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Photocell SensorsOutlineInformation ReferencesWhat are Photocells?How the Photovoltaic effect drives power cellsCell MaterialsLet there be Light…Finishing the CellEfficiency and MaterialsFirst Generation PhotocellsMono Crystalline WafersPoly Crystalline WafersRubber siliconSecond Generation PhotocellsThird Generation PhotocellsSlide 16LimitationsCurrent Research TopicsShow me the money!Other Web ReferencesPhotocell SensorsChris RogersMechatronics – ECE 5320Outline•Photocells Introduced•Materials Used•First, Second and Third Generation Photocells•Limitations•Areas of Research and Future opportunities•Web linksInformation References•Wikipedia -http://en.wikipedia.org/wiki/Solar_cells•HowStuffWorks -http://science.howstuffworks.com/solar-cell1.htm•http://www.pv.unsw.edu.au/Research/3gp.aspblahWhat are Photocells?Photo cells convert photon energy, light, into electricity. All photo cells make use of the photovoltaic effect, which is a two step process.How the Photovoltaic effect drives power cellsThe photovoltaic effect is realized in two steps:1 – Excitation of charge carriers on the surface of the cell2 – Separation of charge carriers to conductive material which can carry currentCell MaterialsSimilar to transistors and diodes, photocell materials are often made up of n-type and p-type silicon materials.N-type materials are semiconductors with an excess of electrons, or negative type.P-type materials are semiconductors with a recess of electrons, and have carrier holes. They get their name from the positive charge, and are hence positive type.Let there be Light…When light hits the sandwiched N-type and P-type, each photon of light releases an extra electron, concurrently creating an extra carrier where the electron was.The extra electrons and carriers force current to flow when an external load is connected.Finishing the CellSilicon is an extremely reflective material, and silicon alone isn’t suited to trap light on it’s surface.Anti-reflective coatings are employed to reduce photon loss through reflection.Image reference:fdashttp://science.howstuffworks.com/solar-cell7.htmA – Glass coverB – Anti-reflective coatingC – Electrical contactD – N-type materialE – P-type MaterialF – Electrical contactEfficiency and MaterialsSilicon is by far the most widely used among photocell materials. Crystalline silicon is put into three categories:-Mono crystalline wafers-Poly crystalline wafers-Ribbon SiliconFirst Generation PhotocellsFirst generation photocells are photocells in the simplest sense– they are nothing but PN junctions that convert sunlight to current.They aren’t very efficient and don’t produce very much current.Mono Crystalline WafersMono crystalline wafers use the Czochralski process to grow silicon crystals by seeding molten silcon.The molten silicon is placed in a vat, and a free moving arm with a seeding silcon crystal is placed in the liquid. As the arm is raised, it spins, using centrifugal forces to move rapidly cooling and crystallizing silicon to the outer edges.Poly Crystalline WafersPoly crystalline wafers are made from carefully cooled and solidified molten silicon ingots.The ingots can be cut in square dimensions, and are valuable for large panel arrays.Rubber siliconRubber silicon is a hybrid of mono and poly crystalline processes. The molten silicon is drawn out, forming thin flat ribbons.These processes are sometimes grown horizontally with substrate material and seeding crystals.Second Generation PhotocellsIn an attempt to improve the current output of first generation photovoltaic cells, the second generation photocells combine several layers of PN junctions and output current.Each layer is tweaked to handle a different wavelength from the sun’s spectrum.Third Generation PhotocellsThird generation photocells attempt to limit the bandwidth of incoming light in order send more usable light to the photocell.Other areas of research seek to tune the bandgap, which tunes the emission wavelength of incident light. This furthers the efficiency of the wafer.Efficiency and MaterialsLimitationsPhoto cells convert, on average, 15% of the sun’s light into electrical energy. Many wavelengths of light emanating from the sun do not have the energy to break electrons free.More area is needed to create more current, however, the larger the arrays of photocell material, the farther current must travel. These long contact lines into the photocell material can be high in resistance.Current Research TopicsSilicon’s biggest stumbling block is that it’s not always pure.Many methods have been worked on to purify silicon and create more mobility for charge carriers, increasing overall electrical current potential.Another major hindrance is cost. Solar panels have a much higher USD$/Watt than other fossil fuels. Money talks.Show me the money!In attempts to cheapen photo cells for power generative purposes, thin film solar cells are a promising area of research. They use less than 1% of raw material compared to wafer based solar cells.Thin film solar cells are amorphous silicon, where nano crystalline and proto crystalline silicon can be obtained. Nano and proto crystalline structures have higher bandgaps, and higher bandgaps mean more energy absorption from the sun, which means more energy production.Other Web References•‘Denim’ Solar Panels–http://www.newscientist.com/article.ns?id=dn3380•Scientists Create New Solar Cell–http://www.sciam.com/article.cfm?chanID=sa003&articleID=0004C094-02CC-1CD0-B4A8809EC588EEDF•Spray On Power Cells–http://news.nationalgeographic.com/news/2005/01/0114_050114_solarplastic.html•Photovoltaics for buildings–http://www.nrel.gov/buildings/pv/factsheets.html•Full Solar Spectrum Photovoltaic Materials Identified–http://www.lbl.gov/msd/PIs/Walukiewicz/02/02_8_Full_Solar_Spectrum.html•Solar Panel


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USU ECE 5320 - Photocell Sensors

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