Complete Report - University of New South Wales

Figure 4.1.3.3 shows the measured I-V curve of the best rear emitter n-PERT cell, Wnrj7-2a,
which has demonstrated an effi ciency of 22.7%. This equals the best reported effi ciency
from n-type silicon substrates [R. King, et al, 21st IEEE PVSC, p.227, 1990, P. Verlinden, 14th
European PVSEC, pp.96, 1997].
1.0
amps
0.8
0.6
0.4
0.2
0.0
Cell ID: Wnrj7-2a
Cell Area: 22.04 cm 2
Temperature: 25.1°C
Voc = 702 mV
Isc = 0.884 A
Jsc = 40.1 mA/cm 2
FF = 0.805
Eff = 22.7%
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
volts
Figure 4.3.1.3: I-V curve of a rear emitter
n-type cell as measured at Sandia National
Laboratories under 100 mW/cm 2 , AM1.5
global spectrum at 25ºC.
It is seen that the cells on thinner substrates demonstrate improved effi ciencies. Also, limited
emitter areas were defi ned by lithography for the last batch, Wnrj7. The emitter boron
diffusion area was terminated about 200 µm from the scribed cell edge. This has signifi cantly
improved the cell fi ll factors to over 80% and hence improved the cell effi ciencies.
4.3.1.4 Spectral Response Measurement and Analysis
The spectral response of the best n-type re-PERT cell, Wnrj7-2a, was measured at Sandia
National Laboratories, with results shown in Figure 4.3.1.4. This cell has internal quantum
effi ciency (IQE) around 98%, compared to 100% for the standard front emitter PERL cells
on p-type substrates. Hence, its current density is only about 1 mA/cm 2 lower compared
to the best p-type PERL cells. The previous 400 µm thick cells had a much lower IQE of only
around 93% and a short-circuit current density about 2 mA/cm 2 lower than the best PERL
cells. It is clear that the 170 µm thin substrates and the modest resistivity of 1.5 Ω-cm have
helped this cell to increase its IQE and Jsc. The IQE for the second batch cell, Wnrj4-3b,
was very close to that from the 170 µm thin cell. Hence, the 270 µm cells and 170 µm thin
cells had similar experimental short-circuit current densities. However, the long wavelength
refl ection from the 170 µm thin cell had been signifi cantly increased compared to that from
the thicker cells. This gives evidence that the 170 µm might be too thin already for the best
light absorption.
Reflectance, EQE, and IQE (%)
100
90
80
70
60
50
40
30
20
10
0
300 400 500 600 700 800 900 1000 1100 1200
Wavelength (nm)
Figure 4.3.1.4: Spectral response of
the best n-type re-PERT cell, Wnrj7-2a,
on a 1.5 µ-cm resistivity, 170 µm thin
n-type substrate.
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