JP3428617B2 - Reflection type liquid crystal display element, driving method thereof, and driving device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflective liquid crystal display device, a driving method thereof, and a driving device.

[0002]

2. Description of the Related Art A reflection type liquid crystal display element does not require a dedicated light source such as a backlight, consumes less power, and can be made thin and lightweight. Therefore, it is used as a display device for small information devices, portable information terminals and the like. Attention has been paid.

As a reflection type liquid crystal display element capable of monochromatic display, one based on a TN (twisted nematic) system is known. A reflective liquid crystal display device of the TN system is loaded with nematic liquid crystal having a positive dielectric anisotropy between two transparent substrates each having a transparent electrode, and the long axis of liquid crystal molecules is vertically moved by controlling an alignment film. A TN cell continuously twisted by 90 ° between the substrates is formed, polarizing plates which are orthogonal to each other are arranged outside the upper and lower substrates, and a reflector is arranged outside the lower polarizing plate.

However, since the TN type reflective liquid crystal display device controls the polarization direction of incident light in one direction by using a polarizing plate, more than half of the incident light is lost, and the reflectance at the time of bright display is shown. Is low and the contrast is low.

As a reflection type liquid crystal display element capable of multicolor display, a combination of a light control layer with a color filter is known. However, when a color filter is used, light loss occurs, so that a sufficient contrast cannot be obtained for a reflective liquid crystal display device that cannot amplify the amount of light like a transmissive liquid crystal display device that uses a backlight. There is.

Therefore, as a reflection type liquid crystal display element capable of monochromatic display or multicolor display without using a polarizing plate or a color filter, one utilizing selective reflection of cholesteric liquid crystal has been proposed.

For example, Japanese Patent Laid-Open No. 3-209425 discloses
As shown in FIG. 7, transparent electrodes 17 and 18 are formed on one surface of the transparent substrates 15 and 16, respectively.
In the meantime, as the display layer 19, a selective reflection layer in which a cholesteric liquid crystal obtained by adding a chiral agent to a nematic liquid crystal having negative dielectric anisotropy is dispersed in a polymer is formed, and the selective reflection layer 19 is driven by the drive circuit 20. It is shown that a single-color display is performed by driving by driving and switching between a selective reflection state when an electric field is applied and a transmission state when no electric field is applied.

Further, as shown in FIG.
Between the respective electrodes 17r on 6r, substrates 16r, 16g
Between the respective upper electrodes 17g and the substrates 16g, 16
between the electrodes 17b on the display layer 19b.
r, 19g, and 19b are the selective reflection layers as described above and selectively reflect red, green, and blue color lights, respectively, and the selective reflection layers 19r, 19g, and 19b are respectively formed into drive circuits. It is shown that multi-color display is performed by the principle of additive color mixing by separately driving by 20r, 20g and 20b.

[0009]

However, in the conventional reflective liquid crystal display device utilizing the selective reflection of the cholesteric liquid crystal described above, the threshold characteristic between the selective reflection state and the transmission state is steep. In addition, in the transition state, since the hue of the spiral structure of the cholesteric liquid crystal unwinds and the hue changes due to the tilt of the spiral axis, it is not possible to realize the luminance change of a specific color within one pixel. Therefore, in order to perform monochromatic gradation display or full color display, it is necessary to use an area gradation method such as a dither method.
There is a problem that the apparent resolution is reduced.

Therefore, according to the present invention, in the reflection type liquid crystal display element utilizing the selective reflection of the cholesteric liquid crystal, it is possible to realize the luminance change within one pixel without accompanying the change of the hue, whereby the area scale is reduced. This is to reduce the apparent reduction in resolution due to the use of the adjustment method.

[0011]

According to the present invention, a pair of substrates each having an electrode, at least one of which is transparent, are provided.
Two cholesteric liquid crystal layers having different threshold electric field intensities that selectively reflect circularly polarized light having the same wavelength in visible light but different rotational directions and different reflectances are laminated.

In this case, the two cholesteric liquid crystal layers can each contain a polymer.

