JPH0311450B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a twisted nematic liquid crystal display device with excellent contrast characteristics. A twisted nematic liquid crystal display device has a basic configuration as shown in FIG. By a rubbing method or the like, two electrode substrates 1 and 2 whose orientations have been controlled are faced to each other so that their orientation control directions are orthogonal to each other, and a seal 3 is formed.
A nematic liquid crystal with positive dielectric anisotropy is injected between the cells to form a liquid crystal cell. The liquid crystal molecules are oriented on the surfaces of the electrode substrates 1 and 2 so that their long axes coincide with the respective orientation control directions, and form a structure twisted by 90 degrees from the substrates 1 to 2. Therefore, when light enters a liquid crystal cell, the plane of vibration rotates 90 degrees along the twist of the molecular arrangement and exits from the liquid crystal cell. When a voltage above a certain value is applied between the electrode and substrate, the liquid crystal molecules rearrange in the direction of the electric field and the twisted structure disappears.
Therefore, the property of rotating the plane of vibration of light is also lost. Polarizing plates 5 and 6 are pasted on both sides of the substrate of this liquid crystal cell and used as a display device. Depending on whether the polarization axes of the two polarizing plates are parallel or orthogonal, there are two types: positive display and negative display. It is possible to display
A negative display is one in which two polarizing plates are pasted so that their polarization axes match, and the light is blocked when an OFF signal is given, and the light is transmitted when an ON signal is given. On the other hand, in the positive display type, the two polarization axes are orthogonal, and when the OFF signal is on, the light is transmitted, and when the ON signal is, the light is blocked. Considering the contrast characteristics in negative display (ratio of light transmittance in light-blocking state and light-transmitting state),
It is desirable that the light transmittance at the time of the OFF signal is low. However, in conventional display devices,
Due to the presence of the non-linear polarization component of the polarizing plate and the elliptical polarization component (component that does not rotate completely 90 degrees) of the liquid crystal cell, the light transmittance during the OFF signal is not small enough. It is known that adding a dichroic dye that absorbs light of a specific wavelength to the liquid crystal is effective in reducing the light transmittance during the OFF signal. However, with this method, the light transmittance during the ON signal also decreases, so the contrast characteristics do not necessarily improve. In addition, when selecting a polarizing plate, it is desirable to use a polarizing plate with a good degree of polarization, but in actual use, in addition to the degree of polarization, the light resistance of the polarizing plate,
It is also necessary to consider heat resistance, wavelength characteristics, etc. Particularly in the case of liquid crystal display devices used in automobile instruments, etc., the usage conditions are severe, and the current situation is that there is no polarizing plate that satisfies reliability and degree of polarization. Therefore, we use a polarizing plate on the front side that has a slightly inferior degree of polarization but has excellent light resistance, or we use a polarizing plate with different wavelength characteristics in order to match the background color of the display device to the conditions of use. There are many things. The present invention provides a twisted nematic liquid crystal display device with negative display with excellent contrast by using different polarizing plates, adding dichroic dyes, and optimizing the installation direction of the polarizing plates and the alignment direction of liquid crystal molecules. A pair of polarizing plates are arranged on both sides of a liquid crystal cell, in which a twisted oriented nematic liquid crystal containing a dichroic dye is held between a pair of electrode substrates, so that the directions of the polarization axes are equal. In a liquid crystal display device, two types of polarizing plates having different degrees of polarization near the maximum absorption wavelength of a dichroic dye added as polarizing plates are used, and the polarizing axis of the polarizing plate having an excellent degree of polarization and the adjacent polarizing plate are used as polarizing plates. This is a liquid crystal display device characterized by aligning the liquid crystal molecules on an electrode substrate in the same direction. In the present invention, since polarizing plates having two types of polarization degrees can be used, the selection can be made taking light resistance, heat resistance, etc. into consideration, and it is possible to obtain a liquid crystal cell with sufficient contrast and background color. It's easy. For example, a polarizing plate with a good degree of polarization generally has good contrast, but has poor light resistance, heat resistance, etc.
The background color also has the disadvantage of becoming noticeable, so by placing a polarizing plate with good light resistance and a slightly inferior degree of polarization on the front side, and placing a polarizing plate with a good degree of polarization on the back side, a relatively superior It is possible to obtain a liquid crystal cell that has contrast, has little background coloring, and has no problem in light resistance. The degree of polarization in the present invention refers to the degree of polarization near the maximum absorption wavelength of a dichroic dye, and is the difference between the transmitted light when the same polarizing plate is arranged in parallel and the transmitted light when it is arranged perpendicularly. It represents a ratio. Since the present invention is a negative display, the polarizing axes of the polarizing plates are arranged in the same direction, and the liquid crystal is twisted by 90 degrees, so on one electrode substrate, the polarizing axis of the polarizing plate and the alignment direction of the liquid crystal molecules match. In the other electrode substrate, the polarization axis of the polarizing plate and the alignment direction of liquid crystal molecules are perpendicular. In this case, the problem is which polarizing plate should match the polarizing axis of the polarizing plate with the alignment direction of the liquid crystal molecules. By matching the alignment direction of liquid crystal molecules, that is, the direction of alignment treatment, a liquid crystal with excellent contrast and less background coloring can be obtained. The liquid crystal cell used in the present invention has electrode substrates formed with transparent electrodes such as In ₂ O ₃ -SnO ₂ formed on transparent substrates such as glass or plastic, and the periphery of the electrode substrates facing each other is sealed, and liquid crystal is sealed inside. Undercoat such as SiO ₂ , Al ₂ O ₃ , etc. as necessary.
A liquid crystal cell with a structure used in various known liquid crystal cells, such as an overcoat such as SiO ₂ or polyimide, two-layer electrodes, placing a semiconductor element on a substrate, metal leads, various spacers, printing for a mask, or forming an opaque layer. is used. In the present invention, as described above, as a method of attaching two different polarizing plates to a liquid crystal cell, FIG.
There are two possible cases shown in parentheses. That is,
A case in which the polarization axis direction a of the two polarizing plates 5 and 6 matches the alignment direction b of the substrate 1 (), and a case in which the polarization axis direction a and the alignment direction c of the substrate 2 match (). As a result of trying these two arrangement methods using a liquid crystal cell to which a dichroic dye has been added, the inventor found that when the polarizing plate 6 has a better degree of polarization than the polarizing plate 5 near the maximum absorption wavelength of the added dye. It has been found that the arrangement of () has lower transmittance during the OFF signal and better contrast characteristics than (), and is desirable for display devices. Further, when the polarizing plate 5 has a higher degree of polarization than the polarizing plate 6, the arrangement method in parentheses is more desirable. In other words, by aligning the polarization axis of a polarizing plate with a good degree of polarization with the alignment direction of the adjacent substrate in the vicinity of the maximum absorption wavelength of the added dye, the light transmittance at the time of the OFF signal is reduced and the contrast characteristics are improved. can be improved. Note that polarizing plates with a good degree of polarization generally have poor light resistance, so when used under outside light, use a polarizing plate with a good degree of polarization as the back side, and use a polarizing plate with a good degree of polarization on the front side. It is preferable to use a polarizing plate. Examples are shown below. In this example, the cell gap is 11 μm,
The liquid crystal is phenylcyclohexane-based Merck ZLI-1565, and the dichroic dye is 600 to 650 nm.
Three types of anthoraquinone dyes with a maximum absorption wavelength in the wavelength range of 1.75% were added. Further, as polarizing plates, Q-10 manufactured by Nitto Denko Corporation was used as the front polarizing plate 5, and QE-10 manufactured by Nitto Denko Corporation was used as the back polarizing plate 6. The light transmittance of two Q-10 sheets at a wavelength of 600 nm is 41% when the polarization axes are parallel, and 1.5% when the polarization axes are perpendicular. In addition, the light transmittance in QE-10 is 38 in parallel.
It was 0.02% at right angle. From this, 600nm
It can be seen that QE-10 is a polarizing plate with a better degree of polarization than Q-10 in terms of wavelength. Table 1 shows the transmission intensity of light in the light-shielding state and the light-transmitting state in the case of arrangement (), (), and arrangement () in which no dye is added, and the ratio of the two showing contrast characteristics. As is clear from the table, it can be seen that the arrangement () in which the dichroic dye is added has the best contrast characteristics. As mentioned above, the twisted film uses two different polarizing plates and adds a dichroic dye.
In a nematic liquid crystal display device, a display device with excellent contrast characteristics and less background coloring can be obtained by arranging the polarization axis direction of a polarizing plate with a better degree of polarization to match the orientation processing direction of an adjacent substrate. be able to.

