Note: Descriptions are shown in the official language in which they were submitted.
2 5 1 , 2 4 0 C A N / W B B
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THERMALLY ADDRESSED LIQUID C~YSTAL DEVICE
FIELD OF THE INVENTION
The invention pertains to thermally addressable
liquid crystal display devices, and more particularly to
thermally addressed visual display devices which use a
light absorption technique to provide a dark image upon a
lighter background.
BACKGROUND OF THE INVENTION
Heretofore, the ability to Eabrica~e large scale
multiplexed liquid crystal displays was very dificult.
This difficulty was primarily due to "cross-talk" efects,
and the necessity to quickly refresh the slow responding
liquid crystal medium. Large scale multiplexed displays
notoriously have had problems with "cross-talk", i.e. t the
unwanted sensitizing o partially selected display
elements. This problem results from the small root mean
square voltage ratio between the "on" and "off" elements
achievable in a large scale multiplexed liquid crystal
display.
As displays become larger, a new problem appears.
Most device effects do not have intrinsic storage. The
display must therefore be repeatedly scanned to update;
this is o~ten with typical display effects done a~ 60 Hz
(per frame). The result ~or large area matrices is a small
25 duty cycle for each individual row or column. Most display
media only partially respond to small duty cycle voltage
information and the resulting effect is only a fraction of
the dc equivalent voltage. The result is low contrast or
brightness. As the display matrix gets larger, the duty
30 cycle gets less and less and optical performance gets
poorer and poorer. The result is a very poor (below
commercial standards) optical performance as the X-Y matrix
gets larger and larger.
These two problems have severely limi-ted
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the ability to provide large scale multiplexed displays,
and to dateJ not one has produced a device which has high
contrast, wide viewing angle, is easy to fabricate, easy to
operate, and which has low cost.
The pre~ent invention has developed a low cost,
large scale multiplexed, visual display device that has
resolved the aforementioned problems, while providiny a new
liquid crys~al device having many advantages over the prior
art.
While the present invention is concerned
primarily with large scale, thermally addressed multiplexed
devices, its new light absorbing method is easily
applicable to devices which are not large scale, and which
do not utilize multiplexing. The subject invention is
15 believed to have wide applica~ion in the field of thermally
addressed liquid crystal displays, and is not considered as
being limited to any particular device or system.
DISCUSSION OF RELATED ART
The invention features certain classes of smectic
20 liquid crystal hosts that have a nematic phase upon
heating. A small percentage of pleochroic dye is added to
the material~ The display is addressed in a thermal
electric mode. For the "on" elements, the liquid crystal
texture is absorbing due to the dye which strongly absorbs
incoming light. The "off" elements and the background have
homeotropic smectic A texture, where the dye exhibits
minimum absorption.
The concept of pleochroic dye switching as the
Guest Host effect in nematic liquid crystal, was first
30 suggested in an article to: G. H. Heilmeier, J. A.
Castellano, and L.A. Zanoni, Mol Crystals and Liquid
Crystals 8, 293 (1969)o
Others have suggested that the liquid crystal
structure can be twist nematic, homogeneous, or
35 homeotropic. Most of these devices using pleochroic dyes
mixed with the liquid crystal material have generally
5137
required external devices such as polarizers or wave plates
to improve the contrast of the image.
Dyes of high order parameter in a cholesteric
liqud crystal host were first suggested in an article to:
D. L. White, G. N. Taylor, J. of Appo Phys. 45 4718 (1974).
~ isplays using this liquid crystal medium have
high contrast and do not require external ploarizers.
These displays have high brightness and a wide viewing
angle not available with ~he field effect twist nematic
lO liquid crystal displays~ Such devices use a cholesteric to
nematic transition efect with liquid crystals of positive
dielectric anisotropy.
In the no field (off) mode, the dye molecules
follow the helical structure of the host and exhibit strong
l5 light absorption. In the on condition, the dye is in a
homeotropic nematic host and the absorption is minimized.
Thus, the display presents a white image against a dark (or
colored) background. A white image against a dark
background is, however, generally not desirable.
