The raw material for silicon, silica, is the most abundant mineral on Earth, including quartz, chalcedony, white sand, and numerous noncrystalline forms. The first step in silicon production is to reduce silica with coke to generate metallurgical-grade silicon,
SiO2 + C Si + CO2. (9.88)
The silicon thus produced is typically 98% pure. For applications in solar cells, at least 99.9999% purity is required, so-called solar-grade silicon. However, many processes can generate silicon with impurity levels less than 10~9, which is needed for high-efficiency solar cells.
Two processes are commonly used. In the Siemens process, high-purity silicon rods are exposed to trichlorosilane at 1150° C. The trichlorosilane gas decomposes and deposits additional silicon onto the rods, enlarging them:
2HSiCl3 -^ Si + 2HCl + SiCl4. (9.89)
Silicon produced from this and similar processes is called polycrystalline silicon. The byproduct, silicon tetrachloride, cannot be reused and becomes waste. The energy consumption in this process is also significant.
In 2006 Renewable Energy Corporation in Norway (REC) announced the construction of a plant based on fluidized-bed technology using silane, taking silicon tetrachloride as the starting point:
3SiCl4 + Si + 2H2 -^ 4HSiCl3,
4HSiCl3 -^ 3SiCl4 + SiH4, (9.90)
SiH4 -^ Si + 2H2.
The purification process takes place at the silane (SiH4) stage. According to REC, the energy consumption of this new process is significantly reduced from the Siemens process. Also, using the almost free hydroelectric power in Norway, REC is expecting to reduce the cost of solar-grade pure silicon to less than $20 per kilogram.
To prepare for solar cell production, pure silicon can go through two alternative processes. For single-crystal silicon solar cells, the Czochraski process or the float – zone process is applied to generate single-crystal silicon ingots. Alternatively, the pure silicon can be melted in an oven to produce polycrystalline ingots.