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Solar cells today are mostly made of silicon one of the most common elements onEarthThe crystalline silicon solar cell was one of the first types to be developed and it isstill themostcommon type in use today They do not pollute the atmosphere and they leave behind noharmfulwaste products Photovoltaic cells work effectively even in cloudy weather and unlikesolarheaters are more efficient at low temperatures They do their job silently and there arenomoving parts to wear out It is no wonder that one marvels on how such a device wouldfunctionTo understand how a solar cell works it is necessary to go back to some basic atomicconcepts In the simplest model of the atom electrons orbit a central nucleus composedofprotons and neutrons each electron carries one negative charge and each proton onepositivecharge Neutrons carry no charge Every atom has the same number of electrons as thereareprotons so on the whole it is electrically neutral The electrons have discrete kineticenergylevels which increase with the orbital radius When atoms bond together to form a solidtheelectron energy levels merge into bands In electrical conductors these bands arecontinuous butin insulators and semiconductors there is an energy gap in which no electronorbits canexistbetween the inner valence band and outer conduction band Book 1 Valence electronshelp tobind together the atoms in a solid by orbiting 2 adjacent nucleii while conductionelectronsbeing less closely bound to the nucleii are free to move in response to an appliedvoltageorelectric field The fewer conduction electrons there are the higher the electricalresistivityofthe materialIn semiconductors the materials from which solar sells are made the energy gap Eg isfairly small Because of this electrons in the valence band can easily be made to jump totheconduction band by the injection of energy either in the form of heat or light Book 4Thisexplains why the high resistivity of semiconductors decreases as the temperature is raisedor thematerial illuminated The excitation of valence electrons to the conduction band is bestaccomplished when the semiconductor is in
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