JP2007178496A - Liquid crystal display element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a reflection type liquid crystal display element enabling an image of high contrast ratio excellent in an angle-of-view characteristic to be displayed even upon high-duty driving. <P>SOLUTION: The liquid crystal display element 1 includes: a liquid crystal cell 2 having a liquid crystal layer of negative dielectric anisotropy held between a pair of vertically aligned substrates; a pair of polarizing plates 3, 4 which hold the liquid crystal cell 2 therebetween and are arranged in a crossed Nichols configuration to each other; a retardation compensation layer 5 disposed between the polarizing plate 3 and the liquid crystal cell 2; and a reflection plate 6 disposed on the polarizing plate 4. The angle made by a fast axis of the retardation compensation layer 5 and an alignment direction of the liquid crystal layer upon voltage application is smaller than the angle made by an absorption axis of the polarizing plate and the alignment direction of the liquid crystal layer upon voltage application, and is more than 0° and is less than 50°. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a liquid crystal display element, and more particularly to a reflective VA mode liquid crystal display element that can also be used for in-vehicle applications.

  In general, a transmissive liquid crystal display element includes an extremely thin liquid crystal layer of about several μm oriented in a predetermined direction, a pair of transparent thin substrates that sandwich the liquid crystal layer, and a polarizer that sandwiches the substrate. And a pair of polarizing plates constituting the analyzer. Here, an electrode patterned in a predetermined shape is formed on the substrate surface on the side where the liquid crystal layer is provided. When a voltage is applied to the liquid crystal layer through this electrode, the alignment of the liquid crystal changes, and the amount or wavelength of light transmitted through the liquid crystal display element changes. Thereby, a desired display can be performed.

  On the other hand, the reflective liquid crystal display element has the same configuration as the transmissive liquid crystal display element described above, but a reflective plate is provided on one side of the liquid crystal display device (for example, the back side that is the anti-viewer side). ing. In particular, in a reflective liquid crystal display element referred to as a so-called “one polarizing plate type”, a circularly polarizing plate is disposed on the viewer side of a liquid crystal panel having the same configuration as the above-described transmissive liquid crystal panel, A reflector is disposed on the side opposite to the viewer. In this case, external light incident from the front side, which is the viewer side, is reflected by this reflector and returned to the viewer side. However, according to the change in the orientation of the liquid crystal due to voltage application via the electrodes, the liquid crystal display element Since the amount or wavelength of light that passes through the viewer and returns to the viewer changes, a desired display can be performed.

  Thus, the liquid crystal display element has a relatively simple structure. In addition, since it is easy to reduce the thickness and weight by selecting components and can be driven at a low voltage, in recent years, it has been actively used as a display element for in-vehicle use as well as consumer use. .

  By the way, liquid crystal display elements are classified into several modes based on the initial alignment state of the liquid crystal layer, the operation state and the alignment state when a voltage is applied, and the like. For example, a VA (Vertical Alignment) mode is used for so-called in-vehicle liquid crystal display elements such as liquid crystal televisions and instrument panels of vehicles such as automobiles (see, for example, Patent Documents 1 and 2). The VA mode is a mode with excellent visibility since it has a high contrast ratio when viewed from the front and a wide viewing angle.

  In the VA mode, a liquid crystal layer having a negative dielectric anisotropy (Δε) whose initial alignment state is substantially perpendicular to the substrate (vertical alignment) is sandwiched between a pair of substrates. It is comprised by pinching with a pair of polarizing plate arrange | positioned so that Nicole may be comprised. When a voltage is applied to the liquid crystal layer through the electrode formed on the substrate surface, the alignment of the liquid crystal changes, and the liquid crystal layer is perpendicular to the electric field, that is, the alignment direction of the liquid crystal is parallel to the substrate. Thereby, the light transmission characteristics determined by the product (Δn · d) of the refractive index anisotropy (Δn) of the liquid crystal and the liquid crystal layer thickness (d) between the portion where the voltage is applied and the portion where the voltage is not applied, , There will be a difference in color. By utilizing this difference, a desired display can be performed.

JP 11-133413 A JP 2000-19518 A

  As described above, in the VA mode, when no voltage is applied, the liquid crystal layer is oriented substantially perpendicular to the pair of substrates that sandwich the liquid crystal layer. For this reason, when no voltage is applied, the light transmitted through the liquid crystal display element is shielded by the action of the polarizing plates arranged in crossed Nicols.

  For example, in the transmissive mode in which a backlight is used in combination, OFF display as a good black display can be obtained by blocking light from the backlight by a liquid crystal display element. When a voltage is applied, alignment changes occur in the liquid crystal, so that light is transmitted between the polarizing plates, and an ON display can be obtained in the liquid crystal display element.

  That is, in the VA mode, it is possible to realize image display by negative display (normally black display) with a high contrast ratio. For this reason, the VA mode is actively used as an active matrix type liquid crystal display element in which an active element such as a TFT (Thin Film Transistor) is provided for each display pixel.

  In addition, the VA mode is paid attention to its high contrast ratio and wide viewing angle characteristics, and its application to a passive matrix liquid crystal display element that performs character display and the like is being actively studied.

  However, when applying the VA mode to a passive matrix type, the slow change in the transmittance (T) of the liquid crystal display element with respect to the applied voltage (V) (so-called slow VT characteristics) may be a problem. is there. In other words, although the viewing angle is narrow, the VT characteristic is steep, and the VT change is steep compared to a liquid crystal mode such as a STN (Super Twist Nematic) mode, which is widely used as a passive matrix liquid crystal display element. Insufficiency can be a problem.

  When an image is displayed by driving a passive matrix liquid crystal display element, it is necessary to set an OFF voltage and an ON voltage in accordance with the duty ratio of the drive. In this case, it is not always possible to simultaneously set 0V, which is no voltage applied as the OFF voltage, and a voltage indicating the maximum transmittance of the liquid crystal display element as the ON voltage.

