JP3205598B2 - Liquid crystal display device and driving method thereof - Google Patents

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 capable of gradation display.

[0002]

2. Description of the Related Art At present, liquid crystal display elements are used in a wide range of fields such as clock processors, calculators, OA equipment such as word processors and personal computers, and pocket televisions. And nematic liquid crystal is sealed between a pair of substrates on which electrodes such as signal lines are arranged.

[0003] As a liquid crystal display device using a nematic liquid crystal, a twisted nematic TN (Twisted Nematic TN) is used.
Liquid crystal display element, super twisted type (Supertw
isted Birefringence Effect (SBE type) liquid crystal display device.

However, a twisted nematic (TN) type liquid crystal display element has a drawback that a drive margin becomes narrower with an increase in the number of scanning lines, and a sufficient contrast cannot be obtained. It is difficult. In order to improve this TN type liquid crystal display device, a super twisted nematic (ST)
N) type liquid crystal display devices and double layer super twisted nematic (DSTN) type liquid crystal display devices have been developed, but the contrast and response speed decrease as the number of scanning lines increases. Capacity is at its limit. In addition, TN type, STN type, DS
A liquid crystal display device using a nematic phase such as a TN type has a large drawback that a viewing angle is narrow.

In these liquid crystal display devices, a thin film transistor (TF) is provided on a substrate to improve contrast.
An active matrix type liquid crystal display element in which T) is arranged has also been developed, and a large-capacity display such as 1000 × 1000 lines has been made possible. However, problems still remain in view angle and response speed.

On the other hand, instead of the conventional nematic liquid crystal,
Ferroelectric liquid crystal display device using ferroelectric liquid crystal such as chiral smectic C phase (NA Clark and ST Lagerw
all, Appl. Phys. Lett., 36 , 899 (1980), JP-A-56-10
No. 7216, U.S. Pat. No. 4,367,924) have been actively developed. Ferroelectric liquid crystal devices have bistability, memory properties,
Although it has excellent characteristics such as high-speed response and a wide viewing angle, it is difficult to display gradation.

Various methods have been proposed for gray scale display of a ferroelectric liquid crystal display element.
3-242624, JP-A-3-243915, Mori, et al., 16th Liquid Crystal Symposium, 3K111 (1990)., Toyoda, et al., 16th Liquid Crystal Symposium, 3K112 (1990)., Matsui, et al. 17th LCD Panel Discussion, 3F3
01 (1991)., K. Nito, et al., Proc. IDRC, 179 (1991).
And the like.

In these methods, an alternating voltage is applied to a monostable ferroelectric liquid crystal display device having no bistability, and the direction of the molecular axis of the liquid crystal molecules is uniquely changed according to the magnitude of the voltage. This is a method of performing gradation display. FIG. 1 is a diagram for explaining this principle. The chiral smectic C liquid crystal is sealed between a substrate having a pair of electrodes, forms a layer structure perpendicular to the substrate, and is oriented substantially parallel to the substrate and inclined at a tilt angle to the layer. FIG. 1 is a diagram showing a part of this state as viewed from above the substrate. Here, 1 represents a liquid crystal layer and 2 represents liquid crystal molecules. When no voltage is applied, the numerator is at position 3 and when a voltage is applied to the numerator, depending on the polarity, the numerator is in four directions if E> 0 and E <0.
If so, move in 5 directions. When a sufficiently high voltage is applied, the molecules move to position 4 or 5, but when the voltage is lower, the liquid crystal molecules stay at intermediate positions. Therefore, for example, the polarization axis of one of the polarizing plates is aligned with the position 3 and the polarization axis of the other polarizing plate is aligned orthogonally thereto.

