JP2014232151A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for allowing a vertical alignment liquid crystal display device to suppress a decrease in display uniformity caused particularly in a high temperature operation and be able to operate by a lower frame frequency.SOLUTION: The vertical alignment liquid crystal display device has a plurality of first electrodes, a plurality of second electrodes, and drive means for supplying a drive voltage to each of the electrodes. The number of the first electrodes is N (N≥32). The drive means performs four-line simultaneous selection driving in which four are simultaneously selected from the N first electrodes and selection voltage and non-selection voltage are supplied, and performs n-line inversion driving in which polarity inversion is performed each time n are selected from the plurality of first electrodes. A bias ratio a when the bias ratio in optimal bias driving in the four-line simultaneous selection driving is set to be √N+1, is set so as to satisfy the condition of (√N+1)-3≤a<√N+1, and the n in the n-line inversion driving is set so as to satisfy the condition of 8≤n≤16.

Description

  The present invention relates to a multiplex-driven vertical alignment type liquid crystal display device.

  Liquid crystal display devices are widely used as information display units in, for example, various consumer and in-vehicle electronic devices. A general liquid crystal display device is configured by disposing a liquid crystal layer made of a liquid crystal material between two substrates arranged to face each other with a gap of about several μm. A vertical alignment type liquid crystal display device is known as one of such liquid crystal display devices.

  In order to realize dot matrix display in a vertical alignment type liquid crystal display device, for example, a multiplex driving method is used. In the multiplex driving method, it is necessary that the electro-optical characteristics are steeper in order to maintain display quality even when the number of scanning lines (number of common electrodes) is increased. With respect to this point, for example, Japanese Patent No. 4641200 discloses that it is effective to make the pretilt angle as close to 90 ° as possible in order to achieve good steepness.

  By the way, when the vertical alignment type liquid crystal display element is operated by multiplex driving as described above, there may be a problem that the display uniformity is remarkably lowered particularly in the bright display portion in a high temperature atmosphere. This display uniformity is caused by display disturbance due to an unintended dark region near the center other than the periphery of the edge portion of the bright display pixel. This dark region is considered to be a phenomenon that occurs because the liquid crystal molecules are aligned in a direction that is not the direction in which the alignment is regulated by the alignment film. Hereinafter, such a phenomenon is referred to as DMA (Dynamic Miss Alignment). Regarding such DMA, for example, Japanese Patent Application Laid-Open No. 2008-281752 discloses a relationship between a pretilt angle of a liquid crystal layer and a frame frequency when each driving waveform is used in a vertical alignment type liquid crystal display device. In this preceding example, a line inversion waveform (A waveform), a frame inversion waveform (B waveform), an n line inversion waveform (C waveform), and a multiple line simultaneous selection waveform are used as drive waveforms.

  The multi-line simultaneous selection driving method has been widely used as an effective method for improving flicker and contrast reduction due to a frame response phenomenon in a liquid crystal display device with high-speed response. For example, in Japanese Patent No. 3,624,738, the non-dispersive type in which the selection periods are continuously arranged within one frame is divided, and the distributed drive in which the selection periods are dispersed and arranged has a flicker and contrast. It has been shown to be effective for improvement. The multi-line simultaneous selection driving method is a driving method in which m scanning lines are simultaneously selected with respect to N scanning lines, but the voltage level applied to the signal line is m + 1. There is a case where the polarity of the applied voltage is frequently inverted, which may cause crosstalk. It is effective to use n-line inversion driving as a method for suppressing the frequency component of the entire driving waveform to be low, and vertical and horizontal streak unevenness can be suppressed.

  In the multiple line simultaneous selection driving method, the bias setting method is the same as that for the waveforms A, B, and C, and the optimum bias is √N + 1. By setting the optimum bias condition, the maximum contrast of the liquid crystal display device can be realized. Many prior art documents show that the multiple line simultaneous selection driving method exhibits its effect when used in a liquid crystal display device having a relatively large number of scanning lines and a high response speed.

