JPH10133195A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a TN-LCD which is greatly improved in a display contrast and the visual field angle characteristic of gradation inversion. SOLUTION: The liquid crystal display device of a normally white mode consisting of a twisted nematic type liquid crystal cell of about 90 deg. in twist angle and two polarizing plates arranged on both sides of the cells is constituted by laminating two kinds of optical compensation layers varying in characteristics; the optical compensation layers (A) having the characters that refractive index anisotropy hardly exists within the film plane and that the refractive index in the film thickness direction is smaller than the refractive index within the plane and the optical compensation layers (B) having the characteristics that the layers have optically positive refractive index anisotropy and that the optical axis inclines by 20 to 70 deg. from the normal direction of the film by at least one layer each in order of the polarizing plate/the optical compensation layer (A)/the optical compensation layer (B)/the liquid crystal cell/the optical compensation layer (B)/the optical compensation layer (A)/the polarizing plate between the liquid crystal cell and the polarizing plates on both sides thereof.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a viewing angle characteristic, in particular,
The present invention relates to a liquid crystal display device with improved display contrast and viewing angle characteristics of gradation inversion.

[0002]

2. Description of the Related Art A liquid crystal display (hereinafter, referred to as LCD) most frequently used at present is a pair of linearly polarizing films arranged so that absorption axes are orthogonal to each other. A liquid crystal sandwiching a nematic liquid crystal having a helical axis in the normal direction of the glass substrate between a pair of glass substrates on which a transparent electrode is formed, and having a twisted angle of about 90 degrees. Twisted nematic LCD (hereinafter, referred to as TN-LCD) in a normally white (hereinafter, referred to as NW) mode with cells interposed therebetween
). In a NW mode TN-LCD, when no voltage is applied, the incident linearly polarized light is rotated by 90 degrees due to the optical rotation of the liquid crystal cell and is emitted, resulting in a white state. When a voltage is applied, the liquid crystal molecules are deposited on the glass substrate. On the other hand, the linearly polarized light which rises up, loses the optical rotation, and is incident is emitted while maintaining that state, so that it becomes a black state. Further, gradation display is performed by utilizing the white state, the black state, and the intermediate state. However, nematic liquid crystals used in LCDs have a rod-like molecular structure and exhibit a large positive refractive index anisotropy in the molecular axis direction. The change differs from the normal direction of the LCD due to the phase difference due to the refractive index anisotropy of the liquid crystal. For this reason, when the display is viewed from an angle deviating from the normal direction of the LCD, the viewing angle characteristic is such that phenomena such as a decrease in contrast and a grayscale inversion in which grayscale display is reversed occur.

Since this viewing angle characteristic is caused by the refractive index anisotropy of the liquid crystal molecules, the liquid crystal molecules have a refractive index anisotropy opposite to that of the liquid crystal for compensating for the phase difference due to the refractive index anisotropy of the liquid crystal molecules. Improvement using the retardation film shown is being studied. A great effect can be obtained by improving the viewing angle characteristic mainly by improving the viewing angle characteristic in black display, that is, in a voltage applied state.
Since the liquid crystal molecules are oriented nearly perpendicular to the glass substrate when voltage is applied, this state is regarded as a positive refractive index anisotropic substance having an optical axis in the normal direction of the glass substrate, and this is compensated for. JP-A-2-015239 and JP-A-3-103823 disclose methods for using a retardation film having an optical axis in the film normal direction and having a negative refractive index anisotropy as the retardation film. ing. However, in an actual LCD, even when a voltage is applied, the liquid crystal molecules near the glass substrate remain inclined at a portion near the glass substrate due to the binding force of the alignment film of the substrate. With a retardation film having an optical axis and negative refractive index anisotropy, it is difficult to sufficiently compensate for the polarization state provided by the liquid crystal cell.
In order to compensate also for such tilted liquid crystal molecules, a method using a retardation film having an optical axis of the retardation film in a direction inclined from the film normal direction and having a negative refractive index anisotropy is also available. It is described in JP-A-63-239421 and JP-A-6-214116. However, in these methods, although the compensation state for the liquid crystal cell in the voltage applied state is improved, the liquid crystal cell in the inclined state increases due to the lower applied voltage, and in the halftone display state in which the liquid crystal cell takes a more complicated alignment form. Since the compensation is incomplete, the viewing angle of the display contrast is improved, but it is difficult to improve the gradation inversion phenomenon. As a result, only incomplete viewing angle characteristics can be obtained. As another method, while having the same positive refractive index anisotropy as the liquid crystal, it is possible to improve the viewing angle characteristics even by using a retardation plate in which the optical axis is inclined from the film normal direction,
JP-A-5-080323, JP-A-7-30640
No. 6 and WO96 / 10773. In these reports, the use of a retardation plate having a positive refractive index anisotropy and having an optical axis inclined from the normal direction of the film is effective in suppressing the grayscale inversion of the liquid crystal cell. However, there is no specific reference to what form the effect is exhibited when used in any form.

