KR101812542B1 - In-Plane Switching Mode Liquid Crystal Display Device And Method Of Driving The Same - Google Patents

Abstract

The present invention provides a plasma display panel comprising first and second substrates facing each other and spaced apart from each other; A liquid crystal layer formed between the first and second substrates, the liquid crystal layer having a first phase delay and a first optical axis; A first polarizer formed on an outer surface of the first substrate; A first retardation film formed on an outer surface of the second substrate and having a second phase retardation equal to the first phase retardation and a second optical axis perpendicular to the first optical axis; A second retardation film formed on the first retardation film and having a third phase retardation of half wave and the third optical axis; And a second polarizer formed on the second phase difference film.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display (LCD)

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display (LCD), and more particularly, to a flat panel display (IPS) liquid crystal display.

Recently, as the age of information society is rapidly progressing, a display field for processing and displaying a large amount of information has been developed.

Particularly in recent years, a flat panel display has been required to meet the era of thinning, light weight, low power consumption, and the like. Accordingly, a thin film transistor liquid crystal display Display: TFT-LCD) has been developed.

Such a liquid crystal display device displays an image by using optical anisotropy and polarization properties of liquid crystal molecules. That is, since the liquid crystal molecules have a thin and long structure and have a pretilt angle that is directional in arrangement, when a voltage is applied to the liquid crystal by artificially changing the line inclination angle of the liquid crystal molecules, Direction can be controlled. Therefore, a desired voltage is applied to the liquid crystal layer to arbitrarily adjust the arrangement direction of the liquid crystal molecules, thereby changing the arrangement of the liquid crystal molecules, and modulating the polarized light by the optical anisotropy of the liquid crystal, thereby displaying a desired image.

At present, active matrix liquid crystal displays (LCDs) in which a thin film transistor and pixel electrodes connected to the thin film transistor are arranged in a matrix manner are receiving the most attention because of their excellent resolution and video realization capability.

In a liquid crystal panel which is a basic element of a general liquid crystal display device, a color filter substrate on the upper side and an array substrate on the lower side are opposed to each other with a predetermined gap therebetween, and a liquid crystal layer containing liquid crystal molecules .

At this time, an electrode for applying a voltage to the liquid crystal layer becomes a common electrode positioned on the color filter substrate and a pixel electrode positioned on the array substrate. When a voltage is applied to the two electrodes, A vertical electric field in the vertical direction is used to control the direction of the liquid crystal molecules located therebetween.

However, as described above, when the common electrode and the pixel electrode are vertically opposed to each other and the liquid crystal layer is driven by the vertical electric field generated in the vertical direction, the characteristics such as the transmittance and the aperture ratio are excellent In order to overcome such disadvantages, an in-plane switching mode (IPS mode) liquid crystal display device using a horizontal electric field has been proposed. However, Mode liquid crystal display device will be described with reference to the drawings.

FIGS. 1A and 1B are cross-sectional views showing an off state and an on state of a conventional planar alignment switching type liquid crystal display device, respectively.

1A and 1B, a planar alignment switching type liquid crystal display 10 includes a first substrate 20 on which a thin film transistor (not shown) is formed, a color filter layer (not shown), and a black matrix A liquid crystal layer 40 formed between the first and second substrates 20 and 30 and a second substrate 30 formed on the outer surfaces of the first and second substrates 20 and 30, And a second polarizing plate (60, 70).

The common electrode 22 and the pixel electrode 27 are alternately formed on the first substrate 20 and the electric field E is generated according to the voltage applied to the common electrode 22 and the pixel electrode 27 And the liquid crystal molecules 40a and 40b of the liquid crystal layer 40 are rearranged in accordance with the electric field E generated.

1A, a common voltage is applied to the common electrode 22, but no pixel voltage is applied to the pixel electrode 27, so that the common electrode 22 and the pixel electrode 27 do not generate an electric field.

Therefore, the first and second liquid crystal molecules 40a and 40b of the liquid crystal layer 40 are not rearranged and maintain their initial alignment state.

1B, a common voltage and a pixel voltage are applied to the common electrode 22 and the pixel electrode 27, respectively, so that the common electrode 22 and the pixel electrode 27 An electric field E is formed.

At this time, an electric field E is generated vertically above the common electrode 22 and the pixel electrode 27, and an electric field E is generated between the common electrode 22 and the pixel electrode 27 in the horizontal direction .

Therefore, the first liquid crystal molecules 40a immediately above the common electrode 22 and the pixel electrode 27 remain in their original state without being rearranged, but the second liquid crystal molecules 40a between the common electrode 22 and the pixel electrode 27 The liquid crystal molecules 40b are rearranged in the same direction as the electric field E.

That is, in the planar alignment type liquid crystal display device 10, the liquid crystal layer 40 between the common electrode 22 and the pixel electrode 27 in the ON state is rearranged in the horizontal direction by the horizontal electric field E, And as a result, the viewing angle is improved.

