JP2001125106A - Ferroelectric liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferroelectric liquid crystal display device attaining an excellent dark state extending for the full temperature range used. SOLUTION: The ferroelectric liquid crystal display device is characterized by holding a ferroelectric liquid crystal cell provided with a ferroelectric liquid crystal, and first and second optical retardation plates between first and second polarizing plates, by utilizing an optical retardation plate of which the optical retardation varies corresponding to the temperature as the second optical retardation plate, by allowing plural alignment positions which are electrically selectable for the liquid crystal alignment in the ferroelectric liquid crystal cell, by essentially containing all the alignment positions in a plane including the substrate normal and by having the alignment positions showing temperature dependence.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferroelectric liquid crystal display device, and more particularly to a ferroelectric liquid crystal display device for compensating for temperature dependence of contrast in the device.

[0002]

2. Description of the Related Art A liquid crystal exhibiting a ferroelectric phase was introduced in 1975.
First synthesized by RBMeyer et al. In the ferroelectric phase, the liquid crystal molecules have a layer structure as shown in FIG. 1, and in each layer, the molecular major axis is inclined from the layer normal direction. Since the azimuth gradually changes from layer to layer, the molecular arrangement has a helical structure. The electric dipole moment is oriented in a direction perpendicular to the plane (inclined plane) containing the layer normal and the molecular long axis, and spontaneous polarization exists in each layer. There is no spontaneous polarization and no polarization domain.
However, in a ferroelectric liquid crystal element that has been subjected to an alignment treatment so that the helical axis is parallel to the substrate and the liquid crystal layer is vertical, when the gap between the substrates becomes about 1 to 2 μm, the helical structure is formed by the action of the substrate interface. It disappears and a polarization domain appears to exhibit ferroelectricity (JP-A-56-107216 or US Pat. No. 4,368,924). Such an element is SSF
LC (surface stabilized ferroelectric liquid crys
tal) (hereinafter referred to as ferroelectric liquid crystal element unless otherwise specified).

In this state, the molecule has two stable states inclined by θc to the left and right from the alignment control axis (rubbing axis) (FIG. 2). In addition, the direction of spontaneous polarization and the two stable states correspond one-to-one as shown in FIG. 2. For example, when the molecule is tilted to the right with respect to the rubbing axis,
The direction of the spontaneous polarization is upward, and when inclined to the left, the direction of the spontaneous polarization is downward (some materials may have the opposite direction). Therefore, when an electric field is applied between the substrates,
Switching occurs due to coupling with spontaneous polarization, and the direction of molecular tilt can be selected according to the direction of the electric field. From an optical point of view, this corresponds to changing one principal axis of the refractive index ellipsoid by 2θc in the plane of the substrate depending on the direction of the electric field.

[0004] A ferroelectric liquid crystal element is arranged between two orthogonal polarizing plates, and the transmission axis (polarization axis) of one of the polarizing plates is oriented in the long axis direction of the molecule in one stable state, that is, the slow axis (fast phase). Axis). When light is incident on this, the light that has been linearly polarized by the first polarizing plate is not affected by the birefringence of the liquid crystal layer, is completely cut by the second polarizing plate, and the amount of transmitted light is zero and dark. State. When an electric field is applied and the other state, that is, the slow axis (fast axis) is shifted by 2θc from the transmission axis of the first polarizing plate, light is transmitted by birefringence. The amount of transmitted light at this time is given by the following equation.

[0005]

(Equation 2) In particular, when θc is 22.5 degrees and Δnd is λ / 2, the maximum transmission intensity is obtained. According to the above principle, a dark state and a bright state can be realized in the ferroelectric liquid crystal element.

However, the inclination angle θc has a temperature dependence as shown in FIG. 5, and is small at high temperatures and large at low temperatures. Therefore, at a certain temperature, by setting the transmission axis of one polarizing plate to be parallel to one stable state of the ferroelectric liquid crystal element, even if a good dark state is obtained,
When the temperature changes, the position of the stable state changes, so that light is transmitted by the action of the birefringence of the ferroelectric liquid crystal element, so that a good dark state cannot be maintained and the contrast is reduced.

[0007] The following methods are known in order to prevent the reduction in contrast. In Japanese Patent Application Laid-Open No. Sho 62-204229, two polarizing plates whose polarization axes are orthogonal to each other are rotatable, and the polarization axes are rotated according to a change in the inclination angle θc, so that the polarization axes of the polarizing plates always coincide with the polarization axes of the polarizing plates. Since one of the stable states of the ferroelectric liquid crystal matches, the dark state can be maintained. In Japanese Patent Application Laid-Open No. 4-186224, a ferroelectric liquid crystal element and a rotatable λ / 2 plate are sandwiched between two orthogonal polarizing plates, and when the temperature changes and the tilt angle θc changes, λ / The good dark state is maintained by the rotation of the two plates.

