JPH0422495B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a liquid crystal device using ferroelectric liquid crystal that can be applied to a liquid crystal display device or a liquid crystal-optical shutter array device.

[Description of prior art]

Conventionally, liquid crystal display elements are well known in which a scanning electrode group and a signal electrode group are configured in a matrix, and a liquid crystal compound is filled between the electrodes to form a large number of pixels to display images or information. . The driving method for this display element is time-division driving, in which address signals are selectively and periodically applied to the scanning electrode group, and predetermined information signals are selectively applied in parallel to the signal electrode group in synchronization with the address signal. It has been adopted.

Most of these that were put to practical use were, for example, “Applied Physics Letters.”
(“Applied Rhysics Letter”) 1971, No. 18(4)
“Voltage Dependant Optical” co-authored by M. Schadt and W. Helfrich, published on pages 127-128.
Activity of a Twisted Nematic Liquid Crystal” (“Voltage”)
Dependent Optical Activity of a Twisted
Nematic Liquid Crystal”)
(twisted nematic) type liquid crystal.

In recent years, the use of bistable liquid crystal elements as an improved version of conventional liquid crystal elements has been proposed by both Clark and Lagerwall in Japanese Patent Application Laid-open No. 107216/1983 and US Patent No.
It has been proposed in the specification of No. 4367924, etc. As a bistable liquid crystal, a ferroelectric liquid crystal having a chiral smectic C phase (SmC ^* ) or H phase (SmH ^* ) is generally used. It has the property of taking either the first optically stable state or the second optically stable state and maintaining that state when no electric field is applied, that is, it has stability and has a response to changes in the electric field. It is expected that it will be widely used in fields such as quick, high-speed, and memory-type display devices.

[Problem that the invention seeks to solve]

However, problems arise when the number of display pixels is extremely large and high-speed driving is required.
That is, the threshold voltage for providing the first stable state in a ferroelectric liquid crystal cell having bistability for a predetermined voltage application time is -Vth ₁ , and the threshold voltage for providing the second stable state is -Vth 1. If the voltage is +Vth ₂ ,
Even if these threshold voltages are not exceeded, if voltage continues to be applied for a long time, the display state written to the pixel (e.g. white state) will be reversed to another display state (e.g. black state) There are things to do. 1st
The figure represents the threshold characteristics of a bistable ferroelectric liquid crystal cell.

Figure 1 plots the application time dependence of the threshold voltage (Vth) required for switching when DOBAMBC (12 in the figure) and HOBACPC (11 in the figure) are used as ferroelectric liquid crystals.

As is clear from Figure 1, the threshold value Vth is dependent on the application time, and the shorter the application time, the more
It is understood that the slope is steep. From this, when applied to an element that has an extremely large number of scanning lines and is driven at high speed, for example, even if a certain pixel is switched to the bright state during scanning, the information signal will always be below Vth from the next scanning onward. It can be seen that if the voltage continues to be applied, there is a risk that the pixel will turn into a dark state during the completion of scanning one screen.

[Means for Solving the Problems] and [Operations] An object of the present invention is to provide a novel method for driving a liquid crystal element that solves the problems in conventional liquid crystal display elements or liquid crystal light shutters as described above. It is in.

Another object of the present invention is to provide a method for driving a liquid crystal element having high-speed response.

Another object of the present invention is to provide a method for driving a liquid crystal device having high density pixels.