FIG. 2 shows the relationship between the voltage of a ferroelectric liquid crystal display device having a polarizing plate as described above and the apparent tilt angle as viewed from above the substrate, and FIG. 3 shows the relationship between the voltage and the amount of transmitted light. From these figures, it can be seen that the apparent tilt angle increases as the voltage increases, and the amount of transmitted light increases. As described above, since the amount of transmitted light changes according to the voltage, gradation display can be performed. It should be noted here that, although it has been stated that the layer structure is formed perpendicular to the substrate and that it is oriented almost parallel to the substrate, the layered shape is used in the actual ferroelectric liquid crystal. The direction of the molecular axis of the liquid crystal molecules is not uniform but twisted from the upper surface to the lower surface of the substrate.

According to the techniques disclosed so far, the use of a monostable ferroelectric liquid crystal cell is described. However, according to the study of the present inventors, both monostable and bistable are used. It has been found that halftone display is also possible.

[0011]

When gradation display is performed in this manner, the liquid crystal molecules return to the position 3 shown in FIG. 1 when the electric field is cut off.
That is, when holding the liquid crystal molecules at a certain position, it is necessary to hold the electric field. Therefore, it is necessary to use an active element such as a TFT.

However, in the case of a ferroelectric liquid crystal, a part of the charge accumulated by the current caused by the impurity ions in the liquid crystal composition and the change in the director profile of the liquid crystal molecules is consumed and applied to the liquid crystal LC. However, there is a problem that the amount of transmitted light cannot be obtained according to the display. Therefore, in order to obtain good display, a liquid crystal material having a small voltage drop, that is, a high charge retention rate is required.

However, most of the ferroelectric liquid crystal compositions which have been generally studied in the past are liquid crystal materials containing a nitrogen-containing heterocyclic ring such as a pyrimidine ring or a pyridine ring, or a functional group such as an ester group or a cyano group. From his research,
It is known that liquid crystal cells are not suitable for driving TFTs in terms of charge retention.

Matsui et al. (17th Liquid Crystal Symposium, 3F301 (199
1).) Each of the ferroelectric liquid crystal materials used contains a functional group such as an ester group or a pyrimidine ring.
Not suitable for T drive. FIG. 8 described in JP-A-3-242624 shows E. coli. Although the drain waveform and the optical response waveform when a TFT is driven using the ferroelectric liquid crystal composition ZLI-4139 manufactured by Merck are described, the charge retention of the liquid crystal cell is very poor.

SUMMARY OF THE INVENTION In view of the above, the present invention provides a ferroelectric liquid crystal display device using a TFT, in which the charge holding ratio of the liquid crystal is increased and gradation display is possible.

[0016]

According to the liquid crystal display device of the present invention, a ferroelectric liquid crystal is provided between a substrate having an active element provided on each of a plurality of pixel electrodes serving as a display screen and a substrate having the electrodes. Pinching and active matrix drive
In a liquid crystal display element for displaying, the ferroelectric liquid crystal is represented by the general formula (1)

[0017]

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(However, R ¹ and R ² each independently represent a linear or branched alkyl group or an alkoxy group having 3 to 16 carbon atoms. X ¹ , X ² , X ³ , and , X ⁴ each independently represent a hydrogen atom or a halogen atom, and at least one of X ¹ , X ² , X ³ and X ⁴ is a fluorine atom.) Or the general formula (2)

[0019]

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(Provided that R ³ and R ⁴ each independently represent a straight-chain or branched alkyl group or alkoxy group having 3 to 16 carbon atoms. X ⁵ , X ⁶ , X ⁷ , X ^8, X ^9, and, X ¹⁰ each independently represent a hydrogen atom or, represents a halogen atom, at least one fluorine of ^{X 5, X 6, X 7} , X 8, X 9, and, X ¹⁰ At least one compound represented by the formula:

Here, R ¹ , R ² and R ³ , R ⁴ in the compounds represented by the general formulas (1) and (2) are represented by the following formulas:
The following specific examples can be given independently of each other.

