KR20040036784A - Reflective liquid crystal display device - Google Patents

Abstract

PURPOSE: A reflective LCD device is provided to differently form size of R, G, and B pixels of a cholesteric liquid crystal color filter, in order to improve white balance by reflectivity differences, thereby realizing good color features. CONSTITUTION: The first and second substrates(110,150) are disposed at certain intervals. The second substrate(150) is made of a transparent insulating material. An optical absorption layer(115) is deposited on the first substrate(110) to absorb light. The first alignment layer(120) made of a polyimide material is deposited on the optical absorption layer(115). A cholesteric liquid crystal color filter(125) is deposited on the first alignment layer(120) in order to reflect light. The first alignment layer(120) orients a liquid crystal molecule of the cholesteric liquid crystal color filter(125) in certain direction. The cholesteric liquid crystal color filter(125) selectively reflects the incident light.

Description

Reflective liquid crystal display device

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

Recently, with the rapid development of the information society, the need for a flat panel display having excellent characteristics such as thinning, light weight, and low power consumption has emerged. Such a flat panel display can be divided into two types according to whether it emits light or not. A light emitting display displays an image by emitting light by itself and displays an image using an external light source. It is called a light receiving type display device. The light emitting display includes a plasma display panel, a field emission display, an electroluminescence display, and the like. A light receiving display includes a liquid crystal display. crystal display).

Among them, liquid crystal display devices are being actively applied to notebooks and desktop monitors because of their excellent resolution, color display, and image quality.

A general liquid crystal display device includes a thin film transistor array substrate called a lower substrate, a color filter substrate called an upper substrate, and a liquid crystal interposed between the two substrates, and is generated by applying voltage to two electrodes formed on the two substrates. By moving the liquid crystal molecules by an electric field, the device expresses an image by the transmittance of light that varies accordingly.

However, as mentioned above, the liquid crystal display does not emit light by itself and thus requires a separate light source. Therefore, an image is displayed by arranging a backlight on the back of the liquid crystal panel and injecting light from the backlight into the liquid crystal panel to adjust the amount of light according to the arrangement of the liquid crystals.

Such a liquid crystal display device is called a transmission type liquid crystal display device. Since the liquid crystal display device uses an artificial back light source such as a backlight, a bright image can be realized even in a dark external environment, but power consumption due to the backlight is consumed. There is a big disadvantage.

In order to compensate for the above disadvantages, a reflection type liquid crystal display device has been proposed. The reflection type liquid crystal display device uses less power than the transmission type liquid crystal display device in the form of adjusting light transmittance according to the arrangement of liquid crystals by reflecting external natural light or artificial light. In the reflective liquid crystal display device, the lower field generation electrode is formed of a conductive material with good reflection, and the upper field generation electrode is formed of a transparent conductive material to transmit external light.

However, since the reflective liquid crystal display device operates by using external light, there is a problem that the luminance is considerably lowered. In detail, because of the characteristics of the reflective liquid crystal display device, external light is reflected by the reflective electrode positioned on the lower substrate after passing through the color filter substrate, and then again through the color filter substrate to represent an image. As the light passes through the filter twice, the transmittance of light drops, thereby lowering the luminance.

In general, the thickness of the color filter is inversely proportional to the transmittance and has a proportional relationship with the color purity. In order to solve the fall of the luminance as described above, there is a method of increasing the transmittance and lowering the color purity by reducing the thickness of the color filter. Due to the nature of the resin used as a filter, there is a limit to manufacturing a constant color filter below a certain thickness.

In order to solve the above problems, a liquid crystal display device having a characteristic of selectively reflecting or transmitting light has been researched and developed.

Generally, liquid crystal molecules have a liquid crystal phase depending on the structure and composition. The liquid crystal phase is affected by temperature and concentration. The liquid crystal or liquid crystal phase most studied and applied so far has been applied to a commercially available liquid crystal display device as a nematic liquid crystal having regularity in which liquid crystal molecules are aligned in a certain direction.

Recently, the liquid crystal being studied in relation to the luminance improvement is a cholesteric liquid crystal, which has a chiral characteristic different from the original phase when the molecular side of the liquid crystal is distorted or the image of the reflected molecule is different. By means of mixing molecules and nematic liquid crystals, it refers to liquid crystals in which a director of the nematic liquid crystal is twisted.

