JP2000314866A - Stn type liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an STN type liquid crystal display device having a double layer cell with improved viewing angle characteristics. SOLUTION: The STN type liquid crystal display device is provided with a first liquid crystal cell having a driving electrode and a second liquid crystal cell for compensating optical retardation of the first liquid crystal cell located so as to be superimposed on the first liquid crystal cell in the same optical path as the first liquid crystal cell. In the second liquid crystal cell, liquid crystal molecules existing at the boundary of two substrates holding the liquid crystal are given a pretilt of which the tilt direction is set so as to make the liquid crystal molecules 4 twist and at the same time be subjected to a splay deformation and besides the tilt direction of the liquid crystal molecules 4 existing at the boundary and the direction of rise of the liquid crystal molecules 4 existing at the central part of the liquid crystal layer of the first liquid crystal cell during driving are arranged to be in parallel with each other.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly to a two-layer STN type liquid crystal display device in which an STN type optical compensation liquid crystal cell and a driving liquid crystal cell (display cell) are arranged in an overlapping manner.

[0002]

2. Description of the Related Art STN-type LCDs (Super-Twis)
ted Nematic Liquid Crysta
l Display) was published in 1984 by T.W. J. Sch
Proposed by effer et al. A two-layer STN-LCD has been proposed as a method of realizing clear black and white display by preventing unnecessary coloring in the STN LCD.

The structure of a two-layer STN-LCD will be described with reference to FIG. FIG. 5 schematically illustrates a cross section of the two-layer STN-LCD when no voltage is applied. The upper STN type liquid crystal cell (hereinafter simply referred to as a cell) 1 is a compensation cell, and the lower cell 2 is a drive cell (display cell). In the compensation cell 1 and the driving cell 2, liquid crystal molecules 4 are arranged between the transparent glass substrates 3 facing each other. As shown in FIG. 5, the liquid crystal molecules 4 have a long axis direction. The twist (twist) direction of the liquid crystal molecules 4 between the upper and lower substrates 3 is defined by a chiral agent added to the liquid crystal, and is reversed in the upper and lower cells. All twist angles are 160-2
It is about 70 degrees. Only the driving cell 2 has a transparent electrode (not shown), and controls the light transmittance by changing the arrangement of the liquid crystal molecules 4 by applying a voltage to the electrode. The upper and lower cells 1 and 2 are sandwiched between two polarizing plates 5a and 5b in a direction of a polarization axis orthogonal to each other.

The compensation cell 1 and the driving cell 2 have the same optical characteristics except for the twist direction of liquid crystal molecules. That is, the upper and lower cells have the same twist angle, pretilt angle, liquid crystal material, liquid crystal refractive index (Δn), cell thickness (d), and the like. In addition, the liquid crystal molecules 4a and 4b facing each other in both cells have their orientation directions orthogonal to each other. Therefore,
Since both cells compensate each other for the optical properties (in-plane anisotropy) of the other cell, when combined with the polarizers 5a and 5b having the orthogonal Nicol arrangement, a high contrast in which optical compensation is completely achieved is achieved. Display of normally black can be realized.

FIG. 6 shows a two-layer STN having a twist angle of 180 degrees.
FIG. 1 is a cross-sectional view of a structure of a liquid crystal display (showing liquid crystal alignment when no voltage is applied). Those having the same reference numbers as in FIG. 5 indicate the same elements. In order to prevent reverse tilt failure at the time of applying a voltage, a pretilt angle is previously given to the liquid crystal molecules in the driving cell 2 so that all the liquid crystal molecules at the center of the cell rise from the same direction. When no voltage is applied, the liquid crystal molecules have a substantially uniform pretilt angle with respect to the entire thickness direction.
In this example, the liquid crystal molecules at the interface are given a pretilt angle by performing a rubbing process so that θm = 2 degrees.

FIG. 7 shows the relationship between the rubbing direction of the rubbing roller and the direction of the pretilt angle θp given thereby. The compensation cell 1 is given a pretilt angle of θp so as to optically compensate the driving cell 2.

