Environmental Engineering Reference
In-Depth Information
Figure 5.4
Polycrystalline silicon for solar cells. Left: Raw silicon. Middle: Silicon blocks. Right: Silicon
wafers.
Photos: PV Crystalox Solar plc.
The silicon is melted down again to produce semiconductor silicon for computer
chips and monocrystalline solar cells. In the crucible process invented by Polish
chemist Jan Czochralski, a silicon crystal is dipped into a crucible with a silicon
melt and then slowly pulled upwards in a rotating movement. The melted silicon
attaches itself to the crystal and a long, round silicon rod is created. In the process
the silicon crystals align in one direction. This creates monocrystalline silicon. Most
of the impurities remain in the melt crucible so that the semiconductor silicon is left
with purities of over 99.9999%.
In the next step, band saws cut the long silicon rods into thin slices, called wafers.
This sawing process produces major waste, and up to 50% of the valuable silicon
material is lost as a result. The alternative is for two thin wires to be pulled through
the liquid silicon melt. With this procedure, thin silicon wafers are formed between
the two wires. Immersion in acid will remove sawing damage from wafers and
smooth the surfaces. Several years ago silicon wafers had a thickness of 0.3 to
0.4mm. To save material and costs, attempts are now being made to reduce the
wafer thickness to under 0.2 mm. Technically, this is a major challenge as the ultra-
thin wafers must not break apart.
The fi nished wafers are exposed to gaseous doping materials. This produces the
p- and n-layers described earlier. A transparent anti-refl ection layer of silicon nitride
less than a millionth of a millimetre thick gives the silicon solar cell its typical blue
colour. This layer reduces the refl ection loss of the silver-grey silicon on the top
side of the solar cell. The darker the cell appears, the less light the cell is refl ecting
(Figure 5.5 ).
The front and back contacts are then applied using screen printing. To reduce the
losses to the opaque front contacts, some manufacturers conceal them under the
surface or try to move them so that they are also on the back of a cell. Although
