JPH0656460B2 - Ferroelectric liquid crystal electro-optical device - Google Patents

Description

The present invention relates to an electro-optical device using a chiral smectic liquid crystal. Liquid crystals are used in various displays, and are often used in displays for watches and calculators due to their excellent characteristics such as small size and thin panel and low power consumption. The liquid crystal used in these displays is a thermotropic liquid crystal and takes various liquid crystal phases within a certain temperature range. The liquid crystal phase is roughly classified into a nematic having no layer (abbreviated as N) and a smectic having a layer (hereinafter referred to as Sm) depending on the presence or absence of a layer structure. Sm is further classified into a uniaxial smectic A phase (SmA) and a biaxial smectic C phase (SmC).

FIG. 1 schematically shows the molecular sequences of N, SmA and SmC. a is N, b is SmA, and c is SmC.

Furthermore, if the liquid crystal molecules have asymmetric carbon and are not racemic, the alignment of the liquid crystal becomes a twisted structure.

N is chiral nematic (N ^* ) and SmC is chiral smectic C (SmC ^* ).

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

Ferroelectric liquid crystal was used in 1975 by Meyer (J.de. Phys. 36, 1
975, 69) et al. And its existence was proved.
At that time, the synthesized liquid crystal is commonly known as DOBAMBC (2-
Methylbutyl P-[(Pn-decyloxybenzylidene) amino] cinnamate), which is still widely used in the research of ferroelectric liquid crystals.

The molecular sequence of SmC ^* can be schematically shown as in FIG.

The molecular axis is inclined by an angle θ with the normal direction of the layer, and this angle is constant in every layer. However, the azimuth angle φ gradually changes depending on the layer, and the molecular orientation forms a spiral structure. The pitch of the spiral varies depending on the liquid crystal, but is usually about several μm.

When SmC ^* liquid crystal is injected into a thin cell of about 1 μm,
SmC in which the spiral structure disappeared and the layers were perpendicular to the cell substrate
It will take the structure of.

Since the SmC ^* liquid crystal has an electric dipole moment perpendicular to the molecular axis, the dipole moments are aligned parallel to the layers in the thin cell. Here, when the electric field is applied upward and downward, the molecule takes a position inclined by ± θ with respect to the normal line of the layer.

By utilizing the birefringence, the two states of ± θ can be made to correspond to light and dark, and can be used as an electro-optical device such as a display.

FIG. 3 shows a schematic view 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.

In other words, (1) μsec-order high-speed response (2) Memory property (3) Desired threshold characteristics Among these characteristics, the high-speed response shows a response of μsec order even in our observation. Also, after applying an electric field to change to any of ± θ states, even if the electric field is cut off, the memory property that maintains that state exists as their claim. However, the desired threshold characteristic was not obtained in our observation. According to our data, Vth, Vs
The value of at showed Vth = 500 (mV) and Vsat = 5 (V).

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

An object of the present invention is to show a new principle and method of time-division driving using SmC ^* , to enable multi-division driving in a range that cannot be realized with TN liquid crystal, and to perform halftone display operation.

Hereinafter, details of the present invention will be described with reference to examples.

FIG. 4 shows an embodiment of a sectional view of a panel using SmC ^* . The transparent electrodes 6 forming the scanning electrodes and the signal electrodes are formed on the inner surfaces of the two transparent substrates 5, and the smectic liquid crystal 8 is sealed between the substrates via the seal portion 9.

Compared with a normal panel structure, the gap is about 2 μm, which is extremely thin (4 μm or less). Also, one of the two polarizing plates 10 is aligned with the molecular axis direction and the polarization direction of the molecule in either of the ± θ states as shown in FIG. 3, and the other is similarly aligned with the molecular axis direction. Or place it at an angle of 90 °.

When a sufficiently high direct current voltage is applied to this panel to bring the molecules into either of ± θ states and then an alternating current is applied to the liquid crystal, when the voltage shown in FIG. Get the change in transmittance.

