JPH11174498A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a reflection liquid crystal display device which has high- speed response and enables bright display. SOLUTION: This liquid crystal display device consists of a liquid crystal cell which is formed by holding chiral neumatic liquid crystals 30 of positive dielectric anisotropy having a natural helix pitch within a range of 1 to 3 times the liquid crystal display thickness in pixel parts between a pair of transparent electrode substrates 21, 22 and is constituted to allow the selection of either one of two quasi stable states as the relaxation state after the occurrence of Frederics transition by impression of voltage, polarizing plates 41 to 42 which are disposed on both outer side of this liquid crystal cell and a reflection plate 51 which is disposed on the further outer side of the one polarizing plate 41. In such a case, the thickness of the liquid crystal layer in the non-pixel parts inclusive of the parts between the pixels is set smaller than the thickness of the liquid crystal display layer in the pixel parts, by which the helix angle of the liquid crystal molecules in an initial state (at the time of no impression of voltage) is varied between the pixel parts and the non-pixel parts.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bistable reflection type liquid crystal display device.

[0002]

2. Description of the Related Art As an operation method of a liquid crystal having a high-speed response, a BTN (Bistable Twist) having bistability is used.
Many proposals have been made on an isotropic nematic (bistable torsion nematic) method, including a liquid crystal display device that drives a pixel formed by a scanning electrode group and a signal electrode group in a time-division manner. ing. Focusing on the alignment state of the liquid crystal molecules and the corresponding optical state, the driving voltage pulse in the pixel section
An ordinary polarizing plate that changes between a first metastable state having a twist angle of (initial twist angle -180 °) and a second metastable state having a twist angle of (initial twist angle + 180 °) The angle arrangement shows a bright state and a dark state, respectively, but since no voltage is applied to the non-pixel portion including between pixels, the alignment state remains at the initial twist angle,
It shows an optical state intermediate between a dark state and a bright state. When BTN is used as a transmissive type, a light shielding layer is formed in the non-pixel portion or the thickness of the liquid crystal layer in the non-pixel portion is reduced in order to prevent a decrease in contrast due to light leakage from the non-pixel portion. It has also been proposed that the orientation state (twist angle) be made darker by making the orientation state (twist angle) larger than the second metastable state (JP-A-6-23).
5920) has not yet been proposed for improving the display brightness when used as a reflection type.

[0003]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a reflection type liquid crystal display device having a high-speed response and capable of displaying a bright image.

[0004]

According to the present invention, a dielectric anisotropy having a natural twist pitch within a range of 1 to 3 times the thickness of a liquid crystal layer in a pixel portion is provided between a pair of transparent electrode substrates. A liquid crystal cell configured so that one of two metastable alignment states can be selected as a relaxed state after a certain chiral nematic liquid crystal is sandwiched and a voltage is applied to cause a Freedericksz transition, and a liquid crystal cell of the liquid crystal cell. A liquid crystal display device comprising a polarizing plate disposed on both outer sides and a reflecting plate disposed further outside than one of the polarizing plates, wherein a thickness of a liquid crystal layer in a non-pixel portion including a space between pixels is reduced. The liquid crystal display device is characterized in that the twist angle of liquid crystal molecules in the initial state (when no voltage is applied) is made different between the pixel portion and the non-pixel portion by setting the thickness to be smaller than the thickness of the liquid crystal layer in the pixel portion. To provide Accordingly, it was possible to solve the above problems.

