JP2005321459A - Color liquid crystal display element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem wherein a conventional ECB liquid crystal element has low light utilizing efficiency and inferior contrast. <P>SOLUTION: A liquid crystal display element has: a first substrate transmitting light; a second substrate disposed to be parallel to the first substrate and reflecting light in the direction to the first substrate; a nematic liquid crystal interposed between the first and the second substrates and having negative dielectric anisotropy; electrodes applying voltage to the nematic liquid crystal; and a retardation plate and a polarizing plate disposed on the outer side of the first substrate. The nematic liquid crystal is aligned nearly vertically to the substrates when no voltage is applied, and aligned obliquely to the substrates when voltage is applied and exhibits hue change by double refraction of transmission light associated with change of an inclined angle. The element is characterized in that the retardation value of the retardation plate is set to be in the range of 1/2 of the wavelength of visible light, the transmission axis of the polarizing plate and the delay phase axis of the retardation plate form an angle θ and the transmission axis of the polarizing plate and the inclination azimuth of the liquid crystal form an angle 2θ+45°. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a liquid crystal display element, and more particularly to a liquid crystal display element capable of color display using birefringence.

  Conventionally, liquid crystal display elements have been widely used in portable information terminals and the like, and most of them are liquid crystal elements having both a reflection function and a transmission function, which are referred to as a reflection type or a transflective type. The reason for this is that it is difficult to ensure outdoor visibility with a transmissive liquid crystal or a self-luminous display element. Therefore, a liquid crystal display element that can use ambient ambient light is advantageous and widely used as a display element used outdoors. Several display principles have been proposed as display elements having a reflective function, but at present, almost all liquid crystal elements that are in practical use are liquid crystal elements using polarizing plates. Full color display can be obtained by continuously modulating the brightness of white, halftone, and black with this polarizing plate type liquid crystal element, and disposing a micro color filter composed of red, green, and blue.

  In the above-described polarizing plate type liquid crystal display element having a reflecting function, a polarizing plate called a broadband circular polarizing plate is often used. A typical configuration of the broadband circularly polarizing plate is that three layers of (1) a linearly polarizing plate, (2) a λ / 2 phase difference plate, and (3) a λ / 4 phase difference plate are laminated. If the transmission axis of the polarizing plate is 0 degree at this time, a uniaxial retardation plate having a phase difference of λ / 2 is disposed on the back side of the polarizing plate, and the optical axis direction θ1 of the λ / 2 plate is Further, a uniaxial retardation film having a phase difference of λ / 4 is disposed on the back side thereof, and the optical axis direction θ2 of λ / 4 is set to 75 degrees. Is. Note that the wavelength λ of light here is mainly around 550 nm where human visibility is high, but it is a representative wavelength of each color in color display such as using a color filter.

  Here, even if θ1 and θ2 are not necessarily set to 15 degrees and 75 degrees as described above, almost desired characteristics can be obtained if they are set so as to satisfy the relationship θ2 = θ1 + 45. That is, when the amount of retardation of the liquid crystal layer disposed on the back surface of the broadband circularly polarizing plate is 0 and a reflective plate is provided on the back surface, visible light is generated by the polarization rotation effect of the broadband circularly polarizing plate. The reflected light can be brought close to zero in almost the entire area. This makes it possible to obtain a reflective liquid crystal element having a high contrast.

  However, since this broadband circularly polarizing plate has a structure in which three films are laminated, in principle, the overall thickness increases and the film weight also increases. Further improvement is required.

  On the other hand, since it passes through three films optically, the light use efficiency at the time of white display is reduced due to the influence of light loss at each film interface.

  For such a problem, Patent Document 1 has a configuration in which a polarizing plate and a retardation plate having a phase difference of λ / 2 are arranged on a liquid crystal panel having a homogeneous alignment or a vertical alignment having a phase difference of λ / 4. It is disclosed. In this configuration, since only two films, a polarizing plate and a λ / 2 plate, are used, it is considered that this is advantageous both physically and optically.

In this configuration, for example, when vertical alignment is used, the display state is white display because the amount of retardation of the liquid crystal layer is zero when no voltage is applied. On the other hand, when a voltage is applied to control the liquid crystal layer to homogeneous alignment, the retardation amount of the liquid crystal layer is λ / 4, and the display state is black display. Of course, the relationship between the optical axis direction θ1 of the λ / 2 plate at this time and the liquid crystal molecule tilt direction θ2 of the liquid crystal layer is set to be θ2 = θ1 + 45. Thus, continuous tone display in a monochrome area can be realized, and full color display can be performed by combining this with a color filter.
JP 2001-66598 A

  Although there are optical advantages as described above, the optical superiority cannot be fully utilized due to light loss due to the use of red, green, and blue color filters in color display. That is, a sufficiently bright color display cannot be performed.

