JPH0473930B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electro-optical device using a chiral smect liquid crystal.

Liquid crystals are used in a variety of displays.
Due to its excellent characteristics such as small and thin panels and low power consumption, it is often used for displays on clocks and calculators. The liquid crystal used in these displays is a thermotropic liquid crystal, which assumes various liquid crystal phases within a certain temperature range. This liquid crystal phase has a layered structure or not, nematic (abbreviated as N) without a layer and smectic (hereinafter referred to as Sm) with a layer.
It is broadly divided into Sm is further divided into uniaxial smectic A (SmA) and biaxial smectic C (SmC).
are categorized.

Figure 1 schematically shows the molecular arrangements of N, SmA, and SmC. a represents N, b represents SmA, and c represents SmC.

Furthermore, if the liquid crystal molecules have an asymmetric carbon and are not racemic, the liquid crystal phase will have a twisted structure.

In N, it is chiral smectic (N ^* ), and in SmC, it is chiral smectic C (SmC ^* )
It is.

In general, SmC ^* not only has a twisted structure, but also
It has a dipole moment in the direction perpendicular to the molecular axis and exhibits ferroelectricity.

Ferroelectric liquid crystal was developed in 1975 by Meyer (J.de.Phys.36,
1975, 69) et al. and their existence was demonstrated. The liquid crystal synthesized at that time was commonly known as DOBAMBC.
It is called (2-methylbutyl P-[(P-n-decyloxybenzylidene)amino]), and is still actively used in research on ferroelectric liquid crystals.

The molecular arrangement of SmC ^* can be schematically shown as shown in Figure 2.

The molecular axis is tilted by an angle θ with respect to the normal direction of the layer, and this angle is constant in all layers.

However, the azimuth angle φ changes little by little depending on the layer,
The molecular orientation gives rise to a helical structure.

The pitch of this spiral varies depending on the liquid crystal, but is usually around several μm.

When SmC ^* is injected into a cell as thin as 1 μm,
The helical structure disappeared and the layers became perpendicular to the cell substrate.
It takes on the structure of SmC.

SmC ^* liquid crystal has an electric dipole moment perpendicular to the molecular axis, so in a thin cell the dipole moments are aligned parallel to the layers. When an electric field is applied upward or downward, the molecules take positions tilted ±θ with respect to the normal to the layer.

By utilizing birefringence, the two states of ±θ can be made to correspond to brightness and darkness, and can be used as electro-optical devices such as displays.

FIG. 3 shows a schematic diagram of a conventional electro-optical device using two polarizing plates.

This driving principle is based on Clark and Lagerwall (Appl.
Phys Lett.36.899.1980).

They further argued that this driving principle has the following characteristics:

That is, (1) high-speed response on the order of μsec (2) memory performance (3) desirable threshold characteristics Among these characteristics, the fast response indicates a response on the order of μsec in our observations.

Furthermore, as they claim, there is a memory that maintains that state even if the electric field is turned off after it is brought into a state of ±θ by applying an electric field.

However, the desired threshold characteristics could not be obtained in our observations.

According to our data, Vth, Vsat are Vth=500 (mV) Vsat=5(V) It showed a value like .

In driving using the voltage averaging method, etc., it is impossible to perform time-division driving such that a voltage of Vsat = 5V is applied to selected points, and a voltage of 500 (mV) or less is applied to non-selected points.

The purpose of the present invention is to present a new principle and method for time-division driving using SmC ^* , and to enable multi-division driving in a range that cannot be achieved with TN liquid crystals.

Hereinafter, the details of the present invention will be explained with reference to Examples.

FIG. 4 shows an embodiment of a cross-sectional view of a panel using SmC ^* .

A transparent electrode 7 constituting a scanning electrode and a signal electrode is formed on each inner surface of two transparent substrates 5, and a sealing portion 9 is formed.
A smectic liquid crystal 8 is sealed between the substrates.

Compared to normal panel structure, gap
It is approximately 2μm and extremely thin (less than 4μm).

Also, as shown in Figure 3, one of the two polarizing plates is aligned with the molecular axis direction of the molecules in either ±θ state and the polarization direction, and the other plate is aligned with the molecular axis direction in the same way. Or place it at a 90° angle.

