KR100956349B1 - Liquid crystal display - Google Patents

Abstract

A first substrate, a pixel electrode formed on the first substrate, a second substrate facing the first substrate, a common electrode formed on the second substrate, and a liquid crystal injected between the first substrate and the second substrate The liquid crystal has a liquid crystal display device in which Δε is -3.0 to -3.6, K11 and K33 are 13x10 ^-12 N or less, and Δn is 0.07 to 0.1.

DCC, response speed, liquid crystal, white voltage, black voltage

Description

Liquid crystal display {LIQUID CRYSTAL DISPLAY}

1 is a cross-sectional view illustrating a liquid crystal display device according to an exemplary embodiment of the present invention, and illustrates an arrangement of liquid crystals in a black mode.

FIG. 2 is a cross-sectional view of a liquid crystal display according to an exemplary embodiment of the present invention and illustrates an arrangement of liquid crystals in a white mode.

3 is a graph illustrating a change in light transmittance with time in a liquid crystal display according to an exemplary embodiment of the present invention, and illustrates a rise time and a fall time.

4 is a graph illustrating a change in light transmittance according to a change in driving voltage applied to a liquid crystal display according to an exemplary embodiment of the present invention.

FIG. 5 is a graph showing that the rise time is delayed due to the dynamic capacitance, and is a graph showing a change in light transmittance generated in the liquid crystal display by applying a driving voltage, for example, a white voltage.

6 is a graph illustrating a phenomenon in which a response time increases as a driving voltage increases in a conventional liquid crystal display.

FIG. 7 is a graph illustrating a change in response time as the driving voltage increases in the liquid crystal display according to the exemplary embodiment. FIG.

<Description of the symbols for the main parts of the drawings>                 

3; Liquid crystal 110; First substrate

190; Pixel electrode 210; Second substrate

220; Black matrix 270; Reference electrode

Tr; Rise time Tf; Fall time

The present invention relates to a liquid crystal display, and more particularly, to a liquid crystal display using a normally black vertical aligned mode.

The liquid crystal display is one of the most widely used flat panel display devices. The liquid crystal display includes two display panels on which a field generating electrode is formed and a liquid crystal layer interposed therebetween. It is a display device which controls the transmittance | permeability of the light which passes through a liquid crystal layer by rearranging.

Such a liquid crystal display device is used not only for displaying still images but also for displaying moving images. However, in moving image display, there is a problem in which afterimages appear remarkably and the moving image appears to pull out the tail. This afterimage problem occurs because the liquid crystal does not respond quickly to changes in the electric field as a polymer material, and after a certain time, the arrangement of the liquid crystal molecules becomes stable. This delay in response time has a problem in that a screen drag phenomenon occurs after leaving an afterimage on the screen.                         

Since the monitor requires a high brightness of 250 cd / m ² or higher, the liquid crystal display of the PVA (Patterned vertical aligned) mode, which is disadvantageous in terms of luminance, uses a method of using six or more lamps or increasing the white voltage to improve luminance. I use it. However, if the white voltage is raised above a predetermined value, the response speed may be delayed.

 To prevent this, we tried to improve the response speed by applying an overdriving method (DCC), which is an overdriving method, at the white voltage corresponding to the on-off gradation. Therefore, the response speed is delayed by the backflow phenomenon, so there is no effect of improving the response speed.

An object of the present invention is to provide a liquid crystal display device in which a response speed is not delayed even at a high driving voltage.

The liquid crystal display device according to the present invention includes a first substrate, a pixel electrode formed on the first substrate, a second substrate facing the first substrate, a common electrode formed on the second substrate, and the first substrate. And a liquid crystal injected between the second substrate and the second substrate, wherein the liquid crystal preferably has a Δε of -3.0 to -3.6, a K11 and a K33 of 13 x 10 ^-12 N or less, and a Δn of 0.07 to 0.1.

In addition, the liquid crystal preferably has negative dielectric anisotropy, and the liquid crystal has a long axis perpendicular to the first and second substrates.

