Xiaohong Gu*, Chiao-Chi Lin, Yongyan Pang, Kathryn ...Engineering Laboratory Xiaohong Gu*, Chiao-Chi...
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Exposure time (day) Accelerated Laboratory Tests Using Simultaneous UV, Temperature and Moisture for PV Encapsulants, Frontsheets and Backsheets Polymeric Materials Group Engineering Laboratory Xiaohong Gu*, Chiao-Chi Lin, Yongyan Pang, Kathryn Connolly, and Joannie Chin Linking Laboratory and Outdoor Exposures INTRODUCTION The use of simultaneous multiple stresses (temperature, moisture, UV radiation) for the accelerated laboratory testing is critical to the development of reliable laboratory test methods that correlate to field test. In this study, the NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) was used for accelerated laboratory testing of PV encapsulants, including ethylene vinyl acetate (EVA), fronsheet fluoropolymers, and polyvinyl fluoride /polyester/EVA (PVF/PET/EVA) backsheet materials. The outdoor exposure was also carried out in Gaithersburg, Maryland. Multiscale chemical, optical, mechanical and morphological measurements were performed to follow changes during accelerated laboratory and outdoor exposures. The degradation mechanism and failure mode of PV materials and components were studied. ACCELERATED LABORATORY EXPOSURE DEVICE NIST Integrating Sphere-based UV Chamber Light Stability, 3 months 8000 8000 700 700 Sp Spec ectra Over ~ 3 m tra Over ~ 3 m 6000 6000 (A) EVA UV/T/RH, individually or in combination, under In Intensi tensity, y, Counts Counts 4000 4000 Laminated EVA 2000 2000 NIST-Patented 2-meter SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) • Chin et al, Review of Scientific Instruments (2004), 75, 4951; Martin and Chin, U.S. Patent 6626053 0 300 300 400 400 500 500 600 600 700 700 Wavelength Wavelength,nm nm • High UV Radiant Exposure (8400 W UV) • 95% exposure uniformity • Visible and infrared radiation mostly removed • Temperature and relative humidity around specimens precisely controlled (25-75°C; 0-95% RH) • Capability for mechanical and electrical loadings • Exposure conditions of 32 chambers can be individually controlled (UV, RH,T) Accelerated Laboratory Exposure (to study effects of simutaneous UV, Outdoor Exposure temperature and moisture on (with monitored weather degradation mechanism of PV parameters) materials/modules) Cumulative Damage Failure Mode Analysis Prediction Model To develop reliable accelerated laboratory test methods that correlate to field test. EXPERIMENTAL Materials SPHERE Exposure UV Irrdiance (200 W/m 2 , 295-480 nm) CaF 2 Substrate (for FTIR , UV-visible and AFM) Different Temperatures (25-85°C) (B) Frontsheet * Different RHs (0-75%) fluoropolymers Outdoor Exposure (C) PVF/PET/EVA backsheets Gaithersburg, MD Photostability of Frontsheet Materials (Fluoropolymer, UV/55 °C/75%RH) Ratio of C-H (1454 cm -1 )/C-F (1040 cm -1 ) UV Transmittance Spectra 100 0 6d ATR-FTIR Relative Intensity Little chemical, optical or morphological changes were 0.5 Dry condition (0% RH) RESULTS FROM LABORATORY EXPOSURE 19d 0.4 0.3 80 25d 32d 38d 60 44d 57d 76d Transmittance (%) 0.2 observed for 80d Effect of Simultaneous UV/T/RH on Degradation of EVA 40 86d 91d 0.1 frontsheet 0.0 fluoropolymers. 0 20 40 60 80 100 96d 20 0 Chemical Changes (FTIR-T) Optical Property Changes Link Indoor and Outdoor 100 200 300 400 500 600 700 800 900 Wavelength (nm) 2920 CH 2 str Typical FTIR Spectra of EVA after UV Exposure for Different Times UV Visible Spectra, UV Vs. No UV (55 C,/75%RH) UV, 55 °C, 0% RH (Indoor) Relative Changes in IR Absorbance (%) 100 80 0 day 1.0 7 day 2920 100 100 CH 2 str Ester C=O str 1241 1370 1465 1740 2850 13 day 0 day 0 day 20 day 0.8 80 7 day 7 day 13 day 1740 Ester C-O str 80 Absorbance 2850 26 day 33 day Transmittance (%) Yellow Index Transmittance (%) IR Intensity