JP2002202526A - Liquid crystal display element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a good black and white display of a liquid crystal display element of a two-layer type laminated with two liquid crystal panels having chiral nematic liquid crystals prepared by adding an optically active material to nematic liquid crystals. SOLUTION: The liquid crystal display element 1 of the two-layer type laminated with two liquid crystal panels 10 and 20 having the chiral nematic liquid crystals prepared by adding the optically active material to the nematic liquid crystals is specified to conditions under which the central wavelength of the selective reflection wavelength in a planar state is 560 to 630 nm on an upper liquid crystal panel 10 side, is 440 and 480 nm on a lower liquid crystal panel 20 side and the refractive index anisotropy Δn of the nematic liquid crystals is 0.18 to 0.30.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-layer liquid crystal display device in which two liquid crystal panels each having a chiral nematic liquid crystal are stacked, and more particularly, to a technique for obtaining a good black and white display. Things.

[0002]

2. Description of the Related Art A liquid crystal display element requires external light because it functions not as a light emitting element but as an optical shutter. There is a transmissive STN liquid crystal display that requires much of the external light from a backlight, but power consumption is increased by turning on the backlight, which is not preferable for a portable information terminal that requires a battery as a power source. For this reason, a reflective STN liquid crystal display that can obtain a high contrast at a relatively low price is often used for a portable information terminal represented by a mobile phone.

[0003] However, even in a reflective STN liquid crystal display, at least one polarizing plate is required, so that the reflectance with respect to external light is limited. In some cases, a display quality with a high brightness cannot be obtained. In addition, there is also a problem that the power consumption is increased due to an increase in the drive voltage and the contrast is reduced due to an increase in the number of drive lines as the definition of display is increased.

Therefore, a liquid crystal display that does not use a polarizing plate for limiting the brightness of the liquid crystal display and does not degrade the display quality even when the definition is increased has been attracting attention.
There is a liquid crystal display using a memory type chiral nematic liquid crystal.

[0005] Chiral nematic liquid crystals are produced by mixing a nematic liquid crystal and an optically active substance. A chiral nematic liquid crystal is sandwiched between a pair of parallel transparent electrode substrates,
When the director of the liquid crystal is arranged so that the center axis (helical axis) of the twist in the twisted structure that rotates at regular intervals is perpendicular to the transparent electrode substrate on average, circularly polarized light corresponding to the direction of the twist Is reflected.

The center wavelength of the reflected light is determined by the distance on the helical axis (helical pitch) during one rotation of the director of the liquid crystal parallel to the substrate surface due to the twist, and the two-dimensional direction parallel to the substrate surface of the nematic liquid crystal. It is the product of the average refractive index at the surface.

Thus, a chiral nematic liquid crystal is
The phenomenon of reflecting circularly polarized light of a specific wavelength by the helical pitch and the refractive index of the liquid crystal is called selective reflection. The liquid crystal arrangement showing this selective reflection is called a planar arrangement, and the average direction of the helical axes of the divided liquid crystal domains is almost perpendicular to the substrate surface.

[0008] The chiral nematic liquid crystal has an arrangement (focal conic) in which the helical axes of a plurality of liquid crystal domains are arranged in a random direction or a non-perpendicular direction with respect to the substrate surface, as another liquid crystal arrangement. You can also. In focal conic, many liquid crystals exhibit a weak scattering state as a whole, and do not reflect light of a specific wavelength unlike selective reflection.

[0009] These two states (planar and focal conic) are stable even when no electric field is applied, and the selective reflection of the planar is bright because no polarizing plate is used, and the planar has a wide viewing angle. A liquid crystal display element using chiral nematic liquid crystal and utilizing its selective reflection functions as a memory type by maintaining its liquid crystal orientation even when no electric field is applied, so that a liquid crystal display element with low power consumption can be obtained. it can.

