JP3394654B2 - Liquid crystal display device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device having bistability. [0002] A bistable liquid crystal display device using a cholesteric liquid crystal is disclosed in Japanese Patent Publication No. 1-51818. This is because a cholesteric liquid crystal having a natural pitch of about twice the liquid crystal layer thickness is sandwiched between liquid crystal layers, and liquid crystal molecules are twisted by about 360 ° in the thickness direction (hereinafter sometimes referred to as a twisted state). A liquid crystal cell capable of switching between two stable alignment states by applying an electric field between a non-state (also referred to as a uniform state) and a pair of polarizing plates arranged so as to sandwich the liquid crystal cell. Is typically composed of In such an arrangement, when the transmission axis of the polarizing plate is perpendicular to the liquid crystal layer, the birefringent color of the liquid crystal layer is observed when the alignment direction of the liquid crystal in the uniform state and the transmission axis of the polarizing plate are shifted by about 45 degrees. Will be. When the optical anisotropy of the liquid crystal is Δn and the thickness of the liquid crystal layer is d, by setting Δnd to about 270 nm, the birefringent color can be made almost white. At this time, birefringence occurs even in the twisted state, but the birefringence at that time is relatively small, so that a color close to black can be obtained.
As described above, in the conventional bistable liquid crystal device using the cholesteric liquid crystal, almost black-and-white display can be obtained, but there is actually light leakage in the black state due to birefringence in the twisted state,
The contrast was not enough. This is a serious problem particularly when color display is performed by providing a color filter, and sufficient color purity has not been obtained. SUMMARY OF THE INVENTION The present invention provides a high-contrast liquid crystal display device which has solved the above-mentioned problems encountered in a conventional bistable liquid crystal device using a cholesteric liquid crystal. The purpose is to: According to the present invention, the alignment process is performed such that the directions of the alignment process are substantially parallel, and the inclination of the liquid crystal at the interface is substantially parallel between the upper and lower substrates. A cholesteric liquid crystal having a natural pitch of about twice the thickness of the liquid crystal layer is sandwiched between a pair of substrates, and a liquid crystal molecule has a twist angle of about 360 ° in the thickness direction and a non-twisted state. A liquid crystal cell that can be switched between two stable alignment states by applying an electric field, and a liquid crystal display element that includes at least a pair of polarizing plates disposed above and below the liquid crystal cell. The technical problem was solved by providing a liquid crystal display element which satisfies the following conditions when the direction of the twist toward the surface is positive. [Number 2] in _{35 ° ≦ φ p ≦ 65 °} -65 ° ≦ φ a ≦ -35 ° 95 ° ≦ φ p -φ a ≦ 110 ° 0.18μm ≦ Δnd ≦ 0.37μm ( Equation, phi _p: Angle formed by the axis of the lower polarizing plate with the direction of the alignment treatment of the lower substrate φ _a : Angle formed by the axis of the upper polarizing plate with the direction of the alignment treatment of the upper substrate) Δn: Refractive index anisotropy d: Liquid crystal layer Here, the direction of the orientation treatment is substantially parallel, which is indicated by R _L in FIG.
And R _U are substantially parallel, and the angle formed by R _L and R _U is 30 ° or less. Further, that the inclination of the liquid crystal is substantially parallel between the upper and lower substrates means that the direction of the alignment treatment is inclined with respect to the substrate, and that the inclination is the same in the upper and lower substrates. The above-mentioned approximately 360 ° refers to a range of 330 ° to 390 °, preferably 340 ° to 380 °. Hereinafter, the structure of the liquid crystal display device of the present invention will be described with reference to the drawings. FIG. 1 shows a configuration of a bistable liquid crystal device using a cholesteric liquid crystal of the present invention. A liquid crystal layer 30 is sandwiched between the lower substrate 11 and the upper substrate 12. 21 and 22 are transparent electrodes for applying a voltage to the liquid crystal layer, 31 and 32 are alignment films for aligning the liquid crystal, and 41 and 42 are polarizing plates. FIG. 2 schematically shows the alignment state of the liquid crystal in the liquid crystal cell. U
Represents a state without twist, P represents a state twisted by 180 °, and T represents a state twisted by 360 °. For the liquid crystal layer, a liquid crystal composition exhibiting a cholesteric liquid crystal phase as a whole, in which a cholesteric liquid crystal is added to a nematic liquid crystal, is preferably used. The liquid crystal is aligned in a direction slightly inclined from the substrate surface by the alignment film. In the configuration shown in this drawing, the tilt of the liquid crystal on the upper and lower substrates is reversed. The natural pitch P of the liquid crystal is set to be approximately twice the thickness d of the liquid crystal layer, and the liquid crystal layer is configured so that the twisted state of approximately 180 ° is stable when the alignment is not regulated by the alignment film.
