JPH06301028A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To improve a visual field angle by eliminating an inversion phenomenon which is a significant fault to a person viewing the liquid crystal display device and the degradation in contrast occurring therein. CONSTITUTION:This liquid crystal display device is formed by disposing an antiparallel cell 21 subjected to an orientation treatment to provide liquid crystal molecules with directivity of the same direction between the liquid crystal display device 22 and a polarizing plate 20 in order to decrease the refractive index anisotropy generated in the thickness direction of the liquid crystal display device.

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, which suppresses the phenomenon that the contrast changes depending on the viewing angle of a person due to the three-dimensional anisotropy of refractive index of liquid crystal and improves the viewing angle. It is used to

[0002]

2. Description of the Related Art A liquid crystal display device using liquid crystal molecules is currently widely used for display of electronic devices because it can be made thin and lightweight.

An optical anisotropic body such as liquid crystal molecules used in this liquid crystal display device at this time is an ellipsoid of refractive index with three-dimensional refractive indices n _x , n _y and n _z as shown in FIG. It is represented by. The birefringence phenomenon is a phenomenon related to a difference in refractive index within a two-dimensional plane when the index ellipsoid is viewed from a specific direction (usually the Z-axis direction). The phase difference of the liquid crystal display device is the birefringence Δn = nx−ny in the ellipse where the Z = 0 plane and the ellipsoid intersect when the refractive index ellipsoid is observed from the Z-axis direction.
Is represented by a value Δn · d obtained by multiplying by the thickness d of the anisotropic body.
However, when this ellipsoid is observed from another direction, its birefringence (nb-na) changes and its optical path length also changes. Therefore, its phase difference is different from that seen from the Z-axis direction. Becomes Further, the liquid crystal molecules have dielectric anisotropy, and the dielectric constant of the liquid crystal molecules is in the long axis direction (X axis direction).
Therefore, when an electric field is applied, a force that tilts the major axis direction of the liquid crystal molecules in the direction of the electric field is generated. A display device is formed by utilizing this dielectric anisotropy.

The twisted nematic mode liquid crystal display device is most widely used because it can produce a liquid crystal display device having features such as low voltage and low power consumption. As shown in FIG. 5, the twist nematic mode liquid crystal display device has a structure in which liquid crystal molecules 35 are interposed between two opposed substrates 31 and 32. The substrates 31 and 32 are transparent electrodes 31b and 32b and alignment films 31c and 32, respectively, on the surfaces of two transparent substrates 31a and 32a made of glass or the like, which face each other.
c are laminated and formed. Liquid crystal molecule 3
The alignment films 31c, 3c are arranged so that the major axis direction of 5 is substantially parallel to the substrate surface and is continuously twisted by 90 ° between the substrates.
The orientation treatment directions of 2c are set to the directions of arrows 33 and 34 so that they are perpendicular to each other when viewed from the substrate surface.

The operation principle of the liquid crystal display device will be described with reference to a model of liquid crystal molecules as shown in FIG. FIG. 7 shows the electrode 4
3 is a model diagram in which liquid crystal molecules 41 are interposed between 3 and 44, and the long axis direction of the liquid crystal molecules 41 is set to be parallel to the electrode surface. FIG. 7A shows the case where the voltage applied to the circuit is zero and the liquid crystal molecule 41 is not applied with an electric field.
The long axis direction of is kept parallel to the electrode surface. Next, in FIG. 7B, when the voltage applied to the circuit is increased, an electric field is applied to the liquid crystal molecules 41, and the major axis direction of the liquid crystal molecules 41 is inclined to the electric field direction to some extent. In FIG. 7C, when the voltage applied to the circuit is further increased, many electric fields are applied to the liquid crystal molecules 41, and the major axis directions of the liquid crystal molecules 41 are tilted so that they are almost in the same electric field direction. . This is because the liquid crystal molecules 41 have a dielectric anisotropy and have a higher dielectric constant in the major axis direction. Therefore, liquid crystal molecules are used in display devices having a function of optical switching.

