Characterization of EVA degradation

processes in Si-based PV modules

by means of spatially-resolved luminescence spectroscopy

1

Degradation of PV modules

Typical construction of a

c-Si PV module

Frontglass EVA

Cells EVA

Backsheet

2

1Degradation scenarios of PV modules Degradation of electrical properties during DH2

Main problems of service life prediction of PV modules:

Low sensitivity of measurement methods

Non-destructive characterization necessary

Occurrence of many different failure modes

1 after Köntges, et al. (2014): Report IEA-PVPS T13-01:2014: 2 Köhl (2014): 29th EUPVSC 5DP.1.3.

A very brief history of Polymer Luminescence

3

Ashby 1961 - Oxyluminescence from polypropylene

Charlesby 1965 - The Identification of Luminescence Centres in Polyethylene and other Polymers

But: Complex changes of luminescence could not be correlated to the degradation of

physical properties!

Since 2008 - Polymer Luminescence at AG Röder: Theory

4

P

Steffen 2016 - Density of States Model of Polymer Luminescence

Polymer Luminescence at AG Röder: Application to PV modules

• Identification spatial luminescence patterns in mini modules and full-scale PV modules after – accelerated aging

– outdoor weathering

• Comparison PV modules of different manufactures to access performance/material differences

• Parallel aging under different conditions (Multiageing Chamber)

• Categorization crack damage of PV modules

• Correlation EVA-Luminescence with mechanical parameters

• Correlation EVA-Luminescence with crosslinking

5

Luminescence upon accelerated aging

UV-laser

spectrometer

lens UV-filter

fiber

Sketch of the optical setup:

dichroic mirror

Luminescence shows a correlation to the aging duration

from the beginning of accelerated aging.

6

sample

2D Luminescence scanning of commercial PV modules

Setup for 2D-photoluminescence detection (scanning) of embedding polymers in PV-Modules

Detail: Scan head with excitation laser and collecting optics

7

16

Luminescence field inspection device

First step of analysis: integration of luminescence spectra and graphical presentation of the location dependent total intensity

Intensity

[a. u.]

Wavelength [nm]

Space on the Scan-axis [a. u.]

Presentation with space-resolution via integration of the spectra

Line-Scan – Total intensity Line-Scan – with spectral information

2. Spatially-resolved measurements on PV-Modules

9

10

3. Luminescence patterns upon accelerated aging

2000h H

2400h DH

4000h DH

1000h UV

1500h UV

2750h DH + 700h UV

Dry heat (H), damp-heat (DH)

Accelerated aging without UV

Accelerated aging with UV

and DH aging followed by UV (DH+UV)

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a) C1

b) C2

c) C4

d) C5

e) C6

f) C7

3. Influence of different materials (accelerated DH aging)

Only manufacturer C1 used SC-EVA Other manufactors used fast/ultrafast cure EVA

DH aging results in similar patterns in most modules

Luminescence of manufacturer C2 does not reduced to zero.

Only manufacturer to use Ce-containing frontglass!

a) C1

b) C2

c) C4

d) C5

e) C6

f) C7

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3. Influence of different materials (accelerated UV aging)

UV aging results in similar patterns in most modules The patterns are fundamentally different from DH aging

Luminescence patterns after outdoor weathering

c-Si- PV-Modules from

7 companies

(data anonymized)

Tropic, Indonesia Desert, Negev

Israel

Moderate,

Germany:Cologne Alpin, Germany:

Zugspitze, 2650 m

4 different weathering sites:

15

14

a) C1

b) C2

c) C3

d) C4

e) C5

f) C6

g) C7

Similar patterns are observed in all modules (e.g. Isreal, 3 years)

Luminescence patterns after outdoor weathering

Patterns are most similar to those of accelerated aging with UV Modules of manufacturer C2 deviate from others (as observed for UV aging). Patterns (and spectra) are distinct for:

1) Exclusive thermal aging

2) UV aging or outdoor weathering

Indonesia 2y Indonesiia 3y

Indonesia 6y

Negev2y

Negev 3y

n.a.

Cologne 2y

Cologne 3y

Cologne 6y

Zugspitze 2y

Zugspitze 3y

n.a.

PV modules of manufacturer C5

5. Influence of outdoor weathering time

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Luminescence intensity upon outdoor weathering

16

Influence of temperature Influence of (UV) radiation

The luminescence intensity correlates with the duration of exposure,

the average temperature and the UV dose

In: Indonesia, Is: Israel, K: Cologne, Z: Zugspitze

Evaluation of the luminescence intensity on the center of the cell (No change of spatial

patterns, no visible yellowing):

Inte

nsity [a

.u.]

Arrhenius-temperature-dose UV-dose [kWh]

Inte

nsity [a

.u.]

Activation energy [kJ/mol]

Corr

ela

tio

n c

oe

ff.

Company Frontglass Encapsulation Cell Backsheet

C1 G1 Solar glass E1 SC EVA poly R1 PVF 170 µm

C2 G2 With Cer E2* FC EVA poly R2* TPT 290 µm

C3 G3 Cer-free E3 UFC EVA poly R3 TPT 350 µm

C4 G4 AR-glass E3 UFC EVA mono R3 TPT 350 µm

C5 G5 Solar glass E4 FC EVA poly R4 PVF 290 µm

C6 G6 Solar glass E5 FC EVA poly R4 PVF 290 µm

C7 G7* n.a. E6* n.a. poly R5* n.a.

5.Correlation of the luminescence intensity of different modules

A good correlation exists for

modules by manufacturers C5 and C6

The materials used by manufacturers C5 and C6 are similar.

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Intensity

Modules by manufacturer C2

show no correlation to all others

1) The luminescence intensity is specific for the

material and the weathering location.

2) A good correlation between modules of

similar composition is observed

3) Differences are observed for different materials

Different materials show differences

In luminesence

c-Si modules from different weathering sites, measurement after 2 years

Application of Luminescene to damage characterization

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transport damage, dendritic fracture

exposure damage (Zugspitze)

„normal“ outdoor exposure

Photoluminescence intensity

Overlay of EL and FL Electroluminescence image (EL)

Luminescence along cracks shows similar reduction like at the cell edges

Schlothauer J., Jungwirth S., Röder B., Köhl M.: Photovoltaics International, 10 (2010) 149-154

7. Evaluation of damages

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Conclusions

• Luminescence intensity is an indicator for ageing time indoor and outdoor

• In PV modules the luminescence is distributed inhomogeneously

• Diffusion processes (mostly O2 ) enable destruction of chromphores in the polymer resulting in

decreased luminescence intensity

• The shape of the spectrum is different for DH and UV ageing

• Complex behaviour of the spectra after combined ageing procedures esp. outdoors

• UV aging and outdoor weathering cause similar luminescence patterns

• Spectral effects of UV and DH ageing can be separated also in case of outdoor weathered

modules

• The EVA luminescence of different manufacturers correlates for different ageing methods, indicating that luminescence can be used to monitor the condition of the encapsulating EVA in PV modules

• Using minimized multi ageing chamber developed at HU Berlin different ageing parameters can

be applied to one mini-module and differences in degradation behaviour can be analysed

• Luminescence can be used for crack inspection (e.g. age of cracks)

• It can be used for PV module inspection: in- and out-door

THANK YOU FOR YOUR ATTENTION

Financial support: Bundesministerium für Wirtschaft Und Energie (FKZ 0325716E) Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit (FKZ 0329978) Sponsored by the industrial partners: Scheuten Solar, Schott Solar, Solarfabrik, Solarwatt, Solar World, Solon

Bernd Litzenburger Norbert Lenck Peter Bentz