World Record for Perovskite Silicon Tandem Solar Cells

In the race for ever higher effi­ciency levels, a team at Helmholtz Zentrum Berlin HZB has once again pulled ahead. The groups of Steve Albrecht and Bernd Stannowski have developed a tandem solar cell made of the semi­conductors perovskite and silicon, that converts 29.15 per cent of the incident light into electrical energy. This value has been offi­cially certified by the CalLab of the Fraunhofer Institute for Solar Energy Systems (ISE) and means that surpassing the 30 per cent efficiency mark is now within reach.

The tandem solar cell was realized on a typical laboratory scale of one square centimeter. However, scaling up is possible. (Source: E. Köhnen, HZB)

While silicon converts mostly the red portions of sunlight into elec­tricity, perovskite compounds primarily utilise the blue portions of the spectrum. A tandem solar cell made of stacked silicon and perovskite thus achieves signi­ficantly higher efficiency than each individual cell on its own. Bernd Stannowski from the HZB Institute PVcomB and Steve Albrecht, who heads a team at HZB, have already jointly set new records for monolithic tandem solar cells on several occasions. At the end of 2018, the team presented a tandem solar cell made of silicon with a metal-halide perovskite that achieved an efficiency of 25.5 per cent. Then Oxford Photo­voltaics Ltd. announced a value of 28 per cent.

Now, the HZB team can report the next record. The value of 29.15 per cent has been certified by the Fraunhofer Institute for Solar Energy Systems (ISE) and now appears in the charts of the National Renewable Energy Lab (NREL), USA. The NREL chart has been tracking the rising effi­ciency levels for nearly all types of solar cell since 1976. Perovskite compounds have only been included since 2013 – and the effi­ciency of this class of material has increased more than in any other material since then.

“We developed a special electrode contact layer for this cell in colla­boration with the group of Vytautas Getautis (Kaunas University of Technology), and also improved inter­mediate layers“, explain Eike Köhnen and Amran Al-Ashouri, doctoral students in Albrecht’s group. The new electrode contact layer also permitted improvement of the perovskite compound‘s composition in the HZB HySPRINT labora­tory. This compound is now more stable when illu­minated in the tandem solar cell and improves the balance of electrical currents contri­buted by the top and bottom cells. The silicon bottom cell comes from Stannowski’s group and features a special silicon-oxide top layer for optically coupling the top and bottom cells.

All processes used to realise this one-square-centi­meter cell are also suitable in principle for large surface areas. Scaling up with the help of vacuum depo­sition processes is very promising, as initial tests have already shown. The realistic practical efficiency limit for tandem cells made of silicon and perovskite is about 35 per cent. Next, the HZB team wants to break the 30 per cent effi­ciency barrier. Albrecht explains that initial ideas for this are already under discussion. (Source: HZB)

Reference: Best Research-Cell Efficiency Chart, National Renewable Energy Lab (NREL), USA

Link: Perovskite Tandem Solar Cells (S. Albrecht), Helmholtz Zentrum Berlin HZB, Berlin, Germany

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