JP2003255397A - Liquid crystal display - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a liquid crystal display wherein an image free from gradation inversion and having high quality can be displayed. <P>SOLUTION: The liquid crystal display provided with a liquid crystal cell comprising a pair of transparent substrates 11 provided opposite to each other and each provided with a transparent electrode 12 on the inner surface side thereof and a liquid crystal layer 14 wherein a liquid crystal material having bend-alignment when the liquid crystal material is driven is held between a pair of the substrates 11, polarizing plates 16 provided on the both sides of the liquid crystal cell, and an optical compensation element 15 provided between the liquid crystal cell and at least one polarizing plate 16 is so designed that the difference R between the front retardation of the liquid crystal layer 14 in 550 nm wavelength when white is displayed and that when black is displayed satisfies 100 nm≤R≤185 nm. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly to an OCB mode liquid crystal display device.

[0002]

2. Description of the Related Art A conventional liquid crystal display device (LCD) is mainly composed of a nematic liquid crystal and a thin film transistor (TFT), particularly a system utilizing a twisted nematic (TN) liquid crystal orientation. However, it has been pointed out that the TN-LCD has a narrow viewing angle and a slow response.

In recent years, a method of improving the viewing angle by inserting an optical compensation element having a hybrid-oriented discotic liquid crystal formed on a plastic film substrate between a liquid crystal cell and a polarizing plate has been reported. There is. However, the improvement by this method is insufficient, and in particular, the gradation inversion and the blackout condition when the TN-LCD is viewed from the lower side and the lower oblique direction are not greatly improved even when the above optical compensation element is used.

Therefore, in order to solve the problem of the viewing angle, a method of changing the alignment state (mode) of the liquid crystal used for display has been devised. For example, IPS (In-Plane
Switching) and MVA (Multi-domain Vertically Alig)
A high-quality display without gradation inversion is obtained by a mode called ned (mode)). However, the slow response in these modes has not been improved.

In order to solve the two problems of wide viewing angle and response speed, OCB (Opticall) utilizing bend alignment is used.
y Compensated Bend) has been attempted to utilize a mode called. FIG. 1 is a diagram schematically showing an alignment state of OCB-mode liquid crystals in a liquid crystal cell. In FIG. 1, 1 is a liquid crystal layer, and 2 is an alignment film formed on the outermost surface on the inner surface side of a substrate sandwiching the liquid crystal layer. As shown in FIG. 1 (a), the alignment state of the liquid crystal when no voltage is applied to the liquid crystal cell is almost symmetrical with respect to the normal direction of the liquid crystal cell, and the liquid crystal is aligned parallel to the substrate surface in the center of the liquid crystal cell. The alignment process is performed so as to obtain an alignment state called splay alignment.
When a high voltage of 10 V or higher is applied to this liquid crystal cell, the liquid crystal is oriented substantially symmetrically with respect to the normal direction of the cell, and the liquid crystal is oriented substantially vertically at the center of the liquid crystal cell and approximately parallel at the substrate interface. A bend orientation is obtained (FIG. 1 (d)). When the voltage is changed in this state, the rising state of the liquid crystal near the center of the liquid crystal cell changes (see FIG. 1 (b),
(C)), the retardation of the liquid crystal layer can be changed. In the OCB mode, this retardation change is replaced with a change in transmitted light intensity by sandwiching a liquid crystal cell between two polarizing plates to perform display. Unlike the TN mode or the like, when the applied voltage is reduced, the bend alignment returns to the splay alignment, so it is necessary to drive within a specific voltage range, and particularly it is necessary to limit the minimum applied voltage.
Since this OCB mode has good bend orientation symmetry, it is easy to perform optical compensation, easily obtain a wide viewing angle, and have a response speed as high as 10 ms or less.

[0006]

However, this O
CB mode is normally white (NW) same as TN
When used in, the gradation inversion occurs in the direction perpendicular to the rubbing direction, which may deteriorate the display image quality. The state of gradation inversion is shown in FIG. FIG. 8 is a diagram showing the applied voltage dependence of the transmittance when the viewing angle direction is changed to the direction orthogonal to the rubbing direction in the conventional OCB mode liquid crystal display device. In FIG. 8, the viewing angle direction normal to the substrate surface of the liquid crystal cell is 0 °, and this viewing angle direction is referred to as the front surface in this specification. The viewing angle direction on the horizontal axis in FIG. 8 is also referred to as a left-right direction in this specification. From FIG. 8, when the line of sight is tilted in the direction perpendicular to the alignment direction of the liquid crystal,
It can be seen that the transmittance curves intersect at around 60 °, and gradation inversion occurs. By eliminating this gradation inversion, it is possible to provide a display with high image quality regardless of the viewing angle direction.

