Patent Application
20140347614
The present invention relates to a liquid crystal display device and more particularly relates to a vertical alignment (VA) mode liquid crystal display device in which liquid crystal domains with axisymmetric alignment are produced upon the application of a voltage.
Liquid crystal display devices have a significantly reduced depth and will dissipate much less power than other kinds of display devices. By taking advantage of these features, liquid crystal display devices have recently been used extensively in various kinds of information devices including laptop personal computers, cellphones, and electronic organizers and videotape recorders with a built camera and an LCD monitor.
A vertical alignment (VA) mode which uses a vertical alignment liquid crystal layer has attracted a lot of attention these days as a display mode that would achieve a high contrast ratio and a wide viewing angle. A vertical alignment liquid crystal layer is generally comprised of a liquid crystal material with negative dielectric anisotropy and a vertical alignment film.
Patent Document No. 1 proposes a vertical alignment mode which is called a “CPA (continuous pinwheel alignment) mode”. In the CPA mode, typically a pixel electrode is divided by a hole and/or a notch at a predetermined position into a plurality of subpixel electrodes. When a voltage is applied to the liquid crystal layer, an oblique electric field is generated in the vicinity of the outer periphery of each of those subpixel electrodes, thereby producing axisymmetrically aligned liquid crystal domains with radially tilted orientations. As a result, a display operation can be carried out at a wide viewing angle.
Meanwhile, Patent Document No. 2 discloses a technique for stabilizing the axisymmetric alignments of liquid crystal molecules in the CPA mode. According to the technique of Patent Document No. 2, the axisymmetric alignment induced by the electrode structure of an active-matrix substrate (i.e., a pixel electrode which has been divided into a plurality of subpixel electrodes) is stabilized by an alignment control structure provided for the counter substrate. The alignment control structure is arranged in a region corresponding to substantially the center of a liquid crystal domain. The alignment control structure may be either a raised portion (which will be referred to herein as an “alignment controlling projection”) or a hole which has been cut through the counter electrode.
On the other hand, a PSA (polymer sustained alignment) technology has been proposed in order to increase the stability of alignment and response speed in the vertical alignment mode (see Patent Documents Nos. 3 and 4, for example). According to the PSA technology, the pretilt direction of liquid crystal molecules is controlled by putting a photo-polymerizable material on an alignment film. The photo-polymerizable material (which is also called an “alignment sustaining layer”) is obtained by introducing a small amount of a photo-polymerizable compound (such as a photo-polymerizable monomer) into a liquid crystal material and irradiating the photo-polymerizable compound with an ultraviolet ray with a predetermined voltage applied to the liquid crystal layer after the liquid crystal panel has been assembled. The alignment state of the liquid crystal molecules when the alignment sustaining layer is formed is sustained (i.e., memorized) by the alignment sustaining layer even after the voltage is removed (i.e., no longer applied). As a result, the stability of alignments and response speed can be increased.
Recently, as smart phones and tablet display devices have become increasingly popular, it has become more and more common these days to design those electronic devices so that an input device such as a touchscreen panel is arranged on an LCD panel. Thus, those electronic devices are more and more often designed to be used by having the user apply stress intentionally onto the surface of the liquid crystal display device.
However, in a situation where a CPA mode liquid crystal display device is used in that way, even if an alignment control structure is provided for the counter substrate, it is still difficult to stabilize the axisymmetric alignment sufficiently. For that reason, if the user puts his or her finger or a dedicated input tool (i.e., a so-called “stylus”) on the surface of the liquid crystal display device, the trace could be left (or recognized) as a kind of display unevenness. Such unevenness (which will be referred to herein as “trace unevenness”) would cause a decline in display quality.
The axisymmetric alignment could be stabilized by not only providing such an alignment control structure for the counter substrate but also adopting the PSA technology. In that case, however, when the LCD panel is fabricated, the process step of forming an alignment sustaining layer should be performed. That is why it would take a longer time and an increased cost to get the manufacturing process done. In addition, even if the PSA technology is adopted, the trace unevenness may not be reduced sufficiently.
The present inventors perfected our invention in order to overcome these problems by providing a CPA mode liquid crystal display device with significantly reduced trace unevenness.
