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Photovoltaic cells

Solar cells are made of materials that directly convert light into electricity. Most of the todays commercially used solar cells are made of silicon (chemical symbol Si). Silicon is a so-called semiconductor. Silicon is found all over the world as sand, which is silicon dioxide (SiO²) also named quartzite. Another application of the semiconductor silicon is found in the microelectronics industry where silicon is used as the base material for chips.

Structure of a solar cell

The silicon solar cells can be of the type single crystalline, multi crystalline or amorphous. The difference between these cells is how the silicon atoms are ordered, the crystalline structure. There is also a difference in efficiency. By efficiency is meant the percentage of sunlight that is converted into electricity. Single and multi crystalline solar cells have almost the same and the highest efficiency with respect to amorphous silicon.

First there is a back contact layer and then two layers of silicon. On top are the front metal contacts with an anti-reflection layer, which gives the solar cell its typical blue colour.

Monocrystalline, polycrystalline and flexible amorphous silicon cells. Photos courtesy of Siemens Solar Industries, Photowatt International S.A. and United Solar Systems Corp., respectively.

The most common photovoltaic (PV) cell material is silicon. It is one of the most abundant elements on earth: sand from the beach is an oxide of silicon. The first commercial PV cells were monocrystalline silicon. Other manufacturing techniques resulted in polycrystalline silicon cells. A monocrystalline cell is made of a single crystal; a polycrystalline cell contains many crystals. Commercial polycrystalline cells are only slightly less efficient than monocrystalline cells and are, therefore, widely used because their cost-performance ratio is similar.

The development of thin-film technologies reduces costs further by decreasing the amount of material needed to make a cell. Amorphous silicon modules require only a thin layer of silicon and can be mass produced. New production techniques led to the manufacture of "multi-junction" amorphous cells, which contain two or three layers of semiconductor. Because of the lower efficiency, modules that are physically larger are needed in order to generate a given amount of power.

Other thin-film technologies have been developed – such as cadmium telluride and copper indium diselenide – and are beginning to appear on the market.

The last decade new types of solar cells of other materials than silicon are being developed. These are for instance thin-film solar cells and CIS (copper indium diselenide) and CdTe (cadmium telluride) solar cells. These cells are beginning to become commercially available.

Watt peak

A solar cell produces electricity when it is exposed to light. Depending on the intensity of the light (the irradiance in W/m²) a solar cell produces more or less electricity: bright sunlight is preferable to shade and shade is better than electric light. To compare solar cells and panels it is necessary to know the so-called nominal power of such a cell or panel. The rated power, expressed in Watt peak or Wp, is a measure of how much energy such a solar panel can produce under optimal conditions.

To determine and compare the nominal power of solar panels, the output is measured under standard test conditions (STC). These conditions are:
- An irradiance of 1,000 W/m²
- Solar reference spectrum AM 1.5 (this defines the type and colour of the light)
- Cell temperature of 25 °C (Importantly, the efficiency of a solar panel drops when the cell temperature rises).

Example:

Information from http://www.mysolar.com and other sources was used.

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