Hereinafter, the principle of the above method will be described with reference to FIGS. 3 (a) and 3 (b). At a certain temperature, one stable state of the ferroelectric liquid crystal element and the optical axis of the λ / 2 plate are aligned with the polarization axis of the one polarizing plate. In this case, the linearly polarized light that has passed through the polarizing plate passes as it is without being affected by the birefringence of the ferroelectric liquid crystal element and the λ / 2 plate.
It is completely cut by the second polarizing plate (FIG. 3A).

When the temperature changes and the tilt angle of the ferroelectric liquid crystal element changes to θc ′, the λ / 2 plate also becomes (θc−θc ′).
Rotate. At this time, the linearly polarized light that has passed through the polarizing plate becomes a linearly polarized light whose direction is rotated by 2 (θc−θc ′) after passing through the ferroelectric liquid crystal element. Further, since this linearly polarized light is rotated by 2 (θc−θc ′) in the opposite direction by the λ / 2 plate, it becomes linearly polarized light having the same orientation as the original linearly polarized light. For this reason, it is completely cut by the second polarizing plate (FIG. 3)
(B)). Thus, even if the temperature changes and the tilt angle of the ferroelectric liquid crystal changes, a good dark state can be obtained over the entire temperature because the λ / 2 plate rotates by the same amount.

In Japanese Patent Application Laid-Open No. 4-186230, a favorable dark state is maintained by using two ferroelectric liquid crystal elements. That is, the first ferroelectric liquid crystal element and the second ferroelectric liquid crystal element having the same temperature dependence of the tilt angle θc are arranged between two polarizing plates whose polarization axes are orthogonal to each other so that their alignment control axes are orthogonal to each other. Are located in The principle that the dark state is maintained by this method is the same as in JP-A-4-186224, and when the temperature changes and the tilt angle of the first ferroelectric liquid crystal element changes to θc ′, Since the tilt angle of the second ferroelectric liquid crystal changes in the same manner, the linearly polarized light having the azimuth of 2 (θc−θc ′) after passing through the first ferroelectric liquid crystal element is converted to the second ferroelectric liquid crystal. When the light passes through the liquid crystal element, it is rotated in the opposite direction by 2 (θc−θc ′), and becomes linearly polarized light having the same orientation as the linearly polarized light after passing through the first polarizing plate. Therefore, the light is completely cut by the second polarizing plate, and the dark state is maintained (FIGS. 4A and 4B).

[0011]

SUMMARY OF THE INVENTION Japanese Patent Laid-Open No. 62-2042
No. 29 requires a device for rotating a polarizing plate,
Further, a sensor for detecting the temperature of the element is required, which increases the manufacturing cost. Japanese Patent Application Laid-Open No. 4-186224 also requires a device for rotating the λ / 2 plate and a sensor for detecting the temperature of the element, which increases the manufacturing cost.

Japanese Patent Application Laid-Open No. 4-186230 does not require such a device. However, even if cells having the same temperature dependence of the inclination angle θc are stacked, the dark state cannot be maintained over the entire temperature. This is because the phase difference between the first ferroelectric liquid crystal element and the second ferroelectric liquid crystal element must be equal, but there is no disclosure regarding this problem.

Further, since the ferroelectric liquid crystal has spontaneous polarization, it has a structure in which the director of the liquid crystal molecules is twisted from the lower substrate toward the upper substrate due to the interaction with the interface. For this reason, when two ferroelectric liquid crystals in which a material having the same direction of spontaneous polarization is injected are stacked, there is a problem that light leakage is increased more than a single ferroelectric liquid crystal due to the twisted structure.

However, in the case of a ferroelectric liquid crystal element for displaying an image, since a high frequency bias voltage is generally applied as a signal voltage, the temperature dependence of the tilt angle is different between when a bias voltage is applied and when no bias voltage is applied. In this case, in order to match the temperature dependence of the tilt angle of the ferroelectric liquid crystal element for temperature compensation with that of the ferroelectric liquid crystal element for image display, the ferroelectric A high frequency bias voltage must also be applied to the liquid crystal element. This leads to an increase in power consumption. An example in which the temperature dependence of the tilt angle differs between when a bias voltage is applied and when no bias voltage is applied is a τ-Vmin mode in which the liquid crystal material has a negative dielectric anisotropy. In the case of this material, the temperature dependence of the tilt angle as shown in FIG.