A liquid crystal display device according to an embodiment of the present invention includes: a plurality of pixels arranged in a matrix pattern; a first substrate on which a pixel electrode is provided for each of the plurality of pixels; a second substrate including a counter electrode arranged to face the pixel electrodes; and a vertical alignment liquid crystal layer interposed between the first and second substrates. At least one liquid crystal domain with axisymmetric alignment is produced when a voltage is applied between the pixel electrode and the counter electrode in each of the plurality of pixels. The second substrate further includes an alignment controlling projection and a plurality of columnar spacers. The projection is arranged in a region corresponding to substantially the center of the at least one liquid crystal domain and induces liquid crystal molecules in the at least one liquid crystal domain to get aligned axisymmetrically. The plurality of columnar spacers includes a first columnar spacer and a second columnar spacer which is lower than the first columnar spacer. The pixel electrode has a length of 35 μm or less as measured along its shorter sides. In at least some of the pixels, the second columnar spacer functions as the alignment controlling projection. And the height of the second columnar spacer is 75% to 92% of that of the first columnar spacer.
In one embodiment, the at least one liquid crystal domain includes a plurality of liquid crystal domains, and the pixel electrode includes a plurality of subpixel electrodes associated with the plurality of liquid crystal domains.
In one embodiment, the first substrate further includes a thin-film transistor provided for each of the plurality of pixels and an interlayer insulating film covering the thin-film transistors. A contact hole is cut through the interlayer insulating film to electrically connect the pixel electrode to the thin-film transistor. And the contact hole is arranged so as to overlap with the alignment controlling projection that is associated with one of the plurality of liquid crystal domains when viewed along a normal to a display screen.
In one embodiment, the thin-film transistor is arranged so as to overlap with the alignment controlling projection that is associated with another one of the plurality of liquid crystal domains when viewed along a normal to the display screen.
In one embodiment, the second substrate further includes a shielding layer, which includes first and second shielding portions which overlap with the contact hole and the thin-film transistor, respectively, when viewed along a normal to the display screen.
In one embodiment, in each of the plurality of pixels, the second columnar spacer functions as the alignment controlling projection that is associated with at least some of the plurality of liquid crystal domains.
In one embodiment, the plurality of pixels includes a pixel in which the first columnar spacer functions as the alignment controlling projection.
In one embodiment, the at least one liquid crystal domain includes only one liquid crystal domain.
In one embodiment, the first substrate further includes a thin-film transistor provided for each of the plurality of pixels, and the thin-film transistor is arranged so as to overlap with the alignment controlling projection when viewed along a normal to the display screen.
In one embodiment, the second substrate further includes a shielding layer, which includes a shielding portion that overlaps with the thin-film transistor when viewed along a normal to the display screen.
In one embodiment, in some of the plurality of pixels, the first columnar spacer functions as the alignment controlling projection and in the other pixels, the second columnar spacer functions as the alignment controlling projection.
In one embodiment, each of the plurality of columnar spacers is either the first or second columnar spacer that functions as the alignment controlling projection, and the second substrate has no additional columnar spacers which are provided anywhere but the region corresponding to substantially the center of the at least one liquid crystal domain.
In one embodiment, the second columnar spacer is lower by approximately 0.5 μm than the first columnar spacer.
Another liquid crystal display device according to an embodiment of the present invention includes: a plurality of pixels arranged in a matrix pattern; a first substrate on which a pixel electrode is provided for each of the plurality of pixels; a second substrate including a counter electrode arranged to face the pixel electrodes; and a vertical alignment liquid crystal layer interposed between the first and second substrates. At least one liquid crystal domain with axisymmetric alignment is produced when a voltage is applied between the pixel electrode and the counter electrode in each of the plurality of pixels. The first substrate further includes a thin-film transistor provided for each of the plurality of pixels. The second substrate further includes an alignment controlling projection and a plurality of columnar spacers. The projection is arranged in a region corresponding to substantially the center of the at least one liquid crystal domain and induces liquid crystal molecules in the at least one liquid crystal domain to get aligned axisymmetrically. The plurality of columnar spacers includes a first columnar spacer and a second columnar spacer which is lower than the first columnar spacer. In at least some of the pixels, the second columnar spacer functions as the alignment controlling projection. And the thin-film transistor is arranged so as to overlap with the alignment controlling projection that is associated with one of the at least one liquid crystal domain when viewed along a normal to a display screen.
