JP2000347160A - Ferroelectric liquid crystal device - Google Patents

Abstract

[PROBLEMS] To simply combine a method of applying an AC electric field to increase an apparent tilt angle in a non-selection period and a method of applying a reset pulse before a selection period, a black state is obtained. The contrast ratio does not rise as expected due to light leakage when switching. SOLUTION: The effective voltage of the AC electric field is represented by θ, the tilt angle of the liquid crystal molecule formed by the layer normal and the molecular axis direction, and the apparent tilt angle of the liquid crystal molecule viewed from the normal direction of the substrate surface,
The tilt angle of the apparent upon applying an AC electric field theta _m, when the tilt angle of the apparent and theta ₀ when no applied AC electric field is set to satisfy the relation of _{θ-θ m ≦ θ m -θ} 0 .

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 device having a high contrast ratio, and is applicable to general direct-view displays, projection projectors, miniature displays, and the like.

[0002]

2. Description of the Related Art At present, for example, a TN (Twisted Nematic) type using a nematic liquid crystal and an STN (Super Twisted) are used.
An isted Nematic) type liquid crystal display device is known. However, these liquid crystal display elements have a problem that the response speed of the electro-optical effect is as slow as the order of milliseconds, so that the display capacity is limited and is not suitable for displaying moving images. Also, the viewing angle is narrow, and it is not suitable for enlargement of the screen. Therefore, in recent years, practical use of a ferroelectric liquid crystal display device as a next-generation liquid crystal display device has been studied.

[0003] In 1975, RBMeyer et al. Discussed the symmetry of molecules and found that if an optically active molecule had a dipole moment in a direction perpendicular to the long axis of the molecule, a chiral smectic C phase (SmC ^* phase) would be used. Is expected to show DOBAMBC (2-methylbutyl p- [p- (decyl
oxybenzylidene) -amino] -cinnamate) and succeeded in confirming the ferroelectricity of liquid crystal for the first time (RBMeyer, L. Liebert, L. Strzelecki and P. kelle)
r: J.Phys. (Paris) (36) (1975) L69).

Further, Clark and Lagerwall describe a phenomenon that when the cell thickness becomes about 1 μm (about the same as the pitch of the spiral), the spiral structure disappears and one of the bistable states is taken according to the direction of the electric field. Was discovered and a surface stabilized type ferroelectric liquid crystal device (SSFLC = Surface Stabilized Ferroelectr
ic liquid Crystal). This is disclosed in
No. 107216, and US Pat. No. 4,367,924.
No. 6,009,036.

[0005] In this SSFLC, spontaneous polarization and an electric field directly interact with each other to generate a driving torque. Therefore, unlike switching using dielectric anisotropy in a normal nematic liquid crystal, the electric field is in the order of μs. High-speed response is possible. Further, the SSFLC has a so-called memory property that maintains the state even when the electric field disappears once it switches to any one of the bistable states, so that it is not necessary to constantly apply a voltage. Furthermore, SSFLC uses two-dimensional in-plane switching (in-plane switching).
Due to the characteristics, a wide viewing angle is also realized in a direct-view display.

Here, the operation principle of the SSFLC will be described with reference to FIG. FIG. 4A is a schematic view showing a movement path of ferroelectric liquid crystal molecules, and FIG. 4B is a projection view as viewed from directly above indicated by an arrow 28 in FIG. In the figure, 2
0 represents a molecule of a ferroelectric liquid crystal.

The ferroelectric liquid crystal molecules 20 are oriented parallel to the substrate and form a layer structure perpendicular to the substrate. The molecules 20 are oriented at a tilt angle θ in the direction of 24 or 25 with respect to the normal direction 23 of this layer. The molecule 20 of the ferroelectric liquid crystal has spontaneous polarization in a direction orthogonal to its long axis (molecular direction). When an external electric field is applied, the vector product of this electric field and the spontaneous polarization Ps in a direction perpendicular to the molecule long axis. And moves on the surface of a cone 26 having a vertex having a double tilt angle 2θ. Since the driving force of this movement is spontaneous polarization, it has a high-speed response of 1/100 to 1/1000 as compared with that using a conventional nematic liquid crystal.

The molecule 20 has two stable states. When the molecule 20 is moved by the positive electric field F to the axis 24 shown in FIG. When moved to 25, a stable state 28
It has the following properties. This stable state is maintained as long as an electric field required for the movement of the molecule is not applied.

If the absorption axis of the polarization slope is made coincident with one of the two stable states 27 and 28, for example, the molecular axis of the stable state 27, this state becomes a black state that does not transmit light, and the other state becomes black. At the time of the stable state 28, it is in a white state where light is transmitted. Then, the stable state of the molecule 20 is operated for each pixel by matrix driving, and a predetermined display state is obtained.

