JP3619506B2 - Liquid crystal display - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
[0002]
[Prior art]
Unlike liquid crystal displays (CRTs) and ELs (electroluminescence), liquid crystal displays do not emit light themselves, so a transmissive liquid crystal display device is used in which a backlight is installed behind the liquid crystal display element for illumination.
[0003]
However, since the backlight usually consumes 50% or more of the total power consumption of the liquid crystal display, a portable information device that is frequently used outdoors or always carried is installed with a reflector instead of the backlight. A reflection type liquid crystal display device that performs display only by itself is also realized.
[0004]
As a display mode used in the reflective liquid crystal display device, a polarizing plate is used in addition to a type using a polarizing plate such as a TN (twisted nematic) mode and a STN (super twisted nematic) mode which are widely used at present. In recent years, a phase transition type guest-host mode capable of realizing bright display has been actively developed and disclosed in, for example, Japanese Patent Application Laid-Open No. 4-75022.
[0005]
[Problems to be solved by the invention]
However, since the phase transition type guest-host mode performs display using light absorption of the dye in the liquid crystal layer in which liquid crystal molecules and the dye are dispersed, the contrast is not sufficient, and the TN (twisted nematic) mode and the STN (super twisted nematic). ) The display quality is significantly worse than that of a liquid crystal display device using a polarizing plate such as a mode.
[0006]
In the case of a liquid crystal display device of parallel alignment or twist alignment, liquid crystal molecules near the center of the liquid crystal layer are inclined in a direction perpendicular to the substrate surface when a voltage is applied, but voltage is applied to the liquid crystal molecules near the surface of the alignment film. Even if it is not perpendicular to the substrate, the birefringence of the liquid crystal layer is far from 0. In the display mode in which black display is performed when a voltage is applied, sufficient black cannot be displayed due to the birefringence of the liquid crystal layer. A sufficient contrast cannot be obtained.
[0007]
TN mode and STN mode liquid crystal display devices are also difficult to say at present with sufficient display quality in terms of brightness and contrast, and further improvements in display quality such as higher brightness and improved contrast are required.
[0008]
In addition, the reflection type liquid crystal display device has a drawback that the reflected light used for display is lowered when the ambient light is dark and the visibility is extremely lowered. There was a problem that the visibility under a clear sky or the like where the light was very bright was lowered.
[0009]
Therefore, a display device combining transmissive display and reflective display has been developed, but there is a problem in that light leakage occurs in black display and a sufficient black level cannot be obtained.
[0010]
[Means for Solving the Problems]
The present invention, whereas the liquid crystal layer is sandwiched between the substrate and the second substrate, a first polarization means above the first substrate the liquid crystal layer of which is provided on the opposite side, and the liquid crystal layer of the other substrate A second polarizing means provided on the opposite surface; a first retardation plate provided between the first polarizing means and the liquid crystal layer; the second polarizing means and the liquid crystal layer; A liquid crystal display device having a second retardation plate provided between the first substrate and the first substrate, wherein the one substrate includes a region having a reflective function and a region having a transmissive function. And the transmission display region are formed separately, the transmission axis of the first polarizing means and the transmission axis of the second polarizing means are orthogonal, and the slow axis of the first retardation plate and the and the slow axis of the second retardation plate is orthogonal, Do more than twice the thickness of the liquid crystal layer of the reflective display region Range, characterized by greater than a thickness of the liquid crystal layer of the transmissive display region and the reflective display region and the thickness of the liquid crystal layer of the.
[0011]
The first retardation plate and the second retardation plate are λ / 4 plates.
[0012]
The angle formed between the transmission axis of the first polarizing means and the slow axis of the first retardation plate is 45 °, and the transmission axis of the second polarizing means and the second position The angle formed with the slow axis of the phase difference plate is 45 °.
[0013]
The operation according to the present invention will be described below.
