JPH0690376B2 - Color liquid crystal display - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a color liquid crystal display device formed by combining a color filter and a liquid crystal, particularly a twisted nematic liquid crystal, and to provide a color liquid crystal display device excellent in contrast and color reproducibility. It is provided.

The present invention is used in color image display devices such as televisions and video monitors, computer terminals, and the like.

[0003]

2. Description of the Related Art Since a liquid crystal display device is thin and can be driven at a low voltage and consumes less power, market needs have recently been rapidly increased as a flat display device. Conventionally, a mono-color type was mainly used, but a color liquid crystal display device using a color filter is about to be commercialized.

Liquid crystal modes used in such a color liquid crystal display device include dynamic scattering (hereinafter abbreviated as DSM),
Twisted nematic (same TN), guest host (same GH), etc. are considered, but here, TN liquid crystal and red (abbreviated as R hereinafter), green (same G), and blue (same B) color filters are combined. A color liquid crystal display device configured as described above will be described as a conventional example.

Before describing the structure of the conventional example, the optical properties of the TN liquid crystal, which is the basic concept of the present invention, will be briefly described.

FIG. 1 shows the display principle of a transmissive TN liquid crystal display device. The liquid crystal 1 and the transparent substrates 2a and 2b form a liquid crystal cell, and the polarizing plates 3a and 3b are arranged so that their polarization axes are parallel to each other. The arrows in the figure represent the traveling direction of incident light and the plane of polarization.

At this time, when no voltage is applied, the liquid crystal cell blocks light, and when a voltage above a certain threshold value (hereinafter abbreviated as V _th ) is applied as shown in FIG. 1A, the liquid crystal cell is _exposed to the electric field in FIG. 1B. As described above, the liquid crystal rearranges (the dielectric constant anisotropy of the liquid crystal is positive), and the incident light passes through the liquid crystal cell as it is. This makes it possible to form bright and dark contrast. The dark state when no voltage is applied as described above is defined as normally black.

It is considered that the transmittance T when no voltage is applied in an optical system as shown in FIG. 1, that is, normally black, is ideally zero, but in reality, due to the optical rotatory dispersion of the TN liquid crystal, The linearly polarized light incident on the cell becomes elliptically polarized light and partially passes through the cell. The transmittance T of the passing light is C.I.
H. Gooch and H.C. A. According to Tarry, the following equation (J.
Phys. D: Appl. Phys. 8,1575 (1
975)).

T = (1 + u ² ) ⁻¹ sin ² [θ (1 + u ² ) ^1/2 ] ……………… (1) However, u = πdΔn / 0λ ……………………………… (2) where d is the thickness of the liquid crystal layer, Δn is the birefringence θ of the liquid crystal, and TN is TN.
Liquid crystal twist angle, λ, represents the wavelength of incident light.

Generally, Δn of liquid crystal has wavelength dependence.
As a liquid crystal in FIG. 2, LIXON 9150 manufactured by Chisso Corporation
As an example, the wavelength dependence of Δn will be shown (following this liquid crystal as a model). Based on the wavelength-dependent data of Δn, the spectral transmission characteristic of the cell having the twist angle θ of 90 ° was calculated by Gooch-Tarry's equation (1) so that d was 5 μm.
FIG. 3 is a plot for the case of and 8 μm.

As can be seen from FIG. 3, the visible region (400
(-700 nm) has a peak transmittance of about 10% and does not completely block light even when no voltage is applied. Further, since the cell has a spectral transmission characteristic as shown in the same figure, it appears reddish purple when d is 5 μm and yellow when it is 8 μm. Just 5
Light of wavelength around 570 nm is blocked at μm, and light of around 440 nm is blocked at 8 μm.

Therefore, if a TN mode liquid crystal is used, there is a problem of light leakage and coloring in a dark state when no voltage is applied. This is not a big problem in the case of displaying a mono color image, but it is a big obstacle in the case of displaying a color image. Based on these, a color liquid crystal display device combined with a conventional color filter will be described.

