Influence of Backsheet on PV Module Reliability - iea …iea-pvps.org/fileadmin/dam/public/report/technical/04... · · 2015-06-15IEA INTERNATIONAL ENERGY AGENCY PHOTOVOLTAIC POWER

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

[email protected]



Content

Requirements for PV module backsheets

State of the art

Backsheet selection criteria

Role of backsheets in PV module

degradation

Reliability of backsheets



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



Selection of backsheet type?

Cost?

Good customer relations?

Experience?

Property profile?

Reliability?

Expected stress factors?

©Taiflex Scientific co., Ltd.



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



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



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



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



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



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



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



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]



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 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



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”



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



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



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]



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



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!

Influence of Backsheet on PV Module Reliability - iea …iea-pvps.org/fileadmin/dam/public/report/technical/04... · · 2015-06-15IEA INTERNATIONAL ENERGY AGENCY PHOTOVOLTAIC POWER

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