Liquid crystal display for reflection operation with a guest-host liquid crystal layer and method of making

A liquid crystal display for reflection operation with a guest-host liquid crystal layer disposed between two cell plates and respective front and rear electrodes, and with a diffusely divergent reflector integrated in the interior of the cell, at least the front side of the cell plate and the front electrode being transparent. The rear electrode and its supply comprise thick layers, and the reflector consists of metal parts insulated against one another and applied to the rear electrode and to the inner surface of the rear cell plate. Also disclosed is a method of producing a liquid crystal display including the steps of printing in a first screen printing process a thick layer having metal parts and a binder and having the configuration of the back electrode and its supply line on the inner surface of the rear cell plate, applying in the subsequent visual region in a second screen printing process a reflector layer with mutually insulated metal parts and a binder to the thick layer and to the inner face of the rear cell plate, printing in a third screen printing process a glass solder impression on the edges of the inner surface of the rear cell plate, mounting a front cell plate having a transparent front electrode disposed on its inner face on the glass solder impression, glass soldering the individual layers formed by the first, second and third printing processes and the front cell plate, and curing the individual layers and the front cell plate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a liquid crystal display for reflection 
operation with a guest-host liquid crystal layer disposed between two cell 
plates and respective front and rear electrodes, and with a diffusely 
divergent reflector integrated in the interior of the cell, at least the 
front side of the cell plate and the front electrode being transparent. 
The invention also relates to a method of producing this liquid crystal 
display. 
2. Description of the Prior Art: 
The principle of cholesterinic guest-host liquid crystal displays is known 
(Appl. Phys. Lett. 13, 91, 1968) and has already been used for 
constructing electro-optic devices (see DT-OS 2,410,557). From Journal of 
Applied Physics, Vol. 45, 1974, p. 4718-4723 a liquid crystal display 
device is known which utilizes pleochroic dyes (guest) and cholestorinic 
liquid crystal mixtures (host). This liquid crystal display is operated by 
reflection and has excellent brightness since external polarizers are not 
used. This publication explains the basic principle of the guest-host 
effect (for example, FIG. 2) and gives information regarding suitable 
liquid crystal mixtures and dyes and measurement results obtained by 
experiment. 
Although it has been shown that in liquid crystal displays having a 
guest-host liquid crystal layer it is possible to dispense with expensive 
polarizers, it has not hitherto been possible for this type of display to 
compete with the successful nematic rotary cells. It has been found that 
in guest-host displays operated by reflection, reading parallax occurs 
through a reflector disposed behind the liquid crystal cell. This has a 
particularly disturbing effect in relatively small displays in which the 
width of the electrode elements is smaller than or comparable to the 
thickness of the cell plates used. Dimension ratios of this kind often 
occur in displays which are intended for use in wristwatches or digital 
voltmeters. 
It therefore appears obvious to integrate the reflector in the interior of 
the cell. Unfortunately, however, new disturbing effects then occur, 
particularly in alphanumeric displays. In this case the integrated 
electrically conductive reflector performs the function of a rear 
electrode and results in not only the activated electrode elements 
themselves but also the connecting lines leading to them becoming visible. 
Since for technological and electrical reasons there is a limit to the 
narrowness of these connecting lines, this disturbing effect prevents the 
appropriate use of displays of this kind. 
As had already been done in the production of nematic rotary cells, both 
the front and the rear electrode have hitherto also been made by a thin 
film technique for guest-host liquid crystal displays. Thus, for example, 
cell plates made of glass were vapor-coated on one side in a high vacuum 
with a conductive transparent layer of SnO.sub.2 or In.sub.2 O.sub.3 of a 
thickness of 2000-5000 A. On these cell plates, referred to as conductor 
glasses, the electrode pattern corresponding to the nature of the 
characters to be displayed and to the activation system of the display was 
then printed on the coating side with acid-resistant protective lacquer by 
a screen printing process. This was followed by an etching process, for 
example etching in a solution of HCl or H.sub.3 PO.sub.4, in which the 
excess parts of the coating were etched away and the coating cleaned by 
rinsing a number of times in water. 
