The NREL researchers used an acoustically cleaved gallium arsenide substrate, which is said to reduce electrical shunt, potentially lowering manufacturing costs. The cell achieved an open-circuit voltage of 1.061 V, a short-circuit current density of 29.9 mA/cm2, and a fill factor of 84.9%.
The production cost of solar cells based on compounds of III-V elemental materials, named after the groups of the periodic table to which they belong, has limited these devices to narrow applications such as drones and satellites, where low weight and high efficiency are more important concerns than the cost of the energy produced.
“Reducing manufacturing costs is considered key for terrestrial applications, and one way to achieve this is the ability to repeatedly reuse the substrate on which cells are grown,” the researchers noted. This was the motivation to develop acoustic chipping, a new process that they claim represents a low-cost reuse pathway for III–V epitaxial growth substrates through substrate recovery and reuse.
In the paper “GaAs Solar Cells Grown on Acoustically Cracked GaAs Substrates at 27% Efficiency”, published recently Joulethe researchers explained that acoustic cracking, also known as Sonic lift-off (SLO), is an experimental process that uses sound waves to control the advance of the crack tip during cracking to prevent facet formation and improve surface smoothness.
“Current technology uses a sacrificial etch layer to lift the cell off the gallium arsenide (GaAs) substrate so the substrate can be reused, but the process takes hours and leaves a residue that requires a polishing step,” the team explained. “Polishing is relatively expensive and limits the potential cost savings of this substrate reuse method. In contrast, cracking takes seconds, creating a controlled fracture in the substrate nearly parallel to its surface. This fracture allows for easy removal of the cell, revealing a new, contaminant-free surface inside the substrate that does not require polishing.”
The academics built photovoltaic devices with a pin structure and a 1 mm thick emitter made of GaAs and selenium (SE), a 1 mm doped GaAs layer and a 1 mm base layer of GaAs and zinc (Zn) on a crack-free SLO substrate without wet etching.
Tested under standard lighting conditions, the master device fabricated 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 duty cycle of 84.9%, with no evidence of nonlinear interference in the cell. reduces the current through the solar cell junction and reduces the voltage of the solar cell.
“These results enable the epitaxial growth of high-performance devices on potentially lower-cost substrates with scale properties,” the researchers said, adding that the acoustic chipping used to build the cells was developed at Arizona State University and is now being commercialized by Phonenix-based startup Crystal Sonic Inc. improves the flatness of the platform.
In July 2022, other NREL researchers developed a III-V solar cell based on a fissile germanium (Ge) substrate. They used sputtered Ge instead of gallium arsenide (GaAs) because the former, commonly used in space applications, is known to reduce many of the problems associated with GaAs spalling.
This solar cell was able to achieve a power conversion efficiency of 23.36% with no cracking defects. It also achieved an open circuit voltage of 1.019 V and a short circuit current density of 28.49 mA cm−2and a fill factor of 80.45%. According to the researchers, these results show that cracked germanium does not need to be restored to a pristine, polished state to achieve high-quality device performance.