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

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impurity-gettering methods which must dissolve the precipitates during a gettering
process.
5.4 Influence of Defect Clusters on Solar Cell Performance
Defect clusters constitute regions of high carrier recombination resulting in low photocurrent
generation, which can be easily observed by performing light beam induced current (LBIC)
mapping. However, their dominant effect occurs through a voltage-degradation mechanism.
Defect cluster regions develop a lower voltage compared to defect-free regions during the
cell operation. Because the entire cell is connected through a common junction, the defect
cluster regions exert a shunting influence on the entire cell. Unfortunately, the shunting
behavior of the defect clusters cannot be evaluated through experimental measurements
alone. It requires a combination of theoretical modeling to include the distributed nature of
the cell and experimental measurements pertaining to local characteristics of the cell. The
next section briefly describes the use of a network model to evaluate the efficiency loss due
to defect clusters. This model was developed previously to describe a nonuniform solar cell.
The detailed discussion of original model can be found in the literature [114].
5.4.1 Theory
Α conventional approach in defect modeling for calculating the influence of defects on
the material quality is to estimate the "average" carrier recombination and express it in
terms of an average minority-carrier lifetime or diffusion length. However, such a
procedure will not work for device modeling, because a solar cell is a distributed device
in which each region "communicates with" all other regions and the entire device "is
sensitive to" the presence of each defect cluster. Hence it is necessary to calculate the