What can be seen from FIG. 8 is that if the voltage at the time of white display is increased and the transmittance at the front (transmittance when the viewing angle direction is 0 °) is decreased, the top solid line in FIG. 8 is used. Since it is not done, gradation inversion is resolved. However, lower power consumption and lower driver IC voltage (resulting in lower cost)
Therefore, it is necessary to set the drive voltage range as low as possible. Therefore, it is necessary to set the voltage for displaying white as low as possible and to find a generally applicable condition in which grayscale inversion does not occur.

The present invention has been made to solve the above problems, and an object thereof is to obtain a liquid crystal display device capable of displaying a high quality image without gradation inversion. is there.

[0009]

A liquid crystal display device according to the present invention includes a pair of transparent substrates provided facing each other and having transparent electrodes on the inner surface side, and a bend alignment during driving between the pair of transparent substrates. A liquid crystal cell containing a liquid crystal layer sandwiching a liquid crystal material, a polarizing plate provided on both sides of the liquid crystal cell, an optical compensation element provided between the liquid crystal cell and at least one polarizing plate, And the difference R in front retardation at a wavelength of 550 nm of the liquid crystal layer when displaying white and when displaying black satisfies 100 nm ≦ R ≦ 185 nm.

The product Δnd of the birefringence Δn of the liquid crystal material at a wavelength of 550 nm and the thickness d of the liquid crystal layer is 750 n.
It is intended to satisfy m <Δnd <1200 nm.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. FIG. 2 is a diagram schematically showing a cross-sectional structure of the liquid crystal display device of the present invention. In FIG. 2, 11 is a transparent substrate, 12 is a transparent electrode, 13 is an alignment film, 14 is a liquid crystal layer made of nematic liquid crystal, 15 is an optical compensation element, and 16 is a polarizing plate. The arrow in FIG. 2 indicates the rubbing direction. The liquid crystal display device of the present invention is provided so as to face each other, and a pair of transparent substrates 11 having transparent electrodes 12 on the inner surface side thereof, and a liquid crystal material exhibiting bend alignment during driving are sandwiched between the pair of transparent substrates 11. And a polarizing plate 1 provided on both sides of the liquid crystal cell.
6 and an optical compensation element 15 provided between the liquid crystal cell and at least one polarizing plate 16.

As the transparent substrate 11, any member may be used as long as it is substantially transparent in the region of the wavelength of light necessary for displaying. For applications such as monitors and televisions, glass substrates that are transparent in the visible range (wavelength of 380 nm to 780 nm) are generally used, but plastic substrates such as polycarbonate that are also transparent in the visible range should be used. You can also

The transparent electrode 12 is generally made of ITO (Indi
um Tin Oxide) is used. Although not shown, a thin film transistor (TFT) which is a switching element for driving is formed on the transparent electrode side on one of the pair of transparent substrates 11 provided with the transparent electrode 12 by a normal semiconductor process. A general configuration of a liquid crystal display device for a monitor or a television has a configuration in which a color filter for performing color display and a black matrix for light shielding are arranged on the transparent electrode side on one transparent substrate.

On the transparent electrode 12 side of the transparent substrate 11, an organic polymer thin film called an alignment film 13 is formed.
Polyimide is generally used as the organic polymer. The thickness of the alignment film 13 is generally about 300 nm to 1200 nm. By rubbing the surface of the alignment film 13, the ability to regulate the alignment direction of the liquid crystal is imparted.
A process called rubbing is performed. A nylon cloth is generally used as the member for rubbing the surface. OCB
In the case of the mode, the rubbing directions of the two transparent substrates 11 are such that the rubbing directions of both substrates are the same when the liquid crystal cell is formed.

It is desirable that the rubbing directions of both substrates are exactly the same. But an exact match is actually difficult. In practice, it is sufficient that the deviation in the rubbing direction is within 30 °, preferably within 10 °, and more preferably within 5 °.

These two transparent substrates 11 are connected to the transparent electrode 12
And, the bonding is performed so that a gap having a specific thickness is formed such that the surface on which the alignment film 13 is formed is inside. The gap is formed by arranging a spherical or columnar resin or silica member called a spacer between the two transparent substrates 11 (not shown). A liquid crystal cell is manufactured by injecting a liquid crystal material into this gap by a capillary phenomenon.

