Electrophoretic Display Technology: The beginnings, the improvements, and a future in flexible electronics Erik Herz May 19, 2006 MSE 542 Materials Science and Engineering Cornell University, Ithaca, NY2Introduction Move over ink on paper and printed shop sign in the window! The death knell is being heard around the industrialized world as electronic paper starts to gain traction as a viable means of conveying and even storing data. This futuristic view of electronic paper and displays may be a few years away from widespread acceptance (remember the coming of the personal computer and paperless society?), but the basic technology and ideas to make it a reality have been available in laboratories for about 30 years. Electrophoretic display technology, since its inception in the 1970s, has progressed slowly to a maturity that now allows it to take full advantage of the things that make it unique in the display arena, including ease of manufacture, low cost, stability, flexibility, and reliability. Though electrophoretic displays are not the only ones available for application in low cost, low energy consumption, flexible and/or portable electronics, their twisted nematic and super twisted nematic counterparts do not offer the same potential for high brightness or high flexibility with the same ease that eletrophoretic particle based displays do.5 Additionally, direct sunlight is often a problem for competing technologies that do not rely on scattering or diffraction phenomena for their contrast and brightness. In this review paper a brief history of display technology up to early electrophoretic displays, the basic principles of image formation, the behavior of light, the electronics necessary for the backplane, and the application of the electrophoretic concept to flexible substrates will be explored. History: Early times and progress for display technology Display technologies have progressed greatly through the ages. Early forms of display were permanent, such as cuneiform clay tablets which served to both display and store information about business transactions thousands of years ago. Three thousand years ago3papyrus leaves and simple colloidal inks were used by Egyptians to provide less permanent, but more portable means of displaying information. Stone inscriptions and graffiti made from simple paints were used during the Roman times to produce long lasting large area displays, but with relatively low data content, low portability, and great difficulty associated with updating the information. Through the centuries since then, up to the 20th century, few improvements were made to display technology save Gutenberg’s printing press. Finally, in the mid-20th century electrified displays started to advance significantly and rapidly. Early displays required relatively high power and yielded relatively low visibility information with limited viewing angles and low resolution, both spatial and spectral. These included most of the cathode ray tube technologies. Static display of information during this period of time continued to be through the use of the time tested ink and paper. In the early 1970s displays began to be produced that incorporated the principles of electrophoresis, or the motion of a charged particle through a liquid medium due to an applied electric field. The general composition of the displays at that time included a suspension of charged particles (containing or consisting of pigments) usually on the order of several microns in diameter in a dyed insulating fluid. The dyed fluid had to have a resistivity greater than 1012 ohm cm and relatively low viscosity (η<5 mN s m-2) to allow for good eletrophoretic mobility of the particles. Solutions with these properties, such as Sudan Red dye in a xylene/perchloroethylene mixture or Oil Blue N dye in toluene were commonly used.1,2 This suspension was sandwiched between two parallel conducting electrode panels, at least one of which needed to be transparent to visible light. The whole array was encapsulated by a glass plate on one side and an insulating substrate on the other with a photocurable adhesive around the edges to seal everything in. Operation consisted of an applied electric field across the parallel4electrodes which would drive the charged particles toward one electrode and allow the viewer to see the light scattered from the particles (i.e. the color of the pigment). If the field is switched, the viewer sees the color of the dyed solvent instead, assuming the dye is in high enough concentration to absorb all light scattered from the particles. Early displays were rigid displays due to the rigid front glass and rear substrate, such as those for alarm clocks that held many advantages over traditional display technologies.3,4 These advantages are inherent to the design and include a thin active matrix that did not require a bulky cathode ray tube or vacuum of any sort to allow it to produce an image. The display has high optical contrast and is easily viewable from almost all angles and does not rely on front- or backlighting except for viewing in the darkest of conditions (and then only front-lighting). There are no drive mechanics that could break , the modules need very low power for the switching operation (i.e. between light and dark), and they could be used for the persistent display of information with zero power consumption. Unfortunately, early displays also suffered from many limiting features that were later overcome, including pigment or particle migration to the electrode edges, floating or settling of particles, bleaching or spectral shift of dyes, and sticking of the particles to one-another or to structures within the active area of the display.3,5 In viewing a schematic, Figure 1, of a simple electrophorectic display cell, it is easy to imagine the reason for at least the floating or migration issues when no barriers existed across the entire cell (vertically in this figure). Almost all of these challenges were associated with image retention, resolution, clarity, contrast, and grey scale.2,45 Figure 1: Schematic of an early electrophoretic display. From Ref. 2. Improvements and The Electrophoretic Ink: To overcome the drawbacks associated with the early displays, many improvements had to be made. Later models included grid barriers to limit the motion of the particles, as shown in Figure 2, the concept of which would be extensively used in the flexible display