JPS6250735A - Liquid crystal display device - Google Patents

Abstract

PURPOSE:To realize a ferroelectric liquid crystal panel which is bright even in an area of thick cell thickness and has a small irregularity in color while reducing birefringence effect by forming a liquid crystal layer of ferrodielectric liquid crystal and forming a twisted structure between upper and lower opposite substrates. CONSTITUTION:The ferrodielectric liquid crystal panel consists of upper and lower polarizing plates 29, glass substrates 30, transparent electrodes 32, ferrodielectric liquid crystal 33, and spacers 34 for making the distance between the opposite substrates constant. Then, the ferrodielectric liquid crystal 33 uses a chiral smetic C phase, chiral smetic I phase, or chiral smetic J phase. The liquid crystal layer is oriented in a constant direction to the substrates 30 by the orientation control films 32 of the opposite substrates 30 to have the twisted structure. The orientation treatment of the orientation control films 32 is carried out by rubbing and the twist angle of the liquid crystal layer is 70-100 deg..

Description

[Detailed description of the invention] Industrial applications The present invention relates to a ferroelectric liquid crystal display device.

2. Description of the Related Art Liquid crystal display devices are widely used because of their light weight, thinness, and low power consumption. However, the normally used nematic liquid crystal has a problem of slow response speed.

Therefore, recently, ferroelectric liquid crystals with fast response speed have been attracting attention.

The conventional ferroelectric liquid crystal described above will be described below with reference to the drawings.

FIG. 2 is a schematic diagram of ferroelectric liquid crystal molecules. Ferroelectric liquid crystals are usually called smectic liquid crystals and have a layered structure. The molecules have a structure tilted by θ with respect to the perpendicular direction of the layer.

Further, ferroelectric liquid crystals are usually composed of optically active liquid crystal molecules that are not racemic.

As shown in Figure 2, ferroelectric liquid crystal molecules have a permanent dipole moment that is spontaneously polarized in the direction perpendicular to the long axis of the molecule, and in the chiral smectic C phase, the conical shape (hereinafter referred to as can move freely outside the cone (called a cone). Also, the vector drawn from the center point 0 of the cone toward the liquid crystal molecules is called a C director.

In the chiral smectic C phase, this C director can move freely outside the cone.

In Figure 2, 5 is a liquid crystal molecule, 6 is a permanent dipole, and 7 is C
8 is a cone, 9 is a layer structure, 10 is a layer normal direction, and 1) is an inclination angle θ.

Next, the operating principle of a ferroelectric liquid crystal will be explained using figures.

Figures 3+al, (b) and (C) show the operating principle of a conventional ferroelectric liquid crystal.

FIG. 3 (a+ is a schematic representation of the orientation of a ferroelectric liquid crystal monodomain cell in a state where no voltage is applied.

In FIG. 3(a), the liquid crystal layer 12 is aligned in a certain direction, but since the spontaneous polarization 13 is free to face in either direction, the molecules 14 are aligned at +θ or aligned with respect to the perpendicular direction 15 of the layer. It is stable no matter which direction you turn. FIG. 3 (bl) is a schematic representation of the state of liquid crystal molecules when an electric field 16 is applied from top to bottom perpendicular to the plane of the paper.

At this time, the spontaneous polarization is aligned in the direction of the electric field, and the molecules are tilted in all +θ force directions. Therefore, the linearly polarized light that has passed through the polarizer is not affected by the birefringence effect and continues as linearly polarized light without changing its direction until it reaches the upper analyzer 1.
Blocked by 8.

As a result, a dark state can be obtained at the peak in Figure 3).

Figure 3 (C1 is a schematic representation of the state of liquid crystal molecules when an electric field 19 is applied from bottom to top perpendicular to the paper surface.At this time, the molecules move in one θ direction, contrary to Figure 3(b). Therefore, the polarization axis of the polarizer and the molecular axis are shifted by 2θ.As a result, due to the birefringence effect, the incident polarized light generally becomes elliptically polarized light, which can be transmitted through the analyzer below. .

The transmitted light intensity I at this time is given by the following equation.

