A collaboration between researchers at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL), the Swiss Center for Electronics and Microtechnology (CSEM) and the École Polytechnique Fédérale de Lausanne (EPFL) shows potential for silicon-based multijunction solar cells.

The research groups created tandem solar cells with record efficiencies of converting sunlight into electricity under 1-sun illumination. The resulting paper, "Raising the One-Sun Conversion Efficiency of III-V/Si Solar Cells to 32.8% for Two Junctions and 35.9% for Three Junctions," appears in Nature Energy. Solar cells made solely from materials in groups three and five of the periodic table have shown high efficiencies, but are more expensive.

Stephanie Essig, a former NREL post-doctoral researcher now working at EPFL in Switzerland, is lead author of the newly published research that details the steps taken to improve the efficiency of the multijunction cell. Co-authors include Timothy Remo, John F. Geisz, Myles A. Steiner, David L. Young, Kelsey Horowitz, Michael Woodhouse and Adele Tamboli, all with NREL; and Christophe Allebe, Loris Barraud, Antoine Descoeudres, Matthieu Despeisse and Christophe Ballif, all from CSEM.

"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," Tamboli said. "It opens the door to develop entirely new multijunction solar cell materials and architectures."

In testing silicon-based multijunction solar cells, the researchers found that the highest dual-junction efficiency came from a tandem cell that stacked a layer of gallium arsenide developed by NREL atop a film of crystalline silicon developed by CSEM. An efficiency of 32.5 percent was achieved using a gallium indium phosphide top cell, which is a similar structure to the previous record efficiency of 29.8 percent announced in January 2016.

The existing photovoltaics market is dominated by modules made of single-junction silicon solar cells, with efficiencies between 17 and 24 percent. The researchers noted in the report that making the transition from a silicon single-junction cell to a silicon-based dual-junction solar cell will enable manufacturers to push efficiencies past 30 percent while still benefiting from their expertise in making silicon solar cells.