  On the other hand, the vertical alignment type liquid crystal display device has a response speed of approximately 200 milliseconds or more when operating 32 or more scanning lines, and the response speed is relatively slow compared to the horizontal alignment type liquid crystal display device. Therefore, the advantages of the multiple line simultaneous selection driving method described above are not enjoyed.

  However, a liquid crystal material having a negative dielectric anisotropy Δε is difficult to make the absolute value of Δε as large as that of a liquid crystal material having a positive dielectric anisotropy Δε. The device has a higher threshold voltage than a horizontal alignment type liquid crystal display device. Since the steepness in the electro-optical characteristics is poor, increasing the apparent steepness by increasing the drive voltage is adopted as one of the methods for improving the display quality.

  In the multi-line simultaneous selection drive method, the selection voltage can be lowered compared to the case of using the A, B, and C waveforms described above, so applying it to a vertical alignment type liquid crystal display device improves contrast and on-transmittance. It is preferable to. Further, it is important to further increase the steepness in the electro-optical characteristics in order to realize a display element having 32 or more scanning lines. Therefore, it is effective that the pretilt angle is as close to 90 ° as possible.

  On the other hand, in the prior example disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2008-281752, display disturbance can be suppressed in the state where the frame frequency is the lowest when the pretilt angle is close to 90 °, when the A waveform is used. . However, the A waveform is not generally used because crosstalk occurs remarkably during high duty driving of 32 or more scanning lines due to problems such as large current consumption.

  The above document shows that it is a driving waveform obtained by the multiple line simultaneous selection driving method that can obtain the effect of suppressing display disturbance after the A waveform. However, only the setting range of the frame frequency with respect to a specific pretilt angle range is shown as the condition, and there is no disclosure or suggestion regarding a condition that can suppress display disturbance more under other conditions. Even in the multi-line simultaneous selection driving method, reducing the frame frequency as much as possible is an important factor for eliminating crosstalk, and it is desired to further reduce the frame frequency from the frame frequency shown in the prior art. .

Japanese Patent No. 4641200 JP 2008-281752 A Japanese Patent No. 3624738

  A specific aspect of the present invention is to provide a technique that enables a lower alignment frequency liquid crystal display device to operate at a lower frame frequency by suppressing a decrease in display uniformity that occurs particularly during high-temperature operation. One.

  A liquid crystal display device according to one aspect of the present invention is provided with (a) a first substrate and a second substrate arranged to face each other, and (b) one surface side of the first substrate, each extending in a first direction. A plurality of first electrodes present, (c) a plurality of second electrodes provided on one surface side of the second substrate, each extending in a second direction intersecting the first direction, and (d) a dielectric (E) a plurality of vertically aligned liquid crystal layers that are configured using a liquid crystal material having a negative rate anisotropy and have a pretilt angle of less than 90 ° and are interposed between the first substrate and the second substrate; Driving means for supplying a driving voltage to the first electrode and the plurality of second electrodes, wherein (f) the number of the first electrodes is N (N ≧ 32), and (g) the driving means is , 4 line simultaneous selection drive for simultaneously selecting four of the N first electrodes and supplying a selection voltage and a non-selection voltage, In the liquid crystal display device, the bias ratio a is set to satisfy the condition of (√N + 1) −3 ≦ a <√N + 1 when the bias ratio at the time of optimum bias driving in the 4-line simultaneous selection driving is set to √N + 1. is there.

  According to the above configuration, the vertical alignment type liquid crystal display device can be operated at a lower frame frequency while suppressing a decrease in display uniformity that occurs particularly during high temperature operation.

  In the above liquid crystal display device, the driving means performs n-line inversion driving that performs polarity inversion every time n of the plurality of first electrodes are selected, and the n in the n-line inversion driving is 8 ≦ n. It is also preferable that the condition is set so as to satisfy the condition of ≦ 16.

  In the above liquid crystal display device, more preferably, the n in the n-line inversion driving is set to 12.

  In the liquid crystal display device described above, more preferably, the operation for polarity inversion is completed within one frame, and the drive waveforms are equal in consecutive frames.

