JP2568508B2 - Ferroelectric matrix liquid crystal display - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device using a liquid crystal exhibiting spontaneous polarization or an optical shutter, and more particularly to a matrix display device and an optical shutter.

2. Description of the Related Art A conventional technique will be described below with reference to the drawings.

Currently, the liquid crystal display device is TN (Twisted Nematic)
The type display method is most widely used. However, as compared with other displays (plasma display, electroluminescence display, etc.), the response speed is inferior in terms of response speed, and no significant improvement has been seen at present.

Recently, a display system using, for example, a ferroelectric liquid crystal as a liquid crystal exhibiting spontaneous polarization has been announced, and has attracted much attention because of its high response speed and the possibility of having a high matrix property.

 First, the ferroelectric liquid crystal will be described with reference to the drawings.

A ferroelectric liquid crystal refers to a liquid crystal that exhibits ferroelectricity.
In order to exhibit ferroelectricity from the theory of crystal symmetry, liquid crystal molecules must first have an asymmetric center, and must have a layered structure and molecules must be tilted in the layer . As a liquid crystal phase satisfying such symmetry, smectic C
A chiral phase, a smectic I chiral phase, a smectic G chiral phase, and the like have been discovered to date and have been recognized as ferroelectric liquid crystal phases. In the present specification, the properties will be described using a smectic C chiral phase having the highest symmetry among ferroelectric liquid crystal phases. FIG. 2 is a schematic view of a ferroelectric liquid crystal molecule. The ferroelectric liquid crystal is usually a liquid crystal having a layer structure called a smectic liquid crystal. The molecules are tilted by θ with respect to the vertical direction of the layer.

In addition, since it has a ferroelectric liquid crystal asymmetric center, it is composed of optically active liquid crystal molecules which are not racemic. As shown in FIG. 2, the ferroelectric liquid crystal molecule has a permanent dipole moment that causes spontaneous polarization in a direction perpendicular to the long axis of the molecule, and the chiral smectic C phase has a structure shown in FIG. It can move freely outside the cone (hereinafter called cone). Further, a vector lowered from the center point O of the cone with respect to the liquid crystal molecules is called a C director.

In FIG. 2, 21 is a liquid crystal molecule, 22 is a permanent pole, and 23 is C
Director, 24 is a cone, 25 is a layer structure, 26 is a layer normal direction, and 27 is a tilt angle θ.

Since ferroelectric liquid crystal molecules have an asymmetric atom,
It has a twist structure. This twist structure is shown in FIG. FIG. 3 shows that a twisted structure exists in the normal direction of the layer.

In FIG. 3, 31 is a liquid crystal molecule, 32 is a permanent dipole moment, 33 is a pitch (L) representing a period of twist, 34 is a layer structure, 35 is a normal direction of the layer, and 36 is a tilt angle θ.

Next, the operation principle of the ferroelectric liquid crystal will be described with reference to the drawings. When the cell thickness (d) of the ferroelectric liquid crystal panel is larger than the pitch (d> L), the influence of the cell substrate surface does not reach the center of the cell, and therefore, the ferroelectric liquid crystal panel usually has a twisted structure. However, when the cell thickness is smaller than the pitch (d>
L) The twisted structure is released by the force of the substrate surface.
There appear two regions (domains) in which the molecule is parallel to the substrate surface. In these two regions, the permanent dipole moments of the molecules are directed in opposite directions, one is directed from the back of the paper to the front, and the other is directed from the front to the back of the paper. This corresponds to the tilt angle of the molecule with respect to the layer normal.

At this time, when an electric field is applied from the back of the paper to the front, the permanent dipole moments are all directed to the direction of the electric field, and all molecules have a tilt angle of + θ as shown in FIG. 4 (b). In this state, the direction of the polarization axis of the polarizer (P) of the polarizing plate is blocked by the analyzer (A) in the major axis direction of the molecule, and a dark state is obtained. When the electric field is applied in the opposite direction, all the molecules have a slope of -θ as shown in FIG. 4 (c), and the linearly polarized light that has passed through the polarizer passes through the analyzer due to the birefringence effect, and a bright state is obtained.

In FIGS. 4 (a), (b) and (c), 41 is the direction of the electric field,
42 denotes a permanent dipole moment of the molecule, 43 denotes a layer structure, 44 denotes a tilt angle θ, and 45 denotes a polarization axis of the polarizer (P) and the analyzer (A).

As described above, light and dark states can be obtained by the positive and negative electric fields.

