WO2017071637A1 - Composite photoalignment layer - Google Patents

Composite photoalignment layer Download PDF

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Publication number
WO2017071637A1
WO2017071637A1 PCT/CN2016/103739 CN2016103739W WO2017071637A1 WO 2017071637 A1 WO2017071637 A1 WO 2017071637A1 CN 2016103739 W CN2016103739 W CN 2016103739W WO 2017071637 A1 WO2017071637 A1 WO 2017071637A1
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Prior art keywords
composite
photoalignment layer
azo dye
thin film
photoalignment
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PCT/CN2016/103739
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French (fr)
Inventor
Man Chun Tseng
Abhishek Kumar Srivastava
Cuiling MENG
Vladimir Grigorievich Chigrinov
Hoi-Sing Kwok
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The Hong Kong University Of Science And Technology
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Application filed by The Hong Kong University Of Science And Technology filed Critical The Hong Kong University Of Science And Technology
Priority to CN202311122049.XA priority Critical patent/CN117406498A/en
Priority to CN201680033894.1A priority patent/CN107710058A/en
Publication of WO2017071637A1 publication Critical patent/WO2017071637A1/en
Priority to US15/785,723 priority patent/US10816855B2/en
Priority to US17/028,464 priority patent/US11294241B2/en

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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1337—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers
    • G02F1/133711—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers by organic films, e.g. polymeric films
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
    • C08F120/00—Homopolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical or a salt, anhydride, ester, amide, imide or nitrile thereof
    • C08F120/02—Monocarboxylic acids having less than ten carbon atoms; Derivatives thereof
    • C08F120/52—Amides or imides
    • C08F120/54—Amides, e.g. N,N-dimethylacrylamide or N-isopropylacrylamide
    • C08F120/58—Amides, e.g. N,N-dimethylacrylamide or N-isopropylacrylamide containing oxygen in addition to the carbonamido oxygen, e.g. N-methylolacrylamide, N-acryloyl morpholine
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
    • C08L101/00—Compositions of unspecified macromolecular compounds
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K19/00—Liquid crystal materials
    • C09K19/52—Liquid crystal materials characterised by components which are not liquid crystals, e.g. additives with special physical aspect: solvents, solid particles
    • C09K19/54—Additives having no specific mesophase characterised by their chemical composition
    • C09K19/56—Aligning agents
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1337—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers
    • G02F1/13378—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers by treatment of the surface, e.g. embossing, rubbing or light irradiation
    • G02F1/133788—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers by treatment of the surface, e.g. embossing, rubbing or light irradiation by light irradiation, e.g. linearly polarised light photo-polymerisation
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1337—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers
    • G02F1/133711—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers by organic films, e.g. polymeric films
    • G02F1/133715—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers by organic films, e.g. polymeric films by first depositing a monomer
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1337—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers
    • G02F1/133711—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers by organic films, e.g. polymeric films
    • G02F1/133726—Surface-induced orientation of the liquid crystal molecules, e.g. by alignment layers by organic films, e.g. polymeric films made of a mesogenic material

Definitions

  • In-plane switching displays, fringe field switching displays, and field sequential color displays based on ferroelectric liquid crystal display have recently become more popular because of their ability to provide relatively high optical quality and resolution, and it is desirable to for display cells to have a fast response time, a wide viewing angle, and high resolution.
  • the use of electrically suppressed helix ferroelectric liquid crystals provides great optical quality (like nematic liquid crystals) , with a relatively fast switching response and a relatively low driving voltage.
  • liquid crystal display cells having fast response, high resolution and high optical contrast may include, for example, fast response photonics devices such as modulators, filters, attenuators, and displays with high resolution requirements (e.g., pico projectors, 3D displays, microdisplays, high-definition televisions (HDTVs) , ultra high-definition (UHD) displays, etc. ) .
  • fast response photonics devices such as modulators, filters, attenuators, and displays with high resolution requirements
  • HDTVs high-definition televisions
  • UHD ultra high-definition
  • the invention provides a composite photoalignment layer for aligning liquid crystal molecules, including: a monomeric material; a photoinitiator; and an azo dye material.
  • the invention provides a method for preparing a composite photoalignment layer for aligning liquid crystal molecules, the method including: mixing, in solution form, a monomeric material, a photoinitiator, and an azo dye material; coating the mixed solution onto a substrate to form a thin film; and exposing the thin film to polarized light to form a solid thin film.
  • the invention provides a composite photoalignment layer for aligning liquid crystal molecules, including: a monomeric material; a thermal initiator; and an azo dye material.
  • the invention provides a method for preparing a composite photoalignment layer for aligning liquid crystal molecules, the method including: mixing, in solution form, a monomeric material, a thermal initiator, and an azo dye material; coating the mixed solution onto a substrate to form a thin film; exposing the thin film to polarized light to impose a single-domain or multi-domain alignment; and heating the thin film to polymerize the monomeric material and form a solid thin film.
  • Figure 1 depicts a schematic example of an exemplary process for preparing a composite photoalignment layer for aligning liquid crystal molecules according to a first exemplary embodiment.
  • FIGS 2A-2B show transmittance against voltage curves (TVCs) for an exemplary twisted nematic (TN) display cell before and after thermal exposure.
  • TVCs voltage curves
  • Figures 3A-3B show the TVCs for an exemplary electrically-controlled birefringence (ECB) nematic display cell before and after thermal exposure.
  • ECB electrically-controlled birefringence
  • Figures 4A-4B show the TVCs for an exemplary TN display cell before and after photo exposure.
  • Figures 5A-5B show the TVCs for an exemplary ECB nematic display cell before and after photo exposure.
  • Figure 6 is an image depicting an example of the optical texture of a multi-domain alignment.
  • Figure 7 depicts a schematic example of an exemplary process for preparing a composite photoalignment layer for aligning liquid crystal molecules according to a second exemplary embodiment.
  • Figure 8 shows the TVCs for an exemplary TN display cell before and after thermal exposure.
  • Figure 9 shows the TVCs for an exemplary ECB nematic display cell before and after thermal exposure.
  • Figure 10 shows the TVCs for an exemplary TN display cell before and after photo exposure.
  • Figure 11 shows the TVCs for an exemplary ECB nematic display cell before and after photo exposure.
  • Figure 12 is a plot showing the time-dependence of residual direct current (RDC) voltage of an exemplary composite photoalignment layer after stress of 10V for 1 hour.
  • RDC residual direct current
  • Figure 13 is an image depicting an example of the optical texture of a multi-domain alignment.
  • the electro-optical modes and pixel structure manipulations needed for certain liquid crystal display cells having fast response, high resolution and high optical contrast may demand highly optimized photoalignment to provide zero pre-tilt angle, large surface uniformity and multi-domain alignment (multi-domain alignment in a pixel improves visual appearance and viewing characteristics) .
  • Conventional photoalignment materials are not able to offer all of these qualities.