-(CH ₂ ) ₂ -CH _3- (CH ₂ ) ₃ -CH _3- (CH ₂ ) ₄ -CH _3- (CH ₂ ) ₅ -CH _3- (CH ₂ ) ₆ -CH _3- ( _{CH 2) 7 -CH 3 - (} CH 2) 8 -CH 3 - (CH 2) 9 -CH 3 - (CH 2) 10 -CH 3 - (CH 2) 11 -CH 3 - (CH 2) 12 - _{CH 3 - (CH 2) 13} -CH 3 - (CH 2) 14 -CH 3 - (CH 2) 15 -CH 3 -CH (CH 3) 2 -CH (CH 3) -C 2 H 5 -CH ( _{CH 3) - (CH 2)} 2 -CH 3 -CH (CH 3) - (CH 2) 3 -CH 3 -CH (CH 3) - (CH 2) 4 -CH 3 -CH (CH 3) - ( _{CH 2) 5 -CH 3 -CH (} CH 3) - (CH 2) 6 -CH 3 -CH (CH 3) - (CH 2) 7 -CH 3 -CH (CH 3) - (CH 2) 8 - _{CH 3 -CH (CH 3) -} (CH 2) 9 -CH 3 _{-CH (CH 3) - (CH} 2) 10 -CH 3 -CH (CH 3) - (CH 2) 11 -CH 3 -CH (CH 3) - (CH 2) 12 -CH 3 -CH (CH 3 _{) - (CH 2) 13 -CH} 3 -CH 2 -CH (CH 3) 2 -CH 2 -CH (CH 3) -C 2 H 5 -CH 2 -CH (CH 3) - (CH 2) 2 - _{CH 3 -CH 2 -CH (CH 3} ) - (CH 2) 3 -CH 3 -CH 2 -CH (CH 3) - (CH 2) 4 -CH 3 -CH 2 -CH (CH 3) - (CH _{2) 5 -CH 3 -CH 2 -CH} (CH 3) - (CH 2) 6 -CH 3 -CH 2 -CH (CH 3) - (CH 2) 7 -CH 3 -CH 2 -CH (CH 3 _{) - (CH 2) 8 -CH} 3 -CH 2 -CH (CH 3) - (CH 2) 9 -CH 3 -CH 2 -CH (CH 3) - (CH 2) 10 -CH 3 -CH 2 - _{CH (CH 3) - (CH} 2) 1 _{1 -CH 3 -CH 2 -CH (CH} 3) - (CH 2) 12 -CH 3 -O- (CH 2) 2 -CH 3 -O- (CH 2) 3 -CH 3 -O- (CH 2 _{) 4 -CH 3 -O- (CH 2} ) 5 -CH 3 -O- (CH 2) 6 -CH 3 -O- (CH 2) 7 -CH 3 -O- (CH 2) 8 -CH 3 - _{O- (CH 2) 9 -CH 3} -O- (CH 2) 10 -CH 3 -O- (CH 2) 11 -CH 3 -O- (CH 2) 12 -CH 3 -O- (CH 2) _{13 -CH 3 -O- (CH 2)} 14 -CH 3 -O- (CH 2) 15 -CH 3 -OCH (CH 3) 2 -OCH (CH 3) -C 2 H 5 -OCH (CH 3) _{- (CH 2) 2 -CH 3} -OCH (CH 3) - (CH 2) 3 -CH 3 -OCH (CH 3) - (CH 2) 4 -CH 3 -OCH (CH 3) - (CH 2) _{5 -CH 3 -OCH (CH 3)} - (CH 2) 6 -CH 3 - _{CH (CH 3) - (CH} 2) 7 -CH 3 -OCH (CH 3) - (CH 2) 8 -CH 3 -OCH (CH 3) - (CH 2) 9 -CH 3 -OCH (CH 3) _{- (CH 2) 10 -CH 3} -OCH (CH 3) - (CH 2) 11 -CH 3 -OCH (CH 3) - (CH 2) 12 -CH 3 -OCH (CH 3) - (CH 2) _{13 -CH 3 -OCH 2 -CH (CH} 3) 2 -OCH 2 -CH (CH 3) -C 2
_{H 5 -OCH 2 -CH (CH 3} ) - (CH 2) 2 -CH 3 -OCH 2 -CH (CH 3) - (CH 2) 3 -CH 3 -OCH 2 -CH (CH 3) - (CH _{2) 4 -CH 3 -OCH 2 -CH} (CH 3) - (CH 2) 5 -CH 3 -OCH 2 -CH (CH 3) - (CH 2) 6 -CH 3 -OCH 2 -CH (CH 3 _{) - (CH 2) 7 -CH} 3 -OCH 2 -CH (CH 3) - (CH 2) 8 -CH 3 -OCH 2 -CH (CH 3) - (CH 2) 9 -CH 3 -OCH 2 - _{CH (CH 3) - (CH} 2) 10 -CH 3 -OCH 2 -CH (CH 3) - (CH 2) 11 -CH 3 -OCH 2 -CH (CH 3) - (CH 2) 12 -CH 3 The following specific examples can be given as examples of the skeleton structure of the compound represented by the general formula (1) having such R ¹ and R ² .

