KR20030077813A - a liquid crystal display using a compensation film - Google Patents

Abstract

PURPOSE: A liquid crystal display device using a compensating film is provided to secure a uniform luminance and high contact rate simultaneously with minimizing the gray level inversion. CONSTITUTION: A liquid crystal display device using a compensating film includes twisted Nematic liquid crystal cells(300), a pair of polarization plates, a light source, and a cholesteric compensating film. The cholesteric compensating film is attached to a front polarization plate(22) facing the liquid crystal cells, and includes a delay layer(132) for delaying light linearly polarized through the front polarization plate by λ/4 to convert the linear polarization to circular polarization, and a cholesteric liquid crystal layer(131) formed of liquid crystal molecules aligned in the spiral structure. The cholesteric liquid crystal layer reflects incident light forming a certain angle with respect to the liquid crystal align direction but transmits the vertically incident light. The liquid crystal layer reflects the light in the wavelength range corresponding to a pitch range of the liquid crystal molecule alignment.

Description

Liquid crystal display using a compensation film

The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device using a compensation film.

In general, a liquid crystal display device has a structure in which a liquid crystal is injected between two substrates on which an electrode is formed, and a light transmission amount is controlled by adjusting the intensity of a voltage applied to the electrode.

In this case, the liquid crystal has birefringence with different refractive indices in the long axis direction and short axis direction of the molecules. The birefringence causes a difference in the refractive index of light depending on the position at which the liquid crystal display device is viewed, so that the linearly polarized light passes through the liquid crystal. However, when the polarization state changes, phase difference occurs, and the amount of light and color characteristics when viewed from a position away from the front are different from those seen from the front. Accordingly, in the liquid crystal display using the liquid crystal material, changes in contrast ratio, color shift, and gray inversion occur according to the viewing angle. In particular, these problems include the arrangement of liquid crystal molecules in a state in which no voltage is applied, the spiral axis of the liquid crystal molecules being parallel to the substrate, and the long axis of the liquid crystal molecules in the state in which the voltage is applied along the direction of the electric field. It is severe in a liquid crystal display having a twisted nematic structure arranged perpendicular to the substrate.

In order to solve this problem, a technique of a twisted nematic liquid crystal display using a phase difference compensation film has been developed as a method of compensating for phase difference generated in a liquid crystal cell. This solves the viewing angle problem by compensating for the change of the phase of light inside the liquid crystal in the opposite direction in the retardation film.

However, even if the viewing angle characteristic is improved by using the phase difference compensation film in this manner, the problem of gray level inversion according to the change of the vertical viewing angle still remains, and there is a problem in that the luminance distribution is uneven with respect to the vertical viewing angle. In addition, even when the compensation film is applied, when the liquid crystal molecules are oriented in two regions in order to improve the viewing angle, only one region in which the phase difference is compensated by the compensation film is compensated. The problem that the reversal is further lowered.

An object of the present invention is to provide a twisted nematic liquid crystal display device capable of minimizing gray scale inversion while ensuring uniform luminance and high contrast ratio.

1 is a cross-sectional view schematically illustrating a structure of a liquid crystal display according to an exemplary embodiment of the present invention.

FIG. 2 is a diagram illustrating an orientation direction of a cholesteric liquid crystal in a liquid crystal display according to an exemplary embodiment of the present invention.

3 is a view showing in detail the reflective action of the cholesteric compensation film in the liquid crystal display according to the embodiment of the present invention,

4A and 4B are graphs showing viewing angles and gray level inversion characteristics of a liquid crystal display including a cholesteric liquid crystal compensation film having a reflectance of 30%.

5A and 5B are graphs illustrating viewing angles and gray level inversion characteristics of a liquid crystal display including a cholesteric liquid crystal compensation film having a reflectance of 35%.

6A and 6B are graphs illustrating viewing angles and gray level inversion characteristics of a liquid crystal display including a cholesteric liquid crystal compensation film having a reflectance of 40%.

7A and 7B are graphs illustrating viewing angles and gray level inversion characteristics of a liquid crystal display including a cholesteric liquid crystal compensation film having a reflectance of 50%.