[0013]

[Function] The cholesteric liquid crystal in which liquid crystal molecules have a spiral structure divides the light incident parallel to the spiral axis into right-handed light and left-handed light, and reflects a circularly polarized light component that matches the twist direction of the helix.
The remaining circularly polarized light component is transmitted.

Therefore, in the above-mentioned conventional reflective liquid crystal display device utilizing selective reflection of cholesteric liquid crystal, the transmittance as shown in FIG. 9 (a) is 0 to several percent depending on the applied voltage. A two-step luminance change is obtained between the state and the selective reflection state in which the reflectance is 50% at the maximum as shown in FIG. 7B, and gradation expression is performed using an area gradation method such as a dither method. In this case, an image is displayed by regarding as many drive pixels as the desired number of gradations as unit display pixels. For example, when displaying 16 gradations, 16 driving pixels are used as unit display pixels.

On the other hand, in the reflective liquid crystal display element of the present invention, as shown in FIG. 6, in the unit display element, two selections made of cholesteric liquid crystals having the same spiral pitch and different twist directions are used. Reflective layer 5,
5'is laminated, and the two selective reflection layers 5 and 5'are laminated.
Since the threshold electric field strength that causes the change in the reflectance of the two selective reflection layers 5 and 5'is set to a different value, the voltage applied between the electrodes 3 and 4 by the drive circuit 8 changes the reflectance of the two selective reflection layers 5 and 5 '. By changing between three appropriate voltage values selected in relation to the electric field strength, the selective reflection layers 5 and 5 ′ can be changed without taking a transition state that causes a change in hue. 2, the two selective reflection layers 5 and 5'are both in the transmissive state, and the unit display element has a transmissive state in which the reflectance is 0% to several percent. As shown in FIG. 5 is a selective reflection state, the other selective reflection layer 5 '
Is a transmissive state, and the reflectance is up to 50% as a unit display element.
And the two selective reflection layers 5 and 5 ′ are both in the selective reflection state, and the unit display element has a maximum reflectance of 100%.
It is possible to obtain a stepwise brightness change.

Therefore, when the gradation expression is performed by using the area gradation method such as the dither method, it is sufficient to display the image by regarding the driving pixels of half the desired gradation number as the unit display pixels. , The apparent resolution is 2 compared with the conventional reflective liquid crystal display device that uses selective reflection of cholesteric liquid crystal.
Can be doubled. For example, when displaying 16 gradations, eight drive pixels may be used as unit display pixels.

Furthermore, since both the right-handed light and the left-handed light can be selectively reflected by the two selective reflection layers 5 and 5 ',
The maximum reflectance can be doubled as compared with the conventional one, and a bright display can be obtained.

[0018]

DETAILED DESCRIPTION OF THE INVENTION

[First Embodiment for Monochromatic Display] FIG. 1 shows an embodiment of the reflective liquid crystal display element of the present invention, which is a case where monochromatic display is possible.

In this embodiment, on one surface of the substrates 1 and 2,
Cholesteric liquid crystals 7 and 7'having electrodes 3 and 4, respectively, between which electrodes 3 and 4 selectively reflect circularly polarized light of the same wavelength with different turning directions, and which have different threshold electric field strengths that cause a change in reflectance. The two selective reflection layers 5 consisting of
Stack 5 '. Spacers 11 and 11 'are inserted in the selective reflection layers 5 and 5', respectively, and a separation substrate 12 is interposed between the selective reflection layers 5 and 5 '.

The substrates 1 and 2 are made of a material having an insulating property and a light transmitting property such as glass or a polymer film,
The electrodes 3 and 4 are formed of a material having conductivity and light transmission such as ITO.

The spacers 11 and 11 'are made of glass or plastic, and the thickness of the selective reflection layers 5 and 5'is controlled to about several .mu.m to 10 .mu.m. It is desirable that the separation substrate 12 is formed of a polymer film having insulating properties and light transmission properties such as polyethylene, polystyrene, and polyethylene terephthalate, and has a film thickness of several μm.

