KR20000042075A - Display device - Google Patents

Abstract

PURPOSE: A display device is provided to obtain a reflection-type or semitransparent display device which improves the optical using rate and has a high luminance. CONSTITUTION: A fixing delaying layer(12) delays the phase of the inputted light between the deflection plate(11) and a deflection reflecting layer(18). A variable delaying layer(15) delays the inputted light according to the applied voltage. A phase difference plate(25) and a deflection plate(26) are disposed on the rear surface of the deflection reflecting layer(18). The phase difference plate(25) converts the inputted light to a circular plate optical substance which is selectively reflected by the deflection reflecting layer(18). According to the display device, a reflection-type or semitransparent display device which improves the optical using rate and has a high luminance can be obtained.

Description

Display element

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

Since the conventional reflective liquid crystal display uses external light, the display screen becomes dark due to lack of 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 transflective plate (half mirror) is used as a reflector for reflecting external light for use as a transmissive liquid crystal display, and a back light is provided on the back of the transflective plate. Semi-transmissive display elements have been applied. However, since the transflective plate has a maximum utilization efficiency of incident light at 50%, the brightness of the display screen is significantly lower than that of the transmissive display element or the reflective display element.

In recent years, the semi-transmissive liquid crystal display element which provided the pinhole for each pixel in the reflecting plate, and arrange | positioned the microlens corresponding to this pinhole is examined by such a problem. In the semi-transmissive liquid crystal display device, when the external light is used, the light reflected from the pinhole of the reflecting plate is removed as a light source, and when the backlight is used, the light that has passed through the pinhole is condensed by a microlens to increase the light utilization efficiency. . However, when external light is used, there is a loss of light in the pinhole, and as a result, the frequency of use as a transmission type is high, resulting in increased power consumption. In addition, since the structure of the reflecting plate is complicated, it is necessary to use an external reflecting plate, and as a result, parallax occurs and the display performance is significantly reduced.

In addition, a so-called front light display device in which a light guide plate is disposed on an observation surface side of a reflective liquid crystal display element and a linear light source is disposed on the side surface of the light guide plate is also studied. However, the surface reflection on the surface of the front light is remarkable, and the display quality such as contrast is significantly reduced.

SUMMARY OF THE INVENTION The present invention has been made in view of the above, and an object of the present invention is to provide a display device that has dramatically improved light utilization by solving a problem of a conventional reflective or transflective display device with a new structure.

1 is a schematic cross-sectional view for explaining the operation of the first embodiment of the present invention, (a) is a state of Von, (b) is a state of Voff,

2 is a schematic view showing the principle of operation of the polarizing reflection layer of the present invention,

3 is a partial plan view showing a configuration of a first embodiment of the present invention;

4 is a partial cross-sectional view of the first embodiment of the present invention;

5 is a sectional view of a TFT of a first embodiment of the present invention;

6 is a partial cross-sectional view of another embodiment of the present invention;

7 is a cross-sectional view of a TFT of another embodiment of the present invention;

8 is a schematic cross-sectional view illustrating another embodiment of the present invention.

<Explanation of symbols for main parts of drawing>

11 --- polarizing plate, 12 --- λ / 4 wavelength plate (fixed retardant layer),

13,14 --- transparent substrate, 15 --- vertically aligned liquid crystal layer (VA layer) (variable retardant layer),

16,17 --- transparent electrode layer, 18 --- cholesteric liquid crystal layer (polarization reflecting layer),

21 --- back light source, 25 --- retarder,

26 --- polarizer, 51 --- retarder,

52 --- Second cholesteric liquid crystal film.

The display device of the present invention for achieving the above object is a structure in which the first polarizing plate, the variable retarder, the polarizing reflection layer, the retardation plate, and the second polarizing plate are sequentially stacked from the observation surface side. . The polarization reflecting layer has a function of selectively reflecting left circularly polarized light component or right circularly polarized light component among incident light reaching the one circumferential surface thereof and transmitting the remaining components.