For example, images can be displayed without reversal even in a viewing angle of about 80 to 85 in the up / down / left / right directions with respect to the front surface of the planar alignment type liquid crystal display device.

The off-state and the on-state of the planar alignment type liquid crystal display device can display black and white, respectively, and will be described with reference to the drawings.

FIGS. 2A and 2B are diagrams showing optical arrangements of an off-state and an on-state, respectively, of a conventional planar alignment switching type liquid crystal display device, in which planes parallel to the first and second substrates are arranged in an x- Axis and a z-axis perpendicular to the x-axis and the y-axis, and a description is made based on the light incident perpendicularly on the plane-aligned switching type liquid-crystal display, that is, the front view angle do.

As shown in Figs. 2A and 2B, in the conventional planar alignment switching type liquid crystal display device (10 in Fig. 1), the first and second polarizing plates 60 and 70 respectively have first and second transparent Axes 62 and 72, respectively.

That is, the first transmission axis 62 of the first polarizing plate 60 forms an angle (x-axis parallel) with respect to the x-axis and the second transmission axis 72 of the second polarizing plate 70 is an x- (Parallel to the y-axis).

Here, the liquid crystal layer 40 has a half wavelength (lambda / 2) retardation, and the director of the liquid crystal molecules of the liquid crystal layer 40 rotates in accordance with the horizontal electric field, The optical axis 42 of the liquid crystal layer 40 in the same direction as the director of the liquid crystal molecules also rotates in accordance with the horizontal electric field.

As shown in FIG. 2A, when the pixel electrode (27 in FIG. 1) of the conventional planar alignment switching type liquid crystal display device 10 is in an off state in which no pixel voltage is applied, A horizontal electric field is not generated between the pixel electrode 22 and the pixel electrode 27, and the liquid crystal molecules of the liquid crystal layer 40 maintain the initial alignment state.

Therefore, the director of the liquid crystal molecules and the optical axis 42 of the liquid crystal layer 40 form an angle of 0 degrees (x-axis parallel) with respect to the x-axis.

Since the first transmission axis 62 of the first polarizer 60 and the optical axis 42 of the liquid crystal layer 40 are parallel to each other, the linearly polarized light passing through the first polarizer 60 passes through the liquid crystal layer 40 While keeping the linearly polarized light state parallel to the first transmission axis 62 without delaying the phase.

As a result, linearly polarized light parallel to the first transmission axis 62 passing through the liquid crystal layer 40 passes through the second polarizer 70 having the second transmission axis 72 perpendicular to the first transmission axis 62 The conventional planar alignment switching type liquid crystal display device 10 displays black.

On the other hand, as shown in FIG. 2B, when the pixel voltage is applied to the pixel electrode 27 of the conventional planar alignment switching type liquid crystal display device 10, the common electrode 22 and the pixel A horizontal electric field is generated between the electrodes 27, and the liquid crystal molecules of the liquid crystal layer 40 rotate in accordance with the horizontal electric field.

Therefore, the director of the liquid crystal molecules and the optical axis 42 of the liquid crystal layer 40 form an angle of 45 degrees with respect to the x axis. The optical axis 42 of the liquid crystal layer 40 and the The angle between the optical axis 42 of the liquid crystal layer 40 at the time of application of the pixel voltage can be defined as a rotation angle?.

At this time, since the first transmission axis 62 of the first polarizer 60 and the optical axis 42 of the liquid crystal layer 40 form an angle of 45 degrees, the linearly polarized light passing through the first polarizer 60 passes through the liquid crystal layer 2) of the liquid crystal layer 40 while passing through the first transmission axis 62 and the second transmission axis 62 and passing through the first transmission axis 62 and the second transmission axis 62, And becomes a linearly polarized light state.

As a result, the linearly polarized light perpendicular to the first transmission axis 62 passing through the liquid crystal layer 40 passes through the second polarizing plate 70 having the second transmission axis 72 perpendicular to the first transmission axis 62 And the conventional planar alignment switching type liquid crystal display device 10 displays white.

In recent years, however, there has been an increasing demand for a display device having a high response speed in order to view a moving picture without degrading image quality such as cross-talk. Particularly, in order to view a three- There is a need for a liquid crystal display device of a planar alignment switching type having a wide viewing angle characteristic to improve the response speed.

The response speed of the plane-aligned switching type liquid crystal display device is determined by the rising time (t _r ) required for increasing the transmittance of the plane-aligned switching type liquid crystal display device from the minimum value to the maximum value by applying the pixel voltage, The rising time t _r and the falling time t _f can be divided into a falling time t _{f and a} falling time t _{f that} are required for the transmittance of the liquid crystal display to change from a maximum value to a minimum value, (1) and (2), respectively.

