JP3944678B2 - Cholesteric liquid crystal display element and cholesteric liquid crystal display device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
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]
[Prior art]
As a flat panel display device, a transmissive liquid crystal display device is widely used in which a liquid crystal element is used as an optical shutter, and display is performed by transmitting or blocking illumination light from a backlight disposed on the back surface thereof. However, the transmissive 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.
[0003]
As a display device that solves this problem, a reflective liquid crystal display device that performs display using reflection of external light without using a backlight is attracting attention, and a cholesteric liquid crystal display device is known as one method. Yes.
[0004]
Since the cholesteric liquid crystal display device is a reflective display device that uses reflection of external light, it does not require illumination power and has low memory consumption, but also has a memory property that can hold display contents without a power source. Therefore, it has features such that an expensive active matrix substrate such as a thin film transistor is not required for driving, a flexible substrate such as a resin substrate can be used, and a clear display with high reflectivity is possible.
[0005]
The cholesteric liquid crystal is composed of rod-like molecules and has a multilayer shape. The molecular long axis is oriented in one direction within 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 optical wavelengths by appropriate selection of materials, in which case the cholesteric liquid crystal selectively reflects visible light. This phenomenon is known as selective reflection of cholesteric liquid crystals.
[0006]
The cholesteric liquid crystal display element using selective reflection of cholesteric liquid crystal 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 light absorption layer 41 is formed on the back surface of the substrate 12 opposite to the observation side (external light incident side).
[0007]
There are three types of alignment states of the cholesteric liquid crystal 31: a planar shown in FIG. 12A, a focal conic shown in FIG. 12B, and a homeotropic shown in FIG. Hereinafter, the planar alignment is referred to as P alignment, the focal conic alignment is referred to as F alignment, and the homeotropic alignment is referred to as H alignment.
[0008]
The P orientation is a state in which the spiral axis is oriented substantially perpendicular to the substrate surface, and colored light in the selective reflection wavelength region is observed. The F alignment is a state in which the helical axis is aligned substantially parallel to the substrate surface, and the cholesteric liquid crystal 31 itself is colorless, and thus the color of the light absorption layer 41 is observed. The H alignment is a state in which the helical structure is unwound and the liquid crystal molecules are aligned perpendicularly to the substrate surface. The cholesteric liquid crystal 31 itself is also colorless, and thus the color of the light absorption layer 41 is observed.
[0009]
Therefore, if the light absorption layer 41 is made black, colored light in the selective reflection wavelength region 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.
[0010]
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 because the initial orientation is P orientation, F orientation, and H orientation, a pulse voltage is applied for a certain time, then the applied voltage is set to zero, and the reflectance of the cholesteric liquid crystal 31 is measured after a certain time. This is a schematic representation of reflectance characteristics.
[0011]
The applied voltage-reflectance characteristics are represented by points A, E, F, and C when the initial orientation is P orientation, and are represented by points D, E, F, and C when the initial orientation is F orientation, When the initial orientation is H orientation, it is represented by points A, E, B, and C.
[0012]
At points B and C, the orientation is H during voltage application, and when the applied voltage is zero, the transition is made to the P orientation, resulting in high reflectivity. Since the point A is in the P orientation, it has a high reflectance. Points D, E, and F have a low reflectance because they are in the F orientation. When the voltage VH is applied, the H orientation (point C) is set regardless of the initial orientation, and when the voltage VL is applied, the F orientation (point E) is set regardless of the initial orientation.
[0013]
When the applied voltage is zero, the P orientation (point A) and the F orientation (point D) are bistable and exhibit a ZFM (Zero Field Memory) effect. Further, when the voltage VB is applied, the H orientation (point B) and the F orientation (point F) are bistable and show a BFM (Bias Fielded Memory) effect.
[0014]
[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 cholesteric liquid crystal display element. In this method, as shown in FIG. 14, a series of drive signals including an erase signal, a selection signal, and a hold signal are applied, and then the applied voltage is set to zero. In US Pat. No. 5,748,277, these erasure, selection, and retention are called preparation, selection, and evolution, respectively.
[0015]
That is, in the dynamic drive method, first, a high voltage pulse is applied as an erasing signal to cause the cholesteric liquid crystal to transition to the H alignment (point C). Next, the voltage VB or VL is applied as a selection signal, and the cholesteric liquid crystal is transitioned 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).
[0016]
The hold signal is necessary to stabilize the F orientation. In other words, when transitioning from the H orientation (point C) to the F orientation (point E), the transition from the TP orientation to the F orientation is completed via a transient state called a transient planar orientation (TP orientation) of the cholesteric liquid crystal. If the applied voltage is made zero before the cholesteric liquid crystal transitions to the P orientation. In order to prevent this, the voltage VB is applied as a holding signal for a certain period of time.
[0017]
By applying the holding signal, the H orientation (point B) or the F orientation (point F) is maintained. Since both of these are colorless, the applied voltage is set to zero and the H orientation (point B) is shifted to the P orientation (point A), causing selective reflection.
[0018]
That is, in the dynamic drive method, the display image is written by using the BFM effect once by the H orientation and the F orientation, then the applied voltage is set to zero, and the H orientation is changed to the P orientation, thereby displaying the display. Visualize the image and use the ZFM effect to hold the display content without power.
[0019]
When displaying a moving image, as shown in FIG. 15, four steps of erasing, selecting, holding, and voltage zero are repeated for each frame. FIG. 15 schematically shows a change in reflectance at the same time when a bright display is performed in frame 1 and a dark display is performed in frame 2 for a certain pixel.
