Photovoltaic solar cells, which directly convert sunlight into
electricity, are made of semiconducting materials. The simplest
photovoltaic cells power watches and calculators and the like, while more
complex systems can light houses and provide power to the electrical grid.
Crystalline Silicon (c-Si)
Crystalline silicon (c-Si) is the leading commercial material for
photovoltaic cells, and is used in several forms: single-crystalline or
monocrystalline silicon, multicrystalline or polycrystalline silicon,
ribbon and sheet silicon and thin-layer silicon.
Common techniques for the production of crystalline silicon include the
Czochralski (CZ) method, float-zone (FZ) method, and other methods such as
casting and die or wire pulling. The removal of impurities and defects in
the silicon is of critical importance, and is addressed with techniques
such as surface passivation (reacting the surface with hydrogen) and
gettering (a chemical heat treatment that causes impurities to diffuse out
of the silicon). Also at issue as the industry grows is the availability
and purity of the solar-grade silicon feedstock.
Although crystalline silicon solar cells have been in existence since
1954, new innovations continue to be developed, including the emitter
wrap-through (EWT) cell and the self-aligned selective-emitter (SASE)
Thin film photovoltaic cells use layers of semiconductor materials only a
few micrometers thick, attached to an inexpensive backing such as glass,
flexible plastic, or stainless steel. Semiconductor materials for use in
thin films include amorphous silicon (a-Si), copper indium diselenide
(CIS), and cadmium telluride (CdTe). Amorphous silicon has no crystal
structure and is gradually degraded by exposure to light through the
Staebler-Wronski Effect. Hydrogen passivation can reduce this effect.
Because the quantity of semiconductor material required for thin films is
far smaller than for traditional PV cells, the cost of thin film
manufacturing is far less than for crystalline silicon solar cells.
Group III-V Technologies
These photovoltaic technologies, based on Group III and V elements in the
Periodic Table, show very high conversion efficiencies under either normal
sunlight or sunlight that is concentrated (see "Concentrating
Collectors" below). Single-crystal cells of this type are usually
made of gallium arsenide (GaAs). Gallium arsenide can be alloyed with
elements such as indium, phosphorus, and aluminum to create semiconductors
that respond to different energies of sunlight.
High-Efficiency Multijunction Devices
Multijunction devices stack individual solar cells on top of each other to
maximize the capture and conversion of solar energy. The top layer (or
junction) captures the highest-energy light and passes the rest on to be
absorbed by the lower layers. Much of the work in this area uses gallium
arsenide and its alloys, as well as using amorphous silicon, copper indium
diselenide, and gallium indium phosphide. Although two-junction cells have
been built, most research is focusing on three-junction (thyristor) and
four-junction devices, using materials such as germanium (Ge) to capture
the lowest-energy light in the lowest layer.
Fabricating Solar Cells and Modules
A variety of technical issues are involved in the fabrication of solar
cells. The semiconductor material is often doped with impurities such as
boron or phosphorus to tweak the frequencies of light that it responds to.
Other treatments include surface passivation of the material and
application of antireflection coatings. The encapsulation of the complete
PV module in a protective shell is another important step in the
Advanced Solar Cells
A variety of advanced approaches to solar cells are under investigation.
Dye-sensitized solar cells use a dye-impregnated layer of titanium dioxide
to generate a voltage, rather than the semiconducting materials used in
most solar cells. Because titanium dioxide is relatively inexpensive, they
offer the potential to significantly cut the cost of solar cells. Other
advanced approaches include polymer (or plastic) solar cells (which may
include large carbon molecules called fullerenes) and photoelectrochemical
cells, which produce hydrogen directly from water in the presence of
Balance of System (BOS) Components
The balance of system (BOS) components include everything in a
photovoltaic system other than the photovoltaic modules. BOS components
may include mounting structures, tracking devices, batteries, power
electronics (including an inverter, a charge controller, and a grid
interconnection), and other devices.
Concentrating photovoltaic collectors use devices such as Fresnel lenses,
mirrors, and mirrored dishes to concentrate sunlight onto a solar cell.
Certain solar cells, such as gallium arsenide cells, can efficiently
convert concentrated solar energy into electricity, allowing the use of
only a small amount of semiconducting material per square foot of solar
collector. Concentrating collectors are usually mounted on a two-axis
tracking system to keep the collector pointed toward the sun.
Building-Integrated Photovoltaics (BIPV)
Building-integrated photovoltaic materials are manufactured with the
double purpose of producing electricity and serving as construction
materials. They can replace traditional building components, including
curtain walls, skylights, atrium roofs, awnings, roof tiles and shingles,
Stand-Alone Photovoltaic Systems
Stand-alone systems produce power independently of the utility grid. In
some off-the-grid locations as near as one-quarter mile from the power
lines, stand-alone photovoltaic systems can be more cost-effective than
extending power lines. They are especially appropriate for remote,
environmentally sensitive areas, such as national parks, cabins, and
remote homes. In rural areas, small stand-alone solar arrays often power
farm lighting, fence chargers, and solar water pumps, which provide water
for livestock. Direct-coupled systems need no electrical storage because
they operate only during daylight hours, but most systems rely on battery
storage so that energy produced during the day can be used at night. Some
systems, called hybrid systems, combine solar power with additional power
sources such as wind or diesel.
Grid-connected Photovoltaic Systems
Grid-connected photovoltaic systems, also called grid interface systems,
supply surplus power back through the grid to the utility, and take from
the utility grid when the home system's power supply is low. These systems
remove the need for battery storage, although arranging for the grid
interconnection can be difficult. In some cases, utilities allow net
metering, which allows the owner to sell excess power back to the utility.
Solar arrays work well for generating power in space and power virtually
all satellites. Most satellites and spacecraft are equipped with
crystalline silicon or high-efficiency Group III-IV cells, but recently
satellites have begun using thin-film amorphous-silicon-based solar
Photovoltaics are expensive to produce because of the high cost of
semiconducting materials. Cost reductions can be achieved by reducing
manufacturing costs. As manufacturing capacity increases, costs of
manufacturing decrease. Manufacturers aim to achieve the break-even cost
for a photovoltaic system, at which the cost of the electricity it
produces is equal to the cost of electricity from an alternative source
plus the cost of delivering this electricity to the site. The distance a
power line needs to be extended to equal the installation cost of a
photovoltaic system is called the break-even distance.
Regulatory and financial incentives, such as tax credits, low interest
loans, grants, special utility rates, and technical assistance to
encourage the installation of photovoltaic systems are all available,
though they vary from region to region.
Performance Characterization and Rating, and Codes and Standards
Performance testing of photovoltaic components, materials, and complete
systems verifies that prescribed specifications are met. Certain
electrical codes and standards also apply to solar electric systems.
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