KR100292242B1 - Reflection type liquid crystal display - Google Patents

Abstract

The present invention provides a linear polarizing plate for converting natural light incident from the outside into linearly polarized light; A phase difference plate positioned below the element to change linearly polarized light into circularly polarized light; A liquid crystal layer for changing a phase of light depending on whether an electric field is applied; A cholesteric liquid crystal color filter having a first state for selectively reflecting light passing through the liquid crystal layer and a second state for transmitting the light; The present invention relates to a reflective liquid crystal display device including a light absorbing layer that absorbs light transmitted through the cholesteric liquid crystal color filter, and is capable of significantly improving color purity than a reflective liquid crystal display device using a conventional color filter. Can maximize the brightness. In addition, since the existing process conditions can be used as it is, the efficiency of the process can be maximized, and the cholesteric liquid crystal color filter can be disposed on the lower surface to minimize the multi-reflection of light on the upper surface.

Description

Reflective liquid crystal display device {REFLECTION TYPE LIQUID CRYSTAL DISPLAY}

The present invention relates to a liquid crystal display device, and more particularly to a reflective liquid crystal display device using a cholesteric liquid crystal color filter.

In general, a liquid crystal display device is a device that displays predetermined information using light, and classifies a light source into a reflective liquid crystal display device and a transmissive liquid crystal display device depending on whether the external natural light or the internal light source is used. The transmissive liquid crystal display device includes a light source such as a backlight therein, and the reflective liquid crystal display device using external light uses reflected light with a reflective electrode embedded therein.

In particular, since the reflective liquid crystal display uses external light, the luminance changes according to the environment. In a general office environment, the reflective liquid crystal display device has a lower luminance than the transmissive liquid crystal display device, so to increase the luminance, the purity of the absorption type color filter that reproduces color is sacrificed.

Hereinafter, a general reflective liquid crystal display device will be described with reference to the accompanying drawings.

FIG. 1 is a cross-sectional view illustrating a general liquid crystal display device, in which first and second substrates 10 and 12 spaced apart from each other by a predetermined interval are arranged, and a liquid crystal 16 is disposed between the two substrates 10 and 12. ) Is located. In addition, a reflective electrode 14 is disposed on the second substrate 12 to reflect external light. At this time, the reflective electrode 14 also functions as a pixel electrode.

The common electrode 18 is positioned on the liquid crystal 16, and the common electrode 18 and the reflective electrode 14 apply an electric field in a vertical direction with the liquid crystal 16 interposed therebetween to form a molecular arrangement of the liquid crystal. Will change.

The color filter 20 is positioned on the common electrode 18 to perform light coloring, and on the first substrate 10, the surface reflection of the light and the mirror reflection of the reflective electrode are reduced and the viewing angle is widened. The scattering film 22 is located. On the scattering film 22, a phase difference plate 24 for converting linearly polarized light into circularly polarized light is positioned. In this case, the retardation plate 24 is a general term for changing the phase of incident light, and a representative one is a λ / 4 plate.

In addition, a linear polarizing plate 26 for converting natural light into linearly polarized light is positioned on the retardation plate 24.

The function and operation of the reflective liquid crystal display device having such a configuration will be described below.

First, external natural light is incident on the liquid crystal display, the incident natural light is converted into linearly polarized light through the linear polarizing plate 26, and the converted linearly polarized light is converted into circularly polarized light through the phase difference plate 24.

The circularly polarized light incident on the liquid crystal layer becomes linearly polarized light while passing through the liquid crystal layer, and the traveling direction of the light is changed while being reflected by the reflective electrode 14, and the reflected linearly polarized light becomes circularly polarized light while passing through the liquid crystal layer again. In addition, a predetermined color is realized while passing through the color filter.

The polarized light passes through the scattering film 22 and the viewing angle is enlarged, and the circularly polarized light is converted into linearly polarized light through the retardation plate 24. The converted linearly polarized light is displayed to the user through the linear polarizing plate 26.

For reference, a state in which light is transmitted when no electric field is applied to the liquid crystal and light is blocked when an electric field is applied is referred to as a normally white mode (hereinafter, referred to as NW mode). On the other hand, light is blocked when no electric field is applied to the liquid crystal, and a state in which light is transmitted when the electric field is applied is referred to as a normally black mode (hereinafter, referred to as NB mode).

