JPS60162225A - Liquid-crystal display element - Google Patents

Abstract

PURPOSE:To attain high time-division driving by specifying the angle of spiral torsion of a liquid-crystal molecule in a range of 90 deg.-160 deg., and shifting the axes of absorption (axis of polarization) of a couple of polarizing plates which are provided in front of and behind it in the orientation direction of liquid-crystal molecules adjacent to an electrode substrate from an electrode substrate. CONSTITUTION:The liquid-crystal molecule is twisted by an angle alpha (90-160 deg.) counterclockwise from the rubbing direction 6 of an upper electrode substrate as a starting point to the rubbing direction 7 of a lower electrode substrate. The angle beta1 between the rubbing direction 6 and the axis 8 of absorption of an upper polarizing plate is equalized to that from the rubbing direction 6 to the twisting direction 10 of the liquid- crystal molecule, and the angle beta2 between the rubbing direction 7 and the axis 9 of absorption of a lower polarizing plate is defined similarly to beta1. The rubbing direction 10 of the liquid-crystal molecule and the angle alpha of torsion are specified according to the rubbing directions 6 and 7 and the kind and amount of rotary polarizing material added to nematic liquid crystal. The angles beta1 and beta2 are 15-65 deg.. Further, the product DELTAn.d of the thickness dmum of liquid crystal and refractive index anisotropy DELTAn is 0.8- 1.2mum. Consequently, high time-division driving characteristics and display characteristics of high quality are obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a liquid crystal display device, and particularly to a field effect liquid crystal display device having excellent time division drive characteristics.

[Background of the invention]

A conventional liquid crystal display element called the twisted nematic type has a 90P twisted helical structure made of nematic liquid crystal with positive dielectric anisotropy between two electrode substrates. A polarizing plate was placed on the outside so that its polarization axis (or absorption axis) was perpendicular or parallel to the liquid crystal molecules adjacent to the electrode substrate. 'In order to align the liquid crystal molecules between two electrode substrates so that they form a 90° twisted spiral structure, for example, a method of rubbing the surface of the electrode substrate in contact with the liquid crystal in one direction with a cloth, the so-called rubbing method, is used. done by. The rubbing direction at this time, that is, the 2-pin direction, is the alignment direction of the liquid crystal molecules. The two electrode substrates that have been oriented in this way are placed facing each other with a gap so that their rubbing directions cross each other at approximately 90 degrees, and the two electrode substrates are adhered with a sealant, and the gap between the two electrode substrates is When a nematic liquid crystal having positive dielectric anisotropy is sealed in the liquid crystal, the liquid crystal molecules are arranged in a spiral structure rotated approximately 90 degrees between the electrode substrates.

Polarizing plates are provided above and below the liquid crystal cell constructed in this way, and the polarization axis or absorption axis of the plates is made approximately parallel to the alignment direction of liquid crystal molecules adjacent to each electrode substrate. Here, the definition of the quantity representing the time-division drive characteristic, which is necessary for the following explanation, will be briefly explained.

FIG. 1 shows typical voltage-luminance characteristics of a conventional liquid crystal display element having a helical structure of liquid crystal molecules twisted by 90 degrees. This makes the reflection line (with respect to the applied voltage) 0%. In general, the voltage at which the relative brightness becomes a factor of 90 is the threshold value v
The voltage corresponding to th, io% is taken as the saturation value Vast and is used as a guideline for the characteristics of the liquid crystal.

However, in practice, if it is 90% or more, the pixel is sufficiently dim and the liquid crystal is not lit, and if it is less than 50%, the pixel is sufficiently dark.
The liquid crystal may be in a lit state, and hereinafter, in this specification,
The voltages at which the relative brightness becomes 90% and 50% are defined as a threshold voltage vth and a saturation voltage Vsat, respectively.

Furthermore, the electro-optical characteristics of the liquid crystal display element vary depending on the viewing direction), and this characteristic limits the field of view in which good display quality can be obtained.

Here, the definition of the viewing angle φ will be explained with reference to FIG. In the figure, the rubbing direction of the upper electrode substrate 11 of the liquid crystal display element 1 is 2, the 2-poling direction of the lower electrode substrate 12 is 3, and the twist angle of the liquid crystal molecules is 4. Also, liquid crystal display element 1
The orthogonal coordinates XY axes are set on the surface of Let the angle formed be the viewing angle φ. In this case, for simplicity, the observation direction 5 is
It is assumed that it is within the Z plane. Moreover, when φ shown in FIG. 2 is positive, contrast becomes high when viewed from such a direction, so such a direction is called a viewing direction.

In Fig. 1, the voltage at which the brightness at angle φ = 10 degrees becomes 90% is vthx, the voltage at which the brightness becomes 50% is Vsatl, and the voltage at which the brightness at angle φ = 40 degrees becomes 90% is vth,
When closing, the rising #) characteristic r, the angular characteristic Δφ, and the time division capability m are defined as shown in the following equation.