In addition, it is preferable to apply the DCC by applying an over-driving voltage to an on-off gray voltage applied between the pixel electrode and the common electrode.

The black voltage applied between the pixel electrode and the common electrode is 0.5 to 1.5V, and the white voltage applied between the pixel electrode and the common electrode is 5.0 to 6.0V.

In addition, the white voltage is about 1.5 to 3 times the saturation voltage, the over-driving voltage applied to the on-off gray voltage is preferably 1.1 times the white voltage.

In addition, the black voltage applied between the pixel electrode and the common electrode is 1/2 times to 1/15 times the threshold voltage, and the white voltage applied between the pixel electrode and the common electrode is about 1.5 times to 3 times the saturation voltage. Is preferably.

In addition, the difference between the white voltage and the black voltage is preferably 4.0V or more.

In addition, it is preferable that the cell gap, which is an interval between the first substrate and the second substrate, is 3.5 µm to 4.0 µm, and the rotational viscosity of the liquid crystal is 100 to 135 mPa · s.

Then, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily implement the present invention. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention.                     

In the drawings, the thickness of layers, films, panels, regions, etc., are exaggerated for clarity. Like parts are designated by like reference numerals throughout the specification. When a part of a layer, film, region, plate, etc. is said to be "on" another part, this includes not only the other part being "right over" but also another part in the middle. On the contrary, when a part is "just above" another part, there is no other part in the middle.

A liquid crystal display according to an exemplary embodiment of the present invention will now be described in detail with reference to the accompanying drawings.

1 is a cross-sectional view showing a liquid crystal display device according to an embodiment of the present invention, showing a liquid crystal display in black mode, and FIG. 2 is a cross-sectional view showing a liquid crystal display device according to an embodiment of the present invention. Is a diagram showing an arrangement of liquid crystals in a white mode.

As shown in FIG. 1, the liquid crystal display according to the exemplary embodiment includes a first substrate 110 and a pixel electrode 190 formed on an upper surface of the first substrate 110. The second substrate 210 is positioned above the first substrate 110 on which the pixel electrode 190 is formed to be spaced a predetermined distance apart. The black matrix 220, the color filter 230, the overcoat layer 250, and the common electrode 270 are formed on the bottom surface of the second substrate 210. The liquid crystal 3 is injected between the first substrate 110 and the second substrate 210. As shown in FIG. 1, in the black mode in which the driving voltage is not applied, the liquid crystal 3 is aligned so that the long axis of the liquid crystal 3 is perpendicular to the horizontal planes of the first and second substrates.                     

As shown in FIG. 2, in the white mode in which the driving voltage is applied, an electric field E is formed between the pixel electrode 190 and the common electrode 270. In this case, the liquid crystal 3 is aligned so that the direction of the electric field E and the long axis of the liquid crystal 3 are perpendicular to each other. As described above, the arrangement of the liquid crystals 3 varies according to the change of the electric field, and the liquid crystal display displays various images.

3 is a view illustrating a change in light transmittance that occurs in a liquid crystal display as a driving voltage is turned on and off in a normally black VA mode, according to time.

In the normally black VA mode, the light transmittance is small in the off state where the driving voltage is not applied, and the light transmittance is increased in the on state where the driving voltage is applied and the electric field affects the liquid crystal 3.

Accordingly, as shown in FIG. 3, in the normally black VA mode, the response time Tre of the liquid crystal display is changed by the driving voltage (ie, the on-off gray voltage) and the pixel electrode 190 and the common electrode 270. Rising time (Tr), which is the time it takes for the liquid crystal 3 to align vertically with the direction of the electric field and reach a stable state when the electric field is caught between the electric field and the time required for the liquid crystal to return to its original stable state. It is defined as the sum of falling time, Tf, which is time. In other words, in the normally black VA mode, the rise time Tr is a time for the liquid crystal to be placed in the electric field by the driving voltage, and the liquid crystal is arranged in the electric field direction to reach a metastable state, which takes the light transmittance from 10% to 90%. The fall time Tf is the time taken for the electric field to disappear and return to the original stable state by the driving voltage off, which is the time taken for the light transmittance to change from 90% to 10%.

