JP2003186019A - Liquid crystal display device and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal display device suppressing generation of a reverse transition dependent on temperature variation and scarcely having any image quality deterioration even if working temperature variation occurs. <P>SOLUTION: The liquid crystal display device is provided with a pair of substrates, a liquid crystal sandwiched between a pair of the substrates and showing a bend alignment in a displaying state, electrodes disposed on each of a pair of the substrates and applying voltage to the liquid crystal, a driving means periodically applying voltage corresponding to display information and voltage to prevent reverse transition to the electrodes and alignment layers disposed on each of a pair of the substrates sandwiching the electrodes between the substrates and the liquid crystal and maintaining ≥8° pretilt angle of the liquid crystal. <P>COPYRIGHT: (C)2003,JPO

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 used for a liquid crystal TV, etc.
CB (Optically Compensated)
The present invention relates to a liquid crystal display device using a birefringence liquid crystal mode.

[0002]

2. Description of the Related Art The OCB liquid crystal mode, which is characterized by high-speed response, is widely used at present in TN (Twisted N).
The liquid crystal mode is more suitable for displaying natural moving images than the liquid crystal mode. Therefore, in recent years, O
The CB liquid crystal mode has become the most promising candidate and is being researched. For the OCB type liquid crystal display device, see "Institute of Electrical Communication, IEICE Technical Report EDI 98-144 199".
See page.

As shown in FIG. 5, the OCB type liquid crystal display device has a basic structure in which the upper and lower substrates 503 coated with an alignment film 502 are rubbed to make the directions parallel. A liquid crystal 501 is sandwiched between the upper and lower substrates 503, and a retardation plate 504 and a polarizing plate 505 are provided outside the upper and lower substrates 503, respectively. By this rubbing process, the liquid crystal molecules closest to the substrate form a certain angle called the pretilt angle with the substrate. Then, it has a bend orientation (FIG. 5b) that is in a state capable of image display and a splay orientation (FIG. 5a) that is not suitable for image display. In order to shift from the splay alignment to the bend alignment, a unique drive such as applying a high voltage for a certain period of time is used. This is called bend transition.

Due to the above-mentioned unique driving, even if the liquid crystal layer is once in the bend alignment, if the state in which the voltage of the predetermined level or more is not applied for a certain period of time continues, the bend alignment cannot be maintained and the liquid crystal layer returns to the splay alignment (hereinafter, referred to as splay alignment). This phenomenon is called "inverse transition phenomenon"). At this time, the pixel in which the reverse transition has occurred looks like a bright spot defect, so it is important to suppress the occurrence of this phenomenon.

Conventionally, the present inventors have devised a driving method for preventing a reverse transition phenomenon (hereinafter referred to as reverse transition prevention driving). In this device, for example, as shown in FIG. 7, a voltage for preventing reverse transition is inserted in one field period in addition to the voltage corresponding to the display image. As another method for preventing the phenomenon of reversal, the present inventors have proposed a method of temperature compensating the reverse transition condition at low temperature. In addition to this, the present inventors have proposed a method of adjusting viscosity in order to reduce temperature dependence at low temperatures.

[0006]

SUMMARY OF THE INVENTION We have found a problem that, under usage conditions at high temperatures, the reverse transition is more likely to occur than at room temperature. FIG. 4 shows a ratio of the pulse width of the reverse transition prevention pulse necessary for the reverse transition prevention to one frame (hereinafter, this ratio is simply referred to as “a ratio”).
It may be said that) temperature dependence. Under the conventional condition where the pretilt angle is relatively low (4 ° in Fig. 4), the reverse transition phenomenon is more likely to occur under high temperature use conditions than under normal temperature use conditions. To be Specifically, about 15% is sufficient at room temperature, but a reverse transition prevention pulse is required to be drastically long at 40 ° C. or higher, 20% at 60 ° C. and 32% at 80 ° C. For this reason, if the reverse transfer prevention drive is performed according to the usage conditions at room temperature, a problem occurs that the reverse transfer phenomenon occurs at high temperatures. Conversely, the reverse transfer prevention drive is performed according to the usage conditions at high temperature. If this is done, there is a problem in that the reverse transition prevention pulse becomes insufficient and the brightness decreases at room temperature.