[0010] In particular, a τ-Vmin mode ferroelectric liquid crystal display has recently attracted attention.
24234). When a monopulse voltage is applied to this ferroelectric liquid crystal material, the pulse width (τ), which is the response time required for all the liquid crystal molecules to completely switch, and the pulse peak value (Vp), which is the voltage, are obtained. A response time-voltage curve (so-called τ-V curve) showing the relationship is
For example, as shown in FIG. 5, it has a minimum value, and a driving method utilizing this characteristic is generally called a τ-Vmin mode.

In the τ-Vmin mode, the force applied to the liquid crystal molecules unrelated to rewriting can be reduced, and the contrast ratio can be increased. A liquid crystal composition having a negative dielectric anisotropy,
Alternatively, it is known that a liquid crystal composition having a large positive biaxial dielectric anisotropy exhibits the τ-Vmin characteristic. Furthermore, this τ-Vmin mode and C2 uniform (C2U)
By combining the orientations, a wide driving voltage region, a wide driving temperature region, a high contrast ratio, and a high-speed response are realized.

Here, the orientation of the ferroelectric liquid crystal will be described. Although the ferroelectric liquid crystal is described as forming a layer structure perpendicular to the substrate, in actuality, as shown in FIG. 6, two kinds of layer structures (chevron layer structure) bent in a “C” shape are used. Is shown. In FIG. 6, reference numeral 30 denotes a liquid crystal molecule;
Reference numeral 1 denotes a pretilt which is an inclination angle of the liquid crystal molecules 31 on the substrate surface from the substrate surface.

From the relationship between the pretilt and the bending direction of the chevron layer structure, the case where the bending direction and the direction of the pretilt 31 are opposite to each other is C1 orientation, and the case where they are the same is C1 orientation.
It is called two orientation. Further, the uniform and the twist indicate whether or not the liquid crystal molecules have a twist in the normal direction of the substrate surface, and an orientation showing an extinction position without twist is called a uniform, and an orientation showing no extinction position with twist is called a twist. For ferroelectric liquid crystal, C1U, CIT, C2U, C2
T can take four orientation states.

Some techniques have been proposed for realizing a high contrast ratio by utilizing the excellent characteristics of the τ-Vmin mode.

For example, in Japanese Patent No. 2640259, a ferroelectric liquid crystal material having a negative dielectric anisotropy is combined with C2 alignment, and the angle between the bistable alignment states in the average molecular axis direction is larger than that in the absence of an electric field. A driving method for applying a sufficient AC electric field has been proposed. Ferroelectric liquid crystal materials with negative dielectric anisotropy have the property of being arranged in parallel with the substrate by the application of an AC electric field with a high frequency. By using this property, a high brightness and high contrast ratio can be obtained. (Prior art).

In Japanese Patent Publication No. 7-50268,
A driving method for applying a pulse (hereinafter, referred to as a reset pulse) for aligning liquid crystal molecules of a pixel in one direction before a selection period has been proposed for a general ferroelectric liquid crystal. This makes it possible to prevent a decrease in memory properties and maintain a good contrast ratio for a long time (prior art).

Further, in Japanese Patent Application Laid-Open No. 6-43433, high-speed response and high contrast ratio are realized by combining C2 orientation, negative dielectric anisotropy, and τ-Vmin mode.

[0018]

However, in any of the above-mentioned prior arts, light leakage that occurs when switching to the black state lowers the contrast ratio and affects the image quality of the conventional TN element and STN element. There is a problem that there is no.

In view of the above, high contrast is achieved by combining the above-described conventional technique of applying an AC electric field and the conventional technique of applying a reset pulse before a selection period (non-selection period). Is also conceivable.

However, if the conventional technique of applying an AC electric field and the conventional technique of applying a reset pulse before a selection period are merely used in combination, the contrast ratio is reduced due to light leakage when switching to a black state. However, when the time division driving method is used, this problem becomes more serious.

The reason will be described below. As described above, the ferroelectric liquid crystal molecules sandwiched between cells having a small thickness of about 1 μm, such as SSFLC, have a layer normal to the smectic layer and a molecular axis direction as shown in FIGS. Apparent tilt angle θ smaller than the tilt angle θ
A position indicating ₀ (± θ ₀ ) is a stable point.

When a DC voltage is applied in such a state,
Due to the interaction between the spontaneous polarization and the electric field, the liquid crystal molecules are arranged so as to approach the direction parallel to the electric field (the position (± θ) indicating the actual tilt angle θ).

In addition, when a high-frequency AC electric field at which spontaneous polarization does not respond is applied to a ferroelectric liquid crystal material having a minimum response time-voltage curve as described above, the effective voltage of the AC electric field is reduced. Accordingly, the liquid crystal molecules are arranged at a position (± θ _m ) showing a value θ _m larger than θ ₀ as a stable point.
This is because the interaction between the dielectric anisotropy and the electric field acts in a direction in which the molecules are arranged in a direction parallel to the substrate.

