II. Material Design

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There are two main processes for creating high efficiency, relatively pure, monocrystalline silicon (or single crystal silicon). The two methods are the Czochralski (CZ) method and the Float Zone method (FZ).

The CZ method uses a crucible, usually lined with SiO₂, to melt chunks of silicon by induction heating to about 1414 ˚C. Then a small poly silicon crystal, called a seed, is placed in the molten silicon. The seed is slowly pulled up, out of the crucible, and rotated, to allow the single cell crystals to grow at a rate of 5 cm/h. Final result is a monocrystalline silicon ingot that ranges from 10, to even 30 cm in diameter. The resulting cylindrical ingot is then cut into wafers by either diamond or wire sawing. For a 4 cm² cell, the efficiency can reach as high as 22% in a lab, but only 14% efficiency for those commercially grown.

It is also possible to use magnetic fields to increase the purity of the crystals from the CZ method. The magnetic fields would allow the monocrystalline silicon to be pulled from the molten silicon without touching the sides of a crucible, where impurities will develop in the silicon.

The FZ method produces the highest quality (or highest purity) of monocrystalline silicon, which has an efficiency of 24.7% in a 4 cm² cell. The FZ method uses radio frequency induction to heat up a purified polycrystalline silicon rod, while wrapping a coil around rod. Then single cell crystals can be pulled from the rod. This method is much more expensive, but as stated above, has a higher efficiency than the CZ method.

References

Hoffman, V. U., & Goetzberger, A. Photovoltaic Solar Energy Generation. Optical Sciences 112. Springer: Heidelberg, 2005. 23-28
Markvart, Tomas, Ed. Solar Electricity. 2^nd ed. John Wiley & Sons: West Sussex, 1997. 46-55