JPH0274919A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To decrease leak light and to obtain high contrast by providing a liquid crystal cell having a liquid crystal layer disposed in such a manner that the nematic liquid crystal added with an optically active substance forms a twisted structure between a pair of electrode substrates having oriented layers and providing a pair of polarizing plates disposed on both sides of the liquid crystal cell. CONSTITUTION:The twist angle alpha of the twisted structure of the liquid crystal layer 20 is 220 to 300 deg.. This twist angle alpha can be regulated by the orientation treatment direction of the oriented layers formed respectively to a pair of the substrates 11, 12 and the kinds, amts., etc. of the nematic liquid crystal and the optically active substance to be added thereto. The product DELTAn.d of the magnitude DELTAn of the refractive index anisotropy of the nematic liquid crystal and the thickness d(mum) of the liquid crystal layer 20 is required to be 0.60 to 1.1mum. The angles (deviation angles) beta and gamma between the polarizing plates 41 and 42 and the orientation direction of the liquid crystal molecules in contact with the surfaces of the substrates 11 and 12 adjacent to the respective polarizing plates satisfy equations I and II. The leak light is decreased in this way and the higher contrast than heretofore is obtd.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a supertwisted nemat
ic) type liquid crystal display device.

[Technology background]

In dot matrix display, in order to increase the display contrast, the normally closed display mode (non-transparent state when off-voltage is applied and transmissive state when on-voltage is applied) is required, in which non-pixel areas are in a dark state (non-transparent state). ) is advantageous.

A blue mode liquid crystal display device has been disclosed as a technology that adopts a normally closed display mode in an STN liquid crystal display device (Japanese Unexamined Patent Application Publication No. 1983-1998).
(See Publication No. 07020).

In this liquid crystal display device, the deviation angles β and γ are 20
~70°, and the sum of β and T (β+γ) is 0°.

[Problem to be solved by the invention]

However, in the liquid crystal display device disclosed in the above-mentioned publication, leakage of light in a dark state is large, especially leakage of blue light is large, resulting in a white display on a blue background, which still has the problem of low contrast.

The present invention has been made based on the above-mentioned circumstances, and its object is to provide a higher contrast than the conventional one in an STN type liquid crystal display device employing a normally closed display mode.

[Means to solve the problem]

In order to achieve the above object, the present invention provides a liquid crystal layer in which nematic liquid crystal added with an optically active substance is arranged to form a twisted structure between a pair of electrode substrates each having an alignment layer arranged opposite to each other. and a pair of polarizing plates disposed on both sides of the liquid crystal cell, and is characterized by satisfying the following conditions (a), (b), and (c).

Condition (a): The twist angle α of the twist structure in the liquid crystal layer is 220 to 300°.

Condition (b): Size Δ of refractive index anisotropy of nematic liquid crystal
The product Δn-a of n and the thickness d (μM) of the liquid crystal layer is 06
Must be 60 to 1.1 μm.

Condition (c): The angle (shift angle) β and T between the polarization axes of a pair of polarizing plates and the alignment direction of liquid crystal molecules in contact with the surface of the electrode substrate adjacent to each of the polarizing plates is expressed by the following formula [
1] and [2] must be satisfied.

Formula ■; Formula ■; β+γ=90゜ [Operation] By satisfying the above conditions (a) to (c), the normally closed display mode is adopted, as will be understood from the explanation of the embodiment described later. In an STN type liquid crystal display device, there is extremely little leakage light and higher contrast can be obtained than conventionally.

[Specific structure of the invention]

Hereinafter, the configuration of the present invention will be specifically explained.

FIG. 1 is an explanatory diagram showing an exploded main part of a liquid crystal display device according to the present invention. In the figure, 11 and 12 are electrode substrates each having an alignment layer (not shown), and at least one of them is translucent.

20 is a liquid crystal layer in which nematic liquid crystal added with an optically active substance is arranged to form a twisted structure. 30 is a liquid crystal cell. Polarizing plates 41 and 42 are arranged facing one side and the other side of the liquid crystal cell 30, respectively.

