Reflective type liquid crystal display device

According to one embodiment, a reflective type liquid crystal display device 11 that can be manufactured at low cost and that can improve a response speed and an aperture ratio is provided. A liquid crystal layer is a positive type liquid crystal layer including vertically-aligned liquid crystal molecules between an array substrate and a counter substrate. The array substrate includes a plurality of pixel electrodes, a plurality of common electrodes, a color filter, and a reflection layer. The color filter is disposed in a lower layer of the pixel electrodes and the common electrodes. The reflection layer is electrically connected to either the common electrodes or the pixel electrodes, and disposed at least in a lower layer of the color filter so as to reflect light having passed through the color filter.

INCORPORATOIN BY REFERENCE

The present invention claims priority under 35 U.S.C. §119 to Japanese Patent Application Nos. 2014-241680 and 2015-141950 filed on Nov. 28, 2014 and Jul. 16, 2015, respectively. The contents of these applications are incorporated herein by reference in their entirety.

FIELD

The embodiments of this invention relate to a reflective type liquid crystal display device which has a liquid crystal layer including vertically-aligned liquid crystal molecules between a first substrate and a second substrate.

BACKGROUND

Conventionally, a TN type (twisted nematic type) reflective type liquid crystal display device, for example, is used as a reflective type liquid crystal display device. In recent years, liquid crystal display devices are achieving higher definition. However, a TN type liquid crystal display device has some problems regarding, for example, slowness in response speed, lower display quality due to electrostatic destruction of a thin film transistor or the like caused by a rubbing treatment process to an alignment film, or the occurrence of so-called edge reverse (disclination) in which liquid crystal molecules existing between pixel electrodes having different polarities are reversed in the case of using a driving method such as a column inversion driving method or the like.

To solve such a problem, with a configuration applying a liquid crystal layer including liquid crystal molecules vertically aligned by use of an alignment film, the liquid crystal molecules are made to fall down in a transverse electric field mode, for example, IPS mode or the like. This is considered to improve response speed and display quality because rubbing treatment to the alignment film is omitted and thus the occurrence of electrostatic destruction is prevented. Further, a configuration of a liquid crystal display device that can provide a higher aperture ratio is desired.

DETAILED DESCRIPTION

A reflective type liquid crystal display device of each of the embodiments includes a first substrate, a second substrate, and a liquid crystal layer. The second substrate is disposed so as to face the first substrate. The liquid crystal layer is a positive type liquid crystal layer including vertically-aligned liquid crystal molecules between the first substrate and the second substrate. The first substrate includes a plurality of pixel electrodes, a plurality of common electrodes, a color filter, and a reflection layer. Each of the common electrodes forms a transverse electric field which makes liquid crystal molecules align between the common electrodes and each of the pixel electrodes. The color filter is disposed in a lower layer of the pixel electrodes and the common electrodes. The reflection layer is electrically connected to either the common electrodes or the pixel electrodes and is disposed in a lower layer of the color filter to reflect the light passing through the color filter.

The configuration of the first embodiment will be described with reference toFIG. 1A,FIG. 1BandFIG. 2.

InFIG. 1A,FIG. 1BandFIG. 2, reference numeral11indicates an active matrix type reflective type liquid crystal display device as a reflective type display device. As an outline, the reflective type liquid crystal display device11includes an array substrate13corresponding to a first substrate as a non-display side substrate, a counter substrate14corresponding to a second substrate as a display side substrate, and a liquid crystal layer15corresponding to an light modulating layer interposed between the substrates13and14. In the reflective type liquid crystal display device11, a gap holding member (spacer) not shown in the drawings for holding a gap is interposed between the substrate13and the substrate14. Further, a sealing member17made of an ultraviolet-curable resin or a thermosetting resin or the like, for example, is provided around the liquid crystal layer15to seal the liquid crystal layer15. In the description below, the reflective type liquid crystal display device11may be simply abbreviated to display device11. Further, the aspect ratio in each ofFIG. 1A,FIG. 1BandFIG. 2is changed in order to provide clearer description schematically.

The array substrate13includes a glass substrate21corresponding to a non-display side substrate body (first substrate body) having light transmitting properties and insulating properties, and further includes on the glass substrate21, a plurality of scanning lines (gate lines)22, a plurality of signal lines (source lines)23, a plurality of thin film transistors24corresponding to switching elements, a reflection layer25, a color filter (CF)26, a plurality of pixel electrodes27, a plurality of common electrodes28, and an (first) alignment film29. That is, the display11has a COA (Color filter On Array) structure.