  • Conventional azo dye alignment layers are able to provide good alignment (with high anchoring energy, small pre-tilt angle, and uniformity over a relatively large area) for liquid crystals in display cells, allowing the liquid crystal display cells to achieve very high pixel resolution.
  • conventional azo dye alignment layers are not stable against chemical, thermal and photo exposure.
  • Exemplary embodiments of the invention provide a composite photoalignment layer for liquid crystals, the composite photoalignment layer including a composite mixing of at least a monomer ( “monomeric material” ) , a thermal free radical initiator ( “thermal initiator” ) or a photoinitiator, and an azo dye material (such as an SD1 azo dye) .
  • a monomer “monomeric material” )
  • thermal initiator” thermal free radical initiator
  • azo dye material such as an SD1 azo dye
  • the composite photoalignment layer provides good alignment characteristics (e.g., low pretilt angle, high polar and azimuthal anchoring energy, low residual direct current (RDC) voltage, high voltage holding ratio (VHR) , low image sticking parameter) , comparable to that of conventional polyimide layers, and meets industry and consumer standards (e.g., with respect to RDC voltage, VHR and anchoring energy) .
  • the composite photoalignment layer is thus suitable for use in a variety of photonic elements and displays, including but not limited to in-plane switching (IPS) and ferroelectric liquid crystal (FLC) displays.
  • IPS in-plane switching
  • FLC ferroelectric liquid crystal
  • a composite photoalignment layer with good alignment characteristics (e.g., high anchoring energy, small pre-tilt angle, and uniformity over a relatively large area) is achieved.
  • the composite photoalignment layer is thus formed in a single step irradiation/exposure, and provides a good and stable photoalignment for liquid crystals.
  • the process starts with a mixture of a monomer, a thermal initiator, and an azo dye material (at concentrations configured to provide stability for the azo dye material without affecting the alignment provided by the photoalignment layer) . Then, in a first step, a preferred orientation of the easy axis of the azo dye photoalignment layer is realized. In a second step, thermal polymerization is performed.
  • Photoalignment provides the ability to realize single-domain or multi-domain alignment with an extremely small pretilt angle in a single step of irradiation/exposure.
  • a single-step photoalignment process with, for example, a patterned wave plate, a multi-domain photoalignment layer may be achieved with highly uniform alignment over a large size.
  • the azo dye material offers only in-plane molecular diffusion from one direction to another, and does not go out of plane, the generated pre-tilt angle is very small.
  • the anchoring energies of the composite photoalignment layer are adjustable by controlling the exposure dosage.
  • exemplary embodiments of the invention are suitable for applications requiring precise control of anchoring energies, including but not limited to, for example, ferroelectric liquid crystal displays.
  • a liquid crystal photoalignment layer shows a preferred alignment direction after being irradiated by polarized light with sufficiently high irradiation energy of certain wavelength (the polarized light imposes an alignment direction on the photoalignment layer) .
  • Photoalignment provides several advantages over conventional rubbing alignment techniques. For example, rubbing may cause mechanical damage or electrostatic charge, which degrades manufacturing yield. Photoalignment avoids mechanical contact with the aligning layer, and thus minimizes such mechanical damage and electrostatic charging (particular advantageous for FLC devices) . Photoalignment is also easier to implement with respect to large substrates and provides better uniformity for high resolution displays. Additionally, photoalignment provides the ability to realize multi-domain alignment on a micro-scale or even on a nano-scale. Furthermore, photoalignment may be utilized with respect to a non-flat surface such as a curved surface or surfaces with microscopic confinements.
  • photoalignment There are several approaches to photoalignment, including for example, the following categories: (1) photoalignment by cis-trans isomerization of azo dye molecules; (2) photocrosslinking of monomers into polymers; (3) photo-degradation of a polymer layer; and (4) photoinduced reorientation of azo dye molecules.
  • photoinduced reorientation of azo dye molecules provides certain advantages—for example, sufficiently high polar and azimuthal anchoring energies for liquid crystal alignment, which may be as strong as a commercial polyimide film based on conventional rubbing; high voltage holding ratio (VHR) and low residual direct current (RDC) voltage is low, which is advantageous for liquid crystal alignment; and very small pretilt angle (e.g., less than 1 degree) , which is advantageous for display modes that require such low pretilt angles, such as the in-plane switching (IPS) mode and derivatives thereof such as the fringe-field switching (FFS) mode.
  • VHR voltage holding ratio
  • RDC residual direct current
  • photoinduced reorientation of azo dyes may be achieved with polarized light over a large range of wavelengths, including for example blue light at 450nm. This allows high power light-emitting diodes (LEDs) to be used as the light source so as to reduce the cost of the photoalignment equipment
  • Photoalignment based on photoinduced reorientation of azo dye molecules is thus able to achieve sufficiently high polar and azimuthal anchoring energy, high VHR, appropriate pre-tilt angles, and uniform alignment. Additionally, photoalignment based on photoinduced reorientation of azo dye molecules is easily rotatable using blue light and provides anchoring energy comparable to a commercial polyimide film with very low pretilt angle. Photoalignment based on photoinduced reorientation of azo dye molecules may be used in a wide range of LC devices, including for example, IPS and FLC displays. Photoalignment based on photoinduced reorientation of azo dye molecules is tunable based on controlling the irradiation energy doses.
  • Photoalignment based on photoinduced reorientation of azo dye molecules is further able to provide a multi-domain alignment with a distinctly defined easy axis of the alignment. Additionally, photoalignment based on photoinduced reorientation of azo dye molecules provides the ability to align nanoscopic domains so as to provide for better viewing, optical and other characteristics of liquid crystal displays.
  • the photo-degradation and instability of conventional azo dye photoalignment layers hinders the deployment of azo dye photoalignment layers in certain real world applications.
  • the easy axis of the azo dye photoalignment layer may change and damage the alignment quality of the display cell.
  • light flux from the backlight of a display system may be strong enough to damage the alignment characteristics of the photoalignment layer within a few hours of operation.
  • the invention provides a composite photoalignment layer for liquid crystals that comprises a monomer, a photoinitiator, and an azo dye material in optimal relative concentrations.
  • the composite photoalignment layer provides good, uniform alignment and is stable after being irradiated by a light source.
  • the concentration of the photoinitator and the monomer are tuned to provide both alignment and stabilization in a single irradiation.
  • the monomer has liquid crystal properties and is a liquid crystalline reactive mesogen;
  • the azo dye is sulfonic dye tetrasodium5, 5'- ( (1E, 1'E) - (2, 2'-disulfonato- [1, 1'-biphenyl] -4, 4'-diyl) bis (diazene-2, 1-diyl)) bis (2-hydroxybenzoate) ( “SD1” ) ;
  • the photoinitiator is 1-hydroxycyclohexyl phenyl ketone. It will be appreciated that in other exemplary implementations, other materials may be used.
