Solar Power: Photovoltaics The Journey From Photons To Electrons

1433 words - 6 pages

IntroductionSolar power is obtained mainly by the use of solar cells. The photovoltaic (PV) effect is the basic physical process through which a PV cell converts sunlight into electricity. Photovoltaics, as the word implies (photo = light, voltaic = electricity), convert sunlight directly into electricity.It is one of the forms of solar cells, in which light falling on a two-layer semi-conductor device produces a photo voltage or potential difference between the layers. This voltage is capable of driving a current through an external circuit and thereby producing useful work.Silicon in solar cellsPhotovoltaic (PV) cells are made of special materials called semiconductors such as silicon, which is currently the most commonly used. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material.Absorbing and wavelength entering the solar cell photons to electronsWhen sunlight strikes a solar cell, only certain wavelengths of light will cause electrons to move within the semiconductor, thereby producing electric current. The energy "band gap" of the semiconductor determines the ideal portion of the light spectrum that will create this effect.The wavelengths of light above the cell's band gap carry more energy. Some of this energy is absorbed to free electrons in the cell, and some of it is absorbed as heat. Light energy below the cell's band gap passes right through the field without affecting electron movement.Overall, only about 55% of the sun's light energy can be converted into electricity.So, the typical solar cell receives the entire solar spectrum. But only the light energy above the cell's energy gap will cause electrons to move and become part of an electric circuit.SiliconLarge numbers of silicon atoms, through their valence electrons, can bond together to form a crystal. In a crystalline solid, each silicon atom normally shares one of its four valence electrons in a "covalent" bond with each of four neighboring silicon atoms. The solid, then, consists of basic units of five silicon atoms: the original atom plus the four other atoms with which it shares its valence electrons.This regular, fixed arrangement of silicon atoms is known as the "crystal lattice." That's what forms the crystalline structure, and that structure turns out to be important to this type of PV cell.Pure silicon is a poor conductor of electricity because none of its electrons are free to move about, as electrons are in good conductors such as copper. Instead, the electrons are all locked in the crystalline structure. The silicon in a solar cell is modified slightly so that it will work as a solar cell.Doping: Phosphorus and boron, P-type silicon and N-type siliconWhen energy is added to pure silicon, for example in the form of heat, it can cause a few electrons to break free of their bonds and leave their atoms. A hole is left behind in each case. These electrons then wander randomly around the crystalline lattice looking...

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