JP2558905B2 - Liquid crystal display - Google Patents

Description

The present invention relates to a liquid crystal display device in which twist nematic liquid crystal is color-compensated.

<Prior Art> Generally, in a liquid crystal display device, when the voltage applied to the liquid crystal layer is less than or equal to a threshold value (threshold voltage) or no voltage is applied, light transmission is closed (black display). In a so-called normally black mode TN-FEM (twisted nematic field effect) type liquid crystal display device in which optical conditions such as a polarizing plate are set in order to obtain good display quality with high contrast, It is important to make the light transmittance in the state of (below threshold voltage) sufficiently low.

However, in the TN-FEM liquid crystal display device, the light transmittance in the normally black mode when no voltage is applied can be approximated by the well-known GOOCH-TARRY formula, and the horizontal axis indicates the retardation value d of the liquid crystal display panel. When Δn is the light transmittance on the vertical axis, it is shown for each wavelength γ = 450 nm, 550 nm, 650 nm as shown in FIG.

The light transmittance greatly changes depending on the value of d · Δn and also changes depending on the wavelength of light. In an actual liquid crystal display device, d.
There is a restriction on the setting of Δn, and it is often set to a value having a high transmittance. Further, even if the optimum value is set at a certain wavelength, light of other wavelengths has a high transmittance, so that it may be perceived as bright or colored by human eyes.

TVs that require high color purity and high contrast
In the case of video image display and the like, this problem has a great influence. TFT-LCD (Thin Film Transis) using TN-FEM liquid crystal
In a liquid crystal display device using a tor, the value of d · Δn is 0.5μ.
In many cases, the contrast of about 100: 1 is obtained in the normally white mode, whereas the value in the normally black mode is about 30: 1, which is the difference in display quality. ing. This difference becomes even larger at a lower value of d · Δn, which has excellent viewing angle characteristics. Therefore, in many TFT-LCD color display devices today, the normally white mode is adopted. However, in the normally white mode, the pixel defects of the TFT element are conspicuous as bright spots, which is a serious problem in production in terms of yield and the like.

In addition, a method is often used in which normally black mode liquid crystal cells are used to control the transmission of light to display pictures, characters, color filters, etc. formed on the back surface of the liquid crystal display panel or in the cells. ing. For example, there are warning displays for automobiles and airplanes, and audio level meters. In these displays, unless the transmittance in the shutter state for closing light is sufficiently low, the display pattern on the back surface appears to be raised, so that the quality is remarkably deteriorated.

In order to solve this problem, a two-layer liquid crystal display device has been developed (Japanese Patent Publication No. 63-53528, etc.). This display device has two liquid crystal cells in which liquid crystal molecules are twist-nematic aligned, one is a display cell for performing a display operation, and the other is a compensating liquid crystal cell for returning elliptically polarized light to linearly polarized light. Is. In a so-called double TN structure (DTN) liquid crystal display device in which liquid crystal cells are superposed for optical compensation, the liquid crystal panel has two layers, and thus there are problems such as thickening and weighting.

<Problems to be Solved by the Invention> Therefore, an optical retardation film (film) for optical compensation is provided.
Has been studied, and various proposals have been made mainly for the case of super twist mode (STN).
However, in the TN mode where the twist angle is around 90 degrees, the conditions for setting the axis of the retardation plate and the retardation value are significantly different from those in the STN-LCD, and there is no effective proposal.

The present invention solves the above-mentioned problems, can be made thinner and lighter than conventional two-layer TN-LCDs and TN-TFT-LCDs, and provides clear white / black display. An object is to provide a liquid crystal display device obtained.

<Means for Solving the Problems> The present invention relates to a twisted nematic layer in which an upper polarizing plate, a 90 ° twist nematic liquid crystal display cell, and a lower polarizing plate having a transmission axis parallel to the upper polarizing plate are laminated in this order. Type liquid crystal display device, an optical retardation plate is provided on at least one of both sides of the liquid crystal display cell, and the axial direction showing the maximum refractive index of the retardation plate is close to the retardation plate. From the direction perpendicular to the long-axis direction of liquid crystal molecules of 10 ~
The retardation plates are arranged so as to be offset by a positive angle of 20 degrees, and the sum of the retardations (Re) of the retardation plate is calculated from the anisotropy of the liquid crystal layer thickness (d) of the liquid crystal display cell and the refractive index of the liquid crystal material. For product (d · Δn) of (Δn), d · ΔN-0.05μ
It is characterized in that m ≦ Re ≦ d · Δn + 0.1 μm.

