WO2006000133A1

WO2006000133A1 - An improved pi-cell liquid crystal display

Info

Publication number: WO2006000133A1
Application number: PCT/CN2004/001207
Authority: WO
Inventors: Hoi-Sing Kwok; Fion Sze Yan Yeung
Original assignee: The Hong Kong University Of Science And Technology
Priority date: 2004-06-26
Filing date: 2004-10-25
Publication date: 2006-01-05
Also published as: CN101023392A; CN100520534C; TWI362524B; TW200600895A

Abstract

The present invention relates to a pi-cell used in a liquid crystal display, wherein the liquid crystal molecules are in a bend state even when no voltage is applied thereto.

Description

AN IMPROVED PI-CELL LIQUID CRYSTAL DISPLAY

FIELD OF INVENTION

The present invention is related to liquid crystal display (LCD) technologies, particularly related to the LCD for increasing the response speed by providing an improved alignment layer.

BACKGROUND OF INVENTION

Liquid crystal displays (LCD) are used in a variety of applications. The optical design of liquid crystal displays (LCD) is an important subject for their application. Ideally a good LCD mode should have excellent contrast, wide viewing angles, high optical efficiency and fast switching speed. One of the major applications is in displaying video signals. This includes LCD televisions (LCD TV). For video rate signal display, the response speed of the LCD has to be fast, otherwise there will be blurring of the moving images. As well, the viewing angle of the LCD TV should be as wide as possible, as in traditional cathode ray tubes. The common requirement is 160° in the horizontal direction and 90° in the vertical direction. For wide viewing angles, the in-plane-switching (IPS) mode and the vertical alignment (VA) mode and their many derivatives are promising for video applications. However, their switching speed needs to be improved from >10ms to 2ms. Thus there is a need for LCD optical modes that can have a fast switching speed as well as a wide viewing angle.

One of the possible candidates for a fast switchinε LCD is the oi-cell. A pi-cell is basically a bend deformation LC cell. The pi-cell was invented in 1986 as a fast liquid crystal switch (P. Bos, US Patent No.4566758: Rapid starting, high-speed liquid crystal variable optical retarder). In its original form as invented, the pi-cell was actually a splay deformation cell which required a bias voltage to transform it into a bend cell or pi-cell. Together with the biaxial compensating film, this pi-cell is later improved and is also called the optically compensated bend (OCB) liquid crystal display (H. Nakamura et al, US Patent No. 6069620: Driving method of liquid crystal display device). The pi-cell basically operates between various bend deformations of the nematic liquid crystal. The application of a voltage changes the degree of bending of the LC director and hence its overall birefringence. Thus the optical property of the pi-cell is basically that of an electrically controlled birefringent (ECB) cell. As such, it can be compensated readily using various optical films to obtain a wide viewing angle. A conventional pi-cell is formed with parallel rubbing of the alignment layers on both sides of the LC cell. The tilt angle on both sides of the liquid crystal cell are tilted toward each other as shown in exaggeration in Figure 2. The possible liquid crystal director alignments that satisfy these boundary conditions are the splay deformation (hereinafter called S-state), the bend deformation (hereinafter called B-state), and the ;r-twist deformation (T-state). The S-cell and the B-cell are shown in Figure 8A and 8B, respectively. Figure 1OA and 1OB show the bend alignments at (A) zero voltage and (B) high voltage. When a high voltage is applied, the B-state becomes the vertical homeotropic alignment (H-state). There have been variants of this basic alignment with the introduction of τi/4 twist and τt/8 twist to the basic structure. In all cases studied, the most stable state is the S-state. A transformation from the more stable S-state to the B-state is required to operate the pi-cell. (See for example, E J Acosta et al, The role of surface tilt in the operation of pi-cell liquid crystal devices, Liquid crystals, vol 27, p 977, 2000; S H Lee et al, Chiral doped optically-compensated bend nematic liquid crystal cell with continuous deformation from twist to twist-bend state, Japanese J of Applied Physics, vol 40, p L389, 2001; S H Lee et al, Geometric structure for the uniform splay to bend transition in a pi-cell, Japanese J Applied Physics, vol 42, p Ll 148, 2003.) Because the S and B deformation are not topologically equivalent, nucleated transformation has to take place. The "conditioning" of the original S-state to the B -state is a major area of research. Various methods have been proposed, including the introduction of protrusions and the addition of chiral dopants. As well, a bias voltage has to be maintained in order for the pi-cell to operate properly in the B-state only. It is an object of the present invention to provide an improved pi-cell. SUMMARY OF INVENTION

In one aspect of the present invention, a liquid crystal alignment layer used in a liquid crystal cell to align liquid crystal molecules is provided. The alignment layer comprises nano-structures of: a. a horizontal alignment material capable of providing a first pretilt angle of the liquid crystal molecules in contact therewith; b. a vertical alignment material capable of providing a second pretilt angle of the liquid crystal molecules in contact therewith; wherein the effective pretilt angle of the liquid crystal molecules in contact with and near the alignment layer can be controlled to have a value of between the first pretilt angle and the second pretilt angle. In a preferred embodiment, the first pretilt angle is between 1-10° and the second pretilt angle is between 80-90°. In a more preferred embodiment, the first pretilt angle is between 1-8°, and the second pretilt angle is between 85-90°. In another preferred embodiment, said alignment layer comprises nano-structures of either the vertical alignment material or the horizontal alignment material. In another preferred embodiment, the nano-structures comprise both the horizontal and vertical alignment materials. In yet another embodiment, the nano-structures are horizontal alignment material. In still another embodiment, the nano-structures are vertical alignment materials. In still another preferred embodiment, the nanostructures are of sizes of 0-1 microns.

In a preferred embodiment, at least one of said alignment materials is a polymer. In a more preferred embodiment, at least one of said alignment materials is selected from a group consisting of polyimide, polystyrene, poly-methyl methacrylate, polycarbonates, polyamic acid, and polyvinyl alcohol. In a better preferred embodiment, at least one of said alignment materials are polyimides. In a still more preferred embodiment, the horizontal alignment material is selected from a group consisting of JALS9203 , AL1454, AL5056, AL3046, JALS-1216, JALS-1217 , SE-7992, SE-7492 and SE-5291 and the vertical alignment material is selected from a group consisting of JALS-2021, JALS-2066, SE-7511L and SE-1211. In a most preferred embodiment, the horizontal alignment material is JALS9203 and the vertical alignment material is JALS2021.

In another preferred embodiment, the weight: weight ratio of the horizontal alignment material to the vertical alignment material is 1 :99 to 99: 1. In a more preferred embodiment, the weight: weight ratio of the horizontal alignment material to the vertical alignment material is 1:4 to 4:1.

In yet another preferred embodiment, the polar anchoring energy on the alignment layers ranges between 5x10-4 J/cm2 to 2.5x10-3 J/cm2.

In another aspect of the present invention, a process of making an alignment layer in a liquid crystal cell is provided, which comprises:

a) dissolving a horizontal alignment material and a vertical alignment material in a solvent to form a homogeneous solution; b) forming a liquid film on a substrate from such a solution; c) curing the film to form a hardened solid film; and d) treating the hardened solid film to obtain a uniform alignment direction. In a preferred embodiment, the horizontal alignment material is capable of providing a first pretilt angle in the alignment layer, and the vertical alignment material is capable of providing a second pretilt angle in the alignment layer. Use of the expression horizontal alignment material throughout the specification refers to horizontal alignment materials that induce liquid crystal molecules in contact therewith to be aligned 0 degrees to the surface thereof as well as horizontal alignment materials that induce liquid crystal molecules in contact with the surface thereof to be aligned at greater than 0 degrees to the surface thereof, for example, horizontal alignment materials that induce liquid crystal molecules in contact with the surface thereof to be aligned at an angle greater than 0 degrees up to about 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0.5 or 0.1 degrees. Use of the expression vertical alignment material throughout the specification refers to vertical alignment materials that induce liquid crystal molecules in contact therewith to be aligned 90 degrees to the surface thereof as well as vertical alignment materials that induce liquid crystal molecules in contact with the surface thereof to be aligned at less than 90 degrees to the surface thereof, for example, vertical alignment materials that induce liquid crystal molecules in contact with the surface thereof to be aligned at an angle less than 90 degrees to about 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 89.5, or 89.9 degrees. In a more preferred embodiment, the first pretilt angle is 1-10° and the second pretilt angle is 80-90°. In a further preferred embodiment, the first pretilt angle is 1-8° and the second pretilt angle is 85-90°.

The horizontal alignment material may be wholly or at least partly miscible with the vertical alignment material.

In another preferred embodiment, the horizontal alignment material is dissolved in a first solvent before mixing. In still another preferred embodiment, the vertical alignment material is dissolved in a second solvent before mixing.

In still another preferred embodiment, the film formed in step b) contains nanostructures of either the vertical alignment material or the horizontal alignment material. In another embodiment, the nanostructures consist both the horizontal and vertical alignment materials. The sizes of the nanostructures are 0-1 micron.