  The liquid crystal display device is more preferably operated in an atmosphere at a temperature of 80 ° C. or lower.

FIG. 1 is a schematic cross-sectional view showing the structure of a liquid crystal display device according to an embodiment. FIG. 2 is a schematic plan view of each first electrode 11 and each second electrode 12. FIG. 3A and FIG. 3B are photographs partially showing the appearance of the liquid crystal display device of the example. FIG. 4 is a table (table) showing the results of external appearance evaluation when the liquid crystal display device of the example is driven with various frame frequencies and bias ratios set. FIG. 5 is a diagram showing the appearance observation evaluation results when the liquid crystal display device of the embodiment is driven with the frame frequency fixed at 125 Hz, the bias ratio set variously, and the n-line inversion number set variously. ).

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a schematic cross-sectional view showing the structure of a liquid crystal display device according to an embodiment. The liquid crystal display device according to the present embodiment shown in FIG. 1 mainly includes a first substrate 1 and a second substrate 2 that are disposed to face each other, and a liquid crystal layer 3 that is disposed between the two substrates. A first polarizing plate 4 is disposed outside the first substrate 1, and a second polarizing plate 5 is disposed outside the second substrate 2. A first viewing angle compensation plate 6 is disposed between the first substrate 1 and the first polarizing plate 4, and a second viewing angle compensation plate 7 is disposed between the second substrate 2 and the second polarizing plate 5. The periphery of the liquid crystal layer 3 is sealed with a sealing material. Hereinafter, the structure of the liquid crystal display device will be described in more detail.

  The first substrate 1 and the second substrate 2 are transparent substrates such as a glass substrate and a plastic substrate, respectively. Spacers 10 are distributed between the first substrate 1 and the second substrate 2. These spacers 10 keep the gap between the first substrate 1 and the second substrate 2 at a predetermined distance.

  The liquid crystal layer 3 is provided between the first electrode 11 of the first substrate 1 and the second electrode 12 of the second substrate 2. In the present embodiment, the liquid crystal layer 3 is configured using a liquid crystal material (nematic liquid crystal material) having a negative dielectric anisotropy Δε (Δε <0). The bold line shown in the liquid crystal layer 3 schematically shows the alignment direction of the liquid crystal molecules when no voltage is applied. As shown in the figure, in the liquid crystal display device of the present embodiment, the alignment state of the liquid crystal molecules in the liquid crystal layer 3 is restricted to monodomain alignment. The pretilt angle of the liquid crystal layer 3 in the present embodiment is preferably close to 90 ° in a range smaller than 90 °, and is set to 89.85 °, for example. Further, the refractive index anisotropy Δn of the liquid crystal layer 3 is, for example, 0.18.

  The polarizing plate 4 and the polarizing plate 5 are arranged so that their absorption axes are orthogonal to each other (crossed Nicol arrangement). In addition, the polarizing plate 4 and the polarizing plate 5 each have an angle of 45 ° in each of the alignment process direction 14 applied to the first substrate 1 and the alignment process direction 13 applied to the second substrate 1. It is arranged to make. Thereby, the absorption axis of each polarizing plate 4, 5 has an angle of 45 ° with respect to the alignment direction of the liquid crystal molecules at the approximate center in the layer thickness direction of the liquid crystal layer 3 defined by the alignment treatment directions 13, 14. Will do.

  The alignment film 8 is provided on one surface side of the first substrate 1 so as to cover the first electrode 11. Similarly, the alignment film 9 is provided on one surface side of the second substrate 2 so as to cover the second electrode 12. Each alignment film 8, 9 is subjected to an alignment process such as a rubbing process. In the present embodiment, as the alignment film 8 and the alignment film 9, one that regulates the alignment state in the initial state (when no voltage is applied) of the liquid crystal layer 3 to the vertical alignment state (vertical alignment film) is used. More specifically, as each of the alignment films 8 and 9, one that can give a pretilt angle that is very close to 90 ° but smaller than 90 ° to the liquid crystal molecules of the liquid crystal layer 3 is used.

