Patent Application
20110128458
The present invention relates to a liquid crystal display device. More specifically, the present intention relates to a display device that is suitably used for liquid crystal display in a transverse bend alignment (TBA) mode.
Liquid crystal display devices are widely used in electronic devices such as monitors, projectors, cellular phones, and portable information terminals (PDA). The display in a liquid crystal display device can be, for example, of transmission, reflection, and transflective type. Among these types, transmission type liquid crystal display devices that employ backlight are mainly used in a comparatively dark environment such as an indoor environment, and reflection type liquid crystal display devices that employ ambient light are mainly used in a comparatively bright environment such as an outdoor environment. Transflective type liquid crystal display devices can be in both the transmissive mode and the reflective mode and can perform mainly the transmissive display indoors and the reflective display outdoors. For this reason, transflective type liquid crystal display devices can ensure high-grade display under various environments, both indoors and outdoors, and are used in large numbers in mobile devices such as cellular phones, PDA, and digital cameras. In the transflective type liquid crystal display devices, for example, a vertical alignment (VA) mode is used as a display mode. In the VA mode, when the applied voltage is OFF, liquid crystal molecules are aligned perpendicular to the substrate surface, and when the applied voltage is ON, the display is performed by causing the liquid crystal molecules to tumble.
However, in the transflective type liquid crystal display devices, the reflected light passes through the liquid crystal layer twice, whereas the transmitted light passes through the liquid crystal layer only once. Therefore, when an optimum cell gap is designed for the reflected light, the transmittance of the transmitted light becomes about half the optimum value. For example, a method for forming a multigap structure in which a cell gap in the transmissive area differs from that in the reflective area, and the thickness of the liquid crystal layer in the reflective area is decreased has been disclosed as a means for resolving the above-described problem. However, this method requires a concavo-convex structure to be provided on the substrate and therefore the structure becomes complex. In addition, high accuracy is required in the manufacturing process.
In addition to the VA mode, an IPS mode and a FFS mode are known as modes suitable for liquid crystal display devices. In the IPS mode and FFS mode, the display is performed by rotating liquid crystal molecules in the substrate plane by a transverse electric field between pairs of electrodes for liquid crystal drive that are provided on one substrate. A transflective type liquid crystal display device operating in the IPS mode has also been disclosed (see, for example, Patent Documents 1 and 2).
Further, a TBA mode is known as a liquid crystal mode in a transverse electric field (see, for example, Patent Documents 3 to 9). In the TBA mode, the display is performed by bending the vertically aligned liquid crystal molecules in the horizontal direction by a transverse electric field created by electrode pairs for liquid crystal drive provided on one substrate. However, a transflective TBA-mode liquid crystal display device has not been disclosed.
Patent Document 1: Japanese Kokai Publication No. 2007-4126
Patent Document 2: International Patent Application Publication No. 2008/001507
Patent Document 3: Japanese Kokai Publication No. S57-618
Patent Document 4: Japanese Kokai Publication No. H10-186351
Patent Document 5: Japanese Kokai Publication No. H10-333171
Patent Document 6: Japanese Kokai Publication No. H11-24068
Patent Document 7: Japanese Kokai Publication No. 2000-275682
Patent Document 8: Japanese Kokai Publication No. 2002-55357
Patent Document 9: Japanese Kokai Publication No. 2001-159759
The present invention was contrived in view of the above circumstances and its object is to provide a TBA-mode liquid crystal display device that is capable of both the reflective display and the transmissive display, without providing a multigap structure.
The inventors have conducted a comprehensive study of TBA-mode liquid crystal display devices that are capable of both the reflective display and the transmissive display, without using a multigap structure, and focused their attention on electrode pairs for liquid crystal drive. Based on the results obtained, the inventors have found that the reflective display and transmissive display can be performed in a TBA mode, without providing a multigap structure, by making the distance between a first electrode and a second electrode disposed in parallel and opposite each other on the same substrate in the transmissive area different from that in the reflective area, and have thus arrived at the possibility of resolving the above-described problem. This finding led to the creation of the present invention.
Thus, the present invention relates to a liquid crystal display device including a first substrate and a second substrate that are disposed opposite each other and a liquid crystal layer that is interposed between the first substrate and the second substrate, and having, in a pixel area, a reflective area where reflective display is performed and a transmissive area where transmissive display is performed, wherein the first substrate has a first electrode and a second electrode disposed parallel to and opposite the first electrode in the pixel area, the liquid crystal layer includes a p-type nematic liquid crystal and is driven by an electric field generated between the first electrode and the second electrode, the p-type nematic liquid crystal is aligned perpendicular to the first substrate and the second substrate when no voltage is applied, a thickness of the liquid crystal layer in the reflective area is substantially equal to a thickness of the liquid crystal layer in the transmissive area, and a distance between the first electrode and the second electrode in the reflective area is different from a distance between the first electrode and the second electrode in the transmissive area.
In accordance with the present invention, a distance between the first electrode and the second electrode in the reflective area is different from a distance between the first electrode and the second electrode in the transmissive area and therefore the intensity of electric field generated in the liquid crystal layer in the reflective area and the intensity of electric field generated in the liquid crystal layer in the transmissive area can be adjusted separately. Therefore, even when the cell gaps in the reflective area and the transmissive area are equal to each other, the phase difference (retardation) of the liquid crystal layer in the TBA-mode liquid crystal display device can be made less in the reflective area, more specifically about half, than in the transmissive area. Thus, in the TBA-mode liquid crystal display device, the retardation of the liquid crystal layer in the reflective area can be set to about half the retardation of the liquid crystal layer in the transmissive area, without providing a multigap structure. As a result, it is possible to realize a TBA-mode liquid crystal display device that is capable of reflective display and transmissive display, without providing a multigap structure.