Recently, high order parameter and ligh~ stable
dyes have become available. Devices using these dyes will
provide viable displays for many applications. However,
they have two major drawbacks which may restrict their
application to simple displays of very low information
25 content only.
These dye displays are very difficult to
multiplex. Even a few rows represent a state oE the art
development. Large size matrix addressing has only been
achieved by adding external non-linear elements to each
30 display elements.
For non-emissive (reflective) displays, a white
image against a dark background is formed. This is
esthetically undesirable, and of limited commercial
utility. Techniques to reverse the îmage contrast to a
35 more pleasing dark against a light background are available,
but the added complication increases the complexity and
cost.
In 1978 C. Tani and T. Ueno discussed the
application of pleochroic dyes to certain smectic liquid
crystals in a scientific paper (~ppl. Phys. Lett. Vol. 33
No. 4, 15 Aug. 1978). The authors, however, specifically
teach aqainst the use of the smetic "Al' phas~ as having
utility in the pleochroic dye system: they indicate that
it has application only in scattering applicaitons such as
in laser addressed light valves~ They concluded that only
materials having smectic H or possibly B phase structure
have useful properties in combination with pleochroic dyes.
Further, they discuss the utilization of slow cooling as
having utility with pleochroic dyes and that rapid cooling
of the elements is ony applicable to light scattering
devices.
The present invention utilizes pleochroic dyas to
produce an absorbing state rather than a scattering state
and uses thermal XY local heating as distinct from the
laser heating as described in other art. Further it
utilizes rapid cooling of the element with liquid crystals
preferentially of the smectic "A" phase. The last factor
is directly against the teaching oE Tani and Ueno, but has
been found to be most effective in this application.
Also recently, a system has been reported in the
French literature, which uses a thermally addressed smectic
"A" crystal medium. Such a system is described in an
article entitled: MATRIX ADDRESSED SMECTIC LIQUID CRYSTAL
DISPLAY: M~ Hareng, S. Le Berre, R. Heh]en, and J. N.
Perbet, Thomson-CSF Laboratoire Central de Rechereches.
Society of Information Display, Conf. Record of 1980
Biennial Display ResO Conf. "Post deadline paper".
Such a system does not use dyes, and employs a
sca~tering light t~chnique, rather than a light absorption
technique as decribed by this invention.
In addition, the system described is embodied in
a very different device than detailed in this invention.
Because of the crucial difference of the light scattering
as compared to light absorption, the device can only be
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viewed through a projection optical system that results in
a very bulky, power intensive system.
While the prior art teaches the use of pleochroic
dyes of high order parameter Eor use in liquid crystal
media, it should also be noted that these dyes are used
primarily to enhance the ligh~ a~fects produced by the
thermal phase transition of the media. The invention by
contrast, relies upon the dye to do most of the light
absorption for the crystal medium; the medium acting as a
vehicle for orientating the dye to develop a light
absorbing stance.
BRIEF DESCRIPTION OF T~E lNv~N~ oN
The invention relates to a thermally addressed
visual device which provides a dark image against a lighter
background. The device comprises a liquid crystal medium
including at least one liquid crystal compound mixed with
at least one coloring agent, generally a pleochroic dye of
high order parameter. The medium has positive dielectric
anisotropy. The medium is thermally sensitive and has a
transition between at least two thermal phases; the lower
thermal phase is a smectic phase. The medium develops two
textures in the smectic phase: a light absorbing texture
and a homeotropic texture. The homeotropic texture is
developed in portions of the medium by sensitizing the
medium as it passes rapidly from an upper thermal phase to
the lower, smectic phase~ The light absorbing texture
develops in the unsensitized portions of the medium as it
goes through the transition to the smectic phase.
The medium is sensitized by applying a voltage to
those portions of the medium to be addressed. The
addressed portions develop a substantially transparent
light state, while the unaddressed portions develop a sub-
stantially light absorbing state. The coloring agent or
dye which is locked within the liquid crystal medium as it
develops its light absorbing texture in the smectic phase
will absorb most of the light passing through the medium;
5 Sl~
^ 6 _
the liquid crystal acting as a vehicle to orient the dye
molecules into a light absorbing position. Electrodes are
provided to sensitize the medium. They are disposed adja~
cent the medium. Heating electrodes are also provided to
5 heat the medium to an upper thermal phaseO In a multi-
plexed device, these electrodes define a matrix of columns
and rows disposed substantially at right angles to each
other, and in different planes.
