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| Sat, 2nd May 2020 12:37:00 |
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The weekend read: New technologies mean new approaches |
Measuring the performance of a solar cell is a tricky affair, and even more so for new technologies such as perovskites and tandem cells. In the laboratory, these are measured over a period of several minutes to ensure accurate characterization. But as technology continues its journey toward commercial production, ensuring an accurate power rating without slowing down the manufacturing process presents a new challenge both for suppliers of flash testing equipment and those working on bringing perovskite solar cells and tandem devices featuring them to market.
There is no shortage of challenges to scaling up perovskite technologies from the laboratory to commercial production. Among these is flash testing and cell characterization: A new technology will require new techniques to measure, and new standards to base these measurements on. "In some ways everyone who is working on perovskites is ahead of the compliance side of testing and certification. Standards and protocols have not really caught up with what will be a new technology," says Chris Case, CTO of Oxford PV, which is currently installing equipment for a 250 MW perovskite/silicon tandem cell manufacturing line in northern Germany.
And for perovskites and tandem cells that incorporate them, it seems a new approach to inline testing may be required. "There are some fundamental differences in our perovskite material's characteristics, including how it responds to testing and how it will respond to some of the certifications," says Case.
Maximum power point
In the research and development setting, cells are measured over a period of minutes. Researchers track a cell's maximum power point (MPP) and ensure that it has stabilized before taking a measurement, rather than the standard IV sweep that is performed for silicon PV measurements.
"Typically, perovskites might show some hysteresis, which means that the efficiency is slightly different whether you scan from low to high or high to low voltage," explains Jan Goldschmidt, head of the Novel Solar Cell Concepts Group at Fraunhofer Institute for Solar Energy Systems. "So then you estimate from the IV curve where your MPP will be, then you set the voltage at the MPP and you vary it a little bit to track the MPP. Then you record the output of your solar cell in the time. This is what we call a stabilized measurement."
This method has become an unofficial standard among universities and research institutes for measuring perovskite and tandem cell efficiencies and is a lot more time consuming than the required measurements for a silicon solar cell. "We characterize perovskite cells and tandems by JV scans with different scan speeds and scan directions followed by MPP tracks over several minutes," says Steve Albrecht, whose group at Helmholtz-Zentrum Berlin currently holds the efficiency record for a perovskite/silicon tandem cell. "For typical silicon cells, no hysteresis is measured, and thus quick scans will usually provide the correct information of the parameters."
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