Further, “parallel” as referred to herein is preferably perfectly parallel, but is not necessarily parallel in the strict sense of the word, and also includes a configuration that can be treated as substantially parallel with consideration for the effect of the present invention. It may also mean parallel to a degree that can be attained when the first electrode and second electrode are designed and formed so as to be parallel, and it goes without saying that an error that can occur in the production process may be also included. Thus, “parallel” as referred to herein includes an error within a range in which the effect of the present invention is demonstrated.
Further, “perpendicular” as referred to herein is not necessarily perpendicular in the strict sense of the word, and also includes a configuration that can be treated as substantially perpendicular with consideration for the effect of the present invention. It may also include an error that can occur in the production process. Thus, “perpendicular” as referred to herein includes an error within a range in which the effect of the present invention is demonstrated.
Further, “substantially equal” as referred to herein is preferably perfectly equal, but is not necessarily equal in the strict sense of the word, and also includes a relationship that can be treated as substantially equal with consideration for the effect of the present invention. It may also mean equal to a degree that can be attained when the first substrate, second substrate, and liquid crystal layer are designed and formed so as to be equal, and it goes without saying that an error that can occur in the production process may be also included. Thus, “substantially equal” as referred to herein includes an error within a range in which the effect of the present invention is demonstrated.
The configuration of the liquid crystal display device of the present invention is not especially limited as long as the above-mentioned components are particularly included. The liquid crystal display device may or may not comprise other components.
The preferred embodiments of the liquid crystal display device in accordance with the present invention will be described below in greater details. The below-described various embodiments may be appropriately combined together.
The distance between the first electrode and the second electrode in the reflective area is preferably larger than the distance between the first electrode and the second electrode in the transmissive area. As a result, in the TBA-mode liquid crystal display device, the intensity of electric field generated in the liquid crystal layer in the reflective area can be made less than the intensity of electric field generated in the liquid crystal layer in the transmissive area. Therefore, the retardation of the liquid crystal layer in the reflective area can be easily set to about half the retardation of the liquid crystal layer in the transmissive area.
The width of the first electrode and the width of the second electrode are substantially equal in the transmissive area and the reflective area (the pixel area). As a result, in the TBA-mode liquid crystal display device, the retardation of the liquid crystal layer in the transmissive area and the retardation of the liquid crystal layer in the reflective area can be easily varied (made different). Therefore, the retardation of the liquid crystal layer in the reflective area can be easily set to about half the retardation of the liquid crystal layer in the transmissive area.
Further, “substantially equal” as referred to herein is preferably perfectly equal, but is not necessarily equal in the strict sense of the word, and also includes a relationship that can be treated as substantially equal with consideration for the effect of the present invention. It may also mean equal to a degree that can be attained when the first electrodes and second electrodes are designed and formed so as to be equal, and it goes without saying that an error that can occur in the production process may be also included. Thus, “substantially equal” as referred to herein includes an error within a range in which the effect of the present invention is demonstrated.
The first electrode and the second electrode are preferably comb-shaped electrodes. As a result, a high-density transverse electric field can be formed between the first electrode and the second electrode and the liquid crystal layer can be controlled with high accuracy.
The pixel is the smallest unit constituting a displayed image. In an active matrix liquid crystal display device with color display, a pixel is usually an area constituted by sub-pixels (monochromatic areas) of a plurality of colors (for example, three colors). Therefore, when the liquid crystal display device in accordance with the present invention is applied to an active matrix liquid crystal display device with color display, the pixel (pixel area) is preferably a sub-pixel (sub-pixel area).
As long as the liquid crystal display device in accordance with the present invention has the above-described features, the control system (liquid crystal mode) thereof is not particularly limited, but the aforementioned TBA mode is preferred.
In accordance with the present invention, it is possible to provide a TBA-mode liquid crystal display device that is capable of both the reflective display and the transmissive display, without providing a multigap structure.
The present invention will be described below in greater detail on the basis of embodiments thereof with reference to the appended drawings, but the present invention is not limited to these embodiments.
A liquid crystal display device of the present embodiment is of a transflective type that uses the so-called TBA system of transverse field systems in which image display is performed by causing an electric field (transverse electric field) in the direction of substrate plane to act upon a liquid crystal layer and controlling the alignment.
a) is a plan schematic view illustrating the configuration of a liquid crystal display panel of Embodiment 1.
A pixel electrode 20 and a thin film transistor (TFT) 26 for switching the pixel electrode 20 are formed in each of a plurality of sub-pixel areas formed as a matrix and constituting a display area (image display area) of the liquid crystal display device of the present embodiment. A plurality of source bus lines 16 are extended from a source driver (data line drive circuit). A source of the TFT 26 is electrically connected to the corresponding source bus line 16. The source driver supplies an image signal to each sub-pixel via the plurality of source bus lines 16. A common electrode 21 provided commonly to the sub-pixels is formed in each sub-pixel area and a common signal is supplied to each sub-pixel.
A plurality of gate bus lines 12 extending from the gate driver (scanning line drive circuit) function as gates of the TFTs 26. Further, scan signals supplied in the pulse-like form to the plurality of gate bus lines 12 at a predetermined timing from the gate driver are successively applied to the TFTs 26 in a linear order. Each pixel electrode 20 is electrically connected to a drain (drain line 18) of the TFT 26. An image signal supplied from the source bus line 16 is applied at the predetermined timing to the pixel electrode 20 connected to the TFT 26 that has been switched ON for a fixed period by the input of the scan signal.