Further, by adjusting the applied voltage, a state in which optical characteristics intermediate between the planar arrangement and the focal conic arrangement can be exhibited after the voltage is applied, and the liquid crystal molecules are aligned in the direction of the electric field during the state change. It can also be homeotropic.

According to a liquid crystal display device using a chiral nematic liquid crystal, three liquid crystal panels having selective reflections of R, G, and B colors are produced by changing the pitch of the liquid crystal, and these are superimposed on each other. Full color display can be performed. In the superposition, usually, when viewed from the display observation surface side, the layers are sequentially superimposed in ascending order of the selective reflection wavelength such as blue, green, and red when viewed from the display observation surface side (for example, see Japanese Patent Application Laid-Open No. 11-6).
No. 4895).

Although not a full-color display, a multi-color display can be performed by a combination of two-layer liquid crystal panels. In this case, a monochrome display can be obtained depending on the selective reflection color to be combined.

[0013]

However, since the color purity of a black-and-white display varies depending on the panel specifications and the liquid crystal material, there are some problems such as turbid display colors and reduced color purity. there were.

[0014]

According to the present invention, a good black-and-white display can be obtained in a two-layer liquid crystal display device in which two liquid crystal panels each having a chiral nematic liquid crystal are laminated.

That is, according to the present invention, a chiral nematic liquid crystal, which is obtained by mixing an optically active substance with a nematic liquid crystal and develops a stable planar state and a focal conic state after an applied voltage according to an applied voltage, is formed by a pair of transparent electrode substrates. A two-layer type liquid crystal panel including a first liquid crystal panel and a second liquid crystal panel sandwiched therebetween, wherein the first liquid crystal panel is disposed on a display observation surface side, and the second liquid crystal panel is disposed on the back surface side. In the liquid crystal display element, the selective reflection wavelength in the planar state is longer on the first liquid crystal panel side than on the second liquid crystal panel side, and the difference between the center wavelengths of the selective reflection wavelengths of the panels is 50 nm or more. It is characterized by being.

The present invention includes several preferred embodiments described below. That is, the center wavelength of the selective reflection wavelength in the planar state is 56% on the first liquid crystal panel side.
0 to 630 nm, and 440 to 48 on the second liquid crystal panel side.
It is preferable that the refractive index anisotropy Δn of the nematic liquid crystal is 0.18 to 0.30 in addition to 0 nm.

The clearing point Tc of the nematic liquid crystal is preferably 70 ° C. or higher (more preferably 90 ° C.).
℃ or more). The dielectric anisotropy Δε of the nematic liquid crystal is 1
It is preferably 0 or more (more preferably 13 or more). Also,
It is preferable that the first liquid crystal panel and the second liquid crystal panel are joined by transparent bonding means having a refractive index of 1.3 to 1.6. Incidentally, the transparent adhesive means may be a double-sided adhesive tape.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an embodiment of a liquid crystal display device according to the present invention will be described with reference to a side view of FIG. 1 and a plan view of FIG.

The liquid crystal display element 1 comprises a laminate of an upper (first) liquid crystal panel 10 on the display observation surface side and a lower (second) liquid crystal panel 20 disposed on the back surface side. Each of the liquid crystal panels 10 and 20 includes a pair of transparent electrode substrates 1.
1, 12; 21 and 22, each transparent electrode substrate is, for example, polycarbonate having a thickness of 0.1 to 1.1 mm,
It may be a plastic substrate made of polyarylate, polyether sulfone, or the like, or a glass substrate.
Note that a black light absorbing layer is provided on the back surface side of the lower liquid crystal panel 20.

