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