[0020]
[About JP-A-10-206899]
On the other hand, Japanese Patent Application Laid-Open No. 10-206899 discloses a driving method of a cholesteric liquid crystal display element in a PCGH (Phase Change Guest Host) mode.
[0021]
A PCGH mode cholesteric liquid crystal display element 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 FIG. The selective reflection wavelength range is set to the infrared wavelength range or longer wavelength range, and a dichroic dye is added to the cholesteric liquid crystal 31 to display by changing the transmittance due to the orientation change of the cholesteric liquid crystal 31. . Since the cholesteric liquid crystal 31 itself does not generate reflected light in the visible range, the light reflecting layer 42 is formed on the back surface of the substrate 12 opposite to the observation side.
[0022]
In this PCGH mode cholesteric liquid crystal display element, when the cholesteric liquid crystal 31 is in the P-orientation or F-orientation as shown in FIG. 16 (A) or (B), the cholesteric liquid crystal is absorbed by light absorption of the dichroic dye contained in the cholesteric liquid crystal 31. When 31 is colored and the cholesteric liquid crystal 31 is H-oriented as shown in FIG. 5C, the cholesteric liquid crystal 31 becomes transparent, and the color of the light reflecting layer 42 is observed.
[0023]
The change in orientation relative to the applied voltage is the same as when no dichroic dye is included. However, the reflectance characteristics with respect to the applied voltage change depending on whether the change in the orientation of the cholesteric liquid crystal 31 is visualized by selective reflection or by the change in the transmittance of the cholesteric liquid crystal 31 due to light absorption of the dichroic dye.
[0024]
FIG. 17 schematically shows the applied voltage-reflectance characteristics in this case. The difference from FIG. 13 is that the high reflectance region of point A does not exist. This is because the P orientation and the F orientation are both colored.
[0025]
That is, when the applied voltage is zero, the P orientation and the F orientation exist as a bistable state regardless of the presence or absence of the dichroic dye, but in the PCGH mode, these orientation states are both colored. No hysteresis is observed on the applied voltage-reflectance characteristics. Therefore, in the PCGH mode, the ZFM effect cannot be used substantially, and only the BFM effect due to the bistability of the H orientation (point B) and the F orientation (point F) generated when the bias voltage VB is applied. It will be used for display.
[0026]
From this point, in the driving method of Japanese Patent Laid-Open No. 10-206899, as shown in FIG. 18, a series of driving signals including an erasing signal, a selection signal, and a holding signal are applied for each frame. FIG. 18 shows a case in which bright display is performed by reflection on the light reflection layer 42 by the H orientation in frame 1 and dark display by coloring by a dichroic dye is performed by F orientation in frame 2 for a certain pixel.
[0027]
That is, in the driving method disclosed in Japanese Patent Laid-Open No. 10-206899, first, a high voltage pulse is applied as an erasing signal to cause the cholesteric liquid crystal to transition to the H alignment (point C). Next, the voltage VB or VL is applied as a selection signal, and the cholesteric liquid crystal is transitioned 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).
[0028]
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 disclosed in Japanese Patent Laid-Open No. 10-206899, display is performed using only the BFM effect by repeating three steps of erasing, selection, and holding.
[0029]
[Problems to be solved by the invention]
However, in the dynamic drive method of US Pat. No. 5,748,277 shown in FIG. 14 and FIG. 15 described above, during the erasing, selection, and holding period, it is either H orientation or F orientation, and selective reflection. The reflected light is generated only during a period in which the applied voltage after the holding signal is zero.
[0030]
Although it depends on the liquid crystal material and the applied voltage, generally, the erasing period requires several milliseconds, the selection period requires 1 millisecond, and the holding period requires ten and several milliseconds, while preventing flicker. Requires a frame period of 30 ms or less.
[0031]
Therefore, in the dynamic drive method, the ratio of time for generating reflected light with respect to the frame period is low, and it is difficult to exceed 50%. Moreover, since the rise time of the reflectance when transitioning from the H orientation to the P orientation is generally as long as 100 to 200 milliseconds, the reflectance is not saturated within the frame period.
[0032]
As a result, the dynamic drive method has a problem that the time-average reflectance at the time of moving image display is low and only a dark display can be obtained at the time of moving image display. In order to avoid this problem, it is conceivable to shorten the longest holding period during the erasing, selection, and holding period. However, as is apparent from frame 2 in FIG. During the zero period, the reflectivity increases and the contrast decreases.
[0033]
On the other hand, in the PCGH mode cholesteric liquid crystal display element shown in FIGS. 16 to 18 and the driving method disclosed in Japanese Patent Laid-Open No. 10-206899, only the selection period and the holding period are involved in the display, and the applied voltage is zero. Since no period is required, the time utilization factor, that is, the ratio of time for generating reflected light with respect to the frame period, can be increased as compared with the dynamic drive method of US Pat. The time average reflectance can be increased.
[0034]
However, the PCGH mode cholesteric liquid crystal display element and the driving method disclosed in Japanese Patent Application Laid-Open No. 10-206899 do not use the ZFM effect caused by the bistability of the P and F orientations when the applied voltage is zero, so that still images are displayed. In order to hold the display contents as in the case of the display, it is necessary to repeatedly write in each frame or to continuously apply the bias voltage VB, and there is a problem that the display contents cannot be held without a power source. .
[0035]
Therefore, the present invention can increase the time utilization rate when displaying a moving image, obtain a bright and high-contrast display, and repeatedly write or continue to apply a bias voltage when displaying a still image. The display contents can be held with no power supply or a small power supply, and the power consumption can be greatly reduced.
