JP5273368B2 - Liquid crystal display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal display of the IPS (In-Plane Switching)-Pro (Provectus) type improved in transmittance by combining am increase of torsion alignment at the pixel center and suppression of domain generated at the pixel end. <P>SOLUTION: An electrode PE close to a liquid crystal layer is of comb tooth shaped structure having connections only at one pixel end and has a structure for suppressing the growth of domain at the tip of the comb tooth shaped structure. The slit width of the comb tooth shaped structure is wider than the width of the electrode, and the alignment of the liquid crystal layer rises from the tip of the comb tooth shaped structure toward the root in an interface close to the electrode. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a liquid crystal display device.

  As the communication speed of portable information devices is increased and the memory capacity is increased, a larger amount of image information and a moving image having a larger number of frames are handled. A display device as an interface is also required to have higher image quality and larger screen than ever before, and further increase the number of pixels. On the other hand, as mobile information devices mature, designs are being reviewed, and thin smart designs tend to be preferred. Accordingly, display devices are also required to be thin.

  The IPS (In-Plane Switching) type liquid crystal display device forms a horizontal electric field mainly composed of parallel components in the liquid crystal plane between the common electrode and the pixel electrode formed on the same substrate, thereby driving the liquid crystal layer. To do. Therefore, in the IPS liquid crystal display device, the alignment change of the liquid crystal layer accompanying the application of an electric field is mainly rotated in the liquid crystal layer. In the VA (Vertically Aligned) method, ECB (Electrically Controlled Birefringence) method, OCB (Optically Compensated Birefringence) method, etc., the change in the orientation of the liquid crystal layer due to electric field application is mainly due to the change in the tilt angle, but the IPS liquid crystal display The tilt angle changes little. Thus, in the IPS liquid crystal display device, the change in the effective value of the retardation due to voltage application is small, and a display with excellent gradation reproducibility can be obtained in a wide viewing angle range. Therefore, the IPS liquid crystal display device can satisfy the demand for higher image quality.

  In addition, the VA method used for small and medium-sized liquid crystal display methods improves viewing angle characteristics by making multi-domain, but at this time, in order to prevent a decrease in transmittance, a quarter wavelength is placed between the liquid crystal panel and the upper and lower polarizing plates. Laminate the plates. The quarter wave plate contributes to the increase in the thickness of the VA liquid crystal display device. However, since the IPS liquid crystal display device does not require a quarter-wave plate, the demand for thinning can be satisfied more.

Since the liquid crystal display device is not limited to the IPS method, the display brightness is determined by the transmittance in addition to the brightness of the light source. As a study example focusing on the elastic constant of the liquid crystal layer for the purpose of improving the transmittance of the IPS system, there is, for example, Patent Document 1 below.
JP 2002-296611 A

  Among the IPS methods, the IPS-Pro (Provectus) method, in which pixel electrodes and common electrodes are stacked via an electrode insulation layer and one electrode shape is a flat (uniform with no gaps), is the IPS method. Among them, it has a high transmittance. This is because arch-shaped lines of electric force are formed so as to connect electrodes in different layers, protrude into the liquid crystal layer, and are distributed to the vicinity of the center of the electrodes. Such an arched electric field connecting electrodes between different layers is called a fringe electric field.

  Focusing on the planar structure of the IPS system, stripe-like electrodes and slits are regularly arranged in the center of the pixel. The liquid crystal layer in the center of the pixel has a twisted orientation when a voltage is applied, and the rotation of the twisted orientation should be increased to improve the transmittance of this portion. Comb electrodes must be connected to each other on at least one of the pixel ends. A planar structure such as the tip of a comb tooth is formed at the pixel end portion without the connecting portion. In any case, since the electrode has a closed structure at the pixel end, the direction of the fringe electric field is distributed so as to rotate 180 degrees.

  For this reason, a fringe electric field that always gives the liquid crystal layer a rotation opposite to the rotation that occurs at the center of the pixel is always generated at a part of the pixel end. If the liquid crystal layer is reversely twisted in the entire thickness direction, a portion where the alignment state hardly changes is generated at the boundary with the normal twisted alignment. Since the transmittance of this part is almost the same as when no voltage is applied, it is observed as a dark line. The dark line is called a domain, and it is necessary to reduce it in order to improve the transmittance at the pixel end.

  A liquid crystal display device mounted on a portable information device has been improved in definition, and accordingly, the pixel size tends to be reduced. When the pixel size is reduced, the contribution of the pixel end portion to the entire pixel is relatively increased. Therefore, in addition to an increase in twisted orientation at the center of the pixel, a decrease in transmittance due to the domain cannot be ignored.

  The problem to be solved by the present invention is to improve the transmittance of the IPS-Pro liquid crystal display device while achieving both an increase in the twisted orientation at the center of the pixel and the suppression of the domain generated at the pixel end.

  A liquid crystal panel having a liquid crystal layer sandwiched between the first substrate, the second substrate, and the first substrate; The liquid crystal panel has a plurality of independently controllable pixels, and each pixel has a pair of pixel electrodes and a common electrode on the display portion on the liquid crystal layer side of the second substrate. The pixel electrode and the common electrode are stacked via an inter-electrode insulating layer, and the planar structure of the pixel electrode and the common electrode that is further away from the liquid crystal layer is a solid flat surface, and a component parallel to the liquid crystal surface is mainly used. In a liquid crystal display device that drives the liquid crystal layer by applying an electric field, the sum of the width of the comb electrode and the width of the slit separating the comb electrode is defined as (comb electrode pitch × interelectrode insulation). The relative dielectric constant of the film / the thickness of the insulating layer between the electrodes is in the range of 81 to 162 and the comb electrode pitch is 2.5 μm to 8.5 μm In the range below, (comb-tooth electrode width / comb electrodes pitch) of 0.3 or more, in the range of 0.5 or less.

  According to the present invention, in the IPS-Pro liquid crystal display device, the twisted orientation of the liquid crystal layer in the main part of the comb electrode is increased, and the domain appearing at the end of the comb electrode is suppressed, and the IPS-Pro liquid crystal display device Can improve the transmittance.

Embodiment 1
A cross section of one pixel of the liquid crystal display device of the present embodiment is schematically shown in FIG. The cut surfaces S1, S2, S3 and S4 in FIG. 2 are shown in FIG. 3 which is a plan view. The liquid crystal display device of this embodiment mainly includes a first substrate, a second substrate, and a liquid crystal layer, and the first substrate and the second substrate sandwich the liquid crystal layer. The first substrate and the second substrate include an alignment film for stabilizing the alignment state of the liquid crystal layer on a surface close to the liquid crystal layer. Further, a means for applying a voltage to the liquid crystal layer is provided on the surface of the second substrate close to the liquid crystal layer.

  The first substrate is made of borosilicate glass with a thickness of about 0.4 mm. Focusing on the part corresponding to the pixel, the first alignment film, flattening film, color filter, and black matrix are arranged from the side close to the liquid crystal layer. They are sequentially stacked. The first alignment film is a polyimide organic polymer film, which is aligned by a rubbing method, and is a horizontal alignment film that imparts a pretilt angle of about 1.5 degrees to an adjacent liquid crystal layer. The flattening film is an acrylic resin, has excellent transparency, and has a function of flattening the unevenness of the base and preventing the penetration of the solution. The color filter is composed of a resist containing a pigment exhibiting red, green, and blue.

  Like the first substrate, the second substrate is made of borosilicate glass, and in order from the side close to the liquid crystal layer, the second alignment film, the pixel electrode, the interelectrode insulating film, the common electrode, the TFT (Thin Film Transistor), scanning wiring, and signal wiring. The second alignment film is a horizontal alignment film similar to the first alignment film. The pixel electrode and the common electrode are both ITO (Indium Tin Oxide), and are excellent in transparency and conductivity, and the layer thickness is 80 nm. The pixel electrode and the common electrode are insulated by an interelectrode insulating film made of silicon nitride (SiN), and the layer thickness of the interelectrode insulating film is 300 nm. The planar shape of the pixel electrode is a comb-like shape joined at the pixel end, whereas the common electrode is a solid planar shape in the transparent portion of the pixel.

  As described above, by separating the pixel electrode and the common electrode by the interelectrode insulating film and sufficiently reducing the film thickness of the interelectrode insulating film, an arch-shaped electric field line is formed between the pixel electrode and the common electrode. The At this time, lines of electric force are distributed mainly through the liquid crystal layer through the interelectrode insulating film, and form a transverse electric field having a component mainly parallel to the substrate plane. This gives a change in alignment peculiar to the IPS system in which the liquid crystal alignment direction changes so as to rotate mainly in the layer plane when a voltage is applied.

