Electro-optical Performance Research Articles

Influence of nano-doping on electro-optical performance of liquid crystals

Influence of nano-doping on electro-optical performance of liquid crystals

Preparation of Highly Durable Reverse-Mode Polymer-Stabilized Liquid Crystal Films with Polymer Walls.

Reverse-mode polymer-stabilized liquid crystal (PSLC) films have wide applications in smart windows for cars as well as buildings and dimming glasses due to their low haze, low energy consumption, and better safety in case of emergency power off. However, PSLC films usually have poor stability of electro-optical properties due to their low polymer content (ca. 5 wt %), and it still remains a challenging task to improve the stability and processability by increasing the polymer content in PSLC as the driving voltage might dramatically increase. In this work, a reverse-mode PSLC film with polymer walls was prepared, which showed excellent stability of electro-optical properties even after 150 000 cycles. The film was prepared through polymerization with a photomask, in which the monomers concentrated on specific areas to form patterned polymer walls. In this way, the polymer content could be increased dramatically and the anchoring effect would not be too strong, thus avoiding a sharp increase in the driving voltage. As a result, the desired reverse-mode film with high stability, relatively low driving voltage, and high contrast ratio was obtained. The effects of monomer compositions, curing temperature, UV light intensity, and the pattern of the photomask on the microstructures, as well as electro-optical performances of the films were carefully studied. This work provides a new idea for the preparation of reverse-mode electrically switchable light-transmittance controllable films with excellent stability and good electro-optical performance, which would broaden their application in smart cars, building windows, and dimming glasses for light management and potential energy saving.

Bistable Cholesteric Liquid Crystal Films with Excellent Electro-Optical Performance and Spacing Stability for Reflective Displays

A bistable cholesteric liquid crystal display offers the Bragg selective reflection over the incidence of light to address various needs in energy conservation, long image storage time, and no backlight; yet, most of the existing technologies still suffer from unstable cell spacing in the curved state. In this work, a bistable cholesteric liquid crystal film with polymer spacer columns for flexible displays was prepared by stepwise UV light polymerization. By investigating the effects of cross-linking agent concentration and liquid crystal content, a formulation composition with optimal electro-optical performance was determined. Then, the influences of different process conditions for mask polymerization on the morphologies of polymer spacer columns and the electro-optical performance were systematically investigated. The results show that films with polymer spacer columns exhibit better electro-optical performance and spacing stability compared to conventional bistable liquid crystal films. Therefore, the bistable liquid crystal films reported in this paper show broad application prospects, especially in flexible displays.

Design, Fabrication and Measurement of Full-Color Reflective Electrowetting Displays

We designed, fabricated and measured full-color, reflective electrowetting displays (EWDs). The display system is composed of three-layer cyan, magenta and yellow EWD elements fabricated with standard photolithographic techniques. The EWDs were driven successfully by the proposed control system and the measurement results show that the electro-optical performance was improved. The aperture ratio of the EWD element can be tuned from 0 to ∼80% as the applied voltage is changed from 0 to 30 V. The response time and the color gamut were measured to be ∼18 ms and ∼58% NTSC, respectively. This paper makes it possible for large numbers of reflective full-color EWDs to be fabricated directly, with advantages of saving power significantly by 85% and no eye irritation compared with LED displays.

Electro-Optic Optimization of Porcine Collagen through Protein-Amicable Supercritical Treatment

The advanced field of bioelectronics bridges the electronic world and biological systems, opening a new area for More than Moore and stirring a wave of exploration in biomaterials. However, previously reported biomaterials usually suffer from poor purity and inferior electro-optic performance, discouraging their further development in bioelectronics. Here, porcine collagen extracted from pigskin through supercritical carbon dioxide is utilized as a dielectric layer in capacitors. A protein-amicable and low-temperature supercritical fluid (LT-SCF) strategy is also explored to enhance the electro-optic figure of merits. With the assistance of supercritical CO2 fluids, free amino acids that might have a detrimental effect on the electro-optic performance of the devices have been eliminated. After LT-SCF, the electrical characteristics involving current–voltage and capacitance–voltage become more stable. The leakage currents of capacitors treated with LT-SCF drop by 15 times. The capacitance and conductance also have declining tendencies, contributing to reduced power consumption. Regarding optical characteristics, transparency and refractive index are also substantially enhanced. And numerous material analyses further verify the role of supercritical CO2 fluids. More notably, the biodegradability of collagen films can be achieved with fast water dissolution. Overall, this low-temperature and protein-amicable supercritical fluid technology brings endless possibilities to explore collagen-based bioelectronics.

Synthesis of new type of nonlinear optical chromophores: The introduction of a novel aromatic amine donor 1-oxajulolidine to enhance the electro-optical activity of organic second-order nonlinear optical materials

Developing new organic second-order nonlinear optical chromophores with a new electron donor is crucial for enhancing performance of electro-optic (EO) materials. A series of new second-order nonlinear optical chromophores (J1, JS and JO) with new electron donor 1-Oxajulolidine have been designed and synthesized. Ref1, Ref2 and Ref3 with N, N-diethylaniline (DEA), 4-anisole and julolidine as electron donor had also been synthesized for comparison. The result of TGA and DSC tests suggested that the decomposition temperature of the chromophores was higher than 260 °C. As the UV–vis absorption spectra of chromophores in solution, maximum absorption (λmax) of the chromophores J1 with 1-Oxajulolidine group presented red-shift compared to Ref1 and Ref2, but blue-shift to Ref3. Nevertheless, chromophore with 1-Oxajulolidine presented more obvious solvatochromic behavior from dioxane to dichloromethane. Density functional theory had been used to calculate HOMO-LUMO energy gaps and β of these chromophores. The r33 values of guest-host EO polymers Ref1/APC, J1/APC, JS/APC and JO/APC with a loading density of 10 wt% were obtained as 7.5, 9.6, 15.3 and 13.5 pm V−1, respectively. The above result clearly indicated that r33 of J1/APC (9.6 pm V−1) was 27% higher than that of Ref1/APC (7.5 pm V−1). The electro-optic coefficient of JS with the same conjugate bridge and acceptor was higher than that of FTC in the same doping concentration. As revealed by the above results, the introduction of the polycyclic structure of 1-Oxajulolidine, containing two electron-rich atoms (N and O), could efficiently enhance the electro-optical performance. Furthermore, λmax of JS in doped film showed blue-shift compared with FTC which exhibited a promising potential for tackling nonlinearity and transparency trade-off in second-order nonlinear optical (NLO) materials.

Engineering Electro-Optic BaTiO3 Nanocrystals via Efficient Doping.

Electro-optic (EO) modulators provide electrical-to-optical signal conversion relevant to optical communications. Barium titanate (BaTiO3 ) is a promising material system for EO modulation in light of its optical ultrafast nonlinearity, low optical loss, and high refractive index. To enhance further its spontaneous polarization, BaTiO3 can be doped at the Ba and Ti sites; however, doping is often accompanied by ion migration, which diminishes EO performance. Here, donor-acceptor doping and its effect on EO efficiency are investigated, finding that La-doped BaTiO3 achieves an EO coefficient of 42 pm V-1 at 1kHz, fully twice that of the pristine specimen; however, it is also observed that, with this single-element doping, the EO response falls off rapidly with frequency. From impedance spectroscopy, it is found that frequency-dependent EO is correlated with ion migration. Density functional theory calculations predict that the ion-migration barrier decreases with La3+ doping but can be recovered with further Mn2+ doping, a finding that prompts to prevent ion migration by incorporating Mn2+ into the Ti-site to compensate for the charge imbalance.

Self-Aligned Liquid Crystals on Anisotropic Nano/Microstructured Lanthanum Yttrium Strontium Oxide Layer.

We propose an efficient alignment method for liquid crystals (LCs). A brush-coating method handles film deposition and LC alignment treatment simultaneously herein, meaning a reduction in the conventional alignment layer treatment process steps. A lanthanum yttrium strontium oxide (LaYSrO) film prepared by the sol–gel process was used for the alignment layer. Topographical details of the brush-coated LaYSrO films (compared with spin-coated films) were investigated by atomic force microscopy. Spin-coated LaYSrO meant that the film formation alone without orientation treatment represented an isotropic surface. On the other hand, the 270 °C-cured brush-coated LaYSrO showed nano/microstructure with directionality. It indicates that brush-hair sweeping induced shearing stress on the sol state of the LaYSrO, which results in surface anisotropy for LC alignment. The uniform LC alignment state was confirmed by polarized optical microscopy and pretilt analysis. The brush-coated LaYSrO shows fine optical transparency compared to plain and indium-tin-oxide coated glasses, and thermal stability up to 150 °C for LC alignment. Competitive electro-optical performances of the brush-coated LaYSrO were verified in a twisted-nematic LC system compared to those of the conventionally used polyimide layer. Consequently, we expect that the brush-coating process can be an innovative technology for LC alignment.

Electro-optical performance of all solid state electrochromic devices with NaF electrolytes

• The NaF film is prepared by e-beam evaporation. • The structure, morphology and optical properties of NaF films are characterized. • The ECDs with the structure of ITO glass/WO 3 /NaF/NiO/ITO are assembled. • The ΔT of the ECD at wavelength of 660 nm is approximately 36.3%. • The t c and t b are 18.2 s and 10.6 s, respectively. Among various energy efficient technologies, electrochromic device can improve energy efficiency via controlling the transmittance of solar energy. However, the high costs, scarce resources on earth, toxicity of Li + based electrolytes still block the commercialization of electrochromic devices. Enlightened by the previous extensively investigated Na + based electrochemical systems, we attempt to study the application of Na + based solid state electrolytes in all solid state electrochromic devices in this study. The structural, morphological and optical performance of NaF films are characterized. The electrochromic devices show fast response (10.6 s for bleaching and 18.2 s for coloring) and high coloration efficiency (112.1 cm 2 C -1 ). This study presents a significant advance in solid state electrolytes and provides new avenues for high-performance electrochromic devices.

The regulation of electro-optical properties and polymer morphology of polymer-dispersed liquid crystal containing POSS microstructure by click chemistry and UV intensity

In this work, the thiol‐ene click reaction was used to prepare polymer-dispersed liquid crystal films by a photo-initiated polymerization reaction. Different content of DET was introduced into the polymer matrix to enhance the electro-optical performance of PDLC. Our results showed that the introducing of DET significantly improved the electro-optical properties of PDLC. The SEM results showed that larger and uniform pores were observed with increasing DET content. And it was found that an increase in UV intensity resulted in a continuous decrease in Toff and an increase in CR for weakening the anchoring effect of the polymer matrix.

Functional Organic Materials for Photovoltaics: The Synthesis as a Tool for Managing Properties for Solid State Applications.

The continuous increase in the global energy demand deeply impacts the environment. Consequently, the research is moving towards more sustainable forms of energy production, storage and saving. Suitable technologies and materials are fundamental to win the challenge towards a greener and more eco-friendly society. Organic π-conjugated materials, including small molecules, oligomers and polymers are a wide and versatile class of functional materials with great potentiality, as they can be used as active matrixes in the fabrication of lightweight, flexible, cheap and large area devices. Their chemical and physical properties, both at a molecular level and mainly in the solid state, are a result of many factors, strictly related to the conjugated structure and functional groups on the backbone, which control the intermolecular forces driving solid state aggregations. The synthesis, through the molecular design, the choice of conjugated backbone and functionalization, represents the first and most powerful tool for finely tuning the chemico-physical properties of organic materials tailored for specific applications. In the present review, we report an overview of our works focused on synthetic methodologies, characterization, structure-properties correlation studies and applications of organic materials designed for energy-involving solid-state applications, organic photovoltaics in particular. The impact of functionalization on electro-optical properties and performance in device are discussed, also in relation to the specific applications.

Improving PDLC electro-optical performance using a photomask multi-step polymerisation method

ABSTRACT Polymer-dispersed liquid crystals (PDLCs) have been intensively studied for their light scattering properties and are of great use in electro-optical display devices. In this paper, high performance PDLCs were prepared by a multi-step polymerisation method using a photomask through a polymerisation-induced phase separation (PIPS) process. The effect of the photomask on the polymerisation of polymerisable monomers was investigated and it was confirmed that the photomask could effectively polymerise the polymerisable monomers in the light-transmitting fraction preferentially to form polymer-dominated spacer columns. In the subsequent complementary polymerisation process, the liquid crystal content of the previously unpolymerised fraction increases, which leads to a decrease in threshold and drive voltage and an increase in contrast of the PDLC film. Compared to conventional polymerisation methods, the multi-step polymerisation method can reduce the threshold voltage to 38.5 V, the drive voltage to 55.8 V and the contrast to 42, which greatly improves the overall electro-optical properties of PDLC, and the formation of spacer columns improves the bonding of the film, offering new possibilities for achieving energy efficient PDLC films and flexible films.

Electromechanical properties and photoluminescence of Er3+-doped PMN-33PT single crystal

Based on the successful growth of large-sized Er3+-doped PMN-PT single crystal, the phase composition, electromechanical properties and photoluminescence of [001]C-oriented Er3+-doped PMN-33PT crystal were investigated in detail. The results show that the crystal coexists in rhombohedral and monoclinic phases at room temperature. The crystal possesses excellent piezoelectric (d33 ∼ 2103 pC/N), dielectric (ε33T ∼ 7589), and electromechanical coupling properties (k33 ∼ 0.91), accompanied by a higher phase transition temperature and larger coercive field compared with that of PMN-33PT single crystal. This indicates an improved electromechanical performance induced by Er3+ doping. Additionally, the crystal achieves upconversion and downconversion luminescence, and a strong dependence of the upconversion emission on temperature is demonstrated. Furthermore, a comparable optical temperature sensing performance based on the fluorescence intensity ratio of the upconversion luminescence is verified. These results demonstrate a promising multifunctional material. The electro-optical coupling performance of the Er3+-doped PMN-33PT crystal is under evaluation.

Steric group-based polymer dispersed liquid crystal composite films with high contrast ratio, low driving voltage and small hysteresis

The unique phase-separated microstructure has endowed polymer dispersed liquid crystals (PDLCs) with outstanding electro-optical performances. However, their applications are hampered by the high driving voltage and low contrast ratio. Attempts to optimize the materials have been made in order to reduce the driving voltage without sacrificing the contrast ratio and other features. Herein, acrylate monomers with different steric end groups were introduced to fabricate PDLCs and their effects on the electro-optical properties and microstructure were investigated. Bipolar droplet configuration was observed in all samples implying tangential anchoring conditions. It was found that the steric end group could affect the morphology of polymer matrix and the electro-optical properties. By carefully adjusting the steric group and doping concentration, the contrast ratio could be enhanced and the driving voltage could be reduced. Besides, hysteresis could also be decreased. The research offers more choices for the construction of PDLCs from material perspective, promoting their potential applications.

The role of nanomesh fibres loaded with BaTiO3 nanoparticles on the electro-optical performance of PDLC devices

In this paper, polymer-dispersed liquid crystal (PDLC) films were prepared from a blend of prepolymer, liquid crystal and BaTiO3 (BTO) nanoparticle-laden nanofibres films by a polymerisation-induced phase separation (PIPS) method. By contrasting the microstructure and electro-optical curves of various proportions of monomer types, it was demonstrated that the hole size of the polymer network had a significant influence on the electro-optical nature of the PDLC films. nanofibres films loaded with different BTO nanoparticle contents were introduced into the liquid crystal mixture specimens, and the alterations in the polymer network, electro-optical nature of these specimens were analysed using SEM images, EDS and electro-optical curves. The electro-optical nature of the nanofibres films doped with PDLCs were found to be improved and the ageing resistance was greatly enhanced. Therefore, the development of PDLC films with faster response times and better electro-optical nature of reticular nanofibres films doped with PDLCs will significantly improve the promising technological applications of PDLC-based equipment for optical windows, displays, energy storage and flexible devices.

Ridge waveguide lasers with vertically stacked quantum wells and tunnel junctions

We report on the fabrication and electro-optical performance of ridge waveguide (RW) lasers emitting near 905 nm based on a vertical layer structure consisting of three single quantum wells (SQWs) as active regions separated by two tunnel junctions (TJs). The single waveguide core is designed in such a way that the third vertical mode is lasing. The width of the ridge is varied between 3 µm and 200 µm. Deep- and shallow-etched ridges were fabricated. The shallow etching stopped slightly above the topmost SQW, whereas the deep etch went through all TJs and stopped slightly above the lowest SQW. For comparison, we also fabricated conventional shallow-etched RW lasers based on a similar vertical structure having only one SQW and no TJs. The TJ RW lasers with deep-etched ridges provide the highest output power, lowest threshold current and best slope efficiency of 2.4 W A−1 for a ridge width of 6 µm. The performance of shallow-etched TJ RW lasers is lowest for small ridge widths but surpasses the performance of the conventional RW lasers when the ridge width is increased. The lateral far field angle of deep-etched TJ RW lasers is about 2° larger compared to the shallow etched ones.

Optically isotropic nano-size encapsulation of nematic liquid crystals with a high-filling factor

• Oil-in-water encapsulation gives nanosize drops that can be coated on substrates. • Optically isotropic film can switch to an anisotropic state by voltage application. • Highly improved electro-optical performances are in a good agreement with extended Kerr effect. The filling factor of LCs in the LC-polymer composite system is a crucial factor for the electro-optic performance of optically isotropic liquid crystal (OILC) devices. The polymerization-induced phase separation method is well-known method to fabricate the OILCs; however, this method spontaneously limits the filling factor by ∼ 40%, which is inadequately low, owing to low-LC concentration in the mixtures. To overcome such intrinsic problem, we propose an encapsulated LC which is fabricated by means of oil-in-water encapsulation. Through this approach, the filling factor of LCs is effectively increased up to 67.8% because the capsules are closely packed. Consequently, we achieve a thin-composite film of nano-size LC encapsulation whose isotropic state is reliably switched to anisotropic state in response to an external electric field with low-operation voltage ( E th = 1.4 V/μm and E op = 4.6 V/μm), fast-response time ( τ on = 0.5 ms, τ off = 2.2 ms), and hysteresis-free electro-optic behavior. The highly improved electro-optic performances of the proposed encapsulated LC device would give versatile applications in blooming free-form flexible displays and various tunable photonic systems.

Physicochemically Constructed Unidirectional Aluminum Bismuth Gallium Zinc Oxide Film for Enhanced Nematic Liquid Crystal System Using a Brush‐Coating Method

We propose a convenient brush coating method for liquid crystal (LC) alignment. This method enables film deposition and an alignment layer treatment process simultaneously. Aluminum bismuth gallium zinc oxide (AlBiGaZnO) is used as the alignment layer. After the curing process, a unidirectional AlBiGaZnO film is formed; its surface morphology and chemical composition were verified using scanning electron microscopy and X-ray photoelectron spectroscopy. This oriented structure of the surface was produced by shear-stress which originated from brush movement. That structure induced a surface anisotropic characteristic and resulted in a uniform LC alignment. The uniform and homogeneous LC alignment state on the film was confirmed using polarized optical microscopy and pre-tilt angle analysis. The brush coated AlBiGaZnO film exhibited excellent thermal budget for advanced LC system. The film exhibited enhanced electro-optical performance with a low operating voltage. These results demonstrate the potential of LC alignment technology via the brush coating method.

Role of semiconductor nanoparticles on electro-optical performance of PDLC devices

ABSTRACT The performance of polymer dispersed liquid crystal (PDLC) can be adjusted according to the needs of the application by controlling its composition ratio and microstructure. The clatter-based PDLC film with polymerisation-induced phase separation was prepared by ultraviolet curing method, and semiconductor nanoparticles were doped in it. The effects of semiconductor nanoparticles doping on the morphology, electrical and optical properties of PDLC films were investigated. Research showed that the addition of semiconductor nanoparticles reduced the rise time, threshold voltage, saturation voltage, and enhances contrast. The development of PDLC doped with semiconductor nanoparticles’ film which has faster rise time, lower threshold voltage, lower saturation voltage and better contrast will significantly increase the potential of PDLC-based devices in smart windows, sensors and flexible equipment application. At the same time, semiconductor nanoparticles had the characteristics of high fluorescence efficiency, wide absorption, narrow emission and bright colours. When PDLC doped with semiconductor nanoparticles is used as smart glass, the optical properties of semiconductor nanoparticles can be used to absorb light in part of the wavelength band and convert it into light in another wavelength band. In addition, it can have variable colours to make the appearance more beautiful while expanding the scope of its use.

Reflectance-enhanced liquid crystal displays and thermochromic multi-color patterning

This paper presents a reflective liquid crystal display (LCD) combined with thermochromic liquid crystal microcapsules to enhance the electro-optical performance of the LCD and to empower new driving modes. The prepared reflectance-enhanced liquid crystal display (Re-LCD) possesses a bilayer structure, with the upper layer being a polymer-stabilized liquid crystal layer prepared by UV-induced phase separation and the lower layer being a thermochromic layer prepared by coating with thermochromic liquid crystal microcapsules. The effect of chiral compound content on the reflection wavelength of liquid crystal center was investigated. After that, the effects of monomer ratio, polymerization time and spin coating speed on the electro-optical properties and morphology of Re-LCD were investigated comprehensively by three sets of experiments. The results show that the combination of LCD and thermochromic materials has a synergistic effect in display, and the prepared Re-LCD shows more than 51% reflectance compared with typical LCD. The multi-field driving function allows the Re-LCD to rapidly switch optical states by force, electric, and temperature fields and obtain multi-color patterns during the driving process. This provides new application possibilities for LCD.