JP2000227590A - Display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To significantly improve the use efficiency of light by disposing a polarizing reflection layer which mostly transmits light in plural small wavelength regions including specified plural wavelengths in a first circularly polarized light component and which mostly reflects light in the regions between the plural small wavelength regions. SOLUTION: In the cholesteric liquid crystal layer 18, the helical pitch of a cholesteric liquid crystal layer 18 changes in the layer thickness direction in such a manner that the product np of the helical pitch p of the layer 18 and the average refractive index n of the cholesteric liquid crystal polymer 19 covers most of the wavelength region in the visible ray region, and that the left-handed circularly polarized light is selectively reflected. However, the device has a structure that transmits left- handed circularly polarized light for the light near the main three wavelengths in the spectrum of the emitted light from a back light Lb. By displaying an image in the display device using the cholesteric liquid crystal layer 18, the reflection to transmission ratio near the region of the main three wavelengths of the back light Lb is 5:95, while in other regions, the ratio is 95:5. In the whole left-handed circularly polarized light emitted from the back light Lb, 95% of the light is transmitted, while 90% of the left-handed circularly polarized light of external light Lf is reflected.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flat display device such as a liquid crystal display device, and more particularly to a reflective display device utilizing external light.

[0002]

2. Description of the Related Art A conventional reflection type liquid crystal display element utilizes external light, and the display screen becomes dark due to insufficient illuminance depending on the use environment, and cannot be used at all in a dark place. On the other hand, in a dark environment, a semi-transmissive reflector (half mirror) is used as a reflector for reflecting external light so that it can be used as a transmissive liquid crystal display element, and a backlight is provided on the back of the semi-transmissive reflector. The transflective display element described above has been applied. However, since the transflective plate has a maximum utilization efficiency of 50% of the incident light, the brightness of the display screen is remarkably inferior to that of the transmissive display element or the reflective display element.

In recent years, a transflective liquid crystal display device in which a pinhole is provided for each pixel on a reflection plate and a microlens corresponding to the pinhole is arranged to solve such a problem has been studied. In this transflective liquid crystal display device, when external light is used, the light reflected from the area excluding the pinhole of the reflector is used as a light source, and when the backlight is used, the light transmitted through the pinhole is collected by a microlens. As a result, the light use efficiency is increased. However, when external light is used, there is a loss of light corresponding to the pinhole, and as a result, the frequency of use as a transmission type is increased, and power consumption is increased. Further, since the structure of the reflection plate is complicated, it is necessary to use an external reflection plate. As a result, parallax is generated, and the display performance is significantly reduced.

Further, a so-called front light type display element in which a light guide plate is arranged on the observation surface side of a reflection type liquid crystal display element and a linear light source is arranged on a side surface of the light guide plate has been studied. However, the surface reflection on the front light surface is remarkable, and the display quality such as contrast is remarkably deteriorated.

[0005]

SUMMARY OF THE INVENTION The present invention solves the problems of these conventional reflective or transflective display elements with a novel structure, and provides a display element with greatly improved light use efficiency. The purpose is to:

[0006]

According to the present invention, there is provided a first polarizing plate which transmits linearly polarized light along its polarization axis, and is disposed behind the first polarizing plate, and modulates incident light according to an applied voltage. A light modulation layer, a polarization reflection layer disposed behind the light modulation layer and selectively reflecting the first circularly polarized light component of the incident light; and a polarization reflection layer disposed behind the polarization reflection layer and having an intensity peak at a plurality of predetermined wavelengths. And a backlight that emits light having: a polarizing reflection layer, the first circularly polarized light component, substantially transmits light of a plurality of small regions including a plurality of predetermined wavelengths, A display element which substantially reflects light in a region between small regions.

[0007]

FIG. 1 shows the structure of a transflective display element 10, and the operation of the transflective display element 10 will be described below with reference to FIG. A polarizing plate 11 is provided on the observation side of the display element.
Are arranged, and a λ / 4 phase difference plate 12 is arranged below the layer. In the lower layer of the λ / 4 retardation plate 12, 2
A liquid crystal element having a vertical alignment type liquid crystal layer 15 sandwiched between glass substrates 13 and 14 is arranged.

The lower glass substrate 14 and the electrode layer 17
A film 18 in which cholesteric liquid crystal is polymerized is formed between the two. FIG. 2 shows how light is transmitted or reflected by the film 18. The cholesteric liquid crystal used for the film 18 assumes that the value np obtained by multiplying the twist pitch p of the liquid crystal molecules 19 by the average refractive index n is equal to the incident light wavelength λ. When the liquid crystal molecules of the cholesteric liquid crystal have a counterclockwise spiral structure, the left-handed circularly polarized light component of the incident light Lf is selectively reflected,
The remaining polarization components are transmitted. When the above value np is equal to the incident light wavelength λ, the cholesteric liquid crystal ideally has a function of reflecting 100% of the circularly polarized light component in the direction equal to the spiral direction (counterclockwise or counterclockwise).
% Transmission. Similarly, the left circularly polarized light component is selectively reflected on the light Lb incident from below.