Referring to the state of the incident light of the reflective liquid crystal display device according to whether the voltage is applied as follows.

FIG. 2 is a state diagram showing a state of incident light of the reflective liquid crystal display when no voltage is applied.

First, the linearly polarized plate 26 is converted into linearly polarized light.

The converted linearly polarized light is converted into circularly polarized light through the retardation plate 24.

The converted circularly polarized light is converted into linearly polarized light through the liquid crystal 16.

The converted linearly polarized light is reflected by the reflective electrode 14 so that the traveling direction of the light is reversed.

The reflected linearly polarized light is converted into circularly polarized light through the liquid crystal 16.

The converted circularly polarized light is converted into linearly polarized light through the retardation plate 24.

FIG. 3 is a state diagram showing a state of light incident on a reflective liquid crystal display when a voltage is applied, and incident light, which is external natural light, is converted into linearly polarized light through the linear polarizing plate 26.

The converted linearly polarized light is converted into circularly polarized light through the retardation plate 24.

The converted circularly polarized light passes through the liquid crystal 16 without any change.

The transmitted circularly polarized light is reflected in the opposite direction to the polarization direction and the traveling direction while contacting the surface of the reflective electrode 14.

The reflected circularly polarized light passes through the liquid crystal 16 without any change.

The transmitted circularly polarized light is converted into linearly polarized light through the retardation plate 24 and absorbed by the linearly polarized light.

The light reflectance according to the incident light of the reflective liquid crystal display device as described above is as follows.

4 is a graph measuring light reflectance according to incident light of a typical reflective liquid crystal display device, in which a horizontal axis represents wavelength λ, a vertical axis represents reflectance, and a dominant wavelength bandwidth having a predetermined bandwidth is A; The other area is referred to as an area B.

As shown, although the light reflectance of the area A is high, the light reflection is also performed in the area B, whereby the purity of the color to be displayed is lowered.

The general reflective liquid crystal display device as described above needs to lower the color purity in order to improve the color filter transmittance, but there is a limit in improving the luminance by simply lowering the color purity.

In addition, since the different layers positioned on the upper substrate have different refractive indices, there is a problem in that multiple reflections of light occur, thereby lowering the light reflectance.

SUMMARY OF THE INVENTION An object of the present invention for overcoming the above problems is to provide a reflective liquid crystal display device having improved luminance without sacrificing color purity by using a cholesteric liquid crystal color filter.

1 is a cross-sectional view showing a cross section of a typical reflective liquid crystal display device.

Figure 2 is a state diagram showing the state of incident light of a typical reflective liquid crystal display when no voltage is applied.

3 is a state diagram showing, for example, the state of incident light of a typical reflective liquid crystal display when a voltage is applied;

Figure 4 is a graph measuring the reflectance of the light according to the wavelength of light transmitted through the color filter of a typical liquid crystal display device.

5 is a cross-sectional view showing a cross section of a reflective liquid crystal display device according to an embodiment of the present invention.

6 is a state diagram illustrating a state of incident light of a reflective liquid crystal display device according to an exemplary embodiment of the present invention when no voltage is applied in the normal white mode.

7 is a state diagram illustrating an example of incident light of a reflective liquid crystal display device according to an exemplary embodiment of the present invention when a voltage is applied in a normal white mode.

8 is a state diagram illustrating an example of incident light of a reflective liquid crystal display device according to an exemplary embodiment of the present invention when no voltage is applied in a normal black mode.

9 is a state diagram showing an example of a state of incident light of a reflective liquid crystal display device when a voltage is applied in a normal black mode.

10 is a graph showing the light transmittance according to the wavelength of incident light of the reflective liquid crystal display device according to an embodiment of the present invention.

FIG. 11 is a graph showing light reflectance according to a wavelength of incident light of a reflective liquid crystal display according to an exemplary embodiment of the present invention. FIG.

