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Solar cells today are mostly made of silicon one of the most common elements on EarthThe crystalline silicon solar cell was one of the first types to be developed and it is still the mostcommon type in use today They do not pollute the atmosphere and they leave behind no harmfulwaste products Photovoltaic cells work effectively even in cloudy weather and unlike solarheaters are more efficient at low temperatures They do their job silently and there are nomoving parts to wear out It is no wonder that one marvels on how such a device wouldTo 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 composed ofprotons and neutrons each electron carries one negative charge and each proton one positivecharge Neutrons carry no charge Every atom has the same number of electrons as there areprotons so on the whole it is electrically neutral The electrons have discrete kinetic energylevels which increase with the orbital radius When atoms bond together to form a solid theelectron energy levels merge into bands In electrical conductors these bands are continuous butin insulators and semiconductors there is an energy gap in which no electron orbits can existbetween the inner valence band and outer conduction band Book 1 Valence electrons help tobind together the atoms in a solid by orbiting 2 adjacent nucleii while conduction electronsbeing less closely bound to the nucleii are free to move in response to an applied voltage orelectric field The fewer conduction electrons there are the higher the electrical resistivity ofIn 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 to theconduction band by the injection of energy either in the form of heat or light Book 4 Thisexplains why the high resistivity of semiconductors decreases as the
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