In order to obtain a pleasing direct viewable
10 display device, the row electrodes are made diffusely
reflective to provide high contrast as well as wide viewing
angle. The reflective electrodes provide for a double pass
of light through the cell enhancing light absorption.
The liquid crystal medium will generally contain
an octyl cyano biphenyl compound and will have two thermal
transitions: between an isotropic and nematic phase, and
hetween the nematic and a smectic phase, in particular a
smectic "A" phase.
More particularly, the liquid crystal will be
comprised of a mixture of cyano biphenyl compounds of the
following formulas:
C8H17 ~ ~ ~ ~ -CN
ClOH21 ~ ~ ~ ~ -CN
In one type of mixture, the octyl cyano biphenyl
can be represented in the mixture in a weight percentage
from approximately 35 to 6S, and the deca cyano biphenyl
can be represented in a weight percentage from approxi-
mately 30 to 60.
In another type of mixture, there can be added ~o
the above materials cyano alkyl oxy biphenyl compounds of
the following formulas:
:~glS87
C8H17 ~ ~ ~ -CN
ClOH21 ~ ~ ~ ~ ~ -CN
The octyl cyano oxy biphenyls can be represented
in the mixture in a weight percentage from approximately 15
5 to 30 and the deca cyano oxy biphenyl in a weight percen
tage ~rom approximately 11 to 26 while the octyl cyano
biphenyl is approximately in the range 20 to 35 and the
deca cyano biphenyl is approximately in the range 24 to 39
by weight
The pleochroic dye can be:
4~4'-N=N-Dimethylamine phenylazo~ azobenzene r and
can have a weight percent range rom 0.5 to 3.0 of the
total composition.
More particularly, the octyl and deca cyano
15 biphenyls may be mi~ed by weight 55.6 and 44.4 percent,
respectively, and the 4(4'-N=N-Dimethylamino phenylazo)
azobenzene can have 1 to 1.5 weight percent of the total
composition.
Also, the octyl, deca, octyl oxy, and deca oxy
20 cyano biphenyls can be mixed by weight percentage 27.5,
31.5, 22.5, and 18.5, respectively. The dye will be mixed
with the above in 0O75~ to 1.75% weight percent of the
total, respectively.
In operating the device, the liquid crystal
25 medium is passed through its thermal transition from its
upper thermal phase to its lower thermal, smectic phase.
The transition must be accomplished reasonably rapidly,
hence rapid thermal pulses are used that heat the liquid
crystal locally but do not signiicantly heat the
30 surrounding glass. Hence the natural cooling period
immediately following the passage of the heat pulse is also
rapid and hence the liquid crystal medium passes through
the nematic phase rapidly. This greatly enhances ~he
optical effect and results in greater contrast.
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Certain portions of the medium are sensitized.
These portions define the background of the medium. These
sensitized portions develop the substantially light trans-
parent state when the medium passes into the smectic ther-
5 mal phase. The remaining unsensitized portions oE themedium develop a light a~sorbing state~ When light
(generally ambient) is passed through the medium, the
unsensitized portions absorb the light to provide a dark
image upon the lighter sensitized background. The
10 addressed portions of the medium may be sensitized in a
chronological sequence.
It is an object of this invention to provide
improved liquid crystal device, method, and liquid crystal
compositions;
It is another object of the invention to provide
an improved large scale, multiplexed, thermally addressed
visual display;
It is a further object of this invention to pro-
vide a highly contrasted dark image on a lighter background
20 or a thermally addressed liquid crystal device.
These and other objects of the invention will
become more apparent and will be better understood with
reference to the following detailed description considered
in conjunction with the accompanying drawings.
25 BRIFF DESCRIPTION OF THE DRAWINGS
Figure 1 is a perspective, exploded, schematic
view of a visual device made in accordance with the
invention;
Figure 2 is a plan schematic view of the device
30 shown in Figure 1, illustrating how an image can be formed
in the liquid crystal medium by a multiplexing technique;
Figure 3 is a graphical illustration of the
chronological sequencing of the row and column electric
waveforms of the device depicted in Figure 1.
