JP2970541B2 - Reflective LCD panel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to relates to a reflection type liquid crystal display panel of simple matrix driving of the low cost that can highly bright contrast display.

[0002]

2. Description of the Related Art Since liquid crystal display elements are thin and light,
It is widely used as a display for portable information terminals. The above liquid crystal is a light-receiving element that does not emit light by itself,
A reflection type liquid crystal element which can be driven at a low voltage of several volts and has a reflection plate on the back surface and illuminates with external light to view a display consumes extremely low power.

However, a reflection type liquid crystal display element is usually used by sandwiching a liquid crystal cell between two polarizing plates and attaching a scattering reflector having a roughened aluminum surface behind the liquid crystal cell. The above-mentioned liquid crystal cell has a very small display capacity such as a 7-segment display, or has a large display capacity even if the display capacity is large.
9 when driven by an active element such as FT
A TN liquid crystal having a twist of 0 degrees is used, and a super-twisted nematic (STN) liquid crystal having a twist alignment of 180 degrees to 260 degrees is used when the display capacity is large by simple matrix driving.

In the case of the above STN liquid crystal, a birefringent effect is obtained by inserting a retardation plate having a birefringence property by stretching a polymer such as polycarbonate or polyvinyl alcohol inside the two polarizing plates. This makes it possible to display in black and white without coloring.

The transmittance of a polarizing plate used in these reflective liquid crystal devices is at most about 45%, the transmittance of polarized light parallel to the absorption axis of the polarizing plate is almost 0%, and the transmittance of perpendicular polarized light is about 0%. Is 90%. In a reflective liquid crystal panel using two polarizing plates, incident light exits through the polarizing plate four times. For this reason, the total transmittance is (0.9) ⁴ × 0.5 = 0.328, which is not higher than about 33%.

Therefore, in order to increase the brightness of the display, a single polarizing plate structure is adopted in which the number of polarizing plates is changed from two to only 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 proposals have been made (for example, see JP-A-7-14646).
No. 9, JP-A-7-84252). in this case,
Since the incident light 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.

[0007]

[0008]

[0009]

The conventional reflection type liquid crystal panel having a single polarizing plate has problems that black display is difficult and that the incident angle and the viewing angle are largely dependent.

[0010]

[0011]

In order to solve the above-mentioned problems, a reflection type liquid crystal display panel according to the present invention is designed so that the reflection type liquid crystal display panel has a size of 180 degrees to 26 degrees.
An ultra-twisted nematic liquid crystal layer twisted by 0 degrees is sandwiched between one polarizing plate and a reflective film, and at least one retardation plate is disposed between the polarizing plate and the liquid crystal layer; Of the retardation plates closest to the polarizing plate, the coefficient Z = (nz-np) / () when the refractive index NP in the slow axis direction, the refractive index ns in the fast axis direction, and the refractive index nz in the thickness direction. When ns-np) is 0.3 or more and 0.7 or less, black display becomes dark and the viewing angle is widened. Preferably, the value obtained by dividing Δn for light at 450 nm by Δn for light at 590 nm is Δn
As the wavelength dispersion value, the difference between the Δn wavelength dispersion value of the liquid crystal layer and the Δn wavelength dispersion value of the retardation plate is smaller than 0.08, more preferably, the orientation direction of the liquid crystal molecules of the liquid crystal layer closest to the polarizing plate and the liquid crystal. When the angle formed by the slow axis of the phase difference plate closest to the molecule is 75 degrees or more and 105 degrees or less, high-contrast, bright, and large-capacity display can be performed.

[0012]

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a reflection type liquid crystal display panel according to an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a sectional view of a reflective liquid crystal display panel according to an embodiment of the present invention. In FIG. 1, a forward scattering film 11 provided by a method for manufacturing a substrate for a liquid crystal panel of the present invention described later is formed on an upper substrate 1 made of glass, and furthermore, indium tin oxide (ITO) is formed on the forward scattering film 11. A transparent electrode film 4 is formed. Further, a mirror reflection film 3 made of aluminum is provided on the lower substrate 2, and the mirror reflection film 3 is patterned in a stripe shape and also functions as an electrode, and is orthogonal to the transparent electrode 4 in a stripe shape. Pixels are formed. Further, a polyimide alignment film 5 is printed on the mirror reflection film 3 and the transparent electrode film 4 and rubbed so as to be twisted by 250 degrees.

