JP2550556B2 - Ferroelectric liquid crystal display element - Google Patents

Description

TECHNICAL FIELD The present invention relates to a ferroelectric liquid crystal display device using a ferroelectric liquid crystal composition.

[Prior Art] In recent years, a ferroelectric liquid crystal display device using a ferroelectric liquid crystal composition has attracted attention. (For example, A.Clark, STLage
rwall, Appl.Phys.Lett. 36, 899 ( 1980)) ferroelectric liquid crystal, a chiral smectic C phase (SmC ^*
Phase), a chiral smectic H phase, or other liquid crystal phase having spontaneous polarization, that is, a characteristic of exhibiting ferroelectricity. In this respect, the characteristics are significantly different from the nematic phase or cholesteric phase conventionally used for liquid crystal display devices. Have.

The ferroelectric smectic phase are known several phases, but since phase application to optical shutter or a dot matrix display or the like is expected is a SmC ^* phase, the following description of the SmC ^* phase An example will be described.

In the SmC ^* phase, liquid crystal molecules form a layered structure, and the long axis direction of the molecules is inclined with respect to the layer normal direction.
In addition, it contains an optically active substance having an asymmetric center in its molecular structure, and the tilt direction of the molecule has a helical structure shifted between layers. Furthermore, the SmC ^* phase has spontaneous polarization in a direction perpendicular to the long axis of the molecule and parallel to the plane of the layer, and the orientation direction of the molecule changes so that the polarity of the spontaneous polarization and the electric field match the external electric field. And can cause optical changes. As a feature of this electro-optical effect, it has been found that a high-speed response of 10 to 1000 times is possible as compared with the effect using a conventional nematic liquid crystal, and that it has a memory property, and a large screen dot matrix display element, It is expected to be applied to high-speed optical shutter devices. However,
On the other hand, there are many technical problems with these excellent features.

[Problems to be Solved by the Invention] The biggest problem is that when a ferroelectric liquid crystal having bistability is switched from one stable state to the other stable state, one pulse signal is stably generated. It is difficult to completely invert the liquid crystal, and it is not possible to invert stably and safely unless the same write pulse signal is applied several times to several tens of times in succession.

Therefore, compared to the ideal case in which the liquid crystal is completely inverted by one pulse signal, the driving method in which the pulse signal must be applied more than once is characterized by the quick response of the ferroelectric liquid crystal. It wasn't alive at all.

Therefore, there has been a demand for a ferroelectric liquid crystal display element in which the ferroelectric liquid crystal is completely inverted from one stable state to the other stable state by one pulse signal.

[Means for Solving the Problem] The present invention has been made to solve the above-described problems, and a liquid crystal composition having ferroelectricity is sandwiched between substrates with electrodes provided with an alignment film. In the ferroelectric liquid crystal display device thus formed, the side of the alignment film in contact with the liquid crystal composition is a polymer alignment film, and the magnitude of spontaneous polarization of the liquid crystal composition is Ps (nC / c
m ² ) and the relative permittivity and thickness of the alignment film are ε and d (Å), respectively, The present invention provides a ferroelectric liquid crystal display device characterized by satisfying the following relational expression.

As a result, it is possible to completely rewrite the screen with a single pulse signal, and it is possible to take advantage of the high-speed response that is a feature of the ferroelectric liquid crystal display element.

FIG. 1 is a schematic diagram schematically showing an example of a ferroelectric liquid crystal display element.

In FIG. 1, 11A and 11B are not shown In ₂ O ₃
It shows a substrate such as glass or plastic provided with a transparent electrode such as —SnO ₂ (ITO) or SnO ₂ , and a polymer alignment film such as polyimide is provided on this transparent electrode. At least one of these alignment films is subjected to a rubbing treatment so as to have an alignment regulating force in one direction. A ferroelectric liquid crystal composition exhibiting ferroelectricity is arranged therebetween, and the liquid crystal molecular layer 12 is oriented so as to be perpendicular to the substrate surface. 13A and 13B
Represents liquid crystal molecules, and these liquid crystal molecules have dipole moments (Ps) 14A and 14B in a direction orthogonal to the molecules.

