Page 182 - A Comprehensive Guide to Solar Energy Systems

184 A COmPrehenSIVe GUIDe TO SOlAr enerGy SySTemS

FIGURE 9.2 Schematic of reactor for electronic grade polycrystalline silicon production.

at temperatures of approximately 500°C. The temperature of silane decomposition is
much lower and, consequently, less energy consuming than the trichlorsilane decompo-
sition.
The final stage involves using dynamic silicon seed spheres in a fluidized bed sustained
by a gas stream of silane and hydrogen. The pure silicon, decomposed from silane, grown
on the silicon spheres and the end product are small granules of polysilicon. The reactor is
schematically shown in Fig. 9.4.
At present approximately 400 × 10 kg of poly c-Si is produced annually; its price has
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decreased from hundreds of US dollars per kilogram in 2008 to less than 20 US dollars per
kilogram. Today approximately 86% of the silicon production is by the Siemens process,
and over 10% is obtained by using the FBr method. It is expected that in the future the
share of FBr technology on the silicon production will increase [4].

9.3 Crystalline Silicon Wafer Fabrication

Silicon is a semiconductor with an indirect band structure. As explained in Section 8.1,
the absorption coefficient relates to the infrared part of solar spectra; a relatively thick
layer (over 250 µm) of c-Si is necessary to absorb all the photons of energy higher than the
bandgap. Such thick layers in the form of wafers of a defined rectangular shape (mostly
square) and resistivity are used in module fabrication. The wafers are usually cut from a
c-Si ingot with a square (or quasi-square) cross section.