Fast-Response Fringe Field Switching LCD with Patterned Common Electrode Daming Xu*, Haiwei Chen*, Shin-Tson Wu*, Ming-Chun Li**, Seok-Lyul Lee**, and Wen-Ching Tsai** *College of Optics and Photonics, University of Central Florida, Orlando, FL 32816, USA ** AU Optronics Corp., Hsinchu Science Park, Hsinchu 300, Taiwan Abstract A fast-response and wide-view fringe-field switching LCD using patterned common electrodes is proposed. By applying a restoring pulse voltage on common electrodes, LC decay process is expedited. The GTG decay time can be reduced by >7X, depending on the applied erasing voltage. This new mode also preserves the wide-view characteristics as the conventional FFS mode. It is attractive for reducing motion blurs. Author Keywords Fringe field switching, liquid crystal display, fast response. 1. Introduction Fringe field switching (FFS) LCD [1-3] has been widely used in high-end display products because of its high transmittance, wide view, and pressure-resistance for touch panels. However, some technical barriers, such as TFT charging time for high resolution LCDs [3], image sticking [4] and slow response time, remain to be overcome. The response time of a FFS LCD is mainly determined by the LC material and cell gap. Compared to turn-on process, which is driven by electric field, the decay process is usually slower since it is mainly governed by the elastic restoring force. Because the twist elastic constant K 22 of LC is relatively small, the decay time of FFS LCD with 3-m cell gap is usually ~20ms [5]. In order to reduce response time, various approaches have been proposed [5-7]. However, each approach has its own pros and cons. Hence, there is urgent need to develop a fast-switching FFS LCD without sacrificing its attractive features. In this paper, we propose a patterned common electrode FFS mode, denoted as PC-FFS, to achieve fast response time. The bright state and the dark state are achieved by applying fringe fields along different directions. With a restoring voltage pulse to expedite the decay process, we are able to achieve >7X faster gray-to-gray (GTG) decay time than conventional FFS LCD. 2. Device Structure Figs. 1(a) and 1(b) depict the top view of a conventional FFS and proposed PCFFS cell, respectively. In the FFS cell, pixel electrodes are stripe-shaped while common electrode is planar. Both are formed on the bottom substrate with a passivation layer in between. However, in the PCFFS cell, the bottom common electrodes are also fabricated to be stripe shape, setting at an angle α w.r.t. the x axis. Same as FFS cells, for the purpose of achieving low driving voltage the homogeneous rubbing angle is set at 10° and 80° w.r.t. the x axis for PCFFS cells using a negative (n-PCFFS) and positive (p-PCFFS) LC, respectively [2]. The cell gap is optimized at Ȝ = 550 nm with dΔn = 360 nm for n-PCFFS and 380 nm for p-PCFFS to obtain high transmittance. To achieve bright state, a driving voltage is applied to the pixel electrodes while common electrodes are grounded. Same as the FFS mode, fringe fields with strong horizontal components are generated here to reorient the LC directors. Hence, the incident light accumulates phase retardation and transmits through the analyzer. During decay process, the pixel electrodes are floated while a restoring voltage pulse is applied between adjacent common electrodes. The electric potential of floated pixel electrodes are spontaneously determined by the restoring voltage [8]. The in-plane field generated by this restoring voltage would exert a strong torque to pull the LC directors back to their initial rubbing [6, 9]. Hence, the decay process is accelerated and faster decay time is obtained. Figure 1. Device structure of (a) conventional FFS and (b) PCFFS modes. 3. Simulation Results: VT Characteristics The device performance are studied and optimized by using commercial simulator TechWiz LCD (Sanayi, Korea) and the electro-optic properties are calculated by the extended 22 Jones matrix method. To make a fair comparison between FFS and PCFFS cells, we use the same device configurations: pixel electrode width W 1 = 2m, electrode gap G 1 = 3m, pretilt angle 2°. The passivation layer between the pixel and common electrodes is Si 3 N 4 ( = 7.5, thickness = 150nm) whereas the alignment layer is 80-nm thick polyimide ( = 3.8). The physical properties of the positive and negative LC materials studied are listed in Table I. The negative LC material UCF-N2 was developed by our group [10], while the positive  LC DIC-LC3 is a commercial material from DIC Japan [11]. TABLE I. Physical properties of two LC mixtures studied (T = 23 °C and  = 550 nm). LC γ 1 (mPa·s) Δε Δn K 11 (pN) K 22 (pN) K 33 (pN) UCF-N2 94.7 -3.8 0.117 15.8 7.2 14.0 DIC-LC3 62.0 9.0 0.111 10.9 5.6 13.8 (a) VT Curves: Fig. 2 shows the simulated voltage-dependent transmittance (VT) curves of FFS and PCFFS cells employing a positive or a negative  LC material. Here, the  angle is set at 10° and -20° for PCFFS cells employing p- and n-PCFFS, respectively. The values of  angle are optimized to achieve the ISSN 0097-966X/15/4502-0652-$1.00 © 2015 SID 43.3 / Daming Xu 652 • SID 2015 DIGEST