  • the process of making the composite photoalignment layer begins with mixing the monomer and azo dye at optimal relative concentrations of 50:50 (since the molecule length of the azo dye and the monomer is approximately the same) . Then, the photoinitiator at 10%wt/wt of the monomer is added to the mixture. It will be appreciated that in other exemplary implementations and that with other materials, other relative concentrations of materials may be used.
  • the concentration of photoinitiator is tuned to optimize the rate of polymerization (e.g., to ensure that polymerization is not completed before photoalignment, which would negatively affect the optical quality) .
  • the concentration of photoinitiator that is added to the mixture may be varied between 1%wt/wt of the monomer to 10%wt/wt of the monomer to optimize the balance between the rate of alignment (to achieve a certain amount of liquid crystal anchoring energy) and the rate of polymerization. Further, based on the relationship between the absorption band of the photoinitiator and the absorption band of the azo dye, different balances between the rate of alignment and the rate of polymerization may be achieved.
  • the photoinitiator absorption band is chosen to match the absorption band of the azo dye (e.g., SD1 azo dye has absorption peaks at 365nm and 450nm) .
  • the absorption band of the photoinitiator is different from the absorption band of the azo dye.
  • the azimuthal anchoring energy of the composite photoalignment layer can be tuned by varying the irradiation energy as well as by balancing the rate of the alignment and the rate of polymerization.
  • a process for preparing a composite photoalignment layer for aligning liquid crystal molecules includes: mixing, in solution form, a monomeric material, a photoinitiator, and an azo dye material; coating the mixed solution onto a substrate to form a thin film; and exposing the thin film to polarized light to form a solid thin film. Exposing the thin film is a single step exposure that provides both alignment and polymerization for the composite photoalignment layer.
  • the photoalignment layer may be coated onto a substrate surface based on a variety of coating techniques, including but not limited to, for example, spin coating, doctor blading, and screen printing.
  • the polarized light may be from a polarized light source having one or more major wavelength components (e.g., such that separate irradiation bands for alignment and polymerization may be used) .
  • Figure 1 depicts a schematic example of this process.
  • a mixture of SD1 azo dye, monomer and photoinitiator, composited in a solvent (e.g., dimethylformamide (DMF) ) is spin coated onto a substrate at stage 101 so as to form a thin film at stage 102.
  • a solvent e.g., dimethylformamide (DMF)
  • the thin film is exposed in a single step exposure that provides both alignment and polymerization for the composite photoalignment layer so as to form a solid thin film having the SD1 molecules and a polymer network formed from the monomers at stage 104.
  • the polymerization of the monomeric material in the composite photoalignment layer causes the composite photoalignment layer to form a solid thin film, and polymerization of the monomeric material provides high liquid crystal anchoring energy (e.g., ⁇ 10 -3 J/m 2 ) .
  • high liquid crystal anchoring energy e.g., ⁇ 10 -3 J/m 2
  • the monomeric material may be fully polymerized in accordance with exemplary embodiments of the invention.
  • the particular level of the anchoring energy may be tuned based on the irradiation dosage.
  • an anchoring energy in the range of 10 -4 J/m 2 to 10 -2 J/m 2 may be achieved (e.g., approximately on the order of magnitude of 10 -4 J/m 2 or 10 -3 J/m 2 ) .
  • the anchoring energy may be tuned within the range of 10 -4 J/m 2 to 10 -2 J/m 2 by adjusting the irradiation dose.
  • the composite photoalignment layer manifests low RDC voltage, e.g., under 10mV.
  • the composite photoalignment layer provides electro-optical characteristics that are the same or similar to conventional polyimide alignment layers.
  • the voltage holding ratio for a planar aligned nematic liquid crystal cell having the composite photoalignment layer is greater than 99%for a frame rate of 60Hz.
  • the composite photoalignment layer provides alignment quality that is comparable to conventional and commercially available alignment layers.
  • the composite photoalignment layer provides an image sticking parameter ( “ISP” ) ratio of 1.01, which is comparable to conventional alignment layers.
  • the image sticking parameter defines how a display panel behaves against a ghost image of a previous frame.
  • the ISP ratio is 1.01 based on application of a stress of 6V being applied to one of two pixels of a cell for 6 hours, with the other pixel being left at 0V, and comparing the transmittance of the two pixels at a stress of 2V.
  • the composite photoalignment layer was demonstrated as being thermally stable in that it did not reveal any traces of degradation after thermal exposure at 100°C for 24 hours in an oven.
  • the transmittance against voltage curves (TVCs) for exemplary display cells having the composite photoalignment layer were unaffected after the thermal exposure.
  • Figures 2A-2B show the TVCs for an exemplary twisted nematic (TN) display cell before and after thermal exposure.
  • Figures 3A-3B show the TVCs for an exemplary electrically-controlled birefringence (ECB) nematic display cell before and after thermal exposure.
  • TN twisted nematic
  • ECB electrically-controlled birefringence
  • the composite photoalignment layer was also demonstrated as being optically stable and did not show any degradation after photo exposure to a light source with intensity 100mW/cm 2 for 1 hour.
  • the TVCs for exemplary display cells having the composite photoalignment layer were unaffected after the photo exposure.
  • Figures 4A-4B show the TVCs for an exemplary TN display cell before and after photo exposure.
  • Figures 5A-5B show the TVCs for an exemplary ECB nematic display cell before and after photo exposure.
  • the alignment quality of the exemplary display cells were also unaffected by the photo exposure, as was apparent from visual inspection.
  • a phase mask is used to provide two or more alignment domains for the composite photoalignment layer.
  • a patterned half wave plate with two domains with characteristic size of 20 ⁇ m is used to provide the phase mask.
  • the phase mask rotates the plane of the impinging light and thereafter the impinging light, with degenerated plane of polarization, exposes the substrate coated with the composite photoalignment layer.
  • the irradiated substrate provides multi-domain alignment that is stable and resistant to thermal and photo exposure, while having high quality optical and electrical parameters.
  • An example of the optical texture of a multi-domain alignment is depicted in Figure 6.
  • the invention provides a composite photoalignment layer for liquid crystals that comprises a monomer, a thermal initiator, and an azo dye material in optimal relative concentrations.
  • the composite photoalignment layer provides good, uniform alignment after being irradiated by a light source and is stable after being heated (e.g., at 230°C for 30 minutes, but it will be appreciated that other times and temperatures can be used) .
  • the concentration of the thermal initiator and the monomer are tuned to provide both a good alignment and stabilization for the alignment.
  • the monomer has liquid crystal properties and is 4- (3-acryloyloxypropyloxy) -benzoesure-2-methyl-1, 4-phenylester;
  • the azo dye is sulfonic azo dye tetrasodium5, 5'- ( (1E, 1'E) - (2, 2'-disulfonato- [1, 1'-biphenyl] -4, 4'-diyl) bis (diazene-2, 1-diyl) ) bis (2-hydroxybenzoate) ( “SD1” ) ;
  • the thermal initiator is 2-cyano-2-propyl dodecyl trithiocarbonate. It will be appreciated that in other exemplary implementations, other materials may be used.