<Operation> In a liquid crystal display device, linearly polarized light that has passed through the lower polarizing plate has elliptically polarizing properties after passing through a liquid crystal layer twisted by 90 degrees, and its major axis direction is rotated by about 90 degrees. , It varies depending on the wavelength. However, due to the action of the optical retardation plate in the present invention, the elliptically polarized light is compensated and the major axis direction is corrected (90 ° rotation is ideal for linearly polarized light), and the light when no voltage is applied in the normally black mode. The transmittance can be reduced and the contrast ratio can be improved.

<Example> An example of the present invention will be described below with reference to the drawings. FIG. 1 shows a basic configuration example of the liquid crystal display device of the present invention. Reference numeral 1 is an upper polarizing plate, and 2 is a retardation plate arranged for optical compensation. 3 and 10 are glass substrates, 4 and 9 are transparent conductive film electrodes (ITO),
Reference numerals 5 and 8 are alignment films, 6 is a liquid crystal composition, and 7 is a sealant, which constitutes a so-called twisted nematic liquid crystal display cell. Reference numeral 11 is a lower deflection plate having a transmission axis parallel to the upper polarizing plate 1. As the retardation plate 2, a birefringent film of polyvinyl alcohol, a polycarbonate film, or the like can be used.

FIG. 2 is a diagram for explaining each optical axis of one embodiment of the present invention. In the case of the structure shown in FIG. ₁ , P ₁ is the major axis alignment direction (direction of the director) of the liquid crystal molecules near the upper substrate (the substrate 3 closest to the retardation plate), and P ₂ is the length of the liquid crystal molecules in the central part of the liquid crystal layer. The axial alignment direction (direction of the director), P ₃ is the direction orthogonal to P _1, and in the case of 90 ° twist, the long axis alignment direction of the liquid crystal molecules near the lower substrate (the substrate 10 that is the farthest in contact with the retardation plate). Become. P ₄
Indicates the setting direction of the maximum refractive index axis (phase difference plate axis) of the phase difference plate 2. Since this structure is a normally black mode, the transmission axis of the lower polarizing plate 11 coincides with the P ₃ direction, and the transmission axis of the upper polarizing plate 1 is parallel to the transmission axis of the lower polarizing plate 2, that is, P 1 The transmission axis is arranged to coincide with the axis orthogonal to the axis.

Example 1 In the TN-FEM liquid crystal display device, the retardation plate 2 was arranged between the upper polarizing plate 1 and the liquid crystal cell, and the axial direction was changed by rotating. After that, the axis of the retardation plate 2 is set to the second
In the figure, the P ₃ direction is 0 degree, and is expressed as an angle (θ) in the positive direction toward the P ₄ direction.

In the above, a TN-FEM liquid crystal display device having the following cell parameters was created.

1) Cell gap (thickness of liquid crystal layer) d: 4.5 μm and 5.5 μm 2) Liquid crystal composition material: Δn (levorotatory); 0.09 and 0.12 composition; phenylcyclohexane-based mixed liquid crystal 3) Cell structure: See FIG.

Using a PVA (polyvinyl alcohol) -based retarder 2 (for example, a product manufactured by Kayapora Co., Ltd.) in this liquid crystal cell, the total Re value of retardation changes from 320 nm to 700 nm at intervals of about 10 to 20 nm. Then, I made a prototype.

(Optimal setting axis angle) FIG. 3 shows the evaluation result of the liquid crystal display device. On the horizontal axis
The change in angle from P ₃ is shown, and the transmittance is taken as 100% on the vertical axis when the phase difference plate 2 is not used. The retardation value (Re) of 2 and d · Δn of the liquid crystal layer are slightly different, but a lower transmittance is obtained between the angle with the set axis of the retardation plate 2 than when the retardation plate 2 is not used. It was found that there is an angle to hold and there is a minimum value.

