NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

IEEE Photovoltaics Specialists Conference 2015 (“IEEE PVSC 42”)

Area 11: Module materials and Component Reliability

#972; 11:00-11:15 Fri, 2015/6/19, Hyatt Regency - New Orleans, Strand 10 (Level 2) 

Degradation in PV Encapsulation Transmittance: An 

Interlaboratory Study Toward a Climate-Specific Test 

David C. Miller1, Eleonora Annigoni2, Amal Ballion3, Jayesh G. Bokria4, Laura S. Bruckman5, David M. Burns6, Xinxin Chen7

Lamont Elliott8

, Jiangtao Feng7, Roger H. French5, Sean Fowler9, Xiaohong Gu10, Peter L. Hacke1, Christian C. Honeker11, 

Michael D. Kempe1, Hussam Khonkar12, Michael Köhl3, Laure-Emmanuelle Perret-Aebi13, Nancy H. Phillips6, 

Kurt P. Scott8, Fanny Sculati-Meillaud2, Tsuyoshi Shioda14, Shigeo Suga

15

, Shin Watanabe15, and John H. Wohlgemuth1

1National Renewable Energy Laboratory (NREL), 15013 Denver West Parkway, Golden, CO 80401, USA

2École Polytechnique Fédérale de Lausanne (EPFL), Rue de la Maladière 71B, Ch-2002 Neuchâtel, Switzerland

3Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstrasse 2, 79110 Freiburg, Germany

4Specialized Technology Resources, Inc. (STR), 10 Water Street, Enfield, CT, USA 06082

5Case Western Reserve University (CWRU), White 538, 10900 Euclid Avenue, Cleveland, OH 44106, USA

6The 3M Company, 3M Center, Building 235-67-15, St. Paul, MN, 55144, USA

7China National Electric Apparatus Research Institute Co., Ltd. (CEI), Guangzhou, 510663, P. R. China

8Atlas Material Testing Technology LLC, 1500 Bishop Court, Mount Prospect, IL 60056, USA, 

9Q-Lab Corporation, 800 Canterbury Road, Cleveland, OH 44145 USA

10National Institute of Standards and Technology (NIST), 100 Bureau Dr., Gaithersburg, MD 20899-8615, USA

11Fraunhofer Center for Sustainable Energy Systems (CSE), 5 Channel Center, Boston, MA 02210, USA

12King Abdulaziz City for Science and Technology (KACST), 17 King Abdullah Road, Riyadh, 11442, Saudi Arabia

13Centre Suisse d'Electronique et Microtechnique SA. (CSEM), Rue Jaquet-Droz 1, Ch-2002 Neuchâtel, Switzerland 

14Mitsui Chemicals, Inc., 580-32 Nagaura-cho, Sodegaura-shi, Chiba, 299-0265, Japan

15Suga Test Instruments Co., Ltd., 5-4-14 Shinjuku, Shinjuku-ku, 160-0022, Tokyo Japan

*Presenter & TG5 US: David.Miller@nrel.gov;

TG5 China: fengjt@cei1958.com (Leo Feng); TG5 Europe: michael.koehl@ise.fraunhofer.de; TG5 Japan: Tsuyoshi.Shioda@mitsui-chem.co.jp 

NREL/PR-5J00-64628
2

Goal and Activities for PVQAT TG5 (UV, T, RH)

•PV safety, qualification tests (61215, 61646, 61730-2) presently prescribe 

up to 137 days equivalent IEC 60904-3 AM 1.5G UV-B radiation dose. 

 This is << 25 years!

•TG5 Goal: develop UV- and temperature-facilitated test protocol(s) that 

may be used to compare PV materials, components, and modules 

relative to field service.

Applications:

•IEC 62892 (climate- and configuration-specific weathering).

1. General UV weathering test.

2. Start of “Leg 2” test series to query delamination.

 (UV→DML→HF10→DH500)

•IEC 62788-7 (PV materials and components weathering).

Accelerated aging test(s) for encapsulation, backsheet, adhesives...
3

Motivation for the kd Interlaboratory Experiment (PVQAT TG5 US)

•Knowing Ea is critical to prescribing & interpreting a UV- and T-mediated test.

•Unfortunately, Ea is not known for the UV degradation of common PV materials.

Goals for the interlaboratory experiment:

1. Quantify Ea.

 Provide a sense of the range of Ea by examining “known bad,” “known good,” 

and “intermediate” material formulations.

2. Determine if there is significant coupling between relevant aging parameters, 

i.e., UV, temperature, and humidity.

What weathering parameters must be considered in a standardized test?

