JPH0456823A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To improve contrast by arranging a double-refraction layer between a pair of polarizers holding a liquid crystal layer between them and setting up the retardation values of areas respectively corresponding to the picture element part and non-picture element part of a liquid crystal cell to respectively different values. CONSTITUTION:The double refraction layer 7 is formed at least on one surface of a pair of polarizers 2, 12 holding the liquid crystal layer 10 between them 2, 12 and the retardation values of the areas of the layer 7 respectively corresponding to the picture element part and non-picture element part of the liquid crystal cell 10 are set up to respectively different values. Especially, it is preferable to adjust the retardation values so that the b;rightness of non-selected picture elements or selected picture elements is equal to that of the non-picture element part at the time of time-dividedly driving the cell 10. Consequently, the contrast of the whole screen can be prevented from being dropped and the quality of display can be improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a liquid crystal display element, and particularly to an STN liquid crystal display device having a structure in which a liquid crystal having positive dielectric anisotropy is twisted by 90 degrees or more in the thickness direction of the liquid crystal layer. It relates to display elements.

[Prior art and problems to be solved by the invention] What are the display modes of liquid crystals that have been used in the past?

It is called a twisted nematic (TN) type, and has a structure in which liquid crystal molecules are twisted approximately 90 degrees between a pair of substrates, and utilizes the rotation of the plane of polarization by the liquid crystal and the disappearance of this effect by voltage application. This display mode was sufficient for low time division driving of watches, calculators, etc., but when high time division driving was used to increase the display capacity, it had the drawbacks of reduced contrast and narrow viewing angles. there were. This is because the ratio of the voltage applied to one selected point and non-selected point approaches 1 when high time-division driving is performed, and in order to obtain a display element with high contrast and a wide viewing angle, the relative transmittance of the element must be 10
% changing voltage■□. Voltage v1 that changes by 50% with respect to
It is necessary to make the steepness γ expressed by the ratio (vsa/v□) as low as possible.

In the case of twisted nematic type, this γ value is about 1.13. In order to reduce this γ value, a method has been proposed in which the twist angle of the liquid crystal molecules is increased and the polarization axis is shifted from the liquid crystal alignment direction, and this method is called SBE mode or STN mode. According to such a system, the γ value can be reduced to 1.1 or less, and high time-division driving of about 1/400 duty is possible.

However, this method uses coloring due to birefringence and its change due to voltage, so it is difficult in principle to display black and white, and the transmitted light or reflected light of the liquid crystal cell is colored. It will be displayed on the background.

A method of using a birefringent medium as a color compensating plate to eliminate coloration of STN type liquid crystal display elements is known (Japanese Patent Laid-Open No. 6
4-519). However, in an STN liquid crystal display element that uses a birefringent layer for color compensation, when the liquid crystal cell is time-divisionally driven, a voltage is also applied to non-selected pixels, so the brightness of the non-pixel area and the non-selected pixels or selected pixels is will be different, and the contrast of the entire screen will decrease.

An object of the present invention is to eliminate the above-mentioned drawbacks found in conventional liquid crystal display elements and to provide a liquid crystal display element having excellent display quality.

[Means and effects for solving the problems] In order to achieve the above object, according to the present invention, a liquid crystal layer made of a liquid crystal composition having positive dielectric anisotropy is formed between a pair of substrates provided with electrodes. The liquid crystal layer is sandwiched between a liquid crystal cell configured to be oriented substantially horizontally with respect to the substrate when no voltage is applied, and to have a twisted structure of 120° or more and 360° or less in the thickness direction of the liquid crystal layer. A birefringent layer is provided at least on one side between the pair of provided polarizers, and the retardation values in regions of the birefringent layer corresponding to pixel portions and non-pixel portions of the liquid crystal cell are different, and in particular, A liquid crystal display characterized in that the retardation of the birefringent layer is adjusted so that the brightness of a non-selected pixel or a selected pixel and the brightness of a non-pixel portion are approximately equal when the liquid crystal cell is driven in a time division manner. An element is provided.

