JP2009080197A - Liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal display device minimized in AC seizure. <P>SOLUTION: The liquid crystal display device includes a first substrate 1 and a second substrate 2 which are mutually opposed. The device further includes a plurality of pixels 10, each of the plurality of pixels 10 including a vertically oriented liquid crystal layer 3 provided between the first substrate 1 and the second substrate 2, a first electrode 4 provided on the liquid crystal layer 3 side of the first substrate 1 and a second electrode 5 provided on the liquid crystal layer 3 side of the second substrate 2. The liquid crystal layer 3 has a pre-tilt angle of not more than 90°, and at least one of the first electrode 4 and the second electrode 5 has at least one slit 11. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device having a wide viewing angle characteristic.

  Conventionally, when the same pattern is continuously displayed on the liquid crystal display for a long time, a phenomenon that the pattern is burned occurs.

For example, the image sticking is most noticeable when an aging test is performed in a state where a white window pattern is displayed on a black background. If a uniform halftone (gray) is displayed on the entire panel after aging, for example, in the normally black mode (black display when no voltage is applied), the part where the white color was displayed (white display) The window part) appears brighter than the part where the black color was displayed (black background part). This is mainly because impurity ions present in the liquid crystal layer are attracted to the white window portion to which voltage is applied and accumulated on the alignment film surface, and a DC component is generated in the white window portion. ing. Therefore, the optimum counter voltage is different between the white window portion and the background black display portion. This can be confirmed from the phenomenon that when the potential of the counter electrode is changed from the state where the burn-in is visible, the burn-in once becomes difficult to be seen and then becomes noticeable again. This burn-in is called “DC burn-in”. Conventionally, in order to prevent DC burn-in, the amount of impurity ions in the liquid crystal layer is reduced, an alignment film material that does not easily adsorb impurity ions, or a combination of such a liquid crystal material and an alignment film material is used. Measures such as finding have been made.
WO2006 / 132369A1

  The present inventor has found that a liquid crystal display device having a pretilt angle of less than 90 ° using a liquid crystal material having a negative dielectric anisotropy for the liquid crystal layer causes burn-in due to a cause other than DC burn-in. Since this burn-in does not result from residual DC, it is referred to as “AC burn-in”. The AC image sticking is considered to be caused by a change in the pretilt angle of liquid crystal molecules that are regulated (anchored) by the alignment film over time when a voltage is continuously applied to the liquid crystal layer.

  The cause of AC image sticking is that the pretilt angle of liquid crystal molecules (hereinafter, the pretilt angle is defined by the rising angle from the substrate surface) is reduced by voltage application, and the VT curve rises more steeply near the threshold value. The inventors speculated. For example, as shown in FIG. 16A, when an aging test is performed in a state in which a white window pattern is displayed on a black background, a voltage is applied to the white window portion, so that the liquid crystal molecules The tilt angle becomes smaller. However, since no voltage is applied to the black back portion, the tilt angle of the liquid crystal molecules does not change. Thereafter, as shown in FIG. 16B, when a uniform halftone is displayed on the entire panel, the white window portion (luminance L1) appears brighter than the black back portion (luminance L2). Even when the potential of the counter electrode is changed, the appearance of image sticking does not change, and it can be seen that this image sticking is not DC image sticking. This phenomenon is conspicuous when a vertically aligned liquid crystal display device is used to display a normally black mode, but it is considered that this phenomenon also occurs in other modes.

  The present invention has been made to solve the above problems, and an object of the present invention is to provide a liquid crystal display device in which burn-in caused by the tilt angle of liquid crystal molecules is improved.

  The liquid crystal display device of the present invention includes a first substrate and a second substrate facing each other, and a plurality of pixels, and each of the plurality of pixels is provided between the first substrate and the second substrate. A vertically aligned liquid crystal layer, a first electrode provided on the liquid crystal layer side of the first substrate, and a second electrode provided on the liquid crystal layer side of the second substrate. A pretilt angle of 85 ° or more and less than 90 ° is exhibited, and at least one of the first electrode and the second electrode has at least one slit.

  In one embodiment, the width of the at least one slit is in the range of 5 μm to 20 μm.

  In one embodiment, the at least one slit is a plurality of slits, and a ratio L / S of a ratio of an interval (L) adjacent to the slit in the width direction with respect to the slit width (S) is L / S. 1 or more and 6 or less.

  In one embodiment, the ratio A1 / A2 between the area A1 of the portion opened by the slit and the area A2 of the display portion of the pixel in which the slit is formed among the plurality of pixels is 0.05 or more and 0.00. 5 or less.

  In one embodiment, the first electrode is a pixel electrode, the second electrode is a common electrode, and the pixel electrode includes a first subpixel electrode and a second subpixel electrode, and at least some intermediate When displaying a tone, an effective voltage applied between the first subpixel electrode and the common electrode is higher than an effective voltage applied between the second subpixel electrode and the common electrode, The at least one slit is formed only in the first subpixel electrode.

