Complete Report - University of New South Wales

As a result of these modifi cations, the open-circuit voltage of EVA cells has been improved to
517mV in 2005. This is believed to be a new world record for homojunction poly-Si thin-fi lm
solar cells on glass. The light intensity versus open-circuit voltage characteristics (“Suns-Voc
curve” [4.4.14]) of the 517-mV poly-Si thin-fi lm solar cell is displayed in Fig. 4.4.9 (left). A twodiode
model fi t (oblique straight lines in the graph) reveals that the cell is clearly limited by n=2
recombination. The Suns-Voc curve can be converted into a “pseudo current-voltage curve”,
as shown in the graph on the right hand side of Fig. 4.4.9. This curve has a good fi ll factor FF
of 0.71.
Fig. 4.4.9: Suns-Voc curve of the 517-mV EVA solar cell (left) and the equivalent pseudo
current-voltage curve (right).
The dopant concentration of the a-Si in our research-style silicon evaporator depends
strongly on the location of the sample within the evaporator during the deposition. This would
be undesirable in solar cell manufacturing lines, but can be exploited in the laboratory for
optimising the dopant concentrations in the fi nished solar cells. As an example, Fig. 4.4.10
compares the contour plots of the emitter dopant concentration (left) and the measured
open-circuit voltage of fi nished EVA cells (right). The black squares indicate the positions where
measurements were taken. A clear correlation can be seen, suggesting that the emitter
doping level in these EVA solar cells needs to be reduced to enhance the voltage of the cells.
This is very likely due to poorer crystallisation, since high dopant concentrations are known to
cause an increased nucleation rate and thus a smaller grain size. A corresponding analysis
of the boron base doping has revealed that the best Voc performance was obtained for the
highest boron doping investigated in this set of experiments.
position y
30
a
b c d
20 0.8
1.0 2.0 3.0
3.0
10
3.4
h
f
e
g
0
-10
i j k l
-20
m n o p
-30
-30 -20 -10 0 10 20 30
position x
position y
30
20
10
0
-10
-20
470
440
-30
-30 -20 -10 0 10 20 30
position x
410
380
Fig. 4.4.10: Contour plots of (left) the relative emitter doping density (as determined by SIMS
and sheet resistance measurements) and (right) the measured Voc of the resulting EVA solar
cells.
60