More specifically, for example, an undercoat layer for flattening not shown in the drawings is provided on the array substrate13, a semiconductor layer not shown in the drawings such as for the thin film transistors24is provided on the undercoat layer, an insulation layer31covering the semiconductor layer is provided, and the scanning lines22(gate electrodes of the thin film transistors24) are provided on the insulation layer31. A gate insulation film32is provided on the scanning lines22, the reflection layer25(having the same potential as that of the common electrodes28) electrically connected to the common electrodes28is provided on the gate insulation film32, an interlayer insulation film33is provided on the reflection layer25, and the signal lines23(source electrodes and drain electrodes of the thin film transistors24) are provided on the interlayer insulation film33. Further, the color filter26is provided on the interlayer insulation film33having the signal lines23, the pixel electrodes27and the common electrodes28are provided on the color filter26, and the alignment film29is provided on the color filter26having the pixel electrodes27and the common electrodes28. The array substrate13may be any substrate having light transmitting properties and insulating properties, such as a synthetic-resin substrate or the like, instead of the glass substrate21.

Each of the scanning lines22is disposed along a horizontal (H) direction and is electrically connected to a driver, not shown in the drawings, provided on the glass substrate21or the like, for example.

Each of the signal lines23, in a state of being insulated from the scanning lines22, is disposed along a vertical (V) direction crossing (orthogonal to) the scanning lines22and is electrically connected to a driver, not shown in the drawings, provided on the glass substrate21or the like, for example. Each of the signal lines23is electrically connected to an external circuit not shown in the drawings. In this embodiment, each of the signal lines23is, for example, set to approx. 0.35 μm thick.

The thin film transistors24are each disposed at locations where the scanning lines22cross the signal lines23. Therefore, the thin film transistors24are disposed in a matrix. The thin film transistors24are disposed so that the gate electrodes face a channel region of the semiconductor layer via the gate insulation film32, and the source electrodes and the drain electrodes are electrically connected to a source region and a drain region of the semiconductor layer, respectively. Further, the gate electrodes of the thin film transistors24are electrically connected to the scanning lines22, the source electrodes thereof are electrically connected to the signal lines23, and the drain electrodes thereof are electrically connected to the pixel electrodes27.

The reflection layer25is a reflection pixel that reflects extraneous light and is made of, for example, aluminum, silver, or a compound or alloy or the like containing aluminum or silver as one component, and is set to a prescribed thickness. The reflection layer25is disposed in a layer located above the scanning lines22(gate electrodes) and a layer under the color filter26so as to face the scanning lines22and the entire surface of the color filter26, and is electrically connected to the pixel electrodes27or the common electrodes28, thereby shielding an unwanted leakage electric field that is directed from the scanning lines22to the liquid crystal layer15. In this embodiment, the reflection layer25is, for example, set to approx. 0.13 μm thick.

The color filter26is disposed in a lower layer of the pixel electrodes27and the common electrodes28and has, for example, filter parts26r,26gand26brespectively corresponding to red (R), green (G) and blue (B), and a light shielding part (black matrix) not shown in the drawings, which partitions the filter parts26r,26gand26binto blocks to shield unwanted light. Each of the filter parts26r,26gand26bis provided to correspond to each of the pixel electrodes27. In this embodiment, each of the filter parts26r,26gand26bis disposed in pixel regions A that are each disposed in a matrix and surrounded by the scanning lines22and the signal lines23. Moreover, the color filter26is surrounded by a black light shielding part35. In this embodiment, the color filter26(filter parts26r,26gand26b) is, for example, set to approx. 2.0 μm thick.

Each of the pixel electrodes27is made of a transparent conductive member, for example ITO or IZO or the like, and is formed into an elongated shape along the direction of the signal lines23, and disposed at each of the pixel regions A.

Each of the common electrodes28is made of a transparent conductive member, for example ITO or IZO or the like, and formed into an elongated shape in the direction of the signal lines23along a boundary between adjacent pixel regions A, A. Further, the common electrodes28are each disposed at a position directly above the signal lines23. That is, the common electrodes28are disposed separated from the pixel electrodes27, while respectively corresponding to each of the pixel electrodes27, wherein the pixel electrodes27and the common electrodes28are alternately disposed along the direction of the scanning lines22. Further, the common electrodes28are formed to have substantially the same width as the pixel electrodes27, and a space between the pixel electrodes27and the common electrodes28is set two times wider or more than the width of the pixel electrodes27and the common electrodes28. In this embodiment, for example, the pixel electrodes27and the common electrodes28are set to approx. 2.5 μm wide and approx. 0.07 μm thick, respectively, and are disposed separated from one another with a 15.0 μm or more space therebetween.

