GB2066498A

GB2066498A - Multi-layered Twisted Nematic Liquid Crystal Display Panel

Info

Publication number: GB2066498A
Application number: GB8040817A
Authority: GB
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 1979-12-20
Filing date: 1980-12-19
Publication date: 1981-07-08
Also published as: DE3048024C2; JPS5688112A; DE3048024A1; GB2066498B

Abstract

A display cell comprises polarisers 7a, 7b (only), between which are disposed adjacent twisted nematic liquid crystal layers 2a, 2b controlled by respective sets of electrodes 3a- 3d. The layers are of opposite twist sense, and the alignments adjacent the common central substrate 1b are mutually parallel, whereby O and E- rays in one layer continue as such in the other layer, producing a high rather than low order interference colour (e.g. if the layer thicknesses differ somewhat) which appears as a neutral grey rather than a variable hue. <IMAGE>

Description

SPECIFICATION Improved Multilayered Twisted Nematic Liquid Crystal Display Panel Background of the Invention The present invention relates to a liquid crystal display cell, and more particularly, to a multilayered twisted nematic liquid crystal display panel.

The convention multi-layered twisted nematic liquid crystal display panels, were known which has at least two layers each containing a liquid crystal material activated by each power supply means. As compared to long-standing singlelayer twisted nematic liquid crystal display panels, they had the following features: (1) providing various different display schemes in an individual display panel; (2) forming a logic which was operated in an electro-optical manner; and (3) increasing the number of display segments according to that of the multi-layers contained therein when, in operating the display panel by a multi-plex driving technique (line scanning driving technique) for displaying purpose, the length of time at which a voltage was applied to each of the display segments, namely, duty factor, was made constant.

However, there were inherent problems that because of the multi-layered structure display contrast of the display panel degenerated to thereby effect substantially practicability of the panel.

Summary of the Invention Accordingly, it is an object of the present invention to provide an improved multi-layered twisted nematic liquid crystal display panel.

It is another object of the present invention to provide an improved multi-layered twisted nematic liquid crystal display panel with a high display contrast.

Other objects and further scope of applicability of the present invention will become apparent from the detail description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirt and scope of the invention will become apparent to those skilled in the art from this detailed description.

To achieve the above objects, pursuant to an embodiment of the present invention, a twisted nematic liquid crystal display is featured by that there is provided a first visible area providing a good display contrast in a first liquid crystal layer is positioned face to face with a second visible area providing a good display contrast in a second liquid crystal layer by transferring extraordinary rays in the first liquid crystal layer in the direction of the extraordinary rays in the second liquid crystal layer or transferring ordinary rays in the first liquid crystal layer in the direction of the ordinary rays in the second liquid crystal layer.

Brief Description of the Drawings The present invention will be better understood from the detailed description given hereinbelow and by accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: Fig. 1 is a cross-sectional view of a multilayered twisted nematic liquid crystal display panel according to the present invention; Figs. 2(A) through 2(E) show examples of conventional orientation direction of liquid crystal molecules in a conventional display panel; Fig. 3 shows explanation of orientation vector foc representing orientation direction of the liquid crystal molecules, with a tile angle; and Figs. 4(A) through 4(E) show examples of orientation direction of the liquid crystal molecules according to the present invention.

Description of the Invention Referring now to Fig. 1 there is indicated a multi-layered twisted nematic liquid crystal display panel of the present invention. This display panel comprises three transparent substrates 1 a, 1 b, and 1 c, two liquid crystal layers 2a and 2b, four transparent electrodes 3a, 3b, 3c and 3d, four orientation layers 5a, 5b, Sc and 5d, two power sources 4a and 4b, spacers 6a and 6b, and two polarizers 7a and 7b.

Selected materials of the three substrates 1 a, 1 b and 1 c may be glass of soda, with a thickness of about 0.2 to about 3mm. They are combined in parallei with one another. On the inner surface of each of the three substrates 1 a, 1 b and 1 c, there is disposed each of the four transparent electrodes 2a, 2b, 2c and 2d made of In203, SnO2 or the like to provide electric energy to each of each of the liquid crystal layers 2a and 2b by each of the two power sources 4a and 4b. Each of the electrodes 3a to 3d is made of In203 etc.

deposited by evaporation, desirably patterned.

Each of the electrodes 3a and 3b is coupled to the power source 4a to drive the liquid crystal layer 2a. Each of the electrodes 3c and 3d is coupled to the power source 4b to drive the liquid crystal layer 2b.

