An international research team of photovoltaics scientists has taken a crucial step toward the industrialization of perovskite silicon tandem solar cells. They demonstrated that passivation of the perovskite top cell is possible in combination with textured silicon bottom cells featuring large pyramid size, which are the current industry standard for solar cells. Additionally, they discovered that the passivation affects the entire perovskite layer—unlike silicon, where surface treatment only influences the upper layers—leading to further efficiency improvements. The researchers from King Abdullah University of Science and Technology (KAUST), University of Freiburg and Fraunhofer Institute for Solar Energy Systems ISE presented their technological and scientific innovations for the passivation of perovskite top cells.
Perovskite silicon tandem solar cells consist of a perovskite top cell and a silicon bottom cell. They represent the next major technological advancement in photovoltaics, as silicon solar cell development is approaching its physical maximum efficiency of 29.4 percent for converting sunlight into electricity.
For large-scale production of these tandem solar cells, using a standard silicon solar cell for the bottom cell would be advantageous, as their manufacturing processes are already well established. These solar cells are textured to increase their surface area, enhancing efficiency, but this texturization also complicates the deposition of the perovskite layer. Achieving high-quality surface passivation of the perovskite top cell on the pyramid-like surface had not yet been accomplished.
"So far, effective passivation has not been fully harnessed on textured perovskite silicon tandem solar cells, with prior success largely confined to flat-front architectures. But we have now managed excellent passivation by depositing 1,3-diaminopropane dihydroiodide on the uneven perovskite surface," said Dr. Oussama Er-Raji, the lead author of the paper and a scientist at Fraunhofer ISE. The passivated tandem solar cells achieved a conversion efficiency of up to 33.1 percent, with an open-circuit voltage of 2.01 volts.
The scientists also observed that the passivation of the perovskite top cell improved the conductivity and thus the fill factor of the cell. They proved that this improvement is due to a deep field effect resulting from the passivation. In silicon solar cells, the passivation acts only close to the surface, whereas in perovskite solar cells, the surface treatment impacts the entire absorber, enhancing its bulk properties.
"This realization provides a solid foundation for all future research in this area," said Prof. Stefaan De Wolf, Professor of Materials Science and Engineering and Applied Physics at KAUST. "It enhances our understanding of the processes occurring in the top cell while converting light into electricity, enabling scientists to leverage this knowledge to develop better tandem solar cells."
"Surface passivation of solar cells is not just a nice-to-have feature; it is an essential booster for their efficiency and stability," adds Prof. Stefan Glunz, Professor of Photovoltaic Energy Conversion at the University of Freiburg and Director of the Photovoltaics Division at Fraunhofer ISE. "For today's silicon solar cells, surface passivation was the key for high efficiencies in industrial production, and it is encouraging that the photovoltaic industry will benefit from these positive effects for perovskite silicon tandem solar cells as well."
The researchers' findings build on work in the Fraunhofer lighthouse project MaNiTU as well as the projects PrEsto and Perle, both funded by the Federal Ministry for Economic Affairs and Energy.