[0036]
[Means for Solving the Problems]
The cholesteric liquid crystal display element of the present invention is
Wavelength range in the visible range Including a cholesteric liquid crystal having a selective reflection wavelength region, and Absorb at least the selective reflection wavelength range A cholesteric liquid crystal layer having dichroism;
This cholesteric liquid crystal layer was provided on the opposite side of the observation side Reflecting at least the selective reflection wavelength region A light reflecting layer;
Shall be provided.
[0037]
In order to make the cholesteric liquid crystal layer have dichroism, a dichroic dye is added to the cholesteric liquid crystal, or a dichroic liquid crystal itself has a dichroic liquid crystal.
[0039]
[Action]
In the cholesteric liquid crystal display element of the present invention, the selective reflection wavelength region of the cholesteric liquid crystal is in the visible region, And Add dichroic dye to cholesteric liquid crystal to add cholesteric liquid crystal layer Absorbs at least the selective reflection wavelength region of cholesteric liquid crystals Give dichroism And On the side opposite to the observation side of the cholesteric liquid crystal layer Reflects at least the selective reflection wavelength region of cholesteric liquid crystal Since the light reflecting layer is provided, (a) the reflection on the light reflecting layer at the H orientation using the BFM effect by the bistability of the H orientation and the F orientation of the cholesteric liquid crystal or the change in transmittance of the cholesteric liquid crystal layer Reflection of selective reflection of cholesteric liquid crystal using the first display mode in which display is performed by selecting the color of the cholesteric liquid crystal layer during F alignment, and (b) the ZFM effect due to bistability of the P and F orientations of cholesteric liquid crystal The second display mode in which the display is performed by selectively reflecting the cholesteric liquid crystal at the P orientation or by selecting the color of the cholesteric liquid crystal layer at the F orientation utilizing the difference in rate can be taken.
[0040]
Therefore, at the time of moving image display, by selecting the first display mode, it is possible to increase the time utilization rate, that is, the ratio of time for generating reflected light with respect to the frame period, and to obtain a bright and high-contrast display. When displaying a still image, the display contents can be held with no power supply or a small power supply without repeatedly writing or continuously applying a bias voltage by shifting from the first display mode to the second display mode. Power consumption can be greatly reduced.
[0041]
DETAILED DESCRIPTION OF THE INVENTION
[Single-layer display element and its drive control method]
(Example of element configuration)
FIG. 1 shows an example of a cholesteric liquid crystal display device according to the present invention, and the cholesteric liquid crystal display device comprises a cholesteric liquid crystal display element 10 and a drive circuit 70 for driving and controlling the cholesteric liquid crystal display element 10. Hereinafter, the cholesteric liquid crystal display element 10 is abbreviated as a display element 10.
[0042]
In this example, the display element 10 is formed with electrodes 21 and 22 on one surface of the substrates 11 and 12, respectively, and the substrates 11 and 12 are opposed to each other with the electrodes 21 and 22 facing inward. In addition, a cholesteric liquid crystal layer 30 in which a dichroic dye 32 is added to the cholesteric liquid crystal 31 is formed, and a light reflection layer 43 is formed on the back surface of the substrate 12 opposite to the observation side.
[0043]
The cholesteric liquid crystal 31 has a wavelength range in the visible range (400 to 800 nm) such as red, green, or blue as a selective reflection wavelength range. Examples of the material include liquid crystal asymmetric carbon compounds such as cholesterol derivatives such as cholesteryl chloride and cholesteryl nonanoate, benzylidene aniline, azobenzene, azoxybenzene, cyanobiphenyl, cyanoterphenyl, phenylcyclohexane, phenylbenzoate, Known nematic liquid crystalline compounds containing chemical structures such as cyclohexylcyclohexane, cyclohexylcarboxylic acid ester, phenylpyrimidine, phenyldioxane, cyclohexylcyclohexane ester, cyclohexylethane, cyclohexene, and tolan, and chiral compounds such as 2-methyl-n-butylcyanobiphenyl A composition to which an asymmetric carbon compound called an agent is added can be used.
[0044]
The dichroic dye 32 is one that absorbs the selective reflection wavelength range of the cholesteric liquid crystal 31, or a black dichroic dye that absorbs the visible wavelength range almost uniformly. As the dichroic dye 32, known dichroic dyes such as azo, anthraquinone and coumarin can be used.
[0045]
If the concentration of the dichroic dye 32 is too low, coloring of the cholesteric liquid crystal 31 during F alignment is insufficient, and conversely, if the concentration is too high, the transmittance of the cholesteric liquid crystal 31 during H alignment decreases (light reflection). The reflectance of the reflection at the layer 43 is reduced), and the reflectance of the selective reflection during the P orientation of the cholesteric liquid crystal 31 is reduced. Therefore, an appropriate range exists for the concentration of the dichroic dye 32. The range varies depending on the intensity of absorption of the dichroic dye 32, the dichroic ratio, the thickness of the cholesteric liquid crystal layer 30 (cell gap), and the like, but generally 1 to 10% is suitable.
[0046]
The cholesteric liquid crystal layer 30 may include components other than the cholesteric liquid crystal and the dichroic dye. For example, the cholesteric liquid crystal layer 30 is 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 fine particles, or conversely, the cholesteric liquid crystal is formed into droplets in the polymer matrix. A dispersed layer or a layer in which a cholesteric liquid crystal is included in a microcapsule may be used.
[0047]
Further, instead of adding the dichroic dye 32 to the cholesteric liquid crystal 31, a dichroic liquid crystal 31 itself may be a dichroic liquid crystal.
[0048]
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.