That is, as shown in FIG. 2, when the cholesteric liquid crystal 19 constituting the polarization reflection layer 18 has a spiral structure of left rotation, the left circularly polarized light component of the natural light Lf incident from the main surface 18f side is It is reflected by the main surface 18f. In addition, the right circularly polarized light component L2 is transmitted to the other main surface 18b side of the incident light. On the other hand, the left circularly polarized light component of the natural light Lb incident from the other main surface 18b becomes the light L2 'reflected from the main surface 18b. In addition, the right circularly polarized light component passes through the polarization reflecting layer 18 with respect to the advancing direction to become the light component L1 '. As described above, the cholesteric liquid crystal ideally reflects the circularly polarized light component in the rotational direction along the spiral direction and transmits the circularly polarized light component in the reverse rotation direction, but when the cholesteric liquid crystal is formed into a thin film, the helical direction It transmits about 10% of circularly polarized light component in the same rotation direction as. Thus, the remaining 90% of the circularly polarized light components in the same rotational direction as the spiral direction are selectively reflected.

In the display element of this invention, when external light enters from an observation surface side, the linearly polarized light component which has a vibration direction according to the polarization axis direction of a polarizing plate is taken out, and reaches a variable retarder. The variable retarder ideally has a fixed retarder layer which delays the phase of the vibration component in the specific direction of the incident light by λ / 4 (λ: incident light wavelength) with respect to the vibration component orthogonal to this, and in the specific direction of the incident light according to the applied voltage. The phase of a vibration component is comprised by the variable retarder layer which delays (lambda) / 2 with respect to the vibration component orthogonal to this.

As the fixed retardant layer, for example, a well-known λ / 4 phase difference plate can be used, and the linearly polarized light transmitted through the polarizing plate is placed in a predetermined rotational direction by arranging the phase axis at an angle of 45 ° with respect to the polarization axis of the polarizing plate. Convert to polarized light. When the phase axis of a retardation plate is arrange | positioned so that it may become an angle of 45 degrees by the right rotation with respect to the polarization axis of a polarizing plate, the circularly-polarized light emitted will become the polarity of a right rotation. Conversely, if the slow axis is arranged so as to form an angle of 45 ° to the left with respect to the polarization axis, the emitted circularly polarized light becomes the polarity of the left rotation.

The variable retardant layer may be a birefringence layer capable of varying the phase difference by voltage control, for example, a vertically aligned homogeneous (VA) liquid crystal layer having negative dielectric anisotropy. Use Since the incident light is emitted without phase modulation in a state where a voltage (first voltage) below the threshold is applied to the VA layer, that is, the liquid crystal layer maintains an initial array perpendicular to the substrate, the polarization of the circularly polarized light remains as it is. maintain. When a voltage above the saturation voltage (second voltage) is applied to the VA layer and arranged in the horizontal direction on the substrate, the vibration component in the specific direction of the incident light is delayed by λ / 2 with respect to the vibration component in the direction orthogonal thereto. The direction of rotation of the circularly polarized light is reversed.

That is, in the present invention, when the variable retardant layer is formed of liquid crystal, phase delay due to the liquid crystal layer occurs relatively when the first voltage is applied and the second voltage is applied. Taking VA liquid crystal as an example, the first voltage indicates that the liquid crystal exhibits an initial alignment state, and the second voltage indicates that the alignment of the liquid crystal is parallel to the substrate.

In addition, since the rotation direction of circularly polarized light cannot be controlled in the liquid crystal layer having high linearity ability, a broadly so-called ECB type (Electrically Controlled Birefringence mode) liquid crystal can be used as the variable retardant of the present invention. In the twisted nematic (TN) liquid crystal, when the twist angle is set to an appropriate value, sufficient birefringence function is obtained.

As an example, a first [lambda] / 4 phase difference plate having a slow axis intersecting at an angle of approximately 45 [deg.] With a right rotation with respect to the polarization axis, a vertically aligned (VA) liquid crystal layer disposed on the rear side thereof, and further disposed on the rear side of the VA liquid crystal layer. When the display device of the present application is constructed using the cholesteric liquid crystal layer twisted to the left, the second phase difference plate disposed on the rear side, and the second polarizing plate, the linearly polarized light reaching the first phase difference plate from the front surface of the display element first It is converted into right circularly polarized light and output.

