The surface of Mars has been explored by rovers powered by III-V solar cells. Credit: National Renewable Energy Laboratory
Inspired by a Mars rover mission 15 years ago, researchers are revamping a technique to produce solar cells.
Scientists from the U.S. Department of Energy’s National Renewable energy Laboratory (NREL) are hoping to refine a technique called hydride vapor phase epitaxy (HPVE) to produce cheaper, more efficient solar cells capable of producing more electricity.
In 2003, NASA sent rovers to Mars that relied on gallium arsenide (GaAs) and Gallium indium phosphide (GaInP) solar panels, which were about the size of a kitchen table and capable of converting about 27 percent of sunlight into electricity. These GaAs and GalnP solar panels—which include semiconductors made from those and similar elements and are collectively called III-V solar cells— are extremely efficient and expensive.
However, to use the same technology on Earth—where the average rooftop solar panel is 15 percent efficient—the costs would be substantial.
“You can only buy gallium arsenide cells, if you’re willing to pay $100 to $300 a watt,” David Young, a senior scientist at NREL with an expertise in silicon solar cells, said in a statement.
III-V solar cells are currently produced using a process called metal organic vapor phase epitaxy (MOVPE), which deposits the elements by layer atop a semiconductor wafer in a time-consuming process.
“You essentially dose pre-engineered chemicals onto a hot wafer, and they will deposit as thin-film layers with the same lattice spacing as the wafer,” said Young. “Multilayer devices are formed by changing the gas mixture to form different compositions of stacked thin films. MOVPE can grow very complicated structures—or devices like solar cells—but it’s expensive and slow.”
HVPE is a process that uses a single chamber where one chemical is deposited on a substrate that was then removed. The chemical is then swapped for another with the substrate returning to the chamber for the next chemical application. The layers grown atop the wafer must line up precisely to avoid defects in the solar cell.
However, this process fell out of favor in the 1960s in favor of MOVPE.
“HVPE is far from new,” Aaron Ptak, a senior scientist who joined NREL in 2001, said in a statement.
It’s been around since the ’50s and ’60s. We fondly refer to it as our brand new, 50-year-old growth technique.”
The new and refined version of HVPE—called dynamic HVPE (D-HVPE)—uses a dual chamber reactor where the substrate moves back and forth between the chambers to greatly reduce the time it takes to make a solar cell. A single junction solar cell that takes an hour or two using MOVPE can be produced in just two minutes using the new HVPE method. This method comes with a 25.3 percent efficiency, just shy of the world record for such a cell, which is 28.8 percent and was grown using MOVPE by a California company “using a fancier structure that we can’t grow,” Ptak said
“It’s actually way higher than we thought we would get in this program,” Ptak said. “When we started this program, we thought, ‘OK, we’re going to make cheap solar cells and they’re going to be kind of bargain-basement solar cells, but they’re going to be good.’ What we have learned during the course of this project is we need to shoot higher. We can shoot higher because the material quality that we’re seeing, the device quality that we’re seeing, is way better than we expected.”
According to Ptak, the military is interested in lighter, more flexible solar cells, as well as drone operators.