In order to solve this problem, in the present invention, a liquid crystal panel in a twisted nematic arrangement is used, and a compensation film made of a cholesteric liquid crystal is disposed on the entire surface outside the polarizing plate.

Here, the compensation film preferably includes a retardation layer for delaying linearly polarized light passing through the polarizing plate and converting it into circularly polarized light.

The liquid crystal molecules of the cholesteric liquid crystal layer are arranged at a pitch in the range of 400-700 nm in the upper and lower directions with respect to the viewing angle, and the reflectance of the cholesteric compensation film is preferably in the range of 10-90%.

Next, embodiments of the liquid crystal display according to the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily implement the present invention.

1 is a block diagram illustrating a configuration of a liquid crystal display according to an exemplary embodiment of the present invention.

As shown in FIG. 1, the structure of the liquid crystal display according to the exemplary embodiment of the present invention is injected between two substrates 100 and 200 facing each other and two substrates 100 and 200 and has a positive dielectric anisotropy. A liquid crystal cell 300 is formed around the liquid crystal material layer 3 and the two substrates 100 and 200 and includes a sealing material 310 for sealing the liquid crystal material layer 3. In addition, the rear polarizer 12 is attached to the rear of the liquid crystal cell 300, the front polarizer 22 is attached to the opposite side, and the liquid crystal panel 300 is disposed between the liquid crystal cell 300 and the rear polarizer 12. The light source 400 for supplying the light source 400 is mounted, and a cholesteric compensation film 13 is attached to the front polarizer 22 facing the liquid crystal cell 300. In this case, the cholesteric compensation film 13 is a cholester made of a retardation layer 132 for delaying linearly polarized light passing through the front polarizer 22 and converting it into circularly polarized light and a liquid crystal molecule arranged in a spiral structure. And a liquid crystal layer 131. Here, the cholesteric liquid crystal layer 131 has a property of reflecting light incident at an arbitrary angle with respect to the arrangement direction of the liquid crystal molecules and transmitting light incident vertically, corresponding to a range of pitches in which the liquid crystal molecules are arranged. Reflect only light in the wavelength range. At this time, the liquid crystal molecules of the cholesteric liquid crystal layer 131 has an upper side of the cholesteric liquid crystal layer 131 with respect to the viewing angle at a pitch in the range of 400-700 nm in order to reflect red, green, and blue light in the upward and downward directions with respect to the viewing angle. And the lower direction. This will be described in detail later with reference to the drawings.

In the liquid crystal cell 300 for a liquid crystal display according to the present invention, the lower substrate 100 has a gate line including a gate line for transmitting a scan signal and a gate electrode connected to the gate line. Here, the gate wiring may include a sustain electrode receiving a voltage such as a common electrode voltage input to the common electrode of the upper substrate from the outside. In addition, the upper substrate 200 includes a data line that crosses the gate line and transmits an image signal, a source electrode connected to the data line, and a drain electrode which is separated from the source electrode and located opposite to the source electrode with respect to the gate electrode. An included data wiring is formed. In addition, each pixel transmits an image signal from the data line through the thin film transistor and the thin film transistor electrically connected to the gate wiring and the data wiring, and a transparent conductive material such as indium tin oxide (ITO) or indium zinc oxide (IZO). Alternatively, a pixel electrode made of an opaque conductive material having a high reflectance is formed.

On the other hand, the upper substrate 200 facing the lower substrate 100 has red, green, and blue color filters sequentially arranged and a common electrode facing the pixel electrode.

Here, the liquid crystal molecules of the liquid crystal cell 300 are in a spirally twisted state until the long axes of the liquid crystal molecules are in parallel with the substrates 100 and 200 while the voltages are not applied to the different substrates 100 and 200. Although it has a twisted nematic structure and is not shown in the drawing, since the liquid crystal molecules are positive in a state where voltage is applied, the long axes of the liquid crystal molecules are arranged perpendicular to the substrates 100 and 200 along the direction of the electric field.