In each of the cells of the upper liquid crystal panel 10 and the lower liquid crystal panel 20, a chiral nematic liquid crystal is put. In the present invention, the selective reflection wavelength of the chiral nematic liquid crystal in the planar state is set to the lower liquid crystal panel 20 side. The upper liquid crystal panel 10 has a longer wavelength than the upper liquid crystal panel 10 and the difference between the center wavelengths of the selective reflection wavelengths of the panels 10 and 20 is 50 nm or more.
The thickness is set to 0 nm or more, and more preferably, the colors presented are in a relation close to complementary colors in order to obtain good white display. Specifically, the upper liquid crystal panel 10 side has 5
The wavelength is set to 60 to 630 nm (yellow) and to 440 to 480 nm (blue), which is shorter than the lower liquid crystal panel 20.

That is, if the upper liquid crystal panel side is set to a short wavelength (440 to 480 nm) by replacing this panel arrangement, the upper liquid crystal panel exhibits a blue or bluish green color.
It is not preferable because the display becomes dark as a whole. Further, in the specification using a liquid crystal having a high refractive index anisotropy value Δn, scattering in the focal conic state of the upper liquid crystal panel having a shorter wavelength is increased, and the black color provided on the back side of the lower liquid crystal panel is increased. If the distance from the light absorption layer is large, the black is backscattered by the focal conic and the black level may deteriorate, which is not preferable.

A chiral nematic liquid crystal can be obtained by adding a predetermined optically active substance to a nematic liquid crystal. Nematic liquid crystals as a host include, for example, biphenyl, tolan, pyrimidine, cyclohexane, difluorostyrene, and the like. A single benzene or a mixture thereof is used.

In order to brighten the display and increase the contrast, the nematic liquid crystal has a refractive index anisotropy Δn of 0.1.
The range of 18 to 0.30, particularly the range of 0.21 to 0.26 is preferred. That is, the refractive index anisotropy Δn is 0.18
If it is less than 1, the reflectance in the planar state is reduced, and the half width of the selective reflection is also reduced, so that the brightness is reduced.
Further, when the refractive index anisotropy Δn exceeds 0.30, the scattering in the focal conic state becomes strong, and the black display state becomes whitish due to the influence of the back scattering, and the contrast with respect to the reflection state of the planar decreases, which is preferable. Absent.

In the present invention, the clearing point Tc of the nematic liquid crystal is preferably 70 ° C. or higher, particularly preferably 90 ° C. or higher. That is, the clearing point Tc of the composition to which the optically active substance is added is:
Generally, it often falls below the clearing point Tc of the nematic liquid crystal. Therefore, in order to increase the clearing point Tc of the composition, it is desirable that the clearing point Tc of the nematic liquid crystal be as high as possible, provided that crystallization or increase in viscosity in a low temperature range is within an allowable range.

The dielectric anisotropy Δε of the nematic liquid crystal is 10
Above, particularly preferably 13 or more. If the dielectric anisotropy Δε is less than 10, the driving voltage may increase, and T
General-purpose inexpensive drive I used for driving N, STN
It is not preferable because C becomes unusable and power consumption increases.

However, if the dielectric anisotropy Δε is too large, the polarity of the nematic liquid crystal becomes too strong, which may cause a problem in reliability and the like.
Considering C, etc., the dielectric anisotropy Δ of the nematic liquid crystal
It is preferable to set ε in the range of 10 to 25.

The amount of the optically active substance added to the nematic liquid crystal is determined by the constant HTP (Helical) of the optically active substance.
The liquid crystal having a blue helical pitch after addition varies from 0.27 to 0.30, depending on the Twisting Power.
μm, yellow liquid crystal is 0.34-0.38 μm
Use the one within the range. The cell gap at this time is preferably 2 to 20 μm, and particularly preferably 3 to 6 μm.

In the present invention, the bonding gap between the upper liquid crystal panel 10 and the lower liquid crystal panel 20 is preferably 100 μm or less, and the bonding means may be a transparent adhesive such as an epoxy-based, acrylic-based, or silicon-based adhesive or a double-sided adhesive. Adhesive tape is used.

