from the other, releasing light. If these OLEDs were constructed from polymers with high flexibility, they could be the basis of lightweight, portable, rollup displays, or displays that could be used on curved surfaces.

Another promising material technology is amorphous oxides. Some amorphous oxides can form thin films that are transparent and electrically conductive, which is why they already serve as the see-through electrode layer in displays and solar cells. It was this combination of qualities that led to the surge in research that began in 1996, when Hideo Hosono and his colleagues at the Tokyo Institute of Technology first noted the merits of amorphous transparent conducting oxides. The biggest problem when amorphous silicon is deposited on flexible plastic is switching and drifts.

Amorphous oxides could do more than simply serve as passive electrodes. They could also replace amorphous silicon as the active semiconducting material in TFTs. The advantages of oxide semiconductors over amorphous silicon are motivating much work in the display industry. Only 2 years after the first oxide-based transistors were reported, Korea’s LG Electronics Co. revealed a prototype OLED display that used indium gallium zinc oxide (IGZO) transistors to drive its pixels. Other companies followed quickly, with oxide-based displays of their own. The U.S. $100 billion flat-panel-display industry has been built on amorphous silicon, and the new materials will have to compete with its 30-year head start. However, amorphous silicon is a mature technology, and most limitations arise from fundamental physical and chemical properties requiring breakthroughs.

Amorphous oxide semiconductors will likely challenge amorphous silicon. When this will happen depends mainly on the development time for a large-scale

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FIGURE C.26 Crystalline, polycrystalline, and amorphous atomic structures. SOURCE: Reprinted, with permission, from Wager, J.F., and Hoffman, R. 2011. Thin, fast, and flexible. IEEE Spectrum 48(5). Copyright 2011 by IEEE.




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