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A new world record efficiency for tandem solar cells has been achieved at the Helmholtz-Zentrum Berlin (HBZ).

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A new world record efficiency of 29.15 percent for tandem solar cells made from perovskite and silicon has been achieved at the Helmholtz-Zentrum Berlin (HZB), working in collaboration with multiple European partners. The new physical processes which the researchers developed could now easily push the cell’s efficiency above 30 percent, according to the team leader Dr Steve Albrecht in a press release.

"The efficiency of 29.15 percent is not only the record for this technology, but also tops the entire ‘Emerging PV’ category in the NREL chart," says Eike Köhnen, a PhD student in Albrecht's team and a co-author of the study which was first published in the journal Science in the foregoing press release. Furthermore, the novel cell combination demonstrated stable performance over 300 hours, without encapsulation.

To achieve this record, the team used a complex composite of perovskite with a 1.68 eV band gap and they focused on optimising the substrate interface. With help from a team at the Kaunas University of Technology, Lithuania, they developed an intermediate layer of organic molecules that self-assemble into a monolayer (SAM), which they then applied to the electrodes to improve the charge flow.

A series of complementary investigations were then performed to analyse the different processes at the interfaces between the perovskite, SAM and the electrode: “In particular, we optimised the so-called filling factor, which is influenced by how many charge carriers are lost on the way out of the perovskite sub-cell,” explains Amran Al-Ashouri, another co-author.

Other project partners included the University of Ljublana, Slovenia, University of Sheffield, UK, the University of Potsdam as well as the Physikalisch-Technische Bundesanstalt (PTB), the University of Applied Sciences in Berlin and the Technical University of Berlin. Dr Albrecht: “Each partner contributes their particular expertise, which is why we were able to achieve this breakthrough together."