Liquid Crystal Photoalignment
Vladimir G. Chigrinov
Hong KongUniversity of Science and Technology
CWB, Kowloon, Hong Kong,
Email:
ABSTRACT
Photoalignment possesses obvious advantages in comparison with the usually “rubbing” treatment of the substrates of liquid crystal display (LCD) cells. The liquid crystal photoalignment is nano-technology, as the thickness of the alignment layer is about 2-15 nm. The photoalignment materials can be very useful for the new generation of the liquid crystals displays.The physical mechanisms and characterization of the photoaligning technique, as well as the application for liquid crystal devices, including displays and photonic deviceswill be reviewed.
INTRODUCTION
Photoalignment has been proposed and studied for a long time. In fact, the subject of light-molecule interactions has been a fascinating subject of research for a long time and is still capturing the imagination of many people. Light is responsible for the delivery of energy as well as phase and polarization information to materials systems. In this particular case, the alignment of the molecules takes place due to a partial ordering of the molecular fragments after a topochemical reaction of a photoselection (Weigert`s effect). While the first photo-patterned optical elements, based on polyvinyl-cinnamate films, appeared in 1977, the technology became a subject of interest for LCD only in the beginning of the 90’s. It was soon shown that these materials could provide high quality alignment of molecules in a LC cell. Over the past twenty years, a lot of improvements and variations have been made for photoalignment. Commercial photoalignment materials are now readily available. Many new applications, in addition to the alignment of LCD, have been proposed and demonstrated. In particular, the application of photoalignment to active optical elements in optical signal processing and communications is currently a hot topic in photonics research.
Photoalignment possesses obvious advantages in comparison with the usually “rubbing” treatment of the substrates of liquid crystal display (LCD) cells. Possible benefits for using this technique include:
(i) Elimination of electrostatic charges and impurities as well as mechanical damage of the surface;
(ii) A controllable pretilt angle and anchoring energy of the liquid crystal cell, as well as its high thermo and UV stability and ionic purity;
(iii) Possibility to produce the structures with the required LC director alignment within the selected areas of the cell, thus allowing pixel dividing to enable new special LC device configurations for transflective, multi-domain, 3D and other new display types;
(iv) Potential increase of manufacturing yield, especially in LCDs with active matrix addressing, where fine tiny pixels of a high resolution LCD screen are driven by thin film transistors on a silicone substrate;
(v) New advanced applications of LC in fiber communications, optical data processing, holography and other fields, where the traditional rubbing LC alignment is not possible due to the sophisticated geometry of LC cell and/or high spatial resolution of the processing system;
(vi) Ability for efficient LC alignment on curved and flexible substrates;
(vii) Manufacturing of new optical elements for LC technology, such as patterned polarizers and phase retarders, tunable optical filters, polarization non-sensitive optical lenses, with voltage controllable focal distance etc.
The technique of azo-dye photoalignment does not involve any photochemical or structural transformations of the molecules. As well, the new photoaligning films are robust and possess rather good aligning properties such as anchoring energies and voltage holding ratios. They can be very useful for the new generation of the liquid crystals devices as well as in new photovoltaic, optoelectronic and photonic devices based on highly ordered thin organic layers. Examples of such applications are light emitted diodes (OLED), solar cells, optical data storage and holographic memory devices. The novel and highly ordered layer structures of organic molecules may exhibit certain physical properties, which are similar to the aligned LC layers.The brief summary of our research results is given below. The results have been published (see the Reference List).
Superthin polarizers, new types of phase retarders, patterned polarizers and retarders
We demonstrated excellent photoalignment capability of the sulfonic azo-dye SD1 for polymerizable liquid crystals (PLC) whose anisotropic films are extensively studied as basic elements of many passive optical devices (retardation films, polarizers, color filters, etc.). These dyes exhibit a high affinity to various substrates and insolubility in polymer liquid crystals (PLC) and organic solvents commonly used for their dilution. Extremely low exposure dose (less than 50 mJ/cm2) is needed to induce an excellent planar alignment of PLC. No sign of alignment deterioration is observed after a photopolymerization action. This altogether implies that new photoalignment materials may be effectively used for the industrial production of optical films based on PLC. They eminently suit for the continuous manufacturing process of such films. Parallel and twisted positive A plates as well as splayed O plates can be produced for the needs of TN and STN LCD compensation. The SD1 layers can also be effectively used to align dyed PLC particularly needed for dichroic polarizers. The alignment of neat and dyed PLC can be easily patterned that is topical problem of patterned compensation films and polarizers requested by modern developments of LCD (multidomain wide viewing angle LCD, transflective displays, 3D imaging, etc.). A simple preparation of photoalignment layers and PLC films suggests to realize manufacturing of optical films in the form of assembly belt. It can be further simplified for flexible substrates, when continuous roll to roll process can be easily arranged. It can be basically used to manufacture any functional film containing aligned PLC layer or a stack of them.