1 = It sin"4θ X5in" (πΔnd/λ) - (1) where Δn
is the anisotropy of the refractive index of the ferroelectric liquid crystal, 1 is the incident light intensity after passing through the polarizer, λ is the wavelength, θ is the tilt angle of the liquid crystal molecules,
d represents the thickness of the liquid crystal layer. This display method is generally called birefringence mode.

When this second is expressed as luminance (Y value), which is the amount of brightness perceived by the human eye, the following equation is obtained.

Y=1/Kolx [S (λ)・I(λ)・
y(λ)ldλ□(2) However, S(λ): Spectral distribution of light source ■ (λ) - Spectral distribution of ferroelectric liquid crystal panel y(λ):
Visibility curve integral range: 380 nm to 700 nm This Y value is An
FIG. 4 shows a plot of d (phase difference) using a calculation formula. At this time, the tilt angle θ was set to 0 = 22.5' to obtain the brightest state.

From FIG. 4, it can be seen that the Y value changes greatly depending on And (phase difference). Further, FIG. 5 shows a plot of the color difference ΔE* (representing color unevenness due to cell thickness unevenness) due to cell thickness unevenness at this time versus And. In FIG. 5, the cell thickness unevenness was calculated assuming 0.2 μm. The color difference was calculated using the CI ELAB uniform color difference space using the following equation.

ΔE*=[(ΔL*)2+(Δa*)2+(Δb*
) t ] l / 2 − (3) However, L * = 1) 6 (Y/ Yo )”' 16a
*-500 [(X/Xo)I/"(Y/Yo)"'] b * = 200 [(Y/ Yo)""-(Z/
Zo)"'] Here, X., Yo, and Zo are the tristimulus values of the reference white surface, and x, y, and z indicate the tristimulus values of the colorimetric object.ΔL*
, Δa*, Δb* are (, *,
It shows the difference between a* and b*, and here, a certain cell thickness (d) and a cell 0.2 μm thicker than that (d+0.2 μm
). From FIGS. 4 and 5, the brightest Δnd (phase difference) with the smallest color difference (color unevenness) is approximately 0.
It can be seen that the diameter is 28 μm. It can be seen that since the birefringence anisotropy (Δn) of ferroelectric liquid crystal is usually 0.13 to 0.18, the cell thickness needs to be very thin, about 1.5 to 2.2 μm. 'Next, the general brightness-voltage curve (hereinafter referred to as
BV curve) is shown in FIG. In FIG. 6, the horizontal axis is time (1), and the vertical axis is voltage (■) or brightness (B). The upper figure is the applied voltage waveform, and the lower figure is the corresponding brightness curve. Fig. 6 will be explained in accordance with the order of the voltage waveforms. First, when a positive voltage is applied, the molecules align in one direction and a bright state is obtained. Next, no voltage is applied (0
In the case of (2), the brightness does not change due to the memory effect and remains in the bright state. Next, when a negative voltage is applied, the molecules align in the opposite direction to when a positive voltage is applied, resulting in a dark state. From this figure, it can be seen that brightness and darkness are given by the polarity of the voltage.

This can also be understood from the principle of ferroelectric liquid crystals.

References (Atsuo Fukuda, Hideo Takezoe 1. Katsumi Kondo: High-speed display using ferroelectric liquid crystal, Optronics, 9
Issue, p. 64, 1983) Problems to be Solved by the Invention However, in the display panel with the above structure, in order to prevent brightness or color unevenness, cell manufacturing requires a very thin and uniform cell thickness of about 2 μm. I had the above problem.

In addition, the brightness of the display panel is determined by the voltage polarity, so if you continue to apply voltages of the same polarity to maintain the bright and dark states, the DC voltage component will darken and the life of the liquid crystal will be shortened. There was a problem in that a voltage (reset pulse) had to be applied separately from the display.

Means for Solving the Problems In order to solve the above problems, the liquid crystal display device of the present invention has a configuration in which ferroelectric liquid crystal has a twisted structure between opposing substrates.

Function The present invention provides a ferroelectric liquid crystal panel that is bright and has less color unevenness by reducing the birefringence effect due to the twisted structure of the ferroelectric liquid crystal.

Hereinafter, the present invention will be explained in detail by taking the chiral smectic C phase as an example.