In yet another preferred embodiment, at least one of said alignment materials is polymer. In a more preferred embodiment, at least one of said alignment materials is selected from a group consisting of polyimide, polystyrene, poly-methyl methacrylate, polycarbonates, polyamic acid, and polyvinyl alcohol. In a better preferred embodiment, at least one of said alignment materials are polyimides. In another embodiment both the horizontal and vertical alignment materials are polyimides. In a most preferred embodiment, the horizontal alignment material is JALS2021 and the vertical alignment material is JALS9203.

In still another preferred embodiment, the first and second solvents are selected from a group consisting of methyl-2-pyrrolidone (NMP), dimethyl formamide (DMF), v -butyrolactone (γBL), Butyl cellosolve (BC) and THF (tetrahydro furan). In a more preferred embodiment, the first solvent comprises γBL, NMP, and BC and the second solvent comprises NMP and BC. In a most preferred embodiment, the first and second solvents are solvents that are contained in the JALS 9203 and JALS2021.

In yet another preferred embodiment, the weight: weight ratio of the horizontal alignment material to the vertical alignment material is 1 :99 to 99: 1. In a more preferred embodiment, the weight: weight ratio of the horizontal alignment material to the vertical alignment material is 1:4 to 4: 1.

In another preferred embodiment, the curing comprises a first baking at 80-120° and a second baking at about 200-250°. In yet another embodiment, the curing is photo curing.

In still another embodiment, the film is formed by spin coating, screen printing, spraying, or inkjet printing.

In yet another embodiment, the rubbing is mechanical rubbing done with a piece of fabric in a fixed direction or by irradiating the surface with an ion beam in vacuum in a fixed direction at a fixed incident angle.

In a further embodiment, the substrate is indium tin oxide coated glass. In a preferred embodiment, the indium tin oxide is patterned into rows and columns in a passive matrix display. In a more preferred embodiment, the substrate consists of an array of thin film transistors in active matrix driving.

In a further aspect of the invention, a process of forming a solution for preparing an alignment layer capable of providing a first pretilt angle between 8° and 85° is provided. The process comprises mixing a horizontal alignment material and a vertical alignment material in a solvent, wherein said horizontal alignment material capable of providing a first pretilt angle in said alignment layer, and said vertical alignment material capable of providing a second pretilt angle in said alignment layer. In a preferred embodiment, the first pretilt angle is 0-10° and the second pretilt angle is 80-90°. In a more preferred embodiment, the first pretilt angle is 0-8° and the second pretilt angle is 85-90°.

In a preferred embodiment, the process comprises mixing commercially available horizontal and vertical alignment materials. In a more preferred embodiment, the horizontal alignment materials are selected from JALS9203 , AL 1454, AL5056, AL3046, JALS- 1216, JALS-1217, SE-7992, SE-7492 and SE-5291 ; the vertical alignment materials are selected from JALS-2021, JALS-2066, SE-7511L and SE-1211.

In another embodiment, said solvent is capable of forming a solution comprising nano-sized droplets. In a preferred embodiment, the droplets are either the horizontal or the vertical alignment materials. In another preferred embodiments, the droplets are both the vertical and the horizontal alignment materials. In the present invention, we also disclose a pi-cell which does not require any bias voltage and which is always in the bend deformation, even at zero bias voltage. There is no stable splay deformation state. Henceforth, this will be called the "no-bias bend" cell (NBB cell). This new pi-cell or NBB cell is therefore very easy to operate. Liquid crystal display made with the NBB cell has a total on-off switching time of less than 2ms, with a tum-on time of less than lms. The optical efficiency has been optimized to nearly 90%. This NBB cell display can also be optically compensated to have wide viewing angles similar to conventional pi-cells. This NBB cell is possible through the invention of a special liquid crystal alignment layer that can produce a stable high pretilt angle in the liquid crystal cell.

The gist of the present invention is the NBB cell. This NBB cell is achieved through the application of alignment layers which can provide large pretilt angles in the range of 30-70 degrees to the liquid crystal molecules.

In yet another aspect of the present invention, a liquid crystal cell is provided, which comprises: (a) two substrates facing each other; (b) a liquid crystal layer sealed between said two substrate; said liquid crystal layer having a thickness between 1 micron to 10 microns; said liquid crystal layer is composed of liquid crystal molecules with a positive dielectric anisotropy, that is, S//> εχ, (c) two alignment layers provided on the inner surfaces of said substrates facing the liquid crystal layer for the purpose of aligning said liquid crystal layer, said alignment layers are in the form of solid thin films with 10-100 nm thickness. In the liquid crystal cell, the solid thin film alignment layers comprise a mixture of a homeotropic (vertical) alignment material and a homogeneous (horizontal) alignment material; said solid thin film alignment layer is capable of providing pretilt angles of 30 degrees to 70 degrees of the liquid crystal molecules in contact therewith.

In a further aspect of the present invention, a liquid crystal display device is provided, comprising (a) the NBB liquid crystal cell as mentioned above; (b) a first polarizer disposed on the back surface of the first substrate. In one embodiment, the first polarizer is an input polarizer. In another embodiment, the liquid crystal device comprises a second polarizer disposed on the back surface of the second substrate. In a preferred embodiment, the second polarizer is an output polarizer. The second polarizer is also known as the analyzer in Figure 3A.

In yet another embodiment, the polarizer angles or and γ, as indicated in Figure 3A, are at plus or minus 35-55 degrees from the rubbing direction of the alignment layers. In still another embodiment, the liquid crystal layer is of thickness between 1 micron to 10 microns. In a further embodiment, the liquid crystal layer is composed of liquid crystal molecules with a positive dielectric anisotropy, wherein S_//> εχ. In a further embodiment, the alignment layers are of the thickness of 10-200 nm.

In still another embodiment, the liquid crystal molecules in contact with the first and the second alignment layers have pretilt angles of 10°-80°. In a preferred embodiment, the liquid crystal molecules in contact with the first and the second alignment layers have pretilt angles of 30°-70°.

In another embodiment, the pretilt angles of the liquid crystal molecules in contact with the first and the second alignment layers are substantially the same. Preferably, the pretilt angles are in the range of 45°-90°.

In yet another aspect of the present invention, a bend state liquid crystal device comprising a liquid crystal cell is provided, which comprises: a. a first substrate having thereon a first alignment layer; b. a second substrate having thereon a second alignment layer; c. a liquid crystal layer sandwiched between said first and second alignment layers, said first alignment layer inducing a first liquid crystal pretilt angle θi with an absolute value in the range of 17°-60°, said second alignment layer inducing a second liquid crystal pretilt angle Θ₂ with an absolute value in the range of 17°-60°; with said angles θj and Θ₂ being opposite in sign as defined in the same coordinate system and said liquid crystal layer being capable of maintaining a stable bend state at zero bias voltage.

In another aspect of the present invention, a bend state liquid crystal device comprising a liquid crystal cell is provided, which comprises: a. a first substrate having thereon a first alignment layer which is treated to impart a pretilt angle θi and an azimuthal angle φj of the liquid crystal molecules in contact with said first alignment layer; b. a second substrate having thereon a second alignment layer which is treated to impart a pretilt angle θj and an azimuthal angle <jh of the liquid crystal molecules in contact with said second alignment layer; c. at least one of said alignment layers being composed of a mixture of vertical alignment materials capable of providing a pretilt angle of 85°-90° and a horizontal alignment material capable of providing a pretilt angle of 0-8°; d. a liquid crystal layer sandwiched between said first and second alignment layers. e. Both the previous two aspects of the present invention include one embodiment, in which θi and Θ2 of the liquid crystal device satisfy the following equation: [K₃₃ - K_u)[sin 2θ_j + sin 2Θ₂ ) + 2(π- 2θj - 2Θ₂

+ K Jj ) = O In the equation, K& is the bend elastic constant and Kn is the splay elastic constant of the liquid crystal.

In a preferred embodiment of both aspects, the θl and θ2 are substantially the same and in the range of 30°-60°. In another preferred embodiment, the θl and Θ2 are 47±5° when K33/Kii=1.3. In yet another embodiment, θl =17±5° and 02 = 60±5 ° when K33/K11=1.3.

In another embodiment of both aspects, at least one of the alignment layers comprises a mixture of horizontal alignment material capable of providing a pretilt angle of the liquid crystal molecules in contact therewith of 0^c-8^c and a vertical alignment material capable of providing a pretilt angle of the liquid crystal molecules in contact therewith 85° -90°. In a preferred embodiment, at least one of said alignment layer comprise nano-sized structures at sizes of 0-1 micron. In another preferred embodiment, the nano-sized structures comprise at least one of the horizontal and vertical alignment materials. In a more preferred embodiment, the horizontal alignment material of the alignment layer is selected from a group consisting of JALS9203, AL1454, AL5056, AL3046, JALS- 1216, JALS-1217, SE-7992, SE-7492 and SE-5291 and the vertical alignment material of the alignment layer is selected from a group consisting of JALS-2021, JALS-2066, SE-7511L and SE-1211.

