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"Germanium-on-Nothing" Solar Cell Fabrication Paves Way for Low-Cost, Industrial-Scale Thin-Film Production

image credit: *Courtesy NREL

Researchers at the Dept. of Energy's National Renewable Energy Laboratory (NREL) and the Korea Advanced Institute of Science and Technology have grown gallium arsenide (GaAs) PV cells on a reusable gallium substrate, paving the way forward for the advent of cheap, industrial-scale production.

The cost of the conventional substrates on which a crystalline wafer made from some other material, such as silicon, are grown typically account for 30% of the cost of multi-junction, III-V type solar PV cells, which are fabricated from elements in columns three and five of the periodic table. Dubbed Germanium-on-Nothing (GON), the new process eliminates the need for a different substrate, which can be reused in the fabrication of more GaAs PV cells.

“Silicon-on-nothing has been known for years, but this is the first time that GON has been demonstrated with a surface sufficiently smooth to allow high-quality epitaxial growth of GaAs,” said NREL's David Young, a senior scientist in NREL's High Efficiency Crystalline Photovoltaics group who co-authored the paper with NREL colleagues John Simon, Kevin Schulte, and Aaron Ptak and Prof. Jihun Oh, a former NREL scientist now a professor at the Korea Institute of Advanced Technology.

The Germanium-on-Nothing process

With GON, a thin layer of GaAs is deposited on top of a germanium wafer with cylindrical pores. The GaAs solar cells are grown in the cylindrical pores. They, along with the surface layer of GaAs of which they are a part, can then be peeled off the substrate and the germanium substrate reused.

"The ratio of the pores’ diameter to depth, coupled with the distance between the pores, allows for the creation of a void (nothing) between the suspended single-crystal layer of germanium and the surface of the germanium wafer. This technique could enable the cost-effective and high-volume production of single and multijunction III-V solar cells," an NREL news report explains.

The GaAs cells are approximately 3 micrometers (µm) (10-6 m) thick and thus very flexible when bonded to a flexible handle like plastic, Young told Energy Central. Furthermore, "III-V materials are well known for their stability in a variety of environments (earth and space). GON might allow the cost to be lowered for flexible III-V cells to enter new markets, especially if low-cost, NREL-developed HVPE (Hydride Vapor Phase Epitaxy) III-V growth is used," Young said.

Using GON, the research team produced a GaAs solar cell with 14.44% energy conversion efficiency. They expect to be able to increase that well above 20% by improving the fabrication process, Young said.

KAIST owns the GON intellectual property. "NREL helped to grow solar cells on the GON technology," Young explained, adding that "NREL and KAIST have applied for further funding to take the technology to the next research level."

The research team's results are detailed in new paper, "Germanium-on-Nothing Technology for Epitaxial Liftoff of GaAs Solar Cells," published in Joule.

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