Figures 4a and 4b ~how a schematic view of two
different light modulating textures developed in the liquid
LS15'7
g
crystal medium of the device of Figure 1, when the medium
passes into its smectic phase; Figure 4a depicts a homeo-
tropic, substantially light transparent texture, and Figure
4b illustrates a substantially light absorbing texture.
5 DETAILED DESCRIPTION OF THE INVENTION
Generally speaking, this invention relates to new
methods, compositions, and visual devices utilizing the
thermal addressing o liquid crystal media. The visual
devices of this invention feature a highly contrasted dark
image on a lighter background O
Where the devices of the invention are multi-
plexed, they are capable of being multiplexed up to a large
number of rows.
This invention provides new displays that incor~
porate pleochroic dyes of high order parameter into a
smectic A liquid crystal material that has a nematic phase
upon heating. By using a thermal electric addressing tech-
nique described hereinafter, this display has major advan-
tages over the previously known dye switching displays
based on the gues~ host effect and the cholesteric to
nematic transition effect.
Smectic A phase is one of the most common liquid
crystal phase~ encountered. Some materials having smectic
A phase also exhibit a nematic phase upon heating. One
example is ~he octyl cyano biphenyl:
C8~17 ~ ~ - ~ -CN
which has a phase transition as follows
20C 32C 40C
The smectic A phase can be aligned homeotropic-
ally, as shown in Figure 4a, if the surface of the display
is treated with materials such as lecithin. In this struc-
ture, the material is transparent.
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There are two forms of thermally addressed
smectic A displays. One type uses a scanning laser beam to
address the display elements. The other type is x y matrix
addressed. The row electrodes are heated sequentially with
electric current and the display is written by applying
voltages on the columns. ~uring the writing process, only
the dots associated to the row where the heating current
has just been removed are affected. In other words, only
the dotswhere the liquid crystal material is rapidly
cooling to the smectic state respond to the column voltage
pulses.
As the liquid crystal material cools rapidly
through the nematic phase to the smectic phase, it can form
two different textures. With a voltage applied on the
15 column, the liquid crystal material is switched to a homeo-
tropic state during the nematic phase and assumes the
homeotropic smectic A texture after cooling is completed~
Without the applied voltage, a light absorbing texture is
developed instead~ Thus, the dots associated with a
20 rapidly cooling row electrode can be written into trans-
parent state or a light absorbing state by applying or not
applying voltages on the columns. The smectic material
used in the inventive display device has positive dielec~
tric anisotropy.
The transition must be accomplished reasonably
rapidly; hence, rapid thermal pulses are used that heat the
liquid crystal locally but do not significantly heat the
surrounding glass. Hence the natural cooling period imme-
diately following the passage of the heat pulse is also
30 rapid and hence the liquid crystal medium passes through
the nematic phase rapidly. This greatly enhances the
optical effect and results in greater contrast. Table 1
below, illustrates this effect:
5~3'~
TA13LE
Impact of Rapid Cool
Rapid Cool(l) Slow Cool(2~
Test# CR Brightness CR Brightness
1 3.1 28% 1.1 29%
2 5.2~ 34% 1.0 33~
3 6.6 29% 1.2 29%
4 8.~ 28% 1~2 29%
8.7 32~ 1.2 31%
(1) Typical rapid cool; greater than 500C per sec.
~2) Typical slow cool; less than 500C per sec.
The present invention, however, must be carefully
distinguished from other similar systems wherein a
scattering texture rather than a light absorbing texture is
developed in the smectic material~ ~isplays developing the
scattering texture are generally not suitable for direct
viewing, and are often used only in projection systems~
The optical contrast developed by a scattering
texture against a transparent texture is similar to ~hose
obtained with the dynamic scattering effect. Under many
commonly encounted illumination conditions, it will not
give a pleasing legible, high contrast image.