  In addition, as shown in FIG. 2, there are many portions where the pixel electrode and the common electrode overlap each other, but since this portion is coupled in parallel to the liquid crystal layer, it is applied to the liquid crystal layer during the holding period. It functions as a transparent holding capacitor that maintains a constant voltage value. Since the storage capacitor is transparent and the liquid crystal layer on the storage capacitor can be driven, both a high aperture ratio and storage characteristics can be achieved.

  As shown in FIG. 3, the stripe of the pixel electrode is parallel to the long side direction of the pixel, and the direction of the slit structure is uniform within the pixel. When the scanning wiring direction is defined as 0 degree and the azimuth angle is defined counterclockwise, the direction of each stripe structure is 90 degrees and the liquid crystal alignment direction is 85 degrees. The stripes are joined on the lower side of FIG. 3, and on the upper side, the tips of the stripes are arranged like the tips of comb teeth.

  FIG. 4 shows an example of a pixel electrode different from this, and the stripe of the pixel electrode is directed in a direction closer to the short side of the pixel. A region where the angle in the direction of the slit structure is 5 degrees and a region where the angle is -5 degrees are present in one pixel, and the area ratio is 1: 1. In these two regions, the rotation directions of the liquid crystal alignment directions at the time of voltage application are different from each other, that is, if one is clockwise, the other is counterclockwise. Each region has a viewing angle direction in which the bright display indicates a yellow hue coloring and a viewing angle direction in which a blue hue coloring is observed. However, since both are observed simultaneously, the coloring is offset. As a result, a more colorless display is obtained in the viewing angle direction.

  As shown in FIG. 3, the signal wiring and the scanning wiring intersect each other, each has a TFT near the intersection, and corresponds to the pixel electrode on a one-to-one basis. A potential corresponding to an image signal is applied to the pixel electrode from the signal wiring through the TFT and the contact hole. Further, the operation of the TFT is controlled by the scanning signal of the scanning wiring. The channel part of TFT consists of an amorphous silicon layer. Alternatively, a polysilicon layer with higher mobility may be formed. The channel portion is covered with a channel portion inorganic insulating film to stabilize the operation, the channel portion organic insulating film is used to flatten the unevenness, and the parasitic capacitance is reduced. Each pixel electrode is rectangular and is controlled independently of each other, and is arranged in a grid pattern on the second substrate. The pixel electrode is connected to the TFT at the through hole.

  For the liquid crystal layer, a liquid crystal material that exhibits a nematic phase in a wide temperature range including room temperature, has a positive dielectric anisotropy, and has a high resistance is used. The voltage drop during the holding period when the pixel writing TFT is turned off is sufficiently small, the transmittance can be maintained during the holding period, and the occurrence of flicker can be prevented.

  After the alignment process is performed on the first alignment film and the second alignment film by a rubbing method, the first substrate and the second substrate are assembled. The gap between the first substrate and the second substrate is uniformly maintained by a post spacer disposed on the first substrate side. The post spacers are substantially cylindrical and are distributed between the pixels. A liquid crystal material is vacuum-sealed in the gap between the first substrate and the second substrate to form the aforementioned liquid crystal layer. The liquid crystal layer has homogeneous alignment, and the alignment direction forms 5 degrees with respect to the scanning wiring. Since the angle formed by the electric field direction and the alignment direction when a voltage is applied is sufficiently large as 85 degrees, a large rotation of the liquid crystal alignment direction occurs when a voltage is applied, and high transmittance can be obtained.

  A first polarizing plate and a second polarizing plate are arranged outside the first substrate and the second substrate, and the first polarizing plate and the second polarizing plate contain iodine dye, Natural light is converted into partially polarized light very close to linearly polarized light by chromaticity. The absorption axes of the first polarizing plate and the second polarizing plate are orthogonal to each other when observed from the plane normal direction, and the absorption axis of the first polarizing plate is parallel to the liquid crystal alignment direction.

  As is clear from the pixel structure of the IPS-Pro liquid crystal display device shown in FIGS. 3 and 4, comb electrodes and slits are regularly arranged at the center of one pixel, but this is cut off at the pixel end. ing. If the distribution of the comb electrode and the slit is different, the distribution of the fringe electric field is also different, so that the change in the orientation of the liquid crystal layer upon voltage application is also different. Specifically, the fringe electric field is directed in a certain direction in the part where the comb-shaped electrodes and slits are regularly arranged. On the other hand, the comb electrode is closed at the tip of the comb structure, and the slit is closed at the joint, and the direction of the fringe electric field is also rotated by 180 degrees and distributed. When improving the transmittance of an IPS-Pro liquid crystal display device, these parts should be separated and measures for improving the transmittance should be considered for each. In the following description, a portion where the comb electrodes and slits are regularly arranged will be referred to as a comb electrode portion, and an end portion thereof will be referred to as a comb electrode end portion. Moreover, the latter is classified into a connection part end part and a tip part.

  In the present embodiment, attention is paid to the comb electrode portion. The liquid crystal layer alignment state of the IPS-Pro liquid crystal display device when a voltage was applied was calculated using a two-dimensional alignment calculation program manufactured by Shintech. The calculation result is expressed as a distribution of tilt angle and azimuth angle in the orientation direction as shown in FIG. The horizontal axis of FIG. 5 indicates the position in the liquid crystal layer thickness direction in the liquid crystal layer, normalized by the liquid crystal layer thickness, the first substrate end is 1, and the second substrate end is 0. The azimuth angle is defined to increase clockwise with the azimuth angle when no voltage is applied being 0 degrees. The three plots in FIG. 5 correspond to three representative portions in the pixel of the IPS-Pro liquid crystal display device. That is, the center of the comb electrode, the center of the slit, and the boundary between the comb electrode and the slit. Overall, the azimuth angle shown in FIG. 5 (a) is larger than the tilt angle shown in FIG. 5 (b), and the liquid crystal layer alignment of the IPS-Pro liquid crystal display device when a voltage is applied. This indicates that the change is represented by rotation in the liquid crystal layer. Further, the azimuth angle increases rapidly in the vicinity of the second substrate having a large fringe electric field strength, and the azimuth angle changes in proportion to the liquid crystal layer thickness between the first substrate edge. This indicates that a twisted orientation is formed between the vicinity of the second substrate and the end of the first substrate.

  Such a liquid crystal layer alignment state is approximately represented by a twisted alignment model shown in FIG. The azimuth angle of the liquid crystal alignment at the first substrate end is the alignment processing direction of the first alignment film. It is assumed that the azimuth angle of the liquid crystal alignment at the edge of the second substrate is deviated from the alignment processing direction of the second alignment film, and twisted alignment occurs between them. In addition, the fluctuation of the tilt angle is ignored, and is assumed to be equal when no voltage is applied.

  From the viewpoint of improving the transmittance, looking at the twisted orientation model, the wave guide mode similar to the TN (Tweited Nematic) method is ideal. That is, if the twist angle is 90 degrees and Δnd is 550 nm, the light with the wavelength of 550 nm, which has the maximum visibility, rotates 90 degrees according to the alignment direction of the liquid crystal layer and reaches the first substrate edge. It becomes linearly polarized light whose vibration direction is parallel to the transmission axis of one polarizing plate, and at this time, the maximum transmittance is obtained.

  However, if Δnd of the liquid crystal layer is increased to 550 nm in order to realize the wave guide mode, a halftone hue change increases. Further, in order to set the twist angle to 90 degrees, the alignment direction of the liquid crystal layer must be parallel to the comb tooth direction, but at this time, the operation of the liquid crystal layer becomes unstable. Thus, although the wave guide mode cannot be realized perfectly, a high transmittance close to the wave guide mode can be obtained if an orientation state close to this is realized. Therefore, it is possible to improve the transmittance while maintaining good hue change of the halftone and the liquid crystal layer operation, and the twisted orientation model provides a guideline for improving the transmittance of the IPS-Pro liquid crystal display device. That is, an increase in twist angle (maximum azimuth angle) and a decrease in tilt angle. Among these, the latter means increasing the effective Δnd while keeping the Δn of the liquid crystal material and the liquid crystal layer thickness constant.

  In order to realize liquid crystal alignment such as a twisted alignment model, it is only necessary to localize the fringe electric field by the edge of the second substrate and increase its strength. This is because if the distribution of the fringe electric field is brought closer to the second substrate end, the portion with the maximum twist angle is closer to the second substrate end. In order to reduce the tilt angle, the fringe electric field may be arranged with a shorter period. The increase in the tilt angle is caused by the alignment of the liquid crystal layer along the arcuate fringe electric field. However, since the fringe electric field is arcuate, tilt angles with different signs are generated at both ends thereof. As a result, bend alignment occurs in the liquid crystal layer along the fringe electric field. When the stress and the bend elastic constant k33 are constant, the tilt angle change per unit length is constant. Therefore, the tilt angle decreases as the comb tooth pitch is reduced.

