Gallium arsenide (GaAs) is a compound semiconductor: a mixture of two elements, gallium (Ga) and arsenic (As). Gallium is a by-product of the smelting of other metals, notably aluminium 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:
• The GaAs band gap 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 in thickness.)
• 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 using aluminium, phosphorus, antimony, or indium have characteristics complementary to those of gallium arsenide, allowing great flexibility in cell design.
• 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 aluminium gallium arsenide. This thin layer allows electrons and holes to be created close to the electric field at the junction.
Drawbacks and Opportunities
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, where the typical concentrator cell is about 0.25 cm2 in area and can produce ample power under high concentrations. In this configuration, the cost is low enough to make GaAs cells competitive, assuming that module efficiencies can reach between 25% and 30% and that the cost of the rest of the system can be reduced. Researchers are also exploring approaches to lowering the cost of GaAs devices, such as fabricating GaAs cells on cheaper substrates; growing GaAs cells on a removable, reusable GaAs substrate; and even making GaAs thin films similar to those of copper indium diselenide and cadmium telluride.
High-Efficiency Concepts and Concentrators
High-efficiency solar cells based on gallium arsenide (GaAs) and related "III-V" materials have historically been used in space applications. Current goals are to take this technology a step further by developing it as a concentrator technology and expanding it to include triple-junction devices. Devices are also being investigated using low-cost substrates (such as glass). The long-term objective for researchers is to establish III-V materials as a competitive terrestrial PV technology by developing the materials science, advancing related science and engineering, coordinating relationships with industry and university partners, and facilitating commercialisation.
Developing Multijunction Technology
Although the gallium indium phosphide (GaInP)/GaAs tandem cell has achieved an efficiency of 30% and is now commercially available for space applications, the cells have not yet been integrated into a concentrator system.