Combining silicon and perovskite based solar cells has resulted in a 25.2% record efficiency. Researchers from Swiss research institutes Ecole Polytechnique Federale de Lausanne (EPFL) and Center for Electronics and Microtechnology (CSEM) claim this manufacturing technique could be directly integrated into existing production lines to lift efficiencies beyond 30%.
One solution is to place two different types of solar cells on top of each other to maximize the conversion of light rays into electrical power. These "double-junction" cells are being widely researched in the scientific community, but are expensive to make. EPFL and CSEM claim they have 'developed an economically competitive solution' by integrating a perovskite cell directly on top of a standard silicon-based cell, which led to a record efficiency of 25.2%. The researchers emphasize that their 'production method is promising, because it would add only a few extra steps to the current silicon-cell production process, and the cost would be reasonable.'
Perovskite's properties have prompted a great deal of research into its use in solar cells over the last few years. Within 9 years, the efficiency of these cells has risen by a factor of six. However, primarily stability issues have prevented the technology from commercial applications so far.
In tandem cells, perovskite complements silicon: it 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 maximize 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," said the study's main authors Florent Sahli and Jérémie Werner. The research paper "Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency" has been published in Nature Materials.
Inorganic layer ensuring optimal micro- and tandem structure
Creating an effective tandem structure by superposing the two materials is no easy task. "Silicon's surface consists of a series of pyramids measuring around 5 microns, 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, a co-author of 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 scientists at EPFL and CSEM have gotten around 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 a thin-film deposition technique called spin-coating, according to the paper. The researchers subsequently heat the substrate to a relatively low temperature of 150°C to crystallize a homogeneous film of perovskite 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.
Rather simple production
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, said Christophe Ballif, head of EPFL's Photovoltaics Laboratory and CSEM's PV-Center: "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," explains
However, the scientists said more research is needed to get the process to move on for commercial use.
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