Plastic Solar Cell Efficiency Hits 6% in U.S. Lab

That percentage may not seem like a huge landmark compared to a photovoltaic (PV) cell achieving an efficiency rating of say 40.7 percent, which was the milestone attained by Spectrolab, Inc. in December 2006. But until two years ago, the highest efficiency ever achieved for plastic solar cells was just three percent.

In 2005, David Carroll, director of the Wake Forest nanotechnology center, and his research group announced they had come close to reaching 5 percent efficiency. Now, a little more than a year later, Carroll said his group has surpassed the 6 percent mark.

"Within only two years we have more than doubled the 3 percent mark," Carroll said. "I fully expect to see higher numbers within the next two years, which may make plastic devices the photovoltaic of choice."

Because they are inexpensive and light weight, especially in comparison to traditional silicon solar panels, researchers have worked for years to create flexible, or "conformal," organic solar cells that can be wrapped around surfaces, rolled up or even painted onto structures.

In theory, plastic solar cells could be used as a replacement for roof tiling or home siding products or incorporated into traditional building facades. These energy harvesting devices could also be placed on automobiles since plastic solar cells are much lighter than the silicon solar panels structures do not have to be reinforced to support additional weight.

In a paper published in the journal Applied Physics Letters, the Wake Forest researchers describe how they have achieved record efficiency for organic or flexible, plastic solar cells by creating "nano-filaments" within light absorbing plastic, similar to the veins in tree leaves. This allows for the use of thicker absorbing layers in the devices, which capture more of the sun's light.

In order to be considered a viable technology for commercial use however, solar cells must be able to convert about 8 percent of the energy in sunlight to electricity. Wake Forest researchers hope to reach 10 percent in the next year, said Carroll, who is also associate professor of physics at Wake Forest.

A large part of Carroll's research is funded by the United States Air Force, which is interested in the potential uses of more efficient, light-weight solar cells for satellites and spacecraft.

Other members of Carroll's research team include Jiwen Liu and Manoj Namboothiry, postdoctoral associates at Wake Forest's nanotechnology center, and Kyungkon Kim, a postdoctoral researcher at the center, who has moved to the Materials Science & Technology Division at the Korea Institute of Science and Technology in Seoul.