Here, t _r is the rise time, t _f is the falling time, ε ₀ is the dielectric constant of air (permittivity), Δε is the liquid crystal permittivity anisotropy (dielectric anisotropy), γ is the viscosity (viscosity), the liquid crystal K ₂₂ is a liquid crystal twist D is the cell gap of the liquid crystal display device, i is the distance between the common electrode and the pixel electrode, E _{input_eff} is the effective input electric field ), E _{th_eff} represents an effective threshold electric field.

Therefore, in order to improve the response speed of the planar alignment switching type liquid crystal display device (i.e., to reduce the rise time t _r and the fall time t _f ), the twist elastic constants K ₂₂ , , The dielectric constant of the liquid crystal, the cell gap (d) of the liquid crystal display device, and the pixel voltage applied to the pixel electrode.

However, there is a limit to the development of a liquid crystal material for the improvement of the response speed by controlling the physical properties, and there is a problem that the electro-optical characteristic deteriorates in the improvement of the response speed due to the decrease of the cell gap (d) and the increase of the pixel voltage.

For example, in order to display black, the liquid crystal layer must maintain a phase retardation of half a wavelength (? / 2). The phase delay is dependent on the refractive index anisotropy? N of the liquid crystal layer and the thickness of the liquid crystal layer ), The refractive index anisotropy (? N) must be increased in order to improve the response speed by reducing the cell gap (d).

However, in general, an increase in the refractive index anisotropy (? N) reduces the optimum viscosity (?) Of the liquid crystal, so that the response speed is not effectively improved.

As can be seen from equations (1) and (2), the application of a high pixel voltage can reduce the rise time t _r but does not affect the fall time t _f , .

The relationship between the cell gap and the pixel voltage and the response speed will be described with reference to the drawings.

3A and 3B are diagrams illustrating calculation results and experimental results on response characteristics of a conventional planar alignment switching type liquid crystal display device, respectively.

As shown in FIGS. 3A and 3B, the response speed is improved as the pixel voltage Vp applied to the pixel electrode in the planar alignment switching type liquid crystal display device increases.

That is, in a planar alignment switching type liquid crystal display device having a cell gap d of 3.4 μm, as the pixel voltage Vp applied to the pixel electrode increases to 8V, 9V, and 10V, the maximum transmittance 0.25) the time of the rise time (t _r) taken to reach up to reduce the response speed is improved.

However, the pixel voltage (Vp) increases though the maximum transmittance (about 0.25), the fall time (t _f) the time taken to reach the minimum transmission (0) on is not decreased.

Therefore, in the planar alignment switching type liquid crystal display device, there is a limit to improvement in the response speed due to the increase of the pixel voltage Vp.

On the other hand, although a liquid crystal display device using a horizontal alignment switching type using a horizontal electric field has excellent viewing angle characteristics as compared with a liquid crystal display device using a vertical electric field, there is a problem that light leakage occurs at a viewing angle inclined with respect to the front viewing angle. .

FIG. 4 is a graph showing luminance according to a viewing angle in a black state of a conventional planar alignment switching type liquid crystal display device.

As shown in FIG. 4, when a conventional planar alignment switching type liquid crystal display device displays black, light leakage occurs at a diagonal viewing angle in a diagonal direction when viewed from the front.

For example, the front viewing angle shows a luminance corresponding to about 3.0 * 10 ^-5 cd / m ² of black, while the azimuthal angle has a polar angle of about 45 °, about 135 °, about 225 °, about 315 ° At an oblique viewing angle of about 70 °, light gaps appear that are brighter than black at about 2.4 * 10 ^-4 cd / m ² .

The light leakage in the oblique viewing angles in the diagonal direction reduces the contrast ratio, and accordingly, there is a problem that the image quality of the conventional planar alignment switching type liquid crystal display device is deteriorated.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and it is an object of the present invention to provide a liquid crystal display device and a liquid crystal display device, It is an object of the present invention to provide a liquid crystal display device.

Another object of the present invention is to provide a planar alignment switching type liquid crystal display device in which the response speed is improved by reducing the cell gap without changing the physical properties of the liquid crystal.

Further, the present invention is characterized in that a second retardation film compensating for a light leakage of an oblique viewing angle is disposed on the first retardation film, so that a high-speed response characteristic and a planar alignment switching with light viewing angle- Type liquid crystal display device according to the present invention.

According to an aspect of the present invention, there is provided a plasma display panel comprising: first and second substrates facing each other; A liquid crystal layer formed between the first and second substrates, the liquid crystal layer having a first phase delay and a first optical axis; A first polarizer formed on an outer surface of the first substrate; A first retardation film formed on an outer surface of the second substrate and having a second phase retardation equal to the first phase retardation and a second optical axis perpendicular to the first optical axis; A second retardation film formed on the first retardation film and having a third phase retardation of half wave and the third optical axis; And a second polarizer formed on the second retardation film.

The first and second phase delays may be equal to or greater than a quarter wavelength (lambda / 4).

Also, the first and second phase delays may be less than a half wavelength (lambda / 2).

The first transmission axis of the first polarizer and the second transmission axis of the second polarizer may be perpendicular to each other.