[0014]
Since the refractive index for both ordinary light and extraordinary light of the birefringent material constituting the retardation plate strongly depends on the wavelength of the light, the phase delay accumulated in the retardation plate of a specific thickness is also the wavelength. Depends on. In other words, in order to give a phase delay of a specific value (for example, λ / 4) to the linear polarization plane of incident light, it can be completely achieved only when a single wavelength light beam having a specified wavelength is incident. Therefore, in the wavelength region where the phase delay of λ / 4 cannot be achieved due to the wavelength dependence of the refractive index anisotropy of the birefringent material constituting the phase difference plate, the light that is transmitted without being blocked by the polarizing means on the output side Occurs, and the dark display is colored.
[0015]
In the present invention, the wavelength dependency of the refractive index anisotropy of the first retardation plate is changed to the second retardation by making the slow axes of the first retardation plate and the second retardation plate orthogonal. The wavelength dependence of the refractive index anisotropy of the plate can be canceled out, and a certain phase difference is satisfied over the entire wavelength band of light. For this reason, coloring of dark display can be improved.
[0016]
In addition, by using a vertically aligned liquid crystal material having a negative dielectric anisotropy for the liquid crystal layer, a state in which the retardation of the liquid crystal layer is almost zero is realized, so that the dark state becomes darker, so that the contrast is increased. Get higher.
[0017]
For example, when parallel alignment liquid crystal is used for the liquid crystal layer, residual retardation occurs even if an attempt is made to reduce the retardation of the liquid crystal layer to 0 by applying a voltage and directing the major axis of the liquid crystal molecules in a direction perpendicular to the electrode. The retardation of the liquid crystal layer does not become zero.
[0018]
Further, in normally black (hereinafter referred to as NB), the contrast ratio hardly changes due to the cell gap change, and a certain degree of margin for the cell gap control can be obtained in terms of productivity.
[0019]
In normally white (hereinafter referred to as NW), which displays white when no voltage is applied to the liquid crystal layer and displays black when a voltage is applied, the applied voltage to the liquid crystal layer that changes to black with respect to the cell gap changes, In an NB that performs black display when no voltage is applied to the liquid crystal layer and performs white display when a voltage is applied, the voltage applied to the liquid crystal layer that changes to white changes with respect to the cell gap change. For this reason, in NW, the contrast ratio changes remarkably due to a change in the cell gap, so that highly accurate cell gap control is required. In addition, since the point defect that has been a bright spot in NW becomes a black spot in NB, an improvement in the yield of non-defective products is expected, and a bright spot-free high-quality display panel can be realized.
[0020]
Also from these things, NB is superior to NW as a display mode of a liquid crystal display device that can be used in any environment.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
(Embodiment 1)
The configuration of the active matrix substrate of Embodiment 1 will be described with reference to FIG.
[0022]
An insulating substrate 1 such as a glass substrate is provided with a reflective electrode 3 formed of a material having high reflectance such as Al and Ta, and a transparent electrode 8 formed of a material having high transmittance such as ITO, and a glass substrate. A counter electrode 4 made of a material having high transmittance such as ITO is provided on an insulating substrate 2 such as ITO, and exhibits negative dielectric anisotropy between the reflective electrode 3 and the transparent electrode 8 and the counter electrode 4. A liquid crystal layer 5 made of a liquid crystal material is sandwiched.
[0023]
A vertical alignment film (not shown) is formed on each surface of the reflective electrode 3, the transparent electrode 8, and the counter electrode 4 in contact with the liquid crystal layer 5. After the alignment film is applied, at least one alignment film is formed on the alignment film. Alignment treatment such as rubbing is performed. Instead of the alignment treatment by rubbing, the alignment may be regulated by optical alignment or electrode shape.
[0024]
The liquid crystal molecules of the liquid crystal layer 5 are aligned with a tilt angle of about 0 degree or about 0.1 to 5 degrees with respect to the vertical direction of the substrate surface by an alignment treatment such as rubbing on the vertically aligned alignment film. Is done.