The situation becomes quite different, however,
when a pleochroic dye of high order parameter is introduced
into the smectic A material. The dye becomes locked into
the liquid crystal, and assumes the orientation of the
liquid crystal molecules. When a randomly oriented texture
is produced due to rapid cooling through the nematic phase
to the smectic phase, the dye molecules in the host absorb
light strongly, transforming it into a light absorbing tex-
ture, either deeply colored or black. In fact, the ran-
domly oriented texture may not scatter light as it usually
does, if liquid crystal materials of very low birefringence
are used. It will, however, strongly absorb the light due
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to the orientation of some of the dye molecules having
their light absorbing axis partially transverse to the
light pathO In the homeotroplc smectic -texture, the dye
molecules have minimum absorp~ion, since they do not absorb
5 light inciden~ upon the edge of their molecular structure.
This texture, therefore, develops a clear tex~ured back-
ground. This results in a high contrast display that is
suitable for direct viewing. No external po:Larizers are
required. The addressiny technique is substantially the
10 same as smectic displays without the dyea
Now referring to Figure 1, an exploded view of a
typical multiplexed, visual display device 10, is
illustrated. The device comprises a liquid crystal medium
11 containing the pleochroic dye, which materlal is
15 disposed between ~wo glass substrate plates 12 and 13,
respectively. The top substrate plate 12 supports a
plurality of column electrodes Cl, C2, C3t etc., which make
up one half of the x y matrix for addressing the liquid
crystal material 11~ The column electrodes are made of
20 electrically conductive, light transmitting material such
as indium tin oxide.
The bottom plate 13 supports a plurality of row
electrodes rl, r~, r3, etc., which make up the remaining
half of the x y matrix. The row electrodes are electric-
25 ally conductive and light reflective, and can be, forexample, silver or aluminum, which can be deposited on the
glass plate 13.
The liquid crystal medium 11 is generally sealed
between the two substrate plates 12 and 13 with the elec~
30 trodes in contact on either side. Light (generally
ambient) is passed through (arrow 18) the glass composite,
as shown.
The physical operation of this display 10 can
best be illustrated with a simple example of a 5 X 7 matrix
35 displaying a character "A", as shown in Figure 2. The rows
of the matrix are tied together at one end to the common 16
and are sequentially heated by applying electric pulses to
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the other ends 17. In time æone 0, (see Figure 3) row 1 is
heated such that -the liquid crystal material over the row 1
electrode r1 is in the isotropic state. In time zone 1,
row 2 electrode r2 is heated. In the meantime, row 1
rapidly cools down and the dots associated with it are
written by applying electric voltage on the column elec-
trodes. In this example, electrodes Cl and C5 have voltage
applied such that the dots rlcl and rlc5 will be in the
clear state. C2, C3, C4 have no voltage applied, and the
dots rlc2, rlc3, rlc4 have a colored light absorbing
texture. During time zone 2, row 3 electrode r3 is heated
and row 2 rapidly cools down, and the voltage on the
columns assume the values corresponding to the l'on'l and
lloff" pattern of dots associated to row 2. The entire wave-
form for displaying a character IIAI', is shown in E'ig. 2.
The colored light absorbing texture associatedwith the "on" dots is metastable and has long relaxation
time of the order of at least months. This texture can be
automatically erased by heating the row during rewriting of
20 the display, or can be erased by applying a voltage on the
columns substantially higher than the wri~ing voltage.
Once the colored light absorbing texture is formed, it will
not be affected by the writing or sensitizing voltage.
This assures that "cross-talk" will not be a problem, and
25 makes possible a large scale matrix display~
The erase-writing process for this display is
very fast. Generally, less than a 100~second writing time
can be achieved. If the display is refreshed at fr times
per second, the total number of rows that can be multi-
30 plexed will be:
r x Tw
Where Tw = the time required to write the row.
With fr = 30 hertz, which is a similar rate as aconventional CRT, and Tw = 100~sec., we have n = 333 rows.
35 Thus, the display can be multiplexed up to a rather large
number of rows.
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In practical display driving, the heating pulse
can be applied over several time zones before the cooling
and writing cycleO This lowers the voltage requirement for
the heating pulses. However, the heating pulse should be
short enough to avoid heat spreading to the neiyhboring
rows and to minimize glass hea~ing that then prevents the
rapid cooling necessary for yood optical images.