When the thickness between the substrates of this liquid crystal display device is made sufficiently thin, the helical structure of the liquid crystal molecules is unraveled and the dipole moment Ps is upward P _A (14
Either A) or downward P _B (14B).

When an electric field E _A or E _B having a certain threshold or more and different polarities is applied to such a liquid crystal display element, the dipole moment is directed upward 14 A or downward corresponding to the electric field vector of the electric field E _A or E _B. 14B, and the liquid crystal molecules are aligned in either the first stable state 13A or the second stable state 13B accordingly.

Liquid crystal molecules have an elongated shape and exhibit refractive index anisotropy in the major axis direction and the minor axis direction thereof. Therefore, for example, a pair of polarizing films should be arranged so that their polarization axes are orthogonal to each other above and below the substrate surface. For example, it is possible to obtain a liquid crystal element whose optical characteristics change depending on the polarity of voltage application.

In the present invention, when switching between the two states is performed by applying a voltage with a pulse width of about 0.1 to 1 msec, all the liquid crystal molecules are inverted by a single pulse, so that the magnitude of the dipole moment of the liquid crystal molecules, that is, spontaneous The relationship between the polarization Ps (nC / cm ² ) and the thickness d (Å) of the alignment film and its relative dielectric constant ε is as follows.

When this value exceeds 750, the probability of reversal with a single pulse gradually decreases, and rewriting failure easily occurs, and finally rewriting becomes impossible. On the other hand, if it is less than 75, the response speed becomes slow and the drive voltage must be increased, which is not practical.

The substrate with an electrode of the present invention, glass, plastic such as In ₂ O ₃ -SnO ₂ (ITO), a transparent electrode such as SnO ₂ is laminated on the substrate, if necessary, in a desired pattern. A patterned product can be used. Of course, as used in a normal liquid crystal display device, a low resistance metal lead is laminated on this transparent electrode, or an insulating layer is provided between the substrate and the electrode.
A color filter, a light-shielding layer, etc. may be laminated, an active element such as a TFT or MIM may be used in combination, or a light-shielding layer may be formed on the electrode.

An alignment film is formed on the substrate with this electrode. In order to improve the alignment of the smectic liquid crystal, at least the polymer alignment film is formed on the surface of the alignment film which is in contact with the liquid crystal. As the polymer orientation film, a film of a well-known polymer material such as polyimide, polyamide, or silicon, which does not adversely affect the liquid crystal and can well align the liquid crystal, can be used.
This alignment film may be a single layer of a polymer alignment film, or may be a laminate of a polymer alignment film and a film of an inorganic material having an excellent insulating property.

As the film of the inorganic material, a film of an inorganic oxide such as silicon oxide, titanium oxide, or aluminum oxide is suitable, and has a great effect of preventing a short circuit between the upper and lower substrates.

When the alignment film has two layers in this way, the above formula (I) may be considered as the sum of the two layers as shown in the following formula (II). In the following formula, d ₁ and d ₂ are the film thicknesses of the inorganic insulating film and the polymer alignment film, respectively, and ε ₁ and ε ₂ are the relative dielectric constants of the inorganic insulating film and the polymer alignment film, respectively.

When the thickness of the polymer alignment film is less than 50Å, the alignment performance of the alignment film becomes insufficient and the uniformity as a liquid crystal display device is likely to be impaired. Therefore, it is preferable that d ≧ 50Å.

Therefore, assuming that the relative permittivity of the polymer alignment film provided on the substrate is about 3, Ps is at most Ps ≦ 45 nC / cm ²
It must be a degree. Considering the fluctuation of the thickness of the polymer alignment film from the viewpoint of work, it is necessary to set Ps ≦ 40 nC / cm ² .

Further, if Ps is smaller than 4 nC / cm ² , the response speed is remarkably reduced, so Ps ≧ 4 nC / cm ² is preferable. Therefore, practically, the spontaneous polarization Ps is 4 ≦ Ps ≦ 40.
It is preferably nC / cm ² .

When Ps is set to a minimum of 4 in terms of response speed,
The thickness d of the polymer alignment film having a relative dielectric constant of about 3 such as polyimide is generally 56 ≦ d ≦ 560Å. From the viewpoint of workability, this is also preferably about d ≦ 500Å.

