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Combining NREL's Deposition Technique with ORNL's Textured Foil

A team of NREL scientists including Howard Branz and Chaz Teplin had developed a way to use a process called hot-wire chemical vapor deposition to thicken silicon wafers with perfect crystal coatings. Using a hot tungsten filament much like the one found in an incandescent light bulb, the silane gas molecules are broken apart and deposited onto the wafer using the chemical vapor deposition technique at about 700°C — a much lower temperature than needed to make the wafer. The hot filament decomposes the gas, allowing silicon layers to deposit directly onto the substrate.

Armed with this new technique, Branz and Teplin searched for ways to grow the silicon on cheaper materials and still use it for solar cells.

They found the ideal synergy when visiting venture capitalists from Battelle Ventures asked them whether they could do anything useful with a breakthrough at ORNL called RABiTS (rolling assisted biaxially textured substrate). It was just the opportunity the two scientists had been seeking.

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.

NREL and ORNL worked to combine their technologies using a small amount of funding from Battelle Ventures. Using the right intermediate “buffer layers” to coat the foil substrates, the researchers were able to replicate the desired foil crystal structure in the silicon layer grown over metal foil (epitaxial growth).

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