As described above, an optical compensator used for improving the viewing angle characteristics of the TN-LCD has been found to be capable of dramatically improving the total viewing angle characteristics including not only the display contrast but also the gradation inversion. Was not in a situation. In view of this situation, the present inventors have conducted intensive studies, and as a result, have found that an optical compensation having an optically positive refractive index anisotropy and a characteristic in which the optical axis is inclined at 20 to 70 ° from the film normal direction. The layer is combined with an optical compensation layer having a characteristic in which the refractive index anisotropy hardly exists in the film plane and the refractive index in the film thickness direction is smaller than the in-plane refractive index. The inventors have found that by using a compensation layer laminated on a polarizing plate and a liquid crystal cell in a specific order, it is possible to greatly improve the viewing angle characteristics of both display contrast and gradation inversion, and have completed the present invention.

[0005]

That is, the present invention provides a normally white mode liquid crystal display device comprising a twisted nematic liquid crystal cell having a twist angle of about 90 ° and two polarizing plates disposed on both sides of the cell. In a normally white mode liquid crystal display device comprising a twisted nematic liquid crystal cell having a twist angle of about 90 ° and two polarizing plates disposed on both sides of the cell, the refractive index anisotropy is in-plane. An optical compensation layer (A) having almost no refractive index and having a characteristic that the refractive index in the film thickness direction is smaller than the in-plane refractive index, has optically positive refractive index anisotropy, and has an optical axis from the film normal direction. An optical compensation layer (B) having a characteristic inclined by 20 to 70 °, and at least one optical compensation layer having different characteristics between the liquid crystal cell and the polarizing plates on both sides thereof. , A polarizing plate / optical compensation layer (A) / optical compensation layer (B) / liquid crystal cell /
The present invention relates to a liquid crystal display device which is laminated in the order of optical compensation layer (B) / optical compensation layer (A) / polarizing plate.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The optical compensation layer (A) used in the present invention, which has almost no in-plane refractive index anisotropy and a refractive index in the film thickness direction smaller than the in-plane refractive index. It is not particularly limited as long as it satisfies the required optical characteristics.For example, the retardation in the film plane is small due to the orientation of the polymer at the time of film formation by the solvent casting method, and in the film thickness direction. cast film or, in the transparent polymer biaxially oriented film of transparent polymer having a refractive index (n _t) is made to have a refractive index structure of less than an average refractive index in the film plane (n _p), JP-a-5
For example, a retardation plate using an inorganic layered compound described in JP-A-196819, a compensator using an alternating multilayer thin film of a substance having a different refractive index described in JP-A-5-249457, or the like can be used.