  In one embodiment, a ratio A1 / A2 between the area A1 of the portion opened by the slit and the area A2 of the display portion of the first subpixel electrode is 0.05 or more and 0.5 or less.

  In one embodiment, in each of the plurality of pixels, the liquid crystal layer has a center in the thickness direction of the liquid crystal layer when a predetermined voltage is applied between the first electrode and the second electrode. A plurality of domains having different tilt directions of neighboring liquid crystal molecules are formed.

  In one embodiment, the plurality of domains are a first liquid crystal domain, a second liquid crystal domain, a third liquid crystal domain, and a fourth liquid crystal domain in which the tilt directions of the liquid crystal molecules are different from each other by an integral multiple of 90 °.

  In one embodiment, each of the plurality of pixels further includes a pair of alignment films formed between the liquid crystal layer and the first electrode and between the liquid crystal layer and the second electrode. The pretilt direction defined by one alignment film and the pretilt direction defined by the other alignment film differ from each other by approximately 90 °.

  In one embodiment, each of the plurality of pixels further includes a pair of photoalignment films formed between the liquid crystal layer and the first electrode and between the liquid crystal layer and the second electrode.

  In one embodiment, the photo-alignment film is a photo-alignment film that causes a photodimerization reaction by irradiation with polarized ultraviolet rays and exhibits the pretilt angle of the liquid crystal layer.

  In the liquid crystal display device of the present invention, not only a vertical electric field but also an oblique electric field (fringe field) caused by the slit is generated by the voltage applied to the liquid crystal layer. Due to the influence of the fringe electric field, the liquid crystal molecules near the slit start to fall before other regions. That is, the transmittance in the vicinity of the slit in the pixel is higher than that in other regions, and the voltage value at which the VT curve starts to rise is reduced in the entire pixel. As a result, the rise of the VT curve becomes smooth and AC burn-in occurs. Black float is less visible.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

  First, an embodiment of a liquid crystal display device according to the present invention will be described with reference to FIGS. As shown in FIG. 1B, the liquid crystal display device of the present embodiment includes a first substrate 1 and a second substrate 2 that face each other. The liquid crystal display device according to the present embodiment includes a plurality of pixels 10, and each of the plurality of pixels 10 is a vertical alignment type liquid crystal layer 3 provided between the first substrate 1 and the second substrate 2. Have Each of the plurality of pixels 10 includes a first electrode 4 provided on the liquid crystal layer 3 side of the first substrate 1 and an alignment film 6 provided on the liquid crystal layer 3 side of the first electrode 4. It has the 2nd electrode 5 provided in the liquid crystal layer 3 side of the 2nd board | substrate 2, and the alignment film 7 provided in the liquid crystal layer 3 side of the 2nd electrode 5. FIG. The alignment films 6 and 7 define the alignment of the liquid crystal molecules 3a in the liquid crystal layer 3, and the liquid crystal molecules 3a whose alignment is defined by the alignment films 6 and 7 exhibit a pretilt angle of 85 ° or more and less than 90 °.

  As illustrated in FIG. 1A, the pixel 10 has a plurality of domains 12 to 15. The domains 12 to 15 are arranged in a matrix of 2 rows and 2 columns in the pixel 10. In addition, in the domains 12 to 15, when a predetermined voltage is applied between the first electrode 4 and the second electrode 5, the tilt directions of the liquid crystal molecules near the center in the thickness direction of the liquid crystal layer 3 are different from each other. Has properties. In the pixel 10, the outside of the display area 16 is a black matrix, and light is blocked.

  In a conventional MVA type liquid crystal display device, the direction in which liquid crystal molecules are tilted when a voltage is applied is defined by an orientation control means such as an electrode slit or a dielectric rib (protrusion) formed on the electrode. The pretilt angle of the liquid crystal molecules on the vertical alignment film during heating was 90 °. In other words, the above-mentioned alignment regulating means was used in order to define the direction in which the liquid crystal molecules vertically aligned with a pretilt angle of 90 ° when no voltage is applied (Japanese Patent No. 2947350). Since the alignment regulating means (slits and ribs) are linear, the alignment regulating force with respect to the liquid crystal molecules becomes non-uniform in the pixel region, and there is a problem that, for example, the response speed is distributed. In addition, since the light transmittance of the region where the slits and ribs are provided is lowered, there is also a problem that the display luminance is lowered.

  On the other hand, in the liquid crystal display device according to the embodiment of the present invention, the pretilt angle defined by the alignment film is less than 90 °, and the pretilt direction is defined by the alignment film itself. When the pretilt direction is defined by the alignment film as described above, the above-described problem in the conventional MVA mode liquid crystal display device does not occur (see WO2006 / 132369A1).