As is clear from the figure, when AC is applied, the optical transmittance vibrates and converges to an intermediate state.

FIG. 7 shows f ₁ , f ₂ , and f ₃ showing changes in optical transmittance when the frequency of the alternating current in the holding period is changed from a high frequency to a low frequency while keeping the applied voltage constant, and V ₁ , V ₂ and f ₃ , respectively. ₂ , V
₃ shows a 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 AC frequency is high and the voltage is low, the change in optical transmittance tends to be small. In other words, the relaxation time is long.

In the present invention, the stable state of the ferroelectric liquid crystal molecules is changed by a DC voltage, and then the relaxation time of the above optical change is lengthened by the application of an AC voltage, and the liquid crystal molecules are as if ± θ.
It seems to have stopped near the state of. This is what is used to display.

That is, it is considered that the molecule to which the AC voltage is applied is stopped while vibrating around b or b'in FIG. It is the driving principle of the present invention to perform display and the like by utilizing this state.

Regarding this characteristic, the panel subjected to the uniaxial orientation treatment has a strong orientation force and immediately returns to the initial orientation state.
A rubbing (PVA means polyvinyl alcohol) or the like has a relatively weak orientation force, and the display state can be maintained by an AC voltage to obtain a good display.

In actual driving, one example of the driving waveform applied to the liquid crystal is as shown in a and b of FIG. Lighting of the pixel on the selected scanning electrode among these waveforms (lighting off or lighting depends on the polarization direction of the polarizing plate, but here it is assumed that the lighting state is obtained by the waveform a in FIG. 14). The waveform shown in FIG. 14a is applied to the selected pixel. At this time, the liquid crystal molecules move to a position a or a ′ in FIG. 5 or a position close to that position when a high voltage Vap is applied, and then, with an alternating voltage having the same positive and negative amplitudes. , B or b '.

In this case, a high voltage Vap is required to select the frequency of the drive waveform.
Therefore, it must be set so that the liquid crystal molecules can move sufficiently to the positions of a and a '.

Moreover, since the relationship between the response time and the voltage shown in FIG. 8 is almost inversely proportional, the frequency can be increased by increasing the voltage. Then, as described above, if the frequency of the alternating voltage applied after applying the voltage capable of inverting the ferroelectric liquid crystal is made high and the amplitude is made short, the state of the liquid crystal molecules can be stably maintained. Type of drive element or type of display (for example, fixed display or video display)
It suffices to set an appropriate drive frequency and drive voltage Vap within a range in which the display state does not deteriorate due to the difference. The drive circuit we used in the experiment was CMOS, and it was driven with a drive voltage Vap of 20V. In addition to CMOS, it can be driven by circuit elements such as FET, bipolar transistor, TTL, VMOS and the like.

Next, the actual drive waveform will be described in more detail.

Selective and non-selective scan electrode drive signals are supplied to the scan electrodes of the liquid crystal panel, and select and non-selective signal electrode drive signals, that is, the following four basic drive waveforms are supplied to the signal electrodes.

φy ... Selected scan electrode signal ▲ ▼ ... Non-selected scan electrode signal φx ... Selected signal electrode signal ▲ ▼ ... Non-selected signal electrode signal From the scan electrode drive circuit to the scan electrode of the liquid crystal panel,
A scanning signal obtained by combining φy and ▲ ▼ is supplied line-sequentially. From the signal electrode drive circuit, φx and ▲ ▼ are applied to the signal electrodes according to the display data in synchronization with the scanning signal.

9 to 12 are examples of basic drive waveforms.
It is considered that a voltage of ± 1/3 Vap is applied when not selected. Also, in the figure, the suffixes of l and d are
It is a symbol corresponding to lighting and non-lighting (either negative or positive is acceptable depending on the orientation of the polarizing plate).

In actual driving, the scanning of lighting and the scanning of non-lighting may be alternately repeated and displayed, and any one of the waveforms l or d in FIGS. 9 to 12 may be used.

