Environmental Engineering Reference
In-Depth Information
about 200
m for high absorptions. So-called light trapping, which reflects the
light in the material, enlarges the path length and reduces the required
thickness.
Further physical details of solar cells as well as descriptions of other solar
cell technologies such as metal-insulator-semiconductor (MIS) cells are not
given here. Details can be found in the literature, for example, Goetzberger et
al (1998); Green (1994); Luque and Heqedus (2003); Marti and Luque (2003).
µ
P
RODUCTION OF
S
OLAR
C
ELLS AND
S
OLAR
M
ODULES
Crystalline silicon solar cells
Various semiconductor materials are suited to solar cell production; however,
silicon is the most commonly used material today. For this reason, only the
process of producing solar cells from silicon is described here.
Silicon can mainly be found in quartz sand (SiO
2
). The following reduction
process extracts silicon from the quartz sand at high temperatures of about
1800°C (3272°F):
(4.28)
The result of this reaction is so-called
metallurgical-grade silicon
(MG-Si) with
a purity of about 98 per cent. Another process for extracting silicon is the
aluminothermic reduction:
(4.29)
However, silicon gained by this process also has significant impurities. Silicon
used by the computer industry is so-called
electronic-grade silicon
(EG-Si) for
the production of semiconductor devices. Its impurity level is below 10
-10
per
cent. This high purity is not necessary for solar cell production, in which
solar-
grade silicon
(SOG-Si) is commonly used. Nevertheless, purification processes
are needed for the production of SOG-Si.
Silicon is mixed with hydrogen chloride or chloric acid (HCl) in the
silane
process
. An exothermic reaction produces trichlorosilane (SiHCl
3
) and
hydrogen (H
2
):
(4.30)
Trichlorosilane is liquid at temperatures of 30°C. Multiple fraction distillations
are used to remove the impurities. The chemical vapour deposition (CVD)
process is used for silicon recovery. Silicon is deposited as a thin silicon rod at
temperatures of 1350°C (2462°F), when the trichlorosilane is brought into
contact with high-purity hydrogen:
