Two-layer guest-host liquid crystal display device

A two-layer guest-host liquid crystal display device is provided, each layer having a guest-host liquid crystal layer sandwiched between a pair of substrates and being provided with an electrode for applying an electric field to said liquid crystal layer, which comprises a first display layer comprising a P-type pleochroic dye material and a first liquid crystal material and a second display layer comprising an N-type pleochroic dye material and a second liquid crystal material.

BACKGROUND OF THE INVENTION 
The present invention relates to a guest-host liquid crystal display and, 
more particularly, to a two-layer guest-host liquid crystal display 
device. 
Guest-host liquid crystal display devices have a guest-host liquid crystal 
layer sandwiched between a pair of substrates each provided with an 
electrode for applying an electric field to the liquid crystal layer. The 
guest-host liquid crystal layer comprises a guest pleochroic dye and a 
liquid crystal host. 
A multistory guest-host liquid crystal display device has been proposed 
where at least two of the guest-host liquid crystal display layers are 
provided to provide mixed coloration. An example of these two-layer 
guest-host liquid crystal displays is shown in FIG. 1. 
The conventional display of FIG. 1 comprised a polarizer 1 where the 
polarization direction is represented by A, glass substrates 2, liquid 
crystal molecules 3, pleochroic dye materials 4 and 4', and two driving 
sources 5. The liquid crystal of the molecules 3 are positive dielectric 
anisotropy nematic liquid crystal. The pleochroic dye materials 4 and 4' 
are P-type dye materials in which each showed a different color. 
When both of the driving sources 5 supplied no voltage to the display, each 
of the liquid crystal molecules 3 and the dye materials 4 and 4' was 
oriented to be parallel with the substrates 2. When white light polarized 
by the polarizer 1 was incident on the display cell, each of the dye 
materials 4 and 4' absorbed peculiar spectra. As a result, since the 
display layers overlapped, their display colors were mixed at no-voltage 
applied portions as the background, thus the mixed display colors were not 
suitable for displays. 
When only one of the two display layers was driven by the driving source 5, 
the dye materials and the liquid crystal materials at positions of any 
voltage applied were oriented to be perpendicular to the substrate 2. 
Little light was absorbed. Then, the display color was the color of the 
dye materials contained within the display layer free of driving voltage. 
When both of the two display layers were driven, the incident white light 
was not absorbed and transmitted so that the display was colorless. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved two-layer guest-host liquid crystal device for showing a 
pleochroic and clear display. 
Briefly described, in accordance with the present invention, a two-layer 
guest-host liquid crystal display device, each having a guest-host liquid 
crystal layer sandwiched between a pair of substrates and each provided 
with an electrode for applying an electric field to the liquid crystal 
layer, comprises a first display layer comprising a P-type pleochroic dye 
material and a first liquid crystal material, and a second display layer 
comprising an N-type pleochroic dye material and a second liquid crystal 
material.

DESCRIPTION OF THE INVENTION 
FIG. 2 shows a schematic representation of a two-layer guest-host liquid 
crystal display device according to the present invention. FIG. 3 shows 
schematic liquid crystal molecule orientation B of the device of FIG. 2 by 
the rubbing method or the like. 
The device of FIG. 2 comprises a polarizer 1 where the polarization 
direction is indicated by A, glass substrates 2, liquid crystal molecules 
3, pleochroic dye materials 4 and 6, and two driving sources 5. The liquid 
crystal of the molecules 3 is positive dielectric anisotropy nematic 
liquid crystal. The liquid crystal molecules 3 are oriented to be 
homogeneous when no voltage is applied. 
According to the present invention, the type of pleochroic dye materials to 
be contained within each of the display layers reverses. When one of the 
two display layers contains P-type pleochroic dye materials, the other of 
the display layers contains N-type pleochroic dye materials. 
P-type pleochroic dye materials are such that, when the dye materials are 
oriented to be parallel with the substrates 2, they absorb peculiar light 
spectra, and when they are oriented to be perpendicular to the substrates 
2, they do not absorb any light spectra. 
N-type pleochroic dye materials are such that, when the dye materials are 
oriented to be parallel with the substrates 2, they do not absorb any 
light spectra, and when they are oriented to be perpendicular to the 
substrates 2, they absorb peculiar light spectra. 
As an example, the pleochroic dye materials 4 are of P-type, such as 
anthraquinone showing blue and the pleochroic dye materials 6 are of 
N-type, such as tetrazine showing red. The types of pleochroic dye 
materials can be exchanged. When the P-type pleochroic dye materials 4 can 
absorb peculiar light spectra, the N-type pleochroic dye materials 6 can 
not absorb any light spectra. 
The liquid crystal of the molecules 3 may be positive dielectric anisotropy 
nematic liquid crystal composed of biphenyl such as commercial name GR-63 
produced by Chisso Corporation, Japan. The N-type dye materials 6 are 
dissolved in a concentration of about 20 weight % while the P-type dye 
materials 4 are dissolved in a concentration of about 2 weight %. When 
sinusoidal AC voltage of about 5 V, 32 Hz is applied to the first display 
layer containing the N-type dye materials 6 and the second display layer 
containing the P-type dye materials 4, the resultant display color is as 
follows: 
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the first the second 
the resultant 
display display display 
layer layer color 
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Off Off blue 
Off ON colorless 
ON Off red 
ON ON red 
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FIG. 4 shows a schematic representation of another two-layer guest-host 
liquid crystal display device according to the present invention. FIG. 5 
shows schematic liquid crystal molecule orientation B of the device of 
FIG. 4 by the rubbing method or the like. Like elements corresponding to 
those of FIG. 2 are indicated by like numerals. 
When no voltage is applied to the display device, the liquid crystal 
molecules are oriented to be parallel with the substrates 2 and are 
twisted between the substrates 2. The same materials as those in the 
device of FIG. 2 are used except that the liquid crystal is a spiral 
nematic liquid crystal comprising a biphenyl nematic liquid crystal, such 
as GR-63, as a principal constituent and a cholesteric liquid crystal, 
cholesteryl nonanoate as an additive of a slight amount. The device of 
FIG. 4 shows good display characteristics similar to that of FIG. 2 when 
the FIG. 4 device is driven under the same condition as the FIG. 2 device. 
Some types of liquid crystal which can be adopted for the present invention 
are as follows: 
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mesophase mode 
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1. a nematic liquid crystal 
homogeneous 
2. a nematic liquid crystal and a 
twisted nematic 
cholesteric liquid crystal of 
a slight amount 
3. a nematic liquid crystal and a 
cholesteric-nematic 
cholesteric liquid crystal of 
phase transition 
a small amount 
4. a cholesteric liquid crystal 
cholesteric-nematic 
phase transition 
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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.