When a sufficiently high DC voltage is applied to this panel to bring the molecules into either ±θ state, and then AC is applied to the liquid crystal, when the voltage shown in Figure 6b is applied,
The change in optical transmittance shown in FIG. 6a is obtained.

As is clear from the figure, when alternating current is applied, the optical transmittance oscillates and converges to an intermediate state. Fig. 7 f ₁ , f ₂ , f ₃ are calculated by keeping the applied voltage constant.
It shows the change in optical transmittance when the frequency of alternating current during the holding period is changed from high frequency to low frequency, and V ₁ ,
V ₂ and V ₃ represent changes in optical transmittance when the applied voltage during the holding period is changed from low voltage to high voltage while keeping the frequency constant.

As shown in FIG. 7, the alternating current frequency is high and the voltage is low, so there is a tendency for the change in optical transmittance to be small. In other words, the relaxation time becomes longer.

In the present invention, the stable state of ferroelectric liquid crystal molecules is changed by a DC voltage, and then the relaxation time of the optical change is lengthened by applying an AC voltage, so that the liquid crystal molecules are in a state close to ±θ. It seems to have stopped. The idea is to utilize this for display purposes.

In other words, the molecules to which an alternating current voltage is applied are thought to remain oscillating around b or b' in FIG. 5. The driving principle of the present invention is to utilize this state to perform display, etc.

Regarding this characteristic, panels treated with uniaxial orientation have a strong orientation force and quickly return to the initial orientation state, but panels with PVA rubbing (PVA refers to polyvinyl alcohol) have a relatively weak orientation force. The display state was maintained using AC voltage and a good display could be obtained.

In actual driving, one example of the driving waveforms applied to the liquid crystal is as shown in a and b in FIG. 17.

Among these waveforms, the pixel on the selected scanning electrode is turned on (whether it is turned off or turned on depends on the polarization direction of the polarizing plate, but for now, the state where the pixel is turned on is obtained by the waveform a in Fig. 17). The waveform of a in FIG. 17 is applied to the pixel that is The movement of the liquid crystal molecules at this time is that when a high voltage Vap is applied, they move to the position a or a' in Figure 5, or a position close to that position, and then an AC voltage with equal positive and negative amplitudes is applied. , b or b'.

In this case, in order to select the frequency of the driving waveform, it must be set so that the liquid crystal molecules can sufficiently move to positions a and a' with a high voltage Vap.

Furthermore, since the relationship between response time and voltage shown in FIG. 8 is linear, the frequency can be increased by increasing the voltage.

The state of the liquid crystal molecules can be stably maintained by increasing the frequency and decreasing the amplitude of the AC voltage applied after applying the voltage capable of inverting the ferroelectric liquid crystal as described above.

Type of driving element or type of display (e.g.
It is sufficient to set an appropriate driving frequency and driving voltage Vap within a range that does not deteriorate the display condition depending on whether the display is fixed (fixed display or moving image display).

The drive circuit we used in the experiment was CMOS,
It was driven with a driving voltage Vap of 20V.

In addition to CMOS, it can be driven by circuit elements such as FET, bipolar transistor, TTL, and VMOS.

FIG. 9 shows the response time as a function of temperature.

Figure 9, Tc-A-T/°C represents the temperature difference from the phase transition temperature Tc in °C, and τ is the response time in units of
Expressed in msec.

Next, examples of actual drive waveforms will be shown to explain the present invention in further detail.

The following four basic drive waveforms are supplied to the scan electrode drive circuit that drives the scan electrodes of the liquid crystal panel and the signal electrode drive circuit.

φy...Selected scanning electrode signal...Non-selected scanning electrode signal φx...Inverted signal electrode signal...Non-inverted signal electrode signal Scanning in which the above φy is combined from the scanning electrode drive circuit to the scanning electrode of the liquid crystal panel. Signals are provided line sequentially. From the signal electrode drive circuit, φx is applied to the signal electrodes as a data signal selected according to display data in synchronization with the scanning signal.

10 to 13 are examples of basic drive waveforms of the present invention.

It is considered that a voltage of ±1/3 Vap is applied when not selected.

