JP2000321556A - Method for transition of alignment state of liquid crystal and method of driving liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control a liquid crystal to be in a OCB mode during an image display period and to improve the response speed by applying a transition voltage higher than the display voltage between first and second electrodes before the display voltage is applied so as to preliminarily cause bend transition in the liquid crystal. SOLUTION: The liquid crystal filled between first and second alignment films has both of splay alignment state and bend alignment state, and when a specified threshold voltage Vc is applied between the first and second electrodes, the energy levels of the splay alignment state and the bend alignment state are exchanged. Then a transition voltage Vt which is higher than the threshold voltage Vc is applied between the first and second electrodes to cause bend transition in the liquid crystal. Once the liquid crystal is changed into the bend state, it does not change again into the spray alignment as far as the voltage applied is decreased enough lower than the threshold voltage Vc. Thereby, in an LCD using an OCB mode, the transition voltage Vt is applied for a transition period T when the power supply is turned on, and then the display voltage in a waveform according to video signals is applied in the image display period.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a liquid crystal display (Li)
Regarding quid Crystal Display (LCD), especially OCB (Optical Controlled Birefringenc) with high liquid crystal driving speed
e) LCD using mode.

[0002]

2. Description of the Related Art An improvement in the moving picture reproduction capability of an LCD and a field sequential LCD (Field Sequential LCD; F)
For practical use of S-LCD), L has a faster response speed.
CD is required.

The response speed of an LCD is the time required for the liquid crystal to change to a driving state after a driving voltage is applied to the liquid crystal. When a voltage is applied to the liquid crystal, the liquid crystal is oriented in a predetermined direction and is driven. However, a certain time is required until the liquid crystal molecules are aligned in the orientation direction, and this time is the response speed. If the response speed is low, 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 a liquid crystal with a faster response speed,
Videos can be displayed more smoothly.

Further, the FS-LCD is a system for displaying a color by rapidly switching light of three primary colors and alternately displaying an image of each color on one pixel. The liquid crystal used for the FS-LCD is required to have a remarkably fast response speed as compared with the liquid crystal used for the color filter type LCD from the principle of operation, and is expected to be put to practical use.

By the way, as a liquid crystal having a fast response speed,
OCB mode liquid crystals have been known for some time. The OCB mode is an LCD system that uses a liquid crystal that bends in alignment with a biaxial optical compensation layer. FIG. 7 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 glass or the like facing each other, and a liquid crystal layer 57 is sealed therebetween. I have. Liquid crystal layer 57
Is a nematic liquid crystal, and the alignment films 55 and 56 are rubbed in a direction substantially parallel to each other, and have a pretilt angle so as to face each other. An optical compensation layer (not shown) is provided thereon and visualized. FIG. 7A shows the electrode 5
3 and 54 are in a state where no voltage is applied. The liquid crystal molecules 57a are oriented in the rubbing direction (parallel to the paper),
The liquid crystal molecules 57a near the alignment films 55 and 56 are oriented in the direction of the pretilt angle. FIG. 7B shows a state where a driving voltage of, for example, 5 V is applied to the electrode 53. The liquid crystal rises due to the applied drive voltage, but the liquid crystal molecules fall at the center of the liquid crystal layer 57. The orientation in the state shown in FIG. 7B is called a splay orientation. FIG. 7C shows a state in which the alignment state of the liquid crystal 57a has changed. The orientation in the state shown in FIG. 7C 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 also stand. The splay orientation and the bend orientation are mutually reversible phase transitions, and the transition from the splay orientation to the bend orientation is called a bend transition.

One of LCDs using bend alignment is OC.
There is B mode. This uses a bend-aligned liquid crystal and a biaxial optical compensation layer. Since the bend alignment has a higher response speed than the liquid crystal mode used for the conventional TN or STN type LCD, the LCD using the OCB mode is suitable for moving image display and FS-LCD.

[0007]

When an LCD is to be manufactured using the OCB mode, the response speed of the splay alignment before the bend transition and that of the bend alignment change drastically. Must be surely transferred.

However, there are still many unclear points regarding the physical mechanism of the bend transition, and there are still many problems to be solved.

Therefore, the present invention provides an L
An object of the present invention is to obtain a LCD having a high response speed by surely causing a liquid crystal to bend-transfer in a CD.

[0010]

SUMMARY OF THE INVENTION The present invention has been made to achieve the above object, and has first and second electrodes formed opposite to each other, and covers the first and second electrodes. Rubbed in substantially the same direction as each other
And a second alignment film, which is sealed between the first and second alignment films, has a splay alignment state and a bend alignment state, and applies a predetermined threshold voltage between the first and second electrodes. When a transition voltage higher than a threshold voltage is applied between the first and second electrodes to a liquid crystal in which the magnitudes of the state energies of the splay alignment and the bend alignment are switched, a liquid crystal alignment state transition method in which the liquid crystal bends. It is.