Hereinafter, the structure of the liquid crystal display device of the present invention will be specifically described with reference to the drawings. FIG. 1 shows a configuration example of a liquid crystal display device of a BTN (Bistable Twisted Nematic) type according to the present invention. A liquid crystal layer 30 is sandwiched between the lower substrate 11 and the upper substrate 12. Reference numerals 21 and 22 denote transparent electrodes for applying a voltage to the liquid crystal layer, and reference numerals 31 and 32 denote alignment films for aligning the liquid crystal. 41 and 42 are polarizing plates, and 51 is a reflecting plate. The liquid crystal layer used here is a chiral nematic liquid crystal having a natural pitch of 1 to 3 times the thickness of the liquid crystal layer and a positive dielectric anisotropy. The liquid crystal is aligned in a direction slightly inclined from the substrate surface by the alignment film subjected to the alignment processing. This inclination angle is preferably about 2 ° to 30 °. If the tilt angle is small, the metastable operation becomes unstable, and good switching becomes difficult. If the tilt angle is too large, the viewing angle dependence of the display characteristics becomes large. In the configuration shown in this drawing, the tilt of the liquid crystal at the interface between the upper and lower substrates is reversed in the initial state. The natural twist pitch p of the liquid crystal is set between 1 and 3 times the liquid crystal layer thickness d. The product Δnd of the birefringence and the thickness of the liquid crystal layer is approximately 以下 or less of the wavelength of the observation light, specifically 0.20 μm to 0.35 μm, preferably 0.22 μm.
It is configured to be in the range of μm to 0.30 μm.
Further, one polarizing plate is disposed so that its light transmission axis forms an angle of approximately 45 ° (35 ° to 55 °) with the orientation direction of the liquid crystal at the interface between the substrates, and the other polarizing plate has the light transmitting axis. The transmission axis is arranged to be symmetric with the light transmission axis of the other polarizing plate with respect to the orientation direction of the liquid crystal at the interface between the substrates.

Next, regarding the switching operation of the BTN, the driving waveform, the orientation state and the optical state, and d /
The relationship of p (liquid crystal layer thickness / natural twist pitch of liquid crystal) will be described. The driving waveform includes a voltage pulse (hereinafter, referred to as a reset pulse) for causing Freedericksz transition, and a voltage pulse (hereinafter, referred to as a second pulse) for selecting one of two metastable states. ) Is applied. The reset pulse is a voltage pulse equal to or higher than a threshold value (Vth) between the initial state and the two metastable states, and the second pulse is selected based on a threshold value (Vc) between the two metastable states. Voltage pulse. When the 2nd pulse voltage is equal to or lower than the critical value, the liquid crystal molecules are turned into a metastable state twisted by 180 ° more from the initial state due to a backflow caused by a sudden relaxation from the reset state (the arrangement of the liquid crystal molecules is a homeotropic state). Become. That is, in the element configuration shown in FIG. 1, since the initial state is about 180 °, the element is in a metastable state twisted by about 360 ° and becomes a dark state when the polarizing plate is arranged. On the other hand, 2nd
It becomes a metastable state with a pulse of about 360 ° twist,
With the general element configuration and the arrangement of the polarizing plates shown here, a dark state occurs. On the other hand, when the peak value of the second pulse is equal to or larger than the threshold value, the backflow is suppressed, so that the liquid crystal molecules are twisted by 180 ° smaller than the initial state, that is, approximately 0 ° in the element configuration shown in FIG. Becomes a metastable state,
In addition, the arrangement of the polarizing plate results in a bright state. On the other hand, when the peak value of the second pulse is equal to or larger than the threshold value, the backflow is suppressed, and the liquid crystal molecules are in a metastable state in which the twist is smaller than the initial state by 180 °, that is, approximately 0 °. In the general element configuration and the arrangement of the polarizing plates shown in the drawing, the light state is obtained.

FIG. 2 schematically shows the relationship between d / p and the 2nd pulse peak value in a metastable state selected after the Freedericksz transition. Reset pulse condition and 2n
When the d-pulse width is fixed, the metastable state selected after the Freedericks transition largely depends on d / p and the applied waveform condition. With respect to d / p, a metastable state of approximately 360 ° twist in a region where d / p is larger and a metastable state of approximately 0 ° in a region where d / p is smaller with a certain d / p value as a boundary, The boundary d / p value changes as shown in FIG. 2 depending on the peak value of the second pulse. Therefore, if the d / p value of the liquid crystal cell in FIG. 2 is defined as the critical value at the intersection with the line indicating the d / p boundary value, the second pulse peak value is set to a voltage larger than the critical value, so that the metastable state becomes approximately 0 °. If the state and the voltage are smaller than the critical value, a metastable state with a twist of approximately 360 ° is obtained, and the two metastable states can be arbitrarily selected.