of 1710 cm -1 800 600 400 200 0 -200 13 day 20 day 26 day 1240 20 day 26 day 0.6 No UV/ 55°C /75%RH UV/55°C /75%RH 60 2920 39 day 60 60 33 day 39 day 33 day 46 day 56 day 0.4 61 day 39 day 46 day 56 day 46 day 56 day 40 40 40 CH 3 bending CH 2 bending 67 day 61 day 67 day 61 day 0.2 67 day 74 day 74 day 74 day 78 day 20 78 day 78 day 1370 20 1465 1 µm x 1 µm Simultaneous UV/T/RH on PVF/PET/EVA Backsheets 20 0.0 200 300 400 500 600 700 800 200 300 400 500 600 700 800 0 Wavelength (%) Wavelength (%) 1000 1500 2000 2500 3000 3500 4000 0 10 20 30 40 50 60 70 80 UV Exposure Time (day) Carbonyl Formation Wavenumber (cm -1 ) Yellowing Index fference Spectra FTIR Di IR Absorbance Difference UV/75%RH UV, 55 °C, 75% RH (Indoor) 1241 1370 1465 1740 2850 0 day 1710 8 0.2 UV, 75%RH 7 day Formation of carbonyl products 13 day Relative Changes in IR Absorbance (%) 100 80 0.0 20 day 26 day 55°C 6 55°C 4 -0.2 33 day 1370 1465 39 day 46 day 56 day -0.4 UV/0%RH No UV, 75%/0% RH 60 2920 61 day 2 67 day -0.6 1240 74 day UV, 0%RH 2850 40 Chain scission 78 day -0.8 Deacetylation 1740 Morphological Change (AFM) 0 Chemical Change (ATR-FTIR) No UV, 75%RH/ 0%RH 2920 0 10 20 30 40 50 60 70 80 20 0 20 40 60 80 -1.0 UV Exposure Time (day) UV Exposure Time (day) 1000 1500 2000 2500 3000 3500 4000 0 Wavenumber (cm -1 ) 0.11 1089 1028 Carbonyl formation may contribute to the yellowing. No significant 0.10 Unexposed 0 10 20 30 40 50 60 70 80 change in C-F Microstructural Changes 0.09 UV Exposure Time (day) region # 0.08 Chain Scission, Loss of 2920 cm -1 Relative Absorbance Changes at 1740 cm -1 (%) Relative Absorbance Changes at 2920 cm -1 (%) 100 100 Outdoor (Gaithersburg, MD) 0.07 10 µm x 10 µm Oxidation Before exposure No UV, 55 ° C/75%RH Absorbance 0.06 Carbonyl formation 0.05 1240 1370 UV/55°C, 0%RH, 76d 100 55°C, 75% RH, UV No UV, 55 °C, 75%RH/ 0%RH 0.04 1465 80 1740 55°C, 0% RH, UV 0.03 IR relative change IR relative change 2850 1720 0.02 2920 60 0.01 UV, 55 °C, 0%RH UV/55°C, 1666 Fresh -0.00 40 75%RH, Wavenumbers (cm-1) Chemical and morphological changes are more 76d severe with combination of UV, T and humidity. 1800 1600 1400 1200 1000 UV, 55 °C, 75%RH 20 0 10 20 30 40 50 60 70 80 0 UV Exposure Time (day) 0 100 200 300 400 UV exposure time (day) Deacetylation, Loss of 1740 cm -1 Change in Bulk Mechanical Property and Fracture Mode UV, 55 ° C/0%RH UV, 55 ° C/75%RH Outdoor (Blocked the Sun ) Tensile Strength vs. Exposure Confocal Images of Tensile Fracture Surfaces No UV, 55 °C, 75%RH/ 0%RH 100 100 µm 250 Stress-Strain Curves EVA * 250 UV, 55 °C, 0%RH 80 Tensile Strength (MPa) 85C Un- 200 Unexposed 150 ductile 100 UV/55°C/0%RH 200 # Stress (MPa) PET 105C exposed 60 120C 150 # PVF 85C,85%RH 50 UV/55°C/75%RH 1240 brittle 40 UV, 55 °C, 75%RH 100 UV,55C,0%RH 1370 EVA * 0 0 UV/55°C, 10 20 30 40 Strain (%) 1465 0 10 20 30 40 50 60 70 80 1740 UV Exposure Time (day) 20 0%RH, PET 50 2850 UV,55C,75%RH 76d The tensile fracture 2920 • Both UV and RH are important factors. 0 0 • Synergistic effect between UV and RH. 0 # PVF * 0 20 40 60 80 100 # modes of the studied 1 µm × 1 µm 100 200 300 400 78 days exposure UV exposure time (day) backsheets change Exposure Time (d) * EVA * UV/55°C, from ductile to brittle Simultaneous UV/T/RH exposure PET 75%RH, during exposure to results in greatest loss of tensile 76d UV/T/RH. strength for backsheet materials. PVF SUMMARY UV radiation was the most important factor for degradation of all studied materials. A RH/UV synergistic effect was observed for EVA and backsheet materials. A fundamental understanding of degradation mechanism under simultaneous multiple stresses is important to develop reliable standardized accelerated tests for PV materials. . 0 20 40 60 80 0 20 40 60 80
Transcript of Xiaohong Gu*, Chiao-Chi Lin, Yongyan Pang, Kathryn ...Engineering Laboratory Xiaohong Gu*, Chiao-Chi...