In one embodiment, the at least one liquid crystal domain includes a plurality of liquid crystal domains, and the pixel electrode includes a plurality of subpixel electrodes associated with the plurality of liquid crystal domains.
In one embodiment, the first substrate further includes an interlayer insulating film covering the thin-film transistors. A contact hole is cut through the interlayer insulating film to electrically connect the pixel electrode to the thin-film transistor. And the contact hole is arranged so as to overlap with the alignment controlling projection that is associated with one of the plurality of liquid crystal domains when viewed along a normal to the display screen.
In one embodiment, the second substrate further includes a shielding layer, which includes first and second shielding portions that overlap with the contact hole and the thin-film transistor, respectively, when viewed along a normal to the display screen.
In one embodiment, in each of the plurality of pixels, the second columnar spacer functions as the alignment controlling projection that is associated with at least some of the plurality of liquid crystal domains.
In one embodiment, the plurality of pixels includes a pixel in which the first columnar spacer functions as the alignment controlling projection.
In one embodiment, the at least one liquid crystal domain includes only one liquid crystal domain.
In one embodiment, the second substrate further includes a shielding layer, which includes a shielding portion that overlaps with the thin-film transistor when viewed along a normal to the display screen.
In one embodiment, in some of the plurality of pixels, the first columnar spacer functions as the alignment controlling projection and in the other pixels, the second columnar spacer functions as the alignment controlling projection.
In one embodiment, each of the plurality of columnar spacers is either the first or second columnar spacer that functions as the alignment controlling projection, and the second substrate has no additional columnar spacers which are provided anywhere but the region corresponding to substantially the center of the at least one liquid crystal domain.
In one embodiment, the plurality of columnar spacers has a lower relative dielectric constant than the liquid crystal layer.
In one embodiment, the first substrate further includes a signal line which is electrically connected to the drain electrode of the thin-film transistor of a pixel belonging to a row of pixels, and the signal line meanders so as to run across respective pixels of its associated row of pixels.
An embodiment of the present invention provides a CPA mode liquid crystal display device which can minimize the trace unevenness.
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. It should be noted that the present invention is in no way limited to the embodiments to be described below.
This liquid crystal display device 100 includes a plurality of pixels which are arranged in a matrix pattern. In
The active-matrix substrate (first substrate) 10 includes a pixel electrode 11 which is provided for each of the plurality of pixels. The pixel electrode 11 is typically made of a transparent conductive material (such as ITO). As shown in
The active-matrix substrate 10 further includes a thin-film transistor (TFT) 12 which is provided for each of the plurality of pixels and an interlayer insulating film 13 which covers the TFT 12. The TFT 12 includes a gate electrode 12g, a source electrode 12s, a drain electrode 12d and a semiconductor layer 12a. A contact hole 13a to electrically connect the pixel electrode 11 to the TFT 12 has been cut through the interlayer insulating film 13.
These components of the active-matrix substrate 10, including the pixel electrode 11, TFT 12 and interlayer insulating film 13 described above, are arranged on a transparent substrate 10a with an electrically insulating property (which is typically a glass substrate).
Specifically, on the surface of the transparent substrate 10a that faces the liquid crystal layer 30, arranged are not only the gate electrode 12g of the TFT 12 but also a scan line GL, a storage capacitor line CsL and a storage capacitor counter electrode 14. The scan line GL is electrically connected to the gate electrode 12g of the TFT 12 and supplies a scan signal to the TFT 12. The storage capacitor line CsL is electrically connected to the storage capacitor counter electrode 14 and applies a storage capacitor counter voltage to the storage capacitor counter electrode 14.