  The plurality of first electrodes 11 are provided on one surface of the first substrate 1. The plurality of second electrodes 12 are provided on one surface of the second substrate 2. Each first electrode 11 and each second electrode 12 are configured by appropriately patterning a transparent conductive film such as indium tin oxide (ITO), for example.

  The driving device (driving means) 15 is connected to each first electrode 11 and each second electrode 12 and supplies a driving voltage with a predetermined driving waveform to each first electrode 11 and each second electrode 12. To do. In the present embodiment, the driving device 15 performs multiplex driving (simple matrix driving).

  FIG. 2 is a schematic plan view of each first electrode 11 and each second electrode 12. As shown in the figure, each first electrode 11 is formed in a stripe shape, and extends in the left-right direction (first direction) in the drawing. Each second electrode 12 is formed in a stripe shape, and extends along the vertical direction (second direction) in the drawing. In the liquid crystal display device of the present embodiment, the first electrodes 11 and the second electrodes 12 are arranged so as to intersect each other in plan view, and the overlapping portions of the first electrode 11 and the second electrode 12 are pixels. The direction 13 of the alignment treatment applied to the alignment film 8 is orthogonal to the extending direction (first direction) of the first electrode 11, and the direction 14 of the alignment treatment applied to the alignment film 9 is the second electrode. 12 is parallel to the extending direction (second direction). The alignment direction 16 of the liquid crystal molecules in the layer thickness direction of the liquid crystal layer 3 is parallel to the alignment processing directions 13 and 14 as shown in the figure.

(Example)
After one side is polished and the SiO ₂ undercoat is applied to the surface, the ITO film is formed into a desired electrode pattern in the photolithography process and the etching process on the glass substrate on which the ITO film is formed. Thus, a first substrate having the first electrode and a second substrate having the second electrode were produced. If necessary, an insulating layer made of SiO ₂ or the like may be formed on a part of the surface of the electrode.

  The first substrate and the second substrate were washed with an alkaline solution, pure water, etc., and then a vertical alignment film was applied by a flexographic printing method and heated in a clean oven at 200 ° C. for 30 minutes. Thereafter, both substrates were rubbed in one position within the substrate surface using a cotton rubbing cloth. Note that the rubbing process may be performed only on one of the substrates.

  A thermosetting sealing material mixed with a rod-shaped glass spacer of about 6 μm was applied to the second substrate in a frame shape by screen printing. In addition, about 5.9 μm plastic spacers were dispersedly arranged on the first substrate by a dry spraying method. Thereafter, the electrode surfaces of both substrates were opposed to each other so that the rubbing directions were anti-parallel, and the sealing material was cured by thermocompression to complete an empty cell. In addition, the said empty cell is produced with the multi-faced mother glass substrate, and obtained one empty cell through the scribe and the breaking process.

  Next, a liquid crystal material having a refractive index anisotropy Δn of about 0.145 and a dielectric anisotropy Δε <0 was injected into the empty cell by vacuum injection. Subsequently, it was pressed so that the cell thickness became more uniform, and an ultraviolet curable resin was applied. Then, after maintaining the press thickness slightly weakened for about several minutes and sucking it in from the injection port, it was sealed by irradiating with ultraviolet rays and cured, and then baked at 120 ° C. for 1 hour.

  After chamfering the external lead electrode terminal, etc., cleaning, laminating a polarizing plate with a laminator so that it is almost crossed Nicol on the back surface of the cell, and then polarizing plate adhesive layer while heating in a vacuum vessel And bubbles between the glass substrate were removed. The pretilt angle measured by the crystal rotation method before bonding the polarizing plates was about 89.85 ° ± 0.1 °.

  The external IC terminal is connected to the driver IC input / output terminal via the anisotropic conductive film through a process of thermocompression bonding the driver IC through the anisotropic conductive film to the external extraction terminal portion, and used as a connection terminal to the external control device. .

