The most important parts of a solar cell are the semiconductor layers, because this is where the electron current is created. There are a number of different materials suitable for making these semiconducting layers, and each has benefits and drawbacks. Unfortunately, there is no one ideal material for all types of cells and applications.

In addition to the semiconducting materials, solar cells consist of a top metallic grid or other electrical contact to collect electrons from the semiconductor and transfer them to the external load, and a back contact layer to complete the electrical circuit. Then, on top of the complete cell is typically a glass cover or other type of transparent encapsulant to seal the cell and keep weather out, and an antireflective coating to keep the cell from reflecting the light back away from the cell.

A typical solar cell consists of a cover glass or other encapsulant, an anti-reflective layer, a front contact to allow the electrons to enter a circuit and a back contact to allow them to complete the circuit, and the semiconductor layers where the electrons begin and complete their voyages.

When photons of sunlight strike a PV cell, only the photons with a certain level of energy are able to free electrons from their atomic bonds to produce an electric current. This level of energy, known as the band-gap energy, is defined as the amount of energy required to dislodge an electron from its covalent bond and allow it to become part of an electrical circuit. The energy that photons possess is called the "photon energy." This energy must be at least as high as the band-gap energy for a photon to free an electron. However, photons with energy higher than the band-gap energy will expend the extra energy as heat when freeing the electrons. So, it's important for a PV cell to be "tuned" (through slight modifications to the semiconductor's molecular structure) to maximize the photon energy. After all, one key to obtaining an efficient PV cell is to convert as much sunlight into electricity as possible.

Effective PV semiconductors have band-gap energies ranging from 1.0 to 1.6 electron-volts (eV). (An electron-volt is equal to the energy an electron acquires when it passes through a potential of 1 volt in a vacuum.) That's because this level of energy is good for freeing electrons without causing extra heat. For example, crystalline silicon's band-gap energy is 1.1 eV.

The photon energy of light, also measured in eV, varies according to the different wavelengths of light. The entire spectrum of sunlight, from infrared to ultraviolet, covers a range of about 0.5 eV to about 2.9 eV. For example, red light has an energy of about 1.7 eV, and blue light has an energy of about 2.7 eV. About 55% of the energy of sunlight cannot be used by most PV cells because this energy is either below the band gap or carries excess energy.

Different PV materials have different characteristic energy band gaps. Photons with energy greater than the band gap may be absorbed to create free electrons. Photons with energy less than the band gap pass through the material or create heat.

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Silicon is still the most popular solar-cell material for commercial applications because it is so readily abundant (it is actually the second most abundant element in the Earth's crust-second only to oxygen!). However, to be useful in solar cells, it must be refined to 99.9999% purity.

In single-crystal silicon, the molecular structure of the material is uniform because the entire structure is grown from the same or a "single" crystal. This uniformity is ideal for efficiently transferring electrons through the material. To make an effective PV cell, silicon is "doped" to make it n-type and p-type. Semicrystalline silicon, on the other hand, consists of several smaller crystals or "grains," which introduce "boundaries." These boundaries impede the flow of electrons and encourage them to recombine with holes and thereby reduce the power output of the cell. However, semicrystalline silicon is much cheaper to produce than single-crystalline silicon, so researchers are working on other ways of minimizing the effects of grain boundaries.

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Amorphous solids, like common glass, are materials in which the 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.

It wasn't until 1974 that researchers began to realize that amorphous silicon could be used in PV devices by properly controlling the conditions under which it was deposited and by carefully modifying its composition. Today, amorphous silicon is commonly used for solar-powered consumer devices that have low power requirements (e.g., wrist watches and calculators).

Amorphous silicon absorbs solar radiation 40 times more efficiently than does single-crystal silicon, so a film only about 1 micron (one one-millionth of a meter) thick can absorb 90% of the usable solar energy. This is one of the most important factors affecting its potential for low cost. Other principal economic advantages are that amorphous silicon can be produced at a lower temperature and can be deposited on low-cost substrates. These characteristics make amorphous silicon the leading thin-film PV material.

The versatility of amorphous silicon is shown in this flexible roof-shingle module developed under a DOE project called Photovoltaics Building Opportunities in the United States (PV:BONUS). The shingle can be built right into new homes where covenants would prohibit more conventional PV modules.

