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

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dependence of the minority carrier lifetime, using I-Ε passivation. This curve shows that
the lifetime maximum was reached much faster and the maximum is higher than previous
values. It is clear that it is necessary to remove the native oxide surface (in this case by
oxidation and dilute HF dip). In order to determine the depth of the native oxide surface,
oxidation was performed in steps.
Figure 4.3 shows the time dependence of the lifetime measurements for first three
steps. The sample was cleaned and oxidized after each set of measurements. Further
oxidation only increased the slope of the curves while the τmax remained the same.
4.2.3 Influence of Illumination
It was also observed that, if the measurements were performed at shorter intervals, the
slope of the curves increased. This is an interesting phenomenon indicating that the I-Ε
surface passivation has a light-activated component. To evaluate this effect, a wafer was
cleaned (using the new oxidation procedure), placed in an I-Ε bag, and exposed to about
0.5 sun intensity from a solar simulator for 15 minutes.
It was found that the lifetime after exposure to light yielded Τmax immediately after
the exposure; furthermore, there was a slow decrease (as shown in Figure 4.4) in τ. This
decrease occurs for all wafers after the measured lifetime reaches a maximum. Figure 4.5
shows a short-term variation of τb for a long lifetime wafer.