  Further, the reduction of the comb tooth pitch is also effective in localizing the fringe electric field at the end of the second substrate, and the fringe electric field is reduced in proportion to the comb electrode pitch. Since the interelectrode insulating film and the liquid crystal layer are coupled in series to the fringe electric field, the interelectrode insulating film is made thinner or the interelectrode electrode is increased in order to increase the strength of the fringe electric field distributed in the liquid crystal layer. What is necessary is just to increase the dielectric constant of an insulating film. However, at this time, the fringe electric field distribution expands in the thickness direction of the liquid crystal layer and is not localized at the edge of the second substrate. If the comb electrode pitch is reduced while maintaining a constant value obtained by multiplying the ratio between the comb electrode pitch and the inter-electrode insulation film thickness by the relative dielectric constant of the inter-electrode insulation film, the fringe electric field distribution is also reduced by the comb electrode pitch and the electrode. Shrink while maintaining a similar relationship with the insulation film thickness. Thereby, this can be localized at the edge of the second substrate while maintaining the strength of the fringe electric field.

  The uniformity of the fringe electric field distribution is important in improving the transmittance in the comb electrode part, and this involves the thickness of the interelectrode insulating film and the relative dielectric constant in addition to the electrode width ratio described later. . When the interelectrode insulating film thickness is reduced and the relative dielectric constant of the interelectrode insulating film is increased to increase the fringe electric field strength inside the liquid crystal layer, the fringe electric field distribution becomes more non-uniform. Further, when these are reduced, the strength of the fringe electric field distributed in the liquid crystal layer is reduced as described above. In order to achieve both the uniformity and strength of the fringe electric field, the value obtained by multiplying the ratio of the comb electrode pitch to the interelectrode insulating film thickness by the relative dielectric constant of the interelectrode insulating film should be kept within a certain range. Since the interelectrode insulating film thickness is 0.2 μm or more and 0.4 μm or less when the interdigital electrode pitch is 5 μm and the relative dielectric constant of the interelectrode insulating film thickness is 6.5, the interdigital electrode pitch and the interelectrode insulating film thickness The optimum range of values obtained by multiplying the ratio by the relative dielectric constant of the interelectrode insulating film is determined to be 81 or more and 162 or less.

  Currently, high definition of medium and small-sized liquid crystal display devices is progressing, and wide VGA (Video Gate Array) specification liquid crystal display devices, which are becoming mainstream as mobile phone monitors, have 800 × 480 × 3 pixels. In addition, the number of pixels is 640 × 480 × 3 in a VGA type liquid crystal display device which is becoming mainstream as a monitor of a digital camera. When such a pixel specification is applied to a liquid crystal display device having a diagonal of about 3 inches, the size of one pixel is 75 μm × 25 μm.

  In such a minute pixel, the transmittance greatly varies depending on the number of comb-tooth electrodes to be arranged. For example, when a comb electrode is arranged in parallel to the long side direction of the pixel, a gap of about 6.5 μm must be arranged at the pixel end in order to prevent color mixing between adjacent pixels, so that the comb electrode can be arranged. The width of the part is 12 μm. If only one comb electrode is disposed in this portion, the transmittance is significantly reduced. That is, at this time, the fringe electric field is only formed in two columns in the pixel, and its distribution is insufficient to sufficiently change the orientation of the liquid crystal layer in the pixel. If the number of comb electrodes is increased to 2, 3, and 4, the fringe electric field also increases to 4, 6, and 8, and the transmittance increases accordingly. Based on the case of a single comb electrode, the transmittance increases 1.52, 1.65, and 1.71 times. Here, the electrode width ratio described later is 0.4, which is the optimum value, and the comb electrode pitches correspond to 8.5 μm, 5.0 μm, and 3.5 μm, respectively. Thus, the increase in transmittance is particularly large when the number of comb electrodes is increased from one to two.

  Here, the color mixture between adjacent pixels is a phenomenon that occurs when the liquid crystal layer of an adjacent pixel to which no voltage is applied causes an orientation change due to the influence of the own pixel. In order to prevent this, it is necessary to set a sufficiently wide gap between the interdigital electrode in the pixel located at the end of the pixel and the pixel boundary, which is the aforementioned 6.5 μm gap.

  The comb electrode pitch has a lower limit due to manufacturing limitations. The common electrode in the lower layer of the interelectrode insulating film is not completely flat, and there are irregularities due to crystal grains or the like generated when ITO is formed. Therefore, if the interelectrode insulating film is thinned, the possibility that the unevenness penetrates the interelectrode insulating film and the common electrode and the pixel electrode are short-circuited increases. At present, the lower limit of the thickness of the interelectrode insulating film is about 0.1 μm, and in order to obtain sufficient transmittance under these conditions, if the relative dielectric constant of the interelectrode insulating film is 6.5 of the SiN film, the comb-tooth electrode The lower limit of the pitch is required to be about 2.5 μm.

  In addition, the ratio between the comb electrode width and the slit width also affects the strength of the fringe electric field. In the IPS-Pro type liquid crystal display device, a voltage is applied between electrodes having one comb-like shape and the other having a solid flat shape, and the outlet of the lines of electric force becomes a slit portion. Therefore, as shown below, the distribution of the fringe electric field becomes the most uniform at an electrode width ratio of 0.4 where the width of the slit portion is slightly wider. The electrode width ratio is defined by {electrode width / (electrode width + slit width)}.

  FIG. 7 shows the results of calculating the BV characteristics by fixing one pitch of the comb electrode to 5 μm and changing the ratio between the electrode width and the slit width. In FIG. 7, the electrode width ratio is changed by 0.1 in the range from 0.1 to 0.9, and the numbers in FIG. 7 indicate the electrode width ratio of each BV characteristic. When the electrode width ratio is 0.4, the BV characteristic is shifted to the lowest voltage side, and the transmittance at the low voltage side of the applied voltage giving the maximum transmittance, for example, the transmittance at 4.5 V is also maximized. If the electrode width ratio deviates from this, the B-V characteristic shifts to the high voltage side as a whole, and the transmittance at 4.5 V also decreases accordingly. Focusing on the transmittance in the low voltage region, for example, the transmittance at 4.5 V, the maximum is obtained when the electrode width ratio is 0.4.

  In addition, when the electrode width ratio is 0.4 and 0.5, the latter BV characteristic is slightly shifted to the high voltage side, but the change is relatively small. Similarly, the change is relatively small when the electrode width ratio is 0.4 and 0.3. However, comparing the B-V characteristics with electrode width ratios of 0.6 and 0.7, it can be seen that the latter is greatly shifted to the high voltage side. Even if the B-V characteristics with the electrode width ratio of 0.3 and 0.2 are compared, the latter is also greatly shifted to the high voltage side.

  The above is also apparent from the electrode width ratio dependence of the transmittance at an applied voltage of 4.5 V shown in FIG. 8 (a). The transmittance at an applied voltage of 4.5 V is maximum at an electrode width ratio of 0.4, and the change around it is gradual. In addition, the BV characteristic of FIG. 7 is corrected to the electrode width ratio dependency of 0.5 Vmax and is shown in FIG. 8 (b). Here, 0.5 Vmax is an applied voltage value that gives a half value of the maximum transmittance of each BV characteristic. 0.5Vmax is minimum at the electrode width ratio of 0.4, and the change around it is gradual. From the above, in the vicinity of the electrode width ratio of 0.4, high transmittance is given in the low voltage range of about 4.5 V applied voltage, and even if the electrode width ratio fluctuates, the change in the BV characteristic is small. However, when the electrode width ratio is increased from 0.4, the change in the B-V characteristic when the electrode width ratio fluctuates increases.

FIGS. 8 to 10 show the orientation change and transmittance distribution of the liquid crystal layer in the IPS-Pro liquid crystal display device. FIG. 9 shows a case where the electrode width ratio is 0.4 and includes a cross section of about 2 pitches as the pitch of the comb electrodes. Regardless of the slit center, comb electrode center, and slit-comb electrode boundary, the orientation change is relatively constant within one pitch of the comb electrode (Fig. 9 (b)), and the transmittance distribution is also the comb electrode. Is relatively constant within one pitch (FIG. 9 (a)). FIG. 10 shows the case where the electrode width ratio is 0.2, and the electric field becomes sparse at the center of the slit, and the change in the orientation of the liquid crystal layer becomes small (FIG. 10 (b)). Also, the transmittance distribution is shown by a solid line in Fig. 10 (a).
It was shown to. The broken line in FIG. 10 (a) is the transmittance distribution when the electrode width ratio shown in FIG. 9 (a) is 0.4. The transmittance is reduced at the center of the slit, which is lower than that of the broken line. FIG. 11 shows a case where the electrode width ratio is 0.7. In contrast to FIG. 10, the electric field at the center of the pixel electrode is sparse. Since the change in orientation of the liquid crystal layer at the center of the pixel electrode is small (FIG. 11 (a)), the transmittance is reduced (FIG. 11 (b)). Thus, by setting the electrode width ratio to a value in the vicinity of 0.4, it is possible to increase the transmittance of a portion away from the end portion in the pixel. As apparent from FIG. 8, the optimum range of the electrode width ratio is 0.3 to 0.5.