At this time, the product Δnd of the birefringence Δn of the liquid crystal material at a wavelength of 550 nm and the thickness of the liquid crystal layer (the length of the gap after injection in the direction of the substrate normal, the gap) d is 750 nm.
It is desirable to satisfy <Δnd <1200 nm. Δn
When d is 750 nm or less, the maximum transmissivity of the liquid crystal display device cannot be made large, resulting in a dark screen. Also, Δnd
However, when the thickness is 1200 nm or more, there is a problem that the color change becomes large when viewed from the front and when viewed from an oblique direction, when the gap d is increased, the response speed becomes slow, and when Δn is increased, the long-term reliability is deteriorated. To do. this is,
This is because a liquid crystal material having a large Δn tends to have a large dielectric constant and a high solubility of ionic impurities. Therefore, when it is used for a long period of time, the impurity concentration in the liquid crystal becomes high and it becomes impossible to exhibit a predetermined display performance. In the present invention, 7
By selecting the birefringence index Δn of the liquid crystal material and the thickness d of the liquid crystal layer so as to satisfy 50 nm <Δnd <1200 nm, it is possible to prevent such a problem from occurring.
It is possible to obtain a liquid crystal display device that can display high-quality images and has excellent long-term reliability.

The elastic constant K ₃₃ for bending deformation and the elastic constant K for splaying deformation of the liquid crystal material to be injected.
The ratio of _11, it is desirable to satisfy _{0.7 <K 33 / K 11 <} 1.4. The smaller K ₃₃ / K ₁₁ is, the easier the bend deformation is, and the lower voltage driving becomes possible. But K ₃₃
When / K ₁₁ is extremely small, that is, when K ₁₁ is large, it becomes difficult to change the alignment state from the splay alignment to the bend alignment by applying a voltage.

Further, it is desirable that the anisotropy Δε of the relative dielectric constant of the injected liquid crystal material satisfies 8 <Δε <20. Δ
The larger ε is, the better the response to voltage is, which is advantageous for low-voltage driving and high-speed response. However, if it is too large, the solubility of the ionic impurities in the liquid crystal becomes high, which causes problems such as a decrease in voltage holding ratio and a decrease in reliability in long-term use.

When the same liquid crystal material is injected into a liquid crystal cell in which the liquid crystal is aligned so that the liquid crystal becomes vertical at the interface of the alignment film and the gap is sufficiently large, the alignment of the liquid crystal near the center of the liquid crystal cell is parallel to the substrate surface by a spiral axis. Has a spiral structure with. The length of one cycle of this spiral structure is called the chiral pitch of this liquid crystal material. When the chiral pitch p of the injected liquid crystal material is larger than p ₀ expressed by [Equation 1], the alignment state of the injected liquid crystal becomes the splay alignment as shown in FIG. In [Equation 1], d is a gap, and Φ is a twist angle when the liquid crystal material is twisted, and is π [rad] in the OCB mode (see FIG. 3). In the normal OCB mode, a liquid crystal material satisfying p> p ₀ is used so that the alignment state when no voltage is applied is the splay alignment.

[Equation 1] [Formula 1]

The chiral pitch p of the injected liquid crystal material is
When it is smaller than p ₀ expressed by [Equation 1], the liquid crystal orientation is twisted along the direction normal to the substrate of the liquid crystal cell, as shown in FIG. In the present invention, a mode in which the initial orientation is twist orientation is also called OCB mode. The present invention, even if the initial orientation is splay orientation,
This is applicable even in the case of twist orientation.

A liquid crystal display device is manufactured by arranging an optical compensation element on at least one side of the liquid crystal cell manufactured as described above and polarizing plates on both outer surfaces thereof. The optical compensation element is provided to correct the viewing angle characteristic that the optical characteristic of the liquid crystal display device changes depending on the viewing direction to a desired characteristic. For this optical compensation element, an element having an optical anisotropy having a minimum absolute retardation value in a specific direction inclined from the normal direction and exhibiting retardation having a polarity different from that of the liquid crystal retardation is effective. . By disposing such an optically anisotropic element on both sides of the liquid crystal cell as an optical compensating element, the retardation of the entire liquid crystal cell is compensated, the transmittance at the time of black display is reduced in a wide range in the line-of-sight direction, and high contrast is achieved. Images can be supplied.

