JP2002122879A - Normally white mode reflective liquid crystal display - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an NW(normally white) mode reflective liquid crystal display capable of affording a bright achromatic display with a high contrast ratio even when a material with large wavelength dispersion of Δn is used. SOLUTION: In the NW reflective liquid crystal display consisting of a substrate 3, a reflection plate 4 formed on the substrate, a transparent substrate 5 arranged on the upper side of the reflection plate, a driving cell 1 composed of a pixel electrode 11 integrated with the reflection plate, a transparent electrode 12 formed on the transparent substrate and a liquid crystal layer 13 for driving arranged between both electrodes, a compensation cell 2 superimposed on the transparent substrate, an optical retardation plate 6 superimposed on the compensation cell and a polarizing plate 7, the retardation of the driving cell and the compensation cell are optimized while being made mutually different.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an NW (normally white) mode reflection type liquid crystal display which displays white when the power is turned off.

[0002]

2. Description of the Related Art Such an NW mode reflection type liquid crystal display comprises, for example, a reflection plate formed on a substrate, as disclosed in Japanese Patent Application Laid-Open No. H11-249165.
A transparent substrate disposed above the reflection plate, a pixel electrode integrated with the reflection plate, a transparent electrode formed on the lower surface side of the transparent substrate, and a driving liquid crystal layer disposed between the two electrodes; And a compensating cell mounted on the upper surface of the transparent substrate, a retardation plate mounted on the compensation cell, and a polarizing plate mounted on the retardation plate The torsion angle of the liquid crystal of the compensation cell is opposite to and equal to the torsion angle of the liquid crystal of the drive cell, and the retardation of the compensation cell is made equal to the retardation of the drive cell. .

[0003]

However, in the above-mentioned conventional NW mode reflective liquid crystal display, the retardation of the driving cell and the compensation cell are set to the same value so that the compensation condition of the driving cell is satisfied. When the wavelength dispersion of the birefringence was large, it was not possible to obtain an achromatic display that was bright and had a high contrast ratio.
In view of the above situation, an object of the present invention is to provide an NW mode reflection type liquid crystal display which is capable of displaying an achromatic color with a high contrast ratio even when a material having a large wavelength dispersion of Δn (birefringence) is used. It is.

[0004]

In order to solve the above-mentioned problems, a substrate, a reflector formed on the substrate, a transparent substrate disposed above the reflector, and a reflector integrated with the reflector are provided. A driving cell composed of a pixel electrode, a transparent electrode formed on the lower surface side of the transparent substrate, and a driving liquid crystal layer disposed between these two electrodes, and a compensation cell mounted on the upper surface of the transparent substrate. In the present invention, in the NW mode reflection type liquid crystal display comprising a phase difference plate mounted on the compensation cell and a polarizing plate mounted on the phase difference plate, the driving cell and the compensation Optimizing while making the cell retardation different.

In other words, the present invention uses a material having a large chromatic dispersion of Δn as long as the common-sense compensation condition that the retardation of the compensation cell and the retardation of the driving cell are made to match is used. From the viewpoint that a colored display cannot be obtained, the idea of individually setting the retardation of the compensation cell and the retardation of the driving cell, in other words, performing an optimization design while making the retardation of the driving cell different from that of the compensation cell. Based on

As a result, by optimizing the retardation of the driving cell and the compensation cell while making them different, Δ Δ
The NW mode reflection type liquid crystal display which is capable of displaying an achromatic color with a bright and high contrast ratio while using a material having a large wavelength dispersion of n can be produced.

The fact that such a material having a large wavelength dispersion of Δn is used is derived from the fact that it is necessary to employ a liquid crystal polymer film having a large wavelength dispersion of current Δn for the compensation cell in order to reduce the thickness and the size. Considering that, it creates significant benefits.

