Next-Wave LCD Technology


by Shin-Tson Wu

After more than three decades spent proving concepts through active research and device development, followed by massive investment in manufacturing technology, the thin-film-transistor liquid-crystal-display (TFT-LCD) industry finally took off and is now dominating the flat-panel-display business. Nowadays, LCDs have become indispensable in our daily lives, their uses range from cell phones, video games, and navigational devices to notebook computers, desktop monitors, large-screen TVs, and data projectors.

It seems as though LCD technology is fairly mature. The most critical issue – viewing angle – has been solved to an acceptable level using multidomain structures and optical-film compensation. The next most frequently mentioned shortcoming – response time – has been improved to 2–5 msec or less through low-viscosity LC material development, overdrive and undershoot voltage methodology, and the thin-cell-gap approach. Motion-image blur has been significantly reduced by using impulse driving, frame insertion, and fast-response liquid crystals. The color shift at oblique viewing angles has been dramatically reduced by using an eight-domain approachvia two transistors. The contrast ratio has exceeded 1,000,000:1 through LED-backlight local dimming. The color gamut would exceed 100% NTSC if RGB LEDs were used. Even with all these technological advances, the cost has also been reduced dramatically through investment in advanced cost-effective manufacturing lines. So what is next? That's a very good question.

One promising advancement involves blue phase, a type of liquid-crystal phase that appears in a very narrow temperature range (1–2°C) between the chiral-nematic and isotropic phases; its molecular structure is made up of double-twist cylinders arranged in a cubic lattice with periods of ~100 nm. For this special LCD issue of Information Display, I invited Prof. H. Kikuchi of the Institute for Materials Chemistry and Engineering, Kyushu University, to write a review paper for general readers so they can have a glimpse of the microscopic blue-phase structures, polymer-stabilized blue phases, and their basic electro-optic properties. A second paper from my research group delves into device physics and macroscopic behavior of the electric Kerr effect from a display-device viewpoint.

Blue-phase liquid crystals have been explored for several decades. However, their mesogenic temperature range was always too narrow for practical applications. But recently this situation changed when a small amount of polymer was embedded to stabilize the LC lattice structure. The polymer-stabilized blue phase showed a reasonably wide mesogenic temperature range, covering room temperature. The research momentum for new blue-phase LCDs has been revitalized worldwide.

At the voltage-off state, the blue-phase liquid crystal appears optically isotropic. As the voltage increases, the induced birefringence increases based on the Kerr effect and the LC refractive-index distribution becomes anisotropic. When the device is sandwiched between two crossed polarizers, the transmittance gradually increases as the voltage increases.

In comparison to conventional nematic LCDs, polymer-stabilized blue phase exhibits four revolutionary features:

(1) It does not require any alignment layer, such as polyimide or inorganic SiO2, which not only simplifies the manufacturing processes but also reduces the cost.

(2) Its response time is in the submillisecond range, which helps to minimize the motion-image blur and, more importantly, enables field-sequential-color displays without color filters if an RGB LED backlight is used. The elimination of the color filters results in several significant impacts: (a) it enhances optical efficiency by ~3x, resulting in lower power consumption if the same display brightness is compared; (b) it increases device resolution by 3x (i.e., crisper images); and (c) it reduces production cost.

(3) The dark state of a blue-phase LCD is optically isotropic so that its viewing angle is wider and more symmetric. Compensation films may or may not be needed, depending on the actual applications.

(4) The transmittance is insensitive to the cell gap, as long as the cell gap exceeds 2–3 μm depending on the birefringence of the LC composite employed. This cell-gap insensitivity is particularly desirable for fabricating large-screen LCDs, in which cell-gap uniformity is a big concern, or single-substrate LCDs.

Although polymer-stabilized blue-phase LCDs hold so much promise, some tough technical issues remain to be overcome before widespread applications can take off. The major challenges are in three areas: (1) The operation voltage is still too high (~50 Vrmsvs. 5 Vrms for conventional nematic LCDs), (2) the transmittance is relatively low (~65% vs. 85% for nematic LCDs), and (3) the meso-genic temperature range is still not wide enough for practical display applications (from –40 to 80°C).

The operating voltage of a blue-phase LCD is primarily governed by the induced birefringence, which, in turn, is dependent on the Kerr constant of the LC composite and the electric-field strength. Therefore, developing new blue-phase LC materials with a large Kerr constant and new device structures for enhancing the horizontal electric-field intensity in the in-plane-switching electrode configuration are equally important.

We hope you enjoy this look into the latest in LCD developments. In the interest of providing practical solutions, our authors placed special emphasis on the approaches for lowering the operating voltage while maintaining a good optical efficiency. •

Shin-Tson Wu is Provost-Distinguished Professor of Optics with the College of Optics and Photonics at the University of Central Florida. He can be reached at swu@creol.