23.3: A Vertical-Field-Driven Polymer-Stabilized Blue Phase Liquid Crystal Displays Yong-Hun Kim 1 , Sung-Taek Hur 3 , Kyung-woo Park 2 , Do Hyuk Park 1 , Suk-Won Choi 3 and Hak-Rin Kim 1,2,4* 1 Department of Sensor and Display Engineering, Kyungpook National University, 1370 Sankyuk-dong, Buk-gu, Daegu, Korea 2 School of Electronics Engineering and Computer Science, Kyungpook National University, 1370 Sankyuk-dong, Buk-gu, Daegu, Korea 3 Department of Advance Materials Engineering for Information and Electronics, Kyung Hee University, 1 Seocheon-dong, Giheung-gu,Yongin-shi, Gyeonggi-do, Korea 4 School of Electronics Engineering, Kyungpook National University, 1370 Sankyuk-dong, Buk-gu, Daegu, Korea Abstract We demonstrate a polymer-stabilized blue-phase liquid crystal device (PS-BPLC), which can be driven by a vertical electric field by transforming normal incident beams into oblique ones. By attaching prism sheets to the cell, the high operating voltage and low transmittance issues on the PS-BPLC employing conventional in-plane field switching modes can be solved simply. The normal brightness can be enhanced by using two prism sheets on top and bottom substrates, respectively. 1. Introduction Polymer-Stabilized Blue Phase Liquid Crystal Displays (PS-BPLC) have drawn much attention as the next generation display technology with many advantages. For example, they have a quick response 10 times faster than the typical Nematic LC (NLC) displays and this enables PS-BPLCs to be used for RGB LED sequential color displays. Also, they do not require alignment layers, which results in less complicate fabrication process [1-4]. Furthermore, the property of optical anisotropy in field-off state guarantees a wide viewing angle [5]. However, this newly emerging display technology has two critical weaknesses that impede PS-BPLCs to be used in the market: one is high operating voltages and the other is low optical transmittance. In this paper, a noble device using a vertical electric field is proposed in order to control transmittance based on low driving voltages and to improve optical transmittance. The bottom prism sheet transforms the light into oblique beams and the top one improves the normal brightness. As a result, the device brightness can be controlled by the vertical electric field-induced retardation in a simpler structure. 2. Motivation BPLC consists of double twisted cylinders according to Meiboom’s model [6]. This BPLC structure is considered optically isotropy because of the randomly distributed LC molecules. BPLC cells usually have a normally black mode under crossed polarizers since light does not experience retardation through the LC layers. Unlike the IPS mode, there is no retardation in the vertical electric field when the beam is normally incident on the cell where the optical axis is parallel to the electric field. Therefore, the transmitted light cannot pass through the crossed polarizers. Hence, BPLC devices generally employ in-plane electric field switching (IPS) method since the amount of retardation increases in the direction of the fields by the fact that the induced retardation axis does not change, which is called Kerr effect. As shown in Figure 1 (a), while vertical fields can be used for NLC devices where the retardation axis is altered by the electric field, PS-BPLCs cannot use the vertical electric field in general. But the BPLCs using the IPS method has low transmittance as a result of the small aperture ratio coming from the peculiar electrode patterns [8]. Moreover, as illustrated in Figure 1 (b), the driving voltage is rather high because of the limitation of shortening the intervals between the electrodes and because of the reduced crossing electric fields in the vicinity of the top substrate [9]. So, it is necessary to invent a novel device structure for new operating mode considering the material constants of the PS-BPLC that possesses a wide range of operation temperature. Based on the motivation explained above, Figure 1 (c) shows a new structure of PS-BPLC cell to simulate its low voltage and high transmittance by patterning the top and bottom electrodes to be corrugated [7]. The intensity of the vertical electric fields formed on the structure is strong enough even near the top electrode and this reorients the LCs in an oblique manner. Thus, the light beams experience sufficient retardation through the LCs and in turn they can pass through the crossed polarizers with low voltage and high transmittance. However, it is difficult to pattern the top and bottom electrodes and even harder to align them as shown in the picture. Thus, a PS-BPLC cell we invented having a relatively simple structure is proposed and analyzed in terms of low voltage and high transmittance by actually measuring those experimentally, not through a simulation. Figure 2 illustrates the structure of the PS- BPLC cell we propose. In order to solve this problem, two prism sheets were attached to the PS-BPLC cell that have top and bottom electrodes structure for vertical electric fields as shown Figure 2. As the light with different incident angles was refracted in the PS- BPLC cell with the two prism sheets, transmittance and viewing angles from different views were measured by changing the driving voltages. 3. PS-BPLC mode with prism sheets 3.1. Analysis of electric optical properties The prism angle was set to be θ p so that the refracted beams were to have θ i incident angles. To fabricate prism sheets, the prism angles for incoming light could be calculated depending on the incident angles to the LC cell. The transmittance for the vertical electric field driven PS-BPLC could also be determined by extraordinary (n e ) and ordinary (n o ) refractive index. Afterwards, using Snell’s Law n 1 sin θ p = n 2 sin (θ p – θ i ) gave the prism angles(θ p ) depending on the incident angles. 298 • SID 11 DIGEST ISSN 0097-966X/11/4201-0298-$1.00 © 2011 SID 23.3 / Y.-H. Kim