[0015] These substrates (although omitted in the figure)
A nematic liquid crystal 7 having a chiral pitch adjusted by adding a chiral agent is injected, with a sealing resin 6 applied and bonded around a spherical spacer having a predetermined particle size. Then, the retardation plates 8 and 9 and the polarizing plate 10 were attached to the outside of the upper substrate. The linearly polarized light passing through the polarizing plate 10 is changed to elliptically polarized light by the two phase difference plates, scattered by the forward scattering film, modulated by the liquid crystal layer 7, specularly reflected by the mirror reflecting film 3, and returned on the return path. Through the same path, the polarizing plate 10 serves as an analyzer to perform bright and dark display. Although the forward scattering film 11 has a function of transmitting and scattering incident light, the phase of incident polarized light is maintained because a material having almost no birefringence is used.

In the configuration of the STN liquid crystal, the display quality greatly changes depending on the optical properties and arrangement of the liquid crystal and the phase difference plate. First, these were examined by optical simulation. In the optical simulation, the optical properties and the arrangement angle of the polarizing plate, the phase difference plate, and the STN liquid crystal are parameters. As optical properties, retardation of liquid crystal and retardation plate = Δnd (λ) is important. In a normal STN liquid crystal having two polarizing plates, Δnd of liquid crystal is 0 at a retardation value of 550 nm, which has the highest visibility. The contrast was highest at around 0.84 microns, which was not significantly changed even when the liquid crystal material and the retardation plate were different.
In the case of a sheet polarizer, it was found that the difference greatly depends on the wavelength dependence of Δn of the liquid crystal and the phase difference plate.

For three types of liquid crystals A, B, and C having different wavelength dependences of Δn as liquid crystal materials, the relationship between the liquid crystal and Δnd of the retardation plate, the plane arrangement angle of the polarizing plate and the retardation plate, and the display quality. First, the case of front incidence and front emission was simulated first. FIG. 2 shows each angle when the liquid crystal panel is viewed from above. The liquid crystal is temporarily fixed in a 250 ° twist right-twisted orientation, the rubbing direction 20 on the upper substrate side and the rubbing direction 21 on the lower substrate side, and the liquid crystal molecules are moved from the rubbing direction 20 on the upper substrate side to the direction 21 on the lower substrate side. It is twisted like the arrow 22 so that it may become reverse.
The angle formed by the orientation direction of the liquid crystal molecules on the upper substrate side, the slow axis of the retardation plate 8 near the liquid crystal layer, the slow axis of the retardation plate 9 near the polarizing plate, and the absorption axis of the polarizing plate 10, respectively. φ1, φ2, φ3, the retardation of the liquid crystal layer is Δnd-LC, the retardation of the phase difference plate 12 is Δnd-f1, the retardation of the phase difference plate 13 is Δnd-f2, and these parameters are set to appropriate initial values. By changing the value sequentially from the values, a configuration was obtained in which the display was closest to black at the off-voltage and white was displayed at the on-voltage.

The wavelength dispersion of Δn is obtained by dividing a value obtained by dividing Δn at 450 nm by Δn at 590 nm based on the wavelength used for measurement.
It was used as an n-wavelength dispersion index. Since the refractive index is roughly expressed by Cauchy's dispersion equation, if it is defined by two wavelengths, it can be defined for almost all wavelengths. The wavelength dispersion values of the liquid crystals A, B, and C = Δn (450 nm) / Δn (590 nm) are each 1.
20, 1.117, 1.06, and 1.092 for the polycarbonate used as the retardation plate.