3. Investigate the spectral requirements for UV light sources, 

 i.e., by comparing specimens aged by Xe-arc, UVA-340, metal-halide.

Is visible light required (e.g., enabling photobleaching)?

Arrhenius representation for rate of ∆ characteristic





 



 −





 





=

RT

n E

d

a

e

T

T k A

0
4

The Materials Used in the kd Experiment

•Discoloration of encapsulation somewhat studied in the literature:

⇒We have a sense of the general rate of degradation.

⇒Use historical (literature) and contemporary formulations.

TPU formulation: not for PV, but chosen as a reference material.

Compare peroxide used for cross-linking (module manufacture).

Compare type or use of UV absorber.

Encapsulation materials being compared in the transmittance (discoloration) experiment. 

•6 Materials examined in interlaboratory study:

INGREDIENT DESCRIPTION MAKER

Elvax PV1400 EVA resin, 33 wt% Vac E. I. du Pont 100 100 100 100 100 N/A

Z6030 silane primer, gamma-methacryloxy propyl 

trimethoxysilane Dow-Corning Corp. 0.25 0.25 0.25 0.25 0.25 ?

TBEC curing agent, OO-Tertbutyl-O-(2-ethylhexyl)-peroxycarbonate Arkema Inc. N/A 1.5 1.5 1.5 1.5 ?

Lupersol 101 curing agent, 2,5-Bis(tert-butylperoxy)-2,5-

dimethylhexane Arkema Inc. 1.5 N/A N/A N/A N/A ?

Tinuvin 329 UV absorber, benzotriazole type BASF Corp. N/A N/A N/A 0.3 N/A ?

Cyasorb UV-531 UV absorber, benzophenone type Cytec Industries Inc. 0.3 0.3 0.3 N/A N/A ?

Tinuvin 770 hindered amine light stabilizer (HALS) BASF Corp. 0.1 0.1 0.1 N/A N/A ?

Tinuvin 123 non-basic aminoether-hindered amine light 

stabilizer (NOR-HALS) BASF Corp. N/A N/A N/A 0.1 0.1 ?

Naugard P anti-oxidant (AO), phosphite containing Chemtura Corp. 0.2 0.2 N/A N/A N/A ?

Designation

(Note)

EVA-A

(known bad, 

"slow cure" )

EVA-B

(improved, 

"fast cure")

EVA-C

(known

good)

EVA-D

(modern)

EVA-E

(no UV

absorber)

TPU

(known

bad)

MASS {g} 
5

Interlaboratory Effort Enables a Wider Range of Study

•Indoor aging is expensive. No institution has all the resources or bandwidth to 

apply the complete set of parameters TG5 wanted to examine.

•Discoloration studied at (14) -volunteer- institutions. 

• Example: compare similar instrument models (e.g., Ci5000 & QSUN XE3).

• Rate of indoor degradation will ultimately be compared against field data to 

determine site-specific acceleration factors.

• Outdoor data will verify the validity of indoor weathering.

• Standard condition (60°C chamber ambient) to compare a broad variety 

of light sources.

• Baseline irradiance of 1.0 W⋅m-2⋅nm-1 at 340 nm for UV sources.
6

Details of the kd Methods and Experiment: Encapsulation Transmittance Test

•Silica/polymer/silica coupon specimens measured using a spectrophotometer 

 (with integrating sphere).

•Measure at specimen center (anaerobic, minimal O2) and periphery (aerobic).

•Analyze: solar-weighted transmittance, yellowness index, and UV cut-off λ. 

User summary:

• Geometry: silica/polymer/silica (3.2 mm/0.5 mm/3.2 mm).

• Size: 2” x 2”.

• Quantity: 3 replicates of 6 materials (pre-conditioned), and 1 reference (not pre-conditioned).

• Aging: 0, 15, 30, 45, 60, 75, 90, 120, 150, 180 cumulative days (indoors).

or 0, 1, 2, 3, 4, 5 years (outdoors).

• Measurements (non-destructive): repeatedly age and measure at each laboratory/test site.

Specimen in sample holder for 

indoor aging at NREL.

Specimens on outdoor rack, 

aging in Golden, CO at NREL.

Visual appearance of the NREL UV 

Suitcase aged specimens at 180 days. 

5 cm 

EVA-A EVA-B EVA-C EVA-D EVA-E TPU
7

UVA-340 vs. Xe: A Comparison of τrsw With Age

Change in τrsw with H for the NREL aged transmittance coupons (center measurements). 

The data points have been connected to guide the eye. 

This presentation: H shown for 295≤λ≤360 nm.