In the chiral smectic C phase, molecules can move freely around the cone as shown in FIG. Therefore, by performing appropriate substrate alignment processing, a twisted structure can be created between the substrates.

Figure 1 shows this twisted structure.

In FIG. 1, numeral 1 represents ferroelectric liquid crystal molecules, and numeral 2 represents a counter substrate. From this, it can be seen that the ferroelectric liquid crystal molecules are twisted while going around the outside of the cone, and the orientations of molecules 3 on the upper substrate and molecules 4 on the lower substrate are shifted by 2θ (θ is the tilt angle).

When the polarizer has such a twisted structure and the polarization axes of the polarizer and analyzer are respectively arranged parallel to the molecules of the upper and lower substrates, the transmittance is expressed by the following equation.

1=I+sin” [θo sin” (1+ u”)
""]/ (1+ u ") - (4) However, U-πΔn, d/θ.λ Here, θ. is the twist angle (i.e. 2θ), and Δn0 is the tilt angle of the liquid crystal molecules when turning around the outside of the cone. The apparent birefringence anisotropy is calculated when considering In addition, ΔE*, which is color unevenness due to cell thickness unevenness, was changed to Δno
Fig. 8 shows a plot against d. At this time, the cell thickness unevenness was set to 0.2 μm. The calculation method is formula (3)
is the same as From FIG. 7, Δno d is around 0.5 μm.

It can be seen that there is a bright region above μm. Furthermore, it can be seen from FIG. 8 that the color difference ΔE* is also small in these areas. The appropriate cell thickness for a ferroelectric liquid crystal with Δn of about 0.15 is approximately 3.6 μm, which is 6.6 μm or more, which is thicker than the conventional birefringence method. be able to.

Next, the display principle of the present invention will be explained using figures.

FIG. 9fa) shows the molecular state of a ferroelectric liquid crystal panel having a twisted structure in a state where no voltage is applied. The ferroelectric liquid crystal molecules 20 are uniformly twisted by 20 along the cone 21 between the upper and lower substrates. Here, for simplicity, the twist angle (2θ) is assumed to be 90 degrees. At this time, the polarization axis 22 of the polarizer is arranged parallel to the liquid crystal molecules of the lower substrate 23. Then, the linearly polarized light that has passed through the polarizer will propagate as linearly polarized light along the twist if the phase difference (Δno d) is large to some extent. (This area is called the wave guide regime.)
The linearly polarized light twisted by 2θ is emitted from the upper substrate 24 along the liquid crystal molecules. At this time, the polarization axis 25 of the analyzer
When placed parallel to the liquid crystal molecules of the upper substrate, the linearly polarized light is emitted without being blocked and a bright state can be obtained. Here, the polarizer and analyzer have an angle of torsion angle (2θ).

FIG. 9 (bl represents the state of molecules when an electric field 26 is applied from the upper substrate to the lower substrate.

Since the ferroelectric liquid crystal molecules have spontaneous polarizable permanent dipoles 27, they tend to align in the direction of the electric field.

Therefore, the twist between the upper and lower substrates is unraveled, and the molecules align in one direction as shown in Figure 9 (bl).At this time, the linearly polarized light that has passed through the polarizer 23 enters the molecules in parallel and is not subjected to birefringence. Keep going without changing direction.

Therefore, the polarization axis of the analyzer on the output side is the torsion angle (2θ=
If the beam is shifted by 90') and the angle is 90 degrees, it becomes a crossed nicol and is blocked, resulting in a dark state.

FIG. 9(C) shows the state of molecules when an electric field 28 is applied from the bottom to the top of the upper and lower substrates. In this case as well, the dipoles tend to align in the direction of the electric field, and as shown in Fig. 9 (b
They will be uniformly aligned in the opposite direction to l. In this case, the linearly polarized light that has passed through the polarizer is incident on the liquid crystal molecules at right angles, and as before, it does not undergo birefringence and travels as it is without changing its direction. Therefore, it is blocked by the analyzer and a dark state is obtained.

In this way, since the brightness and darkness of the display does not depend on the polarity of the voltage, there is no need to provide a voltage (reset pulse) that compensates for the DC voltage component separately from the display.

Example Embodiments of the present invention will be described with reference to the drawings.