As an optical anisotropic element having a specific retardation on the front side and a minimum absolute value of retardation in a specific direction as described above, the direction of the principal axis of the refractive index is the surface of the optical compensation element. is inclined from the normal direction, it is possible to use a biaxial retardation film (the vertical retardation plate c-plate can be handled as a special biaxial retardation film of n _{x =} n _y ).

The OCB mode is generally used to display normally white, that is, black by applying a high voltage. This is because when the voltage for displaying black is high, the area where the liquid crystal stands vertically other than the substrate interface becomes large, so the minimum absolute retardation value in the direction inclined from the normal direction as described above. This is because the liquid crystal layer to be compensated by the optically anisotropic element having is substantially thin, and compensation can be performed by the optically anisotropic element having a small retardation that is relatively easy to manufacture.

When the polarizing plates are used in crossed nicols, the transmission axes (or absorption axes) of the two polarizing plates are orthogonal to each other when viewed from the front, but the transmission axes of one polarizing plate are viewed when viewed obliquely. Unless you look down along the axis (or absorption axis),
The transmission axes (or absorption axes) of the two polarizing plates are no longer orthogonal to each other. Therefore, even if the retardation of the liquid crystal layer is compensated by the optical compensation element, the light leaks because the polarizing plates are not orthogonal to each other when viewed from an oblique direction. This phenomenon can be compensated by disposing a biaxial retardation plate as an optical compensator with its principal axis of the refractive index parallel to the transmission axis (or absorption axis) of the polarizing plate.

Therefore, an optical compensating element in which the above-mentioned biaxial retardation plate is added to the optically anisotropic element having the minimum absolute value of retardation in the direction inclined from the normal direction for compensating the liquid crystal layer is used. As a result, it is possible to reduce the transmittance during black display in a wider range in the line-of-sight direction and supply an image with high contrast.

These optically anisotropic elements constituting the optical compensating element may have different configurations as long as the optical characteristics are substantially equivalent. For example, a-plate and c-plate that are optically equivalent to the biaxial retardation plate may be used. Further, although the above description is for the case where the optical compensation element is attached on both sides of the liquid crystal cell, the optical compensation element may be attached only on one side.

The polarizing plate of the liquid crystal display device is usually made of iodine or 2
A dye having a high chromaticity is used as a light absorber after being adsorbed on a polymer film such as polyvinyl alcohol, and then stretched so that the absorption axis of the absorber is aligned in one direction. The characteristics of this type of polarizing plate include a single transmittance of 40 to 45% (for a monochromatic light having a wavelength of 550 nm) and a wavelength of 550 nm.
The mainstream of which is a parallel transmittance of (for monochromatic light) of about 35 to 40%, and a polarizing plate having such characteristics can be used.

The absorption axes are orthogonal to each other on both sides of the liquid crystal cell having the optical compensation element on at least one side, and the absorption axis has an angle of about 45 ° with the alignment direction (rubbing direction) of the liquid crystal. A liquid crystal display device is manufactured by sticking two polarizing plates as described above.

The liquid crystal display device using the OCB mode drives the liquid crystal between the white voltage and the black voltage after the liquid crystal is oriented in the bend orientation. In this specification, when a voltage within the drive voltage range is applied, the state where the light transmittance (or the amount of transmitted light) when viewed from the front is maximum is defined as white, and the minimum state is defined as black. The voltage for displaying white is called white voltage, and the voltage for displaying black is called black voltage. The change in the alignment state of the liquid crystal at this time is as described with reference to FIG. In the present invention, the difference in the front retardation at a wavelength of 550 nm of the liquid crystal layer at the time of the white voltage and the black voltage at this time is set to 185 nm or less, more preferably 165 nm or less. By doing so, it is possible to suppress the occurrence of grayscale inversion in the horizontal direction.

The reason will be described below. First, the alignment state of the liquid crystal, which has an important influence on the viewing angle characteristics of the display, will be considered. The z-axis is the normal direction (thickness direction) of the liquid crystal cell. The free energy density f of bend orientation is represented by [Formula 2]. Here, θ (z) is the distribution of the tilt angle (angle of the director with respect to the substrate surface), K ₁₁ is the elastic constant for splay deformation, K ₃₃ is the elastic constant for bend orientation, Δε is the relative dielectric anisotropy, and E is It is the electric field strength to be applied, and is expressed as E = V / d, where V is the applied voltage.
In [Formula 2], (z) of θ (z) is omitted.

