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IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
Influence of Backsheet on PV
Module Reliability
Side-Event IEA: PV Module Reliability and System
Performance Analysis
München, 09.06.2015
Dr. Gernot Oreski Polymer Competence Center Leoben GmbH
Roseggerstraße 12
8700 Leoben, Austria
+43 3842 42962 51
oreski@pccl.at
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
Content
Requirements for PV module backsheets
State of the art
Backsheet selection criteria
Role of backsheets in PV module
degradation
Reliability of backsheets
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
Photovoltaic modules
State of the art
Multi-layer laminates to address all requirements
Core layer (PET, PA) mechanical stability, electrical isolation, barrier against
humidity and oxygen
Outer layer (PET, PA, fluoropolymers) Weathering protection
Inner layer (PET, PA, fluoropolymer, EVA) UV protection of core layer,
adhesion to encapsulant
Backsheet: Requirements
Electrical isolation
Protection against weathering
Barrier (humidity, oxygen)
Mechanical stability
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
Selection of backsheet type?
Cost?
Good customer relations?
Experience?
Property profile?
Reliability?
Expected stress factors?
©Taiflex Scientific co., Ltd.
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
PV module degradation – material
interactions
Backsheet Solar cells with encapsulant Glass
Light
O2, H2O, atmospheric
gases, pollutants
Additives, degradation
products, solvents
Metal ions
Electrical current flow
Interactions lead to unintended degradation effects: Yellowing,
corrosion, potential induced degradation, snail trails
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
PV module degradation modes
Corrosion © 3M Snail trails Yellowing
Delamination
Influenced or driven
by permeation
processes
PID[1]
© 3M
[1] Stollwerck, G. et al., „Polyolefin backsheet and new encapsulate suppress
cell degradation in the module“, PVSEC 2013, Paris
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
Oxygen (OTR) and water vapor
transmission rates (WVTR)
OTR and WVTR values depend on
Measurement method
Film thickness
Measurement conditions (temperature, atmosphere,
humidity level)
0
10
20
30
40
50
60
70
80
90
100
58.2
OT
R [
cm
3*m
-2*d
ay
-1*b
ar-1
]
3.9 2.5 3.87.4 6.5
15.7
57.6
B1340 m
0,00
0,01
0,02
0,03
0,04
0,05
Oxtran
0 h
2000 h DH
1000 h CI
Opto-chemical method
0 h
2000 h DH
Mass spectrometer
0 h
1000 h DH
1000 h QUV
23°C/90 % RH
Opto-chemical sensor
0 h
2000 h DH
RT/0 % RH
0.005
0.002
OT
R [
cm
³/m
²day b
ar]
Mass spectrometer
0 h
2000 h DH
1000 h UV
23°C/50 % RH
<0.1 (detection limit reached)
Oxtran
0 h
2000 h DH
1000 h Climate
B3390 m
0,00
0,02
0,04
0,25
0,50
0,75
1,00
1,25
0.04
0.022WVTR ISE
0 h
1000 h DH
1000 h QUV0.002
23°C/90 % RH
23°C/85 % RH
WVTR Mocon
0 h
2000 h DH
1000 h CI
0.6
0.8
WV
TR
[g
*m- ²*
day
-1]
0.64
B1340 m
0,00
0,25
0,50
0,75
1,00
1,25
Permatran
0 h
2000 h DH
1000 h Climate
Mass spectrometer
0 h
2000 h DH
1000 h UV
23°C/90 % RH
23°C/85 % RH
<0.1
WV
TR
[g/m
²day]
B3390 m
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
OTR and WVTR
Influencing factors
Strong temperature dependence
material ranking @ RT ≠ material ranking @ operating temperature
OTR is strongly influenced by relative humidity levels
0,0028 0,0030 0,0032
10-1
100
101
WV
TR
[g
*m-2*d
ay
-1]
inverse temperature [1/K]
TPT
KPK
PPE
AAA
0,0028 0,0030 0,003210
1
102
103
104
AAA 90% rH
AAA 23% rH
OT
R [
cm
3*m
-2*d
ay
-1*b
ar-1
]
inverse temperature [1/K]
TPT
KPK
PPE
85°C 65°C 38°C 85°C 65°C 38°C
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
Formation of Acetic acid
Influence on reliability of PV modules
Oxidation of EVA autocatalytic effect
Yellowing
Low molecular degradation products
Corrosion of PV ribbons
Enhances potential induced degradation by increased ion
mobility [1]
O
CO CH
3
*CH
2
CH2
n
m
CH
2CH
* OHC
O
CH3
*CH
2
CH2
n
m
CH
CH* +
Schematic reaction mechanism
of de-acetylation of EVA
© AE Solar Energy, http://solarenergy.advanced-energy.com
[1] Pingel et al., ” Potential Induced Degradation of solar cells and panels”, Photovoltaic Specialists Conference
(PVSC), 2010 35th IEEE
UV, T, RH
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
