KR100683137B1 - Reflective type fringe field switching mode lcd - Google Patents

Abstract

The present invention discloses a reflective fringe field drive mode liquid crystal display device. The disclosed invention includes a liquid crystal layer comprising several liquid crystal molecules; A first substrate disposed on one side of the liquid crystal layer and having a counter electrode and a pixel electrode for generating a fringe field for driving liquid crystal molecules; A second substrate disposed on the other side of the liquid crystal layer and having R, G, and B color filters formed on an inner side thereof; A first horizontal alignment layer interposed between the liquid crystal layer and the first substrate and having a rubbing axis in a predetermined direction; A second horizontal alignment layer interposed between the liquid crystal layer and the second substrate and having a rubbing axis in a predetermined direction; A polarizer disposed on an outer surface of any one of the first substrate and the second substrate and having a predetermined polarization axis; And a reflecting plate disposed on an outer surface of the other one of the first substrate and the second substrate, wherein the distance d (λ) of the first and second substrates is λ (when driving in a normally black mode). 2n + 1) / 4Δn (λ) (n is an integer, λ: wavelength of incident light, Δn (λ): refractive index anisotropy of liquid crystal molecules).

Cell gap, fringe field

Description

Reflective fringe field drive mode liquid crystal display {REFLECTIVE TYPE FRINGE FIELD SWITCHING MODE LCD}

1 and 2 are cross-sectional views for explaining the operation of the conventional reflective fringe field driving mode liquid crystal display device.

Figure 3 is a graph showing the reflectance according to the retardation for each light source.

4 is a graph showing reflectance according to retardation at all wavelengths.

5 is a cross-sectional view of a reflective fringe field drive mode liquid crystal display device according to the present invention;

(Explanation of symbols for the main parts of the drawing)

10-lower substrate 12-counter electrode

14-gate insulating film 16-pixel electrode

18-first horizontal alignment layer 20-upper substrate

22-Black Matrix 24a, 24b, 24c- R, G, B Color Filter

26-second horizontal alignment layer 30-liquid crystal layer

35-polarizer 38-reflector

The present invention relates to a reflective liquid crystal display device, and more particularly, a reflective fringe field drive mode liquid crystal display device (hereinafter, referred to as a reflective FFS-) capable of improving contrast ratio without the addition of optical means. LCD).

Conventional reflective LCDs employ TN (twisted mematic) LCDs that twist-align nematic liquid crystal compositions having positive dielectric anisotropy. This reflective TN-LCD has a low power consumption and is used for relatively small LCDs such as electronic desk clocks and digital clocks. However, the reflective TN-LCDs have a chronic problem that the viewing angle characteristics are very bad and the contrast ratio is low.

Accordingly, a presently disclosed reflective fringe field drive liquid crystal display device, which has excellent viewing angle characteristics, secures high reflectance and aperture ratio, and does not require optical means such as λ / 4 plate, has been described by the applicants in Korean Patent Application No. 1999-. It was filed as 25214. A conventional reflective fringe field driving liquid crystal display device will be described with reference to FIGS. 1 and 2.

Referring to FIG. 1, the lower substrate 40 faces the upper substrate 60 at a predetermined distance d11 (hereinafter, referred to as a cell gap). A liquid crystal layer 65 including several liquid crystal molecules 65a is interposed between the lower substrate 40 and the upper substrate 60. At this time, the liquid crystal molecules 65a of the liquid crystal layer 65 are nematic, for example, using a material having a positive dielectric anisotropy. In addition, the liquid crystal layer 65 is a normalized black product of the retardation, that is, the product of the refractive index anisotropy (Δn) of the cell gap d11 and the liquid crystal molecules 65a so that the liquid crystal layer 65 itself can serve as a λ / 4 plate. For mode (Normally Black Mode), set it to (2n + 1) λ / 4. In the case of the normally white mode, the retardation of the actual liquid crystal molecules is (2n + 1) so that the effective retardation value is (2n + 1) λ / 4 when a voltage is applied. Adjust in the range of λ / 4 to (2n + 1) λ / 2.