[Table] The above explanation describes a liquid crystal cell with a basic structure, but multiplexing can be achieved by slightly shifting the polarization axes of the polarizing plates without completely matching them, or by slightly shifting the alignment direction instead of 90°. The margin during driving can be improved, and various filters, reflectors, light guide plates, light sources, drive circuits, etc. may be provided outside the liquid crystal cell, and various applications are possible.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a liquid crystal display device, and FIG. 2 is an explanatory diagram showing the polarization axis of the polarizing plate of the present invention and the orientation treatment direction of the substrate. 1, 2...Substrate, 3...Seal, 4...Liquid crystal,
5, 6...Polarizing plate, a...Polarization axis direction of polarizing plates 5 and 6, b...Orientation treatment direction of substrate 1, c...Orientation treatment direction of substrate 2.

Claims (1)

[Scope of Claims] 1 A pair of polarizing plates are arranged on both sides of a liquid crystal cell in which a twisted oriented nematic liquid crystal to which a dichroic dye is added is held between a pair of electrode substrates so that the directions of the polarization axes are equal. In the liquid crystal display device, two types of polarizing plates having different degrees of polarization near the maximum absorption wavelength of the added dichroic dye are used, and the polarization axis of the polarizing plate having an excellent degree of polarization and the electrode substrate adjacent thereto are used. A liquid crystal display device characterized in that the arrangement direction of liquid crystal molecules on the top panel is aligned with that of the liquid crystal molecules on the top panel. 2. The liquid crystal display device according to claim 1, wherein a polarizing plate with an inferior degree of polarization is used as the polarizing plate on the front side, and a polarizing plate with an excellent degree of polarization is used as the polarizing plate on the back side.