[0025]
Here, the reflective electrode 3 is used as an electrode for applying a voltage to the liquid crystal layer. However, the reflective electrode 3 may be used as a reflective plate without being used as an electrode. In that case, the transparent electrode 8 may be extended to the area of the reflector, and the transparent electrode 8 may be an electrode for applying a voltage to the liquid crystal layer 5 in the reflection area.
[0026]
As the liquid crystal material of the liquid crystal layer 5, a liquid crystal material having refractive index anisotropy of Ne = 1.5546 and No = 1.4773 was used.
[0027]
A λ / 4 plate 7 is disposed on the opposite surface of the substrate 2 on the side where the counter electrode 4 is formed, and a λ / 4 plate 10 is disposed on the opposite surface of the substrate 1 on the side where the reflective electrode 3 and the transparent electrode 8 are formed. The slow axis of the λ / 4 plate 10 is set to be orthogonal to the slow axis of the λ / 4 plate 7.
[0028]
A polarizing plate 6 is provided on the surface of the λ / 4 plate 7 opposite to the substrate 2, and a polarizing plate 9 is provided on the surface of the λ / 4 plate 10 opposite to the substrate 1. The axis is inclined 45 degrees with respect to the slow axis of the λ / 4 plate 7, the transmission axis of the polarizing plate 9 is inclined 45 degrees with respect to the slow axis of the λ / 4 plate 10, and the transmission axis of the polarizing plate 6. Is set to be orthogonal to the transmission axis of the polarizing plate 9. In this case, the transmission axis of the polarizing plate is set to 45 degrees with respect to the slow axis of the λ / 4 plate, but the direction of the angle may be either the + direction or the − direction.
[0029]
2A is a schematic plan view of an active matrix substrate of the liquid crystal display device of Embodiment 1, and FIG. 2B is a cross-sectional view taken along line AA of FIG.
[0030]
The active matrix substrate includes a gate wiring 21, a data wiring 22, a driving element 23, a drain electrode 24, an auxiliary capacitance electrode 25, a gate insulating film 26, an insulating substrate 27, a contact hole 28, an interlayer insulating film 29, and a reflective pixel electrode. 30 and a transmission picture element electrode 31 are provided.
[0031]
The auxiliary capacitance electrode 25 is electrically connected to the drain electrode 24 and overlaps with the gate wiring 21 via the gate insulating film 26 to form an auxiliary capacitance.
[0032]
The contact hole 28 is provided in the interlayer insulating film 29 in order to connect the transmissive pixel electrode 31 and the auxiliary capacitance electrode 25.
[0033]
This active matrix substrate includes a reflective pixel electrode 30 and a transmissive pixel electrode 31 in one picture element, and the reflective picture element electrode 30 that reflects light from the outside in one picture element. A part of the pixel electrode 31 for transmission that transmits the part and the light of the backlight is formed.
[0034]
Here, in FIG. 2B, the surface shape of the reflective pixel electrode 30 is illustrated as a plane, but the surface shape may be uneven to improve reflection characteristics. Further, although the picture element electrode is divided into the reflection picture element electrode 30 and the transmission picture element electrode 31, a transflective electrode may be used without being divided.
[0035]
The light transmission state of the reflection mode and the transmission mode in the liquid crystal display device of Embodiment 1 will be described with reference to FIGS.
[0036]
FIG. 3 shows a case where display is performed using a reflective electrode (reflection mode), FIG. 3A shows a case of dark display in which no voltage is applied to the vertical alignment liquid crystal layer, and FIG. The case of white display in which a voltage is applied to the alignment liquid crystal layer is shown. FIG. 4 shows a case where display is performed using a transmissive electrode (transmission mode), FIG. 4A shows a case of dark display where no voltage is applied to the vertical alignment liquid crystal layer, and FIG. Indicates a white display in which a voltage is applied to the vertical alignment liquid crystal layer.
[0037]
The dark display in the reflection mode will be described with reference to FIG.