Referring again to FIG. 1, the operation of the display element 10 will be described. FIG. 1A shows that the power supply 2 is connected to the vertical alignment type liquid crystal layer 15.
It indicates an on state where a voltage is applied from 0, more precisely, a voltage applied state (at the time of Von) equal to or higher than the threshold level of liquid crystal. In this case, the nematic liquid crystal molecules have a homogeneous alignment in which the nematic liquid crystal molecules are arranged in a direction parallel to the substrate from the upper substrate to the lower substrate.

In this state, the light Lf entering from the observation side in the upper part of the figure passes through the polarizing plate 11 and the λ / 4 phase difference plate 12 which is a fixed retarder layer to become clockwise circularly polarized light. Vertical alignment type liquid crystal layer (VA layer)
15 is incident. When the phase is delayed by λ / 2 in the layer 15, the light is converted into left-handed circularly polarized light and reaches the cholesteric liquid crystal layer 18, which is a polarization reflection layer. Therefore, the left-handed circularly polarized light that has reached as described above is reflected by the cholesteric liquid crystal layer 18 and is again converted into right-handed circularly polarized light by the VA layer 15 delaying the phase by λ / 2. This light is again transmitted to the λ / 4 retardation plate 12.
, The light becomes linearly polarized light along the polarization axis of the polarizing plate 11, is output through the polarizing plate 11, and a display in a bright and dark state is obtained.

FIG. 1B shows an off state (including a zero voltage) in which a voltage equal to or lower than a threshold is applied to the vertical alignment type liquid crystal layer 15 (Vo).
ff). In this case, the liquid crystal layer is in a state where the liquid crystal molecules are arranged perpendicular to the substrate and the incident light is not phase-modulated.

In this state, the light Lf incident from the upper side of the drawing is, as in the case of FIG.
The light enters the liquid crystal layer 15 as clockwise circularly polarized light via the / 4 phase difference plate 12, but is not phase-modulated in the same layer and reaches the cholesteric liquid crystal layer 18 as clockwise circularly polarized light.
The clockwise circularly polarized light is transmitted toward the back surface of the display element, and is converted by the phase difference plate 25 into linearly polarized light having a vibration component along the absorption axis of the polarizing plate 26. As a result, no light returns to the observation surface, and a dark display is obtained.

Next, the operation when the surface light source 21 is arranged on the back of the cholesteric liquid crystal layer 18 will be described. The surface light source 21 is a light guide plate 2 formed of an acrylic flat plate or the like.
2, a linear light source 24 arranged on the side surface thereof, and a diffuse reflection layer 23 arranged on the back surface of the light guide plate.

In the state shown in FIG. 1A, that is, at the time of Von, the light Lb output from the surface light source is converted into left-handed circularly polarized light by the polarizing plate 26 and the phase difference plate 25.
0%) passes through the cholesteric liquid crystal layer 18 and the remaining light is reflected. The light that has passed through the cholesteric liquid crystal layer 18 is phase-modulated by the VA layer 15 and converted into clockwise circularly polarized light. And this light is λ / 4 retardation plate 1
2, the light becomes linearly polarized light along the polarization axis of the polarizing plate 11, is output through the polarizing plate 11, and a bright display is obtained.

On the other hand, in the state Voff in FIG. 1B, the counterclockwise circularly polarized light passing through the cholesteric liquid crystal layer 18 is
The signal is output as it is without being subjected to the phase modulation by the A layer 15. When this light passes through the λ / 4 phase difference plate 12, the light becomes linearly polarized light having a vibration direction orthogonal to the polarization axis of the polarizing plate 11, is absorbed by the polarizing plate 11, and a dark display is obtained.

Now, in the display element performing the above-described operation, in the embodiment of the present invention, the value np obtained by multiplying the helical pitch p of the cholesteric liquid crystal layer 18 by the average refractive index n of the cholesteric liquid crystal polymer is visible. The helical pitch changes along the layer thickness direction so as to cover almost the entire optical wavelength range, and the configuration is such that left circularly polarized light is selectively reflected.
The light having a wavelength in the vicinity of the wavelength has a helical structure that transmits left-handed circularly polarized light. The three main wavelengths of light emitted from the backlight in this embodiment are approximately 430n.
m, about 550 nm, and about 610 nm. Then, each of the three main wavelengths is centered on 20
It is assumed to transmit a wavelength range of a width of 30 nm (hereinafter referred to as a slit width).

