JPH06148598A - Driving method for liquid crytal diplay element - Google Patents

Abstract

PURPOSE:To provide a driving method for liquid crystal display element which generates good contrast in a display using an AFLC and which makes a gradation display in three steps possible. CONSTITUTION:A matrix driving liquid crystal display device is structured so that an anti-ferrodielectric liquid crystal with dielectric anisotropy presenting the chylal smectic CA phase is sandwiched between intersecting electrodes, wherein the pulse width modulation drive is such that the width of writing pulse at selection is larger than the width of pulse at the time of non-selection, and at the time of selection, one anti-ferrodielectric state and two ferro-dielectric states capable of being distinguished optically are at the time of selection formed in different positions on the select electrode so that a dielectric torque acts dominantly at the time of non-selection.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving method capable of gradation display of a matrix driving liquid crystal display device using an antiferroelectric liquid crystal.

[0002]

2. Description of the Related Art Liquid crystal display (LCD: LIQUID CRY)
Since STAL DISPLAY) is lightweight and thin, it is widely used as a display for measuring instruments such as pocketable calculators and testers, or a device for displaying figures and characters on a plane for decoration and POP. Recently, thin film transistors have been used as a large-capacity terminal display for televisions, personal computers, and workstations that display moving images in full color.

The origin of the shutter property in these is a so-called twisted nematic mode (TN type), which is a known technique (edited by Kobayashi and Okano, "Liquid Crystal" 1985).
Baifukan).

In order to perform large-capacity, high-quality display with the TN type, it is necessary to form as many active elements as TFTs on the substrate. Since it is difficult to stably form a TFT element on a substrate with high yield, the yield of manufacturing a panel having a size of about 10 inches cannot be improved. On the other hand, a larger LCD having a size of 20 inches or more cannot be manufactured. It has become difficult. Further, in terms of performance, since the TFT element is provided, there is a problem that the aperture ratio is lowered and the image quality becomes dark.

Therefore, in the technical development of LCDs, the technical possibility of displaying a clear image in a matrix without using a TFT is still pursued, and a spurt is nematic (STN type), a ferroelectric liquid crystal display (FLC) is used.
D: FERRO-ELECTRIC LIQUIDCRYSTAL DISPLAY) and anti-ferroelectric liquid crystal display (AFLCD: ANTIFERRO-ELEC)
TRIC LIQUID CRYSTAL DISPLAY) is expected.

In terms of operating principle, the former is similar to the TN type. By devising the twist of the liquid crystal layer, a steeper threshold characteristic is provided. Even in this case, the number of scanning lines is limited to about 400 to 600, which is equivalent to that of the TFT-LCD, and it is considered that the display capacity and the image quality are limited.

The latter two are ferroelectric liquid crystals (FLC: FERR).
O-ELECTRIC LIQUID CRYSTAL) chiral smectic C
Phase (SmC ^* : CHIRAL SMECTIC C PHASE) or antiferroelectric liquid crystal (AFLC: ANTIFERRO-ELECTRIC LIQUID CR
YSTAL) chiral smectic C _A phase (SmC _A ^*)
Since the liquid crystal layer itself has a memory effect, there is no limit to the number of scanning lines in principle, and a large-capacity high-quality display is possible.

For details of these, for example, "Fukuda and Takezoe," Structure and Physical Properties of Ferroelectric Liquid Crystal ", 1990, Corona Co., Ltd.). FLC shows bistability, bright and dark 2
It is difficult to achieve full color because only the state can be taken and gradation does not appear.

Area gray scales, frame gray scales, and combinations thereof are available as means for producing gray scales for such materials. In the former case, one display unit is composed of a plurality of pixels having different areas, but this requires an advanced fine processing technology.
The latter forms one state by scanning a plurality of times and changes the on-off ratio (duty) in it, but high-speed response of the liquid crystal is required. In either method, the number of gradations is limited to about 2 to 4 gradations.

