Flexible Plastic Liquid Crystal Displays
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Flexible Plastic Liquid Crystal Displays John L. West, Greg R. Novotny, Michael R. Fisch, and David Heineman Liquid Crystal Institute, Kent State University Kent, Ohio 44242, USA ABSTRACT Flexible plastic liquid crystal displays (LCDs) offer a number of advantages over traditional glass displays. Roll-to-roll manufacturing of such displays makes possible large size, inexpensive displays. While this type of manufacturing is still in the future, here we report solutions of some of the pressing problems needed to achieve this goal. We utilize liquid crystal effects that do not require polarizers and can therefore utilize commercially available, birefringent substrates such as polyesters and polyethylene terephthalate. We achieve excellent optical and mechanical performance of these flexible displays, even when bent, by creating polymer networks and polymer walls that adhere the front and back substrates and maintain uniform spacing between the substrates. Roll-to-roll processing of flexible plastic displays also requires that the photolithographic techniques used to pattern the electrodes on the current glass LCD’s be replaced by printing techniques and the development of techniques to rapidly etch unwanted electrode material. We must also develop and optimize techniques for rapidly forming polymer walls. Here we report methods of patterning ITO electrodes on flexible plastic substrates using conventional wax transfer printing techniques and rapid etching and cleaning techniques. We also report experiments that elucidate the basic science of rapidly forming polymer walls. We combine these new achievements and understanding and demonstrate their utility by fabricating a flexible plastic LCD using techniques compatible with roll-to-roll manufacturing. Finally, we discuss the critical issues that must be addressed if flexible plastic LCD’s are to reach their full potential. OBJECTIVES AND BACKGROUND Flexible plastic displays offer many advantages compared to displays using glass substrates. They are lighter, not as easily broken, easy to manufacture in non-planar geometries, and have the potential of being made using roll-to-roll processing techniques. Several types of displays are compatible with birefringent plastic substrates: polymer dispersed liquid crystal (PDLC) displays [1], various polymer stabilized cholesteric displays [2], and others [3]. In this work, we utilize reflective cholesterics materials that switch between two stable states: a colored planar state and a transparent focal conic state [4,5]. The planar state is produced by application of a high voltage pulse while the focal conic state is produced by application of a lower voltage pulse. Gray scale can be achieved by application of voltage pulses between those required to produce the colored and transparent states. These bistable cholesteric materials can be used to produce displays that switch between stable reflective and transmissive states. By incorporating a black background, these display switch between black and colored states. Because these dis
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