In the case of driving a display using such a liquid crystal material, in the driving method of applying the AC electric field as a signal voltage, a high-frequency electric field is always applied as a signal voltage during a non-selection period. As a result, the apparent tilt angle increases, and the contrast ratio and the luminance increase.
In this case, one of the polarization directions of the two polarizing plates which are arranged crossed Nicols, is set in accordance with the tilt angle theta _m this apparent.

However, when such a liquid crystal material is applied with a driving method of applying a reset pulse during a non-selection period, which can maintain a high contrast ratio for a long time, an apparent tilt of liquid crystal molecules at the time of applying the reset pulse is obtained. The angle θ _r is the apparent tilt angle θ _m during the non-selection period.
Since it becomes larger, the alignment direction of the liquid crystal molecules deviates from the polarization axis of the polarizing plate, and light transmission occurs. This phenomenon is particularly a factor of lowering the contrast ratio as light leakage in the black state.

[0026]

Means for Solving the Problems The present inventor has paid attention to such problems and made intensive studies, and as a result, has accomplished the present invention. To prevent light leakage caused by the tilt angle theta _r apparent liquid crystal molecules when a reset pulse is applied, the tilt angle theta _r and the signal voltage applied when the apparent tilt angle theta _m is as close as possible to the value of the reset pulse is applied Should be taken. However, if they completely match, the electrical energies of the two become equal, and the action of the reset itself becomes ineffective. Therefore, in general, a relationship of θ _m <θ _r must be established.

Here, the following two cases are considered. 1) If _{θ-θ r> θ r -θ} 0 satisfying this relationship, the tilt angle theta _r when the reset pulse is applied is set to a value close to theta ₀ than the tilt angle theta. As shown in FIG. 8, when the liquid crystal molecules are arranged in the vicinity of θ ₀ , the apparent tilt angle θ _m at the time of applying a voltage shows a relatively large change with respect to the applied voltage. The difference in electrical energy between the two appears as a larger difference in apparent tilt angle. Therefore, in this case, the greater the degree of difference becomes large and light leakage values of theta _m and theta _r.

[0028] 2) if it meets the _{θ-θ r ≦ θ r -θ} 0 This relationship contrary to the above, since the liquid crystal molecules are arranged in the vicinity theta, the electrical energy is changed, the voltage applied The apparent tilt angle θ _m at the time is hard to change.
Therefore, in this case, also the degree of difference is smaller becomes light leakage values of theta _m and theta _r small.

From the above, the condition 2) is preferable.
Considering the request to reduce the difference between the under theta _m and theta _r this condition, equation becomes _{θ-θ m ≦ θ m -θ} 0 is obtained. Therefore, the theta _m when a voltage is applied, θ-θ _m ≦ θ _m
By setting the driving conditions of the liquid crystal so as to satisfy the relationship of −θ ₀ , it is possible to suppress light leakage that occurs when a reset pulse is applied.

As described above, the ferroelectric liquid crystal element of the present invention has a bistable alignment state between the substrate on which the scanning electrode group is formed and the substrate on which the signal electrode group is formed. A ferroelectric liquid crystal element in which a ferroelectric liquid crystal having a response time-voltage curve having a minimum value is sandwiched and pixels are formed at intersections of the scanning electrode group and the signal electrode group, having the following configuration. And

That is, after applying a first pulse voltage to the selected scanning electrode to reset the alignment state of the liquid crystal molecules of the pixel associated with the scanning electrode to one of the above-mentioned bistable alignment states. Driving means for applying a second pulse voltage to set the liquid crystal molecules of the pixel to one of the bistable alignment states, and a period other than when the second pulse voltage is applied to the scan electrode. An AC electric field applying means for applying an AC electric field such that the angle formed in the molecular axis direction between the bistable orientation states becomes larger than when no voltage is applied, and the effective voltage of the AC electric field is
The tilt angle of the liquid crystal molecules formed by the layer normal and the molecular axis direction is θ,
The apparent tilt angle of the liquid crystal molecules viewed from the normal direction of the substrate surface, where the apparent tilt angle when the AC electric field is applied is θ _m , and the apparent tilt angle when the AC electric field is not applied is θ _0, the is characterized in that it is set so as to satisfy the relation of _{θ-θ m ≦ θ m -θ} 0.

According to this, since the driving means applies the first pulse voltage (hereinafter, referred to as a reset pulse) during the non-selection period, the memory property of the ferroelectric liquid crystal is improved, and the driving time is longer. A high contrast ratio can be maintained. In addition, the AC electric field applying means applies an AC electric field to increase the apparent tilt angle, so that the contrast ratio can be improved.

Moreover, since the effective voltage value of the AC electric field in the AC electric field applying means is set so as to satisfy the relationship of θ−θ _m ≦ θ _m −θ ₀ , the liquid crystal is applied by the application of the reset pulse during the non-selection period. Light leakage due to a change in the apparent tilt angle of molecules can be effectively prevented, and the contrast ratio can be further increased.

