JPH0416916A - Liquid crystal display element - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a liquid crystal display element, and more particularly to a birefringent liquid crystal display element such as a super twisted nematic liquid crystal display element using a color compensation plate.

[Prior art and problems to be solved by the invention] The display mode of liquid crystal display elements that has been mainly used in the past is called the twisted nematic (TN) type, in which approximately 90 liquid crystal molecules are arranged between a pair of upper and lower substrates. °It has a twisted structure,
It utilizes the rotation of the plane of polarization by liquid crystals and the disappearance of this effect when voltage is applied. This display method has the advantage that it has an excellent shutter effect because it is a black and white display, and can relatively easily display multiple colors by providing a color filter for each pixel. Due to the poor performance, high time division driving is difficult, and large-capacity displays have problems such as reduced contrast and narrow viewing angles.

Therefore, in order to improve the steepness of the voltage-transmittance characteristic, a method has been proposed in which the twist angle of the liquid crystal molecules is increased and the polarization axis of the polarizing plate is aligned with the alignment direction of the liquid crystal, thereby making use of the birefringence effect of the liquid crystal. 5BE(super t+++1st
ed birefringence effect) or 5TN (super twisted nemat
ic) mode. This method has excellent threshold characteristics, so there is little decrease in contrast even in time-division driving, and it has a wide viewing angle.
Furthermore, it was difficult to create colors in this state.

Recently, in order to reduce the coloring phenomenon in STN mode, two liquid crystal cells with liquid crystal layers with opposite twist directions are stacked, one for driving and the other as a compensator, and the birefringence A two-layer STN liquid crystal display element that compensates for coloration and provides black and white display has been developed. However, this two-layer system has problems in that since two liquid crystal cells are used, the device becomes thick and heavy, and productivity is poor.

These problems can be improved by replacing the compensation cell with a birefringent polymer film (phase plate type monochrome display STN liquid crystal display element). However, this phase plate method has a problem in that the viewing angle becomes narrow. This is due to the fact that the birefringence of the phase plate has a greater viewing angle dependence than that of a liquid crystal cell. As the phase plate, a stretched film made of polyvinyl alcohol, polycarbonate, or the like is used. The viewing angle dependence of the phase plate can be determined by making the refractive index in the thickness direction of the phase plate larger than the minimum value of the in-plane refractive index (Japan Display '89. p3
39; Japanese Patent Application No. 1-32219), it is difficult to embody such a compensating plate even if the stretching conditions are changed with a stretched film of polyvinyl alcohol or polycarbonate, and the structure of the compensating plate itself becomes complicated. . The above document Jap
In order to obtain such a compensator in an Display, a method is used in which a conventionally known phase plate is stacked with vertically aligned liquid crystal cells.

The present invention was made in view of the problems of the prior art as described above, and its purpose is to use a phase plate with a simple structure in which the refractive index in the thickness direction is smaller than the in-plane refractive index. The object of the present invention is to provide an STN liquid crystal display element with reduced viewing angle dependence.

[Means and effects for solving the problem] According to the present invention, a liquid crystal layer having positive dielectric anisotropy that is sandwiched between substrates and is oriented substantially horizontally when no voltage is applied, and polarized light disposed outside the substrates. It is composed of a plate and one or more birefringent layers provided between the liquid crystal layer and the polarizing plate, at least one of which has a refractive index in the thickness direction larger than the minimum value of the refractive index in the plane. There is provided a liquid crystal display element characterized in that at least one of the layers whose refractive index is closer than the minimum value of the in-plane refractive index is made of a liquid crystalline polymer.

Hereinafter, the configuration of the present invention will be explained in detail.

First, the compensating plate used in the liquid crystal display element of the present invention will be described. As shown in FIG. 2, the refractive index of the compensator is represented by the refractive index nx+ny in the film plane (assuming n12) and the refractive index nz in the thickness direction. In a liquid crystal operation mode that utilizes the birefringence of a liquid crystal layer with positive dielectric anisotropy that is oriented approximately horizontally when no voltage is applied, such as STN, the refractive index in the thickness direction (n2) is equal to the in-plane refractive index (ny). The viewing angle can be improved by providing a compensator that is larger than the angle of view. In the present invention, by using a liquid crystalline polymer as a compensating plate, it is possible to easily obtain a compensating plate having the above-mentioned refractive index distribution, which has been difficult in the past. Like low-molecular liquid crystals, liquid crystalline polymers have the characteristic that they can be easily aligned by external fields such as electric fields and magnetic fields, by the influence of contacting substrate interfaces, and by external stress such as abrasion. By selecting an appropriate material, the alignment state in the liquid crystal phase can be reflected in the same phase by aligning at a high liquid crystal phase forming temperature and then rapidly cooling. That is,
Since it can be used in the same phase, it is self-supporting, and the sandwich structure of substrates, which is essential when using low-molecular liquid crystals, is no longer necessary, and a thin compensator plate can be obtained with a simple structure.

