22nd European Photovoltaic Solar Energy Conference, 3-7 September 2007, Milan, Italy
DETECTION AND ISOLATION OF LOCALISED SHUNTS IN INDUSTRIAL SILICON SOLAR CELLS USING
PL IMAGING
Y. Augarten1, M. D. Abbott1, T. Trupke1, R. Bardos1, H. P. Hartmann2, R. Gupta3 , J. Bauer3 and O. Breitenstein3
1 The University of New South Wales, Centre of Excellence for Advanced Silicon Photovoltaics and Photonics, Sydney,
2052, NSW, Australia. Tel: +61 2 9385 4054 Email: y.augarten@student.unsw.edu.au,
2 Deutsche Cell GmbH, Berthelsdorfer Strasse 111A, 09599 Freiberg, Germany
3 Max Planck Institute for Microstructure Physics, Halle, Germany
ABSTRACT: This paper discusses an improved shunt isolation process to improve the efficiency and reverse current
of shunted solar cells using Photoluminescence imaging. The method is applied to twenty shunted multicrystalline
commercial solar cells that were rejected due to a low shunt resistance and the associated high reverse current. The
shunts in eight out of twenty cells were successfully electrically isolated, and the performance of these cells was
recovered to acceptable standards.
Keywords: Photoluminescence, Shunts, Characterisation
contact fingers to minimize the induced shunting. In
order to identify suitable parameters for this combination
of processing, a first experiment was performed on a
non-shunted industrial mc-Si solar cell. Three stages
were used:
1 INTRODUCTION
Efficiency and yield loss due to regions of low shunt
resistance in industrial silicon solar cells is a major
challenge facing PV manufacturers. Shunted cells
significantly reduce average efficiencies, result in
modules with poor low light performance, and reduce the
total yield. This paper presents an update on the
development of a technique to mitigate the influence of
localised shunted regions by effectively isolating such
regions from finished solar cells. The technique was
proposed in [1] and some proof of concept experimental
results were reported in [2]. Electroluminescence (EL)
imaging [3] and Photoluminescence (PL) imaging [4]
were used as fast experimental methods to localise
shunted regions present in industrial screen printed solar
cells. Subsequently a laser scribed groove was used to
electrically isolate the localised shunts from the active
cell area. When applied to a severely shunted industrial
cell a substantial improvement in the efficiency was
achieved (from 9.6% to 13.3%). However, the final
efficiency of the cell was still below the average
efficiency expected for similar cells and the shunt
resistance of the cell was too low to pass hot-spot tests. It
was determined that shunting introduced by the laser
scribe, particularly where the scribe crossed a metal
contact finger, was limiting the potential for
improvement via isolation of the shunt.
Create laser scribed isolation boxes with two
different laser wavelengths (1064nm and
532nnm) and with either a single scribe or a
double pass.
2.
Apply Silver Etch to laser scribed regions and
non laser scribed regions.
3.
Laser Scribe isolation boxes within regions of
silver etching.
At the 532nm laser wavelength the laser was
defocused when two passes were scribed, in order to
reduce the energy density. After individual processing
stages two luminescence images were performed as a
means to test the effectiveness of the various processes in
isolating the area within the scribed squares. PL images
on finished solar cells carried out with the cell at open
circuit conditions (referred to here as PLOC) are normally
unaffected by series resistance effects and therefore
provide information about the local minority carrier
lifetime and local shunts. PL images carried out with
simultaneous current extraction (referred to as PLSC)
provide information about local variations of the series
resistance and can thus be used to assess whether the area
within the above mentioned laser scribed isolation boxes
is successfully isolated from the main part of the cell.
Finally,
dark
lock-in
thermography
(DLIT)
measurements [5] were used to measure the shunts
introduced by the different isolation processes (i.e. by
steps 1-3 above).
The present paper provides an update on the efforts
to improve the shunt isolation process. A combination of
chemical etching of front grid fingers and laser scribing
is investigated as a more gentle process to achieve better
electrical isolation and reduce the shunting induced by
the laser scribing process. This improved technique is
applied to a set of twenty shunted industrial solar cells in
order to assess the potential of inline shunt detection and
subsequent isolation for industrial applications. All
twenty shunted cells investigated initially failed hot-spot
tests that are routinely carried out on every cell by the
manufacturer.
2
1.
