JP2875675B2 - 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. More specifically, the present invention relates to a ferroelectric liquid crystal display device capable of gradation display and a ferroelectric liquid crystal display device driven by using a thin film transistor.

[0002]

2. Description of the Related Art At present, liquid crystal display devices are used in a wide range of fields such as clocks and calculators, OA equipment such as word processors and personal computers, pocket televisions, and the like. It is a thing using.

[0003] as a liquid crystal display device using a nematic liquid crystal, twisted Ted nematic (Twisted
Nematic TN type liquid crystal display device, super twisted wire (Supertwisted Wire)
(Fringence Effect, SBE type) liquid crystal display devices.

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. Have difficulty. Super twisted nematic (STN) to improve this TN type liquid crystal display element
Type liquid crystal display device and double layer super twisted nematic (DSTN) type liquid crystal display device have been developed, but as the number of lines increases, the contrast and response speed decrease, so the display capacity of about 800 x 1024 lines is currently limited. It is. In addition, TN type, STN type, DSTN
A display element using a nematic phase such as a mold has a large drawback that a viewing angle is narrow. In addition, sufficiently good values for contrast and response speed cannot be obtained.

On the other hand, a thin film transistor (TF)
An active matrix type liquid crystal display element with an array of T) has also been developed, which has enabled large-capacity display of 1000 × 1000 lines, etc., and high contrast has been obtained. However, there was a problem in response speed.

[0006] On the other hand, in place of a conventional nematic liquid crystal,
A ferroelectric liquid crystal display device using a ferroelectric liquid crystal such as a chiral smectic C phase (NA Clarkand S. et al.
T. Lagrewall, Appl. Phys. Let
t. , 36 , 899 (1980);
216; U.S. Pat. No. 4,367,924) is being actively developed . The liquid crystal display device of this, unlike the field-effect of the nematic liquid crystal display device utilizing the dielectric anisotropy of liquid crystal molecules, the ferroelectric liquid crystal spontaneous polarization polarity and molecular as polarity and are aligned in the electric field Is a switching liquid crystal display element.

The ferroelectric liquid crystal device is a chiral smectic C
Phase, a chiral smectic F phase, a chiral smectic I phase, or other ferroelectric liquid crystal. These ferroelectric liquid crystals have a helical structure, but when the ferroelectric liquid crystal is sandwiched between liquid crystal cells having a cell thickness smaller than the helical pitch, the helical structure is unwound, and as shown in FIG. an area for stably inclined by inclination angle θ liquid crystal molecules are against the normal smectic layers, a state where an area for stably coexist can be realized inclined by θ in the opposite direction. In FIG.
Z represents the layer normal to the smectic phase, n represents the major axis direction (director) of the liquid crystal molecules, and + represents the direction of spontaneous polarization.
In addition, by applying an electric field in a direction perpendicular to the paper surface in FIG. 1 , the direction of the liquid crystal molecules and their spontaneous polarization can be made uniform, and switching between the two states can be performed by switching the polarity of the applied electric field. It can be performed. With this switching, the birefringent light changes in the ferroelectric liquid crystal in the cell, so that the transmitted light can be controlled by sandwiching the ferroelectric liquid crystal element between two polarizers. Further, even if the application of the voltage is stopped, the alignment of the liquid crystal molecules is maintained in the state before the stop of the application of the voltage by the alignment regulating force at the interface, so that a memory effect can be obtained. In addition, as for the time required for the switching drive, a high-speed response on the order of μsec can be obtained because the spontaneous polarization of the liquid crystal and the electric field act directly.

As described above, the characteristics of the ferroelectric liquid crystal device include bistability, memory characteristics, high-speed response, and the like. In addition, having a very wide viewing angle is a great advantage. Can be.

The ferroelectric liquid crystal device has the above advantages, but one of the major problems is that it is difficult to display a gradation. Various methods have been proposed for gradation display of ferroelectric liquid crystal, but all have problems in practical use. The main methods proposed are shown below. Pixel division method (Proc. IDRC, 111 (1988).) Time division method (Proc. IDRC, 111 (1988).) Charge control method (IEEE Trans. Electron Devices, 36 , 18)
95 (1989).) (J. Appl. Phys., 66 , 1132 (1989).) Electric field gradient method (13th Liquid Crystal Symposium, 138 (1987).) (USP. 4712877). (USP.4802744) Texture method (Proc. SID, 32-2 (1991).) Method using monostable state (JP-A-63-143529)

[0010]

The above-mentioned pixel division method and time division method can obtain 4 gradations, 16 gradations, etc., but it is difficult to obtain infinite gradations. In addition, the charge control method and the electric field gradient method texture method aim at infinite gradation, but all of them aim at controlling the area ratio between two stable states of the ferroelectric liquid crystal element. Considering realistic mass production, it must be said that it is a difficult method. The method using the monostable state uses a monostable ferroelectric liquid crystal element and states that infinite gradation is possible, but only one of the positive and negative voltages is used. For example,
The amount of transmitted light is controlled by applying a positive voltage to a dark state when no electric field is applied. However, since no negative voltage is applied, this is extremely problematic in terms of reliability. .

The present invention has been made under such a circumstance, and provides a new method for realizing an infinite gradation without controlling the ratio between the bistable states of the ferroelectric liquid crystal element. is there.

[0012]

In order to solve the above-mentioned problems, a liquid crystal display device according to the present invention comprises a liquid crystal cell in which a ferroelectric liquid crystal is filled between a pair of substrates having at least an electrode film and an alignment control layer. Before and after, a pair of polarizing plates arranged in a crossed Nicols state was installed, and a liquid crystal display device in which the polarization direction of the polarizing plate was adjusted to one of the extinction positions when no electric field was applied, to 10 Hz.
The above rectangular wave voltage is applied , and apparently according to the value of the voltage.
And continuous tone display is obtained by changing the tilt angle .

Further, the present invention is, on the one substrate of the pair of substrates, a plurality of scanning electrodes and a plurality of signal electrodes are formed in a matrix, thin film transistors at intersections of the scanning electrodes and the signal electrodes Is provided.

Further, according to the driving method of the present invention, in the above-mentioned liquid crystal display device, in the odd-numbered frame, zero or positive signal corresponding to the display required for the signal electrode is synchronized with the turning on of the thin film transistor by sending a signal from the scanning electrode. And applying a zero or negative selection voltage waveform corresponding to the display required for the signal electrode in the even-numbered frames.

[0015]

A pair of polarizing plates arranged in a crossed Nicols state are installed before and after a liquid crystal cell filled with ferroelectric liquid crystal between a pair of substrates having at least an electrode film and an alignment control layer, and when no electric field is applied. By applying a rectangular wave voltage of 10 Hz or more to a liquid crystal display device in which the polarization direction of the polarizing plate is adjusted to one of the extinction positions of the above, the apparent tilt angle is changed according to the value of the voltage. There is provided a ferroelectric liquid crystal display device characterized by obtaining a gradation display.

Conventionally, in a liquid crystal device in which a ferroelectric liquid crystal is filled in a cell having a small cell thickness, it is known that the helix is released and two bistable states appear, and switching between these two states can be performed by an electric field. It was generally believed that an intermediate state between these two states could not be created. However, the inventors have found that an intermediate state can be created in an electric field applied state. FIG. 2 shows the relationship between the tilt angle and the voltage measured by applying a rectangular wave of 0.5 Hz to the ferroelectric liquid crystal cell. The tilt angle is 1/2 of the angle between the extinction positions on both sides of the layer normal. As is apparent from this figure, the tilt angle increases as the voltage increases. It is not impossible to interpret this as an increase in the tilt angle induced by an electric field, but it is more reasonable to interpret it as a change in the apparent tilt angle due to a change in the director profile in the liquid crystal cell. An example of a molecular alignment model in a ferroelectric liquid crystal device is shown in FIG. 3 (M. Koden et al., 2nd Japan-Korea Information Display Joint Study Group, EID91-47 (1991)). FIG. 3A shows C1U (C1
-Uniform) Molecular orientation model in orientation. FIG.
(B) is a molecular orientation model of C2 orientation. In a ferroelectric liquid crystal device, molecules are not evenly aligned from the upper surface to the lower surface of the cell. Molecules are regulated at the interface with the substrate and at the seam surface of the chevron layer, which forces a strained arrangement. Drawing this with the director profile
become that way. FIG. 4A shows a director profile in C1U (C1-uniform) orientation, and FIG. 4B shows a director profile in C2 orientation. Here, the vertical axis represents the cell thickness, and the horizontal axis represents the twist angle (the angle from the layer normal of the director of the liquid crystal molecules when the cell is viewed from above). θ _app is the apparent cone angle. (See FIG. 5), ψ ₀ is the twist angle at the substrate interface,
ψ _IN is the twist angle at the chevron seam. FIG.
In the state described above, the memory angle θ _m is approximately given by θ _m = (ψ ₀ + ψ _IN ) / 2.

FIG. 6 shows an assumed diagram of a director profile when electric fields in the order of E ₁ <E ₂ <E ₃ are applied to this state. FIG. 6A shows a C1U (C1-uniform) orientation, and FIG. 6B shows a C2 orientation. The director profile changes as the electric field increases,
It can be explained that the extinction position changes, and the result of FIG. 2 can be well explained.

By the way, in FIG.
Although the tilt angle changes depending on the voltage in the range of 0.
At 5 V, the tilt angle is already 10 °, and if this is the case, discontinuity will occur in the range of 0 to 0.5 V.
The measurement was performed for a region of 0.5 V or less, and the result of observation with a polarizing microscope at that time is shown in FIG. As shown in FIG. 7A, there is a case where a domain that is oppositely stored is generated by applying an electric field from a uniform state of the entire surface. In each of these regions, the extinction position changes by about 1 to 4 ° depending on the sign of the electric field. Therefore Jo <br/> status of domain inversion does not cause an area can be understood as a state at the time of applying an electric field E ₁ shown in FIG. That is, molecules are not switched at three interfaces in the ferroelectric liquid crystal cell, and only the director profile is changed. Regions undergoing domain inversion is considered to cause the director profile changes, such as when E ₂ is applied in FIG. When two memory states, Up and Down, are provided from the beginning, only the area ratio between the two memory states changes as shown in FIG. 7B. In each of these regions, the extinction position changes by about 1 to 4 ° depending on the sign of the electric field. State of the region does not cause therefore domain inversion can be understood as a state at the time of applying the electric field E ₁ shown in FIG. That is, molecules are not switched at three interfaces in the ferroelectric liquid crystal cell, and only the director profile is changed. The region where domain inversion has occurred is indicated by E ₂ in FIG.
It is considered that the director profile changes as in the case of application. Conversely, when a voltage of 0.5 V or more is applied, it corresponds to the states of E ₂ and E ₃ , and it can be said that a director profile change accompanying switching at the interface. Anyway, continuous gradation cannot be performed as it is.

Then, next, as shown in FIG. 7B, the frequency is set to 0.5 to 60H in a portion where two memory states are mixed.
Observed with a polarizing microscope while applying a voltage of 0 to 10 V while changing the voltage to z, at a frequency of 10 Hz or less, an increase or decrease in the area of the domain occurs with the switching of the polarity. It was determined that no movement of the boundaries of the territory occurred. In fact, FIG.
In the state where the memory is uniformly stored as shown in FIG.
When a rectangular wave voltage of the above frequency is applied, FIG.
The state as in (b) did not occur, and only the brightness changed with the voltage.

Therefore, the polarization direction of the polarizing plate placed in the crossed Nicols state is made coincident with the extinction position of the ferroelectric liquid crystal element in which one memory state is obtained by the application of an electric field.
The transmitted light intensity was measured by applying a rectangular wave of z. The memory angle of the ferroelectric liquid crystal element is set to 1/2 of the angle between the two extinction positions, which is usually smaller than the tilt angle of the liquid crystal. This situation is shown in FIG. The tilt angle θ is sufficient for the electric field (± 20
V) 1/2 of the angle between the extinction positions when applied. FIG. 9 shows the measurement results. As can be seen from the figure, the amount of transmitted light continuously increases as the voltage increases. It can be seen that infinite gradation is possible by using this characteristic. In this method, the applied voltage waveform has no positive or negative bias, and there is no problem in reliability.

[0021]

As a driving method utilizing such characteristics, there is an active matrix driving. In particular, driving using a thin film transistor is preferable. FIG. 10 shows an equivalent circuit of an active matrix liquid crystal display element using a thin film transistor (TFT). When driving the liquid crystal, a signal is sent from the scanning line 1 to apply an electric field to the gate electrode G to turn on the TFT 3. When a signal is sent from the signal line 2 to the source electrode S in synchronization with this, charges are accumulated in the liquid crystal LC4 through the drain electrode D, and the liquid crystal responds by an electric field generated by this.

A specific example of the present invention is described by using one scan electrode G, G
₂ ,. . . , G _n−1 , G _n , G _{n + 1} ,
G _{n + 2,.} . , G _1-1 , G ₁ and k signal electrodes S ₁ , S ₂ ,. . . , Sm, S _{m + 1} ,. . . , S
_A description will be given using a liquid crystal display element as shown in FIG. 11, in which _k-1 and Sk are formed in a matrix, and a ferroelectric liquid crystal is combined with an active matrix substrate in which thin film transistors (TFTs) are arranged at respective intersections. TFT at each intersection
Are connected to the scanning electrodes, and the source electrodes are connected to the signal electrodes. P _1/1 , P _{1/2 ,.} . . , P
_{1 / m} , P1 _{/ m + 1,.} . . , P _{n / 1} ,
P _{n / 2,.} . . , P _{n / m} , P _{n / m + 1,.} . . And the like indicate pixels connected to the drain electrode of the TFT formed at each intersection. FIG. 12 shows drive waveforms for driving this liquid crystal display element.

Firstly, the time t _1, is turned ON TFT sends a signal from the scanning electrode G _1. 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
1 / k-1} , P1 _{/ k} , etc.) are applied from the signal electrode to a zero or positive voltage corresponding to the display required. The time following t ₁ ON the TFT sends a signal from the G _2, sends a signal from the signal electrodes in synchronism with this. Thereafter, similarly, the TFTs connected to the respective scanning electrodes are sequentially turned on.

[0025] Now, after sending a signal from all of the scanning electrode, sends a signal from the time scanning electrode G ₁ of t ₁ again TF
Turn T ON. 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.) to zero or a negative voltage corresponding to the display signal electrodes or we mark pressurizing the sought.
ON the TFT is in the time following _{t 1} sends a signal from the _{G 2}
And a zero or negative signal is sent from the signal electrode in synchronization with this. In the same manner as described above, T
Turn on FT. FIG. 12 shows an example of a voltage waveform applied to the pixel at this time and a change in transmitted light amount at that time.
Large positive and negative electric fields are alternately applied to the pixel P _1/1 for each frame, and this pixel performs white display. Pixel P
Pixel P is a voltage applied to the _1/2 in the first 4 frames
Smaller than the voltage applied to _1/1 , so that P
_1/2 is darker than P _1/1 , and a halftone display is obtained. In the fifth and sixth frames, the applied voltage becomes zero, and this pixel changes to black display. Here, one important thing is the relationship between the method of aligning the polarizing plates and the sign of the electric field application. Figure in the case of 12 must align the polarization direction of the polarizing plate on the extinction position of the memory state occurs upon application of a negative electric field. In other words, if the voltage applied to the pixel is reduced to zero after the positive voltage is applied, white display is obtained when black display is desired. Therefore, in FIG. 12, one set is formed in two frames, and the voltage value must be changed only in the first frame of a pair of frames formed in two frames.

A color display can be obtained by combining color filters.

The use of the ferroelectric liquid crystal device of the present invention as described above has the following advantages.

First, when a black state is required, a high contrast is obtained because no electric field is applied to the liquid crystal. Second
In addition, by changing the voltage applied to each pixel, the amount of transmitted light can be changed, and gradation display can be easily performed.
Third, since the polarity of the applied voltage is switched for each frame, a highly reliable liquid crystal element without bias in electrification can be obtained. In addition, 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.

As a method for forming the alignment treatment layer, there are a rubbing method, an oblique vapor deposition method, and the like. 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. 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. There are a 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 a single rubbing method (a method in which rubbing is performed on only one of the pair of substrates). In the case of the ferroelectric liquid crystal device of the present invention, any alignment method can be used, but a single rubbing method in which a rubbing treatment is performed only on a substrate on which a thin film transistor is not formed 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.

The above-mentioned thin film transistor may be formed by using a-Si as a semiconductor film or poly-Si as a semiconductor film.
It can be used as such have use.

[0031]

【Example】

<Example 1> An insulating film was formed on each of a pair of glass substrates on which a patterned ITO film was formed, and polyimide PSI-A-2001 (manufactured by Chisso Corporation) was spin-coated and rubbed. The pair of glass substrates was bonded with a cell thickness of 2 μm so that the rubbing directions became parallel, and a ferroelectric liquid crystal composition having the composition shown in Table 1 was vacuum-injected. Table 1 shows the physical properties of the liquid crystal composition.

The fabricated ferroelectric liquid crystal cell was set on a polarizing microscope, and the tilt angle was measured at 25 ° C. while applying a rectangular wave of 0.5 Hz. The tilt angle was defined as 角度 of the angle between two extinction positions when a rectangular wave was applied. Figure 2 shows the results.
Shown in It can be seen that the tilt angle changes depending on the voltage. However, when the voltage is 0.5 V or less, the area of the two memory states only increases or decreases as shown in FIG.

<Example 2> The ferroelectric liquid crystal cell used in Example 1 was set in a polarizing microscope,
When observed with a polarizing microscope while applying a rectangular wave of Hz, no movement of the domain boundary was observed.

Example 3 The ferroelectric liquid crystal cell used in Example 1 was set in a polarizing microscope, and the transmitted light intensity was measured at 25 ° C. while applying a 60 Hz rectangular wave. FIG. 9 shows the results.
Shown in It can be seen that the amount of transmitted light changes continuously with the voltage.

[0035]

According to the ferroelectric liquid crystal display device of the present invention,
If, that-out <br/> continuous tone display without using a domain modulation scheme in realization. Further, the ferroelectric liquid crystal of the present invention can be obtained.
Achieved by using ferroelectric liquid crystal that shows two extinction positions easily
Manufacturing process for the liquid crystal display device.
But good in terms of yield, manufacturing cost, reliability, etc.
And is advantageous. In addition, the present invention
Active matrix driving of the electro- optical liquid crystal display device can provide a liquid crystal display device capable of large capacity, wide viewing angle, high contrast, infinite gradation display, and full color display.

[Brief description of the drawings]

FIG. 1 is a schematic diagram for explaining switching of a ferroelectric liquid crystal element.

FIG. 2 is an explanatory diagram showing a relationship between a voltage of a ferroelectric liquid crystal and a tilt angle.

FIG. 3 is an explanatory diagram showing a molecular orientation model of a ferroelectric liquid crystal element.

FIG. 4 is an explanatory diagram showing a director profile of a ferroelectric liquid crystal element.

FIG. 5 is an explanatory diagram for explaining an apparent cone angle.

FIG. 6 is an explanatory diagram showing how a director profile changes when an electric field is applied.

FIG. 7 is a schematic diagram of a polarizing microscope showing a state of switching when a weak electric field is applied.

FIG. 8 is a layout view of the ferroelectric liquid crystal element of the present invention.

FIG. 9 is an explanatory diagram showing a relationship between an applied voltage and a change in transmitted light amount in the ferroelectric liquid crystal element of the present invention.

FIG. 10 is an equivalent circuit diagram for describing an active matrix liquid crystal display.

FIG. 11 is an explanatory diagram for explaining an active matrix type antiferroelectric liquid crystal element of the present invention.

FIG. 12 is an explanatory diagram for describing a driving method according to the present invention.

[Explanation of Signs] 1: Scanning line 2: Signal line 3: Thin film transistor 4: Ferroelectric liquid crystal

Claims (3)

(57) [Claims] 1. A pair of polarizing plates arranged in a crossed Nicols state before and after a liquid crystal cell filled with a ferroelectric liquid crystal exhibiting bistability between a pair of substrates having at least an electrode film and an alignment control layer. The extinction level of the ferroelectric liquid crystal does not match the layer normal, and the polarization direction of the polarizing plate is adjusted to one of the extinction levels when no voltage is applied. A liquid crystal display device, wherein a voltage is applied, and an apparent tilt angle is changed in accordance with the value of the voltage to obtain a continuous tone display. 2. The method according to claim 1, wherein one of the pair of substrates is provided on one of the substrates.
The liquid crystal display device according to claim 1, wherein a plurality of scanning electrodes and a plurality of signal electrodes are formed in a matrix, and a thin film transistor is provided at each intersection of the scanning electrodes and the signal electrodes. 3. A pair of polarizing plates disposed in a crossed Nicols state before and after a liquid crystal cell filled with bistable ferroelectric liquid crystal between a pair of substrates having at least an electrode film and an alignment control layer. Installed, the extinction position of the ferroelectric liquid crystal does not coincide with the layer normal, the polarization direction of the polarizing plate is configured to match one of the extinction positions when no electric field is applied, and the pair of substrates A plurality of scanning electrodes and a plurality of signal electrodes are formed in a matrix on one of the substrates, and an active element is provided at each intersection of the scanning electrodes and the signal electrodes. Apply a square wave voltage,
A method of driving a liquid crystal display device in which an apparent tilt angle is changed according to a value of the voltage to obtain a continuous tone display, wherein a signal is sent from the scan electrode to turn on an active element. Then, in the odd-numbered frames, a zero or positive selection voltage waveform corresponding to the display required for the signal electrode is applied,
A method for driving a liquid crystal display device, wherein a zero or negative selection voltage waveform corresponding to a display required for a signal electrode is applied to a signal electrode in an even frame.