Next, the relationship between the alignment state and the optical state in the initial state, that is, the state where no voltage is applied, and d / p will be described. According to our study using various liquid crystal materials, in the above-described configuration in which the alignment state in the initial state is approximately 180 ° twist, d / p <
At 0.3, the torsion angle is approximately 0 ° and the optically bright state is attained. At d / p> 0.8, the torsion angle is approximately 3 °.
An optically dark state is obtained at a 60 ° alignment state, and 0.3
It is known that in the case of ≦ d / p ≦ 0.8, an intermediate state between the above-described bright state and dark state (rather close to a dark state) can be obtained in an orientation state of about 180 °. The choice of the two metastable states described above with reference to FIG. 2 is in the last, d / p range with an orientation state of approximately 180 ° in the initial state (often between 0.45 and 0.75 ), The non-pixel portion including between pixels in the display device is also set to a value within the above-described d / p range, and is optically close to a dark state, that is, the reflectance is low in the reflection type. It becomes a small dark state. In the present invention, the thickness of the liquid crystal layer in the non-pixel portion of the display device is set to be smaller than the thickness of the liquid crystal layer in the pixel portion so as to satisfy d / p <0.3. The alignment state is always kept at a state where the twist angle is substantially 0 ° (optically bright state), and a bright reflective liquid crystal display device can be obtained.

Means for reducing the thickness of the liquid crystal layer in the non-pixel portion to be smaller than the thickness of the liquid crystal layer in the pixel portion include means other than "pixels formed by overlapping both substrates" on the transparent electrode of at least one of the substrates. In this portion, an insulating structure having a predetermined thickness and high transparency (less loss of transmittance) may be provided. One example of the structure is shown in FIG. Specifically, using a photomask used for forming the transparent electrode 2 on the substrate 1 by a photolithography method,
After exposing the photocurable resin material (for example, an acrylic material) layer applied thereon with a predetermined thickness so that portions other than the transparent electrode portion 2 are cured using the photomask, the uncured portion is removed. do it. As the photocurable resin material, monoacrylate, diacrylate, or a mixture of both, to which a polymerization initiator is added at a predetermined concentration can be used. A high-pressure mercury lamp, a metal halide lamp, or the like is used as an exposure light source.
Exposure may be performed with an exposure amount of about 1 J. The uncured portions can be removed by washing with a suitable organic solvent.

The insulating transparent structure 6 may be formed only on one substrate 1 (one substrate 1 is exposed using two photomasks for both substrates, respectively), or both may be formed. The transparent structure 6 may be formed by exposing each substrate 1 using a photomask for the substrate 1. However, in the latter case, the transparent structure 6
Is formed), it is necessary that the set value of the thickness of the liquid crystal layer in the pixel portion is not more than twice the set value of the thickness of the liquid crystal layer in the non-pixel portion. Otherwise, the former method (the transparent structure 6 is formed only on one substrate) is used. An alignment film 3 is formed on the substrate 1 on which the transparent structure 6 having a predetermined thickness has been formed as described above, an alignment process is performed, the two substrates 1 are overlapped and bonded via a spacer, and further dried. The liquid crystal material is injected later to complete the liquid crystal cell. By arranging polarizing plates on both sides of the liquid crystal cell at a predetermined angle and further providing a reflecting plate outside one of the polarizing plates, a liquid crystal display device is completed.

[0011]

