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Title:
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Doping level effect on sample temperatures in infrared belt furnace firing
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Author(s):
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Published by:
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Publication date:
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ECN
Solar Energy
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30-9-2015
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ECN report number:
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Document type:
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ECN-W--15-054
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Article (scientific)
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Number of pages:
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12
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Published in: Energy Procedia (Elsevier), , 2015, Vol.77, p.665-676.
Abstract:
A firing study has been conducted for a variety of samples with different surface morphologies as well as with different bulk and emitter doping concentrations. In this study the firing settings were fixed and the temperature profile on the sample was measured with thermocouples. We have observed that the measured peak temperature on solar-cell R&D samples in an infrared (IR) conveyer belt firing furnace is strongly dependent on the surface morphology of the sample and its doping level. The surface morphology determines the coupling of IR radiation into the substrate. Samples with a random pyramids (RP) surface texture have reached temperatures that were 150oC higher than that of their polished counterparts. In addition to that, the doping level determines the free carrier absorption (FCA) and thus the heating up due to absorption of IR radiation. We have derived an analytical expression for a Planck-spectrum weighed FCA coefficient that is inversely proportional with the absolute black-body temperature (T) squared. Since the total energy radiated by a black body scales with T4, the radiation absorption is proportional with T2. The total doping level of a sample was quantified by Gtot, the summation of the sheet conductance values [:-1/sq] of the base and emitter region(s). For a sample with a base resistivity of 3 :cm and two emitters of 100 Ohm/sq on either side, yielding Gtot=0.028 :-1/sq, the measured peak temperature was found to be 150oC higher than that of a ‘bare’ (lifetime) sample of 10:cm with Gtot=0.0016 :-1/sq.
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