JP2812601B2 - Liquid crystal display - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a guest-host type liquid crystal display device.

[0002]

2. Description of the Related Art In recent years, applications of liquid crystal display devices to word processors, laptop personal computers, pocket televisions, and the like have been rapidly advancing. In particular, among the liquid crystal display elements, a reflection type liquid crystal display element that performs display by reflecting light incident from the outside is notable because a backlight is not required, so that power consumption is low, and thin and light weight can be achieved. ing.

Conventionally, TN (twisted nematic) and STN (super twisted nematic) systems have been used for reflection type liquid crystal display devices. In these systems, however, the light intensity of natural light is necessarily increased by a linear polarizer. 1/2 will not be used for display, and the display will be dark. To solve such a problem, a display mode has been proposed in which all rays of natural light are effectively used. An example of such a mode is a phase change type guest-host system. (D.L.W.
hit and G. N. Taylor: J. App
l. Phys. 45 4718, 1974). In this mode, a cholesteric-nematic phase transition phenomenon caused by an electric field is used. A liquid crystal composition comprising a cholesteric liquid crystal obtained by adding a chiral substance to a nematic liquid crystal having a positive dielectric anisotropy and a dichroic dye is sealed in a vertically aligned liquid crystal cell. FIG. 2 is a schematic cross-sectional view of such a liquid crystal display device of the phase-change guest-host type. Here, 1 is a transparent substrate having a transparent electrode, 2
Indicates a liquid crystal composition, and a solid line in the liquid crystal composition 7 indicates an alignment direction of liquid crystal molecules.

When no voltage is applied, the liquid crystal composition 7 forms a spiral according to the amount of the added chiral substance. As a result, light passing through the liquid crystal display device is absorbed and colored by the dichroic dye. Next, when a voltage equal to or higher than the threshold is applied, the liquid crystal composition 7 becomes homeotropically aligned, and light passing through the liquid crystal display device becomes colorless without being absorbed by the dichroic dye. A reflection type multi-color display in which a micro color filter is further combined with this method has been proposed (Proceedings).
of the SID Vol. 29,157,19
88).

[0005]

However, in the liquid crystal display device described in the above document, the liquid crystal molecules on the surface of the translucent substrate 1 are vertically aligned. Since the absorption of the liquid crystal composition layer by the dye is eliminated, the transmittance increases and the contrast decreases. FIG. 3 is a schematic diagram when the substrate surface is subjected to horizontal alignment processing. When the substrate surface is subjected to horizontal alignment treatment, the absorption by the dye increases, so the transmittance when no voltage is applied is also low.
When a voltage is applied, the liquid crystal molecules on the substrate surface do not stand sufficiently vertically, so that the transmittance is reduced and a dark display is obtained.

[0006] The present invention is to solve such a problem, and an object of the present invention is to provide a bright liquid crystal display device having a high contrast ratio.

[0007]

SUMMARY OF THE INVENTION The present invention provides a liquid crystal display comprising a chiral nematic liquid crystal obtained by adding a chiral substance to a nematic liquid crystal and a dichroic dye between a pair of translucent substrates each having a transparent electrode on opposing surfaces. Ri Na liquid crystal composition is filled made, the surface of the transparent substrate is a liquid crystal molecules in the liquid crystal composition with a weak anchoring has orientation such that a substantially horizontal, the liquid crystal composition, Toru The angle made with the surface of the optical substrate (
Gillet angle δ) and the natural pitch (p _o) and layer thickness (liquid crystal sets
Following equation between the thickness d) of the layer comprising the formed product: δ / 360-1 / 4 <liquid crystal, characterized by being adjusted so as to satisfy the _{d / p o <δ / 360} + 1/4 A display device is provided.

The liquid crystal composition of the present invention comprises a guest-host type dichroic dye and a chiral smectic liquid crystal. Here, as examples of dichroic dye compounds, those showing magenta include G214 and G241, those showing green are mixtures of G282 and G232, those showing cyan are G282 and G279, and those showing red are G20
G232, G14, which indicate yellow color such as 5, G156
Examples of those exhibiting blue color such as 3, G274 and G277 (both manufactured by Japan Photographic Dye Laboratories) are listed.

The chiral nematic liquid crystal composition as the host of the present invention is prepared by adding a chiral substance to the following known nematic liquid crystal. Known nematic liquid crystals include many compounds and mixtures such as Schiff type, azoxy type, biphenyl type, phenyl ester type, phenylcyclohexane type and pyridine type. Further, as a chiral substance, S-811 (manufactured by Merck), CB-1
1 (manufactured by BDH), CM (manufactured by Chisso) and the like.
The natural pitch (p ₀ ) can be controlled by the amount added to the nematic liquid crystal composition.