  Focusing on the comb-tooth electrode portion of the IPS-Pro liquid crystal display device, the comb-tooth electrode is used to increase the fringe electric field strength in the liquid crystal layer and localize the distribution at the edge of the second substrate based on the twisted orientation model. The parameters of the interelectrode insulating film were obtained. As a result, (comb electrode pitch × dielectric constant of interelectrode insulating film / interelectrode insulating layer thickness) is set in the range of 81 to 162 and the comb electrode pitch is in the range of 2.5 μm to 8.5 μm. And the conclusion that (comb electrode width / comb electrode pitch) should be set in the range of 0.3 to 0.5. By setting the parameters of the comb electrode and the interelectrode insulating film within the above range, the transmittance of the comb electrode portion of the IPS-Pro liquid crystal display device can be improved.

Embodiment 2
In this embodiment, an attempt was made to improve the transmittance of the end portion of the comb electrode. Specifically, we focused on the pixel edge domain and tried to improve the transmittance of the IPS-Pro liquid crystal display device by suppressing it. First of all, we tried to elucidate the cause of pixel edge domain. In the IPS system, the change in liquid crystal alignment that occurs when a voltage is applied is rotation within the liquid crystal layer, and this rotation direction is reversed within the domain. The change in liquid crystal alignment that occurs when a voltage is applied is selected from the clockwise and counterclockwise directions in which the liquid crystal alignment direction is parallel to the fringe electric field with a smaller rotation. This is determined by the relationship between the direction of the fringe electric field and the liquid crystal alignment direction when no voltage is applied.

  The IPS-Pro pixel electrodes are classified into a structure having connecting portions on both sides of the pixel end and a structure having only one side. Hereinafter, the former is called a slit structure and the latter is called a comb-tooth structure. The pixel electrodes shown in FIGS. 3 and 4 have a comb-teeth structure, and corresponding pixel electrodes having a slit structure are shown in FIGS. 12 (a) and 12 (b). In both the slit structure and the comb structure, the fringe electric field direction rotates 180 degrees at the pixel end portion, so that there is always a portion that gives the liquid crystal layer a reverse rotation. 3, 4 and 12 are ideal pixel electrodes assumed when the processing accuracy is sufficiently high, and an acute angle appears at the end of the pixel electrode.

  FIGS. 13A and 13B are enlarged views of pixel electrodes having a comb-tooth structure and a slit structure, and FIG. 13A includes a triangular structure for domain suppression described later. FIGS. 13 (a) and 13 (b) show the rotation direction of the liquid crystal alignment that occurs when a voltage is applied, the solid line is the liquid crystal alignment direction when no voltage is applied, and the broken line is the direction of the fringe electric field. The portions with different liquid crystal alignments are classified by attaching symbols A to D, and the portions that give reverse rotation are shown circled. G is located outside the comb electrode and does not affect the liquid crystal alignment state inside the pixel electrode. In the comb-tooth structure, only the corner vertices B and E are located on the inner side of the pixel electrode and give reverse rotation. Similarly, in the case of the slit structure, only the corner apex B is provided. As described above, when the pixel end portion has an acute angle, the portion that gives reverse rotation should exist only at the vertex of the corner inside the pixel electrode.

  Actually, since the processing accuracy is limited, the edge of the pixel is rounded off with an acute angle. In this case, the rotation direction of the liquid crystal alignment generated when a voltage is applied is shown in FIGS. 14 (a) and 14 (b). Due to the disappearance of the acute angle at the pixel end, B and E are not limited to one point but have a width. Therefore, the part which gives a reverse rotation to a liquid-crystal layer expands compared with the case where it has an acute angle.

  FIG. 15 schematically shows the result of reproducing the transmittance and alignment state of the pixel end portion with an LCD master, which is a three-dimensional liquid crystal alignment calculation program manufactured by Shintech. FIG. 15B shows the liquid crystal alignment state in the sections S5 and S6 shown in FIG. 6, and FIG. 15A shows the transmittance distribution corresponding thereto. A portion surrounded by a thick line is reverse twisted orientation, which appears at the liquid crystal interface close to the pixel electrode. Thus, it is considered that the reverse twisted liquid crystal alignment always exists at the liquid crystal interface close to the pixel electrode. However, if the regulation force from the surrounding normal orientation part is strong, it will not grow any more. The transmittance distribution at this time does not show a clear minimum point as shown in FIG. 15 (a). Even if attention is paid to the relationship with the transmittance distribution, the transmittance distribution related to the distribution of the reverse twist orientation is not observed, so that it is not observed as a domain.

  However, there is a case where the reverse twist orientation overcomes the regulating force from the surrounding normal orientation portion and grows over the entire thickness direction of the liquid crystal layer. This corresponds to the case where the acute angle at the pixel end disappears. FIG. 16 shows the electro-optic calculation result at the pixel end at this time. FIG. 16B shows the liquid crystal alignment state in the S7 and S8 cross sections shown in FIG. 14, and FIG. 16A shows the transmittance distribution corresponding thereto. The reverse twist orientation surrounded by the thick line extends over the entire thickness direction of the liquid crystal layer. In addition, a clear minimum point related to the distribution of reverse twist orientation appears in the transmittance distribution. The boundary between the reverse twisted orientation and the normal orientation portion is observed as a dark line because the change in the orientation of the liquid crystal is small and the orientation is almost the same as when no voltage is applied. As described above, when the acute angle at the end of the pixel electrode disappears, the domain easily appears.

  Here, both the cylindrical model and the nail model are used to show the alignment state of the liquid crystal layer, but the latter is used particularly when the tilt angle is clearly shown in the plan view.

  In the connection portion at the pixel end, no voltage is applied to the liquid crystal layer, so that the transmittance does not increase. For this reason, the comb-tooth structure on one side is more advantageous than the slit structure on both ends for improving the transmittance. In this case, the domains are generated at the tip of the comb-tooth structure and the end of the connecting portion. In the present embodiment, attention is paid to the latter connecting portion end domain. This was reproduced by liquid crystal alignment calculation, and the alignment state inside the connecting portion end domain was observed. FIG. 17 (a) shows the distribution state of the connecting portion end domain.The connecting portion end domain is distributed so as to connect the adjacent pixel electrode and the central portion of the slit, and has a U-shaped shape that lies sideways. Show. FIG. 17 (b) is a diagram showing the alignment state of the connection end domain and the normal part focusing on the azimuth distribution in the alignment direction. The liquid crystal alignment state in the layer plane where the fringe electric field is strongest in the liquid crystal layer is shown. Show. As a result, a twisted orientation opposite to that of the normal portion is generated inside the connecting portion end domain.

  Also, FIG. 17 (d) shows the result of detailed examination of the orientation state inside the domain including the tilt angle. Here, the tilt angle rising toward the right side in FIG. 17 (d) is defined as positive, and the tilt angle rising toward the left side is defined as negative. Inside the domain, there are a positive part and a negative part of the tilt angle, and the rotation angle is maximum at the boundary between them (tilt angle = 0 degree). Further, the areas of the positive part and the negative part of the tilt angle are not equal, and the area of the positive part is larger in FIG. 17 (d). On the other hand, in FIG. 16 (b), the negative part is large. In a similar calculation when the connecting portion is on the left side, the area of the negative portion is larger as shown in FIG. 17 (c). In summary, the tilt angle distribution within the domain is biased, and the dominant tilt angle component is reversed when the connecting portion is on the left side and on the right side.

  Next, the reason why the orientation state inside the connecting portion end domain has the above distribution was considered as shown in FIG. FIG. 18 shows a connecting portion corresponding to two pitches of the pixel electrode whose connecting portion end is at the left end, and FIGS. 18 (a), (b), and (c) show the case where the slit width is wider than the pixel electrode width. Are equal, the pixel electrode width is wider than the slit width. As a result of observing the relationship between the orientation state inside the coupling end domain and the pixel electrode structure, it was found that the boundary between the positive and negative tilt angle regions is located at the boundary between the pixel electrode and the slit as shown by the broken line in FIG. . This part seems to have the maximum rotation angle because the fringe electric field gives the liquid crystal layer a reverse rotation. Further, as described above, the position of the connecting portion end domain is related to the electrode structure, and the U-shaped linear portion that lies down is located at the central portion of the pixel electrode and the central portion of the slit. Since the transmittance of the central portion of the pixel electrode and the central portion of the slit is minimized, it is observed as a dark line although it is not as clear as the connection end domain. The connecting portion end domain is distributed from the central portion of the pixel electrode adjacent to each other to the central portion of the slit, and the boundary between the positive and negative tilt angle regions is located at the boundary between the pixel electrode and the slit. Therefore, if the electrode width and the slit width are not the same, the tilt angle distribution is biased inside the connecting portion end domain as shown in FIGS. 18 (a) and 18 (c). Further, if the electrode width and the slit width are the same, the tilt angle distribution is not biased inside the coupling portion end domain as shown in FIG. 18 (b).