  • the process of making the composite photoalignment layer begins with mixing the monomer and azo dye at optimal relative concentrations of 50:50 (since the molecule length of the azo dye and the monomer is approximately the same) . Then, the thermal initiator at 5%wt/wt of the monomer is added to the mixture. The mixture is further dissolved in a solvent (e.g., dimethylformamide or other polar solvents) . It will be appreciated that in other exemplary implementations and that with other materials, other relative concentrations of materials may be used.
  • a solvent e.g., dimethylformamide or other polar solvents
  • the concentration of the azo dye and monomer combined is 1%wt/wt of the solvent, whereas the concentration of the thermal initiator is 5%wt/wt of the monomer. It will be appreciated that in other exemplary implementations and that with other materials, other relative concentrations of materials may be used.
  • a process for preparing a composite photoalignment layer for aligning liquid crystal molecules includes: mixing, in solution form, a monomeric material, a thermal initiator, and an azo dye material; coating the mixed solution onto a substrate to form a thin film; exposing the thin film with polarized light to impose a single-domain or multiple-domain alignment; and heating the thin film to form a solid thin film. Exposing and heating the thin film may be performed simultaneously as part of a single step or sequentially in separate steps. The thermal polymerization caused by heating the thin film does not affect the alignment properties (such as anchoring energy and surface uniformity) of the composite photoalignment layer.
  • Figure 7 depicts a schematic example of this process.
  • a mixture of SD1 azo dye, monomer and thermal initiator, in solution form is spin coated onto a substrate at stage 701 so as to form a thin film at stage 702.
  • the thin film is exposed in a single step exposure that provides alignment for the composite photoalignment, and at stage 704, the thin film is heated at 230°Cfor 30 minutes, so as to form a solid thin film having the SD1 molecules and a polymer network formed from the monomers at stage 705.
  • the polymerization of the monomeric material in the composite photoalignment layer causes the composite photoalignment layer to form a solid thin film, and polymerization of the monomeric material provides high liquid crystal anchoring energy (e.g., ⁇ 10 -3 J/m 2 ) .
  • high liquid crystal anchoring energy e.g., ⁇ 10 -3 J/m 2
  • the monomeric material may be fully polymerized in accordance with exemplary embodiments of the invention.
  • the particular level of the anchoring energy may be tuned based on the irradiation dosage. For example, an anchoring energy in the range of 10 -4 J/m 2 to 10 - 2 J/m 2 may be achieved (e.g., approximately on the order of magnitude of 10 -4 J/m 2 or 10 -3 J/m 2 ) . In another example, an anchoring energy of approximately 3x10 -3 J/m 2 may be achieved. Further, it will be appreciated that the anchoring energy may be tuned within the range of 10 -4 J/m 2 to 10 -2 J/m 2 by adjusting the irradiation dose.
  • the composite photoalignment layer provides electro-optical characteristics that are the same or similar to conventional polyimide alignment layers.
  • the voltage holding ratio for an electrical controlled birefringence liquid crystal cell having the composite photoalignment layer is greater than 99%for a frame rate of 60Hz.
  • the composite photoalignment layer provides alignment quality that is comparable to conventional and commercially available alignment layers.
  • the composite photoalignment layer was demonstrated as being thermally stable in that it did not reveal any traces of degradation after thermal exposure at 100°C for 24 hours in an oven.
  • the TVCs for exemplary display cells having the composite photoalignment layer were unaffected after the thermal exposure.
  • Figure 8 shows the TVCs for an exemplary TN display cell before and after thermal exposure.
  • Figure 9 shows the TVCs for an exemplary ECB nematic display cell before and after thermal exposure.
  • the alignment quality of the exemplary display cells were also unaffected by the thermal exposure, as was apparent from visual inspection.
  • the composite photoalignment layer was also demonstrated as being optically stable and did not show any degradation after photo exposure to a light source with 400J/cm 2 of energy at a wavelength of 450 nm.
  • the TVCs for exemplary display cells having the composite photoalignment layer were unaffected after the photo exposure.
  • Figure 10 shows the TVCs for an exemplary TN display cell before and after the photo exposure.
  • Figure 11 shows the TVCs for an exemplary ECB nematic display cell before and after the photo exposure.
  • the alignment quality of the exemplary display cells were also unaffected by the photo exposure, as was apparent from visual inspection.
  • the composite photoalignment layer manifests low RDC voltage, e.g., under 10mV in an example where a DC soak of 10V is performed for an hour at 60°C.
  • Figure 12 shows the time-dependence of the RDC voltage of an exemplary composite photoalignment layer after stress of 10V for 1 hour.
  • a phase mask is used to provide two or more alignment domains with distinct alignment directions in neighboring domains for the composite photoalignment layer.
  • the irradiated substrate provides multi-domain alignment that is stable and resistant to thermal and photo exposure, while having high quality optical and electrical parameters.
  • An example of the optical texture of a multi-domain alignment having a checker board pattern with a characteristic size of 20 ⁇ m is depicted in Figure 13.
  • Exemplary embodiments of the invention thus provide a composite photoalignment layer with full polymerization of the monomer, while providing acceptable values for residual DC voltage, image sticking parameter, and voltage holding ratio.
  • a composite photoalignment layer with full polymerization of the monomer provides a minimum and acceptable residual DC voltage value of 0.008 V, a minimum and acceptable image sticking parameter ratio of 1.01, and a minimum and acceptable voltage holding ratio of more than 99%at 60°C and 60Hz frame frequency.

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Abstract

A composite photoalignment layer for aligning liquid crystal molecules includes: a monomeric material; a photoinitiator or a thermal initiator; and an azo dye material. A method for preparing a composite photoalignment layer for aligning liquid crystal molecules includes: mixing, in solution form, a monomeric material, a photoinitiator or a thermal initiator, and an azo dye material; coating the mixed solution onto a substrate to form a thin film; exposing the thin film to polarized light; and, with a thermal initiator, heating the thin film to polymerize the monomeric material and form a solid thin film.

Description

COMPOSITE PHOTOALIGNMENT LAYER
CROSS-REFERENCE TO RELATED APPLICATIONS
This patent application claims priority to U.S. Provisional Patent Application No. 62/285,435, filed October 29, 2015, and U.S. Provisional Patent Application No. 62/493,840, filed July 19, 2016, both of which are incorporated by reference herein in their entireties.
BACKGROUND
In-plane switching displays, fringe field switching displays, and field sequential color displays based on ferroelectric liquid crystal display have recently become more popular because of their ability to provide relatively high optical quality and resolution, and it is desirable to for display cells to have a fast response time, a wide viewing angle, and high resolution. For example, the use of electrically suppressed helix ferroelectric liquid crystals provides great optical quality (like nematic liquid crystals) , with a relatively fast switching response and a relatively low driving voltage.