These liquid crystal compositions are, for example, ZLI-336 as a guest-host liquid crystal composition containing a black dye.
7, ZLI-4756 / 1), ZLI-3521 /
1), ZLI-4714, ZLI-4113, ZLI-
3402/1) (all manufactured by Merck). In this way, a guest (dichroic dye) compound is added to various host (chiral nematic) liquid crystal materials to form a liquid crystal composition, thereby constituting a liquid crystal display device of the present invention.

In the liquid crystal display device of the present invention, at least one of the substrates has an orientation such that molecules of the liquid crystal composition on its surface are substantially horizontal with weak anchoring to the substrate. That is, the orientation of the substrate is imparted by forming the orientation film itself coated on the substrate by rubbing or oblique deposition, or providing a groove suitable for giving the orientation on the surface of the substrate itself. , Can be granted. This will be described in more detail below.

FIG. 7 shows an example of such a liquid crystal display device of the present invention.
The configuration will be described with reference to FIG. Here, the insulating film may be omitted as appropriate, but in the following example, the case where the insulating film is provided will be described. The substrate 9 is formed by forming each layer in the order of a transparent electrode 2a, an insulating film 3a, and an alignment film 4a on a glass substrate 1a as a transparent substrate. Here, the transparent electrode 2a is formed by arranging a plurality of transparent electrodes in a stripe shape so as to be parallel to each other, and the alignment film 4a has a structure in which a uniaxial alignment process is performed.

On the other hand, the substrate 10 is formed on the other side of the glass substrate 1b under the same conditions in which the transparent electrode 2b, the insulating film 3b, and the alignment film 4b are formed in this order. Like the substrate 9, the transparent electrode 2b and the alignment film 4b are formed by arranging a plurality of transparent electrodes in a stripe shape so as to be parallel to each other, and the alignment film 4b is subjected to a uniaxial alignment process. It has a structure.

Next, the substrates 9 and 10 have d / p ₀ (= cell thickness) such that the alignment films 4a and 4b face each other and the twist angle of the liquid crystal composition between the substrates 9 and 10 becomes a desired value. / Natural pitch). Cell thickness d is 2-8
Attach with a seal member 6 at an interval of μm. A liquid crystal cell 11 is manufactured with a liquid crystal composition 7 interposed between these substrates 9 and 10.

Here, the liquid crystal composition 7 is filled.
The thickness (d) is between the upper and lower alignment films, and the thickness (d) is determined by the design and configuration of the liquid crystal display device.
It is about 0.0 μm, and preferably 5.0 to 8.0 μm. FIG. 5 shows a schematic diagram of a method of weakening anchoring on the substrate surface of a light-transmitting substrate and providing an alignment film so that liquid crystal molecules are horizontal to the substrate. As can be seen from the drawing, the oblique vapor deposition of SiO is performed at an angle of 45 ° or more and 80 ° or less with respect to the perpendicular of the substrate with respect to the orientation of the liquid crystal molecules on the surface of the light transmitting substrate 1. At this time, by setting the film thickness to 100 to 6000 °, horizontal alignment with weak anchoring can be obtained.

Another method for weakly anchoring the substrate surface and performing horizontal alignment is to provide a groove structure on the substrate surface instead of forming the alignment films 4a and 4b as shown in FIG. At this time, it is appropriate that the groove structure has a pattern period λ of 0.5 to 3.0 μm and a depth of 20 to 1000 °. Further, the twist angle ([delta]) is defined as the angle of the liquid crystal molecules in the upper and lower substrate surface of the translucent substrate, d / p _o
Varies by.

When the contrast of this display device was measured, it was surprisingly found that only those satisfying the following conditions had high contrast and could provide a bright display. δ / 360-1 / 4 <d / p o <δ / 360 + 1/4

[0018]

[Function] This is because the liquid crystal molecules on the substrate surface are horizontally aligned.
The anchoring is weaker than before.
When no electric field is applied, the liquid crystal molecules on the substrate surface are aligned horizontally.
State will increase the absorption of the dye, at the time of electric field application, group
The liquid crystal molecules on the plate surface have weak anchoring,
To the orientation state approaching the vertical direction, and the absorption of the dye is reduced.
High contrast display is possible.
You.

[0019]

Next, the present invention will be described in detail with reference to examples, but the present invention is not limited to these examples. The liquid crystal display devices of these embodiments are manufactured by the configuration shown in FIG.
FIG. 1 shows a schematic diagram of the operation.

[0020] S-811 as a chiral material ZLI-2327 (Merck) EXAMPLE 1 dielectric anisotropy are mixed with black pigments to a positive nematic liquid crystal (manufactured by Merck) d / P _o ( A predetermined amount was blended so that (cell thickness / natural pitch ratio) = 1.75 to prepare a liquid crystal composition. As shown in FIG. 5, horizontal alignment films 4a and 4b with weak anchoring were obtained by oblique deposition of SiO at 60 ° and film thickness of 500 ° as shown in FIG. Note that the alignment direction of this liquid crystal molecules in relation to the twist angle [delta] and d / P _o δ /
360-1 / 4 <set to 720 ° from 540 ° to satisfy _{d / p o <δ / 360} + 1/4. As a comparative example, vertical alignment and horizontal alignment obtained by rubbing a polymer alignment film having strong anchoring are shown.

