JP2001166340A - Cholesteric liquid crystal display element and cholesteric liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a display device in which the efficiency of time use can be increased and a bright display with high contrast can be obtained for the display screen of animated images, and for the display of static images, the content of the display can be held with no or a little power supply without repeatedly writing or continuously applying a bias voltage. SOLUTION: The cholesteric liquid crystal used has a selective reflection wavelength region in the visible region, to which a black dichroic dye is added. A light-reflecting layer to reflect the light in the selective reflection wavelength region of the cholesteric liquid crystal is formed on the opposite side of the cholesteric liquid crystal layer to the observation side. In the animation mode, a driving signal S1 consisting of a selecting signal to change the cholesteric liquid crystal into homeotropic alignment or focal conic alignment and a holding signal to hold the aligned state selected by the selecting signal are applied on the colesteric liquid crystal layer in each frame. When the static image mode is switched from the animation mode, the voltage on the cholesteric liquid crystal layer is reduced to zero. Therefore, the liquid crystal is subjected to transition from the homeotropic alignment to planar alignment, and the content of the display can be held without power supply by the bistability of the planar alignment and focal conic alignment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a cholesteric liquid crystal display element used as a display panel of various electronic devices and a cholesteric liquid crystal display device including a driving circuit thereof.

[0002]

2. Description of the Related Art As a flat panel display device, a transmission type liquid crystal display device which performs display by transmitting or blocking illumination light from a backlight disposed on the back surface of a liquid crystal element as an optical shutter is widely used. Have been. However, the transmission type liquid crystal display device has a problem that power consumption is large and display is difficult to see under strong external light such as outdoors.

As a display device which solves this problem, a reflection type liquid crystal display device which performs display using reflection of external light without using a backlight has been attracting attention. As one of the methods, a cholesteric liquid crystal display device is used. Are known.

Since the cholesteric liquid crystal display device is a reflection type display device utilizing the reflection of external light, it does not require power for illumination, consumes low power, and can maintain display contents without power supply. It has features such as having memory properties, not requiring an expensive active matrix substrate such as a thin film transistor for driving, being able to use a flexible substrate such as a resin substrate, and having a high reflectance and a clear display. Have.

[0005] Cholesteric liquid crystals are composed of rod-like molecules and have a multilayered shape. The molecular major axis is oriented in one direction in one layer, but the orientation direction is slightly twisted between adjacent layers to form a helical structure as a whole. The period of the helix can be on the order of the optical wavelength by appropriate choice of materials, in which case the cholesteric liquid crystal reflects visible light selectively. This phenomenon is
This is known as selective reflection of cholesteric liquid crystals.

A cholesteric liquid crystal display device utilizing the selective reflection of cholesteric liquid crystal has a cholesteric liquid crystal 31 between two transparent substrates 11 and 12 on which transparent electrodes 21 and 22 are formed, as shown in FIG.
And a light absorption layer 41 is formed on the back surface of the substrate 12 opposite to the observation side (external light incidence side).

There are three types of orientation of the cholesteric liquid crystal 31: planar shown in FIG. 12A, focal conic shown in FIG. 12B, and homeotropic shown in FIG. In the following, the planar orientation is defined as P
The orientation and the focal conic orientation are referred to as F orientation, and the homeotropic orientation is referred to as H orientation.

The P orientation is a state in which the helical axis is oriented substantially perpendicular to the substrate surface, and color light in the selective reflection wavelength range is observed. The F orientation is a state in which the helical axis is oriented substantially parallel to the substrate surface. Since the cholesteric liquid crystal 31 itself is colorless, the color of the light absorbing layer 41 is observed. The H orientation is a state in which the helical structure is unwound and the liquid crystal molecules are oriented perpendicular to the substrate surface. Since the cholesteric liquid crystal 31 itself is also colorless, the color of the light absorbing layer 41 is observed.

Therefore, if the light absorbing layer 41 is made black, color light in the selective reflection wavelength range is observed in the P orientation, and a black appearance is obtained in the F orientation and the H orientation, so that a specific color and black can be displayed. It can be carried out.

The behavior of the cholesteric liquid crystal 31 when a voltage is applied between the electrodes 21 and 22 will be described with reference to FIG. This is based on the applied voltage when the initial orientation is P orientation, F orientation, and H orientation, a pulse voltage is applied for a certain period of time, then the applied voltage is made zero, and after a certain period of time, the reflectance of the cholesteric liquid crystal 31 is measured. It is a diagram schematically showing the characteristics of the reflectance.

The applied voltage-reflectance characteristics are represented by points A, E, F and C when the initial orientation is P orientation, and at points D, E, F and C when the initial orientation is F orientation. When the initial orientation is the H orientation, points A, E, B,
Represented by C.

At points B and C, the orientation is H during the application of the voltage, and when the applied voltage is reduced to zero, the orientation changes to the P orientation, so that the reflectance becomes high. Point A has a high reflectivity because of P orientation. Points D, E, and F have low reflectance because they are in the F orientation. When voltage VH is applied, H orientation (point C) is applied regardless of initial orientation, and when voltage VL is applied, F orientation (point E) is applied regardless of initial orientation.

When the applied voltage is zero, the P orientation (point A) and the F orientation (point D) are bistable, and ZF
The M (Zero Field Memory) effect is shown. When the voltage VB is applied, the H orientation (point B) and the F orientation (point F) are bistable, and BF
The M (Bias Fielded Memory) effect is shown.

[About US Pat. No. 5,748,277] US Pat. No. 5,748,277 proposes a method called a dynamic drive method as a method of driving the above cholesteric liquid crystal display element. ing.
In this method, as shown in FIG. 14, after applying a series of driving signals including an erasing signal, a selection signal, and a holding signal, the applied voltage is set to zero. US Patent No. 5,748,2
In No. 77, these erasure, selection, and retention are referred to as Pr, respectively.
evolution, Selection, Evolu
We call it "tion".

That is, in the dynamic drive method, first, a high voltage pulse is applied as an erase signal to cause the cholesteric liquid crystal to transition to the H orientation (point C). Next, a voltage VB or VL is applied as a selection signal to cause the cholesteric liquid crystal to transition to the H alignment (point B) or the F alignment (point E). Next, a voltage VB is applied as a holding signal to hold the H orientation (point B) or the F orientation (point F).

