JP2012128001A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a reflection type liquid crystal display device which excels in display quality and can reduce a thickness and cost.SOLUTION: A liquid crystal display device comprises an array substrate including a switching element, a pixel electrode connected to the switching element, and a prism reflection layer located above the pixel electrode and composed of prisms formed at a constant pitch, a counter substrate including a counter electrode opposed to the pixel electrode, and a liquid crystal layer held between the array substrate and the counter substrate and containing dichroic dye and liquid crystal molecules. The liquid crystal molecules have refractive index anisotropy in which a refractive index (ne) in a long axis direction thereof is higher than a refractive index (no) in a short axis direction thereof, and the refractive index in the short axis direction of the liquid crystal molecules is substantially equal to a refractive index of the prisms.

Description

  Embodiments described herein relate generally to a liquid crystal display device.

  Flat display devices represented by liquid crystal display devices are utilized in various fields as display devices for OA equipment such as personal computers, information terminals, watches, and televisions, taking advantage of features such as light weight, thinness, and low power consumption. Yes. Among them, a liquid crystal display device using a thin film transistor (TFT) is widely used as a monitor for displaying a large amount of information such as a portable terminal and a computer because of its high responsiveness.

  Such a liquid crystal display device is classified into a transmission type and a reflection type depending on the light source used. The transmission type is a configuration in which an illumination device called a backlight is provided on the back surface of the liquid crystal display panel. The reflection type is a configuration in which a reflection layer is provided on the back side of a liquid crystal layer and external light incident from the outside is used as a light source.

  In recent years, interest in portable information terminal devices such as mobile phones and PDAs (personal digital assistants) has increased, and there has been an increasing demand for reflective liquid crystal display devices that consume less power, are lighter and more portable than transmission types. ing.

Japanese Patent Laid-Open No. 08-152620

  An object of the present embodiment is to provide a reflective liquid crystal display device that has good display quality and can be reduced in thickness and cost.

According to this embodiment,
An array substrate having a switching element, a pixel electrode connected to the switching element, and a prism reflection layer including a prism located above the pixel electrode and formed at a constant pitch, and opposed to the pixel electrode A counter substrate having a counter electrode, and a liquid crystal layer that is held between the array substrate and the counter substrate and includes a dichroic dye and a liquid crystal molecule. The refractive index (ne) has a refractive index anisotropy greater than the refractive index (no) in the minor axis direction, and the refractive index in the minor axis direction of the liquid crystal molecules is substantially equal to the refractive index of the prism. A liquid crystal display device is provided.

FIG. 1 is a plan view schematically showing the configuration of the liquid crystal display device according to the present embodiment. FIG. 2 is a cross-sectional view schematically showing an example of the structure of the liquid crystal display device shown in FIG. FIG. 3 is a cross-sectional view schematically showing another structural example of the liquid crystal display device shown in FIG.

  Hereinafter, the present embodiment will be described in detail with reference to the drawings. In each figure, the same reference numerals are given to components that exhibit the same or similar functions, and duplicate descriptions are omitted.

  FIG. 1 is a plan view schematically showing a configuration of a liquid crystal display device 1 in the present embodiment.

  That is, the liquid crystal display device 1 includes an active matrix type liquid crystal display panel LPN, a drive IC chip 2 connected to the liquid crystal display panel LPN, a flexible wiring board 3, and the like.

  The liquid crystal display panel LPN includes a display area (sometimes referred to as an active area) ACT for displaying an image. This display area ACT is formed of, for example, a plurality of pixels PX formed in a substantially rectangular shape and arranged in an m × n matrix (where m and n are positive integers).

  Such a liquid crystal display panel LPN includes an array substrate AR, a counter substrate CT disposed to face the array substrate AR, a liquid crystal layer LQ held between the array substrate AR and the counter substrate CT, It is configured with.

  The array substrate AR includes a gate line GL extending along the first direction X, a source line SL extending along the second direction Y orthogonal to the first direction X, and the like. The array substrate AR is connected to the gate line GL and the source line SL, and is arranged corresponding to each pixel PX. The pixel electrode is connected to the switching element SW and arranged corresponding to each pixel PX. PE etc. are provided. The counter substrate CT includes a counter electrode CE that faces each of the pixel electrodes PE through the liquid crystal layer LQ.

  The seal material SE is disposed outside the display area ACT, and bonds the array substrate AR and the counter substrate CT together. In the illustrated example, the sealing material SE is formed in a substantially rectangular frame shape and is formed in a closed loop shape that does not have an injection port for injecting a liquid crystal material. However, the sealing material SE is not limited to this example. An injection port may be formed. When the injection port is formed, after the liquid crystal material is injected from the injection port, the injection port is sealed with a sealing material such as an ultraviolet curable resin.