In a liquid crystal display device, a direction having a good contrast and most suitable for display is defined as a normal viewing angle direction. Since the normal viewing angle direction varies depending on the twist angle of the liquid crystal layer and liquid crystal molecules depending on the type of the liquid crystal display device, the direction is specified. Hard to do. When the normal viewing angle direction is described with one liquid crystal molecule, it can be said that it is on an extension line of the long axis direction of the liquid crystal molecule when an operating voltage is applied.

In the liquid crystal display device having the above-mentioned structure, the liquid crystal molecules have the dielectric anisotropy as well as the refractive index anisotropy, so that the contrast is changed by tilting the viewing angle obliquely to the display surface. Has the phenomenon.

[0008]

SUMMARY OF THE INVENTION In a normally nematic mode twisted nematic mode liquid crystal display device in which light is transmitted and white display is performed when no voltage is applied, a characteristic diagram of transmittance and applied voltage is actually measured. Is shown in FIG. FIG. 4 is a vertical view of the display surface of the liquid crystal display device.
As indicated by the solid line L1, the transmittance of light decreases as the applied voltage value increases, and the transmittance becomes almost zero when saturated, and the transmittance remains zero even when the applied voltage is further applied. Therefore, the black portion is displayed in black. However,
When the viewing angle is gradually tilted from above the vertical direction of the display surface over the normal viewing angle direction, as shown by the solid line L2, the transmittance decreases as the applied voltage increases, and when the specific voltage value is exceeded. The phenomenon that the transmittance increases again and then gradually decreases again occurs. Therefore, the black portion of the image looks whitish at a specific angle, which is an inversion phenomenon. In addition, the inversion phenomenon does not occur in other viewing angle directions other than the normal viewing angle direction, but as the viewing angle is deepened, it has a viewing angle characteristic that the contrast ratio of black and white becomes low due to the influence of the refractive index anisotropy. .

The cause of the inversion phenomenon will be described with reference to a model of liquid crystal molecules as shown in FIG. In FIG. 7, observer 4
Reference numeral 2 tilts the viewing angle beyond the normal viewing angle direction of the liquid crystal molecules 41. As shown in FIG. 7A, when the applied voltage is zero,
The liquid crystal molecules remain parallel to the electrodes and appear elliptical to the observer 42. Next, as shown in FIG. 7B, when the applied voltage is gradually increased, the liquid crystal molecule 41 tilts in the direction of the electric field, and there is a moment when the observer 42 looks like a perfect circle. When the voltage is further increased as shown in FIG. 7C, the liquid crystal molecules 41 become substantially parallel to the direction of the electric field and appear again to the observer 42 as an ellipse. This causes a phenomenon in which the transmittance once decreases and again increases. This is a phenomenon that occurs because the refractive index anisotropy of liquid crystal molecules, that is, the refractive index changes as the viewing angle is tilted.

As described above, in the twist nematic mode liquid crystal display device, a reversal phenomenon seen in the direction of the normal viewing angle and a decrease in contrast cause a great obstacle for the viewer.

An object of the present invention is to solve such problems, and an object thereof is to provide a liquid crystal display device having improved viewing angle characteristics and improved display quality.

[0012]

In a liquid crystal display device in which a twisted liquid crystal layer is interposed between a pair of transparent substrates, at least one of the liquid crystal display devices has a transparent substrate surface side,
As an optical compensation cell, one or more anti-parallel cells, which are oriented so that liquid crystal molecules have the same directionality, are provided.

In a twist nematic mode liquid crystal display device in which the twist angle of the liquid crystal layer between the pair of transparent substrates is 90 °, the alignment direction of the anti-parallel cell and the alignment direction of the transparent substrate on the anti-parallel cell side are aligned. The above-mentioned object is achieved by making the orientations of the anti-parallel cells perpendicular to each other in the case of a configuration including a plurality of the anti-parallel cells.

[0014]

In order to improve the viewing angle characteristic of the inversion phenomenon, it is considered that the refractive index anisotropy of liquid crystal molecules should be constant regardless of the viewing angle. That is, as the viewing angle becomes deeper from the vertical direction of the display surface of the liquid crystal display device, the change in the refractive index in the thickness direction of the liquid crystal display device is compensated for, and the viewing angle at which the inversion phenomenon occurs is further increased. The corners should be improved.

