12th Workshop on Crystalline Silicon Solar Cell Materials and Processes, Breckenridge, Colorado, August 2002
Comparison of Shunt Imaging by Liquid Crystal Sheets and Lock-in Thermography
O. Breitenstein, J.P. Rakotoniaina, J. Schmidt*
Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 Halle, Germany
*Institut für Solarenergieforschung Hameln/Emmerthal (ISFH), Am Ohrberg 1,
D-31860 Emmerthal, Germany
1. Introduction
Thermographic techniques are able to image local shunts in solar cells [1-3]. An external bias
is applied to the cell in the dark, and the currents flowing locally through the shunts are leading to
a local heating of the surface of the cells, which may be detected thermographically. Note that
shunts may act in two different ways to solar cells: I. Under forward bias (near the working point
of the cells) shunts are increasing the dark forward current, thus degrading the fill factor and the
open circuit voltage of the cells [4]. II. If a single solar cell in a module is shadowed, it may
become reverse-biased by the other illuminated cells in the string by up to 13 V [2]. If there are
local shunts in this cell being active under reverse bias, in these positions excessive heat may be
produced (hot spots), which may lead to a thermal destruction of the module.
In the past it was generally assumed that shunts should have a linear (ohmic) I-V
characteristic. In this case indeed shunts acting under forward bias would be the same which
produce hot spots under reverse bias, hence reverse bias shunt imaging also would reveal the
dominant shunts acting at the working point. Then for quantitatively estimating the influence of
shunts, it would be sufficient to consider the parallel resistance of the cell measured at low bias.
In recent investigations, however, it has turned out that a large fraction of shunts may show a
non-linear (diode-like) I-V characteristic [5]. In this case reverse-bias shunt imaging would
produce misleading results with respect to the influence of shunts under operation of the cells.
The thermal sensitivity of classical (stationary) infrared (IR) thermography is in the order of
20... 100 mK. This is sufficient to image shunts in silicon cells under a few Volts reverse bias, but
shunts under forward bias usually produce such a low amount of heat that they cannot be imaged
in this way. Lock-in thermography improves the sensitivity of thermal investigations by 2-3
orders of magnitude, since it averages the oscillating thermal signal over thousands of single IR
measurements [3, 5]. Moreover, lock-in thermography suppresses the action of lateral heat
conduction, hence it improves the effective spatial resolution compared to stationary
thermography, and it can be evaluated quantitatively in terms of shunt currents flowing [4].
Unfortunately, lock-in IR thermography is a very expensive technique. The easiest and
cheapest, though quite insensitive, hot spot detection technique is to apply a sufficiently large
reverse bias to the cell and gently glide with your fingers across the surface of the cell. In this
way hot spots can be sensed. With this simple technique is it possible to locate the dominant
shunts acting under reverse bias up to an accuracy of, say, 5 mm. Unfortunately this technique
produces no images. The use of nematic liquid crystal (LC) layers under crossed polarisators is
also known as a cheap thermography technique [6]. For this investigation the sample should be
thermostatted, but it may be hard to ensure a homogeneous covering of the surface, and after
investigation the liquid has to be removed. Recently, Schmidt et al. [7] have demonstrated the
application of polymer-dispersed liquid crystal sheets as an easy-to-use thermographic technique
to detect shunts under reverse bias. These low-cost sheets are sucked together with the cell to a
heat sink, thereby producing images of the dominant shunts under reverse bias. Practically the
same technique is being used also at FhG-ISE by Ballif et al. [8] using commercially available
12th Workshop on Crystalline Silicon Solar Cell Materials and Processes, Breckenridge, Colorado, August 2002
sheets from Edmund Scientific. The aim of this work is to compare this low-cost liquid crystal
sheet thermography technique with the IR lock-in thermography technique on some typical
silicon solar cells.