  Therefore, the liquid crystal display element shows an OFF display (excellent black display) with a very low transmittance when no voltage is applied, and an excellent ON display with an extremely high transmittance by applying the highest possible voltage. Even in the case of a liquid crystal display element in which the change in transmittance due to voltage application is slow, that is, a liquid crystal display element having a VT characteristic in which the transmittance continues to increase gradually according to voltage application, It may be difficult to use as a matrix type liquid crystal display element.

  That is, when applying the liquid crystal display element to a duty-driven passive matrix type, the difference between the set value of the ON voltage and the set value of the OFF voltage that can be ensured according to the duty ratio is naturally determined. For this reason, when the ON voltage is set to a high value with emphasis on the ON display, the OFF voltage may be set to a relatively high value.

  In particular, since the VT change is slow in the VA mode, when the ON voltage is set with an emphasis on the transmittance during ON display, it becomes impossible to set the OFF voltage indicating a sufficiently low transmittance. As a result, when the set OFF voltage is applied, the transmittance increases, so-called “floating” occurs in black display at the time of OFF, and the contrast ratio of the display image may be lowered. On the other hand, when importance is attached to the blackness of the OFF display, the ON transmittance may be lowered, resulting in a dark display.

  Therefore, when applying the VA mode to a duty-driven passive matrix type, it is necessary to improve the steepness of the VT characteristics.

  In the VA mode, in order to solve such problems and improve the steepness of the VT characteristics, a method of setting the retardation (Δnd) of the liquid crystal display element to be relatively high can be considered.

  FIG. 10 is a VT curve schematically showing the influence on the VT characteristic when Δnd is set high in the VA mode. In FIG. 10, when Δnd is set higher, the curve changes from a curve indicated by a broken line to a curve indicated by a solid line. That is, it can be seen from FIG. 10 that by increasing Δnd, the steepness of the VT characteristic is improved particularly in the applied voltage region where the transmittance greatly changes. However, it can also be seen that in such a voltage application region having a value smaller than that of the applied voltage region in which the transmittance changes greatly, a phenomenon in which the transmittance gradually increases as voltage is applied is simultaneously exhibited.

  Therefore, if the VA mode, in which the VT characteristics are improved by setting Δnd to a relatively high value, is applied to a duty-driven passive matrix type, transmission when a set OFF voltage is applied. There is a problem in that the ratio increases, so-called floating occurs during black display, and the contrast ratio of the image decreases.

  In order to increase Δnd of the liquid crystal display element, it is necessary to increase Δn which is the refractive index anisotropy of the liquid crystal or to increase d which is the thickness of the liquid crystal layer. However, any of these parameter changes of the liquid crystal display element is a change in the direction of lowering the response speed of the liquid crystal, leading to a decrease in response speed characteristics in the VA mode. Therefore, it is not preferable to try to solve the problem only by such a method.

  Therefore, in the VA mode, a method other than the method of increasing the retardation value depending on Δn and the thickness of the liquid crystal layer, or a method of improving the VT characteristics by combining with this method is required.

  The present invention has been made in view of these problems. That is, an object of the present invention is a reflective liquid crystal display element that has a steep VT characteristic, can be applied to a passive matrix type, and can realize excellent contrast ratio display and viewing angle characteristics. Is to provide.

  Other objects and advantages of the present invention will become apparent from the following description.

  According to a first aspect of the present invention, a liquid crystal layer having negative dielectric anisotropy is sandwiched between a pair of substrates each having an electrode layer formed on each opposing surface and at least one of which is transparent. Polarized light on the opposite side of the pair of polarizing plates on the opposite side of the liquid crystal cell, the pair of polarizing plates sandwiching the liquid crystal cell, and the opposing surface of the pair of polarizing plates to the substrate A pair of substrates in a liquid crystal display element that displays an image on a display unit by applying a voltage to the liquid crystal layer via an electrode layer of the substrate of the liquid crystal cell. For at least one of the substrates, a vertical alignment film is provided on the surface facing the other substrate, and this alignment film is subjected to an alignment treatment to define the alignment direction of the liquid crystal layer when a voltage is applied. The pair of polarizing plates is one The crossing angle between the absorption axis of the polarizing plate and the absorption axis of the other polarizing plate is 90 ° ± 5 °, and between at least one of the pair of polarizing plates and the liquid crystal cell A retardation compensation layer having retardation, and the angle formed by the fast axis of the retardation compensation layer and the orientation direction of the liquid crystal layer when a voltage is applied is determined by the absorption axis of the polarizing plate and the liquid crystal when a voltage is applied. It is set so as to be smaller than an angle formed with the orientation direction of the layer and more than 0 degree and less than 50 degrees, and an aperture ratio of the display portion is 70% or less.

  According to a second aspect of the present invention, a liquid crystal layer having negative dielectric anisotropy is sandwiched between a pair of substrates, at least one of which is transparent, with electrode layers formed on the opposing surfaces. A liquid crystal cell and a circularly polarizing plate disposed on a viewer side of the pair of substrates opposite to a surface facing the liquid crystal layer, and anti-viewing of the pair of substrates A liquid crystal display element that displays an image on a display unit by applying a voltage to the liquid crystal layer via an electrode layer of the substrate of the liquid crystal cell. For at least one of the substrates, a vertical alignment film is provided on the surface facing the other substrate, and the alignment film is subjected to an alignment process to define the alignment direction of the liquid crystal layer when a voltage is applied. The circularly polarizing plate and the liquid Between the cells, a retardation compensation layer having retardation is provided, and the angle formed by the fast axis of the retardation compensation layer and the alignment direction of the liquid crystal layer when a voltage is applied is the absorption axis of the polarizing plate. It is set so as to be smaller than an angle formed with the alignment direction of the liquid crystal layer when a voltage is applied, and more than 0 degree and less than 50 degrees, and an aperture ratio of the display portion is 70% or less. Is.

  In the first aspect of the present invention, the retardation value of the retardation compensation layer is preferably greater than 0% and not more than 10% of the retardation value of the liquid crystal layer. In the second aspect of the present invention, the retardation value of the retardation compensation layer is preferably larger than 0% and not more than 1.5% of the retardation value of the liquid crystal layer.