  FIG. 2 is a cross-sectional view schematically showing an example of the structure of the liquid crystal display device 1 shown in FIG.

  The array substrate AR is formed by using a first insulating substrate SUB1 having optical transparency such as a glass substrate or a plastic substrate. The array substrate AR includes a switching element SW, a pixel electrode PE, a light absorption layer AB, a prism reflection layer PL, and the like on the side of the first insulating substrate SUB1 facing the counter substrate CT.

  The switching element SW is constituted by, for example, a thin film transistor (TFT). The switching element SW may be a top gate type or a bottom gate type, and includes a semiconductor layer formed of polysilicon, amorphous silicon, or the like, although not described in detail. In the illustrated example, the switching element SW is covered with an insulating layer IS.

  The pixel electrode PE is electrically connected to the switching element SW. The pixel electrode PE is made of, for example, a conductive material such as indium tin oxide (ITO), indium zinc oxide (IZO), aluminum (Al), or silver (Ag). In the illustrated example, the pixel electrode PE is formed on the insulating layer IS, and is connected to the switching element SW through a contact hole formed in the insulating layer IS.

  The prism reflection layer PL is located above the pixel electrode PE. The prism reflection layer PL is composed of prisms PS formed at a constant pitch. The prism PS is formed over substantially the entire display area ACT. Such a prism PS is made of, for example, a transparent resin material. The prism PS is, for example, a triangular pyramid prism having three slopes with an inclination angle of about 45 °, but the shape of the prism PS is not particularly limited to this. For example, the prism PS may be a quadrangular pyramid prism or the like.

  In the present embodiment, the array substrate AR includes the light absorption layer AB on the back side of the prism reflection layer PL. This light absorption layer AB extends over substantially the entire display area ACT. Such a light absorption layer AB is formed of, for example, a resin material colored in black. The surface of the light absorption layer AB is generally flattened.

  In the illustrated example, the light absorption layer AB is disposed on the pixel electrode PE and also on the insulating layer IS. The prism reflection layer PL is disposed on the light absorption layer AB. That is, the light absorption layer AB is disposed between the pixel electrode PE and the prism reflection layer PL. Since the surface of the light absorption layer AB is generally flattened, it is possible to form a prism PS having a uniform shape.

  On the other hand, the counter substrate CT is formed using a second insulating substrate SUB2 having optical transparency such as a glass substrate or a plastic substrate. The counter substrate CT includes a counter electrode CE and the like on the side of the second insulating substrate SUB2 facing the array substrate AR. Although not shown in the figure, this counter substrate CT alleviates the influence of the black matrix partitioning each pixel PX, the color filter layer arranged corresponding to each pixel PX, and the unevenness of the surface of the color filter layer. An overcoat layer or the like may be disposed.

  The counter electrode CE is opposed to each of the pixel electrodes PE. The counter electrode CE extends over substantially the entire display area ACT. Such a counter electrode CE is formed of a light-transmitting conductive material such as ITO or IZO, for example.

  Although not shown, the array substrate AR includes a first vertical alignment film, and the counter substrate CT includes a second vertical alignment film. The first vertical alignment film is disposed so as to cover the surface of the prism PS, and extends over substantially the entire display area ACT. The second vertical alignment film is disposed so as to cover the counter electrode CE and extends over substantially the entire display area ACT.

  The first vertical alignment film and the second vertical alignment film do not require an alignment process step typified by a rubbing process, and a state in which no potential difference is formed between the pixel electrode PE and the counter electrode CE, that is, the pixel electrode When no electric field is formed between the PE and the counter electrode CE, the liquid crystal molecules LM of the liquid crystal layer LQ are respectively transferred to the main surface of the first insulating substrate SUB1 (or the array substrate AR) and the second insulating substrate SUB2 ( Alternatively, it has a characteristic of being oriented substantially perpendicular to the main surface of the counter substrate CT).

  The array substrate AR and the counter substrate CT as described above are arranged to face each other. At this time, a predetermined cell gap is formed between the array substrate AR and the counter substrate CT by a spacer (not shown) (for example, a columnar spacer formed integrally with one substrate by a resin material).

  The sealing material SE is disposed outside the display area ACT. The sealing material SE is bonded to the array substrate AR and the counter substrate CT in a state where a predetermined cell gap is formed. Such a sealing material SE is formed of, for example, an ultraviolet curable resin or a thermosetting resin.