FIG. 4 shows a cell sectional view of a conventional color liquid crystal display device. In FIG. 4, reference numeral 4 designates R.I.s arranged in a matrix as shown in FIG. G and B color filters, 6a and 6b are alignment films for controlling the initial alignment of the liquid crystal 1 when no voltage is applied, and when a voltage is applied between the transparent conductive films 5a and 5b, the molecular alignment of the liquid crystal 1 is changed. It modulates and modulates the light passing through the liquid crystal cell.

At this time, if the liquid crystal corresponding to each of the R, G and B color filters is driven with an appropriate voltage width of V _th or more,
Full color display can be achieved by the additive color mixture of R, G and B. An example of the spectral characteristics of the R, G, B color filters is shown in FIG.

[0015]

Problems to be solved by the invention are the above-mentioned leakage and coloring of light when no voltage is applied.

The contrast ratio is (light transmittance in the bright state)
It is defined by / (light transmittance in dark state), but in the conventional configuration, there is a problem that light leakage when no voltage is applied, that is, in dark state, lowers the contrast ratio.

In the conventional structure, the thickness d of the liquid crystal layer is R,
Since the G and B color filter sections are uniform, for example, when d = 5 μm, as can be seen from FIGS. 3 and 6, when no voltage is applied, the G and R color filter sections block light. However, there is a problem in that the B filter portion does not block light and is colored blue or purple as a whole when no voltage is applied. This is a great obstacle to full-color display.

However, as can be seen from the Gooch-Tarry equation (1), when the thickness of the liquid crystal layer is increased (about 10 μm or more), the transmittance when no voltage is applied is reduced, and accordingly, the coloring is also relatively reduced. Two problems are alleviated. However, when d becomes large, the voltage turns on.
The response time of the liquid crystal to -OFF becomes slow, the viewing angle of the liquid crystal panel becomes narrow, and the color shift due to parallax occurs.
Therefore, the performance of the color liquid crystal display device is extremely poor.

In the color liquid crystal display using the TN mode liquid crystal, in view of the problems of the response time, the viewing angle, and the color misregistration, the thickness of the liquid crystal layer is relatively small (4 to 6 μm), and light of no voltage is applied. It is impossible to solve the two problems of leakage and coloring as long as the conventional configuration in which the thickness of the liquid crystal layer is uniform in the R, G and B portions as shown in FIG.

The present invention has been made in view of the above-mentioned drawbacks of the conventional example, and a color liquid crystal display having a high contrast and excellent color reproducibility by minimizing light leakage and coloring of the liquid crystal cell when no voltage is applied. The purpose is to provide a device.

[0021]

The present invention uses a TN mode liquid crystal and optically optimizes the thickness of the liquid crystal layer corresponding to each color filter of R, G, B, etc.
A color liquid crystal display device having excellent performance is provided by using a white light source having a spectral emission characteristic close to a line spectrum for each wavelength of blue and the color filter in combination. Is.

[0022]

According to the present invention, a vivid and excellent color display can be realized by the above constitution.

[0023]

EXAMPLE A transmissive color liquid crystal display device using TN mode liquid crystal will be described in detail as an example.

Now, a white light source having a spectral intensity as shown in FIG. 7 in accordance with the R, G, and B color filters having the spectral characteristics as shown in FIG. Light]. And R, G, B is 61
The representative wavelengths are 0 nm, 545 nm, and 450 nm.

As can be seen from the spectral radiation characteristic of FIG. 7, the light source is a white light source having characteristics close to a line spectrum for each wavelength of R, G and B, which is different from the embodiment of the present invention. It is very effective, and its effectiveness is diminished in the one having a continuous spectrum such as incandescent lamp or in the monochromatic light source such as EL.