  In addition, the liquid crystal display element in the first aspect and the second aspect of the present invention can be a passive matrix liquid crystal display element.

  According to the present invention, in the VA mode, a retardation compensation layer is provided between the liquid crystal cell and the polarizing plate, and by adjusting the retardation value and the angle between the fast axis and the liquid crystal layer alignment direction at the time of voltage application. As a result, the steepness of the VT characteristic is improved, and as a result, it is possible to display an image with a high contrast ratio that is excellent in viewing angle characteristics even when driven by a duty.

Embodiment 1 FIG.
FIG. 1 is a partial cross-sectional view of a liquid crystal display element 1 in the present embodiment. The liquid crystal display element 1 is assumed to be a VA mode liquid crystal display element.

  Referring to FIG. 1, a liquid crystal display element 1 includes a pair of glass substrates (not shown) each having an electrode layer (not shown) formed on opposite surfaces thereof, and a negative dielectric material sandwiched between the pair of substrates. The liquid crystal cell 2 is provided with an isotropic liquid crystal layer. A pair of viewer-side polarizing plates (crossed Nicols) are disposed on the surface opposite to the surface facing the liquid crystal layer of the pair of substrates constituting the liquid crystal cell 2 and sandwich the liquid crystal cell 2. Hereinafter, it is referred to as an F polarizing plate) 3 and an anti-viewer side polarizing plate (hereinafter referred to as an R polarizing plate) 4. Note that the crossed Nicol arrangement is an arrangement in which the crossing angle between the absorption axis of one polarizing plate and the absorption axis of the other polarizing plate is 90 ° ± 5 ° (hereinafter the same in this specification). .

  A retardation compensation layer 5 having retardation is provided between the F polarizing plate 3 and the liquid crystal cell 2. A reflective plate 6 is disposed on the polarizing plate 4 on the counter-viewer side. In the present embodiment, the phase difference compensation layer 5 may be disposed between the surface on the counter-viewer side of the liquid crystal cell 2, that is, between the R polarizing plate 4 and the liquid crystal cell 2. Further, the phase difference compensation layer 5 may be disposed on both the viewer side and the counter-viewer side.

  In a pair of substrates having an electrode layer formed on the surface thereof that constitutes the liquid crystal cell 2, a vertical alignment process is performed on the surface facing the other substrate. In addition, an alignment treatment for defining the alignment direction of the liquid crystal layer when a voltage is applied is also performed. Thereby, in the liquid crystal layer, the liquid crystal is in an initial alignment state and is in a state slightly tilted in the alignment direction and is substantially perpendicularly aligned. When a voltage is applied, the liquid crystal layer is tilted in a certain direction and operates so as to be parallel to the substrate constituting the liquid crystal cell 2.

  The liquid crystal constituting the liquid crystal layer has a negative dielectric anisotropy, and a voltage is applied to the liquid crystal layer through the electrode layer on the substrate surface of the liquid crystal cell 2 so that the liquid crystal is parallel to the substrate. When operated, the change in the orientation of the liquid crystal layer defined in the orientation processing direction described above occurs, the optical anisotropy of the liquid crystal layer changes, and an image is displayed on the display portion of the liquid crystal display element.

  At this time, regarding the arrangement of the retardation compensation layer 5 having retardation provided between the polarizing plate 3 and the liquid crystal cell 2, the fast axis of the retardation compensation layer and the alignment direction of the liquid crystal layer when a voltage is applied Is preferably set to be smaller than an angle formed between the absorption axis of the polarizing plate and the alignment direction of the liquid crystal layer when a voltage is applied, and more than 0 degree and less than 50 degrees. By providing the retardation compensation layer and making such settings, a display image with a high contrast ratio can be obtained in the liquid crystal display element 1 as described below.

  The retardation magnitude of the retardation compensation layer 5 is preferably greater than 0% and less than or equal to 10% of the retardation value of the liquid crystal layer. More preferably, it is set to be larger than 0% and 8% or less. By performing such a setting in addition to the arrangement angle setting of the phase difference compensation layer, as described below, when the liquid crystal display element 1 is OFF, a pixel portion that is an electrode layer region in the display unit; A difference in appearance with the surrounding area is reduced, and a display image having a natural appearance can be obtained even when the display is turned off. In particular, when it is set to 8% or less, the difference cannot be visually recognized, and the appearance can be further improved.

  The structure of the electrode layer formed on the surface of the substrate constituting the liquid crystal cell 2 is designed to display an image including a character display, and the aperture ratio of the display portion is 70% or less. Accordingly, the peripheral area that is not a pixel portion is large in the display portion, and the difference in appearance between the pixel portion in the display portion and the peripheral region is small when the liquid crystal display element 1 is OFF by setting the phase difference compensation layer 5 as described above. It becomes more important to have a natural appearance.

  Next, an example of a method for manufacturing the liquid crystal display element 1 will be described.

  First, an electrode layer patterned so as to display a desired image is provided on a pair of glass substrates. The electrode layer can be, for example, an ITO (Indium Tin Oxide) electrode.

Next, an insulating film is provided on the glass substrate so as to cover the electrode layer. The insulating film can be, for example, a film made of SiO ₂ —TiO ₂ formed by a sol-gel method.

  Next, an alignment film is formed in the liquid crystal layer so that the liquid crystal is vertically aligned as an initial alignment state. For example, an alignment film having a thickness of about 60 nm may be formed by forming an alignment film material (trade name: JALS-2021) manufactured by JSR Corporation by flexographic printing and baking the substrate at 180 ° C. it can. Next, a rubbing process is performed on the surface of the alignment film to determine the operation direction of the liquid crystal when the electric field is applied.

  Next, the liquid crystal layer is sandwiched between the substrates after the alignment film formation step. At this time, for example, the distance (d) between the substrates can be kept constant by using a resin spacer. As the liquid crystal layer, for example, a layer having a refractive index anisotropy (Δn) of 0.1325 can be used. In this case, when d = 4.0 μm, the retardation (Δn · d) of the liquid crystal cell 2 is 530 nm.

