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New processes may cut the cost of solar cell production

19 September 2012

Fraunhofer researchers are working on the design of new coating processes and thin layer systems that, if successful, could help to reduce the price of solar cells significantly.

Photo courtesy of  Fraunhofer Institute for Surface Engineering and Thin Films IST
Photo courtesy of Fraunhofer Institute for Surface Engineering and Thin Films IST

Researchers at the Fraunhofer Institute for Surface Engineering and Thin Films IST in Braunschweig, Germany are engineering coating processes and thin film systems that have the potential to significantly reduce the production costs of solar cells.

The photovoltaic industry is pinning its hopes on high-efficiency solar cells that can achieve efficiencies of up to 23 percent. These 'Heterojunction with Intrinsic Thin' (HIT) layer cells consist of a crystalline silicon absorber with additional thin layers of silicon.

Until now, manufacturers used the plasma-chemical vapour deposition (plasma-CVD) process to apply these layers to the substrate:  the reaction chamber is filled with silane (the molecules of this gas are composed of one silicon and four hydrogen atoms) and the crystalline silicon substrate.

Plasma activates the gas, thus breaking apart the silicon-hydrogen bonds. The now free silicon atoms and the silicon-hydrogen residues settle on the surface of the substrate. But there’s a problem: the plasma only activates 10 to 15 percent of the expensive silane gas; the remaining 85 to 90 percent are lost, adding significantly to the costs.

The researchers at IST have now replaced this process: Instead of using plasma, they activate the gas by hot wires," explains Dr Lothar Schäfer, department head at IST. “This way, we can use almost all of the silane gas, so we actually recover 85 to 90 percent of the costly gas. This reduces the overall manufacturing costs of the layers by over 50 percent. The price of the wire that we need for this process is negligible when compared to the price of the silane.

“In this respect, our system is the only one that coats the substrate continously during the movement – this is also referred to as an in-line process.” This is possible since the silicon film covers the surface about five times faster than with plasma CVD – and still with the same quality of layer.

At this point, the researchers are coating a surface measuring 50 by 60 square centimetres; however, the process can be easily scaled up to the more common industry format of 1.4 square metres. Another advantage: The system technology is much easier than with plasma-CVD, therefore the system is substantially cheaper. Thus, for example, the generator that produces the electric current to heat the wires only costs around one-tenth that of its counterpart in the plasma CVD process.

In addition, this process is also suitable for thin film solar cells. With a degree of efficiency of slightly more than ten percent, these have previously shown only a moderate pay-off. However, by tripling the solar cells (ie, by putting three cells on top of each other) the degree of efficiency rises considerably.

But there is another problem; because each of the three cells is tied to considerable material losses using the plasma-CVD coatings, the triple photovoltaic cells are expensive. So the researchers see another potential use for their new coating process, which would make the cells much more cost-effective.

Triple cells could even succeed in the long term if germanium were used. However, germanium is also very expensive; in order for it to be a profitable choice, the layers must be applied with as little loss of germanium as possible – by using the hot-wire CVD process, for instance.

The researchers are also working on an alternative approach to the deposition of the conductive grids, moving from ceramic to cheaper metallic tiles.

Scientists will unveil a few of these new processes at the EU PVSEC trade show in Frankfurt from September 25 to 28 (Hall 03, Booth G22).

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