JPH09218403A - Liquid crystal display element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a display element having a high contrast with the bright reflection type liquid crystals of one sheet of deflection plate by specifying the difference between the phase difference of a liquid crystal layer and a phase difference plate when a prescribed voltage is impressed on the liquid crystal layer to a specific ratio of wavelengths nearly over the entire area of visible light wavelengths when the difference is of a specific value with respect to the light of the specific wavelength. SOLUTION: This liquid crystal display element includes the liquid crystal layer, one sheet of the deflection plate 8, a reflection film and the phase difference plate 9. The liquid crystal layer is held by the deflection plate 8 and the reflection film and the phase difference plate 9 is arranged between the deflection plate 8 and the reflection film by approximately intersecting the delay phase axis of the phase difference plate 9 orthogonally with the orientation direction of the liquid crystal layer on a substrate 1 adjacent to the phase difference plate 9. The wavelength distribution of the refractive index of the liquid crystal layer differs from the wavelength dispersion of the refractive index of the phase difference plate 9. The difference between the phase difference of the liquid crystal layer and the phase difference of the phase difference plate 9 is 1/4 times the wavelength in nearly the entire region of the visible light wavelengths when the difference is 138nm with respect to the light of the wavelength 550nm at the time of impressing the prescribed voltage on the liquid crystal layer, by which the black display is made possible.

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 capable of bright and high-contrast display.

[0002]

2. Description of the Related Art Liquid crystal display devices are thin and light, and are therefore widely used as displays for portable information terminals. The liquid crystal is a light-receiving element that does not emit light by itself, and can be driven with a low voltage of several volts. Therefore, a reflection-type liquid crystal element, which is provided with a reflector on the back surface and is illuminated by external light to view a display, has extremely low power consumption.

However, the reflection type liquid crystal display element is usually 2
A liquid crystal cell is sandwiched between a plurality of polarizing plates, and a scattering / reflecting plate with a roughened aluminum surface is attached behind it to be used. As a liquid crystal cell, a TN liquid crystal of 90-degree twist alignment is used when the display capacity is very small, such as 7 segments, or when driven by an active element such as a TFT even if the display capacity is large, a simple matrix is used. When the display capacity is large when driven, STN liquid crystal in which a twist orientation of 180 ° to 260 ° is used. In the case of STN liquid crystal, a polymer such as polycarbonate or polyvinyl alcohol is stretched on the inside of two polarizing plates to insert a retardation plate with birefringence, thereby eliminating coloring due to the birefringence effect. In some cases, the color may be displayed, or conversely, the color due to the birefringence effect may be emphasized for color display (for example,
JP-A-6-301006).

The transmittance of the polarizing plate used in these reflective liquid crystal elements is at most about 45%, the transmittance of polarized light parallel to the absorption axis of the polarizing plate is 0%, and the transmittance of vertical polarized light is 90%. In a reflective liquid crystal panel using two polarizing plates, incident light exits through the polarizing plate four times. Therefore, the total transmittance is (0.9) ⁴ × 0.5 = 0.328, which is never higher than about 33%.

Therefore, in order to make the display brighter, there is a single polarizing plate structure in which the number of polarizing plates is changed from two to one on the front side of the liquid crystal cell and the liquid crystal cell is sandwiched between one polarizing plate and a reflecting plate. Some are disclosed (for example, Japanese Patent Laid-Open No. 7-14646).
9). In this case, since the polarizing plate passes through the polarizing plate only twice, the total transmittance is (0.9) ² × 0.5 = 0.405, and an improvement of about 8% can be expected.

Further, for example, Japanese Patent Application Laid-Open Nos. 6-308479 and 6-308480 disclose a structure in which a retardation plate is provided between a polarizing plate and a liquid crystal in a single polarizing plate structure to perform birefringent color display. ing.

Further, the inventors of the present invention have proposed Japanese Patent Application No. 7-116.
687 discloses a retardation plate having no wavelength dependence and very small dependence on incident angle and viewing angle. It is a laminated retardation plate obtained by laminating a uniaxially stretched polypropylene film and a polycarbonate film having a refractive index in the thickness direction larger than that in the slow axis direction with their slow axes orthogonal to each other.