  Next, the phase difference compensation layer 5 is provided. Specifically, a uniaxial film is disposed on the viewer side surface of the liquid crystal cell 2. At this time, the retardation value set in the uniaxial film can be expressed using the retardation value expressed in the configuration of the liquid crystal display element 1, and the retardation value of the liquid crystal cell 2 is 7.55% of 530 nm. Set to be a value. And the direction of arrangement is set so that the angle formed by the direction of the fast axis of the uniaxial film and the alignment direction of the liquid crystal layer when a voltage is applied is 5 degrees. In the present invention, the phase difference compensation layer may be disposed on the surface on the side opposite to the viewer of the liquid crystal cell, or may be disposed on both the surface on the viewer side and the side facing the viewer.

  Next, the F polarizing plate 3 and the R polarizing plate 4 are installed. Specifically, the liquid crystal cell 2 having the retardation compensation layer 5 disposed on the surface is sandwiched and pasted so that the F polarizing plate 3 and the R polarizing plate 4 are in a crossed Nicols arrangement. At this time, the absorption axis of the F polarizing plate 3 or the R polarizing plate 4 is set so as to form an angle of 45 degrees with the alignment direction of the liquid crystal layer when a voltage is applied.

Although not shown in detail, the liquid crystal display element 1 in the present embodiment has a passive matrix structure. That is, each pixel portion constituting the image display is not provided with a switching element such as a TFT, and a target image is displayed by passive driving using an electrode layer. When the liquid crystal display element 1 was driven at 1/16 duty (1/16 duty) and the characteristics were examined, the ON transmittance was 6.97%. At this time, the minimum transmittance of the liquid crystal display element 1 was 5.7 × 10 ⁻⁶ %, and the voltage indicating the minimum transmittance was set to the OFF voltage.

  Next, an example of the result of evaluating the VT characteristic in the liquid crystal display element 1 of the present embodiment will be described.

FIG. 2 is an example of a VT curve schematically showing the characteristics of the VT characteristic of the liquid crystal display element 1. As shown in FIG. 2, the V-T curve of the liquid crystal display device 1, when going to apply a voltage, at a given applied voltage region, the transmittance becomes smaller than the transmittance T ₁ of the absence of applied voltage. Then, after the lowest transmission value T _2, the transmittance by the application of a further voltage increases rapidly. For this reason, the liquid crystal display element 1 exhibits a very steep VT characteristic. Therefore, the liquid crystal display element 1 can display an image with a high contrast ratio even in passive matrix driving with high duty driving. Note that the value of T ₂ / T ₁ of the liquid crystal display element 1 is 6.73 × 10 ⁻⁵ .

Then, the liquid crystal display device 1 when trying to passive matrix drive, it is preferable to set the vicinity of the voltage value indicating the minimum transmittance T ₂ is turned OFF voltage during driving. By doing so, the degree of shading when OFF is increased, the transmittance is remarkably lowered, and a high level of black display is obtained. On the other hand, when ON, a high transmittance display can be obtained, so that an image display with a high contrast ratio can be realized.

However, as shown in FIG. 2, since the transmittance T ₁ when no voltage is applied and the minimum transmittance T ₂ are different, it has been found that a problem occurs in the display image at the time of OFF.

The problem is that in the liquid crystal display element 1, when the vicinity of the voltage value indicating the minimum transmittance T ₂ is set as the OFF voltage, the area in which the electrode layer in the display unit is formed at the OFF display is OFF. Since the pixel portion to which the voltage is applied and the electrode layer are not provided, the transmittance of the peripheral portion where the liquid crystal layer does not operate in a vertically aligned state is different regardless of whether or not the voltage is applied. It is a problem.

  That is, at the time of OFF, it is preferable that the entire display unit of the liquid crystal display element 1 displays a uniform sunken black. However, since the transmittance of the pixel portion and the transmittance of the peripheral portion are different, the electrode layer There is a possibility that the pixel portion where the pixel is arranged becomes conspicuous, and a non-uniform OFF image display state occurs.

  Such a problem becomes particularly problematic in a display image structure having an aperture ratio of 70% or less, particularly in a display image structure having a small aperture ratio for the purpose of character display. This is the area where the electrode layer is formed in the display area, and the liquid crystal layer operates in the vertical alignment regardless of whether or not voltage is applied to the pixel part where light is transmitted when voltage is applied. This is due to the large area of the peripheral region that does not.

  Therefore, a large number of liquid crystal display elements having the same structure as that of the liquid crystal display element 1 except that only the retardation of the uniaxial film as the retardation compensation layer 5 was changed were manufactured according to the manufacturing method described above. First, the relationship between the difference between the transmittance when no voltage was applied and the minimum transmittance and the retardation of the uniaxial film was examined.

  FIG. 3 is a diagram showing the relationship between the transmittance when no voltage is applied and the minimum transmittance with respect to the retardation developed in the configuration of the liquid crystal display element. In FIG. 3, the x-axis represents the retardation value expressed in the configuration of each liquid crystal display element, which is shown as the retardation value of the liquid crystal cell (in FIG. 3, the panel Δnd. The same applies hereinafter). The value divided by 530 nm is shown. Here, the film Δnd in FIG. 3 indicates a retardation value expressed in the configuration of each liquid crystal display element (hereinafter the same in this specification). On the other hand, the y-axis indicates a value obtained by dividing the minimum transmittance of each liquid crystal display element by the transmittance when no voltage is applied.

  FIG. 3 shows that the minimum transmittance value becomes smaller than the transmittance value when no voltage is applied as the retardation expressed in the configuration of the liquid crystal display element of the present invention, that is, the retardation of the uniaxial film used increases. Therefore, as the retardation of the uniaxial film to be used increases, the minimum transmittance may decrease, but the pixel portion on which the electrode layer is disposed may become conspicuous and a non-uniform OFF image display state may occur. There is.

