Microcrack chapter

MICROCRACKS IN PV MODULE
Dept OF ECE, SREC Page 1
CHAPTER-1
INTRODUCTION
Micro-cracks in a solar cell is an important issue for photovoltaic (PV) modules.
This can cause a long term power loss and effect the reliability of the PV modules. The
cracks in a module can develop at watering or manufacturing of modules or during
transportation or installation of modules. The cracks developed in the young modules
initially not affect the power output much but as with the time the module experience
heat, wind, humidity, mechanical loading the cracks starts affecting the power output
significantly. The small cracks may lead to inactive areas within a cell which are
electrically disconnected. It is very difficult to avoid cracks in modules and it is also
very difficult to quantify its impact on the module output because of lack of
understanding of its behavior during the lifetime of the module. So the cells having
cracks above a limit are rejected before integration of the cell string. This is done by
ultrasonic methods, flux thermography, electroluminescence imaging. Even though we
reject the cells with cracks initially but they may develop during the string and module
production. As it known by various studies that all cracks do not affect the module
output power in the same way and the modules with some cracks also perform will
within its specified power levels, so it is necessary to know the IA crack in a cell may
lead to power loss only if the crack results in a disconnection of cell parts. The exact
effect of cracks are not well known because the growth of cracks depends on the
handling of module, location of module, climate and other environment conditions.
That is why two modules with same amount of crack may give different power output
at two different places.impact of these cracks on power output to reduce the number of
rejected cells and reduce loss of manufacturer.

CHAPTER-2
ORIGIN OF CRACKS
PV cells are made of silicon they are very brittle in nature. The cracks may
develop in the modules very easily. The occurrences of micro cracks in a PV module
can be divided into three categories: during production, during transport and in the
field. The cracks developed during production are because of poor equipment and
inexperienced operator. The wafer slicing during manufacturing, stringing and
embedding processes during the production of cell and module may cause the cell to
crack. The process of stringing has the highest probability of introducing a crack in the
module during manufacturing. The cracks which may develop during production can be
avoided by improving the production process.
After the production of PV module, the other important source which may
introduce cracks is packaging and transportation of module. This can be mitigated by a
good packaging with more protection which helps to reduce damage during
transportation. After this another source is the installation of the module, it is also very
important because a bad installation may develop cracks and also other damage to the
module. Once a crack develops during production there is increased risk during
operation of cell that this short crack may lead to much longer or wider crack. This is
because of the mechanical stress, thermal stress, load due to wind or snow. The hairline
cracks around the busbars may develop during the manual soldering process of joints.
After lamination process, these cracks worsen because of thermal expansion and
pressure of lamination.

CHAPTER-3
CRACK DETECTION TECHNIQUES
As Cracks effects the operation of PV modules so it is necessary to detect them
and analyze their effect. The PV industry requires very fast and effective detection
technique for crack detection and characterization. Various non-destructive methods
have been developed for detection of cracks. Some of them are briefly explained one by
one. Optical transmission: - In this IR portion of the light is used. The Si wafer is placed
above a laser diode or broad spectrum flashlight. And then the CCD camera detects the
transmission through the silicon wafer. The cracks are detected when the infrared light
which passes through wafer is interacted by the cracks present on the wafer. The
minimum size of the crack which can be detected depends on the resolution of CCD
camera. This method is not good for detection of cracks in the finished solar cell. The
reason is the interference caused by the aluminum on the back side of the cell. Fig.3.1 is
showing this method general setup .
.
Fig.3.1 optical transmission

3.1. Infrared ultrasound lock-in thermography:-
The principle of the ultrasound lock-in thermography is that when we fed
ultrasound energy into the wafer then because of the friction at the crack edges heat is
generated. By detecting this heat cracks are detected. The ultrasound energy in feed
periodically into the wafer. A transducer generates the ultrasound energy at a frequency
of 20 KHz. The energy is fed to the silicon wafer by ultrasound coupler. Heat developed
is detected by IR camera and this information is converted into an image by lock-in
thermography.
3.2.Electroluminescence imaging: -
It is a very good way to detect the micro cracks in PV modules. In this a dc
current is supplied to the module to simulate radiative recombination in the solar cell.
As we apply a forward bias across the cell to detect the cracks, this technique is called a
contact technique. It is only used for finished PV modules. A silicon charged coupled
device (CCD) camera is used to detect the luminescence emission from the cell. It is
usually done in a dark environment. EL imaging is one of the best method available to
detect the cracks in PV modules. The cracks in an EL image looks as a dark line in the
cell. It also shows the crystallographic defects in a multicrystal silicon as dark lines.
Because of this reason, the EL image does not tell about cracks automatically and a
person is needed to find out the cracks by observing the EL image. Thus the detection
also depends on the person who is observing, an experienced person can read an EL
image efficiently. As a crack appears as a dark gray line the intensity of grey scale is
constant throughout the length of the gray line. The basic setup for this is shown in
fig.3.2
Fig.3.2 EL Imaging

