How Energy Is Produced - The Renaissance of Renewable Energy - page 106

The amount of current generated by a solar cell depends on the number of electrons that
the photons push off the n-layer. Larger and more efficient cells, or cells exposed to more
intense sunlight, will deliver more electrons.
Types of Solar PV Cells
While the vast majority of modern PV panels are made from silicon, any semiconductor
material would suffice. Silicon has dominated so far because of its abundance in nature
(over 90 per cent of the Earth's crust is composed of silicate minerals, combinations of
silicon and oxygen) and its ubiquity in the electronics industry. The silicon used in PV
cells is crystalline, meaning that its atoms are arranged in an ordered and repeated pattern.
Thanks to this order, electrons can pass easily along the crystal lattice.
The most efficient PV cells are made from large crystals of silicon, 11 cut into wafer-thin
slices. This is known as monocrystalline silicon. The flakes of silicon that are left over
after the crystal has been sliced are used to make polycrystalline silicon (see Figure 4.25 ) .
They are heated just short of their melting point and thus 'welded' together. Since it is
essentially a by-product of monocrystalline silicon, polycrystalline silicon is less expensive
to produce. However, because these wafers do not have an unbroken lattice, they are less
efficient in converting light to electricity than monocrystalline silicon ( Table 4.3 ).
Figure 4.25. A monocrystalline silicon cell (left) ready to be assembled on a solar panel
and the surface of a polycrystalline silicon cell (right). The monocrystalline cells have
characteristic cutoff corners because they are sliced from a cylindrical bar of pure silicon.
Sources: Unknown at Wikimedia Commons; Georg Slickers at Wikimedia Commons.