  Specifically, the liquid crystal display device of the present embodiment uses a vertical alignment film in which the pretilt directions are orthogonal to each other on each substrate, whereby the liquid crystal molecules have a twisted structure (hereinafter referred to as VATN (Vertical Alignment Twisted Nematic)). Mode or RTN (Reverse Twisted Nematic) mode). In the VATN mode, the pretilt direction of the liquid crystal molecules defined by each vertical alignment film is parallel or orthogonal to the absorption axes of a pair of polarizing plates arranged in a crossed Nicol manner via the liquid crystal layer. In the VATN mode, when a sufficient voltage (signal voltage for display of at least the highest gradation) is applied to the liquid crystal layer, the tilt direction of the liquid crystal molecules near the center in the thickness direction of the liquid crystal layer is determined by a pair of alignment films. The two pretilt directions defined are substantially divided in two. When providing domains arranged in 2 rows and 2 columns as shown in FIG. 1A, if the VATN mode is adopted, the total number of alignment treatments (rubbing or light irradiation) for both alignment films is at least 4 times. There is an advantage that you can.

  In the liquid crystal display device in which the pretilt direction is defined by the alignment film, the occurrence of image sticking has been a problem. The present inventor has found a phenomenon of AC image sticking as a cause of image burn-in when the same pattern is continuously displayed on the liquid crystal display for a long time. In this AC image sticking, when a voltage is continuously applied to the liquid crystal layer, the pretilt angle of the liquid crystal molecules regulated by the alignment film decreases (changes in the sleeping direction) with time, and a VT curve (voltage-transmittance). It is considered that the rise of the (curve) is caused by a sharp rise near the threshold. The rise of the VT curve will be described later in detail with reference to FIG.

  As a countermeasure against this AC burn-in, in the liquid crystal display device of this embodiment, at least one of the first electrode 4 and the second electrode 5 has at least one slit 11a to 11d. In the vicinity of the slits 11a to 11d, not only a vertical electric field generated by a voltage applied to the liquid crystal layer 3, but also an oblique electric field (fringe field) caused by the slits 11a to 11d is generated. Due to the influence of the fringe electric field, the liquid crystal molecules 3a in the vicinity of the slits 11a to 11d start to fall before other regions as compared with the case where the slits 11a to 11d are not formed. As a result, the transmittance in the vicinity of the slits 11a to 11d is higher than that in other regions, and the voltage value at which the VT curve starts to rise is small when viewed as a whole pixel. Thereby, the rise of the VT curve becomes smooth, and the black floating due to AC image sticking becomes difficult to be visually recognized.

  Note that the slits 11a to 11d may be formed on only one of the common electrode and the pixel electrode, or may be formed on both. When the slits 11a to 11d are formed in only one of the common electrode and the pixel electrode, the surface on the liquid crystal layer side of the other electrode is flat without any slit or rib.

(Slit direction)
The inventor of the present application simulated which direction the slit is formed in order to more effectively smooth the VT curve. 2A to 2C are diagrams schematically showing the relationship between the direction in which the liquid crystal molecules tilt when a voltage is applied to the liquid crystal layer and the direction in which the slit extends. 2A shows a slit A extending in a direction parallel to the direction in which the liquid crystal molecules (liquid crystal molecules located in the middle layer of the liquid crystal layer) tilt, and FIG. 2B shows a direction perpendicular to the direction in which the liquid crystal molecules tilt. FIG. 2C shows a slit B extending in a direction inclined by 45 ° from the direction in which the liquid crystal molecules are inclined. The slits shown in FIGS. 2A to 2C were formed for each domain in one pixel.

  The values of the light shielding part L and the opening S in the slits A to C are L / S = 32.5 μm / 2.5 μm, 30 μm / 5 μm, 25 μm / 10 μm for the slits A and C, and L / S for the slit B S = 32.5 μm / 2.5 μm, 30 μm / 5 μm, 25 μm / 10 μm, 20 μm / 15 μm.

  2A to 2C, solid arrows indicate the pretilt direction of the liquid crystal molecules whose alignment is regulated by the alignment film formed on the upper substrate, and broken arrows indicate the alignment formed on the lower substrate. The pretilt direction of the liquid crystal molecules whose alignment is regulated by the film is shown. Each shows a pretilt direction with respect to the substrate surface. In this case, since the tilt direction of the liquid crystal molecules on the upper substrate side and the tilt direction of the liquid crystal molecules on the lower substrate side are orthogonal to each other, the liquid crystal molecules exhibit twist alignment when a voltage is applied, and the center of the liquid crystal layer in the layer plane and in the thickness direction The tilt directions of the liquid crystal molecules in the vicinity are aligned in a direction that exactly bisects the tilt directions on the upper substrate side and the lower substrate side.

  As shown in FIG. 2 (d), the polarizer and the analyzer are arranged so that the absorption axis (P) of the polarizer and the absorption axis (A) of the analyzer are respectively above and below (12: 00-6 of the clock face dial). Hour) and left and right (9 to 3 o'clock on the clock face).

