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%.
The 4-year-old company said last week it expects to start production of its high-efficiency solar cells by early next year in its home town of San Jose, California. It is also awaiting word in the next few months on an $80 million loan from the Department of Energy, which would give it favorable financing to expand its current demonstration plant to produce 250 megawatts worth of cells per year, said co-founder Craig Stauffer.
Solar Junction cells are designed to be fitted into concentrating photovoltaic (CPV) solar collectors. Originally used in space, CPV systems concentrate the light hundreds of times using mirrors and lenses onto a small but relatively efficiency solar cell. They are typically mounted on racks to follow the sun in desert areas and are used for installations up to 50 megawatts.
These types of cells, called multijunction cells, achieve those higher conversion rates by using different materials than the traditional silicon cell and multiple semiconductors within a single package. During manufacturing, there are multiple layers of material deposited onto a gallium arsenide substrate, with each layer optimized to convert a different portion of the sunlight’s spectrum. “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,” Stauffer explained. There could be up to 20 layers of material used on each cell which is usually a square of about five millimeters, or just a fraction of an inch.
The main customers for these multijunction cells are CPV solar makers such as Amonix and Concentrix Solar in Germany. But even though this technology has been around for years, it still hasn’t become as established or widely used for wholesale electricity production as regular flat solar panels. Multijunction cells are more complex and expensive. But Stauffer said that the costs of CPV systems with those cells are getting more attractive due to efficiency gains and higher levels of concentration. CPV solar collectors can now concentrate light 1,000 times, compared to 500 times in the past year or two, he said.
Solar Junction expects to stand out from other multijunction suppliers with better reliability in high temperatures and higher efficiency, which Stauffer projects will go over 50 percent in five years as the company adds more layers to capture different wavelengths of light.