Complete Report - University of New South Wales

In silicon wafers that have naturally occurring defects, such as dislocations, grain boundaries
and chemical impurities, the choice of wafer doping type can play a key role in mitigating
the effect that the impurities have on the electrical quality of the wafer. For example, Figure
4.3.2.1a shows the predicted injection-dependent normalized recombination rate for two
Shockley-Read-Hall (SRH) defect species in p-type and n-type materials. Curves A and B
predict the behaviour in p-type material: (A) for a defect located at a single energy level within
the bandgap and (B) the behaviour for a defect distributed over a continuum of energy levels
within the bandgap. Curve C predicts the behaviour for the same defects in n-type material,
while curve D predicts the behaviour of both n-type and p-type material in the absence of
the defect. When viewed along the 1-sun operating loci (“1-sun Vmp”), it is clear that for a
given defect species, the effective recombination rate depends signifi cantly on the doping
type of the wafer. In this case, the recombination in the n-type wafer is one or two orders of
magnitude lower than the p-type wafer.
Figure 4.3.2.1: (a) Predicted injection-dependent normalized recombination rate for n-type
and p-type silicon wafers in the presence or absence of SRH defects; (b) Predicted effect of
SRH defects on the illuminated IV curve for the cases presented in (a).
The signifi cance of the SRH behaviour in n-type and p-type wafers is highlighted in Figure
4.3.2.1b, which shows the predicted illuminated IV curve for the four cases shown in Figure
4.3.2.1a. The strong SRH recombination behaviour degrades the IV performance in the p-
type material signifi cantly, reducing the operating point voltage by as much as 120 mV. The IV
performance is hardly affected in the n-type material.
The DSBC solar cell design, shown in Figure 4.3.2.2a, is ideal for making a direct comparison
of solar cell terminal characteristics of solar cells made side-by-side on n-type and p-type
silicon wafers. The DSBC processing sequence can be applied to either p-type or n-type
wafers without signifi cant changes in the processing sequence - diffusions, oxidations, even
metallisation processes can be applied with good effect to both types of wafers processed in
split batches. Also, the DSBC process uses a heavy boron contact diffusion that is known to
introduce diffusion induced misfi t dislocations into the bulk of the wafer. Such misfi t dislocations
behave similarly to the SRH defects used in the simulations of Figure 4.3.2.1a. Figure 4.3.2.1b
shows a comparison of terminal characteristics of p-type and n-type DSBC solar cells that
were fabricated using the nearly identical fabrication sequences and processed in a split
batch.
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