Therefore, the value of the formula (I) is set to 75 to 750, the spontaneous polarization is set to 4 ≦ Ps ≦ 40 nC / cm ² , and the thickness d of the polymer alignment film is set to 50 ≦ d ≦ 500Å. Preferably.

In the case of the conventional nematic liquid crystal display element, the polymer alignment film is provided in the range of 700 to 2000 Å mainly for the purpose of obtaining the insulating property and reducing the visibility of the transparent electrode. Even if the thickness of the polymer alignment film was variously changed, there was almost no fatal problem in turning on and off the liquid crystal.

On the other hand, in the case of the ferroelectric liquid crystal display element, the values of the spontaneous polarization Ps, the thickness d, and the relative permittivity ε are set as shown in the formula (I) as described above, so that 0.1 to 1 msec. It is possible to easily rewrite the display by applying a single pulse having a pulse width of about the same, and it is possible to rewrite the display at high speed.

In the present invention, the substrate provided with the alignment film thus formed is horizontally aligned by a rubbing method or the like, and a sealing material is applied to the periphery so that the substrates are held in parallel and at regular intervals. , Spacers such as organic beads and alumina particles are sandwiched, and the electrodes are arranged so that the electrode surfaces face each other, and the two substrates are sealed to form a cell. A ferroelectric liquid crystal composition is injected into this cell and the injection port is sealed to complete the liquid crystal cell.

In the above description, a typical liquid crystal cell manufacturing process has been described, and different manufacturing methods may be used. Moreover, a multi-layer liquid crystal cell may be formed by using three or more substrates.

Two polarizing films are arranged outside the cell so that the polarizing films are orthogonal to each other and form a constant angle with the substrate orientation control method. This angle changes depending on the liquid crystal material, the operating temperature of the device, the driving method, etc., and the angle with the best contrast characteristics may be selected. In some cases, the polarization axes of the two polarizing films are arranged so as to be slightly offset from the orthogonal. In some cases.

Further, if necessary, a light source is placed on the back surface of the back substrate so that the light is transmitted to the opposite side. When the reflective type is used, a reflector may be provided outside the polarizing film on the back side.

As the ferroelectric liquid crystal composition used in the driving method of the present invention, a liquid crystal having a liquid crystal phase exhibiting bistability depending on the polarity of an electric field can be used. However, in view of responsiveness, a chiral smectic C phase (SmC ^* phase) is used. A liquid crystal composition having a) is preferred.

The ferroelectric liquid crystal composition used in the present invention is used as a ferroelectric smectic liquid crystal composition by appropriately mixing it with a ferroelectric smectic liquid crystal material, a smectic liquid crystal material or another optically active substance. In addition, in order to adjust viscosity, operating temperature, spiral pitch, etc., and to display in color,
A nematic liquid crystal material, a non-liquid crystal material, a dichroic dye or the like may be added.

As a specific example of this ferroelectric smectic liquid crystal material, p-hexyloxybenzylidene-p'-amino-2-chloro-α-propylcinnamate (HOBACP
C), p-decyloxybenzylidene-p-amino-2
-Methylbutyl-α-Cyan-Cinnamate (DOBAMBC
C) etc. Of course, it may be a single material,
Several materials may be mixed to achieve the property.

Further, as the ferroelectric liquid crystal composition used in the present invention,
A liquid crystal having a smectic A phase (SmA phase) in a temperature range higher than the liquid crystal phase exhibiting ferroelectricity is preferable in terms of bistability symmetry. SmA from isotropic phase (I phase), nematic phase (Ne phase) or cholesteric phase (Ch phase)
When a liquid crystal that directly changes into a ferroelectric liquid crystal phase such as an SmC ^* phase without passing through a phase is used, two types of alignment states in which the direction of the liquid crystal molecular layer is different from the direction of normal alignment control are taken. When these two kinds of orientation states are mixed, the contrast is deteriorated. Therefore, when cooling from I phase, Ne phase or Ch phase to ferroelectric liquid crystal phase such as SmC ^* phase, a DC electric field with a unidirectional polarity is applied. However, it is necessary to take measures such as aligning only one of the two kinds of alignment states. In the stability of the element thus produced, the stable state, which is the same as the polarity of the electric field applied during cooling, is more stable among the first stable state and the second stable state. This leads to a decrease in bistability. On the other hand, in the liquid crystal having the SmA phase, there is only one direction of the liquid crystal molecular layer and no means for applying an electric field is required, and therefore the bistability is symmetrical with respect to the voltage and the bistability is good. .

