microcrack chapter

MICROCRACKS IN PV MODULE

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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.


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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.


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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


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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


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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


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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


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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


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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


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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


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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


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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.


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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


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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


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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


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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


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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.


Dept OF ECE, SREC

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.


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Fig.6.2 DRAK AREA THE DEGRATION


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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.


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CHAPTER 8

REFERENCES

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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

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