FIG. 8 is a plan view showing the relationship between the rubbing directions of the upper and lower substrates of each cell and the polarization axis of the polarizing plate. The case of a 180-degree twist (twist in the direction of liquid crystal molecules between the upper and lower substrates) is shown. FIG. 8A shows a compensation cell 1 of the two-layer cell of FIG.
8 (b) shows the rubbing direction of the upper and lower substrates of the driving cell 2. FIG. In order to realize a display with high transmittance and high contrast, the polarizing plate 5a,
The angle between the polarization axis 5b and the alignment direction of the liquid crystal molecules at the cell substrate interface is usually 45 degrees as shown on the left side of FIG.

When a voltage is applied to the two-layer cell of FIG.
Since the liquid crystal molecules of the drive cell 2 are aligned and rise from the direction of the pretilt angle, the retardation of the drive cell is reduced and the compensation by the compensation cell 1 is broken, so that the two-layer cell is in a light transmitting state (white display).

FIG. 9 shows an example of a characteristic of a change in light transmittance with respect to a change in an applied voltage in the driving cell 2. When the two-layer cell is multiplex driven, the OFF voltage V shown in FIG.
A black-and-white display is obtained by applying the off and ON voltage Von.
The ON voltage Voff and the ON voltage Von are determined by a driving condition such as a duty number, a bias ratio, and a driving voltage value.
The ratio between the ON voltage and the OFF voltage (Von / Voff) decreases as the duty number increases.

[0010]

The two-layer cell shown in FIG.
FIG. 1 shows the viewing angle characteristic (equal contrast curve) when multiplex driving with 1/9 bias is performed at 64 duty.
0 is shown. In the same figure, Φ is the viewing angle of the display screen from the observer
(Azimuth angle), Θ is a polar angle with respect to the substrate surface normal, and C
R indicates a contrast ratio. In this cell, the rising direction of the liquid crystal molecules at the center of the driving cell corresponds to the direction of Φ = 0 degrees. It can be seen that the viewing angle characteristics in the direction passing through Φ = 0 degrees and Φ = 180 degrees, that is, the direction parallel to the alignment direction of the liquid crystal molecules at the center of the driving cell are not good. Especially Φ = 180
In the degree direction, there is a black-and-white inversion area (shaded area in FIG. 10) where the value of the contrast ratio is 1 or less, which makes it hard to see. Considering the cause, it is as follows.

Since the transmittance in the ON state needs to be kept high to some extent, the ON voltage cannot be reduced much. Accordingly, the OFF voltage is also set to a position very close to the voltage at which the transmittance starts to change. When the duty ratio is large, it is necessary to set the OFF voltage to a higher voltage. At such an OFF voltage, the liquid crystal molecules near the center of the driving cell rise considerably as shown in FIG.

FIG. 12 shows the characteristics of the change in the tilt angle of the liquid crystal molecules at the center with respect to the change in the voltage applied to the driving cell. From the characteristics of FIG. 12, it can be seen that the inclination of the liquid crystal molecules in the center of the cell has already started to change at a voltage lower than the voltage at which the transmittance in the voltage-transmittance characteristics of FIG.

The viewing angle characteristic of the two-layer cell in the direction connecting Φ = 0 ° and Φ = 180 ° is determined by the liquid crystal molecules oriented in that direction, that is, the liquid crystal molecules near the center of the driving cell. Liquid crystal molecules near the cell interface. In particular, since the liquid crystal molecules near the center of the driving cell rise with a certain inclination even at the OFF voltage as described above, the change in retardation when the cell is viewed from obliquely above is large. Specifically, when the display surface is viewed from the direction A in FIG. 11, the retardation of the drive cell is larger than when viewed from the front, and is smaller when viewed from the direction B. Therefore, the compensation condition between the compensation cell and the compensating cell greatly shifts depending on the viewing direction, and the viewing angle characteristics in this direction become poor.

If the retardation of the compensating cell also changes in the viewing angle direction in the same manner as the driving cell, the viewing angle characteristics do not deteriorate as described above. However, as shown in FIG. Since the tilt directions of the liquid crystal molecules are opposite to each other via the 180 ° twist, the retardation change with respect to the viewing angle change can be self-compensated in the compensation cell, and the retardation change of the driving cell cannot be compensated.

An object of the present invention is to provide a two-layer cell type STN type liquid crystal display device having improved viewing angle characteristics.