In the present invention, it is necessary that the twist angle α of the twisted structure in the liquid crystal layer 20 is 220 to 300° (
Condition (a)).

If the twist angle α is too small, the intensity of transmitted light changes slowly with respect to the applied voltage, making it impossible to obtain high contrast. On the other hand, when the torsion angle α is excessive,
The on/off response characteristics deteriorate, and the orientation of liquid crystal molecules is disturbed during on/off switching, resulting in a decrease in display quality.

This twist angle α can be defined by the alignment direction of the alignment layer formed on each of the pair of electrode substrates, the type and amount of the nematic liquid crystal layer and the optically active substance added thereto, and the like.

In addition, in FIG. 1, the twist angle α shows a counterclockwise twist in the traveling direction of the incident light, but is not limited to this, and may be a clockwise twist.

In the present invention, the product Δn of the magnitude Δn of the refractive index anisotropy of the nematic liquid crystal and the thickness d (μM) of the liquid crystal layer 20 is
-d is required to be 0.60 to 1.1 μm (condition (5)), preferably 0.7 to 0.91.

When this product Δn−d is too small, the brightness of the display becomes low. On the other hand, when this product Δn-d is excessive, the amount of leaked light increases.

In the present invention, the angle (shift) between the polarizing axes 41A and 42A of the pair of polarizing plates 41 and 42 and the alignment direction of liquid crystal molecules in contact with the surfaces of the electrode substrates 11 and 120 adjacent to the polarizing plates 41 and 42, respectively, is Angle) β and γ must satisfy the following formulas [1] and [2] (condition (c)).

Expression ■.

Formula (2); β+r = 90' The above condition (c) was obtained from the following experimental results.

[Experiment 1] Using an obliquely evaporated SiO film as an alignment film, liquid crystal Z L
A liquid crystal cell with a twist angle α of 270° was prepared using I-2293 (manufactured by Merck & Co., Ltd.). At this time, the pretilt angle θ. was 40°.

By changing the cell thickness d, liquid crystal cells with different Δn-d were manufactured, and for each liquid crystal cell, the deviation angles β and γ of the polarizing plate that provided the maximum contrast in normally closed display mode were determined. Ta.

As a result, the maximum contrast is always β−+T=90
The values of β, the luminance value of transmitted light YI, and the Yorfz contrast ratio Y0°/Y-re are as shown in Figures 5 and 6. .

In other words, the brightness Y of the display. , , become brighter as Δn-d increases, and the contrast ratio is Δn-d =
Q,ll) reaches its maximum near tm, therefore, a bright and high-contrast display requires Δn-d of 0.6 to 1.1μ.
Obtained between Qing. The optimum range of the deviation angle β at this time is -10° to +20°.

[Experiment 2] Polyimide RN-715 (Nippon Kagaku Kogyo ■) was used as the alignment film.
The experiment was carried out in the same manner as in Experiment 1, except that a rubbed film manufactured by Co., Ltd. was used. At this time, the pretilt angle θ0 is 10
It was °. As a result, the optimal deviation angle β, T of the polarizing plate is always when β+T=90°, and the deviation angle β at that time is
The range of values is from -20° to +15°, as shown in Figure 5.
It was °.

[Experiment 3] An experiment was conducted in the same manner as in Experiment 1, except that an obliquely deposited SiO film was used as the alignment film and the twist angle α was 240°. As a result, the optimal deviation angle β, T of the polarizing plate is always β
When 10 γ = 90°, the optimum value of the deviation angle β at that time is around (270° - α)/2, as shown in Figure 7, and around that value -15° to +15 It was in the range of °.

The optimum values of the deviation angles β and T are β+γ=90°, but in reality, a practically sufficient contrast can be obtained within the range β+γ=90°±15°.

In the present invention, the orientation direction of the liquid crystal molecules in contact with the electrode substrate surface refers to the director direction, that is, the direction in which the long axes of the liquid crystal molecules are preferentially oriented (provided that the liquid crystal molecules are at a pretilt angle that is not 0° with the electrode substrate surface). When you have
(projection direction of the director direction onto the electrode substrate surface).