The third optical axis may be parallel to the second transmission axis.

The surfaces parallel to the first and second substrates are defined as x-axis and y-axis perpendicular to each other, and a direction perpendicular to the first and second substrates is defined as a z-axis perpendicular to the x- and y-axes Wherein the first transmission axis is parallel to the x axis, the first optical axis is at an angle of 45 属 with respect to the x axis, the second optical axis is at an angle of 135 属 with respect to the x axis, And the second transmission axis may be parallel to the y axis.

Also, the first and second retardation films may be biaxial films.

The first optical axis is rotated by a rotation angle (?) By application of a pixel voltage,

(λ는 파장, Δnd_{A _ plate}는 상기 제2위상지연, Δnd_{LC _ effective}는 상기 제1위상지연의 실효값)에 따라 결정될 수 있다.
The angle of rotation

can be determined according to (λ is the wavelength, Δnd _{_ A plate} is the second phase delay, Δnd _{LC _ effective} is the effective value of the first phase delay).

As described above, in the planar alignment switching type liquid crystal display device according to the present invention, the first retardation film disposed on the liquid crystal layer and burdening a part of the phase retardation allows high-speed response characteristics .

Further, in the liquid crystal display device according to the present invention, the response speed can be improved by reducing the cell gap without changing the physical properties of the liquid crystal.

Further, in the liquid crystal display device according to the present invention, the second retardation film compensating for the light leakage of the oblique viewing angle is disposed on the first retardation film, thereby ensuring high-speed response characteristics and preventing the light leakage in the oblique viewing angle A wide viewing angle characteristic can be ensured.

FIG. 1A and FIG. 1B are cross-sectional views showing an off state and an on state of a conventional planar alignment switching type liquid crystal display, respectively.
FIGS. 2A and 2B are diagrams showing optical arrangements of an off state and an on state, respectively, of a conventional planar alignment switching type liquid crystal display device.
FIGS. 3A and 3B are diagrams illustrating calculation results and experimental results on response characteristics of a conventional planar alignment switching type liquid crystal display device, respectively. FIG.
4 is a view showing luminance according to a viewing angle in a black state of a conventional planar alignment switching type liquid crystal display device.
5 is a cross-sectional view of a planar alignment switching type liquid crystal display device according to an embodiment of the present invention.
6A and 6B are diagrams showing optical arrangements of an off state and an on state, respectively, of a planar alignment switching type liquid crystal display according to an embodiment of the present invention.
7A and 7B are diagrams illustrating calculation results and experimental results on the response characteristics of a planar alignment switching type liquid crystal display device according to an embodiment of the present invention, respectively.
FIG. 8 is a view showing a poincare sphere showing a polarization state of inclined incident light in a planar alignment switching type liquid crystal display according to an embodiment of the present invention; FIG.
FIG. 9 is a graph showing luminance according to a viewing angle of black of a planar alignment switching type liquid crystal display device according to an embodiment of the present invention. FIG.

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

5 is a cross-sectional view of a planar alignment switching type liquid crystal display device according to an embodiment of the present invention.

5, the planar alignment switching type liquid crystal display device 110 includes a first substrate 120 and a second substrate 130 facing each other and a first substrate 120 and a second substrate 130 facing each other, The first and second retardation films 150 and 160 formed on the second substrate 130, the first polarizer 170 and the second retardation film 170 formed on the lower surface of the first substrate 120, And a second polarizer 180 formed on the second polarizer 160.

A gate electrode 121 and a common electrode 122 are formed on the upper surface (inner surface) of the first substrate 120 and a gate insulating film 123 is formed on the gate electrode 121 and the common electrode 122. An active layer 124 composed of an intrinsic semiconductor layer and an impurity-doped semiconductor layer is formed on the gate insulating layer 123 corresponding to the gate electrode 121, A source electrode 125 and a drain electrode 126 are formed.

A pixel electrode 127 is formed on the gate insulating layer 123 and spaced apart from the common electrode 122. A protective layer 128 is formed on the source electrode 125, the drain electrode 126, and the pixel electrode 127, And the pixel electrode 127 is connected to the drain electrode 126 (not shown).

Here, the gate electrode 121, the active layer 124, the source electrode 125, and the drain electrode 126 constitute a thin film transistor T.

Although not shown in FIG. 5, a gate line and a data line are formed on the first substrate 120 so as to define pixel regions crossing each other. The thin film transistor T is connected to the gate line and the data line, The data signal applied through the data line is applied to the pixel electrode 127 with the pixel voltage Vp.

A black matrix 132 for covering a gate line, a data line and the thin film transistor T is formed on the lower surface (inner surface) of the second substrate 130, and a color filter layer (not shown) is formed under the black matrix 132 and the second substrate 130 134 are formed.

A liquid crystal layer 140 is formed between the protective layer 128 of the first substrate 120 and the color filter layer 134 of the second substrate 130.

