Photovoltaics

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.

Technologies

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) cell.

Thin Films
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 fabrication process.

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 sunlight.

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.

Applications

Concentrator Collectors
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, and windows.

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.

Space Applications
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 panels.

Issues

Cost Issues
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.

Incentives Programs
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.