•Representative solar weighted transmittance (τrsw) evaluated for 300≤λ≤1250 nm. 

UVA-340 

fluorescent UV source Xe UV source

•TPU affected more in Xe (∆τrsw -5.1%) than UVA-340 (∆τrsw -3.4%). 

•EVA-A affected continuously for Xe, delayed for UVA-340. Same ∆τrsw -2.3%. 

•EVA-C improved (∆τrsw 1.0% for UVA-340)!
8

•Yellowness index (YI) calculated from measured τh. 

•Repeatability: YI (0.27) < τrsw (0.63). 

 YI is overt indicator of degradation trends.

UVA-340 vs. Xe: Insights From YI 

UVA-340 fluorescent 

UV source

Xe UV source

•Many instances of ~linear change with time.

•Inflection (∆slope) in some profiles: EVA-A (UVA-340); EVA-C (UVA-340); TPU (Xe).

•Initial yellowing to stable-state for EVA-D (UVA-340 & Xe).

⇒Effects of aging are dominated by interactions between additives, not base material!

Change in τrsw with H for the NREL aged τ coupons (center measurements). 

The data points have been connected to guide the eye.

This presentation: data shown for average of 3 replicates, with error bars for 1 S.D. 
9

UV spectral transmittance for the NREL aged τ coupons (center measurements). 

–u (solid lines) indicates unaged specimens; –a (dashed lines) indicates specimens aged for 6 months.

UVA-340 vs. Xe: Insights From the τ Spectra

•EVA-A, EVA-D, and TPU show a significant rounding of the UV cut-off (shifted ↓, right) 

and increased YI corresponding to ∆visual appearance. 

•Likely explained by chromophore formation (from peroxide, AO, and UV absorber).

UVA-340 fluorescent UV source Xe UV source

•For EVA-B, EVA-C, EVA-E, the UV cut-off wavelength is instead decreased (shifted left) 

and there is an increase in τ. τrsw is increased for EVA-C about ~350 nm.

•Likely explained by the loss of additive(s) with age.

•EVA-A: is UV absorber lost for UVA-340 source? VIS required (photobleaching for Xe)? 
10

Methods of Analysis for the kd Data 

Degradation rate comparison, applied to 

compare UVA-340 and Xe sources . 

Data shown for the NREL aged τ coupons

(center measurements).

•Verify what data may be analyzed. 

e.g., delayed loss of formulation additive.

•Here: consider inflections in ∆τrsw or YI data. 

•Arrhenius analysis used for different 

applied temperatures. (Ea ∝ m⋅R).

•EVA undergoes a melt transition at ~60°C. 

•Example: what kH is required map the 

UVA-340 data parallel to the Xe data? Xe

UVA

H

slope

slope k 340

=

•Degradation rate (linear) or shift 

factor (logarithmic) comparison may be 

used to examine different test 

conditions, e.g., UV sources.
11

[ ]

slope[Xe]

slope UVA kH

340 =

Data shown for NREL aged EVA-A and TPU coupons

(center measurements).

Fowler, “Developing Steady State Exposure Conditions in an ASTM G154 Fluorescent UV Test Chamber for 

Backsheet Materials”, Proc. NREL PVMRW, 2014.

•295≤λ≤360nm may be best criteria between Xe and UVA-340, 

 because UVA-340 provides limited emission 360≤λ≤400 nm.

UVA-340 vs. Xe: Insights From kH Analysis

•kH=0.08 for EVA-A.

•EVA-A not affected early for 

 UVA-340.

•Different aging mechanisms initially 

dominate for UVA-340 or Xe.

•kH=0.6 for TPU (same order of magnitude).

•TUVA-340 ~55°C; TXe ~63°C.

∆T~10°C ⇒ 1.4-1 ⇒ 0.7.

Pickett et. al., Polym. Deg. Stab., 93 684-691 (2008). 

•kH=3.4, 1.1 for EVA-C, EVA-D.
12

•Effect of T examined directly at 3M: same irradiance, RH applied using 

three similar chambers (Ci5000, Xe lamp with Right Light filter).

Effect of Temperature Stands Out in Early Comparison

•Strong effect of aging as 

T is increased.

•Exception: EVA-E 

(no UV absorber). ∆τ~0.

•T coupling anticipated from field 

observation, e.g., increased 

discoloration at local module 

hot spots.

•Ea on order of ~60 kJ⋅mol-1

(0.6 eV) estimated for EVA-A 

from interim experiment. 
13

Summary

•”kd” interlaboratory experiment presently being conducted to provide a 

quantitative basis for UV weathering tests.