On the other hand, AFLC is a material that can assume three stable states from the beginning, and in principle, it is possible to display three gradations within a single pixel within one scanning time. However, it has not been successfully displayed so far.

If gradation can be imparted to the pixel itself, a large increase in the number of gradations can be expected by combining area and frame gradation.

The following is a description of the current state of AFLCs and displays (AFLCDs) using the same according to the present invention.
p102-112, 1992, CMC Co., Ltd. will be described.

The AFLC is normally in the state shown in FIG. 1 (b) (hereinafter AF: anti-ferro) when there is no electric field or it is extremely weak.
And abbreviated). This is a state in which the direction of the spontaneous polarization P (100) is reversed for each smectic layer, and the direction of the director is zigzag. This state is electrically very stable because it has no remanent polarization component as a bulk. On the other hand, in FLC, the polarization direction P
Is not stable if they are all aligned and may exist in large domains with different polarization directions, and it is often difficult to maintain a constant state. In the AF state, the external electric field E (1
Two ferroelectric states (hereinafter abbreviated as F) can be induced in the (01) direction. This state is shown in FIGS. 1 (a) and 1 (c). FIG. 1 shows three possible stable states of AFLC. (Note that the layers in the figure are in the shape of "", which means the chevron structure described later.)

A characteristic of the electro-optical response of AFLC is that it exhibits double hysteresis. As an arrangement of elements for a display, as shown in FIG. 3, normally, the layer direction (300) of the smectic layer in the AF state is made to substantially coincide with the polarizer direction. FIG. 2 is a plot of the optical transmissivity in which a triangular wave of about 1 kHz is applied and synchronized with this. When the voltage is increased from 0 to the positive side,
The AF state (200) is maintained up to Vth, but beyond this, one spontaneous polarization component is gradually inverted, and finally Vst is reached.
The state changes to the F state (202). When the voltage is lowered from now on, the FLC state is maintained up to the voltage Vth 'which is lower than Vth, but below this, the reverse direction component is included,
When the polarity of the electric field changes to negative, everything is in the AF state (20
It becomes 1). This state continues to −Vth, and when it drops below −Vth, the state changes to the F (203) state having the opposite polarity.

Therefore, the direction of the director in the positive F state is + θ direction (301) in FIG. 3 and is −θ on the negative side.
(302) has almost the same transmission of light, and the two F states are optically used as the same state.

The AF state between -Vth and Vth is an intermediate state. This is induced from the F state when a weak DC voltage V (-Vth <V <0, 0 <V <Vth) is applied in addition to the continuous application of the triangular wave. Two F in FLC
Since there is only an LC state and these (−θ and θ here) are used separately, a sufficient contrast can be obtained. However, at the same tilt angle, the reference is exactly in the middle in AFLC, so the contrast is naturally lower than in FLC usage. . Therefore, optimization in the direction of increasing 2θ is necessary.

In actual matrix driving, the AF state and two equivalent F states are used alternately so that no DC component remains. It is possible to use only one of the F states by eliminating the DC component of the voltage waveform, but it appears that both are used due to mechanical pressure problems on the layers.

Typical waveform examples for this purpose are shown in FIG. This is a commonly used voltage modulation type.
The feature is that each frame (the first frame and the second frame in FIG.
(Frame) The DC component is eliminated by inverting the polarities of all basic signals. Even in this case, it is clear that there is no problem from the symmetry of the hysteresis curve in FIG.

The shaded portions 401 and 402 are ON and OFF write signals, but in terms of voltage, | V ₀ + V from FIG.
Writing is possible if _D |> | V _st | and | V ₀ + V _N | <| Vth | are satisfied. In FIG. 4, a is a bias ratio.

Aside from the AF state, which is relatively stable for forming a high-contrast image by matrix driving, the two F states have sufficient memory properties and the bias voltage (400) applied when not selected. ) Must be stable (steep threshold characteristic).