Applications of liquid crystal display cells having fast response, high resolution and high optical contrast may include, for example, fast response photonics devices such as modulators, filters, attenuators, and displays with high resolution requirements (e.g., pico projectors, 3D displays, microdisplays, high-definition televisions (HDTVs) , ultra high-definition (UHD) displays, etc. ) .
SUMMARY
In an exemplary embodiment, the invention provides a composite photoalignment layer for aligning liquid crystal molecules, including: a monomeric material； a photoinitiator； and an azo dye material.
In another exemplary embodiment, the invention provides a method for preparing a composite photoalignment layer for aligning liquid crystal molecules, the method including: mixing, in solution form, a monomeric material, a photoinitiator, and an azo dye material； coating the mixed solution onto a substrate to form a thin film； and exposing the thin film to polarized light to form a solid thin film.
In yet another exemplary embodiment, the invention provides a composite photoalignment layer for aligning liquid crystal molecules, including: a monomeric material； a thermal initiator； and an azo dye material.
In yet another exemplary embodiment, the invention provides a method for preparing a composite photoalignment layer for aligning liquid crystal molecules, the method including: mixing, in solution form, a monomeric material, a thermal initiator, and an azo dye material； coating the mixed solution onto a substrate to form a thin film； exposing the thin film to polarized light to impose a single-domain or multi-domain alignment； and heating the thin film to polymerize the monomeric material and form a solid thin film.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 depicts a schematic example of an exemplary process for preparing a composite photoalignment layer for aligning liquid crystal molecules according to a first exemplary embodiment.
Figures 2A-2B show transmittance against voltage curves (TVCs) for an exemplary twisted nematic (TN) display cell before and after thermal exposure.
Figures 3A-3B show the TVCs for an exemplary electrically-controlled birefringence (ECB) nematic display cell before and after thermal exposure.
Figures 4A-4B show the TVCs for an exemplary TN display cell before and after photo exposure.
Figures 5A-5B show the TVCs for an exemplary ECB nematic display cell before and after photo exposure.
Figure 6 is an image depicting an example of the optical texture of a multi-domain alignment.
Figure 7 depicts a schematic example of an exemplary process for preparing a composite photoalignment layer for aligning liquid crystal molecules according to a second exemplary embodiment.
Figure 8 shows the TVCs for an exemplary TN display cell before and after thermal exposure.
Figure 9 shows the TVCs for an exemplary ECB nematic display cell before and after thermal exposure.
Figure 10 shows the TVCs for an exemplary TN display cell before and after photo exposure.
Figure 11 shows the TVCs for an exemplary ECB nematic display cell before and after photo exposure.
Figure 12 is a plot showing the time-dependence of residual direct current (RDC) voltage of an exemplary composite photoalignment layer after stress of 10V for 1 hour.
Figure 13 is an image depicting an example of the optical texture of a multi-domain alignment.
DETAILED DESCRIPTION
The electro-optical modes and pixel structure manipulations needed for certain liquid crystal display cells having fast response, high resolution and high optical contrast may demand highly optimized photoalignment to provide zero pre-tilt angle, large surface uniformity and multi-domain alignment (multi-domain alignment in a pixel improves visual appearance and viewing characteristics) .
Conventional photoalignment materials are not able to offer all of these qualities. Conventional azo dye alignment layers are able to provide good alignment (with high anchoring energy, small pre-tilt angle, and uniformity over a relatively large area) for liquid crystals in display cells, allowing the liquid crystal display cells to achieve very high pixel resolution. However, conventional azo dye alignment layers are not stable against chemical, thermal and photo exposure.
Exemplary embodiments of the invention provide a composite photoalignment layer for liquid crystals, the composite photoalignment layer including a composite mixing of at least a monomer ( “monomeric material” ) , a thermal free radical initiator ( “thermal initiator” ) or a photoinitiator, and an azo dye material (such as an SD1 azo dye) . By introducing a polymer network into the azo dye material (via thermally-initiated or photoinitiated polymerization) , exemplary embodiments of the invention provide a stabilized composite azo dye photoalignment layer which is stable against ultraviolet light exposure, heat, and other environmental conditions.
The composite photoalignment layer provides good alignment characteristics (e.g., low pretilt angle, high polar and azimuthal anchoring energy, low residual direct current (RDC) voltage, high voltage holding ratio (VHR) , low image sticking parameter) , comparable to that of conventional polyimide layers, and meets industry and consumer standards (e.g., with respect to RDC voltage, VHR and anchoring energy) . The composite photoalignment layer is thus suitable for use in a variety of photonic elements and displays, including but not limited to in-plane switching (IPS) and ferroelectric liquid crystal (FLC) displays.
In a first exemplary embodiment, starting with a mixture of a monomer, a photoinitiator, and an azo dye material (at concentrations configured to provide stability for the azo dye material without affecting the alignment provided by the photoalignment layer) , and by using a single light exposure to provide both photoinduced reorientation of the azo dye material (photoalignment) and polymerization of the monomer, a composite photoalignment layer with good  alignment characteristics (e.g., high anchoring energy, small pre-tilt angle, and uniformity over a relatively large area) is achieved. The composite photoalignment layer is thus formed in a single step irradiation/exposure, and provides a good and stable photoalignment for liquid crystals.
In a second exemplary embodiment, the process starts with a mixture of a monomer, a thermal initiator, and an azo dye material (at concentrations configured to provide stability for the azo dye material without affecting the alignment provided by the photoalignment layer) . Then, in a first step, a preferred orientation of the easy axis of the azo dye photoalignment layer is realized. In a second step, thermal polymerization is performed.
Photoalignment provides the ability to realize single-domain or multi-domain alignment with an extremely small pretilt angle in a single step of irradiation/exposure. Using a single-step photoalignment process with, for example, a patterned wave plate, a multi-domain photoalignment layer may be achieved with highly uniform alignment over a large size. Further, because the azo dye material offers only in-plane molecular diffusion from one direction to another, and does not go out of plane, the generated pre-tilt angle is very small.
Additionally, according to exemplary embodiments of the invention, the anchoring energies of the composite photoalignment layer are adjustable by controlling the exposure dosage. Thus, exemplary embodiments of the invention are suitable for applications requiring precise control of anchoring energies, including but not limited to, for example, ferroelectric liquid crystal displays.
A liquid crystal photoalignment layer shows a preferred alignment direction after being irradiated by polarized light with sufficiently high irradiation energy of certain wavelength (the polarized light imposes an alignment direction on the photoalignment layer) . Photoalignment provides several advantages over conventional rubbing alignment techniques. For example, rubbing may cause mechanical damage or electrostatic charge, which degrades manufacturing yield.  Photoalignment avoids mechanical contact with the aligning layer, and thus minimizes such mechanical damage and electrostatic charging (particular advantageous for FLC devices) . Photoalignment is also easier to implement with respect to large substrates and provides better uniformity for high resolution displays. Additionally, photoalignment provides the ability to realize multi-domain alignment on a micro-scale or even on a nano-scale. Furthermore, photoalignment may be utilized with respect to a non-flat surface such as a curved surface or surfaces with microscopic confinements.