An image signal of a predetermined level that has been written in a liquid crystal layer 30 is stored for a predetermined period between the pixel electrode 20 to which the image signal has been applied and the common electrode 21 facing this pixel electrode 20. In this case, a storage capacitance is formed in parallel with a liquid crystal capacitance formed between these pixel electrode 20 and common electrode 21, for preventing leakage of the image signals stored. The storage capacitance is formed, in each sub-pixel, between the drain line 18 of the TFT 26 and a Cs bus line (capacitance storage line) 13. The common electrode 21 is connected to a common voltage generating circuit and set to a predetermined potential.
The detailed configuration of the liquid crystal display device of the present embodiment will be explained below. The liquid crystal display device of the present embodiment is provided with a liquid crystal display panel 100 and a backlight unit (not shown in the figure) provided at the back surface side of the liquid crystal display panel 100. The liquid crystal display panel 100 is provided with an active matrix substrate (TFT array substrate) 10, an counter substrate 50 facing the active matrix substrate 10, and a liquid crystal layer 30 interposed therebetween.
The counter substrate 50 has a black matrix (BM) layer (not shown in the figure) that blocks the light between the sub-pixels, a plurality of color layers (color filters; not shown in the figure) provided correspondingly to each sub-pixel, and a vertical alignment film 55 provided on the surface on the liquid crystal layer 30 side to cover the aforementioned layers on the one main surface (on the liquid crystal layer 30 side) of a colorless transparent insulating substrate 51. The BM layer is formed from a non-transparent metal such as Cr or a non-transparent organic film such as an acrylic resin including carbon and is formed around the sub-pixel area, that is, in an area corresponding to the below-described gate bus lines 12 and source bus lines 16. The color layer is used to perform color display, formed from a transparent organic film, for example, of an acrylic resin including a pigment, and mainly formed in the sub-pixel area.
Thus, the liquid crystal display device of the present embodiment is a color liquid crystal display device (active matrix liquid crystal display device for color display) provided with the color layer on the counter substrate 50, wherein one pixel is constituted by three sub-pixels outputting color light of R (red), G (green), and B (blue) colors, respectively. The number and types of colors of the sub-pixels constituting each pixel are not particularly limited and can be set appropriately. Thus, in the liquid crystal display device of the present embodiment, each pixel may be constituted, for example, by sub-pixels of three colors, namely cyan, magenta, and yellow, or may be constituted by sub-pixels of four or more colors.
The active matrix substrate 10 has a plurality of gate bus lines 12 that transmit scan signals, a plurality of Cs bus lines 13, a plurality of reflective layers 28, a plurality of source bus lines 16 that transmit image signals, a plurality of TFTs 26 that are switching elements, each being provided for one sub-pixel, a plurality of drain lines 18, each being connected to one TFT 26, a plurality of pixel electrodes 20 provided individually for each sub-pixel, a common electrode 21 provided commonly for the sub-pixels, and a vertical alignment film 25 provided on the surface at the liquid crystal layer 30 side to cover the above-described configuration on one main surface (on the liquid crystal layer 30 side) of a colorless transparent insulating substrate 11.
The vertical alignment films 25, 55 are formed by coating a well-known alignment film material such as a polyimide. The vertical alignment films 25, 55 are usually not subjected to rubbing, but can align the liquid crystal molecules substantially perpendicular to the film surface when no voltage is applied.
The gate bus lines 12 are extended parallel to each other in the left-right direction in the front view of the liquid crystal display panel 100, and the source bus lines 16 are extended parallel to each other in the direction perpendicular to the gate bus lines 12, that is, in the up-down direction in the front view of the liquid crystal display panel 100. The Cs bus lines 13 are extended parallel to the gate bus lines 12, that is, in the left-right direction in the front view of the liquid crystal display panel 100. Thus, the gate bus lines 12 and Cs bus lines 13 are disposed alternately and parallel to each other. In the present embodiment, each sub-pixel area is generally defined as an area surrounded by these gate bus lines 12 and source bus lines 16, and these areas are arranged in a matrix-like configuration.
The configuration of the present embodiment will be described below in greater detail by paying attention mainly to one sub-pixel.
The Cs bus line 13 is disposed so as to pass close to the center of each sub-pixel area, and a high-reflectance reflective layer 28 is provided in an area that is substantially half the sub-pixel area partitioned by the Cs bus line 13. The area that is substantially half the sub-pixel area where the reflective layer 28 has been provided is thus a reflective area R where the reflective display is performed, and the area taking the remaining half of the sub-pixel area where the reflective layer 28 has not been provided is a transmissive area T where the transmissive display is performed. The reflective layer 28 is obtained by patterning a metal film with light reflection ability, such as an aluminum or silver film. It is preferred that the reflective layer 28 has concavities and convexities formed on the surface thereof to provide the layer with light scattering ability. As a result, visibility in reflective display can be improved. The area ratio of the transmissive area T and reflective area R can be appropriately set according to the desired display characteristics.
The pixel electrode 20 is formed from a transparent conductive film such as an ITO film or a metal film such as an aluminum or chromium film. The pixel electrode 20 has a comb-like shape in a planar view of the liquid crystal display panel 100. More specifically, the pixel electrode 20 has a band-like (rectangular in a planar view) trunk portion 20a arranged to lay flatly upon the Cs bus line 13, a plurality of band-like (rectangular in a planar view) branch portions 20b connected to the trunk portion 20a and extended toward the transmissive area T side, and a plurality of band-like (rectangular in a planar view) branch portions 20c connected to the trunk portion 20a and extended toward the reflective area R side. The branch portions 20b and the branch portions 20c are disposed parallel to each other in the up-down direction in the front view of the liquid crystal display panel 100.
