Encapsulant materials and degradation effects ... · Encapsulant materials and degradation effects...
Transcript of Encapsulant materials and degradation effects ... · Encapsulant materials and degradation effects...
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
Encapsulant materials and
degradation effects -
Requirements for new materials
and research trends
IEA PVPS Task 13 Workshop at 29th EU PVSEC
Amsterdam, 23.09.2014
Dr. Gernot Oreski Polymer Competence Center Leoben GmbH
Roseggerstraße 12
8700 Leoben, Austria
+43 3842 42962 51
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
Content
• Introduction
– Requirements for solar cell encapsulants
– State of the art
• PV module degradation
• Trends in R&D
– Polyolefins as replacements for EVA
• Thermo-plastic materials
• Prevention of electro-chemical degradation processes
– Advanced and functional additives
IEA INTERNATIONAL ENERGY AGENCY
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Solar cell encapsulant
Challenges
• Different materials used in PV
modules (glass, polymers, semi-
conductor, metal)
• Different thermal expansion
coefficients of materials used in
PV modules
• Danger of overstressing and
cracking of components
• Different refractive indices of
materials used in PV modules
Requirements
• Mechanical protection
• Structural support and physical
isolation of the cell
• High damping capacity
• Optical coupling
• Refractive index between glass
and AR Coating of solar cell
• Transparency in solar region of
wavelength (300-2500nm)
Low modulus,
Elastomeric material
Ethylene vinyl acetate
(EVA)
IEA INTERNATIONAL ENERGY AGENCY
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Solar cell encapsulant – State of the art
Pro
• High flexibility
• High transparency
• Low cost material
• Good processability
• Good weathering resistance
• > 30 years of experience
Con
• Chemical cross-linking
• Time and energy consuming,
discontinuous module lamination
process
• High amount of stabilizer
• Formation of acetic acid during
oxidation
Chemically cross-
linking ethylene vinyl
acetate (EVA)
Peroxide induced cross-linking
during PV module lamination
Thermo-mechanical stability
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PV module degradation modes
Corrosion of metallic
interconnectors © Dr. Wolfgang Schöppel, 3M
© Dr. Wolfgang Schöppel, 3M
Snail trails Yellowing
Delamination
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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
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Material interactions - photo-oxidation of EVA
CH2CH
2 * CH2
CH *
O
C CH3
O
m
n
CH2CH
2 *n
CH2CH
2 * CH CH *m
n
CH2CH
2 *n
CH2CH
2 * CH2
C *m
n
O
CH2CH
2 *n
CH2CH
2 * CH2
CH *
O
m
n
CH2CH
2 *n
CH4, CO
2, CO
O C
CH3
Norrish I
Norrish II (Deacetylation)
Norrish III
CH3C OH
O
+m
CH3C
O
H+m
CH3C
O
H R +
RH
UV, T
Polyenes
Acetic acid
Ketone
Aldehyde
Czanderna, A.W., Pern, F.J. (1996). Sol. Energ. Mat. Sol. C. 43, 101.
Corrosive
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Material interactions - potential induced
degradation (PID) • Powerloss in Si-PV modules due to leakage current
– Metal ions from glass (Na, Mg, Ca, Al) are main reason for
PID
– Ion mobility increases with temperature and humidity level
– Influence of solar cell encapsulants:
• Solution and transpirt of metal ions due to humidity or acetic acid
• The lower the WVTR, the lower the susceptibility to PID
• Modules using EVA or PVB are very sensitive to PID Potential
© AE Solar Energy, http://solarenergy.advanced-energy.com Pingel et al., ” Potential Induced Degradation of solar cells and panels”, Photovoltaic Specialists Conference
(PVSC), 2010 35th IEEE
Need for
advanced
encapsulants
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Large volume production
• Standard c-Si PV modules
Need for advanced encapsulants
Cost Technology
Driving forces?
Small series production
• BIPV
• Space applications
• Special applications (e.g.