Further, as the ferroelectric liquid crystal composition used in the present invention,
It is preferable to have a Ch phase in a temperature range higher than that of the liquid crystal phase exhibiting ferroelectricity in terms of alignment uniformity. With respect to the method for producing the alignment of the liquid crystal, the element can be produced in a very good orientation by using the method disclosed in JP-A-61-153623.

As a ferroelectric liquid crystal composition, a liquid crystal that has an SmC ^* phase, a Ch phase at a higher temperature, and a helical pitch length (p) in the Ch phase that is four times or more the distance (ds) between substrates is used. It is desirable to use and have an SmA phase between the Ch phase and the SmC ^* phase from the viewpoint of orientation uniformity. Such a ferroelectric liquid crystal composition is obtained by mixing an optically active substance, a smectic liquid crystal material, a nematic liquid crystal material and the like in an appropriate ratio, and a non-liquid crystal additive may be added if necessary. In particular, in order to lengthen the pitch in the Ch phase, it is effective to mix an optically active substance that causes a left helix and an optically active substance that causes a right helix according to the magnitude of the force that causes the helix. .

Normally, the length of the helical pitch in the Ch phase changes with temperature. In order to obtain uniform orientation, it is necessary to satisfy the condition of p> 4ds just above the cholesteric-smectic phase transition point.

However, when the temperature range satisfying this condition is limited to the vicinity of the transition point, the spiral structure unintentionally transitions to the smectic phase when the temperature drop rate is high. In this case, since a uniform orientation cannot be obtained, it is necessary to maintain the temperature satisfying p> 4ds or slow the temperature drop rate until the helical structure is unwound. For this reason, the temperature range in which the helical pitch p is 4 times or more the distance ds between substrates is 5 from the cholesteric-smectic phase transition point.
It is preferably in the range of 0 ° C. or higher, and more preferably in the entire Ch phase temperature range.

It should be noted that the Ch phase referred to here also includes a Ne phase formed by a nematic liquid crystal in which an optically active substance is added to the nematic liquid crystal so as to have a unique pitch.

[Operation] FIG. 2 is a schematic view showing that a bistable ferroelectric liquid crystal display device is aligned in one stable state.
The figure is a schematic diagram showing a state immediately after an electric field is applied to the ferroelectric liquid crystal display element to invert the liquid crystal molecules.

2 and 3, 21A, 21B, 31A and 31B are substrates, 25A, 25B, 35A and 35B are electrodes, 26A, 26B, 36A and 36B are alignment films, 23 and 33 are liquid crystal molecules, 24 and 34. Represents the dipole moment.

In FIG. 2, the dipole moments 24 of the liquid crystal molecules 23 are aligned downward in the figure. When the directions of the dipole moment are aligned, charges Q are inevitably generated above and below the liquid crystal layer as shown by + -in the figure.

Here, since the alignment film is a dielectric, it is polarized by an amount of charge enough to balance the generated charges and cancels each other out. As a result, the electrodes 25A and 25B have the same potential. The electric charge Q generated here has the property of being proportional to the magnitude Ps of the dipole moment of the liquid crystal.

 Next, the polarization of the alignment film will be described.

Since the alignment film plays the role of a condenser,
When the electrode Q is generated, the potential differences V generated above and below the alignment film have the following relationships (III) and (IV).

That is, The voltage generated here has the following meaning.

That is, when the electric field E is applied from the bottom to the top in FIG. 2 in order to invert the liquid crystal molecules, the directions of the dipoles are aligned with the direction of the electric field, and the state shown in FIG. 3 is obtained.

The reversal speed of the liquid crystal molecules is extremely fast, and the reversal is sufficiently completed with a pulse width of 1 msec. At the moment the pulse disappears, the polarization direction of the alignment film with a slow relaxation time remains unchanged as shown in FIG. , The liquid crystal layer has a state in which opposite charges are accumulated.

