JPH04211223A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To provide an electric field effective type liquid crystal display element having an excellent time division driving characteristic. CONSTITUTION:Nematic liquid crystal, which has positive dielectric anisotropy and to which a light emissive material is added, is sandwiched between a pair of upper and lower electrode substrates arranged so as to oppose the rubbing orientation treating surfaces to each other, and forms a spiral structure twisted by an angle within range of 160 degrees to 200 degrees in its thickness direction, and further, polarization axes or absorption axes 8 and 9 of a pair of polarizing plates arranged so as to sandwich this spiral structure are dislocated by an angle within range of 30 degrees to 60 degrees against the liquid crystal molecule array direction on adjacent electrode substrates.

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 field effect liquid crystal display device having excellent time-division drive characteristics.

0002

[Prior Art] A conventional liquid crystal display element called a twisted nematic type has a 90° twisted helical structure made of nectic liquid crystal having positive dielectric anisotropy between two electrode substrates. A polarizing plate was arranged outside the electrode substrate so that its polarization axis (or absorption axis) was perpendicular or parallel to the liquid crystal molecules adjacent to the electrode substrate (Japanese Patent Publication No. 13666/1982).

[0003] In order to align the liquid crystal molecules between two electrode substrates so as to form a spiral structure twisted by 90°, for example, a method of rubbing the surface of the electrode substrate in contact with the liquid crystal in one direction with a cloth, etc. It is done by rubbing method. The rubbing direction at this time, that is, the rubbing direction, becomes 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 manner, and the polarizing axes or absorption axes thereof are 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 is a relative value of the reflected brightness with respect to the applied voltage, and the initial value of the brightness is 100% and the final value (value when the applied voltage is sufficiently large) is 0%. Generally, the voltage at which the relative brightness is 90% is the threshold V
The voltage at which th, 10% is taken as the saturation value Vsat is used as a guideline for the characteristics of the liquid crystal. However, in practice, when the relative brightness is 90% or more, the pixel is sufficiently bright and the liquid crystal is not lit, and when it is 50% or less, the pixel is sufficiently dark and the liquid crystal is lit. %, the voltage at 50% is the threshold voltage Vth, and the saturation voltage Vs, respectively.
Let it be at.

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

[0006] Here, the definition of the viewing angle φ will be explained with reference to FIG. In the figure, an upper electrode substrate 1 of a liquid crystal display element 1
The rubbing direction of 1 is 2, the rubbing direction of the lower electrode substrate 12 is 3, and the twist angle of the liquid crystal molecules is 4. Further, orthogonal coordinates XY axes are set on the surface of the liquid crystal display element 1, the direction of the X axis is defined in a direction that bisects the twist angle 4 of the liquid crystal molecules, the Z axis is defined in the direction normal to the XY plane, and the direction of observation 5 is set. Let the angle formed by this with the Z axis be the viewing angle φ. In this case, for the sake of simplicity, the observation direction 5 is assumed to be within the XZ plane. Further, when φ shown in FIG. 7 is positive, contrast becomes high when viewed from such a direction, and thus such a direction is called a viewing direction.

In FIG. 6, the brightness at angle φ=10 degrees is 9
The voltage that becomes 0% is Vth1, and the voltage that becomes 50% is Vsa.
When t1 is the voltage at which the brightness at angle φ = 40 degrees is 90% and Vth2, the rise characteristic γ and the angle characteristic △φ
and time resolution m are defined as follows.

[0008] The time division drive characteristics of the conventional liquid crystal display element are as follows:
If the refractive index anisotropy of the liquid crystal is △n and the gap between the upper and lower electrode substrates is d, it depends on △n・d, and when △n・d is large (for example, 0.8 μm or more), γ is good. (small),
△φ is bad (small). On the other hand, if △n・d is small (
For example, γ is bad (large) for 0.8 μm or less, and △φ
is good (large). However, when compared in terms of time resolution m, the smaller Δn·d is, the better. Specific examples of the above are shown in Table 1.

[0009]

[Table 1]

[0010] Time-division driving will now be briefly explained using a dot matrix display as an example. As shown in FIG. 8, striped Y electrodes (
A signal electrode) 13 and an X electrode (scanning electrode) 14 are formed on the upper electrode substrate 11, 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 X1, X2,...Xn, X1,
2, . . . Xn are repeated to perform time-division driving. When a certain scanning electrode (X3 in the figure) is selected, the signal electrode 13 is applied to all pixels on that electrode.
Selected or unselected display signals are simultaneously added from Y1, Y2, . . . Ym 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.

