JP5072480B2 - Liquid crystal display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal display device, achieving favorable display quality. <P>SOLUTION: A first optical element OD1 provided on one outer surface of a liquid crystal display panel LPN containing homogeneous aligned liquid crystal molecules includes: a first sheet polarizer 51; a first retardation film RF1 in which nematic liquid crystal molecules are hybrid-aligned along the normal direction in the liquid crystal state and solidified; and a second retardation film RF2 having biaxial refractive index anisotropy. A second optical element OD2 provided on the other outer surface of the liquid crystal display panel includes: a second sheet polarizer 52; and a third retardation film RF3 having a uniaxial refractive index anisotropy. The polarized state of light transmitted through the first optical element OD1 and the liquid crystal display panel LPN is linearly polarized light having the substantially same ellipticity as the light transmitted through the second optical element OD2. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to a liquid crystal display device, and more particularly to a transmissive liquid crystal display device including a liquid crystal layer containing liquid crystal molecules that are homogeneously aligned.

  Similar to a twisted nematic (TN) mode liquid crystal display device, a viewing angle compensation phase difference film is applied to a vertically aligned (VA) mode liquid crystal display device having excellent display characteristics when viewed from the front. Thus, a technique for realizing a wide viewing angle has been proposed (see, for example, Patent Document 1).

In addition, a technique for manufacturing a biaxial birefringent film that can be applied to a liquid crystal display device such as a super twisted nematic (STN) mode has been proposed (see, for example, Patent Document 2).
JP 2005-099236 A JP-A-2005-181451

  In recent years, liquid crystal display devices configured by holding a liquid crystal layer containing liquid crystal molecules that are homogeneously aligned between a pair of substrates have been required to improve display quality such as further improvement of contrast and expansion of viewing angle. On the other hand, it is required to reduce the thickness and cost of the entire apparatus.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a liquid crystal display device which can be reduced in thickness and cost and has a good display quality.

A liquid crystal display device according to an aspect of the present invention includes:
A liquid crystal display panel holding a liquid crystal layer containing homogeneously aligned liquid crystal molecules between a first substrate and a second substrate disposed to face each other;
The first polarizing plate is provided on one outer surface of the liquid crystal display panel, is disposed between the first polarizing plate and the liquid crystal display panel, and transmits light having a predetermined wavelength that passes through the fast axis and the slow axis. A first retardation plate in which nematic liquid crystal molecules are fixed in a hybrid alignment along a normal direction and a predetermined retardation is provided between the first polarizing plate and the first retardation plate A second optical retardation plate having a negative biaxial refractive index anisotropy, and a first optical element,
Light of a predetermined wavelength provided on the other outer surface of the liquid crystal display panel, disposed between the second polarizing plate, the second retardation plate and the liquid crystal display panel, and transmitted through the fast axis and the slow axis. A second optical element having a third retardation plate that gives a predetermined phase difference between them and has uniaxial refractive index anisotropy,
The polarization state of the light transmitted through the first optical element and the liquid crystal display panel is linearly polarized light having substantially the same ellipticity as the light transmitted through the second optical element.

  According to the present invention, it is possible to provide a liquid crystal display device which can be reduced in thickness and cost and has a good display quality.

  A liquid crystal display device according to an embodiment of the present invention will be described below with reference to the drawings. Here, a transmissive liquid crystal display device that displays an image using backlight light (selectively transmits) will be described as an example.

  As shown in FIGS. 1 and 2, the liquid crystal display device is an active matrix type color liquid crystal display device and includes a transmissive liquid crystal display panel LPN. The liquid crystal display panel LPN includes an array substrate (first substrate) AR, a counter substrate (second substrate) CT disposed opposite to the array substrate AR, and a space between the array substrate AR and the counter substrate CT. And a liquid crystal layer LQ held in the liquid crystal layer LQ.

  The liquid crystal display device includes a first optical element OD1 provided on one outer surface of the liquid crystal display panel LPN (that is, the outer surface opposite to the surface facing the liquid crystal layer LQ of the array substrate AR), and a liquid crystal display. A second optical element OD2 provided on the other outer surface of the panel LPN (that is, the outer surface opposite to the surface facing the liquid crystal layer LQ of the counter substrate CT) is provided. Further, the transmissive liquid crystal display device includes a backlight unit BL that illuminates the liquid crystal display panel LPN from the first optical element OD1 side.

  Such a liquid crystal display panel LPN includes a display area DSP for displaying an image. The display area DSP is composed of a plurality of pixels PX arranged in an mxn matrix.

  The array substrate AR is formed using an insulating substrate 10 having optical transparency such as a glass plate or a quartz plate. In other words, the array substrate AR includes m × n pixel electrodes EP arranged for each pixel in the display area DSP, and n scanning lines Y () formed along the row direction of these pixel electrodes EP. Y1 to Yn), m signal lines X (X1 to Xm) formed along the column direction of these pixel electrodes EP, and regions including intersections of the scanning lines Y and the signal lines X in each pixel PX M × n switching elements W, etc.

  The array substrate AR is further connected to at least a part of the scanning line driver YD connected to the n scanning lines Y and the m signal lines X in the driving circuit area DCT around the display area DSP. At least a part of the signal line driver XD. The scanning line driver YD sequentially supplies scanning signals (driving signals) to the n scanning lines Y based on control by the controller CNT. Further, the signal line driver XD supplies video signals (drive signals) to the m signal lines X at the timing when the switching elements W in each row are turned on by the scanning signal based on the control by the controller CNT. Thereby, the pixel electrode EP of each row is set to a pixel potential corresponding to the video signal supplied via the corresponding switching element W.

  Each switching element W is, for example, an n-channel thin film transistor, and includes a semiconductor layer 12 disposed on the insulating substrate 10. The semiconductor layer 12 can be formed of, for example, polysilicon or amorphous silicon, and is formed of polysilicon here. The semiconductor layer 12 has a source region 12S and a drain region 12D on both sides of the channel region 12C. The semiconductor layer 12 is covered with a gate insulating film 14.

  The gate electrode WG of the switching element W is connected to the scanning line Y (or formed integrally with the scanning line Y). Both the gate electrode WG and the scanning line Y are disposed on the gate insulating film 14. These gate electrodes WG and scanning lines Y are covered with an interlayer insulating film 16.