[0038]
The incident light that has entered the surface of the polarizing plate 6 from the upper side of FIG. 3A passes through the polarizing plate 6 and becomes linearly polarized light whose polarization axis coincides with the transmission axis of the polarizing plate, and is incident on the λ / 4 plate 7. .
[0039]
The λ / 4 plate 7 is arranged so that the transmission axis direction of the polarizing plate 6 and the slow axis direction of the λ / 4 plate 7 are 45 degrees, and the light that has passed through the λ / 4 plate 7 becomes circularly polarized light. Become.
[0040]
When an electric field is not applied to the liquid crystal layer 5, the liquid crystal layer 5 using a liquid crystal material exhibiting negative dielectric anisotropy has liquid crystal molecules aligned substantially perpendicular to the substrate surface, and the liquid crystal layer for incident light The refractive index anisotropy of 5 is very small, and the phase difference caused by light passing through the liquid crystal layer 5 is almost zero.
[0041]
Therefore, the circularly polarized light beam that has passed through the λ / 4 plate 7 passes through the liquid crystal layer 5 with almost no loss of the circularly polarized light, and is reflected by the reflective electrode 3 on one substrate 1.
[0042]
The reflected light becomes circularly polarized light whose rotation direction is reversed, passes through the λ / 4 plate 7, becomes linearly polarized light orthogonal to the incident time, and enters the polarizing plate 6.
[0043]
The linearly polarized light that has passed through the λ / 4 plate 7 is linearly polarized light in a direction orthogonal to the transmission axis of the polarizing plate 6 and is absorbed by the polarizing plate 6 and is not transmitted.
[0044]
As described above, when no voltage is applied to the liquid crystal layer 5, dark display is performed.
[0045]
Next, the white display in the reflection mode will be described with reference to FIG.
[0046]
FIG. 3B shows a case where a voltage is applied to the liquid crystal layer 5 and is the same as FIG. 3A until it passes through the λ / 4 plate 7, and a description thereof will be omitted.
[0047]
When a voltage is applied to the liquid crystal layer 5, the liquid crystal molecules aligned in the vertical direction from the substrate surface are slightly inclined in the horizontal direction with respect to the substrate surface, and the circularly polarized light from the λ / 4 plate 7 incident on the liquid crystal layer 5 is It becomes elliptically polarized light due to the birefringence of the molecules, and after being reflected by the reflective electrode 3, it is further influenced by the birefringence of the liquid crystal molecules at the liquid crystal layer 5, and after passing through the λ / 4 plate 7, It does not become orthogonal linearly polarized light but passes through the polarizing plate 6 somewhat.
[0048]
Thus, by adjusting the voltage applied to the liquid crystal layer, the amount of light that can be transmitted through the polarizing plate 6 after being reflected can be adjusted, and gradation display becomes possible.
[0049]
Further, when a voltage is applied from the reflective electrode 3 and the counter electrode 4 to the liquid crystal layer 5 to change the alignment state of the liquid crystal molecules so that the phase difference of the liquid crystal layer 5 is a ¼ wavelength condition, the λ / 4 plate 7 After passing through the liquid crystal layer 5, the circularly polarized light passes through the liquid crystal layer 5 and reaches the reflective electrode 3, and then becomes linearly polarized light orthogonal to the transmission axis of the polarizing plate 6, and again passes through the liquid crystal layer 5 and becomes circularly polarized light. Later, it passes through the λ / 4 plate 7 and becomes linearly polarized light parallel to the transmission axis of the polarizing plate 6, and the reflected light passing through the polarizing plate 6 becomes maximum.
[0050]
FIG. 3B shows the retardation condition of the liquid crystal layer 5 through which the light reflected by the reflective electrode 3 is most transmitted through the polarizing plate 6, and is orthogonal to the transmission axis of the polarizing plate 6 on the reflective electrode 3. The direction is linearly polarized light.
[0051]
Accordingly, when no voltage is applied to the liquid crystal layer 5, there is almost no birefringence in the liquid crystal layer 5, and a dark display is obtained. When a voltage is applied to the liquid crystal layer 5, the light transmittance changes according to the applied voltage, and a gradation display is obtained. Is possible.
