GB2303464A

GB2303464A - Ferroelectric nematic liquid crystal display

Info

Publication number: GB2303464A
Application number: GB9615197A
Authority: GB
Inventors: Sin-Doo Lee
Original assignee: Samsung Display Devices Co Ltd
Current assignee: Samsung SDI Co Ltd
Priority date: 1995-07-20
Filing date: 1996-07-19
Publication date: 1997-02-19
Also published as: JPH0933957A; GB9615197D0; KR970007426A; DE19629551A1; CN1165311A

Description

X 2303464 FERROELECTRIC NEMATIC LIQUID CRYSTAL DISPLAY

BACKGROUND OF THE IlMNTION 1. Field of the Invention

This invention relates to a liquid crystal display, in particular, to a ferroelectric nematic liquid crystal display.

2. Description of the Related Art

Generally, a crystal has positional and orientational orders since the molecules of the crystal is fixed in position and orientation. But both orders disappear when the crystal is melted to become an isotropic liquid. A liquid crystal is a phase or a state different from both the crystal and the liquid An -hat it has only the orientational order or it has the orientational order and the positional order in part.

Since a material in the liquid crystal phase has the orientational order and the molecules of this material have the asymmetrical shapes, it is called to be an anisotropic material having different physical properties to its orientation.

Most liquid crystal molecules have the shapes of thin and long bars. The long axis of the molecule is called the molecular axis and the molecules tend to arrange themselves such that the molecular axes be parallel to each other. The direction along the average molecular axis is represented as a "director" and the degree of the orientational order is measured by the relations between the molecular axes and the director. That is, an order parameter which is the average value of (3 cos 2E) - 1)/2 is used as a measure of the degree of the orientational order, where 0 is an angle between the individual molecular axis and the is director. The typical value of the order paxameter lies in aoout 0.3 to 0. 9, and the order parameter decreases as the temperature of the liquid crystal material increases.

The liquid crystals are classified by the type of order into three categories, i. e., nematic, cholesteric or chiral nematic and smectic liquid crystals.

A nematic liquid crystal has an orientational order but has no positional order. The positions of the molecules of the nematic liquid crystal are out of order, but there exist intermolecular forces causing the molecules to be parallel to each other in the nematic phase. Ferroelectricity is not appeared in the conventional nematic liquid crystals since the molecules possess up-down symmetry so that no net polarization from molecular dipole moments is expected. The nematic liquid crystals are commonly used in displays.

A chiral nematic liquid crystal is often thought to be different from a cholesteric liquid crystal, but these two liquid crystals are not necessarily distinct since their physical properties share some common feature. The two liquid crystals have similar intermolecular forces causing the average molecular axis to rotate in space along the direction perpendicular to the director. This property present in the chiral nematic liquid crystal is called "chirality" and the distance within which the average molecular axis rotates by one turn is "pitch." It is noted that the nonchiral nematic liquid crystal also can have the twisted structure by means of alignment process. The chiral nematic materials are used in displays as well as in a microwave or an electromagnetic field detection.

2 A smectic liquid crystal has a more oidered structure formed into molecular layer than the above two liquid crystals. The smectic liquid crystal has not only the orientational order but also the positional order in part. Therefore, the molecular positions have periodicity in the direction normal to the layers but they do not have long-range positional order in the layer plane.

Among the smectic liquid crystals, tilted chiral smectic liquid crystals show ferroelectricity, for instance, smectic C liquid crystals. Recently, ferroelectric liquid crystals have been extensively studied. Smectic C liquid crystals are those having the molecular axes tilted with respect to the layer normal. The molecules of the smectic C liquid crystal have optical activity so that they form a helical structure along the is layer normal. The smectic C liquid crystal exhibits a spontaneous polarization in a direction perpendicular to the director and the layer normal. The smectic C liquid crystal has rotationary symmetry with respect to the axis perpendicular to the director and inversion symmetry with respect to the surface of the molecular layer. However, if the molecule has a chiral part, the inversion symmetry is broken and the transverse dipole moment produces the spontaneous polarization in the smectic C phase. The macroscopic spontaneous polarization averaged over one period or one pitch is zero since the molecules of the smectic C are helically arranged along the layer normal. Thus, this ferroelectric liquid crystal is called as an improper ferroelectric material. The helical structure can be distorted by an external electric field and completely unwound above a

3 critical field strength, thereby a macroscopic spontaneous polarization being induced. The ferroelectric liquid crystal displays (hereinafter referred to as ferroelectric LCDs) 'have several problems such as the difficulty in alignment and fragile molecular layers.