  When a vertical alignment film is used as the alignment film and no voltage is applied to the liquid crystal layer, the liquid crystal molecules in the layer plane of the liquid crystal layer and near the center in the thickness direction have a tilt angle close to 90 °. When no voltage is applied, the display on the liquid crystal panel is black. When a voltage is applied to the liquid crystal layer, the liquid crystal molecules tilt and light leaks.

  The pretilt angle before AC image sticking occurred was 87.5 °, and after applying stress such as energization aging, that is, the pretilt angle after assuming that image sticking occurred was 87.4 °. That is, a two-dimensional simulation was performed assuming that the tilt angle was reduced by 0.1 ° by energization aging. 3A to 3C show the two-dimensional simulation results.

  FIGS. 3A to 3C show simulation results using a system having slits A to C, respectively. 3A to 3C, the horizontal axis represents the applied voltage, and the vertical axis represents the transmittance calculated by giving 87.4 ° to the pretilt angle, and calculated by giving 87.5 ° to the pretilt angle. A value ΔT divided by the measured transmittance is shown. That is, ΔT is a value obtained by dividing the transmittance of the burned area by the transmittance of the non-burned area, and directly represents the degree of increase in luminance (transmittance) due to burn-in. As shown in FIGS. 3A to 3C, the value of ΔT is larger than 100%, and the region where 87.4 ° is given to the pretilt angle is more transparent than the region where 87.5 ° is given. It can be seen that is higher. It shows that the larger the value of ΔT exceeds 100, the greater the influence of AC image sticking.

  In any of FIGS. 3A to 3C, ΔT of the system (ref) having no slit shows a peak between 2 and 3V. ΔT shows a value exceeding 120% at the maximum. On the other hand, in a system having a slit, it can be seen that ΔT is small in any of the slits A to C if the opening width (S) of the slit is about 5 μm or more. That is, it was found that the AC image sticking was improved in all the systems having slits compared to the system not forming slits. For example, when ΔT of L / S = 25 μm / 10 μm was compared, it was also found that AC image sticking improved in the order of slit B, slit A, and slit C.

(Regarding gradations that tend to cause AC burn-in)
Next, the result of examining the relationship between gradation and AC image sticking by simulation will be described. FIG. 4 is a graph showing the relationship between gradation and ΔT in a system having a slit B. Here, the case of 256 gradations (0 to 255) will be described. In FIG. 4, the vertical axis indicates the value of ΔT, and the horizontal axis indicates the gradation. Note that ΔT in FIG. 4 also shows a value obtained by dividing the transmittance calculated by giving 87.4 ° to the pretilt angle by the transmittance calculated by giving 87.5 ° to the pretilt angle. As shown in FIG. 4, the value of ΔT is large in a relatively low gradation range from about 6/255 gradation to about 30/255 gradation, and the burn-in is large in this range. I understand. For example, it is expected that the burn-in will be reduced when the gradation is about 50/255 gradation or more, but this tendency is the same not only in the simulation but also in the test using the actual panel.

  In the system having a slit of L / S = 25 μm / 10 μm, the value of ΔT near the 30/255 gradation is about half that of the system (ref) having no slit. Thereby, it can be seen that AC image sticking itself is also halved.

(About slit size)
FIG. 5 is a graph showing the result of calculating a VT curve in the front direction using a system having a slit B. FIG. Note that the transmittance Tr on the vertical axis is a value normalized by the transmittance when a voltage of 7 V is applied to a system having no slit. As shown in FIG. 5, it can be seen that when the value of L / S is reduced, the transmittance in the front direction is reduced as compared with a system having no slit. From this result, it is preferable that the value L / S of the ratio of the interval (L) where the slits are adjacent in the slit width direction to the slit width (S) is 1 or more and 6 or less. When the L / S value is 6 or less, AC image sticking can be effectively improved, and when the L / S value is 1 or more, adverse effects due to a decrease in transmittance (brightness loss). Etc.) can be avoided. In particular, when the value of L / S is smaller than 25 μm / 10 μm, the transmittance in the front direction is reduced by about 30% or more as compared with a system having no slit, so the value of L / S is more preferably 2 .5 or more and 6 or less. The specific value of the slit width is preferably in the range of 5 μm to 20 μm.

  Further, if the area of the portion opened by the slit is A1, and the area of the display portion of the pixel in which the slit is formed (the sub-pixel that forms the slit in the case of divided driving) is A2, then A1 / A2 The value is preferably 0.05 or more and 0.5 or less. For example, in the pixel shown in FIG. 1A, the area A1 is an area where the slits 11a to 11d are opened, and the area A2 is an area of a region in the display unit 16 (that is, inside the black matrix). When the A1 / A2 value is 0.05 or more, AC image sticking can be effectively improved, and when the A1 / A2 value is 0.5 or less, an adverse effect due to a decrease in transmittance ( Brightness loss, etc.) can be avoided. A more preferable value of A1 / A2 is 0.1 or more and 0.3 or less.