3
RESULTS
Figures 1-3 show PLOC and PLSC images measured after
each of the processing steps 1-3 described above. In step
1 four sets of six isolation boxes were scribed with
different laser wavelengths and either a single or a
double laser pass across the scribed area. For proper
isolation settings with low shunting and good isolation,
the area within the laser scribed isolation boxes should
IMPROVED SHUNT ISOLATION PROCESS
The improved shunt isolation process uses PL Imaging to
localise the shunted regions, followed by a combination
of Laser Ablation and Wet Chemical Etching of silver
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22nd European Photovoltaic Solar Energy Conference, 3-7 September 2007, Milan, Italy
and etched regions appear brighter in the PLSC image
than after the laser processing. The etching step thus
appears to have reduced the shunting previously present,
most likely by removing silver that had been driven into
the junction by the laser. The direct comparison of the
four etched boxes with the non-etched boxes (e.g. for the
1064nm double pass boxes) indicates a significant
improvement of the isolation process.
Finally the green laser was used to scribe additional
isolation boxes with different laser settings into the
previously etched regions at the top (Fig.3). After the
final scribing step, some isolation boxes are clearly
visible as bright squares in the PLSC image (Fig 3b). The
settings used to scribe these boxes are the most effective,
producing good isolation with low induced shunting. The
low introduced shunting is further confirmed by the
DLIT image shown in Fig.4. Only the 1064nm laser
scribed regions that have not subsequently been etched
show up as strong shunts. As demonstrated in [2] the
shunting appears in that case where the laser passes the
metal fingers. The 532nm laser scribes that have not been
etched do not introduce significant shunting (Fig.4),
however also do not yield sufficient isolation.
appear bright in a PLSC image.
(a)
(b)
Figure 1: Six isolation boxes were written with different
wavelengths and either one or two laser passes. (a) PLOC
image (b) PLSC image measured with 3A current
extraction.
The PLOC and PLSC imaging results from Figs. 1-3 and
the DLIT data were used to identify optimised isolation
settings: (1) Etching followed by a shallow 532nm laser
groove and (2) 1064nm laser scribing with 2 passes
followed by etching.
(a)
(b)
Figure 2: Silver etching was applied to nine areas at the
top of the cell and also to four previously laser written
areas (the left four in each set of six). (a) PLOC image (b)
PLSC image measured with 3A current extraction.
Figure 4: DLIT image measured with 0.5V forward bias,
showing shunts introduced after step 3.
4
(a)
(b)
Figure 3: Isolation boxes written with the 532nm laser
with different laser settings into the previously etched
regions. (a) PLOC image (b) PLSC image measured with
3A current extraction.
APPLICATION TO SOLAR CELLS
The optimised shunt isolation process with the best
settings identified in the above experiments was applied
to shunted industrial solar cells that were rejected by the
manufacturer due to a low shunt resistance and the
associated high reverse current. Twenty mildly to
strongly shunted cells were characterised with PL
imaging. PL images have previously been demonstrated
to reveal the position of shunts as somewhat blurry
regions of reduced luminescence intensity [1] (see e.g.
Fig.8. This blurring effect is caused by lateral currents
flowing from the surrounding cell area into the shunt,
that drag down the local voltage and thereby the
luminescence intensity in those areas. A quantitative
analysis of these blurring effects will be presented at this
conference [6].
The ability to detect shunts and to remove their influence
from the solar cell via isolation methods depends
strongly on the shape, the position and the distribution of
the shunts.
In that case the area within the laser scribed isolation
boxes should appear bright in a PLSC image. Fig.1 shows
that this is not the case for any of the laser settings used
here. The PLOC image indicates that the 1064nm laser
scribe with a double pass introduces significant shunting,
dragging down the local voltage inside the isolation box,
as has been described in [2].
Fig.2 shows images of the cell after silver etching.
The nine bright patches at the top of the cell that are
visible in the PLSC image show that decent isolation is
achieved just by removing the metal fingers, leaving the
etched regions connected to the main part of the cell only
via the emitter. Silver etching was also applied to the
four laser scribed areas on the left hand side of each set
of six laser scribed boxes from step 1. These laser scribed
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22nd European Photovoltaic Solar Energy Conference, 3-7 September 2007, Milan, Italy
observed. A number of point like shunts are visible near
the left hand edge. In addition the edge region appears
dark in parts of the cell which indicates poor edge
isolation.
The edge isolation can be fixed with a single laser scribe
without etching, because no metal fingers need to be
isolated. The point like shunts were isolated by scribing
with a 532nm laser. In this case the isolation resulted in
an improvement in the implied fill factor from 65.0% to
81.4% and in efficiency from 12.4% to 14.4%.