3.3.Photoluminescence imaging:
It is a non-contact method for detection of cracks in PV modules. It takes an
acquisition time of less than a second. The luminescence image of unprocessed wafers
partially processed wafers and the finished solar cells can be taken from this technique.
In this method by using an optical energy source the entire sample surface is
illuminated uniformly. The energy supplied by the equal to or greater than the band gap
energy of the silicon. It creates a large amount of electron-hole pair in the
semiconductor. The image of this photoluminescence is taken by CCD camera using an
IR filter. The luminescence depends on the carrier concentration and recombination
rate. The PL image detects the luminescence, places where there is no crack the
recombination rate is different and the places where the cracks are present there also
recombination rate is different. This difference is because the non-radiative
recombination in high in places where the crack is there and it affects the luminescence
image and it appears dark in the image. The basic setup for this is shown in fig.3.3.
Fluorescence: - Generally the EL method is used to detect the cracks in the PV
modules. The outdoor images taken by this method are of poor quality and it requires
the change of circuit for taking images. So the fluorescence method can be used to
detect cracks. It is useful in detecting cracks in modules with aging. In this the modules
are irradiated by the ultraviolet light and a camera is used to detect the fluorescent light
from the PV module. It gives a great insight into the cracks of a PV module
Fig.3.3 Photoluminescence

3.4.Comparison of various techniques
Method Advantage Disadvantage
Optical transmission
Detect small cracks up to
1um, throughput 1 wafer
per sec.
Used in production stage, inapplicable
for finished cells
Ultrasound lock-in
thermography
Can be used for both
wafers and solar cells
Long acquisition time
Electroluminescence High throughput Interference with other defects, contact
method used only for finished cells
Photoluminescence High throughput,
contactless
Interference with other defects e.g.
scratches
Fluorescence High throughput , also
used for decolourization
Interference with defects
3.4 TABLE 1

CHAPTER 4
CLASSIFICATION OF CRACKS
The cracks which generally appear in the PV modules are of various sizes and
characteristic. For the study of the effect of various types of cracks on the PV modules,
it is necessary to divide the cracks into different type so that the effect of every
individual crack can be understood well. A classification of cracks according to the
orientation is as follows:
No crack: A cell which has no crack is taken as reference. This is shown in
fig.4.1
Fig.4.1 no crack
Dendritic crack: - This crack can present at any part of the cell. It can be
in any direction. These are shown in fig.4.2 . +45/ -45-degree crack: - This name
is given to the cracks because of the orientation of the crack with respect to the
reference cell. It is shown in figure4.3 .
Fig.4.2 dendritic crack Fig.4.3 (a) +45 degree (b) -45 degree

Several direction cracks: The cracks which may appear in all direction are
called several direction cracks. They are shown in fig.4.4. Parallel to bus bar: - The
cracks which are parallel to the bus bars comes under this category. These are shown in
figure4.5 . Perpendicular to bus bars: - These are cracks which are perpendicular to the
bus bars. These are shown in fig.4.6.
Fig.4.4 several direction Fig.4.5 parallel to bus bar
Fig. 4.6 perpendicular to bus bar
Cross line crack: These are line cracks. The name cross line is given because
this kind of crack occurs as a cross line the cell. It is shown in fig.10 .
Fig. 4.7 cross line

In all these types of cracks the cracks which are parallel to bus bars occur mostly. The
relative occurrence of these cracks shown in fig.4.8.
Fig. 4.8 relative occurrences of cracks