There are several approaches to photoalignment, including for example, the following categories: (1) photoalignment by cis-trans isomerization of azo dye molecules； (2) photocrosslinking of monomers into polymers； (3) photo-degradation of a polymer layer； and (4) photoinduced reorientation of azo dye molecules. Among these, photoinduced reorientation of azo dye molecules provides certain advantages—for example, sufficiently high polar and azimuthal anchoring energies for liquid crystal alignment, which may be as strong as a commercial polyimide film based on conventional rubbing； high voltage holding ratio (VHR) and low residual direct current (RDC) voltage is low, which is advantageous for liquid crystal alignment； and very small pretilt angle (e.g., less than 1 degree) , which is advantageous for display modes that require such low pretilt angles, such as the in-plane switching (IPS) mode and derivatives thereof such as the fringe-field switching (FFS) mode. Further, photoinduced reorientation of azo dyes may be achieved with polarized light over a large range of wavelengths, including for example blue light at 450nm. This allows high power light-emitting diodes (LEDs) to be used as the light source so as to reduce the cost of the photoalignment equipment
Photoalignment based on photoinduced reorientation of azo dye molecules is thus able to achieve sufficiently high polar and azimuthal anchoring energy, high VHR, appropriate pre-tilt angles, and uniform alignment. Additionally,  photoalignment based on photoinduced reorientation of azo dye molecules is easily rotatable using blue light and provides anchoring energy comparable to a commercial polyimide film with very low pretilt angle. Photoalignment based on photoinduced reorientation of azo dye molecules may be used in a wide range of LC devices, including for example, IPS and FLC displays. Photoalignment based on photoinduced reorientation of azo dye molecules is tunable based on controlling the irradiation energy doses. Photoalignment based on photoinduced reorientation of azo dye molecules is further able to provide a multi-domain alignment with a distinctly defined easy axis of the alignment. Additionally, photoalignment based on photoinduced reorientation of azo dye molecules provides the ability to align nanoscopic domains so as to provide for better viewing, optical and other characteristics of liquid crystal displays.
However, as mentioned above, the photo-degradation and instability of conventional azo dye photoalignment layers hinders the deployment of azo dye photoalignment layers in certain real world applications. In particular, if a photoaligned display cell is exposed to light, the easy axis of the azo dye photoalignment layer may change and damage the alignment quality of the display cell. Further, light flux from the backlight of a display system may be strong enough to damage the alignment characteristics of the photoalignment layer within a few hours of operation.
In the first exemplary embodiment, the invention provides a composite photoalignment layer for liquid crystals that comprises a monomer, a photoinitiator, and an azo dye material in optimal relative concentrations. The composite photoalignment layer provides good, uniform alignment and is stable after being irradiated by a light source. The concentration of the photoinitator and the monomer are tuned to provide both alignment and stabilization in a single irradiation.
In an exemplary implementation, the monomer has liquid crystal properties and is a liquid crystalline reactive mesogen； the azo dye is sulfonic dye tetrasodium5, 5'- ( (1E, 1'E) - (2, 2'-disulfonato- [1, 1'-biphenyl] -4, 4'-diyl) bis (diazene-2, 1-diyl)) bis (2-hydroxybenzoate) ( “SD1” ) ； and the photoinitiator is 1-hydroxycyclohexyl phenyl ketone. It will be appreciated that in other exemplary implementations, other materials may be used.
In one example, the process of making the composite photoalignment layer begins with mixing the monomer and azo dye at optimal relative concentrations of 50:50 (since the molecule length of the azo dye and the monomer is approximately the same) . Then, the photoinitiator at 10％wt/wt of the monomer is added to the mixture. It will be appreciated that in other exemplary implementations and that with other materials, other relative concentrations of materials may be used.
The concentration of photoinitiator is tuned to optimize the rate of polymerization (e.g., to ensure that polymerization is not completed before photoalignment, which would negatively affect the optical quality) . In various exemplary implementations, the concentration of photoinitiator that is added to the mixture may be varied between 1％wt/wt of the monomer to 10％wt/wt of the monomer to optimize the balance between the rate of alignment (to achieve a certain amount of liquid crystal anchoring energy) and the rate of polymerization. Further, based on the relationship between the absorption band of the photoinitiator and the absorption band of the azo dye, different balances between the rate of alignment and the rate of polymerization may be achieved. In one example, the photoinitiator absorption band is chosen to match the absorption band of the azo dye (e.g., SD1 azo dye has absorption peaks at 365nm and 450nm) . In other examples, the absorption band of the photoinitiator is different from the absorption band of the azo dye.
Additionally, the azimuthal anchoring energy of the composite photoalignment layer can be tuned by varying the irradiation energy as well as by balancing the rate of the alignment and the rate of polymerization.
A process for preparing a composite photoalignment layer for aligning liquid crystal molecules includes: mixing, in solution form, a monomeric material, a photoinitiator, and an azo dye material； coating the mixed solution onto a substrate to form a thin film； and exposing the thin film to polarized light to form a solid thin film. Exposing the thin film is a single step exposure that provides both alignment and polymerization for the composite photoalignment layer. The photoalignment layer may be coated onto a substrate surface based on a variety of coating techniques, including but not limited to, for example, spin coating, doctor blading, and screen printing. The polarized light may be from a polarized light source having one or more major wavelength components (e.g., such that separate irradiation bands for alignment and polymerization may be used) .
Figure 1 depicts a schematic example of this process. As shown in Figure 1, a mixture of SD1 azo dye, monomer and photoinitiator, composited in a solvent (e.g., dimethylformamide (DMF) ) , in solution form, is spin coated onto a substrate at stage 101 so as to form a thin film at stage 102. Then, at stage 103, the thin film is exposed in a single step exposure that provides both alignment and polymerization for the composite photoalignment layer so as to form a solid thin film having the SD1 molecules and a polymer network formed from the monomers at stage 104. In particular, the polymerization of the monomeric material in the composite photoalignment layer causes the composite photoalignment layer to form a solid thin film, and polymerization of the monomeric material provides high liquid crystal anchoring energy (e.g., ～10-3J/m2) . It will be appreciated that the monomeric material may be fully polymerized in accordance with exemplary embodiments of the invention.
The particular level of the anchoring energy may be tuned based on the irradiation dosage. In one example, an anchoring energy in the range of 10-4J/m2 to 10-2J/m2 may be achieved (e.g., approximately on the order of magnitude of 10-4J/m2  or 10-3J/m2) . Further, it will be appreciated that the anchoring energy may be tuned within the range of 10-4J/m2 to 10-2J/m2 by adjusting the irradiation dose.
In an exemplary implementation, the composite photoalignment layer manifests low RDC voltage, e.g., under 10mV.
In an exemplary implementation, the composite photoalignment layer provides electro-optical characteristics that are the same or similar to conventional polyimide alignment layers. In an example, the voltage holding ratio for a planar aligned nematic liquid crystal cell having the composite photoalignment layer is greater than 99％for a frame rate of 60Hz.