According to experiments by the inventor of the present application, even when a liquid crystal polymer film having a large wavelength dispersion of Δn is used for the compensation cell, the retardation of the liquid crystal polymer film and the drive cell are individually optimized to different values. By doing
It has been proved that an achromatic display with a bright display and a high contrast ratio can be obtained irrespective of the wavelength dispersion of the Δn of the phase difference plate and the drive cell. , The wavelength dependence of the spectral reflectance is small, and the reflectance in the bright state is 49.5.
%, And an excellent achromatic color display with a contrast ratio of about 340: 1 is also obtained.

In order to provide an NW mode reflection type liquid crystal display having excellent characteristics as described above, in terms of the driving cell and the compensating cell, the twist angles are equal to each other, and the twist directions are mutually equal. The opposite is proposed by the present invention.

Further, it is proposed based on experimental results that it is advantageous to make the retardation of the driving cell and the compensation cell different by 20% or more as a preferred embodiment of the present invention.

Other features and advantages according to the present invention will become apparent from the following description of embodiments with reference to the drawings.

[0012]

FIG. 1 shows an embodiment of an NW mode reflection type liquid crystal display according to the present invention, which is a retardation plate compensation / single polarizer type reflection type using a liquid crystal polymer film as a compensation cell. The cell structure of a liquid crystal display is shown.

In this case, the driving cell 1 includes a pixel electrode 11, a transparent electrode 12, and a liquid crystal layer (T) disposed between these electrodes.
In this case, the pixel electrode 11 is integrated with the reflection plate 4 formed on the substrate 3, and the transparent electrode 12 is formed of ITO on the lower surface of the transparent substrate 5. A liquid crystal polymer film 20 as a compensation cell 2 is placed on the side of the transparent substrate 5 opposite to the side on which the transparent electrode 12 is formed, and the compensation cell 2 is sandwiched between the transparent substrate 5 and the liquid crystal polymer film 20. The retardation plate 6 is placed on the phase difference plate 6, and finally the polarizing plate 7 is placed on the retardation plate 6. In other words, the compensation cell 2 of the driving cell 1 is replaced by the liquid crystal polymer film 20 having the same molecular orientation as that of the driving cell 1, and compensation is performed by the retardation plate 6. The retardations of the liquid crystal polymer film 20 and the drive cell 1 have different values.

In manufacturing the cell structure according to the present invention, the retardation of the liquid crystal polymer film 20 as the compensation cell 2 and the retardation of the driving cell 1 are set as individual parameters.
Optimum cell conditions are required. At this time, the torsion angle of the driving cell 1, that is, the TN layer 13 is 63 degrees, and other materials and design parameters adopt typical values in the TN liquid crystal. The azimuth of the polarizing plate 7, the azimuth of the retarder 6, the retardation of the retarder 6, the retardation of the liquid crystal polymer film 20, the TN layer 13 Is adjusted.
Here, as examples, two types of TN layers 13 without chromatic dispersion and with chromatic dispersion shown in FIG. 2 and two types of retardation plates 6 without chromatic dispersion and with chromatic dispersion shown in FIG. FIG. 4 shows the spectral reflectance characteristics under the optimal cell conditions obtained by adjusting the above-described parameters under the four combinations of the above.

In FIG. 2, a graph without wavelength dispersion is denoted by a symbol "A", and a graph with wavelength dispersion is denoted by a symbol "B". Is also illustrated. FIG.
In the graph, the graph without wavelength dispersion is denoted by a symbol “a”, and the graph with wavelength dispersion is denoted by a symbol “b”. Further, in FIG. 3, four combinations of the TN layer 13 with no chromatic dispersion and with chromatic dispersion and the retardation plate 6 shown in FIGS. 2 and 3, that is, a code "A-a" (TN with no chromatic dispersion) Combination of layer and retardation plate without wavelength dispersion),
"A-b" (combination of TN layer without wavelength dispersion and retardation plate with wavelength dispersion), "Ba" (T with wavelength dispersion)
Combination of N layer and retardation plate without wavelength dispersion), "B-
The spectral reflectance characteristic graph of the optimal cell condition in each combination marked with “b” (combination of a TN layer with wavelength dispersion and a retardation plate with wavelength dispersion) is illustrated. FIG. 5 shows the contrast ratio under the optimum cell condition in each combination.

