Residual Direct Current Voltage Research Articles

Formation of polymerizable layer on a photo-alignment layer of liquid crystal cell and its image sticking characteristics

The effect for polymerizable layer on a vertical-type photo-alignment layer was investigated from the point of image sticking characteristics. Fabrication process of the polymerizable layer is followings; photo-alignment treatment is firstly carried out by exposing polarized ultraviolet (UV) from the oblique direction. After construction of the liquid crystal (LC) cell, the LC material including monomers is injected, and unpolarized UV is exposed for formation of the polymerizable layer. For avoiding pretilt angle shift during the exposure, fast rate constant of polymerization is necessary, and mixed monomers 4,4’-dimethacryloyl-oxy-biphenyl (4,4’-DMABiph) and 4,4’-dimethacryloyl-oxy-benzilketal (4,4’-DMABzk) are appropriate for obtaining small level of image sticking parameters. As the image sticking parameters, residual direct current voltage (V rDC) and difference in pretilt angle (Δtilt) before and after application of alternate current voltage were evaluated, and the results of V rDC and Δtilt indicated that 6% 4,4’-DMABzk in the mixed monomers 4,4’-DMABiph and 4,4’-DMABzk would be useful for obtaining the small level of image sticking.

Monomolecular vertical alignment layer with room temperature processibility for flexible liquid crystal displays

In this article, we disclose a vertical alignment material using alkylphosphonic acid for liquid crystals that only requires simple room temperature treatments to secure good vertical alignment for liquid crystals. Alkylphosphonic acid forms a monomolecular layer on the substrate providing strong vertical alignment. The proposed alignment layer shows decent electro-optical properties, such as high voltage holding ratio, low residual direct current voltage, low image sticking parameter, etc. Later, we combine the alkylphosphonic acid with photo-rewritable azo-dye to fabricate a hybrid alignment of nematic cells and study its stability and anchoring energy. The material shows great compatibility with flexible substrates. Thus, the proposed alignment layer of alkylphosphonic acid can potentially find application as the alignment layer for flexible liquid crystal displays.

Formation of polymer-sustained-alignment layer of a liquid crystal cell exposed by light-emitting-diode visible light

A new fabrication technique and mixed monomers for a polymer-sustained-alignment liquid-crystal (PSA-LC) cell were proposed in this study. The proposed process for formation of the PSA layer is exposing visible light for polymerization instead of exposing ultraviolet (UV) light. The backlight consisted of light emitting diode (LED) for the LC display (LCD) was used as the source of the visible light, and the mixed monomers were 2,7-dimethacryloyl-oxy-phenanthrene (2,7-DMAPhen) and 4,4’-dimethacryloyl-oxy-benzil (4,4’-DMABzl). Since the monomer 4,4’-DMABzl could generate radicals by a hydrogen abstraction reaction under exposure of the visible light, the PSA layer was formed under the LED-backlight exposure through an intermediary of a polarizer. The fabricated PSA-LC cell showed equal level of voltage holding ratio, residual direct current voltage, and difference in pretilt angle before and after application of an alternate current voltage to the PSA-LC cell fabricated by the UV light exposure. Therefore, we expect that future PSA-LCD would be fabricated without exposure of the UV light.

Image sticking and long-term stability of polymer-sustained-alignment liquid crystal cell produced from the mixed monomers containing phenanthrene- and benzophenon-moieties

As a model of polymer-sustained-alignment liquid crystal display (PSA-LCD), the LC cells having the PSA layers (PSA-LC cells) formed by mixed monomers of 2,7-dimethacryloyl-oxy-phenanthrene (2,7-DMAPhen) and 4,4’-dimethacryloyl-oxy-benzophenon (4,4’-DMABenz) were fabricated, and image sticking parameters and long-term stability under LED-backlight exposure were investigated. Residual direct current voltage (V rDC) and difference in pretilt angle (Δtilt) before and after application of alternate current voltage were evaluated as the image sticking parameters. The V rDC was improved with the addition of a small amount of 4,4’-DMABenz because the concentrations of radical and/or ion in the LC layer had decreased significantly. It was shown that with the addition of 3.8 wt% of 4,4’-DMABenz in the mixed monomers, V rDC can be kept low without increasing Δtilt. Voltage holding ratio was maintained high under the LED-backlight exposure for 1,000 h, indicating that the PSA-LC cell shows high long-term stability. Hence we expect that the proposed PSA-LC cell would be useful for high luminance LCDs.

Physical Properties of Thermally Crosslinked Fluorinated Polyimide and Its Application to a Liquid Crystal Alignment Layer.

This study demonstrated the use of a thermally crosslinked polyimide (PI) for the liquid crystal (LC) alignment layer of an LC display (LCD) cell. Polyamic acid was prepared using 4,4′-oxydianiline (ODA) and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA). The 6FDA−ODA-based polyimide (PI) prepared by the thermal cyclic dehydration of the polyamic acid (PAA) was soluble in various polar solvents. After forming a thin film by mixing trifunctional epoxide [4-(oxiran-2-ylmethoxy)-N,N-bis(oxiran-2-ylmethyl)aniline] with the 6FDA−ODA-based PAA, it was confirmed that thermal curing at −110 °C caused an epoxy ring opening reaction, which could result in the formation of a networked polyimide not soluble in tetrahydrofuran. The crosslinked PI film showed a higher rigidity than the neat PI films, as measured by the elastic modulus. Furthermore, based on a dynamic mechanical analysis of the neat PI and crosslinked PI films, the glass transition temperatures (Tgs) were 217 and 339 °C, respectively, which provided further evidence of the formation of crosslinking by the addition of the epoxy reagent. After mechanical rubbing using these two PI films, an LC cell was fabricated using an anisotropic PI film as an LC alignment film. LC cells with crosslinked PI layers showed a high voltage holding ratio and low residual direct current voltage. This suggests that the crosslinked PI has good potential for use as an LC alignment layer material in advanced LCD technologies that require high performance and reliability.

Low-temperature properties of nematic liquid crystal materials for display

ABSTRACT The orientation of Nematic liquid crystal (NLC) and order degree of LC molecules are easily affected by ambient temperature, so it is of great significance to explore the low-temperature properties of NLC materials to improve the low-temperature performance of LC devices. The dielectric and elastic constant of the NLC material 1M10500-000 under the high and low-temperature conditions were obtained by the dual-cell capacitance method, linear fitting method and numerical fitting, and were compared. The dynamic response of the NLC at different temperatures was tested, and the rotational viscosity was calculated. The experimental results show that the dielectric constant is still proportional to the order parameter at low temperature. The elastic constants k 11 and k 33 and the square of order parameter no longer conform to the proportional relationship at low temperature, and the lower the temperature, the greater the deviation of elastic constants from the proportional relationship. As the temperature decreases, the ratio of k 33/k 11 changes from 2.62 to 2.71, and the rotational viscosity coefficient changes from 5.66 mPa.s to 177.64 mPa.s. Finally, the transient hysteresis phenomenon at low temperature was proposed and explained by the residual direct current voltage.

Stability of liquid crystal systems doped with γ-Fe2O3 nanoparticles* *Project supported by the Natural Science Foundation of Hebei Province of China (Grant Nos. A2019202235 and A2017202004), Research and Practice Project of Hebei Provincial Higher Education and Teaching Reform (Grant No. 2017GJJG018), Research Projects of Undergraduate Education and Teaching Reform in Hebei University of Technology (Grant No. 201802003), the Key Subject

In order to explore the stability of a liquid crystal (LC) system doped with γ-Fe2O3 nanoparticles, the physical properties (clearing point, dielectric properties), electro-optical properties and residual direct-current voltage (RDCV) of the doped LC system were measured and evaluated at different times. First, the temperature was controlled by precision hot stage, and the clearing point temperature of doped LC was observed and measured by a polarized optical microscope. Using a precision LCR meter, we measured the capacitance-voltage curves of the doped LC system at the temperature of 27 °C. The dielectric constant of doped LC was calculated by the dualcell capacitance method. Then, the electro-optical properties of the doped LC system were measured. Finally, the RDCV of the doped LC system was measured and calculated. After five months, the parameters of the doped LC system were re-measured and analyzed under the same conditions to evaluate its stability. The experimental results show that, within five months, the clearing point change rate of doped LC is in the range of 0.24%–1.37%, the change of dielectric anisotropy is in the range of 0.035–0.2, the curves of electro-optical properties are basically fitted, and the change rate of saturated RDCV is about 11.2%, which basically indicate that the LC system doped with γ-Fe2O3 nanoparticles has good stability.

Flexoelectric Effect on Image Sticking Caused by Residual Direct Current Voltage and Flicker Phenomenon in Fringe-Field Switching Mode Liquid Crystal Display

Although transmittance changes like a quadratic function due to the DC offset voltage in FFS mode LCD, its bottom position and flicker minimum DC offset voltage varies depending on the gray level due to the flexoelectric effect. We demonstrated how the influence of the flexoelectric effect changes depending on the electrode width or black matrix position.

Polymer-sustained-alignment liquid crystal cell produced from the monomer containing benzophenone-moiety and its image-sticking property

ABSTRACT As a model of polymer-sustained-alignment liquid crystal display (PSA-LCD), the LC cells having the polymer layers produced from mixed monomers of 4,4ʹ-dimethacryloyl-oxy-biphenyl (4,4ʹ-DMABiph) and 4,4ʹ-dimethacryloyl-oxy-benzophenone (4,4ʹ-DMABenz) were fabricated and investigated image-sticking phenomenon with evaluations of residual direct current voltage (V rDC) and difference in pretilt angle before and after application of alternate current voltage (∆tilt). A rate constant of the polymerisation became large with an addition of a small amount of 4,4ʹ-DMABenz into 4,4ʹ-DMABiph. The V rDC was effectively improved with the addition of a small amount of 4,4ʹ-DMABenz because the concentrations of radical and ion in the LC layer were decreased due to enlargement for the rate constant of the polymerisation. The Δtilt was small enough only below 14% of 4,4ʹ-DMABenz in the mixed monomers of 4,4ʹ-DMABiph and 4,4ʹ-DMABenz. We confirmed that the PSA-LC cell produced from the mixed monomers of 4,4ʹ-DMABiph and 4,4ʹ-DMABenz with the weight ratio of 4,4ʹ-DMABenz between 9% and 14% was useful for obtaining the small level of image sticking.

Novel fabrication method of polymer-sustained-alignment liquid crystal cell formed from the monomer containing benzil-group

ABSTRACT A new fabrication method and its monomer for a polymer-sustained-alignment liquid-crystal (PSA-LC) cell was proposed in this study. The proposed process for formation of a polymer layer is a method to expose visible light for polymerisation of the monomers. As the visible light, a LED-backlight of a LC display (LCD) was used through the intermediary of a polariser. Therefore, the polymer layer was formed by the exposure of the LED-backlight after construction of the LCD. The proposed monomer is 4,4ʹ-dimethacryloyl-oxy-benzil, which can generate a radical by a hydrogen abstraction reaction for an initiation of the polymerisation under exposure of the visible light. The fabricated PSA-LC cell with the proposed method showed high voltage holding ratio, low residual direct current voltage, and comparably small difference in pretilt angle before and after application of an alternate current voltage. Therefore, we can expect that the PSA-LCD would be fabricated without the exposure process of the UV light. Moreover, the PSA-LCD fabricated by the proposed method would be a small level of image sticking.

Characteristics of voltage holding ratio and residual direct current voltage for polymer-sustained-alignment liquid crystal cells formed from mixed monomers containing biphenyl- and benzilketal-groups

For improving process efficiency of polymer-sustained-alignment liquid crystal displays (PSA-LCDs), new photo-initiation monomers (PI-monomers), 2-methoxy-2-methacryloyl-oxy-ethoxy-2-phenylacetophenon (2-MMOEP) and 2,2-dimethacryloyl-oxy-ethoxy-2-phenylacetophenon (2,2-DMOEP), were developed and investigated image sticking characteristics. LC cells having PSA layers formed from a monomer 4,4′-dimethacryloyl-oxy-biphenyl (4,4′-DMABiph) and each PI-monomer were fabricated, and voltage holding ratio (VHR) and residual direct current voltage (VrDC) were clarified as the image sticking characteristics. The VHR and the VrDC were improved with the LC cells having the PSA layers formed from 4,4′-DMABiph and 2,2-DMOEP owing to large rate constant of polymerization. In contrast, the VHR and the VrDC were deteriorated with the LC cells having the PSA layers formed from 4,4′-DMABiph and 2-MMOEP. The deterioration of the VHR and the VrDC was derived from the increase in the concentration of radical and/or ion generated from 2-MMOEP. The results indicate that the number of polymerizable group in the PI-monomer is important, and plural number of polymerizable group in the PI-monomer leads to high VHR and low VrDC.

Polymer-sustained-alignment liquid crystal cell produced from the monomer containing benzilketal-group and its image sticking property

ABSTRACT As a model of polymer-sustained-alignment liquid crystal display (PSA-LCD), the LC cells having the polymer layers produced from mixed monomers of 4,4ʹ-dimethacryloyl-oxy-biphenyl (4,4ʹ-DMABiph) and 4,4ʹ-dimethacryloyl-oxy-benzilketal (4,4ʹ-DMABzk) were fabricated and investigated image sticking phenomenon with evaluations of residual direct current voltage (V rDC) and difference in pretilt angle before and after application of alternate current voltage (∆tilt). A rate constant of the polymerisation became remarkably large with an addition of a small amount of 4,4ʹ-DMABzk into 4,4ʹ-DMABiph. The V rDC was effectively improved with the addition of a small amount of 4,4ʹ-DMABzk because the concentrations of radicals and/or ions in the LC layer were decreased due to enlargement for the rate constant of the polymerisation. The Δtilt was small enough only below 10% of 4,4ʹ-DMABzk in the mixed monomers of 4,4ʹ-DMABiph and 4,4ʹ-DMABzk. We confirmed that the PSA-LC cell produced from the mixed monomers of 4,4ʹ-DMABiph and 4,4ʹ-DMABzk with the weight ratio of 4,4ʹ-DMABzk between 6% and 10% was useful for obtaining the small level of image sticking.

Evaluation and improvement of image sticking in in-plane switching liquid crystal display

Image sticking in liquid crystal display (LCD) can affect the display quality. The saturated residual direct current voltage (SRDCV) can be obtained by analyzing the change in the capacitance of LC cell under the direct current bias, through which image sticking can be evaluated. On this basis, the image sticking was evaluated in the in-plane switching (IPS) cell filled in the LC with negative dielectric anisotropy for different electrode structures, cell gaps (i.e. 3.8, 5.8, and 7.5 µm), and doping concentrations of γ-Fe2O3 nanoparticles (i.e. 0.068, 0.136, 0.204, and 0.272 wt%). The SRDCV decreases under the same conditions by adjusting the ratio of electrode width and electrode gap and it decreases with the cell gap increases. For the thickness of 3.8 µm, the SRDCV initially decreases and then increases as the doping concentration of γ-Fe2O3 nanoparticles increases; while for the thicknesses of 5.8 µm and 7.5 µm, the SRDCV decreases as the doping concentration of γ-Fe2O3 nanoparticles increases. The results show that the structure design of the IPS-LCD and the doping of nanoparticles can effectively improve image sticking and provide useful guidance for enhancing the display quality of LCD.

Novel fabrication process of polymer-sustained-alignment liquid crystal cell and its image sticking property

ABSTRACT A new fabrication process of the polymer-sustained-alignment (PSA) liquid crystal (LC) cell is proposed in this study. The proposed process for fabrication of the PSA-LC cell is in the following: the mixed solution of the vertical polyimide (PI) alignment layer material and the monomer 4,4ʹ-dimethacryloyl-oxy-biphenyl (4,4ʹ-DMABiph) was firstly prepared, and then the PI alignment layer including the monomer 4,4ʹ-DMABiph was prepared on the substrate. The monomer 4,4ʹ-DMABiph was confirmed to be located on the surface of the PI-film by the TOF-SIMS analysis. The polymerisation of the monomer was conducted by exposure of ultraviolet light to the PI-film. Finally, new PSA-LC cell was fabricated by the assembly of the substrates and injection of the LC material. The residual direct current voltage of the PSA-LC cell produced by the proposed fabrication process was significantly improved compared with the PSA-LC cell produced by the conventional process. Therefore, the proposed fabrication process would be useful for the new-type PSA-LCD.

An innovative technique based on voltammetric profiling and current–voltage characteristics for determination of residual direct-current voltage of liquid crystal cells

ABSTRACTGeneration of residual direct-current (DC) voltage (VrDC) induces a serious image sticking of liquid crystal displays (LCDs). In this study, an innovative technique to determine the VrDC of LC cells was proposed from the current-voltage (I-V) curves obtained by application of triangular voltage. The I-V curve shows the maximum and minimum current peaks due to the rotation of the LC director, and the VrDC is determined from the average value of the two peaks. The VrDC values determined from the proposed technique corresponded to the values determined from the flicker minimisation, which is the well-known method to determine the VrDC. The results indicate that the proposed technique is useful for evaluation of the image sticking of the LCDs.

Decreasing the Residual DC Voltage by Neutralizing the Charged Mobile Ions in Liquid Crystals

The decrease of the residual direct current (DC) voltage (Vrdc) of the anti-parallel liquid crystal (LC) cell using silver (Ag)-doped Polyimide (Ag-d-PI) alignment layers is presented in this manuscript. A series of Ag/PI composite thin layers are prepared by spurting or doping PI thin layers with Ag nano-particles, and Ag/PI composite thin layers are highly transparent and resistive. LC are homogeneously aligned between 2.0 mg/mL Ag-d-PI alignment layers, and the Vrdc of the cell that assembled with Ag-d-PI alignment layers decreases about 82%. The decrease of Vrdc is attributed to the trapping and neutralizing of mobile ions by Ag nano-particles. Regardless of the effect of Ag nano-particles on the conductivity of Ag-d-PI alignment layers, the voltage holding ratio (VHR) of the cells is maintained surprisingly. The experiment results reveal a simple design for a low Vrdc LC cell.

Fabrication of vertical alignment liquid crystal cell without forming a conventional polyimide-alignment layer

ABSTRACTA novel preparation method of the vertical alignment polymer film (VAPF) was proposed by polymerisation of the monomers, 2,3-difluoro-4ʹ-[2-(diacryloyl-oxy-hexyl)methyl ester-hexadecyl-oxy]-biphenyl (DFDAMHB) and 2,7-dimethacryloyl-oxy-phenanthrene (DMAPhen), being dissolved in the liquid crystal (LC) material. For obtaining the vertical alignment (VA), the ultraviolet exposure was carried out to the LC cell below the nematic to isotropic transition temperature of the LC material. The VA mode LC cell with the VAPF produced from the monomers DFDAMHB and DMAPhen exhibited enough level of electro-optical and response properties, high voltage holding ratio and low residual direct current voltage.

Enhancement of Image Quality in LCD by Doping γ-Fe₂O₃ Nanoparticles and Reducing Friction Torque Difference.

Improving image sticking in liquid crystal display (LCD) has attracted tremendous interest because of its potential to enhance the quality of the display image. Here, we proposed a method to evaluate the residual direct current (DC) voltage by varying liquid crystal (LC) cell capacitance under the combined action of alternating current (AC) and DC signals. This method was then used to study the improvement of image sticking by doping γ-Fe2O3 nanoparticles into LC materials and adjusting the friction torque difference of the upper and lower substrates. Detailed analysis and comparison of residual characteristics for LC materials with different doping concentrations revealed that the LC material, added with 0.02 wt% γ-Fe2O3 nanoparticles, can absorb the majority of free ions stably, thereby reducing the residual DC voltage and extending the time to reach the saturated state. The physical properties of the LC materials were enhanced by the addition of a small amount of nanoparticles and the response time of doping 0.02 wt% γ-Fe2O3 nanoparticles was about 10% faster than that of pure LC. Furthermore, the lower absolute value of the friction torque difference between the upper and lower substrates contributed to the reduction of the residual DC voltage induced by ion adsorption in the LC cell under the same conditions. To promote the image quality of different display frames in the switching process, we added small amounts of the nanoparticles to the LC materials and controlled friction technology accurately to ensure the same torque. Both approaches were proven to be highly feasible.

Reduction of residual DC voltage via RC matching in LCD

To reduce the residual direct-current (DC) voltage in liquid-crystal display (LCD) using the polymer-stabilized vertical-alignment (PS-VA) mode, the electrical time constant of two or more serially connected dielectric materials, such as liquid crystals (LCs) and silicon nitride (SiNx), is investigated. PSpice simulation of the alternating-current (AC) voltage changes in the two dielectric materials shows mismatch of the time constant, and a resistance cross capacitance (RC) between the two layers is found to be the source of the residual DC voltage. The optimization of the stoichiometry of SiNx is effective for matching the time constant of LCs because it is easy to control the time constant by changing the gas ratio, e.g., SiH4, N2, and NH3, when SiNx is deposited. We suggest that image sticking in the LCD panel can be minimized via RC optimization between the SiNx and LCs by adjusting the stoichiometry of SiNx, which means that the chronic problem of off-axis gamma distortion in the VA mode for large sized, high-resolution TV up to Quad Ultra High Definition (QUHD, 15360 × 8640) can be solved.

UV-induced vertical alignment liquid crystal cell with two species of monomers having vertical-alignment-induced side-chain group and phenanthrene group

ABSTRACTWe propose the method for formation of the vertical alignment polymer film by polymerisation of the monomers being dissolved in the liquid crystal (LC) material. For obtaining the vertical alignment, combination of the two monomers, 4-(4ʹ-octyloxy-biphenyl-4-yloxy)-butyl ester (AOBBE) and 2,7-dimethacryloyl-oxy-phenanthrene (DMAPhen), was found to be useful.The monomer DMAPhen initiates the polymerisation under UV light exposure, and the AOBBE unit induces the vertical alignment without generating any alignment defects. The monomer DMAPhen is useful for maintaining the high voltage holding ratio and low residual direct current voltage after UV light exposure because the molecules of DMAPhen do not remain in the LC layer.