<Explanation of symbols for main parts of the drawings>

30; First substrate 32; Second substrate

34; Light absorption layer 36; Cholesteric Liquid Color Filter

38; Second electrode 40; LCD

42; First electrode 44; Retarder

46; Linear polarizer

The present invention for achieving the above object is a linear polarizing plate for changing the natural light incident from the outside into linearly polarized light; A phase difference plate positioned below the element to change linearly polarized light into circularly polarized light; A liquid crystal layer for changing a phase of light depending on whether an electric field is applied; A cholesteric liquid crystal color filter having a first state for selectively reflecting light passing through the liquid crystal layer and a second state for transmitting the light; It is to provide a reflective liquid crystal display device comprising a light absorption layer for absorbing light transmitted through the cholesteric liquid crystal color filter.

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

For clarity of explanation, components or functions unrelated to the present invention will be omitted.

FIG. 5 is a cross-sectional view of a reflective liquid crystal display device according to an exemplary embodiment of the present invention, and includes first and second substrates 30 and 32 made of a transparent and insulating material. A light absorbing layer 34 for absorbing light is provided under the second substrate 32. At this time, since the light absorbing layer 34 can absorb light, a light absorbing material, for example, a polymer such as a polymer may be applied to the back surface of the second substrate 32.

A cholesteric liquid crystal is coated on the second substrate 32 and patterned to form a cholesteric liquid crystal color filter layer 36. The lower plate is completed by depositing and patterning a transparent conductive metal on the cholesteric liquid crystal color filter layer 36 to form a second electrode 38.

On the other hand, the upper plate and the lower plate manufactured in another process are bonded to each other, and the liquid crystal 40 is injected therebetween. Accordingly, the first electrode 42 is positioned on the liquid crystal 40, and the first substrate 30 is positioned thereon. In addition, a retardation plate 44 is positioned on the first substrate 30, and linear polarizing plates are sequentially stacked thereon.

The liquid crystal layer is transmitted by changing the phase of light incident on the liquid crystal layer according to whether an electric field is applied or not. Electrodes for applying a voltage to the liquid crystal layer are the first and second electrodes, and whether or not the voltage is applied is determined by an external control circuit. Although not shown in the drawings, the liquid crystal display device is formed with a semiconductor device that is turned on / off by an external control signal, recently, a thin film transistor is mainly used.

Here, the cholesteric liquid crystal color filter uses cholesteric liquid crystal as a color filter, causing selective reflection (transmission) of light. That is, the cholesteric liquid crystal reflects only right circularly polarized light and does not reflect left circularly polarized light when its molecular structure is twisted to the right.

In addition, the cholesteric liquid crystal color filter is composed of a plurality of pixels, and each pixel is composed of three subpixels. That is, the light reflected by the cholesteric liquid crystal color filter has red (R), green (G), and blue (B). Unlike the absorption type color filter, the reflected light has a dominant wavelength band. As a result, colors with high color purity are realized.

As is known, every object has a unique wavelength, and the color of the object as perceived by a person is to see the wavelength of light reflected or transmitted by that particular object.

Here, since the selective reflection wavelength band of the cholesteric liquid crystal color filter is determined by the pitch of molecules made of the liquid crystal phase, the wavelength band reflected by the distribution of pitch in one pixel can be adjusted.

On the other hand, the cholesteric liquid crystal color filter and the light absorbing layer used in the embodiment of the present invention may be formed in different positions, or may be formed in a plurality, if necessary, as follows.

First, a plurality of cholesteric liquid crystal color filters may be formed in order to implement more precise colors, and the retardation plate may be formed on a second substrate without being formed on the first substrate. In this case, the cholesteric liquid crystal may be formed. A phase difference plate is formed on the color filter so as to be positioned under the liquid crystal.

In addition, in order to effectively absorb unnecessary light, a plurality of said light absorption layers may be formed, and unlike the above-mentioned, you may form on a 2nd board | substrate.

Referring to the function and operation of the reflective liquid crystal display device having the configuration as described above are as follows.

6 is a state diagram illustrating a polarization state of light incident on a reflective liquid crystal display when no voltage is applied in the normal white mode.

As shown, external natural light is converted into linearly polarized light through the linear polarizing plate 46 of the reflective liquid crystal display device.

The converted linearly polarized light is converted into right circularly polarized light through the retardation plate 44.

