KR100961117B1 - Reflecttive plarizer, backlight unit and liquid crystal display including the same - Google Patents

Abstract

PURPOSE: A reflective polarizer, a backlight unit and a liquid crystal are provided to decrease the deterioration of luminance and the color shift of a side viewing angle by reducing phase delay through the thin film of a reflective polarizer. CONSTITUTION: A thin film cholesteric reflective polarizer comprises a reflection polarized film and a phase delay plate. The reflective polarized film comprises a plurality of cholesteric liquid crystal layers. The reflection polarized film selectively penetrates and reflects the random polarized light. The phase delay plate switches the circular polarization into the linear polarization. The circular polarization is transmitted to the reflection polarized film.

Description

Thin-film cholesteric reflective polarizer, backlight unit and liquid crystal display having the same {REFLECTTIVE PLARIZER, BACKLIGHT UNIT AND LIQUID CRYSTAL DISPLAY INCLUDING THE SAME}

The present invention relates to a thin film type cholesteric reflective polarizer used in a liquid crystal display, and more particularly, to a thin film type that can reduce the color distortion caused by the viewing angle of the backlight unit and the liquid crystal display by thinning the reflective polarizer and at the same time, slimming. Cholesteric reflective polarizer.

In general, a liquid crystal display has a structure in which a liquid crystal is injected between glass substrates on which transparent electrodes are formed, and polarizers are disposed on front and rear surfaces of the glass substrate. The polarizer used in such a liquid crystal display is manufactured by adsorbing iodine or dichroic dye on a polyvinyl alcohol film and stretching it in a predetermined direction.

However, since the polarizer manufactured as described above absorbs light vibrating in one direction and passes only light vibrating in another direction to make linearly polarized light, the efficiency of the polarizer cannot theoretically exceed 50%. The action of the polarizer is emerging as the biggest problem to lower the efficiency of the liquid crystal display.

In order to solve such a problem, the technique which uses the cholesteric liquid crystal which has a circular polarization separation function as a reflection polarizer is disclosed. The cholesteric liquid crystal has a characteristic of so-called selective reflection in which the helical rotation direction and the circular polarization direction of the liquid crystal coincide, and the wavelength reflects only the light of circular polarization such as the spiral pitch of the liquid crystal, and the cholesteric reflective polarizer has such selective reflection By using the characteristic of the liquid crystal display, high efficiency of the liquid crystal display can be achieved by separating and transmitting only specific circularly polarized light among natural light having a certain wavelength band and reflecting and reusing the rest. The circularly polarized light transmitted through the cholesteric liquid crystal is converted into linearly polarized light by passing through the λ / 4 waveplate, and the liquid crystal display having high transmittance is obtained by matching the direction of the converted linearly polarized light with the transmission direction of the absorption polarizing film. Can be. In other words, when the cholesteric liquid crystal and the λ / 4 wave plate are used as a reflective polarizer, there is no loss of light in theory, so the luminance improvement can be expected to be twice that of the conventional polarizer which absorbs 50% of light. will be.

1 is a cross-sectional view of a liquid crystal display equipped with the above-described reflective polarizer, which is roughly divided into a backlight unit 10 and a liquid crystal panel unit 20.

The backlight unit 10 includes a light guide plate 12 that diffuses and emits light incident from the light source 11, and a prism sheet 15 that condenses and emits light incident from the diffusion plate 14 and the diffusion plate 14. ), A reflective polarizing film 16a made of cholesteric liquid crystal to selectively reflect light incident from the prism sheet 15, and a phase delay plate for converting circularly polarized light transmitted through the reflective polarizing film 16a into linearly polarized light. And a reflective polarizer 16 composed of 16b.

In addition, the liquid crystal panel unit 20 includes a liquid crystal 21, a front polarizing film 22 and a rear polarizing film 23 provided on the front and rear surfaces of the liquid crystal 21.

In FIG. 1, reference numeral 13 denotes a reflecting plate, 17 denotes a compensation film, and 24 denotes an absorption type polarizing film.

