Showing papers by "Seung Hee Lee published in 2009"
TL;DR: In this paper, a protrusion electrode structure is proposed to dramatically lower the operation voltage of the emerging blue-phase liquid crystal displays (BP-LCDs), which enables the BP-LCD to be addressed by amorphous silicon thin-film transistors (TFTs).
Abstract: A protrusion electrode structure is proposed to dramatically lower the operation voltage of the emerging blue-phase liquid crystal displays (BP-LCDs). Simulation results indicate that the generated horizontal electric field is not only strong but also penetrates deeply into the bulk LC layer. As a result, a low voltage (∼10 Vrms) and reasonably high transmittance (∼70%) BP-LCD can be achieved. This approach enables the BP-LCDs to be addressed by amorphous silicon thin-film transistors (TFTs). Widespread application of TFT BP-LCDs is foreseeable.
267 citations
TL;DR: In this paper, a partitioned wall-shaped electrode configuration was proposed to achieve uniform induced birefringence between electrodes throughout the entire cell gap, reducing the operating voltage by 2.8× with two transistors.
Abstract: Polymer-stabilized blue phase liquid crystal displays based on the Kerr effect are emerging due to their submillisecond response time, wide view and simple fabrication process. However, the conventional in-plane switching device exhibits a relatively high operating voltage because the electric fields are restricted in the vicinity of the electrode surface. To overcome this technical barrier, we propose a partitioned wall-shaped electrode configuration so that the induced birefringence is uniform between electrodes throughout the entire cell gap. Consequently, the operating voltage is reduced by ~ 2.8× with two transistors. The responsible physical mechanisms are explained.
129 citations
TL;DR: In this article, a photo-curable monomer was mixed in an LC host and a bias voltage was applied to reorient the LC directors within each domain to create a small but well-defined pretilt angle.
Abstract: — The recent development of polymer-induced pretilt angle in multi-domain vertical-alignment liquid-crystal (LC) structures is reviewed. To create a small but well-defined pretilt angle, ∼0.1 wt.% of a photo-curable monomer was mixed in an LC host and a bias voltage was applied to reorient the LC directors within each domain. The monomers are polymerized near the substrate surfaces by UV exposure. The formed polymer layers change the surface pretilt angle of the LC from 90° to about 89° with a defined azimuthal orientation. Consequently, within each domain the LC reorientation direction responding to the external field is well-defined which leads to faster rise time and higher transmittance. This new technology overcomes the long standing problems of conventional MVA devices and is therefore expected to play a dominant role in the future.
92 citations
TL;DR: In this article, the patterned vertical alignment (PVA) liquid crystal (LC) mode shows a wide viewing angle and a perfect dark state at a normal direction, however, it is inevitable to avoid the formation of disclinations and the movement of defect points during stabilization of LC's reorientation.
Abstract: The patterned vertical alignment (PVA) liquid crystal (LC) mode shows a wide viewing angle and a perfect dark state at a normal direction. However, it is inevitable to avoid the formation of disclinations and the movement of defect points during stabilization of LC's reorientation. It is due to fact that the LC directors tilt downward in different directions with collisions between them by the fringe-electric field. Consequently, the transmittance decreases and the response time gets slower. In order to overcome this barrier, the pretilt angles in four different directions are introduced on the substrates utilizing UV-curable reactive mesogen (RM) monomers. According to our studies, concentration of RM, UV curing condition, and applied voltage to the cell are critical to achieve an optimized surface-modified PVA mode which provides the well-defined reorientation of the LCs with respect to an electric field. Moreover, morphological behaviors on surface of substrate depending on curing conditions were investigated in order to confirm the existence of the stabilized polymer.
45 citations
TL;DR: It is demonstrated that the dielectrophoretic force dominated the motion in the colloidal fullerenes by a proper analysis of different electrophoretics parameters.
Abstract: This research focuses on the electrokinetic motion of fullerenes suspended in liquid crystal host medium, which are investigated in the homogeneously aligned nematic liquid crystal cells driven by in-plane field. We investigated the effect of electrophoretic and dielectrophoretic forces and related parameters of the colloidal fullerenes in liquid crystals. The electrophoretic mobility, zeta potential, and critical voltage have been evaluated. Fullerenes suspended in liquid crystal medium migrated toward the positive electrode, but were pulled back in the opposite direction when the polarity was reversed especially at low frequency range $(l5\text{ }\text{Hz})$. At higher electric field and higher frequency ranges, the net displacement of fullerenes has been observed. We demonstrate that the dielectrophoretic force dominated the motion in the colloidal fullerenes by a proper analysis of different electrophoretic parameters. In addition, the electrodynamics of fullerenes was explained by applying the theory of the dielectrophoresis and Schwarz's formula. We propose a model to estimate the density of fullerenes suspended in liquid crystal medium.