The common electrode 21 is also formed from a transparent conductive film such as an ITO film or a metal film such as an aluminum film and has a comb-like shape in the planar view. More specifically, the common electrode 21 has a grid-like trunk portion 21a arranged to lay flatly on the gate bus lines 12 and the source bus lines 16, a plurality of band-like (rectangular in a planar view) branch portions 21b connected to the trunk portion 21a and extended to the transmissive area T, and a plurality of band-like (rectangular in a planar view) branch portions 21c connected to the trunk portion 21a and extended to the reflective area R. The branch portions 21b and the branch portions 21c are disposed parallel to each other in the up-down direction in the front view of the liquid crystal display panel 100.
In this configuration, the branch portions 20b of the pixel electrode 20 and the branch portions 21b of the common electrode 21 and also the branch portions 20c of the pixel electrode 20 and the branch portions 21c of the common electrode 21 have mutually complementary planar shapes and are disposed alternately with a certain spacing. Thus, the branch portions 20b of the pixel electrode 20 and the branch portions 21b of the common electrode 21 and also the branch portions 20c of the pixel electrode 20 and the branch portions 21c of the common electrode 21 are disposed opposite each other and parallel each other in the same plane. In other words, the comb-shaped pixel electrode 20 and the comb-shaped common electrode 21 are disposed opposite each other so that the comb teeth thereof interdigitate together in respective transmissive areas T and reflective areas R. As a result, a high-density transverse electric field can be formed between the pixel electrode 20 and the common electrode 21 and the liquid crystal layer 30 can be controlled with higher accuracy.
The width (length in the short side direction) of the branch portion 20b and the branch portion 20c of the pixel electrode 20 and the width (length in the short side direction) of the branch portion 21b and the branch portion 21c of the common electrode 21 are all substantially equal. From the standpoint of increasing the transmittance, it is preferred that the width of the pixel electrode 20 and the width of the common electrode 21 (the width of the branch portion 20b and the width of the branch portion 20c of the pixel electrode 20 and the width of the branch portion 21b and the width of the branch portion 21c of the common electrode 21) be as small as possible. According to the presently used process rule, the widths may be set, for example, to about 1.0 to 4.0 μm.
The distance between the branch portion 20c and the branch portion 21c provided in the reflective area R is larger than the distance between the branch portion 20b and the branch portion 21b provided in the transmissive area T. More specifically, from the standpoint of making the retardation of the liquid crystal layer 30 in the reflective area R about half the retardation of the liquid crystal layer 30 in the transmissive area T, as will be described hereinbelow, when the width of the pixel electrode 20 and the width of the common electrode 21 (the width of the branch portion 20b and the width of the branch portion 20c of the pixel electrode 20 and the width of the branch portion 21b and the width of the branch portion 21c of the common electrode 21) are equal to each other, it is preferred that the distance between the branch portion 20c and the branch portion 21c provided in the reflective area R be by a factor of 1.1 to 2.0 (more preferably by a factor of 1.3 to 1.8) larger than the distance between the branch portion 20b and the branch portion 21b provided in the transmissive area T.
The TFT 26 is provided close to the intersection portion of the gate bus line 12 and the source bus line 16 and has a semiconductor layer 15 formed from an island-shaped amorphous silicon film that is partially formed inside a planar area of the gate bus line 12, and the source line 17 and the drain line 18 that are formed to overlap partially and planarly the semiconductor layer 15. The gate bus line 12 functions as a gate electrode of the TFT 26 in a position where the semiconductor layer 15 is planarly overlapped. Thus, the TFT 26 is of a channel etch type manufactured by a method in which the semiconductor layer 15 is also somewhat etched when the drain line 18 and the source line 17 are separated and of a reverse stagger type in which the gate bus line 12 that also functions as a gate electrode is provided below (at the insulating substrate 11 side) the drain line 18 and the source line 17.
The source line 17 is branched off the source bus line 16 and has a substantially L-like shape in a planar view that extends to the semiconductor layer 15, thereby connecting the source bus line 16 and the TFT 26. The drain line 18 extends from the semiconductor layer 15 and has an L-like shape in a planar view. This line is connected to the pixel electrode 20 and forms the storage capacitance. More specifically, the drain line 18 has a storage capacitance portion 22 of a substantially rectangular shape in a planar view at the end portion (distal end portion of L-like shape) on the side opposite that of the TFT 26, and the storage capacitance portion 22 is formed to overlap planarly the Cs bus line 13. Further, a storage capacitance having the storage capacitance portion 22 and the Cs bus line 13 as electrodes is formed in a region where the storage capacitance portion 22 and the Cs bus line 13 planarly overlap. The storage capacitance portion 22 is also disposed to overlap planarly the trunk portion 20a of the pixel electrode 20 and is electrically connected to the trunk portion 20a of the pixel electrode 20 by a contact hole 27 provided in the same position.
The cross-sectional structure of the liquid crystal display panel 100 will be described below.