[0052]
The dark display in the transmission mode will be described with reference to FIG.
[0053]
The light emitted from the lower side of FIG. 4A by a light source (not shown) passes through the polarizing plate 9 and becomes linearly polarized light that matches the transmission axis of the polarizing plate 9.
[0054]
The λ / 4 plate 10 is arranged so that the slow axis direction of the λ / 4 plate 10 and the transmission axis direction of the polarizing plate 9 are 45 degrees, and the light that has passed through the λ / 4 plate 10 becomes circularly polarized light. Become.
[0055]
When no electric field is applied to the liquid crystal layer 5, the liquid crystal layer 5 using a liquid crystal material exhibiting negative dielectric anisotropy has liquid crystal molecules aligned substantially perpendicularly from the substrate surface, and the liquid crystal layer 5 is free from incident light. The refractive index anisotropy is negligible, and the phase difference generated when light passes through the liquid crystal layer 5 is almost zero.
[0056]
Therefore, the circularly polarized light emitted from the λ / 4 plate 10 passes through the liquid crystal layer 5 without breaking the circularly polarized light and enters the λ / 4 plate 7.
[0057]
The slow axis direction of the λ / 4 plate 10 and the slow axis direction of the λ / 4 plate 7 are perpendicular to each other, and the circularly polarized light incident on the λ / 4 plate 7 is perpendicular to the transmission axis direction of the polarizing plate 9. The linearly polarized light is incident on the polarizing plate 6.
[0058]
The linearly polarized light emitted from the λ / 4 plate 7 is linearly polarized light in a direction orthogonal to the transmission axis of the polarizing plate 6 and is absorbed by the polarizing plate 6 and does not transmit light.
[0059]
Thus, when no voltage is applied to the liquid crystal layer 5, a dark display is obtained.
[0060]
Next, the bright display in the transmission mode will be described with reference to FIG.
[0061]
FIG. 4B shows a case where a voltage is applied to the liquid crystal layer, and is the same as FIG. 3A until light passes through the λ / 4 plate 10, and the description thereof is omitted.
[0062]
When a voltage is applied to the liquid crystal layer 5, the liquid crystal molecules aligned in the vertical direction from the substrate surface are slightly inclined in the horizontal direction with respect to the substrate surface, and the circularly polarized light from the λ / 4 plate 10 incident on the liquid crystal layer 5 is Due to molecular birefringence, it becomes elliptically polarized light, and after passing through the λ / 4 plate 7, it does not become linearly polarized light orthogonal to the transmission axis of the polarizing plate 6, but passes through the polarizing plate 6 somewhat.
[0063]
Thus, by adjusting the voltage applied to the liquid crystal layer, the amount of light transmitted through the polarizing plate 6 after being reflected can be adjusted, and gradation display becomes possible.
[0064]
Further, when a voltage is applied to the liquid crystal layer 5 to change the alignment state of the liquid crystal molecules so that the phase difference of the liquid crystal layer 5 is a ½ wavelength condition, the circularly polarized light after passing through the λ / 4 plate 7 is It becomes linearly polarized light at a point that is half the cell thickness of the liquid crystal layer 5, and becomes circularly polarized light after passing through the remaining liquid crystal layer 5.
[0065]
When the circularly polarized light emitted from the liquid crystal layer 5 passes through the λ / 4 plate 7, it becomes linearly polarized light parallel to the transmission axis of the polarizing plate 6, and the reflected light passing through the polarizing plate 6 is maximized.
[0066]
FIG. 3B shows the retardation condition of the liquid crystal layer 5 in which the light that has passed through the polarizing plate 9 is most transmitted through the polarizing plate 6.
[0067]
Therefore, when no voltage is applied to the liquid crystal layer 5, there is almost no birefringence in the liquid crystal layer 5, and a dark display is obtained. When a voltage is applied to the liquid crystal layer 5, the light transmittance changes depending on the applied voltage. Key display is possible.
