P-132 / M. S. Kim P-132: Blue Phases Liquid Crystal Cell Driven by Strong In-Plane Electric Field Min Su Kim, Miyoung Kim, Jun Ho Jung, Kyung Su Ha, Sukin Yoon, Eun Gyoung Song, Anoop K. Srivastava, Suk-Won Choi 1 , Gi-Dong Lee 2 , and Seung Hee Lee Department of Polymer Nano-Science and Technology, Chonbuk National University, Chonju, Chonbuk, 561-756, Korea 1 Department of Display Materials Engineering and Materials Research Center for Information Displays, Kyung Hee University, Republic of Korea 2 Department of Electronics, Dong-A University, Republic of Korea Abstract The polymer-stabilized blue phase liquid crystals are in the limelight of liquid crystal display area because of its fast response and wide viewing angle with alignment layer free. The conventional device is driven by in-plane field, which has thin electrodes only on bottom substrate and also due to low Kerr constant of LC, the driving voltage of the device is very high. We propose improved electrode structure which has partition-wall shaped electrodes. The proposed device has maximized horizontal electric field so that the driving voltage is decreased. Further, adoption of two transistors can lower driving voltage. 1. Introduction Nematic phases are now widely used in liquid crystal displays (LCDs), replacing conventional cathode ray tube. Recently, image quality of LCDs has remarkably been improved with wide viewing angle technology such as patterned vertical alignment (PVA) [1], multi-domain vertical alignment (MVA) [2], in-plane switching (IPS) [3], fringe-field switching (FFS) [4-6]. However, all devices require a perfect alignment of LC in one direction in large area, which is troublesome process that decreases yield and lowers image quality. In addition, the response time is not fast enough. As one of solutions, recently new type of LC device associated with polymer stabilized blue phase (BP) LC [7-11] is proposed. The device is driven by in-plane field generated by interdigital electrodes which exists only one substrate like in the IPS device. In this device, the intensity of an electric field is proportional to the distance between electrodes. Therefore, in order to intensify field intensity at given voltage to lower operating voltage, the distance should be reduced. Then in a real pixel whose width is over 100 μm exists many numbers of electrodes. Then, the transmittance of the device could be very low because there is almost no in-plane field above electrodes and thus the light cannot be transmitted there. In other words, transmittance and operating voltage are in trade-off relation. In this paper, we study on device structure that maximizes in- plane field to lower operating voltage while keeping the same transmittance like in the IPS device. 2. Switching principle of the device utilizing Kerr effect In the device associated with polymer-stabilized BP LC, birefringence of the BP LC with cubic structure is zero before bias voltage and then the induced birefringence ∆n induced is occurred by applied electric fields, that is, the device utilizes Kerr effect for electro-optic modulation [12-17]. And the ∆n induced is relevant to dielectric anisotropy of the mixture and it is determined by the equation [18] : ∆n induced = λKE 2 (1) where λ is wavelength from a light source, K is Kerr constant, and E is electric field, respectively. According to previous report [19], K is very important to enhance ∆n induced , however it is in the range of ~ 10 -9 m/V 2 . Therefore in order to have large ∆n induced either LC mixture with large K or strong E is required. In the practical LC device utilizing Kerr effect, the cell is composed of crossed polarizer and polymer-stabilized BP LC driven by in-plane field. Therefore, the normalized transmittance equation can be given as follows : T/T o = sin 2 2ψ(V)sin 2 (πdΔn induced (V)/ λ) (2) Figure 1. Schematic cell structure of FFS mode. Figure 2. Polarizing optical micrographs of polymer-stabilized BP LC cell : (a) and (c) Voltage off, (b) and (d) Voltage on state of FFS and IPS, respectively. (The white spot is defect from its process of polymer-stabilized.) SID 09 DIGEST • 1615 ISSN/009-0966X/09/3903-1615-$1.00 © 2009 SID