Specifically, the term “approximately twice” specifically means that d / P is 0.3 to 0.5, where d is the thickness of the liquid crystal layer and P is the natural pitch of the liquid crystal.
A range of 7 is preferred. The approximately 180 ° is, for example, 15
It refers to the range of 0 ° to 210 °. However, if the tilt of the liquid crystal on the upper and lower substrates is reversed as shown in this figure, the 180 ° twist is accompanied by the splay deformation of the liquid crystal, so that the elastic energy becomes higher and becomes unstable. The state without the rotation and the state where T is twisted by 360 ° are stable. Both orientation states can be switched by the waveform of the applied voltage. FIG. 3 shows a transmission spectrum of a conventional device. In the figure, TS represents the spectrum in the 360 ° twisted state, and US represents the transmission spectrum in the untwisted uniform state. In the US, although almost wavelength-dependent, an almost white spectrum is obtained, and in the TS state, a spectrum close to black is obtained, but the transmittance is high, and light leakage is particularly large in a short wavelength region. This is due to birefringence in the TS state. The present invention reduces the birefringence due to the orientation of the liquid crystal by devising the arrangement of the polarizing plate,
An object of the present invention is to provide a liquid crystal display device having higher contrast. An optical arrangement of the liquid crystal display element of the first configuration example capable of exhibiting such an effect will be described with reference to FIG. In the figure, R _L and R _U are the lower substrate side, respectively.
It indicates the orientation direction of the liquid crystal on the upper substrate side, and P _L and P _U indicate the transmission axes of the lower and upper polarizing plates, respectively. The liquid crystal molecules have a uniform orientation without twist between R _L and R _U or a twist orientation of 360 ° twisted by ωLc in the drawing. φ
_p is _a angle, phi the axis of the lower polarizing plate makes with the direction of the alignment treatment of the lower substrate represents the angle which the axis of the upper polarizing plate makes with the direction of the alignment treatment of the upper substrate. In the present invention, the direction in which the liquid crystal twists from the lower substrate toward the upper substrate is defined as positive. Although the axis of the polarizing plate is represented by the transmission axis, the same effect can be obtained by rotating the upper and lower polarizing plates by 90 ° and replacing the transmission axis with the absorption axis. φ _p needs to be in the range of 35 ° to 65 °, and φ _a needs to be in the range of −65 ° to −35 °. At the same time, the angle ψ (φ _p −φ _a ) formed by the transmission axes (absorption axes) of the upper and lower polarizing plates is set to 95 °.
As described above, it is necessary that the angle be 110 ° or less. FIG. 5 shows that Δnd = 0.275 μm and φ _p
9 shows the dependence of the transmission spectrum of the T state on φ _a when = 45 °. In this case, it phi _a lowest transmittance is obtained at -55 ° vicinity. Similarly, various φ _p
As a result, it was found that the vicinity of φ _p −φ _a = 100 ° gives the darkest state. φ _p −φ _a is 95 °
When the angle is less than the above, light leakage on the short wavelength side becomes remarkable as shown at φ _a = 45 ° in the figure, and when it exceeds 110 °, the light leaks at the longer wavelength side as shown at φ _a = 70 ° in the figure. Light leakage is remarkable. From the above, ψ (φ _p −φ _a ) is 95
It is necessary that it is not less than 110 ° and not more than 110 °. FIG.