  In general, in a liquid crystal display element, particularly in a liquid crystal display element using a vertical alignment state, either white or black is always a no-voltage application state. For example, switching from black to white is switching from a voltage application state to a voltage non-application state. In other words, the response uses the relaxation response of the liquid crystal, and in this case, the response speed is generally slow.

  Many of the drive systems that have been put to practical use as liquid crystal displays use overdrive drive as a method of accelerating the response speed by applying a voltage that is higher or lower than the desired drive voltage for one frame during halftone display. Although generally known and widely used, in switching using the relaxation response of the liquid crystal toward the state where no voltage is applied, since the target voltage is 0 V, a driving method effective for such moving image characteristics is adopted. Can not be done, the response speed will be slow.

The present invention includes a first substrate that transmits light, a second substrate that is disposed in parallel to the first substrate and reflects light in the direction of the first substrate, and between the first and second substrates. A nematic liquid crystal having a negative dielectric anisotropy sandwiched between, an electrode for applying a voltage to the nematic liquid crystal, a retardation plate and a polarizing plate disposed outside the first substrate,
The nematic liquid crystal is oriented substantially perpendicular to the substrate when no voltage is applied, and is tilted with respect to the substrate when a voltage is applied, and the birefringence of transmitted light with a change in the tilt angle A liquid crystal display element exhibiting a hue change due to
The retardation plate has a retardation value set in the range of ½ of the wavelength of light in the visible range, the transmission axis of the polarizing plate and the slow axis of the retardation plate form an angle θ, and the polarization plate The transmission axis of the plate and the tilt direction of the liquid crystal generally form an angle 2θ + 45 °.

  When the angle θ is in the range of more than 5 degrees and less than 25 degrees, even better reflective display characteristics can be obtained.

  The present invention is also advantageous for high-speed response because voltage can be applied to the liquid crystal in both the white display state and the black display state.

  In the above requirements of the present invention, if the angle is approximately before and after this, almost desired characteristics can be obtained. For example, the object of the present invention can be achieved within a range of about ± 3 ° with respect to 2θ + 45 °. The above “approximate angle” refers to this range.

  Next, the best mode of the present invention is shown in FIG.

  The polarizing plate 51, the λ / 2 plate 52, the glass 53, the ITO 54, the alignment film 55, the liquid crystal layer 56, the alignment film 57, the reflecting plate 58, and the glass 59 are arranged in this order.

  In the practice of the present invention, it is preferable to provide a reflective electrode 58 made of a member having reflective characteristics such as an aluminum electrode on the substrate 59 in order to prevent parallax at the time of perspective, and to integrate the liquid crystal panel and the reflective plate. In addition, when used as a direct-view display, a diffuse reflection electrode having an uneven shape may be used as the electrode 58. A forward scattering film may be installed at any position from the transparent substrate 53 to the polarizing plate 51 (not shown). Further, a TFT circuit for applying a voltage to the liquid crystal layer 56 may be provided on the substrate 59 as shown in FIG.

  In the case of a color display element, a color filter may be provided on the substrate 53. About the combination of color filters, you may make sub-pixels with red, green, and blue color filters, respectively, and display them by additive color mixture, but for a brighter color display, Sub-pixels with green and magenta filters are created, and pixels with green filters are displayed for green display, and hue changes using the birefringence effect with pixels with magenta filters for red or blue display You may perform the color display based on. In addition, a combination of red and cyan, a combination of blue and yellow, and the like can be used. Alternatively, only one of the red, green, and blue color filters may be used, and the other two colors may be displayed as the three primary colors by using interference colors.

  Of course, color display using only interference colors may be performed.

  By doing so, it is possible to significantly increase the light utilization efficiency as compared with the conventional color display method using three color filters of red, green, and blue.

  When performing black display, the voltage value is adjusted so that the retardation amount of the liquid crystal layer is λ / 4. In the case of white display, the voltage may be turned off so that the retardation amount of the liquid crystal layer becomes zero, but for speeding up, the voltage is applied so that the retardation amount of the liquid crystal layer becomes λ / 2. White display is preferable.