The transverse electric field formed between two common electrodes28,28having a pixel region A sandwiched therebetween and one pixel electrode27located between the two common electrodes28,28in the pixel region A provides switching (FIG. 1AandFIG. 1B) of the liquid crystal molecules (director)15ain the liquid crystal layer15.

The alignment film29is made of synthetic resin, for example, polyimide or the like. In this embodiment, the alignment film29is, for example, set to approx. 0.07 μm thick.

The insulation layer31is, for example, a silicon oxide film or a silicon nitride film or the like.

The gate insulation film32is, for example, a silicon nitride film or the like.

The interlayer insulation film33is, for example, a silicon oxide film or the like. In this embodiment, the interlayer insulation film33is, for example, set to approx. 0.18 μm thick.

Further, the counter substrate14includes a glass substrate41corresponding to a display side substrate body (second substrate body) having light transmitting properties and insulating properties, and further includes on the glass substrate41a rib-like structure not shown in the drawings, and an (second) alignment film43contacting the liquid crystal layer15while covering the structure. The rib-like structure controls as a control unit the falling direction of the liquid crystal molecules15awhen a voltage is applied. That is, the counter substrate14has no electrode formed thereon, and the liquid crystal layer15is divided by the structure into a plurality of domains at portions respectively corresponding to the pixel regions A. Further, a polarizing plate45is mounted on the opposite side of the liquid crystal layer15of the glass substrate41, that is, on the display side. The counter substrate14may be any substrate as long as it has light transmitting properties and insulating properties, such as a synthetic-resin substrate, for example, instead of the glass substrate.

The alignment film43is made of synthetic resin, for example, polyimide or the like, and the rib-like structure forms the liquid crystal molecules15ain the liquid crystal layer15into a substantially vertical line between the alignment film43and the alignment film29on the side of the array substrate13. In this embodiment, the alignment film43is, for example, approx. set to 0.07 μm thick, substantially the same as the alignment film29.

In the liquid crystal layer15, the liquid crystal molecules15aare vertically aligned between the alignment films29and43. Thus, a positive type liquid crystal layer having a positive dielectric anisotropy is applied in order to make the liquid crystal molecules15afall in the transverse electric field between the pixel electrodes27and the common electrodes28. In this embodiment, the liquid crystal layer15is, for example, set to approx. 2.8 μm thick.

In the above-described display device11, each of the thin film transistors24individually drives each of the pixel electrodes27in accordance with a signal from each of the scanning lines22, and the transverse electric field set between the above pixel electrode27and the two common electrodes28,28having the above pixel electrode27sandwiched therebetween makes the liquid crystal molecules15ain the liquid crystal layer15fall down in accordance with a signal from each of the signal lines23. The polarity of the above pixel electrode27can be selected with the signal lines23located in a lower layer of the two common electrodes28,28, and an appropriate driving method can be used, for example, column inversion driving in which the polarity is inverted every predetermined number of signal lines23for each of the scanning lines22. In this state, extraneous light made incident from the side of the counter substrate14passes through the filter parts26r,26gand26bof the color filter26via the liquid crystal layer15, and then is reflected by the reflection layer25located in a lower layer of the color filter26. This sets the transmittance of the reflected light at each of the pixel electrodes27in accordance with the angles of the liquid crystal molecules15ain the liquid crystal layer15, and the reflected light having passed through the color filter26again and having been emitted to the side of the counter substrate14is displayed as an image.