When the VA liquid crystal layer is applied with the second voltage, that is, in the on state, the right circularly polarized light is converted from the VA liquid crystal layer to the left circularly polarized light to reach the polarized light reflection layer. The polarized light reflecting layer selectively reflects the polarization component of the left rotation according to the principle of FIG. 2. This left circularly polarized light is converted into right circularly polarized light in the VA liquid crystal layer again and is taken out as a linearly polarized light which reaches the retardation plate and vibrates in the same direction as the incident linearly polarized light. Further, the light from the back of the display element is converted into a circularly polarized light component (left rotation circularly polarized light in the above example) in the direction selectively reflected by the second polarizing plate and the second phase difference plate to the polarization reflecting layer. 10% of them pass through the polarized light reflection layer and are converted into circularly polarized light of right rotation in the VA liquid crystal layer. Then, it is taken out as linearly polarized light which reaches the retardation plate and vibrates in the same direction as the incident linearly polarized light.

On the other hand, when the VA liquid crystal layer is applied with the first voltage, that is, in the off state, the right circularly polarized light incident from the front surface of the display element and exiting from the first phase difference plate maintains its rotation direction and reaches the polarized light reflection layer, Permeate through the surface.

Then, the light is converted into linearly polarized light by the second phase difference plate and absorbed by the second polarization plate. In addition, 10% of the polarization components of the left rotation from the back of the display element pass through the polarization reflection layer and the VA liquid crystal layer, and become linearly polarized light which vibrates in a direction orthogonal to the polarization axis of the first polarizing plate by the first phase difference plate. Absorbed by the polarizing plate.

Thus, by the display element having the same configuration, reflective display using external light (incident light from the polarizing plate side) and transmissive display using a back light source can be simultaneously performed.

Further, when the slow axis of the retardation plate is arranged so as to form an angle of approximately 45 ° leftward from the polarization axis, the same operation can be achieved by turning the twist direction of the cholesteric liquid crystal to the right.

As a back light source, the surface light source which arrange | positioned the linear light source on the side surface of the light guide body which consists of transparent flat plates, such as acrylic, can be used, for example. By providing the scattering reflection layer on the back surface of the light guide, the polarization component reflected from the polarization reflection layer is repeatedly reflected between the polarization reflection layer and the scattering reflection layer until it passes through the polarization reflection layer. Therefore, if the loss by the absorption component of the scattering reflection layer is eliminated, the utilization efficiency of the light from the linear light source can be extremely increased.

(Example)

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

1 shows the configuration of the display element 10 in the first embodiment of the present invention. The polarizing plate 11 is arrange | positioned at the observation side of a display element, and the (lambda) / 4 wavelength plate 12 is arrange | positioned at the lower layer. In the lower layer of the? / 4 wavelength plate 12, a liquid crystal element having a vertically aligned liquid crystal layer 15 sandwiched by two glass substrates 13 and 14 is disposed.

A film 18 obtained by polymerizing a cholesteric liquid crystal is deposited between the lower glass substrate 14 and the electrode layer 17. 2 shows a state of light transmitted or reflected by the film 18. The cholesteric liquid crystal used for the film 18 assumes the case where the value np of the twist pitch p and the average refractive index n of the liquid crystal molecule 19 is equal to the incident light wavelength λ. When the liquid crystal molecules of this cholesteric liquid crystal have a helical structure of left rotation, the left circularly polarized light component of the incident light Lf is selectively reflected, and the remaining polarized light component is transmitted. The cholesteric liquid crystal has a function of reflecting ideally 100% of the circularly polarized light component in the same direction (left or right) as the helical direction (left or right) when the value np is equal to the incident light wavelength λ. It actually has about 10% penetration. Similarly, the left circularly polarized light component is selectively reflected on the light Lb incident from the lower side.