[0049]
The electrodes 21 and 22 are formed on the substrates 11 and 12 in a thin film by using a light-transmitting conductive material, for example, indium tin oxide, tin oxide, aluminum-added zinc oxide, or the like by sputtering, vapor deposition, sol-gel method, or the like. Thereafter, it is formed into a desired shape by a photoetching method or the like.
[0050]
The light reflection layer 43 is a layer that reflects a selective reflection wavelength region of the cholesteric liquid crystal 31, a white light reflection layer that substantially uniformly reflects a visible wavelength region, or the like. For example, when the cholesteric liquid crystal 31 selectively reflects red, the light reflecting layer 43 is red or white.
[0051]
The light reflecting layer 43 is formed by applying a paint on the substrate 12, adhering a metal foil, or depositing a metal. Further, the light reflecting layer 43 may be formed on another substrate without being directly formed on the substrate 12 and may be bonded to the substrate 12 together with the substrate. Furthermore, the light reflection layer 43 can also serve as the electrode 22 or the substrate 12. In that case, of course, the electrode 22 or the substrate 12 is not translucent.
[0052]
The display element 10 of the example of FIG. 1 is manufactured as follows, for example. 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 bonded at regular intervals through spacers, thereby forming empty cells. Make it. Next, cholesteric liquid crystal 31 added with dichroic dye 32 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, the light reflecting layer 43 is formed on the back surface of the substrate 12.
[0053]
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 the uniformity thereof. As the alignment film, a resin such as polyimide or polyvinyl alcohol, a low molecular surface modifier such as an alkylammonium compound or an alkylsilane compound, or an inorganic thin film such as SiO can be used. The alignment film is particularly preferably vertically aligned.
[0054]
(Drive control method and display mode)
As a driving control method of the display element 10 by the driving circuit 70, as shown in the frame 1 and the frame 2 in FIG. 4 or FIG. 5, a selection signal for setting the cholesteric liquid crystal 31 to H alignment or F alignment, and this selection signal A driving mode in which a driving signal S1 consisting of a holding signal for holding the orientation state selected by the driving mode is applied between the electrodes 21 and 22, and a driving signal S1 is applied as shown in the frame 3 of FIG. 4 or FIG. Instead, a maintenance mode in which the voltage applied between the electrodes 21 and 22 is zero is selectively taken. Hereinafter, the applied voltage between the electrodes 21 and 22 is simply referred to as an applied voltage.
[0055]
The reason why the applied voltage is set to zero is to hold the display content written immediately before by applying the drive signal S1, as will be described later. However, the applied voltage may not be completely zero and may be set to a certain threshold value or less. The following example shows a case where the applied voltage is zero.
[0056]
When displaying a moving image, the drive signal S1 is applied for each frame, and when displaying a still image while maintaining the display content 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 maintenance mode in which the applied voltage is zero is referred to as a still image mode.
[0057]
Further, in the moving image mode, the BFM effect due to the bistability of the cholesteric liquid crystal 31 in the H orientation and the F orientation is used, and the change in the transmittance of the cholesteric liquid crystal layer 30 is used to reflect or reflect on the light reflecting layer 43 in the H orientation. Display is performed by selecting the coloration of the cholesteric liquid crystal layer 30 in the F orientation, and in the still image mode, the reflectivity of selective reflection of the cholesteric liquid crystal 31 using the ZFM effect due to the bistability of the P orientation and the F orientation of the cholesteric liquid crystal 31 is used. Using the difference, display is performed by selective reflection of the cholesteric liquid crystal 31 at the P orientation or by selection of the color of the cholesteric liquid crystal layer 30 at the F orientation.
[0058]
FIG. 6 schematically shows the applied voltage-reflectance characteristics in this case. Points B and C are H-oriented and have high reflectivity due to reflection by the light reflection layer 43. Points D, E, and F are in the F orientation, and the cholesteric liquid crystal layer 30 is colored by the dichroic dye 32, resulting in a low reflectance. Point A is P-oriented and has high reflectivity due to selective reflection of the cholesteric liquid crystal 31.
[0059]
The selection signal is a signal for setting the cholesteric liquid crystal 31 to H orientation (point C) or F orientation (point E), and its voltage is VH or VL. The holding signal is a signal for holding the orientation state (point B or point F) selected by the selection signal, and its voltage is VB.
[0060]
The transition from the moving image mode to the still image mode is realized by setting the applied voltage to zero after applying the drive signal S1, that is, after applying 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 by reflection on the light reflecting layer 43 is obtained, the applied voltage is zero. By shifting to the P orientation (point A), a bright display by selective reflection of the cholesteric liquid crystal 31 is obtained and a bright display state is maintained.
[0061]
On the contrary, as shown in FIG. 5, when the F orientation (point F) is held by the previous holding signal, that is, when dark display by coloring with the dichroic dye 32 is obtained, the applied voltage is zero. By maintaining the F orientation (point D), dark display by coloring with the dichroic dye 32 is maintained.
[0062]
As described above, by setting the still image mode (maintenance mode), the display contents written in the immediately preceding moving image mode (driving mode) can be held without a power source.
[0063]
In order to make a complete 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 the applied voltage zero sufficiently fast. When this transition speed is slow, the H orientation (point B) transitions to the F orientation (point D).
[0064]
As described above, FIG. 4 shows bright display by selective reflection of the cholesteric liquid crystal 31 by P orientation in the frame 3 in frame 1 and frame 2 in FIG. This is a case where the display is performed and the bright display is maintained following the frames 1 and 2. In the present invention, when the brightly displayed frames are continuous in this manner, there is an advantage that the flicker is hardly generated because the time variation of the reflectance hardly occurs.