In each of the liquid crystals A, B, and C, Δnd-LC is set to 0.1.
From 3 micron to 1.1 micron, the parameter was increased in increments of 0.1 micron, and the other parameters were sequentially changed for each Δnd-LC, and the result of optimization was made so that the emitted light was almost black. , Liquid crystals A, B, and C are shown in the following three tables.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

From the left of the table, Δnd-LC, the retardation Re-f1 of the phase difference plate 8 near the liquid crystal layer, the retardation Re-f2 of the phase difference plate 9, and the above-mentioned set angle φ1,
φ2, φ3, Y value of black state (Y black), and at the right end,
Y value of white state when multiplex drive (Y white)
Is shown. When a voltage is applied to the liquid crystal, the liquid crystal molecules rise, Δnd-LC decreases, and the luminance increases.
When the N liquid crystal is multiplex-driven, the ratio of the ON voltage to the OFF voltage (operation margin) is limited by the drive duty. For example, at the 1/240 duty, which is the condition of the so-called VGA double scan STN,
The operation margin is 1.067. At this time, Δnd-LC at the on-voltage decreases to about 60% of Δnd-LC at the off-voltage. The Y white at the right end indicates the luminance at the ON voltage, or the luminance at that time when it is brightest between ON and OFF. However, here, for simplicity, the transmittance of the polarizing plate is assumed to be 50%, and an ideal polarizing plate is assumed, and interface reflection other than the mirror surface is omitted. Therefore, if the modulation in the liquid crystal panel is ideally performed, the maximum value of the Y value of the emitted light is 50 and the minimum value is 0.

From the results of liquid crystal A (Table 1), Δn
When d-LC is 0.3 nm to 0.5 nm, Y black is 2.0
It sinks below, and in liquid crystal B of (Table 2) 0.4 to 0.96
In the case of the liquid crystal C of micron (Table 3), the Y value was 2 or less at 0.4 to 0.8 micron. That is, when the wavelength dependence of Δn is large as in liquid crystal A, Δnd-LC at which black becomes sufficiently dark decreases, and in liquid crystal B having wavelength dispersion closest to the wavelength dispersion of the retardation plate, Δnd-LC has the largest value. Sometimes black appears. On the other hand, looking at the Y white column, Δn
When d-LC is 0.5 or less, Y white becomes less than 30 and becomes dark as it becomes meaningless to use a single polarizing plate, which is not suitable for high duty multiplex driving. Therefore, in the liquid crystal A, there is no condition in which black sinks and high contrast appears, and white becomes sufficiently bright.
It is not suitable for TN.

In the liquid crystals B and C in (Table 2) and (Table 3),
Even when Δnd-LC is 0.6 or more, black may sufficiently sink, so that bright white can be obtained. In the liquid crystal B in Table 2, Δnd-LC is 0.6 or more, Y white is 30 or more, and
When it is larger, 40 or more Y white is obtained, the white display at the time of voltage application is close to white, and the saturation of black is also almost achromatic, so that black and white display with high brightness, high contrast and low saturation is obtained. Obtained.

In the liquid crystal C shown in Table 3, the color of white display when voltage was applied tended to be slightly yellow, but Δnd-LC
Is 0.6 to 0.8, the Y value of white is 37, and the contrast is as high as 20 or more.

A similar simulation was performed by setting the Δn wavelength dispersion to an intermediate value between the liquid crystals A and B.
When the chromatic dispersion index is larger than 1.17, multiplex driving at a high duty is difficult. The retardation plate is made of polycarbonate and polysulfone (wavelength dispersion value = 1.
When the material is changed to a material having a large wavelength dispersion as in 165), the Δn wavelength dispersion of the liquid crystal suitable for multiplex driving becomes large, and the Δnd-LC of the liquid crystal A becomes 0.8 to 1.1.
Although black sunk at a micron, multiplex driving became possible. However, in liquid crystal C, black sunk when Δnd-LC was less than 0.6 μm, and it was difficult to drive at 1/240 duty. The Δn wavelength dispersion value is 1.08 instead of the liquid crystal C.
When the calculation was performed on the assumption that the liquid crystal of the present invention was used, multiplex driving was possible. From the above, it was found that when the difference between the wavelength dispersion value of the liquid crystal and the wavelength dispersion value of the retardation plate was 0.08 or less, multiplex driving with high duty was possible.
Therefore, polycarbonate which is a retardation plate is 1.092%.
Considering that the liquid crystal layer has a birefringence Δn for light of 450 nm, the birefringence Δn for light of 590 nm.
It is considered desirable to use a liquid crystal material whose value divided by n is in the range of 1.06 to 1.16.