Assuming that the retardation plate is an ordinary 1/4 wavelength plate and sandwiched by one polarizing plate and a reflection plate, the change in the retardation with respect to obliquely incident light is expressed as follows. It is assumed that the main refractive index in the slow axis direction of the ¼ wavelength plate in which the incident polarized light and the direction forming 45 ° are combined is np, and the main refractive index in the fast axis direction orthogonal thereto is ns. The phase difference of a quarter-wave plate when incident polarized light is incident at an incident angle θ in the plane including the slow axis direction and the substrate normal, is the product of the birefringence that decreases with θ and the increasing distance. Is represented by

[0009]

## EQU1 ## {n _p n _s / (n _p ² sin ² θ + n _s ² cos ² θ) ^1/2 −
ns} d / λcosθ, and approximately

[0010]

## EQU2 ## (n _p -n _s ) d cos θ / λ, which decreases in proportion to cos θ as the incident angle increases.

On the other hand, since the birefringence does not depend on the angle in the plane including the molecular short axis and the substrate normal, the phase difference is

[0012]

(3) (n _p −n _s ) d / λ cos θ, which sharply increases in inverse proportion to cos θ.

Thus, for example, when θ = 0 °, λ
Even if it is set to / 4, the phase difference of the phase plate shifts greatly only by tilting the incident angle by about 30 °, and especially in the case of a single polarizing plate, the shift of the phase difference further increases on the return path. The phase compensation by the plate becomes difficult, the contrast becomes extremely low, and the color purity decreases when displaying a birefringent color.

Nagatsuka et al. Disclose a three-dimensional retardation plate in which the refractive index in the thickness direction is adjusted in order to reduce the incidence angle dependence of such a retardation plate (eg, Y. Fujimur).
a, T.Nagatuka, H.Yoshimi and T.Shimomura: SID'91 Dige
st, 35.1 (1991)).

In-plane principal refractive indices np and ns (np
> Ns. The direction of np is called the slow axis and the direction of ns is called the slow axis), and the main refractive index nz in the thickness direction is usually equal to or slightly smaller than ns. When viewed from the front, nz is irrelevant, but when viewed obliquely, the nz component enters the birefringence amount. From the direction corresponding to (Equation 3), if nz is made larger than ns, the birefringence when viewed obliquely becomes small and the optical path becomes long, which is offset and the change in phase difference becomes small.

According to Nagatsuka's simulation

[0017]

In the case of nz = (np + ns) / 2, the dependency of the phase difference on the incident angle can be minimized.

As a typical retardation plate that realizes this,
Nitto Denko's three-dimensional refractive index control retardation film NR
There is Z. The main refractive index nz in the thickness direction is also controlled by devising the method of stretching the polycarbonate.

In Japanese Patent Application No. 7-116687, the inventors of the present invention have proposed a phase difference plate A having a small wavelength dependence of the refractive index and a phase difference having a large wavelength dependence of the refractive index and a phase difference smaller than that of the phase plate A. A laminated retardation plate in which the retardation of the laminated retardation plate monotonically increases in the visible light wavelength range by laminating the plate B with the retardation plate A with their slow axes substantially orthogonal to each other to form a laminated retardation plate, Since the refractive index nz in the thickness direction of the retardation plate B is larger than the refractive index np in the slow axis direction, the retardation plate does not have wavelength dependence and does not have incident angle dependence that satisfies (Equation 4). It has been disclosed that a phase difference plate can be realized.

[0020]

However, the conventional reflection type liquid crystal panel having a single polarizing plate structure has problems that it is difficult to display black and that the incident angle and the viewing angle are largely dependent on each other.

The present invention is intended to solve such conventional problems.

