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Single-Crystalline Thin Film Solar Cells Gallium arsenide (GaAs)
is a compound semiconductor: a mixture of two elements, gallium
and arsenic. Gallium is a by-product 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's been developed for light-emitting diodes (LEDs), lasers, and
other electronic devices that use light.
GaAs is especially suitable for use in
multijunction and high-efficiency solar cells, for several
reasons:
 | 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's amenable to a wide 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.
Design Challenge
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 reducing 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. - Reference U.S. Department of Energy
• Silicon Solar Cells • Polycrystalline Solar Cells • Single-Crystalline Thin Film • Solar Cell Structures • |
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