JP3097498B2 - Liquid crystal display device using liquid crystal having ferroelectric phase - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device using a ferroelectric liquid crystal (including an antiferroelectric liquid crystal) having a ferroelectric phase and / or an antiferroelectric phase, and in particular, can be driven at a low voltage. And a liquid crystal display element having few alignment defects.

[0002]

2. Description of the Related Art Ferroelectric liquid crystal display devices have attracted attention because they can operate at higher speeds, have a wider viewing angle, and have memory properties, as compared with TN liquid crystal display devices and the like.

[0003] A ferroelectric liquid crystal display element is formed by sealing a ferroelectric liquid crystal between a pair of substrates provided with electrodes, and setting a polarizing plate based on the orientation direction of the liquid crystal. I have. In a ferroelectric liquid crystal, when a voltage having a sufficiently large absolute value is applied between electrodes facing each other with a liquid crystal layer interposed therebetween, an average alignment direction of liquid crystal molecules is changed to a first alignment according to the polarity of the applied voltage. One of a first alignment state (first ferroelectric phase) and a second alignment state (second ferroelectric phase) where the average alignment direction of liquid crystal molecules is the second alignment direction. It will be. By controlling this alignment state, transmission / blocking of light is controlled to display an image.

Recently, a ferroelectric liquid crystal has been developed in which the average alignment direction of liquid crystal molecules is intermediate between the first alignment direction and the second alignment direction according to the applied voltage. Also, gradation display has been attempted.

[0005] As one type of ferroelectric liquid crystal, an antiferroelectric liquid crystal having one antiferroelectric phase in addition to two ferroelectric phases has been developed, and some of them have been put to practical use.

[0006]

However, a liquid crystal display device using a liquid crystal having a ferroelectric phase has a problem that the driving voltage is high and the power consumption is large as compared with a TN liquid crystal display device or the like. There is also a disadvantage that alignment conditions are severe and alignment defects are likely to occur.

The present invention has been made in view of the above circumstances, and has as its object to provide a liquid crystal display element using a liquid crystal having a ferroelectric phase which can be driven at a low driving voltage and has few alignment defects. .

[0008]

Means for Solving the Problems] To achieve the above object, a liquid crystal display device using a liquid crystal having a ferroelectric phase of the present invention includes a first substrate on which the first electrode was formed, the first A second substrate on which a counter electrode facing the first electrode is formed; and a second substrate formed on at least one of the opposing surfaces of the first substrate and the second substrate, the sum of the thicknesses being 70 nm or less. An alignment film formed between the first and second substrates, two ferroelectric phases in which liquid crystal molecules are aligned in the first or second direction according to the polarity of the applied voltage, When no voltage is applied, the average alignment direction of the liquid crystal molecules
Orient in a direction almost parallel to the normal direction of the mectic layer
An intermediate alignment state , in which an average alignment direction of the liquid crystal molecules changes continuously between the first direction and the second direction in accordance with a change in applied voltage. When
And a liquid crystal having no alignment memory property .

[0009]

[0010]

According to the liquid crystal display device of the present invention, when no voltage is applied , the average orientation direction of the liquid crystal molecules is controlled by the liquid crystal display.
Orient in a direction almost parallel to the normal direction of the mectic layer
It has an intermediate alignment state, and has an alignment state in which the average alignment direction of the liquid crystal molecules changes continuously between the first direction and the second direction according to a change in applied voltage. ,
Since a liquid crystal having no alignment memory property is used and an alignment film having a thickness of 70 nm or less is formed on at least one substrate, the liquid crystal having a ferroelectric phase is uniformly aligned to reduce alignment defects. Can be.

Further, since the thickness of each alignment film arranged on the substrate to pair toward and 35nm or less, it is possible to drive the liquid crystal display element at a low voltage. Furthermore, the thickness is 1
0 nm or more, or the surface on the alignment film surface
Surface energy should be less than 4 dyne / vm
Thereby, a liquid crystal display element having less alignment defects can be obtained.