[0023]

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Further, the following specific examples can be given as examples of the skeleton structure of the compound represented by the general formula (2).

[0025]

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[0026]

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[0027]

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Further, in the method of driving a liquid crystal display element according to the present invention, in order to form one screen, the pixel electrodes are synchronized with turning on the active elements provided for each pixel electrode in the first frame. A magnitude of applying a selection voltage waveform of one polarity having a magnitude corresponding to a desired display and canceling the voltage to the pixel electrode in synchronization with turning on an active element provided for each pixel electrode in the second frame. And a selection voltage waveform having a polarity opposite to that of the voltage is applied.

[0029]

In the gradation display according to the present invention, the memory property of the ferroelectric liquid crystal is not used, and an active matrix drive in which an active element for holding an electric field is provided in each pixel is used as a driving method.

FIG. 4 shows, as an example, an equivalent circuit of a four-pixel liquid crystal display element using a thin film transistor (TFT) as an active element. The pixel electrodes are arranged in a matrix and apply an electric field to the liquid crystal at positions LC11, LC12, LC21 and LC22. Each pixel electrode is provided with a TFT. When driving the liquid crystal display element, a signal is sent from the scanning line G1, an electric field is applied to the gate electrodes G11 and G12 of the TFT, and the TFT in this portion is turned on. In synchronization with this, the source electrodes S11 and S1 are connected from the signal lines S1 and S2.
When a signal is sent to the pixel 2, an electric field is applied to the pixel electrode through the drain electrode D, and the liquid crystals LC11 and LC12 respond.
Next, this operation is also performed for G2.

When an electric field is applied to the pixel electrode, electric charges are simultaneously stored in the liquid crystal. Therefore, the electric field is continuously applied to the liquid crystal even when the TFT is turned off.

However, a part of the electric charge accumulated by the current caused by the impurity ions in the liquid crystal composition and the change in the director profile of the liquid crystal molecules is consumed,
The voltage applied to the liquid crystal drops, and the amount of transmitted light corresponding to the display cannot be obtained. Therefore, in order to obtain a good display, a liquid crystal material having a smaller voltage drop, that is, a liquid crystal material having a higher charge retention rate is a material more suitable for driving the TFT.

The voltage drop due to impurity ions and the like in the liquid crystal composition can be reduced by using a ferroelectric liquid crystal composition having a large specific resistance. Since the compounds represented by the general formulas (1) and (2) can obtain a large specific resistance value due to a chemical structure not containing an ester group, a cyano group, a nitrogen-containing heterocyclic ring, etc., they contain a large amount of these compounds. By using the ferroelectric liquid crystal composition, a voltage drop due to impurity ions or the like can be reduced.