Interim results:

•Material specific aging behavior ⇒ discoloration follows foremost from the 

formulation additives (Lupersol 101, Naugard P, Tinuvin 770 or Tinuvin 123), 

rather than the base resin.

Regarding Xe and UVA-340 sources: 

•Substantial ∆λcUV, greater ∆YI for EVA-A aged with UVA-340. kH of 0.08.

kH of 0.6 (0.7 ideal), 3.4, and 1.1 observed for TPU, EVA-C, and EVA-D.

•∆ slope (apparent degradation rate) observed for specific combinations of 

UV source and test materials. 

⇒Xe and UVA-340 sources may not always be applied equally for 

accelerated aging of encapsulation.

•Ea on the order of ~60 kJ⋅mol-1 (0.6 eV) is estimated for EVA-A.

•Test T, e.g. 70, 80, or 90 °C, presently being evaluated for UV tests. 
14

Acknowledgements

There has been fantastic participation in TG5. 

Thank you to the many participants for your ongoing support!!! 

•If interested in TG5 activites or the experiments, please contact 

the corresponding regional leader. (See title slide)

•Stay tuned for examination of encapsulant attachment strength. 

(PVQAT TG5 kd “part II”).

NREL STM campus, Dennis Schroeder

This work was supported by the U.S. 

Department of Energy under Contract 

No. DE-AC36-08-GO28308 with the 

National Renewable Energy Laboratory.

Your questions and feedback are 

much appreciated! Please help me 

to cover the important details & 

perspectives.
15

Extra Slides for Reference...
16

Use of REDCap Database for Transmittance Data

• -A LARGE QUANTITY- of transmittance data will be generated for experiment. 

• Case Western Reserve University volunteered the REDcap database to TG5. 

• REDCap comes from the medical research industry.

Benefits: Ensures designed experiments, high data capacity, simultaneous user 

access, automated data quality verification.

• REDCap allows users to view and analyze results in real-time.

Home screen for https://dcru.case.edu/redcap

Transmittance results will be uploaded to 

REDCap using an Excel template file.
17

τrsw: Most Materials More Affected at Center in kd Experiment

•Loss in τrsw less significant at the specimen periphery.

 Example: EVA-A. Significant heterogeneity.

 Exceptions: EVA-C, TPU. Affected relatively uniformly throughout specimen.

•In EVA, a photobleaching effect typically occurs at periphery, but is limited by 

 rate of O2 diffusion to interior. 

Change in transmittance for the NREL UV Suitcase aged specimens. 

Data points connected to guide the eye.

UVA-340 

fluorescent UV source: 

specimen periphery

UVA-340 

fluorescent UV source: 

specimen center
18

Comparing UVA-340 and Xe sources

UV spectral transmittance of the encapsulation materials examined. The subscript 

–u (solid lines) indicates the data for unaged specimens, while the subscript –a 

(dashed lines) indicates the data for specimens aged for 6 months in the NREL UV 

Suitcase (60°C/~7%RH). Results for specimen-periphery (top) and -center 

(bottom).

UV spectral transmittance of the encapsulation materials examined. The subscript 

–u (solid lines) indicates the data for unaged specimens, while the subscript –a 

(dashed lines) indicates the data for specimens aged for 6 months in the NREL 

Ci5000 (60 °C/50%RH). 
19

Major changes: (most overt to least affecting differences between weathered samples)

•The spectral bandwidth was greatly increased (∆λcUV=-64 nm) for EVA-A for the UVA-340 light source but 

not for Xe (∆λcUV=-6 nm). This implies the UV absorber was uniquely degraded or depleted for the 

fluorescent light source. The result is consistent with the UV-VIS fluorescence spectra observed for EVA-A, 

which could be distinguished between the different weathering chambers.

•A more severe rounding of the cut-on profile was observed for EVA-A aged usingUVA-340 (∆YI=13.7) then 

for Xe (∆YI=7.9). A more severe rounding of the cut-on profile was observed for TPU aged using Xe

(∆YI=17.2) then for UVA-340 (∆YI=15.6). Increased yellowness implies the formation of optically absorbing 

chromophore species. These results suggest the action spectrum for TPU includes the full range of UV 

wavelengths (i.e., 300≤λ≤400 nm as supplied by a Xe source). The difference for EVA-A may result from a 

photobleaching process, enabled by the longer wavelengths of UV light present in Xe.