According to the first embodiment described above, the use of the positive type liquid crystal layer15including vertically-aligned liquid crystal molecules15aenables cheap manufacturing, can improve edge reverse likely to occur between pixel electrodes having different polarities in the case of implementing column inversion driving or the like using, for example, a TN type liquid crystal layer, and allows the use of a circular polarizing plate as the polarizing plate45, thus realizing the reflective type liquid crystal display device11having a high transmittance in normally black displaying. Further, as a normal liquid crystal layer used in a vertical alignment (VA) mode, a negative-type liquid crystal layer is used so as to make liquid crystal molecules fall in a vertical electric field. In this embodiment, however, the plurality of pixel electrodes27and the common electrodes28corresponding to the plurality of pixel electrodes27are formed on the array substrate13, and the transverse electric field is formed by a transverse electric field method to make the liquid crystal molecules15aaligned between the pixel electrodes27and the common electrodes28. The use of the positive type liquid crystal layer15having a higher response speed than that of the negative-type liquid crystal layer can improve the response speed thereof. Further, the provision of the color filter26on the array substrate13makes it easier to adjust positioning of the array substrate13and the counter substrate14compared to an example case where the color filter is provided on the counter substrate side. This can suppress reduction of an aperture ratio caused by displacement or the like between the array substrate13and the counter substrate14, thus realizing a high aperture ratio. In addition, the disposement of the reflection layer25in a layer lower of the entire color filter26and upper layer of the scanning lines22, that is, between the color filter26(liquid crystal layer15) and the scanning lines22allows the reflection layer25to shield the unwanted leakage electric field directed from the scanning lines22to the liquid crystal layer15, thus enabling the suppression of a reduction of display quality caused by the leakage electric field.

Further, the counter substrate14does not require any counter electrodes, thus making it easier to adjust positioning of the array substrate13and the counter substrate14.

Moreover, the vertically-aligned liquid crystal molecules15ado not require rubbing alignment treatment to the alignment films29and43. This can prevent electrostatic destruction from occurring due to rubbing and improve yield, thus further reducing cost when manufacturing the display device11.

The widths of the pixel electrodes27and the common electrodes28are substantially the same as each other, and the space between the pixel electrodes27and the common electrodes28is set two times wider or more than the width of the pixel electrodes27or the common electrodes28. This suppresses obstruction of light (incident light and reflection light) by the pixel electrodes27and the common electrodes28, and also reduces the number of not-falling liquid crystal molecules15alocated directly above the pixel electrodes27and the common electrodes28by the transverse electric field, thus achieving low driving voltage, for example, less than 5 V, while providing high transmittance.

Next, a second embodiment will be described with reference toFIG. 3A,FIG. 3BandFIG. 4. Configurations and effects of the second embodiment that are the same as those of the first embodiment are identified by the same signs and the description thereof will be omitted.

In the second embodiment, ribs48are respectively disposed at positions facing each of the pixel electrodes27on the counter substrate14of the aforementioned first embodiment.

Each of the ribs48is formed of a member having a high transmittance and a low dielectric constant. That is, each of the ribs48has light transmitting properties. Each of the ribs48is formed along the center position of each of the pixel regions A and along each of the pixel electrodes27. Therefore, in this embodiment, each of the ribs48is formed in a linear shape. Each of the ribs48has a lower dielectric constant than the liquid crystal layer15; for example, a dielectric constant set to approx. one seventh, preferably 0.1 to 0.2 times the dielectric constant of the liquid crystal layer15. Each of the ribs48has a square shape in cross section, and is set thicker (in height) and wider than each of the pixel electrodes27. Each of the ribs48is, for example, set to 0.7±0.1 μm thick and, for example, 6.0±0.25 μm wide. Preferably, each of the ribs48is 0.3 to 0.4 times wider than the color filter26, and is 0.3 to 0.4 times thicker (in height) than the liquid crystal layer15.

As described above, each of the ribs48having a high transmittance and a low dielectric constant is provided at a position facing each of the pixel electrodes27on the counter substrate14. Thus, when a voltage is applied to the pixel electrodes27, the ribs48disturb electric field vectors between the pixel electrodes27and the common electrodes28, thereby being able to make the liquid crystal molecules15alocated above (directly above) the pixel electrodes27fall. Therefore, reflectance can be improved.

The ribs48that are set thicker and wider than the pixel electrodes27allow the liquid crystal molecules15ato be made to fall down more certainly when a voltage is applied to the pixel electrodes27.

In the second embodiment described above, in the case of using multi-domain pixel regions A for improving a viewing angle or other purposes, each of the pixel regions A may have a bent shape, not necessarily having a rectangle shape. In this case, as shown in the third embodiment ofFIG. 5for example, each of the ribs48is formed in a bent shape along the center position of each of the pixel regions A, thereby providing the same effect as the aforementioned second embodiment.

In each of the embodiments above, any arrangement and colors of the filter parts26r,26gand26bof the color filter26can be set as long as they realize color displaying on the display device11.