JPH0545929B2 - - Google Patents

Description

[Detailed Description of the Invention] [Summary] The present invention solves the conventional problems of slow response speed, narrow viewing angle, and low contrast ratio in large-capacity liquid crystal display elements that can be refresh driven. The helical structure of the cholesteric phase liquid crystal is eliminated by the liquid crystal molecule orientation regulating force of the liquid crystal alignment film on the surface of the substrate that constitutes the display element, and the liquid crystal molecules are aligned in a uniform form with the strong liquid crystal molecule alignment regulating force, and an electric field is applied. By slightly changing the orientation of liquid crystal molecules that have optical rotation, the angle of optical rotation of incident light is changed, and the amount of transmitted light is greatly changed, achieving large-capacity display, high-speed response, wide viewing angle, and high contrast ratio. It is something.

[Industrial application field]

The present invention relates to a liquid crystal display element, and more particularly to a liquid crystal display element for realizing high-speed response, wide viewing angle, and high contrast of a large-capacity liquid crystal display element using refresh drive.

[Prior art and problems to be solved by the invention]

Liquid crystal displays have been developed for a wide variety of uses, from calculators and wristwatches to handheld computers, word processors, and portable televisions. Under these circumstances, liquid crystal display elements have recently been particularly sought after to play a role as a display device in place of the conventional CRT (cathode ray tube). CRTs are light-emitting displays that cause eye strain for workers, and because they are deep, the overall depth of the device increases, and there are design constraints. Panel displays are in high demand. Although liquid crystal displays have begun to be used in some areas as the closest device to this purpose, they currently have not yet caught up with CRTs in terms of functionality. In particular, the display speed is slow at several hundred milliseconds per screen, so it is virtually unusable for video display. In addition, it is possible to display a pseudo-video by reducing the number of rewrites per unit time, but this causes problems such as narrowing the viewing angle and reducing the contrast ratio. There is a strong demand for liquid crystal display elements capable of wide viewing angles and high contrast ratios.

By the way, conventional refresh driven liquid crystal displays include the twisted nematic (TN) type and the super twisted birefringence effect (SBE) type, which is a modification of the TN. In both display modes, as shown in FIG. 4, the liquid crystal molecules are aligned in parallel, and the liquid crystal molecules are continuously twisted from 90 degrees to 270 degrees from top to bottom by an alignment process applied to the upper and lower bases.
This panel is placed between two polarizing films, and display is performed by changing the arrangement of liquid crystal molecules by turning on and off an electric field.

By the way, TN and SBE, which are used in some word processors, handheld computers, etc., are refresh driven and are capable of displaying a certain amount of large capacity. For this reason, these are used as character displays. TN type and
As shown in Figure 4, the SBE type has liquid crystal molecules oriented horizontally to the substrate and twisted from the upper substrate to the lower substrate, and the entire liquid crystal layer has a birefringence effect when light enters the substrate from a direction perpendicular to the substrate. have
It switches the direction of vibration of light. In this way, conventional types of TN and SBE perform display by rotating the plane of vibration of light due to continuous direction changes of a group of liquid crystal molecules, so when a driving electric field is applied, interactions between liquid crystal molecules occur. It takes several tens of milliseconds to several hundred milliseconds for individual liquid crystal molecules to overcome the problem and change direction from horizontal alignment to vertical alignment. In particular, in an SBE capable of displaying a large capacity, the ratio V _S /V NS of the effective voltage V _S applied to a selected point and the effective voltage V _NS applied to a non-selected point can be kept small in time-division drive using matrix _electrodes , i.e. Since the light transmittance rises steeply with respect to the applied voltage, the applied voltage ratio during switching between the selected point and the non-selected point is small, and as a result, the force for changing the direction of the liquid crystal molecules becomes weak and the response becomes slow. This is because the direction of the liquid crystal molecules changes continuously from the upper substrate to the lower substrate, so there is a continuous gradient in the force that orients the liquid crystal molecules along the thickness direction of the liquid crystal layer.As a result, when an electric field is applied, the alignment force This is to gradually change direction from the weak point. Furthermore, SBE, which has a slow response time but is capable of displaying a large capacity with a relatively high contrast ratio, has the disadvantage that it is virtually impossible to perform color display using a color filter method because the background of the display is colored yellow or dark blue.