Further, in the figure, the suffixes l and d are symbols corresponding to lighting and non-lighting (either negative or positive depending on the orientation of the polarizing plate).

In actual driving, scanning of lighting and scanning of non-lighting may be alternately repeated and displayed, and any of the waveforms l or d shown in FIGS. 10 to 13 may be used.

FIGS. 33 to 36 show voltage waveforms of composite voltage pulses applied to pixels using the basic drive waveforms shown in FIGS. 10 to 13, respectively.

The drive waveform shown in FIG. 14 is another example of the basic drive waveform driven by AC pulses, and FIG. 37 shows the voltage waveform of the composite voltage pulse applied to the pixel by this drive waveform.

FIG. 15 shows a basic drive waveform configured to alternately continue the waveforms in FIGS. 10 and 11 to turn on in the first frame and turn off in the second frame, and FIG. 14 respectively show similar basic drive waveforms based on FIG.

FIG. 17 is a diagram showing a specific waveform of a composite voltage pulse applied to a pixel in the case of the driving method in which lighting and non-lighting are alternately performed.

Next, from the four basic drive signals shown in FIG. 10, the drive waveforms to be applied to each pixel shown in FIGS. 17a and 17b are derived. FIG. 31 is a diagram of voltage waveforms sequentially applied to scanning electrodes 1 to 5th. During period τ ₁₁ in the first frame of scan electrode 1 in FIG. 31, selection scan electrode signal φy is applied. Next, after the period τ ₁₂ , a non-selective scan electrode signal is applied. This switching can be performed by a drive circuit or a control circuit that outputs a signal to the scan electrode drive circuit. Next, at τ ₂₁ in the second frame of the scanning electrode 1, the selective scanning electrode signal φy is applied, and after the period τ ₂₂ , the non-selecting scanning electrode signal is applied in the same manner as described above.
Next, in the scanning electrode 2, a non-selection electrode signal is applied in period τ ₁₁ of the first frame, a selection electrode signal φy is applied in the next period τ ₁₂ ,
Thereafter, the non-selection electrode signal is applied again. Second
Similarly, in the frame, the signals are applied in the order of ,φy. In this way, scanning signals are sequentially applied to the scanning electrodes. Although FIG. 31 shows up to the scanning electrodes 5 to which voltage is applied sequentially, the same applies to the case where a larger number of scanning electrodes are provided.

Next, the driving waveform applied to the pixel formed at the intersection of the scanning electrode and the signal electrode will be explained with reference to FIG. FIG. 32 Vy shows the signal waveform of scanning electrode 1 in FIG. 31. φx in FIG. 32 is the same as the inverted signal electrode signal φx and the non-inverted signal electrode signal shown in FIG.

Therefore, φx-Vy or -Vy becomes a composite voltage pulse applied to the pixel with the signal electrode as a reference. φx-Vy in FIG. 32 is a waveform for selecting a pixel and writing one display state, for example, a dark state, and -Vy is a waveform for selecting a pixel and writing the other display state, for example, a bright state. 32nd
φx-Vy and -Vy in the figure are equal to the waveforms in FIGS. 17a and 17b.

In this way, the composite voltage pulse applied to the pixel in FIG. 17 can be derived from the basic drive signal shown in FIG. 10.

This driving method is a driving method in which black and white (lighting and non-lighting) are written in two frames. FIG. 17a shows a signal that changes depending on the frame; for example, black is written in the first half of the scan corresponding to the first frame, and white is written in the second half of the scan corresponding to the second frame. .

That is, in the first frame, a voltage that inverts the ferroelectric liquid crystal molecules to one stable state in which the display state becomes black is applied to the pixels that should be black corresponding to the display information. In the second frame, the remaining pixels that should become white are made white by applying a voltage that inverts the liquid crystal molecules to the other stable state in which the display state becomes white. Since ferroelectric liquid crystal has memory properties, an image is completed by the above two frames.

b in FIG. 18 is a scan selection signal for selecting the first scan line. Signals a and b in FIG.
The figure shows the voltage applied to the pixel when the pixel on the first scanning line selected by the signal b in the figure is made black in a and white in b, for example.

When the second signal b in FIG. 18 is at High level,
Black (lit) and white (not lit) are selected.

