KR101774280B1 - In-Plane Switching Mode Liquid Crystal Display Device And Method Of Driving The Same - Google Patents

Abstract

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 phase difference film formed on the outer surface of the second substrate, the phase difference film having a second phase delay equal to the first phase delay and a second optical axis perpendicular to the first optical axis; And a second polarizer formed on the retardation 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- 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.

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 the off state in which no pixel voltage is applied, 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.

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 electrode 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, there has been an increasing demand for a display device having a high response speed in order to view a moving picture without deterioration in image quality such as cross-talk. Particularly, in order to view a three- A liquid crystal display device of a planar alignment type having a wide viewing angle characteristic is required 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.

--- 식(1)

--- Equation (1)

--- 식(2)

--- (2)

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 the spring constant (twist elastic constant), d is the cell gap of the liquid crystal display device (cell gap) (that is, the thickness of the liquid crystal layer), l is the distance between the common electrode and the pixel electrode, E _{input _ eff} is the effective input field (effective input electric field), E _{th _ eff} represents the effective threshold electric field (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.

SUMMARY OF THE INVENTION The present invention has been made in order to solve the above-mentioned problems, and it is an object of the present invention to provide a liquid crystal display device having a liquid crystal display And an object of the present invention is to provide a 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.

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 phase difference film formed on the outer surface of the second substrate, the phase difference film having a second phase delay equal to the first phase delay and a second optical axis perpendicular to the first optical axis; And a second polarizer formed on the 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.

It is preferable that 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- 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.

The retardation film may be an A plate.

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, the planar alignment switching type liquid crystal display device according to the present invention has a high-speed response characteristic without restricting the phase delay value of the liquid crystal layer by the retardation film disposed on the liquid crystal layer and burdening a part of the phase delay .

Further, the present invention can improve the response speed by reducing the cell gap without changing the physical properties of the liquid crystal.

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 cross-sectional view of a planar alignment switching type liquid crystal display device according to an embodiment of the present invention.
5A and 5B 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.
6A and 6B are diagrams illustrating calculation results and experimental results on response characteristics of a planar alignment switching type liquid crystal display device according to an embodiment of the present invention, respectively.

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

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

4, 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 liquid crystal layer 140 is formed on the first substrate 120 and the retardation film 150 is formed on the second substrate 130. The first polarizer 160 and the second polarizer 150 are formed on the first substrate 120 and the second substrate 130, 170).

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. 4, 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 polarizer 160 is formed on the lower surface (outer surface) of the first substrate 120, a retardation film 150 is formed on the upper surface (outer surface) of the second substrate, and a second polarizer 170 are formed.

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

That is, the liquid crystal layer 140 and the retardation film 150 are arranged to have the same phase retardation and optical axes perpendicular to each other, and the retardation film 150 may be formed of an uniaxial A-plate have.

A plate, a phase difference 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, the retardation film has nx, (nx > ny = nz) or (nx < ny = nz).

The first and second polarizing plates 160 and 170 are arranged to have mutually perpendicular transmission axes.

Specifically, the liquid crystal layer 140 and the retardation film 150 may each have a retardation of more than 1/4 wavelength (? / 4) (? / 4? R <? / 2) which is less than half wavelength (? / 4) and half wavelength (? / 2) of the liquid crystal cell of the prior art, (d) can be reduced. Accordingly, it is possible to improve the response speed of a planar alignment switching type liquid crystal display device without deteriorating 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 phase difference 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 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.

FIGS. 5A and 5B are diagrams showing optical arrangements of an off-state and an on-state, respectively, of a planar alignment switching type liquid-crystal display device according to an embodiment of the present invention, wherein planes parallel to the first and second substrates are perpendicular axis and the y-axis, and the direction perpendicular to the first and second substrates is defined as the x-axis and the z-axis perpendicular to the y-axis.

As shown in Figs. 5A and 5B, in the planar alignment switching type liquid crystal display (110 in Fig. 4), the first and second polarizers 160 and 170 are respectively connected to first and second transmission axes 162, and 172, respectively.

That is, the first transmission axis 162 of the first polarizing plate 160 forms an angle (x-axis parallel) with respect to the x-axis and the second transmission axis 172 of the second polarizing plate 170 is an x- (Parallel to the y-axis).

In this embodiment, the first and second polarizers 160 and 170 are formed so that the first and second transmission axes 162 and 172 are parallel to the x axis and the y axis, respectively. In another embodiment, The first and second polarizers 160 and 170 may be formed such that the transmission axes 162 and 172 rotate in a state of maintaining a perpendicular relation to 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 phase difference film 150 has a second phase retardation R2 equal to the first phase retardation R1 of the liquid crystal layer 140 (R1 = R2) and the first optical axis 142 of the liquid crystal layer 140, And a second optical axis 152 perpendicular to the optical axis.

For example, the retardation film 150 may be 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 is at an angle of 45 degrees with respect to the x axis and the second optical axis 152 is oriented at 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 at 135 degrees relative to the x axis, The retardation film 150 may be formed to have an angle other than 0 degrees.