In view of this situation, display elements using ferroelectric liquid crystals have been proposed to solve the above problems. In other words, the ferroelectric liquid crystal display (FLC) reported by Noel, A.Clark et al.
(Appl. Phys Lett., 36 899 (1980)) It is characterized by aligning chiral smect C or smect H phase liquid crystal parallel to the substrate. By arranging the chiral smect C and smect H liquid crystals in a uniform manner parallel to the substrate, the dipole moments of the individual liquid crystal molecules are aligned upwardly or downwardly toward the substrate, resulting in spontaneous polarization. FLC is a method of switching light by reversing the direction of this spontaneous polarization relative to the substrate upwards and downwards. FLC uses the spontaneous polarization of the liquid crystal as its driving source, so it can respond quickly due to the strong coupling between the external electric field and the spontaneous polarization.Once the spontaneous polarization is reversed, it has a memory function that maintains the previous state unless an electric field of the opposite polarity is applied again. have.

However, since FLC uses chiral smectate C and smectate H liquid crystals that have a layered structure, it is difficult to align individual molecules parallel to the substrate without destroying the layered structure, and even if the desired orientation is not achieved. Even if they were obtained, they had the disadvantage that the layered structure was thermally unstable and easily damaged by heat cycles, mechanical shocks, etc.

[Means for solving the problem, action, and effects of the invention]

The present invention solves all the problems explained above,
We provide a liquid crystal display that has a fast response speed, wide viewing angle, and enables large-capacity display.This liquid crystal display has a helical structure of cholesteric phase liquid crystal with optical rotation due to the strong alignment regulating force of the substrate interface. The long axes of the individual liquid crystal molecules are parallel to each other, at least one of them is oriented in the same direction with respect to the surface of the transparent electrode-attached substrate, and the long axes of the molecules are uniformly parallel, perpendicular, or perpendicular to the surface of the substrate. It is characterized by an inclined arrangement.

Here, the term "long axis of the molecule" means the axis in the longitudinal direction of rod-like liquid crystal molecules.

That is, in the present invention, liquid crystal exhibiting a cholesteric phase is sealed in a narrow cell of a panel gap, and the liquid crystal molecules are controlled by a liquid crystal molecule alignment film that acts almost uniformly from the upper substrate to the lower substrate of the panel. By oriented in a specific direction (for example, parallel to, perpendicular to, or inclined to the substrate) and eliminating the helical structure of the cholesteric phase, large-capacity display and high-speed display are simultaneously possible. In other words, the essence of the liquid crystal display device of the present invention is to forcibly align the cholesteric phase liquid crystal, which originally has a helical structure, in a uniform orientation using an alignment regulating force. This oriented liquid crystal is shown below.
It is called PAC (Parallel Aligned Chiralnematic).

In the present invention, cholesteric phase liquid crystals include cholesteric liquid crystals (steroidal liquid crystals), chiral nematic liquid crystals, nematic liquid crystals, and mixed liquid crystals of cholesteric liquid crystals or chiral nematic liquid crystals (the composition ratio of cholesteric liquid crystals and chiral nematic liquid crystals is the same as that of the entire mixed composition). 3% by weight of
above), or nematic liquid crystals and optically active substances with sufficient compatibility with liquid crystals (for example, l
- menthol or l-adrenaline) (mixing ratio of optically active substance is 3% by weight or more of the entire mixture). Further, the cholesteric phase liquid crystal exhibits a helical pitch size of 1.5 μm or less at room temperature and when no electric field is applied. This is because a liquid crystal with a long helical pitch does not have a sufficiently large optical rotation even if it is sealed in a panel and the spiral is unwound using a strong alignment control force, resulting in insufficient light switching. In general, in a state in which the helix is resolved, the shorter the helical pitch, the higher the optical rotation power, so in order to display effectively, the pitch length needs to be 1.5 μm or less.

In addition, the thickness of the layer (anchoring layer) of the cholesteric phase liquid crystal whose alignment is regulated by interfacial interaction with the liquid crystal alignment film on the substrate surface must be at least 1/3 of the thickness of the entire liquid crystal layer. do.

PAC is unique in that the liquid crystals are arranged uniformly from the top board to the bottom board. That is, since the liquid crystals are uniformly arranged, the liquid crystals in any part respond uniformly to the applied electric field. At this time, if the alignment regulating force for uniformly aligning the liquid crystal is weak, that is, if the thickness of the anchoring layer is thin, the liquid crystal molecules will respond immediately to the applied electric field. -Light transmittance) is lost, making large-capacity display by refresh drive impossible. Therefore, in order to steepen the threshold value and perform large-capacity display, it is necessary to make the anchoring layer as thick as possible to control the movement of liquid crystal molecules in response to the applied electric field. For this purpose, the thickness of the anchoring layer must be 1/1/2 of the entire liquid crystal layer.
Must be 3 or more. This is because if it is 1/3, liquid crystals other than the anchoring layer are also strongly regulated due to intermolecular interactions between liquid crystal molecules. However, it goes without saying that the thicker the anchoring layer, the more effective it is.

