Solar cells still aren't economically competitive with traditional forms of energy generation. So efforts to reduce PV costs are getting a lot of attention. A company called Ampulse Corp. has found a way to make solar cells more economically simply by not thinking like a make of integrated circuits. An IC maker would approach the job of making a solar cell by starting with a silicon wafer and patterning it with features.That's a problem because silicon comprises more than half the cost of a crystalline-silicon solar cell.
Ampulse's approach instead is to grow a thin layer of silicon on a cheaper foil substrate. It only grows enough silicon to react to sunlight, usually tens of microns in thickness. This not only uses less silicon, it also is less energy intensive than having to depend on 1,400°C furnaces to create the crystalline silicon substrate in the first place.
Ampulse's underlying processes were actually developed at DOE's Oak Ridge National Laboratory (ORNL) and National Renewable Laboratory (NREL). Chemical vapor deposition is used to grow silicon on the foil, via a process perfected at NREL. ORNL developed the metal foil with the correct crystal structure to support the growth. A pilot manufacturing line is now in place at NREL's Process Development Integration Laboratory. When kinks have been worked out, Ampulse plans to design a full-scale production line that is a roll-to-roll process using long rolls of metal foil.
NREL's CVD process, called hot-wire chemical vapor deposition, was originally devised to thicken silicon wafers with perfect crystal coatings. It deposits silane gas molecules at about 700°C -- a much lower temperature than needed to make the wafer. The ORNL foil process is called RABiTS (rolling assisted biaxially textured substrate). It forms crystals in the foil able to receive the silicon atoms. Intermediate "buffer layers" coat the foil substrates so silicon can form via epitaxial growth.
Ampulse figures it could potentially produce a 15 to 18%-efficient solar cell -- far more efficient than that available from conventional thin-film PV techniques. Moreover, thinks the resulting cells will come in at less than 50 cents/W, and the capital investment necessary for the equipment to make the cells will be a small fraction of that needed for conventional solar cell lines.
To fabricate solar cells at the PDIL pilot production line, metal foils first sit under quartz lamps that heat them to a temperature of 850°C. The foils are then coated with the necessary buffer layers as they pass through a half-dozen cube-like vacuum chambers. A specially designed chamber is used to grow the key silicon layers. An exposure to atomic hydrogen improves cell electronic properties before a final operation installs cell junction and electrical contacts.
Ampulse says it is currently trying to maximize the cycle speed of the process.
Ampulse Corp.: http://www.ampulse.com/technology/
NREL's account of its work for Ampulse: http://www.nrel.gov/news/features/feature_detail.cfm/feature_id=1804