[0065]
As shown in FIG. 5, it is desirable to add an erasure signal to the drive signal S1 in the moving image mode prior to the selection signal. 5, contrary to FIG. 4, for a certain pixel, in frames 1 and 2, dark display by coloring with the dichroic dye 32 is performed by F orientation, and in frame 3, the same dark display is performed by F orientation, This is a case where the dark display is maintained following the frames 1 and 2.
[0066]
The erasing signal is a high voltage pulse for setting the cholesteric liquid crystal 31 to the H orientation (point C). When the cholesteric liquid crystal 31 is in the H orientation (point C) by the selection signal as shown in FIG. 4, since the selection signal is set to the high voltage VH, it is not necessary to add an erasing signal immediately before the selection signal. In this case, the selection signal also serves as the erase signal.
[0067]
However, as shown in FIG. 5, when the cholesteric liquid crystal 31 is set to F orientation (point E) by the selection signal, that is, when the selection signal is set to the low voltage VL, the selection is performed unless an erasing signal is added immediately before the selection signal. Unless the time width of the signal is set to about several tens of milliseconds, it is difficult to transition to the F orientation (point E). On the other hand, the time width of the selection signal can be shortened to 1 msec or less by adding the erasure signal immediately before the selection signal.
[0068]
Reducing the time width of the selection signal by applying a high voltage pulse as the erasing signal is shown in Japanese Patent Laid-Open No. 10-206899 cited as the prior art, but the display element of Japanese Patent Laid-Open No. 10-206899 In the driving method, as described in the column “C → D” in paragraph 0019 [Table 1] of the publication, the time width of the selection signal still needs about 20 milliseconds.
[0069]
In contrast, in the present invention, the time width of the selection signal can be shortened to 1 msec or less by applying a high voltage pulse as the erasing signal. In JP-A-10-206899, a cholesteric liquid crystal having a long helical pitch whose selective reflection wavelength region is an infrared region or a longer wavelength region is used as the cholesteric liquid crystal. This is because one having a short helical pitch in which the selective reflection wavelength region is in the visible region is used.
[0070]
Further, halftone display is possible by adding an erasing signal. In the absence of an erasing signal, halftone display is difficult because the reflectance depends not only on the selected voltage but also on the previous voltage history. However, since the display is once erased by adding an erasing signal, the reflectance is uniquely determined for the immediately subsequent selection voltage, and an intermediate voltage between VH and VL is applied as the selection voltage. This makes it possible to display halftones.
[0071]
In terms of shortening the time width of the selection signal, an erase signal is not required when the cholesteric liquid crystal 31 is set to H orientation by the selection signal as shown in FIG. 4, but halftone display is also possible, and the drive signal S1. In terms of simplifying the drive circuit 70 by unifying these configurations, it is desirable to apply an erasing signal even when the cholesteric liquid crystal 31 is in the H orientation by a selection signal.
[0072]
When dark display is performed by F orientation as shown in FIG. 5, since it is H orientation during the period of the erasing signal and the selection signal, a spike-like reflectance fluctuation as shown in the figure occurs, which causes flicker and contrast reduction. Become. Therefore, it is preferable that the time width of the erase signal and the selection signal is short.
[0073]
As shown in frame 2 of FIG. 5, the holding signal when shifting from the moving image mode to the still image mode stabilizes the F orientation in the same manner as the holding signal in the dynamic drive method of US Pat. No. 5,748,277. It is necessary to make it. Since the stability of the F orientation becomes higher as the holding signal time is longer, the holding signal time for shifting from the moving image mode to the still image mode may be longer than the holding signal time for repeating the moving image mode. . That is, in the case of FIG. 5, the holding signal may be applied across a part of the frame 3 as indicated by a thick broken line.
[0074]
The example of FIGS. 4 and 5 is a case where the polarity of the drive signal S1 is inverted for each frame. In general, in a liquid crystal display device, such an alternating drive signal is effective for preventing liquid crystal deterioration, threshold voltage fluctuation, and image burn-in.
[0075]
Instead of inverting the polarity of the drive signal S1 for each frame, as shown in FIG. 7, each of the erasing signal, the selection signal, and the holding signal of the drive signal S1 may be configured by a pulse whose polarity is inverted.
[0076]
(Other examples of element configuration)
In FIG. 6, the reflectance at point A is equal to the reflectance at points B and C, but the reflectance at point A is the reflectance of selective reflection when the cholesteric liquid crystal 31 is in the P orientation, Since the reflectances at points B and C depend on the transmittance of the cholesteric liquid crystal layer 30 and the reflectance of the light reflecting layer 43 when the cholesteric liquid crystal 31 is in the H orientation, they generally do not match. In addition to reflectance, display characteristics such as display color, contrast, and viewing angle dependency thereof are usually different between the moving image mode and the still image mode unless they are matched with particular care.
[0077]
Such a difference makes the viewer feel uncomfortable when shifting from the moving image mode to the still image mode, or vice versa. From this point, it is preferable to use a planar-aligned cholesteric liquid crystal having the same selective reflection wavelength region as that of the cholesteric liquid crystal 31 as the light reflecting layer 43.
[0078]
FIG. 2 shows a cholesteric liquid crystal display device in that case. In this example, the display element 10 has the same selective reflection wavelength region as that of the cholesteric liquid crystal 31 between the substrate 12 and the other substrate 45 on the back surface side. A planar aligned cholesteric liquid crystal layer 44 is formed. On the back surface of the substrate 45, for example, a black light absorption layer 46 is formed. However, the substrate 45 can also serve as the light absorption layer 46.
[0079]
A voltage is not applied to the cholesteric liquid crystal 44, and the cholesteric liquid crystal 44 is used as a light reflection layer 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.
