Adhesives in Photovoltaics - European Coatings · Adhesives Used in Photovoltaics • Encapsulants...

Adhesives in Photovoltaics

Dr. Hartmut Henneken Berlin, 23.11.2011

Scale-up Issues towards a 20 MW (250 m Rotor Blade Dimension) Wind Turbine…

www.nolax.com |Adhesives in Photovoltaics | Berlin 23.11.2011

Photo from www.rechargenews.com

To compare dimensions: 20MW PV plant near Cagliari

Picture taken at ECN, Petten, NL


Independent specialist for adhesive bonding technologies

headquarted in Switzerland (Sempach-Station).

Worldwide, the Group employs 328 persons at several

locations in Switzerland, Germany, France, and the

USA.

nolax belongs to the Collano Group

History and Key Figures

1947 Dr. Marcel Ebnöther establishes the adhesive company

Ebnöther AG in Olten/Switzerland

1997 50th anniversary, name change from Ebnöther AG to

Collano Ebnöther AG (and 2001 to Collano AG)

2008 Incorporation of the group companies:

nolax AG, Collano Adhesives AG, Collano Services AG



nolax Core Competencies R&D

Services

Adhesive evaluation

System development

Adhesive development

Technologies

Reactive systems

Hotmelts

Adhesive films

Water-based systems

The Collano group and especially nolax is involved in several commercial and research projects in the field of photovoltaics.

Adhesives Used in Photovoltaics

• Encapsulants Protection of solar cells from environment and

adhesive binding of all module components

• Framing adhesives Edge sealing of PV modules; connecting the module

with the frame

• Junction box adhesives Adhere the junction box to the module

• Back sheets Connecting different layers to each other

• Other (e.g. conductive adhesives, adhesives to connect

wiring, labels,…)


In this presentation, the encapsulants will be covered in detail.

Encapsulants…

• …are adhesives, that are playing an

important role in the PV module setup.

• … are connecting all interfaces of the

PV module to each other and have to

maintain good optical, mechanical and

electrical performance over decades of

operation.

• … are a key player in the lamination

process, which is besides the solar cell

production, significantly influencing

the final cost of the module.


J-Box

Frontsheet

Encapsulant

Backsheet

PV-film

Encapsulant


Known Encapsulant Types

• Ethylene vinyl acetate copolymer (EVA)

• Siloxanes (PDMS)

• Plasticized polyvinyl butyral (PVB)

• Polyethylene ionomers

• Thermoplastic Polyolefins (TPO)

• Thermoplastic polyurethanes (TPU)

• UV curable high vinyl acetate containing EVA (EVM)

… other thermoplastic blockcopolymers

Encapsulant Structures


R

n m

n m

O ONa, Zn

RN O

O

H

n

n m

OO

O O OH

O

n m o

O

SiO

CH3

CH3

n

SiO

H3C

H3C

n

RN O

O

H

Si

CH3

H3C

m

EVA

TPO

Ionomers

PDMS TPU

PVB

… or combinations, e.g. PDMS and TPU blockcopolymers

Production of Thermoplastic Encapsulant


Cast extrusion of encapsulants at nolax (Collano Services GmbH)in Buxtehude/ Germany


EVA Encapsulant

• The dominant encapsulant for c-Si PV today

• More than 25 years in the field, lots of reliability

data available

• Copolymer of ethylene and vinyl acetate (VA) with a

VA content of about 33 % by weight

• High VA content guarantees a greater than 90% light

transmission to the solar cells

• Must be cross-linked to seal the cells and prevent

flow/creep

• Issues with EVA degradation: generate acetic acid,

yellowing in the dark, residual peroxides decompose

into volatiles

PhenolicPhosphonite (0.1% to 0.2%)

Peroxide Decomposer/ Radical Scavenger

Encapsulant Example Formulation In general PV encapsulants are highly formulated

variations of the base resin. An example EVA

formulation for PV was published as followed (EVA

base resin, 96% to 98%):


Photovoltaics International (9th Edition; August 2010; page 172)

Peroxide(1% to 2%) Crosslinker Benzoltriazole

(0.2% to 0.35%) UV Absorber

Hindered Amine Light Stabilizer (HALS) (0.1% to 0.2%)

Decomposes Peroxide Radicals

Trialkoxy Silane (0.2% to 1%)

Adhesion Promoter

Thermoset against Thermoplastic

Thermoplastic encapsulants:

• Potential to decrease cycle times

• Roll-to-Roll lamination possible

• Better reprocessing (e.g. for repairs)

• Easier to recycle

• Require higher process temperatures


Thermoset encapsulants:

• Additional process time for curing

• No danger of creeping

• Lower process viscosity possible

• Shelf life restrictions

Thermoset against Thermoplastic

• Generally, most resins could be prepared as thermoset

or thermoplastic formulations.

• Thermoplastic materials are better for R2R processes,

as junction boxes and connectors could be assembled

and reprocessed offline.

• Though thermoplastics do not need curing time, they

still need to melt, which can take some time,

depending on laminator setup and layer thicknesses.

• UV-curable resins are also available, they behave as

a thermoplastic during lamination and could be cured

in a second step.



Requirements for Encapsulation

• Weatherability (UV, heat, moisture resistant)

• High transparency (over module life time)

• UV-stability, low yellowing

• Good adhesion (with all components)

• Barrier properties (water vapour, oxygen)

• Electric insulation

• Flexibility, no brittleness

• No creeping

• Cost efficiency, availability

• Low shrinkage

• Fit process

• Recyclability

• Flame retardant properties (e.g. for BIPV)

…

Water Ingress due to Diffusivity Rates into Different Encapsulants


M.D.Kempe, NREL, «Review of Polymer Use in Photovoltaics», Plastics in PV Conference, Philadelphia, USA, 20/09/2011

WVTR of some used frontsheet materials are relative high, NO barrier effect:

• ETFE 100 µm = 4 g m-2 d-1

• EVA 500 µm = 20 g m-2 d-1

Short term HF cycling (20 times IEC requirements ) shows that a-Si technology is very resistant to heat and moisture.


Heat and Moisture Resistance

IEC Norm 10 cycles Humidity freeze

-40°C to 85°C, 85% Rh

Courtesy: Flexcell

Heat and Moisture Resistance


Very long term HF cycling (> 100 x times IEC requirements !) shows a gradual power degradation due to mainly 3 phenomena:

Humidity freeze -40°C to 85°C, 85% Rh

Al corrosion from CH3COOH formation: it starts early but little affects the performance

Hydrolysis of PEN and cracking: only at very long exposure time (better than PET)

ITO discoloring and degradation

Courtesy: Flexcell

Transmission

to cells through 3.18 mm glass and 0.45 mm encapsulant

PDMS #1 94.8 ± 0.3%

PDMS #2 94.8 ± 0.3%

PVB 94.3 ± 0.4%

EVA 94.3 ± 0.4%

TPU 93.8 ± 0.3%

Ionomer#1 92.7 ± 0.4%

PDMS #3 92.1 ± 0.3%

Ionomer#2 88.7 ± 0.4%


M.D.Kempe, PVSC 37, Seattle, Washington, June 19-24, 2011

UV Stability


M.D.Kempe, NREL, «Review of Polymer Use in Photovoltaics», Plastics in PV Conference, Philadelphia, USA, 20/09/2011

Shrinkage


This example shows an ETFE/encapsulant/ETFE setup after vacuum lamination, but even these materials could be used to laminate wrinkle less modules by using appropriate laminator settings, even without applying vacuum.

Not only encapsulants, but also polymeric frontsheets, as well as backsheets can show significant shrinking.

No best Encapsulant for all Type of Modules

• Until 1995 PV modules had a more or less similar

setup

• Nowadays different types of solar cells are used or

developed, such as e.g. CIGS, CdTe, DSC, organic

solar cells,…

• There are many new back sheets available

• As front sheets not only glass but also polymers such

as ETFE, PVDF and PMMA are used

the interfaces that need to be connected/sealed have significantly increased


Example: Back Sheet Types and Suppliers


2010: 17 suppliers offered 100 different back sheets; 2011: 22 suppliers offered already 112 types…

Source: K.Brust, Krempel GmbH, 5th SNEC, Shanghai, 2/2011; based on market survey Photon International 9/1010

Development of Solar Cell Technologies


Source: Wikipedia «solar cells», scheme provided by NREL

Encapsulants Should be Selected According to the Specific Module Setup • CIGS cells are much more sensitive to moisture than c-Si

• C-Si wafers are much more sensitive to mechanical stress than

flexible amorphous Si cells

• Not every encapsulant is suitable for every front- or

backsheet (in terms of adhesion or process requirements)

• For R2R lamination thermoplastic encapsulants are better

suited, liquid 2-component systems are in this case difficult

to handle

• Maximum or minimum process temperatures and/or pressure

requirements must fit for all components

• Some materials are more tacky than others, which can be a

disadvantage, but sometimes also improving the handling

• Some encapsulants require vaccum lamination due to bubble

forming, other ones could be laminated at standard pressure


There Cannot be a Single Encapsulant for all Type of Modules

Just from the point of view of adhesion it is very

unlikely that one type of encapsulant is the best choice

(or even working) with every material combination.