automotive)
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Development of new materials
• Technical Challenges
– Thermo-plastic material – no chemical cross-linking
– Thermo-mechanical stability – no creep
– Prevention or reduction of chemical and physical degradation
processes
– Spectral selectivity for enhanced light yield
– Good adhesion to glass, solar cells and backsheet films
– High weathering stability for lifetimes > 25 years
– New accelerated tests and characterization tools for fast and
reliable assessment of new materials
Cost driven development
So far, only direct material costs are counting
Total cost of life has to be considered
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Thermoplastic material
Chemically cross-linked
elastomer
Irreversible covalent bondings (Thermo)-reversible bondings
(Ion and hydrogen bonds, crystallites)
EVA, liquid silicon Ionomers, thermoplastic silicon
elastomers, polyolefines, TPUs
Physically cross-linked
thermoplastic elastomer
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Advantages of thermoplastic materials
• No cross-linking
– Shorter processing times
• No cross-linking agents (peroxides)
– Longer storage times without extra cooling
– Less material interactions
• Roll-to-roll process possible
Steiner, A., Krumlacher, W., „New components for high quality PV modules“, Isovoltaic Technical
Conference, 16.10.2013, Shanghai
Polyolefin Fast cure EVA
Temperature 155°C 150°C
Evacuation 5 min 7 min
Pressing 3 min 1.5 min
Hold / Cross-linking < 2 min 12 min
Processing time < 10 min 20.5 min
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Existing thermoplastic materials
• Low market penetration due to
– Lower transparency and higher material stiffness compared
to EVA
– Higher cost
Material Solar
transmittance
EVA 0.881
TPSE 0.881
Ionomer 0.866
Polyolefin 1 0.827
Polyolefin 2 0.855
300 400 500 600 700 8000,00
0,25
0,50
0,75
1,00
tr
an
sm
itta
nce
[-]
wavelength [nm]
TPSE
Polyolefin 1
Ionomer
EVA
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New approaches for replacement of EVA
• Thermoplastic Polyolefines
– ISOVOLTAIC: Thermoplastic ethylene copolymer
• No cross-linking
• Hydrolysis free, no formation of acetic acid
• Different formulations for use in front and behind solar cells
• Cross-linked Polyolefines
– STR: POE encapsulant (i.e. polyolefin elastomer)
• Replacement of vinyl acetate group
No formation of acetic acid
• Cross-linking necessary
– Solvay: „Polidiemme cross-linked polyethylene“
• Silane cross-linked polyethylene
Cross-linking due to humidity
• Hydrolysis free, no formation of acetic acid
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New approaches for replacement of EVA:
Silane cross-linked PE
• Well established
technology in cable
industry, also used for
backsheets
• Cross-linking at room
temperature and
humidity
– Cross-linking at
storage or transport
of PV modules after
lamination
– Reduced storage
stability of films
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Benefit of Polyolefines: PID resistance
• Influence of solar cell
encapsulant of PID (1)
– EVA showed highest
sensitivity to PID
– Polyolefines and TPSE show
lowest sensitivity to PID
• Chemical formulation and
additives affect PID
sensitivity of EVA(2)
(1) Koch, S. et al., „Encapsulation influence on the potential induced
degradation of crystalline silicon cells with selective emitter
structures“, PVSEC 2012, Frankfurt (2) Stollwerck, G. et al., „Polyolefin backsheet and
new encapsulate suppress cell degradation in the
module“, PVSEC 2013, Paris
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Better utilization of solar spectrum
Aarts, L. (1982, PhD thesis University of Utrecht
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Better utilization of solar spectrum
Hindered amine light stabilizer
UV - Absorber
Anti-oxidant
NH
H O
OO
O
NH
H
OOH
O R1
OOH
O R1
OHO R1
O
P O C9H
19 3
300 400 500 600 700 8000.00
0.25
0.50
0.75
1.00
without UVA
2500ppm UVA
tra
nsm
itta
nce
[-]
wavelength [nm]
Transmission:
0.924 vs. 0.893
Optical properties in the UV
region influenced by stabilizers
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Better utilization of solar spectrum
New developments – higher transparency in the UV region
©Trosifol PVB film, Kuraray Europe, www.kuraray.eu
©Dow Corning
Ketola et al., “Silicones for Photovoltaic Encapsulation”,
presented at the 23rd European Photovoltaic Solar Energy
Conference, Valencia 2008
PVB Silicone
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Better utilization of solar spectrum
New developments – higher transparency in the UV region
(1) Steiner, A., Krumlacher, W., „New components for high quality PV modules“, Isovoltaic
Technical Conference, 16.10.2013, Shanghai
Icosolar Front Encapsulant, © Isovoltaic AG, www.isovoltaic.com
• Increase in
power output
by 0.5% (1)
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Better utilization of solar spectrum
New developments – higher transparency in the UV region
©STR; I. Fidalgo, “New encapsulant options for MWT PV
Cells”, presented at the 5th workshop on MWT solar cell
and module technology, 20.-21.11.2013, Freiburg (D)
• STR Photocap HLT Serie
– UV Cut Off at 305nm
• 3M Solar Encapsulant EVA
Film 9000 and 9100:
– UV Cut Off at 360nm
• Long term stability has to be
confirmed
• Accelerated aging effects due
to missing or lacking
stabilization?
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Summary: Approaches for material
development
• Modified polyethylene copolymers
– Thermoplastic
– No formation of acetic acid
• Enhanced stabilizers / additives
• Material based strategies to prevent or reduce electro-
chemical degradation processes within PV modules
• Optimized optical coupling up to 5% higher module
efficiencies
• Consideration of total cost of life
Better understanding of PV module and material
degradation processes is a precondition for a
successful development of new solar cell
encapsulants
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Thanks to my colleagues Astrid Rauschenbach, Bettina Hirschmann, Marlene Knausz
(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 project “PV Polymer” (FFG Nr. 825379, 3. 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!