The electrical engineering team from NC State focused on the connection junctions between the layers of the stacked cells, also known as multi-junction cells. The research appeared in a paper published in the online September 5 issue of Applied Physics Letters.
Stacked solar consists of layers of cells that can capture a larger portion of the solar spectrum because photons not absorbed by the first cell are transmitted to the second and sometimes third cell, where some of the remaining solar radiation is then absorbed.
One of the limitations of how much solar energy can be converted to electricity is the connecting junctions between the cells. The junctions absorb some portion of the solar energy and also siphon off the voltage produced by the cells, according to the researchers.
“We have discovered that by inserting a very thin film of gallium arsenide into the connecting junction of stacked cells we can virtually eliminate voltage loss without blocking any of the solar energy,” Salah Bedair, a professor of electrical engineering at NC State and senior author of the paper, said in a statement.
Currently, stacked solar can convert about 45 percent of solar energy into electricity, which is far higher than other technologies. Polycrystalline silicon solar modules, for example, are about 15 percent efficient. The goal in multi-junction solar cells is to break the 50 percent conversion efficiency barrier. Sharp currently has a world-record 44.4 percent for its triple-junction solar cell.
Multi-junction cells are used in concentrated solar power (CSP) applications, where lenses are used to concentrate the sun’s energy onto the panels. Concentrating the energy can increase the power of one sun to thousands of suns.
One of the limitations of concentrated solar power is that when the solar energy is intensified to about 700 suns or more, the connecting junctions being to lose voltage, according to the researchers.
“Now we have created a connecting junction that loses almost no voltage, even when the stacked solar cell is exposed to 70,000 suns of solar energy,” Bedair said. “And that is more than sufficient for practical purposes, since concentrating lenses are unlikely to create more than 4,000 or 5,000 suns worth of energy.” Although the novel connecting junctions would have less voltage loss, it is not exactly clear at this time how much more conversion efficiency it would ultimately allow for in commercially-produced cells.
There are various CSP projects being built across the U.S., such as BrightSource Energy’s 370-megawatt Ivanpah project and the Middle East and South Africa are also increasingly active CSP markets.
One of the limitations of CSP is its high cost, but Bedair said that the more effective junctions should help drive down the overall cost because producers could make much smaller cells that produce as much electricity or more compared to larger cells.
Photo: NC State University