When the AC electric field is driven as a signal voltage by the signal electrode, the effective voltage value of the signal voltage is set so as to satisfy the relationship of θ−θ _m ≦ θ _m −θ _0. Has the same effect.

In order to drive the liquid crystal under the above-described conditions, the liquid crystal is required to have suitable properties. That is, at a relatively low voltage, the apparent tilt angle needs to approach the actual tilt angle.

Therefore, the ferroelectric liquid crystal device of the present invention
In the ferroelectric liquid crystal material, the layer normal and the molecular axis direction
Is the tilt angle of the liquid crystal molecules formed by θ, the normal direction of the substrate surface
The apparent tilt angle of the liquid crystal molecules viewed from
Hz, 5 V square wave voltage applied, the apparent h
The tilt angle is θ_m1, The apparent tilt angle when no voltage is applied is θ ₀₁
Then, θ−θ_m1≤θ_m1−θ₀₁That satisfy the relationship
It is preferable to select.

As a result, low-voltage driving of the ferroelectric liquid crystal element becomes possible, heat generation is suppressed without disturbing the orientation of the liquid crystal in the panel surface, and stable driving with low power consumption becomes possible.

The effective voltage value of the AC electric field in the AC electric field applying means is preferably less than 8 Vrms. By setting in this way, the heat generation is not disturbed in the panel surface without disturbing the orientation of the liquid crystal. This enables stable driving with low power consumption.

The orientation state of the ferroelectric liquid crystal in the ferroelectric liquid crystal device of the present invention includes the direction of the pretilt given to the liquid crystal molecules at the interface between the orientation film applied to the substrate and the liquid crystal layer and the smectic layer. An orientation state in which the bending direction of the structure is equal to almost the entire surface of the substrate, that is, a so-called C2 orientation is preferable. C1 with the opposite direction of layer bending
Compared to orientation, C2 orientation is stable in low temperature range and wide temperature range,
It is excellent in that it has a high response speed and a high contrast ratio.

In general, a display using a ferroelectric liquid crystal exhibiting a high-speed response can display a gradation by time division display. In this case, a reset pulse is applied according to the number of divisions. Therefore, the above-described problem caused by the application of the reset pulse appears more remarkably. However, by employing the configuration of the ferroelectric liquid crystal element of the present invention in the driving method for performing the grayscale display by the time division method, the effect is more exerted.

[0041]

Embodiments of the present invention will be described below. FIG. 1 shows an example of the configuration of a ferroelectric liquid crystal device employing the configuration of the ferroelectric liquid crystal device of the present invention. As shown in FIG. 1, the ferroelectric liquid crystal element has two glass substrates 1 and 2 which are bonded to each other with a sealant 10 and face each other, and a liquid crystal layer 9 is interposed between the glass substrates 1 and 2. It is pinched. The liquid crystal layer 9 has a bistable alignment state and, as shown in FIG. 5, is made of a ferroelectric liquid crystal whose response time-voltage curve has a minimum value.

The signal electrodes 3, the scan electrodes 5, the insulating films 4, 6, and the alignment films 7, 8 are formed in stripes on the opposing surfaces of the glass substrates 1, 2.
Pixels are formed at the intersection of the signal electrode 3 and the scanning electrode 5 in a stripe shape.

A polarizing plate 11 is arranged outside the glass substrate 1, and a polarizing plate 12 is arranged outside the glass substrate 2. Polarizing plate 11, glass substrate 1, signal electrode 3,
An electrode substrate 13 is formed by the insulating film 4 and the alignment film 7. Similarly, an electrode substrate 14 is formed by the polarizing plate 12, the glass substrate 2, the scanning electrode 5, the insulating film 6, and the alignment film 8.

The polarizing plates 11 and 12 are arranged in a crossed Nicols arrangement in which the polarizing axes are orthogonal to each other.
Either one of the polarization directions of the 1-12, is set in accordance with the tilt angle theta _m apparent later.

The surfaces of the alignment films 7 and 8 are subjected to a luffing treatment. The interval between the electrode substrates 13 and 14 is set to be smaller than the twist pitch of the ferroelectric liquid crystal constituting the liquid crystal layer 9 along the layer normal direction by a spacer (not shown).

A driving circuit (not shown) for applying a signal to the signal electrode 3 and the scanning electrode 5 to switch the light transmission state of each pixel to perform display is provided to the selected scanning electrode 5.
A reset pulse (first pulse voltage) is applied before the selection period (non-selection period) to change the alignment state of the liquid crystal molecules of the pixel related to the scan electrode 5 to one of the predetermined bistable alignment states. And then the scanning electrode 5
A strobe pulse (second pulse voltage) corresponding to a signal for setting the light transmission state of the upper pixel is applied to set the liquid crystal molecules of the pixel to one of the bistable alignment states. Has become.

