Friday, May 31, 2019

Solar Cell :: essays research papers fc

Solar cellsSolar cells today are mostly made of te, one of the most commonelements on Earth. The crystalline silicon solar cell was one of the firsttypes to be developed and it is still the most common type in use today.They do not infect the atmosphere and they leave behind no harmful wasteproducts. Photovoltaic cells work effectively even in cloudy weather andunlike solar heaters, are more efficient at low temperatures. They do theirjob silently and there are no moving parts to wear out. It is no wonderthat one marvels on how such a device would function.To understand how a solar cell works, it is necessary to go back tosome fundamental atomic concepts. In the simplest model of the atom, electronsorbit a central nucleus, composed of protons and neutrons. each electroncarries one negative charge and each proton one imperative charge. Neutronscarry no charge. Every atom has the same number of electrons as there areprotons, so, on the whole, it is electrically neutral. The electr ons havediscrete kinetic vital force levels, which increase with the orbital radius.When atoms bond together to form a solid, the electron energy levels mergeinto bands. In electrical conductors, these bands are continuous but ininsulators and semiconductors there is an "energy gap", in which noelectron orbits can exist, between the inner valence band and outerconductivity band Book 1. valence electrons help to bind together the atomsin a solid by orbiting 2 adjacent nucleii, while conduction electrons,being less closely bounce to the nucleii, are free to move in response to anapplied voltage or electric field. The fewer conduction electrons there are,the higher the electrical resistor of the material.In semiconductors, the materials from which solar sells are made, theenergy gap Eg is fairly small. Because of this, electrons in the valenceband can easily be made to jump to the conduction band by the injection ofenergy, either in the form of heat or light Book 4. This ex plains why thehigh resistivity of semiconductors decreases as the temperature is raisedor the material illuminated. The excitation of valence electrons to theconduction band is best accomplished when the semiconductor is in thecrystalline state, i.e. when the atoms are arranged in a deadgeometrical formation or "lattice".At room temperature and low illumination, pure or so-called"intrinsic" semiconductors have a high resistivity. just the resistivity canbe greatly reduced by "doping", i.e. introducing a very small amount ofimpurity, of the order of one in a meg atoms. There are 2 kinds of

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