  The source electrode WS and the drain electrode WD of the switching element W are disposed on both sides of the gate electrode WG on the interlayer insulating film 16. The source electrode WS is connected to the signal line X (or formed integrally with the signal line X) and is in contact with the source region 12S of the semiconductor layer 12. The drain electrode WD is connected to the pixel electrode EP (or formed integrally with the pixel electrode EP) and is in contact with the drain region 12D of the semiconductor layer 12. These source electrode WS, drain electrode WD, and signal line X are covered with an organic insulating film 18.

  The pixel electrode EP is disposed on the organic insulating film 18 and is electrically connected to the drain electrode WD through a contact hole formed in the organic insulating film 18. The pixel electrode EP is formed of a light-transmitting conductive material such as indium tin oxide (ITO). The pixel electrodes EP corresponding to all the pixels PX are covered with the alignment film 20.

  On the other hand, the counter substrate CT is formed using an insulating substrate 30 having optical transparency such as a glass plate or a quartz plate. That is, the counter substrate CT includes a counter electrode ET and the like in the display area DSP. The counter electrode ET is disposed so as to face the pixel electrode EP corresponding to the plurality of pixels PX. The counter electrode ET is made of a light-transmitting conductive material such as indium tin oxide (ITO). The counter electrode ET is covered with an alignment film 36.

  The color display type liquid crystal display device includes a color filter layer 34 provided on the inner surface of the liquid crystal display panel LPN corresponding to each pixel. In the example shown in FIG. 2, the color filter layer 34 is provided on the counter substrate CT. The color filter layer 34 is formed of a plurality of different colors, for example, colored resins that are colored in three primary colors such as red, blue, and green. The red colored resin, the blue colored resin, and the green colored resin are disposed corresponding to the red pixel, the blue pixel, and the green pixel, respectively. Such a color filter layer 34 may be arranged on the array substrate AR side.

  Each pixel PX is partitioned by a black matrix (not shown). This black matrix is arranged so as to face wiring portions such as the scanning lines Y, the signal lines X, and the switching elements W provided on the array substrate AR.

  When the counter substrate CT and the array substrate AR as described above are arranged so that the alignment film 20 and the alignment film 36 face each other, a spacer (for example, a resin material) arranged between them is arranged. A predetermined gap is formed by the formed columnar spacer. The liquid crystal layer LQ is composed of a liquid crystal composition including liquid crystal molecules 40 enclosed in a gap formed between the alignment film 20 of the array substrate AR and the alignment film 36 of the counter substrate CT. In this embodiment, the liquid crystal layer LQ includes liquid crystal molecules 40 having a twist angle of 0 deg (homogeneous alignment).

  In the liquid crystal display device according to this embodiment, as shown in FIG. 3, the first optical element OD1 and the second optical element OD2 control the polarization state of the light that has passed through them. That is, the first optical element OD1 controls the polarization state of light passing through it so that light having elliptical polarization or substantially linear polarization is incident. Therefore, the polarization state of the backlight light incident on the first optical element OD1 is converted into a predetermined polarization state when passing through the first optical element OD1. Thereafter, the backlight light emitted from the first optical element OD1 is incident on the liquid crystal layer LQ while maintaining a predetermined polarization state. The emitted light emitted from the liquid crystal display panel LPN has a linearly polarized state.

  Further, the second optical element OD2 controls the polarization state of light passing through the liquid crystal layer LQ so that light having a polarization state of linearly polarized light (or elliptically polarized light that is almost as linearly polarized as possible) enters the liquid crystal layer LQ. Accordingly, the polarization state of the light incident on the second optical element OD2 is converted into a predetermined state, that is, linearly polarized light when passing through the second optical element OD2.

  In other words, the polarization state of the light transmitted through the first optical element OD1 and the liquid crystal display panel LPN is substantially the same ellipticity as the light transmitted through the second optical element OD2 (= minor axis direction amplitude Es / major axis direction amplitude). It is a linearly polarized light having Ep). With such a configuration, the contrast in the normal direction of the liquid crystal display panel LPN can be improved and the viewing angle can be increased.

  Below, each structure is demonstrated more concretely.

  The first optical element OD1 includes one first polarizing plate 51, and a first retardation plate RF1 and a second retardation plate RF2 disposed between the first polarizing plate 51 and the liquid crystal display panel LPN. Configured. In the example shown in FIG. 3, the first retardation plate RF1 is disposed between the first polarizing plate 51 and the liquid crystal display panel LPN (array substrate AR). The second retardation plate RF2 is disposed between the first polarizing plate 51 and the first retardation plate RF1.

  The second optical element OD2 includes one second polarizing plate 52, and a third retardation plate RF3 disposed between the second polarizing plate 52 and the liquid crystal display panel LPN.

  The first polarizing plate 51 and the second polarizing plate 52 applied here have an absorption axis and a transmission axis orthogonal to each other in a plane orthogonal to the light traveling direction. Such a polarizing plate extracts light having a vibrating surface in one direction parallel to the transmission axis, that is, light having a linearly polarized light state, from light having a vibrating surface in a random direction.

  The first retardation plate RF1 applied here is an optically anisotropic retardation plate, and nematic liquid crystal molecules 61 having optically positive uniaxial refractive index anisotropy are converted into a liquid crystal state. The liquid crystal film layer 60 is fixed in a state of being hybrid-aligned along the normal direction (that is, the thickness direction of the retardation plate).

  In such a liquid crystal film layer 60, for example, in the vicinity of the interface on the array substrate AR side, the liquid crystal molecules 61A are aligned so as to form a relatively small tilt angle with respect to the interface (that is, the liquid crystal molecules 61A are substantially aligned with the interface). On the other hand, in the vicinity of the sea surface on the second retardation plate RF2 side, the liquid crystal molecules 61B are aligned so as to form a relatively large tilt angle with respect to the interface (that is, the liquid crystal molecules 61B). Is oriented almost perpendicular to the interface). As such a first retardation plate RF1, an NH film (manufactured by Nippon Oil Corporation) can be applied. Such a liquid crystal film has a function of optically compensating for the retardation of the liquid crystal layer LQ that changes depending on the viewing angle due to the orientation of the liquid crystal molecules 40 included in the liquid crystal layer LQ, and has a function of expanding the viewing angle. This corresponds to the retardation plate.

