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Solar Cell
Solar Cell systems convert sunlight directly into electrical energy.
The backbone of this technology are semiconducting materials such as silicon.

A typical solar cell consists of two differently doped semiconductors. Doping is the controlled introduction of impurities into the host material. Starting out with a pure semiconductor crystal (say, silicon) this is achieved by substituting some of the atoms in the crystal lattice with elements that have one more or less valence electron than the host material (valence electrons are the electrons that determine the chemical behavior of a material, they are located in the outermost orbital shell of the atom). Semiconducting elements have four valence electrons all of which are used for bonding in the crystal lattice. If the doping material has five valence electron there will be one additional, loosely bound electron per dopant atom. These 'free' atoms can move about easily in the lattice and are responsible for an increase in conductivity. Since they have a negative charge the material doped in this way is called an n-type semiconductor. If, on the other hand, the doping material has only three valence electrons the lattice structure will be deficient of electrons and there will be one hole, or positive charge, per dopant atom. Similar to the free electrons above the holes can easily move about in the lattice, again causing an increase in conductivity. Since in this case the free charge carriers are positive this kind of semiconductor is said to be of p-type.

When a p-type semiconductor is joined to an n-type semiconductor, a p-n junction is created. While each side by itself is electrically neutral (there are as many electrons as there are protons) this different for certain areas of the combined configuration. The concentration differences of holes and free electrons between the n- and p-regions produce a diffusion current: electrons flow from the n-side and fill holes on the p-side. This creates a region that is almost devoid of free charge carriers (i.e. free electrons or holes) and is therefore called the depletion zone. In the depletion zero these is a net positive charge on the n-side and a net negative charge on the p-side resulting in an electric field that opposes a further flow of electron. The more electrons move from the n-to the p-side the stronger the opposing field will be and eventually an equilibrium will be reached in which no further electrons are able to move against the electric field. The potential difference of the equilibrium electric field is called the diffusion voltage. It cannot be used externally.

However, when light hits the solar cell the equilibrium conditions are disturbed and the so-called inner photo effect creates additional charge carriers that are free to move in the electric field of the depletion zone.

Holes move towards the p-region and electrons towards the n-region, thus creating an external voltage (no-load voltage) at the cell. The no-load voltage of a solar cell is material dependent and does not depend on the cell's surface area. A Silicon solar cell has a no-load voltage of about 0.5V. Higher voltage can be obtained by connecting individual cell in series.

The Current delivered by a solar cell is proportional to the intensity of the incoming light. Higher currents can be achieved by connecting cells in parallel.

The power of a solar cell depends on the connected electrical load. The maximum power point (MPP) can easily be determined from the power-voltage characteristic of the cell.

The efficiency of a solar cell is temperature dependent. It will decrease with increasing temperature.

Source: h-tec GmbH © h-tec GmbH
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