In general, the nematic liquid crystal phase has regularity in which liquid crystal molecules are aligned in a predetermined direction. In contrast, cholesteric liquid crystals have a layered structure, in which the liquid crystals exhibit regular nematic regularity. However, the arrangement of the interlayer liquid crystals is rotated in one direction, and this rotation causes a difference in reflectance between the layers. This difference in reflectance can show color due to reflection and interference of light.

Rotation of the cholesteric liquid crystal molecules can be seen as a kind of spiral structure. The characteristics of the two structures in this spiral structure are the rotational direction of the spiral and the helical pitch, which is the repetition period of the spiral. The rotation pitch can be understood as the distance until the liquid crystal layer returns to the same arrangement again, and this rotation pitch becomes a variable that determines the color of the cholesteric liquid crystal.

In other words, when all of the helical axes of the cholesteric liquid crystal are arranged in the direction perpendicular to the substrate, only light of a specific wavelength region is reflected according to the helical pitch with respect to the vertically incident light. It has a selective reflection characteristic so that when the rotation pitch is adjusted by region, the color of reflected light is red, green or blue. The reflected central wavelength is a function of the average pitch of the rotational pitch described above and the cholesteric liquid crystal (λ = n (avg) · pitch). (N (avg): average refractive index) For example, the average refractive index is 1.5 When the pitch of rotation of the cholesteric liquid crystal is 400 nm, the central reflection wavelength is approximately 600 nm, which is red. In addition, it can be implemented by giving a pitch of the cholesteric liquid crystal suitable for green and blue.

On the other hand, the cholesteric liquid crystal color filter also determines the polarization state of the reflected light. The direction of circular polarization of reflected light is determined depending on whether the helical structure of the cholesteric liquid crystal is preferential or left. First, the cholesteric liquid crystal reflects the right circular polarization of the pitch. Everyday light can be thought of as the sum of right or left polarized light, and when cholesteric liquid crystals are used, circularly polarized light of certain components can be separated. Considering the fact that the linear polarization characteristics of light are used in the current liquid crystal display devices, the use of such cholesteric liquid crystals can greatly reduce the power consumption by effectively improving the utilization of light as compared to color filters using pigments or dyes. have.

1 is a cross-sectional view of a reflective liquid crystal display device using a cholesteric liquid crystal color filter.

As illustrated, the light absorption layer 15 is formed on the lower substrate 10, and the first alignment layer 20 is formed thereon. A cholesteric liquid crystal color filter 25 is formed on the first alignment layer 20. The cholesteric liquid crystal color filter 25 corresponds to red (R), green (G), and blue (B) for each region. By reflecting light of wavelengths, red, green, and blue are sequentially displayed. Subsequently, a transparent first electrode 30 is formed on the cholesteric liquid crystal color filter 25, and a second alignment layer 35 is formed thereon.

Next, the upper substrate 50 is disposed on the lower substrate 10 at regular intervals, and the transparent second electrode 55 and the third alignment layer 60 are sequentially formed below the upper substrate 50.

The liquid crystal layer 40 is injected between the second alignment layer 35 and the third alignment layer 60, and the liquid crystal molecules of the liquid crystal layer 40 form an electric field between the first electrode 30 and the second electrode 55. The arrangement direction is changed by.

Next, a retardation plate 70 having a retardation value of λ / 4 is disposed on the upper substrate 50, and a polarizing plate 80 is disposed thereon. That is, the cholesteric liquid crystal color filter also serves as a reflector for reflecting color and light, and thus a separate reflector is omitted.

In this case, the cholesteric liquid crystal color filter has an absolute effect on the reflection luminance in the liquid crystal display device using the cholesteric liquid crystal color filter as the color filter greatly affects the reflection luminance in the reflection type liquid crystal display device including the reflection plate. .

The material applied to the cholesteric liquid crystal color filter is poor in thermal stability to form red, green, and blue pixels, and then the red, green, and blue reflectivity gradually decreases through various processes. However, the degree of decrease in reflectance is different for each of the red, green, and blue pixels.

2 is a plan view showing red, green, and blue pixels of a typical cholesteric liquid crystal color filter. The size of the red, green, and blue pixels is constant despite the difference in reflectance.

