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

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where, Δn is the injected carrier concentration.
A schematic of the complete numerical algorithm, proposed by Girisch, allowing
for the calculation of surface velocity rates under the assumption of flat quasi-Fermi
levels in the depletion region and for given values of Q, Δn and V is shown in Figure 3.4
[82].
This formalism was later adopted by Aberle et. a1.[83] who used this theory
successfully to explain the measured injection level dependence of surface recombination
velocity (Sett) for the Si-SiO2 interface.
Low Seff of the PECVD SiO2-passivated Si surfaces is attributed to the
combination of moderately low density of interface states at midgap (Di t= (1—
10)x 1010cm-2eV-1) and a high positive oxide fixed charge density Q0X [(1-10)x 10 11 cm 2]
[84]. The presence of a positive charge leads to a downward band banding (Ís) at the
Si/SiO2 interface. The large ΨΡs lowers surface hole concentration for recombination and
consequently reduces the Se" Therefore, even for a moderately high Di 1, it is possible to
get low Seff with higher QoX. Modeling results of the dependence of Sett on Q 0 for
different interface-state densities are shown in Figure 3.5 for an injection level of
(a) 1014cm-3 and (b)1016cm-3 respectively [84].