[0080]
According to this example, display characteristics such as display color, contrast, and viewing angle dependency thereof can be matched in the moving image mode and the still image mode.
[0081]
In this case, if a right-twisted cholesteric liquid crystal cell and a left-twisted cholesteric liquid crystal cell are stacked as the light reflecting layer, a light reflecting layer with higher reflectivity can be formed.
[0082]
In addition, a polymer liquid crystalline cholesteric liquid crystal film or a film formed on a substrate may be attached to the substrate 12 as a light reflecting layer without using such a cell form.
[0083]
Furthermore, contrary to the above-described example, a special visual effect can be obtained by actively 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 set to red, the dichroic dye 32 is set to black, and the light reflection layer 43 is set to green. Then, display in red and black may be performed.
[0084]
(Matrix drive)
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, but the present invention can also be applied to simple matrix driving and active matrix driving.
[0085]
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, X2, X3,... Of the display element 10 are connected to the signal voltage generation circuit 71, and the scanning lines Y1, Y2, Y3, etc. of the display element 10 are connected to the scanning voltage generation circuit 72. To do. Intersections Y1-X1, Y1-X2,... Y2-X1, Y2-X2,... Of the signal lines X1, X2, X3,.
[0086]
FIG. 9 shows the scanning voltage output from the scanning voltage generation circuit 72 to the scanning lines Y1 and Y2, the signal voltage output from the signal voltage generation circuit 71 to the signal lines X1 and X2, and the pixels Y1-X1 and Y2 in this example. The driving voltage applied to the cholesteric liquid crystal layer of −X1 and the reflectance of the pixels Y1-X1 and Y2-X1 are shown. However, in frame 1, pixel Y1-X1 is darkly displayed, pixel Y2-X1 is brightly displayed, and in frame 2, pixel Y1-X1 is brightly displayed and pixel Y2-X1 is darkly displayed, and then a still image is displayed in frame 3. This is a case of shifting to the mode.
[0087]
As the scanning voltages to the scanning lines Y1 and Y2, voltages Vr, Vs, and Vh are sequentially output corresponding to erasure, selection, and holding, respectively, with the polarity reversed in the frames 1 and 2. The scanning voltage to the scanning line Y1 and the scanning voltage to the scanning line Y2 have the same waveform shape, but the scanning voltage to the scanning line Y2 is set so that the selection period does not overlap between the scanning line Y1 and the scanning line Y2. Then, the scanning voltage to the scanning line Y1 is delayed by the time of the selection period and output.
[0088]
As the signal voltage to the signal line X1, the voltage Vd is output in the selection period for the scanning line Y1, the voltage -Vd is output in the selection period for the scanning line Y2, and the signal voltage to the signal line X2 is scanned. The voltage -Vd is output in the selection period for the line Y1, and the voltage Vd is output in the selection period for the scanning line Y2.
[0089]
Accordingly, the drive voltage applied to the pixel Y1-X1 is Vs−Vd in the selection period for the scan line Y1 in frame 1 and −Vs−Vd = − (Vs + Vd) in the selection period for the scan line Y1 in frame 2. The drive voltage applied to the pixel Y2-X1 is Vs − (− Vd) = Vs + Vd in the selection period for the scanning line Y2 in frame 1, and −Vs − (− Vd in the selection period for the scanning line Y2 in frame 2. ) = − (Vs−Vd).
[0090]
Therefore, by setting the voltages Vs and Vd so that Vs + Vd = VH and Vs−Vd = VL in relation to the voltages VH and VL shown in FIG. 6, the drive voltage applied to a certain pixel becomes Vs + Vd. Or when-(Vs + Vd), the pixel is H-aligned for bright display, and when the drive voltage applied to a pixel is Vs-Vd or-(Vs-Vd), the pixel is F-orientated. Dark display is possible.
[0091]
The signal voltage Vd or −Vd output during the selection period for a certain scan line is also applied to pixels belonging to other scan lines during the erase period or the holding period, thereby causing crosstalk. Therefore, in this case, since the absolute value of the drive voltage applied to the pixels during the erasing period is Vr−Vd or Vr + Vd, the voltage Vr is sufficiently set so that the erasing can be performed even when the absolute value becomes Vr−Vd. The driving voltage applied to the pixel during the holding period is set to Vh−Vd or Vh + Vd, so that the H orientation and the F orientation can be applied when the absolute value is Vh−Vd or Vh + Vd. The voltage Vh is appropriately set so as not to deviate from the bistable state.
[0092]
The number of scanning lines that can be scanned in one frame is given by the frame period / time width of the selection period. The shorter the selection period, the larger the number of scanning 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 after the selection signal.
[0093]
Thereby, even if the selection period is shortened, the cholesteric liquid crystal 31 can be surely set to H orientation or F orientation, and the selection period can be shortened to increase the number of scanning lines that can be scanned in one frame. it can.
[0094]
[Multilayer display element and its drive control method]
The present invention can also be applied to a cholesteric liquid crystal display device that performs multicolor display by laminating a plurality of cholesteric liquid crystal layers.
[0095]
FIG. 3 shows an example of a cholesteric liquid crystal display device in this case. In this example, the display element 10 stacks the liquid crystal cells 50B, 50G, and 50R in this order from the observation side, and forms the white light reflection layer 47 on the back surface of the liquid crystal cell 50R on the side opposite to the observation side. The white light reflecting layer 47 can also serve as the substrate 12 or the electrode 22 on the back surface side of the liquid crystal cell 50R.
