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Silicon crystallization technologies

: Dold, P.


Willeke, G.:
Advances in photovoltaics. Pt.4
Amsterdam: Elsevier, 2015 (Semiconductors and Semimetals 92)
ISBN: 978-0-12-801021-1
Book Article
Fraunhofer CSP ()
Fraunhofer ISE ()
Photovoltaische Module; Systeme und Zuverlässigkeit; Silicium-Photovoltaik; Charakterisierung von Prozess- und Silicium-Materialien; Czochralski; VGF; Zone; silicon; growth

More than 90% of all Photovoltaic (PV) installations are based on crystalline silicon, several hundred thousand tons of which are processed by the solar industry each year. During the last few years, we have seen a huge price reduction in the polysilicon market. But still, the raw material contributes significantly to the total costs and further price reductions might be expected. Today, most polysilicon is produced by the Siemens process, but alternative routes like fluidized-bed reactors or upgraded metallurgical silicon might provide a better cost structure and thus might gain market shares.
The majority of solar silicon is crystallized by the directional solidification method (also called vertical gradient freeze method). This technique is quite robust, easy to handle, and easily scalable. Block sizes between 500 kg and 1 ton are the actual standard. The latest development is the small-grain, high-performance multi. Compared to quasi-mono (mono-like) silicon, the better cost structure and the lower process complexity of the high-performance multi are a clear advantage.
Some 40% of the silicon is crystallized as mono ingots. Right now, Czochralski (Cz) is the standard technology: 8″ or 9″ ingots of cylindrical shape and a length of 1.5–2 m are grown. Since Cz is hardly scalable to larger ingots, the challenge is to reduce cost by accelerating the process, by reducing downtime, and by making a better use of the consumables. Actual trends are multipulling, feeding, continuous pulling, or active crystal cooling to mention just some of them. In particular, for high-efficiency cell technologies like Interdigitated Back Contact (IBC) or Heterojunction with Intrinsic Thin layer (HIT), high-quality n-type material is required.
Finally, the Float-Zone (FZ) technique will be discussed. FZ ingots have at least two orders of magnitude lower oxygen levels compared to Cz material and the corresponding minority carrier lifetimes are very high. Right now, the process suffers by its complexity and the lack of affordable feed rods. Overcoming these limitations, FZ might be an interesting alternative for high-efficiency applications.