A photovoltaic cell is a device that turns light into electric energy.
*Schematic symbol for Photovoltaic cell.
There are three main types of photovoltaic cells.
- monocrystalline cells
- polycrystalline cells
- amorphous cells
Monocrystalline cells are the most expensive to make because they require very pure silicon and involve a complicated crystal growth process. They also have the highest energy efficiency of the three: around 16%.
Polycrystalline cells are less expensive because the cells are not grown in single crystals but in a large block that is then sawn into cells. Their energy efficiency is around 15%.
Amorphous cells are not crystals, but a thin layer of silicon vaporised on a cheap base such as glass. They are relatively cheap, but their energy efficiency is only around 8%. When light is diffuse, the difference in efficiency compared to the other types becomes smaller. This type of cell is typically used as a coating for windows, and on facades.
A common myth is that the production of photovoltaic cells requires more energy than these cells produce in their lifespan. Modern cells typically require two to six years to pay back the energy investment made in them, and their lifespan is around 30 years.
see: Net energy gain
When photovoltaic cells have to be replaced, the silicon can be reused. Only the coating materials need to be renewed.
Photovoltaic cells are often made from refined silicon that is a by-product of semiconductor production. The limited supply of such relatively cheap by-product is an often-overlooked obstacle to the widespread production of photovoltaic cells.
Much research is currently going into photovoltaic cells. The main focus is making them cheaper and more efficient, so they can more effectively compete with other energy sources, including fossil energy. This mostly means development of cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a very common element, but is normally bound in silica sand.
The invention of conductive plastics, (for which Alan Heeger got a Nobel prize) might lead to the development of much cheaper cells that function without silicon.
In 2004, the Fraunhofer Institute reported they had created a particularly small cell with an energy efficiency of 23,5%. Manufacturing this photovoltaic cell would be prohibitively expensive, though. The theoretical maximum for silicon-based cells is 28%.  (http://www.wired.com/news/gizmos/0,1452,45092,00.html)