Types of Silicon Used in Photovoltaics
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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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.
In the Czochralski process, a seed crystal is dipped into a crucible of molten silicon and withdrawn slowly, which pulls a cylindrical single crystal as silicon crystallizes on the seed.
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.
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 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 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. But they have some economic advantages over other materials that make them appealing for use in PV systems.
In 1974, researchers began to realize that they could use amorphous silicon in PV devices by properly controlling the conditions under which it is deposited and carefully modifying its composition. Today, amorphous silicon is common in solar-powered consumer devices that have low power requirements, such as wristwatches and calculators.
Amorphous silicon absorbs solar radiation 40 times more efficiently than single-crystal silicon, so a film only about 1 micrometer—or one one-millionth of a meter—thick can absorb 90% of the usable light energy shining on it. This is one of the chief reasons that amorphous silicon could reduce the cost of PV. It can also be produced at lower temperatures and deposited on low-cost substrates such as plastic, glass, and metal. This makes amorphous silicon ideal for building-integrated PV products such as solar shingles. And these characteristics make amorphous silicon the leading thin-film PV material.
Amorphous silicon does not have the structural uniformity of single- or multicrystalline silicon. Small deviations in this material result in defects such as "dangling bonds," in which atoms lack a neighbor with which they can bond. These defects provide places for electrons to recombine with holes rather than contributing to the electrical circuit. Ordinarily, this kind of material would be unacceptable for electronic devices because defects limit the flow of current. But amorphous silicon can be deposited so that it contains a small amount of hydrogen in a process called hydrogenation. The result is that the hydrogen atoms combine chemically with many of the dangling bonds, essentially removing them and permitting electrons to move through the material.
Instability is the greatest stumbling block for amorphous silicon. These cells experience the Staebler-Wronski effect, in which their electrical output decreases over time when first exposed to sunlight. Eventually, however, the electrical output stabilizes. This effect can result in up to a 20% loss in output before the material stabilizes. Exactly why this effect occurs is not fully understood, but part of the reason is likely related to the amorphous hydrogenated nature of the material. One way to mitigate—though not eliminate—this effect is to make amorphous silicon cells that have a multijunction structure.
Because of amorphous silicon's unique properties, its solar cells are designed to have an ultrathin (0.008 micrometer) p-type top layer, a thicker (0.5 to 1 micrometer) intrinsic middle layer, and a very thin (0.02 micrometer) n-type bottom layer. This design is called a p-i-n structure, named for the types of the three layers. The top layer is made so thin and relatively transparent that most light passes right through it to generate free electrons in the intrinsic layer. The p- and n-layers produced by doping the amorphous silicon create an electric field across the entire intrinsic region, thus inducing electron movement in the i-layer.
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