Here, the reflective polarizing film 16a has a structure in which a plurality of cholesteric liquid crystal layers having different pitches are stacked in multiple stages to selectively reflect random polarized light of red, green, and blue wavelength bands. Each cholesteric liquid crystal layer is known to have a thickness of at least 3 μm or more in consideration of alignment characteristics, reflection efficiency, and the like.

However, as described above, the light transmitted through the cholesteric liquid crystal layer of the corresponding wavelength band is incident on the cholesteric liquid crystal layer having a different pitch so as to selectively reflect the light of the other wavelength band, and thus phase delay occurs. When passing through the layer, it is emitted as an elliptical polarization having a different ellipticity instead of circularly polarized light. Since the ellipticity increases in proportion to the incident angle of light, it causes a problem of generating color shifts not only for the front viewing angle but also for the side viewing angle in the liquid crystal display. .

In order to solve the above problems, a compensation film 17 is interposed between the reflective polarizer 16 and the absorption polarizing film 24 to compensate for the so-called phase difference generated by the liquid crystal. In more detail, since the molecules of the cholesteric liquid crystal layer are generally spiral structures, when the incident angle of light increases, birefringence is increased by increasing the refractive index in one direction, and thus linearly polarized light transmitted through the cholesteric liquid crystal layer has a phase difference. Because it is converted to elliptical polarization, a portion of the light is absorbed, causing a decrease in brightness and color shift, thereby changing the color. Therefore, the compensation film compensates for the phase difference generated in the cholesteric liquid crystal layer to lower the occurrence of luminance and color shift.

However, the use of the compensation film is not able to achieve a slimming of the backlight unit and even the liquid crystal display.

In addition, the phase delay plate 17 stacked on top of the conventional cholesteric liquid crystal layer converts the circularly polarized light incident vertically or obliquely into linearly polarized light, but the light transmitted through the reflective polarizing film 16a is not circularly polarized light. In the case of elliptical polarization, there is a problem in that the absorption type polarizing film 24 provided in the liquid crystal display leads to a decrease in luminance due to light loss and a color shift with respect to the side viewing angle.

The present invention has been made to solve the above-mentioned conventional problems, an object of the present invention is to reduce the occurrence of phase delay through the thinning of the reflective polarizer while reducing the occurrence of color shift with respect to the luminance drop and side viewing angle To provide a thin film cholesteric reflective polarizer.

In addition, the present invention is to provide a slim backlight unit and a liquid crystal display by omitting the compensation film through the use of the thin-film cholesteric reflective polarizer as described above.

In order to achieve the above object, the present invention provides a plurality of cholesteric liquid crystal layers having different pitches so as to selectively transmit and reflect respective random polarizations of the red, green, and blue wavelength bands of the incident visible light. A reflective polarizing film and a phase delay plate for converting the circularly polarized light transmitted by the reflective polarizing film into linearly polarized light, wherein the total thickness of each of the cholesteric liquid crystal layers is formed so as not to exceed 6.3 μm.

The reflective polarizing film may include a first cholesteric liquid crystal layer selectively transmitting and reflecting random polarized light forming the blue wavelength band, and a second collet selectively transmitting and reflecting random polarized light forming the green wavelength band. And a third cholesteric liquid crystal layer for selectively transmitting and reflecting the random polarization constituting the red wavelength band.

In this case, the first cholesteric liquid crystal layer has a thickness of 0.9-2.1 μm, the second cholesteric liquid crystal layer has a thickness of 1.2-2.1 μm, and the third cholesteric liquid crystal layer has a thickness of 1.2-2.1 μm. It is preferable to consist of thickness.

Optionally, the reflective polarizing film comprises an upper cholesteric liquid crystal layer 162d for selectively transmitting and reflecting random polarized light forming at least one wavelength band of incident visible light, and the remaining wavelength band of the incident visible light. It consists of a lower cholesteric liquid crystal layer which selectively transmits and reflects random polarization, and each cholesteric liquid crystal layer preferably has a birefringence of 0.16 not exceeding 0.30.

According to the present invention implemented by the above means, by thinning each of the cholesteric liquid crystal layer that transmits and reflects the wavelength band, the generation of color shift by the viewing angle without compensation film as the occurrence of phase delay is reduced Not only can it be significantly lowered, but also it is a very useful invention that can achieve slimming of the backlight unit and the liquid crystal display.

