The US Department of Energy’s National Renewable Energy Laboratory (NREL), the Swiss Center for Electronics and Microtechnology CSEM (Centre Suisse d’Electronique et de Microtechnique) and EPFL (École Polytechnique Fédérale de Lausanne) in Switzerland have raised the record one-sun conversion efficiency of III–V/Si silicon-based multi-junction solar cells to 32.8% for two junctions and 35.9% for three junctions (‘Raising the one-sun conversion efficiency of III–V/Si solar cells to 32.8% for two junctions and 35.9% for three junctions’, Stephanie Essig et al, Nature Energy 6, 17144 (2017) doi:10.1038/nenergy.2017.144).
Efficiency rates of more than 35% have previously been reached in the laboratory, but with cells that only use expensive materials. The existing photovoltaics (PV) market is dominated by cost-effective modules made of single-junction silicon solar cells, with efficiencies of 17-22%. Like many research centers and industrial players, NREL, CSEM and EPFL are working with multi-junction cells, combining silicon with a cell that absorbs blue light from the sun more efficiently. The transition from a silicon single-junction cell to a silicon-based multi-junction solar cell has the potential to push efficiencies past 30% while still benefiting from the cost-effective manufacturing expertise in making silicon solar cells. In January 2016, the NREL/Swiss team was able to reach 29.8% efficiency, setting their first joint world record.
The CSEM/EPFL/NREL team has now raised their efficiency record to 32.8 % for a dual-junction cell (by combining an NREL GaAs top cell with a CSEM silicon heterojunction bottom cell) and 35.9% for a triple-junction cell (by combining an NREL GaInP/GaAs top cell with a CSEM silicon heterojunction bottom cell). “It shows, for the first time, that silicon-based tandem cells can provide efficiencies competing with more expensive multi-junction cells consisting entirely of III-V materials,” says Adele Tamboli, a senior researcher at NREL. “It opens the door to develop entirely new multi-junction solar cell materials and architectures,” she adds.
“These records show that combining crystalline silicon and other materials is the way forward if we are to improve solar power’s cost/efficiency ratio,” believes Christophe Ballif, director of CSEM’s PV-center and EPFL Photovoltaics laboratory. “It affirms that silicon heterojunction solar cells, when integrated into the structure that we’ve developed, can generate multi-junction cell conversion efficiencies over 32%,” adds Matthieu Despeisse, manager of crystalline silicon solar cells activities at CSEM.
CSEM is working on such silicon-based multi-junction solar cells and is also testing new materials to be used as a top cell in order to optimize the cost/efficiency ratio. For example, it is studying the potential use of perovskite material as part of the European Space Agency (ESA) CHEOPS project.