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SOLAR CELLS
THINNER THAN HUMAN HAIR

University of Florida engineers are pioneering an inexpensive method for making a new breed ofexceptionally thin and cheap solar cells expected to make solar power a more widespread source of electricity in the newmillennium.

The cells, known as "thin-film solar cells," are 100 times thinner and potentially lighter than today's silicon cells. They alsorequire less semiconductor materials, an attribute that opens the door for the cells to be made cheaply and in great quantity.

"The material cost is very minimal," said Sheng Li, a UF professor of electrical and computer engineering and part of afour-member UF faculty team at work on the process. "This is a very promising technology for solar cells."

The UF research comes at a time when the market for solar cells is in a strong growth spurt. For the past several years, theindustry has an annual growth rate of 15 to 20 percent, similar to that of the booming semiconductor and computer industries.Total sales reached the $1 billion mark in 1998, according to industry publications.

Solar cells' advantage is that they produce "green" power without harmful emissions. Also, they can generate power for a houseor small business on site, reducing electrical demand on power plants and electrical grids.

Traditional silicon cells, however, require relatively large amounts of semiconducting materials, making them expensive tomanufacture and driving up their cost in the marketplace. They also are heavy and unwieldy, limiting their potential applications,Li said.

The UF researchers hope to get around these problems through the thin-film cells, which can be created with "pennies' worth"of materials on flexible surfaces such as plastic. Where traditional solar cells use wafers similar to computer chips, thin film cellsuse thin layers of semiconductor material. Thin-film cells, however, have a far more complex chemical structure and are moredifficult to make than traditional cells, attributes that have limited their production and commercialization to date.

Scientists and engineers at more than a dozen universities, government labs and corporations are exploring several differentthin-film solar cell technologies as part of a major initiative sponsored by the National Renewable Energy Laboratory, agovernment lab that seeks to develop renewable energy technologies and improve energy efficiency.

The UF team, part of the "Thin Film Partnership program," is focusing on a technology that uses a compound semiconductorcalled "copper indium diselenide," or "CIS."

The technology involves depositing an extremely thin layer of CIS on a specially prepared material such as glass, Li said. Twoto 3 microns thick, the semiconductor layer is thinner than a human hair and 100 times thinner than conventional solar cells, hesaid.

Researchers elsewhere have demonstrated CIS cells can convert as much as 18 percent of sunlight to electricity, about theefficiency of the most efficient traditional silicon cells, according to the National Renewable Energy Laboratory. But there stillare major challenges to overcome in manufacturing the cells.

"They have a more complicated structure and require more complicated processing," said Tim Anderson, chairman of the UFchemical engineering department and member of the CIS team. "Our role is to better understand the processing and transfer thetechnology to industry."

The UF team also is experimenting with ways to simplify the manufacturing process, Li said. Three years into the six-yearproject, the team has used a simpler, cheaper process to make CIS cells with efficiencies in the range of 8 to 10 percent --good progress toward the project's goal of 13 to 15 percent efficiency, Li said. He added that he expects CIS cells to bewidely available on the market in less than 10 years.

Written by: Aaron Hoover ahoover@ufl.edu. Sources: Sheng Li, shengli@eng.ufl.edu, Tim Anderson, tim@nersp.nerdc.ufl.edu


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