Therefore, in the present invention, an anti-parallel cell is used as an optical compensation cell having the same refractive index anisotropy in order to suppress the change in the refractive index in the thickness direction of the liquid crystal display device. This anti-parallel cell, which is an optical compensation cell, is injected with liquid crystal molecules, so that the long axis direction of the liquid crystal molecules is uniformly unidirectional rather than the twisted nematic mode in which the molecules are twisted in the thickness direction. It is suitable for The liquid crystal molecules of the anti-parallel cell are not perfectly parallel to the substrate surface, but can be tilted to offset and reduce the refractive index anisotropy in the thickness direction. The orientation direction of the anti-parallel cell is perpendicular to the orientation direction of the transparent substrate in contact therewith. By making the alignment direction of the anti-parallel cell perpendicular to the alignment direction of the transparent substrate in contact therewith, the reversal phenomenon can be suppressed and the viewing angle can be widened as compared with the case of parallel to the alignment direction.

The orientation direction of the anti-parallel cell is suitable for each because the twist of the liquid crystal and its optical characteristics differ depending on the liquid crystal display device such as twisted nematic (TN), super twisted nematic (STN), and double super twisted nematic (DSTN). The orientation direction is selected.

Two anti-parallel cells may be used. Particularly, in a twisted nematic liquid crystal display device, the second anti-parallel cell is aligned with the alignment direction of the first anti-parallel cell which is in contact with the liquid crystal display element. The cell orientation direction is vertical. In the liquid crystal display device having this structure, since it is possible to compensate the refractive index anisotropy in the thickness direction in the direction that the first anti-parallel cell could not completely compensate, the inversion phenomenon is less likely to occur and the viewing range is reduced. It becomes wider and the viewing angle characteristics are further improved. Any other orientation will distort the optical system.

As described above, by using the antiparallel cell of the liquid crystal cell having the three-dimensional refractive index anisotropy, the refractive index anisotropy occurring in the thickness direction of the liquid crystal display element can be suppressed.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. In the figure, the same reference numerals represent the same components.

FIG. 1 is a partially exploded perspective view showing the structure of a twist nematic mode liquid crystal display device according to an embodiment of the present invention, which is actually laminated.
Is. The liquid crystal display device is configured by interposing a liquid crystal display element 22 between a pair of polarizing plates 20 and 23, and further disposing an anti-parallel cell 21 which is an optical compensation cell between the polarizing plate 20 and the liquid crystal display element 22. It In the liquid crystal display device having the above structure, light incident from the polarizing plate 23 side passes through the liquid crystal display element 22 and the anti-parallel cell 21, and is emitted from the polarizing plate 20.

Next, each component of the liquid crystal display device will be described. The liquid crystal display element 22 has a thickness 200 of ITO (Indium Tin Oxide) or the like on opposite surfaces of a pair of transparent substrates 1 and 2 made of glass or plastic.
The transparent electrodes 3 and 4 of 0 Å are patterned so as to intersect in a strip shape, and the alignment films 5 and 6 made of polyimide resin and having a thickness of 600 Å are formed on the surface of the transparent electrodes 3 and 4, and plastic beads or the like for keeping the substrate spacing constant The spacers 7 are made to interpose with each other, and the twisted nematic liquid crystal 8 is sealed with a sealing material 9 made of a UV curable resin.
The twisted nematic liquid crystal layer 8 is made of a cloth in which nylon fibers are woven into the alignment films 5 and 6, and the arrows 11 and 12 are used.
Approximately 90 ° spirally by applying orientation treatment
Liquid crystal molecules are arranged in a twisted manner. Transparent electrodes 3, 4
May be switched by any driving method such as a segment type, a simple matrix type, and an active matrix type.

In the liquid crystal display device of the present embodiment, so-called normally white display is performed in which light is transmitted and white display is performed when no voltage is applied. Therefore, the polarizing plates 20 and 23 are arranged such that the arrows 13 and 14 that are the transmission axis directions thereof are the same as the arrows 11 and 12 that are the orientation processing directions of the orientation films 5 and 6, respectively.