  On the other hand, as described in the first embodiment, the transmittance-applied voltage characteristic (BV characteristic) of the IPS-Pro liquid crystal display device depends on the ratio of the electrode width and the slit width. Improves the transmittance. Since the ratio between the electrode width and the slit width is made non-uniform in order to improve the transmittance, the tilt angle distribution is biased inside the connecting portion end domain.

  In the present embodiment, an attempt was made to control the size of the connecting portion end domain with the pretilt angle by utilizing the bias of the tilt angle distribution inside the connecting portion end domain as described above. The principle will be described below. First, the interaction of liquid crystal deformation was considered as shown in FIG. There are torsional deformation and spray deformation in the deformation of the liquid crystal layer. However, if the tilt angle of the upper and lower end surfaces of the liquid crystal layer of interest is 0 degree, the torsional deformation and the spray deformation are independent of each other. FIG. 19 shows a case where the twist angle is 180 degrees as an example of the case where the twist deformation occurs. As shown in FIGS. 19 (a) and 19 (b), if the tilt angle of the upper and lower end surfaces of the liquid crystal layer of interest is 0 °, the twist angle is 0 ° or 180 °. Regardless, spray deformation does not occur. However, if the tilt angle is not 0 degree, the torsional deformation and the spray deformation are related to each other. If the tilt angle is the same sign on the upper and lower end surfaces of the liquid crystal layer, the spray deformation increases as the twist angle increases as shown in FIGS. 19 (c) and 19 (d). If the tilt angle is opposite in the upper and lower end surfaces of the liquid crystal layer, spray deformation decreases as the twist angle increases as shown in FIGS. 19 (e) and 19 (f).

  Based on the above, the relationship between the tilt angle inside the coupling end domain and the pretilt angle at the liquid crystal layer interface close to the electrode was considered as shown in FIG. FIG. 20 shows four combinations in which the tilt angles of the upper and lower end surfaces are positive and negative, respectively, and a case in which both tilt angles are 0 degrees. In this case, the upper and lower end surfaces of the liquid crystal layer of interest are the portions where the fringe electric field is strongest and the liquid crystal interface close to the electrodes. In particular, the latter tilt angle corresponds to a pretilt angle. Twist and spray deformation occur simultaneously in the liquid crystal layer when a voltage is applied, but normally k11 is more than twice as large as k22, so that the twist angle changes to alleviate steep spray deformation. The method of changing the twist angle at this time varies depending on the sign of the pretilt angle. Based on the case where the tilt angle of the upper and lower end surfaces shown in FIG. 20 (c) is 0 degree, when the tilt angle and the pretilt angle inside the connecting portion end domain are the same sign, FIG. 20 (a), (e) As shown in FIG. 5, the twist angle increases to alleviate the spray deformation. In the case of the reverse sign, the twist angle decreases as shown in FIGS. 20 (b) and 20 (d).

  Furthermore, since the alignment direction of the liquid crystal layer changes like a continuum, the size of the connecting portion end domain changes according to the magnitude of the twist angle. As described above, the twisted orientation inside the domain is counter-twisted, so that a portion having no orientation change is formed with the surrounding normal liquid crystal orientation. This part becomes a dark line and is observed as a connecting part end domain, but as the reverse twist angle inside the connecting part end domain increases, the part without orientation change is pushed to the normal orientation side, and as a result, the connecting part end domain expands. To do. As described above, the distribution of the domain is a U-shape that lies sideways, but when the end domain of the connecting portion expands, the straight portion of the U-shape expands. When the reverse twist angle is small, the connecting portion end domain is reduced so that the straight line portion of the U-shape is shortened.

  FIG. 21 collectively shows changes in the size of the connecting portion end domain depending on the pretilt angle when the connecting portion end domain is on the right side and on the left side. The sign in FIG. 21 is the sign of the tilt angle at the strongest part of the fringe electric field. There are a positive part and a negative part in the connecting part end domain, but if the area of both is the same, the effect of expansion and contraction cancels and the size of the connecting part end domain does not depend on the pretilt angle. It does not change. However, in practice, the slit width is set wider than the electrode width in order to improve the transmittance. One of them expands with a sign opposite to the sign of the pretilt angle, and the other contracts with a sign of the forward tilt, but the total size of the connecting end domain is approximately the sum of the two. Therefore, the overall size of the connecting portion end domain is determined by the dominant pretilt angle component.

  When a polyimide-based alignment film is subjected to an alignment process in a rubbing process, a pretilt angle is imparted to the liquid crystal layer. On the other hand, a reduction in the pretilt angle is effective for improving viewing angle characteristics during black display. Considering both, the pretilt angle was set to 1.5 degrees, but even with such a relatively small pretilt angle, the size of the domain changes depending on the order of the signs. In addition, since the absolute value of the pretilt angle has little change on the domain size, attention should be paid to the order of the tilt angle in the strongest part of the fringe electric field and the sign of the pretilt angle for domain control.

  When the connecting portion is on the right side, the portion where the tilt angle is positive in the connecting portion end domain is dominant, so if the rubbing condition is set so that the pretilt angle becomes positive, as shown in FIG. The connecting end domain is reduced. When the connecting part is on the left side, the part with the negative tilt angle is dominant, so if the rubbing condition is set so that the pretilt angle becomes negative, the end part domain of the connecting part as shown in FIG. Shrinks.

  The connection end domain causes a short-time afterimage due to application of a high voltage or external pressure. That is, the pixel end domain is expanded by these stimuli and extends to the center of the pixel. Since it takes time for the extended domain to return to its original size after removal of the stimulus, it becomes a short afterimage. The suppression of the connecting portion end domains by setting the pretilt angle as described above is also effective in preventing afterimages related to these domains.

  The sign of the pretilt angle can be evaluated from the viewing angle characteristics during dark display. The first polarizing plate and the second polarizing plate are arranged so that the absorption axes are orthogonal to each other when observed from the normal direction. However, when this is observed from an oblique direction, as described in T.Ishinabe, T.Miyashita, T.Uchda, Y. Fujimura, non-patent literature Asia Display / IDW'01 Proceedings pages 485-488, two sheets Since the orthogonal relationship of the polarizing plates changes, the transmittance increases. In particular, in the azimuth angle direction that forms 45 degrees with respect to the absorption axis of the polarizing plate, the increase in transmittance when the polar angle is observed is remarkable. That is, in the viewing angle characteristic of the dark display transmittance, the high transmittance region and the low transmittance region are distributed at 90 degree intervals as azimuth angles. FIG. 22 shows the pretilt angle dependency of the viewing angle characteristics during dark display of the IPS liquid crystal display device. In FIG. 22, italic letters indicate polar angles and true letters indicate azimuth angles. When the pretilt angle is 0 degree, the viewing angle characteristics during dark display are symmetric as shown in FIG. 22 (c), but when the pretilt angle is not 0 degree, as shown in FIGS. 22 (a) and (b). Becomes asymmetric. At this time, the direction in which the transmittance becomes larger is the direction in which the pretilt angle rises. In FIG. 22, the liquid crystal layer is homogeneously aligned and the horizontal direction is the liquid crystal alignment direction, and FIGS. 22 (a) and 22 (b) are cases where the pretilt angle is positive and negative, respectively. In order to reduce the transmittance in the viewing angle direction during dark display, a phase plate may be laminated between the polarizing plate and the liquid crystal display panel as described in the non-patent document. The trend of transmittance distribution is maintained.

  Based on the above considerations, we created an IPS-Pro liquid crystal display device with an electrode structure in which the connecting portion shown in FIG. 4 is only on one side, and set the pretilt angle of the liquid crystal layer adjacent to the pixel electrode from the tip of the comb teeth. It was set to stand up toward. In the case of this embodiment, since the connecting portion is on the right side, the rubbing conditions are set so that the pretilt angle becomes positive. Through the above measures, the domain at both ends of the pixel is suppressed, and the transmittance is improved by 5% or more compared to the IPS-Pro liquid crystal display device with a pixel structure that has a connecting part at both ends of the pixel. At the same time, the afterimage phenomenon related to the domain could be prevented.

  On the other hand, for the domain at the tip of the comb tooth electrode, an attempt was made to reduce it by introducing a domain suppression structure at the tip of the comb tooth. Domain suppression structures include an oblique slit structure and a triangular structure, and examples of pixel electrodes having these domain suppression structures are shown in FIGS. 23 and 24, respectively. In both examples, the comb tooth direction is parallel to the long pixel side, and the comb tooth direction is substantially parallel to the short pixel side.