  The liquid crystal layer LQ is held in a cell gap formed between the inner array substrate AR and the counter substrate CT surrounded by the sealing material SE, and is in contact with a first vertical alignment film and a second vertical alignment film (not shown). Yes. The liquid crystal layer LQ contains a dichroic dye (guest dye) DC and contains liquid crystal molecules (host liquid crystal) LM having a negative dielectric anisotropy. The dichroic dye DC is, for example, a black dye. In addition, a chiral material for twist aligning the liquid crystal molecules LM is added to the liquid crystal layer LQ.

  The liquid crystal molecule LM has a refractive index anisotropy (Δn = ne−no) in which the refractive index (ne) in the major axis direction is larger than the refractive index (no) in the minor axis direction. In the present embodiment, for example, the refractive index (ne) is about 1.58, the refractive index (no) is about 1.48, and the refractive index anisotropy (Δn) is about 0.1.

  In the present embodiment, the refractive index (no) in the minor axis direction of the liquid crystal molecules LM and the refractive index of the prism PS constituting the prism reflection layer PL are substantially equal. That is, the refractive index of the material forming the prism PS is about 1.48. Note that the refractive index of the first vertical alignment film (not shown) is substantially the same as the refractive index (no) in the minor axis direction of the liquid crystal molecules LM.

  According to the reflective liquid crystal display device 1 having such a configuration, in the state where no electric field is formed between the pixel electrode PE and the counter electrode CE (OFF state), the liquid crystal of the liquid crystal layer LQ is illustrated as illustrated. Both the molecule LM and the dichroic dye DC are oriented substantially perpendicular to the substrate. At this time, the light incident from the front of the counter substrate CT passes through the liquid crystal layer LQ without being absorbed and travels toward the prism reflection layer PL.

  In such an OFF state, the refractive index of the liquid crystal layer LQ is the refractive index (ne) in the major axis direction of the liquid crystal molecules LM, while the prism PS in the prism reflection layer PL and the first vertical alignment film covering the prism PS The refractive index is substantially equal to the refractive index (no) in the minor axis direction of the liquid crystal molecules LM. The refractive index (ne) is larger than the refractive index (no).

  For this reason, the light that has passed through the liquid crystal layer LQ is totally reflected by the surface of the prism PS (or the interface between the liquid crystal layer LQ and the first vertical alignment film), passes through the liquid crystal layer LQ again, and passes through the liquid crystal layer LQ. Emitted forward. For this reason, the liquid crystal display device 1 is in a bright display (white display) state.

  On the other hand, in a state where an electric field is formed between the pixel electrode PE and the counter electrode CE (ON state), both the liquid crystal molecules LM and the dichroic dye DC of the liquid crystal layer LQ are twisted and substantially horizontal with respect to the substrate. Oriented. At this time, most of the light incident from the front of the counter substrate CT is absorbed by the dichroic dye DC in almost all directions when passing through the liquid crystal layer LQ even in a random polarization state. Furthermore, a part of the light passes through the liquid crystal layer LQ without being absorbed by the dichroic dye DC and travels toward the prism reflection layer PL.

  In such an ON state, the refractive index of the liquid crystal layer LQ is the refractive index (no) in the minor axis direction of the liquid crystal molecules LM, while the prisms PS in the prism reflection layer PL and the first vertical alignment film covering the prisms PS The refractive index is substantially equal to the refractive index (no) in the minor axis direction of the liquid crystal molecules LM.

  Therefore, the light that has passed through the liquid crystal layer LQ passes through the first vertical alignment film and the prism reflection layer PL, and is absorbed by the light absorption layer AB provided on the back side of the prism reflection layer PL. For this reason, the liquid crystal display device 1 is in a dark display (black display) state. Thereby, the normally white mode is realized.

  According to the reflective liquid crystal display device 1 of the present embodiment, the prism reflection layer PL including the prism PS having a refractive index substantially equal to the refractive index in the minor axis direction of the liquid crystal molecules LM is provided. For this reason, under the condition that the refractive index of the liquid crystal layer LQ is the refractive index in the major axis direction of the liquid crystal molecules LM, a difference in refractive index occurs between the liquid crystal layer LQ and the prism PS, and light that has passed through the liquid crystal layer LQ Under the condition that the liquid crystal layer LQ is totally reflected by the prism reflection layer PL and the refractive index of the liquid crystal layer LQ is the refractive index in the minor axis direction of the liquid crystal molecules LM, most of the external light incident on the liquid crystal layer LQ is incident on the liquid crystal layer LQ. Absorbed by the included dichroic dye DC. By such switching, the reflective liquid crystal display device 1 is provided.

