Seok-Lyul Lee

High performance liquid crystal displays with a low dielectric constant material

We report high performance liquid crystal displays (LCDs), including fringe field switching (p-FFS) and in-plane switching (p-IPS), with a small average dielectric constant (e) but positive dielectric anisotropy material. Our low e based p-FFS and p-IPS LCDs offer several attractive properties, such as high transmittance, low operation voltage, fast response time (even at −20°C), which is particularly desirable for outdoor applications of mobile or wearable display devices, and suppressed flexoelectric effect. Combining these advantages with the inherent outstanding features, such as wide viewing angle, no grayscale inversion, negligible color shift, and pressure resistance, the low e LC based p-FFS and p-IPS are strong contenders for next-generation mobile displays, and high resolution and high frame rate TVs.

Image sticking in liquid crystal displays with lateral electric fields

We propose a kinetic model to account for the nonuniform adsorption and desorption processes in fringe field switching (FFS) and in-plane-switching liquid crystal displays. An equation is proposed to describe the generation mechanism of residual DC voltage and good agreements with experiment are obtained. Based on this model, the mechanisms underlying the formation and relaxation processes of residual DC voltage as well as their dependences on offset DC voltage and temperature are investigated. Moreover, the residual DC voltages of FFS cells employing positive and negative dielectric anisotropy LCs are compared and the physics responsible for the observed difference is explained.

A High Performance Single-Domain LCD With Wide Luminance Distribution

In this paper, we review the device structures and present the simulation results of four single-domain LCDs with a quasi-collimated backlight and a free-form optics-engineered diffuser. Such a non-emissive LCD exhibits similar luminance distribution to an emissive OLED, while keeping high transmittance, high contrast ratio over a large (80 °) viewing cone, with low ambient reflection, indistinguishable color shift, and negligible off-axis grayscale distortion.

Analytical equation for the motion picture response time of display devices

Motion picture response time (MPRT) affects the image blurs of thin-film transistor (TFT) liquid crystal displays and organic light emitting diode (OLED) displays. We derive an analytical equation to correlate MPRT with the liquid crystal (LC)/OLED response time and TFT frame rate. Good agreement between our physical model and experimental results is obtained. Based on our model, we find that if the LCs response time is 2 ms or less, then its MPRT is nearly the same as that of OLED, even if OLEDs response time is assumed to be 0. To achieve MPRT comparable to OLEDs, we developed an ultra-low viscosity LC mixture for the vertical alignment mode operation. The measured average gray-to-gray response time is 0.93 ms, and its MPRT at 120 Hz is 6.88 ms. In comparison, OLEDs MPRT is 6.67 ms. To further shorten MPRT, we could either increase the frame rate or reduce the backlight duty ratio. Pros and cons of these approaches are discussed.

A fast-response A-film-enhanced fringe field switching liquid crystal display

We propose a new A-film-enhanced fringe field switching (A-FFS) liquid crystal display whose required dΔn value is only λ/4, which is one-half of a conventional FFS. Fast response time can be achieved by either decreasing the cell gap (d) or choosing a low birefringence (Δn) and low-viscosity liquid crystal. The effect of dΔn mismatch between A-film and FFS cell on contrast ratio is analysed. By choosing a dispersion-matched A-film, we can obtain a reasonably high contrast ratio in the entire visible region.

A Fringe Field Switching Liquid Crystal Display With Fast Grayscale Response Time

A fast-response fringe-field switching (FFS) liquid crystal display using patterned common electrodes is proposed. By applying a restoring pulse voltage on common electrodes, the relaxation process of liquid crystals is expedited by the electric field. The average gray-to-gray decay time is over sevenfold faster compared with the conventional FFS mode.

Enlarging the color gamut of liquid crystal displays with a functional reflective polarizer

We propose to add a functional reflective polarizer (FRP) in the backlight unit to suppress the crosstalk between red, green and blue color filters of a liquid crystal display (LCD) panel. When incorporated with a commercial two-phosphor-converted white light-emitting diode (2pc-WLED), the color gamut of the LCD can be improved from 92% to 115% NTSC standard, which is comparable to the cadmium-based quantum dot (QD) backlight. If a narrow-band color filter is employed, the color gamut can be further enhanced to 135% NTSC. Our design offers an alternative approach to QDs, while keeping low cost and long lifetime. Such a simple yet efficient approach would find widespread applications for enlarging the color gamut of LCDs.

Pixel-by-pixel local dimming for high-dynamic-range liquid crystal displays

We propose a high dynamic range (HDR) liquid crystal display (LCD) with pixel-level local dimming. The device structure consists of a pixelated LCD dimming panel to control the backlight intensity entering the master LCD panel. According to our analysis and test cell experiment, this dual-panel display system possesses exceedingly high contrast ratio (> 1,000,000:1) and high bit-depth (> 14 bits) at merely 5 volts. Meanwhile, to mitigate the Moiré effect induced by the cascaded thin-film transistor (TFT) backplanes, we separate the two LCD panels with a polarization-dependent scattering film. The pros and cons of this HDR display are discussed.

Submillisecond-response liquid crystal for high-resolution virtual reality displays

We report a vertically-aligned liquid crystal display (LCD) device with submillisecond response time, high transmittance, and low operation voltage. The top substrate has a common electrode, while the bottom substrate consists of hole-patterned fringing-field-switching (FFS) pixel electrodes. A negative dielectric anisotropy LC is employed. In the voltage-on state, the LC directors are reoriented by the fringing fields surrounding the hole area and by the longitudinal and fringe fields outside the hole area. After design optimization, we are able to achieve 85% peak transmittance under crossed circular polarizers. During the relaxation process, the standing walls exert a strong restoring force, leading to submillisecond gray-to-gray response time. Moreover, this device enables high resolution density because only one thin-film transistor per pixel is needed and the bottom FFS electrode has built-in capacitor. This device is particularly attractive for the emerging virtual reality displays.

Depolarization effect in liquid crystal displays

We develop a rigorous model to simulate an LCDs contrast ratio (CR) and viewing angle by considering the depolarization effect in thin-film transistor substrate, LC layer, color filter (CF) array, etc. To mitigate the depolarization effect, we propose a new device structure by adding a thin in-cell polarizer between LC layer and CF array. Based on the analysis using our new model, the maximum CR of a multi-domain vertical alignment (MVA) LCD can reach > 20,000:1, while for the fringe-field switching (FFS) mode it can reach > 3000:1. We also discuss other approaches to further enhance the CR. Our model is a powerful tool to analyze the CR degradation mechanism and to guide the future LCD device and material optimizations.