A two dimensional liquid crystal simulation for thin film transistor liquid crystal displays

Abstract

A two-dimensional liquid crystal simulation, whose electrode configuration corresponds to that in a thin film transistor liquid crystal display (TFT-LCD), was carried out. Simulation results show that the lateral field between buslines and pixel electrode forms a reverse tilt domain. This reverse tilt domain leads to the disclination on the pixel electrode. The distance from the pixel electrode edge to this disclination location depends on the dielectric anisotropy and elastic constant for the liquid crystal. A small dielectric anisotropy or large elastic constant makes this distance small.     

Dynamic simulation of Pi-cell liquid crystal displays

The dynamics of π-cell liquid crystal displays was studied by detailed computer simulation. The time evolution of the director configuration and the velocity of flow reveals the mechanism of the fast response of π-cells. The effect of pretilt angle on the dynamics of π-cells was also studied.     

Hysteresis and ferroelectric liquid crystal displays

One cause of the hysteresis in the voltage-transmittance curve is polarization in the alignment layers brought about by the spontaneous polarization of FLC molecules. The hysteresis was reduced by increasing the conductivity of the alignment layers. The structural changes on switching were examined by measuring time-resolved shifts of FT-IR peaks of selected groups. The shifts of selected groups in the IR peak can be attributed to changes in the local dielectric constants of the ambient environment during switching by molecular orbital calculations. The orientation on switching in the SmC* phase was found to be similar to that in the N* phase.     

Dynamic response of super-twisted nematic liquid crystal displays

The dynamics of super-twisted nematic (STN) liquid crystal displays was studied by detailed computer simulation. The time evolution of director configuration and velocity of flow as obtained by solving Ericksen-Leslie hydrodynamic equations. The influence of d/p value and pretilt angle on the dynamic response was also studied. A comparison was also made between twisted nematic and STN liquid crystal displays.     

Electro-optical properties of conic cylindrical-cavities liquid crystal displays

Experimental results have shown that the conic cavities in conic closed-cavity liquid crystal displays (conic CCLCDs) can give a wide viewing angle and grey levels without grey-scale inversion. The purpose of this paper is to extend investigations of conic CCLCDs. In experiments, we describe in detail the fabrication process, microscopic observations and optical measurements for conic CCLCDs. The simulation of director distribution will be based on the tensor form formulation of the free energy density. The differential equations satisfied by the steady-state director distribution are derived from this form of free energy and are solved in conjunction with Poisson's equation. The simulation of the optical propagation and the calculation of the optical transmittance will be based on the extended Jones matrix method. We show by simulation that a normal umbilical structure is the final stable state and that indeed conic CCLCDs can have a wide viewing angle and grey scale without grey-scale inversion. The theoretical results are compared with the experimental results. Transmittance, contrast ratio and viewing angle with various parameters of the LC are measured. Improved iso-contrast contours of the conic CCLCD with a c -plate compensator are also shown. Finally, we have also compared the electro-optical performance and the off axis colour-shift of the CCLCD, the 4-domain VAN device and the 4-domain twist-VAN device.     

Dynamic response of super-twisted nematic liquid crystal displays

The dynamics of super-twisted nematic (STN) liquid crystal displays was studied by detailed computer simulation. The time evolution of director configuration and velocity of flow as obtained by solving Ericksen-Leslie hydrodynamic equations. The influence of d/p value and pretilt angle on the dynamic response was also studied. A comparison was also made between twisted nematic and STN liquid crystal displays.     

Optical transmission of chevron-type ferroelectric liquid crystal displays

Recent X-ray diffraction studies have revealed that the smectic C dielectric in ferroelectric liquid crystal displays quite generally has a chevron structure, instead of the simple bookshelf geometry. The influence of both the tilt of the smectic C layers and the discontinuity of the latter tilt are investigated here. An analytical expression is deduced for the transmission of normally incident light.     

Response speed of in-plane switching mode liquid crystal displays

The response speed of in-plane switching mode liquid crystal displays with three initial director configurations—homogeneous, twisted nematic and 180° super twisted nematic—was investigated via simulations. These simulations studied the relationship between the change in the optical axis and the optical transmission in the three configurations, allowing us to calculate the optical response times. The time-dependent change in the director was calculated using the Erickson-Leslie equations and these two results combined. These results predict that the response time during both the rise and decay periods of a super twist cell is about four times faster than the other two configurations.     

Optimization of film-compensated homogeneous cells for liquid crystal displays

Computer simulations on the display characteristics of polymeric-film-compensated homogeneous liquid crystal cells were performed. Both transmissive and reflective displays were investigated. Results indicate that the biaxial film-compensated reflective displays exhibit small wavelength dispersion, wide viewing angle, high contrast ratio and short response time. Their potential applications in reflective-type colour-sequential projection and direct-view displays are emphasized.     

Preparation and characterization of photo-decomposable ester-containing semi-alicyclic polyimide alignment layers for potential applications in in-plane switching thin-film transistor liquid crystal display devices

Poly(amic acid)s (PAAs) alignment agents have been prepared from the alicyclic dianhydrides, including 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA, I), 1,3-dimethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride (DMCBDA, II), and ester-linked aromatic diamines, including 4-aminophenyl-4′-aminobenzoate (APAB, a) and bis(4-aminophenyl)terephthalate (BPTP, b), respectively. The derived PAAs, including PAA-I_a (CBDA-APAB), PAA-I_b (CBDA-BPTP), and PAA-II_a (DMCBDA-APAB) exhibited the preferable molecular weights, while the PAA-II_b (DMCBDA-BPTP) showed the low one due to the low polymerization reactivity both for the DMCBDA dianhydride and the BPTP diamine. All the PAA solutions except PAA-II_b were adopted as the alignment components to fabricate liquid crystal (LC) minicells with a mode of in-plane switching (IPS). The polyimides (PI) alignment layers derived from the thermal dehydration reaction of the PAA precursors at 230°C for 30 min showed good alignments effects to the LC molecules, which exhibited the pretilt angles (θ_p) from 0.09 to 0.15° after being exposed by the linear polarized ultraviolet light sources with the wavelength of 254 nm. In addition, the PI alignment layers afforded good optoelectronic features to the minicells, including the voltage holding ratio values over 97% at room temperature, and the residual direct circuit voltages lower than 1.0 V. The anchoring energy results indicated that the PI-II_a (DMCBDA-APAB) alignment layer showed the highest interaction with the LC molecules, and thus exhibited the highest threshold voltage (V_th) in the voltage-transmittance (V-T) measurements for the minicells.     

Concept of power consumption saving in transflective liquid crystal displays

This study proposes the concept of power consumption saving in transflective liquid-crystal displays (TR-LCDs). This concept is based on the design of a TR-LCD with three different display modes, namely, transmissive (T), reflective (R) and transflective (TR). Among the three, the TR mode is most often used to operate the designed LCD. To conserve power and extend the battery life, the designed LCD can be switched to operate in T/R mode in an environment with dim/stable ambient light. The highest operation voltages of both T and R modes are less than half of that of the TR mode. Moreover, the strong and weak surface anchoring energies that affect TR-LCD performance and the various pretilt angles of LCs are discussed. Moreover, the operating voltage (3.7 V) to obtain the good dark state of the three modes is independent of the errors of cell gap fabrication and surface anchoring energy, and temperature effect. The concept can be applied to portable electronic gadgets in which power consumption is an important issue.     

Cholesteric liquid crystal displays as optical sensors of barbiturate binding

The influence on the optical properties of cholesteric liquid crystal displays (LCDs) was examined for neutral molecule binding by mesogen/receptors in the mesomorphic phase. The motivation was to prepare neutral molecule sensors that use a colour change to signal analyte binding. A receptor that binds barbiturate analytes was modified with two or one cholesteryl groups to yield compounds 2 and 3, respectively. LCDs were prepared by incorporating one of the receptor/mesogen compounds into a cholesteric LC blend along with a potential H-bonding guest. The optical properties of the LCDs were then determined by measuring the absorbance of the displays. For various LCDs, the colour of the display depended upon several factors: the amount of guest molecule used, the number of cholesteryl side chains on the receptor and the mole concentration of receptor/mesogen in the blend. In particular, complementary host/guest binding of H-bonding analytes by the bis(cholesteryl) receptor 2 in a cholesteric LCD caused a change of up to +70 nm, which was observed by the naked eye as a blue-to-orange colour change. Control experiments confirm that the colour of an LCD is a consequence of molecular recognition in the mesomorphic phase.     

Cholesteric liquid crystal displays as optical sensors of barbiturate binding

The influence on the optical properties of cholesteric liquid crystal displays (LCDs) was examined for neutral molecule binding by mesogen/receptors in the mesomorphic phase. The motivation was to prepare neutral molecule sensors that use a colour change to signal analyte binding. A receptor that binds barbiturate analytes was modified with two or one cholesteryl groups to yield compounds 2 and 3, respectively. LCDs were prepared by incorporating one of the receptor/mesogen compounds into a cholesteric LC blend along with a potential H‐bonding guest. The optical properties of the LCDs were then determined by measuring the absorbance of the displays. For various LCDs, the colour of the display depended upon several factors: the amount of guest molecule used, the number of cholesteryl side chains on the receptor and the mole concentration of receptor/mesogen in the blend. In particular, complementary host/guest binding of H‐bonding analytes by the bis(cholesteryl) receptor 2 in a cholesteric LCD caused a change of up to +70 nm, which was observed by the naked eye as a blue‐to‐orange colour change. Control experiments confirm that the colour of an LCD is a consequence of molecular recognition in the mesomorphic phase.     

Step-shaped electrode for low-voltage and high-optical-efficiency blue-phase transflective liquid crystal displays

A step-shaped electrode design is proposed for Polymer Stabilised Blue Phase (PSBP) Transflective LCD (TR-LCD). This new electrode design is capable of delivering very high optical efficiency (both T and R) of >95% with operation voltage of ~13 V (or >~90% with ~9 V). The step-shaped electrode helps generate strong horizontal electric fields across the whole cell and suppress the formation of dead zone.     

Spontaneous three-dimensional nanostructure formation of perfluoroalkyl terminated liquid crystal: a molecular dynamics simulation study

Structure formation of a perfluoroalkyl terminated liquid crystal molecule was studied by molecular dynamics simulations. Two distinct structures with smectic-C-like layers and with bundles (blocks) of collapsed layers were spontaneously formed depending on the simulation temperatures. The bundles in the latter structure were somewhat positionally ordered (with respect to the small angle spots in its structure function) and orientationally isotropic overall even though the molecules making each bundle were well oriented. These characteristics of the simulated system well correspond to the cubic phase of the real system, and an even more precisely correspond to the proposed cubic structure model with respect to its hierarchical structure.     

Improving the electro-optical properties of bistable reflective cholesteric liquid crystal displays

We have improved the electro-optical properties of a bistable cholesteric liquid crystal display (Ch-LCD) that is driven by a 3+2 dynamic driving scheme (3+2 DDS). The best contrast ratio is achieved at the selection period of 1.2-1.5 ms/line and the temperature range 26-30°C. The suitable preparation period and evolution period for contrast ratio are 40 times and 20 times the selection time, respectively. In the 3+2 DDS, rubbing the homeotropic alignment layer increases reflectivity of the ON state and keeps the reflectivity of the OFF state at the same level, so reflectivity and contrast ratio alike increase as a result. However, in a delayed homeotropic reset driving method, when the homeotropic alignment layer is rubbed, the reflectivity of both the ON and OFF states increases, thus the contrast ratio decreases. The combination of driving method and aligned surface morphology influences the relaxation mechanism in the cholesteric texture. By optimizing panel condition, we have demonstrated an 8.4' foldable VGA Ch-LCD that exhibits high reflectivity and contrast ratio with an addressing speed of around 1.2 ms/line.     

Synthesis and characterization of fluorescent acenequinones as dyes for guest-host liquid crystal displays

Syntheses and spectroscopic properties of alkoxy-substituted para-acenequinones are reported. These compounds showed excellent alignment in nematic liquid crystals as evidenced by polarized UV-vis absorption and fluorescence measurements. [structure: see text]     

Computer simulation of a real liquid crystal

We carried out a computer simulation study of a liquid crystal using semi-empirical atom-atom potentials (the Lennard-Jones 6-12 modification) with nematic 4-ethoxybenzylidene-4-n-butylaniline as an example. The two stages of the calculations were (1) simulation of the structure of an isolated molecule and (2) Monte Carlo simulation of liquid crystal phase. The energy of the system was calculated as the sum over all atomic pair interactions. The molecular structure preferred in the nematic phase is discussed.     

New functional polymers for liquid crystal displays review of some recent developments

Polymeric materials play an important role in the construction and performance of liquid crystal displays. New functional polymers are developed to improve the displays on brightness, power efficiency and viewing angle performance. Polymer films with a controlled molecular structure and architecture provide new polarization optics to convert non‐polarized light into polarized light with higher efficiency than the traditional polarizers based on stretched iodine doped poly(vinyl alcohol). Other films provide a polarization compensation function to maintain an angular invariant net optical retardation of the display device and therewith a better contrast to wider viewing angles. Color generation by non‐absorbing methods is believed to improve on the display brightness. Special control of the liquid crystal alignment by photo‐sensitive orientation layers, polymer protrusions or photo‐formed polymer walls provide multiple director patterns within a single pixel to average out angular LC effects.