Fast-response Liquid Crystal Displays

After about five decades of extensive material research and device development, followed by massive investment in manufacturing technology, thin-film-transistor liquid-crystaldisplay (TFT-LCD) has finally become the dominant flat panel display technology. Nowadays, LCD performances, such as viewing angle, contrast ratio, and resolution, have reached acceptable levels. The remaining major technical challenges are response time, light efficiency, and sunlight readability. Fast response time is desired to reduce motion blur and to enable field sequential color displays using red (R), green (G), and blue (B) LEDs (light emitting diodes) without noticeable color breakup. Sequential RGB colors would eliminate the commonly used spatial color filters which in turn enhances light efficiency and resolution density by ~ 3X. In this dissertation, several new approaches for achieving fast-response LCDs are explored. From material viewpoint, the most straightforward approach for achieving fast response time is to employ a thin cell gap with high birefringence and low viscosity liquid crystal (LC). We investigated the thin cell approach theoretically and experimentally. Voltage shielding effect and anchoring energy effect of alignment layers are found to play important roles on operating voltage and response time. Simulations are carried out to understand the underlying physics and confirm the experimental results quantitatively. Another approach to realize fast response time is to explore novel device configuration. Here, we proposed a dual fringing-field switching (DFFS) mode in which small LC domains are iv formed following the distribution of fringing fields. Therefore, it exhibits submillisecond response time without using thin cell or overdrive/undershoot voltage method. The response time of the DFFS mode is ~20X faster than a conventional vertical aligned LCD. In addition, high optical efficiency is achieved from the complementary top and bottom active LC domains. Two transmissive and one transflective LCDs using DFFS mode are conceived and their electrooptical properties investigated. A shortcoming of DFFS LCDs is their fabrication complexity. To keep the advantages of this fast-response mode while avoiding the requirement of double-TFTs and pixel registration, we modified the device structure to transflective LCD which uses a single TFT in each pixel and vertical aligned positive dielectric anisotropy LC. Two types of electrodes are considered: fringing-field switching (FFS) and in-plane switching (IPS). Besides fast response time and high transmittance, such a transflective LCD shows good sunlight readability. As nematic LC is gradually approaching to its limit in term of response time, polymerstabilized blue phase (PSBP) LCD is emerging. It has potential to become next-generation display because of following revolutionary features: submillisecond response time, no need for alignment layer, good dark state and symmetric viewing angle, and cell gap insensitivity if IPS electrode is employed. In this dissertation, we studied the material-property correlation of Kerr effect-induced birefringence in nano-structured PSBP LC composites. Furthermore, a new device configuration of BP LCD with corrugated electrodes is proposed to solve two critical technical issues: high driving voltage and relatively low transmittance. The on-state voltage can be reduced from ~35 Vrms to ~10 Vrms which will enable TFT addressing, and the transmittance is improved from ~65% to ~85%. This new device configuration will accelerate the emergence of v PSBP LCD. Wide view is another important requirement for a high-end display. Several new LCD configurations with negative A-plate and biaxial plate as phase compensation films are proposed to achieve wide view and broadband operation. The underlying working principles are studied and detailed display performances are included in this dissertation

Dispersion relation on the Kerr constant of a polymer-stabilized optically isotropic liquid crystal

The dispersion relation on the Kerr constant (K) of a polymer-stabilized isotropic phase (PSIP) liquid-crystal (LC) composite is investigated. Our experimental results show that K decreases as the wavelength (lambda) increases. The single-band birefringence dispersion model is used to fit the lambda K values of the PSIP LC composite. Very good agreement between the experiment and physical model is obtained

Transflective display using a polymer-stabilized blue-phase liquid crystal

A wide view, submillisecond response, and single-cell-gap transflective display using a blue-phase liquid crystal (BPLC) is proposed. To balance the optical phase retardation between transmissive (T) and reflective (R) regions, in-plane protrusion electrodes are formed with different gaps in the two regions. This display exhibits reasonably high optical efficiency and well matched voltage dependent transmittance and reflectance curves. Using biaxial films and broadband wide-view circular polarizers, the viewing angle with 100:1 contrast ratio is obtained over the entire viewing cone in the T region, and 10:1 over 50 degrees in the R region. The potential application is emphasized

Low voltage and high transmittance blue-phase liquid crystal displays with corrugated electrodes

A low voltage ( \u3c 10 V) and high transmittance (similar to 85.6%) polymer-stabilized blue-phase liquid crystal (BPLC) display is proposed. The periodic corrugated electrodes generate a strong horizontal field component to induce isotropic-to-anisotropic transition in the BPLC medium through Kerr effect. Moreover, this field is uniformly distributed across the entire LC layer so that the accumulated phase retardation along the beam path is large, resulting in low voltage and high transmittance. This approach enables BPLC to be addressed by amorphous-silicon thin film transistors, which would accelerate its emergence as next-wave display technology

Direct measurement of electric-field-induced birefringence in a polymer-stabilized blue-phase liquid crystal composite

We demonstrate a method to directly measure the electric-fieldinduced birefringence of a polymer-stabilized blue-phase liquid crystal (PS-BPLC) composite. The induced birefringence follows the extended Kerr effect well and is approximately 3X the ordinary refractive index change. The measured data are validated by comparing the simulated and measured voltage-dependent transmittance with an in-plane switching cell. The impact of these results to the material optimization of emerging BPLC displays is discussed

Alignment layer effects on thin liquid crystal cells

Factors affecting the thin cell performance of a liquid crystal cell are analyzed. Examples based on vertically aligned thin cells are given to illustrate these effects. When the cell gap is below similar to 2 mu m, the liquid crystal alignment material, layer thickness, and anchoring energy all play important roles. The first two factors affect the threshold and on-state voltage, while the last one affects the operating voltage and response time. Three reflective liquid crystal cells are studied experimentally. Good agreement between experiment and theory is obtained. (c) 2008 American Institute of Physics

Electro-optics of polymer-stabilized blue phase liquid crystal displays

Electro-optics of polymer-stabilized blue phase liquid crystal displays (BP LCDs) is analyzed and validated experimentally. A numerical model for characterizing and optimizing the electro-optical and display properties of BP LCDs in in-plane switching and fringe field switching cells is developed. The simulated voltage-dependent transmittance curves agree well with the measured results. To lower the operating voltage while keeping a high transmittance, both electrode width and gap, and large Kerr constant make important contributions. A wide-view BP LCD using a single biaxial compensation film is simulated

Negative A-plates for broadband wide-view liquid crystal displays

Negative A-plates fabricated by mechanically stretching polystyrene are studied and their physical properties evaluated. A molecular model is developed to explain the underlying physical mechanisms. An example of a broadband wide-view liquid crystal display including a negative A-plate for phase compensation is illustrated

Submillisecond response nematic liquid crystal modulators using dual fringe field switching in a vertically aligned cell

A fast response nematic liquid crystal (LC) modulator using dual fringe field switching (DFFS) mode is demonstrated. Both top and bottom substrates have pixel and common electrodes to generate complementary fringing fields. The cell consists of a vertically aligned LC layer whose dielectric anisotropy is positive. In a voltage-on state, self-imposed thin LC walls near the center of the slits and electrodes are formed and the surrounding LC molecules exert a strong restoring force. As a result, submillisecond gray-to-gray response time is achieved. Two DFFS cells are used as examples to illustrate the design principles for display and photonic applications. (C) 2008 American Institute of Physics

Extended Kerr effect of polymer-stabilized blue-phase liquid crystals

Electric-field-induced birefringence of a polymer-stabilized blue-phase liquid crystal (BPLC) is investigated. In the low field region, conventional Kerr effect holds. As the electric field increases, the induced birefringence gradually saturates and deviates from Kerr effect. An exponential convergence model, called extended Kerr effect, is proposed to fit the experimental data. Good agreement between experiment and model is obtained. This extended Kerr effect will make a significant impact to the optimization of emerging BPLC display devices