Solar researchers on both sides of the Pacific are looking to space for better solar cells. In separate announcements it has emerged Chinese module manufacturer Jinko Solar and the U.S. National Renewable Energy Laboratory (NREL) are both exploring the production of PV technologies used in space to improve solar power returns back on Earth.
At the NREL, researchers claim to have made a breakthrough in III-V cell technology which they say could bring down the costs of the highly efficient – and very expensive – cells quite significantly. The team said it has grown aluminum indium phosphide (AlInP) and aluminum gallium indium phosphide (AlGaInP) in a hydride vapor phase epitaxy reactor.
Referring to the groups of the periodic table in which such materials are found, III-V solar cells are commonly used in space applications, such as to power satellites or the Mars Rover. More efficient than the silicon wafer-based cells used on earth, they are prohibitively expensive.
An epi-taxing problem
The expense is largely bound up in the two-hours-per-cell metalorganic vapor phase epitaxy (MOVPE) production process, which involves several chemical vapors being deposited onto a substrate in a single chamber.
A partial solution was suggested by the NREL with its dynamic hydride vapor phase epitaxy (D-HVPE) process which reduced the time required to less than a minute per cell. However, the inability to incorporate an aluminum content layer meant cell efficiency dropped.
Using D-HVPE, the NREL made solar cells from gallium arsenide (GaAs) and gallium indium phosphide (GaInP) with the latter working as a “window layer” to passivate the front while permitting light to pass through to the GaAs absorber layer. However, the GaInP layer is not as transparent as the AlInP layer which can easily be grown in a MOVPE reactor.
The world efficiency record for MOVPE-grown GaAs solar cells with AlInP window layers is 29.1%. For GaInP alternatives, the maximum figure for HVPE-grown solar cells is estimated to be 27%.
“There’s a decent body of literature that suggests that people would never be able to grow these compounds with hydride vapor phase epitaxy,” said Kevin Schulte, a scientist in the NREL’s Materials Applications & Performance Center and lead author of a paper highlighting the new research. “That’s one of the reasons a lot of the III-V industry has gone with [MOVPE], which is the dominant III-V growth technique.” Referring to the latest development, Schulte added: “This innovation changes things.”
The NREL team said they had been working to improve the economics of GaAs cells by moving the technology forward incrementally. Firstly, the D-HVPE process reduced costs and now aluminum growth means improved efficiency. With aluminum added to the D-HVPE mix, the scientists said they should be able to reach parity with MOVPE solar cells.
The laboratory last year produced a 25.3% efficient GaAs cell using D-HVPE. Kelsey Horowitz, part of the techno economic analysis group at the NREL’s Strategic Energy Analysis Center, suggested D-HVPE cells made at scale could generate electricity at $0.20-0.80/W, with the help of some tweaks and said applications such as electric vehicle integration, systems for roofs not strong enough to support a silicon PV array, and portable or wearable solar panels could be viable at that cost. “There are these intermediate markets where higher prices can be tolerated,” she said.
“The HVPE process is a cheaper process,” said Aaron Ptak, a senior scientist at the NREL’s National Center for Photovoltaics. “Now we’ve shown a pathway to the same efficiency that’s the same as the other guys but with a cheaper technique. Before, we were somewhat less efficient but cheaper. Now there’s the possibility of being exactly as efficient and cheaper.”
Across the Pacific, Jinko Solar has signed a memorandum of understanding with the Shanghai Institute of Space Power-Sources to jointly develop high-efficiency solar cell technology. The solar manufacturer said it will use a more robust silicon wafer as the supporting substrate and bottom cell.
Jinko did not provide any further detail regarding cell technology but said its high-efficiency solar tech would take advantage of the cheap availability of silicon wafers and would easily transfer into large scale manufacturing.
“The strategic cooperation with [the] Shanghai Institute of Space Power-Sources has great importance,” said Jin Hao, VP of Jinko Solar. “In the future we will continue to increase technical cooperation, leading our industry in the name of technical innovation and providing more efficient solar panels with a wider range of choices for global customers.”
Jinko predicted the new cell technology would prompt a higher conversion rate than current technologies but said more research was needed.
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