Researchers at Germany’s Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) are currently investigating vacuum deposition to fabricate perovskite thin films and contact layers for perovskite-silicon tandem solar cells.
Up until now, the highest efficiency tandem cells, measuring 1 cm² or even smaller, are typically made with a wet chemical spin coating tool. “It is a good method for rapid testing but not scalable to industrial substrate sizes,” Fraunhofer ISE scientist Juliane Borchert told pv magazine.
Another point against spin coating, she noted, is that it cannot be used on pyramidal textured substrates, typical of the front side of silicon solar cells.
Vacuum deposition is one of the two main processing routes for industrial-scale perovskite-silicon tandem solar cells. The other is a solution-based process.
“There is a large variety of deposition methods for perovskite thin films. Some rely exclusively on wet-chemical processes, such as spin coating, blade coating and printing. Others are solvent-free evaporation processes in a vacuum, such as physical vapor deposition, or pulsed laser deposition,” explained Borchert.
The researchers are using M12-sized wafers for their experiments, which are being conducted under a 3-year €6.5 million ($7.0 million) research project called Liverpool.
“In Liverpool, we use a hybrid deposition method that combines the best aspects of both approaches. It is a vacuum evaporation process to deposit the inorganic components and then a second wet chemical step to convert this inorganic scaffold to the complete perovskite film. We target a throughput of several M12 wafers per h, or 40 per day,” said Borchert.
To put the Liverpool research in context, the Fraunhofer ISE has several perovskite-silicon tandem solar cell projects underway. For example, the Pero-Si-Scale project is focused on building a pilot line composed only of scalable processes and technologies, including characterization and testing steps for M12 tandem cells and modules. Both were launched last year in spring and both are funded by the German Federal Ministry for Economic Affairs and Climate Action (BMWK). The Liverpool project runs until April 2026.
In November, a group of researchers at Fraunhofer ISE published a study estimating that the practical power conversion efficiency potential of perovskite-silicon tandem solar cells may reach up to 39.5%.
In late January, the German institute and UK-based Oxford PV announced fabricating a perovskite-silicon tandem solar module with a glass-glass design and a power conversion efficiency of 25%.
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