Is a plot of the Y value with respect to φ _p when φ _p −φ _a is fixed at 100 °. In the twist orientation (T), a good black state was obtained, but φ _p was 35 °.
If the angle is out of the range of about 65 °, the brightness of the uniform orientation (U) decreases. Similarly, for φ _a −
A preferred range is from 65 ° to -35 °. More preferred conditions are that φ _p is in the range of 40 ° to 60 °, φ _a is −60 ° to
The range is -40 °. More preferably φ _p is 45 °
Range of ~55 °, φ _a is in the range of -55 ° ~-45 °. With such a configuration, the birefringence in the twisted state can be greatly reduced. Make a good black and white display,
In order to obtain high contrast, it is necessary to set Δnd of the liquid crystal layer in the range of 0.18 μm to 0.37 μm, and more preferably in the range of 0.2 μm to 0.35 μm.
If Δnd is less than this range, the brightness of the uniform orientation will decrease. In addition, when it is more than this range, coloring occurs in the uniform orientation. The liquid crystal used in the present invention is:
A liquid crystal composition obtained by adding a cholesteric liquid crystal to a nematic liquid crystal having a positive dielectric anisotropy is preferably used. Further, as the nematic liquid crystal, a nematic liquid crystal for dual-frequency driving in which the positive / negative of the dielectric anisotropy changes depending on the frequency can be used. As described above, the natural pitch of the liquid crystal is preferably approximately の of the thickness of the liquid crystal layer. Specifically, when the thickness of the liquid crystal layer is d and the natural pitch of the liquid crystal is P, d
/ P is preferably in the range of 0.3 to 0.7. The alignment of the liquid crystal at the interface of the alignment film is such that the liquid crystal molecules are 0.5 ° to the substrate surface.
A tilt orientation tilted in a range of 30 ° is preferable.
For such an orientation, a conventionally known method such as a method of rubbing a polymer film of polyimide, polyamide, polyvinyl alcohol, or the like, or a method of obliquely depositing a metal oxide can be used. In the above description, the element is described as a transmissive type, but when used as a transmissive type, a backlight unit can be provided on the back surface. Also,
A reflection plate may be provided instead of the backlight unit, and the element may be used as a reflection type. Color display can be performed by using a color filter in combination. Example 1 Polyimide (AL3046 made by Japan Synthetic Rubber) was applied to a glass substrate having a transparent electrode, and a rubbing treatment was performed.
Another substrate subjected to the same treatment and the previous substrate were overlapped via a silica bead spacer so that the alignment treatment surfaces faced each other, and liquid crystal was injected into a gap between the substrates. As a liquid crystal,
Nematic liquid crystal ZL13412-000 made by Merck
(Δn = 0.79), a liquid crystal whose pitch was adjusted to 6.8 μm by adding 1.3% of a chiral nematic liquid crystal S811 manufactured by Merck which induces clockwise twist was used. The thickness of the liquid crystal layer was adjusted to 3.4 μm according to the particle size of the spacer.