FIG. 3 shows a left circular polarization intensity profile of external light and a left circular polarization intensity profile of light emitted from the backlight. When display is performed by a display element using the cholesteric liquid crystal layer 18 of the present embodiment using these lights, a graph of the reflectance and the transmittance as shown in FIG. 4 is obtained. In the region, the reflection: transmission ratio is 5:95, and in the other regions, 95: 5.
Becomes Further, as shown in FIG. 5, 95% of the entire left circularly polarized light emitted from the backlight is transmitted, and 90% of the entire left circularly polarized light of the external light is reflected.
This makes it possible to perform display with good color purity and high luminance.

FIG. 6 shows the relationship between the slit width and the luminance. According to the figure, it can be seen that when the slit width is set to 20 to 30 nm, the sum of the transmission luminance and the reflection luminance becomes maximum.

By using such a display element,
When using external light, a display with extremely high light use efficiency can be obtained together with the case where a light source is used, and a bright display can be obtained.

Further, by forming the polarization reflection layer of the cholesteric liquid crystal layer 18 inside the VA liquid crystal element which is a variable retarder, parallax due to the substrate 14 is eliminated as compared with the case where the polarization reflection layer is arranged on the outer surface of the substrate 14. If the polarization reflection layer is also used as an insulating layer when an active element such as a TFT or MIM is formed on the substrate 14, the manufacturing process can be simplified and the cost can be reduced.

In the above embodiment, the VA liquid crystal element is used as the variable retarder. However, the same effect can be obtained as long as the element can control whether the phase of the incident light is shifted by a half wavelength or not to be phase modulated by an electric field. Needless to say, it can be obtained. For example, as another embodiment, a horizontal alignment type nematic liquid crystal element in which a conventionally known nematic liquid crystal is aligned in parallel with the direction of the substrate may be used, or a twisted nematic liquid crystal element in which the nematic liquid crystal is twisted is used. You may.

In order to obtain a cholesteric liquid crystal layer in which the helical pitch changes as described above, a plurality of types of cholesteric liquid crystal polymer layers having different pitches are laminated, or when a cholesteric liquid crystal material is applied to a substrate and solidified, It is preferable to coat the subsequent film surface with an additive that increases the helical pitch of the cholesteric liquid crystal (for example, a nematic liquid crystal having an infinite helical pitch).

In the above example, it is of course possible to display an intermediate tone by applying an intermediate voltage between Von and Voff as the voltage applied to the variable retarder layer.

In each of the above-described embodiments, the case of operating as a reflective display element by using external light and the case of operating as a transmissive display element by using a backlight are high in both cases. Efficiency can be achieved.

FIG. 7 shows a TFT active matrix type liquid crystal display device configured as a transflective type liquid crystal display device according to a second embodiment of the present invention. FIG. 8 shows the main components, and FIG. 9 shows the TFT element structure of the array substrate. This figure is shown upside down with respect to FIG. 8 in order to explain the structure of the TFT element shown in FIG. The TFT array substrate 13 is formed of an insulating substrate made of glass or the like, and the figure shows a vertical alignment nematic element having a color filter in which the TFT array substrate 13 is disposed on the observation side and the counter substrate 14 is disposed on the backlight side. . A large number of pixel electrodes 30 are provided in the screen display area.
Are arranged in a matrix, and a thin film transistor (TFT) 31 is provided on each pixel electrode 30 as a drive switching element. A signal line 32, a scanning line 34 including a gate electrode 33, and, if necessary, an auxiliary capacitance electrode (not shown) are provided between these pixel electrodes. An oxide film 35, for example, a semiconductor film 36 made of amorphous silicon (a-Si) is sequentially formed from above, and a low-resistance semiconductor film 37 is formed to cover the semiconductor layer. The part constituting the TFT element 31 is covered with a passivation film 38 for protecting the TFT element.

As described above, the gate electrode 33 is formed of the semiconductor film 3
6 is referred to as a bottom gate structure, and external light entering from the array substrate 13 toward the TFT element 31 is blocked by the gate electrode 33 and does not enter the semiconductor film 36. As a result, it is possible to prevent a decrease in the display contrast ratio due to a light leak current generated by light when the display element is used outdoors.

A color filter 3 is provided on the entire surface of the pixel portion.
9 are arranged. The color filter is provided with a contact hole of about 10 μm square. A transparent pixel electrode 30 made of, for example, ITO is provided on the color filter 39.
Are formed for each pixel. The transparent pixel electrode 30 is electrically connected to a source electrode 41 of the TFT via a contact hole 40 provided in the color filter 39.

