Pvi20 Front Cover indd

2
 out of the total of 0.61Ωcm
2
) was attributed to the rear contact.
Front busbar
3%
Front
15%
Front contact
9%
38%
Bulk
7%
28%
(a)
(b)

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Ph o to v o l t a i c s I n te r n at i o n a l
53
Cell 
Processing
For more complicated (arbitrary) layouts, 
SERIS has developed its own software 
called GRIDDLER [10]. This software can 
import metal grid patterns from images 
and perform a meshing and a subsequent 
finite element analysis, for determining (for 
example) the percentage of 
R
s
of the solar 
cell stemming from (1) the grid itself, (2) 
emitter lateral series resistance, (3) back-
surface field lateral series resistance, and 
(4) bulk series resistance. Furthermore, 
a p er turbation analysis allows the 
determination of how the grid patterns can 
be changed with the aim of attaining their 
optimum layouts. 
Fill factor loss analysis 
Taking the measured 
R
s
and 
R
sh
under 
maximum power conditions of the solar 
cell as input parameters, an advanced 
FF
loss analysis [8] can be performed 
by desc r ibing the c u r rent–volt age 
characteristics of the solar cell via the 
two-diode model shown in Fig. 10. The 
FF
of the solar cell is then determined 
by the diode saturation currents 
J
01
and 
J
02
, describing ideal and non-ideal 
recombination in the solar cell, as well as 
by the series and shunt resistances 
R
s
and 
R
sh
. To analyze 
FF
losses, it is important 
to determine the relative contributions of 
these quantities.
An ‘upper limit’ fill factor 
FF
J01
can 
be calculated by assuming only bulk 
recombination, in other words assuming 
no non-ideal second-diode recombination, 
no series resistance and an infinite shunt 
resistance. The loss in 
FF
due to second-
diode recombination currents (non-ideal 
recombination), and to 
R
s
and 
R
sh
, can 
then be calculated [8]. This is illustrated 
in Fig. 11 for the standard industrial 
p-type Al-BSF solar cell of Fig. 6. It is 
clear that 
R
s
is the biggest contributor 
to the 
FF
loss of this cell. However, 
for more advanced, higher-efficiency 
solar cells the contribution of the 
J
02
component becomes larger. For such cells 
it is extremely important to know the root 
cause of the 
FF
losses in order to devise the 
optimal strategy for improvement.
“
The loss quantification 
method enables the largest root 
causes of poor cell performance 
to be focused on first, before 
‘turning knobs’ to fine-tune 
secondary effects.
”
Conclusion
As remarked by Lord Kelvin 140 years ago, 
“To measure is to know.” In this paper it 
has been shown that current, voltage and 
fill factor losses for silicon wafer solar cells 
can be fully quantified by a combination of