Rise time and fall time are determined by the following formula.

Here, η is rotational viscosity, d is a cell gap which is a gap between the first substrate and the second substrate, ε _o is the dielectric constant in the vacuum state of the liquid crystal, and Δε is the dielectric anisotropy of the liquid crystal. to be. The dielectric anisotropy of the liquid crystal has a positive (+) or negative (-) value, and in the normally black mode, a liquid crystal having a dielectric constant anisotropy of the liquid crystal having a negative value is used. When the dielectric anisotropy of the liquid crystal has a positive value, when a voltage equal to or greater than a threshold voltage is applied, the long axis of the liquid crystal molecules is aligned perpendicular to the direction of the electric field.

In addition, V is a driving voltage, and V _TH represents a threshold voltage of the liquid crystal. The threshold voltage (V _TH ) refers to the voltage at the point where the voltage is first applied and the change in light transmittance starts to occur in earnest. If the threshold voltage is large, the voltage to be applied to the liquid crystal is also increased, which may increase power consumption.

The threshold voltage and elastic constant ratio K are expressed by the following equation.                     

Here, K is an elastic constant ratio, and K ₁₁ , K ₂₂ , K ₃₃ are elastic constants and coefficients related to elastic restoring forces for spray, twist and bend deformation, respectively. When the liquid crystal is deformed due to external force acting, there are three deformations: a spray in which the liquid crystal is unfolded, a twist in which the liquid crystal is twisted, and a bend in which the liquid crystal is bent. And the elastic restoring force for each of these deformations.

As shown in the above equation, the rise time Tr is proportional to the rotational viscosity and the square of the cell gap, and is inversely proportional to the difference between the square of the applied voltage and the square of the threshold voltage, and the dielectric constant and dielectric anisotropy of the liquid crystal. Since the threshold voltage is proportional to the square root of the elastic constant ratio K, the rise time Tr is also related to the elastic constant ratio K.

In addition, the fall time Tf is proportional to the rotational viscosity and the square of the cell gap, and is inversely proportional to the square of the threshold voltage, the dielectric constant and dielectric constant anisotropy of the liquid crystal.

Meanwhile, FIG. 4 is a graph showing a change in light transmittance according to a change in driving voltage.

As shown in FIG. 4, the threshold voltage (V _TH ) of the liquid crystal refers to the voltage at the point where the change in the light transmittance starts to occur in earnest. The saturation voltage (Vs) of the liquid crystal is used to increase the light transmittance even when the voltage is increased. The voltage at the point where no change occurs.

In addition, the white voltage Vw refers to a voltage applied to the liquid crystal display to display the brightest color among the driving voltages, for example, a voltage of 256 gray levels, and the black voltage Vb refers to the darkest color among the driving voltages. Refers to a voltage applied to indicate, for example, a voltage of zero gray scale.

In the normally black VA mode, the black voltage Vb is the smallest voltage among the driving voltages, and the white voltage Vw is the largest voltage among the driving voltages.

The liquid crystal used in the liquid crystal display device of the conventional normally black PVA mode has, for example, rotational viscosity of 135 mPa · s, K11 and K33 of 13.6 × 10 ^-12 N and 14.7 × 10 ^-12 N, respectively, and the long-axis dielectric constant of the liquid crystal molecules. Is ε ∥ 3.6, ε 인 which is the uniaxial dielectric constant of the liquid crystal molecules is 7.4, Δ ε which represents the dielectric anisotropy of the liquid crystal is -3.8, ne which is the major axis refractive index of the liquid crystal molecules is 1.5587, no is 1.4764, and Δn indicating birefringence is 0.0822.

In the liquid crystal display of the conventional normally black PVA mode which uses such a liquid crystal, response speed is generally slow. This is because the rise time is delayed by the dynamic capacitance in the domain divided by the pixel electrode having the cutout and the common electrode.                     