Model A crack: - These cracks are those which are present in the cell but not influence
the current flow through the cell. So they do not degrade the performance of cell much.
These cracks have no crack resistance and are still electrically connected to the cell.
This type of crack is shown in fig.4.9
Fig. 4.9 Mode A crack
Model B crack: - The mode B crack affects the current through the cell. It is still
connected to the cell. It has crack resistance. The area of mode B crack is more than
mode A crack. It is shown in fig.4.10
Fig.4.10 Mode B crack

Mode C crack: - These cracks isolate the crack area from the cell and degrade the
power output of modules significantly. It is more critical than other two mode cracks
because it disconnects the crack area from the active cell and the effective area of the
cell decreases. As the current from a cell is directly proportional to the active area of the
cell, so the cell output decreases due to mode C crack. These cracks are shown in the
Fig.4.11
Fig’4.11 Mode C Crack
The mode A crack present in the PV module can affect the power output in
many ways. They may or may not degrade the power. It effects are not well understood
because a mode A crack may change to crack B leading to increased crack resistance
and decreasing power. Also the mode A crack may also change to mode C crack
isolating the crack area from the cell and decreasing the effective area and the power
output significantly. The change of mode A crack to the mode B and mode C crack is
unpredictable this is due to the fact that the mode A crack behaves differently for
different temperature , pressure, stress and other environmental condition. So it is very
difficult to understand the conversion of mode A cracks to other mode cracks with the
aging of the module.

CHAPTER 5
EFFECTS OF CRACKS
The impact of micro cracks on the power output of PV module is not significant in
the initial stage of crack. The reason for this is that initially the crack is electrically
connected with the cell so it will not affect the power because the current is flowing
through it. As the module gets older the crack starts degrading the power of the module
by decreasing the conducting area. Different cracks impact the power differently
depending on their orientation, size, and location. A single crack which leads to the
isolation of cell area effects the power to a higher extent than a number of cracks which
are not electrically separated. We can put cracks based on the orientations into three
categories according to how much they degrade the power. The category I mean low
criticality, category II means moderate criticality, category III means high criticality. A
table (2) showing criticality of different cracks is shown.
Type of crack category
Dendritic III
+45 degree II
-45 degree II
Parallel to busbars III
Perpendicular to busbars I
Cross line II
Several direction III
5.1 TABLE 2 CRITICALITY OF CRACKS

Based on the study done by Kontges the cracks parallel to the busbars have
maximum separated cell area. In their experiment, they found that a substantial number
of cracks parallel to the busbars have no risk of separating cell area so they do not affect
the power much but at the same time some parallel cracks also showed worst case cell
area separation and high impact on the power output.
Diagonal cracks do not impact the power much if the there area is less. It is figured
out in studies that diagonal cracks having an area less than 8% do not degrade the
power output. So we can consider that diagonal crack has very less risk for power
stability of a PV module.
Several direction cracks and the dendritic cracks have a largely isolated cell area.
Their impact on the power is very high as separated cell area is high. So they are very
critical.
To understand how various crack modes impact the power output of PV modules
montages has done an experiment. He has used twelve 60 cell PV module with 15.6 x
15.6 cm2 crack free cells of the same type. He has first taken EL image of modules than
he has done a mechanical load test and again he has taken EL image. After this
humidity freeze test is done by him and again he measured the power output and taken
an EL image. The sequence of steps is shown below fig.5.2
ELELELe
Fig.5.2. sequence of steps for test
EL
Mechanical Load test
EL
Humidity freeze cycle
EL

In his tests he found cell micro cracks impact power loss to a very little extent if they do
not generate inactive area. These are mode A cracks. They found that in a 60 cell PV
module if half of the cells have mode A crack then there is a power loss of about 1%.
They also found that if all cells have mode A crack then the power loss is about 2.5%.
A graph showing the power loss with number of cracks after mechanical load test is
shown in the fig.5.3
Fig. 5.3 POWER LOSS
Another graph fig.5.4 relating power loss with a number of cracks after humidity freeze
cycle is shown below. It shows that the power loss is more for modules having more
number of cracks. They have found a maximum degradation of around 10% in their
test. After humidity freeze test the EL image shows that many modes A crack a has
changed to mode B and mode C. in some cells they have changed to mode B and in
some cells, it has changed to mode C. The change sequence is unpredictable. So it is
important to study their characteristic much deeply

