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Photovoltaic Silicon Cell Basics

August 20, 2013 - 2:19pm

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Silicon—used to make some the earliest photovoltaic (PV) devices—is still the most popular material for solar cells. Silicon is also the second-most abundant element in the Earth's crust (after oxygen). 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 n-type and p-type layers.

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 ways to minimize the effects of grain boundaries.

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Single-Crystal Silicon

To create silicon in a single-crystal state, high-purity silicon must first be melted. It is then reformed or solidified slowly while 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 it starts from a seed, this process is called "growing" a new rod (often called a boule) of single-crystal silicon out of molten silicon.

Several processes can grow a boule of single-crystal silicon. The most established and dependable are the Czochralski (Cz) method and the float-zone (FZ) technique. "Ribbon-growth" techniques are also explained below.

Czochralski Silicon
Illustration of the Czochrakski process for making single-crystal silicon. A crucible with heater coils contains molten silicon. A seed of single-crystal silicon is lowered into the molten silicon and extracted slowly. As it is withdrawn, a cylinder-shaped ingot of silicon forms.
In the Czochralski process, 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.

In the Czochralski process, a seed crystal is dipped into a crucible of molten silicon and withdrawn slowly, which pulls a cylindrical single crystal as silicon crystallizes on the seed.

Float-Zone Silicon

The float-zone process produces purer crystals than the Czochralski method because the crystals are not contaminated by a crucible. In the float-zone process, a silicon rod is set atop a seed crystal and then lowered through an electromagnetic coil. The coil's magnetic field induces an electric field in the rod, which heats and melts the interface between the rod and the seed. Single-crystal silicon forms at the interface and grows upward as the coils are slowly raised.

Once the single-crystal rods are produced (by either the Czochralski or float-zone method), they must be sliced or sawn to form thin wafers. Such sawing, however, wastes as much as 20% of the silicon as sawdust, known as kerf. The wafers are then doped to produce the necessary electric field, treated with a coating to reduce reflection, and fitted with electrical contacts to form functioning PV cells.

Ribbon Silicon

Although single-crystal silicon technology is well developed, the Czochralski and float-zone processes are complex and expensive (as are the ingot-casting processes for multicrystalline silicon). Called ribbon growth, these methods form thin crystalline sheets directly and thus avoid the slicing step required for cylindrical rods.

One ribbon growth technique—edge-defined film-fed growth—starts with two crystal seeds that grow and capture a sheet of material between them as they are pulled from a source of molten silicon. A frame entrains a thin sheet of material when drawn from a melt. This technique does not waste much material, but the quality of the material is not as good as Czochralski and float-zone silicon.

Multicrystalline Silicon

Multicrystalline silicon devices are generally less efficient than those of single-crystal silicon, but they can be less expensive.

Multicrystalline silicon can be produced a variety of ways. The most popular commercial methods involve a process in which molten silicon is directly cast into a mold and allowed to solidify into an ingot. The starting material can be a refined lower-grade silicon rather that the higher-grade semiconductor grade required for single-crystal material. The cooling rate is one factor that determines the final size of crystals in the ingot and the distribution of impurities. The mold is usually square, which produces an ingot that can be cut and sliced into square cells that fit more compactly into a PV module. (Round cells have spaces between them in modules, but square cells fit together better with a minimum of wasted space).

Amorphous Silicon

Amorphous solids, such as common glass, are materials whose atoms are not arranged in any particular order. They do not form crystalline structures at all, and they contain large numbers of structural and bonding defects. Due to its amorphous structure, amorphous silicon does not have an indirect band gap, which limits the absorption in crystalline silicon, and is able to absorb solar radiation more efficiently than single-crystal silicon. Therefore, a film only about 1 micrometer—or one one-millionth of a meter—is needed for thin-film silicon cells.  The amorphous material can also be produced at lower temperatures and deposited on low-cost substrates such as plastic, glass, and metal. 

Several companies were producing amorphous silicon and tandem micro-crystalline silicon thin-film solar panels in 2007-2012, but the module efficiencies could not go much beyond 10%. As CdTe and CIGS thin-film modules increased their efficiencies at a similar cost, amorphous silicon could not compete in the solar module market. The instability due to dangling hydrogen bonds in amorphous silicon (the Staebler-Wronski effect) caused 20% initial degradation in manufactured modules and the defect bands in the material fundamentally limited the voltages that could be achieved in amorphous silicon devices. The tandem architectures, while benefiting from low-cost deposition, were limited by the absorption in thin micro-crystalline silicon bottom layers.

Today, amorphous silicon is used in solar-powered consumer devices that have low power requirements, such as wristwatches and calculators.

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