[0022]

In order to solve the above problems, a liquid crystal display device of the present invention comprises a liquid crystal layer, one polarizing plate, a reflective film and a retardation plate, and the liquid crystal layer is a polarizing plate. A retardation film is sandwiched between the polarizing plate and the reflection film, and the slow axis of the retardation plate is disposed substantially orthogonal to the alignment direction of the liquid crystal layer on the substrate adjacent to the retardation plate. The wavelength dispersion of the refractive index of the liquid crystal layer is different from the wavelength dispersion of the refractive index of the retardation plate, and the phase difference of the liquid crystal layer and the phase difference of the retardation plate when a predetermined voltage is applied to the liquid crystal layer. Difference is wavelength 550n
When the difference is 138 nm with respect to the light of m, the difference is 1/4 times the wavelength over the entire visible light wavelength, thereby enabling black display.

Preferably, the liquid crystal layer is made of homogeneously oriented nematic liquid crystal having a positive dielectric constant, the wavelength dispersion of the refractive index of the nematic liquid crystal is smaller than that of the retardation plate, and the voltage applied to the liquid crystal layer is zero. At this time, the retardation of the liquid crystal layer is 1/4 times the visible light wavelength larger than the retardation of the retardation plate, and the difference between the refractive index nz in the thickness direction of the retardation plate and the refractive index np in the slow axis direction. The product (nz−np) × d of the thickness d of the retardation plate is 69 nm for light of 550 nm.
By setting the thickness to ± 10 nm, the incident angle dependency is improved and the contrast is also improved.

Further, when a predetermined voltage is applied to the liquid crystal layer, the retardation of the liquid crystal layer is 1/4 times the wavelength of visible light, and the average of the retardation plates in the thickness direction is equal to the visible light wavelength. The refractive index nzLC is the average refractive index npLC in the orientation direction of homogeneous alignment.
And the refractive index nsLC in the direction perpendicular to the alignment direction is approximately nzLC = (npLC + nsLC) / 2, and the refractive index nz in the thickness direction of the retardation plate is the refractive index np in the slow axis direction and the fast axis direction. The incident angle dependence is also improved when nz = (np + ns) / 2 with respect to the refractive index ns of.

Originally, in the invention of Japanese Patent Application No. 7-116687, the wavelength dependence and the incident angle dependence were eliminated by the laminated retardation plate. However, in the first liquid crystal display element of the present invention, one of the laminated retardation plates is provided. The use of the liquid crystal layer as the retardation plate realizes a liquid crystal display device in which the total retardation of the retardation plate and the liquid crystal layer does not depend on the wavelength and the incident angle. When the liquid crystal layer is in homogeneous alignment and no voltage is applied, the liquid crystal layer is equivalent to a uniaxial retardation plate, and if a retardation plate is attached to the liquid crystal molecules in the orthogonal direction, the total retardation of the liquid crystal panel will be It is equal to the difference between the phase difference (Δnd) of the liquid crystal layer and the phase difference of the phase plate.

As the retardation plate, the wavelength dispersion of the refractive index is larger than the wavelength dispersion of the refractive index of the liquid crystal, and the phase difference (Δn
If the phase difference of the phase difference plate is reduced by λ / 4 with light of 550 nm from d), the total phase difference will be proportional to the wavelength. While keeping the difference of the phase difference at 550 nm constant,
If the phase difference between the two is changed, the proportional constant of the total phase difference with respect to the wavelength changes. Therefore, if the proportional constant is set to 1, a quarter wavelength plate having no wavelength dependence can be obtained. Therefore,
In the one-sheet polarizing plate configuration, the phase is rotated by ½ wavelength in a reciprocating manner, so that black display is possible.

At this time, the refractive index n in the thickness direction of the retardation plate
Assuming that the product of the difference between z and the refractive index np in the slow axis direction and the thickness d is λ / 8, the total phase difference satisfies (Equation 4).

Alternatively, when the retardation plate and the liquid crystal layer both satisfy (Equation 4), naturally the total retardation is (Equation 4).
And the incident angle dependence is reduced.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. (Embodiment 1) FIG. 1 is a sectional view of an embodiment of a liquid crystal display device of the present invention. Transparent electrodes 3 and 4 made of indium tin oxide (ITO) are formed on an upper substrate 1 and a lower substrate 2 made of glass, and a polyimide alignment film 5 is printed on the transparent electrodes 3 and 4 so that the upper and lower substrates are parallel to each other. The rubbing treatment was performed in the direction of becoming anti-parallel in Nos. 1 and 2. These substrates 1 and 2 were adhered to each other by applying a sealing resin 6 with a spherical spacer of 6.5 μm interposed therebetween.