Figures 19 to 24 show the basic drive waveforms when 1/4 of the applied voltage Vap when selected is applied when not selected, and Figures 40 to 43 show the basic drive waveforms in each of the above figures. The combination of waveforms indicates the voltage waveform of the composite voltage pulse applied to the pixel.

A suffix of l is attached for lighting and d for non-lighting. In actual driving, any combination of lighting and non-lighting signal sequences can be used. However, in this drive, among the pixels on the selected scan electrode, a voltage of 1/2 Vap is applied to a lit pixel when the pixel is not lit, and to a non-lit pixel when the pixel is lit.

In the embodiments shown in Figs. 10 to 24, when unselected,
Among the driving methods in which positive and negative voltages of ±1/a of Vap are applied, this is an example for a=3 and a=4, and generally the driving method in which positive and negative voltages of 1/a are applied is similar. Conceivable. (However, a is any positive number).

This driving method enables multiple divisions that cannot be achieved with TN LCDs. According to the present invention, a large-capacity liquid crystal device can be realized with a simple simple matrix, and an inexpensive, high-quality electro-optical device such as a display can be realized.

When SmC ^* is driven with the above driving waveform, the optical transmittance is as shown in FIG. 25.

To the pixel on the selected electrode among the scanning electrodes,
When a positive or negative Vap is applied, the liquid crystal molecules rotate to the position a or a' in FIG.

After that, the optical transmittance oscillates and attenuates due to the applied alternating current pulses that oscillate equally in positive and negative directions, but the attenuation is greatest immediately after the equal positive and negative alternating current pulses are applied; There is almost no change.

When the number of divisions is large, the time during which scanning electrodes are selected becomes short and the time during which scanning electrodes are not selected occupies most of the time.

In the case of the present invention, the scan electrode groups are selected without interruption (selected continuously so that when the scan control signal of one scan electrode falls, the scan control signal of the next scan electrode rises), so the number of divisions is When is n, one scanning time is t ₀ , and the time t ₁ for selecting one scanning electrode is expressed as t ₁ =t ₀ /n.

Further, the time t ₂ when no selection is made is t ₂ =(N-1)t ₀ /n.

The optical transmittance when an AC pulse is applied during non-selection is driven as described above,
The size hardly changes.

Since this state occupies most of the scanning time, the optical transmittance of this state is perceived by the human eye as pixel contrast.

Therefore, whether the number of divisions is large or small, the contrast remains constant.

In our measurements, in a panel that can currently drive 256 divisions, the contrast did not change much from 8 divisions to 256 divisions.

The SmC ^* saw phenomenon is extremely suitable for multi-segment display compared to the fact that as the number of divisions increases in TN type LCD panels, the difference in effective voltage between selected points and non-selected points disappears and the contrast decreases. It shows that there is.

If the SmC ^* response is possible up to 10 μsec, the scanning electrode groups in the present invention are selected consecutively, so the number of divisions is approximately n=30000 (μsec)/10×2 (μsec)=1500.

However, 30 m sec is the time required for one scan. Moreover, 2 in the denominator indicates that positive and negative voltages are taken during the selected time.

If the liquid crystal responds at the highest speed ever achieved, a panel with about 1500 divisions can be driven.
Further, as described above, it is possible to prevent contrast differences between 1500 divisions and 8 divisions by using the driving method of the present invention.

Here, another advantage of the present invention regarding contrast will be described.

When the cell gap is thinned to about 1μm,
SmC ^* loses its helical structure and the layers align perpendicular to the panel substrate. This is as stated before.

The fact that the layers are perpendicular to the substrate means that the liquid crystal molecules are horizontal to the substrate.

When the molecules in this state are driven by the driving method according to the present invention, they are in the states b and b', which are close to a and a' in FIG. 5, so it is considered that the molecules are approximately parallel to the substrate.

Even when this state is viewed from various viewpoints, there is almost no change in contrast because the molecules are horizontal to the substrate.

On the other hand, in a TN type LCD panel, when the LCD is not lit (positive display), the LCD is not completely horizontal to the substrate, and depending on the viewing angle, it can be considered to be standing, resulting in crosstalk. will occur.