The present invention provides a matrix electrode structure in which a pixel is formed at the intersection of a scanning electrode group and a signal electrode group, and a first a liquid crystal cell having a ferroelectric liquid crystal that produces a stable state in response to an electric field of the other polarity, and a ferroelectric liquid crystal that produces a second stable state in response to an electric field of the other polarity; , scan selection signals having different voltage values, one polarity pulse in the first phase, the other polarity pulse in the third phase, and the voltage value of the one polarity pulse and the voltage value of the other polarity pulse in the second phase. A scan selection signal having an intermediate pulse set to a voltage value between A first optical state is caused by applying a voltage of one polarity that causes a state, and in a second phase, a voltage that simultaneously maintains the first optical state is applied to pixels on the scanning electrode. and in a third phase, a second optical state is achieved by applying a voltage of the other polarity that causes the second stable state to pixels on the scanning electrode other than the selected pixel in the first phase. The first phase, the second phase, and the third phase of the scan selection signal are determined according to input information so as to cause
a first information signal having pulses of one polarity, the other polarity, and one polarity synchronized with the phase; and a first information signal having pulses of one polarity, one polarity, and one polarity synchronized with the first phase, second phase, and third phase of the scanning selection signal; By selectively applying a second information signal having a pulse, writing is performed to a pixel on a scanning electrode selected for scanning, and writing is performed to a pixel on a scanning electrode not selected for scanning,
The present invention is characterized by a liquid crystal device comprising: means for applying an alternating voltage to maintain a written state during scan selection;

Note that the polarities of the voltages applied to the scan electrodes and the signal electrodes are based on the voltages applied to scan electrodes that are not selected for scanning.

〔Example〕

The optical modulation substance used in the driving method of the present invention has at least two stable states, and in particular takes either a first optically stable state or a second optically stable state depending on the applied electric field. . That is, a substance having a bistable state with respect to an electric field, particularly a liquid crystal having such a property, is used.

As ^the liquid crystal having bistability that can be used in the driving method of the present invention, a chiral smectic liquid crystal having ferroelectricity is most preferable.
Phase (SmH ^* ) liquid crystal is suitable. This ferroelectric liquid crystal is described in “Le Journal de Physiove Lutheran” (“Le Journal de Physiove Lutheran”).
36 volumes (L-69), 1975 “Feroelectric Liquid Crystals”
(“Ferroelectric Liquid Crystals”); “Applied Physics Letters” (“Applied
Physics Letters” Volume 36 (No. 11) 1980 “Submicron Second Bistable Electro-Optical Switching in Liquid Crystals”
Bistable Electrooptic Switching in Liquid
“Crystals”); “Solid State Physics” 16 (141) 1981 “Liquid Crystals”
The ferroelectric liquid crystal disclosed in these documents can be used in the present invention.

More specifically, examples of ferroelectric liquid crystal compounds used in the method of the present invention include decyloxybenzylidene-P'-amino-2-methylbutylcinnamate (DOBAMBC), hexyloxybenzylidene-P'-amino- 2-chloropropyl cinnamate (HOBACPC) and 4-o-(2-methyl)
-butylresolcylidene-4'-octylaniline (MBRA8) and the like.

When constructing an element using these materials, the element is supported by a copper block etc. with a heater embedded in it, as necessary, in order to maintain the temperature state such that the liquid crystal compound becomes the SmC ^* phase or SmH ^* phase. can do.

Further, in the present invention, in addition to the above-mentioned SmC ^* and SmH ^*, it is also possible to use a ferroelectric liquid crystal that appears in a chiral smect F phase, I phase, J phase, G phase, or K phase.