SUMMARY OF THE INVENTION 
Accordingly, one object of this invention is to provide a novel guest-host 
liquid crystal display which is feasible for mass production. 
Another object is to provide a liquid crystal display in which electrode 
supply lines are not noticeable. 
A further object is to develop an inexpensive process to integrate the 
production of the rear electrodes in a liquid crystal display into the 
overall display production process. 
These and other objects are attained according to this invention by 
providing a liquid crystal display in which the rear electrode and its 
supply comprise thick layers, and the reflector consists of metal parts 
insulated against one another and applied to the rear electrode and to the 
inner surface of the rear cell plate. Furthermore, these and other objects 
are attained in a method of producing a liquid crystal display comprising 
the steps of printing in a first screen printing process a thick layer 
comprising metal parts and a binder and having the configuration of the 
back electrode and its supply line on the inner surface of the rear cell 
plate, applying in the subsequent visual region in a second screen 
printing process a reflector layer including mutually insulated metal 
parts and a binder to the thick layer and to the inner face of the rear 
cell plate, printing in a third screen printing process a glass solder 
impression on the edges of the inner surface of the rear cell plate, 
mounting a front cell plate having a transparent front electrode disposed 
on its inner face on the glass solder impression, glass soldering in a 
glass soldering process the individual layers formed by the first, second 
and third printing processes and the front cell plate, and curing the 
individual layers and the front cell plate at a maximum temperature of 
500.degree. C. and for a time of from 0.5 to 1.5 hours.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views, and more 
particularly to FIG. 1 thereof, the guest-host liquid crystal display 
consists of a front cell plate 1, a front electrode 2 also disposed on the 
front side, and a back electrode 3 mounted on a rear cell plate 4. Between 
the two cell plates 1, 4 is disposed a liquid crystal layer 5 which is 
tightly closed at the edges by spacers 6. The liquid crystal layer 5 has a 
light-absorbing, helically wound structure 7, which by the application of 
an electric field, for example an alternating current source 12, to the 
supply lines 31 and 32 of the electrodes 2 and 3 can be converted into a 
uniform structure 8 which is either non-absorbent or only partially 
absorbent to light. 
During operation the liquid crystal display is illuminated at the front 
side, this being represented by a light source 10. A viewer 11 is also 
situated at the front side. The light falling on the liquid crystal 
display passes through the region of the non-absorbent structure 8 of the 
liquid crystal layer 5 and is diffusely reflected by a reflector 9 towards 
the viewer 11. 
The liquid crystal layer 5 has a thickness of 15 .mu. and consists of a 
nematic base substance having positive dielectric anisotropy, for example 
a mixture in the weight ratio 1:1:1 of P-butoxy, P-hexyloxy, and 
P-octanoyloxybenzylidene-P-aminobenzonitrile, which base substance 
contains admixtures of about 5 to 15% of an optically active substance, 
for example cholesteryl benzoate, and about 0.2 to 1% of the pleochroitic 
dye, for example indopaenol blue. Mixtures of this kind are known (J. of 
Appl. Phys., 45, 1974, p. 4718-4723) and can be replaced by other mixtures 
having the same or similar physical properties. 
A liquid crystal display according to the invention is seen in FIG. 3. The 
back electrode 3 consisting of individual metal parts 30 is covered over 
the entire visual region by the reflector 9 consisting of metal parts 20 
isolated from one another. Since the individual scale-like metal parts 20 
have lengths of about 2 to 10 .mu.m and in addition are insulated against 
one another, no short-circuits occur between the individual electrode 
segments and their supply lines. The alternating voltage necessary for 
activation, 12 volts 32 Hz, is introduced into the liquid crystal layer in 
the region of the back electrode by way of the metal parts 20. No 
undesirable side-effects, such as activated supply lines or partially 
activated subsidiary regions, etc., now occur. 