At this time, when no voltage is applied to the liquid crystal molecules, the light emitted from the light source 400 passes through the rear polarizer 12 attached to the lower substrate 100, and the polarized light of the polarized light passes through the liquid crystal layer 3. As it passes, it rotates along the twist of the liquid crystal director. The polarization may be rotated by 90 ° by adjusting the dielectric anisotropy of the liquid crystal molecules, the interval between the two substrates 100 and 200, and the pitch of the liquid crystal molecules 3. In this case, if the transmission axes of the two polarizers 12 and 22 are arranged in parallel with each other, the polarized light is blocked by the front polarizer 22 attached to the upper substrate 200 to realize a black state. If the transmission axes of the two polarizing plates 12 and 22 are arranged to be perpendicular to each other, the light passing through the polarizing plate 12 of the lower substrate 100 passes through the polarizing plate 22 of the upper substrate 200 to be in a white state. Of course, when voltage is applied to the liquid crystal molecules, light passes through the rear polarizer 12 attached to the lower substrate 100 and passes through the liquid crystal layer 3 without changing the polarization direction. Here, if the transmission axes of the two polarizing plates 12 and 22 are parallel, the light passes through the polarizing plate 22 attached to the upper substrate 200 to realize a white state. If the transmission axes of the two polarizing plates 12 and 22 are orthogonal to each other, the light passing through the rear polarizing plate 12 of the lower substrate 100 is blocked by the polarizing plate 22 of the upper substrate 200 and becomes dark.

Subsequently, the linearly polarized light passing through the front polarizing plate 22 is converted into circularly polarized light while passing through the retardation layer 132 of the cholesteric compensation film 13, and the light passing through the cholesteric liquid crystal layer 131. The light incident at an arbitrary angle with respect to the arrangement direction of the liquid crystal molecules reflects light in a wavelength band corresponding to 400-700 nm, which is a range of pitches where the liquid crystal molecules are arranged, and is incident light perpendicularly to the arrangement direction of the liquid crystal molecules. Transmits. In this case, the liquid crystal molecules of the cholesteric liquid crystal layer 131 are arranged in the upper and lower directions with respect to the viewing angle, so that a part of the light incident downward is reflected upward and a part of the light incident upward is reflected downward. Therefore, uniform luminance and high contrast ratio can be obtained at upper and lower viewing angles, which will be described in detail with reference to the accompanying drawings.

2 is a diagram illustrating an orientation direction of a cholesteric liquid crystal in a liquid crystal display according to an exemplary embodiment of the present invention, and FIG. 3 is a cholesteric compensation in the liquid crystal display according to the exemplary embodiment of the present invention. It is a figure which shows the reflection effect of a film specifically.

As shown in FIG. 2, the liquid crystal molecules 130 of the cholesteric liquid crystal layer 131 of the cholesteric compensation film 13 in the liquid crystal display according to the exemplary embodiment of the present invention are formed of red, green, and blue pixels. They are arranged in a spiral structure in the upper and lower directions with respect to the viewing angle, respectively.

In this case, as shown in FIG. 3, light incident perpendicularly to the arrangement direction of the cholesteric liquid crystal molecules passes through the delay layer 132 and passes through the cholesteric liquid crystal layer 131. However, a part of the light incident on the cholesteric liquid crystal layer 131 at arbitrary angles (a, b) with respect to the arrangement direction of the cholesteric liquid crystal molecules passes, and the pitch of the cholesteric liquid crystal molecules (400-700 nm) Light in the wavelength band corresponding to the reflection is reflected. At this time, a part of the light incident to the upper side is reflected to the lower side, a part of the light incident to the lower side is reflected to the upper side, and this reflecting action occurs uniformly in the red, green and blue pixels. Therefore, uniform luminance can be obtained at the upper side and the lower side of the viewing angle, and a high contrast ratio can be ensured at the wide viewing angle.

Here, the pitch of the cholesteric liquid crystal is aligned so as to be uniformly arranged in the range of 400-700 nm, the reflectivity of the cholesteric compensation film 13 is preferably in the range of 10-90%, in order to control the reflectance The amount of the lick liquid crystal may be adjusted or the thickness of the cholesteric liquid crystal layer 131 may be adjusted.

