Polymer LEDs could stretch the potential of electronics

Ashley N. Paddock, ashley.paddock@photonics.com

An intrinsically stretchable polymer LED that can be lengthened by as much as 45 percent has the potential to change the direction of the field of stretchable electronics. The new device could be used in wearable electronics, “smart skins” and minimally invasive biomedical devices that can move with the body.

Conventional inorganic electronic devices are brittle yet have a certain flexibility that is achieved using ultrathin layers of inorganic materials – which are either flexible or stretchable – containing discrete LED chips interconnected with stretchable electrodes. However, these devices lack any intrinsic stretchability: Not every part of the devices can stretch.

A creased polymer LED.

“Electronic devices with musclelike stretchability have long been dreamed of or perceived, but have not been feasible due to the requirement that all materials in the devices – electrodes, semiconductors and dielectric – are stretchable,” said Qibing Pei, a professor at the University of California. “While certain polymers (semiconductor and dielectric) can be stretchable, the conventional electrodes – metal, indium tin oxide, conducting polymers – are not stretchable.”

Now, engineers at the UCLA Henry Samueli School of Engineering and Applied Science have demonstrated for the first time an intrinsically stretchable polymer LED. To create it, they developed a simple process to fabricate the transparent devices using single-walled carbon nanotube polymer composite electrodes. The nanotubes’ interpenetrating networks, along with the polymer matrix in the surface layer of the composites, provided low sheet resistance and surface roughness as well as high transparency and compliance.

A blue-light-emitting polymer device at 0 percent, 20 percent and 45 percent strain. Images courtesy of Qibing Pei, UCLA.

The scientists discovered that the metal-free devices could be linearly stretched up to 45 percent, and the composite electrodes could be reversibly stretched by up to 50 percent with little change in sheet resistance. Their findings appeared online July 28 in Advanced Materials (doi: 10.1002/adma.201101986).

“This work hopefully will inspire more research into the development of polymer semiconductors and conductive materials (electrodes) that can be stretched like rubber without significant loss of electrical properties,” Pei said. “Our focus in the immediate next step is to demonstrate stretchable polymer solar cells.”