Figure 5: An example of a rejected cell. The cell has a
large number of material induced distributed shunts.
Isolation of the shunted area would remove
approximately 25% of the cell area, so it is unlikely the
efficiency of the cell would improve.
The shunt isolation removes the shunted regions
electrically from the main part of the cell. While this
reduces (in some cases quite dramatically, as shown
below) the influence of the shunt on the fill factor and
open circuit voltage it also results in the isolated area no
longer being able to contribute to the current. The shunt
isolation therefore works best for cells with a small
number of small sized moderate to strong shunts. i.e.
shunted regions with a small total area.
The cell in Fig.5 is affected by a large number of
distributed point like shunts, caused most likely by
conductive SiC filaments penetrating the wafer that are
often found in cast mc-si wafers [7]. Isolation of the
shunts in that cell would remove too much active cell
area and an efficiency improvement therefore cannot be
expected. Isolation was not attempted on cells with a
shunted area similar to Fig.5.
Fig.6a shows a PL image of a shunted cell that has a
strong shunt in the bottom right corner in the vicinity of a
grid finger. The inset of Fig.6 shows the same image but
plotted on a different color scale, allowing the position of
the shunt to be identified accurately. The optimised
isolation settings were applied to the area around that
shunt. Fig.6b shows a PL image measured after the
isolation, which highlights the fact that the influence of
the shunt is now confined to a much smaller isolated
area. The crack that is visible at the left hand edge was
introduced during cell handling and had no measurable
influence on the cell performance. Fig.7 shows Suns-VOC
measurements [8] performed on that cell before and after
the shunt isolation, demonstrating a dramatic
improvement in the implied fill factor by 20.3% relative
from 69% to 83%. The improvement in cell efficiency,
the latter measured under industrial conditions, was
somewhat more moderate (a relative improvement of
11.8% from 13.5% to 15.1%). The lower relative
improvement of the efficiency compared to the fill factor
as determined from Suns-VOC is caused by the series
resistance, which is not accounted for in Suns-VOC and
which starts dominating the fill factor once the shunt has
been isolated. The excellent implied fill factor of 83%
shows that the isolation process has completely
eliminated the influence of the shunt on the one-Sun cell
performance. Reverse bias measurements at -10V
revealed that the isolation reduced the reverse current
from 15A to 0.7A.
Another example for successful shunt isolation is shown
in Fig.8. In this case two separate shunt mechanisms are
(a)
(b)
Figure 6: PL image of a strongly shunted cell (SSh03)
with a strong local shunt near a grid finger in the lower
right. (a) before shunt isolation, (b) after isolation with
optimised isolation settings. The images are plotted on
the same scale.
Ssh03 - Suns Voc before and after
processing
Before processing
After processing
0
Current (A)
-1
-2
FFbefore = 68.6
FFafter = 82.7
-3
-4
-5
-0.1 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Voltage (V)
Figure 7: Suns-Voc curves of shunted cell Ssh03 before
and after shunt isolation. The fill factor had a relative
improvement by 20.3%, while the relative improvement
in efficiency was only 15.1%. This demonstrates that the
shunt no longer limits the cell’s performance at one Sun,
and has been successfully isolated.
(a)
(b)
Figure 8: PL image of a shunted cell Ssh13 with local
point like shunts and insufficient edge isolation (insets).
(a) before shunt isolation, (b) after isolation with
optimised isolation settings.
The Cell in Fig.9. required both etching and scribing. The
entire cell is significantly affected by a large horizontal
shunt along the bottom busbar, evident in Fig.9. A thick
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22nd European Photovoltaic Solar Energy Conference, 3-7 September 2007, Milan, Italy
Fill Factors are obtained from Suns-Voc data and thus
not affected by series resistance effects.
line of silver paste visible on the surface indicates that
some screen printing paste was inadvertently deposited in
that area and then fired through the SiN to shunt the
junction. Following silver etching and laser scribing,
efficiency was improved from 9.8% to 14.3%. The
isolated region is clearly visible at the bottom of Fig.9b.
The reverse current at -10 Volts was reduced by the shunt
isolation from >15 A to 1.1 A. The Suns -Voc curves
(not presented here) show a relative improvement in the
implied Fill Factor of 67% after processing.