The converted right circularly polarized light is converted into left circularly polarized light through the liquid crystal 40.

The converted left circularly polarized light reflects light through a cholesteric liquid crystal color filter 36 while allowing for selective reflection while maintaining polarization. In this case, the cholesteric liquid crystal color filter 36 is set to reflect the left circularly polarized light.

The reflected left circularly polarized light is converted into right circularly polarized light through the liquid crystal.

The converted right circularly polarized light is converted into linearly polarized light through the linear polarizing plate.

On the other hand, the liquid crystal layer may vary the phase difference according to the design needs, generally used is that can reduce the phase difference of the transmitted light by λ / 4, the liquid crystal layer used in the embodiment of the present invention is transmitted light To convert λ / 2. However, as described above, the phase difference given to the liquid crystal can vary depending on the design needs.

A case where a voltage is applied in the NW mode will be described below.

7 is a state diagram illustrating a case where a voltage is applied in the NW mode, and external natural light is converted into linearly polarized light through the linearly polarized plate.

The converted linearly polarized light is converted into right circularly polarized light through a phase difference plate.

The converted right circularly polarized light is transmitted through the liquid crystal as it is, and the transmitted right circularly polarized light is transmitted through the color filter as it is.

The transmitted right circularly polarized light is absorbed by the light absorption layer formed under the second substrate.

In addition, a case in which no voltage is applied in the NB mode is described below.

8 is a state diagram illustrating a state of incident light when no voltage is applied in the NB mode, and external natural light is converted into linearly polarized light through the linear polarizing plate.

The converted linearly polarized light is converted into left circularly polarized light through the phase difference plate.

The converted left circularly polarized light transmits the liquid crystal without any change.

The transmitted left circularly polarized light is absorbed by the light absorbing plate through the color filter.

Meanwhile, a case in which a voltage is applied in the NB mode will be described below.

FIG. 9 is a state diagram illustrating a state of incident light when voltage is applied in the NB mode, and external natural light is converted into linearly polarized light through the linear polarizing plate.

The converted linearly polarized light is converted into right circularly polarized light through a phase difference plate.

The converted right circularly polarized light is converted into left circularly polarized light through the liquid crystal.

The converted left circularly polarized light is reflected while maintaining the polarization state through the cholesteric liquid crystal color filter, and the light travel direction is reversed.

The reflected left circularly polarized light is converted into right circularly polarized light through the liquid crystal.

The converted right circularly polarized light is converted into linearly polarized light through a retardation plate (λ / 4 plate).

Meanwhile, the transmittance according to the wavelength of incident light incident on the reflective liquid crystal display device according to the present invention will be described as follows.

FIG. 10 is a graph showing selective transmission according to a wavelength of a cholesteric liqid crystal color filter according to the present invention, and FIG. 11 is a graph showing selective reflection. In the former case, the liquid crystal display device according to the wavelength of the cholesteric color filter can be applied to a device displaying yellow, cyan, and magenta, regardless of transmission type or reflection type. At this time, the yellow, cyan and magenta are secondary colors implemented by mixing any two colors among red, blue, and green colors, which are three primary colors ( secondary color). Therefore, in the reflection type liquid crystal display device lacking the amount of light, the secondary color is used because the luminance is relatively higher than that of the three primary colors.

The cholesteric liquid crystal color filter transmits circularly polarized light of the same structure and reflects circularly polarized light of another structure. For example, the left circularly polarized light may be selectively reflected or the right circularly polarized light may be selectively reflected according to the twisted direction of the molecular structure composed of the liquid crystal phase. That is, the cholesteric liquid crystal color filter can be set to selectively reflect only left circularly polarized light or selectively reflect only right circularly polarized light.

This is due to the structural characteristics of the cholesteric liquid crystal molecules, the reflective liquid crystal display device according to the present invention implements a predetermined color by using the characteristics as described above.

As shown, the horizontal axis of the graph represents the wavelength of light and the vertical axis represents the transmittance of light. At this time, the wavelength bandwidth (C) (D) of the light can be widened or narrowed by adjusting the pitch of the cholesteric liquid crystal color filter. As described above, the transmitted light is absorbed by the light absorbing layer.