In yet another embodiment, said liquid crystal layer including liquid crystals having a positive dielectric anisotropy. In a preferred embodiment, the liquid crystal layer having a thickness between 1 micron to 15 microns. In still another embodiment, the alignment layers are solid thin films with a thickness of 10-200 nm.

In another embodiment, the liquid crystal molecules in contact with the first and second alignment layers are tilted in mirror symmetry. In yet another embodiment, the pretilt angles on both sides of the liquid crystal layer are tilted in such a way that the direction of the pretilt angles being parallel when projected onto the surfaces of the alignment layers.

In still another embodiment, at least one of the substrates being an active matrix backplane consisting of thin film transistors arranged in a matrix form.

In yet another embodiment, the treatment of the alignment layers being mechanical rubbing of the alignment layers by a piece of cloth. In still another embodiment, the treatment of the alignment layers being photo-alignment by exposure of the alignment layers to an ultraviolet beam of polarized light. In a further embodiment, the treatment of the alignment layers being irradiation of the alignment layers by an ion beam at an angle in a vacuum chamber.

In another embodiment, the liquid crystal device further comprises an input and an output polarizers. The polarizer axes are disposed at angles of plus or minus 35-55 degrees from the rubbing direction of the alignment layers of said liquid crystal cell. In yet another embodiment, the liquid crystal device further comprises a reflection means disposed outside said liquid crystal cell. In a further embodiment, the liquid crystal device further comprises a reflection means built onto the active matrix backplane inside the liquid crystal cell. In still another embodiment, the liquid crystal device further comprises a compensation retardation film.

According to a further aspect of the present invention, a method for producing a stable bend state in a bend state liquid crystal device is provided. The bend state liquid crystal device includes a first substrate having thereon a first alignment layer, a second substrate having thereon a second alignment layer. The method comprises: a. sandwiching liquid crystals having a positive dielectric anisotropy between the first and second alignment layers; b. inducing a first liquid crystal pretilt angle in the range of 17 ° -60°; c. inducing a second liquid crystal pretilt angle in the range of 17°-60°; and d. aligning the liquid crystals in a bend state that remains stable at zero bias voltage and operating voltages. In one embodiment, the pretilt angles are induced by providing alignment layers comprising a horizontal alignment material capable of providing a pretilt angle of the liquid crystal molecules in contact therewith of 0°-8° and a vertical alignment material capable of providing a pretilt angle of the liquid crystal molecules in contact therewith 85° -90°. BRIEF DESCRIPTION OF THE DRAWINGS Figure IA, IB, and 1C are diagrams illustrating the sectional view of a portion of a liquid crystal displays according to one aspect of the present invention.

Figure 2 is a diagram illustrating the sectional view of a portion of a liquid crystal cell according to another aspect of the present invention.

Figure 3 illustrates a liquid crystal director in Cartesian coordinates.

Figure 3A illustrates the position of a polarizer, analyzer, light, and viewer relative to the liquid crystal cell according to yet another aspect of the present invention. Figure 4 is a flow chart showing a process of making a liquid crystal alignment layer according to yet another aspect of the present invention.

Figure 5A, 5B, and 5C are a set of photographs showing examples of nano- and micro-domains of a solid film formed by liquid crystal alignment agent according to still another aspect of the present invention.

Figure 6A is a chart showing the relationship between the pretilt angles and the concentration of vertical alignment material (JALS 2021) in accordance with the still another aspect of the present invention. The alignment film is formed by spin coating.

Figure 6B is another chart showing the relationship between the pretilt angles and the concentration of vertical alignment material (JALS 2021) in accordance with the still another aspect of the present invention. Here the alignment film is formed by roller printing.

Figure 7 is a chart showing the relationship between the polar anchoring energy and the concentration of vertical alignment material (JALS 2021) in accordance with the still another aspect of the present invention.

Figure 8A and 8B are diagrams showing the (A) splay and (B) bend deformation alignment of the liquid crystal layer, respectively, according to a further aspect of the present invention. Figure 9 is a chart showing the elastic deformation energies of the bend and splay alignment of a liquid crystal layer according to another aspect of the present invention.

Figure 1OA and 1OB are diagrams showing the bend alignments at (A) zero voltage and (B) high voltage according to still another aspect of the present invention.

Figure 11 is a chart showing the relationship between the transmission and voltage of Cell (1) prepared by Example 4 according to yet another aspect of the present invention.

Figure 12A and 12B are oscilloscope traces showing the switching dynamics of Cell (I) prepared by Example 4 according to a further aspect of the present invention.

Figure 13 is a 3-D diagram showing the relationship between the response time (μs), starting level and ending level in percentage, i.e., switching time from gray level to gray level of Cell (I) prepared by Example 4 according to yet another aspect of the present invention.

Figure 14 is a chart showing the relationship between the transmission and voltage of Cell (II) prepared according Example 5 as yet another aspect of the present invention.

Figure 15 is a 3-D diagram showing the relationship between the response time (μs), starting level and ending level in percentage, i.e., switching time from gray level to gray level, of Cell (II) prepared by Example 5 according to yet another aspect of the present invention.

DETAILED DESCRIPTION When two angles are "substantially the same", it means that the two angles differ by less than 5°; preferably less than 3°; most preferably less than 1 °. "Absolute value" herein refers to the numerical value of a real number without regard to its sign. For example, the absolute value of -4 (written -4) is 4. Also called numerical value.

Referring to Figure IA, a portion of a liquid crystal display is shown, comprising a liquid crystal cell 1 and two polarizers 2 and 3. The arrow indicates the light path through the liquid crystal cell. Figure IB shows a transmittive liquid crystal cell with two retardation films 13,14 used to compensate color dispersion and to enhance the viewing angle of the liquid crystal display. The arrow indicates the light path of the liquid crystal cell. Figure 1C shows a portion of a reflective liquid crystal cell, with one retardation Film 13 and an external mirror 12 are provided. Alternatively, the reflective mirror in Figure 1C can be formed inside the liquid crystal cell as part of the pixel structure, for both passive and matrix driven displays. Alternatively, retardation film 14 could be provided instead of 13. The two arrows indicate the light path of the cell.

In Figure 2, a liquid crystal cell when no voltage is applied is shown, comprising a top substrate 4 and a bottom substrate 5; two transparent conductive electrodes 6 and 7, a top alignment layer 8 and a bottom alignment layer 9, a liquid crystal layer 10. Angle θ, formed between the first layer of liquid crystal molecules 1OA and the surface of the bottom alignment layer 9 is called the first pretilt angle. Angle Q₁ formed between the first layer of liquid crystal molecules 1OB and the surface of the top alignment layer 8 is called the send pretilt angle. O₁ and θ, may be substantially the same or different from each other. In this liquid crystal display, the substrates 4 and 5 can be made of glass. One of the glass substrates such as 5 can be an active matrix backplane with a thin film transistor array. Alternatively, one of the substrates such as 4 can be made of glass while the other substrate 5 can be an opaque material such as silicon. An active matrix array can be fabricated on the silicon substrate in this case. If both substrates are made of glass, then the liquid crystal display can be operated in the transmittive or transflective mode. If one of the substrate is opaque, then the liquid crystal display can only operate in the reflective mode with the reflective mirror fabricated as part of the active matrix structure inside the liquid crystal cell.

One of the other essential elements of the first preferred embodiment is the liquid crystal layer 10 which is sandwiched between the said substrates. The thickness of the liquid crystal layer is fixed by having spacers (not shown) inside the liquid crystal cell. The most important parameter for the liquid crystal layer is the pretilt angle at the boundary with the alignment layer. These pretilt angles Q₁ and θ_∑ determine the θ(z) and φ(z) (see Figure 3) of the liquid crystal layer 10 when no voltage is applied. Here we have assumed for simplicity that the angles are dependent only on one variable z which is the distance in the perpendicular direction to the liquid crystal cell as shown in Figure 3. In general, θ and φ can be a function of (x,y,z)- But it does not affect the presentation of this invention here.