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

In the thin film type cholesteric reflective polarizer 160 of the present invention, a plurality of cholesteric liquid crystal layers 162a, 162b, and 162c are laminated to transmit circularly polarized light in one direction while reflecting circularly polarized light in the other direction. And a phase delay plate 164 for converting the circularly polarized light transmitted through the reflective polarizing film 162 into linearly polarized light.

The reflective polarizing film 162 transmits circularly polarized light in one direction of the incident light while reflecting circularly polarized light in the other direction. The reflective polarizing film 162 is a cholesteric liquid crystal in which a very thin molecular layer forms a spiral structure as a whole. Is implemented.

That is, since the cholesteric liquid crystal has a wavelength band for transmitting and reflecting random polarized light according to the length of the helical pitch of the liquid crystal, the reflective polarizing film 162 transmits circular polarization (for example, right circularly polarized light) in one direction. Circularly polarized light (for example, left circularly polarized light) in a direction is formed by stacking a plurality of cholesteric liquid crystal layers 162a, 162b, and 162c having different pitches to be reflected in multiple stages by an adhesive resin.

More specifically, the reflective polarizing film 162 selectively transmits the first cholesteric liquid crystal layer 162a that selectively transmits and reflects the random polarized light of the blue wavelength band among the visible light and the random polarized light of the green wavelength band. A second cholesteric liquid crystal layer 162b for reflecting, and a third cholesteric liquid crystal layer 162c for selectively transmitting and reflecting random polarized light in the red wavelength band, each cholesteric liquid crystal layer ( 162a) 162b and 162c are birefringences of 0.16 to 0.3, more preferably 0.17 to 0.22.

In this case, the first cholesteric liquid crystal layer 162a has a thickness of 0.9 to 2.1 µm, and the second and third cholesteric liquid crystal layers 162a, 162b and 162c have a thickness of 1.2 to 2.1 µm. It is preferable that the total thickness of the reflective polarizing film 162 on which each of the cholesteric liquid crystal layers 162a, 162b, and 162c is laminated is made of 2.7 to 6.3 μm. The stacking order of the cholesteric liquid crystal layers 162a, 162b, and 162c is not limited.

In addition, when a cholesteric liquid crystal having a large birefringence is used, a cover band for incident light is widened. That is, as can be seen from Equation 1 below, the wavelength band width of the cholesteric liquid crystal is increased by increasing the 'n _e -n _o ' value (hereinafter referred to as 'birefringence').

Δλ = 2λ × (n _e -n _o ) / (n _e + n _o )

※ n _e : Normal refractive index, n _o : Abnormal refractive index, λ: Center wavelength

In addition, the cholesteric liquid crystal has a wavelength band for transmitting and reflecting light depending on the length of the helical pitch of the liquid crystal. Accordingly, the cholesteric liquid crystal must include various pitches in order to transmit and reflect all regions of visible light. That is, at least three pitches should be formed in consideration of the transmission and reflectance of the selected wavelength region. For example, when the average refractive index {(n _e + n _o ) / 2} of the cholesteric liquid crystal is 0.15 and the pitch p is 300 nm, the center wavelength λ of the transmission and reflection bands is obtained from Equation 2 below. As can be seen, since the blue wavelength band is 450nm, in order to manufacture the cholesteric liquid crystal reflective polarizing film covering the blue wavelength band, the pitch of 300nm should be formed to a thickness of 900nm or more laminated at least three or more layers.

λ = (n _e + n _o ) / 2 × p = n × p

※ (Note) (n _e + n _o ) / 2 = n: Average refractive index

Looking at the thickness of the cholesteric liquid crystal layer covering the green and red wavelength bands according to Equations 1 and 2, the pitch (p) of the green wavelength band and the red wavelength band is 335 nm and 380 nm, respectively. The thickness of the second cholesteric liquid crystal layer covering the band should be 1.0 μm or more, and the thickness of the third cholesteric liquid crystal layer covering the red wavelength band should be 1.1 μm or more.

In this case, the average refractive index of each cholesteric liquid crystal layer is preferably made of 1.60 ~ 1.65, more preferably the cholesteric liquid crystal layer covering the blue and green wavelength band is 1.63, cholester covering the red wavelength band The Rick liquid crystal layer has an average refractive index of 1.61.