(About pixel division drive)
Next, a description will be given of a result of studying a cell configuration that can suppress a decrease in brightness while reducing AC image sticking.

  As one method for improving the viewing angle, a method called pixel division driving (also referred to as multi-pixel driving) is known (2004-62146 (US Pat. No. 6,958,791)). As shown in FIG. 6, in normal driving, driving is performed by applying a single voltage for each pixel. In contrast, in pixel division driving, one pixel is divided into a plurality of (for example, two) sub-pixels having different brightness. A storage capacitor is provided for each subpixel, and the storage capacitor counter electrode (connected to the CS bus line) constituting the storage capacitor is electrically independent for each subpixel, and a voltage supplied to the storage capacitor counter electrode is Change. Thereby, the effective voltages applied to the liquid crystal layers of the plurality of sub-pixels are different. When this method is used, the viewing angle dependency of the γ characteristic is improved.

  Here, a VT curve when pixel division driving is performed will be described with reference to FIG. FIG. 7 shows a VT curve of a relatively bright subpixel (bright subpixel) and a VT curve of a relatively dark subpixel (dark subpixel). The VT curve of the bright subpixel appears closer to the lower voltage than the VT curve of the entire pixel (shown by the solid line), whereas the VT curve of the dark subpixel is closer to the higher voltage than the VT curve of the entire pixel. Appears. Since the area ratio of the bright subpixel and the dark subpixel shown in FIG. 7 is 1: 1, the VT curve of the entire pixel is an average of the VT curve of the bright subpixel and the VT curve of the dark subpixel. In the present specification and drawings, for example, the division driving in which the ratio of the bright subpixel to the dark subpixel is 1: 1 is referred to as “pixel division driving 1: 1”.

  FIG. 8 shows a VT curve for normal driving, a VT curve for pixel division driving 1: 1, and a VT curve for pixel division driving 1: 2. From FIG. 8, it can be seen that the rise of the VT curve is smoother in the case of pixel division driving than in the case of normal driving. This is because the voltage at which the VT curve of the bright subpixel rises is relatively low, the voltage at which the VT curve of the dark subpixel rises is relatively high, and a difference is provided between these voltages. The reason is that the curve rises smoothly. Furthermore, since the rise of the VT curve is smoother in the case of the pixel division drive 1: 2 than in the case of the pixel division drive 1: 1, if the ratio of the dark subpixel to the bright subpixel is increased, It can be seen that the VT curve becomes gentle. It is not preferable to increase the ratio of the bright sub-pixel to the dark sub-pixel because the VT curve tends to rise sharply.

  Note that the transmittance of the VT curve of the dark sub-pixel finally catches up with the transmittance of the VT curve of the bright sub-pixel, so that no loss of brightness occurs in the liquid crystal panel.

  By combining this pixel division drive with a slit for reducing AC image sticking, a form that can reduce the loss of brightness while smoothing the VT curve was studied.

  A slit B with L / S = 25 μm / 10 μm was formed in the bright subpixel, and the VT curve in the front direction was calculated. Note that the area ratio of the bright subpixel and the dark subpixel in pixel division driving was set to two types of 1: 1 and 1: 2. FIG. 9 shows a VT curve of two types of pixel division driving having a slit and a normal driving VT curve having a slit and not dividing a pixel. As a slit, a slit B with L / S = 25 μm / 10 μm was formed. As can be seen from FIG. 9, in the case of normal driving without pixel division, the brightness (transmittance when 7V is applied) is 30 as compared with the comparative example (ref) that has no slit and does not perform pixel division driving. %, But in the case of pixel division drive 1: 1 and 1: 2, the brightness loss can be improved. For example, in the pixel division drive 1: 2 system, the brightness loss with respect to the comparative example can be reduced to about 12%.

  10 and 11 are graphs showing the relationship between gradation and transmittance (ΔT) in a system having slits and performing pixel division driving. In addition, as a slit, slit B of L / S = 25 μm / 10 μm was formed. In the results shown in FIGS. 10 and 11, the AC image sticking (ΔT) is further reduced as compared with the case of the normal drive shown in FIG. 4 (no pixel division drive, L / S = 25 μm / 10 μm slit B). I understand that. Looking at the same gradation, for example, 30/255 gradation, ΔT was about 105% or more in the case of normal driving, whereas ΔT was about 103% in the case of pixel division driving 1: 2. Is getting smaller.

  As described above, in a liquid crystal panel that performs pixel division driving, by providing a slit in a bright subpixel that rises first, AC burn-in can be reduced and brightness loss can be suppressed.

