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

15
micro-volume defects. Growth of PV multicrystalline (mc) Si materials usually
requires a high pulling rate, so they contain a higher level of vacancies than
interstitials [33].
Dislocations are an example of line defects [(b) above]. Dislocations represent
boundaries between slipped and unslipped regions of a crystal. The formation of
dislocation lowers the total free energy to relieve the tension caused by the
temperature gradient during crystal growth and cooling. Dislocations in silicon may
be dissociated into glide and be involved in the deformation behavior of silicon [34].
As a result of elastic distortions associated with a dislocation, band bending occurs in
its vicinity. Dangling bonds are also created along the core of the dislocation and
introduce energy levels in the bandgap.
Stacking faults [(e) and (h) above], grain boundaries and twin planes are
examples of area defects. Stacking faults arise from excess silicon self-interstitials
generated during oxidation. Grain boundaries are formed during crystal growth. In
polycrystalline silicon material grown by casting process, a large amount of grain
boundaries can be induced. Grain boundaries may be treated as an assemblage of
dislocations whose properties depend on the crystallography of the boundary; and,
their electrical activities are connected with the set of dislocations and constitute a
boundary.
Precipitates [(d) above] and impurity clusters are examples of volume defects.
During processing, contamination by metallic impurities is also present. Unlike the
intentional doping of shallow level impurities, metallic impurities may be
incorporated without notice due their high solubilities in silicon. Transition atoms,
such as Fe, Co, Cr, Ni, Cu, in the silicon lattice are believed to introduce energy levels
in the bandgap [35]. Multicrystalline silicon (mc-Si) solar cells can tolerate iron,