New solar cell coating processes and thin layer systems for cell production are being developed by researchers from Fraunhofer that will lead to significant reductions in the price of solar cells.
The researchers, from the Fraunhofer Institute for Surface Engineering and Thin Films IST in Braunschweig, will showcase some of these new processes at the EU PVSEC trade show in Frankfurt from September 25 to 28.
Commercial high-efficiency solar cells that can reach efficiencies of up to 23 percent have been the goal of the solar industry for awhile now, but until now the production has been somewhat wasteful of the expensive silane gas that it uses, with solar cells reaching above 40% efficiency but being quite costly.
The Fraunhofer news release states: “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.” This is very wasteful — only 10 to 15 percent of the expensive silane gas is activated by the plasma; the other 85 to 90 percent is completely lost, an enormous cost.
So, to rectify this, researchers at IST have completely replaced this process: using hot wires to activate the gas rather than plasma.
“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 happens because the silicon film “grows up at the surface about five times faster than with plasma CVD,” but still with the same quality of layering. Another major advantage of this process is that the system technology is much easier than with plasma CVD, making the system considerably less expensive. As an example, the generator used to produce the electric current that heats the wires costs only about one-tenth of the cost of its counterpart used in the plasma CVD process.
The press release from Fraunhofer continues:
“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.
“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 researchers are intending for both processes to be combined over the long term, ultimately making 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. In principle, these processes should also be suitable for large formats,” states Sittinger.
The processes aren’t production-ready quite yet, though. The researchers estimate that it will take around three to five years for them to become usable in the production of solar cells.
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