  If a slit is provided in the dark subpixel, even if a sufficiently high voltage (for example, about 7 V) is applied, the VT curve of the dark subpixel cannot catch up with the VT curve of the bright subpixel, and the brightness is lost. Therefore, it is not preferable. That is, the slit is preferably provided only in the bright subpixel.

  For example, as shown in FIG. 7, the VT curve of the bright subpixel starts to rise when the effective voltage is about 2V, whereas the VT curve of the dark subpixel rises unless the effective voltage exceeds 2.5 volts. I don't start. The VT curve as one pixel is an average of the VT curves of these two sub-pixels, and the rise of the VT curve becomes gentler than the rise of the VT curve during normal driving.

  In addition, not only a vertical electric field but also an oblique electric field (fringe field) caused by the slit is generated in the liquid crystal layer. Due to the influence of the fringe electric field, the liquid crystal molecules near the slit start to fall before the other regions as compared with the case where the slit is not formed. That is, the transmittance in the vicinity of the slit in the pixel is higher than in other regions, and as a whole, the rise of the VT curve becomes smooth as a result of a decrease in the voltage value when the VT curve starts to rise.

(3D simulation)
Next, the results of a three-dimensional liquid crystal alignment simulation will be described in order to verify the reduction in AC image sticking caused by providing slits.

  12 and 13 are diagrams illustrating an example of one pixel of a model in which a three-dimensional liquid crystal alignment simulation is performed. As shown in FIGS. 12 and 13A to 13D, a 4-domain VATN mode liquid crystal display device was used for the simulation. FIG. 12 shows a transparent electrode (ITO electrode) 17 that is a pixel electrode on the TFT substrate and a display area 16 through which light is actually transmitted. Light is shielded from the outer peripheral portion of the display area 16 by a BM (black matrix) on the CF (color filter) substrate side. Further, four domains 12 to 15 are arranged in a matrix of 2 rows and 2 columns as one pixel, and two slits 11 a to 11 d are formed in each domain 12 to 15. As the slits 11a to 11d, the slit B shown in FIG. 2B was formed, and L / S = 24/8 μm was used. FIG. 13A is a view of the liquid crystal panel as viewed from the upper surface of the panel (upper substrate (CF substrate)), and FIG. 13B is a view of the CF substrate as viewed from the CF surface.

  As the alignment film, a so-called photo-alignment film in which a tilt angle of liquid crystal molecules is expressed by ultraviolet irradiation is used. The alignment treatment of the photo-alignment film is basically performed by irradiating polarized ultraviolet rays from an oblique direction using a striped mask. 13A and 13B, solid arrows indicate the ultraviolet irradiation direction of the photo-alignment film on the CF substrate side, and broken-line arrows indicate the ultraviolet irradiation direction of the photo-alignment film on the TFT substrate side. In this way, two photo-alignment films whose UV irradiation directions are 90 ° different from each other are bonded to the upper and lower substrates.

  In FIG. 13C, solid arrows indicate the tilt direction on the CF substrate side, and broken arrows indicate the tilt direction on the TFT substrate side. In the photo-alignment film used in this embodiment, the ultraviolet irradiation direction and the tilt direction are opposite to each other.

  As shown in FIG. 13C, when a voltage equal to or higher than the threshold is applied, the liquid crystal molecules 20 near the center in the thickness direction of the liquid crystal layer are tilted in an orientation that bisects the alignment processing direction of the upper and lower substrates. Further, as shown in FIG. 13C, a dark luminance line 21 (referred to as a dark line) is generated at the boundary between adjacent domains. FIG. 13D is a diagram showing an image obtained by observing the alignment state with a microscope when a voltage equal to or higher than the threshold voltage is applied, and shows a state where the dark line 21 appears.

  In this simulation, pixel division driving is not considered and normal driving is assumed. Further, in order to simplify the calculation, only one domain of one pixel was taken out and a model was established to perform the calculation. The actual three-dimensional simulation was performed in the area of 56 μm × 67.2 μm shown in FIG. The comparative example (ref) was assumed to have no slit in the same size region. In view of the fact that the calculation takes time and that the AC image sticking is noticeable in the vicinity of the threshold value and the gradation region slightly exceeding the threshold value, the simulation was performed in the voltage range from 0V to 3.5V. However, in order to evaluate the degree of loss of transmittance due to the formation of the slit, only the transmittance when a voltage of 7 V was applied was separately calculated. Further, the calculation was performed with the initial tilt angle (pretilt angle) being 89.0 ° and the tilt angle when burned in being 88.9 °.

  Table 1 shows the results of calculating the transmittance ratio and transmittance when a voltage of 7 V is applied to the region shown in FIG. The transmittance ratio is a relative value when the comparative example (ref) is 100, and the transmittance is the transmittance when air is 100. Table 1 shows not only the calculation result of the region having the slit B (referred to as sample B) but also the calculation result of the region having the slits A and C (referred to as samples A and C, respectively) for comparison. In the sample B (L / S = 24 μm / 8 μm), the transmittance is lost by about 30% compared to the case without the slit (ref). This is equivalent to the value of the two-dimensional calculation result (shown in FIG. 9) calculated previously.