The voltage drop due to the change in the director profile of the liquid crystal molecules occurs for a time corresponding to the response speed of the ferroelectric liquid crystal composition, that is, usually within several tens to several hundreds of microseconds. This is because the time width during which the TFT is turned on is shorter than the time width required for the response of the ferroelectric liquid crystal composition, so that the director profile of the liquid crystal molecules continues to change even after the TFT is turned off, and the spontaneous polarization accompanying the change occurs. This is because current flows for rotation. In order to reduce the current due to the director profile, a ferroelectric liquid crystal composition having a small absolute value of spontaneous polarization can be used to reduce the amount of charge consumed by rotation of spontaneous polarization or a ferroelectric liquid crystal having a fast response speed. It is necessary to make the time width required for changing the director profile of the liquid crystal molecules close to the time width for turning on the TFT using the composition. A ferroelectric liquid crystal composition containing a large amount of the compounds represented by the general formulas (1) and (2) has low viscosity and exhibits a high-speed response even when the absolute value of spontaneous polarization is small. The use of the liquid crystal composition makes it possible to reduce a voltage drop caused by a change in the director profile of liquid crystal molecules.

The driving method of the present invention will be described below.

In the present invention, the absorption axis of the polarizing plate coincides with the stable state of the liquid crystal alignment when an electric field of one polarity is applied, and this state is defined as black. Here, it is assumed that this one-polarity electric field has a negative polarity.

In the first frame, a positive selection voltage waveform having a magnitude corresponding to display is applied in synchronization with turning on the TFT. As a result, the apparent tilt angle of the liquid crystal increases, and the required brightness is obtained. When displaying black, a voltage of 0 V is applied. Since this voltage is held after the TFT is turned off, this brightness can also be held.

Next, in the second frame, a reverse polarity of a magnitude to cancel the electric field applied in the first frame,
That is, a negative voltage waveform is applied. As a result, the liquid crystal shifts to the above-mentioned stable state, and becomes a black state. The first and second
One frame is formed by these two frames, and the average brightness of the two frames is the display brightness. In this way, gradation display is enabled.

As described above, TF is used for the ferroelectric liquid crystal display device.
The use of T has the following advantages. First, since a TFT is used, liquid crystal molecules do not fluctuate due to a bias voltage applied at the time of non-rewriting as in the case of simple matrix driving, so that the amount of transmitted light for black display can be kept low, and high contrast can be obtained. Second, the amount of transmitted light can be changed by changing the voltage applied to each pixel, and gray scale display can be easily performed. There are advantages that the response speed is faster and the viewing angle is wider than an element in which a nematic liquid crystal is combined with a TFT.

Further, according to the present driving method, since there is no charge bias, a highly reliable liquid crystal element can be obtained.

[0041]

【Example】

Example 1 A ferroelectric liquid crystal composition having the composition shown in Table 2 was prepared using the compounds shown in Table 1. Table 2 shows the phase transition temperatures. Compositions 1 and 2 and 3 are all INAC
A phase sequence was shown, showing a smectic C phase at around room temperature.

[0042]

[Table 1]

[0043]

[Table 2]

On the other hand, an insulating film is formed on each of a pair of glass substrates on which a patterned ITO film is formed, and polyimide PSI-A-2101 (manufactured by Chisso Corporation) is spin-coated.
I rubbed. The pair of glass substrates were bonded together so that the rubbing directions were parallel to each other so that the distance between the substrates was 2 μm, and the compositions 1, 2 and 3 shown in Table 2 were vacuum-injected, and And

Spontaneous polarization in these liquid crystal display elements,
The tilt angle and the memory angle were measured. Table 3 shows the results. Here, the tilt angle is an angle which is の of the angle formed by the extinction positions in the two stable states when a liquid crystal display element is arranged between polarizing plates arranged in crossed Nicols and a voltage sufficient for liquid crystal response is applied. The memory angle is the angle formed by the extinction positions in the two stable states when no voltage is applied.

[0046]

[Table 3]

As shown in Table 3, these liquid crystal compositions had 200 μm despite their spontaneous polarization being as small as 2 or less.
High-speed responsiveness of less than sec is shown.