Preferably, it is a thermosetting or light-curing epoxy transparent adhesive having a refractive index of 1.3 to 1.6, and includes a spherical spacer having a particle size of 2 to 100 μm (particularly, 20 to 50 μm). Use a good one. In the case of a double-sided adhesive tape, the thickness is 20 to 100 μm (particularly, 40 to 60 μm).
m) The transparent double-sided pressure-sensitive adhesive tape is preferred.

In any case, if the bonding gap exceeds 100 μm, blurring of display and color mixing may occur due to a problem of parallax. Therefore, the bonding gap is preferably narrow.
The spherical spacer included in the transparent adhesive may be made of resin, glass, or the like, but is preferably a resin-made bead type, and further has a refractive index equivalent to that of the transparent adhesive. Are preferred.

As shown in FIG. 2, in this embodiment, the upper liquid crystal panel 10 has terminal portions 111 and 121 connected to the transparent electrode substrates 11 and 12 on two adjacent sides, respectively. And terminal portions 211 and 221 connected to the transparent electrode substrates 21 and 22 on two adjacent sides of the lower liquid crystal panel 20, respectively.
When stacking, the terminals 111 and 121 of the upper liquid crystal panel 10 and the lower liquid crystal panel 20
The liquid crystal panels 10 and 20 are stacked while being relatively rotated by 180 ° so that the terminal portions 211 and 221 do not overlap in a plan view.

According to this, as shown in FIG. 3, after connecting the TAB substrate 31 to each of the terminal portions 111, 121; 211, 221 via the ACF (anisotropic conductive film), A control board 32 is arranged so as to surround each of the liquid crystal panels 10 and 20, and each TAB board 31 and each control board 32 can be connected.

Example 1 A 90 × 70 mm substrate was used as a transparent electrode substrate using a substrate formed by depositing ITO by sputtering on a 0.2 mm thick polycarbonate plastic substrate having a barrier coat layer made of SiO _{x on} both surfaces. Two cells having the same size were produced. The center wavelength of the selective reflection wavelength in the planar state is 570 in one of the cells.
nm (yellow) chiral nematic liquid crystal is encapsulated to form an upper liquid crystal panel, and the other cell is encapsulated with a chiral nematic liquid crystal having a selective reflection wavelength of 450 nm (blue) in the planar state and a lower liquid crystal panel. did. In addition, the chiral nematic liquid crystal was adjusted as follows. A nematic liquid crystal having a clearing point Tc of 89 ° C., a refractive index anisotropy Δn of 0.234, and a dielectric anisotropy Δε of 16.6 is used. Mixed 85.3 parts of the above nematic liquid crystal and 14.7 parts of the optically active substance. The helical pitch was 0.36 μm. As for the liquid crystal exhibiting the blue selection wavelength for the lower liquid crystal panel, the nematic liquid crystal is used
1.4 parts and 18.6 parts of the optically active substance were mixed. The helical pitch was 0.28 μm. Next, black ink is applied to the back surface of the R plate (the transparent electrode substrate 21 side in FIG. 1) of the lower liquid crystal panel by a screen printing method, and then applied to the F plate (the transparent electrode substrate 22 side in FIG. 1) of the lower liquid crystal panel. Two-component epoxy adhesive (ME-106; 10 manufactured by Bellnox)
g of a curing agent KM1885 mixed at a ratio of 4 g), and a transparent adhesive obtained by adding 0.5 wt% of a resin-based spherical spacer having an average particle diameter of 30 μm to the main agent, and applying the same by screen printing. As shown in FIG. 2, the upper liquid crystal panel was bonded to the lower liquid crystal panel while being rotated by 180 °. The refractive index of the transparent adhesive is 1.
52, the refractive index of the spherical spacer was 1.5. As for the bonding method, it is necessary to pre-install ITO in each liquid crystal panel.
An alignment mark for superposition is formed in advance, an image is taken with a CCD camera, and the alignment mark is positioned such that the alignment mark overlaps. Then, a pressure of 0.2 kg / cm ² is uniformly applied to the liquid crystal panel within the panel surface. The mixture was heated at 90 ° C. for 10 minutes while curing to cure the adhesive.
Then, as shown in FIG. 3, a control board was disposed around each liquid crystal panel, and the control board was connected to a terminal portion of each liquid crystal panel via a TAB board. When this two-layer type liquid crystal display element was turned on, good black and white display was obtained.

