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.