  The domain suppression effect by the domain suppression structure is illustrated in FIG. FIG. 25 (a) shows a case without a domain suppression structure, FIG. 25 (b) shows an oblique slit structure, and FIG. 25 (c) shows a triangular structure. Both are enlarged centering on the tip of the comb tooth and the portion facing the slit portion, and the disappearance of the acute angle that occurs during processing is taken into consideration. The notation of the electric field direction and the orientation direction is the same as in FIGS. 13 and 14, and there is a fringe electric field that causes reverse twisted orientation in the circled portions E and G.

  Of E and G, E is closer to the comb electrode portion, and becomes a domain when the reverse twisted orientation caused by E grows. It is understood that F is the fringe electric field that gives normal orientation and is closest to E, and F is closer to E in the oblique slit structure of FIG. 25 (b) compared to FIG. 25 (a). Similarly, in the triangular structure of FIG. 25 (b), F is closer to E compared to FIG. 25 (a). As a result, E is more strongly subject to the orientation regulating force from F, so that the growth of reverse twisted orientation is suppressed.

  If a domain suppression structure is arrange | positioned at the connection part side, a connection part side domain can also be suppressed. However, if the domain suppression structure is arranged on the coupling part side in addition to the tip of the comb-tooth electrode, the transmittance is reduced to the same level as the slit structure, and the purpose of improving the transmittance cannot be achieved. That is, in the oblique slit structure, since the angle formed by the liquid crystal alignment direction when no voltage is applied and the fringe electric field is small, the rotation angle of the liquid crystal alignment when the voltage is applied is reduced and the transmittance is decreased. Therefore, the domain suppressing structure was not introduced on the connecting part side of the comb-tooth structure, and the connecting part end domain was suppressed by setting the pretilt angle.

  The relationship between the pixel electrode shape and the liquid crystal layer tilt angle is summarized as shown in FIG. The liquid crystal layer tilt angle in FIG. 1 corresponds to FIG. 21B, that is, in the electrode structure in which the connecting portion is only on one side, the pretilt angle should be set so as to rise from the tip of the comb teeth of the pixel electrode toward the root. , Can suppress the linking end domain. By suppressing the domain at the end of the connecting portion and the end of the distal end, the transmittance can be further improved in the present embodiment than in the first embodiment. The introduction of the domain suppression structure is also effective in suppressing the short-time afterimage phenomenon caused by the domain at the end of the connecting portion or the end of the distal end.

Embodiment 3
In the present embodiment, an attempt was made to improve the transmittance of the comb electrode portion of the IPS-Pro liquid crystal display device by optimizing the parameters of the liquid crystal material. As described above, since the orientation change of the IPS-Pro liquid crystal display device when a voltage is applied is a twisted orientation, the torsional elastic constant k22 is involved in the orientation change among the elastic constants. If k22 is reduced, the orientation of the liquid crystal layer can be easily changed, and the BV characteristics shift to the low voltage side. However, at this time, if Δ22 is kept constant and k22 is reduced, Tmax also decreases with the shift of the BV characteristic, so that a sufficient improvement in transmittance cannot be obtained.

  As described in the first embodiment, when the electrode width ratio is set in the vicinity of 0.4, the distribution of the fringe electric field in the pixel becomes the most uniform. However, even in this case, the distribution of the fringe electric field is not completely uniform, and there is no change in the tendency that the fringe electric field is denser at the boundary between the comb electrode and the slit and sparse at the comb electrode center and the slit center. By reducing k22, it becomes easier to change the orientation in the dense part of the fringe electric field, but the orientation change in the dense part becomes difficult to propagate to the sparse part, and the orientation change in the sparse part becomes small. As a result, although the transmittance at the boundary between the comb electrode and the slit is increased, the transmittance at the center of the comb electrode and the center of the slit is decreased, so that the transmittance of the entire comb electrode portion is not improved.

In order to improve the transmittance of the entire pixel when k22 is reduced, Δε reduction is effective. That is, since the force that causes the alignment change in the liquid crystal layer is weakened, a large torsional deformation occurs on the high voltage side where the electric field distribution in the pixel becomes more uniform. From this, the transmittance should be increased while keeping the BV curve in a certain shape, and if the threshold voltage at which the transmittance of the IPS system starts changing when voltage is applied is Vth, the BV curve is characterized by Vth . According to non-patent literature ASIA DISPLAY '95 pages 577-580 by M.Oh-e, M.Ohta, S.Aratani, K.Kondo, Vth is represented by the following equation.
[Equation 1]
Vth = π {k22 / (ε ₀ Δε)} / d (1)

Here, ε ₀ and d are an average dielectric constant and a liquid crystal layer thickness, respectively. Among these, the average dielectric constant is substantially constant in a liquid crystal material that exhibits a nematic phase in a wide temperature range including room temperature and exhibits a high resistance value. In addition, the liquid crystal layer thickness is set to a substantially constant value in order to achieve both sufficient response characteristics and transmittance. Therefore, in order to optimize k22 and Δε to improve the transmittance of the comb electrode portion, it is an effective guideline to keep k22 / Δε within a certain range.

  FIG. 26 shows the result of calculating the Δε dependency of the transmittance at an applied voltage of 4.5 V when k22 is 4.5 pN, 5.0 pN, and 5.5 pN. In FIG. 26, the transmittance is normalized by the value under the standard condition, and k22 and Δε in the standard condition are 6.5 pN and 6.5, respectively. Δε giving the maximum transmittance at k22 = 4.5 pN, 5.0 pN, and 5.5 pN are 4.5, 5.5, and 6.5, respectively, and when k22 / Δε is obtained from these, 1.0 pN, 0.91 pN, and 1.0 pN are obtained. Thus, when the condition for giving the maximum transmittance is expressed by k22 / Δε, it falls within a very narrow range of 0.91 pN to 1.0 pN.

  In any k22, the transmittance changes rapidly with Δε, and the half value is obtained from the half value between the maximum value and the transmittance of the standard condition. When k22 is 4.5 pN, Δε is 3.2 or more and 8.6 or less, when k22 is 5.0 pN, 3.8 or more and 8.4 or less, and when k22 is 5.5 pN, the half width is 4.2 or more and 8.2 or less. From this, the value of k22 / Δε that gives a high transmittance of half or more is 0.52pN or more and 1.41pN or less when k22 is 4.5pN, and 0.60pN or more and 1.31pN or less when k22 is 5.0pN. When k22 was 5.5 pN, it was 0.67 pN or more and 1.31 pN or less. In any k22, k22 / Δε giving a high transmittance is in substantially the same range, and the total of the three calculation results is 0.52 or more and 1.41 or less.

  As described above, by setting k22 / Δε of the liquid crystal material in the range of 0.52 pN or more and 1.41 pN or less, the transmittance of the main comb electrode portion in the IPS-Pro liquid crystal display device can be improved.

Embodiment 4
Response characteristics are also important among the display characteristics of an IPS-Pro liquid crystal display device, and it is more preferable if the response characteristics can be maintained when the transmittance is improved. In many cases, the influence of the comb electrode portion is mainly in one pixel of the IPS-Pro liquid crystal display device, and this also applies to the response time. Therefore, attention is paid to the response time of the comb electrode portion. When a voltage is applied, the liquid crystal layer is in a twisted alignment state, and an elastic constant contributing to the alignment deformation is a torsional elastic constant k22. Therefore, when the voltage V is applied and the response times required for the orientation change when the voltage V is removed are Ton and Toff, respectively, they are expressed by the following equations, respectively.

[Equation 2]
Ton = 4πηd ² / (ε ₀ ΔεV ² -4π ³ k22) (2)
Toff = ηd ² / π ² k22 (3)

Here, η is a viscosity constant. Of the parameters of the liquid crystal layer and the liquid crystal material, η and d contribute to both Ton and Toff.
K22, of which η is a liquid crystal material exhibiting a nematic phase in a wide temperature range including room temperature and having a high resistance value, and takes a substantially constant value when Δn is in the range of 0.08 to 0.12. D is a parameter of the liquid crystal layer, and takes a constant value in combination with Δn which is a physical property value of the liquid crystal material. That is, Δnd should be increased to improve the transmittance, but should be reduced to suppress halftone hue change. In order to achieve both, Δnd must be set to a value in the vicinity of 380 nm. From the above, k22Δn ² can be obtained by expressing the index for maintaining the response characteristics by the physical property value of the liquid crystal material. In order to maintain response characteristics when increasing transmittance by reducing k22, k22Δn ² should be kept constant.

  FIG. 27 shows the results of calculating the transmittance and response time while changing Δε, k22, and Δn. Δε was 4.5, 5.5, 6.5, k22 was 4.5 pN, 5.5 pN, 6.5 pN, and Δn was 0.10, 0.11, 0.12, 0.13. Also, the transmittance and response time are normalized with values obtained under standard conditions. The response time is plotted so as to be shorter toward the right side of FIG. 27, and the transmittance and response time can be improved at the same time in the upper right region from the standard condition. Under standard conditions, k22 is 6.5 pN, and the transmittance increases as this is reduced to 5.5 pN and 4.5 pN. At this time, if Δn is increased to 0.11 and 0.12 at the same time, the response time can be kept substantially constant. Changes in transmittance and response time when k22 is changed to 6.5 pN, 5.5 pN, 4.5 pN, and Δn is changed to .10, 0.11, and 0.12 are indicated by arrows in FIG. Since the arrow points substantially vertically, it can be seen that the transmittance increases while keeping the response time substantially constant.