Example 2 Example 1 was repeated except that a glass substrate having a thickness of 0.3 mm and ITO was formed by sputtering as a transparent electrode substrate for the upper liquid crystal panel and the lower liquid crystal panel. In the same manner as in the above, a two-layer liquid crystal display element was produced. According to the second embodiment, as in the first embodiment, good black-and-white display was obtained.

[0035]

As described above, according to the present invention,
2 laminated two liquid crystal panels having a chiral nematic liquid crystal obtained by adding an optically active substance to a nematic liquid crystal
In the layer type liquid crystal display element, the selective reflection wavelength in the planar state was set to be longer at the upper liquid crystal panel side than at the lower liquid crystal panel side, and the difference between the center wavelengths of the selective reflection wavelengths of the respective panels was set to 50 nm or more. Thereby, a good black-and-white display can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic side view of a liquid crystal display device according to an embodiment of the present invention.

FIG. 2 is a schematic plan view of the liquid crystal display element.

FIG. 3 is a schematic plan view showing a state where a control substrate is connected to the liquid crystal display element via a TAB substrate.

[Explanation of symbols]

 Reference Signs List 1 liquid crystal display element 10 upper liquid crystal panel 11, 12 transparent electrode substrate 111, 121 terminal unit 20 lower liquid crystal panel 21, 22 transparent electrode substrate 211, 221 terminal unit 31 TAB substrate 32 control substrate

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Satoshi Niiyama Asahi Glass Co., Ltd. 1150 Hazawa-cho, Kanagawa-ku, Yokohama-shi, Kanagawa Prefecture (72) Inventor Hikaru Nakagawa 1-2-1, Kamisakabe, Amagasaki-shi, Hyogo Optrex Stock (72) Inventor Masahide Shigemura 1-2-1 Kamisakabe, Amagasaki City, Hyogo Prefecture Opterex Amagasaki Plant F-term (reference) 2H089 HA21 QA16 RA11 SA03 SA09 SA16 5C094 BA03 BA49 CA19 DA03 DA11 JA08 JA11

Claims (5)

[Claims] 1. A chiral nematic liquid crystal comprising a nematic liquid crystal mixed with an optically active substance and exhibiting a stable planar state and a focal conic state after an applied voltage is sandwiched between a pair of transparent electrode substrates. A first liquid crystal panel and a second liquid crystal panel, wherein the first liquid crystal panel is disposed on the display observation surface side, and the second liquid crystal panel is disposed on the back surface side. The selective reflection wavelength in the planar state is the second reflection wavelength.
The first liquid crystal panel has a longer wavelength than the liquid crystal panel, and the difference between the center wavelengths of the selective reflection wavelengths of each panel is 50 n.
m or more. 2. The central wavelength of the selective reflection wavelength is 560 to 630 nm on the first liquid crystal panel side and 440 to 480 nm on the second liquid crystal panel side, and the refractive index anisotropy Δn of the nematic liquid crystal is The liquid crystal display element according to claim 1, wherein the ratio is 0.18 to 0.30. 3. The nematic liquid crystal has a clearing point Tc of 70.
The liquid crystal display device according to claim 1, wherein the temperature is not lower than ℃. 4. The dielectric anisotropy Δε of the nematic liquid crystal.
4. The liquid crystal display device according to claim 1, wherein the number is 10 or more. 5. The liquid crystal according to claim 1, wherein said first liquid crystal panel and said second liquid crystal panel are joined by transparent bonding means having a refractive index of 1.3 to 1.6. Display element.