However, in many cases, the AF state does not show memory characteristics in both AFLC simple substance and mixture. At present, such materials cannot be used at all and cannot be omitted. In addition, even if the memory property is shown, in most cases it is smaller than the structurally allowed value as described later.

In the voltage modulation drive, the bias signal (400)
Due to the perturbation due to, the molecular axis fluctuates in both the F and AF states, and the leak of transmitted light cannot be avoided. This is also a factor that lowers the contrast. A further fatal problem is that there are three stable states and the possibility of three gradation display exists in principle, but this is not utilized at all. This shows that the AFLCD has only the meaning of the complement of the FLCD.

[0023]

[Problems to be Solved by the Invention] As shown above,
Based on the above reality of displays using AFLC,
The points to be solved by the present invention are listed below.

<No contrast> This is because the AF state is a reference, the memory property in the F state is low, and the threshold drive characteristic is bad, so that there is flicker and light leakage in matrix driving by voltage modulation. Responsible.

<The possibility of gradation is not utilized at all.
> As described above, AFLC should be capable of displaying three gradations.

<There are restrictions on usable materials> This is F
This is because there are many monostable materials that have no memory property in the LC state.

[0027]

In order to fundamentally solve the above problems, according to the present invention, an antiferroelectric liquid crystal having a positive dielectric anisotropy exhibiting a chiral smectic C _A phase between the cross electrodes is narrow. In the built-in matrix driving liquid crystal display device, the width of the writing pulse at the time of selection is longer than the width of the pulse at the time of non-selection, and pulse width modulation driving is performed. Of the two distinguishable ferroelectric states at different positions on the selection electrode, or at the time of selection, one or more ferroelectric states optically equivalent to one antiferroelectric state are formed on the selection electrode. All of the above-mentioned problems are solved by forming it in a position so that the dielectric torque acts predominantly when it is not selected.

The present invention will be described in detail below. The problems mentioned above arise from the following points. a) There is no memory property (monostable), or even if it exists, it is weak. b) The threshold characteristic is constantly required within the non-selection time. c) Treat the two F states optically equivalent.

The problems a) and b) are related to the integrity and stability of the memory state, and can be solved only by adopting a driving method that matches the structure of the natural AFLC layer. First, let's talk about this.

According to the recent X-ray diffraction experiment of the smectic layer of FLC and AFLC, the smectic layer is shown in FIG. 5 (b).
Not near the bookshelf structure like "
It has a chevron structure of the same figure (a) (also in FIG. 1) bent in the shape of (Figure 1) (Pee Leeker et al., Physical Review Letters, Volume 59, page 2658).
(1987)).

The reason for this is that the SmC _A layer is formed via the SmA layer in most cases, and the molecules are tilted below the transition temperature, so that the layer spacing d (500) of the smectic layer becomes narrower and the bulk If the volume is kept constant, the length in the direction perpendicular to the substrate extends to compensate for this. For this reason, it is inevitable that they will have to bend. In order to make this situation easy to understand, an enlarged view showing the arrangement of molecules having a chevron structure is shown in FIG.

Therefore, the smectic layer direction (601)
Is not parallel to the glass substrate, but is inclined by a certain finite angle α. In this state, the liquid crystal molecules tend to be as parallel to the substrate as possible, so that the position of the liquid crystal is eventually stable at S ₁ 'and S ₂ ' in FIG. The angle formed by the molecular axis and the substrate is called a pretilt angle, which is smaller than α.

The AF state is such that the directions of (θ) and (-θ) are alternately taken for each layer, but the molecules are unified into one of two F states by the application of the electric field E, but after that, the electric field is changed. E
If the memory is cut off, it will occupy one of the above positions. This position is inside the maximum structurally allowed position (S ₁ and S ₂ ). Normally S
_{At 1} and S ₂ , the contrast is maximized, so the expected contrast cannot be obtained. In some cases, there is no memory property and there is a case of returning to AF or either F state.

