Silver Accentuates Flexible OLED Design

Lynn M. Savage

Improved organic LEDs (OLEDs) and photovoltaic devices are sought after by researchers and commercial product developers because they are lighter and more energy-efficient than standard displays and solar panels, respectively. A bit farther down the wish list are flexible OLEDs, which in theory would enable displays that can be rolled up and put in one’s pocket, or that could be made integral to a document such as a passport.

Achieving an OLED that is flexible enough for such rigorous handling, however, is fraught with challenges. A major detail to work out is what sort of substrate, or backing material, to use for the layers representing the anode, cathode and other LED components. The substrate must be flexible, of course, but it also must be highly transparent and have very low electrical resistance.

Now several teams of investigators working together in South Korea have developed an OLED material that meets all of these requirements.

The scientists — representing Kumoh National Institute of Technology in Gumi, the Korea Institute of Material Science in Changwon, and the Korea Institute of Science and Technology in Seoul — built their flexible OLED with a thin film of silver sandwiched between layers of indium zinc tin oxide (IZTO). Less than 80 μm thick, the stack of layers was deposited onto a 200-μm-thick substrate made of polyethylene terephthalate, or PET.

Field-emission scanning electron microscope images show the results of various thicknesses of silver deposited on a layer of indium zinc tin oxide. Reprinted with permission of Applied Physics Letters.

By itself, a 30-nm-thick layer of IZTO on PET has a resistance of 220 V per arbitrary unit of area — too high for an effective OLED device. Placing a 14-nm-thick layer of silver between a pair of 30-nm layers of IZTO reduced the so-called sheet resistance to 4.99 V/sq.

The researchers also tested various thicknesses of silver layers for transmissivity, finding that a thickness of 8 nm, for example, provided transmittance of 66.8 percent, whereas a 14-nm layer thickness provides 86 percent transmittance. They also found, however, that further increasing the thickness of the silver layer reduced the transmittance, even though the sheet resistance continued to shrink.

The investigators see the sandwiched silver structure as a viable alternative to more typical zincless indium tin oxide for building flexible OLEDs.

Applied Physics Letters, June 2, 2008, 223302.