JP3723380B2 - Liquid crystal display device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a liquid crystal display (LCD), and more particularly to an LCD using an OCB (Optical Controlled Birefringence) mode having a high liquid crystal driving speed.
[0002]
[Prior art]
In order to improve the moving image reproduction capability of the LCD and to put the field sequential LCD (FS-LCD) into practical use, an LCD having a higher response speed is required.
[0003]
The response speed of the LCD is the time required for the liquid crystal to change to the driving state after the driving voltage is applied to the liquid crystal. When a voltage is applied, the liquid crystal is aligned in a predetermined direction and is in a driving state. A certain time is required until the liquid crystal molecules are aligned in the alignment direction, and this time is the response speed. When the response speed is slow, for example, when a moving image is displayed, the previous screen remains, so that the LCD has particularly low moving image display characteristics. If the LCD uses liquid crystal having a faster response speed, the moving image can be displayed more smoothly.
[0004]
The FS-LCD is a method of displaying colors by switching light of three primary colors quickly and displaying images of the respective colors alternately on one pixel. The liquid crystal used in the FS-LCD is required to have a significantly faster response speed than the liquid crystal used in the color filter type LCD because of its operating principle, and is expected to be put into practical use.
[0005]
By the way, OCB mode liquid crystal has been known as a liquid crystal having a high response speed. The OCB mode is a name of a state where the liquid crystal performs bend alignment. FIG. 9 shows an LCD in which first and second electrodes 53 and 54 and alignment films 55 and 56 are formed on transparent substrates 51 and 52 made of opposing glass and the like, and a liquid crystal layer 57 is enclosed therebetween. Yes. The liquid crystal layer 57 is nematic liquid crystal, and the alignment films 55 and 56 are rubbed in a substantially parallel direction with a pretilt angle so as to face each other. An optical compensation layer (not shown) is placed on this and visualized. FIG. 9A shows a state where no voltage is applied to the electrodes 53 and 54. The liquid crystal molecules 57a are aligned in the rubbing direction (the direction parallel to the paper surface), and the liquid crystal molecules 57a in the vicinity of the alignment films 55 and 56 are oriented in the pretilt angle direction. FIG. 9B shows a state in which a driving voltage of 5 V, for example, is applied to the electrode 53. Although the liquid crystal stands by the applied driving voltage, the liquid crystal molecules are tilted at the center of the liquid crystal layer 57. The orientation in the state of FIG. 9B is called splay orientation. FIG. 9C shows a state in which the alignment state of the liquid crystal 57a has changed. The orientation in the state of FIG. 9C is called bend orientation. In the bend alignment, unlike the splay alignment, the liquid crystal molecules at the center of the liquid crystal layer 57 are also standing. Splay alignment and bend alignment are reversible phase transitions, and the transition from splay alignment to bend alignment is called bend transition.
[0006]
One LCD using bend alignment is the OCB mode. This uses a bend-aligned liquid crystal and a biaxial optical compensation layer. Since the bend orientation has a faster response speed than the liquid crystal mode used in the conventional TN or STN type LCD, the LCD using the OCB mode is suitable for moving image display and FS-LCD.
[0007]
[Problems to be solved by the invention]
When an LCD is to be manufactured using the OCB mode, the response speed changes significantly between the splay alignment before the bend transition and the bend alignment. Therefore, it is necessary to bend the liquid crystal in the LCD cell. .
[0008]
However, there are still many unclear points regarding the physical mechanism of bend transition, and there are still many issues to be solved.
[0009]
Accordingly, an object of the present invention is to obtain an LCD having a high response speed by surely bend-transitioning the liquid crystal in the LCD using the OCB mode.
[0010]
[Means for Solving the Problems]
Based on the assumption that the factor preventing the bend alignment diffusion is the region where there is no electric field between the pixels, we made a prototype of an OCB mode LCD panel with a plurality of pixel intervals. It came to make the following invention.
