Emerging blue-phase LCDs
Following more than four decades of active research, device development, and massive investment in manufacturing technology, thin-film-transistor LCDs (TFT-LCDs) have finally taken off, to the point where they dominate the flat-panel display market. LCDs have become indispensable to everyday life. They are used in a range of applications that includes cell phones, computers, TVs, and data projectors.
LCD technology can thus be said to be relatively mature. A problem with the viewing angle has been solved using multidomain structures and optical-film compensation. Response time has been improved to 2–5ms by employing low-viscosity liquid-crystal (LC) material, a special voltage waveform, and a thinner LC layer. Color shifting at oblique-angle viewing has been significantly reduced, as has motion image blur. The contrast ratio currently exceeds 1,000,000:1 through local dimming of the LED backlight. The color gamut would exceed 100% NTSC (National Television Systems Committee standards) if RGB (red, green, blue) LEDs were used, although white currently predominates because of its lower cost. The question is, what is next?
Blue-phase LC (BPLC) exists in a very narrow temperature range (~1–2°C). Its molecular structure consists of double-twisted cylinders arranged in a cubic lattice with periods of ~100nm.1,2 BPLCs have been studied for several decades, but up to now, their mesogenic (LC) temperature range has been too narrow for practical applications. However, when a small amount of polymer is embedded to form an LC composite, the polymer-stabilized BPLC shows a reasonably wide mesogenic temperature range that includes room temperature.3 Consequently, enthusiasm for developing new BP-LCDs has been revived.
We previously showed that, in comparison to conventional, nematic LCDs, polymer-stabilized BP-LCDs exhibit four transformative features.4 First, response time is in the submillisecond range, which helps to minimize motion image blur and, more importantly, enables field-sequential color display without using color filters. Today, RGB LEDs are commonly used as LCD backlighting. Eliminating color filters would triple optical efficiency, resulting in lower power consumption at the same brightness, triple device resolution density (i.e., crisper images), and reduce production costs. A second advantage of the BP is that it requires an alignment layer (typically polyimide or inorganic silicon dioxide), which both simplifies the manufacturing process and reduces costs. Third, the dark state of a BP-LCD is optically isotropic, which means that its viewing angle is wide and symmetrical. Compensation films may or may not be needed. Finally, BP transmittance is insensitive to the cell gap when in-plane electrodes are used, as long as the gap exceeds ~4μm, depending on the birefringence (which relates to contrast ratio) of the LC composite. This cell-gap insensitivity is particularly desirable for fabricating large-panel LCDs in which cell-gap uniformity is a big concern, or single-substrate LCDs for slimness and light weight.
However, major technical issues remain to be tackled for use to be practical. Operation voltage is still too high (~50 versus 5Vrms for conventional nematic LCDs). Transmittance is relatively low (~65 versus 85% for nematic LCDs). The mesogenic temperature range is still not sufficient for practical application (−40 to 80°C). Finally, hysteresis (a factor in grayscale control) and residual birefringence must be addressed for better display performance.
Our research team is designing new BPLC composites and device structures to lower operating voltage and optimize display performance. The BPLC material shows a response time in the submillisecond range.5 Figure 1 illustrates an in-plane-switching (IPS) electrode cell with and without voltage. In the voltage-off state, BPLC appears optically isotropic. As the voltage increases, the LC's refractive-index distribution becomes anisotropic because of the Kerr effect. The induced birefringence is proportional to the incident wavelength, the Kerr constant k, and the square of the applied electric field.6 When the device is sandwiched between two crossed polarizers, transmittance increases gradually as voltage increases.
We also developed a numerical model to study the electro-optics properties of the display.7 Figure 2 shows the protrusion-electrode structure we propose to dramatically lower the operation voltage. Simulation results indicate that the generated horizontal electric field is both stronger and also penetrates deeper into the bulk LC layer compared with traditional IPS structures. As a result, low voltage (~10Vrms) BP-LCDs can be achieved with k=1.27×10−8m/V2 and reasonably high transmittance (~70%).8 This approach enables driving of the BP-LCDs by amorphous-silicon TFTs.
LCDs based on the Kerr effect are attractive because of their fast switching time, symmetric viewing angle, and simple fabrication process. If the challenges involving high voltage and hysteresis can be further addressed, widespread application of TFT BP-LCDs is foreseeable. Currently, our group is working closely with TFT-LCD manufacturers to implement our device designs into prototypes.
Linghui Rao is a PhD candidate. Her research interests include novel LC materials for advanced LCD applications, and BP-LCDs. She has published 11 journal papers and has one pending US patent. She is the president of the Society for Information Display (SID)'s UCF student chapter.
Shin-Tson Wu received his BS degree in physics from the National Taiwan University, and his PhD from the University of Southern California, Los Angeles. He is a Pegasus professor. Prior to joining UCF in 2001, he worked at Hughes Research Laboratories (Malibu, CA) for 18 years. He has co-authored six books, six book chapters, over 350 journal publications, and holds more than 60 issued patents. He is a recipient of the Optical Society of America (OSA)'s Joseph Fraunhofer Award/Robert M. Burley Prize, the SPIE G. G. Stokes Award, and the SID Jan Rajchman prize. He was the founding editor-in-chief of the IEEE/OSA Journal of Display Technology. He is a Fellow of IEEE, SID, OSA, and SPIE.