Fig.5.4Humidiy freeze cycle
. The fig.5.5 showing the change of crack from mode A to another mode.
Fig.5.5 change of mode A crack to B and C

Model A cracks do not affect the series resistance but if the area of mode A
crack is more than 8% it affects the power output. The mode B crack creates an inactive
cell area but as they are still connected to the cell, so they introduce a series resistance
and affect the power output significantly. The study shows that if the resistance
introduced by the crack is of the order of the series resistance then it affects the power
output significantly. If the magnitude of resistance is higher than a mode B crack gives
approximately same output power as mode C which is equal to the completely isolated
inactive area.
It is also found that if in a module a number of cells have cracks than a cell
having 5% larger area compared to other cells determines the power loss of the PV
module. So for most practical cases power loss is determined by the cell having largest
cell area.

CHAPTER 6
CORRELATION OF CRACKS WITH MODULE PARAMETER
We can correlate the effect of crack with location, Pmax degradation. If we
know the location of the crack in a module then we can easily guess the source of the
crack. For this, we divide the module into three zones central, intermediate and
periphery. In all India survey, it is found that most of the cracks are located at periphery
which indicates bad handling of the module. This is shown in the fig.6.1
Fig.6.1 Location of cracks
The micro cracks of a solar cell affect the short circuit current. Mode B and mode C
cracks mainly affect the Inc. The fig.6.2 is showing that with increasing dark area the
degradation in Ic increases. The old modules show high degradation than younger
modules with increasing area because there are other defects also in the old modules.

Fig.6.2 DRAK AREA THE DEGRATION

CHAPTER 7
CONCLUSION
It has been found that a Si wafer cannot degrade the power output of a PV
module by more than 2.5% if the crack does not harm the electrical connection from the
active cell area. A PV module can tolerate up to 8% loss of active area of a cell without
impacting the power output of the module. As the crack affects the long-term power of
a PV module its deeper understanding should be done. To decrease the propagation rate
of crack the modules should be handled carefully. A good way to avoid power loss due
to micro cracks is to avoid cell breakage and use more flexible cell metallization. The
flexible metallization will prevent isolation of cell parts in a cracked cell.

CHAPTER 8
REFERENCES
[1] S. Chattopadhyay, R. Dubey, Vivek K., J. John, C. S. Solanki, Anil K., B. M.
Arora, K. L. Narasimhan, V. Kober, J. Vasi, A. Kumar and O. S. Sastry, All India
survey of PV Module Degradation:2014
[2] Köntges M, Kunze I, Kajari-Schröder S, Breitenmoser X, Bjørneklett B
(2011) The risk of power loss in crystalline silicon-based photovoltaic modules
due to micro-cracks. Sol Energy Mater Sol Cells 95(4):1131–1137.
[3] M. Abdelhamid, R. Singh and M. Omar, "Review of Microcrack Detection
Techniques for Silicon Solar Cells," in IEEE Journal of Photovoltaics, vol. 4, no.
1, pp. 514-524, Jan. 2014.
[4] M. Kšntges, S. Kajari-Schršder, I. Kunze, U. Jahn, “Crack Statistic of
Crystalline Silicon Photovoltaic Modules”, Proc. of 26th EU PVSEC (WIP,
Hamburg, Germany, 2011) 4EO.3.6.
[5] S. Kajari-Schröder, I. Kunze, M. Kontos, Criticality of cracks in PV Modules,
Energy Procedia, 27(2012) pp.658-663.
[6] V. Gade, N. Shiradkar, M. Paggi, and J. Opalewski, "Predicting the long-term
power loss from cell cracks in PV modules," 2015 IEEE 42nd Photovoltaic
Specialist Conference (PVSC), New Orleans, LA, 2015, pp. 1-6, doi:
10.1109/PVSC.2015.7355665
[7] S. Kajari-Schröder, I. Kunze, U. Eisner and M. Köntges, "Spatial and
directional distribution of cracks in silicon PV modules after uniform mechanical
loads," 2011 37th IEEE Photovoltaic Specialists Conference, Seattle, WA, 2011,
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[8]. S. Spataru, P. Hacke, D. Sera, S. Glick, T. Kerekes and R. Teodorescu,
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Orleans, LA, 2015, pp. 1-6.

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