On the back surface of the substrate 2 on the side opposite to the incident side of external light, a light absorbing layer 9 for absorbing a part or all of visible light transmitted through the selective reflection layers 5 and 5'is formed. Electrodes 3,4
Is connected to the drive circuit 8.

The cholesteric liquid crystals 7 and 7'are obtained by adding chiral agents having different rotational directions to nematic liquid crystals, and their respective compositions are adjusted so as to selectively reflect circularly polarized light having the same central wavelength.

As a method of making the threshold electric field intensities that cause the reflectance changes of the two cholesteric liquid crystals 7 and 7 different values, nematics having different dielectric anisotropy or elastic modulus are used for the respective cholesteric liquid crystals 7 and 7 '. A method using liquid crystal or the like can be considered.

When a nematic liquid crystal having a positive dielectric anisotropy is used, the selective reflection layers 5 and 5 ′ are changed from the planar state of the selective reflection state to the focal conic state of the transmission state as the applied voltage increases. Since the structure changes, the characteristics as shown in FIG. 2 are obtained as the change of the reflectance of each selective reflection layer 5, 5 ′ with respect to the voltage applied between the electrodes 3, 4.

Therefore, when the voltage applied between the electrodes 3 and 4 is set between 0 and V1, both the selective reflection layers 5 and 5'become in the selective reflection state as shown in FIG. 6 (c). As a display element, the reflectance is ideally 100%, and when the voltage applied between the electrodes 3 and 4 is set between V1 and V2, as shown in FIG. 6B, the selective reflection layer is formed. 5 is in the selective reflection state, and the selective reflection layer 5'is in the transmission state, and the reflectance is ideally 50% as a display element,
When the voltage applied between the electrodes 3 and 4 is V2 or higher, both the selective reflection layers 5 and 5'become in a transmissive state as shown in FIG. 6A, and the reflectance is ideal as a display element. By changing the voltage applied between the electrodes 3 and 4 to three different values, the reflectance is ideally three levels of 100%, 50% and 0% for the selective reflection color. A brightness change is obtained.

On the other hand, when a nematic liquid crystal having a negative dielectric anisotropy is used, the selective reflection layers 5 and 5 ′ are selectively reflected from the focal conic in the transmissive state as the applied voltage increases. Since the structure changes to the planar state, the characteristics shown in FIG. 3 are obtained as the change in the reflectance of each selective reflection layer 5, 5 ′ with respect to the voltage applied between the electrodes 3 and 4.

Therefore, when the voltage applied between the electrodes 3 and 4 is set between 0 and V3, both the selective reflection layers 5 and 5'are in the transmissive state as shown in FIG. 6 (a). ,
As a display element, the reflectance is ideally 0%, and when the voltage applied between the electrodes 3 and 4 is set between V3 and V4, as shown in FIG. Since the selective reflection state and the selective reflection layer 5 ′ are in the transmission state, the reflectance is ideally 50% as a display element.
When the voltage applied between them is V4 or more,
As shown in (c), both the selective reflection layers 5 and 5'become in a selective reflection state, the reflectance is ideally 100% as a display element, and the voltage applied between the electrodes 3 and 4 is 3%. By changing the values as described above, it is possible to ideally obtain three-level luminance changes of the reflectance of 0%, 50%, and 100% for the selective reflection color.

(Example) As a right-handed cholesteric liquid crystal, a nematic liquid crystal ZLI- having a positive dielectric anisotropy.
69 wt% of 1132 (manufactured by Merck) and 31 wt% of dextrorotatory chiral agent CB15 (manufactured by Merck) were mixed, and a spacer with an adhesive having a diameter of 3 μm was mixed. As a left-handed cholesteric liquid crystal, a nematic liquid crystal ZLI-2116-100 (manufactured by Merck) having positive dielectric anisotropy 70 wt%
And levorotatory chiral agent S811 (Merck) 30 wt%
And a spacer with an adhesive having a diameter of 3 μm was mixed.

A first transparent glass substrate 7059 (manufactured by Corning Incorporated), which is a substrate 2 on which the electrode 4 and the light absorption layer 9 are formed, and which has an ITO transparent electrode vapor-deposited thereon and which is coated with black.
The above-mentioned right-handed cholesteric liquid crystal is dripped on the top surface of the separation substrate 12 while applying heat and pressure.
A thick PET film (manufactured by Toray) was laminated.