Next, through the optical simulation, the viewing angle at which gray level inversion occurs with respect to the reflectance of the cholesteric liquid crystal layer was measured.

Experimental Example

In the experimental example according to the present invention, the liquid crystal panel is in the twisted nematic mode as described in the embodiment, and the luminance of the seven gray levels is changed while changing the reflectance of the cholesteric liquid crystal layer to 30%, 35%, 40% and 50%. The gray scale reversal characteristic was measured according to the change of viewing angle.

4A and 4B are graphs showing viewing angles and gray level inversion characteristics of a liquid crystal display including a cholesteric liquid crystal compensation film having a reflectance of 30%, and FIGS. 5A and 5B are cholesters having a reflectance of 35%. 5A and 6B are graphs showing viewing angles and gray level inversion characteristics of a liquid crystal display including a liquid crystal compensation film, and FIGS. 6A and 6B are viewing angles and gray levels of a liquid crystal display including a cholesteric liquid crystal compensation film having a reflectance of 40%. 7A and 7B are graphs showing viewing angles and gray level inversion characteristics of a liquid crystal display including a cholesteric liquid crystal compensation film having a reflectance of 50%. Here, the horizontal axis of the graph is a viewing angle, and the vertical axis is luminance.

In general, when the cholesteric compensation film according to the present invention is not used, gradation inversion occurs in the range of 25-35 °, and the upper and lower luminances are irregular, and the decrease or increase of the luminance appears sharply. As shown in FIG. 7B, when using the cholesteric compensation film according to the present invention, the viewing angle at which grayscale inversion occurs was well measured in the range of 45 to 75 °, and the luminance was symmetrically with respect to the upper and lower viewing angles. It was measured uniformly and the change in brightness was also measured gently. At this time, in the upper viewing angle, the gray scale inversion was measured in the range of 45 to 65 degrees and the lower viewing angle was in the range of 65 to 75 degrees, and the gray scale inversion occurred when the reflectance of the cholesteric compensation film was 30%. The upper and lower viewing angles are 48 ° and 73 °, respectively, and when the reflectance of the cholesteric compensation film is 35%, the upper and lower viewing angles at which gray level inversion occurs are 46 ° and 70 °, respectively, and the cholesteric compensation film is When the reflectance is 40%, the upper and lower viewing angles at which the gray scale inversion occurs are 47 ° and 70 °, respectively, and when the cholesteric compensation film has the reflectance at 50%, the upper and lower viewing angles at which the gray scale inversion occurs are 63, respectively. And 656 °.

As described above, in the present invention, by using the cholesteric compensation film in the twisted nematic liquid crystal display device, uniform luminance can be obtained at the upper side and the lower side, and gentle luminance change can be obtained. In addition, while ensuring a high contrast ratio, it is possible to secure a wide viewing angle for the gray level inversion generated from the upper side and the lower side.

Claims (5)

A first substrate and a second substrate including a first substrate and a second substrate facing the first substrate, and a liquid crystal layer formed between the first and second substrates and having a liquid crystal material having positive dielectric anisotropy Twisted nematic liquid crystal cell with cotton, A pair of polarizing plates each attached to a first side or a second side of the liquid crystal cell, A light source attached to the polarizing plate on the side facing the first surface of the liquid crystal cell with respect to the polarizing plate, A cholesteric compensation film attached to the polarizing plate on the side facing the second surface of the liquid crystal cell around the polarizing plate and comprising a cholesteric liquid crystal layer made of cholesteric liquid crystal molecules. Liquid crystal display comprising a. In claim 1, The cholesteric compensation film further includes a retardation layer formed between the polarizing plate and the cholesteric liquid crystal layer and converts linearly polarized light passing through the polarizing plate into circularly polarized light. In claim 2, And the liquid crystal molecules of the cholesteric liquid crystal layer are arranged at a pitch in the range of 400-700 nm. In claim 3, The liquid crystal molecules of the cholesteric liquid crystal layer are arranged in the upper and lower direction with respect to the viewing angle. In claim 1, The reflectivity of the cholesteric compensation film is in the range of 10-90%.