Therefore, when each of the cholesteric liquid crystal layers 162a, 162b, and 162c thinned to the thickness as described above is stacked, the phase when the reflected and transmitted light passes through the cholesteric liquid crystal layer having a different wavelength band There is less delay.

The phase delay plate 164 converts the circularly polarized light transmitted through the reflective polarization film 162 into linear polarization and emits the light, and the phase delay plate 164 is formed of a polymer film elongated for λ / 4 phase delay. desirable.

Here, the stretched polymer film includes a thermoplastic polymer, and these thermoplastic polymers may be used alone or in combination of two or more. The thermoplastic polymer is polyolefin (polyethylene, polypropylene, etc.), polynorbornene-based polymer, polyester, polyvinyl chloride, polystyrene, polyacrylonitrile, polysulfone, polyallylate, polyvinyl alcohol, polymethacrylic acid Esters, polyacrylic acid esters, cellulose esters and copolymers thereof.

Alternatively, the phase delay plate 164 of the present invention may be implemented as a nematic liquid crystal.

Hereinafter, the operation and effects of the thin film cholesteric reflective polarizer of the present invention will be described in more detail with reference to Examples.

<Example 1>

A cholesteric reflective polarizer in which a first cholesteric liquid crystal layer having a thickness of 1.5 μm, a second and third cholesteric liquid crystal layer having a thickness of 1.7 μm, and a phase delay plate made of an elongated polymer film are laminated. The liquid crystal display was modeled between the first prism sheet and the second prism sheet included in the backlight unit.

<Example 2>

A cholesteric reflective polarizer in which a first cholesteric liquid crystal layer having a thickness of 1.9 μm, a second and third cholesteric liquid crystal layer having a thickness of 2.1 μm, and a phase delay plate made of an elongated polymer film are laminated. The liquid crystal display was modeled between the first prism sheet and the second prism sheet included in the backlight unit.

Comparative Example 1

A cholesteric reflective polarizer in which a first cholesteric liquid crystal layer having a thickness of 2.2 μm, a second and third cholesteric liquid crystal layer having a thickness of 2.7 μm, and a phase delay plate made of an elongated polymer film are laminated. The liquid crystal display was modeled between the first prism sheet and the second prism sheet included in the backlight unit.

Comparative Example 2

A cholesteric reflective polarizer in which a first cholesteric liquid crystal layer having a thickness of 2.2 μm, a second and third cholesteric liquid crystal layer having a thickness of 2.4 μm, and a phase delay plate made of an elongated polymer film are laminated. The liquid crystal display was modeled between the first prism sheet and the second prism sheet included in the backlight unit.

Comparative Example 3

A cholesteric reflective polarizer in which a first cholesteric liquid crystal layer having a thickness of 2.2 μm, a second and third cholesteric liquid crystal layer having a thickness of 2.7 μm, and a phase delay plate made of an elongated polymer film are laminated. The liquid crystal display was modeled between the first prism sheet and the second prism sheet included in the backlight unit.

Experimental Example 1

The color color coordinates measured through the simulation of the liquid crystal display modeled under the conditions of Examples 1 and 2 and Comparative Examples 1 and 2 are divided into X and Y axes, respectively, as shown in graphs of FIGS. 3A and 3B, respectively. Thus, the deviation of the color coordinates measured from the front (0 °) of the X-axis and the Y-axis accordingly is summarized in Table 1 below.

Here, the solid line S1 of the graph shown in FIG. 3 shows the color color coordinates measured after the random polarization irradiated from the light source passes through the light guide plate, the diffuser plate, and the two prism sheets as the reference example 1. FIG.

X axis Y axis Standard example 1  0  0 Example 1 -0.0021 +0.0015 Example 2 +0.0044 +0.0045 Comparative Example 1 +0.0108 +0.0358 Comparative Example 2 +0.0084 +0.0170

In general, the deviation of the front (0 °) color coordinates of the X-axis and the Y-axis with respect to Reference Example 1 should be less than ± 0.0045 to compensate for the phase difference. According to the results of Table 1 and FIG. In Comparative Examples 1 and 2 in which the thickness of the cholesteric liquid crystal layer exceeds 2.1 μm, not only the curve of the graph changes drastically compared with Examples 1 and 2, but also the graph curve of Reference Example 1, in particular, + 30 ° and -30 °. It can be seen that the distance from the curve in between.