  Further, under the condition that the refractive index of the liquid crystal layer LQ is the refractive index in the minor axis direction of the liquid crystal molecules LM, due to the low dichroic ratio of the dichroic dye DC, for example, part of the extraneous light is Even if the liquid crystal layer LQ passes through the liquid crystal layer LQ without being absorbed by the dichroic dye DC, the refractive indexes of the liquid crystal layer LQ and the prism PS are substantially equal. After passing through the reflection layer PL, it is absorbed by the light absorption layer AB disposed on the back side of the prism reflection layer PL. For this reason, there is almost no light emitted in front of the counter substrate CT. Therefore, a high contrast ratio can be obtained. Thereby, it is possible to obtain a good display quality.

  In addition, since the reflective liquid crystal display device 1 according to the present embodiment has a configuration that does not require a polarizing plate, the light utilization efficiency is high and a bright display can be realized. In addition, since the configuration of the present embodiment does not require a wave plate or a reflector in addition to the polarizing plate, it is possible to reduce the thickness and weight, and to reduce the cost by reducing the number of members. That is, the reflective liquid crystal display device 1 that is light and thin and excellent in portability can be realized at low cost.

  FIG. 3 is a cross-sectional view schematically showing another structural example of the liquid crystal display device 1 shown in FIG.

  The example shown here is different from the example shown in FIG. 2 in that the prism PS of the prism reflection layer PL is formed of a light absorber instead of the light absorption layer AB. In addition, about another structure, it is the same as the example shown in FIG. 2, the same referential mark is attached | subjected and detailed description is abbreviate | omitted.

  The prism reflection layer PL is located above the pixel electrode PE. The prisms PS of the prism reflection layer PL are formed at a constant pitch, and are formed over substantially the entire display area ACT. Such a prism PS is formed of a black resin material which is an example of a light absorber.

  In the illustrated example, the prism PS is formed on the pixel electrode PE and the insulating layer IS, and is covered with a first vertical alignment film that is not illustrated.

  In the reflective liquid crystal display device 1 having such a configuration, in the OFF state, external light incident on the liquid crystal layer LQ is incident on the surface of the prism PS (or the first vertical alignment with the liquid crystal layer LQ), as in the above example. Is totally reflected at the interface with the film), passes through the liquid crystal layer LQ again, and is emitted in front of the counter substrate CT. For this reason, the liquid crystal display device 1 is in a bright display (white display) state.

  On the other hand, in the ON state, most of the external light incident on the liquid crystal layer LQ is dichroic in almost all directions when passing through the liquid crystal layer LQ even in a random polarization state, as in the above example. Absorbed by dye DC. The light that has not been absorbed in the liquid crystal layer LQ passes through the liquid crystal layer LQ, then passes through the first vertical alignment film and the prism reflection layer PL, and is absorbed by the prism PS. For this reason, the liquid crystal display device 1 is in a dark display (black display) state.

  In such a configuration, in addition to the same effect as the example shown in FIG. 2, in addition to providing the light absorption layer AB separately from the prism reflection layer PL, the prism PS itself is used as the light absorber. Thus, it is possible to reduce the thickness and weight, and to reduce the cost.

  In this embodiment, the configuration using the vertical alignment twist type guest-host liquid crystal has been described. However, the present embodiment can be applied to other types of reflective liquid crystal display devices, and the same effect can be obtained. .

  In addition, when the prism reflection layer PL of this embodiment also acts as a retroreflecting plate, it exhibits a particularly high reflectance in the incident direction of external light, but a diffusion layer is provided on the front surface of the counter substrate CT according to the product specifications. It may be provided.

  The operation of the present embodiment is obtained by refractive index matching between the prism reflection layer PL and the liquid crystal layer LQ, and does not necessarily require a combination with the guest-host liquid crystal mode. By using a liquid crystal material having a sufficiently large refractive index anisotropy (Δn) and selecting a prism material having an appropriate refractive index, it is possible to obtain a certain degree of contrast characteristics. However, it is not easy to select a material that exhibits sufficient contrast characteristics as a display device and that satisfies the required conditions regarding characteristics other than the refractive index. Therefore, the configuration applied to the reflective mode such as the guest-host liquid crystal mode described in this embodiment is very effective.

Example 1
The first embodiment corresponds to the liquid crystal display device 1 of the example shown in FIG. First, the switching element SW, the insulating layer IS, and the pixel electrode PE are formed on the glass substrate serving as the first insulating substrate SUB1 by repeatedly forming and patterning an insulating material or a conductive material. Thereafter, the light absorption layer AB is formed of a black resin material. Then, a prism PS is formed using a transparent acrylic resin material having a light refractive index of about 1.48 on the upper layer of the light absorption layer AB, and is used as the prism reflection layer PL. Here, the prism reflection layer PL is formed by arranging triangular pyramid-shaped prisms PS having three slopes with an inclination angle of about 45 ° at a constant pitch.