Accordingly, an object of the present invention is to provide a method and an element which can improve these disadvantages and achieve a good dark state over the entire temperature.

[0016]

In order to achieve the above object, a ferroelectric liquid crystal display of the present invention comprises a ferroelectric liquid crystal between a first polarizing plate and a second polarizing plate. A ferroelectric liquid crystal cell, a first retardation plate, and a second retardation plate sandwiched therebetween, wherein the second retardation plate is a retardation plate whose phase difference changes with temperature; The liquid crystal alignment in the cell has a plurality of electrically selectable alignment positions, each of which is substantially included in a plane including the substrate normal, and whose alignment position has temperature dependence. And

Further, the ferroelectric liquid crystal display device of the present invention, when the counterclockwise direction is positive and the clockwise direction is negative in the light traveling direction, one of the transmission axes of the first and second polarizing plates, The angle θ ₁ formed by the slow axis at one alignment state of the ferroelectric liquid crystal is 0 <θ ₁ <90 °, and the slow axis and the polarization at another alignment state of the ferroelectric liquid crystal are different from each other. The angle θ ₂ formed by the transmission axis of the plate is −90 ° <θ ₂ <0 °, and the angle θ _m formed by the slow axis at the position of the ferroelectric liquid crystal in another alignment state and the transmission axis of the polarizing plate is θ ₂ <θ _m <θ ₁ , the angle θ ₃ between the slow axis of the first retardation plate and the transmission axis of the polarizing plate is + 85 ° <θ ₃ <+ 95 °, and the second angle The angle θ ₄ between the slow axis of the retardation plate and the transmission axis of the polarizing plate satisfies + 130 ° <θ ₄ <+ 140 °.

In the temperature range where the liquid crystal element is used, 1 ° <θ ₁ <20 ° with respect to the above θ ₁ , and more preferably, for θ ₃ , + 85 ° <θ ₃ <+90. °,
Furthermore, the theta _4, characterized in that it is a _{+ 130 ° <θ 4 <+} 135 °.

The phase difference in the second phase difference plate is large at a low temperature and small at a high temperature.

Further, assuming that the phase difference R of the second retardation plate is R _LC (nm), the phase difference of the liquid crystal cell is R _LC (nm).

[0021]

(Equation 3) It is characterized by being.

Preferably, the angle θ _R between the rubbing direction in the ferroelectric liquid crystal cell and the transmission axis of the polarizing plate is set.
Is −90 ° <θ _R <0 °.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to FIGS. The liquid crystal display device according to the embodiment of the present invention has a structure (schematic diagram) shown in FIG. 6, in which a liquid crystal cell, a retardation plate (λ / 4 plate) and a temperature are arranged between two polarizing plates. A phase difference plate whose phase difference changes is sandwiched. Here, the order of the liquid crystal cell, the phase difference plate, and the phase difference plate whose phase difference changes depending on the temperature does not matter.

Regarding the direction of the slow axis of each of these optical elements, for the sake of explanation, it is assumed that the axes are orthogonal to each other as shown in FIG. Light is incident in the minus direction parallel to the Z axis. Regarding the sign of the angle in the figure, the clockwise direction is negative and the counterclockwise direction is positive in the light traveling direction. The first to fourth quadrants are taken as shown in the figure.

Then, the optical axis of each optical element is as follows.

Transmission axis (polarization axis) of first polarizing plate: parallel to x-axis (that is, 0 °) Slow axis of λ / 4 plate: + 90 ° direction from x-axis Phase difference plate whose phase difference changes with temperature : X-axis +135 degree direction Transmission axis (polarization axis) of the second polarizing plate: +90 degree direction from x-axis Slow axis in one stable state of ferroelectric liquid crystal: in the first quadrant The other of ferroelectric liquid crystal Slow axis in the stable state of the liquid crystal: in the fourth quadrant Phase difference of ferroelectric liquid crystal: λ / 2 When the slow axis in one stable state of the ferroelectric liquid crystal is in the first quadrant, it is the dark state. The bright state is when the slow axis in the other stable state is in the fourth quadrant. Further, the slow axis of the λ / 4 plate or the retardation plate whose phase difference changes depending on the temperature may deviate from the above value by about ± 5 degrees.