The liquid crystals have anisotropic physical properties such as the electric and magnetic susceptibilities. As described above, this is because the molecules have the asymmetrical shapes of bars and thus anisotropic intermolecular forces in space.

For example, the electric susceptibility along the director is different from that in the direction perpendicular to the director. Due to the difference in the electric susceptibility, the permittivity is also different in the directions.

Let the permittivity along the director to be el and that in the direction perpendicular to the director to be C2. Then, dielectric anisotropy Ae is defined by Ae = el - C2 Positive dielectric anisotropy is the case that Ae > 0 and negative dielectric anisotropy is the one that Ae < 0. When an electric field is applied to the liquid crystal, an electric displacement

D = 62 E + Ae (nt)n. The electrostatic energy -fDdE -(1/2) e2E2 _ (1/2) Ae(nt) 2.

This electrostatic energy competes with the elastic energy in order to reach a stable state. As a result, the director of the liquid crystal having the positive dielectric anisotropy tends to be parallel to the applied electric field while the director of the liquid crystal having the negative anisotropy tends to be perpendicular to the applied electric field.

LCDs use the anisotropic properties of the liquid crystal.

4 A conventional LCD in the twisted nematic (TN) mode will be described in detail.

FIGS. 1A and 1B show a conventional TN LCD using positive dielectric anisotropy. FIG. 1A illustrates a state in which no electric field is applied across the liquid crystal and FIG. 1B illustrates a state in which the electric field is applied across it.

A liquid crystal having positive dielectric anisotropy is placed between the inner surfaces of two transparent substrates 11 and 12. on the outer surfaces of the substrates 11 and 12, a polarizer 13 and an analyzer 14 are attached, respectively. on the inner surface of each substrate 11 or 12, a transparent electrode 15 or 16 and an alignment layer 17 or 18 are formed. The alignment layers are treated so that the director at one is surface is perpendicular to that at the other, thus making the director continuously to rotate by 90 degrees through the liquid crystal slab.

If the gap between the two substrates 11 and 12 is properly chosen, the polarization of the incident light passing through the liquid crystal slab can be changed in response to the molecular arrangement.

When an external voltage is applied between the transparent electrodes, the molecules in the bulk except for the surface region near the substrates 11 and 12 tend to orient along the direction of the electric field, i. e., to be perpendicular to the substrates 11 and 12. The arrows in FIGS. 1A and 1B represent the director.

Now, the operation of this TN LCD will be described.

4 0 10 In the "OFF" state in which no voltage across the substrates 11 and 12 is applied, the liquid crystal slab acts like a waveguiding plate for the incident light. A light incident on one substrate 11 is linearly polarized on passing through the polarizer 13 and its polarization rotates through the liquid crystal slab. When the polarized light reaches at the other substrate 12, its outcoming polarization rotates by 90 degrees. In the case that the polarization axes of the polarizer 13 and the analyzer 14 are perpendicular to each other, the light passes through the analyzer 14 since the polarization of the light is parallel to the polarization axis of the analyzer 14. However, if the polarization axes of the polarizer 13 and the analyzer 14 are parallel to each other, no light pass through the analyzer 14 since the polarization of the light is perpendicular to the is polarization axis of the analyzer 14.

In the "ON" state in which a voltage is applied between the two substrates 11 and 12, the molecular director becomes to be distorted. Since the liquid crystal has positive dielectric anisotropy, the director tends to orient along the direction of the electric field except for the surface region near the substrates 11 and 12. By controlling the strength of the electric field, the magnitude of the tilt angle of the director is controlled, causing the waveguiding effect of the liquid crystal layer to be changed. Under a sufficiently high electric field, a linearly polarized light through the polarizer 13 reaches at the analyzer 14 without rotation of its initial polarization. If the polarization axes of the polarizer 13 and the analyzer 14 are perpendicular to each other, the light hardly

6 0 a 1.

passes through the analyzer 14. When the polarization axes of the polarizer 13 and the analyzer 14 are parallel to each other, the light passes through the analyzer 14.

Between the OFF state and the ON state the analog gray scale is obtained by controlling the strength of the applied electric field.

The twisted angle of the molecular director in the TN mode is 90 degrees while that in the super-twisted nematic (STN) mode is larger than that in the TN mode, for example, 220 degrees or 270 degrees.

The conventional TN or STN LCD has several problems, one of which is slow response because only the dielectric anisotropy is used in driving.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an LCD having a lower driving voltages and a faster operation speed.