The first and second substrates facing each other and the first and second substrates formed opposite each other on the first and second substrates, respectively.
And a second electrode, first and second alignment films formed over the first and second electrodes and rubbed in substantially the same direction as each other, and sealed between the first and second substrates. And
A liquid crystal whose state energy of the splay alignment and the bend alignment changes when a predetermined threshold voltage is applied between the first and second electrodes; and a display voltage is applied between the first and second electrodes. In a method of driving a liquid crystal display device that performs display by bend-aligning liquid crystal, a liquid crystal display device that bends liquid crystal by applying a transition voltage higher than a threshold voltage between the first and second electrodes. It is a driving method.

Further, the first and second substrates facing each other and the first and second substrates formed on the first and second substrates respectively facing each other.
And a second electrode, first and second alignment films formed over the first and second electrodes and rubbed in substantially the same direction as each other, and sealed between the first and second substrates. A liquid crystal display having a splay alignment state and a bend alignment state, and applying a display voltage between the first and second electrodes to cause the liquid crystal to bend and display. And a method for driving a liquid crystal display device in which a transition voltage higher than the display voltage is applied between the first and second electrodes before the display voltage is applied.

The transition voltage is continuously applied during the transition period, and the transition period is determined according to the transition voltage value.

In an active matrix type LCD, a transition voltage is applied to a common electrode.

[0015]

FIG. 1 is a diagram showing a change in Gibbs energy with respect to a voltage applied to a liquid crystal for explaining a basic principle of a transition method according to the present invention. The solid line indicates the Gibbs energy in the splay orientation and the dashed line indicates the Gibbs energy in the bend orientation. FIG. 2 shows the Gibbs energy of the splay alignment and the bend alignment when the applied voltage is a voltage V1 higher than the threshold voltage Vc.

The 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 having a low state energy is a more stable state. In both the splay alignment and the bend alignment, the energy decreases as the applied voltage increases. This drives the liquid crystal molecules that were stable in the pretilt direction when no voltage was applied,
Either spray or bend orientation. 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 when the applied voltage exceeds Vc. Since the substance has the property of being stable at the lower state energy, while the applied voltage is lower than the threshold voltage Vc,
The splay alignment is more stable, and the liquid crystal molecules are in the splay alignment. 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.

In the OCB mode, a liquid crystal having a bend alignment is used. Even when the applied voltage is simply increased to a voltage higher than the threshold voltage Vc, for example, V1, the transition to the bend alignment (bend transition) is performed.
Is less likely to occur. This is considered to be because a potential barrier PB exists between the splay alignment and the bend alignment as shown in FIG. That is, the applied voltage V
In the case of 1, the bend transition cannot be performed because the voltage is not sufficient to exceed the potential barrier PB of the delta E.
Then, once the liquid crystal undergoes the bend transition beyond the potential barrier PB, the liquid crystal maintains the bend alignment in which the Gibbs energy is lower while the applied voltage is higher than the threshold voltage Vc.

Referring to FIG. 1, when the applied voltage is further increased from V1, the difference between the Gibbs energies in the bend alignment and the splay alignment is further increased. Therefore, the present invention applies a transition voltage sufficiently higher than the threshold voltage Vc in advance before performing display on the LCD, and first causes the liquid crystal in the cell to bend.

First, the structure of the LCD to which this embodiment is applied is the same as that of the conventional LCD shown in FIG. That is, the first and second electrodes 53 and 54 and the alignment films 55 and 56 are formed on the opposing transparent substrates 51 and 52, respectively, and the liquid crystal layer 57 is sealed therebetween. FIG. 3 shows the first electrode 53.
FIG. 10 is a graph showing actual measurement values obtained by continuously applying a constant voltage between the first electrode and the second electrode 54 and measuring the time until liquid crystal between the electrodes undergoes a bend transition. For example, 10 V between the electrodes 53 and 54
, The bend transition of the liquid crystal between the electrodes took about 20 seconds. When the applied voltage was increased, the time required for the bend transition was sharply shortened. When 18 V was applied, the bend transition occurred in about 2 seconds. As described above, by applying a voltage sufficiently higher than the threshold voltage Vc (this is referred to as a transition voltage in this specification), the liquid crystal can bend-transfer and operate in the OCB mode.

It is considered that this is because, by increasing the applied voltage, the energy difference between the bend alignment and the splay alignment is increased, and the number of liquid crystal molecules having energy exceeding the potential barrier PB is increased.