  The longitudinal direction of the second electrode extends in the vertical direction (12 o'clock, 6 o'clock direction) of the liquid crystal display device, and the first electrode extends in the left-right direction (9 o'clock, 3 o'clock direction), and intersects each other. The rubbing direction is the 12 o'clock orientation for the second substrate on the back side, the 6 o'clock orientation for the first substrate on the front side, and the orientation orientation of the liquid crystal molecules in the center of the layer thickness direction of the liquid crystal layer is the 12 o'clock orientation. The direction was set at 6 o'clock. In addition, the liquid crystal display device of the example has pixel dimensions of 320 μm in length, 320 μm in width, and a distance between pixels of 15 μm, 369 second electrodes, and 120 first electrodes. Further, when the liquid crystal display device of the embodiment is multiplex-driven, for example, a multi-line simultaneous selection method (MLS method) disclosed in Japanese Patent Application Laid-Open No. 06-27907 is used. FIG. 3A and FIG. 3B are photographs partially showing the appearance of the liquid crystal display device of the example. Specifically, FIG. 3A is an example of a normal display state, and FIG. 3B is an example of a display state in which display uniformity is significantly reduced.

  Next, the appearance observation evaluation result of display uniformity when the liquid crystal display device of the example is multiplex driven under various conditions will be described. Here, for example, a known (non-dispersive) multiple line simultaneous selection method (hereinafter referred to as MLS method) disclosed in Japanese Patent Laid-Open No. 06-27907 is used. Specifically, the number of first electrodes 11 (the number of scanning lines) N is set to 120, the number of simultaneously selected lines M is set to 4, and a display pattern as shown in FIG. In the same manner, a driving voltage was applied from the driving device 15 and allowed to stand for several minutes, and the appearance of the display state was visually observed.

  FIG. 4 is a table (table) showing the results of external appearance evaluation when the liquid crystal display device of the example is driven with various frame frequencies and bias ratios set. In the table shown in FIG. 4, “×” is a condition in which display disturbance (DMA) occurs, “Δ” is a permissible level but a slight display disturbance occurs, and “◯” is display disturbance. This is a condition that has not occurred almost completely. As shown in the figure, as shown in the prior art, when the frame frequency is increased, a tendency to increase the effect of suppressing display disturbance was observed. The bias a is optimally √N + 1 = 12, but the appearance evaluation is “x” under all the evaluated conditions, and therefore the frame frequency needs to be further increased. Here, the maximum frame frequency is 175 Hz. However, if the frame frequency is set to a higher value than this, even if the crosstalk for bright / dark display becomes intense and display disturbance can be suppressed, display uniformity cannot be realized. On the other hand, when the bias a is lowered from the optimum bias value of 12, the tendency that display disturbance is easily suppressed is observed, and the tendency that display disturbance is easily suppressed even when the frame frequency is decreased is observed. . Therefore, it has been found that the bias a is preferably set to a value lower than the optimum bias, and a value smaller by 3 than the optimum bias is more preferred.

  Although details are omitted, the number of first electrodes 11 (the number of scanning lines) N is set to 64, the number M of simultaneously selected lines is set to 4, and the uniformity of the display state is displayed in the same manner as above in an 80 ° C. atmosphere. In the case of observation, the same tendency as above was observed. In other words, display bias can be suppressed at a lower frame frequency when the bias a is smaller than the optimal bias 9, and the effect of suppressing display disturbance can be further improved by setting the bias a to a value 3 smaller than the optimal bias. Confirmed to increase.

  Next, in the multiple line simultaneous selection driving method in which N = 120 and M = 4 as described above, n-line inversion driving is performed to invert the driving waveform every time n scanning lines are scanned, and the display is performed in an 80 ° C. atmosphere. The result of appearance observation of the state uniformity will be described.