Similarly, the drive waveform shown in FIG. 13 is a drive waveform by an AC pulse.

FIG. 14 is a diagram showing a potential applied to a pixel in the case of the driving method in which the lighting and the non-lighting are alternately performed.

Next, drive waveforms applied to each pixel shown in FIGS. 14A and 14B are derived from the eight basic drive signals shown in FIGS. 9 and 10. FIG. 28 is a voltage waveform diagram sequentially applied from the scan electrode 1 to the fifth electrode. During the period τ ₁₁ in the first frame of the scan electrode 1 in FIG. 28, the selective scan electrode signal φyl in FIG. 9 is applied. Next, after the period τ ₁₂ , the non-selected scan electrode signal φyl 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, the tau ₂₁ in the second frame of the scanning electrode 101, the selection scanning electrode signal φyd of FIG. 10 is applied, the period tau ₂₂ thereafter the same non-selection scanning electrode signal ▲
▼ is applied.

Next, in the scan electrode 2, the period τ _{11 of} the first frame
In FIG. 9, the non-selection electrode signal ▲ in FIG. 9 is applied, the selection electrode signal φyl is applied in the next period τ ₁₂ , and then the non-selection electrode signal ▲ ▼ is applied again.
Similarly, in the second frame, φyd, φyd, ▲
Are applied in this order. In this way, scanning signals are sequentially applied to the scanning electrodes.

FIG. 28 shows the scan electrodes 5 that are sequentially applied, but the same applies to the case where a larger number of scan electrodes are provided.

Next, the combined voltage pulse applied to the pixel formed at the intersection of the scan electrode and the signal electrode will be described with reference to FIG.
FIG. 29 Vy shows a signal waveform of the scan electrode 1 shown in FIG.

The signal electrode signals Vx, ▲ ▼ in FIG. 29 are the selection signal electrode signals φxl, φx shown in FIGS. 9 and 10.
d, and non-selection signal electrode signals ▲ ▼, ▲ ▼ are combined. Therefore, Vx-Vy, or ▲
-Vy is a composite voltage pulse applied to the pixel with reference to the signal electrode. 29. Vx-Vy is a waveform for selecting a pixel and creating a display state of one display, for example, a dark state, and ▲ ▼ -Vy is a waveform for selecting a pixel and creating the other display state, for example, a bright state. is there.

Vx-Vy and ▲ ▼ -Vy in FIG. 29 are equal to the waveforms in FIGS. 14a and 14b. In this way, the combined voltage pulse applied to the pixel of FIG. 14 can be derived from the basic drive signals shown in FIGS. 9 and 10.

This driving method is a driving method for writing black and white (lighting and non-lighting) in two frames. FIG. 14 shows a signal which changes corresponding to a frame. For example, black is written in the first half scanning corresponding to the first frame, and white is written in the latter half scanning corresponding to the second frame.

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

FIG. 15b shows a scanning selection signal for selecting the first scanning line. The signals a and b in FIG. 14 are the pixels on the first scanning line selected by the signal b in FIG.
Represents the voltage applied to the pixel when the pixel is white.

When the second signal b in FIG. 15 is at the high level, black (lighting) and white (non-lighting) are selected. When SmC ^* is driven by the driving waveform as described above, the optical transmittance is as shown in FIG. When positive / negative Vap is applied to the pixel on the selected electrode among the scanning electrodes, the liquid crystal molecules are rotated to the position a or a ′ in FIG. The light and dark reach the highest level.

After that, the optical transmissivity is attenuated while oscillating by the AC pulse that oscillates equally to the positive and negative, but the attenuation is the largest immediately after the AC pulse with the same positive and negative is applied, and changes almost thereafter. There is no. When the number of divisions is large, the scanning electrode selection time is short, and the non-selection time is the majority.

In the case of the present invention, the scan electrode group is continuously selected (when the scan control signal of a certain scan electrode falls, the scan control signal of the next scan electrode is continuously selected so as to rise). Is n, the time t ₁ for selecting one scan electrode is represented by t ₁ = t ₀ / n, where one scan time is t ₀ .

The time t _{2 that} is not selected is Is.

The optical transmittance when the AC pulse in the non-selected state is applied oscillates as described above, but the magnitude hardly changes.

Since this state occupies most of the scanning period, the optical transmittance of this state appears as the pixel contrast in the human eye. Therefore, the contrast becomes constant regardless of whether the number of divisions is large or small.

In our measurement, in the panel which can drive 256 divisions at present, the contrast did not change much from 8 divisions to 256 divisions.

This phenomenon of SmC ^* is suitable for very multi-division display as compared with the fact that the difference in effective voltage between the selected point and the non-selected point disappears as the number of divisions of the TN type liquid crystal display panel increases and the contrast decreases. It indicates that

If the response of SmC ^* is possible up to 10 μsec, the scan electrode group in the present invention is continuously selected, and thus the division number is It will be about.

However, 30 msec is the time required for one scanning.
Further, the denominator 2 indicates that positive and negative voltages are taken during the selection time.

When the liquid crystal responds at the highest speed obtained in the world so far, a panel of about 1500 divisions can be driven, and as described above, it is necessary to prevent the difference in contrast between 1500 divisions and 8 divisions. It is possible by law.

Here, another advantage of the present invention regarding contrast will be described. When the cell gap is reduced to about 1 μm, SmC ^* loses its helical structure and is arranged so that the layers are perpendicular to the substrate of the panel. This is as described above. The layer being perpendicular to the substrate means that the liquid crystal molecules are horizontal to the substrate. When the molecule in this state is driven by the driving method according to the present invention, it is in a state of b, b'close to a, a'in FIG. 5, so it is considered that the molecule is approximately horizontal 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 the TN type liquid crystal display panel, the liquid crystal is not perfectly horizontal with respect to the substrate due to non-lighting (in the case of positive display),
Depending on the viewing angle, it can be considered standing, resulting in crosstalk.

This is known as so-called viewing angle dependence. SmC
^The display according to the present invention using ^* does not have such a viewing angle dependency. In the same way that multi-division changed the conventional wisdom, the electro-optical device using the SmC ^* according to the present invention has epoch-making characteristics such as the contrast does not depend on the viewing angle and the contrast does not change depending on the number of divisions. Can be said to have.

Next, a driving method for obtaining gradation display will be described.

The basic method for gradation display is to modulate the pulse width of ± Vap applied to the pixel on the selected scan electrode to create a halftone.

Focusing on the change in optical transmissivity during driving shown in FIG. 16, when the selection voltage ± Vap ends, the maximum level of light and dark is reached. After that, the light is attenuated, but the optical transmittance when the attenuation almost disappears after a sufficient time has elapsed after the attenuation is proportional to the magnitude of the optical transmittance when the selection voltage ± Vap is applied. It was observed.

By utilizing this phenomenon and adjusting the optical transmittance at the time of selection, gradation display is possible. Since an optical transmittance proportional to the pulse width of the selection voltage ± Vap is obtained, gradation display can be realized by this method.

Examples of drive waveforms are shown in FIGS. 17 to 26. Each will be described.

FIG. 17 shows an embodiment of a waveform to which the suffix l in FIG. 9 is attached, that is, a waveform in which the waveform used for the lighting scan is changed for gradation display. Figures 17 and 9
Only the signal electrode selection signal is different in the figure,
Other signals may be the same. In the embodiment, the selection voltage Vap is modulated by shifting the phase by τm. Selection voltage Vap
If the driving frequency is f, the pulse width τap Becomes Gradation display is performed by adjusting τm according to the halftone level.

FIG. 30 is a diagram specifically showing a composite voltage pulse applied to a pixel by the combination of the basic drive waveforms of FIG. 7.

FIG. 18 shows an example of the voltage actually applied to the liquid crystal from the signal of FIG. FIG. 18a shows a waveform applied to the liquid crystal when the scanning electrode is selected and a selection signal is applied to the signal electrode, and b is a scanning electrode not selected and a nonselection signal applied to the signal electrode. It is a waveform in the case. Contrary to b, c is a waveform when the scanning electrode is not selected and the selection signal is applied to the signal electrode.

The time for which both b and c take ± 1/3 Vap is considered to be equal in b and c.

Similar to FIG. 17, FIG. 19 also shows a waveform obtained by changing the signal for non-lighting scanning in FIG. 10 for gradation display. First
The difference from FIG. 0 is that the selection signal of the signal electrode follows the zero potential portion, and is 2/3 Vap having a width of τm, Vap having a pulse width narrowed by τm, and 1/3 Vap having a width of τm. It consists of pulses. That is, the pulse width of the selection voltage −Vap applied to the liquid crystal is narrowed by τm, and gradation display is performed by adjusting the τm in accordance with the halftone level.

FIG. 31 is a diagram concretely showing a composite voltage pulse applied to a pixel by the combination of the basic drive waveforms of FIG.

Note that FIG. 20 shows the waveform applied to the liquid crystal similarly to FIG. In the figure, a, b, and c indicate the same states as in FIG. 18, and have waveforms in which the polarities of the waveforms in FIG. 18 are inverted.

21 to 23 are examples in which the drive waveform to which an AC pulse having an amplitude of ⅓ Vap is applied when not selected is changed for gradation display. Similar to the above, only the selection signal of the signal electrode needs to be changed. There are two patterns to change.

As shown in FIGS. 21 and 23, when a pulse group having a time width corresponding to the intensity of the display data is used, specifically, as shown in FIG. 21, the time width is τm depending on the intensity of the display data. 1/3 Vap with narrowed Vap and τm width
2 pulses consisting of, and 2 / 3V with a width of τm
The pulse of ap is made up of a total of three pulses arranged with a zero potential in between, and as shown in FIG. 23, Vap and ⅓Va having a width of τm narrowed in time width by τm.
There are two cases, that is, a set of p pulses is composed of a total of four pulses arranged with different polarities, and a case where the phase is changed according to the intensity of display data as shown in FIG.

32 to 34 are views specifically showing a composite voltage pulse applied to a pixel by the combination of the basic drive waveforms of FIGS. 21 to 23.

Next, an embodiment in which the drive waveform which becomes an alternating current having an amplitude of ¼ Vap when not selected is changed for gradation display is shown in FIGS. 24 and 25.
FIGS. 35 and 36, and FIGS. 35, 36, and 37, respectively, are diagrams specifically showing a composite voltage pulse applied to a pixel by the combination.

Similar to the gradation display described above, there are two cases: a case where a waveform composed of four high and low pulses having a time width corresponding to the intensity of display data is used and a case where a waveform having a different phase according to the intensity of display data is used. In each case, the selection voltage ± Vap is modulated by τm.

As described above, there are the following two means for changing the selection signal of the signal electrode in order to obtain the gradation display.

(1) The phase is shifted according to the strength of the display data.

(2) It is composed of 3 to 4 pulses, and each pulse is a voltage pulse having a time width corresponding to the intensity of display data.

The embodiment of (2) above is shown in the general case in FIGS. The upper two (a) and (b) lights up, and the lower (c) and (d)
Is a staircase waveform of a selection signal applied to the signal electrode when is not lit.

V ₁ and V ₂ are the following voltages, respectively.

V ₁ = {1- (2 / a)} Vap V ₂ = (2 / a) Vap a is an arbitrary number, and an AC pulse of ± (1 / a) Vap is applied when it is not selected.

As described above, in the present invention, the voltage pulse applied to the pixel for gray scale display is synchronized with the scan signal applied to the scan electrode, and the phase of the selection signal applied to the signal electrode is changed to the display data. The gradation display of the ferroelectric liquid crystal electro-optical device could be realized by modulating according to the intensity or by making the time width into a waveform consisting of 3 to 4 pulses modulated according to the intensity of the display data. Since the drive waveform of the present invention can be realized with a simple circuit configuration, there is also an advantage that the cost does not increase.

[Brief description 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 around the spiral axis of SmC ^* and the state of a single molecule. Figure 3 shows
It is a schematic diagram which shows the molecular state seen from the substrate direction and the conventional display principle. FIG. 4 is a sectional view of an electro-optical device according to the present invention. FIG. 5 is a schematic diagram showing a molecular state in the electro-optical device according to the present invention. 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 to FIG. 12 show an embodiment in which a positive / negative AC pulse of 1/3 of the selection voltage Vap is applied during non-selection. Figure 13 shows
This is an embodiment in which the signals are composed of AC pulses, while the waveforms of FIGS. 9 to 12 have the same polarity. 14th
The figure shows the voltage actually applied between the liquid crystals when FIGS. 9 to 13 are used. FIG. 15 shows the control signal. FIG. 16 shows changes in optical transmittance when driven. 17 to 23 show the selection voltage V
ap is modulated to perform gradation, and 1 / 3Va when not selected
It is an example of the drive waveform according to the present invention when a positive and negative AC pulse of p is applied. FIGS. 24, 25, and 26 show the implementation of the drive waveforms according to the present invention when positive and negative AC pulses of 1/4 Vap are applied during non-selection, and the selection voltage Vap is modulated for gradation display. Here is an example: FIG. 27 shows an example of the selection signal of the signal electrode for gradation display. FIG. 28 shows the 1st to 5th scanning signals of the scanning electrodes. FIG. 29 shows a drive voltage waveform applied to a pixel in which a scanning signal and a signal electrode signal are combined. 30 to 37 show FIGS. 17, 19, 21, and 22.
FIG. 27 is a diagram specifically showing a composite voltage pulse applied to a pixel by the combination of the basic drive waveforms shown in FIGS. 23, 24, 25, and 26. 1 …… Twin electrode moment 2 …… Liquid crystal molecule 3,4,5 …… Deflection direction 6 …… Electrode 7 …… Alignment film 8 …… Liquid crystal 9 …… Sealant 10 …… Polarizer 11 …… Liquid crystal molecule

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Koji Iwasa 6-31-1, Kameido, Koto-ku, Tokyo Seiko Denshi Kogyo Co., Ltd. (56) Reference JP-A-59-193427 (JP, A)

Claims (1)

[Claims] 1. A substrate having a plurality of scanning electrodes formed on its surface and another substrate having a plurality of signal electrodes formed thereon are arranged in parallel so that the electrodes face each other, and A chiral smectic liquid crystal is sandwiched in the gap, the gap is limited to a spiral pitch of the chiral smectic liquid crystal or less, the two substrates are installed between polarizing plates, and a pixel is formed at each intersection of the scanning electrode and the signal electrode. , A halftone optical state is obtained by utilizing an optical change caused by a change in the molecular arrangement of the chiral smectic liquid crystal when an inversion voltage of the same polarity capable of reversing the stable state of the chiral smectic liquid crystal is applied to the pixel portion. A ferroelectric liquid crystal electro-optical device for outputting, wherein scanning signals are line-sequentially selected and supplied to the scanning electrodes, and display signals are supplied to the signal electrodes. Voltage pulse whose phase is modulated to the intensity of the data, or 3 or 4 pulses, each pulse synchronizing a data signal consisting of a voltage pulse having a time width corresponding to the intensity of the display data with the scanning signal. Applying a combined voltage pulse having a voltage level of the inversion voltage of the chiral smectic liquid crystal and having a time width corresponding to the intensity of display data to the pixel portion during the selection period of the scan electrode. Then, in the non-selection period of the scan electrode, each of the voltage pulses is equal to or lower than the inversion voltage of the chiral smectic liquid crystal,
A ferroelectric liquid crystal electro-optical device is characterized in that the written state is maintained by applying a composite voltage pulse including alternating voltage pulses having different polarities to each other within the scanning period of one scanning electrode to the pixel portion. apparatus.