Solar Cells Essay Research Paper Example For Students

Solar Cells Essay Research Paper Word Count: 1045 today are mostly made of silicon, oneof the most common elements on Earth. Thecrystalline silicon solar cell was one of the firsttypes to be developed and it is still the mostcommon type in use today. They do not pollutethe atmosphere and they leave behind no harmfulwaste products. Photovoltaic cells workeffectively even in cloudy weather and unlike solarheaters, are more efficient at low temperatures. They do their job silently and there are no movingparts to wear out. It is no wonder that one marvelson how such a device would function. Tounderstand how a solar cell works, it is necessaryto go back to some basic atomic concepts. In thesimplest model of the atom, electrons orbit acentral nucleus, composed of protons andneutrons. each electron carries one negativecharge and each proton one positive charge. Neutrons carry no charge. Every atom has thesame number of electrons as there are protons, so,on the whole, it is electrically neutral. Theelectrons have discrete kinetic energy levels, whichincrease with the orbital radius. When atoms bondtogether to form a solid, the electron energy levelsmerge into bands. In electrical conductors, thesebands are continuous but in insulators andsemiconductors there is an energy gap, in whichno electron orbits can exist, between the innervalence band and outer conduction band Book1. Valence electrons help to bind together theatoms in a solid by orbiting 2 adjacent nucleii,while conduction electrons, being less closelybound to the nucleii, are free to move in responseto an applied voltage or electric field. The fewerconduction electrons there are, the higher theelectrical resistivity of the material. Insemiconductors, the materials from which solarsells are made, the energy gap Eg is fairly small. Because of this, electrons in the valence band caneasily be made to jump to the conduction band bythe injection of energy, either in the form of heat orlight Book 4. This explains why the highresistivity of semiconductors decreases as thetemperature is raised or the material illuminated. The excitation of valence electrons to theconduction band is best accomplished when thesemiconductor is in the crystalline state, i.e. whenthe atoms are arranged in a precise geometricalformation or lattice. At room temperature andlow illumination, pure or so-called intrinsicsemiconductors have a high resistivity. But theresistivity can be greatly reduced by doping, i.e. introducing a very small amount of impurity, of theorder of one in a million atoms. There are 2 kindsof dopant. Those which have more valenceelectrons that the semiconductor itself are calleddonors and those which have fewer are termedacceptors Book 2. In a silicon crystal, eachatom has 4 valence electrons, which are sharedwith a neighbouring atom to form a stabletetrahedral structure. Phosphorus, which has 5valence electrons, is a donor and causes extraelectrons to appear in the conduction band. Siliconso doped is called n-type Book 5. On theother 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-typeBook 5. The drawings in Figure1.2 are 2-dimensional representations of n- andp-type silicon crystals, in which the atomic nucleiiin the lattice are indicated by circles and thebonding valence electrons are shown as linesbetween the atoms. Holes, like electrons, willremove under the influence of an applied voltagebut, as the mechanism of their movement isvalence electron substitution from atom to atom,they are less mobile than the free conductionelectrons Book 2. In a n-on-p crystalline siliconsolar cell, a shadow junction is formed by diffusingphosphorus into a boron-based base. At thejunction, conduction electrons from donor atomsin the n-region diffuse into the p-region andcombine with holes in acceptor atoms, producinga layer of negatively-charged impurity atoms. Theopposite action also takes place, holes fromacceptor atoms in the p-region crossing into then-region, combining with electrons and producingpositively-charged impurity atoms Book 4. Thenet result of these movements is the disappearanceof conduction electrons and holes from the vicinityof the junction and the establishment there of areverse electric field, which is positive on then-side and negative on the p-side. This reversefield plays a vital part in the functioning of thedevice. The area in which it is s et up is called thedepletion area or barrier layerBook 4. Whenlight falls on the front surface, photons with energyin excess of the energy gap (1.1 eV in crystallinesilicon) interact with valence electrons and lift themto the conduction band. This movement leavesbehind holes, so each photon is said to generatean electron-hole pair Book 2. In the