[0007] The optical characteristics of the optical compensation layer (A), which has almost no refractive index anisotropy in the plane of the film and has a characteristic that the refractive index in the film thickness direction is smaller than the in-plane refractive index, are determined by the in-plane letter. Deshon value (RA), and the thickness direction retardation value _{(RA '= (n p -n} t) × d, d: film thickness) is defined by. These values are appropriately selected depending on the optical characteristics of the liquid crystal cell to be used and the viewing angle characteristics finally required, but usually RA = 0 to 100 n.
m, RA '= 50-250 nm.

For use in the present invention, the film has an optically positive refractive index anisotropy and has an optical axis of 20 to 70 from the normal direction of the film.
The optical compensation layer (B) having a tilted characteristic is not particularly limited as long as it satisfies necessary optical characteristics. For example, JP-A-4-120512 and JP-A-6-222213 Patent Document 1: Japanese Patent Application Laid-Open No. 7-152035 discloses an inclined sheet in which a thermoplastic resin is oriented by applying an electric or magnetic field or rolling at different peripheral speeds.
Japanese Patent Application Laid-Open No. 7-191216, Japanese Patent Application Laid-Open No. Hei 7-191216, and an inclined alignment layer of a liquid crystalline polymer by fixing an alignment under application of an electric field or a magnetic field or under alignment control by an alignment film. An optically anisotropic layer formed by oblique vapor deposition of an inorganic dielectric or the like on a transparent substrate described in Japanese Patent Application Laid-Open Publication No. H11-157, can be used.

The optical characteristics of the optical compensation layer (B) having an optically positive refractive index anisotropy and having a characteristic in which the optical axis is inclined by 20 to 70 ° from the normal direction of the film, It is defined by the tilt angle (θ) from the normal direction of the liquid crystal cell substrate and the retardation value (RB) measured from the front of the optical compensation layer. These values are appropriately selected depending on the optical characteristics of the liquid crystal cell used and the viewing angle characteristics finally required, but usually θ = 20 to 70 °,
It is used in the range of RB = 20 to 200 nm, preferably θ = 40 to 65 ° and RB = 30 to 100 nm.

The two types of optical compensation layers having different characteristics are provided between the liquid crystal cell and the polarizing plates on both sides of the liquid crystal cell. The polarizing plate / optical compensation layer (A) / optical compensation layer (B) / liquid crystal cell / optical compensation layer ( B) / optical compensation layer (A) / polarizing plate. Each of the optical compensation layers (A) and (B) may be one of the above-described optical compensation layers, or a plurality of optical compensation layers may be used in each layer if necessary.

In the laminated structure of the optical compensation layer of the present invention, the rubbing directions of the substrates on the light incident side and the light emitting side of the liquid crystal cell, and the optically positive refractive index anisotropy disposed on each side. And the optical axis is from 20 to 20 from the film normal direction.
The relationship with the direction of the optical axis of the optical compensation layer (B) having the characteristic inclined at 70 ° greatly affects the effect of improving the viewing angle characteristic. The direction of the optical axis is appropriately selected depending on the optical characteristics of the liquid crystal cell to be used and the viewing angle characteristics finally required. The direction of the optical axis of the optical compensation layer (B) is viewed from the light emission side of the liquid crystal cell. In this case, the direction toward the upper side is positive with respect to the rubbing direction of the liquid crystal cell substrate on the light entrance side and the light exit side of the liquid crystal cell, which are closer to each other, as viewed from the light exit side of the liquid crystal cell. Approximately +180
When set to have a relationship of °, the viewing angle improving effect is most pronounced.

The direction of the absorption axis of the polarizing plate on both sides of the cell is determined by the rubbing of the liquid crystal cell substrate on the side closer to each of the light incident side and the light exit side, depending on the finally required optical characteristics of the LCD. The direction is selected from either substantially perpendicular (E mode) or substantially parallel (O mode).

The retardation value (RA, RA ', RB) and the direction of inclination of the optical axis of the optical compensation layer in the present invention can be obtained by those skilled in the art by measurement based on ordinary methods.