Further, in the drive circuit, the angle formed in the molecular axis direction between the bistable alignment states in a non-selection period other than the selection period in which the strobe pulse is applied to the scan electrode 5 is larger than when no voltage is applied. It is configured to apply such an alternating electric field.

By applying a reset pulse during the non-selection period, the memory properties of the ferroelectric liquid crystal can be improved, a high contrast ratio can be realized for a long time, and an apparent tilt can be realized by applying an AC electric field. By widening the corner, the contrast ratio can be improved.

Further, in the ferroelectric liquid crystal device of the present invention,
The effective voltage value of this AC electric field is
Normal and molecular axis of smectic layer formed by crystalline liquid crystal
The tilt angle of the liquid crystal molecules formed by the direction is θ,
The apparent tilt angle of the liquid crystal molecules viewed from the line direction,
The apparent tilt angle when an AC electric field is applied is θ_m,Alternating current
The apparent tilt angle when no electric field is applied is θ₀To be
And θ−θ_m≤θ_m−θ ₀Set to satisfy the relationship
Have been.

Thus, since the reset pulse is applied during the non-selection period, it is possible to effectively prevent a problem that the apparent tilt angle of the liquid crystal molecules during the non-selection period changes and light leakage occurs due to the change. And the contrast ratio can be increased.

As a ferroelectric liquid crystal material constituting the liquid crystal layer 9 of such a ferroelectric liquid crystal element, the tilt angle of liquid crystal molecules formed by the layer normal and the molecular axis direction is θ, the normal direction of the substrate surface is The apparent tilt angle of the liquid crystal molecules as viewed from
The apparent tilt angle when liquid crystal molecules are applied when a rectangular wave voltage of 5 kHz and 5 V is applied is θ _m1 , and the apparent tilt angle when no voltage is applied is θ
When _01, by selecting the one that satisfies the relationship of _{θ-θ m1 ≦ θ m1 -θ} 01, enables low-voltage driving, without disturbing the alignment of the liquid crystal in the panel surface, heat generation is suppressed, low Stable driving with power consumption becomes possible.

Further, the effective voltage value of the AC electric field in the ferroelectric liquid crystal device of the present invention is preferably set to less than 8 V, so that the orientation of the liquid crystal is disturbed in the panel surface. In addition, heat generation is suppressed, and stable driving with low power consumption becomes possible.

The orientation of the liquid crystal layer 9 in the ferroelectric liquid crystal device of the present invention is preferably C2 orientation, more preferably C2 uniform orientation.

The C2 orientation is superior to the C1 orientation, in which the layer is bent in the opposite direction, in that it is stable in a low temperature range and a wide temperature range, has a high response speed, and has a high contrast ratio. In particular, by adopting the C2 uniform orientation, not only the apparent tilt angle is increased by applying a high-frequency AC electric field to increase the brightness in the bright state, but also the fluctuation of the liquid crystal molecules is suppressed, and the dark state (black state) ) Has the effect of reducing light leakage.

Further, by adopting the structure of the ferroelectric liquid crystal element of the present invention, the contrast ratio can be increased even if gradation display is performed by the time division method. As an example of performing the gradation display by the time division method, when obtaining 16 gradations (4 bits), one frame for writing one screen is divided into 1: 2: 4: 8, and reset for each divided period. A pulse will be input. Therefore, the light leakage there is four times that in the case where time division is not performed, and it is more important to suppress the light leakage itself when a reset pulse is input.

[0056]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a ferroelectric liquid crystal device according to the present invention will be described below along with comparative examples with reference to FIGS.

First, the basic structure of the ferroelectric liquid crystal cell used for the measurement will be described with reference to FIG. The ferroelectric liquid crystal cell has basically the same configuration as that described in the embodiment. The signal electrode 3 and the scanning electrode 5 have a film thickness of 10
It consists of a transparent ITO film of 0 nm. The insulating films 4.6 have a thickness of 100 nm, and the alignment films 7.8 have a thickness of 30 nm. The distance between the electrode substrates 13 and 14 is kept almost uniformly at 1.4 μm by a spacer (not shown).

Example 1 θ = 19.3 at 35 ° C.
°, θ ₀ = 7.7 ° ferroelectric liquid crystal FLC-A,
The DRAMA-3 driving method shown in FIG. 2 (MHAnderson, JRH)
ughes, JCJonesand KGRussell, Conference Record o
f the 1997 International Display Research Conferen
ce, L-40 (1997)) and a selection period of 12 μs (1/36
0 Duty) Equivalent to time division 4 bits. The selection period was the same for all of the following examples and comparative examples), and the signal electrode 3 was driven at an effective voltage value Vd = 6 Vrms.