[0068]
FIG. 5 shows a case where the slow axes of the λ / 4 plate 7 and the λ / 4 plate 10, which are the configurations of the present embodiment, are arranged orthogonally, and a comparative example of the delay of the λ / 4 plate 7 and the λ / 4 plate 10. It is a figure which shows the relationship between the wavelength of the light at the time of black display, and the transmittance | permeability, when arrange | positioning a phase axis in parallel.
[0069]
In the present embodiment, by making the slow axis of the phase difference plate orthogonal, the wavelength dependence of the refractive index anisotropy of the phase difference plate can be offset, and a constant phase difference is satisfied over the entire wavelength band of light. As a result, coloring of the dark display can be improved.
[0070]
Here, the phase difference of the liquid crystal layer 5 having the maximum reflectance in the bright state of the reflection mode is λ / 4, and the phase difference of the liquid crystal layer 5 having the maximum transmittance in the bright state of the transmission mode is λ / 2. Therefore, when the thickness of the liquid crystal layer in the region used as the reflection mode is equal to the thickness of the liquid crystal layer in the region used as the transmission mode, the phase difference of the liquid crystal layer 5 in the region used as the reflection mode is λ / 4, and the transmission mode is The phase difference of the liquid crystal layer 5 in the region to be used cannot satisfy the phase difference of λ / 2 at the same time.
[0071]
That is, when gradation display is performed by changing the phase difference of the liquid crystal layer 5 in the region used as the reflection mode from 0 to λ / 4, the phase difference of the liquid crystal layer 5 in the region used as the transmission mode is also changed from 0 to λ /. Since it changes only up to 4, the transmission mode cannot use light efficiently.
[0072]
Therefore, by changing the thickness of the liquid crystal layer in the region used as the reflection mode and the liquid crystal layer in the region used as the transmission mode, or by changing the voltage applied to the liquid crystal layer in the region used as the reflection mode and the liquid crystal layer in the region used as the transmission mode. Light can be used efficiently in both the reflection mode and the transmission mode. Here, when changing the thickness of the liquid crystal layer in the region used as the reflection mode and the thickness of the liquid crystal layer in the region used as the transmission mode, the thickness of the liquid crystal layer in the region used as the transmission mode is 2 times the thickness of the liquid crystal layer in the region used as the reflection mode. When doubled, the phase difference of the liquid crystal layer 5 in the region used as the reflection mode can be simultaneously satisfied with the phase difference of λ / 4, and the phase difference of the liquid crystal layer 5 in the region used as the transmission mode can be satisfied with λ / 2. Further, even if the thickness of the liquid crystal layer in the region used as the transmission mode is not twice the thickness of the liquid crystal layer in the region used as the reflection mode, the thickness of the liquid crystal layer in the region used as the transmission mode is used as the reflection mode. The use efficiency of light is improved by making the thickness of the liquid crystal layer in the region used as the transmission mode larger than the thickness of the liquid crystal layer in the region used as the reflection mode within a range not exceeding twice the thickness.
[0073]
Here, since the refractive index of the birefringent material constituting the λ / 4 plates 7 and 10 with respect to both ordinary light and extraordinary light strongly depends on the wavelength, the phase accumulated in the wave plate having a specific thickness. The delay is also wavelength dependent. That is, in order to give a phase delay of λ / 4 to the linear polarization plane of incident light, it can be completely achieved only when a single wavelength light beam having a specified wavelength is incident. Therefore, due to the wavelength dependence of the refractive index anisotropy of the birefringent materials constituting the λ / 4 plates 7 and 10, the polarizing plate 6 transmits the light without being shielded in a wavelength region where the phase delay of λ / 4 cannot be achieved. Although light is generated and coloring occurs in dark display, the slow axis of the λ / 4 plate 10 is orthogonal to the slow axis of the λ / 4 plate 7, and the transmission axis of the polarizing plate 6 is that of the polarizing plate 9. By setting so as to be orthogonal to the transmission axis, the wavelength dependence of the refractive index anisotropy of the λ / 4 plate 10 and the wavelength dependence of the refractive index anisotropy of the λ / 4 plate 7 in the transmission mode are set. So that the λ / 4 condition is satisfied over the entire wavelength band. For this reason, coloring of dark display can be improved.