In an exemplary implementation, the composite photoalignment layer provides alignment quality that is comparable to conventional and commercially available alignment layers.
In an exemplary implementation, the composite photoalignment layer, with full polymerization of the monomer, provides an image sticking parameter ( “ISP” ) ratio of 1.01, which is comparable to conventional alignment layers. The image sticking parameter defines how a display panel behaves against a ghost image of a previous frame. In an example, it was demonstrated that the ISP ratio is 1.01 based on application of a stress of 6V being applied to one of two pixels of a cell for 6 hours, with the other pixel being left at 0V, and comparing the transmittance of the two pixels at a stress of 2V.
In an exemplary implementation, the composite photoalignment layer was demonstrated as being thermally stable in that it did not reveal any traces of degradation after thermal exposure at 100℃ for 24 hours in an oven. As shown in Figures 2A-2B and Figures 3A-3B, the transmittance against voltage curves (TVCs) for exemplary display cells having the composite photoalignment layer were unaffected after the thermal exposure. Figures 2A-2B show the TVCs for an exemplary twisted nematic (TN) display cell before and after thermal exposure. Figures 3A-3B show the TVCs for an exemplary electrically-controlled  birefringence (ECB) nematic display cell before and after thermal exposure. The alignment quality of the exemplary display cells were also unaffected by the thermal exposure, as was apparent from visual inspection.
The composite photoalignment layer was also demonstrated as being optically stable and did not show any degradation after photo exposure to a light source with intensity 100mW/cm2 for 1 hour. As shown in Figures 4A-4B and Figures 5A-5B, the TVCs for exemplary display cells having the composite photoalignment layer were unaffected after the photo exposure. Figures 4A-4B show the TVCs for an exemplary TN display cell before and after photo exposure. Figures 5A-5B show the TVCs for an exemplary ECB nematic display cell before and after photo exposure. The alignment quality of the exemplary display cells were also unaffected by the photo exposure, as was apparent from visual inspection.
In an exemplary implementation, during the single step exposure at stage 103 of Figure 1, a phase mask is used to provide two or more alignment domains for the composite photoalignment layer. In an example, a patterned half wave plate with two domains with characteristic size of 20 μm is used to provide the phase mask. The phase mask rotates the plane of the impinging light and thereafter the impinging light, with degenerated plane of polarization, exposes the substrate coated with the composite photoalignment layer. As a result, the irradiated substrate provides multi-domain alignment that is stable and resistant to thermal and photo exposure, while having high quality optical and electrical parameters. An example of the optical texture of a multi-domain alignment is depicted in Figure 6.
In the second exemplary embodiment, the invention provides a composite photoalignment layer for liquid crystals that comprises a monomer, a thermal initiator, and an azo dye material in optimal relative concentrations. The composite photoalignment layer provides good, uniform alignment after being irradiated by a light source and is stable after being heated (e.g., at 230℃ for 30 minutes, but it will be appreciated that other times and temperatures can be used) . The concentration of  the thermal initiator and the monomer are tuned to provide both a good alignment and stabilization for the alignment.
In an exemplary implementation, the monomer has liquid crystal properties and is 4- (3-acryloyloxypropyloxy) -benzoesure-2-methyl-1, 4-phenylester； the azo dye is sulfonic azo dye tetrasodium5, 5'- ( (1E, 1'E) - (2, 2'-disulfonato- [1, 1'-biphenyl] -4, 4'-diyl) bis (diazene-2, 1-diyl) ) bis (2-hydroxybenzoate) ( “SD1” ) ； and the thermal initiator is 2-cyano-2-propyl dodecyl trithiocarbonate. It will be appreciated that in other exemplary implementations, other materials may be used.
In one example, the process of making the composite photoalignment layer begins with mixing the monomer and azo dye at optimal relative concentrations of 50:50 (since the molecule length of the azo dye and the monomer is approximately the same) . Then, the thermal initiator at 5％wt/wt of the monomer is added to the mixture. The mixture is further dissolved in a solvent (e.g., dimethylformamide or other polar solvents) . It will be appreciated that in other exemplary implementations and that with other materials, other relative concentrations of materials may be used.
In an exemplary implementation, the concentration of the azo dye and monomer combined is 1％wt/wt of the solvent, whereas the concentration of the thermal initiator is 5％wt/wt of the monomer. It will be appreciated that in other exemplary implementations and that with other materials, other relative concentrations of materials may be used.
A process for preparing a composite photoalignment layer for aligning liquid crystal molecules includes: mixing, in solution form, a monomeric material, a thermal initiator, and an azo dye material； coating the mixed solution onto a substrate to form a thin film； exposing the thin film with polarized light to impose a single-domain or multiple-domain alignment； and heating the thin film to form a solid thin film. Exposing and heating the thin film may be performed simultaneously as part of a single step or sequentially in separate steps. The thermal polymerization  caused by heating the thin film does not affect the alignment properties (such as anchoring energy and surface uniformity) of the composite photoalignment layer.
Figure 7 depicts a schematic example of this process. As shown in Figure 7, a mixture of SD1 azo dye, monomer and thermal initiator, in solution form, is spin coated onto a substrate at stage 701 so as to form a thin film at stage 702. Then, at stage 703, the thin film is exposed in a single step exposure that provides alignment for the composite photoalignment, and at stage 704, the thin film is heated at 230℃for 30 minutes, so as to form a solid thin film having the SD1 molecules and a polymer network formed from the monomers at stage 705. In particular, the polymerization of the monomeric material in the composite photoalignment layer causes the composite photoalignment layer to form a solid thin film, and polymerization of the monomeric material provides high liquid crystal anchoring energy (e.g., ～10-3J/m2) . It will be appreciated that the monomeric material may be fully polymerized in accordance with exemplary embodiments of the invention.
The particular level of the anchoring energy may be tuned based on the irradiation dosage. For example, an anchoring energy in the range of 10-4J/m2 to 10- 2J/m2 may be achieved (e.g., approximately on the order of magnitude of 10-4J/m2 or 10-3J/m2) . In another example, an anchoring energy of approximately 3x10-3J/m2 may be achieved. Further, it will be appreciated that the anchoring energy may be tuned within the range of 10-4J/m2 to 10-2J/m2 by adjusting the irradiation dose.
In an exemplary implementation, the composite photoalignment layer provides electro-optical characteristics that are the same or similar to conventional polyimide alignment layers. In an example, the voltage holding ratio for an electrical controlled birefringence liquid crystal cell having the composite photoalignment layer is greater than 99％for a frame rate of 60Hz.
In an exemplary implementation, the composite photoalignment layer provides alignment quality that is comparable to conventional and commercially available alignment layers.