This is known as so-called viewing angle dependence. The display according to the present invention using SmC ^* does not have such viewing angle dependence. In the same way that multi-segmentation has completely changed conventional wisdom, the SmC ^* of the present invention has revolutionary characteristics such as no field-of-view dependence and no change in contrast depending on the number of partitions regarding contrast.
It can be said that electro-optical devices using Furthermore, one embodiment of the drive waveform according to the present invention is shown in FIG.

This drive waveform is displayed by scanning the drive waveforms shown in FIGS. 10 to 24 once or a finite number of times, and then maintaining the display in a high impedance state (with the drive circuit turned off). It shows the voltage change applied to the liquid crystal when the liquid crystal is turned on.

In FIG. 26, a indicates the scanning portion and b indicates the high impedance state.

b in Figure 5 in a high impedance state,
The fact that it remains at position b' is clear from the fact that the optical transmittance hardly changes even when the impedance is set to high.

This memory property is long enough, and it is only necessary to scan a when the display changes.

Power consumption in high impedance state is
It is an energy-saving type of drive, and is most suitable for image display where the displayed content does not change much, such as a fixed display.

An example of a drive waveform similar to this drive is shown in the second example.
It is shown in Figure 7.

Similar to the driving method using high impedance shown in FIG. 26, after scanning is performed once or a limited number of times, the driving frequency is increased to maintain the display. If the driving frequency is increased, as shown in Figure 7,
The state of liquid crystal molecules can be maintained more stably.

In the drive shown in Fig. 26, which uses memory through high impedance, the molecular position becomes high impedance and gradually returns to the initial orientation state from the point at which the external regulating force disappears. Get better. The phenomenon of returning to the initial orientation state at high impedance increases as the orientation force becomes stronger, and as the temperature increases, the phenomenon increases.

Since the influence of temperature is especially large, it is more reliable to maintain the display by increasing the driving frequency as shown in FIG.

In the case of the drive shown in FIG. 27, scanning can be carried out normally and only the drive frequency needs to be increased. Furthermore, on the signal electrode side, display data may be output as is, or it may be set to be lit or non-lit.

Regardless of whether the liquid crystal has a positive or negative dielectric anisotropy Δε, by increasing the driving frequency, the molecular state can be held more stably, and as a result, the image quality can be improved. be. In particular, when a negative liquid crystal is used, the application of a high frequency voltage applies a dielectric torque to the liquid crystal molecules that causes them to sag, which also has the effect of improving contrast.

Next, a second embodiment of a drive similar to that shown in FIG.
It is shown in Figure 8.

The difference from the drive shown in Fig. 27 described above is that after one or a finite number of scans, the AC pulse with equal positive and negative voltages applied when not selected is the same as the AC pulse with equal positive and negative voltages, or the AC pulse with equal positive and negative voltages is different. The point is that the display state is maintained by applying pulses.

FIG. 28 shows a drive waveform applied with an AC pulse of ±1/3 Vap.

In the case of this embodiment, the driving waveforms applied to the scanning electrodes and the signal electrodes after scanning are equal amplitude waveforms with different phases as shown in FIG. 29.

In this case, the scanning electrodes are not scanned, and one waveform shown in FIG. 29 is applied to the scanning electrodes, and the other waveform is applied to the signal electrodes, regardless of the data.

FIG. 30 shows an example of the voltage applied to the liquid crystal when using a driving method in which the voltage is set to zero volts after a finite number of scans.

As described above, according to the present invention, frame scans for writing bright states and frame scans for writing dark states are performed alternately, so that all information can be written in two frame scans at the shortest. Further, the written information is held by applying a high frequency AC voltage to the pixel portion after frame scanning.