(Literature: Fukuda, Takezoe, Kondo: High-speed display using ferroelectric liquid crystal, Optronics, No. 9, p. 64, 1983) Also, cells with a smaller cell thickness (d <
In L), the molecule is stable as it is even after the electric field is removed because the twisted structure is usually unwound.
It is said that a so-called memory effect occurs.

When an electric field is applied to such a mono-domain cell, its response time (τ) is given by equation (1).

Here, E represents an electric field, Ps represents spontaneous polarization, and η represents viscosity.

 Next, a matrix display method will be described.

Here, the 1/3 bias method will be described with reference to the drawings as a matrix driving method.

FIG. 5 schematically shows an electrode structure of a matrix display system.

X represents an electrode (scanning electrode) scanned line-sequentially, and Y represents an electrode (signal electrode) to which a signal voltage is applied.

Next, FIG. 6 shows the voltages applied to the X and Y electrodes when the 1/3 bias driving is actually performed.

The points with rounding marks are selected points, and the rest are non-selected points. As shown in FIG. 6, +2 V is sequentially applied to the selected line and 0 V is sequentially applied to the non-selected line, and + V is not selected to the selected line on the signal electrode (Y electrode) side. -V is applied to the line. Then, a voltage of 3 V is applied to the selected point (indicated by a circle), and -V or + V is applied to all other non-selected points. As described above, in the matrix display device, although there is a slight difference depending on the driving method, the bias voltage is basically applied to the non-selected point.

Problems to be Solved by the Invention When a ferroelectric liquid crystal panel is driven in a matrix, a bias voltage is applied to a non-selected point as described above. Therefore, in order for the ferroelectric liquid crystal panel to maintain good display quality, it is necessary that brightness does not change at least even when a bias voltage is applied.

That is, -V, as described by the 1/3 bias method in FIG.
Or, at + V, the light quantity does not change and needs to change to 3V.

After all, in order to perform the matrix driving, a characteristic in which the amount of light does not change up to a certain voltage (hereinafter, referred to as a threshold characteristic) is required.

A conventional ferroelectric liquid crystal panel using a liquid crystal exhibiting spontaneous polarization, for example, has not been found to have excellent threshold characteristics and has a problem of poor matrix display quality.

Means for solving the problem In order to improve the threshold characteristics of the problem, the use of an internal electric field generated by unidirectionally arranged dipole moments by using a liquid crystal material having a large spontaneous polarization is described. Features.

Action By using a liquid crystal material exhibiting spontaneous polarization, an internal electric field is generated inside the cell due to the orientation of the dipole moment. Since the internal electric field opposes the reverse electric field applied from the outside, the liquid crystal panel has a function of exhibiting a distinctive threshold characteristic value.

Example As an example, the optical characteristics of a liquid crystal panel using a ferroelectric liquid crystal material having a large spontaneous polarization were measured. First, the cell configuration will be described. FIG. 7 is a schematic view of a ferroelectric liquid crystal panel by rubbing. Here, 71 is an upper and lower polarizing plate, 72 is an upper and lower glass substrate, 73 is a transparent electrode layer, 74
Is an organic polymer film layer subjected to an alignment treatment, 75 is a ferroelectric liquid crystal layer, 76 is a spacer for keeping a distance (cell thickness) between opposing substrates constant, and 77 is a rubbing direction. Thus, a ferroelectric liquid crystal was sealed between the opposing electrodes to produce a ferroelectric liquid crystal panel.

The orientation method is smectic by rubbing the organic polymer film provided on the glass substrate, injecting liquid crystal, heating the panel to 100 ° C to make it an isotropic liquid, and slowly cooling it slowly (0.6 ° C / min). A monodomain of the C chiral phase was obtained.

 Table 1 below shows the ferroelectric liquid crystal materials used.

As a ferroelectric liquid crystal having a large spontaneous polarization, a mixture (A) of a compound having an optically active 2-octanol group introduced into a chiral portion was used. Smectic C as another compound
A mixture (B) of an ester compound and a pyrimidine compound showing a phase was used.

A is 40wt% and 70wt% respectively as a composition of A and B mixture
%, 90% by weight of the composition.

Here, these compositions will be referred to as follows.

A composition of 40 wt% A-A-40 A composition of 70 wt% A-A-70 A composition of 90 wt% A-A-90 These compositions show spontaneous polarization in Table 2 below. . The measurement of spontaneous polarization was performed using the triangular wave method. The measurement was performed at 15 ° C. below the transition point from the smectic A phase to the smectic C chiral phase.