This liquid crystal layer is characterized by a liquid crystal director with orientations θ and φ, which are the polar and azimuthal angles respectively as shown in Figure 3. θ is also referred to as the tilt angle in the present invention. The tilt angles at the boundary with the alignment layers, θ(0) and θ(d) are called the pretilt angles. The alignment of the director, i.e. the values of θ[z) and φ(z) determines the optical properties of the liquid crystal cell. It determines basically the transmittance or reflectance of the liquid crystal cell. As shown in Figure 3, the director of liquid crystal molecule n is determined by a pretilt angle (tilt angle) #which corresponds to a polar angle of the director n and a pretilt angle direction (twist angle) φ which corresponds its azimuthal angle. The Cartesian coordinate of the director n of liquid crystal molecule is defined:

n=(cos0cos$ cosøsinø, sin6{λ

The director n of of liquid crystal is determined by controlling the pretilt angle θ and the pretilt angle direction f*by carrying out alignment process on the substrate. The transmission or reflectance of light by the liquid crystal cell is determined by the angle of the polarizer αand the angle of the analyzer /as shown in Figure 3A and the alignment condition of the liquid crystal layer 10. The electrodes 6, 7 and the alignment layers 8, 9 are used to control the alignment conditions of the liquid crystal layer 10. The electrodes provide the voltage to control the values of θ(z) and φ(z). The alignment layers and their treatment determine the values of θ(0) Ατιά φ(OJ and θ(d) and φ(d) where d is the thickness of the liquid crystal cell. Essentially^^ is θ, and θ(d) is θ_∑ as mentioned above. They are equivalent symbols. The values of θ(0) and φ(0), and θ(d) and φ(d), together with the elastic Euler's equations, determine the solution of θ(z) and φ(z). The physics of the alignment of the liquid crystal layer is well-known in the art and is well covered in the literature, such as given in the monograph "Electrooptic Effects in Liquid Crystal Materials" written by Blinov and Chigrinov, published by Springer in 1994. θ(0) Αnά φ(0) are known as the easy axes for liquid crystal alignment. It should be noted that the actual alignment directions of the liquid crystal on the surfaces is also dependent on the anchoring energies of the alignment surfaces. The anchoring energy is a measure of how strong the anchoring condition is. If the anchoring energy is large, then it is difficult to deviate from this condition and the alignment angles are given by the easy axes directions. For weak anchoring, the actual angles of the liquid crystals on the surface may deviate from θ(0) and φ(0).

Obviously, the values of θ(0), φ(0), θ(d)&nά φ(d) oτ the alignment of the liquid crystal molecules right near the alignment layers are important in designing the electro-optical properties of the liquid crystal cell. The alignment of the liquid crystal molecules can be achieved by many means and is a well studied problem in liquid crystal physics and engineering. The predetermined alignment conditions are usually achieved, for example, by rubbing the alignment layers 8, 9.

The alignment layers are treated so that they can align the liquid crystal molecules near them. The treatment is mostly done by mechanical rubbing. Sometime other techniques such as photo-alignment or ion beam alignment can be used. For the purpose of the present invention disclosure, we do not restrict ourselves to any one alignment technique. For the discussions below, we use rubbing as an example for the treatment of the alignment layers.

While the rubbing directions on the alignment layers determine φ(0) and φ(d), the values of the pretilt angles Θ(0) and θ(d) ax& determined mostly by the material properties of the alignment layers 8, 9. There are homogeneous alignment materials such as polyimides that can provide pretilt angles of l°-8° for manufacturing twisted nematic (TN) and supertwisted nematic (STN) liquid crystal displays. This kind of material is also called horizontal alignment material. There are also materials that can provide homeotropic alignment with pretilt angles of 85°-90° for manufacturing vertically aligned nematic (VAN) liquid crystal displays. This kind of material is also called vertical alignment material. These homogeneous and homeotropic materials are available commercially. Many inventions have been disclosed on different types of chemicals that can provide horizontal or vertical alignments. But it is noted that these alignment layers can only provide either near horizontal or near vertical alignments. It is impossible to obtain alignment polar angles that are in between. In particular, no known polyimide alignment materials are known that can give a pretilt angle of near 45° is in practical use, although there are several claims that such pretilt angles can be obtained in the past. These polyimide alignment materials have served the liquid crystal display industry well. Large quantities are used for making practical LCD.

In this first preferred embodiment, the alignment layers 8,9 are treated specially to have a high pretilt angle for the liquid crystal layer 10. The coordinate system used to visualize the various angles is shown in Figure 3. This coordinate system is used for describing n, $z) and 6(z) throughout the liquid crystal cell. For a conventional pi-cell, φ(0) = <l(d), and θ(0) (same as θj) and θ(d) (same as fø) have opposite signs. This is achieved simply by parallel rubbing of both the top and bottom alignment layers. In this first preferred embodiment, the pretilt angles are such that a stable pi-cell can be obtained even when no voltage is applied. In other words, the values of θj and 6fe are large enough so that the stable configuration of the cell is a bend-cell, even when no voltage is applied to the liquid crystal cell. This is different from ordinary pi-cells where the zero volt alignment of the liquid crystal cell is a splay cell, and a bias voltage and a conversion method is needed to maintain the bend alignment.

Figure 4 shows an embodiment of the process of making the liquid crystal alignment layer according to one aspect of the present invention. In step 1, a vertical alignment material 40 is diluted with a solvent A that is fully miscible with the vertical alignment material 40, forming a VM solution 42. In step 2, the VM solution 42 and a horizontal alignment material 44 are both dissolved in a solvent C, forming a final mixture 46. In an alternative step 2', the horizontal alignment material 44 is first dissolved in a solvent B, forming a HM solution 45 before mixing with the VM solution 42 in solvent C. In step 3, the mixed solution 46 is applied to a substrate to form a solid film 48. The solid film 48 is then treated by thermal curing, which comprises a pre-baking and a final baking process in step 4 to form a hardened solid film 50. The hardened solid film has a thickness between 10nm-300nm. The hardened solid film 50 is then rubbed in step 5 to produce the desired liquid crystal alignment layer 52. The rubbing can be achieved by mechanical rubbing, which comprises applying a piece of fabric in a fixed direction.

The about-mentioned processes are only embodiments of the present invention. Many variations are available and known to those skilled in the art as alternative ways to implement the present invention. For example, the curing process in step 4 can also be done by photo-curing. The rubbing in step 5 can also be done by irradiating the surface of the alignment layer with an ion beam in vacuum in a fixed direction at a fixed incident angle. Also it is possible that a liquid film 47 is first obtained from solution 46 in an alternative step3\ the liquid film is then treated in step 3" to obtain the solid film 48. The method of drying the liquid film to obtain the solid film can affect the pretilt angle achievable from the above-mentioned process as well.

A conventional pi-cell is formed with parallel rubbing on both sides of the LC cell. The possible director alignments that satisfy these boundary conditions are the splay deformation (S-state), the bend deformation (B-state), and the π-twist deformation (T-state). When a high voltage is applied, the B-state becomes the vertical homeotropic alignment (H-state). In a conventional pi-cell, the pretilt angle is less than 10 degrees. Thus the S-state has lower elastic deformation energy and is therefore more stable. A transformation from this more stable S-state to the B-state is required to operate the pi-cell. (See for example, E J Acosta et al, The role surface tilt in the operation of pi-cell liquid crystal devices, Liquid crystals, vol 27, p 977, 2000; S H Lee et al, Chiral doped optically-compensated bend nematic liquid crystal cell with continuous deformation from twist to twist-bend state, Japanese J of Applied Physics, vol 40, p L389, 2001 ; S H Lee et al, Geometric structure for the uniform splay to bend transition in a pi-cell, Japanese J Applied Physics, vol 42, p Ll 148, 2003.) Because the S and B deformation are not topologically equivalent, nucleated transformation has to take place. The "conditioning" of the original S-state to the B-state is a major area of research. Various methods have been proposed, including the introduction of protrusions and the addition of chiral dopants. As well, a bias voltage has to be maintained in order for the pi-cell to operate properly in the B-state only.

Thus, in the conventional pi-cell, the S-state deformation is more stable than the B-state deformation because of the small pretilt angle. In the present invention, we disclose a new method to make the B-state deformation more stable even at zero bias voltage.

As is well-known, the elastic energy per unit wall area of a no-twist liquid crystal cell is given by the equation

^• =- Y (Kj₁COs² θ+ K₃₃ sin² θ)θ² dz

where Kn and K₃₃ are the splay and bend elastic constants, respectively, θ is the tilt angle which is a function of distance z inside the cell. Under the condition that θ(z) is a nearly linear distribution (It is true for most LC materials), the energies of the B-cell and the S-cell can be calculated. They are shown in Figure 9. It is clearly seen that if the pretilt angle is increased, the B-deformation will be favored. However, in conventional LCD fabrication techniques, it is difficult to make high pretilt angles larger than 10 degrees.

We can estimate readily the pretilt angle needed to form the B-cell. If the pretilt angles are θj and Θ₂ respectively on both sides of the LC layer, then the splay and bend cells will have the same elastic energy if the following condition is satisfied: [K₃₃ - K]]hm2θj +sin2θ₂ )+2{π-2θj -2Θ₂ΪK₃₃ + K₁₁)_^O

If the pretilt angles on both sides of the cell are the same and equal to 0_/, then the above condition can be simplified to: (K₃₃ - K_π)sin2θ] +{π-4θiXκ₃₃ + K_n) = 0

By solving this equation, the condition for the pretilt angle such that the splay and bend deformation energies are the same can be obtained. For example, in MBBA, K₃₃IK1₁-I.3, hence θj is about 47°. It agrees with the graph shown in Figure 9 as well. In general it can be shown that θj is always between 45° and 58° for all values of K₃₃/K_1J. Thus if the pretilt angles on the LC cell is larger than the critical angle, the bend deformation will be more stable than the S deformation. In other words, the LC cell will always be in the B-state. This is a pi-cell under no voltage bias. We shall call this the no-bias bend (NBB) cell. For the case of asymmetric LC cell, the pretilt angle on one side can be smaller than θ_/ and the pretilt angle on the other side will have to be larger than θj. For example, if O₁ is 30°, then according to the above equation Θ2 has to be larger than 67°, if K₃₃IK_n=U.