The other object of the present invention is to provide an LCD made of a ferroelectric liquid crystal which has an advantage of easier alignment over conventional LCDs.

In order to achieve these objects, the present invention uses a ferroelectric nematic liquid crystal.

The existence of the ferroelectric nematic liquid crystal is predicted earlier. For example, "Novel Ferroelectric Fluids", Rolfe G. Petschek and Kimbrly M. Wiefling, Physical Review - 25 Letters 1987 vol. 5 9, No. 3, pp. 345-346; "Ferroelectric Nematic Liquid Crystals: Realizability and Molecular Constraints", P. Palffy-Muhoray, M.A. Lee and Rolfe G.

7 Petschek, Physical Review Letters 1988 vol. 60, No. 22, pp. 2303-2306; and "Ferroelectric nematic liquid-crystal phases of dipolar hard ellipsoids", Marc Baus and Jean-Lois Colot, Physical Review A 1989 vol. 40, No. 9, pp. 5444-5446. The applicant also predicted the existence of the ferroelectric nematic liquid crystal in "Ferroelectric Liquid Crystalline Ordering of Rigid Rods with dipolar Interactionsn. This paper illustrates the phase diagram showing the phase transitions of conventional isotropic -nematic (I-N), nematic-ferroelectric nematic (NFN) and direct isotropic- f erroelectric nematic (I-FN). Recently, the papers showing the existence of the polar nematic liquid crystal, particularly having molecules with permanent dipole moments parallel to their molecular axes, are reported.

The molecular dipole moments of the ferroelectric liquid is crystal orient along the applied electric field, while the dielectric anisotropy makes the molecular axes to be parallel or perpendicular to the applied field depending on the sign.

An LCD according to the present invention comprises a pair of transparent substrates to which an external voltage can be applied, and a nematic liquid crystal layer having ferroelectricity between the substrates.

It is desirable that the molecules of the liquid crystal layer have permanent dipole moments parallel to molecular axes and, in this case, the liquid crystal have positive dielectric anisotropy so that the spontaneous polarization and the positive dielectric anisotropy interact with each other constructively.

The two substrates are treated so as to have homogeneously aligning forces causing the director to be parallel to them. The 8 is 0 00 director an the second substrate makes an angle from 0 degrees to 180 degrees with respect to that on the f irst substrate so that the two substrates produce a twisted angle.

In addition, the liquid crystal layer comprises a chiral dopant which have twisting power for the molecular orientation. The angle between the directors on the first and the second substrates can be then adjusted from 0 degrees to 360 degrees.

The value of the gap between the first and the second substrates divided by the pitch of the liquid crystal layer may be about 0.0 to 1.0. It is preferable that the value of the gap between the first and the second substrates divided by the pitch of the liquid crystal layer is 0.25. Moreover, the optical anisotropy multiplied by the gap between the first and the second substrates is preferably 0.1 microns to 2.0 microns.

On the two substrates, two polarizers are attached, respectively. The angle between the polarization axes of the two polarizers is equal to the twisted angle of the molecular director or + 90 degrees plus the twisted angle of the molecular director. Also, the polarization axes of the two polarizers can be parallel or perpendicular to each other.

The LCD according to the present invention can be operated similarly to the conventional TN LCD.

The molecules maintain the homogeneous alignment in the "OFF" state, so this state is similar to the "OFF" state of the TN LCD. The molecules can have a predetermined pretilt angle.

A linearly polarized light incident on the liquid crystal layer through the polarizer on the first substrate reaches at the polarizer on the second. In the "OFF" state the incident 9 c 0.0 polarization becomes to rotate by an angle corresponding to the twisted angle of the molecular director. If the angle between the polarization axes of the two polarizers is equal to- the twisted angle of the molecular director, the light passes through the polarizer on the second substrate. If the angle is 90 degrees plus the twisted angle of the molecular director, the light cannot pass through the polarizer of the second substrate.

The molecules tend to orient along the electric field in the "ON" state because of the longitudinal dipole moments of the molecules and positive dielectric anisotropy. The tilt angle of the molecular axes would be varied by controlling the strength of the electric field. When the electric field strength is sufficiently high, the most molecules are arranged parallel to the electric field. If the polarization axes of the two is polarizers are parallel to each other, the light passes through the polarizer of the second substrate. But if the polarization axes are perpendicular to each other, the light cannot pass through the polarizer on the second substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the present invention will now be described more specifically with reference to the attached drawings, wherein:

FIGS. 1A and 1B illustrate a TN LCD made of a liquid crystal having positive dielectric anisotropy; and FIG. 2 is a diagram showing the transmittance of the light in the LCD according to the embodiment of the present invention.

