Surface and bulk passivation of multicrystalline silicon solar cells by ...

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5.2 Objective
An electronic model is utilized to understand the nature of defect clusters, their formation
mechanism, their effects on solar cell performance and to assess the limitation on cell
performance posed by defect clusters. New gettering/passivation techniques to achieve
further improvements in cell efficiency and effectively getter defect clusters are
evaluated.
5.3 Characterization of Defect Clusters
Multicrystalline Si used for commercial solar cells is grown either as ribbons or cast in a
crucible and then sliced into wafers by wire sawing. The PV industry has accepted two basic
measures to lower the cost of mc-Si substrates. The first is to utilize substrates with high
impurity content, which result from the use of cheaper, lower-grade feedstock (consisting of
tops and tails, off-spec rejects from the microelectronic industry). The PV industry has
compromised cleanliness of the growth process. Typically, the as-grown material has high
concentrations of C and/or O. It also contains transition metal impurities (such as Fe, Cr) in
levels reaching 1014cm-3, which are detrimental to the minority-carrier lifetime. Typically, the
average minority-carrier lifetime of as-grown material is < 10 μs. The second measure
involves the use of much higher crystal growth speeds as compared to conventional crystal
growth, resulting in higher thermal stresses accompanied by high densities of defects in asgrown
form. Typically, the as-grown material has an average defect density of about
5x105cm-2. A unique feature of current mc-Si wafers is that they contain "defect clusters"—
crystal defects that clump together forming extended defect regions, which remain separated
from each other.