[0012]

First, the structure of a ferroelectric liquid crystal display device according to this embodiment will be described. FIG. 1 is a sectional view of a ferroelectric liquid crystal display device, and FIG. 2 is a plan view of a transparent substrate on which a pixel electrode and an active device of the ferroelectric liquid crystal display device are formed. This ferroelectric liquid crystal display element is of an active matrix type, and as shown in FIG. 1, a liquid crystal cell formed by sealing a liquid crystal 11 between a pair of transparent substrates (eg, glass substrates) 1 and 2. A pair of polarizing plates 1 arranged with the liquid crystal cell interposed therebetween
3 and 14.

In FIG. 1, a lower transparent substrate (hereinafter, referred to as a lower substrate) 1 has a pixel electrode 3 made of a transparent conductive material such as ITO as shown in FIGS. Are connected in a matrix.

As shown in FIG. 2, a gate line 5 is wired between rows of the pixel electrodes 3, and a data line (gradation signal line) 6 is wired between columns of the pixel electrodes 3. Each TFT
4 is connected to the corresponding gate line 5,
The drain electrodes are connected to corresponding data lines 6. The gate line 5 is connected to a row driver 21 and the data line 6 is connected to a column driver 22. The row driver 21 scans the gate line 5 by applying a gate voltage described later. On the other hand, the column driver 22 receives the image data and applies a data signal corresponding to the image data to the data line 6.

In FIG. 1, an upper transparent substrate (hereinafter, upper substrate) 2 is provided with a counter electrode 7 which is opposed to each pixel electrode 3 of the lower substrate 1 and to which a reference voltage V0 is applied. Alignment films 8 and 9 are provided on the electrode formation surfaces of the lower substrate 1 and the upper substrate 2, respectively. The alignment films 8 and 9 are made of, for example, an organic polymer compound containing polyimide as a main component. The organic polymer compound desirably has a small dipole moment. The surfaces of the alignment films 8 and 9 are subjected to an alignment treatment by rubbing in a direction 11C shown in FIG. Further, the alignment films 8 and 9 are formed to have a thickness of 10 to 35 nm in order to reduce the driving voltage and prevent alignment defects, for the reasons described below.

The lower substrate 1 and the upper substrate 2 are adhered to each other at the outer peripheral edge thereof via a frame-shaped sealing material 10. A liquid crystal 11 is sealed in a region surrounded by the substrates 1 and 2 and the sealing material 10.

In the liquid crystal 11, the helical pitch of the chiral smectic C phase is smaller than the distance between the substrates 1 and 2, and
It is a ferroelectric liquid crystal (DHF liquid crystal) having no memory of the alignment state. The liquid crystal 11 has a helical pitch of 700 nm to 400 nm or less (e.g., 400 nm) which is a wavelength in a visible light band.
(300 nm), a spontaneous polarization is large, and the cone angle is about 27 degrees to 45 degrees (preferably 27 degrees to 30 degrees). The thickness of the layer of the liquid crystal 11, that is, the gap length is uniformly maintained at about 1.5 μm by the gap material 12.

The liquid crystal 11 forms a substantially uniform layer structure with the normal of the layer having the layer structure of the chiral smectic C phase substantially oriented in the direction 11 C of the alignment treatment of the alignment film 8. It should be noted that the normal direction of the layer of the layer structure does not always coincide with the direction 11C of the alignment treatment, and is slightly shifted. Since the helical pitch of the liquid crystal 11 is smaller than the substrate pitch, the liquid crystal 11 is sealed between the substrates 1 and 2 with a helical structure. Pixel electrode 3
When a voltage having a sufficiently large absolute value is applied between the liquid crystal 11 and the counter electrode 7, the liquid crystal 11 changes to a first alignment state in which the alignment direction of the liquid crystal molecules becomes substantially the first alignment direction according to the polarity of the applied voltage. The second alignment direction in which the alignment direction of the liquid crystal molecules is substantially the second alignment direction
Is set to any one of the orientation states. Further, when a voltage whose absolute value is lower than the voltage for aligning the liquid crystal molecules in the first or second alignment state is applied between the pixel electrode 3 and the counter electrode 7, the helix of the molecular arrangement of the liquid crystal 11 is distorted, and An intermediate alignment state is obtained in which the average alignment direction is a direction between the first alignment direction and the second alignment direction.

Above and below the liquid crystal display element, a pair of polarizing plates 1 is provided.
3 and 14 are arranged. Transmission axis 13A of polarizing plate 13
Is set substantially parallel to the direction 11C of the above-described alignment processing,
The transmission axis 14A of the polarizing plate 14 is set to be orthogonal to the transmission axis 13A. Note that, in FIG.
A and 11B show the alignment directions (director directions) of the liquid crystal molecules in the first and second alignment states of the liquid crystal 11.

As shown in FIG. 3, the ferroelectric liquid crystal display device in which the polarizing plates 13 and 14 are provided has a structure in which liquid crystal molecules are first or second.
Is the highest in the first or second orientation state in which the liquid crystal molecules are oriented in the orientation directions 11A and 11B (the brightest display).
When the liquid crystal molecules are oriented in the intermediate direction 11C substantially parallel to the normal direction of the smectic phase layer, the transmittance becomes the lowest (the display is darkest).

Next, the alignment film 8 of the liquid crystal display device having the above-described structure.
The relationship between the thicknesses of the pixels 9 and 9 and the voltage applied between the pixel electrode 3 and the counter electrode 7 will be described. FIG. 4 (A) shows that the thickness of each of the alignment films 8 and 9 is 60 nm, and the applied voltage and transmittance when a triangular wave voltage having a relatively long period (about 0.1 Hz) is applied to the pixel electrode 3 and the counter electrode 7. Shows the relationship. FIG.
(B) shows the relationship between the applied voltage and the transmittance when a triangular wave voltage having a relatively long period is applied, with the thickness of each of the alignment films 8 and 9 being 30 nm. The applied voltage width (difference between the maximum applied voltage and the minimum applied voltage) for obtaining substantially the same change in transmittance is 20 V in FIG. 4A and 10 V in FIG. 4B. That is, the thickness of the alignment films 8 and 9 is set to 60 nm.
The driving voltage is reduced to about 1 /
Reduced to 2. The layer thickness of the liquid crystal 11 is about 1.5 μm,
This reduction rate of the driving voltage is much larger than the expected value of the reduction rate of the applied voltage due to the thinning of the alignment films 8 and 9.

FIG. 5 shows the relationship between the thickness of the alignment films 8 and 9 and the driving voltage. As apparent from the figure, the film thickness is 40 nm.
There is a region where the applied voltage changes discontinuously in the range of ５０50 nm. Therefore, in this embodiment, the alignment film 8
9, a thickness of 35 nm or less, for example, 30 n
Adopt m.

On the other hand, when the alignment films 8 and 9 are thinned, the members located below the alignment films 8 and 9, particularly, a material having a high surface energy, such as ITO, may cause the surface of the alignment films 8 and 9 to be damaged. The apparent surface energy γ (polar force component of the surface energy) increases. When the apparent surface energy is increased, the interaction between the alignment film and the liquid crystal molecules is increased, the alignment of the liquid crystal molecules is disturbed, and alignment defects are likely to occur.

FIG. 6A shows the state of occurrence of alignment defects when the surface energy on the alignment film surface is high (8 dynes / cm), and FIG. 6B shows the surface energy on the alignment film surface. (4 dyne / cm) shows the state of occurrence of alignment defects when it is low. In FIG. 6A, the orientation is disturbed, and a large number of minute regions through which light is transmitted despite the applied voltage of 0 are formed. Therefore, the display becomes gray. In contrast, in FIG. 6B, the orientation is stable and the display is black.

Therefore, in order to eliminate the influence of the surface energy of the film formed in the lower layer and to suppress the polar force component of the surface energy on the alignment film surface, in the present invention, the thicknesses of the alignment films 8 and 9 are set. Adopt 10 nm or more. In the case of a color liquid crystal display element, the thickness is preferably 10 nm or more in order to reduce the roughness of the alignment film surface due to the unevenness (surface roughness) of the surface of the color filter or the black mask.

