JP2869451B2 - Liquid crystal display device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly to an STN type liquid crystal having a structure in which a liquid crystal having a positive dielectric anisotropy is twisted by 90 ° or more in a thickness direction of a liquid crystal layer. It relates to a display element.

[Problems to be solved by conventional technology and invention]

The display mode of liquid crystal, which has been mainly used in the past, is called twisted nematic (TN) type, in which liquid crystal molecules are twisted by about 90 ° between a pair of substrates. Utilizing the disappearance of the effect by voltage application. This display mode was sufficient for low-time-division driving such as a clock or a calculator. However, when high-time-division driving was performed to increase the display capacity, there was a disadvantage that the contrast was reduced and the viewing angle was reduced. there were. This is because the ratio of the voltage applied to the selected point and the voltage applied to the non-selected point approaches 1 in the case of high time division driving. In order to obtain a display element with a high contrast and a wide viewing angle, the relative transmittance of the element is 10%. the ratio of the voltage V ₅₀ that changes 50% with respect to the voltage V ₁₀ that varies (V ₅₀ / V ₁₀₎
It is necessary to minimize the steepness γ represented by

In the case of the twisted nematic type, the γ value is about 1.13. In order to reduce the 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, which is called an SBE mode or an STN mode. According to such a method, the γ value can be reduced to 1.1 or less, and high time division driving of about 1/400 duty can be performed.

However, in such a method, it is difficult to perform a monochrome display in principle because coloring by birefringence and a change due to the voltage are used, and the transmitted light or the reflected light of the liquid crystal cell is colored and colored. It will be displayed on the background.

As a color compensating plate for eliminating the coloring of the STN type liquid crystal display element, there are known a method using a liquid crystal cell having a reversed twist direction and an equal twist angle, and a method using a birefringent medium such as a polymer film. JP-A-64-519). However, the conventional color-compensated STN type liquid crystal display element has a drawback that a change in display color and a change in contrast depending on a viewing angle are large.

SUMMARY OF THE INVENTION It is an object of the present invention to eliminate the above-mentioned disadvantages of the conventional liquid crystal display device and to provide a liquid crystal display device having excellent display quality.

[Means and Actions 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 a positive dielectric anisotropy is formed between a pair of substrates provided with electrodes, and substantially without a voltage when a voltage is not applied. A liquid crystal cell that is horizontally oriented and has a twisted structure of 120 ° or more and 360 ° or less in the thickness direction of the liquid crystal layer, and a pair of polarizers provided so as to sandwich the liquid crystal layer. At least one has a polymer liquid crystal layer oriented substantially horizontally with respect to the substrate of the liquid crystal cell, and a birefringent layer having a refractive index in a direction perpendicular to the substrate larger than a refractive index in a direction horizontal to the substrate. A liquid crystal display device characterized by the above is provided.

Next, the configuration of the present invention will be described in detail with reference to the drawings. FIG. 1 is a sectional view showing a configuration example of a liquid crystal display element of the present invention. In this figure, a cell 10 is an STN cell, and substrates 1 and 11 have alignment films 3 and 13 and transparent electrodes 4 and 14 that have been subjected to an alignment process, respectively.
The liquid crystal 6 is sealed between them and is separated from the outside air by the sealant 5. This liquid crystal cell 10 is sandwiched between the first polarizer 2 and the second polarizer 12,
Polymer liquid crystal layer 7 and birefringent layer 8 between substrate 1 and polarizer 2
Are provided to constitute a liquid crystal display element.

When a polymer liquid crystal layer and a birefringent layer are provided between a substrate and a polarizer as in the configuration example of the liquid crystal display element of the present invention, a light-transmitting glass, plastic, or the like is used as the substrate. When using a plastic substrate, reduce the thickness of the substrate.
It is easy to reduce the thickness to 0.2 mm or less, so that the display element can be made extremely thin and lightweight.
Further, there is an advantage that the display does not become a double image because the substrate is thin.

The alignment treatment on the substrate of the liquid crystal display element of the present invention is performed so that liquid crystal molecules are substantially horizontally aligned when no voltage is applied,
The liquid crystal molecules are preferentially aligned along the alignment processing direction. In this case, “substantially horizontal” with respect to the alignment 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 0 ° to 30 °. The orientation can be controlled by oblique vapor deposition or rubbing with a cotton cloth after forming an inorganic or organic film on the substrate. As the alignment films 3 and 13 used in the present invention, those obtained by rubbing a polymer coating such as polyamide or polyimide, or Si
Oblique deposition using O, MgO, MgF ₂ or the like is used.