The important step in the fabrication of such a pi-cell is the preparation of the high pretilt angle alignment layers. Many techniques have been introduced in the past for achieving a high pretilt angle in a LC cell. This includes SiO_x evaporation, ion beams, phototalignment and reverse mechanical rubbing. In our present invention, we combine the pi-cell with a special technique that can produce a high pretilt angle that we have invented.

The method of obtaining such high pretilt angles in this invention is that of rubbing of a specially prepared alignment layer. Liquid crystal cells are aligned by alignment layers 8 and 9. Details about the preparation of these special alignment layers have been discussed above in the description associated with Figure 4. When a vertical alignment material is mixed with a horizontal alignment material in a correct ratio in the solution, it is possible to form an alignment layer that can produce any value of the pretilt angle. Figure 6A and 6B show the results of such an experiment for this embodiment. In that figure, it is shown that by mixing the vertical alignment agent, in this case, a commercial material available from Japanese Synthetic Rubber Company, with a part number of JALS2021 , with a horizontal alignment agent, such as JALS9203, it is possible to obtain any value of pretilt angle from 10° to 80°. The pretilt angles indicated in Figures 6A and 6B are different for the same solution mixture. It indicates that the pretilt angles obtainable is dependent on the method of preparation of the solid alignment layers. In Figure 6A, a spin coating technique is used to prepare the solid alignment layer. In Figure 6B, the method of roller coating followed by rapid heating is used to prepare the solid film. There are many other methods of forming a thin film of the alignment layer from the solution. It is possible, for example, to use screen printing, inkjet printing, dip coating or doctor blade techniques to form the thin film. For the purpose of the present invention disclosure, we shall only use the examples of roller printing and spin coating. But all other techniques are embodied as part of the invention. Suffice it to say that for each method of solid film preparation, there will be a unique dependence of the pretilt angle achievable on the ratio of the solution mixture. Curing of the solid film after the initial preparation can be performed by thermal means such as placing the substrate in a heated oven, or by shining strong ultraviolet light onto the liquid film by photo-curing. If the pretilt angle is larger than 40°, the bend configuration will be obtained even at no voltage situation. If the rubbing alignment condition of the alignment layers is parallel, then a pi-cell or bend-alignment cell is obtained. If the input and output polarizers 2,3 are at 45° and -45° to the rubbing direction of the liquid crystal cell, the transmission of the pi-cell is given by the expression d T = sin² ' j \Δn( z )dz 0 where λ is the wavelength, d is the thickness of the LC layer and Δn is the birefringence of the liquid crystal layer which is dependent on the alignment conditions as Δn( z) = n_e(Θ( z))-n₀

1 _ cos² θ sin² θ n ²(θ) n ² n₀ ²

where n_e is the ordinary refractive index and n₀ is the extraordinary refractive index of the liquid crystal material. As the deformation of θ(z) changes, the transmission of the liquid crystal cell changes as well. This is the basis of an electrical controlled birefringent (ECB) cell. The transmission values of the pi-cell constructed this way can be modeled using standard computer software and is shown in Figure 8A, 8B for the no voltage state and the high voltage state. At high voltage, the homeotropic alignment of the liquid crystal layer can be obtained.

Alignment layers are used routinely to obtain alignment of liquid crystal layers to make liquid crystal displays. There are many alignment materials reported in the literature for this purpose. These materials are mostly polymers that are stable against heat and light. Examples are polyimide (PI), polyvinyl alcohol (PVA), polyester and polyamic acid (PA). These materials are commonly spin coated or screen printed on the substrate 4 and 5. Prebaking and final baking steps are needed to harden and cure the polymeric materials. Some of these polymeric alignment agents can provide a homogeneous alignment condition with a pretilt angle of a few degrees. Some special alignment agents can provide a homeotropic or vertical alignment for the liquid crystals with a pretilt angle of near 90 degrees. Both types of polymers can be coated and cured on the substrates for making alignment layers on the substrates 4 and 5, and are well-known in the art.

In this first preferred embodiment of the present invention, the alignment of the liquid crystal layer remains that of a B-cell for all operating voltages. The switching time is therefore very fast. No conditioning is required to transform the pi-cell from splay to bend deformation is needed as in conventional pi-cells. The pretilt angles on both sides of the liquid crystal layer can be the same or different. In one experimental sample, the pretilt angle is 17 degrees on one side and 60 degrees on the other side. This is a case of extreme inequality of the pretilt angles. The transmission versus voltage characteristics of the experimental cell is shown in Figure 11. The switching time of a prototype device that we have constructed is shown in Figure 12A and 12B. As an indication that the switching can be fast even for intermediate voltages, we plot the switching time for 8 grey scales. The results are shown in Figure 13. It can be shown that the maximum time needed is 3ms, while the fastest time can be much less than 1ms. In this particular example, the values of the various parameters are indicated in Table I.

Table I: Liquid crystal cell parameters

In another example, the pretilt angles on both sides of the liquid crystal layer are made the same. In this case, a pretilt angle of 53 degree is needed. Figure 14 shows the transmission versus voltage curve of such a sample. It is not too different from the asymmetric case. However, due to the smaller total birefringence, the absolute transmission of this sample is lower than the previous one. Figure 15 summarizes the switching time obtained from such a sample. It can be seen that the response times are faster and are generally below 2ms. Table II shows the parameters for this experimental sample. Table II: Liquid crystal cell parameters

In the second preferred embodiment of the present invention, the pretilt angle of the liquid crystal layer is still made to be large using the technique described in the first preferred embodiment. So the alignment of the liquid crystal layer is still that of the bend cell or pi-cell. However, the rubbing directions of the alignment layers are now allowed to be non-parallel for the top and bottom alignment layers. Thus there is allowed to be a certain amount of twist angle to the liquid crystal cell. The purpose of this is so that the cell transmission can be modified and to be that of a twisted nematic cell rather than that of the electrically controlled birefringent cell. This way, the cell gap can be smaller and the response time can be faster. The optical properties of this second embodiment can be optimized using LCD simulation. In this simulation, the cell gap and the twist angles of the LC cell are allowed to change. The requirement is that the contrast should be good and the bright state brightness is close to unity.

In the third preferred embodiment of the present invention, the arrangement of the liquid crystal display is that of a reflective display. As such there is an input polarizer and no output polarizer. Instead a mirror 12 is placed behind the liquid crystal cell as shown in Figure 1C. Alternatively, the mirror can be part of the active matrix backplane for the case of a reflective TFT LCD. The liquid crystal cell is still the same as in the first preferred embodiment. However, since the light beam passes through the cell twice, the birefringence needed is a factor if two less than the first preferred embodiment. This has an important implication to the response speed of the liquid crystal cell. Since the cell gap can now be halved, the speed can be almost 4 times faster, since as a rule, the response time of a liquid crystal cell is inversely proportional to the square of the cell gap. As well, because the path length is now doubled that of the first preferred embodiment, there can be more choices for birefringence Δn of the liquid crystal material.

In all preferred embodiments, it is possible to use retardation films 13,14 to compensate for color dispersion and to enhance the viewing angle of the liquid crystal display. In some cases, just one film is needed. In some other cases, both films are needed on both sides of the liquid crystal cell for exact compensation.

Example 1

Example 1 shows the procedures adopted to prepare an alignment layer capable of providing a pretilt angle of 44 degree.

Materials:

•The horizontal alignment material: purchased from Japan Synthetic Rubber Company (model number: JALS9203), which was in a solution form. (JSR Corporation, 5-6-10 Tsukiji Chuo-ku, Tokyo, 104-8410, Japan.) The solvent in JALS9203 comprises Y -butyrolactone (γBL), methyl-2-pyrrolidone (NMP), and Butyl cellosolve (BC).

• The vertical alignment material: purchased from Japan Synthetic Rubber Company (model number: JALS2021), which was in a solution form. The solvent in JALS2021 comprises methyl-2-pyrrolidone (NMP) and Butyl cellosolve (BC).

• Substrate: an ITO glass coated with electrodes, purchased from Nanbo Company, Shenzhen, China.

Procedures:

0.95 g of horizontal alignment material solution and 0.05 g of the vertical alignment material solution were mixed together and stirred thoroughly. The mixture was applied to the substrate to obtain a soft solid film using spin coating. The spin coating was first operated at 800 rmp for 10 sec and then at 3500 rmp for 100 sec. A soft film consisting of the horizontal and vertical alignment materials was formed with remnant solvents.

In order to drive out all the remnant solvents and to cure the polymers, the coated glass was placed in an oven. It was first baked at 100⁰C for 10 min (soft bake) and then baked at 230⁰C for 90min (hard bake). A hard film, i.e., the alignment layer, was formed.

The surface of the alignment layer was subjected to rubbing treatment using a nylon cloth in such a way that the layer was rubbed in one direction one time.