The liquid crystal display panel 100 is provided with the active matrix substrate 10, the counter substrate 50 disposed opposite the active matrix substrate 10, and the liquid crystal layer 30 interposed therebetween. A retarder 44 and a polarizing plate 42 are stacked in the order of description on the outer surface side (side not facing the liquid crystal layer 30) of the active matrix substrate 10, and a retarder 43 and a polarizing plate 41 are stacked in the order of description on the outer surface side of the counter substrate 50. The retarder 43 is a +λ/4 retarder that imparts the transmitted light with a retardation of about +¼ wavelength. The retarder 44 is a −λ/4 retarder that imparts the transmitted light with a retardation of about −¼ wavelength. By providing the retarders 43, 44, it is possible to match the display characteristics of the reflective display and transmissive display, for example, with those of a normally black mode. Therefore, a wide-angle contrast characteristic can be obtained without using any special features for the device structure or signal processing configuration.
In the present embodiment, a retardation layer that imparts a retardation of about +¼ wavelength or −¼ wavelength may be selectively formed only in the reflective area R at the counter substrate 50 side, instead of using two retarders 43, 44. Further, the liquid crystal display device of the present embodiment may have a viewing angle compensation film or a retarder other than the retarders 43, 44
The active matrix substrate 10 has the transparent insulating substrate 11 made from glass, quartz, or plastic as a base body. The reflective layer 28 formed from a metal film such as an aluminum or silver film is disposed locally in the sub-pixel area at the inner surface side (liquid crystal layer 30 side) of the insulating substrate 11. An interlayer insulating film 23 formed from a transparent insulating material such as silicon oxide is disposed so as to cover the reflective layer 28. The gate bus lines 12 and the Cs bus lines 13 formed from a metal film such as an aluminum film are disposed on the interlayer insulating film 23, and a gate insulator 14 formed from a transparent insulating material such as silicon oxide is disposed so as to cover the gate bus lines 12 and the Cs bus lines 13.
Where the Cs bus lines 13 are formed by using a material with a high reflectivity to have a large width so as to cover the reflective area R, the Cs bus lines 13 can be also used as the reflective layer 28 and the manufacturing process can be simplified.
The amorphous silicon semiconductor layer 15 is formed on the gate insulator 14, and the source line 17 and the drain line 18 formed from a metal film such as an aluminum film are provided so as to be partially placed on the semiconductor layer 15. The source line 17 is formed integrally with the source bus line 16, as shown in
An interlayer insulating film 19 formed from silicon oxide or the like is disposed so as to cover the semiconductor layer 15, source line 17, source bus line 16, and drain line 18. A planarizing film 24 formed from a transparent insulating material such as a photosensitive acrylic resin is disposed on the interlayer insulating film 19, and the pixel electrodes 20 and the common electrode 21 formed from a transparent conductive material such as ITO or a metal film such as an aluminum film are disposed on the surface of the planarizing film 24. The pixel electrodes 20 are electrically connected to the drain lines 18 via the contact holes 27 that pass through the interlayer insulating film 19 and the planarizing film 24 and is located above the drain lines 18. The pixel electrodes 20 are thus partially embedded in the contact holes 27, thereby ensuring electric connection to the drain lines 18. The vertical alignment film 25 from a polyimide or the like is formed to cover the pixel electrodes 20 and the common electrode 21.
The counter substrate 50 has the transparent insulating substrate 51 made from glass, quartz, or plastic as a base body. The BM layer and color layer are provided, as described hereinabove, at the inner surface side (liquid crystal layer 30 side) of the insulating substrate 51. The vertically aligned film 55 from a polyimide or the like is formed to cover the BM layer and color layer. The color layer is preferably partitioned into two areas of different chromaticity inside the sub-pixel area. More specifically, a first color material area is provided correspondingly to a planar area of the transmissive area T, and a second color material area is provided correspondingly to a planar area of the reflective area R. A configuration in which the chromaticity of the first color material area is greater than the chromaticity of the second color material area can be used. As a result, the chromaticity of the display light can be prevented from being different in the transmissive area T where the display light passes through the color layer only once and the reflective area R where the display light passes twice through the color layer, the appearances of reflective display and transmissive display can be matched, and the display quality can be increased.
A planarizing film (undercoat film) formed from a transparent resin material is preferably further laminated on the BM layer and color layer on the liquid crystal layer 30 side thereof in order to planarize unevenness of the configurations. As a result, the surface of the counter substrate 50 can be planarized, thickness uniformity of the liquid crystal layer 30 can be improved, and the non-uniformity of driving voltage in the sub-pixel area and the decrease in contrast can be prevented.
The active matrix substrate 10 and the counter substrate 50 are attached, with a spacer such as plastic beads being interposed therebetween, by a sealing agent provided so as to surround the display area. The liquid crystal layer 30 is formed by sealing a liquid crystal material as a display medium constituting an optical modulation layer in the gap between the active matrix substrate 10 and the counter substrate 50.
The liquid crystal layer 30 includes a nematic liquid crystal material (p-type liquid crystal material) having positive dielectric anisotropy. Liquid crystal molecules of the p-nematic liquid crystal material demonstrate homeotropic alignment when no voltage is applied (when no electric field is generated by the pixel electrode 20 and the common electrode 21) under the effect of an alignment-controlling force of the vertical alignment films 25, 55 of the respective active matrix substrate 10 and the counter substrate 50. More specifically, when no voltage is applied, a long axis of a liquid crystal molecule of the p-type nematic liquid crystal material in the vicinity of the vertical alignment films 25, 55 has an angle of equal to or greater than 88° (preferably equal to or greater than 89°) with respect to the active matrix substrate 10 and the counter substrate 50. Further, the liquid crystal layer 30 is set to substantially the same thickness as the transmissive area T and the reflective area R. Thus, the liquid crystal display panel 100 has a single cell gap.