3 is a spectral graph showing reflectance according to wavelength of each color in a cholesteric liquid crystal color filter having the same size of conventional red, green, and blue pixels. The X and Y axes are represented by wavelength (nm) and reflectance, respectively. The reflection center wavelengths of each color are 600nm, 510nm and 470nm, respectively. Referring to the graph, the green and blue pixels have a reflectance of 0.21 to 0.22, but the red pixel has a low reflectance of about 0.15 compared to the two colors due to particularly poor thermal stability. Since the reflectance of the red pixel is considerably lower than that of the green and blue pixels, the white color is not balanced in the realization of the white color and is biased toward the blue color.

The present invention has been made to solve the above-mentioned problems, the object of the present invention is to form a different size of red, green, blue pixels of the cholesteric liquid crystal color filter to improve the white balance by the difference in reflectance color It is to provide a reflective liquid crystal display device having good characteristics.

1 is a cross-sectional view of a reflective liquid crystal display device using a conventional cholesteric liquid crystal color filter.

2 is a plan view of red, green, and blue pixels of a conventional cholesteric liquid crystal color filter.

3 is a reflection spectrum graph of a conventional cholesteric liquid crystal color filter.

4 is a cross-sectional view of a reflective liquid crystal display device using a cholesteric liquid crystal color filter according to an embodiment of the present invention.

5 is a plan view of red, green, and blue pixels of the cholesteric liquid crystal color filter according to the embodiment of the present invention;

6 is a reflection spectrum graph of a cholesteric liquid crystal color filter according to an embodiment of the present invention.

<Description of Symbols for Main Parts of Drawings>

110: first substrate 115: light absorption layer

120: first alignment layer 125: cholesteric liquid crystal color filter

130: first electrode 135: second alignment layer

140: liquid crystal layer 150: second substrate

155: second electrode 160: third alignment layer

170: phase difference plate 180: polarizer

According to an aspect of the present invention, there is provided a reflective liquid crystal display device comprising: a first substrate; A light absorption layer on the first substrate; A cholesteric liquid crystal color filter disposed above the light absorbing layer and having an area larger than that of the green or blue pixels among the red, green, and blue pixels; A first electrode on the cholesteric liquid crystal color filter; A second substrate spaced apart from the first substrate; A second electrode formed under the second substrate; A retardation plate disposed on the second substrate; A polarizing plate disposed on the retardation plate; And a liquid crystal layer injected between the first and second electrodes.

In addition, a thin film transistor is further provided below the second substrate, and the thin film transistor is connected to the second electrode. The red, green, and red pixels of the cholesteric liquid crystal color filter have a long length of a constant length. And the length of the short axis is longer than that of the green or blue pixel.

In addition, the long axis length of the red, green, and blue pixels of the cholesteric liquid crystal is constant, and the short axis length is characterized in that the ratio of 1.25: 0.75: 1.

Hereinafter, a reflective liquid crystal display using a cholesteric liquid crystal color filter according to an embodiment of the present invention will be described with reference to the accompanying drawings.

4 is a cross-sectional view of a reflective liquid crystal display using a cholesteric liquid crystal color filter according to an embodiment of the present invention.

As shown in FIG. 4, the first and second substrates 110 and 150 are arranged at regular intervals. Here, the second substrate 150 may be made of a transparent insulating material, and the first substrate 110 may be made of a transparent material or may be made of a material having a low transparency.

The light absorption layer 115 for absorbing light is formed on the first substrate 110, and a first alignment layer 120 made of a material such as polyimide is formed thereon. The cholesteric liquid crystal color filter 125 reflecting light having a predetermined wavelength is formed on the first alignment layer 120. Here, the first alignment layer 120 aligns the liquid crystal molecules of the cholesteric liquid crystal color filter 125 in a predetermined direction, and the cholesteric liquid crystal color filter 125 selectively reflects the incident light. Each pixel area is red, green, and blue.

4 and 5, the red, green, and blue pixels have different sizes. When formed with the same size, as shown in FIG. 3, the red reflectance is lower than that of other green and blue reflectances, thereby forming a large pixel size. That is, the long axis length of the red, green, and blue pixels is made constant, and the short axis length is increased from the conventional 1: 1: 1 to 1.25: 0.75: 1, and the short axis length of the green pixel is formed by reducing the short axis length of the green pixel. do.