Further, the rubbing directions in the upper and lower directions were configured to be antiparallel. Further, the liquid crystal display device according to the present invention was constituted by sandwiching the neutral gray polarizing plate between the pair. The transmission axis of the polarizing plate was adjusted so that φ _p = 50 ° and φ _a = -50 °. When the transmission spectrum of this element was measured for the T state and the U state, it was as shown in FIG. 7, and light leakage in the T state was greatly improved as compared with FIG. Examples 2 to 16 and Comparative Examples 1 to 9 In the same manner as in Example 1, liquid crystal display elements shown in Tables 1 and 2 below were produced. In the device shown in the example, better contrast was obtained as compared with the comparative example. Also, let φ _p be 40
° to 60 ° range, phi _a by in the range of -60 ° ~-40 °, better contrast is obtained. Further, by setting Δnd in the range of 0.2 to 0.35 μm, higher contrast was obtained. [Table 1] [Table 2] [Effects] 1. The liquid crystal display element of the present invention is a bistable liquid crystal display element having two stable alignment states, that is, a state in which liquid crystal molecules are twisted approximately 360 ° in a thickness direction and a state in which the liquid crystal molecules are not twisted. By optimizing, light leakage in a black state is significantly improved, and therefore, a very high contrast can be obtained. Further, since the device has high contrast, excellent color reproducibility can be obtained when a color filter array is provided. Further, the liquid crystal display device of the present invention has high time-division driving characteristics and fast response, so that it is particularly suitably used for display applications requiring large-capacity, high-contrast display. 2. The liquid crystal display element of the present invention in which the range of Δnd of the liquid crystal layer is specified can perform bright white display in addition to clear black display.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a configuration of a bistable liquid crystal display device using a cholesteric liquid crystal of the present invention. FIG. 2 is a diagram schematically showing an alignment state of a liquid crystal layer of the liquid crystal display device of FIG. FIG. 3 is a diagram showing a transmission spectrum of a conventional liquid crystal display device. FIG. 4 is a diagram illustrating an optical arrangement of the liquid crystal display device of the present invention. FIG. 5 is a diagram showing the φ _a dependence of the transmission spectrum in the T state (360 ° twisted state) when Δnd = 0.275 μm and φ _p = 45 °. FIG. 6 shows the Y value when φ _p −φ _a is fixed to 100 °.
It is the figure plotted with respect to _p . FIG. 7 is a diagram showing transmission spectra measured for a T state and a U state of the liquid crystal display element of Example 1. DESCRIPTION OF SYMBOLS 11 Lower substrate 12 Upper substrate 21 Transparent electrode 22 Transparent electrode 30 Liquid crystal layer 31 Alignment film 32 Alignment film 41 Polarizer 42 Polarizer U Untwisted state P 180 ° twisted state T 360 ° twisted state R spectrum TS 360 ° twist of the liquid crystal liquid crystal without uniform state of the transmission axis US torsion of the transmission axis P _U upper polarizer orientation process P _L lower polarizing plate in the alignment process R _U on the substrate side of the _L lower substrate side upper polarizer angle of the transmission axis of the light plate ψ φ _p -φ _a ωLc 360 from the angle phi _a upper alignment direction of the liquid crystal of the transmission axis of the lower polarizing plate from the spectrum phi _p lower alignment direction of the liquid crystal of the state ° Twisted twist orientation

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-235920 (JP, A) JP-A-55-142318 (JP, A) JP-A-1-269921 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G02F 1/13-1/141

Claims (1)

(1) A pair of alignment treatments in which the directions of the alignment treatment are substantially parallel and the inclination of the liquid crystal layer at the interface is substantially parallel between the upper and lower substrates. A cholesteric liquid crystal having a natural pitch of about twice the thickness of the liquid crystal layer is sandwiched between the substrates, and two stable alignment states, that is, a state in which liquid crystal molecules are twisted by about 360 ° in the thickness direction and a state in which the liquid crystal molecules are not twisted, are applied to an electric field. In a liquid crystal display element including at least a liquid crystal cell that can be switched by applying a voltage and a pair of polarizing plates disposed above and below the liquid crystal cell, the direction of the twist from the lower substrate toward the upper substrate is positive. A liquid crystal display device satisfying the following conditions when defined. 35 ° ≦ φ _p ≦ 65 ° −65 ° ≦ φ _a ≦ −35 ° 95 ° ≦ φ _p −φ _a ≦ 110 ° 0.18 μm ≦ Δnd ≦ 0.37 μm (in the above formula, φ _p : Angle formed by the axis of the lower polarizing plate with the direction of the alignment treatment of the lower substrate φ _a : Angle formed by the axis of the upper polarizing plate with the direction of the alignment treatment of the upper substrate) Δn: Refractive index anisotropy d: Liquid crystal layer Thick