A similar simulation was performed when there was only one retardation plate, but regardless of the type of liquid crystal and retardation plate,
When the Y value of black was 4 or more, the contrast was less than 10 or less. Therefore, when a uniaxially stretched retardation plate is used, it is desirable to use two or more retardation plates. However, for applications with low contrast, there is a configuration that can be used with a retardation plate.

Further, as a common feature of the structures shown in Tables 1 to 3, the arrangement angle of the retardation plate closer to the liquid crystal molecules is set to the orientation direction of the liquid crystal molecules on the upper substrate closer to the polarizing plate. Horns
It was found that when the angle was from 70 degrees to 110 degrees, black sinking was good and a display with high contrast was obtained.

Although the above examination results are considered only for the light incident on the front and the light exiting from the front, actually, a 1-axis retardation plate of a normal uniaxially stretched polycarbonate (Z coefficient described later is 1.0) is used.
When a trial production of the sheet polarizer STN was performed, the contrast was only about 3 and was very low as compared with the result of the simulation. This is the case of a reflective liquid crystal panel.
In a normal lighting environment, light enters from various angles, is scattered by the scattering film, and the total sum is perceived by the eyes. Therefore, if the incident angle dependence of black display is large, no contrast is expected. Was done.

Therefore, in order to reduce the incident angle dependence, the Z coefficient indicating the relationship between the Δn of the retardation plate and the refractive index in the thickness direction and the incident angle dependence of black luminance were calculated by simulation. Z
The coefficient is defined as a coefficient Z = (n where n is a refractive index in the slow axis direction, ns is a refractive index in the fast axis direction, and nz is a refractive index nz in the thickness direction.
z-np) / (ns-np).

As a basic configuration, the Z coefficients of two retardation plates having a configuration in which Δnd-LC in Table 2 is 0.96 μm are first changed in 0.5 steps from 0.5 to 1.5. Sometimes,
FIG. 5 shows the incidence / emission angle dependence of the reflectance in the black state. However,
In accordance with a method generally performed to improve the threshold characteristic, the setting of Δnd of the liquid crystal layer at no voltage is set to 1.05 μm, and is set slightly larger than Δnd-LC at which a black state is obtained. It was set to be the darkest when applied. FIG. 3 is a characteristic diagram showing a change in reflectance with respect to a voltage of the liquid crystal panel having this configuration. Black drops at a voltage of 2.16 volts. Further, since the incident light is specularly reflected by the mirror surface, the incident angle is equal to the output angle. As shown in FIG. 4, the tilt angle defines the polar angle in the direction 30 in which the molecules in the central part of the liquid crystal layer rise, which is highly angle-dependent, as the input / output angle. The polygonal lines 40 to 43 in FIG. 5 are input / output angle dependence characteristics of the reflectivity. The Z coefficients of the retarders 8 and 9 are (0.5, 0.5), (1.0, 1.0), (1.5, 0.5), The reflectivity in the case of (1.5, 1.5) is shown by taking the incident angle on the horizontal axis and the reflectivity vertically. As can be seen from the figure, the Z coefficient is indicated by a chain line 42 (1.5, 0.
5) and the solid line 40 (0.5, 0.5) have a small angle dependence and a low black level, and the chain line 42 (1.5, 0.5) has the smallest angle dependence. From these results, it is better that the Z coefficient of the retardation plate near the liquid crystal layer is smaller, and that the difference due to the Z coefficient of the retardation plate closer to the polarizing plate is small, but the larger the Z coefficient, the better. Do you get it. On the other hand, FIG. 6 is a characteristic diagram in which the difference due to the angle-dependent Z coefficient of the luminance in the white state when the on-voltage is applied is similarly simulated.
(0.5,0.5), (1.0,1.0), (1.5,0.5), (1.5,1.5)
The reflectance. Also in this case, the angle dependence 50 and 52 where the Z coefficient of the phase difference plate 8 is small are small and bright.