When a white voltage corresponding to 256 gray levels is applied to a specific pixel of the liquid crystal display of the normally black PVA mode, the liquid crystal array changes from vertical to horizontal. Due to the dynamic capacitance caused by the liquid crystal array change, the white voltage applied to a specific pixel of the liquid crystal display during the first frame is about 1V lower than the white voltage actually applied. For example, even if an actual white voltage of 5V is applied, the white voltage applied during the first frame becomes 4V.

Even if voltage is applied to the pixel electrode and the common electrode to be 5V while the liquid crystals are arranged vertically, when the arrangement of the liquid crystals is changed by an electric field, the capacitance of the liquid crystal increases due to the dielectric anisotropy of the liquid crystals. Will drop the voltage. That is, the pixel electrode is placed in a floating state by turning off the thin film transistor after 5V is applied. Therefore, the charge amount Q charged in the pixel electrode is kept constant. However, as the alignment of the liquid crystals is changed, the dielectric constant of the liquid crystals increases, thereby increasing the liquid crystal capacitance between the pixel electrode and the common electrode. As a result, at Q = CV, Q is constant and C increases, so that V decreases. Such characteristics of the liquid crystal capacitor are referred to as dynamic capacitance.

FIG. 5 is a diagram illustrating that the rise time is delayed due to the dynamic capacitance, and illustrates a change in light transmittance generated by a driving voltage, for example, a white voltage, to the liquid crystal display with time.

As shown in FIG. 5, in the conventional liquid crystal display, since the light transmittance reaches 90% only as the second frame, the rise time, which is the time taken for the light transmittance to change from 10% to 90%, is consequently delayed.

As such, dynamic capacitance capture (DCC) is applied to improve the delay time of the rise time when the driving voltage is applied.

The DCC is a method for improving the response speed of the liquid crystal display, and the DCC function will be described in detail below.

The DCC compares the grayscale value of the previous frame and the grayscale value of the current frame for any pixel so that a value larger than the difference is added to the grayscale value of the previous frame. In general, the duration of one frame is 16.7 msec. When voltage is applied across the liquid crystal material in any pixel, it takes time for the liquid crystal material to respond due to the dynamic capacitance. Therefore, a time delay is inevitable for the intended gray scale value to be expressed. The DCC function is a technique to minimize this time delay. For example, when a gray value in a previous frame is '118' for a pixel and a gray value in a current frame is '128', a value larger than '10', which is a difference between the two gray levels, is referred to as a compensation value. Gradation value, for example, '135' is converted as the gradation value of the current frame.

Therefore, the white voltage applied to the first frame due to the dynamic capacity is actually applied by applying an overdriving voltage, which is a voltage about 1V greater than the white voltage to be actually applied by applying DCC, to the on-off gray voltage. Even if it is about 1V lower than the white voltage to be applied, the light transmittance becomes 90% during the first frame to prevent the rise time from being delayed.

However, in this case, a back flow phenomenon occurs because the driving voltage increases. The backflow phenomenon occurs as the driving voltage increases, and at a certain point in the rise time when the driving voltage is applied and the vertically aligned liquid crystal is gradually oriented horizontally in the electric field direction, the liquid crystal is instantaneously oriented at a predetermined angle in the opposite direction. After moving, the liquid crystal is tilted back to its original direction. In this case, the rise time is delayed because of instantaneous movement of the liquid crystal at a predetermined angle in the direction opposite to the direction in which the liquid crystal is to be aligned.

Due to such a backflow phenomenon, a response speed is delayed at a high driving voltage even when DCC is applied in a liquid crystal display device having a liquid crystal having a conventional physical property, and thus there is no effect of improving the response speed.

Therefore, when the driving voltage applied for improving the response speed is lowered to 4.5V or less under the conditions of the liquid crystal as described above, and the DCC is applied, the response time is improved from 25ms to 16ms. Is lowered.