Again, the operation of the display element 10 of the present embodiment will be described with reference to FIG. FIG. 1A shows an ON state in which a voltage is applied from the power supply 20 to the vertically aligned liquid crystal layer 15, and more precisely, a voltage application state (at Von) above the threshold level of the liquid crystal. In this case, the nematic liquid crystal molecules are in a homogeneous orientation 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 incident from the observation side above the drawing is a vertically aligned circularly polarized light that is a variable retardant layer as a right polarization via the polarizing plate 11 and the λ / 4 phase difference plate 12 which is a fixed retardant layer. It enters into a type | mold liquid crystal layer (VA layer: 15). The phase is delayed in the layer 15 by λ / 2, thereby converting to left circularly polarized light and reaching the cholesteric liquid crystal layer 18 which is a polarized light reflection layer. Therefore, the left-handed circularly polarized light reached as described above is reflected by the cholesteric liquid crystal layer 18, and the phase is delayed by λ / 2 by the VA layer 15, so that the right-handed circularly polarized light is converted and outputted. . By passing this light through the λ / 4 phase difference plate 12 again, the light becomes linearly polarized light along the polarization axis of the polarizing plate 11, and is output through the polarizing plate 11, thereby obtaining a bright display.

FIG. 1B shows an off state (including zero voltage) (when Voff) is applied to a vertical alignment liquid crystal layer 15 with a voltage below a threshold. In this case, the liquid crystal layer is in a state where the liquid crystal molecules are arranged perpendicular to the substrate and do not phase modulate incident light.

In this state, the light Lf incident from the upper part of the drawing is a circularly polarized light of right rotation through the polarizing plate 11 and the λ / 4 phase difference plate 12 as in the case of FIG. 1A. Although incident on (15), it does not phase-modulate in the copper layer 15, but reaches the cholesteric liquid crystal layer 18 as circularly polarized light of right rotation. The right-handed circularly polarized light is transmitted to the rear surface of the display element, and is converted into linearly polarized light having a vibration component along the absorption axis of the polarizing plate 26 in the retardation plate 25. As a result, light does not return to the observation surface, and a dark display is obtained.

Next, the operation in the case where the surface light source 21 is disposed on the back surface of the cholesteric liquid crystal layer 18 will be described. The surface light source 21 is comprised by the light guide plate 22 formed by an acryl flat plate, etc., the linear light source 24 arrange | positioned at the side surface, and the diffuse reflection layer 23 arrange | positioned at the back surface of the light guide plate.

In the state of FIG. 1A, that is, Von, the light Lb outputted from the surface light source becomes right-turned circularly polarized light by the polarizing plate 26 and the retardation plate 25, and light having a predetermined ratio (about 10%). ) Passes through the cholesteric liquid crystal layer 18, and the remaining light is reflected. Then, the light passing through the cholesteric liquid crystal layer 18 is phase-modulated by the VA layer 15 and converted into right circularly polarized light. And this light passes through (lambda) / 4 phase difference plate 12, and becomes linearly polarized light along the polarization axis of the polarizing plate 11, it is output through the polarizing plate 11, and the display of a bright state is obtained.

On the other hand, in the state of FIG. 1B, that is, Voff, circularly polarized light of left rotation passing through the cholesteric liquid crystal layer 18 is output as it is without being subjected to phase modulation by the VA layer 15. And this light passes through the (lambda) / 4 phase difference plate 12, and it becomes linearly polarized light which has a vibration direction orthogonal to the polarization axis of the polarizing plate 5, is absorbed by the polarizing plate 11, and the display of a dark state is obtained. .

In this way, when the display element of the same structure is used and external light is used, a display with extremely high light utilization efficiency can be obtained together with the case of using a light source, and bright display is possible.

In addition, by forming the polarization reflecting layer of the cholesteric liquid crystal layer 18 inside the VA liquid crystal element which is a variable retardant, the parallax by the board | substrate 14 is eliminated compared with the case where the polarization reflecting layer is arrange | positioned on the outer surface of the board | substrate 14. In addition, if the polarizing reflection layer is used as an insulating layer in the case of forming active elements such as TFT and MIM elements on the substrate 14, for example, the manufacturing process can be simplified and the cost can be reduced.

Thus, the display element having the same configuration enables reflective display using external light and transmission display using a back light source at the same time.