First, FIG. 10 shows the structure of the ferroelectric liquid crystal panel used in this example. Here, 29 is an upper and lower polarizing plate, 30 is an upper and lower glass substrate, 31 is a transparent electrode, 32 is an organic polymer film subjected to alignment treatment, 33 is a ferroelectric liquid crystal phase, and 34 is a distance between opposing substrates (cell It represents a spacer to keep the thickness constant. In this way, a ferroelectric liquid crystal panel was created by sealing ferroelectric liquid crystal between the opposing electrodes.

The ferroelectric liquid crystal material used in the experiment was an ester-based liquid crystal material exhibiting ferroelectricity in the temperature range from O'C to 58C. The phase transition temperature of the ferroelectric liquid crystal used below is shown.

Cr −m−→S m C* −→Ch −−1→I
s.

～0℃ 58℃ 88℃ Nicode, Cr
: Crystalline phase SmC * : Smectic C Chiral phase Ch : Cholesterinochastic phase Iso : Isotropic liquid Also, the birefringence anisotropy (Δn) of this liquid crystal was 0.14 when measured using a Senarmont type compensator. Ta.

The orientation method involves rubbing the organic polymer film provided on the glass substrate, heating the panel to 100°C after injecting the liquid crystal to make it an isotropic liquid, and then slowly cooling it (0.6°C/m1).
n), a smectic C chiral phase monodomain was obtained.

Next, the directions of the rubbing axis of the ferroelectric liquid crystal panel of this example and the polarization axis of the polarizing plate will be explained using figures. 1st)
In the figure, the alignment treatment is performed by rubbing axis (rl) 35 applied to the upper substrate and rubbing axis (rz
) 36 is made to form an angle twice (2θ) as the rotation angle. This was done to make the twisted structure more stable. The direction of the polarization axis is the upper polarization axis. (p
, ) 37 and the lower polarization axis (pg) 38 were set to be crossed Nicols. For this reason, the deviation angle [α=(90°−θ)/2] between the rubbing axis and the polarization axis was set to be the same on the upper and lower sides, respectively. Since the ferroelectric liquid crystal material used in this example had a tilt angle (θ) of 42 degrees (25° C.), the angle formed by the rubbing axes of the upper and lower substrates was 84 degrees. The upper and lower polarizing plates were set to be crossed nicols (90@”), so the way to attach the polarizing plates was to set the upper and lower polarizing axes to the upper and lower rubbing axes by 3 degrees [α = (9
0@-2θ)/2] so as to shift outward.

Examples, (1) The optical experimental system used for measuring the BV curve is shown in FIG. In FIG. 12, white light emitted from a light source 39 passes through a polarizer 40 and enters a liquid crystal cell 41 as linearly polarized light, passes through an analyzer 42, is focused by a condensing lens 43, and is sensed by a photomultiplier tube 44. , storage oscilloscope 45
It is measured as a BV curve. Note that an arbitrary waveform can be applied to the liquid crystal cell by a programmable pulse generator 46.

In such an experimental system, the B-7 curve of the ferroelectric liquid crystal panel having the above-mentioned configuration was measured. Further, the cell thickness of the ferroelectric liquid crystal panel used was 8 μm.

The obtained BV curve is shown in FIG. In FIG. 13, the horizontal axis is time (1), and the vertical axis is voltage (V) or brightness (B). The upper figure is the applied voltage waveform, and the lower figure is the corresponding brightness curve. To explain Fig. 13 according to the order of the voltage waveforms, first, when a voltage of pulse height + IOV and width of 50 S was applied, the brightness was as low as about 1% (a dark state was obtained. Next, when no voltage was applied) (OV), the brightness was about 26%, and a bright state was obtained.Next, -10V,
When a voltage of 500 m5 was applied, the brightness was about 26% and a bright state was obtained, similar to when a voltage of +1O2 was applied.

This shows that the present invention can display brightness and darkness regardless of the polarity of the voltage.

Example 2 The brightness of ferroelectric liquid crystal panels with various cell thicknesses in the absence of an electric field was measured using a colorimeter. FIG. 14 shows the relationship between cell thickness and brightness.

It can be seen from FIG. 14 that there are bright regions when the cell thickness is around 4 μm and 8 μm or more.