The time division drive characteristic of a conventional liquid crystal display element is expressed as Δn, where Δn is the refractive index anisotropy of the liquid crystal, and d is the gap between the upper and lower electrode substrates.
It depends on od, and if Δn@d is large (for example, 0
．． 8 μm or more), r is good (small), and mu φ is bad (
small). On the other hand, if Δn・d is small (for example, 0.8
μm or less), r is bad (large) and Δφ is good (large). However, when comparing by time resolution m, Δno
The smaller d is, the better. Specific examples of the above are shown in Table 1.

Table 1 Here, time-division driving will be briefly explained using a dot matrix display as an example. As shown in FIG. 3, a striped Y electrode (
The signal electrode) 13 is similarly placed on the upper electrode substrate 11, and the X electrode (
A scanning electrode) 14 is formed, and characters and the like are displayed by turning on or off the liquid crystal at the intersection of the X and Y electrodes. In the figure, n scanning electrodes are xl, x,...
・・X n , Xl, X snow・・・・・・Xn
Time-division driving is performed by repeating line sequential scanning. When a certain scanning electrode (Xs in the figure) is selected, Yl + which is the signal electrode 13 is applied to all pixels on that electrode.
Yl......Yn, select or non-select display signals are added simultaneously according to the signal to be displayed. In this way, lighting or non-lighting of the intersection is selected by the combination of voltage pulses applied to the scanning electrode and the signal electrode. The number of scanning electrodes X in this case corresponds to the number of time divisions.

In conventional liquid crystal display elements, only the time-division driving characteristics illustrated in Table 1 can be obtained, so that the number of time divisions of 32 or 64 is a practical limit.

However, in recent years, demands for improving the image quality of liquid crystal display elements and increasing the amount of displayed information have become stricter, and it has become impossible to satisfy the required specifications.

[Purpose of the invention]

An object of the present invention is to provide a liquid crystal display element that has extremely excellent time-division driving characteristics and good image quality even when the number of time divisions is 32 or more, by adopting a cell structure completely different from that of conventional liquid crystal display elements. There is a particular thing.

[Summary of the invention]

In order to achieve such an object, the liquid crystal display element according to the present invention has a twist angle of the helical structure of liquid crystal molecules in the range of 90 degrees to 160 degrees, and a pair of polarizing plates are provided before and after the helical structure of the liquid crystal molecules. , the absorption axes (or polarization axes) of these polarizing plates are arranged to be shifted by a certain angle with respect to the alignment direction of liquid crystal molecules adjacent to the electrode substrate.

[Embodiments of the invention]

Next, embodiments of the present invention will be described in detail using the drawings.

FIG. 4 shows the alignment direction of liquid crystal molecules (for example, rubbing direction), the twisting direction of liquid crystal molecules, and the absorption axis (polarizing axis) direction of the polarizing plate when the liquid crystal display element according to the present invention is viewed from above. There is. In this case, the liquid crystal molecules are twisted counterclockwise from the rubbing direction 6 of the upper electrode substrate as a starting point by an angle α in the rubbing direction 71 of the lower electrode substrate. Also,
The rubbing direction 6 of the upper electrode substrate and the absorption axis of the upper polarizing plate (
The angle βl with the polarization axis) 8 is the twisting direction 10 of the liquid crystal molecules with the rubbing direction 6 of the upper electrode substrate as the starting point.
(in this case, counterclockwise direction), and the angle β between the rubbing direction T of the lower electrode substrate and the absorption axis (or polarization axis) 9 of the lower polarizing plate is also defined in the same way as β1. do.

It goes without saying that the sum of βl or β3 plus an angle that is an integral multiple of 180 degrees is equivalent to β1 and β2. Hereinafter, the values of β1 and β will be represented by the minimum value of the equivalent angle group.

Furthermore, the twist direction 10 and twist angle α of the liquid crystal molecules are determined by the two-pin direction 6 of the upper electrode substrate, the rubbing direction 7 of the lower electrode substrate, and the type and amount of the optically active substance added to the netic liquid crystal. Ru.

The maximum value of the twist angle α is limited because the lighting state near the threshold value is an orientation that scatters light, and the upper limit is 160 degrees, and the lower limit is due to the remarkable viewing angle dependence of the color of the liquid crystal display element. The limit is 90 degrees.

The angle β between the absorption axis (or polarization axis) 8 of the upper polarizing plate and the rubbing direction 6 of the upper electrode substrate! And the angle β2 formed by the absorption axis (or polarization axis) 9 of the lower polarizing plate and the 2-pin direction 7 of the lower electrode substrate is in the range of 25 degrees to 65 degrees, respectively, considering contrast, brightness, color, etc. It is preferable to set

In addition, in FIG. 4, β1 and β-value are defined assuming that the twisting direction 10 of the liquid crystal molecules is counterclockwise, but even if the twisting direction 10 of the liquid crystal molecules is clockwise, β1 and β
Needless to say, if the atmosphere and direction are set in the clockwise direction, the result will be the same.

