Tandem cells approaching 30% efficiency


A group of scientists at the Helmholtz Zentrum Berlin (HZB) has produced a perovskite/silicon tandem cell measured at 29.15% efficiency, a new world record for the technology.

HZB previously held the efficiency record for PS/Si tandem cell efficiency at 25.5%, before UK/Germany based startup Oxford PV pushed further, producing a 28% efficient cell in late 2018. HZB’s new record has been officially certified by Germany’s Fraunhofer ISE, and the group says it will now be targeting the 30% barrier for this technology.

Optimizing the layers

Key to the group’s achievement were improvements in several of the cell layers. “We developed a special electrode contact layer for this cell in collaboration with the group of Prof. Vytautas Getautis (Kaunas University of Technology), and also improved intermediate layers“, explain Eike Köhnen and Amran Al-Ashouri, doctoral students at HZB.

The two went on to explain that the record breaking cell also featured an improved perovskite composition which increased stability improved the balance of currents delivered by the two cells, and an optimized silicon oxide top layer on the bottom Si cell, which improves optical coupling between the two.

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Diagram showing the structure of the record breaking cell, with intermediate layers inbetween the active perovskite and silicon. Image: Eike Köhnen/HZB

While the cell measures just 1cm² and was produced using laboratory techniques, the group points out the processes used to produce the cell are “suitable in principle for large surface areas”. Without further information, they state that initial tests have shown promising results for vacuum deposition processes to scale up production of these cells.

Pushing for 30%

The next target for the HZB group is to push this efficiency beyond 30% and closer to the technology’s practical limit of around 35%. According to group leader Steve Albrecht, discussions on the best route to achieving this are already underway.

Albrecht’s group at HZB also holds the efficiency record for a perovskite/CIGS tandem cell at 23.26%, and has licensed the technology used in this cell to an unnamed Japanese manufacturer for commercial development.

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