1. Frame 2. Junction box 3. Connectors and PV wire 4. Backsheet 5. Back encapsulant 6. Solar cell 7. Front encapsulant 8. Front sheet 9. Glass

Source: SBM Solar

Organic solar module ©Fraunhofer ISE

Adhesive Testing

• Very small changes in formulation can have large

effects on adhesive performance.

• The same applies for changes in surface chemistry of

the substrates.

• It is very difficult to predict adhesion in detail,

usually it is necessary to do intensive testing.


Adhesive Screening Example


This graph shows a part of a large adhesive screening of base resins (different types/ grades/formulations) laminated on PVDF front sheet film. Generally more polar formulations perform better, but even within one resin, there are very good as well as very bad results found!

Adhesive Strength

• There is no official standard available yet, that describes what adhesive strength would be required for any of the interfaces!

• Some manufacturers use values in the range of 5-10 N/mm peel strength, others are working with 1 N/mm…

• Important is that no delamination occurs during module lifetime, as in this case e.g. humidity could accumulate in cavities and induce rapid corrosion.


Module Lifetime (1)

Solar panels should have a guaranteed performance of 20-

30 years, but manufacturers are facing difficulties

to do it. It is hardly possible to predict the real

effects of outdoor exposure on modern at high

efficiencies operating modules by doing indoor

exposure or ageing test.

There are a lot of standard test procedures that need to

be passed in order to get a certified module, but a

discussion would be to much for this presentation.

Tests include Heat-Freeze Cycles, Damp-Heat Tests, Hail

Testing, Flash Tests, Thermal aging tests,…

…


Module Lifetime (2)

Even though it is a wishful thinking that modules still deliver 80% or more power after 25 years, it is in my opinion maybe not really necessary: Research progress and costs might be reasons that people would like to replace even working modules after 15 years with modern types…



www.pvgum.eu

nolax is Taking Part in the EU-Projekt PV-GUM

http://www.pvgum.eu/

Bitumen: > 50% of all flat roofs in Europe

Goal:

• 10m x 1.2m bituminous PV membrane

• Production by Roll-to-roll process

EU FP-7 cost shared project:

• 01.11.2010 – 31.10.2013

• Consortium: 9 partners

• Total budget 11 M€, EU subsidy 6.2 M€


nolax is Taking Part in the EU-Projekt PV-GUM

www.pvgum.eu

http://www.pvgum.eu/

R2R Lamination of Flexible Solar Cells


The main scope of the PV-GUM project is to

fully integrate flexible thin film PV

modules in the bitumen-based roofing

membrane. A roll-to-roll process and new

equipment is being developed to encapsulate

the solar cell directly on the roofing

membrane.

During this project e.g. Meyer Machines GmbH

had build a prototype vacuum laminator that

is capable to do R2R laminations (no

pictures shown, the photograph shows the R2R

lamination using a non-vacuum setup) and

nolax has gained a lot of experience in

continuous and batch lamination.

Production and application


Adhesion Testing for Batch and Roll Lamination


T2 = T1 + 20°C; HS (High speed) = 6x LS (Low Speed); “+O”: add. heating without pressure WI: Water Immersion at 80°C ST: Special surface treatment of ETFE (standard is the commercially available (also surface treated) material)

Conclusions

• Adhesives are used in many parts of solar modules

• Encapsulation is one of the key adhesive applications

• Many different encapsulants are nowadays available

• Different modules and lamination procedures might require different encapsulants the formerly exclusively used EVA could be NOT suitable in some cases



Thank you for your attention.

Adhesives in Photovoltaics - European Coatings · Adhesives Used in Photovoltaics • Encapsulants...

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Transcript of Adhesives in Photovoltaics - European Coatings · Adhesives Used in Photovoltaics • Encapsulants...