In an exemplary implementation, the composite photoalignment layer was demonstrated as being thermally stable in that it did not reveal any traces of degradation after thermal exposure at 100℃ for 24 hours in an oven. As shown in Figures 8 and 9, the TVCs for exemplary display cells having the composite photoalignment layer were unaffected after the thermal exposure. Figure 8 shows the TVCs for an exemplary TN display cell before and after thermal exposure. Figure 9 shows the TVCs for an exemplary ECB nematic display cell before and after thermal exposure. The alignment quality of the exemplary display cells were also unaffected by the thermal exposure, as was apparent from visual inspection.
The composite photoalignment layer was also demonstrated as being optically stable and did not show any degradation after photo exposure to a light source with 400J/cm2 of energy at a wavelength of 450 nm. As shown in Figures 10 and 11, the TVCs for exemplary display cells having the composite photoalignment layer were unaffected after the photo exposure. Figure 10 shows the TVCs for an exemplary TN display cell before and after the photo exposure. Figure 11 shows the TVCs for an exemplary ECB nematic display cell before and after the photo exposure. The alignment quality of the exemplary display cells were also unaffected by the photo exposure, as was apparent from visual inspection.
In an exemplary implementation, the composite photoalignment layer manifests low RDC voltage, e.g., under 10mV in an example where a DC soak of 10V is performed for an hour at 60℃. Figure 12 shows the time-dependence of the RDC voltage of an exemplary composite photoalignment layer after stress of 10V for 1 hour.
In an exemplary implementation, during the single step exposure at stage 703 of Figure 7, a phase mask is used to provide two or more alignment domains with distinct alignment directions in neighboring domains for the composite photoalignment layer. As a result, the irradiated substrate provides multi-domain alignment that is stable and resistant to thermal and photo exposure, while having  high quality optical and electrical parameters. An example of the optical texture of a multi-domain alignment having a checker board pattern with a characteristic size of 20μm is depicted in Figure 13.
Exemplary embodiments of the invention thus provide a composite photoalignment layer with full polymerization of the monomer, while providing acceptable values for residual DC voltage, image sticking parameter, and voltage holding ratio. In an example, a composite photoalignment layer with full polymerization of the monomer provides a minimum and acceptable residual DC voltage value of 0.008 V, a minimum and acceptable image sticking parameter ratio of 1.01, and a minimum and acceptable voltage holding ratio of more than 99％at 60℃ and 60Hz frame frequency.
All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.
The use of the terms “a” and “an” and “the” and “at least one” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The use of the term “at least one” followed by a list of one or more items (for example, “at least one of A and B” ) is to be construed to mean one item selected from the listed items (Aor B) or any combination of two or more of the listed items (Aand B) , unless otherwise indicated herein or clearly contradicted by context. The terms “comprising, ” “having, ” “including, ” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to, ” ) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the  specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as” ) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.

Claims (20)

  1. A composite photoalignment layer for aligning liquid crystal molecules, comprising:
    a monomeric material；
    a photoinitiator； and
    an azo dye material.
  2. The composite photoalignment layer according to claim 1, wherein the composite photoalignment layer is configured to be exposed to a polarized light source for imposing a single-domain or multi-domain alignment on the composite photoalignment layer and polymerizing the monomeric material to form a solid thin film.
  3. The composite photoalignment layer according to claim 1, wherein the composite photoalignment layer is coated onto a substrate surface.
  4. The composite photoalignment layer according to claim 3, wherein the composite photoalignment layer is configured to be coated onto the substrate surface via spin coating, doctor blading, or screen printing.
  5. The composite photoalignment layer according to claim 1, wherein the photoinitiator is 1-hydroxycyclohexyl phenyl ketone.
  6. The composite photoalignment layer according to claim 1, wherein the monomeric material is a liquid crystalline reactive mesogen.
  7. The composite photoalignment layer according to claim 1, wherein the azo dye material is sulfonic azo dye tetrasodium5, 5'- ( (1E, 1'E) - (2, 2'-disulfonato- [1, 1'-biphenyl] -4, 4'-diyl) bis (diazene-2, 1-diyl) ) bis (2-hydroxybenzoate) .
  8. The composite photoalignment layer according to claim 1, wherein a concentration of the photoinitiator is between approximately 1％ wt/wt and approximately 10％ wt/wt of the monomer material.
  9. A method for preparing a composite photoalignment layer for aligning liquid crystal molecules, comprising:
    mixing, in solution form, a monomeric material, a photoinitiator, and an azo dye material；
    coating the mixed solution onto a substrate to form a thin film； and
    exposing the thin film to polarized light to form a solid thin film.
  10. The method according to claim 9, wherein exposing the thin film is a single step exposure that provides both alignment and polymerization for the composite photoalignment layer.
  11. The method according to claim 9, wherein the polarized light is from a polarized light source having one or more major wavelength components.
  12. A composite photoalignment layer for aligning liquid crystal molecules, comprising:
    a monomeric material；
    a thermal initiator； and
    an azo dye material.
  13. The composite photoalignment layer according to claim 12, wherein the composite photoalignment layer is configured to be exposed to a polarized light source for imposing a single-domain or multi-domain alignment on the composite photoalignment layer, and to be heated for polymerizing the monomeric material to form a solid thin film.
  14. The composite photoalignment layer according to claim 12, wherein the thermal initiator is 2-cyano-2-propyl dodecyl trithiocarbonate.
  15. The composite photoalignment layer according to claim 12, wherein the monomeric material is 4- (3-acryloyloxypropyloxy) -benzoesure-2-methyl-1, 4-phenylester.
  16. The composite photoalignment layer according to claim 12, wherein the azo dye material is sulfonic azo dye tetrasodium5, 5'- ( (1E, 1'E) - (2, 2'-disulfonato- [1, 1'-biphenyl] -4, 4'-diyl) bis (diazene-2, 1-diyl) ) bis (2-hydroxybenzoate) .
  17. The composite photoalignment layer according to claim 12, wherein the monomeric material, the thermal initiator, and the azo dye material are dissolved in a solvent.
  18. The composite photoalignment layer according to claim 17, wherein the concentration of the azo dye material and the monomeric material combined is 1％ wt/wt of the solvent.
  19. The composite photoalignment layer according to claim 12, wherein a concentration of the thermal initiator is approximately 5％ wt/wt of the monomer material.
  20. A method for preparing a composite photoalignment layer for aligning liquid crystal molecules, comprising:
    mixing, in solution form, a monomeric material, a thermal initiator, and an azo dye material；
    coating the mixed solution onto a substrate to form a thin film；
    exposing the thin film to polarized light to impose a single-domain or multi-domain alignment； and
    heating the thin film to polymerize the monomeric material and form a solid thin film.
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Publication number Priority date Publication date Assignee Title
CN109722081A (en) * 2017-10-31 2019-05-07 香港科技大学 It is used to prepare the mixture and method of the homeotropic alignment layer of the light orientation of liquid crystal device
CN109722081B (en) * 2017-10-31 2022-03-18 香港科技大学 Mixtures and methods for preparing photo-aligned homeotropic alignment layers for liquid crystal devices

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