Growing Silicon Crystals Directly on Foil
By BILL SCANLON
Ampulse Corp., working with the Oak Ridge National Laboratory (ORNL) and the National Renewable Energy Laboratory (NREL), has developed a chemical vapor deposition process to grow silicon on inexpensive foil, creating cells just thick enough — 10 microns or less — to convert most of the solar energy into electricity. The process eliminates the waste involved in sawing wafers from massive silicon boules. The company has built a pilot production line in the Process Development Integration Laboratory (PDIL) at NREL.
A team of NREL scientists, including Howard Branz and Chaz Teplin, in 2005 developed a way to thicken silicon wafers with a perfect crystal coating. A hot tungsten filament decomposes the silane gas at about 700°C (1,292°F), allowing silicon layers to deposit directly onto the substrate.
Then in 2007, visiting investors from Battelle Ventures asked Branz and Teplin whether they could use a breakthrough at ORNL called RABiTS (rolling-assisted biaxially textured substrates). If metal foil is to work as a substrate, it must be able to act as a seed crystal so the silicon can grow on it with the correct structure. The RABiTS process forms crystals in the foil that are correctly oriented to receive the silicon atoms and lock them into just the right positions.
Using the correct intermediate buffer layers to coat the foil substrates, the researchers were able to replicate the desired foil crystal structure in the silicon layer grown epitaxially over metal foil.
Battelle Ventures and Innovation Valley Partners then formed Ampulse, and established a $500,000 cooperative research and development agreement (CRADA) with NREL and a similar agreement with ORNL. the company now has 13 employees and six full-time consultants and is currently working with 22 sponsored researchers from two national labs.
“We have the potential to produce a 15 percent efficient solar cell at less than 50 cents per watt with a fraction of the capital investment of other venture-funded [photovoltaics] companies,” said Ampulse CEO Steven Hane. “And that’s due to our R&D collaborations with the national labs.”
The production line at PDIL consists of half a dozen vacuum chambers where foils are overcoated with buffer and silicon layers. It was built to Ampulse’s specifications by Roth & Rau Microsystems of Germany. Quartz lamps heat metal foils to 850°C (1,562°F) and the foils are coated with the necessary buffer layers. After transfer to a specially designed chamber, the key silicon layers are grown. The silicon is then exposed to atomic hydrogen to improve its electronic properties. Finally, solar cell junction and electrical contacts are developed.
“The main thing is that we can grow high-quality silicon layers very fast and without putting much energy into the process. That means the solar cells can turn out much cheaper than the wafer-based cells,” said Branz.
“Our process goes directly from gas to the epitaxial silicon phase, bypassing the growth and sawing phase,” said Mike Colby, Ampulse’s director of planning and logistics. “The goal is to achieve the crystal silicon performance that until now focused on thicker wafers — and without having to use a 1,400°C [2,552°F] furnace.”
Bill Scanlon is a writer in the Public Affairs Office at NREL.