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Kristallijn silicium zonnecellen: thermische behandelingen en nieuwe materialen - THERMAT II
Gepubliceerd door: Publicatie datum:
ECN Zonne-energie 1-1-2005
ECN publicatienummer: Publicatie type:
ECN-C--05-011 ECN rapport
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The TherMat II project investigated the relation between silicon materialquality and solar cell efficiency, and the changes in material which occur as a result of high-temperature process steps. Goals of the project were 1. to find out whether the negative effects of extra impurities in the silicon wafer, e.g. due to the use of new feedstock, can be avoided. 2. to find out which extra impurities and defects are present in Si wafers which are made with modern high-throughput equipment and prototypes, and how these can be eliminated. 3. to look at the effect of short high-temperature process steps. Important results of the project are - new quantitative information on the presence of impurities, and new insight in the segregation phenomena occurring during crystallisation of silicon (chapter 1). - new knowledge on measuring Fe in silicon (chapters 2 and 3). - new insight in the relation between the doping of the silicon and the detrimental effect of impurities (chapters 4 and 5). - characterisation of the detrimental effect of boron-oxygen defect-pairs (chapter 6). - promising results to measure and characterise the passivation of defects with hydrogen, by monitoring interstitial Fe (chapter 7). - insight in the relation between ingot growth, Si material parameters and cell efficiency for standard processing (chapters 8 and 9). - improved gettering of impurities during emitter diffusion (chapter 10). Four practical results are particularly noteworthy: - Improvement of the thermal profile for gettering/diffusion, resulting in carrier lifetime increases of up to a factor 10; - Understanding of the doping-dependence of carrier lifetime in Si, in particular leading to a proposal to use n-type doping for future lower-quality Si wafers; - Quantification of (part of) the detrimental effect of oxygen (due to its interaction with boron) in mc-Si; and - Results which open the way for a better characterisation of the passivation of defects with hydrogen, by monitoring interstitial Fe.

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