The counter electrode 43a and the pixel electrode 46a are disposed on the inner side surface of the lower substrate 40 with the gate insulating layer 44 interposed therebetween so as to generate a fringe field for driving the liquid crystal molecules 65a.

On the other hand, a color filter (not shown) is arranged on the inner surface of the upper substrate 60.

In addition, a first alignment layer 53 is formed on an inner resultant surface of the lower substrate 40, and a second alignment layer 63 is formed on an inner resultant surface of the color filter of the upper substrate 60. The first and second alignment layers 53 and 63 have surfaces for arranging the liquid crystal molecules 65a in a predetermined direction. The first and second alignment films 53 and 63 are horizontal alignment films which are processed so that the liquid crystal molecules have a pretilt angle of 0 to 10 degrees, and each has a rubbing axis. Here, the rubbing axis R1 of the first alignment layer 53 forms a predetermined angle Ψ with the substrate projection line of the fringe field formed between the counter electrode 43a and the pixel electrode 46a. At this time, the angle formed between the rubbing axis R1 of the first alignment layer 53 and the projection line of the fringe field is adjusted within the range of 10 to 85 ° according to the dielectric anisotropy so as to obtain the maximum transmittance. The rubbing axis of the second alignment layer 63 is rubbed to have an anti-parallel, that is, about 180 ° angle difference with the rubbing axis R1 of the first alignment layer 53.

In addition, the polarizer 70 having the predetermined polarization axis 70a is disposed on the outer surface of the upper substrate 60. In this case, the initial state of the screen may be determined according to the arrangement of the polarization axis 70a. For example, when the polarization axis 70a is disposed to coincide with the rubbing axis, the polarization axis 70a is driven in the normally white mode, and the rubbing axis is 45 °. Arranged to achieve normal driving in black mode. In the prior art, for example, a case in normally white mode will be described. In addition, the reflector 75 is disposed outside the lower substrate 40. At this time, the reflector 75 serves to reflect the incident light 180 ° as is known.

The conventional reflective FFS-LCD having such a configuration is operated as follows.

First, if a fringe field is not formed between the counter electrode 43a and the pixel electrode 46b, the liquid crystal molecules 65a in the liquid crystal layer 65 are arranged substantially parallel to the substrate while their major axes coincide with the rubbing axis. Then, as shown in FIG. 1, the natural light 100a becomes incident light 100b traveling in a direction coinciding with the polarization axis 70a by the polarizing plate 70. The incident light 100b passes through the liquid crystal layer 65 arranged such that the rubbing axes R1 and R2 and the long axes of the liquid crystal molecules coincide with each other, and the traveling direction thereof does not change. The incident light 100b passing through the liquid crystal layer 65 is reflected by the reflecting plate 75 to become the reflected light 110a.

As the reflected light 110a passes through the liquid crystal layer 65 again, the traveling direction does not change. Accordingly, the traveling direction of the reflected light 110a coincides with the polarization axis 70a of the polarizer 70 and passes through the polarizer 70. Thus, the screen is in a white state.

On the other hand, when the fringe field F is formed between the counter electrode 43a and the pixel electrode 46a and above the electrode, the liquid crystal molecules 65a are arranged such that the fringe field and the long axis or the optical axis are parallel, so that the cell gap is as described above. Similarly, the liquid crystal layer 65 set to (2n + 1) λ / 4Δn to (2n + 1) λ / 2Δn has the same voltage as (2n + 1) λ / 4 (where n is an integer). Retardation is generated. Then, as shown in FIG. 2, the natural light 200a becomes incident light 200b coinciding with the polarization axis 70a by passing through the polarizer 70. The incident light 200b passing through the polarizer 70 changes its propagation direction by passing through the liquid crystal layer 65 having a retardation of (2n + 1) λ / 4 (where n is an integer), and the right circularly polarized light To incident light 200c. The right circularly polarized incident light 200c is reflected by the reflector to become the right circularly polarized reflected light 210a.