Next, the configuration of the present invention will be explained in detail with reference to one drawing. 1st
The figure is a sectional view showing a configuration example of a liquid crystal display element of the present invention. In this figure, cell 10 is an STN cell, and substrate 1,
11 has alignment films 3 and 13 and transparent electrodes 4 and 14 which have been subjected to alignment treatment, respectively, and are arranged to be spaced apart and face each other, and a liquid crystal 6 is sealed between them, and is shielded from the outside air by a sealant 5. . This liquid crystal cell 10 is sandwiched between a first polarizer 2 and a second polarizer 12, and a birefringent layer 7 is disposed between the substrate 1 and the polarizer 2 to constitute a liquid crystal display element.

When a birefringent layer is provided between a substrate and a polarizer as in the configuration example of the liquid crystal display element of the present invention, a transparent glass, plastic, or the like is used as the substrate. When a plastic substrate is used, it is easy to reduce the thickness of the substrate to 0.2n+m or less, and therefore the display element can be made extremely thin and lightweight. Also, because the board is thin.

A display element with a wide viewing angle can be obtained without displaying double images.

The alignment treatment on each substrate of the liquid crystal display element of the present invention is performed so that the liquid crystal molecules are aligned substantially horizontally when no voltage is applied,
Liquid crystal molecules are preferentially aligned along this alignment treatment direction. In this case, "substantially horizontal" with respect to the orientation of the liquid crystal molecules means that the tilt angle of the liquid crystal molecules with respect to the substrate is in the range of approximately 0" to 30 degrees. This orientation control is performed by using the conventionally known tilt angle with respect to the substrate. This can be carried out by vertical evaporation, or by forming an inorganic or organic film and then rubbing it with a cotton cloth.The alignment films 3 and 13 used in the present invention may be formed by rubbing a polymer film such as polyamide or polyimide. A material formed by oblique vapor deposition using Sin, MgO, MgF2, etc. is used.

As shown in FIG. 2, the birefringent layer 7 of the present invention has a retardation value R1 of regions corresponding to pixel portions and non-pixel portions,
Liquid crystal cell 1 was manufactured with different R2 values, especially liquid crystal cell 1.
Preferably, the retardation is adjusted so that the brightness of a non-selected pixel or a selected pixel and a non-pixel portion when 0 is time-divisionally driven is equal. Such a birefringent layer 7 can be formed between substrates having the same electrode pattern as the display liquid crystal cell, for example, so that the twist direction is opposite to that of the display liquid crystal cell and the twist angle and retardation are equal to those of the display liquid crystal cell. It can be configured by applying a voltage to the liquid crystal cell that is equal to the voltage applied to non-selected pixels of the display liquid crystal cell. It is also possible to apply polymer liquid crystal to areas corresponding to pixel and non-pixel areas of a display liquid crystal cell with different retardations, or to partially change the thickness or refractive index anisotropy of the polymer film. This can also be achieved by changing to .

〔Example〕

Hereinafter, the present invention will be explained in more detail using Examples.
The present invention is not limited to these examples.

(Example 1) An STN cell A was manufactured in which the twist angle of the liquid crystal between the upper and lower glass substrates having striped transparent electrodes was 240°, and the retardation of the liquid crystal layer was 0.87 μs.

The liquid crystal is a nematic liquid crystal ZLI with positive dielectric anisotropy.
2293 (manufactured by Merck & Co.) with chiral nematic liquid crystal 5
811 (manufactured by Merck & Co.) was used. The alignment treatment was performed by rubbing the polyimide film.

Next, the twist angle of the liquid crystal between the upper and lower glass substrates having the same striped transparent electrodes as in cell A is 2406, the retardation of the liquid crystal layer is 0.877a+, and the liquid crystal is a nematic liquid crystal ZtJ2293 (similar to the above). Liquid crystal cell B was prepared using a liquid crystal cell B obtained by adding chiral nematic liquid crystal R811 (manufactured by Merck & Co., Ltd.) to liquid crystal (manufactured by Merck & Co., Ltd.).