Each of the four orientation layers 5a, 5b, Sc and 5d is formed on the surface of each of the four electrodes 3a to 3d and the three substrates 1 a to 1 c to provide orientation of liquid crystal molecules. Each of the layers 5a to 5d may be made of Ski02.

The well-known rubbing method of slant evaporation is applied to complete each of the layers 5a to 5d.

Each of the two liquid crystal layers 2a and 2b comprises nematic liquid crystal mixtures or cholesteric liquid crystal mixtures having a long pitch. Then, the twisted nematic liquid crystal display cell in which the longitudinal axes of the liquid crystal molecules at each surface of the three substrates 1 a, 1 b and 1 c are at right angles to each other. In each of the two liquid crystal layers 2a and 2b, the directions of the longitudinai axes of the liquid crystal molecules are identical to each other.

Around the periphery of each of the substrates 1 a, 1 b and 1 c, there is provided each of the spacers 6a and 6b for sealing each of the liquid crystal layers 2a and 2b. The spacers 6a and 6b may comprise epoxy synthetic resin with glass fiber particles. On the display side and the counter side, each of the polarizers 7a and 7b is provided.

Each of the polarizers 7a and 7b is a linearly polarization filter made of iodine or polyethylene.

In the case of a reflective type display panel, a reflector 8 is additionaliy provided behind the display panel.

With the aid of provision of each of the polarizers 7a and 7b, a display to the viewer 9 is highly enabled by the changes of the liquid crystal molecules in orientation direction in response to power supply by each of the power sources 4a and 4b.

Figs. 2(A) through 2(E) show examples of conventional orientation direction of liquid crystal molecules in a conventional dispay panel having the same mechanical structure, except for optoelectronic properties such as orientation direction, as the display panel of Fig. 1. All the examples are in the counterclockwise optical rotation in the layers 2a and 2b.

More particularly, each of Figs. 2(A) and 2(B) shows those of orientation direction in the liquid crystal layer 2a in contact with the orientation layer 5a and with the orientation layer 5b. Each of Figs. 2(C) and 2(D) shows those of orientation direction in the liquid crystal layer 2b in contact with the orientation layer Sc and with the orientation layer 5d.

In all these drawings, each of characters "r22,, "r:", "r2" and "r," represents an orientation rector provided with the respective liquid crystal molecules. Fig. 3 shows explanation of each vector with a tilt angle A0 against each of the electrodes 3a, 3b, 3c and 3d. In Fig. 3, numeral 2 indicates liquid crystal molecules and numeral 5 indicates either of the orientation layers 5a to 5d.

In the conventional liquid crystal display panel having orientation as specified in Figs. 2(A) through 2(D), a visible area with a display contrast is defined by Fig. 2(E) common to the liquid crystal layers 2a and 2b. When the display panel is viewed along the direction 12 or 13 which does not agree to the Z-axis, interference color remarkably develops at the display area where none of the liquid crystal layers 2a and 2b has an electric power, thereby leading to degradation of the display contrast.

The reason is that difference in light-path lengths by double refraction present between the liquid crystal layers 2a and 2b reduces the display contrast. The double refraction develops by optical rotation dispersion. The light-path lengths by the double refraction are represented by d. An' where d is a thickness of each of the liquid crystal layers 2a and 2b and An' is a component of the double refraction. The values of the light-path lengths are different from each other between the liquid crystal layers 2a and 2b.

More particularly, since the vector r21 in the Fig.

2(A) and the vector r2 in Fig. 2(C) are at the right angle, the components of extraordinary ray and of ordinary ray are altered to each other while ray is passing through the liquid crystal layers 2a and 2b. Accordingly, the resultant light-path length is defined by the formula of ida An'd An'l where da is a thickness of the layer 2a and db is a thickness of the layer 2b.

Usually, the value of da is not equal to that of db. In a combination with the polarizers 7a and 7b' interference color in a low order develops to thereby produce noise visible color and accordingly, reduce the display contrast.

Unlike the above conventional orientation, as features of the present invention, the vectors r: and r2 are different from each other at a substantially 1 800 angle so that extraordinary ray in the liquid crystal layer 2a is transferred in the direction of the extraordinary ray in the layer 2b and, in addition, the ordinary ray in the layer 2a is transferred in the direction of the ordinary ray in the layer 2b.

Figs. 4(A) to 4(E) show examples of orientation direction of the liquid crystal molecules according to the present invention. The directions of the optical rotations are set to be reversed between the liquid crystal layers 2a and 2b.