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Exposure time (day)
Wet condition (75% RH)
Accelerated Laboratory Tests Using Simultaneous UV, Temperature and
Moisture for PV Encapsulants, Frontsheets and Backsheets Polymeric Materials Group
Engineering Laboratory Xiaohong Gu*, Chiao-Chi Lin, Yongyan Pang, Kathryn Connolly, and Joannie Chin
Linking Laboratory and Outdoor ExposuresINTRODUCTION The use of simultaneous multiple stresses (temperature, moisture, UV radiation) for the accelerated laboratory testing is critical to the development of reliable laboratory test methods that correlate to field test. In this study, the NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) was used for accelerated laboratory testing of PV encapsulants, including ethylene vinyl acetate (EVA), fronsheet fluoropolymers, and polyvinyl fluoride /polyester/EVA (PVF/PET/EVA) backsheet materials. The outdoor exposure was also carried out in Gaithersburg, Maryland. Multiscale chemical, optical, mechanical and morphological measurements were performed to follow changes during accelerated laboratory and outdoor exposures. The degradation mechanism and failure mode of PV materials and components were studied.
ACCELERATED LABORATORY EXPOSURE DEVICE
NIST Integrating Sphere-based UV Chamber Light Stability, 3 months
80008000
700700 SpSpecectra Over ~ 3 mtra Over ~ 3 m
60006000
(A) EVA UV/T/RH, individually or in combination, under
InInten
siten
sitty,y,
Coun
tsCo
unts
40004000
Laminated EVA20002000
NIST-Patented 2-meter SPHERE (Simulated Photodegradation via High Energy
Radiant Exposure) • Chin et al, Review of Scientific Instruments (2004), 75, 4951; Martin and Chin, U.S. Patent 6626053
00
300300 400400 500500 600600 700700
WavelengthWavelength,,nmnm
• High UV Radiant Exposure (8400 W UV) • 95% exposure uniformity • Visible and infrared radiation mostly removed • Temperature and relative humidity around
specimens precisely controlled (25-75°C; 0-95% RH) • Capability for mechanical and electrical loadings • Exposure conditions of 32 chambers can be
individually controlled (UV, RH,T)
Accelerated Laboratory Exposure (to study effects of simutaneous UV, Outdoor Exposure
temperature and moisture on (with monitored weather degradation mechanism of PV parameters)
materials/modules)
Cumulative Damage Failure Mode Analysis Prediction Model
To develop reliable accelerated laboratory test methods that correlate to field test.
EXPERIMENTAL
Materials SPHERE Exposure
UV Irrdiance (200 W/m2, 295-480 nm) CaF2 Substrate (for FTIR,UV-visible and AFM) Different Temperatures (25-85°C)
(B) Frontsheet* Different RHs (0-75%) fluoropolymers
Outdoor Exposure (C) PVF/PET/EVA backsheets Gaithersburg, MD
Photostability of Frontsheet Materials (Fluoropolymer, UV/55 °C/75%RH)
Rat
ioof
C-H
(145
4 cm
-1 )/C
-F (1
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cm-1
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UV Transmittance Spectra 100
06d
ATR-FTIR Relative Intensity Little chemical,
optical or morphologicalchanges were
0.5Dry condition (0% RH)
RESULTS FROM LABORATORY EXPOSURE 19d0.4
0.3
80 25d32d38d
60 44d57d76d
Tran
smitt
ance
(%)
0.2 observed for80d
Effect of Simultaneous UV/T/RH on Degradation of EVA 40 86d
91d0.1 frontsheet 0.0 fluoropolymers.
