Encapsulant materials and degradation effects ... INTERNATIONAL ENERGY AGENCY PHOTOVOLTAIC POWER...
Transcript of Encapsulant materials and degradation effects ... INTERNATIONAL ENERGY AGENCY PHOTOVOLTAIC POWER...
IEA INTERNATIONAL ENERGY AGENCY
PHOTOVOLTAIC POWER SYSTEMS PROGRAMME
Encapsulant materials and degradation effects -
Requirements for encapsulants, new materials, research trends
IEA Task 13 Open Workshop Freiburg, 02.04.2014
Dr. Gernot Oreski Polymer Competence Center Leoben GmbH
Roseggerstraße 12 8700 Leoben, Austria +43 3842 42962 51
IEA INTERNATIONAL ENERGY AGENCY
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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
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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)
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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,
discontinouos 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
CH2CH2 * CH2 CH *
O
C CH3O
m
n CH2CH2 *n
CH2CH2 * CH CH *m
n CH2CH2 *n
CH2CH2 * CH2 C *m
n
OCH2CH2 *n
CH2CH2 * CH2 CH *
O m
n CH2CH2 *n
CH4, CO2, CO
O C
CH3
Norrish I
Norrish II (Deacetylation)
Norrish III
CH3C OHO
+m
CH3CO
H+m
CH3CO
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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150 200
temperature [°C]
Film unaged Module unaged Film 2000h DH Module 2000hDH
0,2 W/g
exo
Material interactions – Degradation of Backsheets Crystallisation curves of
unaged and aged backsheet containing PET middle layer • Indicator for hydrolysis No significant changes between backsheets aged as films and aged in PV module • No influence of material
interactions on hydrolysis of PET
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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 electro-chemical 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 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 T [%]
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
tra
nsm
ittan
ce [-
]
wavelength [nm]
TPSE Polyolefin 1 Ionomer EVA
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New approaches for replacement of EVA
• ISOVOLTAIC: Thermoplastic ethylene copolymer – No cross-linking – Hydrolysis free, no formation of acetic acid – Different formulations for use in front of and behind solar
cells
• 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
– Reduced storage stability
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PID resistant materials • Influence of solar cell
encapsulant of PID (1)
– EVA showed highest sensitivity to PID
– Polyolefines and TPSE show lowest sensitivity to PID
• STR: POE encapsulant (i.e. polyolefin elastomer)
– Chemical formulation and additives affect PID sensitivity of EVA
(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 OO
O
O
NH
H
OOH
O R1
OOH
O R1
OHO R1
O
P O C9H19 3
300 400 500 600 700 8000.00
0.25
0.50
0.75
1.00
without UVA 2500ppm UVA
trans
mitt
ance
[-]
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
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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!