Aspiring to improve photovoltaic energy conversion efficiency by using latest advances in nanotechnology, researchers at Aalto University, Finland, are combining black silicon with the method of atomic layer deposition (ALD). As a result, the nanostructures fabricated by plasma dry etching minimise sunlight reflection. Moreover, a conformal, thin passivating layer, deposited onto the nanostructures through ALD, prevents charge carriers from recombining at the surface.
The nanostructured black silicon works very well, in practice preventing all reflection over the full sun spectrum, reports Dr Hele Savin, Assistant Professor in the Department of Micro- and Nanosciences and head of the Electron Physics Group at Aalto University. The ALD coating also works perfectly for surface passivation, the surface recombination velocity for black silicon is as low as for flat surfaces or for conventional pyramids, she says, underlining that there is no tradeoff between reflectance and passivation, and that the black silicon and ALD coating works for multi-crystalline as well as for monocrystalline silicon.
While Salvin believes it is too early to predict the potential impact of this new technique on the future of silicon-based solar technology, she does hope this technology will take over in large scale, thus, we would see higher efficiencies and blacker surfaces in commercial photovoltaic cells.
Savins group is also studying ways to make highly efficient solar cells from impure silicon that conventionally cannot be used for cell production. We have already found ways to use this waste material for efficient solar cells, the scientist says. But the question yet to be studied is if the extra processing steps needed to control the impurities are industrially feasible. The goal would be to fabricate high-performing cells from impure material that would be markedly cheaper than state-of-the-art cells.
When asked when such nanostructure-coated black silicon solar cells emerge in the marketplace, an optimistic Savin says existing prototypes have been fabricated in a lab environment, and next steps include upscaling the process for 156 by 156 wafers as well as studies on the module level and outdoor tests. In principle, there is no special challenge in my mind at the moment that could prevent the upscaling or commercialisation of the technology, Savin says.The cost of manufacturing the nanostructures is a concern, of course. On the other hand, when you are using nanostructures as antireflection, you can skip one process step the growth of a conventional ARC layer thereby, this may compensate for the cost of creatingthe nanostructures.
Lastly, there is one more obstacle Savin and her team want to overcome in their quest for highly efficient black solar cells: Her team is currently using the environmentally hazardous sulfur hexafluoride (SF6) gas to etch the nanostructures. And although this gas will never enter the atmosphere or exit the equipment, she says, We are studying new processes for making black silicon using only environmentally friendly gases.
Written by Sandra Henderson, Research Editor, Solar Novus Today