Abstract: Due to the high energydemands which are followed by the crisis of petroleum, the desire for thefuture lies in the renewable energy resources such as solar energy. In Photovoltaiccells, the mainly used material is Silicon in both crystalline and alsoamorphous form for the fabrication and also used in manufacturing industries.This research papergives the overall overview about the materials and also the processes used forfabricating a solar cell. The aim of this paper is to study the solar cellfabrication technology and also the fabrication of the solar cells.
However,there are a lot of challenges involved such as high manufacturing costs, energyconversion efficiency, uniformity, easy handling and storage etc. In response,solutions have been suggested in terms of both alternatives, manufacturingmethods and materials used in the photovoltaic cells. The paper furtherexplains in detail about the various fabrication processes utilized in themodern era.
The paper ends with contrasting the various techniques and pushesthe idea of using the most efficient solar fabrication processes.INTRODUCTION: Solar energy is the energy generated from theatomic combination in a star, i.e. the sun.
The energy is released when thefusion process takes place. That energy goes through the layers of the sununtil the point when it achieves the surface of the sun, where the light isemitted. Of the transmitted energy that reaches the atmosphere is known as thesolar constant. Solar panels are made up of solar cells which converts light,energy, into electric or electricity form. The earth receives more energy fromthe sun for just one hour than the world which uses in a whole year.
This showsa very high sun based radiation that should be utilized; as a substitutestrategy for the non-renewable energy sources utilized today. Solar panel boards which consist of cellssituated together in modules which mean the solar cells hold a noteworthy partin the panel’s last execution. In solar market today we use commonly threesorts of photo voltaic cells; single and, poly-crystalline cells, thin film.
Withoutany difficulties the cells are separated by their appearance, thin film cellsare sometimes black and even in their shades, single-crystal cells have thickblue color and poly-crystalline cells have various shades of blue. SOLAR CELL CONCEPTS:The most commonsemi-conductive material used in solar cells is silicon where it is importantto separate amorphous (un-structured) and crystalline (ordered) silicon. Monocrystalline cells: Crystalline silicon solar cells representabout 90% of the PV market today. Both crystalline cells have similarperformances; they have high durability and a high expected lifetime of about25 years. Of the two types of crystalline solar cells, the mono-crystallinecells tend to be a bit smaller in size per gained watt but also a bit moreexpensive than the polycrystalline cells. Single-crystalline solar cells arecut from pieces of unbroken silicon crystals.
The crystals are shaped ascylinders and sliced into circular disks of about 1mm. An advantageous propertyof the single silicon crystal cell is that they are not known to ever wear out.Polycrystalline cells: are also ordinarily made from silicon. However, the manufacturing process is somewhatdifferent. Instead for the material to be grown into a single crystal it ismelted and poured into a mould. The mould forms a squared shape and the blockis then cut into thin slices. Since the discs are squared already less or nomaterial has to be cut off and go to waste.
When the material cools down itcrystallizes in an imperfect manner which gives the polycrystalline cells asomewhat lower energy conversion efficiency compared to the single crystallinecells. In consequence the polycrystalline cells are slightly larger in size pergained watt than the single crystal cells are. After the disks of crystallinecells (mono and poly) have been made, they are carefully polished and treatedto repair any damage the slicing might have caused. Thin Films: A more recently developed concept is thethin film solar cell. In principle it is a microscopically thin piece ofamorphous (non-crystalline) silicon, as an alternative to the millimeter-thickdisk, which leads to less used material. Instead of the cell being a componentin itself, the thin film cells are placed directly on a sheet of glass ormetal. Therefore, the cutting and slicing steps of the production process areremoved completely. Furthermore, instead of mechanically assembling the cellsnext to each other they are simply deposited as such on the material sheet.
Silicon is the material most for the thin film cells but some other materialssuch as cadmium telluride may be used. Because the cells are so thin, thepanels can be made very flexible entirely dependent on how the flexible thematerial is that the cells are placed onto. Advantages can bewon from thin film modules compared to the traditional crystalline ones in bothflexibility and weight. They are also known to perform better in poor lightconditions. However, thin film technology offers lower efficiency which means thatfor the same amount of output energy a larger area would be needed. Despite thethin films lower efficiencies, the price per unit of capacity is lower than forcrystalline sells. They also tend to degrade over time because of instabilityin the material structure, making the durability of the panels less certain.From cells to modules Solar cells are built into modules orpanels because the output from a single cell is small while the combination ofmany cells can provide a useful amount of energy.