[0096]
The liquid crystal cell 50B includes a cholesteric liquid crystal layer 30B obtained by adding a yellow dichroic dye 32Y to a cholesteric liquid crystal 31B having a blue wavelength range as a selective reflection wavelength range between the substrates 11 and 12 on which the electrodes 21 and 22 are formed. The liquid crystal cell 50G is formed by adding a magenta dichroic dye 32M to a cholesteric liquid crystal 31G having a green wavelength region as a selective reflection wavelength region between the substrates 11 and 12 on which the electrodes 21 and 22 are formed. The layer 30G is formed, and the liquid crystal cell 50R adds a cyan dichroic dye 32C to the cholesteric liquid crystal 31R having a selective reflection wavelength range between the substrates 11 and 12 on which the electrodes 21 and 22 are formed. The cholesteric liquid crystal layer 30R thus formed is formed.
[0097]
Here, blue, green, and red mean the three primary colors in additive color mixing, and generally indicate colored light having wavelength components of 400 to 500 nm, 500 to 600 nm, and 600 to 700 nm, respectively. Yellow, magenta, and cyan mean the three primary colors in the subtractive color mixture, and are complementary colors of blue, green, and red, respectively. Moreover, white refers to colored light having a wavelength component of 400 to 700 nm.
[0098]
The liquid crystal cells 50B, 50G, and 50R are driven and controlled by the driving circuit 70 independently of each other by the method described above.
[0099]
Therefore, in the moving image mode, the cholesteric liquid crystal layer 30B can select a colorless state when the cholesteric liquid crystal 31B is in the H orientation and a state colored yellow with the dichroic dye 32Y when the cholesteric liquid crystal 31B is in the F orientation, Similarly, the cholesteric liquid crystal layer 30G can select a colorless state and a magenta colored state, and the cholesteric liquid crystal layer 30R can select a colorless state and a cyan colored state. Multi-color display is possible by subtractive color mixture of yellow, magenta, and cyan.
[0100]
On the other hand, in the still image mode, the cholesteric liquid crystal layer 30B has a blue selective reflection state when the cholesteric liquid crystal 31B is in the P orientation and a state colored yellow with the dichroic dye 32Y when the cholesteric liquid crystal 31B is in the F orientation. Similarly, the cholesteric liquid crystal layer 30G can select 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. As a whole, the display element 10 can perform multicolor display by additive color mixture of blue, green and red.
[0101]
The stacking order of the liquid crystal cells 50B, 50G, and 50R may be basically any order, but the liquid crystal cells 50B, 50G, and 50R are observed in this order as in the example of FIG. 3 for the following reason. It is desirable to laminate in order from the side.
[0102]
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 in reducing the saturation. Saturation can be improved by lowering the reflectance in the bottom wavelength region, but in that case, it is better to lower the reflectance in the short wavelength side than in the long wavelength side of the selective reflection wavelength region. High improvement effect.
[0103]
In the display element 10 of the example of FIG. 3, the reflected light in the wavelength region shorter than the selective reflection wavelength region in the reflected light from the green reflective cholesteric liquid crystal layer 30G is applied to the blue reflective cholesteric liquid crystal layer 30B, which is an upper layer. The reflected light in the wavelength region shorter than the selective reflection wavelength region in the reflected light from the red reflective cholesteric liquid crystal layer 30R absorbed by the yellow dichroic dye 32Y contained therein is the green reflective cholesteric liquid crystal layer above this. The magenta dichroic dye 32M included in the layer 30G and the yellow dichroic dye 32Y included in the blue reflective cholesteric liquid crystal layer 30B are absorbed. Therefore, the saturation in the still image mode is improved.
[0104]
Further, 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 region shorter than the selective reflection wavelength region of the lower cholesteric liquid crystal layer. As in the case of providing a color filter on the liquid crystal layer that absorbs the wavelength region shorter than the selective reflection wavelength region, in the still image mode, the color change depending on the viewing angle is suppressed and the viewing angle is expanded. Can do.
[0105]
[Verification by experiment]
Actually, the cholesteric liquid crystal display element of the example of FIG. 1 was manufactured, and the drive control was performed by the method described above by the drive circuit, and the display characteristics were measured.
[0106]
Liquid crystal E44 (manufactured by Merck) /80.6% by weight, chiral agent S811 (manufactured by Merck) /15.5% by weight, chiral agent S1011 (manufactured by Merck) /3.9% by weight are mixed to obtain a reflection spectrum. A green reflective cholesteric liquid crystal having a peak wavelength of 530 nm was prepared. To this, 4.0% by weight of black dichroic dye for liquid crystal S344 (manufactured by Mitsui Chemicals) which absorbs a wavelength range of 400 to 700 nm almost uniformly was added.
[0107]
Next, an alignment film made of vertically aligned polyimide SE7511L (manufactured by Nissan Chemical Industries, Ltd.) was formed on the glass substrate on which the ITO electrode was formed, and a cell with a cell gap of 5 μm was prepared. To this, green reflective cholesteric liquid crystal containing 4.0% by weight of dichroic dye S344 was injected, and the injection port was sealed, and then a ceramic white light reflector was disposed on the back surface.
[0108]
Between the ITO electrodes of the display element thus fabricated, a voltage of 100 V as an erasing signal is 1 msec, a voltage of 40 V for H alignment as a selection signal or a voltage of 0 V for F alignment is 1 msec, and a voltage of 25 V as a holding signal Was applied for 28 milliseconds, and the display element was driven at a frame period of 30 milliseconds.
[0109]
FIG. 11 shows temporal changes in applied voltage and reflectance when dark display and bright display are alternately performed for each frame. Even when the selection period was as short as 1 ms, it was possible to switch between dark display and bright display at high speed. In this case, the time utilization rate (holding time / frame period) is remarkably high at 93% (= 28 msec / 30 msec). In this moving image mode, a white appearance was obtained during bright display and a black appearance during dark display.