[0011]
That is, the present invention is sealed between a plurality of first electrodes provided on a first transparent substrate, a second electrode provided on a second transparent substrate, and the first and second substrates. In the liquid crystal display device including the liquid crystal layer having the OCB mode, the distance between the plurality of first electrodes is 5 μm or less, or 2 μm or less.
[0012]
In addition, a plurality of first electrodes provided on the first transparent substrate, a second electrode provided on the second transparent substrate, and an OCB mode sealed between the first and second substrates In the liquid crystal display device including the liquid crystal layer having the inter-pixel electrode, the inter-pixel electrode is provided between the plurality of first electrodes, and the distance between the inter-pixel electrode and the plurality of first electrodes is 5 μm or less, or 2 μm. The following is a liquid crystal display device.
[0013]
Further, the first electrode is a pixel electrode formed independently for each pixel, a thin film transistor is formed in each pixel electrode, and a signal line and a gate line connected to the thin film transistor are provided between the pixel electrodes. The inter-pixel electrode is a signal line or / and a gate line.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a diagram showing a change in Gibbs energy with respect to a voltage applied to a liquid crystal for explaining the basic principle of the transfer method of the present invention. The solid line shows the energy of the Gibbs with the splay orientation and the alternate long and short dash line with the bend orientation. FIG. 2 shows the Gibbs energy of splay alignment and bend alignment when the applied voltage is a voltage V1 higher than the threshold voltage Vc.
[0015]
Gibbs energy is a state energy that changes depending on the alignment state of the liquid crystal, and it can be said that an alignment state with a low state energy is a more stable state. In both splay alignment and bend alignment, the energy decreases as the applied voltage increases. As a result, the liquid crystal molecules that are stable in the pretilt direction are driven when no voltage is applied, and the liquid crystal molecules are aligned in either the splay or the bend. The Gibbs energy is lower in the splay alignment when the applied voltage is lower than the threshold voltage Vc, and lower in the bend alignment beyond Vc. Since the substance has the property of being stable at a lower state energy, the splay alignment is more stable and the liquid crystal molecules are splayed while the applied voltage is lower than the threshold voltage Vc. That is, the splay alignment is the initial alignment state of the liquid crystal. When the applied voltage is higher than the threshold voltage Vc, the bend orientation is more stable.
[0016]
The OCB mode uses a bend-aligned liquid crystal, but even if the applied voltage is simply increased to a voltage higher than the threshold voltage Vc, for example, V1, the probability of transition to bend alignment (bend transition) is low. This is considered to be because a potential barrier PB exists between the splay alignment and the bend alignment as shown in FIG. In other words, the applied voltage V1 cannot bend because the voltage is not sufficient to exceed the potential barrier PB of the delta E. The liquid crystal once bend-transitioned across the potential barrier PB maintains a bend alignment with a lower Gibbs energy while the applied voltage is higher than the threshold voltage Vc.
[0017]
Now, referring to FIG. 1, when the applied voltage is further increased than V1, the difference in energy between the bend orientation and the splay orientation Gibbs increases more and more. Therefore, according to the present invention, before the display on the LCD, a transition voltage sufficiently higher than the threshold voltage Vc is applied in advance to first bend the liquid crystal in the cell.
[0018]
FIG. 3 is a graph obtained by paying attention to one pixel of the LCD and measuring the time until the liquid crystal between the electrodes continues to bend when a constant voltage is continuously applied between the electrodes. When a voltage of 10 V was continuously applied between the electrodes, the liquid crystal between the electrodes bent and transitioned in about 20 seconds. When the applied voltage was increased, the time required for the bend transition was rapidly shortened. When 18 V was applied, the bend transition occurred in about 2 seconds. In this way, by applying a voltage sufficiently higher than the threshold voltage Vc (in this specification, this is referred to as a transition voltage), the liquid crystal is bend-transitioned to be in the OCB mode.