In the present invention, the deviation angle β refers to the deviation angle of the polarizing plate on the light incident side, and the deviation angle γ refers to the deviation angle of the polarizing plate on the light output side in the case of a transmission type liquid crystal display device. In the case of a reflective liquid crystal display device, it refers to the deviation angle of the polarizing plate on the reflective side of light.

The deviation angle of the polarizing plate is determined based on the direction in which the director direction of the liquid crystal molecules in contact with the surface of the electrode substrate (the direction in which the long axes of the liquid crystal molecules are preferentially oriented) is projected onto the surface of the electrode substrate. This refers to the angle formed by the polarization axis direction of a polarizing plate placed on the substrate side.

In the present invention, the values of the deviation angles β and γ of the polarizing plate are read as values between −90° and +90°. That is, for example, the deviation angle β = 135° is the deviation angle β = -
Equal to 45°.

Here, the direction in which the director direction of liquid crystal molecules in contact with the surface of the electrode substrate is projected onto the surface of the electrode substrate generally coincides with the alignment treatment direction of the alignment layer of the electrode substrate.

Further, the orientation treatment direction refers to the rubbing direction when the orientation treatment is a rubbing method, for example.

The deviation angles β and T of the pair of polarizing plates are defined using either the transmission axis or the absorption axis as the polarization axis of the polarizing plates.

Furthermore, as shown in Fig. 2, when the twist direction of the liquid crystal molecules is counterclockwise in the traveling direction of the incident light, the signs of the deviation angles β and γ are from the director direction of the liquid crystal molecules in contact with the surface of the electrode substrate. If the direction of twist of the liquid crystal molecules is clockwise toward the direction of the incident light as shown in Figure 3, then the direction is counterclockwise. Take the correct direction.

Further, in the present invention, the pretilt angle θ.

That is, the angle θ between the director direction of liquid crystal molecules in contact with the surface of at least one alignment layer and the surface of the electrode substrate having the alignment layer. is preferably 5° or more, particularly 20
° or more is preferable. When this condition is satisfied, the transmission state becomes brighter when an on-voltage is applied, and the change in transmittance with respect to the applied voltage becomes steeper, further improving display performance.

This pretilt angle θ. For example, the magnetic field capacity zero position method (J
, Appl, Phys, 48.1783 (1977
) ) can be measured.

The means for obtaining an electrode substrate having an alignment layer is not particularly limited, and various conventionally known means can be employed. Specifically, the following means are preferred.

(1) For example, a coating of a polymeric material such as imide, amide, polyvinyl alcohol, or phenoxy, which has a nonpolar long chain in the polymer main chain or side chain, is formed on the surface of the electrode substrate, and the surface of this coating is Means for orientation treatment by rubbing with a woven fabric, flocked cloth, cotton-like cloth, etc. made of cotton, vinylon, Tetoron, nylon, rayon, carbon fiber, etc.

(2) For example, a film of an alignment treatment agent made by mixing an appropriate amount of a vertical alignment treatment agent such as a chromium carboxylic acid complex or an organic silane compound with a horizontal alignment treatment agent such as an imide type, amide type, or polyvinyl alcohol type is applied to the surface of the electrode substrate. A means for forming a film and orienting the surface of this film by a rubbing method.

(3) The surface of the electrode substrate is LB (Langmuir-
A means for performing alignment treatment by forming a film (Blodgett).

(4) A method for aligning liquid crystal molecules by forming grating-like grooves in a certain direction on the surface of an electrode substrate by means such as photolithography or anisotropic etching.

(5) Means for orienting the surface by depositing a vapor deposition substance such as 3i0, MgO1MgF2, etc. on the surface of the electrode substrate from an oblique angle.

The nematic liquid crystal is not particularly limited, and various materials can be used. Typical examples are shown below, but the invention is not limited thereto. Further, these substances may be used in combination.