The polymer liquid crystal layer 7 of the present invention has a function of compensating for a change in the polarization state due to the STN cell 10. The polymer liquid crystal layer 7 is oriented substantially horizontally with respect to the substrate 1 and has a structure twisted in the thickness direction or a homogeneous structure without twist. A color compensator using a polymer liquid crystal is industrially advantageous because the production cost is low and the productivity is high as compared with a normal liquid crystal cell.

The birefringent layer 8 has a higher refractive index in a direction perpendicular to the substrate 1 than in a direction parallel to the substrate 1. The birefringent layer 8
It has an action of compensating for a change in the polarization state due to the viewing angle of the color-compensated STN cell 10. The birefringent layer 8 as described above is, for example, a liquid crystal cell or a polymer liquid crystal layer oriented in a direction substantially perpendicular to the substrate 1, a polymer film stretched with polystyrene or the like, or a positive cut out in a direction perpendicular to the optical axis. Crystals having optical anisotropy and the like.

In the configuration example of FIG. 1, the polymer liquid crystal layer and the birefringent layer are provided between the upper substrate and the upper polarizer of the liquid crystal cell. However, the polymer liquid crystal layer and the birefringent layer may be provided between the lower substrate and the lower polarizer. It may be installed on both sides. Further, a polymer liquid crystal layer may be provided on one side of the liquid crystal cell, and a birefringent layer may be provided on the other side. Further, the substrate of the liquid crystal cell and the substrate for forming the polymer liquid crystal layer or the substrate for forming the birefringent layer, the substrate for forming the polymer liquid crystal layer and the substrate for forming the birefringent layer may be used in combination. May be used as a substrate of a liquid crystal cell or a substrate on which a polymer liquid crystal layer is formed.

〔Example〕

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

(Example 1) An STN cell A was prepared in which the twist angle of the liquid crystal on the upper and lower glass substrates having the transparent electrodes in the form of stripes was 240 ° and the retardation of the liquid crystal layer was 0.86 µm. The liquid crystal is a nematic liquid crystal ZLI2293 (Merck) with a positive dielectric anisotropy, and a chiral nematic liquid crystal S811 (Merck).
Was used. The alignment treatment was performed by rubbing the polyimide film.

Next, a polysiloxane-based polymer liquid crystal having a repeating unit represented by the following formula (1) was placed on a glass substrate with a retardation of 0.86 μm, a twist angle of 240 °, and a twist direction opposite to that of cell A. The resultant was applied by a spinner to form a polymer liquid crystal layer B. Next, the polymer liquid crystal layer B was superimposed on the cell A such that the rubbing direction of the upper substrate of the cell A was perpendicular to the orientation direction of the polymer liquid crystal layer B on the glass substrate.

Next, a vertical alignment agent ODS-E is applied to the surfaces of the two glass substrates.
(Made by Chisso), baked, and rubbed to form a vertical alignment film, these two glass substrates are bonded together, and a liquid crystal ZLI4318 (made by Merck) having a negative dielectric anisotropy therebetween. Was sealed, and a liquid crystal cell C having homeotropic alignment between the upper and lower substrates was produced, and was overlaid on the polymer liquid crystal layer B.

Further, the upper and lower polarizers are set such that the transmission axis of the lower polarizer forms 45 ° with the rubbing direction of the lower substrate of the cell A, and the transmission axis of the upper polarizer is orthogonal to the transmission axis of the lower polarizer. Thus, a liquid crystal display device of the present invention was obtained.

In the above liquid crystal display element, the liquid crystal cell A was time-divisionally driven at 1/200 duty and visually observed. As compared with the case where the liquid crystal cell C was not provided, the display performance when viewed from the front did not change, and the display color and contrast did not change. It was found that the change due to the viewing angle was small, and the viewing angle characteristics were improved.

Example 2 A vertical alignment film was formed on the surface of a glass substrate, rubbed, and a polysiloxane-based polymer liquid crystal was applied using a spinner.
A homeotropically aligned polymer liquid crystal layer D was formed.