[0019]
FIG. 4 is a plan view showing the transition from the splay alignment to the bend alignment in the simple matrix LCD. Liquid crystal is sealed in a cell between first and second transparent substrates made of opposing glass, and a plurality of first electrodes 1 extending in a stripe shape in the lateral direction on the first transparent substrate, A plurality of second electrodes 2 extending in a stripe shape in the vertical direction are formed on the second transparent substrate. The region where the first electrode and the second electrode overlap is a pixel region 3 in which an electric field is formed by the voltage applied to the first and second electrodes, and the liquid crystal is aligned here.
[0020]
In FIG. 4 (a), some black dots 4 shown in the cell are transfer factors that trigger bend transfer. The bend transition occurs with this transfer factor 4 as a starting point, and the bend transition diffuses radially around this. FIG. 4B shows this state. In the figure, the hatched region 5 is a region where bend transition has occurred and expands with time centering on the transition factor 4.
[0021]
However, as shown in FIG. 4C, a phenomenon was observed in which the bend transition does not diffuse further after diffusing into the pixel region.
[0022]
The bend transition rate, which is the probability that the bend transition has occurred in the adjacent pixel region, is defined as the bend transition rate = observation point at which the bend orientation has transitioned to the adjacent pixel region / all observation points. FIG. The change of the bend transition rate with respect to the transition voltage when changed to 2 μm, 5 μm, and 11 μm is shown. The diamond mark indicates 2 μm, the square mark indicates 5 μm, and the triangle mark indicates 11 μm.
[0023]
First, the case where the pixel interval indicated by the solid line is 2 μm will be considered. When the applied voltage is 5 V or less, the transition rate is zero. As the applied voltage is increased, the transition rate increases from about 6 V, and when about 8 V is applied, the transition rate is 1, that is, the bend alignment is reliably transferred to the adjacent pixel region.
[0024]
FIG. 6 is a plan view showing the state of the bend transition at this time. The striped first electrode 1 and second electrode 2 are formed in the same manner as in FIG. 4, but the pixel interval d is 2 μm. FIG. 6A shows the same state as FIG. 4A, and the transposable factor 4 is randomly generated. In FIG. 6B, the bend transition diffuses around the transfer factor 4, and the bend transition region 5 ′ spreads over the adjacent pixel region 3. Then, as shown in FIG. 6C, the bend transition region 6 diffuses over the entire cell surface. If the bend transition rate is high, the bend transition region can be diffused over the entire surface. If the bend transition diffuses over the entire surface, the response speed is evenly increased over the entire LCD.
[0025]
Next, in the case of a pixel interval of 5 μm indicated by square marks in FIG. 5, the transfer rate is still 0 at 8V. When the applied voltage is about 9V, the transition rate increases, and when the applied voltage is about 11V, the transition rate is 1. Compared to the case where the pixel interval is 2 μm, in the case of 5 μm where the interval is wider, a higher transfer voltage is required to increase the transfer rate. Furthermore, in the case of an interval of 11 μm indicated by triangular marks, the transition rate was still 0 even when the applied voltage was 10V. If a higher voltage is applied, the transition rate is expected to increase at a certain applied voltage as in the case of 2 μm or 5 μm, but the pixel voltage applied to the LCD is generally about 10V. Accordingly, the distance between the electrodes needs to be 5 μm or less, preferably 2 μm or less. Hereinafter, the distance between the electrodes where the bend transition occurs is referred to as a transition distance.
[0026]
From the above results,
-Bend transition between adjacent pixels is easier to transition as the applied voltage applied between the electrodes is higher,
・ Between electrodes without electrodes, it becomes a barrier to bend transition,
・ The narrower the distance between the electrodes, the easier the bend transition between the pixel areas.
It can be seen that the inter-pixel distance may be not more than the transition distance, that is, not more than 5 μm, preferably not more than 2 μm.