Under standard conditions, k22Δn ² is 0.065 pN, k22 = 5.5 pN, Δn = 0.11 is 0.067 pN, and k22 = 4.5 pN, Δn = 0.12 is 0.065 pN. In any case where the standard condition and almost the same response time are given, k22Δn ² is almost the same value. As described above, the response time can be kept substantially constant by increasing the transmittance while setting k22Δn ² to 0.065 pN. Furthermore, if k22Δn ² is 0.065 pN or more, the transmittance can be improved while the response time is reduced.

  The above results are obtained by the fact that the transmittance does not change when d is reduced while keeping Δnd constant. As described in Embodiment 1, the alignment state of the IPS-Pro liquid crystal display device when a voltage is applied is represented by a twisted alignment model. This indicates that the liquid crystal layer is localized at the edge of the second substrate. It shows that it is driven by a fringe electric field. A torsional orientation is formed between the portion with the strongest fringe electric field and the edge of the first substrate. Even if d is slightly reduced, the alignment regulating force of the first alignment film is the liquid crystal layer in the portion with the strongest fringe electric field. Little effect on rotation angle. When Δn is increased from 0.10 to 0.12 while keeping Δnd constant, the reduction of d is about 0.87 times.

Also, if Δn is greater than 0.12, the viscosity constant increases, and the response time increases. Therefore, 0.065 pN, which is the value of k22Δn ² that keeps the response time obtained here substantially constant, is effective when Δn is 0.12 or less. K22Δn ² η / η ₀ is an effective index for improving the transmittance while maintaining the response time when Δn is 0.12 or more, where η ₀ is a viscosity constant under standard conditions where Δn is 0.12 or less. .

Embodiment 5
In the present embodiment, following the second embodiment, an attempt was made to improve the transmittance of the IPS-Pro liquid crystal display device by reducing the connecting end domain. In the second embodiment, the alignment state of the liquid crystal layer and the electrode structure were optimized. In this embodiment, an attempt was made to reduce the end domain of the connecting portion by optimizing the liquid crystal properties.

  As described above, the liquid crystal layer near the second substrate has the strongest fringe electric field. Therefore, attention was paid to the alignment state of the liquid crystal layer in a plane close to the second substrate and parallel to the interface. FIGS. 17 (c) and 17 (d) schematically show the orientation distribution when the connection end domain is generated. The torsional direction in the domain is opposite to the surrounding normal alignment portion. When this is observed in a direction parallel to the pixel end, the reverse alignment portion and the normal alignment portion are repeatedly distributed. Between the reverse orientation portion and the normal orientation portion is a domain observed as a dark line, and the orientation change hardly occurs. If the liquid crystal alignment directions of the upper part and the lower part of the domain are observed, they are inclined in opposite directions. From this, it can be seen that spray deformation occurs between the reverse orientation portion and the normal orientation portion.

  The repetition cycle of the reverse orientation portion and the normal orientation portion is the same as the comb electrode pitch defined by the sum of the electrode width and the slit width. Since the electrode width and the slit width are 2 μm and 3 μm, respectively, the comb electrode pitch is 5 μm. This is almost the same length as the liquid crystal layer thickness, and the spray deformation that occurs between the reverse alignment part and the normal alignment part is the alignment deformation that occurs in the liquid crystal layer thickness direction when a voltage is applied in the vertical electric field mode liquid crystal display device. Has the same steepness as Therefore, the coupling end domain is easily affected by the elastic constant of the liquid crystal material and can be suppressed by the elastic constant. Since the elastic constant related to spray deformation is k11, increasing k11 increases the orientation energy necessary for the appearance of the coupling end domain, making it difficult for the coupling end domain to appear.

  On the other hand, the spray deformation has an effect of increasing or decreasing the torsional deformation inside the connecting portion end domain as shown in FIGS. 20 and 21, and the effect should be strengthened as k11 is increased. In this embodiment, the planar shape of the pixel electrode is a comb-like shape, the domain at the tip of the comb tooth is suppressed by the domain suppression structure, and the connecting portion end domain rises from the tip of the comb tooth toward the root. Suppressed by setting to. Such tilt angle setting reduces the connecting portion end domain, but acts to enlarge the comb tip end domain. As k11 increases, the domain suppression structure cannot completely suppress the comb tip domain, and the comb tip domain grows and the transmittance starts to decrease. Thus, k11 is not so good that it increases, and there is an optimum range for improving the transmittance.

  The orientation state when a voltage was applied to the pixel electrode having a comb-tooth structure while changing the value of k11 was calculated. The junction end domain decreased with increasing k11 and disappeared. On the other hand, the size of the domain at the tip of the comb teeth is almost constant when the value of k11 is sufficiently small. However, when the value of k11 further increases, the domain expands to the inside of the domain suppression structure.

  Next, two types of liquid crystal materials having different k11 values and substantially the same BV characteristics were prepared, and the k11 value was changed with a finer width by changing the mixing ratio. When the transmittance at an applied voltage of 4.5 V was measured for each liquid crystal material, the results shown in FIG. 28 were obtained. When the value of k11 was 13.5 pN, the transmittance at an applied voltage of 4.5 V was maximized. When the k11 value was 15.5 pN, the transmittance was almost the same as that at 10 pN.

  Thus, in order to improve the transmittance of the IPS-Pro liquid crystal display device, there exists an optimum range for the value of k11. Since k11 of the current liquid crystal material is in a range around 10 pN, the optimum range for the value of k11 is 10 pN or more and 15.5 pN or less in order to obtain a transmittance equal to or higher than this.

Embodiment 6
As described in the third embodiment, k22 out of k11, k22, and k33 is involved in the alignment deformation of the liquid crystal layer in the comb electrode portion. If k22 is reduced while maintaining a constant relationship with Δε, twist deformation is likely to occur, so that the change in the alignment of the liquid crystal layer increases and the transmittance is improved. Further, as described in the seventh embodiment, the coupling end domain can be suppressed by increasing k11 from a value typical of the current liquid crystal material.

  When the direction of the comb electrode is the short side direction of the rectangular pixel, there are many comb end portions along the long side in one pixel. Furthermore, when the pixel size is small, the influence of the coupling end domain on the transmittance cannot be ignored. FIG. 29 (a) shows the result of calculating the transmittance of the liquid crystal material having the elastic constant and dielectric anisotropy in the following conditions 1 to 9 while changing the length of the comb electrode. The transmittance increases with the length of the comb electrode, and this indicates that the end of the comb has a lower transmittance than the comb electrode. Among conditions 1 to 9, condition 1 is a standard condition and is indicated by a broken line in FIG. When each transmittance is normalized and expressed under standard conditions, it is as shown in FIG. 31 (b). Since the normalized transmittance is almost constant in the region where the length of the comb electrode is approximately 100 μm or more, it can be seen that the influence of the end portion of the comb electrode cannot be ignored when the length of the comb electrode is 100 μm or less. .

  The elastic constant and dielectric anisotropy of condition 1 are k11 = 12.9pN, k22 = 6.5pN, k33 = 16.4pN, Δε = 6.5, and condition 2 is k11 = 14.4pN, k22 = 6.5pN, k33 = 16.4 pN, Δε = 6.5, condition 3 is k11 = 15.9 pN, k22 = 6.5 pN, k33 = 16.4 pN, Δε = 6.5, condition 4 is k11 = 12.9 pN, k22 = 5.2 pN, k33 = 16.4 pN, Δε = 6.5, condition 5 is k11 = 14.4 pN, k22 = 5.2 pN, k33 = 16.4 pN, Δε = 6.5, condition 6 is k11 = 15.9 pN, k22 = 5.2 pN, k33 = 16.4 pN, Δε = 6.5, condition 7 is k11 = 12.9 pN, k22 = 5.2 pN, k33 = 16.4 pN, Δε = 5.5, and condition 8 is k11 = 14.4 pN, k22 = 5.2 pN, k33 = 16.4 pN, Δε = The condition 9 is k11 = 15.9 pN, k22 = 5.2 pN, k33 = 16.4 pN, and Δε = 5.5. The electrode shape was a comb-tooth structure, and the width of the connecting portion was 2.0 μm. The comb electrode width and the slit width were 2.0 μm and 3.0 μm, the interelectrode insulating film thickness was 0.3 μm, and the relative dielectric constant of the interelectrode insulating film was 6.5. Further, 1.5 μm on each of the connecting part side and the tip side of the comb electrode is shielded by BM, and this part does not contribute to the transmittance.