FIG. 1 shows an example of a liquid crystal display element of the present invention. A transparent electrode 2 is provided on a glass substrate 1, and an alignment film 3 is attached to the surface of this transparent electrode. This alignment film is formed by applying an organic polymer to the electrodes or depositing an inorganic compound onto the electrodes. A cholesteric phase liquid crystal 5 is sealed in the cell thus constructed to form a panel. Further, during implementation, a polarizing film 4 is attached to the panel.

When an electric field is applied to drive a liquid crystal display element configured in this way, due to the narrow panel gap, the molecular alignment regulating force (anchoring) by the liquid crystal molecular alignment film is applied from the upper substrate to the lower substrate as described above. By working together, all the liquid crystal molecules are aligned in a uniform manner parallel to, perpendicular to, or inclined (slightly inclined in FIG. 1) to the substrate.

When the cholesteric phase liquid crystal whose spiral structure has been eliminated by the alignment regulating force is aligned parallel to, perpendicular to, or obliquely to the substrate to form a uniform state in which the individual liquid crystal molecules are oriented parallel to each other, the liquid crystal molecules are aligned by the alignment regulating force. Fixed with strong force.

When a panel in such a state is placed between two polarizing plates 4 having cross nicols or a cross angle close to crossed nicols, the entire liquid crystal layer is formed by a chiral nematic liquid crystal (cholesteric liquid crystal) having a uniformly arranged optical rotation power. Shows birefringence. As a result, the incident light whose vibration plane is aligned by the polarizing plate is optically rotated by the liquid crystal layer, matches the polarization plane of the second polarizing plate, and is transmitted through this, resulting in a bright state. This state is shown in FIG. 2a. In FIG. 2, 2 indicates a glass substrate. The structure of this glass substrate is the same as that in FIG. 1, and includes a transparent electrode and an alignment film.

On the other hand, when an electric field is applied to a uniformly oriented liquid crystal, the electric field and the induced polarization induced in the liquid crystal are coupled to produce a torque that attempts to rotate the liquid crystal molecules. In conventional TN and SBE, this torque is much stronger than the alignment regulating force that acts at the center of the panel, so the liquid crystal molecules gradually change their orientation from the center toward the substrate interface. However, in PAC, the alignment regulating force is strong at both the center of the panel and the interface of the substrate, so...
An applied voltage of about 10 V is not enough to change the orientation of the molecules (transversely oriented molecules become vertically oriented). However, in PAC, unlike TN and SBE, the liquid crystal molecules themselves have optical rotation ability, so by slightly shifting the liquid crystal molecules from their equilibrium position in the absence of an electric field with a voltage of ~10V, the entire liquid crystal layer undergoes optical rotation. The power changes and becomes a dark state (FIG. 2b), making light switching possible under crossed Nicol conditions. Like this PAC
In this method, light switching is possible by slightly driving liquid crystal molecules with optical rotation power supported by a strong alignment regulating force using a relatively weak electric field, resulting in extremely high speed, high contrast ratio, wide viewing angle, and steep rise. Excellent liquid crystal display becomes possible.

The reasons for the high-speed switching are, firstly, that the degree of change in the orientation of liquid crystal molecules is spatially small, and secondly, the entire liquid crystal layer changes the orientation of individual liquid crystal molecules at the same time.

The reason for the high contrast ratio is that the liquid crystal
This is because unlike TN and SBE, it has a uniform arrangement, which allows for highly efficient optical switching.

In addition, unlike TN and SBE, PAC has a wide viewing angle because the individual liquid crystal molecules that form the liquid crystal layer are in a uniform state, so there is no dependence on the incident angle.
This is because it is smaller than TN and SBE.

In addition, the reason why the steepness is good is that the strong alignment regulating force acts almost uniformly on the entire liquid crystal layer.
This is because the entire liquid crystal molecules move only when the applied electric field exceeds a certain threshold.

Furthermore, as a result, unlike SBE, PAC can display without coloring the display background, making it possible to display full color in combination with a conventional mosaic color filter.

Hereinafter, the present invention will be further explained by examples.
It goes without saying that the present invention is not limited to this example.