[Equation 2] [Formula 2]

The free energy per unit area is obtained by integrating [Equation 2] in the z-axis direction of the liquid crystal cell. The liquid crystal is oriented so that the free energy per unit area is minimized. The problem of finding a function that minimizes the integral value is called a variational problem, and the function of the solution has the characteristic that the quantity called the variational derivative of f takes all 0 within the integration range (Eular equation).

The Euler equation for [Equation 2] is
It is calculated as [Equation 3].

[Equation 3] [Formula 3]

Here, θ (z) is assumed as in [Equation 4]. Here, it is assumed that the center of the liquid crystal cell is z = 0 and that the liquid crystal cell is symmetrically aligned at z = 0. θs is a pretilt angle (tilt angle at the interface with the substrate). The parameter c represents the degree of deformation of the director, and a larger value corresponds to a high voltage application state, that is, a state in which the director is standing up.

[0035]

[Equation 4] [Formula 4]

Next, the solution obtained by numerically solving [Equation 3] was compared with the approximate solution according to [Equation 4]. Figure 4
FIG. 6 is a diagram showing the relationship between the position in the liquid crystal cell and the tilt angle of the director for various liquid crystal materials, obtained from [Equation 3] and [Equation 4]. The parameters (dielectric anisotropy and elastic constant) of the liquid crystal material and the parameter C used for the calculation are shown at the top of each figure. From FIG. 4, the solutions obtained from [Equation 3] are obtained for various liquid crystal material parameters (dielectric anisotropy and elastic constants) and applied voltage.
The solution obtained from [Equation 4] is in good agreement, and [Equation 4] is
It can be seen that the exact solution of [Equation 3] = 0 is accurately approximated.

Next, by substituting θ (z) in [Equation 4] into [Equation 3], [Equation 5] is obtained.

[Equation 5] [Formula 5]

When the trigonometric function of [Equation 5] is expanded by sinh (2cz / d) / sinhc and the first-order term of sinh (2cz / d) / sinhc is taken and is set equal to 0, [Equation 6] becomes can get.

[Equation 6] [Formula 6]

In the liquid crystal cell (within the integration range), | sinh (2cz /
Since d) / sinhc | <1, the first-order term of sinh (2cz / d) / sinhc is the dominant term with the largest absolute value among the terms expanded from [Equation 5] by sinh (2cz / d) / sinhc. is there. Therefore, if [] in [Equation 6] is 0, [Equation 5] is approximately equal to 0. That is, it can be seen that [Equation 4] approximately satisfies the Euler equation (variation of f = 0) and is a function that represents the distribution of the tilt angle of the liquid crystal.

The above [formula 4] indicates that the alignment state of the liquid crystal can be described by the pretilt angle θs and the parameter c.
That is, when the pretilt angle θs is the same, even if the dielectric constant or elastic constant of the liquid crystal is changed, the same alignment state can be obtained by applying the voltage V that gives the same c.

It is difficult to uniformly produce a large pretilt angle θs over a large area, and therefore the pretilt angle is usually set to 10 ° or less. Therefore, π / 2 >> θs, and it can be seen from [Equation 4] that the pretilt angle does not significantly affect the alignment state (and the viewing angle characteristics of the liquid crystal display device).

Assuming that c >> 1, the term of 1 / sinh ² c in [] of [Equation 6] can be regarded as 0, so that c in [] becomes [Equation 7]. Given. Actually, since c is 3 or more and 1 / sinh ² c is 0.01 or less, [Equation 7] is satisfied when c is about 3 or more. Since K ₃₃ / Δε of the liquid crystal material is about 1.0 to 1.8, c = 3 is about 2.0 to 2.7 V when converted to a voltage.

[Equation 7] [Formula 7]

In the OCB mode, when the applied voltage is lowered, the bend orientation changes to twist orientation or splay orientation, so that the applied voltage has a lower limit (see FIG. 1).
Usually, the driving voltage can be lowered by setting the white voltage near the lower limit voltage. This lower limit voltage is usually about 2 to 2.5 V, and it can be considered that [Equation 7] is substantially satisfied within the range of the voltage required for driving.

