Exploring a New Generation of Signage and Graphics Materials
The organic light-emitting diode (OLED) was discovered in the 1970s by Dr. Ching Tang, a scientist at Eastman Kodak Co., who found organic materials that glowed in response to electrical currents. Since then, several companies have developed the technology further, helping it to evolve from its original green color to polychromatic displays and onward to full color.
Ghassan Jabbour, a professor of chemical and materials engineering at Arizona State University, describes OLEDs as simple devices consisting of multilayered structures sandwiched between two electrodes. Jabbour researches organic and hybrid optoelectronics, including OLED technology, and has participated in printing OLEDs on paper, plastic, textiles, glass, and silicon (Figure 5). He says OLEDs are grouped in two camps: small molecule (OLEDs) and polymer organic (PLEDs).
"You can inkjet print polymers, and you can screen print them," he explains. "On the other hand, we were also able to screen print molecules by putting them in polymer hosts. Both versions can be printed if one knows what to do with them. You can print them and make very nice displays."
Jabbour says displays can be quickly made using screen printing or roll-to-roll printing (offset), but the resolution will not be as high as what can be attained when using vacuum thermal evaporation (VTE). We'll discuss VTE a little later.
"You print organics like any ink, and then you print or deposit electrodes," he says. "There is no curing. You let it dry, and that's it."
Environment is a major consideration when screen or inkjet printing OLEDs. Jabbour says printing and encapsulating displays in open air is fine for the purpose of demonstration, but an inert environment around the print-ing area—such as one produced by nitrogen—is necessary to ensure a display's longevity.
Janice Mahon, vice president of technology commercialization for Universal Display Corp. (UCD), Ewing, NJ, says a number of different organic molecules are used to build OLEDs—and several types of OLEDs are made (Figure 6). She explains that the molecules either naturally occur as a crystalline materials or naturally occur in a polymer precursor, typically some type of monomer.
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