4 0 & 0 DETAILED DESCRIPTION OF THE PREFERRED XMBODMM3NTS

The preferred embodiments of the present invention will become apparent from a study of the following detailed description, when viewed in light of the accompanying drawings.

FIGS. 1A and 1B, which also referred to in the description of the conventional art, show an LCD according to the embodiment of the present invention. FIG. 1A illustrates an "ON" state in which no electric field is applied and FIG. 1B illustrates an "OFF" state in which the electric field is applied.

As shown in FIGS. 1A and 1B, two transparent substrates 11 and 12 are placed opposite with each other. The substrates 11 and 12 have transparent electrodes 15 and 16 and alignment layers 17 and 18 are used for producing homogeneous alignment. A ferroelectric nematic liquid crystal layer 10 lies in between the is two substrates 11 and 12. on outer surfaces of two substrates 11 and 12, a polarizer 13 and an analyzer 14 is attached, respectively.

Preferably, the liquid crystal layer 10 has positive dielectric anisotropy and spontaneous polarization parallel to the director and a chiral dopant such as S811 or CB15 which makes the liquid crystal to have chirality may be mixed into the liquid crystal layer 10.

The alignment layers 17 and 18 should enable to align molecules horizontally or homogeneously. They can be f ormed with, for example, surfactants such as alkylphenol and hexadecyltrimethylammonium bromide, polyimides, or alignment absorbants coated by the Langmuir-Blodgett film deposition method. The alignment layers 17 and 18 are rubbed to make the 11 0 a 0 liquid crystal molecules to align homogeneously along a certain direction. It is also possible for the molecules to have some pretilt angle between zero degree to 180 degrees.

The surface treatment, for example, rubbing is performed on either one alignment layer or both the alignment layers 17 and 18. When the treatment is performed on both the alignment layers 17 and 18, the molecular directors on the two alignment layers 17 and 18 can be adjusted with respect to one to the other. It is preferable that the molecular directors is zero degrees to 360 degrees.

Let the gap between the two substrates 11 and 12, more exactly, the distance between the two alignment layers 17 and 18 be d and the pitch of the liquid crystal be p. It is then preferable that d/p is 0.0 to 1.0. When d/p = 0.25, the molecular directors on the two substrates are twisted by 90 degrees through the liquid crystal layer 10.

The operation of the LCD according to the embodiment will be now described.

As shown in FIG. IA, the molecules maintain the homogeneous alignment in the "OFF" state.

In this state, assume that a linearly polarized light through the polarizer 13 and the substrate 11 is incident on the substrate 11 along the surface normal. Then, the polarization of the light rotates according to the twist of the molecular director.

If the angle between the polarization axes of the analyzer 14 and the polarizer 13 is the same as the twisted angle of the molecular director, the polarization of the light reaching at the 12 0 0. ' analyzer 14 is parallel to that of the analyzer 14 and thus the light passes through the analyzer 14.

If the angle between the polarization axes of the analyzer 14 and the polarizer 13 is different by 90 degrees from the twisted angle of the molecular director, the polarization of the light reaching at the analyzer 14 is perpendicular to that of the analyzer 14 and thus the light does not pass through the analyzer 14.

When an electric field is applied to the liquid crystal, the molecular director tends to orient along the field direction since the liquid crystal has the spontaneous polarization parallel to the molecular director. In addition, if the liquid crystal has positive dielectric anisotropy, the tendency of orienting the molecular director parallel to the field direction is is enhanced. If the strength of the electric field is sufficiently high, the molecular axes coincide with the field direction. If the applied field is not sufficient, the molecules experience some degree of orientational distortions. The molecules near the substrates maintain homogeneous alignment since the aligning forces present in the alignment layers 17 and 18 are larger than the electric force. Under a sufficiently high electric field, most of the molecules in the bulk are perpendicular to the substrates 11 and 12. The linearly polarized light through the polarizer 13 reaches at the analyzer 14 without any interference. If the polarization axes of the polarizer 13 and the analyzer 14 are perpendicular to each other, the light hardly passes through the analyzer 14 since the polarization of the light is perpendicular to that of the

1 0.

analyzer 14. But if the polarization axes of the polarizer 13 and the analyzer 14 are parallel to each other, the light passes through the analyzer 14.

