III–V solar cell based on spalled germanium substrate achieves 23.36% efficiency

Researchers from the US Department of Energy’s National Renewable Energy Laboratory (NREL) have developed a III-V solar cell based on a substrate made of spalled germanium (Ge).

They used spalled Ge instead of gallium arsenide (GaAs), as the former, which is commonly used in space applications, reportedly reduces several issues associated with GaAs spalling. “It offers both alignment between a preferred growth orientation and an available cleavage system,” they specified. “Ge largely spalls flatly but the fracture can result in morphological defects, including arrest lines and river lines.”

Arrest lines can have a negative impact on solar cell performance as they tend to lead to device shunting, which reduces the current flowing through the solar cell junction and reduces the voltage from the solar cell. “Arrest line impact on device performance can vary from minimal to complete shunting of the device depending on the severity of the arrest line morphology,” the group explained. “The most evident indicator of arrest line severity is the height profile across the arrest line.”

The solar cell is based on gallium, indium, and arsenide (GaInAs) grown by organometallic vapor phase epitaxy (OMVPE) directly on spalled Ge substrates that undergo minimal surface processing. “By patterning 5 mm × 5 mm cells across entire 50 mm wafers, we isolate regions that are defect-free and those that exhibit different morphological defects,” the scientists said.

Using electroluminescence (EL) and dark lock-in thermography (DLIT) imaging, the academics identified three morphological spalling defect types in the Ge substrates. They associated each spalling defect type with a distinct impact on cell behavior and performance using a variety of functional and physical characterization techniques.

The solar cell was able to achieve a power conversion efficiency of 23.36% where no spalling defects were present. 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 show that spalled germanium does not need to be returned to a pristine, polished state to achieve high-quality device performance.

They described the cell technology in the paper High-Efficiency Solar Cells Grown on Spalled Germanium for Substrate Reuse without Polishing, published in Advanced Energy Materials.

The cost of producing solar cells based on compounds of III-V element materials–named according to the groups of the periodic table that they belong to–has confined these devices to niche applications including drones and satellites, where low weight and high efficiency are more pressing concerns than costs, in relation to the energy produced.