=88.0°, Θ2=88.0°, Θ2−Θ1=0.0
As can be seen from
As also can be seen from
As can be seen from the foregoing description, the liquid crystal display panels 100A and 100B according to an embodiment of the present invention are applicable broadly to any vertical alignment liquid crystal display device operating in the normally black mode.
Embodiments of the present invention are broadly applicable to a liquid crystal display panel and method for fabricating the panel, and are applicable particularly effectively to a vertical alignment liquid crystal display device operating in the normally black mode.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-027125 | Feb 2012 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2013/052749 | 2/6/2013 | WO | 00 |