Moreover, in the above embodiment, the VA liquid crystal element was used as the variable retardant, but it goes without saying that the same effect can be obtained as long as the element can control whether the phase of the incident light is shifted 1/2 wavelength or not by phase modulation. For example, as another embodiment, a horizontally aligned nematic liquid crystal device in which a conventionally known nematic liquid crystal is oriented parallel to the direction of the substrate may be used, or a twisted nematic liquid crystal device in which the nematic liquid crystal is twist-oriented may be used. .

It is of course also possible to apply the display element of the present invention to a color display element. That is, a pixel in which three primary colors of red, green, and blue or yellow, magenta, and cyan complementary colors are disposed between the polarizing plate and the nematic liquid crystal layer, and arranged in a matrix form. By using the electrode for electric field control of the nematic liquid crystal layer in units of pixels, color display by additive color mixing can be performed, and of course, the color filter can be configured inside the cell of the variable retardant.

Such a polarizing reflection layer used in the display element of the present invention is preferably used to exhibit the above-described functions and functions for light of all wavelengths in the visible light region to obtain color display excellent in achromatic white-black display and color reproducibility. For example, in the case where the polarization reflection layer is formed of the cholesteric liquid crystal layer as in each of the above embodiments, the value np of the helical pitch p and the average refractive index n of the cholesteric liquid crystal polymer is the sum of the visible light wavelengths. By having a spiral structure in which the spiral pitch is continuously changed in the layer thickness direction so as to cover the shortest wavelength to the longest wavelength, polarization reflecting capability corresponding to all wavelengths in the visible light region can be obtained.

The rod-shaped polymer constituting the cholesteric liquid crystal has a helical structure, and when light parallel to the helical axis is incident, black reflection of the same wavelength of light in the helical pitch is performed. That is, black reflection is obtained at the same bandwidth (range of wavelength) as the value (np) of the refractive index anisotropy (Δn) and the helical pitch (p), with the light having the same wavelength as the center wavelength as the value np. Further, the refractive index anisotropy (Δn) indicates the difference between the refractive index along the long axis direction and the refractive index along the short axis direction of the rod-shaped liquid crystal polymer, while the average refractive index is 2 of the refractive index along the long axis direction and the refractive index along the short axis direction. Obtained by the square root of sublimation.

However, since the refractive index anisotropy (Δn) of the cholesteric liquid crystal is only 0 to 0.3, and the average refractive index n of the cholesteric liquid crystal is also only 1.4 to 1.6, the black reflection It is difficult to match the central wavelength of to the central wavelength of the visible light wavelength (about 550 nm). Therefore, as described above, changing the spiral pitch of the cholesteric liquid crystal in the layer thickness direction is extremely effective because it obtains a good polarization reflecting ability over the entire visible light region.

In order to obtain a cholesteric liquid crystal layer having such a spiral pitch, two or more kinds of cholesteric liquid crystal polymer layers having different pitches are successively laminated, or when the cholesteric liquid crystal material is applied to a substrate and fixed, The method of coating the film surface with the additive which extends the spiral pitch of a cholesteric liquid crystal (for example, nematic liquid crystal etc. whose spiral pitch is infinite) is preferable.

In the above example, the intermediate display can also be performed by applying an intermediate voltage between Von and Voff as the voltage applied to the variable retardant layer.

In each of the above embodiments, when operating as a reflective display element using external light and when operating as a transmissive display element using a back light source, high light utilization efficiency can be achieved in either case.

Fig. 3 is a TFT active matrix liquid crystal display device constructed as a transflective liquid crystal display device of a second embodiment of the present invention. 4 shows the main components, and FIG. 5 shows the TFT element structure of the array substrate. FIG. 5 illustrates the TFT device structure of FIG. 4, which is shown upside down with respect to FIG. 4. The TFT array substrate 13 is made of an insulating substrate made of glass or the like, and the drawing shows a vertical alignment with 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 rear light source side. A nematic element is shown. A plurality of pixel electrodes 30 are arranged in a matrix in the screen display area, and a thin film transistor TFT 31 is provided in each pixel electrode 30 as a driving switching element. Between these pixel electrodes, a signal line 32, a scanning line 34 including a gate electrode 33, and a storage capacitor electrode (not shown) are provided as necessary. From these, a thermal oxide film 35, for example, a semiconductor film 36 made of amorphous silicon (a-Si) is sequentially formed, and a low resistance semiconductor film 37 is formed covering the semiconductor film. The portion constituting the TFT element 31 is covered by a passivation film 38 for protecting the TFT element.