The holding signal is necessary to stabilize the F orientation. That is, when transitioning from the H orientation (point C) to the F orientation (point E), the cholesteric liquid crystal goes through a transient state called transient planar orientation (TP orientation), and the transition from the TP orientation to the F orientation is completed. If the applied voltage is reduced to zero before the operation, the cholesteric liquid crystal transitions to the P orientation. To prevent this, voltage VB is applied as a holding signal for a certain period of time.

The H orientation (point B) or the F orientation (point F) is held by applying a holding signal. Since these are both in a colorless state, the applied voltage is finally reduced to zero and H The orientation (point B) transitions to the P orientation (point A), causing selective reflection.

That is, in the dynamic drive method, a display image is once written in H orientation and F orientation by using the BFM effect, and then the applied voltage is made zero to make the H orientation shift to the P orientation. Thus, the display image is visualized, and the display content is held without power using the ZFM effect.

When displaying a moving image, four steps of erasing, selecting, holding, and zero voltage are repeated for each frame as shown in FIG. FIG. 15 schematically shows a case where a bright display is performed in frame 1 and a dark display is performed in frame 2 for a certain pixel, and at the same time, a change in reflectance at that time.

[About JP-A-10-206899]
On the other hand, JP-A-10-206899 describes that PCGH (P
1 shows a method of driving a cholesteric liquid crystal display element in a H. Change (Guest Guest Host) mode.

The cholesteric liquid crystal display element of the PCGH mode has a cholesteric liquid crystal 31 sandwiched between two transparent substrates 11 and 12 on which transparent electrodes 21 and 22 are formed, respectively, as shown in a sectional structure in FIG. The selective reflection wavelength range of the cholesteric liquid crystal 31 is set to an infrared range or a wavelength range longer than that, and a dichroic dye is added to the cholesteric liquid crystal 31 so that the cholesteric liquid crystal 31 is
The display is performed by a change in the transmittance due to a change in the orientation. Since the cholesteric liquid crystal 31 itself does not generate reflected light in the visible region, a light reflection layer 42 is formed on the back surface of the substrate 12 opposite to the observation side.

In the cholesteric liquid crystal display device of the PCGH mode, when the cholesteric liquid crystal 31 is in the P or F orientation as shown in FIG.
The cholesteric liquid crystal 31 is colored by the light absorption of the dichroic dye contained in the cholesteric liquid crystal 31, and when the cholesteric liquid crystal 31 is in the H orientation as shown in FIG. 4C, the cholesteric liquid crystal 31 becomes transparent. Forty-two colors are observed.

The change in orientation with respect to the applied voltage is the same as when no dichroic dye is contained. However, whether the orientation change of the cholesteric liquid crystal 31 is visualized by selective reflection,
Depending on whether the cholesteric liquid crystal 31 is visualized by a change in transmittance of the cholesteric liquid crystal 31 due to light absorption of the dichroic dye, the characteristic of the reflectance with respect to the applied voltage changes.

FIG. 17 schematically shows the applied voltage-reflectance characteristics in this case. The difference from FIG. 13 is that the high reflectance area at point A does not exist. This is because both the P orientation and the F orientation are colored.

That is, when the applied voltage is zero, 2
Regardless of the presence or absence of a coloring pigment, the P orientation and the F orientation exist as a bistable state. In the PCGH mode, since these orientation states are all colored states, the applied voltage-reflectance characteristics show hysteresis. Not observed. Therefore, in the PCGH mode, the ZFM effect cannot be substantially used, and only the BFM effect due to the bistability between the H orientation (point B) and the F orientation (point F) generated when the bias voltage VB is applied. It will be displayed by using it.

From this point, in the driving method of Japanese Patent Application Laid-Open No. H10-206899, a series of driving signals including an erase signal, a selection signal, and a holding signal are applied to each frame as shown in FIG. FIG. 18 shows a case in which a certain pixel performs bright display by reflection on the light reflection layer 42 in the H orientation in frame 1 and dark display by coloring with a dichroic dye in the frame 2 in F orientation.

That is, in the driving method disclosed in JP-A-10-206899, first, a high-voltage pulse is applied as an erasing signal to cause the cholesteric liquid crystal to transition to the H orientation (point C). Next, the voltage VB or VL is used as a selection signal.
Is applied to cause the cholesteric liquid crystal to transition to the H orientation (point B) or the F orientation (point E). next,
The voltage VB is applied as a holding signal to hold the H orientation (point B) or the F orientation (point F).

The above is the same as the dynamic drive method of US Pat. No. 5,748,277, but differs from the dynamic drive method in that there is no period during which the applied voltage is zero. That is, in the driving method of Japanese Patent Application Laid-Open No. H10-206899, display is performed using only the BFM effect by repeating three steps of erasing, selecting, and holding.

[0029]

However, FIG.
And the above-mentioned US Pat. No. 5,748,
In the dynamic drive method of No. 277, during the period of erasing, selecting, and holding, whether the orientation is H or F, the reflected light of the selective reflection is not generated, and the reflected light is generated after the application of the holding signal. Only when the voltage is zero.

Although it depends on the liquid crystal material and the applied voltage, it generally requires several milliseconds for the erasing period, one millisecond for the selection period, and ten and several milliseconds for the holding period, while preventing flicker. To do so, the frame period must be 30 ms or less.

Therefore, in the dynamic drive method, the ratio of time during which reflected light is generated to the frame period is low, and it is difficult to exceed 50%. In addition, the rise time of the reflectance when transitioning from the H orientation to the P orientation is generally as long as 100 to 200 ms, so that the reflectance is not saturated within the frame period.

As a result, in the dynamic drive method, the time-average reflectance at the time of displaying a moving image is low.
There is a problem that only a dark display can be obtained. In order to avoid this problem, it is conceivable to shorten the longest holding period during the erasing, selecting, and holding periods. However, as is apparent from the frame 2 in FIG. 15, the applied voltage after the holding signal is reduced. During the period of zero, there is a problem that the reflectance increases and the contrast decreases.

On the other hand, the aforementioned P shown in FIGS.
In the CGH mode cholesteric liquid crystal display device and the driving method disclosed in Japanese Patent Application Laid-Open No. H10-206899, only the selection period and the holding period are involved in the display, and the period in which the applied voltage is zero is unnecessary. 748,2
As compared with the dynamic drive method of No. 77, the time utilization rate, that is, the ratio of the time during which reflected light is generated to the frame period can be increased, and the time average reflectance at the time of displaying a moving image can be increased.