The liquid crystal display elements 1, 2 and 3 are arranged between polarizers arranged in crossed Nicols, and one of them is brought into a stable state by applying an electric field, and the absorption axis of the polarizer is set at the extinction position of the liquid crystal display element. Match. In this state, the transmitted light amount was measured at 28 ° C. while applying a rectangular wave of 60 Hz while changing the magnitude of the voltage. The results are shown in FIG. 7, FIG. 8 and FIG. However, with respect to the liquid crystal display element 2, the measurement was performed with the absorption axis of the polarizing plate coincident with the extinction position in each of the two stable states. (FIGS. 8A and 8B) From these figures, it can be seen that the amount of transmitted light changes continuously as the voltage increases. A gradation display can be performed using this characteristic. 8A and 8B, the characteristics of the amount of transmitted light with respect to the voltage do not match because there is a difference in the stability between the two states. In the case of FIG. FIG. 8B shows the case where the absorption axis of the polarizing plate is matched with the extinction position in the low stability state. Since the change in the amount of transmitted light is not steep with respect to the change in voltage, FIG.
The setting as shown in FIG.

Example 2 The liquid crystal composition 1 was introduced into a cell provided with a TFT.

First, the structure of a cell provided with a TFT and a method of forming the cell will be described.

FIG. 10 is a partial sectional view of a liquid crystal display device having a TFT. The liquid crystal cell of the present embodiment includes a pair of insulating substrates 6 and 7, and a gate electrode 25, a gate insulating film 11, and a channel layer 1 are formed on one of the substrates 6.
2, a TFT (thin film transistor) 31 composed of an etching stopper 13, a contact layer 14, a source electrode 32, and a drain electrode 33 and functioning as a switching element
And the pixel electrode 4 electrically connected to the drain electrode 33
1 is formed. Further, the protective film 17 is formed on the entire surface of the substrate on which the pixel electrode 41 is formed, and the alignment film 19 is formed on the protective film 17. On the other substrate 7, the transparent conductive film 3 and the alignment film 9 are formed. Alignment film 9,1
9 is rubbed after firing. A liquid crystal layer 18 into which the ferroelectric liquid crystal composition 1 is vacuum-injected is sealed between the pair of substrates 6 and 7.

First, T is formed on one substrate 6 by sputtering.
a film is formed and patterned to form 64 gate electrodes 2
5 is formed. Next, a SiNx film, an a-Si semiconductor film, and a SiNx film as a gate insulating film 11, a channel layer 12, and an etching stopper 13 are successively stacked without breaking a vacuum by plasma CVD, and only the uppermost SiNx is first subjected to a predetermined process. Patterned into a shape, and further contact layer 1
As n, n + -a-Si to which phosphorus is added is formed, and this and the a-Si semiconductor film are patterned into a predetermined shape. Next, a Ti film is formed by sputtering, and this and the n + a-Si film are patterned into a predetermined shape to form a source electrode 32 and a drain electrode 33. More IT
An O film is formed by sputtering, and is patterned into a predetermined shape to form a pixel electrode 41.

On the other hand, an ITO film is formed on the substrate 7 by sputtering.

A 0.03 μm PVA film is applied as the alignment films 9 and 19 on the pair of substrates 6 and 7 formed as described above and baked, and only the substrate 7 is uniaxially aligned by rubbing using a rayon-based cloth. Perform processing. These substrates 6 and 7 are bonded together with an epoxy-based seal member via a 2 μm seal spacer (not shown). The ferroelectric liquid crystal composition 1 is introduced between the substrates by a vacuum injection method and sealed.

As a method of treating the alignment films 9 and 19, there is an oblique deposition method in addition to the above-mentioned rubbing method. In the case of mass production of a large-screen liquid crystal display device, the rubbing method is advantageous. In the case of the rubbing method, a rubbing treatment is performed after forming an alignment film, and a parallel rubbing method (a method in which both substrates are rubbed and bonded so that the rubbing directions are the same), an anti-parallel method. Rubbing method (a method in which rubbing is performed on both of a pair of substrates and bonding is performed so that the rubbing directions are reversed), and single rubbing method (a method in which rubbing is performed on only one of the pair of substrates)
There is. In the case of the ferroelectric liquid crystal device of the present invention, any alignment method can be used, but the single rubbing method of rubbing only the substrate on which the thin film transistor is not formed as described above is particularly preferable. The following two reasons can be cited. First, a substrate on which a thin film transistor is not formed is flatter, and uniform rubbing can be easily performed. Second, when a rubbing treatment is performed on a substrate on which a thin film transistor is formed, static electricity generated by the rubbing treatment easily changes characteristics of the thin film transistor and causes dielectric breakdown between wirings.