We fabricated a superthin polarizer using LCP mixture UCL017 (DIC, Japan) and the dichroic azo-dye dye AD1, using rubbed polyimide (PI) layer. The dichroic ratio was 15 at =490 nm. Patterned polarizers and phase retarders using polymerized LC (UCL-011) from DIC (Japan) were also fabricated.
We have demonstrated a photoalignment method for generating a wide range of liquid crystal pretilt angles (20-900) as well as anchoring energy of the pretilt alignment (0.2-1.610-3 J/m2). This method has the advantages of simplicity, wide process window and low exposure dose of deep UV light. It is also time-saving because the baking process is carried out in-situ with the deep UV alignment process. As well it is a non-contact method in which rubbing is not required. Based on this method, we attained reactive mesogene (RM) alignment with pretilt angle controllable in, practically, full range from 0o to 90o. Optically these RM layers act as positive O films, which transform to positive A film and positive C film at pretilt angles 0o and 90o, respectively. The structure of RM layer is preserved in a solidification process. This allowed us to obtain solid O films with controllable tilt angle of optic axis. The prepared O films are confined between two aligning substrates or coated on one aligning substrate. In the first case, RM alignment is uniform along the film normal, while in the second case splayed structure is formed, which depends on the content of RM mixture. In spite of the use of glass substrates in our experiments, the approach is applicable to plastic substrates. Moreover, RM film can be extracted to use separately from the alignment substrates. We believe that the developed method will be useful for optimization of LCD angular characteristics with O compensation films.
We have demonstrated a method for generating a wide range of liquid crystal pretilt angles. This method has the advantages of simplicity, a wide process window, and a low exposure dose of UV light. It is also a nonrubbing method and is based on photoalignment. The process time is short because the baking process is carried out in situ with the UV alignment process.
Photopatterned phase retarder was made for transflective LCD using SD1 photoalignment layer. We have optimized a photo-alignment layer to suit for making in-cell retarders and achieved high thermal stability of the layer comparable to that of conventional rubbed PI alignment layer. Patterning resolution using an optimized photo-alignment was about 5 m. Thermal stability is substantially improved and was comparable to polyimide (PI). Sensitivity was also improved (≦50mJ/cm2). Adhesion and resistance to solvent were good. Basic processes were not changed, so roll-to-roll process of patterned retarders production for transflective LCD is under way.
Vacuum deposition provides optically much more uniform films than spin coating with a higher dichroic ratio. Thick (500-1000 nm) uniform films can be obtained. High CR (35-40) can be achieved in thin films (d<200 nm), while high CR in thick films is still a problem. Superthin internal polarizers based on solid photoaligned dichroic films are envisaged.
Thus we provided a (i) method of manufacturing a photoalignment layer, which comprises coating a material for the photoalignment layer, containing dichroic azodye with one or more sulfo groups promoting good adhesion, on a substrate, and exposing the coated layer to polarized light, thereby imparting the photoalignment function; (ii) photoalignment layer of good quality on different kinds of substrates commonly used in LCD industry; (iii) alignment patterns with desirable shape and size of patterns and desirable orientation of PLC alignment in the film plane; (iv) a photoalignment layer insoluble in PLC and solvents conventionally used to form PLC films; (v) a photoalignment layer providing high quality alignment of PLC with sufficient thermal and photo stability to preserve alignment of PLC during polymerization; (vi) a photoalignment layer with low exposure dose needed to impart alignment function. The anisotropic optical films based on PLC films aligned by the said photoalignment layers may be used as optical retarders and compensators, polarization converters, interference filters, dichroic polarizers and information storage media.
The in-cell retarders were fabricated by in-situ photo polymerization of liquid crystalline monomers. For this, a photoalignment layer was used to align the monomers. We have optimized
materials of photo-alignment layers to improve the thermal stability. Liquid crystalline monomers (UV-curable liquid crystals (UCLs)) were also improved. In addition, the patterned retarders using the optimized photo-alignment layer were developed. The application in a high performance transflective LCD is envisaged.
We extended extension of ion/plasma beam technique for the patterned alignment of reactive mesogens (RM). We showed that this technique can be successfully used for the alignment of the patterning RM films. A variety of prototypes of patterned retarders and dichroic polarizers was prepared.
New materials for photoalignment, photoaligned polarizers and phase retarders. Mechanism of photoalignment
Various new methods were applied to investigate the mechanism of the photoalignment, like spectral ellipsometry and IR spectroscopy. In spectral ellipsometry the Lorentz oscillator model was applied to isotropic and photo-induced anisotropic layers. The resonances were close to absorption ones. Obtained results confirm proposed model of pure reorientation of azo dye molecules under the action of the linearly polarized UV light. The wavelength dispersions of refractive indices of the investigated azo dye mixture as well as the thicknesses of the layers have been accurately extracted and reported. Obtained optical properties of the azo dye SD1/SDA2 mixture can be used for simulation of the optimal parameters of the LC devices. The method of IR spectroscopy helped to explain the photoalignment process from the molecular point of view through hydrogen bonds associates and photoinduced mesophase of azo-dye molecules. The novel considerations on the molecular level comment on the importance of steric factor and linear geometry of the molecule and bring light to the functional role of the specified radical groups at the molecular ends. On the basic of new considerations the photoalignment properties of new azo-dye CD1 were predicted and confirmed in the experiment.