2. Experimental
Two cells have been selected for this comparison. Cell No. 1 was a 19.5 cm2 sized float-zone
silicon solar cell fabricated by means of a high-efficiency cell process. In this cell 4 scratches
have been made at the surface with a diamond scriber in order to produce strong shunts
intentionally. Cell No. 2 was one 5x5 cm2 sized quarter of a 10x10 cm2 commercial
monocrystalline Czochralski silicon solar cell. For the liquid crystal (LC) investigations the cells
were sucked by a vacuum to a thermostatted brass heat sink and covered by the LC sheets. Two
different liquid crystal sheets have been used: On sample No. 1 a non-commercial one having a
temperature range of 25 ... 26 °C was used [9], and on sample No. 2 a commercial one purchased
by Edmund Scientifics having a temperature range of 35 ... 36 °C. The temperature of the heat
sink was stabilized at 25 and 35 °C, respectively, and the colour pattern was photographed. Under
forward bias in both cases no visible signal could be obtained. Under reverse bias, starting from
-3V hot spots became visible, which were fully resolved at -7 ... -10 V reverse bias. Lock-in
thermography investigations have been made at a lock-in frequency of 24 Hz using the TDL 384
M 'Lock-in' thermography system made by Thermosensorik Erlangen [10]. Here the cells were
covered by a thin black-painted plastic IR emitter film vacuum-attached to the surface and
measured at room temperature both under forward and under reverse bias. The acquisition time
for each image was about 30 minutes for the forward bias images and below 1 minute for the
reverse bias ones. Local I-V characteristics have been measured thermally (LIVT [11]) by
repeating the lock-in thermography measurement at different biases and evaluating the results
according to the image integration method, which was described in [4].
3. Results
Figs. 1 and 2 show greyscaled images of the LC investigation and lock-in thermography
images of both samples at reverse bias together with lock-in thermography images measured
under + 0.5 V forward bias. In some images the shape of the cell is indicated by dashed lines. The
colour scale of the LC sheets goes from black across red, yellow and green, to blue.
Unfortunately, in grayscale presentation this colour scale behaves non-monotonically, hence the
blue colour representing the highest temperature appears darker again. Therefore the strongest hot
spots in the LC images are showing a dark dip in shunt position.
In cell No. 1 the four scratches (A through D), which are aligned in cross shape, represent the
dominant hot spots. Under reverse bias these hot spots are visible both in the LC investigation
and in lock-in thermography. However, the spatial resolution of lock-in thermography is clearly
better. Another weak hot spot E is visible in lock-in thermography at the edge of the cell, which
was lying just outside of the photographed region in the LC image. It is visible in both techniques
that hot spot D is the strongest and A the weakest of these four. The lock-in thermography
investigation under +0.5 V forward bias shows that in this cell indeed all hot spots imaged under
reverse bias are also shunts under forward bias.
Cell No. 2 showed a weaker shunting activity than cell No. 1. Therefore a larger reverse bias
of -10 V was applied here in order to see not only the dominant hot spot in LC imaging (Fig. 2,
left). One dominant and two minor hot spots (see arrows) are visible in the LC image, which are
12th Workshop on Crystalline Silicon Solar Cell Materials and Processes, Breckenridge, Colorado, August 2002
all lying in the interior of the cell. This result indicates that the edge of this cell should be wellpassivated. Interestingly, the halo around the dominant spot appears larger here than that around
the spots visible in Fig. 1. Note, however, that the visual appearance of the hot spots in LC
imaging strongly depends on the heating power and on the illumination conditions as well as on
the time left after switching on the power to the cell. The hot spots visible in LC imaging are also
visible with a better spatial resolution in lock-in thermography (Fig. 2, middle). Additionally
some more hot spots become visible here, which remain invisible in the LC image since they are
too weak or are lying in the halo region of the dominant hot spot. The most interesting result is
that the forward bias thermogram of this cell (Fig. 2, right) looks very different to the reverse bias
one. Under forward bias the dominant shunts are lying at the edge of the cell, whereas the
dominant shunt under reverse bias is only a weak one under forward bias. Additionally, there is
some shunting activity under grid lines in the upper right part of the cell, which is totally invisible
under reverse bias. This proves that in this case reverse bias shunt imaging using LC sheets
would not reveal the dominant shunts acting under operation of this cell. This behaviour has been
found also in other solar cells [5], hence in well-processed solar cells an ohmic behaviour of
shunts is rather an exception than the rule.