As is apparent from FIG. 5, the case where the TN layer 13 having a small wavelength dispersion of Δn and the phase difference plate 6 are used (A−a)
), The spectral reflectance is less dependent on wavelength, and 3
Even when the TN layer 13 having a large chromatic dispersion of Δn and the retardation plate 6 having a small chromatic dispersion of Δn are used (combination of Ba), the contrast ratio becomes 40: 1. I have.

Here, the above will be described in more detail while following the optimization procedure. FIG. 6 shows the definition of the axis angle of each cell structural component. R1 and R2 are the orientation directions on the incident side and the reflector side layer surface of the compensation cell 2 (liquid crystal polymer film). , R4 are the alignment directions on the layer surfaces on the incident side and the reflector side of the driving cell 1. Φ1 is the compensation cell 2
R2 is the torsion angle of R1 with respect to R1.
Is the torsion angle of R4 with respect to Β is the azimuth of the transmission axis P of the polarizing plate 7, and γ is the azimuth of the slow axis OP of the retardation plate 6.

First, under the changing range of the material parameters of each cell structure component as shown in FIG. 7, the reflectance in the bright state is 49.0% or more and the display color distance (CIE-x).
The distance between the coordinates of the display color and the coordinates of the D65 standard light source on the y chromaticity coordinates is within 0.05), the reflectivity in the dark state is 2.0% or less, the display color distance is within 0.05, and the display in the half tone. Those having a color distance within 0.05 are selected as good achromatic good cell conditions. The good cell conditions are as follows: (a) the azimuth of the polarizing plate 7 and the retardation of the retardation plate 6;
Azimuth angle and retardation of retardation plate 6, (c) retardation of drive cell 1 and retardation of retardation plate 6,
(D) the retardation of the liquid crystal polymer film 20 and the retardation of the retardation plate 6, (e) the azimuth of the polarizing plate 7 and the retardation of the drive cell 1, and (f) the azimuth of the retardation plate 6 and the retardation of the drive cell 1. (G) the retardation of the liquid crystal polymer film 20 and the retardation of the drive cell 1; (h) the azimuth of the retardation plate 6 and the azimuth of the polarizing plate 7; And (j) the relationship between the azimuth of the retardation plate 6 and the retardation of the liquid crystal polymer film 20. Particularly, in (b), good cell conditions are divided into several cell groups.

Next, the cell condition having the maximum contrast ratio in each cell group is defined as the optimum cell condition of the cell group, and the wavelength dependence of the spectral reflectance characteristic of the optimum cell condition in each cell group is compared. The one with the smallest wavelength dependence is taken as the overall optimum cell condition.

As a result of this experiment, in the case where there is no Δn wavelength dispersion of the liquid crystal, the wavelength dependence of the spectral reflectance characteristic is very small under the optimum cell condition in one group, and it is suitable for color display, and the reflectance in the bright state is 50%. Excellent display characteristics of 0.0% and a contrast ratio of 340: 1 were obtained. The optimum cell conditions and the display characteristics in this case are shown in FIG.

Next, in the case where the Δn wavelength dispersion of the liquid crystal is large, of the discrete cell groups obtained as a result of statistical processing under good cell conditions, the cell condition with the maximum contrast ratio is set to that group. Here, the spectral reflectance characteristics of the two cell groups under the optimal cell conditions are shown in FIG.
Is shown in FIG.

In particular, it can be seen from FIG. 9 that the reflectance when on and the reflectance when off on the long wavelength side are almost flat, the reflectance in the bright state is 49.7%, and the contrast ratio is about 200: 1. It can be understood that it shows. Under the optimum cell conditions at that time, the retardation of the driving cell 1 is 0.28
At μm, the retardation of the liquid crystal polymer film 20 is 0.15 μm, and it is noted that it is shifted by 20% or more. Similarly, even under the optimum cell condition in the group of FIG. 10, the retardation of the driving cell 1 is 0.27.
μm, the retardation of the liquid crystal polymer film 20 is 0.19 μm, which is 20% or more.