Various different shunts were observed in twenty
shunted cells that were randomly picked from the
shunted bin at Deutsche Cell, including poor edge
isolation and also local material and processing induced
shunts. Table 1 summarises the results on cells that were
successfully processed at UNSW to remove the influence
of the shunts, thereby recovering the cell efficiency.
Importantly, all cells that are listed in Table 1 failed the
shunt criteria of the manufacturer prior to processing at
UNSW, whereas after the shunt isolation process they
met these criteria.
6
(a)
A three step process using PL imaging was to
localise and isolate shunted regions in completed
industrial solar cells was successfully applied. The
process was implemented on twenty multicrystalline
solar cells that were rejected from the Deutsche Cell
GmbH production line due to low values of shunt
resistance. In these first experiments, the reverse current
and one-Sun efficiency in eight of the cells were
recovered to acceptable levels.
From the limited statistical data available in this
study, we estimate that approximately half of the shunted
cells typically found in industrial production can be
recovered using the processes described in this work.
Images can be taken in 1 second, and in many cases
shunt isolation can be accomplished using processes
similar to those already used for edge isolation in modern
production lines, which increases the potential for
scalability.
Acknowledgement The Centre of Excellence for
Advanced Silicon Photovoltaics and Photonics is
supported under the Australian Research Council’s
Centres of Excellence Scheme.
(b)
Figure 9. PL image of a cell with a badly shunted edge
caused by a thick strip of silver paste near the bottom
region that is thought to have caused the shunt when it
was fired into the junction. (a) before and (b) after shunt
isolation with optimised settings. Cracks were introduced
during processing, which do not appear to have
significantly affected the final results The images are
plotted on the same scale.
5
SUMMARY OF RESULTS
Cell
Ssh03
Ssh09
Ssh11
Ssh12
Ssh13
Ssh14
Ssh17
Ssh20
Cell
Ssh03
Ssh09
Ssh11
Ssh12
Ssh13
Ssh14
Ssh17
Ssh20
Initial
η
(%)
13.46
13.20
9.81
14.33
12.37
12.78
12.13
14.18
Final η
(%)
15
14.29
14.23
15.15
14.43
14.81
14.42
14.86
Before Processing
Initial
Implied
Irev at FF
10V (A)
15
68.6
14.6
70.6
1.07
48.7
0.3
75.9
<20
65
< 20
65.5
<20
63.8
8.6
76.5
Shunt
Criteria
met
No
No
No
No
No
No
No
No
After Processing
Final Irev
Implied
at -10V
FF
(A)
0.7
82.70
2.2
80.80
1.07
81.70
0.3
82.70
1.6
81.40
2.5
81.00
1.08
81.00
1.7
82.00
Shunt
Criteria
met
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
CONCLUSIONS
REFERENCES
1. T. Trupke, R.A. Bardos, M.D. Abbott, K. Fisher, J.
Bauer, and O. Breitenstein, International Workshop
on Science and Technology of Crystalline Silicon
Solar Cells, Sendai, Japan, 2-3 October (2006).
2. M.D. Abbott, T. Trupke, H.P. Hartmann, R. Gupta,
and O. Breitenstein, Progress in Photovoltaics (2007)
in press.
3. T. Fuyuki, H. Kondo, Y. Kaji, T. Yamazaki, Y.
Takahashi, and Y. Uraoka, 31st IEEE Photovoltaic
Specialists Conference, Orlando, USA (2005).
4. T. Trupke, R.A. Bardos, M.C. Schubert, and W.
Warta, Applied Physics Letters 89 (2006) 044107.
5. O. Breitenstein and M. Langenkamp, Lock-In
Thermography - Basics and use for functional
diagnostics of electronic components. 2003,
Heidelberg: Springer.
6. M. Kasemann, Y. Augarten, T. Trupke, R.A. Bardos,
and W. Warta, this conference.
7. J.P. Rakotoniaina, O. Breitenstein, M. Werner, M.H.
Al-Rifai, T. Buonassisi, M.D. Picket, M. Ghosh, A.
Mueller, and N.L. Quang, 20th EPVSC, Barcelona
(2005).
8. R.A. Sinton and A. Cuevas, 16th EPVSC, Glasgow,
United Kingdom (2000) 1152.
Table 1: Efficiencies and reverse currents of eight of the
eleven shunted cells before and after processing (three
cells were damaged during processing, and are not
shown). The efficiencies are measured by Deutsche Cell,
under industrial measurement conditions. The implied
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