[0014]

According to the present invention, a TN-LCD having significantly improved display contrast and viewing angle characteristics of gradation inversion is provided.
Can be obtained.

[0015]

EXAMPLES The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. Example 1 Based on the method described in Japanese Patent Application Laid-Open No. 5-196819, an optical element having little refractive index anisotropy in the film plane and having a refractive index in the film thickness direction smaller than the in-plane refractive index. A compensation layer (A1) was produced. WO 96/107
No. 73, the optical axis has a positive refractive index anisotropy and the optical axis is 45 ° from the normal direction of the film.
An optical compensation layer (B1) having inclined characteristics was produced. TFT-TN LCD TV (CR
YSTALTRON, 4E-L1), between the liquid crystal cell and the polarizers on both sides of the liquid crystal cell, the above two types of optical compensation layers are provided. The optical compensation layer (B1) / the optical compensation layer (A1) / the polarizing plate were laminated in this order to produce a TN-LCD. At this time, the direction in which the optical axis of the optical compensation layer (B1) is upward when viewed from the light exit side of the liquid crystal cell is closer to the liquid incident side and the light exit side of the liquid crystal cell. It was set so as to have a relationship of + 180 ° with the rubbing direction. In this TN-LCD, the display contrast and the viewing angle characteristics of gradation inversion were both good in all directions, up, down, left, and right, and were far superior to those of the conventional TN-LCD.

Example 2 Based on the method described in WO 96/10773, an optical element having optically positive refractive index anisotropy and an optical axis inclined 65 ° from the normal direction of the film. A compensation layer (B1) was produced. A TN-LCD in which two types of optical compensation layers were laminated was produced in the same manner as in Example 1 except that the above-mentioned optical compensation layer (B2) was used as the optical compensation layer (B). In this TN-LCD, the display contrast and the viewing angle characteristics of gradation inversion were all good in all directions, up, down, left, and right, and were far superior to those of the conventional TN-LCD.

Comparative Example 1 The lamination order of the optical compensation layer was determined by changing the order of the polarizing plate / optical compensation layer (B1).
/ Optical compensation layer (A1) / liquid crystal cell / optical compensation layer (A1)
TN-L in which two types of optical compensation layers were laminated in the same manner as in Example 1 except that the order of / optical compensation layer (B1) / polarizing plate was changed.
A CD was made. In this TN-LCD, the viewing angle of the display contrast in the left and right and downward directions was good, but the upward direction was poor, and the viewing angle characteristics of the grayscale inversion were not very good in all directions.

Claims (2)

[Claims] 1. A normally white mode liquid crystal display device comprising a twisted nematic liquid crystal cell having a twist angle of about 90 ° and two polarizing plates disposed on both sides of the cell. An optical compensation layer (A) having little anisotropy and a characteristic in which the refractive index in the thickness direction of the film is smaller than the in-plane refractive index. An optical compensation layer (B) having a characteristic inclined by 20 to 70 degrees from the linear direction has at least one optical compensation layer having different characteristics between the liquid crystal cell and the polarizing plates on both sides thereof. Polarizing plate / optical compensation layer (A) / optical compensation layer (B) / liquid crystal cell /
A liquid crystal display device comprising: an optical compensation layer (B) / an optical compensation layer (A) / a polarizing plate, which are stacked in this order. 2. Among the two kinds of laminated optical compensation layers, the two optical compensation layers have characteristics of having optically positive refractive index anisotropy and having an optical axis inclined by 20 to 70 ° from the normal direction of the film. The upward direction of the optical axis of the optical compensation layer (B) when viewed from the light exit side of the liquid crystal cell is the rubbing direction of the liquid crystal cell substrate closer to the light incident side and the light exit side of the liquid crystal cell. 2. The liquid crystal display device according to claim 1, wherein the liquid crystal display device is set to have a relation of approximately +180 degrees.