The reflected light 210a becomes the reflected light 210b orthogonal to the polarization axis 70a by retardation of the liquid crystal layer 65 again. Accordingly, the traveling direction of the reflected light 210a is orthogonal to the polarization axis 70a and thus cannot pass through the polarizer 70. As a result, the screen becomes dark. Accordingly, the display can be realized in a normally white manner without having the? / 4 plate.

On the other hand, although not shown in the drawing, in the normally black mode, the retardation value of the liquid crystal before voltage application is set to (2n + 1) λ / 4 (where n is an integer) and the polarization axis of the polarizer 70 ( 70a) and the rubbing axis are arranged to make 45 °.

Then, the light polarized while passing through the polarizer 70 plate becomes circularly polarized light while passing through the liquid crystal layer 65 before voltage application. Thereafter, when the liquid crystal layer 65 passes again through the reflecting plate 75, the light is changed into linearly polarized light perpendicular to the initial linearly polarized light. As a result, the initial voltage application screen becomes dark.

On the other hand, when a voltage is applied, the major axis of most liquid crystals coincides with the transmission axis of the polarizing plate, so that light passing through the polarizer 70a does not change its phase while passing through the liquid crystal layer 65. Subsequently, the phase does not change while passing through the reflecting plate 75 and the liquid crystal layer 65 again, and the screen is in a white state.

However, the conventional reflective FFS-LCD in which the liquid crystal layer 65 also serves as a λ / 4 plate has the following problems.

That is, even though the retardation is adjusted to (2n + 1) λ / 4 so that the liquid crystal layer 65 itself can serve as a lambda / 4 plate, since the wavelength of the incident light is different for each R, G, B pixel, Optimal retardation cannot be provided for each wavelength. For this reason, light leakage occurs in the section which should show dark.

That is, as shown in FIG. 3, the red light (a) having a wavelength of 630 nm, the green light (b) having a wavelength of 550 nm, and the blue light (B) having a wavelength of 480 nm have different wavelengths. Retardation to satisfy the equation is also different. As a result, dark or white colors are realized in different sections. Therefore, when the retardation of the liquid crystal layer 65 is made uniform, optimal contrast cannot be exhibited for each of R, G, and B pixels.

In addition, FIG. 4 is a graph showing reflectance according to retardation at all wavelengths. Referring to FIG. 4, when the retardation is small, the reflectance is 0.05 or less, but as the retardation is increased, the wavelength dependency becomes large, and dark. Hard to realize.

Accordingly, it is an object of the present invention to provide a reflective fringe field drive mode liquid crystal display device capable of improving contrast.

In order to achieve the above object of the present invention, the present invention,

Here, the R, G, B color filters are characterized in that their thickness is different.

(Example)

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

4 is a cross-sectional view illustrating a reflective fringe field driving mode liquid crystal display according to the present invention. Here, the operation of the reflective fringe field drive mode liquid crystal display device of the present invention operates in the same manner as the conventional device, and only the configuration thereof will be described.

First, referring to FIG. 5, the lower substrate 10 and the upper substrate 20 where unit pixels (not shown) are defined are spaced apart from each other by a predetermined distance.

The counter electrode 12 and the pixel electrode 14 are disposed in a known shape on the inner surface of the lower substrate 10 with the gate insulating film 14 interposed therebetween so as to generate a fringe field. The first horizontal alignment layer 18 is formed on the inner surface of the lower substrate 10. The first horizontal alignment film 18 has a predetermined rubbing axis (not shown), which has a predetermined angle with the substrate projection line of the fringe field. More preferably, in order to achieve a maximum transmittance, when the dielectric anisotropy of the liquid crystal molecules to be described later is positive, 45 ° to 90 ° with the substrate projection line of the fringe field, and 0 ° to 45 ° when the negative liquid crystal is achieved. .