In order to obtain a compensator in which n2 is larger than ny, the liquid crystalline polymer may be oriented almost vertically at the liquid crystal temperature. Vertical alignment of a liquid crystalline polymer can be carried out by (1) heating a liquid crystalline polymer having positive dielectric anisotropy to a liquid crystal temperature, and (2) applying an electric field in the thickness direction. At this time, the liquid crystalline polymer can be sandwiched between conductive substrates for voltage application, or one or both sides of the liquid crystalline polymer can be separated from the voltage application electrode. By rapidly cooling the vertically aligned liquid crystalline polymer, a vertically aligned solid liquid crystalline polymer film can be obtained. The liquid crystalline polymer film may be used while being in contact with the substrate, or may be used after being peeled off from the substrate. The above is a case of orientation using an electric field, but the same applies to the case of orientation using a magnetic field. Further, as another method, a method using a vertical alignment agent similar to that used for low-molecular liquid crystals can also be exemplified.

For example, a substrate such as glass is treated with a vertical alignment agent such as a silane compound having a long-chain alkyl group (Chisso's DMOAP, 005-E, etc.) or lecithin, and then sandwiched between two substrates treated with a liquid crystalline polymer. , a vertically aligned liquid crystalline polymer film is obtained by heating to the liquid crystal temperature. If this is cooled to room temperature, a vertically aligned solid liquid crystalline polymer film can be obtained. The liquid crystalline polymer film may be used while being in contact with the substrate, or may be used after being peeled off from the substrate. In this example, the substrate was treated, but if a substrate with low surface tension, such as tetrafluoroethylene, is used, the liquid crystalline polymer can be vertically aligned on the substrate itself.

The liquid crystalline polymer that can be used is particularly preferably one that exhibits thermotropic liquid crystallinity, and preferably exhibits a nematic phase or a smectic phase. Those exhibiting a nematic phase are particularly preferably used since uniform orientation can be easily obtained. Considering the use at room temperature, the temperature at which the liquid crystal phase is exhibited is preferably 60° C. or higher.

Although it can be used at temperatures below 60° C., in this case it is necessary to sandwich the liquid crystalline polymer layer between transparent substrates.

The compensator plate created in this way exhibits positive uniaxiality with the largest refractive index in the thickness direction, and has a positive uniaxial property with the largest refractive index in the thickness direction.
Although the viewing angle dependence of liquid crystal display elements such as CB can be significantly reduced, this compensation plate alone cannot compensate for coloring from the front due to birefringence in liquid crystal display elements such as STN and horizontally aligned ECB. Can not.

Therefore, it is preferable to use this compensator in combination with a compensator for color compensation. As the compensating plate for color compensation, a conventional stretched polymer film made of polyvinyl alcohol, polycarbonate, polypropylene, or the like can be used. In general, these films have the highest refractive index (nx) in the stretching direction, the second largest refractive index (ny) in the direction perpendicular to the stretching direction within the film plane, and the smallest refractive index (nz) in the thickness direction. or approximately equal to ny. By combining such a stretched film with the aforementioned viewing angle compensator made of a liquid crystalline polymer, nz can be made larger than ny for the compensator as a whole. It is preferable that the value range of nz for the compensator plate as a whole satisfies the following formula.

nx) nz>ny If n2 is too large, the viewing angle tends to become narrow, which is not preferable. The relationship in the above equation can be set relatively arbitrarily depending on the combination of the viewing angle compensator and the color compensator.

As another example, a vertically aligned liquid crystalline polymer can be stretched in one direction within the plane to provide birefringence within the plane. In this case as well, it is preferable that the above relationship is satisfied, and it is also possible to combine with other stretched films.