A gate insulating film 15 has been formed so as to cover the gate electrode 12g and the scan line GL. On the gate insulating film 15, arranged are not only the source and drain electrodes 12s and 12d of the TFT 12 but also a signal line SL and a storage capacitor electrode 16. On a portion of the gate insulating film 15 which faces the gate electrode 12g, arranged is the semiconductor layer 12a of the TFT 12 (not shown in
The interlayer insulating film 13 has been formed so as to cover the source and drain electrodes 12s and 12d of the TFT 12 and the signal line SL. The pixel electrode 11 has been formed on the interlayer insulating film 13. The pixel electrode 11 is connected to the storage capacitor electrode 16 through the contact hole 13a and electrically connected to the drain electrode 12d of the TFT 12 via the storage capacitor electrode 16.
The counter substrate 20 includes a counter electrode 21 which is arranged to face the pixel electrode 11. The counter electrode 21 is made of a transparent conductive material (such as ITO). The pixel electrodes 11 are provided for respective pixels independently of each other. On the other hand, the counter electrode 21 is typically a single continuous conductive film which covers the entire display area and is an electrode to be shared in common by every pixel (i.e., a common electrode).
In addition, the counter substrate 20 further includes a color filter layer 22 and a shielding layer 23. The color filter layer 22 includes a red color filter 22R which is provided for a pixel to display the color red, a green color filter 22G which is provided for a pixel to display the color green, and a blue color filter 22B which is provided for a pixel to display the color blue. The shielding layer 23 (of which the outer periphery is indicated by the dotted line in
The counter substrate 20 further includes a plurality of columnar spacers 24, which may be made of a photosensitive resin, for example. Those columnar spacers 24 include a first type of columnar spacers (main spacers) (not shown in
Generally speaking, in a liquid crystal display device of the type that uses columnar spacers, if the density of columnar spacers (i.e., the number of columnar spacers per unit area) were increased to enhance the withstand load thereof, it would be more and more difficult for the cell gap to catch up with the shrinkage of a liquid crystal layer that could occur at a low temperature. As a result, bubbles would be produced in the liquid crystal layer (which phenomenon will be referred to herein as “low-temperature bubbling”). However, if two different types of columnar spacers 24 with mutually different heights are provided and if the cell gap is controlled using basically only the main spacers (i.e., the higher columnar spacers) as is done in this embodiment, the effective spacer density will be defined by only the main spacers, and it will be easier for the cell gap to catch up with the shrinkage of the liquid crystal layer 30. Also, if the cell gap has become narrower due to a load imposed on the LCD panel, then the two substrates will be supported by both the main spacers and the sub-spacers (i.e., the lower columnar spacers) 24s. In that case, the effective spacer density will be defined by both the main spacers and the sub-spacers 24s, and therefore, a high withstand load can be achieved. As can be seen, by providing two types of columnar spacers 24 with mutually different heights, low-temperature bubbling can be suppressed and the withstand load can be increased at the same time.
The components of the counter substrate 20, including the counter electrode 21 and the color filter layer 22 described above, are arranged on a transparent substrate 20a with an electrically insulating property (which is typically a glass substrate). Specifically, the color filter layer 22 and the shielding layer 23 are arranged on the surface of the transparent substrate 20a that faces the liquid crystal layer 30, and the counter electrode 21 is arranged over those layers. And those columnar spacers 24 are dispersed over the counter electrode 21.
The liquid crystal layer 30 is a vertical alignment liquid crystal layer. That is to say, the liquid crystal molecules 31 included in the liquid crystal layer 30 have negative dielectric anisotropy and are aligned substantially perpendicularly to (typically so as to define a pretilt angle of 85 degrees or more with respect to) the surface of the substrate as shown in
In this liquid crystal display device 100, when a voltage is applied between the pixel electrode 11 and the counter electrode 21 in each pixel, a plurality of liquid crystal domains with axisymmetric alignment are produced as shown in
In this embodiment, the length L of the pixel electrode 11 as measured along its shorter sides (see
Also, in this liquid crystal display device 100, each of the plurality of columnar spacers 24 is arranged in a region corresponding to substantially the center of its associated liquid crystal domain and functions as an alignment controlling projection which induces axisymmetric alignment of liquid crystal molecules in the liquid crystal domain. As a result, the axisymmetric alignment of each liquid crystal domain is stabilized by the alignment controlling force of those columnar spacers 24 that function as alignment controlling projections (i.e., thanks to the anchoring effect produced by their surface). In the pixel shown in
It should be noted that the relative dielectric constant of the columnar spacers 24 is suitably lower than the relative dielectric constant of the liquid crystal layer 30 (i.e., that of the liquid crystal material that makes the liquid crystal layer 30) in order to prevent the tilt direction of the liquid crystal molecules 31 defined by the shape of the side surface of the columnar spacers 24 and the direction in which the electric field is going to tilt the liquid crystal molecules 31 when a voltage is applied to the liquid crystal layer 30 from becoming opposite from each other.