However, the PCGH mode cholesteric liquid crystal display element and the driving method disclosed in Japanese Patent Application Laid-Open No. H10-206899 do not use the ZFM effect due to the bistable P orientation and F orientation generated when the applied voltage is zero. In order to maintain the display content as in the case of displaying an image, it is necessary to repeatedly write in each frame or to continuously apply the bias voltage VB, and it is not possible to retain the display content without power supply. There's a problem.

Therefore, according to the present invention, it is possible to increase the time utilization rate when displaying a moving image, to obtain a bright and high-contrast display, and to repeatedly write or apply a bias voltage when displaying a still image. The display contents can be held with no or little power supply without continuing, and the power consumption can be greatly reduced.

[0036]

According to the present invention, there is provided a cholesteric liquid crystal display device including a cholesteric liquid crystal layer having a dichroic property and including a cholesteric liquid crystal having a selective reflection wavelength region within a visible region, and an observation side of the cholesteric liquid crystal layer. And a light reflection layer provided on the opposite side.

In order to make the cholesteric liquid crystal layer dichroic, a dichroic dye is added to the cholesteric liquid crystal, or a dichroic liquid crystal is used as the cholesteric liquid crystal itself.

The cholesteric liquid crystal layer or the dichroic dye can absorb the selective reflection wavelength range of the cholesteric liquid crystal, and the light reflection layer can reflect the selective reflection wavelength range of the cholesteric liquid crystal.

[0039]

According to the cholesteric liquid crystal display device of the present invention,
The selective reflection wavelength range of cholesteric liquid crystal is within the visible range,
The cholesteric liquid crystal layer is provided with dichroism by adding a dichroic dye to the cholesteric liquid crystal, and the light reflection layer is provided on the side opposite to the observation side of the cholesteric liquid crystal layer. Using the BFM effect due to the bistability of the F alignment and utilizing the change in the transmittance of the cholesteric liquid crystal layer, the display is performed by selecting the reflection on the light reflecting layer in the H alignment or the coloring of the cholesteric liquid crystal layer in the F alignment. Selective reflection of cholesteric liquid crystal at the time of P alignment using the display mode of No. 1 and (b) the ZFM effect due to the bistability of the P and F alignments of the cholesteric liquid crystal and utilizing the difference in the reflectance of the selective reflection of the cholesteric liquid crystal. Alternatively, the second display mode in which display is performed by selecting coloring of the cholesteric liquid crystal layer at the time of F alignment can be selectively performed.

Therefore, at the time of displaying a moving image, by selecting the first display mode, it is possible to increase the time utilization rate, that is, the ratio of the time during which reflected light is generated to the frame period, to obtain a bright and high-contrast display. And at the time of still image display, by shifting from the first display mode to the second display mode,
The display contents can be held with no or little power supply without repeatedly writing or continuously applying a bias voltage, and power consumption can be significantly reduced.

[0041]

DESCRIPTION OF THE PREFERRED EMBODIMENTS [Single-Layer Display Element and Driving Control Method Thereof] (Example of Element Configuration) FIG. 1 shows an example of a cholesteric liquid crystal display device of the present invention. It comprises a display element 10 and a drive circuit 70 for driving and controlling the display element 10. Hereinafter, the cholesteric liquid crystal display element 10 is abbreviated as the display element 10.

In this example, the display element 10 includes a substrate 11,
The electrodes 21 and 22 are formed on one surface of the substrate 12, and the substrates 11 and 12 are opposed to each other with the electrodes 21 and 22 facing inward.
The cholesteric liquid crystal layer 30 to which the chromatic dye 32 is added is formed, and the light reflection layer 4 is formed on the back surface of the substrate 12 opposite to the observation side.
Form 3

The cholesteric liquid crystal 31 has a wavelength region in the visible region (400 to 800 nm) such as red, green or blue as a selective reflection wavelength region. Examples of the material include liquid crystalline asymmetric carbon compounds such as cholesterol derivatives such as cholesteryl chloride and cholesteryl nonanoate, benzylidene aniline, azobenzene, azoxybenzene, cyanobiphenyl, cyanoterphenyl, phenylcyclohexane, and phenylbenzoate. Cyclohexylcyclohexane, cyclohexylcarboxylate, phenylpyrimidine, phenyldioxane, cyclohexylcyclohexaneester,
Known nematic liquid crystal compounds having a chemical structure such as cyclohexylethane, cyclohexene, and tolane include 2-
A composition to which an asymmetric carbon compound called a chiral agent such as methyl-n-butyl cyanobiphenyl is added can be used.

The dichroic dye 32 is a cholesteric liquid crystal 3
One that absorbs the selective reflection wavelength region, or a black dichroic dye that absorbs the visible wavelength region almost uniformly. As the dichroic dye 32, known dichroic dyes such as azo, anthraquinone, and coumarin can be used.

If the concentration of the dichroic dye 32 is too low, coloring of the cholesteric liquid crystal 31 at the time of F alignment becomes insufficient.
Conversely, if the concentration is too high, the transmittance of the cholesteric liquid crystal 31 at the time of H alignment decreases (the reflectance of reflection at the light reflection layer 43 decreases), and the reflectance of the cholesteric liquid crystal 31 at the selective orientation at the time of P alignment. Decrease. Therefore, the concentration of the dichroic dye 32 has an appropriate range. The range includes the absorption intensity of the dichroic dye 32, the dichroic ratio, the cholesteric liquid crystal layer 3
Although it depends on the thickness (cell gap) of 0, etc., 1 to 10% is generally suitable.

The cholesteric liquid crystal layer 30 may contain components other than the cholesteric liquid crystal and the dichroic dye.
For example, the cholesteric liquid crystal layer 30 may be a layer in which a polymer or an inorganic compound is dispersed in a cholesteric liquid crystal in the form of a gel, a matrix, or a fine particle, or conversely, the cholesteric liquid crystal may be formed in a droplet in a polymer matrix. It may be a layer in which cholesteric liquid crystal is included in microcapsules or a layer in which cholesteric liquid crystal is dispersed.