As a method for recovering the stable position of the F state to a preferable position, it is also attempted to realize it by controlling the pretilt angle and the tilt angle (θ) by selecting the alignment method and liquid crystal material. However, this obviously limits the material selection range, and it is clear that high pretilt angles and wide tilt angles do not guarantee bistability.

Another problem in this direction is the threshold problem. There is also no guarantee that these positions (S ₁ 'and S ₂ ') will be stable and immobile with respect to the applied voltage (400) when deselected.

It is the most reliable and general-purpose method that does not depend on the liquid crystal species to use the external field to bring the position of the liquid crystal molecules to the optimum position and surely restrain them (no threshold is required). . Since this has already been described by the present inventor, detailed description will be omitted (Japanese Patent Application No. 4-1867).
No. 15, Japanese Patent Application No. 4-219080).

That is, the molecular axis is moved (S ₁ '→ S ₁ or S ₂ ' →
By either stretching the layer or S ₂₎ are, to further constraint there are available dielectric torque is. As is well known in the nematic layer, the dielectric torque is the dielectric anisotropy Δε of the liquid crystal.
The way it works depends on whether the sign is positive or negative. Already known in the case of negative SmC ^* (Jepi Les Peasants et al., Liquid
crystal conference, digest p17 (1984), J.M.G.Ali, SID85, digest128 (1985)), where the chevron structure stretches due to a sufficiently large dielectric torque and α becomes zero, and the molecule becomes a substrate. It is considered to be parallel (see FIG. 5 (c)).

In the case of positive SmC ^* , which is a completely unique idea of the present inventors, the chevron structure remains almost unchanged, and the liquid crystal molecules have the effect of tilting in the direction perpendicular to the substrate. This is true even for SmC _A. By controlling the magnitude of the torque, the restraining effect at the optimum position can be obtained.

If this dielectric torque acts when it is not selected, points (a) and (b) are simultaneously overcome, and means for solving (c) is provided at the same time. Such an effect cannot be expected at all by the voltage modulation described above as the AFLC driving method.

The difference between voltage modulation and non-selection in voltage modulation is the difference in voltage magnitude, and the pulse width is essentially the same. If the pulse width is the same, the response of the spontaneous polarization P is dominant, and the qualitative difference that the dielectric torque exerts its own effect cannot be expressed. It is essential to reduce the pulse width at the time of non-selection to be smaller than that at the time of selection to reduce the contribution of spontaneous branching. For this purpose, pulse width modulation drive is required.

The manner of action of the dielectric torque differs qualitatively depending on whether Δε is positive or negative, but the optical effect is the same. Since the non-selected signal train in pulse width modulation constrains the Dilekda (molecular axis), it is clear that the pulse width modulation does not require the threshold characteristic at the time of non-selection, which was necessary in voltage modulation, and loses its meaning. Is.

The problem is the threshold characteristic within the selected time.
This means that it is possible to write in a stable manner by distinguishing among the three states when selecting. After writing, the state is completely retained by the subsequently applied high frequency pulse. It has already been described by the present inventor that writing can be performed without problems by devising a waveform (Japanese Patent Application No. 4-219080).

Another advantage of pulse width modulation is that it can be applied to monostable materials that are not stable in the F state. That is, when Δε is positive, monostability may induce bistability (when Δε is negative, it is unknown). When the liquid crystal molecules are brought from the F state on the monostable side or the AF state to the unstable F state on the opposite side by a write pulse, the torque is induced by the subsequently applied high frequency pulse, and the state on the opposite side is continuously restricted. It is possible to

As another advantage, an AFLC material whose F state is monostable can be used, which can be said when Δε is positive. Further, in the present invention, high speed writing is possible as compared with the voltage modulation driving. This is because the dielectric torque acts in the direction that assists the movement of molecules, so that the width of the write pulse can be shortened.

