ECN publication
Impurities in silicon and their impact on solar cell performance
Published by: Publication date:
ECN Solar Energy 24-9-2011
ECN report number: Document type:
ECN-O--12-048 Other
Number of pages: Full text:
124 Download PDF  

Most solar cells are made from crystalline silicon. Moreover, these solar cells are the most efficient available on the market. Unfortunately crystalline silicon is expensive and therefore it is difficult to drastically reduce the cost of silicon solar cells. The silicon is not expensive because it is scarce, on the contrary, but because it has to be very pure to make efficient solar cells from it. Surprisingly, nobody knows exactly hów pure and that is why manufacturers play it safe. In other words: they make and use more pure and hence, more expensive silicon than probably necessary. The results described in the PhD-work of Gianluca Coletti of ECN in the Netherlands might change this situation. For the first time the effects of a whole range of (metal) impurities in the silicon starting material (“feedstock”) on the efficiency of modern solar cells are described in detail. This way it becomes possible to purify the silicon in more targeted ways and to optimize the costs. Since “good is good enough” unnecessary steps in the purification processes can thus be avoided. The essence of the new findings described in this thesis is that the negative effects on solar cell performance of impurities in silicon feedstock can be strongly reduced during and by some of the process steps that follow feedstock fabrication, in particular growing large crystal blocks or rods (which are subsequently cut into very thin wafers) and processing wafers into solar cells. Per type of impurity it can now be indicated how much of it can be tolerated in the feedstock if one is to make solar cells of a certain efficiency from it. This is a crucial step towards full specifications of “solar grade” silicon.

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