JP2523982B2 - Liquid crystal display element - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device used for an OA display such as a word processor and a laptop personal computer, and more specifically, a super twisted nematic (hereinafter referred to as STN) type liquid crystal display device. Regarding

2. Description of the Related Art Liquid crystal display devices are characterized by low voltage and low power consumption, and are used for measuring instrument operation panels, wrist watches, television receivers,
It has been adopted in many fields such as computer terminals. The driving method of the liquid crystal display element is roughly classified into an active matrix method and a simple matrix method.

The former is a thin film transistor (TET) for each pixel.
It has a configuration with a switching element such as metal or metal insulator metal (MIM), and it has a problem that the configuration is complicated, the manufacturing cost is high, and it is difficult to create a large-area panel, but it has excellent display quality. Is characterized by.

On the other hand, the latter simple matrix method is not as good as the active matrix method in terms of image quality, but has the characteristics that a large-area panel is easy to manufacture and the manufacturing cost is low. At present, the active matrix method is used for display for television receivers that require high image quality, and the simple matrix method is used for OA displays that require low cost and large screen.

Against this background, the demand for high image quality in the simple matrix system is extremely strong, and the number of scanning lines is gradually increasing. However, as the number of scanning lines (N) increases, the ratio of the on-voltage to the off-voltage α
However, there is a problem in that the value becomes close to 1 and sufficient contrast cannot be obtained. On the other hand, efforts are being actively made to obtain high contrast by further increasing the twist angle of liquid crystal molecules in the panel and making the rising edge of the characteristic in the voltage-transmittance curve steeper. This is Super Twisted by Refringence Effect (SBE) mode, or STN
It is what is called a mode.

It is generally known that the steepness of the voltage-transmittance curve is affected by the elastic constant of the liquid crystal material, the dielectric constant, the anisotropy of the dielectric constant, the binding energy of the liquid crystal-alignment film interface, and the like. And in order to greatly twist the alignment of liquid crystal molecules,
It is necessary to increase the tilt angle of liquid crystal molecules on the surface of the alignment film (hereinafter referred to as tilt angle). However, increasing the tilt angle of the liquid crystal increases the instability of the liquid crystal alignment in terms of energy.

Normally, the liquid crystal display element is optimally designed at room temperature, but the operating temperature range of the liquid crystal display element is usually 0 ° C to 40 ° C.
Alternatively, the temperature is 0 ° C. to 60 ° C., and when the backlight is used, the temperature of the liquid crystal display element is higher than the ambient temperature by nearly 20 ° C., and it is often the case that the liquid crystal display element is used under conditions deviating from the optimum conditions. In particular, in the STN type liquid crystal display element, the elastic energy of the liquid crystal material, the binding energy of the liquid crystal-alignment film interface,
External energy, which is the applied voltage, and energy balance such as energy loss due to driving are important,
If it deviates significantly from the optimum design condition, alignment failure of liquid crystal molecules occurs.

In addition, it is difficult to completely suppress variations in the characteristics of the liquid crystal display element, such as variations in the distance between the glass plates that sandwich the liquid crystal (hereinafter referred to as the panel gap) and variations in the surface state of the rubbing-treated alignment film, during the manufacturing process. It is necessary to consider a certain margin in the design value of the liquid crystal display element.

Usually, this margin is defined by the allowable range of the d / p value which is the ratio of the panel gap (d) and the chiral pitch (P) of the liquid crystal material, and the lower limit thereof does not cause an under twist (low-order twist). It is specified by the minimum d / p value, and the upper limit is the maximum d / p without stripe domain.
Specified by value.

However, a liquid crystal display device having a sufficiently wide margin over a wide temperature range has not been proposed yet.

Problems to be Solved by the Invention The margin is determined by the chilled angle, the liquid crystal regulating force of the alignment film, the twist angle, the anisotropy of the dielectric constant, the elastic constant, etc., and most of them largely change depending on the panel temperature. That is, the margin changes with temperature. Therefore, even if the panel is designed at room temperature, the margin changes in the high temperature region, and it is not uncommon for the defective alignment to occur.

