Perovskite silicon tandem solar cell claims record efficiency

June 12, 2018 // By Nick Flaherty
Perovskite silicon tandem solar cell claims record efficiency
Researchers at EPFL's Photovoltaics Laboratory and the CSEM PV-centre in Switzerland have integrated a perovskite cell directly on top of a standard silicon-based cell, claiming a record efficiency of 25.2% and demonstrating a viable competitor to today's tandem silicon versions.

Their production method would add only a few extra steps to the current silicon-cell production process, and the cost would be reasonable they say in a paper published in Nature Materials.

The efficiency of perovskite solar cells has grown dramatically in recent years, and this can complement the silicon cell for higher overall efficiency. The perovskite material converts blue and green light more efficiently, while silicon is better at converting red and infra-red light. "By combining the two materials, we can maximise the use of the solar spectrum and increase the amount of power generated. The calculations and work we have done show that a 30% efficiency should soon be possible," say the study's main authors Florent Sahli and Jérémie Werner.

Researchers at the European Solliance group have also combined perovskite layers with silicon for tandem cells with higher efficiency. 

However, creating an effective tandem structure is not simple. "Silicon's surface consists of a series of pyramids measuring around 5 microns (shown above), which trap light and prevent it from being reflected. However, the surface texture makes it hard to deposit a homogeneous film of perovskite," said Quentin Jeangros, who co-authored the paper.

When the perovskite is deposited in liquid form, as it usually is, it accumulates in the valleys between the pyramids while leaving the peaks uncovered, leading to short circuits.

The team at EPFL and CSEM avoided that problem by using evaporation methods to form an inorganic base layer that fully covers the pyramids. That layer is porous, enabling it to retain the liquid organic solution that is then added using thin-film deposition via spin-coating. The substrate is then heated to a relatively low temperature of 150°C to crystallize the film on top of the silicon pyramids.

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