[0027]
The above-mentioned point can be similarly applied not only to the simple matrix LCD shown in FIG. 6 but also to an active matrix LCD shown in FIG. FIG. 7A is a plan view of the active matrix LCD, and FIG. 7B is a cross-sectional view of the portion between the pixel electrodes 15. A plurality of data lines 12 are formed on the first transparent substrate 11, and gate lines 13 are formed on the data lines 12 via an insulating film (not shown). On the gate line 13, a pixel electrode 15 is formed for each pixel via an insulating film 14 that is a planarizing film, and an alignment film 17 is formed thereon. On the second transparent substrate 18 disposed to face the first transparent substrate 11, a common electrode 19 is formed to face the plurality of pixel electrodes 15, and an alignment film 20 is formed thereon. . Liquid crystal 21 is sealed between the first and second substrates. A storage capacitor electrode (not shown) is connected to the pixel electrode 15.
[0028]
Now, assuming that the interval between the pixel electrodes 15 is d, the data lines 12 and the gate lines 13 are arranged between the pixel electrodes 15, and the withstand voltage between the pixel electrodes 15 needs to be secured. Is less than or equal to the transition distance. If a transition voltage is applied only to the pixel electrode 15 in a situation where the distance d is larger than the transition distance, the region between the pixel electrodes 15 becomes a barrier for bend transition, and a pixel that does not bend may be generated. Therefore, when applying the transition voltage to the first substrate 11 side, the transition voltage is applied not only to the pixel electrode 15 but also to all the electrodes formed on the first substrate, such as the data line 12, the gate line 13, and the auxiliary capacitance electrode. Apply voltage. By applying a transition voltage to the data line 12 and the gate line 13, the liquid crystal between the pixel electrodes 15 can also bend. However, on the other hand, the wiring for applying the transition voltage becomes complicated, and the transition voltage is also applied to the gate electrode, which may cause breakdown of the thin film transistor 16.
[0029]
Therefore, when a transition voltage is applied to the active matrix LCD, the transition voltage is preferably applied to the common electrode 19. Since the common electrode 19 covers the gate electrode 15, the data line 12, and the gate line 13 widely, the pixel electrode can be obtained by applying a transition voltage to the common electrode 19 and grounding the various electrodes on the first substrate side. An electric field is generated not only between 15 and the common electrode 19 but also between the data line 12, the gate line 13 and the common electrode 15. If an electric field is generated, the diffusion of the bend transition is not hindered, and the entire display screen can be bent more reliably.
[0030]
If the transition voltage is applied in this way, the distance d between the pixel electrodes 15 may be longer than the transition distance, and at this time, the distance d ′ between the pixel electrode 15 and the data line 13 or the gate line 12 is It should just be below a transition distance.
[0031]
Further, as described above, it is difficult to set the distance d between the pixel electrodes 15 to a transition distance of 2 μm or less, but the data lines 12 and the gate lines 13 are formed with the insulating film 14 separated from the pixel electrodes 15. Therefore, it is easy to set the distance d ′ to 2 μm.
[0032]
Furthermore, as shown in FIG. 8, the pixel electrode 15, the data line 13, and the gate line 12 may be formed so as to overlap. As a result, the distance d ′ between the pixel electrode 15 and the data line 12 and the gate line 13 becomes zero.
[0033]
In short, it is important to arrange each electrode as an inter-pixel electrode so that a region where no electrode exists between the pixels does not exist over the transition distance. In other words, the electrodes arranged between the pixels, that is, the inter-pixel electrodes are not limited to the data line 12 and the gate line 13, but may be provided specially, or may be provided by using the auxiliary capacitance electrode. Also good. However, particularly in an active matrix LCD, the data lines and gate lines are arranged in a lattice pattern in all display areas, and are optimal for use as electrodes arranged between pixels.
[0034]
【The invention's effect】
As described above, according to the present invention, since the distance between the first electrodes of the LCD having the OCB mode is set to 5 μm or less, or 2 μm or less, the bend alignment diffuses between the pixels, The liquid crystal can be driven in the OCB mode, and an LCD having a high response speed can be obtained.