Various efforts have been made from the standpoint of molecular design of liquid crystal materials by optimizing elastic constants, dielectric constants, binding energies with alignment films, etc. of liquid crystal materials for the purpose of widening the margin. In addition, in the efforts from the alignment film material, studies have been positively made to widen the margin by increasing the tilt angle, but all of them are efforts to widen the margin. The width of the margin is generally in a trade-off relationship with the steepness of the electro-optical characteristics of the liquid crystal display element, and cannot be unduly increased. There is a limit to such efforts to expand the margin, and rather there is an urgent need to develop a material system that has a moderate margin and has little temperature dependence. However, until now, almost no efforts have been made to improve the temperature dependence of the margin.

The present invention solves the above conventional problems, and an object thereof is to provide a liquid crystal display element having a sufficiently wide margin over a wide temperature range by optimizing a liquid crystal material.

Means for Solving the Problems In order to achieve this object, a liquid crystal display device of the present invention,
In order to match the temperature dependence of the chiral pitch with the temperature dependence of the physical properties of the liquid crystal material such as elastic constant and dielectric constant, the ratio of the chiral pitch P60 at 60 ° C to the chiral pitch P25 at 25 ° C P60 / It has a configuration using liquid crystal containing a liquid crystal mixture having a value of P25 of 1.05 or more and 1.15 or less.

Operation With this configuration, the designed panel gap can always be located near the center of the margin area even if the temperature changes.

 The essential points of the present invention will be described with reference to FIG.

The alignment margin is determined by the tilt angle of the alignment film, surface free energy, twist angle of liquid crystal molecules, dielectric anisotropy, elastic constant and the like. That is, as shown in FIG. 3 (a), if the alignment film material of the liquid crystal material to be used and the surface treatment conditions (film forming conditions, rubbing conditions) are determined, the temperature dependence of the margin expressed in d / p is unique. It is thought to be decided. In the figure, two straight lines represent the upper limit and the lower limit of the margin area, respectively. On the other hand, various chiral materials are added to the actual liquid crystal material, and the temperature dependence of the margin in terms of the margin of the panel gap greatly changes due to the temperature dependence of the chiral pitch. Fig. 3 (b),
(C) is the calculation of the temperature dependence of the margin when assuming that the temperature dependence of the chiral pitch is + 3% and 0% with respect to the temperature change of 10 degrees. Where 25 ° C
The liquid crystal material has a chiral pitch of 13 μm.

As shown in FIG. 3 (b), it can be seen that it is better from the viewpoint of the margin to make the temperature dependence of the chiral pitch larger to some extent than to compensate.

In the present invention, in the STN type liquid crystal display device, the temperature dependence of the chiral pitch is increased to + 5% / 35 degrees to + 15% / 35 degrees, which can significantly suppress the temperature dependence of the alignment margin. it can. That is, according to the present invention, the designed d / p value is always in the center of the alignment margin even when the panel temperature changes, and it is possible to largely eliminate the alignment defect caused by the temperature change. Moreover, the margin of the gap can be significantly increased.

Embodiment One embodiment of the present invention will be described below with reference to the drawings.

In this example, the temperature dependence of the margin was evaluated under a microscope using a wedge cell while controlling the temperature on a hot stage (for example, hot stage FP-82 manufactured by METTLER CORPORATION). At this time, the concentration of each chiral material to be mixed was adjusted so that the chiral pitch p of the liquid crystal material enclosing the wedge-shaped cell was about 13 μm. The lower limit of the margin is defined as the minimum d / p value at which no under-twist (low-order twist) is observed when no voltage is applied. On the other hand, the upper limit of the margin was defined as the maximum d / p value at which no stripe domain was observed when a voltage (60 Hz, rectangular wave) near the threshold voltage was applied to each cell.

(Example 1) A glass substrate having a transparent electrode (for example, an ITO electrode) is coated with a polyimide coating (for example, a polyimide coating RN-721 (4.0 wt.% / NMP solution) manufactured by Nissan Chemical Industries, Ltd.) by a spin coating method. It was applied and cured under the curing conditions of 200 ° C. and 1 hour.
The dry film thickness was 1000Å. After that, labyrinth was performed so that the twist angle of the liquid crystal molecules in the cell was 240 degrees, and then these substrates were bonded to each other by using two types of glass fibers of 4 μm and 8 μm to form a wedge cell.