Therefore, it is assumed that the LIXON 9150 having the wavelength dispersion of Δn as shown in FIG. 1 is used as the liquid crystal material. Based on the equation (1), light of each wavelength of R, G, and B exhibits normally black transmission characteristics according to the thickness d of the liquid crystal layer when no voltage is applied, as shown in FIG. Figure 8
Although the left end of the graph is omitted, the R, G, and B curves monotonically increase as d approaches 0, and all become 1 at d = 0 μm.

As can be seen from this graph, R, G, B
When the thickness of the liquid crystal layer is viewed from the smaller side with respect to each wavelength of light, T = 0 at 5.4 μm, 4.8 μm, and 3.7 μm, and the light is completely blocked by the liquid crystal layer. That is, R,
When the thicknesses of the liquid crystal layers corresponding to the G and B color filters are dR, dG, and dB, respectively, dR = 5.4 μm, d
If G = 4.8 μm and dB = 3.7 μm, the light is completely blocked by each filter portion when no voltage is applied, and the light is blocked over the entire liquid crystal panel, so that the above-mentioned coloring problem cannot occur.

Next, the actual structure and manufacturing method will be described with reference to FIG. First, the color filter layer 4 is formed on the transparent substrate 2b by changing the thicknesses of R, G, and B. As described above, dR, dG, and dB are 5.4 μm and 4.8 μm, respectively.
In order to obtain m and 3.7 μm, for example, the thickness of the filter R portion is set to 1 μm, the G is 1.6 μm, and the B is 2.7 μm.

As a method of forming the color filter 4,
Application, selective removal of organic substances mainly composed of gelatin,
The dyeing is performed by repeating the dyeing three times, but other methods such as screen printing, vapor deposition of dye, and electrodeposition coating are also possible.

A transparent conductive film 5 of In ₂ O ₃ , SnO ₂ or the like is formed on the color filter 4 thus formed.
b is formed, and the alignment film 6b is formed thereon. As the alignment film, an organic material such as polyimide or polyvinyl alcohol is usually applied by spinner, printing, etc. and the surface is rubbed for use. However, even if SiO is applied at a constant angle and vapor deposition, the same function as the alignment film is obtained. Fulfill

On the other transparent substrate 2a, the transparent conductive film 5a and the alignment film 6a are formed by the same method as described above, and dB
Of the transparent substrates 2a and 2b so as to be 3.7 μm (dG and dR are 4.8 μm and 5.4, respectively).
μm. ), The liquid crystal 1 is enclosed in this facing space.

The polarizing plates 3a and 3b are installed parallel or perpendicular to the rubbing direction of the alignment film so that their polarization axes are parallel to each other.

In the above description, in the equation (1), the minimum value that gives T = O

[0034]

[Equation 1]

An example in which the optical path difference (d · Δn / λ) is corrected in the vicinity of, that is, in the vicinity of d of 5 μm in FIG.

On the other hand, in FIG. 8, dG = 10.7 μm and dR = 1
Even at 2 μm and dB = 12.7 μm, the transmitted light of each color becomes zero, and the liquid crystal film thickness difference between them is small. The present invention is such a large region of u

[0037]

[Equation 2]

It is also applicable to When the correction is performed by the combination described above, the size relation of the thickness of the liquid crystal layer corresponding to the R, G, B color filters is different from that in the above-described embodiment.

In the embodiments of the present invention, the color filters have been described as being limited to three types of R, G and B, but the present invention can be similarly applied to the case where four or more types are mixed with other colors. is there. Further, the color filter need not be formed only on one substrate side, but may be formed on both upper and lower substrates, and some colors may be formed on one substrate and other colors may be formed on the other substrate. It may be formed in. In any case d
There is no problem as long as R, dG, and dB are optically optimized values.

The idea of the present invention can be applied to the entire transport of a color liquid crystal display device using TN liquid crystal, and not only simple matrix drive but also one substrate
It does not matter if a non-linear element such as MOSFET, TFT, or MIM is incorporated, or if it is of a transmissive type or a reflective type.

Further, although the description of the present invention is limited to the case of normally black, it can be utilized as it is in the case of normally white which becomes a bright state when no voltage is applied.

