5 Crystalline Silicon Solar Cells: Homojunction Cells
99
The production of pure silicon ingots
3 takes place in four stages [1]:
(a) Production of raw silicon
(b) Preparation of trichlorosilane
(c) Production of high-purity silicon
(d) Production of ingots, either multicrystalline or monocrystalline.
(a) Production of raw silicon
Starting material is silica sand (quartz sand) SiO 2 , which is mined above all in Brazil
and China where the quartz sand is very pure. At ~1800 °C, this silica sand is reduced
with coal (i.e. with carbon C) to metallurgical grade silicon with a purity of 98–99%:
SiO 2 + C → SiO + CO
(5.1)
SiO + C → SiC + CO
(5.2)
2SiC + SiO 2 → 3Si + 2CO
(5.3)
We now have “technical silicon”, e.g. raw or metallurgical silicon with a purity
of 98–99%.
The next three steps (b) to (d) are made with the so-called Siemens process which
was patented by the company Siemens A.G., in 1954.
(b) Preparation of trichlorosilane
Raw silicon is now finely ground; then it is converted in a fluidized bed reactor, with
the aid of hydrochloric acid, to gaseous trichlorosilane. The process temperature for
this step is approximately 300 °C:
Si + 3HCl → H 2 + SiHCl 3
(5.4)
(c) Destillation of Trichlosilane
Trichlorosilane boils at 31.8 °C; it is distilled in tall stainless steel columns. Thereby
a lot of impurities are filtered out. With repeated distillation, the degree of purity can
be increased.
(d) Production of ultrapure silicon
Trichlorosilane is broken down in a reducing atmosphere at around 1000 °C via the
reaction
SiHCl 3 + H 2 → 2Si + SiCl 4 + SiCl 2 + 6HCl
(5.5)
3 This process step is very important, as it accounts for more than half of the total energy invested
in the production of crystalline silicon solar modules.
99
The production of pure silicon ingots
3 takes place in four stages [1]:
(a) Production of raw silicon
(b) Preparation of trichlorosilane
(c) Production of high-purity silicon
(d) Production of ingots, either multicrystalline or monocrystalline.
(a) Production of raw silicon
Starting material is silica sand (quartz sand) SiO 2 , which is mined above all in Brazil
and China where the quartz sand is very pure. At ~1800 °C, this silica sand is reduced
with coal (i.e. with carbon C) to metallurgical grade silicon with a purity of 98–99%:
SiO 2 + C → SiO + CO
(5.1)
SiO + C → SiC + CO
(5.2)
2SiC + SiO 2 → 3Si + 2CO
(5.3)
We now have “technical silicon”, e.g. raw or metallurgical silicon with a purity
of 98–99%.
The next three steps (b) to (d) are made with the so-called Siemens process which
was patented by the company Siemens A.G., in 1954.
(b) Preparation of trichlorosilane
Raw silicon is now finely ground; then it is converted in a fluidized bed reactor, with
the aid of hydrochloric acid, to gaseous trichlorosilane. The process temperature for
this step is approximately 300 °C:
Si + 3HCl → H 2 + SiHCl 3
(5.4)
(c) Destillation of Trichlosilane
Trichlorosilane boils at 31.8 °C; it is distilled in tall stainless steel columns. Thereby
a lot of impurities are filtered out. With repeated distillation, the degree of purity can
be increased.
(d) Production of ultrapure silicon
Trichlorosilane is broken down in a reducing atmosphere at around 1000 °C via the
reaction
SiHCl 3 + H 2 → 2Si + SiCl 4 + SiCl 2 + 6HCl
(5.5)
3 This process step is very important, as it accounts for more than half of the total energy invested
in the production of crystalline silicon solar modules.