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Photovoltaics 101

Solar cells convert sunlight into electricity


Solar photovoltaic panel.

Photovoltaic panel

Photo (c) W.L. Sunshine

Photovoltaic solar technology turns sunlight into electricity. The term photovoltaic is easier to understand when you keep in mind that “photo” means “light” and “volt” is a measure of electricity.

How are photovoltaics used?

A photovoltaic (PV) cell or solar cell is a small semiconductor unit that channels the sun’s energy into electricity. Just one PV cell or a few may be used to power a small device such as a calculator. Several PV cells can be grouped into a module.

One common type of PV module is seen as the flat solar panels attached to signal lights, buoys, and emergency signs on roads.

Another type of PV module is thin film. Thin film photovoltaics have PV cells embedded on the surface of a flexible material such as plastic. They are lighter weight than standard solar panels and some can be rolled up. Thin films are commonly used to power small camping devices such as lanterns. The U.S. military also uses them in encampments to power lights, GPS, and other devices. A thin film module can also be made of glass or heavy metal foil.

All PV modules are oriented to catch as much sunlight as possible. To produce a larger amount of energy, several modules can be grouped into an array. These arrays may be in the form of several rigid panels or in a flexible thin film.

How do photovoltaics work?

When sunlight strikes the PV cells, subatomic particles called photons are absorbed and others called electrons are released. When these free electrons are captured, an electric current results.

Photovoltaics produce direct current that usually is converted to alternating current for use in homes and industry. PV systems require a device called an inverter to change the direct current (DC) output of the solar array into the alternating current (AC) needed for most power applications. Some energy is lost in this conversion from DC to AC.

PV systems can only produce electricity when the sun is shining on them. That's why some PV systems are integrated with a battery that can store electricity for use at night or during overcast days.

What are PV cells made from?

Most PV cells are made from crystalline silicon (either monocrystalline or polycrystalline).

  • Monocrystalline silicon, or single-crystal silicon, is produced by growing a large pure crystal of silicon in a furnace. The pure crystal is then sawed into wafers and assembled in an array. The resulting silicon is highly efficient, but expensive and time-consuming to mass-produce. Further, because the silicon has to be cut prior to assembly, approximately 50 percent of material goes unused, and is therefore wasted. Monocrystalline silicon makes up about 35 percent of world PV production.
  • Polycrystalline silicon, or multi-crystal silicon, is created by casting molten silicon in blocks. While this process is a little less expensive and faster than the process used for growing a single crystal, the resulting multi-crystal silicon is less efficient than its single-crystal counterpart. Polycrystalline silicon makes up about 45 percent of world PV production.

Thin film photovoltaics are generally made with amorphous silicon (silicon in non-crystalline form), cadmium telluride (CdTe), or copper indium selenide/sulfide (CIGS).

What are the advantages of thin film PV modules?

Thin film solar cells are made by depositing a very thin layer of silicon (or another semiconductor substance, depending on the application) on a very thin supporting material, such as glass, plastic, or metal foil. This process is known as chemical vapor deposition.

Although less commonly used than crystalline types, thin film can counterbalance many of the disadvantages associated with crystalline silicon manufacturing because it uses only a fraction of the amount of pure silicon.

Thin plastic film PVs have been available for some time. Recent research has developed a method of printing flexible PV cells on paper and fabric.

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