Next, a method of driving the liquid crystal display device will be described with reference to FIG. FIG. 5 is an equivalent circuit diagram using a TFT. Each pixel of P _{1/1 to} P _{l / k} is provided with a TFT, and these TFTs are connected to _one scanning line G ₁ , G ₂ ,. . . , G _{n -
1} , _Gn , _{Gn + 1} , _{Gn + 2 ,.} . . , G _{l - 1,} G
_l and k signal lines S ₁ , S ₂ ,. . . , S _m , S _{m +
1} ,. . . , S _{k − 1} , S _k . Here, one important thing is the relationship between the method of aligning the polarizing plates and the sign of the electric field application. In the present invention, the absorption axis of the polarizing plate is made to match the stable state of the liquid crystal alignment when an electric field of one polarity is applied, and this state is defined as black. Here, it is assumed that this one-polarity electric field has a negative polarity.

FIG. 6 shows driving waveforms for driving the liquid crystal display element.

First, in the first frame, a signal is sent from the scanning line G _{1 for} a time t ₁ to turn on the TFT.
In synchronization with this, the pixels connected to _{G 1 (P 1/1,} P
_{1/2, P 1 / m} , P 1 / m + 1, P 1 / k - 1, P 1 /
_k , etc.) from the signal electrode. G is in the time following t ₁
A signal is sent from ₂ to turn on the TFT, and a signal is sent from the signal line in synchronization with the TFT. Thereafter, similarly, the TFTs connected to the respective scanning lines are sequentially turned on.

Now, after sending signals from all the scanning lines,
In the second frame, a signal is sent again from the scanning line G _{1 for} the time t ₁ to turn on the TFT. In synchronization with this, G
Pixels connected to the _{1 (P 1/1, P} 1/2, P 1 / m,
P _{1 / m + 1} , P _{1 / k − 1} , P _{1 / k} , etc.) are applied with a reverse polarity of a magnitude that cancels the electric field applied in the first frame, that is, a negative voltage is applied from the signal line. . Next t
The _first time to turn on the TFT sends a signal from the G _2,
In synchronization with this, a zero or negative signal is sent from the signal line. Thereafter, similarly, the TFTs connected to the respective scanning lines are sequentially turned on.

An example of the voltage waveform applied to the pixel at this time and the change in the amount of transmitted light at that time are also shown in FIG. First
In the frame, the apparent tilt angle of the liquid crystal increases according to the magnitude of the voltage, and the amount of transmitted light changes. Where voltage 0
Applying V corresponds to black display. This voltage is TF
Since the brightness is maintained after T is turned off, this brightness can also be maintained.

Next, in the second frame, regardless of the magnitude of the voltage, the liquid crystal shifts to a stable state in which the absorption axes of the polarizers are matched, and becomes a black state. One screen is formed by the first and second frames, and the average brightness of the two frames is the display brightness.

[0062] The example pixel P _1/1 is applied alternately electric field of the positive and negative large value for each frame, the pixel is white display. The voltage applied to the pixel P _1/2 4 first
Less than the voltage applied to the pixel P _1/1 in frame and thus P _1/2 becomes darker display than P _1/1. In the fifth and sixth frames, the applied voltage becomes zero, and this pixel changes to black display. In this way, gradation display is performed.

When the liquid crystal display device having the TFT element was driven by the above-described driving method, an infinite gradation was successfully displayed.

This is an example in which a bistable ferroelectric liquid crystal element is used. However, a monostable element may be used, and a monostable ferroelectric liquid crystal display element has more stable characteristics. Easy to obtain. Example 3 shows a liquid crystal display device using a monostable ferroelectric liquid crystal.

