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Silicon

Silicon is the 14th element on the periodic table and has, over the last half century, become one of the most important elements mined. Each year, roughly 5.75 million metric tons of silicon is mined from the surface of the Earth. Two thirds of that or about 4.6 million metric tons comes from China. Not to worry though, Silicon is the second most abundant element in the crust and the seventh most abundant in the universe. There is a lot to go around.

Silicon, while not the only compound used to build solar panels, is the most prevalent. Silicon, when doped with small quantities of various other materials, is the most useful semiconductor currently known. There are several different forms of silicon that can be used in the construction of solar panels, each providing unique advantages and disadvantages.

Monocrystalline Silicon (Mono-Si)

Mono-Si is the purest form of silicon used in photovoltaic cells. It is one single crystal-lattice structure that has almost no defects. In essence, the structure of a single wafer of monocrystalline silicon in a photovoltaic cell can be thought of as one large, continuous crystal.

The major advantage of mono-Si is that panels made from it can reach efficiencies of up to 17% while thin film and polycrystalline panels can only achieve efficiencies of up to 10%. Despite the efficiency gains, very few solar panels are made with mono-Si due to cost. The attention to detail and length of time involved in the manufacturing process make mono-Si very expensive. Most mono-Si is used in microprocessors where the single crystal structure is essential to operation of the chips.

Polycrystalline Silicon (Poly-Si)

Poly-Si, can be thought of as multiple small crystals together in a single structure. Unlike Mono-Si, where the structure is one continuous crystal, in Poly-Si the material consists of multiple “crystallites.”

Poly-Si is used by both the microprocessor industry and by the solar panel industry. In most cases, rather than being cut and polished as mono-Si is, Poly-Si is deposited on wafer using high pressure, high temperature, and silane (silicon tetrahydride). In 2006, over one half of the world’s supply of poly-Si went to making photovoltaic panels.

Amorphous Silicon (a-Si)

A-Si is the non-crystalline form of silicon. In crystalline silicon, even Poly-Si, the structure of the substance includes long runs of tetrahedrally-bonded atoms of silicon that produce a very well-ordered structure. What separates poly-Si and mono-Si is simply the length of these runs. In mono-Si, the runs are continuous and produce a single, large crystal. In poly-Si, the runs are not all one piece, but they are still quite large.

In amorphous silicon, there are no ordered runs present, but rather there is continuous, random array of silicon that is NOT crystallized. In truth, this amorphous structure is too unstable to allow an electric current to be set up. To overcome this limitation, hydrogen or carbon is added to the mix.

A-Si is generally only useful in solar devices that require very little power, like calculators. They work well in this roll because they are less expensive and simpler to manufacture than other solar alternatives. In larger applications, recent advances have made the cells a little more attractive in specific installations. Among the biggest advantages of a-Si is that wafers can be exceptionally thin and thus they can be stacked atop one another without losses in efficiency. The other advantage is cost.

Microcrystalline Silicon

Microcrystalline silicon, sometimes called nanocrystalline silicon, uses very small crystals that allow it to be more flexible than poly-Si and mono-Si, but more efficient than a-Si. Micro-Si is still in the experimental phases, but combination with a-Si to create micromorphous silicon has shown a great deal of promise in settings where direct sunlight does not fall on the cells.
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