FIG. 11 is a graph showing light reflectance according to a wavelength of a reflective liquid crystal display device of the present invention, where the horizontal axis represents the wavelength of light and the vertical axis represents the reflectance of light. Similarly, the bandwidth E of light (F) can be widened or narrowed by adjusting the degree of distortion of the cholesteric liquid crystal color filter, that is, the pitch. As mentioned above, the reflected circularly polarized light displays predetermined information.

As the cholesteric liquid crystal color filter is set, other colors except for a specific color may be transmitted as shown in FIG. 10, and other colors except for the specific color may be reflected. When reflecting other colors except a specific color, the reflectance graph is drawn as in FIG. 10.

As shown in FIG. 11, only a specific color may be reflected, or alternatively, only a specific color may be transmitted. Even when only a specific color is transmitted, the transmittance graph is drawn the same.

In summary, since the cholesteric liquid crystal color filter formed in the reflective liquid crystal display device of the present invention performs a function of reflecting image information to be displayed, it is set in consideration of reflection characteristics.

On the other hand, the central image of the present invention as described above is not limited to the disclosed embodiment because the cholesteric liquid crystal color filter is applied to the reflective liquid crystal display device to improve the purity and luminance of the color. Accordingly, modifications inferred from the present invention should also be considered to be within the scope of the present invention.

According to the embodiment of the present invention as described above has the following advantages.

First, since the cholesteric liquid crystal color filter is used, light can be selectively transmitted, and unnecessary light can be effectively absorbed by the light absorbing layer, thereby greatly improving luminance.

Second, since color purity does not have to be sacrificed to improve luminance, color purity can be dramatically improved.

Third, because the existing process conditions can be used as it is, it is possible to maximize the efficiency of the process.

Fourth, by placing a cholesteric liquid crystal color filter on the lower surface it can minimize the multi-reflection of light on the upper surface.

Claims (9)

A linear polarizer for converting natural light incident from the outside into linearly polarized light, A retardation plate positioned below and converting linearly polarized light into circularly polarized light; An optically anisotropic medium that changes the phase of light depending on whether an electric field is applied, A cholesteric liquid crystal color filter having a first state for selectively reflecting light passing through the optically anisotropic medium, and a second state for transmitting the light; Light absorbing layer for absorbing light transmitted through the cholesteric liquid crystal color filter Reflective liquid crystal display device comprising a. The method of claim 1, And at least one retardation plate, wherein the retardation plate is a λ / 4 plate. First and second substrates spaced apart by a predetermined interval, A liquid crystal layer disposed between the first substrate and the second substrate, the liquid crystal layer having a first switching state for transmitting while changing a phase of incident light according to whether an electric field is applied, and a second switching state for transmitting without changing the phase of the incident light; , First and second electrodes for applying an electric field with the liquid crystal layer interposed therebetween; A semiconductor device positioned on the second substrate to switch an electrical signal applied to the liquid crystal layer according to an external control signal; A phase difference plate positioned on the first substrate and converting linearly polarized light into circularly polarized light; A linearly polarized layer positioned on the retardation plate to convert external natural light into linearly polarized light; A cholesteric liquid crystal color filter positioned on the second substrate and operating in a first polarization state for reflecting incident light with light of at least one color, and in a second polarization state for transmitting with at least one color of light; A light absorption layer positioned on a rear surface of the second substrate to absorb transmitted light of the cholesteric liquid crystal color filter Reflective liquid crystal display device comprising a. The method of claim 1, And the light absorption layer is formed directly below the cholesteric liquid crystal color filter. The method of claim 1, And the phase difference plate is formed between the linear polarizing plate and the cholesteric liquid crystal color filter. The method of claim 1, The light absorption layer is a reflection type liquid crystal display device made of a light absorbing material. The method of claim 6, The light absorption layer is a reflective liquid crystal display device made of a polymer. The method of claim 1, The cholesteric liquid crystal color filter is at least one reflection type liquid crystal display device. The method of claim 1, The cholesteric liquid crystal color filter is a reflection type liquid crystal display device for changing the wavelength bandwidth by adjusting the pitch of the cholesteric liquid crystal.