As shown in FIG. 5A, when the pixel voltage Vp is not applied to the pixel electrode (127 in FIG. 4) 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. 4) passes through the first polarizing plate 160, becomes a first linearly polarized light parallel to the first transmission axis 162, The linearly polarized light passes through the liquid crystal layer 140 and rotates to become a second linearly polarized light parallel to an arbitrary direction.

At this time, the polarization state of the second linearly polarized light may be determined by the angle between the first transmission axis 162 and the first optical axis 142 and the first phase delay R1.

The second linearly polarized light passes through the retardation film 150 and is changed in polarized state again so that the first phase retardation R1 of the liquid crystal layer 140 and the second phase retardation R2 of the retardation film 150 are equal to each other The first optical axis 142 of the liquid crystal layer 140 and the second optical axis 150 of the retardation film 150 are perpendicular to each other so that the retardation film 150 operates in an optically opposite manner to the liquid crystal layer 140. [

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

As a result, the first linearly polarized light parallel to the first transmission axis 162 passing through the retardation film 150 is transmitted through the second polarizer 170 having the second transmission axis 172 perpendicular to the first transmission axis 162, And the planar alignment switching type liquid crystal display device 110 displays black.

5B, 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].

At this time, since the first transmission axis 162 of the first polarizer 160 and the first optical axis 142 of the liquid crystal layer 140 form the angle (45 占 + σ) of the sum of 45 占 and the rotation angle? The first linearly polarized light having passed through the first polarizing plate 160 is delayed by the effective liquid crystal phase delay _{DELTAnd LC_effective} corresponding to the first phase retardation R1 of the liquid crystal layer 140 while passing through the liquid crystal layer 140 (Not shown) from the first transmissive axis 162. The third linearly polarized light is rotated by the first polarization rotation angle (not shown)

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 third linearly polarized light passes through the retardation film 150 and changes its polarization state again so that the angle formed by the first optical axis 142 of the liquid crystal layer 140 and the second optical axis 152 of the retardation film 150 The phase corresponding to the second phase retardation R2 of the retardation film 150 is delayed from the third linearly polarized light by the second polarization rotation angle (not shown) according to the angle of 135 占 - (45 占 +?) It becomes linear light.

In order for the plane-aligned switching type liquid crystal display 110 to display white, the fourth linearly polarized light must pass through the second polarizing plate 170. To this end, the fourth linearly polarized light is transmitted through the second transmission axis 172 ).

Therefore, when the sum of the first polarization rotation angle from the first linearly polarized light to the third linearly polarized light parallel to the x-axis and the second polarization rotation angle from the third linearly polarized light to the fourth linearly polarized light is 90 degrees, full white can be displayed.

It is necessary to rotate the first optical axis 142 of the liquid crystal layer 140 by the rotation angle σ so that the sum of the first and second polarized rotation angles becomes 90 °, The rotation of the optical axis 142 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).

--- 식(3)

--- (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 retardation film 150, _{Δnd LC_effective} is 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 The liquid crystal display device 110 can display white by applying a pixel voltage Vp capable of rotating the first optical axis 142 of the liquid crystal display device 110 by 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 to apply a pixel voltage Vp to the liquid crystal display device 110 so that the liquid crystal display device 110 displays white.

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 liquid crystal display device 110 may display white by applying a pixel voltage Vp capable of rotating the liquid crystal display panel 142 by 45 degrees.

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. 152 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.

6A and 6B 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.

6A and 6B, the planar alignment switching type liquid crystal display device 110 according to the embodiment of the present invention uses a liquid crystal having the same physical properties as the conventional planar alignment switching type liquid crystal display device, The response speed can be improved by reducing 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 the fall time (t _f ) of Figs. 5A and 5B, respectively.

[Table 1]

[Table 2]

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.

As described above, in the planar alignment switching type liquid crystal display device according to the present invention, the retardation film having the same phase delay as the phase retardation of the liquid crystal layer and having the optical axis perpendicular to the optical axis of the liquid crystal layer is disposed above the liquid crystal layer, The cell gap can be reduced and the response characteristic can be improved without changing the physical retardation of the liquid crystal or the physical property of the liquid crystal.

Such a retardation film can be formed between the second substrate and the second polarizing plate as well as between the liquid crystal layer and the second substrate to simplify the process.

In addition, the retardation film can be applied not only to a planar alignment switching liquid crystal display (IPS mode LCD device) but also to a fringe field switching mode liquid crystal display device (FFS mode LCD device) using a horizontal electric field have.

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: retardation film
160: first polarizer 170: second polarizer

Claims (7)

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 phase difference film formed on the outer surface of the second substrate, the phase difference film having a second phase delay equal to the first phase delay and a second optical axis perpendicular to the first optical axis;
And a second polarizer formed on the phase difference film,
/ RTI &gt;
Wherein the first optical axis is rotated by a rotation angle (?) By application of a pixel voltage,

(? is the wavelength _{,? end A_plate} is the second phase delay _{,? end LC_effective} is the effective value of the first phase delay)
Of the liquid crystal display device.
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,
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 is at an angle of 45 degrees with respect to the x axis, the second optical axis is at an angle of 135 degrees with respect to the x axis, Axis is parallel to the y-axis.
The method according to claim 1,
Wherein the retardation film is an A plate.
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