The alignment treatment was performed by rubbing the polyimide film in the same manner as in Cell A. The chiral nematic liquid crystals of cell A and cell B have opposite twist directions, and the twist directions of the liquid crystal cells are also opposite to each other. Further, the rubbing direction of cell B was set to be perpendicular to the rubbing direction of the upper substrate of cell A when cell B was stacked on top of cell A so that the pixels of each cell were aligned.

Cells A and B are sandwiched so that the transmission axis of the lower polarizing plate makes an angle of 45° with the rubbing direction of the lower substrate of cell A, and the transmission axis of the upper polarizing plate is orthogonal to the transmission axis of the lower polarizing plate. Upper and lower polarizing plates were installed to obtain a liquid crystal display element of the present invention.

Cell A of the liquid crystal display element produced as described above is 1/
When time-division driving was performed with a duty of 200, and a voltage was applied to cell B so that the brightness of non-selected pixels was the darkest,
The retardation of the non-selected pixel in cell A and the retardation of the corresponding pixel in cell B matched, resulting in complete color compensation. In the non-pixel area, since the retardations of cell A and cell B were the same, color compensation was completely performed. Therefore, it was confirmed that the brightness of the non-selected pixels and the non-pixel area matched, and the contrast of the entire screen was improved compared to a normal color-compensated STN type liquid crystal display element.

In addition, if a liquid crystal display element is configured by installing the upper and lower polarizing plates so that their transmission axes are parallel to each other, a voltage should be applied to cell B so that the brightness of non-selected pixels is the brightest. As a result, the contrast was improved in the same way as above.

(Example 2) A print mask P for masking the pixel portion of the cell A of Example 1 and a print mask Q for masking the non-pixel portion were produced. A mask P is placed on a glass substrate, and a polysiloxane-based liquid crystalline polymer having a repeating unit represented by the following formula (1) and a polysiloxane-based polymer having an optically active group having a repeating unit represented by the following formula (II) are prepared. A mixed solution of liquid crystalline polymers was printed so that the retardation was 0°87. However, the direction of twist was opposite to that of cell A, and the twist angles were adjusted to be equal.

Next, I installed Mask Q, and the retardation was 0°83.
~0.85. The polysiloxane polymer liquid crystal was printed so that the twist direction was opposite to that of Cell A, and the twist angles were the same. However, the alignment direction of the polymer liquid crystal on the glass substrate is such that the polymer liquid crystal layer is superimposed on cell A so that the pixel part of cell A and the area of the polymer liquid crystal layer corresponding to the pixel part match. At the same time, the rubbing direction was perpendicular to the rubbing direction of the upper substrate of cell A.

Then, upper and lower polarizing plates were arranged in the same manner as in Example 1 to obtain a liquid crystal display element of the present invention.

The cell of the liquid crystal display element produced as above is 1/
When time-divisionally driven with a duty of 200, the brightness of non-selected pixels and non-pixel areas almost matched, confirming that the contrast of the entire screen was improved compared to a normal color-compensated STN liquid crystal display element. .

(Example 3) The twist angle of 24
0°, retardation 0.83-0.85. , a liquid crystal cell C in which a liquid crystal layer with a twist direction opposite to that of cell A is formed.
was created. The same liquid crystal as in Cell B of Example 1 was used, and the alignment treatment was also performed in the same manner as in Cell B. Further, the rubbing direction was also the same as that of cell B.

Then, install the upper and lower polarizers so that the transmission axis of the lower polarizer makes an angle of 45° with the rubbing direction of the lower substrate of cell A, and the transmission axis of the upper polarizer is orthogonal to the transmission axis of the lower polarizer. ,
A liquid crystal display element of the present invention was obtained.

Cell A of the liquid crystal display element produced as described above is 1/
When time-division driving was performed with a duty of 200 and a voltage was applied to cell B so that the brightness of the selected pixel was equal to the brightness of the non-pixel area, the non-selected pixel was black, the selected pixel was white, and the non-pixel area was also white. became. When a normal color-compensated STN type liquid crystal display element is driven in reverse, the non-pixel area becomes black and the brightness decreases, but in the liquid crystal display element of this example, the non-pixel area becomes white. , the overall brightness of the screen has been improved.

