More Electricity From The Same Area: Passivation Increases The Efficiency Of Perovskite/Silicon Tandem Solar Cells

A special molecular treatment (passivation) can increase the efficiency of perovskite/silicon tandem solar cells to 33.1 per cent and improve the stability of the cells, as demonstrated by scientists from the University of Freiburg, the Fraunhofer Institute for Solar Energy Systems ISE and the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia. The fact that the researchers used tandem solar cells whose base solar cells are made of high-purity silicon (silicon wafers), which is also used in industry, makes it easier to transfer the study results into practice. This is an important step towards the industrialisation of perovskite/silicon tandem solar cells. The results have been published in the journal Science.

In 2024, photovoltaics covered around 14 per cent of Germany’s electricity needs, which signifies an increase of around two per cent from the previous year. Silicon solar cells are the most commonly used worldwide, theoretically converting a maximum of 29.4 per cent – in practice more like 25 per cent – of sunlight into electricity. The photovoltaic industry has almost reached this physical limit. The next major technological innovation is expected to be perovskite/silicon tandem solar cells, which consist of a perovskite top cell and a silicon bottom cell. Scientists at the Fraunhofer Institute for Solar Energy Systems ISE, the University of Freiburg and King Abdullah University of Science and Technology (KAUST) have succeeded in the passivation of the perovskite top cell in combination with the silicon bottom cells:

They coated the perovskite surface evenly with the molecule 1.3-diaminopropane dihydroiodide (PDAI) (passivation), which enabled them to increase the efficiency of the small-format solar cells to up to 33.1 per cent. They also discovered that passivation affects the entire perovskite layer. In silicon, on the other hand, surface passivation only affects the uppermost atomic layers. For their research, the scientists used tandem solar cells based on high-purity silicon wafers, which are also used in industry. This makes it easier to transfer the research results into practice. The journal Science published the study on 4 September 2025.

Crucial steps towards industrialisation
For large-scale production of these tandem solar cells, it would be advantageous to use a standard silicon solar cell for the bottom cell, as its manufacturing processes are already well established. These solar cells are textured, as this increases their surface area and improves efficiency. However, texturing makes it more difficult to apply the perovskite top cell. High-quality surface passivation of the perovskite layer on the pyramid-shaped surface of the silicon had not yet been achieved.

“Until now, effective passivation on fully textured perovskite/silicon tandem solar cells has not been fully realised, with previous successes largely limited to solar cells with flat surfaces. But we have now achieved excellent passivation by depositing 1.3-diaminopropane dihydroiodide on the uneven perovskite surface,” said Dr Oussama Er-Raji, lead author of the paper and postdoctoral researcher at the Department for Sustainable Systems Engineering (INATECH) at the University of Freiburg and Fraunhofer ISE. The passivated tandem solar cells achieved an efficiency of up to 33.1 per cent with an open-circuit voltage of 2.01 volts.

The scientists also observed that 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, passivation only affects the upper layers, whereas in perovskite solar cells, the surface treatment affects the entire layer and increases its efficiency.

“Surface passivation of solar cells is not just a nice extra feature; it is an essential booster for their efficiency and stability,” adds Prof. Dr Stefan Glunz, Full Professor for Photovoltaic Energy Conversion at the University of Freiburg and Head of Division Solar Cells – Development and Characterisation at Fraunhofer ISE. “For today’s silicon solar cells, surface passivation was the key to high efficiency in industrial production, and it is encouraging that the PV industry will also benefit from these positive effects for perovskite/silicon tandem solar cells.”

Dr Juliane Borchert, head of the junior research group for optoelectronic thin-film materials at INATECH at the University of Freiburg and head of the perovskite materials and interfaces group in the photovoltaics department at Fraunhofer ISE, emphasises the interdisciplinary nature of the research project: “Perovskite/silicon tandem solar cells are complex, novel components in which many chemical and physical effects interact. In order to understand and improve them, it is crucial to combine expertise from physics, materials science, chemistry and engineering. It was a pleasure to bring this international team of partners together and work with them to advance the technology.”

The research team’s findings are based on work carried out in the Fraunhofer flagship project ’MaNiTU‘ and in the “PrEsto” and ’Perle” projects, both of which are funded by the Federal Ministry for Economic Affairs and Energy (BMWE). The research work was also part of the junior research group for optoelectronic thin-film materials at the Department for Sustainable Systems Engineering (INATECH) at the University of Freiburg, which is funded by the Vector Foundation.