EXAMPLE 1 On two glass substrates on which a transparent electrode layer (0.1 μm thick) was formed, electrodes were formed by a photolithography method using a photomask for a scanning electrode group and a photomask for a signal electrode group. A pattern was formed (the electrode pattern was a stripe pattern of a scanning electrode having an electrode width of 300 μm and a pitch of 330 μm, and the signal electrode being a stripe pattern of an electrode width of 90 μm and a pitch of 110 μm). On these substrates, an acrylic photocurable resin material (a diacrylate with a polymerization initiator of 3 w
t% added) was formed into a film by spin coating, and after exposure with a high-pressure mercury lamp, the uncured portion was removed with toluene. As a result, the thickness between the electrodes is 0.9
An insulating transparent structure of μm (1.0 μm in thickness from the substrate) was formed. An alignment film was further formed on these substrates. The substrate has a sectional structure shown in FIG. After performing an alignment treatment by rubbing on the alignment film, the two substrates are overlapped with each other through a dispersion of 2.2 μm bead spacers and printing of a sealant so that the alignment treatment directions are different by 180 ° (antiparallel). And dried by heating.
Then, nematic liquid crystal ZLI-1557 made by Merck was used.
(Δn = 0.1147), a liquid crystal composition having a twist pitch (p) adjusted to 4.4 μm by adding a Merck chiral agent S-811 that induces clockwise twist is injected under reduced pressure and sealed. The liquid crystal cell was completed. In this liquid crystal cell, d / p of the pixel portion is 0.50, and d / p of the non-pixel portion is 0.29. Then, two polarizing plates (polarization degree: 99.5%) were provided so as to sandwich the liquid crystal cell. One polarizing plate is arranged so that its transmission axis forms an angle of 45 ° with the direction of the alignment processing of the substrate (same for both substrates), and the other polarizing plate has its transmission axis aligned with the alignment processing direction of the substrate. On the other hand, it was arranged so as to be symmetrical with the transmission axis of the polarizing plate.

In this configuration, when the transmittance of the pixel portion and the non-pixel portion was measured using a microspectrophotometer without applying a voltage, the pixel portion (180 ° twisted alignment state) was measured.
Is 5.3%, and the non-pixel portion (a metastable state at approximately 0 °) is 33.
It was 0%. Next, a voltage pulse having a drive waveform such that all the pixels were in a metastable state of about 0 ° (optically bright state) was applied to the electrodes of both substrates, and the measurement was similarly performed. %, And the non-pixel portion was 33.0%. In this state, when the measurement spot diameter was further set to 5 mm and the transmittance at the center of the display device was measured, it was 33.2%. In this configuration, a diffuse reflection plate is further provided outside one of the polarizing plates, and a voltage pulse having a drive waveform such that all pixels are in a metastable state (optically bright state) of approximately 0 ° is applied to both substrates. When applied to the electrodes, a considerably bright display state was recognized.

Comparative Example 1 The same procedure as in Example 1 was carried out except that the thickness of the insulating transparent structure formed on the substrate was 0.6 μm from the electrode surface (0.7 μm from the substrate). To produce a liquid crystal cell. In this liquid crystal cell, d / p of the pixel portion is 0.50, and d / p of the non-pixel portion is 0.36. A polarizing plate was disposed on this cell in the same manner as in Example 1, and the transmittance of the pixel portion and the non-pixel portion was measured using a microspectrophotometer without applying a voltage. The orientation of twist was 5.3%, and the non-pixel portion was 5.1%. Next, a voltage pulse having a drive waveform such that all the pixels were in a metastable state (optically bright state) of approximately 0 ° was applied to the electrodes of both substrates, and measurement was performed in the same manner.
3%, and the non-pixel portion was 5.1%. In this state, when the measurement spot diameter was further set to 5 mm and the transmittance at the center of the display device was measured, it was 25.9%. In this configuration, a diffuse reflection plate is further provided outside one of the polarizing plates, and a voltage pulse having a drive waveform such that all pixels are in a metastable state (optically bright state) of approximately 0 ° is applied to both substrates. When the voltage was applied to the electrodes, a display state in which the brightness was considerably lower than in the case of Example 1 was recognized.