Next, the occurrence mechanism of the problem of gray scale inversion in the left-right direction will be considered. Polarization ellipticity is χ, principal axis azimuth is ψ
And the Stokes parameters S _{0 to} S ₃ are S
₁ = S ₀ cos 2χ cos 2ψ, S ₂ = S ₀ cos 2χ
sin2ψ, S ₃ = S ₀ sin2χ.
As in the present specification, when the fluctuation of the total light amount S ₀ is not taken into consideration, S ₀ = 1 can be set. The Stokes parameters satisfy the relationship of S ₁ ² + S ₂ ² + S ₃ ² = S ₀ ² and form a sphere with a radius S ₀ . This sphere is called the Poincare sphere. FIG. 5 shows the calculation of the applied voltage dependence of the outgoing polarization state of the OCB mode liquid crystal display device having various Δnd by the 4 × 4 matrix method, and the result is displayed on the Poincare sphere by the Stokes parameter, and this is displayed as S1- It is the figure projected on S2 cross section. The outgoing polarization state is the outgoing polarization state of the liquid crystal display device with the outgoing side polarization plate removed. Further, the total amount of light is standardized to 1, and the viewing angle is 60 °. The characteristics of the liquid crystal material used are shown in Table 1. Also, FIG.
Table 2 summarizes the liquid crystal materials, the gaps d, Δnd, and the white state and the black state c used for the respective drawings (a) to (d). The optical compensator has a contrast ratio of 5 even when viewed from the viewing angle direction of 80 ° in the direction (left-right direction) orthogonal to the rubbing direction.
Designed to achieve the above.

[0045]

[Table 1]

[0046]

[Table 2]

The relative dielectric constant anisotropy and the elastic constant are different between the liquid crystals A and B, but the polarization state of the outgoing polarized light, that is, FIG.
It can be seen that the locus of (a) and the locus of FIG. 5 (b) are on substantially the same curve. Further, it can be seen that the polarization state of the outgoing polarized light hardly changes even if Δnd changes (FIGS. 5B to 5D).

Consideration will be given to the fact that the outgoing polarization state hardly depends on material constants such as relative dielectric anisotropy. First, regarding the polarization state at the black voltage, this makes the transmittance small (specifically, makes it so small that the contrast ratio becomes 5 or more even when the viewing angle is 80 ° in the left-right direction), that is, by the optical compensation element. In FIG. 5, the adjustment is made so that it is as close as possible to the polarization state (state shown by Abs. Axis in FIG. 5) absorbed by the exit side polarizing plate.
The outgoing polarization state at the black voltage hardly depends on the material constants such as the relative dielectric anisotropy (the design is performed).

Next, when the voltage is reduced from the black voltage (that is,
Consider how the output polarization state changes when c is reduced). When the voltage is lowered (that is, when c is reduced), the tilt angle θ of the liquid crystal changes, and the change amount is c of θ.
Can be expressed as a partial differential coefficient ∂θ / ∂c. θ can be written as [Equation 4] as a function of c, and in the region of c> 3, sinhc = coshc
, And sinhc >> 1, ∂θ / ∂c
Is almost independent of c, that is, ∂ ² θ / ∂c ² 〜 0
It can be seen from [Equation 8] that z ₀ is ∂ / ∂
^{a (∂ 2 θ / ∂c 2} ) = 0, satisfy a = 2z / d).
That is, when the voltage is lowered (when c is reduced), the orientation changes similarly regardless of the value of c in the initial black state.

[Equation 8] [Formula 8]

The output polarization state in the black state, which is the initial value, is constant, and the orientation change when the voltage is lowered from that (when c is reduced) hardly depends on c. This is a cause of being almost independent of the material constants such as c and relative dielectric constant anisotropy.

The fact that the outgoing polarization state hardly depends on the material constants such as relative permittivity anisotropy means that the transmittance and the viewing angle characteristics also hardly depend on the material constants such as relative permittivity anisotropy. .

Next, the front transmittance and the transmittance in the viewing angle direction of 60 ° when viewed from the front were obtained by calculation. Figure 6
FIG. 6 is a diagram showing the relationship between the front transmittance obtained by calculation and the transmittance at a viewing angle direction of 60 ° for each liquid crystal cell in Table 2. In FIG. 6, the vertical axis represents the calculation result of the front transmittance, and the horizontal axis represents the calculation result of the transmittance in the viewing angle direction of 60 °. The single-sided transmittance of the polarizing plate used for the calculation was 41.3% and the parallel transmittance was 37.1%, and the transmittance is shown as a value with respect to the parallel transmittance of the polarizing plate. It can be seen from FIG. 6 that although there are slight differences between the liquid crystal cells, they are on almost the same curve regardless of the Δnd of the liquid crystal cells.

As shown in FIG. 6, when the transmittance is small, the transmittance in the 60 ° direction also increases as the front transmittance increases, but the front transmittance is approximately 60% to 70% (see FIG. 6). It can be seen that the gradation inversion occurs due to the decrease in the transmittance in the 60 ° direction at the boundary of the shaded portion).

