New 35.9% Cell Efficiency Record

• Research institutes CSEM, NREL and EPFL have achieved new record conversion efficiencies for silicon based multi-junction solar cells
• For dual-junction solar cells, a 32.8% efficiency was achieved, combining NREL's GaAs top cell to CSEM's silicon heterojunction bottom cell
• For a triple-junction solar cell, 35.9% was reached by combining NREL's GalnP/GaAs top cell to CSEM's silicon heterojunction bottom cell
• Previously, they achieved a record efficiency of 29.8%  for a silicon-based multi-junction cell in January 2016

The Center for Electronics and Microtechnology (CSEM) and the École Polytechnique Fédérale de Lausanne (EPFL), both from Switzerland, together with the US National Renewable Energy Laboratory (NREL) have announced new record conversion efficiencies for III-V/Si multi-junction solar cells – 32.8% for two junction cells and 35.9% for three junction cells. They have beaten their previous 29.8% efficiency record achieved in January 2016.

The three achieved reached a 32.8% efficiency for a dual-junction solar cell by combining NREL GaAs top cell to CSEM's silicon heterojunction (SHJ) bottom cell. For the triple-junction 35.9% record cell, they combined NREL's GalnP/GaAs top cell to CSEM's SHJ bottom cell.

"This achievement is significant because it shows, for the first time, that silicon-based tandem cells can provide efficiencies competing with more expensive multijunction cells consisting entirely of III-V materials," said Adele Tamboli, a senior researcher at NREL. "It opens the door to develop entirely new multi-junction solar cell materials and architectures."

Today's commercial solar modules are generally made from single-junction silicon solar cells, with efficiencies ranging between 17% and 22%. The latest efficiency boost for commercially used cells comes from moving to PERC cells, which improves efficiency by up to 1% (see TaiyangNews PERC Solar Cell Technology Report 2017). The 26.33% record of Japanese company Kaneka for a back-contact heterojunction (BCHJ) silicon cell in September 2016 was a further step in narrowing the already small gap to the theoretical efficiency limit of 29.4% for single-junction crystalline silicon cells.

The transition from a silicon single-junction cell to a silicon-based multi-junction solar cell has the potential to increase efficiencies beyond 30%. This technology still benefits from cost-effective manufacturing expertise in making silicon solar cells.

"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," said 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%," said Matthieu Despeisse, manager of crystalline silicon solar cells activities at CSEM.