  Next, the voltage indicating the minimum transmittance is set to the OFF voltage, and when the OFF voltage is applied, the pixel part to which the OFF voltage is applied is conspicuous with respect to the peripheral part. It was investigated whether or not the viewer was given a non-uniform OFF screen impression.

At the time of OFF, the impression whether the pixel portion to which the OFF voltage is applied to the peripheral portion is noticeable due to the difference in transmittance is recognized as a color difference by the viewer. In the ΔE ^* u ^* v ^* display, when the color difference between the pixel portion and the peripheral portion exceeds 3, the presence of the pixel portion is felt in the OFF display, and a non-uniform image impression is given to the viewer. Here, in the liquid crystal display element 1, the color difference between the pixel portion and the peripheral portion was 1.09.

  Next, the relationship between the color difference between the pixel portion and its peripheral portion with respect to the retardation of the uniaxial film used was examined.

FIG. 4 is a diagram showing the relationship between retardation and color difference that are manifested in the configuration of the liquid crystal display element of FIG. In FIG. 4, the x-axis indicates the retardation value expressed in the configuration of each liquid crystal display element divided by the retardation value of the liquid crystal cell, that is, 530 nm. On the other hand, the y-axis indicates the color difference of each liquid crystal display element by ΔE ^* u ^* v ^* expression.

As shown in FIG. 4, as the retardation that appears in the configuration of the liquid crystal display device of the present invention, that is, the retardation of the uniaxial film to be used increases, ΔE between the pixel portion and its peripheral portion at the OFF time when the OFF voltage is applied ^It can be seen that the color difference in ^* u ^* v ^* expression increases.

In addition, when the value obtained by dividing the retardation value expressed in the configuration of each liquid crystal display element by the retardation value of the liquid crystal cell is 10% or less, the color difference in ΔE ^* u ^* v ^* expression may be 3 or less. Recognize.

  That is, in order to prevent the presence of the pixel portion in the OFF display and avoid giving a non-uniform image impression, the retardation value expressed in the configuration of each liquid crystal display element is set to the retardation value of the liquid crystal cell. It is desirable to set the retardation of the uniaxial film so that the value divided by the value is 10% or less. When it is 8% or less, the color difference is 1.5 or less and the difference cannot be visually recognized.

  Next, 1/16 duty drive is performed using the liquid crystal display element 1 and a liquid crystal display element having the same structure as the liquid crystal display element 1 except that only the retardation of the uniaxial film as the retardation compensation layer 5 is changed. Was used to evaluate the image quality. Specifically, the voltage indicating the minimum transmittance was set to the OFF voltage, and the transmittance when ON by 1/16 duty driving was evaluated.

  FIG. 5 is a diagram showing the relationship between the retardation developed in the configuration of the present liquid crystal display element and the transmittance when ON by 1/16 duty driving. In FIG. 5, the x-axis represents the retardation value expressed in the configuration of each liquid crystal display element divided by the retardation value of the liquid crystal cell, that is, 530 nm. On the other hand, the y-axis indicates the transmittance when ON by 1/16 duty drive.

  From FIG. 5, it can be seen that as the retardation expressed in the configuration of the liquid crystal display element of the present invention, that is, the retardation of the uniaxial film to be used increases, the transmittance at ON by 1/16 duty driving increases. And when the value obtained by dividing the retardation value developed in the configuration of the liquid crystal display element by the value of the retardation of the liquid crystal cell exceeds about 4%, the transmittance at ON time by 1/16 duty driving exceeds 6%. Can provide bright and high-quality image display.

  As described above, according to the present embodiment, in the VA mode, the retardation compensation layer is provided between the liquid crystal cell and the polarizing plate, the retardation value, the fast axis, and the liquid crystal layer alignment direction at the time of voltage application. By adjusting the angle formed, the steepness of the VT characteristic can be improved without increasing the OFF transmittance. Therefore, when duty driving is performed, it is possible to simultaneously realize an image display with a high contrast ratio excellent in viewing angle characteristics and an image display that does not impair uniformity even when OFF.

Embodiment 2. FIG.
FIG. 6 is a partial cross-sectional view of the liquid crystal display element 11 in the present embodiment. The liquid crystal display element 11 is assumed to be a VA mode liquid crystal display element.

  In FIG. 6, a liquid crystal display element 11 includes a pair of glass substrates (not shown) each having an electrode layer (not shown) formed on the respective opposing surfaces, and a negative dielectric difference sandwiched between the pair of substrates. The liquid crystal cell 12 is provided with an isotropic liquid crystal layer. A circularly polarizing plate 13 is disposed on the viewer side of the liquid crystal cell 12, and a retardation compensation layer 14 having retardation is provided between the circularly polarizing plate 13 and the liquid crystal cell 12. Further, a reflector 15 is disposed on the non-viewer side of the liquid crystal cell 12.

  Thus, according to the configuration of the present embodiment in which the polarizing plate to be used is one circularly polarizing plate, compared to the configuration in which two polarizing plates described in the first embodiment are arranged before and after the liquid crystal cell, A liquid crystal composition showing a smaller Δn value can be selected to form a liquid crystal layer, or the liquid crystal layer thickness can be made thinner. All of these methods cause the response speed of the liquid crystal to increase, so that it is possible to display an image with good appearance and excellent response. Furthermore, since only one polarizing plate is required, a liquid crystal display element can be provided at low cost.

  In a pair of substrates that constitute the liquid crystal cell 12 and on which an electrode layer is formed, a vertical alignment process is performed on the surface facing the other substrate. Furthermore, an alignment treatment for defining the alignment direction of the liquid crystal layer when a voltage is applied is also performed. Therefore, in the liquid crystal layer, the liquid crystal is in a state of being slightly tilted in the alignment direction as the initial alignment state and is substantially perpendicularly aligned. When a voltage is applied, the liquid crystal layer is tilted in a certain direction and operates so as to be parallel to the substrate constituting the liquid crystal cell 2.