The plurality of pixels of this liquid crystal display device 100 includes some pixels in which the main spacers function as alignment controlling projections. For example, in the pixel shown in
If the arrangement density of those columnar spacers 24 is represented on an area basis, the main spacers 24m may have an arrangement density of 0.14 to 0.20% while the sub-spacers 24s may have an arrangement density of 0.90 to 10.0%. That is why in a lot of pixels, both of the two columnar spacers 24 are sub-spacers 24s. And in some pixels, one of the two columnar spacers 24 is a sub-spacer 24s and the other columnar spacer 24 is a main spacer 24m. Consequently, in each pixel, the sub-spacer 24s functions as an alignment controlling projection for at least some of the plurality of liquid crystal domains.
In this embodiment, the height Hs of the sub-spacers 24s is set so as to fall within a predetermined range with respect to the height Hm of the main spacers 24m. Specifically, the height Hs of the sub-spacers Hs is 75 to 92% of the height Hm of the main spacers 24m (i.e., 0.75 Hm≦Hs≦510.92 Hm is satisfied). For example, if the height Hm of the main spacers 24m is 3.6 μm (i.e., if the thickness of the liquid crystal layer 30 (or the cell gap) is 3.6 μm), the height Hs of the sub-spacers 24s is approximately 2.7 to 3.3 μm. Also, if the height Hm of the main spacers 24m is 3.1 μm (i.e., if the thickness of the liquid crystal layer 30 (or the cell gap) is 3.1 μm), the height Hs of the sub-spacers 24s is approximately 2.3 to 2.9 μm.
Also, in this embodiment, the contact hole 13a is arranged so as to overlap with an alignment controlling projection (i.e., the sub-spacer 24s) associated with one of the plurality of liquid crystal domains (e.g., one of the two liquid crystal domains in this example) when viewed along a normal to the display screen. Furthermore, the TFT 12 is arranged so as to overlap with an alignment controlling projection (i.e., the sub-spacer 24s or the main spacer 24m) associated with another one of the plurality of liquid crystal domains (e.g., the other of the two liquid crystal domains in this example) when viewed along a normal to the display screen.
By adopting the configuration described above, the liquid crystal display device 100 of this embodiment can minimize the trace unevenness. The reason will be described with reference to a liquid crystal display device 700 as a comparative example shown in
In the liquid crystal display device 700 of the comparative example shown in
In the liquid crystal display device 700, a hole 21a which has been cut through the counter electrode 21 is located in a region corresponding to substantially the center of each liquid crystal domain and functions as an alignment control structure. When a voltage is applied between the pixel electrode 11 and the counter electrode 21, an oblique electric field is generated in the vicinity of the edge of the hole 21 and the axisymmetric alignment of the liquid crystal domain gets stabilized under the alignment controlling force produced by this oblique electric field.
On the other hand, in the liquid crystal display device 800 of the comparative example shown in
In this liquid crystal display device 800, the axisymmetric alignment of the liquid crystal domains gets stabilized under the alignment controlling force produced by the rivets 825 (i.e., thanks to the anchoring effect produced by their surface). If the liquid crystal layer 30 has a thickness of approximately 3 to 4 μm, the rivets 825 may have a height Hp of approximately 1.4 μm, for example.
As can be seen, in the liquid crystal display devices 700 and 800 of these comparative examples, a hole 21a and a rivet 825 are provided as their respective alignment control structures for the counter electrode 21. However, if the liquid crystal display device 700 or 800 is used with stress applied intentionally onto its surface as in smart phones and tablet display devices which have become more and more popular recently, the hole 21a or rivet 825 of the counter electrode 21 would be unable to exert sufficient alignment controlling force to the liquid crystal molecules 31 in the liquid crystal domains and stabilize the axisymmetric alignment sufficiently. Consequently, the trace unevenness would arise.