  When performing white display or black display at this time, overdrive driving to apply a larger (or smaller) voltage in the first frame immediately after display switching from the previous state is a point of response speed. And more preferable.

  Hereinafter, a description will be given based on examples. The configuration described below is used as a common configuration of the present embodiment.

  A reflective electrode made of a 150 nm Al film is formed on a 0.7 mm thick glass substrate to produce a reflective electrode substrate A. A silicon substrate, a plastic substrate, or the like may be used instead of the glass substrate. Next, a transparent electrode made of an ITO film having a thickness of 150 nm is formed on a glass substrate having a thickness of 0.7 mm, and a substrate B is created. A transparent plastic substrate or the like may be used instead of the glass substrate.

  Next, a polyimide alignment film JALS2021 manufactured by JSR Co. was applied to the substrates A and B by spin coating, followed by pre-drying at 80 ° C. for 5 minutes, followed by heating and baking at 200 ° C. for 1 hour, and a film thickness of 50 nm. The polyimide film is formed.

  Subsequently, the polyimide films on the substrates A and B are rubbed with a nylon cloth as a uniaxial orientation process. The rubbing treatment was performed using a rubbing roll in which nylon (NF-77 / manufactured by Teijin Ltd.) was bonded to a roll having a diameter of 10 cm. .

  Subsequently, silica beads having an average particle diameter of 6.5 μm were dispersed as spacers on one substrate, and the substrates were opposed so that the rubbing directions were antiparallel to each other, and a commercially available liquid crystal MLC6608 (manufactured by Chisso Corporation) was used. Injection is performed to obtain a liquid crystal cell.

  A λ / 2 retardation film is pasted on the substrate B of the liquid crystal cell thus obtained. Further, a reflection type liquid crystal display element is obtained by attaching a polarizing plate on the λ / 2 retardation plate. The physical arrangement of the λ / 2 retardation plate at this time and the angle formed by the transmission axis of the polarizing plate and the tilt direction of the liquid crystal molecules are changed according to the embodiment.

[Example 1]
The black reflectance was calculated for the relationship between the λ / 2 plate and the tilt angle of the liquid crystal molecules. The results are shown in FIG.

  FIG. 3A shows that when the voltage of 2.7 V is applied to the liquid crystal layer, the liquid crystal molecule tilt azimuth forms an angle of 75 degrees with respect to the transmission axis of the polarizing plate. The reflectance is shown when the azimuth angle of the optical axis is changed. In this graph, the horizontal axis represents the angle between the optical axis of the λ / 2 plate and the transmission axis of the polarizing plate, and the vertical axis represents the reflectance. As can be seen from this result, the darkest reflectance is shown near 15 degrees.

  Similarly, FIG. 3B shows that when the voltage of 2.7 V is applied to the liquid crystal layer, the liquid crystal molecule tilt azimuth forms an angle of 90 degrees with respect to the transmission axis of the polarizing plate. The reflectance when the azimuth angle of the optical axis of the plate is changed is shown. In this graph, the horizontal axis represents the angle between the optical axis of the λ / 2 plate and the transmission axis of the polarizing plate, and the vertical axis represents the reflectance. As can be seen from this result, the darkest reflectance is shown in the vicinity of 22.5 degrees.

  As for the other angles, the same consideration is made, and if the transmission axis of the polarizing plate and the slow axis of the retardation plate are an angle θ, the transmission axis of the polarizing plate and the tilt azimuth direction of the liquid crystal molecules when a voltage is applied are formed. The darkest display can be obtained when the angle is 2θ + 45 °. In other words, high contrast can be obtained under these conditions.

  3 (a) and 3 (b), even if this angle is not strictly the above relationship, the contrast is within the allowable range if it is within a range of ± several degrees (for example, about ± 3 °). I understand that.

[Example 2]
Based on the above results, when the transmission axis of the polarizing plate and the slow axis of the retardation plate are an angle θ, the angle formed by the transmission axis of the polarizing plate and the tilt azimuth direction of the liquid crystal molecules when a voltage is applied is 2θ + 45. The reflectance was calculated for each of the display states of white display and black display by setting °. The results are shown in FIG. In FIG. 4, the vertical axis represents the reflectance, the horizontal axis represents the wavelength (unit: nm), the solid line represents white, and the wavy line represents black.

