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.
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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.
“Until now, the standard approach for making a perovskite/silicon tandem cell was to level off the pyramids of the silicon cell, which decreased its optical properties and therefore its performance, before depositing the perovskite cell on top of it. It also added steps to the manufacturing process,” said Sahli.
The new type of tandem cell is highly efficient and directly compatible with monocrystalline silicon-based technologies, which benefit from long-standing industrial expertise and are already being produced profitably. “We are proposing to use equipment that is already in use, just adding a few specific stages. Manufacturers won’t be adopting a whole new solar technology, but simply updating the production lines they are already using for silicon-based cells,” said Christophe Ballif, head of EPFL’s Photovoltaics Laboratory and CSEM’s PV-Centre.
Research is continuing in order to increase efficiency further and give the perovskite film more long-term stability.