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

[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,

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



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

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



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



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 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



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 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



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



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



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



Better utilization of solar spectrum

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




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




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





(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!

Encapsulant materials and degradation effects ... · Encapsulant materials and degradation effects...

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