JPH0422494B2 - - 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 arranged in a matrix, and a liquid crystal compound is filled between the electrodes to form a large number of pixels, thereby displaying pixels or information. . The driving method for this display element is time-division driving, in which an address signal is selectively and periodically applied to a group of scanning electrodes, and a predetermined information signal is selectively applied in parallel to a group of signal electrodes 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, 18(4)
M. Schadt published on pages 127-128 of issue
and W. Helfrich, “Voltage Dependant Optical.”
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. +Vth ₂ ,
Even if these threshold voltages are not exceeded, if a voltage continues to be applied for a long time, the display state written to the pixel (e.g., white state) is determined to be a different display state (e.g., black state). Sometimes. 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 Fig. 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; b. Sequentially applying a scan selection signal in which pulses are set to have a longer pulse width than subsequent pulses, and in synchronization with the previous pulse, simultaneously bring about the first stable state in the pixels on the scan electrode selected for scan. A group of signal electrodes is arranged such that a voltage of one polarity is applied, and a voltage of the other polarity that causes the second stable state is selectively applied to pixels on the scanning electrode in synchronization with the subsequent pulse. Then, according to the input information, a bipolar pulse having one polarity and the other electrode pulse or a bipolar pulse having an opposite phase to the bipolar pulse is applied in synchronization with the previous pulse of the scanning selection signal, and after the bipolar pulse is applied. By selectively applying a pulse of one polarity or a pulse of the other polarity in synchronization with the pulse of , writing is performed to the pixels on the scan electrodes that are selected for scanning, and to the pixels on the scan electrodes that are not selected for scanning. ,
A liquid crystal device is characterized in that it has: 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.

That is, the present invention first applies the optical modulation material to a pixel on a selected scanning electrode in the matrix pixel structure in the first and second phases within the writing period.
A voltage is applied to orient the optical modulation material to a second stable state in a third phase, and a voltage is applied to a selected pixel of the pixels in a third phase to orient the optical modulation material to a second stable state, so that the pixels on the scanning electrode It is characterized by being writable.

〔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 Physiove Lutheran” (“Le Journal Physiove Lutheran”).
Letter”) Volume 36 (L-69), 1975 “Fueroelectric 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”
etc., and these disclosed ferroelectric liquid crystals 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 ferroelectric liquid crystals that appear 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 is applied between the electrodes on the substrates 21a and 21b, the helical structure of the liquid crystal molecules 23 is unraveled, and the alignment direction of the liquid crystal molecules 23 is changed so that all dipole moments (P⊥) 24 are directed in the direction of the electric field. can be changed. 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.
As shown in FIG. 3, when an electric field Ea or Eb of different polarity above a certain threshold value is applied to such a cell for a predetermined period of time, the dipole moment will be directed upward 34a or downward 34b in accordance with the electric field vector of the electric field Ea or Eb. The liquid crystal molecules change direction accordingly, 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.

In a preferred embodiment of the present invention, a scanning electrode group is sequentially and periodically selected based on a scanning signal, a signal electrode group facing the scanning electrode group and selected based on a predetermined information signal, and a gap between the two electrodes. When selecting a scanning electrode of a liquid crystal element having at least a liquid crystal that is held at A first phase t ₁ and a second phase t ₂ in which a voltage is applied to provide an electric field in one direction;
(the display state based on the first stable state is _white ).

Preferred specific examples of the driving method of the present invention will be explained with reference to FIGS. 4 to 7.

FIG. 4 is a schematic diagram of a cell 41 having a matrix electrode structure in which a ferroelectric liquid crystal compound is sandwiched between. 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",
Others correspond to "white". 5a and 5b respectively indicate a scan selection signal given to a selected scan electrode and a scan non-selection signal given to other scan electrodes (unselected scan electrodes);
FIGS. 5c and 5d represent information selection signals applied to selected signal electrodes and information non-selection signals applied to unselected signal electrodes, respectively. Figure 5 a~
In d, the horizontal axis represents time and the vertical axis represents voltage.

FIG. 6a shows that on the selected scanning electrode line,
It also represents a voltage waveform applied to a pixel on the selected signal electrode line, and represents a state in which this pixel is written in white.

FIG. 6b represents a voltage waveform applied to a pixel on a selected scan electrode line and on an unselected signal electrode line, and this pixel represents a state in which black is written.

FIG. 6c 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. 6d 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. Further, FIG. 7 shows the voltage waveforms mentioned above in time series.