B
A
D
C
1 cm
E
Fig. 1: Cell No. 1 containing 4 scratches. Left: liquid crystal (LC) image under -7V reverse bias,
middle: lock-in thermogram under -7V reverse bias (0 ... 100 mK), right: lock-in thermogram
under + 0.5 V forward bias (0 ... 5 mK).
1 cm
Fig. 2: Industrial solar cell No. 2. Left: liquid crystal (LC) image under -10V reverse bias,
middle: lock-in thermogram under -10V reverse bias (0 ... 200 mK), right: lock-in thermogram
under + 0.5 V forward bias (0 ... 4 mK).
Finally it should be checked whether the shunts in sample No. 1 are indeed showing a linear I-V
characteristic. Using lock-in thermography local I-V characteristics may be measured thermally
(LIVT [11]). This technique is based on the fact that the thermal signal is proportional to the
dissipated power. The thermal signal phase-shifted by -90° to the pulsed excitation may be
12th Workshop on Crystalline Silicon Solar Cell Materials and Processes, Breckenridge, Colorado, August 2002
4. Conclusions
20
I
region
region
region
region
region
15
10
5
I [mA]
averaged over a sufficiently large region around
a shunt, leading to a quantitative measurement
of the shunt current [4]. The result in Fig. 3
shows that neither the forward nor the reverse
bias characteristics of the shunts in cell No. 1
behave linear. Thus, even in this case, where a
qualitative agreement between forward and
reverse bias shunt investigation has been found,
the reverse bias investigation does not allow to
draw quantitative conclusions as to the action of
these shunts under operation conditions. Even
here the action of the shunts is not sufficiently
described by the parallel resistance of the cell.
A
B
C
D
E
0
-5
-10
-15
-20
-7
-6
-5
-4
-3
-2
-1
0
U [V]
Fig. 3: Measured current (I) and thermally
measured shunt currents of shunts A ... E of
cell No. 1
It has been shown that for reverse bias hot spot investigations LC sheet thermography is an
interesting alternative for lock-in thermography, delivering basically the same results with a
lower spatial and temperature resolution, but at a fracture of the costs. However, these results are
not representative for the behaviour at the working point under forward bias, which can only be
investigated by lock-in thermography. In one of the investigated cells having stronger shunts at
least a qualitative correspondence between the behavior in both polarities has been found, but in
the other cell the behaviour was dissimilar even qualitatively. The quantitative thermal
measurement of local I-V characteristics of shunts is possible only by lock-in thermography.
Thus, LC sheet thermography is sufficient for detecting hot spots under reverse bias, but for
optimizing the fill factor and Voc lock-in thermography has to be used.
This work has been supported by BMWi project No. 0329858D (KoSi). The cooperation with
Thermosensorik GmbH (Erlangen) is acknowledged.
References:
[1] A. Simo, S. Martinuzzi, 21st IEEE PVSC, Kissimee (1990) 800
[2] M. Danner, K. Büchner, 26th IEEE PVSC, Anaheim (1997) 1137
[3] O. Breitenstein, M. Langenkamp, K.R. McIntosh, C.B. Honsberg, M. Rinio, 28th IEEE
PVSC, Anchorage (2000) 124
[4] O. Breitenstein, M. Langenkamp, Proc. 2nd World Conf. on Photovolt. Energy Conv., Vienna
(1998) 1382
[5] O. Breitenstein, M. Langenkamp, J.P. Rakotoniaina, Proc. 17th Eur. Photovolt. Solar Energy
Conf., Munich 2001, 1499
[6] G. Färber, R.A. Bardos, K.R. McIntosh, C.B. Honsberg, A.B. Sproul, Proc. 2nd World Conf.
on Photovolt. Energy Conv., Vienna (1998) 280
[7] J. Schmidt and I. Dierking, Progr. Photovolt: Res. Appl. 9 (2001) 263
[8] C. Ballif, S. Peters, J. Isenberg, S. Riepe, D. Borchert, 29th IEEE PVSC, New Orleans 2002
[9] obtained from M. Pranga, K.L. Czuprynski, S.J. Klosowicz, Warsaw, Poland
[10] www.thermosensorik.com
[11] I.E. Konovalov, O. Breitenstein, K. Iwig, Solar Energy Mat. and Solar Cells 48 (1997) 53