  The liquid crystal constituting the liquid crystal layer has a negative dielectric anisotropy, and a voltage is applied to the liquid crystal layer through the electrode layer on the substrate surface of the liquid crystal cell 12 so that the liquid crystal is parallel to the substrate. When operated, the change in the orientation of the liquid crystal layer defined in the orientation processing direction described above occurs, the optical anisotropy of the liquid crystal layer changes, and an image is displayed on the display portion of the liquid crystal display element.

  At this time, with respect to the arrangement of the retardation compensation layer 14 having retardation provided between the circularly polarizing plate 13 and the liquid crystal cell 12, the fast axis of the retardation compensation layer and the orientation direction of the liquid crystal layer when a voltage is applied. Is preferably smaller than the angle formed between the absorption axis of the polarizing plate and the alignment direction of the liquid crystal layer when a voltage is applied, and more than 0 degree and less than 50 degrees. By providing the retardation compensation layer and making such settings, a display image with a high contrast ratio can be obtained in the liquid crystal display element 11 as described below.

  The retardation magnitude of the retardation compensation layer 14 is preferably larger than 0% and not more than 1.5% of the retardation value of the liquid crystal layer. More preferably, it is set to be greater than 0% and 1.0% or less. By performing such a setting in addition to the arrangement angle setting of the phase difference compensation layer, as described below, when the liquid crystal display element 1 is OFF, a pixel portion that is an electrode layer region in the display unit; A difference in appearance with the surrounding area is reduced, and a display image having a natural appearance can be obtained even when the display is turned off. In particular, when it is set to 1.0% or less, the difference cannot be visually recognized, and the appearance can be further improved.

  The structure of the electrode layer formed on the surface of the substrate constituting the liquid crystal cell 12 is designed so that an image including character display can be displayed, and the aperture ratio of the display portion is 70% or less. Accordingly, the peripheral area that is not a pixel portion is large in the display portion, and the difference in appearance between the pixel portion in the display portion and the peripheral region is small when the liquid crystal display element 11 is OFF by setting the phase difference compensation layer 14 as described above. It becomes more important to have a natural appearance.

  Next, an example of a method for manufacturing the liquid crystal display element 11 will be described.

  First, an electrode layer patterned so as to display a desired image is provided on a pair of glass substrates. The electrode layer can be, for example, an ITO (Indium Tin Oxide) electrode.

Next, an insulating film is provided on the glass substrate so as to cover the electrode layer. The insulating film can be, for example, a film made of SiO ₂ —TiO ₂ formed by a sol-gel method.

  Next, an alignment film is formed in the liquid crystal layer so that the liquid crystal is vertically aligned as an initial alignment state. For example, an alignment film having a thickness of about 60 nm may be formed by forming an alignment film material (trade name: JALS-2021) manufactured by JSR Corporation by flexographic printing and baking the substrate at 180 ° C. it can. Next, a rubbing process is performed on the surface of the alignment film to determine the operation direction of the liquid crystal when the electric field is applied.

  Next, the liquid crystal layer is sandwiched between the substrates after the alignment film formation step. At this time, for example, the distance (d) between the substrates can be kept constant by using a resin spacer. As the liquid crystal layer, for example, a layer having a refractive index anisotropy (Δn) of 0.06625 can be used. In this case, when d = 4.0 μm, the retardation (Δn · d) of the liquid crystal cell 2 is 265 nm.

  Next, the retardation compensation layer 14 is provided. Specifically, a uniaxial film is disposed on the viewer-side surface of the liquid crystal cell 12. At this time, the retardation value set in the uniaxial film can be expressed using the retardation value expressed in the configuration of the liquid crystal display element 11, and the retardation value of the liquid crystal cell 12 is 1.13% of 265 nm. Set to be a value. And the direction of arrangement is set so that the angle formed by the direction of the fast axis of the uniaxial film and the alignment direction of the liquid crystal layer when a voltage is applied is 5 degrees.

  Next, the circularly polarizing plate 13 is provided on the retardation compensation layer 14. In addition, a reflecting plate 15 is provided on the surface of the liquid crystal cell 12 on the side opposite to the viewer.

  Although not shown in detail, the liquid crystal display element 1 in the present embodiment has a passive matrix structure. That is, each pixel portion constituting the image display is not provided with a switching element such as a TFT, and a target image is displayed by passive driving using an electrode layer. When the liquid crystal display element 11 was driven at 1/16 duty (1/16 duty) and the characteristics were examined, the ON transmittance was 13.81%. At this time, the minimum transmittance of the liquid crystal display element 11 was 0.387%, and the voltage indicating the minimum transmittance was set to the OFF voltage.

The liquid crystal display element 11 of the present embodiment also exhibits the same VT characteristics as the liquid crystal display element 1 described in the first embodiment. That is, when gradually applying a voltage, similarly to FIG. 2, in a given applied voltage region, the transmittance becomes smaller than the transmittance T ₁ of the absence of applied voltage. Then, after the lowest transmission value T _2, the transmittance by the application of a further voltage increases rapidly. Therefore, the VT characteristic is very steep. Therefore, the liquid crystal display element 11 can display an image with a high contrast ratio in the passive matrix drive of the duty drive. The value of T ₂ / T ₁ in the liquid crystal display element 11 is 0.915.

If to be passive matrix driven liquid crystal display element 11, it is preferable to set the vicinity of the voltage value indicating the minimum transmittance T ₂ is turned OFF voltage during driving. By doing so, the degree of shading when OFF is increased, the transmittance is remarkably lowered, and a high level of black display is obtained. On the other hand, when ON, a high transmittance display can be obtained, so that an image display with a high contrast ratio can be realized.

