Complete Report - University of New South Wales

Since the rear emitter boron diffusion was over the entire rear surface, the rear emitter cells
have to be scribed into individual cells before measurement, in contrast to standard research
PERL cells which are often measured on the whole silicon wafers without scribing. Hence,
the performances of these scribed rear emitter cells are lower when compared to those unscribed
small-area research cells, due to the extra edge recombination loss and increased
cell area, which is non-optimal for the present silver plated metallisation method. By way
of comparison, the best scribed PERL cell on p-type substrates of the same 22 cm 2 area
demonstrated 23.7% effi ciency [J. Zhao et al, “20 000 PERL Silicon Cells for the ‘1996 World
Solar Challenge’ Solar Car Race”, Progress in Photovoltaics, Vol. 5, pp. 269, 1997], while the
best small 4 cm 2 unscribed cell had a signifi cantly higher effi ciency of 24.7%.
Small-area 4 cm 2 rear emitter cells were initially investigated. However, due to the relatively
larger edge scribing damage contributing to increased total recombination, these small cells
had relatively low Voc around 680 mV. Hence, the present research concentrated upon
large-area rear emitter cells of 22 cm 2 area in an effort to reduce the edge recombination
loss, resulting in high Voc of over 700 mV.
4.3.1.2 Calculated Device Performances with the
Device Simulation Program PC-1D
The device simulation program PC-1D was used to calculate and analyse the cell performance.
The cell Voc and the IQE (internal quantum effi ciency) were the main data to be fi tted in
these calculations. After comparing to the experimental device parameters, the bulk and the
surface recombination parameters were selected as following:
Bulk minority carrier lifetime:
1.1 ms
Front surface recombination velocity: 300 cm/s
Rear surface recombination velocity: 400 cm/s
The substrate resistivity was 0.9 Ω-cm. However, the substrate resistivity was found noncritical
for the calculated device performance if the recombination parameters were not
altered. However, normally the higher substrate resistivity is, the higher the minority carrier
lifetime. In this sense, the substrate resistivity may be very critical to the performance of
the fabricated devices, particularly for these thin cells. Figure 4.3.1.2 shows the calculated
variation of Jsc, Voc and cell effi ciency with the substrate thickness, simulated by PC-1D.
(a) (b) (c)
37.9
716
37.8
714
712
37.7
710
37.6
708
37.5
706
37.4
704
37.3
702
700
37.2
698
37.1
696
37
694
500
400 300 200 100
Cell Thickness, um
0
Jsc, mA/cm2
500
400 300 200 100
Cell Thickness, um
Figure 4.3.1.2: PC-1D simulated variation of (a) J sc , (b) Voc, and (c) cell effi ciency with the
substrate thickness. Reducing the substrate thickness from 400 µm to 200 µm is predicted
to signifi cantly increase all the three parameters.
0
Voc, mV
500
400 300 200 100
Cell Thickness, um
0
22.4
22.3
22.2
22.1
22
21.9
21.8
21.7
21.6
21.5
21.4
Efficiency, %
26