Instead of adding the dichroic dye 32 to the cholesteric liquid crystal 31, a liquid crystal having dichroism may be used as the cholesteric liquid crystal 31 itself.

The substrates 11 and 12 are formed of a translucent insulating material, for example, glass such as borosilicate glass or quartz glass, or a resin such as polyester, polycarbonate, or polyether sulfone.

The electrodes 21 and 22 are provided on the substrates 11 and 12,
A light-transmitting conductive material, such as indium tin oxide, tin oxide, aluminum-added zinc oxide, or the like, is formed into a thin film by a sputtering method, an evaporation method, a sol-gel method, or the like,
It is formed by processing into a desired shape by a photoetching method or the like.

The light reflection layer 43 is formed of the cholesteric liquid crystal 31.
Or a white light reflecting layer that reflects the visible wavelength range almost uniformly. For example, when the cholesteric liquid crystal 31 selectively reflects red, the light reflection layer 43 is red or white.

The light reflection layer 43 is formed by applying a coating material on the substrate 12, bonding a metal foil, or evaporating a metal. The light reflection layer 43 may not be formed directly on the substrate 12 but may be formed on another substrate and adhered to the substrate 12 together with the substrate. Further, the light reflection layer 43
Can also be used as the electrode 22 or the substrate 12.
In that case, of course, the electrode 22 or the substrate 12 is not made translucent.

The display element 10 shown in FIG. 1 is manufactured, for example, as follows. First, an adhesive is applied to the ends of the substrates 11 and 12 on which the electrodes 21 and 22 are formed, leaving a liquid crystal injection port, and the substrates 11 and 12 are adhered at regular intervals via spacers to form empty cells. Make it. Next, the cholesteric liquid crystal 31 to which the dichroic dye 32 is added is injected into the empty cell, and the injection port is sealed. The thickness of the cholesteric liquid crystal layer 30 is about 2 to 20 μm. Finally, a light reflection layer 43 is formed on the back surface of the substrate 12.

An alignment film may be provided at the interface between the electrodes 21 and 22 and the cholesteric liquid crystal layer 30 in order to improve the optical characteristics and electro-optical characteristics of the display element 10 or their uniformity. As the alignment film, a resin such as polyimide or polyvinyl alcohol, a low molecular surface modifier such as an alkyl ammonium compound or an alkyl silane compound, or an inorganic thin film such as SiO can be used. The orientation film is particularly preferably vertically oriented.

(Drive control method and display mode) Drive circuit 70
As shown in the frame 1 and the frame 2 of FIG. 4 or FIG. 5, the drive control method for the display element 10 is a selection signal for setting the cholesteric liquid crystal 31 to the H alignment or the F alignment, and a selection signal based on the selection signal. Drive signal S1 consisting of a holding signal for holding the aligned state
Is applied between the electrodes 21 and 22 and the maintenance mode where the applied voltage between the electrodes 21 and 22 is zero without applying the drive signal S1 as shown in the frame 3 of FIG. 4 or FIG. And are selectively taken. Hereinafter, the applied voltage between the electrodes 21 and 22 is simply referred to as an applied voltage.

The reason why the applied voltage is set to zero is to hold display contents written by the application of the drive signal S1 immediately before, as described later. However, the applied voltage may not be completely zero, but may be lower than a certain threshold. The following example shows a case where the applied voltage is set to zero.

When displaying a moving image, the driving signal S1 is applied to each frame, and when displaying a still image while maintaining the display contents at that time, the applied voltage is set to zero. Hereinafter, the drive mode in which the drive signal S1 is applied is referred to as a moving image mode, and the sustain mode in which the applied voltage is zero is referred to as a still image mode.

Further, in the moving image mode, the BFM effect of the cholesteric liquid crystal 31 based on the bistability of the H and F orientations of the cholesteric liquid crystal 31 is used. The cholesteric liquid crystal layer 30 is displayed by selecting the reflection of the cholesteric liquid crystal layer 30 at the time of F alignment or in the F alignment mode. The display is performed by selectively reflecting the cholesteric liquid crystal 31 at the time of the P orientation or by selecting the coloring of the cholesteric liquid crystal layer 30 at the time of the F orientation by utilizing the difference in the reflectance of the cholesteric liquid crystal layer 30.

FIG. 6 schematically shows the applied voltage-reflectance characteristics in this case. Points B and C are in the H orientation, and have high reflectance due to reflection on the light reflection layer 43. The points D, E, and F are in the F orientation, and the cholesteric liquid crystal layer 30 is colored with the dichroic dye 32 to have a low reflectance.
Point A is in the P orientation, and has high reflectance due to selective reflection of the cholesteric liquid crystal 31.

The cholesteric liquid crystal 31 is set to H
A signal for making the orientation (point C) or the F orientation (point E), and its voltage is VH or VL.
The holding signal is a signal for holding the alignment state (point B or point F) selected by the selection signal, and its voltage is VB.

The transition from the moving image mode to the still image mode is as follows.
This is realized by setting the applied voltage to zero after the application of the drive signal S1, that is, after the application of the holding signal. At this time,
As shown in FIG. 4, when the H orientation (point B) is held by the immediately preceding holding signal, that is, when a bright display is obtained by reflection on the light reflection layer 43, the P orientation is applied by applying no voltage. By transitioning to (Point A), a bright display by selective reflection of the cholesteric liquid crystal 31 is obtained, and a bright display state is maintained.

Conversely, as shown in FIG. 5, when the F orientation (point F) is held by the immediately preceding holding signal, that is, when a dark display is obtained by coloring with the dichroic dye 32, By maintaining the F orientation (point D) at zero applied voltage, a dark display by coloring with the dichroic dye 32 is maintained.

As described above, by setting the still image mode (maintenance mode), the display contents written in the immediately preceding moving image mode (drive mode) can be held without power supply.

In order to completely make the transition from the H orientation (point B) to the P orientation (point A), it is necessary to make the transition from the holding signal to zero applied voltage sufficiently fast. If the transition speed is low, transition from the H orientation (point B) to the F orientation (point D) occurs.

As described above, FIG. 4 shows that, for a certain pixel, in the frames 1 and 2, the light reflection layer 43 is determined by the H orientation.
In the frame 3, bright display is performed by selective reflection of the cholesteric liquid crystal 31 by P alignment, and bright display is maintained following frames 1 and 2. According to the present invention, when frames of bright display continue in this manner, there is an advantage that flicker does not easily occur because there is almost no change in reflectance over time.

