Solar Cell :: essays research papers fc

solar cellsSolar cells today be mostly made of silicon, unrivaled of the most commonelements on Earth. The crystalline silicon solar cell was one of the firstcases to be developed and it is still the most common type in use today.They do not pollute the atmosphere and they communicate behind no harmful wasteproducts. Photovoltaic cells work effectively even in cloudy weather andunlike solar heaters, atomic number 18 more efficient at low temperatures. They do theirjob wordlessly 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 to roughly basic atomic concepts. In the simplest model of the atom, electronsorbit a rally nucleus, composed of protons and neutrons. apiece electroncarries one negative charge and each proton one positive charge. Neutronscarry no charge. Every atom has the very(prenominal) number of electrons as there areprotons, so, on th e whole, it is galvanicly neutral. The electrons get under ones skindiscrete kinetic energy levels, which increase with the orbital radius.When atoms bond unitedly to mannikin a solid, the electron energy levels mergeinto flocks. In electrical conductors, these bands are continuous but ininsulators and semiconductors there is an "energy cattle ranch", in which noelectron orbits can exist, between the inner valency band and outerconduction band Book 1. Valence electrons help to draw together together the atomsin a solid by orbiting 2 bordering nucleii, while conduction electrons,being less closely bound 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 resistivity 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 suffer to the conduct ion band by the injection ofenergy, either in the form of heat or light Book 4. This explains 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 place in a precisegeometrical formation or " lattice".At room temperature and low illumination, pure or so-called" inseparable" semiconductors have a high resistivity. But the resistivity canbe greatly reduced by "doping", i.e. introducing a very small fall ofimpurity, of the order of one in a million atoms. There are 2 kinds of