When the contrast in these four cases where the Z coefficient was changed was examined on an actual panel in a room with normal ceiling illumination, when the Z coefficient was (1.0, 1.0), the contrast was 3.5.
However, when the Z coefficient was (1.5, 0.5), (0.5, 0.5), the contrast was as high as 9.5 and 11, and the same result as the above-described simulation was obtained.

The tendency of the angle dependence depending on the Z coefficient of black and white was the same in other configurations of (Table 1) to (Table 3). The Z coefficient of the retardation plate closer to the polarizing plate was 0.3 to 0.7, and the effect of improving the contrast was remarkable. When the Z coefficient was 0.8 or more, the contrast was as low as less than 5. This was the same even when the number of the phase difference plates was one.

The larger the Z coefficient of the phase difference plate on the side closer to the liquid crystal layer , the better, but worse when the Z coefficient is 2.0 or more. Also, the Z of the retardation plate on the side closer to the liquid crystal layer
Since the difference due to the coefficient is smaller than that of the other retardation plate, it is cheaper to use a Z coefficient of 1.0 when emphasizing cost, so that a Z coefficient of 1.0 to 2.0 is preferable from the viewpoint of cost performance. desirable.

In this embodiment, the STN liquid crystal layer is oriented with a twist of 250 degrees. However, if the orientation is STN, the same result can be obtained even if the twist angle is in the range of 180 to 260 degrees. Can be

Next, the forward scattering film 11 formed by using the method for manufacturing a substrate for a liquid crystal panel of the present invention will be described below.

A solution in which a polyacrylic resin is dissolved in cyclohexanone and a solution in which a polystyrene resin is dissolved in cyclohexanone are mixed and stirred to form a mixed solution, which is applied to glass with a thickness of 5 microns when dried by a blade method. did. The weight percentage of the two resins is 1: 1 and
When the solid content concentration is changed, when the total concentration of the two resin solid contents is 30% or more, the two resins are separated during the stirring, resulting in a large lump of several hundred microns or more. When the mixed solution is applied at a solid concentration of 20% and left at 25 ° C. to evaporate the cyclohexanone, the two resins are separated and one resin becomes fine particles of about 2 μm. Is observed under a microscope to be uniformly dispersed in
Looking through it, cloudiness due to forward scattering was confirmed. It has not been confirmed which resin is in the form of fine particles. After firing at 180 ° C. to increase the degree of polymerization,
The substrate was heated to 180 ° C. by sputtering to form an ITO film on this film, but the degree of forward scattering was hardly changed. When a liquid crystal layer is interposed between the reflective film and the reflective film as shown in FIG. 1, brightness and viewing angle equal to or higher than those of a scattering reflector obtained by vapor-depositing aluminum on a conventional film can be obtained, and backscattering is very small. Therefore, a contrast of about 10 could be secured.

After applying the mixed solution, the substrate is placed on a hot plate at 80 ° C. to evaporate cyclohexanone.
The particle size of the fine particles has become extremely small, and is about 0.3 micron when observed with an electron microscope.
Backscattering became stronger. For this reason, when combined with a reflective film, the brightness was reduced and the contrast was reduced. Also, increasing the solids content of the resin increases the particle size,
Both forward scatter and back scatter weakened. As a result of controlling the evaporation temperature of the solvent and changing the particle diameter, the average diameter of the fine particles is 0.5
When the size was from 3 microns to 3 microns, a forward scattering film having sufficient forward scattering and weak back scattering was obtained. Also, the refractive index of the polyacryl resin is about 1.5, the refractive index of the polystyrene resin is 1.6, and the difference in refractive index is 0.1. Scattered optics simulations show that the difference is too large because the backscattering is enhanced if the difference is too large. A resin other than this embodiment may be used as long as it is a transparent resin having a small birefringence of 0.3 or less and is hardly compatible with each other.