Therefore, since the color coordinates of the liquid crystal display modeled under the conditions of Examples 1 and 2 are closer to the reference example 1 than the color coordinates of the liquid crystal display modeled under the conditions of the Comparative Examples 1 and 2, the front color coordinates are excellent, so that appropriate contrast and luminance are achieved. At the same time, it can be seen that it works widely for the viewing angle.

Experimental Example 2

The color color coordinates measured through the simulation of the liquid crystal display modeled under the conditions of Example 1 and Comparative Examples 1 and 3 are divided into X and Y axes, respectively, and are shown as graphs in FIGS. 4A and 4B, respectively.

Here, the solid line S2 of the graph shown in FIG. 4 shows the color color coordinates measured after the random polarization irradiated from the light source through the light guide plate, the diffuser plate, and the two prism sheets through the simulation as the reference example 2. FIG.

According to the graph of FIG. 4, the color coordinates of the liquid crystal display modeled under the conditions of Example 1 are closer to the reference example 2 than the color coordinates of the liquid crystal display modeled under the conditions of Comparative Example 1, and in particular, from a viewing angle of + 60 ° to The curve is approximated between -40 °. And the color coordinates of the liquid crystal display modeled under the conditions of Example 1 are close to the color coordinates of the liquid crystal display modeled under the conditions of Comparative Example 3, and such features exhibit a curve closer to + 30 ° to + 30 °. As a result, Example 1 showed a graph curve close to the reference example 2, even though there is no compensation film as in Comparative Example 3.

Accordingly, when the results of Experimental Example 1 and Experimental Example 2 are collected and examined, the thickness of any one of the cholesteric liquid crystal layers is 0.9 µm to 2.1 µm, which is a minimum thickness that transmits and reflects random polarization of the wavelength band. When the total thickness of the laminated first cholesteric liquid crystal layer to the third cholesteric liquid crystal layer does not exceed 6.3 μm, the phase delay is reduced even in the absence of a compensation film. A liquid crystal display excellent in reproducibility can be expected.

Meanwhile, FIG. 5 is a cross-sectional view showing another embodiment of the reflective polarizing film 162, wherein the reflective polarizing film 162 selectively transmits random polarized light that forms at least one wavelength band of incident visible light. The upper cholesteric liquid crystal layer 162d for reflecting and the lower cholesteric liquid crystal layer 162e for selectively transmitting and reflecting the random polarized light of the remaining wavelength band of the incident visible light may be implemented. .

For example, the upper cholesteric liquid crystal layer 162d selectively transmits and reflects a random polarization including a blue wavelength band and a portion of the green wavelength band adjacent to the blue wavelength band, and lower cholesteric liquid crystal layer 162e. Is to selectively transmit and reflect random polarized light including the remaining green wavelength band and red wavelength band. In addition, the polarizing film 162 may be implemented by changing the stacking order of the upper and lower cholesteric liquid crystal layers 162d and 162e, and the wavelength band may also be appropriately adjusted.

At this time, each of the cholesteric liquid crystal layers 162d and 162e becomes wider when the birefringence becomes larger according to Equation 1 above. That is, when each of the cholesteric liquid crystal layers 162d and 162e has a birefringence of 0.26 or more not exceeding 0.30, the wavelength band of any liquid crystal layer covers at least 120 nm or more, so that only two liquid crystal layers It covers the full range of visible light.

Here, the upper and lower cholesteric liquid crystal layers 162d and 162e are a mixture of a curable nematic liquid crystal material and a curable chiral material, and the adjustment of the selective reflection wavelength region according to the composition of the two materials. It becomes possible. The curable nematic liquid crystal material and the curable chiral material may be used as long as the liquid crystal material includes a mesogenic group representing a nematic liquid crystal. In addition, the chiral material may be any material having chiral carbon in a conventional nematic liquid crystal, and is not limited to a specific material. Here, the curable includes a material having a thermosetting or photocurable reactive group in its molecular structure. For example, in the case of thermosetting, a combination of monomers including vinyl groups such as vinyl group, acrylic group and methacryl group, or having various reactive groups capable of condensation polymerization may be used. As the photocurable, a reactive group crosslinkable by ultraviolet rays such as a vinyl group, an acryl group, and an aryl group may be used.