Barrier properties: Acetic acid
0,0028 0,0030 0,0032 0,003410
0
101
102
103
104
AAA
KPK
PO
EVA
ace
tic a
cid
tra
nsm
issio
n r
ate
[g
/m²
d]
1/T [1/K]
85°C 65°C 25°C
composition Thickness
[µm]
Activation
energy
[kJ*mol-1]
AAA Co-extruded
polyamide
350 21.2
KPK PVDF-PET-
PVDF
325 16.1
PO Co-extruded
polyethylene
485 15.4
EVA Ethylene Vinyl
Acetate
430 22.4
Highest acetic acid permeation rates were found for EVA, lowest for
PET containing backsheets
Acetic acid permeation rates are higher than WVTR
“Breathable” backsheet supports diffusing out of acetic acid
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
PID prevention and/or retardation
[1] Stollwerck, G. et al., „Polyolefin backsheet and new encapsulate suppress
cell degradation in the module“, PVSEC 2013, Paris
Design matching of backsheet
and encapsulant [1]
Backsheet
Low WVTR
High acetic acid permeation
Encapsulant
Low WVTR
Reduced volume resistivity
Enhanced additives
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
Snail trails [1],[2]
Discoloration of the silver paste of the front
metallization of solar cells
Occurs at the edge of the solar cell and along
usually invisible small cell cracks
Discoloration itself is reported to have no
influence on the performance of the PV module
Cell cracks can reduce the PV module
power
Choice of EVA and backsheet type seems to
be important for the Snail Track occurrence
[1] M. Köntges, I. Kunze, V. Naumann, S. Richter, C. Hagendorf, Schneckenspuren, Snail Tracks, Worm Marks und
Mikrorisse, in: 8. Workshop "Photovoltaik-Modultechnik" TÜV Rheinland, Cologne, Germany, 2011.
[2] S. Meyer, S. Timmel, U. Braun, C. Hagendorf, Polymer Foil Additives Trigger the Formation of Snail Trails in Photovoltaic
Modules, Energy Procedia 55 (2014) 494–497.
Proposed mechanism : Water vapor coming through the backsheet
is claimed to dissolve silver particles which migrate into the
encapsulation on top of the grid finger, where a chemical reaction
within the encapsulation foil results in the typical observed coloring [2]
Module with Snail Tracks [1]
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
Selection of backsheet type!!!
Definition of requested property profile considering
Design Matching to provide optimum module efficiency and avoid
unintended material interactions
Permeation properties (H2O, O2, acetic acid)
Thermo-mechanical properties
Optical properties
Electrical isolation properties
Chemical formulation
Expected stress factors
Climate (moderate, tropical, desert…)
Micro-climate (installation: Building integrated vs. PV power plants in the field)
Experience and good customer relations
Cost Total cost of life
Reliability?
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
IEA Task 13 Report: Performance and
Reliability of Photovoltaic Systems Subtask 3.2: Review of Failure of Photovoltaic
Modules
Download @ http://www.iea-pvps.org/
Delamination and/or cracking of backsheet
Pathways for enhanced ingress of gases and
liquids
Acceleration of other PV module
degradation modes
May cause safety issues due to reduced
electrical insulation
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
Mechanisms of crack formation
[1] W. Gambogi, Y. Heta, K. Hashimoto, J. Kopchick, T. Felder, S. MacMaster, A. Bradley, B. Hamzavytehrany, L. Garreau-Iles, T. Aoki, K. Stika, T. J. Trout, and T. Sample „A
Comparison of Key PV Backsheet and Module Performance from Fielded Module Exposures and Accelerated Tests “, IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 4, NO. 3, 2014
Cracking of PET based backsheets [1]
Cracking of a PVDF layer of
backsheet [1]
Chemical aging processes
Thermo-oxidation
Photo-oxidation
Hydrolysis
Physical aging processes
Post and re-crystallization
Relaxation of orientations
Migration of plasticizers
Swelling
Crack formation due to embrittlement
Significant reduction of maximum strain values
“Changes in
chemical structure
and molar mass
distribution”
“Changes in
polymer
morphology”
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
1) Hydrolysis of PET based backsheets [1],[2]
[1] K. Looney, B. Brennan, „Modelling the correlation between DHT and true field
lifetimes for PET based backsheet“, 5.D0.10.5, EU PVSEC 2014, Amsterdam.