  In the white display state, 3.4 V was applied to the liquid crystal layer, and in the black display state, 2.7 V was applied to the liquid crystal layer.

  4A to 4G show the results when θ = 0, 5, 10, 15, 20, 25, 30, and 35 degrees, respectively.

  From this result, it can be seen that when θ is 0 degree and 5 degrees, the spectral reflectance characteristics in the white display state are mountain-shaped, colored green, and the average light utilization efficiency is low.

  Further, in the black display state, when θ = 25 degrees, 30 degrees, and 35 degrees, the reflectance on the short wavelength side exceeds 10%, and the display becomes bluish instead of black.

  Therefore, it can be seen from the viewpoint of black and white that θ is preferably in the range of more than 5 degrees and less than 25 degrees.

[Example 3]
Using the same element as described above, the interference color effect due to birefringence was calculated. The results are shown in FIG.

  The graph of FIG. 5 shows the locus on the xy chromaticity diagram when the voltage applied to the liquid crystal layer is changed in the range of 0V to 6V. 5A to 5H show the results when θ = 0, 5, 10, 15, 20, 25, 30, and 35 degrees, respectively.

  From this result, when θ is 30 degrees, the value that the x coordinate can take on the xy chromaticity diagram is only 0.4 or less, indicating that red display with high color purity cannot be performed.

  In addition, when the angle was 25 degrees and 35 degrees, a value exceeding 0.4 was obtained, but when the reflected brightness at this time was examined, it was a small value, so that a bright red display was actually made. Absent. Therefore, it can be seen that θ is preferably less than 25 degrees from the viewpoint of color display using interference color due to the birefringence effect.

[Example 4]
Example 4 shows the results of studies on the effect of overdrive described in the specification. As in Example 2, a white display state is a state where 3.4 V is applied to the liquid crystal layer, and a black display state is a state where 2.7 V is applied to the liquid crystal layer.

  When transitioning from the white display state to the black display state, the response speed τ1 when the applied voltage is changed as it is from the 3.4 V application to the 2.7 V application is changed to 3.4 V application to 2.7 V application. Sometimes, after continuously applying 3.4 V, 0 V is applied for one frame (1/60 second), and then the response speed τ 2 when controlling the voltage applied to the device so that 2.7 V is applied, To do. When these τ1 and τ2 are compared, τ2 is faster.

  By the same experiment, it is possible to apply the overdrive driving method when transitioning from the black display state to the white display state.

  In other words, since the applicable range of overdrive can be used from white to black and from black to white, the speed of the device can be increased.

  The color liquid crystal display element of the present invention can also be applied to a transflective reflective portion.

The figure showing the typical structure of the conventional reflection type display element. The figure which shows the TFT circuit structure of the reflection type liquid crystal display element of this invention. The figure which shows the reflectance with respect to the azimuth angle of the optical axis of (lambda) / 2 board. The figure which shows the reflective characteristic in an Example. The figure which shows xy chromaticity diagram in an Example.

Explanation of symbols

51 Polarizing plate 52 λ / 2 retardation plate 53 Transparent substrate 54 Transparent electrode 55 Alignment film 56 Liquid crystal layer 57 Alignment film 58 Reflective electrode 59 Substrate 61 Source line 62 Gate line 63 TFT
64 Reflective electrode

Claims (3)

A first substrate that transmits light, a second substrate that is disposed in parallel to the first substrate and reflects light in the direction of the first substrate, and is sandwiched between the first and second substrates A nematic liquid crystal having a negative dielectric anisotropy, an electrode for applying a voltage to the nematic liquid crystal, a retardation plate and a polarizing plate arranged outside the first substrate,
The nematic liquid crystal is oriented substantially perpendicular to the substrate when no voltage is applied, and is tilted with respect to the substrate when a voltage is applied, and the birefringence of transmitted light with a change in the tilt angle A liquid crystal display element exhibiting a hue change due to
The retardation plate has a retardation value set in the range of ½ of the wavelength of light in the visible range, the transmission axis of the polarizing plate and the slow axis of the retardation plate form an angle θ, and the polarization plate A liquid crystal display element, wherein the transmission axis of the plate and the tilt direction of the liquid crystal form an angle of approximately 2θ + 45 °.   The liquid crystal display element according to claim 1, wherein the angle θ is in the range of more than 5 degrees and less than 25 degrees.   The liquid crystal display element according to claim 1, wherein a voltage is applied to the nematic liquid crystal in both the white display state and the black display state.

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