On the other hand, as a method of dispersing fine particles in a resin as in the conventional example, 0.2 micron of cubic magnesium oxide fine particles were dispersed in an acrylic resin. When about 30%, irregularities were generated on the surface, causing a problem in the alignment of the liquid crystal, and the degree of forward scattering was smaller than that of the scattering film of the present embodiment. When the ratio of the fine particles was further increased, a uniform coating film could not be formed.

As described above, according to the method for manufacturing a substrate for a liquid crystal panel of the present invention, a forward scattering film having good scattering performance can be uniformly and easily formed. In the present embodiment, the scattering film is formed on glass. However, it can be formed on a color filter or provided outside glass.

Also, as shown in FIG. 1, a liquid crystal panel substrate produced by the method of manufacturing a liquid crystal panel substrate of the present invention is a reflection type liquid crystal panel in which liquid crystal is sandwiched between a counter substrate provided with a reflection film. By using this, it is possible to realize a reflective liquid crystal panel with a single bright polarizing plate.
Providing the scattering film in the panel is expected to increase the viewing angle.

[0043]

According to the reflection type liquid crystal display panel of the present invention, an STN liquid crystal layer and a phase difference plate are sandwiched between one polarizing plate and a reflection film .
By compensating for the phase difference plate identify and incidence and emission angle dependence Z coefficient, bright, can realize a high reflection type liquid crystal display device contrast.

[0044]

[Brief description of the drawings]

FIG. 1 is a sectional view of a reflective liquid crystal panel according to an embodiment of the present invention.

FIG. 2 is a plan view of a reflective liquid crystal panel according to an embodiment of the present invention.

FIG. 3 is a diagram showing a voltage-reflectance characteristic of the reflective liquid crystal panel according to the embodiment of the present invention;

FIG. 4 is a perspective view of an evaluation of the angle dependence of the reflective liquid crystal panel according to the embodiment of the present invention.

FIG. 5 is a diagram showing an angle-dependent characteristic of the reflective liquid crystal panel according to the embodiment of the present invention.

FIG. 6 is a diagram showing an angle-dependent characteristic of the reflective liquid crystal panel according to the embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Upper substrate 2 Lower substrate 3 Mirror reflection film 4 Transparent electrode 5 Alignment film 6 Seal resin 7 Nematic liquid crystal 8, 9 Phase difference plate 10 Polarizing plate 11 Forward scattering plate

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G02F 1/1335 G02F 1/1333

Claims (4)

(57) [Claims] 1. A super-twisted nematic liquid crystal layer twisted from 180 degrees to 260 degrees, a single polarizing plate and a reflection film sandwiching the super-twisted nematic liquid crystal layer, and at least one polarizing plate between the polarizing plate and the liquid crystal layer. A liquid crystal display panel having a plurality of retardation plates, wherein a retardation plate closest to the polarizing plate among the retardation plates has a refractive index np in a slow axis direction and a fast axis direction. A reflection type liquid crystal display panel, wherein the Z coefficient = (nz−np) / (ns−np) is 0.3 or more and 0.7 or less when a refractive index ns and a refractive index nz in a thickness direction are used. 2. The first birefringence Δ for light at 450 nm
The value obtained by dividing n1 by the second birefringence index Δn2 with respect to light of 590 nm is defined as Δn wavelength dispersion value, and the difference between the Δn wavelength dispersion value of the liquid crystal layer and the Δn wavelength dispersion value of the retardation plate is smaller than 0.08. The reflective liquid crystal display panel according to claim 1, wherein: 3. A product Δnd of a third birefringence index Δn3 and a thickness d of the liquid crystal layer with respect to light of 550 nm is greater than 0.8 μm and 1.1 μm or less, and the retardation plate is made of polycarbonate. Having a liquid crystal material in which the value obtained by dividing the first birefringence Δn1 for 450 nm light of the liquid crystal layer by the second birefringence Δn2 for 590 nm light is in the range of 1.06 to 1.16. 3. The reflective liquid crystal display panel according to claim 2, wherein: 4. An angle between a direction of orientation of liquid crystal molecules of a liquid crystal layer closest to a polarizing plate and a slow axis of a retardation plate closest to the liquid crystal molecules is 70 degrees or more and 110 degrees or less. Item 4. A reflective liquid crystal display panel according to item 3.