Types of PV Cell
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, - A seed of crystalline silicon is dipped to touch the surface of molten silicon in a crucible and is then slowly
    raised. Silicon solidifies¡ªin the crystal pattern of the seed¡ªas it is pulled from the crucible to form part of the growing boule. The CZ method
    dominates commercial growth of single-crystal silicon and produces laboratory cells with efficiencies as high as 20%.
  • float-zone (FZ) method, A rod of silicon is placed atop a crystalline-silicon seed, and a movable heating coil encircling the rod is slowly
    raised from the seed upward. As the coil is raised, it melts the silicon, which then solidifies in the pattern of the crystal seed below it as the coil
    moves farther upward. FZ silicon has produced cells with a laboratory efficiency as high as 24%. Commercial applications for FZ silicon,
    however, have so far been restricted largely to high-efficiency cells (such as for satellites or concentrator systems). One limiting factor for FZ
    technology is the need for the feed rod of silicon to be smooth, uniform in diameter, and free of cracks. This precludes the use of most
    electronics industry seconds¡ªused by many of the other silicon growth processes in photovoltaic applications¡ªtherefore requiring more
    expensive silicon meltstock and keeping costs high.
  • casting - Casting/Directional Solidification¡ªMolten silicon is either poured into a heated mold or melted in a crucible and allowed to slowly cool
    and solidify as multicrystalline silicon. Simple casting, however, has only relatively modest temperature control for producing quality cells.
    Most current casting operations have therefore added more complex temperature controls, and some have produced cells with a laboratory
    efficiency as high as 17.8%¡ªthe old record broken by the 18.2%-efficient cells made from HEM wafers. Cast silicon is usually made in square
    shapes that are better for packing solar cells into modules. (Both CZ and FZ single-crystal ingots are round.)
  • die or wire pulling
  • Ribbon/Sheet Growth¡ªSeveral variations pull flat "ribbons" or sheets of multicrystalline silicon (one is single-crystal) from a crucible. The
    ribbons or sheets can then be cut directly into semiconductor wafers for solar cells, without the waste and expense of sawing ingots.
    Laboratory cell efficiencies reach 15%. The main drawback seems to be relatively slow production rates.
  • Thin Layer¡ªBecause of the potential for inexpensive production processes, there is considerable interest in thin films of amorphous silicon
    and various nonsilicon semiconductors for solar cells. It may also be possible to grow thin layers of crystalline silicon, and various researchers
    are investigating this option.

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.

 

Monocrystalline Silicon Cells:
Made using cells saw-cut from a single cylindrical crystal of silicon, this is the most efficient of the photovoltaic (PV) technologies. The principle advantage of monocrystalline cells are their high efficiencies, typically around 15%, although the manufacturing process required to produce monocrystalline silicon is complicated, resulting in slightly higher costs than other technologies.


Multicrystalline Silicon Cells:
Made from cells cut from an ingot of melted and recrystallised silicon. In the manufacturing process, molten silicon is cast into ingots of polycrystalline silicon, these ingots are then saw-cut into very thin wafers and assembled into complete cells. Multicrystalline cells are cheaper to produce than monocrystalline ones, due to the simpler manufacturing process. However, they tend to be slightly less efficient, with average efficiencies of around 12%., creating a granular texture

Thick-film Silicon:
Another multicrystalline technology where the silicon is deposited in a continuous process onto a base material giving a fine grained, sparkling appearance. Like all crystalline PV, this is encapsulated in a transparent insulating polymer with a tempered glass cover and usually bound into a strong aluminium frame.


Thin Films

Amorphous Silicon:

Amorphous silicon cells are composed of silicon atoms in a thin homogenous layer rather than a crystal structure. Amorphous silicon absorbs light more effectively than crystalline silicon, so the cells can be thinner. For this reason, amorphous silicon is also known as a "thin film" PV technology. Amorphous silicon can be deposited on a wide range of substrates, both rigid and flexible, which makes it ideal for curved surfaces and "fold-away" modules. Amorphous cells are, however, less efficient than crystalline based cells, with typical efficiencies of around 6%, but they are easier and therefore cheaper to produce. Their low cost makes them ideally suited for many applications where high efficiency is not required and low cost is important.

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.

Other Thin Films:
A number of other promising materials such as cadmium telluride (CdTe) and copper indium diselenide (CIS) are now being used for PV modules. The attraction of these technologies is that they can be manufactured by relatively inexpensive industrial processes, certainly in comparison to crystalline silicon technologies, yet they typically offer higher module efficiencies than amorphous silicon. New technologies based on the photosynthesis process are not yet on the market.

for research on higher-efficiency concentrator photovoltaic (CPV) III-V solar cells

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
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.
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.
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.
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.