All About Silicon

Silicon—used to make some the earliest photovoltaic (PV) devices—is still the most popular material for solar cells. Outranked only by oxygen, silicon is also the second-most abundant element in the Earth's crust. However, to be useful as a semiconductor material in solar cells, silicon must be refined to a purity of 99.9999%.

In single-crystal silicon, the molecular structure—which is the arrangement of atoms in the material—is uniform, because the entire structure is grown from the same crystal. This uniformity is ideal for transferring electrons efficiently through the material. To make an effective PV cell, however, silicon has to be "doped" with other elements to make it n-type and p-type.

Semicrystalline silicon, in contrast, consists of several smaller crystals or grains, which introduce boundaries. These boundaries impede the flow of electrons and encourage them to recombine with holes to reduce the power output of the solar cell. However, semicrystalline silicon is much less expensive to produce than single-crystalline silicon. So researchers are working on other ways to minimize the effects of grain boundaries.

Single-Crystal Silicon
The most widely used technique for making single-crystal silicon is the Czochralski process, in which a seed of single-crystal silicon contacts the top of molten silicon. As the seed is slowly raised, atoms of the molten silicon solidify in the pattern of the seed and extend the single-crystal structure.

After growing the silicon ingot, we must saw it into thin wafers for further processing into PV cells.

To create silicon in a single-crystal state, we must first melt high-purity silicon. We then cause it to reform or solidify very slowly in contact with a single crystal "seed." The silicon adapts to the pattern of the single-crystal seed as it cools and gradually solidifies. Not surprisingly, because we start from a seed, we say that this process is "growing" a new rod (often called a "boule") of single-crystal silicon out of molten silicon.

Several different processes can be used to grow a boule of single-crystal silicon. The most established and dependable processes are the Czochralski (Cz)method and the float-zone (FZ) technique. We also discuss "ribbon-growth" techniques.