On the other hand, the black matrix 22 is disposed on the inner surface of the upper substrate 20 so as to define unit pixels, and the R, G, B color filters 24a, 24b, for each unit pixel surrounded by the black matrix 22 are disposed. 24c) is disposed. In this case, the R, G, B color filters 24a, 24b, and 24c of the present invention are formed to have different heights in consideration of wavelengths different for R, G, and B light sources. That is, since the wavelength of the R light source 24a is 630 nm, the R color filter 24a is formed to have a first thickness t1 to have an optimal retardation. Further, the G color filter 24b is formed with a second thickness t2 that is larger than the first thickness t1 because the wavelength of the G light source is 550 nm smaller than the wavelength of the R light source. The B color filter 24c is formed with a third thickness t3 greater than the first and second thicknesses t1 and t2 since the wavelength of the B light source is 480 nm smaller than the wavelengths of the R and G light sources. By varying the heights of the R, G, and B color filters 24a, 24b, and 24c in this manner, the cell gap, that is, the distance d between the upper and lower substrates 10 and 20, respectively varies. The second horizontal alignment layer 26 is disposed on the surface of the color filters 24a, 24b, and 24c. At this time, the second horizontal alignment layer 26 also has a predetermined rubbing axis, which is 180 ° with the rubbing axis of the first horizontal alignment layer 18.

The polarizer 35 is disposed outside the upper substrate 20, and the polarizer has a predetermined polarization axis. At this time, the arrangement of the polarization axis is changed according to the initial screen state. That is, when operating in the normally black mode, the polarization axis is preferably ± 30 to 50 °, more preferably ± 45 ° to the rubbing axis of the second horizontal alignment layer. On the other hand, when operating in normally white mode, the polarization axis and rubbing axis of a polarizing plate are arrange | positioned or orthogonally.

The reflecting plate 38 is disposed outside the upper substrate 10.

The liquid crystal layer 30 having several liquid crystal molecules is interposed between the upper and lower substrates 10 and 20. The liquid crystal layer 30 may have positive or negative dielectric anisotropy. Here, since the color filters 24a, 24b, and 24c are formed at different heights in consideration of the wavelengths of the R, G, and B light sources, the cell gaps are different for each pixel, so that the retardation (cell gap and refractive index of the liquid crystal layer 30) is performed. Product of anisotropy) is different for each pixel. That is, the region of the R color filter 24a has the largest first cell gap d1, the region of the G color filter 24b has the second cell gap d2 smaller than the first cell gap d1, and the B color filter ( The region 24c has a third cell gap d3 smaller than the first and second cell gaps d1 and d2. In addition, the liquid crystal layer of the present invention has a retardation value of (2n + 1) / 4 in the normally black mode so as to act as a λ / 4 plate.

Accordingly, the cell gap of the normally black mode liquid crystal display device may be defined by the following equation.

d (λ) = λ (2n + 1) / 4Δn (λ) (n is an integer) --- (Equation 1)

d (λ): cell gap

λ: wavelength of incident light, R light source = 630 nm, G light source = 550 nm, B light source = 480 nm

Δn (λ): refractive index anisotropy of liquid crystal molecules

On the other hand, as mentioned in the prior art, when driving in the normally white mode, the actual retardation of the liquid crystal layer is (2n + 1) / 4 to (2n +) so that the effective retardation can have (2n + 1) / 4. It is preferable to have 1) / 2. Therefore, the actual cell gap of the liquid crystal layer in the normally white mode is defined by the following equation.

d (λ) = λ (2n + 1) / 4Δn (λ) to λ (2n + 1) / 2Δn (λ) (n is an integer) --- (Equation 2)

d (λ): cell gap

λ: wavelength of incident light, R light source = 630 nm, G light source = 550 nm, B light source = 480 nm

Δn (λ): refractive index anisotropy of liquid crystal molecules

That is, the cell gap of the present invention is a variable of the wavelength of incident light, and its value is changed by the wavelength of the incident light. This cell gap is adjusted according to the height of the color filter. Therefore, each pixel can have an optimal retardation for each light wavelength.