The liquid crystal that has once bend-transited does not transition again to the splay alignment unless the applied voltage becomes sufficiently lower than the threshold voltage Vc. This is because, as is clear from FIG. 2, the potential barrier PB exists even in the splay orientation. Therefore, the liquid crystal that has undergone the bend transition can be operated in the OCB mode while maintaining the bend alignment as long as the screen display is performed by applying a display voltage in a range not significantly lower than the threshold voltage Vc. In addition, according to our experiment, it was confirmed that the OCB mode is maintained for about several hours even after the applied voltage is set to 0 V in the LCD in which the bend alignment is performed once.

That is, a transition voltage for bend transition is applied once, for example, when the power of the LCD is turned on, and if the liquid crystal is bend-transferred, the display voltage is applied (that is, screen display is performed). During the period, the bend orientation is maintained. Therefore, in the LCD using the OCB mode, as shown in FIG. 4, when the power is turned on (time = 0),
First, a transition voltage Vt is applied during a transition period T, and thereafter, during a screen display period, a screen display is performed by applying a display voltage having a waveform according to a video signal in the same manner as in a conventional LCD. The values of the transition voltage Vt and the transition period T may be determined, for example, with reference to FIG.

To ensure that the liquid crystal undergoes a bend transition,
The transition voltage Vt and the transition period T are plotted in FIG. 3, and the values of both are determined so that the point is located above the solid line shown in FIG. 3. However, if the transition period T is too long, the waiting time until the display of the screen is started becomes long, and during this period, the applied voltage is high, so that the power consumption also increases. In addition, when the transition voltage Vc is high, power consumption increases, and a power supply having a large capacity is required. Therefore, the transition voltage Vt near the solid line
And it is good to set the transition period T. For example, if the transition voltage Vt is set to 15 V, the transition period is only 5 seconds. Further, for example, when used as a monitor of a personal computer, it is not necessary to look at the screen while the OS (Operating System) is running. Therefore, it is possible to secure a long transition period, for example, 15 seconds, and set the transition voltage as low as 11 V. it can. Note that FIG.
Even if the transition voltage Vc and the transposition period T are located in a region below the solid line, the bend transition occurs with a high probability if the region is near the solid line and is not significantly below the solid line. It is feasible.

If the applied voltage is further increased, it is expected that the bend transition will occur more quickly. However, in general, the LCD has a fine structure and a voltage exceeding the withstand voltage between the first electrode 53 and the second electrode 54. Cannot be applied. Furthermore, in order to apply a high voltage, it is necessary to provide an appropriate power supply. For example, when an LCD is used as a monitor of a portable terminal, the size of the device is increased. Therefore, it is not practical to apply a voltage of 20 V or more, and it is necessary to secure a transition time of at least 1 second.

FIG. 5 is a plan view showing an active matrix type LCD. Active matrix LCD
A pixel electrode 11 is formed for each pixel.
Are connected to the data line 13 via the thin film transistor 12. The gate electrode of each thin film transistor 12 is connected to a gate line 14 formed insulated from the data line 13. Further, auxiliary capacitance electrodes and the like (not shown) are also formed. All of the above structures are formed on the first substrate. The common electrode 15 is formed on the second substrate so as to cover the above structure.

The transition voltage is applied to the first electrode, ie, the pixel electrode 11.
And the second electrode, that is, the common electrode 15, and usually, one of them is grounded, and the other is raised or lowered to apply the transition voltage. Now,
As shown in FIG. 1, when the applied voltage is lower than the reversal voltage Vc, the energy of Gibbs is lower in the splay alignment state, so that the bend transition hardly occurs in the region where no voltage is applied. Therefore, when the common electrode 15 is grounded and a transition voltage is applied only to the pixel electrode 11 on the first substrate side, the region on the pixel electrode 11 can bend transition.
In some cases, bend transition does not occur in a region between the two. Therefore, when a transition voltage is applied to the first substrate side, not only the pixel electrode 11 but also the thin film transistor 12 and the data line 1
3, it is necessary to apply a transition voltage to all electrodes formed on the first substrate, such as the gate line 14, the auxiliary capacitance electrode, and the like. Of course, this is not impossible, but the wiring for applying the transition voltage becomes complicated, and a high voltage is applied to the gate electrode as a gate voltage of 20 V. May occur. Therefore, when applying a transition voltage to the active matrix type LCD, it is preferable that each electrode formed on the first substrate is grounded and the transition voltage is applied to the common electrode 15. Since the common electrode 15 is formed to cover the electrode on the first substrate side, if the potential of the common electrode 15 is changed,
A transition voltage can be applied to all regions of the liquid crystal.
At this time, since the first substrate is normally grounded, the potential difference between the first substrate and the common electrode 15 is larger than the potential difference between the first substrate and the pixel electrode 11.