0 20 40 60 80 100
96d 20
0
Chemical Changes (FTIR-T) Optical Property Changes Link Indoor and Outdoor 100 200 300 400 500 600 700 800 900
Wavelength (nm)
2920 CH2 strTypical FTIR Spectra of EVA after UV Exposure for Different Times UV Visible Spectra, UV Vs. No UV (55 C,/75%RH) UV, 55 °C, 0% RH (Indoor)
Rela
tive C
hang
es in
IR A
bsor
banc
e (%
)
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0 day1.0 7 day2920 100 100CH2 strEster C=O str 1241
1370146517402850
13 day 0 day 0 day20 day0.8 80 7 day7 day
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1740Ester C-O str 80
Ab
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UV/55°C /75%RH
60 292039 day 6060 33 day39 day
33 day46 day56 day0.4 61 day
39 day46 day56 day
46 day56 day
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4040CH3 bending CH2 bending 67 day 61 day
67 day61 day0.2 67 day74 day
74 day74 day78 day 20 78 day78 day
1370 201465
1 µm x 1 µm
Simultaneous UV/T/RH on PVF/PET/EVA Backsheets
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0.0 200 300 400 500 600 700 800 200 300 400 500 600 700 800 0Wavelength (%) Wavelength (%)
1000 1500 2000 2500 3000 3500 4000 0 10 20 30 40 50 60 70 80 UV Exposure Time (day)Carbonyl FormationWavenumber (cm-1) Yellowing Index fference SpectraFTIR Di
IR A
bso
rban
ce D
iffe
ren
ce UV/75%RH UV, 55 °C, 75% RH (Indoor) 12411370146517402850
0 day1710 80.2 UV, 75%RH7 dayFormation of carbonyl products 13 day
Rel
ativ
e Cha
nges
in IR
Abs
orba
nce (
%)
100
80
0.0 20 day26 day
55°C 6 55°C
4 -0.2 33 day
1370 1465 39 day
46 day56 day-0.4 UV/0%RH
No UV, 75%/0% RH
60 2920 61 day 267 day-0.6 1240 74 day
UV, 0%RH 2850 40Chain scission 78 day-0.8 Deacetylation 1740 Morphological Change (AFM)0
Chemical Change (ATR-FTIR)No UV, 75%RH/ 0%RH2920 0 10 20 30 40 50 60 70 80 200 20 40 60 80-1.0 UV Exposure Time (day)
UV Exposure Time (day)1000 1500 2000 2500 3000 3500 4000 0Wavenumber (cm-1) 0.11 1089 1028 Carbonyl formation may contribute to the yellowing.
No significant0.10 Unexposed0 10 20 30 40 50 60 70 80 change in C-F
Microstructural Changes 0.09UV Exposure Time (day) region#
0.08
Chain Scission, Loss of 2920 cm-1
Relat
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at 17
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Outdoor (Gaithersburg, MD) 0.07
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mx
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Oxidation Before exposure No UV, 55° C/75%RH
Abso
rban
ce 0.06 Carbonyl formation 0.05
12401370
UV/55°C, 0%RH, 76d
10055°C, 75% RH, UVNo UV, 55 °C, 75%RH/ 0%RH
0.04146580 1740 55°C, 0% RH, UV0.03
IR re
lativ
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IR re
lativ
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2850 17200.02292060
0.01UV, 55 °C, 0%RH UV/55°C,1666 Fresh-0.00
40 75%RH,Wavenumbers (cm-1)
Chemical and morphological changes are more 76d
severe with combination of UV, T and humidity.
1800 1600 1400 1200 1000
UV, 55 °C, 75%RH 20
0 10 20 30 40 50 60 70 80 0UV Exposure Time (day)
0 100 200 300 400 UV exposure time (day)Deacetylation, Loss of 1740 cm-1 Change in Bulk Mechanical Property and Fracture Mode
UV, 55° C/0%RH UV, 55° C/75%RH Outdoor (Blocked the Sun ) Tensile Strength vs. Exposure Confocal Images of Tensile Fracture SurfacesNo UV, 55 °C, 75%RH/ 0%RH 100100 µm250 Stress-Strain CurvesEVA* 250UV, 55 °C, 0%RH 80
Tens
ile S
tren
gth
(MPa
) 85C Un- 200 Unexposed 150
ductile 100 UV/55°C/0%RH
200#
Stre
ss (M
Pa)
PET105C exposed60 120C150# PVF85C,85%RH 50
UV/55°C/75%RH1240 brittle40UV, 55 °C, 75%RH 100 UV,55C,0%RH1370 EVA* 00UV/55°C, 10 20 30 40
Strain (%)14650 10 20 30 40 50 60 70 80 1740UV Exposure Time (day) 20 0%RH, PET50 2850 UV,55C,75%RH 76d The tensile fracture2920 • Both UV and RH are important factors. 0
0• Synergistic effect between UV and RH. 0# PVF*
0 20 40 60 80 100# modes of the studied1 µm × 1 µm 100 200 300 400 78 days exposure UV exposure time (day) backsheets change Exposure Time (d)* EVA*
UV/55°C, from ductile to brittle Simultaneous UV/T/RH exposure PET75%RH, during exposure toresults in greatest loss of tensile 76d UV/T/RH.strength for backsheet materials. PVFSUMMARY UV radiation was the most important factor for degradation of all studied materials. A RH/UV synergistic effect was observed for EVA and backsheet materials. A fundamental understanding of degradation mechanism under simultaneous multiple stresses is important to develop reliable standardized accelerated tests for PV materials.
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