DESIGN IDEAS
❱❱ In studies on the ageing of perovskite
solar cells and the dye cells in the picture, a sufficiently large number of samples is one of the prerequisites for quality
AGEING TESTS FALL SHORT FOR LATEST SOLAR CELL MATERIALS
Researchers at Aalto University have found that only a fraction of stability tests done on new types of solar cells meet proper requirements. Tests lack common standards and should have been done in real-world conditions and in groups of several cells. Perovskite and dye-sensitised solar cells
have been suggested as energy efficient and cost-effective challengers to the silicon solar cells currently on the market. So far, the development of challenger cells has focused mainly on studying and improving their efficiency. In order for the cells to become commercially viable, however, the cells need not just be efficient, but also to have a sufficient life-span. Researchers at Aalto University have
analysed 261 ageing tests conducted on perovskite and dye-sensitised solar cells. Major shortcomings were discovered in both how the results had been reported and how tests had been implemented. “In about half of the ageing studies, the
data was published only for one solar cell. Studying only one cell does not yield a sufficient amount of data to reliably compare how different materials age – that is, how they lose efficiency over time,” says Armi Tiihonen. The researchers also found other
deficiencies in the stability studies. Only a third of tests report the intensities of visible and UV light, the humidity and temperature. Most of the tests – 52 per cent – do not mention the intensity of the UV light. UV light is a significant stress factor for most types of solar cells, shortening cell lifetimes more than pure visible light. About half of the ageing tests had been
performed solely in dark conditions. Only 15 tests had been conducted outdoors and three tests were made using modules comprising several cells connected together. This goes against the principle of testing the cells in conditions in which they will be used, which is a requirement for commercialisation. Inadequately reported test conditions
reduce the reliability of the results and ultimately slow down development of the new solar cell technology. If tests on ageing were implemented in the actual conditions in which the cells are to be used, then the results would be more meaningful. “The field needs common standards. High-quality, well-reported and standardised tests would reinforce the confidence of industry and investors in the technologies,” says Docent Kati Miettunen who directed the work. The researchers have written a detailed
checklist for doing high-quality ageing tests and how to take testing conditions into consideration and how to select the measurements to be carried out during testing. “It would be important to assess in advance how many cells would be needed for a statistically valid result,” adds Tiihonen. The team also proposes organising a
series of conferences for establishing common guidelines and standards for ageing tests. It will require close cooperation with other players in the field. For instance, in silicon cell research, the standards have been set by commercial entities, whereas the standards for new organic solar cells made out of materials such as conductive polymers are the result of efforts by research groups. “Our research is also an open invitation
for discussion. We hope that cooperation among the international research community will increase in future,” Tiihonen concludes. ❱❱ Critical analysis on the quality of stability studies of perovskite and dye solar cells: Armi Tiihonen, Kati Miettunen, Janne Halme, Sakari Lepikko, Aapo Poskela and Peter D Lund, Energy Environ. Sci., 2018, Advance Article, 05 Feb 2018, DOI: 10.1039/C7EE02670F
March 2018 /// Environmental Engineering /// 7
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