In the liquid crystal display device, an anti-parallel cell 21 is arranged as an optical compensation cell between the polarizing plate 20 and the liquid crystal display element 22. Anti-parallel cell 2
The configuration of No. 1 will be described with reference to FIG.

Transparent substrates 53a, 54a made of glass or the like
2,000 made of ITO, etc. on each of the facing surfaces of
Å transparent electrodes 53b and 54b and alignment films 53c and 54c made of polyimide resin and having a thickness of 600 Å are laminated. Liquid crystal molecules are sealed between the transparent substrates 53a and 54a by a sealing material 56 made of a UV curable resin. The alignment films 53c and 54c are formed by the arrow 5 which is a direction of alignment treatment.
By forming 1, 52 so as to face each other in parallel,
The alignment of liquid crystal molecules is uniformly oriented in one direction. The orientation direction 15 of the anti-parallel cell 21 configured as described above,
16 is arranged so as to be perpendicular to the arrow 11 which is the alignment direction on the anti-parallel cell 21 side of the liquid crystal display element 22.

FIG. 4 is a graph showing the applied voltage-transmittance characteristic of the liquid crystal display device. A solid line L1 shows a characteristic curve when the liquid crystal display device provided with the anti-parallel cell is viewed from the upper direction perpendicular to the display surface. The solid line L1 was the same as the result of measurement in the conventional liquid crystal display device and the liquid crystal display device of this embodiment. In the conventional liquid crystal display device, a characteristic curve when viewed from a position inclined by 40 ° from the vertical upper direction to the normal viewing direction with respect to the display surface is shown by a solid line L2. Then, the liquid crystal display device of the present embodiment provided with the anti-parallel cell is arranged in the normal viewing angle direction from the upper direction perpendicular to the display surface.
A solid line L3 shows the characteristic curve when viewed from the tilted position.

In the conventional example, as indicated by the solid line L2, there occurs a phenomenon that the transmittance again increases at a high applied voltage value. However, in the present embodiment, as shown by the solid line L3, the width of the re-increased transmittance at a high applied voltage value is suppressed to be smaller than that in the conventional case. Therefore, in this embodiment, the occurrence of the inversion phenomenon and the decrease in the contrast ratio are improved. Further, the angle at which the reversal phenomenon occurs is larger than in the conventional case, and the viewing angle range of the liquid crystal display device 1 is expanded.

As another embodiment of the present invention, the polarizing plate 2
The number of anti-parallel cells arranged between 0 and the liquid crystal display element 22 may be two. FIG. 2 shows a liquid crystal display device having a structure in which the number of anti-parallel cells is two. The single liquid crystal display device has the same constitutional requirements, but two antiparallel cells 21 and 24 are provided between the polarizing plate 20 and the liquid crystal display element 22. The alignment direction is arranged so as to be perpendicular to the alignment direction 11 on the antiparallel cell 21 side of the liquid crystal display element 22, and the alignment direction of the antiparallel cell 24 thereon is relative to the alignment direction of the antiparallel cell 21. Arrange vertically. By adopting the liquid crystal display device using the two anti-parallel cells of this embodiment, the reversal phenomenon occurs even if the judgment is made only in the normal viewing angle direction as compared with the liquid crystal display device using one anti-parallel cell. It is possible to obtain viewing angle characteristics that are difficult and have good left-right balance. Further, the enlargement of the entire viewing angle and the reduction of the contrast are suppressed, and the inversion phenomenon is less likely to occur.

In this embodiment, the antiparallel cell is provided on the light emitting side, but it may be provided on the incident side.

Further, a transparent electrode made of ITO is provided in the anti-parallel cell, but this can freely change the tilt of the liquid crystal molecules of the anti-parallel cell by simply changing the applied voltage, and a wide variety of liquid crystals can be obtained. Optimal viewing angle characteristics can be obtained in the display device. The applied voltage at that time may be a fixed voltage or may be synchronized with the applied voltage of the liquid crystal display device. It has been confirmed that the viewing angle can be improved even in the configuration in which the transparent electrode is not provided, because the liquid crystal molecules are inclined to the substrate surface to some extent even when no voltage is applied.