  When the influence of the comb electrode end cannot be ignored, it is necessary to suppress the coupling portion side domain and simultaneously increase the twisted orientation in order to improve the transmittance. K11 optimization is effective for suppressing the binding domain, and k22 / Δε is effective for increasing the twist orientation. By using a liquid crystal material that simultaneously satisfies the range of k22 / Δε giving a high transmittance in the third embodiment and the range of k11 giving a high transmittance in the fifth embodiment, the length of the comb electrode is 100 μm or less. The transmittance of the IPS-Pro liquid crystal display device can be further improved.

Embodiment 7
In the pixel electrode shown in FIG. 3, there are two regions in which the rotation direction of the liquid crystal alignment is different from each other when a voltage is applied in one pixel, and a more colorless display is obtained in the viewing angle direction. Thus, when slits having different inclinations are formed in a rectangular pixel electrode with a constant width, the center of the pixel becomes a boundary between the two regions, and an ineffective region where no voltage can be applied to the liquid crystal layer is formed in a triangular shape. There are opaque components such as contact holes, TFTs, and scanning wirings in one pixel. By disposing these components in this portion, the invalid area can be used effectively. On the other hand, the electrode width is widened at the pixel end, and an ineffective region where no fringe electric field is applied to the liquid crystal layer is generated. Such invalid areas can be removed by making the shape of the pixel electric field trapezoid as shown in FIG. Since the shape of the pixel electric field is trapezoidal, the boundary between adjacent pixel electrodes in FIG. 30 has a sawtooth shape, but the scanning wiring passes through the lower layer of the boundary between the two regions in the center of the pixel. The shape of is a straight line.

  Further, in order to suppress the connecting portion end domain, the pixel electrode structure is a comb-tooth structure, and the connecting portion ends are aligned on the same side in all pixels. Specifically, the pixel electrodes in which the connecting portions are arranged on the top side of the trapezoid and the pixel electrodes in which the connecting portions are arranged on the bottom side of the trapezoid are alternately arranged as shown in FIG. As a result, the invalid area due to the connecting portion is reduced in all pixels, and the connecting portion end domain can be suppressed in the same manner as in the first embodiment.

  When the display device of the present invention is used for an interface of a mobile device such as a mobile phone or a digital camera, an image display with high brightness and excellent outdoor visibility can be obtained.

FIG. 6 is a perspective view showing a pretilt angle suitable for suppressing a connecting portion end domain in a pixel electrode having a comb-tooth structure. 2 is a cross-sectional view of one pixel of the liquid crystal display device of Embodiment 1. FIG. FIG. 3 is a plan view showing distributions of various electrodes and wiring of one pixel of the liquid crystal display device of Embodiment 1. It is a top view which shows the pixel electrode which has arrange | positioned two area | regions from which the rotation direction of a liquid crystal alignment differs at the time of a voltage application. It is a figure which shows the azimuth and tilt angle distribution of the orientation direction in a liquid crystal layer at the time of a voltage application. It is a figure which shows a twist orientation model. It is a figure which shows the ratio dependence of the electrode width of a BV characteristic, and a slit width. It is a figure which shows the ratio dependence of the electrode width of a BV characteristic, and a slit width. It is sectional drawing which shows the transmittance | permeability and the distribution within a pixel of a liquid crystal orientation state. It is sectional drawing which shows the transmittance | permeability and the distribution within a pixel of a liquid crystal orientation state. It is sectional drawing which shows the transmittance | permeability and the distribution within a pixel of a liquid crystal orientation state. It is a top view of the pixel electrode which has a connection part on both sides of a pixel end. It is a top view which shows the rotation direction of the liquid crystal alignment which arises at the time of a voltage application in the ideal pixel electrode with an acute angle at a pixel end. It is a top view which shows the rotation direction of the liquid crystal orientation which arises at the time of a voltage application in the pixel electrode from which the acute angle disappeared. It is sectional drawing which shows the transmittance | permeability distribution orientation distribution in the connection part side pixel edge part which the connection part end domain does not come out. It is sectional drawing which shows the transmittance | permeability distribution orientation distribution in the connection part side pixel edge part which the connection part end domain has come out. It is a top view which shows the orientation state in a connection part end domain. It is a top view which shows the relationship between a connection part end domain and an electrode structure. It is a schematic diagram explaining the interaction of twist deformation and spray deformation. It is a schematic diagram explaining the change of the alignment state which arises by the relationship between the tilt angle inside a connection part end domain, and the pretilt angle of the liquid crystal layer interface adjacent to an electrode. It is a schematic diagram explaining the change of the connection part end domain size which arises with a pretilt angle. It is a schematic diagram explaining the relationship between the code | symbol of a pretilt angle and the viewing angle characteristic at the time of dark display. It is a top view which shows the example of the pixel which has a diagonal slit structure. It is a top view which shows the example of the pixel which has a triangular structure. It is a figure which shows the domain suppression effect by a diagonal slit structure and a triangular structure. It is a figure which shows the k22 and (DELTA) epsilon dependence of the transmittance | permeability. It is a figure which shows the relationship between the transmittance | permeability and response time. It is a figure which shows the K11 dependence of the transmittance | permeability. It is a figure which shows the comb electrode electrode length dependency of the transmittance | permeability. It is a top view which shows the example of an arrangement | sequence of the pixel electrode which has arrange | positioned two area | regions from which the rotation direction of a liquid crystal alignment differs at the time of a voltage application.

Explanation of symbols

  CL liquid crystal layer, PE pixel electrode, AD alignment direction, RD alignment processing direction, PL1 first polarizing plate, PL2 second polarizing plate, SU1 first substrate, SU2 second substrate, LL flattening layer, CF color Filter, AL1 first alignment film, AL2 second alignment film, GL scanning wiring, SE source wiring, CE common electrode, CH contact hole, BM black matrix, PCIL interelectrode insulating film, CEIL common electrode insulating film, GIL scanning Wiring insulation film, SL signal wiring, TFT active element, RT reverse twist orientation section, DO connection end domain, DL dark line, EF electric field line.

Claims (7)

A liquid crystal panel having a liquid crystal layer sandwiched between the first substrate, the second substrate, and the first substrate; The liquid crystal panel has a plurality of independently controllable pixels, and each pixel has a pair of pixel electrodes and a common electrode on the display portion on the liquid crystal layer side of the second substrate. The pixel electrode and the common electrode are stacked via an inter-electrode insulating film, and the planar structure of the pixel electrode and the common electrode that is further away from the liquid crystal layer is a solid plane, and the liquid crystal layer of the pixel electrode and the common electrode The electrode on the side is a comb-tooth electrode having a comb-like planar structure with a connecting portion only on one side, and a liquid crystal display that drives a liquid crystal layer by applying an electric field mainly composed of parallel components in the liquid crystal surface In the device
When the sum of the width of the slit separating the width and the comb electrodes of the comb electrodes and comb electrodes pitch, (insulating film thickness between the dielectric constant / the electrode comb electrode pitch × inter-electrode insulating film) 81 or more, 162 in the range below, the comb-tooth electrode pitch 2.5μm or more is in the following range 8.5 .mu.m, Ri small range near than 0.5 a (at the comb-tooth electrode width / comb electrodes pitch) of 0.3 or more,
The liquid crystal display device has a pretilt angle in which the alignment of the liquid crystal layer rises from the tip of the comb-like structure toward the base at the electrode-side interface . Earlier end structure of the liquid crystal layer side of the electrode of the pixel electrode and the common electrode, the liquid crystal display device according to claim 1 slit direction is a structure inclined to the comb-like structure in the center of the pixel. Earlier end structure of the liquid crystal layer side of the electrode of the pixel electrode and the common electrode, the liquid crystal display device according to claim 1 is a structure obtained by adding the triangular tip of the comb-like structure.   If the twist elastic constant of the liquid crystal material is k22 and the dielectric anisotropy of the liquid crystal material is Δε, (k22 / Δε) is in the range of 0.52pN to 1.41pN, and the value of k22 is 4.5pN or more, 5.5pN The liquid crystal display device according to claim 1, which is in the following range. 2. The liquid crystal display device according to claim 1, wherein (k22 × Δn ² ) is 0.065 pN or more and Δn is 0.12 or less, where k22 is the twist elastic constant of the liquid crystal material and Δn is the birefringence of the liquid crystal layer.   2. The liquid crystal display device according to claim 1, wherein k11 is in a range of 10 pN or more and 15.5 pN or less, where k11 is a spray elastic constant of the liquid crystal layer. The liquid crystal layer side of the electrode of the pixel electrode and the common electrode has a planar structure inscribed in trapezoidal and having a structure in which a connecting portion to the base of the trapezoid side, the connecting portion to the trapezoidal top side structure The liquid crystal display device according to claim 1, wherein the two types of structures are alternately arranged.

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