To this cell, a tolan type positive type (dielectric anisotropy) nematic mixed liquid crystal (Δn of 0.148: for light of 550 nm) was injected, and the liquid crystal molecule 7 of FIG.
Was oriented homogeneously. A polarizing plate 8 is attached to the front surface of this cell with the polarization axis oriented in the direction of 45 degrees with respect to the normal to the paper surface, and the retardation plate 9 is attached to the polarizing plate 8 and the upper substrate 1 with the slow axis toward the normal to the paper surface. It was inserted in between, and the aluminum scattering reflector 10 was attached to the back surface.

The phase difference plate 9 is made of polycarbonate 3
FIG. 2 shows the wavelength dispersion of the retardation of the liquid crystal layer and the retardation of the retardation plate when there is no electric field in the dimensional refractive index control retardation plate. The wavelength is plotted on the horizontal axis and the phase difference is plotted on the vertical axis.
The solid line 30b indicates the phase difference of the retardation plate, and the solid line 30c indicates the total phase difference of the liquid crystal layer and the retardation plate. For light of 550 nm, the retardation of the liquid crystal layer is 962 nm and the retardation of the retardation plate is 825 nm. At this time, the total phase difference 30c substantially coincides with the broken line 31 of Δnd = λ / 4, and the quarter wave plate has no wavelength dependence. Therefore, in the non-electric field state, in FIG. 1, incident natural light absorbs one polarized light by the polarizing plate 8, becomes a circularly polarized light by being phase-shifted by ¼ wavelength by the liquid crystal layer and the retardation plate, and becomes by the reflecting plate 10. The light is reflected and becomes linearly polarized light orthogonal to the polarization axis on the return path, and is absorbed by the polarizing plate 8. This enabled black display. When a voltage is applied to the liquid crystal layer, the phase difference becomes smaller, and when the solid line 30a in FIG. 2 goes down and becomes almost equal to the phase difference of the retardation plate, the total phase difference becomes almost zero. It is slightly greenish, but almost white display is obtained. The contrast ratio of white and black was about 10: 1. Further, as the voltage is increased, the phase difference of the liquid crystal layer becomes smaller than the phase difference of the retardation plate, and color display in which the color changes between yellow purple and green can be performed.

The product of the difference between the refractive index nz in the thickness direction of the retardation plate 9 and the refractive index np in the slow axis direction and the thickness is described in Japanese Patent Application No. 7-11.
Similar to 66877, for light of 550 nm, (nz
-Np) x d = 70 nm is used as the phase plate, and at this time, the incident angle dependence is the smallest and
Since the black display does not change even with oblique incident light, the contrast is the highest. (Embodiment 2) The liquid crystal layer and the retardation plate are changed in the same configuration as in Embodiment 1 shown in FIG. The liquid crystal is a cyano-based positive nematic mixed liquid crystal, and Δn is 0.208 (however, 550 nm
(With respect to the light of 1), a material having a very large wavelength dependence of the refractive index was homogeneously aligned as in the first embodiment.
The thickness of the liquid crystal layer is 6.7 microns. The retardation plate 9 is a retardation plate having a retardation of 1066 nm with respect to light of 550 nm of polycarbonate and having a refractive index in the thickness direction adjusted so as to satisfy (Equation 4). When the voltage applied to the liquid crystal layer is increased, the retardation of the liquid crystal layer becomes approximately equal to the retardation of the retardation plate at about 2.3 V, resulting in almost white display.
When the phase difference of the liquid crystal layer was 930 nm at 2 volts, almost black display was obtained. At this time, the retardation in the thickness direction of the liquid crystal layer (nzLC−nsLC) × d is equal to the in-plane retardation (n
It is about half of pLC-nsLC) × d, which satisfies (Equation 4). Here, nzLC, nsLC, and npLC are the average refractive index of the liquid crystal layer in the thickness direction, the ordinary refractive index of the liquid crystal molecules, and the average refractive index of the liquid crystal layer in the rubbing direction, respectively. Since both the liquid crystal layer and the retardation film satisfy (Equation 4), the dependency of the incident angle and the viewing angle on the direction perpendicular to the rubbing direction is small, the black depression is good, and high contrast is obtained. Was given.