As shown in FIG. 5, it is desirable to add an erase signal to the drive signal S1 in the moving image mode prior to the selection signal. FIG. 5 shows the opposite of FIG.
In the frames 1 and 2, the dark display is performed by coloring with the dichroic dye 32 by the F orientation, and the same dark display is performed in the frame 3 by the F orientation, and the dark display is maintained following the frames 1 and 2. .

The erasing signal is generated by setting the cholesteric liquid crystal 31 to H
This is a high voltage pulse for orientation (point C).
As shown in FIG.
Is set to the H orientation (point C), the selection signal is set to the high voltage VH, so that it is not necessary to add the erase signal immediately before the selection signal. In this case, the selection signal also serves as the erase signal.

However, as shown in FIG. 5, when the cholesteric liquid crystal 31 is set to the F alignment (point E) by the selection signal, that is, when the selection signal is set to the low voltage VL, the erasing signal is not added immediately before the selection signal. Unless the time width of the selection signal is set to about several tens of milliseconds, it is difficult to make a transition to the F orientation (point E). On the other hand, by adding the erase signal immediately before the selection signal, the time width of the selection signal can be reduced to 1 ms or less.

The technique of reducing the time width of the selection signal by applying a high-voltage pulse as an erasing signal is disclosed in Japanese Patent Application Laid-Open No. H10-206899 cited as the prior art. In the display element and the driving method of the above, paragraph 0019 of the publication (Table 1)
As described in the “C → D” column therein, the time width of the selection signal still needs to be about 20 ms.

On the other hand, in the present invention, the time width of the selection signal can be reduced to 1 ms or less by applying a high voltage pulse as the erasing signal. this is,
In Japanese Patent Application Laid-Open No. H10-206899, a cholesteric liquid crystal having a long helical pitch whose selective reflection wavelength range is an infrared region or a wavelength region longer than that is used. This is because a short spiral pitch having a region within the visible region is used.

Further, by adding an erasure signal, halftone display can be performed. If there is no erase signal, the halftone display is difficult because the reflectivity depends not only on the selection voltage but also on the previous voltage history. However, since the display is once erased by adding the erasing signal, the reflectivity is uniquely determined for the immediately following selection voltage, and an intermediate voltage between VH and VL is applied as the selection voltage. This makes it possible to display halftones.

In terms of shortening the time width of the selection signal,
As shown in FIG.
In the case where is set to the H orientation, an erasing signal is not necessary, but halftone display is also possible, and the driving signal S1 is unified to simplify the driving circuit 70. It is desirable to apply an erasure signal also to the case where is set to the H orientation.

When the dark display is performed by the F orientation as shown in FIG. 5, the H orientation occurs during the period of the erase signal and the selection signal, so that a spike-like reflectance fluctuation as shown in FIG.
This causes flicker and lower contrast. Therefore, it is preferable that the time widths of the erase signal and the selection signal be short.

As shown in the frame 2 in FIG. 5, the holding signal at the time of transition from the moving image mode to the still image mode is the same as the holding signal in the dynamic drive method of US Pat. No. 5,748,277. Necessary for stabilizing the orientation. Since the stability of the F orientation increases as the duration of the holding signal increases, the duration of the holding signal when shifting from the moving image mode to the still image mode may be longer than the duration of the holding signal when repeating the moving image mode. . That is, FIG.
In this case, the holding signal may be applied to a part of the frame 3 as indicated by a thick broken line.

FIGS. 4 and 5 show the case where the polarity of the drive signal S1 is inverted for each frame. Generally, in a liquid crystal display device, such an alternating drive signal is effective for preventing deterioration of liquid crystal, fluctuation of threshold voltage, and image burn-in.

Instead of inverting the polarity of the drive signal S1 for each frame, as shown in FIG. 7, each of the erase signal, the selection signal, and the hold signal of the drive signal S1 may be constituted by a pulse whose polarity is inverted. .

(Another Example of Element Structure) In FIG.
Is equal to the reflectance at points B and C, but the reflectance at point A is
While the reflectance at the point B or C is the reflectance of the selective reflection at the time of orientation, the reflectance of the cholesteric liquid crystal layer 30 and the reflectance of the light reflection layer 43 when the cholesteric liquid crystal 31 is at the H orientation are different. Generally do not match. In addition to reflectance, display characteristics such as display color and contrast, and their dependence on viewing angle generally differ between the moving image mode and the still image mode unless special attention is paid to matching them.

It is desirable that such a difference be as small as possible, because it makes the observer feel uncomfortable when shifting from the moving image mode to the still image mode or vice versa. From this point, the light reflection layer 4
As 3, it is preferable to use a cholesteric liquid crystal that is planar aligned and has the same selective reflection wavelength range as the cholesteric liquid crystal 31.

FIG. 2 shows a cholesteric liquid crystal display device in that case. In this example, the display element 10 is
A layer of a cholesteric liquid crystal 44 having a planar orientation and the same selective reflection wavelength range as the cholesteric liquid crystal 31 is formed between the cholesteric liquid crystal 31 and the other substrate 45 on the back side. On the back surface of the substrate 45, for example, a black light absorbing layer 46 is formed. However, the substrate 45 can also serve as the light absorbing layer 46.

A voltage is not applied to the cholesteric liquid crystal 44, and the cholesteric liquid crystal 44 is light-reflected by utilizing the fact that the cholesteric liquid crystal 44 is P-oriented and selectively reflects the same color as the selective reflection color of the cholesteric liquid crystal 31. And

According to this example, the display color, contrast,
The display characteristics such as the viewing angle dependency can be matched between the moving image mode and the still image mode.

In this case, if a cholesteric liquid crystal cell having a right twist and a cholesteric liquid crystal cell having a left twist are laminated as a light reflecting layer, a light reflecting layer having higher reflectivity can be formed.

Further, a cholesteric liquid crystal film made of a polymer liquid crystal or a film formed on a substrate may be attached to the substrate 12 as a light reflection layer without using such a cell form.

Further, contrary to the above-described example, a special visual effect can be obtained by positively utilizing the difference between the moving image mode and the still image mode. For example, in the example of FIG. 1, the selective reflection of the cholesteric liquid crystal 31 is red, the dichroic dye 32 is black, and the light reflection layer 43 is green. In this case, a display in red and black may be performed.

