Title: Liquid crystal display element
Abstract: A liquid crystal display element includes array and counter substrates provided opposite to each other. The array substrate is covered with a color filter layer. Pixel electrodes are provided in a matrix form on the color filter layer. The surface of the counter substrate is provided with a common electrode. Alignment films are coated on the pixel and common electrodes. A gap defined between the array and counter substrates are filled with a liquid crystal material to form a liquid crystal layer. The alignment films are processed to have surface energy within the range from 51 to 60 dyn/cm. Such surface energy substantially prevents an image-sticking phenomenon of the liquid crystal display element caused by impurities dissolved into a liquid crystal layer and white or black turbid spots caused by hydrolysis of the alignment films by moisture in the liquid crystal layer.
Patent Number: 6,881,455 Issued on 04/19/2005 to Fukuoka, et al.
| Inventors:
|
Fukuoka; Nobuko (Saitama-Ken, JP);
Yamamoto; Takeshi (Saitama-Ken, JP)
|
| Assignee:
|
Kabushiki Kaisha Toshiba (Tokyo, JP)
|
| Appl. No.:
|
050038 |
| Filed:
|
January 17, 2002 |
Foreign Application Priority Data
| Jan 25, 2001[JP] | P2001-017124 |
| Current U.S. Class: |
428/1.26; 428/1.2; 349/123 |
| Intern'l Class: |
G02F 001//13.37 |
| Field of Search: |
428/1.1,1.2,1.26-1.28,1.3,1.31,1.5
349/123,106
|
References Cited [Referenced By]
U.S. Patent Documents
| 4593977 | Jun., 1986 | Takamatsu et al. | 349/162.
|
| 5190794 | Mar., 1993 | Yoshino et al. | 427/162.
|
| 5608033 | Mar., 1997 | Nihira et al. | 528/353.
|
| 5629056 | May., 1997 | Koike et al. | 428/1.
|
| Foreign Patent Documents |
| 11-119225 | Apr., 1999 | JP.
| |
Primary Examiner: Pyon; Harold
Assistant Examiner: Hon; Sow-Fun
Attorney, Agent or Firm: Pillsbury Winthrop, LLP
Claims
What we claim is:
1. A liquid crystal display element comprising:
a circuit array substrate having pixel electrodes and a resin layer
interposed therebetween;
a counter substrate having a common electrode;
a liquid crystal composition charged in a gap between said circuit array
substrate and said counter substrate; and
first and second polyimide alignment films formed on said pixel electrodes
and said common electrode, respectively,
said first and second polyimide alignment films which have a surface energy
of no less than 51 dyn/cm to prevent an image-sticking phenomenon caused
by said liquid crystal composition and impurities contained in said resin
layer of said circuit array substrate through said first and second
polyimide alignment films, and a surface energy of no more than 60 dyn/cm
to prevent white or black turbid spots caused by impurities contained in
said liquid crystal component.
2. A liquid crystal display element according to claim 1, wherein said
resin layer is a color filter layer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display element and, more
particularly, to a liquid crystal display element to display a high
quality image by substantially preventing deterioration in the display
dignity due to uneven display, white or black turbid spots.
2. Description of the Related Art
A color liquid crystal display element has a color filter layer made of a
resin layer coated on either of an array substrate. The Array substrate
includes a plurality of pixel electrodes connected to a driving element.
Another substrate provided opposite to the array substrate is provided
with an opposite electrode. A gap defined between the two substrates is
charged with a liquid crystal composition. Conventionally, alignment films
are formed on the surfaces of the substrates, respectively, in contact
with the liquid crystal composition to orientate or align liquid crystal
molecules of the liquid crystal composition in predetermined directions.
However, since the resin layer used for the color filter layer in such a
liquid crystal display element contains impurities, the impurities are
dissolved in the liquid crystal layer to cause an image-sticking
phenomenon and defective display so that the display dignity is lowered
remarkably. On the other hand, since the alignment films of the liquid
crystal display is in direct contact with the liquid crystal layer, the
alignment layers are hydrolyzed due to moisture contained in the liquid
crystal layer or moisture soaking from the outside. Such hydrolysis causes
white or black turbid spots and defects on the image display so that the
display dignity is lowered too.
SUMMARY OF THE INVENTION
A first object of the present invention is to solve the aforementioned
problems.
A second object of the present invention is to provide a high display
dignity liquid crystal display element.
A third object of the present invention is to provide a liquid crystal
display element to prevent defective display due to elution of impurities
into the liquid crystal layer and inclusion of moisture therein.
One aspect of a liquid crystal display element in accordance with the
present invention, as means for solving the aforementioned problems,
includes a pair of substrates provided opposite to each other. An
electrode is disposed on either of the substrates. A resin layer is formed
on the surface of at least either of the pair of substrates. Alignment
films are coated over the surfaces of the substrates. The alignment films
each have surface energy of 51 to 60 dyn/cm. A liquid crystal composition
charged in the gap between the pair of substrates with the electrode and
the alignment films provided.
With the aforementioned surface energy of the alignment films, the present
invention can provide a highly reliable liquid crystal display element
with good display dignity because the present invention prevents
impurities from the resin layer from concentrating in predetermined parts
of the alignment films so as to suppress an image-sticking and defects,
and because the present invention also prevents adhesion of moisture to
the alignment films to suppress white or black turbid spots and other
defective display.
The above-stated and other objects and advantages of the invention will
become apparent from the following description when taken with the
accompanying drawings. It will be understood, however, that the drawings
are for purposes of illustration and are not to be construed as defining
the scope or limit of the invention, reference being had for the latter
purpose to the claims appended hereto.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing the principle of the present invention with
respect to the occurrence of defective display depending on the surface
energy of alignment films;
FIGS. 2(a) and 2(b) schematically illustrate a mechanism of a
image-sticking phenomenon;
FIG. 2(a) schematically illustrates the suspension condition of impurities
before application of voltage;
FIG. 2(b) schematically illustrates the concentration of impurities on the
voltage applied part;
FIG. 3 is an illustration showing the trapping condition of impurities due
to an alignment film with high surface energy in accordance with the
present invention.
FIG. 4 shows schematically a cross sectional view of a liquid crystal
display element in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Firstly, the principle of the present invention will be described. A
reliability test has been carried out to analyze causes of defective
display due to impurities and/or moisture in a liquid crystal display
element provided with a color filter. For that purpose the liquid crystal
display element has displayed a black-and-white checker image 10 mm square
on its screen for 1000 hours. There has been investigated the relationship
between the surface energy of polyimide alignment films and defective
display such as an image-sticking phenomenon and white and black turbid
spots. The test results are shown in FIG. 1.
Namely, where the surface energy of the polyimide alignment film has been
50 dyn/cm or less, pixels of the liquid crystal display element have been
incapable of displaying due to an image-sticking phenomenon before the
display time has reached 1000 hours and the display dignity has been
lowered remarkably. On the other hand, where the surface energy of the
polyimide alignment film has been more than 60 dyn/cm, white turbid and
black spots have appeared on the display of the liquid crystal display
element before the display time has reached 1000 hours and the display
dignity has been lowered remarkably, too.
The analyses of defective display mechanism are as follows:
(1) Where the color filter layer is provided in the liquid crystal display
element, impurities contained in the color filter layer are dissolved in
the liquid crystal layer.
(2) Where the surface energy of the polyimide alignment films formed on the
liquid crystal display element is low, impurities 1 dissolved from the
color filter layer are dispersed and suspended in a liquid crystal layer 2
before voltage application as shown in FIG. 2(a).
(3) When a voltage is applied at the time of image display, impurities are
locally concentrated on the alignment films 3 as shown in FIG. 2(b) and
causes an image-sticking phenomenon.
Then, where the surface energy of the alignment films is increased for the
purpose of preventing the image-sticking phenomenon, the impurities 1
dissolved in the liquid crystal layer 2 are trapped on the surfaces of the
alignment films 3 almost uniformly as shown in FIG. 3. As a result, no
impurities are concentrated locally even at the time of voltage
application and, thus, no image-sticking phenomenon is caused. However,
the alignment films 3 with such high surface energy also trap moisture
contained in the liquid crystal layer 2 or soaking from the outside. Thus,
the surfaces of the alignment films 3 are hydrolyzed by such trapped
moisture. Where the surface energy of the alignment films is more than 60
dyn/cm as shown in FIG. 1, white or black turbid spots appear during a
long period of display time.
Therefore, the present invention controls the surface energy of the
alignment films within the range from 51 to 60 dyn/cm to prevent from
causing defective display due to an image-sticking phenomenon and white or
black turbid spots.
With respect to the surface energy of the alignment film, a measured value
is obtained by a method of dropping a waterdrop on the alignment film and
measuring a contact angle between the waterdrop and the alignment film.
Next, on the basis of the aforementioned principle, the present invention
will be explained with reference to the accompanying drawing. FIG. 4 shows
schematically a cross section of a liquid crystal display element 10. The
liquid crystal display element 10 includes an array substrate 11 and a
counter substrate 12 provided opposite to the array substrate via
ball-like spacers 13. The periphery of the liquid crystal display element
is bonded and sealed by a sealing agent 14 made of a thermosetting epoxy
series adhesive ES-550 (made by Mitsui Toatsu Kagaku, Ltd.). A liquid
crystal composition 16 is injected into a gap defined between the
substrates 11 and 12 from an injection port of the periphery and then the
injection port is sealed. Polarization plates 11a and 12a are additionally
put to the array and counter substrates 11 and 12 formed on the glass 24,
respectively.
The array substrate 11 has a thin film transistor (TFT) element 18 for
driving the liquid crystal. The TFT element 18 is provided in the
neighborhood of the intersection point of a scanning line and a signal
line (not shown in the drawing) formed on a glass substrate 17. A color
filter layer 20 made of color layers 20a, 20b, and 20c of R (red), G
(green), and B (blue) are provided adjacent to, and partially on the TFT
element 18 in a stripe shape.
A plurality of pixel electrodes 21 are patterned in a matrix form on the
color filter layer 20. The pixel electrodes 21 are connected to a source
electrode (not shown in the drawing) of the TFT element 18 via through
holes 22 formed in the color filter layer 20. Further, an alignment film
23 with surface energy of 53.5 dyn/cm is formed on the pixel electrodes
21.
A common electrode 26 made of an indium tin oxide (ITO) film is formed on
the counter substrate 12. An alignment film 27 with surface energy of 53.5
dyn/cm is formed on the common electrode 26.
Next, a manufacturing method of the liquid crystal display element 10 will
be described hereinafter. With respect to the array substrate 11, the TFT
element 18 is formed on the glass substrate 17 by repeating a patterning
in an ordinary photolithographic step. As a color layer material, an
ultraviolet curing (setting) acrylic resin resist CR-2000 (made by Fuji
Hunt Technology, Ltd.) with a red pigment dispersed is coated overall on
the substrate 17 by a spinner. Light with a wave length of 356 nm is
irradiated onto the part to be colored red with 100 mJ/cm2 via a
photo-mask. Then, the part is developed in a solution of 1% of KOH for 10
seconds so that a color R (red)) layer 20a is formed.
Color G (green) and B (blue) layers 20b and 20c are formed In the same way
as set forth above. As color layer materials of the color G (green) and B
(blue) layers 20b and 20c, ultraviolet curing acrylic resin resists
CG-2000 and CB-2000 with green and blue pigments dispersed, respectively,
manufactured by Fuji Hunt Technology, Ltd. are used. After the color
filter layer 20 is formed, an ITO film with a thickness of 1500 .ANG. is
deposed by a sputtering process and its patterning is carried out by a
photolithographic step so that the pixel electrodes 21 can be provided on
the color filter layer 20 in a matrix form. Further, the alignment film 23
is coated on the pixel electrodes.
On the other hand, with respect to the counter substrate 12, an ITO film
with a thickness of 1500 .ANG. is formed on the glass substrate 24 by such
a sputtering process, the common electrode 26 is formed, and then an
alignment film 27 is coated on the common electrode 26.
Next, the alignment films 23 and 27 will be described. A polyimide
alignment film material of SE-7492 (made by Nissan Kagaku Kogyo, Ltd.) is
dropped on the pixel electrodes 21 on the side of the array substrate 11
and the common electrode 26 on the side of the counter substrate 12 to
carry out spin coatings, respectively. The overall pixel electrodes 21 and
26 are coated with the polyimide alignment films with a thickness of 500
.ANG. by such a spin coating, respectively. Further, the alignment films
23 and 27 are baked at 180.degree. C. for one hour to set their surfaces
at energy of 53.5 dyn/cm.
The surface energy varies with the material of alignment films and process
conditions. Where, for example, the baking temperature is high and the
baking time is long, the surface energy increases. In this embodiment, the
surface energy is controlled by primarily selecting the material of
alignment films and process conditions.
Thereafter, the alignment films 23 and 27 are orientated or aligned by the
well known method of rubbing. The sealing agent 14 is coated along the
periphery of the alignment film 23 of the array substrate 11. A transfer
material (not shown in the drawing) for applying a voltage to the common
electrode 26 substrate 11 is formed around the sealing agent 14. After the
ball-like spacers 13 are spread on the array substrate 12, the array
substrate 11 and the counter substrate 12 are provided opposite to each
other so as to set the rubbing direction of each of the alignment films 23
and 27 at 90.degree.. The sealing agent 14 is, then, heated and cured.
Thus, liquid crystal cells are formed.
Next, a liquid crystal composition of 16ZLI-1565 (made by E. Merk GmbH.)
with chiral material of 0.1 wt % added is injected into the gap of the
liquid crystal cells through the injection port. Then, the injection port
(not shown in the drawing) is sealed by ultraviolet curing resin. The
polarization plates 11a and 12a are put to the array substrate 11 and the
counter substrate 12 so that the liquid crystal display element 10 is
completed.
A continuous image display test has been made for the liquid crystal
display element 10 manufactured in this way at a high temperature of
50.degree. C. and high humidity of 80% for 1000 hours by using a
black-and-white checker image 10 mm square. As a result, neither
image-sticking phenomenon nor white or black turbid spots have appeared,
and a good quality display image has been obtained.