A polymer liquid crystal layer D was superimposed on the polymer liquid crystal layer B of Example 1, and a polarizing plate was arranged in the same manner as in Example 1 to form a liquid crystal display device. When time-division driving was performed with a duty, the same result as in Example 1 was obtained.

(Example 3) A polystyrene film was uniaxially stretched, and two films were overlapped so that the stretching directions were orthogonal to each other to form a birefringent layer, which was laminated on the polymer liquid crystal layer B of Example 1. In this polystyrene film, the refractive index in the thickness direction of the film was larger than the in-plane refractive index by stretching. In addition, since the two films are stacked so that the stretching directions are orthogonal to each other, changes in the polarization state in the plane cancel each other out. Therefore, even if this film is laminated on the polymer liquid crystal layer B, it can be viewed from the front. The display performance when it did not change.
Next, a polarizing plate was arranged in the same manner as in Example 1 to form a liquid crystal display element. In this liquid crystal display element, when the cell A was driven in a time-division manner at 1/200 dth, the same result as in Example 1 was obtained. .

Example 4 A polymer liquid crystal layer E in which a polysiloxane-based polymer liquid crystal was homogeneously aligned with a retardation of 0.55 μm and having no twist was formed on a glass substrate. Then, the polymer liquid crystal layer E was overlaid on the cell A of Example 1 so that the rubbing direction of the upper substrate of the cell A and the orientation direction of the polymer liquid crystal layer E on the glass substrate were orthogonal to each other.

Next, the homeotropic liquid crystal cell C of Example 1 is superimposed on the polymer liquid crystal layer E, and the transmission axis of the lower polarizing plate forms 45 ° with the rubbing direction of the lower substrate of the cell A. The upper and lower polarizers were arranged so that the transmission axis of the polarizer was at 45 ° with the rubbing direction of the upper substrate of the cell A, thereby producing a liquid crystal display device. In this liquid crystal display element, when the cell A was time-divisionally driven at 1/200 duty, the display performance when viewed from the front did not change and the change in display color and contrast due to the viewing angle was smaller than when the liquid crystal cell C was not provided. It became clear that the viewing angle characteristics were improved.

(Example 5) In Example 4, instead of the liquid crystal cell C, Example 2 was used.
When a liquid crystal display element was manufactured using the polymer liquid crystal layer D of Example 4, the same results as in Example 4 were obtained.

(Example 6) In Example 4, a liquid crystal display device was produced using the uniaxially stretched polystyrene film of Example 3 instead of the liquid crystal cell C, and the same result as that of Example 4 was obtained.

〔The invention's effect〕

In the present invention, in the STN type liquid crystal display element, at least one of the liquid crystal layer and the pair of polarizers, a polymer liquid crystal layer oriented substantially horizontally with respect to the substrate of the liquid crystal cell, and a polymer liquid crystal layer perpendicular to the substrate. By providing a birefringent layer whose refractive index in the direction is larger than the refractive index in the horizontal direction on the substrate, a black-and-white display liquid crystal display element having excellent display quality with little change in display color and contrast due to a viewing angle is provided. Became possible.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a configuration example of a liquid crystal display device according to the present invention. 1,11 Substrate 2,12 Polarizer 3,13 Alignment film 4,14 Transparent electrode 5 Sealant 6 Liquid crystal 7 Polymer liquid crystal layer 8 Birefringent layer

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-63-149624 (JP, A) JP-A-64-519 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G02F 1/133 500 G02F 1/1347 G02F 1/1335 510

Claims (1)

(57) [Claims] A liquid crystal layer comprising a liquid crystal composition having a positive dielectric anisotropy is oriented substantially horizontally between a pair of substrates provided with electrodes when no voltage is applied to the substrate, and the thickness of the liquid crystal layer is increased. In the direction
A liquid crystal cell configured to have a twisted structure of 120 ° or more and 360 ° or less, and at least one between a pair of polarizers provided so as to sandwich the liquid crystal layer, the liquid crystal cell substrate 1. A liquid crystal display device comprising: a polymer liquid crystal layer oriented substantially horizontally; and a birefringent layer having a refractive index in a direction perpendicular to the substrate larger than a refractive index in a direction horizontal to the substrate.