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The NREL researchers used an acoustically thickened gallium arsenide substrate that appears to reduce electrical drift and therefore manufacturing costs.
Researchers at the US Department of Energys National Renewable Energy Laboratory have fabricated a III-V gallium arsenide (GaAs) solar cell based on a spalled GaAs substrate.
The cost of producing solar cells based on compounds of III-V element materials, named after the groups of the periodic table to which they belong, has confined these devices to niche applications, such as drones and satellites, where low weight and high efficiency are more pressing concerns than the cost of energy produced.
“Reducing manufacturing costs is considered key for terrestrial applications, and one way to achieve this is to have the ability to repeatedly reuse the substrate on which cells are grown,” the researchers note. This was the motivation to develop acoustic spalling, a new process they claim represents a low-cost reuse pathway for III-V epitaxial growth substrates with substrate recovery and reuse.
In the article “ GaAs solar cells grown on acoustically spalled GaAs substrates with 27% efficiency ,” recently published in Joule , researchers explain that acoustic spalling, also known as sonic lift-off (SLO), is an experimental process that uses sound waves to control the propagation of the crack tip during spalling in order to suppress faceting and improve surface flatness.
“Current technology uses a sacrificial etch layer that allows a cell to be detached from a gallium arsenide (GaAs) substrate for reuse, but the process takes hours and leaves a residue that requires a polishing step,” the group explains. “Polishing is relatively expensive and limits the potential savings of this substrate reuse method. Instead, spalling takes seconds and creates a controlled fracture in the substrate nearly parallel to its surface. This fracture allows the cell to be easily removed, revealing a new, contaminant-free surface from within the substrate that does not require polishing.”
The researchers built PV devices with a 1-mm-thick nip structure and emitter made of GaAs and selenium (SE), a 1-mm doped GaAs layer and a 1-mm base layer made of GaAs and zinc (Zn) on an SLO substrate as polished, without wet etching.
Tested under standard lighting conditions, the champion device built with this architecture achieved a certified power conversion efficiency of 26.9%, an open circuit voltage of 1.061 V, a short circuit current of 29.9 mA/cm2 and a fill factor of 84.9%, without the cell showing signs of non-linear drift, which reduces the current flowing through the solar cell junction and lowers the cell voltage.
“These results enable epitaxial growth of high-throughput devices on potentially lower-cost substrates with millimeter-scale features,” the researchers note, adding that the acoustic spalling used to construct the cells was developed at Arizona State University and is now commercialized by Phonenix-based startup Crystal Sonic Inc.. “This latest advance uses acoustic spalling, or sound waves, to control fracture, suppress faceting, and improve substrate flatness.”
In July 2022, other NREL researchers developed a III-V solar cell based on a spiked germanium (Ge) substrate. They used weathered Ge instead of gallium arsenide (GaAs), since the former, which is commonly used in space applications, purportedly reduces several problems associated with weathering of GaAs.
This solar cell achieved a power conversion efficiency of 23.36% without detachment defects. It also achieved an open circuit voltage of 1.019 V, a short circuit current density of 28.49 mA cm-2, and a fill factor of 80.45%. According to the scientists, these results demonstrate that it is not necessary to return chipped germanium to a pristine and polished state to achieve high-quality performance from the device. |