[0016]

According to the present invention, there is provided an STN type liquid crystal display device comprising: a first liquid crystal cell having a driving electrode; and the first liquid crystal cell in the same optical path as the first liquid crystal cell. And a second liquid crystal cell for compensating the optical phase difference of the first liquid crystal cell, wherein the second liquid crystal cell applies pretilt to liquid crystal molecules at an interface between two substrates sandwiching the liquid crystal. The tilt direction is set so that the liquid crystal molecules are twisted between the substrates and simultaneously undergoes splay deformation, and the tilt direction of the liquid crystal molecules at the interface and the first tilt direction are set.
The liquid crystal molecules of the liquid crystal cell were arranged so that the rising direction of the liquid crystal molecules at the center of the liquid crystal layer when driving was aligned.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The STN type liquid crystal display device of the present invention devises the arrangement of liquid crystal molecules in a compensation cell, and changes the retardation of the compensation cell, which has been self-compensated conventionally, due to the viewing angle by the change of the driving cell. , The viewing angle characteristic in the direction where the viewing angle characteristic was poor in the past is improved.

According to one embodiment of the present invention, as shown in the cross-sectional view of the two-layer liquid crystal cell in FIG. 1, the pretilt of the liquid crystal molecules in the compensation cell 1 is different from the conventional one. That is, the tilt direction of the liquid crystal molecules at one of the two interfaces of the compensation cell 1 is set to the opposite direction to the conventional one in FIG. 6, and the pretilt at the interface between the upper and lower substrates is directed substantially in the same direction. The liquid crystal molecules in the cell undergo a splay deformation while being twisted. In the case of a 180 ° twist, since the pretilt angles of the upper and lower substrates are substantially parallel, optical anisotropy occurs in this direction. By aligning the tilt direction at the compensation cell interface and the rising direction of the liquid crystal molecules at the center of the driving cell, the liquid crystal molecules at both interfaces in the compensation cell do not self-compensate for the retardation change due to the change in viewing angle in the compensation cell. The compensating cell and the driving cell cooperate with each other, so that the liquid crystal molecules of the compensating cell 1 compensate for the viewing angle dependence of the retardation with respect to the liquid crystal molecules near the cell center of the driving cell 2, thereby improving the viewing angle characteristics. I do. In FIG. 1, the illustration of the transparent electrode and the alignment film is omitted.

FIG. 2 shows the relationship between the rubbing directions of the upper and lower substrates of each cell of the 180-degree twisted two-layer cell of FIG. 1 and the polarization axis of the polarizing plate. FIG. 2A shows the rubbing directions of the upper and lower substrates of the compensation cell 1 of the two-layer cell of FIG. The rubbing directions of the upper and lower substrates of the driving cell 2 are shown in FIG. The angle between the polarization axes of the polarizing plates 5a and 5b and the orientation direction of the liquid crystal molecules at the cell substrate interface is 45 degrees as shown in FIG.

FIG. 3 shows the results of measuring the viewing angle characteristics of the two-layer cell (pretilt angle is 2 degrees) having the structure shown in FIG. 1 under the same conditions as those of the conventional cell shown in FIG. It can be seen that the viewing angle characteristic in the direction connecting Φ = 0 degrees and Φ = 180 degrees, which has conventionally been a problem, has been improved. Considering that the liquid crystal molecules at the center of the drive cell rise considerably at the OFF voltage under this condition, the effect should be more obtained if the pretilt angle of the compensation cell 1 is larger than the pretilt angle of the drive cell.

Based on such a concept, the compensating cell 1 was further manufactured with the pretilt angles of 6 degrees and 14 degrees, and the viewing angle characteristics were measured under the same driving conditions as those of FIG. 1 in combination with the driving cell. The results are shown in FIGS. From this result, the pretilt angle of the compensation cell 1 is 2
Compared with the case of the degree, the improvement of the viewing angle characteristic, which is the effect of the present invention, appears more remarkably. For example, in a cell having a pretilt angle of 14 degrees, as shown in FIG. 4B, a black-and-white inversion area having a contrast ratio of 1 or less existing in the direction of Φ = 180 degrees in the conventional cell of FIG. Disappears, indicating that the viewing angle characteristics in a very wide angle range have been improved.

[0022]

Next, the conditions of the two-layer cell prepared and used for the measurement experiment of the two-layer (D-) STN type liquid crystal display device described above will be described. For comparison, conditions of a two-layer cell having a conventional structure are also shown.