Next, the above-mentioned counterclockwise cholesteric liquid crystal was dropped on this PET film, and while applying heat and pressure, a second ITO electrode was vapor-deposited to form the substrate 1 on which the electrode 3 was formed. A transparent glass substrate was laminated to form a cell.

[Second Embodiment for Single Color Display] FIG. 4
Shows another embodiment of the reflection type liquid crystal display element of the present invention, which is a case where monochromatic display is possible as in the embodiment of FIG.

In this embodiment, the two selective reflection layers 5,
5'is a polymer-dispersed cholesteric liquid crystal (PDCLC: Polymer Dispersed) in which cholesteric liquid crystals 7 and 7'are dispersed in polymers 6 and 6 ', respectively.
Cholesteric Liquid Crystal
l) It has a structure. In this case, the spacers 11 and 11 'and the separation substrate 12 in the case of FIG. 1 are unnecessary.

The selective reflection layers 5 and 5'in this case are formed by an emulsion method and PIPS (Polymerization).
Induced Phase Separation)
Method, TIPS (Thermally Induced)
Phase Separation method, SIPS (S
olvent Induced Phase Sepa
(Ration) method and the like, and a method of phase-separating the polymer and the liquid crystal can be used.

As a method of making the threshold electric field strengths causing the reflectance changes of the two selective reflection layers 5, 5'different, the cholesteric liquid crystals 7, 7'of each layer have different dielectric anisotropies or elastic moduli. A method using a nematic liquid crystal, a method in which liquid crystal droplets dispersed in the polymers 6 and 6 ′ have different sizes in each layer, a method in which a polymer skeleton having a different anchoring effect at the interface is used in each layer, and the like can be considered.

Also in this embodiment, as in the embodiment of FIG. 1, the reflectance is ideally 0% and 50 for each selective reflection color.
%, 100% three-level luminance change can be obtained.

(Example) As a right-handed cholesteric liquid crystal, a nematic liquid crystal ZLI- having a negative dielectric anisotropy.
56 wt% of 2806 (manufactured by Merck) and 44 wt% of dextrorotatory chiral agent CB15 (manufactured by Merck) were mixed. 30 wt% of the obtained uniform solution was mixed with 70 wt% of a 10 wt% concentration PVA aqueous solution, and the mixture was mixed with a mixer at 6000 rpm to 4
It was emulsified by stirring for 5 seconds. The obtained emulsion is degassed, then diluted with water about 3 times to form the light absorption layer 9, the electrode 4 and the substrate 2, and a black-coated PET film with an ITO transparent electrode (High Beam: manufactured by Toray) A selective coating layer 5'having a polymer-dispersed cholesteric liquid crystal structure was formed thereon by applying a film having a thickness of 100 μm using a doctor blade and drying it.

Next, as a left-handed cholesteric liquid crystal, a nematic liquid crystal ZLI-2 having a negative dielectric anisotropy.
806 (Merck) 69 wt% and levorotatory chiral agent S
31 wt% of -811 (manufactured by Merck) was mixed. 30 wt% of the obtained homogeneous solution was mixed with 70 wt% of a 10 wt% concentration PVA aqueous solution, and the mixture was mixed with a mixer at 1500 rpm to 4
It was emulsified by stirring for 5 seconds. The resulting emulsion is degassed and then diluted with water about 3 times to form the electrode 3 and the substrate 1. IT
On a PET film with an O transparent electrode, a selective reflection layer 5 having a polymer-dispersed cholesteric liquid crystal structure was formed by coating with a doctor blade to a thickness of 100 μm and drying.

The two selective reflection layers 5 ', 5 thus obtained were pressure-bonded while applying heat to form a cell.

[Embodiment for Multicolor Display] FIG. 5 shows still another embodiment of the reflective liquid crystal display element of the present invention, in which multicolor display is possible.

