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ELECTROLUMINESCENCE (EL) TEST: AN ESSENTIAL TOOL FOR DETECTING AND IDENTIFYING DIFFERENT TYPES OF DEFECTS IN PHOTOVOLTAIC MODULES PRESENTATION 22-09-2020
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ELECTROLUMINESCENCE (EL) TEST: AN ESSENTIAL TOOL FOR DETECTING AND IDENTIFYING DIFFERENT TYPES OF DEFECTS IN PHOTOVOLTAIC MODULES
PRESENTATION 22-09-2020
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INDEX
1. Introduction
2. International Standard IEC/TS 60904-13 Ed.1.0
3. Influence Biasing Level in EL Measurements
4. Types of Defects detectable by EL technique
5. Classification Types of Cracks by EL
6. EL measurements in the PV plant
7. Conclusions
01 1. Introduction
2. International Standard IEC/TS 60904-13 Ed.1.0
3. Influence Biasing Level in EL Measurements
4. Types of Defects detectable by EL technique
5. Classification Types of Cracks by EL
6. EL measurements in the PV plant
7. Conclusions
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Introduction
• Electroluminescence (EL) measurements are becoming an essential characterization tool
to guarantee the quality of PV modules of different technologies.
• In order to obtain the EL image, the PV module is electrically biased by applying a certain
level of current, and the emitted photons (luminescence) are captured by a camera (Si or
InGaAs)
Power supply
CCD camera
PC
PV module
Dark chamber
Set-up EL measurement system in laboratory CENER technician performing EL
measurements in the field
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Introduction
For EL measurements of crystalline modules (mono and poly) cameras with a Silicon chip
are normally used.
Silicon cameras usually has a very good spatial resolution (e.g. above 2MegaPixels) and an
improving lower price.
Efficiency Response of Silicon CCD detectors in the EL emission range (1000nm-1200nm) is
very low -> filter utilization or dark conditions.
Other types of cameras can be used (InGaAs, Ge):
Better efficiency in the spectral range of the EL
Worst spatial resolution
Much higher economic cost
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Introduction
Very broad fields of application of electroluminescence (EL):
Manufacturing process of PV modules and cells:
Production control
Quality assurance at origin and destination of PV modules
No damage during transport and assembly of the PV modules
Control of guarantees for PV installations
Important part of a global O&M service
Research in R+D+i centers
Supervision at origin
•Quality & production systems
•In-line & out-line testing
•Testing lab. Accredited(1)
•Pre-shipment supervision
Warranty after installation
•Verification of absence of damage after assembly
•Characterization by EL
•Use of special measurement system (CELSOS)
Warranty after transport
•Verification of absence of damage after transport
•Characterization by EL
•Measurement at the arrival of the modules
02 1. Introduction
2. International Standard IEC/TS 60904-13 Ed.1.0
3. Influence Biasing Level in EL Measurements
4. Types of Defects detectable by EL technique
5. Classification Types of Cracks by EL
6. EL measurements in the PV plant
7. Conclusions
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International Standard IEC/TS 60904-13 Ed.1.0
A new international standard IEC/TS 60904-13 Ed.1.0 has just been published to
standardize the methodology for performing electroluminescence (EL) measurements in
the laboratory.
This is a technical specification (TS) that will be part of the IEC-60904 standard:
Photovoltaic devices - Part 13: Electroluminescence of photovoltaic modules
The technical specification sets out the methodology for:
Capture of EL images from PV modules
Image processing to obtain quantitative measurements from the images taken
Guide for the qualitative interpretation of images for the detection and localization of
potential defects
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International Standard IEC/TS 60904-13 Ed.1.0
The directive also sets quantitative references on how to calculate the signal-to-noise
ratio (SNR), and establishes minimum values required depending on the application of the
measurement made:
A) Outdoor measurements: SNR50 ≥ 10
B) Industrial or process control measures: SNR50 ≥ 30
C) Laboratory measurements: SNR50 ≥ 50
The regulations also establish how the cells
are to be numbered in the final results
report
The standard finally establishes clear criteria on
how to classify potential cracks that may appear
in the cells of a PV module
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International Standard IEC/TS 60904-13 Ed.1.0
Finally, the standard also establishes some basic criteria for the qualitative interpretation
of EL images of PV modules based on crystalline silicon.