According to the driving method of the present invention, within the writing period (phase t ₁ + t ₂ + t ₃ ) to a pixel on a selected scan electrode line in a matrix pixel structure, phases t ₁ and t ₂
By setting all or a predetermined portion of the pixels on the line in one display state in at least one of the phases, and then inverting only the pixels selected in phase _t3 to the other display state, By performing line writing and sequentially performing such writing for each scanning electrode line, one screen can be written.

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 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 , −2V 0 at phase t ₂ and −2V 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. 5C, giving -V ₀ at phase t ₁ , V ₀ at phase t ₂ and V ₀ again at phase t ₃ . Also,
The electrical signal given to the unselected signal electrodes is
As shown in Figure 5d, V ₀ at phase t ₁ and V 0 at phase t ₂
-V ₁ at phase t 3 and V ₀ at phase t ₃ .

In this way, the voltage waveforms applied to the selected signal electrodes and the voltage waveforms applied to the unselected signal electrodes alternate in correspondence to the phases t ₁ , t _{2 ,} and t ₃ , and Each alternating waveform has a phase difference of 180° from each other.

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

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

As is clear from FIG. 6a, the pixels on the selected scanning electrode line and the selected signal electrode line exceed the threshold Vth ₂ at phase t ₂ .
A voltage of 3V ₀ is applied to obtain a black display state based on the second stable state of the ferroelectric liquid crystal, and the following phase
At _t3 , a voltage of _{-3V0 exceeding the threshold value -Vth1 is} applied to write a white display state based on the first stable state of the ferroelectric liquid crystal. Furthermore, as is clear from FIG. 6b, the pixels on the selected scanning electrode line and the unselected signal electrode lines receive a voltage of 3V ₀ that exceeds the threshold Vth ₂ at phase t ₁ . The voltage is applied and a black display state is assumed, and in the following phases t ₂ and t ₃ , V ₀ and −V ₀ below the threshold voltage are applied alternately, so the black display state is written. .

The drive signals mentioned above are shown in chronological order as follows.
FIG. 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 applied to pixels A and C shown in Fig. 4. It is a 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 transitions in orientation (a type of crosstalk) occur due to the long-term application of such a weak electric field is influenced by the substrate surface material, roughness, liquid crystal material, etc. Although we have recognized this, we have not yet grasped it quantitatively. However, it has been confirmed that when a uniaxial substrate treatment for aligning liquid crystal molecules is performed, such as rubbing or oblique evaporation of SiO, etc., 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.

Therefore, in the driving method of the present invention, as shown in FIG. 6c and d, the pixels on the unselected scanning electrode lines are supplied with a voltage alternately between -V ₀ and V ₀ below the threshold voltage. Only by applying a waveform, the liquid crystal molecules do not change their alignment state and maintain the display state from the previous scan. Furthermore, the phases t ₁ , t ₂ and
At t ₃ , the voltage alternates between V ₀ and −V ₀ , so
A reversal phenomenon (that is, crosstalk) to another stable state due to one voltage being applied for a long time does not occur. Furthermore, in the present invention, the voltage waveform applied to the pixel _{A or} C is as shown in _FIG . Since the maximum value is 2ΔT which appears in waveform 71 when In addition, in the driving method of the present invention, a pulse width of 3ΔT with phases t ₁ , t ₂ and t ₃ is used.
Since pixels can be written, one screen can be written at high speed.

[Effects of the Invention] According to the present invention, even when a display panel using a ferroelectric liquid crystal element is driven at high speed, a voltage waveform that continues to be applied to pixels on a scan electrode line to which a scan non-selection signal is applied. Since the maximum pulse width of is twice 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 writing scanning 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 Matocris 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. FIG. 7 is an explanatory diagram of voltage waveforms representing the aforementioned signals 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 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, and b. , sequentially applying a scan selection signal in which the previous pulse is set to have a longer pulse width than the subsequent pulse, and in synchronization with the previous pulse, the pixels on the scan electrode selected for scan are simultaneously brought into the first stable state. a voltage of one polarity that causes the second stable state to be applied, and a voltage of the other polarity that causes the second stable state to be applied selectively to pixels on the scanning electrode in synchronization with the subsequent pulse; A bipolar pulse having one polarity and the other polarity pulse or a bipolar pulse having an opposite phase to the bipolar pulse is applied to the signal electrode group in synchronization with the previous pulse of the scanning selection signal, depending on the input information. , by selectively applying a pulse of one polarity or a pulse of the other polarity in synchronization with the subsequent pulse, writing to pixels on the scan electrodes selected for scanning, and writing to pixels on scan electrodes not selected for scanning. to the pixel of
A liquid crystal device comprising: means for applying an alternating voltage to maintain a written state during scan selection;