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Solar cells today are mostly made of silicon, one of the most common elements on Earth.The 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 wouldfunction. To 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 exist,between 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 electrons,being 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 ofthe material. In 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 temperature is raised or thematerial illuminated. The excitation of valence electrons to the conduction band is bestaccomplished when the semiconductor is in the crystalline state, i.e. when the atoms arearranged in a precise geometrical formation or "lattice". At room temperature and low illumination, pure or so-called "intrinsic" semiconductorshave a high resistivity. But the resistivity can be greatly reduced by "doping", i.e. introducinga very small amount of impurity, of the order of one in a million atoms. There are 2 kinds ofdopant. Those which have more valence electrons that the semiconductor itself are called"donors" and those which have fewer are termed "acceptors" [Book 2]. In a silicon crystal, each atom has 4 valence electrons, which are shared with aneighbouring atom to form a stable tetrahedral structure. Phosphorus, which has 5 valenceelectrons, is a donor and causes extra electrons to appear in the conduction band. Silicon sodoped is called "n-type" [Book 5]. On the other hand, boron, with a valence of 3, is anacceptor, leaving so-called "holes" in the lattice, which act like positive charges and render thesilicon "p-type"[Book 5]. The drawings in Figure 1.2 are 2-dimensional representations of n-and p-type silicon crystals, in which the atomic nucleii in the lattice are indicated by circles andthe bonding valence electrons are shown as lines between the atoms. Holes, like electrons, willremove under the influence of an applied voltage but, as the mechanism of their movement isvalence electron substitution from atom to atom, they are less mobile than the free conduction
Solar Energy Sojourner Truth - The Role Of Man In Context To Th Software Piracy
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Solar EnergySolar cells today are mostly made of silicon, one of the most common elements on Earth. The crystalline silicon solar cell was one of the first types to be developed and it is still the most common type in use today. They do not pollute the atmosphere and they leave behind no harmful was
Sojourner Truth - The role of man in context to this women with references to African-American women. In an ever changing world , the evolution of man has been themost drastic in terms of technological, environmental, andemotional advancement. With great expansions in the variousareas mentioned ear
Software PiracyAlmost everyday it seems , software companys keep pumping out brand new software that kills the day befores in that it is more sophisticated and more in tune with the needs of todays superusers , office users , and home users . Yet , at the same time , the software theft industry in g