(1) Cell of Example (Pretilt Angle 2
Degree) Drive cell substrate: Glass substrate with transparent electrode Alignment film: Nissan Chemical Industries SE-150 (pretilt angle 2 degrees) Rubbing: Using rayon rubbing cloth Orientation direction: Same as FIGS. 1 and 2 (180 degree twist) Cell Thickness d: 6.6 μm Liquid crystal: 0.55 wt% of Merck S-811 added as a chiral material to Roriku RDP-00333 (left twist)

Compensation cell substrate: glass substrate (without transparent electrode) Alignment film: Nissan Chemical Industries SE-150 (pretilt angle: 2 degrees) Rubbing: using rayon rubbing cloth Orientation direction: same as FIGS. 1 and 2 (180 degrees) Twist) Cell thickness d: 6.6 μm Liquid crystal: 0.55 wt% of Merck ZLI-3786 added as chiral material to Roriku RDP-00333 (right twist)

Polarizing plate Polarizing plate: G-1220 manufactured by Nitto Denko Polarizing plate arrangement angle: As in FIG.

(2) Example cell (pretilt angle 6
Degree) Drive cell substrate: Glass substrate with transparent electrode Alignment film: Nissan Chemical Industries SE-150 (pretilt angle 2 degrees) Rubbing: Using rayon rubbing cloth Orientation direction: Same as FIGS. 1 and 2 (180 degree twist) Cell Thickness d: 6.6 μm Liquid crystal: 0.55 wt% of Merck S-811 added as a chiral material to Roriku RDP-00333 (left twist)

Compensation cell substrate: glass substrate (without transparent electrode) Alignment film: PIA-2101 manufactured by Chisso (pretilt angle: 6)
Rubbing: Use of cotton rubbing cloth Orientation direction: Same as in FIGS. 1 and 2 (180 ° twist) Cell thickness d: 6.6 μm Liquid crystal: 0.5 g of Merck ZLI-3786 as a chiral material for Roriku RDP-00333. 55wt% addition (right twist)

Polarizing plate Polarizing plate: G-1220 manufactured by Nitto Denko Polarizing plate arrangement angle: As in FIG.

(3) The cell of the embodiment (pretilt angle 14
Degree) Driving cell substrate: Glass substrate with transparent electrode Alignment film: SE-150 manufactured by Nissan Chemical Industries (pretilt angle 2 degrees) Rubbing: Use of rubbing cloth made of rayon Alignment direction: Same as FIGS. 1 and 2 (180 degree twist) Cell Thickness d: 6.6 μm Liquid crystal: 0.55 wt% of Merck S-811 added as a chiral material to Roriku RDP-00333 (left twist)

Compensation cell substrate: glass substrate (without transparent electrode) Alignment film: LQ-1800 manufactured by Hitachi Chemical Co., Ltd. (pretilt angle: 14 degrees) Rubbing: use of nylon rubbing cloth Alignment direction: same as FIGS. 1 and 2 (180 degrees) Twist) Cell thickness d: 6.6 μm Liquid crystal: 0.55 wt% of Merck ZLI-3786 added as chiral material to Roriku RDP-00333 (right twist)

Polarizing plate Polarizing plate: G-1220 manufactured by Nitto Denko Polarizing plate arrangement angle: As in FIG.

(4) Conventional two-layer cell driving cell substrate: glass substrate with transparent electrode Alignment film: SE-150 (pretilt angle 2 degrees) manufactured by Nissan Chemical Industries Rubbing: using rubbing cloth made of rayon Alignment direction: FIG. 6 and FIG. Same as 8 (180 ° twist) Cell thickness d: 6.6 μm Liquid crystal: 0.55 wt% of Merck S-811 as a chiral material added to Roriku RDP-00333 (left twist)

Compensation cell substrate: glass substrate (without transparent electrode) Alignment film: SE-150 manufactured by Nissan Chemical Industries (pretilt angle: 2 degrees) Rubbing: use of rubbing cloth manufactured by rayon Alignment direction: same as FIGS. 6 and 8 (180 degrees) Twist) Cell thickness d: 6.6 μm Liquid crystal: 0.55 wt% of Merck ZLI-3786 added as chiral material to Roriku RDP-00333 (right twist)

Polarizing plate Polarizing plate: G-1220 manufactured by Nitto Denko Polarizing plate arrangement angle: As in FIG.