A high contrast is achieved for the colored or
black image due to the light absorbing character of the dye
10 material vis-a-vis the transparent background~
The contrast is further improved by the reflec-
tive nature of the heating electrodes, which provide a
double light pass back through (arrow 15~ the medium ll,
wherein the unaddressed dye molecules in the light
15 absorbing state (image) can absorb more light as compared
to the addressed transparent background.
The medium ll is depicted in the sensitiæed
(addressed) homeotropic phase in Figure 4a, and is shown in
the unaddressed light absorbing phase in Figure 4b. 1ight
(arrow 20) entering the homeotropic material of Figure 4a,
passes between the liquid crystal molecules 21. The dye
molecules 22 are not light absorbing in this phase r because
they are locked in the crystal to confront the light rays
upon their edge, as shown.
However, in the light absorbing phasev the dye
molecules 22 are locked in the crystal molecules 21 in a
randomly angled pattern, as shown in Figure 4b. In this
phase t the dye molecules 22 will strongly absorb the
impinging light rays 20 to produce an intensely colored or
30 dark imageO
The crystal liquid medium ll can be comprised of
at least one sctyl cvano biphenyl compound.
More particularly, the liquid crystal will be
comprised of a mixture of cyano biphenyl compounds of the
following formulas:
C8H17 ~ - ~ -CN
C10~21 ~ ~ ~ ~ -CN
CllH23 ~ ~ -CN
In one type of mixture, the octyl cyano biphenyl
can be represented in the mixture in a w~ight percentage
from approximately 35 to 55, and the decyl cyano biphenyl
can be represented in a weight percentage from
approximately 30 to 60.
In another type of mixture, the undecyl cyano
biphenyl can be added to the above, and can have an
approximate weight percent range from 15 to 35.
The pleochroic dye can be:
4(4'-N=N-Dimethylamino phenylazo~ azobenzene, and can
have a weight percent range from 0.5 to 3~0 of the total
composition.
More particularly, the octyl and decyl cyano
biphenyls may be mixed by weight 55O6 and 44.4 percent,
respectively, and the 4(4'-N=N-Dimethylamino phenyla~o)
azobenzene can have 1 to 1,5 weight percent of the total
composition,
Also the octyl decyl, and undecyl cyano biphenyls
can be mixed by weight percentage: 44~4; 31.3; and 24.5,
respectively~ The dye will be mixed with the above in 1 to
1,5 weight percent of the total composition.
Other compositions of the liquid crystal medium
can be:
C10H21 ~ ~ ~ ~ - CN 4 - Cyano - 4~ - n -
decyl biphenyl
45% by weight
C8H17 ~ ~ ~ ~ - CN 4 - Cyano - 4~ ~
octyl biphenyl
55%; or
l~lS~';''
- 1.6 -
C8H17 ~ ~ - CN 4 - Cyano - 4~ ~
octyl biphenyl
27~5% by wt.
r~ ,
ClOH21 ~ ~ ~ ~ - CN 4 - Cyano - 4
decyl biphenyl
31.5%
C8H17 ~ ~ ~ - CN 4 - Cyano - 4' -
octoxy biphenyl
22,5~
ClOH21 ~ ~ - CN 4 - Cyano - 4' -
decyloxy biphenyl
18.5%; or
C2H5 - ~ -CN
4' - Ethyl - trans, trans - bicyclohexyl - 4 -
carbonitrile.
To the above liquid crystal compositions, any one
of the following coloring agents may be added:
N ~ - N = N - ~ - N = N - ~ - N = N ~ ~ N 1% by
weight; or
OCH2
c f
2 wt.; or
.
151S'~
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a dye mixture 1.4% by weight of:
C2H5 0,CH2 C2H~
30% wt. ~ N - ~ - N = N - ~ - N = N - ~ - N = N - ~ -N
C~l2 H
54% C4Hg - ~ - CH = N - ~ - N = N - ~ - N = CH - ~ - OC~Hg
O NH - ~ ~ C5
16% C5~110 - ~ - NH
Having thus described the invention, what is desired to be
protected by Letters Patent is presented by the following appended
claims:
