JPH0120724B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a guest-host type color liquid crystal display device, and more specifically, on the surface of one electrode substrate, liquid crystal molecules are arranged almost parallel to the substrate surface, and on the surface of the other electrode substrate, liquid crystal molecules are arranged at an angle. The present invention relates to a phase change type liquid crystal display device. A method of displaying color by adding a pleochroic dye to a cholesteric liquid crystal with positive dielectric anisotropy is already known as a phase change type guest-host display.
This method has advantages over the conventional TN mode, such as a small angular dependence of contrast, a wide viewing angle, and a bright display because no polarizing plate is required. The operating principle of this system is shown in Figure 1. In Fig. 1, 1 is the electrode substrate, 2 is the electrode, 3 is the arrangement state of the liquid crystal composition, 4 is the switch, and 5 is the AC power supply, and when no voltage is applied or a voltage below the threshold voltage value is applied. As shown in the left half of the liquid crystal composition, since the liquid crystal molecules have a spiral structure throughout the liquid crystal composition, the light passing through the liquid crystal layer is absorbed by the pigment, resulting in a colored state. . Next, when a voltage higher than the threshold voltage is applied, the pitch of the spiral rotation of the liquid crystal molecules is extended to infinity as shown in the right half, and the axes of the liquid crystal molecules are aligned almost perpendicularly to the electrode interface, resulting in a transition to a nematic state. At this time, if a dichroic dye that has an electronic transition moment in the visible light region due to light absorption is contained only in the direction of the long axis of the molecule,
Since these dye molecules also align in the same state as the liquid crystal molecules, no light absorption occurs, resulting in a transparent state.
There are two main alignment control methods for this method: vertical alignment, in which the long axis of the liquid crystal molecules is almost perpendicular to the electrode interface, and horizontal alignment, in which the long axis of the liquid crystal molecules is almost horizontal. . Compared to the vertical alignment method, the horizontal alignment method uses less pigment material to obtain the same color density, and the alignment control method uses the rubbing method or oblique evaporation method that has already been established for conventional TN display devices. It is advantageous. From this point of view, the inventors of the present invention have conducted various studies regarding phase change type liquid crystal display devices that employ the horizontal alignment method, and have found that the conditions under which display can always be performed with good contrast, regardless of the radial direction of the liquid crystal composition, are as follows: We have come to the interesting finding that this is related to the angle of inclination of liquid crystal molecules on the surfaces of opposing electrodes.
The present invention has been made based on this knowledge, and provides a display device in which liquid crystal molecules are arranged approximately parallel to the surface of one electrode substrate, and are tilted at an angle on the surface of the other electrode substrate. By adopting this, it is possible to provide a liquid crystal display device that can write and erase with good contrast, and furthermore, the ratio of the liquid crystal layer thickness to the pitch P of the liquid crystal is /2π≦d/P≦/2π+1/4. The present invention provides a device that enables dynamic drive within this range. A rubbing method or an oblique evaporation method is exemplified as a method for controlling the orientation of liquid crystal molecules so that they have an arbitrary inclination with respect to the substrate surface and are aligned in a certain direction.In the rubbing method, the inclination angle α is usually 0 to 8 degrees, 0 to 10 degrees by selecting the deposition direction in the oblique deposition method,
Alternatively, it can be obtained within the range of 20 to 30°. In the rubbing method, liquid crystal molecules are oriented in contact with the substrate on the near side in the rubbing direction, and in the oblique evaporation method, liquid crystal molecules are oriented in contact with the substrate on the side far from the evaporation source. Making the tilt angles of the liquid crystal molecules on the surfaces of both substrates equal is advantageous in manufacturing a liquid crystal display cell because it allows the alignment control method of opposing electrode substrates to be the same; There is a problem. That is, when the tilt angle α is approximately 0° for both substrates, when the voltage is applied and the state changes from the cholesteric coloring state to the nematic transparent state (lighting up), the transition to the transparent state does not occur immediately and the scattering state changes. Due to this, the display contrast is poor. In addition, as shown in Figure 2, when the tilt angle α has a value other than 0° for both substrates, the radial direction of the liquid crystal composition, the rotation angle of the liquid crystal molecules between the two electrode substrates, and the direction of orientation control. Depending on the angle θ formed by
θ, the complexity of selecting the spiral direction arises. On the other hand, in the display device according to the present invention, the liquid crystal molecules are aligned almost parallel and at an angle of 2 degrees or less on the surface of one electrode plate, and aligned at an angle of 5 degrees or more on the surface of the other electrode plate. Since good contrast can always be presented regardless of the direction of rotation, it has the advantage that the orientation control direction and the design of the liquid crystal composition are facilitated. FIG. 3 shows the direction of alignment control according to the present invention, the angle θ that the direction makes between both substrates, and the tilt angle α of the liquid crystal molecules with respect to the substrate surface. The direction shown by the solid line in FIGS. 2 and 3 is on the front electrode substrate surface.
The direction of the arrow shown by the broken line indicates the direction in which the long axes of liquid crystal molecules are aligned on the surface of the back electrode substrate, and the direction in which the liquid crystal molecules are aligned at a tilt angle α with respect to the substrate. ing. The angle θ formed by the direction of orientation control in the present invention refers to the angle formed by the method of orientation control of the back substrate in a counterclockwise direction with respect to the direction of orientation control of the front substrate. That is, as shown in FIG. 2, the angular difference between the long axis directions of liquid crystal molecules centered on the point in contact with the substrates on both substrates, viewed counterclockwise from the front substrate, is defined as θ. In this case, for example, when both substrates are rubbed from the same direction, θ=0°. In addition, the rotation angle refers to the rotation angle of liquid crystal molecules from the alignment control direction of the back electrode substrate to the alignment control direction of the front electrode substrate, and this depends on the relationship between the pitch P of the liquid crystal and the liquid crystal layer thickness d and θ. It will be decided. The liquid crystal composition used in the present invention is a nematic liquid crystal with positive dielectric anisotropy to which a pleochroic dye and a substance giving a helical structure, such as a cholesteric liquid crystal and/or an optically active substance, are added. It is divided into right-handed and left-handed. A right-handed liquid crystal (hereinafter abbreviated as RH) refers to a liquid crystal in which the long axes of liquid crystal molecules are aligned by rotating counterclockwise from the back electrode substrate to the front electrode substrate. (abbreviated) refers to liquid crystals that rotate clockwise and align in the opposite direction. Examples will be described below, and the terms commonly used in the examples are as follows. Left-handed liquid crystal (LH): Nematic liquid crystal with positive dielectric anisotropy (manufactured by Merck & Co.)
ZLI-1285-TNC) and BDH dichroic dye D-
A liquid crystal that exhibits left-handed rotation by adding cholesteryl nonanoate to a liquid crystal containing 2wt% of 16. Right-handed liquid crystal (RH): Nematic liquid crystal with positive dielectric anisotropy (manufactured by Merck & Co., Ltd.)
ZLI-1285-TNC) and BDH dichroic dye D-
A liquid crystal exhibiting right-handed rotation, made by adding the optically active substance CB-15 manufactured by BDH to a liquid crystal containing 2 wt% of 16. Rubbed substrate: A substrate whose electrode substrate surface has been rubbed in a certain direction so that liquid crystal molecules are aligned at an inclination angle of 0 to 2 degrees or 7 to 8 degrees. Oblique evaporation treated substrate: A substrate on which silica is obliquely evaporated 85 degrees from the normal direction of the electrode substrate surface, and the liquid crystal molecules are oriented at 23 to 26 degrees.
When shifted by 60 degrees, the substrates are processed so that they are aligned at an inclination angle of 0 to 2 degrees. Examples and Comparative Examples In both cases, display cells were made using a rubbed substrate and an oblique evaporation treated substrate, and each cell was filled with RH and LH of different pitches to form a display device. Visual observation results of the display state when this display device was statically driven by applying a 100Hz AC rectangular wave between opposing electrodes, and when it was dynamically driven at 100Hz with a 1/2 bias and 1/2 duty waveform. are shown in Tables 1 to 4 in relation to θ. Table 1 shows comparative examples where the rubbing alignment process is performed so that the tilt angles α of the liquid crystal molecules on both substrates are both 1 to 3 degrees, and when θ is π/2 and 3/2π. , Tables 2 to 4 show that due to rubbing, α of one side board is 0 to 2 degrees, and α of the other side is 7 degrees.