A first polarizing plate 170 is formed on the lower surface (outer surface) of the first substrate 120, first and second retardation films 150 and 160 are formed on the upper surface (outer surface) of the second substrate 130, The second polarizing plate 180 is formed on the two-phase difference film 160.

Here, the phase retardation and the optical axis of the liquid crystal layer 140 and the first retardation film 150 are set such that the sum of the total delays experienced by the liquid crystal layer 140 and the first retardation film 150 is "0" .

That is, the liquid crystal layer 140 and the first retardation film 150 are arranged to have the same phase retardation and have optical axes perpendicular to each other, wherein the first retardation film 150 is formed as a biaxial film For example, a B plate (B-plate).

B plate, when the retardation film exists in the xy plane and the x axis and the y axis are in the plane direction of the retardation film and the z axis is in the thickness direction of the retardation film, the retardation film is aligned along the x axis, the y axis, The B plate has two optical axes (nx > ny = nz) or (nx < ny = nz).

The first and second polarizing plates 170 and 180 are disposed so as to have mutually perpendicular transmission axes.

Specifically, the liquid crystal layer 140 and the first retardation film 150 may each have a retardation of more than 1/4 wavelength (λ / 4) (? / 4? R <? / 2) of less than half a wavelength (? / 4) and half wavelength (? / 2), respectively, The cell gap d can be reduced as compared with the related art. As a result, the response speed of the planar alignment switching type liquid crystal display device can be improved without degrading the electro-optical characteristics.

The reason why the cell gap d can be reduced while using the liquid crystal having the same physical properties as the conventional one is that the first retardation film 150 partially borne the phase delay function for the linearly polarized light.

That is, the phase retardation? (?) Of the liquid crystal layer 140 expressed by the product of the refractive index anisotropy? N of the liquid crystal layer 140 and the thickness (i.e., the cell gap d) of the liquid crystal layer 140 It is possible to reduce the increase of the refractive index anisotropy? N with decrease of the cell gap d by setting the value of? / 4? R <? / 2 to a value smaller than the conventional phase delay (R =? / 2) It is possible to prevent a change in physical properties of the liquid crystal.

The phase retardation and the optical axis of the second retardation film 160 are set so as to compensate the polarization state of light incident at an oblique viewing angle in a diagonal direction as viewed from the front.

That is, the second retardation film 160 is disposed so as to have an optical axis parallel to the transmission axis of the second polarizer 180 with a phase retardation of a half wavelength (λ / 2) And may be formed of a biaxial film, for example, a B-plate.

The off-state and the on-state of the planar alignment switching type liquid crystal display device can display black and white, respectively, and will be described with reference to the drawings.

6A and 6B are diagrams showing optical arrangements of the off-state and the on-state, respectively, of the planar alignment switching type liquid-crystal display device according to the embodiment of the present invention, wherein planes parallel to the first and second substrates are perpendicular axis and a y-axis, and a direction perpendicular to the first and second substrates is defined as a z-axis perpendicular to the x-axis and the y-axis, and the front view angle is used as a reference.

As shown in Figs. 6A and 6B, in the planar alignment switching type liquid crystal display (110 in Fig. 4), the first and second polarizing plates 170 and 180 are respectively connected to first and second transmission axes 172, and 182, respectively.

That is, the first transmission axis 172 of the first polarizing plate 170 forms an angle (x-axis parallel) with respect to the x-axis and the second transmission axis 182 of the second polarizing plate 180 is an x- (Parallel to the y-axis).

In this embodiment, the first and second polarizing plates 170 and 180 are formed so that the first and second transmission axes 172 and 182 are parallel to the x axis and the y axis, respectively. However, in other embodiments, The first and second polarizing plates 170 and 180 may be formed such that the transmission axes 172 and 182 rotate in a state of maintaining a perpendicular relationship with each other and are parallel to the other directions.

The liquid crystal layer 140 has a first phase retardation R1 (R1?? / 4) of at least a quarter wavelength (? / 4) The first optical axis 142 of the liquid crystal layer 140 in the same direction as the director of the liquid crystal molecules is also rotated in the horizontal electric field by the horizontal electric field generated by applying the pixel voltage Vp And rotates accordingly.

For example, the liquid crystal layer 140 may be initially oriented such that the first optical axis 142 is at an angle of 45 degrees with respect to the x-axis.

The first retardation film 150 has a second phase retardation R2 equal to the first phase retardation R1 of the liquid crystal layer 140 (R1 = R2) 142 and a second optical axis 152 perpendicular to the optical axis.

For example, the first retardation film 150 may be formed such that the second optical axis 152 has an angle of 135 degrees with respect to the x axis.

The liquid crystal layer 140 is initially oriented so that the first optical axis 142 forms an angle of 45 degrees with the x axis and the first phase difference 152 is formed such that the second optical axis 152 forms an angle of 135 degrees with respect to the x axis. The liquid crystal layer 140 is initially oriented such that the first optical axis 142 forms an angle other than 45 degrees with respect to the x axis and the second optical axis 152 is oriented in the x axis The first retardation film 150 may be formed at an angle other than 135 °.

