Solar Junction is a 4-year-old company spun out of Stanford University that designs high-efficiency, multi-junction solar cells for concentrating photovoltaic (CPV) solar collectors. The National Renewable Energy Laboratory recently certified that its solar cells can operate at 40.9% efficiency, a significantly higher efficiency than typical silicon solar cells that convert sunlight to electricity at an efficiency of about 15-20%.

Multi-Junction Solar Cells

How do these “multi-junction” solar cells convert sunlight into electricity so efficiently? They use “different materials than the traditional silicon cell and multiple semiconductors within a single package.”

“In essence, you have three basic subcell materials that take in some light and pass the rest to the next. They are connected serially inside the device just like battery cells,” Solar Junction co-founder Craig Stauffer explains.

The technology isn’t actually new, but it “hasn’t become as established or widely used for wholesale electricity production as regular flat solar panels” due to the cells’ complexity and price. However, Stauffer says the costs are coming down due to efficiency improvements and higher concentration levels. “CPV solar collectors can now concentrate light 1,000 times, compared to 500 times in the past year or two,” and Stauffer thinks Solar Junction’s solar cells will achieve efficiencies of over 50% in the next five years.

Solar Junction expects to begin production of its multi-junction solar cells sometime in early 2012 in San Jose, California. The company is currently waiting for a decision from the Department of Energy regarding an $80-million loan, “which would give it favorable financing to expand its current demonstration plant to produce 250 megawatts worth of cells per year.”

Solar Junction Not Alone

Along with Solar Junction, other companies are working fast to develop this technology as well. Spire Semiconductor reportedly set a new world record for solar cell efficiency in October 2010. Working with the US National Renewable Energy Laboratory (NREL) under an 18 month incubator project, the company produced a “triple-junction cell” that has apparently achieved a 42.3% conversion efficiency. Other companies have been working on this technology with great success as well.

“That figure beats the previous best of 41.6 per cent, set by Spectrolab last year, after the Fraunhofer Institute for Solar Energy (ISE) had achieved 41.1 per cent in early 2009, and maintains the steady increase in “champion” multi-junction cell performance seen over the past couple of decades.”

Conversion efficiency of solar cells is very important for these solar technologies because the cells are a large proportion of the system cost.

Spire also says that its solar cell technology is now ready for production, “with immediate commercial availability.”

cleantechnica.com