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27.09.2010 Category: Issue 35, Energy Efficient Systems & eMobility, Technology, Fraunhofer IISB, From the institutes

Cheaper silicon wafers for photovoltaics

Researchers at Fraunhofer IISB and Fraunhofer THM have now come up with a way of increasing the yield and material quality in the production of multicrystalline silicon wafers for use in photovoltaics, by using computer simulations and laboratory scale crystallization furnaces.

 

Multicrystalline silicon brick. Photo: SolarWorld
Multicrystalline silicon brick. Photo: SolarWorld
“Virtual furnace” – temperature field in the laboratory test system, calculated using CrysMas. Photo: Fraunhofer IISB
“Virtual furnace” – temperature field in the laboratory test system, calculated using CrysMas. Photo: Fraunhofer IISB

Photovoltaics is one of the most promising responses to the greenhouse effect and climate change in the long term. Several market surveys have shown that the rapid growth in this sector will continue in the next few years despite the current economic crisis.

Silicon – vital for manufacturing solar cells

The semiconductor material silicon plays an important role in converting solar energy into electrical energy: It makes up more than 90 percent of the absorber materials required for the conversion. In crystalline silicon technology, solar cells are mostly manufactured on multicrystalline silicon wafers. These are of a lower quality than monocrystalline wafers, which are used especially in microelectronics, but can also be produced at a much lower cost.

Multicrystalline silicon ingots weighing several hundred kilograms are manufactured from a granular feedstock by means of a melting and crystallization process. The ingots are then divided into silicon bricks, and the individual wafers are sawn off these. The main development targets of this process are the improvement of the material quality, increased cell efficiency, the use of cost-effective raw silicon and an increase in the crystal growth capacity over a given time.

Success thanks to “virtual furnaces”

Both the productivity of the crystallization process and the properties of the wafers are greatly affected by the heating and material transport processes which occur during solidification. Structural defects such as dislocations or grain boundaries, as well as impurities like carbon, nitrogen, oxygen or metals, are responsible for the wafer properties.

This is where the Fraunhofer Institute for Integrated Systems and Device Technology IISB comes in: The Institute, in conjunction with the Fraunhofer Technology Center for Semiconductor Materials THM, is researching a method of optimizing the crystallization process on behalf of one of the world’s leading manufacturers of silicon wafers for use in photovoltaics. A test system is used to recreate the crystallization conditions found in industrial furnaces at laboratory scale, allowing researchers to vary individual process parameters and thus to investigate the effects of these on material properties and wafer yield.

The simulation software CrysMas, which was developed by Fraunhofer IISB and which is used around the world, allows PCs to describe the laboratory system’s heating and material transport processes as a “virtual furnace”. Both the temperature field and the material transport are considered crucial to the simulation. It was demonstrated that convection is responsible for the distribution of impurities within the silicon melt, and that the quantity of these impurities can be predicted in advance.

The researchers were thus able to determine improved processing conditions for the manufacture of multicrystalline silicon ingots in the laboratory system, giving their industrial partner the means of increasing the yield significantly. This contributes to further growth in photovoltaics in the years to come.

Contact:

Dr. Jochen Friedrich
Phone +49 9131 761-269
jochen.friedrich(at)iisb.fraunhofer.de
Fraunhofer Institute for Integrated Systems and Device Technology IISB
Schottkystrasse 10
91058 Erlangen
Germany
www.iisb.fraunhofer.de