EML4930L4

EML4930L4 - Sustainable Energy Science and Engineering...

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S ustainable E nergy S cience and E ngineering C enter Photovoltaic Systems Engineering Solar Cells
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S ustainable E nergy S cience and E ngineering C enter Solar Electricity Solar-thermally generated electricity: Lowest cost solar electric source. Complex collectors to gather solar radiation to produce temperatures high enough to drive steam turbines to produce electric power. For example, a turbine fed from parabolic trough collectors might take steam at 750 K and eject heat into atmosphere at 300 K will have a ideal thermal (Carnot) efficiency of about 60%. Realistic overall conversion (system) efficiency of about 35% is feasible. Photovoltaic energy: The direct conversion of sun light to electricity. The efficiency (the ratio of the maximum power output and the incident radiation flux) of the best single-junction silicon solar cells has now reached 24% in laboratory test conditions. The best silicon commercially available PV modules have an efficiency of over 19%.
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S ustainable E nergy S cience and E ngineering C enter Semiconductor Junction Consider two pieces of a given semiconductor, one doped with donor atoms and the other with acceptor atoms. Suppose that each piece has a plane face and imagine bringing them together at their plane faces. This forms a pn-junction. In practice the junction is manufactured from a single piece of host crystal by varying the doping in different parts of it as the crystal is grown. This produces a transition region between the p-part and n-part that is typically about 1 μ m in width. p-type material - excess holes in the valance band compared with n-type material n-type material - excess electrons in the conduction band compared with the p-type material After the contact is made, it is energetically favorable for some of the excess electrons in the conduction band of the n-type material to cross to the p-type material and annihilate some of the holes there. Consequently, a net negative charge is built up in the p-type material and a net positive charge in the n-type material. Thus an electrostatic potential is set up, and this eventually stop the flow of further electrons across the junction.
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S ustainable E nergy S cience and E ngineering C enter P-type holes doped ion cores electrons + n-type doped ion cores + + + + + + + + + + + + + + + depletion region +ve -ve Charge density distance through crystal Potential distance through crystal Equilibrium potential, φ o In trying to neutralize charges Free electrons in n-type diffuse across junction to p-type and free holes in p- type diffuse to n-type; electrons and holes close to junction recombine. A depletion region (free of mobile charge carriers) develops on either side of the junction with fixed -ve ions on p-side and fixed +ve ions on the n-side. These residue charges prevent further diffusion so that recombination between holes and electrons is inhibited. A potential difference develops across
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EML4930L4 - Sustainable Energy Science and Engineering...

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