We have discovered the azo-dye CD1, which has very similar structure to SD1 and exhibits excellent photo alignment quality both for nematic (TN, STN, VAN, IPS) and ferroelectric LCs. The order parameter S of CD1 is equal to -0.24 at = 394 nm. The azimuthal anchoring energy determined by measuring deviation angle in TN cell filled with LC MLC-12100-000 was about 0.989 10-4 J/m2, which is nearly equal to the anchoring energy obtained for SD1 (1.6 10-4 J/m2). Power of the UV radiation was 3 mW/cm2 and exposure time - 30 minutes. The cells were possessed a high thermal stability of LC alignment. We will have a possibility to tune the alignment properties like (anchoring energy, ability to align LC materials, image sticking, light sensitivity, photo stability, reorientation speed etc.) by preparing the proper weight ratio of the azo-dye compound, if we have the required azo-dye materials, differing in the position and the orientation of the limited number of groups like: –COOH, –NaSO3 and –CF3 to tune the intermolecular interactions. Thus a molecular engineering of the photoalignment layers becomes possible.
We have verified various azo-dye structure and showed the field of the highly effective sulfuric azo dyes is broader than we conceived. We have investigated different azo-dyes that covered almost all the wavelengths starting from 389 nm (SD-1) to 600 nm (Acid Blue). Dyes named BY, CBY and DR and others show excellent photoalignment properties. The detailed investigations on BY is a strong evidence for good alignment on various liquid crystals. Taking into account of high photo and thermal alignment stability one can offer it for various applications. The evidence of these dyes open up the path for using different dyes for effective photoalignment.
Several new azo-dyes were verified aiming at the application in superthin polarizers and phase retarders. The dyes were synthesized by several groups in Ukraine and Belarus. The azo-dyes are capable to cis-trans transformations in dye doped LC mixtures. A new class of LCD is envisaged based on this principle. We plan also to continue our cooperation wit the chemists aiming at new substances with -OH and -COOH terminal groups for a higher stabilization of the photosensitive film for LCD optical elements.
We have developed the photo-alignment layer possessing high thermal stability comparable to that of the conventional polyimide (PI) alignment layer. We have recently found that the performances of the photo-alignment layer on the plastic substrates are different from those on glass substrates. Since the application of the alignment layer to a plastic substrate is important from the practical point of view, the photo-alignment layer and the manufacturing processes have been investigated and optimized to improve uniformity, thermal stability and adhesion.
A new effect of the combined action of an electric field and light on the slow surface dynamics in a layer of nematic liquid crystal (NLC) contacted with a layer of dye pre-treated by UV irradiation was described. After turning off both the electric field and light, extremely slow relaxation of the system to the initial state was observed. This effect depends on a number of control parameters (applied voltage, intensity of light, time of application, dose of a preliminary UV irradiation). The critical slowing down of this process (up to some weeks) via a proper choice of control parameters was established. The physical processes responsible for the combined effect were considered and applied to modify a previously proposed phenomenological model for the electrically induced slow azimuthal rotation of the easy axis of an NLC. The modified model was found to be in a qualitative agreement with the main experimental results. The effect is important as a proper control of anchoring energy of a photoaligned layer is very important for LC device.
We studied both theoretically and experimentally kinetics of photoinduced ordering in azo-dye films. From the absorption-vs-incidence angle curves measured for the probe light linearly polarized parallel and perpendicular to the plane of incidence we obtain dependence of the principal extinction coefficients on the irradiation dose. The transient photoinduced structures were found to be biaxial. The measured irradiation time dependence of the absorption order parameters is interpreted by using the generalized diffusion model.
We studied both theoretically and experimentally switching dynamics in surface stabilized ferroelectric liquid crystal (SSFLC) cells with asymmetric boundary conditions created by a photoaligned layer on only one of the substrates. In these cells the bounding surfaces were treated differently to produce asymmetry in their anchoring properties. It was found that the asymmetry effects were dominated by the polar contribution to the anchoring energy. The polar anchoring parameter was estimated by making a comparison between the results of modeling and the experimental data for various switching regimes.
New types of LC devicess based on photoalignment technology, such as e-paper, transflective and 3D displays and others
A new transflective LCD with single cell gap consisting (OCB) and low twist nematic modes has been studied. Such a configuration does not include double cell gap structure and only one more step UV exposure is needed during photoalignment process to produce domains, thus, the fabrication process is easy. Based on this new configuration, a transflective LCD with high brightness and high contrast could be obtained. Moreover, the TV and RV curves are very close, which means similar gray scale can be obtained under same operating voltage. Very effective transflective LCDs with the same cell gap and optical birefringence, but different twist angles made by photoalignment in transmissive and reflective regions with very high contrast ratio were also proposed. Tranflective LCDs with the single cell gap, same LCD mode, but patterned retarders were also investigated.