Example 2 In the same manner as in Example 1, a photomask for a scanning electrode group and a photomask for a signal electrode group were formed on two glass substrates on which a transparent electrode layer (0.1 μm thick) was formed. Each was used to form an electrode pattern by a photolithography method. An acrylic photocurable resin material (a material obtained by adding 3 wt% of a polymerization initiator to diacrylate) is formed on the substrate for the scanning electrode group by spin coating, and is sequentially exposed using two photomasks. After performing, the uncured portion was removed with toluene. As a result, the thickness 1.1 from the electrode surface is applied to a portion other than the “pixel” formed by overlapping the two substrates.
A μm insulating transparent structure was formed. Figure 4 shows the cross-sectional structure
Shown in After forming an alignment film on both the substrate and the other substrate and performing an alignment process by rubbing, 2.2 μm is formed.
After spraying the m bead spacers and printing the sealant, the two substrates were overlaid so that the alignment treatment directions differed by 180 ° (become antiparallel) and dried by heating. Thereafter, a liquid crystal cell was completed in the same manner as in Example 1. In this liquid crystal cell, d / p in the pixel portion is 0.50, and d / p in the non-pixel portion is
p is 0.25. Then, a polarizing plate was also arranged in the same manner as in Example 1, and the transmittance of the pixel portion and the non-pixel portion was measured using a microspectrophotometer with no voltage applied.
The pixel portion (180 ° twist alignment state) was 5.2%, and the non-pixel portion (substantially 0 ° metastable state) was 33.1%. Next, a voltage pulse having a drive waveform such that all the pixels were in a metastable state (optically bright state) of approximately 0 ° was applied to the electrodes of both substrates, and the same measurement was performed. %,
The non-pixel portion was 32.9%. In this state, when the measurement spot diameter was further set to 5 mm and the transmittance at the center of the display device was measured, it was 33.1%. In this configuration, a diffuse reflection plate is further provided outside one of the polarizing plates, and a voltage pulse having a drive waveform such that all pixels are in a metastable state (optically bright state) of approximately 0 ° is applied to both substrates. When the voltage was applied to the electrodes, a considerably bright display state was recognized as in Example 1.

[0015]

According to the present invention, it is possible to provide a reflective liquid crystal display having a high-speed response and a bright display. Also,
In the present invention, d / p is applied to the non-pixel portion of the display device.
Since it suffices to be <0.3, it is not necessary to precisely control the thickness.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view illustrating a configuration of a liquid crystal display element of Example 1.

FIG. 2 is a diagram illustrating a relationship between d / p and a 2nd pulse peak value regarding a metastable state selected after the Freedericksz transition.

FIG. 3 is a partial perspective view of a substrate of a liquid crystal cell.

FIG. 4 is a partial perspective view of another embodiment of the substrate of the liquid crystal cell.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Transparent electrode 3 Alignment film 6 Insulating structure 11 Lower substrate 12 Upper substrate 21 Transparent electrode 22 Transparent electrode 30 Liquid crystal layer 31 Alignment film 32 Alignment film 41 Polarizer 42 Polarizer 51 Reflector d Liquid crystal layer thickness p Liquid crystal layer Natural twist pitch

Claims (6)

[Claims] 1. A chiral nematic liquid crystal having a positive natural dielectric anisotropy and having a natural twist pitch within a range of 1 to 3 times the thickness of a liquid crystal layer in a pixel portion, between a pair of transparent electrode substrates, A liquid crystal cell configured to be able to select one of two metastable alignment states as a relaxation state after applying a voltage to cause the Freedericksz transition, and polarized light disposed on both outer sides of the liquid crystal cell. A liquid crystal display device comprising a plate and a reflector disposed further outside of one of the polarizing plates, wherein the thickness of the liquid crystal layer in the non-pixel portion including between pixels is greater than the thickness of the liquid crystal layer in the pixel portion. A liquid crystal display device characterized in that the twist angle of liquid crystal molecules in an initial state (when no voltage is applied) is made different between a pixel portion and a non-pixel portion by setting a small value. 2. The ratio of the thickness (d) of the liquid crystal layer in the non-pixel portion to the natural twist pitch (p) of the liquid crystal is d / p <0.3.
The liquid crystal display device according to claim 1, wherein 3. A pair of transparent electrode substrates which have been subjected to an alignment process so that the directions of the alignment process are substantially parallel and the inclination of the liquid crystal molecules at the interface with the substrate is substantially parallel between the upper and lower substrates. 3. The liquid crystal display device according to claim 1, wherein 4. The tilt of liquid crystal molecules with respect to the substrate surface is 2 to 4.
4. The liquid crystal display device according to claim 1, wherein the angle is 30 [deg.]. 5. In one of the two metastable alignment states, the twist angle of liquid crystal molecules in the thickness direction is the twist angle of liquid crystal molecules in the initial state (hereinafter also referred to as φ) + 180 °, and the other is: 5. The liquid crystal display device according to claim 1, wherein the angle is -180 [deg.]. 6. The liquid crystal display device according to claim 5, wherein φ is approximately 180 °.