As described above, according to the liquid crystal display device of this embodiment, the thickness of the alignment films 8 and 9 is set to 10 nm to 35 nm.
nm, the liquid crystal display element can be driven with a low driving voltage. In addition, the liquid crystal can be uniformly aligned, and alignment defects can be reduced.

In the above-described embodiment, an example is shown in which a DHF liquid crystal, which is a ferroelectric liquid crystal, is used as the liquid crystal 11, but the liquid crystal 11 may be an SBF liquid crystal or an antiferroelectric liquid crystal (hereinafter, referred to as AFLC).

In the AFLC, when a voltage having a sufficiently large absolute value is applied between electrodes facing each other with a liquid crystal layer interposed therebetween, the average orientation direction of liquid crystal molecules is set to the first direction in accordance with the polarity of the applied voltage.
A first alignment state (a first ferroelectric phase) in which the liquid crystal molecules have an average alignment direction in a second alignment direction (a second ferroelectric phase) in which a liquid crystal molecule has an average alignment direction in a second alignment direction. When the applied voltage is 0, the liquid crystal molecules in the first alignment direction and the liquid crystal molecules in the second alignment direction have an antiferroelectric phase in which the liquid crystal molecules are alternately arranged for each layer. . The liquid crystal display device using this AFLC can switch the display by switching the alignment state.

As the AFLC, those which can take an intermediate alignment state by the following behavior of liquid crystal molecules or a combined action thereof can be used. (1) The double helical structure drawn by the liquid crystal molecules of the smectic CA ^* phase is distorted by the interaction between the applied voltage and the spontaneous polarization of the liquid crystal molecules, whereby the average orientation direction of the liquid crystal molecules changes continuously. ) The free rotation of the liquid crystal molecules is suppressed by the applied electric field, and the liquid crystal molecules tilt in the direction perpendicular to the applied electric field, so that the average alignment direction of the liquid crystal molecules changes continuously. The effect that the average alignment direction of the liquid crystal molecules changes continuously as the molecules move along the cone by an amount corresponding to the applied voltage, and (4) one of the liquid crystal molecules in the first or second alignment state. Since the portion changes to the second or first alignment state according to the applied voltage, the ratio of the liquid crystal molecules in the first alignment state to the liquid crystal molecules in the second alignment state changes, so that the average of the liquid crystal molecules is changed. Action in which the general orientation direction changes continuously. By using this intermediate orientation state, gradation display becomes possible.

Even when AFLC is used as the liquid crystal 11, the driving voltage is reduced by setting the thickness of the alignment films 8 and 9 to 10 to 35 nm, and the polar force of the surface energy on the alignment film surface is reduced. A liquid crystal display element with few alignment defects can be obtained by suppressing the components.

In the above description, the alignment films 8 and 9 are arranged on both the substrate 1 and the substrate 2, but it is also possible to arrange the alignment film on only one of the substrates 1 and 2, for example.
The ferroelectric liquid crystal and the antiferroelectric liquid crystal have a layer structure of a smectic phase, and are aligned so that the normal direction of the layer substantially matches the direction of the alignment treatment of the alignment film. in this case,
The thickness of this alignment film is 10 nm to 35 or 40 nm. With such a thickness, low-voltage driving can be performed in the same manner as described above, and a liquid crystal display element with few alignment defects can be obtained.

The transmission axis 13A of the polarizing plate 13 and the transmission axis 14A of the polarizing plate 14 may be arranged in parallel. Further, the absorption axis of one polarizing plate 14 is made to coincide with the direction 11C between the first orientation direction 11A and the second orientation direction 11B, and the absorption axis of the other polarizing plate 13 is adjusted to the absorption axis of the one polarizing plate 14. May be orthogonal to. Further, the first orientation direction 1
1A and one of the second orientation directions 11B and the deflection plates 13 and 14
May be parallel or perpendicular to one another, and the optical axis of the other deflecting plate may be parallel or perpendicular to the optical axis of one deflecting plate. Further, the present invention can be applied to an active matrix liquid crystal display element using an MIM as an active element, and can also be applied to a simple matrix type liquid crystal display element.