Design of solar panels isreliant of the type of solar cell that is used. The crystalline cells buildstiff modules that can be integrated between some layers of material-sheets andthen cut in different shapes whereas thin film panels are very flexible, makingthem applicable in other areas. Often solar panels are located on rooftops or in separate constructions where the optimal solar angle isreceived. To make sure the cell loose as little light as possible inreflection, the incident angle is kept at a minimum. The best alternative forthe panels would be perpendicular to the incoming sunlight; which is madecomplicated by the earth moving. Sometimes construction alternatives on roofsare not available or simply undesirable due to glass roofs, flat roofs, smallgardens etc. In such cases a more flexible alternative of solar panels is foundfrom the ones made of thin film cell modules.
FABRICATION TECHNIQUES Physical VaporDeposition: PVD comprises of Evaporation and SputteringMechanisms.Evaporation: Used to deposit thin layers (thin films) ofmetal on a substrate. Some metals films that are easily deposited byevaporation: aluminum, chrome gold, silver, and titanium. Electron Beam Evaporation(commonly referred to as E-beam Evaporation)is a process in which a target material is bombarded with an electron beamgiven off by a tungsten filament under high vacuum. The electron beam causesatoms from the source material to evaporate into the gaseous phase. These atomsthen precipitate into solid form, coating everything in the vacuum chamber(within line of sight) with a thin layer of the anode material. A clearadvantage of this process is it permits direct transfer of energy to sourceduring heating and very efficient in depositing pure evaporated material tosubstrate.
Also, deposition rate in this process can be as low as 1 nm perminute to as high as few micrometers per minute. The efficiency of the materialis high in respect to different techniques and it offers the procedures of structuraland morphological control of the thin films. Because of the very highdeposition rate, this procedure has potential industrial applications forthermal barrier coatings and wears resistant in the aerospace industries, hardcoatings for cutting and tool industries, and electronic and optical films forsemiconductor manufacturing factories. SPUTTERING: Sputteringprocedure includes ejecting material from an “objective” that is a source ontoa “substrate” (for example, a silicon wafer) in a vacuum chamber. This impact iscaused by the bombardment of the objective by the ionization of gases which oftenknown as an inert gas for example, argon. Sputtering is extensively utilized inthe semiconductor devices for the deposition of thin films of various materialsin the integrated circuits.
The Anti-reflection coating is additionally addedby sputtering on the glass for optical applications. Due to the low substratetemperatures utilized, sputtering is a perfect strategy to store metals forthin-film transistors. Maybe the most commonplace results of sputtering arelow-emissivity coatings on glass, utilized as a part of double-pane window assemblies.
The most important advantage of sputtering is that if the materials with veryhigh melting points are very easily sputtered while evaporating these materialsin a Knudsen cell or resistance evaporator is very problematic and complexity.CHEMICALMECHANICAL POLISHING: Chemical mechanical planarization or chemicalmechanical polishing CMP is the process that removes the topography from the polysilicon, silicon oxide and also metal surfaces. It is the most preferableplanarization technique used in the deep sub-micron IC manufacturing industries.The smaller the requested resolution of the structure, the higher is therequest for planarity of the surface. There is a local height variation betweenchip areas of different pattern densities. Chemical mechanical polishing whichis the only technique that performs global planarization of the silicon wafer. Originally CMP is used mainly to planarizethe silicon dioxide inter level the dielectrics of the Silicon dioxide materialdeposited that is thicker than the final thickness requested and these materialare then polished back until the step heights are removed. This results in agood flat surface for the next level.
The process can be repeated for everylevel of wiring that is added.Poly-silicon planarization: Poly-silicon polished easily with almost same types of thepolishers, similar pads and slurries as they are used for the planarization ofsilicon oxide. Applications are typically the polishing of poly silicon plugs,removing the poly silicon from the inter level dielectric and leaving only theplug filled with polysilicon. Poly-silicon planarization can also be used forthe end phase of wafer thinning or just for silicon wafer polishing.PHOSPHOROUSDIFFUSION: Phosphorusdiffusion is presently the first technique for electrode fabrication insemiconducting material (si) electric cell process. The diffusion depends onnumerous factors of that temperature and gaseous environment is mostsignificant .P-type semiconducting material wafers are wide utilized in starindustries and thus diffusion technologies are developed to deposit n-typedoping parts to make the contact. As a result of its low boiling temperature (105.