The trace unevenness arises while a voltage is being applied to the liquid crystal layer 30. In the vertical alignment liquid crystal layer 30, the liquid crystal molecules 31 are vertically aligned orderly while no voltage is being applied. That is why even if the alignment is disturbed to some extent with the pressure applied onto the surface of the panel by the user's finger, for example, the liquid crystal molecules will recover their original alignment state relatively quickly. On the other hand, while the alignment direction of the liquid crystal molecules 31 is being controlled by applying a voltage to the liquid crystal layer 30, the alignment is so disturbed that if the alignment is further disturbed with the pressure applied onto the surface of the panel by the user's finger, for example, the liquid crystal molecules could stay disturbed even after the stress applied is removed. This is the so-called “trace unevenness” phenomenon. The higher the voltage applied to the liquid crystal layer 30 is, the more noticeable the trace unevenness will get.
Hereinafter, it will be described with reference to
As shown in
On the other hand, as shown in
If the trace unevenness has occurred, some liquid crystal molecules 31 stay oriented in a direction that does not agree with the direction defined by the alignment controlling force exerted by the alignment controlling projection (i.e., alignment control structure) 825 in some region of the liquid crystal layer 30 in the pixel as shown in
a) is a micrograph of the liquid crystal display device in the white display state in which trace unevenness has occurred. In this case, the polarizers 40a and 40b are linear polarizers. For example, in the region R1 shown in
As can be seen, in the liquid crystal display devices 700 and 800 of the comparative examples, the hole 21a or rivet 825 of the counter electrode 21 does not have alignment controlling force that is strong enough to eliminate such trace unevenness.
In the liquid crystal display device 100 of this embodiment, the sub-spacer 24s functioning as an alignment controlling projection (or the main spacer 24m in some liquid crystal domains) can stabilize the axisymmetric alignment in each pixel as described above. Since the sub-spacer 24s (and the main spacer 24m) has been formed to be higher than the rivet 825 that the liquid crystal display device 800 of the comparative example has, the sub-spacer 24s can naturally exert the alignment controlling force on a larger number of liquid crystal molecules 31 than the rivet 825 does. That is to say, the sub-spacer 24s of this liquid crystal display device 100 can have sufficiently strong alignment controlling force. Consequently, the liquid crystal display device 100 of this embodiment can stabilize the axisymmetric alignment sufficiently and can suppress the trace unevenness.
In addition, since the liquid crystal display device 100 of this embodiment uses the sub-spacer 24s (or the main spacer 24m) as an alignment controlling projection, there is no need to perform an additional process step of forming alignment controlling projections when the counter substrate 20 is made. On top of that, since there is no need to adopt the PSA technology, the process step of forming an alignment sustaining layer (i.e., the PSA process) does not have to be performed while the LCD panel is being fabricated. As a result, the increase in the cost and time of the manufacturing process that would be involved with the PSA process can be avoided.
From the standpoint of exerting sufficient alignment controlling force, the height Hs of the sub-spacer 24s is suitably 75% or more of the height Hm of the main spacer 24m. Also, to make the sub-spacer 24s perform its expected function fully, the height Hs of the sub-spacer 24s is suitably 92% or less of the height Hm of the main spacer 24m. For these reasons, the height Hs of the sub-spacer 24s is suitably 75 to 92% of the height Hm of the main spacer 24m as in this embodiment.
If the main spacer 24m and the sub-spacer 24s are both made of a photosensitive resin, their heights Hm and Hs could deviate by approximately ±0.2 μm at maximum from their designed values due to variations in various manufacturing process conditions. For that reason, the difference between the designed heights Hm and Hs of the main and sub-spacers 24m and 24s is suitably at least equal to 0.4 μm. Considering this point, it can be said that in order to exert sufficient alignment controlling force, the sub-spacer 24s is suitably lower by approximately 0.5 μm than the main spacer 24m, for example.