Further, although the liquid crystal display device of the twist nematic mode is described in this embodiment, it is a liquid crystal display device such as STN or DSTN because it suppresses the change of the refractive index in the thickness direction of the liquid crystal display device. Good.

Further, in this embodiment, the liquid crystal layer is sandwiched between the glass substrates as the anti-parallel cell. However, a similar optical film such as a film in which liquid crystal molecules are entangled in a polymer or a liquid crystal film is used. The same effect can be obtained by adopting the one having the property.

[0032]

In the transmittance-applied voltage characteristic diagram as shown in FIG. 4, the conventional liquid crystal display device having no anti-parallel cell is tilted in the viewing angle from the direction perpendicular to the display surface beyond the normal viewing angle direction. The characteristic curve at this time is shown by the solid line L2. Similarly, a solid line L3 shows a characteristic curve of the liquid crystal display device of this embodiment provided with the anti-parallel cells when the viewing angle is tilted from the direction perpendicular to the display surface beyond the normal viewing angle direction. As can be seen from the characteristic diagram, the solid line L3, which is the characteristic curve of the present embodiment, suppresses the re-increasing of the transmittance, which is an inversion phenomenon.

As described above, according to the present invention, an anti-parallel cell having liquid crystal molecules having a refractive index anisotropy in the thickness direction, that is, tilted with respect to the substrate surface and having directionality in a fixed direction is provided. By using it, it is possible to cancel the anisotropy of the refractive index generated in the thickness direction of the liquid crystal display element. As a result, it is possible to improve the characteristics such that the inversion phenomenon does not easily occur even if the viewing angle is deepened. The decrease in the contrast ratio due to the inversion phenomenon in the liquid crystal display element is improved, the contrast ratio in black and white display is improved, and the display quality is improved.

By using two anti-parallel cells and making their orientation directions perpendicular, the transmittance-applied voltage characteristic diagram does not change, but the anisotropy of the refractive index in the other thickness directions is compensated. As a result, the inversion phenomenon and the decrease in contrast are suppressed over a wide viewing angle, and the viewing angle is further improved.

[Brief description of drawings]

FIG. 1 is a perspective view of a liquid crystal display device using one anti-parallel cell according to an embodiment of the present invention.

FIG. 2 is a perspective view of a liquid crystal display device using two anti-parallel cells, which is another embodiment of the present invention.

FIG. 3 is a cross-sectional view showing the structure of an anti-parallel cell.

FIG. 4 is a graph showing applied voltage-transmittance characteristics of the liquid crystal display device 1.

5A and 5B are cross-sectional views each illustrating a structure of a liquid crystal display device.

FIG. 6 is a diagram showing refractive index anisotropy of liquid crystal molecules (index ellipsoid).

FIG. 7 is a diagram showing a model of liquid crystal molecules in a liquid crystal display device.

[Explanation of symbols]

 1, 2 Transparent substrate 3, 4 Transparent electrode 5, 6 Alignment film 7 Spacer 8 Liquid crystal layer 9 Sealing material 11, 12 Alignment direction of liquid crystal display element 13, 14 Polarizing axis of polarizing plate 15, 16 Anti-parallel cell alignment direction 20 , 23 Polarizing plate 21 Anti-parallel cell 22 Liquid crystal display device

Front page continuation (72) Inventor Masanori Akito 22-22 Nagaike-cho, Abeno-ku, Osaka, Osaka In the company

Claims (3)

[Claims] 1. A liquid crystal display device in which a twisted liquid crystal layer is interposed between a pair of transparent substrates, wherein at least one transparent substrate surface side of the liquid crystal display device has a direction in the same direction as liquid crystal molecules as an optical compensation cell. 1 anti-parallel cell that has been oriented so that
A liquid crystal display device comprising one or a plurality of sheets. 2. The twist angle of the liquid crystal layer between the pair of transparent substrates is 90 °, and the alignment direction of the anti-parallel cell and the alignment direction of the anti-parallel cell side transparent substrate are perpendicular to each other. The liquid crystal display device according to claim 1. 3. The liquid crystal display device according to claim 2, wherein a plurality of the anti-parallel cells are provided, and the alignment directions of the anti-parallel cells are perpendicular to each other.