The arrangement of optical axes in the liquid crystal display device of the present embodiment is shown in
In the liquid crystal display device of the present embodiment that has the above-described configuration, when an image signal (voltage) is applied to the pixel electrode 20 via the TFT 26, an electric field in the substrate plane direction is generated between the pixel electrode 20 and the common electrode 21, this electric field drives the liquid crystal, transmittance and reflectance of each sub-pixel are changed, and image display is performed.
More specifically, in the liquid crystal display device of the present embodiment, where an electric field is applied, the retardation of the liquid crystal layer 30 is changed due to the distortion of alignment of liquid crystal molecules induced by the formation of electric field intensity distribution inside the liquid crystal layer 30. More specifically, the initial alignment state of the liquid crystal layer 30 is a homeotropic alignment, and where a voltage is applied to the comb-shaped pixel electrode 20 and common electrode 21, a transverse electric field is generated inside the liquid crystal layer 30, and a bend electric field is formed. As a result, as shown in
Thus, the liquid crystal display device of the present embodiment is a TBA-mode liquid crystal display device, and various transmitted light intensity (T)−voltage (V) characteristics can be obtained by changing the width of the pixel electrodes 20 and the common electrode 21 and the distance therebetween.
By contrast, in the liquid crystal display device of the present embodiment, the distance between the branch portion 20c and the branch portion 21c provided in the reflective area R is set larger than the distance between the branch portion 20b and the branch portion 21b provided in the transmissive area T. As a result, the intensity of an electric field generated in the liquid crystal layer 30 in the reflective area R becomes lower than the intensity of an electric field generated in the liquid crystal layer 30 in the transmissive area T. Therefore, although the cell gaps in the reflective area R and transmissive area T are identical, the retardation of the liquid crystal layer 30 in the reflective area R can be made less than, more specifically, about half the retardation in the transmissive area T. Thus, even though a multigap structure is not provided, the retardation of the liquid crystal layer 30 in the reflective area R can be set to about half the retardation of the liquid crystal layer 30 in the transmissive area T. As a result, it is possible to realize a TBA-mode liquid crystal display device in which both the reflective display and the transmissive display can be performed without providing a multigap structure. Thus, in the liquid crystal display device of the present embodiment, the retardation of the liquid crystal layer 30 in the transmissive area T is set to λ/2 and the retardation of the liquid crystal layer 30 in the reflective area R is set to λ/4.
Further, since the width of the branch portion 20b and the branch portion 20c of the pixel electrode 20 is substantially equal to the width of the branch portion 21b and the branch portion 21c of the common electrode 21, the retardation of the liquid crystal layer 30 in the transmissive area T and the retardation of the liquid crystal layer 30 in the reflective area R can be varied (made different) easier. Therefore, the retardation of the liquid crystal layer 30 in the reflective area R can be easily set to about half the retardation of the liquid crystal layer 30 in the transmissive area T.
The display operation of the liquid crystal display device of the present embodiment will be described below.
a) and (b) show an explanatory diagram (on the right side in the figure) illustrating the operation in the reflective display mode (reflective area R) and an explanatory diagram (on the left side in the figure) illustrating the operation in the transmissive display mode (transmissive area T). The explanatory diagram illustrating the operation in the reflective display mode shows how the external light incident from above, as shown in the figure, propagates down, as shown in the figure, reaches the reflective layer, undergoes reflection at the reflective layer, returns to the upper side, as shown in the figure, and becomes display light. The explanatory diagram illustrating the operation in the transmissive display mode shows how the illumination light incident from below propagates upward and becomes the display light.
First, the transmissive display mode (transmissive mode) on the left side in
In the liquid crystal display device of the present embodiment, the light emitted from the backlight is converted into linearly polarized light parallel to the transmission axis 42t of the polarizing plate 42 when the light passes through the polarizing plate 42, and the converted light falls on the retarder 44. Since the retarder 44 is a −λ/4 retarder that imparts the light passing therethrough with a retardation of −¼ wavelength, the linearly polarized light that has passed through the polarizing plate 42 is converted into left-handed circularly polarized light, exits the retarder 44 and falls on the liquid crystal layer 30 of the liquid crystal display panel 100. Where the liquid crystal layer 30 is in the OFF state (non-selective state), the incident light (left-handed circularly polarized light) exits the liquid crystal layer 30 in the same polarization state as that of the incident light and falls on the retarder 43. Since the retarder 43 is a +λ/4 retarder that imparts the light passing therethrough with a retardation of +¼ wavelength, the left-handed circularly polarized light that has passed through the retarder 43 is converted into linearly polarized light parallel to the transmission axis 42t of the polarizing plate 42 and reached the polarizing plate 41. The linearly polarized light that has reached the polarizing plate 41 is oriented perpendicular to the transmission axis 41t of the polarizing plate 41. Therefore, this light is absorbed by the polarizing plate 41 and the sub-pixels demonstrate the black display. Thus, the retardation of the liquid crystal display panel in the transmissive area T in the OFF state (non-selective state) is −λ/4 (retarder 44)+0 (liquid crystal layer 30 in the transmissive area T)+λ/4 (retarder 43)=0 and therefore the black display can be realized in a cross-Nicol state of the polarizing plates 41, 42.