In this way, the structure in which the gate electrode 33 is disposed below the semiconductor film 36 is called a bottom gate structure, and external light that enters the TFT element 31 from the array substrate 13 is the gate electrode 33. Is not incident on the semiconductor film 36. As a result, it is possible to prevent the display contrast ratio from being lowered due to the light leakage current generated by the light when the display element is used outdoors.

In addition, a color filter 39 is disposed in front of the pixel portion. The contact hole 40 of about 10 micrometers is provided in the color filter. On the color filter 39, a transparent pixel electrode 30 made of, for example, ITO is formed for each pixel. The transparent pixel electrode 30 is electrically connected to the source electrode 41 of the TFT via the contact hole 40 provided in the color filter 39.

A wiring electrode of any one of the signal line 32, the scanning line 34, and the storage capacitor line is disposed at the boundary of the transparent pixel electrode 30, and the light of the back light source is transmitted when the transmitted light of the transflective liquid crystal display device is used by the back light source. There is no leakage and the contrast ratio is lowered. On this array, an alignment film (not shown) is further laminated with a predetermined alignment axis.

On the other hand, the polarizing reflection layer 18 is formed in the counter board 14 in predetermined shape. Here, as the polarization reflection layer 18, a film obtained by polymerizing a cholesteric liquid crystal was deposited. In the polarization reflection layer 18, a transparent conductive film of ITO, for example, is laminated as a counter electrode 17 with a predetermined formation. It is preferable that ITO simultaneously perform film-forming and patterning by the means of a normal mask sputter | spatter. Thereby, the process load to the cholesteric liquid crystal layer at the time of ITO formation is extremely small.

Further, an alignment film (not shown) is aligned and laminated. The alignment is a direction in which the liquid crystal molecules are oriented perpendicular to the substrate.

These TFT array substrate 13 and the opposing substrate 14 face each other to form a liquid crystal cell, and the peripheral portions (sealing portions 42) of both substrates are attached by an adhesive (sealing material 43), VA liquid crystal 15 is enclosed in the liquid crystal cell. At this time, the sealant may be applied to a region where the polarization reflection layer 18 of the counter substrate 14 is not formed. The adhesiveness of a sealing agent is bad on the polarizing reflection layer 18, and there exists a possibility that it may cause reliability problems, such as a board | substrate falling with respect to long-term use more than 10,000 hours. Alternatively, the above reliability problem can be avoided by applying an overcoat agent having good adhesion to the sealant on the polarizing reflection layer. The overcoat agent is good, for example, with an acrylic resin usually used for color filters.

The quarter wave plate 12 and the polarizing plate 11 are stacked in this order on the outer surface of the array substrate 13. As the liquid crystal layer of the counter substrate 14, a back light source (not shown) is arranged on the outer surface of the reflection side. In addition, in a large liquid crystal display device having a diagonal screen length of 8 inches or more, a light diffusion film may be provided on the outer surface of the array substrate 1 to enlarge the viewing angle.

The liquid crystal display device of the present embodiment configured as described above is a reflective liquid crystal display device which is driven by a connected external circuit, and enters the ambient light into the liquid crystal display device in a bright place with a suitable light source therein, and displays the light by the reflected light. It is available.

Moreover, it can be used as a transmissive liquid crystal display device which lights a light source on the back of a liquid crystal cell, and displays with the transmitted light in a dark place.

6 and 7 show a TFT type active matrix liquid crystal display device constructed as a semi-transmissive liquid crystal display device of a third embodiment of the present invention.

Parts different from those of the second embodiment described with reference to FIGS. 3 to 5 are as follows, and the other parts are the same.

