Thursday, November 18, 2010
By Aaron Dicks
In the photovoltaic process solar cells are used to covert light into electricity. Solar cells are made up of semiconductor materials such as: silicon, gallium, cadmium telluride and copper indium diselenide. One of the most common materials used in solar cells is silicon.
In the case of crystalline silicon solar cells, substantially pure silicon with high crystal quality is needed to make strong usable solar cells. In the outer shell of a silicon atom it comprises of 4 bonding electrons. In order to form a stable electron configuration, in the crystal lattice two electrons of neighbouring atoms from an electron pair bond. By forming a stable bond with 4 neighbouring electrons silicon achieves its noble gas configuration with 8 out electrons. This electron bond can be broken by light or heat, which enables the electron to move freely and as a result it leaves a hole in the crystal lattice. This is known as intrinsic conductivity.
Intrinsic conductivity cannot be used to produce electricity. The silicon can only produce electricity when impurities (known as doping atoms) are introduced into the crystal lattice. These atoms have one electron more (phosphorous) or one electron less (boron) than silicon in their outer shell. The phosphorous doping method is known as negative doping (n-doping) and the boron doping method is known as positive doping (p-doping).
In the case of n-doping the electron can move about freely in the crystal and as a result can transport electrical charge. On the other hand p-doping has a missing bonding electron for every bonding born atom in the crystal lattice. This enables electrons from silicon atoms to fill the hole caused by the missing bonding electron, creating a new hole elsewhere. The conduction method based on these doping atoms is known impurity conduction.
If both the p and n-doped semiconductor layers are brought together a p-n junction is made. This junction allows surplus electrons from the n-semiconductor to diffuse into the p-semiconductor layer, thus creating an area known as the space charge region. Positively charged doping atoms remain in the n-region of the transition and negatively charged doping atoms remain in the p-region of the transition. An electrical field is then created that is opposed to the movement of the charge carriers, with the result that diffusion does not continue indefinitely. This p-n semiconductor is what is known as a solar cell. Once the solar cell is exposed to light photons are absorbed by the electrons. This contribution of energy breaks electron bonds. The released electrons are pulled through the electrical field into the n-region. The holes that are formed migrate in the opposite direction, into the p-region. This process is what is known as the photovoltaic effect - turning light into electricity.
Copyright (c) 2010 Aaron Dicks
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