From another point of view, on the other hand, by adopting the configuration of the present invention, a change in color tone and a deviation in white balance due to an error in the thickness of the liquid crystal layer during assembly of the liquid crystal cell are extremely small. This is shown in FIG. FIG. 10 shows the transmittance of each color on the green filter when the thickness of the liquid crystal layer corresponding to each filter is optimized for each filter according to the present invention and then the thickness of the liquid crystal layer is changed from the design value due to an assembly error. It is shown in relation to the thickness of the liquid crystal layer. As is apparent from FIG. 10, at the design center value d = 4.8 μm, the transmission is zero for each color light. On the other hand, when d changes from this value, R, G, B
The transmittance of each color light increases almost uniformly. Therefore, the white balance of the liquid crystal cell is maintained and the change in display color tone is small. On the other hand, if the conventional cell structure is adopted, it is obvious from FIG. 8 that the color tone and the like change greatly due to the change in cell thickness.

[0043]

By adopting the above-mentioned structure,
It is possible to provide a color liquid crystal display device having excellent contrast and color reproducibility by eliminating light leakage and coloration when no voltage is applied. This can be realized with a relatively small thickness of the liquid crystal layer by appropriately selecting the liquid crystal material, so that the response time of the liquid crystal is fast, the viewing angle is wide, and there is no color shift due to the field of view. Very good.

From another point of view, even if the thickness of the liquid crystal layer is slightly deviated when the liquid crystal cell is assembled,
The thickness of the liquid crystal layer of each of the R, G, and B color filters is slightly deviated from the optimum design value, which causes a sudden increase in light leakage when no voltage is applied.
The so-called white balance is not disturbed and the liquid crystal cell is not colored.

The present invention is very important for the basic design of a color liquid crystal display device using TN liquid crystal, and its application field is extremely wide.

[Brief description of drawings]

FIG. 1 is a diagram showing a display principle of a TN liquid crystal display device.

FIG. 2 is a diagram showing wavelength dependence of Δn of liquid crystal.

FIG. 3 is a diagram showing an example of spectral transmission characteristics of a liquid crystal cell.

FIG. 4 is a sectional view of a cell of a conventional color liquid crystal display device.

FIG. 5 is a diagram showing an array of R, G, and B color filters.

FIG. 6 is a diagram showing spectral transmission characteristics of R, G, and B color filters.

FIG. 7 is a diagram showing the spectral intensity of a light source .

FIG. 8 is a diagram showing the dependence of the spectral transmission characteristics of R, G, and B wavelengths on the thickness of the liquid crystal phase.

FIG. 9 is a cell sectional view of a color liquid crystal display device according to an embodiment of the present invention.

FIG. 10 is a diagram showing the relationship between the thickness of the liquid crystal layer of the green filter portion of the device of the present invention and the transmittance of each color of light.

[Explanation of symbols]

 1 Liquid crystal 2a, 2b Transparent substrate 3a, 3b Polarizing plate 4 Color filter 5a, 5b Transparent conductive film 6a, 6b Alignment film

 ─────────────────────────────────────────────────── ─── Continuation of front page (56) Reference JP-A-57-62010 (JP, A) JP-A-58-120287 (JP, A) JP-A-58-193523 (JP, A) JP-A-60- 159824 (JP, A) JP 60-159827 (JP, A) JP 60-159830 (JP, A) JP 60-159831 (JP, A)

Claims (1)

[Claims] 1. A liquid crystal layer is sandwiched between a first substrate and a second substrate facing each other, and a plurality of types of color filters having different spectral transmission characteristics are arranged on at least one of the substrates, and a voltage is applied to the liquid crystal layer. And a means for applying light modulation to change the thickness of the liquid crystal layer corresponding to each color filter in accordance with the spectral transmission characteristics of the color filter.
A color liquid crystal display device characterized by using a white light source having a spectral emission characteristic close to a line spectrum for each wavelength of green and blue and the color filter in combination.