Transparent Electrode Key to Flexible Solar Cells

A hybrid transparent electrode that overcomes the “electron-transport bottleneck” of thin films could pave the way for flexible solar cells and color monitors, head-up displays in car windshields and optoelectronics circuits for sensors and information processing.

Transparent electrodes used in today’s touch-screen monitors, cell phone displays and flat-screen TVs use indium tin oxide (InSnO₂), an expensive material that is limited in availability, inflexible and degrades over time, becoming brittle and hindering performance.

The Purdue University electrode, first proposed in 2011 in Nano Letters (doi: 10.1021/nl203041n), is made of graphene draped over silver nanowires.

“It’s like putting a sheet of cellophane over a bowl of noodles,” said David Janes, a professor of electrical and computer engineering. “The graphene wraps around the silver nanowires and stretches around them.”

The concept represents a general approach that could apply to many other materials, said electrical and computer engineering professor Muhammad A. Alam, who co-authored the Nano Letters paper.

Electron microscope images showing a new material for transparent electrodes that might find uses in solar cells, flexible displays for computer and consumer electronics, and future optoelectronics circuits for sensors and information processing. The Purdue University electrodes are made of silver nanowires covered with graphene. (Bottom) A model depicting the “co-percolating” network of graphene and silver nanowires. Courtesy of Birck Nanotechnology Center, Purdue University.

“This is a beautiful illustration of how theory enables a fundamental new way to engineer material at the nanoscale and tailor its properties,” he said.

Such hybrid structures could enable researchers to overcome the electron-transport bottleneck of extremely thin films.

Combining graphene and silver nanowires overcomes drawbacks of each material individually: The graphene and nanowires conduct electricity with too much resistance to be practical for transparent electrodes. Sheets of graphene are made of individual segments called grains, and resistance increases at the boundaries between these grains. Silver nanowires, on the other hand, have high resistance because they are randomly oriented, making for poor contact between nanowires, resulting in high resistance.

“So neither is good for conducting electricity, but when you combine them in a hybrid structure, they are,” Janes said.

Findings show the material has a low “sheet resistance,” or the electrical resistance in very thin layers of material, which is measured in units called “squares.” At 22 ohms per square, it is nearly five times better than InSnO2, which has a sheet resistance of 100 ohms per square.

The hybrid structure also was found to have little resistance change when bent, whereas InSnO2 shows dramatic increases in resistance when bent.

“The generality of the theoretical concept underlying this experimental demonstration — namely ‘percolation-doping’ — suggests that it is likely to apply to a broad range of other 2-D nanocrystalline materials, including graphene,” Alam said.

A patent application has been filed by the university’s Office of Technology Commercialization.

Details of the research appeared in Advanced Functional Materials (doi: 10.1002/adfm.201300124).

For more information, visit: www.purdue.edu