Silicon is one of the most common elements in the world (more than 90% of the earth’s crust is made of some kind of silicon material), and crystalline silicon is the most common material for solar cells. This abundence of material helps even out the cost, which when combined with its semicondusive properties, has made it an industry standard. Silicon is available in several forms before it properly becomes a cell, depending on the structure of the silicon crystals.
Monocrystalline, or single crystal, silicon is produced as circular ingots (also called boules) which are then cut into thin wafers. A cell made from a single crystal wafer is more effective at converting sunlight into energy than other forms of silicon, but it is also much more expensive to process and create cells from.
Polycrystalline, or multi-crystal, silicon is produced as square ingots, which can then be cut into wafers. Cells made using this kind of silicon are much cheaper and waste less silicon because of their square shape, but they are also less efficient than monocrystalline cells.