The second retardation film 160 has a third phase retardation R3 of a half wavelength lambda / 2 (R3 = lambda / 2), and is incident on the second transmission axis 182 of the second polarizer plate 180 And has a parallel third optical axis 162.

For example, the second phase difference film 160 may be formed such that the third optical axis 162 has an angle of 90 degrees with respect to the x axis.

As shown in Fig. 6A, when the pixel voltage (Vp) is not applied to the pixel electrode (127 in Fig. 5) of the planar alignment switching type liquid crystal display device 110, A horizontal electric field is not generated between the liquid crystal layer 122 and the pixel electrode 127 and the liquid crystal molecules of the liquid crystal layer 140 maintain the initial alignment state.

Therefore, the director of the liquid crystal molecules and the first optical axis 142 of the liquid crystal layer 140 form an angle of 45 degrees with respect to the x-axis.

Light emitted from a backlight (not shown) disposed under the first substrate 120 (FIG. 5) becomes a first polarized light that is linearly polarized light parallel to the first transmission axis 172 while passing through the first polarizer 170.

The first transmission axis 172 of the first polarizing plate 170 and the first optical axis 142 of the liquid crystal layer 140 form an angle of 45 degrees so that the first polarized light passes through the liquid crystal side 140, State is changed to become an arbitrary second polarized light.

At this time, the polarization state of the second polarized light may be determined by the first transmission axis 172, the first optical axis 142, and the first phase delay R1.

The second polarized light passes through the first retardation film 150 and is changed in polarized state again so that the first phase retardation Rl of the liquid crystal layer 140 and the second phase retardation Rl of the first retardation film 150 R2 are the same and the first optical axis 142 of the liquid crystal layer 140 and the second optical axis 152 of the first retardation film 150 are perpendicular to each other, the first retardation film 150 is in contact with the liquid crystal layer 140 ). &Lt; / RTI &gt;

Therefore, the second polarized light becomes the first polarized light again while passing through the first retardation film 150.

The first polarized light passes through the second retardation film 160 and the third optical axis 162 of the second retardation film 160 is parallel to the second transmission axis 182 of the second polarizing plate 180, The first polarized light passing through the second retardation film 160 maintains the polarization state as it is.

As a result, the first polarized light parallel to the first transmission axis 172 passing through the second retardation film 160 is transmitted through the second polarizing plate (second polarizing plate) 182 having the second transmission axis 182 perpendicular to the first transmission axis 172 180, and the planar alignment switching type liquid crystal display device 110 displays black.

6B, when the pixel voltage Vp is applied to the pixel electrode 127 of the liquid crystal display device 110, the common electrode 122 and the pixel electrode 127, and the liquid crystal molecules of the liquid crystal layer 140 rotate in accordance with the horizontal electric field.

Accordingly, the director of the liquid crystal molecules and the first optical axis 142 of the liquid crystal layer 140 are rotated by the rotation angle? From the initial alignment state of 45 degrees with respect to the x axis, (45 deg + sigma) of the sum of the rotation angle [theta] and the rotation angle [sigma].

Since the first transmission axis 172 of the first polarizing plate 170 and the first optical axis 142 of the liquid crystal layer 140 form the angle (45 占 +?) Of the sum of 45 占 and the rotation angle? The first polarized light that is linearly polarized light having passed through the first polarizing plate 170 passes through the liquid crystal layer 140 and changes its polarization state to become arbitrary third polarized light.

Here, the polarization state of the third polarized light has an effective liquid crystal phase delay _{DELTAnd LC_effective} corresponding to the angle between the angle between the first transmission axis 172 and the first optical axis 142 and the first phase retardation R1 of the liquid crystal layer 140, Lt; / RTI &gt;

In the planar alignment switching type liquid crystal display, since the liquid crystal molecules rotate on the horizontal plane, the entire liquid crystal does not contribute to the optical function such as the phase delay, and only a part of the liquid crystal contributes to the optical function such as the phase delay. Thus, in an embodiment of the present invention, the rms value may be defined as a value for the optical function that the liquid crystal layer 140 contributes substantially.

The polarized state of the fourth polarized light is different from the polarized state of the third polarized light and the polarized state of the first polarized light 150 and the second polarized state of the first polarized light 150. [ Can be determined by the second phase delay R2.

Here, in order for the plane-aligned switching type liquid crystal display 110 to display white, the fourth polarized light must pass through the second retardation film 160 and the second polarizer 180. To this end, Parallel to the y-axis parallel to the first transmission axis 162 and the second transmission axis 182. [

It is necessary to rotate the first optical axis 142 of the liquid crystal layer 140 by a specific rotation angle σ so that the fourth polarized light becomes linearly polarized light parallel to the third optical axis 162 and the second transmission axis 182 And the rotation of the first optical axis 142 by a specific rotation angle? Is possible by controlling the pixel voltage Vp.

