the display would be the color filter glass rather than the polarizer plastic. Achieving such a substitution would motivate efforts to eliminate the stand-alone cover glass and integrate the protection of a cover into the color filter glass itself.

OLED Displays

OLEDs make use of the electrophosphorescence exhibited by some organic molecules to create light directly at the diode itself when desired; the color of the diode can be adjusted by the choice of organic molecules. Therefore, an array of OLEDs can directly generate the displayed image rather than light a modulator, such as an LCD, to create the displayed image. Because the image is directly generated, light is not wasted by attenuating it with the modulator that creates the image, and these displays can be thinner because generating the image is a single-step process. OLED-based displays can therefore be brighter than LCDs and consume less power.19,20 Two principal ways are used to supply the electrical bias to the individual OLEDs in a display: passive- and active-matrix designs.21 In a passive-matrix scheme, the individual anodes and cathodes are connected by perpendicular conducting strips, and external circuitry applies voltages to the rows and columns that are necessary to determine which pixels will be turned on. At present, the external circuitry required for a passive-matrix design results in higher power consumption than is needed with an active-matrix display. Active-matrix displays incorporate a thin-film transistor (TFT) array as part of the device to supply the power to the individual OLEDs. This biasing scheme uses less power than is needed for the passive-matrix design and has a higher refresh rate, and so it is suitable for large displays.22

Because non-rigid substrates can be used, OLED displays can be more robust than displays that incorporate glass. That makes OLEDs potentially useful in applications in which they will be subjected to rough handling, such as in cell phones or other consumer electronics. Furthermore, in the case of completely flexible substrates, the displays could be used in applications in which no other display currently can be contemplated, such as being integral with an article of clothing.

OLEDs offer several advantages over both LCDs and LEDs for small and large

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19 Kelley, T.W., P.F. Baude, C. Gerlach, D.E. Ender, D. Muyres, M.A. Haase, D.E. Vogel, and S.D. Theisset. 2004. Recent progress in organic electronics: Materials, devices, and processes. Chemistry of Materials 16(23):4413-4422.

20 Iwamoto, M., Y. Kwon, and T. Lee. 2011. Nanoscale Interface for Organic Electronics. Singapore. Hackensack, N.J.: World Scientific.

21 Ju, S., J. Li, J. Liu, P. Chen, Y. Ha, F. Ishikawa, H. Chang, C. Zhou, A. Facchetti, D.B. Janes, and T.J. Marks. 2008. Transparent active matrix organic light-emitting diode displays driven by nanowire transistor circuitry. Nano Letters 8(4):997-1004.

22 For more details about OLED displays, see Appendix C.



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