Next, the means for forming three states will be described. As described above, the three states can be formed by not using the intermediate state of AF as a reference but using one of the F states as a reference, that is, by matching this with the polarizer (dark). If θ is a material of approximately 22.5 degrees, the other F state is bright, and the AF state is an intermediate gray color, and three states are obtained. Figure 7 shows the layout.
You can do like this.

Since the general electro-optical characteristics of the AFLC liquid crystal have already been described, the matrix driving waveform for forming the three states and holding them will be described next. The ratio of the write pulse τw to the high frequency pulse τc when not selected is 2 to 5
The selection and non-selection signals and the three types of data signals in the case of are described in FIGS. The selection signal (i) includes a selection preliminary signal (i + 1), which is turned on, off, or turned off, for example, prior to the formation of the intermediate state (801: FIGS.
(Shaded part 1). By doing so, the scanning time is minimized. (I and i + 1 do not have to be adjacent.)

The problem is formation of the AF state, which is induced from one of the F states with a low DC voltage (800: black portion) in the figure. Although the pulse width τ required for inversion is secured in this portion, the voltage is ½ and induces the AF state. ON state is the shortest pulse width τ sufficient for inversion
Although w (803) is given, the pulse width is τ / 2 or less (804) so that the OFF state is kept in the aligned state, and no inversion occurs.

Whether the signal is not inverted or transferred to the AF state is based on the difference in response characteristics to the signals (800) and (804). In this invention, the adjustment of this part was the most important. Actually, the voltage V, the pulse width τw and the pulse ratio are selected according to the liquid crystal itself. AF induction part (80
If necessary, the voltage 0) can be set to a lower voltage or the width can be lengthened by adjusting (802) of the data signal portion for the intermediate state. (Of course, the changes in the first half of (802) are also included.)

Even when the pulse ratio is 6 or more, the analogy can be easily analogized, so a concrete example of the waveform will be omitted. Also, fine adjustment of individual unit signals and other waveforms are possible, but omitted because they are not essential.

With respect to another content of the present invention, the display principle is improved by applying the dielectric torque to the conventional display principle based on the AF state of FIG. 3 described in the prior art. It is a thing. Since this uses only the two states of AF and F, it can be realized by using a total of two drive waveforms, the middle of FIGS. Since one of the two F states is mainly used in this ON signal, the polarities of all unit waveforms may be inverted for each frame.

[0051]

EXAMPLE An electrode portion was patterned by a photolithography method by a conventional method so that the electrode portions face each other when the glass substrates with the ITO transparent electrode were opposed to each other. After that, a spacer member for adjusting the cell gap was similarly provided outside the electrode portion by the photolithography method, and prebaked in an oven at 140 ° C. to give appropriate rigidity. The material of the spacer portion is “photoresist MP1400 (manufactured by Shipley Corporation)”.
The step difference was approximately 2.0 μm. A polyimide film was formed on the opposite side according to a conventional method and subjected to rubbing treatment. This was post-baked for 1 hour in an oven at about 170 ° C. while being opposed and pressed. As a result, a cell having a gap of 1.8 μm was obtained. After that, the following AFLC was introduced into the cell at about 140 ° C. and cooled to 108 ° C. to obtain an oriented SmC _A ^* phase, which was held at this temperature.

[0052] (Φ represents a benzene ring)

The phase transition of this liquid crystal is as follows.

[0054]

The dielectric anisotropy Δε of this liquid crystal was examined by applying a high frequency of 500 kHz while keeping the cell temperature at about 140 ° C. It was confirmed to be positive because it was observed to change to homeotropic orientation.

The hysteresis curve due to the triangular wave of 500 Hz of this element was the same as in FIG. | Vth | = 10
V, | Vth '| = 4V, and | Vst | = 15V. A
The response speed of F → F and F → F (from θ is −θ) is 0.06m.
s, F → AF was 0.09 ms.