On the other hand, since the transmittance T ₁ when no voltage is applied is different from the minimum transmittance T ₂ , problems that occur in the display image at the time of OFF are as follows. That is, in the liquid crystal display device 11, in the case of setting the near voltage value indicating the minimum transmittance T ₂ as OFF voltage during OFF display, is a by OFF voltage region where the electrode layer in the display portion is formed Since the applied pixel portion and the electrode layer are not provided, the transmittance of the peripheral portion where the liquid crystal layer does not operate in a vertically aligned state is different regardless of whether a voltage is applied. At the time of OFF, it is preferable that the entire display portion of the liquid crystal display element 11 displays a uniform sunken black. However, since the transmittance of the pixel portion and the transmittance of the peripheral portion are different, the arrangement of the electrode layers is different. The provided pixel portion may become conspicuous, and a non-uniform OFF image display state may occur.

  Such a problem becomes particularly noticeable, for example, in a display image structure with an aperture ratio of 70% or less, particularly in a display image structure with a small aperture ratio for the purpose of character display. This is the area where the electrode layer is formed in the display area, and the liquid crystal layer operates in the vertical alignment regardless of whether or not voltage is applied to the pixel part where light is transmitted when voltage is applied. This is due to the large area of the peripheral region that does not.

  Therefore, a large number of liquid crystal display elements having the same structure as that of the liquid crystal display element 11 except that only the retardation of the uniaxial film as the retardation compensation layer 14 was changed were manufactured according to the manufacturing method described above. First, the relationship between the difference between the transmittance when no voltage was applied and the minimum transmittance and the retardation of the uniaxial film was examined.

  FIG. 7 is a diagram showing the relationship between the difference between the transmittance when no voltage is applied and the minimum transmittance with respect to the retardation developed in the configuration of the liquid crystal display element. In FIG. 7, the x-axis indicates the retardation value expressed in the configuration of each liquid crystal display element divided by the retardation value of the liquid crystal cell (shown as a panel Δnd in FIG. 7), that is, 265 nm. Here, the film Δnd in FIG. 7 indicates a retardation value expressed in the configuration of each liquid crystal display element (hereinafter the same in this specification). On the other hand, the y-axis indicates a value obtained by dividing the minimum transmittance of each liquid crystal display element by the transmittance when no voltage is applied.

  FIG. 7 shows that the minimum transmittance value becomes smaller than the transmittance value when no voltage is applied as the retardation expressed in the configuration of the liquid crystal display element of the present invention, that is, the retardation of the uniaxial film to be used increases. Therefore, as the retardation of the uniaxial film to be used increases, the minimum transmittance may decrease, but the pixel portion on which the electrode layer is disposed may become conspicuous and a non-uniform OFF image display state may occur. There is.

  Next, the voltage indicating the minimum transmittance is set to the OFF voltage, and when the OFF voltage is applied, the pixel part to which the OFF voltage is applied is conspicuous with respect to the peripheral part. It was investigated whether or not the viewer was given a non-uniform OFF screen impression.

At the time of OFF, the impression whether the pixel portion to which the OFF voltage is applied to the peripheral portion is noticeable due to the difference in transmittance is recognized as a color difference by the viewer. In the ΔE ^* u ^* v ^* display, if the color difference between the pixel portion and the peripheral portion exceeds 3, the presence of the pixel portion is felt in the OFF display, and a non-uniform image impression is given to the viewer. Here, in the liquid crystal display element 11, the color difference between the pixel portion and the peripheral portion was 2.15.

  Next, the relationship between the color difference between the pixel portion and its peripheral portion with respect to the retardation of the uniaxial film used was examined.

FIG. 8 is a diagram showing the relationship between retardation and color difference that appear in the configuration of the liquid crystal display element of FIG. In FIG. 8, the x-axis indicates the retardation value expressed in the configuration of each liquid crystal display element divided by the retardation value of the liquid crystal cell, that is, 265 nm. On the other hand, the y-axis indicates the color difference of each liquid crystal display element by ΔE ^* u ^* v ^* expression.

From FIG. 8, as the retardation developed in the configuration of the liquid crystal display element of the present invention, that is, the retardation of the uniaxial film to be used increases, ΔE between the pixel portion and its peripheral portion at the OFF time when the OFF voltage is applied. ^It can be seen that the color difference in ^* u ^* v ^* expression increases.

Further, when the value obtained by dividing the retardation value expressed in the configuration of each liquid crystal display element by the retardation value of the liquid crystal cell is 1.5% or less, the color difference in ΔE ^* u ^* v ^* expression is 3 or less. I understand that.

  That is, in order to prevent the presence of the pixel portion in the OFF display and avoid giving a non-uniform image impression, the retardation value expressed in the configuration of each liquid crystal display element is set to the retardation value of the liquid crystal cell. It is desirable to set the retardation of the uniaxial film so that the value divided by the value is 1.5% or less. When the ratio is 1.0% or less, the color difference is further reduced, and the difference cannot be visually recognized.

  Next, 1/16 duty drive is performed using the liquid crystal display element 11 and a liquid crystal display element having the same structure as the liquid crystal display element 11 except that only the retardation of the uniaxial film that is the retardation compensation layer 14 is changed. Was used to evaluate the image quality. Specifically, the voltage indicating the minimum transmittance was set to the OFF voltage, and the transmittance when ON by 1/16 duty driving was evaluated.

  FIG. 9 is a diagram showing the relationship between the retardation developed in the configuration of the present liquid crystal display element and the transmittance when ON by 1/16 duty driving. In FIG. 9, the x-axis indicates the retardation value expressed in the configuration of each liquid crystal display element divided by the retardation value of the liquid crystal cell, that is, 265 nm. On the other hand, the y-axis indicates the transmittance when ON by 1/16 duty drive.

  From FIG. 9, it can be seen that as the retardation expressed in the configuration of the liquid crystal display element of the present invention, that is, the retardation of the uniaxial film to be used increases, the transmittance at ON by 1/16 duty driving increases. And when the value obtained by dividing the retardation value developed in the configuration of the liquid crystal display element by the value of the retardation of the liquid crystal cell exceeds about 1%, the transmittance at ON time by 1/16 duty driving exceeds 13%. It can provide a very bright high-quality image display.

