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Going solar could mean going organic

Despite their advantages, there are many reasons to look for alternatives to existing silicon-based photovoltaic (PV) cells. Heavy, bulky, brittle, and aesthetically compromised, the older designs require clean-room manufacturing facilities, and are made using some less-than-clean materials and processes. Transportation and installation can also be expensive, resulting in higher capital costs.

Thin-film solar cells made of amorphous silicon and other materials address some of the drawbacks of current PV cells, including weight and flexibility—but some of these newer technologies raise environmental and safety questions of their own. More promising, but earlier in development, are organic solar cells, which have the potential to be relatively cheap, easier and cleaner to produce, and more versatile than existing solar technology.

Researchers at the Georgia Institute of Technology have developed a lightweight, flexible, organic photovoltaic cell using pentacene, a polycrystalline organic semiconductor, and the carbon molecule C60. Pentacene is often used in research on transistors, and C60 is in the family of carbon molecules commonly referred to as “buckyballs,” named for their resemblance to Buckminster Fuller’s designs.

The Georgia Tech organic solar cell consists of a glass plate, layers of indium oxide, pentacene, C60, and bathocuproine, and an aluminum electrode. According to lead researcher Bernard Kippelen, a professor of electrical and computer engineering at the university, it can be produced inexpensively and poses no environmental problems throughout its lifecycle.

For designers and builders, the cell’s benefits would include lower transportation costs and easier handling and installation, according to Kippelen. Layered on substrates as thin as a few microns, the cells would conform easily to most roof and wall shapes.

Organic semiconductors, however, are sensitive to moisture and oxygen, and a highly flexible plastic substrate will be needed to provide a sufficient barrier, he added. But while durability is a question mark—they’re unlikely to match the 20-to-30-year life span of silicon-based PV cells—the light weight and low cost of the cells would make frequent replacement feasible. “If you just have to peel them off and put new ones on, it could make sense to change the cells as often as every two years, especially if you can make them by the mile, printing roll to roll,” Kippelin said.

The researchers’ cell has a power conversion efficiency of 3.6 percent, slightly better than the 3.5 percent achieved by most existing organic cells.They expect to raise that to 5 percent soon, said Kippelen, who added that 10 percent efficiency can be achieved within the next few years. Typical silicon PV cells are about 10 to 15 percent efficient, with some high-end cells achieving closer to 30 percent efficiency. Kippelen stressed that a lot of research stands between this early work on organic cells and their widespread use. “Organic materials for semiconducting have only been around for about 10 years,” he said. “The science of these materials is not as advanced as for silicon. It’s difficult to predict what the upper efficiencies are going to be.”

Small versions of the Georgia Tech researchers’ cell—on the order of a square centimeter—could provide power to distributed building sensors or radio frequency identification (RFID) tags within a couple of years. Larger solar panels or rolls of sheeting might be 5 to 10 years away, according to Keppelen.

Environmentally, making organic PV cells poses no significant problems compared to some of the more advanced thin-film solar cells that use harsh chemicals containing cadmium, copper, and arsenic, said Keppelen. “During the manufacture of these cells, people are exposed to nasty chemicals and the process generates toxic waste,” he said. “The materials we’re using are carbon-based and fairly harmless. Photovoltaic technology should be environmentally friendly,” he said.

Ted Smalley Bowen

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