As described above, it is necessary for the compensator according to the present invention to have birefringence within the film plane as a whole in order to perform black and white display. The compensating plate is required to function so as to return the light, which has become elliptically polarized light having different ellipticity and azimuth angle depending on the wavelength by passing through the liquid crystal cell, back to linearly polarized light again. Birefringence (Δn・d) is the in-plane refractive index anisotropy (
It is given by the product of Δn=n)(-ny) and film thickness (d).

To explain the case where only one compensator is used, the preferable range of Δn-d is approximately 0.1 μm to 1.2 μm.
The range is more preferably 0.2ρ to 1.0μs. Outside this range, the display may not become white or black, or the contrast may drop significantly.

The present invention does not particularly limit the liquid crystalline polymer that can be used. - Side chain type or main chain type liquid crystal polymers known for boat fishing can be used.

Next, the structure and operation of a liquid crystal display element using the above compensating plate will be explained.

An example of the structure of the liquid crystal display element of the present invention is shown in a cross-sectional view in FIG. The structure is generally the same as that of a conventional STN type liquid crystal display element, except that a compensating plate is inserted between the substrate and the polarizing plate.

The first light-transmitting substrate 11 and the second light-transmitting substrate 21 are arranged apart and facing each other, and both substrates 11.21 and the outer peripheral seal 1
A liquid crystal is sealed in the space formed by 4 to form a liquid crystal layer 15, and a liquid crystal cell 16 is formed.

The substrate 11 is made of glass or an optically isotropic plastic film. On the inner surfaces of the substrates 11 and 21 are transparent electrodes 12 for applying voltage to the liquid crystal layer 15.
22 and an alignment film 13 for aligning the liquid crystal in a certain direction,
23 is formed. Disposed between the liquid crystal cell 16 and the polarizing plate 27 are a compensating plate 8 made of a liquid crystalline polymer that satisfies the relationship nz)ny, which is a feature of the present invention, and a compensating plate 9 made of a stretched polymer film. These compensating plates 8 and 9 constitute a birefringent layer. In this example, a case will be described in which the compensating plates 8 and 9 are disposed above the liquid crystal cell, but this vertical relationship may be reversed. 17 is a polarizing plate.

The liquid crystal used here is a nematic or cholesteric liquid crystal with positive dielectric anisotropy. It is preferable that the liquid crystal molecules have a twisted structure in the range of 160° to 360° in the thickness direction when no voltage is applied between the upper and lower substrates, forming a so-called STN type cell.

When the twist angle is small, for example when there is no twist (horizontally aligned ECB), the steepness of the voltage-transmittance characteristic is somewhat reduced.

This liquid crystal display element can also be used as a reflective type by disposing a reflective plate on the outside of one polarizing plate. Furthermore, as another configuration example of the present invention, the compensating plate can be incorporated as a component of the polarizing plate itself. In general polarizing plates that utilize the dichroism of iodine and pigments, a stretched film has polarizing ability by adsorbing iodine and pigments, and then two other films are used to protect the stretched film. Although it has a sandwiched structure, a birefringent layer can be disposed between the protective film on the liquid crystal layer side and the stretched film, and the protective film on the liquid crystal layer side can be composed of a birefringent layer. You can also. As described above, the birefringent layer used in the present invention may be placed anywhere between the liquid crystal layer and the polarizer.

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 aligned along this alignment treatment direction. In this case, "substantially horizontal" in terms of the orientation of liquid crystal molecules means that the tilt angle of the liquid crystal molecules with respect to the substrate is approximately in the range of 0° to 30°. This orientation control can be performed by conventionally known oblique vapor deposition or by forming an inorganic or organic film on the substrate and then rubbing it with cotton cloth or the like.

Specifically, polymer coatings such as polyamide and polyimide that have been subjected to rubbing treatment, Sin, MgO1MgF
2 or the like is preferably used.

FIG. 3 shows the definition of angles related to the present invention. D□, D2 are the directions in which the liquid crystal molecules on the first substrate 11 and the second substrate 21 are projected onto the lower substrate 11, and the arrows indicate directions slightly above the substrate 11 when projecting the liquid crystal molecules. It shows the direction you are facing. The liquid crystal layer 15 has D□ and D
2, the liquid crystal molecules have a twisted structure by ω1. In this case, if ω1 is small, the steepness will worsen and the time division characteristics will deteriorate.