In normal gradation inversion, white (maximum transmissivity in the front), black (minimum transmissivity in the front), and black and white are divided into eight gradations, and the display brightness is large or small between these gradations. Judge whether there is a reversal of the relationship. For example, the transmittance on the front side is 100%, 70%, 4 when the transmittance of white is 100%.
Halftones such as 7%, 29%, 17%, 8%, 3.5%, 1%, and black are set, and it is determined whether or not there is a reversal of the magnitude relation of the display luminance between these gradations. . From FIG.
Since gradation inversion occurs on the low voltage side close to white, it is understood that only white and the next gradation (hereinafter, this gradation is referred to as V2) need be considered. Judging by such criteria, if the white front transmittance is set to 76%, the front transmittance at V2 is 76%.
70% of the value is 53%, which means that gradation inversion does not occur although it is barely.

However, a closer examination reveals that gradation inversion occurs between white and V2. Therefore, if it is desired to prevent such gradation inversion, the front transmittance of white is 65%, and more preferably. It can be seen from FIG. 7 that it may be set to 60% or less. This means that the viewing angle is wide, so the optical compensation element is designed to reduce the black transmittance (specifically, the contrast ratio is 5 or more in the left-right direction even when the viewing angle is 80 °). In the case of Marie White's OCB mode, details of the optical characteristics of the liquid crystal material and the optical compensation element,
Generally, it does not depend on the setting of the drive voltage.

However, the index of the white frontal transmittance with respect to the parallel transmittance of the polarizing plate is the transmittance (that is, the ratio of the light amount transmitted through the liquid crystal display device to the light amount of the backlight) as a characteristic of the liquid crystal display device. Is not appropriate as an index for designing a liquid crystal display device, because it cannot be related without considering the parallel transmittance of the polarizing plate, the transmittance of the color filter, and the like.

The front transmittance T of white is the difference R between the front retardations of the liquid crystal layer when white is displayed and when black is displayed,
There is a relation of [Formula 9]. Where λ is the wavelength.

[Equation 9] [Formula 9]

The front retardation of the liquid crystal layer can be measured without being affected by the parallel transmittance of the polarizing plate, the aperture ratio and the transmittance of the color filter. The front retardation of the liquid crystal layer can be determined by making light having a wavelength of 550 nm incident on a liquid crystal cell without an optical compensation element and a polarizing plate and detecting light transmitted through the liquid crystal cell. In this way, the front retardation of the liquid crystal layer can be measured if there is a liquid crystal cell, and the front retardation of the liquid crystal layer when displaying black is the front retardation of the optical compensation element, which is a parameter indicating the characteristics of the optical compensation element. Since it has a clear relation that it is equal to the absolute value of the sum of the above, and it is not necessary to consider the transmittance, it is suitable as an index for designing a liquid crystal display device.

The fact that the front transmittance of white is 76% or less, more preferably 60% or less means that, from a design point of view, the liquid crystal for displaying white and for displaying black at a wavelength of 550 nm. Difference in frontal retardation of layers is 18
This means that the thickness is 5 nm or less, more preferably 155 nm or less. Also, from [Equation 9], it can be seen that the transmittance of white display becomes small if the difference R in the front retardation is too small. Therefore, from a practical point of view, if it is desired to secure a transmittance of 30%, the difference R in the front retardation is 100.
nm or more is required.

As described above, this condition does not depend on the details of the optical characteristics of the liquid crystal material or the optical compensation element, the setting of the driving voltage, etc.
Generally holds. Therefore, at the wavelength of 550 nm,
By designing the difference in the front retardation of the liquid crystal layer when displaying white and when displaying black in such a range, there is no grayscale inversion even if the white voltage is set as low as possible. It is possible to obtain an OCB mode liquid crystal display device capable of displaying a high-quality image.

In the above description, the difference in the front retardation at the wavelength of 550 nm is specified, but it is 550.
The reason why the wavelength is set to nm is that this wavelength is usually used for evaluation. In the case of wavelengths other than 550 nm, [Formula 9] is used.
The front surface retardation difference R may be set so that the transmittance T of 30% or more and 76% or less is satisfied. Also in this case, similarly to the above, it is possible to obtain an OCB mode liquid crystal display device capable of displaying a high quality image without gradation inversion.