This roughly matches the brightness curve (solid line) calculated as described above, and from this it can be seen that the color difference due to the unevenness of the cell thickness is also small.

Effects of the Invention The present invention provides a ferroelectric liquid crystal panel in which the liquid crystal layer has a twisted structure between the upper and lower substrates, thereby reducing the birefringence effect.
It is possible to realize a ferroelectric liquid crystal panel that is bright and has little color unevenness even in areas with thick cells. Furthermore, since brightness and darkness can be displayed regardless of the polarity of the voltage, it is possible to realize a ferroelectric liquid crystal panel that does not require a reset pulse.

[Brief explanation of drawings]

Figure 1 is a schematic diagram showing the twisted structure of ferroelectric liquid crystal, Figure 2
The figure is a schematic diagram showing a cone indicating the operating range of ferroelectric liquid crystal molecules, Figure 3 is a schematic diagram showing a conventional ferroelectric liquid crystal display method, and Figure 4 is a diagram showing a phase difference in a conventional ferroelectric liquid crystal panel. Characteristic diagram plotting brightness against (Δnd), 5th
The figure shows the phase difference (Δnd) in a conventional ferroelectric liquid crystal panel.
), Figure 6 is a graph showing the change in brightness versus voltage in a conventional ferroelectric liquid crystal panel, and Figure 7 is a graph showing the change in brightness with respect to voltage in a ferroelectric liquid crystal panel with a twisted structure. d), Figure 8 is a characteristic diagram in which color difference is plotted against retardation (Δno a) in a ferroelectric liquid crystal panel with a twisted structure, and Figure 9 is a characteristic diagram in which color difference is plotted against the phase difference (Δno a) in a ferroelectric liquid crystal panel with a twisted structure. A principle diagram showing the display principle of a ferroelectric liquid crystal panel, FIG. 10 is a perspective view showing the structure of the ferroelectric liquid crystal panel used in the example,
Figure 1) is a diagram showing the rubbing direction and polarization axis direction of the ferroelectric liquid crystal panel of the example, Figure 12 is a schematic diagram of the optical measurement device used in the example, and Figure 13 is a diagram showing the direction of the rubbing direction and polarization axis of the ferroelectric liquid crystal panel of the example. A diagram showing the change in brightness with respect to voltage of a dielectric liquid crystal panel,
FIG. 14 is a diagram in which the measured brightness is plotted against the cell thickness of a ferroelectric liquid crystal panel having a twisted structure. 1.3.4... Ferroelectric liquid crystal molecules, 2...
...Counter board. Name of agent: Patent attorney Toshio Nakao (1 person) Figure 1 Figure 2 Figure 3 ΔE (Dolbs (0L, υ))
8tN ~
(so 5
5 False Figure 6 Karikai 0.2 (J, 4 6.6 0.θ
/, 0 /, 2 Fig. 8 47'1 侠″2 Q, 4 0.6 1), 8 f, 0
/, 2mu7taLiOLina2 Fig. 10 Fig. 1) Fig. 12 Section 13 Fig. 14 cl (tL Hunger

Claims (6)

[Claims] (1) A liquid crystal display device in which a liquid crystal layer is sandwiched between a pair of substrates having opposing electrodes, characterized in that the liquid crystal layer is a ferroelectric liquid crystal and has a twisted structure between the upper and lower opposing substrates. A liquid crystal display device. (2) The liquid crystal display device according to claim 1, wherein the ferroelectric liquid crystal is a chiral smectic C phase, a chiral smectic I phase, or a chiral smectic J phase. (3) The liquid crystal display device according to claim 1, wherein the liquid crystal layer is oriented in a fixed direction with respect to the substrate by an alignment control film of the counter substrate and has a twisted structure. (4) The liquid crystal display device according to claim 3, wherein the alignment control film is formed by rubbing. (5) The liquid crystal display device according to any one of claims 1 to 4, wherein the liquid crystal layer has a twist angle of 70° to 100°. (6) The liquid crystal display according to claim 1 or 3, wherein the liquid crystal layer is composed of a liquid crystal layer exhibiting pleochroism or a liquid crystal composition in which a pleochroic dye is dissolved. Device.