Silicon-based solar cells Characteristics and production processes ...

Silicon-based solar cells – characteristics and production processes
acceptor, donor and trapping states. The effect of the surface on the kinetics of the
electron processes is described by a quantity called surface recombination velocity S,
defined as [47]:
J
= σ
rυ
r
N
r
(14)
q⋅
∆n
r
S =
e
where: J r – density of the electron or hole current flowing to the surface, for maintaining
the established state in which the excess charge carrier concentration in the volume
equals Δn e ,
N r – number of recombination centres per surface unit,
σ r – collision cross-section of charge carrier capturing,
υ r – thermal velocity of charge carriers,
The surface recombination process taking place through defects is described by
the Shockley-Read-Hall theory, according to which the recombination velocity
increases with the increasing recombination centres N r or free charge carrier
concentration on the surface [48]. The increase of the surface recombination directly
affects the increase of the current I r , causing a drop of the voltage and the current
possible to obtain from the cell [49]. That is why, on the surface of Si, thin layers
of compounds are formed, whose atoms, combining with the surface semi-conductor
atoms, complement their incomplete bondings, that is they passivize the surface. In
practice, the surface passivation is realized through oxidization of the silicon plate
surface in dry oxygen, at the temperature range of 800 o C ÷ 1100 o C. Another
technique is coating the plate with a layer of hydrogenated silicon nitride with the
method of plasma enhanced chemical vapour deposition (PECVD). The Si x N y :H layer,
beside its passivating and protective effect on the Si-surface, is additionally
advantageous in the fact that, during the high temperature metallization process
of the front electrode, the hydrogen atoms released from the layer also passivize the
material in its whole volume, which is a determining factor in the case of mc-Si cells
[50]. The silicon nitride layer deposited by means of PECVD allows for a passivation of
the Si surface, but at the same time, it plays the role of an antireflection coating. The
Si x N y :H layer, due to its electron affinity, is appropriate for the n-type Si. In the case of
the p-type Si surface, more suitable seems to be an Al 2 O 3 layer, which is deposited
with the method of ALD (Atomic Layer Deposition).
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