FIG. 2 schematically depicts an example of a ferroelectric liquid crystal cell. 21a and 21b are In ₂ O ₃ , SnO ₂ or
It is a substrate (glass plate) coated with a transparent electrode such as ITO (indium-tein-oxide).
In between, a liquid crystal of SmC ^* phase, which is oriented such that a liquid crystal molecular layer 22 is perpendicular to the glass surface, is sealed.
The thick line 23 represents liquid crystal molecules,
This liquid crystal molecule 23 has a dipole moment (P⊥) 14 in a direction perpendicular to the molecule. When a voltage higher than a certain threshold value is applied between the electrodes on the substrates 21a and 21b, the helical structure of the liquid crystal molecules 23 is unraveled, and the liquid crystal molecules 23 are aligned so that all dipole moments (P⊥) 24 are oriented in the direction of the electric field. You can change direction. The liquid crystal molecules 23 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and the short axis direction. Therefore, for example, polarizers arranged in a crossed nicol position can be placed above and below the glass surface. For example, it is easily understood that the liquid crystal optical modulation element is a liquid crystal optical modulation element whose optical characteristics change depending on the polarity of applied voltage. Furthermore, if the thickness of the liquid crystal cell is made sufficiently thin (for example, 1μ), the helical structure of the liquid crystal molecules will unravel even when no electric field is applied, as shown in Figure 3, and the dipole moment Pa or Pb will be It takes either an upward direction 34a or a downward direction 34b.
When an electric field Ea or Eb of different polarity above a certain threshold value is applied to such a cell for a predetermined time as shown in FIG. 34b, and accordingly the liquid crystal molecules are either in the first stable state 33a or in the second stable state 3.
3b.

There are two advantages to using such a ferroelectric liquid crystal as an optical modulation element. Firstly, the response speed is extremely fast, and secondly, the alignment of liquid crystal molecules has a bistable state. To explain the second point with reference to FIG. 2, for example, when the electric field Ea is applied, the liquid crystal molecules are oriented in the first stable state 33a, but
This state remains stable even when the electric field is turned off. Furthermore, when an electric field Eb in the opposite direction is applied, the liquid crystal molecules are oriented to a second stable state 33b and the orientation of the molecules is changed, but they remain in this state even after the electric field is turned off. Further, as long as the applied electric field Ea does not exceed a certain threshold value, each orientation state is maintained. In order to effectively realize such fast response speed and bistability, it is preferable for the cell to be as thin as possible, generally 0.5μ to 20μ, especially
1μ to 5μ is suitable.

Preferred specific examples of the driving method of the present invention will be explained with reference to FIGS. 4 to 10.

FIG. 4 is a schematic diagram of a cell 41 having a matrix electrode structure in which a ferroelectric liquid crystal compound is sandwiched in the middle. 42 is a scanning electrode group, and 43 is a signal electrode group. Now, to simplify the explanation,
An example will be shown in which a black and white binary signal is displayed. In Figure 4, the pixels indicated by diagonal lines are "black".
, and others correspond to "white". Fifth
Figures a and b show the scan selection signal applied to the selected scan electrode and the scan non-selection signal applied to the other scan electrodes (unselected scan electrodes), respectively, and Figures c and d show the scan electrodes applied to the selected scan electrode, respectively. It represents an information selection signal applied to a signal electrode and an information non-selection signal applied to an unselected signal electrode. Fifth
In each of the figures a to d, the horizontal axis represents time and the vertical axis represents voltage.

_According to the driving method of the present invention, each of _{the first}
The display state of the pixel between the display state determination phase _t1 , the second display state determination phase _t3 , the first display state determination phase _t1 , and the second display state determination phase _t3 . It has an indeterminate auxiliary phase t ₂ .

In this example, the pulse widths of phases t ₁ , t ₂ and t ₃ are as follows:
Although they are determined to be equal, their pulse widths may be different from each other.

The “display state determination phase” described in this specification is
It means the phase that determines the display state of the pixel in one of the bright state and the dark state within the writing period to the pixel of the selected scan electrode line, and in the present invention, the phase t ₁ is The phase t3 represents the phase that determines the selected pixel among the pixels above to be in a black display state, for example, and the phase _t3 represents the phase that determines the other pixels to be in a white display state.

Further, the "auxiliary phase" described in this specification means a phase in which an auxiliary signal that does not determine the display state of the pixel is applied within the writing period to the pixel on the selected scan electrode line, and the fifth Phase t ₂ in the figure corresponds to this.

Therefore, in the driving method of the present invention, the signal at the phase t ₁ +t _{2 +t 3 shown} in _FIG . A signal at +t ₃ is applied to scan electrodes to which no scan selection signal is applied. and,
The signal electrode group is synchronized with the scanning selection signal in Fig. 5c.
Write signals shown in and d are applied.

