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Both nanoindentation and Raman spectroscopy suggest that high humidity accelerates the photodegradation of backsheet materials in the presence of UV radiation.
Cross-sectional chemical and mechanical profiling using nanoindentation, Raman spectroscopy, and AFM QNM is an effective tool to understand the interfacial property changes of multilayer backsheet films during UV degradation.
Cross-Sectional Chemical and Mechanical Characterization of a
Multilayer Polymeric Backsheet During UV Exposure Peter J. Krommenhoek, Chiao-Chi Lin, and Xiaohong Gu
Polymeric Materials Group, National Institute of Standards and Technology, Gaithersburg, MD, USA
Introduction
In the field, degradation of the polymeric multilayer backsheet can
be detrimental to the efficiency of the photovoltaic (PV) module,
causing catastrophic failure and safety concerns. This is a costly
problem for industry due to the lack of comprehensive knowledge of
multilayer system during weathering.
In this study, cross-sectional characterization techniques were
used to provide structural and property changes of a multilayered
backsheet before and after exposure to accelerated environmental
conditions. New insight into the failure of PV polymeric materials
during accelerated aging is presented.
NIST-Patented 2-meter SPHERE*
Materials and Measurements
* Chin et al, Review of Scientific Instruments (2004), 75, 4951; Martin and Chin, U.S.
Patent 6626053.
Simulated Exposure Characterization • Nanoindentation with continuous stiffness
measurement technique
• Atomic force microscopy with quantitative
nanomechanical mapping
• Laser scanning confocal microscopy
• Raman spectroscopy with laser l = 785 nm
• Sample shape/size: Free standing PET/PET/EVA
backsheet, 19 mm in diameter
• UV exposure: PET outer layer side face to light
• Exposure conditions: Simultaneous UV irradiation,
temperature, and humidity.
• T = 85 °C
• R.H. = 5% (dry) or 60% (wet)
Nanoindentation Depth Profiling of Modulus Change in PPE During UV Exposure
Confocal image of residual indents
Epoxy
Encapsulant/EVA PET core PET
outer
Epoxy
Fresh
• Section of backsheet
was embedded in epoxy
• Sample was faced with
diamond knife using
cryo-microtomy
Cross-Sectional Raman Spectroscopy of
UV-Exposed PPE
Gradient chemical degradation
has been observed on both PET
layer and EVA layer
The wet condition exposure
causes more severe chemical
degradation and yellowing.
PET outer layer
Fresh
Fresh
PET outer
layer surface
PET outer
layer surface
PET core
side
PET core
side
Fresh
Fresh
PET core
EVA side
PET outer
layer side
EVA side
PET outer
layer side
EVA outer layer
surface
pigmented
EVA side
EVA outer layer
EVA outer
layer surface
pigmented
EVA side
Fresh Fresh
Pigmented EVA
EVA outer
layer side
EVA inner
layer side
EVA outer
layer side
EVA inner
layer side
EVA inner layer
Fresh
Fresh
EVA inner
layer surface
pigmented
EVA side
EVA inner
layer surface
pigmented
EVA side
EVA side PET side
Fresh
UV
/T/D
ry (
5%
RH
) U
V/T
/Wet
(60
% R
H)
PET
outer
PET
core outer pigmented
inner epoxy epoxy EVA EVA EVA
High humidity is detrimental to the structure of the
interfacial layers between PET outer/PET core and PET
core/EVA inner.
PE
T O
ute
r P
ET
Co
re
PET Outer/Core Interface 67 d, UV/T/60 % R.H. 67 d, UV/T/5 % R.H. Fresh
EV
A
inn
er
PET Core/EVA Inner Interface 67 d, UV/T/60 % R.H. 67 d, UV/T/5 % R.H. Fresh
PE
T C
ore
Effects of UV Degradation on Interfaces of PPE
Multilayers: Modulus Mapping by AFM QNM
0 50 100 150 200 250 3000
1
2
3
4
5
Mo
du
lus
(GP
a)
Distance (micron)
Fresh
67 d, UV/T/5 % R.H.
67 d, UV/T/60 % R.H.
Fresh 67 d, UV/T/5 % R.H. 67 d, UV/T/60 % R.H.
Ep
ox
y
PE
T O
uter
Epoxy/PET Outer Interface
After exposure, PET outer surface is seen degraded with
increased presence of pigment closer to air/PET interface.
PET/PET/EVA (PPE)
PET outer/core Interface The moduli in PET outer layer increased after exposure,
but no obvious changes observed in PET core.
EVA moduli substantially increased after exposure in high
R.H. due to moisture effect.
PET core/EVA inner Interface
(Note: Due to
elastic recovery
after
indentation, EVA
indents are not
visible.)
Cry
o-m
icro
tom
y
Results and Discussion
Conclusions
engineering la
bo
rato
ry