First, after transferring to the F state, the voltage was cut off and the memory property was examined. In both cases, relaxation from the maximum value was observed, but there was memory. When a high frequency pulse was applied to this state and the frequency at which the constraint effect was exhibited was determined, it was 2 μs. The maximum contrast was obtained when the voltage Vpp was 40V. Next, a test waveform including a three-state forming signal embedded in a high frequency bias signal (1210) as shown in FIG. 12 was applied to this element to examine the change in light transmittance. Write pulse width τw is 0.08
Writing of three states was possible with ms, Vpp = 40V, and pulse ratio = 40. This state is also shown as the light transmittance in FIG.

The lower limit of the writable pulse width τw was 0.05 ms and the voltage was up to 70V.

[0059]

According to the present invention, the display of three gradations having a sufficient contrast on the AFLCD can be performed at a high speed writing, and when combined with the area gradation and the frame gradation, the possibility of full color is opened.

[0060]

[Brief description of drawings]

FIG. 1 is a diagram illustrating an F state and a layer structure of molecules in an AF state which can be taken by AFLC.

FIG. 2 is an explanatory diagram of a voltage-light transmittance relationship of AFLC.
It is explanatory drawing which shows double hysteresis.

FIG. 3 is an explanatory diagram of arrangement of a liquid crystal and a polarizer.

FIG. 4 is an example of a voltage waveform of voltage modulation driving.

FIG. 5 is a cross-sectional view of an SmC layer.

FIG. 6 is a diagram illustrating the positions of molecules in a chevron structure.

FIG. 7 is an explanatory diagram of arrangement of a polarizing plate and elements.

FIG. 8 is a voltage waveform for pulse driving according to the present invention.

FIG. 9 is a voltage waveform for pulse driving according to the present invention.

FIG. 10 is a voltage waveform for pulse driving according to the present invention.

FIG. 11 is a voltage waveform for pulse driving according to the present invention.

FIG. 12 is an explanatory diagram showing the results of an example according to the present invention.

[Explanation of symbols]

100 ... Direction of spontaneous polarization 101 ... Electric field 102 ... Liquid crystal molecule 200 ... Positive ferroelectric DC-induced antiferroelectric state 201 ... Negative DC-induced antiferroelectric state 202 ... Positive DC Ferroelectric state of induction 203 ... Ferroelectric state of DC induction of negative polarity 300, 601 ... Direction of smectic layer 301, 302 ... Direction of director in ferroelectric state 401 ... On Write signal portion 402 ... Off Write signal portion 500 ... Layer spacing of smectic layer 800 ... Signal portion for inducing antiferroelectric state AF from ferroelectric state F801 ... Signal portion for aligning state to OFF state 802 ... Mainly adjustable of intermediate state write signal Part 803 ... ON state write signal 804 ... Non-reversible signal part with short pulse width 1210 ... Via in pulse width modulation Signal

Claims (2)

[Claims] 1. _A width of a writing pulse at the time of selection in a matrix driving liquid crystal display device in which an antiferroelectric liquid crystal having a positive dielectric anisotropy exhibiting a chiral smectic C _A phase is sandwiched between crossing electrodes. Is a pulse width modulation drive that is longer than the pulse width in the non-selected state, and two ferroelectric states that are optically distinguishable from one antiferroelectric state in the selected state are formed at different positions on the selection electrode. A driving method of a liquid crystal display device, wherein the dielectric torque acts predominantly when not selected. 2. _A width of a writing pulse at the time of selection in a matrix driving liquid crystal display device in which an antiferroelectric liquid crystal having a positive dielectric anisotropy exhibiting a chiral smectic C _A phase is sandwiched between crossing electrodes. Is a pulse width modulation drive that is longer than the pulse width when not selected, and when selected, one or more ferroelectric states optically equivalent to one antiferroelectric state are formed at different positions on the selection electrode. , A driving method of a liquid crystal display element, characterized in that dielectric torque acts dominantly when not selected.