Result: The pretilt angle of the alignment layer produced accordingly to Example 1 was 44 degree.

Example 2

Example 2 shows the procedures adopted to prepare an alignment layer capable of providing a pretilt angle of 53 degree.

Materials:

• The horizontal alignment material: purchased from Japan Synthetic Rubber Company (model number: JALS9203), which was in a solution form.

• The vertical alignment material: purchased from Japan Synthetic Rubber Company (model number: JALS2021), which was in a solution form.

• Substrate: an ITO glass coated with electrodes, purchased from Nanbo Company, Shenzhen, China;

Procedures:

0.5 g of the solution of horizontal alignment material, and 0.5 g of the solution of the vertical alignment material were mixed together and stirred thoroughly. The mixture was applied to the substrate to obtain a soft solid film using print coating as follows:

A stainless steel rod of 2cm diameter and 5 inches long was placed on the substrate. A few drops of the mixture were placed underneath the rod until it spreaded out along the contact line between the rod and the surface of the substrate. The rod was then rolled or slided along the substrate surface to form a liquid film. The coated substrate was then placed on a hot plate at 100⁰C for 10 min to drive out all the solvents. It was then put in an oven for hard baking at 230⁰C for 90min. A hard film consisting of vertical and horizontal alignment materials was then obtained.The spin coating was first operated at 800 rmp for 10 sec and then at 3500 rmp for 100 sec. A soft film consisting of the horizontal and vertical alignment materials was formed with remnant solvents.

In order to drive out all the remnant solvents and to cure the polymers, the coated substrate was placed in an oven. It was first baked at 100⁰C for 10 min (soft bake) and then baked at 23O⁰C for 90min (hard bake), forming a hard film, i.e., the alignment layer. The surface of the alignment layer was subjected to rubbing treatment using a nylon cloth in such a way that the layer was rubbed in one direction one time. Result:

The pretilt angle of the alignment layer produced accordingly to Example 2 was 53 degree.

In example 1, the liquid film becomes a soft solid film by spin coating. The solvent is evaporated slowly so that the domains of H and V tend to be larger. Also the ratio of surface areas of H and V domains will favor the material that has a higher solubility in the mixed solvent since the material with a lower solubility will precipitate first.

In example 2, the solidification is fast due to heating on a hot plate. Thus the domains tend to be smaller. The area ratio of the H and V domains will not be affect too much by the different solubility of the materials.

The pretilt angles obtained by the procedures in example 1 and example 2 are different, even for the same mixture of H and V alignment agents. This is because of the different domain structures obtained using the different procedures. Here is a Table listing the pretilt angles obtained for the same mixture using the two different methods.

Results

The pretilt angles of the liquid crystal molecules in contact with the alignment layer were measured by crystal rotation method.

The anchoring energies on the surface of the alignment layer were measured using the method described in the publication of Chigrinov etal (V Chigrinov, A Muravski, H S Kwok, H Takada, H Akiyama and H Takatsu, Anchoring properties of photo-aligned azo-dyes materials, Physical Review E, vol 68 pp 61702-61702-5, 2003.).

The domain structure on the surface of the alignment layer was observed by standard atomic force microscopy and optical microscopy. Figure 8A shows the nano-structures on the alignment layer containing 5% JALS2021. Figure 8B shows the nano-structures on the alignment layer containing 10% JALS2021. Figure SC shows the nano-structures on the alignment layer containing 15% JALS2021.

Example 3

In addition to Example 1 and 2, alignment layers capable of providing a range of pretilt angles were produced based on different proportions of vertical alignment layer (JALS2021) and horizontal alignment layer (JALS9203) and following procedures basically the same as described in Example 1 and Example 2. Details of alignment layers produced by spin-coating are shown in Table 1. Details of alignment layers produced by print-coating are shown in Table 2. Table 1. Alignment layers produced by spin-coating

Table 2. Alignment layers produced by print-coating

The crucial step in the preparation of the new alignment layer is the formation of nano-structures during the solid film formation process. Since the two alignment materials precipitate at different times in the solid film formation process, nano- and micro structures are formed. Figure 5A-5C show examples of the film structure observed under standard atomic force microscopy formed in according with the present invention. In these figures, the lighter area shows the V material. The percentages of V increases from Figure 5A to C. These samples are produce by spin coating process where the transformation from liquid film to solid film is relatively slow and there is more time allowed for segregation of the two alignment materials. In these examples, the features of the nano-structures are of the size of a fraction of a micrometer. In many other cases, the nano-structures can be as small as a few nanometers. The nano-structure is usually of the form of islands of vertical alignment material in a background of the homogeneous alignment material. This structure is determined by the surface tension, surface energies, elasticity and other physical properties of the two materials as well as that of the common solvent. They are all useful for this embodiment.

Using the above-mentioned alignment layer, a liquid crystal cell was prepared, whereby flawless uniform alignment was obtained. The pretilt angle of the liquid crystal molecules are shown in Figure 6A. Figure 6A shows the pretilt angles measured against different concentrations of JALS2021. It can be seen that the pretilt angles can be controlled by the percentage of JALS2021 from about 5° to near 80 ° as the concentration of JALS2021 goes from 0% to about 14%.

The polar anchoring energies of the surface of the alignment layer were also measured and shown in Figure 7. It can be seen that the anchoring energy varies according to the percentage of JALS2021.

The method of preparation of the solid film from the solution mixture is important in determining the nano-structures of the alignment layer and hence the pretilt angle that can be achieved. Figure 8A-C show the optical micrograph of the surface structures of the alignment layers obtained by printing followed by rapid heating to convert the liquid film into a solid film. In this case, the nano-structures are domains consisting of V and H materials. In these pictures, the dark area are the V material while the bright areas are the H material. The percentage of V materials in Figure 8A,B,C are 2.8%, 5.8% and 11.4% respectively. The width of the pictures correspond to 50 microns. Hence it can be seen that the nano-domains are less than 1 micron typically. Figure 9 shows the pretilt angles obtained as a function of the relative concentration of the V material for several coating methods. The case of spin coating has already been seen in Figure 6. It can been seen that the pretilt angle obtained by quite dependent on the method of preparation of the solid film.

Figure 9 shows that for the same solution mixture, quite different pretilt angles are obtained. This is a result of the different nano-structures obtained. In this invention, we do not exhaust all methods of preparing the solid film from the solution mixture. Suffice it to say that for each method of preparation of the solid film from the solution mixture 46, there will be a new set of results of the pretilt angle versue the percentage of the vertical alignment material. The dependence of the pretilt angle achieved on the method of preparation of the solid film can be understood conceptually. Physically, the two types of domains will interact with the liquid crystal molecules. Their alignment forces will compete with each other resulting in an alignment which is intermediate between vertical alignment and horizontal alignment. By varying the relative concentrations of the homeotropic and homogeneous alignment materials, pretilt angles of various values can be produced.

The change in orientation of the liquid crystal layer caused by the above-described nano-domain may be seen as the creation of a uniform alignment layer a short distance away from the alignment surface, as illustrated in Figure 4A. In this illustration, the vertical (V) and horizontal (H) material a mixed but the main as islands adjacent each other in the alignment layer. The final pretilt angle obtained is strongly dependent on the relative area ratio of the V and H materials. In the formation of the solid film from the liquid film, the volume ratio of V vs H may not be the same as the surface ratio of V vs H. In a slow precipitation case, the material with a smaller solubility will solidify first. This solid will adhere to the substrate. Thus the solid precipitate from the material with a higher solubility will appear on top of the entire film. Thus the area ratio of V vs H can be much higher than the actual volume ratio. Depending on the surface tension of the second material which precipitates on the top, it can form domains as schematically indicated in Figures 4C and 4D, or form a network in indicated in Figure 5C. On the other hand, if the formation of the solid is fast, then the nano-structure is more likely to be represented schematically by Figure 4E. Here the area ratio and the volume ratio of V to H will be quite the same.

The alignment layer produced according to the description of the present invention could have many applications. For example, it can be used in a liquid crystal cell of an LCD device. The LCD device using the alignment layer of the present invention has the advantages of improved response speed, wide viewing angles, and excellent image retention properties. Other applications conceivable to those skilled in the art are not detailed herein.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. Although most embodiments of the present invention uses polymer blends mixing two types of polymers that provide vertical and horizontal aligning ability respectively in forming alignment layers, it is understood that other polymers could also be used for preparing alignment layers. For example, in addition to JALS9203, other commercial products can be used as horizontal alignment materials, such as AL 1454, AL5056, AL3046, JALS- 1216, JALS- 1217, SE-7992, SE-7492 and SE-5291. In addition to JALS-2021, other commercial products can be used as vertical alignment materials, such as JALS-2066, SE-7511L and SE-1211.

For another example, a homopolymer having one kind of side-chain structures that provides horizontal alignment ability and another kind of side-chain structures that provide vertical aligning ability can be used. The side chains of the homopolymer can be manipulated in achieving the desired pretilt angles. For another example, copolymers comprising one type of monomers that provides vertical alignment ability and another type of monomers that provides horizontal alignment ability can also be used. The constituent monomers in the copolymers can also be controlled in achieving the desired pretilt angles.