[0033]

As described above, according to the present invention, the average alignment direction of the liquid crystal molecules is continuously changed according to the change of the applied voltage , and the memory of the alignment state is obtained.
Liquid crystal having a ferroelectric phase with no
Since an alignment film having a thickness of 71 nm or less is used for the encapsulated liquid crystal display element, a uniform alignment state with reduced alignment defects can be obtained, and the liquid crystal display element can be driven with a low driving voltage.

[Brief description of the drawings]

FIG. 1 is a sectional view showing the structure of a liquid crystal display device according to one embodiment of the present invention.

FIG. 2 is a plan view showing a configuration of a lower substrate of the liquid crystal display element shown in FIG.

FIG. 3 is a plan view showing directions of transmission axes of upper and lower polarizing plates and alignment directions of liquid crystal molecules.

4A is a graph showing the relationship between the applied voltage and the transmittance when the thickness of the alignment film is 60 nm, and FIG. 4B is the graph showing the applied voltage and the transmittance when the thickness of the alignment film is 30 nm. It is a graph which shows the relationship of.

FIG. 5 is a graph showing a relationship between an alignment film thickness and an applied voltage.

6A and 6B are diagrams for explaining the state of occurrence of alignment defects in liquid crystal molecules. FIG. 6A is an enlarged view showing the state of generation of alignment defects when the surface energy of the alignment film is large, and FIG.
FIG. 4 is an enlarged view showing an alignment defect generation state when the surface energy of the alignment film is small.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Transparent substrate, 2 ... Transparent substrate, 3 ... Pixel electrode, 4 ...
TFT, 5 ... gate line, 6 ... data line, 7 ...
・ Counter electrode, 8 ・ ・ ・ Alignment film, 9 ・ ・ ・ Alignment film, 10 ・ ・ ・ Seal material, 11 ・ ・ ・ Liquid crystal, 12 ・ ・ ・ Gap material, 13 ・ ・ ・ Polarizer, 14 ・ ・ ・ Polarized light Board, 21: Row driver, 22: Column driver

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-29119 (JP, A) JP-A-3-89319 (JP, A) JP-A-2-222930 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G02F 1/1337 G02F 1/141

Claims (4)

(57) [Claims] A first substrate on which a first electrode is formed; a second substrate on which a counter electrode facing the first electrode is formed; a first substrate and the second substrate An alignment film formed on at least one of the opposing surfaces and having a total thickness of 70 nm or less, sealed between the first and second substrates, according to the polarity of the applied voltage. The two ferroelectric phases in which the liquid crystal molecules are oriented in the first or second direction, and the average orientation direction of the liquid crystal molecules when no voltage is applied is the normal to the smectic layer of the liquid crystal.
An intermediate alignment state in which the liquid crystal molecules are aligned in a direction substantially parallel to the direction, and an average alignment direction of the liquid crystal molecules is changed between the first direction and the second direction according to a change in applied voltage. so
Takes a continuously changing orientation state, and the memory of the orientation state
The liquid crystal display device using a liquid crystal having a ferroelectric phase, characterized in that it includes a liquid crystal having no sex, the. 2. The method according to claim 1, wherein the alignment film includes the first substrate and the second substrate.
2. The liquid crystal display device using a liquid crystal having a ferroelectric phase according to claim 1, wherein each of the opposite surfaces of the substrate is formed to a thickness of 35 nm or less. 3. The liquid crystal display device using a liquid crystal having a ferroelectric phase according to claim 1, wherein the alignment film is formed to a thickness of 10 nm or more. 4. The alignment film has 4 dyn on the surface of the alignment film.
The liquid crystal display device using a liquid crystal having a ferroelectric phase according to claim 1, having a surface energy of e / cm or less.