Next, the principle that the dark state is maintained even when the tilt angle θc changes in this configuration will be described using a Poincare sphere. As shown in FIG. 8, the Poincare sphere is a three-dimensional sphere having linearly polarized light having different directions on the equator, circularly polarized light having different directions at the north pole and south pole, and the other part being elliptically polarized light. On the Poincare sphere, the polarization at the point 0 'corresponds to the polarization state of the light that has passed through the first polarizer in FIG. In FIG. 7, the liquid crystal in the first quadrant, the slow axis of which is inclined by θ ₁ from the transmission axis of the first polarizer, causes the polarization at 0 ′ to become the polarization at point A.

Next, the polarized light at the point A is changed to the polarized state at the point B by the λ / 4 plate whose slow axis is inclined by +90 degrees from the transmission axis of the first polarizing plate in FIG. In FIG. 7, when the retardation plate whose slow axis is inclined at +135 degrees from the first polarizing plate and whose phase difference changes with temperature has an appropriate phase difference, the polarization state returns to the point 0 '. Even when the temperature changes and the tilt angle changes, the polarized light that has passed through the λ / 4 plate always comes on the arc NO'S, so that the phase difference plate whose phase difference changes with temperature takes an optimal value. This makes it possible to always return to the original polarization state passed through the first polarizing plate. In this way, the dark state can be maintained even if the tilt angle changes.

<Embodiment 1> A ferroelectric liquid crystal material FDS71 (N. Itoh et al. IDW) is used for the liquid crystal cell shown in FIG.
A cell into which (1998) P205) was injected was used.
The phase difference of this cell is 275 nm, and the wavelength is 550 nm.
Is a λ / 2 plate for the light of The temperature dependence of the tilt angle is obtained when the rectangular wave shown in FIG. 5 is applied. A film having a phase difference of 137.5 nm was used as a λ / 4 plate. An antiparallel rubbing cell into which E8, which is a nematic material, was injected was used as a phase difference plate whose phase difference changes with temperature. By applying a rectangular wave to this and changing the voltage, the molecular orientation of the liquid crystal is changed, and the phase difference is freely changed.

Now, at a temperature of 60 ° C., the ferroelectric liquid crystal is set so that the rubbing direction is -4.5 degrees from the direction of the transmission axis of the first polarizing plate as shown in FIG. 9 (coordinates). Are set as in FIG. 7). Θc at this time was 6.5 degrees. The phase difference of a phase difference plate (nematic cell) whose phase difference changes with temperature was set to 12 nm. FIG. 10 shows a transmission spectrum in the dark state and a transmission spectrum in the bright state.

Next, when the temperature is 45 ° C., θc of the ferroelectric liquid crystal is 11 degrees, and the positions of the dark state and the bright state are as shown in FIG. 11 (the positions in the rubbing direction are the same as those in FIG. 9). . At this time, the phase difference of the phase difference plate whose phase difference changes with temperature was set to 43 nm. At this time, the transmission spectrum in the dark state and the transmission spectrum in the bright state are as shown in FIG.

Next, when the temperature is 0 ° C., θ of the ferroelectric liquid crystal is
c was 15.5 degrees, and the positions of the dark state and the bright state were as shown in FIG. 13 (the positions in the rubbing direction were the same as in FIG. 9). At this time, the phase difference of the phase difference plate whose phase difference changes with temperature was set to 66 nm. At this time, the transmission spectrum in the dark state and the transmission spectrum in the bright state are as shown in FIG.

It can be seen that even when the tilt angle changes, the spectrum in the dark state hardly changes. In the present embodiment, a nematic cell is used as a phase difference plate whose phase difference changes with temperature, and a required phase difference is obtained by applying a rectangular wave. However, as the temperature changes, it is needless to say that the present invention can be applied to optical films and other optical materials whose phase difference naturally changes.
Rather, this is preferable from the viewpoint of device fabrication.

In the first embodiment, the temperature is set to 60 ° C. to 0 ° C.
FIG. 15 shows the result of measuring the contrast while changing
Shown in At this time, the temperature dependence of the phase difference of the phase difference plate whose phase difference changes with temperature is changed as shown in FIG.

The phase difference R of the phase difference plate whose phase difference changes with temperature is given by θ _{1 where the} angle between the transmission axis of the first polarizing plate and the slow axis direction of the liquid crystal in the dark state is θ ₁ .

[0036]

(Equation 4) It is preferred that Here, R _LC is the phase difference (n
m).

<Comparative Example 1> Without using the temperature compensating device described in the first embodiment, as shown in FIG.
The contrast was measured by changing the temperature while setting one of the stable states in S71 to coincide with the transmission axis of the polarizing plate. The temperature dependency of θc is the same as when the rectangular wave in FIG. 5 is applied. The results are shown in FIG. 15 without compensation.