Although in the examples, the alignment materials purchased commercially were already dissolved in solvents, it is possible to select alignment materials and solvents individually as described in Figure 4. Solvents that can be used are those that are miscible with both horizontal and vertical alignment materials. The solvents should be so miscible with the two types of materials such that when the two materials are dissolved in the solvent, a homogeneous solution comprising nano-sized droplets will be formed. When the alignment layer is formed from such solution, nano-sized structures of the two materials could be observed using standard atomic force microscopy. Indratmoko Hari Poerwanto and Gudrun Schmidt-Naake (Telaah, Jilid XXII, No. 1-2, 2001) provided a practical scheme for predicting polymer miscibility. Based on the existing knowledge, it is believed that those skilled'in the art should be able to select the appropriate solvent to dissolve the vertical and horizontal alignment materials with desired miscibility according the present invention.

Example 4

A liquid crystal cell I (Cell I) was made with the following details: Components:

•Liquid crystal: Merck MLC-6080; Nematic; isotropic phase transition temperature: 95 degree; Δε (dielectric anisotropy): 7.2;^" Δn (refractive index anisotropy): 0.2024; Kn: 14.4; K33: 19.1

•The first substrate: purchased from CSG Shenzhen Wellight Conductive Coating Co., Ltd; model number: STN<30Ω; size: 14"xl6"xl.lmm.

•The second substrate: same

•Alignment layer: following Examples 1-3 to prepare alignment layers capable of providing pretilt angles of 17 and 60 degrees.

•UV epoxy: purchased from BOIS Technology Ltd •Spacers: purchased from BOIS Technology Ltd; model number PF70 Procedures: Standard LCD cell fabrication procedure: The glass substrate was washed and coated with alignment layers. The alignment layers were rubbed, forming an empty cell. The cell was filled with the liquid crystal in a vacuum chamber. The ends of the cell were then sealed off with epoxy. The epoxy was cured with UV light. The polarizers were laminated with proper angles: input polarizer angle: 45 degrees; output polarizer angle: - 45 degrees.

Configuration of Cell I:

•Cell gap: 7 micron •Pretilt angles: 17 and 60 degrees

•Alignment of liquid crystal layer: bend state at all operating voltage [please confirm

The second substrate was an active matrix backplane consisting of thin film transistors on a substrate, such as glass arranged in a matrix form. The states (bend or splay) of the cell were confirmed by observation of the cell color, which is related to the retardation of the LC cell. Green color indicates that the cell is in the splay state. White color indicates that the cell is in the bend state. When voltage is applied, white color becomes gray and then black.

In Cell I, the pretilt angle of the liquid crystal molecules in contact with the top alignment layer and the bottom alignment layer are 17 and 60 degrees, respectively. This is a case of extreme inequality of the pretilt angles. The relationship between transmission and voltage of Cell I is shown in Figure 11. The switching time of Cell I that we have constructed is shown in Figure 12. As an indication that the switching can be fast even for intermediate voltages, we plot the switching time for 8 grey scales. The results are shown in Figure 13. It can be shown that the maximum time needed is 3ms, while the fastest time can be much less than lms. The transmission vs voltage curve in Figure 11 (as well as Figure 14) was obtained by using Autronics DMS501 machine. Figures 12A and 12B are oscilloscope traces showing the switching dynamics of Cell (I). The response time in Figure 13 was obtained by using red laser (632nm), fast detector and oscilloscope.

In this Cell 1, the alignment of the liquid crystal layer remains a B-state for all operating voltages, including zero voltage. The switching time is therefore very fast. No conditioning is required to transform Cell I from S-state to B-state as needed in conventional pi-cells.

Example 5 A second liquid crystal cell II (Cell II) was made with the following details:

Components:

•The first substrate: purchased from CSG Shenzhen Wellight Conductive Coating Co., Ltd; model number: STN<30Ω; size: 14"xl6"xl.lmm

•The second substrate: same

•Liquid crystal: Merck MLC-6080; Nematic; isotropic phase transition temperature: 95 degree; Δε (dielectric anisotropy): 7.2; Δn (refractive index anisotropy): 0.2024; Ki i: 14.4; K33: 19.1 •Alignment layer: following Examples 1-3 in preparing an alignment layer capable of providing a pretilt angle of about 53°.

•UV epoxy: purchased from BOIS Technology Ltd

•Spacers: purchased from BOIS Technology Ltd; model number PF50 Procedures:

Standard LCD cell fabrication procedure: The glass substrate was washed and coated with alignment layers. The alignment layers were rubbed, forming an empty cell. The cell was filled with the liquid crystal in a vacuum chamber. The ends of the cell were then sealed off with epoxy. The epoxy was cured with UV light. The polarizers were laminated with proper angles: input polarizer angle: 45 degrees; output polarizer angle: - 45 degrees.

Configuration of Cell (II):

•Cell gap: 5 micron

•Pretilt angles of both alignment layers: 53 degrees; shifted by 45 degrees from each other.

•Alignment of liquid crystal layer: bend state at all operating voltage

Figure 14 shows the transmission versus voltage curve of Cell II. It is not too different from that of Cell I. However, due to the smaller total birefringence, the absolute transmission of Cell II is lower than Cell I. Figure 15 summarizes the switching time obtained from Cell II. It can be seen that the response times are faster and are generally below 2ms. The alignment layers were rubbed in such a way using a standard rubbing machine that the directions of the top alignment layer and the bottom alignment layer were shifted by 45 degrees from each other. Thus there is allowed to be a certain amount of twist angle to the liquid crystal cell. The purpose of this is so that the cell transmission can be modified and to be that of a twisted nematic cell rather than that of the electrically controlled birefringent cell. This way, the cell gap can be smaller and the response time can be faster. The optical properties of this second embodiment can be optimized using LCD simulation. In this simulation, the cell gap and the twist angles of the LC cell are allowed to change. The requirement is that the contrast should be good and the bright state brightness is close to unity.

Example 6 A reflective liquid crystal display I (LCD I) was made with the following details.

Components:

• The first substrate: purchased from CSG Shenzhen Wellight Conductive Coating Co., Ltd; model number: STN<30Ω; size: 14"xl6"xl.l mm

• The second substrate: a piece of glass with a coating of aluminum

• Liquid crystal: MLC-6080; Nematic; isotropic phase transition temperature: 95 degree; Δε (dielectric anisotropy): 7.2; Δn (refractive index anisotropy): 0.2024; Ki i: 14.4; K33: 19.1

• Alignment layer: following Examples 1-3 to prepare alignment layers capable of providing pretilt angles of 50 degrees.

• Mirror: Evaporated aluminum film onto the glass directly. Performed in-house. • UV epoxy: purchased from BOIS Technology Ltd • Spacers: purchased from BOIS Technology Ltd; model number PF50 The procedures of preparing the LCD Cell were the same as described in Examples 4 and 5. In addition, a mirror was placed behind the liquid crystal cell as illustrated in Figure 1C (12). In other examples, the mirror can be placed inside the liquid crystal cell and on top of the substrate as part of the active matrix backplane. The polarizer is an input polarizer placed in a position as illustrated in Figure 1C (2). There is no output polarizer in this example.

Configuration:

•Cell gap: 3 micron •Pretilt angles of both alignment layers: 50 degrees (in opposite signs) •Alignment of liquid crystal layer: bend state at all operating voltage

Since the light beam passes through the cell twice, the birefringence needed is a factor of two less than the example of Cell I. This has an important implication to the response speed of the liquid crystal cell. Since the cell gap can now be halved, the speed can be almost 4 times faster. (As a rule, the response time of a liquid crystal cell is inversely proportional to the square of the cell gap). In addition, because the path length is now doubled than that of Cell I and Cell II, there can be more choices for birefringence Δn of the liquid crystal material.

The alignment layer produced according to the description of the present invention could have many applications. For example, it can be used in a liquid crystal cell of an LCD device. The LCD device using the alignment layer of the present invention has the advantages of improved response speed, wide viewing angles, and excellent image retention properties. Other applications conceivable to those skilled in the art are not detailed herein. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

Claims

CLAIMS 1. A bend state liquid crystal device comprising a liquid crystal cell, comprising: a. a first substrate having thereon a first alignment layer; b. a second substrate having thereon a second alignment layer; c. a liquid crystal layer sandwiched between said first and second alignment layers, said first alignment layer inducing a first liquid crystal pretilt angle θ_\ with an absolute value in the range of 17°-60°, said second alignment layer inducing a second liquid crystal pretilt angle θ₂ with an absolute value in the range of 17°-60°; with said angles θ_\ and θ₂ being opposite in sign as defined in the same coordinate system and said liquid crystal layer being capable of maintaining a stable bend state at zero bias voltage.