Here, a strobe pulse (Vs) is applied to the scan electrode 5 beyond the selection period, and a reset pulse (-Vs / 2) is applied before the selection period. After the reset pulse and the strobe pulse, kickback pulses (± Vs) having opposite polarities are applied.

At this time, the apparent tilt angle θ _m = 14.
0 °, and this value almost coincided with the value when a rectangular wave voltage of 100 kHz and 6 V was applied. (In the following examples and comparative examples, if the effective voltage values are the same, the apparent tilt when a rectangular wave voltage of 100 kHz is applied. The angle substantially matches the apparent tilt angle when driving DRAMA-3). In addition, the phase series of FLC-A,
The phase transition temperatures are as follows. Phase sequence of the FLC-A, the phase transition temperature SmC ^* /68.5℃/SmA/90.7℃/N ^* / 9
8.4 ° C./I Example 2 The FLC-A was driven at an effective voltage value Vd of the signal electrode of 7 Vrms by using the DRAMA-3 driving method. The apparent tilt angle θ _{m at} this time is θ _m =
15.0 °.

[Comparative Example 1] The above FLC-A was replaced with a DRAM.
Using the A-3 driving method, the effective voltage value Vd of the signal electrode is 4 V
Driven at rms. Incidentally, the tilt angle theta _m apparent at this time was θ _m = 11.2 °.

[Comparative Example 2] The above FLC-A was replaced with a DRAM.
Using the A-3 driving method, the effective voltage value Vd of the signal electrode is 5 V
Driven at rms. Incidentally, the tilt angle theta _m apparent at this time was θ _m = 12.7 °.

[Comparative Example 3] The above FLC-A was replaced with a DRAM.
Using the A-3 driving method, the effective voltage value Vd of the signal electrode is 8 V
Driven at rms. At this time, the apparent tilt angle θ _m was θ _m = 15.7 °.

Example 3 θ = 19.3 at 35 ° C.
°, θ ₀ = 7.5 ° ferroelectric liquid crystal FLC-B,
Using the DRAMA-3 driving method, the effective voltage V of the signal electrode
Drive was performed at d = 5 Vrms. The apparent tilt angle θ _{m at} this time was θ _m = 13.7 °. In addition, FLC-B
Are as follows. Phase sequence of the FLC-B, the phase transition temperature SmC ^* /69.2℃/SmA/87.3℃/N ^* / 9
8.3 ° C./I Example 4 At 35 ° C., θ = 18.9 °, θ ₀ =
7.5 ° ferroelectric liquid crystal FLC-C
-3 driving method, the effective voltage value of the signal electrode Vd = 5Vr
ms. The apparent tilt angle θ _{m at} this time is θ
_m = 14.9 °. In addition, the phase series of FLC-C,
The phase transition temperatures are as follows. Phase sequence of the FLC-C, the phase transition temperature SmC ^* /67.1℃/SmA/90.7℃/N ^* / 1
01.6 ° C./I Comparative Example 4 At 35 ° C., θ = 19.9 °, θ ₀ =
7.9 ° ferroelectric liquid crystal FLC-D was added to DRAMA
-3 driving method, the effective voltage value of the signal electrode Vd = 5Vr
ms. The apparent tilt angle θ _{m at} this time is θ
_m = 12.9 °. The FLC-D phase series,
The phase transition temperatures are as follows. Phase sequence of the FLC-D, the phase transition temperature SmC ^* /72.0℃/SmA/83.1℃/N ^* / 9
4.2 ° C./I FIGS. 3B and 3C show Examples 1 to 4 and Comparative Examples 1 to 4.
4 shows optical response waveforms when each sample is always displayed in a black state. FIG. 3A shows a waveform of a signal applied to the scanning electrode 5. In addition, Table 1 shows a summary table including the amount of light (light intensity) of the leakage light at that time. In addition,
The transmitted light intensity shown in Table 1 is the backlight 100
The value is for 00 cd / cm ² .

[0065]

[Table 1]

First, in the same ferroelectric liquid crystal FLC-A, the effective voltage Vd of the AC electric field applied to the signal electrode 3
Are changed, and Examples 1 and 2 and Comparative Examples 1 to 3 are compared.

[0067] The effective voltage value of the AC electric field applied to the signal electrode 3 is 4Vrms, in each sample of Comparative Examples 1 and 2 is 5Vrms does not meet the _{θ-θ m ≦ θ m -θ} 0, 3 As shown in (b), the leakage light upon application of the reset pulse is large, and the contrast ratio is less than 250: 1.

In the ferroelectric liquid crystal cell having the basic configuration as shown in FIG. 1 described above, if a contrast ratio of 250: 1 is realized, the contrast ratio of 150: 1 is obtained in a panel having an actual color filter and the like. Is empirically realized, and this value can be said to be one of the conditions required in the current transmission direct-view type liquid crystal display.