Therefore, the parameters of the driving cell 1 and the compensation cell 2 are individually designed and optimized again, so that the liquid crystal is bright even when there is no chromatic dispersion of Δn and when it is present, and the contrast ratio is excellent. As a result, display characteristics with little wavelength dependence of the spectral reflectance can be obtained. As a result, Δ of the liquid crystal polymer film 20 used as the compensation cell 2
n is large, the liquid crystal layer 1 of the driving cell 1
By individually designing and optimizing the parameters of the liquid crystal polymer film 20 as the liquid crystal polymer film 3 and the compensation cell 2, it is possible to obtain an achromatic display characteristic which is bright with a high contrast ratio and has a small wavelength dependence of the spectral reflectance. It became clear.

In the cell structure of the reflection type liquid crystal display, the reflection plate 4 is preferably a diffusion reflection plate having fine irregularities on its surface, but is not limited to this. Although not shown, when a diffuse reflector is employed, a transparent flat film for absorbing irregularities of the reflector may be provided. When the reflecting plate 4 is a specular reflecting plate, a forward scattering plate, which is a film having weak forward scattering, is provided on the polarizing plate 7, thereby diffusing the light reflected by the specular reflecting plate to reflect the outside world. Should be removed.

[Brief description of the drawings]

FIG. 1 is a diagram of a cell structure in one embodiment of the present invention.

FIG. 2 is a diagram showing wavelength dispersion characteristics of Δn of liquid crystal and liquid crystal polymer film.

FIG. 3 is a diagram showing a wavelength dispersion characteristic of Δn of a phase difference plate.

FIG. 4 is a diagram showing spectral reflectance characteristics under optimum cell conditions.

FIG. 5 is a diagram showing a contrast ratio under optimum cell conditions.

FIG. 6 is a view for explaining the definition of the axis angle of each cell structural component.

FIG. 7 is a diagram showing a change range of each parameter of a component.

FIG. 8 is a diagram showing optimal cell conditions and their spectral reflectance characteristics when there is no wavelength dispersion of Δn of the liquid crystal.

FIG. 9 is a diagram showing optimal cell conditions and their spectral reflectance characteristics when the wavelength dispersion of Δn of the liquid crystal is large.

FIG. 10 shows another example when the wavelength dispersion of Δn of the liquid crystal is large.
Diagram showing two optimal cell conditions and their spectral reflectance characteristics

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Driving cell 2 Compensation cell 3 Substrate 4 Reflector 5 Transparent substrate 6 Retardation plate 7 Polarizer 11 Pixel electrode 12 Transparent electrode 13 Liquid crystal layer (TN layer) 20 Liquid crystal polymer film

Continued on front page F term (reference) 2H049 BA06 BA42 BB03 BC22 2H089 HA24 HA25 QA16 SA04 SA07 TA02 TA14 TA15 TA17 2H091 FA08X FA11X FA14Y FD06 GA02 HA07 JA01 KA03 LA19 LA20

Claims (4)

[Claims] 1. A substrate, a reflection plate formed on the substrate, a transparent substrate disposed above the reflection plate, a pixel electrode integrated with the reflection plate, and a lower surface of the transparent substrate. A driving cell including the formed transparent electrode and a driving liquid crystal layer disposed between the two electrodes; a compensation cell mounted on the upper surface of the transparent substrate; and a position mounted on the compensation cell. An NW mode reflective liquid crystal display comprising a phase difference plate and a polarizing plate mounted on the phase difference plate, wherein the drive cell and the compensation cell are optimized while having different retardations. NW mode reflective liquid crystal display. 2. The NW mode reflection type liquid crystal display according to claim 1, wherein the driving cells and the compensation cells have equal twist angles, and their twist directions are opposite to each other. 3. The NW mode reflection type liquid crystal display according to claim 1, wherein the retardation of the driving cell and the retardation of the compensation cell are different by 20% or more. 4. The NW mode reflection type liquid crystal display according to claim 1, wherein said compensation cell is made of a liquid crystal polymer film.