[0011]

Problems to be Solved by the Invention In conventional liquid crystal display elements, only the time-division drive characteristics as 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.

An object of the present invention is to provide a liquid crystal display element which has extremely excellent time-division drive characteristics and has 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. Our goal is to provide the following.

[0013]

[Means for Solving the Problems] In order to achieve the above object, a liquid crystal display element according to the present invention uses a nematic liquid crystal having positive dielectric anisotropy and added with an optically active substance, which is subjected to a rubbing alignment treatment. The electrode substrates are sandwiched between a pair of upper and lower electrode substrates disposed with their surfaces facing each other to form a twisted helical structure within a range of 160 degrees to 200 degrees in the thickness direction, and the helical structure is sandwiched between them. The polarizing axis or the absorption axis of the pair of polarizing plates is shifted by an angle within the range of 30 degrees to 60 degrees from the alignment direction of the liquid crystal molecules of the adjacent electrode substrate.

[0014]

[Operation] In the liquid crystal display element of the present invention, the twist angle of the helical structure of the liquid crystal molecules is in the range of 160 degrees to 200 degrees, and the absorption axes (or By arranging the polarization axis (polarization axis) at a certain angle with respect to the alignment direction of the adjacent liquid crystal molecules, the applied voltage-light transmittance characteristic curve becomes steeper.
Time resolution is greatly improved.

[0015]

Embodiments Next, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows the alignment direction (for example, rubbing direction) of liquid crystal molecules, the twist direction of liquid crystal molecules, and the absorption axis (or polarization axis) direction of a polarizing plate when the liquid crystal display element according to the present invention is viewed from above. ing. FIG. 2 is a perspective view showing the relationship between them. Parts having the same structure and function as those in FIG. 7 are given the same reference numerals.

The twist direction 10 and twist angle α of the liquid crystal molecules 17 are determined by the rubbing direction 6 of the upper electrode substrate 11, the rubbing direction 7 of the lower electrode substrate 12, and the type and amount of the optically active substance added to the nematic liquid crystal. be done.

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 maximum value is limited to 20
0 degrees is the upper limit, and the lower limit is limited by contrast, and 160 degrees is the limit.

The absorption axis (or polarization axis) 8 of the upper polarizing plate 15 and the absorption axis (or polarization axis) of the lower polarizing plate 16
9 is preferably in the range of 0 degrees to 30 degrees in consideration of contrast, brightness, color, etc. Also, the angle β1 between the absorption axis (or polarization axis) 8 of the upper polarizing plate 15 and the rubbing direction 6 of the upper electrode substrate 11, the absorption axis (or polarization axis) 9 of the lower polarizing plate 16, and the lower electrode Board 1
In consideration of contrast, brightness, color, etc., it is preferable that the angle β2 between the rubbing direction 7 and the rubbing direction 7 is set in the range of 30 degrees to 60 degrees.

Furthermore, the liquid crystal display element according to the present invention exhibits remarkable △n·d dependence, and in terms of contrast, brightness, and color, if it satisfies the condition of 0.8 μm≦△n·d≦1.2 μm. Shows particularly good results. Here, the value of Δn generally has wavelength dependence, and tends to be large on the short wavelength side and small on the long wavelength side. △n used in this specification
The value of is measured at 25°C using He-Ne laser light (wavelength 6328 Å), so the value of Δn・d in this specification will change slightly if measured at other wavelengths. .

[0021] 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. 3 shows the relationship between the structure, that is, the rubbing direction of the electrode substrate, the twist direction and angle of the helical structure of the liquid crystal molecules, and the polarization axis (or absorption axis) of the polarizing plate, and shows the relationship between the rubbing direction of the electrode substrate, the twist direction and angle of the helical structure of the liquid crystal molecule, and the polarization axis (or absorption axis) of the polarizing plate. It is a diagram.

The liquid crystal used was a nematic liquid crystal whose main components were biphenyl liquid crystal and ester cyclohexane (ECH) liquid crystal, and Merck's S811 was used as the optically active substance.
0.5% by weight was added. This mixed liquid crystal △
n is 0.123.

In FIG. 3, the rubbing directions 6 and 7 of the upper and lower electrode substrates are parallel to each other, and the optically active substance S8
11, the twist direction is 10, and the twist angle α is 180.
degree.

The absorption axis 8 of the upper polarizing plate and the absorption axis 9 of the lower polarizing plate are parallel to each other (β3=0 degree), and the angles β1 and β2 formed with the rubbing directions 6 and 7 are both 45 degrees. It is.