On the other hand, if ω is too large, a scattering structure will be generated when a voltage is applied, and the display quality will deteriorate, which is not preferable.

From this, it is particularly preferable that ω1 is 160° or more and 360′ or less. In FIG. 3, when the cell is viewed from the substrate 21 side, the twist direction is clockwise from the substrate 11 to the substrate 21, but depending on the orientation treatment direction and the selection of cholesteric liquid crystal, the twist direction is counterclockwise. It is also possible to do this.

X is the direction in which the in-plane refractive index of the compensator plate is maximum, and has an angle of δ with respect to the liquid crystal molecule alignment direction D2 on the adjacent substrate 21. Furthermore, the transmission axis P2 of the polarizing plate 27 adjacent to the compensation @9 is arranged at an angle of β2 with respect to the X direction. Further, the transmission axis P□ of the polarizing plate 17 forms an angle β□ with the orientation direction D of liquid crystal molecules on the substrate 11. Note that the angle assumes that the direction of the twist of the liquid crystal is positive. δ
The preferable conditions differ depending on whether one compensating plate is used or a plurality of compensating plates are used. The preferred value when using - sheets is δ=-5
It is 0° to 50°.

Although the case where the compensation plate is disposed on one side of the liquid crystal cell has been described above, it is also possible to dispose it on both sides so as to sandwich the liquid crystal cell.

The color produced when a compensating plate is sandwiched between polarizing plates is determined by the turbulence of the compensating plate and the direction of the polarizing plates. Further, the change in color depending on the viewing angle direction is influenced by the viewing angle dependence of the compensating plate adhesive tarpsion. If the viewing angle dependence of the compensating plate adhesive tarpsion is small, the viewing angle dependence of the color will also be small.

Therefore, the smaller the viewing angle dependence of the retardation of the compensation plate used for color compensation of the STN liquid crystal display element, the smaller the viewing angle dependence of the color of the liquid crystal display element. In the liquid crystal display element of the present invention, since the refractive index in the thickness direction of the compensator is larger than the refractive index in the in-plane direction, the color change of the compensator itself due to the viewing angle is small. In some cases, a liquid crystal display element having an extremely wide viewing angle can be obtained. Furthermore, since the compensating plate can be created with a simple configuration, an increase in the dimensions and weight of the element can be suppressed.

〔Example〕

Examples of the present invention will be described below, but the present invention is not limited to these.

(Example 1) A polysiloxane-based liquid crystal polymer having a repeating unit represented by the following formula (1) was treated with a vertical alignment agent 0DS- manufactured by Chisso Corporation.
Sandwiched between a glass substrate coated with E and a Teflon plate,
16 in which the liquid crystalline polymer (1) exhibits a nematic liquid crystal phase
After heating at 5° for 1 hour, it was rapidly cooled to room temperature, and then the Teflon layer was peeled off to obtain a vertically aligned liquid crystalline polymer film about 2 mm thick. Since this liquid crystalline polymer does not exhibit a liquid crystal phase at room temperature, the obtained film was in a glass state. The refractive index of this film was nz=1.68, ny=1.52.

Melting point: 100℃.

Nematic-isotropic transition temperature: 170°C
l,=1.589. ny=1.582. nz=1.58
1. Retardation: (nx-ny) ・d=58
0nm), the refractive index of the entire compensator plate is n1=
1.588. ny=1.581. nz=1.583, and 1)()nz>ny. When the viewing angle dependence of retardation was measured by sandwiching this compensating plate between orthogonal polarizing plates, almost no change in retardation due to viewing angle was observed, as shown as A and a in FIG. 4.

Note that the angles in FIG. 4 are angles from the normal direction, A indicates the case observed from the X direction of the compensating plate, and a indicates the case observed from the X direction.

Next, this compensating plate was installed between the STN type liquid crystal cell and the polarizing plate as shown in FIG. 1 to obtain a liquid crystal display element of the present invention. The liquid crystal cell was constructed as follows.

It has a transparent electrode, the twist angle ω of the liquid crystal between the upper and lower glass substrates is 200°, and the retardation of the liquid crystal layer (the product of the birefringence anisotropy of the liquid crystal molecules and the thickness of the liquid crystal layer) is 0.92.
An STN cell of No. 4 was prepared. The liquid crystal used was a nematic liquid crystal ZLI2:293 (manufactured by Merck & Co., Ltd.) having positive dielectric anisotropy to which chiral nematic liquid crystal 5811 (manufactured by Merck & Co., Ltd.) was added. The alignment treatment was performed by rubbing the polyimide film.