The metal parts 30 of the back electrode may be made of aluminum or of 
precious metals, such as silver, palladium-silver, or gold. 
The reflector 9 has a sheet resistivity of about 10.sup.7 .OMEGA. per 
square, while an oxide coating suffices to insulate the aluminum metal 
particles 20 from one another. 
In order to increase the abrasive resistance of the reflector 9, an 
addition of from 1 to 5% weight of glass solder powder of a grain size of 
from 0.5 to 5 .mu.m has given good results. 
The addition of non-conductive particles 29 of CeO.sub.2, MgO, SiO.sub.2, 
TiO, TiO.sub.2, ZrO.sub.2, Al.sub.2 O.sub.3, whose largest dimension 
corresponds at least approximately to the desired distance between the 
oppositely disposed cell plates 1, 4 produces liquid crystal displays of 
great plane parallelism and reproducible thickness. These particles 29, of 
which a few will be sufficient for each liquid crystal cell, do not bring 
about any visible modification of the optical properties of the display. 
Glass fibers of corresponding diameter and of a length of from 20 to 100 
.mu.m are also suitable for the same purpose. 
Two methods have given good results for the production of a cell plate 4 
coated in accordance with the invention. 
Method A 
In a first conventional screen printing process S.sub.I an electrode 
pattern, corresponding to the nature of the characters to be displayed and 
to the activating system of the display, is printed in the form of a thick 
layer of a thickness of from 5 to 15 .mu.m on a carefully cleaned rear 
cell plate 4 (FIG. 2) of glass or ceramic material, or the like. 
For this first screen printing process S.sub.I an aluminum-bronze ink 
consisting of metal parts 30 (aluminum pigments) and binder has been found 
successful. The weight ratio of metal parts 30 to binder used in this 
mixture is 1:5, (weight ratios of from 1:2 to 1:10 however, have the same 
effect). A solution consisting of one part by volume of nitrocellulose and 
one part by volume of amyl acetate is, for example, suitable as binder. 
The cell plate 4 printed in this manner is then dried in the air and then 
subjected for 30 minutes to a heating process at 530.degree. C. In this 
heating process the binder burns without leaving a residue, the metal 
parts 30 sink onto the cell plate 4, conglomerate to form a layer of a 
thickness of from 1 to 3 .mu.m, and finally sinter together to form 
electrically conductive contacts. 
The previously insulating thick layer has a resistance of about 100 to 
200.OMEGA.-cm, so that it can be used as a rear electrode for a liquid 
crystal display. 
In a second subsequent screen printing process S.sub.II the rear electrode 
3 and the inner surface of the rear cell plate 4 are printed in the visual 
region with aluminum-bronze ink to a thickness of 3 to 20 .mu.m. 
The aluminum-bronze ink mixture used in the first screen printing process 
S.sub.I also gives good results in the second printing process S.sub.2. 
Since they are used to form the reflector 9, however, the aluminum 
pigments are of a mean length of from 2 to 10 .mu.m and assume the 
function of metal parts 20 insulated against one another (FIGS. 2 and 3). 
The individual insulated metal parts 20 consisting of aluminum pigments 
are coated with an insulating oxide layer of a thickness of from 10 to 100 
A, which forms in a normal atmosphere. 
After intermediate drying at 150.degree. C. for about 5 minutes, a glass 
solder impression 6' of a thickness of 15 .mu.m is applied along the outer 
edges of the cell plate 4 in a third screen printing process S.sub.III, 
and intermediate drying is effected, likewise at 150.degree. C. for about 
5 minutes. 
The rear cell plate 4 has thus been prepared to such an extent that the 
front cell plate 1, provided with the front electrode 2 by the 
conventional thin film technique, can be mounted on the glass solder 
impression 6'. After the adjustment of the cell plates 1 and 4 in 
accordance with the shape of the electrodes 2 and 3, a glass soldering 
process, known from the production of nematic rotary cells, is effected in 
a soldering oven at a temperature of from 400 to 500.degree. C. and over a 
period of from 0.5 to 1.5 hours. 