When combining substrates that have been oriented so that the angle is ~8°, or by oblique evaporation of silica, the α of a single-sided substrate can be
An example is shown in which the substrates are oriented so that α is 0 to 2 degrees and the other α is 23 to 26 degrees. is θ=
1/4π, 5/4π, and Table 4 is for θ=π/2, 3/2π, and shows examples according to the present invention. In addition, in each table, the 〇 judgment indicates that the state immediately transitions to a transparent state when ON, and the case that immediately changes to a non-scattering colored state when OFF.
Indicates a case that sometimes becomes a scattering state, or a case where a scattering state or memory property appears for more than 1 second at room temperature when turned off.

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[Table] As shown in the above Examples and Comparative Examples, on the surface of one substrate, the liquid crystal molecules are arranged almost parallel (with an inclination of 2° or less), and with respect to the surface of the other substrate, the molecules are arranged with an inclination of 5° or more. In a display device in which a liquid crystal composition containing a pleochroic dye having a positive dielectric anisotropy and a helical structure is enclosed in a display cell whose orientation is controlled so as to be aligned in a certain direction, In the case of static drive, the conditions that can quickly transition to a transparent state without scattering and a colored state during both lighting and erasing are as follows:
The rotation angle of the liquid crystal molecular axis between the two electrodes was approximately less than 3π. Furthermore, the conditions under which dynamic driving can have good contrast are such that when is large, static driving is possible, and the ratio (d/P) between the pitch (P) of the liquid crystal and the thickness (d) of the liquid crystal layer is / It was found that this is possible only in the range of 2π≦d/P≦/2π+1/4. As described above, the liquid crystal display device according to the present invention has
Since display can be performed with good contrast regardless of θ and the latitude direction of the liquid crystal composition, it is easy to design the orientation control direction and the liquid crystal composition. Since orientation control technology using oblique evaporation of silica or the like can be applied as is, it has the advantage that TN and guest-host display devices can be produced simultaneously on the same production line. Furthermore, since the display device according to the present invention does not require a polarizing plate, it is low cost, has a wide viewing range, can provide a bright display, is free from scattering when turned on and off, has good contrast, and exhibits a fast response. A display device with excellent quality can be provided. or,
Furthermore, since dynamic driving is possible, its utility value is great, such as the ability to increase display functions.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view of a phase change type liquid crystal display device;
The figure shows a perspective view for showing a comparative example, and FIG. 3 shows a perspective view for explaining the present example. 1... Electrode substrate, 2... Electrode, 3... Arrangement state of liquid crystal, 4... Switch, 5... AC power supply.

Claims (1)

[Claims] 1. Orientation control so that on the surface of one electrode substrate, liquid crystal molecules are arranged almost parallel to the substrate surface, and on the surface of the other electrode substrate, they are aligned at an angle and in a fixed direction. Furthermore, two electrode substrates are arranged almost parallel to each other so that their orientation control directions form a constant angle θ, and a pleochroic dye and a nematic liquid crystal with positive dielectric anisotropy are placed between the electrode substrates. When sandwiching a liquid crystal composition to which a substance giving a helical structure is added, the rotation angle (ψ) of the liquid crystal molecules between the two electrode substrates is set to be less than 3π, and the ratio of the thickness d of the liquid crystal layer to the liquid crystal pitch p is , ψ/2π≦d/P≦ψ/2π+1/4, and is dynamically driven. 2. Claim 1, characterized in that the liquid crystal molecules are arranged at an inclination of 2 degrees or less on the surface of one electrode substrate, and arranged at an inclination of 5 degrees or more on the surface of the other electrode substrate. The liquid crystal display device described.