  In discussing birefringence in the liquid crystal layer LQ in which the alignment of the liquid crystal molecules 40 having refractive index anisotropy changes according to the applied voltage and the retardation plate having refractive index anisotropy, the refractive index is relatively The large axis corresponds to the slow axis, and the relatively small refractive index axis corresponds to the fast axis. The slow axis coincides with the vibration plane of the extraordinary ray. The fast axis coincides with the plane of vibration of ordinary rays. When the refractive indices of ordinary rays and extraordinary rays passing through the liquid crystal layer LQ are no and ne, respectively, and the thickness of the liquid crystal layer LQ along the traveling direction of each ray is d, the retardation value of the liquid crystal layer LQ ( The retardation value is defined by Δn · d (nm) = (ne · d−no · d) (that is, Δn = ne−no). For the phase difference plate, main refractive indexes corresponding to three axes orthogonal to each other are applied, and main refractive indexes corresponding to the axes orthogonal to each other in the plane of the phase difference plate are set to nx and ny, respectively. When the main refractive index corresponding to the axis in the normal direction (that is, the thickness direction of the retardation plate) is nz, and the thickness of the retardation plate is d, the front retardation value (front retardation value) of the retardation plate is , R = (nx−ny) · d.

  The first retardation plate RF1 and the second retardation plate RF2 included in the first optical element OD1 and the third retardation plate RF3 included in the second optical element OD2 have a slow axis and a fast phase that are orthogonal to each other. Has an axis.

  That is, the first retardation plate RF1 has, in addition to the above-described viewing angle widening function, the orientation direction (director) of the liquid crystal molecules 61 as a slow axis and the direction orthogonal thereto as a fast axis. It has a function as a phase difference plate that gives a predetermined phase difference (λ is a wavelength, and m is a positive number) between light having a predetermined wavelength (for example, 550 nm) to be transmitted. Yes.

  The second retardation plate RF2 has negative biaxial refractive index anisotropy. The second retardation plate RF2 has a fast axis and a slow axis in its plane, and has a predetermined position between light of a predetermined wavelength (for example, 550 nm) that passes through the fast axis and the slow axis. It has a function as a phase difference plate that gives a phase difference (λ / n phase difference where λ is a wavelength and n is a positive number).

  The third retardation plate RF3 has a uniaxial refractive index anisotropy. The third retardation plate RF3 has a refractive index anisotropy (nx> ny = nz) corresponding to a positive A plate. That is, the third retardation plate RF3 has an Nz coefficient of 1.0 given by Nz = (nx−nz) / (nx−ny). The third retardation plate RF3 has a fast axis and a slow axis in the plane thereof, and has a predetermined position between light of a predetermined wavelength (for example, 550 nm) that transmits the fast axis and the slow axis. It has a function as a phase difference plate that gives a phase difference (λ / n phase difference).

  As such second retardation plate RF2 and third retardation plate RF3, ZEONOR (manufactured by Optes Co., Ltd.), Arton (manufactured by JSR), or the like can be applied.

  In the first optical element OD1, the absorption axis A1 of the first polarizing plate 51, the slow axis D1 in the plane of the first retardation plate RF1, and the slow axis D2 in the plane of the second retardation plate RF2 are set. Each component is arranged so as to have a predetermined angle relationship. That is, the second retardation plate RF2 is disposed on the first polarizing plate 51 so that the slow axis D2 thereof is approximately 45 ° with respect to the absorption axis A1 of the first polarizing plate 51. The first phase difference plate RF1 is disposed on the second phase difference plate RF2 such that the slow axis D1 is substantially 90 ° with respect to the slow axis D2 of the second phase difference plate RF2. When the first optical element OD1 is disposed on the liquid crystal display panel LPN, the first optical element OD1 is configured such that the slow axis D1 of the first retardation plate RF1 having a viewing angle widening function is the liquid crystal molecule 40 of the liquid crystal layer LQ. The direction (the rubbing direction of the alignment film 20 on the array substrate AR side) is substantially parallel to the director, and the direction in which the liquid crystal molecules 61 are hybrid aligned in the first retardation plate RF1 and the rubbing direction of the alignment film 20 on the array substrate AR side are Are arranged in the opposite direction.

  In the second optical element OD2, the third retardation plate RF3 includes a second polarizing plate such that the in-plane slow axis D3 forms approximately 90 ° with respect to the absorption axis A2 of the second polarizing plate 52. 52. When the second optical element OD2 is arranged on the liquid crystal display panel LPN, the second optical element OD2 is a director (array substrate AR) of the liquid crystal molecules 40 of the liquid crystal layer LQ whose slow axis D3 of the third phase difference plate RF3 is set. And the absorption axis A2 of the second polarizing plate 52 and the absorption axis A1 of the first polarizing plate 51 are perpendicular to each other.

  With such a configuration, the first optical element OD1 has a function of converting into elliptically polarized light or a substantially linearly polarized light having a predetermined ellipticity and a function of expanding a viewing angle. The second optical element OD2 has a function of converting into substantially linearly polarized light having an ellipticity substantially equal to the ellipticity of the light transmitted through the first optical element OD1 and the liquid crystal display panel LPN.

  Next, a method for obtaining better optical characteristics, particularly optical compensation at the time of black display will be described. First, the front phase difference value R (RF1) of the first phase difference plate RF1 and the front phase difference value R (RF2) of the second phase difference plate RF2 in the first optical element OD1, and the residual position of the liquid crystal layer LQ in black display. Consider the relationship with the phase difference value R (LQ).

  Here, the residual retardation value R (LQ) of the liquid crystal layer LQ will be described. When a voltage for black display (black display voltage) is applied to the liquid crystal layer LQ, the liquid crystal molecules 40 located at the center (midplane) away from the substrate interface in the cross section of the liquid crystal layer LQ are in the direction of the electric field. They are arranged so that their major axis directions are substantially parallel. For this reason, the front phase difference value of the midplane of the liquid crystal layer LQ can be regarded as substantially zero (nm). However, the liquid crystal molecules 40 that are aligned adjacent to the substrate interface are affected by the alignment regulating force (anchoring) of the interface, have a slow response to voltage, and maintain an initial alignment state. For this reason, the front phase difference value in the vicinity of the substrate interface of the liquid crystal layer LQ is not zero (nm). Therefore, even when a sufficiently high black display voltage for black display is applied to the liquid crystal layer LQ, the front phase difference remains in the liquid crystal layer LQ due to the influence of anchoring at the substrate interface. This is generally called residual phase difference.

  In the present embodiment, (1) the applied liquid crystal layer LQ, the first retardation plate RF1, and the second retardation plate RF2 all have a positive retardation, (2) The director of the liquid crystal molecules 40 in the liquid crystal layer LQ and the slow axis D1 of the first retardation plate RF1 are set substantially parallel to each other. (3) The slow axis D1 and the second position of the first retardation plate RF1. Since the slow axis D2 of the phase difference plate RF2 is set at an angle of about 90 degrees, the total phase difference value combining the phase difference value in the first optical element OD1 and the residual phase difference value in the liquid crystal layer LQ. R (total) can be expressed by the following equation.

R (total) = R (LQ) + R (RF1) −R (RF2)
Here, the front phase difference value of the first phase difference plate RF1 is R (RF1), the front phase difference value of the second phase difference plate RF2 is R (RF2), and the residual phase difference value of the liquid crystal layer LQ is R (RF). LQ).

  In this equation, R (total) becomes zero, that is, by setting the respective phase difference values so that R (LQ) + R (RF1) = R (RF2), the first optical element OD1 and Optical compensation can be realized with the liquid crystal layer LQ. That is, in this embodiment, the first optical element OD1 alone does not convert the polarization state of the backlight light into substantially linearly polarized light, but emits from the liquid crystal display panel LPN in consideration of the residual retardation value of the liquid crystal layer. The polarization state of the emitted light is converted into substantially linearly polarized light (ellipticity <0.1).

  That is, the sum of the residual retardation value R (LQ) of the liquid crystal layer LQ and the front retardation value R (RF1) of the first retardation plate RF1 is the front retardation value R (RF2) of the second retardation plate RF2. By setting to be substantially equal, after the backlight light from the backlight unit BL passes through the first optical element OD1 and the liquid crystal layer LQ during black display, the polarization state (ellipticity < 0.1). Therefore, not only in the case of white display but also in the case of black display, the polarization state of the light that has passed through the first optical element OD1 and the liquid crystal layer LQ is brought close to linearly polarized light having an ellipticity very close to zero. Can do.

  Subsequently, optimization of the second optical element OD2 will be examined.

  That is, the polarizing plate applied in this embodiment has viewing angle characteristics as shown in FIG. That is, isotropic characteristics cannot be obtained for all directions, and the contrast rapidly decreases as the viewing angle increases in the 45 ° -225 ° azimuth and the 135 ° -315 ° azimuth. The third retardation plate RF3 has a function of improving the viewing angle characteristics of the polarizing plate when a polarizing plate having such viewing angle characteristics is applied. Accordingly, the second optical element OD2 can convert light transmitted through the second optical element OD2 into light having a polarization state (ellipticity <0.1) that is almost linearly polarized light.

  When the combination of the first optical element OD1 and the liquid crystal display panel LPN as described above and the second optical element OD2 are applied, the following operation is performed. Here, the vibration plane of linearly polarized light that can be transmitted through the second optical element OD2 is defined as a direction parallel to the Y axis, and the direction orthogonal to the Y axis is defined as the X axis.

  That is, when no voltage is applied to the liquid crystal layer LQ (or when a low voltage is applied to the liquid crystal layer LQ), the polarization state of the light transmitted through the first optical element OD1 and the liquid crystal display panel LPN is a length parallel to the Y axis. It becomes elliptically polarized light having an axis or linearly polarized light parallel to the Y axis. For this reason, the light transmitted through the liquid crystal display panel LPN passes through the second optical element OD2. Thereby, white display is obtained.

  On the other hand, when a voltage is applied to the liquid crystal layer LQ (or when a high voltage is applied to the liquid crystal layer LQ), the polarization state of the light transmitted through the first optical element OD1 and the liquid crystal display panel LPN is a straight line parallel to the X axis. It becomes polarized light. At this time, the ellipticity of the light transmitted through the liquid crystal display panel LPN is substantially equal to the ellipticity of the light that can be transmitted through the second optical element OD2. That is, the plane of vibration of linearly polarized light (parallel to the X axis) transmitted through the liquid crystal display panel LPN is orthogonal to the plane of vibration of linearly polarized light (parallel to the Y axis) that can transmit through the second optical element OD2. For this reason, the light transmitted through the liquid crystal display panel LPN does not transmit through the second optical element OD2. Thereby, a black display with a sufficiently low transmittance can be obtained. Therefore, the contrast can be improved.

  Further, the combination of the first optical element OD1 and the liquid crystal display panel LPN is optically compensated by applying the first retardation plate RF1 and the second retardation plate RF2, and is substantially constant regardless of the viewing angle, particularly during black display. The linearly polarized light having an ellipticity of 5 is obtained. The second optical element OD2 is optically compensated by applying the third retardation plate RF3. In particular, when displaying black, a straight line obtained by combining the first optical element OD1 and the liquid crystal display panel LPN regardless of the viewing angle. It is configured to obtain linearly polarized light having an ellipticity substantially equal to that of polarized light. Therefore, the viewing angle of a high contrast region (particularly, a region having a contrast ratio of 10: 1 or more) can be expanded.

  Next, a specific example of the arrangement of the first optical element OD1 and the second optical element OD2 on the liquid crystal display panel LPN will be considered.

  Here, description will be made based on FIG. 5 in which the liquid crystal display device is observed from the counter substrate CT side. For convenience, an X axis and a Y axis orthogonal to each other are defined in a plane parallel to the main surface of the array substrate AR (or the counter substrate CT), and a normal direction of the plane is defined as a Z axis. In-plane corresponds to a plane defined by the X-axis and the Y-axis. Here, for example, the X axis corresponds to the horizontal direction of the screen, and the Y axis corresponds to the vertical direction of the screen. Further, the positive (+) direction (0 ° azimuth) of the X axis corresponds to the right side of the screen, and the negative (−) direction (180 ° azimuth) of the X axis corresponds to the left side of the screen. Furthermore, the positive (+) direction (90 ° azimuth) of the Y axis corresponds to the upper side of the screen, and the negative (−) direction (270 ° azimuth) of the Y axis corresponds to the lower side of the screen.

  In the liquid crystal display panel LPN, the rubbing direction of the alignment film 20 on the array substrate AR side is set at an angle of 45 ° with respect to the X axis.

  The arrangement of the first optical element OD1 on the liquid crystal display panel LPN is set based on the rubbing direction of the alignment film 20. That is, the first retardation plate RF1 is arranged so that its slow axis D1 is oriented in the direction of 45 ° -225 ° so that it is substantially parallel to the rubbing direction of the alignment film 20. At this time, the hybrid direction of the liquid crystal molecules included in the first retardation plate RF1 is oriented in the direction of 225 ° which is opposite to the rubbing direction of the alignment film 20. Further, the slow axis D2 of the second retardation plate RF2 is disposed so as to be substantially orthogonal to the slow axis D1 of the first retardation plate RF1 (that is, directed to a 135 ° azimuth). Further, the first polarizing plate 51 has, for example, 90 ° -270 ° so that the absorption axis A1 forms an angle of about 45 ° with respect to the slow axes of the first retardation plate RF1 and the second retardation plate RF2. It is arranged to face the direction of

  On the other hand, the arrangement of the second optical element OD2 on the liquid crystal display panel LPN is set based on, for example, the direction of linearly polarized light transmitted through the liquid crystal layer LQ during black display (in this case, the direction parallel to the X axis). The That is, in the second optical element OD2, the second polarizing plate 52 has its absorption axis A2 substantially parallel to the major axis direction of the linearly polarized light after passing through the first optical element OD1 and the liquid crystal layer LQ. Placed in. That is, the 2nd polarizing plate 52 is arrange | positioned so that the absorption axis A2 may face the direction of 0 degree-180 degree. The third retardation plate RF3 is arranged so that its slow axis D3 is oriented 90 ° so as to be substantially orthogonal to the absorption axis A2 of the second polarizing plate 52.

  Here, the optimum conditions in the liquid crystal display device having the above-described configuration will be described.

  First, the optimum range of the Nz coefficient is examined in the second retardation plate RF2 having negative biaxial refractive index anisotropy. When the viewing angle dependence of the contrast ratio was simulated for the liquid crystal display device under the same conditions except for the Nz coefficient of the second retardation plate RF2, the results as shown in FIGS. 6 and 7 were obtained.

  Here, in FIGS. 6 and 7, the center corresponds to the normal direction of the liquid crystal display panel LPN, and concentric circles centering on the normal direction have tilt angles (viewing angles) of 20 °, 40 ° with respect to the normal, It corresponds to 60 ° and 80 °. The characteristic diagram shown here is obtained by connecting regions of equal contrast ratio for each direction. The same applies to the simulation result regarding the viewing angle dependency of the contrast ratio described later.

  From the simulation results shown here, by applying the second retardation plate RF2 having an Nz coefficient of 1.2 or more and less than 1.8, a contrast ratio of 10: 1 is obtained in a viewing angle range within 70 ° in almost all directions. It was confirmed that it was obtained. In addition, when the second retardation plate RF2 having the Nz coefficient in the above range was applied, a contrast ratio of 100: 1 was obtained in a viewing angle range of 50 ° or less in the four directions of the top, bottom, left, and right of the screen.

  More desirably, by applying the second retardation plate RF2 having an Nz coefficient of 1.5 or more and 1.6 or less, a region having a contrast ratio of 10: 1 is enlarged in almost all directions and is 80 ° in almost all directions. It was confirmed that a contrast ratio of 10: 1 was obtained in the viewing angle range within the above range. In addition, when the second retardation plate RF2 having the Nz coefficient in the above range is applied, a region with a contrast ratio of 100: 1 expands in all directions, and a contrast ratio of 100: 1 in a viewing angle range within 50 ° in almost all directions. was gotten.

  When the second retardation plate RF2 having an Nz coefficient of 0.9 or less is applied, the contrast ratio is 10: 1 in the 180 ° azimuth (left side of the screen) and 270 ° azimuth (lower side of the screen). It can be seen that the region is less than the viewing angle range of 80 °. In addition, when the second retardation plate RF2 having an Nz coefficient of 1.8 or more is applied, a region having a contrast ratio of 10: 1 is obtained at 0 ° azimuth (right side of the screen) and 90 ° azimuth (upper side of the screen). Is less than the viewing angle range of 80 °.

  Therefore, the optimum range of the Nz coefficient of the second retardation plate RF2 is 1.2 or more and less than 1.8, and more preferably 1.5 or more and 1.6 or less. By applying the second retardation plate RF2, the optical compensation for the light transmitted through the first optical element OD1 and the liquid crystal display panel LPN is performed. In particular, in black display, a substantially constant elliptical shape is obtained regardless of the viewing angle. Linearly polarized light with a ratio Thereby, the viewing angle can be enlarged.

  Subsequently, based on the relationship between the voltage applied to the liquid crystal layer LQ and the residual retardation R (LQ), the optimum range of the average tilt angle of the liquid crystal molecules in the first retardation plate RF1 will be examined. That is, as shown in FIG. 8, the residual phase difference R (LQ) does not become zero even when a relatively high voltage (for example, 6V) is applied to the liquid crystal layer LQ containing homogeneously aligned liquid crystal molecules. Therefore, optical compensation is performed in consideration of the residual phase difference R (LQ) of the liquid crystal layer LQ. In other words, considering optimization during black display, when a relatively high black display voltage is applied to the liquid crystal layer LQ, the liquid crystal molecules are aligned at a relatively high tilt angle, so the residual retardation value is small. The liquid crystal molecules thus aligned are optically compensated by the first retardation plate RF1 including liquid crystal molecules hybrid-aligned with a relatively small tilt angle. Further, when a relatively low black display voltage is applied to the liquid crystal layer LQ, the liquid crystal molecules are aligned at a relatively low tilt angle, so that the residual retardation value becomes large. The liquid crystal molecules thus aligned are optically compensated by the first retardation plate RF1 including liquid crystal molecules hybrid-aligned with a relatively large tilt angle.

  Here, attention is focused on the average inclination angle of the first retardation plate RF1. The average tilt angle is defined as an angle formed by the main refractive index nz in the depth direction with respect to the normal direction, and is simply given by ((high tilt angle + low tilt angle) / 2 + low tilt angle). Define as a value. In the first retardation plate RF1 described above, for example, as shown in FIG. 3, “high tilt angle” means a liquid crystal that rises with the greatest inclination with respect to the main surface of the array substrate among liquid crystal molecules that are hybrid-aligned. This corresponds to the tilt angle (tilt with respect to the main surface) of the molecule 61B, and the “low tilt angle” refers to the liquid crystal molecule 61A that rises with the smallest tilt with respect to the main surface of the array substrate among the liquid crystal molecules that are hybrid aligned. Corresponds to the tilt angle.

  The first retardation plate necessary for optical compensation of the liquid crystal layer LQ having such a residual retardation value when a black display voltage is set such that the residual retardation value R (LQ) of the liquid crystal layer LQ is 50 nm or more. The average tilt angle of the liquid crystal molecules of RF1 is 35 degrees or more. With such a setting, it is possible to drive at a relatively low voltage, and it is possible to apply a low cost driving circuit with high versatility.

  Subsequently, optimization of the third retardation plate RF3 will be examined. The third retardation plate RF3 compensates for viewing angle characteristics (FIG. 4) due to the first polarizing plate 51 and the second polarizing plate 52. Therefore, the slow axis D3 of the third retardation plate RF3 should be set to either 0 ° or 90 ° with respect to the absorption axis A2 of the second polarizing plate 52. By using a retardation plate with Nz coefficient = 0.5 as the third retardation plate RF3, it is considered that the case where the slow axis D3 is arranged at 0 ° and the case where it is arranged at 90 ° is equivalent. The retardation plate having an Nz coefficient of 0.5 is expensive and has a thickness as thick as 100 microns, which is unsuitable for demands for cost reduction and thickness reduction.

  On the other hand, in the present embodiment, a phase difference plate with Nz coefficient = 1.0 is applied as the third phase difference plate RF3. For such third retardation plate RF3, the slow axis D3 is set to 90 ° with respect to the absorption axis A2, whereby the viewing angle characteristics of the polarizing plate can be compensated. In addition, since the retardation plate with Nz coefficient = 1.0 has high versatility, the cost can be reduced and the thickness is as thin as 30 microns compared with the retardation plate with Nz coefficient = 0.5. Thinning is possible. The application of the third retardation plate RF3 makes it possible to expand the viewing angle.

(Example)
Next, examples of the liquid crystal display device according to this embodiment will be described. This liquid crystal display device is designed as follows, for example.

  For the liquid crystal display panel LPN, the liquid crystal layer LQ is composed of a liquid crystal composition containing homogeneously aligned liquid crystal molecules. For example, MJ041113 (Merck, Δn = 0.065) was applied as the liquid crystal composition. At this time, the director of the liquid crystal molecules 40 (the major axis direction of the liquid crystal molecules) was regulated by the rubbing direction of the alignment film 20 on the array substrate AR side, and was set to make an angle of 45 ° with respect to the X axis. The gap in the liquid crystal layer LQ was set to 4.9 μm. Note that the black display voltage applied to the liquid crystal layer LQ in order to realize black display is set to 4.0 (V). At this time, the residual phase difference value R (LQ) of the liquid crystal layer LQ is 60 (nm). Met.

  In order to compensate for the birefringence caused by the liquid crystal molecules 40, the slow axis D1 (that is, the first retardation plate RF1) of the first retardation plate RF1 is set for the first optical element OD1 to be arranged on the outer surface of the array substrate AR. The orientation direction of the liquid crystal molecules 61 to be configured is set to an orientation (225 ° orientation) substantially parallel to the rubbing direction of the array substrate AR so as to have a relationship of successive compensation. The front retardation value R (RF1) of the first retardation plate RF1 was set to 100 nm. The average tilt angle of the liquid crystal molecules constituting the first retardation plate RF1 was set to 35 degrees. An NH film (manufactured by Nippon Oil Corporation) was applied as the first retardation plate RF1.

  The slow axis D2 of the second retardation plate RF2 is set to an orientation (135 ° orientation) substantially perpendicular to the liquid crystal molecules 40 and the slow axis D1 of the first retardation plate RF1. The front retardation value R (RF2) of the second retardation plate RF2 is the sum of the residual retardation value R (LQ) of the liquid crystal layer LQ and the front retardation value R (RF1) of the first retardation plate RF1. Was set to 160 nm. Therefore, since R (LQ) + R (RF1) = R (RF2) is satisfied, in the front black display, the liquid crystal layer LQ can be optically compensated only by the first optical element OD1. Further, the Nz coefficient of the second retardation plate RF2 was set to 1.6. As such second retardation plate RF2, ZEONOR (manufactured by Optes Co., Ltd.) was applied.

  In order to convert the linearly polarized light that has passed through the first polarizing plate 51 into desired elliptically polarized light or linearly polarized light and enter the liquid crystal layer LQ, the absorption axis A1 of the first polarizing plate 51 has a retardation of the first retardation plate RF1. It is set to an orientation (90 ° orientation) that intersects the phase axis D1 and the slow axis D2 of the second retardation plate RF2 at about 45 °.

  On the other hand, the absorption axis A2 of the second polarizing plate 52 of the second optical element OD2 to be disposed on the outer surface on the counter substrate CT side is an orientation that is substantially orthogonal to the absorption axis A1 of the first polarizing plate 51 (an orientation of 0 °). ).

  In order to improve the viewing angle characteristics of the polarizing plate by the first polarizing plate 51 and the second polarizing plate 52, the slow axis D3 of the third retardation plate RF3 is substantially orthogonal to the absorption axis A2 of the second polarizing plate 52. Set to the direction (90 ° azimuth). Further, the Nz coefficient of the third retardation plate RF3 was set to 1.0. As such third retardation plate RF3, ZEONOR (manufactured by Optes Co., Ltd.) was applied.

  The orientation of the slow axis of the retardation plate and the orientation of the absorption axis of the polarizing plate are defined by the angle formed with the X axis, as shown in FIG. The residual phase difference plate R (LQ) of the liquid crystal layer LQ, the phase difference value R (RF1) of the first phase difference plate RF1, and the phase difference value R (RF2) of the second phase difference plate RF2 are as follows: The value is not limited to the value, and the same result is obtained as long as the relationship of R (LQ) + R (RF1) = R (RF2) is satisfied.

  FIG. 9 shows the light after the backlight passes through the first optical element OD1 and the liquid crystal layer LQ having the above-described configuration when the black display voltage (4.0 V) is applied to the liquid crystal layer LQ in this embodiment. It shows the polarization state. Thus, in the black display state, the light after passing through the first optical element OD1 and the liquid crystal layer LQ has an extremely small amplitude (Es) in the short axis direction relative to the amplitude (Ep) in the long axis direction. The ellipticity is about 0.017, and it was confirmed that it has a polarization state of linearly polarized light. Further, since the major axis direction of this linearly polarized light is almost 0 ° azimuth (X-axis), in order to display a black image with good quality, the absorption axis A2 of the second polarizing plate 2 is oriented to 0 °. Is set.

  In addition, analysis is made on the coincidence between the polarization state of the light after the backlight passes through the first optical element OD1 and the liquid crystal layer LQ and the polarization state of the light after the outside light passes through the second optical element OD2. did.

  FIG. 10 is a characteristic diagram showing the coincidence in the vertical direction of the screen when a black display voltage (= 4.0 V) is applied to the liquid crystal layer LQ in this embodiment. The horizontal axis is the viewing angle in the vertical direction of the screen (that is, the angle formed with respect to the normal), and the vertical axis is the ellipticity at a wavelength of 550 nm as a parameter indicating the polarization state. Further, “A” in the figure corresponds to the polarization state of the light after the backlight is transmitted through the first optical element OD1 and the liquid crystal layer LQ, and “B” is the external light transmitted through the second optical element OD2. It corresponds to the polarization state of the later light.

  Since the liquid crystal molecule orientation of the liquid crystal layer LQ is set to 45 degrees, FIG. 10 shows the viewing angle characteristics in the vertical direction of the screen, but the same viewing angle characteristics are also shown in the horizontal direction of the screen. . In order to realize good viewing angle compensation, the polarization state of the light after the backlight passes through the first optical element OD1 and the liquid crystal layer LQ, and the light after the outside light passes through the second optical element OD2. It is important that the polarization state of each is substantially the same.

  As is clear from FIG. 10, it can be seen that in the present embodiment, both are almost the same regardless of the viewing angle. Further, in this example, it is understood that the substantially linearly polarized light has an ellipticity of less than 0.1, and the polarization state in front of the screen is a linearly polarized light with an ellipticity of almost zero. That is, in this embodiment, linearly polarized light (or elliptically polarized light having a relatively small ellipticity) is mainly used. Thus, the polarization state of light after passing through the first optical element OD1 and the liquid crystal layer LQ is used. It has been confirmed that linearly polarizing is advantageous in view angle characteristics.

  According to the embodiment as described above, when the viewing angle dependence of the contrast ratio was simulated, the result shown in FIG. 11 was obtained. That is, it was confirmed that the viewing angle range with an equicontrast ratio of 10: 1 can realize a sufficiently wide viewing angle of 160 ° in almost all directions of the screen. It was also confirmed that the high contrast region with an equicontrast ratio of 100: 1 was enlarged in all directions.

  Next, a comparative example will be described.

  Comparative Example 1 is configured such that the polarization state of the light after the backlight passes through the first optical element OD1 and the liquid crystal layer LQ is almost circularly polarized with an ellipticity of 0.791. It is. In Comparative Example 1, all other parameters are the same as the configuration of the present embodiment except that the second retardation plate RF2 is included in the second optical element OD2.

  In FIG. 12, the simulation result of the viewing angle dependence of the contrast ratio in the comparative example 1 is shown. That is, in Comparative Example 1, it can be seen that both the regions with the equal contrast ratio of 10: 1 and 100: 1 are narrower than those of the example. This is because the ellipticity changes depending on the viewing angle, and the ellipticity of elliptically polarized light after passing through the first optical element OD1 and the liquid crystal layer LQ, and after the external light passes through the second optical element OD2. This is because the ellipticity of elliptically polarized light deviates as the viewing angle increases.

  In the liquid crystal display device of Comparative Example 2, all the other parameters are the same as in the present embodiment except that the slow axis D3 of the third retardation plate RF3 is set parallel to the absorption axis A2 of the second polarizing plate. The configuration is the same. In FIG. 13, the simulation result of the viewing angle dependence of the contrast ratio in the comparative example 2 is shown. In other words, in Comparative Example 2, it can be seen that the regions having the equal contrast ratio of 10: 1 and 100: 1 are both narrower than those in the example.

  In the liquid crystal display device of Comparative Example 3, all other parameters are the same as the configuration of the present embodiment except that the Nz coefficient of the second retardation plate RF2 is set to 1.0. In FIG. 14, the simulation result of the viewing angle dependence of the contrast ratio in the comparative example 3 is shown. That is, in Comparative Example 3, the region with the equal contrast ratio of 10: 1 is larger than that of Comparative Example 1 and Comparative Example 2, but the region with the equal contrast ratio of 100: 1 is narrower than that of the example.

  In the liquid crystal display device of Comparative Example 4, all other parameters are the same as those in the present embodiment except that the third retardation plate RF3 is not applied. FIG. 15 shows a simulation result of the viewing angle dependence of the contrast ratio in Comparative Example 4. That is, in Comparative Example 4, as in Comparative Example 3, the region with an equal contrast ratio of 10: 1 is larger than Comparative Example 1 and Comparative Example 2, but the region with an equal contrast ratio of 100: 1 is compared with the Example. I can see that it is narrow.

  From the above simulation results, in order to expand not only the region having the equal contrast ratio of 10: 1 but also the region having the equal contrast ratio of 100: 1 in all directions, the backlight light is transmitted through the first optical element OD1 and the liquid crystal layer LQ. The polarization state of the light after transmission is substantially linearly polarized (ellipticity <0.1), the third retardation plate RF3 is applied as the viewing angle compensation of the polarizing plate, and the slow axis D3 is set to the second polarizing plate. The second retardation plate RF2 having a biaxial refractive index anisotropy is applied, and preferably the Nz coefficient is set to 1.5 to 1.6. It is desirable.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the spirit of the invention in the stage of implementation. Further, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, you may combine the component covering different embodiment suitably.

  For example, although the example in which the switching element W is configured by an n-channel thin film transistor has been described, other configurations may be used as long as the same various drive signals can be generated.

  In the above-described embodiment, it is desirable that the first retardation plate RF1 is constituted only by the liquid crystal film layer 60. That is, in the embodiment, the first retardation plate RF1 is configured by the liquid crystal film layer 60 that is in contact with the second retardation plate RF2 and the outer surface of the liquid crystal display panel LPN (that is, the outer surface of the insulating substrate 10 constituting the array substrate AR). Has been. A phase difference plate having a liquid crystal film layer containing liquid crystal molecules hybrid-aligned such as an NH film is a state in which the liquid crystal molecules maintain a predetermined alignment state after applying an alignment treatment on the base film and applying a liquid crystal material. It is obtained by curing with. Triacetate cellulose (TAC) is widely used as the base film. However, since the base film itself has a retardation, it is necessary to compensate in consideration of the retardation of the base film in order to realize good optical compensation. Therefore, optical compensation can be easily realized by applying a base film-less NH film as in the above-described embodiment.

FIG. 1 schematically shows a configuration of a liquid crystal display device according to an embodiment of the present invention. FIG. 2 is a diagram schematically showing a cross-sectional structure of the liquid crystal display device shown in FIG. FIG. 3 is a diagram schematically showing a configuration of a first optical element and a second optical element applicable to the liquid crystal display device shown in FIG. FIG. 4 is a diagram illustrating the viewing angle characteristics of the polarizing plate. FIG. 5 is a diagram for explaining the orientation of the slow axis of each retardation plate shown in FIG. 3 and the orientation of the absorption axis of each polarizing plate. FIG. 6 is a diagram showing the result of simulating the viewing angle dependence of the contrast ratio for the liquid crystal display devices having different Nz coefficients of the second retardation plate shown in FIG. FIG. 7 is a diagram showing the result of simulating the viewing angle dependence of the contrast ratio for the liquid crystal display devices having different Nz coefficients of the second retardation plate shown in FIG. FIG. 8 is a diagram showing a relationship between a voltage applied to a liquid crystal layer including homogeneously aligned liquid crystal molecules and a residual retardation value. FIG. 9 is a characteristic diagram showing the polarization state after the backlight passes through the first optical element and the liquid crystal layer in the liquid crystal display device according to this example. FIG. 10 shows the change in ellipticity depending on the viewing angle of the polarization state after the backlight light is transmitted through the first optical element and the liquid crystal layer, and after the external light is transmitted through the second optical element. It is the characteristic view which showed the matching with the change of the ellipticity depending on the viewing angle of the polarization state. FIG. 11 is a diagram illustrating a simulation result of the viewing angle dependence of the contrast ratio in the liquid crystal display device according to the present embodiment. FIG. 12 is a diagram illustrating a simulation result of the viewing angle dependence of the contrast ratio in the liquid crystal display device according to Comparative Example 1. FIG. 13 is a diagram illustrating a simulation result of the viewing angle dependency of the contrast ratio in the liquid crystal display device according to Comparative Example 2. FIG. 14 is a diagram illustrating a simulation result of the viewing angle dependence of the contrast ratio in the liquid crystal display device according to Comparative Example 3. FIG. 15 is a diagram illustrating a simulation result of the viewing angle dependence of the contrast ratio in the liquid crystal display device according to Comparative Example 4.

Explanation of symbols

  LPN ... liquid crystal display panel, AR ... array substrate, CT ... counter substrate, LQ ... liquid crystal layer, OD1 ... first optical element, OD2 ... second optical element, 51 ... first polarizing plate, RF1 ... first retardation plate, RF2 ... second retardation plate, 52 ... second polarizing plate, RF3 ... third retardation plate, BL ... backlight unit, PX ... pixel

Claims (2)

A liquid crystal display panel holding a liquid crystal layer containing homogeneously aligned liquid crystal molecules between a first substrate and a second substrate disposed to face each other;
The first polarizing plate is provided on one outer surface of the liquid crystal display panel, is disposed between the first polarizing plate and the liquid crystal display panel, and transmits light having a predetermined wavelength that passes through the fast axis and the slow axis. A first retardation plate that is fixed in a state in which a predetermined phase difference is provided between the nematic liquid crystal molecules and the nematic liquid crystal molecules are hybrid-aligned along the normal direction and the average tilt angle of the nematic liquid crystal molecules is set to 35 degrees or more. When the refractive index in the direction perpendicular to each other is arranged between the first polarizing plate and the first retardation plate is nx and ny, and the refractive index in the normal direction is nz Nz coefficient given by Nz = (nx−nz) / (nx−ny) is set to 1.5 or more and 1.6 or less, and has a negative biaxial refractive index anisotropy, A first optical element comprising:
A second polarizing plate provided on the other outer surface of the liquid crystal display panel and having an absorption axis perpendicular to the absorption axis of the first polarizing plate, and disposed between the second retardation plate and the liquid crystal display panel. A third phase difference plate having a uniaxial refractive index anisotropy that gives a predetermined phase difference between light having a predetermined wavelength that passes through the fast axis and the slow axis, and that has an Nz coefficient of 1.0. A second optical element having
The polarization state of the first optical element and the light transmitted through the liquid crystal display panel, Ri linearly polarized der with light substantially the same ellipticity having passed through the second optical element,
In the first optical element, an angle formed between the absorption axis of the first polarizing plate and the slow axis of the second retardation plate is set to approximately 45 degrees, and the slow axis of the first retardation plate and the The director of the liquid crystal molecules contained in the liquid crystal layer is set substantially in parallel, and the angle formed by the slow axis of the first retardation plate and the slow axis of the second retardation plate is approximately 90 degrees. Set,
The sum of the residual retardation value of the liquid crystal layer and the front retardation value of the first retardation plate is substantially equal to the front retardation value of the second retardation plate,
In the second optical element, a liquid crystal display device , wherein an absorption axis of the second polarizing plate and a slow axis of the third retardation plate are substantially orthogonal .   The liquid crystal display device according to claim 1, further comprising a backlight unit that illuminates the liquid crystal display panel from the first optical element side.

Images