[0069] In contrast, the effective voltage value of the AC electric field applied to the signal electrode 3 is 6 Vrms, in each sample of Examples 1 and 2 is 7Vrms _{is, θ-θ m ≦ θ m} -θ 0
, The leakage light is suppressed and the contrast ratio is 2
It is a value of 50: 1 or more.

In the sample of Comparative Example 3 in which the effective voltage value of the AC electric field applied to the signal electrode 3 is 8 Vrms, while satisfying θ−θ _m ≦ θ _m −θ ₀ ,
The orientation of the liquid crystal was disturbed, and black display could no longer be performed. Here, suppose that the effective voltage value of the AC electric field is 8 Vrms.
Even if a high contrast ratio can be obtained with good orientation,
When applied to an actual display, not only is the power consumption large, and the problem of heat generation from the panel surface or the IC circuit becomes significant, which is not practical. Therefore, it is desirable that the driving is performed with the effective voltage value of the AC electric field applied to the signal electrode 3 being less than 8 Vrms.

Then, this time, three liquid crystal materials having different dielectric anisotropies of the liquid crystal material are compared in Examples 3 to 4 and Comparative Example 4 while keeping the effective voltage value of the AC electric field applied to the signal electrode 3 constant. View.

[0072] carried out in descending order of the absolute value of the dielectric anisotropy Example 4, Example 3, has a comparative example 4, the tilt angle theta _m also at the same voltage apparent in this order is closer to the actual tilt angle theta I have. Here, in each of the samples of Examples 3 and 4 satisfying θ−θ _m ≦ θ _m −θ ₀ , FIG.
As shown in (1), light leakage at the time of reset pulse application is small and a contrast ratio of 250: 1 or more is realized, but in the sample of Comparative Example 4 which does not satisfy the conditional expression,
The leakage light is large and a sufficient contrast ratio is not obtained.

When the embodiments 1 to 4 are comprehensively compared, the embodiment 4 in which a high contrast ratio can be obtained at a low operating voltage is the most ideal. One standard is the tilt angle θ,
100kHz, tilt angle theta _m apparent during the rectangular wave voltage is applied 5V, the relationship of the tilt angle theta ₀ apparent when no voltage is applied, the ferroelectric liquid crystal material as a _{θ-θ m ≦ θ m -θ} 0 It can be said that developing and applying it is effective.

[0074]

As described above, in the ferroelectric liquid crystal device of the present invention, the first pulse voltage is applied to the selected scanning electrode to change the alignment state of the liquid crystal molecules of the pixel related to the scanning electrode. Driving means for resetting the bistable alignment state to one of the predetermined states and applying a second pulse voltage to set the liquid crystal molecules of the pixel to one of the bistable alignment states; An AC electric field is applied to apply an AC electric field such that the angle formed in the molecular axis direction between the above bistable alignment states becomes larger than when no voltage is applied during a period other than when the second pulse voltage is applied to the scan electrode. Means, and the effective voltage of the AC electric field, the tilt angle of the liquid crystal molecules formed by the layer normal and the molecular axis direction is θ, the apparent tilt angle of the liquid crystal molecules viewed from the normal direction of the substrate surface And apply the above AC electric field The tilt angle of the apparent theta _m when the, the tilt angle of apparent when not applied the alternating electric field When theta _0, configuration that is set so as to satisfy the relation of _{θ-θ m ≦ θ m -θ} 0 It is.

[0075] Thus, the theta _m when a voltage is applied, theta-theta
By setting the driving conditions of the liquid crystal so as to satisfy the relationship of _m ≦ θ _m −θ ₀ , it is possible to suppress light leakage that occurs when a reset pulse is applied, so that a ferroelectric liquid crystal element having a high contrast ratio can be provided. To play.

When the AC electric field is driven by the signal electrode as a signal voltage, the effective voltage value of the signal voltage is set so as to satisfy the relationship of θ−θ _m ≦ θ _m −θ _0. By doing so, a similar effect is achieved.

The ferroelectric liquid crystal material used in the ferroelectric liquid crystal device of the present invention may be a liquid crystal as viewed from the normal direction of the substrate surface when the tilt angle of liquid crystal molecules formed by the layer normal and the molecular axis direction is θ. The apparent tilt angle of the molecule is 100 kHz,
Assuming that the apparent tilt angle when liquid crystal molecules are applied when a 5 V rectangular wave voltage is applied is θ _m1 and the apparent tilt angle when no voltage is applied is θ ₀₁ , those that satisfy the relationship θ−θ _m1 ≦ θ _m1 −θ ₀₁ are obtained. It is preferable to select, and this enables low-voltage driving of the ferroelectric liquid crystal element, suppresses heat generation without disturbing the orientation of the liquid crystal within the panel surface, and enables stable driving with low power consumption. The effect is also achieved.

The effective voltage value of the AC electric field in the AC electric field applying means is preferably less than 8 Vrms. By setting in this way, the heat generation is not disturbed in the panel surface without disturbing the orientation of the liquid crystal. It is also possible to suppress the power consumption and achieve stable driving with low power consumption.

