JPH0474690B2 - - 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 nematics (N ^* ),
Among SmC, it is chiral smectic C (SmC ^* ).

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 inclined 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 ^* was injected into a cell as thin as 1 μm, the spiral structure disappeared and the layer 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 claimed 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) Vset=5(V) It showed a value like .

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 in driving using a voltage averaging method or the like.

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.

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, the gap is 2μm
It is extremely thin (4 μm or less).

Furthermore, as shown in FIG. 3, one of the two polarizing plates 10 is aligned with the molecular axis direction of molecules in either ±θ state and the polarization direction, and the other plate is aligned with the molecular axis direction in the same manner. or place it at a 90° angle.

When a sufficiently high DC voltage is applied to this panel to bring the molecules into a ±θ state, and then the AC shown in Figure 6b is applied to the liquid crystal, the optical transmittance changes as shown in Figure 6a. Become.

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 retention period is changed from high frequency to low frequency, and V ₁ ,
V ₂ and V ₃ indicate the change in optical transmittance when the applied voltage during the holding period is changed from a low voltage to a high voltage while keeping the frequency constant.

As shown in FIG. 7, when the alternating current frequency is high and the voltage is low, 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.

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.

The response time decreases monotonically as temperature increases. When the temperature rises and the response time becomes shorter, if the drive voltage or drive frequency remains the same as when it was at a low temperature, a display state with many flickers will result.

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 circuit and signal electrode drive circuit that drive the scan electrodes of the liquid crystal panel.

φy...selected scan electrode signal...non-selected scan electrode signal φx...inverted signal electrode signal...non-inverted signal electrode signal A combination of the above φy and φy is transmitted 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 output in synchronization with the above scanning signal.
A data signal selected according to display data is applied to the signal electrode.

In FIG. 10, a pixel is turned off one scanning period ahead of the scanning period to be selected, and depending on the data signal applied to the signal electrode during the period to be selected, the pixel is turned on or off. This is an example showing a basic drive waveform of the present invention for selecting whether to maintain non-lighting without turning on the light.

FIG. 14 is obtained by combining the basic drive waveforms shown in FIG. 10. FIG. 3 is a diagram specifically showing a composite voltage pulse applied to a pixel.

In the embodiment shown in FIG. 10, a voltage signal of φy+1 is applied to the scan electrode in the scan period immediately before the scan electrode is selected, and all pixels on the scan electrode are turned off. When an inverted signal electrode signal of φx is applied to the signal electrode, for the pixel,
-Vap, non-inverted signal When the electrode signal is applied, a composite voltage pulse of -5/3Vap is applied to the pixel.

Next, during the selection period, when φx is applied to the signal electrode, the voltage of Vap is applied to the pixel and it turns on inverted, and when is applied, the composite voltage pulse of 1/3Vap is applied to the pixel. is applied to maintain the non-lighting state without inverting.

FIG. 11 is a diagram specifically showing a composite voltage pulse waveform applied to a pixel in a driving method in which a scan electrode selected next to the selected scan electrode is turned off.

Figure 11a shows a waveform in which a pixel is turned off in the previous scanning period τ ₁ and then turned on in a selected scanning period τ ₂ , and Figure 11b shows a pixel turned on in a selected scanning period τ ₂ . Indicates a waveform that does not work.

Based on the signals shown in FIG. 12, the previous scan electrode and the scan electrode are selected. In addition, a driving method in which all pixels on the next scanning electrode are turned on is also conceivable, which is the opposite of this method.

FIG. 13 shows an example of a driving method in which positive and negative voltages of 1/4 Vap are applied when not selected, and the pixel on the next scanning electrode is not lit.

FIG. 15 is a diagram specifically showing a combined voltage pulse waveform applied to a pixel obtained by combining the basic drive waveforms shown in FIG. 13.

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.

To the pixel on the selected electrode among the scanning electrodes,
When a positive or negative Vap or a voltage higher than Vap is applied, the liquid crystal molecules rotate to the position a or a' in Figure 5, or to a position close to that position, resulting in the best optical brightness and darkness. reach the level.

The optical transmittance oscillates and attenuates due to the 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, and is almost constant thereafter. no change.

When the number of divisions is large, the time during which scanning electrodes are selected becomes short and the non-selection time 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 in the non-selected state oscillates as described above, but
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.

This phenomenon of SmC ^* is extremely unsuitable for multi-segment display, compared to the fact that as the number of divisions of a TN liquid crystal display panel increases, there is no difference in effective voltage between selected points and non-selected points, resulting in a decrease in contrast. It shows that

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 msec 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.
Furthermore, as described above, it is possible to prevent contrast differences between 1500 divisions and 8 divisions by 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 viewing angles, 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 viewing angle dependence and contrast that does not change depending on the number of partitions.
It can be said that electro-optical devices using

As explained above, in the present invention, after all the pixels on the L scan electrodes are brought into the other stable state (for example, non-lit), the display information of the signal electrode is By setting the pixels in one stable state (for example, turning on) and always applying an AC voltage pulse to each pixel during the non-selection period, all pixels of a large-capacity electro-optical device are All information can be written by scanning one frame,
Furthermore, written information can be stably maintained.

As described above, the electro-optical device according to the present invention using SmC ^* is capable of
This is an epoch-making liquid crystal element that breaks the limits of Y matrix type liquid crystal elements. Using this element, it is possible to drive multiple divisions 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 drawings]

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 of SmC ^* around the helical axis. 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. FIG. 10 shows an example of a driving method in which positive and negative alternating current pulses of 1/3 Vap are applied during non-selection in a driving method for erasing all pixels on the scan electrode one ahead of the selected scan electrode. FIG. 11 shows the voltage actually applied between liquid crystals. Figure 12 shows
This is the control signal. FIG. 13 shows an example of a driving method in which positive and negative alternating current pulses of 1/4 Vap are applied when not selected in a driving method for erasing all pixels on the scan electrode one ahead of the selected scan electrode. FIG. 14 shows a composite voltage waveform applied to a pixel based on the combination of the basic drive waveforms shown in FIG. 10. FIG. 15 shows a composite voltage waveform applied to a pixel based on the combination of the basic drive waveforms shown in FIG. 13. 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 smect liquid crystal is sandwiched in the gap, the gap is limited to a helical pitch of the chiral smect liquid crystal or less, 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 section, the scanning electrodes are selected line-sequentially and a scanning signal is supplied, 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 display information is written by applying a composite voltage pulse to the pixel to invert two stable molecular arrangement states of the chiral smectic liquid crystal, during the selection period of the scanning electrode. , when applying a composite voltage pulse of one polarity higher than the operating voltage of the chiral smectic liquid crystal to the pixel portion according to the display information in the data signal to bring one stable molecular arrangement state, A composite voltage pulse of the other polarity, which is equal to or higher than the operating voltage of the chiral smect liquid crystal, is applied to all the pixel portions on the scanning electrodes that precede the selected scanning electrode by L lines (L is a positive integer). A voltage pulse is applied to the pixel portion on the scan electrode other than the selected scan electrode and the scan electrode that precedes the scan electrode by L times. Each voltage is lower than the operating voltage of the chiral smect liquid crystal,
Further, a composite voltage pulse including an alternating current voltage pulse consisting of two voltage pulses having different polarities is applied within a scanning period of one scanning electrode to maintain the molecular alignment state of the chiral smectate liquid crystal. Ferroelectric liquid crystal electro-optical device.