  FIG. 14B shows the calculation results of the VT curves of the comparative example (ref) and sample B when AC image sticking does not occur (pretilt angle 89.0 °). The vertical axis | shaft in FIG.14 (b) has shown the numerical value when the transmittance | permeability at the time of applying the voltage of 7V to a comparative example (ref) is set to 100. FIG. FIG. 14C shows the logarithm of the calculation result shown in FIG. As is clear from FIG. 14C, it was also confirmed by calculation that the rise of the VT curve was smooth by providing the slit.

  In FIG. 14D, ΔT on the vertical axis represents the transmittance when AC image sticking does not occur (pretilt angle 89.0 °), and the transmittance when AC image sticking occurs (tilt angle 88.9 degrees). Shows the percentage of the value divided by. From the result shown in FIG. 14D, the AC image sticking of the sample B can be suppressed to about 1/3 or less compared with the image sticking of the comparative example (ref) even in the three-dimensional calculation in a state closer to an actual liquid crystal panel. I understood.

  Further, a three-dimensional luminance map in one domain will be described with reference to FIG. FIG. 15 shows luminance maps when a voltage of 1.5 V to 3.0 V is applied to the comparative example (ref) and Sample B.

  Note that a luminance map when no voltage was applied was also calculated, and it was confirmed that no light leakage occurred even when a slit was provided. This confirmed that the contrast did not decrease.

  Next, regarding the luminance change in the voltage range from 1.5 V to 3.0 V, in the comparative example (ref), the luminance map is monotonously bright. That is, when the voltage of 1.5 V is applied, the entire surface remains black, and when the voltage of 3.0 V is applied, the entire surface is uniformly brightened. In contrast, in sample B, the vicinity of the slit edge is already bright when 1.5 V is applied. This is presumably because the liquid crystal molecules in the vicinity of the slit collapsed under the influence of the oblique electric field (fringe field) at the slit edge. When the value of the applied voltage is increased, the area away from the slit becomes bright as in the comparative example (ref), but the inside of the slit and the surrounding liquid crystal molecules (inside the slit and the inside of the slit as viewed from the TFT substrate or CF substrate side) The movement of the liquid crystal molecules arranged around is limited and the inside and the periphery of the slit remain dark. Further, it was found that even when the applied voltage was increased to 7 V, the inside and the periphery of the slit remained dark and the white luminance was lowered.

  From the results described above, in Sample B, the transmittance starts to rise before the comparative example (ref), but the degree of increase in the transmittance decreases as the applied voltage increases. As a result, it was also shown in the calculation that the rise near the threshold value of the VT curve becomes gentle. Then, since the rising of the vicinity of the threshold value of the VT curve becomes gentle, AC image sticking is reduced.

  In the conventional MVA type liquid crystal display device, a decrease in luminance due to slits and ribs has been a problem. In this embodiment, the slit is formed, but it is not necessary to form the rib, and the slit may be formed in any one of the pixel electrode and the common electrode. Therefore, the degree of luminance reduction in this embodiment is smaller than the degree of luminance reduction in the MVA type liquid crystal display device, and the influence of the luminance reduction in this embodiment is small.

  In the above description, the liquid crystal display device having the 4-domain VATN mode is shown. However, the present invention may be applied to a mode other than the VA mode, or applied to a mode other than the 4-domain VATN mode. Good. Moreover, it does not necessarily need to have a photo-alignment film. In addition, the liquid crystal display device of the present invention does not necessarily have a TFT.

  For example, the liquid crystal display device of the present invention can also be applied to a general ECB (Electrically Controlled Birefringence) mode, a CPA (Continuous Pinwheel Alignment) mode, and the like. Note that the CPA mode uses a diagonal electric field generated at the edge of the non-solid portion of the pixel electrode when a voltage is applied, by providing the pixel electrode with a non-solid portion (portion without the conductive layer, opening). This refers to a mode for forming a radial tilt alignment (Japanese Patent Laid-Open No. 2002-202511).

  The liquid crystal display device of the present invention has high industrial applicability in that good display characteristics with improved AC image sticking can be obtained.