As described above, the electro-optical device according to the present invention using ^SmC
This is an epoch-making liquid crystal element that breaks the limits of matrix-type liquid crystal elements. Using this element allows for multi-division driving with a simple matrix, greatly reducing the number of driver ICs, and since it is a simple panel that does not use active elements, it is possible to realize an inexpensive large-capacity liquid crystal panel.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of the molecular arrangement of N, SmA, and SmC. FIG. 2 is a diagram schematically showing the molecular arrangement and single molecule state around the helical axis of SmC ^* . FIG. 3 is a schematic diagram showing the molecular state and the conventional display principle as viewed from the direction of the substrate. Figure 4 shows
1 is a cross-sectional view of an electro-optical device according to the present invention. Fifth
The figure is a schematic diagram showing the molecular state in the electro-optical device according to the present invention. FIGS. 6 and 7 show changes in optical transmittance when an AC pulse voltage is applied immediately after applying a DC voltage. FIG. 8 is a diagram showing the relationship between response time and applied voltage. FIG. 9 is a diagram showing the relationship between response time and temperature. FIGS. 10 to 13 show examples in which positive and negative alternating current pulses of 1/3 of the selection voltage Vap are applied during non-selection. Figure 14 shows Figures 10 to 13.
While the figure shows a waveform with the same polarity, this is an example in which the signal is composed of alternating current pulses. Figures 15 and 16 show the actual φY, φX signals that summarize the lighting scan and non-lighting scan.
11, and the case of FIG. 14. FIG. Figure 17 is from Figure 10.
When FIG. 14 is used, it shows the voltage actually applied between liquid crystals. FIG. 18 shows the control signal. Figures 19 to 24 show 1/1 when not selected.
This is an example in which positive and negative alternating current pulses of 4 Vap are applied. FIG. 25 shows changes in optical transmittance when driven. FIGS. 26-28 are examples of the voltage applied to the liquid crystal when using a driving method of floating after a finite number of scans, increasing the frequency, and applying a complete AC pulse. FIG. 29 shows the waveforms applied to the scan and signal electrodes after the finite number of scans of FIG. 28. 30th
The figure shows an example of the voltage applied to the liquid crystal when a driving method with a finite number of scans and zero volts is used. FIG. 31 shows a scanning signal applied to the scanning electrode, and FIG. 32 shows an embodiment showing the waveform of a composite voltage pulse applied to a pixel by combining the scanning signal and the data signal. Figures 33 to 37 are
14 is a diagram showing a composite voltage pulse applied to a pixel by a combination of the basic drive waveforms shown in FIGS. 0 to 14. FIG. Figures 38 to 43 are the same as Figures 19 to 2.
5 is a diagram showing a composite voltage pulse applied to a pixel by a combination of the basic drive waveforms shown in FIG. 4; FIG. 1...Dipole moment, 2...Liquid crystal molecule,
3, 4, 5...Polarization direction, 6...Electrode, 7...Alignment film, 8...Liquid crystal, 9...Sealant, 10...Polarizing plate, 11...Liquid crystal molecule.

Claims (1)

[Claims] 1. One substrate on which a plurality of scanning electrodes are formed and the other substrate on which a plurality of signal electrodes are formed are installed in parallel so that the electrodes face each other, and the substrate A chiral smectic liquid crystal is held in the gap, the substrate gap is limited to less than the helical pitch of the chiral smectic liquid crystal, the two substrates are placed between polarizing plates, and each intersection of the scanning electrode and the signal electrode is a pixel portion is formed in the pixel portion, the scanning electrodes are selected line-sequentially and a scanning signal is supplied, and the scanning signal supplied to the scanning electrode and the data signal supplied to the signal electrode in synchronization with the scanning signal. In the ferroelectric liquid crystal electro-optical device, in which light and dark information is written by applying a composite voltage pulse to the pixel portion to invert two stable molecular arrangement states of the chiral smectic liquid crystal to either one, the scanning electrode A first frame in which bright information is written by applying a composite voltage pulse of one polarity higher than the operating voltage of the chiral smect liquid crystal to the pixel portion according to the bright data in the data signal during the selection period. scanning, and a second step of writing dark information by applying a composite voltage pulse of the other polarity, which is higher than the operating voltage of the chiral smect liquid crystal, to the pixel section in accordance with the dark data in the data signal.
frame scanning is performed alternately, and each of the voltage pulses is lower than the operating voltage of the chiral smect liquid crystal during the non-selection period of the scanning electrode, and the polarity is changed within the scanning period of one scanning electrode. A ferroelectric liquid crystal electro-optical device characterized in that information written in the selection period is maintained by applying a composite voltage pulse including an alternating current voltage pulse to each pixel portion.