  As described above, according to the present invention, in the VA mode, a retardation compensation layer is provided between the liquid crystal cell and the circularly polarizing plate, and the retardation value, the fast axis, and the alignment direction of the liquid crystal layer when a voltage is applied. By adjusting the angle, it is possible to improve the steepness of the VT characteristic without increasing the OFF transmittance. Therefore, when duty driving is performed, it is possible to simultaneously realize an image display with a high contrast ratio excellent in viewing angle characteristics and an image display that does not impair uniformity even when OFF.

  The present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention.

  For example, the present invention can also be applied to a transflective liquid crystal display element using a transflective plate. Specifically, a configuration similar to that of the liquid crystal display element 1 may be used except that the reflector 6 is a transflective reflector in the first embodiment. In this case, by using the backlight together, it is possible to display an image even in an environment where external light is scarce.

  Another example of the configuration of the transflective liquid crystal display element is a liquid crystal cell having the same configuration as that of the liquid crystal cell 2 constituting the liquid crystal display element 1 in the first embodiment, and further includes a transmissive region in the display portion. A liquid crystal cell having a reflective region, and on the electrode layer of the reflective region, a reflective step layer is provided in which the thickness of the liquid crystal layer in the reflective region is approximately half the thickness of the liquid crystal layer in the transmissive region. And a liquid crystal cell sandwiched between a pair of circularly polarizing plates.

3 is a partial cross-sectional view of a liquid crystal display element in Embodiment 1. FIG. 3 is a VT curve of the liquid crystal display element in the first embodiment. FIG. 4 is a diagram showing a relationship between retardation that appears in the configuration of the liquid crystal display element in Embodiment 1, and transmittance and minimum transmittance when no voltage is applied. FIG. 3 is a diagram showing a relationship between retardation and color difference expressed in the configuration of the liquid crystal display element in the first embodiment. FIG. 6 is a diagram showing a relationship between retardation that appears in the configuration of the present liquid crystal display element according to Embodiment 1 and transmittance when ON by 1/16 duty drive. 6 is a partial cross-sectional view of a liquid crystal display element in a second embodiment. FIG. It is a figure which shows the relationship between the retardation expressed with the structure of the liquid crystal display element in Embodiment 2, the transmittance | permeability at the time of no voltage application, and the minimum transmittance | permeability. FIG. 10 is a diagram showing a relationship between retardation and color difference expressed in the configuration of the liquid crystal display element in the second embodiment. It is a figure which shows the relationship between the retardation expressed with the structure of this liquid crystal display element in Embodiment 2, and the transmittance | permeability at the time of ON by 1/16 duty drive. It is a figure which shows the change of the VT characteristic when (DELTA) nd is set high with the liquid crystal display element of VA mode.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,11 Liquid crystal display element 2,12 Liquid crystal cell 3,4 Polarizing plate 5,14 Phase difference compensation layer 6,15 Reflector 13 Circular polarizing plate

Claims (5)

A liquid crystal cell configured by sandwiching a liquid crystal layer having negative dielectric anisotropy between a pair of substrates each having an electrode layer formed on each of the opposing surfaces and being transparent, and sandwiching the liquid crystal cell A pair of polarizing plates, and a reflecting plate disposed on a polarizing plate on a side opposite to the viewer of the pair of polarizing plates, opposite to a surface facing the substrate in the pair of polarizing plates. In a liquid crystal display element that displays an image on a display unit by applying a voltage to the liquid crystal layer through an electrode layer of a substrate of the liquid crystal cell,
For at least one of the pair of substrates, a vertical alignment film is provided on a surface facing the other substrate, and the alignment film is subjected to an alignment treatment so as to define an alignment direction of the liquid crystal layer when a voltage is applied. Has been given,
The pair of polarizing plates are arranged such that the crossing angle between the absorption axis of one polarizing plate and the absorption axis of the other polarizing plate is 90 ° ± 5 °,
A retardation compensation layer having retardation is provided between at least one of the pair of polarizing plates and the liquid crystal cell, and the fast axis of the retardation compensation layer and the alignment direction of the liquid crystal layer when a voltage is applied Is set to be smaller than an angle formed between the absorption axis of the polarizing plate and the alignment direction of the liquid crystal layer when a voltage is applied, and more than 0 degree and less than 50 degrees.
A liquid crystal display element, wherein an aperture ratio of the display portion is 70% or less. A liquid crystal cell formed by sandwiching a liquid crystal layer having negative dielectric anisotropy between a pair of substrates each having an electrode layer formed on each of the opposing surfaces and at least one of which is transparent; A circularly polarizing plate disposed on the viewer side of the pair of substrates opposite to the surface facing the liquid crystal layer, and a reflection disposed on the counter-viewer side substrate of the pair of substrates. In a liquid crystal display element that displays an image on a display unit by applying a voltage to the liquid crystal layer through an electrode layer of a substrate of the liquid crystal cell.
For at least one of the pair of substrates, a vertical alignment film is provided on a surface facing the other substrate, and the alignment film is subjected to an alignment treatment so as to define an alignment direction of the liquid crystal layer when a voltage is applied. Has been given,
A retardation compensation layer having retardation is provided between the circularly polarizing plate and the liquid crystal cell, and an angle formed between the fast axis of the retardation compensation layer and the alignment direction of the liquid crystal layer when a voltage is applied is , Is set to be smaller than an angle formed between the absorption axis of the polarizing plate and the alignment direction of the liquid crystal layer when a voltage is applied, and more than 0 degree and less than 50 degrees,
A liquid crystal display element, wherein an aperture ratio of the display portion is 70% or less.   2. The liquid crystal display element according to claim 1, wherein a retardation value of the retardation compensation layer is greater than 0% and 10% or less of a retardation value of the liquid crystal layer.   The liquid crystal display element according to claim 2, wherein the retardation value of the retardation compensation layer is greater than 0% and 1.5% or less of the retardation value of the liquid crystal layer.   The liquid crystal display element according to claim 1, wherein the liquid crystal display element is a passive matrix liquid crystal display element.

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