The rotation angle [sigma] of the first optical axis 142 of the liquid crystal layer 140 can be calculated by the following equation (3).

Here, σ is the effective rotation angle of the first optical axis 142 of the liquid crystal layer 140, λ is the wavelength, _{Δnd A_plate} is the second phase delay R2 of the first retardation film 150, _{Δnd And LC_effective} represents the effective first phase delay (R1) of the liquid crystal layer 140.

For example, when the first and second phase delays R1 and R2 are quarter wavelength (? / 4), the rotation angle? Is 90 占 and the liquid crystal layer 140 By applying the pixel voltage Vp capable of rotating the first optical axis 142 of the first polarized light to 90 degrees.

When the first and second phase retardations R1 and R2 are 0.3612 wavelength (0.3612?), The rotation angle? Is 76.5 占 and the first optical axis (?) Of the liquid crystal layer 140 142 can be rotated by 76.5 DEG, so that the fourth polarized light can be a linearly polarized light parallel to the y-axis.

When the first and second phase retardations R1 and R2 are respectively half wavelength (? / 2), the rotation angle? Is 45 占 and the pixel electrode 127 is provided with the first optical axis The fourth polarized light can be made to be a linearly polarized light parallel to the y-axis by applying a pixel voltage Vp capable of rotating the light source 142 by 45 degrees.

The fourth polarized light passes through the second retardation film 160 and the third optical axis 162 of the second retardation film 160 is parallel to the second transmission axis 182 of the second polarizing plate 180, The fourth polarized light passing through the second retardation film 160 maintains the polarization state as it is.

As a result, the fourth polarized light perpendicular to the first transmission axis 172 passing through the second retardation film 160 is transmitted through the second polarizing plate (second polarizing plate) 182 having the second transmission axis 182 perpendicular to the first transmission axis 172 180, and the plane-aligned switching type liquid crystal display device 110 displays white.

Accordingly, in the planar alignment switching type liquid crystal display device 110 according to the embodiment of the present invention, the liquid crystal layer 140 is formed to have a first phase delay R1 of at least a quarter wavelength (? / 4) The liquid crystal layer 140 has a second phase retardation R 2 equal to the first phase retardation R 1 of the liquid crystal layer 140 and a second optical axis L 2 perpendicular to the first optical axis 142 of the liquid crystal layer 140. And a third phase retardation R3 of half wavelength lambda / 2 is formed on the first retardation film 150 and the second phase retardation film 150 of the second polarizer 180 is formed on the first retardation film 150. [ A second retardation film 160 having a third optical axis 162 parallel to the transmission axis 182 may be formed to display black and white.

In particular, when the first and second phase delays R1 and R2 are equal to each other, the phase delay (? / 4? (R1 = R2) is less than half the wavelength (? / 4) <? / 2), the cell gap d can be reduced compared with the conventional liquid crystal using the liquid crystal having the same physical properties as those of the conventional liquid crystal display, and thus the response speed of the planar alignment switching type liquid crystal display device Can be improved.

The change of the response characteristic according to the cell gap and the pixel voltage will be described with reference to the drawings.

FIGS. 7A and 7B are diagrams illustrating calculation results and experimental results of the response characteristics of a planar alignment switching type liquid crystal display device according to an embodiment of the present invention, respectively.

7A and 7B, the planar alignment switching type liquid crystal display device 110 according to the embodiment of the present invention uses the liquid crystal having the same physical properties as those of the conventional planar alignment switching type liquid crystal display device, The response speed can be improved by reducing the cell gap d by the cell gap d.

That is, as compared with the conventional planar alignment switching type liquid crystal display device having a cell gap d of 3.4 μm and a pixel voltage Vp of 8 V applied to the pixel electrode, the pixel electrode has a cell gap d of 2 μm, In which the pixel voltage Vp is applied and the pixel voltage Vp of 17V is applied to the pixel electrode with a cell gap d of 1.41 m according to an embodiment of the present invention , The rise time (t _r ) required for reaching the maximum transmittance at the minimum transmittance and the fall time (t _f ) taken for reaching the minimum transmittance at the maximum transmittance are reduced and the response speed is improved .

Table 1 and Table 2 are the numerical values of the rise time t _r and fall time t _f of Figs. 7A and 7B, respectively.

As can be seen from Tables 1 and 2, compared to the rise time t _r and the fall time t _f of the conventional planar alignment switching type liquid crystal display device having the cell gap d of 3.4 μm, Or 1.41 m, the rise time (t _r ) and the fall time (t _f ) of the planar alignment switching type liquid crystal display device according to the embodiment of the present invention are sharply reduced.

Meanwhile, in the planar alignment switching type liquid crystal display device according to the embodiment of the present invention, light leakage at an oblique viewing angle can be minimized by the second retardation film 160, which will be described with reference to the drawings.