This state is extremely unstable, and if the generated voltage V exceeds the threshold voltage of the liquid crystal, it is further inverted and returns to the original state, that is, the state shown in FIG. Become. Moreover, generally, the inversion threshold voltage of the ferroelectric liquid crystal with respect to the DC voltage is considerably lower than that of the conventional TN liquid crystal display element, and inversion occurs even at about 100 mV. Therefore, the magnitude of the generated voltage V must be less than or equal to this threshold voltage, otherwise the voltage will return to the original state when the voltage application is completed.

Being able to rewrite means that the generated voltage V
Is less than or equal to this inversion voltage, so this voltage is 100 mV or less at maximum. Estimating the condition for satisfying the rewritable condition from the above equation (IV) gives the following.

(Here, a is a proportional coefficient) Therefore, a is as follows.

Here, Ps is 15 nC / cm ² and the relative permittivity ε of the polymer alignment film is 3 and the thickness d of the alignment film is changed to 1 msec.
The conditions under which rewriting with one pulse was possible were investigated.

As a result, rewriting is possible when the thickness of the polymer alignment film is 100Å, rewriting is not possible when the thickness is 200Å, and about 150Å
Turned out to be the limit. The value of a at this time was calculated to be 1.33 × 10 ⁻⁴ . Therefore, the following relationship was found.

Further, as a result of combining different Ps, d, and ε and selecting those satisfying the rewritable condition, it was confirmed that the value of V / a still needed to be about 750 or less.

The switching speed of the ferroelectric liquid crystal display element is
It is said that it is proportional to Ps, and it is advantageous to use a small Ps in terms of rewriting described above, but the response speed becomes slow, and the value of the above formula should be 75 or more from a practical point of view. Is preferred.

Ferroelectric liquid crystal display elements have a liquid crystal layer thickness of 0.5 to 2μ.
Since the thickness is as thin as about m, the risk of a short circuit occurring between a pair of opposing electrodes is greater than that of a conventional TN liquid crystal display element. Therefore, it is preferable to form an insulating layer having a sufficient thickness on the electrodes.

However, a polymer alignment film such as polyimide has a low relative permittivity of about 3 to 5, and if a thick insulating layer is formed of such a substance, it is necessary to use a liquid crystal having extremely small Ps from the above conditions. As described above, there is a problem that the response speed is reduced.

Therefore, in order to improve the insulating property, a thin polymer orientation film having a high orientation is used on the surface in contact with the liquid crystal, and an insulation film made of a high dielectric constant material is laminated between the polymer orientation film and the electrode. Would be preferred. Therefore, it is preferable to use, as the insulating film having a high dielectric constant, a film made of an insulating inorganic oxide having a hard film quality and an excellent insulating property. In particular, since titanium oxide has an extremely large relative permittivity, by using it as a material with a relative permittivity of 30 to 50 as a mixture with silicon oxide, the film thickness can be about 10 times that of a polymer alignment film, which adversely affects rewriting. It is possible to give high insulation without giving

[Examples] Examples 1 to 9 and Comparative Examples 1 to 4 Polyimides having relative permittivities of 3 and 4.5 were overcoated on the electrodes of a substrate on which transparent electrodes made of ITO were patterned in a stripe shape, and rubbed to horizontally. A substrate that had been subjected to an orientation treatment was prepared. A sealing material was printed on the periphery of this substrate, two substrates were arranged so that the electrode surfaces of the substrates were opposed to each other, and seal adhesion was performed so that the gap between the substrates was 2 μm to form a liquid crystal cell.

Inside this liquid crystal cell, as a ferroelectric liquid crystal composition,
A liquid crystal cell was completed by injecting a ferroelectric liquid crystal composition having an I phase-Ch phase-SmA phase-SmC ^* phase in this order. Example 1
9 to 9 and the physical properties of these ferroelectric liquid crystal compositions of Comparative Examples 1 to 4, the spontaneous polarization Ps and the relative permittivity ε of the polymer alignment film,
The thickness d is shown in Table 1.

Table 1 shows the results obtained by applying a single pulse having a peak value of 20 V and a pulse width of 1 msec to these liquid crystal cells and examining the inversion characteristics thereof.