FIG. 4 shows the case where d · Δn value = 0.5 μm, which is a general liquid crystal in TN-TFT-LCD, and the retardation value (Re) of the retardation plate 2 is also 0.5 μm. , The angle θ from P ₃ is in the range of 0 to 30 degrees
The transmittance was lower than that without, and the minimum transmittance was obtained at about 15 degrees.

(Optimal Retardation) FIG. 5 shows retardation values (Re) of the retardation plate 2 and measured values of transmittance when the axis of the retardation plate 2 is set at 10 degrees. Retardation value (Re) of retarder 2 on the horizontal axis
Is the light transmittance when the retardation plate 2 is not used on the vertical axis.
The transmittance is taken as%. FIG. 5 shows d.
The retardation value (Re) of the retardation plate 2 is 0.35 μm when the Δn value is 0.4 μm and the setting angles of the polarizing plates 1 and 11 are parallel.
At ~ 0.5μm, it shows lower transmittance than the case where the retardation plate 2 is not used, and the retardation value (Re) is about 0.45μ.
The minimum value of the transmittance was taken near m.

In addition, the d · Δn value of the liquid crystal layer is set to 0.5 μm, and the polarizing plate
When the set angles of 1 and 11 are parallel, as shown in FIG. 6, the retardation value (Re) of the retardation plate 2 is 0.45 μm
The transmittance was low at 0.6 μm, and the transmittance became the minimum value when the retardation value (Re) of the retardation plate 2 was about 0.55 μm.

Other combinations of cell gap d and Δn of liquid crystal material were carried out. FIG. 7 shows the retardation d · Δn of the liquid crystal layer on the horizontal axis and the retardation value (Re) of the retardation plate 2 on the vertical axis. The transmittance without voltage application is lower than that when the retardation plate 2 is not used. In the figure, the circles show the upper limit, and the circles show the lower limit.
The axial angle θ of the phase difference plate 2 is set to three types of 10 degrees, 15 degrees, and 20 degrees.

The retardation value d · Δn of the liquid crystal layer is 0.66 μm,
From the above results including the case of 54 μm, etc., in order to obtain good contrast characteristics (low transmittance at the time of OFF), as compared with the case where the retardation plate 2 is not used, d · Δn−0.05 μm ≦ Re ≦ d · It is preferable to set the retardation value Re that satisfies Δn + 0.1 μm.

Example 2 In the case of TN-TFT-LCD which satisfies the conditions of the above-mentioned example, a TFT-LCD color liquid crystal display device was prepared for confirmation. TF
The T structure was the well-known structure shown in FIG. 8 (inverted stagger type), and the color filter was prepared by the gelatin dyeing method.

FIG. 8 is a configuration diagram of a color liquid crystal display device of TN-TFT-LCD, and FIG. 9 is a plan view of the inverse stagger type TFT-LCD.
The other structure is the same as in FIGS. 1 and 2 except for the liquid crystal display cell portion. Also in this case, it was confirmed that the above-mentioned relationship between "(optimum set angle)" and "(optimum retardation)" was the same.

This will be described in more detail below with reference to FIGS. 8 and 9.

The gate bus line 11 is formed on the transparent substrate 22, and a part of the gate bus line 11 functions as the gate electrode of the TFT. An anodized film 12 is formed on the gate bus line 11, and a gate insulating film 13 is deposited on the entire surface of the anodized film 12. A semiconductor layer 14 is formed on the portion of the gate bus wiring 11 that functions as a gate electrode, with the above-described anodic oxide film 12 and gate insulating film 13 interposed therebetween. A semiconductor layer protective film 15 is formed on the semiconductor layer 14 to protect the semiconductor layer 14.

Further, two contact layers 16 and 16 are provided on the semiconductor layer 14, and the source electrode 1 is provided on each of the contact layers 16 and 16.
7 and the drain electrode 18 are formed. Source electrode 17
Is connected to the source bus line 17 ', and the drain electrode 18 is connected to the pixel electrode 20. TFT formed in this way
The protective film 19 is formed on the protective film 19, and the alignment film 21 is formed on the entire surface of the protective film 19.
Are formed.