[0074]
Furthermore, in order to improve the viewing angle characteristics of the liquid crystal layer 5, another retardation plate is provided between at least one of the polarizing plate 6 and the liquid crystal layer 5 and between the polarizing plate 9 and the liquid crystal layer 5. Good display is achieved in the range.
[0075]
In the first embodiment, vertical alignment liquid crystal is used for the liquid crystal layer 5, but when the alignment of liquid crystal molecules near the substrate surface has a certain tilt angle with respect to the vertical direction of the substrate surface, the liquid crystal layer 5 Even when no voltage is applied, the retardation does not become 0 completely. Therefore, if the retardation of the retardation plate is adjusted by replacing the λ / 4 plate 7 to compensate for this, better dark display can be obtained.
[0076]
When the retardation of α remains in the reflection mode in the liquid crystal layer in which the liquid crystal molecules are substantially oriented in the direction perpendicular to the substrate surface, instead of the λ / 4 plate 7, (λ / 4-α) A retardation plate having retardation may be disposed.
[0077]
In the reflection mode, elliptically polarized light that is shifted from the circularly polarized light by the remaining retardation of the liquid crystal layer is incident on the liquid crystal layer. It passes through the liquid crystal layer and becomes circularly polarized light in a region having a reflection function, and becomes circularly polarized light that is reflected and whose rotation direction is reversed. When the light passes through the liquid crystal layer and exits from the liquid crystal layer, it becomes elliptically polarized light deviated from circularly polarized light. The elliptically polarized light at this time is in a state where the phase at the time of incidence is shifted by 90 degrees. When passing through the retardation plate, it becomes linearly polarized light orthogonal to the transmission axis of the polarizing plate 6.
[0078]
When the reflective display is the main, such as when the reflective pixel electrode is larger than the transmissive pixel electrode, the λ / 4 plate 10 used for the transmission mode display may be left as it is.
[0079]
Therefore, even when the retardation remaining in the liquid crystal layer in the state where the liquid crystal molecules are oriented in the direction perpendicular to the substrate surface cannot be ignored, a high contrast display in the reflection mode can be achieved by arranging a retardation plate in consideration of the retardation. Can be realized.
[0080]
Further, when the retardation of α in the reflection mode and β of the transmission mode remains in the liquid crystal layer, a retardation plate having a retardation of (λ / 4−α) instead of the λ / 4 plate 7 λ / Instead of the four plates 10, a retardation plate having a retardation of (λ / 4- (β-α)) may be disposed.
[0081]
In the transmission mode in which the display is performed with the transmitted light of the region having the transmission function, when the liquid crystal molecules are oriented in the vertical direction of the substrate surface, the elliptically polarized light in the same state as the output light in the reflection mode is emitted from the liquid crystal layer. Thus, the retardation plate having the retardation (λ / 4- (β-α)) is set, and the elliptically polarized light having the retardation has the retardation (λ / 4-α). Since the light is incident on the plate, when it passes through the retardation plate having the retardation of (λ / 4−α), it becomes linearly polarized light orthogonal to the transmission axis of the polarizing plate 6 and dark display with little light leakage is obtained.
[0082]
Therefore, even when the retardation remaining in the liquid crystal layer in the state where the liquid crystal molecules are oriented in the direction perpendicular to the substrate surface cannot be ignored, a high contrast display in the reflection mode can be achieved by arranging a retardation plate in consideration of the retardation. Can be realized.
