Light-Emitting Diode Can Stretch More Than Twice Its Length

Researchers have made a light-emitting diode that remains brightly lit while they repeatedly stretch it (ACS Nano 2014; DOI: 10.1021/nn405887k). Such elastic light emitters could enable new types of devices, such as flexible cell phone screens or wearable electronics.

The key behind the stretchy LED is a transparent electrode composed of a conductive network of silver nanowires dotted with graphene oxide flakes. The graphene oxide acts like solder, wrapping around the junctions between the nanowires and forging the joints together. This soldering ensures that when the electrode stretches, the nanowires don’t rub against each other, which can reduce conductivity, says Qibing Pei, a materials scientist at the University of California, Los Angeles.

To make the electrodes, Pei and his colleagues soaked a piece of glass coated with silver nanowires in a dispersion of graphene oxide flakes. Electrostatic forces make the flakes bond strongly with the nanowire junctions. The researchers then coated the nanowires with a urethane acrylate monomer, which they allowed to cure. When they peeled off the resulting polymer film, the nanowire-graphene-oxide network was embedded. The elastic films are about 5% more transparent and 35% more conductive than the indium-tin-oxide-coated polyethylene films typically used as electrodes for flexible optoelectronic devices, the researchers report.

Finally, the team made an organic LED by sandwiching a white-light-emitting polymer between two of the stretchable conductive films. The researchers were able to pull the device to more than double its original size while it still glowed—but they could do that only once. They could stretch the device 100 times if they limited the pulls to 40% beyond the device’s original size.