By combining color filters, a color display can be obtained.

(Example 3) In Example 2, liquid crystal composition 2 was used as a liquid crystal material, and the absorption axis of the polarizing plate was made to coincide with the extinction position in a state of high stability. When a liquid crystal display element was prepared and operated without the second frame (a frame for canceling the electric field of the first frame) as a drive waveform, infinite gradation could be displayed favorably. The time required for forming one screen was half that of the case of No. 2, and it was not necessary to average the amount of transmitted light in one screen, so that a bright display was obtained.

[0067]

Since the ferroelectric liquid crystal of the present invention has a high response speed and a high charge retention rate, it can be applied to a liquid crystal display device having an active element. Further, by driving a ferroelectric liquid crystal using an active element, a liquid crystal display element capable of large capacity, wide viewing angle, high contrast, and infinite gradation display (full color display) can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing an alignment state of a ferroelectric liquid crystal as viewed from above a substrate.

FIG. 2 is a diagram for explaining characteristics of a ferroelectric liquid crystal display device.

FIG. 3 is a diagram for explaining characteristics of a ferroelectric liquid crystal display device.

FIG. 4 is an equivalent circuit diagram of a liquid crystal display element using a TFT as an active element.

FIG. 5 is an equivalent circuit diagram of a liquid crystal display device showing one embodiment of the present invention.

FIG. 6 is a driving waveform diagram of the liquid crystal display element showing one embodiment of the present invention.

FIG. 7 is for explaining characteristics of a liquid crystal composition used in one example of the present invention.

FIG. 8 is for explaining characteristics of the liquid crystal composition used in one example of the present invention.

FIG. 9 is for explaining characteristics of the liquid crystal composition used in one example of the present invention.

FIG. 10 is a partial cross-sectional view of a liquid crystal display device showing one embodiment of the present invention.

[Explanation of symbols]

 6 Substrate 7 Substrate 25 Gate electrode 12 Channel layer 32 Source electrode 33 Drain electrode 41 Pixel electrode 18 Liquid crystal layer

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Makoto Shiomi 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (72) Inventor Mitsuhiro Mukoden 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka In-house (56) References Tokuyo Hei 2-503444 (JP, A) Tokuyo Hei 2-502914 (JP, A) International Publication No. 92/11241 (WO, A1) (58) Fields surveyed (Int. Cl. ⁷ , DB name) C09K 19/12 C09K 19/42-19/46

Claims (2)

(57) [Claims] 1. An active matrix comprising: a substrate in which an active element is provided for each of a plurality of pixel electrodes serving as a display screen; and a substrate having the electrodes.
In a liquid crystal display element which performs display by driving , the ferroelectric liquid crystal is represented by the following general formula (1): (However, R1 and R2 each independently represent a straight-chain or branched alkyl group or an alkoxy group having 3 to 16 carbon atoms. X1, X2, X3, and X4 each independently represent a hydrogen atom Or, represents a halogen atom, X1, X2, X3,
And at least one of X4 is a fluorine atom. ) Or general formula (2) (However, R3 and R4 each independently represent a linear or branched alkyl group having 3 to 16 carbon atoms or an alkoxy group. X5, X6, X7, X8, X9, and X10 each represent Independently represents a hydrogen atom or a halogen atom; X
At least one of 5, X6, X7, X8, X9, and X10 is a fluorine atom. A liquid crystal display device comprising at least one compound represented by the formula: 2. The method of driving a liquid crystal display device according to claim 2,
In order to form one screen, a one-polarity selection voltage waveform having a size corresponding to the display desired for the pixel electrode is applied in synchronization with turning on the active element provided for each pixel electrode in the first frame. In the second frame, a selection voltage waveform having a polarity opposite to that of the voltage for canceling the voltage is applied to the pixel electrode in synchronization with turning on the active element provided for each pixel electrode in the second frame. Claim 1
The driving method of the liquid crystal display element described in the above.