Fraunhofer develops less expensive processes to produce solar cells

September 29th, 2012 by Fraunhofer Institute for Surface Engineering and Thin Films IST

Fraunhofer IST researchers working to reduce the cost of coating technologies in the production of solar cells (click image to expand - image ©Fraunhofer IST)

Researchers at the Fraunhofer Institute for Surface Engineering and Thin Films IST in Braunschweig, Germany are engineering coating processes and thin film systems aimed at lowering the production costs of photovoltaic solar cells.

A few of these processes were unveiled at the EU PVSEC trade show in Frankfurt in September 2012.

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

Manufacturers have used the plasma-Chemical Vapour Deposition (CVD) process to apply these layers to the substrate but the cost has been high because silane gas is used, which is expensive, and most of it is wasted in the manufacturing process.

Plasma activates the silane in a reaction chamber and breaks apart the silicon-hydrogen bonds. The free silicon atoms and the silicon-hydrogen residues settle on the surface of the substrate, but the plasma activates with only 10 to 15% of silane; the rest is wasted.

Fraunhofer IST researchers have devised a way of activating the gas with hot wires, instead of plasma, which makes use of 85 to 90% of the silane gas, which reduces the overall manufacturing costs of the layers by over 50%.

Dr. Lothar Schäfer, department head at IST, said “the price of the wire that we need for this process is negligible when compared to the price of the silane,”

“Our system, which coats the substrate continuously during the movement, is referred to as an in-line process.”

The silicon film grows up at the surface about five times faster than with the plasma Chemical Vapour Deposition process and produces the same layer quality.

The researchers are coating a surface measuring 50 by 60 square centimetres but can scale the process up to the more common industry format of 1.4 square metres.

Besides the saving on silane gas, the system technology is substantially cheaper. The generator that produces the electric current to heat the wires only costs about 10% of the cost of its counterpart in the plasma CVD process.

This process is also suitable for thin film solar cells.

With a degree of efficiency of slightly more than 10%, thin film solar cells have previously shown only a moderate pay-off, but by putting three cells on top of each other the degree of efficiency spikes up considerably.

Each of the three cells is tied to considerable material losses using the plasma CVD coatings. The new hot-wire CVD coating process would make the cells much more cost-effective.

Triple cells could even succeed over the long term if the rather scarce but highly efficient germanium is used. Germanium is expensive so in order for it to be a profitable choice, one must be able to apply the layers while losing as little of the germanium as possible, perhaps by using the hot-wire CVD process.

Saving 35% in the sputter process for transparent conductive oxide

The power generated by photovoltaic cells has to be able to flow out for it to be used. Usually a contact grid of metal is evaporated onto the solar cells, to allow the current to flow out.

But for HIT cells, this grid is insufficient. Instead, transparent, conductive layers – similar to those in an LCD television – are needed on the entire surface.

This normally happens through the sputter process.  Ceramic tiles, made from aluminum-doped zinc or indium-zinc oxide, are atomized. The dissolved components attach to the surface and produce a thin layer.

The ceramic tiles are expensive so Fraunhofer IST researchers use metallic tiles.  They are 80% cheaper than their ceramic counterparts. An electronic control ensures that the metal tiles do not oxidize because otherwise that would change the manner in which the metal sputters.

“Even though the control outlay is greater, we can still lower the cost of this production process by 35% for 1.4 square metre coatings,” Dr. Volker Sittinger, group manager at IST, said.

The research team intends to combine both processes over the long term, in order to make thin-coated solar cells more cost-effective and more profitable.

“You can produce all silicon layers using the hot-wire CVD, and all transparent conductive layers through sputtering with metal tiles.”

“These processes should also be suitable for large photovoltaic formats,” Dr Sittinger said.

However, it will take another three to five years before these processes can be used in the commercial production of solar cells.

For research reports on the hot-wire CVD process please visit the Fraunhofer IST website.


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