Fraunhofer ISE claims 25.9% efficiency for III-V tandem solar cell

Germany’s Fraunhofer Institute for Solar Energy Systems ISE claims to have achieved a 25.9% conversion efficiency rate for a III-V tandem solar cell grown directly on silicon.

This result marks an improvement on the institute’s previous 24.3% efficiency record.

“This was achieved by replacing a single thin layer within the multi-junction cell,” said researcher Markus Feifel. “A careful analysis of our cells showed that this layer created a potential barrier for electrons moving through the crystal structure.”

Silicon was used as an absorber for the infrared part of the spectrum. Several micrometer-thin layers of III-V compound semiconductors were deposited on top of the silicon layer. All of the cell’s separate absorbers are internally interconnected by additional crystal layers.

The cell is a modified version of a 34.5%-efficient III-V solar cell that is manufactured through a process known as direct wafer bonding, where the III-V layers are first deposited on a aluminum gallium arsenide (GaAs) substrate and then pressed together.

“This technology is very efficient but also expensive,” the scientists said. “For this reason, Fraunhofer ISE has been working for many years on more direct manufacturing processes in which the III-V layers are deposited, or grown directly onto a silicon solar cell.”

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 such 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.