2. The liquid crystal device according to claims 1, wherein said #/ and θi satisfy the equation [K₃₃ -

+ Kn) = O to within 5°; that is to say, in actual practice, the values of the angles θl and Θ2 can deviate from the exact values given by this equation slightly; in particular the angles θl and Θ2 can be allowed to be substantially larger than the exact values given by this equation, for example, to within 5°.

3. The liquid crystal device according to claim 1, wherein θl and θ2 are substantially the same and in the range of 30°-60°.

4. The liquid crystal device according to claim 1, wherein θl and θ2 are 47±5° when K33/K11=1.3.

5. The liquid crystal device according to claim 1, wherein θl =17±5° and θl = 60+5 ° when K33/K11=1.3

6. A liquid crystal device according to claim 1, wherein at least one of the alignment layers comprises a mixture of horizontal alignment material capable of providing a pretilt angle of the liquid crystal molecules in contact therewith of 0°-8° and a vertical alignment material capable of providing a pretilt angle of the liquid crystal molecules in contact therewith 85° -90°.

7. The liquid crystal device according to claim 1, wherein at least one of said alignment layer comprise nano-sized structures at sizes of 0-1 micron.

8. The liquid crystal device according to claim 7, wherein said nano-sized structures comprise at least one of the horizontal and vertical alignment materials.

9. The liquid crystal device according to claims 6, wherein the horizontal alignment material of the alignment layer is selected from a group consisting of JALS92O3 , AL1454, AL5056, AL3046, JALS-1216, JALS-1217, SE-7992, SE-7492 and SE-5291.

10. The liquid crystal device according to claims 6, wherein the vertical alignment material of the alignment layer is selected from a group consisting of JALS-2021, JALS-2066, SE-7511L and SE-1211.

1 1. A liquid crystal device according to claims 1, said liquid crystal layer including liquid crystals having a positive dielectric anisotropy.

12. The liquid crystal device according to claims 1, wherein the liquid crystal layer having a thickness between 1 micron to 15 microns.

13. The liquid crystal device according to claims 1 wherein said alignment layers are solid thin films with a thickness of 10-200 nm;

14. The liquid crystal device according to claims 1, wherein the liquid crystal molecules in contact with the first and second alignment layers are tilted in mirror symmetry.

15. The liquid crystal device according to claims 1, wherein the pretilt angles on both sides of the said liquid crystal layer are tilted in such a way that the direction of the pretilt angles being parallel when projected onto the surfaces of the alignment layers.

16. The liquid crystal device according to claim 1, wherein at least one of the substrates being an active matrix backplane consisting of thin film transistors arranged in a matrix form.

17. The liquid crystal device according to claims 1, where the treatment of the alignment layers being mechanical rubbing of the alignment layers by a piece of cloth.

18. The liquid crystal device according to claims 1, where the treatment of the alignment layers being photo-alignment by exposure of the alignment layers to an ultraviolet beam of polarized light.

19. The liquid crystal device according to claims 1, where the treatment of the alignment layers being irradiation of the alignment layers by an ion beam at an angle in a vacuum chamber.

20. The liquid crystal device according to claim 1, further comprising an input and an output polarizers, wherein the polarizer axes are disposed at angles of plus or minus 35-55 degrees from the rubbing direction of the alignment layers of said liquid crystal cell.

21. The liquid crystal device according to claims 1, further comprising a reflection means disposed outside said liquid crystal cell.

22. The liquid crystal device according to claim 1, further comprising a reflection means built onto the active matrix backplane inside the liquid crystal cell.

23. The liquid crystal device according to claims 1, further comprising a compensation retardation film.

24. A bend state liquid crystal device comprising a liquid crystal cell, comprising: a. a first substrate having thereon a first alignment layer which is treated to impart a pretilt angle θ_/ and an azimuthal angle φj of the liquid crystal molecules in contact with said first alignment layer; b. a second substrate having thereon a second alignment layer which is treated to impart a pretilt angle Θ₂ and an azimuthal angle φz of the liquid crystal molecules in contact with said second alignment layer; c. at least one of said alignment layers being composed of a mixture of vertical alignment materials capable of providing a pretilt angle of 85°-90° and a horizontal alignment material capable of providing a pretilt angle of 0-8°; d. a liquid crystal layer sandwiched between said first and second alignment layers.

25. The bend state liquid crystal device according to claim 24, wherein said azimuthal angles φi and φi are in the range of 0°-45°.

26. The bend state liquid crystal device according to claim 24, wherein said azimuthal angles φi and ^₂ are substantially the same.

27. The liquid crystal device according to claims 24, wherein said θi and θl satisfy the following equation: (K₃₃ - Kji)(sin 2θj + sin 2θ₂ )+2(π~ 2Θ_} -2Θ₂ )[K₃₃ + Kn) = O to within 5°; that is to say, in actual practice, the values of the angles θl and Θ2 can deviate from the exact values given by this equation slightly; in particular the angles θl and Θ2 can be allowed to be substantially larger than the exact values given by this equation, for example, to within 5°.

wherein Ki3 is the bend elastic constant and Ku is the splay elastic constant of the said liquid crystal.

28. The liquid crystal device according to claim 27, wherein θl and θl are substantially the same and in the range of 30^D-60°.

29. The liquid crystal device according to claim 27, wherein θl and 02 are 47±5° when K33/K11=1.3.

30. The liquid crystal device according to claim 27, wherein θl =17±5° and θl = 60±5 " when K33/K11=1.3

31. A liquid crystal device according to claim 24, wherein at least one of the alignment layers comprises a mixture of horizontal alignment material capable of providing a pretilt angle of the liquid crystal molecules in contact therewith of 0^c-8° and a vertical alignment material capable of providing a pretilt angle of the liquid crystal molecules in contact therewith 85° -90°.

32. The liquid crystal device according to claim 24, wherein at least one of said alignment layer comprise nano-sized structures at sizes of 0- 1 micron.

33. The liquid crystal device according to claim 32, wherein said nano-sized structures comprise at least one of the horizontal and vertical alignment materials.

34. The liquid crystal device according to claims 31, wherein the horizontal alignment material of the alignment layer is selected from a group consisting of JALS9203 , AL1454, AL5056, AL3046, JALS-1216, JALS-1217, SE-7992, SE-7492 and SE-5291.

35. The liquid crystal device according to claims 31, wherein the vertical alignment material of the alignment layer is selected from a group consisting of JALS-2021, JALS-2066, SE-7511L and SE- 1211.

36. A liquid crystal device according to claims 24, said liquid crystal layer including liquid crystals having a positive dielectric anisotropy.

37. The liquid crystal device according to claims 24, wherein the liquid crystal layer having a thickness between 1 micron to 15 microns.

38. The liquid crystal device according to claims 24 wherein said alignment layers are solid thin films with a thickness of 10-200 nm;

39. The liquid crystal device according to claims 24, wherein the liquid crystal molecules in contact with the first and second alignment layers are tilted in mirror symmetry.

40. The liquid crystal device according to claims 24, wherein the pretilt angles on both sides of the said liquid crystal layer are tilted in such a way that the direction of the pretilt angles being parallel when projected onto the surfaces of the alignment layers.

41. The liquid crystal device according to claim 24, wherein at least one of the substrates being an active matrix backplane consisting of thin film transistors arranged in a matrix form.

42. The liquid crystal device according to claims 24, where the treatment of the alignment layers being mechanical rubbing of the alignment layers by a piece of cloth.

43. The liquid crystal device according to claims 24, where the treatment of the alignment layers being photo-alignment by exposure of the alignment layers to an ultraviolet beam of polarized light.

44. The liquid crystal device according to claims 24, where the treatment of the alignment layers being irradiation of the alignment layers by an ion beam at an angle in a vacuum chamber.

45. The liquid crystal device according to claim 24, further comprising an input and an output polarizers, wherein the polarizer axes are disposed at angles of plus or minus 35-55 degrees from the rubbing direction of the alignment layers of said liquid crystal cell.

46. The liquid crystal device according to claims 24, further comprising a reflection means disposed outside said liquid crystal cell.

47. The liquid crystal device according to claim 24, further comprising a reflection means built onto the active matrix backplane inside the liquid crystal cell.

48. The liquid crystal device according to claims 24, further comprising a compensation retardation film.

49. In a bend state liquid crystal device, said bend state liquid crystal device including a first substrate having thereon a first alignment layer, a second substrate having thereon a second alignment layer, a method for producing a stable bend state comprising: e. sandwiching liquid crystals having a positive dielectric anisotropy between said first and second alignment layers; f . inducing a first liquid crystal pretilt angle in the range of 17 ° -60 °; g. inducing a second liquid crystal pretilt angle in the range of 17°-60°; and h. aligning said liquid crystals in a bend state that remains stable at zero bias voltage and operating voltages.

50. The method according to claim 49, wherein the pretilt angles are induced by providing alignment layers comprising a horizontal alignment material capable of providing a pretilt angle of the liquid crystal molecules in contact therewith of 0°-8° and a vertical alignment material capable of providing a pretilt angle of the liquid crystal molecules in contact therewith 85° -90°.

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