In the simple matrix type LCD shown in FIG. 6, since the first electrode 21 and the second electrode 22 are equivalent, a transition voltage may be applied to any of the electrodes. Alternatively, voltages of opposite polarities may be applied to the first and second electrodes 21 and 22 so that the combined voltage may be used as a transition voltage.

In this specification, a liquid crystal display device or L
The CD can be implemented in a simple matrix, an active matrix, a transmissive type, a reflective type, or any other type of liquid crystal display device.

[0029]

As described above, according to the present invention, the OC
In an LCD using a liquid crystal having the B mode, a transition voltage higher than the display voltage is applied between the first and second electrodes before the display voltage is applied. In the screen display period, the liquid crystal can be in the OCB mode, and the response speed is high.

The transition voltage is continuously applied during the transition period, and the transition period is determined according to the transition voltage value, so that the bend transition can be reliably performed.

In the active matrix type LCD, since the transition voltage is applied to the common electrode, the wiring for applying the transition voltage is simple.

[Brief description of the drawings]

FIG. 1 is a diagram showing Gibbs energies of 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 a relationship between a transition voltage and a dislocation time.

FIG. 4 is a diagram showing a voltage between electrodes according to the embodiment of the present invention.

FIG. 5 is a plan view showing an active matrix type LCD.

FIG. 6 is a plan view showing a simple matrix type LCD.

FIG. 7 is a cross-sectional view for explaining bend alignment and splay alignment.

[Explanation of symbols]

51, 52: transparent substrate, 53, 54: electrode, 55,
56: alignment film 57: liquid crystal

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H088 JA14 KA30 MA10 2H093 NA07 NA16 NA65 NB22 NC21 ND32 ND60 NF14 5C006 AA01 AF67 BB16 BC03 BC12 FA11 5C080 AA10 BB05 DD08 FF11 GG08 JJ04 JJ05 JJ06

Claims (6)

[Claims] 1. A spraying method comprising the steps of: enclosing a first alignment film and a second alignment film, which are formed so as to cover first and second electrodes disposed opposite to each other and are rubbed in substantially the same direction as each other; Having an orientation state and a bend orientation state,
When the voltage applied between the first and second electrodes is changed, the liquid crystal in which the state energy of the splay alignment and the bend alignment changes at a predetermined threshold voltage is changed. And applying a transition voltage higher than the threshold voltage to bend transition the liquid crystal. 2. A first and second substrate facing each other,
A first and second electrode formed on the first or second substrate to face each other, and a second electrode formed to cover the first and second electrodes and rubbed in substantially the same direction as each other. A first alignment film, a first alignment film, and a splay alignment state and a bend alignment state, which are sealed between the first and second substrates, and apply a voltage applied between the first and second electrodes. When changed, the liquid crystal has a liquid crystal whose magnitude of the state energy of the splay alignment and the bend alignment changes at a predetermined threshold voltage, and applies a display voltage between the first and second electrodes to bend the liquid crystal. In a method for driving a liquid crystal display device which performs display by aligning, a liquid crystal display device includes:
A method for driving a liquid crystal display device, wherein bend transition of liquid crystal is performed by applying a transition voltage higher than the threshold voltage. 3. The first and second substrates facing each other, the first and second electrodes formed on the first or second substrate to face each other, and the first and second electrodes. A first alignment film formed over the first substrate and rubbed in the same direction as the first and second substrates, and a bend alignment state and a splay alignment state enclosed between the first and second substrates. A driving method of a liquid crystal display device having a liquid crystal and applying a display voltage between the first and second electrodes to bend the liquid crystal to perform display, and before applying the display voltage, A method for driving a liquid crystal display device, wherein a transition voltage higher than the display voltage is applied between the first and second electrodes. 4. When the voltage applied between the first and second electrodes of the liquid crystal is changed, the state energy of the splay alignment and the bend alignment change at a predetermined threshold voltage, and the display voltage is changed. 4. The method according to claim 3, wherein the threshold voltage is higher than the threshold voltage. 5. The liquid crystal according to claim 2, wherein the transition voltage is continuously applied during a transition period, and the transition period is determined according to the transition voltage value. A method for driving a display device. 6. The active matrix according to claim 1, wherein the first electrode is a plurality of pixel electrodes formed for each pixel, and the second electrode is a common electrode formed to cover the plurality of pixel electrodes. In the liquid crystal display device, when the transition voltage is applied, a potential difference between the first substrate and the common electrode is larger than a potential difference between the first substrate and the pixel electrode. The method for driving a liquid crystal display device according to any one of claims 2 to 5, wherein: