Display device comprising a liquid crystal display panel having at least two optical compensation films including a positive biaxial film and a negative biaxial film

Abstract

An in-plane switching (IPS) mode liquid crystal display (LCD) device includes a liquid crystal display panel having a first substrate, a second substrate, and a liquid crystal layer between the first and second substrates; a second polarizer on an outer surface of the second substrate, the second polarizer having a second polarizing element; and an in-cell retarder on an inner surface of the second substrate, wherein the in-cell retarder compensates a light leakage in a front direction of the liquid crystal display panel.

Claims

1. A display device, comprising: a liquid crystal display panel having a first substrate, a second substrate, and a liquid crystal layer between the first and second substrates; a second polarizer on an outer surface of the second substrate, the second polarizer having a second polarizing element; and at least two optical compensation films between the second substrate and the second polarizing element to enhance a viewing angle in a diagonal direction of the liquid crystal display panel, the at least two optical compensation films including a positive biaxial film and a negative biaxial film in consideration of a polarization state of the liquid crystal layer, wherein: the positive biaxial film has nz>nx>ny; and the negative biaxial film has nx>ny>nz, and the nx, ny, nz are respective x-direction, y-direction, and z-direction refractive indexes in an x,y,z coordinate system.

2. The device of claim 1, wherein a polarization state of the second polarizing element is parallel to an absorption axis of the second polarization element by the positive biaxial film and the negative biaxial film to prevent a light leakage in the diagonal direction of the liquid crystal display panel, when viewing the liquid crystal display panel in the diagonal direction.

3. The device of claim 1, further comprising an in-cell retarder on an inner surface of the second substrate.

4. The device of claim 3, wherein the in-cell retarder has a retardation value of 15620 nm to prevent a light leakage in a front direction or the diagonal direction of the liquid crystal display panel.

5. The device of claim 3, wherein in the display device having the in-cell-retarder, a light leakage in a front direction or the diagonal direction of the liquid crystal display panel is relatively reduced compared with a display device having no in-cell-retarder.

6. The device of claim 3, wherein the in-cell retarder compensates a light leakage in a direction oblique to a front surface of the liquid crystal display panel.

7. The device of claim 3, wherein the in-cell retarder compensates a light leakage due to an external stress applied to at least one among the first substrate and the second substrate in a direction normal to a front surface of the liquid crystal display panel or in a direction oblique to the front surface of the liquid crystal display panel.

8. The device of claim 3, wherein the in-cell retarder includes a reactive mesogen of a positive A-plate having a retardation value of 15620 nm.

9. The device of claim 3, wherein an absorption axis of the second polarizing element is parallel to a polarized state of light transmitted through the positive and negative biaxial films via the first substrate, the liquid crystal layer, the in-cell retarder, and the second substrate in a direction normal to a front surface of the liquid crystal display panel.

10. The device of claim 3, wherein light transmitted through the positive biaxial film and the negative biaxial film, via the first substrate, the liquid crystal layer, the in-cell retarder, and the second substrate, has a polarized state corresponding to an absorption axis of the second polarizing element, when viewing the liquid crystal display panel from a direction normal to a front surface of the liquid crystal display panel.

11. The device of claim 1, wherein the negative biaxial film is between the positive biaxial film and the second polarizing element.

12. The device of claim 1, wherein: the nz of the positive biaxial film is smaller than 0, and the nz of the negative biaxial film is greater than 1.0; and an optical axis of the positive biaxial film is located between the nz and the ny, and an optical axis of the negative biaxial film is located between the nz and the nx.

13. The device of claim 1, further comprising a first polarizer on an outer surface of the first substrate, the first polarizer having a first polarizing element.

14. The device of claim 13, wherein an absorption axis of the first polarizing element and an absorption axis of the second polarizing element are perpendicular to each other.

15. The device of claim 13, wherein an optical axis of the liquid crystal layer, when no electric field is applied, is parallel to an absorption axis of the first polarizing element.

16. The device of claim 1, wherein the first substrate includes an array substrate, and the second substrate includes a color filter substrate.

17. The device of claim 1, wherein an absorption axis of the second polarization element is parallel to a polarized state of light transmitted through the positive biaxial film and the negative biaxial film in a direction oblique to a front surface of the liquid crystal display panel.

18. The device of claim 1, wherein a polarization state of light of the second polarizing element is parallel to an absorption axis of the second polarization element by the positive biaxial film and the negative biaxial film, when viewing the liquid crystal display panel in a direction oblique to a front surface of the liquid crystal display panel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings, which are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments and together with the description serve to explain the principles of the disclosure. In the drawings:

(2) FIG. 1 is a planar view schematically illustrating a part of an array substrate of a general IPS-mode LCD device;

(3) FIG. 2 is a sectional view of the array substrate illustrated in FIG. 1, taken along the line I-I;

(4) FIGS. 3A and 3B are exemplary views illustrating brightness and viewing angle characteristics in a black state, in a general IPS-mode LCD device;

(5) FIG. 4A is a schematic view illustrating light transmission axes of upper and lower polarizers, which are orthogonal to each other, when viewed from a front direction;

(6) FIG. 4B a schematic view illustrating light transmission axes of the upper and lower polarizers, which are orthogonal to each other, when viewed in a diagonal direction;

(7) FIG. 5 is a sectional view exemplarily illustrating an IPS-mode LCD device in accordance with an example embodiment of the present invention;

(8) FIG. 6 is a sectional view schematically illustrating a structure of a liquid crystal display panel employing an in-cell retarder in the IPS-mode LCD device illustrated in FIG. 5 in accordance with the embodiment of the present invention;

(9) FIGS. 7A and 7B are views illustrating a positive biaxial film and a negative biaxial film as optical compensation films in an IPS-mode LCD device in accordance with an embodiment of the present invention;

(10) FIGS. 8A and 8B are views illustrating an arbitrary elliptical polarization in an orthogonal coordinate system and a corresponding Poincare vector;

(11) FIG. 9 is a view illustrating the Poincare sphere representing a polarized state of light passed through each optical element when viewing the IPS-mode LCD device from a front direction;

(12) FIGS. 10A and 10B are views each illustrating the Poincare sphere representing a polarized state of light passed through each optical element when viewing the IPS-mode LCD device in a diagonal direction;

(13) FIG. 11 is a view illustrating the Poincare sphere representing a light leakage caused due to external stress;

(14) FIGS. 12A and 12B are views illustrating the Poincare sphere representing a reduced state of a light leakage by applying an in-cell retarder in accordance with an embodiment of the present invention;

(15) FIG. 13 is an exemplary view illustrating a simulation result of brightness and viewing angle characteristics in a black (or dark) state in an IPS-mode LCD device in accordance with an embodiment of the present invention; and

(16) FIGS. 14A and 14B are graphs illustrating a light leakage according to a thickness of an in-cell retarder when the IPS-mode LCD device is viewed in front and diagonal directions.

(17) FIGS. 15A, 15B and 15C are exemplary views illustrating the light according to a retardation value of the in-cell retarder.

DETAILED DESCRIPTION OF THE DISCLOSURE

(18) Description will now be given in detail of preferred embodiments of an in-plane switching (IPS) mode liquid crystal display (LCD) device and display device according to the present invention, with reference to the accompanying drawings, such that those skilled in the art to which the present invention belongs can easily practice.

(19) Advantages and features of the present invention and methods of achieving those will be obviously understood with reference to the accompanying drawings and exemplary embodiments to be explained later in detail. Exemplary embodiments of the inventive concepts may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of example embodiments to those of ordinary skill in the art. The same/like reference symbols refer to the same/like components throughout the specification. In the drawings, the thicknesses of layers and regions are exaggerated for clarity.

(20) It will be understood that when an element is referred to as being on another element, the element can be located on the other element or intervening elements may also be present. In contrast, when an element is referred to as being directly on another element, there are no intervening elements present.

(21) Spatially relative terms, such as beneath, below, lower, above, upper and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the drawings. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the drawings. For example, if the device in the drawings is turned over, elements described as below or beneath other elements or features would then be oriented above the other elements or features. Thus, the exemplary term below can encompass both an orientation of above and below.

(22) The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms comprises, comprising, includes and/or including, if used herein, specify the presence of stated steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.

(23) FIG. 5 is a sectional view exemplarily illustrating an in-plane switching (IPS) mode LCD device in accordance with an example embodiment of the present invention. FIG. 6 is a sectional view schematically illustrating a structure of a liquid crystal display panel employing an in-cell retarder in the IPS-mode LCD device illustrated in FIG. 5 in accordance with the embodiment of the present invention.

(24) FIGS. 5 and 6 show a liquid crystal display device as an example for a display device. However, the present invention may not be limited to the liquid crystal display device.

(25) As illustrated in FIGS. 5 and 6, an IPS-mode LCD device 100 in accordance with an embodiment of the present invention may include a liquid crystal display panel 110 to output an image, a first polarizer 105 below the liquid crystal display panel 110, and a second polarizer 115 above the liquid crystal display panel 110.

(26) Here, the liquid crystal display panel 110 includes a color filter substrate 101, an array substrate 111, and a liquid crystal layer (not illustrated) between the color filter substrate 101 and the array substrate 111.

(27) The liquid crystal layer may include nematic liquid crystals which are homogeneously aligned while an electric field is not generated. The liquid crystal layer may exhibit a refractive index profile of nx>ny=nz (here, the refractive indexes within a plane are referred to as nx and ny, and a refractive index in a thickness direction is referred to as nz). Here, in this specification, the case of ny=nz includes both a case where ny and nz are completely the same as each other and a case where ny and nz are substantially the same as each other.

(28) A driving mode using the liquid crystal layer having the refractive index profile may include, for example, an in-plane switching (IPS) mode, a fringe field switching (FFS) mode, or the like.

(29) Here, the IPS mode is a mode of driving nematic liquid crystals, which are homogeneously aligned while an electric field is not present, by a horizontal electric field formed by pixel electrodes and common electrodes, using an electrically controlled birefringence (ECB) effect.

(30) Also, the FFS mode is a mode of driving the liquid crystals according to substantially the same method as the IPS mode. Here, the horizontal electric field of the FFS mode is referred to as a fringe field. The fringe field may be formed by setting an interval between a pixel electrode and a common electrode, which are formed of a transparent conductive material, to be narrower than an interval between upper and lower substrates.

(31) Here, the pixel electrode and the common electrode may have a straight (linear) shape or a zigzag shape. Or, one of the pixel electrode and the common electrode may have the straight shape and the other may have the zigzag shape. Or, one of the pixel electrode and the common electrode may have the straight or zigzag shape and the other may have a rectangular shape. That is, the present invention may not be limited to those shapes of the pixel electrode and the common electrode.

(32) The pixel electrode and the common electrode may be formed of the transparent conductive material. Or, one of the pixel electrode and the common electrode may be formed of an opaque conductive material, for example, copper (Cu), Cu alloy or the like. That is, the present invention may not be limited to the material of forming the pixel electrode and the common electrode.

(33) Here, the embodiment of the present invention illustrates an example of an IPS-mode LCD device. However, the present invention may not be limited to this, but also be applicable to an FFS mode, super-IPS-mode, or reverse TN IPS-mode LCD device.

(34) The color filter substrate 101 includes a color filter 107 having a plurality of sub color filters for rendering red (R), green (G) and blue (B) colors, and a black matrix 106 partitioning the sub color filters and blocking light transmitted through the liquid crystal layer.

(35) An overcoat layer 108 which is formed of an organic material may be formed on the color filter substrate 101 having the color filter 107 and the black matrix 106. The overcoat layer 108 may prevent a flow of dyes and planarize a surface of the color filter 107. Or, the color filter 107, the overcoat layer 108 and the black matrix 106 may be sequentially formed on the color filter substrate 101.

(36) The color filter 107 and the black matrix 106 may be formed in a zigzag shape. That is, the present invention may not be limited to the shape of the color filter 107 and the black matrix 106.

(37) Although not illustrated for the sake of explanation, the array substrate 111 may include a plurality of gate lines and data lines arranged horizontally and vertically to define a plurality of pixel regions, a TFT as a switching element formed on each intersection between the gate line and the data line, and a pixel electrode and a common electrode formed on each pixel region to generate a horizontal electric field.

(38) The configured color filter substrate 101 and the array substrate 111 are assembled (or bonded) to each other in a facing manner, by use of a sealant 140 which is provided along an outer edge of an image display region, thereby configuring the liquid crystal display panel 110. Here, alignment layers 109 and 119 for alignment of the liquid crystal layer are provided on inner surfaces of the color filter substrate 101 and the array substrate 111, respectively.

(39) Here, the liquid crystal display panel 110 according to the embodiment of the present invention includes an in-cell retarder 152 having a positive A-plate and between the color filter substrate 101 and the liquid crystal layer, to prevent a light leakage at a front direction. FIG. 6 exemplarily illustrates a case where the in-cell retarder 152 is formed on a lower layer of the color filter substrate 101, but the present invention may not be limited to this.

(40) The in-cell retarder 152 may be located on an alignment layer 151. The in-cell retarder 152 according to the embodiment of the present invention may include a positive A-plate type of reactive mesogen (RM) whose retardation value is about 15620 nm.

(41) As aforementioned, the first polarizer 105 and the second polarizer 115 are attached onto lower and upper portions of the liquid crystal display panel 110, respectively.

(42) The first polarizer 105 includes a first supporting body 102, a second supporting body 104, and a first polarizing element 103 between the first supporting body 102 and the second supporting body 104. The second polarizer 115 includes a third supporting body 112, first and second optical compensation films 120 and 130, and a second polarizing element 113 between the third supporting body 112 and the first and second optical compensation films 120 and 130.

(43) Here, the first optical compensation film 120 and the second optical compensation film 130 may be sequentially on the liquid crystal display panel 110.

(44) The first polarizing element 103 and the second polarizing element 113 may be formed of polyvinyl alcohol (PVA).

(45) The first supporting body 102 and the third supporting body 112 may be formed of a protection film without retardation, for example, triacetylcellulose (TAC). Also, the second supporting body 104 may be formed of a protection film without retardation for protecting the PVA layer. For example, the second supporting body 104 may be formed of 0-RT (denoting a transformed TAC of which Rth is close to 0 nm, and also called 0-TAC) or cycloolefin polymer (COP). The first polarizing element 103 and the second polarizing element 113 denote films which may convert natural light or polarized light into arbitrary polarized light. Here, the first polarizing element 103 and the second polarizing element 113 may have the function of transmitting one polarization component when dividing incident light into two orthogonal polarization components, and of absorbing, reflecting and/or scattering the other polarization component.

(46) Although there is no limit to an optical film used for the first polarizing element 103 and the second polarizing element 113, examples of such optical film may include a polymer-stretched film having as a main component a PVA-based resin having iodine or dichroic dye, an O-type polarizing element in which liquid crystalline composition having a dichromatic material and liquid crystalline compound is aligned in a predetermined direction, an E-type polarizing element in which lyotropic liquid crystal is aligned in a predetermined direction, and the like.

(47) Here, an absorption axis of the first polarizing element 103 is substantially orthogonal to an absorption axis of the second polarizing element 113. Here, an optical axis of the liquid crystal layer is parallel to the light absorption axis of the first polarizing element 103. On the other hand, as aforementioned, when the first polarizer 105 is located above the liquid crystal display panel 110 and the second polarizer 115 is located below the liquid crystal display panel 110, the optical axis of the liquid crystal layer is parallel to the absorption axis of the second polarizing element 113.

(48) Here, to improve the viewing angle characteristic when viewing the LCD device in the diagonal direction in the present invention, the first optical compensation film 120 and the second optical compensation film 130 are disposed between the second polarizing element 113 and the liquid crystal display panel 110. Here, the first optical compensation film 120 includes a positive biaxial film, and the second optical compensation film 130 includes a negative biaxial film.

(49) Hereinafter, description will be given in detail of the positive biaxial film and the negative biaxial film as the first optical compensation film 120 and the second optical compensation film 130, with reference to the accompanying drawings.

(50) FIGS. 7A and 7B are views illustrating a positive biaxial film and a negative biaxial film which are used as optical compensation films in an IPS-mode LCD device in accordance with an embodiment of the present invention. That is, FIGS. 7A and 7B are views illustrating refractive indexes with respect to the positive biaxial film and the negative biaxial film, respectively.

(51) Retardation films are classified into a uniaxial film and a biaxial film according to the number of optical axes, and also classified into a positive film and a negative film according to a difference between a refractive index in an optical axis direction and refractive indexes in the other directions. For example, the retardation film is classified into a uniaxial film when there is a single optical axis, and into a biaxial film when there are two optical axes. And, the retardation film is classified into the positive film when the refractive index in the direction of the optical axis is greater than the refractive indexes in the other directions, and into the negative film when the refractive index in the direction of the optical axis is smaller than the refractive indexes in the other directions.

(52) The retardation film may be expressed by the refractive index in each direction in a xyz coordinate system. For example, when the retardation film is located on a xy plane, the x and y axes denote in-plane directions of the retardation film, and the retardation film has refractive indexes nx, ny and nz according to the x, y and z axes.

(53) Here, Re (or Rin) refers to an in-plane retardation value (or an in-plane phase difference or retardation) and Rth refers to a thickness-direction retardation value (or a phase difference or retardation in a thickness direction). Also, Nz refers to an index indicating an extent of biaxiality of the retardation film. This can be defined according to Equation 1 as follows.
Re=(nxny).Math.d
Rth=(nxnz).Math.d
Nz=Rth/Re[Equation 1]
Here, d denotes a thickness of a film.

(54) The positive biaxial film (or +B plate), which is an optical film having Nz smaller than 0, has a relation of nz>nx>ny and thus its optical axis is located between nz and ny.

(55) Also, the negative biaxial film (or B plate), which is an optical film having Nz greater than 1.0, has a relation of nx>ny>nz and thus its optical axis is located between nz and nx.

(56) Here, examples of films which can be used as the positive biaxial film and the negative biaxial film may include a uniaxially-stretched TAC, a uniaxially-stretched polynorborene (PNB), a biaxially-stretched polycarbonate (PC), a biaxially-stretched COP, a biaxial LC film and the like.

(57) The first optical compensation film 120 and the second optical compensation film 130 according to the first embodiment having such optical conditions compensate for the broken orthogonality of the first and second polarizers 105 and 115 in the diagonal direction, thereby reducing the light leakage in the diagonal direction. This will be explained in detail using the Poincare sphere.

(58) The Poincare sphere in a polarized state is used in order to geometrically analyze an optical characteristic of a transparent medium.

(59) First of all, the Jones vector can merely exhibit a complete polarization. In order to exhibit more general partial polarization, Stokes parameters which are defined according to the following Equation 2 are used.
S.sub.0=<|E.sub.x|.sup.2>+<|E.sub.y|.sup.2>
S.sub.1=<|E.sub.x|.sup.2><|E.sub.y|.sup.2>
S.sub.2=2|E.sub.xE.sub.y|<cos(.sub.x.sub.y)>
S.sub.3=2|E.sub.xE.sub.y|<sin(.sub.x.sub.y)>[Equation 2]

(60) Here, < > denotes time average. An inequality of S.sub.0.sup.2S.sub.1.sup.2+S.sub.2.sup.2+S.sub.3.sup.2 is established among those four variables, and an equality thereof is satisfied only for a complete polarization.

(61) For the complete polarization, Equation 3 is established among standardized variables s.sub.1, s.sub.2 and s.sub.3 which are obtained by dividing S.sub.1, S.sub.2 and S.sub.3 by light brightness S.sub.0.
s.sub.1.sup.2+s.sub.2.sup.2+s.sub.3.sup.2=1[Equation 3]

(62) This is an equation of the Poincare sphere having a radius of 1 in a three-dimensional space. Here, (s.sub.1, s.sub.2, s.sub.3) denote points of orthogonal coordinates of the Poincare sphere.

(63) Here, in the Poincare sphere, all points on the equator correspond to (represent) linear polarization. The arctic corresponds to right-handed circular polarization and the Antarctic corresponds to left-handed circular polarization. Also, all points of the Northern hemisphere correspond to right-handed elliptical polarization, and all points of the Southern hemisphere correspond to left-handed elliptical polarization.

(64) FIGS. 8A and 8B are views each illustrating an arbitrary elliptical polarization in an orthogonal coordinate system and a corresponding Poincare vector.

(65) As illustrated in FIGS. 8A and 8B, for the Poincare vector P, corresponding to an elliptical polarization in which an azimuthal angle of a long axis thereof is and an elliptical angle is x, a latitude angle is 2, an azimuthal angle is 2 and orthogonal coordinates are (cons(2)cons(2), sin(2)cos(2), sin(2)). If this point is located on the Northern hemisphere, an electric field vector is rotated in a clockwise direction, and if the point is located on the Southern hemisphere, the electric field vector is rotated in a counterclockwise direction. Here, antipodes on the Poincare sphere represent an orthogonally polarized state.

(66) Further, the unitary Jones matrix representing the change in the polarized state when light passes through a transparent medium can be analyzed by a rotation conversion on the Poincare sphere.

(67) FIG. 9 is a view illustrating the Poincare sphere representing a polarized state of light passed through each optical element when viewing the IPS-mode LCD device from a front side.

(68) FIGS. 10A and 10B are views each illustrating the Poincare sphere representing a polarized state of light passed through each optical element when viewing the IPS-mode LCD device in a diagonal direction.

(69) Here, FIG. 10B is a two-dimensional view illustrating a compensation mechanism for a path of light using the Poincare sphere illustrated in FIG. 10A. That is, FIG. 10B is a view illustrating the Poincare sphere illustrated in FIG. 10A when viewing the Poincare sphere from a front side. Although FIG. 10B in the two-dimensional expression illustrates displacement of points before and after a change in the polarized state using arrows, any change in the polarized state of light can be expressed on the Poincare sphere by a rotation by a specific angle around a specific axis, which is determined corresponding to each optical characteristic.

(70) Here, as aforementioned, the Poincare sphere represents all of polarized states of light on a spherical plane. Therefore, the polarized state can be easily predicted by using the Poincare sphere if an optical axis of an optical element and a retardation value are known.

(71) In the Poincare sphere, all points on the equator represent linear polarization. The arctic S.sub.3 corresponds to right-handed circular polarization and the AntarcticS.sub.3 corresponds to left-handed circular polarization. Also, all points of the Northern hemisphere of the other region correspond to right-handed elliptical polarization, and all points of the Southern hemisphere correspond to left-handed elliptical polarization.

(72) Here, referring to FIG. 9, point A and point A refer to an absorption axis of the lower polarizer and a transmission axis of the upper polarizer when viewing the LCD device from the front direction. Point B and point B refer to a transmission axis of the lower polarizer and an absorption axis of the upper polarizer. The polarized states of the upper and lower polarizers are symmetrical to a center 0 of the Poincare sphere and thus perpendicular to each other, thereby realizing a black (or dark) state. That is, as aforementioned, the antipodal points A and B on the Poincare sphere represent orthogonally-polarized states.

(73) However, referring to FIGS. 10A and 10B, when viewing the LCD device in the diagonal direction, the transmission axis A of the upper polarizer and the transmission axis B of the lower polarizer are shifted toward S.sub.2 axis by a predetermined distance, and the absorption axis B of the upper polarizer and the absorption axis A of the lower polarizer are shifted towardS.sub.2 axis by a predetermined distance. Here, since the point A and the point B are not symmetrical to the center 0, the polarized states of the upper and lower polarizers are not perpendicular to each other.

(74) Therefore, the optical compensation films according to the embodiment disclosed herein can be used such that the optical axis of light incident on the upper polarizer can match the absorption axis of the upper polarizer. Here, the polarized state of incident light transmitted through the lower polarizer corresponds to the point B, and the polarized state of light which has been absorbed by the absorption axis of the upper polarizer corresponds to the point B.

(75) That is, when incident light is transmitted through the lower polarizer of which absorption axis is at the point A on the Poincare sphere, the light is linearly polarized and thus is present at the point B. The linearly-polarized light is transmitted through the homogeneous liquid crystal layer. Since the alignment direction of the liquid crystal layer is orthogonal to the polarization direction of the linearly-polarized light, the linearly-polarized light is not changed in phase within the liquid crystal layer. Therefore, light transmitted through the liquid crystal layer is maintained in the same linearly-polarized state, thereby having the polarized state corresponding to the point B.

(76) In this manner, in the IPS-mode LCD device, the light leakage which is caused due to the polarized state being out of the axis in the diagonal direction results from the non-match of the polarized states between the points B and B. Therefore, the optical compensation films of the present invention are used to cause the change in the polarized state of the incident light from the point B to the point B as well as the change in the polarized state of the liquid crystal layer.

(77) Thus, when viewing the IPS-mode LCD device according to the embodiment of the present invention in the diagonal direction, the polarized state of light transmitted through each optical element is represented as follows.

(78) First, a polarized state of light is rotated from point B to point C when the light is transmitted through the positive biaxial film as the first optical compensation film, and then rotated from the point C to point B when the light is transmitted through the negative biaxial film as the second optical compensation film. Therefore, the polarized state (point B) of the light incident on the upper polarizer becomes consistent with the absorption axis of the upper polarizer, and accordingly the light is absorbed, thereby realizing an excellent black state. In detail, when non-polarized light is incident on a first polarizer from a backlight of the LCD device, the light is linearly polarized. Most of the linearly-polarized light is absorbed at the absorption axis (point A) of the first polarizer and the polarized state of the light transmitted through the first polarizer is located at point B. That is, the transmission axis of the first polarizer is located at point B. Here, the absorption axis of the second polarizer is located at point B so as to be spaced apart from the transmission axis of the first polarizer by a distance.

(79) When the light linearly polarized by the first polarizer is transmitted through the positive biaxial film of which optical axis is located at a predetermined region between nz and ny, the polarized state of the light is rotated in a counterclockwise direction around the optical axis of the positive biaxial film so as to be moved from point B to point C.

(80) That is, the linearly-polarized light is rotated in the counterclockwise direction around the optical axis of the positive biaxial direction by 27 times of a value, which is obtained by dividing an available retardation value of the positive biaxial film by green wavelength (550 nm), which affects brightness. Accordingly, the linearly-polarized light is changed into elliptically-polarized light at the point C. The elliptically-polarized light is then incident on the negative biaxial film as the second optical compensation film.

(81) When the elliptically-polarized light by the positive biaxial film is transmitted through the negative biaxial film located at a predetermined region between nz and nx, the polarized state of the light is rotated in a counterclockwise direction around the optical axis of the negative biaxial film. The polarized state of the light is then moved from point C to point B.

(82) That is, the elliptically-polarized light is rotated in the counterclockwise direction around the optical axis of the positive biaxial direction by 27 times of a value, which is obtained by dividing an available retardation value of the negative biaxial film by the green wavelength (550 nm). Accordingly, the elliptically-polarized light is changed into linearly-polarized light at point B. Here, since the point B indicates the absorption axis of the upper polarizer, the incident light is completely absorbed by the upper polarizer, so as to implement an excellent black state.

(83) In this manner, in the embodiment of the present invention, the polarized state of light may be adjusted using the positive biaxial film and the negative biaxial film, to prevent the light leakage in the diagonal direction, thereby preventing the contrast ratio from being lowered.

(84) Meanwhile, as aforementioned, even when viewing the liquid crystal display panel from the front direction, the light leakage is caused due to external stress applied to the liquid crystal display panel. This results from that a glass substrate obtains refractive anisotropy due to interference between a structure and the liquid crystal display panel and stress applied during an array process and a color filter process.

(85) FIG. 11 is a view illustrating the Poincare sphere representing a light leakage caused due to external stress.

(86) As illustrated in FIG. 11, when viewing an IPS-mode LCD device without employing an in-cell retarder from a front direction, the polarized state of light transmitted through each optical element is represented as follows.

(87) First, light is moved from point B to point D when transmitted through a lower glass substrate (namely, the array substrate). This is caused due to the refractive anisotropy of the glass substrate due to external stress. Such retardation value (10 nm) of the glass substrate makes the light path moved from point B to point D.

(88) Afterwards, the light is moved from the point D to point E by the liquid crystal layer having a retardation value in the range of 280 to 350 nm, and then moved from the point E to point F by an upper glass substrate (namely, the color filter substrate).

(89) Here, since the liquid crystal layer has the retardation value of 280 to 350 nm but its alignment direction is perpendicular to a polarization direction of light, which has been linearly polarized at the lower polarizer, the linearly-polarized light is not changed in phase within the liquid crystal layer. However, when the glass substrates has the refractive anisotropy, the light linearly polarized at the lower polarizer is changed into elliptically-polarized light (to be then moved to point D) due to the retardation value of the glass substrate prior to reaching the liquid crystal layer. And, light transmitted through the liquid crystal layer is then elliptically polarized by the retardation value of the liquid crystal layer, and thereafter moved to the point E.

(90) The light is moved from point F to point G when transmitted through the positive biaxial film and the negative biaxial film as the optical compensation films. Hence, the polarized state of the light arrived at the upper polarizer is not consistent with (or does not match) the absorption axis (point B) of the upper polarizer, which causes the light leakage.

(91) FIGS. 12A and 12B are views illustrating the Poincare sphere representing a reduced light leakage by applying an in-cell retarder in accordance with an embodiment of the present invention.

(92) Here, FIG. 12B is a two-dimensional view illustrating a compensation mechanism for a path of light using the Poincare sphere illustrated in FIG. 12A.

(93) As illustrated in FIGS. 12A and 12B, when viewing an IPS-mode LCD device with an in-cell retarder according to an embodiment of the present invention in a front direction, the polarized state of light transmitted through each optical element is represented as follows.

(94) First, when light is transmitted through the lower glass substrate (namely, array substrate), its path is moved from point B to point D.

(95) Afterwards, as aforementioned, the light is moved from the point D to point E by the liquid crystal layer having the retardation value of 280 to 350 nm.

(96) Then, the light is moved from the point E to point F (=point D) by the in-cell retarder according to the embodiment, and then moved from the point F to point G (=point B) by the upper glass substrate (namely, color filter substrate).

(97) Here, the in-cell retarder is formed of RM (Reactive Mesogen) of a positive A plate type whose retardation value is about 15620 nm.

(98) When the elliptically-polarized light by the liquid crystal layer is transmitted through the positive A plate of which optical axis is located at a predetermined region of nx, the polarized state of the light is rotated in a counterclockwise direction around the optical axis of the positive A plate. Accordingly, the polarized state of the light is moved from the point E to the point F.

(99) That is, the elliptically-polarized light by the liquid crystal layer is rotated in the counterclockwise direction around the optical axis of the positive A plate by 27 times of a value, which is obtained by dividing an available retardation value of the positive A plate by the green wavelength (550 nm), thereby being moved to the point F.

(100) The light moved to the point G (=point B) by the upper glass substrate is finally moved to point B, with maintaining the polarized state, when transmitted through the positive biaxial film and the negative biaxial film as the optical compensation films. Therefore, the incident light is completely absorbed at the upper polarizer, thereby exhibiting an excellent black state in the front direction.

(101) According to the present invention, the path of light deviated (changed) due to the glass substrate can be compensated for by the in-cell retarder, such that the light can have the same path of light as that when the glass substrate is isotropic.

(102) FIG. 13 is an exemplary view illustrating simulation results of brightness and viewing angle characteristics in a black state, in an IPS-mode LCD device in accordance with an embodiment of the present invention. Here, the lower polarizer and the upper polarizer are arranged such that their optical absorption axes are orthogonal to each other and the optical axis of the liquid crystal layer is parallel to the optical absorption axis of the lower polarizer.

(103) FIGS. 14A and 14B are graphs illustrating a light leakage according to a thickness of an in-cell retarder when the liquid crystal display panel is viewed in front and diagonal directions.

(104) As illustrated in FIGS. 13, 14A and 14B, the retardation value of the in-cell retarder may be adjusted 156 nm according to light leakage simulation results. The retardation value is a retardation value having the minimum light leakage in the front and diagonal directions. It may be noticed that the minimum light leakage is observed at the retardation value of 167 nm in the front direction, and at the retardation value of 145 nm in the diagonal direction.

(105) Here, it can be understood that the light leakage is reduced by about 73% in the front direction and about 34% in the diagonal direction, as compared with a case without the in-cell retarder.

(106) FIGS. 15A, 5B and 15C are exemplary views illustrating the light according to a retardation value of the in-cell retarder.

(107) Here, FIGS. 15A, 5B and 15C show the amount of the light leakage at the retardation value of 120 nm, 150 nm and 220 nm, respectively. Moreover, in FIGS. 15A, 5B and 15C, the amount of the light leakage may be proportional to the density of the dot used in the hatching.

(108) Referring to FIGS. 15A, 5B and 15C, it can be understood that the minimum light leakage is observed at the retardation value of 150 nm. In case, it can be understood that the light leakage is reduced by about 56%, as compared with above simulation results. That is, it can be understood that the area having the higher density of the dot is minimized at the retardation value of 150 nm. Thus, the light leakage is minimized.

(109) In accordance with one example embodiment of the present invention as described herein, an in-plane switching (IPS) mode LCD device and display device can compensate for a path of light by employing an in-cell retarder having a positive A-plate. Accordingly, the light leakage in a front direction, caused due to external stress, can be reduced by 73% or more, thereby improving image quality. It will be apparent to those skilled in the art that various modifications and variations can be made in the in-plane switching (IPS) mode LCD device and display device of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.