[0035]
In addition, an inter-pixel electrode is provided between the first electrodes, and the space between the pixel electrodes is separated because the inter-pixel electrode and the plurality of first electrodes are 5 μm or less or 2 μm or less. However, it is possible to make the LCD with a fast response speed.
[0036]
Furthermore, since the inter-pixel electrode is a signal line or / and a gate line, it is not necessary to arrange a new electrode, and the cell region can be used effectively.
[Brief description of the drawings]
FIG. 1 is a diagram showing Gibbs energy for bend alignment and splay alignment.
FIG. 2 is a diagram showing a potential barrier between bend alignment and splay alignment.
FIG. 3 is a graph showing the relationship between transition voltage and dislocation time.
FIG. 4 is a plan view showing diffusion of bend transition in a simple matrix LCD.
FIG. 5 is a graph showing the relationship between inter-pixel distance, applied voltage, and bend transition rate.
FIG. 6 is a plan view showing diffusion of a bend transition when a distance between pixels is equal to or less than a transition distance in a simple matrix LCD.
7A and 7B are a plan view and a cross-sectional view showing an active matrix LCD.
8A and 8B are a plan view and a cross-sectional view showing an active matrix LCD.
FIG. 9 is a cross-sectional view for explaining bend alignment and splay alignment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 1st electrode, 2 2nd electrode, 3 Pixel area | region, 4 Transition factor 5 Bend transition area | region, 12 Data line, 13 Gate line 15 Pixel electrode, 19 Common electrode

Claims (8)

A plurality of first electrodes provided on the first transparent substrate and extending in the first direction ;
A plurality of second electrodes provided on the second transparent substrate and extending in a second direction different from the first direction ;
A first alignment film and a second alignment film formed so as to cover each of the first and second electrodes and rubbed in substantially the same direction;
In a simple matrix liquid crystal display device comprising a liquid crystal layer sealed between the first and second substrates and having a bend alignment state and a splay alignment state,
A distance between the plurality of first electrodes is 5 μm or less.   The liquid crystal display device according to claim 1, wherein a distance between the plurality of first electrodes is 2 μm or less. A plurality of first electrodes provided on the first transparent substrate;
A second electrode provided on the second transparent substrate;
A first alignment film and a second alignment film formed so as to cover each of the first and second electrodes and rubbed in substantially the same direction;
A liquid crystal display device including a liquid crystal layer sealed between the first and second substrates and having a bend alignment state;
The liquid crystal display device is characterized in that no electrode-existing region exists between the first electrodes in the direction between the first electrodes by 5 μm or more. A plurality of first electrodes provided on the first transparent substrate;
A second electrode provided on the second transparent substrate and provided across the plurality of first electrodes;
A first alignment film and a second alignment film formed so as to cover each of the first and second electrodes and rubbed in substantially the same direction;
A liquid crystal display device including a liquid crystal layer sealed between the first and second substrates and having a bend alignment state;
An inter-pixel electrode is provided between the plurality of first electrodes,
Applying a transition voltage to the second electrode;
An electric field is generated not only between the first electrode and the second electrode but also between the inter-pixel electrode and the second electrode to bend the liquid crystal layer;
A liquid crystal display device , wherein an interval between the inter-pixel electrode and the plurality of first electrodes is 5 μm or less.   5. The liquid crystal display device according to claim 4, wherein a distance between the inter-pixel electrode and the plurality of first electrodes is 2 μm or less.   The liquid crystal display device according to claim 4, wherein the inter-pixel electrode is formed with an insulating film between the plurality of first electrodes.   The liquid crystal display device according to claim 4, wherein the inter-pixel electrode and at least one of the first electrodes have an overlapping region. The first electrode is a pixel electrode formed independently for each pixel,
A thin film transistor is connected to each pixel electrode,
A signal line and a gate line connected to the thin film transistor are disposed between the pixel electrodes,
8. The liquid crystal display device according to claim 4 , wherein the inter-pixel electrode is the signal line or / and the gate line.

Images