First, the thickness of the counter substrate 14 is as thin as possible and practically as thin as 0.2 mm, and the transparent electrode 17 of the counter substrate 14 is formed with a polarizing reflection layer 18a supplied as a film with an adhesive layer. It is attached to the outer surface opposite to the surface.

Here, as the polarization reflection layer 18a, a film obtained by polymerizing a cholesteric liquid crystal can be used. Since the polarizing reflection layer does not need to be thin-film formed as in the first embodiment, the manufacturing hold is better than in the first embodiment. In addition, in display, the influence by the parallax of about 0.2 mm of board | substrate thickness generate | occur | produces, but when about 0.2 mm, there is no possibility of causing a problem which impairs visibility. Further, the substrate 14 having a thickness of 0.2 mm is used by thinly polishing a 0.7 mm glass substrate or a plastic substrate.

Secondly, the TFT element on the TFT array substrate is different from the first embodiment in that it is a polysilicon TFT element 31a. The light leakage current of the polysilicon TFT element is generally small, and is not a problem even under an external light of tens of thousands of lux (1x).

The polarization reflection layer 18a mentioned above is comprised by the cholesteric liquid crystal film. Coloring by wavelength dispersion of polarization reflecting ability is made by having a spiral structure in which the spiral pitch continuously changes along the layer thickness direction so that the product np of the spiral pitch p and the average refractive index of the cholesteric liquid crystal polymer covers all of the visible wavelengths. Can be controlled.

The rod-shaped polymer constituting the cholesteric liquid crystal has a helical structure, and when light parallel to the helical axis is incident, black reflects the wavelength of light equal to the helical pitch. That is, black reflection is obtained at the same band width (range of wavelength) as the product of refractive index anisotropy (Δn) and spiral pitch (Δnp) with light having the same wavelength as the product np as the center wavelength. Moreover, the refractive index anisotropy (Δn) indicates the difference between the refractive index along the long axis direction and the refractive index along the short axis direction of the rod-shaped polymer, while the average refractive index is the sublimation of the refractive index along the long axis direction and the refractive index along the short axis direction. Obtained by the square root of.

In order to realize this, the spiral pitch of the cholesteric liquid crystal is changed in the layer thickness direction to obtain good polarization reflecting ability over the entire visible light region.

FIG. 8 shows an optical system for improving the light utilization rate of back light source light in the display device of FIG. 1. That is, the retardation plate 51 and the second cholesteric liquid crystal film 52 are disposed between the rear light source 21 and the polarizing plate 26.

The optical system converts a part of the light source light into linear flat light along the flat axis in front of the polarizing plate 26, reflects the rest, and recycles the light. In this embodiment, the second cholesteric liquid crystal film 52 Similar to the polarization reflecting layer 18, it has a spiral structure that is twisted to the left, and the retardation plate 51 has a slow axis parallel to the slow axis of the retardation plate 25.

In this configuration, the right-handed circularly polarized light component of the light source light passes through the second cholesteric liquid crystal film 52, and most of the left-handed circularly polarized light component is reflected by the second cholesteric liquid crystal film 52. About 10% of light passes through the second cholesteric liquid crystal film 52. When the circularly polarized light enters the retardation plate 51, the circularly polarized light component of the right rotation is converted into linearly polarized light having a vibration component parallel to the polarization axis of the polarizing plate 26, and the circularly polarized light component of the left rotation is polarized plate 26. Is converted into linearly polarized light having an oscillation component parallel to the absorption axis. Therefore, only the left-handed circularly polarized light component that is transmitted through the polarizing plate 26 and converted by the phase difference plate 25 is incident on the polarizing reflection layer 18.

About 90% of the left-handed circularly polarized light component is reflected by the polarization reflecting layer 18 and converted into linearly polarized light that is transmitted through the polarizing plate by the phase difference plate 25, and furthermore, by the phase difference plate 51. The liquid crystal film 52 is converted into circularly polarized light. Then, the light reaches the reflecting plate of the rear light source unit and the polarization component is decomposed. By repeating this, the light reflected toward the rear light source can be recycled to increase the utilization efficiency of the light source light.