As described above, the liquid crystal display device of the present invention is
With the single-polarizer configuration, black display and sufficient contrast could be obtained.

In this embodiment, the liquid crystal layer has a homogeneous alignment. However, the present invention is not limited to this, and a twisted nematic alignment or a liquid crystal layer using a birefringence effect such as homeotropic alignment always holds. In the case of twisted nematic alignment, the slow axis of the retardation plate may be perpendicular to the alignment direction of liquid crystal molecules on the substrate closer to the retardation plate,
The relationship of retardation with the liquid crystal layer is the same as that in the above embodiment. In the case of homogeneous alignment, when voltage is applied,
It is sufficient to satisfy (Equation 4) when a voltage is applied, as in the second embodiment, with the liquid crystal molecule tilt direction and the slow axis of the retardation plate being perpendicular.

[0035]

As is apparent from the above description,
In the liquid crystal display device of the present invention, by providing a polarizing plate and a liquid crystal layer with a retardation plate for compensating for wavelength dependence and incident angle dependence, a reflective liquid crystal display having two polarizing plates with higher brightness and higher contrast than the conventional type. The device can be realized.

[Brief description of drawings]

FIG. 1 is a sectional view of a liquid crystal display device according to first and second embodiments of the present invention.

FIG. 2 is a wavelength dependence characteristic diagram of a phase difference according to the first embodiment of the present invention.

[Explanation of symbols]

 1 Upper Substrate 2 Lower Substrate 3, 4 Transparent Electrode 5 Alignment Film 6 Sealing Resin 7 Liquid Crystal Molecules 8 Polarizing Plate 9 Phase Difference Plate 10 Scattering Reflecting Plate

Claims (4)

[Claims] 1. A liquid crystal layer, a polarizing plate, a reflective film,
A retardation plate, the liquid crystal layer is sandwiched between the polarizing plate and the reflection film, the retardation plate between the polarizing plate and the reflection film, the slow axis of the retardation plate the phase difference Arranged substantially orthogonal to the alignment direction of the liquid crystal layer on the substrate adjacent to the plate,
The wavelength dispersion of the refractive index of the liquid crystal layer is different from the wavelength dispersion of the refractive index of the retardation plate, and the difference between the retardation of the liquid crystal layer and the retardation of the retardation plate when a predetermined voltage is applied to the liquid crystal layer. Is 138 nm with respect to light having a wavelength of 550 nm, the difference is 1/4 times the wavelength in almost the entire visible light wavelength. 2. The liquid crystal layer is made of homogeneously oriented nematic liquid crystal having a positive dielectric constant, the wavelength dispersion of the refractive index of the nematic liquid crystal is smaller than the wavelength dispersion of the retardation plate, and the voltage applied to the liquid crystal layer is 2. The liquid crystal display element according to claim 1, wherein the retardation of the liquid crystal layer when it is zero is larger than the retardation of the retardation plate by 1/4 times the visible light wavelength. 3. The product (nz−np) × d of the difference between the refractive index nz in the thickness direction of the retardation plate and the refractive index np in the slow axis direction and the thickness d of the retardation plate is 550 nm light. To 69 nm
The liquid crystal display device according to claim 2, wherein the liquid crystal display device has a thickness of ± 10 nm. 4. When a predetermined voltage is applied to the liquid crystal layer,
The difference between the retardation of the liquid crystal layer and the retardation of the retardation plate is 1/4 times the visible light wavelength, and the average refractive index nzLC of the liquid crystal layer in the thickness direction is the average refractive index npLC of the homogeneous alignment direction. And nzLC = (npLC + nsLC) / 2 with respect to the refractive index nsLC in the direction perpendicular to the alignment direction, and the refractive index nz in the thickness direction of the retardation plate is the same as the refractive index np in the slow axis direction For the refractive index ns in the axial direction, nz = (np + n
s) / 2, The liquid crystal display element according to claim 1.