[0062]

EXAMPLES Example 1. For each of the liquid crystal cells Nos. 1 to 3 in Table 2, the white voltage was set so that the white transmittance on the front surface did not exceed 76%, and the applied voltage dependency of the lateral transmittance at that time was 4 ×. It was calculated by the 4-matrix method. FIG. 7 is a diagram showing calculation results of applied voltage dependence of transmittance in various liquid crystal cells when the viewing angle direction is changed to a direction orthogonal to the rubbing direction. The liquid crystal cell of No. 4 in Table 2 was omitted because it can be seen from FIG. 5D that grayscale inversion does not occur even if the white voltage is lowered to the lower limit. Table 3 summarizes the front transmittance at white display and the difference in retardation between white display and black display. The wavelength is 550 nm,
The calculation was performed by setting the parallel transmittance of 37.1% of the polarizing plate as 100%. It can be seen from FIG. 7 that such design does not cause grayscale inversion in the left-right direction.

[0063]

[Table 3]

[0064]

As described above, in the liquid crystal display device of the present invention, the wavelength of the liquid crystal layer is 5 when displaying white and when displaying black.
The difference R of the front retardation at 50 nm is 5
By setting 100 nm ≦ R ≦ 185 nm at 50 nm, it is possible to display a high quality image in which gradation inversion does not occur.

The product Δnd of the birefringence Δn of the liquid crystal material at a wavelength of 550 nm and the thickness d of the liquid crystal layer is 750 n.
By satisfying m <Δnd <1200 nm, a high-quality image can be displayed and long-term reliability is excellent.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing an alignment state of OCB-mode liquid crystals in a liquid crystal cell.

FIG. 2 is a diagram schematically showing a cross-sectional structure of a liquid crystal display device of the present invention.

FIG. 3 is a diagram schematically showing twist orientation.

FIG. 4 is a diagram showing results obtained by calculating distributions of director tilt angles in a liquid crystal cell for various liquid crystals by using [Equation 3] and [Equation 4].

FIG. 5 is a diagram showing, on a Poincare sphere, the calculation result of the outgoing polarization state in the viewing angle direction of 60 ° in each of the liquid crystal cells shown in Table 2, which is projected on the S1-S2 surface.

FIG. 6 is a diagram showing the relationship between the calculated front transmittance and the transmittance in the viewing angle direction of 60 ° for each liquid crystal cell in Table 2.

7 is a table No. 2 shown in FIG. It is a figure which shows the calculation result of the applied voltage dependence of the transmittance | permeability at the time of changing a viewing angle direction to the direction orthogonal to a rubbing direction in the various liquid crystal cells shown in 1-3.

FIG. 8 is a diagram showing the applied voltage dependency of the transmittance when the viewing angle direction is changed to the direction orthogonal to the rubbing direction in the conventional OCB mode liquid crystal display device.

[Explanation of symbols]

11 transparent electrode, 12 transparent substrate, 14 liquid crystal layer, 15
Polarizing plate, 16 Optical compensation element.

Continued front page    (72) Inventor Kasuko Wakita             2-3 2-3 Marunouchi, Chiyoda-ku, Tokyo             Inside Ryo Electric Co., Ltd. (72) Inventor Tetsuyuki Kuroda             2-3 2-3 Marunouchi, Chiyoda-ku, Tokyo             Inside Ryo Electric Co., Ltd. F-term (reference) 2H049 BA02 BA06 BA26 BA42 BB03                       BB42 BB43 BC02 BC22                 2H088 GA02 HA04 HA15 JA12 KA02                       KA06 KA07 LA06 MA02 MA03                       MA04                 2H091 FA08 FA11 FD10 FD12 GA17                       HA18 KA02

Claims (2)

[Claims] 1. A pair of transparent substrates provided opposite to each other and having a transparent electrode on the inner surface side, and a liquid crystal layer in which a liquid crystal material exhibiting bend alignment during driving is sandwiched between the pair of transparent substrates. A liquid crystal cell, polarizing plates provided on both sides of the liquid crystal cell, and an optical compensation element provided between the liquid crystal cell and at least one of the polarizing plates are provided, and when displaying white and black. Wavelength 5 of the liquid crystal layer when displaying
The difference R of the front retardation at 50 nm is 100.
A liquid crystal display device characterized by satisfying nm ≦ R ≦ 185 nm. 2. The product Δnd of the birefringence Δn of the liquid crystal material at a wavelength of 550 nm and the thickness d of the liquid crystal layer is 750 nm <
The liquid crystal display device according to claim 1, wherein Δnd <1200 nm is satisfied.