Subsequently, a first electrode 130 made of a transparent conductive material is formed on the cholesteric liquid crystal color filter 125, and a second alignment layer 135 made of a material such as polyimide on the first electrode 130. ) Is formed.

Next, a second electrode 155 made of a transparent conductive material is formed under the second substrate 150 on the upper side, and a third alignment layer 160 is formed under the second substrate 150. It may be made of a material such as mead.

Next, the liquid crystal layer 140 is positioned between the second alignment layer 135 and the third alignment layer 160, and the liquid crystal molecules of the liquid crystal layer 140 are formed between the first and second electrodes 130 and 155. The arrangement depends on the electric field.

Next, a quarter wave plate (QWP) 170 is disposed on the second substrate 150. The retardation plate 170 changes the polarization state of light, and has a phase difference value of λ / 4 so that linearly polarized light is converted into circularly polarized light and circularly polarized light is converted into linearly polarized light.

The linear polarizer 180 is positioned on the retardation plate 170, and the polarizer 180 only passes light in a direction parallel to the direction of the light transmission axis.

As mentioned above, the cholesteric liquid crystal color filter 125 causes selective reflection of incident light. Since the selective reflection is determined by the rotation pitch of the cholesteric liquid crystal molecules, the distribution of the rotation pitch in each pixel is different. Thus, different colors of light are reflected from each pixel. Thus, the reflected light becomes red, green, and blue, respectively.

In the liquid crystal display device using the cholesteric liquid crystal color filter 125, the thin film transistor, which is a switching element, and the pixel electrode connected to the thin film transistor are formed on the second substrate 150, so that the second electrode 155 becomes a pixel electrode. It is formed to correspond one-to-one with pixels representing each color of the steric liquid crystal color filter 125, and each of them is connected to a thin film transistor although not shown.

In this case, the size of the second electrode 155, that is, the pixel electrode, is different depending on the size of each pixel of the cholesteric liquid crystal color filter of the first substrate corresponding to one-to-one. That is, the pixel electrode corresponding to each pixel is formed in the same length without changing the major axis length, and the short axis length is long when it corresponds to the red pixel, and the short axis length of the pixel electrode corresponding to the green pixel is shortened to correspond one to one.

FIG. 6 illustrates red, green, and blue reflectances and white reflectances of a cholesteric liquid crystal color filter having a unidirectional length ratio of 1.25: 0.75: 1 of red, green, and blue pixels according to an embodiment of the present invention. Drawing.

By reducing the size of the green pixel, the green reflectance is lower than that of other red and blue, but the white color shifted toward the conventional blue maintains almost the same reflectance at the wavelength from blue to red. White color with excellent white balance and color characteristics without any bias can be expressed.

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

In the reflective liquid crystal display device using the cholesteric liquid crystal color filter according to the present invention, the red, green, and red pixels of the cholesteric liquid crystal color filter are formed differently so that the reflectance is relatively lower than that of the green and red pixels. By improving the reflectance of the white color balance is improved to provide a reflective liquid crystal display device excellent in color characteristics.

Claims (4)

A first substrate; A light absorption layer on the first substrate; A cholesteric liquid crystal color filter disposed above the light absorbing layer and having an area larger than that of the green or blue pixels among the red, green, and blue pixels; A first electrode on the cholesteric liquid crystal color filter; A second substrate spaced apart from the first substrate; A second electrode formed under the second substrate; A retardation plate disposed on the second substrate; A polarizing plate disposed on the retardation plate; Liquid crystal layer injected between the first and second electrodes Reflective liquid crystal display device comprising a. The method of claim 1, A thin film transistor is further provided below the second substrate, and the thin film transistor is connected to the second electrode. The method of claim 1, The red, green, and red pixels of the cholesteric liquid crystal color filter have a long axis having a constant length, and a short axis has a red pixel longer than a green or blue pixel. The method of claim 3, wherein The long axis length of the red, green, and blue pixels of the cholesteric liquid crystal is constant, and the short axis length is a ratio of red: green: blue to 1.25: 0.75: 1.