In the embodiment described above, the twist angle is 1
Although it was 80 degrees, the two-layer STN liquid crystal display device
In general, the present invention can be similarly applied to all twist angles that can be adopted by a two-layer STN liquid crystal display device, for example, about 150 to 270 degrees. In the case other than the 180-degree twist, the liquid crystal molecules at the interface of the compensation cell and the liquid crystal molecules at the center of the driving cell do not become parallel and have a certain angle. The tilt direction of the liquid crystal molecules is reversed from that of the conventional one, so that the liquid crystal molecules in the cell are arranged in such a way that splay deformation occurs, and the tilt direction at the interface of the compensation cell and the rising direction of the liquid crystal molecules at the center of the driving cell are aligned ( The angle between each direction is 90
Degrees or less. Thus, the same effect as in the above-described embodiment can be obtained.

The invention has been described with reference to the embodiments.
The present invention is not limited to these. For example, it will be apparent to those skilled in the art that various modifications, improvements, and combinations are possible.

[0037]

As described above, the two-layer ST of the present invention is used.
In the N-type liquid crystal display device, the orientation direction of the liquid crystal molecules at one interface of the compensation cell is reversed from that of the conventional one, and the pretilt direction at the interface of the compensation cell and the rising direction of the liquid crystal molecules at the center of the driving cell. , Viewing angle characteristics can be improved.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a two-layer cell STN type liquid crystal display according to an embodiment of the present invention.

FIG. 2 is a plan view showing a rubbing direction of a substrate and a polarization axis direction of a polarizing plate of the two-layer cell STN type liquid crystal display device of FIG.

FIG. 3 is a viewing angle characteristic diagram of a two-layer cell STN type liquid crystal display device according to an example of the present invention.

FIG. 4 is a viewing angle characteristic diagram of a two-layer cell STN type liquid crystal display device according to another embodiment of the present invention.

FIG. 5 is a cross-sectional view of a conventional two-layer cell STN liquid crystal display device.

FIG. 6 is a cross-sectional view of a conventional 180-degree twisted two-layer cell STN liquid crystal display device.

FIG. 7 is a diagram showing a relationship between a rubbing direction and a pretilt angle.

FIG. 8 is a plan view showing a rubbing direction of a substrate and a polarization axis direction of a polarizing plate of a conventional 180-degree twisted two-layer cell STN type liquid crystal display device.

FIG. 9 is a graph showing an applied voltage versus light transmittance characteristic of a driving cell.

FIG. 10 shows viewing angle characteristics of a conventional two-layer cell STN liquid crystal display device.

FIG. 11 shows an arrangement of liquid crystal molecules when an OFF voltage is applied to a driving cell.

FIG. 12 is a diagram showing a change in tilt angle with respect to a change in applied voltage of liquid crystal molecules at the center of a driving cell.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Compensation cell 2 Drive cell 3 Glass substrate 4 Liquid crystal molecule 5a, 5b Polarizing plate

[Procedure amendment]

[Submission date] May 26, 1999 (1999.5.2
6)

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] All figures

[Correction method] Change

[Correction contents]

FIG.

FIG. 2

FIG. 3

FIG. 5

FIG. 7

FIG. 4

FIG. 6

FIG. 8

FIG. 9

FIG. 10

FIG. 11

FIG.

Claims (2)

[Claims] 1. A first liquid crystal cell having a driving electrode, and an optical phase difference compensation of the first liquid crystal cell which is arranged in the same optical path as the first liquid crystal cell so as to overlap with the first liquid crystal cell. And a second liquid crystal cell having a second liquid crystal cell, wherein the second liquid crystal cell applies a pretilt to liquid crystal molecules at an interface between two substrates sandwiching liquid crystal, and adjusts the tilt direction between the substrates. The liquid crystal molecules are twisted at the same time and are set to undergo splay deformation, and the tilt direction of the liquid crystal molecules at the interface is aligned with the rising direction of the liquid crystal molecules at the center of the liquid crystal layer of the first liquid crystal cell when driving. STN-type liquid crystal display device arranged like this. 2. The STN-type liquid crystal display device according to claim 1, wherein a pretilt angle of liquid crystal molecules at an interface of the second liquid crystal cell is larger than a pretilt angle of liquid crystal molecules at an interface of the first liquid crystal cell.