•EVA-C was more affected by UVA-340 (∆λcUV=-11 nm for UV Suitcase ) than Xe (∆λcUV=-5 nm for Ci5000) 

spectrum. Transmittance reduction from discoloration (∆YI=2.1 and ∆YI=1.1) was outweighed by the 

increased spectral bandwidth, increasing transmittance (τrsw 0.6% and τrsw 0.4%) for EVA-C in UVA-340 and 

Xe, respectively.

Lesser changes:

•The spectral bandwidth was slightly more increased for EVA-D at its periphery (∆λcUV=-3 nm) than at its 

center (∆λcUV=-1 nm). EVA-D, however, was more discolored at its center (∆YI=2.7 and ∆YI=2.9) than at its 

periphery (∆YI=0.8 and ∆YI=0.8) , for UVA-340 and Xe, respectively. 

Comparing UVA-340 and Xe sources
20

Summary of the results for samples weathered in the NREL UV Suitcase (UVA-340) and NREL Ci5000 (Xe) chambers. 

Comparing UVA-340 and Xe sources
21

kd Experiment Examines Relevant Source Spectra

Overlay of representative common artificial UV sources, relative to the AM1.5 global spectrum.

•Will compare Xe, UVA-340, M-H, and terrestrial light sources for all 

formulations examined.

•Depending on specimen’s action spectrum (damage susceptibility), 

the UV source (e.g., 360-400) could render different results.

•Other base materials or 

components (backsheet) 

may have even stronger 

spectral dependence than 

encapsulation.

•NIST SPHERE experiment 

(passband filters) will 

provide additional insight.

•Also method: ASTM G178.
22

Blacklight image (for E @ 254 nm) for specimens aged for 

180 days in NREL UV Suitcase (UVA-340 source). 

•Anaerobic center and aerobic periphery 

are readily distinguished with a black light.

•Different extent of regions observed on 

EVA’s. TPU is homogenous.

Blacklight image (for E @ 365 nm) for specimens aged for 

180 days in NREL Ci5000 (Xe lamp). Samples are arranged 

in columns: EVA-A, -B, -C, -D, -E, and TPU. 

•Minimal UV-VIS fluorescence observed 

at periphery relative to specimen interior.

Examples of the test configurations for the UV-VIS fluorescence spectrophotometer. The examination region can be confirmed from the spot, 

which appears different (based on its fluorescence) at the center or periphery of the specimen. The size and shape of the discolored 

anaerobic region can be confirmed when the excitation beam is placed at the edge of the silica superstrate.

•The geometry and effect in the specimens 

can be confirmed during the use of the 

fluorescence spectrometer.

Fluorescence Spectroscopy: Additional Insights
23

Effects of Additives in EVA and Polymers

•Representative (normalized) results shown for ~0.01% wt. concentration additive in hexane.

•UV absorbers often feature double absorptance peaks; Anti-Oxidants often characterized by 

absorptance at even shorter wavelengths. 

•UV absorbers often characterized by single thermalization emission peak at ~400 nm, e.g., I168. 

•AOs may characterized by more specific excitation & emission peaks, although a few common 

peak sets were observed here. 

•Fluorescence signature may be very different when multiples additives are present 

(and interacting). 

Optical absorptance spectra for UV absorbers and AOs used in the 

TG5 EVA formulations, including: Cyabsorb UV 531, Tinuvin 123, 

Tinuvin 328, Tinuvin 329, Tinuvin 770; butylated hydroxytoluene

(BHT), and Naugard P. 

Representative fluorescence spectra (excitation on left; emission on 

right) for AOs often used in contemporary polymers, including 

butylated hydroxytoluene (BHT), Irgafos 168, Irganox 245, Irganox

1010, Irganox 1076, and Naugard P. Some additives have multiple 

fluorescence peaks. 
24

Transmittance: Comparison to Historic & Outdoor Data

Localized discoloration of EVA 

(known formulation in module) at the APS site.

Wohlgemuth et. al., Proc IEEE PVSC, 2013, 3260-3265.

•Same & similar formulations in kd

experiment deployed at APS in 1996.

⇒Conclusion: discoloration resulted from 

additive interactions (as in kd experiment). 

•Location specific results 

(center vs. periphery) as in kd experiment. 

Assessing degradation from change in YI.

Reid et. al., Proc SPIE, 2013, 8825-7.

•Historically, yellowness index has been 

used to compare between indoor- and fieldaged encapsulation.

•We examine 2 of the classic formulations, 

using a modern version of the same glass.

•∆YI, EVA-A: ~8 (Xe, 60°C) vs. ~55 (Xe, 70°C).

•TG5 will determine location specific 

acceleration factor for Cleveland, 

Golden, Miami, Phoenix, & Riyadh.