The two substrates 1a and 1b on which the transparent electrodes 2a and 2b are formed are subjected to the above-mentioned alignment treatment, and are bonded together with a sealing member 6 so as to have a cell gap of about 7.7 μm.
A liquid crystal cell was prepared, and the liquid crystal composition 7 was injected to obtain a liquid crystal display device. A voltage was applied to the liquid crystal display device, and a change in transmittance was examined. It is shown in FIG. In the horizontal alignment (2) using the polymer alignment film, the transmittance when no voltage is applied is lower than that in the vertical alignment, but a high voltage is required because the threshold value is increased and the transmittance is saturated. This is because the surface anchoring is so strong that the liquid crystal in the surface layer is not completely vertical. On the other hand, in the vertical alignment (1), although the threshold value decreases, the transmittance when no voltage is applied increases, and the contrast ratio decreases. However, in the horizontal alignment (3) using the SiO film, the transmittance when no voltage is applied is lower than that in the vertical alignment, and the liquid crystal in the surface layer is more likely to be vertical than in the case of the polymer alignment film. For this reason, a display device having a high transmittance when a voltage is applied, a high brightness, and a high contrast ratio is obtained.

Example 2 In Example 1, the obliquely deposited alignment films 4a and 4b were used.
Was formed, a liquid crystal display device having a groove structure formed on the insulating films 3a and 3b was manufactured. In this case, the pattern period λ of the groove shown in FIG.
Å As a result, as in the case of the first embodiment, a display device having a high transmittance at the time of applying a voltage, and having a bright and high contrast ratio was obtained. Also, λ = 0.5 μm to 3.0 μm, d =
A similar effect has been confirmed between 20 ° and 1000 °.

Example 3 The horizontal alignment by the oblique deposition of SiO in Example 1 was performed and d /
A reflection plate was provided on the rear surface of the liquid crystal display device under the condition of P _o = 1.75 to produce a reflection type liquid crystal display device. The contrast ratio was 8, showing good display characteristics.

[0024]

As described above, the liquid crystal display device of the present invention comprises a liquid crystal composition comprising a chiral nematic liquid crystal obtained by adding a chiral substance to a nematic liquid crystal and a dichroic dye. Is encapsulated in a liquid crystal cell that has been subjected to alignment processing so that the substrate is in a horizontal direction and a parallel direction with weak anchoring with respect to the substrate,
The transmittance when no voltage is applied is reduced, and a display with high contrast can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram according to the present invention.

FIG. 2 is a schematic diagram according to a conventional example.

FIG. 3 is a schematic diagram according to a conventional example.

FIG. 4 is a perspective view for explaining a groove structure on a surface.

FIG. 5 is a diagram for explaining a deposition direction.

FIG. 6 is a voltage-transmittance characteristic diagram (corresponding to Examples 1 and 2 of the present invention and a conventional example).

FIG. 7 is a schematic diagram of a liquid crystal display device.

[Explanation of symbols]

 1a, 1b Glass substrate 2a, 2b Transparent electrode 3a, 3b Insulating film 4a, 4b Alignment film 6 Sealing member 7 Guest-host type liquid crystal composition 9, 10 Substrate 11 Liquid crystal display

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-54-98637 (JP, A) JP-A-53-35498 (JP, A) JP-A-56-138712 (JP, A) JP-A-58-1983 28724 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) G02F 1/137 500 G02F 1/1337

Claims (3)

(57) [Claims] A liquid crystal composition comprising a chiral nematic liquid crystal obtained by adding a chiral substance to a nematic liquid crystal and a dichroic dye is filled between a pair of translucent substrates each having a transparent electrode on opposing surfaces. Do Ri Table <br/> surface of the transparent substrate is a weak anchoring the liquid crystal molecules in the liquid crystal composition has a orientation such that the substantially horizontal, the liquid crystal composition, translucent
The angle between the substrate surface (twist angle δ) and the natural pitch (p
_o ) and the layer thickness (thickness d of the layer composed of the liquid crystal composition)
Following equation between: δ / 360-1 / 4 <liquid crystal display device characterized in that it is adjusted so as to satisfy the _{d / p o <δ / 360} + 1/4. 2. A distribution tropism, a liquid crystal display device described in claim 1 that will be granted Ri by the alignment film formed by oblique evaporation method. 3. A distribution tropism, the substrate in contact with the liquid crystal composition itself
The liquid crystal display device described in claim 1, by Ri Ru granted to a groove structure provided on the surface of the.