Crystalline Silicon
Silicon is
the second most abundant element in the Earth’s crust besides oxygen.
Silicon used in solar cell construction is refined to an almost 100%
purity.
Single-crystalline silicon is more efficient at electricity
generation from
solar panels but is
more expensive than poly-crystalline manufacturing. Crystalline cells are
uniformly constructed from slices of a large single crystal ingot. The
orderly arrangement of atoms results in sunlight conversion efficiency.
Poly-crystalline
silicon is composed of many crystals, or grains. Atomic order is
disrupted at grain interfaces, making poly-crystalline silicon less
efficient at converting sunlight power into electricity.
The most widely used
technique for making single-crystalline silicon lowers a seed of
single-crystalline silicon into a vat of molten silicon. As the seed is
raised from the vat, atoms of the molten silicon solidify around the seed,
creating a long, cylindrical silicon ingot with similarly structured
crystals.
Semi-crystalline PV
undergoes a casting process, in which molten silicon is poured into a mold
and solidified into an ingot. The crystalline ingots are then sliced into
thin, fragile wafers that work as PV cells encapsulated between two thick
sheets of glass. The glass allows sunlight to enter PV material while
preventing impact damage.
In the
silicon-film approach, the silicon layer is grown directly on a
ceramic substrate, resulting in a silicon wafer that is one-half the
thickness of a traditional cell.
In silicon string
manufacturing, two parallel strings are pulled through molten
silicon, which spans, then solidifies, between the strings. Both processes
eliminate the inherent cost and waste of sawing an ingot of silicon into
wafers.