Other parameters and characteristics of the created display device
Shown in the table. The number of pixels is 240 × 384 = 92,160, the color filter arrangement method is RGB delta arrangement, twist angle 90 degrees,
The gradation display characteristic is 16 gradations. The liquid crystal material used was phenylcyclohexane mixed liquid crystal, and Δn was 0.09 μm.
belongs to. Two types of cell gap d were prepared, 4.5 μm and 5.5 μm. Therefore, the retardation values d · Δn of the liquid crystal layer are 0.4 μm and 0.5 μm.

The retardation film 2 used was a PVA-based film retardation film (made by Kayapora Co., Ltd.) with a retardation (Re) of 450 nm and 5
It is 50 nm.

Similar to FIGS. 1 and 2, the retardation plate 2 is arranged on the upper side,
The set angle θ of the retardation plate 2 was set to 10 degrees with respect to P ₁ .

Of the normally black mode created in this way
The TN-TFT liquid crystal display device was compared with those of the normally black and normally white modes which do not use a retardation plate. Table 2 shows a comparison of video image display quality.

Looking at the contrast ratio, the conventional normally black mode was "20-30", but it was improved to "40-60", which is about twice the value. Regarding the color characteristics, those in which the conventionally normally black mode was “red tendency” or “slight red tendency” are improved to good results. The overall quality, which was "very inferior" or "inferior", was good, and it was possible to achieve a display quality close to the normally white mode, which is difficult to mass produce.

<Effects of the Invention> According to the present invention as described above, it is possible to make the device thinner and lighter than the conventional two-layer TN-LCD, and to obtain a clear white / black display close to a normally white mode.

[Brief description of drawings]

FIG. 1 is a structural explanatory view of a liquid crystal display device showing an embodiment of the present invention, FIG. 2 is a view showing a relative relationship of each optical axis of the embodiment, and FIG. 3 is a setting angle of a retardation plate and transmission. FIG. 4 is a diagram showing the relationship between the transmittance, FIG. 4 is a diagram showing the relationship between the setting angle of the retardation plate and the transmittance in the other parameters, and FIG. 5 is a relationship between the retardation value (Re) of the retardation plate and the transmittance. Showing the sixth
The figure shows the relationship between the retardation value (Re) of the retardation film and the transmittance for the same parameters. Fig. 7 shows the relationship between the retardation d · Δn of the liquid crystal layer and the retardation value (Re) of the retardation film. Fig. 8 is a configuration diagram of a color liquid crystal display device of TN-TFT-LCD, and Fig. 9 is the same inverted stagger type TF.
FIG. 10 is a plan view of the T-LCD, and FIG. 10 is a diagram showing the relationship between the light transmittance in the normally black mode when no voltage is applied and d · Δn, which is approximated by the GOOCH-TARRY equation. 1, 11: polarizing plate, 2: retardation plate, 3, 10: glass substrate, 4, 9: transparent conductive film electrode (ITO), 5, 8: alignment film, 6: liquid crystal composition, 7: sealing material, P ₁ : the major axis alignment direction of the liquid crystal molecules near the upper substrate (the substrate closest to the retardation plate), P ₂ : the major axis alignment direction of the liquid crystal molecules in the central part of the liquid crystal layer, P ₃ : the direction orthogonal to P ₁ , P _4: the set direction of the maximum refractive index axis of the retardation plate (phase difference plate axis).

Claims (1)

(57) [Claims] 1. A twisted nematic liquid crystal display device in which an upper polarizing plate, a 90 ° twist nematic liquid crystal display cell, and a lower polarizing plate having a transmission axis parallel to the upper polarizing plate are laminated in this order. An optical retardation plate is provided on at least one of both sides of the cell, and the axial direction showing the maximum refractive index of the retardation plate is a direction perpendicular to the liquid crystal molecule major axis direction of the liquid crystal display cell substrate surface adjacent to the retardation plate. From 10 to
The retardation plates are arranged so as to be offset by a positive angle of 20 degrees, and the total retardation (Re) of the retardation plate is calculated from the thickness (d) of the liquid crystal layer of the liquid crystal display cell and the anisotropy of the refractive index of the liquid crystal material. A liquid crystal display device, wherein d · Δn−0.05 μm ≦ Re ≦ d · Δn + 0.1 μm with respect to the product (d · Δn) of (Δn).