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Single-Crystalline Thin Film Used in Photovoltaics
Single-crystalline thin films are made from gallium arsenide (GaAs), a compound semiconductor that is a mixture of gallium and arsenic.
Gallium arsenide (GaAs) is a compound semiconductor, a mixture of gallium and arsenic. 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 has been developed for use in solar cells at about the same time that it has been developed for light-emitting diodes, lasers, and other electronic devices that use light.
GaAs solar cells offer several benefits:
- The GaAs bandgap is 1.43 eV—nearly ideal for single-junction solar cells.
- 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 thick.)
- Unlike silicon cells, GaAs cells are relatively insensitive to heat. Cell temperatures can often be quite high, especially in concentrator applications.
- Alloys made from GaAs and aluminum, phosphorus, antimony, or indium have characteristics that are complementary to those of gallium arsenide, allowing great flexibility in cell design.
- GaAs is highly resistant to radiation damage. This, along with its high efficiency, makes GaAs desirable for space applications.
One of the greatest advantages of gallium arsenide and its alloys as PV cell materials is that it is amenable to a range of designs. A cell with a GaAs base can have several layers of slightly different compositions. This allows 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 has a very thin window layer made of aluminum gallium arsenide. This thin layer allows electrons and holes to be created close to the electric field at the junction.
The greatest barrier to the success of GaAs cells has been the high cost of a single-crystal GaAs substrate. For this reason, GaAs cells are used primarily in concentrator systems, in which a typical concentrator cell measures only about 0.25 cm2 in area but can produce ample power at high concentrations. In this configuration, the cost is low enough to make GaAs cells competitive, assuming that module efficiencies are between 25% and 30% and that the rest of the PV system is cost-effective.
Researchers are exploring several approaches to reduce the cost of GaAs devices. These include placing GaAs cells on cheaper substrates; growing GaAs cells on a removable, reusable GaAs substrate; and making GaAs thin films, similar to those made of copper indium diselenide and cadmium telluride.