In addition, the liquid crystal display element according to the present invention exhibits a remarkable Δn·d dependence, and the contrast and brightness are 0.8μ from the point of view of one color.
When the condition of m≦ΔnIId≦1.2 μm is satisfied, good results are obtained for the ridges. Here, the value of Δn generally has a wavelength dependence of 1, which tends to be large on the short wavelength side and small on the long wavelength side. The value of Δn used in this specification is H
Since the measurements were made using eNe laser light (wavelength 6328X) at 25° C., the value of Δnod in this specification will change slightly if measurements are made at other wavelengths.

Here, the structure and measurement results of a specific example of the liquid crystal display element according to the present invention will be explained. FIG. 5 shows the structure, that is, the rubbing direction of the electrode substrate,
FIG. 2 is a diagram showing the relationship between the twist direction and angle of the helical structure of liquid crystal molecules and the polarization axis (or absorption axis) of a polarizing plate, as seen from above of a liquid crystal display element.

The liquid crystal used is a patented liquid crystal characterized by biphenyl-based liquid crystal and ester cyclohexane (ECH)-based liquid crystal, to which 0.5 weight percent of Merck's 8811 is added as an optically active substance. Δn of this mixed liquid crystal is 0.123
It is.

In FIG. 5, the angle formed by the rubbing directions 6 and 7 of the upper and lower electrode substrates is 140 degrees, and the optically active substance 58
11, the twist direction is 10. The twist angle α is 140 degrees. Also, the rubbing directions 6 and 7 and the absorption axis 8 of the polarizing plate
, 9 is the angle β1. β2 is 45 degrees in both cases.

By changing the thickness d of the liquid crystal layer with the above cell structure, Δn@
Liquid crystal cells with different values of d were produced and their colors and brightness were observed. The results are shown in Table 2.

From this result, it was found that when Δn@d is around 1.00 μm, that is, in the range from 0.90 μm to 1.10 μm, both the brightness and color are at extremely good levels with no problems as a display element. Furthermore, from a more detailed study of Δnod, if there is a relationship shown in Figure 5, then Δn@d
It was found that there is no practical problem in the range of 0.80 μm to 1.20 μm.

Next, Table 3 shows the results of measuring the time division drive characteristics of a liquid crystal cell with Δn@d=1.05 μm. It can be seen that both γ and Δφ9m are significantly improved compared to the conventional liquid crystal display element shown in Table 1.

Table 3 Although the absorption axis was used as the axis of the polarizing plate in FIG. 5, almost the same results were obtained even if the polarization axis was used.

Furthermore, although a biphenyl-based and ECH-based mixed liquid crystal was used in the embodiment, it goes without saying that similar effects can be obtained with other types of nematic liquid crystals having positive dielectric anisotropy. Further, in the above example, the twisting direction of the helical structure was explained as counterclockwise, but it goes without saying that the same effect can be obtained even when the twisting direction is clockwise as shown in FIG.

Also, for optically active substances, if the relationship between the rubbing direction and the twisting direction is maintained as shown in FIGS. 4, 5, and 6,
Needless to say, the type is not limited.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to obtain a liquid crystal display element having high time-division drive characteristics and high quality display characteristics, which were completely impossible in the past.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram showing the voltage-luminance characteristics of a liquid crystal display element used to define time division characteristics, Figure 2 is an explanatory diagram defining the measurement direction of time division drive characteristics, and Figure 3 is an explanatory diagram showing time division drive characteristics. The explanatory diagrams, FIG. 4, FIG. 5, and FIG. 6 are explanatory diagrams showing an example of the relationship among the alignment direction of liquid crystal molecules, the twist direction of the liquid crystal, and the direction of the axis of the polarizing plate in the liquid crystal display element according to the present invention. It is. 1. Liquid crystal display element, 2, 6 --- Rubbing direction of upper electrode substrate, 3, 7 --- Rubbing direction of lower electrode substrate, 4.10 --- Twisting direction of one liquid crystal molecule , 8・
...Absorption axis or polarization axis direction of the upper polarizing plate, 9...
・Absorption axis or polarization axis direction of the lower polarizing plate, 11...
... Upper electrode substrate, 12e... Lower electrode substrate. Figure 1 Seal l1 Roden Noj Mi (*, Instigation, 2) Figure 2 Figure 3 +4 Nono

Claims (1)

[Claims] A nematic liquid crystal having positive dielectric anisotropy and added with an optically active substance is sandwiched between a pair of upper and lower electrode substrates arranged opposite each other, and is twisted within a range of 90 degrees to 160 degrees in the thickness direction. The polarization axis or absorption axis of a pair of polarizing plates that form a helical structure and are provided with this helical structure in between,
Shifting the liquid crystal molecule alignment direction of the adjacent electrode substrate and the above twist direction of the liquid crystal molecules by a value within a range of 25 degrees to 65 degrees,
and the thickness d (μm) of the liquid crystal layer and the refractive index anisotropy △n of the liquid crystal
A liquid crystal display element characterized in that the product Δnod is in the range of 0.8 μm to 1.2 μm.