At the bottom of the STN cell, a neutral gray polarizing plate is installed with its transmission axis 45 degrees with respect to the liquid crystal molecule alignment direction on the lower substrate.
They were arranged so as to form an angle of (β2: 45°). Furthermore, a backlight using cold cathode tubes was installed below this polarizing plate. The compensation plate, which is a feature of the present invention and is placed on top of the liquid crystal cell, has an X direction that is 90° with respect to the rubbing direction of the upper substrate.
It was installed so that it formed an angle (: δ). A neutral gray polarizing plate is placed above the compensator with its transmission axis aligned with the X of the compensator.
Makes an angle of =45° with the direction (β1・-45°)
It was arranged like this.

The liquid crystal display element configured as described above was black when no voltage was applied, turned white when voltage was applied, had small color change depending on the viewing angle direction, and had excellent viewing angle characteristics.

(Example 2) An acrylic liquid crystalline polymer having a repeating unit represented by the following formula (2) was sandwiched between glass substrates with a transparent conductive film and heated to 220°C at which the liquid crystalline polymer exhibits a nematic phase. . Frequency of 50Hz between upper and lower electrodes while maintaining temperature
, a rectangular wave with a voltage of 50 V was applied, and then the sample was rapidly cooled to room temperature. The thickness of the liquid crystalline polymer film was about 2 tones. Conoscopic observation using a polarizing microscope confirmed that the liquid crystalline polymer was oriented perpendicularly to the substrate.

Melting point: 60°C Nematic-isotropic transition temperature: 227°C Phase plate made of the above-mentioned liquid crystalline polymer film stretched polycarbonate (n
z=1.589. ny=1.582. nz=1.581
．． Retardation: (nz-ny)・d=580n
A liquid crystal display element was produced in the same manner as in Example 1 except for using the compensating plate superimposed on (m).

The liquid crystal display element configured as described above was black when no voltage was applied, turned white when voltage was applied, had small color change depending on the viewing angle direction, and had excellent viewing angle characteristics.

(Comparative Example) The viewing angle dependence of retardation was measured using only stretched polycarbonate, and the results were large as shown as B and b in FIG. B shows the case when the compensator is observed from the X direction, and b shows the case when it is observed from the Y direction.6 A liquid crystal display element was prepared in exactly the same manner as in Example 1 using only stretched polycarbonate as the compensator. Although it was possible to display black and white, coloring occurred at viewing angles of 30' or more, resulting in a liquid crystal display element with an extremely narrow viewing angle.

〔Effect of the invention〕

According to the present invention, one or more birefringent layers, at least one of which has a refractive index in the thickness direction that is larger than the minimum value of the in-plane refractive index, are provided as compensators, so that the viewing angle dependence of color is improved. At the same time, the compensating plate can be easily produced, and an increase in the dimensions and weight of the element can be suppressed.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing an example of the structure of a liquid crystal display element of the present invention;
Figure 2 shows the direction of the refractive index of a birefringent film, Figure 3
The figure is an explanatory diagram of the angular relationship between the orientation direction of liquid crystal molecules and the direction of the polarizing plate in the liquid crystal display element of the present invention, and Figure 4 shows the viewing angle dependence of the retardation of the liquid crystal display element of the example of the present invention as a comparative example. It is a diagram shown in comparison. 8.9... Compensation plate 11.21... Substrate 12.22... Transparent electrode 13.23... Alignment film 14... Sealing agent 15... Liquid crystal layer 16... Liquid crystal cell 17.27... Polarizing plate patent Applicant Riko Co., Ltd.

Claims (1)

[Claims] (1) A liquid crystal layer sandwiched between the substrates and having positive dielectric anisotropy that is oriented substantially horizontally when no voltage is applied; a polarizing plate disposed outside the substrate; and at least one layer provided between the liquid crystal layer and the polarizing plate. is composed of one or more birefringent layers whose refractive index in the thickness direction is larger than the minimum value of the in-plane refractive index,
1. A liquid crystal display element, wherein at least one of the layers whose refractive index in the thickness direction is larger than the minimum value of the in-plane refractive index is made of a liquid crystalline polymer.