Trees capture sunlight to produce chemical energy, and they may soon be teaming up with the sun to produce electricity, as well. A team of researchers at Georgia Institute of Technology and Purdue University has created recyclable solar cells using tree-based substrates instead of silicon.
“As more and more solar panels are deployed on rooftops and elsewhere, we asked ourselves if we could make the panels even greener,” said Bernard Kippelen, an engineering professor at Georgia Tech. “Our alternative technology is to replace some of the silicon solar films with organic solar films.”
The researchers have used cellulose nanocrystals, which Kippelen calls “the smallest constituent of a tree,” as the building block of the substrate of the solar cells. By mixing the nanocrystals with glycerol, they created a transparent organic film to hold the organic electronic layers, which convert the sunlight into electrical power. Because the organic film is water soluble, the solar cells can be easily separated for recycling.
“From the very beginning, we wanted to make sure that the solar cells were recyclable,” said Kippelen. “Otherwise we are simply solving one problem – less dependence on fossil fuels, while creating another – a technology that produces energy from renewable sources but is not disposable at the end of its lifecycle.”
According to Kippelen, the organic solar cells achieve a power conversion efficiency of up to 3.8 percent, and he expects they should be able to reach seven to eight percent in a short time. The most efficient silicon-based solar cells reach nearly 20 percent efficiency.
“Our work is a first step in a new direction of trying to make green solar energy even greener,” said Kippelen. “If we find that the path is viable, it could be an interesting new application for high value forest products. It can be produced from standard pulp from the paper industry, which is looking for new applications since demand for paper is going down in the digital age. If people are thinking about deploying solar panels to produce hundreds of gigawatts of power, then we would need millions of tons of cellulose.”
The primary advantage of the technology is also its only problem. Because the organic film dissolves so easily in water, it will likely fall apart when it rains, so the researchers are working on developing a special coating to protect it.
“The active layers that absorb sunlight and convert it to electricity in traditional solar cells are also sensitive to water and oxygen, so they need to be protected anyway. If we have to protect the active layer, we might as well protect the substrate, too,” said Kippelen, who expects it will take another 10-15 years of research before the technology can be commercialized. “When it comes to recycling, you would just have to break that hermetic package to allow water to get in.”