The competition in the photovoltaics market is fierce. When it comes to
price, Asian manufacturers are frequently ahead of the competition by a
nose. Now, Fraunhofer researchers are designing new coating processes
and thin layer systems that, if used, could help to reduce the price of
solar cells significantly.
Scientists will unveil a few of these new processes at the EU PVSEC trade show in Frankfurt from September 25 to 28.
Many people answer with a resounding "yes!" when asked if they want
environmentally-friendly solar cell-based power -- though it should be
inexpensive. For this reason, a veritable price war is raging among the
makers of photovoltaic cells. Above all, it are the cheap products of
Asian origin that are making life tough for domestic manufacturers.
Tough, that is, until now: the researchers at the Fraunhofer Institute
for Surface Engineering and Thin Films IST in Braunschweig are providing
support to these companies. They are engineering coating processes and
thin film systems aimed at lowering the production costs of solar cells
drastically.
The
competition in the photovoltaics market is fierce. When it comes to
price, Asian manufacturers are frequently ahead of the competition by a
nose. Now, Fraunhofer researchers are designing new coating processes
and thin layer systems that, if used, could help to reduce the price of
solar cells significantly. (Credit: Image courtesy of
Fraunhofer-Gesellschaft)
Hot wires instead of plasma
The photovoltaic industry is pinning its hopes particularly on
high-efficiency solar cells that can achieve efficiencies of up to 23
percent. These "HIT" cells (Heterojunction with Intrinsic Thin layer)
consist of a crystalline silicon absorber with additional thin layers of
silicon. Until now, manufacturers used the plasma-CVD process (short
for Chemical Vapor Deposition) 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 with 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, unused.
This involves enormous costs.
The researchers at IST have now replaced this process: Instead of
using plasma, they activate the gas by hot wires. "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,"
explains Dr. Lothar Schäfer, department head at IST. "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 grows up at 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 centimeters; however, the process can be easily scaled up
to the more common industry format of 1.4 square meters. 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 (i.e., by putting three cells on top of each other) the
degree of efficiency spikes up 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 process: the new 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. However,
germanium is also very expensive: 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 -- by using the hot-wire CVD process, for
instance.
Saving 35 percent in the sputter process for transparent conductive oxide
The power generated by photovoltaic cells has to be able to flow out,
in order for it to be used. To do so, usually a contact grid of metal
is evaporated onto the solar cells, which conducts the resulting holes
and electrons. 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, thereby producing a thin
layer. Unfortunately, the ceramic tiles are also quite expensive.
Therefore, the researchers at IST use metallic tiles: They are 80
percent cheaper than their ceramic counterparts. An electronic control
ensures that the metal tiles do not oxidize. Because that would
otherwise 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 percent for 1.4 square meter coatings," says
Dr. Volker Sittinger, group manager at IST.
The research team intends to combine both processes over the long
term, in order to make thin-coated solar cells more cost-effective and
ultimately, more profitable. "You can produce all silicon layers using
the hot-wire CVD, and all transparent conductive layers through
sputtering with metal tiles. In principle, these processes should also
be suitable for large formats," states Sittinger. However, the processes
being used are not production processes quite yet: Even if the
researchers already apply the processes to a countless number of square
centimeters, it will still take about three to five years until they can
be used in the production of solar cells.
Source: http://www.sciencedaily.com/releases/2012/09/120919082933.htm
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