Examples 1 to 3 and Examples 4 to 6 show cases in which the spontaneous polarization ε of the ferroelectric liquid crystal composition was changed while keeping the film thickness d of the polymer alignment film and its relative dielectric constant ε constant. , Example 1,
4, 7 and 8 show the case where Ps and ε were kept constant and d was changed, and Example 9 shows the case where ε of Example 3 was changed.

Further, Comparative Examples 1 and 2 show examples in which Ps · d / ε is out of the range of the present invention by making Ps smaller or larger than Examples 1 to 3, and Comparative Examples 3, 4 Shows an example in which Ps · d / ε is out of the range of the present invention by increasing d.

As is clear from these results, while all of the examples can be rewritten with one pulse,
Comparative Example 1 with a small Ps has an extremely slow response speed, and Ps · d / ε
In all of Comparative Examples 2 to 4 in which the value of was over 750, reversal did not occur by applying the pulse once.

Examples 10 and 11 and Comparative Example 5 Thickness 1000 Å, 2000 Å, 5000 made of titanium oxide and silicon oxide between the polymer alignment film having the constitution of Example 7 and the electrode.
A liquid crystal cell was formed in the same manner as in Example 1 except that the inorganic oxide film (ε = 40) of Å was formed.

As a result, for the 1000 Å and 2000 Å thick inorganic oxide films, both were reversed by one pulse application, but 5000 Å
In the case of the inorganic oxide film of No.

Those provided with these inorganic oxide films had high insulating properties and were unlikely to cause a short circuit between both electrodes.

[Effect] According to the present invention, it is possible to completely rewrite the screen with a single pulse signal having a pulse width of about 1 msec, and it is possible to take advantage of the high-speed response that is a feature of the ferroelectric liquid crystal display device. Therefore, it can be used in various fields such as various information displays and optical shutters.

In addition to the above, the present invention can be applied in various ways within a range that does not impair the effects of the present invention. Two or more of the elements of the present invention can be combined with each other, TN type liquid crystal elements, GH type liquid crystal elements, electro You may combine with a luminescence element etc., and may combine it with a color filter, a color polarizing film, etc.

[Brief description of drawings]

FIG. 1 is a schematic view of an example of a ferroelectric liquid crystal display device of the present invention. FIG. 2 is a schematic diagram showing that the bistable ferroelectric liquid crystal display element is oriented in a constant stable state, and FIG. 3 is an electric field applied to the ferroelectric liquid crystal display element. It is a schematic diagram which shows the state immediately after inverting the liquid crystal molecule. Substrate: 11A, 11B, 21A, 21B, 31A, 31B Liquid crystal molecule layer: 12 Liquid crystal molecule: 13A, 13B, 23, 33 Dipole moment: 14A, 14B, 24, 34 Electrode: 25A, 25B, 35A, 35B Alignment film : 26A, 26B, 36A, 36B

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yukio Endo 13-2 Watatamucho, Kawasaki-ku, Kawasaki (56) References JP 61-62018 (JP, A) JP 61-62023 (JP, A)

Claims (6)

(57) [Claims] 1. A ferroelectric liquid crystal display device comprising a ferroelectric liquid crystal composition sandwiched between substrates with electrodes provided with an alignment film, wherein the side of the alignment film in contact with the liquid crystal composition is polymer aligned. It is a film, and the magnitude of spontaneous polarization of the liquid crystal composition is defined as Ps (nC
/ cm ² ), relative permittivity and thickness of the alignment film are ε and d (Å), respectively, A ferroelectric liquid crystal display device characterized by satisfying the following relational expression. 2. A ferroelectric liquid crystal display device according to claim 1, wherein the magnitude of spontaneous polarization is 4 to 40 nC / cm ² . 3. A ferroelectric liquid crystal display device according to claim 1, wherein the polymer alignment film has a film thickness of 50 to 500 Å. 4. The ferroelectric liquid crystal display device according to claim 3, wherein the polymer alignment film is a polyimide alignment film. 5. The alignment film comprises a polymer alignment film and an inorganic oxide insulating film, the film in contact with the liquid crystal composition is the polymer alignment film, and the film in contact with the electrode is an inorganic oxide insulating film. A ferroelectric liquid crystal display device according to any one of claims 1 to 4. 6. A ferroelectric liquid crystal display device according to claim 5, wherein the insulating film of inorganic oxide is a mixture of silicon oxide and titanium oxide.