Encapsulant materials and degradation effects ... INTERNATIONAL ENERGY AGENCY PHOTOVOLTAIC POWER SYSTEMS PROGRAMME Encapsulant materials and degradation effects - Requirements for

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

[email protected]



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



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)



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



PV module degradation modes

Corrosion of metallic interconnectors

© Dr. Wolfgang Schöppel, 3M

© Dr. Wolfgang Schöppel, 3M Snail trails Yellowing

Delamination



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



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



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



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



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



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



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



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



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



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



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



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



Better utilization of solar spectrum

Aarts, L. (1982, PhD thesis University of Utrecht



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



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




(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)




©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?



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



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!

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Encapsulant materials and degradation effects ... INTERNATIONAL ENERGY AGENCY PHOTOVOLTAIC POWER SYSTEMS PROGRAMME Encapsulant materials and degradation effects - Requirements for