By contrast, where the liquid crystal layer 30 is in the ON state (selective state), the left-handed circularly polarized light incident on the liquid crystal layer 30 is imparted by the liquid crystal layer 30 with a predetermined retardation (λ/2) and converted into the right-handed circularly polarized light that exits the liquid crystal layer 30 and falls on the retarder 43. The right-handed circularly polarized light that has passed through the retarder 43 is converted into the linearly polarized light perpendicular to the transmissive axis 42t of the polarizing plate 42 and reaches the polarizing plate 41. This linearly polarized light has a polarization direction parallel to the transmission axis 41t of the polarizing plate 41 and therefore passes through the polarizing plate 41 and is visible. The sub-pixels thus demonstrate the white display. Thus, the retardation of the liquid crystal display panel in the transmissive area T in the ON state (selective state) is −λ/4 (retarder 44)+λ/2 (liquid crystal layer 30 in the transmissive area T)+λ/4 (retarder 42)=λ/2 and therefore the white display can be realized in a cross-Nicol state of the polarizing plates 41, 42.
The reflective display shown on the right side in
In the reflective display mode, the light incident from above (outside) the polarizing plate 41 is converted into linearly polarized light parallel to the transmission axis 41t of the polarizing plate 41 when the light passes through the polarizing plate 41, and the converted light falls on the retarder 43. Since the retarder 43 is a +λ/4 retarder that imparts the light passing therethrough with a retardation of +¼ wavelength, the linearly polarized light that has passed through the polarizing plate 41 is converted into right-handed circularly polarized light, exits the retarder 43, and falls on the liquid crystal layer 30 of the liquid crystal display panel 100. Where the liquid crystal layer 30 is in the OFF state (non-selective state), the incident light (right-handed circularly polarized light) goes out of the liquid crystal layer 30 in the same polarization state as that of the incident light and reaches a reflector (not shown in the figure), and undergoes reflection. In this case, the rotation direction viewed from the polarizing plate 41 side is reversed, left-handed circularly polarized light is obtained, and this light falls again on the liquid crystal layer 30. Since the liquid crystal layer 30 is in the OFF state (non-selective state), the incident light (left-handed circularly polarized light) exits the liquid crystal layer 30 in the same polarization state as that of the incident light and falls on the retarder 43. The retarder 43 is a +λ/4 retarder and therefore the left-handed circularly polarized light that has passed through the retarder 43 is converted into linearly polarized light orthogonal to the transmission axis 41t of the polarizing plate 41 and reaches the polarizing plate 41. The linearly polarized light is absorbed by the polarizing plate 41 and the sub-pixels demonstrate the black display. Thus, the retardation of the liquid crystal display panel 100 in the reflective area R in the OFF state (non-selective state) is λ/4 (retarder 43)+0 (liquid crystal layer 30 in the reflective area R)+λ/4 (retarder 43)=λ/2 and therefore the black display can be realized in a parallel-Nicol state of the single polarizing plate 41.
By contrast, where the liquid crystal layer 30 is in the ON state (selective state), the incident light (right-handed circularly polarized light) is imparted by the liquid crystal layer 30 with a predetermined retardation (λ/4) and converted into the linearly polarized light orthogonal to the transmissive axis 41t of the polarizing plate 41. In the present embodiment, the distance between the pixel electrode 20 and the common electrode 21 in the reflective area R is set larger than the distance between the pixel electrode 20 and the common electrode 21 in the transmissive area T, and the retardation of the liquid crystal layer 30 in the reflective area R is set to about half the retardation in the transmissive area T. Therefore, as described hereinabove, when the circularly polarized light passes through the liquid crystal layer 30, the light is converted into the linearly polarized light.
The linearly polarized light that exits the liquid crystal layer 30 is reflected by the reflective layer and falls again on the liquid crystal layer 30. Then, the light is again imparted by the liquid crystal layer 30 with a predetermined retardation (λ/4) and converted into right-handed circularly polarized light that exits the liquid crystal layer 30. The right-handed circularly polarized light that exits the liquid crystal layer 30 falls on the retarder 43 and is imparted with a retardation of +¼ wavelength and converted into linearly polarized light parallel to the transmissive axis 41t of the polarizing plate 41. This linearly polarized light reaches the polarizing plate 41. Since the linearly polarized light has a polarization direction parallel to the transmission axis 41t of the polarizing plate 41, this light passes through the polarizing plate 41 and can be viewed, and the sub-pixels demonstrate the white display. Thus, the retardation of the liquid crystal display panel 100 in the reflective area R in the ON state (selective state) is +λ/4 (retarder 43)+λ/2 (two-fold retardation λ/4 of the liquid crystal layer 30 in the reflective area R)+λ/4 (retarder 43)=λ and therefore white display can be realized in a parallel-Nicol state of the single polarizing plate 41.
In the liquid crystal display device of the present embodiment, the distance between the pixel electrode 20 and the common electrode 21 in the reflective area R is set larger than the distance between the pixel electrode 20 and the common electrode 21 in the transmissive area T, and the retardation of the liquid crystal layer 30 in the reflective area R is about half the retardation in the transmissive area T. Therefore, the occurrence of a difference in the essential retardation imparted to the display light between the reflective display using the light that has passed twice through the liquid crystal layer 30 as the display light and the transmissive display using the light that has passed only once through the liquid crystal layer 30 as the display light can be prevented.
The results obtained in simulation measurements relating to the liquid crystal display device of the present embodiment will be described below.
First, merits of the liquid crystal display device of the present embodiment over the liquid crystal display device of the IPS system will be explained. The intensity of transmitted light in a mode in which the birefringence of a liquid crystal cell interposed between orthogonal polarizers is controlled by an electric field can be defined by the following Equation (1).
In Equation (1), I0 stands for an intensity of incident polarized light, θ represents an angle formed by the oscillation directions of the incident polarized light and usual light in a liquid crystal cell, d represents a cell thickness (cell gap), Δn (V) represents a birefringence of the liquid crystal cell under a voltage V, d·Δn represents an optical retardation, and λ represents a wavelength of the incident light.