As described in detail above, according to the present invention, in the reflective FFS-LCD, the cell gaps of the liquid crystal layer are formed to be different from each other according to the wavelength of the light source, and the liquid crystal layer acts as a λ / 4 plate. Optimum retardation is also exhibited at the wavelength. Accordingly, light leakage at dark is greatly reduced, and the contrast ratio is improved.

Claims (8)

A liquid crystal layer comprising several liquid crystal molecules; A first substrate disposed on one side of the liquid crystal layer and having a counter electrode and a pixel electrode for generating a fringe field for driving liquid crystal molecules; A second substrate disposed on the other side of the liquid crystal layer and having R, G, and B color filters formed on an inner side thereof; A first horizontal alignment layer interposed between the liquid crystal layer and the first substrate and having a rubbing axis in a predetermined direction; A second horizontal alignment layer interposed between the liquid crystal layer and the second substrate and having a rubbing axis that is non-parallel to the rubbing axis of the first horizontal alignment film; A polarizer disposed on an outer surface of any one of the first substrate and the second substrate and having a predetermined polarization axis; And A reflection plate disposed on an outer surface of the other of the first substrate and the second substrate, When the distance d (λ) of the first and second substrates is driven in a normally black mode, λ (2n + 1) / 4Δn (λ) (n is an integer, λ: wavelength of incident light, Δn (λ): Refractive index anisotropy of liquid crystal molecules). The reflective FFS-LCD according to claim 1, wherein the polarization axis of the polarizer and the rubbing axis of the second horizontal alignment layer are ± 30 to 50 °. The reflective FFS-LCD of claim 1, wherein the R, G, and B color filters have different thicknesses. The rubbing axis of the first or second horizontal alignment layer is 45 ° to 90 ° with the substrate projection line of the fringe field when the dielectric anisotropy of the liquid crystal molecules is positive, and 0 ° to 45 ° when the negative liquid crystal is used. Reflective type FFS-LCD characterized in that. A liquid crystal layer comprising several liquid crystal molecules; A first substrate disposed on one side of the liquid crystal layer and having a counter electrode and a pixel electrode for generating a fringe field for driving liquid crystal molecules; A second substrate disposed on the other side of the liquid crystal layer and having R, G, and B color filters formed on an inner side thereof; A first horizontal alignment layer interposed between the liquid crystal layer and the first substrate and having a rubbing axis in a predetermined direction; A second horizontal alignment layer interposed between the liquid crystal layer and the second substrate, the second horizontal alignment layer having a rubbing axis that is non-parallel to the rubbing axis of the first horizontal alignment film; A polarizer disposed on an outer surface of any one of the first substrate and the second substrate and having a predetermined polarization axis; And A reflection plate disposed on an outer surface of the other of the first substrate and the second substrate, When the distance d (λ) between the first substrate and the second substrate is driven in normally white, λ (2n + 1) / 4Δn (λ) to λ (2n + 1) / 2Δn (λ) (n is Constant,?: Wavelength of incident light,? N (?): Refractive index anisotropy of liquid crystal molecules). The reflective FFS-LCD according to claim 5, wherein the polarization axis of the polarizer is coincident with or perpendicular to the rubbing axis direction of the second horizontal alignment layer. 6. The reflective FFS-LCD of claim 5, wherein the R, G, B color filters have different thicknesses. The rubbing axis of the first or second horizontal alignment layer is 45 ° to 90 ° with a substrate projection line of the fringe field when the dielectric anisotropy of the liquid crystal molecules of the liquid crystal layer is positive, and the dielectric anisotropy is a negative liquid crystal. In the case of reflective FFS-LCD characterized in that it consists of 0 ° ~ 45 °.

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