[0110]
Next, a bright display by H orientation was performed in the moving image mode, and after the holding signal was finished, the applied voltage was set to zero and the still image mode was set, and a green appearance was obtained. Similarly, dark display by F orientation was performed in the moving image mode, and after the holding signal was finished, the applied voltage was set to zero and the still image mode was set, whereby a black appearance was obtained.
[0111]
【The invention's effect】
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 or display a bias voltage when displaying a still image. The display contents can be held with no power supply or a small power supply without continuing to apply power, and the power consumption can be greatly 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 view showing still another example of the cholesteric liquid crystal display device of the present invention.
FIG. 4 is a diagram for explaining a drive control method according to the present invention.
FIG. 5 is a diagram for explaining a drive control method according to the present invention;
FIG. 6 is a diagram showing applied voltage-reflectance characteristics according to 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 for explaining 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 for a prototype cholesteric liquid crystal display element.
FIG. 12 is a diagram showing a conventional general cholesteric liquid crystal display element.
13 is a graph showing applied voltage-reflectance characteristics of the cholesteric liquid crystal display element of FIG.
FIG. 14 shows the dynamic drive method of US Pat. No. 5,748,277.
FIG. 15 is a diagram showing temporal changes in drive signal and reflectance when a moving image is displayed by the dynamic drive method.
FIG. 16 is a diagram illustrating a PCGH mode cholesteric liquid crystal display element.
17 is a graph showing applied voltage-reflectance characteristics of the cholesteric liquid crystal display element of FIG.
FIG. 18 is a diagram showing temporal changes in drive signal and reflectance in the case of the drive method disclosed in Japanese Patent Laid-Open No. 10-206899.
[Explanation of symbols]
10. Cholesteric liquid crystal display element
11, 12 ... substrate
21, 22 ... Electrodes
30 ... Cholesteric liquid crystal layer
30B: Blue reflective cholesteric liquid crystal layer
30G ... Green reflective cholesteric liquid crystal layer
30R ... Red reflective cholesteric liquid crystal layer
31 ... Cholesteric liquid crystal
31B ... Blue reflective 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 ... Board
46. Light absorption layer
47. White light reflection layer
50B, 50G, 50R ... Liquid crystal cell
70 ... Drive circuit
71. Signal voltage generating circuit
72. Scanning voltage generation circuit
S1 ... Drive signal

Claims (9)

A cholesteric liquid crystal layer having a dichroic liquid crystal containing a cholesteric liquid crystal having a selective reflection wavelength region in a visible wavelength region and absorbing at least the selective reflection wavelength region ;
A light reflecting layer that is provided on the opposite side of the cholesteric liquid crystal layer and that reflects at least the selective reflection wavelength region ;
A cholesteric liquid crystal display device comprising: The cholesteric liquid crystal display element according to claim 1,
A cholesteric liquid crystal display device, wherein the dichroic property of the cholesteric liquid crystal layer is due to a dichroic dye added to the cholesteric liquid crystal. The cholesteric liquid crystal display element according to claim 1 or 2,
A cholesteric liquid crystal display element, wherein the light reflection layer is composed of planar aligned cholesteric liquid crystal having the same selective reflection wavelength region as the cholesteric liquid crystal.   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 are laminated. A cholesteric liquid crystal display element in which a white light reflecting layer is formed on the opposite side of the cholesteric liquid crystal layer from the observation side. The cholesteric liquid crystal display element according to claim 4,
A cholesteric liquid crystal display element in which the blue reflective cholesteric liquid crystal layer, the green reflective cholesteric liquid crystal layer, and the red reflective cholesteric liquid crystal layer are laminated in this order from the observation side. The cholesteric liquid crystal display element according to any one of claims 1 to 5, and a drive circuit for driving and controlling the cholesteric liquid crystal display element.
The driving circuit is configured to reflect the cholesteric liquid crystal display element on the light reflecting layer when the cholesteric liquid crystal is homeotropic aligned, or on the cholesteric liquid crystal layer when the cholesteric liquid crystal is focal conic aligned. A first display mode in which display is performed by selection of coloring, selective reflection of the cholesteric liquid crystal when the cholesteric liquid crystal is in a planar orientation, or the cholesteric liquid crystal layer when the cholesteric liquid crystal is in a focal conic orientation A cholesteric liquid crystal display device that controls driving so that a second display mode in which display is performed by selection of coloring can be selectively performed. The cholesteric liquid crystal display element according to any one of claims 1 to 5, and a drive circuit for driving and controlling the cholesteric liquid crystal display element.
The drive circuit includes a selection signal for changing the cholesteric liquid crystal forming the cholesteric liquid crystal layer of the cholesteric liquid crystal display element into a homeotropic alignment or a focal conic alignment, and maintaining an alignment state selected by the selection signal. A cholesteric liquid crystal display device that selectively takes a drive mode in which a drive signal comprising a hold signal is applied to the cholesteric liquid crystal layer and a sustain mode in which the voltage applied to the cholesteric liquid crystal layer is below a threshold. The cholesteric liquid crystal display device according to claim 7,
The drive circuit is a cholesteric liquid crystal display device that applies an erasure signal to the cholesteric liquid crystal layer for transitioning the cholesteric liquid crystal forming the cholesteric liquid crystal layer to homeotropic alignment prior to the selection signal in the drive mode. The cholesteric liquid crystal display device according to claim 7 or 8,
The cholesteric liquid crystal display device, wherein the driving circuit makes a time width of a holding signal in a driving mode immediately before the sustain mode longer than a time width of a holding signal in another driving mode.

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