As shown in FIG. 15, according to the ferroelectric liquid crystal display device using temperature compensation according to the present invention, it is understood that the contrast can be sufficiently secured over a wide temperature range.

[0039]

As described above, in a ferroelectric liquid crystal display device in which the direction of the slow axis changes with temperature,
By using the ferroelectric liquid crystal display device which has performed the temperature compensation according to the present invention, the contrast can be compensated over a wide temperature range.

[Brief description of the drawings]

FIG. 1 is a structural diagram of a ferroelectric liquid crystal.

FIG. 2 is a diagram showing two stable states and a spontaneous polarization direction when SSFLC is viewed from the upper surface of a cell.

FIG. 3 is an explanatory diagram of a method for compensating contrast in Japanese Patent Application Laid-Open No. 4-186224.

FIG. 4 is an explanatory diagram of a method for compensating contrast in Japanese Patent Application Laid-Open No. 4-186230.

FIG. 5 is a diagram showing the temperature dependence of θc when no bias is applied to the FDS 71 and when a rectangular wave is applied.

FIG. 6 is a sectional view of a cell when the compensation method of the present invention is used.

FIG. 7 is a diagram showing the direction of the slow axis of each optical element in the compensation method of the present invention.

FIG. 8 is a diagram illustrating the principle of a compensation method according to the present invention using a Poincare sphere.

FIG. 9 is an FDS at a temperature of 60 ° C. in the first embodiment.
Shows 71 settings

FIG. 10 is a diagram showing transmission spectra in a dark state and a bright state in the setting of FIG. 9;

FIG. 11 is an FD at a temperature of 45 ° C. in the first embodiment.
It is a figure showing the setting of S71.

FIG. 12 is a diagram showing transmission spectra in a dark state and a bright state in the setting of FIG. 11;

FIG. 13 is an FDS at a temperature of 0 ° C. in the first embodiment.
FIG. 7 is a diagram showing settings of a 71.

FIG. 14 is a diagram showing transmission spectra in a dark state and a bright state in the setting of FIG. 13;

FIG. 15 is a diagram showing the temperature dependence of contrast in Embodiment 1 and the temperature dependence of contrast when the present invention is not used.

FIG. 16 is a temperature characteristic diagram of the phase difference of the phase difference plate in which the phase difference changes according to the temperature in the first embodiment.

FIG. 17 is a diagram showing an optical geometry at a temperature of 35 ° C. in a comparative example.

Claims (5)

[Claims] 1. A method according to claim 1, wherein a first polarizing plate and a second polarizing plate are provided between the first polarizing plate and the second polarizing plate.
A ferroelectric liquid crystal cell including a ferroelectric liquid crystal, a first retardation plate, and a second retardation plate are sandwiched therebetween, and the second retardation plate is a retardation plate whose phase difference changes with temperature. The ferroelectric liquid crystal cell has a plurality of alignment positions where the liquid crystal alignment is electrically selectable, each of the alignment positions is substantially included in a plane including the substrate normal, and the alignment position has a temperature. A ferroelectric liquid crystal display device characterized by dependence. 2. When the counterclockwise direction is positive and the clockwise direction is negative in the light traveling direction, one of the transmission axes of the first and second polarizers is aligned with one of the alignment states of the ferroelectric liquid crystal. The angle θ ₁ formed by the slow axis at the position is 0 <θ ₁ <90 °, and the angle θ ₂ formed by the slow axis and the transmission axis of the polarizing plate at another alignment state of the ferroelectric liquid crystal is −90 ° <θ ₂ <0 °, and the angle θ _m between the slow axis and the transmission axis of the polarizing plate in the other alignment state of the ferroelectric liquid crystal is θ ₂ <θ _m <θ ₁ . The angle θ ₃ between the slow axis of the first retardation plate and the transmission axis of the polarizing plate is + 85 ° <θ ₃ <+ 95 °, and the slow axis of the second retardation plate and the polarization 2. The ferroelectric liquid crystal display device according to claim 1, wherein an angle [theta] ₄ between the transmission axes of the plates is +130 [deg.] <[Theta] ₄ <+140 [deg.]. 3. In a temperature range in which a liquid crystal element is used, θ
Ferroelectric liquid crystal display device according to claim 2, wherein the _first range is _{1 ° <θ 1 <20 °} . 4. The method according to claim 1, wherein a phase difference of the second phase difference plate is large at a low temperature and small at a high temperature.
The ferroelectric liquid crystal display device as described in the above. 5. The phase difference R of the second retardation plate is given by the following equation, assuming that the phase difference of the liquid crystal cell is R _LC (nm). A ferroelectric liquid crystal display device, characterized in that: