A video-speed reflective display based on electrowetting: principle and properties B. J. Feenstra R. A. Hayes I. G. J. Camps L. M. Hage M. T. Johnson T. Roques-Carmes L. J. M. Schlangen A. R. Franklin A. F. Valdes R. A. Ford Abstract — Electrowetting is presented as a novel principle for a reflective display. By contracting a colored oil film electrically, an optical switch is obtained with many attractive properties that make it very suitable for use as a reflective display, for instance, as electronic paper. Firstly, it has the high reflectivity (>40%) and contrast ratio (15) required for a paper-like optical appearance. In addition, the principle shows a video-rate response time (<10 msec) and has a clear route toward a high-bright- ness color display. 1 Finally, the electro-optical response is independent of cell-gap thickness, which will be very beneficial when moving toward a flexible display. Keywords — Electrowetting, reflective displays, video speed, high color brightness. 1 Introduction The display market is currently still dominated by “emis- sive” displays such as CRT’s and LCD’s. However, it is gen- erally agreed that there will be a rapidly increasing demand for reflective displays, mainly driven by the growth in port- able applications. One of the most appealing applications pursued by the industry is the so-called electronic paper. The requirements for reflective displays are clearly very dif- ferent from those for emissive technologies, and there is an emerging consensus that a paper-like look and feel will be essential for their success. This implies that such a technol- ogy should be flexible in addition to its demanding optical properties. Furthermore, the high standards of present-day consumers will require future reflective displays to show video content. Several new principles for reflective displays are being investigated (see, for instance, Refs. 2–5). In nearly all of the emerging principles, color is generated by using an RGB color filter in combination with a B/W switch, or by dividing the area into individual RGB switches. One exception is described in Ref. 5, where a CMY color scheme is proposed. Furthermore, a number of these technologies lack the fast response speed that is required for showing video con- tent. 2,3 Here, we present a novel reflective-display principle, in which the optical switch is driven by the so-called elec- trowetting effect. 6 We introduce the principle behind the optical modulation and show that this principle has the potential for use as a fast, high-brightness color display. Next, we discuss the fabrication of our current test cells in detail. The main body of the text is devoted to the electro- optic properties, where its very attractive optical perform- ance will be emphasized, including the video speed response. Next, we will discuss issues related to the address- ing of electrowetting displays. Finally, we present images of an individually addressed multi-pixel array and show that we can control the oil motion. 2 Principle In Fig. 1, the principle of the reflective electrowetting dis- play is demonstrated. Figure 1(a) shows the optical stack, comprising a white (reflecting) substrate, a hydrophobic insulator, a colored oil layer, and water. In equilibrium the colored oil naturally forms a continuous film between the Revised version of a paper presented at the Twenty Third International Display Research Conference (IDRC ‘03) held September 16–18, 2003, in Phoenix, Arizona, U.S.A. B. J. Feenstra, R. A. Hayes, I. G. J. Camps, L. M. Hage, M. T. Johnson, T. Roques-Carmes, and L. J. M. Schlangen are with Philips Research Laborato- ries, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands; telephone +31-40-2742116, e-mail: johan.feenstra@philips.com. A. R. Franklin, A. F. Valdes, and R. A. Ford are with Philips Research Laboratories, Cross Oak Lane, Redhill Surrey, U.K. © Copyright 2004 Society for Information Display 1071-0922/04/1203-0293$1.00 FIGURE 1 — Electrowetting display principle. Schematic cross section with (a) no voltage applied, therefore a homogeneous oil film is present or (b) dc-voltage applied, causing the oil film to contract. The top view photographs in (c) and (d) demonstrate the corresponding oil retraction obtained with a homogeneous electrode. Journal of the SID 12/3, 2004 293