The PaSodoble consortium set out to push crystalline silicon solar‑cell efficiencies beyond 26 % by creating passivating selective contacts that combine excellent surface passivation with efficient carrier extraction. The project, funded by the German Federal Ministry of Economics and Energy under grant 03 EE1031 B, ran from 1 July 2019 to 31 December 2022 and was carried out jointly by Fraunhofer ISE (project 03 EE1031 A) and Albert‑Ludwig University Freiburg (project 03 EE1031 B). Coordination was led by Prof. Stefan Glunz, with key contributions from Prof. Birgit Esser (organic chemistry), Prof. Ingo Krossing (inorganic and analytical chemistry), and Prof. Fritz Wortelkamp (solar‑cell physics). A close sample‑exchange programme between the university and Fraunhofer ISE was integral to the effort.

The scientific focus was the use of single‑molecule layers of organic dipoles that possess a high permanent dipole moment. Such a monolayer can shift the quasi‑Fermi levels and create a built‑in electric field that selectively extracts electrons while leaving holes to recombine at the surface, thereby reducing parasitic absorption thanks to the large HOMO–LUMO gap of the molecules. The approach can be applied at room temperature from aqueous or ethanol solutions, but controlling the orientation of the dipoles remains a challenge.

Three experimental routes were pursued under subproject AP2. The first employed functionalised alkenes; the second used a metal‑organic Al/Al₂O₃ stack; and the third applied spin‑coated dipole molecules such as L‑histidine. The functionalised alkene and Al/Al₂O₃ approaches yielded minority‑carrier lifetimes in the low‑microsecond range (≈ 1–2 µs), insufficient for the target efficiency. The spin‑coated dipole layer produced improved surface passivation and a lower contact resistance on test structures, yet the minority‑carrier lifetime remained below the 4 µs level required for > 26 % efficiency. In contrast, similar dipole‑based contacts have shown excellent performance in perovskite solar cells, indicating the concept’s broader applicability.

Subproject AP3 focused on characterisation and simulation. The bias‑quasi‑steady‑state photoconductance (bias‑QSSPC) technique was used to measure lifetimes after wet‑chemical, thermal, and HF‑dip oxidation steps, with precursor (H₂AlOtBu)₂. Lifetimes measured at 240, 280, and 430 °C were consistently in the 1–4 µs range, showing no significant improvement. Infrared spectroscopy, including attenuated‑total‑reflectance IR, was performed to confirm the formation of hydrogenated aluminium oxide (HAlO); the expected H‑Al signals were not observed at 240 °C, suggesting incomplete conversion.

Overall, the PaSodoble project advanced the understanding of how single‑molecule dipole layers influence band bending and carrier concentrations in silicon, and it provided a detailed database of lifetimes and contact resistances for various processing routes. While the target minority‑carrier lifetime for > 26 % efficiency was not yet reached, the work established a foundation for further optimisation of dipole orientation and interface engineering, and it highlighted the potential of these concepts for emerging perovskite technologies. The collaboration between Fraunhofer ISE and Albert‑Ludwig University Freiburg, under the guidance of Prof. Glunz and his colleagues, has produced a comprehensive set of experimental results and simulation models that will inform the next generation of passivating selective contacts for high‑efficiency silicon solar cells.