As a result of studying various parameters on the temperature dependence of the reverse transition phenomenon, we have found that the pretilt angle of the liquid crystal is dominant and completed the present invention. By adjusting this pretilt angle, the reverse transition phenomenon was suppressed especially in high temperature use.

An object of the present invention is to provide a liquid crystal display device which suppresses the occurrence of reverse transition depending on the temperature change and causes little image quality deterioration even when the operating temperature changes.

[0009]

A liquid crystal display device according to the present invention which solves the above problems includes a pair of substrates, a liquid crystal sandwiched between the pair of substrates and having a bend alignment in a display state,
The electrodes are provided between the liquid crystal and electrodes provided on each of the pair of substrates, the electrodes applying a voltage to the liquid crystal, the driving means for periodically applying a voltage corresponding to display information and a reverse transition prevention voltage to the electrodes. Thus, each of the pair of substrates is provided with an alignment film for keeping the pretilt angle of the liquid crystal at 8 degrees or more.

The pretilt angle is preferably 12 degrees or more. Further, the pretilt angle is preferably 20 degrees or less, and more preferably 15 degrees or less.

The orientation film is made of polyimide, and the imidization ratio of the polyimide is preferably 60% or more and 80% or less.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the liquid crystal display device of the present invention, the method of detecting the panel temperature and adjusting the reverse transition prevention drive condition when driving the panel was not used. Using the reverse transition prevention driving condition at room temperature, it was decided to drive under the same conditions in all temperature ranges. As a result, the circuit scale was reduced by eliminating the compensation circuit, and the cost was reduced.

FIG. 1 shows the pretilt angle dependency of the reverse transition prevention driving condition at a panel temperature of 80 ° C. in an OCB type liquid crystal panel using a fluorine-based liquid crystal material and a polyimide alignment film. Here, as the reverse transition prevention drive condition,
The ratio of applying a black display voltage for preventing reverse transition to the liquid crystal during one frame (16.6 ms) is shown. It can be seen that the ratio decreases as the pretilt angle increases. That is, it means that the reverse transition phenomenon is less likely to occur, and increasing the pretilt angle is effective in suppressing the reverse transition phenomenon.

Generally, when the voltage applied to the liquid crystal is low, the splay alignment is energetically stable, and when it is high, the bend alignment is stable. The splay alignment is stable when the voltage applied to the liquid crystal is 0 V, but as the voltage is gradually increased, the energy difference between the splay alignment and the bend alignment gradually decreases, and the voltage at which the magnitude relationship is reversed is Exists. This voltage is referred to as a bend-spray critical voltage (hereinafter simply referred to as "critical voltage"). When a voltage equal to or higher than the critical voltage is applied in the situation where the splay alignment and the bend alignment coexist, the bend alignment region grows, and conversely, the splay alignment region grows at this voltage or lower. Eventually, it is integrated only in either domain (spray orientation or bend orientation).

FIG. 2 is a static simulation result showing the relationship between the critical voltage and the pretilt angle. As can be seen from this figure, the larger the pretilt angle is, the lower the critical voltage is. That is, it can be seen that when the pretilt angle becomes large, the bend alignment is more likely to occur, and the reverse transition phenomenon is less likely to occur.

However, it has been clarified in our studies so far that this critical voltage has no temperature dependence (FIG. 6). That is, it can be said that the temperature dependence of the reverse transition phenomenon, which is the object of the present invention, is not the influence of this critical voltage, but is a problem newly discovered by us this time.

Since the temperature dependence does not occur in the driving system which does not use the reverse transition prevention drive, that is, the driving system which obtains the critical voltage, this is the case where the reverse transition prevention drive is adopted.
This is a phenomenon peculiar to the CB liquid crystal display device.

We now find that this is a problem that occurs when the response speed of the liquid crystal becomes extremely fast at high temperature and the dynamic energy relationship between the splay alignment and the bend alignment is lost. I found it. That is, the response speed of the bend orientation is extremely fast, but the response speed of the spray orientation is slow. Since the response of the splay alignment is slow at room temperature, the splay alignment maintains an unstable state immediately after the reverse transition prevention voltage is applied. Therefore, since the splay alignment is relatively unstable, the reverse transition prevention pulse for promoting the bend alignment is sufficient for a short period. However, at a high temperature, the response speed of the splay alignment becomes sufficiently high, so that the splay alignment is stabilized immediately after the application of the reverse transition prevention voltage as in the bend alignment. Then, the reverse transition is likely to occur, so that the width of the reverse transition prevention pulse needs to be widened.

Therefore, in the present invention, the pre-tilt angle is increased and the response speed of the liquid crystal, in particular, the relaxation time of the spray is delayed to mitigate the reverse transition phenomenon at high temperature.

FIG. 3 shows the temperature dependence of the reverse transition prevention driving condition in the liquid crystal panel in which the pretilt angle is changed. Again, the ratio was evaluated as the reverse transition prevention driving condition. It can be seen that the reverse transition prevention driving condition is constant up to a specific temperature, and the ratio becomes higher at that temperature or higher, and the reverse transition phenomenon easily occurs.

When the pretilt angle is 4 °, the panel temperature is 40
It is necessary to widen the ratio of the reverse transfer prevention drive depending on the degree, and there is a problem that the reverse transfer pixel is generated in the drive at the room temperature condition. However, if the pretilt angle is set to 5 °, the panel temperature will be 6
The occurrence of reverse transition is suppressed up to 0 ° C, and the pretilt angle is set to 8
When the temperature is set to °, no problem occurs even when the panel temperature is 80 ° C or higher.

Pretilt angle 1
If it is 2 ° or more, the temperature dependence can be completely eliminated. Further, if the pretilt angle is further increased from here, the ratio of reverse transition prevention drive can be gently decreased. A pretilt angle of 12 ° is 10%, a pretilt angle of 30 ° is 5%, and a pretilt angle of about 50 ° is required to completely set the reverse transition prevention drive ratio to 0%.

Here, the upper limit of the temperature range is set to Tni which indicates the liquid crystallinity limit temperature. For the liquid crystal material used this time, 1
It was 00 ° C.

Recently, the upper limit of the driving temperature range of a liquid crystal panel is required to be high. Especially for in-vehicle monitor applications, it is required to guarantee usage at high temperature, and also for LCD TV applications, it is necessary to strengthen the backlight in order to bring out the brightness. Use guarantee is required. In order to meet these requirements, it is necessary to guarantee the panel temperature up to 80 ° C. For this purpose, a pretilt angle of 8 ° or more is required.

In the future, when the demand becomes more severe, the pretilt angle is set to 1 in order to completely eliminate the temperature dependence.
Must be 2 °.

The present invention is unique to OCB liquid crystal display devices which require high temperature operation. In particular, it is required for a liquid crystal display device having an upper limit of the operating temperature range of 60 ° C. or higher, preferably 80 ° C. or higher. As used herein, the term "use temperature range" refers to the maximum value of the local panel temperature, not the ambient temperature. In this embodiment, the temperature near the backlight is the highest.

In the material system that we have used this time, it is possible to eliminate the temperature dependence of the reverse transition prevention drive condition at a pretilt angle of 8 ° or more. However, even if the alignment film is changed, this pretilt relationship hardly changes. In addition, there is almost no dependence on the liquid crystal material. However, when a fluorine-based liquid crystal is used, the stability of pretilt is extremely good.

In this embodiment, the black display voltage is used as the reverse transition prevention voltage, but it is not limited to this. For example, a voltage higher than the black voltage may be used. To prevent reverse transfer,
The gist of the invention is to periodically apply a voltage equal to or higher than the critical voltage.

The present invention provides a high pretilt to prevent reverse transitions at high temperatures. The following method was effective in achieving a stable high pretilt angle.