(Matrix Driving) Although the above-described example is the case of segment driving in which a driving signal is individually applied to each pixel of the display element 10, the present invention is also applicable to the case of simple matrix driving or active matrix driving. can do.

FIG. 8 shows an example of a cholesteric liquid crystal display device including a driving circuit in the case of simple matrix driving. In this example, the signal lines X1, X
2, X3} are connected to the signal voltage generation circuit 71, and the scanning lines Y1, Y2, Y3 # of the display element 10 are connected to the scanning voltage generation circuit 72. Signal lines X1, X2, X
Intersection Y1 between 3 ° and scan lines Y1, Y2, Y3}
-X1, Y1-X2 {Y2-X1, Y2-X2}
Are each pixel.

FIG. 9 shows the scanning voltage generating circuit 72 of this example.
, Scanning voltages output to the scanning lines Y1 and Y2, signal voltages output to the signal lines X1 and X2 from the signal voltage generation circuit 71, driving voltages applied to the cholesteric liquid crystal layers of the pixels Y1-X1 and Y2-X1, And the pixel Y1-X
1, the reflectance of Y2-X1 is shown. However, in the frame 1, the pixels Y1-X1 are darkly displayed and the pixels Y2-X1 are brightly displayed. In the frame 2, the pixels Y1-X1 are brightly displayed and the pixels Y1-X1 are brightly displayed.
This is a case in which the mode shifts to the still image mode in frame 3 after dark display of 2-X1.

The scanning voltages applied to the scanning lines Y1 and Y2 correspond to the voltages V corresponding to erasing, selecting and holding, respectively.
r, Vs, and Vh are sequentially inverted in frame 1 and frame 2 for output. The scanning voltage applied to the scanning line Y1 and the scanning voltage applied to the scanning line Y2 have the same waveform, but the scanning voltage applied to the scanning line Y2 is set so that the selection periods of the scanning line Y1 and the scanning line Y2 do not overlap. , The scan voltage to the scan line Y1 is output after being delayed by the time of the selection period.

As the signal voltage to the signal line X1, the voltage Vd is output during the selection period of the scanning line Y1, and the voltage −Vd is output during the selection period of the scanning line Y2. Outputs the voltage -Vd in the selection period for the scanning line Y1, and outputs the voltage Vd in the selection period for the scanning line Y2.

Therefore, the driving voltage applied to the pixels Y1-X1 is Vs-Vd during the selection period for the scanning line Y1 in the frame 1, and -Vs-Vd =-(Vs-Vd =-(V) during the selection period for the scanning line Y1 in the frame 2. Vs + Vd)
And the drive voltage applied to the pixels Y2-X1 becomes Vs- during the selection period for the scan line Y2 of the frame 1.
(−Vd) = Vs + Vd, scan line Y2 of frame 2
In the selection period for -Vs-(-Vd) =-(Vs
−Vd).

Therefore, in relation to the voltages VH and VL shown in FIG. 6, Vs + Vd = VH and Vs-Vd = V
By setting the voltages Vs and Vd so as to be L, when the driving voltage applied to a certain pixel becomes Vs + Vd or − (Vs + Vd), the pixel is H-aligned, and a bright display is performed. Drive voltage is Vs-
When Vd or − (Vs−Vd), the pixel is
Orientation allows dark display.

The signal voltage Vd or -Vd output during the selection period for a certain scanning line is also applied to pixels belonging to another scanning line during the erasing period or the holding period, causing crosstalk. Therefore, in this case, the driving voltage applied to the pixel during the erasing period has an absolute value of Vr−Vd.
Or Vr + Vd, so that the voltage Vr is set sufficiently high so that erasing can be performed even when the absolute value is Vr-Vd, and the driving voltage applied to the pixel during the holding period has an absolute value of Vh-Vd. Or Vh + Vd, so that when the absolute value becomes either Vh−Vd or Vh + Vd, H
Voltage V so as not to deviate from the bistable state of orientation and F orientation.
Set h appropriately.

The number of scan lines that can be scanned in one frame is given by the time width of the frame period / selection period. The shorter the selection period, the greater the number of scan lines that can be scanned in one frame. Therefore, as shown in FIG. 10, an auxiliary signal having a voltage lower than the voltage VL may be provided before or after the selection signal or immediately before or immediately after the selection signal.

As a result, even if the selection period is shortened, the cholesteric liquid crystal 31 can be surely set in the H or F alignment, and the selection period can be shortened to increase the number of scan lines that can be scanned in one frame. can do.

[Laminated Display Element and Driving Control Method Thereof] The present invention can also be applied to a cholesteric liquid crystal display device which performs multicolor display by laminating a plurality of cholesteric liquid crystal layers.

FIG. 3 shows an example of the cholesteric liquid crystal display device in this case. In this example, in the display element 10, the liquid crystal cells 50B, 50G, and 50R are stacked in this order from the observation side, and the white light reflection layer 47 is formed on the back surface of the liquid crystal cell 50R on the side opposite to the observation side. White light reflection layer 47
Is the substrate 12 or the electrode 22 on the back side of the liquid crystal cell 50R.
Can also be used.

The liquid crystal cell 50B is a cholesteric liquid crystal in which a yellow dichroic dye 32Y is added to a cholesteric liquid crystal 31B having a blue wavelength region as a selective reflection wavelength region between substrates 11 and 12 on which electrodes 21 and 22 are formed. The layer 30B is formed, and the liquid crystal cell 50G is provided between the substrates 11 and 12 on which the electrodes 21 and 22 are formed, by adding a magenta dichroic dye 3 to a cholesteric liquid crystal 31G having a green wavelength region as a selective reflection wavelength region.
Forming a cholesteric liquid crystal layer 30G to which 2M is added,
The liquid crystal cell 50R is formed on the substrate 1 on which the electrodes 21 and 22 are formed.
Between 1 and 12, a cholesteric liquid crystal layer 30R is formed by adding a cyan dichroic dye 32C to a cholesteric liquid crystal 31R having a red wavelength range as a selective reflection wavelength range.

Here, blue, green, and red mean the three primary colors in additive color mixture.
Color light having wavelength components of 00 nm, 500 to 600 nm, and 600 to 700 nm. Yellow, magenta, and cyan mean the three primary colors in subtractive color mixing, respectively, blue,
It is a complementary color of green and red. Also, white means 400 to 7
Refers to color light having a wavelength component of 00 nm.