During this glass soldering process the binder still present from the 
second screen printing process S.sub.II is burned, once again without 
leaving a residue, but without destroying the mutually insulating oxide 
layer on the aluminum pigments. 
In the soldered liquid crystal cell (FIG. 3) the reflector 9 has a sheet 
resistivity of about 10.sup.7 ohms per square. The sheet thickness of the 
reflector 9 amounts to only from 2 to 5 .mu. because the individual metal 
parts 20 now lie close against one another. 
After the liquid crystal has been introduced into the liquid crystal cell, 
the latter is sealed with a metal solder, as is also customary in the 
production of rotary cells, and is thus ready for operation. 
As already illustrated in FIG. 1, the liquid crystal layer 5 is activated 
by means of an alternating voltage source 12 applied to the electrodes 2 
and 3. As a consequence of the fusing of the aluminum pigments the back 
electrode 3 has good conductivity. The reflector 9 consisting of the metal 
parts 20 insulated against one another is on the other hand 
non-conductive. Because of the insulated metal parts 20 lying close to one 
another, however, good alternating voltage coupling is achieved in a 
direction normal to the cell plates 1, 4 to the back electrode 3 and to 
the liquid crystal layer 5, so that the properties required of the liquid 
crystal display are achieved. 
Because of its rough surface consisting of the individual insulated metal 
parts 20 the reflector 9 is diffusely divergent. This action is achieved 
without surface treatment of the cell plate 4 and without the cost such 
surface treatment would entail. 
Method B 
Method B differs from method A in that screen printing process is performed 
with palladium-silver or gold-bronze ink instead of aluminum-bronze ink. 
Also the single glass soldering process simultaneously serves as a heat 
treatment in the production of the back electrode 3 and of the reflector 
9, and is all that is required. 
The first screen printing process S.sub.I is accordingly carried out with 
precious metal pigments instead of aluminum pigments. Because of the 
higher density compared with aluminum pigments, a mixture ratio of 
precious metal pigment to binder of from 2:1 to 1:2 is necessary. If these 
precious metal pigments are in flare form, the lower ratio is already 
sufficient. This first screen printing process S.sub.I is followed by the 
screen printing processes S.sub.II and S.sub.III in the manner described 
in method A. 
Method B thus provides the advantage that the screen printing processes 
S.sub.I and S.sub.II can be carried out one after the other without a 
time-consuming intermediate heating process. This advantage is however, 
obtained at the cost of a higher price for material for the precious metal 
pigments. 
In both methods increased resistance to abrasion can be given to the 
coatings by adding from 1 to 5% of glass solder powder of a grain size of 
about 1 .mu.m in the first and second screen printing processes S.sub.I, 
S.sub.II. In addition, non-conductive particles 29 can be added to the 
second or third screen printing process S.sub.II or S.sub.III, these 
particles being rearranged by the insulated metal parts 20 after the glass 
soldering process, as shown diagrammatically in FIG. 4, and assuming the 
function of spacers between the cell plates 1 and 4. Since the particles 
29 are added in a very low concentration, since only a few particles are 
sufficient for each liquid crystal cell, and since these particles are 
non-conductive, the optical properties of the display are not disturbed. 
Non-conductive particles 29 of Al.sub.2 O.sub.3 or SiO.sub.2 have given 
good results; particles 29 of CeO.sub.2, MgO, TiO.sub.2, ZrO.sub.2 are, 
however, also utilizable for the same purpose. The largest dimension of 
the particles 29 amounts to 15 .mu.m, which corresponds to the desired 
spacing of the cell plates 1 and 4. The shape of the particles is 
immaterial; glass fibers of a length of from 20 to 100 .mu.m and a 
diameter of 12 .mu.m have also been found satisfactory for the same 
purpose. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.