250 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 3, NO. 3, SEPTEMBER 2007 Fast Switching Liquid Crystals for Color-Sequential LCDs Sebastian Gauza, Xinyu Zhu, Wiktor Piecek, Roman Dabrowski, and Shin-Tson Wu, Fellow, IEEE Abstract—High birefringence and relatively low-viscosity liquid crystal mixtures containing isoth- iocyanato tolane and isothiocyanato terphenyl liquid crystals are developed. A twisted-nematic (TN) 1.6- m thin cell for color-sequential liquid crystal display with 1-ms response time is demonstrated. Index Terms—Color sequential display, high-birefringence liquid crystals, liquid crystal display (LCD), response time. I. INTRODUCTION H IGH-BIREFRINGENCE liquid crystals are attrac- tive for improving the response time of a display device through cell-gap reduction [1]–[3]. In a 90 twisted-nematic (TN) cell, the Gooch–Tarry first minimum leads to the following equation [1]: (1) where is the wavelength. Meanwhile, the decay time is related to the cell gap and visco-elastic coefficient as (2) Fast response time is especially important for color-sequential liquid crystal displays (LCDs) using blinking backlight [2] or primary-color (RGB) light-emitting diodes (LEDs). [3], [4] In the RGB LED-backlit color-sequential LCDs, the pigment color filters can be eliminated, which not only reduces the LCD cost but also triples the device resolution. However, to avoid a color break, the LC response time (gray to gray) should be kept below 5 ms. Commercially available high- TFT-grade LC mixtures usually have . Under such a circumstance, to satisfy the Gooch–Tarry’s first minimum condition would require a cell gap of m [5]. The resultant response time would ex- ceed 5 ms. Thus, there is an urgent need to develop high- and low-viscosity LC mixtures so that the cell gap can be sup- pressed to 2 m or less. The objective here is to formulate a high- nematic mixture which will satisfy the Gooch–Tarry’s first minimum condition using a thin m LC cell. High melting temperature, increased viscosity, and insuffi- cient UV stability are the major concerns for the high- LC Manuscript received February 26, 2007; revised April 16, 2007. This work was supported by the Defense Advanced Research Projects Agency BOSS Pro- gram under Contract W911NF04C0048 and by NATO Security Through Sci- ence under Collaborative Linkage Grant CBP.EAP.CLG 981323. S. Gauza, X. Zhu, and S.-T. Wu are with the College of Optics and Photonics, University of Central Florida, Orlando, FL 32816 USA (e-mail: sgauza@mail. ucf.edu; xzhu@mail.ucf.edu; swu@mail.ucf.edu). W. Piecek and R. Dabrowski are with New Technology and Chemistry De- partment, Military University of Technology, Wojskowa Akademia Techniczna, 00-908 Warsaw 49, Poland (e-mail: rdabrowski@wat.edu.pl; wpiecek@wat. edu.pl). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JDT.2007.900909 Fig. 1. UV absorption spectra of NCS-based tolane and terphenyl compounds. Fig. 2. Emission spectra of a typical RGB LED backlight. materials. Some isothiocyanato tolanes and terphenyls [6] have been found to exhibit high , modest rotational viscosity, and large dielectric anisotropy. Fig. 1 shows the measured absorp- tion spectra of the two compounds employed [7], while Fig. 2 refers to the typical emission spectra of the RGB LED back- lights. From Fig. 2, there is no UV content in the RGB LEDs. As a result, the concern for UV-induced photostability for high- LCs is greatly relieved. A detailed studies of photo and thermal stability of some LC compounds can be found in [7] and [8] The in the range of 0.30–0.40 enables a 1.2–1.6- m cell gap to be used for achieving less than 2-ms response time so that a color-sequential LCD without a color break can be realized. Although such a thin cell is presently still challenging to fabri- cate for large-size panels, it is stretchable for small-size LCDs, e.g., cell phones and PDAs. The 90 TN cell was chosen be- cause of its simplicity, low production cost, and efficient white- light modulation. This device concept can also be extended to TN-based notebook computers where the viewing angle may not be a critical issue. II. EXPERIMENTAL TECHNIQUES All of the electrooptic measurements were carried out using 1.6- m TN cells with ITO electrodes coated in the inner sides 1551-319X/$25.00 © 2007 IEEE