Look both ways for better solar cells

Published online 12 January 2021

Solar cells can now more efficiently capture light that bounces up from the ground and hits their undersides.

Andrew Scott

Michele De Bastiani (2021)
Scientists have fabricated solar cells that efficiently gather energy from light that hits their undersurfaces, in addition to the conventional trapping of direct sunlight. The cells are based on a combination of conventional silicon semiconductor technology and materials called hybrid perovskites.

“[Our solar cell] can perform more efficiently than any commercial silicon-based solar cell,” says Stefaan De Wolf of King Abdullah University of Science and Technology Solar Center in Saudi Arabia. De Wolf and his colleagues developed the technology in collaboration with research groups in Canada, Germany and Italy.

The new system exploits a ‘tandem’ combination of a conventional silicon-based layer and another layer built from perovskite. The tandem structure has been used before, but only to trap direct light.

A substantial quantity of light energy, known as albedo, is scattered and reflected from the ground. Two-sided ‘bifacial tandem’ solar cells could capture much of this otherwise wasted albedo light. 

“Combining the bifacial and tandem concepts opens opportunities for very high power generation at affordable costs,” says De Wolf.

Although layers of silicon and perovskite form the bulk of the new bifacial tandem solar cells, they also incorporate many additional chemical compounds. “The main challenges were in the complex nature of the tandem device, with fourteen materials involved, each of which must be perfectly optimized to consider the effect of the albedo,” says De Wolf.

Professor Christophe Ballif, director of the photovoltaics laboratory at the Ecole Polytechnique Fédérale de Lausanne, Switzerland, who was not involved in the research, comments: “This paper brings a first clear experimental evidence of the interest of bifacial tandem devices.” He points out that the quantitative analysis of performance reported by the researchers will be important for manufacturing stable devices needed for the technology to enter the mass market.

The researchers are now seeking industrial collaborators to move their work towards real world applications.


Bastiani, M. D. et al. Efficient bifacial monolithic perovskite/silicon tandem solar cells via bandgap engineering. Nat. Energy (2021).