8 ?C), attemperature between 850-900 *c within the diffusion chamber, POCI3 is decayinto straightforward phosphorus compounds like P4, P8, P2O5, etc. Thephosphorus diffusion fabrication of crystalline semiconducting materialelectric cell with electrode diffusion, surface passivation and screen printingof conductor ends up in formation of n+ kind electrode at the highest surfaceof the wafer. Phosphorus oxychloride (POCI3) could be a liquid supply thatvaporizes at temperature itself thus it ought to be unbroken in cool place.
Forthe diffusion method, the vapors are administrated by the N2 gas and O2 ispassed through another valve. The reaction takes place, the phosphorousoxychloride reacts with O2 forms P2O5 and so the P2O5 reacts with thesemiconducting material to allow the silicon oxide and therefore thephosphorus. Pre-deposition involves the formation of phosphorus oxide films onthe semiconducting material substrate .throughout installation, phosphorusoxide film acts as an infinite supply for phosphorus diffusion into the sisubstrate. Throughout pre deposition, Phosphorus oxide (P2O5) forms on thesurface of the wafers by the reaction of phosphorus with O2. The P2O5immediately reacts with the semiconducting material by leading to diffusion ofphosphorus and formation of the phosphor silicate glass (PSG). ThePhosphorus atoms placed at the PSG-SI interface penetrate through the SI wafer.
ION IMPLANTATION:The alternative to deposition diffusion is IonImplantation and is utilized to produce a region of dopant atoms deposited intoa silicon wafer of shallow surface. In this process a light emission particlesof impurity ions is accelerated to kinetic energies in the range of tens of kVand is also coordinated to the surface of the silicon. As the impurity atomsenter the crystal, when it is collided they passes their energy to the lattice andfinally it reaches to rest at some average penetration depth, called theprojected range expressed in terms of micro meters (um). Depending on theimpurity and its implantation energy, the range in a given semiconductor mayvary from a few 100angstroms to about 1 um (micro meter). Typical distributionof impurity along the projected range is approximately Gaussian. By performing fewimplantations at various energies, it is possible to synthesize a desiredimpurity distribution, example: auniformly doped region.
A gas containingthe coveted debasement is ionized inside the particle source. The ions are producedand repulsed from their source in a wandering bar that is engaged earlier if goesthrough a mass separator that coordinates just the particles of the desiredspecies through a narrow aperture. A second lens focuses this is fixed by the lightemission which then passes through an accelerator that brings the particles totheir required energy before they strike the objective and become embedded inthe exposed areas of the silicon wafers. The voltages are accelerated from 20kV to as much as 250 kV. In some of the ion implanters, the separations of massoccur after the ions are accelerated with the very high energy.
Because the ionlight emission is quite small, which means they are provided for scanning ituniform across the wafers. For this purpose, the focused ion light emission is scanned electro statically over the surface of the wafer in the objectivechamber. The depth of the penetration of any particular type of ion willincrease with increasing accelerating voltage. The penetration depth willgenerally be in the range of 0.1 to 1.0 micro meters (um).
Results and discussion:Solar cells whichare characterized by their ability to convert sunlight into electricity. LIVtesting is done in the lab to observe the V-I curves of the fabricated solarcell. And also by this testing we can obtain the efficiency of the fabricatedmonocrystalline or polycrystalline cell. Conclusion:-The main objective ofthis research is to fabricate and also study about the mono or poly crystallinesilicon solar cell in the market. Which is why the efficiency of the solar cellis low was accepted and tried to find out the challenges and remedy to improvethe efficiency of the solar cell. The challenges are like to determine the flowrates and timing of the gases in the diffusion chamber, doping process, dopingconcentration and also the optimum temperature zones in the rapid thermalannealing furnace.
Our priority is to find the out the problems for achievinglow efficiency, and also the equipments is to be improved in order to get abetter efficiency.