Note that the rubbing direction 6 of the upper electrode substrate 11
The relationship between the rubbing direction 7 of the lower electrode substrate 12 and the helical structure of the liquid crystal molecules 17 will be explained using the example of FIG. 3 as an example. When the electrode substrate is rubbed, the result is shown in Figure 4.
As shown in , there are two different inclination directions along the rubbing direction.
A nearly periodic repetition of two small slopes is formed. Therefore, in order for the liquid crystal molecules forming a helical structure to be arranged almost parallel between the electrode substrates, it is necessary to make the rubbing directions of the upper and lower substrates almost coincident as shown in FIG.
To improve display image quality.

Next, liquid crystal cells with the above-described cell structure and the thickness d of the liquid crystal layer were changed to change Δn·d were made, and the color and brightness were observed. The results are shown in Table 2.

From this result, it was found that when Δn·d was around 1 μm, both the brightness and color were at a level that would pose no problem as a display element.

From a more detailed study of Δn·d, it was found that when the relationship shown in FIG. 3 exists, there is no practical problem when Δn·d is in the range of 0.7 μm to 1.2 μm.

[0030]

[Table 2]

Next, Table 3 shows the results of measuring the time division drive characteristics of a liquid crystal cell with Δn·d=0.98 μm. It can be seen that γ, Δφ, and m are all significantly improved compared to the conventional liquid crystal display element shown in Table 1.

[0032]

[Table 3]

Although the absorption axis was used as the axis of the polarizing plate in FIG. 3, 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. In the above example, the twisting direction of the helical structure was explained as being counterclockwise, but it goes without saying that the same effect can be obtained even when the twisting direction is clockwise as shown in FIG.

It goes without saying that the type of optically active substance is not limited as long as the relationship between the rubbing direction and the twisting direction is maintained as shown in FIGS. 1, 3 and 4.

[0035]

As described 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]

FIG. 1 is an explanatory diagram showing the relationship among the alignment direction of liquid crystal molecules, the twist direction of the liquid crystal, and the axis direction of the polarizing plate in a liquid crystal display element according to the present invention.

FIG. 2 is an exploded perspective view of essential parts showing the relationship among the alignment direction of liquid crystal molecules, the twist direction of the liquid crystal, and the axis direction of the polarizing plate of the liquid crystal display element according to the present invention.

FIG. 3 is an explanatory diagram showing the relationship among the alignment direction of liquid crystal molecules, the twist direction of the liquid crystal, and the axis direction of the polarizing plate in the first embodiment of the liquid crystal display element according to the present invention.

4 is a cross-sectional view showing the relationship between the alignment direction of liquid crystal molecules, the twisting direction of liquid crystal molecules, and the rubbing direction in the first embodiment shown in FIG. 3. FIG.

FIG. 5 is an explanatory diagram showing the relationship among the alignment direction of liquid crystal molecules, the twist direction of the liquid crystal, and the axis direction of the polarizing plate in a second embodiment of the liquid crystal display element according to the present invention.

FIG. 6 is an explanatory diagram showing voltage-luminance characteristics of a liquid crystal display element used for defining time-division drive characteristics.

FIG. 7 is an explanatory diagram defining measurement directions of time-division drive characteristics.

FIG. 8 is a diagram illustrating the arrangement of an electrode structure for time-division driving.

[Explanation of symbols]

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 liquid crystal molecules, 8... - Absorption axis or polarization axis direction of the upper polarizing plate, 9... Absorption axis or polarization axis direction of the lower polarizing plate, 17... Liquid crystal molecules,
α...Twisting angle of liquid crystal molecules.

Claims (3)

[Claims] [Claim 1] 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 with their rubbed alignment treated surfaces facing each other. The polarization axis or absorption axis of a pair of polarizing plates, which form a twisted helical structure within the range of 160 degrees to 200 degrees in the thickness direction, and are provided with this helical structure in between, is aligned with the liquid crystal molecules of the adjacent electrode substrates. A liquid crystal display element characterized by being shifted by an angle within a range of 30 degrees to 60 degrees with respect to an arrangement direction. 2. The liquid crystal display element according to claim 1, wherein an included angle between the polarization axes or absorption axes of the pair of polarizing plates is within a range of 0 degrees to 30 degrees. 3. The product Δn·d of the thickness d (μm) of the liquid crystal and the refractive index anisotropy Δn of the liquid crystal is from 0.7 μm to 1.2 μm.
2. The liquid crystal display element according to claim 1, wherein the liquid crystal display element is in the range of m.