The orientation state of the ferroelectric liquid crystal in the ferroelectric liquid crystal device of the present invention includes the direction of pretilt given to liquid crystal molecules at the interface between the alignment film applied to the substrate and the liquid crystal layer and the smectic layer. An orientation state in which the bending direction of the structure is substantially the same as the entire surface of the substrate, that is, a so-called C2 orientation is preferable. I have.

In particular, it is more effective to adopt the structure of the ferroelectric liquid crystal element of the present invention in a driving method in which the above-mentioned problem caused by the application of the reset pulse becomes more conspicuous, and a gradation display is performed by a time division method. .

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a schematic configuration of a ferroelectric liquid crystal element as one embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating an example of a DRAMA-3 drive voltage waveform.

FIGS. 3A and 3B show scanning voltage waveforms, and FIGS.
Examples in which the scanning voltage waveform shown in FIG.
9 is a graph showing an electro-optical response at the time of black display of a comparative example.

FIG. 4A is a schematic view showing a movement path of ferroelectric liquid crystal molecules, and FIG. 4B is a projection view of FIG.

FIG. 5 is a graph showing τ-V characteristics of a ferroelectric liquid crystal used in a τ-Vmin mode.

FIG. 6 is a schematic diagram showing a C1 orientation and a C2 orientation in a ferroelectric liquid crystal.

FIG. 7A is a ferroelectric liquid crystal molecule sandwiched between substrates.
It is a schematic diagram showing the movement route of (b), is (a) right above?
The apparent tilt angle θ when no voltage is applied.
₀, Apparent tilt angle θ when applying voltage_mThe actual tilt
FIG. 4 is a schematic diagram showing an angle θ.

FIG. 8 is a graph showing an example of a relationship between an electric field intensity and an apparent tilt angle.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Substrate 3 Signal electrode 5 Scanning electrode 7 Alignment film 8 Alignment film 9 Liquid crystal layer (ferroelectric liquid crystal)

Claims (6)

[Claims] A substrate having a scanning electrode group and a signal electrode group having a bistable orientation state between the substrate and the substrate;
In a ferroelectric liquid crystal device in which a ferroelectric liquid crystal having a response time-voltage curve having a minimum value is sandwiched and pixels are formed at intersections of the scan electrode group and the signal electrode group, a selected scan electrode is After resetting the alignment state of the liquid crystal molecules of the pixel associated with the scan electrode to one of the predetermined states of the bistable alignment state by applying a first pulse voltage, applying a second pulse voltage to the pixel A driving means for setting one of the liquid crystal molecules to one of the bistable alignment states, and a molecular axis direction between the bistable alignment states during a period other than when the second pulse voltage is applied to the scan electrode. And an AC electric field applying means for applying an AC electric field such that the angle formed by the AC electric field becomes larger than when no voltage is applied. Molecular tilt angle theta, a tilt angle of the apparent liquid crystal molecules as viewed from the normal direction of the substrate surface, the tilt angle of the apparent upon application of the alternating electric field theta _m, the tilt angle of apparent when not applied the alternating electric field When the the theta _0, the ferroelectric liquid crystal element characterized in that it is set so as to satisfy the relation of _{θ-θ m ≦ θ m -θ} 0. 2. An apparatus according to claim 1, wherein said AC electric field is driven by a signal electrode as a signal voltage, and an effective voltage value of the signal voltage is θ.
Ferroelectric liquid crystal device according to claim 1, characterized in that it is set so as to satisfy the relation of -θ _m ≦ θ _m -θ _0. 3. The ferroelectric liquid crystal according to claim 1, wherein the tilt angle of the liquid crystal molecule formed by the layer normal and the molecular axis direction is θ, and the apparent tilt angle of the liquid crystal molecule viewed from the normal direction of the substrate surface, 100k
The apparent tilt angle of liquid crystal molecules when a rectangular wave voltage of 5 Hz and 5 V is applied is θ _m1 , and the apparent tilt angle when no voltage is applied is θ _01.
The ferroelectric liquid crystal device according to claim 1, wherein the following relationship is satisfied: θ−θ _m1 ≦ θ _m1 −θ ₀₁ . 4. The ferroelectric liquid crystal device according to claim 1, wherein an effective voltage value of the AC electric field in said AC electric field applying means is less than 8 Vrms. 5. A pretilt direction provided to liquid crystal molecules at an interface between an alignment film applied to a substrate and a liquid crystal layer, and a bending direction of a smectic layer structure of the ferroelectric liquid crystal is substantially equal to the entire surface of the substrate. The ferroelectric liquid crystal device according to claim 1, wherein the device has an alignment state. 6. The ferroelectric liquid crystal device according to claim 1, wherein the gradation display is performed by a time division method.