[0083]
In the first embodiment, the vertically aligned liquid crystal is used for the liquid crystal layer 5, but display can be performed on the same principle using parallel aligned liquid crystal. However, when the parallel alignment liquid crystal is used, the retardation of the liquid crystal layer 5 decreases as a voltage is applied. However, even when the liquid crystal molecules other than the vicinity of the substrate are generally oriented perpendicular to the substrate surface when the voltage is applied, Is hardly moved by an electric field, so that residual retardation is caused by liquid crystal molecules in the vicinity of the substrate. Therefore, when parallel alignment liquid crystal is used rather than using vertical alignment liquid crystal, light leakage occurs at the time of dark display due to the influence of residual retardation, the black level rises, and contrast decreases. Therefore, in order to display the black level similar to the vertical alignment liquid crystal using the parallel alignment liquid crystal, the liquid crystal molecules on the upper and lower substrates so as to cancel the residual retardation due to the liquid crystal molecules in the vicinity of the upper and lower substrates so as to compensate for the residual retardation. Must be oriented or a retardation plate must be added.
[0084]
【The invention's effect】
According to the present invention, by making the slow axes of the first retardation plate and the second retardation plate orthogonal, the wavelength dependence of the refractive index anisotropy of the first retardation plate can be reduced to the second value. In the liquid crystal display device having a reflection function and a transmission function, the transmission mode can be canceled by the wavelength dependence of the refractive index anisotropy of the retardation plate, and satisfy the λ / 4 condition over the entire wavelength band of light. But it can improve the coloring of the dark display.
[0085]
In addition, by using a vertically aligned liquid crystal material having a negative dielectric anisotropy for the liquid crystal layer, a state in which the retardation of the liquid crystal layer is almost zero is realized, so that the dark state becomes darker, so that the contrast is increased. Get higher.
[0086]
Further, in normally black (hereinafter referred to as NB), the contrast ratio hardly changes due to the cell gap change, and a certain degree of margin for the cell gap control can be obtained in terms of productivity.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of a liquid crystal display device according to a first embodiment of the present invention.
FIG. 2 is a plan view of a liquid crystal display device according to Embodiments 1 and 2 of the present invention.
FIG. 3 is a diagram illustrating a light transmission state in a reflection region of the liquid crystal display device according to the first embodiment of the present invention.
FIG. 4 is a diagram showing a light transmission state in a transmission region of the liquid crystal display device according to the first embodiment of the present invention.
FIG. 5 is a diagram showing the relationship between the wavelength of light and the transmittance when black display is performed.
[Explanation of symbols]
1, 2 Substrate 3 Reflective electrode 4 Counter electrode 5 Liquid crystal layer 6, 9 Polarizing plate 7, 10 λ / 4 plate 8 Transparent electrode

Claims (3)

Meanwhile the liquid crystal layer is sandwiched between the substrate and the second substrate, a first polarization means above the first substrate the liquid crystal layer of which is provided on the opposite side, the opposite side to the liquid crystal layer of the other substrate Provided between the second polarizing means provided, the first retardation plate provided between the first polarizing means and the liquid crystal layer, and between the second polarizing means and the liquid crystal layer. In the liquid crystal display device having the second retardation plate ,
The one substrate includes a region having a reflective function and a region having a transmissive function, and is formed by dividing a reflective display region and a transmissive display region in one picture element,
The transmission axis of the first polarizing means and the transmission axis of the second polarizing means are orthogonal, and the slow axis of the first retardation plate and the slow axis of the second retardation plate are Orthogonal ,
A liquid crystal display characterized in that the thickness of the liquid crystal layer in the transmissive display region is larger than the thickness of the liquid crystal layer in the reflective display region within a range not exceeding twice the thickness of the liquid crystal layer in the reflective display region. apparatus. The liquid crystal display device according to claim 1, wherein the first retardation plate and the second retardation plate are λ / 4 plates. The angle formed by the transmission axis of the first polarizing means and the slow axis of the first retardation plate is 45 °, and the transmission axis of the second polarizing means and the second retardation plate The liquid crystal display device according to claim 2, wherein an angle formed by the slow axis is 45 °.

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