22 citations
TL;DR: An anomalous electrokinetic dispersion after contraction of carbon nanotube (CNT) cluster suspended in a vertically aligned nematic liquid crystal (LC) cell is demonstrated and the original morphology of the CNT aggregates was restored after the removal of the field where the elongation was fully reversible.
Abstract: An anomalous electrokinetic dispersion after contraction of carbon nanotube (CNT) cluster suspended in a vertically aligned nematic liquid crystal (LC) cell is demonstrated. CNT aggregates were firstly aligned toward the field line by dielectrophoretic (DEP) torque and secondly they were elongated above a certain threshold field due to interaction between the induced dipole moment of CNTs and external ac electric field. Below a certain breakdown field, the original morphology of the CNT aggregates was restored after the removal of the field where the elongation was fully reversible. Above breakdown electric field, CNT aggregates were ruptured and fragmented into small pieces and consequently CNTs were well dispersed in LC medium.
15 citations
TL;DR: In this paper, the effect of rough surface roughening on the electrowetting performance was investigated and the white area fraction (WA) was calculated by calculating theoretically a measure of optical switching by changing the roughness of the hydrophobic surface.
Abstract: Electrowetting is a new class of reflective display based on electric field controlled movement of oil/water interface across a hydrophobic layer. The focus of this paper is to fabricate electrowetting cells on a rough hydrophobic surface and to study its effect on kinetics of electrowetting. The surface roughening found effective in two ways in the design and operation of the electrowetting device: (i) It enhances the coating of photoresist (PR) on the hydrophobic surface, which is normally difficult due to low surface energy of Teflon and (ii) the roughness changes the contact angle of the liquid (oil), which in turn changes the electro-optic switching behavior of the device. The kinetics of optical switch was checked by calculating theoretically the white area fraction (WA%), which is a measure of optical switching in electrowetting display by changing the roughness of the hydrophobic surface. The present study showed that the optical performance found to increase with the increase of roughness of the hydrophobic surface.
12 citations
TL;DR: In this article, twisted nematic liquid crystal (LC) cells doped with carbon nanotubes (CNTs) were fabricated and their electro-optic characteristics were studied, and the effects of CNTs on voltage-dependent transmittance curves and voltage holding ratio were not found to be so strong.
Abstract: Twisted nematic (TN) liquid crystal (LC) cells doped with carbon nanotubes (CNTs) were fabricated and their electro-optic characteristics were studied. The CNTs with a minute amount of doping did not disturb the liquid crystal orientation in the off and on state. Effects of CNTs on voltage-dependent transmittance curves and voltage holding ratio were not found to be so strong. The response time however was improved as compared to pure LC.
11 citations
01 Jun 2009
TL;DR: In this article, an improved electrode structure which has partition-wall shaped electrodes was proposed to increase the horizontal electric field so as to decrease the driving voltage of the polymer-stabilized blue phase liquid crystal display.
Abstract: The polymer-stabilized blue phase liquid crystals are in the limelight of liquid crystal display area because of its fast response and wide viewing angle with alignment layer free. The conventional device is driven by in-plane field, which has thin electrodes only on bottom substrate and also due to low Kerr constant of LC, the driving voltage of the device is very high. We propose improved electrode structure which has partition-wall shaped electrodes. The proposed device has maximized horizontal electric field so that the driving voltage is decreased. Further, adoption of two transistors can lower driving voltage.
8 citations
TL;DR: In this paper, a single-gap transflective liquid-crystal display driven by a fringe electric field in the transmissive (T) region and a vertical electric fields in the reflective (R) region was designed.
Abstract: A single-gap transflective liquid-crystal display driven by a fringe electric field in the transmissive (T) region and a vertical electric field in the reflective (R) region was designed. In the device, a homogeneously aligned liquid crystal (LC) rotates almost in plane by a fringe field in the T-region whereas the LC tilts upwards by a vertical field in the R-region. A high surface pre-tilt angle of the LC in the R-region is achieved through polymerization of an UV curable reactive mesogen monomer at the surfaces and thus the effective cell retardation in the R-region becomes half of that in the T-region. Consequently, a transflective display driven by a vertical and a fringe electric field with a single cell gap and single gamma curves is realized.
7 citations
TL;DR: It is proved that the proposed VA LC cell can improve the viewing angle and contrast ratio by calculating optical characteristics, particularly in diagonal directions.
Abstract: We propose an optical structure for a vertical alignment (VA) liquid crystal (LC) cell with a wide viewing angle. The proposed LC cell consists of an A-plate and two C-plates for optical compensation. Optical compensation and optimization to eliminate off-axis light leakage in the entire visible wavelength range are performed on a Poincare sphere using the Stokes vector and the Muller matrix method. After optimizing the wavelength dispersion of the retardation films that are used, we prove that the proposed VA LC cell can improve the viewing angle and contrast ratio by calculating optical characteristics, particularly in diagonal directions.
TL;DR: In this paper, the temperature dependence of the electro-optic characteristics of wide-viewing-angle LCD modes, such as inplane switching (IPS), multidomain vertical alignment by patterned electrode (PVA), and fringe field switching (FFS), have been studied, and the results showed that the FFS mode has lower temperature dependence compared to the IPS and PVA modes.
Abstract: As the physical properties of nematic liquid crystals vary with respect to temperature, the performances of liquid crystal displays (LCDs) are highly dependent on temperature. Additionally, it is well known that the electro‐optic characteristics of LCDs, such as transmittance and threshold voltage, also rely on the LCD switching modes. The temperature dependence of the electro‐optic characteristics of the wide‐viewing‐angle LCD modes, such as in‐plane switching (IPS), multidomain vertical alignment by patterned electrode (PVA), and fringe‐field switching (FFS), have been studied, and the results showed that the FFS mode has lower temperature dependence compared to the IPS and PVA modes. Since the liquid crystal (LC) reorients in different ways in each mode, this result is associated with the temperature dependence of LC's bend and twist elastic constants, and also with the position of the main reorientation, either in the middle or on the surface of the LC layer.
TL;DR: In this paper, the Kerr effect was applied to liquid-crystal displays (LCDs) for color-sequential displays with RGB light-emitting diodes (LEDs) for the backlight.
Abstract: is a type of quadratic electro-optic effect caused by an electrical-field-induced ordering of polar molecules in an optically transparent and isotropic medium. Liquid-crystal displays (LCDs) based on the Kerr effect are now emerging because they exhibit the following three distinct advantages: (1) the fabrication process is simple and cost-effective because it does not require any alignment layers, (2) the voltage off-state is optically isotropic, which means that the viewing angle is wide and symmetric, and (3) the switching time is in the sub-millisecond range. Therefore, the technology can be used for color-sequential displays without the use of color filters, resulting in a 3X higher optical efficiency and resolution – provided that RGB light-emitting diodes (LEDs) are used for the backlight.
TL;DR: In this article, the authors calculated the optical transmittance of the In-Plane Switching (IPS) liquid crystal (LC) cell with respect to temperature by considering temperature energy term in the Gibb's free energy.
Abstract: In this paper we calculated optical transmittance of the In-Plane Switching (IPS) liquid crystal (LC) cell with respect to temperature. By considering temperature energy term in the Gibb's free energy, we could achieve the temperature parameters that need to calculate the temperature dependence on the electro-optic characteristics. The calculated temperature parameters permit us to calculate scalar order parameter S with respect to temperature, so that finally we could calculate the optical transmittance of the IPS LC cell as a function of temperature.
TL;DR: In this paper, the authors examined the electro-optical characteristics and stability of a liquid crystal director depending on the electrode patterning of a common electrode on the top substrate in patterned vertical alignment (PVA) mode.
01 Jun 2009
TL;DR: In this article, a single gap transflective liquid crystal display driven by fringe electric field in the transmissive (T) region and vertical electric fields in the reflective (R) region was designed with formation of high pretilt angle larger than 45°through polymerization of an UV curable reactive mesogen monomer at the surface in the R region, the effective cell retardation value becomes half of that in the T region where the LCs are homogenously aligned.
Abstract: A single gap transflective liquid crystal display driven by fringe electric field in the transmissive (T) region and vertical electric field in the reflective (R) region was designed with formation of high pretilt angle larger than 45°through polymerization of an UV curable reactive mesogen monomer at the surface in the R region, the effective cell retardation value becomes half of that in the T region where the LCs are homogenously aligned. Consequently, a transflective display driven by a vertical and fringe electric field with a single cell gap and single gamma curves is realized.
Journal Article•
TL;DR: In this paper, an Optically compensated bend liquid crystal display (OCB-LCD) with high surface tilt angle, which was achieved by polymerization of UV curable reactive mesogen monomer under certain voltage, was manufactured and the cell showed bend state initially.
Abstract: Optically compensated bend liquid crystal display (OCB-LCD) has many application fields owing to its fast response time and wide viewing angle. However, in order to operate the OCB-LCD in bend state, this device needs quick transitions from the initial splay state to bend state. Unlike conventional approach using transient high voltage for the transition, the OCB-LCD with high surface tilt angle, which was achieved by polymerization of UV curable reactive mesogen monomer under certain voltage, was manufactured and the cell showed bend state initially. Electro-optic and electrical characteristics of the cell were analyzed. The cell shows a fast response time owing to high surface pretilt angle and very low residual DC less than 0.1 V although another polymer layer is formed above polymer alignment layers.
TL;DR: In this paper, a correlation between the size of electrode pattern and voltage-dependent transmittance (V-T) has been carried out to get an optimized size of the electrode pattern in order to minimize the non-uniformity in the transmittances.
Abstract: A homogenously aligned liquid crystal device driven by fringe-electric field named as fringe-field switching mode, are being widely used for all kinds of liquid crystal displays due to its excellent wide viewing angle and high transmittance characteristics. One of the characteristics is the voltage-dependent transmittance (V–T), which strongly depends upon the pixel electrode width and distance between them. As a result, if pattern size of the pixel electrode at different positions is not the perfectly matched, the non-uniformity in transmittance does occur because field intensity to rotate liquid crystal would not be same at each and every pixel electrode. In this work, correlation between the size of electrode pattern and V–T characteristic has been carried out to get an optimized size of the electrode pattern in order to minimize the non-uniformity in the transmittance.
01 Jun 2009
TL;DR: In this paper, a pixel structure of viewing angle switchable liquid crystal display associated with fringe-field switching (FFS) mode using one panel is proposed, where one pixel is composed of main and sub-pixel, in which both pixels are formed to generate fringe electric field.
Abstract: We propose a pixel structure of viewing angle switchable liquid crystal display associated with fringe-field switching (FFS) mode using one panel. In the device, one pixel is composed of main- and sub-pixel, in which both pixels are formed to generate fringe electric field. However, the fringe field direction is different each other such that in the main pixel, fringe field rotates liquid crystal (LC) making its optic axis deviating from polarizer axis but fringe field only tilts LC upward without rotating in the sub-pixel region. In this way, light leakage can be generated in the sub-pixel, which can block displayed images in an oblique viewing angle.
01 Jun 2009
TL;DR: In this article, the authors studied the electro-optic characteristics of high performance liquid crystal display using combined fringe and in-plane horizontal electric fields, named as fringe and In-plane switching (FIS), which caused more liquid crystals to reorient above and between the pixel electrodes.
Abstract: We have studied electro-optic characteristics of high performance liquid crystal display using combined fringe and in-plane horizontal electric fields, named as fringe and In-plane switching (FIS). The strong electric fields cause more liquid crystals to reorient above and between the pixel electrodes. Moreover, the operating voltage of FIS mode was decreased by using two thin film transistors (TFTs). To suggest novel structure of FIS mode, electro optic characteristics are analyzed as a function of length between pixel electrodes compared to IPS and FFS mode. Consequently FIS mode shows not only higher transmittance but also lower operating voltage than those of IPS and FFS mode while keeping wide viewing angle.
TL;DR: In this paper, the optimal phase retardation of the FFS mode using a liquid crystal (LC) with negative dielectric anisotropy depending on pixel electrode structure was investigated based on the simulation.
Abstract: The fringe-field switching (FFS) mode has unique characteristics such as high transmittance and wide viewing angle and its electro-optic characteristics strongly depend on pixel electrode width (w) and distance (l) between them. In this study, optimal phase retardation of the FFS mode using a liquid crystal (LC) with negative dielectric anisotropy depending on pixel electrode structure was investigated based on the simulation. The results shows that when the width of the pixel electrode and distance between electrodes are 3 μm and 4.5 μm, respectively, the cell shows transmittance 0.87 with optimal phase retardation value of LC cell 0.36 μ. As w and l are decreased to 1 μ and 1.5 μ, the cell shows transmittance of 0.90 with optimal phase retardation of LC cell 0.40 μ, indicating that the light modulation method is changed according to electrode structure.