On the other hand, the liquid crystal display device having the thin film cholesteric reflective polarizer 160 configured as described above is composed of a backlight unit 100 and a liquid crystal panel unit 200 as shown in FIG.

Here, the backlight unit 100 includes a light source 110, a light guide plate 120 for distributing the light emitted from the light source 110 to an entire area, and light incident from the light guide plate 120 with uniform brightness. The diffusion sheet 140 may be transformed into a surface light source, and two prism sheets 150 may be focused on the light incident from the diffusion sheet 140 and output to the reflective polarization film 160.

The liquid crystal panel unit 200 includes a liquid crystal 210, a front polarizing film 220 and a rear polarizing film 230 provided on the front and rear surfaces of the liquid crystal 210, and the rear polarizing film 230. An absorption type polarizing film 240 is interposed between the reflective polarizers 160.

The present invention is not limited only to the above-described embodiments, and various modifications, changes, substitutions, and additions can be made in various forms without departing from the spirit of the present invention. I can understand. If such improvement, change, replacement, or addition is carried out within the scope of the following claims, it is obvious that the technical idea also belongs to the present invention.

1 is a cross-sectional view of a conventional liquid crystal display.

2 is a cross-sectional view of a liquid crystal display according to the present invention.

Figure 3 (a) and (b) is a graph showing the color coordinates measured through Experimental Example 1 according to the present invention.

Figure 4 (a) and (b) is a graph showing the color color coordinates measured through Experimental Example 2 according to the present invention.

Figure 5 is a cross-sectional view showing another embodiment of a reflective polarizing film according to the present invention.

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

100: backlight unit 160: reflective polarizer

162: reflective polarizing film 164: phase delay plate

200: liquid crystal panel unit

Claims (9)

A reflective polarizing film in which a plurality of cholesteric liquid crystal layers having different pitches are laminated so as to selectively transmit and reflect randomly polarized light having a red, green, and blue wavelength band among incident visible light; A phase delay plate for converting the transmitted circularly polarized light into linearly polarized light, The laminated total thickness of each of the cholesteric liquid crystal layers does not exceed 6.3 μm. The method according to claim 1, The reflective polarizing film may include a first cholesteric liquid crystal layer selectively transmitting and reflecting random polarization forming the blue wavelength band, and a second cholesteric selectively transmitting and reflecting random polarization forming the green wavelength band. A thin film type cholesteric reflective polarizer comprising a liquid crystal layer and a third cholesteric liquid crystal layer for selectively transmitting and reflecting random polarization constituting the red wavelength band. The method according to claim 2, The first cholesteric liquid crystal layer is a thin film type cholesteric reflective polarizer, characterized in that made of a thickness of 0.9 ~ 2.1㎛. The method according to claim 2 or 3, The second cholesteric liquid crystal layer is a thin film type cholesteric reflective polarizer, characterized in that made of a thickness of 1.2 ~ 2.1㎛. The method according to claim 2 or 3, The third cholesteric liquid crystal layer is a thin film type cholesteric reflective polarizer, characterized in that made of a thickness of 1.2 ~ 2.1㎛. The method according to claim 1, The reflective polarizing film includes an upper cholesteric liquid crystal layer 162d that selectively transmits and reflects random polarized light that forms at least one wavelength band among incident visible rays, and random polarized light that forms the remaining wavelength band among the incident visible rays. A thin film type cholesteric reflective polarizer comprising: a lower cholesteric liquid crystal layer 162e for selectively transmitting and reflecting light. The method according to claim 6, Each of the cholesteric liquid crystal layers is a thin cholesteric reflective polarizer, characterized in that the birefringence of 0.26 ~ 0.30. A backlight unit having the thin film cholesteric reflective polarizer of claim 1. Liquid crystal display having a backlight unit of claim 8.

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