crystallinesilicon, electron-hole generation takes placethroughout the thickness of the cell, inconcentrations depending on the irradiance andthe spectral composition of the light. Photonenergy is inversely proportional to wavelength. .udd589f6a8781cc73ba54363732e57486 , .udd589f6a8781cc73ba54363732e57486 .postImageUrl , .udd589f6a8781cc73ba54363732e57486 .centered-text-area { min-height: 80px; position: relative; } .udd589f6a8781cc73ba54363732e57486 , .udd589f6a8781cc73ba54363732e57486:hover , .udd589f6a8781cc73ba54363732e57486:visited , .udd589f6a8781cc73ba54363732e57486:active { border:0!important; } .udd589f6a8781cc73ba54363732e57486 .clearfix:after { content: ""; display: table; clear: both; } .udd589f6a8781cc73ba54363732e57486 { display: block; transition: background-color 250ms; webkit-transition: background-color 250ms; width: 100%; opacity: 1; transition: opacity 250ms; webkit-transition: opacity 250ms; background-color: #95A5A6; } .udd589f6a8781cc73ba54363732e57486:active , .udd589f6a8781cc73ba54363732e57486:hover { opacity: 1; transition: opacity 250ms; webkit-transition: opacity 250ms; background-color: #2C3E50; } .udd589f6a8781cc73ba54363732e57486 .centered-text-area { width: 100%; position: relative ; } .udd589f6a8781cc73ba54363732e57486 .ctaText { border-bottom: 0 solid #fff; color: #2980B9; font-size: 16px; font-weight: bold; margin: 0; padding: 0; text-decoration: underline; } .udd589f6a8781cc73ba54363732e57486 .postTitle { color: #FFFFFF; font-size: 16px; font-weight: 600; margin: 0; padding: 0; width: 100%; } .udd589f6a8781cc73ba54363732e57486 .ctaButton { background-color: #7F8C8D!important; color: #2980B9; border: none; border-radius: 3px; box-shadow: none; font-size: 14px; font-weight: bold; line-height: 26px; moz-border-radius: 3px; text-align: center; text-decoration: none; text-shadow: none; width: 80px; min-height: 80px; background: url(https://artscolumbia.org/wp-content/plugins/intelly-related-posts/assets/images/simple-arrow.png)no-repeat; position: absolute; right: 0; top: 0; } .udd589f6a8781cc73ba54363732e57486:hover .ctaButton { background-color: #34495E!important; } .udd589f6a8781cc73ba54363732e57486 .centered-text { display: table; height: 80px; padding-left : 18px; top: 0; } .udd589f6a8781cc73ba54363732e57486 .udd589f6a8781cc73ba54363732e57486-content { display: table-cell; margin: 0; padding: 0; padding-right: 108px; position: relative; vertical-align: middle; width: 100%; } .udd589f6a8781cc73ba54363732e57486:after { content: ""; display: block; clear: both; } READ: State and Church EssayThe highly energetic photons in the ultra-violet andblue part of the spectrum are absorbed very nearthe surface, while the less energetic longer wavephotons in the red and infrared are absorbeddeeper in the crystal and further from the junctionBook 4. Most are absorbed within a thicknessof 100 ?m. The electrons and holes diffusethrough the crystal in an effort to produce an evendistribution. Some recombine after a lifetime of theorder of one millisecond, neutralizing their chargesand giving up energy in the form of heat. Othersreach the junction before their lifetime has expired. There they are separated by the reverse field, theelectrons being accelerated towards the negativecontact and the holes towards the positive Book5. If the cell is connected to a load, electrons willbe pushed from the negative contact through theload to the positive contact, where they willrecombine with holes. This constitutes an electriccurrent. In crystalline silicon cells, the currentgenerated by radiation of a particular spectralcomposition is directly proportional to theirradiance Book 2. Some types of solar cell,however, do not exhibit this linear relationship. The silicon solar cell has many advantages such ashigh reliability, photovoltaic power plants can beput up easily and quickly, photovoltaic powerplants are quite modular and can respond tosudden changes in solar input which occur whenclouds pass by. However there are still somemajor problems with them. They still cost toomuch for mass use and are relatively inefficientwith conversion efficiencies of 20% to 30%. Withtime, both of these problems will be solvedthrough mass production and new technologicaladvances in semiconductors. Bibliography 1)Green, Martin Solar Cells, Operating Principles,Technology and System Applications. NewJersey, Prentice-Hall, 1989. pg 104-106 2)Hovel, Howard Solar Cells, Semiconductors andSemimetals. New York, Academic Press, 1990. pg 334-339 3) Newham, Michael ,Photovoltaics,The Sunrise Industry, Solar Energy, October 1,1989, pp 253-256 4) Pulfrey, DonaldPhotovoltaic Power Generation. Oxford, VanNorstrand Co., 1988. pg 56-61 5) Treble,Fredrick Generating Electricity from the Sun. NewYork, Pergamon Press, 1991. pg 192-195