〔Example〕

After cleaning and drying the glass substrate with a transparent conductive film (ITO), magnesium fluoride (MgF ₂ ) was deposited on the glass substrate using an ion plating apparatus at a plasma output of 70W. Rub this,
Two substrates were arranged so that the rubbing directions were parallel to each other, and a liquid crystal panel was constructed using alumina (Al ₂ O ₃ ) having a particle size of 0.7 μm as a spacer. on this panel
Nematic mixed liquid crystal Nr2801 manufactured by ROCHE and chiral nematic liquid crystal CB-15 manufactured by BDH.
A cholesteric phase liquid crystal with a weight composition ratio of 80:20 was encapsulated. The spiral pitch of this mixed liquid crystal is
It was 0.7 μm at 25°C. The thickness of the liquid crystal layer (panel gap) was actually measured to be 1.5 μm.
Furthermore, we measured the thickness of the anchoring layer of the alignment film (thickness of the strong alignment regulating force and liquid crystal layer) of the alignment film in this liquid crystal panel using a polarimeter while changing the panel gap. Both were found to be 0.6 μm or 1.2 μm, respectively. The method for measuring the anchoring layer according to this method is reported in detail in a method for measuring the layer thickness of a liquid crystal anchoring layer invented by the present inventors (Japanese Patent Laid-Open No. 169007/1986).

After filling the liquid crystal, set the panel on a polarizing microscope set to crossed nicols, and use a pulse generator to
Apply a 1KHz square wave (±) with a 6V offset (-6V bias voltage applied, 1KHz
± square wave applied), response time to applied voltage (rise τ _r and fall τ _d , τ _r : light transmittance 10−100
The time required for % change (τ _d : time required for light transmittance of 90-0%) was measured. The results are shown in Figure 3. As is clear from this figure, it was confirmed that both τ _r and τ _d showed a response of 180 μS at a drive voltage of about ±6 V. This is due to the conventional refresh drive.
Compared to the single screen response time of TN and SBE (for example, a 640 x 400 pixel panel), which is several hundred milliseconds, approximately
That's 1000 times faster.

In addition, when we measured the rise of the light transmittance with respect to the applied voltage, the so-called rise steepness, we found that TN,
When performing refresh drive using the voltage averaging method similar to SBE, the duty ratio that can achieve a contrast ratio of 4:1 or higher (transmitted light amount ratio) is less than 1/300, and the number of scanning lines of 300 or more can be set for OA It was found that this technology can accommodate the large capacity required for flat panel displays.

Contrast ratio is highly dependent on bias voltage. The maximum contrast ratio was obtained when the bias voltage was +5 V and was 15:1 (transmitted light amount ratio). Also, when no bias voltage is applied, approximately 7:
It was 1.

In addition, when we measured the dependence of the contrast ratio on the incident angle, we found that the incident angle (the angle between the substrate normal and the incident light) at which the contrast ratio was 4:1 or more was 25 degrees (50 degrees on both sides of the normal). I found out that it can be made wider.

〔Effect of the invention〕

As explained above, the present invention uses a cholesteric phase liquid crystal and is configured so that the liquid crystal molecules are aligned in a uniform manner by the liquid crystal molecule alignment regulating force at the substrate interface. Here, the orientation direction is not limited to parallel orientation as described above, but may be either vertical orientation or oblique orientation.

By configuring the present invention in this way, the large display capacity required for OA flat panel displays is at least several hundred times larger than that of conventional liquid crystals, even though it uses the same drive circuits, ICs, etc. as conventional TN and SBE. This makes it possible to display large-capacity video, which was previously impossible, with a high contrast ratio and wide viewing angle.

Furthermore, the PAC of the present invention uses cholesteric phase liquid crystal. Therefore, there is no need to maintain the layer structure as in ferroelectric liquid crystal displays (FLC).
Therefore, the alignment of the liquid crystal can be easily obtained and the alignment state is thermally stable, resulting in high driving stability.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram showing the panel configuration of the PAC, Figures 2a and 2b are diagrams each explaining the driving principle of the PAC, and Figure 3 shows actual measurement data of the response time of the PAC. is a graph, and the fourth
The figure is an explanatory diagram showing the driving principle of a conventional TN. DESCRIPTION OF SYMBOLS 1...Glass substrate, 2...Transparent electrode, 3...Alignment film, 4...Polarizing plate, 5...Liquid crystal molecules.

Claims (1)

[Claims] 1. The strong alignment regulating force at the substrate interface eliminates the helical structure of the cholesteric phase liquid crystal with optical rotation, so that the long axes of the individual liquid crystal molecules are parallel to each other, and at least one of them is transparent. 1. A liquid crystal display element, characterized in that the long axes of molecules face the same direction with respect to the surface of a substrate with electrodes and are arranged uniformly parallel to, perpendicular to, or inclined to the surface of the substrate. 2. The liquid crystal display element according to claim 1, wherein the cholesteric phase liquid crystal has a helical pitch of 1.5 μm or less at room temperature and when no electric field is applied.