Illustrative examples:
Interruption
of
metallization
fingers
during screen
printing
Interaction
soldering flux
with interface
between silicon
and fingers
Different
types of
cracks in the
cells
Typical PID pattern
Conveyor belt marks during
firing process
03 1. Introduction
2. International Standard IEC/TS 60904-13 Ed.1.0
3. Influence Biasing Level in EL Measurements
4. Types of Defects detectable by EL technique
5. Classification Types of Cracks by EL
6. EL measurements in the PV plant
7. Conclusions
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Influence Biasing Level in EL Measurements
EL measurements can be made at different levels of electrical biasing
The captured EL images contain different kind of information about the module's behavior
There are types of defects that manifest themselves more clearly at a certain level of
electrical biasing (e.g., PID is best observed in low-polarization measurements)
Normally, for each module, at least 2 measurements of EL are taken: a 1st image to
IBIAS1≈ISC, and a 2nd image to IBIAS2≈10%*ISC
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
27 29 31 33 35 37 39 41
Cu
rre
nt
(A)
Voltage (V)
~RS ~Rp
M. Ezquer et Al. “New Analytical Software
for Electroluminescence Results Out of
Patterns Indentified after IEC-61215
Standard Tests”, 27th PVSEC - 2012
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Influence Biasing Level in EL Measurements
EXAMPLE: Module with yellowing problems coming from a PV installation
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
27 29 31 33 35 37 39 41
Cu
rre
nt
(A)
Voltage (V)
Evolution of EL image with biasing current variation
PV cell with a shunt
problem (RSHUNT value
significanly lower)
235W 60 (10x6) multicrystalline
156mmx156mm
04 1. Introduction
2. International Standard IEC/TS 60904-13 Ed.1.0
3. Influence Biasing Level in EL Measurements
4. Types of Defects detectable by EL technique
5. Classification Types of Cracks by EL
6. EL measurements in the PV plant
7. Conclusions
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Types of Defects detectable by EL technique
The EL technique is used to detect different types of
defects that may be present in the PV modules:
A. Presence of different types of cracks
B. PID patterns
C. Corrosion by excess of humidity
D. Metallization fingers interruptions
E. Tabbings disconnection
F. Problems with contact resistance,…..
A
B
C D E
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Types of Defects detectable by EL technique
Illustrative Examples of Different Types of Defects in EL images
A
B
C
A: Discontinuity in the fingers
B: Presence of cracks or microcracks
C: Mark due to overheat in the firing process of the cell manufacturing process
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Types of Defects detectable by EL technique
Illustrative Examples of Different Types of Defects in EL images
D E
D: Cracks that make an area of the cell electrically isolated or semi-isolated (-> large increase in
series resistance and even a possible decrease of the Isc value)
E: High series resistance zone due to a high contact resistance
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Types of Defects detectable by EL technique
Illustrative Examples of Different Types of Defects in EL images
F: Defects in the PV cells caused during their manufacture (striation rings originated during silicon ingot
growth, contamination during cell transport –vacuum chucks-….
G: Short-circuit diode or rupture of the electrical continuity of the string
G
F
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Types of Defects detectable by EL technique
Illustrative Examples of Different Types of Defects in EL images
H: 1) Problem in the manufacturing process of the cell (e.g. cell transport during firing process)
2) Problem in the soldering process of the tabbing (e.g. misalignment of the heat application points.
H
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Types of Defects detectable by EL technique
Illustrative Examples of Different Types of Defects in EL images
I: Decrease of the lifetime in the silicon wafer edges, due to
contamination during the silicon ingot growth process (slight importance)
I
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Types of Defects detectable by EL technique
Illustrative Examples of Different Types of Defects in EL images
J: Visual discolored “Trails/Lines" appearing on
the front surface of the module ("Snail Trails")
J
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Types of Defects detectable by EL technique
Illustrative Examples of Different Types of Defects in EL images
K) Failure in the electrical connection of the metallic tabs interconnecting the cells.
K
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Types of Defects detectable by EL technique
Illustrative Examples of Different Types of Defects in EL images
L) Scratch in the backsheet of the module (probably caused during transport or installation of the module).
The electrical insulation of the module could be affected.
L
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Types of Defects detectable by EL technique
Illustrative Examples of Different Types of Defects in EL images
M) Degradation pattern of PID effect (Potential Induced Degradation)
- This kind of defect is seen more clearly in the EL images at low current.
- Important shunts are generated in several cells (usually, closer to the frame) due to high
voltages between internal circuit and frame.
EL high bias (IBIAS=ISC) EL low bias (IBIAS=0.1*ISC)
M
05 1. Introduction
2. International Standard IEC/TS 60904-13 Ed.1.0
3. Influence Biasing Level in EL Measurements
4. Types of Defects detectable by EL technique
5. Classification Types of Cracks by EL
6. EL measurements in the PV plant
7. Conclusions
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Classification Types of Cracks by EL
EL high biasing (IPOL=ISC) EL low biasing (IPOL=0.1*ISC)
Type of crack: A -> Not increase local series resistance
B -> Still connected, but increasing local series resistance
C -> Disconnected, inactive area (decreases cell Isc)
Types of Cracks
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Classification Types of Cracks by EL
Experimental results of
power loss of modules (60
cells) with different
number of cells affected
by type-A cracks
Even with 2/3 of the
cells with type-A cracks,
the power loss does not
exceed 2%.
Cracks type-A
These types of cracks do not isolate or increase the series resistance of any area of the cell.
Intensity of the EL is not affected, even in the high-polarization EL image.
Least severe cracks as they hardly affect the electrical performance of the module.
In the future, they could propagate and become B or C cracks.
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Classification Types of Cracks by EL
Cracks type-B
These types of cracks do not isolate but increase the serial resistance of an area of the cell.