(1) A cyclohexylcarboxylic acid ester compound represented by the following structural formula (where X is R (an alkyl group having 1 to 18 carbon atoms,
The same applies below), OR, CN.

(2) Piphenyl compound represented by the following structural formula (X is R, OR.

represents. ) (3) A phenylcyclohexane compound represented by the following structural formula (Xlt, R, 0RSCN.

(4) A pyrimidine compound represented by the following structural formula (Xli, R5CN.

Y represents R, OR, CN. ) (5) Azo-azoxy system shown by the following structural formula (however, X represents -N=N-1-N=N-1↓ -N=N-) ↓ (6) The following structural formula A benzoic acid ester compound represented by (X is RSRO.

Y is R,0RSCN.

(7) Tolan compounds represented by the following structural formula (however,
X and Y are F and R, respectively.

OR.

represents. ) (8) Ethane-based compound represented by the following structural formula (however,
X and Y are R and OR, respectively.

Z represents H, FSCl, Br or R. )In addition,
The nematic liquid crystal used in the present invention may contain smectic liquid crystal, cholesteric liquid crystal, etc., if necessary.

The optically active substance added to the nematic liquid crystal is generally called a chiral nematic liquid crystal, for example, an ester type, biphenyl type, phenylclohexane type, or azo type having an optically active group represented by the following general formula as a terminal group. A nematic liquid crystal can be used.

R2R2 R, -C"-C, R2,, -1R, -C"-C,, H,
, -0-I R,R.

(However, R4, Rz, and Ra are each an alkyl group or a hydrogen atom, and R,, R2, and R are different from each other.)
Specifically, for example, a compound represented by the structural formula shown below can be used.

-C,H,.

Further, as the optically active substance contained in the nematic liquid crystal composition, chiral smectic liquid crystal, cholesteric liquid crystal, etc. may be used.

It is preferable that such an optically rotating substance is one that can sufficiently shorten the spontaneous twist pitch per unit amount added to the nematic liquid crystal, and by selecting such a substance, the optically rotating substance can be The required amount of the nematic substance to be added can be kept small, preferably 1.5% by weight or less, and as a result, changes in the physical properties of the nematic liquid crystal and the temperature dependence of the spontaneous twist pitch can be made small. Further, in the present invention, in order to further reduce the temperature dependence of the spontaneous twist pitch, a plurality of types of optically active substances whose temperature change coefficients of the spontaneous twist pitch have opposite signs may be used in combination.

〔Example〕

Examples of the present invention will be specifically described below.

FIG. 4 is a schematic diagram of a liquid crystal display device according to the present invention.

In this example, an upper electrode substrate 11 and a lower electrode substrate 12 are arranged facing each other with a gap in between, and the upper electrode substrate 11 has an electrode layer 11B and an alignment layer 11C provided on the inner surface of the support plate IIA. The lower electrode substrate 12 has an electrode layer 12B and an alignment layer 12. on the inner surface of the support plate 12A. It is configured by providing C.

A space between the upper electrode substrate 11 and the lower electrode substrate 12 is sealed by a seal portion 71, and a liquid crystal cell 30 is configured. Inside the liquid crystal cell 30, a plurality of spacers 72 are arranged in a spaced manner, and nematic liquid crystals doped with an optically active substance are arranged in a twisted structure, thereby forming the liquid crystal layer 20. There is.

A front polarizing plate 41 and a rear polarizing plate 42 are provided on the outer surfaces of the upper electrode substrate 11 and the lower electrode substrate 12, respectively.

Both electrode substrates 11 and 12 have translucency.

That is, this example is a transmissive type liquid crystal display device.

Note that in a reflective type liquid crystal display device, a reflective plate may be provided on the outer surface of the rear polarizing plate 42.

Materials constituting the support plates 11A and 12A include glasses such as soda glass, borosilicate glass, and quartz glass; plastic sheets made of uniaxially oriented polyethylene terephthalate, polyether sulfone, polyvinyl alcohol, etc.; aluminum, stainless steel, etc. A metal sheet; etc. can be used.