In general, an alignment film material and a polyimide material are applied by printing on a substrate, which is heated and cured. At this time, a predetermined high tilt angle was obtained when the imidization ratio was 60% to 80%. In order to realize this, the effective heating time of the substrate was between 5 minutes and 20 minutes. If it is less than 5 minutes, there is a problem that stability cannot be obtained, and if it is more than 20 minutes, there is a problem that the pretilt angle becomes small because decomposition and deterioration of the alignment film occurs. A heating time of 5 minutes to 15 minutes was further desired in order to realize a higher tilt.

Furthermore, the rubbing cloth used was best made of cotton. For example, under the condition that a pretilt angle of 12 ° can be realized with a cotton rubbing cloth, only 8 ° can be realized with a rayon rubbing cloth. It is considered that this is because the rubbing of the cotton cloth, which is relatively less damaged, is uniform and achieves a high pretilt.

In the present invention, the temperature dependence of the reverse transition phenomenon is reduced by increasing the tilt. However, there is a limit to the high tilt in terms of uniformity. The upper limit was 15 ° as a practical range. However, up to 20 ° was possible if the occurrence of unevenness was allowed to some extent.

[0033]

INDUSTRIAL APPLICABILITY As described above, by using the present invention, it is possible to suppress the reverse transition phenomenon, which has conventionally been likely to occur in high temperature use, and to make the reverse transition prevention drive condition almost constant. , The reverse transition prevention drive independent of temperature can be performed, and the brightness can be improved.

[Brief description of drawings]

FIG. 1 is a diagram showing the pretilt angle dependence of reverse transition prevention driving conditions.

FIG. 2 is a diagram showing a static simulation result of a relationship between a critical voltage and a pretilt angle.

FIG. 3 is a diagram showing temperature dependence of a reverse transition prevention driving condition in a liquid crystal panel in which a pretilt angle is changed.

FIG. 4 is a diagram showing temperature dependence of reverse transition prevention driving conditions in a liquid crystal panel having a pretilt angle of 4 ° which is a conventional example.

5A is a diagram showing a splay alignment state of the liquid crystal display device according to the embodiment of the invention, and FIG. 5B is a diagram showing a bend alignment state of the liquid crystal display device according to the embodiment of the invention.

FIG. 6 is a diagram showing temperature dependence of a critical voltage in a conventional liquid crystal panel of the present invention.

FIG. 7 is a diagram showing drive waveforms in a drive method for preventing a reverse transition phenomenon.

[Explanation of symbols]

501: Liquid crystal molecule 502: Alignment film 503: substrate 504: Retardation plate 505: Polarizing plate

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yoshinori Tanaka             1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric             Sangyo Co., Ltd. (72) Inventor Kenji Nakao             1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric             Sangyo Co., Ltd. (72) Inventor Shinji Ogawa             1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric             Sangyo Co., Ltd. F term (reference) 2H088 GA02 HA03 JA04 JA09 KA14                       LA07 MA06 MA20                 2H090 HB08Y HD14 KA04 KA07                       LA04 MA11 MA17 MB01                 2H093 NA20 NA43 NB22 ND08 ND44                       NE04 NF04 NF09 NH12 NH14

Claims (5)

[Claims] 1. A pair of substrates, a liquid crystal sandwiched between the pair of substrates and having a bend alignment in a display state, electrodes provided on each of the pair of substrates and applying a voltage to the liquid crystal, A driving unit that periodically applies a voltage corresponding to information and a reverse transition prevention voltage to the electrodes, and a pretilt of the liquid crystal that is provided on each of the pair of substrates so that the electrodes are sandwiched between the liquid crystal and the driving unit. Angle 8
A liquid crystal display device provided with an alignment film which keeps the same or more degrees. 2. The liquid crystal display device according to claim 1, wherein the pretilt angle is 12 degrees or more. 3. The liquid crystal display device according to claim 1, wherein the pretilt angle is 20 degrees or less. 4. The liquid crystal display device according to claim 3, wherein the pretilt angle is 15 degrees or less. 5. The liquid crystal display device according to claim 1, wherein the alignment film is made of polyimide, and the imidization ratio of the polyimide is 60% or more and 80% or less.