Next, a voltage-transmittance curve (hereinafter referred to as a BV curve) was measured using this ferroelectric liquid crystal material.

FIG. 8 shows an optical experimental system used for the measurement of the BV curve. In FIG. 8, white light emitted from a light source 81 passes through a polarizer 82 and is incident on a liquid crystal cell 83 as linearly polarized light. Then, the white light passes through an analyzer 84 and is condensed by a condensing lens 85 to be detected by a photomultiplier 86. , Measured by a storage oscilloscope 87 as a BV curve. An arbitrary waveform can be added to the liquid crystal cell by the programmable pulse generator 88.

In such an experimental system, the BV curve of the ferroelectric liquid crystal panel having the above-described configuration was measured. A ferroelectric liquid crystal panel having a cell thickness of 2.5 μm was used.

FIG. 9 shows a voltage waveform used for measuring the BV curve of the threshold characteristic. After a constant reset pulse 91 is applied in FIG. 9, a variable pulse 92 of opposite polarity for measuring the threshold characteristic is applied. This series of pulses is applied every certain time 93 (frame frequency). First, an example of the BV curve of the threshold characteristic obtained using the ferroelectric liquid crystal composition A-40 is shown in FIG. In FIG. 10, the horizontal axis is time (t), and the vertical axis is voltage (V) or luminance (B). The upper diagram is the applied voltage waveform, and the lower diagram is the corresponding luminance curve. As the variable pulse 101 for measuring the threshold value is gradually increased, in FIG. 10, the dark state caused by the reset pulse gradually becomes a bright state. FIGS. 11 (a) and 11 (b) show plots of the brightness at the point where the variable pulse is applied and the amount of light changes the most with respect to the voltage of the variable pulse. 11A and 11B, the horizontal axis represents voltage, and the vertical axis represents luminance. In FIGS. 11A and 11B, the reset pulse voltage is 25 V, the variable pulse voltage is 0 to 25 V, and the pulse width is 1 ms. FIG. 11 (a) shows that the polarity of the variable pulse is positive and dark to bright, and FIG. 11 (b) shows that the variable pulse has negative polarity and changes from bright to dark. The material used is A-40. Similarly, the ferroelectric liquid crystal composition A-
The results of measuring the threshold characteristics using the 70 and A-90
12 (a) and 12 (b) and FIGS. 13 (a) and 13 (b) respectively.

Table 3 below summarizes the threshold voltage (V _t ) and spontaneous polarization (Ps) of each composition from the preceding figure. Incidentally, the threshold voltage V _t is measured at a temperature under 15 ℃ than the spontaneous polarization as well as the transition point.

The characteristic of the threshold voltage was a voltage when the change in luminance was relatively 10%, and was an average value of the threshold when the polarity of the variable pulse was positive or negative.

From Table 3, the larger the spontaneous polarization, the larger the threshold voltage. 11 (a) and 11 (b), and FIG. 12 (a)
13 (b) and FIGS. 13 (a) and 13 (b), it was found that the larger the spontaneous polarization, the sharper the threshold characteristics, and the better the characteristics for matrix driving.

Next, a simple theoretical calculation was performed to explain the above. Hereinafter, description will be made with reference to the drawings.

FIG. 4 is a diagram schematically showing a state in which dipole moments are aligned in one direction (as shown in FIGS. 4B and 4C).
Figures 14 (a) and (b) show the results. FIGS. 14 (a) and 14 (b) are views as viewed from the cross-sectional direction of the cell. 14 (a) and 14 (b), 141 is an upper substrate, 142 is a lower substrate, 143 is a dipole moment, 144 is a ferroelectric liquid crystal molecule, 145 is a charge of a bipolar plate, 146
Represents an internal electric field.

Since the dipole moments are aligned in one direction, the internal electric field E _f is represented by the equation (1) from the balance of charges.

Q = Ps · S Q = C · V _f C = ε · ε ₀ · S / d E _f = V _f / d, E _f = Ps / ε · ε ₀ − (1) where Q is the charge and Ps spontaneous polarization, S is the electrode area, C is the capacitance, the V _f internal voltage, epsilon is the dielectric constant, epsilon ₀ is the vacuum dielectric constant, d is the cell thickness, the E _f represents the internal electric field.

The dielectric constant of the conventional ferroelectric liquid crystal and a plot of the internal electric field E _f with respect to the spontaneous polarization Ps as about 6 (epsilon) 14
Shown in the figure.