  On the other hand, the counter electrode CE is formed on one surface of the second insulating substrate SUB2 for forming the counter substrate CT. After that, a vertical alignment film is applied on each of the prism reflection layer PL and the counter electrode CE, and both substrates are bonded together in a state where a certain cell gap is formed using an electrically reliable sealing material SE. To fill the liquid crystal material.

  In this embodiment, a twisted GH (guest host) liquid crystal mode is used, and a liquid crystal material including a liquid crystal molecule LM having a negative dielectric anisotropy in which a black dye as a dichroic dye DC and a chiral material are mixed. And the injection port was sealed with an ultraviolet curable resin to produce a liquid crystal display device 1. The liquid crystal material used in this example has a refractive index anisotropy (Δn) of about 0.1 (ne = about 1.58, no = about 1.48).

  According to the reflection type liquid crystal display device 1 produced in this way, a high contrast ratio was obtained.

(Example 2)
The second embodiment corresponds to the liquid crystal display device 1 of the example shown in FIG. First, after forming the switching element SW, the insulating layer IS, and the pixel electrode PE on the glass substrate to be the first insulating substrate SUB1, the light absorption layer is not formed, and a black acrylic resin material is used. A prism PS having a refractive index of about 1.48 is formed as a prism reflection layer PL. Except for this point, the configuration was exactly the same as in the example, and the same material was applied.

  According to the reflection type liquid crystal display device 1 thus produced, a high contrast ratio was obtained as in Example 1.

(Comparative Example 1)
In the reflective liquid crystal display device of Comparative Example 1, the pixel electrode formed of aluminum (Al) was used as the diffuse reflector without providing the light absorption layer and the prism reflection layer. Except for this point, the configuration was exactly the same as in Example 1, and the same material was applied.

  According to the reflective liquid crystal display device of Comparative Example 1, it was confirmed that the contrast ratio was lower than that of Example 1 and Example 2. This is considered to be due to the fact that the external light is not sufficiently absorbed in the ON state and a part of the light is emitted to the front surface of the counter substrate.

(Comparative Example 2)
The reflective liquid crystal display device of Comparative Example 2 has the same configuration as that of Example 1 except that no dichroic dye is added to the liquid crystal material, and the same material is applied.

  According to such a reflective liquid crystal display device of Comparative Example 2, it was confirmed that the contrast ratio was even lower than that of Comparative Example 1.

  As described above, according to this embodiment, it is possible to provide a reflective liquid crystal display device that has good display quality and can be reduced in thickness and cost.

  In addition, although some embodiment of this invention was described, these embodiment is shown as an example and is not intending limiting the range of invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 1 ... Liquid crystal display device LPN ... Liquid crystal display panel AR ... Array substrate SW ... Switching element PE ... Pixel electrode CT ... Counter substrate CE ... Counter electrode LQ ... Liquid crystal layer LM ... Liquid crystal molecule DC ... Dichroic dye AB ... Light absorption layer PL ... Prism reflection layer PS ... Prism

Claims (7)

An array substrate having a switching element, a pixel electrode connected to the switching element, and a prism reflection layer made of a prism located above the pixel electrode and formed at a constant pitch;
A counter substrate having a counter electrode facing the pixel electrode;
A liquid crystal layer that is held between the array substrate and the counter substrate and includes a dichroic dye and a liquid crystal molecule,
The liquid crystal molecules have a refractive index anisotropy whose refractive index (ne) in the major axis direction is larger than the refractive index (no) in the minor axis direction, and the refractive index in the minor axis direction of the liquid crystal molecules is the prism. A liquid crystal display device having substantially the same refractive index as that of the liquid crystal display device.   The liquid crystal display device according to claim 1, wherein the liquid crystal layer contains a black pigment as a dichroic pigment.   The liquid crystal display device according to claim 1, further comprising a light absorption layer on a back side of the prism reflection layer.   The liquid crystal display device according to claim 3, wherein the light absorption layer is disposed between the pixel electrode and the prism reflection layer.   The liquid crystal display device according to claim 1, wherein the prism is formed of a light absorber.   6. The liquid crystal display device according to claim 1, wherein the dielectric anisotropy of the liquid crystal molecules is negative.   7. The array substrate according to claim 1, wherein the array substrate has a first vertical alignment film in contact with the liquid crystal layer, and the counter substrate has a second vertical alignment film in contact with the liquid crystal layer. A liquid crystal display device according to 1.

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