With such a structure of the liquid crystal display element, since the
surface energy of the alignment films 23 and 27 is high, e.g., 53.5
dyn/cm, impurities dissolved into the liquid crystal composition 16 from
the color filter layer 22 are trapped by the alignment films 23 and 27
almost uniformly. As a result, a image-sticking or seizing phenomenon can
be prevented regardless of image display for a long period of time.
Further, since the surface energy of the alignment films 23 and 27 is not
too high and moisture in the liquid crystal composition 16 is not trapped,
the alignment films 23 and 27 are not hydrolyzed, even under the condition
of a high temperature and high humidity, and white or black turbid spots
can be prevented. Therefore, the liquid crystal display element 10,
although the color filter layer 22 includes impurities, such impurities do
not adversely affect the display image quality. The defective display due
to a image-sticking or seizing phenomenon and white or black turbid spots
are substantially prevented, and high display dignity can be obtained.
Comparison Example 1
Further, comparison example 1 will be described. Although the alignment
films 23 and 27 have the surface energy of 53.5 dyn/cm in the
aforementioned embodiment, alignment films in comparison example have
lower surface energy of 50.4 dyn/cm, provided, however, that the pixel and
common electrodes and the other conditions are exactly the same as those
of the aforementioned embodiment. The alignment films with the surface
energy of 50.4 dyn/cm in comparison example 1 are obtained by coating a
polyimide alignment film material of SE-7492 (made by Nissan Kagaku Kogyo,
Ltd.) on the surfaces of the pixel and common electrodes and then by
baking them at 220.degree. C. for one hour.
A continuous image display test has been carried out for the liquid crystal
display element of the comparison example 1 (the alignment films having
surface energy of 50.4 dyn/cm) at high temperature of 50.degree. C. and
humidity of 80% for 1000 hours by using a black-and-white checker image 10
mm square in the same way as for the aforementioned embodiment. Since the
surface energy of the alignment films is lower than the embodiment, the
display has been defective due to an image-sticking phenomenon after about
110 hours and the display dignity has been extremely lowered.
Comparison Example 2
Next, comparison example 2 will be described. In comparison example 2,
alignment films have high surface energy of 62 dyn/cm, the pixel and
common electrodes and the other conditions are exactly the same as those
of the aforementioned embodiment. The alignment films having surface
energy of 62 dyn/cm in comparison example 2 are obtained by coating
polyimide alignment film materials of SE-7492 (made by Nissan Kagaku
Kogyo, Ltd.), on the surfaces of the pixel and common electrodes and then
by baking them at 150.degree. C. for one hour.
When a continuous image display test has been carried out for the liquid
crystal display element (the alignment films having surface energy of 62
dyn/cm) of comparison example 2 at high temperature of 50.degree. C. and
humidity of 80 for 1000 hours by using a black-and-white checker image 10
mm square in the same way as for the aforementioned embodiment. Although
no Image-sticking phenomenon has appeared, since the surface energy of the
alignment films is too high, moisture is much trapped in the injection
port where moisture enters easily and the alignment films are hydrolyzed
by this moisture. White or black turbid spots have been occurred at the
injection port of the liquid crystal composition within about 700 hours.
As a result, the display dignity has been extremely lowered due to
defective display.
The present invention is not limited to the aforementioned embodiments and
my be changed or modified so long as the substance is not deviated
therefrom. For example, where the surface energy is within the range from
51 to 60 dyn/cm, the material of alignment films is not limited and the
manufacturing conditions such as the baking temperature and time may be
optimized.
Further, the structure of a liquid crystal display element may be also
optimized and columnar spacers, for example, may be provided on either of
the array and counter substrates. In this case, the same resin layer as
that used for the color layer may be utilized as a material of spacers. A
transparent or black material to separate one color layer from the the may
be used. Further, the color layer may be provided on the counter substrate
side. However, where the color filter layer is provided on the counter
substrate side, the color filter layer is covered with the common
electrode and the dissolution of impurities on the liquid crystal side is
reduced considerably. As described in the aforementioned embodiment,
however, where the color filter layer is provided on the array substrate
side, impurities of the color filter layer are dissolved much on the
liquid crystal layer side from the gaps between the patterned pixel
electrodes and an image-sticking phenomenon appears easily. As set forth
above, the conditions of alignment films in accordance with the present
invention can effectively prevent such an image-sticking phenomenon in the
even worse case like the latter.
As explained above, according to the present invention, where the surface
energy of alignment films is controlled within a proper range, an
image-sticking phenomenon due to impurities dissolved into the liquid
crystal layer is substantially prevented, and white or black turbid spots
of images due to a hydrolysis of the alignment films by moisture is also
substantially reduced. As a result, since defective display is eventually
prevented, there is provided a liquid crystal display element with good
display dignity.
*