(Example 4) Similarly to Example 2, the retardation of the region corresponding to the pixel portion is 0.83 to 0.85 pm, and the retardation of the region corresponding to the non-pixel portion is 0.6 to 0.85 pm. A polysiloxane-based polymer liquid crystal was printed on a glass substrate so that the result was 7-. direction of twist, angle of twist.

The rubbing direction was the same as that for the polymer liquid crystal layer of Example 2.

Then, upper and lower polarizing plates were installed in the same manner as in Example 3 to obtain a liquid crystal display element of the present invention, and the cell A of this liquid crystal display element was 1/2
When activated in a time-division manner at a duty of 0.00, the same results as in Example 3 were obtained.

(Example 5) A homogeneous cell D was manufactured in which the upper and lower glass substrates having the same striped transparent electrodes as the cell A of Example 1 were homogeneously aligned. LCD is ZLI2
293 was used, and the retardation was set to 0°55 tones.

The alignment treatment was performed by rubbing the polyimide film. Cell D
The rubbing direction was set to be perpendicular to the rubbing direction of the upper substrate of cell A when cell D was stacked on top of cell A so that their pixels matched.

Then, upper and lower polarizing plates were installed in the same manner as in Example 1 to obtain a liquid crystal display element of the present invention, and the cell A of this liquid crystal display element was 1/2
When a voltage was applied to cell D so that the brightness of unselected pixels and the brightness of non-pixel areas were almost equal to each other by time-division driving with a duty of 0.00, the entire screen was brighter than that of a normal color-compensated STN liquid crystal display element. It was confirmed that the contrast was improved.

(Example 6) Similarly to Example 2, the retardation of the area corresponding to the pixel portion is 0.52 to 0.54. A polysiloxane polymer liquid crystal homogeneously aligned so that the retardation of the region corresponding to the non-pixel portion was 0.55 was printed on a glass substrate. The rubbing direction was the same as in Example 5.

Then, upper and lower polarizing plates were installed in the same manner as in Example 1 to obtain a liquid crystal display element of the present invention, and the cell A of this liquid crystal display element was 1/2
When time-division driving was performed at a duty of 0.000, the same results as in Example 5 were obtained.

〔Effect of the invention〕

According to the present invention, in an STN type liquid crystal display element, there is a gap between a liquid crystal layer and a polarizer that corresponds to a pixel part and a non-pixel part so that the brightness of a non-selected pixel or a selected pixel and a non-pixel part are equal. Since a birefringent layer having different retardation values in different regions is provided, the contrast of the entire screen can be prevented from deteriorating and the display quality can be improved.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing an example of the structure of a liquid crystal display element of the present invention;
FIG. 2 is an explanatory diagram of the birefringent layer of the liquid crystal display element of FIG. 1. 1.11...Substrate 2.12...Polarizer 3.13...Alignment film 4.14...Transparent electrode 5...Sealant 6...Liquid crystal 7...Birefringent layer patent Applicant Riko Co., Ltd.

Claims (2)

[Claims] (1) A liquid crystal layer made of a liquid crystal composition having positive dielectric anisotropy is oriented approximately horizontally to the substrates between a pair of substrates provided with electrodes when no voltage is applied, and in the thickness direction of the liquid crystal layer. 120
A birefringent layer is installed at least on one side between a liquid crystal cell configured to have a twisted structure of not less than 360 degrees and a pair of polarizers provided to sandwich the liquid crystal layer, and the liquid crystal 1. A liquid crystal display device characterized in that retardation values in regions of the birefringent layer corresponding to pixel portions and non-pixel portions of a cell are different. (2) The retardation of the birefringent layer is adjusted so that the brightness of a non-selected pixel or selected pixel and the brightness of a non-pixel area when the liquid crystal cell is time-divisionally driven are approximately equal. The liquid crystal display element according to claim 1.