More particularly, each of Figs. 4(A) and 4(B) show those of orientation direction in the layer 2a in contact with the orientation layer 5a and with orientation layer 5b. Each of Figs. 4(C) and 4(D) show those of orientation direction in the layer 2b in contact with the orientation layer Sc and with the orientation layer 5d. According to a preferred embodiment of the present invention, the liquid crystal display panel is completed as follows: The value of the tile angle AO is set to be about 2 to about 5 . The thickness of each of the layers 2a and 2b is set to be about 6 ym. The directions of the optical rotations are set to be reversed between the layers 2a and 2b.In the layer 2a, there is disposed a liquid crystal mixture having a clockwise optical rotation, e.g., biphenyl type ROTN 403 produced by Hoffmann-La Roche Inc.

with about 0.15 weight% of an optically active substance, CB-15 produced by BDH Chemicals Ltd. In the layer 2b, there is disposed a liquid crystal mixture having a counterclock wise optical rotation, e.g., biphenyl type ROTN 403 with about 1.0 weight% of the optical active substance, C-l 5 produced by BDH Chemicals Ltc.

The absolute values of the helical pitches in the layers 2a and 2b are substantially identical to each other in order that the electro-optic property is in harmony with the layers 2a and 2b. Each of the polarizers 7a and 7b may be selected to be L83 produced by Sanritsu Electric Company Ltd. In the case of the reflective liquid crystal display panel, the reflector 8 is additionally provided comprising aluminum plate which has been subject to a sandblast treatment.

Accordingly to the above orientation, there is obtained a shaded area 12 providing a good display contrast in Fig. 4(E). An area in which there is obtained the good display contrast in the layer 2a is positioned to face to face with another area in which there is obtained the good display contrast in the layer 2b. Since the orientation vectors rt and r2 are deviated at substantially 1 800, the resuitant difference of the light-path lengths of the light passing through the both layers 2a and 2b is defined by "da An'+db Ant", thereby generating a high order interference color.

The high order interference color is observed as neutral gray providing an unatractive display.

Preferably, the value of "d An" where d is a thickness of either of the layers 2a and 2b, and An is an index of double refraction, is set to be within about 0.4 to about 0.6 both inclusive or about 1.0 to about 2.0 both inclusive. The term An is defined as "ne-no" where ne is a refractive index of extraordinary ray and nO is that of ordinary ray.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications are intended to be included within the scope of the following

Claims

claims. Claims

1. A twisted nematic liquid crystal display cell comprising: first, second and third substrate means; a first pair of electrode means each formed on the first and the second substrate means, and a second pair of electrode means each formed on the second and the third substrate means; the first and the second substrate means forming a first cavity in combination and the second and the third substrate means forming a second cavity in combination; a first liquid crystal layer disposed in the first cavity and a second liquid crystal layer disposed in the second cavity; polarizer means provided for at least a surface of the first substrate means not carrying the electrode means; the optical rotations of the molecules in the first and the second liquid crystal layers being directed in opposite directions to each other;; the absolute values of helical pitches of the first and the second liquid crystal layers being substantially identical; and an orientation layer formed on each of the first and the second pairs of electrode means for aligning liquid crystal molecules in each of the first and the second liquid crystal layers so that there is provided a first visible area providing a good display cointrast in the first liquid crystal layer is positioned to face to face with a second visible area providing a good display contrast in the second liquid crystal layer by transferring extraordinary rays in the first liquid crystal layer in the direction of extraordinary rays in the second liquid crystal layer or transferring ordinary rays in the first liquid crystal layer in the direction of ordinary rays in the second liquid crystal layer.

2. The display cell of claim 1, wherein orientation vectors representing the direction of liquid crystal molecules oriented in each of the first and the second liquid crystal layers are such that each of the first and the second liquid crystal layers can pass radiation and the orientation vector of the first liquid crystal layer in contact with the second substrate means separating the first and the second cavities is substantially at 1 800 deviation from the orientation vector of the second liquid crystal layer in contact with the second substrate means.

3. The display cell of claim 1, wherein in each of the first and the second liquid crystal layers there is maintained a multiplied value with a thickness and a double refractive index of each of the first and the second liquid crystal layers within about 0.4 to about 0.6 both inclusive of about 1.0 to about 2.0 both inclusive.

4. A twisted nematic liquid crystal display cell substantially as herein described with reference to Figures 1,3 and 4 of the accompanying drawings.