In this embodiment, the two selective reflection layers 5 and 5'shown in FIG. 1 or FIG. 4 are replaced with the selective reflection layers 5r, 5'r, 5g, 5'g, 5b and 5'b. And three unit display elements 10r, 10g, 10 for selectively reflecting red, green, and blue color light, respectively.
Stack b.

However, the substrate 2r is a unit display element 10r,
10 g are common, and the substrate 2 g is a unit display element 10 g, 1
0b, the light absorption layer 9 is formed on the back surface of the substrate 2b on the side opposite to the incident side of external light, and the electrodes 3r, 4r,
3g, 4g, 3b, 4b are respectively connected to drive circuits 8r, 8
g, 8b.

In the reflective liquid crystal display element of this embodiment,
For each color of red, green and blue,
It is possible to realize a change in luminance within one pixel without causing a change in hue, and it is possible to perform multicolor display based on the principle of additive color mixing.

[0044]

As described above, according to the present invention, in the reflective liquid crystal display element utilizing the selective reflection of the cholesteric liquid crystal, it is possible to realize the luminance change in one pixel without the hue change. As a result, it is possible to reduce the apparent reduction in resolution due to the use of the area gradation method, and it is possible to obtain a brighter display.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of a reflective liquid crystal display element of the present invention.

FIG. 2 is a diagram showing a switching operation when a nematic liquid crystal having a positive dielectric anisotropy and a chiral agent are used for the cholesteric liquid crystal in the display device of FIG.

FIG. 3 is a diagram showing a switching operation when a nematic liquid crystal having a negative dielectric anisotropy and a chiral agent are used for the cholesteric liquid crystal in the display device of FIG.

FIG. 4 is a diagram showing another embodiment of the reflective liquid crystal display element of the present invention.

FIG. 5 is a diagram showing still another embodiment of the reflective liquid crystal display element of the present invention.

FIG. 6 is a diagram showing a phase change of two selective reflection layers of the reflection type liquid crystal display element of the present invention.

FIG. 7 is a diagram showing an example of a conventional reflective liquid crystal display element.

FIG. 8 is a diagram showing another example of a conventional reflective liquid crystal display element.

FIG. 9 is a diagram showing a phase change of a selective reflection layer of a conventional reflection type liquid crystal display element.

[Explanation of symbols]

1, 2, 2r, 2g, 2b substrates 3,3r, 3g, 3b electrodes 4,4r, 4g, 4b electrodes 5,5r, 5g, 5b selective reflection layer 5 ', 5'r, 5'g, 5'b selective reflection layer 6,6 'polymer 7,7 'cholesteric liquid crystal 8,8r, 8g, 8b drive circuit 9 Light absorption layer 10r, 10g, 10b Unit display element 11,11 'spacer 12 separation boards

Claims (5)

(57) [Claims] 1. A threshold for selectively reflecting circularly polarized light of different wavelengths having the same wavelength in visible light and having a change in reflectance between a pair of substrates each having an electrode, at least one of which is transparent. A reflection type liquid crystal display device, characterized in that two cholesteric liquid crystal layers having different electric field strengths are laminated. 2. The reflective liquid crystal display element according to claim 1, wherein each of the two cholesteric liquid crystal layers contains a polymer. 3. A display element according to claim 1 or 2, wherein a plurality of display elements selectively reflecting light of different wavelengths in visible light are provided, and a substrate is shared between two adjacent display elements. Alternatively, a reflective liquid crystal display element characterized by being laminated separately. 4. The method for driving a reflective liquid crystal display device according to claim 1, 2 or 3, wherein a unit display device in which the two cholesteric liquid crystal layers are laminated is provided with the threshold electric field of the two cholesteric liquid crystal layers. A display element driving method characterized by driving with a driving voltage that changes between three voltage values selected in relation to strength. 5. A device for driving a reflective liquid crystal display element according to claim 1, 2 or 3, wherein a unit display element in which the two cholesteric liquid crystal layers are laminated is provided with the threshold electric field of the two cholesteric liquid crystal layers. A display element drive device characterized by being driven by a drive voltage that changes between three voltage values selected in relation to strength.