FIG. 14 shows that an internal electric field of about 6 V / μm occurs when the spontaneous polarization is 30 nC / cm ² .

FIG. 15 shows a comparison between the calculated values and the experimental values described above. No.
In FIG. 15, the horizontal axis represents the spontaneous polarization (Ps) and the vertical axis represents the internal electric field (E _f ). The solid straight line represents the calculated value (the relative dielectric constant was calculated as 6), and ● represents the experimental value. Internal electric field is observed (threshold has healed voltage V _t to split the field in the cell thickness measured) and the measured values from Figure 15 the calculated value is almost good agreement is considered to be acting.

From these results, it can be seen that as the spontaneous polarization increases, the internal electric field increases and the threshold characteristics become better. From FIG. 15, it is considered that if the threshold voltage shows a clear value at about 3 V or more in a cell having a cell thickness of about 2.0 μm, the internal electric field becomes clear at about 6 V / μm or more. In order to make the internal electric field 6 V / μm or more, the spontaneous polarization of the ferroelectric liquid crystal material needs to be about 30 nC / cm ² or more at a cell thickness of about 2.0 μm.

Although the above embodiment cited the ferroelectric liquid crystal, the essence of the present invention lies in the relationship between the internal voltage in the panel thickness direction due to the spontaneous polarization of the liquid crystal and the electric field during non-selection during matrix driving. The effect of the present invention can be obtained with any degree of liquid crystal having spontaneous polarization which affects the internal electric field in the panel thickness direction, and is not particularly limited to ferroelectric liquid crystal.

Effect of the Invention The present invention uses a liquid crystal material having a large spontaneous polarization to enhance the matrix property in a matrix liquid crystal display device.
It has been found that by increasing the internal electric field, the threshold characteristics required for matrix driving can be improved.

[Brief description of the drawings]

FIG. 1 is a graph in which measured and calculated values of the internal electric field of spontaneous polarization are plotted, FIG. 2 is a schematic diagram schematically showing the operation principle of the ferroelectric liquid crystal, and FIG. 3 is a twist of the ferroelectric liquid crystal. FIGS. 4 (a), 4 (b) and 4 (c) are schematic diagrams schematically showing the structure of the ferroelectric liquid crystal panel, and FIG. 5 is an electrode configuration of the matrix display panel. FIG. 6 is a schematic diagram showing the state of voltage application by the 1/3 bias driving method, FIG. 7 is a configuration diagram of a ferroelectric liquid crystal panel, and FIG. 8 measures optical characteristics. FIG. 9 is a waveform diagram showing a voltage waveform for measuring a threshold characteristic, FIG. 10 is a characteristic diagram showing an example of measuring an actual threshold characteristic, FIGS. 11 (a) and (b) show A
Characteristic diagram summarizing the threshold characteristics of −40, FIG. 12 (a)
(B) is a characteristic diagram summarizing the threshold characteristics of A-70, FIG.
14A and 14B are characteristic diagrams summarizing the threshold characteristics of A-90, FIG. 14 is a schematic diagram showing an internal electric field when spontaneous polarization is uniformly arranged, and FIG. It is the graph which plotted the calculation value of the internal electric field. 71 ... Top and bottom polarizing plates, 72 ... Top and bottom glass substrates, 73 ...
Transparent electrode layer, 74 ... Oriented organic polymer film layer,
75 indicates a ferroelectric liquid crystal layer, 76 indicates a spacer, 77 indicates a rubbing direction.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Naohide Wakita 1006 Kadoma, Kadoma City Matsushita Electric Industrial Co., Ltd. JP-A-60-156046 (JP, A)

Claims (2)

(57) [Claims] 1. A first electrode group in which a plurality of strip-shaped electrodes are provided on a first substrate in parallel with each other, and a plurality of strip-shaped electrodes on a second substrate are opposed to each other in parallel with each other. And a liquid crystal exhibiting spontaneous polarization is sandwiched between a second electrode group arranged at a predetermined gap so as to be orthogonal to the first electrode group. In a matrix liquid crystal display panel, a line-sequential driving method is performed in which a scanning pulse is sequentially applied to the first electrode group, and a light or dark signal voltage is almost simultaneously applied to the second electrode group in parallel. A matrix liquid crystal display device, wherein the internal electric field in the thickness direction of the panel due to the spontaneous polarization of the liquid crystal is larger than the electric field when the matrix is not selected during the matrix driving. 2. The matrix liquid crystal display device according to claim 1, wherein an internal electric field of the liquid crystal display panel is 6 V / μm or more.