The driving of the liquid crystal cells 50B, 50G, 50R is independently controlled by the driving circuit 70 in the above-described manner.

Therefore, in the moving image mode, the cholesteric liquid crystal layer 30B changes between a colorless state when the cholesteric liquid crystal 31B is oriented in H and a state in which the cholesteric liquid crystal 31B is colored yellow with the dichroic dye 32Y when oriented in F. Similarly, the cholesteric liquid crystal layer 30G can select between a colorless state and a magenta colored state, and the cholesteric liquid crystal layer 30R can select between a colorless state and a cyan colored state. As a whole, multicolor display is possible by subtractive color mixture of yellow, magenta, and cyan.

On the other hand, in the still image mode, the cholesteric liquid crystal layer 30B is colored yellow in the blue selective reflection state in the P orientation of the cholesteric liquid crystal 31B and the dichroic dye 32Y in the F orientation of the cholesteric liquid crystal 31B. In the same manner, the cholesteric liquid crystal layer 30G can select between a green selective reflection state and a magenta colored state, and the cholesteric liquid crystal layer 30R can select a red selective reflection state and a cyan colored state. Can be selected, and the display element 10 as a whole can perform multicolor display by additive color mixture of blue, green, and red.

The order of stacking the liquid crystal cells 50B, 50G, 50R may be basically any order, but for the following reasons, as shown in the example of FIG.
G and 50R are desirably stacked in this order from the observation side.

In general, the reflection spectrum of a cholesteric liquid crystal has a tail on the long wavelength side and the short wavelength side of the selective reflection wavelength region, and this is one factor of a decrease in saturation. Saturation can be improved by lowering the reflectance in the wavelength region of the tail, but in this case, it is better to lower the reflectance in the wavelength region on the short wavelength side rather than the long wavelength side in the selective reflection wavelength region. High degree of effect.

In the display element 10 of the example of FIG. 3, the reflected light in the wavelength range shorter than the selective reflection wavelength range in the reflected light from the green reflective cholesteric liquid crystal layer 30G is converted to the blue reflective cholesteric liquid crystal in the upper layer. In the reflected light from the red reflective cholesteric liquid crystal layer 30R, which is absorbed by the yellow dichroic dye 32Y contained in the layer 30B, the reflected light in the wavelength region on the shorter wavelength side than the selective reflection wavelength region is the upper green layer. The light is absorbed by the magenta dichroic dye 32M included in the reflective cholesteric liquid crystal layer 30G and the yellow dichroic dye 32Y included in the blue reflective cholesteric liquid crystal layer 30B. Therefore, the saturation in the still image mode is improved.

In the display element 10 of the example of FIG. 3, the dichroic dye contained in a certain cholesteric liquid crystal layer absorbs a wavelength range shorter than the selective reflection wavelength range of the lower cholesteric liquid crystal layer. Therefore, as in the case of providing a color filter on the cholesteric liquid crystal layer that absorbs a wavelength range shorter than the selective reflection wavelength range, the color change depending on the viewing angle is suppressed in the still image mode to reduce the viewing angle. Can be expanded.

[Verification by Experiment] The cholesteric liquid crystal display device of the example shown in FIG. 1 was actually manufactured, and the display characteristics were measured by controlling the driving of the cholesteric liquid crystal display device by the driving circuit in the manner described above.

Liquid crystal E44 (manufactured by Merck) /80.6% by weight, chiral agent S811 (manufactured by Merck) /15.5% by weight, and chiral agent S1011 (manufactured by Merck) /3.9% by weight. , Reflection spectrum peak wavelength is 530 nm
A green reflective cholesteric liquid crystal was prepared. In addition, 4
4.0% by weight of a black dichroic dye for liquid crystal S344 (manufactured by Mitsui Chemicals, Inc.) that absorbs the wavelength region of 00 to 700 nm almost uniformly was added.

Next, an alignment film made of vertical alignment polyimide SE7511L (manufactured by Nissan Chemical Industries, Ltd.) was formed on the glass substrate on which the ITO electrode was formed, and a cell having a cell gap of 5 μm was manufactured. A green reflective cholesteric liquid crystal containing 4.0% by weight of the dichroic dye S344 was injected thereinto, the injection port was sealed, and a ceramic white light reflector was disposed on the back surface.

The ITO of the display element thus manufactured was
Between the electrodes, a voltage of 100 V is applied as an erase signal for 1 ms, a voltage of 40 V for H alignment or a voltage of 0 V for F alignment is applied for 1 ms as a selection signal, and a voltage of 25 V is applied as a holding signal for 28 m.
The display element was driven for 30 seconds at a frame period of 30 ms.

FIG. 11 shows the change over time in the applied voltage and the reflectance when dark display and bright display are alternately performed for each frame. Even if the selection period is as short as 1 ms,
It was possible to switch between dark display and bright display at high speed. Time utilization rate in this case (holding period time / frame period)
Is remarkably high at 93% (= 28 ms / 30 ms). In this video mode, white is displayed during bright display,
At the time of dark display, a black appearance was obtained.

Next, a bright display was performed by the H orientation in the moving image mode, and the applied voltage was set to zero after the end of the holding signal to set the still image mode. As a result, a green appearance was obtained. Similarly, dark display was performed by F orientation in the moving image mode, and the applied voltage was set to zero after the end of the holding signal to switch to the still image mode. As a result, a black appearance was obtained.

[0111]

As described above, according to the present invention, it is possible to increase the time utilization rate when displaying a moving image, to obtain a bright and high-contrast display, and to repeatedly write when displaying a still image. The display contents can be held with no power supply or with a small power supply without performing or continuously applying a bias voltage, and power consumption can be significantly reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a cholesteric liquid crystal display device of the present invention.

FIG. 2 is a diagram showing another example of the cholesteric liquid crystal display device of the present invention.

FIG. 3 is a diagram showing still another example of the cholesteric liquid crystal display device of the present invention.

FIG. 4 is a diagram provided for describing a drive control method according to the present invention.

FIG. 5 is a diagram provided for describing a drive control method according to the present invention.

FIG. 6 is a diagram showing an applied voltage-reflectance characteristic in the case of the present invention.

