Dhe developers of solar cells are in a quandary: on the one hand they want to convert as much sunlight as possible into usable electricity, on the other hand they want to keep the costs for the materials and production of the solar cells as low as possible. Two aspirations that are difficult to reconcile. The industry therefore has great expectations of so-called perovskites. These crystalline metal-organic materials can already convert sunlight into electrical energy more efficiently than silicon. In addition, they are readily available and easy to process. In particular, perovskites can be processed into thin layers without much expenditure of energy. For materials researchers, perovskite solar cells could therefore become an initial competitor for the silicon solar cells that dominate the market.
However, perovskite solar cells are still not very stable, which has so far prevented them from being used in practice. Moisture and heat, but above all large temperature differences, cause problems for the material. On a summer’s day, for example, the inside of a solar cell can heat up to 80 degrees. At night, the cell cools down to the outside temperature. Such conditions lead to mechanical stresses that quickly decompose the crystal structure of the perovskites and lead to fatigue. Researchers are trying to get a grip on the weaknesses of perovskite solar cells with various approaches. Solar modules made from this should ultimately be used to generate electricity for at least twenty years – according to the industrial specification – without any significant loss of performance.
Clear voids can be seen at the grain boundaries of an untreated perovskite film (left). These defects can lead to losses and reduce efficiency. With the addition of b-pV2F, the cavities have shrunk (right).
:
Bild: G. Li/HZB
An international research group led by scientists from the Helmholtz Center Berlin has discovered that the solar cells become significantly more stable if the fluorinated polymer b-pV2F – b-poly(1,1-difluoroethylene) – is added during production. “This polymer seems to wrap around the individual perovskite microcrystals in the thin layer like a soft shell and forms a kind of cushion against thermomechanical loads,” explains Antonio Abate, head of research from Berlin.
Scanning electron micrographs have shown that the grains, i.e. the differently aligned crystallites that make up a perovskite crystal, nestle closer together thanks to b-pV2F. Furthermore, the chain-like fluorinated polymer improves the transport of charge carriers, which increases the efficiency of the cell. In fact, cells produced on a laboratory scale showed an efficiency of up to 24.6 percent. For comparison: the best silicon solar cells achieve an efficiency of 25.2 percent.
Solar cells in the stress test
To test the stability of the prototypes, they were repeatedly exposed to temperatures between plus 80 degrees and minus 60 degrees. They then had to withstand 1000 hours of continuous exposure to the strength of the midday sun – which corresponds to a year’s outdoor use. At the end of the stress test, the perovskite solar cells still had 96 percent of their original efficiency, write Abate and his colleagues in the journal Science. The task now is to reduce the losses even further.