NREL post-doc scientist, Rene Rivero, operates a Solar/Wafer Screening Tool for PV Industry. This tool tests a pre-selection of wafers of high fracture strength, improving the yield of Si solar cells by preventing their breakage during cell fabrication. Courtesy NREL.

NREL

Greg Wilson, director of the National Center for Photovoltaics at the National Renewable Energy Laboratory in Golden, CO, reports that his facility has is conducting research in three major areas. For starters, researchers are working in the III-Vs, which refers to semiconductors made from atoms in columns III and V of the periodic table, the most common one being gallium arsenide.

Another part of its research is in conventional single-crystal silicon. “Nearly all of our work at NREL right now is focused on silicon tandems. This involves taking the best cell made and adding a top cell that will harvest another portion of the light spectrum in a two-junction cell,” Wilson says. Many groups around the world are working on this concept, which poses a challenge in getting the cell joined cost effectively, whether you use epitaxy or bonding. NREL is exploring both routes and says the top cell will likely be a III-V.

If done correctly, Wilson says, this has the potential to add about ten points of efficiency to an already high-efficiency silicon part itself. “This approach could produce silicon tandems and have an efficiency of greater than 30 percent.”

The third area is thin-film, which NREL has been exploring for 35 years. Researchers are investigating different materials, including cadmium telluride (the semiconductor that First Solar technology is based on), CIGS (copper indium gallium selenium), and a GIGS substitute known as CZTS, an earth-abundant potential replacement.

Thin-film offers advantages over conventional cells made with crystalline semiconductor material, mainly lower manufacturing costs because it uses a low-temperature process similar to printing instead of the high-temperature vacuum deposition process. It also has the advantage of lighter weight and the ability to be integrated into building materials such as siding, roofing, and tent fabrics.

But as Wilson says, “Silicon isn’t going anywhere. There are a lot of reasons silicon dominates the market today.” Thin-film hasn’t achieved the efficiency silicon has. “There’s another problem with thin-films that’s always out there, and that’s long-term reliability. Silicon has the best, most well-understood degradation behavior over 30 years in the field.” Polycrystalline thin-films degrade faster than silicon, but Wilson thinks that can be solved with more research.