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Illumination & Displays

Fast display mode using a novel liquid crystal

Bent-core molecules should provide fast response, high contrast ratio, wide viewing angle, and continuous gray levels in large displays.
10 July 2007, SPIE Newsroom. DOI: 10.1117/2.1200702.0753

A mere 34 years following their appearance in wristwatches and calculators, liquid crystal display (LCD) technologies now find commercial application in 100-inch TV screens. The displays have obviously matured, but several problems remain to be solved. One of the most serious of these is slow response time, which is inevitable in standard, so-called nematic LCDs. Ferroelectric liquid crystals (FLCs) and antiferroelectric liquid crystals (AFLCs) represent attempts to address this time problem. These materials have many unexpected properties, however, mostly owing to their orderly, or smectic, layer structure, making them unsuitable for application to large LCDs.

The discovery of bent-core LCs opened a new era of LC science.1 Their shape partially hinders free rotation about their long molecular axis, resulting in polarization within layers. Both FLC and AFLC structures occur in a tilted smectic phase.1 A nontilted conventional smectic-A (Sm-A) phase also exists. The phase of the material we use is apparently uniaxial and Sm-A like,2 but not actually this phase. Applying an electric field parallel to the layer easily induces polar order and associated birefringence: see Figure 1(b) and (c).3

Figure 1. (a) Molecular structure of the bent-core liquid crystal. (b, c) Principle of the display mode. In homeotropically aligned cells, molecules align perpendicular to the substrate surfaces: see (c), side view. The molecular dipoles are randomly oriented in the absence of a field: see (b) and (c) left. As the field increases, the dipoles reorient in the direction of the field—see (b) right, top view—which causes the direction of molecular bending to reorient as well: see (c) right. These events induce birefringence, which results in a bright display under crossed polarizers.

We decided to use a novel bent-core LC, shown in Figure 1(a),2 and to align the layers parallel to the substrate. In this way, we were able to avoid layer deformation due to temperature change and achieve a high contrast ratio. This electro-optic performance was obtained as follows. We fabricated homeotropically aligned—i.e., perpendicular to the substrate surfaces—Sm-A-like-phase cells. The uniaxial nature of the phase in the absence of a field results in complete darkness under crossed polarizers, similar to that in a VA (vertical alignment) mode. When we apply an in-plane electric field, dipole reorientation occurs in the direction of the field. Reorientation of the bent direction is associated with that of the dipole, and the induced biaxiality leads to birefringence: Figure 1(b) and (c). Thus, molecules switch ferroelectrically, and fast switching in the range of 100μs is possible, as shown in Figure 2.3

Figure 2. Display performance. It is clear that the electro-optic response is fast and nearly independent of applied field strength. Note that gray-scale display is also possible.

To understand the principle underpinning the switching characteristics, second-harmonic generation (SHG)—frequency doubling—was measured as a function of the electric field. The SHG signal gradually and continuously increased and tended to saturate with greater field strength. The result was analyzed successfully by assuming a two-dimensional Langevin process, where dipoles align to the field direction under the influence of thermal agitation. We found that about 150 molecules switch cooperatively.4

In conclusion, we have proposed and demonstrated a novel LC display mode using a bent-core liquid crystalline Sm-A-like phase that has all the advantages of existing display modes: the high contrast ratio of VA, the wide viewing angle of in-plane switching, the quick response (100μs) of FLCs and AFLCs, and the gray level of V-shaped switching. This development responds to a need for materials that show high biaxiality and exhibit a wide temperature range in the Sm-A-like phase.

Hideo Takezoe, Yoshio Shimbo 
Department of Organic and Polymeric Materials
Tokyo Institute of Technology
Tokyo, Japan

Hideo Takezoe received his DSc degree from Tokyo University of Education in 1975. He is now a professor of organic and polymeric materials at the Tokyo Institute of Technology. He is one of the discoverers of AFLCs (1989) and polar bent-core LCs (1996). He has published more than 500 scientific papers, most of which are related to liquid crystal science and technology.

Yoshio Shimbo is a student working toward his PhD under the guidance of Hideo Takezoe.