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One of the scientific discoveries of the computer semiconductor industry that has shown great potential for the PV industry is thin-film technology. Polycrystalline thin-film devices require very little semiconductor material and have the added advantage of being easy to manufacture. Rather than growing, slicing, and treating a crystalline ingot (required for single-crystal silicon), we sequentially deposit thin layers of the required materials. Several different deposition techniques are available, and all of them are potentially cheaper than the ingot-growth techniques required for crystalline silicon. Best of all, these deposition processes can be scaled up easily so that the same technique used to make a 2-inch x 2-inch laboratory cell can be used to make a 2-foot x 5-foot module (in a sense, a huge cell!). Like amorphous silicon, the layers can be deposited on various low-cost substrates (actually "superstrates," see Transparent Conductors) like glass or plastic in virtually any shape—even flexible plastic sheets.

Single-crystal cells have to be individually interconnected into a module, but thin-film devices can be made monolithically (as a single unit). Layer upon layer is deposited sequentially on a glass superstrate, from the antireflection coating and conducting oxide, to the semiconductor material and the back electrical contacts.

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Gallium arsenide (GaAs) is a compound semiconductor: a mixture of two elements, gallium (Ga) and arsenic (As). Gallium is a byproduct of the smelting of other metals, notably aluminum and zinc, and it is rarer than gold. Arsenic is not rare, but it is poisonous. Gallium arsenide's use in solar cells has been developing synergistically with its use in light-emitting diodes, lasers, and other optoelectronic devices.

GaAs is especially suitable for use in multijunction and high-efficiency solar cells for several reasons:

1. The GaAs band gap is 1.43 eV, nearly ideal for single-junction solar cells.
2. GaAs has an absorptivity so high it requires a cell only a few microns thick to absorb sunlight. (Crystalline silicon requires a layer 100 microns or more in thickness.)
3. Unlike silicon cells, GaAs cells are relatively insensitive to heat. (Cell temperatures can often be quite high, especially in concentrator applications.)
4. Alloys made from GaAs using aluminum, phosphorus, antimony, or indium have characteristics complementary to those of gallium arsenide, allowing great flexibility in cell design.
5. GaAs is very resistant to radiation damage. This, along with its high efficiency, makes GaAs very desirable for space applications.

One of the greatest advantages of gallium arsenide and its alloys as PV cell materials is the wide range of design options possible. A cell with a GaAs base can have several layers of slightly different compositions that allow a cell designer to precisely control the generation and collection of electrons and holes. (To accomplish the same thing, silicon cells have been limited to variations in the level of doping.) This degree of control allows cell designers to push efficiencies closer and closer to theoretical levels. For example, one of the most common GaAs cell structures uses a very thin window layer of aluminum gallium arsenide. This thin layer allows electrons and holes to be created close to the electric field at the junction.

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Today's most common PV devices use a single junction, or interface, to create an electric field within a semiconductor such as a PV cell. In a single-junction PV cell, only photons whose energy is equal to or greater than the band gap of the cell material can free an electron for an electric circuit. In other words, the photovoltaic response of single-junction cells is limited to the portion of the sun's spectrum whose energy is above the band gap of the absorbing material, and lower-energy photons are not used.

One way to get around this limitation is to use two (or more) different cells, with more than one band gap and more than one junction, to generate a voltage. These are referred to as "multijunction" cells (also called "cascade" or "tandem" cells). Multijunction devices can achieve a higher total conversion efficiency because they can convert more of the energy spectrum of light to electricity.

A multijunction device is a stack of individual single-junction cells in descending order of band gap (Eg). The top cell captures the high-energy photons and passes the rest of the photons on to be absorbed by lower-band-gap cells.

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Two very essential parts of a PV cell are the electrical contacts, because they are the bridges that connect the active semiconductor to the external load. The back contact of a cell (away from the sun) is relatively simple and usually consists of a layer of aluminum or molybdenum metal. But the front contact (facing the sun) is more complicated. When placed in sunlight, the cell generates current (flowing electrons) all over its surface. Attaching contacts just at the edges of a cell would not be adequate because of the excessive electrical resistance of the top layer in this configuration (only a small portion of the electrons would make it into the contact). So, the contacts must be made across the entire surface to collect the most current. This is normally done with a metal "grid." Unfortunately, placing a large grid on the top of the cell shades the active parts of the cell from the sun, effectively reducing the cell's conversion efficiency.

Therefore, in designing grid contacts, we must balance electrical resistance losses against shading effects. The usual approach is to design grids with many thin, conductive fingers spreading to every part of the cell's surface. The fingers of the grid must be thick enough to conduct well (with low resistance), but thin enough to block a minimum of incoming light. Such a grid keeps resistance losses sufficiently low, while shading only about 3% to 5% of the surface.

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