FIG. 8 is a view showing a poincare sphere showing a polarization state of inclined incident light of a planar alignment switching type liquid crystal display device according to an embodiment of the present invention, and FIG. 9 is a cross- 1 is a graph showing the luminance according to the viewing angle of black of a plane-aligned switching type liquid crystal display device.

As shown in Fig. 8, the light incident obliquely on the backlight passes through the first polarizing plate (170 in Fig. 6A) and enters the fifth polarized light having a polarization state slightly deviated from the point S1, which is the polarization state of the light vertically incident on the backlight P5).

Thereafter, the fifth polarized light P5 passes through the liquid crystal layer (140 in FIG. 6A) and is changed to a sixth polarized light P6 by changing the polarization state along the first path L1 rotating with reference to the point S2.

Then, the sixth polarized light P6 passes through the first retardation film 150 (FIG. 6A) and changes its polarization state along the path that rotates about the -S2 point. The first retardation film 150 is inclined The sixth polarized light P6 moves backward through the first path L1 and becomes the fifth polarized light P5 again.

Then, the fifth polarized light P5 passes through the second retardation film 160 (FIG. 6A) and the polarized state is changed along the second path L2 rotating with respect to the -S3 point, so that the seventh polarized light P7 Since the seventh polarized light P7 is positioned at the opposite side of the polarized light curtain to the eighth polarized light P8 corresponding to the second transmission axis 182 of the second polarizing plate 180 of FIG. 6A with respect to the inclined incident light, The seventh polarized light P7 is completely absorbed by the second polarizing plate 180 to prevent light leakage.

Therefore, as shown in FIG. 9, the planar alignment switching type liquid crystal display device according to the embodiment of the present invention is applicable not only to the front view angle but also to the oblique viewing angle in the diagonal direction of about 3.0 * 10 ^-5 cd / m ² black And it is possible to display perfect black without light leakage at all viewing angles.

As described above, in the planar alignment switching type liquid crystal display device according to the present invention, the first phase difference film having the same phase delay as the phase delay of the liquid crystal layer and having the optical axis perpendicular to the optical axis of the liquid crystal layer, The second retardation film having the optical axis parallel to the transmission axis of the second polarizing plate with the retardation is disposed on the liquid crystal layer so that the cell gap can be reduced and the response characteristic can be improved without restricting the retardation value of the liquid crystal layer or changing the physical properties of the liquid crystal And it is possible to prevent the light leakage in the oblique viewing angle in the diagonal direction, thereby improving the image quality.

The first and second retardation films may be formed between the second substrate and the second polarizer, and may be formed between the liquid crystal layer and the second substrate to simplify the process.

The first and second retardation films may be formed using a fringe field switching mode liquid crystal display (FFS) mode LCD device using a horizontal electric field as well as an IPS mode LCD device ).

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

120: first substrate 130: second substrate
140: liquid crystal layer 150: first retardation film
160: second retardation film 170: first polarizing plate
180: second polarizer plate

Claims (8)

First and second substrates spaced apart from each other;
A liquid crystal layer formed between the first and second substrates, the liquid crystal layer having a first phase delay and a first optical axis;
A first polarizer formed on an outer surface of the first substrate;
A first retardation film formed on an outer surface of the second substrate and having a second phase retardation equal to the first phase retardation and a second optical axis perpendicular to the first optical axis;
A second retardation film formed on the first retardation film and having a third phase retardation and a third optical axis of a half wavelength;
The second polarizing plate formed on the second retardation film
/ RTI &gt;
Wherein the sum of the total retardation of the liquid crystal layer and the first retardation film in the initial alignment state is zero.
The method according to claim 1,
Wherein the first and second phase delays are quarter wavelength (lambda / 4) or more.
3. The method of claim 2,
Wherein the first and second phase delays are less than a half wavelength (lambda / 2).
The method of claim 3,
Wherein the first transmission axis of the first polarizer and the second transmission axis of the second polarizer are perpendicular to each other.
5. The method of claim 4,
And the third optical axis is parallel to the second transmission axis.
6. The method of claim 5,
A plane parallel to the first and second substrates is defined as an x-axis and a y-axis perpendicular to each other, and a direction perpendicular to the first and second substrates is defined as a z-axis perpendicular to the x-axis and the y-axis Occation,
Wherein the first transmission axis is parallel to the x axis, the first optical axis forms an angle of 45 degrees with respect to the x axis, the second optical axis forms an angle of 135 degrees with respect to the x axis, Is parallel to the y-axis, and the second transmission axis is parallel to the y-axis.
The method according to claim 1,
Wherein the first and second retardation films are biaxial films.
The method according to claim 1,
Wherein the first optical axis is rotated by a rotation angle (?) By application of a pixel voltage,

(λ is the wavelength, Δnd _{_ A plate} is the second phase delay, Δnd _{LC _ effective} is the effective value of the first phase delay)
Of the liquid crystal display device.

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