Moreover, the structure of this optical system is not limited to the said structure, When using the cholesteric liquid crystal film which has a reverse twist, the optical system which has a similar function by rotating the slow-axis direction of the retardation plate 51 by 180 degrees. Can be configured.

As described above, in the display element of the present invention, when operating as a reflective display element using external light and when operating as a transmissive display element using a back light source, a high luminance display screen can be obtained in any case. Therefore, it is not necessary to use the back light source in the reflective display or to increase the brightness of the back light source in the transmissive display, thereby reducing power consumption.

Claims (18)

A first polarizing plate that transmits linearly polarized light along its polarization axis, A variable retarder disposed behind the first polarizing plate and modulating a phase of incident light according to an applied voltage; A polarization reflecting layer disposed behind the variable retarder and selectively reflecting the first circularly polarized light component of a predetermined direction of rotation of the incident light; A retardation plate and a second polarizing plate, which are arranged in sequence behind the polarizing reflection layer and convert light from the rear into the circularly polarized light component of the predetermined direction rotation; The variable retarder converts the linearly polarized light transmitted through the first polarizing plate into the first circularly polarized component when the first voltage is applied, and the second source of reverse rotation with the first circularly polarized component when the second voltage is applied. And converting the polarized light into a linearly polarized light along the polarization axis of the first polarizing plate when the first voltage is applied, and converting the polarized light into the linearly polarized light. And transmitting the transmitted light into linearly polarized light in a direction orthogonal to the polarization axis of the first polarizing plate. The display device of claim 1, wherein the polarization reflection layer is a cholesteric liquid crystal layer. The display device of claim 2, wherein the helical pitch of the cholesteric liquid crystal layer is different depending on a layer thickness direction. The display device according to claim 1, wherein the variable retardant comprises a variable retardant layer that variably modulates the phase of incident light in response to an applied voltage, and a fixed retardant layer that modulates the phase of the incident light by a predetermined amount. The display element according to claim 4, wherein the variable retardant layer is disposed on the polarization reflecting layer side rather than the fixed retardant layer. The display device according to claim 1, wherein the variable retardant layer has a liquid crystal disposed between the electrodes, and the first voltage and the second voltage are applied to the electrode. The display element according to claim 4, wherein the variable retardant layer changes the phase of incident light by λ / 2 when the first voltage is applied than when the second voltage is applied. The display device of claim 7, wherein the variable retardant layer is a twisted nematic liquid crystal device. The display device of claim 7, wherein the variable retardant layer is a vertically aligned nematic liquid crystal device. The display device of claim 7, wherein the variable retardant layer is a horizontally aligned nematic liquid crystal device. The display element according to claim 1, wherein a surface light source is arranged on the other surface side of the polarization reflection layer. The fixed retardant layer is made of λ / 4 phase difference plate, A liquid crystal cell that modulates a phase of incident light arranged so that its slow axis is approximately 45 ° from the polarization axis in a predetermined direction when viewed from the front side of the first polarizing plate, and the polarization reflection layer includes a cholesteric liquid crystal layer. Display element, characterized in that. 13. The display element according to claim 12, wherein the liquid crystal layer cell comprises the electrode layer and two electrode substrates having electrodes formed on respective inner surfaces of the liquid crystal layer and facing the liquid crystal layer. The display device of claim 13, wherein the cholesteric liquid crystal layer is deposited on the surface of the electrode substrate. The display device of claim 14, wherein the cholesteric liquid crystal layer is deposited on an outer surface of the electrode substrate. The display device of claim 15, wherein the cholesteric liquid crystal layer is made of a liquid crystal polymer film. 15. The apparatus of claim 13, wherein one of the electrode substrates includes a plurality of pixel electrodes arranged in a matrix and a switching element for controlling each pixel electrode, and the other electrode substrate has a counter electrode common to the plurality of pixel electrodes. Display device comprising a. The display device according to claim 12, wherein the polarization reflecting layer is made of a cholesteric liquid crystal layer, and a twisting direction thereof is opposite to a rotational direction from the polarization axis to the slow axis.