Electrode layers 11B and 12B are connected to support plates 11A and 12
It is constituted by a transparent electrode made of, for example, an ITO film placed on the surface of A. The transparent electrodes constituting one electrode layer 11B and the transparent electrodes constituting the other electrode layer 12B are arranged at right angles to each other, thereby forming an electrode structure of a matrix type display.

Note that the upper electrode substrate 11 and the lower electrode substrate 12 may further be provided with an insulating layer, a dielectric layer, an alkali ion movement prevention layer, an antireflection layer, a polarizing layer, a reflective layer, etc., if necessary.

<Examples and Comparative Examples> In each of the Examples and Comparative Examples, an STN liquid crystal display device operated in normally closed display mode with the configuration shown in FIG. 4 under the conditions shown in Table 1 below. was created.

Note that the front polarizing plate 41 and the rear polarizing plate 42 are rN
PF-G1220DU” (manufactured by Nitto Denko@). Further, the spacer 72 is made of glass fiber, and the seal portion 71 is made of "Structbond XN-5A".
-Cl (manufactured by Mitsui Toatsu Chemicals).

(Evaluation) Each liquid crystal display device was subjected to an actual display test using multiplex drive with a 1/100 density ratio, and the contrast was examined.

The above results are shown in Table 1 below.

As can be understood from the results in Table 1, the liquid crystal display device according to the present invention can provide high contrast with little leakage light.

On the other hand, in Comparative Example 1, the deviation angle β and T do not satisfy condition (c), which is an essential condition of the present invention.
Contrast is poor.

〔Effect of the invention〕

As explained in detail above, according to the present invention, the condition (a
) to (c), so in an STN liquid crystal display device that adopts a normally closed display mode,
Higher contrast than before can be obtained.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an outline of a liquid crystal display device according to the present invention, and FIGS. 2 and 3 are diagrams showing twist angle α, deviation angle β, and γ.
FIG. 4 is an explanatory cross-sectional view schematically showing the liquid crystal display device according to the present invention, and FIGS. 5 to 7 are graphs showing experimental results to support condition (c). It is. 11.12... Electrode substrate 11A, 12Δ...
・Supporting plates 11B, 12B...electrode layer 11C,i
2C...Alignment layer 20...Liquid crystal layer 30.
...Liquid crystal cell 41.42...Polarizing plate 5],,
52... Orientation layer 71... Seal l 72
... Subesa drilling 1m 1A Pre-4 diagram Polarization axis δ curve 0.6 0.2 +, 0 ,, Self-procedural amendment (
Voluntary) October 18, 1988 Director General of the Japan Patent Office Yoshi 1) Moon Takeshi 1, Indication of the case Patent Application No. 1983-225537 2, Title of the invention Liquid crystal display device 3, Person making the amendment Relationship with the case Patent application Address 1-26-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Name (127) Konica Co., Ltd. 4, Agent 5, Subject of amendment 6, Line 2 of page 9 of the detailed statement of amendment as follows: correct.

Claims (1)

[Claims] (1) A liquid crystal cell comprising a liquid crystal layer in which a nematic liquid crystal added with an optically active substance is arranged in a twisted structure between a pair of electrode substrates each having an alignment layer arranged facing each other, and the liquid crystal A liquid crystal display device comprising a pair of polarizing plates disposed on both sides of a cell, and satisfying the following conditions (a), (b) and (c). Condition (a): The twist angle α of the twist structure in the liquid crystal layer is 220 to 300°. Condition (b): Size Δ of refractive index anisotropy of nematic liquid crystal
The product Δn・d of n and the thickness d (μm) of the liquid crystal layer is 0.6
It should be ~1.1 μm. Condition (c): The angles (shift angles) β and γ between the polarizing axes of a pair of polarizing plates and the orientation direction of liquid crystal molecules in contact with the surface of the electrode substrate adjacent to each of the polarizing plates are expressed by the following formula [
1] and [2] must be satisfied. Formula [1]; [(270°-α)/2]-20°≦β≦[(270°
−α)/2]+20° Formula [2]; β+γ=90°