When the transmittance of a liquid crystal display panel is to be increased, the d·Δn value is set to increase the intensity of the transmitted light with λ of from 550 nm to 650 nm. However, since there is a light wavelength dispersivity in liquid crystals, the liquid crystals usually do not transmit the light uniformly in a range of λ of from 380 nm to 750 nm (visible light range).
The transmittance with respect to different wavelengths was simulation measured with respect to the TBA mode and IPS mode. The results obtained are explained below.
The liquid crystal cell shown in
The results demonstrate that in the IPS mode, where d·Δn is increased from 318 nm to 348 nm, the Y value under an applied voltage of 6.5 V increases from 547 to 553 and the display clearly becomes lighter. However, the comparison of
By contrast, in the TBA mode, where d·Δn is increased from 447 nm to 497 nm, the Y value under an applied voltage of 6.5 V increases from 451 to 459 and the display becomes lighter. The comparison of
When the transmittance is thus increased by increasing the d·Δn value, the transmitted light intensity at λ=380 to 750 nm (visible range) in the TBA mode can be increased more uniformly than in the IPS mode.
Further, in the TBA mode, the transmitted light intensity can be increased more uniformly by adjusting the distance (S) between the electrodes 61, 62.
The comparison of
The above-described results demonstrate that when the liquid crystal display device of the present embodiment uses the same driver as that of the conventional VA-mode (for example, the ASV mode in which liquid crystal molecules are aligned radially, the protrusion provided at the counter substrate serving as a center) transflective type liquid crystal display device, the ideal L/S value in the transmissive area T is 1.5 μm/(7.5 to 10) μm. Thus, in this case, the ideal L/S value in the transmissive area T is obtained when S=7.5 to 10 μm with respect to L=1.5 μm. By contrast, when much importance is attached to the response time, the L/S value in the transmissive area T is preferably set to 1.5 μm/(4 to 7.5) μm. Thus, in this case the preferred L/S value in the transmissive area T is obtained when S=4 to 7.5 μm with respect to L=1.5 μm. However, in this case, a driver that is different from that of the conventional VA mode should be used.
A reflective display characteristic of the liquid crystal display device of the present embodiment will be described below. A transmitted light intensity in a mode in which birefringence of a liquid crystal cell interposed between the parallel polarizers is controlled by an electric field can be generally represented by the following Equation (2). Thus, the reflected light intensity also can be represented by Equation (2) below.
In Equation (2), I0 stands for an intensity of incident polarized light, θ represents an angle formed by the oscillation directions of the incident polarized light and usual light in a liquid crystal cell, d represents a cell thickness (cell gap), Δn (V) represents a birefringence of the liquid crystal cell under a voltage V, d·Δn represents an optical retardation, and λ represents a wavelength of the incident light. Thus, the transmitted light intensity is represented by an equation that differs depending on whether the polarizers are orthogonal or parallel.
Firstly, the results will be explained that were obtained by conducting simulation measurements of a transmission characteristic and a reflection characteristic in a state with a parallel arrangement of polarizers in a TBA-mode liquid crystal display device of a comparative example.
These figures demonstrate that when the L/S value of the reflective area R is set in the same manner as in the transmissive area T, that is, the distance S between the electrodes in the reflective area R is set to the same value as the distance S between the electrodes in the transmissive area T at which good transmissive display has been realized, short-wavelength light leaks when a high voltage is applied and a sufficient reflective characteristic is not obtained.
Since the optical retardation d·Δn differs among the points when the voltage is applied and the results obtained in the transmissive area T and the reflective area R differ depending on whether the polarizers are orthogonal or parallel, the white display and black display are not reversed symmetrically in the transmissive area T and the reflective area R.
Secondly, the results will be explained that were obtained by conducting simulation measurements of a reflection characteristic in a state with a parallel arrangement of polarizers in the TBA-mode liquid crystal display device the present embodiment.
As shown in the figures, where the space between the electrodes 61, 62 is increased from 10 μm to 13 μm, the optical retardation d·Δn and reflected light intensity (R)−voltage (V) characteristic change significantly. A sufficient reflective characteristic can be obtained by setting the distance S between the electrodes in the reflective area R larger than the distance S between the electrodes in the transmissive area T. Further, since the averaged reflected light intensity (R)−voltage (V) characteristic is taken by the human eyes, the liquid crystal display device of the present embodiment makes it possible to recognize uniform light over a range from a short wavelength to a long wavelength, as shown in
With the liquid crystal display device of the present embodiment, the reflective display and transmissive display of excellent quality can be obtained in the TBA mode, without providing a multigap structure. Further, since it is not necessary to provide a concavo-convex structure on the counter substrate side as in the conventional transflective type liquid crystal display device having the multigap structure, the cost can be reduced and a contrast characteristic in the transmissive display can be improved. Further, since it is not necessary to provide transparent electrode or ribs (protrusions for controlling an alignment) on the counter substrate side as in the conventional VA-mode transflective type liquid crystal display device, the cost can be reduced by comparison with that of the conventional VA-mode transflective type liquid crystal display device.
The present application claims priority under the Paris Convention and the domestic law in the country to be entered into national phase to Japanese Patent Application No. 2008-125198, filed on May 12, 2008, the entire contents of which are hereby incorporated by reference into this application.
a) is a schematic plan view illustrating the configuration of a liquid crystal display panel of Embodiment 1, and
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-125198 | May 2008 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/051625 | 1/30/2009 | WO | 00 | 2/14/2011 |