Furthermore, the length L of the pixel electrode 11 as measured along its shorter sides is suitably equal to or smaller than 35 μm as in this embodiment. In a relatively small pixel in which the length L of the pixel electrode 11 as measured along its shorter sides is equal to or smaller than 35 μm, the columnar spacers 24 functioning as alignment controlling projections can easily exert alignment controlling force to the majority of the liquid crystal domains. As a result, the trace unevenness can be suppressed with even more certainty.
Also, for the reasons to be described below, the TFT 12 is suitably arranged so as to overlap with a columnar spacer 24 functioning as an alignment controlling projection when viewed along a normal to the display screen. In the vicinity of the columnar spacer 24, the liquid crystal molecules 31 are aligned substantially perpendicularly to the surface of the columnar spacer 24 even when no voltage is applied, and therefore, light may leak to cause a decrease in contrast ratio in some cases. For that reason, the region with the columnar spacer 24 is suitably shielded from light. Thus, if the TFT 12 is arranged so as to overlap with the columnar spacer 24 functioning as an alignment controlling projection, the region with the columnar spacer 24 can also be shielded from light with the TFT shielding portion 23b to shield the TFT 12 from light. As a result, the percentage of a portion of the entire pixel contributing to the display operation (i.e., its aperture ratio) can be increased and a brighter image can be displayed.
For the same reason, the contact hole 13a is suitably arranged so as to overlap with a columnar spacer 24 functioning as an alignment controlling projection when viewed along a normal to the display screen. If the contact hole 13a is arranged so as to overlap with the columnar spacer 24 functioning as an alignment controlling projection, the region with the columnar spacer 24 can also be shielded from light with the contact hole shielding portion 23a to shield the contact hole 13a from light. As a result, the percentage of a portion of the entire pixel contributing to the display operation (i.e., its aperture ratio) can be increased and a brighter image can be displayed.
The present inventors actually made sample liquid crystal display devices 100 of this embodiment (as Examples #1 through #6) to see whether low-temperature bubbling and trace unevenness occurred or not. The results of evaluation will be described below. For the purpose of comparison, the present inventors also made sample liquid crystal display devices 700 and 800 of the comparative examples (as Comparative Example #1 and Comparative Examples #2 through #6, respectively) to perform similar evaluations. The results will also be described below.
The following Table 1 shows the height of the main spacers 24m (corresponding to the thickness of the liquid crystal layer 30), the type and height of the alignment control structure, the ratio of the height of the alignment control structure to that of the main spacers 24m, the size of the alignment control structure, the arrangement density of the main spacers 24m, and the (vertical and horizontal) pixel sizes with respect to each of Examples #1 through #6 and Comparative Examples #1 through #6. Also, the following Table 2 shows the white voltage, the black voltage, whether the PSA treatment was carried out or not, whether low-temperature bubbling occurred or not, and whether trace unevenness occurred or not with respect to each of Examples #1 through #6 and Comparative Examples #1 through #6.
To determine whether low-temperature bubbling occurred or not, the LCD panel was observed right after a pachinko ball was dropped once from a height of 10 cm at the lower limit temperature of the keeping temperature range specified for that product. The open circle ◯ indicates that the low-temperature bubbling could be suppressed.
On the other hand, to determine whether trace unevenness occurred or not, the decision was made whether the trace disappeared or not after a strong pressure had been applied by a fingertip on the screen in the white display state. The open circle ◯ indicates that trace unevenness could be suppressed. The cross X indicates that trace unevenness occurred. And the open triangle Δ indicates that trace unevenness could be basically suppressed but there was only little margin with respect to the trace unevenness (i.e., trace unevenness occurred depending on dispersion in specification during the manufacturing process of the LCD panel).
As can be seen from Tables 1 and 2, low-temperature bubbling could be suppressed in each of Examples #1 through #6 and Comparative Examples #1 through #6. However, trace unevenness occurred in Comparative Example 1 in which a hole 21a had been cut through the counter electrode 21 as an alignment control structure and in Comparative Examples #4 to #6 in which a rivet 825 was provided as an alignment control structure. In Comparative Examples #2 and #3, even though the rivet 825 was provided as an alignment control structure, trace unevenness could be suppressed, because PSA treatment had been carried out in Comparative Examples #2 and #3.