(A), (b) is the top view and sectional drawing which show the liquid crystal display device of embodiment by this invention. (A)-(d) is a figure which shows typically the relationship between the direction where a liquid crystal molecule inclines when a voltage is applied to a liquid-crystal layer, and the direction where a slit extends. (A)-(c) has each shown the simulation result using the system which has slit AC. 6 is a graph showing the relationship between gradation and ΔT in a system having a slit B. FIG. It is a graph which shows the result of having calculated the VT curve of the front direction using the system which has the slit B. FIG. It is a figure for demonstrating pixel division drive. It is a figure which shows the VT curve of a bright subpixel, and the VT curve of a dark subpixel. It is a figure which shows the VT curve of normal drive, the VT curve of pixel division drive 1: 1, and the VT curve of pixel division drive 1: 2. It is a figure which shows the VT curve of 2 types of pixel division driving which has a slit, and the VT curve of the normal driving which has a slit and does not divide a pixel. It is a graph which shows the relationship between a gradation and the transmittance | permeability ((DELTA) T) in the system which has a slit and performs pixel division driving. It is a graph which shows the relationship between a gradation and the transmittance | permeability ((DELTA) T) in the system which has a slit and performs pixel division driving. It is a figure which shows an example of 1 pixel of the model which performed the three-dimensional liquid crystal orientation simulation. (A)-(c) is a figure which shows an example of 1 pixel of the model which performed the three-dimensional liquid crystal orientation simulation, (d) is the image which observed the orientation state at the time of the voltage application more than a threshold voltage with a microscope. FIG. (A) is a figure which shows the area | region which actually simulated among liquid crystal panels, (b) is a comparative example (ref) and sample B when AC image sticking does not arise (pretilt angle 89.0 degrees). (C) is a graph showing the logarithm of the calculation result shown in (b), and (d) is a case where no AC image sticking occurs (pretilt angle 89.0 °). It is a figure which shows the percentage of the value which divided the transmittance | permeability of this by the transmittance | permeability when AC image sticking has arisen (tilt angle 88.9 degree | times). It is a figure which shows the three-dimensional luminance map in 1 domain. (A), (b) is a figure for demonstrating the phenomenon which arises by AC image sticking.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st board | substrate 2 2nd board | substrate 3 Liquid crystal layer 3a Liquid crystal molecule 4 1st electrode 5 2nd electrode 10 Pixel 11 Slit 12-15 Domain 16 Display area 17 Transparent electrode

Claims (11)

A first substrate and a second substrate facing each other;
A plurality of pixels,
Each of the plurality of pixels is
A vertically aligned liquid crystal layer provided between the first substrate and the second substrate;
A first electrode provided on the liquid crystal layer side of the first substrate and a second electrode provided on the liquid crystal layer side of the second substrate;
The liquid crystal layer exhibits a pretilt angle of 85 ° or more and less than 90 °;
The liquid crystal display device, wherein at least one of the first electrode and the second electrode has at least one slit.   The liquid crystal display device according to claim 1, wherein a width of the at least one slit is in a range of 5 μm to 20 μm. The at least one slit is a plurality of slits;
2. The liquid crystal display device according to claim 1, wherein a value L / S of a ratio of a distance (L) in which the slits are adjacent to each other in a width direction of the slits to a width (S) of the slits is 1 or more and 6 or less.   The ratio A1 / A2 between the area A1 of the portion opened by the slit and the area A2 of the display portion of the pixel in which the slit is formed among the plurality of pixels is 0.05 or more and 0.5 or less. The liquid crystal display device according to claim 1. The first electrode is a pixel electrode,
The second electrode is a common electrode;
The pixel electrode has a first subpixel electrode and a second subpixel electrode,
When displaying at least a certain halftone, the effective voltage applied between the first subpixel electrode and the common electrode is greater than the effective voltage applied between the second subpixel electrode and the common electrode. Higher
5. The liquid crystal display device according to claim 1, wherein the at least one slit is formed only in the first subpixel electrode. 6.   6. The liquid crystal according to claim 5, wherein a ratio A <b> 1 / A <b> 2 of an area A <b> 1 opened by the slit and an area A <b> 2 of the display portion of the first subpixel electrode is 0.05 or more and 0.5 or less. Display device.   In each of the plurality of pixels, the liquid crystal layer has liquid crystal molecules near the center in the thickness direction of the liquid crystal layer when a predetermined voltage is applied between the first electrode and the second electrode. The liquid crystal display device according to claim 1, wherein a plurality of domains having different tilt directions are formed.   The plurality of domains are a first liquid crystal domain, a second liquid crystal domain, a third liquid crystal domain, and a fourth liquid crystal domain, wherein the tilt directions of the liquid crystal molecules are different from each other by an integral multiple of 90 °. Liquid crystal display device. Each of the plurality of pixels further includes a pair of alignment films formed between the liquid crystal layer and the first electrode and between the liquid crystal layer and the second electrode,
9. The liquid crystal display device according to claim 1, wherein a pretilt direction defined by one alignment film and a pretilt direction defined by the other alignment film differ from each other by approximately 90 °.   Each of the plurality of pixels further includes a pair of photo-alignment films formed between the liquid crystal layer and the first electrode and between the liquid crystal layer and the second electrode. The liquid crystal display device according to any one of the above.   The liquid crystal display device according to claim 10, wherein the photo-alignment film is a photo-alignment film that causes a photodimerization reaction by irradiation with polarized ultraviolet rays and develops the pretilt angle of the liquid crystal layer.

Images