The signal line 3 is located at the boundary between the transparent pixel electrodes 30.
2. A wiring electrode of any one of the scanning line 34 and the auxiliary capacitance line is arranged, and when the transmissive light of the transflective liquid crystal display device is used by the backlight, the light of the backlight does not leak to lower the contrast ratio. . An alignment film (not shown) is further provided on the array substrate with a predetermined alignment axis.

On the other hand, the opposite substrate 14 has a polarization reflection layer 18
Are formed in a predetermined shape. Here, the polarization reflection layer 18
Was formed by applying a film obtained by polymerizing a cholesteric liquid crystal. The polarization reflection layer 18 further has a transparent conductive film of, for example, ITO laminated as a counter electrode 17 in a predetermined shape. It is preferable that film formation and patterning of ITO are simultaneously performed by means of ordinary mask sputtering. As a result, the process load on the cholesteric liquid crystal layer during the formation of ITO is extremely small.

Further, an alignment film (not shown) is stacked by performing an alignment process. The orientation is such that the liquid crystal molecules are oriented perpendicular to the substrate. These TFT array substrates 13
And the opposing substrate 14 oppose to form a liquid crystal cell,
A peripheral portion (seal portion) 42 of both substrates is an adhesive (sealant)
43, and the liquid crystal cell has a VA liquid crystal 15
Is enclosed. At this time, the sealing material is
It is preferable to apply it to a region where the polarization reflection layer 18 is not formed. On the polarization reflection layer 18, the adhesion of the sealing material is poor, and there is a possibility that reliability may be caused such that the substrate is peeled off for a long time use of 10,000 hours or more. Alternatively, the reliability problem can be avoided by applying an overcoat agent having good adhesion of the sealing material on the polarization reflection layer. The overcoat agent may be, for example, an acrylic resin usually used for a color filter.

On the outer surface of the array substrate 13, λ / 4
The retardation plate 12 and the polarizing plate 11 are stacked in this order. A backlight (not shown) is arranged on the outer surface of the opposite substrate 14 opposite to the liquid crystal layer. In a medium to large liquid crystal display device having a diagonal screen size of 8 inches or more, a light diffusion film may be provided on the outer surface of the array substrate 13 to increase the viewing angle.

[0032]

According to the present invention, in a transflective display element, a display with high color purity and high luminance can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram showing the configuration and operation of a transflective display element.

FIG. 2 is a schematic view showing the operation principle of a polarization reflection layer.

FIG. 3 is a diagram showing profiles of external light and light emitted from a backlight.

FIG. 4 is a diagram showing a reflectance and a transmittance of the display element according to the embodiment of the present invention.

FIG. 5 is a diagram showing profiles of transmitted light intensity and reflected light intensity of the display element according to the embodiment of the present invention.

FIG. 6 is a diagram illustrating a relationship between a slit width and luminance.

FIG. 7 is a partial plan view showing the configuration of the embodiment of the present invention.

FIG. 8 is a cross-sectional view illustrating a configuration of an embodiment of the present invention.

FIG. 9 is a cross-sectional view of a TFT according to an embodiment of the present invention.

[Explanation of symbols]

 18 cholesteric liquid crystal layer (polarization reflection layer) 21 backlight 11 and 26 polarizing plate 12 λ / 4 wavelength plate (fixed retarder layer) 15 vertical alignment type liquid crystal layer (VA layer) 16, 17 transparent electrode layer 25 ... Phase plate

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H091 FA01Z FA08X FA08Z FA11X FA11Z FA14Y FA14Z FA41Z FB02 FD24 GA06 GA13 GA16 HA07 LA03 LA15 LA16 LA17 LA19 5G435 AA03 AA04 BB12 BB15 BB16 CC09 EE27 EE33 FF00 FF03 FF00 FF03 FF00FF03

Claims (4)

[Claims] 1. A first polarizing plate that transmits linearly polarized light along its polarization axis; a light modulation layer that is disposed behind the first polarizing plate and modulates incident light according to an applied voltage; A polarizing reflection layer disposed behind the modulation layer and selectively reflecting the first circularly polarized light component of the incident light; and a light exiting from the polarization reflection layer and having light intensity peaks at a plurality of predetermined wavelengths. In a display device having a backlight, the polarization reflection layer substantially transmits light of wavelengths of a plurality of small regions including the plurality of predetermined wavelengths in the first circularly polarized light component, and the plurality of small regions are connected to each other. A display element substantially reflecting light in a region between the two. 2. The display device according to claim 1, wherein the plurality of small regions include three regions. 3. The plurality of small areas are each 430n.
2. The display element according to claim 1, wherein the display element includes wavelengths of m, 550 nm, and 610 nm. 4. A method according to claim 1, wherein each of the plurality of small regions has a thickness of 20 nm.
The display element according to claim 1, wherein the width of the display element is from 30 to 30 nm.