On the other hand, in Examples #1 through #6 in which sub-spacers 24s were provided as alignment control structures, no PSA treatment was carried out but trace unevenness could be suppressed. Also, in each of Examples #1 through #6, the height Hs of the sub-spacers 24s fell within the range of 75 to 92% of the height Hm of the main spacers 24m. Thus, it can be seen that the height Hs of the sub-spacers 24s is suitably 75 to 92% of the height Hm of the main spacers 24m. Furthermore, in Example #6 in which the length L of the pixel electrode 11 as measured along its shorter sides was greater than 35 μm, there was little margin with respect to the trace unevenness. Meanwhile, in Examples #1 through #5 in which the length L of the pixel electrode 11 as measured along its shorter sides was equal to or smaller than 35 μm, such a problem did not arise. Thus, it can be seen that the length L of the pixel electrode 11 as measured along its shorter sides is suitably equal to or smaller than 35 μm.
As can be seen from
As can be seen from
In the embodiments described above, when a voltage is applied between the pixel electrode 11 and the counter electrode 21, a plurality of liquid crystal domains are supposed to be produced in each pixel. However, this is just an example of the present invention.
As shown in
In this liquid crystal display device 100′, when a voltage is applied between the pixel electrode 11 and the counter electrode 21, a single liquid crystal domain is produced in each pixel as shown in
It should be noted that the plurality of pixels of this liquid crystal display device 100′ also includes some pixels in which the main spacer 24m functions as an alignment controlling projection as shown in
Even in the liquid crystal display device 100′ shown in
For the same reason as what has already been described for the liquid crystal display device 100, the TFT 12 is suitably arranged as shown in
As can be seen from
It should be noted that the counter substrate 20 suitably has no additional columnar spacers other than the ones provided in a region corresponding to substantially the center of each liquid crystal domain in any of the liquid crystal display devices 100 and 100′ described above. The reason is that if any additional columnar spacers were provided in those regions, the alignments would be disturbed in the vicinity of those additional columnar spacers in the same way as already described with reference to
Furthermore, for the reasons to be described below, each signal line SL of the active-matrix substrate 10 suitably has a zigzag shape which meanders so as to cross respective pixels of its associated row of pixels as shown in
Since there is a parasitic capacitance (i.e., source-drain capacitance) Csd between the pixel electrode 11 and the signal line SL, the potential at the pixel electrode 11 to be retained in each pixel for one frame period varies according to the amplitude of a display signal to be supplied onto the signal line SL. In general, a pattern is formed by lens scan exposure or stepper exposure, for example, during the manufacturing process. However, the position of the signal line SL to be arranged between the pixel electrodes 11 within the panel plane shifts according to the variation in pattern forming accuracy (i.e., arrangement accuracy). If a driving method in which the polarity of the voltage applied to the liquid crystal layer 30 inverts between pixels which are adjacent to each other in the row direction (e.g., the dot inversion drive) is adopted, the difference in parasitic capacitance due to this positional shift may cause a difference in retained voltage between the pixels and may be sensed as unevenness.
If a signal line SL to supply a display signal to the pixel electrode 11 of one pixel is called “its own source” and if a signal line SL to supply a display signal to the pixel electrode 11 of another pixel which is adjacent to the former pixel in the row direction is called “another source”, the potential at the pixel electrode 11 is affected by not only a variation in voltage at its own source but also a variation in voltage at another source.
If the signal line SL has a zigzag shape as shown in
Furthermore, the number of liquid crystal domains to be produced in each pixel upon the application of a voltage does not have to be one or two as described above. But three or more liquid crystal domains may be produced in each pixel as well. Also, if multiple liquid crystal domains are produced, the pixel electrode 11 may be divided by a hole, instead of the notched portion 11b.
An embodiment of the present invention provides a CPA mode liquid crystal display device in which trace unevenness is suppressed. Since the trace unevenness can be suppressed, the liquid crystal display device of the present invention can be used effectively in the display section of a smart phone or a tablet display device.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-005670 | Jan 2012 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2013/050366 | 1/11/2013 | WO | 00 | 7/10/2014 |