6 is a graph illustrating a phenomenon in which a response time increases as a driving voltage increases in a conventional liquid crystal display.

In order to solve this problem, an embodiment of the present invention introduces a liquid crystal having physical properties such that the response speed is not delayed even at a high driving voltage.

That is, in the liquid crystal display according to the exemplary embodiment of the present invention, the response speed is improved by using a liquid crystal having Δε of -3.0 to -3.6, K11 and K33 of 13 × 10 ^-12 N or less, and Δn of 0.07 to 0.1. Let's do it.

In this case, as shown in FIG. 7, even when the driving voltage is increased, the response time does not increase.

In addition, the response speed may be further improved by applying the DCC by applying an overdriving voltage to the liquid crystal display having the liquid crystal properties as described above.

At this time, the black voltage is preferably 0.5 to 1.5V, and the white voltage is 5.0 to 6.0V. The white voltage is about 1.5 to 3 times the saturation voltage, and the overdriving voltage is preferably 1.1 times the white voltage.

And it is preferable that the cell gap d which is the space | interval between a 1st board | substrate and a 2nd board | substrate is 3.5 micrometers-4.0 micrometers, and the rotational viscosity ((eta)) of a liquid crystal is 100-135 mPa * s.

The liquid crystal display may be driven under the following conditions in order to improve not only the response speed but also the luminance of the liquid crystal display having the liquid crystal properties as described above.

The black voltage Vb is set at about 1/2 to 1/15 times the threshold voltage, and the white voltage Vw is set at about 1.5 to 3 times the saturation voltage, so that the white voltage Vw and the black voltage Vb are set. Make sure that the difference with is at least 4.0 V. In this way, when the difference between the black voltage Vb and the white voltage Vw is increased, the luminance is also improved to 250 cd / m ² or more.

Also in this case, it is preferable that the cell gap which is an interval between a 1st board | substrate and a 2nd board | substrate is 3.5 micrometers-4.0 micrometers, and the rotational viscosity of a liquid crystal is 100-135 mPa * s.

Although the present invention has been described with reference to one embodiment shown in the accompanying drawings, this is merely exemplary, and it will be understood by those skilled in the art that various modifications and equivalent other embodiments are possible. Could be. Accordingly, the true scope of protection of the invention should be defined only by the appended claims.

The liquid crystal display according to the present invention has an advantage in that luminance and response characteristics can be improved by using a liquid crystal having physical properties in which the response speed is not delayed even at a high driving voltage.

Claims (8)

First substrate, A pixel electrode formed on the first substrate, A second substrate facing the first substrate, A common electrode formed on the second substrate, A liquid crystal injected between the first substrate and the second substrate, The liquid crystal display device has a liquid crystal display of Δε of -3.0 to -3.6, K11 and K33 of 13 × 10 ^-12 N or less, and Δn of 0.07 to 0.1. In claim 1, And the liquid crystal has negative dielectric anisotropy and the long axis of the liquid crystal is vertically oriented with respect to the first and second substrates. In claim 1, And applying a DCC by applying an over-driving voltage to an on-off gray voltage applied between the pixel electrode and the common electrode. 4. The method of claim 3, The black voltage applied between the pixel electrode and the common electrode is 0.5 to 1.5V, and the white voltage applied between the pixel electrode and the common electrode is 5.0 to 6.0V. In claim 4, The white voltage is about 1.5 to 3 times the saturation voltage, and the over-driving voltage applied to the on-off gray voltage is 1.1 times the white voltage. In claim 1, The black voltage applied between the pixel electrode and the common electrode is 1/2 times to 1/15 times the threshold voltage, and the white voltage applied between the pixel electrode and the common electrode is about 1.5 times to 3 times the saturation voltage. Display device. In claim 6, And a difference between the white voltage and the black voltage is 4.0V or more. In claim 4 or 6, A cell gap, which is an interval between the first substrate and the second substrate, is 3.5 μm to 4.0 μm, and the rotational viscosity of the liquid crystal is 100 to 135 mPa · s.

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