Now, the threshold voltage at the application time ΔT (writing pulse width) to give the first stable state (this is white) in the bistable ferroelectric liquid crystal element is -
Vth _{is 1} , and the second stable state (this is black)
The threshold voltage at the application time ΔT to give +
When Vth is ₂ , the electrical signal given to the selected scanning electrode has a phase (time) as shown in Figure 5a.
-2V ₀ at t ₁ , -V 0 at phase t ₂ and -V ₀ at phase t ₃
It has a pulse that becomes 2V ₀ . Further, the other scanning electrodes are in a grounded state and in an OV state as shown in FIG. 5b. On the other hand, _{the electrical} signal applied to _the selected signal electrode _is shown in _{FIG .} .

Moreover, the electric signal given to the unselected signal electrode is − at phase t ₁ as shown in FIG. 5d.
V ₀ , V ₀ at phase t ₂ and −V ₀ at phase t ₃ .

In this way, the voltage signals applied to the selected signal electrodes and the unselected signal electrodes in the signal electrode group 43 have alternating voltage waveforms corresponding to the phases t ₁ , t ₂ and t ₃ .

In the above, each voltage value is set to a desired value that satisfies the following relationship.

2V ₀ <Vth ₂ <3V ₀ −3V ₀ <−Vth ₁ <−2V ₀ FIG. 6 shows the voltage waveforms applied to each pixel when such an electric signal is applied.

In Fig. 6, a and b are voltage waveforms applied to pixels that are to display black and white, respectively, on selected scanning electrode lines, and c and d in the same figure are voltage waveforms that are applied to pixels that are not selected, respectively. This is a voltage waveform applied to pixels on a scan electrode line.

As is clear from FIGS. 6a and 6b, the pixel on the signal electrode selected by the pixel on the selected scanning electrode line exceeds the threshold value Vth ₂ during the period of phase t ₁ .
A voltage of 3V ₀ is applied, and the display state of the pixel is determined by this phase t ₁ . That is, the ferroelectric liquid crystal corresponding to this pixel is oriented in the second stable state, and "black" is written. During the subsequent phases t ₂ and t ₃ , voltages OV and -VO are applied to the pixels mentioned above, respectively, but since the threshold values -Vth ₁ and Vth ₂ are not exceeded,
It remains in the second stable state. Therefore, at this phase _t1 , the display state of the pixel becomes the second state of the ferroelectric liquid crystal.
One display state is determined based on the stable state.
On the other hand, on the selected scanning conductivity line, the pixels on the unselected signal electrodes are at V ₀ at phase t ₁ ,
At phase t ₂ , it becomes 2V ₀ , but since it does not exceed the threshold voltage,
The previous history remains the same, but the threshold value - at the subsequent phase t ₃
A voltage of −3V ₀ exceeding Vth ₁ is applied, and a transition is made to a “white” display state based on the first stable state of the ferroelectric liquid crystal. Therefore, at the end of the write period for pixels on the selected scan electrode line and on the unselected signal electrode lines, "white" will be displayed. That is, at phase t ₃ , the first phase of the ferroelectric liquid crystal
The white display state is determined based on the stable state of .

Therefore, the present invention provides a pixel writing period (t ₁ + t ₂ + t ₃ )
By providing an auxiliary phase that does not determine the display state of the pixel between the phase that determines "black" display and the phase that determines "white" display, the above-mentioned disadvantages are completely eliminated.

The drive signals described above are shown in chronological order in FIG. 7. S ₁ to _{S 5} are electrical signals applied to the scanning electrodes, I ₁ and I ₃ are electrical signals applied to the signal electrodes, and A and C are electrical signals applied to pixels A and C shown in FIG. This is the voltage waveform.

Now, the mechanism of switching by an electric field in a ferroelectric liquid crystal in a bistable state is not necessarily clear from a microscopic perspective, but in general, it is switched to a predetermined (first) stable state by a strong electric field for a predetermined period of time. If the electric field is left in a state where no electric field is applied after that, it is possible to maintain that state almost semi-permanently, but if the electric field is so weak that it does not switch for a certain period of time (in the example explained earlier,
(corresponding to voltages below Vth), if an electric field of opposite polarity is applied for a long time, the orientation state will transition again to the opposite (second) stable state, resulting in the correct orientation. Situations may arise where the display or modulation of information is not achievable. The present inventors believe that the ease with which the orientation state transfer (a type of crosstalk) occurs due to the long-term application of such a weak electric field is due to the material of the substrate surface,
Although it has been recognized that it is affected by roughness, liquid crystal material, etc., it has not yet been determined quantitatively.
However, it has been confirmed that when uniaxial substrate processing for aligning liquid crystal molecules, such as rubbing or oblique evaporation of SiO, etc., is performed, the tendency for the above-mentioned transition to occur tends to increase.
It was also confirmed that this tendency was stronger at higher temperatures.

In any case, in order to achieve correct information display and modulation, it is preferable to avoid applying an electric field in a fixed direction for a long time.

At the aforementioned phase _t1 , the pixels on the selected scanning electrode line and the pixels on the selected signal electrode line are determined to have a black display state based on the second stable state of the ferroelectric liquid crystal. At the phase _t3 , the pixels on the selected scanning electrode line and the pixels on the unselected signal electrode line are determined to display white based on the first stable state of the ferroelectric liquid crystal. At this time, signals with a voltage waveform of V 0 are applied to the selected signal electrodes at phases t ₁ and t _3, respectively, and signals with a voltage waveform of V ₀ are applied to the unselected signal electrodes at phases t _{1 and t 3} .
At t ₃ , a signal with a voltage waveform of −V ₀ is applied. Therefore, when the phases t ₁ and t ₃ are continuous, for example, after the -V ₀ waveform, which is a white write signal, is applied to the signal electrode, black writing is subsequently designated. If the V ₀ waveform continues to be applied, the V ₀ waveform will continue to be applied to the pixels written to white by the above-mentioned white write signal, and the above-mentioned white writing will continue in the middle of scanning one screen. This causes the inconvenience that the inserted pixels are inverted to black. Therefore, in the driving method of the present invention, the voltage waveforms applied to pixels on unselected scanning electrode lines are
As shown in , since V ₀ and −V ₀ which alternate at phases t ₁ , t ₂ and t ₃ respectively do not exceed the threshold voltage, it is due to the above-mentioned reversal development (i.e. crosstalk). Inconveniences can be prevented.
Moreover, in the present invention, the voltage waveform applied to the pixel A or C changes the writing pulse to ΔT as shown in FIG.
(At this time, the pulse width of phases t ₁ , t ₂ and t ₃ can be set to ΔT). Since the maximum is 2ΔT, which appears in waveform 71, even if the voltage margin during driving is set wide, The above-mentioned reversal phenomenon can be completely prevented. Furthermore, in the driving method of the present invention, the phases t ₁ ,
Since one pixel can be written with a pulse width of 3ΔT having t ₂ and t ₃ , one screen can be written at high speed.

8 to 10 represent another embodiment of the driving method of the present invention. FIG. 8a shows the scan selection signal applied to the selected scan electrode line, and FIG. 8b shows the scan selection signal applied to the selected scan electrode line.
This is a scan non-selection signal applied to unselected scan electrode lines. Figure 8c is an information selection signal applied to the selected signal electrode line, and Figure 8d is an information selection signal applied to the selected signal electrode line.
is an information non-selection signal applied to unselected signal electrode lines. FIG. 9a shows a voltage waveform applied to a pixel on a selected scanning electrode line and a pixel on a selected signal electrode line, and FIG. 9b shows a pixel on a selected scanning electrode line. It is also a voltage waveform applied to pixels on unselected signal electrode lines. FIG. 9c shows a voltage waveform applied to a pixel on an unselected scanning electrode line and a pixel on a selected signal electrode line, and FIG. 9d shows a voltage waveform applied to a pixel on an unselected scanning electrode line. , and represents a voltage waveform applied to a pixel on an unselected signal electrode line. FIG. 10 shows these signals in time series. Pixel A or C shown in FIG.
By simply applying a voltage waveform alternating between -V ₀ and V ₀ when scanning is not selected, the above-mentioned inversion phenomenon can be completely prevented. In particular, in this drive example, the write pulse is set to ΔT as shown in Figure 10.
Since the maximum voltage is 3ΔT which appears in the waveform 101, the voltage margin during driving can be set wide without causing the above-mentioned inversion phenomenon. In addition, in this example, the phase has phases t ₁ , t ₂ and t ₃
Since one screen can be written with a pulse of 3ΔT, one screen can be written at high speed.

〔Effect of the invention〕

According to the present invention, even if a display panel using a ferroelectric liquid crystal element is driven at high speed, the maximum pulse width of the voltage waveform that continues to be applied to the pixels on the scan electrode line to which the scan non-selection signal is applied is Since it is twice or three times the pulse ΔT during writing, it is possible to effectively prevent the phenomenon in which the display state is reversed to another display state during the writing scan of one screen.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the threshold characteristics of a ferroelectric liquid crystal. FIGS. 2 and 3 are perspective views schematically showing a ferroelectric liquid crystal element used in the present invention. FIG. 4 is a plan view of the matrix pixel structure used in the present invention. FIGS. 5a to 5d are explanatory diagrams showing voltage waveforms of signals applied to the electrodes, respectively. FIGS. 6a to 6d are explanatory diagrams showing voltage waveforms of signals applied to pixels, respectively. Figure 7 shows
FIG. 2 is an explanatory diagram of a voltage waveform representing the above-mentioned signal in time series. FIGS. 8a to 8d are explanatory diagrams showing voltage waveforms of different signals applied to the electrodes, respectively. FIGS. 9a to 9d are explanatory diagrams showing voltage waveforms of different signals applied to pixels, respectively. Figure 10 shows
FIG. 7 is an explanatory diagram of a voltage waveform representing the above-mentioned other signal in time series.

Claims (1)

[Scope of Claims] 1a A matrix electrode structure in which pixels are formed at intersections of a scanning electrode group and a signal electrode group, and a matrix electrode structure arranged between the scanning electrode group and the signal electrode group and resisting an electric field of one polarity. a liquid crystal cell having a ferroelectric liquid crystal that produces a first stable state in response to an electric field of the other polarity and a second stable state in response to an electric field of the other polarity; A scanning selection signal having voltages of different voltage values in the order of: one polarity pulse in the first phase, the other polarity pulse in the third phase, and the voltage value of the one polarity pulse and the other polarity pulse in the second phase. A scan selection signal having an intermediate pulse set to a voltage value between the voltage value of creating a first optical state by applying a voltage of one polarity that creates a stable state of one, and simultaneously holding the first optical state in a pixel on the scanning electrode in a second phase; applying a voltage and, in a third phase, applying a voltage of the other polarity that causes the second stable state to occur in pixels other than the selected pixel in the first phase on the scanning electrode; A first phase, a second phase and a third phase of the scan selection signal are determined in response to input information to produce an optical state.
a first information signal having pulses of one polarity, the other polarity, and one polarity synchronized with the phase; and a first information signal having pulses of one polarity, one polarity, and one polarity synchronized with the first phase, second phase, and third phase of the scanning selection signal; By selectively applying a second information signal having a pulse, writing is performed to a pixel on a scanning electrode selected for scanning, and writing is performed to a pixel on a scanning electrode not selected for scanning,
A liquid crystal device comprising: means for applying an alternating voltage to maintain a written state during scan selection;