[2] G. Oreski, G. Wallner, „Aging mechanisms of polymeric films for PV encapsulation”,
Solar Energy 79 (2005) 612–617
0 20 40 60 80 100 1200
25
50
75
100
125
str
ess [
MP
a]
strain [%]
85°C 85H
unaged
1000h
2000h
PET-SiOx
0
25
50
75
100
125
PET
Reduction of molar mass
Strong embrittlement
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
2) Post-crystallization of PVDF
Polymorphism: PVDF has 4 crystal
modifications
α-PVDF: trans-gauche TG+TG-
β-PVDF: all-trans TTTT
γ-PVDF: G+TTT oder G-TTT
δ-PVDF: TG+TG+ oder TG-TG-
Preferred conformation:
α-PVDF kinetic favored
β-PVDF thermo-dynamic favored
Production:
α-PVDF: crystallization from melt
β-PVDF: stretch forming of α-PVDF
[1] G. Oreski, G. Wallner, „Aging mechanisms of polymeric films for PV encapsulation”,
Solar Energy 79 (2005) 612–617
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
50 100 150 200 250
temperature [°C]
he
at
flow
[m
W]
unaged
PVDF
2000h
2mW TG
post-crystallisation
Accelerated aging at 85°C (damp heat
conditions) is above glass transition of
PVDF
Enhanced chain mobility lead to strong
increase in degree of crystallinity from 13 to
30% [1]
Strong embrittlement
Significant reduction of
maximum strain values
Formation of cracks due to expansion during
thermal cycling [2]
1500 1250 1000 7500.0
0.5
1.0
1.5
85°C / 85% RH
ab
so
rba
nce
[-]
wave number [cm-1]
unaged
2000h
PVDF
2) Post-crystallization of PVDF [1], [2]
[1] G. Oreski, G.M. Wallner, Aging mechanisms of polymeric films for PV encapsulation, Solar Energy 79
(2005) 612–617.
[2] W. Gambogi, et al. „Comparison of Key PV Backsheet and Module Performance from Fielded Module
Exposures and Accelerated Tests “, IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 4, NO. 3, 2014
Cracking of a PVDF layer of
backsheet [2]
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
Role of backsheet in PV module reliability
Main role of backsheet laminates are
Electrical isolation and
Barrier against atmospheric gases (O2, H2O)
and
Weathering protection of inner layers
Delamination and cracking of backsheet
laminate can lead to
Faster ingress of moisture and oxygen
Enhanced physical and chemical material
degradation processes (embrittlement, micro-
cracks, discoloration, corrosion…)
Further delamination within the PV module
Decrease of electrical power output
Safety issues
Cracking of backsheet
Delamination
Long term stability of backsheets is one main factor for reliability and
durability of PV modules
Design matching with other PV module components under consideration
of expected stress factors increase performance and reliability
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
Thanks to my colleagues Astrid Rauschenbach, Bettina Hirschmann, Marlene
Knausz, Antonia Mihaljevic (PCCL) and Prof. Gerald Pinter (University of Leoben) for
the support within this project.
This research work was performed at the Polymer Competence Center Leoben (PCCL)
within the projects “PV Polymer” (FFG Nr. 825379, 3. Call “Neue Energien 2020”, Klima-
und Energiefonds) and “Analysis of PV Aging” (FFG Nr. 829918, 4. Call “Neue Energien
2020”, Klima- und Energiefonds) in cooperation with the Chair of Materials Science and
Testing of Plastics at the University of Leoben. The PCCL is funded by the Austrian
Government and the State Governments of Styria and Upper Austria.
Thank you for your attention!
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