  FIG. 5 is a diagram showing the appearance observation evaluation results when the liquid crystal display device of the embodiment is driven with the frame frequency fixed at 125 Hz, the bias ratio set variously, and the n-line inversion number set variously. ). In FIG. 5 as well, “×” indicates a condition in which display disturbance (DMA) occurs, “△” indicates an acceptable level but slight display disturbance, and “◯” indicates a display disturbance almost completely. It is a condition that has not occurred. Although the appearance observation state was evaluated when the line inversion number n was changed by a multiple of 4 at each bias setting value, the n line inversion number capable of suppressing display disturbance was disturbed except when the bias a was set to 9. There wasn't. Further, it was found that when the bias a is 9, display disturbance can be suppressed when the n-line inversion number is 8, 12, or 16. When the n line inversion number is 8 and 16, it is an acceptable level, but a slight crosstalk may be observed, and when the n line inversion number is 12, the display uniformity is the best.

  In addition, in the above-described n-line inversion driving, a driving method for ending the inversion operation within one frame and returning the inversion operation to the initial state in the first scan of the next frame was also confirmed. This driving method is a method of matching driving waveforms in consecutive frames when the scanning line number N is not divisible by the line inversion number n, and has an effect of suppressing display flicker and crosstalk, for example. Even when the scanning line number N is divisible by the line inversion number n, there is an effect of suppressing the direct current component from being superimposed. Based on the observation result shown in FIG. 5, it was confirmed whether or not the display disturbance suppression effect was obtained when the bias a was set to 9. As a result, an effect of suppressing display disturbance when the line inversion number n is 8, 12, and 16 is obtained, and particularly when the line inversion number n is 8 and 16, crosstalk is suppressed and display uniformity is improved. I found out that

  In the above-described embodiments, appearance observation was performed in an atmosphere at 80 ° C. However, an actual liquid crystal display device needs to be operated in a wide temperature range, and driving conditions can be changed by setting each temperature. For example, in an operation at room temperature, settings such as lowering the frame frequency, changing the bias, changing the number of n-line inversions, and the like may be performed. In any case, the above-described preferred conditions can be applied in an atmosphere of at least 80 ° C. or lower temperature.

  In addition, this invention is not limited to the content of embodiment mentioned above, In the range of the summary of this invention, it can change and implement variously. For example, the value of the number of scanning lines N is not limited to the above example, and N may be 32 or more.

1: First substrate 2: Second substrate 3: Liquid crystal layer 4: First polarizing plate 5: Second polarizing plate 6: First viewing angle compensation plate 7: Second viewing angle compensation plate 8, 9: Alignment film 10: Spacer 11 : First electrode 12: second electrode 13, 14: direction of alignment treatment 15: driving device 16: alignment direction of liquid crystal molecules in the layer thickness direction of the liquid crystal layer

Claims (5)

A first substrate and a second substrate disposed opposite to each other;
A plurality of first electrodes provided on one side of the first substrate, each extending in a first direction;
A plurality of second electrodes provided on one surface side of the second substrate, each extending in a second direction orthogonal to the first direction;
A vertically aligned liquid crystal layer that is configured using a liquid crystal material having a negative dielectric anisotropy and has a pretilt angle of less than 90 ° and is interposed between the first substrate and the second substrate;
Driving means for supplying a driving voltage to the plurality of first electrodes and the plurality of second electrodes;
Including
The number of the plurality of first electrodes is N (N ≧ 32),
The driving means performs four-line simultaneous selection driving for simultaneously selecting four of the N first electrodes and supplying a selection voltage and a non-selection voltage;
The liquid crystal display device, wherein the bias ratio a is set to satisfy the condition of (√N + 1) −3 ≦ a <√N + 1 when the bias ratio at the time of optimum bias driving in the 4-line simultaneous selection driving is set to √N + 1. The driving means performs n-line inversion driving that performs polarity inversion every time n of the plurality of first electrodes are selected.
The liquid crystal display device according to claim 1, wherein the n in the n-line inversion driving is set to satisfy a condition of 8 ≦ n ≦ 16.   The liquid crystal display device according to claim 2, wherein n in the n-line inversion driving is set to 12. 4.   4. The liquid crystal display device according to claim 2, wherein the operation for performing the polarity inversion is completed within one frame, and the drive waveforms are equal in consecutive frames.   The liquid crystal display device according to claim 1, wherein the liquid crystal display device is operated in an atmosphere at a temperature of 80 ° C. or lower.