Environmental Engineering Reference
In-Depth Information
distillation ensures compliance with the extreme purity requirements. Afterwards
silicon is again obtained by pyrolysis of the purified trichlorosilane. In appropriate
pyrolysis reactors within a reducing atmosphere the trichlorosilane is decomposed
at hot bars. Elementary silicon is separated as polycrystalline material. The ob-
tained "poly-silicon" meets the requirements of "SeG-Si" (semiconductor grade
silicon) and shows grit sizes within the µm range.
Up to now the photovoltaic industry was able to use silicon of polysilicon qual-
ity for the production of standard products. This polysilicon quality did not meet
the requirements of the semiconductor industry, but is still sufficient for solar cell
manufacturing. Due to the decreasing growth rates of the semiconductor industry
on the one hand and the strongly increasing production of the photovoltaic indus-
try on the other this kind of "off grade" material is expected to become more and
more scarce in the years to come. Therefore some countries already develop alter-
native purification methods for metallurgical grade silicon with the aim to produce
cost-efficient "solar silicon". Such developments have already been conducted on
a global scale in the early eighties of the last century; however, they had been
stopped due to the competition of "off grade silicon".
Polycrystalline silicon serves as base material for the provision of silicon
mono-crystals. The standard process applied for the production of such mono-
crystals is the Czochralski process (Cz process). Within a shielding gas atmos-
phere polysilicon is melted down in a crucible (see Fig. 6.9). A seed crystal is
dipped into the molten silicon and is again removed slowly by continuous turning,
while precisely controlling the temperature gradients so that cylindrical mono-
crystal bars are obtained. By cutting these bars with the help of a wire saw thin
(250 to 300 µm) mono-crystal silicon wafers are obtained. However, sawing
(standard technology) wastes up to 50 % of the (expensive) material. The semi-
conductor industry uses such silicon wafers to produce integrated electrical cir-
cuits and subsequently subdivides the individual wafers into different functional
"chips" to be used in computers and other electronic devices. Within the photo-
voltaic industry these wafers are used to manufacture mono-crystalline silicon
solar cells. Therefore the circular wafers are additionally trimmed to obtain square
plates to allow for a better space utilisation and thus for enhanced surface specific
module efficiencies.
The Czochralski process produces inappropriate wafers to manufacture solar
cells with record efficiencies of 25 %, since too many crystal imperfections and
impurities occur. Such high tech solar cells require mono-crystals manufactured
by the float zone process, which is much more sophisticated and more costly
compared to the Czochralski method. The float zone process is thus unsuitable for
a mass production of solar cell.
Besides single-crystal or mono-crystalline wafers also "poly-crystalline" wafers
are successfully used by the photovoltaic industry. For this purpose, polysilicon is
melted down and cast into ingot moulds to solidify slowly and adjusted. These
poly-crystalline blocks (with grain sizes ranging from the mm to the cm range) are
cut into quadratic poly-crystalline plates by sawing. However, the cheaper manu-
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