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Transparent electrode paves way for flexible solar cells

Posted: 27 May 2013     Print Version  Bookmark and Share

Keywords:Purdue University  electrode  solar cells  flexible displays  optoelectronic 

Researchers at Purdue University have created a new type of transparent electrode that could be used in solar cells, flexible displays for computers and consumer electronics and future "optoelectronic" circuits for sensors and information processing.

The electrode is made of silver nanowires covered with a material called graphene, an extremely thin layer of carbon.

"The hybrid material shows promise as a possible replacement for indium tin oxide, or ITO, used in transparent electrodes for touch-screen monitors, cell-phone displays and flat-screen televisions," the researchers said.

"If you try to bend ITO it cracks and then stops functioning properly," said Purdue University doctoral student Suprem Das.

Combining graphene and silver nanowires in a hybrid material 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 like a jumble of toothpicks facing in different directions. This random orientation makes 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. "The graphene is draped over the silver nanowires. It's like putting a sheet of cellophane over a bowl of noodles. The graphene wraps around the silver nanowires and stretches around them."

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 five times better than ITO, which has a sheet resistance of 100 ohms per square.

Moreover, the hybrid structure was found to have little resistance change when bent, whereas ITO 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 nanocrystaline material, including graphene," Muhammad A. Alam, a Purdue professor of electrical and computer engineering.





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