Affected area looks dark in EL images, and with lower intensity of the area in the low-biasing
image, but greater than the background noise.
Its influence on the power depends on the affected area, and the series resistance value
(medium-high importance).
In the future they could propagate and become cracks-C and be a more important problem.
B B
Power loss simulation results of a PV
module as a function of the "semi-
isolated" area and the series resistance
of the "fractured" area
The power loss depends greatly on the "semi-
insulated" area and the degree of electrical
insulation
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Classification Types of Cracks by EL
Cracks type-C
These types of cracks are the most critical since they completely isolate an area of the cell.
The affected area looks dark in both EL images, and the intensity of the area isolated by the
cracks is zero (equal to the background noise) in both polarizations.
Its influence on the power loss of the module depends significantly on the percentage of the
affected area.
If the percentage of isolated cell area is less than 7%-8% -> there is no power loss.
Power loss simulation results of a PV
module as a function of the percentage
of cell area that is electrically "isolated”
C C
∆PMAX ≈ 0, if
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Classification Types of Cracks by EL
Cracks type-C
Power loss of the module depends on the
percentage of isolated area of the cell with the
largest inactive area
A single cell with isolated area can cause the
complete disconnection of one of the strings
C C
PV module
simulation
with a C-type
cracked cell
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Classification Types of Cracks by EL
CENER’s program for Crack Analysis
C C
EL high biasing (IBIAS=ISC)
EL low biasing (IBIAS=0.1*ISC)
CENER has developed its own image analysis program for EL images.
The program allows, among other things, to detect and classify the presence of cracks in
different cells, as well as to quantify the percentage of isolated cell area.
06 1. Introduction
2. International Standard IEC/TS 60904-13 Ed.1.0
3. Influence Biasing Level in EL Measurements
4. Types of Defects detectable by EL technique
5. Classification Types of Cracks by EL
6. EL measurements in the PV plant
7. Conclusions
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EL measurements in the PV plant
Motivation:
Market increasingly demands the possibility to perform the
characterization of PV modules by means EL within the PV
plant.
Advantages:
No need to disassemble the modules
Transport of the modules is avoided
Reduces the risk of possible damage to the module during the entire process (disassembly, transport, unpacking, measurement, reassembly...)
Electrical biasing of the complete string
No need to stop the power production of the installation
Conclusion:
Carrying out on-site EL measurements means important
reduction in the economic cost, as well as in the risk of
damage during the whole process.
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EL measurements in the PV plant
Technological challenge: Solar irradiance in the EL emission
range of crystalline silicon modules is much higher than the
intensity of the emitted EL
Solutions:
InGaAs cameras
Analog filters
Digital filters
Performing the measurements during the night
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EL measurements in the PV plant
CENER has developed a new characterization system named CELSOS:
CENER’S ELECTROLUMINESCENCE SYSTEM ON-SITE
InGaAs InGaAs ó SiInGaAs ó Si
InGaAs ó Si
CELSOS allows collection of EL measurements to PV
modules in the field, without disassembling them
from their location, without having to disconnect
them, and during any day-time.
Integration of high-performance measurement
equipment (camera, power supply, filters,...)
Controlling program implemented in CENER
Own software for treatment and advanced
analysis of EL images obtained
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EL measurements in the PV plant
Example of real application of CELSOS:
Detection of modules with new cracks after being installed in the trackers
(Direct comparison with EL image captured after reception or in factory)
EL after reception EL after installation EL after
reception:
Zoom
EL after
installation:
Zoom
EL image captured on-site
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EL measurements in the PV plant
Example of real application of CELSOS:
Detection of modules with scratches in the backsheet after being installed in the trackers
(Direct comparison with EL image captured after reception or in factory)
EL after reception EL after installation
EL after
reception:
Zoom
EL after
installation:
Zoom
EL image captured on-site
07 1. Introduction
2. International Standard IEC/TS 60904-13 Ed.1.0
3. Influence Biasing Level in EL Measurements
4. Types of Defects detectable by EL technique
5. Classification Types of Cracks by EL
6. EL measurements in the PV plant
7. Conclusions
www.cener.com
Conclusions
Electroluminescence (EL) measurements have emerged as an essential characterization
tool to guarantee the quality of PV modules of different technologies.
Broad fields of application of EL technique along all the entire value chain, from
manufacturing to the final installation of the modules in the plant.
New standard IEC/TS 60904-13 Ed.1.0 has been published to standardize the methodology
for performing electroluminescence (EL) measurements (laboratory).
EL technique is used to detect almost all the types of defects that may be present in the
PV modules.
Using different biasing levels during the EL measurements maximizes the information that
can be extracted.
Market increasingly demands the possibility to perform the characterization of PV modules
by means EL within the PV plant.
CENER has developed a new characterization system (CELSOS) and a special analytical
software to obtain good quality EL images in the field, 24 hours per day.
[email protected] www.cener.com
[email protected] [email protected]
Thank you very much for your attention!!