FIG. 7 is a diagram illustrating another example of a drive signal.

FIG. 8 is a diagram illustrating an example of a cholesteric liquid crystal display device in the case of simple matrix driving.

FIG. 9 is a diagram provided for describing a drive control method in the case of simple matrix drive.

FIG. 10 is a diagram illustrating another example of a drive signal.

FIG. 11 is a diagram showing experimental results of a prototype cholesteric liquid crystal display device.

FIG. 12 is a view showing a conventional general cholesteric liquid crystal display device.

FIG. 13 is a diagram showing an applied voltage-reflectance characteristic of the cholesteric liquid crystal display device of FIG.

FIG. 14 illustrates the dynamic drive method of US Pat. No. 5,748,277.

FIG. 15 is a diagram showing a temporal change of a drive signal and a reflectance in displaying a moving image by a dynamic drive method.

FIG. 16 is a diagram showing a cholesteric liquid crystal display device in a PCGH mode.

FIG. 17 is a diagram showing an applied voltage-reflectance characteristic of the cholesteric liquid crystal display element of FIG.

FIG. 18 is a diagram showing a time change of a driving signal and a reflectance in a case of a driving method disclosed in Japanese Patent Application Laid-Open No. H10-206899.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Cholesteric liquid crystal display element 11,12 ... Substrate 21,22 ... Electrode 30 ... Cholesteric liquid crystal layer 30B ... Blue reflection cholesteric liquid crystal layer 30G ... Green reflection cholesteric liquid crystal layer 30R ... Red reflection cholesteric liquid crystal layer 31 ... Cholesteric liquid crystal 31B ... Blue reflection Cholesteric liquid crystal 31G ... Green reflective cholesteric liquid crystal 31R ... Red reflective cholesteric liquid crystal 32 ... Dichroic dye 32Y ... Yellow dichroic dye 32M ... Magenta dichroic dye 32C ... Cyan dichroic dye 43 ... Light reflecting layer 44 ... Cholesteric liquid crystal (light reflection layer) 45 ... Substrate 46 ... Light absorption layer 47 ... White light reflection layer 50B, 50G, 50R ... Liquid crystal cell 70 ... Drive circuit 71 ... Signal voltage generation circuit 72 ... Scan voltage generation circuit S1 ... Drive signal

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Taketo Hikiji 430 Sakai Nakaicho, Ashigarakami-gun, Kanagawa Prefecture Inside Green Tech Nakai Fuji Xerox Co., Ltd. F term (reference) 2H088 FA29 GA03 GA10 GA13 HA06 HA08 HA21 JA06 MA02 MA06 MA07 MA20 2H091 FD06 GA01 GA11 GA13 HA08 JA02 LA15 LA17 LA19 2H093 NC03 NC34 ND04 ND10 ND13 ND17 ND39 NE01 NF06 NF11

Claims (10)

[Claims] 1. A cholesteric liquid crystal layer having cholesteric liquid crystal having a selective reflection wavelength region within a visible region and having dichroism, and a light reflecting layer provided on the opposite side of the cholesteric liquid crystal layer from the observation side. Cholesteric liquid crystal display device. 2. The cholesteric liquid crystal display device according to claim 1, wherein the dichroism of the cholesteric liquid crystal layer is based on a dichroic dye added to the cholesteric liquid crystal. 3. The cholesteric liquid crystal display device according to claim 1, wherein said cholesteric liquid crystal layer or said dichroic dye absorbs said selective reflection wavelength range, and said light reflection layer reflects said selective reflection wavelength range. Cholesteric liquid crystal display device. 4. A cholesteric liquid crystal display device according to claim 3, wherein said light reflecting layer is made of a planar-aligned cholesteric liquid crystal having the same selective reflection wavelength range as said cholesteric liquid crystal. 5. A blue reflective cholesteric liquid crystal layer containing a yellow dichroic dye, a green reflective cholesteric liquid crystal layer containing a magenta dichroic dye, and a red reflective cholesteric liquid crystal layer containing a cyan dichroic dye. A cholesteric liquid crystal display device in which a white light reflecting layer is formed on the side opposite to the observation side of the three cholesteric liquid crystal layers. 6. The cholesteric liquid crystal display device according to claim 5, wherein the blue reflective cholesteric liquid crystal layer, the green reflective cholesteric liquid crystal layer, and the red reflective cholesteric liquid crystal layer are:
A cholesteric liquid crystal display device stacked in this order from the observation side. 7. A cholesteric liquid crystal display device according to claim 1, and a drive circuit for driving and controlling said cholesteric liquid crystal display device.
A first display mode in which reflection is performed on the light reflecting layer when the cholesteric liquid crystal is homeotropically aligned, or display is selected by selecting coloring of the cholesteric liquid crystal layer when the cholesteric liquid crystal is in focal conic alignment. And a second display mode for performing display by selective reflection of the cholesteric liquid crystal when the cholesteric liquid crystal is in a planar orientation, or by selection of coloring of the cholesteric liquid crystal layer when the cholesteric liquid crystal is in a focal conic orientation. And a cholesteric liquid crystal display device that controls the driving so as to be able to selectively take the following. 8. A cholesteric liquid crystal display element according to claim 1, and a drive circuit for driving and controlling the cholesteric liquid crystal display element, wherein the drive circuit forms the cholesteric liquid crystal layer of the cholesteric liquid crystal display element. A drive mode in which a selection signal for causing the liquid crystal to be in a homeotropic alignment or a focal conic alignment, and a drive signal including a holding signal for holding an alignment state selected by the selection signal, are applied to the cholesteric liquid crystal layer, A cholesteric liquid crystal display device that selectively takes a maintenance mode in which a voltage applied to the cholesteric liquid crystal layer is equal to or lower than a threshold. 9. The cholesteric liquid crystal display device according to claim 8, wherein the drive circuit is configured to cause the cholesteric liquid crystal forming the cholesteric liquid crystal layer to transition to homeotropic alignment in the drive mode prior to the selection signal. A cholesteric liquid crystal display device for applying an erasing signal to the cholesteric liquid crystal layer. 10. The cholesteric liquid crystal display device according to claim 8, wherein the drive circuit makes a time width of a hold signal in a drive mode immediately before the sustain mode longer than a time width of a hold signal in another drive mode. Cholesteric liquid crystal display device.