Next, four types of A, B, C, and D (for example, chiral materials CNL-611, CNL-632, CNL-646, and CNL-649 manufactured by Agus Argus Co., Ltd.) are selected as chiral materials, each of which is a liquid crystal material for STN. (For example, STN liquid crystal material ZLI-4335 manufactured by Merck) was mixed and adjusted so that the chiral pitch at 25 ° C. was 12.7 μm. Then, these liquid crystal materials were injected into each wedge cell to measure the temperature dependence of the margin.

Table 1 shows the temperature dependence of the chiral pitch of the STN liquid crystal material in which 1 wt.% Of each chiral material A, B, C, D was mixed. The measurement results of the temperature dependence of the margin are shown in FIGS. 1 (a) to 1 (d). The measurement was performed under two temperature conditions of 25 ° C, 40 ° C and 60 ° C. In the present embodiment shown in FIGS. 1 (a) to 1 (d), the size of the chiral pitch at 25 ° C. is fixed to 12.7 μm, so the width of the margin is directly measured as the allowable range of the liquid crystal layer thickness. It is possible to In FIG. 1, the range of the margin is shown by the length of a straight line. The lower limit of the margin area (on the lower twist side) was measured on a sample that was once cooled to a predetermined temperature after the liquid crystal layer was once brought into an isotropic phase state.

As shown in FIG. 1, (a), (b), (c),
The temperature dependence of the chiral pitch increases in the order of (d), but the center value of the panel gap does not change. Reducing the temperature dependence of the chiral pitch of the liquid crystal material used in this way is not preferable for improving the temperature dependence of the margin. From FIG. 1, it can be seen that the use of a liquid crystal material having a P60 / P25 value of 1.05 to 1.15 as the temperature dependence of the chiral pitch of the liquid crystal material is extremely effective in improving the temperature dependence of the margin. The value of P60 / P25 is 1.05
When it is smaller than or larger than 1.15, the margin at high temperature and the margin at room temperature are less overlapped with each other, and the panel gap margin is reduced.

(Example 2) A polyimide coating (for example, a polyimide coating PSI-2001 (4.0 wt.
%, NMP solution)) by spin coating method, and 200 ℃, 1
It was cured under the curing condition of time. The dry film thickness was 1000Å. After that, rubbing was performed so that the twist angle in the cell of the liquid crystal molecules was 240 degrees, and then these substrates were bonded using two types of glass fibers of 4 μm and 8 μm to form a wedge cell.

Next, a mixture of a chiral material E (for example, chiral material CNL-625 manufactured by ADEKA ARGUS) and F (for example, chiral material S-811 manufactured by Merck) at a weight ratio of 1: 1 is a liquid crystal material for STN ( For example, it was mixed with STN liquid crystal material ZLI-2293) manufactured by Merck & Co., Inc. and adjusted so that the chiral pitch at 25 ° C. was 13.0 μm. Then, this liquid crystal material was injected into a wedge cell to measure the temperature dependence of the margin. The results are shown in FIG. The temperature dependence of the chiral pitch of this liquid crystal material was P60 / P25 = 1.08.

In the embodiment shown in FIG. 2, even if the cell temperature changes, the center of the margin has substantially the same panel gap value, and the temperature dependence of the margin can be greatly reduced.

As described above, the size and temperature dependence of the chiral pitch vary slightly depending on the type of liquid crystal material used, but it is possible to design the size and temperature dependence arbitrarily by combining various chiral materials. Needless to say.

Further, although the width of the margin largely changes depending on the type of the alignment film, the contribution of the alignment film to the temperature dependency is smaller than that of the liquid crystal material.

EFFECTS OF THE INVENTION As is clear from the above examples, the liquid crystal display element of the present invention is one in which the temperature dependence of the chiral pitch of the enclosed liquid crystal material is set to an appropriate size, and the temperature dependence of the margin is It can be greatly reduced and its practical value is extremely large.

[Brief description of drawings]

1 (a) to 1 (d) are diagrams showing the temperature dependence of the margin of the liquid crystal display element in one embodiment of the present invention, and FIG. 2 is the margin temperature of the liquid crystal display element in the other embodiment of the present invention. FIGS. 3 (a) to 3 (c), which show the dependence, are diagrams for explaining the main points of the liquid crystal display element of the present invention.

Claims (1)

(57) [Claims] 1. A liquid crystal display device using a liquid crystal containing a liquid crystal mixture having a ratio P60 / P25 of a chiral pitch P60 at 60 ° C. to a chiral pitch P25 at 25 ° C. of 1.05 or more and 1.15 or less.