The.transmittance of a monochromatic light depends on the gap d between the two substrates 11 and 12, the optical anisotropy An, and the wavelength A of the incident light. The transmittance T T = 1 - sin 2 (7rvr( +U2) /2) / (,+U2) where u = 2d(An/1).

is The transmittance has maxima when u = 0, ih, AS, hS,... The first maximum occurs when u=h, the second maximum occurs when u=vf5, and so on. Since the wavelength of the visible light is in the range between 0.4 and 0.7 microns, it is desirable that dAn is less than 2 microns, especially 0.08 to 2 microns if the first to the third maxima is used.

FIG. 2 shows the transmittance according to the embodiment of the present invention compared with that according to the conventional art. The twisted angle of the molecular director is 90 degrees, the dielectric anisotropy Ae is + 5, the thickness of the liquid crystal layer, i. e., the gap is 5 microns, and the angle between the polarization axes of the polarizer and the analyzer is 90 degrees. The graph in FIG. 2 shows that the LCD according to the present embodiment has transmittance with varying the voltage, compared with that of the conventional TN LCD. The transmittance according to the present invention varies more drastically than the conventional TN LCD as the spontaneous polarization P becomes larger.

As described above, the present invention comprises two 14 substrates and a ferroelectric nematic liquid crystal layer. The present invention has the advantages of the small driving voltage and so the larger operation speed over the conventional TN'LCD. In addition, the present invention does not have difficulty of alignment pertained to the conventional ferroelectric smectic C LCD.

It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of the present invention. Accordingly, it is not intended that the scope of the claims append hereto be limited to the description as set forth herein, but rather that the claims be construed as encompassing all the features of patentable novelty that reside in the present invention, including all features that would be treated as is equivalents thereof by those skilled in the art which this invention pertains.

is

Claims

1. A liquid crystal display comprising: a first and a second transparent substrates to which an electric field is applicable; and a nematic liquid crystal layer having ferroelectricity between the first and the second transparent substrates.

2. A liquid crystal display as claimed in claim 1, wherein the liquid crystal layer has spontaneous polarization parallel to the average molecular axis.

3. A liquid crystal display as claimed in claim 1 or 2, wherein the liquid crystal layer have positive dielectric anisotropy.

4. A liquid crystal display as claimed in claim 1, 2 or 3, wherein the first and the second substrates have homogeneously aligning forces causing the molecular director to be aligned along any two directions, respectively.

5. A liquid crystal display as claimed in claim 4, wherein the molecular director to be aligned due to the aligning force of the first substrate makes an angle 0 degrees to 360 degrees with the molecular director to be aligned due to the aligning force of the second substrate so that the molecular director on the first substrate makes a twisted angle with respect to the molecular director on the second substrate.

16

6. A liquid crystal display as claimed in any preceding claim, wherein the value of the gap between the first and the second substrates divided by the pitch of the liquid crystal layer is 0.0 to 1.0.

7. A liquid crystal display as claimed in claim 6, wherein the value of the gap between the first and the second substrates divided by the pitch of the liquid crystal layer is 0.25.

8. A liquid crystal display as claimed in any preceding claim, wherein the optical anisotropy of the liquid crystal layer multiplied by the gap between the first and the second substrates is 0.1 microns to 2.0 microns.

9. A liquid crystal display as claimed in any preceding claim, further comprising two polarizers attached to the first and the second substrates, respectively.

10. A liquid crystal display as claimed in claim 9, wherein the angle between the polarization axes of the two polarizers is equal to the twisted angle of the molecular director.

11. A liquid crystal display as claimed in claim 9, wherein the angle between the polarization axes of the two polarizers is 90 degrees plus the twisted angle of the molecular director.

12. A liquid crystal display as claimed in claim 9, wherein the polarization axes of the two polarizers are 13 parallel to each other.

13. A liquid crystal display as claimed in claim 9, wherein the polarization axes of the two polarizers are perpendicular to each other.

14. A liquid crystal display as claimed in any preceding claim, wherein the liquid crystal layer comprises a chiral dopant which have a twisting power of orienting the molecules to be twisted.

15. A liquid crystal display as claimed in any preceding claim, wherein the molecular director on the first substrate makes an angle 0 degrees to 360 degrees with respect to the molecular director on the second substrate.

16. A liquid crystal display as claimed in any preceding claim, further comprising a phase retardation plate attached to one of the first and the second substrates.

17. A liquid crystal display substantially as described herein with reference to the accompanying drawings.

i